Investigation of Acoustical and Mechanical

Properties of Epoxy Based Natural Fibre Composites

Muhammad Khusairy Bin Bakri

(4222644)

A thesis submitted in fulfilment of the requirement for the degree of

Master of Engineering (by Research) (MEng)

Faculty of Engineering, Computing & Science

Swinburne University of Technology, Sarawak Campus

August/2015

i

ABSTRACT

Noise is unwanted sound, which is harmful to our auditory system. Sound absorbing

materials are used in reducing, eliminating or insulating the noise. Existing sound

absorbing materials are mostly made from asbestos and synthetic materials, which

are known to be harmful for the environment and health. Thus, this has motivated the

initiative to develop alternative materials. In this research, natural waste product such

as banana, luffa and oil palm fibre were blended with epoxy to form natural fibre

composites. Mechel, and Delany and Bazley empirical models were used as a

preliminary theoretical analysis in order to gain an understanding of the effect of

thickness, fibre size, flow resistivity and types of fibre on sound absorption. The

modelling showed that banana, luffa, and oil palm have almost similar sound

absorption behaviour. Based on the empirical models, it showed that with increasing

fibre diameter, the flow resistivity decreased, but the sound absorption still increased

as long as the porosity is high. Furthermore, with increasing material thickness, an

increase in the sound absorption is expected. Lastly, the modelling results showed

that the sound absorption, increased with increasing frequency. In the case of

acoustical and mechanical characterisation, it was revealed that the fibre loading and

alkaline treatment have a significant effect. It was found that the sound absorption

coefficient for the composites tested increased with increasing fibre loading. The

result also showed that the treated fibre gives better sound absorption compared to

the untreated counterpart. The treatment caused removal of certain wax, oil, and

certain structure (i.e. such as hemicellulose, cellulose, and lignin), which improved

the tensile and yield strength of the natural fibre composite materials. Chemical

treatment caused break, stretch or bend in the H, OH and CH bond structure of the

fibres which were validated by using Fourier transform infrared spectral analysis.

Furthermore, the changes on the surface roughness and the structure of the fibre can

be seen using scanning electron microscope and light/optical microscope. It showed

that the treated fibre have a rougher surface and larger lumen structure than the

untreated fibre. Based on the result of acoustical and mechanical test, it was

concluded that the novel composites developed would find new applications for

agricultural waste and reduced oil-derived polymers.

ii

ACKNOWLEDGEMENT

First of all, I am grateful to The Almighty God for establishing me to complete this

research and thesis. I wish to express my sincere thanks to Swinburne University of

Technology, Sarawak Campus and Universiti Malaysia Sarawak for providing

me with all the necessary facilities.

I would like to express my gratitude to my supervisors and collaborators, Dr.

Elammaran Jayamani, Dr. Soon Kok Heng, Dr Basil Wong, Professor Dr. Sinin

Hamdan and Dr. Md Rezaur Rahman for their support and supervision in pursuing

this research and provided me with an extended vision of the research. Furthermore,

their valuable comments, helpful advice, technical guidance and valuable feedback

throughout the development of the work especially related to the research.

I would like to express my gratitude to Swinburne University of

Technology, Sarawak Campus for providing the financial support for my

international conferences. Furthermore, I would like to express my gratitude to

Universiti Malaysia Sarawak for the collaboration in the international journal

publications and providing me with their lab facilities. I would also like to thank the

lab technician Mr. Fadillah Johari, Mr. Thomas Manggie and other Staff

Member at Swinburne University of Technology, Sarawak Campus and

Universiti Malaysia Sarawak for their provision of the experimental facilities and

technical assistance.

Finally, I wish to express my special thanks to My Family for their great

encouragement throughout my research, thesis and supporting funds, as well as my

friends and counterparts, Mr. Toh Ming Hui, Mr. Kong Wai Loon, Mr. Tan Yi

Zhou, Mr. Kelvin Yong, Mr. Mark Tee Kit Tsun, Mr. Aaron Isaac Kuek, Mr

Jackson Wong, Mr. Akshay Kakar, Mr. Win Ko Min, Dr. Lee Ker Chia, Mr.

Mohd Elfy Mersal, Mr. Izzul Islam Sazali, Mr. Abang Adib Izzaniq Abang

Iskandar, and Mr. Mohamad Azhar Zulkifli Handeri for their continual supports

and encouragement.

iii

AUTHOR DECLARATION

I hereby declare that this thesis entitled “Investigation of Acoustical and

Mechanical Properties of Epoxy Based Natural Fibre Composites” is the result of

my own research work except for quotations and citations which have been duly

acknowledged. I also declare that is, it has not been previously or concurrently

submitted for any other master students at Swinburne University of Technology

(Sarawak Campus).

Name: Muhammad Khusairy Bin Bakri

ID: 4222644

Date: 30th

August 2015

iv

LISTS OF JOURNAL PUBLICATIONS AND CONFERENCES

Journal Papers:

Bakri, MKB, Jayamani, E, Kok Heng, S, & Hamdan, S, 2015, ‘Reinforced

Oil Palm Fiber Epoxy Composites: An Investigation on Chemical Treatment of

Fibers on Acoustical, Morphological, Mechanical and Spectral Properties’, Materials

Today: Proceedings, vol. 2, no. 4-5, pp. 2747-2756.

Bakri, MKB, Jayamani, E, Kok Heng, S, Hamdan, S, & Kakar, A, 2015, ‘An

Experimental and Simulation Studies on Sound Absorption Coefficient of Banana

Fibers and Their Reinforced Composites’, Journal of Biomimetics, Biomaterials and

Biomedical Engineering, vol. 0, no. 0, pp. 0. (Accepted and Reviewed – Waiting for

Publication)

Bakri, MKB, Jayamani, E, Hamdan, S, Rahman MR, & Kakar, A, 2015,

‘Fundamental Study on the Effect of Alkaline Treatment on Natural Fibers Structures

and Behaviours’, ARPN Journal of Engineering and Applied Sciences, vol. 0, no. 0,

pp. 0 (Accepted and Reviewed – Waiting for Publication)

Jayamani, E, Hamdan, S, Rahman, MR, Kok Heng, S, & Bakri, MKB, 2014,

‘Processing and Characterization of Epoxy/Luffa Composites: Investigation on

Chemical Treatment of Fibers on Mechanical and Acoustical Properties’,

BioResources, vol. 9, no.3, pp. 5542-5556.

Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2014, ‘Comparative

Study of Dielectric Properties of Hybrid Natural Fiber Composites’, Procedia

Engineering, vol. 97, no. 1, pp. 536-544.

Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2014, ‘Investigation

of Fiber Surface Treatment on Mechanical, Acoustical and Thermal Properties of

Betelnut Fiber Polyester Composites’, Procedia Engineering, vol. 97, no. 1, pp. 545-

554.

Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2015, ‘Study of

Sound Absorption Coefficient and Characterization of Rice Straw Stem Fibers

Reinforced Polypropylene Composites’, BioResources, vol. 10, no. 2, pp. 3378-3392.

v

Jayamani, E., Hamdan, S, Rahman, MR, Bakri, MKB, & Kakar, A, 2015,

‘An Investigation of Sound Absorption Coefficient on Sisal Fiber Poly Lactic Acid

Bio-Composites’, Journal of Applied Polymer Science, vol. 132, no. 34, pp. 1-8.

Jayamani, E, Hamdan, S, Rahman, MR, & Bakri, MKB, 2015, ‘Dielectric

Properties of Lignocellulosic Fibers reinforced Polymer Composites: Effect of Fiber

Loading and Alkaline Treatment’, Materials Today: Proceedings, vol. 2, no. 4-5,

pp.2757-2766.

Jayamani, E, Hamdan, S, Ezhumalai, P, & Bakri, MKB, 2015, ‘Investigation

on Dielectric and Sound Absorption Properties of Banana Fibers Reinforced Epoxy

Composites’, Jurnal Teknologi, vol. 0, no. 0, pp. 0 (Accepted and Reviewed –

Waiting for Publication)

Jayamani, E, Hamdan, S, Rahman MR, Bakri, MKB, Kok Heng, S, & Kakar,

A, 2015, ‘Analysis of Natural Fiber Polymer Composites: Effects of Alkaline

Treatment on Sound Absorption’, Journal of Reinforced Plastics and Composites vol.

0, no. 0, pp. 1-9.

Jayamani, E., Bakri, MKB, Kok Heng, S, and Kakar, A, 2015, “Acoustical,

Thermal and Morphological Properties of Zein Reinforced Oil Palm Empty Fruit

Bunch Fiber Bio-Composites,” Journal of Applied Polymer Science, vol. 0, no. 0, pp.

0-0. (Under Review)

Book Chapter:

Jayamani, E, Hamdan, S, & Bakri, MKB, 2015, ‘Sustainable Noise Control

Materials Based on Bamboo/Unsaturated Polyester Composites: Analysis of Factors

Affecting the Sound Absorption Coefficients’ Apple Academic Press, New Jersey.

(Accepted and Reviewed – Waiting for Publication)

Conferences Attended:

Bakri, MKB, Jayamani, E, Kok Heng, S, & Hamdan, S, (2015), ‘Reinforced

Oil Palm Fiber Epoxy Composites: An Investigation on Chemical Treatment of

Fibers on Acoustical, Morphological, Mechanical and Spectral Properties’, (4th

International Conference on Materials Processing and Characterization, ICMPC 2015,

14th – 15th March 2015, Organized by Department of Mechanical Engineering,

vi

Gongkaraju Rangaraju Institiute of Engineering & Technology, GRIET, Hyderabad,

India). – Paper Presentation for Reinforced Oil Palm Fiber Epoxy Composites: An

Investigation on Chemical Treatment of Fibers on Acoustical, Morphological,

Mechanical and Spectral Properties

Bakri, MKB, Jayamani, E, Kok Heng, S, & Hamdan, S, (2015), ‘An

Experimental and Simulation Studies on Sound Absorption Coefficients of Banana

Fibers and Their Reinforced Composites’, (Recent Advances in Nano Science and

Technology, 2015, RAINSAT 2015, 8th – 10th July 2015, Organized by Sathyabama

University in association with Central Leather Research Institiute , Chennai, India). –

Certificate of Presentation for An Experimental and Simulation Studies on Sound

Absorption Coefficient of Banana Fibers and Their Reinforced Composites.

Paper Award:

Jayamani, E, Hamdan, S, Kok Heng, S, Rahman, MR, Bakri, MKB, & Kakar,

A, 2015, ‘The Effect of Natural Fibres Mercerization on Natural

Fibres/Polypropylene Composites: A Study of Thermal Stability, Morphology and

Infrared Spectrum’, (International Conference on Material, Mechatronics,

Manufacturing, and Mechanical Engineering 2014, ICMMMM 2014, 24th – 25th

October 2015, Organized by International Postgraduate Network (IPN.org), Kuching,

Sarawak, India). – Best Paper Award for The Effect of Natural Fibres Mercerization

on Natural Fibres/Polypropylene Composites: A Study of Thermal Stability,

Morphology and Infrared Spectrum

vii

TABLE OF CONTENTS

ABSTRACT i

ACKNOWLEDGEMENT ii

AUTHOR DECLARATION iii

LISTS OF JOURNAL PUBLICATIONS AND CONFERENCES iv

TABLE OF CONTENTS vii

LIST OF FIGURES xii

LIST OF TABLE xv

CHAPTER 1 1

1.0 INTRODUCTION 1

1.1 SOUND 1

1.2 NOISE 3

1.3 SOUND ABSORBING MATERIAL 4

1.4 TYPE OF SOUND ABSORBER 7

1.5 SOUND ABSORPTION MECHANISM 8

1.6 MAIN AIM OF RESEARCH 9

1.7 OBJECTIVES OF RESEARCH 9

1.8 RESEARCH FLOWCHART 10

CHAPTER 2 11

2.0 LITERATURE REVIEW 11

2.1 OVERVIEW 11

2.2 FACTORS AFFECTING SOUND ABSORPTION OF ABSORBING

MATERIALS 11

2.2.1 FIBRE SIZE 12

2.2.2 POROSITY 13

2.2.3 FLOW RESISTIVITY 14

2.2.4 DENSITY 15

2.2.5 THICKNESS 15

2.2.6 TORTUOSITY 16

2.2.7 COMPRESSION 17

2.2.8 SURFACE IMPEDANCE 18

2.2.9 PLACEMENT/DESIGN 18

2.2.10 TEMPERATURE 19

viii

2.3 EMPIRICAL MODEL FOR SOUND ABSORBING MATERIALS 19

2.3.1 EMPIRICAL MODEL FOR THE FLOW RESISTIVITY 20

2.3.1.1 MECHEL MODEL 20

2.3.1.2 BIES & HANSEN MODEL 21

2.3.1.3 GARAI & POMPOLI MODEL 22

2.3.2 EMPIRICAL MODEL FOR THE SOUND ABSORPTION

COEFFICIENT 22

2.3.2.1 DELANY& BAZLEY MODEL 22

2.3.2.2 GARAI & POMPOLI 23

2.4 METHODS TO MEASURE SOUND ABSORPTION COEFFICIENT 24

2.4.1 REVERBERATION METHOD 24

2.4.2 STANDING WAVE METHOD 25

2.4.3 TWO-MICROPHONE TRANSFER FUNCTIONS METHOD 26

2.5 COMPOSITE CHARACTERISATIONS 28

2.5.1 ACOUSTICAL PROPERTIES 28

2.5.2 MECHANICAL PROPERTIES 31

2.5.3 CHEMICAL MAPPING ANALYSIS BY FOURIER TRANSFORM

INFRARED SPECTROSCOPY (FTIR) 34

2.5.4 MICROSTRUCTURE ANALYSIS 36

2.6 FIBRE TREATMENT 36

2.7 SUMMARY AND RESEARCH GAP 37

CHAPTER 3 39

3.0 METHODOLOGY 39

3.1 OVERVIEW 39

3.2 SOUND ABSORPTION MATERIALS 40

3.3 EMPIRICAL MODELS USED 41

3.4 CALCULATIONS 42

3.5 MATLAB EMPIRCAL MODELLING 44

3.6 MATERIAL PREPARATION 45

3.7 FIBRE PREPARATION 45

3.8 FABRICATION OF NATURAL FIBRE SOUND ABSORPTION TEST

SPECIMENS 48

3.9 FABRICATION OF NATURAL FIBRE EPOXY COMPOSITE

MATERIALS 48

ix

3.10 DESIGN CRITERIA FOR TWO-MICROPHONE TRANSFER

FUNCTION IMPEDANCE TUBE METHOD 49

3.10.1 THE SOUND SOURCE 50

3.10.2 THE TEST SPECIMEN 50

3.10.3 THE CONSTRUCTION OF TUBE 50

3.10.4 THE MICROPHONES 51

3.11 FABRICATION OF THE TWO-MICROPHONE TRANSFER

FUNCTION IMPEDANCE TUBE METHOD 52

3.12 CALIBRATION OF THE TWO-MICROPHONE TRANSFER

FUNCTION IMPEDANCE TUBE METHOD AND SOUND ABSORPTION

CALCULATION 55

3.13 SOUND ABSORPTION TEST 58

3.14 TENSILE TEST 59

3.15 LIGHT/OPTICAL MICROSCOPE 59

3.16 SCANNING ELECTRON MICROSCOPE 60

3.17 FOURIER TRANSFORM INFRARED SPECTROSCOPY 61

CHAPTER 4 62

4.0 RESULTS AND DISCUSSION 62

4.1 EMPIRICAL MODELLING 62

4.1.1 BANANA FIBRE 62

4.1.1.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF

BANANA FIBRE ON SOUND ABSORPTION 62

4.1.1.2 EFFECT OF THICKNESS OF BANANA FIBRE ON SOUND

ABSORPTION 65

4.1.2 LUFFA FIBRE 67

4.1.2.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF

LUFFA FIBRE ON SOUND ABSORPTION 67

4.1.2.2 EFFECT OF THICKNESS OF LUFFA FIBRE ON SOUND

ABSORPTION 70

4.1.3 OIL PALM FIBRE 72

4.1.3.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF OIL

PALM FIBRE ON SOUND ABSORPTION 72

4.1.3.2 EFFECT OF THICKNESS OF OIL PALM FIBRE ON SOUND

ABSORPTION 74

4.2 EXPERIMENTATION 76

x

4.2.1 ACOUSTICAL PROPERTIES OF BANANA, LUFFA, OIL PALM

FIBRE (SOUND ABSORPTION ANALYSIS) 76

4.2.2 BANANA FIBRE EPOXY COMPOSITES 80

4.2.2.1 MORPHOLOGICAL OF BANANA FIBRE EPOXY

COMPOSITES (SCANNING ELECTRON MICROSCOPE AND

LIGHT/OPTICAL MICROSCOPIC ANALYSIS) 80

4.2.2.2 ACOUSTICAL PROPERTIES OF BANANA FIBRE EPOXY

COMPOSITES (SOUND ABSORPTION ANALYSIS) 82

4.2.2.3 MECHANICAL PROPERTIES OF BANANA FIBRE EPOXY

COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS) 86

4.2.2.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS

OF BANANA FIBRE EPOXY COMPOSITE 88

4.2.3 LUFFA FIBRE EPOXY COMPOSITES 92

4.2.3.1 MORPHOLOGY OF LUFFA FIBRE EPOXY COMPOSITES

(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL

MICROSCOPIC ANALYSIS) 92

4.2.3.2 ACOUSTICAL PROPERTIES OF LUFFA FIBRE EPOXY

COMPOSITES (SOUND ABSORPTION ANALYSIS) 94

4.2.3.3 MECHANICAL PROPERTIES OF LUFFA FIBRE EPOXY

COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS) 96

4.2.3.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS

OF LUFFA FIBRE EPOXY COMPOSITES 98

4.2.4 OIL PALM FIBRE EPOXY COMPOSITES 101

4.2.4.1 MORPHOLOGY OF OIL PALM FIBRE EPOXY COMPOSITES

(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL

MICROSCOPIC ANALYSIS) 101

4.2.4.2 ACOUSTICAL PROPERTIES OF OIL PALM FIBRE EPOXY

COMPOSITES (SOUND ABSORPTION ANALYSIS) 104

4.2.4.3 MECHANICAL PROPERTIES OF OIL PALM FIBRE EPOXY

COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS) 107

4.2.4.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS

OF OIL PALM FIBRE EPOXY COMPOSITES 108

4.3 COMPARISONS BETWEEN BANANA, LUFFA AND OIL PALM

FIBRE AND ITS COMPOSITES 111

4.3.1 COMPARISONS BASED ON EMPIRICAL MODEL 111

4.3.2 COMPARISONS BASED ON EXPERIMENTATION 112

4.3.3 COMPARISONS BASED ON EMPRICAL MODEL AND

EXPERIMENTATION 113

xi

CHAPTER 5 115

5.0 CONCLUSIONS AND FUTURE WORKS 115

5.1 CONCLUSIONS 115

5.2 APPLICATIONS 116

5.2 FUTURE WORKS 117

REFERENCES 118

APPENDICES 134

GANTT CHART FOR RESEARCH 134

SODIUM HYDROXIDE DATASHEET 135

EPOXY RESIN PART A DATASHEET 145

EPOXY RESIN PART B DATASHEET 148

xii

LIST OF FIGURES

Figure 1: Typical structure of human ear (Cheremisinoff, 1996) 2

Figure 2: The typical behaviour of absorption of three different type of sound

absorber (Jacobsen et al., 2011) 7

Figure 3: The flowchart of the overview research 10

Figure 4: The flowchart showing the overview of literature review 11

Figure 5: Standing wave method 25

Figure 6: The two-microphone transfer function impedance tube method 27

Figure 7: Composites schematic views (Mohanta and Acharya, 2013) 33

Figure 8: Flowchart of methodology 40

Figure 9: Creating a new M-file on MATLAB 44

Figure 10: Banana fibre: (a) untreated, and (b) treated 47

Figure 11: Luffa fibre: (a) untreated, and (b) treated 47

Figure 12: Oil palm fibre: (a) untreated, and (b) treated 47

Figure 13: Hydraulic press machine 49

Figure 14: Mould for: (a) tensile test, and (b) sound absorption test (circular

cavities) 49

Figure 15: Schematic diagram of two-microphone method 52

Figure 16: The two-microphone transfer function impedance tube test rig 53

Figure 17: Sinusoidal sound wave for microphone A and B 54

Figure 18: Fast Fourier transform graph of sound wave: (a) real part (the

incidence peak), and (b) imaginary part (the reflective peak) 54

Figure 19: Tensile test machine 59

Figure 20: Light/optical microscope 60

Figure 21: Equipment for morphological test (a) scanning electron microscope,

and (b) sputter coating machine 61

Figure 22: Infrared spectrometer 61

Figure 23: The effect of fibre diameter of banana fibre on the sound absorption

(5 mm thickness) 63

Figure 24: The effect of material thickness of banana fibre on the sound

absorption (32 µm fibre diameter) 66

Figure 25: The effect of fibre diameter of luffa fibre on the sound absorption (5

mm thickness) 68

xiii

Figure 26: The effect of material thickness of luffa fibre on the sound

absorption (19 µm fibre diameter) 71

Figure 27: The effect of fibre diameter of oil palm fibre on the sound absorption

(5 mm thickness) 73

Figure 28: The effect of material thickness of oil palm fibre on the sound

absorption (22 µm fibre diameter) 75

Figure 29: The sound absorption coefficient: (a) untreated banana, luffa, and oil

palm fibre without epoxy (5 mm thickness), and (b) treated banana, luffa, and oil

palm fibre without epoxy (5 mm thickness). 78

Figure 30: Comparison between untreated and treated sound absorption

coefficient: (a) banana fibre (5 mm thickness) (b) luffa fibre (5 mm thickness),

and (c) oil palm fibre (5 mm thickness). 79

Figure 31: SEM micrographs of banana fibres: (a) tangled banana fibres, (b)

hollow lumen structure, (c) untreated banana fibre, and (d) treated banana fibre 81

Figure 32: Light/optical micrographs of banana fibre: (a) untreated banana fibre

(b) treated banana fibre 82

Figure 33: The effects of fibre loading on sound absorption for banana epoxy

composites: (a) composites of untreated fibre (5 mm thickness), and (b)

composites of treated fibre (5 mm thickness) 83

Figure 34: Illustration of tortuosity in fibre 84

Figure 35: Illustration of type of pores in hollow fibre 85

Figure 36: The effects of chemical treatment on banana fibre for banana fibre

epoxy composites at 15 wt% (5 mm thickness) 86

Figure 37: The effects of banana fibre loading on: (a) tensile strength, and (b)

yield strength 88

Figure 38: Fourier transforms infrared spectroscopy analysis of: (a) untreated

banana fibre; and (b) treated banana fibre 89

Figure 39: SEM micrographs of luffa fibres: (a) randomized tangled luffa fibres,

(b) hollow lumen structure, (c) untreated luffa fibre, and (d) treated luffa fibre 93

Figure 40: Light/optical micrographs of luffa fibre: (a) untreated luffa fibre (b)

treated luffa fibre 94

Figure 41: The effects of fibre loading on sound absorption for luffa epoxy

composites: (a) composites of untreated fibre (5 mm thickness), and (b)

composite of treated fibre (5 mm thickness) 95

xiv

Figure 42: The effects of chemical treatment on luffa fibre for the luffa fibre

epoxy composites at 20 wt% (5 mm thickness) 96

Figure 43: The effects of fibre loading on: (a) tensile strength, and (b) yield

strength 97

Figure 44: Failure of the luffa fibre epoxy composite. Failure on the epoxy has

initiated whereas a significant portion of the fibre is still intact 98

Figure 45: Fourier transforms infrared spectroscopy analysis of: (a) untreated

luffa fibre; and (b) treated luffa fibre 99

Figure 46: SEM micrographs of oil palm fibres: (a) untreated oil palm fibres, (b)

treated oil palm fibres, (c) hollow lumens structure of oil palm fibre, and (d)

agglomeration of oil palm fibre 102

Figure 47: Light/optical micrographs of oil palm fibre: (a) untreated oil palm

fibre (b) treated oil palm fibre, and (c) fracture part of oil palm fibre epoxy 103

Figure 48: The effects of fibre loading on sound absorption for oil palm epoxy

composites: (a) composites with untreated fibre (5 mm thickness), and (b)

composites with treated fibre (5 mm thickness) 105

Figure 49: The effects of chemical treatment on oil palm fibre for oil palm fibre

epoxy composites at 15 wt% (5 mm thickness) 106

Figure 50: The effects of oil palm fibre loading: (a) tensile strength, and (b) yield

strength 107

Figure 51: Fourier transforms infrared spectroscopy analysis of: (a) untreated oil

palm fibre, and (b) treated oil palm fibre 109

xv

LIST OF TABLE

Table 1: Speed of sound in different mediums (Salvato et al., 2003) 2

Table 2: Typical noise levels in different environments 4

Table 3: Value of the eight coefficient equation from new impendence model

(NMI) compare with Delany & Bazley model value (Garai and Pompoli, 2005) 23

Table 4: Summary of acoustical properties for natural fibre 30

Table 5: Summary of acoustical properties of natural fibre composites 31

Table 6: Luffa fibre epoxy composites mechanical properties (Mohanta and

Acharya, 2013) 32

Table 7: Summary of mechanical properties of natural fibre composites 34

Table 8: The properties of natural fibre materials 41

Table 9: Maximum recommended frequency based on microphone size diameter

(ASTM E1050-12, 2012) 51

Table 10: Properties of the banana fibre and the respective flow resistivity at

different fibre diameters 63

Table 11: Properties of the banana fibre and flow resistivity value used in

empirical calculations 65

Table 12: Properties of the luffa fibre and the respective flow resistivity at

different fibre diameters 68

Table 13: Properties of the luffa fibre and flow resistivity value used in empirical

calculations 70

Table 14: Properties of the oil palm fibre and the respective flow resistivity at

different fibre diameters 73

Table 15: Properties of the oil palm fibre and flow resistivity value used in

empirical calculations 75

Table 16: The characterisation of banana fibre using infrared spectral analysis 91

Table 17: The characterisation of luffa fibre using infrared spectral analysis 100

Table 18: The characterisation of oil palm fibre using infrared spectral analysis 110

1

CHAPTER 1

1.0 INTRODUCTION

1.1 SOUND

A simple common definition of sound is something that can be heard by the animal

and human auditory system (Talty, 1988) which includes the auditory parts of

sensory system and sensory organs (i.e. the ears). In scientific terms, sound was

created because of the variation of oscillation wave due to the changes in pressure in

a medium that composed of different sets of frequency range, that can be diagnosed

both by animal or human auditory system that later be transmitted to brain to be

analysed. This was described by Cory (2005), Crocker and Arenas (2007) and Talty

(1988) in their publication of fundamental of sound. Sound travels, transfers or

transmits in different types of medium such as air, water and solid from one to

another except in vacuum medium. The sound wave is a transverse wave as in a solid

medium. If the sound wave is in other mediums apart from solid, it is then

transmitted in the form of longitudinal waves and with some mixture of transverse

waves.

