Patent GB-923504-A (GB)
Title: Improvements in methods and apparatus for manufacture of porous diaphragms
Inventor: MAIRE JACQUES
IPC: B05D1/00
Filed: 1959-09-15
Abstract: <PICT:0923504/IV(a)/1> <PICT:0923504/IV(a)/2> <PICT:0923504/IV(a)/3> <PICT:0923504/IV(a)/4> Porous diaphragms comprising a metal gauze e.g. of nickel, stainless steel, bronze and silver, having a porous polytetrafluoroethylene coating is produced by depositing a powder formed by diffusing an impregnating agent into the midst of the polytetrafluoroethylene, reducing to powder and subjecting to continuous agitation, the agent having a viscosity so chosen as to impregnate the powder in a homogeneous manner without agglomerating the powder, on an intermediate supporting band so as to constitute a uniform coating, eliminating the excess impregnating agent in the coating, e.g. by the application by rolling of an absorbent material against the coating, transferring the coating from the band to the metal gauze by passing the gauze, while applied against the coating, between two cylinders pressed one against the other, washing the coated gauze with an appropriate solvent, e.g. perchlorethylene, acetone, benzene, trifluorochloroethane or light petroleum spirit, and finally drying.  The drying is carried out under such conditions, e.g. temperature and airflow, that the speed of drying is substantially constant.  The impregnated powder may be spread over a band of absorbent material, e.g. Kraft paper, which may be previously impregnated, at least in part, by the agent to be absorbed, before being transferred by rolling on to the intermediate supporting band. The impregnating agent may be formed by a substance with a viscosity between 0,8 and 100 centipoises at 30 DEG C. and may be paraffin or petroleum jelly oil, Mayoline oil, light and heavy naphtha solvents, alkyl-aryl polyether alcohols, gas-oil, medium aromatic solvents constituted by paraffinic and aromatic hydrocarbons in equal parts, benzol extraction oils and butyl phthalate.  The agent may be brought to the required viscosity by dilution in a volatile solvent which can be eliminated by evaporation, e.g. trifluorochloroethane, ethyl alcohol, water or acetone.  The impregnated powder may contain between 20 and 70% by weight of impregnating agent.  The intermediate supporting band may be constituted by regenerated cellulose.  The operations of depositing the powder on the supporting band, and of transferring the coating to the gauze, may be carried out in a single or in two separate stages.  Apparatus for producing the porous diaphragm comprises a rotary drum with a horizontal shaft containing the powder to be impregnated, and means for spraying the impregnating agent into the drum, in a continuous or non-continuous manner, by means of compressed air or inert gas.  The apparatus comprises working cylinders pressed one against the other, delivery rollers, tensioning rollers and receiving rollers having parallel shafts, respecttively intended for the absorbent material, supporting band and metal gauze, means for delivering and distributing the impregnated powder and means for driving the rollers and cylinders. As shown in Figs. 1 and 2, impregnated powder 0 from hopper 1 is spread by distributer 2 over sheet of Kraft paper 3 from feed roller 4.  Sheet 3, driven by receiving roller 5 and stretched by roller 6, passes together with a regenerated cellulose band 9 from feed roller 10, driven by receiving roller 11 and stretched by roller 12, between two working cylinders 7 and 8 rotating in opposite directions to transfer the film of powder to sheet 9.  In a second stage, rollers 11, and 13 which delivers metal gauze 14, are put in the place of rollers 10 and 4 respectively, and the process is repeated to transfer the coating to the gauze.  The device may comprise three cylinders, one of which is common to the two operations, the other cylinders being pressed against the first cylinder.  As shown in Fig. 3, the layer of polytetrafluoroethylene powder 0 is transferred from Kraft paper sheet 3 to the regenerated cellulose band 9 between cylinders 7 and 8, and thence from sheet 9 to gauze 14 between cylinders 17 and 18.  Again, as shown in Fig. 4, the rolling operations may be simultaneous, the layer of impregnated powder 0 on regenerated cellulose band 9 being transferred to gauze 14 while the gauze is squeezed between the sheet of Kraft paper 3 and the coated face of sheet 9 by cylinders 7 and 8.  The section of the band coated and the section of band to be coated are disposed just before reaching the zone of tangency of the cylinders, on the same side with respect to the plane of tangency of the cylinders.