CS248039B2 - A base material suitable for making a multi-layer protective and / or decorative coating - Google Patents

Abstract

A base material suitable for the production of a multilayer protective and/or decorative coating on the surface of a substrate, comprising a film-forming substance, a volatile liquid medium for this film-forming substance and pigment particles dispersed in this medium, which is characterized in that the system of the film-forming substance and the volatile liquid medium is formed by a dispersion of cross-linked microparticles of acrylic addition polymers in an aqueous medium, which have a diameter in the range of 0.01 to 10 μm, are insoluble in the aqueous medium used and are stable to coarse flocculation, and this dispersion has a pseudoplastic or thixotropic character. The base material according to the invention is of particular importance for the production of so-called metallises. In this case, the base material contains a metal pigment, for example aluminum flakes. Either solvent-based or water-based materials can be used as the covering coating materials. The resulting coatings have the same surface quality and metallised effect, the so-called "flip", as coatings made from coating materials on a fully solvent basis.

Description

The invention relates to a primer usable in the application of protective and decorative coatings to surfaces, in particular to the surface of automobile bodies.

The use of coating materials containing coke pigments is well known, especially in the automotive industry. These so-called "metallics" exhibit different light reflectances depending on the viewing angle. In order to achieve the highest degree of this effect, the so-called "flip", great attention must be paid to the composition of the coating material, both the resin-forming resin and the liquid medium. It can be difficult to design a single material that would meet the above requirement and at the same time show the high degree of gloss after finishing, which is usually required for automobiles. For this reason, one of the processes proposed for the production of metallic coatings was a process involving the application of two coatings.

The first coating that is sprayed onto the substrate surface is a basecoat containing a metallic pigment. This coating is formulated to achieve optimum flip. A non-pigmented topcoat is sprayed over the basecoat to achieve the desired degree of gloss without modifying the characteristics of the basecoat in any way.

A fundamental criterion for a rapid two-layer metallization system consisting of a base coat and a clear top coat is that the base film coating must be able to resist the solvents contained in the material from which the clear top coat is formed when the top coat is applied. This is important to prevent disruption of the distribution of the metallic pigment and thus deterioration of the flip. Furthermore, it is highly desirable that the base coat has this property without having to undergo a long intermediate drying or curing.

In known basecoat, clearcoat systems, in which both the basecoat and the topcoat are solvent-based, this requirement is achieved in most cases by using an additive capable of imparting a gel-like character to the freshly formed base film. Cellulose acetobutyrate has been used as this additive. The transition between the relatively low viscosity that the basecoat must have in the spray gun and this gel-like character is facilitated by the fact that the liquid diluent contained in this material contains volatile components that preferentially evaporate during the journey from the spray gun to the substrate.

In view of the desire to prevent atmospheric pollution, considerable interest has been focused in recent years on coating compositions in which water is used as the diluent instead of organic diluents. A considerable number of such compositions have been proposed for use in the automotive industry. However, it has not been possible to use aqueous compositions as primers in basecoat/clearcoat systems with success until now. One factor which tends to prevent the achievement of this objective is the fact that it is extremely difficult to effectively evaporate the diluent from the primer between the spray gun and the substrate in a controlled manner, except by very expensive humidity control in the environment in which the spraying is carried out. It has now been surprisingly found that satisfactory water-based coating compositions based on an aqueous dispersion of a crosslinked polymer microgel can be obtained.

The subject of the invention is a base material suitable for the production of a multilayer protective and/or decorative coating on the surface of a substrate, comprising a film-forming substance, a volatile liquid medium for this film-forming substance and pigment particles dispersed in this medium, characterized in that the system of the film-forming substance and the volatile liquid medium comprises a dispersion of cross-linked microparticles of acrylic addition polymers in an aqueous medium, which have a diameter in the range from 0.01 to 10 μπι, are insoluble in the aqueous medium used and are stable to coarse flocculation, wherein this dispersion has a pseudoplastic or thixotropic character.

Crosslinked polymer microparticles can be formed from various types of polymer. Of particular interest in this application are acrylic addition polymers derived from one or more alkyl esters of acrylic or methacrylic acid, or mixtures thereof with other ethylenically unsaturated monomers. Suitable acrylic or methacrylic acid esters are methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate. Other suitable copolymerizable monomers include vinyl acetate, vinyl propionate, acrylonitrile, styrene and vinyl toluene. Since the polymer is to be crosslinked, a minor amount of a monomer that is polyfunctional with respect to polymerization, for example ethylene glycol dimethacrylate, allyl methacrylate or divinylbenzene, may be added to the monomers from which the polymer is prepared. Alternatively, minor amounts of two other monomers carrying pairs of chemical groups which can be reacted together either during or after polymerization may be added to these monomers. Examples of such pairs of mutually reactive groups include an epoxy group and a carboxyl group (e.g., the glycidyl methacrylate-methacrylic acid system), an anhydride group and a hydroxyl group, or an isocyanate group and a hydroxyl group.

The chemical composition and degree of crosslinking of the polymer of the microparticles may be such that the polymer has a glass-to-rubber transition temperature Tg either lower than room temperature, in which case the particles have a rubbery character, or higher than room temperature, in which case the particles are hard and glassy.

As already mentioned, it is necessary that the polymer microparticles are dispersed in the base material in a state that is stable against coarse flocculation, i.e. in a state where even at low solids content the dispersion contains few aggregates of a larger number of particles, if any at all. However, this does not exclude the possibility of a slight degree of flocculation of the particles, especially at higher solids content. This state can be achieved, for example, by steric stabilization, i.e. by forming a wall of chains of another polymer around the particles, which are solvated by the aqueous medium of the coating material. These chains therefore have an unfolded configuration. The term “solvated” in this context means that these polymer chains would actually be soluble in said aqueous medium if they formed independent molecules. However, since these chains are attached to the microparticles at one or more points, the steric barrier remains permanently attached to the microparticles. Spherically stabilized polymer microparticles can be conveniently prepared by dispersion polymerization of the respective monomers in an aqueous medium in the presence of a spherical stabilizer. The stabilizer is amphipathic in nature, i.e. it contains in the molecule two basic polymer components with different properties: one component is formed by a polymer chain that is solvated by the aqueous medium and the other is formed by a polymer chain that is not solvated by this medium and as a result is anchored on polymer microparticles that are by definition insoluble in aqueous medium. Suitable dispersion polymerization procedures are described in published British patent application of the company ICI number 2 039 497A. The aqueous medium in which the polymerization is carried out consists of water mixed with a volatile organic co-solvent.

