Journal of Reinforced Plastics and Composites OnlineFirst, published on August 19, 2008 as doi:10.1177/0731684408092440
Relationship of Rheological Study with
Morphological Characteristics of Multicomponent
(Talc and Calcium Carbonate) Filled
Polypropylene Hybrid Composites
S. S. JIKAN, M. S. F. SAMSUDIN, Z. M. ARIFF, Z. A. M. ISHAK AND A. ARIFFIN*
School of Materials & Mineral Resources, Engineering Campus
Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Selatan
Penang, Malaysia
ABSTRACT: Polypropylene (PP) copolymer reinforced with talc, CaCO3, or multicomponent fillers
(a combination of talc and CaCO3) were compounded in a Brabender Plasti-CorderÕ internal mixer.
These compounds then experienced an extrusion process by means of a capillary rheometer. The
rheological and morphological (SEM) properties of the extrudate were analyzed to investigate the
effect of shear stress, filler type, and temperature. The rheological studies revealed that the
incorporation of multicomponent fillers has increased the melt viscosity. However, different ratio of
filler type demonstrates no significant effect. It is also shown that the system appears pseudoplastic
over one range of shear rates but dilatant over another. The presence of filler seems to have a
pronounced influence on dilatant flow in filled PP. Observation on SEM analysis at high shear stress
showed that large CaCO3 agglomerates are seen to be randomly dispersed whereas talc particles are
more uniformly distributed and oriented to flow direction, suggesting better mixing.
KEY WORDS: talc, calcium carbonate, polypropylene, multicomponent, pseudoplastic, dilatant.
INTRODUCTION
improve the competitiveness of Polypropylene (PP) in engineering
applications, its properties have to be modified by introducing a reinforcing filler.
Among the reinforcing fillers, talc and calcium carbonate (CaCO3), which fall in the group
of mineral fillers, are the most commonly used in PP [1–4]. It is well known in the
rheological world that a small amount of the addition of such filler could cause significant
changes in the rheological properties such as the viscosity of the base resin [2,5]. Thus it is
vital to understand their rheological behavior before introducing any new polymeric
product to the outside world. There have been few studies of rheological properties on PP
reinforced with single mineral filler [4,6]. Kim and White [2] made an attempt to study the
rheological properties of suspensions of talc, CaCO3, and their mixtures in a polystyrene
(PS) melt and indicated that the viscosity of mixed particle (talc and CaCO3) compound is
generally higher than the CaCO3 compounds but lower than the talc compounds.
I
N ORDER TO
*Author to whom correspondence should be addressed. E-mail: [email protected]
Figures 1–5 appear in color online: http://jrp.sagepub.com
Journal of REINFORCED PLASTICS
AND
1
COMPOSITES, Vol. 00, No. 00/2008
0731-6844/08/00 0001–11 $10.00/0
DOI: 10.1177/0731684408092440
ß SAGE Publications 2008
Los Angeles, London, New Delhi and Singapore
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However, there is no previous study on the rheological properties of mixed (talc and
CaCO3) filled PP. Thus, the study on rheological properties of talc, CaCO3, and a mix of
these filler compounds in PP has been carried out in order to investigate the effects on
temperature, pressure, shear history, and filler loading by the shape of the flow curve. The
purpose of this study is to develop a rheological understanding of multicomponent filled
PP as this could help us to optimize the processing condition of this new polymeric material.
EXPERIMENTAL
Materials
The thermoplastic used in this study was polypropylene (PP) copolymer resin grade ProFax SM-240 supplied by Titan PP Polymers (M) Sdn. Bhd. The SM-240 was received in
pellet form with a specific density of 0.894 g/cm3, and melt flow index (MFI) of 25 g/10 min
was obtained when measured according to ASTM D1238-90b, at 2308C and with 2.16 kg
load. The two important fillers used were untreated calcium carbonate (CaCO3) grade
OMYACARB 3-SA and talc (chemical formula Mg3Si4O10.(OH)2). They were provided
by Malaysian Calcium Corporation Sdn. Bhd. and Chung Chemicals Sdn. Bhd.,
respectively. The physical properties of fillers were determined using Malvern
Mastersizer. The filler specifications [7] are listed in Table 1.
All experiments were carried out based on 30 wt% of filler loading. The compositions
for all single filler and multicomponent filler composites studied in this study are presented
in Table 2. A Brabender Plasti-CorderÕ model PLE 331 with internal mixer head was used
in compounding the samples. The temperature of the mixing chamber was set at 1908C and
the rotor speed was 50 rpm with total compounding time of 12 min.
