Compositions comprising solution styrene butadiene rubber (SSBR) are used in such items as tire treads often in combination with a second rubber such as cis-butadiene rubber (BR) or natural rubber (NR). The characteristics of the compositions such as heat build up (HBU), rolling resistance, grip, and/or tear are believed to be heavily influenced by the composition of the SSBR, e.g., the styrene and vinyl content of the incorporated butadiene. Typically, an increase in styrene content, vinyl content, or both may improve properties like grip, tear, and/or braking performance but worsen properties like HBU, rolling resistance and/or abrasion resistance.
Various methods and compositions have been tried in attempts to alter the resulting polymer and improve characteristics such as wet grip and abrasion balance. As EP1637353 describes, mixtures of oil extended high styrene, low vinyl SSBR with cis polybutadiene, silica, processing oil and organosilicon compounds may be used in an attempt to balance these properties. However, additional compounds are still needed which have improved grip and rolling resistance behavior when combined with natural rubber and/or carbon black as filler.
DE3108583 proposes to polymerize styrene and butadiene in such a way that a part of the monomers with a higher ratio of styrene than the final ratio of the target polymer is put into the reaction zone containing solvent. Simultaneously, a styrene/butadiene mixture containing the same or lower weight ratio of styrene than the targeted polymer is charged over a certain time to the reaction zone. The resulting polymer is coupled with a di-functional coupling agent. The coupling efficiency is about 50%. Unfortunately, the resulting polymer has a higher styrene content on the chain ends and a lower styrene content in the middle of the polymer chain. Such styrene blocks often result in deficient HBU and rolling resistance properties.
DE2247470 proposes using a dilithio initiator and charging the monomer mixture to the reaction zone at a higher or equal rate than the polymerization growth rate. Unfortunately, such dilithio initiators are not cost-effective and the resulting polymer often has small styrene blocks on the end of the polymer chain. Such blocks may result in insufficient rolling resistance and/or HBU.
In DE1300239 a faster reacting monomer is charged to a reaction zone which contains an initiator, solvent, and a monomer mixture consisting of a specific ratio of both monomers which is maintained during the polymerization. Unfortunately, the resulting polymer has a styrene content which is only slightly increased during the long polymerization time.
Accordingly, improved compositions of SSBR compositions are needed. It would be desirable if such improved compositions exhibit sufficient or improved balance of characteristics such as rolling resistance, HBU, abrasion, grip, and/or tear. It would further be desirable if such compositions could be made in an efficient, cost-effective manner.
Advantageously, improved compositions of SSBR and methods of making them have been discovered that meet many of the aforementioned needs and also have other desirable characteristics.
In one embodiment, a composition of the present invention comprises a styrene butadiene rubber. The styrene butadiene rubber is characterized before any vulcanization by one or more of the following: (a) two or more portions in one polymer chain which are incompatible with each other; or (b) two or more glass transition temperatures that vary by at least about 6° C.; or (c) two or more δ (SBR) values which differ by at least more than about 0.65 (J/cm3)0.5; or (d) two or more portions which vary in styrene content by at least about 20 weight percent based on the total weight of rubber. The styrene butadiene rubber comprises at least a portion which is compatible with a second rubber (e.g., a rubber used in a tire tread composition) and a portion which is not compatible with said same second rubber.
In another embodiment, the present invention relates to a method of making a styrene butadiene rubber. The method comprises first charging styrene and butadiene into a reactor in a first molar ratio of styrene:butadiene in the presence of a monolithio initiator, a randomizer, and a solvent under polymerization conditions sufficient to obtain a first portion of styrene butadiene rubber with a monomer conversion in one embodiment of higher than about 80%, in another embodiment higher than about 88%, and in another embodiment higher than about 99%. Next, styrene, butadiene, or a mixture thereof is charged into the reactor in a second molar ratio in the presence of said first portion of styrene butadiene rubber, randomizer, and solvent under polymerization conditions sufficient to obtain a second portion of styrene butadiene rubber by polymerization to a final monomer conversion of higher than about 95%, in another embodiment higher than about 98%, and in another embodiment higher than about 99% based on the total charged monomer. The first and second molar ratios are selected so as to produce styrene butadiene rubber characterized by one or more of the following: a) two or more portions in one polymer chain which are incompatible with each other; or (b) two or more glass transition temperatures that vary by at least about 6° C.; or (c) two or more δ (SBR) values which differ by at least more than about 0.65 (J/cm3)0.5; or (d) two or more portions which vary in styrene content by at least about 20 weight percent based on the total weight of the rubber. The styrene butadiene rubber comprises at least a portion which is compatible with a second rubber (e.g., a rubber used in a tire tread composition) and a portion which is not compatible with said same second rubber.