Opacifying polymer particles

Abstract

An aqueous dispersion of pigmented opacifying polymer particles, the particles comprising: i. an inorganic core comprising dispersed inorganic pigment particle and a dispersant ii. a first layer of base-swellable polymer comprising acid groups encapsulating the core iii. a second layer of non-base-swellable polymer encapsulating the first layer wherein the dispersant comprises or consists of hydroxyl moieties; and carboxyl groups derived from itaconic acid.

Claims

1. An aqueous dispersion of pigmented opacifying polymer particles, the particles comprising: i) an inorganic core comprising a dispersed inorganic pigment particle and a dispersant, ii) a first layer of a base-swellable polymer comprising acid groups encapsulating the core, and iii) a second layer of a non-base-swellable polymer encapsulating the first layer; wherein the dispersant is a polymer having a weight average molecular weight (Mw) of at least 1000 Daltons, and comprises hydroxyl moieties, and carboxyl groups derived from itaconic acid.

2. The aqueous dispersion according to claim 1, further comprising an intermediate polymer layer at least one of between the pigment particle and the base-swellable polymer or between the base-swellable polymer and the non-base swellable polymer.

3. The aqueous dispersion according to claim 2 comprising an intermediate polymer layer between the pigment particle and the base swellable polymer layer and which is free of an intermediate polymer layer between the base swellable polymer layer and the non-base swellable polymer layer.

4. The aqueous dispersion according to claim 2 wherein the intermediate polymer layer has an acid value of from 0.1 to 65 mg KOH/g of polymer.

5. The aqueous dispersion according to claim 1 wherein the pigment particle is a titanium dioxide particle.

6. The aqueous dispersion according to claim 1, wherein the dispersed inorganic particles have a mean particle diameter of from 75 to 300 nm.

7. The aqueous dispersion according to claim 6, wherein the acid value of the dispersant is from 65 to 400 mg KOH/g non-volatile (non-vol.) dispersant.

8. The aqueous dispersion according to claim 1, wherein the base swellable polymer is derived from one or more monomers selected from the group consisting of methacrylic acid, acrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, cinnamic acid, fumaric acid, and beta carboxy ethyl acrylate.

9. The aqueous dispersion according to claim 1, wherein the non-base swellable polymer is non-film forming at ambient temperature.

10. The aqueous dispersion according to claim 1, wherein the Fox Tg of the non-base swellable polymer is at least 60 C.

11. The aqueous dispersion according to claim 1, wherein said non-base swellable polymer comprises monomers selected from the group consisting of styrene, alpha methyl styrene, tert-butyl styrene, vinyl toluene, methyl methacrylate, acrylonitrile, methacrylonitrile, and mixtures thereof.

12. The aqueous dispersion according to claim 11, wherein the non-base-swellable polymer contains 90-100% styrene monomer.

13. A coating composition comprising the aqueous dispersion according to claim 1.

14. A substrate coated with the coating composition according to claim 13.

15. A method of preparing the aqueous dispersion according to claim 1, comprising: i) dispersing an inorganic pigment in an aqueous medium using a dispersant of acid value at least 65 mg KOH/g of dispersant, wherein the dispersant is a polymer having a weight average molecular weight (Mw) of at least 1000 Daltons and comprises carboxyl acid moieties of differing pKa derived from itaconic acid, and hydroxyl groups, wherein the pH during dispersion is greater than the pKa of the carboxyl moiety having the lower pKa and at least 0.25 pH units above or below the iso-electric point of the pigment, to form a stable aqueous dispersion of the pigment in the aqueous medium; ii) adjusting the pH of the pigment dispersion to below the pKa of the acid moiety comprising the base swellable polymer of iii while satisfying the pH criteria of i; iii) forming a first layer of a base swellable polymer on the pigment particles by polymerising monomers comprising acid groups; iv) forming a second layer of a non-base swellable polymer; v) optionally polymerising further monomers to form intermediate polymer layers having an acid value between 0 and 65 mg KOH/g polymer between at least one of the pigment and the first layer or the first and second layer; vi) at least partially neutralising the first polymer, using a base, at a temperature above the Tg.sub.eff of the second layer of non-swellable polymer whereby the first layer of polymer is caused to swell; and, vii) cooling the dispersion to a temperature below the Tg.sub.eff of the second layer.

