Cure Mechanism of a Modified Nitrile Epoxy Adhesive
By using a n adhesive for structure bonding of metal-to-metal honeycomb sandwich, structural plastic laminates and fiber-reinforced composites is gaining in popularity as a fastening method in all spheres of engineering and industry. Since bonding can be carried out at relatively lower temperatures than the traditional welding techniques, and no holes are needed as in riveted and bolted joints, a more uniform stress takes place when the chemically bonded laminates are loaded. However, experience has told us that the mechanical properties and performance of adhesive bonds are strongly dependent on the curing conditions.' Higher temperatures or longer curing times may result in brittle adhesive layers and reduce the impact strength. On the other hand, the curing reaction may not be fully complete, and the mechanical properties may change during use if the temperature is too low. Therefore, an understanding of the curing mechanism of an adhesive is essential to control and optimize mechanical properties and performance of a structure-bonded laminate.
A variety of experimental techniques has been developed to study the kinetics of thermosetting reaction^.^^ One is to use the differential scanning calorimeter to measure the heat generated during the cure. This method has been employed by a number of workers to investigate the cure mechanism of epoxy, polyester, and phenolic resin^.^-'^ Prime expressed the reaction kinetics by a simple nth-order Kamal and co-workers proposed a very realistic model to describe the dependence of the rate of cure on time and temperature.+12 Their empirical expression consists of two reaction-rate constants and two reaction-order parameters. This model has been proven satisfactorily for describing the experimental data for a liquid epoxy resin and for injection-molding grade epoxy, phenolic, and polyester resins."I6 However, in all these studies, the kinetic parameters were determined by means of data fitting using a complicated nonlinear, least-square method. In order to overcome this deficiency, Ryan and Dutta recently proposed a rapid estimation technique for the determination of these parameters.I6 However, the total reaction order for curing was assumed to be a constant in their analysis.
In this short note, we attempt to explore the feasibility of using Kamal's model to determine the curing kinetics of a modified nitrile epoxy adhesive. In addition, as an extention of Ryan and Dutta's work, a quick and simple estimation procedure is proposed to determine the kinetic parameters, if the empirical mathematical equation for reaction has one reaction-rate constant and two reaction-order parameters.