GB2280192A - Brighter polymer-modified particulate inorganic opacifiers - Google Patents

Abstract

Brighter polymer-modified particulate inorganic white opacifiers for use in coating compositions (especially paints for application at ambient temperatures to buildings or their fittings or furnishings) are achieved by the selection of anatase or zinc sulphide as the opacifier (optionally with an optical brightener) and offsetting their lower opacifying power relative to conventional rutile by using polymer modification to create a barrier to agglomeration which enables the opacifier to be used more effectively. Discontinuous polymer barriers are preferred which comprise polymer particles of a pre-determined shape and size attached to the opacifier particles.

Description

BRIGHTER POLYMER-MODIFIED PARTICULATE INORGANIC OPACIFIERS This invention relates to brighter polymermodified particulate inorganic white opacifiers to a process for producing them and to coating compositions containing them.

Coating compositions (especially paints) generally comprise at least an organic film-forming material, a particulate inorganic white opacifier and a carrier liquid which evaporates as the coating composition dries. The film-forming material may be present either as a solution in the carrier liquid (e.g. the so-called "solvent-borne" paints) or as a colloidally stable dispersion of particles in the carrier liquid (e.g. the so-called "emulsion" or "latex" paints). The opacifier is present as an essentially colloidally stable dispersion of particles in the carrier liquid. As the carrier liquid evaporates, the film-forming material forms a film which binds opacifier particles and any other non-volatile ingredients of the composition.Conventionally the opacifier particles have a number average particle size of from 100 to 400nm (preferably 200 to 300 nm) and they should be well dispersed in the coating composition.

White opacifiers are pigments which enable a dried coating to hide marks on the surface to which the coating has been applied. If colours other than white are wanted, the white opacifier will be used in combination with other pigments which impart the wanted colour. Apart from white lead (now regarded as too poisonous for general use), zinc sulphide was probably the earliest modern white opacifying pigment.

It was then largely displaced by the titanium dioxides which have higher opacifying powers, that is to say they are better at hiding marks on a surface to which the coating has been applied.

Two forms of titanium dioxide have been used commercially in coating compositions. They are anatase and rutile. Anatase was the first form to become commercially available, possibly because lower temperatures are used in its manufacture. However, rutile has the higher density and refractive index and so it has a higher opacifying power. It is often said that rutile can be around 30% more effective as an opacifier than anatase.

The early availability of anatase meant that it was the form of titanium dioxide originally used in paints.

However, during the 1940's rutile became available and its greater effectiveness as an opacifier meant that it began to displace anatase from paints. Both forms of titanium dioxide can promote the degradation of many of the organic film-forming polymers used in paints, but during the 1950's it was discovered that this degradation could be substantially reduced by coating the particles with hydratable oxides such as alumina, silica, zirconia and/or tin oxide. Accordingly, during the 1950's coated rutile displaced anatase from paints suitable for application at ambient temperatures to surfaces of the type found on buildings or vehicles or their fittings or furnishings.

Coated rutile is still the form of titanium dioxide used in such paints today even though it imparts an unwelcome yellowish tinge to the dried coatings which is perceived by the human eye as detracting from the brightness OL the coatings when seen alongside a standard barium sulphate white. The first part of the objective of this invention is to minimise this perceived loss of brightness.

During the 1960's, the use of optical brighteners (also called fluorescent whitening agents) developed.

Optical brighteners are fluorescent compounds which absorb near-ultra-violet light (300 to 400 nm) and then fluoresce emitting visible light so making the substrate from which they are emitting appear brighter. Optical brighteners are used to brighten the appearance of paper, soaps (i.e.

detergents), textiles and also thermoplastics, see the opening paragraph of Chapter 14 of the third edition of the "Plastics Additives Handbook" edited by R. G chter and H Muller and published in 1990 by Hanser Publishers of Munich. The contents of this Chapter 14 are herein incorporated by reference. Optical brighteners ought to provide a convenient means for offsetting the loss of brightness in paints arising from the use of rutile as an opacifier but nevertheless no commercial use of optical brighteners in paints of the type used on buildings has occurred. The problem is that the rutile titanium dioxide pigments used in modern paints strongly absorb most of the ultra-violet light in the waveband 330 to 400 nm which light is needed to excite suitable optical brighteners.

In order to overcome the problem of near-ultra-violet light being absorbed by titanium dioxide pigment, European Patent Application EP 0 497 507A (published in 1992, the contents of which are herein incorporated by reference) proposed the use of voided polymer particles as an alternative to opacifying pigments such as titanium dioxide. Organic polymers do not absorb significant amounts of near-ultra-violet light so virtually all the light remains available to excite the optical brightener.

Alternatively, it has been proposed to incorporate optical brightener into a clear varnish (i.e. a varnish not containing titanium dioxide opacifier) and then to paint a coat of the varnish onto a dried coat of paint which does contain rutile white opacifier. This achieves a brighter effect because most of any incident ultra-violet light is absorbed and fluoresced by the optical brightener in the clear coat of varnish before it can reach the rutile.

However such use of a varnish on top of an already applied coat of paint does of course necessitate an extra painting operation which adds to the expense of painting.

