Renewable resource-based epoxy resins derived from multifunctional poly(4-hydroxybenzoates)

Abstract

Herein we report on the preparation and cure of epoxy resins derived from renewable resources such as 4-hydroxybenzoic acid, glycerol, pentaerythritol, dipentaerythritol, trimethylolpropane, and sorbitol, aiming at substituting bisphenol A-based epoxy resins. Subsequent to the catalytic transesterification of polyols with ethyl-4-hydroxybenzoate, derived from bioethanol and 4-hydroxybenzoic acid, the resulting poly(4-hydroxybenzoate) phenolic resins (PHB-phenol) are glycidized with epichlorohydrin, obtained from glycerol. The epoxy functionality of these liquid ester-functional epoxy resins (PHB-epoxy) varies between 2 and 5. The PHB epoxy viscosities decrease with increasing epoxy functionality. In dicyandiamide-mediated thermal cure, property profiles similar to that of bisphenol A-based epoxy resins are obtained. The monofunctional glycidyl ether of ethyl-4-hydroxybenzoate (EHB-epoxy), formed as a by-product when PHB-phenol is not purified, can remain in the PHB-epoxy resins, serving as a reactive diluent. The incorporation of ester-functional epoxy resins does not impair the water-resistance of epoxy resins. Contrary to the flexible epoxy resins based upon epoxidized plant oils and glycidyl derivatives of glycerol, sugars, and polyfunctional fatty acids, the glycidyl ethers of the corresponding poly(4-hydroxybenzoates) and most especially their blends with conventional bisphenol A-based epoxy resins afford much higher stiffness, strength and glass temperatures, thus meeting the demands for applications as components of structural adhesives.