Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use


Composition: This discussion will be limited to kaolin, the most common form of clay added to paper as a filler. (Bentonite, a generic term for montmorillonite clay, is a microparticle, and it is described elsewhere in this mini-encyclopedia.) Kaolin is a hydrated silica-aluminate. Deposits of kaolin particles have been built up by natural geologic processes. In one of these processes the kaolinite first becomes released by glaciation. Rivers carry silt in the glacial runoff and deposit it in shallow seas. Larger particles settle first, then the finer ones, usually at a different location. By choosing their locations, clay producers have access to a range of particle sizes and brightness of kaolin particles. High quality deposits of kaolin, having moderately high brightness and uniform particle size are found all over the world, but especially in China, Georgia, Brazil, and England. Anionic dispersants such as phosphates and acrylates are usually added to clay as it is collected and processed. The particles are typically irregular hexagons with a layered underlying structure. The size is highly variable; filler clays usually are selected with an average (SEM) particle size of about 1 to 3 micrometers. When the kaolin is first released it is likely to contain dark and abrasive impurities. Processing steps can include sieving, centrifugal cleaning, magnetic removal of dark contaminants, and bleaching. After these processes the particles of kaolin is still likely to resemble "books," comprised of layers. This is the most common type of clay used for paper filling, especially when it is important to maintain strength at a given filler level. Extended agitation of clay slurries in the presence of hard ceramic balls (of visible size, much larger than the kaolin particles) causes the kaolin to delaminate into thinner platelets. This is delaminated clay.

Function: Besides reducing the cost of paper, clays are especially useful for creation of gloss (in highly filled, supercalendered papers), for increasing resistance to air flow (in the case of delaminated clays), and for imparting a moderate decrease in the friction coefficient of paper (especially when added to the size-press solution).

Strategies for Use: The main concerns are (a) how high a filler level is best, and (b) how to achieve an adequate retention. A uniform dispersion of clay can be obtained with conventional dispersing equipment, e.g. a Cowles mill. Also it is possible to get bulk delivery of clay slurries with solids levels in the neighborhood of 70%. The price that one pays for the convenience of using pre-slurried clay is that it contains anionic dispersants. These can make retention more difficult. Coating grades of clay are especially difficult to handle at high levels in the wet end due to the combination of dispersants, higher surface area, and smaller particles that are harder to filter from the water as the paper is being formed. The content of clay in paper usually is limited by decreasing strength properties and decreasing caliper at a given smoothness and basis weight. Clay tends to produce dense paper, especially when it is calendered. Unlike calcium carbonate fillers, clay products can be used at any pH. Clay will form agglomerates if prematurely mixed with such additives as cationic starch, alum, retention aids, and the like. Intentional agglomeration of filler is a potential way to increase the strength of paper at a given filler content; however this technology has not become widely used. Most retention aid systems are effective with clay. Ashing of the paper at 900 oC drives off waters of hydration. If a paper sheet is assumed to contain only hydrous kaolin (not calcined clay), then the percent ash needs to be divided by 0.86 to estimate the percent of clay originally present in the paper.

Cautions: Normal precautions regarding dust need to be observed when clay is received as a dry powder.

Kaolin clay products typically have wide particle size distributions.   Particle size distributions of clay products commonly used for paper filling and coating

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, .