The mixture of water and co-solvent is capable of dissolving monomers, most or all of which would be substantially insoluble in water alone. These procedures include the additional requirement that the polymerization be carried out at a temperature that is at least 10 degrees Celsius higher than the glass transition temperature of the polymer to be formed and the requirement that at no time should a separate monomer phase be present in the system. The amphipathic steric stabilizer may be added to the polymerization mixture as a pre-formed substance, or may be formed in situ during the polymerization from a polymer that is soluble in an aqueous medium and that is capable of copolymerizing with any of the polymerized monomers, or that, after hydrogen elimination, can undergo graft copolymerization with any of the polymerized monomers. The sterically stabilized microparticle dispersions obtained by these processes are very suitable for the production of the base materials used according to the invention, since the organic co-solvent can be separated from them by distillation without impairing the stability of the dispersed phase, to give a product in which the continuous phase consists exclusively of water.

Alternatively, a dispersion of polymer microparticles can be obtained by aqueous emulsion polymerization of suitable monomers. In this case, flocculation stability is achieved by the presence on the particles of electrically charged species derived from a water-soluble ionic surfactant and/or a water-soluble ionizable polymerization initiator. Such polymerization processes are extensively described in the literature.

Polymer microparticles can further be produced by a non-aqueous dispersion polymerization of monomers, after which the resulting polymer is transferred to an aqueous medium. Such a process is described in published British patent application ICI No. 2,006,229A. In the first step of this process, a sterically stabilized dispersion is formed in a non-aqueous liquid of a polymer that is insoluble in both the non-aqueous liquid and water, using any of the processes known for the production of such dispersions, in the second step, one or more monomers are polymerized in the dispersion thus obtained in the presence of a spherical stabilizer, from which a second polymer can be formed, which is itself soluble in the desired aqueous medium at a suitable pH value, and finally the resulting composite polymer microparticles are transferred from the non-aqueous medium to an aqueous medium.

The above discussion is focused on the case where the crosslinked polymer microparticles are composed of addition polymers, which are the most suitable type of polymer for this purpose. However, it is also possible for the microparticles to be formed from a condensation type polymer, for example, a polyester prepared from a polyhydric alcohol and a polyhydric acid. Suitable polyhydric alcohols are ethylene glycol, propylene glycol, butylene glycol, 1,6-hexylene glycol, neopentyl glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, glycerol, trimethylolpropane, trimethylolethane, pentaerythritol, dipentaerythritol, tripentaerythritol, hexanetriol, oligomers of styrene and allyl alcohol (for example, the product of Mansanto Chemical Company "RJ 100"), condensation products of trimethylolpropane with ethylene oxide or propylene oxide (such as the commercially available products known as "Niax" tritols). Suitable polycarboxylic acids are succinic acid (or its anhydride), adipic acid, azelaic acid, sebacic acid, maleic acid (or its anhydride), fumaric acid, muconic acid, itaconic acid, phthalic acid (or its anhydride), isophthalic acid, terephthalic acid, trimellitic acid (or its anhydride) and pyromellitic acid (or its anhydride). Crosslinking of polycondensates is achieved by adding substances with a functionality higher than 2 to the starting reaction mixture, although in this case it can be difficult to ensure that all molecules are actually crosslinked. This is because the molecules formed in the polycondensation are characterized by a broad molecular weight distribution compared to the molecules of addition polymers.

The methods described above for the preparation of polymer microparticles by the addition-type polymerization of monomers in an aqueous medium are not generally applicable to condensation-type monomers, since water has an inhibitory effect on the condensation reaction. Microparticles of condensation polymers, however, can be readily prepared by dispersion polymerization in a non-aqueous medium by the methods described in British Patents Nos. 1,373,531, 1,403,794 and 1,419,199 to ICI. These patents also describe the preparation of crosslinked microparticles. These microparticles can then be subjected to a second polymerization stage in a dispersion in a non-aqueous medium, carried out by the method described in British Patent Application Publication No. 2,006,229A cited above, after which the microparticles are transferred to the selected aqueous medium.

The term "aqueous medium" herein refers to either water alone or water in admixture with a water-miscible organic liquid, such as methanol. The aqueous medium may also contain water-soluble substances added to adjust the pH of the matrix, as described in more detail below.

The pigment particles which are dispersed in the aqueous medium of the base material according to the above definition may have a size in the range of 1 to 50 μΐη and may be any pigments commonly used in coating materials for the surface treatment of substrates, such as inorganic pigments, for example titanium dioxide, iron oxide, chromium oxide, lead chromate and carbon black, organic pigments, for example phthalocyanine blue and phthalocyanine green, carazole violet, anthrapyrimidine yellow, phthalavanthrone yellow, isoindoline yellow, indathrone blue, quinacridone violet and perylene red. For the purposes of the invention, the term pigment also includes conventional fillers and extenders, such as talc or kaolin.

The invention is of particular importance in the case of base materials containing metal flakes as a pigment, which are used for the production of bright metallic coatings, so-called metallises, mainly on the surface of car bodies. Suitable metal pigments are in particular aluminum flakes and copper bronze flakes. However, the invention also offers advantages in painting to permanent shades, as described in more detail below. In general, pigments of any kind can be added to the base materials in an amount of from 2 to 100 %, based on the total weight of the material. If pigmentation with metal pigments is used, their amount is preferably in the range of from 5 to 30 % by weight, based on the total weight of the material.

These pigments, whether metallic or otherwise, can be incorporated into the base materials using known pigment dispersants suitable for use in aqueous systems.

The presence of crosslinked polymer microparticles in the basecoat composition gives the film of this composition the desired ability to withstand the subsequent application of the topcoat composition without damage to the film or pigmentation, especially pigmentation with metallic pigments. Without the use of these microparticles, a satisfactory basecoat/clearcoat system cannot be produced.

