Preparation of polymeric particles

Abstract: A process for the manufacture of core/shell polymer particles comprises forming a core particle containing acid functionality, a first shell around said core of a copolymer of a nonionic monoethylenically unsaturated aromatic monomer and a copolymerisable polar monomer in an amount of at least 15% by weight of the copolymer and having a solubility in water at 20°C of at least 1% by weight and a second shell around said first shell of a polymer or copolymer of a nonionic monomer having a solubility in water at 20°C of less than 1% by weight and swelling the particle with a non-volatile alkali to generate one or more vesicles. Preferably each shell contains a polymer or copolymer of styrene.

Description: This invention relates to the preparation of polymeric particles and particularly to vesiculated core/shell polymer particles. Hitherto polymer particles containing a void or voids have been known for use as opacifying agents in coating compositions and other products (See, for example, EP 0 321 096). Such opacifiers can be made by a process involving sequential emulsion polymerization of particles comprising an ionic core containing acid groups and a sheath polymer around the core. Such core/shell polymer can be swollen in hot aqueous basic solutions to produce particles that contain voids which are filled with water when the particles are still in the form of an aqueous dispersion. On drying the voids should become filled with air, thus enhancing the opacifying properties of the particles. While useful, such products have some disadvantages that are overcome by the products of this invention. Particularly a reduced opacity from that most desired was believed to be associated with less than ideal encapsulation of the core by the sheath which tended to result in rupturing of the shell when the particle was subsequently swollen to produce a vesicle or vesicles. By this invention we have devised a method for ensuring that the core of said unswollen core/shell particles is adequately centered within the sheath so that undesired rupture of particles during the swelling step is significantly reduced. Another disadvantage of the products of the type described above is a tendency for coatings based on polymer emulsions and containing such particles as opacifying agents to have inadequate gloss. This invention discloses procedures to produce substantially unruptured, swollen void-containing particles, the use of which does not affect the gloss of water-based coatings adversely. Clearly there is a need for a core/shell polymer particle of improved core centering and gloss when used in a paint. According to the invention a process for the manufacture of vesiculated core/shell particles comprises:- forming an aqueous emulsion of at least one ethylenically unsaturated monomer with acid functionality, polymerising said unsaturated monomer to form core particles of polymer forming an aqueous dispersion of said core particles and a monomer mixture of a nonionic monoethylenically unsaturated aromatic monomer and a copolymerisable polar monomer in an amount of at least 15 per cent by weight of the total weight of the monomer mixture and said copolymerisable polar monomer having a solubility in water at 20°C of at least 1 per cent by weight polymerising said monomer mixture to form a first shell on said core particles adding to an aqueous dispersion of said core particles having a first shell a further nonionic monomer which has a solubility in water at 20°C of less than 1 per cent by weight and polymerising said further monomer to form a second shell on the said particles, and mixing the core/shell particles so obtained with a non-volatile fixed or permanent base to swell the cores and generate therein one or more vesicles. The products of the process of this invention have fewer ruptured particles and hence are better opacifiers than those made according to the same process in which the first, intermediate shell layer has been omitted. When used as opacifiers in latex paints the resulting dried compositions have better gloss than products made according to the same process in which the second, outer shell layer has been omitted. The production of the core particles is usually accomplished by sequential emulsion polymerization in aqueous media. In the most preferred embodiment of this invention particles of a preformed latex or "seed" polymer dispersed in water are increased in size by polymerization of the required monomers to form one or more successive polymer shells onto these preformed particles. Care must be taken in these subsequent polymerization stages to ensure that the later stage polymers are deposited on these preformed particles and do not form new particles or are not embedded within these polymer particles that have been made in the previous stage. The procedures of this invention are effective in preventing the formation of "new generation" particles wherein a second or later polymerization stage results in a product which is not deposited on the preformed particles. Measurement of the particle size