Hollow polymer latex particles

Abstract

This invention relates to polymer particles that are acid-free in the swellable polymer core and to hollow polymer particles derived therefrom, and to a process for the production of these particles. The acid-free core advantageously decreases the polymer particle's affinity to water. Applicants have discovered that non-carboxylated latexes having a variety of Tg's will swell at temperatures higher than the Tg of the polymer. These latex particles are useful as opacifying plastic pigments for various coatings and particularly as gloss-enhancing plastic pigments for paper and paperboard coatings. Preparation of the hollow polymer particle comprises the use of (meth)acrylate ester monomers, vinyl esters of carboxylic acids or a mixture thereof which are hydrolyzable and swellable at higher temperatures in alkaline environments. The particles may be encapsulated with a hard shell.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to hollow polymer latex particles and a process for the production of the particles which are useful as opacifying plastic pigments for various coatings and particularly as gloss-enhancing plastic pigments for paper and paperboard coatings.

2. Prior Art

Hollow particles are known to be useful as opacifying agents in coating applications such as in paint or paper coating. They also improve paper coating gloss. The use of the hollow latex in such coatings reduces the need for expensive pigments, such as TiO.sub.2, without adding excessive and undesirable weight to the coating. The hollow latex particle provides opacity because the hollow structure in a latex particle scatters light more efficiently than a corresponding particle of uniform density. The light scattering properties are related to the difference in refractive index between the shell and the internal void. Such hollow latexes have additional utility in areas other than coatings such as in processes involving microencapsulations to prepare, for example, control release agents.

U.S. Pat. No. 4,427,836 and U.S. Pat. No. 4,594,363 disclose two different processes for making hollow latexes by swelling core-shell polymer particles having acid-containing cores with a volatile base or a permanent base. U.S. Pat. No. 4,880,842 describes several disadvantages of having high levels of copolymerized acid in the core polymer and discloses a process for making hollow latexes by introducing a non-polymeric acid to an early stage of the multi-stage polymer particles instead of copolymerizing acid to make swellable cores.

These known methods for preparing hollow latexes require cores containing acid or acidic monomers to enable swelling to occur at room temperature. The existence of acid in the polymer disadvantageously increases the polymer's affinity to water. The sensitivity to the effects of water undesirably renders the coating into which the latex is formulated sensitive to moisture as well; an undesirable property in coating applications. An acidic core is also thought to impart polarity to the polymer and because of such polarity the polymer is thought to not adsorb surfactants well rendering the polymer unstable or prone to coagulation or aggregation in larger particles. The water sensitivity of the core or high affinity for water by the core is thought to make it difficult to encapsulate the core with the shell polymer because the core tends to remain at the polymer-water interface on the surface of the particle rather than be encapsulated by the shell.

These known processes for preparing hollow polymer particles often also require a post-encapsulating step with crosslinked polymers, that is, the acid-containing cores are encapsulated with an uncrosslinked shell, the core is swelled, then the shell can be encapsulated with crosslinked polymers.

It would be desirable to have improved hollow polymer particles with acid-free hydrophilic cores and a process for their production.

SUMMARY OF THE INVENTION

The present invention relates to a hollow polymer particle derived from a polymer particle comprising

A. an acid-free hydrophilic core copolymer phase made of (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and one or more crosslinking monomers; the core copolymer being base-hydrolizable at temperatures greater than the T.sub.g of the copolymer; and

B. an intermediate shell copolymer phase which is compatible with the hydrophilic core copolymer phase, the intermediate shell copolymer made of (meth)acrylate monomers having a T.sub.g for their copolymer greater than 50.degree. C.; and one or more vinyl acid monomers; and, optionally, one or more monovinyl aromatic monomers; and, optionally, one or more crosslinking monomers; the intermediate shell copolymer also being compatible with

C. an outer rigid shell copolymer which is hydrophobic and crosslinked, the copolymer having a T.sub.g greater than about 80.degree. C., the outer shell copolymer made of one or more monovinyl aromatic monomers; and one or more vinyl acid monomers; and one or more crosslinking monomers; and, optionally, (meth)acrylate monomers having a T.sub.g for their copolymer greater than 50.degree. C.

