GB761080A - Improvements relating to microporous thermoplastic materials - Google Patents

Abstract

<PICT:0761080/IV(a)/1> <PICT:0761080/IV(a)/2> <PICT:0761080/IV(a)/3> Microporous thermoplastic sheet is rendered dimensionally stable at comparatively high temperatures, e.g. 90 DEG C., by passing it through a hot zone at a temperature above the softening point and approaching the temperature of manufacture, whilst permitting shrinking or swelling in all directions, then rapidly cooling to below the softening point, e.g. by cold water or air at atmospheric temperature. The temperature of the hot zone and time of treatment therein may be increased beyond that necessary to produce dimensional stability in order to modify the pore size of the microporous material. The microporous material may be of polyvinyl chloride and may be produced as described in Specification 565,022, [Group IV]. Three methods of effecting the heating and cooling are illustrated in Figs. 1-3 respectively. As shown in Fig. 1, a roll 1 of microporous sheet material whose pores are waterfilled is cut into pieces about 3 ft. long and 1 ft. wide by knife 2 and forwarded by endless conveyer belt 4, of perforated stainless steel, past steam jets 6. The pieces then pass under sponge rubber roller 8, cold water sprays 9 and are stacked at 10. During one minute's exposure to steam, a 3 ft. long sheet of microporous polyvinyl chloride may lose about 1/2 in. in length and show slight gains in width and thickness. The sheets are then dried, preferably at a temperature not exceeding 60 DEG C., and are dimensionally stable up to 90 DEG C. In an alternative arrangement, Fig. 2, a continuous sheet is fed by driven rollers 13, 14 and passed along the surface of water heated to 95 DEG C. The speed of the driven take-off rollers 15 is adjusted so that the sheet is not under tension. The sheet is in contact with the hot bath for 1 minute and is then cooled by air at room temperature and passed through a drier 16 whose temperature does not exceed 60 DEG C. In a second alternative, Fig. 3, the stabilization treatment is carried out on a material already dried. The continuous sheet 17 is passed through a drier 18 at 70 DEG C. and chamber 19 with heaters 20 to maintain a temperature of 95 DEG C. The take-off mechanism 21 is adjusted to leave a loop 22 so that the material is in the heating chamber for 2 1/2 minutes. The material is cooled by the atmosphere and reeled on drum 23. As an example, microporous sheet 0.010 inch thick, of 5 parts polyvinyl chloride and 4 parts polypropylene sebacate was heated to 80 DEG C. for 2 1/2 minutes in the chamber 19, Fig. 3. The sheet may be reduced 20 per cent longitudinally, without alteration of pore size; by heating to 95 DEG C. for 2 1/2 minutes the material shrinks 30 per cent in length, 10 per cent in width and the average pore size of 3 microns was reduced to 1 micron. Specifications 727,679 and 728,600 also are referred to.