WO2014083242A1

WO2014083242A1 - Titanium dioxide pigment

Info

Publication number: WO2014083242A1
Application number: PCT/FI2013/051112
Authority: WO
Inventors: Kaarina HEIKKILÄ
Original assignee: Sachtleben Pigments Oy; Sachtleben Chemie Gmbh
Priority date: 2012-11-28
Filing date: 2013-11-27
Publication date: 2014-06-05
Also published as: FI125473B; EP2925819A1; FI20126250A

Abstract

The present invention relates to a method for processing a titanium dioxide pigment and to said pigments and uses thereof. The method comprises the steps of providing an aqueous suspension of titanium dioxide core particles to an elevated temperature, and introducing a silicon containing compound into said slurry the pH of which is made alkaline, and decreasing the pH of said slurry to a value of about 6 for precipitating the silicon containing layer onto said titanium dioxide core, and introducing subsequently an aluminum containing compound into said slurry the pH of which is acidic, and increasing the pH of said slurry to a neutral value. Subsequently, a phosphate containing compound is introduced to said slurry, the resulting slurry is filtered into a suspension cake, and an organosilane compound is introduced to said suspension and the obtained pigment product is dried. Optionally, phosphate containing compound is added after filtration and washing into silica-alumina coated pigment slurry. Improved colour stability is obtained in polycarbonate plastics plated prepared using above mentioned titanium dioxide pigments.

Description

Titanium dioxide pigment Field of the invention

The present invention relates to a titanium dioxide pigment. It further relates to a method for manufacturing said titanium dioxide pigment and to plastics con- taining said titanium dioxide pigment.

Background Art

Titanium dioxide pigments are used in a number of applications. The most demanding applications from the viewpoint of product requirements are plastics produced at high temperatures and outdoor paint systems.

Engineering plastics are a line of thermoplastics which differ from standard plastics in their mechanical and thermal properties as well as in their chemical stability and optical properties. Examples for engineering plastics are polycarbonate, polyamide, polyester, polyoxymethylene, acrylonitrile-butadiene- styrene, polyethylene terephthalate, polybuthylene terephthalate, acrylonitrile styrene acrylate, styrene acrylonitrile, polymethyl methacrylate, thermoplastic polyurethane as well blends of polycarbonate with above mentioned plastics. The requirements for durability of titanium dioxide pigments in engineering plastics are high contrary to commodity plastics such as polyethylene, polypro- pylene, polyvinyl chloride, polystyrene or polyurethane.

Polycarbonate (PC) is commonly used in electronic components, construction materials, data storage and automotive, aircraft and security components as well as domestic appliances. Polycarbonate is a predominantly amorphous, transparent, hard-elastic plastic with low water absorption. Polycarbonate has useful features such as good mechanical properties, high toughness, temperature resistance and a high heat deflection temperature. Furthermore, it is easy to process. Many and diverse potential applications of polycarbonate confront this polymer with great challenges. The employment of additives and fillers is crucial in order to achieve the desired properties of products.

The requirements for a T1O2 pigment used in engineering plastics, especially in polycarbonate, comprises good optical properties, good processability, thermal and colour stability in the plastic matrix. Most of the commercial pigments offered for use in polycarbonate do not fulfil these requirements, especially the excellent colour stability. In addition, these pigments often are produced using special process steps with high energy consumptions and/or low capacity.

When polycarbonate is coloured with pigments or fillers, the nature of surface coating on the pigment/filler surface plays an important role. Type, thickness and density of the surface coating layer influences meaningfully several important properties including colour stability and optical properties of plastic matrix. Polycarbonate itself has very low water absorption. This is required also of the surface coated T1O2 pigment applied.

A major part of the commercial T1O2 pigments are surface coated with inorganic and organic agents. The reasons for this step are the improvement of dis- persibility, colour stability and hiding power. Those feature in coated T1O2 pigments are clearly better in comparison to uncoated T1O2 ones. In addition, plastics and paints containing surface coated T1O2 pigments have a better colour stability and weather durability than the corresponding systems pigmented with uncoated T1O2 pigments.

Hydrated oxides of silicon and aluminum are commonly used as inorganic coating substances in T1O2 pigments. Also some other chemicals are em- ployed in order to improve applicability of the T1O2 pigments for special purposes.

Titanium dioxide is an attractive pigment material for several applications. However, depending on the ambient conditions this material may suffer from oxidation state changes and/or photochemical reactions providing a greyish or yellowish colour hue.

Degradation reactions take place on the surface of T1O2 pigment in the presence of moisture and oxygen. Even after drying of the pigment/filler and the polycarbonate, OH groups in the surface of T1O2 pigment can cause damages in the polycarbonate during production and/or during its further processing.

The role of titanium dioxide in photochemical degradation is twofold. By absorbing ultraviolet light it protects the polymer matrix. On the other hand due to its photo catalytical nature, chemical reaction can take place between titanium dioxide and polymer resulting in degradation of the matrix. In order to hinder or suppress these undesired changes a suitable coating combination is needed for the titanium dioxide material. In addition, chemical and thermal stability are required from the organic additives used. It is known that the addition of discordant siloxanes can counteract damaging or degrading of the polycarbonate in such cases. The high processing temperatures for the engineering plastics material complicate the colour stability issues. Processability at high temperatures set also requirements on the ΤΊΟ2 pigment.

