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

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Water is an essential component of a coating color making it possible to mix the

components of a coating color, e. g., so that all the pigment particles are separated

from each other, which is impossible in the dry state. Water also makes it possible

to transport the color elsewhere and apply it onto the base paper so that the coating

color remains uniformly dispersed. As water evaporates from the coating layer, the

coating layer consolidates when the binder forms bridges between pigment parti-

cles and base paper. Coating colors should contain only as much water as the flow

properties need, in order to save energy and costs for drying. The solid contents of

the coating colors can be as high as about 70 wt.%.

The composition of coating colors resembles that of paints, containing similar

components. Of course, there are differences in detail: the additives may be totally

different, there are different kinds of binders, and paper coatings are white while

most paints are colored so the pigments are different, etc. One major difference

between paints and coating colors is the amount of binder: paints contain much

more binder than do coating colors. This is partly due to their different objectives:

The purpose of painting is to improve the looks of the surface to be painted, and

also to create a protective layer. This latter function demands that the paint layer

should be nonporous in most cases. This is achieved by using sufficient binder to

completely fill the spaces between the pigment particles.

As for the coating colors, other than to improve the looks of the paper, the

purpose of their application is to achieve the desired properties on the paper sur-

face, the most important being the printing properties. The coating layer should be

strong enough to resist the stresses of the printing process; the surface strength of

the coating dictates the allowable minimum amount of binder. For example, offset

printing inks are tacky, which requires a certain z-strength, or pick-strength, of the

coating. On the other hand, increasing the amount of binder has a negative effect

on several coating properties, and excess binder may cause quality concerns, e.g.,

low opacity and gloss, or glueability problems. Therefore adding more binder to

the coating than the surface strength requires should be avoided; there is also a

cost consideration. However, the coating process and base paper absorbency can

make high binder levels necessary.

3.6.9.2 Market Situation and Future Trends

The worldwide consumption of paper and board will grow by approximately 2 to

2.5% for the next two decades at least. The development and fast worldwide expan-

sion of electronic media has led to a certain shift in paper qualities and challenged

the development of new paper and board qualities. In particular, pigment coated

grades have participated most in this new competition in the area of communica-

tion and packaging. Because of their significantly better printability, their more

aesthetic attractiveness and their more valuable feel, the growth rate of coated

paper and board will be twice as high as that of paper and board in general (Table

3.1).

Printed paper is a cost efficient medium with a high capacity for information,

universally easily available and fully recyclable. Paper and board based packaging

3 Chemical Additives102

material is also cost efficient, aesthetically attractive and environmentally friendly.

Most pigments are significantly cheaper than chemical pulps so increasing both

the proportion of coated paper in general and the coating layer(s) compared to

fibers (Fig. 3.12) is an important economic factor. In 2004 coated paper and board

accounted for 17% of the total worldwide paper production of 350 V 10

t and will

grow by 2014 to 21% of the then 430 V 10

t paper and board production. The

proportion of coated paper to board is ca. 80% to 20 % and the proportion of

coating layer compared to the total area weight for paper is significantly higher

(30–65%) than for board (5–20 %). The future trends for coated papers will involve

more specific paper products, further development of coating technology, ongoing

progress in printing technology, environmental issues, development of coating

color raw materials and the globalisation of paper companies.

Table 3.1 Further predicted overproportional growth of coated

paper and board production worldwide.

Production

(10

1994 2004 2014 Growth rate

1994–2004

p. a.

Growth rate

2004–2014

p. a.

Paper and Board,

World

280 350 430 2.3% 2.1%

Coated 42 60 90 3.6% 4.2%

Share of total 15% 17% 21%

Fig. 3.12 Predicted increasing proportion of fillers and

coatings vs. fibers/base paper.

3.6 Functional Chemicals

103

3.6.9.3 Components of Coating Colors

3.6.9.3.1 Pigments

With respect to mass the most important component is the pigment with a total

amount of almost 18 V 10

t per year dry material worldwide. There may be only

one kind of pigment in a coating color, or more commonly a combination of

several types e.g. clay, calcium carbonate, talc, titanium dioxide (more details see

2.2.2). The share of pigment in the dry coating is about 85–95 wt. %.

