Study of two types of styrene butadiene rubber in tire tread compounds

Polymer Testing 20 (2001) 539–544 www.elsevier.nl/locate/polytest

Material Properties

Study of two types of styrene butadiene rubber in tire tread compounds Pham Thi Hao 1, Hanafi Ismail *, Azanam S. Hashim School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Received 1 August 2000; accepted 2 November 2000

Abstract The ratio of vinyl butadiene and styrene groups in styrene butadiene rubber (SBR) structures is a crucial factor that affects the inherent rubber characteristics such as glass transition temperature, Tg, hysteresis, strength, etc. In this paper, two types of SBR (Krynol 1721 and Buna VSL 5025-1) which contain higher ratios of these two groups, were blended with natural rubber (SMR 5) and compared with a blend of general purpose SBR (Krynol 1712) and SMR 5. The results show that the blends with the two rubbers possess a markedly lower resilience (i.e. higher hysteresis) than that of the general purpose SBR. Besides resilience, other properties of the compounds, Mooney viscosity, scorch time, cure time, tensile strength, tear strength, and ageing resistance were also investigated. At a similar blending ratio of 50:50, blends with Krynol 1721 and Buna VSL 5025-1 show markedly lower rebound resilience while other mechanical properties are considered acceptable. This preliminary investigation indicates that the two rubbers are suitable for wet grip improvement. Subsequently, the ratio of Krynol 1721 and Buna VSL 5025-1 in the blends was varied from 30 to 70 phr. As the ratio of the rubbers is increased, a reduction in rebound resilience is also observed and the effect of Buna VSL 5025-1 is more pronounced than Krynol 1721. The result is consistent with the higher Tg of the former. Mooney viscosity, scorch time, cure time and ageing index (based on tensile strength) are increased but there is a slight drop in tensile strength and tear strength.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Styrene butadiene rubber; Rubber–rubber blend; Tire tread; Mechanical properties; Curing characteristics

1. Introduction Rubber blends are used in tire rubber compositions due to three reasons: lowering cost, processing improving, and modifying the final product performance. In tire tread, natural rubber (NR) and SBR have long been blended for these reasons [1]. Among many requirements for a tire tread compound, good wet grip is one of the most important properties. Although many factors affect compound performance, the

* Corresponding author. Tel: +60-4-657-7888 (ext 2214); fax: +60-4-657-3678. E-mail address: [email protected] (H. Ismail). 1 On study leave from The Southern Rubber Industry Co., Ho Chi Minh City, Vietnam.

inherent characteristics of the rubbers are the basic controlling factors [2,3]. Several studies [2,4,5] have been reported on the tire wet skid resistance from the viewpoint of the viscoelastic properties of the tread rubbers. A good correlation has been found between the wet grip property and the glass transition temperature (Tg), the dynamic loss angle (tan d), or the rebound resilience of the rubber compounds. The higher the Tg of the rubber, the higher the hysteresis and, thus, better the wet grip property. In this study two rubbers, Krynol 1721 and Buna VSL 5025-1, were evaluated against Krynol 1712 (general purpose SBR) in order to improve the wet grip property of a motorcycle tire tread compound (the control compound) which contains Krynol 1712 and NR at a 50:50 ratio. Krynol 1721 and Buna VSL 5025-1 are also SBR but their Tg values are significantly higher than that of Krynol 1712 due to the higher styrene and vinyl-butadiene contents in

0142-9418/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 0 ) 0 0 0 7 3 - 8

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Table 1 Rubber characteristics[6] Rubber

Vinyl-butadiene contenta (%)

Krynol 1712 (SBR 1712) Krynol 1721 (SBR 1721) Buna VSL 5025-1(VSL) 50±4 a b

Styrene contenta (%)

Oil content (phr)

Mooney viscosityb Tg (°C) ML(1+4) 100°C

23.5 40.0 2562

37.5 37.5 37.5

51±4 54±4 50±5

⫺51 ⫺32 ⫺20

Content of base polymer. ASTM D 1646.

Table 2 Compound recipes of different rubber blendsa Ingredients (phr)

1

2

3

SMR 5 SBR 1712 SBR 1721 VSL

50 50

50

50

different ratios of the two rubbers in blends. The concentration of each ingredient used in the blends is basically the same as in the control recipe (recipe 1) except for the curatives. The sulphur level in these blends was reduced to 2 phr from 2.3 phr because at high levels blooming could occur during the processing stage [8]. Therefore, CBS was increased up to 1.5 phr to balance the reduction in sulphur concentration.

