Thermally reversible copolymers and process for the preparation thereof

#The present invention is directed toward polymer compositions comprising copolymers of N-(alkoxymethyl)acrylamides, dienes or diene-containing rubbers and metal salts. The amide-metal polymer compositions are cross-linked via metal ions and are thermally reversible in solution or bulk. Such copolymers exhibit improved green strength and can additionally be blended with synthetic rubber to improve green strength. When blended with natural rubber, equivalent amounts of the latter are saved without sacrifice of the green strength thereof.

#An important requirement for a tire body ply is that it have good green strength in order to prevent tearing or excessive sagging during the building operation as well as subsequent conveying and/or storing of the green tire. Additionally, high green strength will help to prevent cord movement during tire molding. Normal emulsion SBR containing 23% by weight bound styrene has poor green strength. Upon stretching, the material necks down rapidly and shows no strain hardening. By comparison, natural rubber strain crystallizes giving it excellent green strength. Improving the green strength of SBR rubber without adversely affecting its processability can be achieved by practice of the present invention.

#Resinous materials of N-(isobutoxymethyl)acrylamide, styrene-butadiene or styrene-isoprene and an aldehyde are disclosed in U.S. Pat. Nos. 3,037,963 and 3,079,434 and can be utilized to form coating films having good flexibility, recoat adhesion and resistance to marring, staining, heat, detergents and corrosion. Neither patent discloses the coagulation of a polymer latex comprising these ingredients with metal salts nor any suggestion by which the green strength of synthetic rubber such as SBR could be improved.

#FIGS. 1-6 present various stress-strain curves for copolymers of the present invention compounded with a conventional SBR and, for comparison, a conventional high green strength SBR and a natural rubber, all of which shall be explained in greater detail hereinbelow; and

#FIGS. 7 and 8 present additional stress-strain curves for copolymers of the present invention demonstrating the effect of cross-linking with metal ions in gum rubber and compounded stocks comprising SBR-IBMA polymers.

#The polymer composition of the present invention comprises a copolymer of an N-(alkoxymethyl)acrylamide, a diene and optionally a monomer containing a vinyl group. Suitable acrylamides having the formula ##STR1## can be employed wherein R is a straight or branched alkyl chain having from 1 to about 20 carbon atoms with 3 to 8 carbon atom chains being preferred. One particularly useful acrylamide we have employed for the work reported herein is N-(isobutoxymethyl)acrylamide, hereinafter abbreviated IBMA, the R group being isobutyl.

#With respect to the other monomers, those that are polymerizable in emulsion systems can be employed. Generally, diene monomers having from about four to 20 carbon atoms could be employed with butadiene or isoprene being preferred. Additionally, halogenated or other substituted emulsion polymerizable monomers could be employed. Monomers containing vinyl groups that can be employed include monovinyl aromatic compounds having from eight to about 20 carbon atoms and vinyl compounds having three to about five carbon atoms and a nitrile group. Examples of the first type would include styrene and alpha-methylstyrene as well as other alkyl substituted or halogenated emulsion polymerizable monomers. In a preferred embodiment, butadiene and styrene will be selected to form the styrene-butadiene copolymer, SBR, which is, in turn, copolymerized with IBMA and subsequently cross-linked with metal ions. Examples of the second type of vinyl compound would be acrylonitrile and methyl methacrylate. Butadiene and acrylonitrile can be employed to form a copolymer, nitrile rubber which is, in turn, copolymerized with IBMA or other acrylamide and cross-linked with metal ions.

#Regarding composition of the copolymer, the amount of acrylamide employed can range from about 0.5 to about 11 percent by weight with about 0.5 to 6 percent being preferred; the amount of diene can range from about 10 to about 90 percent by weight with about 70 to 80 percent being preferred and, the amount of the monomer containing a vinyl group can range from about 0 to about 90 percent by weight with about 20 to 30 percent being preferred. The vinyl monomer can be absent, wherein the copolymer of diene and acrylamide is formed, or it can be a monovinyl aromatic such as styrene or it can be the nitrile species as well as mixtures of the two. Number average molecular weight of the copolymers of the present invention can range from about 5,000 to about 500,000. Preferred number average molecular weights of the diene-vinyl-acrylamide copolymer can range from about 80,000 to about 150,000 while for the diene-acrylamide copolymer the range is from about 70,000 to about 250,000.