Curing epoxy resins with anhydrides. Model reactions and reaction mechanism

Abstract

The mechanism of acid curing of epoxy resins catalyzed with tertiary amines was investigated by using model systems composed of phenylglycidyl ether and benzoic acid or acetic acid anhydrides in the presence of benzyldimethylamine. The reaction was studied by NMR spectrometry, liquid chromatography, and ozone absorption. The main findings are that (1) the tert‐amine is bound chemically and irreversibly during the reaction under the formation of a quaternary ammonium salt magnified image and (2) 1‐phenyloxypropanediol‐2,3‐dibenzoate or diacetate is the main reaction product. The suggested reaction mechanism involves initiation in which the tertiary amine reacts with the epoxy group, giving rise to a zwitterion that contains a quaternary nitrogen atom and an alkoxide anion; the latter immediately reacts with the anhydride and quaternary salt is formed. In a later stage the carboxy anion of the quaternary salt reacts first with the epoxy group, then with the anhydride. By this reaction diester is formed and the carboxy anion is regenerated.

Isoconversional method to explore the mechanism and kinetics of multi-step epoxy cures

“…This alternating anionic copolymerization controls propagation of the polymer chain. The mechanism for this major channel was first proposed by Matejka et al 14) and most recently refined by Leukel et al 15) Trappe et al 16) found that the initiation rate is significantly slower than the rate of propagation. Assuming that the initial stages of epoxy-anhydride cure are controlled by initiation, we may assign the value of L 20 kJ N mol -1 to the activation energy of initiation.…”

A simple model to explain the complex kinetic behavior of epoxy/anhydride systems

“…1 Propagation occurs through the alternating addition of epoxy and anhydride functionalities to the growing chain. But as active epoxides (alkoxides) at chain ends react at a faster rate than active anhydrides (carboxylates), 2,3 an almost simultaneous addition of an epoxy/anhydride couple to the growing chain takes place. Therefore, the propagation step may be described by a single rate constant.…”

Investigação da cinética de cura por calorimetria diferencial exploratória (DSC) de resinas epóxi preparadas a partir de óleo de soja epoxidado com diferentes anidridos e aminas terciárias

“…O mecanismo de polimerização epóxi-anidrido na presença de uma de amina terciária como catalisador é bastante complexo, mas a maioria dos autores concorda que o mecanismo de reação é aniônico (Figura 2) [30] . Pode-se dizer que a reação é iniciada pela reação entre a amina terciária com o grupo epóxi formando um "zwitterion" que apresenta em sua estrutura um átomo de carbono quaternário e um anion alcóxido (reação 1) e que o processo de propagação ocorre através de uma acilação seguida de uma esterificação (reação 2 e 3).…”