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Morphology and melt rheological behaviour of short-sisal-fibre-reinforced SBR composites
Composites Science and Technology 60 (2000) 1737±1751
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Morphology and melt rheological behaviour of short-sisal-®bre-reinforced SBR composites R. Prasantha Kumar a, K.C. Manikandan Nair a, Sabu Thomas a,*, S.C. Schit b, K. Ramamurthy b a
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills PO, Kottayam-686 560, Kerala State, India b Central Institute for Plastics Engineering and Technology, Guindy, Chennai, India Received 9 February 1999; received in revised form 6 January 2000; accepted 1 March 2000
Abstract The melt-¯ow behaviour of untreated and treated short-sisal-®bre-reinforced styrene±butadiene rubber (SBR) composites were analysed by using an Instron capillary rheometer. The eects of ®bre breakage, length, concentration and shear-rate/stress on melt viscosity have been studied. The ®bre breakage was analysed before and after extrusion and the polydispersity index(PDI) was estimated. It was found that these composites behave as pseudo-plastic materials. At low shear rates, the short ®bres increase the viscosity much more than at high shear rates. There was an increase in viscosity upon chemical treatments owing to the strong interfacial adhesion between the ®bre and the rubber matrix. The dependence of melt viscosity on temperature, ¯ow behaviour index, n0 , melt elasticity, extrudate distortion and deformation of these composites were analysed. Finally, die-swell measurements were carried out to understand the elastic eects. The extruded samples were analysed by optical and electron microscopy in order to study the surface morphology and extrudate deformation of these composites. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Sisal ®bre; Styrene±butadiene rubber; Composites; Rheology; Bonding agent; Interfacial adhesion
1. Introduction The ease of processability of a rubber compound is essential for the fabrication of an end product, since it undergoes dierent types of shear during every stage of processing [1]. The need for rheological studies is essential in making a logical choice of the polymer and its processing conditions [2]. The modern theories of rheology of elastomers have been related to the processing characteristics of rubber and molecular structure [3±5]. The study of the rheological behaviour of polymer composites is very important for fabrication of end products. It is seen that at higher shear rates, ®bre orientation takes place because of the convergent, divergent and shear ¯ows. The ®bres orient along the ¯ow direction during convergent ¯ow and at 90 to the direction in a divergent ¯ow [6]. Setua et al. [7] and Murthy et al. [8] have observed that short-®bre-reinforced elastomer composites behave as pseudoplastic materials. Goettler et al. [9] have reported * Corresponding author. Tel.: +91-481-598015/598303; fax: +91481-561190. E-mail address: [email protected] (S. Thomas).
the rheological and extrusion behaviour of short-®brereinforced rubber compounds. Kutty et al. [10] have found the dependence of temperature on the shear viscosity of short-®bre-®lled composites. Melt elastic parameters like die swell ratio were found to increase with increasing shear rate and decrease with ®bre loading [11]. This is due to the irreversible orientation of the ®bres in the matrix during extrusion. Goettler [12] has found that the ®bres raise the viscosity of the composite due to the higher pseudoplasticity induced by the ®bres. Czarnecki et al. [13] have worked on the shear ¯ow rheological properties, ®bre damage, and mastication characteristics of short-®bre-reinforced polystyrene melts. Recently, Thomas and co-workers have extensively studied the eect of short ®bres on the rheological behaviour of composites of natural rubber [14±15] and PS (polystyrene) [16] respectively. Recently, the authors have reported on the mechanical [17±19], processing [20], electrical [21] and transport [22] behaviour of shortsisal-®bre reinforced styrene±butadiene (SBR) composites. However, there seems to be no previous studies made on the rheological and extrusion behaviour of shortsisal-®bre-reinforced SBR composites. In this paper, we
0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00057-9
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report our observations found on the rheological and extrusion behaviour of untreated, treated and bonding agent added short-sisal-®bre-reinforced SBR composites with special reference to the eect of ®bre breakage and ®bre distribution, ®bre length and ®bre concentration on shear viscosity.
2.2.2. Treatment of ®bres 2.2.2.1. Water treatment. Fibres of length 6 mm were immersed in water at 28 C for 1 h. These ®bres were then washed several times with water and were later dried in an air oven at 70 C for 2 days and kept in polyethene bags. These ®bres were designated and used as untreated ®bres.
2. Experimental
2.2.2.2. Acetylation. Fibres were immersed in 18% NaOH solution at 28 C for 1 h . These ®bres were washed several times with water and 0.1 N HCl. The ®bres were then soaked in glacial acetic acid for 1 h at the same temperature. Later they were soaked again in acetic anhydride containing Con.H2SO4. The resulting ®bres were washed and dried in an air oven.
2.1. Materials Synthetics and chemicals, Bareilly, Uttar Pradesh State, India supplied Synaprene (SBR-1502). Sisal-®bre (Agave sisalana) was obtained from a local processing unit situated in the state of Tamil Nadu. The chemical constituents of sisal-®bre are reported in Fig. 1 [23]. It was reported that the density [24] and the average diameter of the ®bre are 1.45 g/cmÿ3 and 0.1212 mm respectively. All other chemicals such as hexamethylene tetramine and resorcinol used were of laboratory reagent grade. 2.2. Methods 2.2.1. Fibre distribution analysis The mixes were dissolved in toluene and the length of the extracted ®bres was measured under the optical microscope. The variation of average ®bre length with the volume loading of ®bre and the changes in the average ®bre length before and after extrusion at dierent shear rates was analysed.
2.2.2.3. Benzoylation. The mercerised ®bres were agitated with 10% NaOH and 50 ml benzoyl chloride for 15 min. The dried ®bres were then soaked in ethanol for 1 h, washed and dried in an air oven at 70 C. 2.2.3. Preparation of composites Mixes were prepared in a two-roll laboratory open mixing mill (150300 mm), at a nip gap of 1.25 mm. These mixes were compounded according to the basic formulation given in Table 1. All the ingredients were added in the mix according to the mixing sequence [17]. 2.2.4. Rheological measurements The rheological studies of untreated and treated short-sisal-®bre-reinforced SBR composites were carried out using an Instron Capillary Rheometer Model
Fig. 1. Chemical constituents of sisal ®bre.
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Table 1 Formulation of mixes (from A to H2) Ingredients
Mixes
Synaprene Sulphur Stearic acid Zinc oxide Silica (Pptd) Hexaa Resorcinol CBSb TDQc Sisal ®bre 1. Untreated 2. Acetylated 3. Benzoylated a b c
A
B
C
D
E
E1
E2
E3
E4
F1
F2
G1
G2
H1
H2
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 1 2 2.5 1 1
100 2.2 2 5 1.5 2.5 5 1 1
100 2.2 2 5 3 1.5 3 1 1
100 2.2 2 5 5 2.5 5 1 1
100 2.2 2 5 ± 2.5 5 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± 2.5 5 1 1
100 2.2 2 5 ± 2.5 5 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± 2.5 5 1 1
± ± ±
20 ± ±
20 ± ±
20 ± ±
± ± ±
5 ± ±
10 ± ±
15 ± ±
15 ± ±
± 5 ±
± 10 ±
± 15 ±
± 20 ±
± ± 5
± ± 15
Hexamethylene tetramine. N-cyclo hexyl benzothiazole sulphenamide. 2,2,4-Trimethyl 1,2-dihydroxy quinoline polymerized.
3211 [25]. A capillary of L/D ratio 20 and an angle of entry 90 were used in all experiments. All studies were done in the shear range of 3.68±1226.9 sÿ1, and a temperature range of 90±130 C due to the instrument limitations. The shear stress, w , increases linearly with distances, r, from the centre line and is given by: w
P r 2lc
1
where P is the pressure drop across the length, lc , of capillary tube. The true wall shear stress was calculated by, F w 4Ap lc =dc
2
where F is the force on the plunger (N), Ap crosssectional area of the plunger (mm2), and lc (mm) and dc (mm) the length and diameter of the capillary respectively. The apparent shear rate was calculated by using the equation: :
w;a 8V=dc But;
V
3 VXH db2 60 dc3
4
where V is the mean velocity of the liquid ¯owing through the capillary, VXH , the cross head speed in cm/min, db , the diameter of the barrel, dc , the diameter of the capillary. The correct wall shear rate may be found by using the Rabinowes correction:
3 n0 1 8V :
w;a 4n0 dc
5
The factor 3n0 1=4n0 is the Rabinowes correction applied to calculate the true shear rate. But;
n0
d ln w d ln a
6
: i.e. n0 is the slope of the graph of ln w vs ln w;a . Thus wall shear rate for pseudoplastic materials is, therefore: 2 3n0 1 VXH db2 :
w;a 15 4n0 dc3
7
Thus, apparent viscosity, a is calculated by, : na w = w;a
8
2.2.5. Die swell and extrudate morphology The extrudates diameters were measured at dierent times and were kept for 24 h to attain equilibrium before ®nal readings were recorded, using a travelling microscope. The die swell ratio can be calculated from the formula, de 9 Die swell ratio dc where de and dc are the diameter of the extrudate and the capillary, respectively.
