Journal of Thermoplastic Composite Materials 2013 Nurul 627 39

8/11/2019 Journal of Thermoplastic Composite Materials 2013 Nurul 627 39

1/14

http://jtc.sagepub.com/Composite Materials

Journal of Thermoplastic

http://jtc.sagepub.com/content/26/5/627Theonline version of this article can be found at:

DOI: 10.1177/0892705711427345December 2011

2013 26: 627 originally published online 6Journal of Thermoplastic Composite MaterialsMS Nurul and M Mariatti

compositesEffect of thermal conductive fillers on the properties of polypropylene

Published by:

http://www.sagepublications.com

at:can be foundJournal of Thermoplastic Composite MaterialsAdditional services and information for

http://jtc.sagepub.com/cgi/alertsEmail Alerts:

http://jtc.sagepub.com/subscriptionsSubscriptions:

http://www.sagepub.com/journalsReprints.navReprints:

http://www.sagepub.com/journalsPermissions.navPermissions:

http://jtc.sagepub.com/content/26/5/627.refs.htmlCitations:

What is This?

- Dec 6, 2011OnlineFirst Version of Record

- May 23, 2013Version of Record>>

by guest on September 7, 2014jtc.sagepub.comDownloaded from by guest on September 7, 2014jtc.sagepub.comDownloaded from
http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/content/26/5/627http://jtc.sagepub.com/content/26/5/627http://jtc.sagepub.com/content/26/5/627http://www.sagepublications.com/http://jtc.sagepub.com/cgi/alertshttp://jtc.sagepub.com/cgi/alertshttp://jtc.sagepub.com/subscriptionshttp://jtc.sagepub.com/subscriptionshttp://www.sagepub.com/journalsReprints.navhttp://www.sagepub.com/journalsReprints.navhttp://www.sagepub.com/journalsPermissions.navhttp://jtc.sagepub.com/content/26/5/627.refs.htmlhttp://jtc.sagepub.com/content/26/5/627.refs.htmlhttp://online.sagepub.com/site/sphelp/vorhelp.xhtmlhttp://online.sagepub.com/site/sphelp/vorhelp.xhtmlhttp://jtc.sagepub.com/content/early/2011/12/02/0892705711427345.full.pdfhttp://jtc.sagepub.com/content/26/5/627.full.pdfhttp://jtc.sagepub.com/content/26/5/627.full.pdfhttp://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://online.sagepub.com/site/sphelp/vorhelp.xhtmlhttp://jtc.sagepub.com/content/early/2011/12/02/0892705711427345.full.pdfhttp://jtc.sagepub.com/content/26/5/627.full.pdfhttp://jtc.sagepub.com/content/26/5/627.refs.htmlhttp://www.sagepub.com/journalsPermissions.navhttp://www.sagepub.com/journalsReprints.navhttp://jtc.sagepub.com/subscriptionshttp://jtc.sagepub.com/cgi/alertshttp://www.sagepublications.com/http://jtc.sagepub.com/content/26/5/627http://jtc.sagepub.com/


2/14

Article

Effect of thermal

conductive fillers onthe properties ofpolypropylenecomposites

MS Nurul and M Mariatti

Abstract

We investigated the effects of various fillers such as carbon nanotube (CNT), syn-thetic diamond (SND), boron nitride (BN), and copper (Cu) on the properties ofpolypropylene (PP) composites. The thermal conductivity and stability of PP wereenhanced upon the addition of thermally conductive fillers. Youngs modulus increasedwith filler loading, while tensile strength increased at up to 2 vol.% then decreasedwith elongation in all filler types. The morphology of the composite samples showedagglomeration and void content in PP/Cu composites, leading to the deterioration ofthermal and mechanical properties at high-volume loading. Findings indicate that PP/CNT has better thermal and mechanical properties compared with the other typesof fillers.

Keywords

Thermal properties, tensile properties, conductive fillers, polypropylene, composites

IntroductionConductive polymer composites (CPCs) are among the versatile materials that can be

used in several applications such as self-regulated heating, electromagnetic shielding,

vapor sensing, and bipolar plates in the fuel cell.1 Reinforcement of polymers with con-

ductive fillers such as carbon nanotube (CNT), silica, synthetic diamond (SND), silicon

School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong

Tebal, Penang, Malaysia

Corresponding author:

M Mariatti, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering

Campus, 14300 Nibong Tebal, Penang, Malaysia.

