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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”

3.6.1
Coloring Materials (Dyes)

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 106 €) 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 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 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 fixation. 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.
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.

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 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 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.
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 TiO2 absorbs UV, thus OBA cannot be used in conjunction with high TiO2 loading.
Substances of different chemical composition and different biological origin are used for fluorescent whitening of paper. Derivatives of diaminostilbene disulfonic acid have proved the most popular in industry because of their fastness properties. The active ingredient content is generally between 20 and 27 %. Their central part is disulfonated diaminostilbene. The types differ in the number of sulfonic acid groups in the side groups. The high-substantivity types with only 2 sulfo groups make up about 11 % of the market while tetrasulfonated derivatives with medium substantivity are about 80 %. The rest is low-substantive, hexasulfonated types.
The paper industry today needs fluorescent whitening agents to obtain high degrees of whiteness at reasonable cost, especially if the FWA (OBA) are combined with shading dyestuffs. This in turn leads to sharper contrasts in the printed image and thus helps to reduce the toner consumption of copiers, for example. Similarly, the color brilliance of color prints is improved. For application in the stock, high or medium-substantivity types are generally used, being 1 % of the commercial-grade product. For surface application in the size or film press, medium or low-sub-stantivity types are used, the normal addition being 1.5 % and in exceptional cases up to 3.5 %. Low or medium-substantivity FWA are also the preferred products for coating application, the amount required being up to 1.5 % up to 3.5 % in excep¬tional cases. A maximum amount of FWA can be used in each application as the shade becomes greener. This will lead to a graying effect on the brightness.

3.6.3
Chelating Agents – Complexing Agents [3–5]
The presence of heavy metals negatively influences many production processes and paper properties. In particular, they interfere with the bleaching agents in chemical and mechanical pulp production, reducing their effect, so the task of chelating agents is to counteract this detrimental impact. Chelating agents can also reduce or stop uncontrolled decomposition of a hydrogen peroxide solution, which would result in loss of bleaching effectiveness.

Chelating agents that contain amino and carboxyl groups mask metal ions effec¬tively. Chemical compounds of this type are nitrilotriacetic acid (NTA), ethylene¬diaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA). Other complexing agents include the soluble salts of oxalic acid, citric acid, tartaric acid, gluconic acid, amines, and ammonia.
Both the demand for increased brightness and the technology trend to elemental chlorine-free and total chlorine-free bleached pulps has led to an increased use of chelating agents today, dominated by DTPA and EDTA.
In view of environmental protection, there exists a controversial discussion about the biodegradability of chelating agents. The results of newer investigations show that, under certain conditions, EDTA and, with some restrictions, DTPA, are fairly biodegradable. Their degree of biodegradability is still under discussion. It is strongly influenced by pH (optimum at pH 8.5–9), by ultraviolet light and by the effectiveness of adapted microorganisms. Up to 90 % biodegradability of EDTA is reached in some pulp and paper mills.

Of less importance are chelating agents based on polyphosphates, phospho-nates, and hydroxycarboxylates as well as nitrogene free products like citrate, ta¬trate and gluconate.

3.6.4.1 Fundamental Aspects of Paper Sizing

Sizing makes the native fiber network hydrophobic and thus prevents or reduces the penetration of water or other aqueous liquids into the paper. Sizing prevents the spreading and strike through of ink or printing colors. Papermaking fibers have a strong tendency to interact with water. This property is important for the development of strong interfiber hydrogen bonds, especially during drying, and is also the reason why paper loses its strength when rewetted. A high absorbency is important for a few paper grades such as toweling and tissue. Also corrugated medium paper must be “absorbent” to a certain degree to convert properly in the corrugating process. On the other hand such properties are disadvantageous for many paper grades e. g. liquid packaging, top layer of corrugated board, writing and printing papers and most of the specialty papers. The water and liquid absorb¬ency can be reduced by the addition of sizing agents to the paper stock and/or by their application to the paper surface.
Since the invention of paper about 2000 years ago, it has been treated or sat¬urated with mucilage and rice starch for surface glazing properties, so that people could write on it. After 1280 A. D. animal glue was the principal sizing agent. Alum (potassium aluminum sulfate) was used to harden the applied glue. Alum rosin sizing entered the scene in 1806 (Moritz Friedrich Illig). In 1876, paper-maker’s alum, Al2(SO4)3, was introduced. Since the 1950s the various forms of rosin size (paste, dispersed, fortified), alkyl ketene dimmer size (AKD), alkenyl succinic anhydride size (ASA) and polymers mainly based on styrene acrylate and styrene maleinate (PSA) have come to the market. Today, beside starch for strength improvement (Fig. 3.2) and polymer binders for paper coating, sizing agents are the most important quality improving additives in paper manufacturing. Amongst the specialty chemicals, sizing agents represent a share of 10 % in dry mass (Fig. 3.3.) and a total worldwide turnover of approximately 950 V 106 €.

3.6.4.2 Product Classes
The main classes of chemicals and their consumption as a proportion of the total worldwide usage of 350 000 tons dry p. a. is shown in Fig. 3.10. Because of the trend to neutral or slight alkaline papermaking and the increasing use of calcium carbonate as a filler and coating pigment, the proportion and also the absolute volume of rosin will decline further and the synthetic products will steadily grow.
 
3.6.4.2.1 Rosin-based Sizing Agents
These are still the most widely used of all wet-end sizes, because they are cheap to produce and are produced from regenerative raw materials. They are mainly used in acid and much less in pseudo-neutral pH conditions. They have the advantage that the degree of sizing can be controlled very easily, and the sizing effect is fully cured when the paper leaves the machine. In practical use there are different preparations of rosin, for which the following terms are used:
. • Free rosin: Resin acids whose carboxyl groups are in the completely protonated form.
. • Dispersed size: High (95–100 %) free rosin dispersion of resin acids. The dis¬persed size particles are stabilized by surfactants, starch, polymers, or pro¬teins.
. • Rosin soap size: Rosin size composed of the sodium salts of resin acids. Formed by neutralizing (saponifying) resin acids with NaOH or some other sodium base, such as Na2CO3. Different forms of soap sizes are available, including paste, extended, and dry sizes.
. • Extended size: A 50 % solids product consisting of a 50:50 mixture of completely neutralized resin acids and urea.
. • Fortified size: Rosin size that has been reacted with fumaric acid or maleic anhy¬dride to form the Diels-Alder adduct. The reaction product contains extra car¬boxyl groups and produces more efficient sizing response than the unreacted resin acids. It is used as a starting material for other size products.
. • Dry size: A 100 % neutralized, dry rosin size product that dissolves easily in water.
. • Paste size: An 80 % neutralized rosin size product marketed as a 70–80 % solids paste.

3.6.4.2.2 Synthetic Sizing Materials
These were only developed in the second half of the last century. Driving forces have been the increasing use of the cost and performance effective calcium carbonate as filler and coating pigment and also the need to produce paper with high and permanent paper strength. To enable its use the paper has to be produced under alkaline conditions. Other paper grades must have very hard sizing in order to resist edge wicking in liquid packaging applications or photographic base paper, or to exhibit hot water resistance for gypsum board. Many mills also seek an alter¬native sizing procedure to the wet-end application, e. g. in the form of a surface treatment in the drying section, to obtain higher efficiency.

. • Alkyl Ketene Dimer (AKD) was the first synthetic size and appeared in the patent literature in 1953. Structurally, alkyl ketene dimers are unsaturated lactones. In the manufacturing process a frequently used synthesis route is: the acid chloride of a carboxylic acid is prepared (fatty acids, C16–22 homologs treated with, e. g. phosphorus trichloride, phosphorus pentachloride, thionyl chloride or phos¬gene), followed by intermolecular lactone ring condensation via a labile inter¬mediate carboxylic acid obtained by dehydrohalogenation of the acid chloride with triethanolamine in an organic solvent. Newly developed processes work without any solvent. To handle and to use AKD in a paper mill, the wax must be converted into tiny particles (0.5–2 mm) dispersed in water. The emulsification is usually effected in a hot (75–90 °C) solution containing the cationic starch stabi¬lizer (alternatively a cationic polymer e. g. polydadmac, polyvinylamine) and a small amount of surfactant, e. g. sodium lignin sulfonate, to which are fed AKD flakes. After melting the AKD, the mixture is forced through a microfluidizer and cooled. Small amounts of a promoter (cationic polymer with low molecular weight and high charge density) and biocide can also be included. The solid content of the commercial products varies between 6 and 30 %. Some scientists believe that AKD reacts with the hydroxy groups of cellulose to form b-keto esters.

. • Alkenyl Succinic Anhydride (ASA), as a synthetic sizing agent for paper, was first synthesized in 1974. It is composed of an unsaturated alkenic hydrocarbon back¬bone coupled to succinic anhydride. It is usually manufactured in two stages: First an unsaturated linear or branched olefin, e. g. a 1-alkene, is isomerized by moving its double bond randomly from its a-position. This will yield an ASA product that is liquid at room temperature, a condition important for easy emul¬sification of the size on application in the paper mill. In the second step, the mixture of isomerized alkenes is reacted with maleic anhydride to produce the ASA raw material. Normally ASA is a yellowish oily product. This 100 % active substance can be stored as such for a long time but must be well protected from water or humidity.
. • Polymeric Sizing Agents (PSA) are mainly based on styrene-acrylates, polyure¬thanes or styrene-maleic acid anhydride. They are either water-based dispersions or aqueous solutions. These products were developed around 1960. Since that time an increasing number of size-presses for strength improvement with starch have been installed. The styrene-maleinate polymers are anionic, aqueous solutions with a solid con¬tent of 20–30 % and an alkaline pH (mainly containing the ammonium salt). They are specially designed for surface application. Their advantages are very good compatibility with all other anionic chemicals (e. g. native and anionic starches, CMC, optical brighteners, anionic polyacrylamides) and high stability in the size-press or in water doctors. The disadvantages of styrene-maleinate polymers are that the sizing effect depends on a certain amount of alum in the base paper, the improvement of printability on wood-free white papers is lim¬ited, and these products are not suitable for wet-end addition (internal sizing). In respect of this, the cationic styrene-acrylate dispersions are much more com¬monly used. They possess high paper stock affinity and the sizing effect is largely independent of the alum content and pH in the stock suspension. There¬fore they can be used for nearly all paper grades, depending on the cationic density of the polymer. With increasing cationic density a reduction in the opti¬cal brightening and the paper brightness will occur. Most products of this class are suitable for wet-end and surface application. The sizing effect is developed within the paper machine. The sizing is improved by intense contact drying on hot metal surfaces (e. g. drying cylinders). The sizing obtained is resistant to acids and alkalis. The surface properties of the paper are improved, especially pick strength and printability and linting and dusting are also reduced.

