Selecting Curing Agents for Structural Epoxy Adhesives (Part 2): Elevated Temperature Cure

Introduction

This is the second part of two articles that describe common epoxy adhesive curing agents and their selection process. This current article focuses on epoxy adhesive systems that require elevated temperature curing conditions. The first part provided information regarding curing agents that are used to achieve cure at room temperature or below.

A variety of curing agents will react with epoxy resins to provide crosslinked adhesives. There are several curing agents that are commonly used, as shown in Table 1, but there are also many others. These curing agents are often divided into low or room temperature curing types and elevated temperature curing types. This article will describe the mechanisms behind elevated temperature curing epoxy adhesives and how to go about selecting the proper curing agent for a specific application.

The choice of a particular curing agent or catalyst depends on the processing requirements (e.g., viscosity, pot life, application method, curing temperature, rate of reactivity, mix ratio) and the end-use requirements (e.g., thermal and chemical resistance, shear strength, toughness) of the cured adhesive. The curing agents along with the epoxy resin will determine the type of chemical bonds and the degree of crosslinking that will occur. The first article in this series provides rationale for selecting either a room temperature or an elevated temperature curing epoxy adhesive.

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One-part INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (1K) epoxy adhesives require curing at elevated temperatures. However, they generally provide greater strength and heat / chemical resistance than room temperature curing epoxy adhesives. On the negative side, elevated temperature curing epoxy adhesives are more rigid and this limits their elongation and toughness which becomes noticeable as lower peel and impact strengths In 2K epoxy systems, curing can take place at room temperature depending on the curing agent, but better properties generally result if heat is used.

Characteristics of Elevated Temperature Curing Agents

Room temperature curing cannot, in principle, achieve the same degree of crosslinking as is obtained by curing at elevated temperatures. At elevated temperatures the epoxy resin and curing agent molecules are mobile, and there is a greater potential for reaction than at room temperature. Figure 1 shows the glass transition temperatures of common epoxy formulations cured at room and elevated temperatures.

Figure 1: Glass transition temperatures of several epoxy formulations cured at room and elevated temperatures.

Another reason for higher Tg with elevated temperature curing systems is that the hydroxyls along the epoxy chain can react. At ambient temperature, reaction between an epoxy group and a hydroxyl group proceeds very slowly. Hence, the diepoxy group, which is formed when the epoxy group reacts with an amine, generally cannot enter i