Development of Epoxy Resins without Hydroxyl Groups in the Molecular Chain

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Epoxy resins without hydroxyl groups after curing are expected to have low water absorption and dielectric constant. They are desirable as high-performance electric and electronic materials. Therefore, various novel curing agents were developed for epoxy curing, and properties of cured resins such as mechanical and dielectric properties and water absorption were examined in detail.When esters of 1,3,5-trihydroxybenzene with carboxylic acids were used as trifunctional active esters, the reactivity for epoxy curing was decreased with the bulkiness of carboxylic acid residues. The glass transition temperature (Tg) shifts to lower temperatures with increasing length of alkyl chain of carboxylic acid residues but is shifted to higher temperatures by branching the alkyl moiety or incorporating a phenyl group into ester.The active esters as curing agents improve the water resistance and electric properties of cured resins because no hydroxyl groups are left after curing. However, the epoxy resins cured with active esters have lower Tg values compared to that cured with conventional curing agents. Therefore, novel multifunctional active esters with internal ester bonds were prepared and used as curing agents utilizing the insertion reaction of internal ester groups into epoxy groups. The Tg of cured resins were improved and the maximum value of 142ﾟC was attained.Trimethylsilyl ethers of phenol novolak and cresol novolak were used as curing agent, the Tg of cured resins was as low as 110ﾟC but the water absorption was decreased to 1/10 of that cured with conventional curing agents.