16. The aqueous dispersion according to claim 4, wherein the intermediate polymer layer comprises sodium styrene sulphonate.

17. The aqueous dispersion according to claim 1, wherein the dispersed inorganic particles have a mean particle diameter of from 225 to 275 nm.

18. The aqueous dispersion according to claim 1, wherein the Fox Tg of the non-base swellable polymer is from 90 to 140 C.

19. The aqueous dispersion according to claim 7, wherein the base swellable polymer is derived from one or more monomers selected from the group consisting of methacrylic acid, acrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, cinnamic acid, fumaric acid, and beta carboxy ethyl acrylate.

20. The aqueous dispersion according to claim 8, wherein said non-base swellable polymer comprises monomers selected from the group consisting of styrene, alpha methyl styrene, tert-butyl styrene, vinyl toluene, methyl methacrylate, acrylonitrile, methacrylonitrile, and mixtures thereof.

Description

(1) FIG. 1 shows an idealised structure of an opacifying particle comprising the invention where (1) is a pigment particle, (2) is a layer of base-swellable polymer in its unswollen state and (3) is the layer of non-base-swellable polymer.

(2) FIG. 2 shows the same structure as in FIG. 1 at basic pH with the base-swellable polymer (2) swollen with water.

(3) FIG. 3 shows an idealised structure of an opacifying particle comprising the invention where (1) is a pigment particle, (2) is a layer of base-swellable polymer in its unswollen state, (3) is the layer of non-base-swellable polymer and (4) and (S) are intermediate layers.

BASE SWELLABLE POLYMER

(4) The role of the base swellable layer is to react with a base once it has been at least partially overcoated with a rigid outer shell to form a hydrophilic ionomeric layer. This layer causes surrounding water to be drawn in by osmosis and to swell the polymer.

(5) The base swellable polymer is preferably derived from one or more monomers selected from the group consisting of methacrylic acid, acrylic acid crotonic acid, maleic acid or maleic anhydride, itaconic acid, cinnamic acid, fumaric acid and beta carboxy ethyl acrylate.

(6) This base swellable polymer layer should preferably contain sufficient acid monomer to give an acid value of from 100 to 450 mg KOH/g of polymer more preferably 130 to 260 mg KOH/g of polymer. This corresponds to from about 15 to 70 wt % and 20 to 40 wt % of methacrylic acid calculated on the total weight of base-swellable polymer.

(7) The base-swellable-polymer layer can also contain non-polymerisable acid if desired. Suitable non-polymerisable acid monomers include C.sub.6-C.sub.12 aliphatic moncarboxylic acids and aromatic acids such as benzoic acid.

(8) The base-swellable-polymer layer may optionally be crosslinked with a multifunctional unsaturated monomer such as divinyl benzene, diallyl methacrylate, ethylene glycol dimethacrylate, butane diol dimethacrylate or allyl methacrylate. Preferably, from 0.1 to 5 wt. %, based on the weight of base swellable polymer, of said multifunctional unsaturated monomer is used, more preferably from 0.1 to 1.0 wt % is used. Allyl methacrylate is particularly preferred as a crosslinker as it promotes grafting between the different polymer layers.

(9) The Fox Tg of the base-swellable-polymer layer is preferably from 0 to 170 C., and more preferably from 20 to 150 C.

(10) The thickness of the base-swellable-layer is from 2 to 20 nm unswollen, and from 10 to 200 nm swollen. By thickness is meant as measured by the increase in radius of the particle.

(11) Non-Base-Swellable Polymer

(12) The non-base swellable polymer must fulfil two roles. Firstly, during the neutralisation step of the base-swellable polymer it must expand, without rupturing, in order to accommodate the increased volumethus, it must be chosen to have an effective Tg below the temperature used during the neutralisation step. Secondly, during drying and loss of water, the non-base-swellable polymer must be rigid enough to prevent collapse of the void. The polymer should preferably have a high modulus.

(13) Preferably the non-base-swellable polymer is non-film forming at ambient temperature. Non-film forming polymers have a Tg.sub.eff above ambient temperature.

(14) A good guide to modulus is the glass transition temperature, Tg. Generally, as Tg increases, modulus also increases and we have found this can be used as a useful guide to determine the Tg of the polymer which resists collapse and film formation.