The procedure involved preheating of PP matrix in the Brabender Plasti-CorderÕ ’s
mixing chamber for 3 min without rotation and after preheating the rotors started to speed
Table 1. Materials specification.
Sample
Filler particle
Talc (T)
CaCO3 (CC)
PP (wt%)
Density
(g/cm3)
Talc (wt%)
Hardness
(Mohs scale)
CaCO3 (wt%)
Mean particle
diameter (km)
2.79
2.70
1
3
6.3
4.5
Table 2. Formulation comprising single filler and hybrid
filler composites.
Unfilled PP
0T30C
5T25C
10T20C
15T15C
20T10C
25T5C
30T0C
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at 50 rpm. At 5 min, the multicomponent fillers were carefully added to the mixing
chamber within 30 s. Each compound was sheared at constant rotational speed and the
processability behavior of the compound was measured and studied until the torque and
melt temperature reached their steady state. After 12 min, the rotors were stopped. Finally,
the molten compound was quickly removed from the chamber and sheeted through a
laboratory scale two-roll mill at 2.0 mm nip setting and then cut into small particles for
rheological test.
Testing
The rheological properties of the composites were tested according to JIS K 7210–1976
(reference test) and JIS K 6719–1977 with Shimadzu capillary rheometer model CRT-500
(constant shear stress). The flow rate, apparent viscosity, and apparent shear rate of the
individual sample measured over a range of shear stress of 4.9–39.2 MPa, was determined
at three different temperatures (180, 200, and 2208C). This study involved a fixed flat-entry
capillary die with a diameter of ¼ 1 mm and L/D (length over diameter) ratio of 10. First,
the apparatus was preheated up to the desired testing temperature. The sample of
approximately 1.5 g was then loaded into the cylinder and tamped down by the piston.
The sample was primarily preheated in the cylinder for 5 min in order to allow the sample
to melt and consolidate. When the preheating period came to the end, the weight cylinder
was lowered to apply a load to the sample. Consequently, the melted sample was extruded
by flowing out through the die and named as an extrudate. The results were then
calculated by a programmed software installed in the rheometer apparatus and were
automatically printed out. Second, these extrudates were carefully collected in order to
further study morphological and thermal analyses.
RESULTS AND DISCUSSION
Mixing Studies
The Brabender Plasti-CorderÕ torque values were measured as a function of time for
5T25C, 15T15C, and 25T5C (with virgin or unfilled PP as a control) at 1908C with a rotor
speed of 50 rpm. The shear force and temperature (above melting point) subjected to the
system leads to a destruction of all the primary particles, hence developing the compound
into a homogenous state which is reflected in a torque–time curve [8]. The results obtained
in this study are reported in Figure 1. On loading the chamber with unmelted PP,
the torque increases rapidly and then gradually decreases as PP starts to melt, until it
reaches a stable torque level (minimum torque value) at which the PP melt reduces to a
constant value of viscosity or is believed to be fully melted. In this case, a fully melted PP
is developed in about 8 min of mixing. On the contrary, for all multicomponent filled PP,
two significant peaks of the respective melt compound are clearly observed before reaching
the stabilization level of torque. The first peak of multicomponent filled PP is much
lower than that of virgin PP due to the reduction in charged weight of PP [9]. As the
mixing continues, the melt viscosity is reduced, leading to a decrease in torque value.
At this stage, the compound can effortlessly be deformed due to the easing of the polymer
molecules mobility. After 5 min of shearing, the filler is loaded into the chamber and a
significant increase in torque can be observed. As a result, another peak is formed.
This scenario can be explained by the presence of fillers, which restrict the mobility of
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40
Unfilled PP
35
5T25C
Torque (Nm)
30
15T15C
25T5C
25
20
15
10
5
0
0:00
2:24
4:48
7:12
9:36
12:00
Time (min)
Figure 1. Mixing torque–time curves for unfilled PP and multicomponent filled PP (5T25C, 15T25C, and
25T5C) at 1908C and 50 rpm.
polymer molecules [10]. Furthermore, the powdery state of fillers is capable of increasing
the melt viscosity, thus giving rise to the torque value.
