Use of reinforcing silica in model sidewall compounds: effects of carbon black type, polymer type and filler level.
Consumers expect to have long-wearing, trouble-free tires. They also expect to have extensive choices in technical designs tailored to their driving preferences, such as high performance tires, all season tires or tires designed for on/off road applications. Therefore, secondary characteristics of tires have become increasingly important, especially black sidewall appearance and the contribution of the tire to reduced fuel consumption. The goal of programs to improve sidewall appearance is to maintain a black, glossy rubber surface over the lifetime of the tire. Studies to reduce the surface-discoloring bloom apparent on in-service tires protected with para-phenylenediamine antiozonants have been reported. Benko, Evans and coworkers [refs. 11 and 12] have used microencapsulation techniques that slow the niigration of the antiozonant to the surface of the rubber to increase resistance to ozone aging and to decrease the sidewall discoloration. Evans et al. [ref. 13] have shown that a reduction in the discoloration rates of sidewall compounds results when using microphase rubber domains having a high solubility for paraphenylenediamines. Waddell et al. [refs. 14 and 15] reported the use of precipitated silica to improve the physical properties, including resistance to ozone aging, of a model black sidewall compound and the potential to reduce sidewall discoloration through reduction in the paraphenylenediamine level. They also showed that the crack-free lifetime of the model sidewall compound upon exposure to 50 parts-per-hundred-million (pphm) of ozone related directly to reduced hysteresis of the black sidewall compound. Approaches to completely replace the para-phenylenediamine antiozonants with a non-discoloring antiozonant have had limited success. The most prevalent approach to achieve a high-gloss black sidewall over the life of a tire is to use a saturated-backbone polymer, such as ethylene-propylene-diene terpolymers (EPDM), butyl or halogenated butyl rubbers (IIR, CIIR, BUR), or polyisobutylene rubber [refs. 16-18] alone, or in conjunction with cis-polyisoprene (IR, NR) or polybutadiene (BR) rubber.
The increasing environmental movement has had a significant effect on the tire tread composition [refs. 19 and 20] since the tread contributes from 25% to 50% of the rolling resistance to the tire [ref. 21]. As advances in tire design and tread compounding, such as the recently introduced silica-filled tread formulation [ref. 22], continue to provide lower rolling resistance tires, reducing the hysteresis of the tire sidewall will increase in importance. The sidewall contributes from 5% to 20% of the rolling resistance of the tire [ref. 21], and can be changed relatively independently of tire design so that changes in compound hysteresis directly affect the tires rolling resistance. Waddell and Evans [ref. 15] have reported that use of precipitated silica lowers the hysteresis of a model black sidewall formulation containing natural and polybutadiene rubbers, as well as advantages for mixing the silica into the BR polymer phase prior to blending with the NR.
Interest in extending the lifetime and lowering the hysteresis of sidewall fonnulations containing various carbon black types and polymer blends by using precipitated silica has directed this work. Experiments were performed to relate the hysteresis contribution of precipitated silica to a model NR/BR black sidewall compound formulated using a range of carbon black types, and to measure the physical properties and ozone resistance of these compounds. Results show that use of up to 16 phr of a precipitated silica with a BET nitrogen adsorption value of 145 [m.sup.2]/g produced black sidewall compounds with improved tear strength and cut growth resistance. The hysteresis of the compound was directly related to total filler level used in the fortnulation. For compounds containing reinforcing and highly-reinforcing carbon blacks, the hysteresis of the compound could be reduced by partial replacement of carbon black with precipitated silica. Designed experiments were performed to explore the use of precipitated silica in a model EPDM/N/BR sidewah forinulation. Improvements in compound physical properties, particularly tear strength and cut-growth resistance, were noted for the use of precipitated silica.
Rubber compounds were niixed using a two-stage process in a laboratory internal mixer with the ingredients added in the order shown in table 1. Compounds were milled into sheets on a two-roll mill. Cure properties were measured using a moving-die rheometer and samples were press-cured at 150[degrees]C to a time cor-responding to [T,sub.90]plus 5 minutes plus appropriate mold lag for each sample, unless otherwise specified. Test procedures and equipment used to measure the physical properties of the compounds are shown in table 2. Ozone aged specimens were rated versus a standard set of photographs using a 1-10 scale [ref 14] with a zero rating representing no cracking, and a ten rating representing cracking just prior to complete breaking of the sample.
