Stock material or miscellaneous articles – Hollow or container type article – Nonself-supporting tubular film or bag
Patent
1989-06-09
1991-10-22
Herbert, Jr., Thomas J.
Stock material or miscellaneous articles
Hollow or container type article
Nonself-supporting tubular film or bag
1563082, 1563099, 428 359, 428458, 428480, B65D 100, B32B 1508
Patent
active
050594605
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for producing a laminated metal sheet, and to laminated metal sheet Produced by such a process.
Lamination of polymer materials to metal sheet such as metal strip is a well known and well documented technique. The resultant laminates have many applications such as for example in the manufacture of can bodies and can ends for containers for foodstuffs and beverages, and aerosol containers.
Polyester coatings are frequently used to coat metal sheet in order to impart good corrosion resistance to the metal sheet. It is usual to try to coat the metal sheet with a polyester resin which has a crystalline and oriented structure since such polyester films have low permeability to oxygen, water and steam. However, it has been found that it is not easy to achieve adhesion of such crystalline, biaxially oriented polyester film to a metal sheet.
One solution to this problem is to laminate the biaxially oriented polyester to the metal sheet by use of processing conditions which require heating the metal sheet to high temperatures thereby to melt at least part of the biaxially oriented polyester. Such proposals are contained, for example, in GB 759876 and GB 2123746. However, while this solution has certain advantages, it relies on very close control of metal strip temperature during the lamination and lamination rolls of highly resistant materials for the very high temperatures necessary to laminate biaxially oriented polyethylene terephthalate film to metal strip.
An alternative solution is to provide an intermediate layer of adhesive between the crystalline polyester layer and the metal sheet to which that layer is to be adhered. This type of solution to the problem is disclosed, for example. in GB 2164899 which requires the application of an epoxy resin adhesive to the metal sheet for the application of a polyester resin tilm. This allows the lamination process to be performed at a lower lamination temperature but produces a coating with relatively poor formability in deep drawing operations and is a relatively expensive way of overcoming the said problems.
Similarly, GB 1501353 describes a blend of a polyester, an .alpha.-olefin copolymer and an epoxy compound for the purpose of formulating an adhesive suitable for use in laminating thermoplastic resins to a metal substrate. GB 2055687 describes a laminate formed by heat bonding a film of a biaxially oriented polyester film to a metal sheet by use of an adhesive layer, the adhesive layer being made of a polyblend which incorporates a high melting point polyester and a low melting point polyester. The blend of polyesters may incorporate a polyolefin resin.
None of the above-mentioned patents are concerned with the problems which arise in deep drawing cans from polyester-metal laminates.
Conventional adhesives can be used to bond metal sheet and film. For example, isocyanate-based adhesives may be coated onto the polyester film or metal sheet before the two are brought into contact in a lamination nip.
In practice, if one attempts to manufacture cans from laminates according to GB 2123746 and 2164899, typically by deep-drawing a disc of 189 mm diameter to a can of height 100 mm and diameter 65 mm (a size widely used in the canning industry), severe disruption of the polyester coating is observed leading to substantial loss of coverage of the metal by the coating. As a result the shelf life of the resultant container when packed with food products is found to be substantially reduced so that such containers are unacceptable from a commercial point of view. The coating disruption in deep drawing is a consequence of the forming process extending the laminated oriented film beyond its elongation limit, resulting in fracturing of the oriented polyester. This phenomenon affects oriented polyester coatings as described in GB 2123746 and 2164899 and those produced and laminated by other techniques in which attention is not paid to the coating's tensile properties.
Lamination of a polyester to a metal substrate is also described in G
REFERENCES:
patent: 2861022 (1958-11-01), Lundsager
patent: 4362775 (1982-12-01), Yabe et al.
patent: 4493872 (1985-01-01), Funderburk et al.
Heyes Peter J.
Middleton Nicholas J.
CMB Foodcan plc
Herbert, Jr. Thomas J.
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