- Email: [email protected]
Oil palm wood flour reinforced epoxidized natural rubber composites: The effect of filler content and size
Eur. Polym. 1. Vol. 33, No. 10-12, pp. 1627-1632, 1997 0 1997 Elsevier Science Ltd. All rights reserved
Printed in &at
Britain
0014-3057/97$17.00 + 0.00
PII: SOO14-3057(!97)00020-7
OIL PALM WOOD FLOUR REINFORCED EPOXIDIZED NATURAL RUBBER COMPOSITES: THE EFFECT OF FILLER CONTENT AND SIZE HANAFI
ISMAIL,* H. D. ROZMAN, R. M. JAFFRI and Z. A. MOHD ISHAK School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia (Received 14 August 1996; accepted in final form 23 October 1996)
Abstract-The effect of filler content and size on curing characteristics and mechanical properties of oil palm wood flour (OPWF) reinforced epoxidized natural rubber (ENR) composites has been studied. The
cure (rm)and scorch times of all filler size decreasewith increasing OPWF content. At any filler content, larger OPWF particle size show shorter tw and scorch time. The torque values increase with an increase in fibre content and OPWF with smallestparticle size showsthe highest torque. Increasing OPWF content in ENR compound resulted in reduction of tensile strength and elongation at break but increased tensile modulus, tear strength and hardness. Again, the composites filled with smaller OPWF size showed higher tensile strength, tensile modulus and tear strength. 0 1997 Elsevier Science Ltd
INTRODUCTION
Natural and synthetic rubbers in vulcanized form are used to prepare various rubber products such as mechanical goods, hoses, soles, V-belts, seals, gaskets, tyre treads, etc. In the vulcanization of rubber, carbon black and silica are the main fillers used in the compounding recipes. They are normally used up to 50 phr. In spite of being well known for their capabilities as reinforcements, i.e. they impart strength and stiffness to the vulcanized rubber, these fillers are relatively expensive. Therefore, considerable R & D efforts are being carried out to investigate the possibility of replacing these fillers with the target to reduce the cost of rubber products while maintaining their desired properties. The use of lignocellulosic material in composites especially wood appears to gain importance, particularly for low cost/high volume applications. This development has occurred because lignocellulosic-derived fillers have several advantages compared with inorganic fillers, i.e. lower density, greater deformability, less abrasiveness to equipments and, of course, lower cost. Moreover, lignocellulosic based fillers are derived from a renewable resource. Xanthos [1] studied the effects of processing conditions on the mechanical properties of injection moulded polypropylene composites containing 30 and 40% wood flour. He obtained comparable properties to those of the talc and calcium carbonate filled polypropylene. The use of wood fibres as reinforcement in medium density polyethylene (MDPE) was reported by Raj et al. [2]. Myers et al. [3] investigated the influence of maleated polypropylene and composition variables on the properties of wood flour/polypropylene composites. Zaini et al. [4] reported the effect of filler content and size on the *To whom all correspondence should be addressed.
mechanical properties of polypropylene/oil palm wood flour composites. They found that the composites filled with larger sized filler show higher modulus, tensile and impact strength, particularly at high liller content. Other studies carried out on wood-based fillers have been reported by a number of workers [5-6]. As indicated from the various literatures cited above, most of the research works involving biocomposites have been devoted on the combination of lignocellulosic fillers and/or fibres with both amorphous and semi-crystalline thermoplastics matrices. To date, not many studies on the mechanical properties of lignocellulose filled rubbers appear to have been published. Thus, it is the aim of this contribution to assess the potential utilization of lignocellulose filler in rubber composites. In the present study, a relatively new type of wood-based filler is investigated. The filler used is derived from oil palm tree (Hais guineensis) component, called, empty fruit bunch (EFB). This material is the by-product of palm oil industry and consists of about 65% of holocellulose and 25% of lignin [7]. The effect of filler content and size on the curing characteristics and mechanical properties of oil palm wood flour (OPWF) filled epoxidized natural rubber (ENR) composites will be reported. Scanning electron microscopy (SEM) studies are carried out to determine the failure mechanism of the composites. EXPERIMENTAL
Compounding ingredients and formulation
Table 1 shows the formulation used in this study. Epoxidized natural rubber (grade ENR 50) was purchased from Kumpulan Guthrie Sdn. Bhd. Oil palm empty fruit bunch (EFB) in fibrous form was obtained from Sabutek, Sdn. Bhd., Teluk Intan, Perak, Malaysia. EFB fibres were ground into three sizes, 270-500, 180-270 and 75-180 pm. Other chemicals such as sulphur, zinc oxide, stearic acid and n-cyclohexylbenthiazyl sulphenamide (CBS), were all
1627
1628
H. Ismail et al. Table 1. Typical formulations
Materials
% (phr)
ENR 50 Stearic acid Zinc oxide Flectol H CBS Sulphur Fillers”
100 2.0 5.0 1.0 0.5 2.5 0, 15, 30, 40, 50
*
aThree different sizes were used: 27G-500, 18G-270 and 75-180 pm.
r,
0 270-5OOpm + 180-270pm * 7%180pm
2 4g 2-
purchased from Bayer(M) Ltd. The Flectol H (poly-1,2-dihydro-2,2,4&methylquinoline) was supplied by Monsanto Company. All materials were used as supplied.