An object of a refinement of this invention is to provide an inorganic opacifier for coating compositions which can be used beneficially with an optical brightener which absorbs light in the waveband 330 to 400 nm.

There are only limited opportunities for improving the optical properties of a coating by using large amounts of opacifier. The use of large amounts of inorganic opacifier not only increases the cost of a coating composition, but more unfortunately it also results in an increased risk of agglomeration of opacifier and agglomeration causes a loss of opacifying power. Furthermore, large amounts of opacifier also result in an inevitable reduction in the sheen and coin mar resistance of the eventual dried coating. ("Coin mar" is the marking of a dried coating when the milled edge of a coin or similar metal object is rubbed against the surface of the coating). Reduction in sheen and coin mar resistance occurs because the surfaces of the inorganic opacifier particles are both highly irregular and hard and some of them are found in or just below the surface of the dried film where they are sufficiently accessible to affect adversely sheen and coin mar resistance. The opacifier particles have sometimes been mixed with other particles, both organic and inorganic and especially other inorganic particles known as "extender" particles. Such mixing was done for various reasons including the purpose of spacing apart the opacifier particles to inhibit their tendency to agglomerate in liquid dispersions.These other particles did not bond chemically to the surfaces of the opacifier particles and so whilst they inhibited agglomeration, they did not affect the irregularity or hardness of the surfaces of the opacifier particles and hence they did not lead to any improvements in sheen and coin mar resistance. On the contrary, many extender particles are themselves irregular and hard and so they too reduce sheen and coin mar resistance.

More recently, proposals have been made to encapsulate opacifier particles in organic polymer by for example polymerising monomer in water in the presence of the particles and under conditions such that the polymer formed coats the surface of the particles. The polymer coating therefore creates a physical barrier around the particles which is firmly attached to the particles. Two such proposals are made respectively in European Patent Specification EP 0 392 065A and in United States Patent Specification US 4 771 086 (the contents of both of which are herein incorporated by reference). The polymer barrier inhibits agglomeration of the opacifier particles so avoiding the loss of opacifying power which would otherwise occur if the surfaces of two adjacent unmodified particles were allowed to approach each other closely.In addition, provided that the polymer barrier is thick enough, total encapsulation will also render the irregular and hard titanium dioxide surface less irregular and softer so improving sheen and coin mar resistance.

Total encapsulation by a polymer coating of a suitable thickness is useful, but it can also be wasteful of polymer because thick layers are needed. In addition, polymerisation in the presence of the opacifier particles complicates a commercial polymerisation prdcess in many ways. For example, the already expensive polymerisation vessel has to be fitted with means for handling abrasive dispersions of inorganic polymer particles. Also a large proportion of the capacity of the vessel is of course occupied by the opacifier particles and so the volume available for the polymerisation reactants is much reduced leading to a corresponding loss in polymerisation capacity.

European Patent Specification EP 0 337 672A (the contents of which are herein incorporated by reference) describes a process for modifying the surfaces of opacifier particles by polymerising monomer in water in the presence of the particles under conditions such that the polymer formed deposits only onto one or more small and separate portions of the particle surfaces but is nevertheless firmly attached to the particles. The result is that the opacifier particles are not fully encapsulated, but instead each carries one or more bonded nodules of polymer. In this way a discontinuous polymer barrier is formed which allows, use of the polymer to be more economical but at the cost of producing nodules whose shapes are irregular and unpredictable. EP 0 337 672A also remains dependant on a polymerisation performed in the presence of the opacifier particles with its attendant disadvantages.

An object of a further refinement of the invention is to enhance the achievement of the first objective by providing a more effective polymer barrier.

In its broadest aspect, this invention provides a brighter polymer-modified (including copolymer-modified) particulate inorganic white opacifier comprising particles of inorganic white opacifier of number average particle size (Dt) 100 to 400 nm attached to an organic polymer barrier which inhibits agglomeration of the opacifier particles wherein the inorganic opacifier is selected from anatase and/or zinc sulphide. It is important that a polymer barrier be present to inhibit agglomeration and so offset at least some of the loss in opacity incurred by using anatase or zinc sulphide as the white opacifier instead of rutile whereupon the selection of anatase or zinc sulphide avoids the yellowish hue associated with rutile and in fact leads to the dried coating having a slightly blueish hue which the eye perceives as being even brighter.Moreover, neither anatase nor zinc sulphide absorb significant amounts of near-ultra-violet light, so the use of one or moreCoptical brighteners is beneficial.

Accordingly, in a refinement of the invention, the opacifier particles are used in combination with at least one optical brightener.

Pigment-grade anatase titanium dioxide is currently supplied as particles having a number average particle size of from 100 to 200 nm (usually 120 to 160 nm). It would be preferred to use particles having a number average particle size of at least 200 nm but not more than 300 nm and most preferably 200 to 250 nm in order to achieve better opacity. The anatase white opacifier preferably comprises from 5 to 25 volt of the non-volatile content of the paint.

Non-volatile content may be measured according to ASTM Test D-2697-86. Each particle is also preferably coated with at least one hydratable inorganic oxide such as alumina, silica, zirconia and/or tin oxide, usually in amounts of up to 20 wtt of the weight of the particle.