In another embodiment this invention relates to a process for preparing a hollow polymer particle comprising

A. emulsion polymerizing (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and one or more crosslinking monomers to form an acid-free hydrophilic core copolymer phase which is base-hydrolizable at temperatures greater than the T.sub.g of the copolymer;

B. emulsion polymerizing (meth)acrylate monomers having a T.sub.g for their copolymer greater than 50.degree. C.; and one or more vinyl acid monomers; and, optionally, one or more monovinyl aromatic monomers; and, optionally, one or more crosslinking monomers to form an intermediate shell copolymer phase which is compatible with the hydrophilic core copolymer phase;

C. emulsion polymerizing one or more monovinyl aromatic monomers; and one or more vinyl acid monomers; and one or more crosslinking monomers; and, optionally, (meth)acrylate monomers, vinyl esters of carboxylic acid monomers or a mixture thereof having a T.sub.g for their copolymer greater than 50 C. to form an an outer rigid shell copolymer phase which is hydrophobic and crosslinked, the copolymer having a T.sub.g greater than about 80.degree. C., the outer shell copolymer being compatible with the intermediate shell copolymer; and

D. at a temperature from about 100.degree. C. to about 150.degree. C., expanding the particle produced in C. by exposing the particle to a strong alkaline solution in an amount from about 0.75 to about 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, to produce a hollow polymer particle; or at a temperature from about 100.degree. C. to about 150.degree. C., expanding and encapsulating the particle produced in C. by

i. mixing the particle with an encapsulation mixture comprising a monovinyl aromatic monomer, a crosslinking monomer, an initiator and a strong alkaline solution containing from about 0.75 to about 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, and ii. subjecting the mixture resulting from i. to simultaneous expansion and polymerization conditions to produce a hollow polymer particle which is encapsulated in a hard shell; or at a temperature from about 100.degree. C. to about 150.degree. C., expanding the particle produced in C. by i. exposing the particle to a strong alkaline solution in an amount from about 0.75 to about 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, and then, at a temperature from about 75.degree. C. to about 150.degree. C., encapsulating the hollow polymer particle by ii. emulsion polymerizing an encapsulation mixture comprising a monovinyl aromatic monomer and a crosslinking monomer in the presence of the expanded particles to produce a hollow polymer particle which is encapsulated in a hard shell.

Within the scope of this invention is a coating composition comprising the hollow polymer particle; a binder; and, optionally, a filler; and, optionally, a pigment; and a process for the preparation of a coating composition comprising mixing together the hollow polymer particle; a binder; and, optionally, a filler; and, optionally, a pigment.

DETAILED DESCRIPTION OF THE INVENTION

The applicants have discovered that non-carboxylated latexes having a variety of T.sub.g 's will swell at higher temperatures. The mechanism entails the use of acrylate and methacrylate monomers, herein defined collectively as (meth)acrylates, which are hydrolyzable and swellable at higher temperatures in alkaline environments. Such mechanisms for making swollen latexes provide for a wide variety of latexes which are suitable for many end uses.

The latex can be prepared by stages which create a core and shell structure which subsequent to neutralization forms a hollow polymer particle. Therefore, the latex requires a swellable core and a shell sufficiently deformable to enable the core to swell but sufficiently hard to withstand deformation, thus creating a hollow particle on drying. The latex polymer particles of the present invention can be prepared by using batch or semi-continuous emulsion polymerization.

The degree of swelling by the polymer can be measured by viscometry or hydrodynamic chromatography (HDC).

The lack of an acidic core reduces the polarity of the polymer. Without such polarity the polymer is thought to adsorb surfactants well making them stable and less prone to coagulation or aggregation in larger particles. The lack of water sensitivity of the core is also thought to render the core receptive to being surrounded by the shell because the core is not attracted to the polymer-water interface on the surface of the particle and therefore is surrounded by the shell.

The present process also enables production of hollow polymer particles having crosslinked shells without a post-encapsulating step with crosslinking monomers. Thus, the acid-free cores can be encapsulated with crosslinked polymers directly and the cores can be hydrolyzed and swelled with bases such as NaOH at elevated temperatures while stretching the crosslinked shells. This is in contrast to other processes in which the procedure is to encapsulate an acid containing core with an uncrosslinked shell, swell the core, then crosslink the shell.

"Hollow polymer latex particle", as used herein, is meant to include a latex particle which is not completely solid. Such particle morphology can include various void structures such as uniform microvoids or hemispherical particles with voided centers. The more preferred hollow polymer latex particles are those having a centered void and a particle wall thickness aspect ratio of inner diameter to outer diameter of from about 0.1 to about 0.9. The hollow polymer latex particles having a centered void have greater utility in coatings than those structures having microvoids or hemispherical particles with voided centers.

The more specific hollow polymer particle contemplated by the present invention is derived from a polymer particle which comprises a hydrolyzable, neutralizable core as a seed, an intermediate copolymer layer with a relatively high glass transition temperature (T.sub.g) that is compatible with the copolymer of the seed, and a rigid copolymer shell which is hydrophobic, crosslinked and is also of a copolymer having a relatively high T.sub.g. The polymer particle is then hydrolyzed at a temperature higher than the T.sub.g of the copolymer of the shells to produce the hollow polymer particles. The terms "hollow latex", "hollow polymer particle" and "hollow copolymer particle" and "hollow polymer latex particle" are all used herein to indicate the polymer particle produced by emulsion polymerization followed by high temperature neutralization to produce expansion. Optionally, these expanded hollow polymer particles can be further encapsulated with a more crosslinked rigid copolymer, if a greater degree of dimensional stability is needed during drying.

Latex Monomers

Representative monomers useful to produce hollow latexes of the present invention include acrylate monomer, methacrylate monomer, monovinyl aromatic monomer, aliphatic conjugated diene monomer, vinylidene halide or vinyl halide monomer, vinyl esters of carboxyl acids containing from 1 to 18 carbon atoms, such as vinyl acetate or vinyl stearate, methacrylonitrile and acrylonitrile. The monoethylenically unsaturated carboxylic acid monomer is used in the shell polymers but is nonexistent in the core polymers. Various mixtures of these monomers are also employable. Crosslinking agents could also be used to decrease the swellability of the polymer or for various other conventionally known reasons for the addition of crosslinking agents to polymers.

The term "(meth)acrylate" monomers is meant those monomers which are used to prepare the swellable copolymers of the present latexes by the processes of the present invention including conventionally known acrylates such as alkyl esters of acrylic acid represented by the formula CH.sub.2 .dbd.CHCOOR, wherein R is from 1 to 16 carbon atoms and substituted alkyls represented by the formulas --CH.sub.2 Cl; --CH.sub.2 CH.sub.2 OH; and ##STR1##

The term "(meth)acrylate" monomer(s) as used herein is meant also to include the monovinyl acrylate and methacrylate monomers. The (meth)acrylates can include esters, amides and substituted derivatives thereof. Generally, the preferred (meth)acrylates are C.sub.1 -C.sub.8 alkyl acrylates or methacrylates.

Examples of suitable (meth)acrylates include methyl acrylate, ethyl acrylate, butyl acrylate, hexyl acrylate, 2-ethyl hexyl acrylate, octyl acrylate and isooctyl acrylate, n-decyl acrylate, isodecyl acrylate, tert-butyl acrylate, methyl methacrylate, butyl methacrylate, hexyl methacrylate, isobutyl methacrylate, isopropyl methacrylate as well as 2-hydroxyethyl acrylate and acrylamide. The preferred (meth)acrylates are methyl acrylate ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate, octyl acrylate, isooctyl acrylate, methyl methacrylate and butyl methacrylate.

The term "monovinyl aromatic" monomer, as used herein, is meant to include those monomers with a radical of the formula ##STR2## wherein R is hydrogen or an alkyl having from 1 to 4 carbon atoms, attached directly to an aromatic nucleus containing from 6 to 10 carbon atoms, including those wherein the aromatic nucleus is substituted with alkyl or halogen substituents. The preferred monovinyl aromatic monomers are styrene and vinyltoluene.

The term "aliphatic conjugated diene", monomer, as used herein, is meant to include compounds such as 1,3-butadiene, 2-methyl-1,3-butadiene, piperylene (1,3-pentadiene), and other hydrocarbon analogs of 1,3-butadiene. Other monomers inclusive as aliphatic conjugated dienes are halogenated compounds such as 2-chloro 1,3 -butadiene.

"Vinylidene halides" and "vinyl halides" suitable for this invention include vinylidene chloride and vinyl chloride, which are highly preferred Vinylidene bromides and vinyl bromide can also be employed.

The term "monoethylenically unsaturated carboxylic acid" monomer, as used herein, is meant to include those monocarboxylic monomers such as acrylic acid, and methacrylic acid; dicarboxylic monomers such as itaconic acid, fumaric acid, maleic acid, and their monoesters.

The C.sub.3 -C.sub.8 .alpha.,.beta.-ethylenically unsaturated carboxylic acid monomer contemplated include monomers represented by the formula: ##STR3## where R is H and R' is H, C.sub.1 -C.sub.4 alkyl, or --CH.sub.2 COOX; R is --COOX and R' is H or --CH.sub.2 COOX; or R is CH.sub.3 and R' is H; and X is H or C.sub.1 -C.sub.4 alkyl.

