COLLOIDAL ZINC OXIDE

Abstract: A colloidal-sized agglomerated zinc oxide material having an average agglomerate size below 100 nanometers (nm) and method for production are disclosed. Preferably, the agglomerate size is below 60 nm. The colloidal zinc oxide material will transmit more than 75 %, and preferably more than 85 % of the light in the visible range of the spectrum (i.e., light between 390 and 800 nm in wavelength) and less than 5 % and preferably less than 3 % of light having a wavelength in the ultraviolet range between 190 and 390 nm when tested at 0.1 weight percent in mineral oil. The material is formed by first heat treating zinc carbonate to produce zinc oxide. This zinc oxide is then combined with a polyacrylic acid dispersant and milled to produce a colloidal sized zinc oxide material.

Description: COLLOIDAL ZINC OXIDE BACKGROUND OF THE INVENTION The present invention relates to a superfine parti- culate colloidal-size zinc oxide UV absorber and a process for the manufacture thereof. The colloidal zinc oxide has a high degree of absorption of ultraviolet light and a high degree of trans ittance of visual light. DESCRIPTION OF RELATED ART The use of various materials as UV absorbers is known in the art. Organic UV absorbers include benzophenone, octyl salicylate, benzotriazole, PABA (para-aminobenzoic acid) , etc. Inorganic UV absorbers include Ti0 2 , Zn0 2 , etc. Problems with the organic UV absorbers include skin irritation, photode- gradation, and incompatibility with polymeric compounds. The inorganic UV absorbers also have problems including relatively large particle size and consequent opaqueness and whitening when incorporated into compounds. Recently, attempts have been made to produce inorganic UV absorbers with smaller part¬ icle sizes in an attempt to reduce the whitening effect of the compounds. However, most zinc oxides still have ultimate particle sizes in excess of 200 nanometers (nm) and agglom¬ erate sizes even larger. Such zinc oxides have insufficient clarity for many uses. Thus, zinc oxide has found limited use in sunscreens and UV blockers because of its great ability to whiten any composition in which it is incorporated. Accord- ingly, it would be useful to develop a zinc oxide which would be sufficiently fine to be transparent in the visible light range, yet still opaque in the UV range in order to block UV light. Other attempts at making smaller sized zinc oxide have been made. Specifically, Japanese unexamined patent applica¬ tion publicatic. 3-199121 to Nishihara, et al. published August 30, 1991, pertains to a superfine particulate zinc oxide powder capable of UV absorption. Specifically, Nishi¬ hara, et al. discussed the production of a superfine zinc oxide powder having an average particle size of 0.01 to 0.03 millimicrons. However, Nishihara's zinc oxide agglomerates to form much larger agglomerate sizes. Accordingly, that material is also whitening, and does not achieve the objective of the present invention, namely a zinc oxide material which is light transmitting in the visible light range, yet light absorbing in the UV range. SUMMARY OF THE INVENTION The present invention relates to a colloidal-sized agglomerated zinc oxide material having an average agglomerate size below 100 nm, and method for production. Preferably, the agglomerate size is below 60 nm. The colloidal zinc oxide material will transmit more than 75%, and preferably more than 85%, of the light in the visible range of the spectrum (i.e., light between 390 and 800 nm in wavelength) and less than 5% and preferably less than 3% of light having a wavelength in the ultraviolet range between 190 and 390 nm, when tested at 0.1 weight percent in mineral oil. The material is formed by first heat treating zinc carbonate to produce zinc oxide. This zinc oxide is then combined with a dispersant and milled to produce a colloidal- sized zinc oxide material. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a photomicrograph of prior art zinc oxide material before milling. Figure 2 is a photomicrograph of the colloidal zinc oxide of the present invention. Figure 3 is a graph of the trans ittance of light by the colloidal zinc oxide of the present invention. Figure 4 is a graph of the effect of UV radiation on the color of white polyester/cotton fabric variously treated with flame retardant and the colloidal zinc oxide of the present invention. Figure 5 is a graph of the effect of UV radiation on the yellowness of white polyester/cotton fabric variously treated with flame retardant and the colloidal zinc oxide of the present invention. Figure 6 is a