CA1328331C

CA1328331C - Hollow acrylate polymer microspheres

Info

Publication number: CA1328331C
Application number: CA000615455A
Authority: CA
Inventors: Joaquin Delgado
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1988-11-30
Filing date: 1989-09-29
Publication date: 1994-04-05
Anticipated expiration: 2011-04-05
Also published as: JPH02194079A; BR8906061A; ZA897828B; KR900007990A; JP2940875B2; NO175154B; AU619350B2; DE68926846T2; KR0155353B1; AU4295189A; NO894064D0; US5045569A; ES2088899T3; DK599989A; MX169793B; NO894064L; EP0371635A2; DE68926846D1; EP0371635B1; DK599989D0

Abstract

Abstract Hollow, polymeric, acrylate, infusible, inherently tacky, solvent-insoluble, solvent-dispersible, elastomeric pressure-sensitive adhesive microspheres having an average diameter of at least about 1 micrometer.
Preferred microspheres are those wherein a majority of the hollow microspheres contain at least one of interior void having a diameter of at least 10% of the diameter of the microsphere.
These hollow microspheres are useful as repositionable pressure-sensitive adhesives. The invention also provides pressure-sensitive adhesives consisting essentially of such hollow microspheres. Aqueous suspensions of these microspheres, processes for their preparation, spray repositionable pressure-sensitive adhesive compositions, and microsphere-coated sheet materials are also provided. Surprisingly, hollow microspheres of the invention show reduced or even eliminated adhesive transfer, in comparison with prior art repositionable pressure-sensitive adhesives which are based on solid microspheres.

Description

132~33~
80LLOW ACRYLATE POLY~ER ~ICROSPBERES

Field of the Invention This invention relate~ to hollow, polymeric, acrylate, infu6ible, inherently tacky, ela6tomeric, 10 solvent-di6persible, 601vent-in601uble microsphere6, to proces~es for their preparation, and to their u~e a6 repositionable pressure-6ensitive ~dhe6ive6.

Background of the Invention Description of the Related Art Solid, inherently tacky, elastomeric microspheres are known in the art to be useful in repositionable pre6sure-cen6itive adheGive applications. A~ used herein, the term "repositionable" refer6 to the ability to be repeatedly adhered to and removed from a 6ubstrate without sub6tantial 10s6 of adhesion capability. Microsphere-ba6ed adhe6ive6 are thought to perform well in 6uch application6 at least in part due to their "6elf-cleaning~ character, wherein 6ub6trate contaminants tend to be pu~hed a6ide and trapped between the micro6phere6 a~ the adhe6ive i6 applied. Upon removal, the adhe6ive can then 6till present a relatively uncontaminated 6urface for reapplication to the 6ub6trate. However, problem6 with micro6phere 10s6, 39 i.e., microsphere tran6fer to the ~ubstrate, and the resultant need for use o~ a primer or binder have been recognized in the art.
Numerou6 references concern the preparation and/or u6e of inherently tacky, elast3meric acrylate poly~eric micro6pheres which are ~olid in nature. Such ~phere6 and their use in aero601 adhesive ~y6tems having repositionable propertie6 are disclo~ed in U.S. Pat. No.
3,691,140 ~Silver). The~e micro6phere6 are prepared by aqueou6 6u6pension polymerization of alkyl acrylate , .

., , , :, ,: . . , monomers and ionic comonomers, e.g., sodium methacrylate, in the presence of an emulsifier, preferably an anionic emulsifier. The use of a water-soluble, substantially oil-insoluble ionic comonomer is critical to preventing 5 coaqulation or agglomeration of the microspheres.
U.S. Pat. No. 4,166,152 (saker et al.) describes solid, inherently tacky (meth)acrylate microspheres which are prepared from non-ionic alkyl acrylate or methacrylate monomer(s) in the presence of both an emulsifier and an 10 ionic suspen6ion stabilizer having an interfacial tension sufficient to prevent microsphere agglomeration. Such ~icrospheres are also disclosed in U.S. Pat. Nos. 4,495,318 and 4,598,112 (Howard), where the preparative methods involve the use of a nonionic emulsifier or a cationic 15 emulsifier. All three patents disclose utility as a "reusable adhesive". -U.S. Pat. No. 4,786,696 (Bohnel) describes a suspension polymerization process for preparing solid, inherently tacky (meth)acrylate microspheres which does not 20 require the use of either an ionic comonomer or an ionic suspension stabilizer in order to prevent agglomeration.
Rather, the pro~ess requires agitation of the vessel charge prior to the initiation of the reaction sufficient to create a suspen6ion of monomer droplets having an average 25 monomer droplet size of between about 5 and about 70 micrometers. In addition to (meth)acrylate monomer, a minor portion of a non-ionic, vinylic comonomer such as, e.g., acrylic acid may be included to modify the "tacky nature" of the microspheres.
U.S. Pat. No. 3,620,988 (Cohen) discloses a ~ethod of preparing ~bead-type polymers" which involve~ the use of a water-in601uble polymeric thickening dispersing agent. The method can be applied to produces pres~ure-sen6itive adhesives in the form of coatable bead 35 ~u~pen~ions, the adhesive6 comprising a high ~olids suspension/disper~ion of a lightly cro~slinked polymer of a higher alkyl acrylate and a tackifier.
U.S. Pat. No. 4,735,B37 (Miyasaka et al.) discloses a detachable adhesive sheet having an adhesive :- , . - ;, .
.

.

.

