Pigments as Fillers
Mineral fillers have long been used in papermaking. While they can be found in different paper and board grades, printing and writing papers represent by far the most dominant area for their application. Initially, they were added to increase the weight of the sheet and to improve the writing properties. Today, there are several further functional reasons for the use of fillers in printing and writing papers.
188.8.131.52 Why Use Fillers?
Fillers are applied to the paper mainly
. • to improve the optical properties, such as brightness and opacity
. • to improve the smoothness of the sheet surface (i. e. decreased roughness, espe¬cially after calendering)
. • to improve the sheet formation by filling the voids between the fiber matrix
. • to enhance printability in the various printing processes due to a more uniform paper surface, higher opacity and better ink receptivity. The latter resulting in reduced printing ink penetration, wicking and ink strike-through to the opposite side of the sheet
. • to improve the dimensional stability of the paper as most fillers remain inert when wetted, unlike the natural fibers usually used in papermaking.
. • to improve the permanence of the paper (alkaline papermaking, calciumcarbo¬nate (CaCO3) filler)
As regular mineral fillers are, in general, lower in price than typical papermaking fibers, replacing fiber by mineral fillers usually provides better papermaking eco¬nomics. Therefore, efforts are made to include as much filler in the paper as the technological demands can support.
The amount of filler (loading) in the paper has of course a strong impact on the sheet properties obtained. Figure 2.9 shows the influence of increased filler load¬ing on some important paper properties. The results shown are based on a pilot study, using natural ground calcium carbonate (GCC) as filler in a wood-free fiber furnish. The basis weight was 80 g m–2. In general, the influence of the filler at increased loading is magnified in both directions, desired and undesired. In partic¬ular the main location and distribution of the filler in the sheet has a major influ¬ence on the end performance. In the final sheet, the effect of a filler is dependent not only on the intrinsic properties of the filler particles, but also on the extrinsic influence of the filler on the fiber network. This includes for example the disrup¬tion of the fiber network by the filler.
The limits in terms of filler loading are set primarily by an accompanying reduc¬tion in stiffness and insufficient strength properties (tensile strength, tear resis¬tance, internal bond and surface strength). Sufficient surface strength is partic¬ularly important in offset printing or, for example, in copier machines. Inade¬quately fixed filler on the paper surface can result in dusting (blanket piling) in the offset printing process or contamination of the copy machine. Typical filler loading levels in different European paper grades are given in Table 2.3.
Overall, the wood-free uncoated paper grades mentioned are loaded mostly with primary (fresh) filler only. The coating base papers are filled with coating pigment
Table 2.3 Typical filler loading levels in European printing and writing paper.
Paper Filler loading (%)
wood-free uncoated 12–26
(copy, stationery, offset)
wood-free coating base 12–20
(single, double, triple coated)
wood-containing uncoated 24–38
(super calendered, magazine, catalogues)
wood-containing coating base 8–12
(LWC, MWC, HWC)
2.2 Non-fiber Raw Material
coming from the coated paper broke plus some primary filler. The wood-contain-ing uncoated papers can contain recycled pigment (originating from the use of recycled fiber furnish) plus primary filler or just 100 % primary filler. Today news¬print is often made from recycled fibers, sometimes combined with virgin fibers. Newsprint, made from recycled fibers, contains substantial amounts of recycled pigment and, if sensible and possible, also some fresh filler, reaching up to 18 % total filler loading. Newsprint paper based on virgin fiber only, carries almost no filler in the paper. However, more recently, virgin fiber newsprint paper can be found in the paper market, loaded with up to 12 % chalk as filler.
Additionally worth mentioning is a development which increases the filler load¬ing level effectively through the paper surface. GCC filler is added to the film press starch solution and penetrates together with the starch into the sheet. Thereby the wet end system is not affected at all and, at moderate loading increase, paper stiffness and strength are also unaffected. Paper caliper and bulk are reduced as in a conventional internal filler loading increase. Porosity is dramatically reduced by this method, allowing potentially also for some reduction in fiber refining
184.108.40.206 Choice of Fillers
The choice of fillers usually takes into consideration primarily the desired perform¬ance in a specific paper grade, the mill internal situation, logistical possibilities and consequences, the general availability of the product, and the costs, including the external and internal logistics.
Optimizing one type of filler for a specific paper grade usually results in a com¬promise between the obtainable properties with this filler. In cases where one single pigment does not meet all the requirements, filler combinations are applied. There is a continuous trend towards fully or partially replacing specialty pigments by regular or modified regular fillers. Table 2.4 provides an overview of the range of regular fillers and specialty pigments applied in the production of certain paper grades.
Of great importance are the potential interactions of the fillers with wet end furnish components like retention aids, starch, sizing agents, dyes etc. Therefore, the selection of the filler type and grade needs also to be made in the light of these possible interactions.
220.127.116.11 Characterization of Fillers
Chemical composition, particle morphology, particle size and particle size distribu¬tion, brightness, refractive index, specific surface, particle charge and abrasiveness are commonly used to characterize papermaking fillers. Table 2.5 summarizes some chemical and physical data of fillers and fibers. More detailed information is given in the following paragraphs.
Table 2.4 Paper grades and range of fillers and/or specialty pigments applied.
