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
1997-12-16
2004-06-22
Sergent, Rabon (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C252S182240, C524S198000, C525S399000, C525S453000, C525S456000, C525S457000, C528S028000, C528S045000, C528S049000, C528S060000, C528S065000, C528S070000, C528S071000, C528S073000, C528S085000, C560S025000, C560S026000, C560S115000, C560S158000
Reexamination Certificate
active
06753386
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of a particular class of oligomeric polyols to form high solids coatings having reduced viscosity as well as resistance to environmental factors such as acid rain and ultraviolet light. Polyurethane polyols are prepared by reacting a polyisocyanate with both a compound having a single functional group reactive with isocyanate, such as a monofunctional alcohol or monofunctional thiol, and with a diol or triol which reacts substantially only-single-endedly with an isocyanate. Mixtures of nonfunctional polyurethanes and polyurethane polyols are also taught.
2. Background of the Invention
Many of the high-performance automotive high-solids coatings presently in use are based on polymeric systems containing polyester or acrylic polyols. In typical single-component coatings, wherein all of the coating ingredients are combined into one storage stable mixture, the polyester or acrylic-polyol component is typically crosslinked with melamine (aminoplast resin) under heat cure conditions of about 250 degrees F. or above to provide a thermally cured coating. In typical two-component systems, such polyols are combined with a suitable isocyanate shortly before application to the surface to be coated and the combination is cured at temperatures ranging from about 70 degrees F. to about 280 degrees F.
Currently, the automotive industry is using basecoat/clearcoat coatings in ever increasing amounts. In such systems, a pigmented coating is applied over appropriate primers and the coating system is completed by applying an unpigmented clear topcoat over. the pigmented basecoat. It is also desirable that such coating systems comply with VOC regulations, which typically require that the clearcoat have volume solids in excess of 50 percent (for a high solids type). Simultaneously, due to the deterioration of our environment, the automotive industry has been searching for coatings systems which, after curing/drying, are acid rain resistant.
To obtain high solids while maintaining acceptable coating formulation viscosity for spray application, the industry has tended to decrease the number average molecular weight (Mn) of the film forming polymers and to increase the amount of crosslinker, thereby obtaining a cured coating having adequate hardness, gloss, impact strength, appearance and exterior durability. Typical coating formulations use a melamine or other amino resin as the crosslinker. Increased amounts of monomeric melamine crosslinkers reduce the formulation viscosity. As the amount of amino resin is increased, the acid rain resistance of these coatings is compromised. At this time the automobile manufacturers consider improved resistance of automotive finish coatings to environmental etching (acid rain) to be a high priority. It is believed that ester bonds in an acrylic melamine or polyester melamine coating are weak points in the crosslinked resin network, susceptible to acid catalyzed hydrolysis.
Current high solids automotive topcoats, whether they be monocoats or the more modern basecoat/clearcoats, are predominantly oligomeric acrylic polyols crosslinked with melamine-formaldehyde resins. Modern topcoats of this type form visually appealing, high gloss films and are designed to retain high levels of gloss after extensive accelerated weathering and Florida exposure. In recent years, further improvement in durability has been obtained by the use of basecoat/clearcoat systems, where the clearcoat acts as a screen to protect the pigmented film.
There has been a general reduction in the pH, and an increase in the concentration of electrolytes, in rain water, creating “acid rain”. Probably as a result of the combination of these factors, a new problem has evolved in automotive topcoat technology which is generally referred to as acid or environmental etching. The defect appears as a grainy water spot pattern seen predominantly on horizontal surfaces. An in depth study of the problem by General Motors workers indicates that acidic components in a wetting event (dew or rainfall) react with calcium, a common constituent of dirt. As droplets evaporate, calcium sulfate precipitate forms on horizontal surfaces around the droplet perimeters. Subsequent washing removes the precipitate, but scars remain. It is generally observed that the problem is most conspicuous on dark, freshly painted surfaces in warmer and more polluted environments. The normal crosslinking at the surface of a coating induced by exposure to UV radiation and oxygen may eventually protect the film. Thus, the problem is largely one that occurs on automobile dealers' lots. Frequently, etched cars must be repainted before they can be sold. One major U.S. manufacturer estimates the cost of environmental etching to exceed 50 million per year.
A considerable amount of work has been done related to coatings containing polyurethane polyols. One way to make polyurethane polyols is to react a diisocyanate or a multifunctional isocyanate with a significant stoichiometric excess of a diol. After the reaction is complete, the excess of diol is removed, preferably by distillation. The obvious disadvantage of this method of making low molecular weight polyurethane polyols is that the distillation of the diols is inconvenient and it is not possible to use diols of high molecular weight (which cannot be distilled off) unless they are later recrystallized. Also, molecular weight control is difficult in such processes because even at the stoichiometric excess, a limited number of hydroxyl groups on the same diol molecules will react with the isocyanate, giving chain extensions beyond the intended low molecular weight polymers. This results in broad molecular weight distributions. U.S. Patents describing the production of polyurethane polyols by using stoichiometric excess of diols include: U.S. Pat. No. 4,543,405 to Ambrose, et al.; issued Sep. 24, 1985; and U.S. Pat. No. 4,288,577 to McShane, Jr., issued Sep. 8, 1981.
Crosslinked coatings based on polyurethane polyols of this type have been described in U.S. Pat. Nos. 4,548,998 to Chang, et al., issued Oct. 22, 1985; 4,540,766 to Chang et al., issued Sep. 10, 1985; and 4,485,228 to Chang et al., issued Nov. 27, 1984. The coatings based on these compositions offer good flexibility and hardness balance.
Another class of similar coating polymeric systems is based on urethane-modified polyesters. The polymeric systems are prepared by reacting a polyisocyanate with an excess of diol and then using this resulting mixture as a polyol reactant for carrying out a conventional polyester condensation involving acids, diols, triols and so on. Alternatively, hydroxyl terminated conventional polyesters can be extended with isocyanates.
Typical U.S. patents describing such polymeric systems include: U.S. Pat. No. 4,605,724 to Ambrose et al., issued Aug. 12, 1986; U.S. Pat. No. 4,540,771 to Ambrose et al., issued Sep. 10, 1985; U.S. Pat. No. 4,530,976 to Kordomenos et al., issued Jul. 23, 1985; U.S. Pat. No. 4,533,703 to Kordomenos et al., issued Aug. 6, 1985; U.S. Pat. No. 4,524,192 to Alexander et al., issued Jun. 18, 1985; and U.S. Pat. No. 4,533,704 to Alexander et al., issued Aug. 6, 1985. These patents describe methods of making the polymers and their use in coatings.
Japanese Patent 82-JP-115024, assigned to ASAHI Chemical IND KK, discloses a method of preparing an isocyanate terminated prepolymer wherein the isocyanate termination groups have different reactivity. The isocyanate terminated prepolymer is prepared by reacting two types of polyisocyanate having different reactivities with diols having two kinds of hydroxyl groups of different reactivity. The resulting prepolymer is subsequently crosslinked/cured using moisture or another source of hydroxyl groups.
U.S. Pat. No. 3,576,777 discloses the use of polyurethanes prepared from organic diisocyanates and glycols in conjunction with unsaturated oil-modified alkyd resins for preparing thixotropic paints. Small quantities of monoisocyanates and monoalcoho
Wagstaff Ian
Walker Frederick Herbert
Yahkind Alexander Leo
Akzo Nobel N.V.
McGillycuddy Joan M.
Sergent Rabon
Vickrey David H.
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