_3_ 132 8~31 layer containing "elastic micro-balls", wherein the microballs partially protrude from the surface of the adhesive layer. The microballs may or may not be tacky.
They can be derived from, e.g., (meth)acrylate monomer and 5 an -olefinic carboxylic acid monomer via suspension polymerization in an aqueous medium. However, no details as to the nature of the surfactants utilized, etc., are disclosed. The microballs and an adhesive are disper~ed in solvent, ~ixed, and coated, with the ratio of adhesive to 10 microballs being from about 1:10 to about 10:1. This ratio i8 disclosed to be critical in order that all microballs in the final product, including those protruding from the surface, are completely covered with the adhesive. A range of 1,000 to 150,000 pieces per square centimeter is 15 di6closed as preferred.
DE 3,544,882 Al INichiban) describes crosslinked microsphereE compo6ed of 90 to 99.5 weight percent of (meth)acrylate ester and 10 to 0.5 weight percent of vinyl type monomer, e.g., acrylic acid, having a reactive 20 functional group through which crosslinking is achieved by reaction with an oil-soluble crosslinking agent. The microspheres are prepared by dispersing in water a solution ~in organic solvent) of copolymer prepared by known methods such as solution, bulk, emulsion, or suspension 25 polymerization. ~However, the reference notes that in cases where emulsion or ~uspension polymerization is u6ed with water as a dispersion medium, it is not neces6ary to make a new aqueous dispersion.) When tacky, the spheres are said to be useful in spray or coated sheet form as "removable 30 adhesiven. The stated purpose of the invention is to provide microspheres having a uniform particle size, but it is also stated that the ~icrospheres may contain other monomers such as vinyl acetate, ~tyrene, acrylonitrile, methacrylonitrile, etc., "...to prevent partial transfer of 35 the adhesive when the carrier ~backing) is pulled away from the subfitrate... n .
U.S. ~at. Nos. 4,645,783 and 4,656,218 (Xinoshita) di6clo6e a "repeatedly usable and releasable ~heet" coated with an aqueous suspension of microspheres , . , . , -.
.
,~. ~,, . : : : , , ~ - : - ~ - .:
., , ~ .
, .,, ,, . . . ...... ~,.
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-4~ 13283~
obtained by aqueous suspension polymerization (in the presence of a protective colloid comprising casein as a main ingredient) of one or more alkyl(meth)acrylate esters, one or more a-monoolefin carboxylic acids, and one or more 5 other vinyl monomers. The microsphere6 are preferably interspersed with finer polymer particles prepared by polymerization of one or more vinyl monomers in an aqueous medium. These fine polymer particles are said to be "...
effective in improving the anchorage to the adherend and 10 the adhesion to the substrate after the aqueous 6uspen6ion prepared in accordance with the present invention is applied to the substraten.
U.S. Pat. No. 3,B57,731 (Merrill et al.) and EP 209337 tSmith & McLaurin) both address problems with 15 microsphere adhesive tran~fer. The former discloses sheets coated with the tacky elastomeric copolymer microspheres of the Silver patent and a binder material which provides sockets in which the microspheres are held by predominately mechanical forces. The latter 6tates that microsphere adhesives could be put to more demanding applications if it were not for the drawback of adhe~ive transfer. Tacky, elastomeric microspheres are then described which have a composition formed from non-ionic monomers alone or together with a proportion of ionic comonomers. The microspheres further comprise an adhesion promoting monomer having functionality which remains unreacted during polymerization of the monomers and is available for subsequently binding the micro6pheres through electrostatic interaction or chemical bonding to a substrate or binder-coated 6ub6trate. Preferably, the microspheres are derived from at least one alkyl acrylate or methacrylate ester.
In view of the foregoing, it i5 an object of this invention to reduce or eliminate problems with microsphere adhesive tr~nsfer without the need for a ~eparate binder material or for inclusion of an additional adhesion-promoting monomer.
It is a further object of this invention to provide an elastomeric microsphere-ba~ed, repositionable .. . -' , . . ';: : :
; .

.:
.
' ~ .
.

13283~
pressure-sensitive adhesive which exhibits greater shear adhesion for a given coating weight of adhesive. Thus, the adhesive can support heavier objects.
It is a further object of this invention to 5 provide an elastomeric microsphere-based, repositionable pressure-sensitive adhesive which exhibits greater peel adhesion for a given coating weight of adhesive. This yields a greater amount of tack for an equal weight of microspheres.
It has now been discovered that these objects, and others, which will become apparent from the following discussion may be achieved by preparing microsphere~ which, in addition to being inherently tacky, elastomeric, infus-ible, solvent-insoluble, and solvent-dispersible, are also 15 hollow.

Summary of the Invention This invention provides hollow, polymeric, acry-20 late, inherently tacky, infusible, solvent-insoluble, solvent-dispersible, elastomeric pressure-sensitive adhe6ive microspheres having diameters of at least about one micrometer. Preferred hollow microspheres contain one or more interior voids having diameters at least 10% of the 25 of the hollow microspheres. These microspheres are useful as repositionable pressure-sensitive adhesives.
The invention also provides pressure-sensitive adhecives based on the hollow microspheres, aqueous suspensions of these microspheres, processes for their 30 preparation, spray repositionable pressure-sensitive adhesive composition~, and pressure-sensitive adhesive-coated sheet materials.
Surprisingly, pressure-sensitive adhesives based on hollow microspheres of the invention show reduced or 35 even eliminated adhesive transfer, in comparison with prior art repositionable pressure-sensitive adhesives which are based on solid microspheres. The hollow microspheres of this invention are, in effect, "self-priming" and, thus, require neither ~eparate primer or binder material nor an : :: . . : ~

