Patent Description:
Titanium oxide (TiO<NUM>) is the mostly commonly used opacifying pigment in the paint industry due to its very high refractive index. Nevertheless, TiO<NUM>, is the most expensive component in paint; moreover, its manufacture requires high energy consumption and poses potential environmental hazardous risks. As regulatory agencies around the world are promoting legislation designed to place warning labels on products containing TiO<NUM>, an additional urgency for greatly reducing the concentration of TiO<NUM> in consumer products such as architectural paints has arisen.

Acceptable opacifying performance (hiding) in paints can be achieved in the absence of TiO<NUM>, by substituting TiO<NUM> with large amounts of extender to obtain above-critical pigment volume concentration formulations. However, inasmuch as acceptable opacity is achieved through the creation of air voids arising from insufficient binder to form effective films, the resultant coating suffers from poor scrub resistance.

Although opacifying performance can also be enhanced by addition of opaque polymer particles, the ability of these organic opacifying agents to boost opacity is limited by their inherently lower index of refraction with concomitant inferior coating properties at high concentrations. Consequently, opaque polymer particles are not a substitute for TiO<NUM>; their usage has been limited to an ancillary role to reduce the loading of TiO<NUM> required to achieve acceptable hiding and maintain performance in architectural coatings.

<CIT> discloses a composition comprising an aqueous dispersion of a thickener and composite particles comprising phosphorus acid functionalized polymer particles adsorbed to the surfaces of TiO<NUM> particles, wherein the phosphorus acid functionalized polymer particles have a core-shell morphology wherein the core protuberates from the shell.

<CIT> discloses opaque polymers functionalized with phosphorous acid groups.

<CIT> discloses aqueous formulations comprising at least one film-forming (co)polymer, particles with a core and at least one shell different from the core and optionally at least one hydrophobic agent.

<CIT> discloses an opacifying pigment encapsulated in polymer including a pigment particle, an aminophosphorus acid-functional first polymer having been used to disperse the pigment particle in an aqueous medium and a second polymer that at least partially encapsulates the dispersed pigment particle.

Accordingly, it would be an advance in the art to discover a pigmented coating composition that is substantially free of TiO<NUM> with acceptable hiding and scrub resistance performance.

This invention addresses a need in the art by providing a waterborne composition comprising an aqueous dispersion of first and second multistage polymer particles, wherein each of the first and second polymer particles comprises:.

wherein the second multistage polymer particles further comprise:
c) a polymeric binder layer superposing the shell, which polymeric binder layer has a Tg of not greater than <NUM> and comprises structural units of at least one monoethylenically unsaturated monomer;.

The present invention addresses a need in the art by providing a composition that substantially reduces, and in some instances, eliminates the requirement of TiO<NUM> as an opacifying pigment in paint formulations.

The present invention is a waterborne composition comprising an aqueous dispersion of first and second multistage polymer particles, wherein each of the first and second polymer particles comprises:.

The water-occluded core comprises from <NUM>, preferably from <NUM>, more preferably from <NUM>, and most preferably from <NUM> weight percent, to <NUM>, preferably to <NUM>, more preferably to <NUM>, and most preferably <NUM> weight percent structural units of a salt of a carboxylic acid monomer based on the weight of structural units of monomers in the core.

As used herein, the term "structural units" refers to the remnant of the recited monomer after polymerization. For example, a structural unit of a salt of methacrylic acid, where M+ is a counterion, preferably a lithium, sodium, or potassium counterion, is as illustrated:
<CHM>.

Examples of suitable carboxylic acid monomers include acrylic acid, methacrylic acid, itaconic acid, and maleic acid.

The water-occluded core further comprises from <NUM>, preferably from <NUM>, more preferably from <NUM>, more preferably from <NUM>, and most preferably from <NUM> weight percent to <NUM>, preferably to <NUM>, more preferably to <NUM>, and most preferably to <NUM> weight percent structural units of a nonionic monoethylenically unsaturated monomer based on the weight of structural units of monomers in the core. Examples of nonionic monoethylenically unsaturated monomers include one or more acrylates and/or methacrylates such as methyl acrylate, ethyl acrylate, n-butyl acrylate, t-butyl acrylate <NUM>-ethylhexyl acrylate, methyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, isobornyl methacrylate, lauryl methacrylate, and cyclohexyl methacrylate; and one or more monoethylenically unsaturated aromatic compounds such as styrene, α-methylstyrene, and <NUM>-t-butylstyrene. A preferred nonionic monoethylenically unsaturated monomer is methyl methacrylate.