3.6.4.3 Application Advice
. • Rosin: The average addition rate is 1 % solid calculated on paper, the individual rate depending on the required degree of sizing. With these products this degree can be adjusted very precisely and is fully obtained directly after the paper has been dried. The product is usually added to the stock suspension upstream of the headbox and then fixed chemically, in the simplest case with aluminum sulfate (alum). The retention of rosin size is in the region of 70 % if the usual fixing agents are employed. Rosin sizes are not usually suitable for surface ap¬plications.
. • AKD: As little as 0.005–0.008 % reacted AKD is believed to yield sufficient monomolecular coverage of the paper stock, while dosage rates are considerably higher and typically vary from 0.05 to 0.2 %, calculated as solid AKD wax based on paper stock. The stability to hydrolysis of most commercial products has been improved significantly in recent years. The ketone resulting from the hydrolytic decomposition of AKD is a solid (melting point 80–85 °C) which can be retained in the sheet but does not contribute to sizing. The risk of the formation of deposits is relatively low. For cost efficient sizing, the curing (orientation of AKD particles) must be initiated appropriately because chemical reaction alone is not sufficient. The curing can be improved by drying the paper web at high contact temperatures and to a very low moisture content e. g. before the size press, typically to 1–3 %, especially on fast-running machines. Another way to speed up the curing before converting is to carry out AKD sizing in the presence of a so-called promoter resin, e. g. polyamide epichlorhydrin, modified polyethyleni¬mine, polyvinylamine.
. • ASA: This will not dissolve in water and, prior to application in papermaking, it must be emulsified on site at the paper mill. There, a small amount (3–6 %) of an activator is added, plus cationic starch and a synthetic cationic polymer which serve as stabilizers. Activators are surface-active agents that promote effective emulsification at low mechanical energy. Emulsion stability is best with syn¬thetic polymer, and sizing efficiency is best with cationic starch, improving with the amount of starch used. The ratio of starch to ASA is usually in the range 2:1 to 4:1. Emulsification is primarily carried out in a continuously operated special automated equipment, and can be affected by both low and high shear proce¬dures. Particle sizes in the range 0.5 to 2 mm can be obtained. Emulsification imparts a cationic charge on the oil droplets which helps to increase the stability as well as improving ASA retention on anionic fibers and sizing efficiency.

Linear hydrocarbons are more hydrophobic than branched ones. However, the greater the hydrophobicity of the ASA molecule, the more difficult is the prepa¬ration of a stable quality ASA emulsion. ASA undergoes the normal reactions of acid anhydrides. Of particular interest in conjunction with sizing is the reaction with alcoholic hydroxy groups to yield an ester, and the hydrolysis with water. Both reactions occur in the papermaking system. ASA is highly reactive, and the reactions proceed rapidly and irreversi¬bly. Although this would provide satisfactory development of sizing on a paper machine, the hydrolysis of ASA is undesirable. The hydrolysis accelerates with pH, time, and temperature and leads to deposits and runnability problems on the paper machine. In order to limit the hydrolysis of ASA emulsions, the pH can be lowered immediately after the emulsification by addition of aluminum sulfate. High solids retention is extremely important in ASA sizing. If this is guaranteed and ASA is applied correctly, a relatively small dosage of 0.1–0.2 % based on solid furnish is required to produce the desired degree of sizing. Careful selec¬tion of retention aid is therefore an important part of the ASA sizing process. The great advantage in favor of ASA is the high rate of cure in comparison with AKD size. The reaction with cellulose hydroxy groups takes place rapidly in the dryer section of the paper machine at less than 5 % web humidity. More than 90 % of the attainable sizing potential is, in most cases, achieved before the size press unit. The reaction rate is influenced by the same parameters as the hydrol¬ysis, that is primarily by pH, previous residence time, and stock temperature. The best conditions obviously depend upon the balance between the reaction rates with cellulose hydroxy groups and hydrolysis of ASA, and optimum sizing can be achieved at neutral rather than at higher pH, depending on the full set of system conditions.

• PSA: For internal sizing (wet-end addition) quantities of 0.5 to 1 % (solid product on dry paper stock) are required to obtain a hard sized sheet, depending on the nature of the stock. The product is best metered continuously to the stock sus¬pension before the pressure screens. The polymer must be prediluted with an in-line mixer to a concentration of 1–5 %. A cationic retention aid, e. g. modified polyethylenimine, cationic polyacrylamide or polyvinylamine improves the fixa-tion/retention and therefore gives a better sizing effect.
For surface application, first the compatibility with the other chemicals used in the preparation and also the shear stability have to be checked (e. g. with starch, optical brightener, dyes). It is advisable to meter the undiluted polymer by means of a piston pump to the circuit of the size-press, e. g. before the feed pump. To achieve a good effect, the PSA must penetrate sufficiently into the paper, e. g. an adequate quantity of sizing solution must be taken up. Quantities of 2–4 g l–1 solid polymer in combination with 40–80 g l–1 starch are normally sufficient to achieve a hard sized paper sheet; 1–2 g l–1 solid polymer is suffi¬cient for final sizing of a paper that has been presized internally.
 
3.6.4.4 Requirements and Measurements of Sized Papers
The purpose of sizing is to modify the surface of the fibers to control penetration of aqueous liquids into the paper. The penetration usually correlates with the ab¬sorbency, repellency (hydrophobicity), and spreading of the aqueous liquid con¬cerned. Control of these important properties may be required for three pur¬poses:
1. 1. Control of the penetration rate of the aqueous phase in a converting operation such as size press treatment, coating, glueing (influences also the machine runnability).
2. 2. Control of liquid absorption or wetting in a printing process.
3. 3. Control of the serviceability of many grades of paper and board, e. g. milk/juice carton, packaging papers, wallpaper, printing writing papers, etc.

Besides a certain degree of water and ink resistance the papermakers and their customers often look for improvements in dimensional stability, surface strength (pick and rub), internal bond, linting, dusting, stiffness, smoothness, porosity, or friction coefficient, depending on the converting process and use of the paper.
Several test methods are used to measure the degree of water and ink resis¬tance:
. • Water drop absorption: the time required for defined droplets of water to be ab¬sorbed by paper.
. • Ink flotation test: the time required for writing ink to penetrate through a floating piece of paper and to change the color of the upper surface.
. • Hercules sizing test: the decrease in reflectance of the opposite side of a paper sheet which has been covered with a given amount of ink.
. • Cobb test: the amount of water absorbed by a given area of paper in a specified period of time.
. • Pen and ink feathering test: the extent of spreading of lines drawn on a paper sample with a steel pen and ink.
. • Contact angle: the tangential angle from the horizontal which the base of a drop of liquid develops when carefully placed on a paper sheet.
. • Edge penetration/edge wicking test: the lineal penetration of liquid in the in-plane dimension of the paper.

3.6 Functional Chemicals

Dry-Strength Resins (DSR) [3, 9–11, 22–24]
Some of the specific mechanical properties important for paper are tensile strength, tearing resistance, folding endurance, bending resistance, burst and sur¬face strength, internal bond and compression resistance. It is generally accepted that the major factors contributing to dry strength development in paper manu¬facture include Van der Waals forces, hydrogen bonding and ionic bonding. Be¬cause of the special effect of water on paper strength, it is common to distinguish between “dry” and “wet” strength properties of paper. The two subjects are ob¬viously related, but wet strength will be treated as a separate topic. Paper strength is affected by many furnish and process variables. On the furnish side, longer softwood fibers produce stronger papers than shorter hardwood fibers. Fillers re¬duce strength. Alkaline pH conditions in the wet-end produce stronger papers, especially after aging, than do acid pH conditions. On the process side, both in¬creased refining and wet pressing increase paper strength. The basic factors that influence paper strength are: individual fiber strength, interfiber bond strength, the number of interfiber bonds (bonded area) and the distribution of fibers (sheet formation). While the first factor cannot be influenced by strength additives, the remaining three factors can be strongly influenced by such products.
Many water soluble, hydrogen bonding polymers will act as dry strength ad¬ditives. In fact, wood fibers contain their own natural dry strength additive in the form of hemicelluloses. It is well-known that the removal of hemicelluloses from wood fibers makes it more difficult to develop their bonding characteristics.
. • Starch: In general starch derivatives represent the most common and by far the largest amount of dry strength additives (Fig. 3.1).

. • Vegetable Gums: Much less important but also used are water soluble vegetable gums, such as locust bean gum and guar gum. These highly hydrophilic poly¬mers have chemical structures which are similar to cellulose, enabling them to participate in extensive hydrogen bonding with fiber surfaces. The natural mate¬rials are nonionic and are not retained by fibers to any extent. Consequently, successful commercial products all have cationic groups attached to the main chain, which increases the attraction between gum molecules and fibers and results in improved polymer retention. Due to the combination of high retention and effective dry strength enhancement only 0.1–0.35 % of the material has to be added in most instances.

. • Polyacrylamide Resins: The fully synthetic DSR are of growing importance. In Japan they already form the largest proportion of the total consumption of dry strength resins. Their worldwide consumption is an average of 300 g dry poly¬mer per ton of paper or 3 % of the total amount of specialty chemicals (see Fig. 3.3). In the group of synthetic dry strength resins the polyacrylamide-based products are still dominant today. Since anionic polyacrylamides are negatively charged, they are not directly attracted to paper fibers. A cationic substance, such as alum or polyamide epichlorohydrin resin, must be used to promote their retention. To avoid the need for a cationic promoter, it is possible to incorporate cationic groups (e. g. methacryuloyloxethyl trimethyl ammonium methosulfate, dimethyldiallyl ammonium chloride, vinyl benzyl trimethyl ammonium chlo¬ride, 3-acrylamido-3-methyl butyl trimethyl ammonium chloride) directly into the polyacrylamide backbone by copolymerization. When used as dry strength additives, typically 10 % of the monomers will be charged and their molecular weight will be between 100 000 and 500 000. This range is low enough that the polymers will not bridge between particles and cause flocculation, and high enough to retard migration of the polymer into the fiber pores with concomitant loss of activity.