Paper grade Filler/specialty pigment
wood-free uncoated PCC, GCC, PCC/GCC, talc/GCC (Asia only),
(copy, stationery, offset) amorphous silicate, calcined clay
wood-free coating base GCC, PCC
(single, double, triple coated)
wood-containing uncoated clay, clay/GCC, clay/PCC,
(super calendered, magazine, catalogues) recycled pigment/clay/GCC or PCC
calcined clay, amorphous silicate
wood-containing coating base GCC, Talc, PCC
(LWC, MWC, HWC)
Newsprint/Directory Recycled pigment, GCC, PCC,
amorphous silicate, calcined clay, (TiO2)
high opaques TiO2, Zinc sulfide, PCC, amorphous silicate
(thin print) calcined clay
decoration paper TiO2, TiO2/Talc, TiO2/calcined clay or silicate
cigarette paper PCC
white top liner GCC, PCC, calcined clay
Tappi brightness is measured at a wavelength of 457 nm. Fillers with high pigment brightness, like GCC or PCC, are in demand for the production of high quality printing and writing papers. Figure 2.10 compares the Tappi brightness of various fillers of different geographical origin as well as of highly bleached chemical and mechanical pulps (dotted lines). The brightness range of the different types of filler is given by the raw material quality used and the various processes (mechanical classification, magnetic separation, flotation, grinding, bleaching, precipitation etc.) applied.
18.104.22.168.2 Refractive Index
The refractive index indicates the extent to which a light beam is deflected when passing from vacuum into a given substance. The refractive index is given by the filler’s chemical composition and molecular structure. Powders are usually meas¬ured in a refractive index matching liquid. The material, ground into a fine pow¬der, is immersed in a series of liquids until scattering and reflectance disappear,
i. e. the material becomes invisible. Of course the powder should not be soluble in the liquid. It can be tested either using a laser beam or by studying the turbidity with the eye or a turbidimetric method. The more the particles differ from the medium (i. e. the more their refractive indices differ), the more light will be scat¬tered by the particles. If there is no difference at all, no light will be scattered. However, as calcium carbonate is birefringent (double refraction), the method de¬scribed is not ideally suitable for this specific mineral. The higher the refractive index developed in the paper, the higher the amount of reflected and/or scattered light, which results in an increase in paper opacity. While titanium dioxide TiO2 shows an extraordinarily high refractive index (rutile 2.75, anatase 2.55), other common fillers exhibit indices of around 1.55–1.65 (zinc sulfide 2.39). For compar¬ison, the refractive index of cellulose, as used in papermaking, is about 1.60.
22.214.171.124.3 Particle Morphology
The structure of a filler can be observed and characterized best by scanning elec¬tron microscopy (SEM). The particle morphology has an influence on light scatter¬ing via the number and size of air microvoids in the sheet. For different morpholo¬gies, there is a different optimum for light scattering in terms of particle size. The particle morphology has an impact also on the packing of the filler particles in flocculates usually formed during the papermaking process. The results are mor¬phology related differences in sheet drainage, drying behavior, and in paper prop¬erties such as bulk, porosity, ink receptivity, strength, dusting, etc. The sheet sur¬face roughness and paper gloss after calendering, as well as the sheet compressi¬bility (important in rotogravure printing) are also influenced by the morphology of the filler or specialty pigment applied. Figure 2.11 shows SEM pictures of various typical fillers and specialty pigments currently applied in the paper industry.
126.96.36.199.4 Particle Size and Particle Size Distribution
There are different methods (sedimentation, laser scattering/diffraction etc.) to determine the particle size distribution of a filler. However, the results obtained by the different methods are usually not directly comparable. Even comparing pig¬ments with very different morphology by just one single method, does not provide correctly comparable particle size distribution curves. The sample preparation (state of dispersion) also has a great influence on the outcome of the individual measurement. Figures 2.12 and 2.13 show some typical particle size distribution curves (measured by the sedigraph method) of GCC and PCC based fillers.
Fig. 2.14 Scanning electron micrograph pictures of natural ground calcium carbon¬ate filler and chemical pulp fibers in a paper sheet (source: OMYA).
Figure 2.14 indicates the size relation between the GCC filler particles and the fibers in the sheet. As is clearly demonstrated, the filler particles are much finer than the fiber and, in particular, much smaller than the voids between the fibers.
In the papermaking process, most of the original filler particles form agglomer¬ates or flocculates, by addition of different flocculating wet end additives. The degree of flocculation can be optimized by careful handling of the wet end chem¬istry. To obtain a highly uniform sheet, any excessive flocculation, of course, needs to be avoided.
In general the optical properties provided by a filler are also strongly influenced by its particle size and particle size distribution. Finer fillers, within limits, as well as steeper particle size distribution curves (more particles of similar optimum size) produce more light scattering, hence more opacity. Table 2.6 gives some typical light scattering ranges for major fillers, specialty pigments and also, for compar¬ison, for some virgin pulps. However, finer fillers exhibit a more negative impact on paper strength properties than relatively coarse pigments of the same weight proportion. Non-platy large particles tend to be released out of the paper surface more easily than smaller particles resulting in, for example, more blanket piling in the offset printing process or the contamination of a copy machine.