0
Fig.
I
of tensile,
tear strength
and hardness
The various rubber compounds were compression moulded at 140°C according to their respective tw, into sheets. All samples were tested in the mill direction in the measurement of mechanical properties. Dumb-bell and crescent test pieces according to IS0 37 and IS0 34, respectively, were then cut out. Tensile and tear tests were carried out on Monsanto Tensometer, TIO according to BS 903: Part A2 and BS 903: Part A3, respectively at SOOmm/min cross-head speed. The test for hardness was carried out by using the Shore type A Durometer according to ASTM 2240. All tests were conducted at room temperature (25°C).
Scanning
I
I
40
50
Table 2. Cure time (tug), scorch time and maximum torque of OPWF-filledENR vulcanizateswith particle sizes 27&5oO~m 0
15
30
40
50
13.59 3.54
10.16 1.52
10.14 I .48
10.03 1.40
9.30 1.32
8.43
10.41
11.84
12.65
14.55
electron
microscopy
Examination of the fracture surface was carried out using a scanning electron microscope (SEM) model Leica Cambridge S-360. The objective was to obtain some idea on the mode of fracture. The fracture ends of the tensile specimens were mounted on aluminium stubs and sputter coated with a thin layer of gold to avoid electrical charging during examination. Rubber-fibre
interactions
Lorenz and Parks equation [8] has been applied to study the rubber-fibre interaction. According to this equation:
where Q is defined as grams of solvent per gram of hydrocarbon and is calculated by Q=
tw (min) Scorch time (min) Maximum torque (dNm)
I
strength of OPWF-filled ENR vulcanizates.
Mixing was carried out on a laboratory size (160 x 320mm) two-roll mixing mill (Model XK-160) in accordance to the method described by ASTM D 3184-80. The respective cure times at 140°C as measured by tw were then determined using a Monsanto Rheometer, model MDR 2000. The scorch times, torque, elastic modulus, etc. were also determined from the rheograph.
% Filler (phr)
I
20 30 Filler content (phr)
I. The effect of filler content and particle size on tensile
Sample preparation
Measurement
I
10
swollen weight - dried weight original weight x lOO/formnla weight’
(2)
The subscripts f and g in equation (1) 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 Qf/Q, values, the lower will be the extent of interaction between the fibre and the matrix.
Table 3. Cure time (r~), scorch time and maximum torque of OPWF-filled ENR vulcanizates with particle sizes 180-270 pm % Filler (phr) rw (min) Scorch time (min) Maximum torque (dNm)
0
15
30
40
50
13.59 3.54
10.51 2.42
10.20 2.22
10.15 2.20
10.08 2.15
8.43
10.77
12.87
13.79
17.26
Table 4. Cure time (r90), scorch time and maximum torque of OPWF-filled ENR vulcanizates with particle sizes 75-180 pm % Filler (phr) fm (min) Scorch time (min) Maximum torque (dNm)
0
15
30
40
50
13.59 3.54
10.57 2.53
10.48 2.28
10.39 2.23
11.25 2.16
8.43
10.80
13.59
16.04
19.26
*75-180pm 2.01 0
I 10
I I 20 30 Filler content (phr)
I 40
I 50
Fig. 2. The effect of filler content and particle size on M300 of OPWF-filled ENR vulcanizates.
1629
The effect of filler content and size 80 r
4
30
t 20-
4301 0
I 10
I 20
I 30
I 40
I 50
Fig. 3. Relationship between elongation at break, filler content and particle size of OPWF-filled ENR vulcanizates. AND DISCUSSION
Curing characteristics
Tables 2-4 summarize the value of tw, scorch time and maximum torque of OPWF-ENR composites with different particle sizes. The tw and scorch time of all particle sizes decrease with increasing OPWF content, i.e. it shows enhancement in cure rate. However, at any filler content, OPWF with larger particle size show shorter tw and scorch time. The cure enhancement can be associated to the filler-related parameters such as surface area, surface reactivity, particle size, moisture content and metal oxide content. In general, a faster cure rate is obtained with fillers having a low surface area, high moisture content and high metal oxide content [9]. Butler and Freakley [lo] found that cure rate is directly related to the humidity and water content of the rubber compound. However, in the present study, the cure enhancement of larger particle size of OPWF is probably due to its lower surface area. Tables 2-4 also indicate that the addition of OPWF increases the torque value of the ENR compound. The increment in the torque with fibre content is due to the presence of more fibres which impart more restriction to the deformation and consequently increase the OPWF-ENR composites stiffness [1 11. The maximum torque of all the 30
r
3
2
25-
c
+ 180-270um ff 75- 180nm 15
0
I 10
I 20
I 30
I 40
I 50
Filler content (phr) Fig.
