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Emulsion styrene-butadiene rubber

Imported: 23 Feb '17 | Published: 22 Oct '02

Howard Allen Colvin, Michael Leslie Senyek

USPTO - Utility Patents

Abstract

This invention discloses an emulsion styrene-butadiene rubber (SBR) that can be employed in manufacturing tire tread formulations that have rolling resistance and treadwear characteristics that are similar to those made with solution SBR but with improved traction characteristics. Thus, the emulsion SBR of this invention is superior in many respects for use in tire tread compounds to conventional solution SBR and conventional emulsion SBR. This invention more specifically discloses a styrene-butadiene rubber composition which is comprised of repeat units which are derived from styrene and 1,3-butadiene, wherein the styrene-butadiene rubber composition has a number average molecular weight as determined by thermal field flow fractionation which is within the range of about 50,000 to 150,000, and wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 1.8 to 3.9. The subject invention further reveals a styrene-butadiene rubber composition which is comprised of repeat units which are derived from styrene and 1,3-butadiene, wherein a plot of log frequency versus storage modulus of the styrene-butadiene rubber composition crosses over a plot of log frequency versus loss modulus of the styrene-butadiene rubber composition at a log frequency within the range of 0.001 radians per second to 100 radians per second when conducted at 120° C. using parallel plate geometry in the dynamic oscillation frequency sweep of the styrene-butadiene rubber.

Claims

1. An emulsion styrene-butadiene rubber composition having improved characteristics for utilization in tire tread rubber formulations which is comprised of (I) a high molecular weight styrene-butadiene rubber having a weight average molecular weight of at least about 300,000 and (II) a low molecular weight styrene-butadiene rubber having a weight average molecular weight which is less than about 280,000; wherein the ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 80:20 to about 25:75; wherein the-bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 5 percentage points; wherein the styrene-butadiene rubber composition is made by coagulating a blend of a latex of the high molecular weight styrene-butadiene rubber and a latex of the low molecular weight styrene-butadiene rubber; and wherein the latex of the high molecular weight styrene-butadiene rubber and the low molecular weight styrene-butadiene rubber are made by a process which comprises (1) charging water, a soap system, a free radical generator, 1,3-butadiene monomer and styrene monomer into a first polymerization zone; (2) allowing the 1,3-butadiene monomer and the styrene monomer to copolymerize in the first polymerization zone to a monomer conversion which is within the range of about 15 percent to about 40 percent to produce a low conversion polymerization medium; (3) charging the low conversion polymerization medium into a second polymerization zone; (4) charging an additional quantity of 1,3-butadiene monomer and an additional quantity of styrene monomer into the second polymerization zone; and (5) allowing the copolymerization to continue until a monomer conversion of at least about 50 percent is attained to produce the latex of styrene-butadiene rubber.

2. An emulsion styrene-butadiene rubber composition as specified in claim 1 wherein the high molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 20 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 80 weight percent 1,3-butadiene.

3. An emulsion styrene-butadiene rubber composition as specified in claim 2 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 1 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 99 weight percent 1,3-butadiene.

4. An emulsion styrene-butadiene rubber as specified in claim 3 wherein the high molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 80 to about 160.

5. An emulsion styrene-butadiene rubber as specified in claim 4 wherein the low molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 2 to about 40.

6. An emulsion styrene-butadiene rubber composition as specified in claim 5 wherein the high molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 20 weight percent to about 30 weight percent styrene and from about 70 weight percent to about 80 weight percent 1,3-butadiene.

7. An emulsion styrene-butadiene rubber composition as specified in claim 6 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 3 weight percent to about 10 weight percent styrene and from about 90 weight percent to about 97 weight percent 1, 3-butadiene.

8. An emulsion styrene-butadiene rubber composition as specified in claim 6 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 40 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 60 weight percent 1,3-butadiene.

9. An emulsion styrene-butadiene rubber composition as specified in claim 7 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

10. An emulsion styrene-butadiene rubber composition as specified in claim 9 wherein the high molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 90 to about 150.

11. An emulsion styrene-butadiene rubber composition as specified in claim 10 wherein the low molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 3 to about 30.

12. An emulsion styrene-butadiene rubber composition as specified in claim 11 wherein the high molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 100 to about 130.

13. An emulsion styrene-butadiene rubber composition as specified in claim 12 wherein the low molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 5 to about 20.

