Decreasing the Photosensitivity of Titanium IV Dioxide

This invention concerns a process for treating TiO.sub.2 pigments to reduce their sensitizing effect, on for example, the photostability of synthetic fibers such as cellulose acetate, polyesters and polyolefins. The utility of TiO.sub.2 as a pigment is of course universally recognized in the fields of surface coatings, plastics, paints and fibers. Not so well advertised however, to the layman, is its extensive use as a catalyst in a wide variety of reactions such as the oxidation of cumene to phenol and acetone. Incident to its ability to catalyze reactions is its tendency in the presence of sunlight to catalyze degradation of the polymer which it is pigmenting, resulting in premature failure of the product in such properties as elongation and the like.

Objectives of this invention include the development of a process for treating TiO.sub.2 to reduce its ability to sensitize the photodegradation of polymers, and to produce sunlight-resistant cellulose acetate yarn.

These and other objects hereinafter appearing have been attained in accordance with the present invention through the discovery that treating TiO.sub.2 pigments with water solutions of manganous, cobaltous, or cerous salts, especially acetate, wherein evaporation of the water leaves the pigment coated with the salt, greatly reduces the photosensitizing properties of TiO.sub.2. Incorporating the treated TiO.sub.2 pigment into polymeric materials particularly cellulose acetate yarn markedly diminishes the photodegradation normally anticipated. In contrast, the use of oxides of such metal gives greatly inferior results.

The useful salts have the formula (R-CO.sub.2).sub.2 Me where Me is Ce.sup.+.sup.3, Mn.sup.+.sup.2 or Co.sup.+.sup.2 and R is a linear or branched alkyl group having from 1 to 12 carbons, preferably 1 to 3 carbons. Such alkyl groups are, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, and the like.

The process for treating the TiO.sub.2 is generally that of coating the pigment with the salt dissolved in water, although organic solvents can be used as shown in Table I. The important factor is that the solvent must wet TiO.sub.2 and be removable. The useful concentration range of the manganese, cobalt or cerium in the actual coating on the TiO.sub.2 is from about 0.1% to 20.0% by weight, based on the weight of the TiO.sub.2, with from about 1.0% to about 10.0% being preferred.

TiO.sub.2 pigments treated as herein described can be used in polymers wherever outdoor weatherability is desired. Examples of uses are in yarn and synthetic fibers usually at concentrations of from about 0.01 to about 5.0% by weight, molding compositions of high polymers, paints, powder coatings and wire coatings. The polymer is not restricted to cellulose acetate but includes polyesters, polyamides, polyolefins, ABS, poly(vinyl alcohol), poly(vinyl chloride), polyacrylonitrile and modacrylic compositions, polycarbonates, regenerated cellulose and the like.

Baker, reagent grade, anatase TiO.sub.2 was treated by preparing a slurry of the pigment with a water solution of various acetic acid salts. The water was evaporated on a steam bath to leave a dry TiO.sub.2 coated with approximately 3% by weight of the salt. Tests for photosensitivity were conducted by mixing 2 g. of the pigment with 8 ml. isopropanol in a 16.times. 150-mm. culture tube. The tube was flushed with oxygen, capped with a Teflon-lined screw cap and exposed to light from a 1,000-watt mercury arc. During the irradiation period the tubes were agitated to keep the pigment in suspension. Control samples containing no TiO.sub.2 and untreated TiO.sub.2 was also tested. After a period of irradiation, lasting from 5-200 hr., the tubes were removed, the isopropanol clarified by centrifuging, and analyzed for acetone by gas chromatography; acetone is the oxidation product of isopropanol. It was established that no acetone was formed by light in the absence of TiO.sub.2.

The results of the tests, summarized in Table 1 indicate that while several salts tend to reduce the photosensitivity of TiO.sub.2, manganese, cobalt and cerium acetate are outstanding in their effectiveness.

In another series of tests, the effect of the salts in terms of metal concentration, on the desensitization of rutile and anatase TiO.sub.2 was determined by the process described in Example 1 employing salt concentrations to give the desired metal content in the coatings. The results, summarized in Table 2, show the effect of the metals over the range of 0.1 to 10% by weight of the metal.

The effect of cerous acetate on the desensitization of TiO.sub.2 was compared with that of zinc diethylphosphate. The procedure described in Example 1 was used. The results, summarized in Table 3, indicate that cerous acetate is quite superior to zinc diethylphosphate in desensitizing the TiO.sub.2 pigments Unitane OR-650 and Unitane OR-450.

The effects of cerous acetate and zinc diethylphosphate on the desensitization of Degussa P-25 TiO.sub.2 were compared by the procedure described in Example 1. The results, shown in Table 4, indicate that cerous acetate is an effective desensitizer for Degussa P-25 pigment.

Pigments treated with cerous acetate were incorporated at the indicated concentrations in cellulose acetate fiber by admixing the treated pigment with an acetone dope of cellulose acetate and spinning fibers in a commercial spinning cabinet. The fibers thus produced were exposed to 3100A irradiation from a fluorescent lamp and the physical properties of the fibers measured versus irradiation time. The results, shown in Table 5, show that the yarn compositions containing cerous acetate treated TiO.sub.2 were more stable to ultraviolet light than those containing untreated TiO.sub.2 or no TiO.sub.2.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.