188.8.131.52.5 Specific Surface Area
Usually the specific surface area is measured by the nitrogen-adsorption method (BET: Brunauer, Emmet, Teller). The particle fineness, the particle size distribution and the particle morphology are, depending on the structure, indirectly reflected in the specific surface area of a filler. Finer nonstructured fillers exhibit a higher specific surface than coarser ones. There is a general direct correlation of the specific surface area of a filler and, for example, the internal sizing agent demand. Internal sizing agent is applied to the wet end in order to make the paper more hydrophobic. The specific surface area of regular paper fillers ranges between 4 and 12 m2 g–1. Typical papermaking fibers exhibit a specific surface area between 1 and 2 m2 g–1, while fiber fines show specific surface areas of 6–8 m2 g–1. As one consequence, increasing the filler loading level usually significantly increases the additive demand, i. e. sizing agent, dyes etc. Specialty pigments applied as filler can reach specific surface areas as high as 70–80 m2 g.
184.108.40.206.6 Particle Charge
The fillers applied need to be understood as part of the rather complex wet end chemistry in the papermaking process. The electrostatic charge (zeta potential), which surrounds the pigment, can be anionic or cationic. Depending on the type of filler, its origin, the specific chemicals applied in the production process and the dispersing treatment, the charge density can be on a quite different level. To obtain a good stabilization of a filler slurry, the dispersant applied needs to be highly charged. Typically the salts of polyacrylic acid are applied for this purpose of elec¬trostatic stabilization as these provide the pigment surface with a distinct anionic charge. Low molecular weight and high charge density polydadmac, modified poly¬ethylenimine or polyvinlyamine etc. can be used, for example, to develop a distinct cationic slurry stabilization.
The charge density of a filler slurry is of particular importance in the context of selecting the most suitable retention aid system. Today there are many retention systems available and the most suitable one has to be found for each individual filler application. The ordered addition of multi-polymer retention aid systems and, in particular, so-called microparticle systems offers efficient tools to retain anionic stabilized filler pigments at high production speeds effectively. Cationically charged, partly self-retaining, fillers are no longer much in use, one reason being the potential for quenching the commonly applied negatively charged optical brightening agents
The abrasion potential of fillers is influenced by the particle structure, the particle fineness and the particle hardness. Relatively coarse platy pigments tend to be less abrasive than non-platy ones of similar size. Increasing the filler fineness de¬creases the wire abrasion potential significantly. This is particularly pronounced when testing non-platy pigments. Large size non-platy impurities such as quartz increase the wire abrasion excessively. Any pigment which is harder than the syn¬thetic wire, commonly installed in the paper machine, potentially initiates wear on the wire.
There are different laboratory methods of determining the potential abrasive¬ness of a filler. The most modern unit is the Einlehner AT 2000 abrasion tester. This instrument works by using a cylindrical ceramic body with specific slots and a wire made of synthetic filament. Figure 2.15 compares different paper fillers on a global basis by this specific method. Again, because of differences in the raw material used, the processes applied and the differences in particle fineness, abra¬sion ranges are presented as being indicative only.
Practical experience collected in recent years has shown, that wire abrasion is greatly influenced also by the conditions on the paper machine. In general, lower filler retention potentially increases wire wear. The wire drag load should be mini¬mized, for example, by a low number of stationary drainage elements and opti¬mized low vacuum loads. The dry suction box ceramic surfaces should display a low surface porosity and should be well maintained .
220.127.116.11 Main Mineral Fillers
The main mineral fillers (in terms of quantity applied) are kaolin (hydrous), GCC, PCC and talc.
Figure 2.16 depicts the global consumption of the different main types of virgin fillers applied in papermaking. The data refer to the market situation in the year 2004. The share given for talc includes the use of talc as filler as well as its applica¬tion as a pitch control agent. Globally in 2004, the paper industry consumed 29 million tons of pigment for coating and filling. As indicated in Figure 2.16, for filling purposes alone, 11.4 million metric tonnes were used.
With the use of recovered paper in papermaking (e. g. for newsprint, SC-B, board, etc.) a substantial amount of secondary pigment is transferred into the new paper. This recycled pigment also acts as a filler. However, as the pigment is mostly agglomerated and the composition varies by nature, the resulting impact on paper quality is somewhat inconsistent. Recycled pigment cannot meet the quality and the range of functionality, as provided by the specifically designed virgin fillers.
18.104.22.168.1 Kaolin (hydrous)
This is used today predominantly in wood-containing uncoated papers (super cal¬endered magazine papers, catalogues etc.) . Kaolin deposits can be found at a number of major sites around the globe as a result of metamorphosis granite outcrops. The largest ones, from which paper fillers are extracted, are located on the South East coast of the United States (mainly secondary deposits) and in Corn¬wall in the South West of the United Kingdom (primary deposits). Primary kaolins are those which are found at the places where they were originally formed and are accompanied by the original matrix including partially altered and residual materi¬als. Secondary kaolins have been moved from their place of origin and by pro¬gressive sedimentation become deposited at a different location. Other kaolin de¬posits of commercial interest occur in Russia, Central Europe (Czech Republic, Germany, France, Spain) (all primary), Australia (secondary), China (primary, sec¬ondary) and Brazil (secondary). Due to the presence of a very high ratio of kaolin, exploiting a secondary deposit source requires less processing than drawing from a primary deposit.
