Abstract:
A pearlescent pigment consists essentially of C glass flakes having a layer comprising rutile titanium dioxide or iron oxide thereon. A hydrous layer of the rutile forming titanium dioxide or iron oxide is formed on the glass flakes and the resulting coated flakes are calcined.

Description:
BACKGROUND OF THE INVENTION 
     Imparting a pearlescent luster, metallic luster and/or multi-color effects approaching iridescent can be achieved using a nacreous or pearlescent pigment which comprises a metal oxide-coated platelet. These pigments were first described in U.S. Pat. Nos. 3,087,828 and 3,087,829 and a description of their properties can be found in the Pigment Handbook, Vol. I, Second Edition, pp. 829-858, John Wiley &amp; Sons, N.Y. 1988. 
     The oxide coating is in the form of a thin film deposited on the surfaces of the platelet. The oxide in most wide spread use at present is titanium dioxide. The next most prevalent is iron oxide while other usable oxides include tin, chromium and zirconium oxides as well as mixtures or combinations of oxides. 
     The coating of the metal oxide on the platelet must be smooth and uniform in order to achieve the optimum pearlescent appearance. If an irregular surface is formed, light scattering occurs, and the coated platelet will no longer function as a pearlescent pigment. The metal oxide coating must also adhere strongly to the platelet or else the coating will be separated during processing, resulting in considerable breakage and loss of luster. 
     During the preparation of these coatings on the platelets, particles which are not attached to the platelet may form. These small particles cause light scattering and impart opacity to the pigment. If too many small particles are present, the pearlescent appearance may be reduced or lost. The addition of these metal oxide coatings to a platelet so that the luster, color and color homogeneity are maintained is a very complex process, and to date, the only platy substrate which has achieved any significant use in commerce is mica. 
     A wide variety of other platy materials have been proposed for use as a substrate for forming these pearlescent pigments. These include non-soluble inorganic materials such as glass, enamel, china, porcelain, natural stones or other silicaceous substances, metal objects and surfaces of organic polymer materials such as polycarbonate. See, e.g., U.S. Pat. Nos. 3,123,485, 3,219,734, 3,616,100, 3,444,987, 4,552,593 and 4,735,869. While glass has been mentioned as a possibility on many occasions, for instance in U.S. Pat. No. 3,331,699, commercial pearlescent products are not made using glass and experience has shown that products made using glass as the platelet substrate have rather poor quality. 
     Said U.S. Pat. No. 3,331,699 discloses that glass flakes may be coated with a translucent layer of particles of a metal oxide having a high index of refraction, such as titanium dioxide, provided there is first deposited on the glass flakes a nucleating substance which is insoluble in the acidic solution from which the translucent layer of metal oxide is deposited. The patent teaches that the nature of the glass is not critical, but that the presence of the nucleated surface is critical. It is further stated that there are only a small number of metal oxide compounds which are insoluble in the acidic solution and capable of forming a nucleated surface on the glass flakes; tin oxide and a fibrous boehmite form of alumina monohydrate are the only two such materials disclosed. As demonstrated in the examples below, products prepared according to the teachings of this patent are poor in quality. 
     U.S. Pat. No. 5,436,077 teaches a glass flake substrate which has a metal covering layer on which is formed a dense protective covering layer of a metal oxide such as a titanium dioxide. In this patent, the nature of the glass is unimportant, the metallic coating provides the desired appearance and the overcoating of the metal oxide is present to protect the metallic layer from corrosive environments. 
     It has now been determined that there is a method for preparing smooth, uniform coatings of metal oxides on glass flakes which adhere to the glass flakes to yield high quality pearlescent pigments and it is accordingly the object of the present invention to provide such a method and to provide such metal oxide coated glass flake pearlescent pigments which result from that method. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a pearlescent pigment and to a method for the production of such a pigment. The resulting pigment can be used in any application for which pearlescent pigments have been heretofore used such as, for example, in cosmetics, plastics, inks and coatings including solvent or waterborne automotive paint systems. 
    
