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

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3.4.3

Starch Application

As can be seen from Fig. 3.5, 80% of total starch consumption in the paper in-

dustry is used for surface application, either for surface sizing (62 %), or for spray-

ing (3%) on the wet paper web or between two layers in board manufacturing, or

as a coating binder (15%). The main role of surface size is to promote surface

properties, e.g. strengthen the paper surface and to bind particles such as fibers

and pigments more strongly to the surface. Additionally, the starch is expected to

add internal strength to the sheet through a liquid penetration in the z-direction.

Opposite to wet-end application, where starch retention is crucial, starch applied to

the surface is generally 100 % retained.

For production of woodfree uncoated and coated fine papers up to 40 kg starch

per ton of paper are applied. 3 to 10 kg starch is added at the wet end, with the aim

of internal strength improvement and retention increase. The major share of the

starch is added to the sheet in surface treatment. A mass balance on a typical fine

paper machine has shown, that more than 90% of the added starch is retained in

the final paper product. Losses occur mainly during the sheet forming process in

the wire section due to insufficient retention. Starch which is not held back in the

paper is discharged with the process effluents to the waste water treatment plant,

where a complete biodegradation process follows.

Packaging paper, e.g. testliner and corrugating medium, made from 100 % re-

covered paper, can only be produced economically and in the required quality by

adding cost effective dry-strength agents, i.e. biosynthetic starch products. There-

fore these papers are produced with an average starch consumption of 40 kg t

–1

mainly by surface application. A further 25 kg (t corrugated box board)

–1

is applied

as an adhesive in the converting plant. This means that a high amount of starch is

returned to the production process via recovered papers, where it is nearly not

retained in the paper sheet. Therefore this uncontrolled starch quantity leads to a

considerable load in the white water circuit (usual COD levels from 5.000 to

30.000 (mg O

) l

–1

) and finally also in the waste water. By applying starch with a

size-press to the paper surface, the paper strength will be increased by 30 to 60 %.

Additional energy is necessary for extra drying of the paper web and the paper

machine runnability may decrease.

To improve productivity and reduce production costs, effective and well con-

trolled starch application at the wet end is required (see Fig. 3.6). New practical

experience shows that a starch-polyvinylamine complex can be an alternative to a

surface treatment by size-press for strength improvement. A holistic picture of the

application of starch based on analytical investigations (in produced paper, exhaust

air, clarified waste water and soil) shows that starch is an extraordinary, envir-

onmentally friendly additive with a diversified functionality.

3 Chemical Additives72

3.5

Aluminum Compounds [3, 9]

Aluminum compounds are widely used in the paper industry: in rosin sizing, as

drainage and retention aids, and as fixing agents for anionic trash (dissolved or-

ganic compounds e.g. extracts from wood fibers, degraded starch). Important ap-

plications are the treatment of circuit water and effluents or the thickening of

sludge, where the flocculation and precipitation effects of aluminum ions are uti-

lised. The most frequently applied aluminum compounds are aluminum sulfate,

polyaluminum chloride (PAC), polyaluminum hydroxidechloride sulfate, and re-

cently also (poly)-aluminum nitrate and polyaluminum nitrate sulfate. Aluminum

compounds have a favorable price-performance ratio. An analysis of the alumi-

num concentration in the production of a wood-containing printing paper (at neu-

tral pH, with 0.45% aluminum sulfate added to the white water) showed an alumi-

num level in the water circuit (at a pH of approx. 7.5) of about 0.1 mg l

–1

, max-

imum 0.2 mg l

–1

(determined by UV/VIS spectroscopy). This indicates that the

paper retains nearly all of the aluminum added in the form of aluminum hydrox-

ide. The anionic counterions sulfate and chloride, however, are nearly fully dis-

charged into the effluent. The paper retains the share of ions dissolved in its

residual moisture at the end of the drying section. Sulfate and chloride ions are

nonvolatile, i.e. they are not released into the atmosphere. The loss via paper-

making residues is negligible. The two anions may be considered environmentally

harmless. As sulfate promotes the corrosion of concrete, there may be statutory

discharge limits for this substance (500 mg sulfate l

–1

). There are usually no re-

strictions, however, on the discharge of chloride.