4. The
0
I 10
I 20
I 30
I 40
I 50
Filler content (phr)
Filler content (phr)
RESULT!3
10
0 270~5OOnm + 180-270bm * 75-180um
effect of filler content and particle size on tear strength of OPWF-filled ENR vulcanizates.
Fig. 5. The effect of filler content and particle size on hardness of OPWF-filled ENR vulcanizates. vulcanizates shows that OPWF with smallest particle size has the highest torque. It seems from the results that the trend is affected by the particle size of the fillers. According to Ishak and Bakar [ 121the smaller the particle size, hence the larger surface area, the greater the interaction between the filler and rubber matrix. Thus, a higher restriction to molecular motion of the macromolecules is expected. In other words, the addition of fillers of a smaller size tends to impose extra resistance to flow. Mechanical properties
Figure 1 shows the effect of filler content and particle size on tensile strength of OPWF-ENR composites. It can be seen that the tensile strength of the composites decreases steadily with filler content. This rather poor strength properties may be attributed to the geometry of the OPWF fillers. For irregularly shaped fillers, the strength of the composites decreases due to the inability of the filler to support stresses transferred from the polymer matrix. The presence of such fillers is evident from our SEM study that will be dealt with in the forthcoming section. The results obtained are in agreement with the commonly observed data in other particulate filler systems. The smaller particle size filler exhibits higher tensile strength especially at content below 30 phr. The better filler dispersion and filler-matrix interaction may be the two main factors responsible for the observed trend. Similar observations have been reported by Bigg [13] and Fuad et al. [14] for other filled composites. Figure 2 shows the modulus at 300% elongation (M300) of the OPWF-ENR composites increases steadily with increasing filler content. This observation highlights the fact that the incorporation of fillers into the rubber matrix can improve the stiffness of the composites. As expected, the smaller-sized fillers gave higher M300 values than larger filler size [15-161. Murty et al. [17] found that modulus value increases when the fibre concentration is increased for natural rubber-jute, SBR-jute, SBR-glass and natural rubber-glass composites. As can be seen from Fig. 3, the elongation at break, EB drops continuously with the addition of OPWF filler into ENR compound. It is interesting to note
1630
H. lsmail it al
Fig. 6(a)
that the EB of the smaller filler size composites is higher than the larger one up to a filler content of 30 phr. At 50 phr, the EB of all composites converge to more or less similar values, irrespective of filler sizes. This trend may be attributed to two main factors. Firstly, below 30 phr, the better filler dispersion of the smaller OPWF fillers in the ENR 50 matrix reduces the tendency of filler-filler interactions from taking place. Thus, the samples can be elongated to a much higher value. However, at the highest filler content, i.e. 50 phr, the degree of filler-filler interactions became more prominent even in the case of the smallest filler size composites. Consequently, dramatic reduction in EB was observed. Secondly, there is another related factor. i.e. the interface that needs to be considered. The increment in filler content will eventual resulted in the
reduction of the deformability of a rigid interface between the filler and the rubber matrix 141. Figure 4 shows the effect of filler content and particle size on tear strength of the composites. It can be seen that the tear strength increases with increasing filler content. The smallest filler, i.e. particle size of 75-180 pm, has a slightly higher tear strength than the bigger filler. The increment in tear strength may be due to the hinderance imposed by the short fibre to the tear paths, thus making crack propagation more difficult [ 181.Akhtar et al. [ 191also found that the tear strength increased with increasing fibre content in their work with silk fibre reinforced polyethylene-rubber blends. A similar trend is also observed in the case of hardness (Fig. 5). As the filler content increases the composites became stiffer and harder. Thus, an
Fig. 6(b)
The effect of filler content and size
1631
Fig. 6(c) increase in the hardness with increasing filler content was obtained. Chakraborty et al. [20] have obtained similar results. As expected, at any fibre content, there was no significant effect of filler size on hardness.
composites at three different filler contents and 180-270 pm particle size are shown in Fig. 6. Figure 6(a) provides a clear indication for the occurrences of fibre bundles pull-out in the case of composite with the lowest filler content. The bundle of diameter N 50 pm can be seen on the crack plane. The presence Fractography and failure mechanisms of such fibre bundles is expected to give poor It is known that for short fibre reinforced contribution to the strength of the ENR vulcanizates. composites, the failure mechanisms are dominated by Perhaps significant improvements in strength fibre debonding and fibre pull-out processes [l 11. property can be obtained if the OPWF fibres are to Some qualitative evidences for the failure mechanbe dispersed uniformly as fines fibres (i.e. high fibre isms can perhaps be obtained from the fractography aspect ratio) in the matrix. study using the scanning electron microscopy (SEM) Several studies involving commercial fibres [21] technique. The SEM micrographs for OPWF-ENR have indicated the enhancement in mechanical
Fig. 6(d) Fig. 6. SEM micrograph of OPWF-filled ENR vulcanizates after tensile fracture at various filler content (particle size: 180-270 pm). (a) 15 phr (mag. 150 x ); (b) 30 phr (mag. 150 x ); (c) 50 phr (mag. 150 x ); (d) 50 phr (mag. 50 x ).
