Polymer blend is a mixture of two or more polymers which typically possesses
the combination of the polymers properties. Polymer blending is considered
as the most versatile and economic way to engineer a material with complex desired
properties (Saion et al., 2005; Moldovan
et al., 2008; Yousif et al., 2006).
Polypropylene (PP) was blended with Polycarbonate (PC) with the prime objective
to improve poor impact strength of PP while having sufficient stiffness. Upon
the successful effort, research on PP/PC blends has moved beyond impact strength
and stiffness, as other properties of the blends were also studied. For instance,
thermal properties of PP/PC had been the subject of several studies and improvement
in PP thermal stability was reported (Chand and Hashmi,
1999; Renaut et al., 2005). However, characterization
data of PP/PC blend is still lacking and issues such as water absorption of
the blend needs to be comprehended.
In PP/PC blend, PC is a polar polymer which is susceptible to chemical reaction
with polar solvent such as water and oxygen in carbonate group, CO3,
found in PC makes it more prone towards water absorption (Merdas
et al., 2002). Besides that, one of the mechanisms for water penetration
into polymer composite is governed by capillary action, in which water molecules
flow into the interface between matrix and reinforcement. This mechanism is
particularly important when the interfacial adhesion between the matrix and
reinforcement is weak, especially in the case of polymer blend composite (Kushwaha
and Kumar, 2010). Previously, amorphous PC is blended with semi-crystalline
polyethylene terephthalate (PET) to improve solvent and chemical resistance.
This is one of the examples of how polymer blending can improve specific properties
of a material and produce a more useful and commercially attractive product.
Materials having oxygen or oxy-hydrogen groups are very susceptible to water
absorption, whereas plastics containing only hydrogen and carbon, such as polyethylene,
polystyrene and PP are extremely water resistant. In addition, PP also provides
excellent resistance to organic solvents, degreasing agents and electrolytic
attack (Tripathi, 2002). Therefore, blending PP with PC
was expected to reduce PCs affinity to water. In summary, this research
is aimed to characterize the water absorption properties of PP/PC/PP-g-MA polymer
blends. PP-g-MA as compatibilizer has been suspected to improve interfacial
adhesion between PP and PC phase (Renaut et al.,
2005) and is intended in this work to reduce water absorption caused by
Water absorptions threats: Mechanical, physical, chemical and
dimensional properties could suffer from detrimental effect because of water/moisture
absorption of polymers (Choqueuse et al., 1997;
Hng et al., 2011). Some polymers swell
and soften while some even dissolve in water such as nylon and polyvinyl alcohol
(Merdas et al., 2002). In the case of swelling
and soften, molecular mobility is increased through the absorption of water.
By the crowding of solvent molecules, polymer structure will open up and swell,
leading to increase in spacing between polymer molecules. This will lower the
secondary bonding and result in less resistance to applied stress from the decrease
in intermolecular friction, allowing for easier translational motion (White,
Water absorption in polymers also poses a threat during processing. Even with
0.2% water absorption, polymer could suffer from hydrolytic degradation during
melt processing, resulting in significant loss of mechanical and physical properties.
Hydrolytic degradation is a chemical reaction which occurs at high temperature
with some polymers in the presence of water. It causes primary bonds in the
molecular chains to be broken down severely, thus reducing molecular weight.
Hence for a hydrophilic polymer such as PC, preparation or pre - treatment before
compounding such as drying is a very important step taken to ensure its physical
and mechanical properties will not be reduced.
Kinetics of water absorption: Water and moisture penetration into composite
materials is governed by four different mechanisms (Karmaker,
1997; Gaudichet-Maurin et al., 2008). The
first one is the diffusion of water molecules inside the microgaps or free volume
between polymer chains. Next is the capillary transport of water molecules into
the space between matrix and reinforcement phase due to the imperfection interfacial
bonding between the two phases, especially when they are of different polarities
and immiscible with each other. Then, the presence of voids in matrix composite
from compounding process that allow for water penetration. Recently, solubility
is cited as another mechanism, especially in amorphous polymers of low to moderate
hydrophilicity (Gaudichet-Maurin et al., 2008).
Despite of all the water absorption mechanism, many researches (Kushwaha
and Kumar, 2010) claimed that the overall effect can be conveniently modeled
by using only the diffusion mechanism. Diffusion mechanism can be divided into
three cases according to Eq. 1 (Kushwaha
and Kumar, 2010):
where, Mt is the composites
moisture content at time t, Ms is the moisture content at equilibrium,
k is a constant of interaction between the composite and water and n is a constant
which indicates the diffusion case. The value of n = 0.5 when the diffusion
follows Ficks law (case 1),
n = 1 when the moisture equilibrium in the composite is rapidly reached and
maintained with increasing time (case 2) and n value in between 0.5 and 1 for
the anomalous diffusion (case 3).