distributions of the dispersed polymer at every stage of the sequential polymerization process ensures that the desired encapsulation of the preformed particles has indeed occurred. Particle size distribution is conveniently and reliably measured by use of a disk centrifuge such as that described by J.D. Hildreth and D. Patterson in J. Doc. Dyers Color., 80, 474 (1960). A procedure for following the course of sequential emulsion polymerisation and the swelling of particles has been described by Cooper A.A., Devon MS and Rudin A. in Journal of Coatings Technology, vol. 61, No. 769 pp 239-244, 1989. The core polymer may be made by aqueous emulsion polymerization of one or more ethylenically unsaturated monomers containing a carbon-carbon double bond and a carboxylic acid group. It is preferred, but not essential, that the carbon-carbon double bond in such monomers contains a carbon atom that carries two hydrogens. Such monomers are generally more reactive in polymerizations than the corresponding acidic monomers in which both carbons of the double bond are bonded to substituents other than hydrogen. Suitable acidic monomers include acrylic acid, methacrylic acid, itaconic acid, aconitic acid, maleic acid or anhydride, fumaric acid, crotonic acid, monoesters of the dibasic acids mentioned above and the like. Methacrylic acid is the preferred acidic component in this invention. The core polymer may be produced by the polymerization of one or more such acidic monomers. In preferred embodiments, however, an acidic monomer or mixture thereof is copolymerized with one or more ethylenically unsaturated monomers that contain no ionizable groups. Examples of such ethylenically unsaturated monomers include methyl methacrylate, styrene, vinyl toluene, alpha-methylstyrene, ethylene, vinyl acetate, vinyl chloride, acrylonitrile, (meth)acrylamide, various alkyl or alkenyl esters of methacrylic or acrylic acid, various alkyl and alkenyl esters of vinyl alcohol. In general core polymers containing at least 10 per cent and preferably at least 20%, by weight of acid-containing monomers are particularly useful in the present invention. In an optional, but preferred process for making the core/shell polymers of this invention the first stage in the emulsion polymerization sequence is the production of a so-called seed latex, which provides nuclei for the subsequent growth of the core polymer particles. The polymer of the seed latex may contain an acidic vinyl monomer, but this is not essential for the purposes of this invention. The particle size of the seed latex is preferably less than 100 nanometres. The acid-containing core polymer particle, whether produced in a single stage or a multiplicity of stages, usually has a particle size of from 100 to 1000 nanometres and preferably between from 100 to 500 nanometres. Such measurements refer to the sizes of the latex particles in the unswollen condition. As stated in the process of the invention the core particles are then formed into an aqueous dispersion containing the monomers to form the first shell. The shell is formed from a nonionic monoethylenically unsaturated aromatic monomer and a copolymerisable monomer which has a solubility in water at 20°C of at least 1 per cent by weight. Also the copolymerisable monomer should be present in an amount of at least 15% by weight of the total weight of the monomer mixture. Preferably the monomers are free of free-acid groups so that the shell has no acid groups after formation. Examples of these nonionic monoethylenically substituted monomers in which a phenyl or other aromatic group is attached to one of the carbon atoms of the carbon-carbon double bond are styrene, alpha-methylstyrene, vinyl toluene, ring substituted derivatives of styrene, of alpha-methylstyrene and of vinyl toluene and mixtures thereof. The copolymerisable monomer is chosen from those having a solubility in water of at least 1 per cent and examples of such are methyl methacrylate, acrylonitrile and methacrylamide. Methyl methacrylate is preferred. The amount of the said copolymerisable monomer is at least 15 per cent by weight and usually from 25% to 80% of the total weight of the total monomer mixture. The amount of the styrenic-type monomers i.e. the nonionic monoethylenically substituted monomers is usually greater than 20 per cent by weight of the total monomer mixture and preferably from 40% to 85% by weight. The monomer mixture can also contain a so-called "soft" monomer or monomers together with the so-called "hard" monomers i.e the styrene-type to lower the softening temperature of the first shell and thereby facilitate subsequent swelling. Specific examples of soil monomers include butyl acrylate, ethyl acrylate, isobutyl acrylate, the various ethyl hexyl acrylates, butadiene, isoprene, lauryl methacrylate and similar long chain esters of acrylic and methacrylic acids and combinations thereof. Dibutyl maleate may also be used. Butyl acrylate is preferred. Amounts of such soft monomers can vary according to the particular desired softening temperature but usually the amount will be at least 2% by