One of the most common sources of sound is a speaker. When a speaker is on,

it creates shock wave and vibration motion on the surface area of the solid diaphragm

of the speaker. The motion on the surface area of the solid diaphragm forces air

molecules to vibrate and move. Thus, these create a fluctuating up and down (or back

and forth) pressure in the surrounding air (Talty, 1998). Similar fluctuating wave can

also be created in pulsating gases in-car exhaust system or moving fluids in flowing

water in a river (Wakefield Acoustics Ltd., 2004).

It is known that the structural mechanisms of human hearing in the inner and

middle parts of the ears are very sensitive to the rapid change of fluctuating air

pressure. A rapid change of fluctuating air pressure caused the vibration in the inner

and middle parts of the ears. This vibration is also called as hearing sensation. This

sensation produces a signal that later being transcended and transmitted into the brain

to be analysed and recognized as sound. Figure 1 shows the typical structure of the

human ear. A sound can travel in any directions radiated from its source. According

to Wakefield Acoustics Ltd. (2004), the stronger the source of the sound created, for

2

example through the motions; the bigger the pressure waves, and hence the louder

the sound, thus faster vibration of the source and the higher sound frequency.

Ear Drum

Hammer

Anvil

Eustachian Tube

Ossicles

Cochlea

Fluid

Nerve

MIDDLE

EAR

INNER

EAR

OUTER

EAR

Organ of Corti

Figure 1: Typical structure of human ear (Cheremisinoff, 1996)

A normal young person normally can receive approximately 15 Hz to 16 kHz

range of audible frequency. However, the sensitivity of normal hearing can detect

sound frequency approximately 3 kHz to 6 kHz and at a pressure approximately 20

µPa. When comparing this to an atmospheric pressure (101.3 kPa), it may vary in

fractional variation approximately 2x10-10

(Vér and Beranek, 2006). Based on

Crocker and Arenas (2007), the sound level is normally expressed in term of Decibel

(dB) and sound frequency is measured in terms of cycles per second or Hertz (Hz).

As the sound wave enters different mediums, the speed will vary accordingly. The

sound that move, transfer or transmitted either fast or slow is dependent on the

density of the medium it enters or produces. Table 1 shows the speed of sound in

different mediums.

Table 1: Speed of sound in different mediums (Salvato et al., 2003)

Medium Speed

ms-1

fps

Air (at 21°C) 344 1129

Wood (soft) 3350 10991

Concrete 3400 111155

3

1.2 NOISE

In terms of unsophisticated understanding, noise was considered as unwanted sound

that can be heard by human or animal. According to Cheremisinoff (1996), noise is a

result of discordant sound because of the change and non-periodic vibration in a

medium, in term of physical and mechanical views. Unfortunately, the definition of

noise is completely dependent on the hearer level of understanding. A noise can

create an emotional feeling as responses, depending on the level of consciousness

and subconsciousness of the feeler (Cheremisinoff, 1996). A certain variation and

level of sound causes the hearer to feel anger, frustrated, distracted and stressed and

this effect cause changes in physiological and psychological emotion. Thus, the noise

effect can affect our behaviour, health, environment, communication and etc.

(Cheremisinoff, 1996).

Hearing damage is normally results from the long exposure to noise in a

certain duration and intensity. Hearing loss can either be permanent or temporary,

depending on the situation and sensitivity of hearing. In general, it is believed that

the exposure to loud noise can cause significant temporary hearing loss or threshold

shift that may lead to permanent hearing loss for prolonged or recurring exposure.

Unfortunately, the exact relationship between permanent and temporary hearing loss

could not be clarified. Hearing loss in the high frequency ranges might seriously

affect the way and understanding of speech. Thus, distinguish the consonants of

speech that carried information on the higher frequencies of hearing is very

important (Cheremisinoff, 1996).

Although evidence to support non-auditory on health effects cases may not be

as complete as the hearing loss cases, there should be a concern regarding on this

matter. This is because noise can alter the normal functioning of the endocrine,

neurological and cardiovascular systems. It may cause a rise in blood pressure and

affect the in-equilibrium disorder in body system, constriction of blood vessels, and a

change in heart rhythm. Noise may produce similar effects, or compound effects of,

other more common stresses; as studies performed on animals to prove such

phenomena. However, it is not clear enough to support, whether a change in

physiology and psychology are permanent due to recurring noise (Cheremisinoff,

1996). The effects on behavioural may range from the dulling response to certain

4

specify auditory signals in noisy environments that might produce a frustration,

falling to a sensitizing of annoyances that normally would be ignored.

Noise can magnify the minor exasperation especially in a work environment.

Furthermore, it is known that human performance is affected by the presence of

noise, especially those who involved in information gathering, processing and

analysing. It is acknowledged that prolonged exposure to noise could affect the

health. The issue of exposure to noise and its undesirable effect on health is only

limited to the workplace, such as industrial and office work. Technological advances

have provided a lot of new design and engineering system such as engines, tires and

subways that contributed a lot of noise to the environment. Sometimes, household

noise sources can be as severe as industrial sources. For example, household

machinery such as chain saw, lawn mowers, hand tools, televisions, stereos, fans and

air conditioners are considered as noise sources. Table 2 shows the typical noise

levels in different environments. These noise levels are sometimes left unmonitored

and uncontrolled. It may subsequently affect the unregulated and unprotected

occupants. Thus, these noise levels may be significant in causing hearing impairment.

Table 2: Typical noise levels in different environments

Source of Noise Sound Level (dB) References

Iron factory 100.4 Ali (2011)

Power plant 91.5 Ali (2011)

Residential areas (in city) 55 Ali (2011)

School yard 55 Sarantopoulos et al. (2014)

Roadside 70 Sarantopoulos et al. (2014)

1.3 SOUND ABSORBING MATERIAL

The different structure of sound absorbing materials, including porous structure,

hollow structure, multi-dimensional size and length structure or solid composite

materials have their unique sound absorbing characteristics upon interacting with a

variety of sound frequency and intensity. Thus, in order to understand the

development of sound absorbing material, a brief review on the asbestos based sound

absorbing materials and the reason it was replaced with more advanced alternatives

are given below.

5

Asbestos was the first materials that being used as sound insulating or sound

proofing materials. Asbestos is also considered as fibrous mineral materials which is

not easily burn and corrode. It is easily deformed with other materials and it is

generally considered as a good electrical insulator. According to National

Toxicology Program (2011), the term asbestos is the common name for a specific

generic mineral that occurs in the mineral that consists of natural occurring mineral

fibre such as actinolite, chrysotile, crocidolite, tremolite, amosite and anthophyllite.

According to National Toxicology Program (2011) and National Park Service (1999),

the first application of asbestos is reported in the early 1800’s in the United States in

line with the booming of industrial revolution in the country. Although at the first

stage introduction of asbestos was very challenging and rare, it managed to reach its

highest peak demand in the 1940-1975 eras. Asbestos was widely used in building as

fire insulator and sound proof product for roof and ceiling in school, houses, office

and plant.

Unfortunately, during the introduction stage of asbestos to industry, there is a

lack of understanding on the harmfulness of asbestos toward human, animal and

environmental health. After a decade, because of the advancement in the sound

proofing technologies and research, the threat of asbestos started to reveal. The

asbestos materials when reacts with air or water in a long period of time may release

small particle poisonous fibrous molecules from the fibre materials which volatile

and yet insoluble. In other words, it means that the small particle poisonous fibrous

molecules released are unreactive and has a slow degradation rate in the environment

(Clement International Corporation, United States Public Health Service, and United

States Agency for Toxic Substance and Disease Registry, 2002).

The small particle released from the asbestos material caused health problem

when it is inhaled for a long period of time. This will cause the deformation of small

growth, which is called a cancer. According to Churg et al. (1984), McDonald and

McDonald (1997) and Bernstein et al. (2005), the extreme bio-persistence,

inflammatory and strong scouring action when inhaled, asbestos (especially tremolite)

is considered to be co-responsible for lung cancer and mesothelioma even when it is

present as a contaminant phase embedded within other asbestos fibres such as

chrysotile and also within non-asbestos materials, as for example talc and vermiculite.

6

With the increasing awareness on the effect on health posed by asbestos

material, most usage and applications of asbestos material have been banned in most

countries. According to Kogel et al. (2006), countries within the European Union

have banned the use of asbestos. Tight restriction in importing and exporting causes

some factories to stop asbestos production and eventually has to terminate their

operation due to the low demand on asbestos. However, some company in the

industry manages to pursue other alternative material such as synthetic fibre

materials made from artificial materials. As a result, the related industry of synthetic

fibres or artificial fibres has expanded during their introduction as the main substitute

for asbestos fibres. It is known that there are more than hundred types of synthetic

fibre in the world today.

According to Rouette (2001), synthetic fibres are often manufactured with

cellulose as starting material that commonly made from natural polymers. Due to the

large demand and the urgent need to replace asbestos material, the worst effect

possessed by synthetic fibre was taken slightly due to the lack of on this area. After a

research conducted in lab, it was found that the synthetic fibre was having similar

behaviour, characteristic and hazardous properties as asbestos material, specifically

the inhalation of synthetic fibre may cause injury in the lung and formation of growth

called cancer, according to Su and Cheng (2009). It is also known that the usage of

synthetic fibre is bad for the environment because it has slow degradation and

deterioration rate. After many health and environmental cases about synthetic fibre

and asbestos reported, the research and development of alternative sound absorbing

materials based on natural fibre are emerging. The replacement of synthetic fibre

with natural fibre materials can help to reduce the health and environmental problem

cause by synthetic fibre and asbestos fibre materials. It is known that natural fibre is

affordable, renewable, process able, recyclable, sustainable and biodegradable

(Fragomeni and Venkatesan, 2010).

A lot of researchers reported for natural fibre composite materials, however,

most of them only reported the acoustic behaviour of the material without taking

consideration of other properties (i.e. mechanical, chemical, morphological and etc.).

These properties, especially the findings of the mechanical properties, the availability

of certain chemical functional group in the fibre-polymer interface, and the

morphological information enable further interpretation of the structure-properties

7

relationship. Thus, research on using natural fibre composite material as alternatives

in the application, such as sound proof insulation materials with constructive

structure is very promising.

1.4 TYPE OF SOUND ABSORBER

Sound absorber can be referred to the model or method of the sound being absorbed

in the material. It can be categorized into three types of sound absorber which are

porous absorber, panel absorber (membrane) and resonators (Helmholtz). Figure 2

shows the typical behaviour of absorption of three different types of sound absorber.

There are a lot of common porous absorber materials, such as carpet, fibrous mineral

wool, open-cell form and etc. Generally, this type of material allows air or sound

wave to flow inside of the material structure which comprises of many cavities,

interstices and channel. It also can be categorized as cellular, granular and fibrous

according to Rey et al. (2012). Throughout this report, it studies focuses on fibrous

porous composite material.

Figure 2: The typical behaviour of absorption of three different type of sound

absorber (Jacobsen et al., 2011)

Panel absorber is characteristically a non-rigid or non-porous type of sound

absorber which modelled over an airspace that vibrated in a flexure mode in response

to sound pressure exerted by adjacent of air molecules. This panel normally takes on

honeycomb structure and has a smooth membrane cover that help to shield it from

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 500 1000 1500 2000

Sou

nd

Ab

sorp

tio

n C

oe

ffic

ien

ts (α

)

Frequency (Hz)

Porous Absorber Helmholtz Resonator Panel Absorber

8

humidity. Because of the characteristic that it has, according to Kang & Fuchs (1999),

this panel absorber type is also being used as silencers or as surface treatment in

architectural applicants. Panel absorber is also called as membrane absorber.

Resonators used to absorb sound in a low and narrow-type frequency band

range. Resonators type of sound absorber normally has the characteristic like

perforated material and opening end (holes and slots) materials. An example of

resonators is Helmholtz resonators. Helmholtz resonators have a connecting cavity in

the main duct that connected in series that produce two resonances at low-frequency.

Due to some limitations in Helmholtz resonator, it is not efficient in absorbing and

controlling noise that does not have tonal characteristics (Tang et al., 2012).

1.5 SOUND ABSORPTION MECHANISM

When sound is produced, it generates a vibration in the molecule of air. The

vibration in the air molecule causes the energy be transferred from one molecule to

another molecule in the air. When it reaches to the absorber material, it will transfer

the energy into the sound absorbing material from air medium to solid medium

through the collision particle of air and sound absorbing material. The energy

transfers from air to the sound absorbing material causes the sound absorbing

material to vibrate as well. The vibration and collision between the sound absorbing

material and air cause friction which generates heat.

Therefore, almost all the heat generated is absorbed by the sound absorption

material and some of them are reflected back in the air. According to Cox and

D’Antonio (2009), the flow of sound wave expands and contracts when flowing

through irregular pores. Furthermore, there is a change in the direction of flow and

this change causes the sound wave to lose momentum. The perfect sound absorber

will absorb almost all the noise created and reflects a minimal quantity of sound.

According to Vér and Beranek (2006), the sound energy degenerated after being

absorbed by the sound absorbing material before it reaches a surface due to reflection

from the rigid backed of the material. This also implies that the porous materials used

should be sufficiently thick in order to reduce the noise reflected.

9

1.6 MAIN AIM OF RESEARCH

The aim of this project is to study the novel sound absorbing materials, specifically

those based on agricultural waste/product natural fibre and polymer matrix

composite material, and understand their absorbing mechanism by using empirical

modelling and experimentation.

1.7 OBJECTIVES OF RESEARCH

In order to achieve the main aim, the following objectives have to be fulfilled:

Empirical modelling for predicting sound absorption coefficient of fibres.

Design and fabrication of the test rig and subsequently characterize the sound

absorption coefficient of novel sound absorbing composites.

Fabrication of agricultural waste natural fibres reinforced polymer matrix

composites.

Investigation on the cause or factor (i.e. fibre loading, chemical treatment and

etc.) that affecting the sound absorption of natural fibres and composite

material.

Evaluation on the effects of fibre-polymer interface adhesion on acoustical,

mechanical, spectral, and morphological properties of composites.

10

1.8 RESEARCH FLOWCHART

Investigation of Acoustical and

Mechanical Properties of Epoxy Based

Natural Fibre Composites

Literature Review

Research Proposal

Higher Research Degree Committee

Approval

No

Yes

For Empirical Modelling For Experimentations

Aim and Objectives

Identifying Empirical Model For Sound

Absorbing Materials: Delany and Bazley Garai and Pompoli Mechel Bies and Hansen

Modelling Sound Absorbing Materials Using

MATLAB

Fabrication of The Two-Microphone Transfer

Function Impedance Tube (ASTM E1050-12)

Fabrication of The Natural Fibre Composites

Calibration and Improvement of The Two-

Microphone Transfer Function Impedance Tube

(ASTM E1050-12)

Sound Absorption Test (ASTM E1050-12)

Light/Optical Microscopic Test (ASTM E2015-04)

Tensile and Yield Test (ASTM D638-14)

Scanning Electron Microscopic Test (ASTM E2015-04)

Fourier Transform Infrared Spectroscopy Test (ASTM, E168-06 and ASTM E1252-98)

Results and Discussions

Conclusions

Future Works

Thesis PreparationJournal Publications and

Conferences

Figure 3: The flowchart of the overview research

11

CHAPTER 2

2.0 LITERATURE REVIEW

2.1 OVERVIEW

In this literature review, the understanding of the current knowledge, including the

substantive findings, either in term of theoretically or methodologically in the aspect

of sound absorbing materials and related influence on the sound absorption will be

reported. This literature review covers the factors that influence the sound absorbing

material needs to be identified and the underlying fundamental needs to be

comprehended. Apart from that, the uses of empirical models to predict the sound

absorption coefficient were also covered. Figure 4 shows the flowchart of literature

review covered.

Literature Review

Empirical Models to Predict Sound Absorption

for Materials

Experiment Validation and

Characterisation

Empirical Models to Predict Sound Absorption Coefficient: Delany & Bazley Garai & Pompoli

Fibre Size Porosity Flow Resistivity Density Thickness Tortuosity Compression Surface

Impedance Placement/

Design Temperature

Reverberation Method

Two-Microphone Transfer Function Impedance Tube Method

Standing Wave Method

Factors Affecting Sound Absorbing

Materials

Empirical Models to Predict Flow Resistivity: Mechel Bies & Hansen Garai & Pompoli

Acoustical Mechanical Spectral Morphological

Type of Sound Absorption Testing

Treatment

Figure 4: The flowchart showing the overview of literature review

2.2 FACTORS AFFECTING SOUND ABSORPTION OF ABSORBING

MATERIALS

Sound absorption occurs when the sound wave passes through a material and a

reduction in the acoustic energy is resulted. The materials capable of reducing

acoustic energy significantly are also called as sound absorptive materials. The

change in the parameters (i.e. porosity, density and etc.) is an important thing that

12

needs to be understood deeply because it can influence the properties of the materials

to decide the outcome of sound absorption properties of material such as the

absorption coefficient. Sound absorbing materials were normally used to control an

acoustic environment into a certain level of an enclosed volume by reducing the

amplitude of the reflected waves. Common factors, including fibre size, porosity,

flow resistivity, density, thickness and etc. that affect sound absorption of the

materials will be investigated and discussed.

2.2.1 FIBRE SIZE

Fibre sizes may vary depending with materials that were usually used. The changes

in the fibre size can either be the changes in the size of the fibre diameter or the size

of the fibre length itself. It is also one of the main factors that influenced the sound

absorption properties of porous materials. The effect due to changes in fibre size can

be observed through related studies reported in the literature. Lee and Joo (2003)

managed to conduct an experiment based on the usage of three different types of

non-woven fabric materials that were made of recycled polyester fibres that having

the same fibre length but different in denier. The fibre deniers used were 1.25 den, 2

den and 7 den in diameter with a 38 mm in length. For binding purposed, low

melting point polyester staple fibre was used, that having a 6 den in diameter with 42

mm in length. Denier (den) is the unit measurement of the fibre linear mass density

of fibre size. Lee and Joo (2003) concluded that the fine fibre content increases the

sound absorption coefficient values of the materials due to an increase in the airflow

resistance by mean of friction of viscosity through the vibration of the air.

Koizumi et al. (2002) also reported that an increase in the sound absorption

coefficient coherent with a decrease in fibre diameter size. The reason behind this

was due to that sound waves can easily move within the thin fibre compared to the

thick fibres. Meanwhile, according to Sun et al. (1993), by using fine fibre denier,

more fibres are needed to fulfil or reach equal of its original fibre content or same

volume density. This resulted in higher airflow resistance and more tortuous path. A

study by Koizumi et al. (2002) also showed that the fine fibre denier that range from

1.5 to 6 dpf (denier per filament) performed better acoustically compared to the

coarse denier fibres. Furthermore, it is also reported that the micro denier fibres (less

13

than 1 dpf) can provide a dramatic increase in the acoustical performance of sound

absorption in the materials (Koizumi et al., 2002).

2.2.2 POROSITY

The simplest definition of porosity is an empty void or space in materials. According

to Allard et al. (1989), the porosity of materials is defined as the ratio of the volume

of the voids in the porous materials to its total volume. In terms of mechanical and

mathematical perspective, porosity is the fraction of the volume of medium voids (i.e.

air). Porosity normally traps gas (i.e. air), liquid (i.e. water) or solid (i.e. starch) form

of materials. The porosity in materials is higher, especially for materials which are

having a foam type structure or open pores such as porous silicon and porous rubber.

Furthermore, according to Shoshani and Yakubov (2001), for designing an absorber

of non-woven webs which has a high sound absorption coefficient, the structure of

the absorber should have increasing porosity along the direction of propagation of

the sound wave. Based on Canham (1997), porosity can be identified in many ways

and it depends on the shape and size packed inside a material. The formula for

porosity can be identified as below:

Porosity, 𝜙 =𝑉v

𝑉t (1)

Whereas, 𝑉v is the volume of medium voids (i.e. air) in the acoustical materials (m3),

and 𝑉t is the total volume of the acoustic materials (m3).

According to Sagartzazu et al. (2008), the values of porosity are limited in the

range of 0 to 1. In other word, it is measured from 0 % to 100 %. It is known that

some porous absorber like polymer foams and fibrous polymer, the porosity can lie

between the ranges of 0.95 to 0.99. The effect of porosity can be supported through

research conducted by Knapen et al. (2003). In their research, the porosity was

manipulated and the effect on the sound absorption coefficient against frequency was

investigated. The high porosity in a material caused the material surface contact with

sound wave to increase and causes more acoustical energy being dissipated. Other

than that, the number, size and types of pores are important factors that should be

considered while studying sound absorption mechanism in porous materials. Thus, to

allowed sound dissipation by friction, the sound waves have to enter the porous

14

materials. In other words, there should be enough pores within the material and on

the material surface for the sound to pass through and get dampened.

2.2.3 FLOW RESISTIVITY

A determination on how much the air can enter a porous absorber and the resistance

to the air flow from the entering the structure is called flow resistivity. Thus, flow

resistivity is one of the most important aspects that influence the characteristics of

sound absorption in materials. It is also known that the specific flow resistance per

unit thickness of the materials influences the sound absorbing properties of fibrous

materials. Interlocking within the fibres in non-woven materials provides the friction

that resists acoustic wave motion. Generally, when a sound wave enters these

materials, the amplitude will decrease as friction happens when the waves travels

through the tortuous and rough passages. The acoustical or sound wave energy was

converted into heat energy as sound waves energy pass through the frictional rough

and tortuous passage of the materials (Conrad, 1983). Friction quantity can be

expressed by the resistance of the materials to airflow that is called airflow resistance

and it is defined as the following:

Flow Resistivity, 𝑅f =∆𝑃

∆𝑇𝑢 (2)

Where, ∆𝑃 is the differential sound pressure across the thickness of the sample (Pa),

∆𝑇 is the incremental thickness (m), and 𝑢 is the particle velocity through the sample

(ms-1

).

Based on Crocker and Arenas (2007), the high flow resistivity materials had a

high resistance of air flow through the materials. This showed that materials with

high flow resistivity sometimes tend to reflect more sound than absorbed it.

Furthermore, according to Ren and Jacobsen (1993), because of the impedance and

propagation constant characteristic, which normally describes as the acoustical

properties of porous materials, the flow resistance of the materials were governed to

a great extent. For a porous material, the airflow resistivity per unit thickness is

proportional to the shear viscosity coefficient of the fluid (i.e. air) involved and

inversely proportional to the square of pore size characteristics of materials. Thus,

for fibrous materials with given porosity, this showed that the flow resistivity per

unit thickness is inversely proportional to the square of fibre diameters (Uno Ingard,

15

1994). Meanwhile, according to Cox and D’Antonio (2009), flow resistivity may

vary between common porous absorber, thus this can imply that it is one of the most

important factors in the research.

2.2.4 DENSITY

Density is considered to be the important parameter that influences the behaviour of

sound absorption coefficient in the materials. According to the studies done by

Koizumi et al. (2002), there was an increase of sound absorption coefficient as the

material density increased, especially in the middle and higher frequency. Similar

results were shown by Ballagh (1996) research on wool materials with varying bulk

density of the materials. Based on both results, it showed that the low density type of

material absorbs more sound of low frequency compare to the higher density

materials that normally absorb middle and high frequency. Furthermore, the material

starts to perform well with a big margin or gap range in higher density starting from

500 Hz and above. This is because the energy loss will increase as the surface

friction increases in a denser material which allows an increase in the sound

absorption coefficient. Moreover, when the apparent density is high, the quantity of

fibres per unit area increases. The sound absorption coefficient increases as the

energy loss increases when the surface friction increases. The density of a material

composite can be determined according to the formula:

Density, 𝜌 =𝑊a

𝑊a+𝑊w−𝑊b(𝜌water) (3)

Where, 𝑊ais the specimen weight in air (kgm-3

), 𝑊w is the partly immersed wire

specimen holder weight (kgm-3

), 𝑊b is the specimen weight fully immersed in

distilled water along with the partly immersed wire specimen holder (kgm-3

) and

𝜌water is the density of distilled water at testing temperature (kgm-3

).

2.2.5 THICKNESS

There is an enormous amount of study done for porous materials, especially on the

sound absorption behaviour and the effect of its thickness. According to Coates and

Kierzkowsld (2002), an effective sound absorption of a porous absorber was

achieved when the thickness of the materials is around one tenth of its incident sound

wavelength by following the rule of thumb. Meanwhile, according to Hirabayashi et

16

al. (1995), peak absorption might occur at one quarter incident sound wavelength at

the resonance frequency. Thus, these showed that there is a relationship between

thickness either at low, medium or high frequency sound absorption. According to

Ibrahim and Melik (1978), as the material increases in the thickness, at the low

frequency, the sound absorption will increase indirectly. However, at certain high

frequency, the sound absorption showed an inconsistent effect.

The effects of thickness against sound absorption were also included in

Hakamada et al. (2006) report. According to Hakamada et al. (2006), based on their

experiment on the three samples of material with different thickness at 1 mm, 2 mm

and 20 mm, the result clearly showed the effect of thickness difference on the sound

absorption coefficient. It showed that the thicker the materials, the more sound would

be absorbed. Material thickness can enhance the acoustic impedance for porous

materials (Huang et al., 2008). It can be seen clearly in Fatima and Mohanty (2011)

results, where the natural rubber latex jute composites showed higher sound

absorption coefficients in thick composites as compared with thin composites.

Meanwhile, Jiang et al. (2012) also reported that the chlorinated polyethylene (CPE)

and seven-hole hollow polyester fibres (SHPF) composites show improved sound

absorption when the thickness increased. Ersoy and Küçük (2009) also reported that

a linear increase in sound absorption coefficient can be observed in the sample

results, as the thickness increased.

2.2.6 TORTUOSITY

In general, the measurement of the elongated passage way through the pores in

certain thickness of the sample was called tortuosity. However, there is a lot of

definition describing tortuosity. According to Yamashita et al. (2009), tortuosity was

defined as a parameter in the internal structure of porous materials that represent in

the quantity of sound waves that enter the materials air space. Meanwhile, according

to Knapen et al. (2003), tortuosity is the internal structures that influence the

acoustical properties of the material. According to Wassilieff (1996), tortuosity is the

measurement of how far the pores deviate from the normal, or twist bout the material

axis. Tortuosity equation can be defined as below:

Tortuosity, 𝜏 = 𝜙𝑅s

𝑅f (4)

17

Where, 𝜙 is the porosity, 𝑅s electrical resistivity of the saturated sample (N.s.m-4

)

and 𝑅f is the resistivity of the fluid used (N.s.m-4

).

Based on Sakagami et al. (1996) report, it appears that the optimum sound

absorption is achieved by the less tortuous materials rather than the more tortuous

materials. Knapen et al. (2003) stated that the more tortuous the materials, the lower

absorption coefficient it had. Higher tortuosity causes larger fluctuations in the sound

absorption coefficient. The fluctuations will start to manifest from the lower

frequencies as the tortuosity increased. However, Sakagami et al. (1996) stated that

tortuosity only had a small effect on the noise transmission coefficient. Horoshenkov

and Swift (2001) reported that the tortuosity only affects the location about a quarter

wavelengths of its peaks, whereas the flow resistivity and porosity may involve in

affecting the height and width of the peaks. Furthermore, the value of tortuosity

might determine the high frequency response of sound absorbing porous materials.

