Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Utility Patent
1998-12-02
2001-01-02
Sellers, Robert E. L. (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...
C428S418000, C428S458000, C428S461000, C428S463000, C525S117000, C525S170000, C525S207000, C525S454000, C525S533000
Utility Patent
active
06169150
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to curable coating compositions, more particularly to coating compositions that are curable at low or ambient temperatures.
The present invention involves improvements that are particularly applicable to automotive refinish coatings, but will have utility in others areas as well, where adhesion of coatings to bare metal is a concern. When repairing a defect in the original coating of an automobile may involve sanding or grinding out a defect by mechanical means, or completely stripping an entire part or the whole vehicle of its original coating layers. This sanding or grinding typically results in bare metal being exposed, which usually requires coating with a primer composition prior to a finish coating being applied to the repaired area.
Automotive body parts are sometimes made of cold-rolled steel and/or steel which has been surface-treated with a zinc containing material (e.g., galvanized or galvanealed). Adhesion of coating compositions to these zinc coated steel substrates can be problematic. Attempts to modify coating compositions to improve adhesion to zinc-containing surfaces sometimes been found to cause adhesion to cold-rolled steel to be impaired. One approach to this adhesion problem involves the use of chrome-containing wash primers prior to application of the primer. This undesirably entails an additional step in the refinish painting process. Furthermore, chrome containing materials may require special handling for environment purposes.
Because of these shortcomings in the prior art approaches, it would be desirable to provide coating compositions that exhibit enhanced adhesion to cold rolled steel as well as to galvanized and galvanealed steel substrates without requiring the use of chrome-containing wash primers.
A coating composition that attains the objective of adhesion to cold rolled and zinc treated steel is disclosed in the aforesaid U.S. patent application Ser. No. 08/720,937, abandoned, and U.S. patent application Ser. No. 08/928,533, U.S. Pat. No. 5,891,960. The coating composition is comprised of a film forming polymer which is the reaction product of (a) a polymer or compound containing pendant and/or terminal hydroxyl or epoxy functional groups and (b) citric acid or citric acid anhydride. These coating compositions are disclosed as employing polyisocyanate or aminoplast type curing agents. For automotive refinish applications that usually require curing at ambient temperatures, the polyisocyanate curing agents are typically employed.
Further improvements have been sought in these citric acid-containing coating compositions. In particular, attempts have been made to shorten the length of time required for the applied coating to become “tack-free” (i.e., the time required before the coating may be sanded). The ability to sand a primer coating as soon as possible after application is an important consideration in optimizing the productivity of automotive refinishing operations. Such an improvement is the subject of the aforesaid U.S. patent application Ser. No. 09/145,691, wherein it is disclosed that shortened tack-free times of citric acid-containing coating compositions can be attained by using anhydride curing agents in place of the polyisocyanate curing agents. The use of the anhydride curing agent was found to accelerate tack-free times while maintaining adhesion to zinc treated steel substrates without the need for chrome containing wash primers. To maintain the desired adhesion to cold-rolled steel substrates, it was found desirable to include a polyisocyanate in addition to the anhydride curing agent.
It would be desirable to provide the short tack-free times of the anhydride cured coatings and also maintaining adhesion to both zinc treated steel and cold rolled steel without the need for a polyisocyanate.
SUMMARY OF THE INVENTION
It has now been discovered that an anhydride curable coating composition can be provided with accelerated tack-free times as well as good adhesion to both cold-rolled steel and zinc treated steel substrates without the use of a chrome-containing wash primer. The coating compositions of the present invention are comprised of (a) an adduct containing carboxyl groups and (b) an anhydride curing agent. The adduct (a) is the reaction product of (i) a compound, polymer or oligomer containing pendant and/or terminal hydroxyl or epoxy groups, and (ii) a material selected from the group consisting of tartaric acid, malic acid, adipic acid, succinic acid, and anhydrides thereof, wherein the carboxyl group of the adduct is in a terminal and/or pendant position and is derived from the reaction of the hydroxyl groups with the acid or anhydride and/or the reaction of the epoxy groups with the acid or anhydride. Advantageously, no isocyanate-containing curing agent is needed in this composition to attain the desired adhesion properties.
Compositions containing adduct (a) made from each of tartaric acid, malic acid, adipic acid, succinic acid exhibited some improved adhesion under certain conditions compared to a composition containing no such adduct, but overall, those made from tartaric acid were clearly superior to the others. All four acids, when incorporated into the adducts described above, improved adhesion to both cold rolled and electrogalvanized steel under dry conditions, although adhesion to cold rolled steel was best with the tartaric acid embodiment. When tested after exposure to humidity, the tartaric acid embodiment was superior to the other three acids in adhesion to both cold rolled and electrogalvanized steel. After humidity exposure, the malic, adipic, and succinic acid embodiments showed some improvement in adhesion to electrogalvanized steel but not as much as the tartaric acid embodiment (the improvement for adipic was slight). There was some improvement in adhesion to cold rolled steel after humidity exposure for the adipic acid embodiment (but not as much as for tartaric acid), slight improvement for the succinic acid embodiment, and a reduction in adhesion for the malic acid embodiment. Thus, for refinishing automobiles and other applications where the cured coating must withstand a wide range of conditions, the tartaric acid embodiment is uniquely qualified. In view of the structural similarity of all four of these acids, the superiority of tartaric acid is unexpected.
In addition to curable coating compositions, the present invention also encompasses the cured coatings resulting therefrom, as well as articles (especially metallic substrates) carrying the cured coating.
DETAILED DESCRIPTION OF THE INVENTION
In making the adduct (a) containing carboxyl groups in accordance with the present invention, reactant (i), a compound, polymer or oligomer containing pendant and/or terminal hydroxyl or epoxy functional groups, is reacted with a dicarboxylic acid reactant (ii). Reactant (i) may be selected from a variety of materials, examples of which include polyepoxides, acrylic polymers containing epoxy or hydroxyl groups, polyester polymers containing epoxy or hydroxyl functional groups, and oligomers containing epoxy or hydroxyl groups.
The polyepoxides used as reactant (i) in the practice of the present invention may be saturated or unsaturated, cyclic or acyclic, aliphatic, alicyclic, aromatic or heterocyclic as are known to those skilled in the art. Also, the polyepoxides may contain substituents such as halogen, hydroxyl, and ether groups. Examples of polyepoxides are those polymers having a 1,2-epoxy equivalency greater than one and preferably about two, that is, polyepoxides which have on an average basis two epoxy groups per molecule as are well known in the art. Preferred polyepoxides are polyglycidyl ethers of cyclic polyols. Particularly preferred are polyglycidyl ethers of polyhydric phenols such as bisphenol A. These polyepoxides can be produced by etherification of polyhydric phenols with epihalohydrin or dihalohydrin such as epichlorohydrin or dichlorohydrin in the presence of alkali. Examples of polyhydric phenols are 2,2-bis(4-hydroxyph
Carney Joseph M.
Connelly Bruce A.
Swarup Shanti
Millman Dennis G.
PPG Industries Ohio Inc.
Sellers Robert E. L.
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