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

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 (νe) 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 (νt) was theoretically calculated assuming complete reaction between the functional groups. The ratio, νe/νt 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.