3.6.9.3.2 Dispersants

In the dry form, pigment particles form clusters, in which the parts of the clusters

are more or less tightly attached to each other. More tightly attached ones are called

“aggregates”. Aggregates and primary particles together can also form clusters that

are less tightly bound – so-called “agglomerates” or “flocs”. The purpose of dis-

persing is to make a dispersion where neither agglomerates nor aggregates exist

and only primary particles are present. Primary particles are evenly distributed in

water, and the system stays stable for a certain time.

Disruption of aggregates, disaggregation, is typically an irreversible process, af-

ter which particles cannot be bound as tightly as they used to be. Instead of form-

ing aggregates, disaggregated particles tend to form agglomerates. Disruption of

agglomerates is called “deflocculation”. Deflocculation is a reversible process. Ag-

glomerates re-form when the force for deflocculation is removed.

The process of dispersion can be divided into three stages: wetting, disruption of

particle clusters and stabilization. Wetting means that all the external surfaces of

pigment particles must come into contact with water. Air must also be displaced

from the internal surfaces between pigment particles in agglomerates and ag-

gregates. Wetting is usually not a problem with paper pigments except for talc

which is not spontaneously wet by water. When talc is dispersed, a separate surfac-

tant is required to provide proper wetting. Disruption of particle clusters, disaggrega-

tion, is accomplished by mechanical energy. Disruption can be performed using

crushing mills, kneading mixers, or high-speed mixers. Crushing mills are needed

when the shear forces induced by mixers are not sufficient for disaggregation.

Although dispersing can be performed in mixers, there is a certain difference

between mixing and dispersing. Mixing does not change the size and surface area

of particles, while dispersing changes both. Stable pigment dispersions with the

highest solid content can be achieved only by using dispersants. When pigment

clusters are broken down by dispersing, the surface area of pigment in the disper-

sion increases, there is more surface for particles to interact with each other, and

the viscosity in the dispersion increases rapidly. Use of dispersants stabilizes de-

flocculated particles in the dispersion and hinders their interaction. The dispersant

must be mixed with water at once, when the breakdown of pigment clusters starts.

Only then can interaction and therefore reagglomeration of particles be avoided.

For this reason, dispersant is added to water at the beginning of dispersing, even

before the pigment is added.

3 Chemical Additives104

Reagglomeration is avoided if particles are kept far enough from each other. Two

general principles are considered as stabilization mechanisms: electrostatic stabili-

zation and steric stabilization. When both occur simultaneously, it is called “elec-

trosteric stabilization”. In electrostatic stabilization, the charges of particle surfaces

are made to be the same sign. Dispersant is adsorbed onto the particle surface, and

thus the surface gets a highly localized charge with the same sign as that of the

dispersant. A negative charge on the surface, when anionic dispersant is used,

creates a cloud of positive counterions around the particle (the so-called “electric”

or “ionic” double layer). The closer to the surface, the more localized are the coun-

terions. At a greater distance from the particle, in a continuous water phase, neg-

ative and positive ions are in balance. The counterion cloud acts as a stabilizer,

creating repulsive forces between particles. Electrostatic stabilization is the most

commonly used stabilization method in paper pigment dispersions. To act as a

good stabilizer, the dispersant must be highly charged. Examples of these kinds of

dispersants are salts of polyacrylic acid and polyphosphates. In steric stabilization,

particle surfaces are covered with uncharged polymer, the chain of which extends

into the water. When two particles with polymers on them approach each other,

they cannot approach too closely because the polymer chains would overlap, and

this is not entropically favored. Thus polymers create a steric hindrance against

particle interaction. Dispersants, which work as steric stabilizers, are also called

“protective colloids”. Examples of these are starches and polyvinylalcohols. A good

example of a dispersant, which works as an electrosteric stabilizer, is carboxy

methyl cellulose (CMC).

The viscosity minimum is the optimum dosage of dispersant. After the opti-

mum the viscosity is slowly increased. Viscosity decreases when dispersant is

added, due to the increased stability of the dispersion. The stability is a maximum

at the viscosity minimum. The viscosity starts to increase after the optimum be-

cause additional dispersant can no longer be adsorbed onto the pigment surface,

and thus stays in the water increasing its electrolyte concentration and decreasing

the stability. Overdosing of dispersant therefore must be avoided. Pigment type,

dispersant type, pH, and additional components all affect the dispersant dosage

required. The smaller the particle size the larger the total surface area of pigment,

and the more dispersant is needed for stabilization. Generally it can be said that

the required dispersant dosage is in the range 0.1 to 0.5% of dry pigment.

