Rheological behavior of short sisal fiber-reinforced polystyrene composites

Composites: Part A 31 (2000) 1231–1240 www.elsevier.com/locate/compositesa

Rheological behavior of short sisal fiber-reinforced polystyrene composites K.C.M. Nair a, R.P. Kumar a, S. Thomas a,*, S.C. Schit b, K. Ramamurthy b a

School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills. P. O, Kottayam, Kerala-686 560, India b Central Institute for Plastics Engineering and Technology, Guindy, Chennai 600 032, India Received 30 November 1998; accepted 19 April 2000

Abstract The rheological behavior of short sisal fiber-reinforced polystyrene composites containing short sisal-fiber has been studied using an Instron capillary rheometer. The effect of fiber length, fiber loading, shear rate, shear stress and temperature on the rheological behavior of the composites was studied. Unlike other short fiber-reinforced thermoplastics at lower temperature the melt viscosity of polystyrene(PS)-sisal composites are lower than that at higher temperatures. At 1808C the viscosity of the composite is governed by wall-slip, which decreases the viscosity and at 1908C the viscosity is governed by fiber melt interaction that increases the viscosity. The morphology of the extrudate was studied using optical and electron microscopy. q 2000 Elsevier Science Ltd. All rights reserved. Keyword: Sisal-fiber composites

1. Introduction The incorporation of short fibers into thermoplastics and elastomers to achieve cost reduction [1] and to improve mechanical properties has become increasingly important in recent years. The rheological behavior of the materials is important for selecting the processing conditions to fabricate polymer products. A number of investigations on the rheological behavior of short fiber-reinforced thermoplastics and elastomers have been reported [2–5]. The rheological behavior of aramid, glass and cellulose fiberreinforced polystyrene (PS) melts were reported by Czarnecki et al. [6]. Usually, the incorporation of short fiber in thermoplastics and elastomers increases the melt viscosity and may result in unusual rheological effects. A decrease in melt viscosity as a result of the incorporation of short fibers was also reported [7]. A decrease in the melt viscosity of coir/natural rubber (NR) composites at higher fiber loading was reported by Geethamma et al. [8]. The influence of fiber length and fiber loading is more pronounced at lower shear rate than at higher. The rheological behavior of short jute fiber composites has been studied by Murthy et al. [9]. The melt viscosity of glass fiber filled polyethylene (PE) and polypropylene was studied by Becraft and Metzner [10] and it was found that there is a significant increase in visc* Corresponding author. Tel.: 191-481-561800; fax: 191-481-561190. E-mail address: [email protected] (S. Thomas).

osity with fiber loading at low shear rates and little change in viscosity at higher. Recently, our research group reported on the rheological behavior of short sisal fiber-reinforced NR [11] PE [12] composites and short pineapple fiber-reinforced PE composites [13]. Recently, short sisal fibers were used as good reinforcing material for PS and the mechanical properties of untreated and treated fiber-reinforced composites were reported in detail [14] by these authors. However, no study has been reported so far on the rheological behavior of these composites. Hence, in the present article we report on the rheological behavior of short sisal fiber-reinforced PS composites. The effects of temperature, fiber concentration, fiber length and shear rate on the melt flow behavior of composites is investigated. This study is very important for optimizing the processabilty of sisal fiber-reinforced composites by extrusion and injection molding techniques.

2. Experimental 2.1. Materials: Polychem Ltd, India, supplied polystyrene (POLYSTRON 678 SF-1, crystal grade) and sisal fiber was obtained from local sources. The fiber was cleaned and chopped into the desired length ranging from 2 to 6 mm. Table 1 gives the physical properties of PS and sisal fiber.