Enhancing the colour stability may not lead to deterioration of the other desired properties of ΤΊΟ2 pigment applied like hiding power or dispersibility. Further issue is the required mechanical properties of the pigmented plastics material.

It is known practice to improve the colour stability of ΤΊΟ2 pigments by applying an amorphous, dense S1O2 layer and, a further layer of AI2O3. The classical surface coating methods for T1O2 operate in batch mode, in which context an aqueous suspension of T1O2 particles is mixed with a solution of the coating substance in a mixing tank, and the pH value is adjusted in a way that the substance is deposited on the particle surface. Methods of this kind are disclosed, for example, in patent publications US 3,437,502 or EP 0409879. Another surface coating method for T1O2 comprises continuously adding various chemicals into aqueous suspension of T1O2 particles step by step in a pipeline. This kind of method is described in the patent publication US 4,125,412. These publications illustrate a preparation of amorphous, dense silica coating followed by alumina deposition resulting in improved colour stability for T1O2 pigments.

The applicant's previous patent EP 0406194 discloses a process for coating a T1O2 pigment with hydrated oxides of phosphorus, zirconium and aluminum. The process comprises the following steps: (1 ) an acidic hydrolysable titanium compound, (2) a water soluble phosphate and (3) an acidic hydrolysable zirconium are added to the titanium dioxide pigment dispersion. The slurry is made alkaline with base and (4) a water soluble hydrolysable alkaline aluminum compound is added. The alkaline slurry formed is neutralized with an acid and the coated titanium dioxide pigment is recovered after filtration and washing.

US 2009/297852 A1 discloses a method for coating a T1O2 pigment, wherein an aqueous suspension of T1O2 base material is mixed with sodium hexameta- phosphate as dispersant. The suspension obtained is disagglomerated using zirconium dioxide grinding media. Aqueous sodium silicate solution is added to the suspension. The obtained suspension is stirred and after stirring for 30 minutes, sodium silicate solution is added and finally AI2O3 in the form of sodium aluminate solution.

For incorporation in polymers the pigment particle surface is usually additionally treated with an organic substance to improve dispersibility and processability as disclosed, for example, in US 7,01 1 ,703.

US 6,576,052 discloses titanium dioxide pigment wherein titanium dioxide particles are coated with an aluminum phosphate compound and a hydrolysate of an organo-silane compound without essentially increasing the water content of the pigment defined by Karl Fisher method. This pigment is produced from aqueous titanium dioxide slurry whereto first e.g. orthophosphoric acid together with aluminum sulfate was added. Subsequently e.g. hexylmethoxysilane was added. The method includes pH and temperature control during and after additions.

US 2010/01251 17 A1 discloses a surface-treated T1O2 pigment wherein a S1O2 layer, an AI2O3 layer and an organic layer are applied consecutively around the pigment particles. The organic layer contains at least one compound from the group comprising H-siloxanes, silicone oils and organically functionalized pol- ysiloxanes.

Summary of the invention An object of the present invention is to provide a pigmented plastics material which has a stable whiteness and low yellowish shade i.e. excellent colour stability and good mechanical properties.

A further object is to provide a titanium dioxide pigment which is suitable for use in said plastics material and able to provide the enhanced colour perfor- mance and stability.

A yet further object is to provide a process for the production of said titanium dioxide pigment having the improved optical characteristics and compatibility with said plastics material.

The present invention provides a titanium dioxide pigment as set out in claim 1 , and a method for coating titanium dioxide particles in order to produce said pigment as depicted by claim 12. This titanium dioxide pigment is suitable for use in plastics materials. The present invention further provides a plastic including said pigment as depicted by claim 17.

Commercial titanium dioxide pigments designed for demanding applications usually are surface coated with inorganic and organic agents. This is to enhance their dispersability, colour stability and hiding power in varying applications. The performance of the surface coated pigment incorporated into a matrix such as binder depends on both the pigments characteristics and the matrix characteristics resulting in the overall visual appearance and stability thereof. The discolouring of polycarbonate plastics due to production at high temperature is successfully hindered or suppressed using the coating of the present invention on the titanium oxide. It is possible to incorporate large enough quantities of the present pigment into plastics homogenously in order to result in the desired hiding power and colour intensity. The compounded plastics show good processability, high enough thermostability and good mechanical properties. The pigments of the present invention contain reduced amount of OH groups on the pigment surface detrimental to subsequent processing of engineering plastics.

Furthermore, in the coating of the present invention commonly used inorganic chemicals are applied which provides both a safety and an economical advantage compared to more complicated chemicals. The obtained whiteness values measured from the plastics plates are better those obtained using the pigments presently on the market. Advantageously, no post treatments, such as thermal stabilization, are required which decrease energy, production and investment costs and increase capacity.

Moreover, it is possible to manufacture the coating of the present invention in a large industrial scale, especially taking advantage of the suitability for continuous processing, and especially, as an alternative to batch wise processing. A high capacity and throughput can be reached without increasing the pro- cessing equipment size considerably.

The coating of the present invention offers an opportunity to use titanium oxide of varying origin. The versatile solution provided by the inventive coating is able to overcome the fluctuations depending on the inherent properties of the varying titanium dioxide base material. Figures

Figure 1 shows the colour brightness values (L^*) measured from PC plates pigmented with reference pigments (A and C) and with the pigment according to the present invention (B) as discussed in table 7. Figure 2 shows colour tone values (b^*) measured from PC plates pigmented with reference pigments (A and C) and with the pigment according to the present invention (B) as discussed in table 7.