Today, the most commonly used pigment dispersants are polyacrylate salts. Usu-

ally they are low molecular weight polymerization products of acrylic acid, which

have been neutralized with sodium or ammonium hydroxide. They are very resis-

tant towards different types of attack like high pH, high temperature, or high shear

forces. Polyphosphates were previously used as dispersants. They are effective, but

lack hydrolytic stability during storage of the dispersion; they easily hydrolyze to

orthophosphates, which have no deflocculation power. This causes an increase in

the viscosity of the dispersion. The longer the polyphosphate chain, the more

effective it is as a dispersant. Tetrasodium pyrophosphate, Na

, is the simplest

polyphosphate that can be used as a dispersant. Sodium tripolyphosphate,

, is widely used as a dispersant and as a builder in detergents. Other

3.6 Functional Chemicals 105

anionic polymeric dispersants are lignin sulfonic acid salts and formaldehyde con-

densation products with aromatic sulfonic acids. Because of the nature of sulfonic

acid, these types of dispersants bear the maximum anionicity over the whole pH

range typically used in pigment dispersions. Lignin sulfonates are made from

native lignin. Their effectiveness as a dispersant depends on their purity and de-

gree of sulfonation. Lignin sulfonates can be used as a dispersant with hydro-

phobic surfaces. Their disadvantages are poor thermal stability, tendency to foam

and they bring a high load of detrimental substances to the papermaking process.

Formaldehyde condensation products with aromatic sulfonic acids have an aro-

matic backbone and are also effective dispersants with hydrophobic surfaces.

Nonionic polymers, which can be used as stabilizers are, e.g., starches, polyvinyl

alcohols, and polyacrylamides. Nonionic polymers work as protective colloids;

their mechanism of stabilization is steric stabilization. Carboxy methyl cellulose

bears a small anionic charge along the chain. However, it is often considered to act

as a protective colloid. Actually, carboxy methyl cellulose can be considered to use

both its protective colloid properties and its charge in stabilizing, thus acting as an

electrosteric stabilizer.

The charge of pigment dispersants is usually anionic but, in some applications,

cationic dispersants are preferred. They are seldom needed in coating color pig-

ments, but they are beneficial in pigment dispersions meant for filler applications

or for specialty coating applications. Cationic dispersants are typically cationically

charged polymeric compounds e.g. modified polyethylenimines, polyvinylamines.

Usually they do not act as effectively as anionic dispersants.

3.6.9.3.3 Binders

Binders are the second most abundant component in the coating color after the

pigments with a total amount of approximately 3 V 10

t of dry substances. Many

different binder types are used (Fig. 3.13). The most important aims for the appli-

cation of binders in coating colors are the binding of pigment particles to base

paper, the binding of pigment particles to each other, partial filling of voids be-

tween pigment particles (porous coating structure) and affecting the viscosity and

water retention of the coating color.

An ideal binder can be characterized by good binding power, good water reten-

tion properties, ease of mixing or dissolving in water, general ease of handling,

good compatibility with other coating components, low or desired effect on the

viscosity of the coating color, good mechanical and chemical durability, good opti-

cal and mechanical properties, nonodorous and harmless to health, low tendency

to foaming, resistant to bacteria, constant quality properties, low price and good

availability. Again, as with pigments, there is no single binder, which can meet all

these requirements. Latexes meet many of them; however, they often need a co-

binder or thickener to adjust the rheology and water retention to the desired

level.

There are three kinds of binders in coating, the (main) binder, the co-binder and

the sole-binder. A sole-binder is a single binder that alone can perform all the

3 Chemical Additives106

desired binder functions in a coating. Usually the binder systems consist of a

combination of two binders, in which the (main) binder is responsible for the

binding function. The co-binder is used to affect the rheology and water retention

properties of the coating color. Its dosage is smaller than that of the main

binder.

