Tear and processing behaviour of short sisal fibre reinforced styrene butadiene rubber composites

Polymer International 38 (1995) 173-182

Tear and Processing Behaviour of Short Sisal Fibre Reinforced Styrene Butadiene Rubber Composites

R. Prasantha Kumar & Sabu Thomas*

School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills PO, Kottayam-686 560, Kerala, India

(Received 20 April 1995; accepted 24 May 1995)

Abstract: The tear failure and processing characteristics of short sisal fibre reinforced styrene butadiene rubber (SBR) composites were investigated. Tear strength was examined with special reference to the effects of fibre length, fibre orientation, fibre concentration and bonding agent. It was observed that the tear strength depends on all the above factors. The tear failure mechanism was analysed from fractographs taken using a scanning electron microscope (SEM). During tear testing, the composites failed by a shearing process. Microscopic examination of cracks propagating in SBR composites revealed that the amorph- ous SBR matrix developed cracks, leaving ligaments of rubber attached to the broken fibres. The rubber particles were stretched as the crack opened and failure occurred at large critical extensions. It was observed that an increase in the concentration of fibres increased the tear strength in both longitudinal and transverse directions. The tear strength values were almost three to four times higher than those of the unfilled vulcanizates under similar conditions. In order to analyse the processing behaviour, the green strength, mill shrinkage and Mooney viscosity of the compounds were determined. Finally, the polymer-filler interaction was studied using the Lorenz-Park and Kraus equations.

Key words: sisal fibre, styrene butadiene rubber (SBR), tear strength, bonding agent, interfacial adhesion.

1 INTRODUCTION the material as well as the end-structure. The uses of these composites are increasing in sports goods such as

Composites are a combination of two or more materials fishing rods, golf shafts, hockey sticks and tennis on a macro-scale having a recognisable interface. The rackets. With the availability of wide varieties of basic constituents of these materials are (1) a matrix as a polymer composites, it becomes important to select binder and (2) a filler (fibrous or non-fibrous) as them carefully to meet the specific end-use require- reinforcement. Fillers with both plastics and rubbers ments. provide a new class of physically modified high strength Short fibre elastomer composites are a class of materials. As fibrous fillers improve the strength several rapidly growing materials because of their improved times of both plastic and rubber materials, these com- physical and mechanical properties, easy processability posites have found uses in structural applications.' and economic advantages. These materials bridge the

The essential characteristic of composites in solving gap between conventional elastomers and fibres by modern problems is the possibility of custom designing combining the stiffness of short fibres with the elasticity

of rubber. The major applications of these composites * To whom correspondence should be addressed. are in tyre treads,2 roofing^,^ hoses: sheeting~,~ V-belts,6

Polymer International 0959-8103/95/$09.00 0 1995 SCI. Printed in Great Britain 173

174 R. Prasantha Kumar, S . Thomas

industrial rubber products’ and complex shaped arti- cles.* These materials usually undergo tearing during their application. Therefore, it is important to study the tear resistance of these composites. The introduction of a fibrous filler influences the tear behaviour in two ways: (1) by contributing a strengthening structure and (2) by influencing the basic tear process.

Recently, from this laboratory, several studies have been reported by Thomas and coworker^^-^^ on the mechanical properties of short fibre reinforced polymer composites. The morphology and mechanical properties of short sisal and coir fibre reinforced natural rubber composites have also been studied by Varghese et al.I7 and Geethamma et al.,” respectively. De and coworker^'^^'^ reported the mechanical properties of short jute fibre reinforced elastomer composites. The mechanical properties of short sisal fibre reinforced SBR composites were reported earlier from this labor- a t ~ r y . ~ ~ Several workers have reported on the mechanism of reinforcement. Kraus and Medalia have made excellent reviews on the theories of reinf~rcement~’.~~ and mechanisms of tearing of rubbers.30

In the present paper, the tear and processing behaviour of short sisal fibre reinforced styrene butadiene rubber composites are studied with special reference to the effects of fibre length, fibre orientation, fibre concentration and bonding agent. The rubber-fibre interactions are also studied by swelling measurements. The tear properties are correlated with the morphology of the composites.

