Curing characteristics of epoxy resins filled with cellulose and oxidized cellulose

Abstract

Gel time and Brookfield viscosity studies of epoxy resins containing varying concentrations of cellulose, periodate oxidized cellulose, chromic acid oxidized cellulose, and ethylene glycol were carried out at 120°C using the tertiary amine hexamethylene tetramine as catalyst. It was found that the hydroxyl groups of cellulose and oxidized cellulose did not only act as accelerators for the tertiary amine catalyzed polymerization of the epoxy resin, but evidence was also found which pointed to the actual participation of the cellulosic fillers in the crosslinking reaction leading to the thermoset epoxy polymer. The gel times of the epoxy curing reaction decreased as the concentrations of the cellulosic fillers increased. The change in viscosity with time (curing characteristics) curves for 40 parts of cellulose or oxidized cellulose per hundred parts of resin (phr) were similar to that of 10 phr ethylene glycol, while the curve for 20 phr cellulose-filled epoxy resin was almost identical to the curve for 10 phr periodatefilled resin. These differences are explained on the basis of the percentage of hydroxyl groups in the various filler molecules that are available for their role as accelerators. The possibility of greater participation of oxidized cellulose in the network formation with the epoxy resin than ordinary cellulose has also been surmised on the basis of its increased viscosity build-up after the gel time is achieved.