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Chapter 5 Melt Rheological Behaviour of PAL FnDPE Composites
The results presented in this chapter have been published in Polymer, 37,5421 (1996).
5.1 Introduction
I n recent years, cellulose fibre reinforced thermoplastic composites have found
wide use in structural and non structural applications because of their superior
mechanical properties. The choice of suitable processing conditions is guided
mainly by the rheological behaviour of composites. A number of investigations on
the rheological behaviour of short fibre reinforced thermoplastics and elastomers
have been reported.'" Incorporation of fillers in thermoplastics will increase the
melt viscosity which may result in unusual rheological effects. Studies on the
rheological behaviour of filled thermoplastics and their applications have been
reported. The shear flow along the die can cause significant misalignment of fibres
and the fibre, orientation at the exit of the die is a function of the flow rate and lld
ratio. Crowson et UI. ' .~ reported in detail the flow behaviour of short glass fibre
reinforced thermoplastics during injection moulding. Composites viscosib is
considerably influenced by fibre loading and fibre length at lower shear rate than
at higher shear rate . The rheological behaviour of short jute fibre composites has
been studied by Murthy el Melt flow behavlours of short sisal fibre in different
polymer systems were reported by Thomas and co- worker^.'^"'
The present chapter deals with the rheological characteristics of short PALF
reinforced LDPE composites. The effects of fibre loading, fibre length, chemical
treatments and temperature on melt viscosity have been investigated. The
experimental viscosity values were compared with theoretical predictions. Shear
stress-temperature superposition was done to predict the melt viscosities of the
composite materials. Finally, master curves were generated using capillary and
melt flow index data.
5.2 Results and discussion
5.2.1 Effect of shear rate and fibre loading on viscosity
The dependence of fibre loading and shear rate on melt viscosity is shown
in Figure 5.1. The curves are typical of pseudoplastic materials. It is clear that the
viscosity of the system increases with fibre loading. The increase in viscosity is
more predominant at lower shear rate than at higher shear rates. In general, the
pseudoplastic behaviour of polymeric systems can be explained in two ways4 (a) If
a system of asymmetric particles, which are initially randomly dispersed, is
subjected to shear, the particles tend to align themselves with the major axis, in the
direction of shear, thus reducing the viscosity. The degree of alignment is a
function of the deformation rate. At low shear rates, there is only a slight depiuture
from randomness but at higher shear rates, particles are almost completely oriented.
(b) In highly solvated systems, when chemical interaction among polymer particles
exists, with increase of shear rate, the solvated layers may be sheared away
resulting in decreased viscosity.
0.1 L 1 10 100 1000 10000
SHEAR RATE (s')
Figure 5.1. Variation of melt viscosity (q) with shear rate ( y ) of PAL.F/LDPE composites at different fibre loading (Fibre length 6 mm, temperature 125°C).
The fibre filled system exhibits higher viscosity than the pure polymer at all
shear rates. In filled system, fibres will perturb the normal flow of polymer and
hinder the mobility of chain segments in flow. Crowson et UI . ' .~ reported the effect
of converging flow in the die entrance region which will result in high fibre
alignment. This will occur when a fluid passes from a wide to a narrow cross
section. At low shear rates fibres are disoriented. The probability of fibre-fibre
collision is much higher for disoriented fibres. This collision increases with fibre
loading and as a result viscosity increases. But as the shear rate increases, most of
the shearing takes place closer to the tube wall and the fibres will align along the
tube axis. Therefore, the probability of fibre-fibre collision is much lower and so
the increase in viscosity with fibre content is less at high shear rates. Goldsmith
and aso on" have also observed radial migration of filler particles towards the
capillary axis during shear flow. if this occurs during the flow of fibre-filled
thermoplastics, the region where most of the shear takes place may virtually be
fibre free. This could be a reason for the small dependence of viscosity on fibre
concentration at high shear rates. WU" also has reported that migration occurs
during the extrusion of glass fibre filled poly(ethy1ene terephthalate). The very
close viscosity values a! higher shear rates, for filled and unfilled thermoplastics, is
an important factor in explaining the successful exploitation df these materials in
injection moulding technology, since very little additional power will be required
to mould the filled materials (Figure 5.1).