H. Ismail er al.
1632 1.2 r
0
IO
20
30
40
50
Filler content (phr) Fig. 7. The effect of tiller content and particle size on Qr/Qs of OPWF-filled ENR vulcanizates.
properties of short-fibre composites can be achieved through good dispersion of fibres in the matrix. In the present context, it is believed that the polarity of lignocellulosive OPWF tillers is a strong factor which influence the agglomeration of the fillers into bundles. As expected, the dispersion and wettability of the OPWF fillers worsen with increasing filler content [Figs 6(b) and (c)l. The poor bonding quality between the constituent phases has resulted in serious fibre bundle pull-out, as indicated by formation of large holes (5 3C70 pm in diameter) on the crack plane. This mode of failure explains why the tensile strength of the composites decreases steadily with filler content. The SEM micrograph shown in Fig. 6(d) provides further direct evidence for the presence of irregular-shaped OPWF fillers, which give rise to the poor strength properties of the composites. Rubber-Jibre
interactions
Figure 7 shows the relationship between Q/Q8 and filler content in the OPWF-ENR composites. It can be seen that irrespective of particle size of fillers, Qt/Qs values show increasing trend with filler content. Since the higher the Qr/Q# values, the lower will be the extent of interaction between the fibre and the matrix, it is clear that the increasing filler content has weakened the rubber-fibre interactions. This result supports the proposed mode of failure as discussed in a previous section (SEM). However, at any fibre content, the smallest particle size of tiller shows the lowest Qr/Q, values, which mean better rubber-tibre interactions. This result thus compliments our earlier observation that OPWF-ENR composites with the smallest particle size show the best overall mechanical properties, i.e. tensile strength, tensile modulus and tear strength than larger particle size. CONCLUSIONS
1. There is a cure enhancement of OPWF-ENR composites with increasing filler content. At any tiler
content, OPWF with larger particles size show shorter cure and scorch times. 2. The maximum torque increase with increasing fibre content and OPWF with smallest particles size show the highest torque values. 3. Addition of OPWF in ENR compounds increases tear strength, modulus and hardness, but decreases tensile strength and elongation at break. Smaller particle size of OPWF gives better mechanical properties. 4. The SEM micrographs show the occurrences of fibre bundles pull-out in the OPWF-ENR composite. As the filler content increases the dispersion and wettability of the composite worsen and consequently the strength of composite decreases steadily. Acknowledgements-The authors wish to express their thanks to Universiti Sains Malaysia (USM) and Sabutek Sdn. Bhd. (Malaysia) for their financial support.
REFERENCES I. Xantos, M.. Plast. Rubber Process. Appl., 1983, 3, 223. 2. Raj, R. G., Kokta, B. V., Groleau, G. and Daneault, C., PI&t. Rubber Process. Appl., 1989, 11, 215.
3. Mvres. G. E.. Chahvadi. I. S.. Coberlv. C. A. and Eker; D. S., mr. J.Polym. Mater., 199i, 15, 21. 4. Zaini, M. J., Fuad, M. Y. A., Ismail, Z., Mansor, M. S. and Mustafah, J., Polym. Int., 1996, 40, 51. 5. Yamaguchi, Y. and Nakayama, Y., US Patent 3, 704, 161, 1972. 6. Maldas, D. and Kokta. B. V., Comp. Interfaces, 1993, 1, 87. 7. Husin, M., Zakaria, Z. Z. and Hassan, A. H., Workshop Proc. Palm Oil Res. Inst. Malaysia, 1987, 11, 7. 8. Lorenz, 0. and Parks, C. R., J. Polym. Sci., 1961, SO, 299. 9. Wagner, M. P., Rubb. Chem. Technot., 1976, 49, 704. 10. Butler, J. and Freakley, P. K., Rubb. Chem. Technol., 1991, 65, 374.
11. Prasanta Kumar, R., Geethakumari Amma, M. L. and Thomas, S., J. Appl. Polym. Sci., 1995, Ss, 597. 12. Ishak, Z. A. M. and Bakar, A. A., Eur. Polym. J., 1995, 31, 259. 13. Bigg, D. M., Polym. Comp., 1987, 8, 115. 14. Fuad. M. Y. A.. Ismail. Z.. Ishak. Z. A. M. and Omar, A. K:, Eur. Poiym. J.,. 1995, 31,.885. 15. Leidner, J. and Woodhams, R. T., J. Appl. Polym. Sci., 1974, 18, 1639. 16. Maiti, S. N. and Lopez, B. H., J. Appl. Polym. Sci., 1992, 44, 353. 17. Murty, V. M. and De, S. K., Rubb. Chem. Technol., 1982, 55, 287. 18. Prasantha Kumar, R. and Thomas, S., Polym. Inr., 1995, 38, 173. 19. Akhtar, S., De, P. P. and De, S. K., J. Appl. Polym. Sci., 1986, 32, 5123. 20. Chakraborty, S. K., Setua, K. K. and De, S. K., Rubb. Chem. Technol., 1982,55, 1286. 21. Folkes, M. J., Short Ftbre R&forced Thermoplastics.
