Cure Characteristics and Mechanical Properties of Butadiene Rubber/Whole Tyre Reclaimed Rubber Blends P.A. Nelson and S.K.N. Kutty* Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin.682022, India. Fax: -91-484-542747 E mail: Sunil @ cusat.ac.in Received: 16 January 2002 Accepted: 9 April 2002 ABSTRACT Cure characteristics and mechanical properties of Butadiene rubber/ reclaimed rubber blends were studied. Minimum torque, scorch time, and cure time decreased with the increase in reclaim content. Tensile strength, tear resistance, ultimate elongation, cure rate and compression set increased where as resilience and abrasion loss decreased with reclaim loading. The (maximum minimum) torque was reduced at higher loading of reclaim. Heat build up was higher for the blends containing higher amounts of reclaimed rubber. Ageing resistance of the blends was inferior to that of the gum compounds. INTRODUCTION The uncontrolled exploitation of natural resources has caused much damage to the environment in terms of solid and air pollution - the major pollutant being the products from industries and transport vehicles. Used tyre constitutes one group of materials that pose great threat to environment, as they are hardly degradable by biological method. One effective method is to recycle them into reclaimed rubber, which can be reused as an economic source of rubber hydrocarbon. These can be blended with virgin rubbers for many practical purposes. Reclaiming of scrap rubber by mechanical (1,2) and chemical process (3,4) has received much attention. One process involves thermo mechanical degradation of a rubber vulcanizate network (5). Another process is *To whom all correspondence should be sent 85
P.A. Nelson and S.K.N. Kutty cryomechanical comminution (6-9). The chemical process for reclaiming rubber includes devulcanization or destructive distillation (10). Crane and Kay (11) have reported that scrap rubber vulcanizate could be depolymerised to a product known as Depolymerised Scrap Rubber (DSR), which could be useful as a rubber compounding ingredient and as a fuel oil extender. It has been reported that rubber powder can mix with virgin rubber (12,13), but there was significant drop in the tensile strength at the lower levels of the addition of scrap rubber (14,15). An improvement of the method could be achieved by a compounding the tyre scrap with new rubber and sulfur and subsequent vulcanization of the compound (16-18). Phadke et al. (19) have reported that reclaimed rubber vulcanizate were inferior to those of the control mixes. They also exhibited poor processing characteristics and physical properties, which could be improved by blending with fresh rubber. Sreeja and Kutty have reported the use of reclaim in SBR, NBR and NR (20). However a systematic study of the blend of reclaimed rubber (WTR) and butadiene rubber (BR) has not been reported so far. Butadiene rubber is a general-purpose rubber widely used in tyres and belts. Partial replacement of virgin BR with reclaimed rubber without loosing its mechanical properties can reduce the cost. In this study the cure characteristics and the mechanical properties of BR/WTR blends are reported. EXPERIMENTAL Materials Used Butadiene rubber, CISAMER 220 was obtained from Indian Petrochemical Ltd., Baroda and its Mooney viscosity ML (1+4) was 45. Reclaim rubber, WTR, was obtained from Kerala Rubber and Reclaims, Mamala, Kerala, India. The characteristics of WTR used are given in the Table 1. Zinc oxide, stearic acid, mercaptobenzothiazoledisulphide (MBTS), tetramethylthiuramdisulphide (TMTD), and sulphur were obtained from Sameera Enterprises Kottayam. Antioxidant 4020 i.e. [N (1,3,dimethyl butyl)n phenyl p-phenylene diamine] was obtained from Bayer India Ltd. Processing Formulation of the mixes is given in the Table 2. The mixes were prepared on a laboratory size two roll (150x 330 mm) mill as per ASTM 86
Table 1 Characteristics of reclaim rubber Property Value Acetone extract (%) 15 Carbon black (%) 30 Gel content (%) 68 M ooney Viscosity [ML (1+4)] at 100 C 24 Particle size 30 mesh Table 2 Formulations of the mixes I ngredients Mix. No. A B C D E F BR 100 90 80 70 60 50 Reclaim rubber - 20 (22)* 40 (50)* 60 (85)* 80 (133)* 100 (200)* Note: BR = Butadiene rubber, ZnO - 5 phr, Stearic acid - 2 phr, 4020-1 phr, MBTS - 0.8 phr, TMTD - 0.4 phr, Sulphur - 2 phr are common to all mixes *Amount of reclaim rubber as percentage of BR D 3182(1989). As the WTR contains 50% rubber hydrocarbon only, for every 10 parts of virgin BR replaced by 20 parts of WTR are to be added as formulations is based on parts per hundred rubber (phr). The corresponding percentage values of WTR with respect to BR is also indicated in the table. Cure characteristics were determined by using a Goettfert Elastograph model 67.85 at 150 o C. Vulcanization was carried out at 150 o C under a pressure of 180 kg/cm 2 in an electrically heated hydraulic press. For thicker samples, sufficient extra cure time was given so as to get the same extent of cure. The different mechanical properties of the vulcanizate were tested according to ASTM standards. Tensile and Tear properties were measured using a tensile tester from Lloyd Instruments, LRX PLUS, according to ASTM D 412. The tear strength was measured as per ASTM 87