weight of the weight of the monomer mixture. A useful measure of the softening temperature of the sheath polymer is its glass-to-rubber transition temperature, which is denoted here as Tg. Tg may be measured by a variety of methods that are well known in the industry. Such methods are summarised in textbooks of polymer science such as "Elements of Polymer Science and Engineering", by Alfred Rudin, published by Academic Press in 1982. The Tg of a copolymer may be approximated by the equation: where Tg is the glass-to-rubber transition temperature of the copolymer and Tg a and Tg b are the Tg values for homopolymers of monomer a and monomer b, which are present in the comonomer mixture with weight fractions of W a and W b , respectively. The Tg of the sheath polymer used in this invention should be above room temperature and preferably be within the range 60°C and 100°C. The polymerisation reactions involved in the present invention are initiated with initiating agents that are generally similar to those employed in conventional emulsion polymerizations. As is common to such processes, preferably water-soluble initiators are used. Examples are tert-butyl hydroperoxide, hydrogen peroxide, an alkali metal (sodium, potassium or lithium) or ammonium persulphate or a mixture of such initiator with a reducing agent to form a redox system. Examples of reducing agents include alkali metal or ammonium metabisulphites, hydrosulphites or hyposulphites, or sodium formaldehyde sulphoxylate or various metal ions, such as iron, chromium and copper, that may exist in a plurality of valence states. The concentration of initiator may be from 0.01 to about 2 per cent by weight of the monomers used in the polymerization and an approximately similar range of reducing agent may be used, in a redox system. The molecular weight distribution of the polymer formed in any of the polymerization steps may be controlled through use of a free radical chain transfer agent, such as a mercaptan, mercaptoethanol ethanolamine, carbon tetrachloride, carbon tetrabromide or isooctyl mercaptopropionate. Nonionic or anionic emulsifiers may be used, either alone or together. In the final latex, it is preferable that there should be present both an anionic surfactant and a non-ionic surfactant, for best polymerization of the monomer(s). The choice of mixed surfactants, along with the proper selection of initiator and polymerization temperature, facilitates the production of a stable latex and substantially complete absence of residual monomers, the presence of which would make the latex product obnoxious. Suitable anionic surfactants include sodium dodecyl benzene sulphonate. Suitable non-ionic surfactants include ethoxylated alkyl phenols, where the alkyl group is nonyl, octyl, decyl, or similar substances that are known in the industry. After the formation of the first shell there is polymerised onto the core/shell particles a second shell. It is preferred that this occurs immediately on completion of polymerisation of the first shell. The second or outer shell is formed by forming an aqueous dispersion of the core/shell particles having the first shell and a nonionic monomer having a solubility of less than 1% by weight in water at 20°C. These monomers usually will be chosen from the said nonionic monoethylenically aromatic substituted monomers described hereinbefore which are used to form the first shell. Preferably the monomer is styrene or a styrenic type and the monomer or monomer mixture is free of acid groups. Also there can be polymerised with the styrene or styrenic type monomer an amount of a so-called "soft" monomer chosen from those hereinbefore described and in an amount of, preferably, at least 2% by weight of the monomer mixture. The monomer mixture used to form the second shell will preferably be free of monomers which have been described herein as those having a solubility of at least 1% weight in water at 20°C. Usually the monomers to form the second shell are added to the aqueous dispersion consecutively to those forming the first shell and preferably there is no time interval between the first and second additions, or at most, only a very short time interval. After polymerisation of the second shell unswollen core/shell particles are obtained having a size of say 150 to 4000 nanometres and preferably between 200 and 2000 nanometres. The unswollen core/shell particles are then heated to swell the particles to introduce one or more voids or vesicles in the particle. The swelling of the core/shell particles is effected through the use of non-volatile fixed or permanent bases. Typically the swelling is effected at an elevated temperature e.g. of from 50°C to 120°C more preferably from 85°C to 100°C using an inorganic base such as an alkali metal hydroxide, bicarbonate or other similar base. Sodium hydroxide and potassium hydroxide are eminently suitable for use as the base. If desired, the swelling can be effected under superatmospheric pressure. Optionally an additional hard shell can be polymerized on said particles after the swelling step. The swollen core/shell particles