EMPTY LATEX PARTICLES.

Abstract

A coating composition comprising hollow latex particles, wherein each of said hollow latex particles has a hollow interior and comprises: optionally a core comprising a hydrophilic component, at least one intermediate shell comprising, as polymerized units, one or more hydrophilic monoethylenically unsaturated monomers, one or more nonionic monoethylenically unsaturated monomers, or mixtures thereof, and an outer shell comprising a polymer having a Tg of at least 60 °C, and containing one or more functional groups selected from the group consisting of 1,3-dikete, amino, ureide, urea, hydroxyl, polyether, silane, phosphate, epoxy, fluorocarbon, aldehyde, ketone, acetoacetyl, or combinations thereof; wherein the hollow latex particles are non-film-forming and opaque, and wherein the composition further comprises film-forming latex particles.

Description

This application relates to latex particles and emulsion polymerization processes for producing such particles. In particular, this application relates to aqueous emulsion polymerization processes for preparing "hollow" or "empty" latex particles and the latex particles prepared therefrom, which are useful as non-film-forming opacifiers. BACKGROUND OF THE INVENTION Paints and coatings play an important role in preserving, protecting, and beautifying the objects to which they are applied. Architectural paints are used to decorate and extend the lifespan of interior and exterior surfaces of residential and commercial buildings. "Hollow latex" (i.e., empty latex particles) has been developed that is non-film-forming for use as opacifiers in paints and other coatings. As such, it is typically used as a complete or partial replacement for other opacifiers such as titanium dioxide. Known processes for preparing hollow polymer particles that include a separate swelling stage occurring after the polymerization of the core and shell layers, or between the formation of the shell layers, require specific timing coordination of those stages, which is difficult to achieve at a commercial plant scale. If the timing of these stages is not ideal, the final particle geometric structures are compromised. Poor timing coordination of these stages can result in shell thickness, void diameter, particle size, and particle morphology (such as the formation of penetrating pores) that produce undesirable product performance. BRIEF DESCRIPTION OF THE INVENTION The present invention provides a process for forming empty latex particles, wherein the process includes contacting multi-stage emulsion polymer particles comprising a core, at least one intermediate shell, with a swelling agent, and polymerizing an outer shell after said contact with the swelling agent, wherein: the core comprises a hydrophilic component; qj i / ηη / ζζηζ / E / γίΛΐ the at least one intermediate shell comprises, as polymerized units, one or more hydrophilic monoethylenically unsaturated monomers, one or more non-ionic monoethylenically unsaturated monomers, or mixtures thereof, the outer shell comprises a polymer having a Tg of at least 602C, the core and the at least one intermediate shell are brought into contact with the swelling agent in the presence of less than 0.5% monomer based on the weight of the polymer particles in a multi-stage emulsion, and; substantially all the swelling occurs during the polymerization of the outer shell. DESCRIPTION OF THE FIGURES Figure 1 schematically illustrates an example process that can be used to obtain polymer particles in a multi-stage emulsion. Figure 2 is a typical scanning transmission electron (STEM) micrograph of particles prepared according to the process described in this document. DETAILED DESCRIPTION OF THE INVENTION The process of the present invention avoids the timing coordination problems associated with the separate swelling and polymerization stages of the outer shell by adding a swelling agent with less than 0.5% monomer present in the emulsion, and then adding the outer shell monomer in such a way that substantially all the swelling occurs during the polymerization of the outer shell. The inventors were thus unexpectedly able to perform the swelling during the polymerization of the outer shell, thereby avoiding the timing coordination problems discussed above, while obtaining empty latex particles with desirable performance characteristics. Furthermore, the inventors were able to use more environmentally friendly swelling agents than those emitting volatiles such as ammonia. The empty latex particles prepared by the process of the present invention can be characterized as non-film-forming. "Non-film-forming" means that the empty latex particles will not form a film at or below ambient temperature, or, in other words, will only form a film at temperatures above ambient temperature. For the purposes of this specification, ambient temperature is taken to be in the range of 152°C to 452°C. Thus, for example, when incorporated into an aqueous coating composition, applied at a temperature of qj / / nn / zznz / E / YiAi to the substrate, and dried or cured at or below ambient temperature, the empty latex particles do not form a film. The empty latex particles generally remain as discrete particles in the dried or cured coating.Empty latex particles are capable of functioning as opacifiers; that is, when added in sufficient quantity to a coating composition that would otherwise be transparent upon drying, they render the dried coating composition opaque. The term “opaque” indicates that the refractive index of a coating composition is higher when the empty latex particles of the present invention are present in the coating composition compared to the same coating composition without the empty latex particles of the present invention, where the refractive index is measured after the coatings are touch-dry. The term “outer shell polymer” refers to the outer layer of the particle of the present invention after swelling. The empty latex particles prepared by the process of the invention generally comprise a hollow interior and an outer shell enclosing the hollow interior, although, as will be explained in more detail later, one or more additional layers may be present between the outer shell and the interior void of each particle. Generally speaking, the empty latex particles may have a diameter of at least 200 nm, at least 250 nm, at least 300 nm, at least 350 nm, or at least 400 nm and a diameter of no more than 1200 nm, no more than 1000 nm, no more than 700 nm, no more than 650 nm, no more than 600 nm, no more than 550 nm, or no more than 500 nm. The hollow interior generally has a diameter of at least 100 nm, at least 150 nm, or at least 200 nm, but is normally no more than 1000 nm, no more than 800 nm, no more than 600 nm, no more than 500 nm, or no more than 400 nm in diameter.The thickness of the layers surrounding the hollow interior, including the outer shell and any additional layers present, is typically 30 to 120 nm. In some embodiments, the particles may have a diameter greater than 