Pigment grade zinc sulphide is available as "Sachtolith" which is predominantly zinc sulphide or as lithopone which is a mixture of zinc sulphide and barium sulphate. "Sachtolith" is supplied by Sachtleben GmbH of Duiberg, Germany. "Sachtolith" is more expensive than lithopone but has a greater opacifying power. The number average particle sizes of commercially available "Sachtoliths" and of the zinc sulphide component of and lithopones lie in the ranges 150 to 400 nm, but the preferred range is 200 to 300 nm for both. The barium sulphate component of the lithopone preferably has a number average particle size of from 600 to 800 nm. The zinc sulphide is preferably used in the same concentrations as anatase.

A typical optical brightener is usually a colourless compound and it should absorb a high proportion of any incident ultra-violet light in the waveband 330 to 400 nm and then fluoresce a substantial proportion of it as light of wavelength above 400 nm. Fluorescence from a dried coat of the paint enables the coat to emit more light than would be the case if the coat emitted only reflected and scattered light with the result that the fluorescence causes the coat to appear brighter to the human eye provided of course that the coat is illuminated by natural light or by artificial light containing ultra-violet light in the waveband 330 to 400 nm.

Useful optical brighteners are mentioned in Chapter 14 of G chter and Muller's "Plastics Additives Handbook" and EP 0 497 507A. Typical examples include triazinephenylcoumarins, benzotriazole-phenyl coumarins, naphthotriazole-phenylcoumarins and bis-benzoxazoles, for example 2,21- (2,5 - thiophenediyl) -bis (5- (1,1-dimethyl ethyl)) benzoxazole of the graphic formula:

Preferred optical brighteners have a peak absorption lying in the range 350 to 400 nm and combinations of two or more optical brighteners having peak absorptions at different wavelengths can be beneficial. For example an optical brightener having a peak absorption in the waveband 350 to 370 nm might be usefully combined with a different one having a peak absorption in the waveband of upwards from 370 to 390 nm. The optical brightener is preferably used in amounts of from 0.1 to 10 micromoles per g of nonvolatile content of the paint and the most preferred range is from 1 to 5 micromole/g. Non-volatile content may be measured according to ASTM Test D 2697-86.

The use of a polymer barrier which totally encapsulates the opacifier particles or which comprises irregularly shaped nodules can be wasteful of polymer and so a further refinement of this invention provides a polymer-modified particulate anatase or zinc sulphide comprising particles of anatase and/or zinc sulphide having a number average particle size (Dt) of from 100 to 400 nm attached to a discontinuous polymer barrier e comprising particles of organic polymer having a particle size which allows them to be accommodated around the anatase or zinc sulphide particles wherein a) the particles of polymer are pre-formed prior to their attachment to the anatase or zinc sulphide particles, b) the particles are pre-formed either by (i) a free radical initiated aqueous emulsion or dispersion polymerisation performed in the presence of a water-soluble compound which during the course of the polymerisation bonds chemically to the polymer as it is being formed, or (ii) a polymerisation which is followed by the chemical bonding of a water-soluble compound to the polymer and c) the water-soluble compound is a polymeric material which is chemically bondable to the polymer, which contains at least one moiety adsorbable onto a surface of the titanium dioxide particles and preferably which has a weight average molecular weight of at least 1500 prior to attachment to the polymer.

Pre-formation of the polymer particles in the absence of the opacifier particles results in the formation of predictably (usually essentially spherically) shaped solid particles having a pre-determinable range of particle sizes. Because the shape and size of the particles is predetermined before their attachment to the anatase or zinc sulphide, their ability to pack around the opacifier particles is pre-determined by their preformed shape and size. In particular they do not touch or closely approach more than a small proportion of the surfaces of the opacifier particles but because of their regular shape and size, they provide maximum projection from those surfaces so that they are still able to function as a discontinuous barrier.The barrier inhibits agglomeration and lessens if not totally prevents the reductions in sheen and coin mar resistance consequent on introducing opacifier particles into a coating composition. This enables polymer to be used very efficiently and further offset the loss of opacifying power consequent on the selection of anatase or zinc sulphide as the white opacifier.

In this further refinement, the polymer particles are attached to the anatase or zinc sulphide particles via the water-soluble compound which also contributes to the colloidal stability of an aqueous dispersion of the polymer-modified opacifier particles by imparting at least some steric stabilisation. The water-soluble compound therefore serves as a coupling agent which brings and holds together the two different types of particle. It is not clear whether the water-soluble moieties make actual contact with the surfaces of the opacifier particles or whether they are held very close to but spaced slightly apart from the surfaces. Presumably they would be held in such spaced relationship by a balance of ionic, steric and van der Waals' forces.However whatever the mechanism of attachment may be, the attachment is strong enough to enable the modified opacifier particles to be present as a colloidally stable aqueous dispersion which retains very useful stability even when subjected to conditions (especially temperature variations) normally encountered in the manufacture, storage and use of water-borne paints.

The water-soluble compound must be chemically bonded to the polymer and preferably attachment is via a covalent bond though bonding via salt formation is possible.