graph of the effect of UV radiation on the color of polypropylene variously treated with flame re¬ tardant and the colloidal zinc oxide of the present invention. Figure 7 is a graph of the effect of UV radiation on the yellowness of polypropylene variously treated with flame retardant and the colloidal zinc oxide of the present invention. Figure 8 is a graph of the effect of UV radiation on the color of polypropylene variously treated with flame re¬ tardant and the colloidal zinc oxide of the present invention. Figure 9 is a graph of the effect of UV radiation on the yellowness of polypropylene variously treated with flame retardant and the colloidal zinc oxide of the present invention. DETAILED DESCRIPTION OF THE INVENTION Generally, industrial quantity zinc oxide is made according to one of two production methods. These are termed the "French process" and the ".American process." In the French process, zinc metal is vaporized in large containers by external heating. In an adjoining off-take pipe or com- bustion chamber, the vapor is burned off in the air to produce zinc oxide powder. In the American process, oxidized ores of roasted sulfide concentrates are mixed with anthracite coal and smelted. This process reduces the ore to metallic zinc vapor. The vapor is then burned to produce zinc oxide. Recently, other methods for producing zinc oxide in fine particles have been developed. Particularly, as dis¬ closed in Nishihara, et al., superfine zinc oxide is produced from a starting solution of ammonium carbonate or hydrogen ammonium carbonate with aqueous zinc chloride or zinc sulfate solution added. The addition of the zinc chloride or zinc sulfate produces a zinc carbonate precipitate which is filtered off and dried. The dried zinc carbonate is then thermally decomposed to produce a superfine zinc powder. However, the powder is in a dry agglomerated condition with a weight mean agglomerate size in excess of 10 microns. Thus, the material transmits little or no visible light and has a high whitening effect. The material of the present invention can be produced by heating zinc carbonate to around 300°C to produce an agglomerated zinc oxide. These agglomerates are then broken down to agglomerates with weight mean diameters less than 100 nm by milling with a dispersant for several hours. Generally, milling can be accomplished in two to three hours, depending on the desired agglomerate size and the mill speed. Higher mill speeds will reduce the milling time required. By in¬ creasing the milling time, material with weight mean diameters as low as 20 nm may be produced. The resulting agglomerates are sufficiently small to transmit visible light. The zinc oxide composition of the agglomerates allows the absorption of ultraviolet light, resulting in a superfine, nearly invis¬ ible zinc oxide UV absorber. The zinc oxide of the present invention mixes well with both hydrophilic and oleophilic materials, as evidenced by its miscibility with water and mineral oil. The zinc oxide is also compatible with octyl salicylate and other common chemical sunscreens. The material is thus useful in a large variety of products including sunscreen, plastic, glass, cosmetics, fibers, textiles, and rubber or paint formulations. Further, if some pigmentation is desired, the particles of zinc oxide may be milled for a shorter period of time to produce larger agglomerate particles which will lend whitening pigmentation to any composition. Generally, agglomerates below 100 nm in size should be largely transmissive in the visible range and should have little whitening effect. This transmission of visible light drops off dramatically with agglomerate sizes above 130 nm. In processing material, it should be kept in mind that a longer time period in the mill will result in smaller agglomerated zinc oxide particles, provided the milled mate¬ rial includes a dispersant which will prevent reagglomeration 5 of the milled particles. The practical limit to this is the ultimate particle size of the zinc oxide particles, which is about 10 to 30 nm. As used herein, ultimate particle size means the diameters of the individual zinc oxide particles which may comprise an agglomerate. Similarly, "agglomerate 0 size" refers to the size of the agglomerated individual zinc oxide particles. EXAMPLE 1. Zinc oxide was prepared by heating 140 grams of zinc carbonate (obtained from J. T. Baker Analyzed Reagent of New 15 Jersey) at 300°C for 18 hours to produce 100 grams of agglom¬ erated zinc oxide. A quart size adaptation of the Bureau of Mines Turbomill (from Redhill Grinder, Chicago Boiler, Inc. of Chicago) was