Volume 3, Number 4
Oct 1990

New & Little-Known Components of Paper

Hollow-Sphere Pigment (HSP). Rohm and Haas introduced a coating ingredient in 1984 that improves opacity by. increasing the number of solid/air interfaces that refract and scatter the light. It consists of an aqueous styrene/acrylic polymer dispersion in the form of hard sub-micron hollow spheres filled with water. The water evaporates, but the spheres do not collapse either from drying or from subsequent calendering. HSP is used in the coatings of paper on which the Metropolitan of Art prints its art reproductions. (Refs.: C.P. Hemenway et al., 1984 TAPPI Coating Conference; W.J. Haskins and D.I. Lunde, Pulp and Paper May, 1989.)

Sulfuric Acid. This is commonly added to both alkaline and acid stocks to regulate pH. The pH of kraft pulp, since it is made with alkaline chemicals, may have to be adjusted for use in an acid system; and recycled fiber, if it is too alkaline, may cause foam when it is used to make a paper at a low pH. Even stock sized with alum and rosin may need to be more acidic to make it run better. Sulfuric acid is used so widely for this purpose that there are blends of alum and sulfuric acid on the market. (Ref.: B. Wortley, PIMA March 1990 and TAPPI J. April 1990.)