EFFECTS OF BIODEGRADABLE POLYPROPYLENE ......polypropylene additive covered in this study are 0.25, 0.5, 1.0, 1.5 and 2.0; the recommended weight precentage by Biosphere Plastic is

http://iaeme.com/Home/journal/IJMET 109 [email protected]

International Journal of Mechanical Engineering and Technology (IJMET)

Volume 9, Issue 5, May 2018, pp. 109–121, Article ID: IJMET_09_05_014

Available online at http://iaeme.com/Home/issue/IJMET?Volume=9&Issue=5

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

EFFECTS OF BIODEGRADABLE

POLYPROPYLENE ADDITIVE ON THE IMPACT

STRENGTH AND SPHERULITES GROWTH

RATE OF ISOTACTIC POLYPROPYLENE

Mubarak Y.A

Chemical Engineering Department, The University of Jordan, Amman-11942-Jordan

ABSTRACT

The effect of a biodegradable polypropylene additive on the nucleation intensity,

spherulite growth rates, and the impact strength of isotactic polypropylene was

studied by means of polarized light microscopy and impact testing. A single screw

extruder was used to prepare polypropylene/biodegradable additive composites in

which the weight % of the additive was varied between 0.25 and 2. It has been found

that the addition of a biodegradable additive to isotactic polypropylene matrix

increases the intensity of the spherulites at all covered isothermal crystallization

temperature in the range from 125 to 145oC. In comparison with the neat isotactic

polypropylene spherulites, much smaller spherulites were obtained at all

crystallization temperatures for the isotactic polypropylene/biodegradable additive

composite. The obtained results show that the presence of the biodegradable additive

enhances spherulite growth rate at low crystallization temperatures (below 135oC)

while the effect of this additive is almost negligible at high crystallization temperature

(above 135oC). The enhancement in polypropylene’s spherulites growth rate was

attributed to the reduction in polypropylene’s viscosity by the addition of the

biodegradable additive. At higher crystallization temperatures, the viscosity of molten

polypropylene is already low and the addition of the additive did not decrease it

further and hence did not increase the growth rate. In general, the addition of the

biodegradable polypropylene additive reduced the impact strength and this reduction

approaches 55% when 2 wt% of additive is added to iPP. The high intensity of

nucleation and the small size of the final spherulites within the composite increase the

brittleness of the composite and hence decrease the impact strength.

Keyword: isotactic polypropylene, composite, spherulite, growth rate, biodegradable

additive, crystallization.

Effects of Biodegradable Polypropylene Additive on the Impact Strength and Spherulites Growth

Rate of Isotactic Polypropylene


Cite this Article: Mubarak, Y.A, Effects of Biodegradable Polypropylene Additive on

the Impact Strength and Spherulites Growth Rate of Isotactic Polypropylene,

International Journal of Mechanical Engineering and Technology, 9(5), 2018,

pp. 109–121.

http://iaeme.com/Home/issue/IJMET?Volume=9&Issue=5

1. INTRODUCTION

The increased knowledge of polymers has resulted Polypropylene (PP) is a thermoplastic

made from the combination of propylene monomers. PP is tough and yet flexible and classed

as semi-rigid, some of the most significant properties of polypropylene are chemical

resistance, elasticity and toughness, fatigue resistance, insulation, transmissivity, dimensional

stability and processability, it is extremely resistant to heat. It has a wide range of uses,

including clear film packaging, carpet fibers, house wares, ropes, labelling, banknotes,

stationary, reusable containers, loudspeakers, automotive components, laboratory equipment,

thermal underwear [1].

Polypropylene (PP) resins are one of the fastest-growing commodity thermoplastic resins

in the world. Experts predict that the growth rates of polypropylene demand could be as high

as 8.3 % annually [2]. Demand for polypropylene is estimated to grow to 130 million tonnes

worldwide by 2023 [3]. As the use of the material widens so does the amount of waste

disposed of into the environment [4]. Once in the environment, plastic waste is subjected to

solar radiation, UV rays, heat, which affect their surface as well as to some extent their bulk

properties. This deterioration or degradation process, however, is extremely slow and may

take decades [5]. Polypropylene shows resistance to biodegradation since it is highly

hydrophobic, has high molecular weight, lacks of an active functional group and has a

continuous chain of repetitive methylene units [6].

Information reported on biodegradation of PP is scarce in the literature; the following is a

brief summary of some of the published studies. Using soil organisms, biodegradation of

polypropylene/starch or polypropylene/cellulose blends has been reported. The authors

reported that the organisms can easily degrade starch or cellulose leaving behind the polymer.