1200 nm with a hollow interior diameter greater than 1000 nm. Typically, the empty latex particles will be approximately spherical, although oblong, oval, teardrop, or other shapes are also possible. Particles with penetrating pores are undesirable and do not occur in any substantial quantity (e.g., less than 0.5% of particles on average) when the process disclosed herein is implemented. The dimensions and morphology of the particles are determined by examining STEM images.The percentage of particles with penetrating pores, i.e., those with large pores visible in STEM images connecting the hollow core to the external surface of the multi-stage emulsion polymer particles, is determined by counting the particles with penetrating pores (if any) as visualized in STEM images as a percentage of the total number of particles in a representative sample. The process of the present invention includes a multi-stage emulsion polymerization process. The process includes the formation of a core comprising a polymer of at least one hydrophilic monoethylene unsaturated monomer, at least one intermediate shell, and an outer shell comprising an outer shell polymer. The multi-stage emulsion polymer particles can be brought into contact with a swelling agent, such as a base, capable of swelling the core, particularly in the presence of water. Unlike previously known processes, the process of the present invention combines swelling with the polymerization of the outer shell. This is accomplished by adding a swelling agent in the presence of less than 0.5% monomer based on the weight of the multi-stage emulsion polymer particles, and provided that substantially all the swelling occurs during the polymerization of the outer shell.The use of the term “substantially all swelling occurs during outer shell polymerization” indicates that most of the swelling occurs during outer shell polymerization and that little or no swelling occurs during the addition of the swelling agent in the presence of less than 0.5% monomer based on the weight of the polymer particles in the multi-stage emulsion. In some embodiments, less than 10% or less than 5% of the swelling will occur during the addition of the swelling agent, with the remainder occurring during outer shell polymerization.The percentage of swelling that occurs during the formation of the outer shell compared to the addition of the swelling agent is determined by comparing the average size of the hollow cores, as observed in STEM images of the multi-stage emulsion polymer particles obtained after the addition of the swelling agent, with the size of the hollow cores of the multi-stage emulsion polymer particles obtained after the addition of the outer layer. In some embodiments, the swelling agent may be added before the formation of an intermediate layer, and swelling may occur during the formation of the intermediate layer, with an outer layer being added after swelling. A monomer level of less than 0.5% monomer can be achieved during the addition of the swelling agent by adding a sufficient amount of polymerization initiator before contact with the swelling agent to reduce the amount of monomer present during contact with the swelling agent to less than 0.5% monomer based on the weight of the polymer particles in the multi-stage emulsion. Other polymerization induction methods may also be used. Swelling is substantially avoided during the addition of the swelling agent and until the outer shell monomer is added by selecting an intermediate layer and swelling agent combination that minimizes penetration of the swelling agent into the core of the polymer particles in the multi-stage emulsion.For example, one or more intermediate layers can be crosslinked by adding a crosslinking agent, and a swelling agent containing sodium can be used. Polymerization inhibitors are preferably avoided. The swollen core causes the intermediate and outer shells to expand, so that when the polymer particles are subsequently dried and / or re-acidified, the shells remain enlarged in volume, creating a void within the particle as a result of the shrinkage of the swollen core. Each empty latex particle may contain a single void. However, in other embodiments of the invention, individual empty latex particles may contain a plurality of voids (for example, an empty latex particle may contain two or more voids within the particle). The voids may be interconnected by pores or other passageways. The voids may be substantially spherical in shape, but may also take other forms such as empty channels, interpenetrating networks of void and polymer, or sponge-like structures. The process of the present invention can be carried out using a batch process where the product of one step is used in the following step. For example, the product of the core step can be used to prepare the product of the next step, whether it be an outer shell step or an intermediate encapsulation polymer step. Similarly, the shell step is prepared from the product of the core step or, when there are one or more encapsulation polymer steps, from an intermediate encapsulation polymer step. The core component of polymer particles in a multi-stage emulsion is generally located at or near the center of such particles. However, in one embodiment, the core may coat and surround a seed comprising a polymer different from the polymer used to prepare the core. In this embodiment, for example, the seed may comprise a polymer that is non-hydrophilic; that is, the seed polymer may be a homopolymer or copolymer of one or more non-ionic, monoethylenically unsaturated monomers such as methyl methacrylate. In one embodiment, the seed polymer is a methyl methacrylate homopolymer that is resistant to swelling by the swelling agent used to swell the core. The seed typically has a particle size of approximately 30 to approximately 200 nm or approximately 50 to approximately 100 nm.To form the core, the seed can be coated with another polymer comprising at least one hydrophilic monoethylenically unsaturated monomer, optionally in combination with at least one non-hydrophilic monoethylenically unsaturated monomer such as an alkyl (meth)acrylate and / or an aromatic vinyl monomer. However, sufficient hydrophilic monoethylenically unsaturated monomer must be used so that the resulting polymer is capable of being swollen with a swelling agent such as an aqueous base. In one embodiment, for example, the polymer used to coat the seed and provide the core component is a copolymer of methyl methacrylate and methacrylic acid, the methacrylic acid content of the copolymer being approximately 30 to approximately 60 percent by weight. The core comprises a hydrophilic component that provides a degree of swelling sufficient for the formation of voids or gaps. In some embodiments, the hydrophilic component is provided as a hydrophilic monomer used to prepare the core polymer (i.e., a polymer used to obtain the core includes polymerized units of a hydrophilic monomer in an amount effective to convert the core polymer into a hydrophilic one). In other