Variable frequency microwave (VFM) processing facilities and application in processing thermoplastic matrix composites

Variable frequency microwave (VFM) processing facilities and application in processing thermoplastic matrix composites

Microwave processing of materials is a relatively new technology advancement alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. Factors that hinder the use of microwaves in materials processing are declining, so that prospect for the development of this technology seem to be very promising. The two mechanisms of orientation polarisation and interfacial space charge polarisation, together with dc conductivity, form the basis of high frequency heating. Clearly, advantages in utilising microwave technologies for processing materials include penetration radiation, controlled electric field distribution and selective and volumetric heating. However, the most commonly used facilities for microwave processing materials are of fixed frequency, e.g. 2.45 GHz. This paper presents a state-of-the-art review of microwave technologies, processing methods and industrial applications, using variable frequency microwave (VFM) facilities. This is a new alternative for microwave processing.

Submitted abstract: This paper extends the range of applications for Variable Frequency Microwave (2 – 18 GHz) (VFM) facilities to thermoplastic composites. Five thermoplastic polymer matrix composites are processed and discussed, including 33% by weight random carbon fibre reinforced polystyrene [PS/CF (33%)], and low density polyethylene [LDPE/CF (33%)]; 33% by weight random glass fibre reinforce polystyrene [PS/GF (33%)], low density polyethylene [LDPE/GF (33%)] and nylon 66 [Nylon 66/GF (33%)]. Bond strengths of the joints were shear tensile tested and results were compared with those obtained using fixed frequency (2.45 GHz) microwave processing. The primer or coupling agent used was 5-minute two-part adhesive containing 100% liquid epoxy and 8% amine, which was more readily microwave reactive than the composites themselves. The VFMF was operated under software control, which provided automatic data logging facilities.

Industrial applications of microwaves are relatively new technology. Factors that hinder the use of microwaves in materials processing are declining, so the prospects for the development of this technology seem to be very promising.1 The mechanisms that govern the energy distribution process during microwave joining of materials include dipole friction, current loss and ion jump relaxation. This results in a relatively uniform heat distribution throughout the entire exposure to microwave irradiation, immediately in front of rectangular or circular waveguides. 2,3,4,10 The fast heating rate encountered using microwave energy can thus lead to reduced processing time and consequent energy efficiency. These advantages have encouraged the development of facilities for joining a range of thermoplastic composites autogenously and heterogeneously. In the heterogeneous mode, at room temperature, transparent materials, including a range of thermoplastic and thermosetting resins can be bonded using two part adhesives cured at fast rates when exposed to focused microwave irradiation.3,5,10

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