Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2001-08-27
2003-02-04
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C523S406000, C523S407000, C523S409000, C523S412000, C525S329700, C525S330200
Reexamination Certificate
active
06514619
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an aqueous resin composition excellent in adhesion and corrosion resistance and also relates to a coated metal material coated with the aqueous resin composition and having a cured coated film excellent in adhesion to the surface of the metal material and also excellent in corrosion resistance, flexibility, and can end opening properties.
BACKGROUND OF THE INVENTION
Hitherto, as metal can materials, metals such as aluminum, tin plate, tin-free steel, etc., have been used. In the case of using these metals as cans, a coated film is usually formed on the inside and outside surfaces of cans for preventing the occurrence of corrosion. As a coating material for the inside surfaces of cans, coating materials of an epoxy/phenol resin-base, an epoxy/amino resin-base, or of a vinyl chloride organosol-base have usually been used because of being excellent in the points of adhesion and corrosion resistance.
However, the coating materials as described above contain organic solvents and thus have particularly possibilities of causing air pollution and deteriorating working atmosphere as well as the danger of causing a fire and an explosion. Of these coating materials, since the vinyl chloride organosol-base coating material was pointed out as being a possible generating source of dioxins, the activity of avoiding the use of vinyl chloride resin has been quickly accelerated. In view of the circumstances, there have been variously proposed, as means for avoiding these problems, methods of dispersing in water a so-called self-emulsifiable epoxy resin obtained by introducing a segment having an emulsifying force into the molecule by modifying an epoxy resin with an acrylic resin.
For example, JP-A-53-1228 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method of stably dispersing, into an aqueous medium containing a base, a grafted epoxy resin obtained by polymerizing a monomer mixture containing a carboxyl group-containing vinyl monomer using a free radical generating agent such as benzoyl peroxide, etc., in the presence of an epoxy resin.
JP-A-55-75460 and JP-A-56-109243 each discloses a method of dispersing, into an aqueous medium in the presence of ammonia or an amine, a partial reaction product having excessive carboxyl groups obtained by reacting an acrylic resin and an aromatic epoxy resin having a relatively high molecular weight.
JP-A-55-3481 and JP-A-55-3482 each discloses a method of neutralizing, with a base, a carboxyl group-containing vinyl polymer-modified epoxy resin substantially having no oxirane group of an epoxy resin, which is obtained by esterifying a carboxyl group-containing vinyl polymer with an epoxy resin in the presence of an amine-base esterification catalyst, and dispersing the neutralized product in water.
Furthermore, JP-A-57-105418 and JP-A-58-198513 each discloses a method of copolymerizing: a low molecular weight compound having an epoxy group and an acryloyl group in one molecule, obtained by partially reacting an aromatic epoxy group and (meth)acrylic acid; and a monomer mixture containing acrylic acid or methacrylic acid, and neutralizing the copolymer with a base to disperse in water.
These aqueous resin compositions are excellent in the point of general food hygiene, but on the other hand, when the resin compositions are used for particularly a can end, it is difficult to simultaneously satisfy the opening property of the can end and the processing corrosion resistance. In addition, opening of a can end is carried out by pulling up the tab portion of a pull top tab or a stay-on-tab of the can end, but there can occur the phenomenon that the coated film is not cut along the score portion and a part of the coated film remains at the open portion, and in the worst case, the can end cannot be opened. Such characteristics of the can end is called can end opening properties.
Recently, JP-A-4-351682 discloses an aqueous resin composition using a bisphenol F-type epoxy resin for improving particularly the opening property of a can end, wherein a carboxyl group-containing self emulsifiable epoxy resin is dispersed in an aqueous medium. Also, JP-A-9-169948 and JP-A-11-181349 disclose, for the purpose of use for ends, methods of modifying an epoxy resin, obtained by copolymerizing a bisphenol A-type epoxy resin and a bisphenol F-type epoxy resin, with a carboxyl group-containing acrylic resin, and neutralizing the modified epoxy resin with a base to disperse in water. Also, JP-A-11-199827 discloses a method of using a mixture of a bisphenol A-type epoxy resin and a bisphenol F-type epoxy resin. As can be seen from these inventions, by using a bisphenol F-type epoxy resin, the opening property can be improved to some extent. However, there occurs a problem that owing to the deformation (doming) of a can end caused by a sterilizing process after filling a high carbonated beverage, microcracks, which are fine cracks of a coated film, generate and from the cracks, a corrosion proceeds. The performance of a coating film, which does not cause the microcracks and the corrosion accompanied therewith even with respect to such a deformation of an end, is called secondary processing corrosion resistance.
JP-A-8-302275 discloses an aqueous resin dispersion comprising an acryl-modified epoxy resin and an acrylic resin containing at least 40% of an acrylic monomer, a homopolymer of which has a glass transition temperature of not higher than 0° C. However, when a mono-basic carboxylic acid monomer is not substantially used in the acrylic resin containing at least 40% of the acrylic monomer component, a homopolymer of which has a glass transition temperature of not higher than 0° C., even by adding this acrylic resin to the acryl-modified epoxy resin and neutralizing it with an amine, etc., in order to disperse the resin in an aqueous medium, the acrylic resin is separated and becomes a graininess form, whereby the acrylic resin cannot stably exist in the aqueous resin dispersion. Also, even when a mono-basic carboxylic acid monomer is used in the acrylic acid, the acrylic acid reacts with the acryl-modified epoxy resin at baking to become compatible with the epoxy resin. Therefore, although the improving effect for the secondary processing corrosion resistance at an external shocked portion may be obtained a little, there is a large problem that the opening property of the can end cannot be improved, which is another important problem.
Furthermore, JP-A-9-227824 discloses an aqueous coating material composition obtained by mixing an aqueous dispersion of a carboxyl group-containing modified epoxy resin and an emulsion resin obtained by emulsion polymerizing an ethylenically unsaturated monomer in the presence of an aqueous dispersion of a carboxyl group-containing acrylic resin. However, such a technique has a problem that satisfactory excellent opening property of the can end and secondary processing corrosion resistance cannot be attained unless the ethylenically unsaturated monomer is appropriately selected paying attention to the glass transition temperature of its homopolymer and an attempt is made so that the particle sizes of the emulsion resin made of the ethylenically unsaturated monomer is optimized and homogenized.
In addition, the above-described publication merely discloses coated film properties as for the aqueous resin compositions for can coating and does not mention about the application property. In spite of that the application property for coating resin compositions for can coating on metal materials, in the case of particularly an aqueous dispersion, has importance as high as or higher than the coated film properties in certain meanings, there has been no proper means for controlling flowability to the present.
As described above, it is the actual circumstances that as an aqueous resin composition for can coating, particularly, a coating material for the inside surface of a can end, it has not yet been found an aqueous resin composition which is e
Itoh Yoshiki
Kojima Takashi
Shimada Hiroaki
Yamano Yoshihiro
Armstrong Westerman & Hattori, LLP
Aylward D.
Dainippon Ink and Chemicals Inc.
Dawson Robert
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