Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-06-07
2001-08-21
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S166000, C525S194000, C525S377000, C525S386000, C525S934000, C524S091000, C524S102000, C524S116000, C524S117000, C524S229000
Reexamination Certificate
active
06277917
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to curable thermosetting powder coating compositions for clear coats of a multilayered coating composite on substrates. The composite has at least a base coat layer with a clear coat layer over the base coat layer. Additionally, the composite can have a primer coating layer beneath the base coat layer. Such powder clear coat layers and multilayer coating composites are useful in painting motor vehicles.
Solid particulate coating formulations referred to in the industry as “powder coatings” are finding increasing use in painting metal substrates both as primer coatings and as top or clear coats of the base coat and clear coat composite coatings. The automotive industry provides corrosion protection and decorative appearance for motor vehicles by multilayered paint composites on the various types of surfaces of motor vehicles. The finishing layers of this composite usually include the popular base coat clear coat composites. The base coat is a pigmented layer and the clear coat is a nonpigmented or only slightly pigmented layer that is applied separately to the base coat and cured to some degree with the base coat. To provide improved coating composites for motor vehicles, the industry is seeking solutions to the problem of abrasive chipping of the paint from the action of road dirt and debris, like sand and gravel, that may strike areas of the vehicle such as the hood and lower portions of the vehicle such as rocker panels. These strikes can be with considerable impact velocity to result in unaesthetic chipping of the clear coat which can expose one or more underlying layers of the multilayered coating composite, which can also contribute to rusting.
Chipping in a multilayered coating composite can involve several failure mechanisms such as adhesive failure between layers of the multilayered composite or cohesive failure within a layer. To obtain good protection against chipping damage, the main underlying layers of the multilayered composite should have good intercoat or interlayer adhesion. Typically, clear coats which are generally the outermost layer of the multilayered coating composite typically assist in providing the properties of good appearance and environmental protection from etch, scratch and UV degradation along with good intercoat adhesion with the base coat.
The powder clear coats are becoming more widely used for their advantages in application from their lower organic solvent emissions. These coatings typically have very low levels of volatile solvents, i.e., on the order of two percent or slightly higher but generally much less than other paint systems. Generally, powder coating manufacturers have focused on the appearance, protection, and processability features of powder coatings to extract the environmental benefits of these coatings rather than any contribution of these coatings to chip resistance of the multilayered coating composite.
For instance, U.S. Pat. Nos. 5,270,391 and 5,407,706 show thermosetting curable powder coatings having epoxy functional acrylic copolymers in blends where the powders have good storage stability and give coatings with good appearance properties. The resin blends are of either a high softening point glycidyl-containing acrylic with a low softening point glycidyl-containing acrylic, or of different viscosities.
It is an object of the present invention to provide a clear coating composition which contributes to improved chip resistance of the multilayered coating composite of which the clear coat is a part without a detrimental effect on the appearance properties of the coating. Another object of the present invention is to provide improved stability of the powder and outstanding durability of the clear coat.
SUMMARY OF THE INVENTION
In accordance with the present invention, a curable thermosetting powder clear coating composition is provided for a clear coat of a base coat and clear coat composite coating with improved chip resistance. This is accomplished by the curable clear coating composition having: I) as the predominant film-forming polymer of the powder coating at least one epoxy functional acrylic polymer or copolymer (hereinafter referred to as “epoxy acrylic polymer”), and (II) polycarboxylic acid crosslinking agent where the ratio of epoxy functionality of (I) to acid functionality of (II) is about 1:0.6 to 1.4. Generally, the amount of the at least one particulate film-forming epoxy acrylic polymer is from about 60 to about 99.99 percent by weight based on the solids of the curable clear powder coating composition. Generally, the amount of the polycarboxylic acid crosslinking agent is in the range of about 10 to about 40 weight percent based on the weight of solids of the curable clear powder coating composition.
When a single epoxy acrylic polymer is the predominant film-forming polymer, the polymer has a number average molecular weight (“Mn”) range, a range of glass transition temperature (Tg), and a range of epoxy content all as shown in Table A below. When a blend of more than one epoxy acrylic polymer is the predominant film-forming polymer of the curable clear coating composition, two of the epoxy acrylic polymers in the blend have the Mn, Tg, and epoxy content of one and the other epoxy acrylic polymers are as shown in Table A as “A” and “B”.
TABLE A
Mn (grams)
Tg ° C.
Epoxy Content
2
Single Epoxy
about 1000
about 30 to about
35 to 85
Acrylic
to about
60° C. as measured or
Polymer
5500
50 to 85° C.
1
Blend of
A) about
about 40 to about 85
1
40-60
Epoxy Acrylic
1000 to
Polymers
about 10,000
B) about 500
about 35 to about 125
1
50-100
to about
3000
1
As calculated by the Acrylic Glass Transition Temperature Analyzer from Rohm and Haas Company which is based on the Fox equation.
2
(GMA) As weight percent of the monomers to prepare the epoxy acrylic polymer.
In the blend, two epoxy acrylic polymers have relatively low molecular weights and one polymer has a higher epoxy content within the higher range shown in Table A. If the epoxy acrylic polymer B of Table A has the epoxy content of 50 weight percent, the epoxy acrylic polymer A has a lower epoxy content within the range of 40 to almost 50. Also, when the higher epoxy content epoxy acrylic polymer has an epoxy content from 50 to 60, the epoxy content of the lower epoxy content epoxy acrylic polymer is in the range from 40 up to almost the value of epoxy content for the higher epoxy content epoxy acrylic polymer. Of course, with the epoxy content of greater than 60 for the higher epoxy content epoxy acrylic polymer, the epoxy content for the lower epoxy content epoxy acrylic polymer is in the range of from about 40 to 60. Also, the higher epoxy content epoxy acrylic polymer usually has the lower molecular weight between the two polymers “A” and “B” in the blend. Generally, the ratio of the low epoxy content polymer to the higher epoxy content polymer is in the range of 90:10 to 10:90.
The one or more epoxy acrylic polymers typically are formed by addition polymerization under free radical initiated conditions from at least the following: (i) about 20 to about 100 percent by weight of the total monomers of a glycidyl functional copolymerizable ethylenically unsaturated monomer; and (ii) up to about 65 percent by weight of one or more copolymerizable ethylenically unsaturated monomer or mixture of monomers. The exact amount of each monomer (i) and (ii) depends on the desired epoxy content for the epoxy acrylic polymer and on whether one or a blend of the epoxy acrylic polymers is used as the predominant film-forming polymer for the powder coating composition. With a single polymer the particular amounts of (i) and (ii) are from about 35 to about 85 percent by weight of (i) and about 15 to 65 percent by weight of (ii) monomers. The monomers of (ii) that are free of glycidyl functionality are of a type and in an effective amount with the other monomers to yield the desired Tg for the epoxy acrylic polymer. The percents by weight for the monomers of (i) and (ii) are based on the total weight of (i) and (
Allen Karen B.
Furar John M.
Humbert Kurt A.
Jurgetz Anton
Kania Charles M.
Altman Deborah M.
PPG Industries Ohio Inc.
Short Patricia A.
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