The most important industrial use of kaolin as a filler mineral is in the paper industry where it is used in both filler and coating applications.  However, the use of kaolin is under pressure from other competing minerals, notably ground and precipitated calcium carbonate (GCC and PCC).

The paper industry can be broadly divided into the paper/paperboard and pulp sectors.  The main trend within the industry is the faster growth in paper and paperboard output compared to pulp mainly due to increased recycling rates in many countries.  Wastepaper and filler consumption is continuing to rise, as are developments in deinking which allow a greater variety of paper to be recycled.  Fillers are only used in the paper and paperboard sector of the industry.

1.1Paper manufacture

Paper is composed of thin sheets of interlaced cellulose fibre, generally derived from wood.  The strength of the paper comes from the hydrogen bonding formed when the cellulose fibres are pressed together in the presence of water.  Paper can be divided into mechanical and chemical (woodfree) types, depending on the method by which the cellulose fibres are prepared and the lignin (the “woody” material in plants) is removed. 

The paper industry can be divided into two parts, paper and paperboard.  At a simplistic level the primary difference between the two is that paper is one sheet while paperboard is a laminate of several.  The raw materials for each include a fibre source, virgin pulp and/or recycled paper, fillers and a variety of chemicals.  The precise formula will dependent upon the properties required in the final product.  Thus a writing or printing paper will be made so as to be opaque and accept ink without smearing.  A tissue paper on the other hand will be designed so as to absorb liquid while retaining physical strength.

Fillers such as kaolin may be used both in the production of the sheet itself and in any subsequent treatment (finishing) of the sheet.  The choice of which mineral type(s) to use will be determined, in part, by the conditions under which the paper sheet is formed.  When the final product will be printed or written on, the paper must be designed so as to accept ink without allowing that ink to run or smear.  As cellulose fibres have a natural tendency to absorb water and many solvents, the paper sheet has to be sized in order to limit this tendency.

Historically this sizing was achieved by the use of rosin, a by-product of paper pulp manufacture, which was precipitated onto the cellulose fibres by alum (aluminium sulphate) in acidic conditions.  These acidic conditions limited if they did not prohibit, the use of carbonate fillers such as GCC and PCC and kaolin was the filler of choice.

During the 1960s alkaline sizing systems were introduced.  While their widespread adoption took many years to effect, today they are the dominant systems in many paper-making regions.  A consequence has been that carbonate fillers have been able to take market share from kaolin.  The switch has been made on the grounds of both lower cost-in-use and improvements to paper qualities.

Paper made in acidic conditions retains some acidity, this limits the use of carbonate fillers in any coating (finishing) operations.  The switch to alkaline paper-making has removed those limits so that carbonate fillers have also taken market share in the paper finishing sector.

1.2Competing filler and coating pigments in paper manufacture

Kaolin was formerly the most widely used mineral filler and coating pigment in paper manufacture but in the last two decades has been steadily supplanted by calcium carbonate.  The two causes for this have been the conversion from acid to alkaline papermaking and demand for brighter and bulkier paper.  

The switch from acid to neutral production methods has enabled paper producers to use calcium carbonate, which is less expensive and brighter than kaolin.  Neutrally sized paper can have higher mineral filler loadings than acid sized paper so calcium carbonate slurries are used in preference to kaolin slurries because of their higher solids content.  

The use of talc minerals as a filler in paper is restricted to areas where local sources are available or where there are particular technical requirements in paper mills.  Talc is used principally as a coating and pitch control agent rather than as a filler pigment. 

Kaolin generally has a lower surface area and internal porosity than calcium carbonate so interferes less with fibre-to-fibre bonding resulting in a stronger product.  Blending these two minerals together can produce a material with exactly the required specifications.  For example, calcium carbonate produces a product with a “flat” finish so kaolin can be added as a “glossing” agent to improve the finish of the product.

1.3Paper filling

Minerals are added to paper as a sheet of paper composed entirely of cellulose pulp has an irregular surface and is sometimes insufficiently opaque for many printing and writing purposes.  For the majority of printing and writing papers, a smoother and more opaque sheet is required.  This can be achieved by filling the voids in the cellulose fibre web with non-fibrous mineral fillers or by coating the sheet with similar materials.

Ideally, filler minerals consumed in paper should be very white, soft, non-abrasive and free from impurities, especially grit, to minimise wear and tear on paper making machinery.  In addition, the particle shape of coating minerals should be flat so that it can be formed into a smooth sheet by calendering, as a smooth surface is essential to ensure a good printing surface.  A filler mineral should also be chemically inert so as not to interfere with chemicals used in the sizing process.

Mineral fillers reduce the mechanical strength of a paper.  This is offset by improved paper printing characteristics, increased brightness and opacity.  Mineral fillers and pigments extend cellulose fibres and thereby reduce unit product costs.  In the case of woodfree uncoated paper, the cost of the fibres per unit weight can be up to five times higher than that of the pigment.  In double-coated woodfree paper, where binders add to production costs, fibre costs can still be up to three times higher than those of fillers.

The most commonly used minerals in filling applications are kaolin, calcium carbonate (both ground and precipitated forms) and talc.  Other minerals including barytes, diatomite and gypsum are also used, but in far smaller quantities.

Water washed kaolin is the preferred type of kaolin for paper filling applications, as it is readily dispersible in water.  Air floated kaolin generally has a lower brightness than water washed grades and tends to be used in less expensive paper products as a result.