Green epoxy synthesized from Perilla frutescens: A study on epoxidation and oxirane cleavage kinetics of high-linolenic oil

Abstract

Despite the growing interest in epoxidized vegetable oils (EVOs) as sustainable alternatives to petroleum-based epoxies, their high epoxy equivalent weight (EEW) (>250 g/eq) hinders use in high-strength applications. This study reports the synthesis of a sustainable EVO that possesses EEW of ∼164 g/eq – comparable to that of diglycidyl ether of bisphenol-A (DGEBA). Non-edible perilla oil, extracted from the non-edible seeds of Perilla frutescens, was used to synthesize sustainable green epoxy via Prilezhaev epoxidation reaction at 40, 50 and 60 °C for 8 h. Perilla oil was chosen due to its higher number of chemically modifiable groups (>90%), i.e. ∼62–65 wt. % linolenic acid (C18:3), ∼13–15 wt. % linoleic acid (C18:2) and ∼12–15 wt. % oleic acid (C18:1) moieties. Rates of epoxidation and side reactions were estimated by experimentally determining iodine, epoxy and α-glycol value of epoxidized perilla oil (EPeO) extracted at regular time intervals during synthesis. Kinetics of post-oxirane cleavage was studied at aforementioned reaction temperatures and times by reacting EPeO with formic acid in weight ratio of 1:3. Reaction kinetics studies for both epoxidation and ring opening reactions (pseudo) showed reaction rate constants in the order of 10−6 L mol−1 s−1 (for both reactions) and activation energies of 20.10 kJ/mol (epoxidation) and 43.11 kJ/mol (ring opening) respectively. Thermodynamic parameters – such as activation enthalpy, activation entropy, and Gibbs’ free energy of activation – were calculated for both epoxidation and oxirane ring cleavage reactions. All the thermodynamic parameters were observed to be lower for epoxidation reaction compared to post-oxirane cleavage reaction. Enthalpy of activation was found to be ∼18 kJ/mol for epoxidation reaction while it was ∼56 kJ/mol for post-oxirane cleavage reaction.