Low density polyethylene film has high usage appeal for a wide range of applications. As an example, it is widely used for shrink wrapping of articles or combinations of articles where the film after being stretched or oriented during manufacture is wrapped about an object and shrinks upon the application of heat, closely conforming to the wrapped article and furnishing protection while having sufficient transparency to provide a view of the article. Other uses include a wrap for bread, confections, baked goods, and other food products, packages for liquids enclosed in a box frame structure, bags for produce, soft goods, i.e., fabrics and the like (replacing kraft paper in many instances), a wrap for frozen food, ice bags, heavy wall bags, drum liners, trash bags, etc.
The appeal of polyethylene for these applications is based on a number of factors. The density of polyethylene has a range from about 0.910 to about 9.970 grams per cubic centimeter. When extruded and produced as a blown film, low density polyethylene (density of about 0.910 to about 0.928) results in a yield of about 42,000 sq. inches per pound for a 0.7 mil thickness film. Using 1 mil. film for comparison purposes, the yield of polyethylene is 30,000 sq. inches per pound and is significantly higher than other similar films. For example, cellulose acetate has an area factor of 21,000 to 22,000 sq. inches per pound per mil, nylon 6 is 24,500, high density polyethylene is 29,000. A copolymer of ethylene and vinyl acetate is 29,000, polystyrene is 26,300, and polyvinylchloride (plasticized) is 20,000 to 23,000. Thus, polyethylene and copolymers of ethylene and vinyl acetate give yields of film on the order of as much as 11/2 times the yield of other plastic films.
The tensile strength of polyethylene film without any additives is adequate for most packaging purposes, although lower than that of cellulose acetate, Nylon 6, or polystyrene films, and is comparable to the strength of polyvinylchloride film.
Elongation (In percentage) of uncompounded polyethylene film is much higher than that for cellulose acetate, polystyrene, or Nylon 6, and polyethylene or copolymers of ethylene with vinyl acetate exceed the elongation of polyvinylchloride.
In the case of tear strength, polyethylene and EVA copolymers exceed that of nearly all other thermoplastic films and are suitable for packaging in this regard.
Water Absorption for films of polyethylene and EVA copolymers is very low, as is polyvinylchloride, but is markedly lower than the water absorption for cellulose acetate, Nylon 6, or polystyrene films.
Water vapor transmission for polyethylene film is lower than cellulose acetate, Nylon 6, and polystyrene by a marked degree, and lower than polyvinylchloride. This is an important physical property for a thin packaging material especially in films less than 1 mil in thickness.
Polyethylene and especially EVA copolymers have high permeability to CO.sup.2 gases compared with all of the above films and is also high in permeability to oxygen. This property is especially meaningful for thin films when packaging fresh meats and produce.
Even though polyethylene has many desirable properties for packaging purposes, some of its properties can be improved for all or certain packaging applications. For example, polyethylene alone does have a haze and could be improved when greater contact clarity with a product to be packaged is desired. Polyethylene could also have a greater gloss to give it a better appearance. The heat sealing temperature range for low density polyethylene films of about 1 mil and less is a comparatively narrow range and would make a better packaging material if it could be a broader range and preferably a lower temperature. Heat sealability can be improved by increased burn through resistance. For some uses a better hand or softness and more cling and tack are desirable improvements.
Another disadvantage of polyethylene when it is desired to improve its properties for packaging purposes is its property of being inert. Possibly because it is relatively insoluble or non-compatible, it is difficult to add and incorporate additives, such as, plasticizers, into polyethylene, without their bleeding or exuding to the surface, if added, over a relatively low amount. This is true, even if the additives are intimately admixed throughout the polyethylene.
Thus, while polyethylene and its EVA copolymers are desirable for many film uses their use for many applications can be greatly improved by the use of appropriate additives alone or in combination.