EFFICIENT USE OF FLUORESCENT WHITENING AGENTS AND SHADING COLORANTS IN THE PRODUCTION OF
WHITE PAPER AND BOARD

Author

Liisa Ohlsson1 and Robert Federer2

Organisation and address

1Ciba Specialty Chemicals, Business Line Paper, Water and Paper Treatment, Grazerstrasse 42a, A-8101 Gratkorn, Austria
2Ciba Specialty Chemicals, Business Line Paper, Water and Paper Treatment, Postfach, CH-4002 Basel, Switzerland

email

liisa.ohlsson@cibasc.com and robert.federer@cibasc.com

Keywords

FWA, shading pigments, whiteness, FDA compliance

ABSTRACT

The use of Fluorescent Whitening Agents and shading colorants is a widespread technology and is extremely important for all producers of white paper and board.

A novel whiteness concept for production of white paper is introduced with three examples from the production of uncoated fine paper where combined use of a unique FWA slurry and special pigment colorants has proved to be highly effective.

For liquid and dry food packaging papers and boards, the use of a complete whiteness package is now available. Whiteness of the paper and board can be increased considerably by using FDA compliant pigments and Fluorescent Whitening Agents.

1 BACKGROUND

As the whiteness levels are still increasing in most grades of white paper, from improved newsprint to coated board, the correct use of Fluorescent Whitening Agents (FWAs) and shading colorants are of increasing importance. The correct choice of product and application point is critical. In addition to a new development in the field of FWAs, this paper also shows how to produce high white paper and board by using novel concepts, which enable the paper manufacturer to save in treatment cost for whiteness and shade.

2 THEORETICAL BACKGROUND FOR WHITENESS

All natural material, like fibers, pigments etc. have a yellowish tint. The bleaching of pulp further removes lignin and other non-fibrous materials, but residual lignin and other colored substances cause the yellowish tint of the chemical pulp. For the papermaker, the use of FWA and shading colorants are the main tools to eliminate this yellowish hue and to improve paper quality in terms of appearance.

FWAs convert invisible ultraviolet radiation at 300-400 nm to visible blue light at 400-500 nm. Using FWAs is a convenient way to increase the reflectance (and thus the lightness) of paper and simultaneously to move the shade from yellow to blue. This subtle tint change makes the paper look even whiter.  

Violet or blue shading colorants reduce or eliminate the yellow hue in pulp with a different mechanism. The shading components absorb yellow and red light. The human eye perceives these papers to be whiter than paper with a yellowish hue. Shading colorants can be used both to control the final shade of the paper or board and to reduce the level of FWA required in achieving the target whiteness.

The main types of FWAs used in the paper industry are derivatives of diaminostilbene di-sulphonic acid. The number (2, 4 and 6) of sulphonic acid groups (-SO3H) is used to classify these products since product properties and application of the different classes exhibit distinct differences. The various other substituents in the FWA compound are of minor importance with regards to the fluorescence, but significantly affect application properties and substantivity. High affinity di-sulphonic FWAs are mainly used in the wet end, but also find their use in coating application. Traditional tetra-sulphonic FWAs are widely used in the wet end, at the size press and in coating. Hexa-sulphonic FWAs are applied only at the size press or in coating application for high whiteness levels, due to their low affinity to fibers.

3.1 WHITENESS CONCEPT - UNCOATED FINE PAPER

The whiteness and brightness of copy paper has increased steadily over the last years. A few years ago a CIE Whiteness of 170 was thought to be an impossible goal. Today, an increasing number of paper mills are producing white papers with a whiteness above 160 and approaching the magical 170 mark. 

In the scale of ISO Brightness, this corresponds to figures between 110 and 112, depending on the type and amount of used shading colorants, see Figure 3.1.

Figure 3.1

Even if the demand for wood free uncoated grades has not been growing to the same extent as coated papers, especially the higher white copy paper grades continue to grow at a rate above the average.

This development sets new demands for raw materials and wet end chemistry. In order to increase the base brightness, high bright fillers like PCC and marble, and highly bleached pulps are used. Fluorescent Whitening Agents and shading colorants are used to enhance the whiteness from the base level and are essential in producing high white papers.

In high white copy papers, the cost of these additives account for on average 2-3% of the total selling price of tonne of paper. Optimization of chemicals enhancing optical properties can give considerable cost savings. 

