PIGMENTS FOR COATING

Dr. Surendra P. Singh

Ideal pigment • Chemically stabile and

low solubility in water • High reflectance at all

wavelengths - brightness and whiteness

• Free from impurities • Appropriate particle size

and particle size distribution

• High refractive index - good opacity

• Low binder demand

• Good flow properties as an aqueous suspension Mixes easily with water (dispersability)

• Good glossing properties

• Compatibility with other coating components

• Low density • Non-abrasive • Low water absorption • Cheap

Classification of pigments Main pigments: major fraction of the pigment part Special pigments: similar to main pigments, but with limited applications Additional pigments: minor fraction of the pigments - as a rule of thumb, < 10%.

Main pigments Kaolins, Ground calcium carbonates (GCC), Talcs

Special pigment Gypsum, Barium sulfate

Additional pigments

Precipitated calcium carbonates (PCC), Calcined kaolins, Plastic pigments, Alumina trihydrates, Titanium dioxides

Particle size and size distribution • In practice, dp = a few µm • Customary to express as percentage of particles smaller

than 2 µm rather than average particle size. • For "normal" sized pigments, this figure is around 80%.

For "fine" pigments, it is over 90% and, for "coarse" pigments, less than 70%.

• Particle size influences paper smoothness, gloss, ink absorption etc – a specific particle size is desired for each application.

• A high amount of fines can influence binder demand, ink absorption, ink setting, and ink density.

Particle shape • The basic particle shapes are:

– Spherical or cubic, most isometric – Rod-like or needle-like – Platy.

• Real shapes are often complicated • By mixing pigments with different particle shapes

in favorable proportions, the porosity and bulkiness of the coating layer can be increased. It is quite common to mix kaolin and ground calcium carbonate.

• The particle shape depends on the crystalline structure of the mineral, which in turn depends on the chemical composition.

Light Scattering • For most effective scattering, the particle diameter

should be roughly one half of the wavelength of the light to be scattered. If the particles are too small, the light waves pass the particles without bending. If the particles are too large, scattering of light by diffraction is ineffective due to fewer particles per unit weight.

• If the refractive indices of a medium and a particle are the same, no scattering of light occurs.

• The refractive indexes of kaolin clay, talc, gypsum, and calcium carbonate are nearly the same. The differences in opacity between pigments are due to the structure of the coating and not to the pig-ment material itself.

Hardness and Abrasivity • Hardness impacts the abrasivity of the pigment slurry or coating color.

• Moh's scale of hardness: 1: Talc, 2: Rock salt or gypsum, 3: Calcite, 4: Fluorite, 5: Apatite, 6: Feldspar, 7: Quartz, 8: Topaz, 9: Corundum and 10: Diamond.

• Besides hardness, abrasivity depends on the amount and size of the impurities.

• Particle size and distribution also have effect on abrasivity

• Kaolin and talc have relatively low abrasivity even when the amount of impurities is high, because these minerals are soft and platy and not sharpedged. GCC is harder and more abrasive than talc and clay.

• PCC can be produced having a very low amount of impurities. Kaolin usually contains a higher impurity content than the carbonate. If kaolin contains a high quartz content, it can be very abrasive. Among TiO2 pigments, anatase grades are regarded as less abrasive than rutile grades and chloride pigments more abrasive than their sulfate equivalents.

Coating color rheology • Coating color rheology depends largely on the rheology of the pigment(s)

used. However, the presence of additives such as binders, co-binders, water, and retention aids will modify the flow characteristics, particularly if there are chemical interactions between the various components.

• The size, size distribution, specific surface area, surface chemistry, and shape of the particles affect the viscosity of the pigment slurry.

• Generally, low viscosity at relatively high solid content can be maintained if the particle size is big, the size distribution is wide, and particle shape is roundish.

• In general, the dilatancy (shear thickening) of the coating color is observed at lower rate of shear when the particles are platier. Due to rheology limitations linked to lamellarity, current coating talc slurries can only reach solids content of 65%.

• Aggregation of pigment particles, e.g., by calcination of clay, to form a relatively narrow size distribution of larger particles also increases viscosity.