(15) Preferably, the Fox Tg of the non-base-swellable polymer is at least 60 C., more preferably from 90 to 140 C.

(16) Furthermore, the polymer is preferably not significantly plasticised by water as this will cause it to soften and deform, especially as the paint dries. Preferably it is hydrophobic as this reduces water plasticisation and it has the added benefit that it improves water resistance of coatings containing the opacifying polymer particles of the present invention. More preferably the solubility of the monomers making up the non-base-swellable polymer should be less than 2 g/100 g water at 20 C.

(17) Suitable monomers to make the non-base-swellable polymer include styrene, alpha methyl styrene (as a monomer in a copolymer), tert-butyl styrene, vinyl toluene, methyl methacrylate, acrylonitrile, methacrylonitrile and copolymers of these with lower Tg monomers.

(18) Preferably the polymer contains 90-100% styrene monomer. It may also contain from 0 to 20% acidic moieties, more preferably from 0 to 10%. These are especially beneficial because it helps the non-base-swellable layer adhere to the base-swellable polymer layer. The presence of the acid moieties also helps with the swelling stage by helping to facilitate passage of the neutralising base through the rigid outer shell during the neutralisation step.

(19) The acid value of the non-base-swellable polymer layer is preferably less than 130, more preferably less than 100 even more preferably less than 65 mg KOH/g non vol. polymer.

(20) The thickness (i.e. the radius) of the non-base-swellable polymer layer is preferably from 15 to 150 nm, more preferably from 30 to 75 nm. Of course the thickness of the non-base-swellable polymer reduces as the base-swellable polymer swells because it has a greater area to encapsulate.

(21) Optional Intermediate Polymer Layer

(22) The optional intermediate polymer layer is not base-swellable.

(23) Preferably, the acid value of the optional intermediate polymer layer is from 0.1 to 65.0 mg KOH/g of polymer, more preferably from 10 to 65 mg KOH/g of polymer. The presence of some acid functional monomers is thought to help the passage of base, especially inorganic base such as NaOH, through the intermediate layer. Suitable acid functional monomers are as hereinbefore described in relation to the base swellable polymer layer. Additionally, monomers comprising strong acid moieties such as sulphonate and phosphate can be used. Suitable examples include sodium styrene sulphonate.

(24) The Fox Tg of the intermediate polymer layer is preferably between 30 and 100 C. and more preferably between 50 and 90 C.

(25) The thickness of the optional intermediate polymer layer is 5 to 50 nm before swelling of the base swellable polymer layer, more preferably 10 to 30 nm.

(26) Without being bound by this, it is thought that the optional intermediate polymer layer at least partially or preferably fully encapsulates the base swellable polymer layer in order to make it easier for the far less polar rigid outer shell to overcoat the base swellable polymer in a more uniform manner.

(27) Swelling Stage

(28) The base swellable polymer layer is swollen by raising the pH using, for example, either volatile bases, including ammonia or amine, or non-volatile bases, for example alkali metal hydroxides such as sodium hydroxide. Of course, the polymer is not actually swollen by base but rather by the aqueous medium or water comprising the continuous phase of the dispersion. Following addition of the base, the neutralised acid moieties on the polymer become hydrophilic and the aqueous phase or water is drawn in by osmosis creating the driving force for swelling.

(29) In order for the water ingress to proceed unhindered, the non-base-swellable non-film-forming polymer must be extensible during the neutralisation stage. It is necessary that the addition of the base is carried out at a temperature above the effective glass transition temperature of the non-base-swellable non-film-forming polymer in order to accommodate the increase in volumes accompanying the water ingress without rupturing.

(30) An aqueous medium comprises at least 50 wt % of water, the remainder comprising organic solvents, preferably water compatible solvents.

(31) In the absence of a plasticising material for the polymer, the effective Tg is the same as the Fox Tg. Where a plasticiser is present the effective Fox Tg is lower than the Fox Tg.

(32) Preferably, the addition of base is at a temperature at least 5 C., more preferably from 5 to 20 C. above the effective Tg of the polymer.

(33) For example where the base swellable layer is overcoated with an intermediate polymer layer of Tg less than the process temperature, the base can be added to the dispersion at elevated temperature (>Tg of the intermediate layer) and after the swelling is complete and the pH has dropped, a rigid hydrophobic layer of polymer is polymerised on top of this. Alternatively, the addition and polymerisation of the monomer can occur whilst the swelling is occurring.