Figure 3 depicts two alternative processing schemas according to the present invention. Detailed description of the invention

The titanium dioxide particles to be coated may originate from any desired manufacturing process. Preferably, the titanium dioxide originates from the commercial sulfate process or from the commercial chloride process. These processes are described in detail in Volume I of Pigment Handbook, publ. John Wiley & Sons (1988) pp. 1 1 -16. The use of the surface coating of the present invention renders the origin of the titanium dioxide particle less relevant and the differences based on process technology may thus be levelled.

In the first aspect of the present invention a method for processing a titanium dioxide pigment is provided comprising the subsequent steps from a to h. The preferred processing schemas are illustrated in figure 3.

In one embodiment the feed of the T1O₂ core particles comprises discharge from the calciner of a sulfate process. In another embodiment the feed of the T1O₂ core particles comprises burner discharge from a chloride process.

In step a an aqueous suspension of titanium dioxide core particles is provided and the suspension is subjected to elevated temperature, preferably to at least 30 °C. Preferably, the titanium dioxide core particles are at least 85%, more preferably 95% and most preferably 98 %, by weight of rutile form. The feed is advantageously wet milled. The aqueous suspension slurry has preferably a concentration from 300 to 1500 g T1O₂/I, more preferably from 500 to 1200, and most preferably 700 - 1000 g ΤΊΟ2/Ι water. The temperature of the slurry is preferably elevated to at least 30 °C, more preferably the elevated temperature is from 30 °C to 100 °C, most preferably from 40 °C to 80 °C, such as from 50 °C to 70 °C. The reactivity of the inorganic surface coating compounds is in- creased by increasing the reaction temperature. However, in a reaction temperature approaching 100 °C the evaporation of water becomes excessive and energy consumption is unfavourably increased. The pH of the titanium dioxide slurry is at this point adjusted to preferably more than 8, more preferably to more than 9. If the increasing pH causes increase in viscosity dispersing agents may further be added into the slurry. Preferably, the dispersants are selected from the group of water soluble silicon compounds, such as sodium metasilicate, and/or amino alcohols, such as monoisopropanolamine. The amount of the dispersing agent is preferably about 0.1 to 0.4 % by weight for the silicon compound calculated as S1O2, or by weight of the amino alcohol. If the T1O2 core originates from the chloride process, typically, there is no need to add dispersants.

In step b a silicon containing compound is introduced into said slurry the pH of which is made alkaline, preferably the pH is at least 1 1 . The silicon containing compound is preferably alkaline water soluble silicate, more preferably an alkali metal silicate, most preferably water glass. This silicon containing compound is preferably added as an aqueous solution into the suspension or slurry of the titanium dioxide particles. The concentration of the silicon containing compound in an aqueous solution is preferably from 50 to 300 g/l calculated as S1O2, more preferably from 50 to 150 g/l.

In step c the pH of said slurry, obtained from step b, is adjusted step by step to a value of preferably about 6 for precipitating the silicon containing layer onto said titanium dioxide core. Adjustment of the pH from the alkaline value, pref- erably to about 6, is performed by an acid, preferably an inorganic acid, such as sulfuric acid (H₂SO₄). Adjusting the pH from an alkaline value, preferably to below about 9.5, leads to precipitation resulting in gel like, dense amorphous silicon oxyhydrate i.e. gelatinous precipitate. After precipitation the pH is further decreased preferably into a value of 6 or less to favour Al solubility, and the slurry is further mixed to ensure a uniform composition of the slurry, preferably for a few minutes, such as 15 min. By the expression "step by step" is meant that at least two different approaches may be used. In one embodiment this process step is performed in a batch type vessel i.e. reactor. In this embodiment the pH is gradually adjusted as a function of time by addition of the reaction chemicals and/or pH controlling agents such as acids or bases. In another embodiment the process step is performed in continuous mode. The slurry is continuously transferred from one vessel to another for gradually changing the pH of the transported slurry at each vessel. Subsequently, in step d an aluminum containing compound is introduced into the slurry from step c. This slurry contains titanium dioxide core particles coated with the amorphous, dense, silicon oxyhydrate layer. The aluminum containing compound to be introduced is preferably an acidic aluminum salt, more preferably a salt selected from the group consisting of aluminum sulfate, alu- minum chloride and alkali aluminate, most preferably this salt is aluminum sulfate. The aluminum containing compound is added as an aqueous solution having a concentration from 50 to 300 g/l calculated as AI2O3, preferably from 50 to 200 g/l. The pH of the aluminum containing compound in aqueous solution is acidic or, if alkali aluminate is introduced, pH is kept at less than 4 by adding acid simultaneously. The addition of the aluminum containing compound results in further decrease of pH of the slurry, preferably the pH of the slurry is decreased to a value of less than 4, more preferably to a value less than 3. The decrease of the pH of the slurry may be assisted by addition of an acid, preferably a mineral acid, most preferably sulfuric acid, if necessary.