The use of polyaluminum nitrate and polyaluminum nitrate sulfate is fairly new.

The two compounds help to prevent or combat malodours caused by the conver-

sion of sulfate into hydrogen sulfide under anaerobic conditions. The results of

mill trials suggest that nitrate is converted into elemental nitrogen because no

nitrate could be detected in the effluent – nitrate was found only in the white water.

According to information from mills using large quantities of polyaluminum ni-

trate, a minor discharge of nitrate into the effluent can occur [3].

3.6

Functional Chemicals

All chemical additives which are used to obtain certain properties of the finished

paper, e. g. color, improved resistance to ink or water, gloss, printability, strength,

are called “functional chemicals” (see Fig. 3.3.).

3.6 Functional Chemicals 73

3.6.1

Coloring Materials (Dyes) [3, 9–11]

3.6.1.1 General

Historically, dyes are the oldest class of synthetic specialty chemicals employed in

the production of paper. Dyes are added to paper at a statistical average rate of only

150 g chromophore per 1 ton of paper and board worldwide. The turnover in dyes

(total 500 V 10

€) accounts for 5% of the specialty chemicals applied to paper but

only for 1,5% in view of solid mass addition (Fig. 3.2 and 3.3).

Colorants are used in paper manufacture for a variety of reasons, including eye

appeal, color coding, and brand identification. The selection of suitable colorants

depends for instance upon end-use requirements, physical and chemical proper-

ties, and handling characteristics. Coloring is done to achieve the following two

general goals:

• to produce paper having a given color or shade

• to produce white paper having a desired tint.

Coloring is rather tricky:

• Process variables may interfere with dyes.

• Specific dyes react differently in different furnishes.

• Dyes may adversely interact with other additives – primarily through charge

interaction – and interfere with them.

Additional parameters have to be considered:

• In the case of food contact, bleeding has to be assessed according to DIN EN

646

• For advertisement use the light fastness is important

• Dyes have to meet the toxicological and ecological requirements, e.g. German

“Technical Regulations of Risky Substances” and EU guideline 2002/61/EG

(19. 07. 2002). Therefore, no listed aromatic amines are released after reductive

cleavage of any of the azo-bonds.

3.6.1.2 Classes of Coloring Material

3.6.1.2.1 Anionic Direct Dyes

These are the dominant class of dyes. They account for 52% of the total worldwide

turnover, followed by basic dyes with 28 % share (Fig. 3.7). Anionic direct dyes are

the sodium salts of azo dyes containing sulfo groups to give water solubility. An-

other group are copper phthalocyanines which also contain sulfo groups (Fig. 3.8).

They have a high affinity to bleached chemical pulps, and often do not need addi-

tional fixing agents, unless very deep shades with good bleed fastness are required

(e.g. deeply colored napkins). Suitable fixatives are condensation products of or-

ganic amides with formaldehyde, however, they are not very well suited for the

dyeing of wood-containing stocks because the incrusting components (binding

3 Chemical Additives74

substances) of wood prevent uniform dyeing. Therefore a mottling tendency is

occasionally observed with stock mixtures (bleached chemical and mechanical

pulp; recovered paper). The lightfastness is usually good, but the colors attainable

may not be as brilliant as with acid and basic dyes.

3.6.1.2.2 Cationic Direct Dyes

These retain the planar molecular structure of acid direct dyes, but cationic groups

have been incorporated in the structure. This is to increase their affinity to paper

fibers. They are moderately adsorbed on bleached lignin-containing stocks, and

even in the case of deep shades, alum or a fixative is usually not required. These

dyes produce fairly good bleed fastness. Combination dyeing with anionic direct

dyes can be carried out to improve the fixation of the dyes on the paper machine

Fig. 3.8 Chemical structure of two direct dyes.