Table 1 - model sidewall compounds
Natural rubber (CV 60] 50 phr Butadiene rubber (BR1220 50 Carbon black 36-50 Silica 0-16 Napthenic processing oil 10 Stearic acid 2 Paraffinic wax 1 6-ppd antiozonant 3 Diary-ppd antioxidant 1 Hydrated-trimethylquinoline 2 Zinc oxide 3 Sulfur 1.8 Accelerator (MOR) 1
Table 2 - test methods and equipment used for rubber property measurement
Rubber property Test method Equipment Cure ASTM D2084-92 Monsanto MDR2000 Flex fatigue ASTM D-813-87 DeMattia Rebound USO 4662-1986 Zwick 5109 Hardness ASTM D2240-91 Zwick 5109 Stress/strain ASTM D412-87 Instron 4204 Abrasion ASTM D2228-88 Pico abrader Tear strength PPG CD-25-43 Instron 4204 Ozone resistance ASTM D3395-91 OREC 0900-64 modified to include an on/off cycle [ref. 6] Dyn. properties ASTM D2231-87 Rheometrics RDAll using rotational concentric shear
Data were analyzed using SAS, GLM, REG and ANOVA procedures [ref 23] to determine significant variables and responses and to model the response surface.
To evaluate the effects of precipitated silica addition versus the addition of various carbon black types, a PPG model sidewall compound formulation [ref 14], shown in table I was employed. The base level of total filler was 36 phr. Carbon black, silica or both fillers were added incrementally in a mixture design [ref 24] to deterniine the effect of each filler on compound physical properties.
Experimental EPDM compounds were compared to a model tire black sidewall compound from the R.T. Vanderbilt Rubber Handbook [ref 25], shown in table 3.
Results and discussion
Effect of silica versus various carbon blacks
The carbon blacks shown in table 4 were used in the model sidewall compound (table 1) to study the effects on compound physical properties of adding precipitated silica in place of the various carbon blacks. The silica has a nitrogen adsorption surface area ([N.sub.2]SA) by the BET (1 point) method of 147 [m.sup.2]/g and an absorption of dibutylphthalate (DBP) of 197 mL/100g of silica. To maintain equal volume for the mixture experiment, approximately 1.1 phr of precipitated silica was added for each phr of carbon black replaced. Physical properties of the 50 phr carbon black compounds are shown in table 5 and physical properties of compounds containing 42 phr carbon black and 9 phr silica are shown in table 6. Comparison of the data in tables 5 and 6 shows that the substitution of 9 phr of precipitated silica for 8 phr of carbon black increases compound elongation to break by 13 to 17%, increases tear strength by 15 to 83% and increases cutgrowth resistance by up to 71%, reduces hysteresis (G" at 30[degrees]C) by up to 27%, and improves the resistance to ozone cracking by up to 24%. Compound hardness, break strength and modulus at 300% strain are reduced for all compounds. All data were analyzed with alpha = 0.05 and beta = 0.10 for comparisons.
Table 7 summarizes the statistically different responses of precipitated silica versus carbon black addition to the model sidewall compound. Compound tear strength, cut-growth resistance, crack resistance in exposure to ozone, and G" are respectively shown in figures 1-4 versus addition of silica and/or carbon black to a 36 phr carbon black base-level compound. These data show that the use of precipitated silica in a black sidewall compound can result in a compound with improved tear strength and cut-growth resistance for all carbon black types, and improved resistance to cracking in exposure to ozone for the more reinforcing N-220, N-330 and N-351 carbon blacks. Finally, the hysteresis of the compound, measured by the G" @ 2% strain can be reduced by using, precipitated silica. While no specific equations exist to predict the effects of lowering the sidewall hysteresis on tire performance, such as those introduced by Futumara [refs. 26 and 27] for the effects of tread compound hysteresis on tire rolling resistance, the lower hysteresis of the black sidewall compound is expected to result in fuel savings, especially for large tires that use highly-reinforcing carbon blacks.
