Density functional theory study on mechanisms of epoxy‐phenol curing reaction

Abstract

A comprehensive picture on the mechanism of the epoxy-phenol curing reactions is presented using the density functional theory B3LYP/ 6-31G(d,p) and simplified physical molecular models to examine all possible reaction pathways. Phenol can act as its own promoter by using an addition phenol molecule to stabilize the transition states, and thus lower the rate-limiting barriers by 27.0-48.9 kJ/mol. In the uncatalyzed reaction, an epoxy ring is opened by a phenol with an apparent barrier of about 129.6 kJ/mol. In catalyzed reaction, catalysts facilitate the epoxy ring opening prior to curing that lowers the apparent barriers by 48.9-50.6 kJ/mol. However, this can be competed in highly basic catalysts such as amine-based catalysts, where catalysts are trapped in forms of hydrogen-bonded complex with phenol. Our theoretical results predict the activation energy in the range of 79.0-80.7 kJ/mol in phosphine-based catalyzed reactions, which agrees well with the reported experimental range of 54-86 kJ/mol.

Density functional theory study of mechanism of epoxy‐carboxylic acid curing reaction

“…Epoxy (denoted as E), carboxylic acid (RCOOH), tertiary amine catalyst (R3N) can form three hydrogen-bonded precursors namely acid-acid, acid-epoxy, and amine-acid and some are found to be involved in a number of epoxy-acid curing reaction pathways. Similar to our previous study of the epoxyphenol curing reaction, [3] the uncatalyzed epoxy-acid reaction can also follow two possible pathways as illustrated in Scheme 1. First is the isolated pathway (designated as C-I), wherein epoxy reacts with an acid curing agent alone.…”

“…An amine catalyzed epoxy‐acid reaction can occur in three possible pathways. First, the R 3 N catalyst acts as a base and forms a hydrogen‐bonded complex with a carboxylic acid to enhance its nucleophilicity prior to reaction with the epoxy ring in similar pathways as for the uncatalyzed reaction . These isolated and self‐promoted catalyzed pathways, designated as C‐IH and C‐PH, are presented in Scheme .…”

Concurrent curing kinetics of an anhydride-cured epoxy resin and polydicyclopentadiene

“…The alternating copolymerization of epoxy and anhydride monomers catalyzed by a tertiary amine is commonly assigned the mechanism detailed in Scheme S1 of the Supporting Information. (34)(35)(36)(37) The tertiary amine attacks an epoxide group on the DGEBA molecule, creating an alkoxide anion. This alkoxide anion then reacts with an anhydride to form a carboxylate anion as the propagating species.…”

Preparation and properties of soybean oil–based curing agents for epoxy resin

“…Only by reacting with different kinds of curing agents are three‐dimensional and highly crosslinked networks developed to provide superior mechanical properties and good thermal and dimensional stabilities. Various curing agents such as phenols, acid anhydrides, carboxylic acids, amines, imidazoles, and others have been widely used . Among them, amines play extremely important roles in their applications due to their curing at room temperature and high reactivity.…”

“…Various curing agents such as phenols, acid anhydrides, carboxylic acids, amines, imidazoles, and others have been widely used. [5][6][7][8][9] Among them, amines play extremely important roles in their applications due to their curing at room temperature and high reactivity. The performances of the cured products are largely dependent on the chemical structures of the amine curing agents.…”