Field of the Invention

This invention relates to tacky, polymeric, organic solvent-insoluble, elastomeric, pressure-sensitive adhesive microspheres, prepared from vinyl ester monomers, to processes for their preparation, and to their use as pressure-sensitive adhesives.

Background of the Invention

Tacky, elastomeric microspheres are known to be useful in repositionable pressure-sensitive adhesive applications. As used herein, the term "repositionable" refers to the ability to be repeatedly adhered to and removed from a substrate without substantial loss of adhesion capability. Microsphere-based adhesives are thought to perform well in such applications at least in part due to their "self-cleaning" character, wherein substrate contaminants tend to be pushed aside and trapped between the microspheres as the adhesive is applied. Upon removal, the adhesive can then still present a relatively uncontaminated surface for reapp cation to the substrate.

Numerous references concern the preparation and/or use of inherently tacky, elastomeric acrylate polymeric microspheres which are solid in nature. The composition, preparation and uses of such microspheres, along with their positionable properties, are disclosed in U.S. Pat. Nos. 3,691,140 (Silver); 4,166,152 (Baker et al.); 4,495,318 (Howard); 4,598,112 (Howard); 4,786,696 (Bohnel); DE 3,544,882 Al (Nichiban); 4,645,783 (Kinoshita); and 4,656,218 (Kinoshita). Such solid, acrylate tacky microspheres are typically the suspension polymerization products of acrylate or methacrylate esters and free radically copolymerizable polar monomers. These

polymerizations are performed in the presence of a variety of emulsifiers, surfactants, stabilizers and/or under particular process conditions which induce the formation and prevent the agglomeration of the spheres.

Hollow, polymeric, acrylate, inherently tacky, infusible, solvent- insoluble, solvent-dispersible, elastomeric pressure-sensitive adhesive microspheres having diameters of at least about one micrometer have been described in U.S. Patent Nos. 5,045,569 (Delgado) and 4,988,567 (Delgado). Delgado teaches that small amounts of a high Tg vinyl ester such as vinyl acetate can be used in the composition provided that the resultant polymer has a Tg below -20 C. Preferred hollow microspheres contain one or more interior voids having diameters at least 10% of the hollow microspheres. The pressure-sensitive adhesives based on the hollow microspheres show reduced or even eliminated adhesive transfer in comparison with repositionable pressure-sensitive adhesives which are based on solid microspheres. The disclosed microspheres are useful as repositionable pressure-sensitive adhesives and in repositionable spray pressure-sensitive adhesive compositions. U.S. Pat. No.

5,053,436 (Delgado) and U.S. Pat. No. 3,691,141 (Silver et al.) both respectively disclose spray pressure sensitive adhesives using solid and hollow acrylates microspheres.

Other than vinyl acetate, vinyl esters have found limited use in pressure- sensitive adhesive formulations. Examples of non-mi crosphere pressure-sensitive adhesives containing vinyl esters include U.S. Patent Nos. 3,751,449 (Gobran et al.); 4,296,017 (Eichelseder et al); and 3,519,587 (Bergmeister et al.). In general, such vinyl esters are not used due to their lack of availability, high cost and poor rate of reactivity and slow polymerization under free radical conditions when compared to acrylate, methacrylate and maleate esters. In addition, the polymers prepared therefrom are hydrolytically less stable than acrylic polymers. For these reasons vinyl esters have not been specifically utilized as major components in tacky, elastomeric microsphere compositions.

Summary of the Invention

A need, thus, exists for new classes of tacky microspheres based upon different monomer systems. This invention provides tacky, polymeric, organic solvent- insoluble, elastomeric, vinyl ester containing pressure-sensitive adhesive microspheres typically having diameters of at least about one micrometer. In addition, the microspheres of the invention are organic solvent swellable. These microspheres are useful as repositionable pressure-sensitive adhesives.

The present invention also provides pressure-sensitive adhesives comprising these microspheres. More specifically, these inherently tacky, polymeric, organic solvent-insoluble, elastomeric, pressure sensitive adhesive microspheres comprise: a (co)polymer having a Tg of less than about -IO C comprising the polymerization product of:

(a) about 50 to about 100 percent by weight of at least one free radically polymerizable monomer comprising vinyl ester monomers wherein a polymer prepared from the monomers have a Tg of less than about -10 C; and

(b) about 0 to about 20 percent by weight of at least one polar monomer copolymerizable with the monomer of element (a) and monomer of element (c) if included; and (c) about 0 to about 50 percent by weight of a nonpolar free radically polymerizable vinyl monomer, copolymerizable with the monomer of element (a) and element (b), if included wherein the weight percentages of (a), (b) and (c), are based upon the total weight of the (co)polymer; and

(d) about 0 to about 0.15 equivalent weight percent to a multifunctional crosslinking agent based upon the total weight of the monomers plus the crosslinking agent.