, . ~ ' ' ~ " '~' " ' 1328331 ~

additional, adhesion-promoting monomer. It has also been discovered that, relative to prior art systems, greater shear and peel adhesion for a given coating weight of adhesive can be achieved by using hollow microspheres.
This invention also provides a pressure-sensitive adhesive consisting es6entially of these hollow microspheres. More specifically, the pressure-sensitive adhesive consists essentially of hollow, polymeric, acrylate, inherently tacky, infusible, ~olvent-insoluble, 0 solvent-dispersible, elastomeric microspheres comprising:
a) at least about 85 parts by weight of at least one alkyl acrylate or alkyl methacrylate e~ter;
and b) up to about 15 parts by weight of at least one polar monomer, a majority of the microsphere~
having one or more interior void6 having a diameter of at least about 10% of the diameter of the microsphere.
Aqueous suspensions of these hollow microspheres 0 may be prepared by a two-step emulsification process comprising the steps of:
a) forming a water-in-oil emulsion of an aqueous solution of polar monomer(sJ in oil phase monomer(s);
b) forming a water-in-oil-in-water emulsion by di6persing the water-in-oil emulsion into an aqueous phase; and c) initiating polymerization preferably by application of heat ~or radiation).
Aqueous ~uspensions of hollow microspheres wh$ch contain moderately ionized polar monomer(s) may also be prepared by a simpler ("one-~tep") emulsification process comprising aqueous suspension polymerization of at lea~t one alkyl acrylate or alkyl methacrylate ester monomer and at least one non-ionic polar monomer in the pre6ence of at lea6t one emulsifier which is capable of producing a water-in-oil emulsion inside the droplets, as defined below, which is substantially stable during emulsification and polymerization. Both methods produce an aqueou6 .. : , ,; , . , ~ : .
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132833~

~uspe~sion of monomer droplets which upon polymerization become microspheres, a majority of which have at least one interior cavity that, upon drying, becomes a void.
The following terms have these meanings as used 5 herein:
1. The term "droplet" means the liquid 6tage of the micro6pheres prior to the completion of polymerization.
2. The term "cavity" mean~ a space within the wall6 of a droplet or microsphere when 6till in the 10 6u6pen6ion or di~per6ion medium prior to drying, and thu6 containing whatever medium wa~ used.

3. The term "void" mean~ an empty space completely within the walls of a polymerized microsphere.

4. The term "hollow" means containing at least 15 one void or cavity.
All percents, parts, and ratios described herein are by weight unless specifically 6tated otherwise.

Detailed Description of the Invention Alkyl acrylate or methacrylate monomers useful in preparing the hollow microspheres and pres6ure-sensitive adhesives of this invention are those monofunctional unsaturated acrylate or methacrylate e~ters of non-tertiary alkyl alcohols, the alkyl groups of which have from 4 to 25 about 19 carbon atom6. Such acrylates are oleophilic, water emul6ifiable, have re6tricted water 601ubility, and as homopolymers, generally have glas6 transition temperatures below about -20C. Included within this class of monomers are, for example, isooctyl acrylate, 4-methyl-2-pentyl acrylate, 2-methylbutyl acrylate, i~oamyl acrylate, ~ec-butyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, isodecyl methacrylate, isononyl acrylate, isodecyl acrylate, and the like, singly or in mixture~.
Preferred acrylates include isooctyl acrylate, isononyl acrylate, issamyl acrylate, i~odecyl acrylate, 2-ethylhexyl acrylate, n-butyl acrylate, fiec-butyl acrylate, and mixtures thereof. Acrylate or methacrylate or other vinyl monomers which, as homopolymers, have glass transition temperatures higher than about -20C, : . . - ., .: : - . . . : . .
- - :. . , .:,. . . .

.. ~ : -132833~
e.g., tert-butyl acrylate, isobornyl acrylate, butyl methacrylate, vinyl acetate, N-vinyl pyrrolidone, acrylamide, and the like, may be utilized in conjunction with one or more of the acrylate or methacrylate monomers 5 provided that the glass transition temperature of the resultant polymer is below about -20C.
Polar monomers suitable for copolymerization with the acrylate or methacrylate monomers are those polar monomers which are both somewhat oil-soluble and water-10 soluble, resulting in a distribution of the polar monomerbetween the aqueous and the oil phases.
Representative examples of suitable polar monomers include moderately ionized polar monomers such as acrylic acid, methacrylic acid, itaconic acid, crotonic 15 acid, maleic acid, fumaric acid, ~ulfoethyl methacrylate, 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-1-sulphonate, N,N-dimethyl-N-(~-methacryloxy-20 ethyl) ammonium propionate betaine, trimethylamine methacrylimide, 1,1-dimethyl-1-~2,3-dihydroxypropyl)amine methacrylimide, and the like. Preferred polar monomer6 are monoolefinic mono- and dicarboxylic acids, salts thereof and mixtures thereof.
The hollow microspheres of this invention and the pressure-sen6itive adhesives made therefrom compri~e ~t lea~t about 85 parts by weight of at least one alkyl acrylate or alkyl methacrylate ester and correspondingly, up to about 15 parts by weight of one or more polar monomers. Preferably, at least one polar monomer is included in the composition, but hollow microspheres may al~o ~e prepared u$ing acrylate or methacrylate monomer(6) alone or in combination only with other vinyl monomers, e.g., vinyl acetate. However, when methacrylate monomer alone i8 utilized, a cro~linking agent, infra, mu~t be in~luded. For most polar monomers, incorporation of from about 1 part to about 10 partg by weight is preferred, as this ratio provides hollow microspheres with balanced pressure-sensitive adhesive properties.

