Changing pigments or bleaching technology to improve brightness and print quality compared

By Using Technology, Improved SCA Grades Can Target LWC

Much of the growth of SC paper, and more specifically SC-A paper (filler levels of 25% to 35%), has been in rotogravure grades used in catalogs. Some of this growth has come at the expense of lightweight coated (LWC) wood-containing grades. This growth has been fueled by improvements in SC paper quality, as well as a decrease in price differentials between SC and LWC Paper.³ As the price differential decreased, it’s driven printers to evaluate SC-A papers; the improved quality of this paper has kept these customers. With these improvements, use of SC-A has begun to move beyond catalogs and into magazines that are printed rotogravure. SC-A paper is even beginning to target the offset-printed magazine market.

For growth in SC-A demand to continue broader success in the rotogravure magazine market as well as offset magazine applications will be needed. Success in the magazine markets will require that SC-A quality be raised again by improving brightness and more importantly improving print properties. This paper will examine some of the technologies that are available to the SC producer that should assist them in meeting these challenges.

SC PAPER TECHNOLOGY TRENDS. There are various technologies that can be used to change key sheet properties. The key properties are brightness and printibility.

Brightness: Improving SCA brightness and printability has been a focus over the last several years. Papermakers have available to them different tools to achieve improved brightness, including pigments and bleaching. Upgrading bleaching systems from those based on reductive bleaches, such as hydrosulfite, to those based on oxidative methods, such as peroxide bleaching, can increase the brightness of the final sheet by 5 to 6 units, i.e. brightness of 68 versus 73. However, this increase in brightness comes with a loss in opacity as a result of a decreased light absorption coefficient of the pulp, and at a higher cost (Table 1). In addition, this gain in brightness can require a capital investment as high as $15 million depending on the capacity needed.⁴ While peroxide bleaching is being used, the rush to this technology has been limited due to the high costs.

An alternate method to achieve higher SC paper brightness is to convert the paper machine to neutral running conditions and use high brightness calcium carbonate as the primary filler pigment.While alkaline darkening of pulps bleached with reductive bleaches has been a concern, running these systems at neutral pH’s coupled with the high filler loadings used in SC-A papers, have proven that alkaline darkening issues can be overcome. Acid tolerant technology was introduced in the early 1990s, and a new technology that limits the calcium solubility and virtually eliminates the acid demand of the precipitated calcium carbonate (PCC) pigment has recently been introduced (Figure 1.).⁵ This new technology also helps make neutral wood-containing paper machines more user friendly by significantly reducing the deposits on machines that have been observed with the traditional scalenohedral PCC’s (S-PCC).These improvements have allowed PCC to be a viable alternative to improve brightness and opacity cost effectively. Theoretical data comparing various approaches optical properties and costs illustrate the different approaches.⁴

FIGURE 1:Newer PCC technology reduces acid demand vs. traditional pigment.

Rotogravure Printability. With the cost and brightness advantages shown in Table 2, conversions of SC paper to PCC should be rapid. And while there have been a few conversions in the middle 1990s, conversions to date have been limited by concern over printability. The use of a highly structured standard scalenohedral PCC in wood containing grades can present challenges in terms of running the machine, i.e. calcium deposits, strength issues, pH control, and can also present significant challenges in meeting printing demands.

To achieve the desired level of print quality in rotogravure both smoothness and porosity control must be achieved. Smoothness is needed for efficient ink transfer in the press and porosity control for ink hold out. When using a standard scalenohedral PCC, optical performance is clearly enhanced, and smoothness is in the desired range vs. a sheet made with a filler clay-calcined clay mixture. However, the porosity of this sheet filled with the S-PCC is significantly higher (Table 3). Higher sheet porosity leads to a higher level of ink absorption and thus a less desirable printed look than the sheet made with kaolin. One solution is to maintain a certain level of kaolin in the sheet to control porosity, but this approach reduces the optical advantages of using a PCC.

To address these shortcomings, pigment technology must be reassessed. Traditionally, pigment technology has focused on maximizing the performance of the pigment and then adapting papermaking technology to accommodate the pigment. Sometimes this is successful, as in the case of PCC for used for wood free paper. But other times the performance characteristics of the pigment are lost due to incompatibility with the other components of the system. This has been observed when narrow particle size kaolins are used in conjunction with narrow particle size PCC’s and ground calcium carbonate (GCC) in coatings. In order to get the most out of the high value pigments they must be designed to work with the rest of the system.Designing components of the total system to work with one another can control print characteristics.At times this has resulted in pigments that by themselves would not be the first choice for use, but in the system give the best results. This design approach is also applicable to designing performance in uncoated paper. ^6,7

Using this approach, alternative methods to control porosity without sacrificing optical properties were examined. Changing the morphology of the pigment away from the traditional highly structured scalenohedral crystal has made it possible to reduce porosity and yet still maintain the desired optical properties. This new approach can produce porosity values in the range of a typical acid sheet with 100% of the filler being PCC (Figure 2).

FIGURE 2:Adjusting PCCpigment design can lower SC paper porosity.

This produces better ink hold out and results in improved print quality. Other benefits of a sheet filled with 100% PCC can now be realized along with the improved print quality (Table 4). Thus, by eliminating the need to add lower brightness kaolin to the sheet for porosity control, the full brightness potential of the PCC can be realized.

