Heat-shrink film

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. 

Polyethylene shrink film wrapping of cormgated ftberboard trays is in common use outside of the United States. Equipment erects the trays, fills the trays with primary packages such as cans or jars, wraps the grouping in shrink film, and heat shrinks the combination. Shrink film wrapping keeps primary and secondary packaging materials clean and dry. 

In order to reduce the tendency of the film to shrink oriented film may be annealed at about 100°C whilst under tension immediately after drawing. The film is often coated with another polymer sueh as a vinylidene ehloride-based copolymer. This both improves the barrier properties and improves the heat scalability. 

Polyolefins Heat-shrink films and tubing Cross-linking... 

No-wear high-performance floors for high-traffic areas Heat-shrink films and tubing Curing of adhesives, coatings and inks... 

Specific gravity it is heated. Shrink film gives good protection to the products packaged and has excellent clarity. The density (mass per unit volume) of any material divided by that of water at a standard temperature, usually 4 °C. Since water s density is nearly 1.00 g/cc, density in g/cc and specific gravity are numerically nearly equal. 

An iron disilicide material is suitable for the sensing device because it has a good heat resisting property when a porosity is lower than 8 vol. % [3], It is also well known that the thick film hardly shrink during sintering because of restriction of a substrate. 

Oriented films are usually coated with a heat sealant which seals at a temperature below the orientation temperature, otherwise the film will shrink in the heat seal zone, crystallise and possibly become distorted (cockle) and brittle. Oriented films can be heat set by special treatments. 

One of the widely used effects of irradiation of PE is the so-called memory effect [Charlesby, 1960, 1987 Silverman, 1992]. PE is irradiated at RT in an inert atmosphere, and heated above the melting point, then stressed and given a particular shape. During the subsequent cooling, the crystallization fixes the imposed shape. However, upon reheating, the crystals melt and the cross-linked PE returns to its original unstressed shape. This memory effect is the basis of the production of many heat-shrink products, e.g., food-packaging film and electrical connectors [Bradley, 1984 Silverman, 1992]. 

The linear dimensions of the films refers to their width and thickness. The thickness of the films is an important parameter as it reflects stability of molding regimes and defines their strength. The measurement procedure for linear dimensions of films is specified in both national and international standards [139-142]. Relative changes in linear dimensions of heat-shrink films under elevated temperatures are characterized by a shrinkage parameter... 

Alternatively, in the double-bubble process, a cast tube is blown into a large bubble with simultaneous stretching in both directions to produce a balanced film. This process is used to make heat-shrink films as well as standard BOPP films. 

The film can then be fiattened and wound up as a tube or slit to obtain a single sheet. A polyvinylidene chloride film obtained in this way is heat-shrinkable at temperatures above SO C. If a heat-stabilised film is desired, an annealing step is applied by re-inffating the film and leading it through a radiant-heating tunnel (d). The third set of nip rolls (c) rotates at the same rate as the second set, and the air pressure in the bubble prevents the film from shrinking in the transverse direction. This is very effective... 

The source of compression in the replicated network is a volume contraction of the electrochemically deposited material on heating. This is confirmed by thickness measurements of flat films (Fig. 2.22) before and after high temperature processing. Since the film cannot shrink in the lateral direction (without fracturing from the substrate), all compression is taken up by vertical compression. 

Cross-finking increases melt viscosity, which improves the foaming process. It is used to produce heat-shrink film and tubing. It increases environmental stress-crack resistance, and it retains hot strength beyond the melting point of the crystalline phase. These improvements are particularly useful in wire and cable, pipe insulation, gasketing, and sports and orthopedic equipment. 

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. 

Similar to tubing, film or tape can also be made to have heat shrinking characteristics. Prestretched polyvinytidene fluoride from Du Pont (Teldlar) is the most commonly used shrink film because it shrinks readily on apptication of heat. It is often used in filament or tape winding operations to maintain pressure on the winding during cure. 

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. 

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