Holik H. (ed.) Handbook of Paper and Board

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holes, wrinkles, creases, edge nicks, turnovers, blade stripes, etc. Detection and

counting of dirt specks are done online today. With low-viscosity colors, upstand-

ing fibers in the surface cause pimples because the color climbs up these lumps of

coating. Faults such as whiskers on the surface, loose fibers, coarse fibers, dust,

lint, and fuzz, are usually only determined by inspection.

7.9.9

Brightness and Whiteness

Brightness and whiteness are highly dependent on the coating composition, pig-

ment type, coating amount and the use of optical brightening agents (see section

3.6.9.3.5.4). Fiber type and basis weight also influence optical properties. Bright-

ness measurements are taken over a range of wavelengths by using instruments

conforming to ISO 2469/2470. Such instruments commonly found in use are the

Elrepho 3000 (Datacolour/Zeiss) and the MacBeth White-Eye. The American paper

industry uses a standard brightness instrument that has 45° illumination and 0°

viewing (TAPPI Test Method T 452).

Whiteness is a comprehensive term used to express the visual impact of near-

white surfaces by means of a single value. Numerous equations have been devel-

oped to this end, and CIE (Commission Internationale de l’Eclairage) whiteness is

the most common in practical use. Whiteness is also used to denote a more com-

prehensive expression of color and color shade by use of the CIE color coordinates

L*, a*, and b* (or CIELAB). Three reflectance figures L*, a*, and b* are calculated

from measurements using a standard D65 light source. Here the UV content of

the D65 illuminant must be accurately controlled to ensure reliable assessment of

surfaces containing optical brightening agents. The coordinates a* and b* meas-

ure color. Positive figures for a* express redness, negative figures greenness, and

positive figures for b* indicate yellowness, negative figures blueness. L* is a per-

centage which measures luminance on a scale where black is zero and pure white

is 100%. One can measure color coordinates as well as brightness with the same

instrument, e.g., with a Hunterlab colorimeter or any other equipment. The calcu-

lations are complex and require the aid of computerized systems. Color coordi-

nates are currently also measured and controlled by fixed-point or moving on-line

equipment in paper machines.

7.9.10

Opacity

This is a measure of the amount of light transmitted through paper. When no light

is transmitted, the opacity is 100%. The measuring principle is based on compar-

ison of reflectance for a single paper sheet over a black background compared to an

opaque stack of paper samples. As opacity increases with increased absorption and

scattering of light, the following parameters are of prime importance for opacity:

basis weight, filler content in base paper, degree of calendering, type and treatment

of pulp, as well as coat weight and coating color components, especially the type of

7 Coating of Paper and Board372

pigment used (fine pigments increase scattering). Opacity is measured in accor-

dance with ISO 2471 and TAPPI Test Method T 425.

7.9.11

Mottling

There are several types of mottling: first color mottling, wet repellance mottling

and back-trap mottling. First color mottling results from unevenness of the phys-

ical surface structure of the coated sheet (roughness, pore structure) and from the

unevenness of optical surface properties (brightness, gloss). Wet repellance mot-

tling occurs when the fountain solution layer between coating and ink interferes

with ink transfer. Back-trap mottling leads to uneven ink setting in the coating

layer. Mottling can be tested during normal print conditions either in a print house

or research environment. For its investigation large areas (5 V 5cm

) of solid and

halftone are needed. For checking first color mottling, packing is removed from

the first, third and fourth unit: no water under the cyan ink and no back-trapping

after ink transfer means no water or back-trap mottling. To test wet repellance

mottling, packing is removed from the first unit: if there is no water under the

cyan ink then no water mottling is given. For back-trap mottling packing is re-

moved from the third and fourth units: when no back-trapping after ink transfer

occurs then no back-trap mottling is given.