contain voids which when the particles are in the form of an aqueous dispersion are filled with liquid water. On drying the voids should become filled with air thus conferring on the particles their opacifying properties. The particles can be used in a wide variety of applications such as aqueous paint systems, plastic compositions and many others where opacity is an important consideration. The invention is illustrated in the following Examples. EXAMPLE 1 (Comparative) (a) Emulsion Polymerization of Seed Polymer water 1018.0g anionic surfactant (Alcolac® Inc. DS-10) 6.05g ammonium persulphate initiator 3.35g t-butyl hydroperoxide 0.31g sodium formaldehyde sulphoxylate 0.21g monomer emulsion: water 298.5g butyl acrylate 466.4g methyl methacrylate 386.4g methacrylic acid 44.9g anionic surfactant (Alcolac® Inc. DS-10) 0.6g In a 5 litre stainless steel reactor, water and initiator were combined with stirring at 85°C. Monomer emulsion was added at 5 g/min. 35 minutes after monomer addition was complete the emulsion of seed polymer particles was cooled to 40°C and filtered through a 200 mesh screen. (b) Emulsion Polymerization of Core Polymer water 43.6g ammonium persulphate initiator 1.3g seed polymer emulsion 22.0g monomer emulsion: water 72.5g methyl methacrylate 96.6g methacrylic acid 35.17g butyl acrylate 43.5g anionic surfactant (Siponate DS-10) 0.3g In a one litre reactor, water, initiator and seed polymer were combined with stirring at 85°C. Monomer emulsion was added at 1.2 g/min. 30 minutes after monomer addition was complete 0.2g ammonium persulphate was added to the reactor. The reactor temperature was maintained at 85°C. The emulsion of core polymer particles was cooled to 35°C and filtered through a 200 mesh screen. Number average particle size was determined to be 170 nm. (c) Emulsion Polymerization of Core/Shell Polymer water 180.0g ammonium persulphate initiator 1.4g core polymer emulsion 104.0g monomer emulsion: water 70.0g methylmethacrylate 100.0g styrene 142.0g butylacrylate 13.0g anionic surfactant (Siponate DS-10) 0.8g nonionic surfactant (Alkasurf® NP 40) 2.4g In a one litre glass reactor, water, initiator and core polymer were combined with stirring at 84°C. The monomer emulsion was added at 0.85 g/min. One hour after monomer addition was complete, the emulsion of core/shell particles was cooled to 35°C and filtered through a 200 mesh screen. (d) Sodium Hydroxide Treatment of Core/Shell Particles water 13.0 ml core/shell emulsion 180.0g anionic soap (Dowfax® 2A1) 0.9g 10% aqueous sodium hydroxide 13.0g To the anionic soap and core/shell emulsion the aqueous sodium hydroxide was added over a 1 hour period at 88°C, then cooled to 35°C. Number average particle size of NaOH treated particles was 469 nm. EXAMPLE 2 (Comparative) (a) Emulsion Polymerization of Seed Polymer water 1018.0g anionic surfactant (Alcolac® Inc. DS-10) 6.05g ammonium persulphate initiator 3.35g t-butyl hydroperoxide 0.31g sodium formaldehyde sulphoxylate 0.21g monomer emulsion: water 298.5g butyl acrylate 466.4g methyl methacrylate 386.4g methacrylic acid 44.9g anionic surfactant (Alcolac® Inc. DS-10) 0.6g In a 5 litre stainless steel reactor, water and initiator were combined with stirring at 85°C. Monomer emulsion was added at 5 g/min.; 35 min. after monomer addition was complete the emulsion of seed polymer particles was cooled to 40°C and filtered through a 200 mesh screen. (b) Emulsion Polymerization of Core Polymer water 2582.0g ammonium persulphate initiator 7.7g seed polymer emulsion 131.4g monomer emulsion: water 429.3g methyl methacrylate 572.0g methacrylic acid 450.0g butyl acrylate 257.6g anionic surfactant (Siponate DS-10) 1.78g In a 5 litre stainless steel reactor, water, initiator and seed polymer were combined with stirring at 83°C. Monomer emulsion was added at 5 g/min. 30 minutes after monomer addition was complete 0.6g t-butyl hydroperoxide and 0.41 g sodium formaldehyde sulphoxylate was added to the reactor. The reactor temperature was maintained at 83°C. The emulsion of core polymer particles was cooled to 40°C and filtered through a 200 mesh screen. Number average particle size was determined to be 160 nm. (c) Emulsion Polymerization of Core/Shell Polymer water 1225.0g ammonium persulphate initiator 2.1g core polymer emulsion 828.0g monomer emulsion water 500.0g methyl methacrylate 1032.0g styrene 591.0g butyl acrylate 159.0g anionic surfactant (Dowfax® 2A1) 11.3g nonionic surfactant (Alkasurf® NP40) 15.5g In a 5 litre stainless steel reactor, initiator and core polymer were combined with stirring at 85°C. Monomer emulsion was added at 5 g/min. 30 minutes after monomer addition was completed 0.7 g t-butyl hydroperoxide and 0.4 g sodium formaldehyde sulphoxylate was added to the reaction. The reactants were stirred for an additional 30 minutes. The emulsion of core/shell particles was cooled to 40°C and filtered through 200 mesh screen. Number average particle size was determined to be 337 nm. (d) Sodium Hydroxide Treatment of Core/Shell Particles (single layer shell) water 105.0g core/shell emulsion 3800.0g anionic soap (Dowfax® 2A1) 63.0g 