(34) Alternatively, a rigid hydrophobic layer of polymer can be polymerised directly onto the base swellable polymer layer, followed by plasticising this polymer layer with monomer (in order to reduce its effective Tg). This can be achieved by (a) stopping the addition of initiator, (b) optionally adding a free radical inhibitor (e.g. monomethyl ether hydroquinone, MEHQ), (c) using a non-homopolymerisable monomer, or (d) using a monomer with a ceiling temperature below the operating temperature (e.g. alpha methyl styrene). The base is then added to the dispersion at elevated temperature as before and after the swelling is complete and the pH has dropped, the polymerisation is continued (e.g. by adding further initiator and monomer) and a rigid hydrophobic layer of polymer is polymerised on top of this. Of course, a plasticising solvent can be also be used but is far less preferable as, since, depending on its boiling point, it may well contribute to VOC.

(35) Polymerisation Method

(36) The polymerisation steps of the present invention are carried out using a sequential emulsion polymerisation process in the presence of inorganic pigment particles, preferably titanium dioxide, dispersed in aqueous medium, preferably water.

(37) By sequential is meant that monomer mixtures of different composition are polymerised one after the other. In the simplest example of such a method, particles may be made having a first polymer or core region differing in composition from a second or shell polymer region. Of course, the particles may have more than two polymer regions.

(38) Preferably, the polymers are made using unconstrained free radical polymerisation methods, more preferably using free radical emulsion polymerisation methods. Even more preferably free radical initiators are used.

(39) The monomers are preferably emulsified in water and surfactant and fed into the reactor vessel over a period of from 1 to 6 hours, preferably from 1 to 3 hours. Consecutively, the free radical initiator is fed into the reactor.

(40) It is advantageous to have a delay between the various monomer feeds. In the non-RAFT type polymerisation used in this invention, the growing chains are very short-lived. The delay, therefore, ensures that any growing chains of the previous polymerisation terminate and stop growing before the next monomer mixture is polymerised.

(41) Preferably the titanium dioxide pigment particles are dispersed in water in the form of primary particles (i.e. with the minimum level of agglomerates). More preferably, the dispersed pigment has a mean particle diameter of from 150 to 300 nm, even more preferably from 200 to 300 nm and most preferably from 225 to 275 nm as measured by dynamic light scattering.

(42) The polymerisation is carried out in the absence of RAFT chain transfer agents, preferably in the absence of all types of controlled radical polymerisation.

(43) Processing Conditions

(44) It is preferable to run the polymerisation under conditions that encourage control of the morphology, e.g. low free monomer levels and levels of surfactant that are adequate for giving stability but not so high as to cause nucleation and stabilisation of non pigmented latex particles. As a corollary to this, it may be useful to make adjustments to lower the pH of the dispersion of inorganic pigment particles formed after step i) of the processfor instance, by the addition thereto of an acid such as hydrochloric acidand allow for pH equilibration before the polymerization of the base swellable polymer.

(45) The invention will now be illustrated by the following examples, in which the abbreviations used are defined below.

(46) The abbreviations below have the following meanings

(47) AMA Allyl Methacrylate

(48) AMPS 2-Acrylamido-2-Methylpropane Sulfonic Acid DELETE ???

(49) AMS Alpha-Methyl Styrene DELETE ???

(50) BA Butyl Acrylate

(51) BMA Butyl Methacrylate

(52) DI water Deionised water

(53) DMAEMA Dimethylaminoethyl Methacrylate DELETE ???

(54) DVB Di-Vinyl Benzene DELETE ???

(55) HEMA Hydroxyethylmethacrylate

(56) IA Itaconic Acid

(57) MAA Methacrylic Acid

(58) MAM Methacrylamide

(59) MMA Methyl Methacrylate

(60) SSS Styrenesulfonic Acid Sodium salt hydrate

(61) ST Styrene

Example 1

Anionic Dispersant Solution (D1)

(62) A solution of anionic polymer dispersant was produced according to the process described below and in Table 1, and having a monomer composition MMA:HEMA:IA:MAM of 15:45:36:4 by weight.

