GB2109802A - Emulsion polymerization - Google Patents

Abstract

A stable aqueous polymeric adhesive latex has a dispersed polymer phase comprising particles having integral core and outer shell portions formed by the emulsion polymerization of a shell phase monomer system containing at least 60% acrylate monomer in the presence of core particles comprising polymer or polymer-sub-polymer- monomer species obtained by emulsion polymerization of mono- olefinic monomer e.g. styrene and/or methyl methacrylate. Each of the emulsion polymerizations is preferably carried out with the same stabilizer, emulsifier and catalyst system. The latex product comprises a major portion of core-reinforced shell polymer enhancing adhesive utility and is suitable for forming into transfer tapes.

Description

SPECIFICATION Emulsion polymerization method and pr.oduct This invention relates to emulsion polymerization and particularly to an emulsion polymerization process which provides a stable, aqueous polymeric latex, the dispersed polymer phase comprising particles having an inherently reinforced core-shell structure. The invention further relates to latices produced by such a process and adhesive articles prepared from such latices.

To be effective pressure sensitive adhesive should have good modulus characteristics, e.g.

cohesive strength, as well as adequate adhesion to a variety of surfaces. Many of the monomers typically used in preparing adhesive latices often require the incorporation of less tacky, but relatively high modulus, monomer materials, also referred to as high Tg fesin-forming monomers.

Increase in polymer modulus characteristics is often at the expense of the essential tack and adhesion properties sometimes necessitating the use of compensating auxiliary tackifiers; however, the use of such materials may entail significant risk of latice destabilisation, impairment of essential modulus characteristics or alteration in the aging characteristics, e.g. UV stability thereby proving largely self-defeating. Generally, modulus-enhancing monomers, e.g. one or more of methyl methacrylate, styrene, acrylonitrile, and methylacrylate, are included with the latex-forming monomer system, composed for example of copolymerizable acrylates, and thus become incorporated into the acrylate polymer chain.The amount of methyl methacrylate or other high Tg resin-forming monomer used for such purposes is usually and necessarily quite small to avoid or minimize detackification of the product latex.

Thus, an object of the invention is to provide an emulsion polymerization process for producing latices in which the foregoing disadvantages have been eliminated or at least substantially reduced to provide a product having good stability, adhesion and modulus properties.

According to the present invention there is provided an emulsion polymerization process for preparing an aqueous, stable polymer latex composed of emulsified particles having an inner or core phase of high Tg resinous polymer and a polymeric outer or shell pHase integral with said core phase, the process comprising contacting at a temperature of from 50 to 850C a dispersion of core phase particles, being the polymerizate in emulsion form of one or more high Tg resin-forming mono-olefinic monomers, with an addition-polymerizable, shell phase-forming monomeric composition comprising at least 60% acrylate monomer, in the presence of an effective amount of polymerization catalyst and a stabilizing amount of emulsifying agent, to produce a latex having a solids content of from 40 to 60%, the weight ratio of said monomeric composition to core phase particles being from about 3:1 to 10::1, a pH of from 2 to 8 being maintained throughout the process. The process according to the present invention enables the use of relatively large amounts of reinforcing, high modulus monomer for adhesive latex formation with little or no adverse effect upon essential latex properties, e.g. viscosity.

It is essential that the acrylate monomer phase undergo emulsion polymerization in the presence of a preformed, emulsified dispersion of core particles, the latter preferably having been prepared by an emulsion polymerization process and more preferably by such a process employing stabilizer and catalyst identical with those used in the polymerization of the acrylate monomer composition to form the shell phase. In certain embodiments to be explained in detail later, the core phase particles comprise both polymer and sub-polymer species corresponding to partial, e.g. about 20% or more, conversion of high Tg monomeric reactants.Owing perhaps to increased reactivity levels as well as molecular weight distribution of the product, acrylate-partial polymerizate contacting is capable of producing a latex having high shear properties, the latter being particularly manifest when specific types of emulsifiers to be described in detail later are employed. In these embodiments, shell phase monomer in the initial stages of the polymerization process is relatively rich in the core monomeric material, the latter being a mixture of polymer, monomer and partial polymerizate as indicated.

Accordingly, amounts of polymeric product resulting from the reaction of the core monomeric material are found as a uniformly dispersed phase in the shell portion of the product latex particles. Whether the core particles available for interaction with the acrylate monomer phase be fully or partly polymerized at the time acrylate phase polymerization is initiated-and the process of the invention includes either case-the core portion comprises a uniformly dispersed phase, this being clearly demonstrated by the bluish cast of the dry polymer film when cast onto 1 mil MYLAR and analyzed by light scattering (Tyndall effect).

It thus appears, although the Applicants do not wish to commit themselves, that due to polymer chain entanglement and/or grafting and/or other stable bonding mechanism, the respective core and shell phase polymerizates become uniformly and mutually dispersed at least in the region of their common interface. The high-TG core phase thus reinforces the acrylate phase and the total structure can be regarded in a sense as a "quasi" block copolymer, this again being indicated by light scattering analysis. Regardless of the bonding mechanism actually involved, the outer shell phase becomes integral with the core phase in the manner described, thereby providing a stable latex product useful for various adhesive purposes.

Factors affecting carrying out the process according to the present invention include the manner of monomer addition, i.e. whether directly or as an aqueous pre-emulsion with one or more other ingredients such as catalyst and monomer; the catalyst-emulsifier system, i.e., the choice of amount and type to determine specific properties of the latex product; the type and amount of shell phase monomeric composition in relation to core phase particles; the pH values best conductive to effective stabilizer function and the choice catalyst type and amount, as factors affecting the achievement of the optimum contact of ingredients within the reaction vessel.

As will be made evident, one or more of the aforementioned parameters can be determined in a given instance to enhance specific properties of the latex product, e.g. adhesion to steel, polyethylene and the like or shear-resistance for SMACNA (Sheet Metal Air-Conditioning National Association) tape applications. In all cases, the polymeric latices of the invention are useful for transfer tapes.

The "seed" polymerization method described herein affords significant advantage over "nonseed" systems wherein all monomer is provided as a single phase, i.e. wherein all of the core monomer would be included as a component of the acrylate phase. In the latter case, a core-shell structure is not obtained as demonstrated by actual experiment. Consequently, one of the signal advantages of the present invention, i.e. core reinforcement of shell phase polymer, is not obtained.In the present "seed" polymerization method, greater proportions of core monomer, based on total amount of acrylate monomer, can be used to enhance the modulus characteristics of the latex product by the aforedescribed reinforcement mechanism, with little or no adverse effect on latex viscosity and stability, the latter term primarily connoting the capacity of the latex to retain its dispersed structure for prolonged periods without significant coagulum formation. Thus, the latex product is generally low in floc and possesses viscosity fully compatible with commercial adhesive application. Typically, the product has enhanced cohesive strength as measured by standard corrugator lap splice testing as described in U.S.Patent 4,204,023; and is capable of forming high-strength adhesive bonds with a variety of surfaces even though applied as a relatively thin film, for example of a thickness of 0.7 mii.