7.9.12

Print Unevenness

This means unwanted variation in density, color or gloss in print. The size and

intensity of the variations are measured. Print density variations (graininess

0.5–2 mm; mottle 2–8 mm) are measured from an even black and/or cyan tone of

at least 40% and as an average of eight images with a size of 51 V 51 mm

7.9.13

Taste and Odor

When paper is used for packaging purposes, no off-taste or off-odor from the

package must contaminate the product itself. Odor and taste from paper can arise

from a number of sources such as wood resins in mechanical pulp or residual

chemicals in pulp making, or the paper may have internal biological activity, which

can also produce odorous substances. The most sensitive instrument available to

measure the odor and taste of a substance is a human being. Members of trained

panels assign numerical ratings and record their impression of tainting flavors or

volatile odors experienced. The test methods available are the triangle test (the test

states if differences between specimens are found, DIN 10 951), the pair test (a test

describing the differences between specimens, DIN 10 954) and the Robinson test

(a taste test identifying how much taint a paper has given to a taste medium;

chocolate very commonly being used, DIN 10 955). Beside the trained panel, gas

7.9 Measurements of Coated Surface 373

chromatography (GC) and mass spectrometry (MS) are used to identify the origi-

nating chemical compound. The so-called “electronic nose”, an original invention

from the military world, has also been utilized to detect off-odors.

7.10

Coating Machines

Martin Tietz

7.10.1

Overview

Coating machines are separate machines in which a previously produced dry base

paper is unwound, coated, dried, and then reeled again (Fig. 7.24). In contrast to

coating applicators placed on-line in a paper machine (Section 6.8), they are usu-

ally called “off-line coaters” or “OMC” (off-machine coaters).

In a paper mill, one off-line coater is usually associated with one paper machine.

Sometimes, however, one OMC receives paper from different paper machines. In

most cases, the off-line coater has the same width as the corresponding paper

machine (except for the edge trim). Some coaters with half width are in operation,

where the jumbo reels from the paper machine are split, and the two halves are

coated separately.

Off-line coating has the advantage that the paper machine can continue produc-

tion during machine stops on the off-line coater. These stops can be for instance

due to coating color changes, which is especially important for machines with a

very broad product range. Since off-line coaters operate faster than the paper ma-

chine to which they belong, the time efficiency of the entire line is higher than for

on-line concepts.

7.10.2

Applicators

The actual coating process usually comprises two steps: (i) the application of the

coating medium onto the paper or board and (ii) the metering of the coating

medium to the desired quantity (or coat weight). If the metering is done directly on

the paper after the application, the process is called direct coating or blade coating.

If, however, metering is done before the transfer of the coating medium to the

paper or board, the process is called indirect coating or film coating.

Fig. 7.24 Off-line coater (source: Voith).

7 Coating of Paper and Board

374

7.10.2.1 Direct Coating

In direct coating, the web is supported by a backing roll and the coating medium is

fed in excess onto the web. After a certain distance, defining the “dwell time”, the

coat layer is metered down to the final thickness, being the desired coat weight.

In the “roll applicator” or “LDTA” (long dwell time applicator), feeding is done

with a roll, which draws the coating medium from a pan onto the web (Fig. 7.25).

The applicator roll and the backing roll form a nip. The nip pressure and, conse-

quently, the nip gap are adjustable, determining the amount of coating medium

fed to the web. The nip load also yields an external penetration pressure which,

together with the capillary pressure, causes penetration of some coating medium

or components thereof into the paper or board web. This is to a certain extent

desirable, because it anchors the coating to the base paper and gives surface

strength. Excessive penetration, however, would reduce the gain in surface proper-

ties, such as smoothness or gloss.

Penetration of the coating into the base paper can be controlled with the “water

retention” of the coating medium. At elevated machine speeds, e.g. above approx-

imately 1500 m min

–1

, the splitting of the coating color between applicator roll and

paper at the nip exit shows irregularities, which negatively influence the homoge-

neity of the coat layer. Also, the relatively short nip and the considerable penetra-

tion caused by the nip load cause runnability and quality problems. To overcome

these limitations, the application of the coat medium to the paper can be per-

formed with a free jet. In a “free jet applicator” (Fig. 7.26), the jet usually has a

thickness of less than one millimeter and a length of a few centimeters. The result-

ing coat layer (still before final metering) is much more homogeneous than that of

a roll applicator, and, since the external pressure at the impingement point is

considerably less than in the roll nip, the penetration of the coating medium into

the base paper is reduced. This improves the so-called “coating holdout”, which

results in a better surface quality of the coated paper.