50 50

a Other ingredients: N220=35.0; N330=35.0; ZnO=3.0; stearic acid=3.0; IPPD=1.0; aromatic oil=31.8; paraffinc wax=1.0; CBS=1.0; TMTD=0.1; Vulkalent G=0.16; sulphur=2.3.

their structure [6]. Rebound resilience has been used as an indicator of hysteresis (hysteresis is 100% minus the resilience) [7]. The other properties studied were tensile properties, tear strength, compression set, abrasion resistance, cure characteristics and Mooney viscosity.

2.3. Mixing and preparing of samples Compounds were prepared on a 6 in×12 in laboratory mill. Natural rubber (SMR 5) was masticated prior to blending due to the higher viscosity compared with SBR. Testing samples were cured at 150°C in a hot press according to the cure time (t90) obtained from Monsanto Rheometer M2000.

2. Experimental 2.4. Testing 2.1. Materials Natural rubber (SMR 5) was obtained from the Malaysian Rubber Board. Three types of SBR: Krynol 1712, Krynol 1721, Buna VSL 5025-1 and other chemicals were supplied by Bayer (M) Sdn. Bhd. Carbon black N220 and N330 were purchased from Cabot (M) Sdn. Bhd. Table 1 shows the rubber characteristics. 2.2. Compounding The recipes employed to study the effect of different types of rubber are given in Table 2. Table 3 shows the

All testing has been done according to the standard procedures by using equipment available in the laboratory. The tests reported are tensile properties, i.e. modulus, tensile strength and elongation at break (Monsanto Tensometer T10, according to BS 903: Part A2), tear strength (Monsanto Tensometer T10, according to BS 903: Part A3), ageing index based on tensile strength (24 h at 70°C), resilience (Wallace Dunlop Tripsometer according to BS 903: Part A8), compression set (ASTM D395, method B at 25°C), and abrasion resistance (Akron abrader according to BS 903: Part A9).

Table 3 Recipes showing various ratios of SBR 1721 and VSL in blends with SMR 5a Ingredients (phr)

S1

S2

S3

S4

S5

V1

V2

V3

V4

V5

SMR 5 SBR 1721 VSL

70 30

60 40

50 50

40 60

30 70

70

60

50

40

30

30

40

50

60

70

a

Other ingredients: N220=35.0; N330=35.0; ZnO=3.0; stearic acid=2.0; IPPD=1.5; aromatic oil=30; antilux=1.0; CBS=1.5; sulphur=2.0.

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Table 4 Cure characteristics and other properties of different blends Properties

1

2

3

Mooney viscosity, ML(1+4) at 100°C (MU) Cure characteristics at 150°C Minimum torque, ML (dNm) Maximum torque, MH (dNm) Scorch time, T10 (min) Cure time, T90 (min) Other mechanical properties Ageing index at 70°C×24 h (%)a Tensile strength (MPa) Tear strength (kN/m) Rebound resilience (%) Compression set (%) Abrasion resistance (cm3/1000 revs)

40

43

47

2.2 15.1 6.4 13.6

2.2 15.8 6.4 13.2

2.1 14.5 6.5 13.1

102 19.56 108.5 41 12.2 0.130

102 18.42 93.2 35 13.5 0.130

101 16.89 95.2 31 14.2 0.133

a

Based on tensile strength.

Fig. 1.

Stress–strain curve of recipes 1, 2 and 3.

3. Results and discussion 3.1. Evaluation of SBR 1721 and VSL in tire tread compound SBR 1712 in the control compound (compound 1) was replaced by SBR 1721 and VSL weight for weight; other

ingredients are kept the same. Table 4 shows the Mooney viscosity, cure characteristics and other mechanical properties of the three compounds. Stress versus elongation for the three compounds is shown in Fig. 1. It is observed that the cure characteristics of these compounds, relative to each other, are not much different. In contrast, there is a significant difference in other

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properties such as tensile strength, tear strength, resilience and compression set. Stress–strain properties of compounds 1 and 2 are comparable but compound 3 is less stiff, with tensile stress at a similar elongation lower than the other two compounds (Fig. 1). Compounds 2 and 3 have significantly lower tear strength than compound 1 but their rebound resiliences are much lower than compound 1. Consistent with the resilience behavior, the compression set of these compounds increases in the expected order. For abrasion resistance, the three compounds are quite comparable. It can be seen that significantly lower resilience can be achieved by replacing SBR 1712 in the control compound with SBR 1721 or VSL. Therefore, SBR 1721 and VSL can be used to improve the wet grip property of tire tread compounds. However, other properties such as tensile strength and tear strength are affected. 3.2. Effect of rubber ratio in blends In this section the concentration of SBR 1721 and VSL in the blends is changed from 30 to 70 phr. The Mooney viscosity, cure characteristics and other properties of the two sets of blends are shown in Tables 5 and 6, respectively. It is obvious that the Mooney viscosity is increased as the ratio of each synthetic rubber increases. As the other ingredients in the recipes are similar, the differences in Mooney viscosity could be attributed to the inherent dif-