The adhesive bond may be further enhanced by the use of commerciai tackifiers such as Foral Emulsion Tackifiers.

Monomeric compositions suitable for shell phase polymer formation generally comprise one or more addition-polymerizable mono-olefins, at least 60% and preferably at least 75% by weight of any monomer mixture being acrylate monomer, and any remainder being composed of copolymerizable polar monomer such as acrylic acid (AA) and/or self-curing monomer such as isobutoxymethacylamide (IBMA), the latter being present in amounts up to about 1% and preferably from 0.05 to 0.5% by weight of the shell phase monomer mixture. Particularly preferred for purposes of preparing pressure sensitive adhesives are monomer mixtures comprising at least 75% 2-ethylhexylacrylate with up to 20% ethylacrylate, 5% methyl acrylate and 5% acrylic acid. The afore-mentioned acrylates inherently form solid, tacky-soft polymers as is known in the art.Other useful acrylates generally comprise the esters of acrylic acid and primary and secondary alkanols containing about 4 to 12 carbons. The polymethacrylates are generally less tacky and of a relatively rigid, rubbery texture or feel and in this sense tougher than the corresponding polyacrylates. Methyl methacrylate (MMA), for example, in an amount of up to about 6% of the shell monomer mixture, may be used if desired to increase the cohesive strength of the product polymer latex.

Emulsifiers useful as stabilizers herein generally comprise amphoteric, or anionic, materials often referred to as surfactants. In some cases, the stabilizer may have both anionic and nonionic moieties as do certain anionic/nonionic sulfosuccinates available commerically, e.g. the material available from American Cyanamid Co. as AEROSOL 501. Preferred stabilizers include (A) The half maleate reaction product of maleic anhydride (MA) with an ethoxylated C10C, linear alcohol or ethoxylated alkyl phenol, e.g. octyl or nonyl phenol containing 4 to 9 moles condensed ethylene oxide, the half maleate having the formula: HOOCCH=CHCOO(CH2CH2O)XR, wherein R is linear C8-C30 alkyl or the moiety

R1 being o-, m- or p-octyl or nonyl, x is from 4 to 9 and n is 1 to 3. These half maleates are prepared by reacting the indicated starting materials at a temperature of from about 80 to 1O00C in a bulk, solvent-free system usually in equimolar reactant proportions, as described in copending US Application in the name of Edward Witt, Serial No. 317,207 filed November 2, 1981, entitled Emulsion Polymerization stabilizers, the reievant disclosure of which is hereby incorporated by reference; (B) Ethoxylated alkali metal (e.g. sodium or potassium) or ammonium C9-C18 alkyl ether sulfates containing 2 to 1 5 moles condensed ethylene oxide, e.g. the ethoxylated (3 moles) sodium tridecylether sulfate supplied as SIPEX EST by Alcolac Chemical Co.;; (C) The reaction product of maleic anhydride, di-(C1-C4) alkylaminoethanol, chloracetamide and ammonium C8C28 alkyl sulfate as described in U.S. Patent 3,925,442, being an amphoteric species; and (D) an anionic/nonionic sulfosuccinate such as the aforementioned AEROSOL 501. The stabilizer is used in a relatively small amount which is effective in any event to stabilize the latex product, the amounts ranging generally from 0.5 to 6 phm (parts per hundred parts monomer). Optimum amounts in a particular case are related to, for example, the type of emulsifier, the pH of the core particle as well as the shell monomer phase, and the type of catalyst used, i.e. ionic vis-à-vis nonionic or ionically inactive species, as well as to the properties desired in the latex product.However, the pH is of primary importance.

Thus the pH selected should in any event be substantially non-interfering with respect to the stabilizing as well as other functional effects of the emulsifier. For example, in the case of the half maleates (A), it is generally recommended that the pH in the reaction medium, and of course, in the shell monomer phase, be maintained at a pH greater than 5.0. As explained in the above-mentioned copending application, the half maleates become increasingly less water-soluble below the stated pH value and this can impair their function; but with certain monomers, greater latitude is permitted.In general, the following emulsifier proportions and pH ranges of the reaction medium, give the best results for the emulsifiers (A)-(D): pH Proportions (phm)(fl) (A) 6 to 7 3 to 6 (B) 3to7 2to4 (C) 2to4 2to4 (D) 3 to 5 0.5 to 1.5 abased on monomer (core or shell) present in that phase but excluding stabilizer.

As used herein, the term "stabilizing amount" is to be accorded a significance consistent with the foregoing ranges.

Catalysts suitable for initiating shell phase polymerization of the acrylate monomer system under the conditions described are well known in the art, preferred species including the peroxy type, e.g. tbutyl hydroperoxide (TBHP-90) supplied for example as a 90% solution, and alkali metal persulfates, e.g. potassium persulfate. Catalyst amounts are usually quite small and need be effective only to initiate the polymerization reaction, such amounts generally ranging from 0.2 to 0.4 phm (shell or core monomer phase). Particularly effective results are obtained by the use of a co-catalyst reductant usually in the form of an aqueous solution. In the case of persulfate initiator, sodium bisulfite solution in a small amount providing a 0.4 to 1 weight ratio with respect to the persulfate is recommended.In the case of TBHP initiator, a sulfoxylate reductant system comprising for example Fe(NH4)2(SO4)2. 6H2O and NaSO2CH2OH . 2H2O in amounts providing, respectively, a 0.005:1 to 0.01:1, and a 0.5:1 to 0.8:1, weight ratio with respect to TBHP is recommended. The temperature for shell phase polymerization is generally from 60 to 750C.

Core phase particles useful in the present invention are obtained by the emulsion polymerization of one or more high Tg resin forming mono-olefinic monomers including styrene, methylmethacrylate, acrylonitrile, vinyl chloride and methylacrylate, with preferred polymerizates being derived from the polymerization of styrene and/or methyl methacrylate. In a particular preferred embodiment, the total core monomer is entirely constituted by styrene.

