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
1999-08-11
2002-04-23
Mulcahy, Peter D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S221000, C525S222000, C525S403000, C525S404000, C525S408000, C525S528000, C525S529000, C525S530000
Reexamination Certificate
active
06376608
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a curable powder film-forming composition having improved resistance to chemical attack such as acid, as well as multi-component composite coating compositions comprising a pigmented or colored base coat and a transparent or clear topcoat.
BACKGROUND OF THE INVENTION
In recent years, powder coatings have become increasingly popular in the automotive coatings industry because these coatings are inherently low in volatile organic content (VOC), which significantly reduces air emissions during the application process. Epoxy condensation polymers, vinyl chloride polymers and acrylic resins have been used as the main film-forming polymers for these coatings. The advantage of the acrylic resin system over the epoxy condensation polymers and vinyl chloride polymers is that acrylic resins provide superior outdoor durability, are more resistant to heat and have better solvent and chemical resistance. The use of glycidyl functional acrylic resins provide such desired properties.
There is a need to apply powder coatings compositions in the automotive original equipment manufacturing (OEM) process. In today's competitive market, automobile manufacturers are placing stricter requirements on coatings properties and performance. Good exterior durability, acid etch and water spot resistance, and excellent gloss and appearance are typical examples. Currently, these properties an be achieved using epoxy-acid cured liquid coatings. However, liquid coatings have significantly higher VOC levels than powder coatings which essentially have zero VOC levels.
Although epoxy-acid systems are known for powder coatings, they are presently not widely used commercially for automotive OEM clear coatings in color-plus-clear composite systems because of inferior appearance such as yellowing, poor flow, and poor cured film properties. Poor appearance is frequently due to the composition of the base coat used underneath the powder clear coat. Attempts to improve cured film properties such as chemical resistance have included addition of auxiliary curing agents or other resinous additives, which may lead to gas evolution or aggravation of yellowing.
U.S. Pat. No. 5,710,214 discloses a curable powder film-forming composition comprising a glycidyl functional acrylic copolymer, a polycarboxylic acid and a blocked polyisocyanate, present at about 17 to 34 percent by weight so as to participate significantly in crosslinking mechanisms. Powder film-forming compositions containing blocked polyisocyanates at such high levels may be prone to yellowing, and when used in a multi-component composite coating composition over a waterborne base coat, may exhibit undesirable gas evolution.
It is desirable to provide an epoxy-acid powder coating composition and a color-plus-clear composite coating system utilizing an epoxy-acid powder clear coating and having improved appearance, flow, stability, and cured film properties such as chemical resistance without the drawbacks of the prior art.
SUMMARY OF THE INVENTION
In accordance with the present invention, a curable powder film-forming composition is provided. The composition comprises a solid, particulate mixture of the following ingredients:
(a) about 55 to about 80 percent by weight based on the total weight of resin solids in the film-forming composition, of an epoxy functional copolymer having a T
g
of about 77° F. to about 158° F. (about 25° C. to about 70° C.) formed by polymerizing under free radical initiated polymerization conditions:
(i) about 25 to about 75 percent by weight based on the total weight of (i) and (ii) of a glycidyl functional ethylenically unsaturated monomer, and
(ii) about 25 to about 75 percent by weight based on the total weight of (i) and (ii) of a copolymerizable ethylenically unsaturated monomer or mixture of monomers free of glycidyl functionality;
(b) about 15 to about 35 percent by weight based on the total weight of resin solids in the film-forming composition, of a polycarboxylic acid; and
(c) about 1.0 to about 5.0 percent by weight based on the total weight of resin solids in the film-forming composition, of a polyisocyanate at least partially capped with a capping agent. Note that the total amounts of (a), (b), and (c) may not equal 100 percent, particularly when used at preferred levels, due to the presence of resinous additives.
Also provided is a multi-component composite coating composition comprising a base coat deposited from a pigmented film-forming composition and a transparent top coat applied over the base coat wherein the transparent top coat is deposited from a clear film-forming composition and is the curable powder film-forming composition described above.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”.
DETAILED DESCRIPTION OF THE INVENTION
The curable powder film-forming composition of the present invention comprises (a) an epoxy functional copolymer, (b) a polycarboxylic acid, and (c) a polyisocyanate at least partially capped with a capping agent.
The epoxy functional copolymer is prepared by copolymerizing one or more glycidyl functional ethylenically unsaturated monomers such as glycidyl acrylate, glycidyl methacrylate, 3,4-epoxycyclohexylmethyl(meth)acrylate, 2-(3,4-epoxycyclohexyl)ethyl(meth)acrylate, or allyl glycidyl ether with an ethylenically unsaturated monomer or mixture of monomers free of glycidyl functionality. Glycidyl methacrylate is the preferred epoxy functional monomer.
In the practice of the invention, the glycidyl functional monomer is typically copolymerized with at least one other monomer having a T
g
greater than about 200° F. (about 93° C.). By this is meant that a homopolymer of a given monomer has a T
g
of at least about 93° C. T
g
as used for the monomer herein refers to actually measured values. Differential scanning calorimetry (DSC) can be used to determine T
g
(rate of heating is 18° F. (10° C.) per minute, T
g
taken-at-the first inflection point). A high T
g
monomer is important because it prevents caking and instability problems associated with powder coatings. Suitable monomers include methyl methacrylate and styrene.
In addition to the glycidyl functional ethylenically unsaturated monomer and high T
g
monomers, a different copolymerizable ethylenically unsaturated monomer or mixture of monomers can also be used to prepare the epoxy functional copolymer of (a). Examples of such monomers include esters of acrylic acid or methacrylic acid such as ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, and so forth. Cyclic esters such as cyclopentyl methacrylate, cyclohexyl methacrylate, methylcyclohexyl-methacrylate, trimethylcyclohexyl methacrylate, norbornylmethacrylate, norbornylmethyl methacrylate, isobornyl methacrylate, and their corresponding acrylates, as well as hydroxyalkyl esters such as 2-hydroxyethyl acrylate or methacrylate, 2-hydroxypropyl acrylate or methacrylate may also be used. In addition, vinyl aliphatic or vinyl aromatic compounds other than alpha-methyl styrene dimer which is considered a chain transfer agent, such as acrylonitrile or methacrylonitrile can be used.
The copolymer can be prepared by traditional free radical initiated polymerization techniques using suitable catalysts which include organic peroxides and azo type compounds and chain transfer agents such as alpha-methyl styrene dimer and tertiary dodecyl mercaptan.
The copolymer usually contains between (i) about 25 to about 75 percent by weight, based on weight of the copolymer, of the glycidyl functional ethylenically unsaturated monomer and usually between (ii) about 25 to about 75 percent of a copolymerizable ethylenically unsaturated
Barkac Karen A.
Maier Douglas W.
Martin Roxalana L.
Altman Deborah M.
Cannoni Ann Marie
Mulcahy Peter D.
PPG Industries Ohio Inc.
LandOfFree
Curable powder film-forming composition having improved... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Curable powder film-forming composition having improved..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Curable powder film-forming composition having improved... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2902024