2.2.7 COMPRESSION

The effect of compression can play a vital role in the field of sound absorption or

acoustical absorption. The results of manipulating porous materials such as fibrous

materials or cellular materials can create different variants of physical parameters

that can affect sound absorption. The compression of materials caused the sound

absorption may vary in different porous materials. According to Castagnede et al.

(2000) and Wang et al. (2008), the sound absorption properties decreased as the

fibrous mats or porous materials being compressed. Under compression condition,

the various fibres in the mat were brought nearer to each other without deformation

(without any change in fibre size) (Castagnede et al., 2000). The compression also

resulted in a decrease in thickness and indirectly cooperates with other physical

variation (Castagnede et al., 2000).

Based on the study by Castagnede et al. (2000), compression caused an

increase in flow resistivity and tortuosity, and indirectly decreased the shape factor

(include characteristic length) and porosity. Thus, this sometimes caused

inconsistency in sound absorption. According to Wang et al. (2008), the large

compression rate caused the effect on the sound absorption at the lower frequency, as

the final thickness of porous materials is invariable. Unfortunately, these physical

parameter variations in compressed materials do not caused the reason for the drop in

18

sound absorption directly because it normally related to the decrease in sample

thickness based on the experiment Castagnede et al. (2000) conducted. According to

Keshavarz and Ohadi (2013), compression may either give negative or positive

effects on sound absorption coefficient depending on the compression method.

2.2.8 SURFACE IMPEDANCE

Jayaraman (2005), has investigated the influenced of surface impedance on sound

absorption by using air laid samples with different configurations. It can be seen that

there is an increase in the sound absorption coefficients when the surface impedance

of the materials were altered. Significantly, the surface impedance can influence the

sound absorption by altering the tortuous path and porosity of the material. Higher

sound resistivity was expected when the material with a given layer of thickness

produced higher dissipation. The reason behind this was due to the surface

impedance that increased at the same time with resistivity that caused a greater

amount of reflection, especially on the surface layer that indirectly produces a lower

absorptive capability. Moreover, according to Simón and Pfretzschner (2004), the

whole process must be frequency dependent, as the lower frequency bands on the

necessary layer thickness increases, the resistivity will decrease.

2.2.9 PLACEMENT/DESIGN

According to Everest (2001), the different types of sound absorbers used in different

places such as sides, ends and ceilings at different axial modes (can either in

transverse, vertical or longitudinal axis) may influence the sound absorption of

materials. Typically, absorbing materials placed near corners or along edges of the

room surfaces demonstrated effective sound absorption, especially in a rectangular

room. Taking example of speech studios, the absorbing materials should be placed at

head height on the walls, so that it will be more effective at higher audio frequencies.

At a certain point, the sound absorption can be twice more effective as the same

material placed elsewhere if it were applied at lower portions of high walls (Everest,

2001). Moreover, the material surfaces should never face each other.

If there is an air cavity behind the materials, the materials will act as a

membrane of a certain mass. This design is frequency dependent. The air inside the

materials cavity is analogous to mechanical spring. Significantly, the sound

19

absorption of materials is enhanced by embedding an air gap between the rigid back

surface and the sample (Hakamada et al., 2006). In the research work by Hakamada

et al. (2006), it was observed that the air gap increase when the thickness decrease.

Aso and Kinoshita (1965) concluded that the greatest absorption coefficient in a thin

sample of large porosity slightly decreased as in the depth of the air space increased.

Besides, their study revealed that the absorption coefficient in a thick sample is

almost constant, independent with the profundity of its air space.

2.2.10 TEMPERATURE

Srivastava et al. (2006) and Harris (1966) examined the effect of temperature on the

mineral sound absorption and the outcomes showed that sound absorption was

affected when there was a change in temperature. Furthermore, Srivastava et al.

(2006) had established a theoretical connection between the thermal conductivity

using a least-square regression method and the noise reduction coefficient. These

relations were done under the distinctive temperature condition.

2.3 EMPIRICAL MODEL FOR SOUND ABSORBING MATERIALS

It is known that the sound absorbing material properties can be anticipated by

microstructural and phenomenological models. However, the validity limited to a

certain condition. In the empirical models formulation, the theoretical considerations

were not generally considered, nor are the inside structure of the material taken into

account (Ballagh, 1996).

In the past few years, there were several models developed (Qunli, 1988;

Dunn and Davern, 1986) based on the model and principles laid down by Delany and

Bazley (1970). These models were subsequently extended by Mechel (Mechel and

Ver, 1992). Impedance and propagation coefficient characteristic of the material can

be predicted by taking the flow resistivity of the materials and normalized it by the

frequency of desired. From the complex quantities, another prediction can be made

for acoustic properties. Although, this method was used extensively, however, some

materials like natural wool are outside the limits of the available empirical models. It

is best to use microstructure models in some of the cases (Ballagh, 1996). It is

because that it is more detailed in knowledge as the consideration of physical

properties and internal structure of the material were taken in consideration to predict

20

the acoustic performance. The models used normally correlated with such parameters

like tortuosity, porosity, shape factors, and flow resistivity. There is a lot of

development on different models over the past few years by Attenborough (1982),

Rayleigh (1896), Biot (1956), and Zwikker and Kosten (1949). Other complex model

structure used shape factors also have been modelled by Allard and Champoux

(1992), and Johnson et al. (1987).

2.3.1 EMPIRICAL MODEL FOR THE FLOW RESISTIVITY

The influenced of flow resistivity in a material for sound absorption is significant.

The increased and decreased of sound absorption coefficient are normally related

with the change of flow resistivity that profoundly in the materials. Thus, flow

resistivity plays a role in modelling sound absorption. It is known that the flow

resistivity can be manipulated by either make it as constant or variable in the formula

in modelling of sound absorption coefficient. The model to determine the flow

resistivity can be taken from Mechel (2008), Garai and Pompoli (2005), and Bies and

Hansen (1980). The flow resistivity is normally derived from related calculation and

measurement based on the fibre radius, porosity and the bulk density of the materials

relates.

2.3.1.1 MECHEL MODEL

Based on Mechel (2008) models, the derivation of the relationship between the fibre

and the sound incidence that is perpendicular to it is by having the same radii for

flow resistivity. Normally, the prediction equations were based upon the fibre radius

and bulk density or material porosity of the materials. As stated below, the equations

based on a model created by Mechel (2008):

Flow Resistivity, 𝜎 = 10.56 (𝜂

𝑟2) (

𝜇1.531

𝑟2(1−𝜇)3) (5)

and

Flow Resistivity, 𝜎 = 6.8 (𝜂

𝑟2) (𝜇1.296

𝑟2(1−𝜇)3) (6)

Where, 𝜂 is the viscosity of the medium (i.e. air viscosity for 1.84 × 10−5𝑃𝑎. 𝑠), 𝑟 is

the radius of the fibre (m) and 𝜇 is the massitivity (𝜇 = 1 − 𝜀, where 𝜀 is the porosity

of the material). Equation 5 and 6 can be simplified as below:

21

Flow Resistivity, 𝜎 =10.56𝜂(1−𝜀)1.531

𝑟2𝜀3 (7)

and

Flow Resistivity, 𝜎 =6.8𝜂(1−𝜀)1.296

𝑟2𝜀3 (8)

Where, 𝜀 is the porosity of the material and 𝑟 is the radius of fibre (m).

As for fibre diameter range from 6 µm to 10 µm, Equation 5 was used and for

a fibre diameter range from 20 µm to 30 µm, Equation 6 was used. According to

Oldham et al. (2011), it stated that for material having a small amount of binder or no

closed cell, the porosity, 𝜀 is given by the equation below:

Porosity, 𝜀 ≈ 1 −𝜌b

𝜌m (9)

Where, 𝜌b is material bulk density (kgm-3

) and 𝜌m, is the material matrix density

(kgm-3

).

2.3.1.2 BIES & HANSEN MODEL

Bies & Hansen (1980) model is based on the materials which have the same matrix

materials with the bulk density as the main functions of porosity. According to Bies

and Hansen (1980), the flow resistivity can be expressed as below:

Flow Resitivity, 𝜎 =𝐾

𝑑2𝜌m−1.53 (10)

Where, 𝐾 is the constant value (3.18 × 10−9), 𝑑 is the fibre diameter (m), and 𝜌𝑚 is

the bulk density of the material (kgm-3

). The model was revised by Oldham et al.

(2011) and is given as below:

Flow Resistivity, 𝜎 =3.18×10−9

4𝑟2𝜌−1.53 (11)

Where, 𝑟 is the radius of fibre (m). Based on Oldham et al. (2011), the expression

above was measured and obtained by using fibreglass and fibre mineral with a matrix

material density approximately 2600 kgm-3

.

22

2.3.1.3 GARAI & POMPOLI MODEL

According to Oldham et al. (2011), the improved Garai & Pompoli model is based on

Bies & Hansen model that is for fibre with a variable diameter (i.e. between 18µm to

48µm), a mean value (i.e. around 33µm) with a matrix density material

(approximated 1350 kgm-3

). Below is the equation based on Garai and Pompoli

(2005):

Flow Resistivity, 𝜎 =28.3×10−9

4𝑟2𝜌−1.404 (12)

Where, 𝑟 is the radius of fibre (m). The equation created by Garai and Pompoli (2005)

before the update is based on Bies & Hansen model formula as below:

Flow Resistivity, 𝜎 = 𝐴𝜌m𝐵 (13)

Where, 𝜌mis the bulk density of the material (kgm-3

), and both A and B are the free

parameters of the materials. It is best to decide an ideal value for A and B. Garai and

Pompoli (2005) managed to obtain A and B values as 25.989 and 1.404 by using

least-square best fitting method.

2.3.2 EMPIRICAL MODEL FOR THE SOUND ABSORPTION

COEFFICIENT

There are two models which predict sound absorption coefficients were presented by

Garai and Pompoli (2005) and Delany and Bazley (1970).

2.3.2.1 DELANY& BAZLEY MODEL

According to Oldham et al. (2011), Delany & Bazley model were obtained by a

simple power-law relations by best-fitting a big amount of experimental data for a

range of fibrous porous absorbers. The data obtained were normalized based on

dimensional variable (which is the frequency and the flow resistance) and

represented in power-law relations. By that, they manage to create the empirical

formula below.

Characteristic Impedance, 𝑍c = 𝜌0𝑐 (1 + 0.0571 (𝜌0𝑓

𝜎)

−0.754

− 𝑗0.087 (𝜌0𝑓

𝜎)

−0.732

)

(14)

23

Complex Wave Number, 𝑘c =𝜔

𝐶(1 + 0.0978 (

𝜌0𝑓

𝜎)

−0.754

− 𝑗0.187 (𝜌0𝑓

𝜎)

−0.732

)

(15)

Where, 𝜌0 is the air density (kgm-3

), 𝜎 is the flow resistivity (Nsm-4

), 𝑓 is the

frequency (Hz), and 𝜔 = 2𝜋𝑓 is the angular frequency (rads-1

). The equation, is

limited in the range of 0.01 < 𝜌0𝑓

𝜎 < 1. Based on Delany and Bazley (1970), it was

shown that the propagation of sound was influenced by two complex quantities

which are characteristic impedance, 𝑍 = 𝑅 + 𝑗𝑋 and coefficient propagation 𝛾 =

𝛼 + 𝑗𝛽.

2.3.2.2 GARAI & POMPOLI

Garai & Pompoli model is based on Delany & Bazley improved formula. Garai and

Pompoli (2005) claimed that the empirical formula made by Delany and Bazley

(1970) is not suitable for other material like polyester fibre material. The equations

are derived from the best-fit model’s value from the new impedance model (NMI)

for polyester fibre materials. As shown in Table 3, the comparison between Delany

& Bazley result and the eight coefficient value of polyester material are given.

Table 3: Value of the eight coefficient equation from new impendence model

(NMI) compare with Delany & Bazley model value (Garai and Pompoli, 2005)

Model 𝑪𝟏 𝑪𝟐 𝑪𝟑 𝑪𝟒 𝑪𝟓 𝑪𝟔 𝑪𝟕 𝑪𝟖

Delany

and

Bazley

0.057 0.754 0.087 0.732 0.189 0.595 0.098 0.700

NMI 0.078 0.623 0.074 0.660 0.159 0.571 0.121 0.530

The new equation was then rewritten so it can be related in the range of 0.05<𝜌0𝑓

𝜎

<8.4 based on the power-law relationship from data obtained by Garai and Pompoli

(2005) according to Oldham et al. (2011). Below is the simplified equation:

Real Part of Characteristic Impedance, 𝑍R = 𝜌0𝐶0 [1 + 𝐶1 (𝜌0𝑓

𝑟)

−𝐶2

] (16)

Imaginary Part of Characteristic Impedance, 𝑍I = −𝜌0𝐶0 [𝐶3 (𝜌0𝑓

𝑟)

−𝐶4

] (17)

Real Part of Propagation Constant, 𝛾, 𝛼 =2𝜋𝑓

𝐶0[𝐶5 (

𝜌0𝑓

𝑟)

−𝐶6

] (18)

24

Imaginary Part of Propagation Constant, 𝛾, 𝛽 =2𝜋𝑓

𝐶0[1 + 𝐶7 (

𝜌0𝑓

𝑟)

−𝐶8

] (19)

Where, 𝜌0 is the air density (kgm-3

) and 𝑓 is the frequency (Hz).

From the equation, at normal incidence, the sound absorption coefficient for a

rigid backed fibrous layer can be obtained by using the equation below:

Surface Impedance, 𝑍l = (𝑍R + 𝑖𝑍I)[coth(𝛼 + 𝑖𝛽) 𝑙] = 𝑍𝑙R + 𝑖𝑍𝑙I (20)

Sound Absorption at Normal Incidence, 𝛼𝑛 =4𝑍𝑙𝑅𝜌0𝐶0

|𝑍𝑙|2+2𝜌0𝐶0𝑍𝑙R+(𝜌0𝐶0)2 (21)

Where, 𝑙 is the layer thickness (m) and 𝜌0 is the density (kgm-3

). From the equation

above, it showed that the simplified procedure can be used in most applications,

where usually the best fit is based on the values of both propagation constant and

characteristic impedance that works usefully in noise control purposes (Delany and

Bazley, 1970; Dunn and Davern, 1986). Furthermore, according to Gardner et al.

(2003), in analogous ways, the neural network of the measured values of the surface

impedance and absorption coefficient was obtained and resulting in optimized on the

sound absorption coefficient.

2.4 METHODS TO MEASURE SOUND ABSORPTION COEFFICIENT

Reverberation and impedance tube are two methods used to measure sound

absorption coefficient. Meanwhile, the impedance tube method is divided into two

sub-methods, which are the standing wave and two-microphone transfer function

method. The method mentioned generally use concentrate ratio of absorbed sound

energy to incident energy. Thus, one method focussed on this research that is the

two-microphone method.

2.4.1 REVERBERATION METHOD

Reverberation is a method that uses the concept of echoes in receiving and hearing

the sound in certain locations from a sound source as the outcome of reflections

occurs in a compartment or room. The method like reverberation in a room is being

considered in terms of reverberation time. The factor that affected reverberation time

is the sound absorption coefficient of the room interior surface and the room size.

According to Chen (2011), the reverberation time was concluded at the time requisite

25

for reflections of the direct sound to attenuate by 60 dB below the level of the direct

sound. In general and based on Jambrosic et al. (2008), the testing procedure of

reverberation is to measure the reverberation time in an empty room. The specimen

sample is placed in an empty room and the reverberation time is taken again

repeatedly. Then the average of the reverberation time obtained is used to calculate

the absorption performance of the test sample.

2.4.2 STANDING WAVE METHOD

The standing wave method is created earlier compared to the other known methods.

Based on this method, sound absorption is measured though measuring the sound

speed using a tube, (Oldfield, 2006). One of the method standing waves is standing

wave ratio (SWR). It is when the traversing microphone used to determine the

magnitude and the location of sequential minimum and maximum of the standing

wave pattern in the tube. This will deduce the normal acoustic impedance and

reflection coefficient. In other words, the main fundamental concept of the standing

wave method is to create a standing wave in the tube where the source of sound is

being transmitted in the tube before sound wave is reflected at the end side of the

tube where the specimen is mounted at. The microphone is being moved to utilize the

measurement of the amplitude to get the minimum and maximum of the standing

wave which helps in the calculation of sound absorption coefficient as in relation to

the change in frequency. Figure 5 shows the impedance standing wave diagram.

Standardize standing wave method normally standardized in accordance with ISO

10534-1 (1996) and ASTM C384-04 (2011).

Sound Source

Specimen

Moveable Microphone

Figure 5: Standing wave method

26

According to the concept of standing wave method, when the source of sound

is being switched on, it creates a standing wave. The pressure point for the minimum

and maximum can be found by moving back and forth (up and down) along the tube

by using a microphone. The microphone is then connected to a voltmeter to get a

reading of measurement. The computation of the sound coefficient of the specimen

requires data based on the phase and the magnitude of standing wave relative to the

specimen location. Due to the varieties of position and value needed to be noted

separately for each related frequency, the standing wave method will consume a lot

of time. However, the result of the standing wave method is still reliable and easy to

be interpreted.

2.4.3 TWO-MICROPHONE TRANSFER FUNCTIONS METHOD

According to Huiping et al. (2006), the sound absorption can be calculated

experimentally. The two-microphone method is also known as the two-microphone

transfer function impedance tube method. According to ISO 10534-2 (1998), the

two-microphone transfer function method has replaced the standing wave ratio

method that is popular during the introduction of it. This is because the two- transfer

function impedance tube method has several advantages against standing wave ratio

method. This method uses an impedance tube that acts as a waveguide cylinder for

plane wave which is generated by the source of sound. The sound pressure is

measured at two locations where the two microphones are located near the specimen

sample as shown in the Figure 6. According to Suhanek et al. (2008), the frequency

of this method is depending on the size of the diameter of the tube and the distance

between the microphone positions. According to Lefebvre et al. (2007), the

impedance of an object was evaluated when the measurement was taken at two

different locations along a waveguide connected to the object in the two-microphone

transfer function impedance tube.

According to Putra et al. (2013), in their reported work, two microphones

method was employed to test the coefficient of sound absorption for bio-degradable

sample based on sugar cane fibres. The sugar cane is dried and cut off before being

heated at 80 °C for five minutes to remove the remaining water. Binders like

polyester and polyurethane are used during fabrication of test specimens with

different composition of sugar cane fibre. After that, a hot-pressed mould is used

27

with different pressure to create different levels of porosity. By using the two-

microphone method, the result showed that the binder with up to 40 % of the total

composition displayed no effect on the performance of the absorption regardless of

different type of the binder in the sugar cane. At a frequency of 3500 Hz, the fibres

started to show a good result in coefficient of absorption (α > 0.5). The absorption

kept increasing until it reaches 0.78 at 4500 Hz. From the research made by Putra et

al. (2013), it managed to provide a lot of information on the fabrication of bio-

composites material. Furthermore, the result obtained showed that the two-

microphone method is reliable to characterise the sound absorption properties of

composite material.

Two-Microphone

Sound Source

Specimen

Figure 6: The two-microphone transfer function impedance tube method

The impedance tube method is based on the mathematical theory of one

dimensional planar pressure wave propagation. According to Seybert (1988), this

technique is driven by random sound in the tube. Thus, this method gives more

advantage in creating fast measurement of the normal incidence wave as a change in

parameters either by using small samples that are easy to disassemble or assemble.

Furthermore, the results demonstrated by Horoshenkov et al. (2007) shows that the

result obtained varied at different laboratory conditions. The advantage of the

transfer function impedance tube is that the apparatus is smaller and therefore it can

become more practical for small test specimens. On the other hand, it allows the

surface impedance to be determined in addition to the sound absorption (Oldham et

al., 2011). However, the disadvantages are that the sound absorption can only be

measured at normal incidence wave and uncertainties are introduced when measuring

28

the sound absorption of materials having porous structure. Thus, other related

characterisation needed to be done to reduce the uncertainties.

2.5 COMPOSITE CHARACTERISATIONS

Several characterisation needs to be done in order to understand the overall

performance of composite materials. It is best worthwhile to gain a thorough

understanding on the development of these materials and their characterisation

techniques.

2.5.1 ACOUSTICAL PROPERTIES

Khedari et al. (2003), Khedari et al. (2004), and Zulkifli et al. (2008) have reported

successful fabrication of a composite particle board using agricultural wastes.

Furthermore, according to Koizumi et al. (2002), specimens of bamboo fibre

demonstrated almost similar sound absorption properties as glass wool fibres. The

bamboo fibreboard materials with enclosed surface yielded higher sound absorption

properties than the plywood materials, although it had a similar density. Yang et al.

(2003) also obtained a similar result in randomly cut rice straws and wood particles

composite boards. In the frequency range of 500 Hz to 8000 Hz, it tended to exhibit

higher sound absorption properties as compared with fibreboard, plywood and

particleboard. Thus, these studies implied that there is a great attention in using

natural fibres to create composite materials. The reason behind this is due to the

superior performance that reinforced natural fibre composites compared to the

reinforced synthetic fibre composites, especially in the consideration of lightweight,

combustible, recyclable and biodegradable. According to Avella et al. (2000),

because of the good physical properties that it have, natural fibre composites have

been ranked among high performance composites that have an advantages in the

environment and economy.

Sound absorbing materials are chosen based on the frequency range it can

control, materials, types and dimensions (Simón and Pfretzschner, 2004). Taking the

example of poly (l-lactic) acid reinforced ramie fibre, at the frequency of 250 Hz to

1600 Hz, the sound absorption coefficient obtained were from 0.089 to 0.353 (Chen

et al., 2010). Furthermore, Zou et al. (2010) also showed a higher sound absorption

coefficient from 0.03 to 0.2 within the range of 300 Hz to 1800 Hz in the

29

polypropylene reinforced with wheat straw as compared with polypropylene

reinforced with jute fibre. Reddy and Yang (2011), showed a higher sound

absorption coefficient form 0.06 to 0.8 for the range of 1000 Hz to 5000 Hz for zein-

jute composites as compared with polypropylene-jute composites within the

frequency range from 1000 Hz to 5000 Hz. Composite boards made from rice straw

and wood particle reinforced commercial urea formaldehyde showed better sound

absorption coefficient in the frequency range of 500 Hz to 8000 Hz as compared to

plywood, particleboard and fibreboard (Yang et al., 2003). A higher sound

absorption coefficient at the frequency in the range of 2000 Hz to 8000 Hz were also

seen in commercial polyurethane reinforced rice straw and waste tire particle

composite that also show superior performance than fibreboard, particleboard and

rice straw-wood particle composite board (Yang et al., 2004).

Nowadays, raw materials from agricultural waste are widely being used to

replace synthetic fibres and wood-based materials for acoustic absorption purposes.

In Malaysia for example, plenty of agricultural waste such as coconut, rice husk, and

oil palm frond fibre are expected to be useful in various material applications that

can be used in the near future (Mohd Nor et al., 2004). As similar to wood particle or

fibre, agricultural lignocellulose fibres such as wheat straw, oil palm frond and rice

straw can be easily crushed into particles or chips. Thus, this show it is a good

substitute for wood-based raw materials. The advantage of these fibres is that they

are cheaper, abundant, less toxic, nonabrasive and renewable. Thus, through research,

acoustical materials based on this renewable resource can lead to viable alternatives

to conventional materials for future and current applications (Nick et al., 2002;

Asdrubali, 2006). Table 4 and Table 5 summarized the acoustical properties of raw

natural fibre and composites available in the research literature.

30

Table 4: Summary of acoustical properties for natural fibre

Materials Fabrication

Methods

Sound

Absorption

Coefficient

Specimen

Parameters References

Bamboo Compression 0.02 - 0.4 (at

3000 Hz)

Thickness of 50

mm

Koizumi et al.

(2002)

Wood Compression 0.1 (at 4000

Hz)

Thickness of

25mm

Wassilieff

(2003)

Cellulose

(isofloc)

-

1.10 (at 3000

Hz) Thickness of 60

mm Asdrubali

(2006) Sheep wool

0.80 (at 3000

Hz)

Mineral wool 0.90 (at 3000

Hz)

Thickness of 40

mm

Arenga

pinnata -

0.40 - 0.80 (at

5000 Hz)

Thickness of 10 -

40 mm and

diameter of 28

mm

Ismail et al.

(2010)

Cotton Compression 0.50 (at 500

Hz)

Thickness of 50

mm

Oldham et al.

(2011)

Kapok - 0.67 - 0.99 (at

4000 Hz)

Thickness of 20

mm - 60 mm,

density of 8.3 -

41.7 kg/m3, and

combined

diameter of 30

mm and 60 mm

Xiang et al.

(2013)

Coir

Compression

0.78 (at 4000

Hz) Thickness of 20

mm and combined

diameter of 100

mm and 28 mm

Fouladi et al.

(2013)

Sugarcane

Grass 0.70 (at 4000

Hz)

Corn 0.90 (at 4000

Hz)

31

Table 5: Summary of acoustical properties of natural fibre composites

Materials Fabrication

Methods

Sound

Absorption

Coefficient

Specimen

Parameters References

Rice straw/

wood urea

formaldehyde

composites

Hot

compression

(500 psi 140

°C)

0.05-0.5 (at

8000 Hz)

Thickness of 10 mm,

diameter of 25 mm,

and fibre loading of 0

– 30 wt%,

Yang et al.

(2003)

Hemp brick

(66%)

-

0.50 (at

2000 Hz) Thickness of 300 mm

Asdrubali

(2006)

Polystyrene

(PST)

0.20 (at

3000 Hz) Thickness of 40 mm

Recycled latex +

coco

0.50 (at

3000 Hz)

Rami poly (L-

lactic acid)

composites

Hot

compression

0.01-0.02

(at 1800

Hz)

Thickness of 3mm,

diameter of 110 mm

and various grade of

PLLA.

Chen et al.

(2010)

Wheat straw/

polypropylene

composites

Hot

compression

0.03-0.23

(at 3000

Hz)

Thickness of 3.2 mm,

fibre loading of 40 -

80 wt%, and

diameter of 63 mm

Zou et al.

(2010)

Polyester fibres

polyethylene

composites

Hot

compression

0.03-0.35

(at 10000

Hz)

1st condition:

Thickness of 1 - 3

mm and fibre loading

of 0 - 20 wt% ,

2nd

condition:

Thickness of 20mm

and diameter of 60

mm

Jiang et al.

