Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2002-12-20
2004-03-09
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S098000, C525S940000, C524S575000
Reexamination Certificate
active
06703449
ABSTRACT:
FIELD OF THE INVENTION
This invention is concerned with improved block copolymers. More particularly, it is directed to block copolymers having both polystyrene and low vinyl content polydiene hard blocks to give improved resistance to organic solvents.
BACKGROUND OF THE INVENTION
Block copolymers of styrene and conjugated dienes such as butadiene and isoprene are well known. Such block copolymers are often selectively hydrogenated so that they contain blocks of polystyrene and blocks of hydrogenated polydiene. The most common configuration for these polymers is the general structure A-B-A where each A is a styrene polymer block and B is a conjugated diene polymer block. These polymers are generally referred to as thermoplastic elastomers because they behave as a vulcanized rubber below their softening point but act as a thermoplastic melt above the softening point and, moreover, even after being raised to such a melt temperature and thereafter cooled, these polymers resume the properties of an elastomer. The diene used to make the polydiene block in these polymers generally must have a relatively high vinyl content, i.e. at least about 25% and preferably up to about 40% by weight because if the vinyl content is lower than that, when the polymer is hydrogenated, the polydiene block is not sufficiently elastomeric in nature.
These polymers exhibit an excellent range and variety of physical properties, including the strength and elastomeric properties described above. However, they have certain limiting characteristics, one of the most serious of which is their sensitivity to organic solvents and particularly to relatively volatile hydrocarbons. Contact with such solvents causes dissolution of the polymer or at least an undesirably high degree of swelling depending on the circumstances and species of the solvents involved as well as upon the particular block copolymers so exposed. It would be highly desirable to eliminate or minimize this solvent sensitivity so as to broaden the utility of these block copolymers.
Several attempts have been made to make slightly different polymers which exhibit improved resistance to solvent attack. Japanese published application JP06306127 describes oil resistant hydrogenated block copolymers which comprise a block of polybutadiene with a vinyl content of above 80% and number average molecular weight of 3,000 to 30,000, an isoprene or isoprene-butadiene block with a vinyl content above 40% and a number average molecular weight of 30,000 to 200,000, and a vinyl aromatic hydrocarbon (styrene) block with a molecular weight of 40,000 to 300,000. These polymers are disadvantageous because the high vinyl or isoprene-butadiene mixed block offer no physical reinforcement in the presence of solvents. Japanese published application 06306128 describes oil resistant hydrogenated 40,000 to 300,000 molecular weight block copolymers of the A-B-A structure, wherein the A blocks are blocks of butadiene with a vinyl content of above 80% and a number average molecular weight of 3,000 to 30,000 and the B block is an isoprene or isoprene-butadiene block with a vinyl content above 40% and a number average molecular weight of 30,000 to 200,000. These polymers are disadvantageous because they are very weak unless the A block molecular weight is 20,000 to 30,000 wherein they are very difficult to manufacture because they have extremely high solution viscosities and again the high vinyl blocks offer no physical resistance to the effects of solvents.
U.S. Pat. No. 3,670,054 describes hydrogenated block copolymers having reduced solvent sensitivity which have the structure C-A-B-A-C wherein each A is a vinyl aromatic hydrocarbon (styrene) block having a molecular weight of 7,500 to 100,000, B is an elastomeric conjugated diene polymer block having a molecular weight of 25,000 to 200,000 and having a vinyl content of 35 to 55%, and each C is a polybutadiene block having a molecular weight between 1,000 and 15,000 and having a vinyl content of less than 25% by weight. These polymers are disadvantageous because they are easily gelled during processing and production by crystallization of both endblocks of the molecule.
SUMMARY OF THE INVENTION
The present invention provides block copolymers which have reduced solvent sensitivity and which overcome the foregoing disadvantages. In one embodiment of the present invention, there is provided an asymmetric block copolymer of the structure A-B-C-A wherein each A block is formed of a vinyl aromatic hydrocarbon, preferably styrene, and has a weight average molecular weight of 5,000, preferably 7,500, to 50,000, B is a polybutadiene block having a weight average molecular weight of 1,000 to 15,000 and a vinyl content of less than 25% by weight, and C is an elastomeric conjugated diene polymer block having a weight average molecular weight of 25,000 to 200,000 and has a vinyl content of 30 to 90%, preferably 35 to 80%, and most preferably 35 to 70% by weight. In the second embodiment of the present invention, there is provided a hydrogenated block copolymer of the formula A-B-C-B-A wherein A, B, and C have the definitions set forth above.
DETAILED DESCRIPTION OF THE INVENTION
The term “vinyl content” refers to the fact that a conjugated diene is polymerized via 1,2-addition (in the case of butadiene—it would be 1,2 or 3,4 addition in the case of isoprene). Although a pure “vinyl” group is formed only in the case of 1,2 addition polymerization of 1,3 butadiene, the effects of 1,2 or 3,4 addition polymerization of isoprene (and similar addition for other conjugated dienes) on the final properties of the block copolymer will be similar. The term “vinyl” refers to the presence of a pendant vinyl group on the polymer chain. The purpose here is to introduce chain branching and thereby structural irregularity which yields a suppression of crystallinity.
The endblocks of these novel block copolymers are polymer blocks of styrene. Other vinyl aromatic hydrocarbons, including &agr;-methylstyrene, various alkyl-substituted styrenes, alkoxy-substituted styrenes, vinyl naphthalene, vinyl toluene, and the like, can be substituted for styrene and are especially included in this invention. The butadiene used herein must produce a polymer block with a low vinyl content. In other words, the percent of 1,2 addition of the butadiene should be less than 25% by weight, preferably 1 to 10%. When the vinyl content of the polybutadiene block is below 25%, when it is hydrogenated it forms a hard, crystallizable block very similar to polyethylene. It is relatively resistant to solvent attack and assists the polymer as a whole in this regard as discussed in more detail below. The conjugated diene used herein for the internal hydrogenated elastomeric C block must produce a polymer block with a relatively high vinyl content. The percent of 1,2 addition of butadiene or 1,2 or 3,4 addition of isoprene must be in the range of 30 to 90%, preferably 35 to 80%, and most preferably 35 to 70%, because in this range, this polymer block will be elastomeric in nature and thus will give the polymer itself its elastomeric character.
The strength exhibited by the styrene-hydrogenated diene-styrene block copolymers of the prior art is in theory explained by the presence of polystyrene “domains” which form in the polymer. The polystyrene blocks from different molecules associate together and this physical crosslinking provides the mechanism for strength in styrenic block copolymers. These polystyrene domains, however, are glassy in nature. As such they are susceptible to swelling and dissolution by solvents. Even small amounts of solvents in the glassy polystyrene domains results in significant reductions in physical strength. These strength-forming domains become plasticized.
Crystalline polymers are known to be resistant to dissolution by solvents. While the solvents swell the amorphous segments of partially crystalline polymers like polyethylene, the crystalline segments require heating to lose their structural order and strength.
However, because of crystalline polymers'
Hoxmeier Ronald James
Masse Michael Alan
Kraton Polymers U.S. LLC
Teskin Fred
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