The polymeric shell of the first and second polymer particles preferably has a Tg in the range of not less than <NUM>, more preferably not less than <NUM>, and most preferably not less than <NUM>, and preferably not greater than <NUM>, and most preferably not greater than <NUM>. As used herein, Tg refers to the glass transition temperature as calculated by the Fox equation.

Preferably, the shells of the first and second polymer particles comprise structural units of methyl methacrylate, styrene, α-methylstyrene, isobornyl methacrylate, lauryl methacrylate, or cyclohexyl methacrylate. In one embodiment, the shell comprises at least <NUM>, more preferably at least <NUM>, and most preferably at least <NUM> weight percent structural units of styrene. In another embodiment, the shell comprises from <NUM> to <NUM> weight percent structural units of styrene and from <NUM> to <NUM> weight percent structural units of any or all of methyl methacrylate (<NUM> to <NUM> weight percent), cyclohexyl methacrylate (<NUM> to <NUM> weight percent), methacrylic acid (<NUM> to <NUM> weight percent), and the multiethylenically unsaturated monomer, allyl methacrylate (ALMA, <NUM> to <NUM> weight percent).

The polymeric shells of the first and second polymer particles may also further comprise structural units of other multiethylenically unsaturated monomers such as divinyl benzene (DVB), trimethylolpropane trimethacrylate (TMPTMA), or trimethylolpropane triacrylate (TMPTA).

As used herein, "polymeric binder" refers to a polymeric material that is film forming on a desired substrate, with or without a coalescent. In one aspect, the Tg of the polymeric binder as calculated by the Fox equation is not greater than <NUM>; in another aspect, not greater than <NUM>, in another aspect, not greater than <NUM>, and in another aspect not less than -<NUM>, and in another aspect not less than -<NUM>.

Examples of suitable polymeric binder materials include acrylic, styrene-acrylic, vinyl esters such as vinyl acetate and vinyl versatates, and vinyl ester-ethylene polymeric binders. Acrylic binders comprising structural units of methyl methacrylate and structural units of one or more acrylates such as methyl acrylate, ethyl acrylate, n-butyl acrylate, or <NUM>-ethylhexyl acrylate, are especially preferred, as are styrene-acrylic binders.

Preferably, the weight-to-weight ratio of structural units of monomers of the core to the shell in the first and second multistage polymer particles is in the range of <NUM>:<NUM> to <NUM>: <NUM>. Preferably, the weight-to-weight ratio of the polymer binder to the sum of the structural units of monomers of the core and the shell in the second multistage polymer particles is in the range of from <NUM>:<NUM>, more preferably from <NUM>:<NUM>, and most preferably from <NUM>:<NUM>, to preferably <NUM>:<NUM>, more preferably to <NUM>:<NUM>, and most preferably to <NUM>:<NUM>.

Preferably, the weight-to-weight ratios of the first multistage polymer particles to the second multistage polymer particles is in the range of from <NUM>:<NUM>, more preferably from <NUM>:<NUM>, more preferably from <NUM>:<NUM>, and most preferably from <NUM>:<NUM>, to preferably <NUM>:<NUM>, more preferably to <NUM>:<NUM>, more preferably from <NUM>:<NUM>, and most preferably to <NUM>:<NUM>.

In one aspect, the z-average particle size of the first polymer particles is preferably in the range of from <NUM> to <NUM>; in another aspect, the z-average particle size of the first polymer particles is preferably in the range of from <NUM>, more preferably from <NUM>, and most preferably from <NUM>, to preferably <NUM>, more preferably to <NUM>, and most preferably to <NUM>. As used herein, z-average particle size refers to particle size as determined by dynamic light scattering, for example by a BI-<NUM> Plus Particle Size Analyzer (Brookhaven).

The z-average particle size of the second polymer particles is in the range of from <NUM>, more preferably from <NUM>, most preferably from <NUM>, to preferably <NUM>, more preferably <NUM>, and most preferably to <NUM>.

The composition of the present invention can be conveniently prepared by mixing an aqueous dispersion of first multistage polymer particles with an aqueous dispersion of second multistage polymer particles. The aqueous dispersion of first multistage polymer particles can be prepared by methods known in the art, for example, as disclosed in <CIT> and <CIT>. Examples of commercially available dispersions of first multistage polymer particles include ROPAQUE™ Ultra Opaque Polymers, AQACell HIDE <NUM> Opaque Polymers, and ROPAQUE™ TH-<NUM> Hollow Sphere Pigments. (ROPAQUE is a Trademark of The Dow Chemical Company or its Affiliates. ) The aqueous dispersion of second multistage polymer particles can be prepared as described in <CIT>. An example of a preferred method of preparing the dispersion of second multistage polymer particles is shown in Intermediate Example of the Example section.