10% aqueous sodium hydroxide 300.0g To the anionic soap and core/shell emulsion the aqueous sodium hydroxide was added over a 1 hour period at 91°C, then cooled to 40°C. Number average particle size of NaOH treated particles was 447 nm. EXAMPLE 3 (Invention) (a) Emulsion Polymerization of Seed Polymer water 478.0g anionic surfactant (Alcolac® Inc. DS-10) 2.8g ammonium persulphate initiator 1.6g t-butyl hydroperoxide 0.15g sodium formaldehyde sulphoxylate 0.1g monomer emulsion: water 140.1g butyl acrylate 219.2g methyl methacrylate 197.0g methacrylic acid 5.6g anionic surfactant (Alcolac® Inc. DS-10) 0.2g In a one litre glass reactor, water and initiator were combined with stirring at 85°C. Monomer emulsion was added at 1.2g/mm. 30 minutes after monomer addition was complete the emulsion of seed polymer particles was cooled to 40°C and filtered through a 200 mesh screen. (b) Emulsion Polymerisation of Core Polymer water 2402.0g ammonium persulphate initiator 5.6g seed polymer emulsion 138.9g monomer emulsion: water 405.0g methyl methacrylate 639.5g methacrylic acid 422.3g butyl acrylate 144.5g anionic surfactant (Siponate DS-10) 3.2g In a 5 litre stainless steel reactor, water, initiator and seed polymer were combined with stirring at 85°C. Monomer emulsion was added at 5 g/min. 30 minutes after monomer addition was complete 1.1g ammonium persulphate were added to the reactor. The reactor temperature was maintained at 85°C. The emulsion of core polymer particles was cooled to 40°C and filtered through a 200 mesh screen. (c) Emulsion Polymerization of Core/Shell Polymer water 1714.0g ammonium persulphate initiator 2.3g core polymer emulsion 549.8g monomer emulsion (1st shell layer): water 287.3g methyl methacrylate 412.5g styrene 675.0g butyl acrylate 59.3g anionic surfactant (Siponate DS-10) 3.8g nonionic surfactant (Alkasurf® NP-40) 15.0g monomer emulsion (2nd shell layer): water 255.0g styrene 480.0g butylacrylate 24.0 anionic surfactant (Siponate DS-10) 2.3g nonionic surfactant (Alkasurf® NP-40) 9.0g In a 5 litre stainless steel reactor, water, initiator and core polymer were combined with stirring at 85°C. First and second shell monomer emulsions were added consecutively at 5 g/min. One hour after monomer addition was complete, the emulsion of core/shell particles was cooled to 40°C and filtered through a 200 mesh screen. (d) Sodium Hydroxide Treatment of Core/Shell Particles (double layer shell) core/shell emulsion 150.0g 10% aqueous sodium hydroxide 9.4g Dowfax® 2A1 0.2g To the core/shell emulsion the sodium hydroxide solution was added and soaked at room temperature for 22h. The Dowfax 2A1 was added. The mixture was heated to 90°C and maintained at 89-90°C for 30 minutes. It was then cooled to room temperature and was filtered using a 200 mesh screen. Number average size of swollen particles was 495 nm. EXAMPLE 4 (Invention) (a) Emulsion Polymerization of Core Polymer water 450.0g ammonium persulphate initiator 1.3g anionic surfactant (Siponate DS-10) 0.1g monomer emulsion: water 70.0g methyl methacrylate 120.0g methacrylic acid 80.0g butyl acrylate 28.0g anionic surfactant (Siponate DS-10) 0.3g carbon tetrabromide 0.15g In a one litre reactor, water and initiator were combined with stirring at 83°C. Monomer emulsion was added at 1.2 g/min. 30 minutes after monomer addition was complete 0.2 g ammonium persulphate was added to the reactor. The reactor temperature was maintained at 83°C. The emulsion of core polymer particles was cooled to 35°C and filtered through a 200 mesh screen. Number average particle size was determined to be 170 nm. (b) Emulsion Polymerisation of Core/Shell Polymer water 130.0g ammonium persulphate initiator 1.1g core polymer emulsion 70.0g monomer emulsion (1st shell layer): water 32.0g methyl methacrylate 50.0g styrene 70.0g butyl acrylate 8.1g anionic surfactant (Siponate DS-10) 0.4g nonionic surfactant (Alkasurf® NP-40) 1.9g monomer emulsion (2nd shell layer): water 30.0g styrene 80.5g butyl acrylate 4.5g anionic surfactant (Siponate DS-10) 0.25g nonionic surfactant (Alkasurf® NP-40) 1.2g In a one litre reactor, water, initiator and core polymer were combined with stirring at 83°C. First and second shell monomer emulsions were added consecutively at 0.85 g/min. One hour after monomer addition was complete, the emulsion of core/shell particles was cooled to 35°C and filtered through a 200 mesh screen. (c) Sodium Hydroxide Treatment of Core/Shell Particles (double layer shell) water 18.0g core/shell emulsion 150.0g 10% aqueous sodium hydroxide 11.0 ml To the core/shell emulsion the aqueous sodium hydroxide was added dropwise over a 1 hour period at 90°C, then cooled to 40°C and filtered through a 200 mesh screen. When examined by transmission electron microscopy the swollen particles of Examples 3 and 4, with compressed double shells as taught by this invention, were seen to have voids that were better centered than the single shell materials of Examples 1 and 2. Examples 3 and 4 contained significantly fewer non-spherical and ruptured particles and the particles contributed to the opacity of paint films when