Fig. 3.7 Classes of coloring materials in 2004 (market shares

at world wide turnover of 500 V 10

€).

3.6 Functional Chemicals

and to reduce the contamination of the circuit process water and the waste water.

In most cases combination dyeing leads also to an improved bleed fastness.

3.6.1.2.3 Basic Dyes

These are the salts (chlorides, hydrochlorides, sulfates, and oxalates) of color bases.

They are soluble in acid aqueous solutions, and this is often the reason for the use

of acetic acid to produce concentrated dye solutions. Such liquid dye formulations

often contain 5–25% acetic acid and 5–20% of a glycol derivative for stability

reasons, along with ca. 25% chromophor. The chromophors have a very high affin-

ity for paper fibers, especially to unbleached and mechanical pulps (lignin-contain-

ing fibrous material) and also for fillers. This is due to their high cationic charge

density, corresponding to a 99 % fixation in the finished paper. Salt formation gives

rise to very stable color lakes that are insoluble in water.

Anionic fixatives, e.g., the sodium salt of condensation products of formalde-

hyde and naphthalene sulfonic acid, also form color lakes of this type with cationic

dyes. They are used to improve the fixation of basic dyes on bleached pulps that

contain only small amounts of lignin. Better dye fixation always results in richer

colors, improved bleed fastness, and less effluent staining.

Basic dyes are mainly used in the production of packaging papers (liner, tes-

tliner, corrugating medium; 55 % of total volume), for tinting of wood-containing

printing papers (20%) and colored, wood-containing printing/writing papers

(15%).Within the class of basic dyes, brown grades have the greatest volume. The

main reason for this is that recovered paper is nearly the sole source of packaging

paper in many countries. Brown dyes in testliner should imitate unbleached kraft-

liner, enhance the visual appearance and characterize certain qualities. Two typical

representatives of these dyes are “Basic Brown 1”, an azo-dye, and “Basic Brown

19”, a methine dye, corresponding to the international color index (Fig. 3.9). The

strong cationic charge centers of both dyes give a very high affinity to the paper

stock. Because of the molecule doubling the methine dye shows even better fixa-

Fig. 3.9 Chemical structure of two basic brown dyes.

3 Chemical Additives

tion. Basic dyes are brilliant and tinctorially strong, resulting in good fastness to

water and steam on groundwood-containing furnishes. Disadvantages are poor

affinity to bleached furnishes, mottling tendency (i.e., parts of the paper stock or

stock mixture are dyed more deeply than others), and poor lightfastness.

3.6.1.2.4 Acid Dyes

These are all water-soluble salts (usually in the form of the sodium or potassium

salt) of colored organic acids which dissociate in water to form colored anions.

Most acid dyes are azo dyes and are similar to direct dyes. These two groups

overlap with no distinct boundary. Acid dyes generally contain more acid groups

which gives rise to their greater solubility in water, compared to direct dyes.

Acid dyes have little affinity to paper fibers. Although the dye molecules pene-

trate well into the capillaries of the fibers, they must be fixed with aluminum

sulfate (pH ca. 4.5) and cationic fixing agents, e.g., condensation products of di-

cyandiamide with formaldehyde, polyamines, polyvinylamines, in order to achieve

a satisfactory dye yield. Further disadvantages are relatively strongly colored waste

water/effluent and poor bleed fastness. Advantages of acid dyes are good solubility,

no tendency to mottle on stock mixtures, and they are very well suited to dip and

surface dyeing. Acid dyes are mainly used for (certain) writing and printing pa-

pers, decorative crepe paper, and carbon paper. The importance of acid dyes for

paper dyeing has decreased tremendously because of their poor cost/performance

ratio and the above mentioned waste water problems.

