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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