1,127,973. Coated sheet material. POLYMER PROCESSING RESEARCH INSTITUTE Ltd. 4 Feb., 1966 [3 June, 1965], No. 5011/66. Heading B2E. [Also in Division B5] A method of producing a polymer paper as hereinafter defined comprises the steps of: improving the molecular orientation of a film of thermoplastic polymer by uniaxially or biaxially after-stretching; and pressing fine solid matter as hereinafter defined into the film under pressure while heating the film to a temperature at which softening occurs and substantially preventing any shrinkage, whereby the molecular orientation in three dimensions is improved. A polymer paper is defined as polymeric film material having the properties usually associated with conventional cellulosic paper as regards writability and printability. Fine solid matter is defined as organic or inorganic, soluble or insoluble fine material, such as ground stone particles, clay, talc, chalk, baryta, or fine short natural or synthetic fibres as foir example wood pulp or tissue paper. Fine solid matter pressed into the film may be embedded in the interior of the film and this technique may be followed by heat setting and/or calendering. Suitably, the film is after-stretched (i.e., stretched at its softening point after extrusion or inflation and coagulation, and not simultaneously with extrusion or inflation), to between 3 and 8 times its original area, the thickness of the film following such after-stretching preferably being between 10 and 20 microns. The fine solid matter may comprise organic and/or inorganic small particles, with or without the addition of sizing materials or adhesives, or fine short fibres of wood pulp, natural fibres or synthetic fibres, or tissue paper. The fine solid matter may also comprise a waterinsoluble metal oxide, clay or talc, ground stone powder or pigment. The film is shaped from raw material of thermoplastic polymer by a wet, dry or melt process. Materials specified include polyvinyl chloride, polypropylene, polyethylene, which are exemplified, and in addition polystyrene, polyvinyl alcohol, polyvinylidene chloride, polyacrylonitrile, polyesters, e.g. polyethylene terephthalate, polyamides, e.g. nylon 6 and cellulose acetate. Films are made from these polymers by conventional means, and are then after-stretched. Advantageous methods for such stretching are described in Specifications Nos. 1,050,082, and 1,094,309. The third step of the invention is that of pressing and embedding into one or both sides of the film a fine solid material, while heating it to its softening point and preventing shrinkage as much as passible. Such pressing and embedding may be performed: (i) by pressing and embedding the fine solid matter in to the film and allowing it to remain; (ii) by pressing and embedding the fine solid matter into the film and subsequently removing it by solution; and (iii) by pressing and embedding the fine solid matter into the film and stripping it off by a physical means. The softening point of the film in this third step is that of the stretched film, and varies with the type of polymer, and on the degree of molecular orientation. In general, the lower the processing temperature, the higher the required embedding pressure. Lower processing temperatures may be used where the film contains residual solvent. When the fine solid matter to be pressed and embedded is small in quantity, and a thin layer is formed, pressing and embedding on a smooth flat film can be carried out with the fine solid matter alone, but when a large amount of fine solid matter is to be pressed and embedded, it is desirable to use a method which includes coating the surface of film with a concentrated dispersion prepared by incorporating fine solid matter, sizing agent, into an emulsion or suspension of a polymer having a softening point lower than that of the film, drying the coated film, and passing it through rollers to effect squeezing and embedding while heating at a softening point. If the fine solid matter comprises wood pulp, artificial or natural fine short fibres, or tissue paper, they can be used alone, or may be compounded with a filler, a sizing agent, a suspension, emulsion or melt of polymer, adhesive, &c., and the resultant mixture coated on films, dried or coagulated and squeezed while heating at the softening point. A writing material having a high waterproofness may be made using a hydrophobic polymer; conversely with use of water-soluble polyvinyl alcohol film, a water-soluble polymer paper as hereinabove defined may be made. The methods for pressing fine solid matter into stretched films include such treatment as electrostatically adhering fine solid matter in advance to films, coating the film with a liquid prepared by mixing fine solid matter with an adhesive and drying the coating, or passing films between embossing rolls to give microsenpinally rough surfaces, coating them with a concentrated dispersion of fine solid matter and drying the coating, which is followed by the treatment in which pressing and embedding are finished by heating at the softening point of the film. Shrinkage of film in both longitudinal and transverse directions is prevented by keeping the stretched films in contact with the curved surfaces of a number of rolls revolving in a heating chamber or while sandwiched between two belts and indirectly contacting the rolls in the composite belts thus formed. Metal or fabric belts may be used, or a set of rolls one of which is made of metal and the other of which is surfaced with paper or cotton. In an alternative technique, fine solid matter may be pressed into the film with a heat-press having flat plates as soon as the stretched films are heated up to a softening temperature, but processing must then be carried out intermittently. Films so made incorporating fine solid matter may be subjected to further processing, e.g. pressing, pressing and embedding, and other paper process techniques known in the industry. Specified treatment temperatures include 120-145‹C for high density polyethylene, 100-130‹C for polyvinyl chloride, 100-200‹C for polyvinyl alcohol containing a very small proportion of water, 130-160‹C for polyacrylonitrile containing less than 10% by weight of a comonomer, and 180-250‹C for nylon 6 or polyethylene terephthalate. In examples: (1) a tubular film of polyvinyl chloride plasticised by copolymerising with 10% by weight of vinyl acetate is biaxially afterstretched and is coated with an aqueous ermulsion containing polyvinyl chloride and calcium carbonate. The coated film is then pressed at 100 Kg/sq.cm. and 110‹C for two minutes. A white translucent film is obtained, the coated material being firmly pressed and embedded into the film, and may be written on with pencil. A film having similar properties may be obtained by pressing in a dry calcium carbonate powder, under the same conditions. (2) In Example 2, a biaxially oriented polypropylene film is coated with an aqueous emulsion containing polyvinylidene chloride, pulp fibres, and baryta powder, and dried. Heating and pressing are carried out as in Example 1, at a temperature of 165‹C. In the resultant polymer paper, the pulp fibres are pressed and embedded into the film. Printability of this product is similar to conventional paper. (3) In Example 3, an afterstretched film of polyethylene is subjected to a surface oxidation treatment and pinholes imparted to it by pricking with a porcupine roller. The treated film was then placed upon a felt and a thin paper pulp layer which had been prepared upon a screen from a stiff pulp mixture and to which an adhesive was added, was transferred to the stretched film upon the felt and vacuum suction applied from the backside of the felt. After drying, pressing and embedding while heating, a polymer paper having a surface composed of an extremely thin paper layer, was obtained. (4) In Example 4, a fine sodium bicarbonate powder was spread upon a biaxially afterstretched polyvinyl chloride film and pressing and embedding carried out at 115‹C with the use of a heat-press. The resultant pressed film was washed with water, most of the sodium bicarbonate was dissolved out but a rough and irregular surface embedded with fine powder remained upon the surface of the film. In this way, a white polymer paper with a translucent appearance to light scattering was obtained. (5) In Example 5, sand paper having fine mesh particles is put upon a biaxially after-stretched polyethylene film and pressing carried out at 100‹C using a heat press. When the resultant pressed film was stripped off from the sand paper, the microscopic irregular pattern produced by pressing with fine particles of sand paper remained on the film surface, to give a white polymer paper having a translucent appearance.