SE502708C2 - Process for making pigments with composite structure - Google Patents

Abstract

The invention relates to a method for the production of pigments having a composite structure. It is characterized in that to a dispersion of core particles a solution containing complex forming or precipitating anions and a solution containing complex forming or precipitating cations is added so that a layer of a sparingly soluble complex or salt is formed on the core particles whereupon a protecting film is applied on the particles.

Description

15 20 25 30 502 708 2 Pigments are most often produced by precipitation and crystallization in aqueous solutions. In order to obtain the desired particle size, the pigment usually has to be ground, sieved and sieved. To obtain the highest possible brilliance and luster, contaminants should be thoroughly washed away. For some organic pigments, this means very complicated processes that involve production times of many weeks, even several months.

One method for producing cheap pigments, especially for wall and so-called lime paints, is to precipitate a basic cationic dye on the surface of silicates and aluminum silicates, such as kaolin, zeolites and permutite; DRP 407618, 415203, 416462, 416463, 429484, 472975 and BP 462968. The process for producing this type of pigment involves making a dough from the substrate, eg kaolin clay, and a little water, spraying on an acidic solution, usually acetic acid, of the dye, stir well and then dry the dye immediately.

The main function of the pigment is to dye such end products as ink and paint, plastic products and synthetic fibers.

Organic pigments are usually the most expensive component in the system and it is obvious that they try to obtain the highest color value of the pigments in different areas of use.

Organic pigments, which are usually crystalline, are of molecular size when formed by precipitation in aqueous solution, but the molecules grow rapidly into crystals.

Pigment crystals that form very close to other crystals attract each other and form aggregates. The crystals in the aggregates are held together by van der Waals forces, which act strongest at distances less than 1 nm. Further processing produces agglomerates, which are larger accumulations (even flocculate) of the primary pigment particles, i.e. crystals and aggregates of crystals. 10 15 20 25 30 35 502 708 3 It is well known in the industry that the color strength of a pigment increases with further grinding. It is therefore considered that the pigment strength of a pigment increases as the particle size decreases. It turns out, however, that for some pigments the color strength goes through a maximum and then decreases with decreasing particle size. In order to obtain the highest satisfactory color value of pigments, it is therefore necessary to disperse the pigment particles so that the dispersion consists mainly of primary particles, ie crystals or aggregates of crystals, with optimal particle size, which can vary with the color tone, and a minimum of loosely held agglomerates. or flocculate.

Swedish patent 468090, 9101678-2 describes a pigment with a composite structure. It is characterized in that it comprises a core coated with one or more layers of dyes, each of which in turn is coated with an optically transparent film which is insoluble or sparingly soluble in water or organic solvents.

TECHNICAL PROBLEM: The above-mentioned and other known pigments can in themselves be satisfactory in many contexts; However, they are expensive to manufacture and many have the undesirable property that they can dissolve when washed or bleed into the substrate they are in. It has therefore always been a desire to be able to manufacture the pigments cheaper or to be able to use denx in a lot. small doses while maintaining the desired staining and to be able to produce pigments that in all contexts are washable and blood-free.

THE SOLUTION = By means of the present invention it has been possible to eliminate the disadvantages of the known pigments and to provide a new process for producing a pigment with a new type of structure, a composite pigment which is characterized in that to a dispersion of core particles with a size of up to 2000 nm simultaneously or sequentially set a solution containing a complexing or precipitating anion and a solution containing a complexing or precipitating cation so that a layer of a sparingly soluble complex / salt with a thickness of up to 50 nm is formed on the core particles, after which a film with a thickness of less than 100 nm, preferably less than 10 nm, is applied to the particles by a film-forming material added after the complex coating.

The further features of the present invention are set out in the subclaims which also comprise a pigment prepared by the invention.

Thus, one of the objects of the present invention has been to provide a process for producing a pigment with a high color value at a low material cost.

A further object of the present invention has been to provide a process for producing a pigment consisting of discrete particles having an average diameter up to 2000 nm and having a narrow particle size distribution within this range.