1359-835X/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S1359-835 X( 00)00 083-X

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Table 1 Physical properties of polystyrene and sisal fiber Physical property

Polystyrene

Sisal fiber

MFI (g/10 min) Density (g/cc) Softening point(8C) Elongation at break (%) Cellulose content (%) Lignin content (%) Tensile strength (Mpa) Young’s modulus (Mpa)

15 1.05 100 9 – – 34.9 390

– 1.4 – 4–9 85–88 4–5 450–700 7000–13,000

The chopped fibers were washed with water and dried at 708C in an oven, before the composites were made. 2.2. Preparation of composites The composites were prepared by a solution mixing technique as reported in the previous paper [14]. In this method, fiber was mixed with viscous slurry of PS in toluene that was prepared by adding toluene to a melt of the polymer. The melt is then dried and made into small pieces by hand. The small pieces were then completely dried in a vacuum oven at 1008C for 48 h, cooled, and then charged into rheometer for measurements. As this method involves no mechanical cutting or mixing, the chances for fiber break up is very less. Neat PS is also subjected to a similar process and the resulting material was used as unfilled PS. Composites containing 10, 20 and 30wt% of sisal fiber were prepared using fibers of length 2, 4 and 6 mm. These composites were denoted by symbols

U106, U206, U306, etc. In these notations, the first letter denotes the nature of the fiber, e.g. U—Untreated. The first and the second digits together denote the weight percentage of the fiber and the third digit denotes its length. For example, U106 indicates a composite that contains 10% untreated fiber and has a length of 6 mm. 2.3. Rheological measurements The melt rheological measurements were carried out using an Instron capillary rheometer model 3211 at different plunger speeds in the range of 0.02–20 cm/min. The capillary used was made of tungsten carbide with a length to diameter (L/D) ratio of 40:52 and an angle of entry at 908. The sample for testing was loaded inside the barrel of the extrusion assembly and forced down into the capillary using a plunger. After giving a residence time of 5 min the melt was extruded through the capillary at predetermined plunger speeds. The initial position of the plunger was kept constant in all the experiments and shear viscosities at different shear rates were obtained from a single charge of the material. The measurements were carried out at two different temperatures viz. 180 and 1908C. The shear stress at different plunger speeds were calculated using the equation

taw ˆ

F 4Ap …lc =dc †

…1†

Where F is the force on the plunger, Ap the cross-sectional area of the plunger and lc and dc are the length and diameter of the capillary, respectively. The shear rate at the wall is calculated using the equation r_w ˆ

2 …3n 0 1 1† db2 VxH 15 4n dc3

…2†

where VxH is the plunger speed in cm/min and db and dc are the diameter of the barrel and capillary, respectively. The factor …3n 0 1 1†=4n is the Rabinowich correction applied to calculate the shear rate at wall of non-Newtonian fluids. The value of n 0 , the flow behavior index is given by, n0 ˆ

d ln tw d ln g_ w

…3†

In addition, the values of n 0 are obtained through regression analysis of the values of t aw and g_ ; obtained from experimental data. The melt viscosity, h , is calculated as t h ˆ aw …4† g_ 2.4. Extrudate morphology Fig. 1. Fiber length distribution before and after extrusion through the capillary, at three different shear rates (54, 541 and 1804 s 21).

The surface characteristics and distortion of the extrudate

K.C.M. Nair et al. / Composites: Part A 31 (2000) 1231–1240

Where, X Ni Li Ln ˆ Ni

Table 2 Poly Dispersity Index (PDI) of fibers before and after extrusion (a) Before extrusion Ln 5.687

Lw 5.746

PDI 1.01

(b) After extrusion Sample Shear rate (s 21) 54 Ln Lw PD1 U106 4.58 5.18 1.13 U206 4.19 4.68 1.11 U306 4.29 4.84 1.12