embodiments, the hydrophilic component is an additive to the core (e.g., the hydrophilic component may be blended with a non-hydrophilic polymer). In further embodiments, the hydrophilic component is present both as an additive incorporated into the core and as a hydrophilic polymer that is part of the core. In some embodiments, the hydrophilic component is an acid-containing monomer or additive, such as a monomer or additive bearing carboxylic acid functional groups. In some embodiments, one or more of the polymers used to prepare the core can be converted into a swellable component after the polymer has already been prepared. For example, a polymer containing vinyl acetate units can be hydrolyzed to form a core polymer containing sufficient hydroxyl groups to make the polymer swellable. The hydrophilic component of the core can be provided by polymerization or copolymerization of one or more monoethylenically unsaturated monomers bearing a hydrophilic functional group such as a carboxylic acid group or some other type of ionizable functional group. In some embodiments, such a monoethylenically unsaturated monomer is copolymerized with at least one nonionic monoethylenically unsaturated monomer. Examples of hydrophilic monoethylenically unsaturated monomers useful for preparing the core polymer include monoethylenically unsaturated monomers containing acid functionality, such as monomers containing at least one carboxylic acid group, including acrylic acid, methacrylic acid, acryloxypropionic acid, methacryloxypropionic acid, itaconic acid, aconitic acid, maleic acid or anhydride, fumaric acid, crotonic acid, monomethyl maleate, monomethyl fumarate, monomethyl itaconate, and the like. In certain embodiments, the hydrophilic monoethylenically unsaturated monomer is acrylic acid or methacrylic acid. Examples of hydrophilic non-polymeric components that may be present in the core include compounds containing one or more carboxylic acid groups such as aliphatic or aromatic monocarboxylic and dicarboxylic acids, such as benzoic acid, m-toluic acid, p-chlorobenzoic acid, o-acetoxybenzoic acid, azelaic acid, sebacic acid, octanoic acid, cyclohexanecarboxylic acid, lauric acid, and monobutyl phthalate, and the like. The hydrophilic monoethylenically unsaturated monomer may be present in the core polymer in amounts of, as polymerized units, approximately 5 to approximately 80, approximately 10 to approximately 80, approximately 20 to approximately 80, approximately 30 to approximately 70, approximately 30 to approximately 60, approximately 40 to approximately 60, or approximately 30 to approximately 50 percent by weight, based on the weight of the core polymer. The core polymer may additionally contain recurring units derived from non-ionic monomers. Examples of nonionic monomers that may be present in polymerized form in the swellable core polymer include aromatic vinyl monomers such as styrene, α-methylstyrene, p-methylstyrene, t-butylstyrene or vinyltoluene, olefins such as ethylene, vinyl acetate, vinyl chloride, vinylidene chloride, (meth)acrylonitrile, (meth)acrylamide, (C1-C20) alkyl or (C3-C20) alkenyl esters of (meth)acrylic acid, such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, benzyl (meth)acrylate, lauryl (meth)acrylate, oleyl (meth)acrylate, palmyl (meth)acrylate, stearyl (meth)acrylate and the like. The core polymer may also contain polyethylene unsaturated monomers in amounts, as polymerized units, from 0.1 to 20 percent. Examples of suitable polyethylene unsaturated monomers include co-monomers containing at least two polymerizable vinylidene groups, such as α,β-ethylenically unsaturated monocarboxylic acid esters of polyhydroxylated alcohols containing 2–6 ester groups. Such co-monomers include alkylene glycol diacrylates and dimethacrylates, such as, for example, ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycol diacrylate, propylene glycol diacrylate, and triethylene glycol dimethylacrylate; 1,3-glycerol dimethacrylate; 1,1,1-trimethylolpropane dimethacrylate; 1,1,1trimethylolethane diacrylate; pentaerythritol trimethacrylate; 1,2,6-hexanetriacrylate; sorbitol pentamethacrylate;methylene-bis-acrylamide, methylene-bis-methacrylamide, divinylbenzene, vinyl methacrylate, vinyl crotonate, vinyl acrylate, vinylacetylene, trivinylbenzene, trialyl cyanurate, divinylacetylene, divinylethane, divinyl sulfide, divinyl ether, divinylsulfone, diallylcyanamide, ethylene glycol divinyl ether, diallyl phthalate, divinyldimethylsilane, glycerol trivinyl ether, divinyl adipate; dicyclopentenyl (meth)acrylates; dicyclopentenyloxy (meth)acrylates; unsaturated esters of monodicyclopentenyl glycol ethers; allylic esters of α,β-unsaturated mono- and dicarboxylic acids having terminal ethylenic unsaturation, including allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl itaconate, and the like. Multi-stage emulsion polymer particles may contain one or more layers of intermediate encapsulating polymer. The intermediate encapsulating polymers partially or completely encapsulate the core. Each encapsulating polymer layer may be partially or completely encapsulated by another encapsulating polymer layer. Each encapsulating polymer layer may be prepared by performing emulsion polymerization in the presence of the core or a core encapsulated by one or more encapsulating polymers. The intermediate encapsulating polymer layer may function as a compatibilizing layer, sometimes called a bonding or connecting shell layer, between other layers of the multi-stage emulsion polymer particles; for example, an intermediate encapsulating polymer layer may help adhere the outer shell to the core.An intermediate encapsulation polymer layer can also be used to modify certain characteristics of the final empty latex particles. At least one encapsulation polymer intermediate may contain, as polymerized units, one or more hydrophilic monoethylenic unsaturated monomers and one or more nonionic monoethylenic unsaturated monomers. The hydrophilic monoethylenic unsaturated monomers and the nonionic monoethylenic unsaturated monomers useful for making the core are also useful for making such an encapsulation polymer intermediate. Generally, however, the encapsulation polymer intermediate contains a lower proportion of hydrophilic monomer than the core polymer, so that the encapsulation polymer intermediate swells less when it comes into contact with the swelling agent. Other encapsulation polymer intermediates may contain, as polymerized units, nonionic monoethylenic unsaturated monomer and little or none (e.g.,less than 5% by weight) of hydrophilic monoethylenically unsaturated monomer. Intermediate encapsulation polymers may further include crosslinking agents such as alkylene glycol diacrylates and dimethacrylates, such as, for example, ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycol diacrylate, propylene glycol diacrylate, and triethylene glycol dimethylacrylate; 1,3-glycerol dimethacrylate; 1,1,1-trimethylolpropane dimethacrylate; 1,1,1-trimethylolethane diacrylate; pentaerythritol