The core phase emulsion polymerization is preferably carried out utilizing emulsifier and catalyst reductant identical with those to be employed in effecting the shell phase polymerization. Proportions of these materials on the basis of total core phase monomer are generally equivalent to those described for the shell phase polymerization. Decreased amounts are of course involved since the amount of core phase monomer constitutes but a fraction of the total amount of shell phase monomer.

The selection of pH in a particular instance is based essentially on the relevant governing factors discussed in connection with shell phase polymerization. Polymerization is best effected at a temperature of from about 60 to 750C, although in most cases, operation within the lower 60 to 650C range is adequate, particularly with styrene monomer. As indicated previously, relatively large amounts of core phase monomer can be effectively used herein enabling correspondingly greater realization of the favourable modulus characteristics inherent in such materials, and thus greater shell phase reinforcement. The ratio of total shell phase to total core phase monomer, or alternatively core particles, may vary from about 3:1 to 1O:1,with a range offrom 5:1 to 8:1 being preferred.

Optional ingredients to achieve particular effects include FORAL, a well known tackifier and DAXAD-1 1, a dispersant comprising the sulfonated reaction product of formaldehyde and naphthalene and supplied by Dewey and Almy, a Division of W. R. Grace Co. FORAL enhances the latex adhesion property, i.e. increases its adhesive aggressiveness, whereas DAXAD-1 1 generally reduces floc levels.

However, care should be exercised with the latter material since it may in some cases tend, particularly when used with AEROSOL 501 type stabilizers, to reduce the shear properties of the latex product, perhaps due to a reduction in particle size, and thus molecular weight, of the latex polymer.

Generally, shell phase polymerization is effected by adding monomer, catalyst, reductant, stabilizer etc. to a preformed emulsion of the aforedescribed core particles, with any required pH adjustment being effected by addition of base such as ammonium hydroxide. In most cases, monomer, emulsifier and catalyst (excluding reductant which is separately added) are combined as an aqueous pre-emulsion (2.5:1 to 3:1 water) for addition to the polymerization reaction mass.

Also, total catalyst, monomer, stabilizer, etc., may be initially charged or 'added sequentially at predetermined intervals throughout the course of the polymerization. One such procedure found to be effective particularly in the case of persulfate initiated core and shell monomer systems, involves continuous addition of monomer and catalyst, preferably non-emulsified, over the course of the reaction, with stabilizer being added at e.g. in a total amount of 1/4 or 3/4 of total monomer feed in equal portions.

Shell phase monomer is contacted with emulsified core particles when the latter has achieved a degree of polymerization corresponding to at least about 20% conversion of precursor monomeric reactants. In actual runs, it is found that half maleate (A) as well as ethoxylated alkyl sulfate (B) stabilized systems utilizing TBHP catalyst and preceeding as described generally produce substantially fully polymerized systems approximately one hour after polymerization commences. Under similar conditions, an AEROSOL 501 stabilized system exhibited only about 28% conversion. However, shell phase polymerization effected in contact with the partial polymerizate is found to produce a shear resistant latex and thus forms a valuable feature of the invention.

To assure high conversion and formation of a fully polymerized latex product and particularly with emulsifier (B)- and (C)-stabilized systems, post catalyst addition may be advisable wherein a small amount of catalyst and reductant, constituting about 0.01 to 0.1 of that added during the core-shell forming polymerization, is added to the reaction mass approximately 1/2 hour after completion of said major polymerization. In this manner, unreacted monomer or sub-polymer species is confined to a minimum, if not eliminated.

It should be understood that core-phase polymerization is preferably effected in accordance with the criteria governing shell phase polymerization as to pH, ingredient proportions, process temperature and the like. The core phase particles may be produced utilizing stabilizer and catalyst differing in type and amount from those used in the shell phase polymerization or vice-versa. Furthermore such core particles may be obtained by post-emulsification of a polymerizate produced by other than an emulsion process. However, to attain those beneficial effects unique to the interphase coaction of the ingredients herein specified, it is recommended that each of the core and shell phases be provided in the manner described.

The following Examples are given for purposes of illustration only and are not to be considered as limiting the scope of the invention; all parts are by weight. (A)-stabilized systems are evaluated in Examples 1-8 for the effect of pH and stabilizer levels on latex floc and conversion (Table 1); increasing seed polymer concentration (Example 7); using polymethylmethacrylate as the seed polymer (Exampie 8); adhesion properties of transfer tapes prepared with the tested latices (Table 2); corrugator lap splice testing (Table 3); adhesion to polyethylene (Table 4), both tackifier loaded and non-loaded samples being tested; comparison with non-seeded polymerizates (Table 5); and SMACNA tape utility.

Example 1 Core or seed phase particles are obtained by polymerizing the following composition: (a) Parts Styrene 1 5.00 H2O 62.75 Stabilizer* 0.75 DAXAD-1 1 0.50 TBHP-90 0.0273

Fe(NH4)2(SO4)2. 6H O reductant 0.000197 NaSO2CH2OH. 2H2O 0.02 at a temperature of 60-650C, pH being adjusted to 6.3-6.5 with NH4OH. A conversion of 53.5% is obtained in 1/2 hour and 100% conversion in 1 hour. The latter latex sample is taken for use.

A shell phase composition is prepared as follows:

(b) Parts 2-EHA 80.00 EA 16.00 MA 1.9 AA monomer 2.0 IBMA pre-emulsion 0.1 H2O 35.00 Stabilizer of (a) 5.00 TBHP-90 1 0.18 Fe(NH4)2(SO4)2. 6H201 reductant 0.0014 NaSO2CH2OH.2H2O solution 0.12 H2O J 15.00 *Reaction product of maleic anhydride and Tergitol 1 5-S-5 (Union Carbide)-ethoxylated (5 moles) linear C11-C15 alcohol.

**Sodium formaldehyde sulfoxylate (SFS) Shell phase polymerisation is effected at 68-700C by first forming the monomer pre-emulsion consisting of the monomer, stabilizer, and catalyst (oxidant). The monomer pre-emulsion and reductant solution are then added continuously to the 1-hour part (a) polymerizate over 16 to 2 hours, pH being maintained at 6.3-6.5 throughout. The ratio of total shell monomer to core monomer is 100:15. A stable latex product with negligible pre-floc formation is obtained wherein a majority of the particles comprise the aforedescribed core-shell structure, the core layer comprising a uniformly dispersed phase as indicated by light scattering analysis. Testing of the latex indicated good adhesion to polyethylene and steel.