Another object of the invention has been to make a process for producing a pigment which may be in the form of spheres, rods or plates. Yet another object of the invention has been to make a process for producing a pigment which does not bleed either in water or in organic solvents or in various binders. Another object of the invention has been to provide a process for producing pigments whose surface can be modified so that it is either hydrophilic or organophilic.

Another object of the invention has been to produce pigments dispersed in water with a dry content of up to 60% by weight with a simple and cost-effective process.

The core Cores according to the present invention are mainly solid materials with such properties that they can be dispersed into particles of colloidal size, i.e. up to 2000 nm, in water.

The kernels should be solids consisting of inorganic or organic material that is insoluble in water. By "insoluble in water" is meant that at most 0.1% of the material is soluble in water at 25 ° C.

Very suitable as cores according to the present invention are particles of silicic acid, particles of silicic acid whose surface has been modified with aluminum so that strongly acidic negatively charged aluminosilicate seats are formed in the surface, and particles of aluminosilicate with a narrow particle size distribution centered in the range 8-2000 nm. dispersed in water.

Particles of silicic acid dispersed in water, so-called silicic acid sols, have a surface charge of 0 at pH 1.8-2.0, which is the zero charge point of silicic acid. The surface charge which is negative at pH> 2.0 increases monotonically in absolute value with increasing pH.

The surface charge of particles with aluminosilicate seats in the surface also increases in absolute value with pH but has a significant negative charge even at pH 2.

Other very suitable cores are particles of polymers, prepared for example by emulsion polymerization, with a narrow particle size distribution centered in the range 10-1000 nm, dispersed in water. Dispersions of this kind are usually called polymer latex as the particle size is in the range 100-5000 nm. This type of dispersion of polymer particles has been prepared by emulsifying monomers into ultrafine droplets by means of emulsifiers. After completion of the polymerization, the emulsifiers remain attached to the surface of the polymer particles and give them an electric charge which is usually negative.

Other cores that can be used according to the present invention are natural metal silicates such as various clays, such as kaolin, bentonite, attapulgite and halloysite, natural rod-shaped metal silicates such as chrysotile asbestos, palygorskite, crocidolite and. wollastonite, as well as mica-type flake-like silicates such as vermiculite, muscovite, biotite, talc and antigorite. Various finely divided forms of, more or less hydrated, alumina, such as, for example, pseudoboehmite and boehmite, can also be used as cores according to the present invention.

The surface of the cores must be of such a nature that the particles can attach dyes of various kinds in such an amount that their surface can be completely covered. The surface charge of the nuclei is usually negative and becomes more negative with increasing pH. If adsorption of a certain dye requires a positive charge on the surface of the core, this can be achieved by reversing the charge to positive by means of a positive charge substance. Suitable positively charged substances are water-soluble cationic polymers, for example polymers containing quaternary ammonium groups, of low or medium molecular weight; less than 100,000. Other suitable positively charged substances are basic salts of metals such as aluminum and chromium. Examples of such basic salts are basic aluminum chloride, basic chromium chloride, basic chromium sulphate, basic aluminum acetate and basic aluminum formate. Requires another type of dye that the surface of the core is electrically neutral or, for example, only slightly negatively charged to the highest. possible adsorption, this can be achieved by adjusting the pH to correspond to or slightly above the zero charge point for the surface of the core. With particles of silicic acid as nuclei, this means that the pH should be between 2 and 4. If a slightly positive charge is required on the surface of the particles for maximum adsorption, one way of achieving this is to adjust the pH to just below the zero charge point of the surface.

The dye substance Dyes are intensely dyed substances that are used to dye different substrates. The dye is derived from chromophoric groups in the dye molecules. These chromophoric groups can be used to classify dyes. Thus, there are, for example, azo dyes, anthraquinone dyes, sulfur dyes, indigo dyes, triphenylmethane and xanthan dyes, and phthalocyanine dyes.

