A new family of optically optimised mineral fillers is allowing the production of a new generation of ultrathermic films which can be tailor-made to suit climatic conditions and light levels. It is widely known that vegetables grown under tunnels or greenhouses covered with the so called thermic films (plastic films opaque to the infrared (IR) radiation, specially between 7 and 14 micrometers) have a greater vegetative development, obtain earlier harvests, are of greater quality and have a higher yield. These advantages are greater as the plastic cover is more opaque in the mentioned interval of infrared radiation and, at the same time, more transparent to the visible part of the solar spectrum, i.e. light used for the photo-synthetic process. Otheradditional advantages of these films are the reduced risk from frost when the greenhouse is not heated, or an important reduction of the energy consumption when any system of heating is used; both improvements are due to lower heat losses by radiation.Therefore, these covers keep the inside of the greenhouse at a thermally higher level than normal films. Most films or plastic sheets which appear transparent to the visible light are, to a certain extent, opaque to the IR radiation mentioned, so partially prevent its transmission. However, the most used material for this application, LDPE films, are very transparent to these wavelengths. In order to reduce this disadvantage, the industry has developed compounds that prevent, at least partially, the loss of heat from the interior of the greenhouse to the sky. The low price of the raw material, its good processability (films of up to 20 meters wide can be obtained), its high mechanical resistance, its resistance to UV radiation and its little elasticity are some of the reasons why this material is widely used in horticulture.
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