In step e the pH of said slurry is increased to a neutral value, preferably to at least 5, more preferably to at least 6, most preferably to at least 7, such as 7.5. But the pH is required to be below 9, preferably below 8.5 in order to avoid dissolution of the aluminum coating. This adjustment is made by addition of an alkali solution, preferably a hydroxide or carbonate solution or mixture thereof, most preferably by addition of a mixture thereof to enhance the favourable structural formation of the Al compound produced. Increasing the pH value of the slurry leads to precipitation of aluminum oxyhydrate onto the surface of the titanium dioxide core particle coated with the silicon oxyhydrate. Optionally, the silicon oxyhydrate and aluminum oxyhydrate coated titanium dioxide particle is washed at this stage (step e^*) for removal of excess soluble species such as sodium or sulfate ions. After precipitation of aluminum containing compound and before the final introduction of the organic silane coating layer a phosphorus containing layer is introduced in between these two layers. The amount of phosphorus is low ena- bling two different treatment approaches as depicted by figure 3 solid and dashed lines comprising steps f and g or f^* and g^*.

In step f, a phosphate containing compound is introduced into said slurry at a pH range from 6 to 8.5. The phosphate containing compound is preferably a water soluble phosphate salt, phosphoric acid or disodium hydrogen phosphate, more preferably an alkaline phosphate salt, most preferably sodium hexametaphosphate. The amount of the phosphate containing compound to be added is small, preferably resulting in P₂O₅ amount less than 3 % by weight of the pigment. This small amount does neither essentially change the pH of the slurry nor provide a disturbing amount of additional salts. Addition of a phosphate containing compound leads to precipitation or formation of aluminum phosphate onto the surface of the titanium dioxide core particle coated with the silicon oxyhydrate and aluminum oxyhydrate. The density of the surface structure is thereby increased. Experimental data has shown a clear change in e.g. oil absorption properties when the surface has been phosphorus treated.

In step g the resulting coating layer is let to mature during an extended mixing period of preferably at least 15 min, more preferably at least 30 min, partially dependent on the volume of the product to be coated. The resulting slurry thus obtained from step f is filtered after step g. The removal of excess water and soluble salts by filtering results in formation of a suspension cake.

Optionally, the filtrate is washed with water to remove excess salts (step g). The filtrated and optionally washed suspension cake preferably has a solids content of more than 20 % by weight, preferably more than 30 %, more preferably more than 35, such as over 40 %. The removal of salts by washing is preferably performed by either consequently washing the filtrate cake for at least 10 times, preferably 15 times with an amount of water comparable with the filtered amount. Or, the amount of salts may be monitored and the washing is guided by the analysis result. In an alternative embodiment for the preparation of ΤΊΟ2 pigment with desired colour stability and durability properties steps a to e are applied as described above. In step f^* the resulting coating layer consisting of dense silica and alumina is let to mature, preferably during an extended mixing period of at least 15 min, more preferably at least 30 min, partially dependent on the volume of the product to be coated. Next, the silica-alumina coated ΤΊΟ2 pigment is filtered and washed. In step g^* the phosphate containing compound is added and after mixing, preferably for 15 min, aluminum oxyhydrate reacts with phosphate containing chemical forming aluminum phosphate. No further filtering or washing steps are needed before drying. See the dashed line in figure 3 for this processing schema.

It is anticipated without being bound by any theory that the precipitated phosphate is reacting with the surface of the aluminum oxyhydrate layer on the par- tides forming a mixed aluminum phosphorus compound and/or aluminum phosphate. Due to the thinness of the formed layers it is difficult to characterize these layers even qualitatively from the multilayered particles formed. Simulation tests have been performed to study the reactions and their outcome. These studies have shown aluminum and phosphorus to react resulting in a precipitate having e.g. different porosity properties than pure aluminum precipitate.

In one embodiment of the present invention the density of the resulting titanium dioxide pigment surface comprising a core particle consisting of titanium dioxide having a multilayer coating thereon comprising a silicon containing compound layer, an aluminum containing compound layer, a phosphorus containing compound layer is increased compared to a titanium dioxide pigment surface without the phosphorus containing compound layer. This density change can be seen in the varying properties of the pigment, such as in filtra- tion and washing times during the production process, and in specific surface area (BET) values, total pore volume and average pore radius of the coated pigment. As the surface of the pigment particles become denser, the filtration time is increased at least 60 %, preferably from 60 to 70 %. The washing time is increased at least 70 %, preferably from 70 to 80%, as depicted in more de- tail by e.g. the results of example 1 . The specific surface area (BET) is decreased at least by 30 %, preferably from 30 to 45 %, more preferably from 35 to 40%. The total pore volume is decreased at least by 45 %, preferably from 45 to 70 %, more preferably from 50 to 65 %. The average pore radius is decreased at least by 25%, preferably from 25 to 35 %, more preferably from 30 to 35 %. The differences become less pronounced when a further organic coating is provided on the pigment particle surface which levels out the density of the coating.

Subsequently, in step h an organosilane compound is introduced. Depending on the previous step organosilane compound is according to one embodiment introduced to said filtrated suspension free of soluble salts. According to an- other embodiment the organosilane compound is added into the suspension if optionally no additional filtration is needed after addition of a phosphate containing compound. The obtained coated titanium dioxide pigment product is dried. Typically, the product is dried using spray or belt dryer. The final pigment product may be ground, preferably by a jet mill with moderate efficiency.

