Method of forming a two-layer coat

Details Method of forming a two-layer coat Method of forming a two-layer coat

2001-10-30

2004-01-13

Barr, Michael (Department: 1762)

Coating processes

Applying superposed diverse coating or coating a coated base

Synthetic resin coating

C427S384000, C427S385500, C427S386000, C427S412100, C427S500000, C427S514000, C427S551000, C427S553000

Reexamination Certificate

active

06676997

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a novel and useful method of forming a 2-way coat. More specifically, the invention pertains to a method of applying, onto a thermoplastic resin base, a coating composition containing: an acrylic resin obtained by copolymerization of a specific monomer mixture; a coloring agent; and an organic solvent, drying the resulting coating to form a colored layer, applying, onto the colored layer, an energy beam curable overcoat composition containing an acrylic resin obtained by copolymerization of a specific monomer mixture, and then curing the overcoat composition by exposure to energy beams, thereby forming a 2-way coat featuring excellent abrasion resistance, interlayer adhesion between the colored layer and the overcoat composition, and adhesion with the thermoplastic resin base.
BACKGROUND OF THE INVENTION
Since thermoplastic resin-molded articles obtained from an ABS resin, poly(methyl methacrylate) resin, polycarbonate resin or polystyrene resin, or a polymer alloy thereof are light in weight and have excellent mechanical physical properties, they have been used for a wide variety of applications such as household electric appliances, automobiles and communication apparatuses.
These molded articles however involve a serious defect such as insufficient abrasion resistance on their surface. With a view to protecting the surface of the molded articles such as mobile phone and to imparting them with good appearance, a method of applying various energy beam curable coating compositions, which are cured by exposure to ultraviolet rays or electron beams, has been studied as a coating method for them from the viewpoints of labor saving, improvement in working efficiency and improvement of productivity, each in the application step (JP-A-63701 and JP-A-11-124515 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”)).
In an ultraviolet ray curing system, when such an energy beam curable coating composition is used as a pigmented coating, the pigment absorbs most of ultraviolet rays necessary for curing and ultraviolet rays sufficient for curing are not fed to radical polymerizable unsaturated double bonds, leading to insufficient curing. It was therefore difficult to apply, to the surface of the molded article of a thermoplastic resin, an ultraviolet ray curable coating composition colored with a pigment incorporated therein. Accordingly, a 2-way coating system by applying a pigmented coating on the surface of the molded article of a thermoplastic resin and then applying a transparent type ultraviolet ray curable hard coating composition onto the layer of the pigmented coating, is therefore adopted for coloring of the molded article of a thermoplastic resin.
For example, JP-A-49-92152 discloses a film forming method comprising applying onto a base an undercoat composition comprising an acrylic thermoplastic resin, applying an active energy beam curable coating composition and then exposing it to active energy beams. This patent aims to improve air dryness of the undercoat composition and hence adhesion of the undercoat composition to the base by incorporating a vinyl monomer or an organic solvent having a boiling point of 160° C. or less in the undercoating composition. Such a technique makes it possible to improve the adhesion of the undercoat composition to the base to some extent. When the undercoat composition is a pigmented coating containing a vinyl monomer and a pigment and it is cured by exposure to ultraviolet rays, however, sufficient adhesion with the base material cannot be attained, because the pigment absorbs most of the ultraviolet rays necessary for curing, which prevents sufficient curing of the vinyl monomer.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a very useful method of forming a 2-way coat capable of forming a film excellent in abrasion resistance, interlayer adhesion between a colored layer and an overcoat composition and adhesion with a thermoplastic resin base.
Other objects and effects of the invention will become apparent from the following description.
With a view to attaining the above-described objects, the present inventors made extensive investigations. As a result, it was found that a film excellent in abrasion resistance, interlayer adhesion between the colored layer and overcoat composition and adhesion with the thermoplastic resin base can be obtained by applying a coating composition (1) containing a specific acrylic resin (A), a colorant (B) and an organic solvent (C) onto a thermoplastic resin base, drying the resulting coating to form a colored layer, applying thereto an energy beam curable overcoat composition (2) which contains, as essential components, a compound (D) having at least three (meth)acryloyl groups and a molecular weight of 250 to 1,800 and a specific acrylic resin (E), said compound (D) amounting to 50 wt. % or greater of the overcoat film-forming components, and then curing the overcoat composition by exposure to energy beams. The invention is based on this finding.
Specifically, the present invention provides a method of forming a 2-way coat, which comprises the steps of:
applying (1) a coating composition onto a thermoplastic resin base and drying said coating to form a colored layer, said coating composition (1) comprising:
(A) an acrylic resin obtained by copolymerization of a monomer mixture comprising n-butyl (meth)acrylate and methyl (meth)acrylate, each in an amount of at least 30 wt. %;
(B) a colorant; and
(C) an organic solvent;
applying (2) an energy beam curable overcoat composition onto said colored layer, said overcoat composition (2) comprising:
(D) a compound having at least three (meth)acryloyl groups and having a molecular weight of 250 to 1,800; and
(E) an acrylic resin obtained by copolymerization of a monomer mixture comprising at least 20 wt. % of methyl (meth)acrylate, said compound (D) amounting to at least 50 wt. % of overcoat film-forming components; and
curing said applied overcoat composition by energy beam irradiation.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
As the coating composition (1) to be used in the invention, any coating composition containing, as essential components, an acrylic resin (A) obtained by copolymerization of a monomer mixture comprising n-butyl (meth)acrylate and methyl (meth)acrylate, each in an amount of 30 wt. % or greater, a colorant (B) and an organic solvent (C) may be employed. Among them, particularly preferred are coating compositions produced using an acrylic resin obtained by copolymerization of a monomer mixture containing n-butyl (meth)acrylate and methyl (meth)acrylate, each in an amount of 35 wt. % or greater and in total, 80 wt. % or greater. As the above-described acrylic resin (A), preferred are those having a number-average molecular weight (Mn) of 5,000 to 100,000 and a weight-average molecular weight (Mw) of 10,000 to 200,0000, especially a number-average molecular weight (Mn) of 10,000 to 70,000 and a weight-average molecular weight (Mw) of 20,000 to 150,000.
In the present invention, the terms “number-average molecular weight (Mn)” and “weight-average molecular weight” means, respectively, number-average molecular weight in terms of polystyrene conversion and weight-average molecular weight in terms of polystyrene conversion, each measured by using, as a measuring instrument, Millennium 32 of GPC system of Waters Corp.; as an analysis column, Ultrastyragel 500 Å, 10
3
Å, 10
4
Å, 10
5
Å and 10
6
Å in series; and as a detector, an RI detector.
Methyl (meth)acrylate contents of the monomer mixture, which is to be used for obtaining the acrylic resin (A), less than 30 wt. % are not preferred, because the glass transition temperature of the resulting acrylic resin (A) is lowered, thereby causing following problems: (i) it retards dry tack of the resulting coating composition (1) and dust tends to adhere to the surface in a coating line, (ii) abrasion resi

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