14. An emulsion styrene-butadiene rubber composition as specified in claim 13 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

15. An emulsion styrene-butadiene rubber composition as specified in claim 8 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

16. An emulsion styrene-butadiene rubber as specified in claim 15 wherein the high molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 90 to about 150.

17. An emulsion styrene-butadiene rubber as specified in claim 16 wherein the low molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 3 to about 30.

18. An emulsion styrene-butadiene rubber as specified in claim 17 wherein the high molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 100 to about 130.

19. An emulsion styrene-butadiene rubber as specified in claim 18 wherein the low molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity which is within the range of about 5 to about 20.

20. An emulsion styrene-butadiene rubber as specified in claim 19 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

21. An emulsion styrene-butadiene rubber as specified in claim 3 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 400,000 to about 1,750,000.

22. An emulsion styrene-butadiene rubber as specified in claim 21 wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 70,000 to about 270,000.

23. An emulsion styrene-butadiene rubber composition as specified in claim 22 wherein the high molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 20 weight percent to about 30 weight percent styrene and from about 70 weight percent to about 80 weight percent 1,3-butadiene.

24. An emulsion styrene-butadiene rubber composition as specified in claim 23 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 3 weight percent to about 10 weight percent styrene and from about 90 weight percent to about 97 weight percent 1,3-butadiene.

25. An emulsion styrene-butadiene rubber composition as specified in claim 23 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 40 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 60 weight percent 1,3-butadiene.

26. An emulsion styrene-butadiene rubber composition as specified in claim 24 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

27. An emulsion styrene-butadiene rubber as specified in claim 26 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 1,000,000 to about 1,500,000.

28. An emulsion styrene-butadiene rubber as specified in claim 27 wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 120,000 to about 260,000.

29. An emulsion styrene-butadiene rubber as specified in claim 28 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

30. An emulsion styrene-butadiene rubber composition as specified in claim 25 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

31. An emulsion styrene-butadiene rubber as specified in claim 30 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 1,000,000 to about 1,500,000.

32. An emulsion styrene-butadiene rubber as specified in claim 31 wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 120,000 to about 260,000.

33. An emulsion styrene-butadiene rubber as specified in claim 32 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

34. An emulsion styrene-butadiene rubber composition which is comprised of (I) a high molecular weight styrene-butadiene rubber having a number average molecular weight which is within the range of about 200,000 to about 1,000,000 and (II) a low molecular weight styrene-butadiene rubber having a number average molecular weight which is within the range of about 20,000 to about 150,000; wherein the ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 80:20 to about 25:75; wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 5 percentage points; wherein the styrene-butadiene rubber composition is made by coagulating a blend of a latex of the high molecular weight styrene-butadiene rubber and a latex of the low molecular weight styrene-butadiene rubber; and wherein the latex of the high molecular weight styrene-butadiene rubber and the low molecular weight styrene-butadiene rubber are made by a process which comprises (1) charging water, a soap system, a free radical generator, 1,3-butadiene monomer and styrene monomer into a first polymerization zone; (2) allowing the 1,3-butadiene monomer and the styrene monomer to copolymerize in the first polymerization zone to a monomer conversion which is within the range of about 15 percent to about 40 percent to produce a low conversion polymerization medium; (3) charging the low conversion polymerization medium into a second polymerization zone; (4) charging an additional quantity of 1,3-butadiene monomer and an additional quantity of styrene monomer into the second polymerization zone; and (5) allowing the copolymerization to continue until a monomer conversion of at least about 50 percent is attained to produce the latex of styrene-butadiene rubber.

35. An emulsion styrene-butadiene rubber composition as specified in claim 34 wherein the high molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 20 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 80 weight percent 1,3-butadiene.

36. An emulsion styrene-butadiene rubber composition as specified in claim 35 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 1 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 99 weight percent 1,3-butadiene.

37. An emulsion styrene-butadiene rubber as specified in claim 36 wherein the high molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 300,000 to about 970,000.

38. An emulsion styrene-butadiene rubber as specified in claim 37 wherein the low molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 50,000 to about 130,000.

39. An emulsion styrene-butadiene rubber composition as specified in claim 38 wherein the high molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 20 weight percent to about 30 weight percent styrene and from about 70 weight percent to about 80 weight percent 1,3-butadiene.

