Filler and Coating Pigments for Papermakers - WAKAOLIN

FILLER AND EXTENDER USES

Filler and Coating Pigments
for Papermakers

Ian Wilson

INTRODUCTION Table 1. Paper and paperboard production by region in 2003

Historians attribute the Egyptians in the Nile Delta with preparing Region Production, kt % of World Production
the first type of paper, from a marsh grass called Cyperus papyrus. (rounded up)
They cut the plant’s stem into thin strips, softened them in the river, Asia 110,585
and then pounded them into thin sheets that were then left to dry in Europe 104,093 32
the sun. The resulting sheets were ideal for writing on, and Egyp- North America 100,280 31
tians, Greeks, and Romans used them for record keeping, spiritual Latin America 30
texts, and works of art. Papyrus is the basis of the word paper. The Australasia 16,254
father of papermaking is T’sai Lun, who in 105 AD experimented Africa 3,871 5
with a wide variety of materials and refined the process of macerat- 3,672 1
ing the fiber of plants until each filament was completely separate. Total 1
He mixed the individual fibers with water in a large vat with a 338,755 100
screen that was submerged and then lifted up through the water,
catching the fibers on its surface; when dried, this thin layer of Adapted from Paperloop 2004.
intertwined fiber became what we today call paper. The art of
papermaking spread to the Middle East and to Egypt, where it of production (Paperloop 2004), led by the United States (24%),
replaced papyrus in the 9th century, followed by Morocco and then China (12%), Japan (9%), Canada (6%), Germany (6%), Finland
Spain (about 1150) and France (1190). (4%), Sweden (3%), South Korea (3%), France (3%), Italy (3%),
and all other countries (27%).
Industrial minerals play a major part in the manufacture of
modern paper. Originally, because they were less expensive than The growth of the Chinese market from 1992 to 2003 was dra-
fiber, minerals such as calcium carbonate (chalk) and kaolin were matic, especially when compared with Japan (Figure 1): the growth
used as fillers to reduce production costs. Although cost is still an rate was 13.1% in China, and the rate decreased by 1.3% in Japan.
important factor, minerals have become “functional fillers” that The Chinese growth was mainly driven by major companies invest-
impart specific properties to paper, such as improved printability, ing in new paper mills in China (including Asia Pulp and Paper
brightness, opacity, and smoothness. In paper coating, minerals are Company [APP], UPM-Kymmene, Stora Enso, and Oji Paper) com-
used as white pigments to conceal the fiber, thereby improving bined with the fast-growing, apparent consumption per capita—in
brightness, whiteness, opacity, and smoothness. 2003 paper consumption was 36 kg in China, an increase of 24%
over 2002. This is still well below the levels of the United States
PAPER INDUSTRY STRUCTURE with 301 kg, the United Kingdom with 207 kg, as well as many
other countries; and just ahead of Indonesia and India with 23 kg
World Production and 6 kg, respectively.

In 2003, world production of paper and paperboard was 339 Mt Fibrous Plant Materials Used in Papermaking
compared with 239 Mt in 1990. A split of this production for 2003
(Table 1) indicates similar levels from North America, Europe, and Although almost any plant material can be used for papermaking,
Asia. Between 1990 and 2003, North America’s share of the world very few are used because a number of factors determine what
market fell from 37% to 30%, whereas Asia increased from 25% to makes a good raw material:
32% as both Western and Eastern Europe decreased from 33.7% to
31%. The world average annual growth in the production of paper • The plant must be abundant, inexpensive, and, if a waste prod-
and paperboard during the period from 1990 to 2002 was +2.8%, uct, of little use to others. It must grow in an accessible place
with Asia showing the largest growth of +5.2%. Of the 339 Mt and should grow quickly.
(Table 1), Asia is now the leading producer with 32%, just overtak-
ing North America and Europe with 30% and 31%, respectively. • It must contain a high proportion of cellulose fibers, and its
The top ten producing countries in the world now account for 73% structure must allow the fibers to be isolated from the rest of
the plant material with reasonable ease and without undue
expenditure of chemicals or heat.

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1288 Industrial Minerals and Rocks

45,000 2002–2003 +13.1% caustic soda technology. If successful, Malaysia has the potential to
40,000 produce 3 Mtpy of pulp from EFB alone.

35,000 Main Paper Grades

Ktpa 30,000 2002–2003 –1.3% Table 2 summarizes the main types of paper grade, their fiber com-
25,000 position, pigment filler and coating loading, and their end uses.
Japan Coated paper is coated on one or both sides with a mix of clay and
carbonates to create a high quality printing surface. Coated paper
20,000 can be fine, lightweight, medium weight, or machine finished.
Uncoated fine paper is used principally for printing and writing.
China Woodfree, freesheet, or fine paper is paper used by the graphic
industry for writing, including office paper such as photocopying
15,000 and laser printing paper; these may be coated or uncoated.
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
Leading Producers of Paper and Paperboard
Adapted from Paperloop 2004.
The paper industry continues to consolidate, with the top 10 com-
Figure 1. Growth in paper market for China versus Japan, 1993 to panies accounting for an increasing proportion of world paper and
2003 board capacity. Production by the 10 leading companies in 2003
accounted for 27.9% of global output (Table 3).
• The fiber itself when isolated should be suitable for paper-
making, which means that it should be long and strong, and In 2001, Finnish-owned Stora Enso increased its capacity to
develop strength on beating. At the same time, it should be 15.1 Mtpy and International Paper closed down 2.2 Mtpy of its
capable of being bleached to a good color without undue loss less-efficient capacity; in the same year, UPM-Kymmene acquired
of strength. Haindl, a leading European producer of publication paper. Publica-
tion paper includes newsprint, coated, and uncoated papers
Of the many species of plants in the world, the following meet (mainly lightweight coated [LWC] and SC). This further consoli-
the above requirements and are commonly used in papermaking: dation increased UPM-Kymmene’s capacity from 8.285 Mtpy to
11.705 Mtpy by 2003. UPM-Kymmene is now the world’s largest
• Seed hairs—cotton manufacturer of magazine paper (LWC and SC) with annual
capacity of 5.465 Mt, or about one-quarter of the global market for
• Bast fibers—flax, hemp, jute, ramie magazine paper.

• Wood fibers—coniferous and deciduous woods MAJOR PIGMENTS USED IN PAPERMAKING

• Leaf fibers—esparto, manila, sisal The major pigments used in printing and writing paper (P&W;
includes SC, MFC, WFU, and WFC) are calcium carbonate (pre-
• Grasses—bamboo, bagasse cipitated calcium carbonate [PCC] and ground calcium carbonate
[GCC]), kaolin, and others (talc, TiO2, and others). Harris (2004)
Potential new pulp sources are being developed all the time. estimated that 30 Mt of these major pigments were used worldwide
Malaysia, for example, announced in March 2003 that the world’s
first oil-palm–based pulp plant would be set up in Sabah (Borneo)
with the capacity to produce 25,000 t of pulp. A new pulping
method using empty fruit bunches (EFBs), which are currently a
waste product from the palm oil industry, will be developed using

Table 2. Paper grades, fiber composition, pigment and coating loading, and end uses

Paper Grade Fiber Raw Material Pigments End Uses
De-inked pulp and/or mechanical pulp
Newsprint De-inked pulp and/or mechanical pulp Filler loading up to 12%, originally Newspapers, inserts, flyers (advertising)
from de-inked pulp
Specialty newsprint Mechanical and chemical pulp Filler loading <10%; specialty Newspaper supplements, newspapers,
Books, papers pigments can be used as well books, directories, advertising
High brightness
Filler loading up to 35% Multicolor magazines, catalogs,
Supercalendered (SC) papers supplements, inserts, advertising materials;
SC B, A, and A+ grades Filler loading up to 10%; coating used in gravure and offset printing
25%–30% of paper weight
Coated mechanical papers Mechanical and chemical pulp Magazines, catalogs, supplements, books,
(also called machine-finished Chemical pulp Filler loading up to 10%; coating advertising materials
coated [MFC] paper) typically from 20%–35% of paper
weight Office papers, writing papers, envelopes,
Woodfree uncoated (WFU) direct mail, magazines, books, advanced
papers Filler loading up to 25% materials
Magazines, brochures, direct mail, annual
Woodfree coated (WFC) Chemical pulp, possible to use some Filler loading up to 15% and reports, books, advertising materials;
paper; also coated fine paper higher quality books, reports
(art printing paper) chemi-thermomechanical pulping (CTMP) double/triple coating Label papers, label release papers, food
Specialty papers wrapping, packaging
Chemical pulp Filler load and coating dependent on Sacks, bags, wrapping and packing,
Kraft papers Chemical pulp grade envelopes

No pigments

Source: Haarla 2002.