P.A. Nelson and S.K.N. Kutty D 624 (die C). The Abrasion resistance of the blend was measured using a DIN abrader as per DIN 53516 and values were expressed as volume loss per hour. Compression set at constant strain was measured according to ASTM D 395-86 method B. The compression set was carried out at 70 o C As per ASTM D 395-86 method B. Resilience was measured according to ASTM D2832-88 using a vertical rebound Resilience tester from Modex Industries. The heat built up test was carried out using a Goodrich Flexometer as per ASTM D 623-99 method A. The test samples were preconditioned at oven temperature for 20 minutes. The heat developed at the base of the sample was measured using a thermocouple. The temperature rise at the end of the specific time interval (20 minutes) was taken as heat build up. For ageing resistance studies, samples were aged in an air oven for 48 hours at 70 o C (ASTM D 573-88). The tensile and tear properties were measured after completion of ageing. RESULTS AND DISCUSSION Cure Characteristics Figure 1 shows the variation of minimum torque and (maximum minimum) torque ( T) with reclaim loading. Minimum torque, a measure of the stock viscosity, shows a gradual reduction with respect to reclaim loading. This increased flowability of the mixes can be attributed to the plastizicer present in the matrix. The reclaim contains 15% of plasticizers. Increasing reclaim loading increases plastizicer content of the mixes, and hence the minimum torque decreases. The (maximum minimum) torque value shows a marginal reduction beyond 20 parts of reclaim content. The ( T) value is a measure of cross-links that are formed during the vulcanization reaction. The lower level of cross links at higher reclaim loading may be attributed to the fact that the reclaim is already partially cross linked matrix and hence there is relatively less reaction sites available for further cross linking. Figure 2 gives a plot of cure time and scorch time versus reclaim loading. The cure time is reduced gradually from 8.5 minutes to 4.4 minute at 50-phr-reclaim loading. The reduced cure time indicates an increased rate of cure reaction. The reclaimed rubber normally contains some amount of unreacted curatives or cross-linked precursors, which can increase the cure rate (Figure 3) and hence reduce cure time and scorch time. 88
Figure 1 Variation of minimum torque and (maximum- minimum )torque with reclaim loading Figure 2 Variation of cure time and scorch time with reclaim loading 89
P.A. Nelson and S.K.N. Kutty Figure 3 Variation of cure rate with reclaim loading Mechanical Properties Figure 4 shows the variation of tensile strength with reclaim loading. Tensile strength shows almost a linear increase from 1.2 MPa to 5.7 MPa at 50 parts of reclaim loading. The increase in tensile strength may be attributed to the presence of reinforcing filler in the reclaim. Reclaimed rubber contains about 30% of carbon black filler (Table 1). Similar results have been reported in the case of NBR-Reclaim blends by Sreeja and Kutty (20). Figure 5 gives a plot of ultimate elongation versus reclaim loading. A linear increase in the ultimate elongation of the blend with increase in reclaim content points to increased extensibility of the matrix. This may be resulting from the presence of about 15% of plasticizers in the reclaim rubber (Table 1). Tear strength increases with increase in reclaim loading (Figure 6). Increased tear strength in case of blends indicates the reinforcing capacity of the reclaim rubber, arising from the presence of carbon black filler. Figure 7 shows a plot of abrasion loss versus reclaim loading. There is a drastic reduction in the abrasion loss at 20 parts of the reclaim beyond which further increase in reclaim content has only a marginal effect in the abrasion loss. The increased abrasion resistance with the increased 90
Figure 4 Variation of tensile strength with reclaim loading Figure 5 Variation of ultimate elongation with reclaim loading 91
P.A. Nelson and S.K.N. Kutty Figure 6 Variation of tear resistance with reclaim loading Figure 7 Variation of abrasion loss with reclaim loading 92