In addition to this basic property, it is also desirable that the dispersion of insoluble microparticles should have a pseudoplastic or thixotropic character. This means that the apparent viscosity of the dispersion varies according to the degree of shear stress to which the dispersion is subjected, in particular that the apparent viscosity under conditions of low shear stress is higher than under conditions of high shear stress. The change in viscosity which occurs when the applied shear stress is changed may be instantaneous or may require a finite time interval which is nevertheless within the range of the viscosity measurement. The reason why the dispersion on which the base material is based is required to have this character is most obvious in the case where the material contains metal flakes as a pigment. In such a case, it is desirable that the total concentration of non-volatile solids present be relatively small. This allows for significant shrinkage of the base film after application to the substrate and during drying, which results in the correct orientation of the metal flakes and the achievement of an optimal flip effect. However, when the base material is to be applied to the substrate by spraying, it is necessary that it has a sufficiently low viscosity to ensure effective atomization in the spray gun. However, once the material hits the substrate, its viscosity should be sufficiently high to prevent it from running off, forming curtains or uneven distribution and orientation of the metal flakes, despite the fact that the loss of water and other solvents by evaporation between the spray gun and the substrate is only low (due to the high ambient humidity).

These pseudoplastic properties are often documented by stating the apparent viscosity values η a in Pa.s at a selected shear rate D[s ^-1 ]. In the case of the base materials with metallic pigments used according to the invention, this value should be at a non-volatile solids content below 30% by weight, preferably lower than 0.05 Pa.s at a shear rate D = 10,000 s- ¹ s and more than 2 Pa.s at a value of D — 1 s ' ¹ . It is even more advantageous if the base material has a value 77a lower than 0.025 Pa.s at D = 10,000 s ^-1 and more than 4 Pa.s at D '== 1.0 s ¹ . In the case of base materials colored to permanent shades with pigments other than metal flakes, the preferred value is 73a, again at a solids content below 30%, less than 0.1 Pa.s at D = - 10,000 s ^_] and more than 1.0 Pa.s at D = 1.0 s ^_1 . However, it is in the nature of pseudoplastic or thioxotropic behavior that it cannot be fully and precisely defined by a few selected viscosity/shear stress data. Much depends on the actual viscosity measurement method used. The above values should therefore not be considered as strict limits to be observed in order to achieve the desired effects of the invention. They are rather approximate guidelines and a person skilled in the art can easily determine by simple practical tests whether a particular dispersion or base material made therefrom has a sufficient degree of pseudoplasticity or thioxotropy.

There are various ways in which a base dispersion can be given a pseudoplastic or thioxotropic character. In some cases no special precautions are required. This may be the case, for example, where the microparticles have been prepared by the above-mentioned process described in published British patent application number 2 006 229A. In this process, the polymer microparticles themselves are first produced by dispersion polymerisation of suitable monomers in a non-aqueous medium and then polymerisation of further monomers is carried out to produce a second substantially uncrosslinked polymer which is hydrophilic in character, meaning that it is inherently soluble in the aqueous medium in which the resulting dispersion is to be formed at the appropriate pH.

In reality, however, not all of the second polymer dissolves in the aqueous medium when the product obtained by the two-stage polymerization in a non-aqueous medium is transferred to an aqueous medium. A significant portion of the second polymer remains associated with the polymer microparticles and the microparticles are thereby stabilized in the dispersion in the aqueous medium. This associated polymer can at the same time impart pseudoplastic or thioxotropic properties to the aqueous dispersion. Other monomers which are suitable for the second stage of polymerization are, for example, hydroxyalkyl esters of acrylic or methacrylic acid, a monoester of acrylic or methacrylic acid and a polyglycol, such as polyethylene glycol, a monovinyl ether of such a polyglycol, or vinylpyrrolidone, optionally in admixture with smaller amounts of non-hydrophilic monomers, such as methyl methacrylate, butyl acrylate, vinyl acetate or styrene.

Alternatively or additionally, the desired solubility in aqueous media can be achieved by using as the main monomer component an acrylic-type ester containing basic groups, for example, dimethylaminoethyl methacrylate or diethylaminoethyl methacrylate. These groups are then converted to salt groups by reaction with a suitable acid, for example, lactic acid, dissolved in aqueous media. The second polymer can further be derived from comonomers containing a substantial amount of a polymerizable carboxylic acid, such as acrylic acid or methacrylic acid, in which case this polymer can be soluble in aqueous media containing a dissolved base, such as dimethylaminoethanol. In general, the second polymer can therefore be nonionic, anionic or cationic in character.

In the case where the polymer microparticles have been produced by a known aqueous emulsion polymerization process, a second inherently water-soluble polymer can be produced by further polymerization in the same aqueous medium and in the presence of the microparticles. The monomers used are monomers capable of providing a polymer containing acidic salt-forming groups which can impart water solubility to the polymer. Suitable monomers are therefore polymerizable carboxylic acids such as acrylic or methacrylic acid, optionally together with minor proportions of non-hydrophilic monomers such as methyl methacrylate and also hydrophilic monomers which provide water-insoluble homopolymers, for example hydroxyethyl methacrylate and hydroxypropyl methacrylate.

Where the polymer microparticles have been prepared by the aqueous dispersion polymerization process described in the above-cited British Published Application No. 2,039,497A, a second water-soluble polymer may conveniently be formed by further polymerization in the same medium. Suitable monomers are those mentioned in the preceding paragraph as well as basic monomers such as dimethylaminoethyl methacrylate, from which water-soluble salts can be formed.

Not all water-soluble polymers produced in situ in the presence of microparticles by any of the methods described above can impart the desired pseudoplastic properties to the base coating compositions. The appropriate polymer composition can be arrived at by simple testing, for example, by measuring the viscosity at selected shear rates, or by actually applying the compositions to the substrate.

Instead of preparing a suitable water-soluble polymer in situ, this polymer can be added as a pre-formed separate component to the aqueous dispersion of microparticles. Suitable polymers are those which, when dissolved in an aqueous medium, even at low concentrations, cause a significant increase in the viscosity of the composition. For example, one or more thickeners which are well known and which are used in coating compositions based on aqueous polymer latexes can be added. However, not all thickeners are suitable for this purpose, since some of them are not able to impart the necessary pseudoplastic properties to the compositions to which they are added. On the other hand, some thickeners which, when dissolved in an aqueous medium themselves, do not have these properties, can impart these properties to the dispersion of microparticles, through interactions between them and the microparticles (for example, through hydrogen bonds or interactions of polar groups). One commercially available thickener that has proven to be very suitable is "Acrysol" ASE 60 (Rohm and Hass).