• Polyvinylformamide/Polyvinylamine Resins (PVF/PVAm): Recently developed new types of polymers in the form of polyvinylformamide and polyvinylamine are coming into use. All these water soluble polymers contain primary amino groups that can form hydrogen bonds with surface cellulose molecules in fibers and improve interfiber bonding. These polymers enable the papermaker to achieve combinations of paper properties that cannot be obtained through refin¬ing alone. For example, strength properties can be improved without affecting sheet bulk and/or appearance properties. The new DSRs are produced by polymerizing vinylformamide and then hydro¬lyzing it. This results in a chain type macromolecule with primary amino groups without using an additional monomer. These polymers can be varied within a very wide range of molar mass and charge density to optimize the performance. Medium or low charged polyvinylamines of medium molecular length give high performing dry strength resins. Further improvements in cost-performance can be achieved by product combinations, e. g. low/middle charged cationic poly¬vinylamine plus low molecular, middle/high charged anionic polyacrylamide or cationic polyvinylamide plus anionic polyvinylformamide. These products have no remaining monomers, and do not contain formalde¬hyde or organically bound chlorine, therefore they do not release chlorine to the effluent. The environmental advantages of this new group of DSRs is underlined by the fact that they have been approved by the German health authorities (BfR-Bundesinstitut für Risikobewertung) for food packaging paper and board. PVF and PVAm products also conform to the requirements of the United States Food and Drug Administration (FDA) regulation 21 CFR 176.170 (Components of paper and board in contact with aqueous and fatty foods), according to its cur¬rent status. It may be added at levels of up to 1.5 % solid polymer, expressed as a proportion of the dry, finished paper. Polyacrylamide- and polyvinylamine-based DSRs are mainly supplied as ready to use aqueous solutions or emulsions (10–40 %) or as water soluble powders that must be dissolved prior to use. No other preparatory steps are necessary. The most beneficial way of application is to meter them continuously to the thick paper stock at a point of thorough mixing, e. g. at stock dilution. Addition rates of 0.1 to 0.5 % of solid material are adequate for most uses. Excessive use can overcationize the stock suspension and reduce the effectiveness of the DSR and of other cationic additives.

Wet Strength Resins (WSR) [2, 9–11, 21–24]
Certain types of paper can only fulfil their purpose if they have adequate wet strength. Such papers include, for example, filter papers, hygienic papers, papers for bags, label papers, wallpapers, laminate base papers, packaging papers for moist goods and all papers which, in the course of further processing and use, risk breaking when rewetted. The required wet strength (up to 50 % of the dry paper strength can be retained) is obtained with the aid of wet-strength resins. For ex¬tremely high wet-strength properties the most common WSR are urea formal¬dehyde resins (UF-resins) and melamine formaldehyde resins (MF-resins), These chemicals need acid pH conditions and the presence of alum in the papermaking process. For neutral pH conditions polyamide-epichlorohydrin resins (PAE-resins) are mainly used (e. g. for hygiene and laminate papers); polyethylenimine products are used for specialty papers such as industrial filter papers and shoe board.
The total consumption of wet strength resins, together with insolubilizers for coating (see 3.6.9.3.5.2), accounts for about 0.07 % of the worldwide paper produc¬tion or 7 % of all specialty chemicals, calculated on the active ingredient (Fig. 3.3). Over the past years, consumption of PAE-resins has increased overproportionately. This is partly due to the trend from acid manufacturing conditions to the neutral pH range, where polyamide-epichlorohydrin resins are more effective than urea formaldehyde and melamine formaldehyde resins. However, the increasing im¬portance of PAE-resins is no doubt also largely a result of the formaldehyde con¬troversy of the early eighties. PAE-resins account today for about 45 %, urea for¬maldehyde resins 15 %, melamine formaldehyde resins 10 %, glyoxal resins 15 % and the remaining 15 % are others, e. g. ammonium zirconium carbonate (as in¬solubilizer) and newly developed products e. g. polyvinylamines.

There are two theories regarding the mechanism of wet strength. The first states that the wet strength effect is due, at least in part, to a reaction between the resin and the cellulose, which leads to the formation of ether bonds. The second theory assumes that the wet-strength resins crosslink on exposure to heat in the dryer section to form a three-dimensional network, wrap themselves around the points where the fibers intersect and thus protect the points of intersection from water penetration and swelling. Given the short contact times with the steam-heated cylinder surfaces (less than a second in the case of the yankee cylinder used in hygienic paper production), wet-strength agents require a high level of reactivity to allow crosslinking to take place and bonds to form. At the same time, the wet-strength resins have to have a selective effect if they are not to react with the surplus hydroxy groups in the paper stock suspension. Therefore a healthy balance between reactivity and selectivity has to be found, so that the chemical reactions (crosslinking, formation of covalent bonds) are not completed at the end of the paper manufacturing process but continue during storage until maximum wet strength is reached one to three weeks later. This gradual curing should not be looked upon as a disadvantage as it is essential for good recycling of the paper machine broke.

3.6.6.1 Melamine-Formaldehyde Resins
In most instances, melamine is made from a basic product such as cyanamide. The melamine molecule is then condensed with formaldehyde to form a series of methylol melamines, e. g. monomethylol melamine and hexamethylol melamine. On introduction to a papermaking system, the melamine formaldehyde product can crosslink with itself forming an ether link or a methylene link, as well as crosslinking with a cellulose carboxyl group to form the covalent bond, both of which contribute to wet strength. The advantages of the melamine-formaldehyde resins are that they lead, with similar addition rates to UF-resins, to wet tensile strength levels up to 50 % and to even higher wet bursting strength. MF-resins also provide a very high alkaline resistance, therefore such products are mainly used for label papers and banknote base papers.

3.6.6.2 Urea-Formaldehyde Resins
The formation of aqueous solutions of urea-formaldehyde condensation products involves the stepwise reaction of urea with formaldehyde, and the first step is undertaken at pH 7–8 to form dimethylol urea. Further reaction to a controlled degree with formaldehyde forms a condensation product in aqueous solution, which can be stored and transported. Urea-formaldehyde and melamine-formal-dehyde resins are delivered in aqueous solutions with solid contents of 12 % (MF) and 40 % (UF) as well as in powder form (MF). They are mainly applied at the wet end, but they can be also used via surface application in the paper machine. Suita¬ble feeding positions for the continuous wet-end addition are between the stock consistency controller and the mixing pump, shortly before final stock dilution, where an optimum of mixing is guaranteed. The UF-resins are the least expensive ones. An important area of application is in the manufacture of sack paper for shipping and cement packaging. With addition rates of 0.5 to 3 %, calculated on dry paper stock, a wet-strength level of up to 40 % of the dry-strength figure can be achieved.

3.6.6.3 Epoxidised Polyamide Resins
The chemistry of the production of polyamide resin is very similar to the original process by which nylon was produced. In the Nylon 66 process a dicarboxylic acid, such as adipic acid is reacted with a six carbon amine, for example hexamethylene¬diamine, to produce a synthetic fiber. In the case of polyamide resin, a dicarboxylic acid is reacted or condensed with an amine such as diethylenetriamine to form an amino polyamide. The secondary amine groups of this water soluble polymer are then reacted with epichlorohydrin to form the aminopolyamide epichlorohydrin intermediate. This is then crosslinked to build molecular weight whilst maintain¬ing solubility. The polymerization reaction is terminated by dilution and acidification.

The amount of polyamide-epichlorohydrin resin required in hygienic paper pro¬duction is between 0.1 and 4 % dry substance, calculated on the paper. These resins are supplied in the form of aqueous solutions with a solids content of 12 to 25 %. They are effective in the pH range 5 to 8, although the best wet-strengthen-ing effect is obtained in the neutral or slightly alkaline range; they are therefore often called neutral wet-strength resins. In the majority of cases, PAE-resins are added to the stock, preferably just before the last stock pump in front of the head-box. This ensures that the fiber/resin bond is not impaired by high shear forces. Depending on the amount added, the relative wet strength can be increased to over 35 % without significantly reducing the absorbency of the paper. Wet-strength res¬ins in general also increase the dry strength of paper (i. e. tear and burst). They also have favorable effects on the dry and wet abrasion resistance of paper. Additionally the retention of fillers and fines is increased. A special effect of PAE-resins, even with small quantities, is to form a coating on the crepe cylinder of a hygiene paper machine to control the adhesion of the paper web on the cylinder.

The disadvantages are that the degree of whiteness is less stable than with UF and MF resins, and the AOX problematic. Much effort was put into developing “low-AOX” and recently also “AOX-free” polyamide-epichlorohydrin resins with no detectable amount of byproducts. At the same time, chlorine-free wet-strength agents have also been developed, e. g. modified glyoxals and polvinylamines, but often with higher costs. In the longer term, workplace safety aspects could lead to the application of these new products becoming successfully established in the market.

3.6.6.4 Glyoxalated Polyacrylamide Resins
These products are prepared by crosslinking a low molecular weight polyacryla¬mide (PAM) with glyoxal. The PAM is normally a copolymer of acrylamide and a quaternary ammonium cationic monomer which is prepared in aqueous solution. This results in a cationic polymer which is attached to pulp fibers. The cationic backbone is then crosslinked with sufficient glyoxal to react with most, but not all, of the PAM backbone amide groups. On storage, the resin continues to crosslink and can ultimately gel. In order to achieve the desired stability, paper mills dilute the resin on receipt. At 25 °C, a 10 % solution will gel in about 8 days, whereas a 6 % solution will take about 65 days to gel at room temperature.
There is strong evidence that glyoxalated PAM imparts wet strength primarily through covalent bond formation between the resin and the fibers. It can be taken for granted that there is some intermolecular crosslinking within the resin but, in order to function, there needs to be at least some fiber-resin-fiber bonds within the fiber-fiber bonded area. The reaction of glyoxalated PAM with cellulose is rapid at neutral pH 6–8 and even more rapid at acidic pH 4–6, resulting in 80–100 % of the potential wet strength. Ageing or curing of the paper gives little or no additional wet strength. The reaction is reversible in the presence of water and a resin-treated paper gradually loses wet strength on prolonged soaking. This temporary wet strength can be sufficient for some paper grades, e. g. paper towels, and also dur¬ing the paper manufacturing process when the the sheet is passing through a size press or coater. Glyoxalated PAM resins also contribute significantly to the dry strength of treated paper.