2 EXPERIMENTAL

2.1 Materials

Sisal, a monocotyledonous plant, belongs to the family Agave Sisalana. Commercially, the fibres are extracted from the long leaves. The fibres are thick-walled with a narrow lumen and have pointed ends. Due to the high degree of variability of the size of the lumen at a partic- ular cross-sectional position, the fibres are treated as a technical fibre.31 They are extensively grown in the southern-most part of India, especially in the states of Tamil Nadu and Kerala. The chemical constituents and physical properties of the fibres3’ are shown in Table 1. The raw fibres, which were collected from the local processing unit situated in the state of Tamil Nadu, were washed with water to remove undesirable materials and dried in an air oven at 70°C for 5 h, before the mixing process.

Styrene butadiene rubber (SBR-1502) was synthesized by Synthetics and Chemicals, Bareilly, U.P., India, under the trade name Synaprene. The properties of the raw rubber are given in Table 2. A combination of hexamethylenetetramine and resorcinol of laboratory

TABLE 1. Properties of sisal fibres

Chemical constituents (%) Cellulose 78 Hemicellulose and pectins 10 Lignin 8 Wax 2 Ash 1

Physical properties Diameter (mm) Density (g/cm”) Water absorption (24 h) (%) Tensile strength (MN/m2) Young‘s Modulus (GN/m2) Size of cells (mm) Cell length (/)/cell breadth (d) Cell wall thickness (pm) Micro-fibrillar angle (degrees) Aspect ratio (L/D)

0.1 21 2 1.45

530-630 17-22

150

0.5-4 (long) 100

20-25 20-60

12.5

reagent grade was used as a two-component dry bonding system. All other chemicals used were of com- mercial grade.

2.2 Composite preparation

Short sisal fibres of different lengths, 2, 6 and lOmm, and loading, 5, 10, 15, 20 and 35 parts per hundred rubber (phr), were incorporated into the styrene butadiene rubber matrix on a laboratory two-roll open mixing mill (1 50 mm x 300 mm). Compounding was done with a basic formulation (Table 3) using a conventional vulcanization system.

The nip gap, the mill-roll speed ratio, sequence of addition of ingredients, time and temperature of mixing,

TABLE 2. Properties of SBR-1502 (raw rubber)

Chemical constituents (%) Min. Max.

Styrene Volatile matter Organic acid Soap Ash Antioxidant

21.5 25.5

4.75 7 0.75

0.5 1.5

0.5 1.5

Physical properties Mooney viscosity, 46 58

[ML(1 +4)] at 100°C Tensile strength (kg/cm2) 21 0 Elongation at break (%) 320 Modulus at 300% (kg/cm2) 70 170

130 200 150 200

Cure time (min) 35

POLYMER INTERNATIONAL VOL. 38, NO. 2, 1995

Short sisal fibre reinforced SBR composites 175

TABLE 3. Basic formulation of Mixes A to L

Ingredients Mix

A B C D E F G H I J K L

SBR-1502 Sulphur Stearic acid Zinc oxide Resorcinol Hexaa CBSb TDQ" Sisal fibre (untreated)

(Fibre length, mm) Cure time (min) Scorch time (min)

100 100 2.2 2.2 2 2 5 5

- - 1 1 1 1

35 2

29.1 14 14 6.5

-

-

100 2.2 2 5 - - 1 1 35 6 17.1 6.25

100 2.2 2 5 - - 1 1 35 10 18.1 10

100 2.2 2 5 - - 1 1 5

6 19 7

100 100 2.2 2.2 2 2 5 5 - - - - 1 1 1 1 10 15

6 6 17.5 18 6.5 7

100 2.2 2 5 - - 1 1 20

6 17.1 7.1

100 100 100 2.2 2.2 2.2 2 2 2 5 5 5 - - - - - - 1 1 1 1 1 1 15 20 25

10 10 10 25.2 22.3 25.2 10.5 9 10.2

100 2.2 2 5 5 2.5 1 1 35 6 6.06 1

a Hexamethylenetetramine. N-Cyclohexyl benzothiazole sulphenamide. Polymerized 2,2,4-trimethyl-1,2-dihydroquinoline,

and number of passes were kept the same for all mixes. Initially, SBR was masticated for 5min and then the compounding ingredients and bonding agent were incorporated according to the basic formulation.

