GB609927A - Improvements in or relating to the production of heat-sealable, transparent, cellulosic sheets and films - Google Patents

Abstract

A heat-sealable transparent cellulosic film comprises a transparent, smooth substantially non-porous, non-fibrous, water-sensitive cellulosic sheet having incorporated therein a polyvinyl alcohol, the said polyvinyl alcohol being a hydroxylated polyvinyl product containing in addition, ester, ether, acetal or ketal groups, the numerical hydroxyl content being between 70 and 96 per cent and being sufficiently large to render the said product soluble in water. The polyvinyl alcohol of the type described may thus be a partially hydrolysed ester such as the acetate, propionate, lactate, etc.; a partially hydrolysed ether such as the methyl, ethyl, glycollic acid ethers etc.; a partially hydrolysed acetal such as the formal, acetal, butyral, glyoxal, etc.; or a partially hydrolysed ketal such as is derived from acetone, methyl ethyl ketone, or diethyl ketone. Co-polymers such as those of polyvinyl acetate or other ester with polyvinyl chloride, methacrylic acid or ethylene may also be hydrolysed. The films or sheets which are rendered heat-sealable are primarily those of regenerated cellulose, but other materials such as lowly substituted cellulose ethers, e.g. glycol-, methyl-, ethyl-cellulose and cellulose glycollic acid may be used. The polyvinyl alcohol of the type described is incorporated in the cellulosic sheet to an extent between 0.1 and 1.0 per cent based on the weight of the air-dry finished sheet. The incorporation is effected by passage of the film, preferably in the gel state, through a bath containing an aqueous dispersion of the polyvinyl alcohol, which may contain in addition softening agents for the sheet or film, hardening agents for the polyvinyl alcohol and a water-soluble condensing agent to be used in conjunction with the hardening agent. Hardening agents include condensable and polymerisable water-soluble thermosetting resins such as urea-formaldehyde, phenol-aldehyde, melamine-aldehyde and the like and such compounds as chromic nitrate, chromic acetate, and boric acid. The treated sheet is passed through squeeze rollers or the like to remove excess dispersion and dried for example by passage over heated rollers. The film may be coated as a base with a heat-sealing surface coating or with a moisture-proof, heat-sealable, surface coating (see Group VIII). In an example a gel regenerated cellulose film prepared from viscose by known methods is passed through a bath containing an aqueous dispersion comprising 11 per cent glycerol and 1 per cent of a polyvinyl alcohol prepared by partial hydrolysis of polyvinyl acetate until the numerical content of hydroxyl groups is 80 per cent and the saponification number is 150. The film is passed through squeeze rolls and dried by contact with heated rollers at 60 DEG -90 DEG C. In a further example a similar film from viscose is impregnated with a dispersion as in the previous example containing in addition 0.5 per cent of condensable water-soluble urea-formaldehyde of the kind sold under the Registered Trade Mark "Uformite 414" and 0.05 per cent of maleic acid, and dried in the usual way. The film of this example is coated in a further example in two stages, the first being the application of a sub-coating comprising partly condensed urea-formaldehyde-isobutanol resin, para-toluene sulphonic acid and high viscosity nitro-cellulose dissolved in a mixture of isobutanol, ethyl acetate and toluene, and the second a coating comprising nitrocellulose, dewaxed dammar, dibutyl phthalate, dicylohexyl phthalate, an aryl sulphonamide formaldehyde resin and paraffin wax dissolved in a mixture of ethyl alcohol, ethyl acetate and toluene. Tests for moisture proofing and for measuring the heat-seal bond strength are described. Specifications 446,383, 545,523, 552,527 and 560,715 are referred to.