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For SEM, the peripheral and cross-sectional surfaces of the extrudates were sputter-coated with gold within 24 h of testing. 3. Results and discussion The calculations of melt viscosity and the shear rate of a pseudoplastic material depends on several assumptions: (i) ¯uid ¯ow is parallel to the axis, (ii)velocity of any ¯uid element is a function of radius only, (iii) ¯uid is incompressible, (iv) velocity of the ¯uid is zero at the wall, (v) ¯uid ¯ow is isothermal and (vi) all energy is consumed within the capillary itself. 3.1. Fibre breakage Fibre breakage was analysed from the ®bre length distribution curve and from the polydispersity index (PDI). Fig. 2 shows the ®bre length distribution for compound G2 before and after milling. The short sisal ®bres have not undergone severe breakage, although high shear forces were generated during the time of mixing. The initial 6 mm length of the ®bres is converted to varying sizes. It can be observed that the maximum breakage is seen between 5 and 6 mm in length. During extrusion, the ®bre again undergoes breakage and the percentage of ®bre of less than 5 mm length increases considerably. It is clear from Fig. 3(a) that the maximum in the range 5±6 mm ®bre length before extrusion is shifted to 0±1 mm after extrusion at a shear rate of 1226.9 sÿ1. The increase in the breakage of ®bres is due to the result of the ®nal break-up of kinked ®bres during extrusion. As the shear force increases, the percentage of breakage increases dramatically, and at a shear rate of 1226.9 sÿ1 more than 40% of the ®bres are of less than 2 mm length. The increase of PDI value as shown in Table 2 with the increase of shear rate also revealed the ®bre breakage. On analysing Fig. 3(b) it is seen that the most probable length of the ®bre in composite decreases with shear rate. 3.2. Eect of ®bre length The eect of ®bre length on the melt viscosity of SBR composites at 120 C is shown in Fig. 4. A series of mixes containing 20 phr short sisal ®bre of length 2, 6 and 10 mm were used to study the eect of ®bre length. Generally, an increase in viscosity on increasing the ®bre length is expected. It is seen that the melt viscosity increases with ®bre length from 2 to 10 mm. The ¯ow behaviour of composites containing ®bres having a length beyond 10 mm was dicult to study. The viscosity increases with ®bre length at low shear rates. But at higher shear rates, the ®bre alignment and ®bre
distributions occur simultaneously and, therefore, increase in viscosity is less predominant here. It is dicult for ®bres having higher ®bre lengths to become oriented in the direction of ¯ow and the distribution or dispersion of ®bres also is poor at higher ®bre length. This is due to the entanglement of ®bres at higher ®bre content. Thus ®bres having shorter length can be more easily aligned and dispersed than those having higher lengths along the direction of ¯ow, i.e. SBR composites containing short sisal ®bres having shorter length show low viscosity than having higher length. This fact is clearly shown in the Fig. 5. As the ®bre length increases the dispersion of the ®bre becomes poor and it will cause agglomeration of short sisal ®bres in the SBR matrix. This increases the melt viscosity of the composite. 3.3. Eect of shear stress and ®bre loading on melt viscosity The variation of melt viscosity of sisal-SBR composites with shear stress at dierent ®bre loading at 120 C is shown in the Fig. 6. Like all other ®bre reinforced elastomer composites, the melt viscosity of SBR composites reduces with the increase in shear stress. The increase in viscosity with increasing shear stress is attributed with the orientation of the polymer molecules and the ®bres in the direction of shear. These curves are typical of pseudoplastic materials, which show a decrease in viscosity with increasing shear stress. All the mixes have been modelled using the power-law equation [26], : K n
10
where n is the power law index and k the consistency index. The power-law model was found to be the most suitable equation to model this behaviour. The slight non-linearity from the power-law model is due to the variation in the extent of ®bre orientation in the matrix and also due to wall slip [27]. For a given stress, the viscosity of SBR composites was found to increase with ®bre loading. The increase in viscosity of the composites with increasing ®bre loading is due to the interaction between ®bre±®bre and ®bre±matrix. 3.4. Eect of chemical modi®cation of ®bres Several researchers [28±32] have proved that strong interfacial bonding at the interface can be obtained by dierent chemical treatments. 3.4.1. Acetylation The cellulosic hydroxyl groups in the lignocellulosic sisal ®bre are not so reactive, since they form strong hydrogen bonds. The addition of 18% caustic soda
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solution treatment destroys the hydrogen bonding in cellulosic hydroxyl groups and cause partial removal of lignin. This makes the ÿOH groups more reactive (Scheme 1). The hydrogen atom on acetyl carbon atom becomes more reactive due to the presence of a carbonyl group. This reacts with active sites on SBR to form chemical links, thereby improving adhesion between ®bre and rubber matrix. The hypothetical mechanism showing the acetylation reaction on the ®bre is shown in Scheme 2. The mechanism of acetylation has been explained by Chand et al. [33].
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3.4.2. Benzoylation The hypothetical mechanism showing the benzoylation reaction on the alkali treated ®bre is shown in Scheme 3. The ÿOH group of the lignocellulosic sisal ®bre reacts with benzoyl chloride. The extractable materials such as lignin, waxes, etc. were removed by the addition of 18% caustic soda solution. Considerable attention has been paid to see that the ®bres do not lose their physical properties and ®brous nature during this treatment. The reaction proceeds through any hydroxyl group attached to the cellulose backbone, since the oxygen linkage is a hydrolysable one. As per the IR studies made on the ®bre after benzoylation, it is observed that as a result of esteri®cation of the hydroxyl group, the hydroxyl vibration absorption at 3400 cmÿ1 is found to be decreased. The absorption bands at 1950, 1600 and 710 cmÿ1 indicate the presence of aromatic rings. The ester group is identi®ed by the presence of peaks at 1725 and 1300 cmÿ1.
Table 2 Fibre length distribution index of sisal ®bres before and after extrusion Shear rate sÿ1
Ln
Lw
Ln/Lw
5.08
1.08
4.3 3.58 3.38
1.15 1.43 1.45
Before extrusion 4.68 After extrusion
Fig. 2. Eect of ®bre length distribution before and after milling of SBR composites.
12.269 122.69 1226.91
3.72 2.5 2.32
Fig. 3. (a) Eect of ®bre length distribution of sisal ®bres after extrusion. (b) Eect of ®bre length on increasing shear rates at 120 C.
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Fig. 4. Eect of shear rate on melt viscosity at dierent ®bre lengths.
Fig. 6. Eect of shear stress on melt viscosity of mixes A, E2, E3 and E4 at 120 C.
the maximum viscosity followed by acetylated ®bre containing mix G1 and it decreases on increasing shear rate. The increase in viscosity is due to the better bonding with the ®bre and rubber due to chemical modi®cation. The better interfacial adhesion between the bezoylated sisal ®bre and SBR matrix is shown in Scheme 3. Schematic models of the interfaces after acetylation and benzoylation treatments are shown in Fig. 8(a) and (b). 3.6. Eect of temperature
Fig. 5. Eect of ®bre length on melt viscosity at dierent shear rates at 120 C.
3.5. Viscosity of chemically modi®ed composites The eects of chemical treatments on the melt viscosity of treated SBR composites at a temperature of 120 C are discussed in the Fig. 7. The ®gure shows the eect of hydration, acetylation and benzoylation on the melt viscosity of the composites. On analysing the ®gure, it is seen that the benzoylated ®bre containing mix H2 has
During processing, the elastomers undergo considerable change due to the eect of temperature. At all ®bre loading, the melt viscosity of the composites decreases on increase of temperature. This is due to several factors such as (a) accelerated molecular motion due to theavailability of greater free volume and the decrease of entanglement density, (b) weak intermolecular interactions. In the Arrhenius expression, () is related to absolute temperature (T) by the following equation [34]. AeD E=RT
11
where A is a constant characteristic of the polymer, E is the activation energy for viscous ¯ow, and R is the universal gas constant. The eect of temperature on melt viscosity of the SBR composites at dierent loading of ®bres at four dierent temperatures at a shear rate of 36.807 sÿ1 is shown in Fig. 9. These plots are treated as the semi logarithmic Arrhenius plots, i.e. log vs 1=T at ®xed shear rate of 368.07 sÿ1. From the slopes of these linear plots, E values were estimated. The E
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Scheme 1.
