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
1999-11-22
2002-05-28
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C524S832000, C524S833000, C524S845000, C524S555000, C528S363000, C525S450000, C525S451000, C525S452000, C525S453000, C523S335000, C526S075000, C526S078000, C526S079000, C526S318000, C526S318200, C526S318300, C526S319000, C526S321000, C526S325000
Reexamination Certificate
active
06395827
ABSTRACT:
The present invention relates to an aqueous crosslinkable coating composition comprising an aqueous dispersion of a certain hydroxy-functional and acid-functional vinyl polymer(s) and an organic polyisocyanate.
It is known in the art to employ aqueous two-component crosslinkable coating compositions comprising an aqueous dispersion (usually an aqueous latex or aqueous solution) of a vinyl polymer (usually an acrylic polymer) which is both hydroxyl and acid functional, and an organic polyisocyanate, for the provision of clear or pigmented coatings (e.g. paint coatings) which become cured (i.e. crosslinked) on removal of the aqueous carrier medium. The vinyl polymer provides the polymeric binder material of the composition, while the polyisocyanate acts as a crosslinker material for reacting with the hydroxyl groups in the polymer after coating formation to provide curing of the coating via covalent urethane linkages. The acid groups in the vinyl polymer provide or assist aqueous dispersion stability (or, if in sufficient amount and/or of a sufficiently low pK, water solubility) and enhanced substrate adhesion of the resulting coating, and may also contribute to curing by reaction with the polyisocyanate. Such compositions are considered an advantageous improvement over organic solvent-based compositions using hydroxyl-functional polymer and polyisocyanate components because of the entire or substantial replacement of environmentally suspect organic solvents with water as the carrier medium for the composition.
For example, patent publications such as AU 41135/89 (equivalent to EP 0358979), U.S. Pat. Nos. 4,618,390, 4,442,145, 5,304,400, and AU 9470010, disclose coating compositions based on a hydroxyl-functional and acid-functional acrylic polymer and an organic polyisocyanate, the acrylic polymer being in aqueous solution and/or dispersion, and the polyisocyanate normally being present in emulsified form in the acrylic polymer dispersion. The preferred monomer(s) providing the acid functionality is invariably acrylic acid and/or methacrylic acid (being used in all the worked examples), although other carboxylic acids such as crotonic acid, itaconic acid, maleic acid or anhydride, phthalic acid or anhydride, or their half esters, have also been mentioned; sometimes unsaturated sulphonic acids are also mentioned as being usable, normally in addition to the use of an unsaturated carboxylic acid monomer.
We have, nevertheless, encountered a problem when forming films from such prior art carboxylic acid-containing compositions, viz that the resulting cured films often tend to contain entrapped carbon dioxide bubbles which mars their appearance and protective capability (the carbon dioxide bubbles becoming entrapped in the films during the curing process). This is particularly a problem found when using more than very low levels of acid monomers such as the hitherto invariably employed acrylic acid or methacrylic acid when preparing the acid-bearing polymer (e.g. above about 1.5 to 3 weight % of such monomer(s) based on the total weight of monomers used for polymerisation); such more than minimal levels are, nevertheless, normally desirable to promote adequate dispersion stability and resulting coating substrate adhesion. The problem is also more apparent the thicker the resulting coatings that are prepared. For example, in the paper by Vandervoorde and Brinkman, “Isocyanate Free Two Component Waterborne Coatings”, presented during “High Performance Waterborne Coatings”, Forum de la Connaissance, Paris, Nov. 26-27, 1996, it is mentioned that, in contrast to all-acrylic aqueous systems, aqueous two pack polyurethane systems (i.e. aqueous vinyl/acrylic polymer plus polyisocyanate systems) develop a strong tendency for gas-bubble formation when applied in thick layers (e.g. up to 130 &mgr;m as used for the all acrylic system).