trimethacrylate; 1,2,6-hexanetriacrylate; sorbitol pentamethacrylate; methylenebis-acrylamide, methylene-bis-methacrylamide, divinylbenzene, vinyl methacrylate, vinyl crotonate, vinyl acrylate, vinylcetylene, trivinylbenzene, trialyl cyanurate, divinylacetylene, divinylethane, divinyl sulfide, divinyl ether, divinylsulfone, diallylcyanamide, ethylene glycol divinyl ether, diallyl phthalatedivinyldimethylsilane, glycerol trivinyl ether, divinyl adipate; dicyclopentenyl (meth)acrylates; dicyclopentenyloxy (meth)acrylates; unsaturated esters of monodicyclopentenyl glycol ethers; allylic esters of α,β-unsaturated mono- and dicarboxylic acids having terminal ethylenic unsaturation, including allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl itaconate, and the like. The outer shell is polymeric and may, for example, be composed of a thermoplastic polymer. The outer shell polymer has a glass transition temperature (Tg) above room temperature, typically at least 60°C, at least 70°C, at least 80°C, or at least approximately 90°C. The Tg of the outer shell polymer may be, for example, from 60°C to 140°C. Although the outer shell polymer may be a homopolymer, it is more typically a copolymer comprising polymerized recurrent units of two or more different monomers, especially ethylenically unsaturated monomers such as those capable of being polymerized by free-radical polymerization. The outer shell polymer is further characterized by carrying one or more different types of functional groups, particularly reactive, polar, chelating, and / or heteroatom-containing functional groups.These functional groups can be varied and selected as desired to modify certain characteristics of the empty latex particles, such as wet adhesion, scrub resistance (washability), stain resistance, solvent resistance, and blocking resistance properties of a coating composition that includes the empty latex particles. For example, functional groups can be selected from 1,3-dikete, amino, ureide, and urea functional groups, and combinations thereof. Suitable 1,3-dikete functional groups include acetoacetate functional groups, which can correspond to the general structure -OC(=O)CH2C(=O)CH3. Suitable amino functional groups include primary, secondary, and tertiary amine groups. The amino functional group can be present in the form of a heterocyclic ring. The amino functional group can, for example, be an oxazoline ring.Other types of functional groups useful in the present invention include, for example, hydroxyl (-OH), silane (for example, trialkoxysilyl, -Si(OH)3), phosphate (for example, PO3H and salts thereof), fluorocarbon (for example, perfluoroalkyl such as trifluoromethyl), polyether (for example, polyoxyethylene, polyoxypropylene), and epoxy (for example, glycidyl). In one embodiment, the functional group contains a Lewis base such as the nitrogen atom of an amine. In another embodiment, the functional group contains a hydroxyl functional group. The functional group may be reactive; for example, the functional group may be capable of reacting as an electrophile or a nucleophile. The functional group, or a combination of functional groups in close proximity to one another, may be capable of complexation or chelation. Functional groups can be introduced into the outer shell polymer by various means. In one embodiment, the functional groups are introduced into the outer shell polymer during polymer formation, for example, by polymerization of one or more polymerizable monomers bearing the desired functional groups (hereinafter referred to as “functionalized monomers”). Such polymerization can be carried out as a copolymerization in which one or more functionalized monomers are copolymerized with one or more non-functionalized monomers. Monomers bearing the functional groups described herein can be added at any stage in the preparation of the multi-stage emulsion, provided that the polymers bearing such functional groups reside at least partially or completely within the outer shell polymer of the particles after swelling. For example, the outer shell polymer may be a copolymer of an aromatic vinyl monomer (e.g., styrene) and an ethylenically unsaturated, free-radical polymerizable monomer containing a functional group such as a 1,3-dikete, amino, ureide, urea, hydroxyl, silane, fluorocarbon, aldehyde, ketone, phosphate, or polyether group. The copolymer may contain one or more additional types of comonomers, such as alkyl (meth)acrylates (e.g., methyl methacrylate). The proportions of different monomers may be varied as desired to impart certain characteristics to the resulting outer shell polymer. Typically, the copolymer contains 0.1 to 10 wt% of ethylenically unsaturated, free-radical polymerizable monomer(s) containing the functional group(s). Such a copolymer may further comprise 80-99.9% by weight of an aromatic vinyl monomer such as styrene and 0-10% by weight (e.g., 0.1-10% by weight) of an alkyl (meth)acrylate such as methyl methacrylate. The ethylenically unsaturated, free-radical polymerizable monomer may contain a (meth)acrylate group (i.e., acrylate or methacrylate) or a (meth)acrylamide group (i.e., acrylamide or methacrylamide). Such (meth)acrylate and (meth)acrylamide groups are capable of participating in free-radical copolymerization with the aromatic vinyl monomer. Allylic groups may also be used to provide a polymerizable site of unsaturation. qj i / nn / zznz / E / YiAi Imidazolidinone (meth)acrylic monomers such as 2-(2-oxo-1-imidazolidinyl)ethyl (meth)acrylates and N-(2-(2-oxo-1-imidazolidinyl)ethyl (meth)acrylamides can be used as, for example, comonomers. Other suitable ethylenically unsaturated, free-radical polymerizable monomers containing functional groups useful in the practice of the present invention include, without limitation, acetoacetoxy (meth)acrylates (e.g., acetoacetoxyethyl methacrylate, AAEM), allyl acetoacetate, derivatized methacrylamides such as methyloxalated diacetone (meth)acrylamides, aminoalkyl (meth)acrylates (including dialkyl and monoalkyl aminoethyl (meth)acrylates), and polymerizable aziridinyl monomers. ethylenically unsaturated (such as those described, for example, in U.S. Patent No. Q3,719,646,(incorporated herein by reference in its entirety for all purposes). Other suitable ethylenically unsaturated free-radical polymerizable monomers containing useful functional groups include hydroethylethylene methacrylate (HEEUMA) and aminoethylethylene methacrylate (AEEUMA). The ethylenically unsaturated free-radical polymerizable monomer may contain a plurality of functional groups in each monomer molecule; for example, the monomer may carry two or more urea and / or ureide groups per molecule, such as the compounds described in U.S. Patent No. 6,166,220 (incorporated herein by reference in its entirety for all purposes). Illustrative examples of particular ethylenically unsaturated free-radical polymerizable monomers suitable for use in the present invention in functionalized monomers include, but are not limited to, acrylate and aminoethyl