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
2001-06-22
2002-08-06
Niland, Patrick D. (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, C524S817000, C524S839000, C524S840000, C525S123000, C525S126000, C525S266000, C525S291000, C525S440030, C525S454000, C525S455000, C525S535000, C525S536000, C526S287000, C528S364000
Reexamination Certificate
active
06429256
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an aqueous coating composition comprising:
a) a polyol or a mixture of polyols having at least two hydroxy groups per molecule and:
i a content C of carboxylate- and/or carboxylic acid-functional groups and
ii a content S of sulfonate- and/or sulfonic acid-functional groups, and
b) one or more cross-linkers capable of reacting with hydroxy groups.
The invention also relates to a polyol which is particularly suitable for use in such a composition, the polyol having carboxylate- and/or carboxylic acid-functional groups with a content C and sulfonate- and/or sulfonic acid-functional groups with a content S. In this respect, carboxylate groups are defined as neutralized carboxylic acid groups and sulfonate groups are defined as neutralized sulfonic acid groups.
BACKGROUND OF THE INVENTION
European patent EP-B 0 537 568 discloses an aqueous composition comprising a dispersion of a polyol consisting of a polyester resin containing sulfonate groups and, optionally, carboxylate group in specified amounts. The sulfonate- and carboxylate-functional groups serve to make the polyol soluble in water. However, in the disclosed polyester polyol, the carboxylate groups, if present at all, play a minor role. In the disclosed polyester resins, the sulfonate groups are dominant. This has as a result that the initial curing is very slow.
The article “The rheological properties of an aqueous acrylic dispersion suitable for automotive waterborne basecoats” by Hiromi Harakawa c.s. in
Progress in Organic Coatings
34, 1998, pages 84-90, discloses some coating compositions which are specifically designed for use as a metallic base coat for the automotive industry. After application, such a base coat is generally overcoated with a transparent top coat. One of the disclosed compositions in this publication comprises a sulfonate- and carboxylate-functional polyol, as well as an internally cross-linked latex, and a melamine formaldehyde curing resin. The internally cross-linked latex gives the coating composition a suitable structure to fixate the orientation of aluminium flakes during curing. This hinders the flow of the coating composition, which makes it unsuitable for other purposes, such as use as a top coat where excellent levelling, smoothness, and gloss are required. The polyol comprising the sulfonate and carboxylate groups has a high acid number of 40 mg KOH/g resin (ca. 0.72 mmol/g resin). As a result of the high acid content, such a coating shows a very low humidity resistance.
PCT patent application WO 94/03516 discloses an aqueous coating composition comprising carboxylate-functional polyols which are neutralized with amines. A major drawback to such compositions is that hydrophobic polyisocyanates cannot be stirred into the polyol component by hand. Polyisocyanates rendered hydrophilic by built-in polyethylene oxide segments are easily stirred in by hand, but combined with a carboxylate and/or carboxylic acid stabilized polyol and neutralizing amines, they do not lead to the formation of coating films of sufficient humidity resistance. Another drawback to coatings comprising polyols with carboxylate and/or carboxylic groups is their tendency to form surface defects, especially with thick layers, such as gas bubbles, pinholes, and craters. This is a result of the reaction of carboxylate and/or carboxylic acid groups with isocyanate groups, which produces amide linkage and formation of CO
2,
Furthermore, unlike in the case of solvent-borne compositions, an acceptable balance of long potlife and fast curing is very difficult to achieve.
An aqueous system comprising polyols with sulfonate groups is disclosed in PCT patent application WO 97/19118. It was found that in the case of aqueous emulsions of acrylic polyol stabilized solely with sulfonate groups, the formation of gas bubbles and pinholes is largely reduced, even with thick layers. The potlife of such coating compositions is long and the films show a good resistance to solvents and humidity. However, curing is slow and the film smoothness is significantly lower than for solvent-borne systems.
German Offenlegungschrift DE-A 44 10 557 discloses an aqueous two-component coating composition comprising a polyol having carboxylate and/or sulfonate groups. Similar compositions are also disclosed by the European patent applications EP-A 0 358 979 and EP-A 0 557 844. None of these publications, however, discloses specific ratios of the carbonate and sulfonate groups.
The object of the invention is a stable aqueous coating composition which shows good flow properties directly after application in combination with faster and better curing in comparison with the known systems stabilized by carboxylate or sulfonate.
SUMMARY OF THE INVENTION
This object of the invention is achieved with an aqueous coating composition comprising:
a) a polyol or a mixture of polyols having at least two hydroxy groups per molecule and:
i a content C of carboxylate- and/or carboxylic acid-functional groups and
ii a content S of sulfonate- and/or sulfonic acid-functional groups, and
b) one or more cross-linkers capable of reacting with hydroxy groups.
DETAILED DESCRIPTION OF THE INVENTION
When, in the inventive aqueous coating composition, the total content of the sulfonate and/or sulfonic acid and carboxylate and/or carboxylic acid groups C+S is between 0.09 and 0.6 mmol/gram polyol, and the ratio C/S has a value between 0.5 to 20, these measures result in a somewhat delayed curing, so the paint can still flow directly after application. This gives an exceptionally good and smooth film appearance, also in thick layers. Levelling is excellent, and no surface defects, such as gas bubbles, pinholes or craters, occur. Formed films show a high gloss, excellent clarity, good hardness, and good chemical resistance, e.g., to solvents or gasoline. Moreover, coating compositions according to the current invention are colloidally stable aqueous compositions which provide high-quality coatings of high aesthetic quality without sacrificing technical characteristics, such as hardness, solvent resistance, and water resistance. The potlife and curing rate are easily controllable by the addition of catalysts.
The preferred combination of carboxylate and/or carboxylic acid and sulfonate and/or sulfonic acid groups depends on the intended use. If fast curing is required and the film's appearance is of less importance, the carboxylate and/or carboxylic acid content should be chosen higher. If, on the other hand, the film's appearance and a good hardness are more important than fast curing, the sulfonate and/or sulfonic acid content may be raised.
Particularly satisfying balances between film appearance and curing speed are found when the ratio C/S is within the preferred range between 0.5 and 10 or, more preferably, between 1 and 5. Optimum results are obtained if the ratio C/S is between 1.5 and 3.
Preferably, the total acid content of the carboxylate and/or carboxylic acid groups and the sulfonate and/or sulfonic acid groups C+S is between 0.18 and 0.6 mmol/gram of polyol. Optimum results are obtained if the total acid content C+S is between 0.18 and 0.36 mmol/gram of polyol.
Preferably, the polyols or polyol mixture has a hydroxy value between 50 and 300 mg KOH/gram polyol, more preferably between 50 and 200 mg KOH/gram polyol. Most preferably, the hydroxy value is between 75 and 200 mg KOH/gram polyol. The polyols have two or more hydroxy groups per molecule, preferably three or more.
The object of the invention can advantageously be achieved with a polyol having a content C of carboxylate- and/or carboxylic acid-functional groups and a content S of sulfonate- and/or sulfonic acid-functional groups, the total content C+S of the sulfonate and/or sulfonic acid and carboxylate and/or carboxylic acid groups being between 0.09 and 0.6 mmol/gram solid polyol, preferably between 0.18 and 0.36 mmol/gram of polyol, and the ratio C/S having a value between 0.5 to 20, preferably between 1.5 to 3. With such a polyol, a
Brinkman Egbert
Egmond Robert Van
Otte Rob Adriaan
Vandevoorde Paul Marie
Akzo Nobel N.V.
Niland Patrick D.
Parker Lainie E.
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