Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-02-16
2001-08-28
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S522000, C523S526000, C523S527000, C523S127000, C523S169000, C523S167000, C523S375000, C523S375000, C523S437000, C523S440000
Reexamination Certificate
active
06281272
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to waterborne coating compositions, and more particularly to waterborne coating compositions containing fatty acid-containing polyesters and having improved appearance and improved humidity resistance at low baking temperatures.
BACKGROUND OF THE INVENTION
Several trends within the automotive industry point to the need for waterborne coatings which have good physical properties and appearance when baked at lower than normal cure temperatures. One trend within the industry has been the effort to reduce atmospheric pollution due to volatile organic solvents emitted during the painting process. One approach to this end has been to develop waterborne coating compositions which contain less organic solvents. Another way to reduce emissions and to cut energy expenditure is to lower the temperature at which the coating is baked or cured; less fuel is required to heat an oven to a lower temperature and consequently there is less carbon dioxide emitted to the atmosphere.
Another trend is the increasing use of automobile body parts made of thermoplastic olefins which deform at normal bake temperatures. Such parts are usually painted in separate operations using coatings which are different from the coatings used to paint the metallic parts of the automobile body.
Defects in automotive topcoats which occur during application are usually repaired separately and with a minimum amount of damage to the original coating. In order to avoid overbaking the original coating, it is desirable to have repair coatings which can be cured at lower bake temperatures. Currently, it is common in automotive plants to use different coatings for metallic parts and plastic parts and to repair defects, even though all are being applied to the same automobile. Since it is difficult for coatings with different compositions to have the same appearance, it is highly desirable to have one universally applicable waterborne coating that will perform well for different applications at both low and normal cure temperatures. If different coatings must be used for the different applications, then it is desirable that the coatings have compositions as similar as possible.
Waterborne automotive coatings containing water dispersible fatty acid-containing polyesters or alkyds are known. U.S. Pat. No. 5,412,023 teaches the use of a polyester polyol containing isomerized soybean fatty acid with an acid value of 34. However, this patent specifically teaches that the polyester polyol must contain sufficient ionic groups to be stably dispersed into water. U.S. Pat. Nos. 4,851,460 and 5,342,882 teach the use of a water dispersible polyester with an acid value of 86.5 made from epoxidized linseed oil. When acrylic monomers are polymerized in the presence of such a dispersion, the result is a polyester/acrylic dispersion that is stabilized by the ionic functionality of the hydrophilic polyester. In none of these cases is there any indication that resins made from fatty acid-containing polyesters would be suitable for coatings cured at low baking temperatures.
In order to decrease damage to automotive coatings due to the effects of acid rain, some coatings manufacturers have been making modifications to topcoats that make them more hydrophobic. Such modifications can cause a decrease in adhesion when another coating is applied to them to repair defects. The coating used to repair defects must be adapted to overcome any loss in adhesion caused by the hydrophobic nature of the coating being repaired.
Unfortunately, as the baking temperature for a coating is decreased, the rate and degree of curing is also decreased. This is particularly true for coatings which are cured using aminoplast crosslinking agents. A low degree of cure can result in a weakening of the physical properties of the coating, and in particular, a reduction in the degree of cure can result in a decrease in the cohesive and adhesive strength of the coating, especially when the coating is subjected to a humid environment.
Therefore, in order to obtain the advantages of a low bake temperature, a means must be found to overcome the loss of physical properties associated with a lower degree of cure. In addition, a low temperature cure waterborne coating must still have all the characteristics required of an automotive coating, such as smoothness, metallic flake orientation, and broad application latitude.
SUMMARY OF THE INVENTION
In accordance with the present invention, a waterborne coating composition is provided comprising pigment, a crosslinking agent, and a polymeric film-forming resin. The polymeric film-forming resin is a latex which comprises a dispersion of polymeric microparticles prepared by forming a mixture in an aqueous medium of an ethylenically unsaturated monomer or mixture of ethylenically unsaturated monomers with greater than 10 percent by weight of a substantially hydrophobic, saturated fatty acid-containing polyester, the percent by weight being based on weight of ethylenically unsaturated monomer(s) and fatty acid-containing polyester.
The fatty acids from which fatty acid-containing polyesters are prepared are long chain aliphatic saturated carboxylic acids derived from naturally occurring fats and oils. The fatty acid-containing polyester has a number average molecular weight greater than about 600 as measured by gel permeation chromatography using a polystyrene standard, and is prepared from about 20 to 80 (preferably between 20 to 50) percent by weight of a fatty acid, based on total weight of the fatty acid-containing polyester. The mixture of ethylenically unsaturated monomer(s) and fatty acid-containing polyester is particularized into microparticles by high stress techniques followed by polymerizing the polymerizable species to form the polymeric microparticles which are stably dispersed in the aqueous medium.
The present invention provides a waterborne coating composition, comprising: a) a dispersion of polymeric microparticles in an aqueous medium wherein the microparticles are prepared by (i) forming a mixture in the aqueous medium of ethylenically unsaturated monomers and a substantially hydrophobic, saturated fatty acid-containing polyester, wherein the amount of fatty acid used to prepare the polyester ranges from about 20 to 80 weight percent of the total weight of the fatty acid-containing polyester, the fatty acid-containing polyester containing functional groups which are capable of reacting with a crosslinking agent; and (ii) particularizing the mixture into microparticles by high stress techniques followed by (iii) polymerizing polymerizable species within the microparticles to produce the polymeric microparticles; the amount of the fatty acid-containing polyester in the dispersion being at least 10 and up to 70 weight percent based on weight of the ethylenically unsaturated monomers and fatty acid-containing polyester, and the amount of the polymeric microparticles in the coating composition being from 20 to 80 weight percent based on total weight of resin solids in the coating composition and b) a crosslinking agent.
Another aspect of the present invention is a method for coating an elastomeric substrate, comprising the steps of: (a) applying a layer of the above waterborne coating composition upon a surface of an elastomeric substrate to form a film on the surface of the substrate; and (b) heating the coated substrate at a temperature of less than 93° C. (200° F.) such that the waterborne coating composition is cured.
Yet another aspect of the present invention is a method for repairing a coating defect on the surface of a coated substrate, comprising the steps of: (a) applying the above waterborne coating composition to the defect; and (b) heating the coated substrate at a temperature of less than 93° C. (200° F.) to cure the waterborne coating composition.
DETAILED DESCRIPTION OF THE INVENTION
The film-forming waterborne coating composition of the present invention comprises a dispersion of polymeric microparticles in an aqueous medium as a polymeric film-forming resin. T
Baldy Heidi M.
Faler Dennis L.
Rardon Daniel E.
Terrago Gina M.
Altman Deborah M.
Egwim Kelechi C.
PPG Industries Ohio Inc.
Wu David W.
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