Abstract:
A pearlescent pigment comprising substrate particles having an adherent layer of rutile titanium dioxide thereon is prepared by effecting the deposition of hydrous titanium dioxide on the substrate particles after contacting the particles at elevated pH with titanium tetrachloride.

Description:
BACKGROUND OF THE INVENTION 
     Nacreous or pearlescent pigments (also more recently know as “effect pigments”) which are titanium dioxide coatings on mica or glass or other substrates are well known. The pigments exhibit pearl-like and/or iridescent effects from their reflection and transmission of light. The titanium dioxide coating is actually transparent to light. However, because the coatings are extremely smooth and have a high index of refraction, they follow the laws of thin films. Part of the light which strikes each platelet is reflected and part transmitted to lower platelets where multiple reflections can occur. These multiple reflections from lower layers give a sense of depth which simulates the real pearl. Also, if the thickness of the titanium dioxide layer is controlled, interference of light occurs and the platelets act as optical filters separating light into two components. A color is seen by reflection and a complementary color by transmission. 
     Pearlescent pigments are used extensively in a variety of applications including plastic incorporation, automotive coatings and in cosmetics. The pearlescent pigments which are titanium dioxide coated on a mica substrate have a high index of refraction. The pigments are normally dispersed in mediums such as paint films, or nail enamel films which, when fully cured, have an index of refraction of about 1.5. The index of refraction of the pearlescent pigment must therefore be considerably higher than 1.5 if reflectivity of light is to occur. This high index of refraction is provided by the titanium dioxide layer whose index can vary between about 2.4 and 2.7. The mica substrate on which the titanium dioxide is coated has an index about 1.5 and therefore, does not take part in any reflectivity when incorporated in a film. The index of other substrates is similar. The rutile form of titanium dioxide has a higher index than the anatase form and as a result, the rutile modification will have greater reflectivity than the anatase form. Therefore, the rutile modification of titanium dioxide in a pearlescent pigment is more desirable than the anatase modification. 
     There are many other reasons for preferring the rutile modification. The rutile modification is more stable in outdoor weathering than is the anatase modification. The rutile modification of a titanium dioxide coated substrate results in a product which has better luster and reflectivity, better color and color homogeneity and also contains fewer small particles. In the processing stage during the formation of the titanium dioxide on the substrate, particles which are not attached to the substrate may form. These small particles, which resemble pigmentary TiO 2 , cause light scattering. If too many small particles are present, the pearlescent appearance may be lost or diminished. The process for coating substrate in the rutile crystalline form results in very few small particles compared to the anatase form. 
     The formation of pigmentary TiO 2  in the rutile form is known. In most reported cases, the primary concern is the formation of the rutile structure since coatings are not made on a substrate such as mica. Other considerations which are of primary importance in the formation of high quality pearlescent pigments which include the maintenance of extremely smooth surfaces, uniformity of thickness and color homogeneity are not pertinent. 
     In contrast to the formation of pigmentary TiO 2 , mica is, as well known, anatase directing. Thus, if mica is coated with a layer of hydrous titanium dioxide and then subjected to the normal processing methods which include washing, drying and calcining usually from 750° C. to 900° C., the titanium dioxide which is formed is in the anatase form. The presence of the mica causes the TiO 2  to orient in the anatase crystalline form. Such pigments have been described for example in Quinn et al. U.S. Pat. No. 3,437,515 issued Apr. 8, 1969 and Rieger et al U.S. Pat. No. 3,418,146 issued Dec. 24, 1968 and Linton U.S. Pat. No. 3,087,828. 
     If a rutile crystalline form is desired, the use of additives has been required. Most often, a layer of hydrous tin oxide is first precipitated on the surface of the mica followed by a layer of hydrous titanium dioxide. When this layered combination is processed and calcined, the titanium dioxide is oriented in the rutile form. This is described in detail in U.S. Pat. No. 4,038,099 and also U.S. Pat. No. 4,086,100. Other methods of forming rutile TiO 2  on mica substrates using tin oxide are also described. 
     Although many additives can aid in the formation of rutile TiO 2  per se, the formation of rutile TiO 2  on mica requires a very special additive. The coating of TiO 2  on the mica must be smooth and uniform. If an irregular surface is formed, light scattering takes place and the pigment no longer functions as a pearlescent pigment. The coating of TiO 2  must also adhere strongly to the mica or else the coating of TiO 2  will be separated from the mica during processing, resulting in considerable breakage and loss of luster. It is also necessary that the luster, color and color homogeneity be maintained. Small particle formation must be suppressed. Otherwise, the small particles will scatter light and diminish the pearlescent luster as was mentioned previously. An additive which is used must therefore perform many functions besides being a rutile crystalline director. It has been difficult to find an additive (other than tin) which can orient the TiO 2  to the rutile modification while still maintaining quality and all of the other desirable characteristics. 
     U.S. Pat. No. 5,433,779 teaches that if a small concentration of Fe and one or more of Zn, Ca and Mg ions are introduced into the coating prior to the start of the precipitation of hydrous titanium dioxide on mica, the precipitation proceeds as if a layer of hydrous tin oxide had been added. Complete rutile formation is achieved. 
     U.S. Pat. No. 6,056,815 teaches a process of making rutile titanium dioxide coated mica by first coating the mica platelet substrate with a titanium initiating layer by precipitating a Ti(III)salt solution in the presence of a constantly agitated platelet suspension. The precipitation is preferably accomplished by adjusting the pH to about 2-3, preferably about 2.4-2.8. A pH significantly lower than 2 results in a coating with an anatase structure or, at best, a mixture of anatase and rutile, while a pH significantly higher than 3 is likely to cause agglomeration. Thereafter, hydrous titanium dioxide is precipitated on the Ti(III) layer from a titanium tetrachloride bath. This process requires the use of a Ti(III) compound that is very expensive, sensitive and difficult to handle. The hydrolysis of TiCl 3 , for example, has to be strictly controlled under an inert atmosphere to prevent oxidation and pH control during the TiCl 3  hydrolysis is critical, and additionally because of these considerations, the commencement of the TiCl 4  addition must be done under an inert atmosphere as well. 
