Vesiculated Polymer Granules Containing TiO.sub.2

This invention relates to a form of TiO.sub.2 pigment. More particularly, it relates to TiO.sub.2 pigment composed of polymer granules containing TiO.sub.2 particles and micro-voids, to a method for preparing such granules, and to coating compositions containing them.

TiO.sub.2 has a high refractive index for its density, which makes it an outstanding pigment for coatings.

This high refractive index can be enhanced to an even higher level, and the hiding power of the TiO.sub.2 correspondingly improved, if the TiO.sub.2 particles are incorporated in polymer granules which contain micro-voids. Such granules are commonly called "vesiculated" granules and are described in U.S. Pat. No. 3,923,704 to Gunning and Lubbock.

Vesiculated granules are ordinarily prepared by the well-known method of first making a slurry of TiO.sub.2 in water, using a hydrophobic surfactant as a dispersing aid. This slurry is then added, with vigorous agitation, to a solution of a polymer, ordinarily but not necessarily a polyester, in a liquid cross-linking agent such as styrene. This gives an emulsion of water in the polymer/cross-linking agent solution. In this step, the TiO.sub.2 particles are flushed from the water phase to the polymer phase. This emulsion is then itself emulsified in water, and the polymer cross-linked, usually by heat, to give an aqueous slurry of vesiculated granules containing TiO.sub.2 particles. For maximum enhancement of refractive index, the TiO.sub.2 particles should be in the polymer matrix, and not in the voids or on the granule surfaces.

Some commercial TiO.sub.2 slurries contain hydrophilic surfactants for stability. When these slurries are used to prepare vesiculated granules in the manner just described, a significant portion of the TiO.sub.2 particles is ultimately found either in the voids or on the granule surfaces, and much of the refractive index enhancement otherwise possible is lost.

It has now been found that such TiO.sub.2 slurries can be successfully used to prepare vesiculated granules if the slurries contain a phosphate ester represented by the structure

where

R is an aliphatic hydrocarbon radical of 4-16 carbon atoms, an octylphenol radical or a nonylphenol radical,

n is 4-12,

x is 1 or 2,

y is 2 or 3 and

z is H, Na or K.

When these phosphate esters are used, most of the TiO.sub.2 particles in the resulting vesiculated granules are in the cross-linked polymer matrices, not in the voids or on the granule surfaces. These granules will have diameters of 1-50 microns (as measured optically against a calibrated standard), and will contain 1-60%, by weight of the granules, of TiO.sub.2 particles (as determined by ashing); 5-95%, by volume of the granules, of voids (as determined by densitometry); and 0.5-4.0%, by weight of the TiO.sub.2, preferably 1.8-2.2%, of a phosphate ester of structure (I). (The identity of the ester, and the amount present, is determined by quantitative extraction of the granules with acetone, removal of acetone from the extract, separation of the esters from the other extractables by high pressure liquid chromatography, and identification of the ester by infrared spectroscopy.)

The vesiculated polymer granules of the invention are prepared from an aqueous slurry of TiO.sub.2 which contains a hydrophilic surfactant stabilizer such as, for example, a polycarboxylic acid. Such a slurry may also contain amines for pH control and defoaming agents. The TiO.sub.2 in the slurry can be any of the rutile variety, prepared by either the chloride process or the sulfate process.

To this slurry is added 0.5-4.0%, preferably 1.8-2.2%, by weight of the TiO.sub.2, of a phosphate ester of structure (I). Mixtures of esters can also be used. A product preferred for this use is "Monofax 831", sold by Mona Industries of Paterson, N.J. This product is a 75/25 weight mixture of esters represented by the structure

and

respectively. This mixture will be referred to hereinafter as Ester Mixture A.

Vesiculated polymer granules may then be prepared from this slurry, using the materials and procedures shown in the previously mentioned U.S. Pat. No. 3,923,704, which is incorporated into this application to describe polymers, cross-linking agents and adjuncts which may be used, as well as the manipulative steps and process conditions.

The polymer preferred for use is "Polycor 939-X-300", sold by Cook Paint and Varnish Co., Kansas City, Missouri, and believed to be a polyester conventionally prepared from fumaric acid, phthalic anhydride and propylene glycol in molar proportions of approximately 3:1:4.

The slurry of vesiculated granules of the invention which results when these procedures are followed can be used directly to prepare a coating composition by mixing it with a suitable film-forming component, using known techniques. Alternatively, the granules can be separated from the slurry by filtration or centrifugation and may then be used in any of the conventional ways.

In the following description, all parts are by weight.

Two hundred parts of a slurry containing

were placed in a vessel.

To the slurry were then added, with stirring,

The slurry was stirred for five minutes and to it was then added, with stirring, a solution of

The slurry was stirred for one minute and was then added to a vigorously stirred solution of

The resulting emulsion was vigorously stirred for 2 minutes, slowly stirred for 15 minutes more, and then added to a vigorously stirred solution, at 90.degree.-95.degree. C. of

Stirring of the mixture was then reduced to low, heating was stopped, and then stirring continued for 10 minutes.

The mixture was then cooled to ambient temperature, to give a slurry of vesiculated polymer granules, substantially all of whose TiO.sub.2 particles were in the polymer matrix.