Scheme 2.
Scheme 3.
values provide valuable information on the sensitivity of the material towards the changes in temperatures. The E of the composite melt increases with increase in the ®bre concentration, indicating that the melt viscosity of the composite is greater temperature sensitive than that of the gum compound. The estimated E values presented in Table 3 support this fact. Thomas et al. [14,15] have already reported similar behaviour for sisal/coir ®bre reinforced NR composites. Fig. 10 shows the eect of melt viscosity with temperature on dierent chemically modi®ed ®bre reinforced composites at a shear rate of 368.07 sÿ1. Among the treated mixes, the benzoylated ®bre containing mix H2 showed highest slope value showing the higher activation energy, and it indicates that it has greater sensitivity to temperature among all other mixes (Table 4). 3.7. Eect of relative viscosity
Fig. 7. Eect of shear rate on melt viscosity of sisal/SBR composites containing chemically modi®ed ®bres at a temperature of 120 C.
In the case of polymer composites, the relative viscosity is calculated as the ratio of the shear viscosity of the ®lled compounds to that of un®lled compounds. The variation of relative viscosity with volume fraction of
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Fig. 8. (a) Schematic model of the interface after acetylation treatment. (b) Schematic model of the interface after benzoylation treatment.
®bres at a shear rate of 3.68 sÿ1 is shown in Fig. 11. When the temperature is increased from 90 to 120 C, the relative viscosity of the composite decreases. For the dierent temperatures studied, it can be seen that the relative viscosity increases with ®bre loading. But the magnitude of the increase show a decreasing trend as the temperature is elevated with ®bre loading. At 110 and 120 C, the rate of change of relative viscosity is uniform over the entire range of ®bre loading whereas at 90 and 100 C the rate of change shows a sharp increase. At higher ®bre loadings and at higher temperatures, the scorch eect causes the ®bres to be less oriented as compared with their behaviour at a lower temperature. This decreases the relative viscosities of composites containing the same level of ®bre loading, as the temperature is increased.
3.8. Flow behaviour index (FBI) The pseudoplastic materials have n0 values less than unity and a high value of n0 indicates a low shear thinning nature. The eect of ®bre length on the ¯ow behaviour index of the composites is presented in Fig. 12. No remarkable eect was noticed upon the increase of ®bre length. Fig. 13 shows the eect of volume percentage of ®bres on the ¯ow behaviour index of sisal-SBR composites. From the ®gure, it is seen that the ¯ow behaviour index of sisal/SBR composites was found to be less than unity, indicating the pseudoplastic nature of the system. It is also noted that the n0 values decrease upon increasing the ®bre content in the system. The increase of pseudoplasticity arises due to the orientation of ®bres during its ¯ow through the capillary.
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3.9. Die swell and extrudate distortion
Fig. 9. Eect of temperature on melt viscosity of mixes A, E1, E3 and E4 at a shear rate of 36.807 sÿ1.
Table 3 Activation energy of sisal/SBR composites at dierent ®bre loading at a shear rate of 36.807 sÿ1 Mixes
Volume of ®bres (% )
Activation energy (kJ/ K/mol)
E E1 E2 E3
0 5 10 15
2.707 4.841 5.867 7.973
Fig. 10. Eect of temperature on melt viscosity of treated mixes at a shear rate of 36.807 sÿ1.
Die swell measurements at 120 C for three dierent shear rates were made and shown in Table 5. The die swell increases slightly with increase of temperature and shear rate in the case of ®bre reinforced rubber compounds. But the die swell is greatly reduced by the addition of ®bre and the eect is negligible beyond 10phr loading. These results are agreement with earlier observations made by Chan et al. [35]. At the same ®bre loading, the die swell ratio increases with shear rate and it is observed that the chemical treatment on the ®bres does not show much remarkable change in the die-swell. Optical photograph of the SBR composites extruded at 120 C at three dierent shear rates is shown in Fig. 14. The presence of ®bre reduced the extrudate distortion at high shear rates and at 15 phr loading, the distortion is remarkably low. It has been also observed that the distortion is negligible at 90±110 C. It is evidenced from the optical photograph that the presence of ®bre reduced the extrudate distortion at high shear rates and at higher loading of ®bres (mixes E4, G1 and H2).
Table 4 Activation energy of treated sisal/SBR composites at same ®bre loading at a shear rate of 368.07 sÿ1 Mixes
Type of chemical treatment
Activation energy (kJ/ K/mol)
E4 G1 H2
Untreated Acetylated Benzoylated
2.734 2.872 3.063
Fig. 11. Eect of volume of ®bres (%) on relative viscosity of SBR composites at a shear rate of 3.68 sÿ1.
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Fig. 12. Eect of ®bre length on ¯ow behaviour index of SBR composites.
Fig. 13. Eect of ®bre loading on ¯ow behaviour index of SBR composites. Table 5 Variation of die swell ratio (de/dc) of SBR composites for dierent shear rates at 120 C Samples
A E E1 E2 E3 E4 G1 G2 H2
Shear rate (sÿ1) 3.6807
3.6807
3.6807
1.45 1.46 1.40 1.38 1.15 1.14 1.06 1.05 1.05
1.49 1.47 1.42 1.39 1.16 1.14 1.08 1.06 1.06
1.52 1.51 1.48 1.39 1.16 1.13 1.08 1.07 1.07
3.9.1. SEM studies on extrudate morphology The extrudates show smooth surfaces and uniform diameter at low shear rate. But on increasing shear rates, the extrudates exhibit surface irregularity. Fig. 15(a), (b) and (c) shows the SEM micrographs of
1226.91 1.61 1.58 1.55 1.40 1.15 1.13 1.07 1.06 1.06
extrudates E, F1 and G1 at dierent shear rates 36.807, 368.07 and 1226.91 sÿ1 respectively. The surface morphology of these composites extruded at dierent shear rates show an increase in the surface discontinuity with increasing shear rates. The irregularity at the periphery of the mixes increases as the shear rate increases. The
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Fig. 14. Optical photograph showing the extruded distortion of mixes A, E1, E2, E3, E4, G1, G2, H1 and H2.
surface discontinuity is associated with the migration of ®bre towards the periphery of the extrudates. Fig. 16(a) and (b) shows the cross-sectional view of the extrudates E2, and E3 at a constant shear rate of 368.07 sÿ1 containing 10 and 15 phr loading of ®bres. It is observed that the ®bres are well oriented along the direction of extrusion. At low shear rate the ®bres are mainly concentrated at the periphery, whereas at medium shear rates they form a uniform dispersion throughout the matrix. The unequal retroactive forces experienced by the components in the composite result in the redistribution of the ®bre and its consequent migration to the periphery of the extrudate [35]. It is observed that short sisal ®bres, which are dispersed uniformly at medium shear rate, are concentrated at the core region along the direction of capillary ¯ow. When the shear rate is increased from 36.807 to 368.07 sÿ1, the redistribution of short ®bres takes place and more and more ®bres were concentrated at the core of the extrudates (E2 and E3). It is also clear from these ®gures that dispersion of short ®bres increased with treated ®bre concentration. Micrographs, Fig. 17(a) and (b), of composites containing treated ®bres show increased orientation of short ®bres due to the increased ®bre±rubber interactions. G2 is extruded at a shear rate of 36.8 sÿ1 while G1
Fig. 15. SEM of the surface morphology of dierent extrudates at dierent shear rates 36.807, 368.07 and 1226.91 sÿ1 respectively: (a) Egum; (b) F1 (5 phr); (c) G1 (15 phr).
is extruded at a shear rate of 1226.1 sÿ1. In the case of G2, the ®bres are concentrated at the periphery while in the case of G1, the ®bres are concentrated at the core of the extrudate. 3.10. Theoretical modelling Guth et al. [36] has proposed the following equation to explain the viscosity of the oriented composite: ÿ c un 1 0:6fc 1:62f 2 c2
12
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Fig. 16. SEM of the cross-sectional view of the extrudates at a shear rate 368.07 sÿ1: (a) E2-10 phr; (b) E3-15 phr.