In addition to that, the adhesion of the organisms to the surface of the polymer is also

increased by the presence of these carbohydrates or fillers.

Kaszmarek et al. [7] irradiated polypropylene composites containing 5–30% cellulose and

then composted in garden soil in laboratory conditions. Photo- and bio-induced changes in

samples were studied using reflectance infrared spectroscopy (ATR-FTIR) and tensile tests

while the destruction of surface morphology was observed by scanning electron microscopy.

Compared to processes occurring in pure PP, it was found that photo- and bio-induced

changes in PP/cellulose compositions are accelerated. The mechanical properties of the

sample tested are lower than those for PP alone but the influence of cellulose amount on the

mechanical strength of compositions is insignificant.

In another study samples of polypropylene (PP) filled with a biodegradable additive

marketed under the Bioeffect trademark, were subjected to an outdoor soil burial test for 21

months was carried out by Ribes-Greus et al. [8]. Characterization by thermogravimetry

reveals that the biodegradable additive is more susceptible to degradation process rather than

the PP matrix. Changes in the crystalline morphologies and activation energies of the

relaxation process were confirmed by thermal analysis. The analysis of the relaxation spectra

shows that the interfacial and crystalline regions of the PP matrix are quite affected by the

degradation process. It has also been found that changes in the crystallinity and the

mechanical behavior of the samples take place in different stages.

Mubarak. Y.A


The biodegradability in the soil of mixtures of polypropylene and a starch-based

biodegradable additive has been studied by Morancho et al. [9]. In order to reveal the effect of

UV radiation; the mixtures were photo-oxidized before biodegradation. The results presented

showed changes in the crystallinity of the samples and in their crystallization kinetics. Photo-

oxidation was found to reduce the crystallinity of the mixtures while degradation in soil

increases it. Also, it is reported that while biodegradation tended to increase the thermal

stability of the starch units and did not affect the polypropylene, the photo-oxidation tended to

decrease the thermal stability of the mixture.

Over the past 20 years, more and more emphasis has been placed on green and being

environmentally conscious in the creation of industrial products and solutions. Throughout

this time, there have been many different attempts aimed at creating the most environmentally

friendly plastic products, and this has brought the introduction of biodegradable plastic.

Producers of biodegradable plastic’s additives claim that the addition of these additives

enhances the ability of plastic to biodegrade in anaerobic and aerobic environments. Plastic,

when placed into active microbial environments begin, to decompose at very slow rates by

microorganisms. Although considerable literature addresses the biodegradation of low and

high-density polyethylene reports on biodegradation of PP are very scarce.

Biodegradable plastic additive enhances the ability for the plastic product to decompose

by microorganisms. Products that have been treated with the biodegradable plastic additive

can see results of biodegradability in landfills, anaerobic digestion systems, and aerobic

facilities [10]. On the other hand; the presence of these additives may alter the thermal,

morphological, and mechanical properties of the polymer. There is a lot of published research

about the effects of different additives on PP properties. Hattotuwa, et al. [11] compared the

mechanical properties of rice husk powder filled polypropylene with talc filled polypropylene

composites while Sanadi, et al. [12] used recycled newspaper fibers as reinforcing fibers to

improve the impact and tensile properties of polypropylene. Polypropylene/Silica

nanocomposites mechanical properties were studied by Garcia, et al. [13] and Hernández, et

al. [14] studied the impact properties of polypropylene /styrene-butadiene-styrene block

copolymer (PP/SBS) blends. Also, Yang, et al. [15] investigated the influence of impact

modifier on the microstructure and physico-chemical and mechanical properties of

polypropylene crystallized at elevated pressures. To the best of our knowledge, the effect of

biodegradable additives on the intensity and growth rate of iPP spherulites has not been

studied or published before. The present work investigates, presents, and discusses the effect

of biodegradable polypropylene additive from Biosphere Plastic (BPA) on the nucleation

intensity, growth rate of polypropylene spherulites, and the impact strength. In another paper,

the whole mechanical properties will be investigated and discussed.

2. MATERIALS & EXPERIMENTAL PROCEDURES

2.1. Materials

Isotactic polypropylene (iPP) homopolymer grade SABIC PP 575P for Injection Molding was

used in this study. This PP is manufactured to be used for injection molding and its typical

applications include housewares articles, caps, closures, containers and toys. It has a density

of 905 kg/m3 and a melt flow index of 11 g/10 min. SABIC PP 575P is free of any nucleating

agent and has a processing temperature within the range of 220 to 240oC.