40. An emulsion styrene-butadiene rubber composition as specified in claim 39 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 3 weight percent to about 10 weight percent styrene and from about 90 weight percent to about 97 weight percent 1,3-butadiene.

41. An emulsion styrene-butadiene rubber composition as specified in claim 39 wherein the low molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 40 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 60 weight percent 1,3-butadiene.

42. An emulsion styrene-butadiene rubber composition as specified in claim 40 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

43. An emulsion styrene-butadiene rubber composition as specified in claim 42 wherein the high molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 650,000 to about 930,000.

44. An emulsion styrene-butadiene rubber composition as specified in claim 43 wherein the low molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 55,000 to about 110,000.

45. An emulsion styrene-butadiene rubber composition as specified in claim 44 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

46. An emulsion styrene-butadiene rubber composition as specified in claim 41 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

47. An emulsion styrene-butadiene rubber as specified in claim 46 wherein the high molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 650,000 to about 930,000.

48. An emulsion styrene-butadiene rubber as specified in claim 47 wherein the low molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 55,000 to about 110,000.

49. An emulsion styrene-butadiene rubber as specified in claim 48 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

50. An emulsion styrene-butadiene rubber as specified in claim 1 wherein the high molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 200,000 to about 1,000,000.

51. An emulsion styrene-butadiene rubber as specified in claim 50 wherein the low molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 20,000 to about 150,000.

52. An emulsion styrene-butadiene rubber as specified in claim 21 wherein the high molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 650,000 to about 930,000.

53. An emulsion styrene-butadiene rubber as specified in claim 52 wherein the low molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 70,000 to about 110,000.

54. An emulsion styrene-butadiene rubber as specified in claim 27 wherein the high molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 650,000 to about 930,000.

55. An emulsion styrene-butadiene rubber as specified in claim 54 wherein the low molecular weight styrene-butadiene rubber has a number average molecular weight which is within the range of about 70,000 to about 110,000.

56. A styrene-butadiene rubber composition which is comprised of repeat units which are derived from styrene and 1,3-butadiene, wherein the styrene-butadiene rubber composition has a number average molecular weight as determined by thermal field flow fractionation which is within the range of about 50,000 to 150,000 and wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 1.8 to 3.9.

57. A styrene-butadiene rubber composition which is comprised of repeat units which are derived from styrene and 1,3-butadiene, wherein a plot of log frequency versus storage modulus of the styrene-butadiene rubber composition crosses over a plot of log frequency versus loss modulus of the styrene-butadiene rubber composition at a frequency within the range of 0.001 radians per second to 100 radians per second when conducted at 90° C. to 120° C. using parallel plate geometry in the dynamic oscillation frequency sweep of the styrene-butadiene rubber, wherein the styrene-butadiene rubber composition has a number average molecular weight as determined by thermal field flow fractionation which is within the range of about 50,000 to 150,000 and wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 1.8 to 3.9.

58. A styrene-butadiene rubber composition as specified in claim 56 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

59. A styrene-butadiene rubber composition as specified in claim 58 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

60. A styrene-butadiene rubber composition as specified in claim 59 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

61. A styrene-butadiene rubber composition as specified in claim 60 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

62. A styrene-butadiene rubber composition as specified in claim 61 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

63. A styrene-butadiene rubber composition as specified in claim 62 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

64. A styrene-butadiene rubber composition as specified in claim 57 wherein a plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.001 radians per second to 10 radians per second.

65. A styrene-butadiene rubber composition as specified in claim 64 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.01 radians per second to 5 radians per second.

66. A styrene-butadiene rubber composition as specified in claim 65 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.05 radians per second to 1 radians per second.

67. A styrene-butadiene rubber composition as specified in claim 66 wherein the storage modulus is less than the loss modulus at a frequency of 0.1 radians per second and wherein the storage modulus is greater than the loss modulus as a frequency of 100 radians per second.

68. A styrene-butadiene rubber composition as specified in claim 67 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

69. A styrene-butadiene rubber composition as specified in claim 68 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

70. A styrene-butadiene rubber composition as specified in claim 69 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.001 radians per second to 10 radians per second.

71. A styrene-butadiene rubber composition as specified in claim 70 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

72. A styrene-butadiene rubber composition as specified in claim 71 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

73. A styrene-butadiene rubber composition as specified in claim 72 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.01 radians per second to 5 radians per second.

