Oriented plastic film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), is widely used as a packaging material for a variety of products, including foodstuffs.
In many instances plastic film coated with a metal has replaced metallic foils, such as aluminum foil, for end uses such as labels and decals, solar window film, packaging films, decorative products and microfilm. Among other uses, the metallic foils serve to hinder the onset of oxidative rancidity of the products either prepared in or served with certain edible oils by preventing the passage of visible and ultraviolet light. PET, when coated with a thin metallic layer, provides a strong, flexible packaging material with the desirable oxygen and moisture barrier properties common to PET packaging applications, along with the visible and ultraviolet light barrier properties common to metallic foil packaging applications at a greatly reduced cost. Unfortunately, the metal to film bond can fail in many applications due to the flexibility of a plastic film substrate layer, thereby causing the metal to be removed from the film.
One approach for improving the adhesion of the metal to PET film is to heat the coated film as disclosed in British Pat. No. 1,370,893. The drawback to this method is the experimentation which must be performed in order to determine the proper temperature and time elements for this procedure before any metallized film may be produced. Another approach has been to coat polyester film with a layer comprising a random copolymer of ethylene with from 0.5 to 15 weight percent of a 3 to 6 carbon alpha-mono-olefin as disclosed in European Publication No. 0,023,389. Although PET films with this type of coating display a certain improvement in metal adhesion over uncoated PET, they also display an undesirable coating pattern. For a discussion of the possible mechanisms of metal adhesion to polymer surfaces, see:
(1) Burkstrand, J. M., "Chemical Interactions at Polymer Metal Interfaces and Correlations with Adhesion," Fifth Annual Meeting of the Adhesion Society, Mobile, Ala., Feb. 22-24, 1982; Published by the Adhesion Society, Mobile, Ala., 1982, pp 10a-10c;
(2) Burkstrand, J. M., "Metal-Polymer Interfaces: Adhesion and X-ray Photoemission Studies," Journal of Applied Physics, Volume 52, No. 7, July 1981, 4795-4800; and
(3) Burkstrand, M. J., "Hot Atom Interactions with Polymer Surfaces," Journal of Vacuum Science and Technology, 21(1), May/June, 1982, 70-73.
It is known to provide polyester or copolyester coatings derived from sulfoisophthalic acid as in U.S. Pat. No. 4,252,885 or coatings composed of from 20 to 60 molar percent of isophthalic acid, from 6 to 10 molar percent of sulfoisophthalic acid and the remainder terephthalic acid as in European Publication No. 0,029,620. Both of these prior art disclosures, though, are addressed to the production of photographic films.
It is also known to provide an aqueous dispersion of a polymer composed of terephthalic and/or isophthalic acid, a saturated aliphatic dicarboxylic acid such as sebacic acid, and from about 0.5 to about 2.5 molar percent of a sulfomonomer such as sodium 5-sulfoisophthalic acid with a di- or tri-glycol as in U.S. Pat. No. 3,563,942. Similar materials are also disclosed in U.S. Pat. No. 3,779,993 and are taught as being useful as hot melt adhesives. Each of these patents teaches the use of such copolyesters as adhesives for various substrates such as paper, metal and plastic film, and broadly suggest the construction of laminates of various plastic, paper and metal layers using the copolyesters as bonding agents. However, neither patent discloses specific copolyester formulations which provide surprisingly enhanced adhesion to plastic film substrates of metal applied by a metallizing technique.
Accordingly, it is an object of this invention to provide an oriented self-supporting plastic film material having enhanced adhesion to metal applied by a metallizing technique.
Another object of this invention is to provide a coating material for biaxially oriented polyethylene terephthalate film material which renders film more adhesive to coatings applied by a metallizing technique.
A further object of this invention is to provide a metallized plastic film of enhanced quality suitable for packaging and solar applications.