Careful examination of Figure 1 shows that upon the addition of fillers, the 5T25C
compound exhibits slightly higher torque peak with respect to that of 25T5C. This
observation suggests that the degree of resistance to deformability of PP matrix [9]
increases due to the higher Mohs hardness of CaCO3 which is approximately 3, whereas
talc having only Mohs hardness of 1 (see Table 1), and the tendency of the CaCO3 particles
to agglomerate with each other, also provide pronounced increase on the torque value.
When all the primary filler particle network completely break down and have uniform
distribution in the matrix, the torque value reduces and stabilizes. A further increase in
mixing time revealed that the system of 25T5C exhibits slightly higher stabilization level of
torque. It is believed that talc has the ability to increase the viscosity of the system as
examined by SEM and that compounds with higher content of talc are observed to be
densely packed. Thus, introducing more rigidity and resistance to deformability leads to
higher stabilization of torque value.
Flow Curve Studies
The influence of filler content on the rheological behaviors of single component filled PP
and multicomponent filled PP on a log–log scale can be seen in Figure 2(a) and (b).
In this study, all the single component filled PP and multicomponent filled PP have a
decrease in shear stress with shear rate of approximately 35% (over a range of 102–104 s1)
with respect to that of unfilled PP. This fact is attributed to the substitution of PP matrix
which consists of molecules with more rigid filler particles. The incorporation of filler is
also believed to restrict the molecules’ mobility [10,11]; hence, leading to a dramatic
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(a)
100
Unfilled PP
Apparent shear stress (MPa)
5T25C
15T15C
25T5C
10
1
10
100
1000
10000
100000
Apparent shear rate (s−1)
Apparent shear stress (MPa)
(b) 100
10
15T15C
30T0C
0T30C
1
10
100
1000
10000
100000
Apparent shear rate (s−1)
Figure 2. Relationship between apparent shear stress and shear rate of (a) unfilled PP and multicomponent
filled PP (5T25C, 15T25C, and 25T5C), (b) single component filled PP (30T0C and 0T30C) and
multicomponent filled PP (15T15C) at 1808C (L/D ¼ 10).
change in shear stress with shear rate of filled PP (representing single component filled PP
and multicomponent filled PP). However, the unfilled PP demonstrates dilatant level, _d at
1.8 104 s1. Above this level, the shear rate value seems not to exhibit any significant
change with an increase in apparent shear stress.
A comparison was made between sample 30T0C and 0T30C giving the result that
sample 30T0C, which has the highest ratio of talc, showed a slight increase in shear stress
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1000
Apparent shear viscosity (Pa.s)
0T0C
15T15C
30T0C
0T30C
100
10
10
100
1000
10000
100000
Apparent shear rate (s−1)
Figure 3. Relationship between apparent shear viscosity and shear rate of unfilled PP, single-component
filled PP (30T0C and 0T30C) and multicomponent filled PP (15T15C) at 1808C (L/D ¼ 10).
with shear rate if compared to that of sample 0T30C (see Figure 2(b)). The effect of talc on
shear stress with shear rate indicates the ability of talc to promote stiffness and rigidity to
the PP melt. As the shear stress increases, 30T0C melt becomes more rigid due to the platelike structure of talc which influences good interaction between filler and matrix, thus
leading to restriction in chain mobility [1]. Unlike CaCO3, which has a spherical shape, the
ability of such fillers to have better filler–matrix interaction is fairly low due to the
formation of larger cavities inbetween filler and matrix. Therefore rigidity of 0T30C melt is
much lower than that of 30T0C.
As can be seen in Figure 2(a) and (b), the flow curves of all systems are dependent on the
addition of filler and the molecular chain characteristic of PP. When the shear stress of
29.4 MPa and above are applied to PP melt, the values of shear rate show a slight increase
which indicates that at this stage the shear rate is virtually independent of the shear stress.
This scenario is believed to be due to the molecular chain orientation of the PP system and
manifests itself as an aloft sweep of the curve; a sign of shear-induced crystallization. In
this case PP is semi-crystalline polymer; therefore, a large-scale crystalline structure in PP
will enhance the motion of the polymer chains (which hold the molecules together) as well
as raising the disentanglement of the polymer chains due to reduction in the mobility of
molecules. As reported by Pogodina et al. [12], the polymer chains of PP align themselves
parallel to the flow direction.