Most of the time, Ficks law will govern the process in water absorption
of polymer composite. In this case, when plotted in a graph, the mass of absorbed
water increases linearly with square root of time, then the rate slowly decreases
until equilibrium plateau reached (Dhakal et al.,
2007). Ficks law is given by Eq. 2 (Kushwaha
and Kumar, 2010):
where, h is specimen thickness and D is the diffusion coefficient which represents
the ability of water molecules to penetrate inside the composites. The higher
D value, the higher the value of maximum water absorption. Higher D value also
might suggest the presence of high void content in the composite (Kushwaha
and Kumar, 2010).
MATERIALS AND METHODS
PP used was Propelinas G425, produced by Polypropylene Malaysia (PETRONAS)
and has Melt Volume-flow Rate (MVR) of 11.0 cm3/10 min while PC was
from the trade name Panlite®
grade L-1225Y, manufactured by Teijin Kasei America Inc. (Teijin Chemicals),
with MVR value of 11.0 cm3/10 min (300°C/1.2 kg). The compatibilizer
selected was PP-g-MA from the brand Sigma-Aldrich, with molecular weight of
9100 by gel permeation chromatography (GPC) and maleic anhydride content of
Sample fabrication: PC was dried at 95°C for 12 h prior to compounding
to minimize hydrolytic degradation. Pellets of PP, PC and PP-g-MA were mixed
according to designated weight fractions. The blends, containing between 0-35%
of PC and 5% compatibilizer were compounded by twin- screw extruder at 250°C
and 100 rpm before formed into standard shapes by compression molding, with
temperature ranging from 190-250°C.
Microscopy: Scanning Electron Microscope (SEM) was used to study the
morphology of the blends. Specimens were cryogenically fractured and plated
with thin gold layer as preparation.
Water absorption test: Water absorption test was done according to ASTM
D570. Specimens were molded in the shape of cylinder with 50.8 mm diameter and
3.2 mm thick and for each blend composition three specimens were produced. The
picture of the specimens is presented in Fig. 1. Before immersion,
the specimens were conditioned by drying in an oven for 24 h at 50°C, cooled
in desiccators and immediately weighed to the nearest 0.0001 g. The immersion
took place in distilled water at 23°C and after 24 h, they were taken out
from the distilled water, wiped with dry cloth and weighed to the nearest 0.0001
g. Upon weighing, the specimens were immersed again in distilled water and re-weighed
again after 7 days. After that, the weighing were done per two-week period and
halted once the weight increase was less than 5 mg, shown by three consecutive
weighing. Finally, the samples were dried in an oven for 3 days at 50±3°C
and then weighed again.
|| Water absorption specimens cooled in desiccators
Data after the drying are called the corrected values. This was done to cater
the effect of samples dissolution, if any occurred. Similar steps were repeated
for immersion at 100°C.
RESULTS AND DISCUSSION
Water absorption: The results of water absorption at 23°C are shown
in Fig. 2. The water absorption value, M percentage, is expressed
in wt.% and calculated by Eq. 3:
with Mu is the wet weight of specimen while Md is the
initial weight of dry specimen. The results in Fig. 2a show
that all the samples exhibited the same pattern. Rate of absorption is high
at the beginning, then slowly decreasing and finally remains relatively constant.
When it becomes constant, the sample is regarded as substantially saturated.
The absorption summary is tabulated in Table 1. When Ficks
diffusion behavior is plotted in a graph, the mass of absorbed water increases
linearly with square root of time, then slowly the rate decreases until equilibrium
plateau is reached (Dhakal et al., 2007), as
can be observed in Fig. 2b.
Generally, it can be observed that as PC content increases, water absorption
also increases. PC and water are polar polymer and polar solvent, respectively.
Therefore, solvent-solute interaction, namely like dissolves like
will occur. PC also contains oxygen in carbonate group, CO3 which
is susceptible to water absorption. Crowding of water molecules at PC polymer
chain causes the polymer structure to deform and open up, leading to higher
free volume (White, 2006). With higher free volume,
more water could penetrate in and this is the reason PC has the highest water
absorption value. Increasing PC content in PP/PC/PP-g-MA blends will cause the
blends to have polar charge, hence increasing its water absorption.
Besides the normal diffusion mechanism, it is suspected that capillary transport
of water molecules into the space between PP matrix and PC reinforcement phase
potentially occurred. This absorption mechanism is important, especially when
interfacial adhesion is weak (Kushwaha and Kumar, 2010).
This is due to the imperfection interfacial bonding between the two phases since
they are of different polarities and immiscible with each other (Karmaker,
The highest level of absorption among the blends was 0.260 wt.% which is still
considered as small. Therefore, it can be deduced that the interfacial adhesion
between PP matrix to PC reinforcement is higher than the polar bonding of the
|| Plot of water absorption at 23°C versus, (a) Immersion
time and (b) Immersion time square root
|| Water absorption value at 23°C immersion
Otherwise, the water absorption level could have been higher.