Traditionally, a generic tetra-sulphonic fluorescent whitening agent has been used both in the wet end and size press application. This approach can be convenient, i.e. saving storage space and requiring simple dosing set-up, but it is not always the most cost-effective option. In higher white shades, a hexa-sulphonic FWA is additionally applied at the size press to obtain the required whiteness level. Anionic direct dyes and basic dyes have been used for shading of white paper to give the extra lift to whiteness as well as to adjust chromaticity co-ordinates.   

A new approach combines the wet end application of highly efficient Ciba® TINOPAL® NP Slurry FWA and Ciba® IRGALITE® shading pigments with the optimized use of tetra- or hexa-sulphonic FWA at the size press.

A laboratory study indicates that the new approach is also efficient when the aim is to up-grade whiteness, see Figure 3.2. On constant dosages of FWA and shading colorant, the new concept reaches higher CIE Whiteness and especially higher ISO Brightness.

Figure 3.2

TINOPAL NP Slurry is a urea free di-sulphonic FWA. This new development in the field of FWAs has an environmentally friendly formulation without any urea, solvents or solubilisers. This gives an advantage when maintaining Nitrogen, BOD, and COD levels under limits agreed with local environmental authorities.

Due to its high substantivity, TINOPAL NP Slurry is highly effective in stock addition. The amount of FWA needed to obtain a certain whiteness level is in most cases lower than for other types of FWAs. The high affinity of this FWA makes a late addition possible, which permits the exact control of the process, where whiteness and brightness are quickly corrected by a change in the FWA dosage level.

The low anionic charge and excellent affinity to pulp ensures that TINOPAL NP Slurry does not interfere with cationic additives in the stock. Excellent retention of this FWA also decreases the amount of FWA discharged to the effluent plant.

IRGALITE pigments used in shading of white paper are specially manufactured organic pigments in an aqueous anionic formulation. Typically they have high light stability and do not lose their effect over time. Due to their pure and bright color, they have positive influence on brightness compared to direct dyes, as can be seen in Figure 3.3. 

Figure 3.3

Several paper machine conversions have shown that this technology can give noticeable cost savings and increase the efficiency of the papermaking process especially in the production of high white grades.

3.1.1 CASE 1

TINOPAL NP Slurry was introduced in this mill with IRGALITE pigments during year 2000. The fine paper machine of capacity of 20 t/h produces several white grades from a furnish containing spruce with various hardwood pulps and PCC. This machine used a traditional concept of tetra-sulphonic FWA in the wet end and film press.

The following example is taken from a grade with ISO Brightness of 113 (CIE Whiteness 160). During the trial, a clear trend of decreasing FWA amount required in the wet end was observed when the shading dye was changed to IRGALITE pigment and FWA to TINOPAL NP Slurry.

The mill benefited from substantially lower treatment cost for whiteness. Additional benefits were a visually more brilliant shade due to bright IRGALITE pigments and lower environmental load due to urea-free formulation TINOPAL NP Slurry. Figure 3.4 illustrates the use of FWA and shading colorants as well as the total cost of whitening.

Figure 3.4

3.1.2 CASE 2

An envelope grade with an ISO Brightness of 108 (CIE Whiteness 155) is produced on a 15 t/h paper machine. For this grade, the addition concept of FWAs was changed. Before conversion, the mill was using tetra-sulphonic FWA both on wet end and size press. Direct dyes were used as shading components. After conversion, same paper quality could be produced using only wet end addition of TINOPAL NP Slurry and IRGALITE pigments to finalize the shade.

The change to lower anionic FWA decreases the cationic demand in white water. See Figure 3.5. Furthermore, broke has lower anionicity due to removal of tetra-sulphonic FWA from size press. This permits easier and more stable control for wet end (retention and internal sizing) especially when producing high white grades.

Figure 3.5

Additionally, the ISO Brightness and L* value could be increased by 2 and 1 points respectively. Decreased production cost can be seen in Figure 3.6.

Figure 3.6

3.1.3 CASE 3

This case study presents a fine paper machine with a production of 14 t/h copy paper with CIE Whiteness of 162. The furnish consisted of a pine/birch pulp mixture with PCC as a filler.

A trial was carried out in order to optimize the use of FWAs and shading colorants on this high white grade. The main result was the replacement of hexa-sulphonic FWA at the size press with a tetra-sulphonic type. This was made possible by introducing high affinity TINOPAL NP Slurry to the wet end together with the new IRGALITE shading concept.

