Table of Contents
Kaolin is a white, naturally occurring, hydrated aluminum silicate that is formed by the weathering of feldspar. Commercially significant deposits of paper grade kaolin are located in Georgia and South Carolina in the southeast United States, the Cornwall District of England, Brazil, and Australia. Kaolin deposits are classified as primary or secondary deposits depending on their location when formed. Primary deposits are found at the location where they were formed, while secondary deposits have been transported from their original place of formation.
The kaolin and paper industries use particle size distribution to differentiate between kaolin grades. Particle size distribution is determined using sedimentation techniques and is reported as a weight percentage of particles finer than a given particle size. The weight percent finer than two microns is the accepted classification point in the kaolin industry. Therefore, an ultra fine kaolin has 98% of its particles by weight, finer than two microns; a number one fraction kaolin is 90% finer than two microns; a number two fraction kaolin is 80% finer than two microns and so on.
Composite pigments are the most recent development in pigment technology and are produced by agglomerating two dissimilar minerals via the addition of a chemical structuring agent. Currently available composite pigments are comprised of varying amounts of calcined or hydrous kaolins in conjunction with titanium dioxide.
Performance of Kaolin-Based Pigments in End-Applications
Paper filling applications involve the addition of mineral pigments to a suspension of pulp fibers in water such that the filler material is retained in the fiber matrix and incorporated in the finished paper product.
The brightness and opacity results in Figures 2 and 3, respectively, demonstrate the aesthetic improvements achieved by the use of filler pigments in an uncoated freesheet application.
It is important to note that while the calcined kaolin produces significantly higher brightness and opacity than the other kaolin-based fillers, it is not used in commercial applications at the levels shown due to cost and abrasion concerns, especially at excessive filler loadings. Calcined kaolins are generally used at significantly lower levels (less than 5-6%) as a sole filler pigment or at even lower levels (1-3%) as part of multi-component filler pigment systems.
The superior print-through performance of calcined kaolin is again attributed to its porous, bulky particle morphology. Calcined kaolin particles have a greater propensity to accept and retain printing inks thereby retarding their penetration into the sheet and improving print-through.
The coefficient of friction results in Figure 9 are another example of modifying paper functional properties via filler addition. The results show calcined kaolin is the pigment of choice to increase coefficient of friction. If an application requires lower coefficient of friction, the chemically structured, delaminated or water-washed kaolins should provide the required performance.
All paper producers seek ways to lower production costs and improve profitability. The use of filler pigments is a primary means to generate cost savings. In general, filler pigments are less expensive than fiber. To illustrate cost savings generated by substituting filler for fiber, let’s assume fiber costs $300 per ton and a standard kaolin-based filler costs $100 per ton.
As with any rule, there is always an exception. In this case it is calcined kaolins. Due to their specialized production requirements, calcined kaolins are more expensive than some fiber sources. But calcined kaolins are unique.
The coating of paper and paperboard involves the application of a coating formulation comprised of pigments and binders to a basestock to improve the aesthetic properties of brightness, gloss and opacity, but more important, to improve printabillty and print quality.
Kaolin Performance in High Coat Weight Applications
End use applications for coated paper having heavy coat weights (20 – 35 gsm C2S coat weight) include book paper, annual reports, automobile brochures and coated paperboard grades. The coating formulations used in the production of these grades vary considerably. However, kaolin-based pigments such as high glossing, number one, number two and delaminated kaolins still represent the primary component of the coating formulations.
solids level of delaminated kaolins are limited to 67-68%. The viscosities of the four coating colors are listed in Table 3 and show the same trends as the clay slurry viscosity results. The high glossing kaolin has the highest Brookfield viscosity and lowest Hercules high shear viscosity. The number one kaolin and number two kaolin are intermediate to the high glossing kaolin. The delaminated kaolin has inferior rheological properties compared to the standard hydrous kaolins as reflected in the lower solids content required to achieve equivalent viscosity.
The brightness results track the block brightnesses of the pigments with the number one and delaminated kaolins producing higher sheet brightness in accordance with their higher block brightnesses. The opacity results are essentially equivalent.
Kaolin Performance in Low Coat Weight Applications
Coated paper produced with low coat weights (12 – 18 gsm C2S coat weight) are referred to as lightweight coated papers or LWC. Magazines, retail catalogues and advertising inserts and flyers are examples of end-use applications for LWC. The reasons for coating LWC grades are the same as other coated papers, to improve printability and aesthetic properties.
Heliotest results show the chemically structured kaolin provides substantially better rotogravure printabillty than the delaminated kaolins as a result of its smoother, bulky coating structure. In summary, the lightweight coated paper results illustrate that chemically structured and delaminated kaolins provide the enhanced basestock coverage and improved performance properties that are necessary to produce high quality LWC offset and rotogravure grades.
Performance of Kaolin-Based Pigments as Additives in Coating Applications
As previously discussed, kaolin-based pigments are the predominant component of most coating formulations, but some kaolin-based pigments such as calcined kaolins and composite pigments are used as additive pigments to provide specific optical or functional properties.
In addition to its optical properties and demonstrated effectiveness as a titanium dioxide extender, incorporating calcined kaolin into coating formulations increases the bulk, smoothness and porosity of coating structures to produce improvements in fiber coverage, rotogravure printability, ink receptivity, blister resistance, water dissipation and reduced binder migration.
Products are available with different particle morphologies, particle size distributions, rheological properties, brightness, etc., and selecting a specific product for a given application requires careful consideration. The selection process must be effectively coordinated by the paper producer and with their pigment supplier(s) to identify the appropriate product to provide the required performance. Pigment selection will become more difficult as the range of kaolin-based pigments expands as new products are introduced and paper and paperboard producers face new demands for improved performance and higher quality from their customer.