Very suitable dyes according to the present invention are cationic water-soluble dyes of the type azo, thiazolazo, anthraquinone, triarylmethane, methine, thiazine, oxazine, acridine and quinoline.

Other very suitable dyes are anionic water-soluble dyes such as azo, anthraquinone, triarylmethane, azine, xanthene, ketone imine, nitro, nitroso and quinoline.

Deposition of dye on core In Swedish patent 468090, 9101678-2 it is stated that dye is applied to the core, by; eg one. cationic dye is adsorbed on negatively charged nuclei, eg particles of silicic acid at pH above 8, under equilibrium conditions.

The ratio between the amount of dye on the particles and the amount of dye on the particles and the amount of dye in solution corresponds to a point on the isotherm for adsorption of the dye on silica nuclei. The color strength of the pigment depends on how much dye can be adsorbed on the core.

This amount of dye depends on the strength of the interaction between core and dye, the arrangement of the dye molecules on the surface of the core and on the specific surface of the core particles. The lower the latter, ie the smaller the core particles, the more dye is adsorbed per gram of core.

According to the present invention, the dependence of the dye strength on high equilibrium adsorption of dye on the surface of the core is eliminated by depositing, precipitating the dye on the surface of the core in the form of a sparingly soluble complex compound or a salt.

A cationic organic dye consists of a colored organic cation and a colorless anion, which may be organic or inorganic. Similarly, an organic anionic dye consists of a colored organic anion and a colorless cation, which is often a metal ion. To form a colored complex on the surface of the core, for example, the colored cation of a cationic dye in aqueous solution may be combined with a colored or uncolored anion with which the cation forms a sparingly soluble complex which precipitates and is attached to the surface of the core. Similarly, the colored anion of an anionic dye in aqueous solution can be combined with a colored or colorless cation with which the anion forms a sparingly soluble complex which precipitates and adheres to the surface of the core.

Many cationic organic dyes form in water solution insoluble compounds with anionic organic dyes.

Thus, a cationic organic dye in aqueous solution can be precipitated quantitatively by adding an aqueous solution of an anionic organic dye. When the ratio of the amounts of cationic and anionic dye is in a certain range, the aqueous phase above the colored complex of anionic and cationic dye is very weakly colored by dye corresponding to the solubility of the complex under the experimental conditions.

Other anions that form sparingly soluble complexes with cationic dyes are the anionic species found in alkali silicate solutions of various ratios, anionic water-soluble polymers, and organic colorless anions such as salicylate and naphthoate ions.

Cations other than those found in cationic dyes and which can form sparingly soluble complexes with anionic dyes are the polycations present in aqueous solutions of basic metal salts such as basic aluminum chloride and cationic polymers. Still other cations that form insoluble salts with anionic dyes are ions of earth metals, ie Ca, Ba, Sr, Mg and ions of such metals as AL, Cr, Mn, Fe, Co, Ni, Cu.

To deposit a complex of a cationic dye and / or an anionic dye on the surface of a core, aqueous solutions of the components of the colored complex are added at a carefully controlled rate and with very good stirring to a dispersion of nuclei.

The cores suitably consist of particles of silicic acid, aluminosilicate, alumina, polymer, etc.

The membrane The membrane of the pigment of the present invention consists of metal oxide, preferably amorphous silicic acid or alumina hydrate, which is bonded to the covering layer of dye adsorbed on the surface of the core. The surface of, for example, amorphous silicic acid is dense and hydrated. A film of silicic acid 10 has all the properties associated with amorphous silicic acid.

It has low solubility in water and is insoluble in most organic solvents. The surface of the membrane contains OH groups, so-called silanol groups, and is therefore hydrophilic. Because these groups are reactive, they can be used to: modify the surface of the membrane so that it becomes organophilic. In the same way as for silicic acid sols, the charge of the film is dependent on pH and is negative at pH above 2. The surface of the film can therefore also be made organophilic by adsorbing cationic surfactants or polyelectrolytes. P g; Due to the nature and ability of the skin to modify its surface, pigments of the present invention can be dispersed in water as well as organic solvents.