(2012)

Flax/epoxy Compression

(laminated)

0.11 (at

2000 Hz) Thickness of 3 mm

and diameter of 100

mm

Prabhakaran

et al. (2014) Glass/flax/epoxy

Compression

(laminated)

0.15 (at

2000 Hz)

2.5.2 MECHANICAL PROPERTIES

The reason that bio-composite was preferred is due to their environmental

friendliness and carbon dioxide free (Saw et al., 2013). Besides that, natural fibres

were also known for their low cost, renewability, biodegradability and etc. that

would benefit to the society. Specimen like luffa reinforced epoxy composite had

been tested for mechanical tensile testing as part of research conducted by Mohanta

and Acharya, (2013). The composite was fabricated with different layer from single,

double and triple layer for testing under mechanical testing. The weight composition

32

was varied from 8 wt%, 12 wt% and 18 wt% with a ratio of 1:10 hardener to epoxy.

The result showed that there was an increase in tensile test for double and single

layer composite and decrease in triple layer as shown in Table 6.

Table 6: Luffa fibre epoxy composites mechanical properties (Mohanta and

Acharya, 2013)

Luffa Fibre Layer Fibres Composition (wt%) Tensile Strength (MPa)

0 Neat epoxy 13.50

SL 7 16.29

DL 13 16.76

It is understandable that the fibre strength and fibre content are the main

influences that affect and enhance the strength of composite material itself as the

different load applied to it. However, there was an argument on the result, whereas

the triple layer composite did not meet the requirement performance on the strength

where the single and the double layer composite outperformed triple layer

counterpart. Nevertheless, the result obtained by Mohanta & Acharya (2013) is

useful as a guideline for our research. Figure 7 shows the typical schematic diagram

for the composites reported in their research

Natural fibre reinforced composites properties depend on certain parameters

such as a fibre aspect ratio, stress transfer at interface, volume fraction of the fibres,

fibre-matrix adhesion, orientation, structure, fibre chemical composition, defects,

micro fibril angle, cell dimensions, and mechanical properties (Faruk et al., 2012).

The properties of natural fibre can differ due to the method of fabrication, testing,

fibre, and moisture conditions. Thus, it is important to know that certain mechanical

properties of natural fibre composite materials. It is known that the tensile strength is

more sensitive to the matrix properties. This is because that the modulus is dependent

on the fibre properties. Thus, to improve the tensile strength, low stress concentration,

good fibre concentration and orientation, high fibre aspect ratio and fibre wetting in

the matrix phase can help to determine the tensile modulus. According to Van Der

Jagt and Beukers (1999), the deformation characteristics of composites in tension are

normally governed by the reinforced fibre strength and the behaviour between the

fibre and the matrix interface bonding.

33

Epoxy Resin + Hardener

Luffa Fibre

Epoxy Resin + Hardener

Epoxy Resin + Hardener

Luffa Fibre

Epoxy Resin + Hardener

Luffa Fibre

Epoxy Resin + Hardener Epoxy Resin + Hardener

Luffa Fibre

Epoxy Resin + Hardener

Luffa Fibre

Epoxy Resin + Hardener

Luffa Fibre

Epoxy Resin + Hardener

Single Layer Composite Double Layer Composite Triple Layer Composite

Figure 7: Composites schematic views (Mohanta and Acharya, 2013)

The use of chemical treatments of the fibres can help to improve the

interfacial bonding and surface roughness that promote strong bonding between

fibres and matrix. Sathishkumar et al. (2012) examined the mechanical properties of

reinforced snake grass fibre polyester composites by using a simple lay-up technique

with various fibre length and volume fraction. It was noted that the optimum tensile

and flexural properties was obtained at the fibre length of 30 mm and 150 mm.

Venkateshwaran et al. (2011) also investigated the polyester reinforced with

randomly oriented banana fibres fabricated using a simple lay-up method with

different fibre length of 5 mm, 10 mm, 15 mm and 20 mm with various weight

fractions of 8 %, 12 %, 16 % and 20 %. It showed that the 10 mm and 15 mm fibre

length at the weight fraction of 12 % composites have exhibited a maximum tensile

strength. Table 7 summarized the mechanical properties for natural fibre composites

available in the research literature.

34

Table 7: Summary of mechanical properties of natural fibre composites

Materials Fabrication

Methods

Maximum

Tensile

Strength

Specimen

Parameters References

HDPE/

henequen

Hot

compression

21 MPa – 27

MPa

Various types of

fibre surface

treatment

Herrera-

Franco and

Valadez-

González

(2005)

Rice straw/

PHBV

copolymer with

hydroxyvalerate

Melt mixing 5 MPa – 6

MPa

Fibre loading of

20 - 30%, and

thickness of 3.5

mm

Buzarovska

et al. (2008)

Kenaf/

Polyurethane

Composites

Hot

compression

85 MPa – 20

MPa

Various

treatment

concentration

and additive (i.e.

alkaline

treatment,

pMDI)

El-Shekeil et

al. (2011)

Luffa/Epoxy

Composites Hand-lay up -

Various layer

(i.e. single,

double, triple),

and thickness of

5 mm

Mohanta and

Acharya

(2013)

Asian palymyra/

epoxy

composites

Hot

compression

14 MPa – 27

MPa

Volume

percentage of 20

- 80 %

Balakrishna

et al. (2013)

Jute/ PLA

composites

Injection

moulding

28 MPa –

90.7 MPa

Various fibre

lengths (i.e.

short and long).

Thickness of 8

mm

Arao et al.

(2015)

2.5.3 CHEMICAL MAPPING ANALYSIS BY FOURIER TRANSFORM

INFRARED SPECTROSCOPY (FTIR)

An effective analytical tool used to identify the functional groups that present in the

fibre structure is the Fourier transforms infrared (FTIR) spectroscopy. In FTIR, the

infrared radiation is directed pass through the sample. Thus, this cause the bond in

the functional groups to vibrate vigorously depending on the reactivity of the bond to

infrared radiation and the absorbance of the radiant energy in a specific frequency

range (from strong to weak frequency and vice versa). In relation to the vibration

frequencies, the energy absorption produced peaks and the samples for molecular

35

fingerprints can be generated. The sample fingerprint can be presented in either

transmission or absorbance versus frequency curve, where the peaks explain the

molecular structure of the fibres (Kabir et al., 2012). Generally, for composites, the

FTIR spectra were presented in the mid infrared region of 4000 cm-1

to 400 cm-1

.

Furthermore, it is also known that the mid infrared spectrum can be divided into four

regions and the nature of the functional group’s presence is determined based on the

region where it is located. According to Stuart (2005), the four regions are the

fingerprint region (1500 cm-1

to 600 cm-1

), the double bond region (2000 cm-1

to

1500 cm-1

), the triple-bond region (2500 cm-1

to 2000 cm-1

) and X-H stretching

region (4000 cm-1

to 2500 cm-1

). In the vibration due to infrared radiation, the X-H

stretching region (4000 cm-1

to 2000 cm-1

) may involve O-H, C-H and N-H

stretching.

To determine the presence of methylene or methyl groups in a sample, the

exhibition of the C-H stretching band due to vibration is used as a diagnostic tool to

show the location of the groups. Meanwhile, because of the high force constants on

the bonds, the region of 2500 cm-1

to 2000 cm-1

falls for triple bond stretching

absorption. Where, C≡ C bond peak absorptions are seen in the region of 2300 cm-1

and 2050 cm-1

and C≡ N bond occurs in between the region of 2300 cm-1

and 2200

cm-1

. The double bond region is also called as a principle band region in the 2000

cm-1

to 1500 cm-1

is due to C=O and C=C stretching. It is known that the C=C double

bond groups are useful to identify the group for alkenes whereas the general range

for this vibration located in the region of 1680 cm-1

to 1630 cm-1

. Almost all the

natural fibre composites FTIR spectrum shows various bands at aliphatic

hydrocarbon. Aliphatic hydrocarbon does not contain any aromatic ring. Three

common groups of hydrocarbons are alkynes, alkenes and alkanes (Smith, 1998).

Among the three of these groups, alkanes are the simplest type of hydrocarbon.

Alkanes contained carbon-hydroxide (C-H) and carbon-carbon (C-C) single bond

molecular groups. Meanwhile, alkenes contained a carbon=carbon (C=C) double

bond molecular groups. It also called as olefins groups. While, alkynes contained

carbon≡ carbon (C≡ C) a triple bond molecular group. Thus, this contributes the

factor that affected the property enhancement of natural fibre composites (in term of

interaction between the fibre and polymer). In this context, FTIR analysis gives the

insight about the interactions between the fibre and polymer.

36

2.5.4 MICROSTRUCTURE ANALYSIS

The best way to examine the surface morphology either for the fibre or the fracture

structure of the natural fibre composites are by using scanning electron microscope

(SEM) and light/optical microscope. The SEM images showed two different opposite

morphological situations. It either shows (Fornasieri et al., 2011):

(a) The domain structure of dispersed fibre in the polymer matrix and the

interfacial interaction between the fibre and the polymer that prevented the

fibre released from the matrix, and/or

(b) The poor interfacial interaction between polymer and fibre that caused the

fibre released from the polymer matrix.

Meanwhile, for light/optical microscope, it showed the absorption of polymer inside

the fibre and the physical structure of fibre.

According to Reddy et al. (2010), by using SEM, it revealed that the addition

of coupling agent increased the surface adhesion between the fibres and polymer

matrix interface that further showed an increased in the mechanical properties of the

composites. Thus SEM showed that the surface modifications improved the adhesion

between fibre and polymer matrix. Verma et al. (2013) also observed the increased in

adhesion between fibre and polymer matrix by using SEM. SEM also showed the

weak interaction between fibre and polymer matrix as can be seen in SEM analysis

of Perinovic et al. (2010). SEM and light/optical microscope also help in showing the

lignocellulose fibre hollow structure, the cell walls of fibres, spirally oriented

cellulose in the hemicellulose and lignin matrix. The constituents at the outer surface

of the cell wall are a layer of lignocellulose material and waxy substances that bond

the cell to its adjacent neighbours (Jacob et al., 2005).

2.6 FIBRE TREATMENT

Treatment is used to remove impurity in the materials especially fibres. Normally it

involves using chemicals as reactant. One of the common treatments was called

mercerization or alkaline treatment. The alkaline treatment helps to create better

adhesion between the fibre and the binder and thus improves better bonding and

interlocking reactions. Through research conducted by Demir et al. (2006), they have

used pre-treated luffa fibre with 0.1M sodium hydroxide (NAOH) solution for 20

37

minutes at boiling temperature before re-treating the fibre with silane-coupling agent.

Then, it was mixed with polypropylene (PP) composite which contained 2 wt% to 15

wt% of treated and pre-treated luffa fibre. Then it was hot pressed at 190 °C at 100

bars for 10 minutes before cooling down to 40 °C in 6 minutes under the same

pressure.

Through scanning electron microscope (SEM), it showed that 15 wt%

untreated luffa fibre showed the presence of the void between the matrix and fibre

which indicated a poor adhesion. It also showed that fracture happened due to matrix

failure and not from fibre breakage. However, the 15 wt% treated luffa fibre showed

better fibre-matrix adhesion although void presence still remained. From the result

published by Demir et al. (2006), it can be concluded that alkalisation treatment of

natural fibre improved the adhesion and created a better mechanical properties of

reinforced composite material. However, our main focus is on the acoustical

properties of luffa fibre reinforced composite. Hence, the effect of treatment on luffa

fibre is still unknown as none literature was found.

2.7 SUMMARY AND RESEARCH GAP

A considerable amount of literature regarding the acoustic property of fibrous

materials has been published. However, very little literature reports on the modelling

of acoustic property using empirical model. Furthermore, although there was a lot of

treatment used to treat the fibre but only a few of the researches used the alkaline

treatment (i.e. sodium hydroxide) as treatment agent. To date, there is very few

systematic studies conducted to investigate the sound absorption behaviour for

natural fibre composites. On the other hand, the effect of fibre chemical treatment on

the acoustic property is seldom reported in the literature. The important parameters

affecting the acoustic property include the fibre types, porosity, fibre loading,

chemical treatment, and etc. Thus, more tests are needed to fully explore the

properties of natural fibre composites. The current study is done in order to

understand the behaviour and performance of banana, luffa and oil palm and its

composites. In addition, SEM and chemical mapping analysis via FTIR were

performed in order to obtain microstructural and fibre-polymer matrix interface

information. It is believed that these findings could give an insight on how the

microstructure and interface adhesion affect the mechanical and acoustic properties.

38

The efforts devoted to the understanding of these materials may enhance

understanding of sustainable materials as noise reduction elements in future. Natural

waste fibres, such as banana, luffa, and oil palm have considerable potential to be

used as structural material to replace wood in construction industry. Thus, this led to

the present work to investigate the acoustic and mechanical properties of natural

fibre composites.

39

CHAPTER 3

3.0 METHODOLOGY

3.1 OVERVIEW

The main aim and objectives of this research is to understand the behaviours and

characteristics of sound absorption properties in sound absorbing materials such as

natural fibre composites. Both experimentation and modelling are required to achieve

the aim. MATLAB was used to run an empirical model for sound absorption

coefficient of fibre. The model created was based on combinations of two empirical

models, (i.e. the Delany and Bazley, and Mechel models). The reason behind the

combination of these two models is to create more input on the variable and constant.

By using this empirical model, the flow resistivity (in which is a function of fibre

size) and sound absorption can be predicted. For the experimentation, the two-

microphone transfer function impedance tube method was used to validate the

modelled sound absorption coefficient incorporating the effect of fibre loading,

treatment, and fibre type (this includes fibre orientation and structure). Thus, the

fabrication of the test rig needs to be done accordingly to an established standard.

In this research, a thermoset polymer resin, epoxy was used as the matrix

materials. Meanwhile, lignocellulose type of fibres including the banana, oil palm

and luffa fibre were used as reinforcing materials. The lignocellulose fibres used

were chemically treated while untreated fibres were used as an experimental control

in order to observe the effect of treatment. The untreated and treated fibres were then

used to prepare natural fibre composite materials. These natural fibre composite

materials were then used to examine the potential of using lignocellulose fibre as

reinforcement and as sound absorption materials. Several tests were used to

determine the effect of certain factors that affects the sound absorption. Sound

absorption coefficients were evaluated using the two-microphone transfer function

impedance tube fabricated in house. The mechanical property was characterized

using tensile test. It is crucial to check if the mechanical property is not deteriorated

in order to realize any practical application. The scanning electron microscope was

used to analyse the morphological properties that influenced the sound absorption.

Fourier transform infrared spectroscopy (FTIR) helped to evaluate the compatibility

at the molecular level. Figure 8 shows the methodology workflows.

40

For Empirical Modelling For Experimentations

Empirical Model for Sound Absorbing Materials:

Mechel Delany and Bazley

Modelling of Sound Absorbing Materials Using

MATLAB

Fabrication of The Two-Microphone Transfer

Function Impedance Tube (ASTM E1050-12)

Fabrication of The Natural Fibre Composites

Calibration and Improvement of The Two-

Microphone Transfer Function Impedance Tube

(ASTM E1050-12)

Sound Absorption Test (ASTM E1050-12)

Light/Optical Microscopic Test (ASTM E2015-04)

Tensile and Yield Test (ASTM D638-14)

Scanning Electron Microscopic Test (ASTM E2015-04)

Fourier Transform Infrared Spectroscopy Test (ASTM E168-06 and ASTM E1252-98)

Results and Discussions

Methodology

Figure 8: Flowchart of methodology

3.2 SOUND ABSORPTION MATERIALS

There are three types of natural fibre materials that were used in this research. They

are the banana, luffa and oil palm fibre. Sound absorption modelling was conducted

on all three of these natural fibre materials. It is to predict these natural fibre

materials having the highest sound absorption coefficient. The reason behind this is

to understand the independent behaviour of standalone natural fibre materials before

the fabrications of natural fibre composite materials through modelling. Therefore,

before conducting any experimental test, it is best to know the basic properties of

each natural fibre material. The natural fibre materials that were selected and used in

this research are expected to help to bring down the level of pollution in addition to

exhibiting good, economical, sustainable, and degradable quality. To create sound

absorption materials based on natural fibre materials, the main properties such as the

41

fibre diameter, bulk density, matrix density and porosity need to be identified. This

data are available in the literature. Table 8 shows the data tabulated for natural fibre

materials.

Table 8: The properties of natural fibre materials

Material

Range of

Fibre

Diameter

(µm)

Mean of

Fibre

Diameter

(µm)

Bulk

Density

(kg/m3)

Matrix

Density

(kg/m3)

Porosity References

Banana 14-50 32 154 1325 0.884

Sfiligoj

Smole et al.

(2013),

Tholkappiyan

et al. (2014),

and Kulkarni

et al. (1983)

Luffa 8-30 19 - - 0.930

Laidani et al.

(2012), and

Saeed and

Iqbal (2013)

Oil Palm 8-36 22 - - 0.636

Abdul Khalil

et al. (2012),

and

Evbuomwan

et al. (2013)

3.3 EMPIRICAL MODELS USED

Based on the literature reviews, there are a few methods to calculate the flow

resistivity and sound absorption coefficient of the natural fibre materials. In this

research, in order to calculate the flow resistivity of the natural fibre materials,

Mechel empirical model was used. The reason is because the Mechel model is

derived from the diameter range applicable to the natural fibre involved in this

research. According to Oldham et al. (2011), the diameter range for Mechel model

covered from 6 µm to 30 µm. This can be confirmed and shown in the obtained

tabulated data in Table 8. From Table 8, it shows that the luffa, banana and oil palm

fibre diameter are relatively closer to the lower limit of Mechel empirical models.

According to Oldham et al. (2011), there is not much different between the two

predicted values based on Garai and Pompoli, and Mechel empirical model. However,

there are slightly different values expected when the fibre diameter reached the

42

middle range value or above. Unfortunately, this slight difference does not affect the

sound absorption coefficient significantly.

Meanwhile, Delany and Bazley empirical model were used to calculate the

sound absorption coefficient of the natural fibre materials. Although Garai and

Pompoli empirical models were also used to calculate the sound absorption, the

limits between the two empirical models were different. Delany and Bazley

empirical model have a limit range in between 0.01 <𝜌0𝑓

𝜎< 1 and for Garai and

Pompoli empirical model have a limit range in between 0.05 <𝜌0𝑓

𝜎< 8.4. Thus, all

the computed results based on the Delany and Bazley model were validated using the

results obtained from the Garai and Pompoli model. According to Oldham et al.

(2011), the accuracy of Delany and Bazley empirical model is more accurate if the

value of the fibre diameter is acceptable in its 𝜌0𝑓

𝜎 range. Whereas, the Garai and

Pompoli model is more suitable for bigger range results, based on the Mechel

empirical model for the flow resistivity.

3.4 CALCULATIONS

Before applying any data into the MATLAB, it is best to simplify and identified all

the parameter that related to the sound absorption empirical model calculation. In

both modelling, the data obtained in Table 8 were inputted into the empirical models

to compute the flow resistivity and sound absorption. For modelling the effect of

flow resistivity toward material sound absorption, all variables were held constant

except for the fibre diameter, since this will create a variable change in the flow.

Meanwhile, for modelling the effect of material thickness, the mean diameter and the

flow resistivity are held constant while the thickness is taken as variable.

The flow resistivity, 𝜎 of natural fibre materials is related to the material

porosity, bulk density and fibre radius. The major factor that affected the sound

absorption for a wide range of porous or fibrous materials is the flow resistivity. This

relationship can also be seen in the empirical model presented by Mechel (Mechel

and Ver, 1992). For sound incidence perpendicular to the direction of the fibre

materials, the flow resistivity is given by Equation 8. The fibre size indirectly

influences the flow resistivity value obtained. The radius of the fibre, 𝑟 is given by:

43

𝐹𝑖𝑏𝑒𝑟 𝑅𝑎𝑑𝑖𝑢𝑠, 𝑟 =𝑑

2 (22)

Whereas, 𝑑 is the diameter of the fibre (m).

Assuming there is no closed cells in the natural fibre material and having a

small amount of binder, the porosity is given by Equation 9. It is known that

common sound absorbing materials especially porous materials have many varieties

and forms. It is known that the material porosity depends upon their pore structure.

Commonly, sound absorbers are highly homogenous and porous with the structure

that consists of fibrous and open cell foam. Thus, many empirical models were

created based upon the work of Delany and Bazley empirical model. The

characteristic impedance can be identified using Equation 14 by taking the air

density at 1.20 kgm-3

and speed of sound at 340.29 ms-1

. Meanwhile, to determine

the complex wave number, Equation 15 was used.

Based on the Delany and Bazley empirical model that is Equation 14 and

Equation 15, the characteristic impedance, 𝑍c and complex wave number, 𝑘c needs

to be solved since it involved an imaginary number. To remove this imaginary

number, it is best to apply a constant and variable value in it that indirectly simplifies

the empirical model by eliminating the imaginary number. The constants applied in

the equation are the speed of sound value, 𝑐, frequency, 𝑓, density of air, 𝜌0 and the

angular frequency, 𝜔 . Meanwhile, the variable that can be added in is the flow

resistivity value. After the simplified equation of 𝑍c and 𝑘c been obtained, the

simplified equation will be applied in the surface impedance equation below:

Surface Impedance, 𝑍s = −𝑗𝑍c cot(𝑘c𝑡) (23)

Where, 𝑡 is the thickness (m) of natural fibre materials or the thickness for single

layer backed rigid porous absorber.

The set of thickness in the equation is depending on the factor it was going to

be tested. Thus, the thickness of the samples, the complex wave number and the

characteristic impedance may indirectly influence its absorption characteristics. The

surface impedance will be substituted in the pressure reflection coefficient equation

below:

44

Pressure Reflection Coefficient, 𝑅 =𝑧s−𝜌0𝑐

𝑧s+𝜌0𝑐 (24)

It is known that the intensity of the plane wave reflected from the surface of

absorbent materials tend to be much smaller than the incident wave by a factor of

|𝑅2|. The sound absorption coefficient, α is thus given by:

Sound Absorption Coefficient, 𝛼 = 1 − |𝑅2| (25)

3.5 MATLAB EMPIRCAL MODELLING

MATLAB software was used to generate and run the empirical model to obtained

predicted sound absorption coefficient. MATLAB is the high level mathematical

languages and interactive software that can help shorten the calculation time for

empirical modelling. The MATLAB modelling was carried out based on the

simplified equation in Section 3.3 and Section 3.4 that is a combination of Mechel

and Delany and Bazley empirical models, and natural fibre materials data in Table 8.

The modelling will help to investigate the influence of change in flow resistivity as a

change in fibre diameter and change in thickness against the sound absorption

coefficient of the material to be tested. Furthermore, the reason MATLAB being

used to calculate the flow resistivity and the sound absorption coefficient of the

material is because it can handle imaginary numbers. Figure 9 shows the new file on

MATLAB.

Figure 9: Creating a new M-file on MATLAB

45

When MATLAB empirical modelling software started, a new file needs to be

created. After new file was created, all the empirical models and data were added to

the new file. All variables and constant value are added in the file whether it is the

change in flow resistivity due to a change to the diameter size or a change in the

thickness. For modelling the effect of thickness, the flow resistivity is set to a

constant value, whereas the thickness of fibre materials is taken as a variable.

Meanwhile, for modelling the effect of flow resistivity, the thickness of the materials

is set to a constant and the flow resistivity is set as a variable. Finally, the file being

saved and run to get the result of sound absorption coefficient against the frequency

for various thickness and flow resistivity. All the data obtained from the modelling

will be plotted using a spreadsheet (i.e. Microsoft Excel).

3.6 MATERIAL PREPARATION

Pre-mixed types of epoxy resin with product code ‘BBT 7892 A’ and epoxy hardener

with product code ‘BBT 7892 B’ were supplied by Borneo Indah (Malaysia) Sdn.

Bhd, one of the branch suppliers for epoxy (the main supplier was Berjaya Bintang

Timur (Malaysia) Sdn. Bhd). This product contained bisphenol-A for part ‘A’ resin

and polyoxypropylene diamine and diethylenetriamine for part ‘B’ hardener. These

types of epoxy resin had a low reactivity, slow curing and yellowish in colour. The

oil palm fibres were directly obtained from local agricultural and industrial waste

from Federal Land Consolidation and Rehabilitation Authority (FELCRA) (Malaysia)

Bhd. Oil Palm Mill Centre at Kota Samarahan, Sarawak, Malaysia. Luffa fibre and

banana fibre were obtained from local farms in Kuching, Sarawak, Malaysia.

Malaysia. Caustic soda types of sodium hydroxide (NaOH) with product code

‘S/4920/AP1’ were supplied by Fisher Scientific, UK. The pellet forms caustic soda

of sodium hydroxide completely soluble in water, high alkalinity (before soluble in

water) and odourless.

3.7 FIBRE PREPARATION

Musa paradisiaca or musa balbisiana is the scientific name of bananas that were

commonly found in Malaysia, especially in Borneo. The banana fibres were

extracted from the tree trunks. Banana fibres were also called as pseudo stem fibres.

The tree trunk of banana was cut down and sliced into small long rectangles sizes up

to 10 to 15 cm long. Banana tree trunks were then cleaned with water to remove dirt

46

and sap colour. Later it was dried and shredded using shredding machine before

being chopped into small sizes varied from 1 to 10 mm long. Elaeis guineensis is a

scientific name of oil palms that were used. The oil palm fibres were usually

extracted from the oil palm empty fruit bunches wastes in agriculture and industries.

The oil palms empty fruit bunches wastes were then pressed before sent into

decorticator for shredding the fibres. A decorticator was also used to remove the skin

and bulk of the empty fruit bunches. Later, the shredding oil palm fibres transferred

into the drum dryer to remove the moisture by 15 %. Lastly, the oil palm fibres being

sieved to remove the dust and the fibres were sorted into different sizes. The fibre

sizes obtained and used were normally varied from 1 to 10 mm long. Luffa

cylindrical is the scientific name for luffa. The luffa fibres were usually extracted

either two ways, by either naturally drying on the plant itself or by cutting it when it

had matured and then allowing it to dry under the sun. Once it dried, the hardtop was

removed by cutting off the hard top shell with scissors. The seed inside the luffa was

removed by striking the luffa against hard objects. It also indirectly removed the skin.

Later, the luffa was soaked and sprayed with clean water to remove the sap colour.

The luffa fibres were then dried before chopped into smaller sizes varied from 1 to

10 mm long.

Two sets of composite, with either untreated or treated fibres, for each sample

were fabricated. For untreated samples, the luffa, oil palm and banana fibres were

rinsed with distilled water and dried in an oven at 60 °C for 48 hours. For the treated

samples, the luffa, oil palm and banana fibres immersed in a 5 wt% sodium

hydroxide solution at room temperature for 24 hours. The sodium hydroxide

solutions were obtained by dissolving the pellet forms of caustic soda with distilled

water. Immersing the fibre with sodium hydroxide solution enabled the removal of

impurities and increased the surface roughness of the fibre. The immersed luffa, oil

palm and banana fibres in the sodium hydroxide solution were later cleaned with

distilled water and dried in an oven at 60 °C for 48 hours. The drying oven with

model name ‘ECOCELL EC55’ and brand ‘MMM Group’ equipped by Fisher

Scientific, UK was used in the process. Figure 10, Figure 11, and Figure 12 shows

the untreated and treated banana, luffa and oil palm fibre.