Biodegradable polypropylene additive (BPA) in pellets form was supplied by Biosphere

Plastic LLC-USA. It is used to enhance the biodegradation of plastic by adding in hydrophilic

parameters to the polymer chain. This allows the microbial enzymatic action to reduce the

structure of the polymer by utilizing macromolecules within the plastic polymer.




2.2. Preparation of iPP/Biodegradable Additive Composites

In order to ensure good mixing BPA particles were mixed physically with SABIC iPP 575P

then fed to the extruder’s hopper by direct addition. A single screw extruder (Axon ab10 Mini

Extruder-Sweden) with a 10 mm screw diameter, 20×D L/D-ratio was used to prepare the

iPP/BPA composites of different compositions. The weight percentages of biodegradable

polypropylene additive covered in this study are 0.25, 0.5, 1.0, 1.5 and 2.0; the recommended

weight precentage by Biosphere Plastic is 2.0. Extruder’s temperature zones were set at

170°C near the feeder, 185 and 195°C in the middle zones, and 210°C at the die [16]. On exit

from the extruder’s die (the point that gives the final shape), the extrudate passed through a

water trough at a temperature of 25oC for further cooling and solidification. The cooled laces

are chopped into small granules using a pelletizing machine (Axon Pelletizer) equipped with a

steel blade [17]. The obtained PP/BPA composites granules were dried in an oven at 100°C to

remove any moisture.

2.3. Crystallization and Growth of iPP Spherulites

A polarized light microscopy (PLM) (ML9430-Meiji Techno-Japan microscope) equipped

with a (Mettle FB82-USA) hot stage and a Sony digital camera were used to study the

morphology and to measure the growth rates of melt-crystallized iPP/BPA composites.

Polarized light microscope samples were cut from the prepared iPP/PBA composites and then

sandwiched between two microscope cover glasses. Samples were first melted at a heating

rate of 10°C/min passing the measured melting temperature of SABIC PP 575P (Tm ≈ 164°C),

pressed into a thin film, and then kept for 3 minutes at a temperature of 200°C to erase melt

memory effects. On termination of the 3 minutes period of time, the molten samples were

cooled rapidly to the required crystallization temperature at a rate of 40°C/min, the

temperature was maintained during the period of time required to complete the crystallization

process [18-20]. A Sony digital camera fixed on top of the microscope tube and connected to

the PC by a TV card along with a video recorder software were used to record the

crystallization process. On completion of the crystallization process, images were captured

and then analyzed by measuring the spherulite diameter as a function of time.

2.4. Preparation of Impact Testing Samples

Six sets of iPP/BPA composites (0, 0.25, 0.5, 1, 1.5, and 2 wt % BPA) for the impact testing

were prepared by hot compression moulding technique using a (65x65x3) mm steel square-

shaped mould and a heat-resistant, over-head projector transparency sheets to form a non-

stick layer between the composites and the compression platens. The samples were

compressed at 220 ͦC and 158 bars for 4 minutes then naturally cooled; they were then cut

using a VLS 6.6 Versa Laser System into (63.5x12.7x3) mm samples. A manual Notcher

(Ceast 6530) was used to create a 45, 2.5 mm notch at the centre of each sample [21]. The

prepared impact samples were analyzed via a 6545 Ceast Izod Impact Tester. The sample was

centred for testing, and a 7.5 J pendulum hammer was used to hit the sample. The energy

needed for breaking each of the samples was recorded for further calculations. Approximately

10 replicates of each sample were tested; the average of these results was used to represent the

final result.

3. RESULTS AND DISCUSSION

The effect of the addition of biodegradable polypropylene additive on both the morphology

and the spherulites growth rate will be presented and discussed in the following sections

Mubarak. Y.A


3.1. Effect of BPA iPP Morphology

Isotactic polypropylene and iPP/BPA composite crystals morphology was investigated by

conducting isothermal crystallization using a controlled hot stage and a polarized light

microscope. The range of crystallization temperature covered lies between 145 and 125oC.

As a result of the investigation, it has been observed that only the monoclinic α phase

crystals were obtained for both neat iPP and iPP/BPA composite for all crystallization

temperatures coved in this study. The hexagonal β crystals were not possible to be obtained at

any of the covered crystallization temperatures. Figures 1 and 2 show neat PP and 98 wt%

PP/2 wt% BPA samples during isothermal crystallization at different crystallization

temperatures, both figures reveal that only the monoclinic phase exists. It is clearly seen that

the biodegradable polypropylene additive used here has high nucleation efficiency when

compared with neat iPP, the number of spherulites exists per same unit area is higher and the

final size of these spherulites is much smaller than those spherulites grow in neat iPP. It

seems that this BPA plays a rule of a nucleating agent which enhances the high nucleation

rate and increases the intensity of the spherulite [22-24]. As the number of nuclei increases,

the spherulites will impinge at an early time in a limited space, this could end up in a smaller

spherulite size without a well-defined spherulite structure, and this can be clearly seen in Fig.

