Synthesis and thermal characterization of some hardeners for epoxy resins based on castor oil and cyclic anhydrides

Abstract

Castor oil, a triglyceride ester of ricinoleic acid containing up to 2.7 hydroxyl groups per molecule, was converted with three cyclic anhydrides into esters with carboxyl in their structures. The obtained products have potential as crosslinking agents for epoxy resins. The chemical structures, thermal properties, degradation mechanisms and gaseous composition obtained during thermal decomposition of the products in nitrogen atmosphere (up to 600 °C) were assessed with the aid of thermogravimetric (TG), Fourier transform infrared (FT-IR) and mass spectroscopy (MS) analyses. The kinetic parameters (pre-exponential factor and activation energy) were evaluated from the data recorded at three heating rates, in a first instance by using the isoconversional method of Friedman (FR). Based on the curve shapes of activation energy versus conversion degree it can be concluded that the thermal degradation processes take place through multi-step kinetics. The most probable thermal decomposition mechanism was obtained by using the multivariate linear regression (MLR) method included in “Thermokinetics-3″ software and is described by a process that takes place in three steps. The main evolved gases during thermal degradation were identified from the FT-IR and MS spectra. The gaseous mixture contained water, carbon dioxide, some saturated and unsaturated hydrocarbons moieties, aromatic compounds and carboxylic derivatives.