Lw ˆ 541 Ln 4.75 5.20 4.58

Lw 5.32 5.45 5.05

PD1 1.12 1.04 1.10

1804 Ln 5.04 4.43 4.65

Lw 5.46 4.85 5.15

PD1 1.08 1.09 1.11

X Ni L2i Ni Li

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…6†

…7†

Here L w is the weight average fiber length; L n is the number average fiber length and Ni is the number of fibers having length Li. The low values of PDI indicate a narrow fiber distribution before and after extrusion. The fiber distribution curve (Fig. 1) however, indicates a slight shift to the lower aspect ratio after extrusion. 3.2. Effect of fiber loading and shear rate on viscosity

were studied using optical and electron microcopies. The extrudate were fractured under liquid nitrogen and the morphology of extrudate cross section was studied using scanning electron microscopy. 2.5. Die swell behavior The extrudate were carefully collected as they emerged from the capillary die, taking care to avoid any deformation. The diameter of the extrudate was measured after 24 h of extrusion using a travelling microscope. The die swell is calculated as Di/D where Di is the diameter of the extrudate and D is the diameter of the capillary. 2.6. Fiber breakage analysis Fiber breakage analysis was done by dissolving the PS matrix in toluene and measuring the length of the fibers using an optical microscope. 3. Results and discussion 3.1. Fiber breakage analysis Fig. 1 shows the fiber length distribution of PS-sisal composites before and after extrusion through the capillary at three different shear rates at 30% fiber loading. During extrusion, there is a chance for fiber breakage due to high shear stress. However, in the present case, Fig. 1 shows that most of the fibers retain their original length after extrusion. This is attributed to the flexible nature of the cellulose. This is in agreement with the works reported by Czarnecki et al. [6]. However, the percentage of fibers having most probable length decreases slightly during extrusion. This is associated with the breakage of fibers due to higher shear rates. Table 2 gives the PDI(Poly Dispersity Index), based on the length of 100 fibers before and after extrusion. PDI is given by PDI ˆ

Lw Ln

…5†

Fig. 2a shows the variation of melt viscosity of PS composites with shear rate and fiber loading at 1808C. These curves are typical of pseudoplastic materials, which show a decrease in viscosity with increasing shear rate. The reasons for the variation of viscosity with shear rate are well established. All the systems investigated have been found to obey the power law relationship viz.

h ˆ K…g†n21 Where n is the power law index and K the consistency index. In general, the incorporation of fibers in polymer systems increases the viscosity and goes on increasing with fiber content. At low concentration levels, the viscosity is expected to increase rapidly with increasing concentration of the fibers because of the rapidly increasing collisions between particles as they become packed more closely to each other. However, at a critical concentration level, random packing ceases to be possible and further increase in fiber concentration leads to a more orderly anisotropic structure of the fibers in suspension, and these may now slide readily past one another. Hence, above the critical concentration level, further increase in fiber concentration progressively decreases the viscosity of the system until very high concentration levels of fibers are reached [15] The increase in viscosity is found to be more predominant at lower shear rates where fiber and polymer molecules are not completely oriented. The addition of fiber to a polymer system will perturb the normal flow of the polymer and will hinder the mobility of chain segments in flow. As the fiber content increases this phenomenon becomes more predominant and hence the viscosity increases further. In the case of PS-sisal fiber composites Fig. 2a and b shows that at 1808C (Fig. 2a) the viscosity follows the order PS , U206 , U106 , U306 and at 1908C (Fig. 2b), PS , U106 , U206 , U306: This can be explained as follows. The incorporation of the fibers in the PS may introduce two effects: (1) increased fiber–matrix interaction that increases the viscosity; and (2) increased wall-slip [5,7] due to the presence of longitudinally oriented fibers along the wall melt interface, which decreases the viscosity. As the fiber

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Fig. 2. Variation of the melt viscosity of PS composites PS, U106, U206, U306) with shear rate at: (a) 1808C; and (b) 1908C.

