A paper substitute, which may be coated, comprises a thin compressed sheet of foamed, substantially closed cellular plastic material. Before, during or after compression the sheet may be (1) treated with a filler, e.g. clay, titanium dioxide, talc, lignocellulosic materials, calcium sulphate and mixtures of calcium sulphate and alumina; (2) coated with a casein solution of clay, satin white, titanium dioxide or pigment followed by treatment with alkali to render the casein insoluble; or (3) treated with wax. After compression the paper substitutes may be (1) heat glazed; (2) bleached; (3) impregnated with resins, asphalts, tars and pitches and (4) surface conditioned as by staining and application of sizing, e.g. glue, gelatin, rosin, starch or dextrin. In the examples, the paper substitutes are (12) coated with a mixture of straight run asphalt, a diglycidyl ether of 2,2-bis-(4-hydroxyphenyl) propane and diethylene triamine as curing agent (13) coated with paraffin wax, coal tar, coal tar pitch, alkyd resin or pine tar coating and (15) coated with a mixture of a diglycidyl ether of 2,2-bis-(4-hydroxyphenyl) propane and m-phenylene diamine to give sheets which are subsequently laminated.ALSO:A paper substitute comprises a thin compressed sheet of foamed, substantially closed cellular plastic material, the material before compression having a Young's modulous at room temperature of at least 105 degrees per cm.2 and the compression being such as to effect a permanent change in the material, the pores in the compressed sheet having an average thickness which is less than the thickness of the sheet. Numerous plastic materials are specified of the following main types:-polyolefins and other polymers of ethylenically unsaturated monomers (including homopolymers, copolymers, graft copolymers and block copolymers), alkyd resins, polyurethanes, epoxy materials, polycarbonates, phenolic resins, urea-formaldehyde resins and cellulose derivatives. Before, during or after pressing the sheet may be (1) treated with a filler, e.g. clay, titanium dioxide, talc, ligno-cellulosic materials, calcium sulphate and mixtures of calcium sulphate and alumina; (2) coated with a casein solution of clay, satin white or titanium dioxide followed by treatment with alkali to render the casein insoluble; and (3) treated with wax, or heat glazed (after pressing). Fibres such as seed hairs, e.g. cotton; stem fibres, e.g. linen, jute or manilla; leaf fibres, e.g. straw, pineapple or esparto; fruit fibres; wood fibres; nylon or glass fibres may be incorporated into the foamed materials during or after preparation. The paper substitutes may be impregnated with resins, asphalts, tars and pitches or may be bleached, e.g. with hydrogen peroxide, or surface conditioned such as by staining and application of sizing, e.g. glue, gelatin, rosin, starch or dextrin. In the examples, paper substitutes are prepared from (1) expanded cellulose acetate (2) rigid polyurethane foam prepared from hexamethylene diisocyanate and a long chain polyol or from toluene diisocyanate and a long chain polyester of adipic acid and ethylene glycol (3) foamed polystyrene (9) a foamed glycidyl polyether of 2,2-bis(4-hydroxy-phenyl)propane (10) a foamed polycarbonate obtained by reacting phosgene and 2,2-bis-(4-hydroxy-phenyl)propane (11) a foamed polyvinyl chloride, foamed polyethylene, a foamed alkyd obtained by reacting glycerol with phthalic anhydride and t-butyl benzoic acid, and a foamed urea-formaldehyde resin. In Example 12 paper substitutes prepared in the earlier examples are coated with a mixture of a straight run asphalt, a diglycidyl ether of 2,2-bis-(4-hydroxy-phenyl)propane and diethylene triamine as curing agent.ALSO:A paper substitute which may be a filter paper comprises a thin compressed sheet of foamed, substantially closed-cellular plastic material, the material before compression having a Young's modulus at room temperature of at least 105 dynes per cm2 and the compression being such as to effect a permanent change in the material, the pores in the compressed sheet having an average thickness which is less than the thickness of the sheet. For use as a filter paper the compressed foamed plastic is preferably leached with a solvent such as a mixture of benzene and isopropyl alcohol. After filtration, the paper can be removed by dissolving in a suitable solvent and the filtered material recovered. "Thickness" is explained in the Specification.ALSO:A paper substitute comprises a thin compressed sheet of foamed, substantially closed cellular plastic material, the material before compression having a Young's modulus at room temperature of at least 105 degrees per cm2 and the compression being such as to effect a permanent change in the material, the pores in the compressed sheets having an average thickness which is less than the thickness of the sheet. Numerous plastic materials are specified of the following main types: polyolefins and other polymers of ethylenically unsaturated monomers (including homopolymers, copolymers, graft copolymers and block copolymers), alkyd resins, polyurethanes, epoxy materials, polycarbonates, phenolic resins, urea-formaldehyde resins and cellulose derivatives. Before, during or after pressing the sheet may be (1) treated with a filler, e.g. clay, titanium dioxide, talc, ligro-cellulosic materials, calcium sulphate and mixtures of calcium sulphate and alumina; (2) coated with a casein solution of clay, satin white or titanium dioxide followed by treatment with alkali to render the casein insoluble; and (3) treated with wax, or heat glazed (after pressing). Fibres such as seed hairs, e.g. cotton; stem fibres, e.g. linen, jute or manila; leaf fibres, e.g. straw, pineapple or esparto; fruit fibres; wood fibres; nylon or glass fibres may be incorporated into the foamed material during or after its preparation. The paper substitutes may be impregnated with resins, asphalts, tars and pitches; they may also be bleached, e.g. with hydrogen peroxide, or surface conditioned such as by staining and application of sizing, e.g. glue, gelatin, rosin, starch or dextrin. In the examples, paper substitutes are prepared from (1) expanded cellulose acetate (2) rigid polyurethane foam prepared from hexamethylene diisocyanate and a long chain polyol or from toluene diisocyanate and a long chain polyester of adipic acid and ethylene glycol (3) foamed polystyrene (9) a foamed glydidyl polyether of 2, 2-bis (4-hydroxyphenyl) propane (10) a foamed polycarbonate obtained by reacting phosgene and 2, 2-bis-(40hydroxyphenol) propane (11) foamed polyvinyl chloride, foamed polyethylene, a foamed alkyd obtained by reacting glycerol with phthalic anhydride and tert-butyl benzoic acid, and a foamed ureaformaldehyde resin.