ISHS Acta Horticulturae 87: Symposium on Potential Productivity in Protected Cultivation
WEATHERING RESISTANCE OF POLYETHYLENE AND PVC FILMS USED IN PROTECTED CULTIVATION |
| Author: | A. Ram |
| DOI: | 10.17660/ActaHortic.1978.87.2 |
| Abstract:
Low-density polyethylene (LDPE) and plasticized PVC, both in the form of thin films, represent the most popular glazing material for modern greenhouses and tunnels. Extensive research efforts were undertaken in order to enhance the weathering resistance of these polymers. The film is expected to retain its optical and mechanical properties throughout the crop season, while a multi-season solution is frequently desired. The plastic film is exposed to ultra-violet radiation (mainly in the summer), to oxidative conditions and high temperatures as well as to cyclic temperature and humidity changes and vibrations caused by winds. The mechanism of failure is different in both polymers, but a high molecular weight and narrow molecular-weight-distribution will always be preferred. After choosing the right polymer grade, the role of specific additives is crucial for extended lifetime. In the case of LDPE the major protective additives are ultra-violet absorbers (UVA) and antioxidants. They neutralize the high-energy induced by the UV portion of the light (causing chain shortening) and retard oxidation of the polymer during processing and exposure to atmosphere. All processes are highly accelerated at elevated temperatures. The major damage due to oxidation, crosslinking and chain-scission is observed by embrittlement of the film, which is best measured by the decrease of ultimate elongation. The latter can also be correlated with the increase of carbonyl-number (amount of reacted oxygen). The chemical structural response is represented by decrease in the molecular-weight average, narrower molecular weight distribution and loss of long-chain branches. In choosing the proper additives, the role of compatibility with the polymer and elimination of premature exudation to the atmosphere should be carefully controlled. A synergistic composition of two light stabilizers, one acting as a true absorber and the second as an energy quencher, has been proven very beneficial. Other combinations are currently studied. In the case of plasticized PVC, the proper choice of heat and light stabilizers and in particular the plasticizers is of paramount importance. In contrast to LDPE, PVC is little affected by oxidation, while being sensitive to UV and thermal energy, resulting in chemical decomposition and chain scission. In addition to the loss of mechanical properties, light transmittance is gradually diminishing unless properly protected. The plasticizer may exude to the surface and diffuse into the surrounding causing early embrittlement. After prolonged exposure studies (natural and accelerated weathering) several compositions were chosen promising endurance of over one year in a country of hot climate. The disadvantage in using PVC films stems from their higher price and limited range of width. Our present study deals with the separation of climatic components (UV light, heat, oxygen, wind), and the search for better understanding of the mechanism of weather resistance of polymeric systems. | |
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