Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-05-29
2002-10-01
Woodward, Ana (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C525S123000, C528S075000
Reexamination Certificate
active
06458885
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a curable coating composition particularly adapted for use as a clear top coat on automobiles to provide fast drying speed. More particularly, the type of coating involved is that which includes as the primary binder constituents an acrylic polyol and a curing agent, typically a polyisocyanate.
Automobiles are sometimes coated with a plurality of layers, typically a pigmented base coating composition onto which is subsequently applied a different top coating composition. The top coat is frequently referred to as “clear,” that is, substantially without pigment, but minor amounts of pigments or other materials that affect the appearance of the automobile may be included. Acrylic polyol based top coatings are particularly useful for their compatibility with a wide variety of base coating compositions.
A major factor in the productivity of an automobile painting operation is the speed with which the top coat dries to a condition that is no longer tacky. “Drying” in this context does not entail completing the curing reactions, but rather an initial surface hardening due primarily to solvent evaporation. Although acrylic polyol based coating compositions have a desirable combination of properties, it has been desired to shorten the dry time of this type of coating. It was known to increase drying speed by selecting monomers that increase the T
g
(glass transition temperature) of the acrylic polyol. In particular, increasing the methacrylate monomer content of the acrylic polyol was known as a way to increase T
g
(U.S. Pat. Nos. 5,279,862 and 5,324,953). Attempts to further increase drying speed, especially under ambient conditions, have heretofore not been as successful as desired because increased drying speed was generally been accompanied by an undesirable degradation in the appearance of the final coating. It is therefore an objective of the present invention to shorten drying times of the acrylic polyol/isocyanate type of coating composition while continuing to provide the other properties required of automotive applications.
French Patent Publication No. 2 452 500 discloses generally the use of glycidyl compounds reacted with acrylic acid or methacrylic acid as a component of acrylic polyols.
SUMMARY OF THE INVENTION
The rapid-drying coating composition of the present invention contains as the primary binder components (A) 50 to 80 weight percent of an acrylic polyol more fully described below, and (B) 20 to 50 weight percent of a curing agent reactive with hydroxyl groups, such as a polyisocyanate. The weight percentages are based on the total resin solids weight of (A) plus (B). The particular acrylic polyol of (A) is the free radical reaction product of vinyl monomers, of which 0.5 to 15 percent by weight (based on total resin solids weight of the monomer mixture from which acrylic polyol A is made) is an acrylic compound having the structure:
where R
1
=H or CH
3
,
R
3
is H or an alkyl group,
R
4
is an alkyl group, and
R
5
is an alkyl group containing at least four carbon atoms.
DETAILED DESCRIPTION
The coating composition of the present invention comprises two components that are maintained as separate packages until immediately prior to use. One component (A) contains acrylic polyol, and the other component (B) contains a curing agent reactive with the hydroxyl groups in the first component. Preferably, the curing agent includes a polyisocyanate, but may comprise other known curing agents reactive with hydroxyl groups such as melamine-formaldehyde compounds and anhydrides. The reaction product of components (A) and (B) comprises the primary film-forming portion of the coating composition. Optimized commercial embodiments of the coating composition may optionally include additional ingredients such as diluents (e.g., solvents and/or reactive resinous diluents), catalysts, ultraviolet light absorbers or stabilizers, and pigments as are conventional in the art. These additional ingredients may be included in either or both components. Such acrylic based coating compositions are particularly useful for automotive finishes, especially as clear top coats over a pigmented base coat. The clear coatings are very advantageous as clear coats over a variety of base coats. In a particular commercial application of the present invention, the coating composition is intended for use in refinishing automobiles, wherein drying of the film-forming polymer is effected at or near ambient temperatures.
The acrylic polyol is made by addition polymerization of at least two types of unsaturated monomers, at least one of which includes the structure (I) above. Preferably, additional hydroxyl functionality is provided by including in the monomer mixture another monomer having a hydroxyl group. Most commonly, several other monomers are included to provide desired physical properties to the final coating.
Based on total resin solids weight of the monomer mixture, the acrylic polyol of the present invention may be synthesized from a monomer combination that comprises: (a) 0.5 to 15 weight percent of a compound of structure (I); (b) 0 to 45 weight percent hydroxy-containing unsaturated monomers such as hydroxyalkyl acrylates and methacrylates; and (c) from 40 to 98 weight percent of other ethylenically unsaturated copolymerizable materials such as non-functional alkyl acrylates, alkyl methacrylates, and vinyl aromatic monomers. The preferred weight percentages of these monomers are 1 to 10 percent of (a), 20 to 40 percent of (b), and 50 to 80 percent of (c).
The monomer (a), i.e., the compound of structure (I), comprises an acrylate or methacrylate in which the esterifying group is the residue of a glycidyl group which is, in turn, bound to a terminal group that includes a branched alkyl group, preferably a tertiary alkyl group. At least one of the alkyl branches in the terminal group may include a chain four or more carbon atoms in length. Preferably the branched alkyl group includes at least eight carbon atoms. A branched alkyl structure in the terminal group is believed to be particularly advantageous for attaining optimal results with the present invention. Surprisingly, the substantial chain length of monomer (a) and its inclusion of alkyl groups of substantial length permit the acrylic polyols into which it is polymerized to have very rapid drying rates without a substantial loss of hardness in the final coating that might otherwise be expected of such a structure. Good gloss and flow properties are also attained with the preferred embodiments.
Monomer (a) may be synthesized by reacting acrylic or methacrylic acid with a monoepoxide having substantial hydrocarbon chain length, such as commercially available epoxidized alpha olefins of the formula:
where R
6
includes a branched alkyl group having at least 6 carbon atoms, preferably at least 8 carbon atoms. Polyepoxies such as certain of the commercially available family of EPON products may be used if partially defunctionalized to form monoepoxies. Epoxies with little or no aromatic content are preferred for automotive coatings. Preferably, the terminal group in the esterifying group of monomer (a) itself includes an ester group, in which case the monomer may be the reaction product of acrylic acid or methacrylic acid and CARDURA E (a glycidyl ester of Versatic acid sold by Shell Chemical Company). Versatic acid is a proprietary, synthetic blend of isomers of saturated tertiary alkyl monoacid having nine to eleven carbon atoms. The (meth)acrylic and CARDURA E reaction yields the following structure:
where the groups are as defined above. Alternatively, monomer (a) with an ester-containing terminal group may be produced from the reaction of glycidyl acrylate or glycidyl methacrylate with a long chain organic acid such as Versatic acid, neodecanoic acid, or isostearic acid. The branched structures of the preferred acids are believed to be advantageous.
Examples of suitable hydroxy-containing unsaturated monomers of group (b) in the monomer mixture employed to make acrylic polyo
Claar James A.
Kania Charles M.
Stachel Kenneth J.
Stengel Sandra R.
Thomas Stephen J.
Miles Jacques B.
Millman Dennis G.
PPG Industries Ohio Inc.
Woodward Ana
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