Email: [email protected]

Journal of Thermoplastic Composite

Materials

26(5) 627639

The Author(s) 2011

Reprints and permissions:sagepub.co.uk/journalsPermissions.nav

DOI: 10.1177/0892705711427345

jtc.sagepub.com

by guest on September 7, 2014jtc.sagepub.comDownloaded from
http://www.sagepub.co.uk/journalsPermissions.navhttp://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://www.sagepub.co.uk/journalsPermissions.nav


3/14

nitride, boron nitride (BN), copper (Cu), ferrite, bronze, and aluminum nitride can be

adapted to satisfy the required characteristics of CPCs.25

Thus, CPCs are emergingas one of the most economical and effective ways to cope with thermal management

issues.6

In general, the effectiveness of reinforcing fillers in composites is inversely propor-

tional to the size of the filler. Previous studies reported that the absorption energy of a

smaller particle is higher than that of a larger particle due to its high surface energy.7

Jung et al.8 and Boudenne et al.9 proved that the nano-sized conductive fillers in com-

posites give better thermal conductive and stability characteristics, since smaller parti-

cles have better interaction and can more easily form the conductive path than the

micron-sized particles.

Moreover, the geometry of the particle is an important factor in achieving the optimal

properties of composites. A greater surface-to-volume ratio of filler results in greater

effectiveness. Volume fraction is another factor that affects the effectiveness of the rein-

forcing filler; it should be as high as 20 by vol.% to afford satisfactory conductivity prop-

erties. However, filler loading at higher content is generally required to yield these

positive effects of fillers. This would detrimentally affect some important properties

of the polymers matrix, including processability, appearance, density, and ageing

performance.10

In this study, we investigated the effects of four types of conductive fillers, specifi-

cally CNT, SND, BN, and Cu, in polypropylene (PP) composites. The correlationsbetween filler loading ranging from 0 to 4 vol.% and thermal and mechanical properties

of these composites were investigated.

Experimental

Materials

Homopolymer PP (Titanpro 6431) is a commercial product from Titan Polymer (M) Sdn.

Bhd, with a melt index of 7 g/10 min and a density of 0.9 g/cm3. CNT, SND, BN, and Cu

were supplied by Shenzhen Nanotech Port Co., Ltd, Heyuan Zhong Lian Nano-

technology, TaijiRing Nano-products, and Sigma Aldrich, respectively. The properties

of these fillers are presented in Table 1.

Table 1.Typical properties of thermal conductive fillers used in the study.

Properties (units) SND CNT BN CU

Thermal conductivity (W/mK) 2000 2000 300 385

Particle size distribution (nm) 56 2645 3043 379492

Mean particle size (d50) 5.5 69 36 434

Density (g/cm3) 3.3 1.3 2.2 8.9

Shape Sphere Tube Sphere Sphere

BN: boron nitride, CNT: carbon nanotube, CU: copper, SND: synthetic diamond.

628 Journal of Thermoplastic Composite Materials 26(5)

http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/http://jtc.sagepub.com/


4/14

Sample preparation

Conductive nanofillers were dried in oven at 100C for 3 h to remove moisture before

mixing with PP ranging at 1, 2, 3 and 4 vol.% of filler loading. Compounding between PPand fillers was performed in a two-roll mill heater at a constant temperature of 185C and

at 50 rpm for 20 min. Then, the composite sheet was compression molded in an electri-

cally heated hydraulic press at 185C and subsequently cooled at 1000 psi for 3 min.

Filler characterizations

Particle size of the fillers was measured by Nanophox particle size analysis, model

NX0064. Data on particle size distribution were presented as cumulative distribution as a

function of particle size. Thermal stability of the filler was determined by thermogravi-

metric analysis (TGA)/differential thermal analysis (DTA) using Linseis model L75/04.Fillers were heated from room temperature to 800C at a heating rate of 10C/min.