We have now discovered new two-pack aqueous vinyl polymer/polyisocyanate compositions which, in contrast to many of the prior art systems discussed above, may be applied to form thick coatings, e.g. up to 200 &mgr;m or even 300 &mgr;m dry thickness, without incurring any formation of carbon dioxide bubbles.
According to the present invention there is provided an aqueous crosslinkable coating composition comprising:
(A) an aqueous dispersion of at least one vinyl polymer which is hydroxyl- and acid-functional, said aqueous dispersion being a dispersion of said at least one vinyl polymer in a liquid carrier medium at least 50% by weight of which is water, and in which:
(i) said at least one vinyl polymer has a hydroxyl value within the range of from 8 to 250 mg KOH/g of solid polymer;
(ii) said at least one vinyl polymer has an acid value within the range of from 2 to 55 mg KOH/g of solid polymer, where the contribution of at least 30% of the acid value results from the acid groups of polymerised units in the vinyl polymer of a monocarboxylic acid(s) of formula:
CH
2
═CR
1
—Q—CO
2
H (1)
where R
1
is H or lower alkyl of 1 to 5 carbon atoms and Q is a non carboxyl-containing spacer group providing a spacer chain of at least 3 atoms, wherein the spacer group Q is of the formula:
[—C(═O)—X—(CH
2
)
m
—]
k
(2)
where X is —O— or —NH—, m is at least 1, preferably 2 or 3, and k is at least 1, preferably 1, 2, 3, 4, or 5; and
(B) at least one organic polyisocyanate.
There is also provided according to the invention a method of coating a substrate, which method comprises applying a coating composition as defined above to a substrate, causing or allowing the aqueous carrier medium of the composition to be removed, and developing crosslinking of the coating that has been applied to the substrate.
There is further provided according to the invention a crosslinked coating which has been derived from a coating composition as defined above.
There is further provided according to the invention a coated substrate which has been prepared by applying a coating composition as defined above to a substrate, causing or allowing the aqueous carrier medium of the composition to be removed, and developing crosslinking of the coating that has been applied to the substrate.
By an aqueous dispersion of the vinyl polymer(s) of component (A), is meant herein a dispersion of the component in a liquid carrier medium of which water is the principal component (at least 50% by weight, more usually at least 80% by weight of the carrier medium). The vinyl polymer component may be dispersed as an aqueous emulsion (that is to say, an aqueous latex), suspension or solution. However, the at least one vinyl polymer of component (A) is usually in the form of colloidally dispersed particles, i.e. in the form of an aqueous latex.
It has thus been found that the crosslinkable coating composition of this invention, which includes a hydroxyl- and acid-functional vinyl polymer(s) as defined in which at least 30% of the acid value thereof results from polymerised units in the polymer of a monocarboxylic acid(s) of the above-defined formula, rather than solely from hitherto a conventionally-employed unsaturated acids such as acrylic and methacrylic acid, allows one to form coatings (which may be very thick if desired) without any unsightly porosity resulting from gas bubble formation. The invention coating compositions are particularly useful for providing bubble-free coatings of dry thickness within the range of from 140 to 300 &mgr;m.
The acid value range quoted above refers to the acid groups when in their free (i.e. unneutralised) form although such groups may be (in part or in total) in neutralised form in the invention compositions, and indeed this is often preferred.
With regard to the carboxyl-containing monomer of formula (1), R
1
is preferably H or methyl, and is more preferably H. The spacer group Q preferably provides a spacer chain of at most 20 atoms. A preferred range for the spacer chain of Q is therefore 3 to 20 atoms.
It is especially preferred in one embodiment of the present invention that the unsaturated carboxyl-containing monomer(s) of formula (1) is
Colstee Erik Hendrik
Nabuurs Tijs
Overbeek Gerardus Cornelis
Padget John Christopher
Pears David Alan
Avecia Limited
Lee Rip A.
Pillsbury & Winthrop LLP
Wu David W.
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