In order to produce low-density crosslinked foam of superior properties having a uniform cell structure from POEs/LDPE blends, the crosslinking efficiency of resin, behavior of crosslinking sensitizer, mechanical properties according to the irradiation dose, and compressive strength in different cell sizes have been examined. When POE/LDPE (70/30) was irradiated at 3 Mrad, the gel content was 18.5%, but the addition of 0.3 phr of TATM only increased gel content up to 36%. The lower the melt index of the resin, the more the gel fraction increase was confirmed. Although tensile strength increased up to a certain point in proportion to the gel content, the decrease of tensile strength was observed over the fixed gel content. The effect of polyfunctionality was investigated by analyzing the gel fraction in accordance with the number of double bonds in molecules of crosslinking sensitizers. As a result of the measurement of changes on compressive strength according to the sizes of different foam cells, having the same degree of density, bigger cells showed stronger compressive strengths when compressed at 25%, while foams containing smaller cells showed superior compressive strengths when compressed over 50%.
The conventional cross-linking method using an electron beam for polyolefin foams is highly efficient, but the drawbacks are: costly equipment, nonuniform crosslinking, uneven dispersion of chemical foaming agents and additives, and relatively low gel content, which affect physical properties of the foam. In this paper, low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) were blended in an 80/20 ratio, and this noncrosslinked foam was exposed to various doses of electron beam irradiation in air. The foam was made using isobutane as the blowing agent. It has been found that foam with uniform cell structure was not achieved in the case of precrosslinked foam with more than 40% gel fraction. On the other hand, noncrosslinked foams are easily crosslinked to a gel content of about 57% with an electron beam irradiation under optimum conditions. The gel content of the postcrosslinked foams increases with increase in the irradiation dose, beam intensity, irradiation temperature and with the crosslinking coagent content. As derived from experimental studies, beam intensity seems to be the most essential parameter in regard to gel content homogeneity. Noticeable changes in mechanical properties, e.g. tensile strength, tear strength, compressive strength and hardness, were also observed. This paper also presents an experimental study on the manufacture of low-density open-cell foam.
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