(63) TABLE-US-00001 TABLE 1 Dispersant Polymer (P1) Material Weight (g) Solvent (A) Iso-Propanol 15.17 DI water 5.06 Monomer (B) Methyl Methacrylate 7.05 Hydroxyethylmethacrylate 20.97 Itaconic Acid 16.68 Methacrylamide 1.91 n-Octyl Mercaptan 1.05 VAZO 67 1.46 Iso-Propanol 22.58 DI Water 7.75 Initiator (C) VAZO 67 0.11 Iso-Propanol 0.24 DI Water 0.14 Total 100.17

(64) The solvent mixture (A) was loaded to a reaction vessel. 25% of the mixture (B) was then added and the temperature increased to 65 C. under a blanket of nitrogen.

(65) The mixture was then allowed to exotherm to 82 C. and held for 15 min. The remainder of the mixture (B) was then fed over 2 h at 85 C. The mixture (C) was then added and the mixture held at reflux temperature (80 to 85 C.) for 2 h.

(66) A Dean Stark was then adapted to the reaction vessel and the Iso-Propanol was distilled off. As described in Table 2, (D) and (E) were then added while stirring at high speed at 75 C. to form the polymer solution (D1) at 27.3 wt % solids.

(67) TABLE-US-00002 TABLE 2 Anionic Dispersant Solution D-1 Material Weight (g) Polymer (P1) 33.50 DI Water (D) 62.50 Ammonia (E) 4.00 Total 100.00

(68) Aqueous dispersion of titanium dioxide MB-1

(69) The anionic dispersant solution, D1, was used to make an aqueous dispersion of titanium dioxide according to the recipe and method below.

(70) TABLE-US-00003 TABLE 3 Material Weight (g) D-1 23.01 DI Water 106.24 Tipure R-706 420.75 Total 550.00

(71) 23.01 g of the dispersant solution (D1) were diluted in 106.24 g of water. 420.75 g of Tipure R-706 were then dispersed in the solution obtained using a high speed disperser operating at 1,500 rpm for 20 min.

(72) Aqueous Dispersion of Opacifying Polymer Particles

(73) An aqueous dispersion of opacifying polymer particles was made using the titanium dioxide dispersion prepared in Table 3. The method and ingredients used are shown in Table 4.1 and 4.2

(74) TABLE-US-00004 TABLE 4.1 TiO2 dispersion with first polymer layer (ME-1) Material Weight (g) Aqueous charge (A) MB-1 261.90 Sodium Dodecyl Benzene Sulphonate 1.46 Solids adjust (B) DI water 258.68 Initiator Catalyst (C) 0.1% Iron II Sulphate aqueous solution 1.12 1% Ethylene Diamine Tetra Acetic Acid aqueous solution 0.08 Initiator Feed (D) Tert-Butyl Hydroperoxide (70% active) 0.37 DI water 3.27 Reductant Feed (E) Ascorbic Acid 0.20 DI water 15.04 Optional non-base swellable polymer layer (F) DI water 5.38 Sodium Dodecyl Benzene Sulphonate 0.64 Butyl Methacrylate 10.93 Methyl Methacrylate 6.25 Styrenesulfonic Acid Sodium salt hydrate 1.93 Total 567.69

(75) The TiO.sub.2 dispersion (A) was charged to a reaction vessel, diluted with DI (B), purged with nitrogen and the temperature raised to 50 C. The catalyst (C) was then added followed two minutes later by the initiator (D). The reductant (E) and the monomer mixture (F) were then fed in linearly over 30 min at 50 C.