where f is the ratio of longest to the shortest diameter of the ®bre, c, the volume fraction of the ®bre, un , the viscosity of the un®lled mix, and c , the viscosity of the composite. The theoretical and experimental viscosity of sisal/ SBR composites at 100 and 120 C at dierent shear rates are shown in Fig. 18(a) and (b). Fig. 18(a) shows that at 100 C, the experimental values are higher than the theoretical value in all loading. On increasing the concentration of ®bres, the ®bre-matrix interaction increases and thereby diculty arises in the better alignment and orientation of ®bres. Therefore, the experimental viscosity is higher than the predictions. Fig. 18(b) also shows that the experimental viscosities are higher than the theoretical values at all ®bre loading. On increasing temperature, the viscosity is governed by ®bre±matrix interactions and misalignment of ®bres in ®bre distribution, which lead to higher viscosity than the theoretically predicted one. Fig. 19 shows the comparison of experimental and theoretical relative viscosities at dierent volume fraction of ®bres and dierent temperatures at a shear rate of 36.807 sÿ1. The relative viscosity increases upon the increase of the volume fraction of ®bres. It is also seen that the experimental relative viscosity is higher than the theoretical one. This is due to the increased interfacial adhesion between the ®bre and matrix, i.e. the better
Fig. 17. SEM of cross-sectional view of the extrudates of treated composites at shear rates 36.807 and 1226.91 respectively: (a) G2-20 phr; (b) G1-15 phr.
bonding occurred in the interface, if not, it could be due to the entanglement of ®bres in the composite. According to Kamal et al. [37], the relative viscosity and concentration relations represent semi-empirical extensions of dilute suspension theories through the inclusion of 2v . r 1 v k v 2
13
where r is the relative viscosity of the suspension and v is the term for the ®rst order interactions among ®bres. Thus the variation of relative viscosity of composites with ®bre loading at dierent temperature and shear rate can be ®tted into a second-degree polynomial type of equation [38], i.e. f =g 1 K1 Vf K2 V2f
14
where K1 and K2 are functions of rate of shear and temperature, f and g , the viscosities of the ®lled and the gum compound respectively and Vf , the volume fraction of the ®bre in the composite. The relative viscosities of sisal/SBR composites have been ®tted into the
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Fig. 18. (a) Comparison of experimental and theoretical melt viscosities at dierent shear rates of mix H2 (a) 100 C; (b) 120 C.
Fig. 19. Comparison of experimental and theoretical relative viscosities at dierent volume fraction of ®bre and dierent temperatures (shear rate 36.807 sÿ1). Table 6 Comparison of theoretical and experimental viscosity of mix H2 Shear rate (sÿ1)
Temperature ( C) 90
3.6807 36.807 368.07
100
110
120
K1
K2
K1
K2
K1
K2
K1
K2
5.9384 2.6537 0.1196
ÿ12.741 ÿ0.0857 1.5170
0.6109 0.5726 8.4956
29.59 23.62 ÿ68.80
0.1350 16.871 ÿ0.0390
ÿ26.84 ÿ381.26 106.98
ÿ5.536 5.863 0.034
133.3 ÿ10.80 ÿ99.37
second-degree polynomial equation and the values of K1 and K2 at dierent shear rate and temperature are given in Table 6. Some K1 values are negative, which are attributed to the wall slip occurred during extrusion.
4. Conclusions The rheological behaviour of short-sisal-®brereinforced SBR composite has been studied. The eects
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of shear rate, ®bre loading, temperature and the extent of interface bonding on the viscosity of the system have been examined in detail. The sisal ®bres undergo severe breakdown during extrusion. All the system showed pseudoplastic behaviour showing the decrease of viscosity with increasing shear rate. The incorporation of treated ®bres increased melt viscosity and the viscosity enhancement was due to the increased interfacial adhesion between the ®bre and matrix due to the chemical treatments. Of the various chemical treatments, benzoylated ®bre reinforced composites showed maximum viscosity among all mixes. The ¯ow behaviour index of the composites was found to decrease and attain a steady value on increasing the concentration of treated ®bres. The relative viscosity of the composites was also found to increase with increase of ®bre loading. The extrudate distortion and die swell behaviour of SBR composites were reduced by the addition of short sisal ®bres. Short ®bres present in the matrix prevent the shape distortion of the extrudates and the increase in ®bre content decreased the shape distortion. Die swell values decreased substantially with the incorporation of short ®bres. SEM photographs indicated that the increase of short ®bres creates discontinuity in the polymer matrix. The ®bres were found to be concentrated more on the periphery at low shear rates and on the core at high shear rates. On comparison of theoretical melt viscosity with the experimental values, it is seen that the experimental viscosity is greater than the theoretical value as a result of better interfacial adhesion and misalignment of the ®bres. Moreover, the variation of relative viscosity of composites with ®bre loading at dierent temperatures and shear rates has been ®tted into a second-degree polynomial type equation. Acknowledgements One of the authors (R.P.K.) would like to thank the Council of Scienti®c and Industrial Research (CSIR), New Delhi for the ®nancial assistance awarded in the form of a research fellowship. His thanks are also due to Mr. Murukesan, Technical Assistant, Rheological laboratory, CIPET, Guindy, Chennai. Finally the authors thank the referee for the valuable comments. References [1] Norman RH, Johnson PS. Processability testing. Rubb Chem Technol 1981;54:493. [2] Brydson JA. Flow properties of polymer melts. 2nd ed. ILIFFE Books, 1982 (Chapter 1). [3] White JL. Elastomer rheology and processing. Rubb Chem Technol 1969;42:257.
[4] White JL. Processability of rubber and rheological behaviour. Rubb ChemTechnol 1977;50:163. [5] White JL, Tokita N. Elastomer processing and applications of rheological fundamentals. J Appl Polym Sci 1967;11:321. [6] Crowson RJ, Folkes MJ, Bright PF. Rheology of short glass ®bre reinforced thermoplastic and its application to injections mouldding. I. Fiber motion and viscosity measurements. Polym Eng Sci 1980;20(14):825. [7] Setua DK. Rheological behaviour of short silk ®ber ®lled rubber compound. Int J Polym Mat 1985;11:67. [8] Murty VM, Gupta BR, De SK. Rheological behaviour of natural rubber compounds ®lled with short jute ®bers. Plast Rubb Proc & Applications 1985;5(4):307. [9] Goettler LA, Leib RI, Lambright AJ. Short ®bre reinforced hose Ð a new concept in production and performance. Rubb Chem Technol 1979;52:838. [10] Kutty SKN, De PP, Nando GB. Rheology of short Kevlar ®brereinforced thermoplastic polyurethane. Plast Rubb Proc & Applications 1991;15:23±9. [11] Tanner RI, Melt elastic nature on rheology. J Polym Sci A-2 1970;14(8):2067. [12] Goettler LA. In: Bhowmick AK, Stephens HL, editors. Hand book of elastomers. Marcel Dekker, 1988. p. 221. [13] Czarnecki L, White JL. Shear ¯ow rheological properties, ®ber damage, and mastication characteristics of aramid-, glass and cellulose ®bre-reinforced polystyrene melts. J Appl Polym Sci 1980;25:1217. [14] Varghese S, Kuriakose B, Thomas S, Premalatha CK, Koshy AT. Rheological behaviour of short sisal ®bre reinforced natural rubber composites. Plast Rubb Comp Proc Appln 1993;20(2):93. [15] Geethamma VG, Ramamurthy K, Janardhan R, Thomas S. Melt ¯ow behaviour of short coir ®bre reinforced natural rubber composites. Int J Polym Mat 1996;32:147. [16] Manikandan Nair KC, Prasantha Kumar R, Ramamurthy K, Thomas S. Rheological behaviour of short sisal ®bre reinforced polystyrene composites Composites Part A, (in press). [17] Prasantha Kumar R, Geethakumari Amma ML, Thomas S. Short sisal ®bre reinforced styrene butadiene rubber composites. J Appl Polym Sci 1995;58:597. [18] Prasantha Kumar R, Thomas S. Short ®bre elastomer composites: eect of ®bre length orientation and loading. Bull Mat Sci 1995;18(8):1021. [19] Prasantha Kumar R, Thomas S. Natural ®bre reinforced elastomer composites: eect of chemical modi®cation and bonding agent. Polymer, (in press). [20] Prasantha Kumar R, Thomas S. Tear and processing behaviour of short sisal ®bre reinforced styrene butadiene rubber composites. Polym Intl 1995;38(2):173. [21] Prasantha Kumar R, Thomas S. Dielectric properties of short sisal ®bre reinforced styrene±butadiene rubber composites. J Polym Eng, (in press). [22] Prasantha Kumar R, Thomas S. Studies on the interfacial adhesion of sisal ®bre/styrene±butadiene rubber composites by restricted equilibrium swelling. Rubb Chem Technol, (in press). [23] Barkakatty BC. Some structural aspects of sisal ®bre. J Appl Polym Sci 1976;20:2921. [24] Mukherjee PS, Satyanarayana KG. Structure and properties of some vegetable ®bres: part I. Sisal ®bre. J Mater Sci 1984;19: 3925. [25] Working manual for instron capillary rheometer, Model 3211. [26] Hans CD. Rheology in polymer processing. New York: Academic Press, 1968. [27] Goldsmith H, Mason, L. In: Eirich FR, editor. The micro rheology of dispersion in rheology theory and applications. Academic press, New York, 1967 (Vol.4). [28] Callow HJ. Acetylation of jute. J Text Inst 1952;29:1423. [29] Varghese S, Kuriakose B, Thomas S, Koshy AT. Studies on