When homogeneity in mixing had been achieved, short sisal fibres of different lengths and concentrations were added to the compound to produce a variety of mixes. The fibres were added at the end of the compounding process in order to prevent fibre breakage and to promote better fibre dispersion and orientation.

During the milling operation, shear forces oriented the short fibres along the grain direction. These shear forces also caused fibre breakage. The extent of this breakage was calculated for the composite^.^^ On repeated passing through the mill rolls, maximum orientation of the short fibres took place.

2.3 Physical testing of samples

Mill shrinkage of the compound was determined by cutting a piece of a compounded sheet from the open mill as per ASTM standard D 1917-89. F 0 1 d i ~ ~ has developed a method for green strength measurement, in which the surface tack of the compound is eliminated by pressing the samples at 120°C for 2min between alu- minium foils in a hydraulic press having electrically heated platens.

The curing characteristics of the mixes were studied using an oscillating disk rheometer (Monsanto Rheome- ter R-100). Mooney viscosity [ML(1 + 4)] at 100°C of the mixes was determined in a Mooney viscometer. The samples were vulcanized at 150°C using the cure times determined from the rheographs. The samples for tear strength measurements were punched out from sheets

(15cm x 15cm x 0-15cm) along (longitudinally oriented fibres) and across (transversely oriented fibres) the grain direction (Fig. 1). The measurements were done using unnotched 90" angle test pieces of both orientations at a cross-head speed of 50cm/min using a Uni- versal Testing Machine (Zwick-1475) in accordance with ASTM D 624-81.

Tear failure surfaces of the composites were examined with a scanning electron microscope. The fractured ends of the samples were sputter-coated with gold within 24 h of failure using a fine coat ionizer JFC-1100. The micrographs were taken using a Jeol scanning electron microscope model JSM 35 C.

Hardness of the composites containing varied lengths and loading of fibres was measured at room temperature using a Shore-A hardness tester according to

Swelling studies were conducted using circular shaped samples, having a diameter of 1.9 mm, punched

ASTM D 2240-8 1.

FORCE I

b FURCE

i f UIfCt

LONGITUDINAL TRANSVERSE Fig. 1. Tear specimens showing orientation and fractured sur-

faces of the SBR composite.



out of the tensile sheets. The samples were allowed to swell for 72 h in toluene.

3 RESULTS AND DISCUSSION

3. I Morphology of fibre surface

Figure 2 shows an SEM micrograph of the surface structure of raw sisal fibre, while Fig. 3 shows the surface structure of water-treated fibre. By water treatment, the binding material, lignin, which holds the cells together, was partially removed from the fibre surface. In both cases the surfaces are characterised by the vesti- gial attachments of the parenchymatous cells in which the fibre is embedded in the leaf. No changes in the surface morphology are visible in the partially de- lignified fibre except that the surface is rougher after water treatment.

Fig. 2. SEM micrograph showing the surface structure of raw sisal fibre ( x 600).

Fig. 3. SEM micrograph showing the surface structure of water-treated sisal fibre ( x 780).

3.2 Breakage of fibres

Generally, fibres undergo breakage during mixing due to the high shear forces experienced in the process. The extent of breakage of fibres was calculated using the fibre length distribution curve and distribution index.j4 The fibre length distribution curves of sisal fibres before and after milling are shown in Fig. 4. The breakage of sisal fibres was low because of its intrinsically flexible nature.

The number-average and weight-average fibre lengths can be defined as

Ni Li L,=C-- Ni

N~ L: L w = C - - Ni Li where L, is the number-average fibre length, zw is the weight-average fibre length, and Ni is the number of fibres having length Li .