The amounts of silicon, aluminum, phosphorus and organosilane compounds to be used are in comparison with the amount of titanium dioxide pigment preferably from 0.5 to 6 % by weight calculated as S1O2, more preferably from 1 .0 to 4.0 %, most preferably from 1 .5 to 3.0 %, for the silicon containing com- pound; preferably from 0.5 to 6 % by weight calculated as AI2O3, more preferably from 1 .0 to 4.0 %, most preferably from 1 .5 to 3.0 %, for the aluminum containing compound; preferably from 0.1 to 3.0 % by weight calculated as P2O₅, more preferably from 0.15 to 2.5 %, most preferably from 0.2 to 1 .5 %, for the phosphorus containing compound; and preferably from 0.02 to 2.0 % by weight of carbon, more preferably from 0.05 to 1 .5 %, most preferably from 0.1 to 1 .2 %, for the organosilane compound.

The process of the present invention leads to a coated T1O2 pigment having improved colour stability and, optionally, provides better mechanical properties in the application tests compared to the tested commercially available pigments. The process is independent of the process for the titanium dioxide core particles i.e. sulfate or chloride process and the operational mode of the inorganic coating process i.e. continuous or batch surface coating. The inorganic surface coating chemicals are commonly available and the handling of used silicon, aluminium and phosphorous compounds is well known and thus safe. No additional thermal treatment is needed after the coating process. The amount of OH groups is found to be low thus promoting the stability in the pol- ycarbonate application. The manufactured pigment is well suited for use in pigmentation of engineering plastics, especially polycarbonate plastics.

In the second aspect of the present invention a titanium dioxide pigment is provided. Preferably, this pigment is made by the above described method. The titanium dioxide pigment of the present invention comprises a core particle consisting of titanium dioxide having a multilayer coating thereon. This multilayer coating comprises silicon containing compound layer, an aluminum containing compound layer, a phosphorus containing compound layer and an out- er organosilane layer in this order on the surface of said core.

The silicon containing compound layer of the titanium dioxide pigment of the present invention comprises silicon-oxygen compound, preferably amorphous silicon oxyhydroxide. The amount of silicon containing compound is preferably from 0.5 to 6.0 % by weight of said pigment, more preferably from 1 .0 to 4.0 %, most preferably from 1 .5 to 3.0 % calculated as S1O2. The silicon oxyhydroxide is manufactured preferably from water glass by precipitation from the alkaline pH side.

The aluminum containing compound layer of the titanium dioxide pigment of the present invention residing on top of the silicon gel layer comprises an alu- minum-oxygen compound, preferably aluminum oxyhydrate. This aluminum oxyhydrate is preferably manufactured from aluminum sulfate. The total aluminum content of the pigment comprising the aluminum containing compound layer is from 0.5 to 6.0 % by weight of said pigment, more preferably from 1 .0 to 4.0 %, most preferably from 1 .5 to 3.0 % calculated as AI2O3 and referred to the total pigment amount.

The phosphorus containing layer on top of the silicon oxyhydrate and the aluminum containing layer of the titanium dioxide pigment according to the present invention comprises preferably aluminum phosphate.

In one embodiment of the present invention the aluminum containing layer fur- ther comprises aluminum phosphate and/or an alkaline phosphate salt. The phosphate layer deposited after aluminum oxyhydrate layer may at least partly react with the aluminum oxyhydrate layer or at least the surface of said layer. Subsequently, the aluminum containing layer and the phosphorus containing layer may become intermixed at least at the interphase providing a mixed layer and said layers overlap at least partly with each other. As a result preferably at least part of the aluminum containing compound layer is converted into aluminum phosphate during manufacture.

The amount of phosphorus in the titanium dioxide pigment of the present in- vention is preferably from 0.1 to 3.0 % by weight, preferably from 0.2 to 2.5 %, more preferably from 0.3 to 1 .5 %. The phosphorus containing layer is preferably manufactured from sodium hexametaphosphate.

On top of the titanium dioxide core coated with the silicon, aluminum and phosphorus containing layers an organosilane layer is provided. This outer layer comprises preferably siloxane or alkyl silane compound, more preferably an amino-functional oligosiloxane or alkoxy alkyl silane, preferably ethoxy alkyl silane, such as ethoxy octyl silane. The amount of said organosilane layer is preferably from 0.05 to 1 .5% by weight of the pigment calculated as carbon content, preferably from 0.1 to 1 .2 %. The titanium dioxide pigment according to the present invention has a titanium dioxide core which is preferably in rutile form. The core titanium dioxide crystal size (i.e. primary particle size) is preferably from 120 to 700 nm, preferably from 150 to 300 nm, more preferably from 170 to 250 nm. The core titanium dioxide has preferably a specific surface area (BET) value less than 40 m²/g, more preferably less than 25 m²/g, most preferably less than 10 m²/g.

The titanium dioxide pigment according to the present invention has preferably a total pore volume of less than 150 μΙ³^, preferably less than 50 μΙ³^, at the surface. The average pore radius is preferably less than 200 A, more preferably less than 150 A, most preferably less than 100 A. Moisture of the titanium dioxide pigments in accordance to this invention is preferably less than 0.4 %, more preferable less than 0.3 %, most preferable less than 0.25 %.

A following method is applied for measuring the photo activity of titanium dioxide pigments: The pigment sample is exposed to visible light or ultra violet light radiation. Decomposition of gaseous molecules such as isopropanol to acetone and CO2 or intermediates is determined using FTIR. The values of acetone formation for the samples of the present invention T1O2 pigment is less than 150 ppm/h, preferable less than 100 ppm/h and more preferable less than 50 ppm/h.

In the third aspect of the present invention plastics is provided comprising a titanium dioxide pigment according to the present invention as described above or manufactured as described above.