Acrylic or methacrylic acid or a mixture thereof with itaconic or fumaric acid are included as well as crotonic and aconitic acid and half esters of these and other polycarboxylic acids such as maleic acid. Although such monomers are contemplated as monomers of the shell stages of the present invention, such acids are not contemplated to be part of the core stage.

The term "crosslinking" monomer is meant to include monomers conventionally known in the art as useful for crosslinking polymerizable monomers. Examples of such monomers typically include di- or trifunctional monomers such as divinyl benzene, ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylol propane trimethacrylate, allyl methacrylate or diene functional monomers such as butadiene. The crosslinking monomer can be present in the hollow polymer particles of the present invention in the core or shell phases.

High-temperature expandable latexes can be made by conventional emulsion polymerization processes including batch or semi-continuous and either seeded or non-seeded emulsion polymerization.

Compositions of Hollow Polymer Particle Phases

A. The Core Phase

The core or seed of the present hollow polymer particles is a copolymer which does not comprise monoethylenically unsaturated carboxylic acid monomer but is swellable at temperatures greater than the T.sub.g of the core copolymer. Monomers most suitable for the core include any base-hydrolyzable (meth)acrylic ester monomers. The core copolymer is typically crosslinked and is to be hydrolyzable with alkaline solutions at temperatures greater than the T.sub.g of the copolymer.

The most preferred composition of the core is ethyl acrylate, methyl acrylate and allyl methacrylate. The ratios preferred for the monomers are based on 100 parts by weight of the core total as follows: from about 49 to about 80 parts ethyl acrylate; from about 20 to about 50 parts methyl acrylate; and from about 0.05 to about 1 parts allyl methacrylate. To enable the total number of parts to work out to 100 parts a skilled artisan would understand that if allyl methacrylate is present in an amount of 0.05 parts, the ethyl acrylate will be present in an amount of about 79.95 parts. The most preferred composition for the core is 59.5 parts ethyl acrylate, 40 parts methyl acrylate and 0.5 parts allyl methacrylate, methyl acrylate being the more readily hydrolyzable moiety. The core should be present in the latex particle in an amount of from about 2 to about 15 parts by weight based on the total weight of the latex. The more preferred amount of the core to the total latex particle is about 3 parts to about 10 parts.

B. The Intermediate Shell Phase

The intermediate shell is an intermediate copolymer layer with a relatively high T.sub.g compared to the core and is made up of monomers that are compatible with the copolymer of the seed or core. The intermediate shell can comprise the somewhat hydrophilic monomers such as ethyl methacrylate, methyl methacrylate and the hydrophobic harder monomers such as styrene, vinyl toluene, .alpha.-methyl styrene.

The preferred composition of the intermediate shell is a copolymer of methyl methacrylate, styrene, methacrylic acid and allyl methacrylate or divinyl benzene. The ratios preferred for the monomers based on 100 parts by weight of the total intermediate shell are as follows: from about 65 to about 99 parts methyl methacrylate; from 0 to about 30 parts styrene; from about 1 to about 3 parts methacrylic acid; and from 0 to about 2 parts allyl methacrylate or divinyl benzene. The most preferred composition for the intermediate shell is 72.5 parts methyl methacrylate; 25 parts styrene; 2 parts methacrylic acid; and 0.5 parts allyl methacrylate or divinyl benzene.

The intermediate shell should be present in the latex in an amount from about 15 to about 50 parts by weight based on the total weight of the latex. The most preferred amount of the intermediate shell to the total latex particle is about 25 parts

C. The Outer Shell Phase

The outer shell is a rigid copolymer which is hydrophobic, crosslinked and is also of a copolymer having a relatively high T.sub.g. The composition of the outer shell will preferably incorporate styrene; methyl methacrylate; methacrylic acid; acrylic acid and allyl methacrylate or divinyl benzene. The ratios preferred for the monomers are based on 100 parts by weight of the total outer shell as follows: from about 67 to about 99 parts styrene; from 0 to about 25 parts methyl methacrylate; from about 1 to about 3 parts methacrylic acid; from 0 to about 2 parts acrylic acid; and from about 0.2 to about 10 parts allyl methacrylate or divinyl benzene. The crosslinking monomer can be desirably tapered from 0.1 part to 10 parts for balanced expansion and dimensional stability of hollow latexes. The most preferred composition for the outer shell is 94 parts styrene, 2 parts methacrylic acid and 2 to 5 parts allyl methacrylate or divinyl benzene.