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[30] [31] [32]
[33]
natural rubber±short sisal ®bre composites. Ind J Nat Rubb Res 1991;4(1):55. Bledzki AK, Reihmane S, Gassan J. Properties and modi®cations of vegetable ®bers for natural ®ber composites. J Appl Polym Sci 1996;59:1329. Mitra BC, Basak RK, Sarkar M. Studies on jute-reinforced composites, its limitations, and some solutions through chemical modi®cations of ®bers. J Appl Polym Sci 1998;67:1093. Geethamma VG, Mathew KT, Lakshminarayanan R, Thomas S. Composite of short coir ®bres and natural rubber: eect of chemical modi®cation, loading and orientation of ®bre. Polymer 1998;39(6±7):1483. Chand N, Varma S, Khazanchi AC. SEM and strength
[34] [35] [36] [37] [38]
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characteristics of acetylated sisal ®bre. J Mater Sci Lett 1989; 8:1307. Mckelvey JM. Polymer Processing. New York: John Wiley, 1962. Chan Y, White JL, Oyanagi Y. In¯uence of glass ®bres on the extrusion and injection moulding characteristics of polyethylene and polystyrene melts. Poly Eng Sci 1978;18:268. Guth EJ. Rheological behaviour of composites. J Appl Phys 1945;16:20. Muttel AT, Kamal MR. In: Utracki EK, editor. Two phase polymer systems. Hanser Publishers, New York, 1991. p. 311. Roy D, Gupta BR. Rheological behaviour of short carbon ®ber®lled thermoplastic elastomer based on styrene-isoprene block copolymer. J Appl Polym Sci 1993;49:1475.
www.elsevier.com/locate/compscitech
Morphology and melt rheological behaviour of short-sisal-®bre-reinforced SBR composites R. Prasantha Kumar a, K.C. Manikandan Nair a, Sabu Thomas a,*, S.C. Schit b, K. Ramamurthy b a
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills PO, Kottayam-686 560, Kerala State, India b Central Institute for Plastics Engineering and Technology, Guindy, Chennai, India Received 9 February 1999; received in revised form 6 January 2000; accepted 1 March 2000
Abstract The melt-¯ow behaviour of untreated and treated short-sisal-®bre-reinforced styrene±butadiene rubber (SBR) composites were analysed by using an Instron capillary rheometer. The eects of ®bre breakage, length, concentration and shear-rate/stress on melt viscosity have been studied. The ®bre breakage was analysed before and after extrusion and the polydispersity index(PDI) was estimated. It was found that these composites behave as pseudo-plastic materials. At low shear rates, the short ®bres increase the viscosity much more than at high shear rates. There was an increase in viscosity upon chemical treatments owing to the strong interfacial adhesion between the ®bre and the rubber matrix. The dependence of melt viscosity on temperature, ¯ow behaviour index, n0 , melt elasticity, extrudate distortion and deformation of these composites were analysed. Finally, die-swell measurements were carried out to understand the elastic eects. The extruded samples were analysed by optical and electron microscopy in order to study the surface morphology and extrudate deformation of these composites. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Sisal ®bre; Styrene±butadiene rubber; Composites; Rheology; Bonding agent; Interfacial adhesion
1. Introduction The ease of processability of a rubber compound is essential for the fabrication of an end product, since it undergoes dierent types of shear during every stage of processing [1]. The need for rheological studies is essential in making a logical choice of the polymer and its processing conditions [2]. The modern theories of rheology of elastomers have been related to the processing characteristics of rubber and molecular structure [3±5]. The study of the rheological behaviour of polymer composites is very important for fabrication of end products. It is seen that at higher shear rates, ®bre orientation takes place because of the convergent, divergent and shear ¯ows. The ®bres orient along the ¯ow direction during convergent ¯ow and at 90 to the direction in a divergent ¯ow [6]. Setua et al. [7] and Murthy et al. [8] have observed that short-®bre-reinforced elastomer composites behave as pseudoplastic materials. Goettler et al. [9] have reported * Corresponding author. Tel.: +91-481-598015/598303; fax: +91481-561190. E-mail address: [email protected] (S. Thomas).
the rheological and extrusion behaviour of short-®brereinforced rubber compounds. Kutty et al. [10] have found the dependence of temperature on the shear viscosity of short-®bre-®lled composites. Melt elastic parameters like die swell ratio were found to increase with increasing shear rate and decrease with ®bre loading [11]. This is due to the irreversible orientation of the ®bres in the matrix during extrusion. Goettler [12] has found that the ®bres raise the viscosity of the composite due to the higher pseudoplasticity induced by the ®bres. Czarnecki et al. [13] have worked on the shear ¯ow rheological properties, ®bre damage, and mastication characteristics of short-®bre-reinforced polystyrene melts. Recently, Thomas and co-workers have extensively studied the eect of short ®bres on the rheological behaviour of composites of natural rubber [14±15] and PS (polystyrene) [16] respectively. Recently, the authors have reported on the mechanical [17±19], processing [20], electrical [21] and transport [22] behaviour of shortsisal-®bre reinforced styrene±butadiene (SBR) composites. However, there seems to be no previous studies made on the rheological and extrusion behaviour of shortsisal-®bre-reinforced SBR composites. In this paper, we
0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00057-9
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report our observations found on the rheological and extrusion behaviour of untreated, treated and bonding agent added short-sisal-®bre-reinforced SBR composites with special reference to the eect of ®bre breakage and ®bre distribution, ®bre length and ®bre concentration on shear viscosity.
2.2.2. Treatment of ®bres 2.2.2.1. Water treatment. Fibres of length 6 mm were immersed in water at 28 C for 1 h. These ®bres were then washed several times with water and were later dried in an air oven at 70 C for 2 days and kept in polyethene bags. These ®bres were designated and used as untreated ®bres.
2. Experimental
2.2.2.2. Acetylation. Fibres were immersed in 18% NaOH solution at 28 C for 1 h . These ®bres were washed several times with water and 0.1 N HCl. The ®bres were then soaked in glacial acetic acid for 1 h at the same temperature. Later they were soaked again in acetic anhydride containing Con.H2SO4. The resulting ®bres were washed and dried in an air oven.
2.1. Materials Synthetics and chemicals, Bareilly, Uttar Pradesh State, India supplied Synaprene (SBR-1502). Sisal-®bre (Agave sisalana) was obtained from a local processing unit situated in the state of Tamil Nadu. The chemical constituents of sisal-®bre are reported in Fig. 1 [23]. It was reported that the density [24] and the average diameter of the ®bre are 1.45 g/cmÿ3 and 0.1212 mm respectively. All other chemicals such as hexamethylene tetramine and resorcinol used were of laboratory reagent grade. 2.2. Methods 2.2.1. Fibre distribution analysis The mixes were dissolved in toluene and the length of the extracted ®bres was measured under the optical microscope. The variation of average ®bre length with the volume loading of ®bre and the changes in the average ®bre length before and after extrusion at dierent shear rates was analysed.