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that at 1808C the viscosity is governed by wall-slip, which decrease the viscosity and at 1908C by polymer melt–fiber interaction, which increases viscosity. The reason for the reduction in viscosity of the composites at higher shear rate may be explained as follows: The increase in viscosity of the composite is due to the collision between fiber–fiber and fiber–matrix. At low shear rate, the fibers are in a disoriented manner and the probability of a fiber–fiber collision is much higher. This collision increases with fiber loading and therefore viscosity increases. However, as the shear rate increases, most of the shearing of fiber takes place close to the wall and therefore the fibers will strongly align along the tube axis. Therefore, the probability of fiber–fiber collision is much less, hence the increase in viscosity with fiber content is less at higher shear rate. The radial migration of filler particles towards capillary axis during shear flow [16] was also reported in literature. If this occurs during the flow of fiber filled thermoplastics, the region where most of the shear takes place may be virtually fiber free. This may also be the reason for the small dependence of viscosity on fiber concentration at higher shear rate. 3.3. Effect of fiber length

Fig. 3. SEM photographs of the cross section of PS sisal composite (U206) at a shear rate of 54 s 21 at: (a) 1808C; and (b) 1908C.

loading or temperature increases, these two effects balance giving constant viscosity or one dominates over the other to increase the viscosity. At lower temperature, the effect of fiber orientation is more predominant and the viscosity is controlled by wall-slip and therefore the viscosity decreases.. The effect of fiber orientation is more predominant at 20% fiber loading than at 10 and this accounts for the lower viscosity value of U206 compared to U106. At 30% fiber loading the effect of the fiber–melt interaction dominates over the wall-slip and accounts for the highest viscosity of U306 composite. However, at 1908C (Fig. 2b) the viscosity follows the order PS , U106 , U206 , U306 as expected and in this case the viscosity is governed by fiber–melt interaction, which increases the viscosity. The distribution of the fiber in the extrudate is clear from the SEM photographs of the cross section of U206 composite at a shear rate of 545 s 21 (Fig. 3a and b). From Fig. 3 it is clear that at 1808C most of the fibers are oriented along the periphery of the extrudate (Fig. 3a) and at 1908C (Fig. 3b) the fibers are uniformly distributed along the extrudate. This means

The variation of melt viscosity of PS-sisal composite with fiber length is given in Fig. 4 Composites containing 20% fiber and a length of 2, 4 and 6 mm were used to study the effect of fiber length. The fiber length was limited to 6 mm as it was difficult to study the flow behavior of composites containing fibers having a length beyond 6 mm. At all shear rates studied, composites containing 4 mm fibers show the highest viscosity, composites containing 6 mm fibers show the least viscosity and those containing 2 mm fibers show a viscosity value that lies in between the two values. The decrease in viscosity on increasing fiber length is unusual. However, one possible explanation is as follows. As discussed earlier, the melt viscosity of the composite is governed mainly by two factors, viz.: (i) the fiber orientation that controls the wall-slip; and (ii) fiber–matrix interaction. Another factor that controls the viscosity is the distribution of fiber in the composite. A better distribution of the fiber reduces the viscosity of the composite. In the case of 6 mm fiber composite, the chance of orientation is maximum and leads to the increased wall-slip. The fiber–matrix interaction is lower in this case as the number of fibers is less and this accounts for the lowest viscosity of 6–mm fiber composite. Moreover, the long fibers at higher volume fractions may form themselves into a nematic structure, with a lot of local alignment, and the shorter fibers may rotate more as individuals. The locally aligned structure has less resistance to shearing and leads to an overall lower viscosity in the case of long fiber composites. As the fiber length decreases the chances for fiber orientation decreases and fiber–matrix interaction increases. This accounts for the higher value of viscosity in the cases of 4 and 2 mm fiber composites.

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However, the fiber distribution will be maximum in the case of 2 mm fiber composites, which reduces the viscosity. That is, the effect of fiber distribution will overshadow the effect of fiber orientation and fiber– matrix interaction in the case of 2 mm fiber composites, and these factors lead to a decrease in the viscosity of 2 mm fiber composite as compared to 4 mm fiber composite. 3.4. Effect of temperature

Fig. 4. Variation of the melt viscosity of PS-composite with fiber length at different shear rates.