5.2.2 Effect of shear stress and fibre loading on viscosity
The dependence of viscosity of PALF/LDPE composites on shear stress for
different fibre loading is shown in Figure 5.2. It is observed that in all w e s the
viscosity decreases with an increase in shear stress due to the onentation of the
fibre and polymer molecules in the direction of extmsion. I t may be noted that no
yield stress is exhibited by PALFLDPE composites, unllke glass and other
cellulose fibre-filled stern.^^"^
0.1 1 2 20
SHEAR STRESS x 10.' (Pa)
Figure 5.2. Variation of melt viscosity (q) with shear stress (r) of PALFLDPE composites at different fibre loading (Fibre length 6 mm temperature 12S°C).
For a given shear stress, viscosity of the composite increases with fibre
loading. This is true for all shear stresses. In the low shear stress region, there
would be little deformation, and strong interaction between fibres and polyethylene
due to the agglomeration of fibres would result in a high viscosity. As the level of
shear stress increases, fibres will align along the direction of flou and consequently
viscosity decreases.
5.2.3 Effect of fibre length
Figure 5.3 shows the effect of fibre length on the melt viscosity of
PALFLDPE composites. It can be noticed that the viscosity marginally increases
upon the increase of fibre length from 2 to 10 mm. At higher fibre length the
dispersion of fibre is not so good and at the same time it is difficult to orient the
fibres in the direction of flow. This is associated with the fibre entanglement at
higher fibre loading. Fibres having shorter length are more easily aligned and
distributed along the direction of flow than longer fibres.
0.1 I 10 100 1000 10000
SHEAR RATE (st)
Figure 5.3. Variation of melt viscosity (11) with shear rate ( y ) of PALFLDPE composites at different fibre length (Fibre loading 20%. temperature 125OC).
5.2.4 Effect of chemical treatments
Studies on the composite materials have shown that the bondlny between
the reinforcing fibre and the matrix has a significant effect on the properties of the
composite. Good bonding at the interface can be achieved by modifying the fibre-
matrix interface with various surface reactive additives or coupling agents 16'" In
order to enhance the fibre-matrix interaction PALF is treated with various reagents.
The effect of chemical treatments on viscosity of PALFILDPE composite at a
temperature of 125OC is shown in Figure 5.4. It is clear that the viscosity of the
composite increases as a result of chemical treatment Sllane treatment
(Silane A172) enhances adhesion at the polymer-fibre interface, and in the process
the viscosity is increased. As a result of silane treatment a strong interaction is
induced at the fibre-matrix interface and hction between the polymer and fibre is
increased." This results in an increase of viscosity.
0.4~ I I 10 100 1000 10000
SHEAR RATE (s')
Figure 5.4. Variation of melt viscosity (q) with shear rate ( y ) of PALFLDPE composites for different fibre treatments (Fibre length 5 mm, temperature 125°C).
The increase in viscosity of PMPPIC treated fibre composite is due to the
increased interaction between fibre and matrix as discussed earlier The peroxide
treated composites also show higher viscosity than the untreated composites. This
is due to the fact that during processing in the presence of peroxides, grafting of
polyethylene on to cellulose fibres occurs by combining cellulose and poiyethylene
radicals. Other reactions possible during the processing of composites with
peroxides are increase in molecular weight and crosslinking of polymer matrix by
combining polyethylene radicals.I9 All these lead to the increase in melt viscosity:
The increase in viscosity of NaOH treated composite IS attributed to the
increased mechanical interlocking between fibre and matrtx During alkali
treatment the fibre surface becomes rough due to the removal of waxy materlal
present on the surface. The rough surface topography provides better mechanical
interlocking with the polyethylene matrix. Among the various treatments viscosity
increases in the following order, untreated < NaOH < silane < DCP < BPO <
PMPPIC.
