Polyurethane Elastomer Produced by Reaction Between a Triazole-Stabilized Isocyanate Prepolymer and a Diamine

Urethane elastomers are produced in large quantities from various isocyanate terminated prepolymers by reaction with a hindered diamine. Historically, 4,4'-methylene bis (o-chloroaniline) (hereinafter "MOCA") has probably been the most widely used hindered amine. However, MOCA is a carcinogen suspect agent, and, as a consequence, taking the necessary safety precautions to safeguard the health of workers substantially increases the cost involved in producing elastomers using MOCA as the hindered amine. Another hindered amine that has also seen considerable use is 4,4'-methylene bis (methyl-anthranilate) (hereinafter "MBMA").

So far as is known, unhindered diamines have not heretofore been used for reaction with an isocyanate terminated prepolymer to produce an elastomer. Pot life is short, only a few minutes, even when MOCA and MBMA are used as the diamines. In a recent experiment wherein an attempt was made to produce a urethane elastomer from an isocyanate terminated prepolymer and an unhindered diamine the pot life was estimated to be five seconds; the specific unhindered diamine used was 4,4'-methylene dianiline (hereinafter "MDA").

The present invention is based upon the discovery that an isocyanate terminated prepolymer can be stabilized by reaction with benzotriazole* or a tolyl triazole, and that a mixture of the stabilized prepolymer with a diamine has a longer pot life than does a mixture of the unstabilized prepolymer with that diamine. When a hindered diamine is used in producing a urethane elastomer according to the invention, e.g. MOCA or MBMA, the pot life is increased substantially so that the labor required for carrying out the mixing operation can be minimized, and the prepolymer is preferably stabilized with from 0.1 to 0.5 equivalent of the triazole per equivalent of isocyanate in the prepolymer. On the other hand, when the diamine is unhindered, urethane elastomers which, so far as is known, had not been made are the result of practicing the instant invention, and it is usually preferred that the prepolymer be stabilized with from 0.5 to 1.0 equivalent of the triazole per equivalent of isocyanate in the prepolymer.

The following Examples, which are presented solely for the purpose of illustrating and disclosing the invention, set forth the best presently known mode.

An isocyanate-terminated prepolymer was prepared from 1 mole of a poly(oxytetramethylene)glycol having a molecular weight of 1000 and a melting temperature of 50.degree. C. (hereinafter "PTMG 1000") and 2 moles of tolylene diisocyanate (hereinafter "TDL"). The specific TDL used was an 80/20 blend of 2,4- and 2,6-isomers. The isocyanate (hereinafter "NCO") content of the prepolymer, determined by di-n-butyl amine titration, was 6.2 percent. Benzotriazole (hereinafter "BT") was then melted, vacuum degassed, and mixed with a sample of the prepolymer which had been preheated to 75.degree. C. The BT was added in the proportion of 0.25 equivalent (mole) thereof per isocyanate equivalent of the prepolymer. The resulting mixture was stirred at 80.degree. C. for 1 hour to insure complete reaction between the BT and the free NCO groups of the prepolymer. The stabilized prepolymer which resulted, at 80.degree. C., was mixed with MBMA, which had been vacuum degassed at 150.degree. C., mixing temperature 140.degree. C., at an NCO: NH.sub.2 ratio of substantially 1:1. The resulting composition was found to have a pot life of 20 minutes at 100.degree. C.; it was vacuum degassed and poured into a mold preheated to 100.degree. C. The mold, which had a central cavity 6 inches by 6 inches by 0.06 inch in depth, was then covered by a flat plate and placed in a hydraulic press where a compressive force of 10,000 pounds per square inch was applied to the mold and plate. After approximately one hour in the mold at 100.degree. C. the partially cured elastomer was removed from the mold and transferred to an oven where it was cured for an additional 19 hours at 100.degree. C.

After conditioning at room temperature and 50 percent relative humidity for one week the elastomer was found to have the following physical properties:

Other urethane elastomers according to the invention, and controls in which no triazole was used, have been produced by the method described above in Example 1. The starting materials used, the ratio of equivalents of triazole used to equivalents of NCO, the mixing temperatures, the curing conditions, the pot life and the physical properties of the various elastomers, determined by the tests identified above, are set forth in Table 1, below. The following abbreviations, all used in Table 1, have the indicated meanings:

The curing was for approximately 1 hour in a mold at the temperature reported in Table 1 followed by oven curing at that temperature for the remainder of the time for each elastomer. In each case, the prepolymer was produced by reacting two moles of the indicated isocyanate with one mole of the polyol.

Substantially the procedure described above in Example 1, except that the prepolymer:diamine blend was merely poured into a pan mold after degassing, was used to produce urethane elastomers according to the method of the invention from a prepolymer made from TDL and a polyester which was hydroxy-terminated. The specific prepolymer had an NCO content of approximately 3.5 percent by weight; it is commercially available under the trade designation "Vibrathane 6025". Data concerning the elastomers are set forth in Table 11, below:

It is known e.g., in U.S. Pat. No. 3,721,645, that triazoles other than BT and TT are capable of reaction with NCO groups, and that the reaction is reversible in the sense that apparently normal cure occurs at an elevated temperature. It will be apparent that an equivalent amount of any of the other triazoles could be substituted for BT and TT in each of the foregoing Examples, and with similar results. However, BT and TT are the preferred triazoles because of availability, low volatility, anti-corrosive properties and price.

The curing times reported in the foregoing Examples are those which were actually used. It has been found, however, that the 18 to 20 hour curing times used in many instances are unnecessary in the sense that substantially the same physical properties result after a shorter cure. In most cases, a cure time of three to six hours has been found to be adequate.

Various changes and modifications can be made, as will be apparent to one skilled in the art, from the specific embodiments described above without departing from the spirit and scope of the invention as defined in the following claims: