Throughout the world, paper mills produce large quantities of paper intended for use in printed publications. These producers use mineral pigments as fillers in the sheet as well as in the formulations that are sometimes used to coat publication papers. These pigments are a cost-effective means for improving paper opacity and, especially brightness, which is particularly important in providing a good visual print appearance. When manufacturers of woodfree papers switched from clay pigments to precipitated calcium carbonate (PCC), they were able to take full advantage of the brightness and opacity that PCC provided. Today, papermakers producing mechanical fiber containing papers are keenly interested in increasing sheet brightness for improved printability. Typically, these manufacturers bleach to improve the brightness of the fiber and of the finished product. However, as fiber is bleached more heavily, the cost of manufacturing rises and opacity is subsequently lowered. Both are undesirable, and lower opacity impacts poorly on printability. By using PCC in conjunction with patented AT® PCC technology, from Specialty Minerals Inc. (SMI), for use in acid systems, papermakers are able to achieve higher brightness and opacity, lower environmental burden and improve overall economics. For many manufacturers of mechanical papers, PCC has become the most attractive and, in some cases, the only alternative to accomplish this.
A sheet of paper filled or coated with PCC exhibits high brightness and opacity because of the shape (morphology) of the PCC particles. Specific particle shapes cause a high degree of internal reflection and refraction as incident visible light repeatedly hits multiple pigment-air interfaces. PCC is better suited to this purpose than most other minerals due to its narrow particle size distribution and uniform particle shape. We precisely control these characteristics in order to maximize the mineral surface available for light scattering. But optimizing particle size and shape to enhance light scattering requires other system optimizations as well. When papermakers use PCC as a filling or coating pigment, a large number of sub-micrometer pores are created within the paper structure. This micro-porous matrix is highly efficient at scattering light. However, a second natural consequence is that a filled or coated paper containing a large proportion of PCC tends to readily absorb fluids. Printers observe this on offset printing presses, when the fluids in the inks and fountain solutions are absorbed to a higher degree than usual. The extent of this absorption is very difficult to quantify, and does not significantly affect print quality. However, during extended printing press runs, which often involve hundreds of thousands of impressions, printers sometimes encounter an increased frequency of press cleaning, or increased buildup of contaminants on the press equipment. Printers using rotogravure presses sometimes find that inks do not spread as well as for other pigments, and this can cause decreased print quality.
We know that by optimizing the entire printing system, including inks and/or fountain solutions, printers can accommodate the increased absorbency of PCC containing papers so that no negative effects are observed. We recommend this comprehensive approach in dealing with printing issues, as it is much more effective than adjusting the printing parameters individually.
When printing on offset presses, a fountain solution must have the correct viscosity, be able to emulsify in the ink and be removed from non-image areas. Fountain solution concentration and feed rate through the press must be maintained in order to ensure adequate protection of non-image areas. Commonly used inexpensive additives, such as isopropyl alcohol replacements and anti-piling agents, tend to increase viscosity and improve emulsification. We recommend consulting fountain solution suppliers for the best cost-effective remedy. Each supplier has their own formulations that work best with their products.
Inks having appropriate emulsification capabilities also help ensure problem-free operation. SMI has demonstrated on many commercial presses that it is possible to achieve the correct ink-fountain solution balance by minor adjustments of the fountain solution and/or the ink, and by ensuring appropriate “water” carrying capability of the printing plates. Our printing specialists are at your disposal for more detailed discussions of potential issues and solutions to your printing issues.
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