Covalent bonding is preferably achieved by choosing a water-soluble compound which generates a radical moiety when exposed to the action of free radicals generated by a free radical initiator during an aqueous polymerisation reaction used to form the polymer particles. This causes the water-soluble compound to bond to the polymer as it is being formed. Alternatively the water-soluble polymer may comprise a moiety which can be induced to chemically bond onto an already formed polymer. For example, the watersoluble compound may comprise a group (such as a carbon to carbon double bond) which likewise generates a radical moiety when exposed to the action of free radicals generated by irradiation or the decomposition of a compound which is decomposable to produce free radicals.It is also possible to chemically bond the water-soluble compound to an already formed polymer by means of pairs of co-reactive moieties, one member of a pair belonging to the watersoluble compound whilst the other member belongs to the polymer. Examples of co-reactive moiety pairs include epoxide/carboxylic acid pairs, epoxide/amine pairs and carboxylic acid/amine pairs, the latter pairs being examples of bonding by means of ionic salt formation.

Certain polymeric water-soluble compounds having a weight average molecular weight over 1500 are particularly suitable for use as water-soluble compounds since they generally comprise chains which are comfortably long enough to act as convenient coupling agents between the polymer particles and the opacifier particles. Examples include polymers and co-polymers (including salts, analogues and derivatives) of the following monomers namely acrylamide, acrylic and methacrylic acids, hydroxyalkyl (especially hydroxyl ethyl) acrylates and methacrylates, aminoalkyl acrylates and methacrylates, vinyl pyridine, vinyl pyrrolidone, vinyl and styrene sulphonic acids.Particular polymeric water-soluble compounds contain chains of poly (ethylene imine, poly (ethyoxylate), poly (vinyl alcohol), cellulose ethers such as hydroxyalkyl celluloses (including hydrophobically modified variants), alkylhydroxyalkyl celluloses, carboxyalkyl celluloses and carboxyalkylhydroxy-alkyl celluloses. Still further polymeric water-soluble compounds include water-soluble or water-reducible polyesters and polyurethanes or starch derivatives such as acetates, hydroxyalkyl and carboxyalkyl starches or ionic starch derivatives such as phosphate, sulphate and aminoalkyl or polysaccharides such as xanthan and guar gum and gum arabic.Certain preferred watersoluble compounds have weight average molecular weights well in excess of 20 000 and include the water-soluble cellulose ethers such as hydroxyethyl cellulose, hydrophobically modified hydroxyethyl cellulose (especially when the number average particle size of the polymer particles is below 225nm), and salts of carboxymethyl cellulose together with polymers and copolymers of acrylamide, vinyl alcohol, vinyl pyrrolidone and acrylic acid. Other preferred polymeric water-soluble compounds contain polyethoxylate chains having a weight average molecular weight of preferably from 1500 to 5000 which form esters with unsaturated carboxylic acid and of which poly(ethylene glycol) methacrylates are particularly useful examples. The water-soluble compounds quite often hinder homo-aggregation of the polymer-particles and contribute steric stabilisation to the colloidal stability of an aqueous dispersion of the polymer particles.

If the polymer particles are composed of a polymer whose minimum film-forming temperature is below 300K, it is preferred to choose a polymer whose surface characteristics are such that

where 71-3 is the interfacial energy of the opacifier particle surface/water interface 71-2 is the interfacial energy of the opacifier particle surface/polymer interface 72-3 is the interfacial energy of the polymer particle surface/water interface and Vp and Vt represent the relative volumes of respectively, a polymer particle of the number average particle size Dp and an opacifier particle of the number average particle size D,and

For convenience, the factor (γ1-3-γ1-2)γ;2-3 will be referred to as the " factor". By choosing a polymer whose surface characteristics allow its factor to meet the above relationship, it is possible to reduce the proportion of the surface of the opacifier particles which is touched by polymer after the attachment of the polymer particles which in turn means that the efficiency of utilisation of the polymer is increased. This is particularly so when the Y factor is less than zero. If a Y factor lies between 1 and -1 it can be conveniently measured by the technique described in United States Patent Specification US 4 997 864 or European Patent Specification EP 0 327 199A, the contents of both of which are herein incorporated by reference.These references explain that by use of the Young-Dupre equation, the Y factor can be shown to be equal to the cosine of the contact angle (as shown in Figure 4) for a particle 34 of polymer on a opacifier surface 33 in water 30. The Young-Dupre equation is of course only valid if e lies between 0 and 1800. It is explained in more detail on pages 24 and 25 of the book "Polymer Surfaces" by B W Cherry published in 1981 by Cambridge University Press and the contents of these pages are herein incorporated by reference. Accordingly for minimal touching of the surface by polymer, it is preferred that cos e be less than zero which means that e cannot be greater than just below zero.

Total non-distortion of the polymer particles in water is ensured if the Y factor is equal to or less than -1 and total non-distortion means that the polymer particles when in water do not spread over the surfaces of the opacifier particles.

