It is well recognized that in certain applications, heat shrink films provide a number of distinct advantages over other types of materials. The most prominent applications for heat shrink films are in labels and other packaging applications, electrical wire or cable, and protection of buried pipeline or other equipment.
In many of these applications, the addition of an adhesive is required to provide sealing, better film conformation, or a multitude of other functions. The adhesive may be applied as an integrated layer on the shrink film itself or may be applied as a separate component during application of the film.
Although the adhesive provides added functionality, some shrink film products have sufficient cling properties without the use of an adhesive. This reduces cost and complexity of the process and eliminates a potential source of contamination when it comes to recycling products such as packaging.
This article will review the types of shrink film products on the market today and anticipate future products and applications. A discussion will be provided on the shrink films most commonly utilized, how the heat shrink film is made, and how it is utilized in practice. The requirements for the adhesive systems that are used with these products are then discussed, and finally the common types of adhesives systems are explained.
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Shrink film, also known as shrink wrap, is material made of specially processed plastic film. When heat is applied, the film will shrink tightly over whatever it is covering. Heat can be applied by manual electric or gas gun or the product and film can pass through a heat tunnel oven. A shrink film can be made to shrink in one direction (unidirectional or mono-directional) or in both directions (bidirectional).
Shrink film is applied over or around the intended item, often by automated equipment. It is then sent through a heat tunnel or oven for shrinking. Manual heat guns can be used on large items or on small production numbers.
Shrink wrap can be supplied in several forms. Flat roll stock can be wrapped around a product. Here an adhesive may be required to provide a seam at the end of the rolling operation and before the product is exposed to elevated temperatures. Center folded film is also used. In this format the film is supplied on a roll, once cut to length the film is folded in half, the product is placed in the center portion, the remaining three edges are sealed to form a bag, and the package is then heated and shrunk. Another popular form is continuous tubing where the product is placed in the tubing, sealed, and processed.
Shrink film is commonly used as an over wrap on many types of packaging, including cartons, boxes, beverage cans and containers, and integrated pallet loads. It is also commonly used to insulate, seal, and repair electrical wiring. It has even been used to seal environmental hazards, such as asbestos, lead, and other loose material for safe removal. A newer application is as a corrosion resistant covering on buried pipeline. In many of these applications, adhesives are either applied to the shrink film as an integrated product or a separate adhesive material is required to make seams and overlaps. In some applications, such as when shrink films are used for certain packaging applications or when it is used to cover boats and other products for winter storage, an adhesive may not be required.
One of the most important decisions in a shrink wrap application is the type of film to be used. Film choices are made on the percentage of shrink required, desired finish, end-use requirements, applications considerations, and environmental footprint such as energy use and waste.
The most commonly used shrink film is polyolefin. It is available in a variety of thicknesses, clarities, strengths, and shrink ratios. Polyvinyl chloride (PVC) polyethylene terephthalate, polycarbonate films are also used. Even biobiased polylactic acid films and expanded polyolefin foam films have also been used as shrink film. Coextrusions and laminations are also available for specific mechanical and barrier properties such as for shrink wrapping food.
A performance comparison of common shrink films is provided in Table 1. The typical properties of common shrink films are given in Table 2.
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