(76) TABLE-US-00005 TABLE 4.2 Material Weight (g) Aqueous charge ME-1 567.69 Solids Adjust (G) DI Water 154.89 Initiator Catalyst (H) 0.1% Iron II Sulphate aqueous solution 1.48 1% Ethylene Diamine Tetra Acetic Acid aqueous solution 0.17 Sodium Dodecyl Benzene Sulphonate 4.93 Initiator Feed (I) Tert-Butyl Hydroperoxide 1.19 DI Water 11.18 Reductant Feed (J) Sodium Formaldehyde Sulfoxylate 0.42 DI Water 11.13 Base swellable layer stage (K) DI Water 7.08 Sodium Dodecyl Benzene Sulphonate 3.26 Methyl Methacrylate 16.01 Butyl Acrylate 16.01 Methacrylic Acid 16.01 Allyl Methacrylate 0.05 Solids Adjust (L) DI Water 234.84 Initiator Catalyst (M) 0.1% Iron II Sulphate aqueous solution 1.13 1% Ethylene Diamine Tetra Acetic Acid aqueous solution 0.13 Initiator Feed (N) Tert-Butyl Hydroperoxide 0.92 DI Water 8.63 Reductant Feed (O) Sodium Formaldehyde Sulfoxylate 0.32 DI Water 8.46 Optional Intermediate Polymer layer (P) Methyl Methacrylate 33.10 Butyl Acrylate 3.68 Aerosol MA80 0.87 DI Water 8.83 Non film forming outer shell (Q) Methyl Methacrylate 55.21 Aerosol MA80 1.31 DI Water 13.24 Initiator/Feed (R) Ammonium Persulphate 0.34 DI Water 19.51 Ammonia addition (S) Ammonia 45.00 Total 1247.00

(77) ME-1 and Solids Adjust (G) were charged to a reaction vessel. The dispersion was then adjusted to pH 4.1 by addition of hydrochloric acid (1 mol/L) and stirred for 10 min to allow for pH equilibration. After addition of (I), the pH was checked again and readjusted if necessary to 4.1 by addition of hydrochloric acid (1 mol/L). The solution was purged with N.sub.2 and the temperature adjusted to 50 C. (J) and (K) were fed in linearly over 30 min, before the solid adjust (L) was added.

(78) The temperature was then kept at 50 C. while adding (M) and (N); (0) and (P) were then fed over 30 min.

(79) The dispersion was then heated up to 90 C. and (Q) and (R) were fed into the vessel.

(80) The mixture was then allowed to cool to room temperature; it was filtered to remove any grit. The filtered dispersion was then stirred at 90 C. and ammonia (S) added over 30 min. The stirring was continued for 4 hours at 90 C., before the dispersion was allowed to cool and was filtered.

Example 2

(81) As for Example 1, with the following formulation changes:

(82) TABLE-US-00006 Base swellable layer stage (K) DI Water 7.13 Sodium Dodecyl Benzene Sulphonate 3.29 Methyl Methacrylate 20.78 Butyl Acrylate 13.07 Methacrylic Acid 14.53 Allyl Methacrylate 0.05 Optional Intermediate Polymer layer (P) Styrene 36.78 Aerosol MA80 0.87 DI Water 8.83 Non film forming outer shell (Q) Styrene 55.21

(83) All other process and formulation steps remaining identical.

Example 3

(84) As for Example 1, with the following formulation changes:

(85) TABLE-US-00007 Base swellable layer stage (K) DI Water 7.02 Sodium Dodecyl Benzene Sulphonate 3.24 Methyl Methacrylate 11.84 Butyl Acrylate 16.75 Methacrylic Acid 19.06 Allyl Methacrylate 0.05 Non film forming outer shell (Q) Styrene 54.79 Aerosol MA80 1.30 DI Water 13.14

(86) All other process and formulation steps remaining identical.

Example 4

(87) As example 2, with the following formulation changes:

(88) TABLE-US-00008 Base swellable layer stage (K) DI Water 7.08 Sodium Dodecyl Benzene Sulphonate 3.26 Methyl Methacrylate 16.01 Butyl Acrylate 16.01 Methacrylic Acid 16.01 Allyl Methacrylate 0.05

(89) All other process and formulation steps remaining identical.

Example 5

(90) As for example 1, with the following formulation changes:

(91) TABLE-US-00009 Non film forming outer shell (Q) Styrene 56.18 Aerosol MA80 1.31 DI Water 13.24

(92) All other process and formulation steps remaining identical.

Example 6

(93) As for example 1, with the following formulation changes:

(94) TABLE-US-00010 Optional Intermediate Polymer layer (P) Methyl Methacrylate 31.72 Butyl Acrylate 5.06 Aerosol MA80 0.88 DI Water 8.83 Non film forming outer shell (Q) Styrene 53.44 Aerosol MA80 1.36 DI Water 12.81

(95) All other process and formulation steps remaining identical.

