Coating processes – Interior of hollow article coating – Metal base
Patent
1994-08-18
1996-03-19
Marquis, Melvyn I.
Coating processes
Interior of hollow article coating
Metal base
427386, 4273881, 523404, 525108, 525109, 525119, B05D 722
Patent
active
055002463
DESCRIPTION:
BRIEF SUMMARY
DETAILED DESCRIPTION OF THE INVENTION
1. Industrial Field of the Invention
This invention relates to a resin composition for water-based coating which comprises an aromatic polyol resin and an aromatic epoxy resin as main components, and to a water-based coating composition. More specifically, the invention relates to a resin composition for water-based coating and a water-based coating composition capable of forming an excellent coating film as a coating composition for metals, particularly coating for the inner surface of a can and precoat metal (PCM) coating.
2. Prior Art and its Problem
In the case of solvent-based coatings which use an organic solvent, the use of an organic solvent is now being restricted more than before from a viewpoint of resource and energy saving and environmental preservation. For this reason, it is desired to shift to coatings which use a water-based solvent.
Particularly, epoxy resin coatings have been mainly studied to shift can coating for coating metal surfaces, precoat metal (PCM) coating and corrosion-resistant coating to coating using a water-based solvent. Various methods for improving the water solubility of epoxy-resins have been proposed.
For example, Japanese Patent Laid-Open Publication Nos.1228/1978 and 1285/1978 disclose a method which comprises the steps of polymerizing an acrylic monomer including a carboxyl group-containing monomer using a radical initiator such as benzoyl peroxide in the presence of an epoxy resin, grafting from the aliphatic hydrocarbon site of the epoxy resin, and dispersing the resulting polymer into water using a basic compound such as ammonium and amine.
However, the method described above has the defects that it is necessary to enhance graft ratio to achieve a highly stable emulsion and that a large volume of radical initiator such as benzoyl peroxide which is expensive and involves high risk of explosion must be used at the time of production.
To solve the above problems, Japanese Patent Laid-Open Publication No.3481/1980 discloses a method which comprises the steps of esterifying a carboxyl group-containing vinyl polymer with an epoxy group of an epoxy resin in the presence of an amine-based esterifying catalyst to achieve an epoxy resin modified by the carboxyl group-containing vinyl polymer and having substantially no epoxy group, neutralizing the resulting epoxy resin with a base, and dispersing the epoxy resin into water.
However, since a reaction is continued until the epoxy group of the epoxy resin is perfectly consumed in the method described above, the higher molecular weight and high crosslinking of the resin cannot be avoided. Particularly, can coating is inferior in the hardness of its coating film and flexibility that can stand processing and hence, has a problem with the processability of its coating film.
In addition, when these compositions are used in coating for the inner surface of a can, a low-molecular weight compound derived from an epoxy resin and an acrylic resin as main components elutes into the contents of the can, thus causing a food sanitary problem. Furthermore, to prevent the elution of such a low-molecular weight compound, it is necessary to perfectly harden the coating film. For this purpose, it is necessary to bake it at a high temperature. Therefore, the method has the problems that a large amount of energy is required for baking and baking speed becomes slow.
The inventors recognized the defects of the above prior arts, and invented and as a water-based coating that can overcome these defects, previously proposed a composition for water-based coating (Japanese Patent Laid-Open Publication No.285,636/1992), which comprises an ammonium salt and/or an amine salt of a reaction product (C) between an aromatic polyol resin (A) and a carboxyl group-containing acrylic resin (B), the weight ratio of component (A) to component (B) being in the range from 95/5 to 5/95 in terms of solid content.
PROBLEM TO BE SOLVED BY THE INVENTION
The present inventors have further conducted studies to develop a water-base
REFERENCES:
patent: 3294865 (1966-12-01), Price
patent: 4302373 (1981-11-01), Steinmetz
patent: 4579888 (1986-04-01), Kodama et al.
patent: 4963602 (1990-10-01), Patel
Harada Taira
Morizono Kenichi
Aylward D.
Marquis Melvyn I.
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