THE USE OF ACRYLIC SHEETS FOR GREENHOUSES

This paper is given from the point of view of the engineer endeavouring to develop new types of permanent rigid plastic coverings for the growing of crops.

It is hoped that these structures will give the grower greater yields and more economical production. In addition, greenhouses can be made which will be more resistant to adverse climatic conditions such as high winds and heavy snow loadings.

An ideal greenhouse, irrespective of glazing materials must meet the following requirements :

1. Above all things it must admit the maximum light available in a spectrum suitable for plant growth.
2. Heat losses must be must be minimised for economy of operation.
3. The house must be structurally sound, capable of withstanding snow and wind loading, and the glazing material must have good weathering and impact properties.
4. The greenhouse system must be an overall viable financial proposition, taking into account capital and installation costs, running and maintenance costs and the output of produce grown in the house.

Summarising these four points, (1) Light = growth, (2) Economy of operation, (3) Sound structurally, (4) Financially sound.

In some parts of the world greenhouses are of course only a temporary winter requirement, whereas in other parts efficient houses with climatic control are necessary all the year round.

Designs have changed considerably in recent years, with high standards of living people are demanding all the year round vegetables and flowers which may be more easily grown locally even if by part artificial light, rather than transported considerable distances from climatic areas where growth conditions are more favourable.

Development work is towards more sophisticated houses and ancillary equipment, and one need only mention the tower greenhouses developed by RUTHNER in Vienna to realise there is great scope for an entirely new concept of a greenhouse.

We have already said that the covering material must admit a maximum amount of light in a spectrum suitable for crop growth. The correlation between plant growth and light admission has not been fully investigated but a number of research works consider that 1 % increase in light admission is worth 1 % extra growing capacity under European light conditions at about lattitude 52°, and it is quite likely that this figure will increase at higher latitudes where light values are lower.

Light admission to a house is a combination of the glazed area and the construction of the house, in particular the shape of the house and the restriction imposed by the presence of glazing bars. It is this area in which the use of plastic sheet materials score as large sheets can be used bent to a curvature with a minimum of glazing bars.

The requirements for a plastic sheet are therefore :

1. High light transmission in the appropriate waveband.
2. Good weathering properties.
3. Availability in large area sheets.
4. Adequate rigidity.
5. High impact strenght.

Of all plastic sheets that are available, only three are probably worth considering, namely, glass reinforced polyester sheet, clear rigid PVC sheet and acrylic sheet.

Glass reinforced polyester sheet although adequate for many outdoor applications suffers from a reduction in light transmission after a period of outdoor exposure and the surface tends to become rough exposing the glass filaments and producing ideal conditions for dirt retention.

Clear rigid PVC has most of the desirable properties for a greenhouse but in every case they are slightly inferior to acrylic sheet.