Effect of silica in EPDM polymer blends
Experiments were conducted to determine the effects of the addition of precipitated silica to an EPDM/NR/BR-blend black sidewall fonnulation. Four EPDM polymers with high weight percent ethylidine norbornene (ENB) to insure adequate co-cure with diene polymers were evaluated in a NR/BR/EPDM formulation as shown in table 8. All compounds were cured at 160[degrees]C. The polymers contained varying molecular weight distribution, Mooney viscosity index, ethylene content and oil extension levels. A 100 phr paraffinic oil-extended polymer with 59% ethylene, and 7.5% ENB was selected for further study based on hardness and modulus values suitable for a black sidewall compound and the highest (stress @ break X strain @ break) product as detailed by von Hellens [ref. 28].
A series of compounds was cured with three systems: peroxide cure, hybrid peroxide/sulfur cure and sulfur vulcanization. All compounds were cured at 165[degrees]C to accommodate the peroxide vulcanization system. Comparison of the results to those of the control diene compound (table 3) is shown in table 9. The most appropriate cure system for a black sidewall compound is the sulfur vulcanization system, which is particularly evident from the unacceptably low cut-growth resistance values for the peroxide and hybrid cure systems.
A series of designed experiments was carried out using the EPDM/NR/BR black sidewall formulation [ref. 28] shown in table 10 to determine the effects of silica level in the sidewall compound and the polymer phase (EPDM, NR/BR, EPDM/NR/BR) into which the silica was added. Increased modulus (39%) and hardness values, faster cure times (12%) and lower rebound values were obtained upon the addition of precipitated silica to the EPDM phase versus the addition of silica to the NR/BR phase, as shown in table 11. Furthermore, compound tear strength was increased by 50%, cut growth resistance was increased 66% and ozone resistance was improved for the compound containing precipitated silica added to the EPDM phase.
Figure 5 shows rubber samples removed from the ozone chamber after dynamic cycling in 50 pphm ozone for eight days. Extensive surface cracking and a brown surface discoloration are evident for the Vanderbilt control compound, table 3. Samples containing 15 phr of precipitated silica in the EPDM/NR/BR sidewall compound (table 10) maintained their glossy appearance over the lifetime of the ozone-aging test. When silica was added to the NR/BR phase, surface cracking has proceeded to a severe level. However, when silica was added to the EPDM phase, no surface cracking was evident. A detailed analysis of the physical property responses of the compounds from the mixture design shown in table 10, covering the addition of up to 18 phr of precipitated silica:
* into the NR/BR phase;
* into the EPDM phase; and
* added after blending of the NR/BR and EPDM phases, was performed. There were no significant differences at the 95% confidence level between adding silica to the NR/BR phase or adding silica to the blended NR/BR/EPDM polymer. Statistically significant differences due to the addition of silica to the individual phases, EPDM versus NR/BR, are shown in table 12. The addition of silica to the EPDM phase of the system improves resistance to ozone and cut growth, even diough the compounds have higher hardness and modulus values. Silica addition to the EPDM phase also leads to slightly faster cure time and significantly improved tear strength when compared to silica addition to the diene rubber phase.
Precipitated silica use enhances the durability and performance of tire black sidewall compounds containing a variety of carbon blacks. Statistically designed compounding studies of silica, and carbon black type and level detailed the improvements in physical properties. Increases in compound tear strength from 15-83%, cut growth resistance up to 71% and resistance to ozone aging up to 24%, and reductions in hysteresis up to 27% were attained by using precipitated silica. Plots show the significant improvements in black sidewall durability properties that can be attained depending upon the carbon black type used.
The effects of using EPDM polymers and of phase niixing of precipitated silica into individual polymers or the polymer blend prior to carbon black addition showed that phase mixing of precipitated silica into the EPDM polymer reduced cure time by 12%, increased modulus at 300% strain by 39%, increased tear strength by 50%, increased cut growth resistance by 66% and improved ozone resistance.