substituted for part of the titanium dioxide content of semi-gloss coatings formulations based on acrylic or vinyl-acrylic latex polymers. Similar results were observed when swollen particles with double shells were produced by variations of the foregoing procedures of the invention, as illustrated by Examples 5-7. EXAMPLE 5 (a) Emulsion Polymerization of Core/Shell polymer particles (double shell) water 130.0g ammonium persulphate initiator 1.1g core polymer emulsion from Example 4 70.0g monomer emulsion (1st shell layer): water 32.0g methyl methacrylate 50.0g styrene 70.0g butyl acrylate 8.5g anionic surfactant (Siponate DS-10) 0.4g nonionic surfactant (Alkasurf® NP-40) 1.9g monomer emulsion (2nd shell layer): water 30.0g styrene 80.5g butyl acrylate 4.5g anionic surfactant (Siponate DS-10) 0.25g nonionic surfactant (Alkasurf® NP-40) 1.25g In a one litre reactor, water, initiator and core polymer were combined with stirring at 83°C. First and second shell monomer emulsions were added consecutively at 0.85 g/min. One hour after monomer addition was complete, the emulsion of core/shell particles was cooled to 35°C and filtered through a 200 mesh screen. Number average particle size was determined to be 316 nm. (b) Sodium Hydroxide Treatment of Core/Shell Particles (double layer shell) water 11.5 ml core/shell emulsion 190.0g 10% aqueous sodium hydroxide 14.25 ml To the core/shell emulsion the sodium hydroxide solution was added and soaked (22 hours) overnight at 40-50°C, then, was heated and maintained at 90°C for 1 hour. It was cooled to 40°C and filtered using a 200 mesh screen. EXAMPLE 6 (a) Emulsion Polymerization of Seed Polymer water 478.2g anionic surfactant (Alcolac® Inc. DS-10) 2.8g ammonium persulphate initiator 1.6g t-butyl hydroperoxide 0.15g sodium formaldehyde sulphoxylate 0.1g monomer emulsion: water 140.1g butyl acrylate 219.2g methyl methacrylate 197.0g methacrylic acid 5.6g anionic surfactant (Alcolac® Inc. DS-10) 0.2g In a one litre glass reactor water and initiator were combined with stirring at 85°C. Monomer emulsion was added at 1.2 g/min. 60 minutes after monomer addition was complete the emulsion of seed polymer particles was cooled at 40°C and filtered through a 200 mesh screen. (b) Emulsion Polymerization of Core Polymer water 420.0g ammonium persulphate initiator 1.0g seed polymer emulsion: 25.1g monomer emulsion: water 73.2g methyl methacrylate 115.6g methacrylic acid 76.3g butyl acrylate 21.1g anionic surfactant (Siponate DS-10) 0.52g ethanol 8.0g In a one litre reactor, water, initiator and seed polymer were combined with stirring at 85°C. Monomer emulsion was added at 0.85 g/min. 30 minutes after monomer addition was complete 0.2 g ammonium persulphate was added to the reactor. The reactor temperature was maintained at 85°C. The emulsion of core polymer particles was cooled to 35°C and filtered through a 200 mesh screen. (c) Emulsion Polymerization of Core/Shell Polymer water 286.0g ammonium persulphate initiator 0.85g core polymer emulsion 80.3g monomer emulsion (1st shell layer): water 51.6g methyl methacrylate 74.25g styrene 121.5g butyl acrylate 10.67g anionic surfactant (Siponate DS-10) 0.68g nonionic surfactant (Alkasurf® NP-40) 2.7g monomer emulsion (2nd shell layer): water 45.9g styrene 86.4g butyl acrylate 4.32g anionic surfactant (Siponate DS-10) 0.41g nonionic surfactant (Alkasurf® NP-40) 1.62g In a one litre reactor, water, initiator and core polymer were combined with stirring at 85°C. First and second shell monomer emulsions were added consecutively at 0.85 g/min. One hour after monomer addition was complete, the emulsion of core/shell particles was cooled to 35°C and filtered through a 200 mesh screen. EXAMPLE 7 (a) Emulsion Polymerization of Core Polymer water 420.0g ammonium persulphate initiator 1.0g seed polymer emulsion (from Example 6) 25.1g monomer emulsion: water 73.2g methyl methacrylate 115.6g methacrylic acid 76.3g butyl acrylate 21.1g anionic surfactant (Siponate DS-10) 0.52g carbon tetrabromide 2.21g carbon tetrachloride 1.1 ethanol 8.0g In a one litre reactor, water initiator and seed polymer were combined with stirring at 85°C. Monomer emulsion was added at 1.2 g/min. 30 minutes after monomer addition was complete 0.2 g ammonium persulphate was added to the reactor. The reactor temperature was maintained at 85°C. The emulsion of core polymer particles was cooled to 35°C and filtered through a 200 mesh screen. (b) Emulsion Polymerization of Core/Shell Polymer water 286.0g ammonium persulphate initiator 0.85g core polymer emulsion 80.3g monomer emulsion (1st shell layer): water 51.6g methyl methacrylate 74.25g styrene 121.5g butyl acrylate 10.67g anionic surfactant (Siponate DS-10) 0.68g nonionic surfactant (Alkasurf® NP-40) 2.7g monomer emulsion (2nd shell layer): water 45.9g styrene 86.4g butyl acrylate 4.32g anionic surfactant (Siponate DS-10) 0.41g nonionic surfactant (Alkasurf® NP-40) 1.62g In a one litre reactor, water, initiator and core polymer were combined with stirring at 85°C. First and second shell monomer emulsions were added consecutively at 0.9 g/min. One hour after monomer