Results of immersion in 100°C distilled water for 168 h are shown in Fig.
3. After samples drying in oven for 3 days at 50±3°C, corrected
values were determined. The immersion data and corrected values are tabulated
in Table 2. Except for 60/35/5, all the other blends showed
reduction in water absorption level compared to pure PP. Similar to the previous
result of water immersion at 23°C, as PC content increases in the blends,
the water absorption also becomes greater. The trend of blends showing lower
absorption value compared to PP at elevated temperature can be traced back to
the blends improved thermal stability.
Ficks diffusion Eq. 1 is used for analysis purpose.
From the equation, the agreement towards Ficks law is governed by n value.
To determine the value of n, Eq. 1 was modified into Eq.
4 (Kushwaha and Kumar, 2010):
|| Plot of water absorption at 100°C versus, (a) Immersion
time and (b) Immersion time square root
|| Water absorption value at 100°C immersion
Figure 4 shows an example of fitting experimental data according
to Eq. 4. The values of parameter n obtained from curve fitting
are summarized in Table 3. It can be observed that the values
of n for immersion at 23°C are ranging from 0.205-0.403. They are below
0.5 which is the value of n for when the diffusion follows Ficks
law. The closer n value to 0.5, the more water diffusion follows Ficks
diffusion law. In the meanwhile, immersion in 100°C distilled water also
shows the same trend, where the values of n are still lower than 0.5. However,
the n values are lower than that of 23°C immersion. The range of n values
According to Ficks law, the ability of water molecules to penetrate inside
a composite can be represented by diffusion coefficient, D.
|| Summary of n values for immersion at 23 and 100°C
|| Fitting of experimental data to find n value for 90/5/5 composition
The diffusion property was obtained by determine the first slope of water absorption
(in wt.%) curve versus square root of time, then D was calculated using Eq.
5 (Dhakal et al., 2007):
where, k is the initial slope of water absorption weight gain versus square
root of time, h is the thickness of specimen which is 3.2 mm in this case and
finally Ms is the equilibrium water absorption value.
The values of D are summarized in the Table 4. The higher
D value, the higher the value of maximum water absorption will be. For immersion
at 23°C, generally the D values for PP/PC/PP-g-MA blends are showing increment
as PC content increases, except for 60/35/5 composition. This is in agreement
with the actual result of their water absorption, where higher PC content will
lead to higher water absorption. For immersion at 100°C, the values of D
are no longer showing correlation with the water absorption of the blends. This
is because water absorption kinetics of the blends in 100°C is deviating
quite far from the ideal Ficks
diffusion law. It also suggests some other mechanism of absorption occurred
apart from diffusion.
When comparing the effect of temperature towards water absorption, it can be
concluded that at 100°C the absorption value is much higher than that of
23°C. The same trend occurs for diffusion coefficient, D, value. This suggests
that different sorption kinetics took place when the samples were immersed in
100°C, compared to 23°C. The high and fast water absorption may be attributed
to degradation in the matrix - reinforcement interface region, caused by moisture
and elevated temperature. After the degradation, the damage in the matrix-reinforcement
phase makes the water transport mechanism even more active (Comyn
and Marom, 1985).
|| The 5000x magnification of PP/PC/PP-g-MA (a) 90/5/5, (b)
80/15/5, (c) 70/25/5 and (d) 60/35/5 by SEM
|| Summary of diffusion coefficient (D) values for immersion
at 23 and 100°C
|| Results of PC fibers diameter from SEM
Morphology: SEM was used to observe the morphology of compression molded
samples. The micrographs are shown in Fig. 5 while PC fibers
diameter results are tabulated in Table 5.
The morphologies show that PC existed as fibers, dispersed throughout PP matrix.
Generally, as PC content in compatibilized blend increases, the fibers
diameter range also increases. This is true except for 60/35/5 composition that
exhibited the smallest range among the compatibilized blends. It is suspected
that as PC amount increases, the chances for PC fibers to coalesce with each
other increases, thus bigger fiber diameter will be produced.
Except for 60/35/5 composition, all other blends showed lower water absorption
than pure PP when immersed in 100°C distilled water. The reduction of water
absorption compared to pure PP was up to 39.6%, showed by 90/5/5 composition.
The improvement in the thermal stability of the blends was proven to be beneficial
in accelerated aging of water immersion. In the meanwhile, immersion at 23°C
demonstrated that PP/PC/PP-g-MA blends were having water absorption values in
between PP and PC. As PC content was increases, water absorption becomes greater.
In general, water absorption at 23°C is still considered low, despite the
weak interfacial adhesion of the blends that can cause high water absorption.
The authors would like to thank the project supervisors, laboratory technologists
and the University for making this research possible.