Preferably the copolymer referred to above consists essentially of (a), (b), (c) and (d), and most preferably consists of (a), (b), (c), and (d).

Preferably the pressure sensitive adhesive micropheres consist essentially of said aforementioned copolymers, most preferably consists of said aforementioned copolymers.

The invention also provides for microspheres of this invention which are hollow, microspheres of this invention which are solid, methods of making these microspheres, aqueous suspensions and solvent dispersions of these microspheres, spray repositionable pressure-sensitive adhesive compositions, and microsphere coated sheet materials. When hollow, preferred hollow microspheres contain one or more interior voids having diameters at least 10% of the hollow microspheres. The microspheres of the invention have a glass transition temperature (Tg) of less than about -10°C, preferably about -100°C to about -20°C. If the glass transition temperature of the microspheres rises above -10°C, then the microspheres will have reduced tack and elasticity.

Aqueous suspensions of hollow microspheres may be prepared by a two-step emulsification process comprising the steps of:

(a) forming a water-in-oil emulsion by combining (i) an Aqueous Phase I comprising water and, optionally, at least one free radically polymerizable polar monomer; with (ii) an Oil Phase II comprising at least one free radically polymerizable vinyl ester monomer, an emulsifier having an HLB value of below about 7, optionally at least one free radically polymerizable nonpolar vinyl monomer wherein a polymer prepared from all the monomers would have a Tg of less than about -10°C, and optionally at least one multifunctional crosslinking agent;

(b) forming a water-in-oil-in-water emulsion by dispersing the water-in-oil emulsion into an Aqueous Phase II comprising water and an emulsifier having a hydrophilic-lipophilic balance value of at least about 6; and

(c) initiating polymerization; wherein all or part, if used, of the polar monomer(s) and/or nonpolar monomer(s) and/or crosslinking agent(s) is alternatively added to the water-in-oil-in-water emulsion after polymerization of the water-in-oil-in-water

emulsion is initiated, but before 100% to conversion to polymer of the monomers of said water-in-oil-in-water emulsion occurs.

Aqueous suspensions of hollow microspheres which may contain polar monomer(s) may also be prepared by a simpler ("one-step") emulsification process comprising the steps of:

(a) forming droplets by mixing in any order together

(i) at least one free radically polymerizable vinyl ester monomer, wherein a polymer prepared from the monomer(s) would have a Tg of less than about -IO C; (ii) optionally at least one free radically polymerizable polar monomer;

(iii) optionally at least one free radically polymerizable nonpolar vinyl monomer; and

(iv) at least one emulsifier which is capable of forming a water-in-oil emulsion inside the droplets, the emulsion being substantially stable during emulsification and polymerization;

(v) an aqueous medium; and,

(vi) optionally at least one multifunctional crosslinking agent; and (b) initiating polymerization.

Aqueous suspensions of hollow microspheres may also be prepared by a modification of the "one-step" emulsification process comprising the steps of: (a) forming droplets by mixing together

(i) at least one free radically polymerizable vinyl ester monomer, wherein a polymer prepared from the monomer(s) would have a Tg of less than about -10 C,

(ii) optionally a portion of, if used, of at least one free radically polymerizable polar monomer;

(iii) optionally a portion, if used, of at least one free radically polymerizable nonpolar vinyl monomer;

(iv) at least one emulsifier which is capable of forming a water-in-oil emulsion inside the droplets, the emulsion being substantially stable during emulsification and polymerization, and

(v) an aqueous medium; and

(vi) optionally at least one multifunctional crosslinking agent; and

(b) initiating polymerization; and, (c) adding all or the remaining portion of polar monomer(s) and/or nonpolar monomers, if used, or a portion of the multifunctional crosslinking agent, if used, prior to the 100% conversion of the monomer(s) contained in the droplets.

Aqueous suspensions of solid microspheres may be prepared by an analogous "one-step" emulsification process comprising the steps of: (a) forming droplets by mixing together:

(i) at least one free radically polymerizable vinyl ester monomer, wherein a polymer prepared from the monomer would have a Tg of less than about -10 C,

(ii) optionally at least one free radically polymerizable polar monomer;

(iii) optionally at least one free radically polymerizable nonpolar vinyl monomer;

(iv) at least one suspension stabilizer; (v) an aqueous medium; and (vi) optionally at least one multifunctional crosslinking agent;

(b) initiating polymerization; and

(c) adding all or any remaining portion of optional polar monomer(s) and all or the remaining portion of nonpolar monomer, if used, and adding

all or any remaining portion of the multifunctional crosslinking agent prior to the 100% conversion of the monomer contained in the droplets.