Research Studies Press, Whey, Chichester, 1982.
Printed in &at
Britain
0014-3057/97$17.00 + 0.00
PII: SOO14-3057(!97)00020-7
OIL PALM WOOD FLOUR REINFORCED EPOXIDIZED NATURAL RUBBER COMPOSITES: THE EFFECT OF FILLER CONTENT AND SIZE HANAFI
ISMAIL,* H. D. ROZMAN, R. M. JAFFRI and Z. A. MOHD ISHAK School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia (Received 14 August 1996; accepted in final form 23 October 1996)
Abstract-The effect of filler content and size on curing characteristics and mechanical properties of oil palm wood flour (OPWF) reinforced epoxidized natural rubber (ENR) composites has been studied. The
cure (rm)and scorch times of all filler size decreasewith increasing OPWF content. At any filler content, larger OPWF particle size show shorter tw and scorch time. The torque values increase with an increase in fibre content and OPWF with smallestparticle size showsthe highest torque. Increasing OPWF content in ENR compound resulted in reduction of tensile strength and elongation at break but increased tensile modulus, tear strength and hardness. Again, the composites filled with smaller OPWF size showed higher tensile strength, tensile modulus and tear strength. 0 1997 Elsevier Science Ltd
INTRODUCTION
Natural and synthetic rubbers in vulcanized form are used to prepare various rubber products such as mechanical goods, hoses, soles, V-belts, seals, gaskets, tyre treads, etc. In the vulcanization of rubber, carbon black and silica are the main fillers used in the compounding recipes. They are normally used up to 50 phr. In spite of being well known for their capabilities as reinforcements, i.e. they impart strength and stiffness to the vulcanized rubber, these fillers are relatively expensive. Therefore, considerable R & D efforts are being carried out to investigate the possibility of replacing these fillers with the target to reduce the cost of rubber products while maintaining their desired properties. The use of lignocellulosic material in composites especially wood appears to gain importance, particularly for low cost/high volume applications. This development has occurred because lignocellulosic-derived fillers have several advantages compared with inorganic fillers, i.e. lower density, greater deformability, less abrasiveness to equipments and, of course, lower cost. Moreover, lignocellulosic based fillers are derived from a renewable resource. Xanthos [1] studied the effects of processing conditions on the mechanical properties of injection moulded polypropylene composites containing 30 and 40% wood flour. He obtained comparable properties to those of the talc and calcium carbonate filled polypropylene. The use of wood fibres as reinforcement in medium density polyethylene (MDPE) was reported by Raj et al. [2]. Myers et al. [3] investigated the influence of maleated polypropylene and composition variables on the properties of wood flour/polypropylene composites. Zaini et al. [4] reported the effect of filler content and size on the *To whom all correspondence should be addressed.
mechanical properties of polypropylene/oil palm wood flour composites. They found that the composites filled with larger sized filler show higher modulus, tensile and impact strength, particularly at high liller content. Other studies carried out on wood-based fillers have been reported by a number of workers [5-6]. As indicated from the various literatures cited above, most of the research works involving biocomposites have been devoted on the combination of lignocellulosic fillers and/or fibres with both amorphous and semi-crystalline thermoplastics matrices. To date, not many studies on the mechanical properties of lignocellulose filled rubbers appear to have been published. Thus, it is the aim of this contribution to assess the potential utilization of lignocellulose filler in rubber composites. In the present study, a relatively new type of wood-based filler is investigated. The filler used is derived from oil palm tree (Hais guineensis) component, called, empty fruit bunch (EFB). This material is the by-product of palm oil industry and consists of about 65% of holocellulose and 25% of lignin [7]. The effect of filler content and size on the curing characteristics and mechanical properties of oil palm wood flour (OPWF) filled epoxidized natural rubber (ENR) composites will be reported. Scanning electron microscopy (SEM) studies are carried out to determine the failure mechanism of the composites. EXPERIMENTAL
Compounding ingredients and formulation
Table 1 shows the formulation used in this study. Epoxidized natural rubber (grade ENR 50) was purchased from Kumpulan Guthrie Sdn. Bhd. Oil palm empty fruit bunch (EFB) in fibrous form was obtained from Sabutek, Sdn. Bhd., Teluk Intan, Perak, Malaysia. EFB fibres were ground into three sizes, 270-500, 180-270 and 75-180 pm. Other chemicals such as sulphur, zinc oxide, stearic acid and n-cyclohexylbenthiazyl sulphenamide (CBS), were all
1627
1628
H. Ismail et al. Table 1. Typical formulations
Materials
% (phr)
ENR 50 Stearic acid Zinc oxide Flectol H CBS Sulphur Fillers”
100 2.0 5.0 1.0 0.5 2.5 0, 15, 30, 40, 50
*
aThree different sizes were used: 27G-500, 18G-270 and 75-180 pm.
r,
0 270-5OOpm + 180-270pm * 7%180pm
2 4g 2-
purchased from Bayer(M) Ltd. The Flectol H (poly-1,2-dihydro-2,2,4&methylquinoline) was supplied by Monsanto Company. All materials were used as supplied.