' :' ~: . ~ :

9 13283~1 Aqueous suspensions of the hollow micro6pheres may be prepared by a "two-step" emulsification process which first involves forming a water-in-oil emulsion of an aqueous solution of polar monomers in oil pha~e monomer, 5 i.e., at least one acrylate or methacrylate ester, u6ing an emulsifier having a low hydrophilic-lipophilic balance (HL~) value. Where it is desirable not to include a polar monomer, water may be mixed directly with the oil phase monomer, i.e., acrylate or methacrylate ester, and 10 emulsifier to form the water-in-oil emulsion. Suitable emul~ifier~ are those having an ~LB value below about 7, preferably in the range of about 2 to about 7. Examples of such emulsifiers include sorbitan monoleate, sorbitan trioleate, and ethoxylated oleyl alcohol such as Bri jTM 93, 15 available from Atlas Chemical Industries, Inc. Thus, in thiC fir6t 6tep, oil phase monomer(6), emul~ifier, a free radical initiator, and, optionally, a cros61inking monomer or monomers as defined below are combined, and an aqueous 601ution of all or a portion of the polar monomer(6) i6 20 agitated and poured into the oil pha6e mixture to form a water-in-oil emulgion. A thickening agent, e.g., methyl cellulose may al60 be included in the aqueou6 phase of the water-in-oil emul6ion. In the second 6tep, a water-in-oil-in-water emulsion i8 formed by di6persing the 25 water-in-oil emul6ion of the fir6t step into an aqueous pha6e containing an emulcifier having an HLB value above about 6. The aqueou6 phase may al60 contain any portion of the polar monomer(e) which was not added in step one.
Example6 of such emul6ifier6 include ethoxylated 60rbitan 30 monooleate, ethoxylated lauryl alcohol, and alkyl 6ulfate6.
In both 6tep6, when an emul6ifier i6 utilized, it6 concentration should be greater than it6 critical micelle concentration, which is herein defined a8 the minimum concentration of emul~ifier neces~ary for the formation of 35 micelle6, i.e., ~ubmicroscopic agqregation6 of emul~ifier molecule6. Critical micelle concentration i6 61ightly different for each emul6ifier, u6able concentration6 ranging from about 1.0 x 10-4 to about 3.0 moles/liter.
Additional detail concerning the preparation of ' ' : ~ . '.~., . ............................ .. ' ~ ' ': , ~ '.
..
' water-in-oil-in-water emulsions, i.e., multiple emulsions, may be found in various literature references, e.g., Surfactant Syctems: Their Chemistry, Pharmacy, & ~iology, (D. Attwood and A. T. Florence, Chapman & Hall Limited, New 5 York, New York, 1983). The final process step of this method of the invention involves the application of heat or radiation to initiate polymerization of the monomers.
Suitable initiators are those which are normally suitable for free radical polymerization of acrylate monomers and 10 which are oil-soluble and of very low 601ubility in water.
Examples of such initiators include thermally-activated initiators such as azo compounds, hydroperoxides, peroxides, and the like, and photoinitiators such as benzophenone, benzoin ethyl ether, and 2,2-dimethoxy-2-15 phenyl acetophenone. Use of a water-soluble polymerization initiator causes formation of substantial amounts of latex.
The extremely small particle size of latex particles renders any significant formation of latex undesirable. 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, preferably up to about 5 percent.
Agueous suspensions of hollow microspheres which eontain moderately ionized polar monomer(s) may also be prepared by a "one-step" emulsification proces6 comprising aqueous suspension polymerization of at least one alkyl acrylate or alkyl methacrylate ester monomer and at least one moderately ionized polar monomer in the pre~ence of at lea6t one emulsifier capable of producing a water-in-oil emulsion inside the droplets which is sub~tantially stable during emulsification ~nd polymerization. As in the two-step emulsification process, the emulsifier is utilized in concentration6 greater than its critical micelle concentration. In general, high HLB emulsifiers are required, i.e., emul~ifiers having an HLL value of at least about 25, will produce stable cavity-containing droplets during the polymerization, and are suitable for use in this one-step process. Examples of such emulsifiers include alkylarylether fiulfates such as sodium alkylarylether ;.

132833~
sulfate, e.g., Triton~M w/30, available from Rohm and Haas, alkylarylpolyether sulfates ~uch as alkylarylpoly~ethylene oxide) sulfates, preferably those having up to about 4 ethyleneoxy repeat units, and alkyl sulfates such as 5 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, preferably those having up 10 to about 4 ethyleneoxy units. Alkyl ~ulfates, alkyl ether sulfates, and alkylarylether ~ulfates are preferred as they provide a maximum void volume per microsphere for a minimum amount of surfactant. Polymeric stabilizers may also be present but are not necessary.
The composition may also contain a crosslinking agent such as a multifunctional (meth)acrylate, e.g., butanediol diacrylate or hexanediol diacrylate, or other multifunctional crosslinker such as divinylbenzene. When used, crosslinker(s) is (are) added at a level of up to 20 about 1.0 percent, preferably up to about 0.5 percent, of the total polymerizable composition.
The hollow microspheres of the invention are nor~ally tacky, elastomeric, insoluble but swellable in organic solvents, and small, typically having diameters of 25 at least 1 micrometer, preferably in the range of about 1 to about 250 micrometers. The voids typically range in size up to about 100 micrometers or larger. The majority of the hollow microspheres prepared by the methods of this invention contain at least one void with a void diameter 30 which is at least about 10% of the diameter of the microsphere, preferably at least about 20%, more preferably, at least about 30%.
Following polymerization, an aqueous su~pension of the hollow microspheres is obtained which is ~table to 35 agglomeration or coagulation under room temperature conditions. The suspension may have non-volatile 601ids contents of from about 10 to about 50 percent by weight.
Upon prolonged standing, the suspension separates into two pha~es, one phase being aqueous and substantially free of .