The plastics of the present invention is preferably engineering plastics, more preferably selected from the group of polycarbonate, polyoxymethylene, poly- amide, polyester and acrylonitrile-butadiene-styrene, acrylonitrile-butadiene- styrene, polyethylene terephthalate, polybuthylene terephthalate, acrylonitrile styrene acrylate, styrene acrylonitrile, polymethyl methacrylate and thermoplastic polyurethane. Most preferable, the plastics is polycarbonate or a blend of above mentioned plastics therewith.

The amount of the titanium dioxide pigment is from 1 to 20 % by weight of the pigmented plastics, preferably from 1 .5 to 10 %, more preferably from 1 .5 to 5 %.

In one embodiment the titanium dioxide pigment is included into a plastics, preferably plastics comprising polycarbonate, masterbatch, which may be further processed into various plastics compositions and articles or end products.

A polycarbonate article pigmented with the titanium dioxide pigment according to the present invention shows good optical properties in view of colour stability and whiteness.

In a preferred embodiment the colour tone b^* values of the polycarbonate articles such as plates pigmented with the titanium dioxide pigment according to the present invention are increased less than 8 %, but preferably less than 5 %. An improvement in brightness L^* values of the plates are observed. The decrease in L^* value is less than at least 0.4 %, preferably less than 0.2 %, is obtained. Colour coordinate values of the plates given as x,y,z decreased less than 1 % for x, less than 1 % for y and less than 1 .5 % for z. This indicates a shift to a whiter colour hue compared to commercially available PC plates with pigments or compared to PC plates with pigments without the phosphorus containing layer. An indication for the good mechanical properties PC plates pigmented with various titanium dioxide pigments are attained by the melt flow index test.

QUV - accelerated weathering test - gives information of colour stability of the PC plates various exterior conditions. The fourth aspect of the present invention provides processing of the pigmented plastics.

In one embodiment the plastics, preferably polycarbonate plastics, are first dried in a drier, preferably SOMOS drier, for a few hours, such as 3 h, at a temperature over 100 °C. The desired amount of titanium dioxide pigment of the present invention in ground form is added into the plastics powder and mixed to ensure good uniformity of the pigment throughout the plastics matrix. This mixture is heated in the moulding process to a temperature of at least 250 °C, preferably at least 300 °C, to enable fusion of the plastics. The titanium dioxide pigment is required to withstand the moulding temperature of the plas- tics. The weight ratio of the pigment to plastics may be at least 1 .5:100, preferably 2:100.

In a preferred embodiment finely micronized titanium dioxide powder is mixed less than a minute with polycarbonate plastics and subsequently injection moulded at a temperature of about 300 °C in order to provide a pigmented plate.

The invention is further illustrated by the following non-limiting examples.

Examples

Example 1.

A titanium dioxide pigment was prepared by surface coated titanium dioxide particles using the following sequence.

Titanium dioxide slurry was prepared according to EP 0406194 B2. This preparation method provides rutile titanium dioxide particles having an appropriate particle size and a specific surface area (BET) value of less than 10 m²/g.

The wet milled T1O2 slurry having a concentration of 220 g T1O2/I was intro- duced into a 10 I vessel with a jacket heater. The obtained 1500 g batch of ΤΊΟ2 was heated to 70 °C while continuously stirring with a laboratory stirrer. Mixing was continued during the whole surface coating sequence in order to ensure an even distribution of the treatment chemicals. The pH of the slurry was increased to a value more than 1 1 by addition of 2.1 % by weight calcu- lated as S1O2 of sodium water glass solution. The resulting slurry was mixed for 5 min. Next, pH of the slurry was decreased with sulfuric acid stepwise to 6 with sulfuric acid within 60 min. Next, aluminum sulfate (2.3 % by weight calculated as AI2O3) was introduced and the slurry was mixed for 5 min. The pH of the slurry was less than 2. The pH of the slurry was than increased using NaOH up to 6 whereby aluminum oxyhydroxide was precipitated. Next, water soluble hexametaphosphate, Calgon, 0.5 % by weight calculated as P2O₅, was introduced into the slurry. After mixing for 5 min, pH was adjusted by addition of Na2CO3 into 8. This slurry was mixed for 30 min whereby the phosphorus is reacted with the aluminum coating forming partly aluminum phosphate. The resulting slurry was filtered for 3 - 5 min and washed with 20 litres of ion- exchanged water for 1 - 2 h in order to remove water soluble salts. This results in a suspension with a solids content of about 40 % by weight. Amino functional siloxane compound (several commercial providers, e.g. from Wacker or Momentive) is added into the suspension the amount of which is adjusted by determining the carbon content. A value of 0.4 % of carbon was aimed at. The resulting suspension was dried in an air circulation oven for 20 h at 105 °C. The dried powder was then jet-grounded in a laboratory jet mill using a moderate efficiency.

Comparative Example 1

A pigment without phosphorus was manufacture according to Example 1 whereby no water soluble hexametaphosphate, Calgon, was added during the surface coating.

Table 1 shows selected analyses of silica-alumina coated T1O2 vs. silica- alumina-phosphorus coated T1O2. Table 1 .

Example 2

A titanium dioxide pigment was prepared by modifying the surface coating of ti- tanium dioxide particles using the following sequence.