3.6.1.2.5 Colored Pigments

These fall into three classes: natural inorganic pigments, synthetic inorganic pig-

ments, and synthetic organic pigments. The third class is by far the most im-

portant for the paper industry. The natural inorganic pigments such as ocher, terra

sienna, or umber are of little significance at present. The synthetic inorganic pig-

ments (iron oxide, cadmium, chromium oxide) the organic pigments (azo and

polycyclic), and the metal complex pigments (phthalocyanine) are the important

ones.

In the pigments the colorant is present in a water insoluble, finely dispersed

form. The binding of the pigment to the fibers is improved by rosin sizing and the

use of aluminum sulfate. The pH of the system must be kept below 5.0 in order to

keep the alum in an active form for retention. Fixing agents and effective retention

aids must be employed during papermaking in neutral or slightly alkaline systems.

The lightfastness of pigments is excellent and mottling does not usually occur. The

low tinctorial value of the colored pigments means that high loading levels are

required to achieve strong colors, which leads to a weakening of the sheet and to

high costs. Therefore these pigments are only used for specialty papers where a

very high lightfastness is essential, e. g. bookkeeping and label papers, document

and laminate base papers, lightfast printing and writing papers.

3.6 Functional Chemicals 77

Colored pigments are applied more often in surface coloring and coating for-

mulations where they are superior to soluble dyes, because they do not follow the

water into the paper sheet when the starch or size or pigment coating is applied.

3.6.1.3 Dyeing Processes

3.6.1.3.1 Stock Dyeing

This is also called internal dyeing and is the most widely used paper dyeing proc-

ess. Because of clean working conditions and the most efficient usage, the dyes are

now mostly added continuously and fully automatically into the stock flow. More

seldomly the dyes are metered batchwise in the pulper or mixing chest.

The choice of dye and the fixing and dyeing conditions largely depend on the raw

materials used in papermaking (recycled fibers, stone groundwood, TMP, CTMP,

unbleached or bleached chemical pulp, type and portion of filler) and on its prepa-

ration process e.g. a higher degree of beating of the pulp results in a deeper

coloring. Fillers increase the required amount of dyes because they absorb dyes

and, at the same time, reduce the coloration owing to their covering power and

brightness. Fillers also lead to an increase in the two-sidedness of the paper

sheet.

In continuous dyeing, the point of addition is also determined by a few factors

e.g. high consistency dyeing at a stock consistency of 3–4 % (before mixing with

white water ahead of the headbox) and alternatively low consistency dyeing at a

stock consistency of 0.5–1.5% (in front of the mixing pump or pressure screens).

The pH conditions are very important. The addition of aluminum sulfate usually

promotes the absorption of dyes and yields less colored waste water and effluent.

In general, there is a trend towards paper production in the neutral or alkaline pH

range. These conditions need dyes with a very good affinity to the paper stock in a

neutral medium and/or very effective fixatives and retention aids.

The two stock dyeing processes have the following advantages and disadvan-

tages:

Batch addition has the advantage of thorough mixing of the additives with the

paper stock and optimal fixation due to longer contact time between the fiber and

dye. The disadvantages are that the time required for color correction and color

change is relatively long (loss of productivity). The handling of the dyes is more

problematic with regard to clean working conditions and to an exact and regular

metering control system. An integration of a continuous on-machine dye shade

measurement with the metering of the dyes is not possible (less productivity).

Continuous addition has the advantages of a short length in the stock line that

must be cleaned when the color is changed, and of less broke because the desired

shade is attained more quickly (higher productivity). However, a lower color yield

(low contact time) is obtained for intensely colored papers. The more complex

equipment required for this dyeing process must be taken into consideration as

well. On the other hand, control of the shade of the paper produced by continuous

color measurements in the paper machine, and fast adjustment of the feeding

pumps, lead to less broke and thus higher productivity.