3.

Celli et al. [25] reported that, in isothermal conditions, for crystallization temperatures

varying between 123 and 138oC, the number of crystallites per unit area does not depend on

crystallization time and temperature. Instead, at small undercooling, the total number of nuclei

per unit area remains independent of crystallization time but decreases with increasing

temperature.

Figure 1 PLM photomicrographs of neat iPP during isothermal crystallization at A) 125 B) 130 C)

135 D) 140 E) 145oC. [Magnification = 133X].




Figure 2 PLM photomicrographs of 98 wt% iPP/2 wt% BPA composite during isothermal

crystallization at A) 125 B) 130 C) 135 D) 140 E) 145oC. [Magnification = 133X].

Figure 3 PLM photomicrographs of neat iPP (A, B, C, and D) 98 wt% iPP/2 wt% BPA composites (E,

F, G, and H) during isothermal crystallization at 125 (A, E), 130 (B, F), 135 (C, G) and 145oC (D, H)

[Magnification = 133X].

Mubarak. Y.A


3.2. Effect of BPA on iPP Spherulites Growth Rates

The diameter of the growing spherulites was monitored during solidification by real- time

recording then using video snapshot software was used to take photomicrographs appropriate

intervals of time. Some of the photomicrographs for neat iPP during isothermal crystallization

at 145oC are presented in Fig. 4. Figures 5 to 7 show typical examples for the relationship of

spherulite diameter (D) versus crystallization time (t) for neat iPP and 98 wt% iPP/2 wt%

BPA composite measured isothermally at crystallization temperatures of 125, 130, 135, 140,

and 145oC. Spherulites growth rates are reported at these high temperatures only because it

was so difficult to observe the growth rates under the PLM at lower crystallization

temperatures, especially for the iPP/BPA composites because the nucleation rate is very high

and hence very small spherulites are obtained.

Figures 5 to 7 reveal that the diameter of neat iPP and iPP/BPA composite spherulites

increases linearly with time until impingement occurs. This is well accepted since

homopolymer PP spherulites grow linearly at a given fixed temperature [26, 27]. Since at high

crystallization temperatures the nucleation density of PP spherulites is low and the growth

rate is small, then less number and larger spherulites diameter will be obtained. In addition to

that longer crystallization time is required to complete the solidification process. As a result of

this difference in crystallization time and spherulites size at high (≥ 135oC) and at low (<

135oC) crystallization temperatures, Figures 6 and 7 are used to represent the obtained results

at high temperature and low temperatures for iPP/BPA composites with different BPA

concentrations. Only a short period of crystallization time is presented in Fig. 7 because the

difference in spherulites diameter is almost negligible compared to those diameters at lower

crystallization temperatures. Showing up this short period of crystallization time at higher

crystallization temperatures will show the differences between spherulites diameter.

Figure 4 PLM photomicrographs of neat iPP during isothermal crystallization at 145oC after A) 60 B)

240 C) 420 D) 600 E) 780 F) 960 min. [Magnification = 133X].




It is clearly seen that the slope of the obtained curves decreases with the increase of

crystallization temperature due to the reduction in the crystallization driving force and a

decrease in supercooling. A comparison between neat iPP and 98 wt% iPP/ 2 wt% BPA

spherulite diameters at different crystallization temperatures is presented in Figure 8. Since

the final spherulites size for iPP/BPA is much smaller than those spherulites for neat iPP, only

a short period of time is presented in the figure for crystallization temperatures above 130oC.

It is concluded that the presence of the BPA particles within iPP enhanced spherulites

nucleation and growth, especially at low crystallization temperatures.

Spherulite growth rate (G) is generally measured at isothermal conditions, by monitoring

the variation of the spherulite diameter (D) as a function of time (t). At a fixed crystallization

temperature, equation (1) shows that the slope of the line is linear and its slope gives the value

of G [28]

𝐺 =𝑑𝐷

𝑑𝑡 1

Figure 5 Spherulite diameter of neat iPP samples crystallized isothermally at different crystallization

temperatures.

Figure 6 Spherulite diameter of iPP/BPA of different compositions isothermally crystallized at 125,

130, and 135oC.

