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synopsisThe mechanism of cure has been studied in coating systems comprising blends of ammonium salts of acrylic acid-acrylic ester copolymers and alkoxymethylmelamines.Experiments were conducted both on the resin components themselves and on model compounds having analogous chemical structures. Curing of the resin begins with loss of ammonia to regenerate free carboxyl groups, rather than loss of water to form amides.The allcoxy group is eliminated, and ester crosslinks are formed between the carboxyl and the methylolmelamine. Base binding studies on cured films show that the extent of crosslinking depends on the exact composition of the resin system and the presence or absence of an acidic catalyst. These variables can be adjusted to provide essentially quantitative crosslinking in the cured coating, as shown by the complete disappearance of unreacted carboxyl groups.

SynopsisStatic and dynamic properties were studied in a series of polyurethane elastomers as a function of selected compositional variables such as curative system, curative level, catalyst level, and curing temperature. A number of physical properties including swelling ratio, density, glass transition temperature, stress-strain behavior, and thermal conductivity were also measured on these elastomers. The selected variables affect dynamic mechanical properties as well as heat buildup. A good correlation was noted between the loss modulus and the heat generation. The loss modulus and the heat generation decrease with decreasing curative level. The elastomers cured with a mixture of trio1 and diamine give lower loss modulus and heat buildup than those cured with diamine alone. These responses are believed due to the increase in covalent crosslinks. The observed low heat generation of the elastomer cured with 0.2 phr azelaic acid as a catalyst level was also attributed to the high crosslink density. The curing temperature, in the range investigated, appears to have very little effect on the properties. Thus, the choice of formulation variables, especially the use of diamine-trio1 blends, provides an effective means of minimizing heat generation in dynamic applications of polyurethane elastomers.

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SynopsisThe curing of hexakis(methoxymethy1)melamine (HMM) alone and in blends with functionally substituted acrylic polymers was monitored by means of torsional braid analysis, infrared spectroscopy, weight change, and analysis of the volatile by-products. HMM alone, when heated with a strong acid catalyst, forms thermoset condensation polymers which evidently have the same methylene-bridged structure as conventional melamine-formaldehyde resins; the major elimination product is methylal. When HMM is blended with an acrylic polymer containing methacrylic acid units, the acrylic chains are crosslinked by elimination of methanol and formation of methylene ester linkages between the acid groups and the melamine. The crosslinking reaction proceeds with or without strong acid catalysis and is many times faster than the self-condensation of HMM. Acrylic polymers containing primary hydroxyl groups (derived from 2-hydroxyethyl methacrylate) also undergo acid-catalyzed reaction with HMM, and at a rate so much faster than analogous carboxylated polymers, that storage stability of the catalyzed resin becomes a serious problem. If a weaker catalyst, such as phthalic anhydride, is used, the curing reaction is very much slower. I n order to develop the maximum number of new linkages, according to any of the experimental criteria, all of these systems must be heated for longer times, or a t higher temperatures, than are customarily used in thermosetting resin technology. Thus the known utility and durability of such acrylic/melamine resins are achieved with only a fraction of the total number of crosslinks which are potentially capahle of formation.