The effect of temperature on the viscosity of polymers is important as the polymers undergo considerable temperature changes during their processing. Generally, the viscosity decreases with temperature at all fiber loading. This is due to the accelerated molecular motion at higher temperature due to the availability of greater free volume and also due to the decreasing entanglement density and weaker intermolecular interactions at higher temperature. Fig. 5 shows the variation of melt viscosity of the composite at 180 and 1908C, at different shear rates. Generally, the viscosity decreases with temperature and the same trend is

Fig. 5. Variation of melt viscosity of PS sisal composite at 180 and at 1908C.

K.C.M. Nair et al. / Composites: Part A 31 (2000) 1231–1240 Table 3 Flow behavior index of PS-sisal composites Material

PS U106 U206 U306

Flow behavior index (n 0 ) 1808C

1908C

0.456 0.301 0.346 0.231

0.386 0.191 0.200 0.234

observed with pure PS too. However, PS-sisal composites show a reverse tendency, i.e. the viscosity of the composites increases with temperature. As discussed earlier, the viscosity of the composite is controlled by the fiber–matrix interaction that increases the viscosity, and increased wallslip due to the presence of longitudinally oriented fibers along the wall–melt interface, which decreases the viscosity. From Fig. 3a and b it is clear that at 1808C the fibers are oriented along the periphery of the extrudate and at 1908C fibers are distributed more uniformly throughout the extrudate. This means that at higher temperature (1908C) it is the fiber–matrix interaction that controls the viscosity and at lower temperature(1808C) the wallslip controls the viscosity. In fact, the fiber–matrix interaction at 1908C increases the viscosity, and wallslip at 1808C decreases the viscosity. This explains the unusual behavior of PS-sisal composites.

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3.5. Flow behavior index (n 0 ) The values of flow behavior index as a function of fiber content at two different temperature, viz. 180 and 1908C is given in Table 3. Non-Newtonian pseudoplastic materials have n 0 values less than unity and a high value of n 0 indicating a low non-Newtonian or low pseudoplastic nature of the system. In the case of PS-sisal composites, the n 0 values were found to be less than unity indicating the pseudoplastic nature of the system. As the fiber loading increases, the values of n 0 decrease indicating more pseudoplastic nature for the composite at high fiber loading. This increased pseudoplasticity is due to the orientation of the fibers. 3.6. Die swell ratio The increase in the diameter of the extrudate as it comes out of the capillary is known as the die swell. This phenomenon occurs as a result of the orientation of polymer molecules as they are sheared while passing through the die of the extruder. As the melt comes out of the die, reorientation and recovery of the molecules occur and these lead to the die swell. Fig. 6 shows the variation of die swell as a function of fiber loading at two different shear rates—122 and 1224 s 21, respectively. There is a sharp decrease in die swell ratio upon the addition of 10% fiber, followed by a leveling off at higher fiber loading. At the same fiber loading the die swell ratio increases with shear rate. These observations are

Fig. 6. Variation of die swell ratio as a function of fiber loading at different shear rates (122 and 1224 s 21).

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Fig. 7. (a–d) Optical photographs of the extrudates (PS, U106, U206, U306) at 1808C.

in agreement with the earlier studies [12,13] and the reasons for these behaviors are already well established. In the case of short fiber composites, during the flow through the capillary orientation of polymer molecule and fibers takes place. As the composites come out of the capillary polymer, molecules retract by recoiling effect. The fibers being non-elastic exert little retractive forces. The unequal retractive forces experienced by the two components of the composite can lead to redistribution of fibers. Since the molecules at the periphery undergo maximum deformation, the retractive forces on these molecules will also be higher. This leads to the migration of fibers to the periphery of the extrudate. Thus, the retractive force that is mainly responsible for the die swell is utilized for the migration and reorientation of fibers, and this accounts for the reduced die-swell of short fiber filled composite. 3.7. Extrudate morphology The surface morphology of the extrudates is governed by a number of factors, which include die swell, thermal contraction of the polymer, surface effects between the polymer and metal, flexing of fibers near the surface of the extruded filaments, expansion of gas bubbles in the