charged with 660 grams of glass beads (Potters AO 50 beads from Potters Industries, Inc., Valley Forge, 20 Pennsylvania), 180 milliliters of water, and 7.5 grams of Accusol 445N dispersant (a polyacrylic acid available from Rohm and Haas Chemical Company, Philadelphia, Pennsylvania) . The mill was set to approximately 800 rpm and the 100 grams of zinc oxide was sifted into the mill. After the zinc oxide 25 was thoroughly wet out, the mill agitation speed was increased to 1600 rpm, and milling continued for 2 hours and 45 minutes. Thereafter an additional 2 grams of Accusol were added and milling was maintained for 15 minutes. During the milling process, cooling was applied to maintain the temperature below 30 40°C. The resulting colloidal zinc oxide particles were measured with a Coulter N4 particle size analyzer to determine the weight mean diameter. The colloidal zinc oxide particles were determined to have a weight mean diameter of 80 nm. EXAMPLE 2. 35 In accordance with the procedure described above, colloidal zinc oxide having a weight mean diameter of 58 nm was produced by increasing the milling time to 4 hours and 30 minutes. This material had a surface area of 51 m 2 /gm and a calculated ultimate particle size based on surface area of 20 nm. The weight mean diameter was measured to be 58 nm with a Coulter N4 analyzer. The weight mean diameter used through- out these experiments refers to the mean diameter based on the number of particles of a measured size (as opposed to the weight of particles of a given size) . In such a calculation, the increased weight of particles with larger diameters must be taken into account since 1 milligram of 100 nm diameter particles has fewer particles than 1 milligram of 50 nm dia¬ meter particles. This measurement is performed automatically by the Coulter analyzer. Furthermore, in these experiments, it was determined that 95% of particles in a sample of mate¬ rial will fall within an 80 nm range around the weight mean diameter. Thus, in this case where the weight mean diameter if 58 nm, 95% of the particles in this sample had actual diameters between 18 and 98 nm. The material from Example 2 was tested to determine the transmittance of light of various wavelengths. Three samples of the material were prepared in mineral oil at concentrations of 0.1 weight percent, 0.3 weight percent, and 2.4 weight percent. The samples were introduced into square tubular blanks having a standard 1 centimeter square path length and tested to determine transmittance of light having wavelengths in the range of 190 nm to 890 nm (1900 angstroms to 8900 angstroms) . Pure mineral oil was used as a reference. The results of this testing are shown in Figure 3. As may be seen from Figure 3, transmittance of light in the visible range (390 to 800 nm) is excellent with the 0.1 weight percent sample. This transmittance decreases with increasing concentration of zinc oxide in the mineral oil, as may be expected. However, all three samples show excellent UV blocking at all wavelengths of UV radiation. Generally, the UV spectrum below 390 nm wavelength of light is divided into three sections, UVA (320 to 390 nm) , UVB (290 to 320 nm) and UVC (190 to 290 nm) . All three samples can be seen to transmit less than 5% of the UV light across the entire UVA, UVB and UVC spectrum. Thus if additional visible light transmittance is desired, the zinc oxide content can be reduced below 0.1 weight percent without significantly affect¬ ing the UV blocking characteristics of the resulting mixture. The colloidal zinc oxide of Example 2 was used in a series of tests to determine the UV blocking ability of the zinc oxide under varying conditions. The tests were performed by adding zinc oxide to a polypropylene fiber formulation, or as a surface treatment to a cloth, where the cloth or fiber included an organic flame retardant. The organic flame re¬ tardant tended to cause yellowing upon exposure to UV light. The zinc oxide was used to try to prevent this exposure and consequent yellowing. The samples were all white. The cloth was a polyester/cotton blend fabric, and the polymer was virgin polypropylene. In the cloth study, a white polyester/cotton blend fabric was treated variously with N22 flame retardant (anti¬ mony pentoxide and pentabromodiphenyl oxide) (Sample 2) , N22 + 14 parts per 100 colloidal oxide of the present inven- tion (Sample 3) , N22 + 23 parts per 100 colloidal zinc oxide of the present invention (Sample 4) , N24 flame retardant (a blend of antimony pentoxide, a brominated chlorinated olefin from Dover Chemical Corporation of Dover, Delaware, and