The polydispersity index can be taken as a measure of fibre length distribution. Table 4 shows the fibre length distribution index of untreated sisal fibres before and after mixing. The value of the polydispersity index (L,\L,) remains almost the same, which indicates that little fibre breakage occurred during the milling oper- ations. It can be seen that the majority of fibres had

80

60

- BEFORE MILLING ; - AFTER MILLING

-A- 2 mm -* 6 mm -X- 10 mm

2 4 6 8 10 12

FIBRE LENGTH (mm ) Fig. 4. Fibre length distribution curves.

POLYMER INTERNATIONAL VOL. 38, NO. 2. 1995

Short sisal jibre reinforced SBR composites

TABLE 4. Distribution index of fibre length

Sisal fibre (optimum fibre length 6 rnrn)

Before milling After milling

qn (mm) 5.9 4.63 kW (_""I 5.98 5.06

1.01 4 1.093 L WIL n

fibre lengths between 5 and 6mm (Table 4). The initial aspect ratio of the mixes was also maintained (Table 1).

3.3 Critical fibre length

The minimum fibre length required for the fibre stress to be equal to the fibre ultimate strength at its mid- length is termed the critical fibre length.35 For a given fibre diameter and fibre/matrix interfacial condition, the critical fibre length, I , , is calculated from the following equation :

fu

2% I, = - df (3)

where f, is the ultimate fibre strength, is the interfacial shear stress (assumed to be constant), and df is the fibre diameter.

When If < I , , the maximum fibre stress may never reach the ultimate fibre strength. Here, either the fibre/ matrix interfacial bond, or the matrix, may fail before fibres achieve their potential strength. In this case, there will be no fibre failure. Instead, interfacial bond failure followed by fibre pull-out from the matrix takes place. When If > I,, the maximum fibre stress may reach the ultimate fibre strength over much of its length. Here fibre failure occurs. When I , = I,, effective fibre reinforcement occurs in the composite by utilizing the maximum strength of the fibres. Maximum load transmittance from the matrix to the fibre is possible at this critical fibre length.

The addition of fibres to the SBR matrix increases the torque value of the compound. The longer fibres ( I , > 1,) impart more restriction to deformation, while the shorter fibres (If c I,) create friction and heat generation in the composite due to the increase in the number of fibre ends.

The effect of fibre length and orientation on the tear strength of the SBR composites is shown in Fig. 5. For both orientations, tear strength increased with increase in fibre length. The increase was higher in the case of longitudinally oriented samples than transversely oriented ones. The tear strength showed a maximum value at 6 mm fibre length. Interaction between the short sisal fibre and SBR matrix reaches its maximum value at this fibre length. Increase in fibre length beyond 6 mm marginally reduced the strength of the composite due to fibre entanglement resulting in poor dispersion.

'I 70 $ 1 v 60

A LONGITUDINAL

0 TRANSVERSE

GUM

177

4 6 8 10 0 2

FIBRE LENGTH (rnrn )

Fig. 5. Effect of fibre length and orientation on tear strength of SBR composites.

Further increase in fibre length beyond 10mm caused difficulty in incorporating the fibres into the SBR matrix, since the dispersion of these fibres was not uniform due to increase in fibre-fibre interaction.

3.4 Fibre orientation

Two modes of orientation were considered : (a) longitudinal (fibres oriented along the grain direction) and (b) transverse (fibres oriented across the grain direction) (Fig. 1). The longitudinally oriented fibre composites showed greater tear strength than the transversely oriented ones due to efficient load transmittance from the matrix to the fibre (Fig. 5). Composites having longitudinally oriented fibres show brittle type fracture behaviour, while transversely oriented fibres show ductile behaviour with a higher extent of plastic deformation. SEM micrographs of the tear fronts of Mixes L and C (Figs 6 and 7) are for longitudinal and transverse orientation of the fibres, respectively. In the case of Fig. 6, the fibre ends may be seen protruding from the SBR matrix. This confirms the longitudinal fibre orientation. Figure 7 shows the fibres aligned across the direction of the tear forces, indicating the transverse orientation.

