Transmission Efficiency of plastic Films Part 1

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TRANSMISSION EFFICIENCY OF PLASTIC FILMS An important factor is that of dispersion, to develop colour strength and maximum optical properties. Dyes are not normally a problem because of their soluble nature, but pigments need to be properly wetted by the plastic or by suitable vehicles. Poor dispersion can lead to processing difficulties with thin films. Improvements have been made through surface treatment of pigments or through better grinding techniques. 7. HEAT STABILISERS The high temperatures encountered during film extrusion can cause thermal decomposition of certain plastics. This follows a similar path to the decomposition reactions caused by oxidation. There is a close relationship between the two reactions, thermal decomposition being catalysed by oxygen and oxidation being catalysed by heat. Some polymers such as low density polyethylene and the nylons are stable enough not to require heat stabilisers, but PVC, high density polyethylene and polypropylene may require significant amounts. A wide range of heat stabilisers is available, the most common being barium/cadmium, organotin and lead compounds. The choice in any particular instance is governed by the temperatures likely to be encountered, the duration of protection required, the amount of oxygen present and the presence or absence of antioxidants. The resin Slip additives act as external lubricants and the materials normally used are fatty acid amides. In addition to solving problems in film handling equipment, slip additives are also useful in applications where sheaves of film are to be dispensed. 8. UV SCREENING AGENTS UV radiation has a harmful effect on many products, particularly on certain foodstuffs. Where these effects are likely to be significant they can be greatly reduced by the addition of UV screening agents to the film wrapper or sachet. These screening agents act by absorbing or reflecting the UV radiation. The reflected radiation is dispersed harmlessly but absorbed UV radiation is converted to radiation of a different wavelength and this might have an effect on the plastic. If the converted wavelength is in the visible or infra-red spectrum then it is relatively harmless but shorter wavelengths, such as different areas of the UV spectrum, could have a deleterious effect on the film itself In most food packaging applications, however, the time of exposure to UV is likely to be small and the effect on the plastic should be negligible. UV screening is achieved either by pigments or by colorless organic compounds that absorb the radiation. One example of a bag incorporating a UV screen is the one used for pre-packed potatoes. This is often tinted a light brown color to prevent ‘greening’ of the potatoes. The most effective pigment is carbon black which gives optimum protection at concentrations of about 2—3%. Films intended for outdoor use, particularly in the tropics, are often protected in this way. 68
9. UV STABILISERS The material must absorb UV light in the desired region. TRANSMISSION EFFICIENCY OF PLASTIC FILMS These are added for the purpose of protecting the film itself and operate by interfering with the degradation chain reactions started, or catalysed by, the radiation. It has been established that the breakdown of polymeric materials under exposure to UV radiation involves a light-catalysed free radical mechanism. This results in rupture of the bonds along the polymer chain and degradation of the physical properties of the polymer. In practice one or more of the following effects can result from exposure of unstabilised polymers to UV energy sources. Discoloration due to the formation of conjugated structures or complex radicals. A reduction in tensile or impact strength. Dimensional changes. Surface crazing or cracking. Powdering. The various polymers differ in their sensitivity to UV radiation. Cellulose nitrate, for example, deteriorates quite rapidly whereas poly(methyl methacrylate) is quite resistant. The requirements for a UV stabiliser are as follows. It must be stable under processing conditions. It must be compatible with the polymer. It must be chemically inert. There must be no toxic hazard. Materials commonly used as UV stabilisers include benztriazole derivatives, benzophenone derivatives and metallic complexes, such as nickel or cobalt salts of substituted phenols, alkylphenones and dithiophosphinates. Most of the UV stabilisers mentioned are highly effective at low concentrations and are used at levels of 0.1% or less. 6.4 Manufacturer and Suppliers: The different manufacturers of adhesives are as following, ZHEJIANG HENGLI COMPOUND MATERIAL CO., LTD. AL TECHNOLOGY, INC. PERSON ADHESIVE, INC CLARIENT PVT LTD. 69
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Transmission efficiency of Plastic
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C O N T E N T S S. No. CHAPTERS Pag
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The strategy for the project, TRANS
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TRANSMISSION EFFICIENCY OF PLASTIC
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Bags and Sacks 22% Bags and sacks 2
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2.1 EXTRUSION OF FILM: TRANSMISSION
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Film casting (slit-die extrusion):
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Transmission Efficiency of plastic Films Part 1

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