a PVC latex) (Sample 5) , and N36 flame retardant (also a blend of antimony pentoxide, a brominated chlorinated olefin from Dover Chemical Corporation, and a PVC latex) (Sample 6) . Untreated fabric was used as a control (Sample 1) . The variously treated cloth samples were then irradiated with intense UV radiation and tested at 20-hour intervals for yellowing and color change. Testing was performed on a Minolta CR-200 Chroma Meter with an open nose piece. The results are re¬ ported in standard CIE 1976 L*a*b* format. Total color difference between the initial measurement and after irradia¬ tion is reported as delta E*. Yellowing is indicated by the yellowness index YI as defined in ASTM E313. The results are shown in Table 1 and Figures 4 and 5. Referring now to Figures 4 and 5, it can be easily seen that the overall color change of the fabric treated with flame retardant upon irradiation with UV light was minimized by the addition of colloidal zinc oxide to the treated cloth. Since it is largely the flame retardant which effects the color change upon UV exposure, the untreated cloth showed little or no yellowing upon exposure. The cloth treated with flame retardant can be seen to produce severe yellowing and color change upon exposure to UV light even at only 20 hours. Fourteen parts per 100 colloidal zinc oxide reduced but did not eliminate this yellowing and color change. However, 23 parts per 100 colloidal zinc oxide was sufficient to essen¬ tially eliminate the color change in the fabric associated with the flame retardant treatment, evidencing the excellent UV blocking properties of the colloidal zinc oxide. The polypropylene study was conducted using pure virgin polypropylene as a control, and with the addition of various amounts of NYACOL EAM 3-8 flame retardant [antimony pentoxide and bis(3,5-dibromo, 4-dibromo propyloxyphenyl) sulfome in polypropylene, available from PQ Corporation, Valley Forge, Pennsylvania] with various amounts of colloidal zinc oxide. The samples were irradiated with UV light and color change and yellowing (yellowness index) were recorded at various intervals up to 1000 hours of irradiation. The results can be seen from Table 2 and Figures 6 to 9. Sample 1 is virgin polypropylene, Sample 2 is polypropylene with 2% EAM flame retardant, Sample 3 is polypropylene with 5% EAM flame retardant, Sample 4 is polypropylene with 2% EAM flame retardant and 2% colloidal zinc oxide, Sample 5 is poly- propylene with 2% EAM flame retardant and 5% colloidal zinc oxide, Sample 6 is polypropylene with 2% EAM flame retardant and 10% colloidal zinc oxide, Sample 7 is polypropylene with 2% EAM flame retardant and 0.5% colloidal zinc oxide, and Sample 8 is polypropylene with 5% EAM flame retardant and 5% colloidal zinc oxide. The polypropylene including a flame retardant without colloidal zinc oxide showed severe yellowing and color change Table 2 upon exposure to UV radiation. This yellowing was reduced by the addition of even 0.5% of colloidal zinc oxide, and was minimized or eliminated by the addition of zinc oxide at 2% or greater, indicating the effectiveness of colloidal zinc oxide as a UV blocker. Figures 8 and 9 are similar to Figures 6 and 7, but show only three samples each, to facilitate comprehension of the data. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalence thereof by those skilled in the art to which this invention pertains.

Claims: CLAIMS We claim: 1. A zinc oxide agglomerate having a weight mean diameter of less than 100 nanometers (nm) . 2. The zinc oxide material of Claim 1 wherein said weight mean diameter is between 20 and 60 nm. 3. The zinc oxide material of Claim l wherein said weight mean diameter is less than 60 nm. 4. The zinc oxide material of Claim 1 wherein said material transmits less than 5% of light having a wavelength between 225 nm and 375 nm, and transmits more than 75% of light having a wavelength between 390 nm and 800 nm when tested at 0.1 weight percent in mineral oil. 5. A method of making a zinc oxide agglomerate comprising: a. heating zinc carbonate to produce zinc oxide agglomer¬ ates; and b. milling said agglomerates in the presence of a surfactant for a sufficient period of time to reduce the size of said agglomerates to below 100 nm weight mean diameter. 6. The method of Claim 5 wherein said agglomerates are milled at 1600 rpm for at least 2-1/2 hours. 7. The method of Claim 5 wherein said surfactant is a polyacrylic acid.

Inventors: MYSZAK EDWARD A JR, MYSZAK, EDWARD, A., JR.
Applicants: PQ CORP, PQ CORPORATION
Application year: 1995
Main classification: C01G9
IPC codes: C01G9