Example 7

(96) As for example 1, but with the following changes:

(97) TABLE-US-00011 Material Weight (g) Aqueous charge (A) MB-1 262.81 Sodium Dodecyl Benzene Sulphonate 1.47 Solids adjust (B) DI water 259.58 Initiator Catalyst (C) 0.1% Iron II Sulphate aqueous solution 1.12 1% Ethylene Diamine Tetra Acetic Acid aqueous solution 0.08 Initiator Feed (D) Tert-Butyl Hydroperoxide (70% active) 0.37 DI water 3.28 Reductant Feed (E) Sodium Formaldehyde Sulfoxylate 0.20 DI water 15.09 Optional non base swellable polymer layer (F) DI water 5.40 Sodium Dodecyl Benzene Sulphonate 0.65 Butyl Methacrylate 11.28 Methacrylic Acid 0.39 Methyl Methacrylate 5.98 Styrenesulfonic Acid Sodium salt hydrate 1.96 Solids Adjust (G) DI Water 155.43 Initiator Catalyst (H) 0.1% Iron II Sulphate aqueous solution 1.48 1% Ethylene Diamine Tetra Acetic Acid aqueous solution 0.17 Sodium Dodecyl Benzene Sulphonate 4.95 Initiator Feed (I) Tert-Butyl Hydroperoxide 1.19 DI Water 11.22 Reductant Feed (J) Sodium Formaldehyde Sulfoxylate 0.42 DI Water 11.17 Base swellable layer stage (K) DI Water 7.10 Sodium Dodecyl Benzene Sulphonate 3.27 Methyl Methacrylate 20.70 Butyl Acrylate 13.02 Methacrylic Acid 14.47 Allyl Methacrylate 0.05 Solids Adjust (L) DI Water 235.66 Initiator Catalyst (M) 0.1% Iron II Sulphate aqueous solution 1.14 1% Ethylene Diamine Tetra Acetic Acid aqueous solution 0.13 Initiator Feed (N) Tert-Butyl Hydroperoxide 0.92 DI Water 8.66 Reductant Feed (O) Sodium Formaldehyde Sulfoxylate 0.32 DI Water 8.49 Optional intermediate Polymer layer (P) Methyl Methacylate 33.21 Butyl Acrylate 3.69 Aerosol MA80 0.87 DI water 8.86 Non film forming outer shell (Q) Styrene 55.40 Aerosol MA80 1.31 DI water 13.29 Initiator/Feed (R) Ammonium Persulphate 0.34 DI Water 19.58 Ammonia (S) Ammonia 40.81 Total 1246.98

(98) All process steps remain unchanged.

Example 8

(99) Same as example 1, but with the following formulation changes:

(100) TABLE-US-00012 Optional non base swellable polymer layer (F) DI water 5.42 Sodium Dodecyl Benzene Sulphonate 0.65 Butyl Acrylate 4.18 Methyl Methacrylate 13.54 Styrenesulfonic Acid Sodium salt hydrate 1.97 Base swellable layer stage (K) DI Water 7.13 Sodium Dodecyl Benzene Sulphonate 3.29 Methyl Methacrylate 20.78 Butyl Acrylate 13.07 Methacrylic Acid 14.53 Allyl Methacrylate 0.05 Non film forming outer shell (Q) Styrene 55.61 Aerosol MA80 1.32 DI Water 13.34 Ammonia addition (S) Ammonia 36.17

(101) All other formulation and process steps remaining unchanged.

Example 9

(102) Same as example 5, with the following formulation changes.

(103) TABLE-US-00013 Optional non base swellable polymer layer (F) DI water 5.38 Sodium Dodecyl Benzene Sulphonate 0.64 Styrene 0.39 Butyl Methacrylate 10.93 Methyl Methacrylate 6.69 Styrenesulfonic Acid Sodium salt hydrate 1.93

(104) All other formulation and process steps remaining unchanged.