3.6.6.5 Other Wet-Strength Resins
. • Polyethylenimine (PEI) was the first effective WSR used under neutral/alkaline pH conditions in papermaking without influencing the absorbency of the paper. The PEI manufacturing process is described in Section 3.7.1.1. The effective mechanism of PEI formation is somewhat different from the resins. PEI devel¬ops wet strength without curing the paper and the level of wet strength that can be attained is less than with the thermosetting resins. It has been proposed, that PEI functions by creating stronger interatomic bonding, rather than by forming homo- or co-crosslinked networks. The amine cationic groups responsible for wet strength have dissociation constants of around 12 and the retention and performance of PEI is best at pH 7–9. The reasons why PEI has not been used more extensively as a wet-strength resin are higher costs than with thermoset¬ting resins and because it causes yellowing and loss of brightness in white print¬ing and writing papers.
. • Polyvinylamines are a relatively new group of wet-strength resins. These products are environmentally friendly, their use does not result in any negative ecological impact (see also Section 3.6.5 on Dry Strength Resins). Their cost-performance ratio is at present less favorable than the conventional WSR in most cases.
. • Polyisocyanate is another new type of WSR, up to now with very little practical use.
. • Dialdehyde Starch (DAS) also has the potential for crosslinking cellulosic hydroxy groups in paper to give temporary wet strength. DAS is essentially a highly modified starch in which the vicinal hydroxy groups (at the C-2 and C-3 carbons) are selectively attacked by periodic acid, severing the C-2 to C-3 bond to form dialdehyde starch. The aldehyde groups are not present to any extent as free aldehydes, but rather as hemiacetals or as hemialdals. Since the linkages in these compounds are weak, dialdehyde starch reacts as if the aldehydes were free, permitting its use as a reactive polyaldehyde capable of reaction through hydroxy amino or imino groups.

3.6.7
Additives for Recovered Fiber Processing
For economic and environmental reasons the use of recovered paper as raw mate¬rial has already reached a high level and will grow further. For its preparation specific chemicals are required, depending on the quality of the recovered papers and the necessary properties of the produced paper grades.

3.6.7.1 Additives for Repulping
These chemicals are intended to facilitate the repulping of recovered papers, espe¬cially for mixed and brown grades e. g. packaging material, corrugated boxes. To get an easier and faster defibration the addition of nonfoaming wetting agents
(e. g. nonionic surfactants) and dispersants is used. Wetting agents reduce the surface tension. These products are predominantly surface active, such as sulfonated oils, alkyl sulfates, alkyl sulfonates, alkyl aryl sulfonates, or ethoxylated prod¬ucts based on nonylphenol. Dispersants used for repulping are mainly condensa¬tion products of formaldehyde and a naphthalene sulfonic acid as the sodium salt, or the sodium salt of a polycarboxylic acid. These products have high dispersing capacity for pitch, waxes, bitumen, etc. which otherwise would adversely affect the whole repulping and papermaking process as well as the paper quality.

The appli¬cation of such products preferably takes place in the pulper in undiluted form with amounts of 0.1 to 0.5 %, calculated on oven dry paper stock.
Repulping of wet-strength paper always requires more energy than repulping normal unsized or sized paper. Depending on the type of wet-strength resin used, different methods and chemicals are employed. When UF (urea formaldehyde) resins or MF (melamine formaldehyde) resins have been used, repulping was effected in an acid medium with sulfuric acid and/or alum at elevated temperature above 60 °C in the pulper for 15–30 min. If the wet strength of the recovered paper is based on polyamine-type chemicals, defibering of this paper also requires high-energy pulping at an alkaline pH value >10 by addition of sodium hydroxide. Other useful additives are hypohalous acid and persulfate salts.

3.6.7.2 Additives for Deinking
Newspapers, weekly and monthly magazines, brochures and office papers are used as raw material to produce graphic papers, tissue or the top ply of white board. In the “deinking process” first the printing ink has to be detached from the paper surface then the released ink has to be removed from the pulp slurry, either by flotation or by washing or by a combination of the two. Whereas in Europe flota¬tion is most commonly used and washing is only used for special deinked pulp (DIP) qualities, in North America washing is more common.
The removal of ink from recovered paper is determined by the type of ink binder and the chemicals used during pulping. For wood containing recovered paper grades the most important chemical is sodium hydroxide (NaOH) with addition rates of 0.5–2 % (calculated on oven dry paper stock) to adjust the pH to 10–11. NaOH eases the detachment of the ink particles from the fibers as saponifiable binders in the ink are saponified by NaOH and the fibers swell substantially in this environment. However, sodium hydroxide solution also causes yellowing of the fibers, particularly of mechanical pulp. In order to prevent this, hydrogen peroxide (H2O2) is used as a bleaching chemical, which also has a saponifying effect. In addition, 1–5 % water glass (sodium silicate) is added to stabilize the hydrogen per¬oxide and to prevent the ink particles from redepositing on the fiber.
Additionally 0.1–0.2 % of a nonfoaming wetting agent (e. g. nonionic alkyphenol polyethylene glycol ether) can support the removal of the ink particles from the printed recovered paper. Since hydrogen peroxide is more effective at higher con¬centrations, the pulping process is carried out at stock consistencies of 12–20 % (high consistency pulper or drum pulper). Soaps and fatty acids are used in addition rates of 0.5–1.2 % as dirt collectors and flotation agents. They form calcium soaps with hard water or with the calcium carbonate from the coating or filler of the recovered paper. Calcium chloride must be added if the water is not sufficiently hard. For higher quality papers, further bleaching with sodium dithionite and/or hydrogen peroxide may follow. Coated papers can be deinked more easily because the inks are fixed only on top of the coating layer.
For woodfree recovered paper grades the dosage of deinking chemicals (NaOH, soap, fatty acid, nonfoaming wetting agent) can usually be reduced substantially. Because deinked pulp (DIP) is very often used in paper grades of high brightness, bleaching with hydrogen peroxide, sodium dithionite and/or formamidine sulfinic acid (FAS) is more important and bleaching is often performed in two or more stages.
The effluent/white water quality has to be controlled very exactly. COD increases with increasing pH. For flotation deinking of recovered paper the total chemical costs have a relatively high proportion of 15–20 % of the overall DIP production costs. Yield rates up to 93 % for newsprint production are attainable.
In wash deinking after saponification the ink particles together with pigments and fillers of the recovered paper are removed by washing the pulp slurry through wire supported by the addition of 0.1–0.5 % of a dispersing agent. Here large effluent streams are produced and solid losses are high, with yield rates of only 60 to 70 % being not unusual. So flotation deinking, or a combined flotation-washing process, is also gaining a foothold in North America.

3.6.8
Additives for Specialty Papers [13, 14]
Specific functional chemicals are indispensable in the production of specialty pa¬per grades. Such grades are expected to display a very heterogeneous range of properties. Specialty paper grades account for only ca. 4 % of worldwide paper production, but the proportion of chemicals applied for their production is mostly significantly higher than for conventional papers. A number of specialty papers with their required properties and applied additives are described below .

3.6.8.1 Photographic Base Paper
Paper for photographs must carry a uniform emulsion coating, it must resist the development solution in the development bath, it must be perfectly clean for a clean image, and it cannot contain any inhibitors to the photochemical process like iron, copper, or sulfur. It must even be free of radioactive traces, which cause photographic reactions and spots in the image. The paper, which must have a stable white color, is made of clean bleached pulp. The necessary high dry and wet opacity will be obtained with titanium dioxide (TiO2), chalk and low molecular weight polymers (e. g. polyacrylamides). In order to obtain resistance against the reagents and rinsing water, including edge and dimensional stability, the base paper needs a strong stock-sizing with behenyl diketene and high wet-strength with polyamine-polyamide-epichlorohydrin resin. Additional the paper web is dip sized with gelatin, polyvinyl alcohol (PVA), polyacryl amide (PAM), and modified starch before calendering. Most photographic papers for color prints are extrusion coated with an opaque plastic film (e. g. polyethylene) to improve the impermeability.

3.6.8.2 Banknote Papers
These papers have to avoid forgery, must be durable and resistant to wetting, folding and aging. Therefore they are produced under alkaline conditions with high wet and dry strength. To achieve these properties, polyamide-epichlorohydrin resins, polyacrylamides and/or aminoplast resins are used, together with strong stock sizing with AKD (alkyl keten dimer) and surface sizing with proteins plus crosslinking agent (glutaraldehyde). For security reasons these papers will be marked by mingle colored fibers with the paper stock and/or by using uncolored reactive dyes, which create a certain color when they react with an acid or an alkali.

3.6.8.3 Laminate Papers: Décor Paper, Pre-impregnated Foils
Décor paper is made for white or colored décor, often imitating wood finishes. It needs a high wet and dry opacity and very high lightfastness, which is obtained by using titanium dioxide (TiO2) plus low molar mass polymers (polyamine-poly-amide-epichlorohydrin resins, polyacrylamides). Also high wet strength without loss of absorbency is demanded and achieved by relatively high addition rates of polyamine-polyamide-epichlorohydrin resins. Colored décor papers with very high fastnesses are produced with inorganic and organic colored pigments.
Pre-impregnated foils are used where the surface requirements for resistance and closedness do not require more expensive high-density laminates. The im¬pregnation will be made on the paper machine, using a modified size press. Typ¬ical resins for this application are UF (urea formaldehyde) or MF (melamine for¬maldehyde) resins with a very low content of free formaldehyde. To achieve good flexibility and printability of these foils, additional polymer dispersions (e. g. sty¬rene acrylates) are used.

3.6.8.4 Filter Papers
There is a wide range of filter papers for various purposes. The use of porous paper for filtering and separation ranges from the use of filter cartridges for engine protection to dust pouches for vacuum cleaners and air conditioning, and from tea bags and coffee filters to reagent carriers and filter disks for laboratory use. Their common denominator is the requirement for a controlled, high porosity. Depend¬ing on the intended use, additional required characteristics are the resistance to different media and/or temperatures, stiffness, and cleanliness. Specific character¬istics of filter papers are their resistance to flow, their filtration efficiency, and their dust-holding capacity.
The controlled porosity together with very high wet and dry strength is obtained by using polyamine-polyamide-epichlorohydrin resins together with a low molar mass polyacrylamide or polyethyleneimine, polyisocyanate, or polyvinylamine.

