ES314092A1 - Improvements relating to microporous plastics sheet material - Google Patents

Abstract

A microporous plastics sheet is made by mixing together a plastics material, a solvent therefore, and a particulate pore forming material to produce a dough, forming the dough into a strip, heating the strip to such a temperature (the "Thermal Shock Temperature") that the particles of pore forming material are exposed within the strip and substantially all the solvent is driven off, immersing the strip in an aqueous bath to leach out the exposed pore forming particles, and finally drying the porous strip thus formed. The plastics material may be plasticized or unplasticized PVC, low or high density polythene, polypropylene or polyurethane, the "Thermal shock temperature" being not less than 130 DEG C., 150 DEG C., 130 DEG C., 150 DEG C., 170 DEG C. and 185 DEG C. respectively. The pore forming particles may be salts of alkali or alkaline earth metals or ammonium salts, e.g. sulphates, chlorides and bicarbonates. In one embodiment (see all Figures, not shown) ground ammonium sulphate is mixed with powdered PVC at a station (30) and the mixture introduced into a twin screw mixer (60) during the passage through which a solvent and a minute proportion of a melting agent is added through a feed pipe (113), the dough thus formed being subsequently de-aereated. The de-aereated dough is then fed either directly to an extrusion die or, as in the example, via a granulator 17 and storage hopper, to an extrusion die, whence it is formed into a ribbed sheet. The sheet is then passed through successively hotter regions in a hot air oven (see Fig. 3) and heated to the required temperature, after which it passes through a series of water baths each successive one being higher than the next (see Figs. 4 and 5) so that a counter current of water is produced. The now porous sheet is finally passed to a drying oven (160) having successively cooled zones, to form the finished strip. Material suitable for clothing, battery separators or filters may be made.