It follows from the above that any inherently water-soluble polymer which is associated with the polymer particles or which is added to the dispersion of microparticles in order to impart pseudoplastic or thixotropy properties to the dispersion must be non-crosslinked in nature; however, such a polymer may nevertheless, if desired, be of the crosslinkable type. That is to say, it may contain chemically reactive groups by means of which it may be crosslinked in the application of the base material with preferably also the top coating material. The crosslinking may optionally take place with the aid of an added crosslinking agent. The polymer may therefore contain, as has already been shown, hydroxyl or carboxyl groups derived from monomers bearing these groups and these groups may be used for subsequent crosslinking! using an amino resin, for example a methylated melamine formaldehyde condensate, which is soluble in an aqueous medium.

It follows from the above description that the base coating composition may consist exclusively of polymer microparticles, pigment particles, an aqueous medium in which both groups of particles are dispersed and an inherently water-soluble polymer which imparts pseudoplastic properties to the composition. This is true, but it is much more advantageous if the composition also contains a film-forming polymer which is soluble in an aqueous medium. The presence of a film-forming polymer ensures that, after the base coating has been applied to the substrate and the aqueous medium has evaporated, there is a substance present on the substrate which can coalesce and fill the voids between the microparticles to form a coherent, sufficiently integrated film in the second stage of the process according to the invention. This function may in fact be fulfilled by a portion of the inherently water-soluble polymer which is present in the composition in order to impart pseudoplastic or thioxotropic properties to it. Since the amount of such polymer required for this purpose is usually small, it may be appropriate to supplement it with one or more other water-soluble film-forming substances which are added to the composition. These substances may optionally be chemically reactive with the components already present. The composition may therefore contain oligomeric substances which can be converted into high molecular weight products after application of the composition, but which do not themselves contribute significantly to the viscosity of the composition before application. Examples of these substances include low-volatility diols such as 2-ethyl-1,3-hexanediol, low-molecular-weight propylene glycols, low-molecular-weight adducts of ethylene oxide with dihydric or trihydric alcohols such as neopentyl glycol, bisphenol A, cyclohexanedimethanol, glycerol and trimethylolpropane, /S-hydroxyalkylamides such as N.N.N^NMetrakis-(,/3-hydroxyethyljadipamide and cyclic amides and esters such as ε-caprolactam and ε-caprolactone. If these substances are not sufficiently soluble in pure water, they should be soluble in an aqueous medium containing water together with an organic liquid that is miscible with water. Each of these oligomeric substances can be converted into a high-molecular-weight polymer after application of the base material to the substrate by using their hydroxyl groups or other reactive groups to form crosslinks using a polyfunctional reagent also present in the material. Amino resins soluble in the aqueous medium of the mass are particularly suitable for this purpose, especially melamine formaldehyde condensates, such as hexa(alkoxymethyl)melamines and their low molecular weight condensates.

Instead of containing components that form a film-forming polymer upon application to the substrate, the base coating composition may contain a preformed water-soluble acrylic polymer that does not impart pseudoplastic properties to the composition, or may contain particles of uncrosslinked polymer in dispersion that are stabilized in a manner similar to the crosslinked microparticles present. Each of these alternative components may optionally contain functional groups, such as hydroxy groups, by which it may be crosslinked after application to the substrate using a crosslinking agent, such as an amino resin.

The relative proportions of the various components of the base material may vary within wide limits and the optimum proportions of each individual system are often best determined experimentally, but some general principles can nevertheless be given which may serve as a guide. It is particularly important to take into account that if the amount of polymer microparticles is too high in relation to the other film-forming substances present in the material, the material does not contain sufficient substance to fill the gaps between the microparticles. In such a case, after the subsequent application of a clear topcoat, the topcoat material tends to sink (fall) into the gaps of the base film, resulting in a loss of gloss. If, on the other hand, the microparticle content is too low, the particles may not provide the base film with the required degree of resistance to attack by the solvent contained in the clear topcoat material. To some extent, the lower amount of microparticles can be compensated for in this respect by allowing the base film to dry longer before applying the clear topcoat, but this reduces one of the main advantages to be obtained with the method according to the invention. A satisfactory content of microparticles is usually in the range of 5 to 80% by weight, based on the total non-volatile content of the base material.

The optimum content of microparticles, however, depends to some extent on whether the pigment present in the base material is metallic or non-metallic. For materials containing metallic pigments, the preferred range of the content of microparticles is 40 to 75% by weight, based on the above-mentioned basis. When using conventional colorfast pigments, the preferred range of the content of microparticles is somewhat lower, i.e. 10 to 50% by weight, based on the above-mentioned basis, since in order to achieve a suitable opacity at a sufficiently small film thickness, it is usually necessary to use higher amounts of these pigments. By reducing the amount of microparticles, it is possible to avoid a too high volume fraction of the total amount of dispersed substances, which could result in the formation of a porous base film. When applying a transparent topcoat to such a porous film, the topcoat material could flow (seep) into the base coating, which results in poor gloss of the topcoat film.

As regards the thickener or the second polymer which imparts pseudo-plastic properties to the composition, it can generally be stated as a guide that their amount in the base composition may vary from 0.3 to 50% by weight, based on the total non-volatile matter content of the composition. The amount of other film-forming substances present in the composition may range from 0 to 30% by weight and, when a crosslinker such as aminophosphoric acid is present, its amount may also be up to 30% by weight, based on the total non-volatile matter content of the base composition. The base composition may also optionally contain a catalyst for crosslinking to take place after the composition has been applied to the substrate. This catalyst may be a water-soluble acidic compound such as p-toluenesulfonic acid, orthophosphoric acid, maleic acid or another strong carboxylic acid such as tetrachlorophthalic acid. Instead of the acid, heat-labile salts of such an acid with a volatile amine can also be used.

The base coating compositions according to the invention are suitable for use in a method of producing a multilayer protective and/or decorative coating on the surface of a substrate, which is the subject of Czechoslovak patent No. 226,427. Details relating to the application of the base material to the substrate and further processing of the applied film are given in this patent.

Since the primers according to the invention are water-based, they have the advantage of eliminating or significantly reducing the disadvantages associated with atmospheric pollution inherent in solvent-based materials. Their main importance lies in the fact that a short time after application to the substrate is sufficient for their drying and yet it is achieved that when applying a topcoat to the base film there is no tendency for the base coat to mix with the topcoat or for the base film to dissolve with the topcoat. If the primers according to the invention are used in the production of so-called metallics, these advantages are achieved without sacrificing good orientation of the metallic pigments. In the case of so-called permanent shades, the base film is subject to a much smaller extent to the effect known as "bubbling" compared to water-based coatings that do not contain polymer microparticles.

