The Precipitated Calcium Carbonate (PCC) Advantage for Coated Mechanical (Groundwood) Papers
Coated mechanical papers are made from base sheets, which consist mainly of mechanical pulps. These sheets are then coated with various minerals and chemicals to form a smooth, bright surface that enhances ink holdout. These usually light weight coated (LWC) papers are used in magazines, catalogs and inserts and can be printed via offset or rotogravure printing processes. In North America, LWC papers are also classified as coated #4 or #5 papers.
The performance of a pigment on the surface of a paper is related to five major particle parameters: shape, size, size distribution, surface chemistry and surface area. Specialty Minerals Inc. (SMI) crystal-engineered precipitated calcium carbonate (PCC) pigments can deliver these properties, which combine to control the surface and brightness of the coating layer including optical properties, smoothness, bulk, stiffness and printability. Working together with the papermaker, our scientists create not only coating formulations but, more importantly, also provide the ability to meet the cost-efficient demands of tomorrow's paper markets for high quality, brighter papers. Specialty Minerals PCC helps LWC papermakers reduce premium pigment (TiO2) usage. Many mills can reduce bleaching costs as well.
Today, LWC papers are coated with Opacarb® coating grade PCC from SMI to enhance both optics and printability. With a range of aragonite particle sizes and narrow particle size distributions, Opacarb® PCCs are tailor made to maximize light scatter and fiber coverage, which provides an optimal coating structure for porosity control and ink receptivity. Color print images are sharp and vibrant.
Because the optical properties of coated mechanical papers are strongly influenced by the base sheet, the composition of filler pigments is critical. Whether incorporated into the base sheet as virgin filler or through coated broke, PCC provides significant quality improvements to coated mechanical papers.
Specialty Minerals Offers a Broad Range of Precipitated Calcium Carbonate (PCC) Products
Opacarb® PCC family sets the standard for aragonite shaped coating grade PCC in the world today. With a narrow particle size distribution and a choice of specific aragonite particle sizes from 0.4 microns to 0.6 microns, it provides exceptional coated paper performance. Opacarb® PCC delivers outstanding performance at high levels (80 parts) in the coating formulation, developing high gloss and smoothness while enhancing brightness and whiteness. Opacarb® PCC is suitable for both offset and rotogravure formulations at all coater speeds.
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.