0
Fig.
I
of tensile,
tear strength
and hardness
The various rubber compounds were compression moulded at 140°C according to their respective tw, into sheets. All samples were tested in the mill direction in the measurement of mechanical properties. Dumb-bell and crescent test pieces according to IS0 37 and IS0 34, respectively, were then cut out. Tensile and tear tests were carried out on Monsanto Tensometer, TIO according to BS 903: Part A2 and BS 903: Part A3, respectively at SOOmm/min cross-head speed. The test for hardness was carried out by using the Shore type A Durometer according to ASTM 2240. All tests were conducted at room temperature (25°C).
Scanning
I
I
40
50
Table 2. Cure time (tug), scorch time and maximum torque of OPWF-filledENR vulcanizateswith particle sizes 27&5oO~m 0
15
30
40
50
13.59 3.54
10.16 1.52
10.14 I .48
10.03 1.40
9.30 1.32
8.43
10.41
11.84
12.65
14.55
electron
microscopy
Examination of the fracture surface was carried out using a scanning electron microscope (SEM) model Leica Cambridge S-360. The objective was to obtain some idea on the mode of fracture. The fracture ends of the tensile specimens were mounted on aluminium stubs and sputter coated with a thin layer of gold to avoid electrical charging during examination. Rubber-fibre
interactions
Lorenz and Parks equation [8] has been applied to study the rubber-fibre interaction. According to this equation:
where Q is defined as grams of solvent per gram of hydrocarbon and is calculated by Q=
tw (min) Scorch time (min) Maximum torque (dNm)
I
strength of OPWF-filled ENR vulcanizates.
Mixing was carried out on a laboratory size (160 x 320mm) two-roll mixing mill (Model XK-160) in accordance to the method described by ASTM D 3184-80. The respective cure times at 140°C as measured by tw were then determined using a Monsanto Rheometer, model MDR 2000. The scorch times, torque, elastic modulus, etc. were also determined from the rheograph.
% Filler (phr)
I
20 30 Filler content (phr)
I. The effect of filler content and particle size on tensile
Sample preparation
Measurement
I
10
swollen weight - dried weight original weight x lOO/formnla weight’
(2)
The subscripts f and g in equation (1) 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 Qf/Q, values, the lower will be the extent of interaction between the fibre and the matrix.
Table 3. Cure time (r~), scorch time and maximum torque of OPWF-filled ENR vulcanizates with particle sizes 180-270 pm % Filler (phr) rw (min) Scorch time (min) Maximum torque (dNm)
0
15
30
40
50
13.59 3.54
10.51 2.42
10.20 2.22
10.15 2.20
10.08 2.15
8.43
10.77
12.87
13.79
17.26
Table 4. Cure time (r90), scorch time and maximum torque of OPWF-filled ENR vulcanizates with particle sizes 75-180 pm % Filler (phr) fm (min) Scorch time (min) Maximum torque (dNm)
0
15
30
40
50
13.59 3.54
10.57 2.53
10.48 2.28
10.39 2.23
11.25 2.16
8.43
10.80
13.59
16.04
19.26
*75-180pm 2.01 0
I 10
I I 20 30 Filler content (phr)
I 40
I 50
Fig. 2. The effect of filler content and particle size on M300 of OPWF-filled ENR vulcanizates.
1629
The effect of filler content and size 80 r
4
30
t 20-
4301 0
I 10
I 20
I 30
I 40
I 50
Fig. 3. Relationship between elongation at break, filler content and particle size of OPWF-filled ENR vulcanizates. AND DISCUSSION
Curing characteristics
Tables 2-4 summarize the value of tw, scorch time and maximum torque of OPWF-ENR composites with different particle sizes. The tw and scorch time of all particle sizes decrease with increasing OPWF content, i.e. it shows enhancement in cure rate. However, at any filler content, OPWF with larger particle size show shorter tw and scorch time. The cure enhancement can be associated to the filler-related parameters such as surface area, surface reactivity, particle size, moisture content and metal oxide content. In general, a faster cure rate is obtained with fillers having a low surface area, high moisture content and high metal oxide content [9]. Butler and Freakley [lo] found that cure rate is directly related to the humidity and water content of the rubber compound. However, in the present study, the cure enhancement of larger particle size of OPWF is probably due to its lower surface area. Tables 2-4 also indicate that the addition of OPWF increases the torque value of the ENR compound. The increment in the torque with fibre content is due to the presence of more fibres which impart more restriction to the deformation and consequently increase the OPWF-ENR composites stiffness [1 11. The maximum torque of all the 30
r
3
2
25-
c
+ 180-270um ff 75- 180nm 15
0
I 10
I 20
I 30
I 40
I 50
Filler content (phr) Fig.
4. The
0
I 10
I 20
I 30
I 40
I 50
Filler content (phr)
Filler content (phr)
RESULT!3
10
0 270~5OOnm + 180-270bm * 75-180um
effect of filler content and particle size on tear strength of OPWF-filled ENR vulcanizates.
