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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.
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Effects of Biodegradable Polypropylene Additive on the Impact Strength and Spherulites Growth
Rate of Isotactic Polypropylene
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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.
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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.
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Effects of Biodegradable Polypropylene Additive on the Impact Strength and Spherulites Growth
Rate of Isotactic Polypropylene
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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
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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].
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Effects of Biodegradable Polypropylene Additive on the Impact Strength and Spherulites Growth
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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].
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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].
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Effects of Biodegradable Polypropylene Additive on the Impact Strength and Spherulites Growth
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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
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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.
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Effects of Biodegradable Polypropylene Additive on the Impact Strength and Spherulites Growth
Rate of Isotactic Polypropylene
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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
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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 Additive on the Impact Strength and Spherulites Growth
Rate of Isotactic Polypropylene
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