132833~

polymer, the other phase being an aqueous suspension of microspheres having at least one cavity, which, upon drying, becomes a void. Both phases may contain a minor portion of small latex particles. Decantation of the 5 microsphere-rich phase provides an aqueous suspension having a non-volatile solid~ content on the order of about 40-50 percent which, if shaken with water, will readily redisperse. If desired, the aqueous suspension of hollow microspheres may be utilized immediately following 10 polymerization to provide inherently tacky pre6sure-6ensitive adhesive coatings. The 6uspen6ion may be coated on suitable flexible or inflexible backing materials by conventional coating techniques sush as knife coating or Meyer bar coating or use of an extrusion die.
Alternatively, the aqueous suspension may be coagulated with polar organic solvents such as methanol, with ionic emulsifiers having a charge opposite to that of the emulsifier used in the polymerization process, or with ~aturated salt solutions, or the like, followed by washing 20 and drying. The dried hollow microspheres, with sufficient agitation, will readily disper6e in common organic liquids such as ethyl acetate, tetrahydrofuran, heptane, 2-butanone, benzene, cyclohexane, and esters, although it i~ not possible to resuspend them in water. Solvent 25 disper~ions of the hollow microspheres may also be coated on suitable backing material6 by conventional coating technique6, as described above for aqueous suspen6ions.
Suitable backing materials for the aqueous or 601vent based coatings include paper, plastic films, 30 cellulose acetate, ethyl cellulose, woven or nonwoven fabric formed of ~ynthetic or natural material~, ~etal, metallized polymeric film, ceramic sheet material, and the like. Primer6 or binder~ may be u6ed, but they are not required.
Suspensions or disper6ions of the hollow microsphere6 in a liquid medium, e.g., water or an organic liquid as described above, may be sprayed by conventional techniques without cobwebbing or ~ay be incorporated in aero~ol containers with 6uitable propellants such as - , --13- 1 328~31 alkanes, alkenes, or chlorofluorocarbons, e.g., Freons~M.
The hollow microspheres of the invention provide a repositionable pre6~ure-sensitive adhesive, i.e., a pressure-sensitive adhesive having a degree of adhesion 5 which permits separation, repositioning, and rebonding.
Useful aerosol formulae have a golids content of from about 5~ to about 20%, preferably from about 10% to about 16~.
The pressure-sensitive adhesive properties of the 10 hollow microspheres may be altered by addition of tackifying resin and/or plasticizer. Preferred tackifiers for use herein include hydrogenated rosin esters commercially available from companies 6uch as Hercules Inc., under such trade names as ForalSM, and Pentalyn M.
15 Individual tackifiers include ForalSM 65, ForalSM 85, and ForalSM 105. Other useful tackifiers include those based on t-butyl styrene. Useful plasticizers include dioctyl phthalate, 2-ethyl hexyl phosphate, tricresyl phosphate, and the like.
It is also within the scope of this invention to include various other components, cUch as pigments, fillers, stabilizers, or various polymeric additives.
The pressure-6ensitive adhe6ives of the invention have been found to show little or no microsphere transfer, 25 thereby reducing or even eliminating the transfer problems di6closed by the prior art. These pressure-sensitive adhesives also provide greater peel and shear adhesion for a given coating weight than do prior art repositionable pressure-sensitive adhesives which are solid microsphere-30 based These and other aspects of the invention areillustrated by the following examples which should not be viewed as limiting in ~cope.
TEST METIIODS

Microsphere Transfer ~ n area of coated sheet material was marked and observed using an optical microscope. The number of microspheres within the area were counted and this number :
.. . . - , - :

13283~

designated "Y". The marked area of the coated sheet was then adhered to KromekoteTM paper, a commercially available paper for the printing industry, for a few seconds and then removed. The marked area was again observed with an opti-5 cal microseope, and the number of microspheres remaining inthe area were counted and this number designated ~zn. Per-cent microsphere transfer is defined a~ 100 time6 the ratio of the difference between the number of microspheres initi-ally present (~n the marked area of the coated sheet) after 10 coating an~ the number of microsphere~ remaining in the marked area after each adhesion and removal from the paper substrate (Y - Z) to the number of microspheres initially present in the area just after coating.
lOO(Y-Z) 15 Percent transfer ~
y Peel Adhesion Peel adhesion is the force required to remove a coated flexible sheet material from a test panel measured at a specific angle and rate of removal. In the examples, this force is expressed in gram~ per centimeter (cm) width of coated ~heet. The procedure followed is:
A strip 1.27 cm in width of the coated sheet is applied to the horizontal surface of a clean glass test plate with at lea~t 12.7 lineal cm in firm contact. A 2 kg hard rubber roller is used to apply the ætrip. The free end of the coated strip is doubled back nearly touching itself so the angle of removal will be 180. The free end is at-tached to the adhe6ion tester scale. The glass test plate i6 clamped in the jaws of a tensile te~ting machine which is capable of moving the plate away from the scale at a constant rate of 2.3 meters per minute. The scale reading in grams is recorded as the tape is peeled from the glass 6urface. The data is reported as the average of the range of numbers observed dur~ng the test.

Shear Strength The shear strength i~ a measure of the cohe6ive-ness or internal strength of an adhe6ive. It is ba~ed upon the amount of force required to pull an adhesive strip from - . ~
- : . :
: . ~
.
", . ~ , . ~: ,;

1~8331 a standard flat surface in a direction parallel to the sur-face to which it has been affixed with a definite pressure.
It is measured in minutes required to pull a standard area of adhesive coated sheet material from a stainless steel 5 test panel under stress of a constant, standard load.
The tests were conducted on strips of coated sheet material applied to a stainless steel panel such that a 1.27 cm by 1.27 cm portion of each strip was in firm con-tact with the panel with one end portion of the tape being 10 free- The panel with the coated strip attached was held in a rack such that the panel formed an angle of 178 with the extended tape free end which was tensioned by application of a force of 200 grams applied as a hanging weight from the free end of the coated strip. The 2 less than 180 is 15 used to negate any peel forces, thus insuring that only the shear forces are measured, in an attempt to more accurately determine the holding power of the tape being tested. The time elapsed for each coated film to ~eparate from the test panel was recorded as the ~hear strength.