Fig. 5. The effect of filler content and particle size on hardness of OPWF-filled ENR vulcanizates. vulcanizates shows that OPWF with smallest particle size has the highest torque. It seems from the results that the trend is affected by the particle size of the fillers. According to Ishak and Bakar [ 121the smaller the particle size, hence the larger surface area, the greater the interaction between the filler and rubber matrix. Thus, a higher restriction to molecular motion of the macromolecules is expected. In other words, the addition of fillers of a smaller size tends to impose extra resistance to flow. Mechanical properties
Figure 1 shows the effect of filler content and particle size on tensile strength of OPWF-ENR composites. It can be seen that the tensile strength of the composites decreases steadily with filler content. This rather poor strength properties may be attributed to the geometry of the OPWF fillers. For irregularly shaped fillers, the strength of the composites decreases due to the inability of the filler to support stresses transferred from the polymer matrix. The presence of such fillers is evident from our SEM study that will be dealt with in the forthcoming section. The results obtained are in agreement with the commonly observed data in other particulate filler systems. The smaller particle size filler exhibits higher tensile strength especially at content below 30 phr. The better filler dispersion and filler-matrix interaction may be the two main factors responsible for the observed trend. Similar observations have been reported by Bigg [13] and Fuad et al. [14] for other filled composites. Figure 2 shows the modulus at 300% elongation (M300) of the OPWF-ENR composites increases steadily with increasing filler content. This observation highlights the fact that the incorporation of fillers into the rubber matrix can improve the stiffness of the composites. As expected, the smaller-sized fillers gave higher M300 values than larger filler size [15-161. Murty et al. [17] found that modulus value increases when the fibre concentration is increased for natural rubber-jute, SBR-jute, SBR-glass and natural rubber-glass composites. As can be seen from Fig. 3, the elongation at break, EB drops continuously with the addition of OPWF filler into ENR compound. It is interesting to note
1630
H. lsmail it al
Fig. 6(a)
that the EB of the smaller filler size composites is higher than the larger one up to a filler content of 30 phr. At 50 phr, the EB of all composites converge to more or less similar values, irrespective of filler sizes. This trend may be attributed to two main factors. Firstly, below 30 phr, the better filler dispersion of the smaller OPWF fillers in the ENR 50 matrix reduces the tendency of filler-filler interactions from taking place. Thus, the samples can be elongated to a much higher value. However, at the highest filler content, i.e. 50 phr, the degree of filler-filler interactions became more prominent even in the case of the smallest filler size composites. Consequently, dramatic reduction in EB was observed. Secondly, there is another related factor. i.e. the interface that needs to be considered. The increment in filler content will eventual resulted in the
reduction of the deformability of a rigid interface between the filler and the rubber matrix 141. Figure 4 shows the effect of filler content and particle size on tear strength of the composites. It can be seen that the tear strength increases with increasing filler content. The smallest filler, i.e. particle size of 75-180 pm, has a slightly higher tear strength than the bigger filler. The increment in tear strength may be due to the hinderance imposed by the short fibre to the tear paths, thus making crack propagation more difficult [ 181.Akhtar et al. [ 191also found that the tear strength increased with increasing fibre content in their work with silk fibre reinforced polyethylene-rubber blends. A similar trend is also observed in the case of hardness (Fig. 5). As the filler content increases the composites became stiffer and harder. Thus, an
Fig. 6(b)
The effect of filler content and size
1631
Fig. 6(c) increase in the hardness with increasing filler content was obtained. Chakraborty et al. [20] have obtained similar results. As expected, at any fibre content, there was no significant effect of filler size on hardness.
composites at three different filler contents and 180-270 pm particle size are shown in Fig. 6. Figure 6(a) provides a clear indication for the occurrences of fibre bundles pull-out in the case of composite with the lowest filler content. The bundle of diameter N 50 pm can be seen on the crack plane. The presence Fractography and failure mechanisms of such fibre bundles is expected to give poor It is known that for short fibre reinforced contribution to the strength of the ENR vulcanizates. composites, the failure mechanisms are dominated by Perhaps significant improvements in strength fibre debonding and fibre pull-out processes [l 11. property can be obtained if the OPWF fibres are to Some qualitative evidences for the failure mechanbe dispersed uniformly as fines fibres (i.e. high fibre isms can perhaps be obtained from the fractography aspect ratio) in the matrix. study using the scanning electron microscopy (SEM) Several studies involving commercial fibres [21] technique. The SEM micrographs for OPWF-ENR have indicated the enhancement in mechanical
Fig. 6(d) Fig. 6. SEM micrograph of OPWF-filled ENR vulcanizates after tensile fracture at various filler content (particle size: 180-270 pm). (a) 15 phr (mag. 150 x ); (b) 30 phr (mag. 150 x ); (c) 50 phr (mag. 150 x ); (d) 50 phr (mag. 50 x ).
