Degradation Studies of Polyurethanes Based on Vegetable Oils. Part 2. Thermal Degradation and Materials Properties

Polyurethanes (PUs), produced by cross-linking with a di-isocyanate hydroxylated vegetable oils (rapeseed and sunflower), have been examined from the viewpoints of their thermal degradation and their materials properties, and how both of these aspects are influenced both by the addition of TiO₂ filler and by UV irradiation.

The thermal decomposition of the PUs was investigated using thermal gravimetric analysis (TGA) and IR spectroscopy: the decomposition of a foam of the PU based on rapeseed oil showed complex kinetics, being apparently single-stage and of the first order at 973 K but with at least two separate stages at other temperatures. Thermolysis of PUs as films resulted in the production of carbonyl groups (v 1793 cm⁻¹) suggesting formation of an acid anhydride. Pyrolysis in vacuo produced volatiles which could be separated and examined by IR and NMR spectroscopy. Reaction mechanisms for the thermal decomposition of the PUs are proposed.

The materials properties of the PUs, with and without TiO₂ filler, have been examined by differential scanning calorimetry (DSC), dynamical mechanical thermal analysis (DMTA), tensile testing and measurement of scratch resistance and Shore D hardness. Addition of TiO₂ improves systematically the scratch resistance and hardness of the PUs, but the glass transition temperature T_g showed a more complicated dependence on the TiO₂ loading, initially falling and then increasing. Prolonged UV irradiation of TiO₂-loaded samples increased the value of T_g at all loadings, suggesting increased levels of cross-linking on irradiation. The most dramatic effect of adding TiO₂ was on the shape of the stress-strain curves: at 10% loading clear ‘soft-but-tough’ behaviour is evident. The values of tan δ and the storage modulus are also strongly affected by addition of TiO₂, with and without UV-irradiation.

From the viewpoint of utilisation of these PUs, their thermal stability up to 350°C and the beneficial effects of adding TiO₂ make them promising materials.