Aqueous suspensions of solid microspheres may be prepared by a "one- step" emulsification process comprising the steps of: (a) forming droplets by mixing together:

(i) at least one free radically polymerizable vinyl ester monomer, wherein a polymer prepared from the monomer would have a Tg of less than about -10°C,

(ii) optionally at least one free radically polymerizable nonpolar monomer;

(iii) at least one emulsifier having a hydrophilic-lipophilic balance value of less than about 25;

(iv) an aqueous medium; and

(v) optionally at least one multifunctional crosslinking agent; (b) initiating polymerization; and

(c) adding all or any remaining portion of nonpolar monomer(s), if used, and all or any remaining portion of multifunctional crosslinker, if used, prior to the 100% conversion of the monomer contained in the droplets.

The following terms have these meanings as used herein: 1. The term "droplet" means the liquid stage of the microspheres prior to the completion of polymerization.

2. The term "cavity" means a space within the walls of a droplet or microsphere when still in the suspension or dispersion medium prior to drying, and thus containing whatever medium was used. 3. The term "void" means an empty space completely within the walls of a polymerized microsphere.

4. The term "hollow" means containing at least one void or cavity.

5. The term "solid" means not hollow; that is, void-free or cavity- free.

6. The term "elastomeric" has been described, for example, as, "... applying to amorphous or non-crystalline materials that can be stretched to at least twice their original length and which will retract rapidly and forcibly to substantially their original dimensions upon release of the force." [S.L. Rosen, Fundamental Principles of Polymeric materials. Wilev: New York, p. 314 (1982 ]

7. The term "organic solvent-insoluble" in reference to a polymeric material refers to a polymeric material which is not totally dissolved on a molecular level in common organic solvents.

8. The term "solvent swellable" in reference to a polymeric material refers to a polymeric material that swells in solvent to an extent larger than its original dimension and forms a dispersion consisting, substantially, of individual particles.

All percents, parts, ratios, etc. described herein are by weight unless indicated otherwise.

Detailed Description of the Invention

Free Radically Polymerizable Vinyl Ester Monomers

Useful ester monomers are oleophilic, water emulsifiable, have restricted water solubility (less than about lg/per lOOg of water at 25°C), and as homopolymers, generally have glass transition temperatures below about - 10°C.

Vinyl ester monomers useful according to the invention are those of the general formula:

O

II CH2=C-O-C-CH ₂ -R ¹

I H

wherein R s selected from the group consisting of linear or branched alkyl groups having 1 to 12 carbon atoms. Such vinyl esters include but are not limited to those selected from the group consisting of vinyl 2-ethylhexanoate, vinyl caprate,

vinyl laurate, vinyl pelargonate, vinyl hexanoate, vinyl propionate, vinyl decanoate, vinyl octanoate, and other monofunctional unsaturated vinyl esters of linear or branched carboxylic acids comprising 3 to 14 carbon atoms which as homopolymers have glass transition temperatures below about -10°C. Preferred vinyl ester monomers include those selected from the group consisting of vinyl laurate, vinyl caprate, vinyl-2-ethylhexanoate, and mixtures thereof.

Additionally, cycloaliphatic- and phenyalkyl-substituted vinyl esters, such as those described in U.S. Patent No. 3,751,449 (Gobran et al.) having the formula set forth immediately below, can also be used in the microspheres of the present invention.

O R ²

II I

CH ₂ =CH-O-C-CH-R ³ where R ² is a cycloaliphatic or aromatic group (e.g., phenyl), and

R3 is a hydrogen atom or lower alkyl (e.g., with 1 to 6 carbon atoms).

Free Radically Polymerizable Polar Monomers

The free radically polymerizable polar monomers useful in the present invention are both somewhat oil-soluble and water-soluble, resulting in a distribution of the polar monomer between the aqueous and the oil phases.

Representative examples of suitable polar monomers include but are not limited to those selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, sulfoethyl methacrylate, N-vinyl pyrrolidone, N-vinyl caprolactam, 2-vinyl-4,4-dimethyl-2-oxazolidinone, t-butyl acrylamide, dimethyl amino ethyl acrylamide, N-octyl acrylamide, and ionic monomers such as sodium methacrylate, ammonium acrylate, sodium acrylate, trimethylamine p-vinyl benzimide, 4,4,9-trimethyl-4-azonia-7-oxo-8-oxa-dec-9-ene-l-sulphonate, N,N-dimethyl-N-(beta-methacryloxy-ethyl) ammonium propionate betaine, trimethylamine methacrylimide, l,l-dimethyl-l-(2,3-dihydroxypropyl)amine

methacrylimide, mixtures thereof, and the like. Preferred polar monomers include those selected from the group consisting of monoolefinic monocarboxylic acids, monoolefinic dicarboxylic acids, acrylamides, N-substituted acrylamides, salts thereof, and mixtures thereof. Examples of such preferred polar monomers include but are not limited to those selected from the group consisting of acrylic acid, sodium acrylate, N-vinyl pyrrolidone, and mixtures thereof.

