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
1999-09-24
2001-10-23
Yoon, Tae H. (Department: 1714)
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...
C524S507000, C524S539000, C524S602000, C524S845000
Reexamination Certificate
active
06306956
ABSTRACT:
The present invention relates to stable aqueous polyester dispersions useful as primers. More particularly, the aqueous polyester dispersions balance relatively high molecular weights and low hydroxyl numbers to provide a primer coating that effectively improves chip resistance of paint coatings overlaying the primer while maintaining adhesion and low levels of volatile organic compounds and low viscosities.
BACKGROUND OF THE INVENTION
Automotive and other durable coatings are complex multilayer systems which rely on certain performance criteria at each level for overall success. The bulk of the responsibility for providing improved chip resistance lies with the primer surface layer. However, improving chip resistance properties of automotive coatings or other types of protective or decorative durable coatings is clearly a system-dependent task. For many systems, there appears to be several key ingredients to achieving incremental improvements. A chip-resistance test is by nature high in stress, impact, and shear, which intuitively generates hypotheses based on the softness, or rubber-like characteristics of the primer layer. Any contributions from the primer to increase brittleness eventually may lead to reduced chip performance. This can be related to the degree of cross linking. Very low and very high levels of cross linking in such systems lead to poor chip resistance. In the case of low cross linking, the coating does not possess the strength properties necessary to withstand the impact and shear forces applied by the gravel. The base molecular weight is usually too low to provide appropriate elastic properties associated with light cross linking. The result is flow and scission on gravel impact. An extremely high cross link density affords large numbers of covalent cross links which impart brittleness to the coating, and failure is based on the inability of the coating to absorb and evenly distribute these concentrated forces applied at the gravel impact. The material neither flows, nor elastically deforms, but instead suffers brittle fracture. Further, cross linkers often cause VOCs by virtue of VOC by product release from the cross link reaction between the cross linkers and polymeric resin. Also complicating systems which use separate cross linking agents is the propensity for the cross linking agent to react with itself. For example, self-condensation of melamine cross linkers is a potential competing reaction, which may result in local areas of high concentration of highly cross linked melamine resin. This would produce localized brittleness which may result in chip or adhesion failures. Another key contributor to successful chip resistance is the adhesion of the primer to the substrate, and the subsequent adhesion of another paint coating to the primer (collectively intercoat adhesion). A major contributor to this effect is the functionality of the primer resin-polar group functionality including hydroxyl and carboxyl that play a key role in interactions with the substrate and other paint layers.
In order to obtain the optimal chip-resistant properties, the coatings overlying the primer and the primer base resin must each deliver a balance of properties to the final coating. The easiest way to maximize mechanical properties of the primer resin is to increase molecular weight. Increases in molecular weight, however, generally increase the viscosity of such resins. While high viscosity resins may be cut with organic solvents, this results in an undesirable increase in volatile organic compounds (VOCs).
SUMMARY OF THE INVENTION
The present invention is directed to an aqueous dispersion of a polyester salt and polymeric vehicles which include the aqueous dispersions of a polyester salt. The polymeric vehicle is particularly effective for providing coating binders in cured primer coatings (the cured primer paint coating film). The primer coating compositions (also known as formulated coating compositions) are useful for multilayer paint coating films which have at least two layers of paint film including the cured primer coating composition film. The use of the primer polymeric vehicles of the present invention in conjunction with one or more additional overlaying top coating film layers provide a multiple layer coating system having improved adhesion and chip resistance.
The primer polymeric vehicle of the invention permits the use of less cross linker to cross link the polyester salt. The use of less cross linker results in the release of less VOCs upon cross linking and a lower cross linker density in the cured coating binder film of in the cured primer coating composition. The reduced cross link density with the relative high molecular weight of the polyester salt provides improved chip resistance. While not intending to be bound by a theory, the polyester salt of the present invention has a higher molecular weight which enhances chip resistance and needs less cross linker (which also enhances chip resistance) to cure into a useable film than a polyester which is compositionally the same (and is not a metal or ammonium salt), but has a lower molecular weight. These lower molecular weight polymers are used in high solids systems and are dispersed in an organic solvent or mixed organic/water solvent system. Moreover, the molecular weight of the polyester in the latter high solids system can not be increased without also increasing the viscosity of the polymer dispersion or solution (increasing molecular weight inherently increases viscosity in such systems). Hence, the polymeric vehicle of the present invention includes a water dispersible polyester salt having unique combination of high molecular weight, a relatively low hydroxyl value or number, a unique ratio of hydroxyl value to number average molecular weight and a polyester with an acid value which permits formation of the water dispersible salt.
The polyester salt is cross linkable to form a cured coating binder in a primer paint coating film which is chip resistant when it is part of a multilayer paint coating system which has at least two paint film layers including the cured primer paint coating film layer. The cured coating binder of the primer of the invention also renders the multilayer paint coating system chip resistant. Hence, the polymeric vehicle of the primer coating composition balances the hydroxyl value and number average molecular weight of the polyester salt in the aqueous dispersion to provide VOC reduction through the use of an aqueous system with reduced amount of cross linker as well as improved chip resistance and adhesion for the cured primer paint coating film of the primer coating composition.
For enhancing chip resistance of coatings systems in which primers of the invention are used, the desired hydroxyl number is generally inversely related to the polyester molecular weight. By way of example, the polymeric vehicle for the primer coating composition or formulated coating comprises an aqueous dispersion of a polyester salt which is the residue of a polyester having a number average molecular weight of about 1500 Daltons and a hydroxyl value of about 90. In such aspect, the polyester has a hydroxyl equivalent weight of about 600. This aqueous dispersion is effective for providing the polymeric vehicle with less than about 5 weight percent organic solvent. At a higher molecular weight, the polymeric vehicle for the primer coating composition or formulated coating comprises an aqueous dispersion of a polyester salt which is the residue of a polyester having a number average molecular weight of about 2800 Daltons and a hydroxyl value of about 50. In such aspect, the polyester has a hydroxyl equivalent weight of about 1100. This aqueous dispersion is effective for providing the polymeric vehicle with less than about 5 weight percent organic solvent.
The chip resistance of the multilayer paint coating system which includes the coating binder of the primer of the invention, is superior to the chip resistance of a multilayer paint coating system which includes a primer which has compositiona
Kemp Lisa Kay
Knight, Jr. Michael Charles
Nothnagel Joseph Leo
Spilman Gary Eugene
Fitch Even Tabin & Flannery
McWhorter Technologies Inc.
Yoon Tae H.
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