    
     DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, a pearlescent pigment is formed by establishing a hydrous layer of titanium and/or iron oxides on glass flakes and thereafter calcining the coated flakes provided that the glass flakes employed are C glass flakes and when the hydrous layer is titanium, the procedure is a rutilizing procedure. 
     Glass flakes are desirable in the industry because they are very resilient and can be optically attractive as well. The glass is primarily composed of SiO 2  and Al 2  O 3  and can also include ZnO, CaO, NaO, B 2  O 3 , Na 2  O and K 2  O as well as FeO and Fe 2  O 3 . The glass flakes are made by stretching a molten glass into thin sheets, beads or glass tubes followed by crushing the glass into flakes. The flakes have a size and shape mimicking the mica platelets used in the TiO 2  and Fe 2  O 3  -coated mica pearlescent pigments and thus have an average particle size in the range of about 1 to 150 microns and a thickness of about 0.1-10 microns. 
     Glass can be classified as A glass, C glass or E glass. The A glass is a soda-lime glass and is commonly used to make windows. It contains more sodium than potassium and also contains calcium oxide. C glass, also known as chemical glass, is a form of glass which is resistant to corrosion by acid and moisture. It contains zinc oxide which makes the flakes more resistant to chemical destruction. E glass or electrical glass is, as the name implies, designed for electronic applications and although it is very stable at high temperatures, it can be susceptible to chemical attack. The following table shows the composition of several commercial samples of A, C and E glasses in weight percent. 
     
                       TABLE 1______________________________________       A      C          E     EType        Glass  Glass      Glass Glass______________________________________SiO.sub.2   72.5   65-70      52-56 52.5Al.sub.2 O.sub.3       0.4    2-6        12-16 14.5CaO         9.8    4-9        20-25 22.5MgO         3.3    0-5        0-5   1.2B.sub.2 O.sub.3       0.0    2-7         5-10 8.6Na.sub.2 +K.sub.2 O       5.8     9-13      &lt;0.8  &lt;0.5ZnO         --     1-6        --    --FeO/Fe.sub.2 O.sub.3       0.2    --         --    0.2______________________________________ 
    
