Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-10-02
2004-09-07
Mullis, Jeffrey (Department: 1711)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S375000, C427S386000, C427S388200, C525S088000, C525S09200D, C525S09200D, C525S096000
Reexamination Certificate
active
06787188
ABSTRACT:
This invention relates to a coating composition, its preparation and use. Metal food and drink containers, often referred to as cans, are usually coated on the inside to prevent reaction between the contents and the metal from which the can is formed. Such reaction leads both to unwanted deterioration of the can and also potentially damaging effects on the contents, particularly in terms of changes in quality and taste. Without an interior coating, most cans of food or drink would not remain usable for very long. The coating is often applied to the flat metal by roller coating before the can is formed and then dried or cured in a stoving operation. The can is then formed from the flat metal by a drawing process before being filled with food or drink and finally sealed up.
The coatings are required to have very good flexibility, adhesion, sterilisation resistance and stability properties. Flexibility and adhesion are essential if the coating is to remain intact during the can formation process when the coated flat metal sheet is drawn into the form of the can. When the cans are filled with food, the contents are usually sterilised by heating the sealed can to temperatures of around 130° C. for 1 to 2 hours (depending on the nature of the food). The coating is then in direct contact with the contents of the can for a considerable period of time which could be many years. During sterilisation and subsequent storage, the coating is required to maintain its integrity so as to prevent corrosion of the metal can and to prevent metal migration into the can contents. Additionally, the coating must not impair the contents by releasing unwanted material or by altering the flavour or appearance. These resistance properties impact not only on the shelf life of the product but also on public health and safety. Thus, there are particularly stringent and specific requirements of coating compositions for can interiors which are different from those for other coatings.
One known type of coating composition for cans is based on an epoxy resin. Epoxy resin compositions comprise an epoxy resin and, optionally, a crosslinker such as a phenolic resin dissolved or dispersed in an organic liquid. In those compositions containing a crosslinker, the crosslinker reacts with the epoxy groups on the epoxy resin during the stoving operation so as to form a crosslinked final coating. In known compositions the epoxy resin contains bisphenol A diglycidyl ether (abbreviated to BADGE), a commonly available liquid epoxy resin of low epoxy equivalent weight. Health concerns have arisen over the level of BADGE appearing in food supplied in cans which have been coated on the inside with epoxy coatings which invariably may contain some unreacted BADGE. Low molecular weight BADGE exists at high levels in low molecular weight commercial epoxy resins while low level of BADGE exists in high molecular weight epoxy resins. The concern is that all of the BADGE does not react with the crosslinker and that some of the residual free BADGE can leach out of the coating and into the food. As a result of these concerns, a limit on the level of free BADGE in the final cured coating for the interior of food cans has been proposed based on the amount of free BADGE in the coating and an assumption that all of this could theoretically migrate into the food. The current proposal is a limit on the quantity of free BADGE in the coating such that the contents would contain no more than 1 part per million (ppm) of BADGE if all of the BADGE were to migrate from the coating to the contents. This very low level of free BADGE is not easy to achieve by simple modifications of the existing formulations. The problem is particularly acute in smaller cans which have a larger interior surface area, and thus more coating, in relation to the volume of contents. The problem is to formulate coatings suitable for cans which meet the requirements for very low (less than 1 ppm) or zero (non-detectable) levels of BADGE or similar low molecular weight epoxy-based materials, appearing in food, while retaining or improving on all the other required characteristics of flexibility, adhesion and sterilisation resistance.
European Patent Application EP-A-0 111 986 discloses pigmented coating compositions based on an epoxy-polyester block copolymer in which the polyester component is prepared by polycondensation of terephthalic acid and/or isophthalic acid and a difunctional hydroxy compound having 2-24 carbon atoms. European Patent Application EP-A-0 399 108 discloses similar compositions in which the polyester is the condensation product of a carboxylic diacid and a dihydroxy compound in which the components are non-aromatic. However neither of these types of polymer are suitable for use in can coatings because neither gives the required cured film combination of flexibility, adhesion and sterilisation resistance and compatibility. In accordance with this invention, the cured film problems have been resolved along with extremely low levels of free BADGE by the use of a particular epoxy-polyester block copolymer in combination with fatty acids.
According to the present invention, provided is a coating composition comprising an organic liquid carrier in which is dispersed or dissolved a mixture of organic film forming components comprising by weight:
i) from 50% to 100% of an epoxy-polyester block copolymer consisting of the reaction product of an epoxide terminated epoxy resin and a preformed carboxyl functional polyester polymer,
ii) from 0% to 10% of organic monocarboxylic acid, preferably fatty acid, and
iii) from 0% to 50% of a crosslinker,
where the sum of (i) and (ii) is 100%, characterised in that the preformed polyester polymer is the reaction product of one or more polyols, predominantly diol, with dicarboxylic acid or their anhydrides, where the dicarboxylic acids comprise by weight (a) 20% to 45% of an aromatic polycarboxylic acid or its anhydride, (b) 55% to 80% cyclohexane dicarboxylic acid, and (c) 0% to 10% other difunctional carboxylic acid, where the sum of (a), (b) and (c) equals 100%, and where the epoxy-polyester is optionally further reacted with organic monocarboxylic acid.
It has been found that this use of a combination of an aromatic polyfunctional carboxylic acid or its anhydride and cyclohexane dicarboxylic acid in making the polyester gives rise to unexpectedly improved properties in the final film, particularly better adhesion, sterilisation and flexibility when compared to the use of either aromatic or aliphatic acids alone to produce BADGE-free or low-BADGE (containing less than 1 ppm) can coating compositions.
The resulting epoxy-polyester copolymer is the reaction product of an epoxy resin with a carboxyl functional polyester consisting of residue of the carboxyl functional polyester esterified with the residue of the epoxy resin, where the copolymer contains between 1 and 20 and preferably between 1 and 10 ester polymeric units. In preferred aspects of the invention, organic aliphatic monocarboxylic acid, preferably fatty acid, is further reacted with the epoxy-polyester polymer. The organic liquid carrier can be one or more organic liquids in which the epoxy-polyester block copolymer can be dissolved or dispersed. Typical organic liquids are aromatic solvents commercially available as Solvesso 100□ or Solvesso 150™ from Exxon.
Referring now to the epoxy prepolymer, suitable epoxy resins are aromatic or aliphatic epoxy resins with aromatic epoxy resins being preferred. Useful epoxy resins are predominantly linear chain molecules comprising the coreaction product of polynuclear to dihydroxy phenols or bisphenols with halohyrdrins to produce epoxy resins containing preferably two epoxy groups per molecule. The most common bisphenols are bisphenol A, Bisphenol F. bisphenol S and 4,4′-dihydroxybisphenol, with the most preferred being Bisphenol A. Halohydrins include epichlorohydrin, dichlorohydrih and 1,2-dichloro 3-hydroxypropane, with the most preferred being epichlorohydrin. Preferred epoxy resins comprise the coreacti
Imperial Chemical Industries plc
Mullis Jeffrey
Stachel Kenneth J.
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