Examples 24 The effects of pH and stabilizer concentration on the stability of the reaction are tested. Example 1 is repeated with pH of the core and shell phases as well as stabilizer concentration (shell phase) being varied as indicated. The results are summarized in Table 1.

Table 1 Example No.

2 3 4 5 6 Stabilizerconc. 4 4 4 5 6 Core pH 8.5 7.2 6.0 6.0 6.3 Shell pH 7.7 7.3 6.1 6.1 6.3 % Floc 1.3 2.6 3.6 nil nil % Conversion > 99 > 99 > 99 > 99 > 99 At 4 parts stabilizer (Examples 2--4), floc levels decrease with increasing pH. However, at high pH (Example 2), floc level is still relatively high.

Increasing stabilizer concentration from 4 to 5 phm significantly reduces floc (3.6 to nil) at the same pH. (Examples 4 and 5). Floc level thus seems to be more sensitive to stabilizer concentration than to pH within the limits tested. In all cases, conversions of > 99% were obtained.

The effects of increasing the amount of core, i.e., reinforcing monomer in the seed or core phase is evaluated in Example 7.

Example 7 Example 5 is repeated but increasing the styrene core polymer concentration to 30 phm based on shell phase monomer. The product latex (49% solids) indicated 99% conversion and a 5.1% floc level.

As indicated in Table 2, physical properties of the related product were not adversely affected by the increase in core phase concentration.

As indicated in Example 8, polymethyl methacrylate can replace polystyrene as the core phase polymer without appreciable adverse effects.

Example 8 Example 5 is repeated but employing polymethyl-methacrylate as the core or seed polymer. The product latex (51.3% solids) indicated 98% conversion and nil floc. Properties (Table 2) are equivalent to those obtained with the polystyrene core.

Samples for transfer tape testing and adhesive evaluation (Table 2) are prepared by hand casting the latex products of Examples 5, 7 and 8 onto release paper and Mylar base followed by air drying and curing for 1 minute at 1 490C.

Samples thus prepared provide a film which was found to have a modulus sufficient to maintain film integrity during slitting and application. When the tape (polymer film) is attached to a substrate followed by lifting the film with the liner removed, the polymer film broke cleanly with little elongation, i.e. no rubber band effect. This clearly shows the improved film modulus of latices prepared in accordance with the invention.

Transfer tapes prepared from the latex of Example 5 were produced on the pilot spreadline.

The latex was evaluated with and without Foral 105-50W tackifier (30 php). The latex viscosity was adjusted using Reichhold 68-710 latexthickener(1 1.68 mils/kg. of latex). Compounding with the Foral 1 05-50W emulsion increased the aggressiveness of the adhesive to the release paper, allowing the use of normal score slitting. The preferred product was produced with the Foral compounded latex system (Sampie (d), Table 2).

Table 2 Adhesive properties of transfer tapes* Latex of Parts Parts Foral Thickness Probe Adhes. steel 65.5 C Creep** Sample Ex. No. styrene/MMA 105-50W (mm) tack, g (g/cm) Hrs.

(a) 7 30/0 - 0.309 - 1.345 100+ (b) 5 15/0 - 0.406 526 1.705 100+ (c) 5 15/0 30 0.406 893 4.127 100+ (d) 5 15/0 30 0.457 953 4.306 100+ (e) 8 0/15 - 0.482 - 1.345 100+ *Adhesives laminated to 0.025 mm Mylar for testing.

**Applied stress of 77.5 g/cm.

Additional 10 gallon and 200 gallon batches gave similar results. The tapes were also evaluated for corrugator splicing properties using Allied Container Kraft paper. All tapes passed the 43.9 g/cm2, 60 minute test at 1 500 C. Sample (d) was also evaluated at greater stresses (Table 3) where the time to failure, Tf, of the splice is presented.

Table3 Corrugator lap splice test Force (g/cm2) Tf(minutes) 43.9 > 60 87.8 > 60 109.7 1.0 175.6 0.3 A tape product, produced with a mixture of emulsifying agents comprising (1) a sulfonated diphenyl ether and (b) a sulfonated reaction product of formaldehyde and naphthalene such as described in U.S. Patent 4,204,023, was found in its development to fail at 8 minutes under an applied stress of 82.31 g/cm2 at 1 500 C. Thus, it is seen that the inherent reinforced core-shell system possesses a greater cohesive strength. At the same temperature approximately the same stress (82.31 vs. 87.8 g/cm2), the present latex is superior by a factor of more than 7-8 (time-to-failure) to the tape sample.

Adhesion to polyethylene is evaluated as follows: The polyethylene utilized as the substrate was 0.29 mm black VISQUEEN film. The indicated latex was cast onto 4 mil Edison polyethylene (R sheet).

The results are summarized in Table 4.

Table 4 Latex of +ForaP USP +Forala) Foray Ex. 5 105-50W 4202023 105-50W Ex. 8 105-50W Thickness (mm) 0.056 0.048 0.041 0.051 0.046 0.051 Adhes. black Visquem (g/cm) 223 379 89 167 78 234 Adhes. backing (g/cm) 223 50 89 - - - Adhes. glass (g/cm) 223 858 267 - - - Adhes.steel(g/cm) 345 635 256 323 138 401 a)tackifier @ 30 php (parts per hundred parts of total latex polymer) The above clearly demonstrates the superior adhesive properties of the instant latex compositions and particularly those as embodied in Example 5. The somewhat inferior adhesion of the non-tackified MMA (Example 8) suggests perhaps a somewhat high intimacy of the shell and core MMA polymer portions.It might therefore be concluded that an abnormally high content of the nontacky resin polymer is dispersed in the shell portion and/or is positioned within the shell phase in such manner as to partly displace the tack effects of the polyacrylate.

To demonstrate its enhanced adhesion to polyethylene, the instant core-shell latices are compared with non-seeded systems.

The Example 5 latex tested as reported in connection with Table 2, is used as the basis of comparison. A latex sample is prepared solely from the shell phase composition of Example 5, i.e., excluding the core phase composition entirely. A third latex sample is similarly prepared employing the following as the sole monomer composition, wherein the styrene employed as the core monomer is merely added to the acrylate monomer phase to produce a compositional equivalent.

Monomer Parts 2-EHA 69.6 Styrene 13.0 EA 13.9 MA 1.6 AA 1.8 IBMA 0.1 The Samples are designated 5-1, 5-2 and 5-3 respectively in Table 5.

Each of the Samples was cast onto polyethylene and evaluated for physical properties.