Composites characterizations

Flow behaviors of samples were determined using Dynisco Polymer Test model 4004

following the method described in American Society for Testing and Materials (ASTM)

D 1238-90b with a load of 2.16 kg at 230C and a melt time of 360 s. Cutting samples

within an interval of 10 s were weighed and melt index values were calculated in g/10 s.

Physical ashing test was performed according to ASTM D2584 to determine filler weightfraction (Wf) in the composites after compounding. Void content was determined from a

relationship between the theoretical density and the experimental density of the compo-

sites. Thermal conductivity was tested using a hot disc thermal constant analyzer model

TPS 2500 according to ASTM D792-98. The heat source was placed between two

4 4 8 mm samples and connected to thermal conductivity detector. TGA was per-formed using model Perkin Elmer Pyris TGA-6. The sample was heated from room tem-

perature to 600C at 10C/min in a nitrogen environment. Melting and crystallization

behavior of the composites was studied, employing differential scanning calorimeter

(DSC) using a Perkin-Elmer DSC-6 at a heating rate of 10C/min. Melting temperature

Tm and crystallization temperature Tc were derived from endothermic and exothermic

peak temperatures. The degree of crystallinity Xc was calculated from heat of fusion

by taking 207 J/g as the enthalpy to crystallize 100% PP.11 Tensile test was conducted

by Instron 3366 with gauge length of 50 mm and speed of 50 mm/min according to

ASTM D 638-98. The morphology of tensile fracture specimens was captured by ZEISS

SUPRA 35 VP field emission scanning electron microscope (FESEM).

Results and discussion

Melt flow index

Figure 1 illustrates the decreasing trends of melt flow index (MFI) as the conductive

filler loading was increased. These trends were expected because the incorporation of

Nurul and Mariatti 629



5/14

fillers hinders polymer flow and increases the viscosity of composites. PP/CNT exhib-

ited the lowest MFI due to the high aspect ratio of CNT, leading to strong intermolecularinteraction between the nanotubes. In contrast, the greater size of Cu (micron-sized)

resulted in a higher MFI value, which slightly increased at high Cu loading (i.e. 3 and

4 vol.%). This trend can be attributed to the metallic properties of Cu, such that it is able

to induce and catalyze the degradation of polymer composites. In addition, the heat

energy absorbed by Cu will spread to the surrounding PP matrix. Thus, the polymer

chains will be cut down, allowing MFI to increase.12

Tensile properties

The correlation between average tensile strength and void content of PP and PP com-

posites is presented in Figure 2. SND and BN systems exhibited higher tensile strength

compared with CNT and Cu systems. The maximum tensile strength was observed at

2 vol.%, after which a decreasing trend was observed. Tensile strength was reduced at

higher nanofiller loading due to strong interactions between particleparticle rather than

particlematrix. This trend is supported by the increasing void content as filler content

was increased. In the CNT- and Cu-filled PP systems, a decreasing trend in tensile

strength compared with that of PP was observed. This may be related to the large particle

size of Cu, which functions as a defect, and the high void content in the two-composite

systems. Increasing void content could cause detrimental effects on mechanical proper-

ties that create stress concentration and inhibit stress transfer from the matrix to the fil-

ler.1316 The distribution of fillers in the PP matrix at 4 vol.% was revealed by sectional

fractography of tensile test by SEM (Figure 3). PP surface (Figure 3a) was dramatically

Filler loading (vol.%)

0 1 2 3 4

Meltflowindex(g/10s)

0

5

10

15

20

25

PP/CNT

PP/SND

PP/BN

PP/CU

Figure 1.Melt flow index (MFI) curves of polypropylene (PP) and PP composites as a function of

filler loading.




6/14

changed by the presence of thermal conductive particles. SND and BN in Figure 3(b) and

(c) were well dispersed; the filler appears to be embedded in the PP matrix, suggestingthat the tensile strength of these systems is high. The worst dispersion and distribution

were observed in PP/Cu composites, as indicated by the presence of agglomerations and

voids in Figure 3(d). CNT was poorly distributed but was well dispersed in PP matrix

(Figure 3e). These properties of PP/Cu and PP/CNT are responsible for the increased

stress behavior and the ineffective transfer of load applied in PP composites, leading

to decreased tensile strength of CNT and Cu systems.

