WO1994021733A1

WO1994021733A1 - Pigments having good scattering properties and process for the production thereof

Info

Publication number: WO1994021733A1
Application number: PCT/SE1994/000202
Authority: WO
Inventors: Jan-Erik Otterstedt; Orvar Erik Otterstedt; Per Johan Sterte; Peter Greenwood
Original assignee: Kompopigment Ltd.
Priority date: 1993-03-15
Filing date: 1994-03-11
Publication date: 1994-09-29
Also published as: SE501035C2; AU6293894A; SE9300839L; SE9300839D0

Abstract

The invention relates to a pigment having a high light scattering ability. It is characterized in that it comprises a core having a size exceeding 100 nm which is covered with a sealing layer of titanium dioxide (TiO2) and a film thereon of, for example, amorphous silicic acid. The invention also comprises a method for the production of a pigment having a high light scattering ability which includes stepwise covering of the core particles which are present in the form of a sole with titanium dioxide by hydrolysis of titanium compounds and finally covering by means of, for example, silicic acid from a de-ionized alkali silicate.

Description

TITLE:

PIGMENTS HAVING GOOD SCATTERING PROPERTIES AND PROCESS FOR THE PRODUCTION THEREOF.

TECHNICAL FIELD:

The present invention relates to a pigment having high light scattering ability and relates more specially to such a pigment comprising titanium dioxide and a method for the production of such pigments.

PRIOR ART:

Pigments which are based on titanium dioxide are well known. Titanium dioxide is virtually the dominating white pigment in different applications. Its supreme opacity and covering ability compared to other kinds of white pigments, when it is used in coverings, depend on the high refraction index of the titanium dioxide.

For titanium dioxide pigments as for other pigments the rule exists that full colour strength of the pigment, when it is dispersed, is obtained only with particles being less than 100 nm. If, however, a higher scattering ability of the pigment is desired, the particle size must be appreciably higher and in the range of 300 nm. The properties of the titanium dioxide are such that the pigment has a broad size distribution of the particles with an amount of material which is larger than 300 nm but also a large amount of material below 300 nm. The titanium dioxide also flocculates very easily when it is present in the shape of small particles and when it is used together with water soluble polymers.

Due to the different physical properties of the titanium dioxide, attempts to modify the titanium dioxide pigment have been made so that it shall be more suitable for different applications. Examples thereof are described in for instance, the American patents 3 244 639 and 2 885 366.

TECHNICAL PROBLEM:

Due to the different properties of the titanium dioxide particles the problem exists that it is very difficult to disperse titanium dioxide to a predestinated desired particle size which is thereafter maintained during a succeeding treatment and admixing into a carrier substance. Typical titanium dioxide particles have, as mentioned above, a very large size distribution and they agglomerate very easily which results in that particles of a certain size will not be brought about in a simple way. As pigment particles of small size give high colour strength whereas particles of greater size give higher light scattering it is of course difficult or impossible to attain a pigment having the right particle composition by typical titanium dioxide particles so that a high colour strength or a large scattering ability can be chosen. This means that more titanium dioxide pigment than what is absolutely necessary to obtain the desired colour strength and/or colour distribution must be used.

A further problem with the titanium dioxide is that the surface of the particles catalyses destruction of many binders for white or white pigmented colours, especially under influence of light. THE SOLUTION:

Accordingly, there has existed for a very long time a desire to be able to produce a titanium dioxide pigment having the desired size distribution of the pigment particles with regard to light strength and light scattering ability and which has a surface which does not catalyze the destruction of neighbouring material. According to the invention a pigment has therefore been attained having a large light scattering ability which is characterized in that it comprises a core which does not consist of mica and which is substantially round and has a size exceeding 100 nm and which is covered by a layer of titanium dioxide (Ti0₂) which in turn, is covered by a film of amorphus silicic acid, aluminium silicate or a metal oxide, such as aluminium oxide having a thickness of 1-100 nm, preferably 3-25 nm.

According to the invention the core may consist of silicic acid, organic polymers, kaolin, atapulgite, bδhmite, bayerite or gibbsite.

It is further suitable that, according to the invention, the layer of titanium dioxide has a thickness of 10-100 nm.