addition was complete, the emulsion of core/shell particles was cooled to 35°C and filtered through a 200 mesh screen. The products of Examples 6 and 7 were treated with alkali to effect swelling as described below. Core/Shell emulsion 140.0g 10% aqueous sodium hydroxide 9.21g To the core/shell emulsion the sodium hydroxide solution was added and soaked at room temperature for 1.5 h. The mixture was heated to 90°C and was maintained for 30 minutes at 90°C. It was then cooled to room temperature and filtered using a 200 mesh screen. EXAMPLE 8 (Invention) (a) Emulsion Polymerization of Seed Polymer water 382.5g anionic surfactant (Siponate DS-10) 2.89g ammonium persulphate initiator 2.0g monomer emulsion: water 112g butyl acrylate 175.4g methyl methacrylate 157.6g anionic surfactant (Siponate DS-10) 0.2g In a one litre glass reactor water and initiator were combined with stirring at 85°C. Monomer emulsion was added at 1.2 g/min. Thirty minutes after monomer addition was complete the emulsion of seed polymer particles was cooled to 40°C and filtered through a 200 mesh screen. (b) Emulsion Polymerization of Core Polymer water 460g potassium persulphate initiator 1g seed polymer emulsion 16.21g monomer emulsion: water 63g methyl methacrylate 94.05g methacrylic acid 66.4g butyl acrylate 42.3g anionic surfactant (Siponate DS-10) 0.504g iso-octyl-3-mercapto propionate 3.0g In a one litre glass reactor, water, initiator and seed polymer were combined with stirring at 85°C. Monomer emulsion was added at 0.8 g/min. After monomer addition was complete the reactor temperature was maintained at 85°C for 60 minutes. The emulsion of core polymer particles was cooled to 40°C and filtered through a 200 mesh screen. (c) Emulsion Polymerization of Core/Shell Polymer water 295g potassium persulphate initiator 1.0g core polymer emulsion 140g monomer emulsion (1st shell layer): water 50g methyl methacrylate 134g styrene 82g butyl acrylate 6.5g anionic surfactant (Siponate DS-10) 1.71g monomer emulsion (2nd shell layer): water 49g styrene 96.5g butylacrylate 3.7g anionic surfactant (Siponate DS-10) 0.9g In a one litre reactor, water, initiator and core polymer were combined with stirring at 85°C. First and second shell monomer emulsions were added consecutively at 0.85 g/min. One hour after monomer addition was complete, the emulsion of core/shell particles was cooled to 35°C and filtered through a 200 mesh screen. (d) Sodium Hydroxide Treatment of Core/Shell Particles (double layer shell) core/shell emulsion 150g 11.26% aqueous potassium hydroxide 15.34g To the core/shell emulsion the potassium hydroxide solution was added at 80°C over a 15 minute period. The mixture was heated to 90°C and maintained at 89-90°C for 30 minutes. It was then cooled to room temperature and was filtered using a 200 mesh screen. (e) Preparation of swollen particles with outer polymer layer core/shell emulsion 280g 12.4% aqueous potassium hydroxide 25.58g ammonium persulphate initiator 0.18g monomer emulsion: water 5g styrene 15g anionic surfactant (Siponate DS-10) 0.096g To the core/shell emulsion in a one litre reactor potassium hydroxide solution was added at 80°C over a 15 minute period. The mixture was heated to 90°C and maintained at 89-90°C for 30 minutes. The temperature was then lowered to 75°C and initiator was added. The monomer emulsion was added at 75-73°C over 55 minutes. It was held at reaction temperature for a further 30 minutes then cooled to room temperature and filtered using a 200 mesh screen. (f) Preparation of swollen particles with crosslinked outer polymer layer core/shell emulsion 220g 14% aqueous potassium hydroxide 17.74g potassium persulphate initiator 0.25g monomer emulsion water 18g styrene 12.1g divinyl benzene 0.24g anionic surfactant (Siponate DS-10) 0.036g To the core/shell emulsion in a one litre reactor potassium hydroxide solution was added at 80°C over a 15 minute period. The mixture was heated to 90°C and maintained at 89-90°C for 30 minutes. The mixture was then cooled to 61°C and initiator was added. The monmer emulsion was added over 55 minutes. The mixture was held at reaction temperature for a further 30 minutes then cooled to room temperature and filtered using a 200 mesh screen. To determine inherent opacity of the swollen particle, it was mixed with a polyvinyl acetate-based latex in the proportion nine parts by mass of wet latex (55% solids) to one part by mass of wet swollen particle (37.5% solids). The blend was drawn onto an opacity chart with a 3 mil Bird applicator. When the film had dried, opacity was determined using a Hunterlab model D25-9 tristimulus colorimeter to compare the reflectance (Y-value) over the black and white halves of the chart. The ratio Y black /Y white was the contrast ratio. It described the opacity of the film. The swollen particles were tested in this way. The original swollen particle (d) produced an opacity of 15%. The swollen particle with an uncrosslinked polystyrene outer layer (e) produced an opacity of 17% while the swollen particle with a crosslinked polystyrene outer shell (f) produced a contrast ratio of 20%.