The aqueous dispersion of the first and second multistage polymer particles of the present invention form opaque polymer particles or hollow sphere polymer particles (also known OPs or HSPs) upon application of the dispersion onto a substrate followed by evaporation of the water occluded in the core. As such, the composition of the present invention is useful as opacifiers and binders in paint formulations, especially paint formulations where it is desirable to reduce, and even eliminate the loading of TiO<NUM>. It has surprisingly been discovered that the combination of binder coated opaque polymer particles and non-binder coated opaque polymer particles gives superior hiding and scrub resistance, as compared to a dispersion containing opaque polymer particles and distinct binder particles that do not superpose the opaque polymers.

The composition may include other materials such as rheology modifiers, dispersants, defoamers, surfactants, coalescents, extenders, and inorganic pigments. ZnO is a particularly useful pigment that can be used as a replacement for TiO<NUM> - and a supplement or partial replacement for the first and second multistage polymer particles - in the composition of the present invention. Preferably, the composition of the present invention comprises less than <NUM> weight percent TiO<NUM>. In another aspect, the composition comprises <NUM> weight percent TiO<NUM>.

The hiding performance was characterized by the S/mil as follows. Three draw-downs were prepared using a <NUM>-mil Bird draw down bar and one draw-down was prepared using a 25mil Bird draw down bar for each paint on Black Release Charts. The drawdowns were allowed to dry overnight. Using a template, <NUM>" x <NUM>" rectangles were cut out with an X-ACTO knife on each chart. Five replicated-reflectance measurements were collected using a XRite reflectometer in each of the scribed areas. The y-reflectance was measured in five different areas of the draw down and the average y-reflectance recorded. Kubelka-Munk hiding value S is given by <MAT>.

Where X is the average film thickness of the thin films, R is the average reflectance of the thick film (<NUM> mil) and RB is the average reflectance over black of the thin film (<NUM> mil). X can be calculated from the weight of the film (Wpf), the density (D) of the dry film; and the film area (A). Film area for a <NUM>" x <NUM>" template was <NUM> in<NUM>.

The scrub resistance test was based on the ISO <NUM>. Drawdowns were made on black vinyl scrub charts with a <NUM>-mil Dow applicator in a controlled temperature and humidity room and then dried for <NUM> d. The drawdown charts were weighed before and after the scrub test (and dried overnight) to determine the weight loss on an analytical balance before the scrub test was run. The scrub test was run on a Pacific Scientific Abrasion Tester using <NUM>% DS-<NUM> as scrub media and Scotch Brite <NUM>+ Ultra Fine Hand Pad as the scrub pad. Prior to the test, the scrub media was spread on the coating surface with a soft brush, and the scrub pad was saturated with the scrub media to a final total mass of <NUM>. The scrub test was run for <NUM> cycles, immediately after which the scrubbed panel was rinsed with water. The panel was allowed to dry overnight and the charts were re-weighed. The weight loss was then used to calculate the film thickness loss.

Pigment volume concentrations are calculated by the following formula: <MAT> where binder solids refers either to the contribution of polymer from the styrene-acrylic binder layer of the Intermediate Example, or to binder from Acronal S <NUM> Styrene Acrylic Binder, or both. OP refers to the contribution of the volumes of the first multistage polymer particles and the core: shell portion of the second multistage polymer particles.

In the following Example, Core #<NUM> refers to an aqueous dispersion of polymer particles (<NUM> MMA/<NUM> MAA, solids <NUM>%, z-average particle size of <NUM>) prepared substantially as described in <CIT>.

A <NUM>-liter, four necked round bottom flask was equipped a paddle stirrer, thermometer, N<NUM> inlet and reflux condenser. DI water (<NUM>) was added to the kettle and heated to <NUM> under N<NUM>. Sodium persulfate (NaPS, <NUM> in <NUM> water) was added to vessel immediately followed by Core #<NUM> (<NUM>). Monomer emulsion <NUM> (ME <NUM>), which was prepared by mixing DI water (<NUM>), Disponil FES-<NUM> emulsifier (<NUM>), styrene (<NUM>), methacrylic acid (<NUM>), linseed oil fatty acid (<NUM>), acrylonitrile (<NUM>), and divinyl benzene (<NUM>), was then added to the kettle over <NUM>. The temperature of the reaction mixture was allowed to increase to <NUM> after <NUM> and allowed to increase to <NUM> after <NUM>. Upon completion of the ME <NUM> feed, the reaction was cooled to <NUM>.

