Improvements in or relating to the production of water-sensitive non-fibrous sheet-like or film-like materials

摘要

The permanent shrinkage characteristic of water-sensitive, non-fibrous, sheet-like or film-like material containing not more than 10 per cent of moisture is reduced by applying thereto a surface coating comprising a water-insensitive film-forming substance, subjecting the coated material to the action of water to raise the moisture content of the base to an amount sufficient to release internal strains therein, removing the absorbed water while maintaining the material under little or no tension, and then removing the surface coating. Permanent shrinkage is defined as non-recoverable shrinkage which occurs when the material is subjected to relatively low humidity conditions. The coating substance, dissolved in a volatile solvent, may be applied to both sides of the sheets by a method such as dipping or spraying, solvent is evaporated by heat and the coated sheet is subjected to the action of liquid water or water vapour until it attains a moisture content such as 35 per cent and this moisture is removed, as by heated rolls, the material being at first allowed to shrink freely and it may then be subjected to slight tension until the moisture is reduced to 6 per cent. The non-fibrous sheet-like material so treated may be lowly substituted cellulose ether such as methyl or ethyl cellulose or cellulose glycollic acid; lowly substituted cellulose ester such as the acetate, butyrate or aceto-butyrate; polyvinyl alcohol, or regenerated cellulose which may contain a softener such as glycerin. The coating material as described in several examples may include nitrocellulose, ethyl cellulose, chlorinated rubber, paraffin wax, dammar resin, modified rosin, dibutyl phthalate, zinc stearate, urea-formaldehyde isobutanol resin, paratoluene sulphonic acid, polyisobutylene, chlorinated paraffin, cyclised rubber and ester gum modified with maleic anhydride; solvents for this coating material comprise toluol, ethyl acetate, butyl acetate, ethyl alcohol, isobutyl alcohol and acetone. Specifications 483,399, 522,643, 522,644, 535,378 and 538,280 and U.S.A. Specification 2,017,866 are referred to.