The invention also comprises a method for the production of pigments having large light scattering ability and it is characterized in that a solution or dispersion of a titanium compound being hydrolysable in acid environment is added to a sole of colloidal core particles which do not consist of mica and which are substantially ball-shaped and have a particle size exceeding 100 nm, whereby, after completed hydrolysis when the core particles have been covered with a sealing layer of titanium dioxide, a film of an amorphus silicic acid, aluminium silicate or a metal oxide, such as aluminium oxide, having a thickness of 1-100 nm, preferably 3-25 nm, is applied at pH 8-11.

According to the invention the titanium compound used may be an organic compound such as tetra alkyl titanates or tetraphenyl titanates, preferably tetra alkyl titanates containing alkyl chains having less than 18 carbon atoms, preferably less than 5 carbon atoms.

According to the invention inorganic titanium compounds can also be used such as titanium chloride (TiCl₄) whereby HCl formed during the hydrolysis is continuously removed or neutralized. The removal of hydrochloric acid can be carried out by rinsing through a membrane filter.

It is according to the invention advantageous that the film forming material consists of ionized alkali silicate.

To cover the particles layered with titanium dioxide by a film it is according to the invention important that the particles are recharged before the covering with the film.

DETAILED DESCRIPTION:

Pigment particles according to the present invention are preferably ball-shaped and consist of an inner core on which a layer of titanium dioxide is covered. On this layer of colorant a film is arranged. The core consists in preferred cases of silicic acid having a diameter less than 2000 nm. It may also consist of a polymer or some other solid substance. The layer of titanium dioxide has normally a thickness of 5-100 nm whereas the film has a thickness of 1-100 nm, preferably 3-25 nm. The film consists in preferred cases of amorphus silicic acid. Both the core, the titanium dioxide layer and the protecting film will be described below in more detail.

THE CORE:

Cores according to the present invention are primarily solid materials having such properties that they can be dispersed to particles of colloidal size, which means less than 2000 nm, in water. The cores should be solid substances consisting of inorganic or organic materials which are insoluble in water. By the expression "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 the surface of which has been modified by aluminium so that strongly acid negatively charged sites of aluminium silicates are formed in the surface, and particles of aluminium silicate having a narrow size distribution of the particles centred in the range of < 2000 nm dispersed in water.

Other very suitable cores are particles of polymers produced through emulsion polymerization having a narrow distribution of particle size centred in the range of < 1000 nm dispersed in water. Dispersions of this kind are usually called polymer latex when the particle size is in the range of 100-5000 nm. These kind of dispersions of polymer particles have been produced by emulgating monomers to ultra fine droplets by means of emulsifying agents. After completed polymerization, the emulsifying agents remain attached on the surface of the polymer particles. THE LAYER OF TITANIUM DIOXIDE:

According to a preferred embodiment of the invention cores of silica soles which contain water sufficiently for hydrolysing and depositing of hydrolysable titanium compounds on the silicic acid are used. If an organic titanium compound is used it is of less importance whether this is added before or after the addition of a suitable amount of water in the sole. The titanium compound may for example be added to a completely water free organosole and water can thereafter be added to the sole containing the titanium compound for hydrolysing of the titanium compound and covering thereof on the particles of the silicic acid. If for example 1 mole hydrolysable organic titanium compound is added to a water-free silicic acid sole, at least 1 ol water has to be added afterwards for hydrolysing of the titanium compound and fixing of it on the particles of the silicic acid.

The hydrolysable organic titanium compounds which may be used for covering the silicic acid particles may be chosen from a number of commercially available titanium compounds. Hydrolysable organic titanium compounds such as titanium alkoxides, complexis, polymers and salts of organic acids may be used. Titanium compounds containing nitrogen or silica or such having titanium-carbon bounds are also usable. The preferred titanium compounds are titanium esters, especially tetra alkyl or tetra alkyl esters. As examples of tetra substituted esters of titanium tetramethyltitanate, tetraethyltitanate, tetraisopropyl- titanate, tetraethenbutyltitanate and tetraphenyltitanate may be mentioned. Among these especially tetraisobutyl- titanate is preferred.

