Articles
INFLUENCE OF DIFFERENT COVERING MATERIALS USED AS SINGLE OR DOUBLE LAYER ON THE BIOCLIMATE UNDER NON HEATED PLASTIC GREENHOUSE
Utilisation of modified PE and EVA in Tunisia has only permitted an average increase of the minimum air temperatures with about 0.7°C, while soil temperatues taken at-20 cm raised with 1 °C in comparison to a normal long life PE covering (Driss Z., 1981 ; Touayi M. and Driss Z., 1984 ; Knani H., 1984). Simular results have been obtained in France by Brun et Lagier (1983), where actinothermical indices and minimum temperatures were the highest und EVA copolymer with additives, followed by EVA and modified PE, while coextruded EVA/PE gives still better results than normal longlife PE. Utilisation of PVC sheets resulted also in good results (Brun et Laberche, 1975), but high prices of this covering material seems to be the major factor of its non utilisation up to now.
In view of obtention of better results under PE greenhouse Laberche et al. (1978) experimented the temporary use of small PE, EVA and PVC plastic tunnels under artisanal PE greenhouse and this permitted an average elevation of the minimum air temperatures with 1.6°C, 2°C and 2.7°C respectively.
Similar results were obtained by Martinez and Gonzalez (1981) in Spain.
In Tunisia the utilisation of double layer PE on a Filclair PE permitted an average increase with about 0.7–1°C of air temperature taken at 1.5 m of height in comparison to the control single layer greenhouse, while for the soil temperatures at-20 cm there was also an increase during the winter period, but a decrease during the spring period (Mougou et Verlodt, 1978). Results obtained in Monastir (Tunisia) under double layer PE, with a separation of 60 cm between the two PE films of 180 μm thichness each, indicated a temperature raise of about 2–4°C, depending upon the outdoor climatic couditions, but temperatures were taken at a height of only 30 cm (De Muynck and Van De Velde, 1984).
On the other hand a better technology of both the resine producer and the extruder permits the production of new high quality films with ameliorated physical performances.
After the radicalar PE there are now also linear PE films