47

Figure 10: Banana fibre: (a) untreated, and (b) treated

Figure 11: Luffa fibre: (a) untreated, and (b) treated

Figure 12: Oil palm fibre: (a) untreated, and (b) treated

(a) (b)

(a) (b)

(a) (b)

48

3.8 FABRICATION OF NATURAL FIBRE SOUND ABSORPTION TEST

SPECIMENS

Both untreated and treated raw fibres were filled in the mould. The moulds used for

sound absorption test has circular cavities of 25±0.5 mm in diameter and 5±0.5 mm

depth that complies with ASTM E1050-12, (2012) standard. The raw fibres were

compressed using hydraulic hot/cold press machine with model code ‘LS-22071’

equipped by Lotus Scientific (Malaysia) Sdn. Bhd. under a pressure of 100 psi (0.69

MPa) (i.e. same as normal human finger/palm press pressure). The moulds were then

left in the press machine for 30 minutes under standard room temperature 24±1 °C to

let the fibre maintained in circular form. Figure 13 shows the hydraulic hot/cold

press machine and Figure 14 (a) and (b) show the moulds used.

3.9 FABRICATION OF NATURAL FIBRE EPOXY COMPOSITE

MATERIALS

Epoxy was then mixed with the hardener at a ratio of 4:1 resin to hardener (4 amount

of part ‘A’ and 1 amount of part ‘B’). Both untreated and treated fibres, acts as fillers

being mixed together with epoxy achieved 5 wt%, 10 wt%, 15 wt% and 20 wt% fibre

loading. The moulds used for sound absorption test has circular cavities of 25±0.5

mm in diameter and 5±0.5 mm depth that complied with ASTM E1050-12, (2012)

standards. Meanwhile, tensile mould with a width of 13.0±0.5 mm and thickness of

3.2±0.4 mm (<7 mm) were also fabricated according to ASTM D638-14, (2014)

standards. The moulds were waxed before filled with the mixture of composite

ingredients. The releasing wax contained carnauba that used to prevent the sample

stick and stuck in the mould when removing the samples out from the mould.

Mixture of epoxy and fibres were then poured into the mould and compressed by

using hydraulic hot/cold press machine with model code ‘LS-22071’ equipped by

Lotus Scientific (Malaysia) Sdn. Bhd. under a pressure of 1000 psi (6.89 MPa). The

moulds were then left in the press machine for 24 hours under standard room

temperature 24±1 °C for curing purposes. Figure 13 shows the hydraulic hot/cold

press machine and Figure 14 (a) and (b) show the moulds used.

49

Figure 13: Hydraulic press machine

Figure 14: Mould for: (a) tensile test, and (b) sound absorption test (circular

cavities)

3.10 DESIGN CRITERIA FOR TWO-MICROPHONE TRANSFER

FUNCTION IMPEDANCE TUBE METHOD

Fabrication of the two-microphone transfer function impedance tube is required for

characterizing sound absorption materials. Thus, the sound source, tube, test

specimen and microphone need to be constructed, fabricated and designed according

to certain criteria as required by the ASTM E1050-12 (2012).

(a) (b)

50

3.10.1 THE SOUND SOURCE

According to ASTM E1050-12 (2012), the sound source must be supplied with

constant power over the range of frequency desired. It can be connected to the

impedance tube either by using a straight tampered, co-axially, or exponential

transition in between the tube and sound source. To reduce the effect of sound

leaking, it is best to isolate and seal the outer tube, the sound source cover and

transition path.

3.10.2 THE TEST SPECIMEN

The specimen must be in shape with the tube cross section area, based on ASTM

E1050-12 (2012). The specimen must be fitted ideally to the tube (i.e. not too loose

that there is a gap between specimen; and not too tight that there is a bulge at the

centre). It is recommended to seal any gap between specimen and the tube wall with

modelling clay or petroleum jelly. The test specimen is fitted normal to the tube axis.

At least two specimens need to be made and tested in the same mounting conditions.

The average results of tests were taken to improve the accuracy of the result in order

to minimize the potential variability in the surface quality of the specimen which can

influence the acoustic response. The thickness is allowed to be varied depending on

the test scope (ASTM E1050, 2012). The specimen specification used in this study is

reported in Section 3.9 and Section 3.13.

3.10.3 THE CONSTRUCTION OF TUBE

By referring to ASTM E1050-12 (2012), the tube must be constantly straight end-to-

end with the negligence of the type of sound transmitted either it is in rectangular or

circular shape interior. The tube material can be in a form of plastic, metal, wood or

cement as long as the inside surface is free of dust, non-porous, solid and smooth.

Sealing the tube with smooth coating will sustain the low sound attenuation. The

source of sound should be capable of generating both plane wave and non-plane

wave sound (i.e. desired and undesired wave) at the same time. Thus, the tube must

be long (i.e. the length of the distance from the sound source to the receiver which is

the microphone should be at least three times the tube diameter) to make sure that the

plane wave is developed when it reached the receiver microphone and the test

specimen. This setup will reduce the non-planar wave intensity to a minimum. The

51

tube diameter can be calculated by taking consideration of upper working frequency

by referring to the working frequency range as below:

𝑓l < Operating Frequency, 𝑓 < 𝑓u (26)

𝑑 <𝐾𝑐

𝑓u (27)

Where, 𝑓u is the upper working frequency of the tube (Hz), 𝑓l is the lower working

frequency of the tube (Hz), while 𝐾 has the value of 0.586 and 𝑐 is the speed of

sound (m/s) (ASTM E1050-12, 2012). The details of the impedance tube used in this

study are reported later in Section 3.11.

3.10.4 THE MICROPHONES

As stated in ASTM E1050-12 (2012), the microphone diameter should be smaller

than the spacing of microphone ports so that the spatial average is at a minimum at

high frequency across the face diaphragm. Furthermore, it must be lesser by twenty

percent of the highest wavelength working frequency. For high sensitivity

microphone, larger diameter of microphone can be used especially for large tube at

low frequency. The maximum frequency limits recommended are tabulated in Table

9 at room temperature based on ASTM E1050-12 (2012). Although the large spacing

between microphones can improve the accuracy of the measurement, it is still limited

by the equation below for spacing between the two-microphones:

Microphone Spacing, 𝑠 ≪𝑐

2𝑓u (28)

Where, 𝑐 is the speed of sound (m/s) and 𝑓u is the highest working frequency (Hz). It

is advised that the maximum microphone spacing, 𝑠 to be 80 % of 𝑐

2𝑓u.

Table 9: Maximum recommended frequency based on microphone size

diameter (ASTM E1050-12, 2012)

Nominal Diameter

(inch.)

Diaphragm Diameter

(mm)

Maximum Frequency

(Hz)

1 22.70 3000

0.5 12.2 5600

0.25 5.95 11500

To calculate the small distance between the specimen and the nearest

microphone, two identical microphones must be mounted depending on the surface

52

characteristics of the specimen (either it is asymmetrical or flat surface). For

specimen with a flat surface, the nearest distance the microphone can be placed in

front of the specimen is within one-half of the tube diameter in order to maintain the

best signal-to-noise ratio. However, for asymmetrical surface, the minimum distance

between the microphone and the specimen must be at least two times the tube

diameter (ASTM E1050-12, 2012). The specification of the microphone used in this

study is reported later in Section 3.11.

3.11 FABRICATION OF THE TWO-MICROPHONE TRANSFER

FUNCTION IMPEDANCE TUBE METHOD

The best method used to calculate the sound absorption coefficient of materials is by

using the two microphone transfer function impedance tube method. In order to

fabricate the test rig for the two-microphone transfer function impedance tube

method, all of the criteria mention in ASTM E1050-12 (2012) was used as a standard

reference. Figure 15 shows the schematic diagram of the two-microphone transfer

function impedance tube method.

Microphone BLoudspeaker

Microphone A

Sample

Frequency Generator

Sound Analyzer (PicoScope)

PC Oscilloscope Software Computer

Amplifier B Amplifer APower Supply

Switchable

Cylindrical Tube

Sound Wave Generated

Figure 15: Schematic diagram of two-microphone method

53

SPECIMEN HOLDER

AMPLIFIER A AND B

FREQUENCY GENERATOR

POWER GENERATORPICOSCOPE

INSULATED TUBE PIPE

SPEAKER

COMPUTER

MICROPHONE A & B

Figure 16: The two-microphone transfer function impedance tube test rig

Figure 16 shows the actual two-microphone transfer function impedance tube

test rig. The frequency generator (GW Instek SFG-2110 Synthesized Function

Generator) is connected to the speaker (8 Ω, 50 W) to generate a sinusoidal wave at

different frequency in acoustic range. Then, the end of the tube is connected with two

microphones, A and B. The function of the two microphones is used to detect the

sound transmitted in the tube. Each microphone is connected to an amplifier which is

also connected to a stable power supply of 9 V ~ 12 V direct current (GW GPC-3030

D.C Power Supply). Lastly, both amplifiers is connected to a PicoScope (PC

Oscilloscope ADC-216) that obtained the reading and converted it to digital form

wave which can be seen in the PicoScope software on the computer. From sinusoidal

sound wave, fast Fourier transforms (FFT) graph were obtained. It is plotted in

decibel (dB) versus frequency (Hz) graph. The two highest peaks generated, which is

the real part (the incidence peak) and imaginary part (the reflection peak) was

obtained. Figure 17 shows the sinusoidal wave graph and Figure 18 shows the real

part (the incidence peak) and imaginary part (the reflective peak).

54

Figure 17: Sinusoidal sound wave for microphone A and B

Figure 18: Fast Fourier transform graph of sound wave: (a) real part (the

incidence peak), and (b) imaginary part (the reflective peak)

The tube for the test rig was made from bronze and it is cylindrical, straight,

non-porous, dust free, has smooth inner and outer surface, and thick. This helps to

maintain low attenuation. Attenuation is the gradual loss in intensity throughout the

medium that disturb the propagation wave and the reading obtained on the

microphone. The tube was insulated using synthetic sound proof layer that prevents

outer noise or sound entering the tube. A sound source was placed at one end and the

sample holder at the other end. The two sets of microphone were mounted at two

(a)

(b)

55

different locations along the wall of the tube. For data processing and acquisition, a

two-channel, digital frequency analysis device was used. The working frequency

range is given by:

𝑓lower < Working Frequency, 𝑓working < 𝑓upper (29)

Where, flower is the lower working frequency of the tube (Hz) and fupper is the upper

working frequency of the tube (Hz). To maintain the plane wave generated in the

tube, the upper limit is given by:

Upper limit, 𝑓upper <𝐾𝑐

𝑑 (30)

Where, K is constant (0.586), c is the speed of sound (m/s) and d is the diameter of

the tube (m).

For accuracy of the measurements, a large spacing between the microphones

is desired. Where, the microphone spacing must be less than the shortest wavelength

of desired frequency. The microphone spacing, s is given by:

Microphone Spacing, 𝑠 ≪𝑐

2fupper (31)

According to ASTM E1050-12 (2012), the maximum spacing of the two

microphones must be 80 % of the equation. The frequency range for the two-

microphone transfer function impedance tube test rig has the working frequency

range of 500 Hz to 6000 Hz. Although the apparatus can support from 200 Hz to

6000 Hz, from the range from 500 Hz to 6000 Hz is selected in order to reduce

inconsistent data from lower frequency because of the lower sound level created by

the sound source.

3.12 CALIBRATION OF THE TWO-MICROPHONE TRANSFER

FUNCTION IMPEDANCE TUBE METHOD AND SOUND ABSORPTION

CALCULATION

Calibration needs to be done in order to get better enhancement on the results in the

sound absorption. It may be done in both physical and theoretical ways. The

calibration must follow the criteria stated in ASTM 1050-12 (2012). One of the best

ways to get an accurate result in obtaining good sound absorption coefficient is by

56

using a sound insulator. Sound insulator was used to insulate the impedance tube and

the outer cover speaker. It also helps to provide enhancement for the test rig. The

insulation of the tube is done to restrict the sound generated by the speaker to be

leaking away from the impedance tube and to prevent outside, external sound or

noise from disrupting the signal detected by the two microphones. Furthermore, it

also used to avoid airborne flanking transmission to the microphone. Insulation for

the speaker reduces the resonances of the air column in the impedance tube that were

arises when the mechanical impedance of the speaker membrane or diaphragm is at

high frequency. As referred to the ASTM E1050-12 (2012) criteria, the sound

produce from the speaker must be in respective position that can be detected by both

microphones which is at least 10 dB greater than the noise from the background

which is the environment area. Apart from that, the usage of sound insulation can

also reduce distortion produce by the speaker.

Another method of calibration that was done on the two microphones method

is by interchanging the location of the two microphones. By interchanging the

location of the two microphones, it rectifies the phase mismatch and internal

amplitude between both two microphones according (Wolkesson, 2013). This

technique is also called switching technique. Nevertheless, the switching of the two

microphones needs to be handled with care to get the exact location of normal

configuration as mentioned in ASTM E1050-12 (2012). Furthermore, the reading

from the interchange and original location of the two microphones were also

corrected by using a calculation as stated in ASTM E1050-12 (2012) indirectly for

small mismatch.

The frequency range is taken from 500 Hz to 6000 Hz. It is known that the

speed of air changes depending on the temperature. The speed of sound is given by:

Speed of Sound, 𝑐 = 20.047√273.15 + 𝑇 (32)

Where, T is the temperature (°C) such as room temperature. In this case, calibrations

need to be done to reduce error and link two different measurements. For calibration,

to find the complex amplitude mismatch is given by:

Complex Amplitude Mismatch, |𝐻C−| = (|𝐻−I|/|𝐻−II|)

1

2 (33)

57

Complex Amplitude Mismatch, |𝐻C−| = [(𝐻r

−I)2 + (𝐻i−I)2] × [(𝐻r

−II)2 + (𝐻i−II)2]

1

4

(34)

Where, Hr-I is the complex real part (incidence peak data) from the microphones

reading before the switch, Hi-I is the complex imaginary part (reflective peak data)

from the microphones reading before the switch, Hr-I is the complex real part

(incidence peak data after the switch) from the microphones reading after the switch

and Hi-II

is the complex imaginary part (reflective peak data after the switch) from

the microphones reading after the switch. Meanwhile, calibration to find the complex

phase mismatch is given by:

Complex Phase Mismatch, 𝛷c− =

1

2(𝛷−I − 𝛷−II) =

1

2tan−1

[(𝐻i−I𝐻r

−II)+(𝐻r−I𝐻i

−II)]

[(𝐻r−I𝐻r

−II)−(𝐻i−I𝐻i

−II)]

(35)

After calibration, the values for the real and imaginary part of the microphone

is corrected by finding the complex real part and complex imaginary parts that are

given by:

Complex Real Part, 𝐻r =1

𝐻c− [(𝐻r

− cos(𝛷c−)) + (𝐻i

− sin(𝛷c−))] (36)

Complex Imaginary Part, 𝐻r =1

𝐻c− [(𝐻r

− cos(𝛷c−)) + (𝐻i

− sin(𝛷c−))] (37)

In the meantime, to complex reflection coefficient part is given by:

Complex Reflection Coefficient, 𝐷 = 1 + 𝐻r2 + 𝐻i

2 − 2[𝐻r cos(𝑘𝑠) + 𝐻i sin(𝑘𝑠)]

(38)

Where, s is the microphone spacing and, k is the attenuation constant that is given by:

Attenuation Constant, 𝑘 =2𝜋𝑓

𝑐 (39)

The complex imaginary reflection is given by:

Complex Imaginary Reflection, 𝑅i =2𝐻r sin[𝑘(2𝑙+𝑠)]−sin(2𝑘𝑙)−(𝐻r

2+𝐻i2) sin[2𝑘(𝑙+𝑠)]

𝐷

(40)

Meanwhile, the complex real reflection part is given by:

58

Complex Real Reflection, 𝑅r =2𝐻r cos[𝑘(2𝑙+𝑠)]−cos(2𝑘𝑙)−(𝐻r

2+𝐻i2) cos[2𝑘(𝑙+𝑠)]

𝐷

(41)

Lastly, the sound absorption coefficient is given by:

Sound Absorption Coefficient, 𝛼 = 1 − |𝑅|2 = 1 − 𝑅r2 − 𝑅i

2 (42)

MATLAB was used to generate and run the calculation to obtain the sound

absorption coefficient of banana epoxy composites. MATLAB can help to complete

with less time.

3.13 SOUND ABSORPTION TEST

The sound absorption coefficients of the natural fibre composites were tested in the

acoustic frequency range of 500 Hz to 6000 Hz for acoustical testing. The sound

absorption coefficients were obtained from two-microphone transfer function

impedance tube test rig fabricated in house, according to ASTM E1050-12 (2012)

standards. The inner diameter of the impedance tube is 25 mm and therefore the

circular specimens of the same size were fitted into the tube. The thickness of the

specimen is 5 mm. It should be noted that specimen thickness has a significant effect

to the measured sound absorption coefficient even though the material formulation

has not been changed. This was reported by Ismail et al. (2010). Therefore, the

measured sound absorption coefficient based on this thickness are expected to be

lower than those reported in the literature which have specimens thickness in the

range of 20 to 300 mm. Charts of sound absorption coefficients of different

composition in terms of fibre content and chemical treatment were plotted and

comparisons were made. A sample was tested for each fibre loading. The standard

deviation for repeated tests (i.e. 5 run for each sample) is around ± 2 %. It is known

that the transfer function estimates are made from sample records of finite duration

and frequency resolution, and susceptible to random and bias errors. The random

error (i.e. distortion and etc.) generally kept low by creating frequency smoothing,

that is by averaging together the results of the test repeatedly (ASTM E1050-12,

2012). Meanwhile, for bias error, this may arise from uncorrected phase and

amplitude mismatch. Thus, these errors were corrected by using the calibration

calculations mentioned in Section 3.12 and as stated in ASTM E1050-12 (2012).

59

3.14 TENSILE TEST

The tensile tests were conducted using a universal testing machine with model code

‘UTM107’ according to ASTM D638-14 (2014) standards. The tensile test machine

was originally built and equipped by T-Machine Technology (Taiwan) Co. Ltd. Five

good samples were tested for each weight loading. The results of the five tests were

then averaged. Figure 19 shows the tensile test machine.

Figure 19: Tensile test machine

3.15 LIGHT/OPTICAL MICROSCOPE

Samples were taken from each of the untreated luffa, epoxy, oil palm epoxy and

banana epoxy composites and the treated counterparts. Images for each of the

samples were taken at the breaking point and the middle section of the natural fibre

composites. Samples were typically tested using industrial microscope with model

code ‘Eclipse 3x2 LV140’ equipped by Nikon (Japan) Corporation. Test preparations

were typically done according to the ASTM E2015-04 (2014) standards. Figure 20

shows the light/optical microscope.

60

Figure 20: Light/optical microscope

3.16 SCANNING ELECTRON MICROSCOPE

For the scanning electron microscope, the samples were sliced and mounted on

aluminium stubs. Then, they were fine coated by sputtering the samples with metal

particle coating (i.e. gold) for a one to two minutes to a thickness about 10 nm under

0.1 torr and 18 mA by using auto fine coaters with the model code of ‘JFC-1600’

equipped by JEOL (Japan) Ltd. The morphological studies of the untreated and

treated natural fibre composites were clearly observed using a Hitachi Analytical

Table Top SEM (bench top) with model code ‘TM-3030’ equipped by Hitachi High-

Technologies Europe (Germany) GmbH. It used a field emission gun and with

accelerating voltage of 5 kV and 15 kV to collect images of the surface of

composites. The test preparations were coherence with ASTM E2015-04 (2014)

standards. Figure 21 (a) shows the scanning electron microscope and Figure 21 (b)

shows the spurred coated machine.

61

Figure 21: Equipment for morphological test (a) scanning electron microscope,

and (b) sputter coating machine

3.17 FOURIER TRANSFORM INFRARED SPECTROSCOPY

The Fourier transform infrared spectroscopy (FTIR) was conducted using a

spectroscopy with the model name of ‘IRAffinity-1’ equipped by Shimadzu (Japan)

Corporation. The scanning was conducted in the wavenumber range of 400 to 4000

cm-1

for both untreated and treated fibre samples. Test prepared according to ASTM

E168-06 (2006) and ASTM E1252-98 (2013) standards. An infrared spectrum of the

wavenumber range was collected and the functional groups of fibre were identified.

Figure 22 shows the devices used to obtain the spectral data.

Figure 22: Infrared spectrometer

(a) (b)

62

CHAPTER 4

4.0 RESULTS AND DISCUSSION

4.1 EMPIRICAL MODELLING

Several tests were carried out by manipulating the fibre characteristics in order to

understand the basic relationship between the sound absorption coefficient and those

parameters under investigation. These tests included the change in flow resistivity,

the change in fibre size, and the change in thickness. Three types of fibres were used

and analysed using the Mechel, and Delany and Bazley empirical models (Section

3.2 to Section 3.5). The fibres considered were banana, luffa and oil palm fibre.

Other factors which might influence the sound absorption coefficient were also

discussed and analysed.

4.1.1 BANANA FIBRE

4.1.1.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF BANANA

FIBRE ON SOUND ABSORPTION

Figure 23 shows the effects of the fibre diameter of banana fibre on sound absorption.

The results were obtained by using a combination of Mechel, and Delany and Bazley

empirical models. As shown in Figure 23, the sound absorption, increased with

increasing flow resistivity which is the results of decreasing fibre diameter. It is

observed that at the lower frequency range (i.e. from 500 Hz to 2000 Hz), with

increasing fibre diameter, the sound absorption exhibited an increasing trend.

Meanwhile, at medium to higher frequency range (i.e. from 2000 Hz to 6000 Hz),

with decreasing fibre diameter, the sound absorption is increased. It was also

observed that the gap between the lower to medium frequency is nearer and the gap

between the medium to higher frequency is greater as the sound absorption increases

due to increase in frequency. A crossover point can be observed at 2000 Hz. At the

frequency of 6000 Hz, the fibre size of 20 µm has the highest sound absorption

coefficient (i.e. 0.64) whereas the fibre size of 50 µm has the lowest sound

absorption coefficient (i.e. 0.32). Beyond 2000 Hz, with increasing frequency, the

sound absorption coefficient for all fibre sizes exhibits increasing trends. Table 10

shows the properties of banana fibre and the flow resistivity values used in modelling

the sound absorption. The porosity and the fibre material thickness were taken as a

63

constant, whereas the fibre diameter and the flow resistivity were manipulated, when

predicting the sound absorption coefficients using the empirical models.

Table 10: Properties of the banana fibre and the respective flow resistivity at

different fibre diameters

Type of

Fibre Thickness Porosity Fibre Diameter Flow Resistivity

Banana 0.005 m 0.884

20 µm 111410 Pa.s.m-2

30 µm 49517 Pa.s.m-2

40 µm 27853 Pa.s.m-2

50 µm 17826 Pa.s.m-2

Figure 23: The effect of fibre diameter of banana fibre on the sound absorption

(5 mm thickness)

At the lower frequency range from 500 Hz to 2000 Hz, a higher sound

absorption coefficient is observed for the larger fibre diameter. This is because a

larger fibre diameter, the pore within the fibre lumen structure is expected to be

larger if compared with fibre of a smaller diameter, even though the flow resistivity

can be lower. Meanwhile, at the frequency range of 2000 Hz to 6000 Hz, it showed

the higher sound absorption coefficient is expected for larger diameter fibre. This is

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 1000 2000 3000 4000 5000 6000

Sou

nd

Asb

orp

tio

n C

oe

ffic

ien

t (α

)

Frequency (Hz)

20µm 30µm 40µm 50µm

64

because the higher flow resistivity values at larger fibre diameters have a more

significant effect compared to the case at the lower frequency range. According to

Ballagh (1996), the fibre diameter size is inversely proportional to the flow resistivity.

Thus, this support the behaviour of the high flow resistivity happened at a lower fibre

diameter or vice versa. In other words, the increase in the fibre diameter size will

eventually reduce the flow resistivity and vice versa.

By comparing the sound absorption coefficient with other parameters used in

the empirical model calculations, with increasing complex wave number, 𝑘c and

characteristic impedance, 𝑍c , a minor change in the flow resistivity value of the

Mechel empirical model was expected. A low pressure reflection coefficient, 𝑅 was

produced because of the higher surface impedance, 𝑍s (Oldham et al., 2011). Thus,

with a lower reflection coefficient exhibited by a material, a higher sound absorption

coefficient is expected. However, at medium and higher frequencies, the exhibition

of higher sound absorption coefficient is ascribed to the small fibre diameter that

produced a comparatively larger impact on the flow resistivity value of the Mechel

empirical model which subsequently caused a higher sound absorption. According to

Koizumi et al. (2002), the reason behind the high sound absorption coefficient is that

the decrease in the fibre diameter indirectly increases the number of fibre per unit

area inside the samples. Moreover, the energy loss also increases with increasing

surface as a result of higher fibre per unit area.

The relationship between the fibre diameter and the flow resistivity may be

used to indicate the permeability of a medium through the sound absorbing materials.

A material with high flow resistivity will be less permeable to sound. The reason

behind this is that the sound wave that projected onto the fibrous materials was

highly resisted to enter, move or pass throughout the materials. This characteristic of

the fibrous materials tends to reflect a certain amount of sound wave or air flow.

Usually, the fibre agglomeration forms a network or web that acts as a natural fibre

filter. This network or web had an interlocking characteristic between the fibres that

not only provides strength, but also help to become a natural resistance to the

acoustic wave motion (Seddeq, 2009). In general, as sound entered the materials, the

amplitude started to decrease due to the friction, as the result of waves tried to enter,

move or pass through the tortuous path (Conrad, 1983). Tortuosity is a measurement

of elongation path through the pores that created by the fibres, especially on the fibre

65

lumen structure, the fibre hollow structure or the structure created by fibre

agglomeration forms. As the matter of facts, Knapen et al. (2003) described that the

internal structure of the materials may influence the sound absorption coefficient of

the materials.

4.1.1.2 EFFECT OF THICKNESS OF BANANA FIBRE ON SOUND

ABSORPTION

Figure 24 shows the effects of material thickness of banana fibre on the sound

absorption coefficient. The results were obtained by using a combination of Mechel,

and Delany and Bazley empirical models. Based on Figure 24, it showed that the

increases in sound absorption were due to the increasing in the material thickness. It

was observed that at the lower to medium frequency range (i.e. from 500 Hz to 4000

Hz), with increasing material thickness, the sound absorption followed the same

trend. Whereas, at a higher frequency range (i.e. from 4000 Hz to 6000 Hz), there

was a drop in sound absorption for thickness of 0.020 m. This behaviour does not

correlate with the basic theoretical concept and understanding. This is because, as the

materials become thicker, it should absorb more acoustic energy as compared to the

thinner materials. Beyond 4000 Hz, for banana fibre material of thickness more than

0.015 m, the fibre sound absorption coefficient stabilized in the range of 0.90 to 0.99.