74. A styrene-butadiene rubber composition as specified in claim 73 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

75. A styrene-butadiene rubber composition as specified in claim 74 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

76. A styrene-butadiene rubber composition as specified in claim 75 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.05 radians per second to 1 radians per second.

77. A styrene-butadiene rubber composition as specified in claim 76 wherein the storage modulus is less than the loss modulus at a frequency of 0.1 radians per second and wherein the storage modulus is greater than the loss modulus as a frequency of 100 radians per second.

78. An emulsion styrene-butadiene rubber composition as specified in claim 34 wherein the high molecular weight styrene-butadiene rubber is comprised of repeat units which are derived from about 5 weight percent to about 20 weight percent styrene.

79. An emulsion styrene-butadiene rubber composition which is made by a process which comprises coagulating a latex composition which is comprised of (a) water, (b) an emulsifier, (c) a high molecular weight styrene-butadiene rubber having a weight average molecular weight of at least about 300,000 and (d) a low molecular weight styrene-butadiene rubber having a weight average molecular weight of less than about 280,000; wherein the ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 80:20 to about 25:75; and wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 5 percentage points.

80. An emulsion styrene-butadiene rubber composition as specified in claim 79 wherein the styrene-butadiene rubber is dried subsequent to the coagulation.

81. An emulsion styrene-butadiene rubber composition as specified in claim 79 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 10 percentage points.

82. An emulsion styrene-butadiene rubber composition as specified in claim 81 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 400,000 to about 1,750,000 and wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 70,000 to about 270,000.

83. An emulsion styrene-butadiene rubber composition as specified in claim 82 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 15 percentage points.

84. An emulsion styrene-butadiene rubber composition as specified in claim 83 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 1,000,000 to about 1,500,000 and wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 120,000 to about 260,000.

85. An emulsion styrene-butadiene rubber composition as specified in claim 84 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

86. An emulsion styrene-butadiene rubber as specified in claim 84 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

87. An emulsion styrene-butadiene rubber composition as specified in claim 75 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by 5 percentage points to 40 percentage points.

88. An emulsion styrene-butadiene rubber composition as specified in claim 82 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by 10 percentage points to 30 percentage points.

89. An emulsion styrene-butadiene rubber composition as specified in claim 85 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by 15 percentage points to 25 percentage points.

90. A styrene-butadiene rubber as specified in claim 56 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.13 to 0.19 and a tan delta at 60° C. which is within the range of 0.06 to 0.120 after being cured in a rubber blend containing 70 parts by weight of the styrene-butadiene rubber, 30 parts by weight of high cis-1,4-polybutadiene rubber, 7.5 parts of highly aromatic processing oil, 70 parts by weight of N220 carbon black, 2 parts by weight of zinc oxide, 0.8 parts by weight of a paraffin wax, 3 parts by weight of a microcrystalline wax, 1.15 parts by weight of para-phenylene diamine antioxidant, 1.2 parts by weight of N-cyclohexyl-2-benzothiazole sulfenamide, 0.3 parts by weight of tetramethyl thiuram disulfide and 1.45 parts by weight of sulfur.

91. A styrene-butadiene rubber composition as specified in claim 90 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.14 to 0.18 and a tan delta at 60° C. which is within the range of 0.08 to 0.11.

92. A styrene-butadiene rubber composition as specified in claim 90 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.15 to 0.17 and a tan delta at 60° C. which is within the range of 0.09 to 0.10.

93. A styrene-butadiene rubber as specified in claim 57 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.13 to 0.19 and a tan delta at 60° C. which is within the range of 0.06 to 0.120 after being cured in a rubber blend containing 70 parts by weight of the styrene-butadiene rubber, 30 parts by weight of high cis-1,4-polybutadiene rubber, 7.5 parts by weight of highly aromatic processing oil, 70 parts by weight of N220 carbon black, 2 parts by weight of zinc oxide, 0.8 parts by weight of a paraffin wax, 3 parts by weight of a microcrystalline wax, 1.15 parts by weight of para-phenylene diamine antioxidant, 1.2 parts by weight of N-cyclohexyl-2-benzothiazole sulfenamide, 0.3 parts by weight of tetramethyl thiuram disulfide and 1.45 parts by weight of sulfur.

94. A styrene-butadiene rubber composition as specified in claim 93 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.14 to 0.18 and a tan delta at 60° C. which is within the range of 0.08 to 0.11.

