1. Field of the Invention
The present invention relates to an aqueous dispersion coating material containing an aqueous dispersion of polyester resin as a film-forming constituent, and more particularly relates to an aqueous dispersion coating material capable of forming a film having a high level of water resistance and mechanical strength.
2. Description of the Prior Art
In recent years, from the viewpoint of environmental preservation and resource saving, the conversion toward an aqueous coating material which uses water as a diluent from the conventional so-called solvent coating material which uses an organic solvent as a diluent is being pushed forward. On the other hand, the required performance for a coating film has become greater recently, and, actually even an aqueous coating material is required to have excellent water resistance and strength capable of exhibiting durability over a long period of time.
Conventional aqueous coating materials can be largely classified into two types, namely, water-soluble coating materials which contain a water-soluble resin as a binder constituent and water dispersion coating materials which use a water-insoluble resin in its dispersed state. Out of these types, the water-soluble coating material needs to contain a large number of highly hydrophilic carboxyl groups or sulfonic acid groups in the resin and, therefore, is lacking in both the water resistance and the mechanical strength of the coated film. Because of this, the aqueous dispersion coating material has been studied mainly in recent years.
The following aqueous dispersion coating materials are currently known,
(i) A so-called emulsion coating material which uses a dispersing element which can be obtained an emulsification/polymerization method which uses surfactants and emulsifiers.
(ii) An aqueous dispersion coating material (for example, Japanese Patent Application Lay-Open No. 212250/1982) which uses a so-called self-emulsification aqueous dispersing element which is able to be emulsified in water by incorporating sulfonic acid groups and polyalkylene glycols into the high molecular hydrophobic resin.
(iii) An aqueous dispersion coating material (for example, Japanese Patent Application Lay-Open No. 11141/1979) which uses an aqueous dispersing element obtained by a so-called mechanical emulsification method, where a water-soluble resin of low polarity is mixed with a hydrophobic resin and contacted with water and the hydrophobic resin is emulsified in water by utilizing the emulsification action of the water soluble resin as a high molecular weight surfactant.
However, the emulsion coating material of the Item (1) out of these aqueous dispersion coating materials is excellent with respect to the mechanical strength of the film by the introduction of the high molecular weight resin, but, on the other hand, is weak in water resistance because of the residues of the strongly hydrophilic surfactant and emulsifier in the coated film. The self-emulsification aqueous dispersion type coating material in Item (ii) is better in both the water resistance and in mechanical strength as compared with conventional water-soluble coating materials but fails to exhibit a high level of water resistance because it is unavoidable for the highly hydrophilic sulfonic acid groups and polyalkylene glycols to not have an adverse influence on its water resistance. The aqueous dispersion coating material obtained by the mechanical emulsification method in Item (iii) is excellent with respect to water resistance because a resin of low polarity is used as a binder constituent and the usual surfactants are not used but is limited in the emulsification of high molecular hydrophobic resins which have a high adhesive force, and therefore is not always sufficient in terms of the mechanical strength of a coated film.
As described above, all of the conventional aqueous coating materials have failed to satisfy both aspects of a high level of water resistance and high mechanical strength.