Kaolin processing involves purification of the kaolin-containing raw material by several techniques including mechanical classification, grinding, bleaching, mag¬netic separation and flotation. Kaolin is supplied as a slurry or dried in powder form (spray dried or in crumbles). Figure 2.17 shows a generalized flow sheet for the wet processing of kaolin, based on primary and secondary deposits.
The aspect ratio or platyness of kaolin is strongly dependent on the geophysical nature of the deposit. The aspect ratio expresses the relationship of the major diameter to the platelet caliper. Secondary kaolins tend to be of lower aspect ratio; some Brazilian examples, however, are exceptions to this rule. Primary Cornish kaolin, for instance, has an aspect ratio of 25–40:1, compared to Georgian kaolin with an aspect ratio of 12–25:1). The aspect ratio distribution throughout the vari¬ous size ranges is also an important attribute of kaolin, with the presence of bento¬nite or montmorillonite at the fine end of the particle size distribution being a particular feature of some clays. The hexagonal crystalline platelets of kaolin pro¬duce a high gloss of the finished paper after calendering. This gloss development also depends on the degree of delamination, i. e. the extent to which the platelet agglomerates or stacks are broken into individual platelets. Due to its chemical composition, kaolin can be used as a filler in both acid and alkaline papermaking environments.
The brightness of regular filler kaolin is distinctly lower than most CaCO3 based fillers. For this reason, in recent decades, kaolin has been largely replaced globally by CaCO3 fillers in wood-free uncoated papers. Due to the continued growth of the higher brightness SC paper market (super calendered uncoated magazine paper, catalogues etc.), kaolin is being increasingly combined with or replaced by high brightness calcium carbonate based fillers in this area.
22.214.171.124.2 Natural Ground Calcium Carbonate (GCC)
This is predominantly applied in wood-free uncoated paper, mechanical uncoated papers (usually in combination with kaolin), coating base papers, newsprint and white top liner board.
Natural CaCO3 constitutes the most frequently occurring type of sedimentary rock on our planet. It covers about 1 % of the earth’s crust. Natural CaCO3 occurs in three major geological modifications. Chalk was and is formed in the oceans through biomineralization and the reactions of calcium salts with the CO2 in the air. By geological transformation (pressure) and thermal metamorphosis (heat and pressure), it is modified into limestone and marble.
Natural CaCO3 for the paper industry is processed at numerous locations around the globe: for example, in North America (Vermont, Canada, British Co¬lumbia, Alabama), in Europe (Norway, Finland, France, Spain, Germany, Austria, Italy, Turkey), in the Far East (South Korea, Taiwan, Indonesia), in Australia and New Zealand. In addition there are active plants located in Latin America (Mexico, Chile, Brazil), Russia, China and South Africa .
GCC fillers are produced by pre-washing the raw material, followed by grinding, fine grinding and screening the product. Undesired impurities in the raw material are removed by magnetic separation and flotation. Figure 2.18 shows a typical production process flowchart for GCC fillers, based on limestone or marble raw material.
The production, shipping and application of GCC fillers in wet (slurry) form has become by far the most preferred practice. GCC filler slurries exhibit a solids content of 65–72 % (by weight) and are usually stabilized by using an anionic grinding and dispersing agent. Specifically, cationically stabilized GCC filler prod¬ucts are also available.
The structure of GCC filler is rhombohedral. Because of the high brightness demand, GCC fillers based on limestone and marble are preferred by the paper industry. Lower brightness chalk is increasingly used as a filler in the production of regular newsprint, where there is less demand for brightness. The fineness of GCC fillers for paper is generally much greater than kaolin based fillers, partic¬ularly those sourced from primary kaolin deposits. This is required, for example, for obtaining high light scattering, low abrasiveness and low dusting out of the paper surface in the printing process.
CaCO3 is soluble under acid conditions, and therefore requires a near-neutral or slightly alkaline pH papermaking environment . To be able to use CaCO3 as a filler and/or as a coating pigment, numerous paper and board mills around the globe have converted their wet end systems from acid to neutral and slightly alka¬line pH. Historically, paper was mostly produced at acid pH (< pH 7). Paper pro¬duced in such a manner decomposes relatively quickly and therefore presents a serious problem for the conservation of documents in libraries etc. Complicated and expensive measures are applied to save valuable documentation printed, un¬fortunately, on acid made paper. Wood-free paper, produced in the slightly alkaline mode and containing some CaCO3 filler for buffering, exhibits a dramatically improved permanence and can be stored for hundreds of years under regular condi¬tions. Figure 2.19 depicts the solubility of CaCO3 relative to the pH of the envi¬ronment.
126.96.36.199.3 Modified Natural Ground Calcium Carbonate (GCC)
In a more recent development, finely ground GCC has been specifically modified into a pigment with a completely different morphology and consequently different properties. Figure 2.20 shows modified GCC particles in their original state and after compressing (calendering). It should be mentioned that the modified GCC also exhibits an extraordinarily high specific surface area of up to 80 m2 g–1 BET. Providing high brightness, easy gloss development and good printability in offset and rotogravure, this modified GCC has already found its way into the commercial production of SC (super calendered) paper. As specialty pigment, it is applied in conjunction with regular CaCO3 fillers .
