Patent GB-549668-A (GB)
Title: Improvements in or relating to light polarizers and process of making same
IPC: G02B5/30
Filed: 1939-10-17
Abstract: 549,668. Treating films ; dyeing cellulose derivatives and regenerated cellulose ; polyvinyl acetal resins. INTERNATIONAL POLAROID CORPORATION. Oct. 17, 1939, No. 28058. Convention date, Oct. 29, 1938. Drawings to Specification. [Classes 2 (ii), 2 (iii) and 15 (ii)] [Also in Groups XX and VIII] A method of preparing a dichroic lightpolarizing sheet comprises stretching while in the " rubber-elastic " state a previously formed sheet of a transparent high-polymeric plastic material whose molecules contain one or more strongly polar groups, the stretch being in one direction only and substantially to the limit of the rubber-elastic extensibility, the sheet being dyed or stained at one stage in the process and being maintained in the stretched condition. The term " rubber elastic state " is defined as the state in which certain materials show a high reversible elastic limit, for example 500 per cent. or more, and a low modulus of elasticity, and may be stretched to a considerable degree without permanent distortion. Cellulose acetate may be rendered rubber elastic by heating, and cellulose esters and ethers as well as regenerated cellulose by treatment with a swelling agent, whilst in other cases both heating and treatment with a swelling agent may be necessary. The stretching produces molecular orientation, the molecules being substantially parallel in polarizers made according to the invention. In the preferred manner of carrying out the invention a sheet of a cellulose derivative, such as cellulose acetate or ethyl cellulose, or of regenerated cellulose, or a sheet of a vinyl compound such as a vinyl acetal resin, is rendered rubber elastic and is stretched substantially to its elastic limit, in which state it is held or set so that the deformation is retained. Either before or after stretching, the sheet is dyed with a suitable dye such as a direct cotton or a mordant dye, or is stained by iodine or bromine, or a metal such as mercury, silver, gold, copper, arsenic, bismuth, antimony, selenium or tellurium. The resulting sheet exhibits high dichroism, which may be made to extend over substantially the whole spectrum by the use of several dyes, the bands of maximum dichroism of which together cover the spectrum. Many cotton dyes may be used, such as National Erie Black GXOO, Amanil Black, Amanil Fast Black, Tintex Black and Tintex Purple, and the dyeing may be carried out in aqueous solution. Other suitable dyes are referred to. Regenerated cellulose sheets may be dyed or stained with iodine by soaking them in a saturated solution of zinc chloride containing potassium iodide and after removal of excess fluid subjecting them to iodine vapour. The sheet while still moist is then stretched in air at a temperature slightly above 100‹C. To prevent action of moisture on the stain the sheet may be-secured between glass plates by a suitable adhesive such as vinyl acetate plasticized with dibutyl phthalate. Bromine may be similarly used for staining. Sheets of regenerated cellulose may be rendered rubberelastic, either before or after dyeing, by treatment with a swelling agent such as a water solution of sodium hydroxide and methanol, with or without acetone, after which they are extended substantially to the rubber-elastic limit and are maintained in this condition either by laminating to suitable supports or by removal of the swelling agent in a water or acid bath while still stretched. Certain of the cellulosic plastics, such as plasticized cellulose acetate, may alternatively be rendered rubberelastic by heating. Staining with metals may be performed by immersing the sheets in solutions of a reducible salt of the metal and then reducing it, before or after stretching the sheet, by a reducing agent such as sodium hydrosulphite. Suitable compounds are copper sulphate, silver nitrate, gold chloride, arsenious acid anhydride, antimony potassium tartrate, bismuth subnitrate, selenous acid, stannous chloride, nickelous sulphate and tellurous acid. Mercury salts may be reduced by heating the sheet, or the staining may be performed by exposing the sheet after treatment with a swelling agent to mercury vapour. Metallic platinum may be used as a stain by treating the sheet with a solution of potassium chloroplatinite, ferric oxalate and ferric chlorate, then exposing it to ultraviolet radiation and treating it with potassium oxalate. Bismuth may be used by immersing the sheet in a solution of bismuth subnitrate in dilute hydrochloric acid, then drying it and wiping it with a concentrated solution of hydrazine hydrate. The sheets may be formed from a vinyl plastic, such as vinyl acetal resin, which may be used by forming a mixture of the resin dissolved in butanol, ammonium iodide dissolved in "Cellosolve " (Registered Trade Mark), and a solution of iodine in butanol, casting it on a glass plate, stripping the sheet thus formed, stretching it and maintaining the stretch until the sheet is substantially free from solvent. In another method the resin sheet is stained in a saturated solution of ammonium iodide and iodine in xylene. The sheet, which is swelled by the xylene, is then stretched and dried whilst the stretch is maintained. The resin sheets may be dyed before stretching by exposure to iodine fumes or by a direct cotton dye. The dye may be employed in a 50-50 solution of water and denatured alcohol with a little soap. Alternatively a solution of the vinyl compound in methanol may be dyed with the dye, and a film cast on to glass, which film is subsequently stripped, stretched and dried.