95. A styrene-butadiene rubber composition as specified in claim 93 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.15 to 0.17 and a tan delta at 60° C. which is within the range of 0.09 to 0.10.

96. A styrene-butadiene rubber composition as specified in claim 90 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

97. A styrene-butadiene rubber composition as specified in claim 96 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

98. A styrene-butadiene rubber composition as specified in claim 91 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

99. A styrene-butadiene rubber composition as specified in claim 98 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

100. A styrene-butadiene rubber composition as specified in claim 92 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

101. A styrene-butadiene rubber composition as specified in claim 100 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

102. A styrene-butadiene rubber composition as specified in claim 93 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

103. A styrene-butadiene rubber composition as specified in claim 102 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

104. A styrene-butadiene rubber composition as specified in claim 103 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.001 radians per second to 10 radians per second.

105. A styrene-butadiene rubber composition as specified in claim 94 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

106. A styrene-butadiene rubber composition as specified in claim 105 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

107. A styrene-butadiene rubber composition as specified in claim 106 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.01 radians per second to 5 radians per second.

108. A styrene-butadiene rubber composition as specified in claim 95 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

109. A styrene-butadiene rubber composition as specified in claim 108 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

110. A styrene-butadiene rubber composition as specified in claim 109 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.05 radians per second to 1 radians per second.

111. A styrene-butadiene rubber composition as specified in claim 110 wherein the storage modulus is less than the loss modulus at a frequency of 0.1 radians per second and wherein the storage modulus is greater than the loss modulus as a frequency of 100 radians per second.

112. A styrene-butadiene rubber as specified in claim 56 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.18 to 0.40 and a tan delta at 60° C. which is within the range of 0.09 to 0.16 after being cured in a rubber blend containing 70 parts by weight of the styrene-butadiene rubber, 30 parts by weight of high cis-1,4-polybutadiene rubber, 7.5 parts of highly aromatic processing oil, 70 parts by weight of N220 carbon black, 2 parts by weight of zinc oxide, 0.8 parts by weight of a paraffin wax, 3 parts by weight of a microcrystalline wax, 1.15 parts by weight of para-phenylene diamine antioxidant, 1.2 parts by weight of N-cyclohexyl-2-benzothiazole sulfenamide, 0.3 parts by weight of tetramethyl thiuram disulfide and 1.45 parts by weight of sulfur.

113. A styrene-butadiene rubber composition as specified in claim 112 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.20 to 0.30 and a tan delta at 60° C. which is within the range of 0.10 to 0.14.

114. A styrene-butadiene rubber composition as specified in claim 112 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.23 to 0.27 and a tan delta at 60° C. which is within the range of 0.10 to 0.13.

115. A styrene-butadiene rubber as specified in claim 57 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.18 to 0.40 and a tan delta at 60° C. which is within the range of 0.09 to 0.16 after being cured in a rubber blend containing 70 parts by weight of the styrene-butadiene rubber, 30 parts by weight of high cis-1,4-polybutadiene rubber, 7.5 parts by weight of highly aromatic processing oil, 70 parts by weight of N220 carbon black, 2 parts by weight of zinc oxide, 0.8 parts by weight of a paraffin wax, 3 parts by weight of a microcrystalline wax, 1.15 parts by weight of para-phenylene diamine antioxidant, 1.2 parts by weight of N-cyclohexyl-2-benzothiazole sulfenamide, 0.3 parts by weight of tetramethyl thiuram disulfide and 1.45 parts by weight of sulfur.

116. A styrene-butadiene rubber composition as specified in claim 115 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.20 to 0.30 and a tan delta at 60° C. which is within the range of 0.10 to 0.14.

117. A styrene-butadiene rubber composition as specified in claim 115 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.23 to 0.27 and a tan delta at 60° C. which is within the range of 0.10 to 0.13.

118. A styrene-butadiene rubber composition as specified in claim 112 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

119. A styrene-butadiene rubber composition as specified in claim 118 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

120. A styrene-butadiene rubber composition as specified in claim 111 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

121. A styrene-butadiene rubber composition as specified in claim 120 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

122. A styrene-butadiene rubber composition as specified in claim 114 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

123. A styrene-butadiene rubber composition as specified in claim 122 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

124. A styrene-butadiene rubber composition as specified in claim 115 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

125. A styrene-butadiene rubber composition as specified in claim 124 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

126. A styrene-butadiene rubber composition as specified in claim 125 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.001 radians per second to 10 radians per second.