AD~ESIVE-SOLVENT EVALUATI~NS

Sprayability The composition to be tested is sprayed out of an 25 aerosol can onto aluminum foil at 22C and an immediate visual evaluation i6 made. Sprayability is considered ~good" when there is a wide pattern of finely atomized spray with no streaming. Sprayability is "fair" when the ~pray is coarse with a narrower pattern and occa~ional 30 qlobs are pre~ent in the spray.

Spray Adhesive Transfer A light coat of the composition is 6prayed at 22C onto fitandard white copier paper. At a set time after 35 application, the adhesive side of the paper is briefly contacted with an acetone-wiped plate glass specimen u6ing hand pressure. The paper is then peeled off and the glass is held up to a light source to visually determine the amount of adhesive transfer from the paper to the glass.

:

, : :
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' . :'' : ~ ' ,, - - : , ..

- ,; , .. .
-, 1~28~1 For an adhesive to be evaluated as having no adhesive transfer means that no microspheres are seen on the sub-strate when viewed with the naked eye.

Soak-In on Paper A light coat of the adhesive composition is 6prayed onto standard white copy paper and the opposite side of the paper is immediately evaluated visually for ~oak through. "Low~ soak-in means that 0-10% of the sprayed 10 area soaks through to the reverse side of the paper, "moderate" soak-in equals about 50~ soak~ through, ~high"
soak-in equals 90~ or higher soak-in.

EXAMPLES

Example 1 In a one-liter resin reactor equipped with mechanical stirrer, condenser, and inlet-outlet lines for vacuum and argon, 450 grams of deionized water, 141 grams 20 Of i~ooctyl acrylate, 0.04 gram of 1,4-butanediol-diacrylate, 9.0 grams of acrylic acid and 0.5 gram of benzoyl peroxide were charged. Vacuum was applied to evacuate the reactor atmosphere, and the reactor was then purged with argon. The aqitation was set to 400 rpm and 25 when the initiator had dissolved, 1.5 grams of ammonium lauryl sulfate (StandapolSM A, Henkel AG) were added. The temperature of the reactor was raised to 60C and main-tained at ~uch temperature for 22 hours. An argon purge was maintained during the polymerization. After the 22-hour period, the suspension was ~llowed to cool to room temperature. The reactor wa6 then emptied and the su6pen~ion filtered. Optical microscopy revealed hollow microspheres from about 4 to about 90 micrometers in diameter suspended in water. The majority of the micro-~phere6 contained a central cavity having a cavity diameter of at ~east ~0% of the diameter of the micro~phere. After drying in a vacuum oven~ the microspheres were microtomed.

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13~8~31 Scanning electron microscopy also showed that the micro-spheres contained large central voids having diameters of from about 2 to about 65 micrometers.

Examples 2-11 These examples illustrate the use of different polar monomers and initiators to prepare hollow, tacky elastomeric microspheres using the general equipment and one-step emulsification techhnique outlined in Example 1.
10 Details of the compositions are listed in Table I. In all cases 1.5 grams of ammonium lauryl sulfate (Standapol~n A, ~enkel AG) were used. The reactor temperature was 60C in all the following examples except in Examples 6 and 11 where 50C was used.

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Examples 12 to 17 These examples illustrate the use of various alkyl (meth)acrylate ester monomers (see Table II~ for the preparation of hollow, tacky, elastomeric microspheres 5 using acrylic acid as the polar monomer and 1.5 grams of ammonium lauryl 6ulfate as the surfactant. The polymerization equipment and polymerization technique used were those described in Example 1.

Table II
Exa-ple Alkyl ~ethacrylate Acrylic acid Initiator Nu~ber (g) (Benzoyl Peroxide) 12141 g 2-ethyl-hexyl 9.0 0.5 g 15 acrylate 13144 g n-butyl acrylate 6.0 0.5 g 16144 g isononyl acrylate 6.0 0.5 g 15135 g isooctyl acrylate 3.0 0.5 g 20 12 g methyl methacrylate 16141 g lauryl acrylate 9.0 0.5 g 17 127.5 q isooctyl acrylate 4.5 0.5 g 18 g vinyl acetate 25 Examples 18 to 24 These examples illustrate the use of various surfactants a6 well as various multifunctional monomers in the preparation of hollow, tacky elastomeric microspheres (see Table III). In all cases 0.5 gram of ben~oyl peroxide 30 wa6 used except in Example 20 where 0.5 gram of lauryl peroxide was 6ubstituted. Example 24 illustrates the u6e of a combination of a nonionic surfactant and an anionic ~urfactant.

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1328~31 Table I I I
Example Monomer~ Surfactant Number 5 18 141 g isooctyl acrylate 1.5 g sodium lauryl 9.0 g acrylic acid sulfate 19 141 g isooctyl acrylate 1.5 g sodium hexadecyl 9.0 g acrylic acid sulfate 73.5 g isononyl acrylate 1.6 g triethanolamine 73.5 g 2-ethyl hexyl lauryl sulfate acrylate 3.0 g acrylic acid 0.1 g 1,6-hexanediol diacrylate 21 120 g i600ctyl acrylate 1.5 g ammonium lauryl 24 g isononyl acrylate ether sulfate 6.0 g acrylic acid 0.1 g 1,4-butanediol diacrylate 22 141 g isooctyl acrylate 14 g TritonTM W/30 9.0 g acrylic acid 23 141 g isooctyl acrylate 1.5 g ammonium lauryl 9.0 g acrylic acid sulfate 0.08 g divinyl benzene 24 146 g isooctyl acrylate 1.5 g ammonium lauryl 2.0 g acrylic acid sulfate 1.0 g methacrylic acid 1.0 g SiponateTM
Y500-70**
*Triton~M W/30 i6 a tradename for a 27% aqueous solution (also containing 27 percent 2-propanol), of sodium alkylaryl ether sulfate available from Rohm and Haas Company.
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Examples 25C to 27C
These are comparative examples. When a ~urfactant with a ~LB value les~ than about 25 was used in the 35 one-~tep polymerization process, tacky, ela~tomeric micro~pheres having no voids were formed. Polymerization equipment and polymerization technique used were those de~cribed in Example 1. Benzoyl peroxide ~0.5 gm) was u~ed . . ~ . ; . : . . , - ~
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Table IV