When the kettle temperature reached <NUM>. , an aqueous mixture of ferrous sulfate and EDTA (<NUM>, <NUM> wt. % FeSO<NUM>, <NUM> wt. % EDTA) was added to the kettle. When the kettle temperature reached <NUM>, co-feeds including a solution of t-butylhydroperoxide (t-BHP <NUM>) and NaPS (<NUM>) mixed with DI water (<NUM>), along with a separate solution of isoascorbic acid (IAA, <NUM> in <NUM> water) were both added simultaneously to the kettle at a rate of <NUM>/min.

Two min after the charging of the co-feed solutions, ME <NUM>, which was prepared by mixing DI water (<NUM>), Disponil FES-<NUM> emulsifier (<NUM>), butyl acrylate (<NUM>), methyl methacrylate (<NUM>), <NUM>-ethylhexyl acrylate (<NUM>), acetoacetoxyethyl methacrylate (<NUM>) and methacrylic acid (<NUM>), was added to the kettle over <NUM> while allowing the temperature to rise to <NUM> without providing any external heat. Upon completion of ME <NUM> addition, the co-feed solutions were stopped and the batch was held for <NUM> at <NUM>-<NUM>. A solution of NH<NUM>OH (<NUM>, <NUM> wt. ) mixed with DI water (<NUM>) was then added to the kettle along with hot (<NUM>) DI water (<NUM>).

ME <NUM>, which was prepared by mixing DI water (<NUM>), Disponil FES-<NUM> emulsifier (<NUM>), butyl acrylate (<NUM>), methyl methacrylate (<NUM>), and <NUM>-hydroxy TEMPO (<NUM>), was fed to the kettle over <NUM>. Immediately after the ME <NUM> feed addition was complete, NH<NUM>OH (<NUM>, <NUM> wt. ) mixed with DI water (<NUM>) was added to the kettle over <NUM>. When NH<NUM>OH addition was complete, the batch was held for <NUM>. The addition the co-feed solutions was resumed at <NUM>/min until completion, whereupon the dispersion was cooled to <NUM>. While cooling, additional co-feeds including a solution of t-BHP (<NUM>) in DI water (<NUM>), along with a separate solution of IAA (<NUM>) in water (<NUM>) were both added simultaneously to the kettle at a rate of <NUM>/min. Upon completion of addition of the second co-feed, the dispersion was filtered to remove any coagulum. The filtered dispersion had a solids content of <NUM>%. The S/Mil was measured to be <NUM> with collapse of <NUM>%.

Table <NUM> illustrates paint formulations with first and second multistage polymer particles. In the following Table, Opaque Polymer refers to ROPAQUE™ Ultra EF Opaque Polymer (<NUM> wt. % solids), Defoamer refers to Foamstar A34 Defoamer, Coalescent refers to Texanol Coalescent, Thickener refers to Natrosol <NUM> MHR Thickener, ZnO refers to ZOCO <NUM> ZnO, Extender refers to and Dispersant refers to TAMOL ™ <NUM> Dispersant. (TAMOL is a Trademark of The Dow Chemical Company or its Affiliates. In each formulation, the volume solids was <NUM>%.

Tables 2A and 2B illustrates the comparative paint formulations. Binder refers to Acronal S <NUM> Styrene Acrylic Binder (<NUM> wt. % solids), TiO<NUM> refers to Kronos <NUM> TiO<NUM> slurry (<NUM> wt. %) and Extender refers to Omyacarb UF CaCO<NUM> extender.

Claim 1:
A waterborne composition comprising an aqueous dispersion of first and second multistage polymer particles, wherein each of the first and second polymer particles comprises:
a) a water-occluded core comprising from <NUM> to <NUM> weight percent structural units of a salt of a carboxylic acid monomer and from <NUM> to <NUM> weight percent structural units of a nonionic monoethylenically unsaturated monomer; and
b) a polymeric shell having a Tg in the range of from <NUM> and <NUM>;
wherein the second multistage polymer particles further comprise:
c) a polymeric binder layer superposing the shell, which polymeric binder layer has a Tg of not greater than <NUM> and comprises structural units of at least one monoethylenically unsaturated monomer;
wherein the weight-to-weight ratio of structural units of monomers in the water-occluded core to the shell in the first and second multistage polymer particles is in the range of <NUM>:<NUM> to <NUM>:<NUM>;
the weight-to-weight ratio of the polymer binder to the sum of the shell and the structural units of monomers in the core in the second multistage polymer particles is in the range of <NUM>:<NUM> to <NUM>:<NUM>;
the weight-to-weight ratio of the first multistage polymer particles to the second multistage polymer particles is in the range of from <NUM>:<NUM> to <NUM>:<NUM>;
the z-average particle size of the first polymer particles is in the range of from <NUM> to <NUM>; and
the z-average particle size of the second polymer particles is in the range of from <NUM> to <NUM>.