A list of titanium compounds which can be used as starting materials follows below. ALCOXIDES AND AROXIDES:

Ti(OC₄H₉-sec)₄

Ti(OC₄H₉-tert)₄ Ti(OCH₂CH₂H₉)₄

Ti(OC₅H_π)₄

Ti(OC₅H_n-tert)₄

Ti[OC(CH₃)₂C₂lH₅]₄

(CH₂=CHCH₂0) ₃TiOC₃H,,-iso (CH₂=CHCH₂0)₂Ti(OC₉H_n-iso)₂

( Iso-C₃H,,0) ₃TiOCH₂CH=CH₂

(Iso-C₃H_nO)₂TiOC₃H₇-iso

(Iso-CjHuOJ_jTiOC-H_j

(Iso-C₅H₁₁0)₃TiOC₃H₇ Iso-C₃H_πOTi(OC₃H₇)_g

Ti[OCH(C₂H₃)₂]₄

Ti[OCH(CH₃)C₃H₇-iso]₄

Ti[OCH(CH₃)C₃H₇]4

Ti (OCH₂C₄H₉-sec) ₄ Ti(OC₆H₄CH₃-o)₄

Ti(OC₆H₄CH₃-m)₄

Ti(OC₆H₄CH₃-p)₄

Ti(OC_gH₁₇-iso)₄

Those compounds which are especially preferred are tetra alkyltitanates and tetraphenyltitanates. .Among these especially tetra alkyltitanate containing alkyl chains which are shorter than 18 carbon atoms, preferably shorter than 5 carbon atoms, are preferred.

Although it is a critical feature of the titanium compounds which are to be used according to the present invention that they shall be able to be hydrolysed to be fixed to the silicic acid particles, it is however not necessary that these titanium compounds are only monomer ic. Any titanium compound which can be hydrolysed further can be used. It has been shown that partially hydrolysed organic titanium compounds which can react further and polymerize after the hydrolysing also can be used according to the present invention.

It has been shown that partial prepolymers of titanium compounds such as tetra alkyltitanates can effectively cover the silicic acid particles with titanium dioxide. Prepolymerized tetrabutyltitanatester can for instance be used.

A combination of partially hydrolysed or hydrolysable organic titanium compounds may be used as reactants when the silicic acid particles are covered. The prepolymerized partially hydrolysed titanate esters may be combined with other monomeric hydrolyseable organic titanium compounds and thereafter be covered on the silicic acid particles.

As mentioned above, an acid silicic acid sole is contacted with a hydrolysable organic titanium compound and thereafter heated to 50-100°C. The resulting titanium silicic acid sole is then stable if it is maintained in this acid environment.

It is necessary during the covering reaction that the silicic acid sole and the resulting titanium covered silicic acid is stabilized by an addition of some acid substance which lowers the pH in the sole mixture to below 2. Mineral acids such as hydrochloric acid and nitric acid e t c are preferred. It can also be necessary to add further acid to maintain pH at or below this upper limit of 2. Preferably during the covering operation the silicic acid sole is maintained at a pH from 0,5-2 until the addition of the organic titanium compound is complete. Although a suitable titanium-covered silicic acid sole can be produced by addition of the whole amount of hydrolysable titanium compound in one go followed by heating of the solution in one step, it is preferred that the active titanium compound is divided in 2-4 equal parts. Each such part is added separately to this silicic acid sole and every addition is followed by a heating step. The number of heating steps depends on the number of portions of titanium which are added and which may vary from 2-4.

It is most preferable to add the titanium compound in three parts. Each part is added separately to the silicic acid sole and followed by a heating step which then will be three. Each heating step is carried out at temperatures from 50-100°C, preferably 60-80°C. The duration of the heating may vary from a quarter of an hour to three hours, preferably from half an hour to two hours.

The number of heating steps and corresponding addition portions of the titanium compounds depends on the amount of titanium which is to be covered on the silicic acid particles. It has been shown that long term stable titanium-covered silicic acid soles containing from 5-10% titanium (Ti0₂) based on the weight of the silicic acid (Si0₂) , can be produced by one single heating step. If 10- 20% titanium is wanted, two heating steps are preferred. If finally more than 20% titanium is wanted, at least three heating steps following each addition of equal parts of hydrolysable titanium compound are preferred.

During the additions of the titanium compound it is suitable to vigorously stir the silicic acid sole in a suitable way. The time for the addition of the titanium compound may vary depending on the size of the batch in question. The addition time varies usually from 5-60 minutes. The stirring may be maintained up to one hour after completed addition of the titanium compound, but it is preferred that it is terminated in 1-10 minutes.