5.2.5 Effect of temperature
Substantial temperature changes occur at various stages durlng moulding
and it is important to study the effect of temperature on viscosity. In the present
study, flow curves are made at 125, 135 and 14S°C. Figure 5.5 shows the variation
of viscosity of the composites with temperature at different fibre loading at two
shear rates. It is clear that viscosity decreases with increase of temperature for the
filled and unfilled systems. .4t higher temperature, the molecular motion is
accelerated due to the availability of greater free volume and also due to decreasing
entanglement density and weaker intermolecular interactions. The decrease in
viscosity is more predominant at low shear rate than at high shear rate This may
be due to the high residence time of the melt in the barrel of the capillary rheometer
at low shear rate.
0.2 I
120 130 140 150
TEMPEARATURE ('C)
Figure 5.5. Variation of melt viscosity (q) with temperature at different shear rates and at different fibre loading (Fibre length 6 mm).
To further understand the influence of temperature on the viscosity of
composites, Arrhenius plots at constant shear rate are made (Figure 5.6). 111 this
figure the logarithm of viscosity is plotted against reciprocal temperature. Linear
plots are obtained. The activation energy of a materid provides valuable
information on the sensitivity of the material towards the changes in temperatures
The higher the activation energy the more temperature sensitive the material w ~ l l
be. The activation energies of the system at different fibre loading are given in
Table 5.1. The flow of composites is dependent on the quantrty of fibres prescnt in
the system as well as on the temperature and shear rate, i.e the presence of fibre
restricts molecular motion.
Figure 5.6; Plot of log q versus 1/T.
Table5.1. Activation energy of PALFILDPE composites at different fibre loading
System Activation energy (cal/mol) I LDPE I 702 I LDPE + 10 wt % fibre I 397 I LDPE + 20 wt % fibre I 298 I LDPE + 30 wt % fibre I 546 I
In the case of composites containing treated fibre a different trend is
observed (Tables 5.2 and 5.3). There is an increase of viscosity when the
temperature is increased from 125 to 135'C. This behaviour is exhibited by
isocyanate treated and peroxide treated composites at low shear rates. But at high
shear rates this behaviour disappears because the residence time of melt in the
barrel is less.
Table 5.2. Viscosity of untreated and treated composites at varlous temperatures (Fibre loading 20%).
Table 5.3. Viscosity of peroxide treated composites at vanous temperatures (Fibre loading 200h).
This behaviour is due to the fact that at higher temperature,
crosslinking is expected to take place during extrusion. In order to analyse the
extent of crosslinking, the crosslink density of the composites 1s detenn~ned by
equilibrium swelling method using the equation,20
where p, = density of composite, V = molar volume of solvent and + = volume
fraction of polymer in the solvent swollen sample which is given by
where w = weight of sample, d = weight after drying the sample, f = fraction of
insoluble component and & = weight of solvent. The interaction parameter x is
given
where p = lattice constant (0.34), V. = molar volume of solvent, R = gas constant,
T = temperature, p = solubility parameter of solvent and p, = solubility parameter
of the composite. As the effective bonding between the matrix and fibre increases
the restriction to swelling increases. The degree of crosslink~ng ('I) has been
detennined from M, values as 1/2M,. The differences in crosslink density values of
composites at two different temperatures are shown in Table 5.4. It is clear that the
crosslink density of peroxide treated composite is higher at 135OC. Therefore it can
be concluded that the increase in viscosity of the composite with temperature is due
to the increased crosslink density of the system.
Table 5.4. Crosslink density values of composites at two different temperatures (Fibre loading 200h).