The outer shell should be present in the latex in an amount from about 50 to about 83 parts by weight based on the total weight of the latex. The most preferred amount of the outer shell to the total latex particle is about 75 parts.

A preferred embodiment of the hollow polymer particle is that wherein

A. the core copolymer phase is from about 2 to about 15 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the acid-free hydrophilic core copolymer phase, the core is made of from about 99 to about 99.99 parts (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and from about 0.01 to about 1 part of one or more crosslinking monomers;

B. the intermediate shell copolymer phase is from about 15 to about 50 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the intermediate shell copolymer phase, the intermediate shell is made of from about 65 to about 98 parts (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.; and from about 1 to about 3 parts vinyl acid monomers; and from 0 to about 30 parts monovinyl aromatic monomers, and from 0 to about 2 parts crosslinking monomers;

C. the outer rigid shell copolymer phase is from about 50 to about 83 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the outer shell copolymer phase, the outer shell is made of from about 67 to about 99 parts monovinyl aromatic monomers; and from about 1 to about 3 parts vinyl acid monomers; and from about 0.5 to about 10 parts crosslinking monomers; and from 0 to about 25 parts (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.

Preparation of the Hollow Latexes

The latexes can be made by means of emulsion polymerization. The core or seed is prepared first and the shells are polymerized subsequently. The expansion of the latexes to produce the hollow morphology can occur as a post-polymerization step.

The temperature during the emulsion polymerization will be anywhere from 50.degree. C. to 150.degree. C.; preferably 70.degree. C. to 100.degree. C. The polymerization time can be from 2 to 10 hours depending upon the pressure and temperature.

The expansion step is achieved by exposing the latex to a strong alkaline solution, such as sodium hydroxide, in an amount of from about 0.75 to about 1.5 equivalents of base, based on all the acids in the shell phases and the more easily hydrolyzable acrylate esters in the core, such as methyl acrylate. The expansion of the hollow latexes should occur between 100.degree. C. to 150.degree. C.; preferably between 110.degree. C. to 140.degree. C. When the crosslinking of the shells is greater, the temperature of the expansion step should also be greater. Solvents can aid the swelling in the expansion step. The expansion time can range from about 0.5 to about 10 hours; preferably from about 2 to about 5 hours.

Paper Coatings

Paper coating colors are formulated with clay, a pigment such as the hollow polymer latex particles of this invention, and styrene butadiene (S/B) latex binder. To avoid coating placement problems associated with paper substrates, the coatings are made on polyester films by using a Meyer Rod to obtain a desired thickness of the coatings. They are calendered typically at 150.degree. F., 1000 pli, and 3 nips, then are measured for gloss and opacity.

EXAMPLE

A hydrolyzable, expandable core latex is prepared by charging into a two gallon reactor approximately: 2158 grams (g) of water; 0.6 g of sodium persulfate, 2 g of a 45 percent solution of an alkylated diphenol oxide disulfonate surfactant; and 43 g of a 1 percent active aqueous pentasodium diethylene triamine pentacetate solution. The monomer feed comprising 1267 g (59.5 parts) of ethyl acrylate, 852 g (40 parts) of methyl acrylate and 11 g (0.5 part) of allyl methacrylate is then simultaneously con-added over 4 hours to the reactor with an aqueous feed comprising: 47.3 g of a 45 percent solution of an alkylated diphenol oxide disulfonate surfactant; 6.4 g of sodium persulfate; 213 g of GAFAC.TM. RE-610 (10 percent) (GAF Corporation's phosphate ester of nonionic surfactant) and 852 g of water. Upon completion of the con-add, the core latex is allowed to polymerize to completion, the latex is then cooled. The latex is found to be 1250 .ANG. in diameter, 39.53 percent solids, and pH 2.49.

The shells are prepared by charging to a reactor: 2.5 parts based on solids of the seed or core prepared above; approximately 2099 grams (g) of water; 0.6 g of sodium persulfate; 135 g of the core latex prepared above (2.5 parts) (39.53 percent) and 41 g of a 1 percent active aqueous pentasodium diethylene triamine pentacetate solution.