An alumina hydrate film has all the properties associated with alumina hydrate. It has low solubility in water and is insoluble in most organic solvents. The surface of the membrane contains OH groups and is therefore hydrophilic. In the same way as for colloids. particles of alumina hydrate, the charge of the membrane depends on the pH. At low pH it is positive while at high pH it is negative. In a pH range where the surface of the membrane is positive, it can be made organophilic by adsorbing organic surfactants or polyelectrolytes.

The thickness of the membrane can vary within wide limits. In order to be called a film, i.e. a tightly cohesive layer over the adsorbed layer of dye, the film must consist of at least 5-10 molecular layers of metal oxide, eg silicic acid, SiO2, or alumina hydrate, Al2O; ~ XHZO, which corresponds to a thickness of about 2-4 nm. The main purpose of the film is to protect the layer of dye adsorbed on the core so that the pigment does not bleed when dispersed in various media. With a thickness of the membrane of a few nanometers, the pigment particles according to the present invention will from a chemical point of view be perceived by the environment as particles of silicic acid or alumina hydrate and possess the properties of such particles in terms of solubility, dispersibility, stability. against precipitation or flocking etc.

Application of the film to the adsorbed layer of dye Using silicic acid as the film, it is deposited on the surface of the dye from a supersaturated solution of monomeric silicic acid at a rate which increases with the degree of supersaturation. Since the deposition of silicic acid on the surface of the dye layer is a condensation polymerization reaction, it is catalyzed by hydroxyl ions and deposition always takes place at pH above 8.

The critical step in applying the film is the formation of the first very thin layer of silicic acid on the surface of adsorbed dye. In order for this layer to form, the surface of the adsorbed dye layer must be susceptible to silicic acid. Different dyes are susceptible to silicic acid to varying degrees. Thus, anionic dyes can be made more susceptible to silicic acid by adsorbing polycations of aluminum or chromium or cationic surfactants such as water-soluble polymers containing quaternary ammonium groups on the surface of the dye layer before the deposition of silicic acid begins.

A convenient and practical source of silicic acid that can be used to form a film according to the present invention is alkali water glass, eg sodium water glass of ratio 3.3 (ratio - SiO 2 / Na 2 O). This can be used to apply a film by first making a mixture of water of alkali water glass and particles consisting of cores with adsorbed covering layer of dye. In this case, the concentration of alkali ions must be kept so low that the ions do not cause coagulation or precipitation of the particles. The pH of the mixture is typically between about 11 and 12, preferably between 11.0 and 11.7. Dilute acid, such as hydrochloric acid, is added until the pH drops to 9-10. During the acidification, monomeric silicic acid is formed which forms a film on top of the layer of adsorbed dye. The rate of addition of acid must be so low that colloidal silicic acid does not have time to form. The maximum rate of addition of acid depends on the temperature and the surface of the particles, ie the surface of the nuclei plus the adsorbed layer of dye, in the mixture.

So-called "active silicic acid", i.e. a decationised solution of 3.3 ratio sodium water glass with a SiO2 content of 3-5% by weight, can also be used to apply a film. The small particles of SiOU with a diameter around Zrmp in the eluate from the ion exchanger have a relatively high solubility and when they are suddenly introduced into a system consisting of a slurry of nuclei with adsorbed layer of dye at pH 8-10 the system becomes supersaturated with monomeric silicic acid which is deposited on the surface of the particles, i.e. on the layer of dye. Addition of active silicic acid must take place so slowly that the monomeric silicic acid formed has time to diffuse to adjacent particles and be deposited on their surface instead of forming silicic acid sprouts which would rapidly grow into colloidal size silicic acid particles. The maximum rate of addition of active silicic acid depends on the pH, temperature and surface area of the particles, i.e. the surface area of the nuclei plus the adsorbed layer of the dye, in the sludge.