2.2.2.3. Benzoylation. The mercerised ®bres were agitated with 10% NaOH and 50 ml benzoyl chloride for 15 min. The dried ®bres were then soaked in ethanol for 1 h, washed and dried in an air oven at 70 C. 2.2.3. Preparation of composites Mixes were prepared in a two-roll laboratory open mixing mill (150300 mm), at a nip gap of 1.25 mm. These mixes were compounded according to the basic formulation given in Table 1. All the ingredients were added in the mix according to the mixing sequence [17]. 2.2.4. Rheological measurements The rheological studies of untreated and treated short-sisal-®bre-reinforced SBR composites were carried out using an Instron Capillary Rheometer Model
Fig. 1. Chemical constituents of sisal ®bre.
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Table 1 Formulation of mixes (from A to H2) Ingredients
Mixes
Synaprene Sulphur Stearic acid Zinc oxide Silica (Pptd) Hexaa Resorcinol CBSb TDQc Sisal ®bre 1. Untreated 2. Acetylated 3. Benzoylated a b c
A
B
C
D
E
E1
E2
E3
E4
F1
F2
G1
G2
H1
H2
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 1 2 2.5 1 1
100 2.2 2 5 1.5 2.5 5 1 1
100 2.2 2 5 3 1.5 3 1 1
100 2.2 2 5 5 2.5 5 1 1
100 2.2 2 5 ± 2.5 5 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± 2.5 5 1 1
100 2.2 2 5 ± 2.5 5 1 1
100 2.2 2 5 ± ± ± 1 1
100 2.2 2 5 ± 2.5 5 1 1
± ± ±
20 ± ±
20 ± ±
20 ± ±
± ± ±
5 ± ±
10 ± ±
15 ± ±
15 ± ±
± 5 ±
± 10 ±
± 15 ±
± 20 ±
± ± 5
± ± 15
Hexamethylene tetramine. N-cyclo hexyl benzothiazole sulphenamide. 2,2,4-Trimethyl 1,2-dihydroxy quinoline polymerized.
3211 [25]. A capillary of L/D ratio 20 and an angle of entry 90 were used in all experiments. All studies were done in the shear range of 3.68±1226.9 sÿ1, and a temperature range of 90±130 C due to the instrument limitations. The shear stress, w , increases linearly with distances, r, from the centre line and is given by: w
P r 2lc
1
where P is the pressure drop across the length, lc , of capillary tube. The true wall shear stress was calculated by, F w 4Ap lc =dc
2
where F is the force on the plunger (N), Ap crosssectional area of the plunger (mm2), and lc (mm) and dc (mm) the length and diameter of the capillary respectively. The apparent shear rate was calculated by using the equation: :
w;a 8V=dc But;
V
3 VXH db2 60 dc3
4
where V is the mean velocity of the liquid ¯owing through the capillary, VXH , the cross head speed in cm/min, db , the diameter of the barrel, dc , the diameter of the capillary. The correct wall shear rate may be found by using the Rabinowes correction:
3 n0 1 8V :
w;a 4n0 dc
5
The factor 3n0 1=4n0 is the Rabinowes correction applied to calculate the true shear rate. But;
n0
d ln w d ln a
6
: i.e. n0 is the slope of the graph of ln w vs ln w;a . Thus wall shear rate for pseudoplastic materials is, therefore: 2 3n0 1 VXH db2 :
w;a 15 4n0 dc3
7
Thus, apparent viscosity, a is calculated by, : na w = w;a
8
2.2.5. Die swell and extrudate morphology The extrudates diameters were measured at dierent times and were kept for 24 h to attain equilibrium before ®nal readings were recorded, using a travelling microscope. The die swell ratio can be calculated from the formula, de 9 Die swell ratio dc where de and dc are the diameter of the extrudate and the capillary, respectively.
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For SEM, the peripheral and cross-sectional surfaces of the extrudates were sputter-coated with gold within 24 h of testing. 3. Results and discussion The calculations of melt viscosity and the shear rate of a pseudoplastic material depends on several assumptions: (i) ¯uid ¯ow is parallel to the axis, (ii)velocity of any ¯uid element is a function of radius only, (iii) ¯uid is incompressible, (iv) velocity of the ¯uid is zero at the wall, (v) ¯uid ¯ow is isothermal and (vi) all energy is consumed within the capillary itself. 3.1. Fibre breakage Fibre breakage was analysed from the ®bre length distribution curve and from the polydispersity index (PDI). Fig. 2 shows the ®bre length distribution for compound G2 before and after milling. The short sisal ®bres have not undergone severe breakage, although high shear forces were generated during the time of mixing. The initial 6 mm length of the ®bres is converted to varying sizes. It can be observed that the maximum breakage is seen between 5 and 6 mm in length. During extrusion, the ®bre again undergoes breakage and the percentage of ®bre of less than 5 mm length increases considerably. It is clear from Fig. 3(a) that the maximum in the range 5±6 mm ®bre length before extrusion is shifted to 0±1 mm after extrusion at a shear rate of 1226.9 sÿ1. The increase in the breakage of ®bres is due to the result of the ®nal break-up of kinked ®bres during extrusion. As the shear force increases, the percentage of breakage increases dramatically, and at a shear rate of 1226.9 sÿ1 more than 40% of the ®bres are of less than 2 mm length. The increase of PDI value as shown in Table 2 with the increase of shear rate also revealed the ®bre breakage. On analysing Fig. 3(b) it is seen that the most probable length of the ®bre in composite decreases with shear rate. 3.2. Eect of ®bre length The eect of ®bre length on the melt viscosity of SBR composites at 120 C is shown in Fig. 4. A series of mixes containing 20 phr short sisal ®bre of length 2, 6 and 10 mm were used to study the eect of ®bre length. Generally, an increase in viscosity on increasing the ®bre length is expected. It is seen that the melt viscosity increases with ®bre length from 2 to 10 mm. The ¯ow behaviour of composites containing ®bres having a length beyond 10 mm was dicult to study. The viscosity increases with ®bre length at low shear rates. But at higher shear rates, the ®bre alignment and ®bre
distributions occur simultaneously and, therefore, increase in viscosity is less predominant here. It is dicult for ®bres having higher ®bre lengths to become oriented in the direction of ¯ow and the distribution or dispersion of ®bres also is poor at higher ®bre length. This is due to the entanglement of ®bres at higher ®bre content. Thus ®bres having shorter length can be more easily aligned and dispersed than those having higher lengths along the direction of ¯ow, i.e. SBR composites containing short sisal ®bres having shorter length show low viscosity than having higher length. This fact is clearly shown in the Fig. 5. As the ®bre length increases the dispersion of the ®bre becomes poor and it will cause agglomeration of short sisal ®bres in the SBR matrix. This increases the melt viscosity of the composite. 3.3. Eect of shear stress and ®bre loading on melt viscosity The variation of melt viscosity of sisal-SBR composites with shear stress at dierent ®bre loading at 120 C is shown in the Fig. 6. Like all other ®bre reinforced elastomer composites, the melt viscosity of SBR composites reduces with the increase in shear stress. The increase in viscosity with increasing shear stress is attributed with the orientation of the polymer molecules and the ®bres in the direction of shear. These curves are typical of pseudoplastic materials, which show a decrease in viscosity with increasing shear stress. All the mixes have been modelled using the power-law equation [26], : K n
10
where n is the power law index and k the consistency index. The power-law model was found to be the most suitable equation to model this behaviour. The slight non-linearity from the power-law model is due to the variation in the extent of ®bre orientation in the matrix and also due to wall slip [27]. For a given stress, the viscosity of SBR composites was found to increase with ®bre loading. The increase in viscosity of the composites with increasing ®bre loading is due to the interaction between ®bre±®bre and ®bre±matrix. 3.4. Eect of chemical modi®cation of ®bres Several researchers [28±32] have proved that strong interfacial bonding at the interface can be obtained by dierent chemical treatments. 3.4.1. Acetylation The cellulosic hydroxyl groups in the lignocellulosic sisal ®bre are not so reactive, since they form strong hydrogen bonds. The addition of 18% caustic soda
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solution treatment destroys the hydrogen bonding in cellulosic hydroxyl groups and cause partial removal of lignin. This makes the ÿOH groups more reactive (Scheme 1). The hydrogen atom on acetyl carbon atom becomes more reactive due to the presence of a carbonyl group. This reacts with active sites on SBR to form chemical links, thereby improving adhesion between ®bre and rubber matrix. The hypothetical mechanism showing the acetylation reaction on the ®bre is shown in Scheme 2. The mechanism of acetylation has been explained by Chand et al. [33].