[1.] R.S. Bhakuni, S.K. Mowdood, W.H. Waddell, I.S. Rai and D.L. Knight, "Tires" in Encycl. Polym. Sci. Eng., 16, 834 (1989). [2.] R.W. Layer and R.P. Lattimer, Rubber Chem. Technol., 63, 426 (1990). [3.] R.W. Layer, Rubber Chem. Technol., 39, 1584 (1966). [4.] R.P. Lattimer, E.R. Hooser, H.E. Diem and C.K. Rhee, Rubber Chem. Technol., 53, 1170 (1980). [5.] R.P. Lattimer, E.R. Hooser, R.W. Layer and C.K. Rhee, Rubber Chem. Technol., 56, 431 (1983). [6.] W.H. Waddell, K.A. Benzing, L.R. Evans, S.K. Mowdood, J.M. McMahon, R.H. Cody., Jr., and J.A. Kinsinger, Rubber Chem. Technol., 64, 622 (1991). [7.] W.H. Waddell in "Applications of analytical techniques to the characterization of materials," D.L. Perry, ed., Plenum, New York, 1991. [8.] W.H. Waddell, K.A. Benzing, L.R. Evans and J.M. McMahon, Rubber Chem. Technol., 65, 411 (1992). [9.] S.D. Razumovskii and L.S. Batashova, Rubber Chem. Technol., 43, 1340 (1970). [10.] E.R. Erickson, R.A. Berntsen, E.L. Hill, P. Kusy, Rubber Chem. Technol., 32, 1059 (1959). [11.] D.A. Benko, L.R. Evans, J.G. Gillick, W.H. Waddell, B.A. Metz, B.F. Benton, G.E. Pickett and W.R. Kruman, U.S. Patent 4,895,884, January 23, 1990. [12.] L.R. Evans, D.A. Benko, J.G. Gillick, W.H. Waddell, Rubber Chem. Technol., 65, 211 (1992). [13.] L.R. Evans, W.H. Waddell, F.W. Harris, D.A. Benko, U.S. Patent 5,023,287, June 11; 1991. [14.] W.H. Waddell, J.B. Douglas, T.A. Okel and L.J. Snodgrass, Rubber World, 208 (3), 21 (1993). [15.] W.H. Waddell and L.R. Evans, "Tire technology international 1993," L.J.K. Setright, ed., UK & International Press, Westcott, U.K., 1993, pp. 64-71. [16.] A.J.M. Summer and H. Fries, Kautsch. Gummi. Kunstst., 45 (7), 558-561, (1992). [17.] F.C. Cesare, Rubber World, 201 (3), 14-17 (1989). [18.] D.D. Flowers, J.V. Fusco and D.S. Tracey, Rubber World, 209 (6), 32 (1994). [19.] W.M. Hess and W.K. Klamp, Rubber Chem. Technol., 56, 390 (1993). [20.] D.G. Vera, R.O. Simpson, J. Bergh, "The environmental tire," presented at the ACS, Rubber Division Meeting, Louisville, KY, May 19-22, 1992. [21.] D.J. Schuring (Ed.), Rubber Division Symposium 1, Rubber Division, ACS, Akron, OH, Lancaster Press, 1983. [22.] R. Rauline, U.S. Patent 5,277,425, July 13, 1993. [23.] Sas Institute Inc. SAS/STAT User's Guide, Cary, NC, SAS Institute, Inc., 1988, p. 1028. [24.] J.A. Cornell, "Experiments with mixtures: Designs, models and the analysis of data," 2nd Ed., Wiley & Sons, New York, 1990. [25.] W.H. Waddell, R.S. Bhakuni, W.W. Barbin and P.H. Sandstrom, "Pneumatic tire compounding" in The Vanderbilt Rubber Handbook, 13th ed., R.F. Ohm, ed., R.T. Vanderbilt Company, Inc., Norwalk, 1990, pp. 595-611. [26.] S. Futamura, Tire Science Technol., 1, 2 (1990). [27.] S. Futamura, Rubber Chem. Technol., 64, 57 (1991). [28.] W. von Hellens, "Effect of EPDM characteristics on cure state and co-vulcanization in NR/EPDM belnds" presented at the ACS, Rubber Division Meeting, Nashville, TN, November 3-8, 1992.
