Synthesis of styrene-isoprene rubber

Details Synthesis of styrene-isoprene rubber Synthesis of styrene-isoprene rubber

2000-09-01

2001-11-06

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S856000, C525S271000, C525S313000, C525S332300, C525S332900, C526S065000, C526S073000, C526S087000, C526S088000, C526S340000

Reexamination Certificate

active

06313216

ABSTRACT:

BACKGROUND OF THE INVENTION
It is desirable for a tire to exhibit good traction characteristics on wet and dry pavements, and for the tire to provide good treadwear and low rolling resistance. In order to reduce the rolling resistance of a tire, rubbers having a high rebound can be utilized in making the tires' tread. Tires made with such rubbers undergo less energy loss during rolling. The traditional problem associated with this approach is that the tire's wet traction and wet skid resistance characteristics are compromised. This is because good rolling resistance which favors low energy loss and good traction characteristics which favor high energy loss are viscoelastically inconsistent properties.
In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-isoprene rubber (SBR), polyisoprene rubber, and natural rubber are commonly used in automobile tire treads formulations. Styrene-isoprene rubber is included in tire tread formulations primarily to improve the traction characteristics of the tire without greatly compromising tread-wear or rolling resistance.
The versatility of solution SBR (SSBR) synthesis relative to the synthesis of emulsion (ESBR), including control of molecular weight, macrostructure, microstructure, and functionalization, is well established (see Hirao, A.; Hayashi, M. Acta. Polym. 1999, 50, 219-231, and references cited therein). Performance advantages arising from this versatility have led to an acceleration of the replacement of emulsion SBR in the fire industry, and an expansion in the market for random, low vinyl SBR for use in tire compounds (see Autcher, J. F.; Schellenberg, T.; Naoko, T. “Styrene-Butadiene Elastomers (SBR),” Chemical Economics Handbook SRI-International, November, 1997). These developments have stimulated interest in developing technology for commercial production of random, low vinyl solution SBR.
Although anionic initiated synthesis of random medium vinyl solution SBR and random high vinyl solution SBR is easily accomplished by the addition of Lewis bases, these polar modifiers promote randomization at the expense of increased vinyl content (see Antkowiak, T. A.; Oberster, A. E.; Halasa, A. F.; Tate, D. P. J. Polym. Sci., Part A-1, 1972, 10, 1319). Due to the large differences in monomer reactivity ratios of isoprene and styrene, measures must be taken to promote random incorporation of styrene into low vinyl solution SBR In the absence of such measures, the polymerization leads to a tapered block copolymer with inferior elastomeric performance characteristics (see U.S. Pat. No. 3,558,575).
British Patent 994,726 reports that it is possible to produce random solution SBR by manipulating monomer polymerization rates via control of monomer concentrations throughout the polymerization process without the use of polar modifiers. For solution SBR, this requires that the polymerization proceed in a styrene rich medium throughout the polymerization. In continuous polymerizations the issues associated with maintaining constant monomer concentration ratios while increasing conversion become quite complex.
U.S. Pat. No. 3,787,377 reports that alkali metal alkoxides (NaOR) can be used as polar modifiers in the copolymerization of styrene and isoprene to randomize styrene incorporation without significantly increasing the vinyl content of the rubber. However, alkali metal alkoxide modifiers are so effective that they may actually increase the rate of polymerization of styrene to the extent that it is depleted before the polymerization is complete (see Hsieh, H. L.; Wofford, C. F. J. Polym. Sci., Part A-1, 1969, 7, 461-469). Furthermore, there is typically some undesired increase in vinyl content over what would be expected from an unmodified polymerization (see Hsieh, H. L.; Wofford, C. F. J. Polym. Sci., Part A-1, 1969, 7, 449460).
SUMMARY OF THE INVENTION
A method to prevent the formation of tapered block solution SIR in unmodified polymerizations using standard continuous stirred tank reactors (CSTRs) has been developed. This method involves charging all of the styrene and part of the isoprene being polymerized into a first polymerization zone. The first polymerization zone is typically a continuous stirred tank reactor. The amount of styrene charged into the first polymerization zone will typically be at least 2 percent more than the amount of styrene bound into the styrene-isoprene rubber being synthesized. It is important for a conversion within the range of about 60 percent to about 95 percent to be attained in the first polymerization zone. Additional isoprene monomer is charged into a second polymerization zone, such as a second continuous stirred tank reactor. Typically from about 5 percent to about 40 percent of the total amount isoprene charged will be charged into the second polymerization zone. It is also important for a isoprene conversion of at least about 90 percent to be attained in the second polymerization zone and for the total conversion (styrene and isoprene) to be limited to a maximum of about 98 percent in the second polymerization zone.
This invention more specifically discloses a process of synthesizing random styrene-isoprene rubber having a low level of branching and a low vinyl content which comprises: (1) continuously charging isoprene, styrene, an alkyl lithium initiator, and an organic solvent into a first polymerization zone, (2) allowing the isoprene and styrene to copolymerize in the first polymerization zone to total conversion which is within the range of about 60 percent to about 95 percent to produce a polymer cement containing living styrene-isoprene chains, (3) continuously charging the polymer cement containing living styrene-isoprene chains and additional isoprene monomer into a second polymerization zone, wherein from 5 percent to 40 percent of the total amount of isoprene changed is charged into the second polymerization zone, (4) allowing the copolymerization to continue in the second polymerization zone to a conversion of the isoprene monomer of at least 90 percent, wherein the total conversion of styrene and isoprene in the second polymerization zone is limited to a maximum of 98 percent, (5) withdrawing a polymer cement of random styrene soprene rubber having living chain ends from the second reaction zone, (6) killing the living chain ends on the random styrene-isoprene rubber, and (7) recovering the random styrene-isoprene rubber from the polymer cement, wherein the copolymerizations in the first polymerization zone and the second polymerization zone are carried out at a temperature which is within the range of about 70° C. to about 100° C., and wherein the amount of styrene charged into the first polymerization zone is at least 2 percent more than the total amount of styrene bound into the random styrene-isoprene rubber. The living chain ends on the random styrene-isoprene rubber can optionally be killed by the addition of a coupling agent, such as tin tetrachloride.
The present invention also reveals a cement of living styrene-isoprene rubber which is comprised of an organic solvent and polymer chains that are derived from isoprene and styrene, wherein the polymer chains are terminated with lithium end groups, wherein the polymer chains have a vinyl content of less than 10 percent, wherein less than 5 percent of the total quantity of repeat units derived from styrene in the polymer chains are in blocks containing five or more styrene repeat units, and wherein the molar amount of polar modifier in the cement of the living styrene-isoprene rubber is at a level of less than 20 percent of the number of moles of lithium end groups on the polymer chains of the living styrene-isoprene rubber. Such cements of living styrene-isoprene rubber made by the process of this invention can be easily coupled because they contain very low levels of polar modifiers.
In cases where the polymerization is carried out in the presence of a significa

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