The binders can be classified by their origin and solubility in water:

• Soluble in water:

– Starches

– Proteins

– Cellulose derivatives: ethers e.g. carboxy methyl cellulose

– Polyvinyl alcohol (PVA, PVOH)

• Insoluble in water:

– Carboxylated styrene butadiene latexes (XSB Latex)

– Styrene acrylate latexes (SA Latex)

– Polyvinyl acetate latexes (PVAc Latex)

Water-soluble binders give better water retention for the coating layer than latexes

that are not soluble in water. They also affect the rheological properties of the

coating colors, making them more viscous, and also shear thinning (pseudo-plas-

tic) and thixotropic. Some synthetic co-binders have a similar effect. Carboxylation

of SB-Latex means incorporation of small amounts of unsaturated carboxylic acids

such as acrylic acid, methacrylic acid, maleic acid, to improve considerably the

compatability of these highly hydrophobic polymers with the other coating com-

ponents.

3.6.9.3.3.1 Derivatives of Natural Polymer Binders

In the initial phases of paper coating, i.e. the late 19th century till nearly the first

half of the 20th Century, these binder types were used exclusively. All are hydro-

Fig. 3.13 Market shares of binders for paper and board

coating in 2004 (total of ca. 3 V 10

t of dry materials world-

wide). PVAc = polyvinyl acetate, SA = styrene acrylate, XSB =

carboxylated styrene butadiene.

3.6 Functional Chemicals

107

philic. The most widely employed binders of this type are starch and its derivatives

with a consumption of approximately 800 000 tpa worldwide at present. Additional

natural binder types are cellulose ethers such as carboxymethyl cellulose, in the

United States, hydroxyethyl cellulose, soybean protein, and, to a very small extent,

casein and alginates. Natural binders act as protective colloids that prevent the

flocculation of the pigments; they increase the viscosity and water retention of

coating colors and give the coat a higher stiffness. Natural binders have a relatively

low water resistance (wet pick). They are mainly used in combination with syn-

thetic polymer binders and/or hardening agents (see Section 3.6.9.3.6).

Starch derivatives of various types are used in mixtures with other binders. For

example, oxidized starches are usually employed together with polymer disper-

sions. Hydrolyzed starches exhibit high stability in solution, good binding power,

and good flow behavior. Hydrolyzed, esterified starches exhibit good stability in

solution, high binding power, and increased reactivity towards wet-strength ad-

ditives (hardening agents) such as urea- or melamine- formaldehyde resins. Hy-

drolyzed, etherified starches exhibit the same properties as the esterified deriva-

tives. However, in contrast to the esterified derivatives, they can be used at pH

values above 8 without the risk of saponification. Starch derivatives that contain

phosphate and amino groups are compatible with cationogenic substances such as

satin white. The phosphate groups react with multivalent metal ions, such as alu-

minum ions, which leads to a certain degree of water resistance. The amino

groups react with aldehydes, which enhances the activity of hardening agents.

Cellulose Derivatives: Pure carboxymethyl cellulose (CMC) coats of 0.5–3 g m

–2

increase the grease resistance and printability of paper. Depending on the use,

various mixtures of low- and high-viscosity CMC are employed. Pigment-contain-

ing CMC coats, which can be applied on the size press, contain up to 10% semi-

technical, low-viscosity CMC or salt-free, purified CMC. CMC is usually processed

together with other natural or synthetic binders. Above all, CMC increases the

effectiveness of optical brighteners (see 3.6.9.3.5.5) and the water retention of the

coating mixture. Water retention is so high that the addition of other binders that

promote water retention in the coat is only necessary to a lesser extent. In general,

the amounts of CMC employed are 0.3–1.5% based on the pigment, and very low

viscosity types are preferred. This gives an adequate coating color viscosity, even if

the solids content of the coating color is high. Types of CMC that are soluble in

cold water are rapidly becoming established because they do not require dissolving

at elevated temperature. The presence of satin white pigment can cause strongly

interfering coagulations in the coating color.