188.8.131.52.4 Precipitated Calcium Carbonate (PCC)
This is also predominantly applied in wood-free uncoated paper, wood-containing uncoated paper (in combination with kaolin), wood-free coating base paper, direc¬tory grades and white top linerboard . In order to control porosity and burning rate, PCC is also widely used as a filler in the manufacture of cigarette paper.
One very important raw material for the production of PCC is a suitable deposit of natural CaCO3. Only rather few limestone deposits meet the stringent demands for the production of high quality PCC. Carefully selected limestone is calcined at 800–900 °C to obtain calcium oxide (CaO or quicklime), requiring energy and re¬leasing CO2. The quality and uniformity in quality of the lime used has an im¬mense influence on the quality of the final PCC product. The addition of water (exothermic reaction) produces calcium hydroxide (Ca(OH)2 or slaked milk of lime). The usually applied carbonation process consists of bubbling CO2 through the slaked milk of lime. At the end of the process there exists once again CaCO3, now in the form of PCC. The process can be manipulated, within limits, to influ¬ence particle shape and fineness. PCC is often produced in plants located on-site at the paper mill, but there are also many so-called off-site production units. A gener¬alized flow sheet for the production of off-site manufactured PCC is presented in Figure 2.21.
Various investigations and development work have been carried out in order to produce PCC directly in, for example, a high consistency stock. However, so far there has been no major breakthrough into the commercialization of such proc¬esses [9, 10].
The morphologies of PCC used as fillers are commonly scalenohedral (rosette-shaped), rhombohedral (cubic-shaped) or aragonite (needle-shaped). The type of morphology is defined by process parameters like temperature, pressure, reaction speed, additives etc. The particles can be arranged as individual discrete, clustered or agglomerated. These different arrangements represent an additional tool to influence the overall pigment performance.
Currently the most common PCC morphology applied around the globe is the clustered scalenohedral one. This rosette-shaped type of PCC provides increased caliper and bulk to the paper (compared to kaolin or GCC). However, as a consequence of this higher bulk, strength properties are reduced and the sheet be¬comes distinctly more open and permeable. An extra high bulk providing PCC in general tends also to exhibit a lower light scattering potential. Combining different carefully selected morphologies assists in the optimization of specific paper prop¬erties. Although particle morphology and fineness can be influenced largely in the PCC production process, in the end a suitable compromise addressing the differ¬ent properties desired by the papermaker has to be found.
CaCO3 in the form of PCC, as true for GCC, is naturally soluble under acid conditions and therefore requires a near neutral to slightly alkaline pH environ¬ment. Residual calcium hydroxide Ca(OH)2 in the PCC can require extra measures for pH control of the final product and the paper mill wet end system. Despite reported developments in providing so-called acid tolerant PCC, intended to re¬main stable against dissolution under acid (< pH 7) conditions, papermakers, it seems, continue to make the conversion to slightly alkaline or alkaline paper¬making when considering the use of CaCO3.
This is applied in wood-free uncoated paper in Asia Pacific, often combined with GCC. In Europe it is used in a few cases as a primary filler in wood-containing coating base paper and wood-containing uncoated paper, in the latter case in com¬bination with kaolin.
Talc is found naturally throughout the world within an assembly of various min¬erals and as a secondary pigment. It is a hydrous silicate mineral that is very soft (Moh’s hardness 1, compared with pure CaCO3 being typically 3 and diamond 10) and has a low abrasiveness. It is chemically inert, very platy, water insoluble, orga¬nophilic and hydrophobic. As a result of the pronounced platelet structure found in pure grades, the use of such talc as filler provides a high smoothness and gloss to the paper at calendering. Due to its hydrophobic nature, talc has some limits in the use of offset printing papers when used alone, and so should be used judi¬ciously in combination with other hydrophilic minerals such as CaCO3 or kaolin. Talc has particular merit when used in rotogravure printing papers.
Micronised talc is applied to control pitch and stickies, originating during wood fiber processing depending upon the variety of fiber in use. Talc for sticky control can also be found in paper mills processing recovered paper. Talc particles are adsorbed on the surface of the resin or the sticky component and render these interfering substances harmless by covering them or being incorporated into them.
184.108.40.206.6 Gypsum (Calcium Sulfate)
This is used only on a very small scale as a filler in printing and writing papers. In recent years, gypsum as filler has been replaced by GCC at various locations. Gyp¬sum on the other hand is largely applied in the manufacturing of gypsum board. Gypsum board is made up with a high percentage of calcium sulfate material and lined with reinforcing sheets of paper. It is used in building construction for walls and ceilings.
Gypsum can be sourced from either natural deposits or as a byproduct, for example in the production of fertilizer or citric acid. Another gypsum byproduct originates from the desulfurization of flue gases emitted by power plants operating on the basis of fossil energy. Although available in huge amounts, so far all at¬tempts to make this byproduct technically and economically useable as paper filler, have been unsuccessful. Gypsum may exist as different chemical structures (CaSO4 2H2O dihydrate, CaSO4 1/2H2O hemihydrate, CaSO4 anhydrite) and in general exhibits a high solubility, more or less independent of the pH environment . Figure 2.22 compares the solubility of gypsum and calcium carbonate versus the pH.