     Thus, while there are additives other than tin for forming the rutile modification of titanium dioxide on a substrate such as mica while still maintaining all other desirable characteristics, the use of tin oxide is the most prevalent method used to make in commercial rutile titanium dioxide coated micas. 
     There are, however, two major disadvantages to the use of tin to make rutile TiO 2  coated substrates such as mica. The first is that tin oxide is not permitted in polymer compositions which are to be used in contact with food. Thus, any high quality pearlescent or interference pigment which contains tin oxide cannot be used to color the polymer film. The second is that in some countries, the presence of tin oxide may not be permitted in cosmetic products. Cosmetic manufacturers are therefore faced with a choice of either formulating cosmetic products destined for such countries with anatase only products and having a second line of the same products for the rest of the world formulated with rutile products or having a single anatase product line for the entire world. The result is that polymer formulations in contact with food and cosmetic lines to be used worldwide use anatase products even though the rutile TiO 2 -coated products have better color, color homogeneity and luster. 
     It is therefore the object of this invention to provide a pearlescent pigment of a titanium dioxide coated substrate in which the titanium dioxide is in the rutile crystalline form and in which tin has not been used to promote rutilization. A further object of this invention is to provide a rutile TiO 2 -coated substrate which has the same advantages and characteristics of the tin containing product which includes luster, color, color homogeneity and few small particle formation during manufacture. These and other objects of the invention will become apparent to one of ordinary skill in this art from the following detailed description. 
     SUMMARY OF THE INVENTION 
     This invention relates to high quality TiO 2 -coated micaceous pearlescent pigments. If hydrous titanium dioxide is deposited from a titanium tetrachloride solution onto a micaceous slurry at an initially relatively high pH and thereafter at a relatively low pH, the precipitation of hydrous titanium dioxide on the mica proceeds as if a layer of hydrous tin oxide had been added. Complete rutile formation is achieved. Both the pearl color and also the interference colors are formed that have the same quality and characteristics as the tin oxide-containing counterparts. 
     DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, high quality TiO 2 -coated substrate pearlescent pigments are formed in which the TiO 2  is in the rutile form as a result of the use of TiCl 4  and the use of a rutile directing additive(s) or coating is not necessary. 
     A pearlescent pigment which comprises particles having an adherent layer of rutile titanium dioxide and which does not contain tin or other elements as a rutile promoter is achieved by precipitating the hydrous titanium dioxide on a substrate such as mica from a TiCl 4  bath. The procedure is generally the same as that employed to form a TiO 2 -coated substrate with the exception of pH control. 
     In the coating process, the substrate such as mica is dispersed in water, which is preferably distilled. Although muscovite mica is the preferred mica because of its white color, other micas can be used which include phlogopite, lipidolite or synthetic micas. Other substrates such as glass, kaolin, aluminum, bismuth oxychloride, silica and the like can also be used. 
     The average particle size of the substrate such as mica which is used can vary from an average particle size of about 3 microns to an average particle size of about 100 microns. The concentrations of the substrate in the water can vary from about 5% to 25%. The generally preferred concentration varies between about 10% to 20%. 
     After the substrate is dispersed in the water, it is placed in an appropriate vessel and the pH of the slurry is adjusted using a base such as NaOH to at least 8, more preferably, about 10 or more, and most preferably about 10 to 11. The TiCl 4  is then introduced. 
     The TiCl 4  is employed in the form of a solution in any convenient solvent although an aqueous solution is preferred. The pH of this solution is acidic and it needs to be added to the highly basic slurry such that the pH remains at a value of at least 8, more preferably about 10 or more, and most preferably about 10 to 11. This can be accomplished by metering the acidic solution into the basic slurry at an appropriate rate. If desired, additional base can be added to the combined materials. It is preferred, although not necessary, to conduct the contacting at elevated temperature of about 60-90° C. and preferably about 70-80° C. 
     After a period of time, the pH is then lowered to below about 2, and preferably to about 1.4 to 1.6, for example by addition of a suitable acidic reagent such as a 1:1 HCl:water solution. The period of time can be varied considerably and a suitable period determined by a few brief trial runs. In general, the time will be about 0.25 to 2 hours, and preferably about 0.5 to 1 hour. 
     The pH at which the hydrous titanium dioxide is precipitated is important. Above about pH 2, complete rutilization will not occur. Below that value, it is dependent on the particular system although complete rutilization is formed at about pH 1.6 and a pH below about 1 should be avoided. Generally, the pH should be at least about 1.4. 
     Other than the modifications noted above, the procedure to form the additive free, rutile titanium dioxide-coated substrate pigment is conventional. 
     The products of this invention can find use in, without limitation, the cosmetic, automotive and industrial sectors. For example, in cosmetics these pigments can be used in the lip/eye area and for all external and rinse off applications. Thus, they can be used in lip gloss, lip rouge (cream), lipstick, mascara cake/cream, nail enamel, nail enamel remover, perfume lotion, pre-electric shave lotion, shampoos of all types (gel or liquid), shaving cream (concentrate for aerosol, brushless, lathering), skin glosser stick, skin makeup (liquid, pomade) sunscreen lotion, leg makeup, insect repellent lotion, hair spray, hair groom, face powder (loose/pressed), eye shadow (stick, powder, pomade, liquid, cream, pressed), eye liner, dusting powder, cologne stick, cologne (emollient), cologne, bubble bath, body lotion (moisturizing, cleansing, astringent, analgesic), after-shave lotion and after-bath milk. The pigments of this invention have an unlimited use in all types of automotive (exterior) and industrial coating applications (interior or exterior). These pigments can be used in mass tone or as styling agents to spray paint all types of automotive or non-automotive vehicles. Similarly, they can be used with clay/formica/wood/glass/metal/enamel/ceramic and other non-porous or porous surfaces. They can be incorporated into plastic articles geared for the toy industry or the home. These pigments can be used to improve the look of rubber, vinyl/marble flooring, vinyl siding and all other vinyl products. Also, these colors can be used in modeling hobbies of all types, including ships, airplanes, trains and automobiles. This list of uses is intended to be illustrative rather than limiting. 
    
