Orientation shrink-film

Oriented polypropylene film (OPP) may be classified as heat-set and non-heat-set, blown and tentered, coextmded and coated. Orientation improves the cold-temperature resistance and other physical properties. Heat-set biaxially oriented polypropylene film (BOPP) is the most widely used protective packagiag film ia the United States. It is used to wrap bakery products, as lamination pHes for potato and com chips, and for pastas and numerous other flexible pouch and wrapping appHcations. Nonheat-set OPP is used as a sparkling, transparent shrink-film overwrap for cartons of candy. 

Other special features include shrink films, stretch films and cling films. These features are, however, unlikely to be used in combination materials although oriented plastics are used in cold formed blister packs. 

Unoriented random PP copolymer films are soft and are easy to heat seal. Oriented PP films (both homopolymer and copolymer) have improved strength, clarity, and gloss (brightness) compared to unoriented films. Applications for oriented PP include shrink wrap (records, toys, games, hardware items, frozen foods, and cigarette wrap). A 7% ethylene copolymer is often used as a heat-seal layer in food packaging. 

To make shrink film, a polymer film can be oriented at an elevated temperature and the orientation frozen by rapid cooling. When the film is subsequently heated, the molecular memory of the polymer causes it to attempt to return to its original dimensions. Lightly cross-linked materials are often used to increase the tendency to shrink. In that case, electron beam irradiation of the plastic film produces free radicals, which then react to produce cross-links between adjacent molecules. The presence of these cross-links means the material will no longer become liquid and flow at its normal melting temperature. That, in turn, allows the shrink film to be exposed to high temperatures, at or above its former melt temperature, without flow, so these elevated temperatures can be used to promote shrinking. 

Freezing in of orientations is made use of with what are called heat-shrink films Heat-shrink films are stretched or drawn biaxially, and these films are used to solidly encapsulate mold components. The wrapped goods are then warmed, whereby the film expands slightly below the glass transition temperature (for amorphous polymers) or the melting temperature (for crystalline polymers). At the transition temperature, however, the molecular segments become mobile the molecules tend to assume their unperturbed dimensions and the film shrinks in upon itself. 

Shrink film n. (1) Films which shrink when heated and are useful for packaging articles. (2) The pre-stretched or oriented film used in shrink packaging. 

There are two major markets for irradiated film. The first is packaging film for food and small parts. The irradiated film has improved strength and reduced perme-abihty. When oriented the film shrinks to conform to the contours of the product and has crystal clarity. The second market is heat shrink tubing for electrical connectors. Here the polyethylene is used together with cross-linked polyethylene to form a protective sleeve. The interlayer is polyethylene, which, when heated, flows to fill voids and completely encapsulates the wire. The outer layer is cross-linked polyethylene, which shrinks to form a compact protective sheet over the connector. 

Figure 16.2 shows P t,T) of one of the experiments. The tracking curve appears smooth and demonstrates the reliability of the tilt-angle determination. Tilt-angle variation is an issue because the oriented PP film is heated until it becomes a viscous melt. Therefore, the material shrinks and bends in the synchrotron X-ray beam. 

Metal contact strips are used to attach the ends of the assembled battery to the battery terminals (e.g., 9-V transistor battery). The orientation of the stack subassembly (cathode up or anode up) is only important for each manufacturer s method of assembly. The use of contact strips allows either design mode. The entire assembly is usually encapsulated in wax or plastic. Some manufacturers also sleeve the assembly in shrink film after waxing. This aids the assembly process cleanliness and provides additional insurance against leakage. Cost, ease of assembly, and process efficiencies usually dictate the orientation during the assembly process. 

Oriented polyolefin films will shrink substantially on initial heating... 

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