The polymer particles may be particles of any organic polymer to which water-soluble compound can be chemically bonded. However it is convenient to choose particles of a polymer (including copolymer) obtainable by a free-radical initiated emulsion or dispersion polymerisation of monomers (including mixtures of monomers) which are polymerisable in water or a mixture of water and water-miscible insolvents such as aliphatic alcohols by means of a free-radical initiated reaction. The choice of free-radical initiated emulsion or dispersion polymerisations allows bonding of appropriate water-soluble compounds to take place conveniently during the polymerisation. The polymers may be either film-forming or non-film-forming at ambient temperatures, that is to say they may have a minimum film forming temperature of either above or below 300K.

Polymers having minimum film-forming temperature of below 275K are especially welcome because they avoid the need to add organic coalescing solvent to the water. Such solvent is becoming increasingly environmentally unwelcome.

Examples of suitable monomers include vinyl esters, especially vinyl acetate or vinyl "Versatate" also alkyl (especially methyl, ethyl and n-butyl) esters of unsaturated carboxylic acids such as acrylic or methacrylic or fumaric or maleic acids, unsaturated carboxylic acids such as acrylic or methacrylic acids, unsaturated acid anhydrides such as maleic anhydride, monovinylidine aromatics especially styrene, vinyl toluene or vinyl pyridine, alkenes and halogenated alkenes such as ethylene, propylene, vinyl chloride, vinylidene chloride and tetrafluorethylene, unsaturated nitriles, dienes and (for use only in copolymerisations) minor amounts of hydroxyl or amino alkyl (especially ethyl) esters of unsaturated carboxylic acids such as acrylic or methacrylic acids, epoxy compounds such as glycidyl methacrylate and also sulphonate. Examples of suitable free radical initiators include ammonium persulphate, azobis-isobutyronitrile, ammonium azobis-cyanovalerate dibenzoyl, peroxide, tertiary butyl peroxy-2-ethyl hexanoate and redox couples such as Vinyl "Versatate" is the vinyl ester of so-called "Versatic" acid which is a mixture of aliphatic monocarboxylic acids each containing an average of 9, 10 or 11 carbon atoms and is commercially available from the Shell Chemical Company of Carrington, England.

tertiary butyl hydroperoxide/sodium formaldehyde sulphoxylate, hydrogen peroxide/ascorbic acid, hydrogen peroxide/ferrous salt and systems comprising the cerium 4 cation, Ce4+, such as ceric ammonium nitrate.

For optimum reduction of agglomeration and improvement in sheen and coin mar resistance, it is preferred to use certain number ratios of polymer particles to opacifier particles. When the number average particle size Dp of polymer particles is larger than the number average particle size Dt of the opacifier particles, (i.e. Dp is greater than Dt), the number ratio of polymer particles to opacifier particles is desirably at least 3:1 and preferably at least 4:1. For opacifier particles having a given number average particle size Dt which comprise a given volume fraction f of the combined volumes of the opacifier particles and polymer particles, there is a preferred maximum permissable number average particle size, Dp== for the polymer particles which should not be exceeded.

It is given by the equation:

where N is 3 or preferably 4 and [Op] where f = = volume fraction TiO2 [Op] + [Polymer] and where [Op] equals the total volume of particles and [Polymer] equals the total volume of polymer particles in the modified opacifier.

For example, if the number average particle size Dt of the opacifier particles is 300nm and their volume fraction f is 0.18, then the number average particle size Dp of the polymer particles must not exceed 345nm and preferably it should not exceed 313nm.

When the polymer particles are smaller than or equal in size with the opacifier particles (i.e. Dp 5 Dt), the number ratio of polymer particles to titanium dioxide particles should preferably exceed 3.64 D 2 . - + 1 :1 or 3:1 20 Dp whichever gives the greater number of particles.

Preferably this number ratio should exceed 3.64 D 2 . - + 1 :1 or 4:1 10 Dp whichever gives the greater number particles.

Many commercially useful colloidally stable aqueous dispersions of opacifier particles contain particles whose number average particle size lies in the range 200 to 350nm. It is preferred to select a balance of particle sizes and opacifier weight fractions so that the number of polymer particles attached to each opacifier particle is from 4 to 30 and such that the number average particle size of the polymer particles lies in the range 50 to 500nm.

This invention also provides a more refined process for producing a brighter polymer-modified particulate inorganic opacifier comprising particles of anatase or zinc sulphide attached to a discontinuous polymer barrier comprising particles of organic polymer in which process polymer is attached to particles of anatase or zinc sulphide which are present as a colloidally stable aqueous dispersion and have a number average particle size (Dt) of from 100 to 400nm wherein the process comprises a) providing in water a polymeric water-soluble compound which is chemically bondable to the polymer and which contains at least one moiety adsorbable onto the anatase and/or zinc sulphide particles and which preferably has a weight average molecular weight of at least 1500 prior to attachment to the polymer b) preparing a colloidally stable aqueous dispersion of polymer particles by (i) performing a free-radical initiated aqueous emulsion or dispersion polymerisation in the absence of the opacifier particles but in the presence of the water-soluble compound whereby water-soluble compound chemically bonds to the polymer as it is being formed or (ii) providing a colloidally stable aqueous dispersion of polymer particles and chemically bonding water-soluble compound onto them or (iii) chemically bonding water-soluble compound onto polymer particles and then colloidally stably dispersing the particles in water and c) mixing the colloidally stable aqueous dispersion of polymer particles with a colloidally stable aqueous dispersion of anatase and/or zinc sulphide particles whereupon on mixing, polymer particles spontaneously attach to particles of anatase and/or zinc sulphide to produce a colloidally stable dispersion of the polymer-modified opacifier particles.

