GB573574A - Improvements in the finishing and coating of fibrous materials - Google Patents

Abstract

Fibrous materials are impregnated or coated with an aqueous solution of a polyvinyl alcohol, an aldehyde, and an acidic catalyst, the amount of aldehyde being sufficient only to insolubilize the polyvinyl alcohol with which it reacts preferentially, so that any cellulosic material present is not reacted with by the aldehyde, and the materials are then dried and heated so as to form a water-insoluble polyvinyl acetal on or in the fibres. The fibrous material may be in the form of yarns or threads, or knitted, woven, or felted sheets or fabrics (including paper). The treatment may be applied for filling, coating, or stiffening the fibrous materials. Coated materials are resistant to water and impervious to war gases, e.g. mustard gas in vapour or liquid form. Compound materials having a gas-proof interlayer may be produced by coating one or both of the component webs with the solution, drying the coated web or webs, applying a further coating to one of the webs, superimposing the other web, pressing the webs together, and then drying and heating. Two coated webs may be combined without additional adhesive coating, by damping the coated sides of the two webs, pressing them together, drying and heating. A web may be coated on both sides with the polyvinyl alcohol-aldehyde solution and then combined in the damp state by pressure and heat with an uncoated web on each side. A web or fabric may be impregnated or coated uniformly or in selected areas only, by printing, stencilling, or the like. Instead of a simple polyvinyl alcohol there may be used a water-soluble partial alcohol such as a polyvinyl ester-alcohol, acetal-alcohol, or ester-acetal-alcohol. When a partial polyvinyl alcohol is used the treated materials have a greater resistance to swelling in water than when a simple polyvinyl alcohol is used. When formaldehyde is used as the aldehyde the proportion may be as low as 2.5 per cent or as high as 40 per cent of the weight of polyvinyl alcohol. Instead of formaldehyde there may be used acetaldehyde, propaldehyde, butaldehyde, or polymers thereof; or substitution products such as chloral, chloral hydrate, and aldol; or unsaturated aliphatic aldehydes such as acrolein, crotonaldehyde, and citral; or dialdehydes such as glyoxal; or aromatic aldehydes such as benzaldehyde and alkyl-, halogen-, nitro-, hydroxy-, and alkoxy-substituted benzaldehydes; cinnamic aldehyde; or heterocyclic aldehydes, such as furfural. The acid catalyst may be lactic, citric, tartaric, oxalic, acetic, formic, chloracetic, an aromatic sulphonic acid, hydrochloric or sulphuric acid. Instead of a free acid, a substance which develops acid on heating may be used, e.g. a salt of ammonia, or of an aliphatic or aromatic amine such as hexamethylene tetramine. The impregnating or coating solution may contain polyhydric alcohols, starches, gums, synthetic resins or their pre-condensates, proteins, plasticisers, water-repelling agents, mineral fillers, carbon, mineral and organic pigments, soluble colours, and fire-proofing agents. The heat treatment may be effected at 60 DEG -200 DEG C. For impregnating webs and fabrics an impregnating mangle may be used, the excess of liquid being squeezed out by means of rollers. Drying may be effected by means of heated cylinders or by a current of hot air. For coating webs or fabrics, the solution may be in the form of a paste. It may be applied in excess and the excess may be scraped off by a blade or similar device, or it may be applied in controlled quantity by means of an engraved roller. The web or fabric may be treated with substances such as starch, gum, protein, natural or synthetic resins, or water-repelling agents, which modify the penetration of the coating solution subsequently applied. In an example, a mercerised cotton lawn is impregnated with an aqueous solution containing polyvinyl alcohol, glycerine, a little formaldehyde, and monochloracetic acid. Excess is removed, and the fabric is dried on hot cylinders at 105 DEG C. and then treated with soap solution. The fabric is stiffened. A cotton net fabric is dipped in an aqueous solution of polyvinyl alcohol, formaldehyde, and ammonium sulphate. The fabric is dried without squeezing and then heated to 160 DEG C. A semi-transparent film on a net foundation is obtained and it may be used as a glass substitute in windows. A soft, absorbent paper web is dipped in a similar solution, dried, and heated. A parchment-like product of high wet strength is obtained. Heavy cotton canvas is coated on one side with an aqueous solution of polyvinyl alcohol, glycerol, aldol, and hexamethylene tetramine lactate. The coating is dried on a hot cylinder with polished surface. A glazed coating, resistant to abrasion and water is obtained. Two pieces of linen canvas are coated with an aqueous solution of a polyvinyl ester-acetal-alcohol containing acetyl, ethylidene, and hydroxyl groups, formaldehyde, glycerol, and oxalic acid. After drying, one of the pieces is coated a second time and applied to the coated face of the other piece under pressure. The compound fabric is dried and heated. A gasproof material is obtained. A cotton fabric is waterproofed with an emulsion of paraffin wax, and then coated twice with an aqueous solution of a polyvinyl-ester-acetal-alcohol, polyglycerol, dimethylol urea, formaldehyde, and hexamethylene tetramine oxalate. It is dried after each coating and then heated at 150 DEG C. A flexible coating resistant to mustard gas and to swelling in water is obtained.