The apparent shear viscosity of unfilled PP and filled PP is plotted against shear rate at
constant temperature of 1808C on a log–log scale, as shown in Figure 3. The incorporation
of filler into PP has resulted in a significant rise in shear viscosity of approximately 25%
with respect to that of unfilled PP (only up to the level before unfilled PP reaches the
dilatant behavior).
At higher shear rates, samples 30T0C and 0T30C exhibit similar flow behavior, showing
that shear viscosity decreases with increase in shear rate irrespective of filler type. As the
shear rate increases, the dispersion of filler particles in the PP matrix increases, allowing
more mobilization of polymer matrix molecular chains. The only difference that could be
shown between 30T0C and 0T30C is a slight decrease in shear viscosity value with
increasing shear rate given by 0T30C if compared to that of 30T0C. This is due to the
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fact that CaCO3 is loosely embedded in PP matrix and these factors contribute to less
resistance to deformation, hence decreasing the viscosity of the respective filled PP that
corresponds to the results obtained in mixing studies. Since the flow curves of all filled
PP exhibit a relatively similar trend, only samples of 5T25C, 15T15C, and 25T5C have
therefore been selected to be the candidates to further investigate the rheological behavior
of these multicomponent filled PP systems, while virgin PP is used as a control.
In order to differentiate the influence of talc and CaCO3 on the microstructure of filled
PP, a cross-section of extrudate surfaces of 25T5C and 5T25C was examined by means of
SEM. The result shows that the external stress subjected on the system has changed the
talc orientation to a more organized structure. It is quite positive that talc has a strong
tendency to be oriented in chorus with polymer matrix following the flow direction when
processed by the extrusion method. Talc is deeply embedded in the PP matrix due to its
plate-like shape, thus giving rise to filler–matrix interaction that justifies the significant
improvement in flow instability [1]. Furthermore, 25T5C is densely packed if compared to
5T25C suggesting an increase in viscosity with increasing talc content as evidenced in the
flow curves (Figure 3).
The SEM result revealed poor interaction of CaCO3 particles with PP matrix, as
evidenced from the existence of voids between matrix and CaCO3 agglomerates. The
CaCO3 particles tend to be exposed and loosely distributed on the cross-section surface.
Effect of Temperature
It might be expected that the dilatant behavior of multicomponent filled PP (5T25C,
15T15C, and 25T5C) depends very closely on the extrusion temperature. Figure 4 represents the temperature dependence of multicomponent filled PP at different shear rate and
shear stresses range from 5–40 MPa.
It is noted that the flow of all systems at 1808C exhibit similar behavior in which
shear rate increases linearly with an increase in the apparent shear stress. However, at
higher temperatures (200 and 2208C), all systems show a tendency to behave dilatantly.
Dilatant phenomenon become visible at shear stress of 29.4 MPa at a tested temperature of
2208C, whereas it is delayed to 34.3 MPa for 2008C. The dilatancy of filled PP has been
discussed in an earlier section. When the system tends to be dilatant, the shear rate sustains
its value, although the shear stress is gradually increased and the dilatancy is shifted
to appear one point earlier towards higher temperature. It should be noted here
that from 29.4 to 34.3 MPa and beyond, the shear rate value is almost independent to
the temperature.
It is now important to investigate the flow curve of both systems, between unfilled and
multicomponent filled PP when tested at the highest temperature of 2208C. The plot
of apparent shear viscosity versus shear rate (Figure 5) may give constructive information
on the change in the flow behavior of both systems.
It is clearly seen that the viscosity of all multicomponent filled PP exhibits a
similar result as all flow curves converge and superimpose on each other. The only
difference that could be observed was a slight increase in shear viscosity of 25T5C with
respect to that of 5T25C. At the early stage of experiment, the shear viscosity decreased
linearly with increasing shear rate. However, as the flow of all systems approached a shear
rate of 1.8 104 s1, the viscosity started to increase, indicating the onset of dilatancy
behavior. At this point, the randomly entangled polymer chains and filler particles align
themselves into highly ordered configurations parallel to each other in order to permit the
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(a)
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Apparent shear stress (MPa)
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180°C
200°C
220°C
1
10
Apparent shear stress (MPa)
(b)
100
1000
10000
Apparent shear rate (s−1)
100000
100
10
180°C
200°C
220°C
(c)
100
Apparent shear stress (MPa)
1
10
10
100
1000
10000
Apparent shear rate (s−1)
100000
180°C
200°C
220°C
1
10
100
1000
10000
100000
Apparent shear rate (s−1)
Figure 4. Temperature dependence of apparent shear stress – apparent shear rate plots of sample
(a) 5T25C, (b) 15T15C, and (c) 25T5C.