The invention is further illustrated by the following examples, in which parts and percentages are always meant as parts and percentages by weight. The examples are illustrative only and do not limit the scope of the invention in any way. The apparent viscosity values of the base stocks were determined in the examples using two different instruments. The values of -qa at a shear rate of 10,000 ^s_1 were measured using a cone and plate viscometer modified to measure viscosity in the range of 0 to 0.2 Pa. s at the shear rate indicated. This instrument is described in the Journal of the Oil and Colour Chemists ¹ Association, July 1966, in an article by CH Monk and is manufactured by Research Equipment (London) Limited.

The values of r,a at a shear rate of 1.0 s" ¹ were measured using a concentric cylinder viscometer, "Theomat 30" with "A" cup and weight. Each sample was subjected to a shear rate of 660 s ^_1 until a constant shear stress value was reached and immediately the shear rate was changed to 1.0 s ^_1 , the shear stress was measured and the viscosity was calculated from these data. The "Rheomat 30" device is manufactured by Contraves AG in Zurich.

Example

AND.

Preparation of a dispersion of polymer microparticles in an aliphatic hydrocarbon

35.429 parts of heptane are introduced into a reactor equipped with a stirrer, thermometer, reflux condenser and a device for dosing monomer into the reflux distillate. The heptane is heated to reflux (95 to 96 °C) and a mixture of the following components is added:

methyl methacrylate 5.425 parts, azodiisobutyronitrile 0.420 parts graft copolymer as stabilizer (33% solution described below] 1.984 parts

The reactor contents are held at reflux temperature for 30 minutes to create a "batch" polymer dispersion, and then the following mixture of ingredients is added to the reflux distillate. The mixture is added at a constant rate over a period of 3 hours.

methyl methacrylate 25.000 parts allyl methacrylate 0.775 ¹ part azodiisobutyronitrile 0.338 parts graft copolymer as stabilizer (33% solution described below) 5.316 parts

After the end of the dosing, the reactor contents are maintained at reflux temperature for another hour, 12.874 parts of heptane are added and the mixture is heated to reflux temperature again. A pre-formed mixture of the following composition is introduced into the refluxed distillate at a constant rate for 1 hour:

methyl methacrylate 3,883 work butyl acrylate 3,066 work hydroxyethyl acrylate 2,044 work acrylic acid 1,226 work azodiisobutyronitrile 0.071 work graft copolymer as stabilizer zator (33% solution described further) 2,149 work

After the entire amount of this mixture has been added, the reaction mixture is maintained at reflux temperature for 1 hour. A stable dispersion of crosslinked polymer microparticles is obtained, which has a total non-volatile solids content of 43.5 to 44.5% and a non-volatile solids content insoluble in any polar solvent (so-called gel content) of 34.5 to 35%.

The graft copolymer stabilizer used in the above process is prepared as follows: 12-hydroxystearic acid is autocondensed to an acidity value of about 31 to 34 mg KOH/g (corresponding to a molecular weight of 1,650 to 1,800 J) and then reacted with an equivalent amount of glycidyl methacrylate. The resulting unsaturated ester is copolymerized in a 2:1 weight ratio with a mixture of methyl methacrylate and acrylic acid in a 95:5 weight ratio. The copolymer is used as a 33% solution in a mixture of ethyl acetate (11.60%), toluene (14.44%), an aliphatic hydrocarbon boiling at 98 to 122°C (61.29%) and an aliphatic hydrocarbon boiling at 138 to 165°C (12.67%).

B.

Conversion of polymer microparticles into dispersion in an aqueous medium

Demineralized water 72.308 parts butoxyethanol 10.332 parts dimethylaminoethanol 0.529 parts is introduced into a reactor equipped with a stirrer, thermometer and a device for removing volatile solvent by distillation.

The reactor contents are heated to 100°C and 46.497 parts of the microparticle dispersion from stage A are introduced into the reactor at a rate such that the heptane contained in the dispersion is removed by distillation without being significantly concentrated in the reactor. The time required for this operation is about 2 hours. In the process, 29 to 30 parts of a distillate consisting mainly of heptane and some water are separated.

The product obtained is a stable aqueous dispersion of polymer microparticles, which has a non-volatile solids content of 20 to 22% and a pH of 7.2 to 7.5.

C.

Preparation of aluminum pigment concentrate

Aluminum paste (metal content %J 5.8 parts butoxyethanol 2.9 parts) is placed in a stirred mixing vessel.

The ingredients are mixed together for 15 minutes, then 2.9 parts of butoxyethanol are added at a constant rate over 30 minutes and the mixture is then further mixed for 1 hour. Then _4.84 parts of hexamethoxymethylmelamine are added, mixing is continued for another hour, then 1.93 parts of hexamethoxymethylmelamine and 0.97 parts of butoxyethanol are added and the mixture is further mixed for an hour.

D.

Preparation of the base coating material

The following components are mixed together for 1 hour: aluminum concentrate from grade C 19.34 parts microparticle dispersion from grade B 79.61 parts hexamethoxymethylmelamine 0.46 parts dimethylaminoethanol salt of p-toluenesulfonic acid, 10% solution in demineralized water 3.47 parts

The base material thus obtained has the following properties:

Solids content:

27.2%

Apparent viscosity -zja:

0 _; 35 Pa . s at shear stress rate D 1 s ^_l

-0.006 Pa.s at shear stress rate D —- 10,000 s' ¹

Preparation of acrylic polymer for clear topcoat

Into a reactor equipped with a stirrer, thermometer and reflux condenser, xylene 22.260 parts aromatic hydrocarbon with a boiling point of 190 to 210 °C 10,000 parts

The mixture is heated to reflux temperature (142 to 146 °C) and a pre-formed mixture of the following composition is added to it over a period of 3 hours:

styrene ethyl acrylate 2-ethylhexyl acrylate hydroxyethyl acrylate acrylic acid cumene hydroperoxide

21.49 parts

4.51 parts

13.75 parts 10.05 parts 0.49 parts 1.41 parts

The reactants are maintained at reflux temperature for another 2 hours and then isobutyl alcohol xylene is added.

12.72 parts 3.32 parts

A clear polymer solution is obtained, which has a solids content of 50%.

F.

Preparation of a solvent-based clear topcoat

The following ingredients are mixed together:

polymer solution from grade E 53.3 parts butyl melamine formaldehyde resin as a 67% solution in butanol 26.5 parts dipentene 5.0 parts flow-improving polymer as a 10% solution in xylene 0.1 parts isobutyl alcohol 2.0 parts xylene 13 1 part

A clear solution with a solids content of 44.4% is obtained. Its viscosity is 40 s (British Standard B4 cup at 25°C).