    
     Various non-limiting examples are set forth below to further illustrate the present invention. In these, as well as throughout the balance of this specification and claims, all parts and percentages are by weight and all temperatures are in degrees Centigrade unless other indicated. 
     EXAMPLES 1-9 
     A coating procedure was adopted in which 200 grams of muscovite mica having an average particle size of about 18 microns (by laser light scattering) were dispersed in 1 liter of water and heated, with stirring at 250-300 rpm, to 74° C. The pH of the slurry was adjusted to 10 using a 35% aqueous sodium hydroxide solution. Then 10 ml of a 50% aqueous TiCl 4  solution was added at 2 ml/min while maintaining the pH constant at 10. Forty minutes after the addition was complete, the pH was lowered to 1.6 using aqueous HCl and additional 50% aqueous TiCl 4  solution was added at 1 ml/min while maintaining the pH constant at 1.6. The titanium introduction was continued until either a white pearl or interference colors which include gold, red, blue and green had been reached. When the desired end point was achieved, the slurry was filtered on a Buchner Funnel and washed with additional water. The coated platelets were then dried and calcined at 850-900° C. Samples were analyzed for the percentage of rutile and anatase that was present in each sample by x-ray diffraction. The results are shown in Table 1. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Color 
                 % Anatase 
                 % Rutile 
               