Density • Low density pigments are preferred

because they will result in thicker and more voluminous coatings for the same coat weight. TiO2 pigments have higher specific gravity than most other coating pigments; 3.9 for anatase and 4.2 for rutile, while it is 2.6 for clay and 2.7-2.8 for calcium carbonate. Gypsum has 10%-15% lower density than that of kaolin and calcium carbonate.

KAOLIN

• Kaolin is one of the most widely occurring minerals. Kaolinite, the principal constituent of kaolin is a layered aluminosilicate having the chemical formula: AI4Si4O10(OH)8

Kaolins a. Kaolin stack b. English kaolin c. North American

kaolin d. Brazilian kaolin from

Yari area e. Brazilian kaolin from

Capim f. Calcined kaolin

Physical properties of English coating kaolins

Type

Brightness ISO

Percent < 2µm

Slurry solids (wl.%*)

High Bright Ultrafine 88.0 92 - Ultrafine 86.5 92 - High Bright Fine 87.5 80 66.5 Fine (SPS) 85.5 80 66.5 Medium 83.5 65 66.0 Rotogravure 1 83.0 65 62.5 Rotogravure 2 79.8 53 -

* Solids at which slurries are typically supplied

Physical properties of typical North American coating kaolins.

Brightness GE**

Percent < 2µm

Slurry solids (wl.%*)

High-brightness NO.1 (ultrafine) 90-92 95-100 70 NO.1 90-92 90-94 70 NO.2 90-92 80-86 70 Regular brightness NO.1 (ultrafine) 86-88 94-98 70 NO.1 86-88 90-94 70 NO.2 85-87 78-84 70 NO.3 85-86 73-75 70 Delaminated Fine Particle 87-89 96-98 69.5 Regular Particle 87-89 96-98 67.5 Coarse Particle 84-86 45-55 63

* Solids at which slurries are typically supplied.

Shape of clay particles • Kaolin particles are platy (pseudo-hexagonal ).

Average aspect ratios can vary between 10:1 and 80:1. Secondary kaolins from the Georgia deposit tend to be somewhat less platy (aspect ratios: 6:1 - 20:1) than English kaolins.

• Brazilian kaolins from the Capim area have a very low amount of fine particles below 0.2 µm. The deposit at the Yari River shows the finest grades at particles below 2 µm.

Calcined, structured, and engineered kaolins

• Kaolins can be aggregated by either thermal or chemical means. The aim is twofold:

1. Increase in light-scattering: Particles smaller than ≈ 0.3 µm do not scatter light. Aggregation of ultrafines into larger particles enhance light scatter

2. The intra-particle pores formed by aggregation are capable of absorbing printing inks besides scattering light – improve strike-through.

• As an alternative to aggregation, the particle size distribution can be "engineered" to maximize the number of particles in the optimum particle size range for light scatter - with few coarse or ultra fine particles.

Thermal aggregation • Heating to about 550°C-1100°C using large

multiple hearth or rotary kilns • Dehydroxylates the kaolin and converts it to a

noncrystalline aluminum silicate sintering ultrafine particles into aggregates – with open but rigid structure.

• The end products normally have very steep size distributions; typically coating grades have most particles in the size range 1 to 5 µm.

• Calcined clays have different rheological properties from conventional hydrous clays and slurry solids are generally limited to a maximum of about 50 % by weight.

Chemical structuring of clay • Aggregating through chemical routes - normally

involving the use either of polymers or of precipitation reactions to form an insoluble binder.

• Increased scattering coefficient resulting from a more optimized particle size distribution does lead to improved optical properties.

• Aggregates tend to be somewhat weaker than calcined clays.

• Less abrasive than calcined kaolin • Worse rheology than standard hydrous clays

GROUND CALCIUM CARBONATE • Natural CaCO3 occurs mainly as the mineral calcite in

various rock forms: chalk, limestone, and marble. All consist of individual rhombohedral calcite crystals.

• Chalk: Loosely layered sedimentary rock of biogenic origin. Age: about 80-110 million years.