Claims: A process for the manufacture of vesiculated core/shell particles comprising forming an aqueous emulsion of at least one ethylenically unsaturated monomer with acid functionality, polymerising said unsaturated monomer to form core particles of polymer, forming an aqueous dispersion of said core particles and a monomer mixture of a nonionic monoethylenically unsaturated aromatic monomer and a copolymerisable polar monomer in an amount of at least 15 per cent by weight of the total weight of the monomer mixture and said copolymerisable polar monomer having a solubility in water at 20°C of at least 1 per cent by weight, polymerising said monomer mixture to form a first shell on said core particles, adding to an aqueous dispersion of said core particles having a first shell a further nonionic monomer which has a solubility in water at 20°C of less than 1 per cent by weight and polymerising said further monomer to form a second shell on the said particles, and mixing the core/shell particles so obtained with a non-volatile fixed or permanent base to swell the cores and generate therein one or more vesicles. A process according to claim 1 characterised in that the amount of said copolymerisable polar monomer is from 25% to 80%. A process according to claim 1 or 2 characterised in that the said polar monomer is free of free-acid groups. A process according to claim 1, 2 or 3 characterised in that said nonionic monomer employed to form said first shell or said second shell is styrene, alpha-methylstyrene, vinyl toluene, ring substituted derivatives of styrene, of alpha-methylstyrene and of vinyl toluene and mixtures thereof. A process according to any one of claims 1 to 4 characterised in that the amount of said nonionic monomer used to form the first shell is greater than 20 per cent by weight of the total monomer mixture employed to form the first shell. A process according to any one of claims 1 to 5 characterised in that the monomer mixture used to form the second shell is free of said copolymerisable polar monomer. A process according to any one of claims 1 to 6 characterised in that said mixture of monomers used to form said first shell, or said second shell or both contains a soft monomer selected from the class consisting of butyl acrylate, ethyl acrylate, isobutyl acrylate, an ethyl hexyl acrylate, butadiene, isoprene, lauryl methacrylate, a long chain ester of acrylic acid, a long chain ester of methacrylic acid, dibutyl maleate and mixtures thereof. A process according to claim 7 characterised in that the amount of the soft monomer is at least 2% by weight of the monomer mixture in which said soft monomer is present. A process according to any one of claims 1 to 8 characterised in that said core is formed by polymerising an acidic monomer selected from the class consisting of acrylic acid, methacrylic acid, itaconic acid, aconitic acid, maleic acid, maleic acid anhydride, fumaric acid and crotonic acid. A process according to any one of the preceding claims characterised in that the core is a copolymer of an unsaturated monomer selected from the class consisting of methyl methacrylate, styrene, vinyl toluene, alpha methyl styrene, ethylene, vinyl acetate, vinyl chloride, acrylonitrile, acrylamide, methacrylamide, alkyl esters of methacrylic acid and of acrylic acid and of vinyl alcohol and alkenyl esters of methacrylic acid and of acrylic acid and of vinyl alcohol. A process according to any one of the preceding claims characterised in that the core particles are formed by polymerising said required monomers on preformed seed polymer particles dispersed in water. A process according to any one of the preceding claims characterised in that the said base is an alkali metal hydroxide. A process according to any one of claims 1 to 11 characterised in that the said base is an alkali metal bicarbonate. A vesiculated core/shell polymer particle comprising a core containing acid functionality of a polymer or copolymer of an ethylenically unsaturated monomer, a first shell around said core and a second shell characterised in that the first shell around said core is formed of a copolymer of a nonionic monoethylenically unsaturated aromatic monomer and a polar monomer in an amount of at least 15 per cent by weight of said copolymer and said polar monomer having a solubility in water at 20°C of at least 1 per cent by weight, the second shell is formed of a polymer or copolymer of a nonionic monomer having a solubility in water at 20°C of less than 1 per cent by weight, and said core/shell particles having at least one vesicule formed by reaction with a non-volatile fixed or permanent base.

Inventors: CHIP GERALD KWONG, GHARAPETIAN HRIRE, RUDIN ALFRED, CHIP,GERALD KWONG, GHARAPETIAN, HRIRE, RUDIN, ALFRED
Applicants: TIOXIDE GROUP LTD, TIOXIDE GROUP LIMITED
Application year: 1991
Main classification: C08F285
IPC codes: C08F285, C08F2, C08J3