Filler and Coating Pigments for Papermakers 1289

in 2002 (Harris 2004), and Figure 2 shows the breakdown of use. Table 3. World’s 10 leading producers of paper and paperboard in
Of the 30 Mt of pigment used, 18 Mt was for coating pigments and 2003
the split was GCC (53%), kaolin (35%), PCC (9%), and others
(3%; this was mainly talc, titanium dioxide, and a few others). The Position Company Output, ktpy
remaining 12 Mt is used as a filler.
1 Stora Enso 13,960
ROLE OF PIGMENT PROPERTIES IN PAPERMAKING 2 International Paper 13,844
3 UPM-Kymmene 10,232
For the papermaker, the critical pigment properties are 4 Svenska Cellulosa (SCA)
5 Georgia Pacific 9,725
• Physical properties (also includes optical properties such as 6 Weyerhaeuser 8,843
brightness, yellowness, and shade; coverage; ink absorption; 7 Oji Paper 8,558
and others) 8 Nippon Unipac Holding 7,900
9 Smurfit Stone Container Corporation 7,835
— Particle size and shape 10 Abitibi Consolidated 7,307
6,421
— Particle-size distribution Total 94,625

— Aspect ratio (platiness or blockiness—kaolin can be platy Adapted from Paperloop 2004.
and blocky but calcium carbonate is generally rhombohe-
dral [hexagonal] for marble [crystalline limestone]) Talc, TiO2, Calcium Carbonate (CC)
and Others 55%
• Pigment moisture (for pigment handling)
7% 16.5 Mt
• Pigment hardness—abrasiveness (wear on wire, doctor and 2.1 Mt
slitter wearing) GCC
Kaolin 38%
• Residues, impurities, contamination—origin can be from pro- 38% (70% of CC)
cessing the industrial mineral, from transportation, and from 11.4 Mt 11.5 Mt
other sources (“runnability”* in paper machine or coating or
calendering causes streaks and breaks, which are expensive) PCC
17%
ROLE OF FILLERS IN PAPER (30% of CC)
5 Mt
Fillers are highly desirable in printing papers because they increase
the opacity, raise the brightness, and generally improve printing Source: Harris 2004.
properties. The main types of mineral filler for acid papers are talc,
hydrous kaolin, calcined kaolin, precipitated silicas and silicates Figure 2. World pigment use in P&W paper (2002)
(PSS), and titanium dioxide. For neutral/alkaline papers, talc,
hydrous kaolin, calcined kaolin, PSS, titanium dioxide, GCC, and quality. An understanding of filler interactions with retentions aids,
PCC are used. The use of fillers is important when opacity is sizing agents, cationic starch, and the dynamics of the wet and
needed at a low-basis weight, and they are invaluable in packaging forming systems is required (Jopson and Moore 2004).
grades where low permeability is combined with opacity to protect
food from light. There are many fillers: barite, GCC (based on The main driving force for filler in fine papers is to substitute
chalk, limestone, and marble), PCC, kaolin, pyrophyllite, mica, more expensive fiber with filler. No filler is capable of producing
gypsum, plastic pigment, satin white, alumina, and titanium diox- maximum light scattering for brightness and opacity without hav-
ide. The properties of the filler derive from the ability of the filler ing any detrimental impact on wet web strength and sheet physical
particles to refract and backscatter light through the surface of the properties. The best pigments for brightness and opacity debond
sheet. If the filler is not evenly dispersed through the sheet and floc- fibers the most because of their inherent high surface area.
culates in small clumps, then the optical efficiency of the filler will
be reduced. If the z-direction distribution of the filler is uneven, Kaolin, calcium carbonate (GCC and PCC), and talc are the
then the sheet may appear two-sided. The whiteness of the filler most widely used mineral fillers, with regional variations depend-
relates to the dominant wavelength of the light. Excluding some ing on local resources available. Table 4 lists the pigments used as
uncoated book-publishing grades, papers tend to have a blue white- filler in different printing paper applications. In the United States,
ness. This requires adding a blue or violet dye to shift the shade of PCC is widely used as filler because of the wide availability of
the paper into the desired region of the spectrum. limestone; the lime produced from limestone is converted into PCC
at a satellite plant adjacent to a paper mill. Filler pigments must
Because dyes reduce brightness, high-brightness filler must be have a high degree of whiteness, a high index of refraction, small
used or optical brightening agents added for premium grade, such particle size, low solubility in water, and low specific gravity. It is
as company stationery and direct mail papers. The importance of also important that the filler be chemically inert to avoid reactions
avoiding filler flocculation emphasizes the point that fillers are not with other components in the sheet and in the papermaking system.
simply inert optical entities but interact with other additives, not The filler should contain a minimum of impurities, and the grit con-
only in terms of their own distribution but also to influence sheet tent must be low to avoid excessive wear of the wire and other pro-
structure such as formation, bulk, pore structure, and surface topog- cessing equipment such as cutting blades. Furthermore, unless the
raphy (texture). Aside from their optical effects, fillers or filler filler has very unusual properties, it must be inexpensive.
blends can be used to improve aspects of product uniformity and
Hydrous Kaolin
* Newspapers and magazines require long runs on the printing presses, and
several factors affect how well a paper will run on the press. Runnability Kaolin was the usual filler used in Europe and the United States up
describes a paper’s ability to hold ink on its surface consistently and to until the 1990s when the use of PCC in the United States and GCC
absorb ink uniformly, along with its dimensional stability and its surface
texture.

1290 Industrial Minerals and Rocks

Table 4. Filler pigments in different printing paper applications

Type of Paper Ash Content, % Clay, % GCC, % PCC, % Talc, %
Can be used
Newsprint* <15 <10 <10 <20
<20 <10
Machine-finished specialty (MFS) <15 <10 <10 <10 <10
<10 <10
Supercalendered offset (SCO) <35 <35 <10 <10 <10
<22
Supercalendered rotogravure (SCR) <35 <35 <10 <15 Pitch talc

Lightweight coated offset (LWCO)† <10 <10 <10

High-brightness lightweight coated (HB LWC)† <12 <10 <10

Medium-weight coated (MWC)† <10 <10

WFU† <22

WFC† <15 <15

Adapted from Haarla 2002.
* Most of the ash comes from recycled fiber in newsprint, if the main raw material is recycled paper.
† A part of the ash comes from coated broke in coated paper.