(105) Spreading Rates

(106) The spreading rate to achieve a contrast ratio of 95% and 98% was evaluated by converting examples 1-3, 5-7 and 9 of the dispersion to model paints according to the formulations shown in Table A

(107) TABLE-US-00014 TABLE A Example 1 2 3 5 6 7 9 wt (g) wt (g) wt (g) wt (g) wt (g) wt (g) wt (g) Opacifier dispersion 46.72 50.01 49.19 32.85 47.26 44.66 48.90 of example indicated above Film forming latex 35.50 34.30 33.03 33.57 32.83 33.94 33.64 VA:BA Thickener 1.50 1.50 1.49 1.48 1.51 1.51 1.50 Water 15.98 13.89 15.99 31.79 18.10 19.59 15.66 AMP 95 0.30 0.30 0.30 0.30 0.30 0.30 0.30 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 PVC 29.57 31.95 34.43 33.35 34.80 32.66 33.27 TiO.sub.2 PVC 8.09 8.72 8.74 8.64 8.84 8.53 8.62 Density 1.11 1.11 1.11 1.11 1.11 1.11 1.11 Volume solids 25.00 25.00 25.00 25.00 25.00 25.00 25.00 (vol %) Weight solids 32.54 32.52 32.49 32.51 32.48 32.47 32.47 (wt %)

(108) Table B shows the recipes of two standard paints at PVC 9 and 10%.

(109) TABLE-US-00015 TABLE B Standard A Standard B Standard TiO.sub.2 slurry A Standard TiO.sub.2 slurry B Weight (g) Weight (g) TiO2 slurry (76.5 wt % 9.30 10.30 TiO2) ASP 170 slurry (65 wt % 21.72 20.83 clay) VA-BA latex 30.18 30.08 Thickener 1.50 1.49 DI 37.00 36.99 AMP 95 0.30 0.30 100.00 100.00 34.80 34.82 TiO2 PVC % 9.00 10.00 Density 1.20 1.20 Volume solids % 25.00 25.00 Weight solids 37.51 37.70

(110) The TiO.sub.2 and clay slurry recipes as used in Standard A and B are shown below

(111) TABLE-US-00016 TiO.sub.2 slurry ASP 170 slurry wt % wt % Water 22.80 34.30 Biocide (25% solids) 0.11 Dispersant (45% solids) 0.30 0.35 (Polyacrylic acid) Antifoam 0.20 AMP 95 0.04 Biocide (22% solids) 0.23 Antifoam (60% solids) 0.19 TiO.sub.2 76.48 Hydrous Clay 65.00 Total 100.00 100.00

(112) The spreading rates are shown in Table C

(113) TABLE-US-00017 TABLE C Spreading Rate Spreading Rate (m2/L at (m2/L at 95% 98% contrast Run Sample TiO.sub.2 PVC PVC contrast ratio) ratio) 1 Example 1 8.09 29.57 7.3 4.5 2 Example 2 8.72 31.95 7.5 4.5 3 Example 3 8.74 34.43 7.3 4.2 4 Example 5 8.64 33.35 7.3 4.4 5 Example 6 8.84 34.80 7.3 4.3 6 Example 7 8.53 32.66 7.2 4.2 7 Example 9 8.62 33.27 7.3 4.3 12 Standard A 9.00 34.80 6.3 3.8 13 Standard B 10.00 34.82 7.0 4.4

(114) As can be seen, at 95% contrast ratio all of the examples containing opacifying particle of the invention have better opacity, as evidenced by the increased spreading rates, compared to standard A and B at PVC of 9 and 10%, even though the inventive compositions are at PVC significantly lower than these. Even at 98% contrast ratio all examples of the invention have a better spreading rate than the standard A at PVC of 9%.

(115) Effect of Intermediate Polymer on pH Stability of a Pigment Dispersion

(116) Onto the pigment dispersion, MB-1 (as described in Table 3) was polymerised various intermediate polymer comprising sodium styrene sulphonate.

(117) The resulting dispersions were allowed to cool and hydrochloric acid was added, whilst stirring, to reduce the pH. The minimum pH reached before flocculation was noted.

(118) Replacing sodium styrene sulphonate monomer with methacrylic acid monomer in the intermediate polymer resulted in flocculation on addition of hydrochloric acid.

(119) TABLE-US-00018 .sup.1Intermediate .sup.2SSS content of polymer/ intermediate Minimum pH reached Example wt % polymer/wt % before flocculation 1 9.6 10.0 3.78 2 13.0 15.0 3.53 3 9.6 20.0 1.40 4 6.0 20.0 3.05 5 6.0 15.0 Flocculated at pH 4.3 6 9.6 10.0 3.05 .sup.1wt % based on pigment .sup.2SSS = sodium styrene sulphonate