Multi-pigment formulations contribute to the rise of matte LWC: formulations for matte coatings in acid systems have been developed and tested.
These market drivers, combined with new multi-pigment mineral formulations that overcome traditional obstacles in acid papermaking, will allow U.S. paper producers to undergo a rapid transition to matte grade paper production within the decade.
NEUTRAL LOOKING MATTE
The predominant coated paper produced in the United States is lightweight-coated, usually produced in acid systems and usually supercalendered (1,3). Many matte-coating formulations used in higher quality grades use significant percentages of calcium carbonate pigments to allow calenderability while maintaining low gloss levels.
However, acid systems used in most LWC production can tolerate only low levels of calcium carbonate. This fact, combined with the use of supercalenders in many LWC mills, leaves few options for matte production. There is a need for engineered coating pigment formulations to allow use in supercalendered acid systems while providing good optics, good printability and low sheet gloss.
An experiment was designed to minimize uncertainties related to coatweight, calendering, printing and measuring variations. To determine "optimum" results, a technique called quality function analysis was employed to identify optimal parameters from a variety of competing responses (4).
For example, when coated papers are subjected to greater calendering pressure, they typically become smoother and glossier, but they will lose optical scatter and thus have lower brightness and opacity. To arrive at an optimal calendering condition, you must compare smoothness, gloss, brightness and opacity. That is difficult since each type of response has its own scaling and can have very different intrinsic values, such as cost. Opacity might be controlled using Ti[O.sub.2], loss might be controlled with plastic pigment, and brightness and smoothness might be controlled with coatweight. Depending on the application, smoothness also might be viewed as more desirable than optical responses. Thus, the scaling and desirability of each attribute must be well understood to arrive at an optimal solution and is typically decided by individual mill requirements.
TESTING KAOLIN/CARBONATE COMBINATIONS
An experimental design performed at the IMERYS Technology Center in Sandersville, Georgia, USA included three levels of calcium carbonate and three particle sizes of kaolin. The calcium carbonate was a coarse precipitated grade for use in matte coatings. The kaolins were experimental delaminated kaolins for which shape varied slightly with particle size. Table I shows some physical properties of the pigments. Coating pigment formulations are shown in Table II. The formulations used a binder system with 8 parts each of starch and latex. Coating was on a 44 g/[m.sup.2] acid LWC basestock using a Helicoater 2000 at a speed of 1000 m/rain, with a target coat weight of 10 g/[m.sup.2]
Samples were calendered with a Beloit sheet-fed laboratory supercalender to dual gloss targets of 25 and 30. All paper property measurement data were interpolated to a constant gloss of 28. Sheet property measurements included ISO brightness and L, a, b color space coordinates, printer's opacity, 75 [degrees] sheet and print gloss, and Parker Print-Surf S10 roughness. A Prufbau press with heatset magenta ink and a Sinvatrol hot air dryer to set the ink were employed for printing. Table III shows sheet property measurement results. We interpolated sheet properties to a constant sheet gloss of 28 and print gloss to a constant ink density of 1.3. Because of the broad range of pigments used, calendering loads required to achieve the dual gloss targets varied widely. The calendering loads varied by greater than a factor of five to maintain the target sheet gloss.
Two different quality functions were calculated to help illustrate the versatility of this technique, shown in Table IV. Quality function A has been calculated with equal weighting of brightness, Hunter b, printer's opacity, roughness, and print gloss. In this calculation, each property is normalized so that the best value is given a 1 and the worst value is given a O. The quality is then the sum of the normalized values. Similar terms have been used to calculate quality function B. However, in this example, roughness has been given a weight of 1, print gloss has been given a weight of 0.5 and all other properties have been given weights of 0.25. Thus, the best (lowest) value of roughness has been given a 1 and the best (highest) value of print gloss has been given a 0.5. These different property weightings, chosen arbitrarily, resulted in a different ranking of the formulations. The B version might be applicable where a production line has poor roughness and inferior print gloss and must improve those particular properties, perhaps to the detriment of other properties.
To determine time influence of pigment physical properties and formulation ratios on the quality functions, we performed response surface regression analyses. Results of the analyses were significant, with significance levels of 0.024 and 0.016 for quality functions A and B. The adjusted R (2) values, the part of the variation in the data accounted for by the model, were 0.97 and 0.95. The observed and predicted values of the overall qualities are shown in Figure 1.
[FIGURE 1 OMITTED]
Table V gives standardized coefficients for the pigment formulation parameter effects on the overall qualities. The standardized coefficients take into account the range of variation of the independent variables. Thus, they describe the relative amount of variation in the dependent variable that each term in the model can produce. For quality function A, the kaolin particle size has about a 300% greater influence on quality than the CaC[O.sub.3] level. However, the CaC[O.sub.3] level has about a 30% greater influence on quality function B than the kaolin particle size. This shows that desired quality parameters can have profound effects on which coating pigment formulation variables should be used to optimize paper quality.
For the North America market, a method has been illustrated to formulate matte coatings in acid systems. The quality of such matte grades were predicted from calcium carbonate levels and the particle size of kaolins.
Within the constraints of this investigation, different qualities were obtained with calendering loads varying by a factor of five, while maintaining a targeted sheet gloss.
Table I. Pigment physical properties. Property Kaolin A Kaolin B Kaolin C Calcium Carbonate GE Brightness 86.4 86.9 87.3 96.1 Sedigraph PSD % 39 52 64 56 < 2 [micro]m Table II. Coating formulations. Starch 1 2 3 4 5 Kaolin A 100 0 0 0 80 Kaolin B 0 0 0 90 0 Kaolin C 0 100 80 0 0 Calcium Carbonate 0 0 20 10 20 Table III. Sheet property measurement results. Coating # 1 2 3 4 5 Calender Pressure (kN/m) 107 39 85 104 206 Brightness 67.9 69.5 69.8 69.0 68.8 Hunter b 6.6 6.2 6.2 6.4 6.5 Printer's Opacity 85.9 87.1 86.8 86.1 85.8 Parker Print-Surf 1.62 1.82 1.69 1.63 1.55 S10 Roughness ([micro]m) Print Gloss 44.9 54.8 55.4 51.5 46.8 Table IV. Overall qualities. Coating # 1 2 3 4 5 Overall Quality A 0.8 3.8 4.2 2.7 2.0 Overall Quality B 0.8 1.2 1.7 1.3 1.3 Table V. Standardized coefficients for the pigment formulation parameter effects on the overall qualities. Parameter Quality Function A Quality Function B Kaolin PSD @ 2 0.95 0.6 mm % [CaCO.sub.3] 0.3 0.78
The author would like to acknowledge the contribution of Susan F. Boss in gathering market information.
1. "2001 Coated Printing Papers Survey," American Forest and Paper Association, Washington, D.C., USA, August 2002.
2. Personal communication, Martin Glass, EMGE & Co., Truro, Cornwall, United Kingdom, October 2002.
3. "Fisher Pulp and Paper Database," Fisher International, Inc., Norwalk, CT, USA, 1999.
4. "Quality Function Deployment: Integrating Customer Requirements into Product Design," Yoji Akao (editor), Productivity Press, Inc., New York, NY, USA, May 1990.
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|Title Annotation:||Practical Solutions|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||May 1, 2003|
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