in Europe emerged. The main use of kaolin fillers in the last decade GCC because its particles pack less efficiently on the surface com-
has been in engineered products. The new range of filler content for pared to the platy and broad particle distribution kaolin (Bown
SCA paper is based on the production of a high-aspect-ratio (often 1991; Lorusso 2002; Hiorns and Nesbitt 2003). Although it is clear
>40) filler and a controlled particle-size distribution of about 50 wt % that coating color formulation and coating conditions have poten-
<2 µm and a minimum of 5% >10 µm. These high-aspect-ratio tially greater influence on the coated paper quality, the filler content
kaolin products, with a brightness of ISO 82, provide good opacity of the base paper has a profound effect on coating runnability and
to the base paper. They are particularly useful in SCA papers where paper quality.
the platy kaolin with calendering gives a sheet that can compete
with LWC papers. The development of a range of platy kaolins has Calcined Kaolin
been at the expense of the less expensive kaolin filler.
Calcined clay is used in small amounts in newsprint and as an addi-
Over the past two decades, additional processes have been tion to other fillers to replace more expensive titanium dioxide to
developed to improve the quality of the kaolin from the deposits in improve opacity. With the onset of calcined clay as filler in news-
Cornwall in the United Kingdom. These deposits have been well print, it is common now to have newspapers with color photographs
documented in the literature (Exley 1959, 1976; Sheppard 1977; not visible through the sheet because of its increased opacity.
Halliday 1980; Allman-Ward et al. 1982; Alderton and Rankin
1983; Bray and Spooner 1983; Allman-Ward et al. 1985; Bristow Calcium Carbonate
and Exley 1994; Bristow 1995; Manning, Hill, and Howe 1996;
Scott, Hart, and Smith 1996; Psyrillos, Manning, and Burley 1998; The conversion from acid to alkaline papermaking techniques and
Psyrillos et al. 1999; Bristow et al. 2000; Thurlow 2001; Bowditch, the demand for brighter and bulkier paper have been the main driv-
undated). For brightness enhancement, superconducting magnets ers behind the increased preference for calcium carbonate over
have been introduced alongside high-intensity magnetic separators kaolin. This switch has eroded the share of the market held by kaolin
(HIMSs), froth flotation, selective flocculation, and selective separa- because paper producers partly substitute its use with calcium car-
tion processes to remove abrasive materials such as quartz and feld- bonate, which is less expensive and often brighter. Neutrally sized
spar. The most significant processing development over the last paper can have higher mineral filler loadings than acid-sized paper,
decade, however, has been the production of delaminated clays from so calcium carbonate slurries are preferred over kaolin slurries
vermiform or stacky kaolinites so that approximately 60% of the because of their higher solids content. Table 5 compares the proper-
processed Cornwall kaolin is now delaminated. The flow process at ties of kaolin and calcium carbonate, both precipitated and ground.
low-abrasion plants involves flotation to remove contaminants (feld- Although calcium carbonate is generally brighter than most com-
spar, quartz, and mica) and a sand grinder (known as such because it mercial kaolin, new grades of kaolin for use in paper coating have a
once used a round resistant sand as the grinding medium; a ceramic brightness of more than 90%.
bead is now the preferred medium) for delamination.
Ground Calcium Carbonate
The aspect ratio of the resultant delaminated kaolin is an impor-
tant parameter, and Imerys developed a stop-flow conductivity mea- Table 6 lists some of the advantages of GCC compared to kaolin
surement instrument that gives a shape factor (called a factor because usage in alkaline woodfree papermaking. Kaolin was once the most
aspect ratio is not actually measured). The method is known as PAN- widely used filler in paper manufacture, but the last two decades
ACEA (particle assessment [by] natural alignment [and] conductivity have seen a steady increase in the use of calcium carbonate. The
effect analysis). Measurements of shape factor are made online and conversion from acid to alkaline papermaking and the demand for
help to control the process. Historically, aspect ratio was measured brighter paper have been the main reasons for this change.
using the transmission electron microscopy (TEM) platinum shadow-
ing technique, which was a very lengthy process relying on the parti- Precipitated Calcium Carbonate
cle thickness being proportional to the shadow length—the thinner
the platelet of kaolin, the narrower the shadow. World PCC capacity is approximately 6.2 Mtpy, of which almost
three-quarters is used in paper. Nearly all of the PCC in paper is
The base paper pigment has an important bearing on the sub- used as a filler, and the largest market is the United States. As
sequent coating pigment application. The influence of the base papermakers transferred to alkaline technology, the number of sat-
paper structure is more noticeable when the paper is coated with ellite PCC plants has increased significantly since the first U.S.
plant opened in 1986. By 2000, some 80 plants had been installed

Filler and Coating Pigments for Papermakers 1291

Table 5. Comparison among properties of kaolin, PCC, and GCC in papermaking

Property Kaolin GCC PCC

Brightness 80%–85% (some 90%) >90%–96% 90%–97%
Particle size Naturally 2 µm Requires grinding Manufactured fine
Opacity Excellent Moderate at high load High at high load
Loading levels 20%–30% 20%–30% Limited to 20%
Sheet strength Good Excellent Moderate
Bulking Moderate Good Good
Absorption Low Low High
Chemical reactivity Inert Unstable in acid environments Unstable in acid environments
Flexibility Filler/coating Alkaline-only filler/coating Mainly filler
Processing Extensive Grinding/sizing Energy intensive
Availability Restricted Geologically plentiful Satellite plants
Price Low (North America) Low (Europe) Based on cost-effectiveness

Source: Harben 1998.

worldwide, with almost 50 in North America alone; the industry is Table 6. Advantages of GCC compared to kaolin in alkaline
dominated by MTI with 54 plants, followed by Huber Engineered woodfree papermaking
Materials with 12 and Imerys with 6.
Paper
PCC manufacturing in Europe is shared between five major
producers with an estimated total production capacity of more than • Brightness—GCC has a higher brightness than clay; lower optical
2.0 Mtpy in 2003, including products for paper and other appli- brightening agent (OBA) demand without alum
cations. The 5 companies are Huber Engineered Materials with
6 plants, Specialty Minerals Inc. (SMI) with 13 plants, Omya with • Strength—tends to be higher without high amounts of alum; filler loadings
4 plants, Solvay with 6 plants, Schaefer Kalk with 3 plants, and higher
Imerys with 1 plant. At the time of this printing, Huber is in the pro-
cess of selling their 6 PCC plants. The competitiveness of an on-site • Permanence—absence of alum benefits aging properties of paper
PCC plant is primarily influenced by the size of the plant (economics
of scale) and by the CO2 content in the gas source from the host Process
paper mill. This means that an on-site satellite PCC plant has to have
a certain minimum size and be supplied with a gas of a certain mini- • Refining—30% energy savings in refining under slightly alkaline conditions
mum CO2 content in order to be economically viable. For example,
an on-site PCC plant has to process a minimum of 20,000 tpy to be • Drainage—rhombohedral shape of GCC drains better than platy kaolin
economically justified, corresponding to the demand from paper pro-
duction of 100,000 tpy of uncoated paper with a 20% filler level. • Drying—dries better than clay because of slightly more hydrophobic nature
of GCC
In Europe, PCC has shown by far the strongest growth rate
and since 1995 has continuously taken market share from GCC and • Water—better drainage and lower bacteriological activity reduce water
other fillers. The growth of PCC is likely to continue with further demand
penetration as filler into the WFU paper segment, although at a
slower rate as the market matures. • pH stability—strong buffering action; GCC keeps pH level stable at 7.2 to
8.4
Some paper mills that had been using GCC derived from
chalk in Europe were among the first to capitalize on the added soft, organophilic, chemically inert, and platy—are reasons why it
brightness that could be achieved using PCC. Several PCC satellite today is used as filler in many different kinds of paper. The organo-
plants have come onstream in Asia since the mid-1990s, including philic surface helps reduce dye consumption and two-sidedness in
plants in Thailand, Indonesia, Japan, China, South Korea, and colored paper. Two-sidedness in colored paper is the term used to
Malaysia. New satellite plants have also been built in South Africa describe the difference in color characteristics between the top side
and in South America. and the bottom side of the sheet. The difference can be either in
shade or in strength, or both. Contributing factors are finish, chemi-
PCC is now making some inroads as filler in groundwood (SC cal additives, points of addition, order of addition, and colorant
and LWC) papers, and this represents the largest remaining poten- characteristics. With careful control of all these factors, however,
tial market. This market is currently dominated by kaolin and talc, two-sidedness can be eliminated to result in a uniform sheet.
however, especially in European rotogravure paper. MTI has
invested much research into developing acid-tolerant PCC, allow- The relatively coarser particle-size distribution of talc (com-
ing its entry into the groundwood paper sector, for which it now has pared with other pigments) leads to better retention in the sheet and
several satellite plants. lower effect on the paper’s strength properties. Improved dewatering,
less wire abrasion, higher retention, a longer life of cutting knives,
Talc and fewer core breaks in SCR printing are typical advantages in the
process provided by talc. In addition, the pitch and sticky control
The choice of filler in paper is driven by cost reduction and quality function reduces tacky deposits and improves paper machine run-
improvement. For talc, which is more expensive than some other nability. The paper quality itself is influenced by talc with increased
pigments, improving the paper quality and the papermaking pro- smoothness, better printability, deeper color in colored papers, and a
cess itself is the dominate driving factor. The properties of talc— lower impact on strength properties than other fillers.