G.

Application of basecoat and clear topcoat to substrate

The metal plate is provided with a layer of primer, then a leveling coat, then two coats of the primer containing the metal pigment described in step D are applied by spraying (the materials are used without further dilution) at a temperature of 22°C and a relative humidity of 39%. The first coat is allowed to dry for 2 minutes between the two applications. The material feed rate into the spray gun is 400 ml/min.

After the second coat of basecoat has been applied, the board is air-dried at 25°C and then two coats of clear topcoat as described in step F are applied. (The clear topcoat is thinned with xylene to a viscosity of 45 before application and according to the British Standard using a B 3 cup at 25°C. J The second topcoat is applied to the damp first coat; the first coat is allowed to dry for only 2 minutes between applications. After a final three-minute dry period, the board is fired for 30 minutes at 125 to 130 ^° C.

The resulting silvery metallic coating has excellent flip, good pigment orientation and an appearance comparable to the highest flip coatings produced using fully solvent-based coating systems. Gloss and intercoat adhesion are good and there is no bleeding (sinking) of the clearcoat into the basecoat.

Example 2

The metal plate is provided with a layer of primer, then a leveling coat, then two coats of the primer containing the metal pigment described in step D of example 1 are applied by spraying (the materials are used without further dilution) at a temperature of 25 °C and a relative humidity of 58%. The first coat is allowed to dry for 2 minutes between the two applications. The material feed rate through the cutting gun is 400 ml/min.

After the second coat of primer has been applied, the board is dried for 10 minutes at 35 to 42 degrees Celsius and then two coats of the clear topcoat described in Step F of Example 1 are applied. (The clear topcoat is thinned with xylene to a British Standard 45s viscosity using a B3 cup at 25 ^°C, C before application.) The second topcoat is applied to the damp first coat; the first coat is allowed to dry for only 2 minutes between applications. After a final three-minute dry period, the board is fired for 30 minutes at 125 to 130 ^° C.

The coating thus obtained has the same excellent properties as the coating described in the example

1.

Example 3

AND.

Preparation of aqueous dispersion of polymer microparticles

The following mixtures of substances are prepared:

(1) mixture of monomers

methyl methacrylate 18,350 work allyl methacrylate 1,340 work styrene 4,700 work butyl acrylate 18,800 work methacrylic acid 1,410 work primary octyl mercaptan 0.159 work ammonium sulfate adduct nonylphenol and 5 moles ethylene oxide 0.185 work

(2) initiator solution ammonium persulfate 0.130 parts demineralized water 4.010 parts (3) surfactant solution methyl methacrylate 20.000 parts ammonium sulfate adduct of nonylphenol and 5 moles of ethylene oxide 20.000 parts

The following is introduced into a reactor equipped with a stirrer, thermometer, reflux condenser and a device for the controlled introduction of two separate liquid charges:

demineralized water 47.641 parts surfactant solution (3) 0.100 parts

The batch is heated to 80-85°C, then 2,000 parts of monomer mixture (1) are added and the resulting mixture is kept at 80-85°C for 15 minutes. Then 1.068 parts of initiator solution (2) are added and the reaction mixture is kept at the same temperature for 20 minutes. Then at a constant rate over 5 hours, the following is added:

monomer mixture (1) 42.944 parts hydroxyisopropyl methacrylate 2.350 parts

During the same five-hour period, 3.672 parts of the initiator solution are fed separately to the reactor at a constant rate. The reaction mixture is maintained at 80-83°C for 1 hour and then cooled to room temperature. A stable aqueous dispersion of crosslinked polymer microparticles is obtained. The dispersion contains 49.6% solids and the content of non-volatile solids insoluble in any organic solvent is 44.5%.

B.

Preparation of the base coating material

Mixture (4J

2.563 parts of a commercially available thickener known as Acrysol ASE 60 (Rohm and Hass Co.) are mixed with sufficient 25% dimethylaminoethanol solution in demineralized water to adjust the pH to 7.65. Additional demineralized water is then added to bring the mixture to a total weight of 23.956 parts.

Mixture (5j

A mixture of 29.299 parts of the microparticle dispersion from step A and 18.614 parts of demineralized water is adjusted to pH 7.65 by adding a sufficient amount of a 25% solution of dimethylaminoethanol in demineralized water. Then 23.956 parts of mixture (4) prepared as described above are mixed into the resulting mixture.

In a running blender, add:

aluminum paste (metal content 65%) 3.133 parts

2-butoxyethanol 15.193 parts

The batch is stirred for 15 minutes, then added:

hexamethoxymethylmelamine 3.696 parts polypropylene glycol (average molecular weight 400) 2.464 parts and mixing is continued for another hour. Then mixture (5) 71.869 parts demineralized water 1.623 parts are added and the mixture is mixed for another hour. The base material thus obtained has the following properties:

solids content:

205 percent

Apparent viscosity -qa:

3.46 Pa. s at shear rate D = 1 s ¹ 0.042 Pa. s D — 10,000 s” ¹

C.

Preparation of acrylic polymer for clear topcoat

42.20 parts of isopropyl alcohol are placed in a reactor equipped with a stirrer, thermometer and reflux condenser. The isopropyl alcohol is heated to reflux temperature (84°C) and then the following mixture of substances is added to it at a constant rate over a period of 3 hours:

methyl methacrylate 19.85 work butyl acrylate 24.80 work hydroxyethyl methacrylate 2.48 work acrylic acid 2.48 work isopropyl alcohol 7.45 work benzyl peroxide (60% paste in dimethylphthalate) 0.64 work Reactive components ss maintain another 2 ho dins

at reflux temperature. A polymer solution containing 50 ^% solids is obtained. 28.87 parts of isopropyl alcohol are removed from the solution by direct distillation and 1.86 parts of dimethylaminoethanol and then 80.35 parts of demineralized water are added to the residue with stirring. The distillation of the azeotropic mixture of isopropyl alcohol and water is then continued until a temperature of 96 to 98°C is reached, and the distillate is replaced by the addition of demineralized water. The total amount of distillate separated is 118.83 parts and the total amount of demineralized water added additionally is 114.65 parts. An aqueous solution of acrylic polymer containing 33.5% solids is obtained.

D.

Preparation of a clear water-based coating material.