Free Radically Polymerizable Nonpolar Vinyl Monomers

Free radically polymerizable nonpolar vinyl monomers which, as homopolymers, have glass transition temperatures higher than about -10°C, e.g., tert-butyl acrylate, isobornyl acrylate, butyl methacrylate, vinyl acetate, acrylonitrile, mixtures thereof, and the like, may optionally be utilized in conjunction with one or more of the vinyl ester monomers provided that the glass transition temperature of the resultant copolymer is below about -10°C. Similarly, other free radically polymerizable nonpolar vinyl monomers which, as homopolymers, have glass transition temperatures lower than about -10°C, such as alkyl acrylate and methacrylate monomers, are useful in preparing the microspheres and pressure-sensitive adhesives of this invention. Such alkyl acrylate and methacrylate monomers are those monofunctional unsaturated acrylate and methacrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which preferably have from about 4 to about 14 carbon atoms. Examples of such monomers include but are not limited to those selected from the group consisting of isooctyl acrylate, 4-methyl-2-pentyl acrylate, 2-methylbutyl acrylate, isoamyl acrylate, sec-butyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, isodecyl methacrylate, isononyl acrylate, isodecyl acrylate, and mixtures thereof.

Prefeπed acrylate monomers include those selected from the group consisting of isooctyl acrylate, isononyl acrylate, isoamyl acrylate, isodecyl acrylate, 2-ethylhexyl acrylate, n-butyl acrylate, sec-butyl acrylate, and mixtures thereof.

Component Ranges

The microspheres of this invention and the pressure-sensitive adhesives made therefrom comprise a copolymer comprising about 50 to about 100 percent by weight of at least one free radically polymerizable vinyl ester monomer; optionally, about 0 to about 20 percent by weight of one or more polar monomers, (typically about 0.1 to about 20 percent, if included); and optionally about 0 to about 50 weight percent of a nonpolar free radically copolymerizable vinyl monomer (typically about 0.1 to about 50 percent by weight, if included), based upon the total weight of the copolymer. Preferably, the pressure-sensitive adhesive microspheres comprise a copolymer comprising about 60 to about 100 percent by weight of free radically polymerizable vinyl ester monomer; optionally, about 0 to about 15 percent by weight of at least one polar monomer (typically about 0.1 to about 15 percent, if included); and optionally about 0 to about 30 weight of nonpolar free radically copolymerizable vinyl monomer (typically about 0.1 to about 30 percent, if included) based upon the total weight of the copolymer. Most preferably, the pressure-sensitive adhesive microspheres comprise a copolymer comprising about 70 to about 100 percent by weight of free radically polymerizable vinyl ester monomer; optionally about 0 to about 10 percent by weight of polar monomer (typically about 0.1 to about 10 percent by weight, if included), and optionally about 0 to about 20 percent (typically about 0.1 to about 20 percent by weight, if included) by weight of nonpolar free radically copolymerizable vinyl monomer, based upon the total weight of the copolymer.

Preferably, at least one polar monomer is included in the composition, but microspheres may also be prepared using free radically polymerizable vinyl ester monomer(s) alone or without a polar monomer in combination only with other free radically polymerizable nonpolar vinyl monomers, such as isooctyl acrylate, vinyl acetate, etc. Most preferably, at least about 1 percent to about 10 percent by weight polar monomer is included as this ratio provides microspheres with balanced pressure-sensitive adhesive properties.

Two-Step Method of Preparing Hollow Microspheres

Aqueous suspensions of the hollow microspheres of the invention may be prepared by a "two-step" emulsification process which first involves forming a water-in-oil emulsion of an aqueous solution of water and, if used, polar monomer(s), in the oil phase monomer (i.e., at least vinyl ester monomer and optional nonpolar monomer), using an emulsifier having a low hydrophilic-lipophilic balance (HLB) value. Where it is desirable not to include a polar monomer, an aqueous phase may be mixed directly with the oil phase monomer (i.e., vinyl ester monomer and optional nonpolar monomer), and emulsifier to form the water-in-oil emulsion. The hollow morphology of the microspheres of the invention can be determined through the choice of emulsifiers used in this "two-step" process. Suitable emulsifiers for the use in the first step of the two step preparation of hollow microspheres are those having HLB values below about 7, preferably in the range of about 2 to about 7. Examples of such emulsifiers include but are not limited to those selected from the group consisting of sorbitan monooleate, sorbitan trioleate, ethoxylated oleyl alcohol (such as Brij 93, available from Atlas Chemical Industries, Inc.), and mixtures thereof. It is noted that if a two-step method is followed in which the water-in-oil-in-water emulsion is unstable, solid microspheres could result. In the first step for preparing hollow microspheres, oil phase monomer(s), emulsifier, a free radical initiator, and optional crosslinking monomer or monomers as defined below are combined, and an aqueous solution comprising water and, if used, polar monomer(s) is agitated and poured into the oil phase mixture to form a water-in-oil emulsion. A thickening agent, such as methyl cellulose, may also be included in the aqueous phase of the water-in-oil emulsion. In the second step, a water-in-oil-in-water emulsion is formed by dispersing the water-in-oil emulsion of the first step into an aqueous phase containing an emulsifier having an HLB value above about 6. Examples of such emulsifiers include but are not limited to those selected from the group consisting of ethoxylated sorbitan monooleate, ethoxylated lauryl alcohol, alkyl sulfates, and mixtures thereof. In both steps, when an emulsifier is