In overall, the banana fibre material thickness of 0.015 m has the highest sound

absorption coefficient (i.e. 0.97) whereas the banana fibre material thickness of 0.005

m has the lowest sound absorption coefficient (i.e. 0.46). With increasing frequency,

the sound absorption coefficient followed the same trend. Table 11 shows the

properties of banana fibre and the thickness values used in these empirical

calculations. In this case, the porosity, the mean fibre diameter and the flow

resistivity were taken as constants whereas the fibre material thickness was varied.

Table 11: Properties of the banana fibre and flow resistivity value used in

empirical calculations

Type of

Fibre Thickness Porosity Fibre Diameter Flow Resistivity

Banana

0.005 m

0.884 32 µm 43521 Pa.s.m-2 0.010 m

0.015 m

0.020 m

66

Figure 24: The effect of material thickness of banana fibre on the sound

absorption (32 µm fibre diameter)

It is known that a thicker material tends to produce a higher sound absorption

coefficient. Based on Zent and Long (2007), thicker samples caused a higher sound

absorptions or vice versa (as thinner samples caused lower sound absorptions). This

may be true for other type of fibre materials based on the empirical model observed.

However, based on Figure 24, it shows the opposite way of understanding. It is

observed that the sound absorption coefficient of banana fibre with a lower thickness

of 0.015 m can achieve a higher sound absorption coefficient at higher frequency

ranges. The reason behind this was due to the porosity of banana fibre has a value

close to the matrix density or the bulk density. This explanation can be supported

based on Ren et al. (2011), it stated that material with low porosity or a very close

porosity with the matrix density and its bulk density will tend to have a higher sound

absorption coefficient ranging from medium to high frequency at low thickness with

the same material used. Apart from that, when the thickness of the material sample

increased, the improvement in the sound absorption performance at the lower to

medium frequency range is more significant compared to the result in the higher

frequency range. A similar result is reported in another study. According to Ren et al.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 1000 2000 3000 4000 5000 6000

Sou

nd

Asb

orp

tio

n C

oe

ffic

ien

t (α)

Frequency (Hz)

0.005m 0.010m 0.015m 0.020m

67

(2011), there is little influence of thickness to the sound absorption performance at

high frequency range, especially for fibre thickness in the range of 0.010 m to 0.020

m. The results for banana fibre as in Figure 24, supports the conclusion drawn by

Ren et al. (2011). In conclusion, the basic understanding that a thicker material has a

higher sound absorption coefficient only applied at lower frequency. This theory is

further supported by the work conducted by Ibrahim and Melik (1978). They showed

that the increased sound absorption with increasing thickness happened at low

frequency. However, at higher frequency, the thickness of the materials had no

significant effect on the sound absorption.

4.1.2 LUFFA FIBRE

4.1.2.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF LUFFA

FIBRE ON SOUND ABSORPTION

Figure 25 shows the effect of the fibre diameter of luffa fibre on sound absorption.

The results were obtained by using a combination of Mechel, and Delany and Bazley

empirical models. Based on Figure 25, it can be observed that the fibre diameter has

a significant effect on the sound absorption. It is also observed that at the lower

frequency range (i.e. from 500 Hz to 1500 Hz), with increasing the fibre diameter,

the sound absorption coefficients increased slightly. Meanwhile, at medium to higher

frequency (i.e. from 1500 Hz to 6000 Hz), with decreasing as the fibre diameter, the

sound absorption followed an increasing trend, which is a result of increased flow

resistivity. The results gap of each sound absorption coefficient between the lower to

medium frequency is nearer as the frequency increased until at a certain point it will

start to overlap with each other. Once it overlaps, the results gap of each sound

absorption coefficient is greater, especially at medium to higher frequency. The fibre

size of 10 µm has the highest sound absorption coefficient (i.e. of 0.74), whereas the

fibre size of 25 µm has the lowest sound absorption coefficient (i.e. 0.40). Table 12

shows the properties of luffa fibre and the flow resistivity values used in the

empirical calculations. In this case, the porosity and the fibre material thickness were

taken as constant when comparing the sound absorption coefficient of luffa fibre

material of various fibre diameters.

68

Table 12: Properties of the luffa fibre and the respective flow resistivity at

different fibre diameters

Type of

Fibre Thickness Porosity Fibre Diameter Flow Resistivity

Luffa 0.005 m 0.930

10 µm 198240 Pa.s.m-2

15 µm 88105 Pa.s.m-2

20 µm 49559 Pa.s.m-2

25 µm 31718 Pa.s.m-2

Figure 25: The effect of fibre diameter of luffa fibre on the sound absorption (5

mm thickness)

It can be seen that at the lower frequency, ranging from 500 Hz to 1500 Hz,

the sound absorption coefficient increased with increasing fibre diameter. Although

the value of the flow resistivity is lower for larger fibre diameter, it helps to maintain

the porous lumen structure in the fibre as compared to the smaller diameter fibre. If

compared with banana fibre, luffa fibre had higher porosity which caused the shifted

of the crossover point that is 2000 to 1500 Hz for luffa fibre. Furthermore, the

frequency range of 1500 Hz to 6000 Hz, it showed a higher sound absorption

coefficient because of the higher flow resistivity for luffa fibres. With a decreased

fibre diameter, the more porous structure of the sound absorbent is expected. Thus, it

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indirectly increased the sound absorption coefficient. Furthermore, according to

Ballagh (1996) and Seddeq (2009), the fibre diameter is inversely proportional to the

flow resistivity. Thus, this support the behaviour of high flow resistivity is due to the

decreased in fibre diameter.

Furthermore, compared with banana fibre, it is observed that the fibre

diameter of luffa fibre is smaller and hence more porous. This caused luffa fibre to

have a higher sound absorption. The relationship between the fibre diameter and flow

resistivity may be used to indicate the permeability of a medium through the sound

absorbing materials. Materials with high flow resistivity will be less permeable to

sound. The reason behind this is that the sound wave that projected onto the fibrous

materials was highly resisted to enter, move or pass throughout the materials. This

characteristic of the fibrous materials tends to reflect a certain amount of sound or air

flow. Usually, the fibre agglomeration forms a network or web that acts as a natural

fibre filter. This network or web had an interlocking characteristic between the fibres

that not only provides strength, but also help to become a natural resistance to the

acoustic wave motion (Seddeq, 2009).

In general, as sound entered the materials, the amplitude decreased due to the

friction, as the result of waves tried to enter, move or pass through the tortuous path

(Conrad, 1983). As already known, tortuosity is a measurement of elongated path

through the pores that created by the fibres, especially in the fibre lumen structure,

fibre hollow structure and/or structure created by fibre agglomeration forms. As a

matter of fact, Knapen et al. (2003) described that the internal structure of the

materials may influence the sound absorption coefficient of the materials. The luffa

fibre has a larger pore compared to the banana fibre. In fact, the banana fibre is

flatter than the luffa fibre if the dimensions of both types of fibres are compared.

Thus, it is expected that the sound absorption for the luffa fibre is better than banana

fibre. Furthermore, the density of luffa fibre is different to banana fibre. The density

caused a change in porosity of the fibre itself. A lower ratio of density caused

increases in the porosity whereas a higher ratio of density caused decreases in the

porosity.

70

4.1.2.2 EFFECT OF THICKNESS OF LUFFA FIBRE ON SOUND

ABSORPTION

Figure 26 shows the effects of the material thickness of the luffa fibre on the sound

absorption coefficient. The results were obtained by using a combination of Mechel,

and Delany and Bazley empirical models. Based on Figure 26, it showed that the

increases in the sound absorption were due to the increasing material thickness. It

was observed that at the lower to medium frequency range (i.e. from 500 Hz to 3500

Hz), with increasing material thickness, the sound absorption followed the same

trend. Whereas, at the higher frequency range (i.e. from 3500 Hz to 6000 Hz), there

was a drop in the sound absorption for the thickness of 0.020 m. However, this

behaviour does not correlate with the basic theoretical concept and understanding.

This is because, as the materials become thicker, it should absorb more sound energy

if compared to the thinner materials. At high frequency more than 3500 Hz, for fibre

material having thickness more than 0.015 m, the sound absorption coefficient

stabilized in the range from 0.90 to 0.99. Overall, the fibre material thickness of

0.015 m has the highest sound absorption coefficient (i.e. 0.99), whereas the fibre

material thickness of 0.005 m has the lowest sound absorption coefficient (i.e. 0.50).

With increasing frequency, the sound absorption coefficient followed the same trend.

Table 13 shows the properties of luffa fibre and the thickness values used in these

empirical calculations. In this case, the porosity, the mean fibre diameter size and the

flow resistivity were taken as constants whereas the fibre material thickness was

varied.

Table 13: Properties of the luffa fibre and flow resistivity value used in

empirical calculations

Type of

Fibre Thickness Porosity Fibre Diameter Flow Resistivity

Luffa

0.005 m

0.930 19 µm 54913 Pa.s.m-2 0.010 m

0.015 m

0.020 m

71

Figure 26: The effect of material thickness of luffa fibre on the sound

absorption (19 µm fibre diameter)

It is known that a thicker material tends to produce a higher sound absorption

coefficient. Based on Zent and Long (2007), thicker material samples created caused

higher sound absorptions or vice versa (as thinner samples caused lower sound

absorptions). This may be true for other type of fibre materials based on the

empirical calculations observed. However, based on Figure 26, it shows the opposite

way of understanding. It is observed that the sound absorption coefficient of luffa

fibre with a lower thickness of 0.015 m can achieve a higher sound absorption

coefficient at higher frequency ranges. The reason behind this was due to the

porosity of luffa has a value closed with to the density. This explanation can be

supported based on Ren et al. (2011), it stated that material with low porosity or a

very close porosity with the density will tend to have a higher sound absorption

coefficient ranging from medium to high frequency at low thickness with the same

material used.

Apart from that, when the thickness of the sample material increased, the

improvement in the sound absorption performance at lower to medium frequency

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range is more significant compared to the result in the higher frequency range. A

similar result is reported in another study. According to Ren et al. (2011), there is

little influence in the change of thickness to the sound absorption coefficient

performance at high frequency range, especially for fibre thickness in the range of

0.010 m to 0.020 m. The results obtained in Figure 26, supports the conclusion

drawn by Ren et al. (2011). In conclusion, the basic understanding a thicker material

has a higher sound absorption coefficient only applied at low frequency. This theory

is further supported by the work conducted by Ibrahim and Melik (1978). They

showed that the increased sound absorption with increasing thickness happened at

low frequencies. However, at higher frequency, the thickness had no significant

effect on the sound absorption.

4.1.3 OIL PALM FIBRE

4.1.3.1 EFFECT OF FLOW RESISTIVITY AND FIBRE SIZE OF OIL PALM

FIBRE ON SOUND ABSORPTION

Figure 27 shows the effects of the flow resistivity and the fibre diameter of the oil

palm fibre on the sound absorption. The results were obtained by using a

combination of Mechel, and Delany and Bazley empirical models. As shown in

Figure 27, the sound absorption increased, with decreasing flow resistivity. The

behaviour is different than the banana fibre and the luffa fibre due to its solid

structure and also because of the low porosity value it has. It is also observed that at

the lower frequency range (i.e. from 500 Hz to 2000 Hz), there is instability of sound

absorption. It showed that the fibre of 20 µm had a higher sound absorption in the

range of 500 Hz to 1250 Hz and followed by 25 µm diameter in the frequency range

of 1250 Hz to 2000 Hz. Meanwhile, at medium to higher frequency range (i.e. from

2000 Hz to 6000 Hz), as the fibre diameter increased, the sound absorption increased.

It was also observed that the gap between the lower to medium frequency is nearer

and the gap between the medium to higher frequency is greater as the sound

absorption increases due to increase in frequency. The fibre size of 30 µm had the

highest sound absorption coefficient (i.e. 0.65), whereas the fibre size of 15 µm had

the lowest sound absorption coefficient, (i.e. 0.28). Generally, as the frequency

increased, the sound absorption coefficient also increased. Table 14 shows the

properties of the oil palm fibre and the flow resistivity values used in the empirical

73

calculations. The porosity and the fibre material thickness were taken as constants

whereas the fibre diameter and flow resistivity were manipulated, when predicting

the sound absorption coefficients using the empirical models.

Table 14: Properties of the oil palm fibre and the respective flow resistivity at

different fibre diameters

Type of

Fibre Thickness Porosity Fibre Diameter Flow Resistivity

Oil Palm 0.005 m 0.636

15 µm 2333600 Pa.s.m-2

20 µm 1312600 Pa.s.m-2

25 µm 840080 Pa.s.m-2

30 µm 583390 Pa.s.m-2

Figure 27: The effect of fibre diameter of oil palm fibre on the sound

absorption (5 mm thickness)

At the lower frequency range from 500 Hz to 2000 Hz, it can be observed

that there is instability in sound absorption caused by the tortuosity in the fibre

materials. Random tortuosity caused the sound absorption for 15 µm, 20 µm, 25 µm

and 30 µm to be overlapped with each other and caused the fluctuation in sound

absorption. Furthermore, it is known that the structure of oil palm is denser and

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contains more oil compared to other fibres. These physical and chemical differences

affect the sound absorption behaviour. The almost-solid structure caused the fibre to

act efficiently in increasing the apparent density and hence the sound absorption

behaviour. Knapen et al. (2003) described that the internal structure of the materials

may influence the sound absorption coefficient. Meanwhile, at the frequency range

of 2000 Hz to 6000 Hz, it showed that the higher sound absorption coefficient is

because of the lower flow resistivity values at bigger diameter size. At an increasing

fibre diameter, the structure of the fibre became denser and hence the highest sound

absorption was obtained.

Furthermore, according to Ballagh (1996), the fibre diameter size is inversely

proportional to the flow resistivity. Thus, this support the behaviour of the higher

flow resistivity happened at lower fibre diameter and vice versa. In other words, as

the increase in the fibre diameter size will eventually reduce the flow resistivity and

vice versa. According to the studies done by Koizumi et al. (2002), there was an

increase of sound absorption coefficient as the material density increased, especially

in the middle and higher frequency. Similar results were shown by Ballagh (1995)

research on wool materials at various densities of the materials. Based on both results,

it showed that the less density type of material absorbs more sound at low frequency

compare to the higher density materials that normally absorbed at middle and high

frequency.

4.1.3.2 EFFECT OF THICKNESS OF OIL PALM FIBRE ON SOUND

ABSORPTION

Figure 28 shows the effects of the material thickness of the banana fibre on the sound

absorption coefficient. The results were obtained by using a combination of Mechel,

and Delany and Bazley empirical models. Generally, the increases in the sound

absorption were due to increases in the material thickness. However, based on Figure

28, it showed the material thickness does not affect the sound, especially at medium

to higher frequency. It is observed that at the lower frequency range (i.e. from 500

Hz to 1500 Hz), as the material thickness increased, the sound absorption increased.

Meanwhile, at medium to higher frequency range (i.e. from 1500 Hz to 6000 Hz), the

results of 0.010 m, 0.015 m and 0.020 m fibre overlapped. This behaviour does not

correlate with the basic theoretical concept and understanding. This is because, as the

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materials becoming thicker, it is expected to absorb more sound as compared to

thinner materials. The fibre material thickness of 0.005 m has the highest sound

absorption coefficient (i.e. 0.45), whereas the fibre material thickness of 0.010 m,

0.015 m and 0.020 m have the lowest sound absorption coefficient (i.e. 0.43).

Generally, as the frequency increased, the sound absorption coefficient also

increased. Table 15 shows the properties of oil palm fibre and the thickness values

used in these empirical calculations. In this case, the porosity, the mean fibre

diameter size and the flow resistivity were taken as constants whereas the fibre

material thickness was varied.

Table 15: Properties of the oil palm fibre and flow resistivity value used in

empirical calculations

Type of

Fibre Thickness Porosity Fibre Diameter Flow Resistivity

Oil Palm

0.005 m

0.636 22 µm 1084800 Pa.s.m-2 0.010 m

0.015 m

0.020 m

Figure 28: The effect of material thickness of oil palm fibre on the sound

absorption (22 µm fibre diameter)

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It is known that the thicker material tends to produce higher sound absorption

coefficients. But for oil palm fibre, the sample showed an opposite. It is observed

that the sound absorption coefficient of oil palm fibre with a lower thickness (i.e.

0.005 m) can achieve a higher sound absorption coefficient at medium to high

frequency range. The reason behind this may be due to the fibre agglomeration. The

fibre agglomeration is not expected for the luffa fibre and the banana fibre. The

degree to which the material is consolidated or compacted also becomes the main

factor that may affect the results due to further agglomeration. According to Oldham

et al. (2011), the relatively poor sound absorption may also because of the uneven

nature of the fibres and the difficulty to compact the sample that caused or resulting

in high porosity that indirectly give an opposite result than expected. This is probably

due to the diameters of these fibres differing considerably from those one that the

predictive models have been developed and the possibility that the surfaces of the

very uneven fibres might also contain micro-pores.

4.2 EXPERIMENTATION

Several tests were done to investigate the relationship and the effect of chemical

treatment on sound absorption, mechanical, spectral and morphological properties.

Furthermore, the interrelationship between properties was also investigated.

Composites were fabricated using moulds, with epoxy as matrix and three types of

natural fibre were used as fillers. The fibres are banana fibre, luffa fibre and oil palm

fibre.

4.2.1 ACOUSTICAL PROPERTIES OF BANANA, LUFFA, OIL PALM

FIBRE (SOUND ABSORPTION ANALYSIS)

Figure 29 shows the sound absorption of raw untreated and treated banana, luffa and

oil palm fibre. The fibres were compressed until it reached 5 mm thickness without

adding any adhesive. The compression was done carefully in order to maintain the

original structure of the raw natural fibre as mentioned in Section 3.8. According to

Piggott (2013), the structure of the fibre will be maintained if the pressure is less than

1MPa (less than 150 psi). Based on Figure 29, the sound absorption coefficients

increased with increasing frequency. It is observed that raw luffa fibre had better

sound absorption frequency range of 500 Hz to 2500 Hz, and 3200 Hz to 5000 Hz.

Meanwhile, raw banana fibre showed the highest sound absorption coefficient at the

77

frequency of 5000 Hz to 6000 Hz. The increase in the sound absorption is believed to

be related to the structure of the raw fibre. Natural fibre had a hollow lumen structure

that tends to absorb entities (i.e. nutrition, water, and etc.). Based on Pickering et al.

(2015), most of hydrophilic hollow structure tends to absorbed more entities than the

hydrophilic solid structure due to greater number of open and through pores in the

fibre as illustrated later in Figure 34 in Section 4.2.2.2. A larger hollow lumen

structure on the fibre is believed to absorb sound better. This speculation is

consistent with the result shown in Figure 29. The raw luffa fibre has larger hollow

lumen structure compared to the raw oil palm and banana fibre. The fluctuation of

sound absorption curves in Figure 29 is believed to be the results of the raw fibres

having a wide range of diameters in contrast to an uniform diameter as reported by

Fouladi et al. (2011) in his study of coir fibre sound absorption. According to Fouladi

et al. (2011), the coir fibre has a varied diameter from 0.1 mm to 0.4 mm. Although

different types of fibre were used in this study, it is likely the fibres have a wide

diameter range. The wide distribution of fibre diameter caused the flow resistivity to

be varied significantly especially the samples were exposed to sound emission at

varying frequency.

Similar fluctuations behaviour of sound absorption is reported by Ismail et al.

(2010). Furthermore, based on their research, the sample with 10 mm thickness can

only achieved up to 0.3 - 0.4 sound absorption coefficient. Thus, based on their

sample, it is expected that by using 5 mm thickness, the sample should show the

same sound absorption properties as the raw banana, luffa and oil palm fibre given in

Figure 29 (i.e. fall in the range of sound absorption coefficient of 0.1 - 0.2). Other

than that, among the three types of raw fibre used, raw luffa fibre has a 3D-

interconnected structure which will be shown in Figure 40 (a) in Section 4.2.3.1. The

raw luffa fibre has the main stems interconnected by side branches that work as

natural occurring network. Another reason for higher sound absorption of raw luffa

fibre was due to the tortuosity of the fibre. Tortuosity will increase the sound

absorption path that will lead to high sound absorption. The raw fibre surface

structure also played an important role. For example, the raw oil palm fibre had a lot

of impurities (i.e. such as wax, dry oil and etc.), which caused the surface of the raw

fibre to be smooth. Compared to raw luffa fibre, the fibre has a bit rougher than the

raw banana and oil palm fibres. Furthermore, increased in sound absorption may be

78

due to high density of raw luffa fibre. According to Zulkifli et al. (2008), it showed

that raw oil palm had a higher sound absorption coefficient than raw coir fibre due to

higher density.

Figure 29: The sound absorption coefficient: (a) untreated banana, luffa, and

oil palm fibre without epoxy (5 mm thickness), and (b) treated banana, luffa,

and oil palm fibre without epoxy (5 mm thickness).

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Figure 30: Comparison between untreated and treated sound absorption

coefficient: (a) banana fibre (5 mm thickness) (b) luffa fibre (5 mm thickness),

and (c) oil palm fibre (5 mm thickness).

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Based on Figure 30, it is observed that there are no significant changes at

lower frequency of 500 Hz to 2000 Hz for all fibres. The only significant changes

happen in the middle and upper frequency of 2000 Hz to 6000 Hz, It is also observed

that the treated luffa and oil palm fibre has the sound absorption coefficients

increased by 10 - 20 %, while the treated banana fibre has the sound absorption

coefficient decreased by 10 - 20 %. The reason behind the low sound absorption

coefficient may be due to shrinkage of banana fibre, which resulted decreased in

fibre size. The fibre is taken from pseudo stem part, which is known to be sensitive

with the chemical reactions. Shrinkage of fibre causes reduction in size of pores.

Meanwhile, for the treated luffa and oil palm fibre, the alkaline treatment cause

increased in size of pores and fibre size, which resulted in increased sound

absorption coefficients. The comparison between empirical model and experiment

were discussed in Section 4.3.

4.2.2 BANANA FIBRE EPOXY COMPOSITES

Composites of epoxy and banana fibre were fabricated at 5 wt%, 10 wt%, 15 wt%

and 20 wt% fibre loading. The properties such as acoustical, mechanical and the

infrared spectral of absorption as well as the morphological were investigated and

analysed. Besides that, the comparisons between the untreated and the treated banana

fibre were conducted.

4.2.2.1 MORPHOLOGICAL OF BANANA FIBRE EPOXY COMPOSITES

(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL

MICROSCOPIC ANALYSIS)

Figure 31 (a), (b), (c) and (d) shows the scanning electron microscope (SEM)

micrographs of the composites. It is found that the banana fibres are made up of a

bundle of hollow sub-fibres which have a lumen structure. Banana fibre contains

numerous connected air pores or air cavities. These air cavities are believed to

contribute to the sound absorption of the composites. From Figure 31 (a), it is

obvious that the fibre were entangled in the composite. Thus, this concludes that

there were fibres network structures in the composites (Jiang et al., 2012). However,

raw fibres have dry and liquid substance (i.e. oil, wax and etc.) and impurities that

may fill the structure and covering the fibre as interpreted from Figure 31 (c). The

alkaline treatment caused the re-construction of a hollow lumen structure by

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removing the dry and liquid (i.e. oil, wax and etc.) substance impurities inner and

outer surface of the fibre. Thus, this will indirectly create rough surface on the fibre

as interpreted from Figure 31 (d).

Figure 31: SEM micrographs of banana fibres: (a) tangled banana fibres, (b)

hollow lumen structure, (c) untreated banana fibre, and (d) treated banana

fibre

When the sound wave energy generated projected onto the surface of the

composite structure, the air motion, vibration and compression caused friction within

the microspores walls and limited the movement of air molecules. According to Chen

et al. (2010), because of the viscous and frictional forces, some parts of the energy

created were converted to heat energy and cause reduction in the sound energy

projected on the composites. As can be seen in Figure 31 (b), the alkaline treatment

allowed absorption of epoxy into the hollow lumen structure due to removal of a

certain impurities on the surface and inner structure of fibre. This indirectly

(a) (b)

(c) (d)

Smooth

surface

Rough

surface

Epoxy was absorbed

inside the lumen

structure

Twisted

fibre

82

improved the mechanical properties of banana fibre indirectly. Figure 32 (a) and (b)

shows the light/optical micrograph of both the untreated and treated banana fibres. It

can be seen that the untreated banana fibre structure is more intact compared to the

structure of treated fibre. The alkaline treatment caused the banana fibre to have an

uneven structure. Thus, this caused a certain reduction in the cross sectional area of

the fibre. The untreated fibres have a rough surface which easily that traps air and

this feature tends to improve sound absorption. However, it will not absorb too much

sound as expected if the presence of long path tortuosity within the raw fibre itself

was being eliminated through treatment.

Figure 32: Light/optical micrographs of banana fibre: (a) untreated banana

fibre (b) treated banana fibre

4.2.2.2 ACOUSTICAL PROPERTIES OF BANANA FIBRE EPOXY

COMPOSITES (SOUND ABSORPTION ANALYSIS)

Figure 33 (a) and (b) shows the effect banana fibre loading of composites, on the

sound absorption coefficient. The results were obtained using the two-microphone

transfer function impedance tube method according to ASTM E1050-12 (2012)

standards. By comparing the acoustics behaviour of the raw fibre and the composites,

blending the raw fibre with epoxy resin has reduced the sound absorption coefficient

to a lower level (i.e. from 0.18 to 0.12 at frequency of 5000 Hz to 6000 Hz). The

reason that caused the low sound absorption coefficient may be due to the reduction

in the quantity of fibre used. Lower sound absorption coefficient were obtained

because the fibre involved are less than 20 wt% than the raw fibre is which is 100

wt%. The other reason that caused the low sound absorption coefficient may be due

to process/method used to fabricate the samples. The compression caused the epoxy

(a) (b)

20 µm 20 µm

83

being forced into the hollow lumen of the banana fibre as shown earlier in Figure 31

(b) in Section 4.2.2.1.

Figure 33: The effects of fibre loading on sound absorption for banana epoxy

composites: (a) composites of untreated fibre (5 mm thickness), and (b)

composites of treated fibre (5 mm thickness)

It can be observed form Figure 33 (a) and (b) that the sound absorption

coefficient increases with increasing fibre loading. According to Markiewicz et al.

(2011), the sound absorption is defined as the ratio of the acoustical wave energy that

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84

is absorbed by the composites to the total energy incident to the sample. The

reduction in the sound pressure is due to the absorption of sound wave within the

material. It is believed that the sound energy is converted to heat energy. The

fluctuating behaviour at 2000 Hz, 3000 Hz, and 4000 Hz was due to the specific

characteristic (i.e. tortuosity path) of banana fibre epoxy that tends to reflect and

absorbing sound at that frequency (Yang et al. 2003) as illustration in Figure 34.

Another factor that might influence the fluctuation was due to different fibre

diameter size, uneven structure and crumpled structure of the banana fibre. Overall, it

was observed that banana epoxy composites with 15 wt% tend to absorb more sound

energy than other composites.