The hydrolysable organic titanium compound may be added as such in its natural physical shape or it can be dissolved in a suitable organic solvent in which it is not reactive. The solvent which is to be used as carrier for the titanium compound must be free from every trace of water for avoiding hydrolysis and polymerization of the titanium compound before it comes into contact with the silicic acid sole. Usual solvents such as methanol, ethanol, isopropylalcohol, buthanol and other organic alcohols may be used as solvents. Benzene, toluene, xylene, e t c can also be used.

If the above mentioned method is followed, a titanium covered silicic acid sole can be produced which has in the dispersed phase titanium covered silicic acid particles in an amount of from 1,0-60% of the weight of the final sole. Water or possible organic polar liquid can be used at the beginning of the continuous phase of the silicic acid sole or can be added later in the system. Mixtures of the above mentioned can also be used. Additionally, the final compositions can be concentrated by evaporation of the solvent in known ways.

Also inorganic titanium compounds such as titanium tetrachloride may be used for covering the silicic acid particles. It is then suitable that a dispersion of suitable cores is adjusted, for example silicic acid cores having a size of 200 nm in water, to pH of 0,5-1. The solid content of the dispersion may vary from 5-20 weight%. To this dispersion TiCl₄ is added under vigorous stirring at a speed of 0,10-0,15 g TiCl₄ per hour and per gramme cores. In water having a pH of 0,5-1 TiCl₄ is quickly hydrolysed to titanium dioxide and hydrochloric acid, HC1. It is essential that the HCl is removed at the same speed as it is formed so that pH is maintained at 0,5-1. This can be brought about by filtering the reaction mixture through a membrane filter which does not let 200 nm particles pass through at the same time as water is added to the reaction mixture with the same speed as HCl is removed from the system. Another way to maintain pH in the range of 0,5-1,0 is to neutralize this hydrochloric acid as it is formed by continuous or batch-wise addition of a strong anion- exchanger resin in the shape of hydroxyl ions which is removed by filtering when the addition of TiCl₄ has ceased. Addition of TiCl₄ is interrupted when the desired amount of Ti0₂ has been deposited on the cores.

When depositing Ti0₂ on cores of silicic acid, the first mono layer of Ti0₂ is formed by Ti-O-Si-bonds with the surface. When a mono layer of Ti0₂ has been formed around the particles, the succeeding addition of TiCl₄ results in Ti-O-Ti-bonds until all added Ti-atoms have been consumed.

Electron microscopic studies of the titanium dioxide covered silicic acid sole shows that the sole is not only a mixture of titanium dioxide and silicic acid. The electron microscope shows discrete particles having a diameter equal to the starting silicic acid particles plus the added titanium dioxide. If the soles only contained a mixture of silicic acid and titanium dioxide, there would be an irregular random distribution of differently large sized particles of the two starting materials in the sole. The thickness of the titanium dioxide layer varies between

10 and 100 nm.

The titanium dioxide covered silicic acid sole is finally covered with a film of, for example, silicic acid on the titanium dioxide layer. The charge of the titanium dioxide covered silicic acid particles is positive at a pH of 0,5-2. This charge can be reversed by mixing the titanium dioxide covered silicic acid sole with a precalculated amount of a solution of water glass under vigorous stirring. When the addition is completed the mixture shall have a pH of 8-11. Active silicic acid in the form of de- ionized water glass of pH 2-3 is added to the now negatively charged sole of titanium dioxide covered silicic acid at a temperature of 80-100°C and pH 9-10. pH is maintained at 9-10 by adding either continuously or batchwise diluted sodiumhydroxide or a Na-silicate solution.

A critical step when covering titanium dioxide covered silicic acid sole is the recharging with de-ionized Na water glass. When this has been completed, every particle is surrounded by a thin layer of negatively charged silicic acid. Thereafter, the building of the film occurs in that monomeric silicic acid from the super saturated solution is deposited on and reacts with the first thin negatively charged layer of silicic acid.

Covering of titanium dioxide covered silicic acid soles can also start by raising the pH of the sole from 0,5-2 to 8-11 by addition of sodium hydroxide or a water glass solution. Thereafter, de-ionized Na water glass is added to the titanium dioxide covered sole during stirring in such a way that the temperature is maintained at 80-100°C and pH 8-11 and at such a speed that all added silicic acid is deposited, building a film on the surface of the particles of silicic acid soles covered with titanium dioxide and lasts until a film of the desired thickness has been built up.