Xc also influences the mechanical properties of composites.17,18 Theoretically, the

mechanical strength of a crystalline polymer is determined by its crystalline structure.

Table 2 presents the Xc

, Tm

, andTc

values for PP and PP composites in this study. At

4 vol.% filler loading, Xc of SND and BN was higher than that of PP because the

crystalline region acts as a physical crosslink that enhances the tensile strength of PP

composites. In contrast, CNT and Cu systems exhibited low Xc values since the

superficial area interferes with crystal growth, thus leading to reduced tensile strength

of the composites.19,20 Tm values were not significantly changed by the addition of

conductive fillers and an increase in filler loading. This may be attributed to the

maintenance of the flexibility of the polymer chain even when fillers are dispersed in

the polymer matrix.8 Tc values of composites were higher than that of PP and were

within the range of 110135C, indicating that the conductive fillers can act as

nucleating agents.Figure 4 illustrates the correlation of Youngs modulus and filler content (Wf) of the

PP and PP composites. In general, the trends markedly increased with respect to the Wf.

The highest Youngs modulus were found in CNT followed by BN, SND, and Cu fillers,

Tensilestrength(MPa)

26

28

30

32

34

36

38

40

PPPP/CNT

PP/SNDPP/BN

PP/CU

Voidcontent(%)

0

5

10

15

20

25

30

35


0 1 2 3 4

Figure 2. Tensile strength and void content of polypropylene (PP) and PP composites as a function

of filler loading. Bar graph refers to the tensile strength and line plot refers to the void content,

respectively.




7/14

with increases of up to 31%, 27%, 25%, and 9% from that of PP, respectively. Incor-

poration of rigid and stiff reinforcement into the polymer enhanced the stiffness of the

polymer composites. Higher rigid filler content increased the Youngs modulus signif-

icantly. The PP/CNT system exhibited the highest maximum Youngs modulus due to

the high aspect ratio of CN, which leads to greater stiffening compared with particulate

composites. The lower interfacial area of the sphere shape of SND, BN, and Cu results in

lower Youngs modulus compared with CNT system. The Cu system had the lowest

Youngs modulus because of the large particle size of Cu, which results in less inter-

action between fillerfiller and fillermatrix. Figure 5 illustrates the trends of descending

Figure 3.Scanning electron microscope (SEM) micrograph of the 4 vol.% filler loading at 5 K

magnifications. (a) Polypropylene (PP), (b) PP/synthetic diamond (SND), (c) PP/boron nitride (BN),

(d) PP/copper (CU), and (e) PP/carbon nanotube (CNT).




8/14

elongation at break with addition of stiff reinforcement, which decreases the ductility of

the matrix.

Thermal conductivity

Figure 6 presents the thermal conductivity of PP composites at room temperature. We

found that the thermal conductivity of composites increased monotonically from that of

PP and increased directly with increased filler amount. This ascending trend may be

attributed to the ease of heat transfer obtained by increasing contact in the composites.

CNT was the most effective filler for enhancing thermal conductivity, followed by SND,

Cu, and BN; this trend seems to follow the hierarchy of thermal conductivities of the

Y

oung'smodulus(GPa)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Weightfraction(%)

0

5

10

15

20

25

30

35

PPPP/CNT

PP/SNDPP/BN

PP/CUFiller loading (vol.%)

0 1 2 3 4

Figure 4.Youngs modulus and weight fraction of polypropylene (PP) and PP composites as a

function of filler loading. Bar graph refers to the Youngs modulus and line plot refers to the weight

fraction, respectively.

Table 2.DSC and thermal interface resistance (Ri) data for PP and PP composites filled at 4 vol.%

of CNT, SND, BN, and CU.

Composites Xc(%) Tm(

C) Tc(

C) Ri(nm2

/w K)

PP 34.5 164.4 118.4 -

PP/CNT 30.0 163.9 123.7 0.98

PP/SND 38.7 163.9 124.3 0.14

PP/BN 42.4 164.0 125.8 1.3

PP/CU 33.9 163.9 120.7 1.1

BN: boron nitride, CNT: carbon nanotube, CU: copper, DSC: differential scanning calorimeter, PP:

polypropylene, SND: synthetic diamond.