The monomer feeds comprising 376 g (18.1 parts) of methyl methacrylate, 130 g (6.3 parts) of styrene, 10 g (0.5 parts) of methacrylic acid and 3 g (0.1 part) of allyl methacrylate is then con-added over 75 minutes to the reactor simultaneously with an aqueous feed comprising 46 g of a 45 percent solution of an alkylated diphenol oxide disulfonate surfactant; 10 g of sodium persulfate; 207 g of GAFAC.TM. RE-610 (10 percent); 10 g of a 20 percent solution of sodium hydroxide and 829 g of water (the aqueous stream is a continuous stream for both the first and second monomer streams); upon completion of the first monomer stream, a second monomer stream is con-added over 225 minutes with the aqueous feed; the second monomer stream comprising 1515 g (73.1 parts) of styrene, 31 g (1.5 parts) of methacrylic acid and 8 g (0.4 parts) of allyl methacrylate. Upon completion of the con-add, the solution is allowed to polymerize to completion and the latex is cooled. The latex is determined to be 4192 .ANG. in diameter, 39.66 percent solids, and pH 3.17.

The evaluation of the swelling of the particle is done by determining the swell ratio (SR) of the latex. The swelling ratios are measured by hydrodynamic chromatography. The swelling ratio can also be measured by viscosity measurements.

The particle size distribution of the latex by HDC prior to hydrolysis/swelling is measured and the latex is then exposed to one equivalent of sodium hydroxide per each acid and methyl acrylate in the particle at 120.degree. C. for three hours. The percent volume increase of the latex is about 30 percent indicating a swelling of the latex will enable the latex to occupy a greater volume than the unswelled latex. The core measurement indicates a swelling of approximately thirteen times its original size. The average particle size increases from about 4190 Angstroms to about 4580 Angstroms.

The opacifying power of the unexpanded latex prepared above and the same latex expanded which is a hollow polymer latex particle is tested as follows:

The base (unexpanded) latex and the expanded latex (hollow polymer particle) are blended with a styrene butadiene (S/B) latex binder at a weight ratio of 30/70, respectively, and these blends are formulated at about 45 percent solids. Coatings are made on 2-mil (50.8 .mu.m) polyester films with No. 30 Meyer Rod (a wire-wound rod) and their thickness is about 1 mil. (25.4 .mu.m) Before the drawdown, the polyester film surface is cleaned with a soap solution. The opacity of these air-dried latex blend coatings is measured on BNL-2 Opacimeter in terms of a contrast ratio (TAPPI Testing Procedures T-425). The opacity of the base latex-containing coating is found to be 20.4, while the expanded latex-containing coating is 50.1. These results clearly demonstrate that the expanded latex particles are acting as opacifying hollow polymer latex particles.

Alternatively, the above unexpanded base latex is mixed with varying amounts (50-100 parts per 100 parts of the base latex) of styrene and divinyl benzene mixtures (S/DVB: 95/5-85/15) and an oil-soluble initiator such as tertiary perbenzoate at a level of 0.5 part per 100 parts monomer mixture and is subject to the simultaneous expansion and polymerization at 120.degree. C. for 5 hours. These hollow polymer latex particles made by this process exhibit good opacity.

In another case, the above unexpanded base latex is first expanded, then encapsulated with different amounts of styrene and divinyl benzene mixtures at 90.degree. C. These hollow polymer latex particles made by this post-encapsulation also exhibit good opacifying properties.

A hollow polymer latex particle prepared as above is evaluated for paper coating properties such as gloss and opacity, then is compared with a comparative polystyrene plastic pigment of the same particle size as that of the hollow polymer latex particle. Paper coating colors are formulated as follows: 90 parts No. 1 clay, 10 parts plastic pigment, and 15 parts styrene butadiene (S/B) latex binder at 60 percent solids and pH 7-8. These two coatings containing the above hollow polymer latex particle and comparative polystyrene plastic pigment, respectively, are made on 2 mil (50.8 .mu.m) polyester films cleaned with a soap solution by using No. 20 Meyer Rod to obtain about 1 mil (25.4 .mu.m) thick coatings. They are calendered at 150.degree. F. (65.6.degree. C.), 1000 pli, and 3 nips, then are measured for gloss and opacity. The results are shown in Table I.

TABLE I______________________________________ TAPPI 75.degree. TAPPICoating Composition Gloss Opacity______________________________________Polystyrene PP 84.5 71.8(comparative example -not an example of theinvention)Example - hollow latex 97.5 73.4particle______________________________________ TAPPI 75.degree. Gloss by TAPPI Testing Procedures T480 TAPPI Opacity by TAPPI Testing Procedures T425

The above results clearly show that the hollow polymer latex particle improves both paper coating gloss and opacity.

