Vegetable oil-derived polyether-polyester thermosets: Solvent-free synthesis and mechanical properties

Vegetable oils differing in the number and kind of reactive chemical sites were investigated as potential feedstock in reactions involving epoxy rings to produce sustainable polymeric thermosets. Sustainable polyether-polyester matrices were synthesized through the mixing of three different vegetable oils (castor, tung and sunflower oil) with maleic anhydride to promote their chemical activation, and subsequently induce the reaction with polyethylene glycol diglycidyl ether (PEGDGE). The interactions between double bonds and/or hydroxyl groups present in vegetable oils, anhydrides and epoxy rings within a solvent-free medium promote the expected chemical crosslinking, yielding polymeric thermosets that encompass the Green Chemistry tenets. Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry tests were employed to validate the chemical reactions taken place during the synthesis, as well as to monitor the kinetics of curing. Moreover, a rheological characterization was conducted to assess the influence of both the vegetable oil and the ratio of reactive components on the ultimate mechanical properties. Although chemical crosslinking was suitably attained in all the systems studied, a more reinforced network with values of the storage modulus (G’) of 54⋅10^5 Pa was obtained in those based on tung oil possessing the higher quantity of reactive functional sites; meanwhile, the presence of hydroxyl groups within the vegetable oil structure led to the production of more flexible thermosets (G’ of 6.2⋅10^5 Pa).