220.127.116.11 Specialty Filler Pigments
18.104.22.168.1 Calcined Clay
This is a specialty filler, applied to the sheet mainly in order to increase light scattering/opacity and to reduce the ink print-through potential. Typical areas for the application of calcined clay are lower basis weight uncoated wood-containing papers (i. e. telephone directories, low basis weight newsprint and SC paper etc.).
In the calcination process of kaolin, the water of hydroxylation (14 %) is first driven off at temperatures of 500–700 °C. On continued heating up to 900–1000 °C the particles begin to fuse together into secondary particle aggregates, accompa¬nied by an increase in brightness. Further agglomeration results in tertiary parti¬cles. The final result is a large number of kaolin-air interfaces and relatively high internal pore volume leading to increased light scattering and opacifying proper¬ties.
2.2 Non-fiber Raw Material
22.214.171.124.2 Titanium Dioxide
This is a specialty and particularly effective filler. It is applied in specialty papers, often at low basis weights, with particularly high demand in dry and wet opacity and sheet brightness. TiO2 can be found in thin print paper, i. e. bible paper, high opaque grades, label paper and decoration paper.
Titanium is the fourth most abundant chemical element found on earth. In nature, titanium occurs only in the form of oxides or mixed oxides with other elements. Mineable deposits are generally of volcanic origin. The titanium dioxide pigment industry uses between 90 and 95 % of the global titania ore extracted.
Titanium dioxide is a synthetic material, commercially produced by the sulfate or the chloride process. It exists in three modifications with different crystal lattice structures and therefore different physical properties. The modifications are rutile, anatase and brookite. Only rutile (hexagonal close packing of the oxygen atoms) and anatase (cubic close packing of the oxygen atoms), which differ from each other in some of their physical properties, are of technical importance. The typical refractive index for rutile is 2.75 and 2.55 for anatase (cf. CaCO3 1,58). In compar¬ison to other pigments TiO2 exhibits also an extraordinarily high density
(3.9 g cm–3 anatase, 4.2 g cm–3 rutile). Rutile gives higher brightness (but less blue whiteness) and opacity, while anatase supports optical brightening agents better as it absorbs less ultraviolet light. Compared with the rutile form, anatase has a lower Moh’s hardness of 5.5 to 6.0 instead of 6.5 to 7 .
Even at only low addition levels, TiO2 has a remarkable effect on paper opacity due to its unmatched high refractive index and high light scattering potential. Because of its high refractive index, TiO2 is the only pigment, which also provides high opacity when the paper is wet (label paper). The extra high refractive index is also important for providing opacity in the manufacturing of decoration paper. In the production process of this specialty paper, resin penetrates completely throughout the sheet structure, thereby filling up the voids and diminishing pore structure volume with resulting reduced light scattering. Because of the extreme fineness (average particle size 0.2–0.25 mm) and the complex chemistry in applica¬tion (for example high amounts of cationic wet strength agent etc.) TiO2 filler pigments can be difficult to retain in the paper web.
Due to its high cost, TiO2 is often used in combination with extenders. These extenders are other less costly minerals (for example Al(OH)3, synthetic silicate and silica, barium sulfate), which are capable of replacing TiO2 to a limited extent while producing similar results. The effect of providing wet opacity to the sheet by TiO2, however, cannot be reached by using extenders alone. No extender matches the extra high refractive index of TiO2. The closest to TiO2 in terms of refractive index is zinc sulfide with a value of 2.37.
126.96.36.199.3 Amorphous Silicates and Silica
These are also specialty pigments, mostly used in lower basis weight newsprint and directory paper. These pigments are added in small percentages (sometimes applied in combination with regular paper fillers), to enhance paper brightness and opacity as well as absorption properties, e. g. reduction of print-through.
Amorphous silicates are produced by destabilization of soluble silicates to yield amorphous discrete particles in varying degrees of aggregation. The high specific surface area in combination with an organophilic surface is responsible for the excellent ink adsorption characteristics. Amorphous silicate also has the potential to increase somewhat the bulk of the paper sheet.
In a recent development, Mg-Al-silicate was coprecipitated with a rhombohe-dral-shaped precipitated calcium carbonate as core, as one single product. This development has already found its way into commercial application. One target is to reduce the ink print-through in lower basis weight newsprint or directory paper at lower cost than regular Mg-Al-silicates .
188.8.131.52.4 Aluminum Trihydrate (Hydrated Alumina)
This represents a specialty filler which contributes to paper brightness, ink re¬ceptivity and acts as a flame retardant. The effect on flame retardancy can be explained by the 35 % of bound water based on material weight. The bound water is released at temperatures above 150 °C. The raw material source for the produc¬tion of aluminum trihydrate (ATH) is bauxite. Bauxite is a blend of Al2O3, Fe2O3, SiO2, H2O, TiO2 and other minerals. To produce ATH it is necessary to stabilize the alumina content and to separate out the other minerals. This is done by the so-called Bayer process. After final filtration, the clear liquor of sodium aluminate is seeded with specially prepared fine crystals of ATH. This seeding causes the so¬dium aluminate to decompose to ATH or Al(OH)3 which forms a precipitate.