     In the practice of the present invention, the C or chemical type glass is preferred. While metal oxide coatings an A or E glass can be prepared, the resulting pigments do not have the quality of the products as C glass and hence have limited commercial value. When TiO 2  coated products are prepared, anatase or rutile crystal modifications are possible. The highest quality and most stable pearlescent pigments are obtained when the TiO 2  is in the rutile form. Also the glass used can influence the crystal form of the titanium dioxide coating. For instance, when E glass is used, the resulting crystal phase is primarily anatase. In order to obtain rutile, an additive must be used which can direct the TiO 2  to the rutile modification. 
     The coating of the glass flakes with titanium dioxide or iron oxide generally follows procedures known in the art for the formation of TiO 2  -coated or iron oxide-coated mica. Mica is anatase directing and, as noted earlier, most glass also appears to direct titanium dioxide coatings to the anatase crystalline form. At least some rutile formation is necessary to obtain higher quality and more stable products. 
     In general, the procedure involves the dispersing of the glass flake particulate and combining that dispersion with a precursor which forms a hydrous titanium oxide or iron oxide coating on the flakes. 
     In the coating process, the glass flakes are dispersed in water, which is preferably distilled. The average particle size which is preferably used can vary from an average of about 3 microns to an average of about 100 microns although larger flakes can also be used if so desired. The concentration of the glass flake in water can vary from about 5% to 30% although the generally preferred concentration varies between about 10% to 20%. 
     After the glass is dispersed in the water and placed in an appropriate vessel, the appropriate titanium or iron source materials are introduced. The pH of the resulting dispersion is maintained at an appropriate level during the addition of the titanium or iron by use of a suitable base such as sodium hydroxide to cause precipitation of the hydrous titanium dioxide or hydrous iron oxide on the glass flakes. An aqueous acid such as hydrochloric acid can be used for adjusting the pH. The coated platelets can, if desired, be washed and dried before being calcined to the final pearlescent pigment. 
     The source of the iron is preferably ferric chloride although any other iron source known in the prior art can be employed. The source of the titanium is preferably titanium tetrachloride although, similarly, other sources known in the art can be employed. If desired, layers of titanium and iron can be deposited sequentially. 
     In the case of titanium dioxide, the modifications of the foregoing procedure to realize a rutilization procedure are known in the prior art. In one procedure, a layer of hydrous tin oxide is first precipitated on the surface of the glass flakes followed by the layer of hydrous titanium dioxide. When this layered combination is processed and calcined, the titanium dioxide is oriented in the rutile form. The procedure is described in detail in U.S. Pat. Nos. 4,038,099, which is incorporated herein by reference. An alternate procedure involves the deposition of the hydrous titanium dioxide on the glass flakes in the presence of iron and calcium, magnesium and/or zinc ions without the use of tin. This is described in detail in U.S. Pat. No. 5,433,779, the disclosure of which is hereby incorporated by reference. 
     In order to further illustrate the invention, various non-limiting examples are set forth below. 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 otherwise indicated. 
     EXAMPLES 1-4 
     A coating procedure was adopted in which 100 grams of C glass flakes (RCF-140 from Nippon Sheet Glass) having an average particle size of about 140 microns (by laser light scattering) were dispersed in 750 ml of water. Iron and zinc were introduced in the form of 1 ml of a 39% aqueous solution of ferric chloride and 7 ml of a 9% aqueous zinc chloride solution. The pH of the slurry was adjusted to 3.0 using a 35% aqueous sodium hydroxide solution and the slurry heated to a temperature of 76° C. The pH was then lowered to 1.6 by addition of hydrochloric acid and a 40% aqueous solution of titanium tetrachloride was added at a rate of 100 ml/hour while the pH was maintained at 1.6 by the addition of 35% aqueous sodium hydroxide. The titanium introduction was continued until an appearance of either a white pearl or the interference colors gold, red and blue 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 600° C. 
     Microscopy evaluation of the resulting pigment confirmed that the platelets are coated with a smooth homogeneous layer of TiO 2 . 
     In addition, the luster and color of the resulting pigments were evaluated visually and instrumentally using drawdowns on a hiding 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 and luster when it is examined specularly, while the coating on the white portion displays the transmission color when it is viewed at non specular angles. The drawdowns were prepared by incorporating 12% pigment in a nitrocellulose lacquer and applying this suspension to the black and white chart with a Bird film applicator bar. 
     When these cards were examined visually, pearlescent pigments with good luster and color intensity were observed. The appearance characteristics of these pigments were further characterized by determining the wavelength at which the reflectivity is a maximum and a minimum and the color described in terms of L*a*b*. The L*a*b* data characterizes the appearance of a product in terms of its lightness-darkness component symbolized by L*, a red-green component represented by a* and a yellow-blue component symbolized by b*. These measurements were made by using a goniospectrophotometer (GK-111 from Datacolor, Inc.). 
     Besides the appearance measurements, three of the pigments were also analyzed for the percentage of rutile and anatase that was present in each sample by x-ray diffraction. These results are all summarized in Table II. 
     
                       TABLE 2______________________________________          Crystal                 GK-111 12% Black 45/0Example  COLOR   λmax                 λmin                      Form   L*    a*   b*______________________________________2      Gold    570    700  Rutile 88.64 6.73 1.683      Red     630    540  Rutile 89.25 17.52                                        -5.784      Blue    670    570  Rutile 85.03 -3.32                                        -3.95______________________________________ 
    
     In all cases, a high-quality, high-luster rutile titanium dioxide-coated glass flake pearlescent pigment was obtained. 
     EXAMPLE 5-11 
     One hundred grams of C glass flakes having an average particle size of 140 μm (RCF-140 from Nippon Sheet Glass) were dispersed in 333 ml of distilled water. This dispersion was heated to 74° C., and the pH adjusted to 1.6 using dilute Hydrochloric acid. Then 7 ml of an 18% Stannous Chloride solution were slowly added. After the addition of the tin, a 40% aqueous solution of titanium tetrachloride was added at a rate of 100 ml/hr. The pH was maintained at 1.6 during the addition of the tin and titania by simultaneously adding a dilute aqueous solution of sodium hydroxide. The titania addition was continued until either a white pearl, interference colored gold, red, blue or green were observed. When the desired end point was reached, the slurry was filtered, washed with additional water and calcined at 600° C. 
     The resulting products were examined microscopically to verify that the TiO 2  was attached to the glass flakes in the form of a smooth homogeneous layer. When drawn down, a series of vibrant, high quality colors were observed. 
     The color data and X-ray diffraction results for these products are summarized in Table 3. A high quality, high luster, rutile titanium dioxide coated glass flake pearlescent pigment was obtained in each case. 
     