Offset Printability. While rotogravure is SC-A’s primary market, continued growth of this grade will require that it meet the needs of offset printers as well. Today a limited amount of offset is run on acid sheets using SC inks.For SC-A paper to compete with coated mechanical grades in the offset market, a focus on print quality will be required. One ranking for the requirements for coated mechanical (#5 web grades) and SC grades is shown (Table 5).⁸ These data show that the two grades overlap in terms of requirements for offset printing. For SC-A grades to be used in place of a coated mechanical papers the SCA grade must have qualities that exceed the minimum requirements.This level of high quality performance will be needed to prevent picking and linting on the offset press that can occur when using high tack inks.

To assess the offset print quality of SC-A paper produced with PCC, the print characteristics of an acid sheet were compared to a neutral sheet produced with a traditional S-PCC. The results shown in Figure 3 indicate that the acid sheet produces a lower slope, i.e. built ink tack at a slower rate, than the sheet produced with an S-PCC. While both sheets had the pick strength that would meet the requirements of a coated sheet as described in Table 5, (>400gm/cm), the sheet containing the S-PCC would fail first on the press because of the high rate that ink tack was building on press. The sheet utilizing the new PCC technology designed for use in rotogravure printing, also built ink tack on the press at a faster rate than the acid sheet although it was closer to the acid sheet performance than the sheet made with the S-PCC. All three of these sheets printed adequately on offset presses using SC inks. However, given the high rate of ink tack build of the sheets produced with PCC, they would be likely to fail if used with offset inks designed for coated paper.(Note: Offset inks used for LWC paper are generally higher in tack than that used SC paper).

FIGURE 3: Acid sheet builds ink tack slower than traditional S-PCCsheet.

One method to improve the printability on an offset press using higher tack inks would be to increase the pick strength of the paper. This could be accomplished using wet end starch or different refining approaches. However, none of the papers tested had low pick strength. Rather the sheets that were examined built ink tack quickly which would cause the sheet to reach its failure point earlier. Thus, an alternative solution would be to re-examine the pigments in the system and adjust the pore structure of the paper to slow the ink set rate. Indeed other studies have shown that changes in the pore structure of the paper greatly influence the rate at which the ink tack is built on press.^6,7,9 This has become a common approach in coated papers for addressing printability concerns. By using this approach for SC-A paper, offset printability could be enhanced more cost effectively by eliminating the need for additional chemical or energy usage.By adjusting the PCC pigment properties to help change the paper structure, the ink set rate can be reduced while still maintaining other properties. New PCC technologies can yield ink set rates equivalent to that of an acid sheet (Figure 4). Using design of experiment techniques (DOE) further control of the ink set rate can be envisioned and new technology to create a high quality offset SC-A sheet appears achievable. For the SC-A producer this opens the opportunity to explore new markets that have been reserved only for coated papers.

FIGURE 4: Ink set rate can be adjusted to match acid based sheets with new pigment approach.

Improving Acid SC-A Paper. Options to improve printability are not limited to those utilizing PCC. Significant improvements can still be made in acid sheets. This could be a viable option for those sheets not needing a significant improvement in brightness, or where other factors prevent conversion to neutral papermaking. For instance, it is quite well know that the addition of calcined clay to a SC sheet improves its brightness, opacity and print properties such as show through. Further improvements can be obtained by using silicate-based pigments in place of calcined clay. Various improvements can be obtained as the filler combinations are changed in an acid SC-A sheet (Table 6).This wide degree of performance allows those making acid SC-A paper to change the quality of the paper to meet their customer’s needs as well without having to convert to neutral running conditions.

Conclusion. As the SC paper market continues to grow, new solutions will be needed to continue to fuel the expansion. Over the last decade properties on SC paper have continually become more demanding. This trend does not appear ready to change anytime soon. The solutions that will be needed over the next five years to meet these increasing quality demands will not be found by looking only at the traditional pigments. They will require that new pigments be designed specifically for the SC markets that take into account their needs. Enhanced printability can be obtained in acid grades by changing the pigment and paper structure to create improved ink transfer and better ink hold out.For neutral machines PCC can be designed to work with the rest of the system allowing the desired brightness and opacity gains to be obtained while maximizing offset and/or rotogravure printability and minimizing acid demand. For SC-A offset grades to compete with coated mechanical grades, new pigments that can address the printability issues such as controlled ink set rate will be required. These technologies coupled with a design approach that looks at the whole system will allow the papermaker to maximize the performance on machine.Design strategies to create the solutions for tomorrow are available today.

The author would like to recognize the work that was performed by paper applications team in Copenhagen, Denmark, and Macon, Georgia, U.S. The author would also like to thank Klaus Lundén, Tommy Bisgaard, and Peter Tøttrup for their discussions and input.

References:

1. Robert D. Ball, “Growth and Acceptance of Supercalend-ered Papers,” Paper and Marketing Distribution Trends, Pulp and Paper , 1994, p. 89.

2. Jaakko Poyry, Market Growth Estimate

3. RISI Long Term North American Pulp and Paper Review, July 1999.

4. Simons Consulting Group, 1998.

5. US Patent #’s 5043017, 5156719.

6. Douglas Carter, Cynthia Goliber, Joseph Ishley, Joanna Barfield, and Joachim Drechsel, 1999 Tappi Coating Conference Proceedings.

7. Douglas Carter, Paperage, May 1998.

8. Nancy Plowman Associates, Massachusetts

9. Donigan, D.W., Ishley, J.N. and Wise, K.J., TAPPI, Vol. 80, No. 5, 1997.

Douglas Carter is Global Business Leader-Paper, Huber Engineered Materials