Abstract:
The present invention provides a novel manufacturing process for producing a high surface area USP grade titanium dioxide in the nano-anatase form. In a manufacturing method aspect of the present invention, a process of producing a high surface area USP grade TiO 2  nano-anatase base is provided. The method involves steps of: a) phosphorus doping of the titanium hydrate obtained in hydrolysis of a titanium compound; b) drying and calcination of the doped paste, thereby producing a high surface area USP grade titanium dioxide in the anatase crystal form with nano-particles suitable for UV screens and cosmetics.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is generally directed to the manufacturing of USP grade titanium dioxide in the anatase crystal form. More specifically it is directed to a manufacturing process producing high surface area USP grade titanium dioxide anatase nano-particles derived from the process, which are suitable for use in sun screen lotions, cosmetics and products requiring high level of optical transparency to the visible light. 
       BACKGROUND OF THE INVENTION 
       [0002]    Titanium dioxide is known in many different crystal forms but only three of them are commonly found in the nature-rutile, anatas and brookite. 
         [0003]    Nanosized titanium dioxide is a material of an increasing importance in the cosmetic industry. Nano-TiO 2  is perfectly inert and its special characteristics, such as absorption of UV light, make it a desirable material in UV screen lotions and other cosmetic products. It is often used in combination with nano-sized zinc oxide which also absorbs UV light. 
         [0004]    Small particle size of TiO 2  is critical for the optical transparency of the products. Particles above 70 nm in size scatter visible light causing the “white nose” effect. 
         [0005]    While the USP grade nanosized rutile has been used in the cosmetic lotions for over a decade, the use of anatase and brookite has been limited. The thermodynamically stable rutile can be manufactured with the particle size in the required nano-range, meeting the desired USP specifications at the same time. Nano-rutile is produced by several industrial processes, for example by the diluted gas phase chloride process or via titanates. 
         [0006]    Anatase and brookite are thermodynamically unstable. Particles of anatase rapidly sinter, and the particle size distribution is quickly moving outside the nano-range at temperature higher than 600° C. There is a noticeable conversion of nano-anatase into large rutile crystals above this temperature. 
         [0007]    Large particle size and high volume of volatiles are two major technical obstructions that disqualify the anatase crystalline materials from UV screens and other cosmetic applications. The existing USP grade anatase pigment products are heavily sintered into large aggregates of about 300 nm in size. Generally, particles in these products are by one order of magnitude larger than,is the desirable size for the UV screens in cosmetics. Light scattering on these pigmentary particles eliminates the materials from UV screen applications requiring the optical transparency in the visible light. Additionally, they are often contaminated by a fraction of pigmentary rutile. 
         [0008]    Nanosized anatase made by the low-temperature processes contains usually a significant portion of volatile impurities. It is common that materials processed at 500° C. show 5 percent weight loss when heated to 800° C. Materials prepared at 350° C. may have over 20 weight percent of volatiles, while the USP limit is only one weight percent. The USP product must be over 99 percent titanium dioxide, showing less than 1 weight percent loss on the ignition at 800° C. for 2 hours (LOI). 
         [0009]    For these reasons, manufacturing of a pure high surface nano-anatase meeting the USP specifications in the desired quality is not possible. 
         [0010]    An object of the present invention is to provide a manufacturing method for production of USP grade high surface area nano-anatase base. A further object is to provide the USP grade nano-anatase base suitable for sun screens and cosmetic products. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides an economical process for production of a high surface area titanium dioxide USP grade nano-anatase base, which can further be modified and used as the UV filter in the cosmetic products. This product can also be utilized as an UV protective material in applications requiring the optical transparency in the visible light (e.g. epoxy, plastics and resin filler). 
         [0012]    In a method aspect of the present invention, a process of producing a high surface area USP grade titanium dioxide base, consisting of nano-particles in the anatase crystal form, is provided. 