Composite
PMPPIC treated
BPO treated
5.2.6 Shear stress-temperature superposition master curve
Crosslink density
I I
The application of shear rate-temperatre superposition in predicting melt
viscosities was recently reported by ~ n a n d . ~ ' The values of superposition shift
factors (m) are obtained by choosing a shear stress value at the reference
temperature of 135OC and shifting the corresponding points on the flow curves for
other temperatures to coincide with these shear stresses. I h e values of a, are
calculated from the following equation:
125°C
1.08 lo4
1 .OO x lo4
The master curve is constructed by plotting the product rra-r vi:nuc- y (Figure 5.7).
It is clear that the curves at different temperatures shift to a single reference
temperature curve. The superposition of the data is achieved over the whole range
of temperature and shear rate. The average shift values for a reference temperature
of 13S°C are shown in Figure 5.8 as log a= versus 1 R .
135°C
1 40 r l o '
1.35 x lo4
1.20 lo4 DCP treated 9.40 x 10" .
"" LDPE . . . . 10%
+- 125'C 135°C 145°C
SHEAR RATE (s-')
Figure 5.7. Shear stress versus shear rate superposition master curves at different fibre loading at different temperatures.
Figure 5.8. Plot of log a.~. versus l/T.
5.2.7 Comparison with theoretical prediction
For non-spherical partrcles ~ u t h " proposed an equat~on to prcdlct the
viscosity of oriented composites:
q c = q.. (1+0.6fc + 1 .62fZc2) ( 5 . 5 )
where f = ratio of longest to shortest diameter of fibre, c = volume fraction of fibre,
qY. = viscosity of unfilled system and q, = viscosity of composite
The theoretical and experimental viscosities of PALF filled polyethylene
composites are shown in Figure 5.9. It is clear that experimental viscosity 1s higher
than theoretical viscosity. This is associated with the disorientation of fibres as
well as the fibre orientation distribution.
- - - THEORETICAL - EXPERIMENTAL
*LDPE 410% *20% +30%
Si
g 20 ? S? X
G rn 0
2 5 5 W Z
0.2 1 I 10 100 1000 10000
SHEAR RATE (s')
Figure 5.9. Comparison of theoretical and experimental viscosity of PALFiLDPE composites at different fibre loading.
5.2.8 Fibre breakage analysis
The fibre length distribution of composites before and after extrusion
through the capillary at three different shear rates is given in Figure 5.10. Owing to
the high shear stresses during extrusion there is a possibility of fibre breakage.
From the figure it is clear that about 70% of fibres retain the original length (6 mm)
after extrusion through the injection moulding machine (nozzle dlameter 4 mm).
During extrusion at high shear rates, the fibre undergoes breakage. Only about
40% of fibres are of 6 mm length at a shear rate of 1640 <I. At all shear rates the
most probable length is 6 mm. It is interesting to note that this value decreases
linearly with increase of shear rate (Figure 5.1 1). This is associated with the
breakage of fibres at high shear rates.
80
*BEFORE EXTRUSION * 1 6 4 <'* 164 s"* 1640 1 -- !
FIBRE LENGTH (mm)
Figure 5.10. Fibre length distribution curve
40 1
0 400 800 1200 J
1600
SHEAR RATE (s')
Figure 5.1 1. Plot of most probable length versus shear rate ( j. ).
The fibre length distribution can also be represented in the same way as
molecular weight distribution. The number and weight averages are represented
as:
where i n = number average fibre length, i , = weight average fibre length and Ni
= number of fibres having length Li. The ratio, t , / c n , the polydispersity index is
taken as a measure of fibre length distribution. The fibre length distrihutlon
becomes broader as the value i , / i n increases. The values of E,,i,and - L,/E, based on 150 fibres extracted from the composites before and after
extrusion are given in Table 5.5. The polydispersity index remains the same before
and after extrusion at all shear rate values.
