Patent GB-648895-A (GB)
Title: Improvements in or relating to methods and apparatus for converting thermo-softeningmaterial to the cellular condition
Filed: 1947-08-27
Abstract: <PICT:0648895/IV (a)/1> <PICT:0648895/IV (a)/2> A body of synthetic rubber or synthetic resin containing a sponging agent, preferably sodium bicarbonate, is converted to the cellular condition by subjecting the material to the effects of a high-frequency electrostatic field, the material being unconfined other than by a layer or two separate layers of freely-flowable, finely-divided dry dielectric material disposed about the body, which may be introduced between electrodes spaced apart so that the space between them is incompletely filled thereby, which electrodes are preferably parallel and form part of an oscillatory circuit.  The body may be placed between plies of a porous fibrous sheet material preferably textile fabric or paper, and may travel on a support between the spaced electrodes.  In a preferred apparatus for carrying out the process of the invention there are provided a bed 11 of electric insulating material such as phenol-formaldehyde resin, hard rubber or wood and a similar bed 13 to serve as a table upon which the work is assembled. Adjacent the ends of the beds 11, 13 pulley rolls 14, 15 carry a conveyer belt 16, whose upper reach 17 travels in close contact over the beds 11, 13.  Another table-like member 21 of insulating material is supported so as to be substantially parallel and co-extensive with the bed 11, and two electrodes 30, 31 are secured to the bed 11 and member 21 respectively.  Pulley rolls 22, 23 carry an upper conveyer belt 24 which is driven at the same speed as belt 16.  A hopper 40 is provided for laying down a thin layer 42 of soapstone on the belt 16 and on top of layer 42 is laid a ply 44 of textile fabric or paper from a roll 43.  A calendered sheet 46 of synthetic rubber or synthetic resin containing a blowing agent is laid down from roll 45, and a third roll 47 lays down a second ply 48 of paper or textile fabric on top of the sheet of rubber or resin.  A second hopper 49 then lays down a layer 50 of soapstone on the three plies of material.  In the first stages of travel of the material between the electrodes there remains an unfilled space between the top layer 50 of soapstone and the belt 24 but as the belts advance the material between the electrodes the rubber or resin expands slowly until the layer 46 has expanded sufficiently to fill the vacant space. Sideboards may be provided to restrain the transverse expansion of the sheet 46.  At the discharge end of the apparatus a wind-up roll 60 is provided for the sponge slab and rolls 52 and 59 for the fabric or paper layers.  The soapstone falls into a hopper 53 or is directed to a shoot 57 by a scraper blade 56.  In a modification (Fig. 4, not shown) the upper conveyer belt may be dispensed with.  In a second modification one or both the conveyer belts are of metal, preferably copper or aluminium, and also serve as the electrodes.  The frequency of the current used is preferably greater than one megacycle.  The sponge material produced may be used for the production of gaskets, washers and vibration and sound dampener strips.  The material treated may be a urea-formaldehyde or phenol-formaldehyde resin in the A-stage or B-stage, a polymer or copolymer of vinyl halides or esters, vinylidene halides, acrylic esters, methacrylic esters, and styrene, polychloroprene or a copolymer of a butadiene-1,3 hydrocarbon with a compound containing a single olefinic double bond copolymerizable therewith.