methacrylate,acrylate and dimethylaminopropyl methacrylate, 3-dimethylamino-2,2-dimethylpropyl-1-acrylate and methacrylate, 2-N-morpholinoethyl acrylate and methacrylate, 2-N-piperidinoethyl acrylate and methacrylate, N-(3-dimethylaminopropyl)acrylamide and methacrylamide, N-(3-dimethylaminomethylacrylamide and methacrylamide, N-(4-morpholinomethyl)acrylamide and methacrylamide, vinylimidazole, vinylpyrrolidone, N-(2-metachromyloxyethyl)ethylenurea N-(2-methacryloxyacetamidoethyl)-N,allylalkylethyleneurea, N-methacrylamidomethylurea, N-methacryloylurea, 2-(1-imidazolyl)ethyl methacrylate, 2-(1-imidazolidine-2-one)ethyl methacrylate, N-(methacrylamido)ethylurea, glycidyl (meth)acrylates, hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylates, gamma-(meth)acryloxypropyltrilacoxyslanes, N,N-dimethyl(meth)acrylamides, diacetone(meth)acrylamides, ethylene glycol (meth)acrylate phosphates, polyethylene glycol (meth)acrylates,polyethylene glycol methyl ether (meth)acrylates, diethylene glycol (meth)acrylates and combinations thereof. In another embodiment of the invention, a precursor polymer is first prepared and then reacted to introduce the desired functional groups, thereby providing the outer shell polymer. For example, amine functional groups qj i / nn / zznz / E / YiAi can be introduced into the outer shell polymer by reacting a precursor polymer bearing carboxylic acid groups with an aziridine. In this example, the precursor polymer can be a polymer prepared by polymerizing an ethylenically unsaturated carboxylic acid such as (meth)acrylic acid, optionally together with other monomers such as alkyl (meth)acrylates and / or aromatic vinyl monomers (e.g., styrene). Suitable free radical initiators for the polymerization of the monomers used to prepare the multi-stage emulsion polymer particles may be any water-soluble initiator suitable for aqueous emulsion polymerization. Examples of free radical initiators suitable for the preparation of the multi-stage emulsion polymer particles of this application include hydrogen peroxide, tere-butyl peroxide, alkali metal persulfates such as sodium, potassium, and lithium persulfate, ammonium persulfate, and mixtures of such initiators with a reducing agent. The amount of initiator may be, for example, from 0.01 to 3 percent by weight, based on the total amount of monomer. In some embodiments, a redox initiator is used in the polymerization system. In a redox free-radical initiation system, a reducing agent can be used in conjunction with an oxidizing agent. Suitable reducing agents for aqueous emulsion polymerization include sulfites (e.g., alkali metal metabisulfite, hydrosulfite, and hyposulfite). In some embodiments, sugars (such as ascorbic acid and isoascorbic acid or an alkali metal (iso)ascorbate salt) could also be suitable reducing agents for aqueous emulsion polymerization. In a redox system, the amount of reducing agent can be, for example, from 0.01 to 3 percent by weight based on the total amount of monomer. Oxidizing agents include, for example, hydrogen peroxide and persulfates, perborates, peracetates, ammonium or alkali metal peroxides and percarbonates, and a water-insoluble oxidizing agent such as, for example, benzoyl peroxide, lauryl peroxide, t-butyl peroxide, t-butyl hydroperoxide, 2,2'-azobisisobutyronitrile, t-amyl hydroperoxide, t-butyl peroxyneodecanoate, and t-butyl peroxypivalate. The amount of oxidizing agent can be, for example, from 0.01 to 3 percent by weight, based on the total amount of monomer. The temperature for free-radical polymerization is typically in the range of approximately 102°C to 100°C. For persulfate systems, the temperature can range from approximately 602°C to approximately 1002°C. In redox systems, the temperature can range from approximately 30°C to approximately 100°C, from approximately 30°C to approximately 60°C, or from approximately 30°C to approximately 45°C. The type and quantity of initiator can be the same or different at the various stages of multi-stage polymerization. One or more emulsifiers, or non-ionic or ionic surfactants (e.g., cationic, anionic), may be used alone or together during polymerization in order to emulsify the monomers and / or to keep the resulting polymer particles in a dispersed or emulsified form. Examples of suitable nonionic emulsifiers include tert-octylphenoxyethylpoly(39)-ethoxyethanol, dodecyloxypoly(10)ethoxyethanol, nonylphenoxyethylpoly(40)ethoxyethanol, polyethylene glycol 2000 monooleate, ethoxylated castor oil, fluorinated alkyl esters and alkoxylates, polyoxyethylene(20)-sorbitan monolaurate, sucrose monocotate, di(2-butyl)phenoxypoly(20)ethoxyethanol, hydroxyethylcellulose-polybutyl acrylate graft copolymer, dimethylsilicon-poly(alkylene oxide) graft copolymer, poly(ethylene oxide)-poly(butyl acrylate) block copolymer, and propylene oxide-ethylene oxide block copolymers. 2,4,7,9-tetramethyl-5-decyno-4,7-diol ethoxylated with 30 moles of ethylene oxide, N-polyoxyethylene(20)lauramide, N-laurylN-polyoxyethylene(3)amine and poly(10)ethylene glycol dodecyl thioether. Examples of suitable ionic emulsifiers include sodium lauryl sulfate, sodium dodecylbenzenesulfonate, potassium stearate, sodium dioctyl sulfosuccinate, sodium dodecyldiphenyloxide disulfonate, ammonium salt of nonylphenoxyethyl poly(l)ethoxyethyl sulfate, sodium styrene sulfonate, sodium dodecyl allyl sulfosuccinate, palmitic acid, palmitoleic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, fatty acid mixtures (e.g., fatty acid from linseed oil), sodium or ammonium salts of ethoxylated nonylphenol phosphate esters, sodium octoxynol-3-sulfonate, sodium cocoylisarcocinate, sodium 1-alkoxy-2-hydroxypropyl sulfonate, aolefin (C14 -Ci6)sodium sulfonate, hydroxyalkanol sulfates, tetrasodium N-(1,2-dicarboxyethyl)-Noctadecyl sulfosuccinamate,disodium N-octadecylsulfosuccinamate, disodium polyethoxy alkylamido sulfosuccinate, disodium nonylphenol ethoxylated semi-ester of sulfosuccinic acid and the sodium salt of tert-octylphenoxyethoxypoly(39)ethoxyethyl sulfate. One or more emulsifiers or surfactants are generally used at a level of zero to 3 percent based on the weight of the monomers. The emulsifier or surfactant may be added before the addition of any monomer charge, during the addition of a monomer charge, or a combination thereof. Suitable swelling agents are generally bases, including volatile bases such as ammonia, ammonium hydroxide, and volatile lower aliphatic amines such as morpholine, trimethylamine, and triethylamine, as well as carbonates, bicarbonates, and the like. Fixed or permanent bases such as sodium hydroxide, potassium hydroxide, lithium hydroxide, zinc-ammonium complex, copper-ammonium complex, silver-ammonium complex, strontium hydroxide, barium hydroxide, and the like may also be used. Solvents, such as ethanol, hexanol, octanol, and the solvent Texanol®, and those described in U.S. Patent No. 4,594,363, may be added to aid in the penetration of the fixed or permanent base. In some embodiments, the swelling agent is ammonia or ammonium hydroxide. An