The silica-alumina surface coated T1O2 slurry having a concentration of 35 w-% was introduced into a 10 I vessel with a jacket heater. The preparation of silica- alumina surface coating was prior carried out according to the sequence presented in Example 1 . A 1500 g batch of T1O2 slurry was heated to 70 °C while continuously stirring with a laboratory stirrer. Mixing was continued during the whole modified surface coating sequence in order to ensure an even distribution of the treatment chemical . Water soluble hexametaphosphate, Calgon (0.5 % by weight calculated as P2O5), was introduced into the slurry having a pH value of 8. The pH value was not adjusted. This slurry was mixed for 30 min whereby the phosphorus is reacted with the Al coating forming partly aluminum phosphate. No filtration or washing step was carried out. Amino functional si- loxane compound is added into the suspension the amount of which is adjusted by determining the carbon content. A value of 0.6 % of carbon was aimed at. The resulting suspension was dried in a rotating oven for 4 h at 150 °C. The dried powder was not jet milled. Comparative Example 2

A pigment without phosphorus was manufacture according to Example 2 whereby no water soluble hexametaphosphate (Calgon) was added.

Table 2 shows selected analyses of silica-alumina coated ΤΊΟ2 vs. silica- alumina-phosphorus coated ΤΊΟ2.

Table 2.

Example 3

The example 3 was prepared according to the same procedure as the example 1 with the exception that no organic additive was used. The suspension was dried in a circulation oven for 4 h at 300 °C. The dried powder was not jet milled.

Comparative sample 3

A pigment without phosphorus was manufactured according to Example 3 whereby no water soluble hexametaphosphate, Calgon, was added during the surface treatment.

Table 3 shows selected analyses of silica-alumina coated T1O2 vs. silica- alumina-phosphorus coated T1O2 when no organic additive is used. Table 3.

Tests and test results

Filtration and washing times Filtration and washing times were determined using the following procedure: Two different kinds of filter papers were wetted and placed into filtering funnels. Suction was put on. From a 10 litre vessel the surface coated pigment slurry was decanted into four filtering funnels. During filtration a cake was built on the filter paper. The time was taken down. This is called a filtration time. Five litre of ion-exchanged water was decanted stepwise into each filtering funnel. When no visible water was available, the time was taken down. This is called a washing time. The results of the filtration and washing times of Example 1 and 2 are presented in table 3. The long filtration and washing times of Example 2 (silica, alumina and alumina-phosphate coated T1O2 pigment) indi- cates a dense coating.

Table 4 shows filtration and washing times of silica-alumina coated T1O2 vs. silica-alumina-phosphorus coated T1O2. Table 4.

Specific surface area

Specific surface of area, total pore volume and average pore radius of T1O2 pigments were determined according to BET (Braunauer-Emmet-Teller) theory using a NOVA 3200 device.

Table 5 shows specific surface area, total pore volume and average pore radius of silica-alumina coated T1O2 vs. silica-alumina-phosphorus coated T1O2.

Table 5.

Specific surface area (BET), total pore volume and average pore radius values in Table 5 show, that the T1O2 pigment prepared with a silica, alumina and alumina-phosphate layers (Example 1 ) provides a denser coating than the reference T1O2 pigment having only a coating consisting of silica and alumina. Therefore the colour stability in polycarbonate matrix of the Example 1 T1O2 pigment is improved. When the phosphorus chemical was added into the silica-alumina coated, filtered and washed slurry without additional washing step (Example 2), the effect on specific surface area of ΤΊΟ2 pigment was about the same.

Table 6 shows specific surface area, total pore volume and average pore radi- us of silica-alumina coated T1O2 vs. silica-alumina-phosphorus coated T1O2 when no organic additive is used.

Table 6.

Specific surface area (BET), total pore volume and average pore radius values in Table 6 show, that the T1O2 pigment prepared with a silica, alumina and alumina-phosphate layers (Example 3) provides a denser coating than the reference T1O2 pigment having only a coating consisting of silica and alumina (Comparative Example 3) .

The result is in line with the results obtained in Example 1 and 2 but the de- crease in BET, total pore volume and average pore radius values is more pronounced without the organic coating.

Optical properties of polycarbonate plates pigmented with Ti0₂

The stability of T1O2 pigments were tested by preparing polycarbonate injection mouldings with a pigmentation level of 2 % by weight. The polycarbonate used was Makrolon 2405 PC from Bayer. Polycarbonate was dried using SOMOS dryer at 120 °C for 3 h. 784.0 g of polycarbonate (98.0 %) and 16.0 g of T1O2 pigment (2.0 %) were mixed using a shaker for 30 s. Plates were pressed using injection moulding equipment Engel ES240/40HL. Optical properties (X, Y Z and Yl as well as L^*, a^* and b^* values) of plates pigmented with ΤΊΟ2 pigments were measured with Hunterlab Ultrascan XE (Table 7 and 8). A polycarbonate plate pigmented with the Comparative sample 3 was included in both test series. Table 7 and figures 1 and 2 show the colour values measured from PC plates pigmented with silica-alumina coated T1O2 (fig. 1 B and 2B) vs. silica-alumina- phosphorus coated T1O2 (fig. 1A and fig. 2A) in comparison to values measured from commercial samples (fig. 1 C and 2C).

Table 7.

