Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: TiO2. Pigmentary titanium dioxide, as used by the paper industry, usually consists of semi-spherical particles having diameters all very close to the range 0.25 to 0.4 micrometers. Most of this is delivered to paper mills in the form of aqueous slurries having solids levels near to 70%. This may seem to be an impractically high number until one considers the fact that the mineral is much denser than water, so the volume fraction is much lower than the mass fraction. Dispersants used in stabilizing the aqueous slurries can contribute a very high negative colloidal charge to the system. In addition, the surface or the bulk phase of TiO2 products is often treated so that it contains a minor amount of such materials as aluminum, silicon, or phosphorous, and such treatments also affect the colloidal charge.
Function: To increase the opacity of paper products, the brightness of white-top linerboard and similar products, and brightness of some paper products.
Strategies for Use: The important thing to remember about titanium dioxide particles is that they increase opacity by scattering light. Their ability to scatter light depends on their being present in the sheet as separate particles, not clumps. This means that the TiO2 product should be well dispersed before it is added. Premature mixing of titanium dioxide slurry with alum or other cationic materials should be avoided in order to minimize self-agglomeration of the pigment. Good optical efficiency usually can be achieved by first adding the pigment at a place where it becomes well mixed with the furnish, and then adding a retention aid. Even better optical efficiency can be achieved in some cases if enough highly charged cationic polymer is added before the TiO2 to create cationic sites on the surfaces of fibers and fines. The amount has to be optimized, since an excess of cationic polymer in the solution merely will agglomerate the TiO2 to itself. Some of the negative features of titanium dioxide are (a) relatively high abrasiveness, and (b) absorption of ultraviolet light, reducing the effectiveness of fluorescent whitening agents. Though TiO2 is often used to meet brightness goals, this approach tends to be expensive relative to other options such as increased bleaching and use of calcium carbonate filler, etc.
Cautions: Titanium dioxide can create quite a mess, but, with effort, it can be cleaned up. Even trace amounts of TiO2 in effluent water will tend to make it turbid.
PLEASE NOTE: Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification.
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This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, firstname.lastname@example.org .