127. A styrene-butadiene rubber composition as specified in claim 116 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

128. A styrene-butadiene rubber composition as specified in claim 127 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

129. A styrene-butadiene rubber composition as specified in claim 128 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.01 radians per second to 5 radians per second.

130. A styrene-butadiene rubber composition as specified in claim 117 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

131. A styrene-butadiene rubber composition as specified in claim 130 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

132. A styrene-butadiene rubber composition as specified in claim 131 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.05 radians per second to 1 radians per second.

133. A styrene-butadiene rubber composition as specified in claim 132 wherein the storage modulus is less than the loss modulus at a frequency of 0.1 radians per second and wherein the storage modulus is greater than the loss modulus at a frequency of 100 radians per second.

134. A styrene-butadiene rubber as specified in claim 56 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.20 to 0.40 and a tan delta at 60° C. which is within the range of 0.03 to 0.10 after being cured in a rubber blend containing 70 parts by weight of the styrene-butadiene rubber, 30 parts by weight of high cis-1,4-polybutadiene rubber, 33.75 parts by weight of highly aromatic processing oil, 60 parts of silica, 10 parts of N330 carbon black, 2 parts of polymerized 1,2-dihydro-2,24-trimethylquinoline antioxidant, 3 parts of zinc oxide, 1.25 parts of para-phenylene diamine antioxidant, 1.8 parts of N-cyclohexyl-2-benzothiazole sulfenamide, 1.45 parts of diphenylguanidine accelerator and 1.6 parts of sulfur.

135. A styrene-butadiene rubber composition as specified in claim 134 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.22 to 0.35 and a tan delta at 60° C. which is within the range of 0.04 to 0.08.

136. A styrene-butadiene rubber composition as specified in claim 134 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.25 to 0.27 and a tan delta at 60° C. which is within the range of 0.05 to 0.07.

137. A styrene-butadiene rubber as specified in claim 57 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.20 to 0.40 and a tan delta at 60° C. which is within the range of 0.03 to 0.10 after being cured in a rubber blend containing 70 parts by weight of the styrene-butadiene rubber, 30 parts by weight of high cis-1,4-polybutadiene rubber, 33.75 parts by weight of highly aromatic processing oil, 60 parts of silica, 10 parts of N330 carbon black, 2 parts of polymerized 1,2-dihydro-2,24-trimethylquinoline antioxidant, 3 parts of zinc oxide, 1.25 parts of para-phenylene diamine antioxidant, 1.8 parts of N-cyclohexyl-2-benzothiazole sulfenamide, 1.45 parts of diphenylguanidine accelerator and 1.6 parts of sulfur.

138. A styrene-butadiene rubber composition as specified in claim 137 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.22 to 0.35 and a tan delta at 60° C. which is within the range of 0.04 to 0.08.

139. A styrene-butadiene rubber composition as specified in claim 137 wherein said styrene-butadiene rubber composition has a tan delta at 0° C. which is within the range of 0.25 to 0.27 and a tan delta at 60° C. which is within the range of 0.05 to 0.07.

140. A styrene-butadiene rubber composition as specified in claim 134 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

141. A styrene-butadiene rubber composition as specified in claim 140 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

142. A styrene-butadiene rubber composition as specified in claim 135 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

143. A styrene-butadiene rubber composition as specified in claim 142 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

144. A styrene-butadiene rubber composition as specified in claim 136 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

145. A styrene-butadiene rubber composition as specified in claim 144 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

146. A styrene-butadiene rubber composition as specified in claim 137 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 60,000 to 145,000.

147. A styrene-butadiene rubber composition as specified in claim 146 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.0 to 3.8.

148. A styrene-butadiene rubber composition as specified in claim 147 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.001 radians per second to 10 radians per second.

149. A styrene-butadiene rubber composition as specified in claim 138 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 75,000 to 140,000.

150. A styrene-butadiene rubber composition as specified in claim 149 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.1 to 3.7.

151. A styrene-butadiene rubber composition as specified in claim 150 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.01 radians per second to 5 radians per second.

152. A styrene-butadiene rubber composition as specified in claim 139 wherein the number average molecular weight as determined by thermal field flow fractionation which is within the range of about 90,000 to 135,000.