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3.4.2. Benzoylation The hypothetical mechanism showing the benzoylation reaction on the alkali treated ®bre is shown in Scheme 3. The ÿOH group of the lignocellulosic sisal ®bre reacts with benzoyl chloride. The extractable materials such as lignin, waxes, etc. were removed by the addition of 18% caustic soda solution. Considerable attention has been paid to see that the ®bres do not lose their physical properties and ®brous nature during this treatment. The reaction proceeds through any hydroxyl group attached to the cellulose backbone, since the oxygen linkage is a hydrolysable one. As per the IR studies made on the ®bre after benzoylation, it is observed that as a result of esteri®cation of the hydroxyl group, the hydroxyl vibration absorption at 3400 cmÿ1 is found to be decreased. The absorption bands at 1950, 1600 and 710 cmÿ1 indicate the presence of aromatic rings. The ester group is identi®ed by the presence of peaks at 1725 and 1300 cmÿ1.
Table 2 Fibre length distribution index of sisal ®bres before and after extrusion Shear rate sÿ1
Ln
Lw
Ln/Lw
5.08
1.08
4.3 3.58 3.38
1.15 1.43 1.45
Before extrusion 4.68 After extrusion
Fig. 2. Eect of ®bre length distribution before and after milling of SBR composites.
12.269 122.69 1226.91
3.72 2.5 2.32
Fig. 3. (a) Eect of ®bre length distribution of sisal ®bres after extrusion. (b) Eect of ®bre length on increasing shear rates at 120 C.
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Fig. 4. Eect of shear rate on melt viscosity at dierent ®bre lengths.
Fig. 6. Eect of shear stress on melt viscosity of mixes A, E2, E3 and E4 at 120 C.
the maximum viscosity followed by acetylated ®bre containing mix G1 and it decreases on increasing shear rate. The increase in viscosity is due to the better bonding with the ®bre and rubber due to chemical modi®cation. The better interfacial adhesion between the bezoylated sisal ®bre and SBR matrix is shown in Scheme 3. Schematic models of the interfaces after acetylation and benzoylation treatments are shown in Fig. 8(a) and (b). 3.6. Eect of temperature
Fig. 5. Eect of ®bre length on melt viscosity at dierent shear rates at 120 C.
3.5. Viscosity of chemically modi®ed composites The eects of chemical treatments on the melt viscosity of treated SBR composites at a temperature of 120 C are discussed in the Fig. 7. The ®gure shows the eect of hydration, acetylation and benzoylation on the melt viscosity of the composites. On analysing the ®gure, it is seen that the benzoylated ®bre containing mix H2 has
During processing, the elastomers undergo considerable change due to the eect of temperature. At all ®bre loading, the melt viscosity of the composites decreases on increase of temperature. This is due to several factors such as (a) accelerated molecular motion due to theavailability of greater free volume and the decrease of entanglement density, (b) weak intermolecular interactions. In the Arrhenius expression, () is related to absolute temperature (T) by the following equation [34]. AeD E=RT
11
where A is a constant characteristic of the polymer, E is the activation energy for viscous ¯ow, and R is the universal gas constant. The eect of temperature on melt viscosity of the SBR composites at dierent loading of ®bres at four dierent temperatures at a shear rate of 36.807 sÿ1 is shown in Fig. 9. These plots are treated as the semi logarithmic Arrhenius plots, i.e. log vs 1=T at ®xed shear rate of 368.07 sÿ1. From the slopes of these linear plots, E values were estimated. The E
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Scheme 1.
Scheme 2.
Scheme 3.
values provide valuable information on the sensitivity of the material towards the changes in temperatures. The E of the composite melt increases with increase in the ®bre concentration, indicating that the melt viscosity of the composite is greater temperature sensitive than that of the gum compound. The estimated E values presented in Table 3 support this fact. Thomas et al. [14,15] have already reported similar behaviour for sisal/coir ®bre reinforced NR composites. Fig. 10 shows the eect of melt viscosity with temperature on dierent chemically modi®ed ®bre reinforced composites at a shear rate of 368.07 sÿ1. Among the treated mixes, the benzoylated ®bre containing mix H2 showed highest slope value showing the higher activation energy, and it indicates that it has greater sensitivity to temperature among all other mixes (Table 4). 3.7. Eect of relative viscosity
Fig. 7. Eect of shear rate on melt viscosity of sisal/SBR composites containing chemically modi®ed ®bres at a temperature of 120 C.
In the case of polymer composites, the relative viscosity is calculated as the ratio of the shear viscosity of the ®lled compounds to that of un®lled compounds. The variation of relative viscosity with volume fraction of
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Fig. 8. (a) Schematic model of the interface after acetylation treatment. (b) Schematic model of the interface after benzoylation treatment.
®bres at a shear rate of 3.68 sÿ1 is shown in Fig. 11. When the temperature is increased from 90 to 120 C, the relative viscosity of the composite decreases. For the dierent temperatures studied, it can be seen that the relative viscosity increases with ®bre loading. But the magnitude of the increase show a decreasing trend as the temperature is elevated with ®bre loading. At 110 and 120 C, the rate of change of relative viscosity is uniform over the entire range of ®bre loading whereas at 90 and 100 C the rate of change shows a sharp increase. At higher ®bre loadings and at higher temperatures, the scorch eect causes the ®bres to be less oriented as compared with their behaviour at a lower temperature. This decreases the relative viscosities of composites containing the same level of ®bre loading, as the temperature is increased.
3.8. Flow behaviour index (FBI) The pseudoplastic materials have n0 values less than unity and a high value of n0 indicates a low shear thinning nature. The eect of ®bre length on the ¯ow behaviour index of the composites is presented in Fig. 12. No remarkable eect was noticed upon the increase of ®bre length. Fig. 13 shows the eect of volume percentage of ®bres on the ¯ow behaviour index of sisal-SBR composites. From the ®gure, it is seen that the ¯ow behaviour index of sisal/SBR composites was found to be less than unity, indicating the pseudoplastic nature of the system. It is also noted that the n0 values decrease upon increasing the ®bre content in the system. The increase of pseudoplasticity arises due to the orientation of ®bres during its ¯ow through the capillary.
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3.9. Die swell and extrudate distortion
Fig. 9. Eect of temperature on melt viscosity of mixes A, E1, E3 and E4 at a shear rate of 36.807 sÿ1.
Table 3 Activation energy of sisal/SBR composites at dierent ®bre loading at a shear rate of 36.807 sÿ1 Mixes
Volume of ®bres (% )
Activation energy (kJ/ K/mol)
E E1 E2 E3
0 5 10 15
2.707 4.841 5.867 7.973
Fig. 10. Eect of temperature on melt viscosity of treated mixes at a shear rate of 36.807 sÿ1.
Die swell measurements at 120 C for three dierent shear rates were made and shown in Table 5. The die swell increases slightly with increase of temperature and shear rate in the case of ®bre reinforced rubber compounds. But the die swell is greatly reduced by the addition of ®bre and the eect is negligible beyond 10phr loading. These results are agreement with earlier observations made by Chan et al. [35]. At the same ®bre loading, the die swell ratio increases with shear rate and it is observed that the chemical treatment on the ®bres does not show much remarkable change in the die-swell. Optical photograph of the SBR composites extruded at 120 C at three dierent shear rates is shown in Fig. 14. The presence of ®bre reduced the extrudate distortion at high shear rates and at 15 phr loading, the distortion is remarkably low. It has been also observed that the distortion is negligible at 90±110 C. It is evidenced from the optical photograph that the presence of ®bre reduced the extrudate distortion at high shear rates and at higher loading of ®bres (mixes E4, G1 and H2).
Table 4 Activation energy of treated sisal/SBR composites at same ®bre loading at a shear rate of 368.07 sÿ1 Mixes
Type of chemical treatment
Activation energy (kJ/ K/mol)
E4 G1 H2
Untreated Acetylated Benzoylated
2.734 2.872 3.063
Fig. 11. Eect of volume of ®bres (%) on relative viscosity of SBR composites at a shear rate of 3.68 sÿ1.
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Fig. 12. Eect of ®bre length on ¯ow behaviour index of SBR composites.