In this way, organic polymer particles are pre-formed in the absence of anatase and/or zinc sulphide and then firm attachment of the pre-formed polymer particles to opacifier particles is achieved without having needed to perform a polymerisation in the presence of the opacifier particles.

Polymer modification performed in this way produces a discontinuous polymer barrier which is especially efficient at avoiding agglomeration of anatase and/or zinc sulphide particles and so largely offsets the lower opacifying power of these opacifiers.

The aqueous dispersions of polymer particles used in the more refined process must be stable, that is to say they must be capable of remaining dispersed for at least 24 hours. Dispersions having this degree of stability are well known to the paint trade where they are referred to as colloidally stable emulsion polymers or latexes. Sometimes the presence of the chemically bonded water-soluble compound imparts sufficient steric stability to render polymer particles colloidally stable but often the presence of surfactants will be needed. Where the polymer dispersion is prepared by a polymerisation performed in the presence of the chemically bondable water-soluble compound, it is preferable to have surfactant present during the polymerisation. Preferable surfactants may be ionic or non-ionic.They generally have molecular weights of below 1500 and usually below 1000 and they do not chemically bond to the polymer particles. Di(ethyl-hexyl) sodium sulphosuccinate is a suitable anionic surfactant and nonyl phenol poly(ethyoxylate)s with for example from 20 to 50 ethoxylate units are suitable non-ionic surfactants. Care should be taken to limit or avoid the presence of surfactants or any other material which is not chemically bonded to the polymer particles and which has an adsorbability onto the anatase or zinc sulphide particles which is similar to or greater than that of the adsorbable moieties of the bonded polymeric water-soluble compound.

Failure to do so could in some circumstances result in an unacceptable reduction in the attachability of the watersoluble compound to the opacifier particles. Fortunately it requires only a simple trial and error test to determine whether or to what extent the presence of any such nonbonded material is tolerable.

The colloidally stable aqueous dispersion of anatase and/or zinc sulphide particles used in the more refined process should preferably meet the standards customary in the paint trade. Aqueous coating compositions are customarily made using particles of opacifier which are well dispersed since (over the practical range of concentration, say up to 20 vol) the effectiveness of the opacifier particles increases with the quality of dispersion. Good dispersions can be obtained by choosing opacifier particles having a number average particle size of 100 to 400nm (preferably 200 to 300 nm) and dispersing them in water in the presence of the usual (preferably ionic and especially carboxylate containing) surfactants, and/or pigment dispersants using for example a high speed mixer.Many suitable pigment dispersants are commercially available but they are proprietary materials of unpublished composition. Many are anionic and may be simple acid or amine salts whereas others are polyelectrolytes having a weight average molecular weight of over 2,000. The most common anions are carboxylate, phosphate and occasionally sulphate. However cationic and non-ionic dispersants could also be used subject to the caution mentioned above that a non-ionic surfactant should not be too competitive in its absorbability onto the opacifier particles. Where the anatase particles are coated with hydratable oxides, the choice of an appropriate dispersant will be influenced by the nature of the oxides.For example, if the hydratable oxide is alumina or alumina-rich, a suitable dispersant would be a sodium or ammonium salt of a polymer or copolymer containing polymerised unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic anhydride optionally copolymerised with other unsaturated monomers. Polyphosphates may be used with silica or silica-rich coatings. The dispersions are customarily stable for at least 24 hours (though sometimes slow stirring may be required) and so they are usually described as "colloidally stable".

The stable dispersions of opacifier particles may be mixed with the stable dispersions of polymer particles simply by pouring one dispersion into the other, preferably whilst stirring. Whether it is preferable to pour the opacifier dispersion into the polymer dispersion or vice versa will depend on the natures of a particular pair of dispersions and so both sequences should be tried to determine the better. On mixing, polymer-modified particulate anatase and/or zinc sulphide forms spontaneously and provided that the two dispersions are themselves colloidally stable, a colloidally stable dispersion of polymer-modified opacifier particles will be formed. The stability of the polymer and opacifier dispersions ensures that homo-aggregation of polymer particles and of opacifier particles is hindered and usually substantially prevented whilst the presence of the adsorbable moieties in the attached water-soluble compound encourages heterocontact between polymer particles and opacifier particles.

The opacifier particles and polymer particles will almost certainly have a range of particle sizes and so it may not always be possible to derive the full benefits of using the more refined process for creating the polymer barrier which means that the reductions in agglomeration and the improvements in sheen and coin mar resistance will not be total. Nevertheless, worthwhile improvements can often be achieved. Sometimes a plurality (for example 2 to 6) of polymer-modified opacifier particles may agglomerate together. This is not a disadvantage because their ability to lessen the reductions in agglomeration and to improve sheen and coin mar resistance is not affected.