An economically advantageous way to produce active silicic acid and to use it according to the present invention is partly to neutralize a soluble silicate such as sodium or potassium silicate with an acid such as sulphur acid or hydrochloric acid in the pH range of 8-11. This acidifying occurs preferably in the presence of the titanium dioxide- covered particles of silicic acid which shall be provided with a film of silicic acid.

Production of active silicic acid can also occur in the absence of the titanium dioxide covered particles of silicic acid provided that the active silicic acid is added to these before they have become inactive by polymerizing to larger particles.

pH in the system should be maintained between 8 and 11. This can be done by addition of suitable proportions of either acid or a base depending on the ratio of alkali to silicic acid, Si0₂ in the alkali silicate used.

The content of alkali metal ions should be kept below 0,3 M to avoid flocculating or coagulation. When active silicic acid is added in the form of freshly de-ionized alkali water glass, a control of the alkali metal ions does not create any problem. When active silicic acid is formed in situ by simultaneously adding acid and alkali metal silicate, it is desirable to use silicate having a ratio as high as possible and, if necessary, to dilute the reaction mixture to suppress the molarity of the alkali metal ions. The content of alkali metal ions can also be maintained below 0,3 M by instead of using an acid using a kation exchanging resin, preferably a strong one in hydrogen ion form which is filtered off when the addition of alkalimetal silicate has ceased.

The temperature of the reaction mixture is preferably maintained between 80 and 100°C. At temperatures substantially below 80°C it can be difficult to build a film at a practical speed. The speed at which the film is formed depends on pH, the content of alkali metal ions, the temperature and the specific surface of the titanium dioxide covered silicic acid particles. The film of silicic acid is formed quickly at pH around 10 in the presence of sodium ions. Active silicic acid forms a film quicker at higher temperatures and the connection between speed and temperature can roughly be estimated by the observation that many reaction speeds are doubled for each increase of the temperature by 10°C in the above mentioned temperature range.

The invention is not limited to the embodiments shown but can be varied in different ways within the scope of the claims.

Claims

CLAIMS :

1. Pigment having high light scattering ability, c h a r a c t e r i z e d in that it comprises a core which does not consist of mica and which has a size exceeding 100 nm and which is covered with a covering layer of titanium dioxide (Ti0₂) which, in turn, is covered with a film of amorphous silicic acid, aluminium silicate or a metal oxide such as aluminium oxide having a thickness of 1-100 nm, preferably 3-25 nm and which is substantially ball shaped.

2. Pigment according to claim 1, c h a r a c t e r i z e d in that the core consists of silicic acid, organic polymers, kaolin, atapulgite, bohmite, bayerite or gibbsite.

3. Pigment according to claim 1, c h a r a c t e r i z e d in that the titanium dioxide layer has a thickness of 10-100 nm.

4. Method for the production of pigment having a high light scattering ability and which is substantially ball shaped according to any of the claims 1-3, c h a a c ¬ t e r i z e d in that a solution or a dispersion of a titanium compound being hydrolysable in acid environment is added to a sole of colloidal core particles which do not consist of mica and which have a particle size exceeding 100 nm, whereby, after performed hydrolysis when the core particles are covered with a covering layer of titanium dioxide, a film of an amorphous silicic acid, aluminium silicate or a metal oxide such as aluminium oxide having a thickness of 1-100 nm, preferably 3-25 nm is applied at pH 8-11.

5. Method according to claim 4, c h a r a c t e r i z e d in that tetra alkyl titanate or tetraphenyltitanate, preferably tetra alkyl titanates containing alkyl chains having less than 18 carbon atoms, more preferably less than 5 carbon atoms, is used as a titanium compound.

6. Method according to claim 4, c h a r a c t e r i z e d in that TiCl₄ is used as a titanium compound whereby HCl formed by the hydrolysis is continuously removed or neutralized.

7. Method according to claim 6, c h a r a c t e r i z e d in that the hydrochloric acid is removed by rinsing through a membrane filter.

8. Method according to any of the claims 4-7, c h a r a c t e r i z e d in that the film forming material consists of de-ionized alkali silicate.

9. Method according to any of the claims 4-8, c h a r a c t e r i z e d in that the particles covered with titanium dioxide are recharged before covering with the film.