Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2001-06-21
2002-11-19
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
Reexamination Certificate
active
06482474
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to polyurethane coating compositions, their preparation and use.
BACKGROUND
Coatings used for the repair painting of motor vehicles generally comprises several layers of different coating compositions. The initial coating is usually a primer coating. The primer coating provides adhesion to the substrate, which can be bare metal or can be existing primer or topcoat or a mixture of these depending on the nature of the repair. The primer is also formulated so as to provide a relatively thick layer of coating which can be sanded (also known as ‘flatted’) using sand- or glasspaper so as to obliterate minor imperfections in the substrate and so provide a perfectly flat surface for subsequent layers of coating. Over the primer, a topcoat is applied which itself can comprise more than one type of coating such as a basecoat and a clearcoat.
One class of coating compositions which is well known for use in primers comprises a hydroxyl functional polymer, such as a polyester or acrylic polymer, and a polyisocyanate. These two components react together after application to the substrate to form a polyurethane coating. These compositions are often thus referred to as ‘polyurethane coating compositions’ although strictly the polyurethane is only formed when the coating crosslinks. Polyurethane primers of this general type have been known for at least 20 years.
Examples of prior art polyurethane coating compositions are disclosed in U.S. Pat. No. 4,273,912; Chem Abs. 109 (10), 1988; WPI database, week 9012, XP-002094192; GB2187196 and WPI database, week 9715, XP-002094193. None of these documents however discloses the use of a coating composition comprising a solution in a volatile organic solvent of a hydroxy functional polymer and polyisocyanate comprising a mixture of an aliphatic polyisocyanate having an average isocyanate functionality of 2.5 or more, and diphenylmethyl diisocyanate.
One problem which we have found with using polyurethane primers is that when they are sanded down in a particular spot, they tend to chip around the edge of the primer coating, where it meets bare metal or existing paint rather than producing a smooth edge. Although small, this chipping is sufficient to present a noticeable defect in the final topcoat and is unacceptable in producing high quality spot repairs on cars. We have now discovered a solution to this chipping problem by using primer compositions which contain an aliphatic polyisocyanate having an average isocyanate functionality of 2.5 or more, and a hydroxyl functional polymer, in which the polyisocyanate comprises a proportion of diphenylmethane diisocyanate, also known as MDI
SUMMARY OF THE INVENTION
According to the present invention there is provided coating composition comprising a solution in a volatile organic solvent of;
i) A polyisocyanate, and
ii) a hydroxyl functional polymer,
the polyisocyanate comprising a mixture of an aliphatic polyisocyanate having an average isocyanate functionality of 2.5 or more, and diphenylmethane diisocyanate.
DETAILED DESCRIPTION OF THE INVENTION
The volatile organic solvent can be any solvent which will dissolve the polyisocyanate, and the hydroxyl functional polymer. It can be an aliphatic or aromatic hydrocarbon such as Solvesso 100™, toluene or xylene, an alcohol such as butanol or isopropanol, an ester such as butyl acetate or ethyl acetate, a ketone such as acetone, methyl isobutyl ketone or methyl ethyl ketone, an ether, an ether-alcohol or an ether-ester or a mixture of any of these.
Preferably the compositions contain less than 500 g/l of volatile organic solvent based on the total composition, more preferably less than 480 g/l still more preferably less than 420 g/l and most preferably less than 250 g/l.
Polyisocyanates are compounds having two or more isocyanate groups per molecule. Aliphatic polyisocyanates are well known in the coatings art. The isocyanate groups can be blocked but unblocked isocyanates are preferred.
The aliphatic polyisocyanate has an average isocyanate functionality of 2.5 or more isocyanate groups per molecule. Suitable polyisocyanates include the isocyanurate trimers and allophanates of aliphatic diisocyanates as well as the reaction products of these diisocyanates with polyols. Examples of aliphatic diisocyanates include tetramethylxylene diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate. Polyols are compounds having three or more hydroxyl groups. Suitable polyols include trimethylol propane, glycerol and pentaerythritol. Many such aliphatic polyisocyanates are commercially available, for example under the Desmodur trade mark from Bayer or the Tolonate trade mark from Rhodia.
Diphenylmethane diisocyanate (MDI) is generally sold as a mixture of two isomers, 4,4′- and 2,4-diphenylmethane diisocyanate. MDI is commercially available, for example as Suprasec MDI and MDI-based prepolymers from ICI Polyurethanes. Preferably the polyisocyanate comprises 1 to 99% by weight of MDI, more preferably 1 to 500% and most preferably 20 to 50%.
Polyisocyanates are preferably used in an amount such that the ratio of isocyanate groups on the polyisocyanate to the total number of hydroxyl groups on the hydroxyl functional polymer is in the range 0.7:1 to 3:1, more preferably 1:1 to 3:1.
The hydroxyl functional polymer can be any conventional hydroxyl functional film-forming polymer (including oligomers) known in the coatings art. Preferably the hydroxyl functional polymer has a hydroxyl value of 5 to 500, more preferably 50 to 250.
Conventional film forming polymers include acrylic addition polymers, epoxy polymers, alkyd polymers, polyether polymers or polyester polymers. Preferred hydroxyl functional polymers are polyesters and acrylic addition polymers.
Hydroxyl functional acrylic addition polymers are derived from polymerisable ethylenically unsaturated monomers such as vinyl or acrylic monomers and comprise functional units and structural units. Whenever referred to herein, the term acrylic monomer refers to esters of acrylic or methacrylic acid. The term (meth) acrylate refers to both the acrylate and methacrylate equally and the term (meth) acrylic acid refers to acrylic or methacrylic acid equally.
Functional units are derived from hydroxyl functional vinyl or acrylic monomers. An example of a hydroxyl functional vinyl monomer is vinyl alcohol. Examples of hydroxyl functional acrylic monomers are hydroxy ethyl (meth) acrylate, hydroxy butyl (meth) acrylate and hydroxy propyl (meth) acrylate.
Other examples of suitable hydroxyl functional monomers are the reaction products of glycidyl (meth) acrylate with monocarboxylic acids, such as versatic acid and the reaction product of (meth) acrylic acid with monoepoxy compounds such as Cardura E™ (the glycidyl ester of versatic acid; from Shell).
Structural units are derived from monomers which do not have any functional groups which will react with the crosslinker. Examples of monomers which do not have any functional group which will react with the crosslinker are non-functional vinyl monomers and alkyl esters of (meth) acrylic acid.
Examples of suitable non-functional vinyl monomers are styrene and tertiary butyl styrene, preferably tertiary butyl styrene. Examples of suitable alkyl esters of (meth) acrylic acid are C
1-12
alkyl esters such as methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate, t-butyl (meth) acrylate, n-propyl (meth) acrylate and isobornyl (meth)acrylate.
It is also possible to include certain monomers which carry groups other than hydroxyl groups, such as carboxylic acid groups, amine groups and epoxy groups. An example of a monomer carrying a carboxylic acid group is (meth) acrylic acid An example of a monomer carrying an amine groups is tertiary-butyl aminoethyl (meth) acrylate. An example of a monomer carrying a glycidyl group is glycidyl (meth) acrylate.
It is also possible to react certain functional groups with other compound so as to modify the polymer. For example, acid groups on the polymer can be reacted with glycidyl fun
Fenn David Robert
Lam Peter Kwok Hing
Cameron Erma
Miles Jacques B.
PPG Industries Ohio Inc.
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