This invention further provides a coating composition containing the colloidally stable aqueous dispersion of brighter polymer-modified particulate inorganic white opacifier. In particular the coating composition containing the dispersion may be an emulsion or latex paint which may also contain optical brightener. The coating composition may also contain conventional additional ingredients such as colourants, coalescing solvents, antifoaming agents, biocides and extenders subject to the caution that extenders may have their own adverse effect on sheen and/or mar resistance. The coating composition may also comprise purposely added film-forming polymer and in fact such an addition will be essential if the modifying polymer is not or is not sufficiently film-forming at ambient temperatures.In the case of emulsion or latex paints, the purposely added film-forming polymer will comprise a stable dispersion of polymer particles.

The coating composition may be made by mixing the colloidally stable dispersions of opacifier particles and polymer particles dispersions together either in the absence of any other added particulate ingredients of the composition (in which case the modified opacifier will contain no added inorganic particles) or in the presence of some or all of them. Extender particles may interfere with the attachment of polymer particles to opacifier particles but on the other hand, if the extender has a surface to which the adsorbable moiety of the water-soluble compound can attach, then the effect of the further refinement of this invention can also be used to reduce any loss of sheen and coin mar resistance caused by irregular and/or hard surfaces on the extender or indeed any other irregular hard particulate ingredient.If the modified opacifier is made in the absence of desirable added ingredients, then the stable dispersion of modified particles may be subsequently mixed with these other ingredients.

Examples of polymer-modified particulate anatase according to this invention are illustrated by the following description which refers to Figures 1 to 4 of the accompanying drawings of which Figure 1 shows in diagrammatic section two examples of polymer-modified particulate anatase, Figure 2 shows in diagrammatic section an example of polymer-modified particulate anatase in which a plurality of modified titanium dioxide particles are agglomerated together, Figure 3 shows in diagrammatic section an example of polymer modified particulate anatase where the number average particle size (Dp) of the polymer particles is greater than the number average particle size of the titanium dioxide particles.

Figure 4 shows the location of contact angle 6.

Figure 1 shows examples 1 or 5 of polymer-modified particulate anatase comprising particles 4 of organic polymer attached to or in the vicinity of an irregular hard surface 3 of a particle 2 of anatase. Each particle 4 touches or is close to only a small portion of surface 3 but offers the maximum projection available from a polymer particle made by an emulsion or dispersion polymerisation.

Actual modified particles similar to modified particles 1 and 5 can be seen in Figure 3.

Figure 2 shows an example 11 of polymer-modified anatase comprising particles 14 of organic polymer attached to or in the vicinity of irregular hard surfaces 13 of three anatase particles 12. Polymer particles 14a attach to more than one anatase particle 12 and so form an agglomerate.

Figure 3 shows an extreme example of a polymermodified anatase in which the polymer particles 24 have a greater particle size than the anatase particle 22. All three polymer particles 24 have a particle size which just enables them to attach to the irregular hard surface of anatase particle 22. Again each polymer particle touches or is close to only a small area of the anatase surface.

Figure 4 shows the location of contact angle 6 for a particle 34 adjacent the surface 33 of a anatase particle 32 immersed in water 30.

The attachment of the polymer particles around the particles of anatase largely offsets the loss in opacity which would occur were unmodified anatase to be used instead of conventional rutile. It also reduces the loss in sheen and coin mar resistance caused by the presence of opacifier particles in a dried film of coating composition.

The viscosity of the coating composition is also improved as is the scrub resistance of the dried film.

Claims (27)

1. A brighter polymer-modified particulate inorganic white opacifier comprising particles of inorganic white opacifier of number average particle size 100 to 400 nm attached to an organic polymer barrier which inhibits agglomeration of the opacifier particles wherein the inorganic opacifier is selected from anatase and/or zinc sulphide.

2. A modified opacifier as claimed in Claim 1 in combination with at least one optical brightener.

3. A modified opacifier as claimed in Claim 1 or combination as claimed in Claim 2 comprising particles of anatase and/or zinc sulphide having a number average particle size of from 100 to 400 nm attached to a discontinuous polymer barrier comprising particles of organic polymer having a particle size which allows them to be accommodated around the anatase or zinc sulphide particles wherein:: a) the polymer particles are pre-formed prior to their attachment to the anatase or zinc sulphide particles, b) the polymer particles are pre-formed either by (i) a free radical initiated aqueous emulsion or dispersion polymerisation performed in the presence of a water-soluble compound which bonds chemically to the polymer as it is being formed, or (ii) a polymerisation which is followed by the chemical bonding of a water-soluble compound to the polymer and c) the water-soluble compound is a polymeric material which is chemically bondable to the polymer and which contains at least one moiety adsorbable onto the opacifier particles.

4. A modified opacifier or combination as claimed in Claim 3 wherein the water-soluble compound has a weight average molecular weight of at least 1500 prior to bonding to the polymer.

5. A modified opacifier or combination as claimed in Claim 3 or Claim 4 where the number average particle size of the polymer particles lies in the range 50 to 500nm.

