Bio-based thermoset composites from epoxidised linseed oil

Bio-based thermoset composites were prepared from epoxidised linseed oil (ELO) using bio-derived crosslinkers. The use of Pripol 1009 (a dimerised fatty acid derived from natural oils and fats) as a crosslinker yielded homogenous transparent films. The inclusion of catalysts, in particular, 4-dimethylaminopyridine (DMAP), demonstrated a significant improvement in the mechanical properties of the resins. An infrared spectroscopic study coupled with modulated differential scanning calorimetry revealed the epoxide ring opening, followed by etherification occurred during the curing process. The optimum DMAP catalyst loading was 0.5-1 % with respect to the total resin weight.

The optimised formulation consisting of ELO, Pripol and DMAP were subsequently combined with starch or modified starch in order to improve the resin properties. Normal corn starch, high amylose corn starch and their acid hydrolysed derivatives were included in the formulation. The addition of starch improved the mechanical properties of the films with high amylose starch yielding a film with the most desirable properties.

Expansion of high amylose corn starch (gelatinisation and retrogradation) yielded a high surface area material. The formulation with 20% wt. of gelatinised starch yielded a film with 227% improvement in tensile strength and 166% enhancement in Young’s modulus, compared to those with no added starch. Moreover, expanded starch granules uniformly dispersed in the polymer matrix, resulting in a complete disappearance of phase separation. This was attributed to better interfacial adhesion of porous expanded starch and the polymer matrix. Thermal analysis revealed retardation in the cure process in the presence of starch, however the hydroxyl groups of starch were likely to enhance the extent of curing, as indicated by the higher total enthalpy of reaction. Furthermore, these bio-based composites demonstrated excellent thermal stability. Esterification of expanded starch dramatically decreased the water uptake of the resins however, the mechanical properties were compromised, owing to low thermal stability of the esterified starch.