material, regions of local alignment, stress variations resulting from fiber concentration inhomogeneities, and velocity profile rearrangement at the exit [10]. The optical photograph of the surfaces of the extrudate of pure PS and PSsisal composites U106, U206 and U306 are given in Fig. 7a–d, respectively. It is observed that the 10% fiber filled composites show the maximum distortion and non-uniformity in diameter. The neat PS shows a uniform diameter and maximum die swell. Addition of 10% fiber causes maximum distortion and non-uniformity in diameter. As the fiber loading increases to 20%, the extrudate becomes smooth with a reduction in diameter. This is also clear from the SEM of the pure PS and filled composites (Fig. 8a–d). The distortion of the extrudate is governed by the strength of the melt, and when shear stress exceeds the strength of the melt, maximum distortion will occur. Our earlier studies indicated that in the case of PS-sisal fiber composite, the addition of 10% fiber reduces the strength of the composite [14] and therefore shear stress may exceed the strength of the composite. This accounts for the maximum deformation in the case of 10% fiber composite. As the fiber loading increases further, the strength of the composite increases, and this may exceed the applied shear stress. The strength of pure PS may also be higher than the applied

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Fig. 8. (a–d) SEM photographs of the extrudates (PS, U106, U206, U306) at a shear rate of 54 s 21 at 1808C.

stress. This accounts for the minimum distortion of the extrudate in the case of pure PS, U206 and U306 composites. The deformation of the extrudate was found to increase with shear rate at all the fiber loading studied. Fig. 9a and b show the surface of the extrudate containing 20% fiber at shear rates of 18 and 1800 s 21, respectively. These figures show surface irregularities at lower shear rates. As the shear rate is increased the surface becomes smoother. From Fig. 9a, it is clear that at lower shear rate the fibers are mainly oriented along the periphery of the extrudate. As the shear rate increases from 18 to 1800 s 21, the fibers are dispersed more uniformly (Fig. 9b). The migration and redistribution of the fiber is attributed to the unequal retractive forces experienced by the components in the composite.

4. Conclusions The rheological behavior of short sisal fiber-reinforced PS composites has been studied as a function of fiber loading, fiber length, shear rate and temperature. These composites exhibit pseudoplastic behavior and can be represented

by the power law equation. At 1908C the addition of fibers increases the viscosity of the system and increase in viscosity is sharper at 30% fiber loading. However at 1808C the viscosity follows the order PS , U206 , U106 , U306 and this can be explained based on the wall-slip effect. Generally, an increase in temperature decreases the viscosity of the polymer system. However, PS-sisal composites show the reverse tendency, i.e. the viscosity of the composite increases with temperature. This may be due to the increased interaction between the fibers and polymer molecules, at higher temperature. The increased wall-slip due to the presence of longitudinally oriented fibers along the wall–melt interface at lower temperatures, also contribute to this effect. The flow behavior index of the sisal filled PS composite at a given temperature was found to be lower than that of pure PS suggesting a higher degree of pseudoplasticity for the composite. However, the variation of the flow behavior index was not linear. The die swell ratio showed a sharp decrease at 10% fiber loading, followed by a leveling off at higher fiber loading. The maximum distortion and non-uniformity of the extrudate was observed at 10% fiber loading and becomes uniform at higher fiber loading. The SEM studies showed that at a low shear rate

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[2]

[3]

[4]

[5] [6]

[7]

[8]

[9]

[10]

[11]

[12] Fig. 9. SEM photographs of the surface of U206 at 1808C with: (a) a shear rate of 18 s 21; and (b) a shear rate of 1800 s 21.

most of the fibers are oriented along the periphery of the extrudate. At higher shear rates the fibers are uniformly distributed.

[13]

[14]

[15]

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