Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: When papermakers use the word "filler" they are talking about finely divided mineral products, usually in the size range of about 0.5 to 5 micro-meters. The most important of them are calcium carbonate and kaolin clay. Sometimes people also include titanium dioxide in the category of "filler" even though the particles tend to be smaller (e.g. 200-300 nm) and the term "filler" does not seem sufficiently dignified for a material that is so expensive. Some papermakers in places such as Asia and Finland regularly use talc filler having average particle sizes larger than 5 mm; this is possible because talc happens to be very soft and non-abrasive. Most fillers can be received by paper mills either as dry powders or as slurries. Precipitated calcium carbonate (PCC) is often prepared at a plant adjacent to the paper mill.
Function: Reduction of materials cost per unit mass of paper, increased opacity, increased smoothness.
Strategies for Use: The type of paper to be produced will have a huge bearing on the choice of filler, blends of fillers, and their level in the product. One thing to keep in mind is that fillers tend to interfere with inter-fiber bonding, reducing the strength of paper. The debonding effect of fillers is closely related to the mechanism by which they can contribute to brightness and opacity. The reason is that the fillers increase the amount of air-solid interfaces within the paper. Rays of light passing through paper are refracted or scattered only when they encounter a change in refractive index, such as occurs at air-solid interfaces. The most general rules for use of fillers include (a) making sure that the material is fully dispersed into individual particles before it is added, (b) mixing it with the furnish at a location that doesn't adversely affect other additives, and (c) retaining it. The first goal and the last sometimes can be in conflict with each other, especially if a large amount of dispersant has been used to create a stable suspension. The most serious adverse interactions to avoid (item "b") include mixing calcium carbonate with acidic materials, and allowing any coagulant or flocculant to contact the filler slurry before it is mixed with pulp. Various groups have explored the possibility of intentionally agglomerating fillers to improve paper strength. These strategies often seem promising, but they may be difficult to control.
Cautions: Dry filler materials are a potential source of dust; appropriate equipment has to be used to disperse them.
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 .