Microwave techniques for the preparation of polymer foams

A novel microwave processing technique for the heating and foaming of ethylene vinyl acetate (EVA) copolymers with different percentages by weight vinyl acetate content (6, 18 and 28 %) was investigated as an alternative to conventional production techniques. The foam formulations comprised ethylene vinyl acetate copolymer, an organic peroxide crosslinking agent and a blowing agent based on azodicarbonamide. Thermal characterisation was performed on the formulation components to determine the likely foam processing conditions. Dielectric characterisation was also performed in order to determine the dielectric response of materials as a function of both frequency (1-10 GHz) and temperature (25 - 250 °C). The results showed that each of the formulation components had sufficiently high loss at 2.45 GHz to allow heating; it was also shown that dielectric loss increased with increasing frequency. The loss factor of the EVA base polymers increased with increasing vinyl acetate content which was attributed to the increase in polymer polarity. As temperature was increased, the loss of the EVA polymers increased. This was a result of increased molecular freedom due to polymer viscosity reduction and melting of the crystalline regions of the polymers. Initial microwave foaming was performed on EVA 206 (6 % vinyl acetate) using a section of rectangular waveguide as the microwave applicator. A 30 W microwave amplifier was used as the power source. It was found that foaming took place predominantly in the centre of the samples and the foamed portions appeared to be encapsulated in a thick unfoamed skin which was confirmed by analysis of detailed images obtained using a scanning electron microscope. This was attributed to heat loss from the sample surface. The main thrust of this work concentrated on using a newly developed combination microwave / hot air oven for the batch processing of EVA foams using variable frequency (200 W) microwave heating. Supplemental hot air heating (170 °C) was used to reduce the thickness of the foam skin encountered earlier. Crosslinking of the base polymers was investigated using conventional and microwave heating. It was found that the microwave heated samples had lower gel content than those heated conventionally. It was found that microwave heated foams could be produced that were comparable with conventionally heated samples. Six microwave heating frequencies were used to evenly distribute the microwave field throughout the samples. The densities of the EVA 328 (28 % vinyl acetate) microwave heated foams were found to be comparable with the conventionally heated EVA 328 foams. The foams based on EVA 218 (18 % vinyl acetate) had slightly higher densities than the conventional foams which was attributed to their lower polarity and hence lower microwave heating efficiency. Foams based on EVA 328 had lower densities than conventional samples (at the upper expansion temperature range) and showed less degradation. Compression testing revealed that, generally, the microwave heated foams had slightly higher Young's modulus than the conventional foams which was attributed to their higher density. Scanning electron microscopy was used to examine the cellular structure of the conventionally and microwave heated foams. Cell sizes for the microwave foams were marginally larger than the conventional foams suggesting that localised overheating of the samples due to non uniform microwave field distribution was occurring.