H. Ismail er al.
1632 1.2 r
0
IO
20
30
40
50
Filler content (phr) Fig. 7. The effect of tiller content and particle size on Qr/Qs of OPWF-filled ENR vulcanizates.
properties of short-fibre composites can be achieved through good dispersion of fibres in the matrix. In the present context, it is believed that the polarity of lignocellulosive OPWF tillers is a strong factor which influence the agglomeration of the fillers into bundles. As expected, the dispersion and wettability of the OPWF fillers worsen with increasing filler content [Figs 6(b) and (c)l. The poor bonding quality between the constituent phases has resulted in serious fibre bundle pull-out, as indicated by formation of large holes (5 3C70 pm in diameter) on the crack plane. This mode of failure explains why the tensile strength of the composites decreases steadily with filler content. The SEM micrograph shown in Fig. 6(d) provides further direct evidence for the presence of irregular-shaped OPWF fillers, which give rise to the poor strength properties of the composites. Rubber-Jibre
interactions
Figure 7 shows the relationship between Q/Q8 and filler content in the OPWF-ENR composites. It can be seen that irrespective of particle size of fillers, Qt/Qs values show increasing trend with filler content. Since the higher the Qr/Q# values, the lower will be the extent of interaction between the fibre and the matrix, it is clear that the increasing filler content has weakened the rubber-fibre interactions. This result supports the proposed mode of failure as discussed in a previous section (SEM). However, at any fibre content, the smallest particle size of tiller shows the lowest Qr/Q, values, which mean better rubber-tibre interactions. This result thus compliments our earlier observation that OPWF-ENR composites with the smallest particle size show the best overall mechanical properties, i.e. tensile strength, tensile modulus and tear strength than larger particle size. CONCLUSIONS
1. There is a cure enhancement of OPWF-ENR composites with increasing filler content. At any tiler
content, OPWF with larger particles size show shorter cure and scorch times. 2. The maximum torque increase with increasing fibre content and OPWF with smallest particles size show the highest torque values. 3. Addition of OPWF in ENR compounds increases tear strength, modulus and hardness, but decreases tensile strength and elongation at break. Smaller particle size of OPWF gives better mechanical properties. 4. The SEM micrographs show the occurrences of fibre bundles pull-out in the OPWF-ENR composite. As the filler content increases the dispersion and wettability of the composite worsen and consequently the strength of composite decreases steadily. Acknowledgements-The authors wish to express their thanks to Universiti Sains Malaysia (USM) and Sabutek Sdn. Bhd. (Malaysia) for their financial support.
REFERENCES I. Xantos, M.. Plast. Rubber Process. Appl., 1983, 3, 223. 2. Raj, R. G., Kokta, B. V., Groleau, G. and Daneault, C., PI&t. Rubber Process. Appl., 1989, 11, 215.
3. Mvres. G. E.. Chahvadi. I. S.. Coberlv. C. A. and Eker; D. S., mr. J.Polym. Mater., 199i, 15, 21. 4. Zaini, M. J., Fuad, M. Y. A., Ismail, Z., Mansor, M. S. and Mustafah, J., Polym. Int., 1996, 40, 51. 5. Yamaguchi, Y. and Nakayama, Y., US Patent 3, 704, 161, 1972. 6. Maldas, D. and Kokta. B. V., Comp. Interfaces, 1993, 1, 87. 7. Husin, M., Zakaria, Z. Z. and Hassan, A. H., Workshop Proc. Palm Oil Res. Inst. Malaysia, 1987, 11, 7. 8. Lorenz, 0. and Parks, C. R., J. Polym. Sci., 1961, SO, 299. 9. Wagner, M. P., Rubb. Chem. Technot., 1976, 49, 704. 10. Butler, J. and Freakley, P. K., Rubb. Chem. Technol., 1991, 65, 374.
11. Prasanta Kumar, R., Geethakumari Amma, M. L. and Thomas, S., J. Appl. Polym. Sci., 1995, Ss, 597. 12. Ishak, Z. A. M. and Bakar, A. A., Eur. Polym. J., 1995, 31, 259. 13. Bigg, D. M., Polym. Comp., 1987, 8, 115. 14. Fuad. M. Y. A.. Ismail. Z.. Ishak. Z. A. M. and Omar, A. K:, Eur. Poiym. J.,. 1995, 31,.885. 15. Leidner, J. and Woodhams, R. T., J. Appl. Polym. Sci., 1974, 18, 1639. 16. Maiti, S. N. and Lopez, B. H., J. Appl. Polym. Sci., 1992, 44, 353. 17. Murty, V. M. and De, S. K., Rubb. Chem. Technol., 1982, 55, 287. 18. Prasantha Kumar, R. and Thomas, S., Polym. Inr., 1995, 38, 173. 19. Akhtar, S., De, P. P. and De, S. K., J. Appl. Polym. Sci., 1986, 32, 5123. 20. Chakraborty, S. K., Setua, K. K. and De, S. K., Rubb. Chem. Technol., 1982,55, 1286. 21. Folkes, M. J., Short Ftbre R&forced Thermoplastics.
Research Studies Press, Whey, Chichester, 1982.