3.6.8.5 Imitation Parchment (Food Packaging)

Some of these papers have to be water-resistant and pore free, which is why micro¬crystalline wax, cationic styrene-acrylic emulsions, alkenyl succinic aldehyde (ASA) plus cationic starch, and styrene-maleic-anhydride (SMA) are used. To achieve resistance to oil and fat mainly perfluorinated alkyl acids, carboximethyl¬cellulose (CMC), alginates and stearyl-melamines are applied.

3.6.8.6 Aquarelle Board
These boards must be resistant to fading, neutral, and stable. The finish varies from a coarse surface for painting to a very smooth surface for graphic work. High opacity and uniform surface structure will be achieved by using chalk (calcium carbonate) together with a low-molar-mass polymer e. g. modified polyethylenei¬mine, polyamine, polyvinylamine. Cationic styrene-acrylic emulsions plus carboxy methyl cellulose (CMC), starch and/or low molar mass polyacrylamides or poly¬vinyl formamides lead to controlled uptake of water and oil as well as high stiffness and rattle. For rub out resistance, surface sizing with starch and styrene-acrylic emulsion is applied.

3.6.8.7 Carbonless Copying Paper
The dominating principle for carbonless copy is that an emulsion of a specific oil
e. g. diisopropylnaphthalene (DIPN) together with color formers (reactive dyes) is encapsulated in microcapsules (e. g. gelatine, aminoplast resins) applied as a coat¬ing on the backside of the copying paper (CB-coated back). Through the pressure of writing, the microcapsules are broken, and the color former solution flows to wet the front side coating of a receiving sheet (CF-coated front). The front side coating reacts with the color former, forming an image. The pressure sensitive coloring side (CB) has to be resistant to abrasion. Microcapsules from gelatine or polyurea or melamine-formaldehyde condensation products contain solvent e. g. diisopropylnaphthaline (DIPN), or isopropyl-butylbiphenyl, or phenylmethane¬ethane plus reactive dyes (color former). Polymer dispersions of styrene-butadiene are used as binder for the surface application of the microcapsules. The coating formulation of the reactive receiving side (CF) consists of activated bentonite or phenolic resin or zinc salicylate plus styrene-butadiene binder.

3.6.8.8 Ink-jet Papers
Ink-jet is a noncontact printing method, since no part of the printing device other than ink contacts the paper at the moment of ink transfer. A sharp, detailed printed image (no wicking and bleeding), high color density and no strike-through will be obtained by a hydrophilic paper surface using a coating color with silica gel, polyvinylalcohol (PVA) and/or carboxy methyl cellulose (CMC) plus low-molar-mass cationic polymer e. g. modified polyethyleneimine, polyvinylamine, conden¬sation product of organic amides with formaldehyde, polyacrylamide. The base paper should be hard-sized with alkyl ketene dimer (AKD), or alkenyl succinic aldehyde (ASA), or rosin size.

3.6.8.9 Fire-resistant Papers
Papers with fire-resistant properties are used for wallpaper, decoration paper, Chi-nese/Japanese lamps and partition walls. Flame retardants are added either at the wet end or by surface treatment in the paper production process. They either release incombustible gases on heating, which prevent the entry of atmospheric oxygen, or when heated produce a nonflammable melt that surrounds the paper. Chemicals for this purpose include calcium chloride, magnesium chloride, dia¬mmonium ethyl phosphate, and mixtures of zinc borates, antimony oxides, and organic haloid salts as well as inorganic bromides and oxybromides.

3.6.8.10 Anticorrosion Papers
These papers prevent the rusting of iron parts and the tarnishing of silver, alumi¬num, and copper. Generally they have to be produced in alkaline wet-end condi¬tions in the absence of any acid and alum. Additionally the paper has to be impreg¬nated or coated with chemicals that inhibit corrosion, e. g., sodium nitrite or so¬dium benzoate. The paper is coated by deposition of the chemicals from the vapor phase.

3.6.8.11 Abrasive Base Papers
Abrasive papers which are coated with an abrasive grit in a binder are used for belts in heavy grinding machines, as sheets for grinding by hand, in vibrating hand-held grinding machines, or disks in rotary machines. There are specific grades for wet grinding and for dry finishing by hand. The base paper must be strong enough to resist the forces in use, give a good anchoring of the grit, and suit the coating operation. High amounts of anionic and cationic starches, often to¬gether with carboxy methyl cellulose are used to obtain high strength properties. Additional polymer dispersions based on styrene acrylates or styrene butadiene are applied at the wet end of the paper machine and/or by on-machine or off-machine impregnation. With wet-end addition of these polymers effective fixation and re¬tention in the paper stock with alum and/or cationic polymers e. g. polyethyleni¬mines, polyamines, polyvinylamines are necessary.

3.6.8.12 Papers with Barrier Properties
Paper or board have almost no barrier properties against penetrants like moisture, water vapor, oxygen and other gases, aroma, grease and fat. To provide protection against outside influences as well as protection against loss of features from in¬side, a specific barrier coating is required. To make paper and board suitable as a barrier packaging material, the barrier layer has to be applied either by wax im¬pregnation, lamination with films e. g. PE (polyethylene) or aluminum foil or the extrusion of molten polymers, The most favorable and economic method is the on-or off-machine coating of paper and board with an aqueous system, e. g. an aque¬ous polymer dispersion. According to the laws of Henry and Fick, polymers are needed whose chemical nature is quite the opposite of the penetrant. So the most hydrophobic polymers suit as a barrier against hydrophilic penetrants like mois¬ture vapor whereas the most hydrophilic polymers protect against hydrophobic penetrants like oxygen or some solvents.

Suitable polymer dispersions are based on vinylidene chloride, acrylic esters, styrene-butadiene, polyurethane, polyethyl-ene-acrylic acid, or acrylic acid-acrylonitrile. A minimum of water vapor and oxy¬gen transmission can be reached with polyvinylidene chloride. Acrylics offer an excellent barrier against aromas like terpenes and hydrocarbons as well as fat and a moderate oxygen barrier combined with good water resistance. As a barrier poly¬mer, styrene-butadiene (S/B) offers moderate moisture and vapor protection as well as good water repellence. The aroma barrier to fruity esters is quite good. The S/B coatings are readily sealable with high sealing strength, correlating with the sealing temperature.
New in the market are hydrophobically modified styrene-butadiene dispersions which show an outstanding moisture vapor barrier, very similar to the moisture vapor barrier of a polyethylene film. The water resistance and the aroma barrier to esters are also very good

• The sealability of the modified S/B coat is better than the unmodified one. Polyurethane (PUR) coatings have very high permeation resis¬tance to moisture and vapor. For aroma, they offer a moderate or even good barrier against fruity esters and terpenes and an excellent barrier against hydrocarbons. The fat and oil resistance is good. PUR has extraordinary sealing properties and thermoactivability, which means, after thermal activation, the coating is cold seal¬able for about half a minute and afterwards no agglutination will occur. Polyethyl-ene-acrylic acid is also a coating with excellent sealing properties and, in contrast to PUR, offers a very good moisture and vapor barrier and an excellent aroma barrier against fruity esters. The surface is highly water repellent.

The aqueous polymer solution based on polycarboxylic acid derivative is an excellent oxygen barrier; it exceeds polvinyl dichloride (PVdC), but the coating is sensitive to water and moisture. The aroma barrier is also very good as long as the coating is dry.
Very high grease and oil resistance together with water and alcohol repellency are obtained by fluorinated acrylic copolymers which are completely miscible in water. These products can be applied either by surface or by internal application.

3.6.9.1 General Aspects
The paper coating process was first developed in the USA late in the 19th century, but did not find broader application until the middle of the 20th century. Since that time the European paper industry has become a leader in coating technology. Coated papers suit the highest requirements as regards printability. In paper and board coating an aqueous suspension, called coating color, is applied to one side of the sheet (mainly in the case of board) or to both sides (mainly for printing papers). After application of the required amount, the coating is dried and finished. In finishing, the coated paper and board achieve their smoothness and gloss poten¬tial. Coating is done either in the paper machine (on-machine coating) or in a separate step after the base paper production (off-machine coating). It is desirable that, besides filling the cavities, the coating also covers the highest lying fibers on the base paper surface (Fig. 3.11). There are methods that tend to favor filling of the cavities while higher spots remain covered by only thin or practically no coat¬ing (equalizing or leveling coat, e. g. with blade equipment). Some other methods give a coating of more or less uniform thickness, thus also covering the highest spots on the base paper surface, yet the cavities remain only partially filled (con¬tour coat, e. g. roll applicators).

Coating colors consist of several components, white pigments (e. g. clay, calcium carbonate, talc, titanium dioxide) and so-called binders (e. g. starch, latexes) being the most important as regards volume and cost. Further specific additives influ¬ence and control the applicable solid content, the rheology, water retention and immobilization of the coating color during the coating process (e. g. dispersant, co-binders, thickeners), and others influence the physical and optical surface struc¬ture and properties of the coating layer (e. g. associative thickeners, lubricants, hardening agents, fluorescent whitening agents, defoamers, degassing agents). These components are described in more detail in Section 3.6.9.3.

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.

 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 106 t and will grow by 2014 to 21 % of the then 430 V 106 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.
 
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 106 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.

3.6 Functional Chemicals
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, Na4P2O7, is the simplest polyphosphate that can be used as a dispersant. Sodium tripolyphosphate, Na5P3O10, is widely used as a dispersant and as a builder in detergents. Other 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 106 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 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¬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,

3.6 Functional Chemicals
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 106 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 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
3.6 Functional Chemicals

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 3.3 Co-binders and thickeners in paper and board
coating (source: E. Lehtinen, Helsinki University of
Technology.
CMC = carboxy methyl cellulose, HEC = hydroxyethyl cellulose,
PVOH = polyvinyl alcohol, OBA = optical brightening agent.

Table 3.4 Main requirements placed on co-binders (source: BASF). OBA = optical brightening agent.
 
3.6 Functional Chemicals
thetic binders. So natural products, principally starch and soy protein, are nowa¬days only used as co-binders. Casein is only used in cast coating because it has some special features which make it difficult to replace with synthetic products. Alginates and hydroxyethyl cellulose (HEC) are mainly used in the United States and Japan, and have not been adopted to any significant extent in Europe.