The following substances are mixed together: polymer solution from the above grade C 75.16 parts hexamethoxymethylmelamine 6.29 parts butoxyethanol 12.26 parts demineralized water 5.50 parts dimethylaminoethanol salt of p-toluenesulfonic acid (pH 7.6) 0.79 parts

The clear coating mass thus obtained contains 31.5% solids and has a viscosity of 0.05 Pa.s.

E.

Application of basecoat and clear topcoat to substrate

The metal plate is prepared with a primer, then a leveling coat is applied and then three coats of the base coating material with metallic pigment described in step B are sprayed. The coating material is used without further dilution and spraying is carried out at a temperature of 25 ‘C and a relative humidity of 51 %/o. The second coat is applied 1 minute after the first coat, the material feed rate into the spray gun is 400 ml/min.

After the third coat of primer has been applied, the board is dried for 10 minutes at 35 to 42 ^° C. Three coats of clear topcoat as described in step D are then applied. The coats are applied without intermediate drying into the damp previous coat, as there is a 2 minute waiting period between the individual coats and a 3 minute waiting period after the third coat. The board is then first heated to 70 ^° C and finally fired for 30 minutes at 150 ^° C.

The resulting coating has excellent flip, good pigment orientation and an appearance comparable to the highest flip coatings produced using fully solvent-based coating systems. Gloss and intercoat adhesion are good and there is no bleeding (sinking) of the clearcoat into the basecoat.

Example 4

AND.

Preparation of aqueous dispersion of polymer microparticles

A reactor equipped with a stirrer, thermometer, reflux condenser and a device for the controlled introduction of two separate liquid charges is charged with

demineralized water 28,030 pieces and then a pre-prepared mixture containing methyl methacrylate ammonium sulfate adduct 0.029 parts nonylphenol and 5 moles of ethylene oxide 0.029 parts The reactor contents are stirred heats up 80 to 85 °C and a mixture of the following ingredients is added in advance: ready butyl acrylate 0.069 parts methyl methacrylate 0.069 parts

The reaction mixture is maintained at 80 to 85 °C for 15 minutes and then a mixture of demineralized water 0.67 parts ammonium persulfate 0.021 parts

The reactor contents are maintained at 80 to 85°C for a further 20 minutes and then a previously prepared mixture of butyl acrylate 10.75 parts methyl methacrylate 10.189 parts allyl methacrylate 0.686 parts ammonium salt of nonylphenol sulfate adduct and 5 moles of ethylene oxide 0.081 parts is fed into the reactor at a constant rate for 3 hours.

During the same three-hour period, a solution of 0.037 parts ammonium persulfate in 4.985 parts demineralized water is simultaneously fed into the reactor at a constant rate.

After the addition of both of the above-mentioned charges is complete, the reactor contents are maintained at 80 to 85°C for 1 hour. Then 34.716 parts of demineralized water are added, the temperature is adjusted back to 80 to 85°C, and the following pre-prepared mixture is added to the mixture at a constant rate over the course of 1 hour:

methacrylic acid 0.950 work butyl acrylate 2,035 work hydroxyethyl acrylate 1,357 work methyl methacrylate 0.950 work ammonium salt of sulfate adduct nonylphenol and 5 moles ethylene oxide 0.028 work

During the same one-hour period, a solution of 0.019 parts ammonium persulfate and 0.016 parts sodium borate in 0.59 parts demineralized water is also simultaneously fed into the reactor at a constant rate.

After the addition of both of the above-mentioned charges, the temperature of the reaction mixture is maintained at 80 to 85 °C for another hour. The mixture is then rapidly cooled. A stable aqueous dispersion of polymer microparticles is obtained. The dispersion has a total non-volatile solids content of 30 % and a non-volatile solids content insoluble in organic solvents of 27 %.

B.

Preparation of the base coating material

The pH of the dispersion obtained in step A is adjusted to 8.0 by adding dimethylaminoethanol and 54.15 parts of this dispersion are introduced into the mixer. Then the following substances are added to the mixer in the following order:

demineralized water 18.91 parts butoxyethanol 8.12 parts aluminum pigment concentrate described in Example 1 (C) 18.02 parts

The mixture is stirred for 1 hour. A base material with the following properties is obtained:

Solids content:

25.8 percent

Apparent viscosity Tja:

0.20 Pa. s at shear stress rate D -- 1 s ^-1

0.02 Pa. s at shear rate D = 10,000 s ^-1

C.

Application of primer and clear coat to substrate

Two coats of the base material containing the metal pigment described in step B (without further dilution) are sprayed onto the metal plate prepared with the base material (primer) and the leveling coating at a temperature of 22°C and a relative humidity of 39%. The material described in step F of example 1 is then applied as the topcoat. The procedure for applying both the base material and the clear topcoat is otherwise identical to the procedure described in example 1, step G.

The results achieved are similar to those achieved in Example 1, Grade G. Comparative Example A

AND.

Preparation of silvery metallic base material without microparticles

Add the following to the blender while stirring:

aluminum paste (metal content 65% j 6.0 parts

2-butoxyethanol 18.7 parts

The batch is stirred for 30 minutes, then hexamethoxymethylmelamine is added in 4.3 parts of polypropylene glycol (average molecular weight

400) 2.9 parts and the mixture is further stirred for 1 hour. Then, with stirring over a period of 30 minutes,

51.2 parts of an aqueous dispersion of acrylic polymer with a solids content of 33.5%, prepared as described in Example 5 C below, and then 16.9 parts of demineralized water.

The method described above produces a base material which has the same pigment to binder ratio, the same hexamethoxymethylmelamine to total non-volatiles ratio and a very similar apparent viscosity ηβ at D = 10,000 ^s_1 (0.03 Pa.s) as the value given for the base material produced by the process according to the invention described in Example 1. The apparent viscosity of the coating material not containing microparticles at high shear rate shows that the material is suitable for spray application onto a substrate. However, its apparent viscosity at low shear rate D = = 1 ^s_1 is only about 0.1 Pa.s, which shows that the material has weak pseudoplastic or thioxotropic properties.

The base material is applied to the plate and a clear acrylic topcoat is applied to the base coating as described in Example 1G. The composition of the clear topcoat corresponds to that given in Examples IE and F. The silvery metallic coating thus obtained has a very low "flip" and shows signs of inhomogeneous pigment distribution and orientation. In addition, blisters form on the coating during firing.