Table 5 Comparison of polymers for adhesion 5-1 5-2 6-3 Latex of Example Acrylate phase of Styrene-loaded 5 Example 5 crylate phase Non- Non- Non tackified Tackifieda) tackified Tackifieda' tackified Tackifieda Thickness, (mm) 0.056 0.048 0.051 0.048 0.051 0.051 Adh. Visqueen (g/cm) 223 34 15 22 9 22 Adh.steel(g/cm) 31 57 18 36 16 29 a)Tackified with 30 php Foral 1 05-50W-(hydrogenated resin ether) The non-seeded polymers in Samples 5-2 and 5-3, and the latter in particular, are clearly seen to have lower adhesion properties than the ExampleS polymer.Apparently, with the 5-3 sample the airpolymer interface is influenced by the dispersed polystyrene, which perhaps affects the contact angle, i.e., wetting of the polyethylene.

An important aspect of the present inherently reinforced core-shell structure is that the adhesion property (steel) is superior to the acrylate phase, Sample 5-2 and the non-seeded styrene loaded acrylate phase, Sample 5-3. Thus the present system can contain a much greater percentage of a high Tg monomer without losing properties, i.e. up to about 30 phm (shell phase) Table 2-Sample (a), as compared to the approximately 1 5 phm of Sample 5-3. As demonstrated, at the identical styrene loading, the present latex is markedly superior.

The present latices were found to possess high shear strength when evaluated in the corrugator shear test as well as high (334 g/cm) adhesion to polyethylene. The polymer, however, was lacking somewhat with respect to SMACNA long term shear requirements, holding less than the required 6 hrs at a stress of 43.86 g/cm2 at 22.20C.

Neither an increase of the IBMA (isobutoxymethylacrylamide) from 0.3 to 1.0 parts nor the inclusion of additional styrene monomer (1 part) to the shell phase caused an improvement to the shear properties. Apparently, the molecular weight and/or the distribution of the half maleate stabilized polymer is inadequate to obtain the high SMACNA shear properties required.

The polymerization recipe for the class of stabilizers designated (D) is similar to that employed with the half maleate (A) emulsifiers except for the use of lower stabilizer quantities and pH levels in both core and shell phase polymerization. In preferred embodiments as with the (A) stabilizers, the same stabilizer, catalyst and reductants are used in both core and shell phase polymerization.

In (D)-stabilized systems, it is found at the start of the acrylate feed, i.e., about one hour after initiation of the core phase, conversion of the styrene monomer is about 28%. Thus, the shell phase is initially rich in styrene, diminishing as the polymerization proceeds. It is hypothesized that this structure as well as the polymer molecular weight distribution contributes to the high shear properties of the (D)-stabilized polymer latex product.

The (D)-stabilized system was found to be particularly suitable for use in producing SMACNA and transfer tapes. Adhesions to polyethylene were found to be moderate as compared with (A)stabilized systems.

In the following Examples both the core and shell phase stabilizer concentrations are varied and effects upon conversion, solids and floc levels (Table 6) and adhesion properties (Table 7) measured.

Effects of the use of fully polymerized seed polymer (Table 8), copolymer seed (Table 9), persulfate catalyst (Table 10) are given as are transfer tape utility (Table 11) and polyethylene adhesion (Table 12).

Examples 9-18 Core particles are obtained by polymerizing the following composition: (a) Parts Styrene 15.00 H20 65.15 Aerosol 501 0.15 TBHP-90 0.027 Fe(NH4)2(SO4)2. 6H2O 0.000204 SFS 0.02 at a temperature of 60-650C. pH being adjusted to 4 4.5 with NH4OH.

A shell phase composition is prepared as follows:

(b) Parts 2-EHA 80.00 EA 16.00 MA 1.90 AA 2.00 monomer IBMA 0.10 pre-emulsion H2O 36.125 Aerosol 501 1.125 TBHP-90 0.182 Fe(NH4)2(SO4)2 6H2O 0.00136 Reductant SFS 0.12 H2O 1 5.00 - solution Polymerization of the shell phase is effected by adding monomer, stabilizer, and catalyst (oxidant) in the form of an aqueous emulsion to (a), reductant being separately added.Temperature of the reaction mass ingredients is maintained at 65-700C.

Table 6 9* 10 11 12 13 14 15 16 17 18 Conc. Aerosol core (phm) 1.0 1.0 1.0 1.1 1.17 1.33 1.0 0.83 0.67 0.5 Conc. Aerosol Shell (phm) 1.0 1.0 1.125 1.125 1.125 1.125 1.25 1.25 1.25 1.25 % Solids 48.6 47.1 46.5 47.8 44.0 47.2 49.3 48.3 47.6 39.0 % Conv. 97.0 96.8 93.6 96.9 90.5 99.6 99.0 96.6 99.0 91.0 % Floc 0.6 2.6 0.6 1.3 2.5 5.2 3.8 0.5 4.8 13.0 *0.5 phm DAXAD 11 Low floc levels were obtained with the addition of Daxad (Example 9). With 1.0 phm Aerosol 501 in the shell phase, the monomer pre-emulsion was not stable, breaking within 100m of completion of the addition, unless mechanically maintained. With increasing emulsifier concentration in the shell, and 1.0 phm in the core, the floc level exhibited a minimum at 1.125 phm in the shell.The floc level was found to increase with increasing surfactant concentration in the core, and a constant concentration of 1.125 phm in the shell. Increasing the surfactant concentration in the shell phase to 1.25 phm, a minimum floc value was obtained with a concentration of 0.83 phm in the core phase.

The various polymers were compounded with 60 php Foral 105-50W and cast onto SMACNA aluminum foil, air dried and heat treated for e' @ 1 490C, Table 7.

Table 7 Physical properties (varying surfactant concentration) Latex of Example No.

9* 10 11 12 15 16 Conc. Aerosol-core (phm) 1.0 1.0 1.0 1.1 1.0 0.83 Conc. Aerosol-shell (phm) 0.0254 0.0254 0.0255 0.0285 0.0317 0.0317 Thickness (mm) 1.8 0.7 1.9 2.0 1.9 Adh. steel (g/cm) 936 970 724 713 981 791 Adh. backing (g/cm) 880 992 1003 - - Long term shear (6 hrs., 22.2"0, 43.86 g/cm2) Fail Pass Pass Pass Pass Pass High temp. shear (6 hrs., 65.5"0, 4.38 g/cm2) Fail Pass Pass - Low temp. shear (6 hrs., 4.40C, 21.93 g/cm2) Pass Pass Pass - - Short term shear (6 hrs., 22.2"0, 21.93 g/cm2) Pass Pass Pass - *0.5 phm Daxad 11 The polymers produced with Aerosol 501 were found to pass SMACNA specifications.The addition of Daxad to the recipe, perhaps in part due to a reduction in particle size and thus molecular weight, was found to decrease the shear properties. Example 11, (1.125) parts surfactant in the shell phase) was found to pass all the shear and adhesion SMACNA specifications even though the coating thickness was found to be only 0.018 mm.