Fig. 13. Eect of ®bre loading on ¯ow behaviour index of SBR composites. Table 5 Variation of die swell ratio (de/dc) of SBR composites for dierent shear rates at 120 C Samples
A E E1 E2 E3 E4 G1 G2 H2
Shear rate (sÿ1) 3.6807
3.6807
3.6807
1.45 1.46 1.40 1.38 1.15 1.14 1.06 1.05 1.05
1.49 1.47 1.42 1.39 1.16 1.14 1.08 1.06 1.06
1.52 1.51 1.48 1.39 1.16 1.13 1.08 1.07 1.07
3.9.1. SEM studies on extrudate morphology The extrudates show smooth surfaces and uniform diameter at low shear rate. But on increasing shear rates, the extrudates exhibit surface irregularity. Fig. 15(a), (b) and (c) shows the SEM micrographs of
1226.91 1.61 1.58 1.55 1.40 1.15 1.13 1.07 1.06 1.06
extrudates E, F1 and G1 at dierent shear rates 36.807, 368.07 and 1226.91 sÿ1 respectively. The surface morphology of these composites extruded at dierent shear rates show an increase in the surface discontinuity with increasing shear rates. The irregularity at the periphery of the mixes increases as the shear rate increases. The
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Fig. 14. Optical photograph showing the extruded distortion of mixes A, E1, E2, E3, E4, G1, G2, H1 and H2.
surface discontinuity is associated with the migration of ®bre towards the periphery of the extrudates. Fig. 16(a) and (b) shows the cross-sectional view of the extrudates E2, and E3 at a constant shear rate of 368.07 sÿ1 containing 10 and 15 phr loading of ®bres. It is observed that the ®bres are well oriented along the direction of extrusion. At low shear rate the ®bres are mainly concentrated at the periphery, whereas at medium shear rates they form a uniform dispersion throughout the matrix. The unequal retroactive forces experienced by the components in the composite result in the redistribution of the ®bre and its consequent migration to the periphery of the extrudate [35]. It is observed that short sisal ®bres, which are dispersed uniformly at medium shear rate, are concentrated at the core region along the direction of capillary ¯ow. When the shear rate is increased from 36.807 to 368.07 sÿ1, the redistribution of short ®bres takes place and more and more ®bres were concentrated at the core of the extrudates (E2 and E3). It is also clear from these ®gures that dispersion of short ®bres increased with treated ®bre concentration. Micrographs, Fig. 17(a) and (b), of composites containing treated ®bres show increased orientation of short ®bres due to the increased ®bre±rubber interactions. G2 is extruded at a shear rate of 36.8 sÿ1 while G1
Fig. 15. SEM of the surface morphology of dierent extrudates at dierent shear rates 36.807, 368.07 and 1226.91 sÿ1 respectively: (a) Egum; (b) F1 (5 phr); (c) G1 (15 phr).
is extruded at a shear rate of 1226.1 sÿ1. In the case of G2, the ®bres are concentrated at the periphery while in the case of G1, the ®bres are concentrated at the core of the extrudate. 3.10. Theoretical modelling Guth et al. [36] has proposed the following equation to explain the viscosity of the oriented composite: ÿ c un 1 0:6fc 1:62f 2 c2
12
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Fig. 16. SEM of the cross-sectional view of the extrudates at a shear rate 368.07 sÿ1: (a) E2-10 phr; (b) E3-15 phr.
where f is the ratio of longest to the shortest diameter of the ®bre, c, the volume fraction of the ®bre, un , the viscosity of the un®lled mix, and c , the viscosity of the composite. The theoretical and experimental viscosity of sisal/ SBR composites at 100 and 120 C at dierent shear rates are shown in Fig. 18(a) and (b). Fig. 18(a) shows that at 100 C, the experimental values are higher than the theoretical value in all loading. On increasing the concentration of ®bres, the ®bre-matrix interaction increases and thereby diculty arises in the better alignment and orientation of ®bres. Therefore, the experimental viscosity is higher than the predictions. Fig. 18(b) also shows that the experimental viscosities are higher than the theoretical values at all ®bre loading. On increasing temperature, the viscosity is governed by ®bre±matrix interactions and misalignment of ®bres in ®bre distribution, which lead to higher viscosity than the theoretically predicted one. Fig. 19 shows the comparison of experimental and theoretical relative viscosities at dierent volume fraction of ®bres and dierent temperatures at a shear rate of 36.807 sÿ1. The relative viscosity increases upon the increase of the volume fraction of ®bres. It is also seen that the experimental relative viscosity is higher than the theoretical one. This is due to the increased interfacial adhesion between the ®bre and matrix, i.e. the better
Fig. 17. SEM of cross-sectional view of the extrudates of treated composites at shear rates 36.807 and 1226.91 respectively: (a) G2-20 phr; (b) G1-15 phr.
bonding occurred in the interface, if not, it could be due to the entanglement of ®bres in the composite. According to Kamal et al. [37], the relative viscosity and concentration relations represent semi-empirical extensions of dilute suspension theories through the inclusion of 2v . r 1 v k v 2
13
where r is the relative viscosity of the suspension and v is the term for the ®rst order interactions among ®bres. Thus the variation of relative viscosity of composites with ®bre loading at dierent temperature and shear rate can be ®tted into a second-degree polynomial type of equation [38], i.e. f =g 1 K1 Vf K2 V2f
14
where K1 and K2 are functions of rate of shear and temperature, f and g , the viscosities of the ®lled and the gum compound respectively and Vf , the volume fraction of the ®bre in the composite. The relative viscosities of sisal/SBR composites have been ®tted into the
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Fig. 18. (a) Comparison of experimental and theoretical melt viscosities at dierent shear rates of mix H2 (a) 100 C; (b) 120 C.
Fig. 19. Comparison of experimental and theoretical relative viscosities at dierent volume fraction of ®bre and dierent temperatures (shear rate 36.807 sÿ1). Table 6 Comparison of theoretical and experimental viscosity of mix H2 Shear rate (sÿ1)
Temperature ( C) 90
3.6807 36.807 368.07
100
110
120
K1
K2
K1
K2
K1
K2
K1
K2
5.9384 2.6537 0.1196
ÿ12.741 ÿ0.0857 1.5170
0.6109 0.5726 8.4956
29.59 23.62 ÿ68.80
0.1350 16.871 ÿ0.0390
ÿ26.84 ÿ381.26 106.98
ÿ5.536 5.863 0.034
133.3 ÿ10.80 ÿ99.37
second-degree polynomial equation and the values of K1 and K2 at dierent shear rate and temperature are given in Table 6. Some K1 values are negative, which are attributed to the wall slip occurred during extrusion.
4. Conclusions The rheological behaviour of short-sisal-®brereinforced SBR composite has been studied. The eects
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of shear rate, ®bre loading, temperature and the extent of interface bonding on the viscosity of the system have been examined in detail. The sisal ®bres undergo severe breakdown during extrusion. All the system showed pseudoplastic behaviour showing the decrease of viscosity with increasing shear rate. The incorporation of treated ®bres increased melt viscosity and the viscosity enhancement was due to the increased interfacial adhesion between the ®bre and matrix due to the chemical treatments. Of the various chemical treatments, benzoylated ®bre reinforced composites showed maximum viscosity among all mixes. The ¯ow behaviour index of the composites was found to decrease and attain a steady value on increasing the concentration of treated ®bres. The relative viscosity of the composites was also found to increase with increase of ®bre loading. The extrudate distortion and die swell behaviour of SBR composites were reduced by the addition of short sisal ®bres. Short ®bres present in the matrix prevent the shape distortion of the extrudates and the increase in ®bre content decreased the shape distortion. Die swell values decreased substantially with the incorporation of short ®bres. SEM photographs indicated that the increase of short ®bres creates discontinuity in the polymer matrix. The ®bres were found to be concentrated more on the periphery at low shear rates and on the core at high shear rates. On comparison of theoretical melt viscosity with the experimental values, it is seen that the experimental viscosity is greater than the theoretical value as a result of better interfacial adhesion and misalignment of the ®bres. Moreover, the variation of relative viscosity of composites with ®bre loading at dierent temperatures and shear rates has been ®tted into a second-degree polynomial type equation. Acknowledgements One of the authors (R.P.K.) would like to thank the Council of Scienti®c and Industrial Research (CSIR), New Delhi for the ®nancial assistance awarded in the form of a research fellowship. His thanks are also due to Mr. Murukesan, Technical Assistant, Rheological laboratory, CIPET, Guindy, Chennai. Finally the authors thank the referee for the valuable comments. References [1] Norman RH, Johnson PS. Processability testing. Rubb Chem Technol 1981;54:493. [2] Brydson JA. Flow properties of polymer melts. 2nd ed. ILIFFE Books, 1982 (Chapter 1). [3] White JL. Elastomer rheology and processing. Rubb Chem Technol 1969;42:257.
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