High energy rate of joining thermoplastic composites using microwaves
Creators: name: Ku, Harry S.
Publisher: name: Swinburne
Year: 2024

description: One of the most important areas of the application of industrial microwave technology for materials processing is curing polymers and polymer based composites, and this is currently in a state of considerable flux. Curing of thermosetting composites in multi-mode applicators and joining of a range of thermoplastics in ridged waveguides have been some of the major achievements in the field. This research extends the applications horizon of microwaves in the area of random fibre reinforced thermoplastic composites joining and places emphasis on the development of facilities and methodologies aiming at maximising bond quality. It discusses the fixed frequency microwave equipment used, including a 0.8 kW variable control power generator operating at 2.45 GHz, waveguide and a tuning piston designed for obtaining a standing wave at the seam of the butted and lapped test pieces. The effect of power input and cycle time is detailed together with the underlying principles of test piece material interactions with the electromagnetic field. The process of heterogenous joining, with filler material, or primer, of 33% by weight of random glass fibre reinforced nylon 66, polystyrene (PS) and low density polyethylene (LDPE), thermoplastic composites is mentioned together with the process of autogenous joining of 33% by weight of random carbon fibre reinforced polystyrene (PS) and low density polyethylene (LDPE). The weldability dependence on the dielectric constant, dielectric loss, and hence loss tangent of these materials is also described and investigated. Measurements of the dielectric properties were carried out using the waveguide transmission, probe kit and resonant cavity techniques. X-ray photoelectron spectroscopy for surface analysis was carried out to assess if the filler material had diffused into the parent thermoplastic composite materials. Bond strength measurements of the welded thermoplastic composites were obtained using tensile shear tests. A variable frequency microwave (VFM) facility was also used to join the thermoplastic composites and bond strength results obtained were compared with those procured by the fixed frequency microwave configuration. From the measured data of LDPE/GF (33%), PS/GF (33%) and nylon 66/GF (33%), it was found that at relatively high temperatures and frequencies, these materials became less reflective and permitted more microwave energy to be absorbed. At the same time, their dielectric loss factor increased with temperature and frequency and enabled more of the absorbed energy to be converted to heat. Therefore, within limits, higher temperatures and higher frequencies were more suitable for microwave-assisted joining of the three materials tested. As to the primer/filler, only liquid rapid Araldite was found to be the most favourable in microwave-assisted joining of fibre reinforced thermoplastic (FRTP) composites because of its considerably high value of loss tangent. Another important finding was that the dielectric probe method was found to be the recommended technique to use for measurements of ε' and tan δ in microwave joining of materials out of all the methods applied. However, for low loss materials, the values of tan δ need to be acquired through the resonance cavity method.