| 摘要 |
Porous bodies are produced using a thermoplastic material and an expanding agent by means of an apparatus provided with two screws enclosed by a casing and in co-operation with one another, the screws, between an intake for the starting material and an outlet for the treated material, forming zones of mutually varying shapes for bringing about different treatment steps, the material during its passage through the interspaces between the screws and the casing of the apparatus being subjected by a plurality of kneading discs to a plurality of kneading operations with components of movement in axial, radial and peripheral directions. The thermoplastic material is supplied with expanding agent in one of the zones and subjected in the interspaces between the screws and the casing to a gelatinization, kneading and mixing process under pressure and heat so that it expands, having left the apparatus, into a porous state. Preferably the various kneading operations are of different durations and intensities. There may be some return flow of the material towards the intake during its passage between some of the kneading discs. Before the addition of expanding agent the material may be subjected to an increase in temperature and pressure, then to a first kneading operation in order to effect a gelatinization and then to a further increase in pressure. The material may attain its highest temperature before the addition of the expanding agent, the temperature then being allowed to fall gradually until the material leaves the apparatus. The kneading discs may act in radial and peripheral directions whilst the screw sections impart a component of motion in an axial direction. The thermoplastic materials specified are polystyrene, polyethylene and polyvinyl chloride. The expanding agent is preferably gaseous at room temperature for example methyl chloride. |