Twisted elongation path

Twisted fibre on it axis

Figure 34: Illustration of tortuosity in fibre

At high fibre loading, the banana fibres were more compact within the epoxy

matrix. It is speculated that the banana fibre were packed closer to each other within

the matrix. This compact structure caused a reduction in the size and the volume of

the air void fractions within the composites. This indirectly produced narrower

passages for sound wave and increase in the tortuosity of sound propagation. Thus, it

will cause the sound to travel in a longer distance than the usual distance. Jiang et al.

(2012) showed similar results in their study of the acoustic properties of seven-hole

hollow polyester fibres (SHPF). At increasing fibre loading, the sound absorption

coefficient of the SHPF composites shows an increasing trend. Furthermore,

Markiewicz et al. (2011) also reported that the sound absorption coefficients of PP-

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long hemp fibre composites, PP-crumble hemp plant composites, PP-rapeseed straw

kaszub composites, and PP-long flax fibre composites increased significantly with

the change in the fibre loading, especially in the medium and high frequencies of

sound. Moreover, porosity is another factor that may influence the sound absorption.

Beside the size of the pores, the number and types of pores also influenced the sound

absorption of the materials (Seddeq, 2009). Figure 35 shows types of pores for

hollow lumen structure. Meanwhile, some pores tend to propagate the sound and

dampened it. At fibre loading of 20 wt%, it is found that the composites have the

sound absorption coefficient lowered than the 15 wt%. The reason behind this may

be due to the difference of the microstructures between the two composites. For the

composite at 20 wt% fibre loading, the hollow lumen structure is collapsed, thus this

shorted the sound to travel inside the natural fibre epoxy composites. This

characteristic is not observed for the 15 wt% fibre loaded composite.

Closed pores

Through pores

Inner hollow

structure

Open pores

Blind pores

Rough surface

Outside of the fibre

Figure 35: Illustration of type of pores in hollow fibre

Figure 36 shows the effects of the chemical treatment on sound absorption of

banana fibre epoxy composite. Based on Figure 36, it shows that the alkaline

treatment of the banana fibre decreased the sound absorption coefficient of banana

fibre epoxy composites. It was observed that composite at 15 wt% untreated banana

fibre loading tends to absorb more sound compared to the counterpart with treated

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banana fibre. Significant difference in sound absorption can be seen in the frequency

ranges from 2000 Hz to 3000 Hz; and from 5000 Hz to 6000 Hz. This is because the

chemical treatment using sodium hydroxide and heat treatment cause the fibre to

shrink, and hence affects the structure of banana fibre. Changes in the structure of

banana fibre caused the elimination of liquid-structure (i.e. moisture) inside the

banana fibre. Thus, this caused the epoxy to absorb into the fibre easily. The

absorption of epoxy caused the closing in the porous structure of the fibre and hence

reduces the porosity. Furthermore, according to Fouladi et al. (2011), the mix

between the matrix and the fibre during fabrication causes the matrix and the fibre

act as one part of the materials itself. This indirectly will create a less porous

structure with lower tortuosity for sound propagation.

Figure 36: The effects of chemical treatment on banana fibre for banana fibre

epoxy composites at 15 wt% (5 mm thickness)

4.2.2.3 MECHANICAL PROPERTIES OF BANANA FIBRE EPOXY

COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS)

Figure 37 (a) and (b) shows the effect of fibre loading of the composite on the tensile

strength and yield strength. Figure 37 (a) and (b) also compares the tensile and yield

strength for the untreated and the treated banana epoxy composites. The results were

obtained by using the tensile testing machine according to ASTM D638-14 (2014)

standards. Based on Figure 37 (a) and (b), it shows that increasing in the fibre

loading has a decreased tensile and yield strength of banana epoxy composites. The

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87

optimum fibre loading is at 5 wt%. At 5 wt% fibre loading, the composite has the

highest tensile and yield strength followed by 20 wt%, 10 wt% and lastly 15 wt%.

The increased in the tensile and yield strength is due to the inclusion of fibres (Liu et

al., 2009). However, because the banana fibre has a flatter structure unlike other

natural fibres which are rounded, the fibre caused a different effect on the composite

tensile and yield strength. The randomized distribution of the banana fibre inside the

epoxy matrix during the moulding may cripple of fibre structure, crumples the fibre.

It is speculated that this randomization caused the tensile and yield strength to

behave in the opposite way as expected for other types natural fibre composite. The

mechanical properties of composites normally depend on the fibre matrix interaction.

It is known that when the fibre loading is higher, the resin will be insufficient to

completely wet the fibre. However, in this case, the overlapping between the flat

surfaces of the fibres may cause incomplete wetting and caused the reduction in the

tensile and yield strength test results.

Another factor that caused the decrease in the tensile and yield strength was

due to the improper fracture of the composite. This happened as the flat banana fibres

started to break first before the epoxy started to break. Improper fracture was also

caused due to the small holes, void or internal crack caused by poor dispersion of

fibre inside the composites. However, according to Boynard & D’Almeida (2000),

this may be considered as controlled fracture which does not break suddenly like

normal fracture. According to Prasad et al. (2014), it may be also because of the poor

dispersion of the fibres. Other than that, the weak banana fibre itself caused the

decreasing in the composites strength. The composites based on the treated banana

fibre have a higher tensile and yield strength compared to the counterpart with

untreated banana fibre. The alkaline treatment modified the structure of banana fibre.

It caused the surface to be rough. It also removes moisture and change in the

molecular structure of fibre. The two effects later indirectly increased the fibre

strength (Balakrishna et al., 2013).

88

Figure 37: The effects of banana fibre loading on: (a) tensile strength, and (b)

yield strength

4.2.2.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS OF

BANANA FIBRE EPOXY COMPOSITE

Figure 38 shows the FTIR spectral of the untreated banana fibres and the treated

banana fibres. In Figure 38 the region of the broad absorption band at 3500 cm-1

to

3000 cm-1

for the both treated and untreated banana fibres are characterized with O-H

stretching and H- bonding structure that contains phenols, alcohols and water

functional group. The O-H stretching, and H- bonding broad absorption band in the

regions is decreased after alkaline treatment. The decreased of the broad absorption

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Untreated Banana Epoxy Treated Banana Epoxy

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Untreated Banana Epoxy Treated Banana Epoxy

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(a)

89

band promote addition of two small peaks for the treated banana fibres at 3431.36

cm-1

and 3300.20 cm-1

. The decrease of functional group phenolic or aliphatic

hydroxyl in the fibre was due to the reaction with sodium hydroxide which promotes

almost hydroxyl free bond structure. A small peak at 2924.09 cm-1

and 2870.08 cm-1

for the untreated banana fibre was attributed to the C-H stretching and O-H

stretching of alkanes (cellulose and lignin) and carboxylic acids. According to Saw et

al. (2013) small peak in the region of the C-H stretching includes the functional

group of methyl and methylene (cellulose and lignin). These peaks were reduced due

to alkaline treatment.

Figure 38: Fourier transforms infrared spectroscopy analysis of: (a) untreated

banana fibre; and (b) treated banana fibre

(a)

(b)

90

Table 16 shows the characterisation of the untreated banana fibres and the

treated banana fibres as extracted from Figure 38. For the untreated banana fibre, it

shows that the small peak band at 2403.30 cm-1

is characterized as the C≡ C

stretching and C≡ N stretching of the functional group of nitriles and alkynes. The

peaks at 1720.50 cm-1

and 1635.64 cm-1

are characterized as the C=C stretching of

the functional group of alkenes (lignin). The small peaks at 1323.17 cm-1

and

1247.94 cm-1

are characterized as the C-H bending of the functional group of alkanes

(cellulose, hemicellulose and lignin). The peak at 1018.41 cm-1

is characterized as

the C-O stretching of the functional group of alcohols (cellulose, hemi-cellulose and

lignin) and the small peak at 883.40 cm-1

is characterized as C-H bending, C-H rock,

N-H wags of the functional group of aromatic rings (out-of-plane), alkanes, 1°, 2°

amines (lignin). Compared with the treated fibre in Figure 38 (b), there were a

reduction in the intensity of the absorption bands at 1720.50 cm-1

and 1635.64 cm-1

and shifting of the absorption peak to 1662.64 cm-1

that are characterized as C=C

stretching from the functional group, alkenes, and another shifting from 1323.17 cm-1

and 1247.94 cm-1

to 1419.61 cm-1

which are characterized as C-H bending from the

functional group, alkanes due to the reaction of sodium hydroxide. The alkaline

treatment caused removal of lignin, hemicellulose and cellulose structure. Thus, this

created hollow lumen structure and rough surface of banana fibre that tends to give

better absorption of epoxy resin into the pore and surface. Better absorption of epoxy

caused greater reduction in sound absorption coefficient. Thus, this caused reduction

in the sound absorption for the treated composites as compared with raw banana fibre

and untreated banana fibre epoxy composites. The process also caused a swelled

structure due to the changes in the structure, morphology, mechanical and dimension

properties; whereas some part of the fibre tend to absorbed more entities (i.e. epoxy)

than the other part of it due to change in diameter (Bledzki and Gassan. 1999).

Swelling increased the banana fibre diameter size and porosity that increased the

sound absorption coefficient.

Due to alkaline treatment, the alkaline sensitive hydroxyl groups (O-H bond

structure) present in the natural fibre molecules was broken. It then reacted with

water, phenols or alcohols molecule groups (H-O-H bond structure) and moved in or

out from the fibre structure depending on the characteristic of fibre toward the

reaction of alkaline. Thus, the remaining of the reactive molecules indirectly formed

91

the fibre cell of -O-Na bond structure between the cellulose molecular chains (John

and Anandjiwala, 2008). Thus, some hydrophilic hydroxyl groups were reduced and

the fibre moisture resistance properties were increased. Some of the impurities,

pectin, hemicelluloses, hemi-cellulose, lignin, wax and oil that covered the fibre

surface were removed as due to alkaline treatment (Li et al., 2007). Alkaline

treatment also caused fibrillation that broke the composites fibre bundle into smaller

fibres and reduced the fibre diameter and length. An increased in the aspect ratio of

the fibre led to enhanced interface adhesion between fibre and matrix due to higher

surface roughness (Maiti et al., 2014). The sound absorption, mechanical, water

absorption and thermal behaviours of the composites are expected to improve

significantly by this treatment. According to Kabir et al. (2012), the treated fibres are

known to have lower lignin content than the untreated fibres. Furthermore, the

chemical treatment also partially removes the oil and wax that cover fibres and

distension of crystalline cellulose order (Kabir et al., 2012).

Table 16: The characterisation of banana fibre using infrared spectral analysis

Bond - Functional Group

Untreated banana

fibre (Wavenumber,

cm-1

)

Treated banana

fibre

(Wavenumber, cm-1

)

O-H stretching , Free hydroxyl -

Alcohol, Water, Phenols 3797.34, 3684.04 3855.70, 3728.40

O-H stretching , H- bonded, -

Alcohol, Water, Phenols 3284.77 3431.36, 3300.20

C-H stretching, O-H stretching -

Alkanes (CH; CH2; CH3),

carboxylic acids

2920.23, 2870.08 -

C≡ C stretching, C≡ N stretching –

Nitriles, Alkynes 2403.30 2418.74, 2287.58

C=C stretching - Alkenes (lignin) 1720.50, 1635.64 1662.64

C-H bending - Alkanes (cellulose;

hemi-cellulose; lignin) 1323.17, 1247.94 1419.61

C-O stretching - Alcohol

(cellulose; hemi-cellulose; lignin),

Carboxylic acids, Esters, Ethers.

1018.41 1029.99

C-H bending, C-H rock, N-H wag -

Aromatic rings (out-of-plane ),

Alkanes, 1°, 2° Amines (lignin)

883.40 844.82

92

4.2.3 LUFFA FIBRE EPOXY COMPOSITES

Composites of epoxy and luffa fibre were fabricated at 5 wt%, 10 wt%, 15 wt% and

20 wt% fibre loading. The properties such as acoustical, mechanical and the infrared

spectra of absorption as well as the morphology were investigated and analysed.

Besides, the comparisons between the untreated and the treated luffa fibre were

conducted.

4.2.3.1 MORPHOLOGY OF LUFFA FIBRE EPOXY COMPOSITES

(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL

MICROSCOPIC ANALYSIS)

Figure 39 (a), (b), (c) and (d) shows the scanning electron microscope (SEM)

micrographs of the composites. A single luffa fibre is made up of a bundle hollow

sub-fibre as can be seen in Figure 39 (b). Meanwhile, according to Yang et al. (2012),

the sub-fibre cell wall is made up of millions nano-fibrils luffa fibre. It also contains

numerous connected air pores or air cavities. The nano-sized fibril would lead to

extra vibration that caused higher energy dissipation. The distribution of fibres in the

composites also produced a porous structure that promoted sound absorption. It is

speculated that the porous structure is the major contribution for sound absorption.

From Figure 39 (a), it is obvious that the fibre were randomly entangled in the

composite. Thus, this concludes that there were fibres network structures in the

composites (Jiang et al., 2012). However, raw fibres have oil, wax or other impurities

that may fill in the hollow lumen structure and covering the fibre as can be

interpreted from Figure 39 (c). The alkaline treatment caused the re-construction of a

hollow lumen structure by removing the oil, wax or other impurities inner and outer

surface of the fibre. Thus, this will indirectly create rough surface on the fibre as

interpreted from Figure 39 (d). Figure 40 (a) and (b) shows the light/optical

micrographs of luffa for untreated and treated.

93

Figure 39: SEM micrographs of luffa fibres: (a) randomized tangled luffa fibres,

(b) hollow lumen structure, (c) untreated luffa fibre, and (d) treated luffa fibre

As the sound wave projected incident on the surface of the porous luffa epoxy

composite structure, the air compression and motion tend to cause vibration and

friction between microspores walls and restrict the movement of the air. Because of

the friction and viscous forces, some parts of the sound energy are converted to heat,

which could subsequently cause sound energy attenuation. Likewise, the heat loss

caused by the heat exchange between the air, microspores and the microspore wall

will also cause sound energy attenuation. Due to the bonding between luffa and

matrix, the sound tends to reflect as it hit the composite wall because of its heavy

bonding and tighter tangle arranged structure. The higher reflection and lower sound

absorption is due to the dense layer created by the pectin, hemicellulose, and other

low-molecular substance on the surface and internal structure of the fibre in the

untreated version of the composites. According to Yilmaz et al. (2012), the treatment

may also cause the fibre flow resistivity to be reduced (as this created a better bond

between matrix and fibre), which makes it one of the primary factors influencing the

reduction in the sound absorption coefficients.

Tangled

fibre

Hollow lumen

structure of fibre

Smooth

surface

Rough

surface

(a) (b)

(c) (d)

94

Figure 40: Light/optical micrographs of luffa fibre: (a) untreated luffa fibre (b)

treated luffa fibre

4.2.3.2 ACOUSTICAL PROPERTIES OF LUFFA FIBRE EPOXY

COMPOSITES (SOUND ABSORPTION ANALYSIS)

Figure 41 (a) and (b) shows the sound absorption coefficient of untreated and treated

luffa epoxy composites. Comparing the acoustics property of the raw luffa fibre and

the composites, it showed that blending epoxy resin with the raw luffa fibre reduced

the sound absorption coefficient (i.e. from 0.16 to 0.08 at the frequency of 4000 Hz

to 6000 Hz). The reason that caused the low sound absorption coefficient may be due

to the reduction in the quantity of fibre used. Lower sound absorption coefficient

were obtained because the fibre involved are less than 20 wt% than the raw fibre is

which is 100 wt%. Other than that, the reason for the low sound absorption

coefficient can be related to the process/method used to fabricate the samples. For

the range of fibre loadings concerned, it can be observed that increasing fibre content

resulted in higher sound absorption coefficients at increasing frequency. Figure 41 (a)

and (b) also show that the sound absorption of 20 wt% untreated and treated luffa

epoxy composite have a higher sound absorption coefficient compared to other fibre

loading. At high fibre loading, it is expected that the composite is compacted with

fibres, and this structure should have reduced air voids and makes the air passages

much narrower and more tortuous. Thus, this caused the sound wave to travel at a

longer distance. This resulted in the reduction of sound energy (Huang et al., 2013).

This was also supported by Jiang et al. (2012), whereas there is an increase in the

sound absorption coefficient as the seven-hole hollow polyester fibre content

increased.

(a) (b)

20 µm 20 µm

95

Figure 41: The effects of fibre loading on sound absorption for luffa epoxy

composites: (a) composites of untreated fibre (5 mm thickness), and (b)

composite of treated fibre (5 mm thickness)

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Figure 42 shows the comparison between untreated and treated luffa epoxy

composites. Chemical treatment was used to remove the moisture and impurity of the

fibre, and promotes better adhesion of the epoxy. This alkalisation or chemical

treatment indirectly gave a big impact on the sound absorption properties of the

material. Specifically, the treated luffa epoxy composite an exhibits higher sound

absorption if compared to the untreated luffa fibre reinforced epoxy composite. The

soaking of luffa fibre in alkaline during the chemical treatment and the drying

process using an oven, caused swelling of the fibre and hence increase sound

absorption property.

Figure 42: The effects of chemical treatment on luffa fibre for the luffa fibre

epoxy composites at 20 wt% (5 mm thickness)

4.2.3.3 MECHANICAL PROPERTIES OF LUFFA FIBRE EPOXY

COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS)

From Figure 43 (a) and (b), it is clear that there was an increase in the tensile and

yield strength of both the untreated and treated luffa epoxy composites. The

maximum tensile and yield strength were achieved at the fibre loading of 15 wt% for

the untreated luffa epoxy composites. For the treated luffa epoxy composite, the

optimum fibre content occurred at 10 wt% fibre. The increase in tensile and yield

strength of the composites was due to the increase in the fibre loading (for the both

types of fibre) in the composites. The increase in the fibre content caused the load to

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97

be distributed uniformly within the composites. After reaching the optimum fibre

loading, the tensile and yield strength decreased at increasing fibre loading. This

reduction after the optimum point is because of the decrease in the binder content.

The decrease in the epoxy content was due to excessive luffa fibre in the composites

at high fibre loading. According to Liu et al. (2009), the increase in the tensile

strength was due to the presence of fibres dispersed within the matrix that allowed

uniform distribution of stress and load on the materials. Also, a decrease in the

tensile and yield strength after the optimum level was due to the high excessive of

fibre in the materials, which contributes to the initiation of cracks. Cracks have been

reported to cause non-uniform stress transfer due to the fibre agglomeration within

the fibre matrix (Liu et al. 2009).

Figure 43: The effects of fibre loading on: (a) tensile strength, and (b) yield

strength

0

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98

On the other hand, the highest strength is obtained at the treated fibre loading

of 10 wt%. The lignocellulosic characteristic (lignin, hemicellulose, and cellulose

removal) of was affected by alkaline treatment. This is supported by the other work

from Balakrishna et al. (2013), specifically that the chemical treatment removed the

moisture and impurity substance in the fibre and hence increased the strength.

Besides, the decrease in the tensile and yield strength is more drastic for composites

with treated luffa fibre. The reason for the decrease in the tensile and yield strength is

the low matrix-filler interface. This happens as the matrix binder started to break

before the fibre failed, as can be seen in Figure 44. According to Boynard and

D’Almeida (2000), this type of failure or fracture is considered as controlled fracture.

Specifically, the failure will not occur suddenly.

Figure 44: Failure of the luffa fibre epoxy composite. Failure on the epoxy has

initiated whereas a significant portion of the fibre is still intact

4.2.3.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS OF

LUFFA FIBRE EPOXY COMPOSITES

Figure 45 shows the FTIR spectral of the untreated luffa fibres and the treated luffa

fibres. In Figure 45 the region of the broad absorption band at 3500 cm-1

to 3000 cm-1

for the both treated and untreated luffa fibres are characterized with O-H stretching

and H- bonding that contains phenols, alcohols and waters functional group (Gañán

et al. 2008). Based on Ramadevi et al. (2012) and Saw et al. (2013), the O-H

stretching and H- bonding was ascribed to the presence of carbohydrates (cellulose

99

and hemicellulose). The O-H stretching, and H- bonding broad absorption band in

the regions is decreased after alkaline treatment. The decreased of the broad

absorption band promote addition of two small peaks for the treated luffa fibres at

3861.49 cm-1

and 3597.24 cm-1

. The decrease in functional group, phenolic or

aliphatic hydroxyl in the fibre was due to the reaction with sodium hydroxide which

promotes almost hydroxyl free bond structure. A small peak at 2918.30 cm-1

for the

untreated luffa fibre was attributed to the C-H stretching and O-H stretching of

alkanes (cellulose and lignin) and carboxylic acids. According to Saw et al. (2013)

small peak in the region of the C-H stretching includes the functional group of

methyl and methylene (cellulose and lignin). These peaks were reduced due to

alkaline treatment.

Figure 45: Fourier transforms infrared spectroscopy analysis of: (a) untreated

luffa fibre; and (b) treated luffa fibre

(a)

(b)

100

Table 17: The characterisation of luffa fibre using infrared spectral analysis

Bond - Functional Group Untreated luffa fibre

(Wavenumber, cm-1

)

Treated luffa fibre

(Wavenumber, cm-1

)

O-H stretching , Free hydroxyl -

Alcohol, Water, Phenols 3782.41

3861.49, 3732.26,

3597.24

O-H stretching , H- bonded, -

Alcohol, Water, Phenols 3305.99 3321.42

C-H stretching, O-H stretching -

Alkanes (CH; CH2; CH3),

carboxylic acids

2918.30 2906.73

C≡ C stretching, C≡ N stretching –

Nitriles, Alkynes 2146.77 2276.00

C=C stretching - Alkenes (lignin) 1734.01, 1612.49 1658.78

C-H bending - Alkanes (cellulose;

hemi-cellulose; lignin)

1423.47, 1323.17,

1244.09 1506.41, 1431.18

C-O stretching - Alcohol

(cellulose; hemi-cellulose; lignin),

Carboxylic acids, Esters, Ethers.

1029.99 1024.20

C-H bending, C-H rock, N-H wag -

Aromatic rings (out-of-plane ),

Alkanes, 1°, 2° Amines (lignin)

898.83 -

Table 17 shows the characterisation of the untreated luffa fibres and the

treated luffa fibres as extracted from Figure 45. For the untreated luffa fibre, it shows

that the small peak band at 2146.77 cm-1

is characterized as the C≡ C stretching and

C≡ N stretching of the functional group of nitriles and alkynes. The peaks at 1734.01

cm-1

and 1612.49 cm-1

are characterized as the C=C stretching of the functional

group of alkenes (lignin). The small peaks at 1423.47 cm-1

, 1323.17 cm-1

and

1244.09 cm-1

are characterized as the C-H bending of the functional group of alkanes

(cellulose, hemicellulose and lignin). The peak at 1029.99 cm-1

is characterized as

the C-O stretching of the functional group of alcohols (cellulose, hemi-cellulose and

lignin) and the small peak at 898.83 cm-1

is characterized as C-H bending, C-H rock,

N-H wags of the functional group of aromatic rings (out-of-plane), alkanes, 1°, 2°

amines (lignin). Compared with the treated fibres in Figure 45 (b), there were

reduction in the intensity of the absorption bands at 1734.01 cm-1

and 1612.49 cm-1

and shifted of the peak 1658.78 cm-1

that are characterized as C=C stretching from

functional group alkenes and another reduction in the intensity of the absorption

101

bands and shifting from 1423.47 cm-1

, 1323.17 cm-1

, and 1244.09cm-1

to 1506.41 cm-

1 and 1431.18cm

-1 are characterized as C-H bending from the functional group,

alkanes, due to the reaction of sodium hydroxide. Furthermore, there was a

reduction in the intensity of the bands at 898.83 cm-1

characterized as C-H bending,

C-H rock, N-H wag of the functional groups of aromatic rings (out-of-plane), alkanes,

1°, 2° amines (lignin). This caused disappearing of smell from the luffa fibre.

The alkaline reaction caused the removal of lignin, hemicellulose and

cellulose structure. Thus, this hollow lumen structure and rough surface of luffa fibre

are believed to give better absorption of epoxy resin (i.e. on the surface and through

the pores). Although, better absorption of epoxy caused reduction in the sound

absorption coefficient, the multi-layered cells of the luffa fibre prevented the epoxy

to be absorbed due to its high resistivity and high tendency toward alkaline treatment

(i.e. whereas luffa fibre contained sub-layered structure within the fibre structure).

Thus, this caused increased in the sound absorption inside the treated luffa fibre

epoxy composites as compared with untreated luffa fibre epoxy composites. The

process also caused a swelled structure due to the changes in the structure,

morphology, mechanical and dimension properties; whereas some part of the fibre

tend to absorbed more epoxy than the other part of it (Bledzki and Gassan. 1999).

Swelled increased the banana fibre diameter size and porosity that increased the

sound absorption coefficient.

4.2.4 OIL PALM FIBRE EPOXY COMPOSITES

Composites of epoxy and oil palm fibre were fabricated at 5 wt%, 10 wt%, 15 wt%,

and 20 wt% fibre loading. The properties such as acoustical, mechanical, and the

infrared spectra absorption as well as the morphology were investigated and analysed.

Besides, the comparisons between the untreated and the treated oil palm fibre were

conducted.

4.2.4.1 MORPHOLOGY OF OIL PALM FIBRE EPOXY COMPOSITES

(SCANNING ELECTRON MICROSCOPE AND LIGHT/OPTICAL

MICROSCOPIC ANALYSIS)

Figure 46 (a), (b), (c) and (d) shows the scanning electron microscope (SEM)

micrographs of the composites. As shown in Figure 46 (a), the fibre has a smooth

102

outer surface layer. From the perspective of the interface strength, this may cause

poor adhesion between the epoxy matrix and the fibres. Meanwhile, Figure 46 (c)

shows the hollow lumen structure of oil palm fibre after treatment. Figure 46 (b)

shows the treated oil palm fibre epoxy composites with the interface of fibre and

matrix.

Figure 46: SEM micrographs of oil palm fibres: (a) untreated oil palm fibres, (b)

treated oil palm fibres, (c) hollow lumens structure of oil palm fibre, and (d)

agglomeration of oil palm fibre

The alkaline treatment caused the smooth outer surface layer to be removed

together along with the oil and impurities. In addition, this treatment caused the fibre

surface to be rough, cripple and tear out. The structure on the outer surfaces

promotes better adhesion of fibre and epoxy. The cripple and tear out fibre caused

interlocking within the fibre and epoxy. Meanwhile, the rough surface created

friction that decreases the movements of fibre agglomeration. Thus, this resulted in

the high tensile and yield strength properties of the composites. Also, there are

potholes and small bubbles observed. These defects are also reported in the published

Smooth

surface

Better adhesion

between fibre and

matrix

Rough

surface

Void/bubbles

Hollow

lumen

structure

Fibre

(a) (b)

(c) (d)

103

works of Matta et al. (2014) and Nayak (2014). Nayak et al. (2014) stated that the

traces of the potholes or the small bubbles were resulted from to the hand lay-up

fabrication technique. The mixing technique of the pre-mixed epoxy with fibre

contributed in likely to produce the potholes or small bubbles. This happened when

the vigorously stirred epoxy caused the air being trapped. This problem can be

alleviated by letting the mixture settle for a couple of minutes before being

compression moulded with pressure. As seen in the SEM images in Figure 46 (d), the

alkaline treatment which caused the fibre cripple and tear out also contributed to

potholes or small bubbles. The cripple and tear out fibre may trap the air indirectly.

