Studies on allophanate–urethane networks based on hydroxyl terminated polybutadiene: effect of isocyanate type on the network characteristics

Abstract

Polyurethane networks containing allophanate groups, based on hydroxyl terminated polybutadiene and various diisocyanates, such as toluene diisocyanate, 4,4 ′ di(isocyanatocyclohexyl) methane and isophorone diisocyanate were synthesized at various NCO/OH equivalence ratios ( r-values) ranging from 1.0 to 1.5. Mechanical properties such as tensile strength, stress at 100% elongation, equilibrium relaxation modulus increase and % elongation at break decreases with r-value. Crosslink density ( ν e) for the networks was determined using swell methods. Crosslink density values calculated from equilibrium relaxation modulus were found to be close to those obtained from swell data. Crosslink density ( ν t) was theoretically calculated assuming complete reaction between the functional groups. The ratio, ν e/ ν t decreases with r-value. Linear fits with good correlation coefficients were obtained between network parameters and mechanical properties. Dynamic mechanical studies show that the r-value does not influence the glass transition temperature. Thermal decomposition studies using thermogravimetric technique indicate that the decomposition is in multiple stages.