Casein has been used as a central binder type in the past, but nowadays is of only

marginal importance in cast coatings. For its application it must be present in the

dissolved state. It is dissolved by the addition of alkali (e.g., ammonia, sodium

hydroxide, borax, or sodium carbonate) either separately in a cooker (up to 70 °C)

or with the pigment in a kneader. The casein concentration is limited to ca. 20 %

due to its high viscosity. The limit of processability is shifted to ca. 33% by the

addition of urea or dicyanodiamide, which reduces the viscosity and increases the

storage stability of casein solutions. The mixing of casein solutions with pigments,

3 Chemical Additives108

especially China Clay, can cause a “shock” phenomenon (a rapid increase of vis-

cosity).

Soybean protein is a very important binder type, especially in North America. It

has properties very similar to those of casein. Isolated soybean proteins are hydro-

lyzed, isoelectric proteins. They are used in the form of alpha and delta proteins

with four different viscosities (extra low, low, medium, and high). The viscosity

refers to the dissolved soybean protein, the solvent of choice being aqueous ammo-

nia (26 Bé). Proteins dissolved in this way exhibit very low sensitivity to water after

drying. Like casein, soybean protein is mostly used as a mixture with polymer

dispersions. This combination permits the preparation of coating colors with high

solids content and a relatively low viscosity. Solids contents of ca. 60%, suitable for

blade coaters, can be achieved.

3.6.9.3.3.2 Synthetic Latex Binders

The ongoing development of coating equipment and applicators in connection

with the increasing production speed (actual process up to 2.000 m min

–1

; pilot

coaters up to 3.300 m min

–1

) has forced the development of new binder types,

which meet the changed requirements better. In Europe the first styrene-butyl acryl-

ate dispersion (SBA) was polymerized by Badische Anilin- und Soda-Fabrik, today

BASF, in 1929. In the following years a partial substitution of the natural polymer

binders began and from the1950s onwards an extensive and continuous growth in

market demand has been observed. The corresponding pioneer work in the USA

took place in Dow Chemicals, where, from 1944 onwards, the production and

market introduction of carboxylated styrene-butadiene latexes (XSB) started. Polvinyl

acetate latexes (PVAc) as a coating binder were first researched and introduced to

the market in 1955, also in the USA, where a higher proportion than in other

world regions is still used. The actual worldwide market demand for these three

main types of synthetic coating binders comes to approximately 2 V 10

t p.a. as

dry material with approximately 75% XSB-, 15 % SA-, 7 % PVAc- and 3% other

latexes. These synthetic latex binders made it possible for the first time to attain a

high solid content at low viscosity, a prerequisite for modern high-speed coating

machines. In addition, polymer dispersions give the coat a higher water resistance,

better flexibility, higher gloss, and better printability. The products employed today

are aqueous dispersions of polymers, usually stabilized with anionic emulsifiers.

The solid content of the dispersions is generally ca. 50%. Very often, the polymers

are copolymers of several monomers, e.g., styrene, butadiene, acrylic esters, vinyl

acetate, and acrylonitrile. Apart from these main monomers small amounts of

auxiliary monomers, such as acrylamide, acrylic acid, maleic acid, and methacryla-

mine are also added to modify the dispersion properties.

Styrene-butadiene dispersions lead to varying film hardness, depending on the

proportion of styrene used. Approximately equal proportions of styrene and buta-

diene result in binders which provide a relatively soft film and a very good pigment

binding capacity. Disadvantages are the odor of the dispersion and the tendency of

the films to yellow when exposed to light. Acrylate dispersions are specialties and

3.6 Functional Chemicals 109

of high importance for impressive prints. Butyl acrylate is mainly copolymerized

with styrene or vinyl acetate. The ratio of the soft component (butyl acrylate) to the

hard one (styrene or vinyl acetate) determines the application characteristics of the

dispersion. In general, acrylate dispersions have an excellent brightness and age-

ing resistance and are less odorous. Apart from these two most important groups

of polymer dispersions, vinyl acetate homo- or copolymers have gained acceptance

in paper coating plants, especially in the USA. These products generally have a

lower binding power, but provide very hard and porous coats, and have an excellent

ageing resistance. Polymer dispersions based on methacrylates and copolymers of

vinyl acetate and ethylene are less important in paper coating.