3 Chemical Additives78

A specialty of stock dyeing is the so-called tinting, mainly used for printing and

writing papers. This procedure basically consists of counteracting the slight yellow

tinge of all paper stocks by adding a violet dye or a combination of pure blue and a

brillant red dye, which leads to a slightly blue shade. The human eyes perceive this

shade as more bright.

3.6.1.3.2 Surface Coloring

Here liquid dyes are added e.g. during the size preparation for size press or film

press application. Other additives e.g. starch, synthetic sizing agents, optical

brighteners are also applied in this way. For such product combinations negative

interaction of any kind must be avoided. An essential prerequisite for uniform

dyeing is adequate and, above all, uniform absorbency of the paper. The advan-

tages of this process are: quick changeover of shades, the possibility of only one-

paper-side dyeing (typical paper grades for this are liner and testliner), absence of

dyes in the water circuit, and, in the case of papers with higher basis weights

(> 80 g m

–2

), the saving of dyes. Nevertheless, compared with stock dyeing – the

classical dyeing process – surface dyeing has gained acceptance only in individual

cases because really uniform dyeing of the paper is difficult to achieve. It is occa-

sionally advisable to combine stock and surface dyeing, e. g. to correct two-sided-

ness. The bleed fastness of surface dyed papers is generally lower than for internal

dyed papers.

3.6.1.3.3 Dip Dyeing

A small group of specialty papers, called effect papers (flower crepe paper, tissue

paper), is noted for its intense brilliant shades. The paper is passed through a

dipping bath containing an aqueous solution of the dye or dye combinations. The

excess dye liquor is pressed off between two rolls and the wet paper is creped, if

required, before drying. Acid dyes are usually used because they have high sol-

ubility and bright shades. The low affinity of these dyes for fibers results in uni-

form dyeing, even in the case of papers with greatly varying fiber composition. The

bleed fastness of dip dyed paper is poor, corresponding to that of surface-colored

paper and even poorer than stock dyed paper.

3.6.1.3.4 Surface Coloring by Coating

In the usual coating process, the surface of the paper or board is covered with

white pigments. In the case of colored coatings, the starting material is also a white

pigment coating mixture, and the desired shade is attained by adding a dispersion

of a colored organic or inorganic pigment. This coloring method and these colored

pigments are mainly used for specialty paper and board e.g. for labels, documents,

impressive image brochures and packaging materials. E.g. for a bronze-glazed

paper surface aluminum or brass powder is added to the coating color, which

produces a silver or gold effect or, in combination with soluble dyes, a metallic

3.6 Functional Chemicals 79

effect. Water-soluble dyes can be used only to a limited extent because, even with

the use of fixatives, bleeding cannot be prevented (migration into the base paper

and into moist surfaces). Also, the inadequate lightfastness compared to pigments

limits the use of these dyes.

3.6.1.4 Requirements of Colored Paper and Board

Depending on the intended purpose of the paper, different fastness properties are

required:

3.6.1.4.1 Light Fastness

This is defined as the fastness of a dyed paper to the action of light. It is deter-

mined by both the dye used and the raw materials of the paper. The degree of

lightfastness is specified by a test method according to DIN 54 003, which is also

used in the textile industry. Originally lightfastness was tested by exposing the

dyed material to sunlight under defined conditions. Today artificial light with a

radiation spectrum similar to sunlight is used (Xeno test apparatus or Fade-Ome-

ter). The lightfastness cannot be given as an absolute value but can only be ex-

pressed in relation to a standard which is exposed simultaneously. In the textile

and paper industry, the Blue Wool Scale is used as a standard for comparison. It

consists of blue dyeings with lightfastness ratings from 1–8. Rating 1 signifies the

lowest, and rating 8 the highest lightfastness. A dyed paper with a lightfastness of

1 will change its shade after one hour “sunlight” exposure. However, the shade will

not fade completely until it has been exposed for several hours, depending on the

depth of shade and the fibrous material. Since paper is normally not subjected to

such severe exposure, a lightfastness of 1 is sufficient for all short-lived paper

grades e.g. newsprint, magazines and grades based on mechanical pulps (e. g. for

coating base paper, notepad) and/or mixed recovered paper (e. g. for liner, test-

liner). A lightfastness of 3 corresponds to a resistance of several days exposure.