Polystyrene latex was prepared in 99+% conversion. The preformed polystyrene latex was then used as the seed (15 phm) for the standard shell phase polymerization. Aerosol 501, at levels of 1.0 and 1.25 phm were evaluated, Examples 19 and 20. The samples were compounded and tested for basic properties with results given in Table 8.

Table 8 Effect of preformed seed latex Ex. 19 Ex. 20 Parts Aerosol 501 1.0 1.25 % Solids 47.4 49.3 % Conversion 97 99 % Floc 2.2 0.4 Adh. steel (g/cm) 981 925 Long term shear Fail Fail The floc level was found to be greatly diminished with the higher surfactant level 0.4 vs 2.2%.

Although the adhesion values are equivalent to that obtained in the in situ synthesis, i.e. partially polymerized styrene core particles, the use of a preformed seed latex apparently differs in the polymer micro structure in that the polymers failed in shear strength. Use of preformed polystyrene probably results in less interaction between the seed and shell phases. Addition of the acrylic phase to the in situ polystyrene would find "live" i.e. reactive polystyrene chains and should therefore be in a more intimate relationship and leading to perhaps a greater graft fraction (C--C bond as well as chain entanglement) and thus higher shear (cohesive) strength.

Styrene-methylmethacrylate (83/1 7) copolymer was evaluated as the core phase polymer, Example 21. The concentration of Aerosol 501 was maintained at 1.0 and 1.125 phm in the seed and shell phases respectively. The polymer was found to pass the screening properties (adhesion and long term shear) as the results of Table 9 indicate.

Table 9 Effect of copolymer seed polymer Ex. 21 Seed Sty/MMA-(83/17) % Solids 47.4 % Conversion 98 %Floc 3.1 Thickness (mm) 0.079 Adh. steel (cm/g) 858 Long term shear Pass As in the styrene core polymerization, the Sty/MMA copolymer of this Example was found at the start of the acrylic feed to have a low conversion, 36%.

The use of K2S208/NaHAS03 initiator was investigated using 1.0 phm and 1.13 phm of the surfactant in the core and shell phases respectively. Relatively low conversion and high floc values were obtained.

However, high conversion polymer was obtained in a system in which the monomer was not preemulsified, Example 22. No pH adjustment of the batch was made. The polymerization of the styrene seed (1.0 phm surfactant) was initiated with 0.25 phm K2S2O8 and 0.2 phm NaHSO3. The monomer mix was then added continuously, along with K2S2O8 solution (0.25 phm) to the reactor. The surfactant (Aerosol 501) was added incrementally during the shell phase polymerization at 1/4 and 3/4 of monomer feed (0.5 phm each injection). Although the floc level was found to be high (3.7%), the properties were evaluated for comparison purposes, Table 1 0. The adhesion-shear properties were found not to be altered with either the change in catalyst or polymerization conditions.

Table 10 Effect of persulfate initiation Ex. 22 % Solids 47.0 % Conversion 98.0 % Floc 3.7 Thickness (mm) 0.059 Adh. steel (g/cm) 1003 Long term shear Pass The Aerosol 501 stabilized latex system was found to be suitable for the production of a transfer tape. Example 9, with and without Daxad 11 was compounded with 30 php Foral 105-50W and hand cast onto release paper. The polymer film possessed sufficient strength to be useful in a transfer tape configuration. Physical properties, including the corrugator splice test, were evaluated, Table 11.

Table 11 Transfer tape Daxad- 1 1 No Daxad- 1 1 Thickness (mm) 0.053 0.050 Adh. steel (applied to 0.025 mm Mylar, g/cm) 479 461 Creep, (hrs) (63.50C, 77.5 g/cm2) 100+ 100+ Corrugator splice (1 500C, 43.86 g/cm2) Pass Pass The latices of Examples 9 and 21 were tested for adhesion to polyethylene using Edison R sheet (polyethylene film) as the backing and designated Samples 12-A and 12-B respectively (Table 12). A further latex Sample (12-C) is prepared solely from the shell phase composition of Example 1 9.

Another latex Sample is prepared (12-D) employing as the sole monomer the composition of Sample 5-3 (Table 5). Each of Samples 12-C and 12-D is cast onto Edison R sheet (polyethylene film) as the backing. Adhesion results are summarized in Table 12.

Table 12 Adhesion to Visqueen polyethylene 12-A 12-B 12-C 12-D Aerosol 501 (phm) 1.0 1.125 1.125 1.5 Seed polymer Sty Sty/MMA - - Adh. steel (g/cm) 290 189 212 256 (+30 php Foral 105-50W) 345 379 334 401 Adh. "Visqueen" (g/cm) 56 100 111 45 (+30 php Foral 105-50W) 145 189 212 156 The Aerosol 501 system was found to have little adhesion to the "Visqueen" polyethylene.

Essentially no difference was observed between the seeded and unseeded polymers. Thus with the Aerosol 501 system, the adhesions to the polyethylene are apparently a "minimum" value depending upon the polymer composition.

Latices obtained with systems stabilized with poly-ethoxylated alkylether sulfates, as typified by SIPEX EST comprising ethoxylated (3 moles) sodium tridecylether sulfate, when compounded with 30 php Foral 105-50W, have polyethylene adhesion properties comparable to the (A) half-maleatestabilized systems. The latices are also useful for transfer tapes. Shear properties, however, are inferior to the (D)-sulfosuccinate-stabilized systems (Aerosol 501).

With these stabilizers, it is advisable to include a small amount of the shell phase monomer catalyst (oxidant)-stabilizer pre-emulsion in the core phase monomer composition to facilitate initiation of the polymerization reaction. Usually, from about 1 to 3%, of the shell phase pre-emulsion is sufficient for such purposes. Thus, it is found that initiation rapidly commences with the addition of 1.7% of the shell monomer emulsion to the seed charge. Approximately 100% conversion of seed monomer is obtained within 1 hour after polymerization commences.The effect of pH on the (B) stabilized polymerization process is evaluated in the following Examples: Examples 23-28 Core phase particles are obtained by polymerizing the following composition: Parts Acrylate emulsion of Example 1 (b) 1.7 Styrene 15.00 H2O 65.79 Sipex EST-30 0.338 TBHP-90 0.055 Fe(NH4)2(SO4)2. 6H2O 0.000408 SFS 0.04 at a temperature of 60-65 C, pH being maintained at 4-4.5.