The final metering of the applied coating is often performed with a blade. With

this blade, most of the coating medium is removed and returns to the working

tank. The coat layer remaining on the paper or board is evenly distributed. The

blade pressure ensures that the surface voids of the web are filled with coating.

Due to the blade geometry, the coated web has a good smoothness. The blade

usually has a thickness of less than one millimeter, typically approximately

0.4 mm. The stick-out length (or unsupported length) is usually a few centimeters.

Fig. 7.25 Roll applicator (source: Voith).

7.10 Coating Machines

375

The operating angle between blade and paper web is usually 20–35°. Under these

conditions, the blade is considered a “stiff blade”, because its bending is negligible

with respect to the operating principle. If, however, a blade geometry is chosen

where the bending of the blade results in an operating angle close to 0°, the mode

is called “bent blade”. The bent blade gives – in comparison to the stiff blade – an

even smoother surface but it is more prone to surface defects, such as blade

scratches, and is more difficult to operate. Since the web has a considerably re-

duced roughness after coating, the coat layer itself must be nonuniform in caliper

to even out the base paper roughness. On certain grades, especially board, this

nonuniform layer thickness can be seen as inhomogeneous coverage or mottling.

If coverage is desired rather than smoothness, metering can be done with a rod

rather than with a blade. Rod metering is typical for the pre-coat of board and for

specialty papers where the base web has to be evenly covered with a specialty

coat.

In most cases, the metering blade (or rod) is placed against the same backing roll

as the coat applicator. The angle between point of application and point of meter-

ing is then approximately 60°. This results in a dwell length of 400–600 mm, de-

pending on the backing roll diameter. The resulting “dwell time” depends strongly

on the machine speed. It is in the range of 10–200 ms.

A “short dwell time applicator” (or SDTA) was tried in order to reduce the dwell

time to much shorter values. This SDTA consists of a closed chamber which incor-

porates the coating feed and the metering blade. However, vortices in the chamber

caused a streaky appearance of the coat layer at elevated machine speeds. There-

fore, the SDTA has not received much attention outside North America.

7.10.2.2 Indirect Coating

As an alternative to direct coating, a film press can be used for the application of

coating media. The film press was derived from the size press, which is used for

the application of starch or size solutions. Today, a wide variety of coating media

are applied, including pigment dispersions with high solids content. In a film

press, a film of the coating medium is formed and metered on a large diameter

roll. This roll forms a nip with another roll. The paper or board web passes this nip

Fig. 7.26 Jet applicator (source: Voith).

7 Coating of Paper and Board

376

and picks up a certain portion of the film. Application can be – but does not have to

be – simultaneous on both sides. A schematic drawing of a film press is shown in

Section 6.7.

Metering is performed using metering rods. These can be either smooth or

profiled. Profiled rods provide a certain volume of coating medium due to the open

cross section in the profile. A deeper or coarser profile gives a higher coat weight

than a fine profile. The application weight is mainly adjusted by choosing an ade-

quate profile. Fine tuning of the application weight can be done by modifying the

rod pressure. Profiled metering rods are mainly used for low viscosities (i.e. low

solids contents of the coating medium) and low machine speeds. With increasing

viscosity and speed, the rod loses contact with the roll due to the hydrodynamic

forces of the coating medium, comparable to aquaplaning. Then, the application

weight depends less on the profile and more and more on the hydrodynamic

conditions, such as rod pressure, rod diameter, viscosity and speed. Consequently,

a smooth metering rod is used. Typical rod diameters are 14–38 mm for smooth

rods. Larger diameters yield higher application weights. Higher viscosities and

machine speeds require smaller rod diameters for the same application weight

than lower ones. The coating medium is not transferred completely to the web. A

certain amount remains on the roll and returns to the application unit where it is

mixed with fresh coating medium. The transfer ratio depends on the acceptance

behavior of the web, on the properties of the coating medium and – to a limited

degree – on the surface properties of the roll. For starch, where the absorptivity of

the web is high, the transfer ratio can be more than 90%. For pigment coats, it can

be as low as 50%.