         [0013]    The process involves the steps of: a) introducing phosphorus doping to the wet or dry titanium hydrate [Ti(OH) 4 ], obtained preferably by hydrolysis of titanium sulphate or oxysulphate, wherein the phosphorus doping is phosphoric acid, a mineral acid containing phosphorus in the molecule, an inorganic or organic water soluble phosphate, or an inorganic or organic water soluble compound containing phosphorus in the molecule, wherein the level of introduced phosphorus doping is in the range from 0.1 to 0.5 weight percent on the TiO 2  basis, providing a P-doped intermediate product; b) drying of the P-doped intermediate product and calcination in the temperature range from 600 to 900° C. for a time period 1 to 24 hours, thereby producing a high surface area USP grade nano-anatase base. 
         [0014]    In the product aspect of the present invention, a USP grade nanoanatase base with the specific surface area greater than 20 m 2 /g and smaller than 75 m 2 /g, determined from the nitrogen adsorption and desorption isotherms at 77 K by the BET method, is provided. The USP base weight loss on ignition at 800° C. for 2 hours is smaller than 1 weight percent. The average particle size of the USP base, determined from the X-Ray diffraction crystal domain size using the Scherrer equation, is smaller than 40 nm and greater than 15 nm. 
         [0015]    The USP base is further dispersed by wet milling, surface treated to block the photocatalytic activity of titanium dioxide, and optionally dried and micronized to achieve the desired morphology of the powder product. 
         [0016]    The dispersion of the surface treated product or the dry and optionally micronized powder are re-dispersed in alkyl benzoate, oil, or another organic or aqueous media to make a ready-to-use intermediate product for cosmetics or applications requiring high optical transparency in the visible light. 
     
    
     
       DESCRIPTION OF THE DRAWINGS  
         [0017]      FIG. 1 . is a general flow sheet, showing the steps of one embodiment of the process of the present invention, where the product is the high surface area USP grade nano-anatase base. 
           [0018]      FIG. 2 . is a general flow sheet, showing the steps of another embodiment of the process of the present invention, where the USP grade nano-anatase base is further processed to obtain a desirable intermediate product that can be incorporated into the cosmetic products as an UV screen. 
           [0019]      FIG. 3 . shows a graph describing the original particle size and the specific surface area of the 0.1 weight percent phosphorus doped nano-anatase, produced at different temperatures in the range 500-800° C. It expresses the weight loss on ignition of these materials at 800° C. for 2 hours. 
           [0020]      FIG. 4 . shows a graph describing the particle size development of the 0.45 weight percent phosphorus doped nano-anatase, produced at different temperatures in the range 650-850° C., and their weight loss on ignition at 800° C. for 2 hours. 
           [0021]      FIG. 5 . shows scanning electron microscope images of the nano-anatase USP base materials with the 0.45 weight percent of phosphorus content on the TiO 2  basis, prepared in the temperature range 650 to 850° C. 
           [0022]      FIG. 6 . shows scanning electron microscope images of anatase with different phosphorus content of 0.00; 0.10 and 0.45 weight percent after the LOI test at 800° C. for 2 hours. 
           [0023]      FIG. 7 . shows the X-ray diffraction (XRD) pattern of the USP nano-anatase base doped with 0.3 weight percent of phosphorus on the TiO 2  basis, prepared at 720° C./10 hours. 
           [0024]      FIG. 8 . shows TEM images of the USP nano-anatase base doped with 0.3 weight percent of phosphorus, prepared at 720° C. for 10 hours, before (A) and after (B) wet milling. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Present invention provides a method for economical mass-production of TiO 2  USP grade base consisting of nanoparticles of anatase. The method is particularly suitable for the sulfate process used for the production of the TiO 2  pigment; however, any other source of the titanium hydrate, such as titanium hydrate originating from the hydrolysis of titanium chloride or oxychloride, can be used. 
         [0026]    The method of the present invention is generally described in reference to  FIG. 1 . The flow sheet illustrates manufacturing of the USP nano-anatase base starting with the hydrolysis of titanium oxysulfate, making titanium hydrate ( 1 ), cleaning the titanium hydrate [Ti(OH) 4 ] from the sulfuric acid ( 2 ), iron and other impurities ( 3 ), in the repetitive washing and filtration step ( 10 ), where the titanium hydrate cake is re-pulped several times and washed with diluted sulfuric acid and deionized water. 