                       TABLE 3______________________________________                GK-111          λmax.                  λmin.                        Crystal                              ResultsExample  Color   (nm)    (nm)  Form  L*   a*   b*______________________________________5      Pearl   400     700   Rutile                              73.55                                   -1.28                                        -2.596      Gold    630     700   Rutile                              61.17                                   0.69 13.057      Orange  670     470   Rutile                              59.75                                   8.00 10.278      Red     700     490   Rutile                              60.47                                   8.95 2.859      Violet  400     520   Rutile                              61.37                                   12.04                                        -8.8610     Blue    440     580   Rutile                              60.09                                   0.96 -9.9511     Green   500     670   Rutile                              60.06                                   -9.44                                        0.41______________________________________ 
    
     EXAMPLES 12-20 
     75 grams of C-glass flakes having an average size of 100 μm were dispersed in 300 ml of distilled water. The dispersion was heated to 76° C. and the pH adjusted to 3.2 with dilute hydrochloric acid. A ferric chloride solution was added to the suspension at 0.2 ml/min while maintaining the pH at 3.2 using dilute sodium hydroxide. The ferric chloride addition was continued until the desired color was observed. At the appropriate end point, the slurry was filtered, washed with water and calcined at 600° C. to yield an Fe 2  O 3  coated glass flakes. 
     The resulting products were examined microscopically verifying that the Fe 2  O 3  is attached to the glass flakes as a smooth, homogeneous coating. 
     Since Fe 2  O 3  has an inherent red color, glass flakes coated with this oxide have both a reflection color and an absorption color. The interference color is from the interference of light, while the absorption color is due to the absorption of light. The reflection color will change from gold to red to blue to green as increasing amounts of iron (III) oxide are coated on the glass flakes. As more iron (III) oxide is added, even thicker coatings of Fe 2  O 3  are obtained which yield another series of interference colors known as the second observable interference colors. These second colors have even higher color intensity than the first colors along with higher coverage. If the coating process is continued even further, a third series of interference colors can be obtained. 
     When these iron oxide coated glass flakes were drawn down, a series of vivid, high quality colors were observed. Color data from these drawdowns was obtained and is summarized in Table 4. 
     
                                           TABLE 4__________________________________________________________________________ MI 39%     Interference             λmax.                  λmin.Example FeCl.sub.3     Color   (nm) (nm)                      L* a* b*__________________________________________________________________________12    48  1st Orange             665  400 75.64                         13.50                            21.6313    66  1st Red 700  540 64.49                         10.72                            2.3614    78  1st Violet-Blue             700  600 66.03                         -2.66                            0.3515    90  1st Green             570  660 70.69                         -5.34                            8.2816    102 2nd Orange             583  400 72.02                         1.33                            14.3017    126 2nd Red 620  400 61.06                         7.86                            6.1418    138 2nd Violet-Blue             590  400 56.17                         3.48                            2.3519    150 2nd Green             700  400 54.24                         -0.31                            2.0820    174 3rd Orange             590  400 51.46                         1.48                            2.67__________________________________________________________________________ 
    