Example ~ono~ers 8urfactant Nu~ber 25C 141 9 isooctyl acrylate 1.5 g sodium dodecyl 9.0 g acrylic acid benzene sulfonate 10 26C 141 g isooctyl acrylate 1.5 9 sodium dodecyl 9.0 g itaconic acid benzene sulfonate 27C 141 g isooctyl acrylate 7.0 g Triton x200*
9.0 g acrylic acid **TritonTM X-200 is a tradename for a 28 weight percent aqueous suspension of sodium alkylaryl polyether 6ulfonate 15 available from Rohm and Haas Company.

Example 28 The following example illustrates the preparation 20 of hollow pressure-sensitive adhesive microspheres using a two 6tep emulsification process. S~x grams of ammonium acrylate were dissolved in 450 grams of deionized water. A
water-in-oil emulsion was prepared in an OmniTM mixer by stirring lOO grams of the above-mentioned aqueous solution 25 with 144 grams of isooctyl acrylate containing 3 ~rams of Span 80~M, ~orbitan monoleate available from ICI Americas, Inc. and 0.5 gram of benzoyl peroxide. The remaining ~mmonium acrylate aqueous solution was placed in a re6in reactor similar to that described in Example 1 and 1.5 30 grams of ammonium lauryl ~ulfate were added. The agitation wa6 ~et to 400 rpm. The oil-in-water emulsion prepared previously was added to the reactor. The temperature was increased to 60C and maintained for 22 hours. After the 22-hour period, the 6uspension wa6 allowed to cool to room 35 temperature. The reactor wa~ emptied and the suspen~ion filtered. Optical microscopy of the suspension 6howed micro6phere6 having diameters of from about 4 micrometers to about 30 micrometers, a majority containing internal cavities.

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Ex ample 29 This example illustrates the use of a thickening agent in the aqueous phase of the water-in-oil emulsion for 5 the preparation of hollow microspheres by the two-step emulsification process.
The emulsification and polymerization equipment were those described in the previous example. An aqueous solution of 6 grams of ammonium acrylate in 450 grams of 10 water was prepared. Two grams of methyl cellulose were added and dis601ved into 100 grams of this aqueous 601ution. A water-in-oil emul6ion of the aqueou~ 601ution of the ammonium acrylate and methyl cellulose in 144 gram6 of isooctyl acrylate containing 5.75 grams of "Span 80" and 15 0.5 gram of benzoyl peroxide was prepared as described in Example 29. The water-in-oil emul6ion was poured into the reactor containing the rest of the ammonium acrylate aqueous solution and l.S grams of ammonium lauryl sulfate.
The temperature of the reactor was 60C when the 20 water-in-oil-emulsion was poured, and the agitation was 300 rpm. The reactor was kept at 60C for 22 hours. At the end of this period, the suspension was treated as in Example 23. The diameter of the cavi~y containing micro6pheres was in the range of from about 4 to about 40 25 micrometers~
Examples 30 to 33 Examples 30 to 33 were executed following the procedure described in Example 27. Materials and 30 conditions used and diameter of the resulting micro6pheres are ~pecified in Table V.

Example 34 This example illu6trates the use of a 35 photoinitiator.
A ~olution of 2.5 grams of N-(3-6ulfopropyl)--N-methacryloxy ethyl N,N-dimethyl ammonium betaine and 150 qrams of deionized water was prepared in a three-neck 200cc Morton flask. ~hirty-five cc of the aqueou6 ~olution ~. ,: , : , .

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of the betaine were emulsified for 15 minutes into 47.5 grams of isooctyl acrylate containing 0.02 gram of butanediol diacrylate, 4 grams of "Span 80" and 0.125 gram of 2,2-dimethoxy-2-phenylacetophenone in an Omni~M mixer to 5 form a water-in-oil-emulsion. The water-in-oil-emulsion was ~lowly added to the reactor. ~he reactor contained the rest of the aqueous solution of the betaine, 1.172 grams of Span 20 and 0.33 gram of Tween 80SM, ethoxylated sorbitan monoleate available from ICI Americas, Inc. while stirring.
10 Fifteen minutes after the completion of the addition of the water-in-oil emulsion, 10 cc of the water-in-oil-in water emulsion were taken from the reactor and placed in a transparent rectangular glass cell. The cell was then irradiated for 10 minutes with ultraviolet light. After 15 the irradiation period, the suspension was recovered and observed in the optical microscope. Microspheres having diameters of from about 1 micrometer to about 20 micrometers were observed. These microspheres had cavities in their interior. The majority of the 20 microspheres contained at least one void having a void diameter of at least 10% of the diameter of the microsphere.

