Precipitated Calcium Carbonate (PCC) Provides Quality Improvement in all Coated Mechanical Papers
Precipitated calcium carbonate (PCC) is well known for good runnability of high solids coatings, but it is especially well known for high performance. Two different mechanisms lead to its high performance coating structure and optical efficiency. Scientists have correlated increases in coating structure to the simple concept of maximizing the number of particles of the “right” particle size and shape that can scatter light effectively. The optimum particle size for maximizing the light scattering of PCC is 0.4 - 0.6 microns. In addition to providing precise control over the crystal structure, our PCC precipitation process also allows regulation over many other product attributes including average particle size and particle size distribution.
In selecting pigments for a coating formulation, control over all three pigment properties (particle shape, size and distribution) determines the final coating structure, its optical efficiency, and the resulting coated paper performance. Specifically, particle shape can improve coating structure through physical hindrance while average particle size and particle size distribution can provide coating structure through controlled consolidation. This is supported by particle packing theory which states that particles narrowly distributed in size will not pack as efficiently as those widely distributed in size. The average particle size and narrow size distribution can also provide optical efficiency by maximizing the number of optically active particles, those with a particle size between 0.4 and 0.6 microns.
The following “Broken Star” chart illustrates the performance benefits of PCC in a light weight coated (LWC) paper coating formulation. This SMI study compared an aragonitic Opacarb® A40 PCC/kaolin clay blended topcoat with a narrow particle size-distribution ground calcium carbonate (GCC)/kaolin clay topcoat. Coatings were applied over a standard LWC mechanical pulp base paper.
In this example, Opacarb® A40 PCC demonstrates a performance advantage over narrow particle distribution GCC. In a standard LWC formulation, Opacarb® A40 PCC can be used at significantly higher levels to improve brightness, opacity, smoothness and printability while not detracting from sheet gloss. As shown, 80 parts of Opacarb® A40 PCC compares with 60 parts of GCC at equal sheet gloss, but with much better optical properties and print performance.
Regarding coater runnability, a non-spherical particle shape and a narrow particle size distribution can lead to high performance in coated paper but also can lead to slightly increased coating high-shear viscosity. This correlation between a non-spherical shape and increased coating high-shear viscosity is widely recognized. On the other hand, a non-spherical shape may not lead to poor runnability, but might lead to slightly lower coating solids.
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