Thermal welding of an unstable thermoplastic facilitated by a diffusion-promoting interlayer

A study into the feasibility of thermal welding of an unstable thermoplastic has been undertaken. A heater wire embedded in a diffusion promoting interlayer has been used to accelerate interdiffusion between two poly(vinylchloride) (PVC) plaques. Interlayers consisted of a compatible vinyl resin and a plasticiser. Both normal resistance and an isothermal induction process were used as heat sources, with lap shear testing used to determine the strength of such systems. Vickers hardness testing has been used to ascertain the extent of diffusion and immersion diffusion testing was used to find the activation energy for the process. Micro thermal analysis (MTA) in conjunction with laser induced mass analysis (LIMA), ultra-violet fluorescence microscopy and microscopic infrared techniques were used to study degradation. It has been found that the use of an interlayer allows thermal welding of PVC without deleterious degradation. The concentration and type of plasticiser was found critical in producing a strong weld. Low concentrations of plasticiser did not cause sufficient diffusion and high concentrations of plasticiser in the interlayer produced a weak interface; the optimum amount was dependent on the diffusion coefficient of the plasticiser. Fast fusing plasticisers resulted in higher lap shear strength because they cause a greater extent of diffusion during a constant welding time than slower fusing plasticisers. Degradation products were detected in proximity to the heater wire. Resistance heating was found to cause an exponential increase in degradation closer to the wire while isothermal heating produced a degradation profile with a plateau region next to the heater wire.