3.5 Effect of fibre content

Figure 8 shows the effect of volume percent of fibre and orientation on tear strength. The tear strength increased with the volume of fibre for both orientations. For longitudinal samples, the tear strength increased rapidly


178 R. Prasantha Kumar, S. Thomas

Fig. 6. SEM micrograph showing the longitudinal orientation of sisal fibres in the composite (Mix C).

with fibre loading. For transverse samples, there was only a marginal increase. The sharp increase in tear strength for longitudinal orientation is due to the obs- truction caused by the short fibres to the tear paths making propagation more difficult. The value of tear

of sisal fibres in the composite (Mix C).

n I I I Y

0 5 10 15 20

VOLUME OF FIBRE ( % )

Fig. 8. Effect of volume (%) and orientation of fibres on tear strength of SBR composites.

strength reached a maximum at 35 phr loading of fibres (Mix C).

3.6 Effect of bonding agent Mix L with bonding agent added showed a higher torque value and a shorter cure time than all the other mixes. This is due to the fact that adhesion between the fibre and the rubber matrix is improved. The conden- sation reaction of hexamine and resorcinol creates a resin which binds the fibre and rubber matrix firmly in the composite. A hypothetical mechanism for the reaction is shown in Scheme 1.

From Tables 5 and 6, it can be seen that tear strength is a strength, green strength and hardness reached a maximum with the addition of bonding agent (Mix L).

TABLE 5. Processing characteristics

Property Orientationa Mix

A C L

Green strength ( M Pa) L 0.29 1.020 1.216 T 0.248 0.49 0.562

Mill shrinkage (%) 30 5 1 Mooney viscosity 33.5 35 55.5 [ML(I +4)] at 100°C

a L-longitudinal; T-transverse.


Short sisal Jibre reinforced SBR composites 179

0 I

- Y C &

Scheme 1

RESIN

SBR

crack initiation. As uniaxial stress is applied, failure ini- tiates and propagates from the apex, allowing more and more stress to concentrate at the tip of the advancing crack. Consequently, the rubber near to and along the crack paths of the fracture surface undergoes large plastic deformation. Analysis of the morphological fea- tures of fracture surfaces by SEM is an important tool for observing the surface morphology of fibres, the cause of crack initiation and the failure process in composite materials. The fracture surface of the tear specimen for Mix L, which had the highest tear strength of all the samples, exhibits lower pull-out of fibres from the matrix (Fig. 9).

On comparing the surface morphology of raw fibres, water-treated fibres and those with bonding agent added, it can be seen that rubber particles adhere to the surface of the fibre stripped out of the composite during tear failure tests (Fig. 9).

Figure 10 shows an SEM micrograph of Mix C. The fracture surface shows deep cracks which have developed during tear failure. This micrograph is evidence of the weak interfacial adhesion between sisal fibre and SBR matrix in the composite. Figure 11 shows an SEM of the fracture front of a tear sample of the same mix, C. Interestingly, the fibre pull-out is more prominent.

However, resilience and elongation at break decreased with the addition of bonding agent. 3.8 Processing characteristics

On analysing Table 5, it is found that processing characteristics such as green strength, mill shrinkage and Mooney viscosity are influenced by the addition of fibres. The green strength is defined as the strength of the unvulcanized rubber. This showed an increase in both longitudinal and transverse directions. The presence of bonding agent further enhanced the property.

