Method of forming coating

Details Method of forming coating Method of forming coating

2001-02-26

2004-04-27

Cameron, Erma (Department: 1762)

Coating processes

Applying superposed diverse coating or coating a coated base

Synthetic resin coating

C427S410000, C524S500000, C525S187000

Reexamination Certificate

active

06726961

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of forming a composite coating film on car bodies and so on and a composite coating as obtainable by the method.
PRIOR ART
Generally, coatings for automobiles are rich in organic solvent content and, therefore, the organic solvents evaporate in large amounts in the step of coating or baking for curing. As one of the measures for reducing the number of steps of treating them, studies have been made to formulate coatings in the water-based form.
Thus, for instance, Japanese Kokai Publication Hei-07-53913 discloses a water-based coating composition which comprises a resin obtained by neutralizing at least part of a polymer derived from an amide group-containing, ethylenically unsaturated monomer, an acidic group-containing, ethylenically unsaturated monomer and a hydroxyl-containing, ethylenically unsaturated monomer, together with an aqueous dispersion of carboxyl-containing acrylic resin particles. However, not only this but also the so-far known water-based base coat coatings in general are inferior in flip-flop properties of the metallic coating film, in particular, to solvent base coating compositions.
It is an object of the present invention to provide a method of forming a composite coating film having high flip-flop properties by applying a water-based base coat coating to a substrate, then applying a clear top coat coating thereonto and curing both coatings simultaneously while controlling the interlayer imbibing or inversion of coating films under high humidity conditions.
SUMMARY OF THE INVENTION
The present invention provides a method of forming a composite coating film by applying a water-based base coat coating to a substrate and then applying a clear top coat coating thereonto,
wherein said water-based base coat coating comprises:
a polyether polyol having at least 0.02 primary hydroxyl group, on average, per molecule, a number average molecular weight of 300 to 3,000 and a water tolerance of not less than 2.0; and a resin emulsion obtained by emulsion polymerization of an &agr;, &bgr;-ethylenically unsaturated monomer mixture having an. acid value of 3 to 50 and containing at least 65% by weight of a (meth) acrylate ester whose ester moiety contains 1 or 2 carbon atoms.
The invention also provides a method of forming a composite coating film as defined above,
wherein said polyether polyol has at least one primary hydroxyl group in each molecule and has a hydroxyl value of 30 to 700, and further said polyether polyol preferably has at least three hydroxyl groups.
The invention further provides a method of forming a composite coating film as defined above,
wherein said water-based base coat coating contains a polyester resin or an alkyl resin in each-molecule.
In a further aspect, the invention relates to a composite coating film as obtainable by the above method.
DETAILED DESCRIPTION OF THE INVENTION
In the following, the invention is described in further detail.
Water-based Base Coating
The water-based base coat coating to be used in-the method of forming a coating film according to the invention comprises a polyether polyol having at least 0.02 primary hydroxyl group, on average, per molecule, a number average molecular weight of 300 to 3,000 and a water tolerance of not less than 2.0; and a resin emulsion obtained by emulsion polymerization of an &agr;, &bgr;-ethylenically unsaturated monomer mixture having an acid value of 3 to 50 and containing at least 65% by weight of a (meth) acrylate ester whose ester moiety contains 1 or 2 carbon atoms. The water-based base coat coating may contain a curing agent, another or other film-forming resins, a color pigment, a luster pigment, and/or another or other additives.
The polyether polyol to be contained in the water-based base coat coating has at least 0.02 primary hydroxyl group, on average, per molecule, a number average molecular weight of 300 to 3,000 and a water tolerance of not less than 2.0. By containing this polyether polyol, the coating films can be improved in flip-flop properties, water resistance and chipping resistance.
When a polyether polyol having less than 0.02 primary hydroxyl group, on average, per molecule is used, the resulting coating films will show low water resistance and low chipping resistance. The number of primary hydroxyl groups per molecule is preferably not less than 0.04. More preferably, each molecule has at least one hydroxyl group. From the viewpoints of water resistance and chipping resistance in coating films, it is preferred that the number of hydroxyl groups, inclusive not only of primary but also secondary and tertiary hydroxyl groups be preferably at least 3. From the hydroxyl value viewpoint, the hydroxyl value is preferably 30 to 700. When the hydroxyl value is below the lower limit, the curability will be low and the resulting coating films will have low water resistance and low chipping resistance. Above the upper limit, the resulting coating will be low in stability and the coating films obtained will show decreased water resistance. A hydroxyl value of 50 to 500 is particularly preferred.
When a polyether polyol having a number average molecular weight less than 300 is used, the resulting coating films will show decreased water resistance. When that molecular weight exceeds 3,000, the resulting coating films will show decreased curability and chipping resistance. A preferred range is 400 to 2,000. The molecular weight of such a polymeric material as referred to in the present specification is determined by GPC based on the styrene polymer standards.
On the other hand, the use of a polyether polyol having a water tolerance less than 2.0 results in low water dispersibility and poor coating film appearance. A water tolerance of not less than 3.0 is particularly preferred.
The term “water tolerance” is used herein to evaluate the degree of hydrophilicity and a higher value thereof means a higher level of hydrophilicity. The value of water tolerance, so referred to in the present specification, is determined by dispersing 0.5 g of the polyether polyol in 10 ml of acetone in a 100-ml beaker at 25° C., gradually adding deionized water to this mixture using a buret and determining the amount (ml) of deionized water required to cause the mixture to become turbid. This amount (ml) of deionized water is reported as the water tolerance value.
When a hydrophobic polyether polyol, for instance, is tested by this method, the polyether polyol is initially compatible well with acetone and, upon addition of a small amount of deionized water, becomes incompatible with acetone, with the result that the measurement system becomes turbid. Conversely, in the case of a hydrophilic polyether polyol, a polyether polyol higher in hydrophilicity requires a larger amount of deionized water for causing turbidity. In this way, this method can determine the extent of hydrophilicity/hydrophobicity of a polyether polyol.
The content of the above polyether polyol on the resin solids basis is preferably 1 to 40% by weight, more preferably 3 to 30% by weight. Above the upper limit, the resulting coating films will show low water resistance and low chipping resistance. Below the lower limit, the appearance of the coating films will become poor.
As the above polyether polyol, there may be mentioned compounds derived from an active hydrogen-containing compound, such as a polyhydric alcohol, a polyhydric phenol or a polybasic carboxylic acid, by addition of an alkylene oxide. The active hydrogen-containing compound includes, for example, water, polyhydric alcohols (dihydric alcohols such as ethylene glycol, diethylene glycol, trimethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-dihydroxymethylcyclohexane and cyclohexylene glycol, trihydric alcohols such as glycerol, trihydroxyisobutane, 1,2,3-butanetriol, 1,2,3-pentanetriol, 2-methyl-1,2,3-propanetriol, 2-methyl-2,3,4-butanetriol, 2-ethyl-1,2,3-butanetriol, 2,3,4-pentanetriol, 2,3,4-hexanetriol, 4-propyl-3,4,5-heptanetriol, 2,4-dimethyl-2,3,4-pentanetriol,

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