utilized, its concentration should be greater than its critical micelle concentration, which is herein defined as the minimum concentration of emulsifier necessary for the formation of micelles, i.e., submicroscopic aggregations of emulsifier molecules. Critical micelle concentration is slightly different for each emulsifier, usable concentrations ranging from about 1.0 x 10"^ to about 3.0 moles/liter. The final process step of the method of the invention involves the application of heat or radiation (such as ultraviolet radiation, etc.) to initiate polymerization of the monomers. One skilled in the art would be able to select an appropriate amount of heat and/or radiation depending on the initiator selected.

One-Step Method of Preparing Hollow Microspheres

Aqueous suspensions of hollow microspheres which may contain polar monomer(s) may also be prepared by a "one-step" emulsification process comprising aqueous suspension polymerization of at least one vinyl ester monomer and optional nonpolar monomer and, optionally, at least one polar monomer in the presence of at least one emulsifier capable of producing a water-in-oil emulsion inside the droplets which is substantially stable during emulsification and polymerization. As in the two-step emulsification process, the emulsifier is utilized in concentrations greater than its critical micelle concentration. In general, emulsifiers formulated at this concentration will produce stable cavity-containing droplets during the polymerization, and are suitable for use in this one-step process. Examples of such emulsifiers include but are not limited to those selected from the group consisting of alkylarylether sulfates such as sodium alkylarylether sulfate, e.g., Triton W/30, available from Rohm and Haas; alkylarylpolyether sulfates such as alkylarylpoly(ethylene oxide) sulfates; alkyl sulfates such as sodium lauryl sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, and sodium hexadecyl sulfate; alkyl ether sulfates such as ammonium lauryl ether sulfate, and alkylpolyether sulfates such as alkyl poly(ethylene oxide) sulfates; alkylarylpolyether sulfonates such as alkylarylpoly(ethylene oxide) sodium sulfonate (e.g., Triton X-200, commercially available from the Rohm and Haas Co.); alkyl

benzene sulfonates such as sodium p-dodecylbenzene sulfonate (e.g., Siponate DS -10, commercially available from Alcolac, Inc.); alkyl sulfosuccinates, such as Aerosol OT, a dioctyl ester of sodium sulfosuccinic acid commercially available from American Cyanamid Process Chemicals Dept; and mixtures thereof. Emulsifiers selected from the group consisting of alkyl sulfates, alkyl ether sulfates, alkylarylether sulfates, and mixtures thereof are preferred as they provide a maximum void volume per microsphere for a minimum amount of surfactant. Nonionic emulsifiers, e.g. Siponic Y-500-70 (ethoxylated oleyl alcohol, commercially available from Alcolac, Inc.) and Pluronic PI 03 (block copolymer of polypropylene oxide and polyethylene oxide commercially available form BASF Corporation), can also be utilized alone or in conjunction with anionic emulsifiers, and mixtures thereof. Polymeric stabilizers may also be present but are not necessary.

One-Step Method of Preparing Solid Microspheres Aqueous suspensions of solid microspheres may be prepared by a

"one-step" emulsification process comprising an aqueous suspension polymerization of at least one vinyl ester monomer; at least one polymeric stabilizer, such as poly(vinyl alcohol); optionally, at least one polar monomer; optionally, at least one nonpolar monomer, and/or optionally a crosslinking monomer. It is believed that other polymeric stabilizers, such as those described in U.S. Patent No. 4,166,152 (Baker et al.) (e.g., carboxy-modified polyacrylamides, carboxy-modified celluloses, quaternary amine-substituted celluloses, etc.), and other steric or electrosteric polymeric stabilizers, including but not limited to those selected from the group consisting of polyoxyethylene, polyacrylic acid, polymethacrylic acid, polyacrylamide, polyvinyl pyrrolidone, polyethylene imine, polyvinyl methyl ether, salts thereof, and mixtures thereof, would also be useful according to the present invention.