ferences in characteristics of each rubber. Nielsen [9] reported that any factor which increases the Tg of the polymer tends to increase viscosity. In the case of SBR, styrene groups in SBR 1721 and VSL have caused a marked effect on the reduction of molecular chain flexibility due to their bulkiness compared to SMR 5. Therefore, the Mooney viscosity increases as the ratio of the synthetic rubber is increased. At a similar blend ratio, VSL shows a more pronounced increase in Mooney viscosity than the SBR 1721. This might be due to the higher fraction of vinyl groups in VSL. Vinyl-butadiene groups, in the form of lateral groups, are more bulky compared with cis- or trans-butadiene in the main chains, which reduces the rubber chain flexibility and consequently increases the viscosity. It is also observed that scorch time (T10) and cure time (T90) are increased as the concentration of synthetic rubber increases. This is mainly due to the lower unsaturation of SBR compared to SMR 5 [10]. In SBR 1721 and VSL, butadiene groups comprise just 60% and 75% by weight, respectively. Only these groups are capable of reacting with sulphur in a crosslinking reaction because they possess double bonds, whereas in SMR 5 every isoprene group can take part in the crosslinking process. For tensile properties, in the case of SBR 1721, the tensile strength of the blends is considered to be not much affected by changing the rubber ratio if taking into account the error of measurement. For VSL, it is observed that there is some effect; when the ratio of VSL

Table 5 Properties of compounds with different ratio of SBR 1721 Properties

S1

S2

S3

S4

S5

Mooney viscosity, ML(1+4) at 100°C (MU) Cure characteristics at 150°C Minimum torque, ML (dNm) Maximum torque, MH (dNm) Scorch time, T10 (min) Cure time, T90, (min) Other mechanical properties Modulus at 100% elongation, M100 (MPa) Modulus at 300% elongation, M300 (MPa) Tensile strength (MPa) Elongation at break (%) Ageing index at 70°C×24 h (%)a Tear strength (kN/m)

42

46

47

56

62

a

2.3

2.5

2.4

2.8

2.9

18.5

18.4

16.9

18.2

18.1

4.9 10.5

5.1 10.6

5.6 11.3

5.8 12.5

6.0 13.4

2.8

2.7

2.6

2.7

2.6

10.1

10.1

9.7

10.1

10.2

18.5 540 95

18.6 530 99

19.2 560 97

18.2 510 100

17.6 490 101

115.8

109.9

98.7

96.8

90.1

Based on tensile strength.

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Table 6 Properties of compounds with different ratio of VSL Properties

V1

V2

V3

V4

V5

Mooney viscosity, ML(1+4) at 100°C (MU) Cure characteristics at 150°C Minimum torque, ML (dNm) Maximum torque, MH (dNm) Scorch time, T10 (min) Cure time, T90 (min) Other mechanical properties Modulus at 100% elongation, M100 (MPa) Modulus at 300% elongation, M300 (MPa) Tensile strength (MPa) Elongation at break (%) Ageing index at 70°C×24 h (%)a Tear strength (kN/m)

44

52

59

63

66

a

2.1

2.4

2.5

2.8

2.8

17.5

17.4

17.2

17.1

16.1

4.9 9.9

5.1 10.1

5.4 11.2

5.7 12.1

6.3 13.5

2.5

2.4

2.5

2.4

2.4

8.7

8.4

8.4

8.3

8.2

18.0 590 97

17.2 580 96

17.2 550 98

16.1 560 99

15.7 550 101

98.1

99.2

93.7

87.2

70.0

Based on tensile strength.

is increased up to 70 phr, the tensile strength dropped about 2.5 MPa. It is also observed that at the similar rubber ratio, SBR 1721-based blends have a higher tensile strength than VSL-based ones. The superior tensile strength of the former can be explained by the difference in their Tg. Borders and Juve [11] have found that, for amorphous rubbers, the tensile strength was proportional to the difference between the test temperature T and Tg. At the similar testing temperature T, SBR 1721 with a lower Tg will give a higher tensile strength, as observed. Ageing index based on tensile strength of the two sets of blend is increased as the ratio of synthetic rubbers is increased. This is due to the difference in heat resistance property of SMR 5 and the two synthetic rubbers. According to Brydson [12], weak links in the main chain is one of the factors which influences the heat resistance of a rubber under oxidative conditions. For these rubbers, the carbon–carbon double bonds present in the main chains can be seen as the weak links because of their tendency to be attacked by oxygen, ozone and other agents. SMR 5 contains a higher ratio of double bonds in molecular structure, therefore it has poorer heat resistance. Tear strength is reduced with the increase in ratio of synthetic rubbers. This result can be attributed to the reduction of SMR 5 portion in the blends. Due to the crystallization that occurs on stretching at the tear tip, the tear strength of SMR 5 is greatly enhanced compared with SBR [13]. The poor tear strength of VSL blends compared with SBR 1721 blends at the same blend ratio