Claims

1. A hollow polymer particle derived from a polymer particle comprising

A. an acid-free hydrophilic core copolymer phase made of (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and one or more crosslinking monomers; the core copolymer being base-hydrolizable at temperatures greater than the T.sub.g of the copolymer; and

B. an intermediate shell copolymer phase which is compatible with the hydrophilic core copolymer phase, the intermediate shell copolymer made of (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.; and one or more vinyl acid monomers; and, optionally, one or more monovinyl aromatic monomers; and, optionally, one or more crosslinking monomers; the intermediate shell copolymer also being compatible with

C. an outer rigid shell copolymer which is hydrophobic and crosslinked, the copolymer having a T.sub.g greater than about 80.degree. C., the outer shell copolymer made of one or more monovinyl aromatic monomers; and one or more vinyl acid monomers; and one or more crosslinking monomers; and, optionally, (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.

2. The hollow polymer particle of claim 1 wherein

A. the core copolymer phase is from 2 to 15 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the acid-free hydrophilic core copolymer phase, the core is made of from 99 to 99.99 parts (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and from 0.01 to 1 part of one or more crosslinking monomers;

B. the intermediate shell copolymer phase is from 15 to 50 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the intermediate shell copolymer phase, the intermediate shell is made of from 65 to 98 parts (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.; and from 1 to 3 parts vinyl acid monomers; and from 0 to 30 parts monovinyl aromatic monomers; and from 0 to 2 parts crosslinking monomers;

C. the outer rigid shell copolymer phase is from 50 to 83 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the outer shell copolymer phase, the outer shell is made of from 67 to 99 parts monovinyl aromatic monomers; and from 1 to 3 parts vinyl acid monomers; and from 0.5 to 10 parts crosslinking monomers; and from 0 to 25 parts (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.

3. The hollow polymer particle of claim 2 wherein

A. the core phase is made from 49 to 80 parts ethyl acrylate; from 20 to 50 parts methyl acrylate; and from 0.05 to 1 parts allyl methacrylate based on 100 parts by weight of the core phase;

B. the intermediate shell phase is made from 65 and to 99 parts methyl methacrylate; from 0 and to 30 parts styrene; from 1 to 3 parts methacrylic acid; and from 0 to 2 parts allyl methacrylate or divinyl benzene based on 100 parts by weight of the total intermediate shell,; and

C. the outer shell phase is made from 67 to 99 parts styrene; from 0 to 25 parts methyl methacrylate; from 1 to 3 parts methacrylic acid; from 0 to 3 parts acrylic acid; and from 0.2to 10 parts allyl methacrylate or divinyl benzene based on 100 parts by weight of the total outer shell.

4. The hollow polymer particle of claim 1 further comprising an encapsulating shell.

5. A coating composition comprising the hollow polymer particle of claim 1; a binder; and, optionally, a pigment; and, optionally, a filler.

6. A process for preparing a hollow polymer particle comprising

A. emulsion polymerizing (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and one or more crosslinking monomers to form an acid-free hydrophilic core copolymer phase which is base-hydrolizable at temperatures greater than the T.sub.g of the copolymer;

B. emulsion polymerizing (meth)acrylate monomers having a Tg for their copolymer greater than 50.degree. C.; and one or more vinyl acid monomers; and, optionally, one or more monovinyl aromatic monomers; and, optionally, one or more crosslinking monomers to form an intermediate shell copolymer phase which is compatible with the hydrophilic core copolymer phase;

C. emulsion polymerizing one or more monovinyl aromatic monomers; and one or more vinyl acid monomers; and one or more crosslinking monomers; and, optionally, (meth)acrylate monomers, vinyl esters of carboxylic acid monomers or a mixture thereof having a Tg for their copolymer greater than 50.degree. C. to form an an outer rigid shell copolymer phase which is hydrophobic and crosslinked, the copolymer having a T.sub.g greater than 80.degree. C., the outer shell copolymer being compatible with the intermediate shell copolymer; and

D. at a temperature from 100.degree. C. to 150.degree. C., expanding the particle produced in C. by exposing the particle to a strong alkaline solution in an amount from 0.75 to 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, to produce a hollow polymer particle; or at a temperature from 100.degree. C. to 150.degree. C., expanding and encapsulating the particle produced in C. by

i. mixing the particle with an encapsulation mixture comprising a monovinyl aromatic monomer, a crosslinking monomer, an initiator and a strong alkaline solution containing from 0.75 to 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, and

ii. subjecting the mixture resulting from i. to simultaneous expansion and polymerization conditions

to produce a hollow polymer particle which is encapsulated in a hard shell; or at a temperature from 100.degree. C. to 150.degree. C., expanding the particle produced in C. by

i. exposing the particle to a strong alkaline solution in an amount from 0.75 to 1.5 equivalents of base, based on all the acids in the shell phases and the hydrolyzable monomers in the core phase, and then, at a temperature from 75.degree. C. to 150.degree. C., encapsulating the hollow polymer particle by

ii. emulsion polymerizing an encapsulation mixture comprising a monovinyl aromatic monomer and a crosslinking monomer in the presence of the expanded particles to produce a hollow polymer particle which is encapsulated in a hard shell.