growth of spherulites. Deformation and orientation of filler particles when subjected
to different shear stress have been analyzed using SEM. Cross-sections of extrudate
surfaces of composites were examined to study the effect of different shear stresses
at 2208C. Multicomponent filled PP (15T15C) was selected as the representative samples
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1000
Unfilled PP
Apparent shear viscosity (Pa.s)
5T25C
15T15C
25T5C
100
10
100
1000
10000
100000
Apparent shear rate (s−1)
Figure 5. Relationship between apparent shear viscosity and apparent shear rate of unfilled PP and
multicomponent filled PP (5T25C, 15T25C, and 25T5C) at extrusion temperature of 2208C (L/D ¼ 10).
for this examination. The CaCO3 particles have a tendency towards formation of agglomerates due to their irregular shapes. The particles are loosely embedded in PP matrix and
large voids exist between matrix and filler particles, creating weak filler–matrix interaction.
Much lower voids were observed for 15T15C subjected to shear stress of 9.8 MPa, which
relates to an external stress being subjected to the system. However, it does not show any
sign of development in the filler orientations. In fact, the agglomeration of particles is still
in evidence, which indicates that the shear stress applied is insufficient to disperse the
CaCO3 agglomerates.
When subjected to higher shear stress, the morphologies exhibit a significant contrast.
At 24.5 MPa, the cross-section of extrudate surface shows more oriented melt. The voids
are much smaller and fillers are more uniformly distributed and dispersed as the shear
stress increases, suggesting improved mixing. The size distribution of CaCO3 before
extrusion was then shifted to smaller particle size, showing approximately 4 mm which is
similar to as-received CaCO3 size (see Table 1). These obvious differences are clearly
related to the molecular and filler orientation exhibited by the system. Nonetheless,
CaCO3 particles are still loosely embedded due to their weak interaction with PP matrix.
This scenario is observed after the system is subjected to 29.4 MPa at which the
fluid appears to behave as a dilatant fluid (see the earlier discussion on flow curve studies).
An observation was made by Nagasawa et al. [13] that the crystallization phenomenon in
tubular blown film is due to the fact of molecular orientation agreeing with the results
obtained in this experiment. That is, a change in orientation of molecules controls the
growth rate of crystallites and leads to spherulite formations.
The presence of spherulites in the polymer melt has changed its physical structure and
created a stiffer environment to the melt flow, giving rise to viscosity. Once solidified, these
molecules remain aligned due to the growth of crystallites which fold the molecules within
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the spherulites [14] and consequently keep the polymer chain from falling apart. The
interest in this physical structure is that the aptitude of this sheared composite enhances its
anisotropic structure. A composite which has this class of structure exhibits similar
mechanical properties in all directions [15]. According to many studies reported on shearinduced crystallization [16,17], this phenomenon is vital knowledge for almost all polymer
processing in extrusion, fiber spinning, injection molding and film blowing.
CONCLUSIONS
The single component filled PP and multicomponent filled PP were prepared by melt
compounding using a Brabender Plasti-CorderÕ internal mixer. The results revealed that
the addition of fillers had drastically increased the torque value and then decreased
gradually as soon as the compound had reached its homogenous state.
The rheological studies have shown that the apparent shear rate of all systems increases
with increasing apparent shear stress. However, all flow curves exhibit a sign of shearinduced orientation, gd, at 1.8 104 s1. At this stage, the flow curves of the systems
shift to higher viscosity and retain the shear rate value. Above gd, the shear rate of all
systems fall at the same range and the shear rate values seem to be self-governed from the
temperature and pressure. Nonetheless, different filler type and filler ratio do not show a
definitive effect on the shape of the flow curves. Higher temperature (2008C) and shear
stress (429.4 MPa), however, have greatly influenced the flow curve by changing the
pseudoplastic behavior into dilatant behavior. The morphological studies of the given
systems were observed by SEM. The results revealed a good interaction between talc
particles and PP matrix, whereas CaCO3 seemed to be exposed and loosely embedded in
the PP matrix. It was also observed that the improvement in mixing of multicomponent
filled PP was achieved in the extrusion at high shear stress (424.5 MPa) when compared
with that at lower shear stress. The observation implies that sufficient pressure is needed to
disperse the particle agglomerates into individual particles.
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