Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-02-20
2002-10-29
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C428S457000, C428S458000, C428S480000, C428S482000, C428S035800, C528S272000, C528S302000, C528S363000
Reexamination Certificate
active
06472480
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates generally to a coating composition and more particularly concerns a coating composition for the interior surface of a can.
It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal closure or container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance can be imparted to the metal article, or to a metal substrate in general by coating the metal substrate with a corrosion-inhibiting coating.
Coating compositions for the inside of beer, beverage and food cans must be approved for direct food contact. The fundamental function of the inside coatings on cans and ends is to protect the packed product to maintain its nutritional value, texture, color and flavor when purchased and used by the consumer. To meet these requirements, the organic film must be free of any material which might extract into the packed product and must maintain its integrity over the product recommended shelf life. The ingredients in the can must not make contact with the metal surface. On the whole, current commercial coating compositions for cans and ends, have met all these performance criteria. The precoated metal sheets are subjected to severe elongative and compressive stresses during the can forming process of the fabrication of can ends. The integrity of the respective coating must be maintained during all the specific fabrication operations. The easy open ends, for two-piece beverage cans for example, are stamped from precoated aluminum coil and are fed through a series of presses where the rivet, scaring and tab making steps occur. Coatings for ends must be highly flexible to withstand this series of operations.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. However, improving one particular advantageous property is often achieved at the expense of another advantageous property. Investigators have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal closure or container must be able to withstand the solvating properties of the packaged product. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products by imparting an off-taste to the product.
Epoxy resins are widely used in coating compositions for application to the interior surfaces of food cans. Bisphenol A diglycidyl ether (Badge) is widely used as a monomer for epoxy resins. When epoxy resins made from bisphenol A diglycidyl ether are employed in coating compositions that are used on the interior surfaces of food cans, residual bisphenol A diglycidyl ether can be extracted into the food, and for this reason, epoxy resins containing bisphenol A diglycidyl ether and used for this application have come under scrutiny by various regulatory agencies. Thus, it is highly desirable to develop a polyester resin that can be cross-linked by a hardener to provide an alternative to the currently employed combination of an epoxy resin and a crosslinking hardener.
Polyester resins possess many of the essential properties such as flexibility, adhesion to metal and corrosion resistance, that are needed in a resin that is employed in a can coating for the interior surface of a can, and additionally are free of bisphenol A diglycidyl ether. When cross-linked with suitable hardeners such as a phenoxy group-containing or amino group-containing resin, polyester resins can form excellent coatings. However, the critical shortcoming of conventional polyester resins for use in coatings of the interior surfaces of cans is that their ester bonds are susceptible to hydrolysis, especially during the retort process after the food is canned.
Japanese laid open patent application (Kokai) No. 4-39370 which was laid open on Feb. 10, 1992 discloses a resin composition for can coating materials characterized in that a polyester resin (A) and an alkyl etherified amino formaldehyde resin (B) are compounded in a proportion of (A)/(B)=95-60/5-40 (weight ratio) in which the dicarboxylic acid component is 20 to 80 mol % of isophthalic acid, 80 to 20 mol % of aromatic dicarboxylic acid such as naphthalene dicarboxylic acid other than isophthalic acid, 0 to 50 mol % of aliphatic dicarboxylic acid of 2 to 12 carbon atoms, the glycol component is 25 to 99 mol % of glycol containing at least one or two or more side chain alkyl groups as indicated by general formula (I) below, 1 to 75 mol % of glycol with 8 to 12 carbon atoms and 0 to 50 mol % of other alkyl glycols, and, in which, further, polycarboxylic acids and/or polyols that are trivalent or of higher valence are present in an amount of 0.5 to 10 mol % relative to the total acid component.
wherein, R
1
and R
2
are alkyl groups with 1 to 6 carbon atoms. However, the resin composition is disclosed as being applicable as a coating for the exterior surface of a can.
OBJECTS OF THE INVENTION
It is therefore a general object of the present invention to provide an improved coating composition for the interior surface of a can which solves the aforementioned problem and meets the aforementioned need.
More particularly, it is an object of the present invention to provide an improved coating composition for the interior surfaces of a can which employs a polyester resin cross-linked with a phenoxy group-containing or amino group-containing resin that is stable to hydrolysis or sterilization.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.
SUMMARY OF THE INVENTION
The aforesaid objects are achieved by the coating composition of the present invention for application to an interior surface of a can, comprising a phenoxy group-containing or amino group-containing resin cross-linking agent in an amount in the range of from about 2 to about 45 weight percent of the coating composition, a polyester resin in an amount in the range of from about 6 to about 59 weight percent of the coating composition, and a nonaqueous carrier in the range of from about 35 to about 80 weight percent of the coating composition, wherein the polyester resin comprises a condensation product formed from (a) from about 2 to about 48 mole percent of isophthalic acid, (b) from about 2 to about 48 mole percent of a naphthalene dicarboxylic acid or alkyl ester thereof, (c) from about 0 to about 10 mole percent of trimellitic anhydride or trimethylolpropane, and (d) from about 30 to about 60 mole percent of neopentyl glycol.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A coating composition of the present invention, after curing, provides a cured coating composition that effectively enhances corrosion inhibition of metal substrates, such as, but not limited to, aluminum, iron, steel and copper. The present coating composition, after curing, also demonstrates excellent adhesion to the metal substrate, excellent chemical resistance and scratch resistance, excellent flexibility, and excellent stability to hydrolysis and sterilization.
In general, the coating composition of the present invention for application to an interior surface of a can comprises a phenoxy group-containing or amino group-containing resin that serves as a cross-linking agent in an amount in the range of from about 2, preferably from about 4, more preferably from about 8, to about 45, preferably to about 40, more preferably to about 33 weight percent of the coating composition, a polyester resin of this invention in an amount
BP Corporation North America Inc.
Henes James R.
Kanady Mary Jo
Ribar Travis B.
Seidleck James J.
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