Examples 1 to 4 illustrated the application of the process of the invention to the production of metallic coatings. The following example illustrates its application to coatings pigmented with conventional colorfast pigments.

Example 5

AND.

Preparation of the rubbed white pigment base

The following ingredients are ground together in a ball mill for 16 hours:

titanium dioxide, as pigment 31.3 parts

2-butoxyethanol 18.9 parts demineralized water 18.9 parts dimethylaminoethanol 0.2 parts hexamethoxymethylmelamine 7.7 parts

The resulting rubbing base, which has a particle size below 0.5 μΐη, is then diluted with 11.6 parts 2-butoxyethanol and 11.6 parts demineralized water.

B.

Preparation of the rubbed blue pigment base

The following ingredients are ground together in a ball mill for 16 hours:

phthalocyanine blue pigment 12.9 parts

2-butoxyethanol 23.8 parts demineralized water 23.8 parts dimethylaminoethanol 0.2 parts hexamethoxymethylmelamine 9.7 parts

The resulting rubbing base, which has a particle size below 0.5 µm, is then diluted with 14.8 parts of 2-butoxyethanol and 14.8 parts of demineralized water.

C.

Preparation of an aqueous solution of acrylic polymer

The following ingredients are mixed together:

methyl methacrylate 19.9 parts butyl acrylate 24.8 parts hydroxyethyl methacrylate 2.5 parts acrylic acid 2.5 parts isopropyl alcohol 7 4 parts benzoyl peroxide 0.7 parts

A mixture of 15.0 parts of the above mixture and

42.2 parts of isopropyl alcohol are introduced into a flask equipped with a stirrer, thermometer, reflux condenser and a device for adding liquid charge at a controlled rate. The contents of the flask are heated to reflux temperature (84°C) and the remainder of the above mixture (42.8 parts) is introduced into the flask over a period of 3 hours at a constant rate. The reaction mixture is refluxed for a further 2 hours, whereby a polymer solution containing 5!1.0% solids is obtained. 1.8 parts of dimethylaminoethanol are then added to the solution, the mixture is heated again to reflux temperature, the reflux condenser is switched to descending and a total of 33 parts of distillate are distilled off over a period of 10 hours and 85.0 parts of demineralized water are added. The aqueous solution of acrylic polymer which is finally obtained contains 33.5% solids.

D.

Preparation of blue primer

The following ingredients are mixed in the order listed:

white pigment base rub described in paragraph A 52.25 parts blue pigment base rub described in paragraph B 8.39 parts microparticle dispersion described in Example 1 B 14.62 parts acrylic polymer solution described in paragraph G 24.33 parts

10% aqueous solution of p-toluenesulfonic acid neutralized to pH 7.6 by adding 0.41 part of dimethylaminoethanol

The resulting base coating composition has a viscosity of 1.6 Pa.s at a shear rate of 1 s- ¹ and 0.053 Pa.s at a shear rate of 10,000 s ^-1 .

E.

Application of basecoat and clear topcoat to substrate

The metal plate is provided with a primer base coat, a levelling compound coat and two coats of the blue primer described in paragraph D. The blue primer, used without further dilution, is sprayed onto the surface at a temperature of 22°C and a relative humidity of 39%, with a 2 minute delay between the two coats. After the second coat of primer, the plate is blown with air at 25°C and then two coats of the clear topcoat described in Example 1 F are applied. The clear topcoat is diluted with xylene to a viscosity of 45 s measured according to the British Standard in a B 3 cup at 25°C before spraying. The second coat of topcoat is applied to the still damp first coat, since the delay between the first and second coats is 2 minutes. After applying the second coat, the coating is allowed to dry for 3 minutes and then the board is fired for 30 minutes at 125 to 130 °C.

The resulting coating has good opacity and gloss and there is no bleeding of the clear coating material into the base film.

Comparative example B

AND.

Preparation of a blue base coating material containing no polymer microparticles

The following ingredients are mixed together in the order listed:

white pigment base rub described in Example 5 A 54.8 parts blue pigment base rub described in Example 5 B 8.8 parts hexamethoxymethylmelamine 0.3 parts acrylic polymer solution described in Example 5 C 35.7 parts

101% aqueous solution of p-toluenesulfonic acid neutralized to pH 7.6 by adding 0.4 parts of dimethylaminoethanol

The resulting base material has a viscosity of 0.1 Pa.s at a shear rate of 1 s ^-1 and 0.083 Pa.s at a shear rate of 10,000 s ^_1 , i.e. it has a very weak pseudoplastic or thioxotropic character.

B.

Application of basecoat and clearcoat to substrate

The procedure described in Example 5 E is repeated, except that instead of the primer described in Example 5 D, the primer described in paragraph A above is used. In this case, considerable blistering of the base coat is observed, i.e. the air entrained in the base coat is separated in the form of bubbles which deform the surface of the film and damage its smooth appearance. In addition, longer delays than 3 minutes must be used between the application of the second coat of primer and the first coat of clear topcoat, since otherwise the clear topcoat disrupts the base film, which has a detrimental effect on the final appearance of the board.

Claims (5)

A matrix suitable for producing a multi-layered protective and / or decorative coating on a substrate surface, comprising a film former, a volatile liquid medium therefor and pigment particles dispersed therein, characterized in that the film former and volatile liquid medium system comprises The dispersion of the crosslinked microparticles of acrylic addition polymers in an aqueous medium having a diameter ranging from 0.01 to 10 µm is insoluble in the aqueous medium used and is stable to coarse flocculation, the dispersion having a pseudoplastic or thixotropic character. A matrix according to claim 1, characterized in that the polymeric microparticles are formed from acrylic addition polymers derived in particular from one or more alkyl esters of acrylic or methacrylic acid. 3. The matrix according to claim 1, wherein said matrix comprises a metal pigment and has a non-volatile solids content of less than 30% by weight. and has an apparent viscosity below 0.05 Pa. s at a shear rate of 10,000 s ^-1 and more than 2 Pa. . s at a shear rate of 1 s ^-1 . 4. The matrix of any one of claims 1 and 2, characterized in that it comprises a non-metallic pigment, having a non-volatile solids content below 30% by weight. and has an apparent viscosity below 0.1 Pa. s at a shear rate of 10,000 s ^-1 and more than 0.5 Pa. . s at a shear rate 1 sec ^_1. 5. A matrix according to any one of claims 1 to 4, characterized in that it comprises from 5 to 80% by weight. % of polymeric microparticles, based on the total non-volatile matter content of the mass.