Study on curing kinetics of epoxy-amine to reduce temperature caused by the exothermic reaction

Highlights

• FT-IR, non-isothermal and isothermal DSC are used to study curing kinetics.

• Acid amide structures of amine hardener reduce exothermic temperature of curing.

• Lower exothermic temperature results in more homogenous mechanical properties.

Abstract

Diglycidyl ether of bisphenol A (DGEBA) epoxy resin is cured by two typical diethylene triamine-derived curing agents which have a versamid structure (VDETA) and a butyl ether structure (BDETA) in their molecules respectively, and the exothermic curing kinetics of epoxy/amine is studied by non-isothermal, isothermal DSC and in-situ FT-IR methods. It is found that VDETA hardens epoxy resin more gently with little heat accumulation while BDETA leads to explosive increase of temperature caused by the exothermic reaction, and the dramatic temperature gradient heavily decreases homogeneity of mechanical properties. Kinetic studies reveal that VDETA containing acid amide structure is favorable to reduce the temperature and increase the homogeneity of the mechanical properties.

Introduction

Epoxy resins, as one of the most important thermosetting polymers, exhibit unique properties such as excellent adhesion to various substrates, high modulus, high temperature performance, low shrinkage and good corrosion resistance [1]. Epoxy resins have attracted great interest in various fields, such as protective coatings [[2], [3], [4]], adhesives [5,6], high-performance composites [[7], [8], [9], [10], [11]], and insulating materials [12]. In the above-mentioned applications, a curing agent is necessary to harden epoxy resins from monomers or oligomers. Amines, anhydrides, carboxylic acids and phenols are mostly used curing agents, and primary and secondary amines are the largest class of hardeners (50 %) [[13], [14], [15], [16], [17]]. Due to their extensive applications, many theoretical and experimental efforts have been focused on the design of amine hardeners [18,19], curing kinetic study of epoxy/amine reactions [[20], [21], [22], [23]] and improvement of mechanical performance [24,25], but little work is devoted to the effect of curing agent structure on exothermic behaviour.

Although heat from exothermic curing does not influence applications for coatings, adhesives and other thin materials in which the reaction heat can be dissipated easily, the nature of high thermal resistance of epoxy resins results in dramatic increase of temperature caused by the exothermic reaction which significantly affects the uniformity of final thick products, especially for the production of large-scale composites. According to the reports, ring-opening of epoxy/amine is able to result in a exothermic heat of about 100 kJ/mol [26,27] and the specific heat capacity of epoxy resins ranges between 1.3–1.5 J/g/K [28]. The calculation shows that the maximum temperature caused by the exothermic reaction for an ideal adiabatic process with full conversion reaches 338∼390℃, which is high enough to have a dramatic effect on the usage of epoxy resins [28]. Apart from the possibility of burn and matrix degradation caused by the quite high temperature, low thermal conductivity can lead to large thermal and cure gradients, further resulting in detrimental residual stresses. Thus, it is of high significance to ascertain the relationships between amine structures and heat-releasing behaviours during curing.

In this report, curing kinetics of epoxy/amine is examined in an effort to reveal solutions to reducing temperature caused by the exothermic reaction. A versamid amine (VDETA) and a butyl ether modified amine (BDETA) derived from diethylene triamine (DETA) are employed as curing agents for a liquid diglycidyl ether of bisphenol A (DGEBA). Although epoxy resins are commonly cured at constant temperature, the inner temperature increase induced by reaction would drive the curing reaction into non-isothermal kinetics. Thus, we have described both dynamic differential scanning calorimetry (DSC) curing and isothermal DSC curing model of two hardeners which show dramatically different curing behaviours, and the applicability of isothermal model for kinetic studies is clarified. A three-step elementary reaction mechanism rather than hydroxyl group autocatalysis is developed to understand the kinetic model from DSC data. The fundamental understanding of the mechanism and kinetics is highly beneficial to the use of epoxy resin in large scale.