A shell phase composition is prepared as follows:

Parts 2-EHA 80 EA 16 MA 1.9 AA 2 monomer IBMA 0.1 pre-emulsion H2O 40.25 Sipex EST-30 2.25 TBHP-90 0.182 Fe(NH4)2(SO4)2.6H2O 0.00136 reductant reductar SFS 0.12 # solution solution H2O 15 Polymerization of the shell phase is effected by adding monomer, stabilizer and catalyst (oxidant) in the form of an aqueous emulsion to (a), reductant being separately added. Temperature of the reaction mass ingredients is maintained at 65-70 C.

The effect of pH on the polymerization was examined. The samples were also evaluated for both SAMCNA and adhesion properties, Table 13. The samples were compounded with 60 php Foral 10550W for SMACNA, and 30 php Foral 105-50W for polyethylene adhesion.

Table 13 Effect of pH on Sipex EST stabilized polymerization Example No.

23 24 25 26 27 28 pH 2.8 4.1 4.1 6.3 6.3 4.1 Sipex EST, phm 2.25 2.0 2.25 2.25 2.25 2.25 Seed Sty Sty Sty Sty Sty Sty/MMA % Solids 48.1 48.6 48.2 47.9 48.6 47.6 % Conv. 99.0 99.0 98.0 97.0 97.2 97.0 % Floc 3.3 0.9 0.6 nil nil 0.3 SMACNA properties* Thickness (mm) - 2.3 2.4 2.2 - 2.4 Adh. steel (g/cm) - 71 70 91 - 64 Long term shear - Fail Fail Fail - Fail "Visqueen" adhesion** Thickness (mm) - 0.046 - 0.046 0.046 0.051 Adh. steel (g/cm) - 256 - 267 290 223 (+Foral) - 357 - 424 401 323 Adh.Visqueen (g/cm) - 156 - 167 178 178 (+Foral) - 245 - 290 290 323 *Compounded with 60 php Foral 105-50W, cast onto SMACNA aluminium foil **Compounded with 30 php Foral 105-50W, cast onto "EDISON R SHEET" polyethylene film.

All samples tested for SMACNA shear were found to have inadequate strength indicating a different micro structure from that obtained with Aerosol 501. The adhesion to polyethylene was found to approach that obtained with the (A)-stabilized system. Essentially no effect was observed with respect to the composition of the seed polymer (styrene vs. styrene/MMA copolymer).

Example 25 was scaled up into the 10 gallon reactor (Example 29) and evaluated for adhesion, Table 5. Samples were cast onto EDISON R SHEET (polyethylene).

Table 14 Example 29 % Solids =47.9 % Conversion =97.2 % Floc =nil pH =4.1 RVT Brook. Visc. No. 2 @ 100 =220 cps Adh.

"Visqueen" Foral Thickness Adh. steel polyethylene 105-50W (mm) (g/cm) (g/cm) 1.5 334 189 30 1.5 379 312 A latex Sample, 1 5-A (Table 1 5) is prepared solely from the shell phase composition of Example 25. A further latex Sample, 1 5-B, is prepared using as the sole monomer, the composition of Sample 5-3 (Table 5). Each of the Samples was cast onto EDISON R SHEET (polyethylene) with the following results.

Table 15 Effect of non seeding on properties 15-A 15-B % Solids 50.2 43.7 % Conversion 99.6 88.6 % Floc 0.3 0.6 Adh. steel (g/cm) 189 201 (+30 php Foral 105-50W) 312 334 Adh. "Visqueen" (g/cm) 114 134 (=30 php Foral 105-50W) 201 1 56 The unseeded polymers were found to decrease in adhesion to polyethylene and to be equivalent to that obtained with the Aerosol 501 stabilized polymers. As with the (A)-stabilized polymer, the present seeded polymer structure, perhaps in conjunction with the ethoxylated surfactant, resulted in an improved adhesion to polyethylene.

Efforts to analyze the (A), (B) and (D) stabilizer systems with respect to latex particle size and gelswelling index proved to be inconclusive.

All polymer systems were found to contain, within experimental error, essentially the same geiswelling index. (6575% gel and a swelling index of 30-45 as measured in toluene with a contact time of 24 hrs.). The particle size (Dw) of the various latices were also found to be essentially equivalent. 3000-3500A0. The particle size distribution (not measured) is most likely in variance as observed by the coloration of the latices (Tyndall).

Examples 30-34 Systems stabilized with (C), the reaction product of maleic anhydride, dimethylaminoethanol and chloroacetamide mixed 1:1 mole ratio and ammonium lauryl sulfate, are evaluated at stabilizer concentrations of 3 and 6 phm in the seed phase and 1.5 and 3 phm in the shell phase. The results are summarized in Table 16.

Table 13 (C) stabilized polymers Example No.

30 31 32 33 34 Seed polymer Sty Sty Sty MMA MMA (C) (seed), phm 3 6 3 6 3 (C) (shell), phm 3 3 1.5 3 1.5 % Solids 47.7 47.7 44.0 46.8 47.6 % Conv. 99 99 99 97 99 % Floc 2.3 2.0 9.8 1.9 6.0 Adh. steel (g/cm) - 267 201 323 223 Creep (66.5 C, 77.5 g/cm) - - 100+ - 100+ Lap splice (66.50C, 43.86 g/cm2) - pass pass pass pass *Adh. polyethylene (g/cm) 189 - 156 - *Compounded with 30 php Foral 105-50W The floc level was found to be higher than that obtained in the prior examples. The polymer obtained was found to possess the improved modulus property and to pass the corrugator lap splice test ( > 60 min. 66.50C, 43.86 g/cm2). Adhesion to polyethylene, however, was found to be relatively low.There is no apparent difference in adhesion property with the use of styrene or methylmethacrylate core polymer.

Example 35 A poly (2EHA/EA/MA/AA--80/1 5/2/2), styrene core (15 parts), system was produced with 3 phm of the aforedescribed (C) stabilizer in the seed and 6 phm in the shell. The self cure monomer IBMA (isobutoxy-methacrylamide) is omitted.

The polymerization proceeded to full conversion with negligible floc formation. The latex was cast onto 1 mil Mylar for evaluation of adhesion properties and release paper (transfer tape) for Corrugator Lap Splice evaluation, Table 1 7.

