Greenhouse Films

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Currently, the low-density polyethylene (LDPE) homopolymer and the ethylene-vinyl acetate (EVA) copolymers are the most common plastics materials used for greenhouse cover films applications [4]. However, in Japan, plasticized (PVC) is used to a larger extent. The growing importance of plastics films for greenhouse covers is best illustrated by a few figures with greenhouse films worldwide was approximately 60,000 ha [1]. In 1980 the surface area covered with worldwide greenhouse films was 80,000 ha [1] and reached 220,000 ha in the 1990s [2]. Spain went through a tremendous evolution for the agricultural surface-covered greenhouse: from 15,000 ha in 1983 [3] to an area of 28,000 ha in the 1990s [2]. Although there is some leveling off in the total area covered with greenhouses in southern Europe, the development is likely to continue in other parts of the world, particularly in South America.

The main types of polyethylene films used as greenhouse covers include the following:

1. LDPE is enriched by needed and measured quantities of UV stabilizers that ensure the films a life of some months to 2-3 years.

2. EVA is PE containing different doses of vinyl acetate (VA). The higher the quantity of VA used, the better the absorption for long infrared (IR) (5-35 ^m); nevertheless, the mechanical properties may be not quite satisfactory.

3. Thermic IR or modified PE is produced from compounds enriched with special "charges" that absorb other parts of the long IR radiation than VA does.

4. Thermic PE contains low quantities of VA and simultaneously some thermic charges. They possess the advantages of the EVA and of the thermic PE but do not have their disadvantages.

5. Linear PE seems to present some advantages interesting in agriculture [5].

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