The influence of the biodegradable particles (BPA) on the rate of radial growth (G) of iPP

is illustrated in Fig. 9. According to what presented in the figure, G values decrease with

increasing crystallization temperature for both the neat iPP and the iPP/BPA composites of all

compositions used in the present study. In general, for melt crystallization, higher

crystallization temperatures can result in the decrease of the degree of super cooling and

Mubarak. Y.A


consequently the decrease of growth rate. It is clearly seen that the effect of the BPA particles

on the growth rate iPP at high crystallization temperatures (> 135oC) is almost negligible,

while the difference in growth rates is significant at lower crystallization temperatures and the

influence increases by increasing the wt % of the BPA.

Figure 7 Spherulite diameter of iPP/BPA of different compositions isothermally crystallized at 140

and 145oC.

Figure 8 Comparison between spherulite diameters of neat iPP and iPP/2 wt% BPA isothermally

crystallized at different temperatures.

It is well known that increasing the temperature will decrease the viscosity of the molten

polymer and hence allows polymers chain to move freely and faster, but on the other hand,

the supercooling, nucleation, and growth rate will be reduced at high temperature. It seems

that in addition to its nucleation efficiency, the addition of BPA to iPP reduces the viscosity

further. The effect of this reduction in viscosity appears clearly at low crystallization

temperature and allows for higher growth rates when compared with neat iPP as shown in Fig.

9 at 125 and 130oC.

Although the viscosity of iPP is lower at high temperature and with the presence of the

biodegradable polypropylene additive there was a further reduction in the viscosity but the

effect of the low supercooling overcome this reduction in viscosity and did not improve or

accelerate the growth rate of iPP. At high crystallization temperatures, no significant effect of

BPA particles on the growth rate of iPP crystal can be noticed. Thus it becomes clear that the

reason of enhancement of overall crystallization rate is due to the presence of BPA as a

nucleation agent, and has nothing to do with the spherulite growth rate at higher temperatures.




Figure 9 Neat iPP and iPP/BPA spherulites growth rates as a function of isothermal crystallization

temperature.

PLM work of Ning et al. [29] showed a constant spherulite growth rate and a decreased

spherulite size at given isothermal crystallization temperature, suggesting that nucleation and

growth of a spherulite are two independent processes. It is proposed by Bryant et al. [30] that

a spherulite originates from a single nucleus and that growth proceeds thence in a statistically

radical fashion until all crystallizable domains are utilized or until growth is arrested due to

increased viscosity of the medium.

3.3. Effect of BPA on the Impact Strength of iPP Composites

It is clearly seen from Figure 10 that the impact resistance of isotactic polypropylene

decreased by the addition of the biodegradable additive. At low weight percentages of BPA (

0.5), the impact strength of the composites presented little reduction while for weight

percentages of 1.0 the reduction in the impact strength was observed to be over 50%. It

seems that the incorporation of the BPA improves stiffness and tensile strength of iPP but

reduces the toughness leading to poorer impact strength [31]. It is also noticed that iPP/2 wt

% BPA composites were brittle compared to the composites with less BPA wt %.

Figure 4 Notched Izod Impact Strength of iPP/BPA composites as a function of BPA wt %.

It is known that impact properties are strongly dependent on filler-matrix adhesion, which

dictates the energy transfer from matrix to filler particles [32]. Since the BPA was used in this

study without any addition of any agents to improve the bonding between BPA particles and

iPP, it is expected to have significant changes in iPP composite impact properties. However,

at higher BPA concentrations and with poor particle distribution and a large number of weak

Mubarak. Y.A


sites due to poor adhesion, this will be reflected in a decrease in iPP composite impact

strength. Also, at higher concentration of BPA, some agglomerations may exist and this

resulted in a weak interfacial adhesion between the matrix and the BPA particles and hence

decreases the impact resistance of the composite.

4. CONCLUSIONS

It is concluded that the biodegradable polypropylene additive (BPA), when added to isotactic

polypropylene matrix, behaves exactly the same as a nucleating agent in terms of increasing

the nucleation sites and hence the number of spherulite per unit area. At any isothermal

crystallization temperature and compared with the neat PP, the intensity of spherulites per unit

area is greater for PP/BPA composites. Also, the final spherulites size is much smaller.

It has been found that the addition of the biodegradable polypropylene additive (BPA) to

iPP increases spherulites growth rate at low crystallization temperature significantly (Tc




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EFFECTS OF BIODEGRADABLE POLYPROPYLENE ......polypropylene additive covered in this study are 0.25, 0.5, 1.0, 1.5 and 2.0; the recommended weight precentage by Biosphere Plastic is