alkali metal hydroxide such as sodium hydroxide is preferred because it does not emit volatiles.The swelling agent may be in the form of an aqueous liquid or a gaseous medium containing a volatile base. The compositions of the outer shell and any intermediate encapsulation layers may be selected to be permeable to the swelling agent at room temperature or at a moderately elevated temperature. In one embodiment, the swelling agent is brought into contact with the multi-stage emulsion polymer particles at a temperature somewhat lower than the glass transition temperature of the outer shell polymer. For example, the contact temperature may be 5 to 20°C, 10 to 30°C, or 540°C lower than the Tg of the outer shell polymer. The hydrophilic core component swells when the multi-stage emulsion polymer particles are subjected to a basic swelling agent that permeates the intermediate shells of the multi-stage emulsion polymer particles in the presence of the outer shell polymerization monomer. In one embodiment of the invention, the hydrophilic core component is acidic (having a pH less than 6). Treatment with a basic swelling agent in the presence of the outer shell polymerization monomer neutralizes the acidity and increases the pH of the hydrophilic component to more than 6, or at least approximately 7, or to at least approximately 8, or at least approximately 9, or at least approximately 10, or to at least approximately 13, thereby producing hydration swelling of the hydrophilic core component.The swelling, or expansion, of the core may involve the partial mixing of the outer periphery of the core into the pores of the inner periphery of the layer immediately adjacent to the core (such as the outer envelope or an intermediate encapsulation envelope) and also the partial increase or bulging of such adjacent layer and the entire particle in general. The weight ratio of the core to the outer shell can generally be, for example, in the range of 1:5 to 1:20 (e.g., 1:8 to 1:15). To reduce the dry density of the final empty latex particles, the amount of outer shell relative to the amount of core should generally be decreased; however, sufficient outer shell must still be present so that the core remains encapsulated. Methods previously described in the subject for producing empty latex particles may be adapted for use in the present invention, provided that the processes are modified to include the addition of a swelling agent in the presence of less than qj / / ηη / ζζηζ / E / γίΛΐ 0.5% monomer based on the weight of the polymer particles in the multi-stage emulsion; and substantially all of the swelling occurs during polymerization of the outer shell. Previously known methods subjected to such modification may include those described, for example, in U.S. Patent Nos. 4,427,836; 4,468,498; 4,594,363; 4,880,842; 4,920,160; 4,985,469; 5,216,044; 5,229,209; and 5,273,824, each of which is incorporated herein by reference in its entirety for all purposes. For example, particles can be prepared according to the present invention by incorporating the functional monomers described herein into the outer shell of the particles described in the following examples: (1) Examples 0-14 of U.S. Patent No. 4,427,836, (2) Examples 0-12 of U.S. Patent No. 4,468,498, (3) Examples 1-4 of U.S. Patent No. 4,594,363, (4) Examples 1-IX of U.S. Patent No.s4,880,842, (5) Examples 1-13 of U.S. Patent No. 24,920,160, (6) Examples 1-7 of U.S. Patent No. s4,985,469, (7) Examples 1-7 of U.S. Patent No. s5,216,044, (8) Examples 1-8 of U.S. Patent No. s5,229,209, and (9) Examples 1-50 of U.S. Patent No. s5,273,824. Figure 1 schematically illustrates an example process that can be used to prepare multi-stage emulsion polymer particles and empty latex particles according to the invention. In step 1, methyl methacrylate (MMA) is homopolymerized to form seed 1 as a small particle comprising poly(methyl methacrylate). Seed 1 is coated with a methyl methacrylate / methacrylic acid copolymer layer by copolymerization of methyl methacrylate (MMA) monomer and methacrylic acid (MAA) monomer to provide core 2 (step 2). In step 3, the encapsulating polymer layer 3 is formed by copolymerization of styrene (S) monomer and methyl methacrylate (MMA) monomer. During the formation of the encapsulation layer 3, the polymerization initiator is added before the addition of the swelling agent to reduce the amount of monomer present to less than 0.5% monomer based on the weight of the polymer particles in a multi-stage emulsion. A multi-stage emulsion polymer particle 7 is obtained following step 3. The multi-stage emulsion polymer particle 7 is contacted with aqueous sodium hydroxide in step 4. Although sodium hydroxide is added during step 4, substantially all the swelling of the particle occurs during the polymerization of the monomer in step 5. The sodium hydroxide acts as a swelling agent during the copolymerization of the styrene monomer (S) or the copolymerization of the styrene monomer and a functionalized monomer (S / FM), such as an imidazolidinone (meth)acrylic monomer. The carboxylic acid functional groups of the MMA / MAA copolymer of core 2 are at least partially neutralized and core 2 swells in volume as a result of the absorption of water qj i / nn / zznz / E / YiAi by the neutralized core 2.The increased volume of the core 2 pushes the encapsulating polymer layer 3 and the outer shell 4 outward, increasing the overall diameter of the polymer particle in the multi-stage emulsion. Drying of the particles in step 6 yields the empty latex particle 6. The empty latex particle 6 is characterized by having an outer shell 4 surrounding the hollow interior 5. Residues of the seed 1 and core 2 may still be present within the hollow interior 5. In this embodiment, the encapsulating polymer layer 3 is capable of functioning as a connecting shell between the core 2 and the outer shell 4, which is formed in step 4 by copolymerization of a styrene monomer (S) and a functionalized monomer (FM) such as an imidazolidinone (meth)acrylic monomer. In other embodiments, the functionalized monomer may be omitted, and the encapsulating polymer layer 3 may be omitted.More than one encapsulation polymer layer may be present between the core layer 2 and the outer envelope 4, if desired. Empty latex particles according to the present invention are useful in coating compositions, such as water-based paints and paper coatings. Empty latex particles according to the present invention can impart enhanced gloss, luminosity, and opacity to paper coating formulations to which they are added. Therefore, empty latex particles according to the present invention can impart opacity to aqueous coating compositions, such as paints, to which they are added. Furthermore, the wet adhesion of coating compositions can be improved by including empty latex particles according to the present invention, especially where the outer shell polymer contains functional groups selected from the 1,3-diketoid, amino, ureide, and urea functional groups. For example, a coating