In recent years the requirements for coating binders have become more and

more diverse. Changes in raw materials for paper production and/or production

process conditions as well as new requirements for paper characteristics and print-

ing technologies have forced the development of tailor-made binders with very spe-

cific property profiles. Such products are not generally different from XSB- or SBA-

latexes, as regards the principles of physics and chemistry. But with the know-how

of correlation and influence factors of monomers, functional monomers, process

auxiliaries, functional additives, process parameters and the interactions with dif-

ferent pigments, together with a flexible polymerization unit, it is possible to opti-

mize, very flexibly and quickly, a binder in respect of binding power, stiffness, print

evenness, print gloss, blister resistance, low yellowing and coater runnabilty.

Apart from the two-component systems, binder + co-binder, synthetic sole binders

which do not require a co-binder have been available since the early 1960s. Syn-

thetic binders are low-viscosity aqueous dispersions which usually do not influ-

ence properties such as viscosity and water retention that are important for the

flow behavior of the coating. For this reason, these “multipurpose dispersions” are

used alone in only a few cases. They are usually mixed with co-binders, which are

responsible for adjusting the flow properties of the coating color. On addition of

alkali, these products develop the required viscosity and water retention, but retain

their dispersion form. Special binders of this type have become popular, especially

for illustration paper that is produced in large amounts and used in rotogravure or

web offset printing. Table 3.2 lists the various binder systems, their influence on

the production of coating colors, the rheological properties of the coating color, and

the coating properties.

3.6.9.3.4 Additives Influencing the Properties and Processing of the Coating Color

It is desirable that coating colors have moderate pseudoplastic, shear thinning flow

behavior. Too drastic shear thinning can cause excessive water penetration into the

base paper and, accordingly, loss of binder. To adjust the rheological properties and

water retention of a coating color, synthetic co-binders and thickeners are used.

They have a profound effect on the runnability of a coating color, and hence of the

coating machine, on account of their thickening action, their characteristic rheol-

ogy at different shear rates, and their water retention. The mechanisms for dissolv-

ing, thickening, water retention, and rheology of synthetic co-binders are very

3 Chemical Additives110

similar to those of synthetic thickeners. Co-binders and thickeners can be roughly

divided into “natural” and “synthetic” products. Natural products are proteins (ca-

sein, soy protein) and polysaccharides (starch, alginates, cellulose ethers) and syn-

thetic products are e.g. acrylic copolymers, polyvinyl alcohol, polyvinyl acetate and

associative thickeners (Table 3.3).

3.6.9.3.4.1 Co-binders

These are primarily used to adjust the viscosity and water retention of coating

colors to the required levels and to modify their rheology according to the demands

of particular coating techniques. But they should also offer some additional advan-

tages over the thickeners in order to justify the higher recipe costs, e.g. binding

power, activation of optical brighteners (Table 3.4).

Natural products, like casein, starch, and soy protein used as main binders in the

past, also impart the necessary viscosity and water retention to coating colors.

However, they were unable to satisfy the increasing demands that were placed on

runnability and coating quality, and they have gradually been superseded by syn-

Table 3.2 Comparison of various Binder Systems

Binder system Production Rheological behavior Coating properties

Polymer dispersion alone

(rarely used)

simple mixing process usually unfavorable variable via type of disper-

sion; waterproof, glossy coat

attainable; no activation of

optical brighteners

Polymer dispersion +

watersoluble cobinder,

e.g., casein, starch,

CMC (very frequent)

mixing and dissolving

processes, sometimes

separately with heating

largely controllable

via cobinder

variable via single compo-

nents; limited water resis-

tance and gloss; full activa-

tion of optical brighteners

Polymer dispersion +

synthetic cobinder

(frequent)

integrated mixing/

dissolving processes,

no heating

controllable via

cobinder

limited variability; water-

proof and glossy coat attain-

able; full activation of opti-

cal brighteners

Polymer dispersion +

viscosity regulating additives

(in special cases)

integrated mixing/

dissolving processes,

no heating

limited controllability variable only via type of dis-

persion; good water resis-

tance and gloss, no activa-

tion of optical brighteners

Special dispersion

(for publication papers)

integrated dissolving/

mixing processes,

no heating

slight controllability limited variability; improved

water resistance with addi-

tives; good gloss attainable;

optical brighteners can be

activated only via auxiliary

components

3.6 Functional Chemicals 111