Photochromic Composition

The invention relates to photochromic compositions and in particular to photochromic compositions which can be combined with various types of moldable or extrudable plastics and plastisol including, for example polyvinyl chloride, acrylates, methacrylates or polyethylenes.

Inorganic photochromic compositions are known in the art. For example, U.S. Pat. No. 3,329,648 issued July 4, 1967 to Chopoorian and U.S. Pat. No. 3,317,321 issued May 2, 1967 to Chopoorian both disclose an inorganic photochromic composition consisting of titanium dioxide (TiO.sub.2) activated by ferric oxide (Fe.sub.2 O.sub.3). In addition these patents further disclose that this inorganic photochromic composition could be dispersed in a vinyl polymer or a polyester. However, the inorganic mixture of titanium dioxide and ferrous oxide produced has a relatively soft crystalline structure which would not disperse uniformly throughout the plastisol.

Accordingly, it is an object of this invention to provide an inorganic photochromic composition which overcomes the problems of the prior art. In particular, it is an object of the invention to provide an inorganic photochromic composition having a crystalline structure which is harder than the prior art inorganic photochromic compositions. The increased hardness permits the composition to be milled to a finer structure which can be thereafter dispersed more uniformly throughout the plastisol to produce more even photochromic properties.

In accordance with the invention a photochromic composition is formed which consists essentially of titanium dioxide, ferrous oxide and lead nitrate; the ferrous oxide being in the range of about 2% to 10% by weight of the titanium dioxide and the lead nitrate in the range of about 0.1% to 1% by weight of the titanium dioxide. A photochromic plastisol is formed by mixing this photochromic composition with an appropriate plastisol.

These and other objects and features of the invention will become more apparent from the detailed examples provided below.

The basic photochromic material is titanium dioxide powder which is available in its water dispersible anatase crystalline form from many sources including New Jersey Zinc Corporation. Titanium dioxide is also available in a rutile crystalline form however, this form of titanium dioxide will not produce the desired photochromic composition. The powdered titanium dioxide is dispersed into distilled water to create a slurry. The amount of water is not critical, however, too much water will increase the time required for the subsequent evaporation step described below. Up to 10% by weight of ferric oxide is then added to the slurry; the amount of ferric oxide depending upon the degree of photochromic activity desired. Finally up to 1% by weight of lead nitrate is added to the slurry.

The slurry is throughly mixed and rapidly dried, for example in a flash drier, to drive off the excess water. Thereafter, the dried mixture is placed in a furnace typically at 1000.degree. C. for approximately fifteen minutes. This high temperature firing insures that the ferric oxide is reduced to ferrous oxide (FeO) and that the lead nitrate is reduced to lead oxide and becomes integral with the crystal structure. Firing temperature is important to proper formation of the photochromic material since too high a temperature can alter the crystalline structure of the titanium dioxide while at too low a temperature the ferric oxide or lead nitrate might not enter the crystal lattice The presence of lead oxide insures a more homogeneous photochromic composition with a harder crystalline structure which can be reduced to sub-micron particles by conventional milling procedures such as micropulverization or jet milling.

The foregoing composition is photochromic and will darken to a deep tan when exposed to sufficient light. Typically, the composition will darken after exposure to sunlight for approximately fifteen to thirty seconds and will return to its normal color within about fifteen minutes after removal from sunlight. It has also been found that the composition will darken when exposed to other light sources, such as an incandescent lamp.

In one specific example, 1000 grams of titanium dioxide was added to distilled water to make a slurry. To the slurry was added 100 grams of ferric oxide and 10 grams of lead nitrate. The excess water was evaporated in a flash dryer and the resulting mixture calcined for fifteen minutes at 1000.degree. C.

If has been found that the photochromic composition is particularly useful when incorporated into a plastic material which is then used to form dolls which will appear to tan when exposed to sunlight. For example, mixing of up to fifty (50%) by weight of the photochromic composition into the plastic material can be advantageously accomplished using conventionally techniques. Flesh tones can be provided by the use of suitable commercially available dies and/or other pigments. Other substances such as viscosity depressants, antioxidants, fillers, inert diluents and the like may also be used.

For example, to form a doll, a slurry of plastisol or other plastic material and photochromic material is injected into a mold which is rotated in a multiplicity of planes thereby causing the slurry to adhere to the walls of the mold. The mold temperature is elevated to cure the slurry. Upon separating the mold, a soft flexible doll which conforms to the interior of the mold can be removed.