Other Filler Pigments

Other mineral systems are employed—for example, titanium diox-
ide. Titanium dioxide is used more as a specialty chemical to impart

1292 Industrial Minerals and Rocks

specific end-use properties rather than as filler because of its cost. It medium). Even in conventional offset lithographic grades, careful
is used primarily for its opacity properties, particularly in light- development has been undertaken over many years to optimize the
weight papers such as paper used for bibles. ink transfer and drying characteristics of the coatings. In electro-
photographic printing grades, controlling surface resistivity and
A new specialty filler in newsprint is Zeocros PF, produced by toner adhesion is critical in coatings development. In the Indigo
Ineos Silicas, a leading global supplier of silica, silicate, and zeolite unit with its wet toner system, surface chemistry, specifically acid-
products. Zeocros PF has a high ISO brightness of 97; small, angular base interaction, is critical in image transfer to the paper surface.
particles with efficient light-scattering surfaces; and a tight particle- Matching the pigment formulation to the imaging systems is an
size distribution. In newsprint it raises opacity, improves printability, increasingly important part of product development, especially in
reduces print through/strike through, and enhances brightness. A 1% higher value coated paper grades.
filler addition will give 0.6 points of opacity increase. Paper with
Zeocros also prevents ink penetration into the paper. The coating formulation must also be optimized for runnabil-
ity on a high-speed coating line. This requires careful attention to
Gypsum, or calcium sulfate, is an abundant mineral formed nat- rheology (viscosity at high and low shear). Such parameters are
urally and from industrial by-products (such as flue gas desulfured important not only to coating transfer and metering but also to uni-
gypsum and phosphogypsum). It generally has the disadvantage of formity of the coating in terms of coverage of the fiber in the base
high solubility, and it also tends to plug machine felts. Calpak C20 is sheet. This is vital to print quality. The rheology of the coating is
filler derived from gypsum in Spain and is being developed by determined by the interactions between the pigment particles them-
Kemira. It is a bright filler for a wide range of pH and sizing systems. selves and between pigment and binder under the influence of coat-
The production of filler gypsum, however, is limited in the world. ings solids content and temperature. Common binders include
styrene-butadiene rubber (SBR) lattices, starch, acrylics, or vinyl
Diatomaceous earth is not filler in the sense of the other pig- polymers. The rheology can be manipulated by water-phase thick-
ments but is normally used for pitch control (0.5%–1.5% on pulp), eners such as carboxyl methyl cellulose (CMC) and associative
to improve formation, and to increase the rate of drainage, though thickeners such as alkali-swellable acrylates or hydrophobically
one serious disadvantage is its abrasiveness. modified urethanes.

THE ROLE OF PIGMENTS IN PAPER COATING In most paper coating, the pigment concentration is higher
than the critical pigment binder concentration (CPVC) that is famil-
Application of Coating Pigments iar in paint formulations. A blade coating for offset lithographic
printing would contain 100 parts pigment to 14 parts binder at
A coating layer on a paper surface can bring the following improve- 60%–65% solids. A coating for air knife or rod coater application
ments to the paper sheet: would be about 30%–40% solids. The objective is to create a
microporous coating than can facilitate ink transfer. For folding car-
• Cover the base paper fibers to give a uniform surface ton board, acrylic and polyvinyl acetate binders are used to increase
porosity to permit the use of adhesives on the coated surface.
• Make the paper whiter Exceptions to the higher pigment binder ratio are found in barrier
coating. Here pigments are used not for their optical properties but
• Contribute to the paper’s opacity to increase the tortuosity of the diffusion path of oils, grease, or
water through the barrier. Pigments with high aspect ratios such a
• Give the desired finish—gloss, silk, or matte platy clay and talc find applications in this area. The addition level
is generally about 30–40 parts pigment to 100 parts emulsion poly-
• Give the desired printing properties mer, well below the CPVC. A balance has to be struck between
adding pigment to boost barrier properties and preserving coating
A coating mixture, known as a coating color, is normally a mix- elasticity, so that the paper and board can be folded without crack-
ture of a mineral with a binder to stick the mixture to the paper, a vis- ing the barrier layer.
cosity modifier to assist in the application to the paper surface, and
often other chemicals that improve the shade of the coating and its Table 7 lists pigments used in the coating process and their
printing properties. An important characteristic for any coating color basic properties. Kaolin and GCC are the major coating pigments,
is good rheological response during metering and application and an accounting for some 90% of the total. Basically, GCC and kaolin
ability to retain water during application. An ideal coating mixture are blended in a wide range of differing coating formulations,
needs to be fluid during application, yet immobilize quickly after depending on the type of paper being manufactured.
metering. An ideal coating should cover all the paper fibers. Fiber
coverage is often achieved by using high coatweights, coarse parti- Kaolin
cles, platy particles, and coating mixtures that immobilize rapidly.
Kaolin has a platy morphology that is still required for a large num-
In paper coating, minerals are used as white pigments to con- ber of paper applications, particularly in lightweight coated papers.
ceal the fiber, thereby improving brightness, whiteness, and opac- The trend in recent years has been to combine different minerals in
ity, as well as smoothness. If applied by a blade coater, the one coating formulation. Kaolin can be mixed with GCC, with
pigmented layers impart a fairly constant surface layer, but of vary- PCC, and more recently, with talc, to obtain improved performance.
ing depth as the surface voids and pits of the base sheet are filled in. If a choice is to be made between kaolin and GCC for coating, the
There is a growing understanding that the filler in the base paper papermaker considers the solids percentage (the higher the solids,
can have a great influence on the behavior of the coating pigment. the less drying of paper necessary), paper brightness, paper opacity,
Application methods such as the air knife and, to a degree, the film fiber coverage, paper gloss, and print gloss. For high-brightness
press or metering size press provide a coating of more uniform paper, GCC is used; but for fiber coverage, paper gloss, and print
thickness, which follows the larger-scale surface contours of the gloss, the platy nature of kaolin is preferable. Kaolin is widely used
sheet to give a varying surface level. Such contour coating methods in paint as an extender, and the calcined grades give higher opacity
are particularly useful in applying a ground coat to uneven sub- than a hydrous type. There are regional trends, with the United
strates such as folding carton board and corrugating liners.

