In the German Democratic Republic the area of greenhouses made of polyethylene film increased fivefold during the period of 1966 to 1969. At present there are production enterprises that manage 2–4 ha and cooperative unions that have over 15 ha of plant growing structures, covered with polyethylene film.
This development generates important tasks with a view to promoting the productive forces and the further consolidation of the cooperative system in the German Democratic Republic.
The effect of light on plastics and the elimination of its consequences are an important applied-technical problem.
Its importance grows with the expanding use of plastics as greenhouse construction element.
It is certain that the hard plastics, first of all in the form of polyvenylchloride and its copolimerizers, and in the form of GFP will be extensively adopted since their qualities were improved in the last few years.
They are distinguished by more favourable economic indicators.
Structures of these materials are a necessity because the further concentration of greenhouse areas, covered with polyethylene film, is restricted by the high labour input in its annual replacement.
Bearing this in mind, the Institute for Vegetable Growing at Grossbeeren conducted investigations concerning the adoption of hard plastics in vegetable growing since 1962.
The studies for determining light conditions and yield capacity were carried out during the 1963–1964 period through 1968 in greenhouses 12 m and 18 m wide, made of GFP covering units (fig. 1 and 2).
In both single span greenhouses the roof consists of glassfiber reinforced polyester in the form of covering units.
Two covering units of GFP in the case of 12 m wide greenhouse and four covering units in the 18 m wide greenhouse span the structure as an arch.
Equal, 1.2 mm thick GFP-units have been used for both lateral walls of the greenhouses.
The other indicators are presented in table 1.
Nonsaturated polyester resin "Buna G" with a layer of glass wool (alkaline type 216 St 450 g/sqm) was used in making the GFP covering units.
Since the polyester resin is prone to yellowing under the influence of ultraviolet rays, a stabilizer was added to the resin, which stabilizer acts against ultraviolet light in the form of 2-oxy-4-methoxy benzophenone and 2,4-dioxybenzophenone.
The covering units were given special protective coating by applying gelcoat layers (layers of bone resin).
It was found that light transluscence (degree of transluscence in directed perpendicular light) in these GFP covering units in the newly built greenhouse ranged between 73 and 79 per cent.
The reason for the relatively low degree of light transluscence of these covering units is the fact that the polyester resin "Buna" is not colourless.
It is yellowish and