153. A styrene-butadiene rubber composition as specified in claim 152 wherein the styrene-butadiene rubber has a light scattering to refractive index ratio which is within the range of 2.2 to 3.0.

154. A styrene-butadiene rubber composition as specified in claim 153 wherein the plot of log frequency versus storage modulus crosses over a plot of log frequency versus loss modulus at a frequency within the range of 0.05 radians per second to 1 radians per second.

155. A styrene-butadiene rubber composition as specified in claim 154 wherein the storage modulus is less than the loss modulus at a frequency of 0.1 radians per second and wherein the storage modulus is greater than the loss modulus at a frequency of 100 radians per second.

156. A styrene-butadiene rubber latex which is comprised of (a) water, (b) an emulsifier, (c) a high molecular weight styrene-butadiene rubber having a weight average molecular weight of at least about 300,000 and (d) a low molecular weight styrene-butadiene rubber having a weight average molecular weight of less than about 280,000; wherein the ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 80:20 to about 25:75; and wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 5 percentage points.

157. A styrene-butadiene rubber latex as specified in claim 156 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 10 percentage points.

158. A styrene-butadiene rubber latex as specified in claim 157 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 400,000 to about 1,750,000 and wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 70,000 to about 270,000.

159. A styrene-butadiene rubber latex as specified in claim 158 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by at least 15 percentage points.

160. A styrene-butadiene rubber latex as specified in claim 159 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 1,000,000 to about 1,500,000 and wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 120,000 to about 260,000.

161. A styrene-butadiene rubber latex as specified in claim 160 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

162. A styrene-butadiene rubber latex as specified in claim 160 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

163. A styrene-butadiene rubber latex as specified in claim 156 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by 5 percentage points to 40 percentage points.

164. A styrene-butadiene rubber latex as specified in claim 158 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by 10 percentage points to 30 percentage points.

165. A styrene-butadiene rubber latex as specified in claim 161 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber differs from the bound styrene content of the low molecular weight styrene-butadiene rubber by 15 percentage points to 25 percentage points.

166. A styrene-butadiene rubber composition as specified in claim 57 wherein the plot of log frequency versus storage modulus of the styrene-butadiene rubber composition crosses over a plot of log frequency versus loss modulus of the styrene-butadiene rubber composition at a modulus which is within the range of about 5000 Pascals to about 100,000 Pascals.

167. A styrene-butadiene rubber composition as specified in claim 57 wherein the plot of log frequency versus storage modulus of the styrene-butadiene rubber composition crosses over a plot of log frequency versus loss modulus of the styrene-butadiene rubber composition at a modulus which is within the range of about 10,000 Pascals to about 50,000 Pascals.

168. A styrene-butadiene rubber composition as specified in claim 57 wherein the plot of log frequency versus storage modulus of the styrene-butadiene rubber composition crosses over a plot of log frequency versus loss modulus of the styrene-butadiene rubber composition at a modulus which is within the range of about 12,000 Pascals to about 40,000 Pascals.

169. An emulsion styrene-butadiene rubber composition which is made by a process which comprises coagulating a latex composition which is comprised of (a) water, (b) an emulsifier, (c) a high molecular weight styrene-butadiene rubber having a weight average molecular weight of at least about 300,000 and (d) a low molecular weight polybutadiene rubber having a weight average molecular weight of less than about 280,000; wherein the ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight polybutadiene rubber is within the range of about 80:20 to about 25:75; and wherein the bound styrene content of the high molecular weight styrene-butadiene rubber is at least about 10 weight percent.

170. An emulsion styrene-butadiene rubber composition as specified in claim 169 wherein the styrene-butadiene rubber composition is dried subsequent to the coagulation.

171. An emulsion styrene-butadiene rubber composition as specified in claim 169 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber is at least about 15 weight percent.

172. An emulsion styrene-butadiene rubber composition as specified in claim 171 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 400,000 to about 1,750,000 and wherein the low molecular weight polybutadiene rubber has a weight average molecular weight which is within the range of about 70,000 to about 270,000.

173. An emulsion styrene-butadiene rubber composition as specified in claim 172 wherein the bound styrene content of the high molecular weight styrene-butadiene rubber is at least about 20 weight percent.