It has also been found that aqueous suspension of solid microspheres not incorporating any polar monomer may be prepared by a similar "one-step" emulsification process which incorporates emulsifiers having HLB values lower than

about 25 rather than polymeric stabilizers in the formation of these microspheres. Examples of emulsifiers having HLB values lower than 25 include but are not limited to those selected from the group consisting of alkylarylpolyether sulfonates such as alkylarylpoly (ethylene oxide) sodium sulfonate χ-200, commericially available from the Rohm and Haas Co.); alkyl benzene sulfonates such as sodium p- dodecylbenzene sulfonate (e.g., PolystepTM Biosoft LAS-50, commericially available from Alcolac, Inc.); alkyl sulfosuccinates, such as Aerosol^M OT, a dioctyl ester of sodium sulfosuccinic acid commercially available from American Cyanamid Process Chemicals Dep ; and mixtures thereof. All of these preparative methods may be modified by withholding the addition of all or part, if used, of the polar monomer(s) and/or nonpolar monomer(s), if used, and/or multifunctional crosslinking agent, if used, until after polymerization of the water-in-oil-in-water emulsion is initiated. This may be done provided that the withheld components are added to the polymerizing mixture prior to its 100% conversion of monomers to polymer in the emulsion. This processing flexibility allows the formulator to add any portion of the optional polar monomer(s) and/or optional free radically polymerizable vinyl monomers and/or any portion of the multifunctional crosslinking agent at any convenient point in preparing the pressure-sensitive adhesive microspheres of this invention.

Initiators

Suitable initiators are those which are normally suitable for free radical polymerization of free radically polymerizable monomers and which are oil-soluble and of very low solubility in water (i.e., typically less than about 1 g per lOOg of water at 20°C). Examples of such initiators include but are not limited to those selected from the group consisting of thermally-activated initiators such as azo compounds, hydroperoxides, peroxides, and the like, and photoinitiators such as benzophenone, benzoin ethyl ether, and 2,2-dimethoxy-2-phenyl acetophenone, and the like, and mixtures thereof. Use of a water-soluble polymerization initiator causes formation of

substantial amounts of latex. The initiator is generally used in an amount ranging from about 0.01 percent up to about 10 percent by weight of the total polymerizable composition (i.e., monomers, and optional crosslinking agent), preferably up to about 5 percent.

Crosslinking Agents

The composition from which the microspheres of the invention are made may also contain a multifunctional crosslinking agent. The term "multifunctional" as used herein refers to crosslinking agents which possess two or more free radically polymerizable ethylenically unsaturated groups. Useful multifunctional crosslinking agents include but are not limited to those selected from the group consisting of acrylic or methacrylic esters of diols such as butanediol diacrylate, triols such as glycerol, and tetrols such as pentaerythritol. Other useful crosslinking agents include but are not limited to those selected from the group consisting of polyvinylic crosslinking agents, such as substituted and unsubstituted divinylbenzene; and difunctional urethane acrylates, such as Ebecryl 270 and Ebecryl 230 (1500 weight average molecular weight and 5000 weight average molecular weight acrylated urethanes, respectively - both available from Radcure Specialties), and mixtures thereof. When used, crosslinker(s) is (are) added at a level of up to about 0.15 equivalent weight %, preferably up to about 0.1 equivalent weight %, of the total polymerizable composition (i.e. monomers plus optional crosslinking agent). The "equivalent weight %" of a given compound is defined as the number of equivalents of that compound divided by the total number of equivalents in the total composition, wherein an equivalent is the number of grams divided by the equivalent weight. The equivalent weight is defined as the molecular weight divided by the number of polymerizable groups in the monomer (in the case of those monomers with only one polymerizable group, equivalent weight - molecular weight). The crosslinker can be added to any phase at any time before 100% conversion. Preferably it is added before initiation occurs. Crosslinking can

alternatively occur via exposure to an appropriate radiation source, such as gamma or electron beam radiation.