Coating using the size press is a growing practice, producing
paper that bridges the gap between uncoated and full-blown blade-
coated products. An extension of this is using high surface area sil-
ica or calcium carbonate pigments to provide dye-receptive coat-
ings for inkjet paper (pigment acts as an acceptor of the image

Filler and Coating Pigments for Papermakers 1293

Table 7. Properties of pigments used in coating paper

Pigment Composition Refractive Specific Dry Brightness, Average Particle Size Crystal Form
Index Gravity % D50, µm

Kaolin Al2O3•2SiO2•2H2O 1.55 2.65 70–91 at least 70% <2 Pseudo-hexagonal
Calcined clay Al2O3•2SiO2 1.62 2.70 90 ~2, aggregate Chunky aggregate
Natural GCC CaCO3 1.49–1.66 2.72 Rhombohedral
PCC CaCO3 90–96 0.8–1.5 Scalenohedral (C)
Calcite (C) 1.49–1.66 2.72 >95 0.1–0.2 Rhombic (C)
Talc Aragonite (A) 1.53–1.68 2.94 Acicular (A)
Gypsum 3MgO•4SiO2•H2O 2.75 85–90 ~50% <2 Monoclinic, platy
Titanium dioxide CaSO4•2H2O 1.57 2.34 85–90 at least 70% <2 Monoclinic, prism
TiO2, rutile (R) 1.52 4.20 97–98 Tetragonal
Alumina TiO2, anatase (A) 2.70 3.90 98–99 0.2–0.5 Tetragonal
Satin white Al(OH)4 2.55 2.42 98–99 0.2–0.5 Monoclinic, platy
Blanc fixe Calcium sulfo-alumina complex 1.57 1.55 0.3–1.0 Acicular
Zinc sulfide BaSO4 1.46 4.3–4.5 >90 ~90% <2 Orthorhombic
Zinc oxide ZnS 1.69 3.98 98 0.2–2.0 Generally hexagonal
Plastic pigment ZnO 2.37 5.65 97–98 0.3–0.5 Hexagonal
Polystyrene 2.01 1.05 97–98 0.3–0.5 Spherical
1.59 >97 0.1–0.5

Adapted from Dean 1997.

States still relying dominantly on kaolin for coating, followed by Table 8. Summary of some No. 1 and No. 2 properties based on
PCC and GCC. In Europe and Asia, the trend has been toward brightness and particle size distribution of coating clays from the
GCC, no doubt because of the proximity of high-quality marble United States, Brazil, and Australia
deposits in such places as Carrara in Italy and Ipoh in Malaysia.
Product TAPPI Brightness, GE % Particle Size,
The world kaolin market of high-quality beneficiated kaolin (unless stated otherwise) % <2 µm
was estimated at 25 Mt for 2003 (Wilson 2003, 2004b).The major
producing kaolin companies worldwide (Table 8) are led by Imerys U.S. Clays 90–94
with 25% of the market. Leading kaolin-producing countries are 96–100
the United States, mainly based on the sedimentary deposits in No. 1 high bright 90–92 90–94
Georgia, with 36%; United Kingdom, 10%; Brazil, 9%; and other No. 1 fine high bright 90–92 95–99
countries, 45%. Brazil has shown the most significant growth and is No. 1 86.5–88 90–100
expected to soon overtake the United Kingdom. Large reserves of No. 1 fine 87–89 80–84
high-quality coating kaolin discovered in the Amazon Basin have Range of No. 1 products 86.5–92 80–84
been developed over the past 20 years. These deposits are all sedi- No. 2 high bright 90–92 80–84
mentary in origin and are widespread throughout parts of the Ama- No. 2 85.5–87 54–62
zon Basin. The main operations in the Amazon Basin are CADAM Range of No. 2 products 85.5–92 75–81
(now owned by Companhia Vale do Rio Doce [CVRD], which is Coarse delaminated 85–87 83 minimum
the largest exporter of iron in the world); Para Pigmentos SA Delaminated 88–90 54–83
(PPSA; now 100% owned by CVRD); and Rio Capim Caulim Delaminated high glossing 88–90
(RCC; 100% owned by Imerys). In 2005, these three companies Range of delaminated clays 85–90 98
had an installed capacity of 2.25 Mtpy, split between CADAM 80
(0.8 Mt), PPSA (0.6 Mt), and RCC (0.85 Mt). Sales in 2001 Brazilian (Amazon)
brought in revenue of US$200 million. Proven reserves of kaolin 90
are put at >500 Mt, with CADAM having 270 Mt of ultrafine clay CADAM—Premier clay 89 (ISO) 90
(98 wt % <2 µm); PPSA, 110 Mt of platy clay at 82–85 wt % <2 µm PPSA—Century 90–91
(excluding other reserves that CVRD controls in the same region); RCC (Capim)—Imerys 80
and RCC, with 120 Mt of platy-type kaolin at 78–94 wt % <2 µm. 90.5 80
Future expansions based on these large high-quality reserves are Engineered pigments (fine) 90.5
planned with CADAM aiming to produce 1 Mtpy by 2007; with Capim DG 85
PPSA, 1 Mtpy; and RCC, 1 Mtpy by mid-2005. CVRD is emerging Capim GP 90.5 85
as the second largest kaolin company in the world, following 89.0 85
Imerys. Coating high-brightness
(delaminated)
The world coating clay market is estimated at 8 Mt, of which
the major suppliers are the United States, Brazil, and the United Capim NP
Kingdom, with some production from Australia, China, the Czech Capim CC
Republic, Bulgaria, Germany, and France. Coating clays based on
sedimentary kaolin sequences in the United States (Georgia), and in Australia (Pittong, Victoria)
the Amazon Basin show a wide range of properties but generally
EckaPlate (HB) S—delaminated 86 (ISO)

EckaPlate HB—delaminated 85 (ISO)

EckaCote—delaminated 84 (ISO)

1294 Industrial Minerals and Rocks

Table 9. Range of Imerys coating clays from the United Kingdom, United States, and Brazil

United Kingdom, ISO brightness % United States, GE brightness % Brazil, GE brightness %
Coating regular brightness Engineered pigments
Engineered pigments
Suprastar—88.5 Fine #1 Astra Glaze—88.0 Capim DG—90.5
Supraprint—88.5 #1 Premier—88.0 Capim GP—90.5

Coating high brightness Delaminated Coating high-brightness delaminated
Suprawhite—95 88.0 Astra-Plate—86.0 Capim NP—90.5
Suprawhite—80 87.5 Capim CC—89.0

Coating regular brightness
SPS—85.5

Ultra platy coating
Supraplate—86.5
Suprasmooth—65 83.0

Table 10. Range of Thiele Kaolin Company’s calcined, coating, and filler clays from the United States

Grade Description TAPPI Brightness, Sedigraph, Viscosity* Maximum Residue, % Moisture Solids,
GE % % <2 µm % <325 mesh pH† dry 1.5% maximum

Calcined Clays 0.010

Kaoclay Calcined high-brightness 92.0 minimum 86–92 Not available 0.010 7.0–8.0 50–52
Kaoclay 80 coating and filler 92.5 minimum 86–92 Not available 50–52
0.010 6.5–7.5
Calcined low-brightness 80–83 0.010 4.0–6.0
coating and filler 0.010
0.010
Coating Clays 0.010
0.010
Kaogloss 90 No. 1 high bright 90–92 90–94 300 0.010 6.0–8.0 69–71
Kaobrite 90 No. 2 high bright 90–92 90–94 300 0.010 6.0–8.0 69–71
Kaofine 90 No. 1 fine high brightness 90–92 96–100 200‡ 0.020 6.5–8.0 69–71
Kaogloss No. 1 86.5–88 90–94 300 0.010 6.5–7.5 69–71
Kaobrite No. 2 86.5–87.0 80–84 300 0.010 6.5–7.5 69–71
Kaofine No. 1 fine 87–89 95–99 300 6.5–8.0 69–71
Kaowhite Delaminated 88–90 75–81 450§ 0.010 6.5–7.5 67–68
Kaowhite S Delaminated high glossing 88–90 83 minimum 450§ 0.15 6.5–7.5 67–68
Kaowhite C Coarse delaminated 85–87 54–62 350** 0.30 6.5–7.5 62–64
Kaoprint Top coat high bright 90.5–92.5 89–94 500†† 6.0–8.0 68–69
Lopaque M Base coat low bright 76–82 88–96 300 6.5–8.0 69–71

