coating paper and board

1. Coating paper& Board
IN CHEMICAL PERSPECTIVE
By Audrey Zahra

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3. Coating in the production process
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4. From wood
to coated
paper
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5. Coated paper types
• Two-sides and one-side coated papers
– Matt coated papers
– Demi-matt (silk) coated papers
– Gloss coated papers
– Gussgestrichene Papiere
• Coating is possible on both woodfree and mechanical papers
– Woodfree
– Mechanical
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6. Coating process
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7. Coating ingredients
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8. Application types
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9. Coating types
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10. Coating components
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11. Objectives of
Coating
• The main purpose of coating is to improve the surface
quality of paper or board. The quality improvement can be
aimed at optical properties such as brightness, gloss or
opacity, at tactile properties such as smoothness, but,
most importantly, at printability and print image quality.
• The application of (usually white) pigments to the base
paper surface enhances the brightness of the paper. The
coat layer reduces the penetration of ink into the paper
sheet. Therefore, the ink does not spread as much and the
print image is clear and sharp.
• The benefits of applying a coating layer become very
apparent when comparing paper surfaces with different
coatings. The SEM (scanning electron microscope)
micrograph of an 80g m–2 woodfree base paper shows
multiple layers of intersecting fibers (Fig).
Fig. Comparison of base paper, precoat, topcoat uncalendered and topcoat calendered (source: Omya).
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12. Pigments as Fillers
Fillers are applied to the paper mainly
• to improve the optical properties, such as brightness and opacity
• to improve the smoothness of the sheet surface (i. e. decreased roughness, especially after calendering)
• to improve the sheet formation by filling the voids between the fiber matrix
• to enhance printability in the various printing processes due to a more uniform
paper surface, higher opacity and better ink receptivity. The latter resulting in
reduced printing ink penetration, wicking and ink strike-through to the opposite
side of the sheet
• to improve the dimensional stability of the paper as most fillers remain inert
when wetted, unlike the natural fibers usually used in papermaking.
• to improve the permanence of the paper (alkaline papermaking, calciumcarbonate (CaCO3) filler)
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13. Characterization of Fillers
Chemical composition, particle morphology, particle size and particle size distribution, brightness, refractive index,
specific surface, particle charge and abrasiveness are commonly used to characterize papermaking fillers.
(source: Holik Bert).
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14. Brightness
Fig. Brightness (Tappi R-457) of various paper fillers and highly bleached mechanical and chemical pulp (source:OMYA).
Tappi brightness is measured at a wavelength of 457 nm. Fillers with high pigment brightness, like GCC or PCC, are in
demand for the production of high quality printing and writing papers. Figure above compares the Tappi brightness of
various fillers of different geographical origin as well as of highly bleached chemical and mechanical pulps (dotted
lines). The brightness range of the different types of filler is given by the raw material quality used and the various
processes (mechanical classification, magnetic separation, flotation, grinding, bleaching, precipitation etc.) applied
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15. (source: Holik Bert).
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16. Coating Pigments
Coating improves the surface quality of paper and board, resulting in higher brightness,
smoothness, and gloss as well as better opacity and generally significantly
improved printability. Pigments are the main coating components for improving
the surface properties of coated paper and board grades. Usually they account for
80 – 95% of the total dry coating weight or, as a volume fraction, 70 – 80% of the
solid material of the coating. This is based on a pigment density of about 2.5 g cm-3
and a density of the rest of the solid material in the coating of about 1.0 gcm−3.
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17. Particle
Morphology
• The structure of a filler can be
observed and characterized best by
scanning electron microscopy (SEM).
The particle morphology has an
influence on light scattering via the
number and size of air microvoids in the
sheet. The results are morphology
related differences in sheet drainage,
drying
• behavior, and in paper properties such
as bulk, porosity, ink receptivity, strength,
dusting, etc. Figure shows SEM pictures
of various typical fillers and specialty
pigments currently applied in the paper
industry.
(source: Holik Bert).
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18. (source: Holik Bert).
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19. (source: Holik Bert).
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20. (source: Holik Bert).
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21. Pigment Characteristics: Aspect Ratio, Particle Size, and Particle Size Distribution
(source: Holik Bert).
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22. (source: Holik Bert).
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23. GCC allows reduced binder requirements and drying energy savings. The
development of finer-grade carbonates with particle sizes down to 99% <2
µm and special PSD curves covers a broad spectrum of whiteness, paper
and print gloss, as well as opacity. The rhombohedric structure allows for
high solid contents of pigment slurries (up to 76%) and coating colors with
excellent runnability even at the highest coating speeds of >1800 m min1.
This also explains the overproportional growth of GCC as a coating
pigment during the last decades, whereas kaolin stagnates and talc shows
constant growth, especially for gravure coating colors.
(source: Holik Bert).
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24. PCC allows for the production of coating pigments with specific morphologies by controlled synthesis. Particle size,
PSD, and particle shape can be controlled. The surface properties of the calcium carbonate particles can also be
changed if needed. Commercial PCC pigments typically have CaCO3 contents higher than 97%. The remainder is
MgCO3 (magnesium carbonate) and other residues. Industrially, it has been found practical to produce PCC near
the paper mill by using satellite plants.
(source: Holik Bert).
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25. The characteristics generally differentiating talcs from delaminated Georgia
kaolin clays (the United States) or Cornish kaolin clays (the United Kingdom) are
hydrophobicity, low cohesion between the crystal layers inside the talc particles,
resulting in extremely low friction coefficient, softness, and low abrasivity in spite
of a large particle diameter.
Talc
Gypsum
Gypsum consists of calcium sulfate crystallized with different
degrees of hydration, all of which can be referred to as gypsum. However, in most
cases, gypsum refers to calcium sulfate in the dihydrate form and the other forms
are distinguished from each other by additional names (e.g.. hemihydrate, calcined
gypsum, and stucco gypsum). The solubility of gypsum pigment is high, so that
the water circulation of a paper machine are enriched not only with Ca ions from
internal broke but also when gypsum-containing paper is recycled.
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26. Plastic pigments are used in paper coatings to provide
a surface with the desired appearance and printability. If properly chosen and
formulated, these pigments provide a coating with surface smoothness, brightness, and
opacity, as well as a balance of ink holdout and ink receptivity upon which to print.
Organic pigments, commonly referred to as plastic pigments, are used as partial
replacements for inorganic pigments to improve the optical and print properties of coated
paper and paperboard.
Plastic pigments
Satin white (calcium sulfoaluminate) is still of some importance
in paper coating, especially if it comes to cast coating. This pigment has
very fine needlelike particles, is extra white, and has a low density. It increases ink
absorption and gloss. The disadvantages of satin white are its sensitivity to increase
in temperature and decrease in pH, and its high adhesive demand.
Satin white
Titanium dioxide pigments show extremely high refractive indices. This and their whiteness,
that is, their high reflectance in the visible region of light, as well as their optimal particle size,
make them the most effective white pigments. Titanium dioxide exists in three crystal forms:
anatase, rutile, and brookite. The first two are stable and therefore of commercial importance.
Titanium dioxide pigments are used in paper coatings to increase the opacity of coated paper.
Titanium dioxide
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27. Coating machine
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29. Surfaces
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33. Thank you

34. References:
Holik, Herbert. 2006. Handbook of Paper and Board. WILEY-VCH Verlag GmbH & Co KGaA.
Germany.
https://www.sappi.com/
https://www.sappi.com/color-management-100-percent-solution