"Use of reinforcing silica in model sidewall compounds: "Effects of carbon black type, polymer type and filler level" is based on a paper given at the April 1994 Rubber Division meeting. "Alternate approach to study carbon black" is based on a paper given at the January 1995 Akron Rubber Group meeting. "Performance-driven black selection system" is based on a paper given at the October 1994 Rubber Division meeting. "Viscoelastic characterization of polyethylacrylate" is based on a paper given at the October 1994 Society of Rhelogy
Table 3 - Vanderbilt handbook control sidewall compound [ref. 24)
Natural rubber (CV 60) 50 phr Butadiene rubber (BR1220) 50 N330 carbon black 50 Napthenic processing oil 10 Sulfonic acid processing aid 2 Stearic acid 2 Paraffinic wax 3 6-ppd antiozonant 2 Hydrated-trimethylquinoline 2 Zinc oxide 3 Sulfur 1.75 Accelerator (OBTS) 1 176.75
Table 4 - carbon blacks evaluated versus silica
Astm grade DBP,mL/100g [N.sub.2]SA(BET),[m.sup.2]/g N-220 112 121 N-330 102 80 N-351 120 73 N-550 117 41 N-660 91 34
Table 5 - physical properties of 50 phr carbon black-containing sidewall compounds
Black N-220 N-330 N-351 N-550 N-660 Property
ML, dNm 3.6 3.2 3.1 2.3 2.2 MH, dNm 20.2 18.5 19.4 17.5 16.2 T[S.sub.2], minutes 7.9 6.2 7.2 6.5 5.6 [T.sub.90], minutes 14.4 13.1 13.2 12.9 10.7 Breaking strength ,MPa 25.7 25 25.7 21.4 19.7 Elongation @ break, % 633 580 567 601 573 Modulus @ 20% MPa 0.7 0.7 0.7 0.7 0.7 Modulus @ 300% MPa 8.4 7.6 11.0 7.5 7.3 Hardness @ 23[degrees]C 58 57 58 55 53 Rebound @ 23[degrees]C. % 54.2 56.2 57 60.1 67.6 Tear strength, N/mm 11.9 8.7 9.6 16.0 9.2 Cut growth @ 54,000 18.6 16.7 17.5 10.2 6.9 cycles, mm G' @ 2%, 30[degrees]C, MPa 2.62 2.15 2.20 1.84 1.18 G" @ 2%. 30[degrees]C, MPa 0.41 0.27 0.22 0.22 0.10 Ozone rating @ 10 days 10 9.1 9.3 7.5 6.1
Tables 6- physical properties of sidewall compounds with 42 phr carbon black and 9 phr silica
Black N-220 N-330 N-351 N-550 N-660 Property
ML, dNm 3.6 3.2 3.1 2.3 2.2 MH, dNm 18.1 17.2 17.4 17.5 15.3 T[S.sub.2], minutes 7 6.6 6.8 6.5 5.9 [T.sub.90], minutes 14.5 13.9 13.6 12.9 12.7 Breaking strength, MPa 24.6 25.2 24.0 20.1 19 Elongation @ break, % 719 684 659 699 671 Modulus @ 20%, MPa 0.5 0.6 0.6 0.5 0.5 Modulus @ 300%, MPa 6.1 6.8 7.6 6 5.4 Hardness @ 23[degrees]C 54 53 53 51 49 Rebound @ 23[degrees]C, % 53.0 54.6 55.0 62.3 65 Tear strength, N/mm 15.8 11.2 11.2 18.4 16.9 Cut growth @ 54,000 17.2 12.4 20.0 6.2 2.0 cycles, mm G'@ 2%, 30[degrees]C, MPa 2.36 1.70 1.60 1.41 1.01 G" @ 2%, 30[degrees]C, MPa 0.30 0.20 0.19 0.21 0.1 Ozone rating @ 10 days 8.5 7.9 9.3 7.9 5.6
Table 7 - response of physical properties of sidewalk compound to addition of silica vs. carbon black (base level of 36 phr carbon black)
Carbon black N-220 N-330 N-351 N-550 N-660 Property