Microsphere Diameter The microspheres of the invention are normally tacky, elastomeric, organic solvent-insoluble but swellable in organic solvents, and small, typically having diameters of at least about 1 micrometer, preferably in the range of about 1 to about 300 micrometers. When the microspheres are hollow, the voids typically range in size up to about 100 micrometers or larger. Following polymerization by any of these one-step or two-step processes, an aqueous suspension of the hollow or solid microspheres is obtained which is stable to agglomeration or coagulation under room temperature conditions (i.e., about 20 to about 25°C). The suspension may have a non-volatile solids contents of from about 10 to about 50 percent by weight. Upon prolonged standing, the suspension separates into two phases, one phase being aqueous and substantially free of polymer, the other phase being an aqueous suspension of microspheres. Both phases may contain a minor portion of submicron latex particles. Decantation of the microsphere-rich phase provides an aqueous suspension having a non-volatile solids content on the order of about 40 to about 50 percent which, if shaken with water, will readily redisperse. If desired, the aqueous suspension of microspheres may be utilized immediately following polymerization to provide inherently tacky pressure-sensitive adhesive coatings. The suspension may be coated on suitable flexible or inflexible backing materials by conventional coating techniques such as knife coating or Meyer bar coating or use of an extrusion die. Once dried, the microspheres, with sufficient agitation, will readily disperse in common organic liquids such as ethyl acetate, toluene, tetrahydrofuran, heptane, 2-butanone, benzene, cyclohexane, and esters Solvent dispersions of the microspheres may also be coated on suitable backing materials by conventional coating techniques, as described above for aqueous suspensions.

Suitable backing materials for the aqueous or solvent based coatings include but are not limited to those selected from the group consisting of paper, plastic films, cellulose acetate, ethyl cellulose, woven or nonwoven fabric formed of synthetic or natural materials, metal, metallized polymeric film, ceramic sheet material, and the like. Primers or binders may be used thereon.

Suspensions or dispersions of the microspheres in a liquid medium, e.g., water or an organic liquid as described above, may be sprayed by conventional techniques without cobwebbing or may be incorporated in aerosol containers with suitable propellants including but not limited to those selected from the groups consisting of alkanes, alkenes, chlorofluorocarbons, e.g., Freon halocarbon propellents (commercially available from E.I. du Pont de Nemours & Co., Inc.), and mixtures thereof. Useful aerosol formulae have a solids content of from about 5% to about 20%, preferably from about 10% to about 16%.

The pressure-sensitive adhesive properties of the microspheres may be altered by addition of tackifying resin and/or plasticizer. It is also within the scope of this invention to include various other components, such as pigments, neutralizing agents such as sodium hydroxide, etc., fillers, stabilizers, or various polymeric additives. Preferably, the pressure-sensitive adhesive of the invention consists essentially of the microspheres of the invention, more preferably consists of the microspheres of the invention. Test Methods Tack

The tack of sheets coated with the microspheres of the invention was measured with a Polyken Probe Tack tester (available from Kendall Company) according to American Society for Testing and Materials Test Method ASTM

D2979-88. Microspheres of the current invention were coated onto 1.5 mil primed polyester film, yielding a dried adhesive coating thickness of 1 to 2 mils. After cleaning the probe with ethyl acetate using a lint-free cloth, a 2 cm x 2 cm sample of the adhesive coated sheet was placed on the annular ring weight of the Polyken apparatus.

The tack was then measured and recorded using a 10 mm stainless steel probe having a diameter of 0.4975 cm with a speed of 0.5 cm/second and dwell time of 1 second.

Glass Transition Temperature (Tg) Glass transition temperatures were measured using Differential Scanning

Calorimetry at a heating rate of 20 C per minute.

Abbreviations and Tradenames

The following abbreviations and tradenames are used herein.

AA acrylic acid Acrysol Acrysol A3, polyacrylic acid available from Rohm and Haas Co. which has been neutralized with NH OH

BDDA 1,4-butanediol diacrylate

Biosoft Polystep Biosoft LAS-50, sodium dodecyl benzene sulfonate available from Stepan Co.

DVB divinyl benzene HDDA 1,6-hexanediol diacrylate IOA isooctyl acrylate ITA itaconic acid

NA not available or not measured Siponate Siponate DS 10, sodium p-dodecylbenzene sulfonate available from Alcolac, Inc.

Span 80 Span 8θTM. sorbitan monooleate available from

ICI Americas, Inc.

Standapol Standapol^M^ ammonium lauryl sulfate available from Henkel AG V2EH vinyl 2-ethylhexanoate

VA vinyl acetate

Vinol 350 VinolT^350, polyvinyl alcohol available from Air Products

Vinol 205 VinolTM 205, polyvinyl alcohol available from

Air Products

VL vinyl laurate

Examples

The following Examples further illustrate but do not limit the invention. All parts, percentages, ratios, etc., in the Examples and the rest of the specification are by weight unless indicated otherwise. Microspheres consisting of a predominance (i.e. at least about 50 percent by weight) of at least one free radically polymerizable vinyl ester were prepared and examined for tack, diameter, Tg and morphology.

Examples 1 and 2

These examples illustrate the preparation of hollow tacky microspheres by a one step emulsification method.