Dyes with this lightfastness rating can be used for most paper articles, provided

that the paper stock, too, has approximately the same lightfastness e. g. for all

kinds of high grade printing and writing papers. Dyed paper with a lightfastness

rating of 5 does not undergo any change in shade, even on exposure to direct

sunlight for several weeks, this rating is needed e.g. for document paper, photo-

graphic paper, laminating base paper.

3.6.1.4.2 Bleed Fastness

This is required for paper and board that are used for food-packaging and tissue

papers (napkins and hygienic papers). According to the regulations, tests must be

carried out in each case to determine whether and to what extent dye can migrate

from a colored paper onto the packed foodstuff or to human skin. The paper may

come into contact with water, dilute fruit acid, grease, oil, alcohol or alkali. A test

specimen is placed between two uncolored glassfiber papers moistened by dipping

3 Chemical Additives80

into the test solution. This sandwich is then placed between two glass plates of the

same size. The whole is then wrapped airtight in a polyethylene film and loaded

with a certain weight. The specimen is kept in this condition for 24 h at 20 °C.

After drying, the coloration of the glassfiber paper is compared with the Grey Scale

to assessing the fastness to bleeding according to DIN 54 002. It should be empha-

sized that the bleed fastness is not only dependent on the dye but also on the

fibrous material and the type of dye fixation. A reliable prediction can be made

only on the basis of tests with the paper in question. For napkins and hygiene

papers a number of direct dyes are suitable and for wood-containing papers and

testliner a good bleeding fastness can be obtained with basic dyes.

3.6.1.4.3 Other Properties

Rub resistance is required for album and wrapping paper and for cover board.

Direct dyes are suitable for this purpose.

Acid resistance is required for parchment and vulcanized fiber base paper, writ-

ing, and printing paper. Organic and inorganic pigments, selected representatives

of all colorant groups, are suitable for this purpose.

Solvent resistance is required for labeling paper (packaging of perfumes, medi-

cines, and spirits). Special pigments are suitable for this purpose.

Heat resistance is required for cable and core paper and laminate paper. Organic

and inorganic pigments are suitable for this purpose.

3.6.2

Optical Brightening Agents (OBA) – Fluorescent Whitening Agents (FWA) [2, 9, 11]

OBAs, also called FWAs, represent 3% by value and only 1 % of the total dry

amount of specialty chemicals (see Fig. 3.2 and 3.3). They increase the whiteness/

brightness of paper and are preferably added to the stock. They are very effective

when used with highly bleached pulps, and much less effective, or even ineffective,

when applied to unbleached chemical pulps and mechanical pulps. OBA are also

used in surface applications such as surface sizing and paper coatings.

OBA absorb light in the ultraviolet spectrum range (below 370 nm) and re-emit

the light in the visible blue range (peaking at 457 nm). This results in a fluorescent

effect with bright white in daylight masking the inherent yellowness of the raw

materials. Any material that absorbs ultraviolet light will lower the efficiency of

fluorescent whiteners. For example, lignin absorbs ultraviolet light and the higher

the lignin content of the pulp, the less effective is the OBA. Hence, mechanical

pulps and unbleached pulps are less susceptible to whitening with OBA. Some

filler clays tend to counteract the fluorescence and reduce the effect of OBA. Fillers

such as calcium carbonate and aluminum trihydrate reflect ultraviolet light,

thereby enhancing the effect of OBA. A high pH (above 6) also helps to achieve

maximum whiteness. On the other hand TiO

absorbs UV, thus OBA cannot be

used in conjunction with high TiO

3.6 Functional Chemicals 81