         [0027]    Phosphorus doping ( 4 ) is then introduced to the washed titanium hydrate. Usually, the phosphorus doping is uniformly mixed into the clean titanium hydrate paste during the last re-pulping step ( 10 ), producing the P-doped intermediate product. Subsequently this P-doped intermediate product is separated from the slurry ( 20 ). Most often the separation occurs in the last filtration cycle of the step ( 10 ). The step ( 20 ) is often identical with the last cycle of the step ( 10 ). The introduced level of phosphorus is in the range from 0.1 to 0.5 weight percent on the TiO 2  basis. Optimally the phosphorus level is between 0.4 and 0.45 weight percent on the TiO 2  basis. Phosphoric acid is mainly used as the phosphorus doping. 
         [0028]    Doping of a dry titanium hydrate is also possible. 
         [0029]    The P-doped intermediate product is then dried ( 30 ). Drying of the filtered paste of the P-doped intermediate occurs preferably in the first zone of a rotary calciner just before it enters the calcination zone. The P-doped titanium hydrate slurry can also be dried without the previous filtration. In this case, drying is preferably conducted in a spray drying operation. 
         [0030]    The dried P-doped intermediate product is then calcined ( 40 ) in the temperature range of 600-900° C. for the time period 1 to 24 hours producing the high surface area USP nano-anatase base ( 5 ). The specific surface area of the USP base, determined from the adsorption isothermes and calculated by the BET method, is greater than 20 m 2 /g and smaller than 75 m 2 /g. Most often the specific area of the USP base is between 25 and 30 m 2 /g. The average particle size of the individual TiO 2  crystals is greater than 15 nm and smaller than 40 nm, determined from the XRD crystal domain size using the Scherrer equation. Typically the average particle size is the between 20 and 30 nm. 
         [0031]    The flow sheet in  FIG. 2  describes the addition of the phosphorus doping ( 1 ) to the titanium hydrate paste, obtainable by the hydrolysis of titanium oxysulfate, separation ( 10 ) and drying ( 20 ) of the P-doped intermediate, and calcination of the dried product ( 30 ). The calcination occurs in the temperature range from 600 to 900° C. for the time period 1 to 24 hours. Typically the calcination conditions are between 700° C. and 850° C. for 2 to 10 hours. 
         [0032]    The high surface USP nano-anatase base ( 2 ) is further processed in wet milling with the ceramic milling media, such as zirconia beads. Organic or inorganic surfactants are used in order to break the nano-anatase aggregates. The dispersed slurry of mostly individual particles contains less than 5 percent of oversized aggregates larger than 100 nm ( 40 ). The TiO 2  specific surface area significantly increases during the milling procedure. In some cases it can be the double. 
         [0033]    The slurry of milled particles is further surface treated with silica and alumina to block the photocatalytic activity of TiO 2  ( 50 ). The coating is usually achieved by a slow deposition process during co-addition of hydrochloric acid and sodium silicate or sodium aluminate. The coating temperature is slightly below the boiling point of water, typically 80-95° C. 
         [0034]    The surface treatment may involve alkyl silanes in combination with the silica-alumina coating, or without it. Silane surface treatment is often provided during the wet milling of the USP base, where the silane is used as the dispersant. 
         [0035]    The surface treated USP base is then washed to remove all side-products from the surface treatment. Typically, a rotary filter combining the filtering procedure with the washing is used ( 60 ). The washed and filtered product is further dried ( 70 ), micronized ( 80 ) and re-dispersed ( 90 ) in alkyl benzoate, oil, milk or other media to receive a ready-to-use product for cosmetics ( 4 ). 
         [0036]    The micronized product can often be re-dispersed directly in the cosmetic product. The wet paste obtained in the step ( 60 ) can also be directly incorporated into the cosmetic products, such as body milk, by redispersion of the wet coated USP material in the cosmetic composition. 
         [0037]    The following examples illustrate, but do not limit, the present invention. 