     EXAMPLES 21-23 
     The TiO 2  coatings also produce a series of interference colors as the thickness of the TiO 2  layer on the glass increases. Thin TiO 2  coatings produce a whitish reflection which appears pearly or silver. As the TiO 2  coating layer becomes thicker, gold, red, blue and green interference colors are observed. As the coating becomes even thicker, a series of second observable color is observed. These second colors have more color intensity and hiding than the first colors described in the Examples above. 
     These second colors were prepared by dispersing 50 gm of the glass flakes used in Examples 1-11 in 333 ml of distilled water. The pH was adjusted to 1.6 with dilute HCl, and the suspension was heated to 74° C. 7 ml of 18% stannous chloride were added followed by the addition of 40% titanium chloride at a rate of 0.33 ml/min. The pH was maintained at 1.6 by simultaneously adding dilute sodium hydroxide. The titania addition was continued until the desired color was observed. The slurry was filtered, washed with water and the resulting presscake calcined at 600° C. to yield a TiO 2  coated glass flakes. 
     When drawn down, the resulting products have higher color intensity and more coverage than their comparable first observable interference colors. Color data from these drawdowns is summarized in Table 5. 
     
                       TABLE 5______________________________________Inter-                      TiO.sub.2                             GK-111ference     λmax.               λmin.                       Crystal                             DataExample  Color 2nd           (nm)    (nm)  Form  L*   a*   b*______________________________________21     Gold     580     430   Rutile                               67.89                                    -3.54                                         10.8822     Orange   640     480   Rutile                               65.14                                    6.13 8.9823     Red      680     510   Rutile                               63.26                                    7.96 1.02______________________________________ 
    
     EXAMPLES 24-28 
     For comparative purposes, several examples of the aforementioned U.S. Pat. No. 3,331,699 were repeated. 
     In the first instance, examples 1 and 2 of the patent in which glass was treated first with tin and then with titanyl sulfate were reproduced using E glass (REF-140 from Nippon Sheet Glass). The resulting products were examined using an optical microscope. The coatings were not smooth, and very little of the TiO 2  was actually attached to the surface of the glass flakes. When drawn down on the black and white cards, the resulting products had low luster, and exhibited no real interference effect. The patent&#39;s example 1 states that if a small portion of a dried sample was re-slurried in water, it would exhibit a lustrous sparkle. Accordingly, small quantities of the samples were redispersed in distilled water. The reproduced sample from example 1 exhibited nothing except a milky suspension while the sample from Example 2 displayed a pale purple colored flake. The reproduced product of example 1 was calcined and a mixture of anatase and rutile titanium dioxide was observed, but the product quality was poor. 
     Example 6 of the U.S. Pat. No. 3,331,699 was also reproduced twice and resulted in a sequence of interference color pigments. In one reproduction, the glass flake was a C glass (RCF-140 of Nippon Sheet Glass) and in the other reproduction it was E glass (REF-140 of Nippon Sheet Glass). When the pigments produced in these two examples were examined microscopically, some of the flakes were uncoated, and for those flakes that were coated, the coatings were rough, contained many cracks and in some cases the coatings were peeling away from the surface of the glass. The pigments produced on the C glass were superior to their counterparts produced on the E glass. The C glass products were approximately 30% rutile and 70% anatase while the E glass products were almost exclusively anatase. 
     Example 12 of the U.S. Pat. No. 3,331,699, in which glass flakes were treated with tin and then with iron, was repeated using E glass (REF-140 of Nippon Sheet Glass). The resulting product was a rust colored powder that exhibited primarily an absorption color effect. When a drawdown was prepared, the product exhibited very low luster and reflectivity. The products prepared in Examples 12-20 above were vastly superior. 
     EXAMPLES 29-30 
     The procedure of Examples 5-8 was repeated except that an E glass was substituted for the C glass. The TiO 2  coating was made which had a white, pearly appearance. The resulting E glass product had a titanium dioxide coating which was primarily anatase while the C glass products had 100% rutile titanium dioxide coatings and were much higher product quality. The color data is presented in Table 6. 
     
                       TABLE 6______________________________________                 GK-111 12%Ex-   Interference           λmax.                   λmin.                         Crystal                               Black 45/0ample Color     (nm)    (nm)  Form  L*    a*  b*______________________________________29    Pearl     400     700   Rutile                               73.64                                    -0.94                                         -2.3630    Gold      400     700   Anatase                               73.44                                    -0.55                                         0.21______________________________________ 
    
     Various changes and modifications could be made in the process and products of this invention without departing from the spirit and scope thereof. The various embodiments which have been disclosed herein were for the purpose of further illustrating the invention but were not intended to limit it.