Carboxymethyl cellulose (CMC) is a versatile product with an all-round range of properties, and is popular in many regions. CMC improves the water retention efficiently. By choosing the optimum grade, the water retention can be adjusted to the individual needs, which are dependent on the coating conditions and the coat¬ing color formulations. In formulations with kaolin clay, the differences in water retention between different CMC grades are much smaller than in colors based on calcium carbonate. One reason for this is that CMC quickly builds up a network structure with clay particles, which imparts extra water retention. With calcium carbonate pigment, the water retention is more dependent on the viscosity of the water phase. A higher molecular weight and higher viscosity type of CMC is needed to give good water retention for coarse calcium carbonate-based precoat¬ings. Low molecular weight and low viscosity type CMC grades give good perform¬ance for fine clay-based coatings. Lower molecular weight and lower viscosity type CMC grades are closer to Newtonian5 type behavior, while higher molecular weight and higher viscosity type CMC grades are more pseudoplastic. CMC is mechanically stable in high shear conditions and compatible with all common types of coating raw materials.

Synthetic products comprise polyvinyl alcohol (PVOH), polyvinyl pyrrolidone (PVP) in combination with PVOH, acrylic copolymers, and associative thickeners. Due to the high degree of carboxylation of acrylate ester dispersions, they turn into colloidal dispersions upon addition of alkali. Thus the laborious and energy con¬suming dissolving and cooking processes involved in the use of natural binders are avoided. The binding power of PVOH exceeds that of all other binders used in paper coating, nevertheless it has gained only limited acceptance. This is mainly due to the fact that the application of large amounts of polyvinyl alcohol lead to rheological problems on the coating machine during processing. PVOH is a solid compound, which is composed of a hydrocarbon chain bearing hydroxy groups on every second carbon. Depending on the extent of the polyvinyl acetate hydrolysis, more or less acetyl groups remain attached to the chain. The stereochemical struc¬ture of polyvinyl alcohol, the direction in which the OH groups/acetyl groups point, is already fixed during vinyl acetate polymerization. Like most free radical induced reactions, the PVOH polymer shows an atactic structure.

This means that the functional groups are randomly oriented. Their molecular weight (MW) and degree of hydrolysis primarily characterize polyvinyl alcohols. In practice, further features such as tacticity, branching, average length, and distribution of residual acetyl group sequences play a minor role only. Since PVOH is fully soluble in water, its viscosity under defined conditions is taken as a proportional measure of its molecular weight. Coating grades range from 3 mPa s (very low MW) to 6 mPa s (low MW). PVOHs with viscosities higher than approximately 6 mPa s should not be used on coating machines. The degree of hydrolysis is based on the measurement of the ester value and indicates how much mole percent of the basic polyvinyl acetate is “saponified” to PVOH. For coating purposes, a degree of hy¬drolysis is selected from a range of 88 % (partially hydrolyzed) through 99 % (fully hydrolyzed). At the same degree of hydrolysis, higher concentrations or lower tem¬peratures lead to an increase in viscosity. Given the same MW, fully hydrolyzed grades display a higher viscosity than do partially hydrolyzed grades due to in¬creased hydrogen bonding. Going from about 97 to 100 mol % hydrolysis, the crystallinity of the polymer increases considerably, which has an impact on the solid state in particular. One apparent change is the reduction in cold water sol¬ubility of the PVOH.

The choice of other monomers for synthetic co-binders is not restricted to acrylic acid, methacrylic acid, and esters such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, and ethyl methacrylate. Other functional monomers such as acrylonitrile, acrylamide, and vinyl acetate can also be used, so there is a great scope for varying the chemical composition of synthetic co-binders. The pro¬portion of carboxylic acids to the other monomers is usually lower, with the result that synthetic co-binders are less anionic and they adsorb more readily on the surfaces of clay pigments. Important differences of synthetic co-binders compared to the thickeners (see below) are their shorter chain length and their low propor¬tion of acids. This explains the low thickening effect of synthetic co-binders and hence they can be added to coating colors in larger quantities. They are usually added at rates of 0.5–3 parts per 100 parts of pigment, expressed as solids. These relatively high levels of addition and the presence of functional monomers can have a substantial effect on the properties of the coating.

3.6.9.3.4.2 Thickeners
The main function of thickeners is to adjust the viscosity of the coating color to the desired level and to impart the necessary degree of water retention (Table 3.5). Thickeners must be able to interact strongly with water molecules if they are to increase the water retention of coating colors. They also need to interact with other ingredients of coating formulations, especially pigments, in order to display a thickening effect. The nature and strength of these interactions depend on the chemical composition of the polymer. Thickeners also need to display pronounced pseudoplastic flow. High runnability depends on a combination of these features. Most synthetic thickeners are supplied in the form of aqueous, acidic dispersions (which are often erroneously referred to as “emulsions” in the literature) or alka¬line solutions of synthetic polymers. Some are supplied in the form of true water-in-oil emulsions, but they are much less common.
The main products used as thickeners are CMC, PVOH (both already described in Section 3.6.9.3.4.1), acrylic copolymers and associative thickeners. The acrylic copolymers are nonionic monomers and acrylic acid or methacrylic acid. The non¬ionic monomers are mainly esters of acrylic acid (principally methyl acrylate and ethyl acrylate) and methacrylic acid (principally methyl methacrylate), and acryla¬mide. The monomers that are selected need to be fairly hydrophilic and polar in Table 3.5 Main requirements of thickeners order that the polymer is able to dissolve in aqueous coating colors and interact with other polar coating ingredients such as pigments. Strongly hydrophobic mon¬omers such as styrene, butadiene, and ethylene are hardly ever used because they interfere with the interaction between the polymer and water molecules.

Poly-acrylic acid can also be used as a thickener in its dissociated form, i. e., as the sodium or ammonium salt, if its molar mass is high enough. Polymers have to be able to dissolve before they can exert a thickening effect. Most natural products have to be heated and converted to make them capable of being dissolved, whereas acrylic copolymers are soluble on account of the alkali ions contained in the coat¬ing color. When addition of alkali increases the pH of the dispersion, the carboxyl groups dissociate and donate a proton, which causes them to become anionic. The formation of anionic charges along the polymer chain causes it to stretch owing to mutual repulsion, and water molecules are attracted to the polymer chain and become attached to it. The dispersed thickener particles then dissolve, which al¬lows them to unfold their effects.

It only takes a few minutes to dissolve them completely. Nevertheless, it is important to ensure that sufficient alkali is available in the coating color because the thickener dispersion consumes alkali and the stability and viscosity of the coating color can suffer if the pH is allowed to drop. Another important point to be considered is the sensitivity of the dissolved poly¬mer to electrolytes. Polyvalent cations such as Ca2+, Mg2+, Al3+, and Fe3+ can have a detrimental effect on the performance of these products by occupying the sites of anionic charge on the polymer.
The interaction between the polymer and water has a pronounced effect on the viscosity of the aqueous phase as well as on the water retention of the coating color. Like all hydrocolloids, synthetic thickeners bind a large number of water molecules along their polymer chains, with the result that their diameter and volume in¬crease and they occupy a greater space in the aqueous phase (Fig. 3.14). Apart from this hydrodynamic mechanism, there are a number of other mechanisms by which the thickeners are able to restrict the mobility of the aqueous phase. The high molar mass and the high degree of structural order cause a large increase in the volume of the dissolved polymers under hydrodynamic forces. The anionic charges along the length of the polymer also have the effect of stiffening the poly¬mer chain by causing it to unfurl and stretch out because the charges repel each other. The mobility of the aqueous phase is lower because of the stiffness and extension of individual polymer chains and the crosslinking between different polymer chains.

The maximum degree of internal crosslinking is obtained as the result of the associative interaction between the hydrophobic side chains (see Sec¬tion 3.6.9.3.4.3). The common feature of all these effects is that the mobility of the aqueous phase is reduced owing to intramolecular and intermolecular crosslink¬ing and the viscosity is increased due to the increase in the volume of the polymer. The pronounced thickening effect of thickeners is the result of their interaction with pigment particles. Acrylic and cellulosic thickeners have a high affinity for pigments because the polar functional groups of the thickener molecule are at¬tracted by the polar surfaces of the pigment. The polymer chains are adsorbed on the surface of the pigment particles and bind them together by means of a bridg¬ing mechanism, leading to a higher degree of crosslinking within the entire sys¬tem. The thickening effect, i. e., the increase in viscosity at low shear, is principally the result of the thickener forming “bridges” by means of adsorption.
 
3.6.9.3.4.3 Associative Thickeners
These have a long history of use in paints. They consist of an essentially hydro¬philic, water-soluble polymer with strongly hydrophobic terminal groups or side chains. The hydrophobic terminal groups are aliphatic or aromatic hydrocarbons, and they are completely insoluble in water. They are joined to the main polymer backbone by means of a hydrophilic spacer, which ensures that they remain flexi¬ble. Their structure is similar to that of surfactants, and they also tend to join together in water to form micelles. This associative interaction between the hydro¬phobic side chains increases the degree of internal coordination of the whole sys¬tem, with the result that the coating color has a very high viscosity at low shear (Fig. 3.15). The association between the hydrophobic groups is the result of van der Waals’ forces, which are very weak and easily overcome. If the coating color is subjected to shear, they quickly lose their attraction for each other and no longer form a network. The micelles break down and the thickener molecules are ori¬ented in the direction of flow, which causes a reduction in the viscosity of the coating color. However, because the hydrophobic constituents are no longer asso¬ciated with each other, a consequence of the low viscosity of the coating color under high shear is that its water retention is much lower.

The associative thickeners can be divided into three different categories accord¬ing to the chemical composition of the hydrophilic polymer backbone (Table 3.6).
1. HEUR (Hydrophobic Ethoxylated Urethanes): These thickeners consist of rel-atively short-chain, ethoxylated polyurethanes that have been terminated with hy¬drophobic substituents. The associative interaction between the hydrophobic chain ends causes them to join together to form long chains, and the viscosity increases accordingly. Another feature of these products is that they also interact with the dispersed polymer binder particles. There are variants in which the main polymer backbone consists of alternating hydrophilic and hydrophobic blocks. HEUR-type associative thickeners have long been used in the paints industry but they have been less successful in coating colors because of their lack of affinity for pig¬ments.
Table 3.6 Types and properties of associative thickeners used for coating (source: BASF).
 
