The molecular structure of epoxy resins has been studied to improve the fracture toughness of epoxies at cryogenic temperatures. The mechanical and the thermal properties were investigated at cryogenic temperatures together with the molecular structure of epoxy resins. It was found that for the improvement of fracture toughness at cryogenic temperature, the thermal stress should be reduced that is reducing the thermal contraction was needed. To reduce the thermal contraction, the epoxy having high density and rigid molecular structure has been thought to be desirable. Such systems, however, were found to be brittle and the cracks were introduced during the cooling down process and sometimes even during the curing process. Consequently the another molecular design concept to improve the toughness of epoxy without decreasing dimensional stability is needed. The two-dimensional linear polymer of epoxy resin has been found to show superior properties at cryogenic temperature to the three-dimensional cross-linked epoxy resins. Referring the cryogenic properties of two dimensional epoxy resin the molecular design of epoxy resin for cryogenic use is discussed.
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