Table 5.5. Fibre length distribution index for PALF
5.2.9 Extrudate morphology
Figures 5.12a-5.12~ show the cross section of extrudate contrun~ng 200/0
fibre at shear rates of 16.4 and 164 and 1640 s-' respectively. The fibres which are
concentrated at the periphery at low shear rate (Figure 5.12a) are dispersed
uniformly at medium shear rate (Figure 5.12b). However, at high shear rate fibres
are aligned along the direction of flow and concentrated at the core region
(Figure 5.12~). Figures 5.13a and 5.13b show the surface morphology of the
extrudates at different shear rates. It is clear from these figures that surface
discontinuity increases with shear rates. Most of the extrudates have smooth
surfaces and are of uniform diameter at low shear rates. But at high shear rates the
extrudates exhibit surface irregularity.
- L,/L,,
1.03
1.03
1 0 4
1.08
- LW
5.68
5.51
5.56
5.14
Fibre
Before extrusion
After extrusion at shear rate of
16.4 s-I
164 i'
1640 s'l
- L ~ ,
5.53
4.33
5.33
4.73
Figure 5.12. Scanning electron micrographs of the cross section of extrudate at different shear rares; (a) 16.4 s-' (fibres are concentrated at the periphery), (b) 164 s-' (fibres are uniformly dispersed), and (c) 1640 s" (fibres are concentrated at the core region).
(a) Ib) Figure 5.13. Scanning electron niicrographs of surface morphology of extnidate at
different shear rates; (a) 16.4 s-' and (b) 1640 s-'.
5.2.10 Flow behaviour index
The effects of fibre loading and temperature on the flow behaviour index
(n') are shown in Table 5.6. For all the systems, n' values are less than unity,
characteristic of the pseudoplastic nature of the composites. The degree of
pseudoplasticity of the composite increases with fibre loading and with
temperature. The high pseudoplasticity arises due to the orientation of fibres. The
flow behaviour index of treated fibre composites at 20% fibre loading is given in
Table 5.7. The pseudoplasticity of treated composites is higher than that of
untreated composites except in the case of silane treated composite.
I LDPE I 0.417 I 0.403 I 0.414 1
Table 5.6. Flow behaviour index (n') of PALFILDPE composites at different fibre loading.
System
Table 5.7. Flow behaviour index (n') of treated PALFLDPE composites (Fibre loading 20%).
PE + 20% fibre
PE + 30% fibre
n'
1 Untreated 1 0.314 I 0.300 I 0.299 I
1 2S°C
0.314
0.294
Treatment
I PMPPIC 1 0.287 I 0.258 I 0.226 I I Silane I 0.3 12 I 0.355 I 0.363 I
13S°C
0.300
0.290
n'
14S°C
0.299
0.289
12S°C
BPO
DCP
13S°C 1 45OC
0.281
0.288 0.288 1 :.::: 1 0.290
5.2.11 Correlation between melt flow index (MFI) and capillary rheometer data
Although MFI may be a good indicator for studying the effect of processing
history of the polymer, it cannot be correlated directly with processing behaviour
since the values of temperature and shear rate employed in the MFI test differ
substantially from those encountered in the actual process. Shenoy el ol.'.'"
developed a method to estimate the rheograms from a knowledge of MFI for
polyolefms, styrenics and cellulosics. Bhagawan el d2' combined the MFI data
and capillary rheometer data to provide master curves for silica and black filled
thermoplastic 1.2-polybutadiene rubber.
F~gure 5.14 shows the master curve of composite obta~ned by correlating
MFI (Table 5.8) and capillary rheometer data at different fibre loading Using a
plot of q x MFI/p versus y x p/MFI (p = density of composite), it is shown that the
four curves are unified as a single curve. The advantage of such a master curve is
that simple evaluation of MFI would be adequate to generate viscosity versus shear
rate curves without the use of sophisticated rheological instruments.
Table 5.8. MFI values of PALFILDPE composites ( d l 0 mln) at 1445°C
System
LDPE
100h fibre
20% fibre
30% fibre
MFI
180
43.7
12 3
4.54
1,000
A LDPE 10% X 20% 9 30%
L
Figun 5.14. Viscosity master curves for different PALFILDPE systems at 14S°C
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