174. An emulsion styrene-butadiene rubber composition as specified in claim 173 wherein the high molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 1,000,000 to about 1,500,000 and wherein the low molecular weight polybutadiene rubber has a weight average molecular weight which is within the range of about 120,000 to about 260,000.

175. An emulsion styrene-butadiene rubber composition as specified in claim 174 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight polybutadiene rubber is within the range of about 70:30 to about 30:70.

176. An emulsion styrene-butadiene rubber as specified in claim 174 wherein the weight ratio of the high molecular weight styrene-butadiene rubber to the low molecular weight polybutadiene rubber is within the range of about 60:40 to about 40:60.

177. An emulsion styrene-butadiene rubber as specified in claim 169 wherein the low molecular weight polybutadiene rubber has a Mooney ML 1+4 viscosity that differs from the Mooney ML 1+4 viscosity of the high molecular weight styrene-butadiene rubber by at least 50 Mooney points.

178. An emulsion styrene-butadiene rubber as specified in claim 169 wherein the low molecular weight polybutadiene rubber has a Mooney ML 1+4 viscosity that differs from the Mooney ML 1+4 viscosity of the high molecular weight styrene-butadiene rubber by at least 70 Mooney points.

179. An emulsion styrene-butadiene rubber as specified in claim 169 wherein the low molecular weight polybutadiene rubber has a Mooney ML 1+4 viscosity that differs from the Mooney ML 1+4 viscosity of the high molecular weight styrene-butadiene rubber by at least 80 Mooney points.

180. An emulsion styrene-butadiene rubber composition which is made by a process which comprises coagulating a latex composition which is comprised of (a) water, (b) an emulsifier, (c) a high molecular weight polybutadiene rubber having a weight average molecular weight of at least about 300,000 and (d) a low molecular weight styrene-butadiene rubber having a weight average molecular weight of less than about 280,000; wherein the ratio of the high molecular weight polybutadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 80:20 to about 25:75; and wherein the bound styrene content of the low molecular weight styrene-butadiene rubber is at least about 10 weight percent.

181. An emulsion styrene-butadiene rubber composition as specified in claim 180 wherein the styrene-butadiene rubber composition is dried subsequent to the coagulation.

182. An emulsion styrene-butadiene rubber composition as specified in claim 180 wherein the bound styrene content of the low molecular weight styrene-butadiene rubber is at least about 15 weight 10 percent.

183. An emulsion styrene-butadiene rubber composition as specified in claim 182 wherein the high molecular weight polybutadiene rubber has a weight average molecular weight which is within the range of about 400,000 to about 1,750,000 and wherein the low molecular weight polybutadiene rubber has a weight average molecular weight which is within the range of about 70,000 to about 270,000.

184. An emulsion styrene-butadiene rubber composition as specified in claim 183 wherein the bound styrene content of the low molecular weight styrene-butadiene rubber is at least about 20 weight 25 percent.

185. An emulsion styrene-butadiene rubber composition as specified in claim 184 wherein the high molecular weight polybutadiene rubber has a weight 30 average molecular weight which is within the range of about 1,000,000 to about 1,500,000 and wherein the low molecular weight styrene-butadiene rubber has a weight average molecular weight which is within the range of about 120,000 to about 260,000.

186. An emulsion styrene-butadiene rubber composition as specified in claim 185 wherein the weight ratio of the high molecular weight polybutadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 70:30 to about 30:70.

187. An emulsion styrene-butadiene rubber as specified in claim 185 wherein the weight ratio of the high molecular weight polybutadiene rubber to the low molecular weight styrene-butadiene rubber is within the range of about 60:40 to about 40:60.

188. An emulsion styrene-butadiene rubber as specified in claim 180 wherein the low molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity that differs from the Mooney ML 1+4 viscosity of the high molecular weight polybutadiene rubber by at least 50 Mooney points.

189. An emulsion styrene-butadiene rubber as specified in claim 180 wherein the low molecular weight styrene-butadiene rubber has a Mooney ML 1+4 viscosity that differs from the Mooney ML 1+4 viscosity of the high molecular weight polybutadiene rubber by at least 70 Mooney points.

190. An emulsion styrene-butadiene rubber as specified in claim 180 wherein the low molecular weight styrene-butadiene rubber has a Mooney-ML 1+4 viscosity that differs from the Mooney ML 1+4 viscosity of the high molecular weight polybutadiene rubber by at least 80 Mooney points.