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The first twenty years of the third millennium could be termed the "Age of the White Forest" based on the potential role of minerals in papermaking. Minerals cost only about one-quarter as much as fiber and impart properties to paper not otherwise possible.
HIGHER QUALITY, LOWER COST. The marketplace demands paper that exhibits life-like color, sharp text, sheet flexibility, strength, lighter weights, and speedy commercial and home printing. However, customers, consumers, etc., expect to pay little or no premium for a product that can do all this. Several key advances are helping to produce sheets of paper to meet these needs.
Scientific advances in the understanding of particle shape and size are creating a new generation of kaolin and calcium carbonates. These products enhance fiber performance and combine with other additives and chemicals to produce better formulations at lower cost. Painting coatings on micro-thin layers would not be possible without engineered "super kaolin" and "super carbonate." Also, much has been learned in only the past few years that allows clay to be made with specific particle size and shape to produce desired sheet properties. In some cases, less super kaolin can be used, or a boost in the use of other minerals can achieve a particular objective. Knowledge is also growing about multi-pigment combinations to hit higher levels of sheet performance and improve coating productivity.
PAPER DESIGN. Nano-structures are the underlying reason for the increased power of images on paper. Improved paper imaging capabilities, which provide a nano-scale view of fiber and other components in paper and paper coatings, offer insight that is helpful in creating future grades of paper. Advances in the creation of new composites on the nano-structure level are transforming paper, and the coating is a key component. Super pigments are playing an ever-increasing role in helping papermakers build properties into the sheet. Nano-technology developments in fiber science, minerals, and other additives provide new tools to put order and structure into the designs of a sheet.
COATING FROM THE INSIDE. Compared to copy paper, highly filled grades such as supercalendered SC-A grades with 35 % to 40% filler have more filler on the surface than in the middle. Suction foils cause minerals to migrate to the top and bottom of the sheet and act as an internal coating. Less filler in the middle means fiber can hold the sheet together and makes the base sheet look more like a coated sheet while enhancing gloss and printability.
Careful selection of pigment shape and size also produces the best drainage and allows for better sheet consolidation in the press section. Engineered clay can provide higher sheet solids off the press section, thus producing better wet web strength, gloss, and printability.
ADVANCES IN COATING. Another key is the evolution of coating technology. The current move is to fountain jet applicators that spray on the coating and doctor with a blade. The jet applicator avoids pressure pulses common with roll applicators and short dwell heads that squeeze water out of the coating. The jet system is more forgiving of engineered or narrow particle-size distribution (NPS) pigments that tend to dewater faster. The metered size press (MSP) is another key development that can be put on machine and is forgiving of weak sheets. The MSP allows uncoated mills to produce value added coated grades, and they are of special interest for producers looking to make ultra lightweight-coated groundwood.
NEW WORLD OF PAPER GRADES. The selection and manufacture of particles with the right size and shape is the first step toward a new world of paper grades. Very narrow particle size fractions allow use of differently shaped particles as compared to conventional kaolins, which have so many particles present they fill up the spaces in the sheet. This masks the benefits possible by mixing various shapes. Today, NPS clays are creating new possibilities in coating engineering. These products make it possible to use technologies developed in the world of liquid crystals and colloid chemistry. One example of this would be to allow a self-assembly of the components to take place.
The ultimate development will be a coating that self-organizes into glossy high quality sheets, much the same way that an automotive body develops gloss as it is painted. This will require different types of formulations and pigments, as well as novel coating systems, such as the curtain coater, to develop a high gloss film. The challenge will be to micro-engineer a high gloss coating with sufficient porosity for printing. Part of this solution will also require development of different coating systems.
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