4.11. Photodegradation of plastics
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- B. Brenndorfer
The most common use of plastics in solar collectors and dryers is as a transparent cover allowing incident radiation to pass through and impinge on an absorber surface - or on the commodity being dried. Used in this way' plastics must be able to withstand elevated temperatures, high levels of insolation' high humidities, wind loading and the effects of heavy rain over long periods of time. Low cost, low density, and good optical properties make some plastics very suitable for use in solar collectors and dryers. Stemming from its convenience as a building material, black polythene also finds use as an absorber material. However its shiny surface - a characteristic of the manufacturing process - is not conducive to high absorption efficiency.
Photo-degradation in plastics is caused by the ultra violet (UV) component of solar radiation, that is radiation of wavelength from 0.295 to 0.400 um. This is absorbed by some plastics' and causes the breakage of bonds in the polymers leading to photo-oxidation.
The physical effects of photo-degradation vary from loss of transmissivity and discolouration to crazing of the surface and embrittlement of the plastics. Loss of transmissivity will result in a lowering of the efficiency of a collector or drier and crazing or embrittlement will render the plastic more prone to damage by wind and rain. Degradation of plastics occurs more rapidly at higher temperatures and thus deterioration is often worst at hot-spots such as points where the plastic is supported or attached to the framework.
Polymers vary greatly in their resistance to weathering, some such as polymethylathacrylate (PMMA) and polytetrafluosoethylene (PTFE) are transparent to UV radiation and hence not susceptible to photodegradation, others such as polyethylene deteriorate rapidly in UV radiation. The resistance of polymers such as polyethylene to UV degradation can be improved by the use of additives which absorb the UV radiation and remain stable, or by coating with another polymer which is opaque to UV radiation.
A wide range of clear plastic sheet and film with properties suitable for use in solar energy applications which also have good resistance to weathering is now avaiolable. Plastics commonly used for glazing in solar collectors include PMMA, polycarbonate (FC), glass-fibre reinforced polyester (GRP), polyvinylfluoride (PVF), fluorinated ethylene propylene copolymer and polyester film (FEP).
PMMA is used in the form of a rigid sheet which can be thermoformed to different shapes. ICI have subjected "Perspex" to outdoor exposure over a period of 10 years in the UK with only a 4% loss in transmissivity. A similar time under tropical conditions resulted in only slight discolouration. PC sheet is sesceptible to micro-cracking when exposed to outdoor weathering, even in the standard UV stabilised form and so existing commercial polycarbonate glazing material is not adequately protected against UV radiation. GRP is light, has a high light transmissivity and good impact resistance. When protected either by stabilising to resist UV degradation or by coating with PVF film this material shows good resistance to weathering. In unprotected forms the surface of the sheet develops micro-cracks and develops fibre prominence' both of which decrease its light transmissivity.
PVF film is used both as a protective coating for other plastics to increase their resistance to UV and also on its own in solar collectors. Tedlar PVF is quoted by Du Pont as having excellent weatherability with no discolouration or significant loss of transmission after prolonged exposure. FEP exhibits good weathering characteristics' for their FEP film Du Pont claim that all its properties remained constant for 5 years during continuous exposure in Florida. Polyester films also possess good transmissivity and stability to UV radiation.
As the long term stability of polymers to UV radiation can vary greatly with additives used, great care should be taken when choosing a plastic for prolonged exposure under the conditions found in solar collectors. Specifying the polymer will not always be sufficient. In order to achieve the length of service of which UV resistant plastics are capable, methods of attaching the plastic to the framework, commonly used in simple agricultural systems, such as stapling or nailing' are unsatisfactory as they create point of stress where the material is likely to fail.
When attaching plastic sheet to the framework a method should be chosen which will distribute any stresses on the sheet as evenly as possible over its whole length or width.
The most simple method of achieving this is to attach the sheet by means of battens running the length and width of the sheet. Sharp edges on the battens and framework should be avoided where possible. Use of a soft compressible material between the batten and the plastic will give a more weatherproof seal as well as giving a more even stress distribution, especially in cases where the framework has an uneven surface.
A groove and caulk strip with the caulk strip clamped to the framework along its length also ensures an even distribution of forces on the polymer sheet. This method however requires a higher level of skill than the previous method of attachment.
The properties of these polymers are summarised in the table below but in general, their availability in Third World countries will depend on the presence of a manufacturing agent or stockist within the country.
1. British Cellophane Limited - Duraphane (polyethylene)
2. ICI - Perspex (PMMA)
- Melinex (Polyester)
3. Du Pont - Tedlar (PVF)
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