1. 2. HASE (Hydrophobic Alkali-Swellable Emulsions): The hydrophilic polymerbackbone consists of an alkali-soluble polyacrylate, and they do not differ from the other acrylic-based thickeners described above in this respect. The most commonly used monomers are ethyl acrylate, acrylic acid, and methacrylic acid. The hydro¬phobic side chains are attached to the polymer backbone by means of a polyethyl¬ene oxide spacer. The associative thickening effect can be controlled by varying the ratio of associative monomers to conventional monomers in the polymer back¬bone, the hydrophobicity of the polymer (i. e., the chain length of the aliphatic hydrocarbons), and the number of ethylene oxide units in the spacer. The inter¬molecular association between the hydrophobic side chains themselves and be¬tween the side chains and the dispersed binder particles is responsible for the very high viscosity that can be achieved. The relatively high molar mass of the acrylic polymer backbone and its high affinity for pigments mean that these products can be used in paper coating, but the water retention level is lower than with conven¬tional thickeners.
2. 3. ACT (Associative Cellulosic Thickeners): Products in this category consist of cellulose ethers with hydrophobic substituents. The most important product in this group is hydrophobic modified hydroxyethyl cellulose (HMHEC) which also has a high affinity for styrene-butadiene binders and clay pigments. The applica¬tions open to these products are comparable to those for HASE products. The low water retention can be compensated for by the speed with which the filter cake forms on the surface of the base paper. Associative thickeners bring about a very high increase in viscosity, and so they only need to be added in very small amounts. This is an economic advantage over conventional thickeners, but only if the water retention of the coating color is unimportant. Other products need to be added to boost the water retention of the coating color if, for instance, highly absorbent paper is coated with a precoat or if coarse CaCO3 is used as the pigment.

3.6 Functional Chemicals
Table 3.7 Comparison of conventional and associative thickeners (source: BASF).
 
It has to be noted that the results obtained for water retention with static methods do not correlate with the water retention in practice under dynamic conditions. The great advantage of associative thickeners is that they are much more pseudo-plastic than conventional products, and coating colors have a lower viscosity at high shear (Table 3.7). This gives better runnability, and less blade pressure need be applied in order to control the coat weight. Associative thickeners cause a large, immediate increase in viscosity when the shear applied to the coating is released, and this can lead to a more bulky coating being formed with a superior optical appearance.

3.6.9.3.4.4 Lubricants
Lubricants are used in the coatings of paper and board for a number of reasons. They improve the runnability of the coating color by reducing the friction between the machine and the coating color, and also by reducing the friction between the base paper and the coating unit. This can be seen for example as fewer scratches in the coating and longer lifetime of coater blades. They enhance the plastic deforma¬tion of the dry coating in the supercalender by preventing the cracking of the soluble binder film that leads to dusting. This also improves gloss. During cal¬endering, lubricants migrate from the coating onto the hot calender rolls forming a monolayer on the rolls and thus preventing the sticking of the coating to the rolls that causes build-ups. Different kaolins have different dusting tendencies, and dusting can often be solved by using a lubricant. The most commonly used lu¬bricant is calcium stearate. It is produced by reacting stearic acid with calcium hydroxide, followed by emulsification and processing to a 50 % aqueous disper¬sion. The critical properties of the calcium stearate dispersion are mechanical im¬purities and free Ca2+, a good dispersion contains a minimum amount of both. Particle size and shape are important for antidusting properties. The optimum size is 5–10 mm and the shape should be platy. The platy shape enables the stearate to gather on the pigment, plasticize the surface of the dry coating, and reduce the dusting at calenders and printing machines. Wax emulsions are mostly emulsions of paraffin waxes, microcrystalline waxes, or polyethylene waxes. These are the oldest group of lubricants in paper and board coating. These emulsions give good runnability but have less effect as antidusting agents than does stearate. Particle size is small and the dry solids content of the emulsion is usually 20 %-30 %. Soy lecithin/oleic acid blends are a new group of substances used as lubricants. Poly¬ethylene and polypropylene glycols are used in blends with calcium stearate or alone as a lubricant. These lubricants are claimed to have influence on the rheology and flow properties of the coating color.

3.6.9.3.4.5 Defoamers/Deaerators
In the coating process, entrapped air or gas can give rise to production problems and quality loss. The major part of the air seems to get mixed into the coating color during the coating application and circulation. Such air is mostly dispersed as small bubbles that, due to the high viscosity of some coating colors, may remain in the color until drying. Higher coating speeds (2000–3000 m min–1) and total solid content of coating formulations possibly contribute to more stable micro-foam. Formation of foam is a topic in many industrial processes including the coating of paper and board. Foam is formed through an interaction of mechanical forces and physical chemical properties of the chemicals. From a physical chemical point of view, foam can be defined as a dispersion of air or a certain gas in a liquid or fluid medium. The formation of foam always involves a substantial increase in surface area between the dispersed air or gas and the liquid. Foams in general are in¬herently unstable and are susceptible to decay with time. On the other hand, it is well known that under certain circumstances foams can be quite stable and may persist over long periods of time. Pure liquids and liquid mixtures, which do not contain a surface-active ingredient, are not able to produce stable foams. Surface-active ingredients or surfactants reduce the surface tension of the liquid and, in consequence, the surface energy renders the system more stable than without surfactants. Besides surface activity, other parameters like foam structure, lamel¬lae thickness, foam drainage, surface rheology, and elasticity contribute to the stabilization or destabilization of foams. These parameters are dependent on time, pH, the presence of surfactants, polymers, proteins, and/or salts as well as the chemical composition and the physical properties of the liquid. Foam generation can occur at different levels of mechanical energy input to the liquid system. Dif¬ferent foam volumes can be generated using the same liquid composition but with different mechanics involved.
Foam reduction, defoaming or deaeration can be achieved by various means. Apart from pressure reduction or mechanical influences (e. g., skimmer or separa¬tion grids), special chemical additives are mainly applied to control the foam. In general, it can be stated that there is a need for formulations of specific defoamers for each specific foaming liquid system.

In those cases where surfactants are already a part of the chemical composition of the liquid medium, high-foaming surfactants may be replaced by low-foaming surfactants, e. g., methyl- or butyl-capped alkyl- or fatty alcohol ethoxylates or pro¬poxylate or polymeric materials like polyoxyethylene or polyoxypropylene or both as block copolymers. In contrast to these water-soluble defoamers, usually insolu¬ble hydrophobic additives are widely used as very efficient defoamers. Among these should be mentioned hydrocarbon/fatty acid/ester or wax blends, (poly) siloxanes, fluorocarbons, dispersions of solid particles (hydrophobic silica, organic microwaxes, etc.) in hydrocarbons or (poly)siloxanes. The chemical composition of the defoamer and the particle size distribution of the dispersed additive in the liquid medium determine the efficiency of these defoamers. In these defoamers, the fluid component serves as a dispersal aid for the solid, crystal-like particles. When added to the foam, the hydrophobic liquid component spreads out on one side of the foam lamella surface, producing a nonzero contact angle to the sur¬rounding water phase. After further drainage of the foam lamella, the hydrophobic drop finally bridges the lamella and causes lamella breakdown due to the high contact angles and ongoing drainage.

In this respect, the dispersed solid particles enhance these processes and improve the efficiency of the defoamer system to a great extent. Foam control agents are usually applied during coating preparation. In systems where the pigments, binder(s) and the other additives are made up simultaneously, the antifoam agent is added before the pigment and adhesive. Normal dosages lie in the range 0.05 to 0.2 % on dry coating solids. Defoamers are sometimes added pre-diluted and should be properly mixed into the system to ensure an even distribution throughout the volume. Spots in the finished coating layer, commonly referred to as fisheyes and birdeyes, can have a variety of causes. Such deficiencies can be related to the type of antifoam or to too high a dosage of antifoam, where the foam control agent counteracts the complete spreading or wetting of the paper surface at these positions.

3.6.9.3.5 Additives Influencing the Quality and Printability of the Paper Surface
3.6.9.3.5.1 Co-binders and Thickeners
The structure formed by the pigments plays an important part in determining the physical properties of the coating and, because co-binders and thickeners have a high affinity for pigments, they play an important part in influencing the structure of the coating by controlling dewatering. The physical properties of the coating that are affected most by the use of synthetic co-binders and thickeners are the smooth¬ness and porosity and the brightness of coatings that contain fluorescent whiten¬ing agents. Careful attention needs to be paid to all these factors when the paper or board is to satisfy all the market demands made on its gloss and print gloss, ink uptake and holdout, glueability, brightness and opacity, water resistance, and ab¬sence of mottling. Synthetic co-binders also help to bind pigments to each other.
They largely contribute to the pick strength of the coating. The amount of binder employed in the coating can be reduced if large proportions of synthetic co-binder are employed. Generally speaking, two parts of co-binder can be used to replace one part of binder. They also increase the water resistance of coatings. Once they have formed a film, they become firmly integrated within the structure of the coating and are no longer soluble in water.