In this case, both wet end and size press FWAs were replaced by products with lower anionicity. This decreased the cationic demand in the wet end considerably and gave better control on retention. Time for grade changes on and off this high white grade was reduced.

Better lightfastness of the paper was reported, which was mainly due to the excellent light stability of the chosen IRGALITE pigments.

Significant cost savings were obtained as can be seen in Figure 3.7.

Figure 3.7

3.2 WHITENESS CONCEPT - FOOD PAPER AND BOARD

The use of chemical additives for liquid and dry food packaging is regulated under various standards, the most commonly used being set by BgVV and FDA. Until now, the list of FDA compliant colorants for paper and board has been limited. In 2001, the FDA added a new colorant, IRGALITE Violet RM to the list. This allows the use of an effective shading package also in white food board, especially liquid packaging board.

FDA compliant colorants IRGALITE Violet RM and IRGALITE Blue R-L can be applied either separately or as a combination in the wet end. They not only control the shade but also improve the visual whiteness when added in correct dosages. Moreover, for some paper and board grades additional whiteness can be achieved by using an FDA compliant TINOPAL ABP type FWA. Increased whiteness gives liquid or dry packaging paper and board a visually enhanced appearance and consequently, an improved print quality. This can be either sharper contrast in the black-and-white printed image or improved color brilliance in color prints.

In Figures 3.8 and 3.9, the effect of IRGALITE Violet RM and IRGALITE Blue R-L is illustrated on a 100% bleached furnish in neutral conditions. At the addition rate of 120 g/t, whiteness of the furnish was increased by 7 CIE Whiteness points for the pure bleached furnish and 9 points for furnish treated with FDA compliant TINOPAL ABP-Z liquid. The control of chromaticity coordinates by these shading pigments is demonstrated in Figure 3.9.

Figure 3.8

Figure 3.9

IRGALITE pigments are effective in all furnish types and in all layers of a multi-layer board. The amount used in shading of white paper or board depends upon the initial brightness of the furnish and on the shade target desired. Normal application would involve shading the white top ply, but in most cases addition of shading pigments into the inner plies is feasible as well. The recommended addition level for individual IRGALITE pigment dispersion is between 20 and 200 g/t on final paper or board. In the production of multi-layer board the addition level can be higher in a certain layer or layers depending on the basis weight of the final board.

4 SUMMARY

For the papermaker, the use of FWA and shading colorants is the main means to overcome the natural yellowish hue of the pulp and to improve paper quality in terms of appearance. Violet and blue shading colorants can be used both to control the final shade of the paper or board and to reduce the level of FWA required in achieving the whiteness effect.

For production of white wood free papers, the wet end use of unique urea-free TINOPAL NP Slurry, in combination with selected IRGALITE shading pigments has shown to be a very efficient way of reducing production cost or increasing whiteness.

Optimized and correct use of TINOPAL NP Slurry and IRGALITE shading pigments in combination gives trouble-free shade control and the potential for further whiteness increases for all white paper grades. Paper mills using this concept have reported reduced overall FWA consumption and better control of retention. Quality benefits like a more brilliant shade, higher light-fastness and better environmental profile are added to more cost effective whitening.

A new comprehensive, FDA compliant, shading package has recently become available. It enables food packaging paper and board producers to gain even higher whiteness levels and better print quality, without having to compromise on meeting exacting standards set by regulatory authorities. 

LITERATURE

A. Siegrist, C. Eckhardt, J. Kaschig, E. Schmidt, "Optical Brighteners", Ullmann's Encyclopedia of Industrial Chemistry, 1991, 153 - 176

F. Muller, D. Loewe, B. Hunke, "Fluorescent whiteners – New discoveries regarding their properties and behavior in paper", Paper South Africa, April 1993, 4 – 22

O. Jokinen, P. Rohringer, "Optical brighteners for high white papers", World Pulp&Paper Technology, 2001, 17-18

J. Markert, J. Ohler, "Hohe Weissgrade bei gestrichenen Papieren – kombinierter Einsatz von optischen Aufhellern und Nuancierfarbstoffen", Wochenblatt für Papierfabrikation, 9/1989, 378-381

Copyright © Ciba Specialty Chemicals Inc. 2002

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