Table 17 Physical properties of non curing polymer Example 35 Thickness (mm) 0.038 Probe tack (g) 460 Adh. steel (g/cm) 312 Quick stick (g/cm) 201 Creep (66.50C, 77.5 g/cm2) 1.2 Lap splice (66.50C, 43.86 g/cm2) Fail The seeded polymer with no cure mechanism was found to fail the lap splice test ( > 1 hr.

required). The polymer did, however, have sufficient strength to produce a satisfactory transfer tape. It is apparent that both the seeded structure and self-cure monomer are required for optimum properties.

Examples 36 s 37 Employing a styrene seed (7.5 parts) a (2EHA/VA/MA/AA 65/16.5/1 5.5/2) shell polymer was produced using 3 phm of an (A) type stabilizer comprising the reaction product of maleic anhydride and IGEPAL CO-630 (GAF)-(9 moles ethylene oxide) nonylphenoxy(ethyleneoxy)ethanol and the aforedescribed sulfoxylate-hydroperoxide catalyst system, at a pH of 6 with negligible floc formation, Example 36. An additional sample was produced with a styrene seed (15 phm) and with K2S208 catalysts. Approximately 10% floc was obtained (Example 37). The two samples were evaluated for physical properties, Table 1 8.

Table 18 Ex.36 Ex. 37 Thickness (mm) 0.056 0.033 Adh.steel(g/cm) 212 256 Quick stick (g/cm) 145 212 Creep (hrs) (66.50C, 77.5 g/cm2) 100+ 100+ Lap splice (66.50C, 43.86 g/cm2) - Pass ( > 60 min) Both polymers were found to possess the higher modulus required for use in a transfer tape configuration. The polymer of Example 37 was found to possess good cohesive strength, passing the lap splice test without a post cure. This clearly shows that the properties (e.g. splicing) depend upon the "gel" structure and/or composition of the shell polymer.

As indicated by the foregoing Examples, the core-shell latex structures stabilized as described herein have improved modulus properties with respect to both a non-seeded acrylic shell structure and the structure obtained by including total core monomer in the shell phase monomer composition.

Regardless of the particular stabilizer used, the resulting latices are found to be useful in the manufacture of transfer tape without a reinforcing membrane. The physical properties of the latex product are in part determined by the stabilizer used, particularly as regards adhesion to steel and polyethylene and SMACNA tape utility. It further appears that latex properties are in large part determined by the composition of the shell phase, although influenced to some extent by the composition of the seed polymer. However, the composition of the core polymer as well as stabilizer influences the adhesion to polyethylene and steel.

The polymer product produced with Emulsifier (A) was found to be most useful when used to produce a diaper tape. The tape so produced was found to be repositionable when tested on disposable diapers yet to have sufficient holding power to the outer polyethylene film to be serviceable.

Claims (15)

Claims

1. An emulsion polymerization process for preparing an aqueous, stable polymer latex composed of emulsified particles having an inner or core phase of high Tg resinous polymer and a polymeric outer or shell phase integral with said core phase, the process comprising contacting at a temperature of from 50 to 850C a dispersion of core phase particles, being the polymerizate in emulsion form of one or more high Tg resin-forming mono-olefinic monomers, with an addition-polymerizable, shell phaseforming monomeric composition comprising at least 60% acrylate monomer, in the presence of an effective amount of polymerization catalyst and of emulsifying agent as stabilizer, to produce a latex having a solids content of from 40 to 60%, the weight ratio of said monomeric composition to core phase particles being from about 3:1 to 10::1, a pH of from 2 to 8 being maintained throughout the process.

2. A process according to Claim 1, wherein the core particles have been produced by an emulsion polymerization process utilizing catalyst and stabilizer identical with those used for the shell phase polymerization.

3. A process according to Claim 1 or 2, wherein said stabilizer comprises amphoteric or anionic, or anionic/nonionic emulsifying agent.

4. A process according to Claim 1, 2 or 3, wherein said core particles comprise polymer, monomer and sub-polymer species corresponding to at least a 20% conversion of said one or more resin-forming olefinic monomers.

5. A process according to any preceding claim, wherein the shell forming monomer, polymerization catalyst and stabilizer are combined and added to the reaction medium as an aqueous pre-emulsion.

6. A process according to any preceding claim, wherein said stabilizer is selected from (A) the half maleate reaction product of maleic anhydride and an ethoxylated C10-C16 linear alkanol or ethoxylated C8-C9 alkyl phenol containing 4 to 9 moles condensed ethylene oxide; (B) ethoxylated alkali metal or ammonium C9-C18 alkylether sulfate containing 2 to 15 moles condensed ethylene oxide; (C) the reaction product of maleic anhydride, di-(C1-C4) alkylaminoethanol, chloracetamide and ammonium C8C28 alkyl sulfate; and (D) an anionic/nonionic sulfosuccinate.

7. A process according to Claim 6, wherein said stabilizer comprises said reaction product (A) and the pH of the reaction medium is maintained at a value of from 5 to 8.

8. A process according to Claim 6, wherein said stabilizer comprises said alkylether sulfate (B) and the pH of said reaction medium is maintained at a value of from 3 to 8.

9. A process according to Claim 6, wherein said stabilizer comprises said reaction product (C) and the pH of said reaction medium is maintained art a value of from 2 to 6.

10. A process according to Claim 6, wherein said stabilizer comprises said sulfosuccinate (D) and the pH of said reaction medium is maintained at a value of from 3 to 7.

11. A process according to any preceding claim, wherein said core particles comprise polystyrene, polymethylmethacrylate or a copolymer of styrene and methyl methacrylate.

12. A process according to any preceding claim, wherein said shell phase comprises, on the basis of total monomer content, at least 60% 2-ethylhexyiacrylate, with up to 20% ethylacrylate, 5% methyl acrylate, 5% acrylic acid and 1% isobutoxymethacrylamide.

1 3. A process according to any preceding claim, wherein said polymerization catalyst comprises tertiary butylhydroperoxide or sodium, potassium or ammonium persulfate.

14. A process according to any preceding claim, wherein said core particles have been produced by the emulsion polymerization of a monomer composition including from 1 to 5% of the aqueous preemulsion of monomer employed in the shell phase polymerization.

15. A process according to any preceding claim, wherein before initiation of shell phase polymerization and prior to completion thereof, additional catalyst composition is added to the reaction medium to maximize conversion.

1 6. A latex when produced by a process as claimed in any preceding claim.

1 7. An adhesive article formed from latex as claimed in Claim 16 or comprising a base coated therewith.