Transmission Efficiency of plastic Films Part 1

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TRANSMISSION EFFICIENCY OF PLASTIC FILMS to create a cell. The melt temperature has to be carefully controlled at this point, therefore, because it affects the melt strength. Films produced in this way have some of the attributes of paper, e.g. appearance handle, stiffness and some retention of creasing. Densities are of the order of 600 g/l (37.5 lb/ft 3 ). 2.7 VACUUM METALLIZATION: Although not strictly a printing process, vacuum metallization is a means of decoration. The fundamental principle of this process is the heating of aluminum to a high enough temperature to vaporize it. In practice this necessitates temperatures in the range 1500—I800C. One way of achieving this is to hang short pieces of aluminum wire onto a tungsten filament which is then heated electrically. This is only suitable for short heating cycles and so is used for metalizing plastics moldings on a batch system. For longer cycles such as are needed for the metallization of film, the aluminum wire is fed onto a block of metal, usually titanium. The tantalum is heated by holding it in a carbon crucible across which is arced a high voltage current. Vaporization of the aluminum causes minute particles to be ejected from the surface in all directions. The operation must be carried out in a vacuum in order that the metallic particles can reach the surface to be metalized Certain films may have to be de-gassed before vacuum metalizing because they contain moisture or other volatile constituents, such as plasticizers, which could otherwise cause difficulties during the process. If the normal vacuumisation does not completely remove them before metallization commences then their continued out given interferes with adhesion of the aluminum giving dull and incompletely anchored coatings. Even if the volatile constituents are completely removed they can make vacuumisation difficult and cause contamination of the pumping system. Films which need degassing include regenerated cellulose and cellulose acetate. Since this degassing may also remove other, perhaps important, constituents of the film a preferable method of solving the problem may be to seal the surface of the film with a lacquer. It is important, of course, that the lacquer itself should not contain anything likely to interfere with the vacuum. If colored metallic effects are required, the film can be self-colored or a colored lacquer can be applied after metalizing. One of the most important things to note about vacuum metallisation is that any faults in the underlying film are not hidden by the very thin film of metal deposited. Indeed, faults may even be enhanced. The lacquer film can help to hide substrate faults but can, of course, contribute faults of its own as, for example, ‘orange peel’, or ‘fish eyes’. Correct tensioning during unwind or re-reeling is also important, especially with very thin plastics films because creasing will cause uneven deposition of the aluminum. Cracking and wrinkling of the aluminum layer can also occur when a metalized film is used to form a heat sealable material by extrusion coating it with 20
TRANSMISSION EFFICIENCY OF PLASTIC FILMS polyethylene. The problem arises because of the high temperature of the molten polyethylene during the coating process. This is much higher than the temperature encountered during heat sealing, for which the metalized materials were designed. Such heat sealable laminates are required in order to reduce costs vis-d-vis laminated materials based on aluminum foil. Another problem of mealiest films when used as substitutes for aluminum foil is their higher tear strength. This is apparent when they are used as inner bags in cartons and as soup pouches, where they are more difficult to open by tearing than aluminum foil. One possible answer to this problem may be the use of mono-oriented films with low tear strength in one direction. Materials of high stiffness, too, are often required for high speed packaging in bags and pouches. Plastics films are inferior to aluminum foil in this respect but work is proceeding on the development of metalized coextruded films to give improved stiffness. Although vacuum metalizing is often carried out for purely decorative reasons, it is increasingly being used to improve the barrier properties of the base film. Such films are sometimes used as competitors to aluminum foil. The barrier properties of a number of metalized films are given in Table below with the properties of the base films in parentheses. Metalized films are also less liable than aluminum foil to loss of barrier properties when flexed, according to work carried out in Japan (see Table). PERMEABILITIES OF METALLISEI FILMS WITH METAL LAYER (APPROX. 500 A) Film LDPE (30 micron) Cast PP(25 micron) Biaxial oriented PP (25 micron) Polyester (12 micron) PVDC coated polyester (12 micron) Water vapor transmission (g/m 2 /24 h) 0.8 (17) I (15) 1—2 (6) 0.7 (46) 0.5 (11) Oxygen Transmission (cm 3 /m 2 /24 h) 38 (5000) 47 (3600) 20 (1600) 16 (58) 08 (9) Practical packaging tests using potato crisps have also confirmed the improvements in barrier properties conferred by vacuum metallization. Storage tests were carried out at 95% relative humidity and temperatures of 22, 30 and 38°C for periods up to 33 days. Moisture content changes were measured and organoleptic properties were monitored by measuring peroxide value (hydro peroxide radicals are formed by light) and hexanal concentration (breakdown product of inloleic acid). Under all conditions, 21
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Butyl, GRS, polyurethane, neoprene
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Transmission Efficiency of plastic Films Part 1

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