Heat-keeping property of plastic materials as a covering of plastic tunnels

Abstract

Experiments were planned to ascertain the practical use of plastic materials as a substitute of straw mat, which has been used mostly as an outside covering of plant beds for protection from low night temperature.The air and soil temperatures inside the tunnel coverings with various plastic materials, were compared with those of straw mat covering during the early spring season.The plastic materials used in this study were as follows.A-mat: Foamed polyethylene sheet, 0.35mm thick.B-mat: Wadded polyethylene sheet, made from double layers of 0.05mm polyethylene film, with inserted Tevilon (PVC) fibre, 0.25mm thick, between them.C-mat: Laminated polyethylene film, made from double layers of 0.05mm polyethylene film, and re-inforced with polyethylene net.D-mat: Laminated polyethylene film, made from triple layers of 0.05mm polyethylene film.E-mat: Laminated polyethylene film, made from two layers of ordinary and one layer of corrugated polyethylene film, each 0.5 mm thick.F-mat: PVC film 0.1mm thick, filling aluminium foil.G-mat: Laminated polyethylene film, made from one layer of 0.05mm ordinary film and one layer of 0.025mm pneumatic polyethylene film, containing many small air-filled knobs.The best heat-keeping was obtained with straw mat on a clear night, that is, heat loss under such covering was less than under other plastic mats.On rainy and snowy nights, however, every plastic mat showed higher heat-keeping property than straw mat. Minimum inside temperature of the PVC tunnel covered with moist straw mat was occasionally lower than in beds without cover.The amounts of sunlight transmitted through straw mat and F-mat were less than under other coverings, so the soil temperatures under PVC tunnels covered with them were lower than under other mats.Using pneumatic mat (G-mat), the inside temperatures were higher with the increase of air volume, on cloudy or rainy days, but there were no significant differences among them on clear nights.The amounts of heat insulating material inserted between two layers of polyethylene film (B-mat), influenced transmittability of solar radiation. So they showed high heat-keeping propertiy, according to their quantities, on clear than on rainy or snowy nights.In addition to heat-keeping property as a covering material, light-transmittability, water-proof-ness, tear strength and aging resistance should be considered.