Filler Clays

Kaofill HB Delaminated filler clay 87–89.5 77–83 Not available 6.5–7.5 68–69.5
Not available 6.5–7.5 69–71
Kaofill Coarse filler clay 83.0 minimum 50 minimum Not available 6.5–8.0 69–71

EG-44 Fine filler clay 81.0 minimum 88 minimum

* Brookfield viscosity. No. 1 spindle, 20 rpm, typically @ 70% solids.
† Slurry as shipped, dry @ 20% solids.
‡ Number 2 spindle.
§ Number 3 spindle @ 67.5% solids.
** Number 3 spindle @ 63% solids.
†† Number 2 spindle. As shipped.

are clays with a high brightness and good runnability. There are Table 10 shows a full range of kaolin products covering cal-
also ranges of platier clays that are either found naturally or can be cined clays, coating clays, and filler clays for the U.S. producer
delaminated from stacky kaolinite found in the deposits. Thiele Kaolin Company.

A range of clays from the United States, Brazil, and Australia New kaolin deposits are being identified in many parts of the
is shown in Table 8 and indicates the range of products, whether world, including Ukraine, Suriname, China, and Australia.
they have been delaminated or not, and the particle-size distribution. Although the coating kaolin market will be dominated by Brazilian
and U.S. clays, there is the opportunity for potential new ventures
Imerys, as the largest producer of kaolin with 25% of the to enter the market. Australia is seen as a likely source of high-
world capacity of 25 Mtpy, is the only international kaolin company quality coating kaolin, with deposits in Western Australia being
to produce coating clays from the United States, the United King- developed. W.A. Kaolin Holdings Pty (WAK) has acquired the
dom, and Brazil; Table 9 shows their range of products.

Filler and Coating Pigments for Papermakers 1295

>500 µm, >250 µm, <500 µm, Disperse Mixed Standard and High
+53 µm, <250 µm Brightness Matrices

<53 µm feed Cyclones HC Overflow Dynocone
Centrifuge
Calcination HC Underflow Dynocone
@ 850˚C and Underflow Dynocone
Sand ground @ SG @ 35 Overflow
1,150˚C 35 & 70 Kwh/t and 70 Kwh/t
Magnet, bleach
Magnet, bleach Magnet, bleach
Dyno
o/f gr

Calcined Delaminated Delaminated Delaminated Coating
Clay Products Product C Product D
Product A Product B
90.0 90.4
91.8 94.0 89.1 (ISO Brightness) 89.6

Figure 3. Scanning electron microscopy (SEM) of the WAK Figure 5. Processing of the WAK kaolin with brightness of various
Australian kaolin dynocone underflow showing kaolinite stacks products

Figure 4. SEM of WAK-delaminated dynocone overflow platy kaolin For the Chinese market, the high-brightness clays may be
blended with GCC in a mix of 70% GCC and 30% kaolin. The West-
deposits evaluated in great detail by CRA/Rio Tinto in the Wick- ern Australian kaolin can be processed to give a high-brightness
epin Area, 180 km southeast of Perth. The firm has drilled 621 delaminated (HBD) coating clay and also as a high-brightness coat-
boreholes in the area, amounting to almost 20,000 m of core. All of ing clay (Figure 5). Table 11 compares properties of the WAK with
this core has been evaluated and models of the deposit prepared. Brazilian and U.S. sources, showing the high-brightness values of the
Proved reserves of 100 Mt have been identified with the potential WAK clay.
for an additional 300 Mt from 274 km2. Full-scale pilot-plant trials
were planned for late 2005, leading to a decision on whether a plant Some Chinese kaolin is suitable for coating clay but, at
will be constructed. The deposit is kaolinized granite with a high present, it supplies only the coated board market, not coated paper.
kaolin yield of 50% at the <45-µm refining level; by comparison, A deposit supplying the coated board market is Maoming in
kaolinized granite from Cornwall shows a 15% yield, reflecting the Guangdong Province (Zhang et al. 1982; Yuan and Murray 1993;
different levels of feldspar in the basement granites–gneisses of Wilson, Halls, and Spiro 1997; Wilson 2004a).
Western Australia and the high-level granites of southwest
England. Detailed characterization studies have been carried out in Kaolin that has been carefully processed to give a controlled
U.K., Japanese, and U.S. laboratories, and the potential for high- particle-size distribution is known as engineered kaolin. Ultrafine
brightness coating clay has been evaluated in helicoating trials in (less than 0.1 µm) particles are removed to improve light scattering
Finland and elsewhere. This processing has involved cyclone and (brightness). These particles are too fine to affect light scatter.
centrifuge separation with the coarse booklets of kaolinite from the Removing such particles does not affect sheet gloss because parti-
underflows (Figure 3) being subjected to delamination to give a cles from 1 µm to 0.1 µm have the greatest influence on this prop-
platy product (Figure 4). erty. The disadvantage with engineered kaolin is that the spaces left
by the fine particles that have been removed must be filled with
water, leading to lower weight-percent solids and less water reten-
tion. Because of its higher light scattering, engineered pigments
will give improvements in coated sheet brightness and opacity
(Table 12).

The shape of the kaolinite particle also influences the coated
sheet properties. Pigment shape refers to aspect ratio: the ratio of
particle height to width. For a pigment with the same average parti-
cle size as measured by a sedigraph, a platy or high-aspect-ratio
pigment will impart more fiber coverage when coated. This
improved coverage often will result in a smoother coated sheet,
which is extremely important for holding dot quality in rotogravure
printing. The disadvantage of platy pigments is that the large plate-
like particles do not easily flow among one another, giving rise to
lower weight-percent solids. The same plate-like characteristics
mean, however, that under a coating blade, platy pigments do not
lose their water easily.

Ground Calcium Carbonate

Roskill Information Services (2002) estimated that the demand for
GCC in papermaking was 15 Mtpy in 2002, with Europe dominat-
ing at 67%, followed by Asia and Oceania (22%), North America
(8%), and others (3% for Middle East, Africa, and South America).

1296 Industrial Minerals and Rocks

Table 11. Comparison of Australian clays with Brazilian and U.S. It can be seen that Europe is a major user of GCC based
coating products mainly on marble deposits and also some chalk. Most of the coating
market is supplied by products derived from marble deposits, with
HBD Coating Clays for Blending with GCC Omya controlling 75% of the total market and Imerys, Provencale,
(HBD clays—fine particle size distribution, platy, glossy) Reverte, and others accounting for the rest. A major growth market
for GCC is now Asia, of which China is at the forefront with many
Australia Brazil United States new projects involving satellite GCC plants (for example, APP has
a 500,000-tpy GCC plant in Dagang).
Product name HBD SBD Capim SP Hydragloss 90
Deposit Wickepin Wickepin Capim Georgia Using GCC as a coating pigment gives technical and eco-
Company Imerys Huber nomic benefits such as high brightness, coating at higher solids,
Wt % <2 µm WAK WAK 91 98 lower binder demand, good runnability, and improved printability.
Wt % <1 µm 94 94 70 98 In some applications, the advantage of high-brightness coating pig-
Wt % <0.5 µm 76 81 42 92 ments leads to lower hiding power and reduced opacity. Therefore,
Wt % <0.25 µm 41 56 19 61 adding opacifying pigments to the coating color to reach the
ISO brightness 13 28 89.5 88.8 required opacity is common practice. Pigments with high bright-
ISO yellowness 90.5 88.4 4.0 4.2 ness, broad particle-size distribution, and average particle size of
Shape factor 3.3 3.5 25 22 many of the GCCs currently used in coating are not optimal for
27 25 opacity.

