Patent GB-567878-A (GB)
Title: Improvements in or relating to impregnated fibrous materials
IPC: C08J5/06
Filed: 1942-11-24
Abstract: A fibrous material in the form of a sheet is stabilized against dimensional change due to alteration of humidity by impregnating with an aqueous solution or dispersion of a low molecular weight crystalloidal condensation product (derived from formaldehyde and a component adapted ultimately to yield with formaldehyde or thermosetting synthetic resin) so as to incorporate not more than 10 per cent of the condensation product within the fibres, further condensing the material within the fibres, and subsequently forming the fibrous material into a mass with a thermoplastic bonding agent. In a modification, loose fibres are so impregnated and any bonding agent is used.  Thermoplastic bonding agents referred to are natural and artificial rubber resins.  The untreated material may be either cellulosic or non-cellulosic and in massive form (e.g. wadding or felt), or in sheets or webs (e.g. wood veneer, paper, or textiles). High density paper may be used. The moisture content of the untreated material may be adjusted to the equilibrium value for the relative humidity that the finished product has to withstand.  The material is treated with solution containing from 5 to 10 per cent, by weight of a low molecular weight phenol-formaldehyde or ureaformaldehyde resin. A condensation accelerator may be applied to the impregnated fibre, or may be mixed with the condensation product immediately before its application to the fibre, or applied to the fibre already impregnated with the condensation product.  The fibrous material may be cured by heat, with or without pressure, and without an accelerator present mere ageing may suffice.  The curing step may comprise merely the application of a dissolved or dispersed accelerator to the impregnated material, the drying step prior to the curing being omitted and drying being effected afterwards.  A mass of water swellable fibres may be impregnated with a solution prepared by dissolving urea in neutralized 40 per cent formaldehyde solution, allowing to condense for 3 to 20 hours at ordinary temperature, diluting with water and adding ammonium dihydrogen phosphate.  The mass is squeezed and dried, preferably at less than 100 DEG  C., and heated for 2 minutes at 120 DEG  C. or 1 minute at 140 DEG -150 DEG C.  The pH of the dried material should be 4.5.  The ammonium dihydrogen phosphate may be replaced by ammonium tartrate, when the final heating should be for 2 minutes at 170 DEG  C.  The material is then washed in warm soap solution containing sodium carbonate, rinsed, and dried.  In example 1, cellulosic fibrous material, treated as described, is treated with a solution of a polymethacylate resin, dried and bonded together under heat and pressure.  In example 2 the fibres are impregnated with a phenolformaldehyde resin, hydro-extracted, dried below 100 DEG  C. and heated for 3 minutes at 170 DEG  C.  The mass is brought into the form of a lap and a number of sheets are bonded together using a thermoplastic resin as bonding agent.  In example 3, paper or fabric sheets are impregnated with low molecular weight urea-formaldehyde resin which is hardened in situ, and the dried material is impregnated with a solution of a urea-formaldehyde thermosetting resin and formed into a laminated product.