Some bauxite ore deposits exhibit a rather high purity and brightness. Such material can be converted directly into filler grades. The physical difference be¬tween a fine precipitated ATH and a fine ground ATH is small.
184.108.40.206.5 Other Fillers
It must be mentioned, that there are several other industrial minerals being used directly or indirectly as paper fillers. These materials include barytes, barium sul¬fate, calcium sulfite, zinc oxide, zinc sulfide, diatomaceous earth, mica, bentonite and pyropholite. Synthetic organic fillers are, for example, based on polystyrene or urea-formaldehyde. Local availability, cost and the need for obtaining certain spe¬cial properties are the deciding factors in the selection of these materials. However, the total volume is quite small in comparison to the major fillers and specialty pigments, as described above.
There is a continuous demand for technical and economic development in the various paper grades. In this context, the quality and choice of filler is also often reconsidered. Brightness, opacity and surface properties like smoothness, uni¬formity, surface strength and printability will continue to be of great importance. To address these demands, further specific pigment developments and the combined application of different filler grades can be expected. Costructuring different pigments and coprecipitation could represent suitable tools to reach some of the functional targets. Investigations on how to further increase the filler loading in various paper grades are constantly ongoing. Maintaining stiffness properties at increased loading and cost effectiveness are some of the challenges. At the already very high filler loading levels as practiced in Europe, each % point of further in¬crease represents a particularly significant success. Filling through the surface of the sheet could be an additional tool for completely new developments. Positioning the fillers at a specific location within the z-direction of the sheet (multilayering) is in an early stage of investigation. Replacing more expensive specialty pigment applications by regular fillers is always of interest. Calcium carbonate based fillers will penetrate the paper market even further, as for example currently observed in the area of higher brightness SC paper.
Much emphasis is given today on seeking the potential of nanotechnology in the area of papermaking. It remains to be seen if nano per se will provide innovative products, but there are materials available now, primarily in structured forms of PCC or surface modified GCC that provide nanoscale features on the surface of otherwise microparticles. Properties so far indicate high absorption potential, and their use in digital inkjet printing and as specialty fillers are being evaluated intensively [14, 15].
220.127.116.11 General Overview
Coating improves the surface quality of paper and board resulting in higher bright¬ness, smoothness and gloss as well as better opacity and generally significantly improved printability. Pigments are the main coating components for improving the surface properties of coated paper and board grades. Usually they account for 80–95 % of the total dry coating weight or, as a volume fraction, 70–80 % of the solid material of the coating. This is based on a pigment density of about 2.5gcm–3 and a density of the rest of the solid material in the coating of about 1.0gcm–3. In 2004 a total of approximately 18 V 106 tons of coating pigments were used worldwide in the paper and board industry, which means nearly 2/3 of the total usage of fillers and coating pigments. Kaolin (Clay) has been the biggest coating pigment in the past, but its share dropped to 42 % in 2004 while natural ground calcium carbonate (GCC) took over the No. 1 position with 51 % share. Recently precipitated calcium carbonate (PCC) with a present share of 3 % switched from the status as a special coating. Pigments also largely determine the cost of the coating. A sizeable number of white pigments are available for surface finishing in the paper and board industry. A large majority of these are of natural origin, being physically and chemically homogenous minerals formed by inorganic processes. Organic products like plastic pigments may also be used in coating colors in special cases.
Central properties of pigments are particle size and size distribution, particle shape and shape distribution, refractive index, light scattering and light absorption and density. The properties of coating color and the final coating can be influenced and explained to a great degree by these factors. The following simplifies how different quality parameters of coated paper can be improved by changing specific pigment properties:
. • better gloss by increase in platiness and/or decrease in particle size
. • higher opacity by increase in refractive index and/or decrease in particle size
. • higher brightness by decrease in light absorption
. • higher porosity and ink absorption by decrease in packing (less compact pig¬ment layer), e. g. through mixing of different particle shapes
18.104.22.168 Main Coating Pigments [16–19]
22.214.171.124.1 Kaolin Clays
Kaolin clays in paper coating have probably been used since the second half of the 19th century when paper coating was developed. Since that time they played a dominant worldwide role as a coating pigment almost until today. Kaolin is one of the most widely occurring minerals. Kaolinite, the principal constituent of kaolin is a layered aluminosilicate having the chemical formula Al4Si4O10(OH)8. Kaolin is an intrinsically valuable coating pigment because of its platy particle shape, good color (white or near white), and the relative ease with which it can be processed to a fine particle size.
126.96.36.199.2 Ground Calcium Carbonate (GCC)
GCC as a coating pigment was successfully introduced in 1973. It marked an epochal step towards lower production costs and improved paper and print quality by the successful development of “high solid” coating in the 1980s. Herewith GCC allows reduced binder requirements and drying energy savings. The development of finer-grade carbonates with particle sizes down to 99 % < 2 mm and special particle size distribution curves covers a broad spectrum of whiteness, paper and print gloss, as well as opacity. High solids coating colors offer excellent runnability even at the highest coating speeds of > 1800 m min–1. This also explains the over proportional growth of ground calcium carbonate as a coating pigment during the last three decades, whereas kaolin is stagnating and talc is showing constant growth, especially for gravure coating colors.
