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
2000-06-28
2002-04-16
Wu, David W. (Department: 1713)
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...
C524S461000, C524S513000, C524S539000, C524S566000, C524S923000, C525S007000, C525S007100, C525S069000, C525S167500
Reexamination Certificate
active
06372840
ABSTRACT:
BACKGROUND OF THE INVENTION
Dispersed polymeric microparticles produced by non-aqueous dispersion techniques are well known. The polymer within the microparticles may be crosslinked, in which case they are sometimes referred to as microgel, and these crosslinked microparticles find use as rheology control agents in coating compositions, particularly for automobile coatings. The presence of the crosslinked microparticles reduces the tendency of wet coatings to sag prior to cure and, when the coating includes metallic pigments, has a beneficial effect on the alignment of the pigment. Both of these effects have the end result of producing a finish that is more attractive and is substantially free of defects.
Methods for preparing crosslinked polymeric microparticles are known in the art, an example of which is disclosed in U.S. Pat. No. 4,147,688 (Makhlouf et al.). Conventionally, the microparticles are prepared by free radical addition polymerization of monoethylenically unsaturated monomers in a hydrocarbon dispersion medium, in the presence of a steric stabilizer polymer. The steric stabilizer is typically characterized by a polymer in which one portion affiliates itself with the monomers and their polymerization product and another portion affiliates itself with the hydrocarbon dispersion medium. Since the hydrocarbon dispersion medium is often a non-polar, liquid aliphatic hydrocarbon, steric stabilizers often include a segment or side chains that are solvated by such non-polar liquids. Thus, in the aforesaid U.S. Pat. No. 4,147,688, the preferred steric stabilizer comprises a comb-type polymer having an acrylic backbone with polyester side chains. These polyester side chains are poly(12-hydroxystearic acid) reacted with glycidyl methacrylate, for example, to provide unsaturation for copolymerization into the acrylic backbone.
In order to provide greater versatility to non-aqueous dispersions, the prior art has suggested using combinations of different steric stabilizers. For example, U.S. Pat. No. 5,025,060 (Yabuta et al.) discloses a stabilizer based on poly(12-hydroxystearic acid) used in combination with another stabilizer which is an acrylic polymer with unsaturated side chains or terminal groups. In another example, U.S. Pat. No. 5,468,801 (Antonelli et al.) also discloses a stabilizer based on poly(12-hydroxystearic acid) used in combination with a second stabilizer which is an acrylic macromolecule grafted onto the microparticle core polymer by means of terminal unsaturation.
A non-aqueous dispersion using an alkyd type of steric stabilizer is disclosed in U.S. Pat. No. 5,516,820 (Babjak et al.). The alkyd is made from a vegetable oil, pentaerythritol, maleic anhydride, and phthalic anhydride, and would be expected to have a branched polyester structure.
In some coating formulations in which microparticles are used for rheology control, conditions are encountered that are more polar than is optimum for the functioning of conventional steric stabilizers. When these conditions are encountered, dispersions relying on conventional steric stabilizers such as those based on poly(12-hydroxystearic acid) can become de-stabilized. In that case, microparticles that come out of dispersion create the visible coating defect known as “seeds.” It would be desirable to provide enhanced stabilization of microparticles in these situations.
It has also been found that the effectiveness of stabilizers based on poly(12-hydroxystearic acid) sometimes varies. It has been discovered that this problem is attributable to variations in the poly(12-hydroxystearic acid) itself as received from the supplier.
SUMMARY OF THE INVENTION
The present invention provides dual stabilization to microparticle dispersions, that is, a combination of two different steric stabilizers is used. One stabilizer may be the conventional acrylic comb type with side chains that are solvatable in the hydrocarbon dispersing medium. These side chains may comprise the commonly used poly(12-hydroxystearic acid). The other stabilizer is a hyper-branched polyester having hydrophobic side chains and a polar backbone. This combination has been found useful for enhancing microparticle stability in coating formulations having relatively polar constituents. As a result, coating formulations may include these polar constituents as dictated by other performance requirements without increasing the incidence of seeding caused by microparticle destabilization. Additionally, it has been discovered that the combination of stabilizers of the present invention masks the variations that occur in poly(12-hydroxystearic acid) from suppliers.
DETAILED DESCRIPTION
The basic process of forming gelled microparticles in the present invention may be essentially the same as that described in U.S. Pat. No. 4,147,688, a process which comprises the free radical addition copolymerization of alpha, beta-ethylenically unsaturated monocarboxylic acid, at least one other copolymerizable monoethylenically unsaturated monomer and a crosslinking monomer selected from the group consisting of (1) epoxy group-containing compound and (2) a mixture of alkylenimine and organoalkoxysilane. This free radical polymerization takes place in the presence of the polymeric steric stabilizers and the hydrocarbon dispersing liquid. The dispersing liquid is selected to be a solvent to the monomers but not to the cross-linked acrylic polymer particles. The reaction is carried out at elevated temperatures such that the dispersion polymer first forms and then is crosslinked; usually at temperatures between about 50° C. and 150° C.
Examples of alpha, beta-ethylenically unsaturated monocarboxylic acid which may be used are acrylic acid, methacrylic acid, ethacrylic acid, alpha-chloroacrylic acid, crotonic acid isocrotonic acid, tiglic acid and angelic acid. The preferred alpha, beta-ethylenically unsaturated monocarboxylic acids are acrylic acid and methacrylic acid. Methacrylic acid is especially preferred. The amount of alpha, beta-ethylenically unsaturated monocarboxylic acid employed in the process of the invention is usually in the range of from about 0.5 percent to about 15 percent by weight of the monomers used in the copolymerization process.
Various other monoethylenically unsaturated monomers may be copolymerized with the acid monomer in the process of this invention. Although essentially any copolymerizable monoethylenic monomer may be utilized, depending upon the properties desired, the preferred monoethylenically-unsaturated monomers are the alkyl esters of acrylic or methacrylic acid, particularly those having from about 1 to about 4 atoms in the alkyl group. Illustrative of such compounds are the alkyl acrylates, such as methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate and the alkyl methacrylates, such as methyl methacrylate, ethyl methacrylate, propyl methacrylate and butyl methacrylate. Other ethylenically unsaturated monomers which may advantageously be employed include, for example, the vinyl aromatic hydrocarbons, such as styrene, alpha-methyl styrene, vinyl toluene, unsaturated esters of organic and inorganic acids, such as vinyl acetate, vinyl chloride and the like, and the unsaturated nitriles, such as acrylonitrile, methacrylonitrile, ethacrylonitrile, and the like. From about 70 percent to about 99 percent by weight of such monoethylenically unsaturated monomers, based on the weight of monomer solids can be utilized.
Crosslinking can be provided in the microparticle polymer by including a crosslinking monomer selected from the group consisting of (1) epoxy group-containing compounds and (2) mixtures of alkylenimine and organoalkoxysilane. A particularly preferred class of epoxy group-containing compounds that may be utilized for crosslinking are monoepoxide compounds that additionally contain ethylenic unsaturation. Illustrative of such compounds are, for example, glycidyl acrylate and glycidyl methacrylate.
For disclosure regarding the various alkylenimines and organoalkoxysilanes that can be utilized as crosslinkers, reference may be
Mayo Michael A.
Shalati Mohamad D.
Steppan Susana C.
Altman Deborah M.
Egwim Kelechi C.
Millman Dennis G.
PPG Industries Ohio Inc.
Wu David W.
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