Paper brightness

Table XVII. Coefficients of Correlation by Rank for Reflective Brightness (Papers in Study No. Ill)...

Ultra White 90. [EngelM Kaolin high brightness paper ingment... 

ZCA USP-1 Zoco 100 Zoco 103 Zoco 124 Zoco 600 Zoco 603 Zoco 604 Zoco 624 Zoco 627 Zoco 672 pigment, high-brightness paper coatings Carbital 90... 

If the m.p. is not quite satisfactory, dissolve the o-nitroplienol in hot alcohol (or methylated spirit) under reflux, add hot water drop by drop until a cloudiness just appears, and allow to cool spontaneously. Filter off the bright yellow crystals and diy between filter paper. 

Optical Properties. Brightness, or visual whiteness of paper, can be defined as the degree to which light is reflected uniformly over the visible spectmm. Since pulp and typical impurities tend to be yellowish, blue dye is sometimes added in addition to appropriate fillers. The percentage reflectance is usually measured in the blue end of the spectmm at or near 457 nm (14). 

The abihty of fillers to improve paper brightness increases with their intrinsic brightness, surface area, and refractive index. According to the Mie theory, this abiUty is maximum at an optimum filler particle size, about 0.25 pm in most cases, where the filler particle size is roughly one-half the wavelength of light used for the observation. 

These thermoplastic pigments found application in a much wider range of finished products. New, large-volume applications included coated paper for labels and point of purchase signage, gravure for soap box cartons and bright textiles for fashion, and safety applications. 

Pigment-coated printing papers usually are required to have high brightness to achieve contrast between the printed and unprinted areas. The coatings frequendy have a glossy surface. There also is a demand for duU-coated papers and for dull papers upon which glossy ink films can be printed. 

White Hydroxide. Tlie soda sinter process appHed to bauxite or bauxite residue produces a hydroxide that is completely free from organic coloring matter and is very wliite. A value of more than 95% is obtained on the GE brightness scale relative to Ti02 as followed in the paper (qv) industry. Tliis compares to about 70% on the same scale for the nomial Bayer product. Tlie wliite hydroxide is preferred in the paper, toothpaste, and artificial marble industries. 

Whiteness and Brightness. Photometric instmments, originally developed by the paper industry, are used for these measurements. Values are compared against standard white pigments such as BaSO, Ti02, or MgO. 

Sulfite pulps have properties that are desirable for tissues and top quaHty, fine papers. Because sulfite pulping is not as versatile as kraft pulping, various options have been developed, and the choice of a specific process is dependent on individual mill situations. The unbleached pulp has high (60+) brightness compared to kraft pulp and is easily bleached. However, it is limited to select wood species. The heartwood of pine, Douglas fir, and cedars are not easily pulped. Additionally, pulps produced from hardwood have limited economic value because of low strength. 

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