is the result of the higher content of vinyl in the former. According to Day and Futamura [14], an increase in vinyl-butadiene resulted in a drop in tear strength. Rebound resilience of the two sets of blend is decreased with increasing concentration of SBR 1721 and VSL, as shown in Fig. 2. According to a definition in ASTM D945-48T; resilience is the ratio of energy given up on recovery from deformation to the energy required to produce the deformation. Resilience is related to the flexibility of molecular chains; the more flexible the molecular chains the better the resilience. SMR 5 molecular contains more than 99% cis-1,4-polyisoprene [10]. It is free of bulky side groups (such as phenyl groups) or other groups that raise the rotational energy; thus, their chain backbones are very flexible. In contrast, SBR possesses a certain ratio of styrene groups in its structure; these bulky side groups will increase the rotational energy and result in reduction in chain flexibility. Therefore, SMR 5 has superior resilience or lower hysteresis than SBR. In Fig. 2, it is also observed that for resilience reduction, VSL is more pronounced than SBR 1721. This result is consistent with the observation in the previous section and with the higher Tg value of VSL.

4. Conclusions 앫 At a similar blend ratio of 50:50 using SBR 1712:SMR 5 as the reference, blends with SBR 1721

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Fig. 2.

Relation between rebound resilience at 25°C and concentration of SBR 1721 and VSL.

and VSL show a significant reduction in rebound resilience. VSL causes a more significant effect on resilience reduction than SBR 1721. This observation is consistent with the higher Tg of the former. 앫 As the concentration of SBR 1721 and VSL in the blends is increased from 30 to 70 phr, a reduction in rebound resilience is also observed and again the effect of VSL is more pronounced. Other properties of the blends are also affected by the increasing ratio of the two synthetic rubbers. Mooney viscosity, scorch time and cure time increased. Ageing index based on tensile strength is slightly better but tensile and tear strengths are poorer. Acknowledgements The authors would like to express their appreciation to The Southern Rubber Industry Co. (Casumina) for financial support and for permission to publish this work. Particular thanks are due to Bayer (M) Sdn. Bhd. and Degussa Taiwan Ltd for kindly supplying the rubbers and chemicals. References [1] D.R. Paul, S. Newman, Polymer Blends, Vol. 2, Academic Press Inc, New York, 1978.

[2] I.R. Gelling, Influence of tread polymer on traction, rolling resistance, and wear properties of tires, in: B.T. Kulakowski (Ed.), Vehicle–Road Interaction, ASTM STP 1225 ASTM, Philadelphia, 1994, pp. 107–108. [3] A. Ahagon, T. Kobayashi, M. Misawa, Rubber Chem. Technol. 61 (1988) 14. [4] K.A. Grosch, Rubber Chem. Technol. 49 (1976) 890. [5] R.M. Russell, Tire Technol. Int. 14 (1993) 19. [6] Bayer AG, http://www.rubber.bayer.com/ (1988). [7] L.P. Smith, The Language of Rubber, Butterworth Heinemann Ltd, London, 1993. [8] W.W. Barbin, in: A.K. Browmick et al. (Eds.), Rubber Products Manufacturing Technology, Marcel Dekker Inc, New York, 1994, p. 526. [9] L.E. Nielsen, Mechanical Properties of Polymer, Van Nostrand Reinhold Co, New York, 1962. [10] D.C. Blackley, Synthetic Rubbers: Their Chemistry and Technology, Applied Science Publishers, London, 1983. [11] A.M. Borders, R.D. Juve, Ind. Engng Chem. 38 (1946) 1066. [12] J.A. Brydson, Rubber Chemistry, Applied Science Publishers, London, 1978. [13] A.N. Gent, in: J.E. Mark (Ed.), Science and Technology of Rubber Academic Press, New York, 1994, p. 420. [14] G.L. Day, S. Futamura, Paper No. 22, in: Meeting of Rubber Division Americal Chemical Society, New York, 1986.