Figure 47: Light/optical micrographs of oil palm fibre: (a) untreated oil palm

fibre (b) treated oil palm fibre, and (c) fracture part of oil palm fibre epoxy

Figure 47 (a), (b) and (c) show light/optical microscope images of oil palm

epoxy composites. In Figure 47 (a), it shows that the untreated oil palm fibre had less

adhesion to the epoxy matrix if compared to the treated fibre in Figure 47 (b). The

treated oil palm epoxy composite showed better adhesion between the epoxy matrix

and the fibres because of alkaline treatment. The removal of impurities and oil on the

fibres caused the fibres to become highly adhesive to the matrix because of the

hollow structures of the fibre. Furthermore, the drying process of the fibres caused

the moisture into the fibres to be removed and increased the span of absorption of the

fibres. Although the untreated fibres were dried too, the impurities and the oil

(a) (b)

(c)

20 µm 20 µm

20 µm

104

prevented and reduced the adhesion. Figure 47 (c) shows the fracture surface of oil

palm fibre epoxy. These types of fractures showed that the epoxy was semi-

amorphous (Odegard and Bandyopadhyay, 2011). The traces of epoxy at the fibre

showed that fibre absorbed a certain amount of epoxy and resulted in a more brittle

behaviour.

4.2.4.2 ACOUSTICAL PROPERTIES OF OIL PALM FIBRE EPOXY

COMPOSITES (SOUND ABSORPTION ANALYSIS)

Based on the acoustics behaviour of the raw oil palm and the composites in Figure 48

(a) and (b), it showed that blending epoxy resin with the raw fibre caused a reduction

to the sound absorption coefficient from 0.0.12 to 0.09 at frequency range of 4000

Hz to 6000 Hz. The reason that caused the low sound absorption coefficient may be

due to the reduction in the quantity of fibre used. Lower sound absorption

coefficients were obtained because the fibre quantity involved is less than 20 wt%.

These fibre loadings were low if compared to the sample fabricated at 100 wt% raw

fibre. The reason for this reduction is the same to the case of banana fibre and luffa

fibre reported in Section 4.2.2.2 and Section 4.2.3.2. The compression moulding

process is believed to force the epoxy resin into the hollow lumen structure of the oil

palm fibre.

Considering the effect of fibre it can be observed that higher fibre content

exhibits a higher sound absorption coefficient at increasing frequency. It is also

shown that the sound absorption of the composite at 15 wt% fibre loading has a

higher sound absorption coefficient than the others. The increase in the sound

absorption coefficients is believed to relate to the microstructure as shown earlier in

Figure 46 (b) in Section 4.2.4.1. At increasing fibre loading, the structure of the

composites becomes more compact. In addition, the amount of fibres that brought

nearer to each other caused a decreased in the size and volume of air voids. Thus,

these indirectly created narrower air passages and increased the tortuosity. However,

at 20 wt% of fibre loadings for both untreated and treated composites, the sound

absorption coefficients were found to decrease. This is because of the shrinkage of

fibres as a consequent of being compressed nearer to each other.

105

Figure 48: The effects of fibre loading on sound absorption for oil palm epoxy

composites: (a) composites with untreated fibre (5 mm thickness), and (b)

composites with treated fibre (5 mm thickness)

Besides that, the shrinkage caused the hollow fibre structure to collapse. The

hollow structure of the fibre is expected to trap and absorb sound that entered the

composites. Thus, in other words, the sound waves created tends to bounce back if

the hollow lumen structure is collapsing. Apart from that, the increasing sound

absorption with increasing fibre loading can be supported by the research done by

Jiang et al. (2012) on seven-hole hollow polyester fibre (SHPF) composites. The

0

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0 1000 2000 3000 4000 5000 6000

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)

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5wt% 10wt% 15wt% 20wt%

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(a)

(b)

106

SHPF composites showed an increased in sound absorption coefficients as the SPHF

content increased. Another factor that contributed in the sound absorption

coefficients was the random orientation distribution of oil palm fibres within the

composites. Randomized in size and length was also another reason that contributes

in the sound absorption coefficients. Indirectly, this created a different angle of fibre

orientation in the composites. Based on Chen et al. (2010) research, the sound

absorption of ksFIB/PLLA was comparably higher to PLLA and ksFAB/PLLA

because of no specific orientation and uniform distributed of ksFIB in the composites.

When the sound waves hit the surface of the composites, it dispersed in the material

at a random direction and thus being absorbed after being reflected at different angles

by the fibres in the composites.

Figure 49: The effects of chemical treatment on oil palm fibre for oil palm fibre

epoxy composites at 15 wt% (5 mm thickness)

Based on Figure 49, in terms of the effects of fibre treatment, it is observed

that treated oil palm epoxy composites absorbed more sound compared to untreated

oil palm epoxy composites. Sodium hydroxide causes the oil, impurities and fatty

acids being removed completely. According to Li et al. (2007), the hydrogen bonds

in the network structure of the fibre were disrupted when reacted with sodium

hydroxide. The alkaline treatment increased the fibre surface roughness. Furthermore,

according to Prasad et al. (2014), sodium hydroxide exaggerated the de-

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107

polymerization of cellulose. The rough surfaces caused friction which promotes

sound absorption. The treatment also initiated better adhesion of oil palm fibre and

epoxy. Besides that, the rough surface of the oil palm fibres indirectly allowed the air

to be trapped and created small bubbles that cause disruption of sound propagation in

the composites by creating a medium that reflected sound within the composites

itself.

4.2.4.3 MECHANICAL PROPERTIES OF OIL PALM FIBRE EPOXY

COMPOSITES (TENSILE AND YIELD STRENGTH ANALYSIS)

Figure 50: The effects of oil palm fibre loading: (a) tensile strength, and (b) yield

strength

0

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8

10

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5wt% 10wt% 15wt% 20wt%

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Untreated Oil Palm Epoxy Treated Oil Palm Epoxy

(a)

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5wt% 10wt% 15wt% 20wt%

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a)

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Untreated Oil Palm Epoxy Treated Oil Palm Epoxy

(b)

108

Figure 50 (a) and (b) shows the results of average tensile and yield strength for

untreated and treated oil palm epoxy composites. It can be observed that the tensile

and yield strength increased with increasing fibre loading. The highest strength is

observed at fibre loading 10 wt%. The increased in the tensile and yield strength was

because of the reinforcing effects of fibres (Liu et al. 2009).

According to Liu et al. (2009), the dispersed fibre allowed uniform

distribution of load and stress within the materials. However, the tensile and yield

strength started to show a decrease trend at fibre loading beyond 10 wt% because of

the excessive amounts of fibre contents. According to Prasad et al. (2014), excessive

fibre loading could reduce the uniformity in fibre dispersion. Furthermore, the

excessive fibre loading also promoted initiation the cracks. The cracks caused non-

uniform stress transfer. The similar explanation is also proposed by the published

work by Liu et al. (2009). Another reason for the decreased in the tensile and yield

strength was due to premature fracture of epoxy matrix components of the

composites. Specifically, the epoxy started to fracture while the fibre is still intact.

According to Boynard and D’Almeida (2000), this fracture mode was known as

controlled fracture. As a contrary, a sudden and complete fracture will pose a safety

issue. Besides that, the treated oil palm epoxy showed a higher tensile and yield

strength than the untreated counterpart. This was because of the removal of moisture

and impurity of fibre that increased the fibre strength (Balakrishna et al. 2013).

4.2.4.4 FOURIER TRANSFORM INFRARED SPECTRA ANALYSIS OF OIL

PALM FIBRE EPOXY COMPOSITES

Figure 51 (a) and (b) shows the FTIR spectral of the untreated oil palm fibres and the

treated oil palm fibres. According to Khan et al. (2005), the peak locations shown in

the absorption spectra of the oil palm fibre are characteristics of its constituents. In

Figure 51 (a) and (b), the region of the broad absorption band from 3500 cm-1

to

3000 cm-1

for the untreated oil palm fibres and the treated oil palm fibres are

characterized with O-H stretching and H- bonding that contains functional groups of

phenols, alcohols and waters (Gañán et al. 2008). Based on the published works of

Ramadevi et al. (2012) and Saw et al. (2013), the O-H stretching and H- bonding was

due to the presence of carbohydrates (cellulose and hemicellulose). The intensity of

the O-H stretching, and H- bonding broad absorption band is decreased after alkaline

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treatment. Two smaller peaks at 2916.37 cm-1

and 2848.86 cm-1

for the untreated

luffa fibre were attributed to the C-H stretching and O-H stretching of the functional

group of alkanes (cellulose and lignin) and carboxylic acids. According to Saw et al.

(2013), the smaller peak in the region of the C-H stretching represents the functional

group of methyl and methylene (cellulose and lignin). This peak is reduced after

alkaline treatment.

Figure 51: Fourier transforms infrared spectroscopy analysis of: (a) untreated

oil palm fibre, and (b) treated oil palm fibre

Table 18 shows the characterisation of the both untreated and treated oil palm

fibres and the treated oil palm fibres as extracted from Figure 51. For the untreated

oil palm fibre, it shows that the peak at 1708.93 cm-1

is characterized as the C=C

stretching of the functional group of alkenes (lignin). The small peaks at 1369.46 cm-

1, 1313.52 cm

-1, and 1240.23 cm

-1 are characterized as the C-H bending of the

functional group of alkanes (cellulose, hemicellulose and lignin). The peaks at

(a)

(b)

110

1159.22 cm-1

and 1031.92 cm-1

are characterized as the C-O stretching of the

functional group of alcohols (cellulose, hemi-cellulose and lignin) and the small peak

at 896.90 cm-1

is characterized as C-H bending, C-H rock, N-H wags bond structure

of the functional group of aromatic rings (out-of-plane), alkanes, 1°, 2° amines

(lignin). Compared with the treated fibre in Figure 51 (b), there were reduction in the

intensity of the absorption bands at 1159.22 cm-1

and 1031.92 cm-1

and shifting of

the absorption peak to 1033.85 cm-1

that are characterized as C-O stretching of the

functional group alcohols (cellulose; hemi-cellulose; lignin), carboxylic acids, and

esters, as a result of reaction with sodium hydroxide. Furthermore, there was a

reduction in the intensity of the bands at 896.90 cm-1

characterized as C-H bending,

C-H rock, N-H wag of the functional groups of aromatic rings (out-of-plane), alkanes,

1°, 2° amines (lignin) which caused disappearing of smell from the oil palm fibre.

Table 18: The characterisation of oil palm fibre using infrared spectral analysis

Bond - Functional Group

Untreated Oil Palm

Fibre (Wavenumber,

cm-1

)

Treated Oil Palm

Fibre

(Wavenumber, cm-1

)

O-H stretching , H- bonded, -

Alcohol, Water, Phenols 3340.71 3344.57

C-H stretching, O-H stretching -

Alkanes (CH; CH2; CH3),

carboxylic acids

2916.37, 2848.86 2922.16

C=C stretching - Alkenes (lignin) 1708.93 1639.49, 1598.99,

1512.19

C-H bending - Alkanes (cellulose;

hemi-cellulose; lignin)

1369.46, 1313.52,

1240.23

1460.11, 1419.61,

1369.46, 1267.23

C-O stretching - Alcohol

(cellulose; hemi-cellulose; lignin),

Carboxylic acids, Esters, Ethers.

1159.22, 1031.92 1033.85

C-H bending, C-H rock, N-H wag -

Aromatic rings (out-of-plane ),

Alkanes, 1°, 2° Amines (lignin)

883.40 -

The alkaline reaction caused the removal of lignin, hemicellulose and

cellulose from the surface and some in the internal structure. Thus, this created

hollow lumen structure and rough surface of oil palm fibre that tends to give better

absorption of epoxy resin in the fibre and on the surface of the fibre. Although, better

absorption of epoxy caused reduction in the sound absorption coefficient, the multi-

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layered cells of the oil palm fibre prevented the epoxy to be absorbed due to its high

resistivity and high tendency toward alkaline treatment. Thus, this caused reduction

in the sound absorption for the treated composites as compared to the raw oil palm

fibre and untreated oil palm fibre epoxy composites. The process also caused a

swelled structure due to the changes in the structure, morphology, mechanical and

dimension properties; whereas some part of the fibre tend to absorbed more entities

(i.e. epoxy) than the other part of it (Bledzki and Gassan. 1999). Swelled increased

the banana fibre diameter size and porosity that increased the sound absorption

coefficient. These factors are believed to influence the sound absorption of the

composites.

4.3 COMPARISONS BETWEEN BANANA, LUFFA AND OIL PALM FIBRE

AND ITS COMPOSITES

The purpose of the empirical modelling is to identify the potential effect of the

various types of raw natural fibre on sound absorption. As there is no suitable

empirical model for natural fibre composites, parametric study was done on those

empirical models for raw fibre in order to identify the significant parameters

affecting sound absorption. It is acknowledged that the raw fibre and natural fibre

composites exhibit different structure and formulation. Thus, the sound absorption

behaviour of the materials is expected to be different. However, it would be

fascinating if the composites showed consistent trend to the case of raw fibre. In this

section, the findings from empirical modelling will be compared and discussed with

experimental results.

4.3.1 COMPARISONS BASED ON EMPIRICAL MODEL

For effect of fibre size, it is found that luffa fibre has a higher sound absorption

coefficient (i.e. 0.74 at 6000 Hz for fibre size of 10 µm) compared to the banana

fibre (i.e. 0.64 at 6000 Hz for fibre size of 30 µm) and oil palm fibre (i.e. 0.64 at

6000 Hz for fibre size of 20 µm). Luffa fibre has a smaller diameter compared to

banana and oil palm fibre. The smaller diameter caused higher flow resistivity. It also

allowed more fibre to be filled in specific specimen mould. The porosity is

influenced by the number and size of pores inside the fibre. Larger pores in the fibre

will resulted in a more porous fibre. By comparing banana fibre and oil palm fibre,

both of them were having the similar result of sound absorption coefficient (i.e. 0.64

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at 6000 Hz). The oil palm fibre has a smaller diameter than the banana fibre. Smaller

diameter caused more fibre can be filled in the specimen even though the oil palm

fibre is less porous than banana fibre. Thus, it is speculated the abovementioned

reasons caused the increased in the sound absorption of oil palm fibre to be similar to

the banana fibre.

For effect of thickness, it is found that banana and luffa had a higher sound

absorption coefficient (i.e. 0.99 at 6000 Hz for thickness of 0.015m) compared with

oil palm fibre (i.e. 0.44 at 6000 Hz for thickness of 0.015m). The reason behind this

low sound absorption for oil palm fibre was due to the lower porosity value of oil

palm fibre, which causes the fibre to be less sound absorbing. Comparing the sound

absorption trend for banana and luffa fibre, luffa fibre tends to absorb more sound

than the banana fibre. It can be seen that luffa fibre achieved higher sound absorption

at the frequency range from 500 Hz to 3000 Hz if compared to the banana fibre. The

reason behind this was due to the small fibre diameter of luffa fibre (i.e.19 µm)

which is smaller than banana fibre (i.e. 32 µm). Smaller fibre diameter tends to have

higher flow resistivity. Even though the luffa fibres have a lower porosity, it is

believed that the smaller fibre diameter of luffa is the main reason for its better sound

absorption compared to banana fibre and oil palm fibre.

4.3.2 COMPARISONS BASED ON EXPERIMENTATION

For acoustical properties (i.e. sound absorption analysis), it is found that the raw

luffa fibre has a higher sound absorption coefficient than raw banana and oil palm

fibre at the frequency range from 500 Hz to 5000 Hz. As it reached higher frequency,

banana is observed to have a higher sound absorption coefficient. The reason behind

this may be due to natural swelling on the fibre. However, as it mixed with epoxy

resin and fabricated into composites, all of the raw fibres sound absorption

coefficients were reduced by 20-40%. It is found that the highest reduction happened

to the raw luffa fibre. The reason behind this may be due to the nature of luffa fibre

that tends to absorb entities (i.e. water, nutrients and etc.). The nature of luffa fibre is

known to be highly porous which resulted in increase of absorption properties, which

allowed it to become denser after absorption. Furthermore, the fibre has a 3-D

interconnected structure which is believed to be related to the increase in sound

113

absorption. This allowed the sound to be redirected in any direction inside the

composites.

As for natural fibre composites, it is found that banana fibre epoxy

composites has a higher sound absorption coefficient (i.e. 0.12 at 5500 Hz for 15 wt%

untreated banana fibre loading) than luffa fibre epoxy composites (i.e. 0.08 at 6000

Hz for 15 wt% untreated luffa fibre loading) and oil palm fibre epoxy composites (i.e.

0.08 for 15 wt% untreated oil palm fibre loading). The reason behind the higher

sound absorption observed in Section 4.2 may be due to the banana fibre used in the

fabrication of composites has higher density than luffa and oil palm fibre. For the

treated natural fibre epoxy composites, the sound absorption was found to increase

for luffa and oil palm fibre. However, for banana fibre, the alkaline treatment caused

the sound absorption to decrease. The chemical treatment caused structural change

that increased the size and number of pores in the fibre especially to luffa and oil

palm fibre. Furthermore, the surfaces of the fibre were found to be rough as can be

seen in the micrographs in Section 4.2 after the removal of oil, wax and impurities as

verified by FTIR. Surface roughness caused better adhesion that indirectly increased

the tensile and yield strength of the natural fibre composites. It is also known that

chemical treatment caused changes in the H-, C-H, and O-H bonding and related

functional groups at the fibre surface. Alkaline treatment caused stretch, break and

re-bond to the molecular structures. Taking luffa fibre as an example, the reduction

of intensity of O-H bond at 3500 cm-1

to 3000 cm-1

cause breaking of O-H bond

which cause an addition of an extra peak for treated fibre was due to the removal of

waterish (hydroxide) structure. This caused the fibre to be more absorbent which

may cause better absorption. Thus, this shows that the chemical treatment improved

the strength and sound absorption of the composites.

4.3.3 COMPARISONS BASED ON EMPRICAL MODEL AND

EXPERIMENTATION

Based on the empirical and experimental results, it showed that the sound absorption

coefficient of banana, luffa and oil palm fibre for were varied due to the change in

porosity and etc. It is found that the sound absorption predicted for empirical model

is higher than the experimentation by 20 % – 30 %. Meanwhile by comparing the

raw fibre with natural fibre composites, the results were dropped by 20-30%. There

114

are a few reasons behind this difference. It is well known that the natural fibre used

in the experiment or in real-world applications does not have a wide distribution of

fibre diameter. It is expected that the fibre diameter will vary depends on the part of

the crop it is derived. The natural fibre size depends on the nutrient the plant gained

from the soil. On the other hand, the empirical model considers the fibre to have a

uniform diameter which may be obtained from literature or measurement, which is

based on fibres from different sources (Oldham et al. 2011). Moreover, the variation

of fibre diameter will create random flow resistivity due to wide and narrower path

for sound to travel which resulted in the fluctuating pattern in sound absorption as

shown in Section 4.2.2, Section 4.2.3 and Section 4.2.4. Another reason for the

difference is that the porosity of an individual fibre is different to another fibre in the

same batch based on results obtained in literature. Furthermore, some empirical

model used different ways to predict and obtained the porosity values, whereas the

empirical model porosity valued was measured by the total change in volume

(Oldham et al. 2011, Mechel 2008). Although there are a few differences between

the results of the empirical modelling and experimentation, it still identifies the

significant parameters influencing the sound absorption, including fibre size, fibre

types and etc. Other than that, it can be used as early prediction and be used as basic

concept of understanding of relationship of sound absorption between theoretical and

experimental result. It is able to predict the sound absorption of banana, luffa and oil

palm fibre. Since the model does not assume a materials structure and formulation

other than fibrous materials, it cannot be used in prediction of natural fibre composite

sound absorption behaviour.

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CHAPTER 5

5.0 CONCLUSIONS AND FUTURE WORKS

5.1 CONCLUSIONS

Based on the empirical calculations conducted, it can be concluded that the higher

the flow resistivity, the higher the sound absorption coefficient. It showed that luffa

fibre had a higher sound absorption (i.e. 0.74 at 6000 Hz for fibre size of 10µm) than

banana fibre (i.e. 0.64 at 6000 Hz for fibre size of 30 µm) and oil palm fibre (i.e. 0.64

at 6000 Hz for fibre size of 20 µm). This is due to the higher flow resistivity as fibre

diameter is smaller. The material thickness has a direct effect on the sound

absorption. An increase in the thickness of the materials will result in higher sound

absorption coefficient. It showed that luffa fibre and banana fibre have the higher

sound absorption (both fibre having 0.99 at 6000 Hz for thickness of 0.015m) than

oil palm fibre (i.e. 0.44 at 6000 Hz for thickness of 0.015) at increasing thickness.

Although the modelling gave different values to the experiment results, it showed a

comparable trend sound absorption. Specifically, for the fibre studied, the sound

absorption coefficients increased with increasing frequency.

Meanwhile, for the natural fibre polymer composites, the samples exhibited

almost similar trend at all frequencies, specifically the sound absorption coefficient

for natural fibre polymer composites increased with increasing frequency. Compared

with the results obtained for raw natural fibre, the sound absorption coefficients were

20 - 30 % lower. For the case of the composites, the sound absorption coefficients

increased with increasing fibre loading from 5 wt% to 15 wt%. The fibre loading has

an effect on the structure of the composites. Specifically, at excessive fibre loading,

the lumen structure of the fibre is found to be collapsed. Thus, this structure resulted

in lower sound absorption. It is speculated that, the collapsed of hollow lumen

structure was due to over-packed fibre in the specimens due to compression.

Further experimentation on the composites revealed that the treated fibre with

alkalisation altered the characteristics of the natural fibre. Alkalisation caused the

fibre surface to be rough due to the removal of impurities and wax from the fibre and

subsequently improved the interfacial adhesion between the fibre and matrix.

Alkalisation treatment separated the micro-fibril structure and reduced the fibre

116

diameter because of the removal of cement materials (i.e. cellulose, hemicellulose,

lignin and etc.). Reduction in the diameter increased the flow resistivity, which led to

higher sound absorption. It also increased the effective surface area available to

adhere with the matrix and hence improved the interfacial adhesion. Furthermore,

chemical treatment caused increased porosity within the fibre. It is observed that the

treated natural fibre composites have increased in sound absorption, tensile and yield

strength (depending on the behaviour of the fibre towards alkalisation).

Through the chemical mapping analysis via FTIR, it was observed that

alkaline treatment caused re-construction of fibre structure and removal of certain

impurities that prevented or decreased the tensile and yield strength and sound

absorption. The mapping confirmed that the alkalisation resulted in removal of

lignin, cellulose and hemicellulose. The SEM revealed the micro-phase separation in

the composites, the porous structure of the natural fibre bundle and the distribution of

natural fibres in the polymer matrix composites. These special structures and the

fibre distribution are believed to be the main reasons for better sound absorption.

5.2 APPLICATIONS

The applications of sound absorbing materials are not only limited in the studio or

lab applications. It can also be applied in housing and automotive industries. In

industrial environments, the sound created from noise sources (i.e. engine, water and

drain pipes, generator, etc.) that can cause a noise problem resulting in sound

disruption, or even worse, hearing damage. An effective solution to this problem is to

enclose the noise source in order to reduce noise pollution without hindering its

operations. With proper design, it can meet a variety of processes and applications

from small to large item if structural design. For condominium/apartment and home

theatre design, natural composites can be used to replace asbestos or fibreglass

materials applications (i.e. roof deck, ceiling, woofer box, etc.).

It can also be used as undercoat layer. For marine and petroleum applications,

the uses of natural composites can help reduce the noise created from the engine

room (i.e. exhaust, HVAC system, etc.). Furthermore, it has and advantages of

restraining from corrosion due to salt water (i.e. non-corroding and lower electrical

conductivity). In automotive, road noise and engine noise can be reduced greatly

with the application natural composites as sound absorbing material. This application

117

has the potential to reduce the noise level in the vehicle. It also provides better

performance for car audio system. The applications are covered from the doors,

interior walls, panels and etc. Other than that, it can be used in interior design of

aircraft. The new materials are safer compared with the traditional absorbing

materials produced in the market. Furthermore, it is lighter and more technologically

optimized. In addition, the concept of environmental friendly, recyclability, and

sustainable materials will be an important part of the marketing the sound absorbing

natural fibre polymer composite materials.

5.2 FUTURE WORKS

In this research, a compression moulding technique was used to fabricate the

composites. However, there are other existing manufacturing processes for polymer

composites (i.e. extrusion, reaction injection moulding and etc.). Thus, final

conclusion can be drawn from there after analysed. The results provided in this thesis

can act as a base for the optimization of the composites. From this work, it was found

out that chemical treatment such as alkaline modified the fibre. It improves the

performance of the composites. However, there are other chemical treatments that

can be used such as silane, acetylation, acylation, isocyanates, permanganate and etc.

that can be used. In the current tests for morphology, the current measurement is

limited in micrometre (µm). This work can be further extended to the other smaller

measurement (i.e. nanometre and etc.). Thus, the extended study thereafter will

clarify in extending the behaviour of the fibre polymer composites. Current research

only focused on the materials itself without consideration of design work and

placement of the materials. Thus, in future, by using the existing materials, a

prototype can be fabricated and its design can be custom made so that it may replace

existing materials in the markets.

118

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APPENDICES

GANTT CHART FOR RESEARCH

ID Task Name2014 2015

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

Simulations MATLAB

Fabrication of Luffa Epoxy Composites

Sound Absorption Testing for Luffa Epoxy Composites

Tensile and Yield Strength Testing for Luffa Epoxy CompositesScanning Electron Microscope and Light/Optical Microscope Test for Luffa Epoxy Composites

Fourier Transform Infrared Spectroscopy Test for Luffa Epoxy Composites

Fabrication of Banana Epoxy Composites

Sound Absorption Testing for Banana Epoxy Composites

Tensile and Yield Strength Testing for Banana Epoxy Composites

Scanning Electron Microscope and Light/Optical Microscope Test for Banana Epoxy CompositesFourier Transform Infrared Spectroscopy Test for Banana Epoxy Composites

Fabrication of Oil Palm Epoxy Composites

Sound Absorption Testing for Oil Palm Epoxy Composites

Tensile and Yield Strength Testing for Oil Palm Epoxy CompositesScanning Electron Microscope and Light/Optical Microscope Test for Oil Palm Epoxy CompositesFourier Transform Infrared Spectroscopy Test for Oil Palm Epoxy Composites

Journal Publication

Conference ICMMMM2014

Conference ICMPC2015

Conference RAINSAT2015

Thesis Preparation

135

SODIUM HYDROXIDE DATASHEET

136

137

138

139

140

141

142

143

144

145

EPOXY RESIN PART A DATASHEET

146

147

148

EPOXY RESIN PART B DATASHEET

149

150

151

152

153

154