Non-Delaminated HB Coating Clays (HB clays—blocky) The larger companies such as Omya and Imerys are now pro-
ducing a range of products for which the GCC is manufactured to
Australia Brazil optimize the opacity. The new products are produced by a slightly
different processing technique: the particle-size distribution and the
Product name HB Century Amazon 90 average particle size are moved according to the theory of light
Deposit Wickepin Capim Jari scattering toward that of the ideal opacifying pigment. The process-
Company PPSA ing involves techniques to give steeper curves and in some cases
Wt % <2 µm WAK CADAM removing finer particles. The percentage of solids at which the pro-
Wt % <1 µm 90 82 98 cessing of the GCC is carried is also important.
Wt % <0.5 µm 75 61 96
Wt % <0.25 µm 49 33 83 Omya has developed a new pigment called Hydrocarb CC.
ISO brightness 15 10 46 Using the theory of light scattering, Omya designed a pigment with
ISO yellowness 89.5 89 88 a tailor-made (engineered) mean particle size and particle-size dis-
Shape factor 3.5 5.0 tribution. For high opacity, the target is a narrow particle-size distri-
12 4.0 15 bution with a mean diameter between 0.6 and 0.8 µm. Using a
11 modified grinding technology, Hydrocarb CC has been manufac-
tured with a narrower particle distribution than Hydrocarb 90.
Table 12. Effect on paper brightness and opacity of engineered Comparing Hydrocarb 90 and the new pigment shows that the nar-
kaolin and GCC row particle size leads to a higher wet void volume and a lower spe-
cific surface for Hydrocarb CC. This difference is explained by the
Coating Pigment Pigment Paper Paper reduced amount of fine particles, which results in a more open and
(magazine paper) Brightness Brightness Opacity porous surface for an increase in ink receptivity. Hydrocarb CC
produced from marble shows a higher brightness and opacity on the
Kaolin 88 70.8 85.2 finished paper compared to Hydrocarb 90 produced from the same
Engineered kaolin 88 72.8 85.8 source of calcium carbonate.
Ground marble (GCC) 95 71.4 84.0
Engineered GCC 95 73.4 85.5 The choice of mineral pigment used by the papermaker will
depend on a number of factors related to the paper grade being pro-
Adapted from Gentile 2003. duced. These include fiber production costs, mineral pigment price,
paper optical properties, and strength. It is significant that the
Table 13. Coating pigment choice between kaolin and GCC based papers considered the best in relative quality terms have the highest
on properties* mineral content, whether it is by filler content or coating or both.
Minerals add value to paper.
Color Paper Paper Fiber Paper Print
Gloss Table 13 shows a choice between kaolin and GCC, based on
Solids Brightness Opacity Coverage Gloss the paper properties. Here GCC is clearly better for color solids and
paper brightness but has poorer paper gloss and print gloss than
Coarse GCC ++ + – + –– –– kaolin. The platy nature of the kaolin gives better fiber coverage
(especially the coarse platy type for SCA papers), and the fine platy
Fine GCC ++ + – – – –– kaolins give good paper gloss and print gloss compared to GCC.

Engineered ++ + + –– Today the pigments used in paper coating are many and varied
GCC in their properties. Blending has become a common feature with
precoats often being a coarser GCC (about 60–70 wt % <2 µm) fol-
Blocky kaolin + – + ++ + lowed by a topcoat with a fine GCC (90–95 wt % <2 µm) blended
with either a blocky or platy kaolin dependent on the type of sur-
Coarse platy – – + ++ + face required. Very fine clays, such as Amazon Premium, are often
blended with 20–50 wt % fine GCC to give a higher brightness
kaolin surface with enhanced printing properties. Various blends from

Fine platy – + + + ++
kaolin

Engineered + + + ++
kaolin

* Plus signs indicate most advantageous property in coating; minus signs
indicate least advantageous property in coating. Blank cells indicate no
particular advantage or disadvantage.

Filler and Coating Pigments for Papermakers 1297

Table 14. Typical blends of kaolin and GCC used in LWC and MWC paper

Blend of Kaolin and GCC

Paper Type Type of Kaolin Type of GCC Comments
LWC Single coated, 60-gsm paper
50% U.S. #1 (blocky) 50% 90 wt % <2 µm Single coated
MWC/woodfree—gloss 80% Platy clay 20% 90 wt % <2 µm Base paper, 90 gsm
Gloss, precoat
Gloss, topcoat None 100% 60–75 wt % <2 µm Base paper, 90 gsm
30% U.S. #1 blocky 70% 90–95 wt % <2 µm
MWC/woodfree—matte
Matte, precoat None 100% 60–75 wt % <2 µm
Matte, topcoat 50% Fine platy kaolin 50% 90 wt % <2 µm

Table 15. Soft calcined clay products for the paper industry

Product Properties Brightness, % Size, wt % <2 µm Product Form

Luminex Ultra-high brightness calcined Engelhard 80–90 Dry
Ansilex 93 High brightness calcined 86–90 Dry
Ansilex Standard brightness calcined 95.8–96.5 86–90 Dry, 51% solids
Excaliber Standard brightness, high opacity 92.5–93.5 86–92 Dry
90.0–92.5
Hycal Calcined 80.0–82.0 86–96 Dry
Hubertex Calcined Huber NR* Dry, slurry
Hubertex D Calcined, high solids slurry NR*
92.0–94.0 Slurry
Alphatex Calcined 92.0–93.5 93
Alphatex HP Calcined 92.0–93.0 91 50% solids, dry
Opacitex Calcined Imerys 88 50% solids, dry
Deltatex Calcined 92
Liner-fil 300 Calcined 92.5 91 Dry
Astra-Plex Calcined composite 92 NR 59% solids, dry
80 50% solids, dry
Kaocal High brightness calcined 92.5 86–92
Kaocal 80 Standard brightness 92.5 86–92 56% solids

* NR = no results. NR 50%–52%
Thiele 50%–52%

92–83
80–83

pigments such as GCC, PCC, kaolin, and talc are continuously United States and Brazil. Calcined clay might well be used at the
being developed to give the papermaker a wide choice. Table 14 10% level in the precoat.
shows some typical blends of kaolin and GCC used for various
types of paper. Calcined Clay

Unlike the situation in other areas where large players such as Soft calcined clays are loosely aggregated as a result of fluxing that
Imerys, Engelhard, Huber, Thiele (United States); CADAM and occurs on the edges of the particles. This aggregation results in par-
PPSA (Brazil; for kaolin); Omya and Imerys (for GCC); and Spe- ticles of nominally larger sizes but with entrained air voids that give
cialty Minerals dominate, the Chinese paper pigment market has a rise to good light-scattering characteristics. These properties are
larger number of producers for all pigments, with no single com- widely used in the paper industry to provide good opacity, ink
pany having a sizeable market share. Development of PCC and immobilization, and light-scattering effects. Table 15 shows the
GCC in China will be dependent on local sources of limestone and main calcined products sold into the paper industry, which is the
marble, respectively, and also the logistics of delivering marble for largest consumer of soft calcined clay products. These are all soft
satellite GCC plants. calcined products that are not reactive like metakaolins but have
been agglomerated by heating. They have point-contact adhesion,
The largest paper mill in China is the APP operation at which gives the kaolin platelets a three-dimensional structure; it is
Dagang. Here the pulp is imported from Indonesia and the base this structure that is of benefit to the papermakers: they enhance the
paper is manufactured using PCC produced from lime from local optical and printing properties of paper. These products are used in
limestone in a satellite plant at the paper mill. The precoat is a GCC rotogravure applications where low levels of calcined clay improve
at approximately 65 wt % <2 µm, followed by a topcoat that is optics and structure the coating to provide good fiber coverage and
95 wt % <2 µm GCC, mixed with 30% imported kaolin from the