7. The process of claim 6 wherein

A. the core copolymer phase is from 2 to 15 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the acid-free hydrophilic core copolymer phase, the core is made of from 99 to 99.99 parts (meth)acrylate monomers, vinyl ester monomers of carboxylic acids or a mixture thereof, and from 0.01 to 1 part of one or more crosslinking monomers;

B. the intermediate shell copolymer phase is from 15 to 50 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the intermediate shell copolymer phase, the intermediate shell is made of from 65 to 98 parts (meth)acrylate monomers having a T.sub.g for their copolymer greater than 50.degree. C.; and from 1 to 3 parts vinyl acid monomers; and from 0 to 30 parts monovinyl aromatic monomers; and from 0 to 2 parts crosslinking monomers;

C. the outer rigid shell copolymer phase is from 50 to 83 parts by weight of the hollow polymer particle, and, based on 100 parts by weight of the outer shell copolymer phase, the outer shell is made of from 67 to 99 parts monovinyl aromatic monomers; and from 1 to 3 parts vinyl acid monomers; and from 0.2 to 10 parts crosslinking monomers; and from 0 to 25 parts (meth)acrylate monomers having a T.sub.g for their copolymer greater than 50.degree. C.

8. The process of claim 7 wherein

A. the core phase is made from 49 to 80 parts ethyl acrylate; from 20 to 50 parts methyl acrylate; and from 0.05 to 1 parts allyl methacrylate based on 100 parts by weight of the core phase;

B. the intermediate shell phase is made from 65 and to 99 parts methyl methacrylate; from 0 and to 30 parts styrene; from 1 and to 3 parts methacrylic acid; and from 0 and to 2 parts allyl methacrylate or divinyl benzene based on 100 parts by weight of the total intermediate shell; and

C. the outer shell phase is made from 67 to 99 parts styrene; from 0 to 25 parts methyl methacrylate; from 1 to 3 parts methacrylic acid; from 0 to 3 parts acrylic acid; and from 0.2 to 10 parts allyl methacrylate or divinyl benzene based on 100 parts by weight of the total outer shell.

9. A process for the preparation of a coating composition comprising mixing together the hollow polymer particle produced by the process of claim 6; a binder; and, optionally, a filler; and, optionally, a pigment.

10. The particle of claim 1 wherein the core copolymer phase comprises ethylacrylate, methylacrylate and allyl methacrylate.

11. The particle of claim 1 wherein the intermediate shell copolymer comprises methyl methacrylate, styrene, methacrylic acid, and allyl methacrylate or divinylbenzene.

12. The particle of claim 11 wherein the crosslinking monomer is allyl methacrylate.

13. The particle of claim 1 wherein the outer shell copolymer comprises styrene, methyl methacrylate, methacrylic acid, acrylic acid, and allyl methacrylate or divinylbenzene.

14. The particle of claim 1 wherein the outer shell copolymer comprises styrene, methacrylic acid and allyl methacrylate or divinylbenzene.

15. The particle of claim 4 wherein the encapsulating shell comprises a copolymer phase made of a monovinyl aromatic monomer and one or more crosslinking monomers.

16. The particle of claim 15 wherein the monovinyl aromatic monomer is styrene and the crosslinking monomer is divinylbenzene or allyl methacrylate.

17. The particle of claim 16 wherein the crosslinking monomer is allyl methacrylate.

18. The particle of claim 16 wherein the crosslinking monomer is divinylbenzene.

19. The particle of claim 1 wherein the monovinyl aromatic monomer is styrene.

20. The polymer particle of claim 1 wherein the core copolymer phase comprises ethylacrylate, methylacrylate, and allyl methacrylate; the intermediate shell copolymer comprises methyl methacrylate, styrene, methacrylic acid, and allyl methacrylate or divinylbenzene; and wherein the outer rigid shell copolymer comprises styrene, methyl methacrylate, methacrylic acid, acrylic acid, and allyl methacrylate or divinylbenzene.