| 摘要 |
<p>A foil, for use as a cover for a plant forcing bed, a cloche, or as a rooflight for a coach, whose translucence varies reversibly with the temperature and/or the intensity of illumination falling thereon, comprises a film composed of a compound of partly acetalized polyvinyl-alcohol plus water mixed with at least two hydrated salts having different anions and the same or different cations, said film being coated on one or both sides with a permeable plastics material. The mixture of the hydrated salts is present in such high concentration that the partial vapour pressure of the partly acetalized film is reduced to below ambient pressure thus enabling the transition point (the point at which the film becomes cloudy) to be raised into the useful temperature range (e.g. 20-30 DEG C.). A mixture of two more of the following hydrated salts may be used: a chloride, bromide, iodide, or nitrate or lithium, calcium or magnesium, and a perchlorate of sodium or magnesium. The permeability of the inside layer in the case of agricultural soils, i.e. the layer nearest the earth, may be of a higher permeability than the layer directly exposed to the weather, which latter layer may be polyethylene terephthalate or polymethyl methacrylate or polystyrene. The plastics layers may be lacquers based on cellulose acetate, polyvinyl acetate, polyvinyl acetal, polyacrylic acid ester, copolymers of vinyl chloride and vinyl acetate or natural substances, such as gelatin, which have been rendered insoluble. Instead of applying the plastics layers to the film, the film material, in solution in aqueous alcohol, may be cast, brushed, sprayed or spread on one or both sides of a plastics layer and a lacquer spread thereon.</p> |