The pre-metered films usually have thicknesses of 7–20 mm (or ml m

–2

). Lower

values would require very high rod pressures and also coverage of the web would

be insufficient. Higher values are not meaningful since the web has a limitation

with respect to coating acceptance. If the amount of pre-metered coating medium

is too high, the surface of the coated web appears uneven, with an “orange peel”

character. At elevated machine speeds, the film split at the nip exit can create a fine

mist of coating medium. This mist deposits on machine parts or even on the paper

or board web. Since this misting increases with film thickness, it is the major

limitation for the application weight at high speed. For typical applications, mist-

ing becomes a limitation above 1500 m min

–1

. At 1800 m min

–1

, for example, coat

weights above 7 g m

–2

are difficult to achieve without disturbing misting.

7.10.2.3 Curtain Coating

Besides film and blade coating, curtain coating is used for the application of coat-

ing media onto paper surfaces. Although curtain coating is a very old application

technique, going back to the end of the 19th century, it has only been used for

coating of paper since the 1990s. Curtain coating is mainly used for specialty

papers. Specialty coats are usually very expensive. Therefore, the application

amount must be kept to an absolute minimum. On the other hand, a certain

amount is required to achieve the desired function of the coat. In a curtain coater,

7.10 Coating Machines 377

a thin film of coating medium is formed which falls by gravity onto the paper

surface. The only metering device for the coating color is the nozzle slice. The slot

opening is about 20 to 50 times larger than the actual coating color thickness.

Thickness is reduced by gravity forces while the jet is moving towards the web and

by the shear forces exerted by the high speed difference between the jet and the

running web (Fig. 7.27). Due to the homogeneity of the film, the coverage of the

paper by the coating medium is very uniform.

Unlike film and blade coaters, the application is contact-free. This considerably

reduces the risk of web breaks and, therefore, improves runnability. It is expected

that the significance of curtain coating will increase.

7.10.3

Application Concepts

Typical coat weights for graphic papers are given in Fig. 7.28, plotted against the

basis weight of the base paper. The coat weight is typically 25–40 % of the final

basis weight of the paper.

Wood-containing papers are mainly single-coated (LWC). Coat weights are usu-

ally between 6 and 10 g m

–2

per side. Traditionally, the coat is applied with a blade

coater. More modern installations use a film press for the single coat.

At higher basis weights (MWC), two coat layers may be applied per paper side.

Here, the top coat is usually a blade coat. The pre-coat may be either a film or a

blade coat.

Woodfree papers cover a very wide basis weight range. Depending on the basis

weight, the application can be single coat, double coat or triple coat. The coat

weight per layer can vary in a wide range between approximately 8 and 20 g m

–2

For board grades, the coat weight depends on the requirements of the final

product and on the quality of the base board. Coverage, brightness and smooth-

ness are the main properties that are improved by the coating. Depending on the

required coat weight, two or three coat layers are applied.

Due to their functions, different formulations are used for the individual coat

layers. For instance, pigments with high opacity are used in the first layers,

whereas very fine pigments with good smoothness potential are used for the top

coat.

Fig. 7.27 Principle of curtain coating (source: Voith).

7 Coating of Paper and Board

378

7.10.4

Drying

After its application, the wet coat must be dried. Three different drying principles

are used: radiation (infrared), convection (hot air), and contact (cylinders).

Infrared drying yields a very good energy transport, whereas mass transfer is

inferior. Therefore, infrared is especially suitable for heating the paper or board

web to temperatures where considerable drying occurs. Heating of the infrared

radiators can be by electricity or gas. Gas-heated infrared dryers reach higher tem-

peratures and are usually cheaper. Electrical infrared dryers are easier to control

and are, therefore, often used for moisture profiling in the cross machine direc-

tion. It depends very much on the local conditions at the mill site, such as availabil-

ity and prices, which heating principle is used for infrared drying.

Air flotation dryers are widely used for the drying of coat. In an air-flotation

dryer, hot air is blown onto the paper surface. Air temperature can be as high as

350 °C, sometimes even higher. The impingement velocities of the air can exceed

40ms

–1

. The impingement effect gives a very effective heat and mass transfer

between air and paper surface. The blowing nozzle sections in the dryer are alter-

nately placed on one side of the paper and on the other side. This gives a sinusoidal

web run through the dryer with a web-stabilizing and wrinkle-avoiding effect.

