Transmission Efficiency of plastic Films Part 1

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TRANSMISSION EFFICIENCY OF PLASTIC FILMS Tests are usually carried out by exposing samples of the film to light of a specified wavelength, or combination of wavelengths, for a given time and noting the effect on various properties such as tensile strength or the various optical properties. The intended end use is of importance when assessing the suitability of the film as the type of radiation may be very different. In addition to the effect of sunlight it is important in many applications to know the effect of shop or display cabinet lighting. The screening effect of glass is also important when packs are to be on display in shop windows. In the case of colored films, exposure, daylight or artificial light may cause undesirable color changes. EFFECT OF TEMPERATURE HIGH TEMPERATURES: One consequence of a high temperature on a thermoplastic film may be to soften it - and - if high temperatures are likely to be encountered during use then a plastic with an appropriately high softening point must be chosen. For boil in the bag packs, for instance, films with a softening point above the boiling point of water are required. High temperatures may also lead to other Undesirable changes, especially in the case of plasticized film, where loss of plasticizer can be accelerated leading to brittleness of the film. LOW TEMPERATURE Low temperature behavior of film is also important in many applications, including the packaging of frozen foods. Some films, such as polypropylene and plasticized PVC, suffer a loss in impact strength at temperatures around 0°C although oriented polypropylene is very much better in this respect. Where low temperature use is envisaged, it is preferable to carry out tests on the actual package since any stresses and strains in the film may influence the behavior. 5.5 IDENTIFICATION OF FILM MATERIAL The most reliable method of identifying a plastics film is by infra-red spectroscopy but the equipment is expensive and requires skilled operation. There are many simple tests, however, and if they are applied in combination surprisingly good results can be obtained, with experience. If the material to be identified is a laminate then it is necessary to separate it into its component parts before applying any tests. SEPARATION OF LAMINATE PLIES This is best carried out by refluxing the sample with a suitable solvent. The choice of solvent is dictated by the adhesive used for laminating but if this information is not available the best solvent must be found by experiment. Table shows some of the commonest laminating adhesives together with possible solvents. 56
Adhesive Solvent (s) Wax Starch Casein latex Vinyl (PVA, PVAc, etc.) Low density polyethylene PHYSICAL PROPERTY TESTS TRANSMISSION EFFICIENCY OF PLASTIC FILMS Acetone, carbon tetrachloride, toluene Water An emulsion of Duponol (obtainable from DuPont Chem. Company), toluene, ammonia and water Acetone, carbon tetrachioride, toluene Toluene (at >60°C) Films can be separated into two classes on the basis of their extensibility. A sample strip should be taken in the hands and stretched gently, but firmly, noting the approximate amount of stretch. Extensible films (classified as those stretching more than 100%) include low density polyethylene, high density polyethylene polypropylene, stretch PVC, polyvinyl alcohol and rubber hydro chloride. Nonextensible films include cellulose acetate, polyester, regenerated cellulose, nylon, and oriented polystyrene and polyvinylidene chloride. Stiffness is another property which can often give some information as to the material used. Low density polyethylene, polyvinylidene chloride some vinyls and some grades of rubber hydrochloride are limp and soft to the touch and make very little noise when crumpled. Regenerated cellulose, cellulose acetate, nylon, polypropylene, polyester, rigid PVC and some grades of rubber hydrochloride are stiffer, harder to the touch and make more noise when crumpled. Oriented polystyrene is even stiffer and when rattled it has a metallic ring. Tear strength, or rather, tear propagation resistance, can also yield useful information. The film should be nicked with a knife or razor blade and then torn. Materials with low tear strength include regenerated cellulose and cellulose acetate while nylon and polyester are slightly higher. Materials with a high resistance to tear propagation include PVC, polyvinylidene chloride, low density polyethylene, and rubber hydrochloride. 57
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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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