Using alumina hydrate as time, it is deposited on the surface of the dye by simultaneously adding to a dispersion of dyes coated with dye according to the present invention solutions of an aluminum salt and alkali so that the pH of the system is such that alumina hydrate precipitates effectively on the dye. A convenient and practical source of alumina hydrate that can be used to form a film of the present invention is so-called basic aluminum salt, for example basic aluminum chloride of the empirical formula Al 2 (OH) 5 Cl. This can be used to apply a film by first making a solution of basic aluminum chloride in water. If this solution is added to a dispersion of dyes coated with dye at the same time as a solution of alkali, eg NaOH, so that the pH of the mixture becomes such that alumina hydrate precipitates effectively on the surface of the dye layer, then a dense, impermeable alumina hydrate film is formed.

Characterization methods Two factors that greatly affect the properties of pigments according to the present invention are particle size and particle size distribution. For measuring particle size and particle size distribution, we have used the instrument Malvern Z-sizer IIc (Malvern Instruments Ltd.), which measures these properties by means of dynamic scattering of laser light.

A prerequisite for a colloidal suspension to be stable is that the particles repel each other by having the same and sufficiently large electrostatic charge on the surface. With decreasing charge, the risk of aggregation through coagulation increases. The magnitude and sign of the charge at the effective interface between particles and surrounding medium, the z-potential, for pigments and cores of the present invention were also measured with the Malvern Z-sizer IIc instrument (Malvern Instruments Ltd.). Prior to measurement, the particles were dispersed in a 10 mM solution of NaCl.

The Z potential is defined as the electric potential of the electric bilayer around a particle at a distance equal to the thickness of the immobile liquid layer around the particle. Products made according to the present invention have also been studied by scanning electron microscopy (SEM). This technology provides valuable information about the appearance of the pigment particles, aggregation and the possible presence of particulate by-products. For analysis with SEM, we have used a JEOL JSM-5200 Scanning Electron Microscope.

The term "composite structure" refers to a particulate matter which is composed of several layers.

Claims (7)

10 15 20 25 30 502 708 15 Process for the production of pigments with a composite structure, characterized in that a solution containing a complexing or precipitating anion and a solution containing a complexing or precipitating cation are added to a dispersion of core particles with a size of up to 2000 nm simultaneously or sequentially so that a layer of a sparingly soluble complex / salt with a thickness of up to 50 nm is formed on the core particles, after which a protective film with a thickness of less than 100 nm, preferably below 10 nm is applied to the particles of a film-forming material added after the complex coating. Process according to Claim 1, characterized in that the complex-forming or precipitating cations consist of cations in organic dyes, cations in various metal salts, e.g. Al, Cr, Cu, Mn, Fe, Co, Ni, polycations in basic metal salts e.g. aluminum salts, and cationic polymers, as well as ions of earth metals such as Ca, Ba, Sr, Mg. 3. Process according to claim 1, characterized in that the complexing or precipitating anions consist of anions in anionic organic dyes, anionic species in alkali silicates, anionic water-soluble polymers, organic anions in organic salts, organic colorless anions such as 1: n salicylate anions. 4. A method according to claim 1, characterized in that the protective film consists of silicic acid, alumina, aluminosilicate or an organic polymer. 10 15 20 502 708 16 5. A method according to any one of claims 1-4, characterized in that the membrane-forming material consists of alkali silicate and that the membrane precipitates on the particles by lowering the pH of the mixture after the addition of the alkali silicate from about 11-12 to about 9-10. by the addition of acid. 6. A process according to any one of claims 1-5, characterized in that the membrane-forming material consists of deionized alkali silicate and that the membrane is deposited on the particles by deionized alkali silicate and alkali hydroxide simultaneously being added to the dispersion of particles at such rates that pH is kept at 8-1, preferably 9-10. 7. A method according to any one of claims 1-6, characterized in that the membrane-forming material consists of an aluminum salt and that the membrane is deposited on the particles by adding aluminum salt and alkali hydroxide simultaneously or sequentially to the dispersion of the particles at such rates that the pH is maintained at such a value that alumina hydrate precipitates effectively on the dye.