               
                   
               
             
             
               
                 Pearl 
                 0 
                 100 
               
               
                 Gold 
                 0 
                 100 
               
               
                 Red 
                 0 
                 100 
               
               
                 Blue 
                 0 
                 100 
               
               
                 Green 
                 0 
                 100 
               
               
                   
               
             
          
         
       
     
     For comparison purposes, a number of different pigments were prepared at various thicknesses by the same procedure but without adding the TiCl 4  at pH 10. The x-ray diffraction results are shown in Table 2 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Color 
                 % Anatase 
                 % Rutile 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Pearl 
                 95 
                 5 
               
               
                 Gold 
                 86 
                 14 
               
               
                 Blue 
                 63 
                 37 
               
               
                 Green 
                 60 
                 40 
               
               
                   
               
             
          
         
       
     
     The luster quality of the pigments produced in the foregoing fashion are determined by reflectance measurements made on standard drawdowns on a hiding power chart (Form 2-6 Opacity Charts of The Leneta Company), half of which is black and half of which is white. A coating on the black part of this chart displays the reflection color when it is examined by specular reflection while the coating on the white portion displays the transmission color when it is viewed at an angle which is not equal to the angle of incidence. 
     The standard drawdowns are prepared by suspending 3% pigment in a nitrocellulose lacquer which contains 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Nitrocellulose RS type 15-20 sec. 
                 2.9% 
               
               
                   
                 Nitrocellulose RS type 30-40 sec. 
                 6.6 
               
               
                   
                 Isopropanol 
                 5.1 
               
               
                   
                 Amylacetate 
                 44.8 
               
               
                   
                 n-Butyl acetate 
                 37.6 
               
               
                   
                 Mono-butoxydiethylene glycol 
                 3.0 
               
               
                   
                   
                 100.0% 
               
               
                   
                   
               
             
          
         
       
     
     The two grades of nitrocellulose provide the desired combination of solids content and a viscosity of approximately 2000 centipoises at 25° C. The mono-butoxy-diethylene glycol is used to prevent “blushing” or clouding of the lacquer film by condensation of water vapor from the atmosphere. 
     The drawdowns are made with a Bird film applicator which produces a wet film of approximately 0.003 inch (about 0.008 cm) thickness on the hiding power chart held firmly against a Bird vacuum plate. The spectrophotometric curve of the sample is determined with a Leres Trilac spectrophotometer using an angle of incidence of viewing of 15° to the normal. The reflectance is measured relative to a pressed cake of barium sulfate. Reflectance at the maximum (R max ) and the average reflectance are measurements of pearlescent or nacreous luster. The wavelength at the maximum is an indication of color, although the entire curve is required to describe the color completely. 
     In all cases, the procedure of the invention produces products with quality equal to or exceeding the products which contained Sn. All the products were 100% rutile with no anatase present. 
     EXAMPLES 10-11 
     Example 1 was repeated twice changing the pH of the mica slurry to either 5 or 7. In both cases, the titanium dioxide was found to be a mixture of anatase and rutile. The amount of anatase was greater when the slurry had a pH of 5 
     EXAMPLE 12 
     Example 1 was repeated except that the mica is replaced with the same amount of glass platelets. A rutile TiO 2 -coated glass effect pigment is realized. 
     Various changes and modifications can be made in the products and process of the present invention without departing from the spirit and scope thereof. The various embodiments which have been disclosed herein were set forth for the purpose of illustrating the invention but were not intended to limit it.