3.7 Fractograph y studies

In tear failure, the test specimen is subjected to a uniaxial tensile stress. The angular geometry of the sample causes the concentration of large amounts of stress at the apex, which functions like a notch. A triaxial stress is produced at the apex, which acts as a precursor for

TABLE 6. Properties of mixes

Mix Orientation Tear Tensile Elongation Modulus at Shore-A Resilience strength strength at break 10% & 20% hardness (%I ( W m ) (MPa) (%I elongation

A L 13.89 2.36 373 0.37 0.53 43 76.1 0

E L 19.10 2.1 0 80 0.92 1 *45 58 66.73

F L 21.07 3.39 40 2.07 3.03 60 65.55

G L 26.80 3.45 30 2.45 3.25 63 64.59

H L 28.1 0 3.95 27 3.20 3.91 71 63.58

C L 46.50 6.71 23 5.70 6.38 80 61.58

L L 56.25 8.1 0 15 7.80 - 93 58.26

T 9.94 2.07 288 0.36 0.50 43

T 14.92 1.42 221 0.45 0.64 55

T 16.77 1.56 180 0.54 0.79 59

T 17.79 1.64 126 0.70 1.01 68

T 19.08 1.72 81 0.85 1 -22 70

T 23.30 2.24 69 1.22 1.75 81

T 36.66 2.30 65 1.29 1.82 90

a L-longitudinal ; T-transverse.



Fig. 9. SEM micrograph of a sisal fibre stripped out of Mix L showing rubber particles on the fibre surface.

The addition of short fibres reduces the elasticity of rubber. This is clearly seen from the values of mill shrinkage reported in Table 5. The resistance to shrinkage also depends on fibre characteristics. The addition of hexamine-resorcinol as a bonding agent further reduced the mill shrinkage values.

The Mooney viscosity CML(1 + 4)] at 100°C increased upon the addition of fibres and is a maximum in the case of Mix L containing bonding agent. Scorch time and cure time, however, decreased sharply in the presence of bonding agent (Table 3).

3.9 Rubber-fibre interactions

The interaction between rubber and fibre has been studied by several researchers. The polymer-filler interaction in silica-filled epoxidized natural rubber has been studied by Alex et al.,36 and Chakraborty et al.37 have

Fig. 10. SEM micrograph showing deep cracks formed during tear failure due to the weak interfacial adhesion between fibre

and rubber in the composite (Mix C).

Fig. 11. SEM micrograph showing large holes developed on the fracture surface of the composite due to the pull-out of

fibres during tear failure (Mix C).

investigated the rubber-filler interaction in silica fibre and jute fibre filled carboxylated nitrile rubber composites. The interaction between rubber and filler in carbon black filled natural rubber and styrene butadiene rubber vulcanizates has been analysed by Bhowmick & Neogi3' The effects of aliphatic solvents on short sisal fibre reinforced natural rubber composites have been investigated by Varghese et aL3' The following equations have been applied to study rubber-fibre interactions.

3.9.1 Lorenz & Parks equation?' According to this equation,

(4) Qf - = ae-' + b Qg

where Q is defined as grams of solvent per gram of hydrocarbon and is calculated by

Swollen weight - Dried weight (5 ) = Original weight x 100/Formula weight

The subscripts f and g of eqn (4) refer to filled and gum vulcanizates, respectively. z is the ratio by weight of filler to rubber hydrocarbon in the vulcanizate, while a and b are constants. The higher the QJQ, values, the lower will be the extent of interaction between the fibre and the matrix.

Table 7 shows Qf/Q, values for variable fibre loadings in the SBR matrix. The ratio QJQ, has maximum values in the case of Mixes C and G. The lowest QJQ, value for Mix L confirms that maximum fibrerubber interaction has occurred between the sisal fibre and the SBR matrix in the presence of bonding agent.

3.9.2 Kraus equation?' The rubber-fibre interaction can also be assessed using the Kraus equation :

K*Kf = 1 - m ( W - 6) (6)

POLYMER INTERNATIONAL VOL. 38, NO. 2,1995

Short sisal fibre reinforced SBR composites 181

TABLE 7. QJQ, values of the

mixes

Mix Q,/Q,

E 0.9087 F 0.9496 G 1.01721 C 1.01224 L 0.5599

TABLE 8. V,,/V,, values of the mixes

Mix Vro/Vrf

E 0.922 7 F 0.9593 G 1.0260 C 1.0366 L 0.641 5

where 4 is the volume fraction of filler and m is a constant characteristic of the filler and indicative of the degree of adhesion.