composition may contain, in addition to water, empty latex particles according to the present invention, one or more film-forming latex polymers (e.g., an acrylic (A / A) latex and / or a vinyl acrylic (V / A) latex), and, if desired, any of the additives or other components normally employed in such latex coating compositions, such as coalescing solvents, biocides, pigments, fillers, opacifiers other than empty latex particles (e.g., titanium dioxide, CaCO3), thickeners, leveling agents, pH adjusting agents, surfactants, antifreeze agents, and the like. The empty latex particles may be present in such coating compositions at levels of, for example, 0.5 to 10 percent by weight. The film-forming latex polymer used in combination with the empty latex particles of the present invention may also be selected to contain functional groups that help modify or enhance certain characteristics of the coating composition, such as wet adhesion, rub resistance, solvent resistance, stain resistance, or the like. For example, the film-forming latex polymer may be a polymer prepared by polymerizing a so-called wet adhesion monomer, optionally in combination with one or more other types of comonomers. The wet adhesion monomer may be, for example, an ethylenically unsaturated compound carrying a functional group of urea, ureide, 1,3-diketoid, amino, or other such functional group.Such functionalized film-forming latex polymers are well known in the art and are described, for example, in U.S. patents Nos. 23,935,151; 3,719,646; 4,302,375; 4,340,743; 4,319,032; 4,429,095;. qj / / nn / zznz / E / YiAi Functional group pair Functional group A Functional group B 4,632,957; 4,783,539; 4,880,931; 4,882,873; 5,399,706; 5,496,907; and 6,166,220, each of which is incorporated herein by reference in its entirety for all purposes. In one embodiment of the invention, a film-forming latex polymer is selected for use in a coating composition in combination with the non-film-forming empty latex particles described herein, wherein the film-forming latex polymer contains functional groups capable of interacting with functional groups present in the outer shell of the non-film-forming empty latex particles in such a way as to provide a crosslinking effect. Such a crosslinking effect can result when the coating composition is applied, for example, to a substrate surface and dries. This interaction normally takes place through a chemical reaction between the two types of functional groups, producing the formation of covalent bonds, although the interaction could alternatively result from a non-covalent association such as complexation or salt formation.The opacity and solvent resistance of the coating can be enhanced, for example, by using such a functionalized film-forming latex polymer and functionalized non-film-forming empty latex particles in combination with each other. Examples of functional group pairs capable of interacting with each other are as follows. Functional group A may be present in the empty, non-film-forming latex particles (as part of the outer shell polymer), and functional group B may be present in the film-forming latex polymer component. Alternatively, functional group A may be present in the film-forming polymer, and functional group B may be present in the outer shell of the empty, non-film-forming latex particles. 1 Carbonyl Hydrazide 2 Epoxy Amine 3 Oxazoline Aldehyde 4 Acetoacetyl Amine qj / / ηη / ζζηζ / Ε / γίΛΐ In another embodiment of the invention, the coating composition is formulated to contain one or more non-polymeric compounds bearing two or more functional groups per molecule capable of interacting with the functional groups present in the outer shell of the empty, non-film-forming latex particles. Such non-polymeric compounds can thus also serve as crosslinking agents. For example, where the outer shell contains acetoacetate groups, a non-polymeric compound containing a plurality of primary amine groups in each molecule can be used. In another embodiment, the functional groups present on the outer shell of the empty, non-film-forming latex particles are selected so that they are capable of condensing with each other, thus forming bonds between different particles. For example, the outer shell may carry -Si(OH)3 functional groups that can undergo a dehydration reaction to form a siloxane bond (e.g., Si(OH)2-O-Si(OH)2-). In a preferred embodiment, the process does not include ammonia as a swelling agent, i.e., converting the process into “Ammonia Udder. EXAMPLE (HYPOTHETICAL) Multi-stage emulsion polymer particles and empty latex particles are prepared by the process shown in Figure 1. In step 1, methyl methacrylate (MMA) is homopolymerized to form seed 1 as a small particle comprising poly(methyl methacrylate). Seed 1 is coated with a methyl methacrylate / methacrylic acid copolymer layer by copolymerization of methyl methacrylate (MMA) monomer and methacrylic acid (MAA) monomer to provide core 2 (step 2). In step 3, the encapsulating polymer layer 3 is formed by copolymerization of styrene (S) monomer and methyl methacrylate (MMA) monomer. During the formation of the encapsulation layer 3, polymerization initiator is added before the addition of swelling agent to reduce the amount of monomer present to less than 0.5% monomer based on the weight of the polymer particles in the multi-stage emulsion.A multi-step emulsion polymer particle 7 is obtained following step 3. The multi-step emulsion polymer particle 7 is contacted with aqueous sodium hydroxide in step 4. Although the sodium hydroxide is added during step 4, substantially all the particle swelling occurs during monomer polymerization in step 5. The sodium hydroxide acts as a swelling agent during the copolymerization of styrene monomer (S) or copolymerization of styrene monomer and a functionalized monomer (S / FM), such as an imidazolidinone (meth)acrylic monomer. The carboxylic acid functional groups of the core 2 MMA / MAA copolymer are at least partially neutralized, and core 2 swells in volume as a result of water absorption by the neutralized core 2.The increased volume of the core 2 pushes the encapsulating polymer layer 3 and the outer shell 4 outward, increasing the overall diameter of the polymer particle in the multi-stage emulsion. Drying of the particles in step 6 yields an empty latex particle 6. The empty latex particle 6 is characterized by having an outer shell 4 surrounding the hollow interior 5. Seed residue 1 and the core 2 may still be present within the hollow interior 5. In this embodiment, the encapsulating polymer layer 3 is capable of functioning as a connecting shell between the core 2 and the outer shell 4, which is formed in step 4 by copolymerization of a styrene monomer (S) and a functionalized monomer (FM) such as an imidazolidinone (meth)acrylic monomer. In other embodiments, the functionalized monomer may be omitted, and the encapsulating polymer layer 3 may be omitted.More than one encapsulation polymer layer may be present between the core layer 2 and the outer envelope 4, if desired.