1298 Industrial Minerals and Rocks

missing dot performance. Calcined clays are also used in newsprint gonite. Using crystal-engineered PCC can also improve paper
applications to improve the opacity and reduce the consumption of machine productivity.
titanium dioxide.
Imerys produces a broad range of PCC products for filling,
Talc precoating, and top/single coating, applicable in all paper- and
board-manufacturing processes. Imerys’s PCC coating products
In Europe, the two main printing processes are rotogravure and include Opti-Cal Print (rhombohedral), with a mean particle size
offset. The offset process poses problems for the use of talc (mps) of 0.5–0.7 µm and a brightness of 95–98 GE. Opti-Cal Matte
because the transfer of the image uses a hydrophilic/hydrophobic is rhombohedral with an mps of 2.0–2.2 µm and Opti-Cal Gloss is
action, which is largely incompatible with an organophilic min- aragonite with an mps of 0.3–0.5 µm and a brightness of 95–98 GE.
eral like talc. The rotogravure process, however, prints by direct Imerys was chosen by M-real, one of Europe’s foremost producers
contact between the image cylinder and the paper. Producing of P&W papers (ranked first in WFC paper, and second in WFU
printing cylinders is expensive, and, therefore, rotogravure print- paper), as the supplier of calcium carbonates for its paper mill in
ing is suitable only for large-volume magazines, mail order cata- Husum, Sweden, which has a capacity of 620 ktpy. Under a long-
logs, or flyers. Key requirements for rotogravure papers are good term contract, Imerys completed construction in 2005 of a satellite
spool formation and smoothness. Good spool formation is essen- PCC plant at M-real’s Husum facility.
tial if the printing presses are to run without constant stops. In the
early 1970s, papermakers exclusively used platy English and Ger- Blanc Fixe
man clays, and then added U.S. delaminated clays. In the late
1970s, the research department at Finminerals (now Mondo Min- Blanc fixe (meaning stable white) is a synthetic barium sulfate that
erals) began working with large Finnish customers to develop talc is precipitated with a defined particle size from highly purified
grades suitable for coating; in 1981 they launched their product. solutions of barium salts and sodium sulfate. Because of its chemi-
This was followed by Luzenac introducing a rotogravure-coating cal production process, blanc fixe is free of impurities such as
talc. The talc works in two ways to improve rotogravure paper. quartz. The feed material for the process is chemical-grade barite
First, its low coefficient of friction allows the large spools to be (BaSO4), which is generally low in silica. Much of the chemical-
produced with a constant tension, which reduces paper breaks; grade barites used in Europe for blanc fixe production come from
and second, because talc is very platy, it helps improve the China. With its hardness of 3 on the Mohs scale, blanc fixe has low
smoothness and therefore gives better printability. Talc has suc- abrasion and exhibits an extremely high light reflectance in broad
cessfully established itself as a major coating mineral for rotogra- ranges of the spectrum, not only in the visible but also well into the
vure paper (Whiteley 2002). ultraviolet and infrared ranges. Blanc fixe is generally 99% BaSO4
with an average particle size of 3 µm, a lightness/color of 98.5 (on
Precipitated Calcium Carbonate the International Commission on Illumination [CIE] L*a*b* color
system), and a pH of 9. It is used both in pulp and as a pigment in
PCC is well accepted as a filler pigment and is now beginning to coating because of its color and low binder requirements. It is used
be used as a coating pigment. Currently, PCC accounts for less in small amounts in coated art papers and in photographic papers,
than 5% of coating pigments used but the market is developing, led where it was applied to the base to improve the surface before the
by SMI, Imerys, and Omya. SMI, part of Minerals Technologies, light-sensitive photographic emulsion is applied. The development
pioneered the concept of the satellite plant for PCC and now has of extrusion coating involving special plastics that do not absorb the
more than 50 plants in operation around the world. PCC is valued processing chemicals, however, has generally replaced this precoat,
for its high brightness and light-scattering characteristic in paper especially for color printing paper.
filling and coating applications. PCC is produced through a reac-
tion process that uses very pure calcium carbonate crystals and Satin White
water. The crystals can be produced in a variety of different shapes
and sizes, depending on the specific reaction process that is used. Satin white, a calcium sulfo-aluminate, is one of the oldest known
The two main raw materials required are quicklime and carbon pigments for paper. Manufacture involves reacting alum with
dioxide. Because quicklime is readily available in many areas of slaked lime at a controlled temperature. Satin white forms acicular
the world, it can be delivered to the satellite plant at a reasonable crystals that are often 1–2 µm long and 0.1–0.2 µm in cross section.
cost. In most paper mills, carbon dioxide is available to be used This shape imparts an open, bulky structure to the coating that has a
from a mill flue gas. This makes PCC economically attractive for major influence on its optical properties; in particular, it is responsi-
the paper mill. ble for high-gloss development during calendering as well as giving
a high print gloss, high ink receptivity, high bulk, and good cover-
For paper coating, SMI developed several PCC product ing power. A 4-g/m2 coating of satin white would have a similar
lines—ALBAGLOS, OPACARB, and JETCOAT. ALBAGLOS has covering power to a 10-g/m2 clay coating. Satin white, however,
a precisely controlled particle-size distribution, good high-shear has a very high dispersant demand because of calcium ions in solu-
rheology, and good ink hold-out, and it can be used at high levels to tion. It also leads to high adhesive demand if the satin white level
improve paper brightness and opacity while maintaining sheet exceeds 10%. The main applications for satin white are in high-
gloss. OPACARB has an acicular shape that provides good smooth- quality art paper and high-quality lightweight offset grades.
ness, gloss, brightness, and opacity owing to a narrow particle-size
distribution and a mean particle size of 0.40 µm. JETCOAT is the Titanium Dioxide
latest PCC coating pigment and is designed specifically for coating
inkjet and other nonimpact papers. SMI has developed six different TiO2 offers the highest opacity and hiding power of all coating pig-
commercial crystal shapes of PCC that give a wide variety of crys- ments, giving good optical density, excellent brightness, and very low
tal structures that offer different performance characteristics grit levels (0.0010–0.0050 wt % of <325 mesh). High brightness,
(brightness, opacity, and bulk) to a sheet of paper. These shapes are combined with superior light scattering, provides excellent bright-
scalenohedral calcite, spherical calcite, prismatic calcite, clustered ness and whiteness to paper and paperboard products. TiO2 provides
acicular aragonite, rhombohedral calcite, and discrete acicular ara- the best opacifying performance available to the paper industry
because of highly efficient light scattering. The higher the TiO2 g/m2

Filler and Coating Pigments for Papermakers 1299

40 deposits. New deposits of anatase and rutile are being discovered
and developed, which means no shortage of titanium dioxide.
Wireless Application Protocol (WAP) Mobile Phones
The real value of minerals used in papermaking lies in their
30 Expansion of the Internet capability to further improve printing results and enable the paper
or board substrate to be a good printing surface. Developing higher
Satellite TV CD ROMs brightness, whiteness, and opacity remains a challenge, especially
Mini-Computers Laser Printers as papers and boards trend to lower basis weights and sheet thick-
ness. The digital revolution has enabled almost everyone to access
20 Color PCs text and pictures that can then be used to produce an infinite variety
Mainframe TV of printed media. This capability has created new requirements for
Cable TV paper and board surface properties than can be satisfied not only by
TV, Radio Computers surface-applied pigments but also by the use of fillers to improve
10 Cinema base sheet uniformity. Developing filled barrier coatings is also an
P&W Papers emerging opportunity.

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