(7) Volume of the filler

= Total volume of the recipe

V, is the volume fraction of rubber, which was calculated from the equilibrium swelling data using the following equation :

where D is the weight after drying out, f is the fraction of insoluble components, T is the weight of sample before the experiment, pr is the density of rubber, ps is the density of solvent, and A, is the weight of the imbibed solvent. When eqn (6) is applied to rubber containing reinforcing fillers, it is found that V,, of the rubber phase in the swollen gel (corrected for the volume of filler, since the filler is assumed not to swell) is always much higher than for the pure gum, Vro. So the ratio V,,/V,, decreases with increase of filler loading. The ratio represents the degree of restriction of the swelling of the rubber matrix due to the presence of filler.

From Table 8, it is seen that the presence of fibres causes an increase in the values of V,,/V,, because of pockets of solvent forming around the fibres. Because of

the hydrophilic nature of the sisal fibre and the hydro- phobic nature of the styrene butadiene rubber, many pockets of solvent will be found at the interface of the rubber and fibre.

The lowest V,,/V,, value of Mix L is associated with enhanced fibre-rubber adhesion in this composite due to the presence of bonding agent. On comparing Mix C with Mix L, it can be inferred that the amount of solvent sorbed by a composite at equilibrium swelling is less for the composite containing bonding agent. The rate of swelling of unbonded composites is faster than that of the gum compound, As a result of the large number of voids at the interfaces, the swelling is higher in composites containing no bonding agent. Hence an increase in the values of V,,/V,, is seen for Mixes E, G and C.

Figure 12 shows photographs of samples of Mixes A, C and L swollen in toluene. Mixes A and C exhibit very high swelling in comparison with Mix L. Because of the poor bonding between fibre and matrix, composite Mix C swells both in diameter and thickness just like the gum compound (Mix A). In the case of Mix L, the solvent penetrates into the polymer only through the thickness because of the strong interface created by the bonding agent. It should be noted that, although the samples were all circular before swelling, that for Mix L was no longer circular after swelling, although the opposing surfaces remained planar and parallel. Sample

Fig. 12. Photograph showing the swelling behaviour of Mixes A, C and L.



L became oval due to preferential swelling in the thickness direction.39

4 CONCLUSIONS

The tear failure and processing behaviour of short sisal fibre reinforced styrene butadiene rubber composites have been evaluated with reference to the effect of fibre length, fibre orientation, fibre loading and bonding agent. Sisal fibres of 6 mm fibre length were found to be the most effective for maximum reinforcement of the SBR matrix. Increase in fibre content increased the tear strength in the longitudinal direction more than in the transverse direction. The maximum tear strength was shown by the mix containing bonding agent. SEM studies of the tear fracture fronts indicated fibre orientation, fibre pull-out and fibre-matrix adhesion. The SEM studies also gave an insight into the mechanism of failure.

The incorporation of short sisal fibres into the SBR matrix greatly influenced the processing characteristics of the composite by increasing plastic deformation and decreasing rubbery elastic deformation. Better processing characteristics were shown by Mix L because of the presence of bonding agent. Kraus and Lorenz & Parks equations were used to investigate the fibrerubber interactions. The maximum restriction to swelling was shown by Mix L containing 35 phr of fibres and bonding agent, This has been explained by increased interaction between fibre and rubber because of the presence of bonding agent. A hypothetical reaction scheme has been suggested to explain the mechanism of action of the bonding agent. Photographs of swollen samples revealed that swelling occurred only in the thickness direction for Mix L because of high fibrerubber adhesion.

ACKNOWLEDGEMENTS

R.P.K. is grateful to Mr N. T. Madhu for his assistance in reading and correcting this manuscript. His thanks are due to Dr C. V. Asokan, Lecturer at this university, for his valuable suggestions. He is also grateful to Mr T. Ouseph, Deputy Secretary, Rubber Board, Kottayam, for his valuable help in carrying out this research.

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Tear and processing behaviour of short sisal fibre reinforced styrene butadiene rubber composites

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