Effect of reactive diluent (MMA) on properties of urethane methacrylates

Abstract

Urethane methacrylate prepolymers were prepared by reacting hydroxyl terminated polybutadiene (HTPB, mol. wt 3000), tolylene diisocyanate (TDI) and 2-hydroxyethyl methacrylate (HEMA) in molar ratios of 1:2:2 and 1:1.5:1. Reactions were carried out at room temperature (35°C) using stannous octoate as catalyst. Second order rate constants for the uncatalysed reaction of TDI (1 mol) with HTPB (1 mol) in tetrahydrofuran (THF) were also determined and an activation energy of 28.2 kJ mol −1 was found in the temperature range of 40–61°C. Eight copolymer compositions based on urethane methacrylate prepolymer and 10, 30, 50 and 70% (w/w) of methyl methacrylate (MMA) were utilized for sheet fabrication. 1% azobis(isobutyronitrile) (AIBN) was used as initiator. An increase in the concentration of MMA in the copolymer or an increase in the prepolymer molecular weight resulted in an increase in turbidity and density of sheets. The increased turbidity may be attributed to the immiscibility of components, i.e. hydrocarbon backbone of HTPB and polar urethane methacrylate and methyl methacrylate moieties. An increase in the room temperature tensile strength, Young's modulus and storage modulus and a decrease in elongation was observed with an increase in MMA content or by a decrease in the prepolymer molecular weight. A three step decomposition was observed in TG traces of these copolymer sheets in a nitrogen atmosphere.