ML, dNm H[*] MH, dNm L[**] L L L L T[S.sub.2], minutes [T.sub.90], minutes H H H Breaking strength, MPa L L L L Elongation @ break, % H H H H H Modulus @ 300%, MPa L L L L L Hardness @ 23[degrees]C L L L L L Rebound @ 23[degrees]C, % L L L L L Tear strength, N/mm H H H H H Cut growth @ 54,000 cycles, mm L L L L L G' @ 2%, 30[degrees]C, MPa L L L L G" @ 2%, 30[degrees]C, MPa L L L Ozone rating @ 10 days L L L
[*]H = 1.1 phr of silica increases value of property to a greater extent than 1 phr of black [**]L = 1.1 phr of silica increases value of property to a greater extent than 1 phr of black
Table 8 - EPDM polymer selection
Polymer A B C D
ENB content, weight % 7.5 10.5 8 8 Ethylene content, weight 59 49 52 52 Oil content, phr 100 15 Molecular weight distribution M B M N NR (CV60) 45 45 45 45 BR 1203 15 15 15 15 EPDM 40 40 40 40 Extender oil 40 7.5 Napthenic processing oil 0 2.5 10 10 Sulfonic acid procetsing aid 2 2 2 2 Vanfre AP-2 1 1 1 1 Stearic acid 2 2 2 2 Zinc oxide 3 3 3 3 Dicumyl peroxide 2.6 2.6 2.6 2.6 Stress x strain @ break 3,897 3,069 3,006 3,146 Modulus @ 100%, MPa 1.8 4 4 3.9 Hardness @ 23[degrees]C 56 68 68 68
Table 9 - EPDM cure system selection
Cure system Peroxide Hybrid Sulfur Control CBS accelerator 0 0.5 1 1 Dicumyl peroxide 2.6 1.3 0 0 Sulfur 0 0.9 1.8 1.75 Stress x strain @ break 1,104 2,166 7,873 15,561 Modulus @ 1 00%, Mpa 2.7 2.3 1.5 1.6 Hardness @ 23[degrees]C 57 56 47 51 Cut growth resistance, Failed[*] Failed 18.4 18.4 mm @ 54,000 cycles
[*]Crack progresses to sample width (25.4 mm) prior to completion of test
Table 10 - EPDM sidewall formulation, silica addition to EPDM or NR/BR phase
EPDM A 100 phr Silica 0 - 18 NR (SMR5, CV60) 40 BR 1220 10 Silica 0 - 18 N330 36 - 50 Sulfonic acid processing aid 2 Vanfre AP2 1 Stearic acid 2 Zinc oxide 3 Sulfur 1.75 CBS accelerator 1
Table 11 - physical properties of EPDM/NR/BR sidewall compounds
Phase silica added to EPDM NR/BR Property
ML,dNm 1.8 1.5 MH, dNm 18 16.5 T[S.sub.2], minutes 3.4 4.1 [T.sub.90], minutes 7.2 8.2 Breaking strength, MPa 14.7 11.3 Elongation @ break, % 391 536 Modulus @ 20%, MPa 0.6 0.5 Modulus @ 300%, MPa 8.2 5.9 Hardness @ 23[degrees]C 54 48 Rebound @ 23[degrees]C, % 57.6 62 Tear strength, N/mm 9.1 6.0 Cut growth @ 54,000 cycles, mm 6.2 18.4 Ozone rating @ 10 days 4 5
Table 12 - Change in rubber physical property by addition of silica to rubber phase
Property Change[*] MH, dNm +0.02 T[S.sub.2], minutes -0.06 [T.sub.90], minutes -0.03 Breaking strength, MPa +0.04 Elongation @ break, % -7.07 Modulus @ 300%, MPa +0.12 Hardness @ 23[degrees]C +0.04 Tear strength, N/mm +0.32 Cut growth @ 54,000 cycles -0.32 Ozone rating @ 10 days -0.1
[*] Calculated change when silica is added to the EPDM phase versus added to the NR/BR phase
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|Author:||Waddell, Walter H.|
|Date:||Jun 1, 1995|
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