188.8.131.52.3 Precipitated Calcium Carbonate (PCC)
PCC allows the production of coating pigments with specific morphologies by controlled synthesis. Particle size, particle size distribution, and particle shape can be controlled. The surface properties of the calcium carbonate particles can also be changed if needed. Commercial PCC pigments typically have CaCO3 contents higher than 97 %. The remainder is MgCO3 (magnesium carbonate) and other residues. Industrially, it has been found practical to produce PCC near the paper mill by using what are called satellite plants. This saves inbound transportation costs because the necessary CO2 gas (40 % of the final product weight) can be taken from a local source and because calcium oxide can be delivered to the PCC satellite plant in a dry form. Outbound savings are possible as well because the produced coating or filler pigments can be delivered in a slurry form to a paper mill, possibly through a pipe.
184.108.40.206 Special Pigments
Talc as a coating pigment was introduced industrially in Finland and France in 1982. Since then, talcs have become widely used in light weight coated (LWC) gravure and coated fine papers in offset formulations. Recently another use has been found for talc in the coating of special papers. In some applications, e. g. coating paper for gravure printing, talc can technically replace clay if it is economi¬cally viable and vice versa due to the similar platy form of the particles. The charac¬teristics generally differentiating talcs from delaminated Georgia kaolin clays (US) or Cornish kaolin clays (UK) are hydrophobicity, low cohesion between the crystal
2.2 Non-fiber Raw Material
layers inside the talc particles, resulting in extremely low friction coefficient, soft¬ness and low abrasivity in spite of a large particle diameter. The brightness of European coating talcs varies from ISO brightness 82 to 88, depending on the origin.
Gypsum consists of calcium sulfate crystallised with different degrees of hydra¬tion, all of which can be referred to as gypsum. However, in most cases, gypsum refers to calcium sulfate in the dihydrate form, and the other forms are distin¬guished from each other by additional names (e. g., hemihydrate, calcined gyp¬sum, and stucco gypsum). The rate of solubility of gypsum of pigment fineness is high, so the solubility easily reaches equilibrium in, for example, the water circula¬tion of a paper machine. A saturated gypsum solution contains approximately
2.1 g l–1 of CaSO4, which equals 2.5 g l–1 as dihydrate and 580 mg l–1 as a calcium concentration. Gypsum in itself is a neutral salt that, unlike carbonate, does not buffer the pH of circulation water. When it comes to solubility, it makes no dif¬ference whether gypsum pigment is used in an acidic or neutral papermaking process. The major advantages of gypsum pigment include brightness, bulk and offset printability. Additional optical brighteners used with gypsum are very ef¬fective.
220.127.116.11.3 Plastic Pigments
Plastic pigments are used in paper coatings to provide a surface with the desired appearance and printability. If properly chosen and formulated, these pigments provide a coating with surface smoothness, brightness, opacity, as well as a balance of ink holdout and ink receptivity upon which to print. Organic pigments, com¬monly referred to as “plastic pigments”, are used as partial replacements for in¬organic pigments to improve the optical and print properties of coated paper and paperboard. There are two general classes of plastic pigments used in the prepara¬tion of coatings for paper and paperboard: solid bead and hollow sphere. Both are available in a variety of particle sizes and compositions and, in the case of hollow spheres, in a range of void volumes. All plastic pigments are supplied as polymeric particles dispersed in water. In the case of hollow-sphere pigments, those particles are water-filled spheres. During drying the water diffuses through the shell, leav¬ing an air-filled core – hence the term “hollow sphere”. Plastic pigments are used in paper coatings wherever improvements in finishing efficiency, sheet gloss, or print gloss are important. They are used extensively in coated folding boxboard, woodfree coated printing papers, and they are also used in LWC and ULWC pa¬pers.
18.104.22.168.4 Satin White
Satin white (calcium sulfoaluminate) is still of some importance in paper coating. This pigment has very fine particles; it is extra white, and has a low density. It increases ink absorption and gloss. The disadvantages of satin white are its sensi¬tivity to increases in temperature and decreases in pH, and its high adhesive de¬mand.
22.214.171.124 Additional Pigments
126.96.36.199.1 Titanium Dioxide
Titanium dioxide pigments show extremely high refractive indices. This, and their whiteness, i. e., their high reflectance in the visible region of light, as well as their optimal particle size, make them the most effective white pigments. Titanium dioxide exists in three crystal forms: anatase, rutile, and brookite. The first two are stable and therefore of commercial importance. Titanium dioxide pigments are used in paper coatings to increase the opacity of coated paper. To ensure good optical efficiency of titanium dioxide in coatings, TiO2 pigments must be properly dispersed in the coating color. Undispersed aggregates or agglomerates diminish the optical efficiency of TiO2 pigments. Rutile slurry pigments are usually the most effective titanium dioxide grades in paper coatings because they have the highest TiO2 content and refractive index, as well as optimal particle size and particle size