Example 1 In a one-liter resin reactor equipped with mechanical stiπer and inlet- outlet lines for vacuum and argon, 450 grams of distilled and deionized water and 6.0 grams of Standapol^M A (Ammonium lauryl sulfate from Henkel AG) were charged and heated to 65°C. Next, 0.71 gram of Lucidol™-70 (70% benzoyl peroxide commercially available from Atochem North America, Inc.) was dissolved in a mixture of 144 grams of vinyl laurate, 6.0 grams of acrylic acid, and 0.04 gram of 1-4 butanediol diacrylate. When the Lucidol -70 was dissolved, the monomer mixture was added to the reactor while stirring at 400 RPM. Vacuum was applied to evacuate the reactor atmosphere and the reactor was then purged with argon. The temperature

of the reactor was maintained at 65°C for 15 hours. An argon purge was maintained during the polymerization. After the 15 hour period, the suspension was allowed to cool to room temperature. The reactor was emptied and the suspension filtered. Optical microscopy showed hollow microspheres about 30 microns in diameter containing multiple cavities of about 15% of the diameter of the microspheres.

Example 2

The same procedure described in Example 1 was followed with the exception that 144 grams of Vynate^M 2EH (vinyl 2-ethylhexanoate available from Union Carbide) were used instead of the vinyl laurate. The temperature of the reactor was maintained at 65°C for 22 hours. At the end of the polymerization, optical microscopy showed hollow microspheres about 30 microns in diameter containing multiple cavities of about 15% of the diameter of the microspheres. Example 3 The following example illustrates the preparation of hollow tacky microspheres using a two step emulsification method:

First, 0.75 gram of Standapol *- A was dissolved in 150 grams of distilled and deionized water and charged into a one-liter glass reactor equipped with a mechanical stirrer and inlet-outlet lines for vacuum and argon. A water-in-oil emulsion was prepared in an Omni mixer by stirring 75 grams of distilled and deionized water with

75 grams of vinyl laurate, 1.50 grams of Span 80~M (sorbitan monooleate available from ICI Americas, Inc.) and 0.36 gram of Lucidol ^τ ^-70. The water-in-oil emulsion was added to the reactor containing the aqueous solution of StandapolTM A. The reactor was heated to 75°C while stiπing at 400 RPM. The reactor was kept at 75°C for 22 hours and an argon purge was maintained during this time. After the 22 hour period, the suspension was allowed to cool to room temperature. The reactor was emptied and the suspension filtered. Optical microscopy showed hollow microspheres

ranging from 2 to 70 microns in diameter containing multiple cavities of at least 10% of the diameter of the microspheres.

Examples 4 and 5 The following examples illustrate the preparation of solid tacky microspheres.

In Example 4 a polymeric stabilizer was used. In Example 5 a solid microsphere which did not incorporate polar monomer was made using a surfactant with an HLB less than 25. Example 4 In a one-liter glass reactor equipped with a mechanical stiπer and inlet- outlet lines for vacuum and argon, 3 grams of Vinol^M 350 (polyvinyl alcohol available from Air Products) were dissolved in 225 grams of distilled and deionized water. The reactor was heated to 75 °C, its atmosphere evacuated by applying vacuum and refilling with argon. When the temperature of the reactor reached 75°C, a mixture of 75 grams of Vynate™-2EH and 0.36 grams of LucidoH-^70 were added. The agitation was set at 400 RPM. The reactor temperature was maintained at 75 °C for 22 hours. An argon purge was maintained during the polymerization. After this time period the suspension was allowed to cool down to room temperature. The reactor was emptied and the suspension filtered. Optical microscopy showed solid microspheres of about 90 microns in diameter.

Example 5

Solid microspheres were prepared as in Example 4, only that 2.17 grams of Polystep Biosoft LAS-50 (sodium dodecyl benzene sulfonate available from Stepan Co., HLB=approximately 19) were used instead of polyvinyl alcohol. In addition, 0.019 gram of divinylbenzene was used along with the Vynate-2EH. The

polymerization was carried out at 75°C for 30 hours. After polymerization, optical microscopy showed solid microspheres of about 70 microns in diameter.

Examples 6 to 10 The following examples, made in accordance with Example 1 and in the proportions stated in Table 1, illustrate the preparation of different tacky microspheres. In all cases 225.0 grams of distilled and deionized water was used in the recipe. All polymerizations were carried out at 75°C.

Examples 11 and 12

The following examples, made in accordance with Example 4 and in the proportions stated in Table 1, illustrate the preparation of solid tacky microspheres using an alternative polymeric stabilizer (Example 12) and a polar monomer and an alternative polymeric stablilizer (Example 11).

* Vinol 350 and Vinol 205 are polymeric stabilizers.

While this invention has been described in connection with specific embodiments, it should be understood that it is capable of further modification. The claims herein are intended to cover those variations which one skilled in the art would recognize as the chemical equivalent of what has been described here.