Brightness paper fillers

Fig. 2.10 Brightness fTappi R-457) of various paper fillers and highly bleached mechanical and chemical pulp (source OMYA).

These are also specialty pigments, mostly used in lower basis weight newsprint and directory paper. These pigments are added in small percentages (sometimes applied in combination with regular paper fillers), to enhance paper brightness and opacity as well as absorption properties, e. g. reduction of print-through. 

Paper fillers - High-brightness, low-abrasion airfloated, water-washed and delaminated kaolins are used as pulp extenders in acid-sized paper, where they improve opacity, smoothness and ink receptivity. Partially calcined clay is used as a Ti02 extender. 

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. 

Fig. 2. (a) Brightness improvement obtained by the use of kaolin as a filler in paper and (b), opacity improvement obtained as pulp fibers are replaced with various kaolins. In both cases, A represents fine-particle calcined clay B, high brightness No. 1 kaolin C, coarse-particle water-washed kaolin and D,... 

Many inert pigments (often known as fillers) are incorporated into paper in addition to the cellulosic fibres. They may be added to improve certain optical properties—in particular opacity and brightness—or simply as a cheap replacement for costly fibre. The two most common pigments are kaolin (china clay) and chalk (limestone), but talc and speciality pigments such as titanium dioxide are also used. The particle size for general purpose fillers is normally expressed as an equivalent spherical diameter (esd) and this is determined from sedimentation data. Values for the common paper-... 

Amorphous silica has become increasingly important in paper technology. It cannot be used alone because it affects coating rheology and adhesive demand. But used in combinations with other fillers, it improves the brightness, opacity, and ink receptivity of paper. Similarly, talc is used as a 10 to 40% replacement for other fillers to improve paper surface smoothness and ink receptivity. Talc is also used in place of clay if clay is not locally available. 

Chemical composition, particle size, particle shape, specific gravity, surface area, refractive index, brightness, absorptivity, and wettability are the most important criteria used in selecting a filler for the paper industry. For further reading, a recent monograph on the use of fillers in paper industry is an excellent source of practical information. ... 

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