Claims

1. A coating composition comprising hollow latex particles, wherein each of said hollow latex particles has a hollow interior and comprises: (i) optionally a core comprising a hydrophilic component, (ii) at least one intermediate shell comprising, as polymerized units, one or more hydrophilic monoethylenically unsaturated monomers, one or more non-ionic monoethylenically unsaturated monomers, or mixtures thereof, and (ii) an outer shell comprising a polymer having a Tg of at least 60 °C, and containing one or more functional groups selected from the group consisting of 1,3-dikete, amino, ureide, urea, hydroxyl, polyether, silane, phosphate, epoxy, fluorocarbon, aldehyde, ketone, acetoacetyl, or combinations thereof; wherein the empty latex particles are non-film forming and opaque, and wherein the composition further comprises film-forming latex particles.

2. The coating composition according to claim 1, wherein at least a portion of the film-forming latex particles contains functional groups that interact with functional groups present in the polymer of the outer envelope.

3. The coating composition according to claim 1, wherein said film-forming latex is an acrylic latex and / or a vinyl acrylic latex.

4. A paint composition comprising the coating composition according to claim 1, further comprising one or more coalescing solvents, biocides, pigments, fillers, titanium dioxide, calcium carbonate, thickeners, leveling agents, pH adjusting agents, surfactants or antifreeze agents.

5. The coating composition according to claim 1, wherein the outer shell of the empty latex particle comprises a polymer containing phosphate functional groups.

6. The coating composition according to claim 1, wherein the polymer of the outer shell of the empty latex particle is a copolymer of an aromatic vinyl monomer and an ethylenically unsaturated polymerizable monomer containing one or more of said functional groups and combinations thereof.

7. The coating composition according to claim 6, wherein the vinyl aromatic monomer is styrene.

8. The coating composition according to claim 6, wherein the ethylenically unsaturated polymerizable monomer contains a (meth)acrylate or (meth)acrylamide group.

9. The coating composition according to claim 6, wherein the copolymer contains from 0.1 to 10% by weight of the polymerizable ethylenically unsaturated monomer.

10. The coating composition according to claim 6, wherein the polymerizable ethylenically unsaturated monomer is selected from the group consisting of acetoacetoxy (meth)acrylates, allyl acetoacetate, methyloxalated diacetone (meth)acrylamides, aminoalkyl (meth)acrylates, and polymerizable ethylenically unsaturated aziridinyl monomers and combinations thereof.

11. The coating composition according to claim 1, wherein the at least one intermediate envelope is an encapsulating polymer layer present between the hollow interior and the outer envelope.

12. The coating composition according to claim 1, wherein the at least one intermediate shell comprises hydrophilic monomer in a lower proportion than any core polymer present.

13. The coating composition according to claim 1, wherein the empty latex particles comprise a first intermediate layer comprising a copolymer of methacrylic acid, styrene, and methyl methacrylate.

14. The coating composition according to claim 13, wherein the empty latex particles comprise a second intermediate layer comprising copolymerized methyl methacrylate and styrene.

15. The coating composition according to claim 1, wherein the core is present and comprises methyl methacrylate, and / or copolymerized methacrylic acid and methyl methacrylate.