3.6.9.3.5.2 Insolubilizers
There are many chemically different types of insolubilizers or crosslinkers, but they all have the same function – to add water resistance to the coated paper surface. Water resistance is particularly important in offset printing, but also for wallpaper, label paper, poster paper and in the storage of board packages. In dou-ble-coated boards, crosslinkers are used in the precoating to impart water resis¬tance against the topcoat. The water resistance can be measured as wet rub and wet pick or seen as less pick, print mottle, or binder migration. The water sensitiv¬ity of paper and board coating originates from the fact that water-soluble binders tend to lose their binding power in contact with water and dissolve. This water sensitivity of binders can be described as the amount of O-atoms in the molecule (in hydroxy and carboxyl groups). The water sensitivity can be decreased by cross-linking the soluble binders with insolubilizers or by building an insoluble net around the binders.
Traditional insolubilizers in paper and board coating are based on formaldehyde and its amino compounds (melamine, urea) or on glyoxal. Imidazoline derivatives are also used as crosslinkers. Next-generation insolubilizers are based on zirco¬nium; the product most widely used is ammonium zirconium carbonate (AZC).
Glyoxal is the simplest aliphatic dialdehyde. It is an effective crosslinker of starch, and gives the coating an immediate curing. The crosslinking mechanism is a reaction with the hydroxy groups. Glyoxal first reacts with one starch molecule and then with another one, leading to crosslinking of two starch molecules. Glyoxal is a less effective crosslinking agent for synthetic water-soluble binders. Some of its derivatives, such as condensate products with urea or ethylene urea can be used here. Glyoxal is ineffective at pH >8.5. Difficulties have resulted with viscosity build-ups due to crosslinking in the wet form.
Melamine-formaldehyde (MF) and urea-formaldehyde (UF) resins have been used in paper coating since 1940. In the 1970s, methylated MF and UF resins for the most part replaced these resins. Melamine-formaldehyde (MF) resins and urea-formaldehyde (UF) resins have reactive imino- (>NH) and methylol (>N–CH2–OH) groups. Methylated MF and UF resins have also some functional methoxymethyl groups (>N–CH2–O–CH3). These groups undergo reactions with paper coating binders: the hydroxy group of starch and polyvinyl alcohol, and the carboxyl groups of latexes. Both UF and MF resins can also self-condensate. The reaction of MF/ UF and their derivatives is an acid catalyzed condensation reaction, which requires a certain temperature and pH. The curing reaction takes up to two weeks to complete.
3.6 Functional Chemicals

Environmental pressures against formaldehyde-based resins, higher pH in coat¬ing formulation, and the need for faster curing are the reasons why ammonium zirconium carbonate (AZC) is more and more used in the paper industry. AZC reacts with carboxyl and hydroxy groups in the coating, hydroxy groups in starch and PVA, carboxyl groups in latexes, and oxidized starch. The reaction takes place when ammonia is evaporated and water is removed on drying. AZC forms cova¬lent bonds with carboxyl groups and weaker hydrogen bonds with hydroxy groups. High pH does not affect the reaction. Other zirconium-based crosslinkers are po¬tassium zirconium carbonate and zirconium acetate. The reaction mechanism is sim¬ilar for all these products.
In coatings the amount and type of binder present determines the addition level of crosslinkers. It is recommended to use 5 %-10 % crosslinker based on total dry binder. In general, starch-based coatings require a higher amount than coatings based on synthetic binders. It is important to optimize the dosage of crosslinker. An excessive amount can result in cracking problems or even in increased, instead of decreased, solubility of water-soluble coating components. A crosslinker should be added to the coating color as the last ingredient.

3.6.9.3.5.3 Tinting (Shading)
In many cases, the yellowish color of the coating clay does not satisfy market requirements. Apart from the addition of pigments such as titanium dioxide, which increase brightness, blue-violet dyes (shading dyes ) are often used at a con¬centration of about 2 g per 100 kg of pigment to attain a bluish white coating surface which appears brighter to the eye. Not only basic dyes, but also consider¬ably more lightfast direct dyes and pigments may be used. If the brightness re¬quired is very high, optical brighteners are also added.

3.6.9.3.5.4 Optical Brightening Agents (OBA)
The brightness of paper and board has increased dramatically during recent years. The brightness of pulp, fillers and coating pigments is not high enough to reach these brightness targets. Therefore there is a need to use a coating additive called an optical brightener. These products are also known under the names optical whitening agents, optical bleaching agents, or fluorescent whitening agents (FWA).
Fluorescence is a phenomenon where the molecules of a fluorescent substance become electronically excited by absorbing light energy and then emit this energy at a higher wavelength. Fluorescence is usually restricted to compounds with large conjugated systems containing p-electrons. Most of the OBAs on the market are derivatives of bis(triazinylamino)stilbene. Only the trans-isomer exhibits strong fluorescence, the cis-isomer is nonfluorescent. The OBAs used in the paper in¬dustry are natrium salts and are thus water soluble. There are three types of optical brighteners used in the paper industry, all based on the stilbene molecule. The main difference is the number of solubilizing sulfonic groups. Disulfonated OBAs have two sulfonic groups; the two other substituents could be hydrophilic groups. This OBA has a very good affinity but limited solubility and is mostly used in the wet-end. The most commonly used OBAs are the tetrasulfonated types. Tetra-sulfonated OBAs are versatile products because of their characteristics of medium affinity and good solubility. They can be used in most applications in the paper industry: wet-end, size-press, and coating. The hexasulfonated OBAs are special¬ties used mostly in coatings where high brightness is required.
OBAs absorb ultraviolet radiant energy at 300–360 nm and re-emit the energy in the visible range, mainly in the blue wavelength region. This increases the amount of light emitted, resulting in higher brightness or whiteness. Because the reflected light is bluish, the yellow shade of paper is compensated, contributing to making the paper look still whiter. Whiteness is defined as the measured reflectance of light across the visible spectrum including color components. Brightness again is defined as the reflectance of light at the wavelength 457 nm without color in the measurement. To measure the brightness or whiteness of paper and board con¬taining OBAs requires an instrument having a known amount of UV in the illumi¬nation. The test method used increasingly is the CIE whiteness (SCAN method P66) instead of the traditional ISO brightness, which does not define the illumi¬nant. An increase in OBAs at lower concentrations results in an increase in white¬ness • As the concentration goes up to 1.5 parts of dry pigment there is no more gain in whiteness when adding more tetrasulfonated OBA. This is called the sat¬uration point or the graying point. The hexasulfonated OBA actually has no gray¬ing point because of its high solubility.

3.6.9.3.5.5 OBA Carrier
Optical brighteners work only when they are fixed to a carrier. A good carrier is linear and contains OH- or other hydrophilic groups. Linearity increases the con¬tact between the carrier and OBA so that physical bonds (such as hydrogen bonds and van der Waals forces) can be formed between the OBA and the hydrophilic groups of the carrier. Carriers in a coating color are, among others, starch, CMC, or PVA. By adding one of these products in coating color, higher brightness can be achieved.
UV exposure causes yellowing of brightened paper, but a good carrier increases the light stability. Furthermore the carrier has an influence on the migration of OBA; the more efficient the carrier, the better the resistance of OBA to the mi¬gration.

3.6.9.3.5.6 Influencing Opacity
The main contribution to opacity has to come from the pigments. Synthetic prod¬ucts are usually less opaque than natural products and this can be a problem, especially when coating ULWC (ultra light weight) grades of paper. Products with a very pronounced thickening effect are only added in small quantities, so they hardly have any effect on opacity.

3.6.9.3.5.7 Influencing Smoothness and Gloss
The gloss of paper largely depends on its smoothness. High gloss depends on the evenness of the topology of the paper surface. Like all hydrocolloids, co-binders and thickeners have an effect on the smoothness and the gloss of the paper. They are able to migrate in the wet coating, and migration can lead to them becoming unevenly distributed on the surface. They can also absorb water and swell, which also impairs the smoothness of the paper. Synthetic co-binders are usually less detrimental to gloss than natural products. One of the reasons for this is that natural products are able to absorb moisture and swell after they are dried, whereas acrylic polymers are much less sensitive to moisture once they have dried to form a film. Another reason is that acrylic polymers are highly thermoplastic, and respond very well to calendering. The films formed by natural products are thermosetting, and so they are not deformed as easily under heat and pressure in the calender nip.

3.6.9.3.5.8 Influencing Porosity, Print Gloss and Glueability
The gloss of the printed paper is determined by the smoothness of the paper surface and the ink holdout. In turn, the ink holdout is mainly determined by the porosity and the chemical and physical structure of the coated surface. Coatings need to be porous on a microscopic scale so that the soluble component of printing inks is able to penetrate the paper and dry more quickly. The intensity and bril¬liance of the printed image and the ink consumption depend on the pigments staying on the surface. The hydrophobicity of the coated surface and the surface tension both influence the ink uptake.
Coatings that contain synthetic thickeners and co-binders are more hydrophobic and take up more ink, but their porosity can have a detrimental effect on the print gloss. Synthetic co-binders give rise to an open-pored structure, which can in¬crease the rate of ink absorption and give a lower print gloss. The task here is to find the best compromise between the pore size distribution and the hydropho¬bicity of the coated surface in order to obtain the highest possible print gloss without prolonging the drying time of the ink too much. An open-pored structure is often highly desirable in multiple coats applied to board because this guarantees high ink absorption.
Another important aspect governed by the porosity of the coating is the glue-ability of coated board. Here the affinity of the adhesive also plays an important role. High porosity can be obtained by using binders based on vinyl acetate, and it is for this reason that large quantities of vinyl acetate binders are used in the United States to coat folding boxboard, in spite of disadvantages such as low bind¬ing power, poor printability and build up of stickys in the wet-end of the paper or board machine. The approach taken in Europe is to use styrene-acrylic and sty-rene-butadiene binders, which give a more compact, less porous coating, and to add special co-binders that form a porous film and aid glueability. The porosity of the films formed by acrylic-based products and their compatibility with acrylic and acetate-based adhesives give very good glueability when they are applied to folding boxboard.

3.6.9.3.5.9 Influencing Printability
The evenness of the printed image is a very important criterion for quality. The acceptance of coated paper by printers depends on its runnability in the printing process and its printability in terms of an even image that is free of defects.
The printability of paper is principally determined by the surface of the coating, and its structure and topology, pore size and pore size distribution, chemical na¬ture, and homogeneity all play an important part in influencing the interaction between paper and ink. The binder has the greatest influence on printability, but co-binders and thickeners also play a part and they have to be adapted to the printing process.
In offset litho, mottling is mainly caused by the uneven distribution of coating ingredients at the surface of the coating. There are different types of mottling depending on the causes and the way in which the mottling manifests itself. In gravure printing, missing dots can be avoided if the paper has a very smooth surface and a defined microscopic roughness.
In both printing processes, it can be assumed that co-binders and thickeners with very high water retention will have a detrimental effect on print quality. Very high water retention prolongs the drying time of the wet coating and the various different ingredients in the coating migrate at different rates, which causes the binder, co-binder, and thickener to become unevenly distributed. This patchiness gives rise to mottling in offset litho printing processes. Low web speeds and low drying capacity tend to exacerbate this problem. Excessively high water retention can also give rise to a rough, uneven coating which can cause missing dots in gravure printing.
There are no hard and fast rules with co-binders and thickeners when it comes to avoiding mottling. All natural and synthetic products can cause mottling under adverse conditions. Coatings that contain starch are known to have a high ten¬dency to mottle, even if starch is only contained in the precoat. Soy protein behaves similarly. Synthetic products are not usually prone to mottling, although CMC can give rise to missing dots in gravure printing processes.