Opaque white pigment

White Pigments. Opaque white pigments commonly used in inks, in order of decreasing opacity, ate titanium dioxide and zinc oxide. TiO is by fat the most popular white pigment. Mixtures of whites ate often made with the various colored pigments to add opacity or to make pastel colors. 

The most widely used opaque white pigment was basic carbonate white lead, manufactured by a process little different from that known by the early Greeks in 400 B.C. Other commonly used pigments were zinc oxide, dating back to 1770 zinc sulfide, first produced in 1783 lithopone, developed in 1847 and basic sulfate white lead, which first appeared in 1855. 

The basic scientific concepts necessary to establish the absolute opacity, particle size distribution, and optimum average particle size of opaque white pigments were among the litter of "machinery" still waiting to be used. 

It was significant of change, however, that within 10 years time the traditional "standard" of the coating industry, basic carbonate white lead, had disappeared from a tabulation of opaque white pigments. 

Today the consequences are evident. Spurred by the price increases secured by titanium dioxide s apparent domination of the opaque white pigment field, solid and microvoid polymeric "pigments" and highly efficient spacing extenders are being offered—all designed to reduce the industry s consumption of titanium dioxide by increasing its efficiency. 

Stewart (1950) Stewart, A. Lead pigments and paints Official Digest of the Federation of Paint and Varnish Production Clubs 311 (1950) 1100-1113 SHEG (1985) Stieg, F.B. Opaque White Pigments in Coatings Applied Polymer Science, ACS Symposium Series 285, 2nd ed., less, R.W. Poehlein, G.W. (eds.) American Chemical Society, Washington (1985) 1249-1269... 

This occurs naturally as a white solid in various crystalline forms, in all of which six oxygen atoms surround each titanium atom. Titanium dioxide is important as a white pigment, because it is nontoxic. chemically inert and highly opaque, and can be finely ground for paint purposes it is often prepared pure by dissolving the natural form in sulphuric acid, hydrolysing to the hydrated dioxide and heating the latter to make the anhydrous form. 

In this application, the process analyzer is used in the vis-NIR spectral region to measure the clear top layer on a co-extruded polymer film. The bottom layer is pigmented to an opaque white color and its thickness cannot be determined by this method. Prior to the installation of the fiber-optic spectroscopy system, film samples were measured manually in the laboratory by a subtractive scheme. First, the total thickness of a sample was measured on a manual profilometer. The top layer of the polymer was removed with methylene chloride. The sample was then repositioned on the profilometer as closely as possible to the originally measured spot and the thickness of the second white layer was determined. The thickness of the top layer was then determined by difference. 

Zinc oxide (ZnO) is widely used as an active filler in rubber and as a weatherability improver in polyolefins and polyesters. Titanium dioxide (TiOj) is widely used as a white pigment and as a weatherability improver in many polymers. Ground barites (BaS04) yield x-ray-opaque plastics with controlled densities. The addition of finely divided calcined alumina or silicon carbide produces abrasive composites. Zirconia, zirconium silicate, and iron oxide, which have specific gravities greater than 4.5, are used to produce plastics with controlled high densities. 

The natural color of plain UHMWPE is opaque white. Of course, it can be loaded with pigments. 

Polyethylene is translucent to opaque white in thick sections, opacity increasing with density. Relatively clear film can be extruded from polyethylene, especially if it is quenched rapidly. The plastic accepts pigmentation readily. Most coloring is performed using dry-blend techniques. Color dispersion devices are required to ensure thorough mixing of resin and pigment. 

Printing onto Colored Substrates — Many substrates onto which coding is required are not that suitable for printing with the standard black ink or indeed other dye-based inks, as the colors are too dark for the inks to show up. In this case, the inks need to be formulated with opaque colorants and these will usually be either pigments white, yellow, or blue. Sometimes the formulation will contain a mixture of white pigment for opacity and a dye to color the white. 

Due to the risk of breakage, glass containers are more and more being replaced by plastic containers. It has been shown that white pigmented, opaque, low-density polyethylene, containers can provide complete photo protection to certain photosensitive products, such as dexamethasone- and predni-solone phosphate-eye drops (6). 

---