Application of Variable Frequency Microwave (VFM) to Adhesives

Abstract

Microwave processing of adhesives is a relatively new technology alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. Alternative in the sense that most adhesives are normally cured in ambient conditions or in ovens. However, the most commonly used facilities for microwave processing of materials operate on fixed frequency microwaves (FFM), e.g., 2.45 GHz. This paper presents a review of microwave technologies, processing methods and industrial applications, using variable frequency microwave (VFM) facilities. The technique offers rapid, uniform and selective heating over a large volume at a high energy coupling efficiency. This is accomplished using a preselected bandwidth sweeping around a central frequency by employing tunable frequency sources. Successful applications of these modern facilities include finding out the optimum cavity conditions of glass or carbon fibre reinforced thermoplastic matrix composites, and of adhesives, e.g., two-part five-minute Araldite, and the joining of the above-mentioned composite materials with, or without, primers. Finding out the optimum cavity conditions of a material has helped identify the best frequency range to process the material using microwave energy and by means of the VFM facility. Microwave energy has been used to rapidly cure several types of two-part epoxy based adhesives, e.g., Araldite. Bond strengths obtained using variable frequency microwave (VFM) techniques are compared with adhesive joints cured in fixed frequency microwave (FFM) conditions.