Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1998-11-19
2002-12-17
Pyon, Harold (Department: 1772)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S182000, C156S307100, C156S309900, C156S322000, C156S308200, C156S309900, C428S418000, C428S425800, C428S458000, C428S462000, C428S466000, C428S521000, C427S228000, C427S226000
Reexamination Certificate
active
06494983
ABSTRACT:
The present invention relates to compositions for promoting the bonding of shaped thermoplastic elastomer articles to metallic substrates. More particularly, it relates to metallic food container closures specially adapted to use in high temperature filling, sterilization, and retort processing environments, including firmly adherent thermoplastic elastomer sealing gaskets or liners.
Vessel closures for use in food containers include a closure shell formed of either metal or plastic provided with a liner or gasket on the inner facing surface of the closure shell. The liner or gasket provides a hermetic seal between the closure member and the vessel opening. In the past, poly(vinyl chloride)-based liner formulations have been used to provide closure seals. The use of poly(vinyl chloride) resin-based compound liners is currently being discouraged for a number of reasons.
Recently, poly(vinyl chloride) (PVC) resins have received adverse EPA publicity, due to incineration, land fill, and recyclability concerns. PVC resin based plastisols conventionally employed as the closure gasket or liners, now interfere with the recyclability of both the plastic closure shell and the thermoplastic polyester bottle or container.
To overcome the shortcomings with prior art PVC-based liners and gaskets, a search is currently underway to provide substitute non-PVC type liner materials. Early efforts have focused on hot melt compositions, such as those described in U.S. Pat. No. 4,032,492 and 4,085,186. The compositions proposed include rubbery block copolymers based on styrene and butylene or ethyl vinyl acetate copolymers employed in combination with low molecular weight hydrocarbon oils, waxes, plasticizers, and other additives. The hot melt formulations generally possess low melting or softening points ranging from 70° to 125° C. In some food processing and packaging environments and applications, hot filling and pasteurization conditions are frequently carried out at temperatures above 70° to 125° C. Moreover, in hot fill, high retort food filling operations, in addition to elevated temperatures, internal vacuums of as high as 15-26 inches of mercury (Hg) are realized, which cause problems for low temperature softening sealing or gasketing materials. The proposed hot melt compositions generally cannot maintain the compressive set values and cut-through resistance values necessary to provide satisfactory hermetic seals under these high temperature processing conditions.
An additional requirement for liner and gasketing compositions is that they must possess good to excellent adhesion to the closure substrate to minimize the gasket or compound liner movement and cut through during hot fill and retort conditioning. The maintenance of hermetic seals during processing, case packing, shipping, and prolonged storage periods are all essential to successful food packaging.
More recently, it has been proposed to employ thermoplastic elastomer products to provide hermetic sealing structures for various plastic or metallic food vessel closures. Thermoplastic elastomers are thermoplastic processable polymer materials possessing easy processability and rubbery mechanical performance characteristics. Thermoplastic elastomers, often referred to as TPEs, possess a number of processing advantages over earlier rubber materials because thermoplastic elastomers may be extruded and molded to shape and used with little or no extra compounding, vulcanization, or heating steps and the recycling of scrap and the ability to use common plastics processing tools and methods is a distinct advantage. Thermoplastic elastomers possess satisfactory high temperature rubbery performance characteristics to be used as liner gaskets for food closures. However, they are difficult to satisfactorily bond to metal closure materials. For this reason, they have not been readily employed.
Another effort at providing non-PVC based liner and gasketing formulations has been to employ polypropylene polymers and copolymers as the liner compound or gasketing material. Adhesion of the polypropylene liner materials to metal substrates and polymer substrates also ran into some early difficulties. For example, in U.S. Pat. No. 4,034,132, it is disclosed that the adhesion of a propylene polymer to an enamel-coated metal surface such as is provided on a foil pull tab on a container opening is improved by incorporating an adhesion-promoting amount of carboxyl modified polypropylene resin in the metal coating enamel. In U.S. Pat. No. 4,478,677, it is disclosed that the adhesion of a heat sealed polypropylene lined aluminum foil pull tab tape strip to an enamel coated metallic surface and opening, provided with an enamel coating information based on an epoxy resin, an aminoplast resin and a carboxylated polypropylene resin is further improved and made satisfactory by the addition of a butene polymer, such as polyisobutylene, into the enamel coating composition prior to its application to the metal surface.
Other efforts more directly related to bonding thermoplastic elastomer gasketing materials to metal or plastic closures are described in U.S. Pat. No. 5,060,818 wherein adhesion of the TPE gasket to the closure is promoted by incorporating a low temperature melting point liquid paraffin resin and a polypropylene resin into a thermoplastic elastomer formulation prior to injection molding or shaping the elastomer for placement in the vessel closure. Paraffin-modified formulations may be suitable for low temperature packaging operations but they generally cannot be used in high temperature processing conditions because paraffin softens at temperatures of about 250° F.
In addition to modifying the thermoplastic elastomer compositions per se, prior to molding or shaping to form the gasket or liner U.S. Pat. No. 5,060,818 additionally states that if an epoxy phenolic type coating is applied to the surfaces of a metallic closure, the bonding of the line to the inner side of the vessel closure may be promoted by applying a separate layer of an adhesive which contains an oxidized polyethylene resin or an acid modified olefin resin including a carboxyl modified polypropylene resin.
In our laboratories it has been found that the adhesion of shaped thermoplastic elastomer articles to metallic substrates can be improved by the addition of a carboxyl modified polyolefin resin adhesion promoter to an enamel coating composition comprising an epoxy resin and a phenolic resin. Incorporating the carboxylated polyolefin adhesion promoter for TPE materials into the enamel coating composition avoids the need to apply a separate adhesive layer comprising the carboxylated polyolefin resin to obtain satisfactory adhesion of TPE materials. While enamel compositions comprising an epoxy resin, a phenolic resin and carboxylated polyolefin resin provide these advantageous properties, it is desirable to provide coatings based on these resins better flexibility and corrosion resistance.
In accordance with this invention, and to overcome the shortcomings of the prior art arrangements, it is an object of the present invention to provide lidded, stoppered, threaded, capped, or lined metallic closures for vacuum or pressure type products requiring low orders of gas or liquid permeation with a functional hermetic seal.
It is another object of the invention to provide metallic closures provided with liner structures capable of maintaining a hermetic seal under vacuum pressure, pasteurization, hot fill, and retort processing conditions.
It is a further object of the invention to provide new and improved gasketed closures which avoid the use of PVC-based materials.
It is still another object of the invention to provide closures with non-PVC based extrusion or injection processable thermoplastic elastomers which do not require post-vulcanization to impart functional hermetic sealing closure gasket under pasteurization and sterilization conditions.
It is a further object of the present invention to provide thermoplastic elastomer-lined metallic closures exhibiting functional torque release p
Graue F. William
Scaletta Joyce M.
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Miggins Michael C
Pyon Harold
White Cap, Inc.
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