Example 35C
This example illustrate8 the fact that use of a ionic monomer in the one-step emulsification process yields solid tacky microspheres rather than hollow. In a one-liter resin reactor equipped with mechanical ~tirrer, condenser, and inlet-outlet lines for vacuum and argon, 450 30 grams of deionized water, 144 grams of isooctyl acrylate, six grams of ammonium acrylate, 0.5 gram of benzoyl peroxide and 1.5 grams of ammonium lauryl ~ulfate were charged as described in Example 1 under identical condition8 of temperature and agitation. A su~pension was 35 recovered from the reactor. Optical microscopy ~howed microsphere~ of from about 4 to about 40 micrometer8 in diameter in the ~uspension. The~e microspheres had essentially no internal cavities.

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Example 36 This example compares the adhesive properties of 5 sheet material coated with hollow, tacky pressure 6ensitive adhesive microspheres and with solid tacky pressure-sensitive adhesive microspheres.
Microspheres prepared in Example 2 (94:6 isooctyl acrylate:acrylic acid hollow microspheres), Example 25C
10 (94:6 isooctyl acrylate:acrylic acid solid microspheres), and Example 35C (96:4 isooctyl acrylate:ammonium acrylate non-hollow microspheres) were dispersed in heptane (5~
microspheres) and coated with a knife coater on 49 micro-meters thick unprimed cellulose acetate film at a coating 15 weight of 4.1 g/m2. The coated samples were dried at room temperature and conditioned overnight at a constant temperature of 22.2C and 50% relative humidity. The coated samples were then tested for adhe&ive transfer, peel adhesion, and shear strength. Test results are shown in Table VI.

Table VI
Microsphere ~ Transfer % Tran6fer Peel Shear 25 Sa-ple after one after t~o Adhe6ion Strength adhe6ion ~dhe6ion6 (g/cm) (~inute~) and one and two removal removal~
Example 2 6.0 18.6 204 248.0 ~hollow) Example 25C 48.7 76.9 178.6 59.5 ~olid) Example 35C 76.4 92.7 153.4 35.0 (solid) As can be observed in the test results 6hown above, the 6heet material coated with hollow microsphere6 exhibited much lower adhesive tran6fer, higher peel ~dhesion, and higher shear strength than either the 6heet material coated with ~olid microspheres of the same polymer , ' ' ' ' . ,.... '': ~ . . :

composition or the sheet material coated with isooctyl acrylate:ammonium acrylate solid microspheres.

Example 37 This example illustrates the use of hollow pressure-sensitive adhesive microspheres of the invention in aerosol adhesive ~ystems. The microspheres were tested in various solvents for sprayability, paper soak-in and adhesive transfer. As the results in Table VII show, the 10 microspheres provided good sprayability with low adhesive transfer in a wide variety of solvents.

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Claims

1. Hollow, polymeric, acrylate, inherently tacky, infusible, solvent-insoluble, solvent-dispersible, elastomeric pressure-sensitive adhesive microspheres having a diameter of at least 1 micrometer.

2. Hollow, polymeric, acrylate, inherently tacky, infusible, solvent-insoluble, solvent-dispersible, elastomeric pressure-sensitive adhesive microspheres according to claim 1 wherein a majority of said microspheres contain at least one interior void having a diameter of at least about 20% of the diameter of said microspheres.

3. Hollow, polymeric, acrylate, inherently tacky, infusible, solvent-insoluble, solvent-dispersible, pressure-sensitive microspheres according to claim 1 comprising:
a) at least about 85 parts by weight of at least one alkyl acrylate or alkyl methacrylate ester, and b) correspondingly, up to about 15 parts by weight of at least one polar monomer.

4. The hollow microspheres of claim 3 wherein the alkyl acrylate is selected from the group consisting of isooctyl acrylate, 2-ethyl hexyl acrylate, isononyl acrylate, isoamyl acrylate, isodecyl acrylate and butyl acrylate, and the polar monomer is selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, and salts thereof.

5. A particulate pressure-sensitive adhesive consisting essentially of the hollow microspheres of claim 1 or claim 3.

6. A repositionable spray pressure-sensitive adhesive comprising the hollow microspheres of claim 1 and a liquid medium therefor.

7. A repositionable spray pressure-sensitive adhesive according to claim 6 further comprising a propellant selected from the group consisting of alkanes, alkenes, and chlorofluorocarbons.

8. A sheet material having coated on at least a portion thereof the pressure-sensitive adhesive of claim 5.

9. A process for preparing an aqueous suspension containing the hollow microspheres of claim 1 comprising the steps of:
a) forming a water-in-oil emulsion of a water phase selected from the group consisting of water and aqueous solutions of at least one polar monomer in at least one oil phase monomer selected from the group consisting of alkyl acrylate and alkyl methacrylate esters;
b) forming a water-in-oil-in-water emulsion by dispersing the water-in-oil emulsion into an aqueous phase containing an emulsifier having a hydrophilic-lipophilic balance value of at least 6; and c) initiating polymerization.

10. A process according to claim 9 wherein the water-in-oil emulsion further comprises an emulsifier having a hydrophilic-lipophilic balance value of less than about 7.

11. A process according to claim 9 wherein polymerization is initiated by means of exposure to radiation.

12. A process according to claim 9 wherein polymerization is initiated by means of exposure to heat.

13. A process for preparing an aqueous suspension containing the hollow microspheres of claim 1 comprising the steps of:
a) forming droplets by mixing together i) at least one monomer selected from alkyl acrylate esters and alkyl methacrylate esters, ii) at least one moderately ionized polar monomer, and, iii) at least one emulsifier which is capable of producing a water-in-oil emulsion inside said droplets, said emulsion being substantially stable during emulsification and polymerization.
b) initiating polymerization.

14. A process according to claim 13 wherein said emulsifier has an HLB value of at least 25.

15. A process according to claim 13 wherein polymerization is initiated by means of exposure to radiation.

16. A process according to claim 13 wherein polymerization is initiated by means of exposure to heat.