9/14

filler (refer to Table 1). The correlation with thermal interface resistance would influencethe effectiveness of the phonon to pass through in the composites systems. The thermal

resistance at the interface between the matrix and the filler, known as Kapitza resistance

(Ri), was analyzed according to Eq. (1).21

Elongationatbreak(%

)

2

3

4

5

6

7

8

9


0 1 2 3 4

PPPP/CNT

PP/SNDPP/BN

PP/CU

Figure 5.Elongation at break of polypropylene (PP) and PP composites as a function of filler loading.

Thermalconductivity

(W/m.K)

0.22

0.24

0.26

0.28

0.30

0.32

0.34

0.36

PP/CNT

PP/SND

PP/BN

PP/CU


0 1 2 3 4

Figure 6.Thermal conductivity of the polypropylene (PP) composites as a function of filler loading.




10/14

Kc Km KmL

2RiKfL

vf

3 1

whereKc,Km, andKfare the thermal conductivity of the composite, matrix, and filler,

respectively;Ri is the interfacial thermal resistance; L is the length of filler assumed

at diameter d50; and vf is the volume fraction taken at 4 vol.% filler loading. The

predicted values of Ri are summarized in Table 2. Lower thermal resistance was

exhibited by the SND and CNT systems due to their high thermal conductivity.

However, the CNT filler can produce higher thermal conductivities at identically lower

filler content due to its high aspect ratio, so that it is able to form a conductive network

for easier phonon-dominated ballistic heat transport compared with the spherical SND.

HighRiwas observed in the Cu and BN systems due to their low thermal conductivity.

However, the PP/Cu system exhibited minimum thermal conductivity at 2 vol.% load-ing only, with decreasing values obtained with further addition of filler loadings. This

is related to the poor adhesion and poor dispersion and distribution of Cu seen in SEM

morphology (Figure 3d). Variations in agglomeration size, high void content, and the

lack of contact between particles suggest that Cu particles were relatively nonhomo-

genously dispersed in the matrix. This subsequently resulted in low heat transfer in the

Cu system, which led to low thermal conductivity of the composite. In contrast, the

nearly uniform size of particles indicating good dispersion in the PP matrix (Figure

3b, c, and e) resulted in better thermal interaction in CNT-, SND-, and BN-filled

PP composites.

Thermogravimetry analysis

The TGA curves for PP and PP composites at 4 vol.% filler loading are presented

in Figure 7. The curve shows single-step degradation where it shifted to the right

(i.e. higher temperature) with the addition of filler. This indicates that PP composites

achieve a stabilization effect through the barrier effect of filler loading, which hinders

volatilization of bulk samples into gas phase.22 TGA curves reveal that composites

are stable at up to 350C, with weight reduction of around 0.5%. The TGA profile can

be clearly depicted by the derivative weight % (DTG) curve in Figure 8. The points

where degradation starts shifted to a higher temperature in composites compared

with PP. TGA trends of the composite materials are supported by the TGA analysis

of fillers (Figure 9), which revealed weight reduction in fillers as indicated by the

minus () sign in the y axis as a function of temperature. CNT exhibited the highestcurve, suggesting that CNT has better thermal stability compared with the other

fillers. Weight reductions at different temperature (Table 3) follow the sequence of

CNT, SND, Cu, and BN. As shown in Figure 7, increasing filler loading leads to

increased thermal behavior of the composites due to the higher thermal stability of

fillers compared with the matrix. In general, all composite systems exhibited slightly

similar weight residue, which explains why fillers are retained without decomposition.

Most of the fillers will decompose at very high temperatures, while PP will be com-

pletely degraded.




11/14

ConclusionsIn this study, we performed characterization of fillers and investigation of the effect of

thermally conductive fillers on the mechanical, flow, and thermal properties of PP

composites. Findings suggest that CNT has better thermal properties compared with

Temperature (C)

300 350 400 450 500 550

Weightloss(%)

0

20

40

60

80

100

PP

PP/CNT1

PP/CNT4

PP/SND4

PP/BN4

PP/CU4

Figure 7.Thermogravimetric analysis (TGA) curve of polypropylene (PP) and PP composites as a

function of temperature. The numbers 1 and 4 refer to 1 and 4 vol.% of filler loading, respectively.