6. A modified opacifier or combination as claimed in any one of Claims 3 to 5 in which the water-soluble compound is chosen from one or more of celluloses and/or cellulose ethers and/or polymers and copolymers of acrylamide, vinyl alcohol, vinyl pyrrolidone and acrylic acid and/or polymers containing poly (ethoxylate) chains.

7. A modified opacifier or combination as claimed in Claim 6 wherein the cellulose or cellulose ether is a hydroxyethyl cellulose, a carboxylmethyl cellulose or a hydrophobically modified hydroxyl ethyl cellulose.

8. A modified opacifier or combination as claimed in Claim 6 or Claim 7 where in the number average particle size of the polymer particles is less than 225nm.

9. A modified opacifier or combination as claimed in any one of Claims 3 to 8 wherein the preformed polymer has a minimum film-forming temperature of at least 300K.

10. A modified opacifier or combination as claimed in any one of Claims 3 to 8 wherein the pre-formed polymer has a minimum film-forming temperature of below 300K.

11. A modified opacifier or combination as claimed in Claim 10 wherein the polymer has a minimum film-forming temperature of less than 275K.

12. A modified opacifier or combination as claimed in any Claims 3 to 11 wherein the modified opacifier contains no inorganic particles other than those comprised in the pigment grade opacifier particles.

13. A modified opacifier or combination as claimed in any one of Claims 3 to 12 wherein the modified opacifier is associated with other inorganic particles modified by attachment of the same type of polymer particles.

14. A process for producing a polymer-modified particulate inorganic opacifier comprising particles of anatase or zinc sulphide attached to a discontinuous polymer barrier comprising particles of organic polymer in which process polymer is attached to particles of anatase or zinc sulphide which are colloidally stably dispersed in water and have a number average particle size (Dt) of from 100 to 400nm wherein the process comprises a) providing in water a polymeric water-soluble compound which is chemically bondable to the polymer and which contains at least one moiety adsorbable onto the anatase and/or zinc sulphide particles, b) preparing a stable aqueous dispersion of polymer particles by (i) performing a free-radical initiated aqueous emulsion or dispersion polymerisation in the absence of the opacifier particles but in the presence of the polymeric water-soluble compound whereby water-soluble compound chemically bonds to the polymer as it is being formed or (ii) providing a colloidally stable aqueous dispersion of polymer particles and chemically bonding water-soluble compound onto them or (iii) chemically bonding water-soluble compound onto polymer particles and then colloidally stably dispersing the particles in water and c) mixing the colloidally stable aqueous dispersion of polymer particles with the colloidally stable aqueous dispersion of anatase or zinc sulphide particles whereupon on mixing polymer particles spontaneously attach to the opacifier particles to produce a colloidally stable dispersion of the polymer-modified opacifier particles

15. A process as claimed in Claim 14 for making a modified particulate opacifier as claimed in any one of Claims 3 to 13.

16. A process as claimed in Claim 14 or Claim 15 wherein the polymer particles in the colloidally stable aqueous dispersion are stabilised with the help of a non-ionic surfactant which is not chemically bonded to the polymer and which is less adsorbable onto the opacifier particles than are the adsorbable moieties of the water-soluble compound.

17. A process as claimed in Claim 16 wherein the polymer particles are stabilised with the help of ionic surfactant.

18. A process as claimed in any one of Claims 16 to 17 wherein the stable dispersion of opacifier particles is stabilised by ionic surfactant and/or dispersant.

19. A process according to Claim 18 wherein the ionic surfactant and/or dispersant comprises carboxylate moieties.

20. A process according to any one of Claims 14 to 19 wherein the mixing of the stable dispersions is performed in the absence of inorganic particles other than those present in the pigment grade opacifier.

21. A process according to any one of Claims 14 to 20 wherein the stable polymer dispersion is provided with a pH of at least either 2 pH units above the isoelectric point of the stable dispersion of opacifier particles if the opacifier particles are anionically stabilised or at least 2 pH units below their isoelectric point if the opacifier particles are cationically stabilised.

22. A modified particulate opacifier optionally in combination with optical brightener as claimed in any one of Claims 1 to 13 or as made by a process as claimed in any one of Claims 14 to 21 when present as a colloidally stable dispersion in water.

23. A coating composition containing modified particulate opacifier optionally in combination with optical brightener as claimed in any one of Claims 1 to 13 or as made according to a process as claimed in any one of Claims 14 to 21 or a dispersion as claimed in Claim 22.

24. A coating composition as claimed in Claim 23 wherein the opacifier particles comprise from 10 to 35% by volume of the total solids content of the composition.

25. A coating composition as claimed in Claim 23 or 24 wherein the composition contains colloidally stably dispersed particles of film-forming polymer unattached to the opacifier particles.

26. A coating composition according to Claim 25 wherein the polymer-modified particulate opacifier is modified by polymer having a minimum film-forming temperature of above 300K.

27. The use of a modified opacifier optionally in combination with optical brightener as claimed in any one of Claims 1 to 13 or made by a process as claimed in any one of Claims 14 to 21 or as a dispersion as claimed in Claim 22 to improve the brightness and the sheen and/or coin mar resistance of a dried film of a coating composition containing anatase and/or zinc sulphide.