• Limestone: Stronger layered sedimentary rock of biogenic origin. The crystal size is between chalk and marble. Age: about 110-150 million years.

• Marble: Metamorphic carbonate rock formed through tectonic changes (recrystallization) of limestone. Age: about 300-500 million years.

Attributes of ground calcium carbonate.

Refractive index Mohs' hardness Density (g/cm3) pH of a 10% suspension Solubility Particle-size Brightness

1.48 - 1.66 3 2.6 - 2.8 About 9 Dissolves under acidic conditions 40- 98% < 2 µm 80-96%

Product characteristics of GCC • GCC is preferred to improve the rheological behavior, to

increase the brightness of the paper, and to reduce costs. • Because of the dense packing, the binder level is lower

(compared with clays) even for the extremely fine calcium carbonates.

• Thus, the important reasons for using ground CaCO3 as a coating pigment are: – Favorable rheological properties – High coating solids – Better runnability (coater) – Energy savings (coater) - Lower binder demand – High brightness – Better optical brightener efficiency – Good print qualities and high print gloss.

PCC • High brightness, whiteness, light scattering, and

bulking effect - good fiber coverage, adjustable ink setting properties - good printability, and low blistering tendency.

• The improved printability occurs because we can select the proper particle size, size distribution, and shape to obtain the desired particle packing and pore-size distribution within the coating layer.

• In practice, this is accomplished by producing specific morphologies via controlled synthesis.

Simplified PCC process 1. Calcination: CaCO3 + energy → CaO + CO2

2. Slaking: CaO + H2O → Ca(OH)2 + energy Screening easily removes impurities from the limestone deposit

since the impurities are much larger than the calcium hydroxide particles. For this reason, commercial PCC pigments typically have CaCO3 contents higher than 97%: The remainder is MgCO3 and other residues

3. Carbonation: Ca(OH)2 + CO2 → CaCO3 + H2O + energy The usual sources of CO2 are stack gas of a power plant, recovery

kiln, or lime kiln. In this process step, we can control the particle size, particle size distribution, and particle shape. Also, surface properties of the calcium carbonate parti-cles can be changed if so needed.

Particle shape

• PCC can be obtained as aragonite's needle-like particle form (high aspect ratio) or calcite form.

• Aragonite is beneficial in paper coating - good fiber coverage and loose coating layer packing with these needles.

• Prismatic calcite, which has a lower aspect ratio than aragonite, can be used to adjust other paper properties, e.g., paper gloss.

PCC pigment

Aragonitic Prismatic

Physical properties of the major PCC coating pigments.

Kaolin Talc GCC PCC ISO -brightness (%) 86 85 93 95 b* -value (%) [Ratio of yellowness to blue-ness 3.5 3.0 1.0 0.8

90% < (µm) 3.0 6.0 2.0 0.8-3.0 Av particle size (µm) 0.7 2.0 0.8 0.4-2.0 Sp surface area (m2/g) 6 5.0 11 4-11 Slurry solids (%) 67-72 67-70 74-78 71-75

Specific gravity 2.65 2.71 2.71 2.71-2.83

Refraction index 1.55-1.57

1.55-1.60

1.49-1.66 1.49-1.67

Note: Kaolin clay, GCC, and talc have rather typical property values; whereas, PCC has a range values according to the different PCC grades.

Coating applications of PCC • Fine PCC (0.3-0.4 µm) is the best choice in high glossing double- or

triple-coated papers. • Coarser PCC (0.6-0.8 µm) recommended for precoating - to open

the structure – more light-scattering voids. With this open structure, water retention can be a concern and it may be necessary to compensate by adding some long chain length CMC or synthetic thickener to the coating color.

• With the narrow particle size PCC, it is possible to produce matte papers having good smoothness, low paper gloss, high printed gloss, and sufficient ink setting, which is a combination of properties known to be very difficult to produce.

• PCC with high aspect ratio and narrow particle size distribution mounts more rheological challenges than roundish and wide particle size distribution pigments like GCC. Yet PCC has lower viscosity than delaminated clay at the same solid content.