Derivativeweight%

(%/m)

30

20

10

0

PP

PP/CNT1

PP/CNT4

PP/SND4

PP/BN4

PP/CU4

Temperature (C)

350 400 450 500 550

Figure 8.Derivative weight percentage (DTG) of polypropylene (PP) and PP composites as a

function of temperature. The numbers 1 and 4 refer to 1 and 4 vol.% of filler loading.




12/14

other conductive fillers. Results demonstrate that CNT, SND, BN, and Cu particles

variably affect the properties of PP composites. In general, the MFI of composites

decreased with increased filler loading due to the ability of fillers to hinder plastic flow.

PP/CNT exhibited the greatest thermal conductivity and thermal stability due to the high

aspect ratio of CNT, which facilitates the formation of bridges for phonon transformation

compared with the spherical fillers. However, entanglements of CNT lead to stress

concentration, resulting in reduced tensile properties. In general, the overall thermal

properties of composites improved with filler addition. For particulate fillers, lowerd50results in higher tensile strength, Youngs modulus, higherRi, and lower thermal con-ductivity values. The thermal conductivity andRi of the composite materials generally

seems to follow the hierarchy of thermal conductivities of the filler. Cu with the highest

d50showed poor thermal and tensile properties due to the agglomeration and voids which

0 200 400 600 800

Delta-M(mg)

25

20

15

10

5

0

CNT

SND

BN

CU

Temperature (C)

Figure 9.Thermogravimetric analysis (TGA) curve for conductive fillers used as a function of

temperature.

Table 3.Weight reduction (mg) in conductive fillers at 100 and 500C.

Conductive fillers

Weight reduction (mg)

At 100C At 500C

CNT 0.9 7.0

SND 1.7 8.6

CU 3.1 12.7

BN 4.0 13.2

BN: boron nitride, CNT: carbon nanotube, CU: copper, SND: synthetic diamond.




13/14


14/14

15. Naganuma T, Naito K, Kyono J and Kagawa Y. Influence of prepreg conditions on the

void occurrence and tensile properties of woven glass fiber-reinforced polyimide composites.

Compos Sci Technol2009; 69(14): 24282433.

16. Rutz BH and Berg JC. A review of the feasibility of lightening structural polymeric compo-sites with voids without compromising mechanical properties.Adv Colloid Interface Sci2010;

160(1-2): 5675.

17. Hartikainen J, Hine P, Szabo JS, Lindner M, Harmia T, Duckett RA, et al. Polypropylene

hybrid composites reinforced with long glass fibres and particulate filler. Compos Sci Technol

2005; 65(2): 257267.

18. Nurazreena, Hussain LB, Ismail H and Mariatti M. Metal filled high density polyethylene

compositeselectrical and tensile properties. J Thermoplast Compos Mater 2006; 19(4):

413425.

19. Zhao Y-Q, Lau K-T, Kim J-K, Xu C-L, Zhao D-D and Li H-L. Nanodiamond/poly (lactic

acid) nanocomposites: Effect of nanodiamond on structure and properties of poly (lactic acid).Compos B Eng2010; 41(8): 646653.

20. Kang CH, Yoon KH, Park Y-B, Lee D-Y and Jeong S-S. Properties of polypropylene compo-

sites containing aluminum/multi-walled carbon nanotubes. Compos A Appl Sci Manuf2010;

41(47): 919926.

21. Razavi-Nouri M, Ghorbanzadeh-Ahangari M, Fereidoon A and Jahanshahi M. Effect of car-

bon nanotubes content on crystallization kinetics and morphology of polypropylene.Polym

Test2009; 28(1): 4652.

22. Mukhopadhyay A, Otieno G, Chu BTT, Wallwork A, Green MLH and Todd RI. Thermal and

electrical properties of aluminoborosilicate glass-ceramics containing multiwalled carbon

nanotubes.Scripta Mater2011; 65(5): 408411.


Documents

Journal of Thermoplastic Composite Materials 2013 Nurul 627 39