Oriented films heat shrinkable

Heat-shrinkable sheets (thickness 0.040 to 0.120 inch, or 1 to 3 mm) and films (thickness 0.001 to 0.020 inch or 0. 025 to 0.5 mm) are fabricated from many of the same materials as shrinkable tubing.91 They are produced by extrusion as a tube, sheet or blown film. Irradiation is done by the equipment shown in Figure 8.10. Orientation (stretching) after irradiation can be done by several methods, namely by differentially heated and driven rolls (in the machine direction) or by a tenter frame (see Figure 8.11) in the transverse direction. If desired, biaxial stretching can be done. 

Hot blown plastic films usually do not possess sufficient orientation to be heat shrinkable at temperatures below 100°C. For certain shrink packaging applications low temperature shrinkage is essential and can be obtained by biaxial orientation below the melting point. Radiation crosslinking was found to facilitate orientation of these copolymers greatly because of the much greater bubble stability which results from crosslinking. 

In other reports, a heat shrinkable (oriented) film was described in which the base resin was a copolymer of chlorotrifiuoroethylene and up to 5% of another monomer such as vinylidene fluoride. This film could be shrunk by the application of heat (about 2 minutes at 150°C), thus it was useful as a covering. To orient the film, it was wound around a pair of parallel rolls (similar to Fig. 6.26) in which the second roll (B in Fig. 6.26) moved at a faster rate than the first roll (A in Fig. 6.26). Heat was required to assist the stretching process and, to accomplish this, the rolls were heated to a temperature in the range of 85 °C to 130°C. The stretched film was cooled below the second transition temperature of the polymer while it was held under tension. Stretching the film in the... 

DuPont films of Teflon PFA are tensiled (uniaxi-ally oriented) and heat toughened to increase mechanical performance, dimensional stability, and impermeability. DuPont T films are available as heat-stabilized or heat-shrinkable. They are highly transparent and weather resistant. One class is surface treated for printing or adhesive bonding to various substrates. 

Thermal shrinkage, accompanying release of moulded-in orientation, can be the cause of severe warping of pofymer mouldings when heated, but desirable effects can also be achieved. Shrink-wrap polymer films have caused a minor revolution in methods of packaging. Heat-shrinkable polymer tubes and cable insulation are currently finding wide application. 

Shrink Film (1) Films which shrink when heated and are usefid for packaging articles. (2) The prestretched or oriented film used in Shrink Packaging. See also Heat-Shrinkable Film. 

Oriented, heat shrinkable, poly(vinylidene chloride) (PVDC) films are widely used for packaging purposes, particularly for packaging food. However, vinylidene chloride copolymers need to be plasticized so that they can be extruded and stretched into oriented films at commercial rates. The greater the proportion of plasticizer, the lower the viscosity and the easier the polymer is to extrude and orient, as well as better the abuse resistance of the final product. 

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