Traditionally, the first material used for greenhouse cover was glass. As an alternative cover material to glass, oiled paper was tried in the Netherlands during the late 18th century and was in common use in Japan well into the 20th century (Norton, 1992). After the Second World War, plastic materials became more readily available. From the time clear plastic materials were first produced on a commercial scale, their potential for replacing glass in agricultural facilities has been recognized. Nowadays, PVC and polyethylene films are attached internally to the greenhouse framework, thus creating an insulating air gap between the outer cover and the protected artificial environment. Polyethylene is very popular for agricultural applications because it is available in wider sections than most other films and is of low cost, despite its short lifetime of about a year when exposed to typical weather conditions. Additionally, because polyethylene is the most common plastic film used, data for light transmission through this material are readily available.
Generally, plastic materials have inferior light transmission properties compared with glass. Additionally, since they degrade when exposed to heat and ultraviolet light, their useful life is much shorter, typically a few years compared with decades for glass. Condensation on the inner surface of the cover, which under some conditions could persist during the day, reduces the light transmission. This reduction is more pronounced with plastics than with glass because of the higher angle of conduct between the water bubbles and the plastic, leading to a higher proportion of reflected light. The advantages of plastic materials, however, are their low specific mass and high strength, requiring a lightweight structure and lower cost, resulting in lower initial investment.
Although polyethylene is the most widely used plastic film in agriculture, other materials are available, such as polymers containing fluorine compounds, whose radiation transmission properties and resistance to aging are superior to those of polyethylene films. These, however, are more expensive than polyethylene.
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