       Example 1 
       [0038]    Titanium hydrate paste was treated with 0.1M phosphoric acid to adjust the concentration of phosphorus in the TiO 2  material to 0.1 weight percent. The doped paste was mixed well and then dried. The dried intermediate was subsequently calcined at 500, 600, 700 and 800° C. for 10 hours. The calcined samples were analyzed using the required USP procedures. Further the samples were evaluated for the particle size and the particle size distribution to determine suitability of the materials for the sun screens and applications requiring optical transparency.  FIG. 3  shows a plot with the results. Importantly, materials with the small particle size did not pass the criteria of the weight loss on ignition at 800° C. Materials meeting the USP specifications were too coarse and heavily sintered into large aggregates to suit the cosmetic applications. There was only a narrow window of the calcination conditions between 725-750° C. at the 10 hour calcination time, producing the USP nano-base. 
       Example 2 
       [0039]    Titanium hydrate paste was treated with  1 M phosphoric acid to adjust concentration of phosphorus in the TiO 2  material to 0.45 weight percent. The doped paste was mixed well and dried. The dried intermediate was subsequently calcined at 650, 675, 700, 725, 750, 775, 800 and 850° C. for 10 hours. The calcined samples were analyzed according to the required USP analytical procedures. Further the samples were evaluated for the particle size and the particle size distribution to determine suitability of the materials for sun screen cosmetics and applications requiring optical transparency to the visible light.  FIG. 4  shows a plot with the results. The materials all met the USP specifications. The weight loss on ignition at 800° C. (LOI) was less than 1 weight percent for all materials, including the 650° C. sample. The particle size was optimal for use in transparent cosmetics. The 850° C. material is already fused into coarser aggregates. Corresponding SEM pictures of the products are in  FIG. 5 . The particle size does not change much with the temperature. Shortening of the 850° C. calcination time can produce the USP base with the particle size distribution better suitable for cosmetics. The 650° C. material with the specific surface area of 50 m 2 /g and the particle size around 20 nm represents the lower limit of this production technique. 
         [0040]    The low weight loss effect is not completely understood. The low loss on ignition at 800° C. of materials made at temperatures lower than 700° C. must be somehow associated with the content of phosphorus. 
       Example 3 
       [0041]    Titanium hydrate paste was treated with diluted phosphoric acid to adjust the concentration of phosphorus to 0.3 weight percent in the TiO 2  material. The doped paste was mixed well and then dried. The dried intermediate was subsequently calcined at 500, 600, 700, 725 and 800° C. for 10 hours. The calcined samples were analyzed according to the required USP analytical procedures. Further, the samples were evaluated for the particle size and-particle size distribution to determine suitability of the materials for applications requiring optical transparency to the visible light. Materials calcined above 700° C. all met the USP specifications and their particle size was suitable for their use in cosmetics. Materials calcined at 700° C. and lower did not meet the USP specifications. They did not pass the criteria of the weight loss on the ignition at 800° C.  FIG. 6  shows SEM images of materials prepared in 700° C./10 hour calcination with different levels of phosphorus after the weight loss on ignition at 800° C. for 2 hour test (LOI). The impact of phosphorus on the particle size and aggregation of materials is significant. 
       Example 4 
       [0042]    The dry titanium hydrate paste from the Example 3, doped with 0.3 weight percent of phosphorus, was calcined at 720° C. for 10 hours. The calcined material passed the USP specifications, including LOI.  FIG. 7  shows the XRD pattern of the material. It is phase pure TiO 2 -anatase crystalline material with the average particle size of 27 nm determined from the XRD pattern by the Scherrer calculation. The nano-base was further wet milled with YSZ 0.5 mm milling media for 60 minutes.  FIG. 8A  shows a Transmission Electron Micrograph (TEM) of the material before milling. The USP nano-anatase base forms large, lightly sintered aggregates before milling. These clusters of nano-anatase particles are often one micron in size.  FIG. 8B  shows a TEM image of the dispersed material. The light aggregates were broken apart almost to the primary particles. The size of remaining aggregates was reduced under 50 nm. The specific surface area increased from 30 m 2 /g to 45 m 2 /g. The particle size distribution after milling provided good transparency to the visible light. Surface of the milled material was further treated with silica and alumina coating to eliminate the TiO 2  photocatalytic activity, and the product was further dispersed in water and oil. These products can be directly integrated into the sun screens showing high UV light absorption, excellent optical transparency to the visible light and no harmful photocatalytic activity.