X Y Z L* a* I b* Yl

Example 1 81.7 86.5 87.5 94.5 - 0.5 j 3.7 6.2

Comparative example 1 80.7 85.5 86.0 94.1 - 0.5 j 4.0 6.7 commercial sample 79.5 84.0 84.7 93.4 - 0.2 j 3.9 6.9

The results presented in Table 7 demonstrate that the b^* value of polycarbonate plate pigmented with the pigment prepared according to the invention (Example 1 ) is lower e.g. improved in comparison to that of comparative ex- ample 1 and that of Control plate containing commercial titanium pigment (commercial sample). There is also a clear improvement in the L^* value. These results indicate that by using the coating treatment of Example 1 (2.1 % by weight calculated as S1O2, 2.3 % by weight calculated as AI2O3 and 0.5 % by weight calculated as P2O5) gives improved colour stability of polycarbonate plate.

Table 8 shows colour values measured from PC plates pigmented with silica- alumina coated T1O2 vs. silica-alumina-phosphorus coated T1O2. Table 8.

Table 8 similar results of the b^* value of polycarbonate plate pigmented with the pigment prepared according to the invention (Example 2) is lower e.g. im- proved in comparison to that of Comparative example 2. There is also a clear improvement in the L^* value. Improved colour stability of polycarbonate plate can also be achieved by adding 0.5 % by weight calculated as P2O₅ into washed, surface coated pigment with an amount of 2.1 % by weight calculated as S1O2, 2.3 % by weight calculated as AI2O3.

Claims

1 . A titanium dioxide pigment, comprising a core particle consisting of titanium dioxide having a multilayer coating thereon comprising a silicon containing compound layer, an aluminum containing compound layer, a phosphorus containing compound layer and an outer organosilane layer in this order on the surface of said core.

The titanium dioxide pigment according to claim 1 , wherein the silicon containing compound layer comprises silicon-oxygen compound, preferably amorphous silicon oxyhydroxide.

The titanium dioxide pigment according to claim 1 or 2, wherein the amount of the silicon is from 0.5 to 6 % by weight calculated as S1O2 of said pigment, preferably from 1 .0 to 4.0 %, more preferably from 1 .5 to 3.0 %.

4. The titanium dioxide pigment according to any one of the claims 1 -3, wherein the aluminum containing compound layer comprises an aluminum-oxygen compound, preferably aluminum oxyhydrate.

5. The titanium dioxide pigment according to any one of the claims 1 -4, wherein the amount of aluminum is from 0.5 to 6 % by weight calculated as AI2O3 of said pigment, preferably from 1 .0 to 4.0 %, more preferably from 1 .5 to 3.0 %.

6. The titanium dioxide pigment according to any one of the claims 1 -5, wherein the aluminum containing layer further comprises aluminum phosphate and/or an alkaline phosphate salt.

7. The titanium dioxide pigment according to any one of the claims 1 - 6, wherein said aluminum containing compound layer and said phosphorus containing layer at least partly overlap each other.

8. The titanium dioxide pigment according to any one of the claims 1 -7, wherein the phosphorus containing layer comprises aluminum phosphate.

9. The titanium dioxide pigment according to any one of the claims 1 -10, wherein the amount of phosphorus in said pigment is from 0.1 to 3.0 % by weight calculated as P2O₅, preferably from 0.2 to 2.5 %, more pref- erably from 0.3 to 1 .5 %.

10. The titanium dioxide pigment according to any one of claims 1 -9, wherein the organosilane layer comprises siloxane or alkyl silane compound, preferably an amino-functional oligosiloxane or alkoxy alkyl compound.

1 1 . The titanium dioxide pigment according to any one of claims 1 -10, wherein the amount of said organosilane is from 0.05 to 1 .5% by weight calculated as total carbon content of the pigment, preferably from 0.1 to 1 .2 %.

12. A method for processing titanium dioxide pigment, comprising the steps of

a) providing an aqueous suspension of titanium dioxide core particles to an elevated temperature, preferably to at least 30 °C, and

b) introducing a silicon containing compound into said slurry the pH of which is made alkaline, preferably the pH is at least 1 1 , and

c) decreasing the pH of said slurry, preferably to a value of about 6, for precipitating the silicon containing layer onto said titanium dioxide core, and

d) introducing subsequently an aluminum containing compound into said slurry the pH of which is acidic, preferably less than 4, and e) increasing the pH of said slurry to a neutral value, preferably to at least 5, and

f) and g) introducing subsequently a phosphate containing compound to said slurry, preferably at a pH range from 6 to 8.5, and filtering the resulting slurry into a suspension cake, and subsequently

h) introducing an organosilane compound to said suspension, and drying the obtained pigment product.

13. The method according to claim 12, wherein the elevated temperature in step a) is from 40 to 80 °C, preferably from 50 to 70 °C.

14. The method according to claim 12 or 13, wherein the pH in step b) adjusted by addition of water soluble silicate, preferably alkali metal s cate.

15. The method according to any one of the claims 12-14, wherein the pH in step d) is adjusted by addition of an acidic aluminum salt and/or by addition of an acid, preferably sulfuric acid.

16. The method according to any one of the claims 12-15, wherein the pH in step e) is adjusted, preferably with an alkali solution, to a value from 5 to 9.

17. A plastic comprising a titanium dioxide pigment according to any one of the claims 1 -1 1 or a titanium dioxide pigment manufactured by the method of any one of the claims 12-16.

18. The plastic according to claim 17, wherein the plastic is an engineering plastic, preferably polycarbonate.

19. The plastic according to the claims 17-18 wherein the amount of said titanium dioxide pigment is from 1 to 20 % by weight.