Cylinder drying of coat is possible, as soon as the freshly coated surface is dry

enough that the coating color does not stick on the cylinder surface. Therefore,

drying cylinders are only found after a certain amount of contact-free drying, i.e.

Fig. 7.28 Typical coat weights for graphic papers. Basis weight

includes base paper + coat weight (source: Voith).

7.10 Coating Machines

379

towards the end of the dryer section after a coater station. In modern coating

machines, cylinder drying contributes only a small amount to the total drying

capacity, sometimes less than 20 %. Contact drying has usually the lowest specific

costs. Furthermore, the cylinder group is an excellent fix point for the web run.

When no cylinders are used, a separate fix point between the coater stations has to

be included, such as an S-wrap or a pull stack.

In some machines, the last cylinders are used to cool down the web rather than

for drying. After drying, the web can reach temperatures well above 70 °C and does

not cool sufficiently before the next coater station. Too high a web temperature

causes excessive penetration of the coating color into the web so cooling provides a

higher product quality.

7.11

Coated Paper and Board Grades (for more details see Chapter 11) [4, 7]

Werner Auhorn

Table 7.14 gives an overview of coated paper and board grades as related to the

main components of base paper, basis weight, coat weight, brightness, and their

main usage.

Figure 7.29 illustrates the relative positions of coated, mechanical pulp dominat-

ing papers in basis weight and brightness. Figures 7.30 and 7.31 present typical

end uses for coated mechanical papers and for coated fine papers worldwide.

Fig. 7.29 Classes of wood-containing coated printing papers

and their positioning according to basis weight (base paper +

coating layers) and brightness (source: H. Paulapuro, Dep. of

Forest Products Technology, Helsinki Univerity of Technology).

7 Coating of Paper and Board

380

Table 7.14 Overview of coated paper and board grades and

their most import characteristics.

Paper/board grades Base Paper

main fiber

furnish

Basis weight

base paper

(g m

–2

)

Coat weight

per side

(g m

–2

)

No. of

Coating

Layers

ISO

brightness

(%)

Main usages of paper/board grades

Printing Papers (both sides coated):

ULWC (ultra light weight coated) MP, CP, DIP 25–35 5–7 single 69–72 catalogs, magazines

LWC (light weight coated) MP, CP, DIP 35–45 6–12 single 72–78 (84) magazines, inserts

MFC (machine finished coated) MP, CP, DIP 35–60 5–15 single, double 72–78 magazines, catalogs

FCO (film coated offset) MP, DIP 30–45 4–10 single 72–78 inserts, flyers, advertising

MWC (medium weight coated) MP, CP, CP 45–65 12–25 double 80–88 special magazines, advertising

HWC (high weight coated) MP, CP, CP 65–100 25–35 double, triple 82–90 magazine covers, advertising

WFC (woodfree coated fine papers) CP 60–120 25–35 single, double, triple 88–98 magazines, image brochures, labels

Art Paper CP 60–150 20–40 double, triple 94–98 illustrated books, calendars, image

br.

Board Grades (mainly top side only coated):

FBB (folding box board) DIP, RP, MP, CP 135–425 12–35 single, double, triple 72–86 packaging of food, book covers

WLC (white lined chipboard) DIP, RP, MP, CP 170–425 10–30 single, double 76–92 shoe and wine boxes, protection

packaging

SBS (solid bleached sulfate) BHSP, BSSP 120–250 15–25 double 84–98 packaging of chocolate, cigarettes,

cosmetics

LPB (liquid packaging board) BSSP, UBSSP 280–330 10–15 single 76–86 milk and juice packaging

Abbreviations: MP = mechanical pulp; DIP = deinked pulp;

CP = chemical pulp; RP = recovered paper; BHSP = bleached

hardwood sulfate pulp; BSSP = bleached softwood sulfate pulp;

UBSSP = unbleached softwood sulfate pulp

7.11 Coated Paper and Board Grades 381