reclaim load can also be attributed to the presence of reinforcing filler in the reclaim. A low gum strength matrix can be reinforced more efficiently with the carbon black filler. Thus there is a drastic reduction in the abrasion loss even at 20 parts of reclaim loading. Figure 8 shows the variation of compression set with reclaim loading. This marked increase in the set values at higher reclaim loading may be resulting from combined effect of filler, plasticizer and elevated temperature (70 o C), all of which reduce the elasticity of the matrix. Low elastic matrices facilitate irreversible flow under stress, resulting in higher set values. The reduced elasticity is also evident from Figure 9, which is a plot of resilience versus reclaim loading. As expected, the resilience decreases with the addition of the reclaim. Figure 10 shows a linear increase in heat build up with the reclaim loading. The increase in heat build is again due to the presence of reinforcing filler present in the reclaim matrix. The filler - matrix interface is an area of energy dissipation. More is the filler, more is the interface area and hence more is the dissipation of energy by heat due to friction. Figure 8 Variation of compression set with reclaim loading 93
P.A. Nelson and S.K.N. Kutty Figure 9 Variation of resilience with reclaim loading Figure 10 Variation of heat build up with reclaim loading 94
Ageing Resistance Table 3 shows the tensile strength of the blends before and after ageing. In all the cases the tensile strength value is found to be reduced after ageing. The percentage of retention values, calculated as the ratio of tensile strength after and before ageing, shows a gradual reduction from 100% to 74% at a reclaim content of 50 parts. A synthetic matrix with better resistance to degradation, the BR gum compound (mix A) gives a percentage retention of 100. Whereas in the case of blends, the presence of reclaim which is relatively more prone to degradation, lowers the retention values. Sreeja and Kutty have reported similar result in the case of NBR/ Reclaim blend (20). A similar trend is also observed in the case of retention of tear resistance on ageing (Table 4). Table 3 Tensile strength of mixes before and after the ageing M ix No. Tensile strength ( MPa) Before ageing After ageing Percentage retention A 1. 2 1. 2 100 B 1. 8 1. 5 83 C 2. 4 1. 9 79 D 3. 1 2. 4 77 E 4. 1 3. 1 76 F 5. 7 4. 2 74 Table 4 Tear resistance of mixes before and after the ageing M ix No. Tear resistance (N/mm) Before ageing After ageing Percentage retention A 5. 1 4. 8 94 B 8. 5 6. 8 80 C 13. 1 10. 1 77 D 17 12. 6 74 E 22. 4 16. 2 72 F 22. 9 16. 4 71 95
P.A. Nelson and S.K.N. Kutty Table 5 shows the retention values of ultimate elongation of the mixes A - F. Ultimate elongation values of the aged sample are lower than that of the unaged sample. This is in agreement with the reduced tensile strength values, of the aged samples. Table 5 Ultimate elongation of mixes before and after the ageing M ix No. Ultimate elongation (%) Before ageing After ageing Percentage retention A 75 75 100 B 150 105 70 C 217 133 66 D 263 166 63 E 320 187 62 F 338 249 61 CONCLUSIONS From the above study, the following conclusions were drawn. Cure characteristics such as minimum torque, (maximum minimum) torque, scorch time, cure time and cure rate of the butadiene rubber were affected by the incorporation of the reclaimed rubber. While minimum torque, (maximum minimum) torque, scorch time, cure time, and resilience, were reduced, cure rate, compression set at constant strain, heat build up, tensile strength, tear strength, ultimate elongation and abrasion resistance were increased with the reclaim content in the blend. The ageing resistance of the blend was inferior to that of the gum compound. REFERENCES 1. N.R.Barton, Waste age, 61 (May June 1972) 2. N.R.Barton and J.A. Koutsky, Chem.Engi.News 52 (6) 21 (1974) 3. A.Ratcliffe, Chem.Eng., 79 (7), 62 (1972) 4. A.A.Harshaft, Environ. Sci. Technol., 6 (5), 412 (1942) 5. T.C.P. Lee and W.Millns, US Patent 4, 046, 834 (1977) (to Gould Inc.) 96
6. M.W. Bindhulph, Conserv. Recycl. 1, 169 (1977) 7. D.J. Zolin, N.B. Frable and J.F.Gentlecore in Meeting of Rubber Division, A.C.S. 1977; RubberChem. Technol., 51, 385 (1978) abstract 8. L.E. Peterson, J.T. Moriarty and W.C. Bryant in Meeting of Rubber Division, A.C.S. 1977; RubberChem. Technol., 51, 386 (1978) abstract 9. M.C. Kazanowicz, E.C. Osmundson, J.F. Boyle and R.W. Savage in Meeting of Rubber Division. A.C.S. 1977; RubberChem. Technol., 51, 386 (1978) abstract 10. J.A. Backman, G. Grane, E.L. Kay, and J.R. Laman, Rubber Age, 104 (4), 43 (1973) 11. G. Crane and E.L Kay, RubberChem.Technol., 48, 50, (1975) 12. N.P. Chopey, Chem.Eng. 80 (20), 54 (1973) 13. R.H. Wolk, Rubber Age, 104, 103 (1972) 14. M.D. Burgogne, G.R. Leaker and Kretic, RubberChem. Technol., 49, 375 (1976) 15. H. Fumiya, Nippon Gonnu Kyokaishi, 51, 169 (1978) 16. U.I. Vatazhima, L.A. Panferova, F.A. Drannukova, A.B Korovnikov and L.G. Rubinsteim, USSR Polim. Stroit. Mater., 37, 39 (1974) 18. S. Euchriro, W. Shokei and S. Takeo, Japan, Kokai 75, 60, 581 (1975) 18. O. Katsuhiro, N. Toshihide and N. Saburo, Japan Kokai, 75, 34 (1975) 19. A.A. Phadke, A. K. Bhatacharya, S.K. Chakraborthy and S.K.De, Rubber Chem. Technol., 56, 726 (1983) 20. Sreeja and Kutty S K N, Polym. Plast. Technol. Eng. 39 (3) (2000) 501 97