Coating pigments: from size and shape to nanotech.

Worldwide coating pigment consumption was 17.6 million tons in 2004. There are significant regional differences in coating pigment usage, as shown in the following graphs from a presentation by Peter Burri at the 2005 TAPPI Coating Conference: (1) (For the CD-ROM from the Coating Conference, see "Additional Resources" at left.)

ADVANCES IN PIGMENT TECHNOLOGIES

Coating pigment suppliers are developing new processing and treatment technologies to improve performance and value. For many years, kaolin clay was specified as either regular or high brightness and categorized by particle size distribution. Then, delaminated clays were developed to improve coverage. Calcined kaolin--thermally structured clay--was developed as an extender for titanium dioxide. Chemically structured clays were developed to improve coverage. Platy Brazilian clays offered high aspect ratio pigments to improve surface coverage.

Clay suppliers have used size engineering and shape to improve coverage and performance and are now focused on nano engineering of pigment morphology. Particle morphologies are now controlled in nano dimensions to optimize light interaction with the coating structure.

Ground calcium carbonate (GCC) has traditionally been conceptualized as a spherical pigment with particle size distribution controlled by grinding methods. It provided the advantage of natural high brightness. The coating grades of GCC were rated by percentage finer than 2 microns with 60 and 90 being the standards. GCC suppliers have used size engineering to produce products with improved coverage and glossing. New products include chemically treated and structured specialty GCC. The combination of size engineering and chemical treatment produces nano surfaced microparticles with discrete pore size distribution. This decouples capillarity from permeability and optimizes ink and coated paper surface interaction.

Precipitated calcium carbonate (PCC) has seen a trend away from rhombohedral (a spherical pigment) to aragonite PCC (an acicular pigment). Fine particle size aragonite PCC has good rheology and can provide excellent glossing and smooth coated surfaces. The newest PCC product is calcite, developed for coating ink jet papers at 40% coating solids on standard coating equipment.

The following sections explore other key pigments:

Talc is a soft, platy hydrophobic pigment that is especially beneficial in rotogravure papers--a process that is in more widespread use in Europe. North American LWC roto producers began using talc several years ago. In the past few years, the potential benefits of using 10-20 parts in coated matte offset paper have been recognized. Talc improves the matte effect while maintaining ink gloss, reduces ink scuff, and provides a silky feel. The benefits of using talc in label papers at 20-50 parts include longer die life and reduced dusting--plus an anti-blocking effect.

Titanium Dioxide has the highest refractive index of all coating pigments. It is widely used in coated unbleached kraft and coated recycled boxboard. It is also used in LWC and ULWC paper coatings to improve opacity. Two processes produce titanium dioxide. The older sulfate process was used to produce both anatase and rutile titanium dioxide crystals. The newer chloride process produces rutile at lower cost and higher quality. The last anatase production in the U.S. shut down in 2004. (2)

Plastic Pigments are submicron polystyrene spheres introduced in 1969 as 100-500 nanometer solid sphere pigments to improve glossing. Hollow sphere pigments were developed in the '80s. Although water-filled in the wet state, they were air-filled after the coating was dried--providing glossing and increased opacity. Porous hollow sphere pigments were developed in the '90s. The newest hollow sphere developments include larger particles (1000-1500 nanometers) combined with binding capacity. They are also available as cationic charged shell particles for digital imaging coated papers.

New Non-Mineral Starch Based Pigments may replace mineral pigments. (3) This work is at the research and pilot stages. Pilot coating trials at Western Michigan University showed starch pigment compares favorably to calcium carbonate and kaolin clay in brightness, opacity, gloss, and roughness. Mixtures of starch pigment with GCC and clay gave even better gloss and opacity.

New Pigments for Coated Ink Jet Papers and other digital imaging papers are undergoing rapid development. High quality coated ink jet papers are generally made using high surface area silica pigments. These pigments provide excellent quality but have process limitations. Silica pigments cannot be dispersed at above 25% solids, and they have very high binder demand. These factors generally result in ink jet coatings being applied at <20% solids--impractical for on machine coating, especially by metered size press.

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Several new specialty pigments are used for ink jet coating. One is the calcite PCC product, which can be coated at up to 40% solids. Another is a specially-treated kaolin clay that has a surface area of 45 m2/g and can be dispersed at 58-62% solids--facilitating coating at higher solids. (4) A modified natural zeolite (5) may also work in this application. Zeolite has a cage-like structure facilitating ink absorption to improve image quality. This zeolite can be dispersed at 54% solids and coated at up to 50% solids by blade or metered size press coating.

REFERENCES

1. P. Burri, "Properties and Performance Attributes Between Coated Papers in N.A., Eur. and Asia," 2005 TAPPI Coating Conference.

2. R. W. Johnson, "Performance of Ti[O.sub.2] Pigments in Coated Paper Applications," 2005 TAPPI Coating Conference

3. M. K. Joyce, J. S. Saari, K. Kataja, H. Mikkonen, S. Peltonon and P. K. Qvintus-Lieno, "Coating trial results with non-mineral bases pigments," 2005 TAPPI Coating Conference.

4. P. B. Malla and S. Devisetti, "Novel Pigment for High Solids Ink Jet Coating," PTS Conference, Munich, Germany, October 2004.

5. U.S. Patent 6,616,748 and U.S. Patent 6,679,973.

ABOUT THE AUTHOR

Charles P. (Chuck) Klass heads a consulting firm, Klass Associates Inc., Redington Beach, Florida, USA. He is also Adjunct Professor of Paper Engineering, Chemical Engineering & Imaging at Western Michigan University. He is a TAPPI Fellow and a member of the Solutions! Editorial Board. Contact him by email at [e-mail]

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CHARLES KLASS, KLASS ASSOCIATES, INC.

WHAT YOU WILL LEARN

* Consumption of coating pigments in different world regions.

* Advances in clay, GCC, and PCC coating pigments.

* Trends in other key coating pigments.

ADDITIONAL RESOURCES

* 2005 TAPPI Coating Conference Proceedings CD-ROM, ISBN: 159510-0857, Member Price: $55.00, Non-Member Price: $80.00. To order the fully searchable CD, type the following Product Code into the search field on www.tappi.org: COATINGCD-05, or call TAPPI Member Connection at 1 800 332-8686 (US); 1.800 446 9431 (Canada); [phone] (International). The 2005 TAPPI Coating Conference was held April 17-20, 2005 in Toronto, Ontario. These proceedings contain papers presented throughout the conference.

* "Coated Markets Improve, Technology Marches On," by Charles Klass, Solutions!, April 2005. Product Code: 05APRS026 (See ordering information in item above).

* "Coated paper and board: grades in transition," by Charles Klass, Solutions!, May 2004. Product Code: 04MAYS036.

Clay    64%
GCC     26%
PCC      5%
Others   5%

2004 North American Coating Pigment Consumption by Type

Total consumption = 4.3 million tons

Note: Table made from pie chart.

Talc     3%
Clay    26%
GCC     66%
PCC      2%
Others   3%

2004 European Coating Pigment Consumption by Type

Total consumption = 8.0 million tons

Note: Table made from pie chart.

Clay    39%
GCC     55%
PCC      5%
Others   1%

2004 Asia-Pacific Coating Pigment Consumption by Type

Total consumption = 4.1 million tons

Note: Table made from pie chart.