Components and Properties of Coating Colors
Coating colors have a very simple composition, although the interactions between ingredients are far from simple. A coating color consists primarily of pigments dispersed in water, plus binders, cobinders and additives (see also Section 3.6.9.). Calculations are, as a rule, based on the dry product, even if the actual ingredients deployed are mainly liquid commercial products with differing dry contents (pro¬portion of active substances).
The basis of calculation is 100 parts of pigment, to which all additions are referred (Table 7.2). As an example 24 kg of commercial product binder must be added to 100 kg of dry pigment to obtain 12 parts of dry binder in the formulation if the synthetic binder has 50 % dry content.
Table 7.2 Example of an offset coating formulation.
100.0 Coating pigment
0.5 Additives (e. g. calcium stearate, wax, optical brightener)
pH is set to 8.5 by adding sodium hydroxide solution (NaOH),
and the solid content to 65 %–70 % by adding water.
1 The term parts refers to the parts by weight addition of a component based on 100 parts by weight of pigment
Pigment is the most abundant component in the coating, so pigment is naturally the most important factor affecting the properties of the coating (see also Section 2.2.2). An ideal pigment would have the following properties:
. • appropriate particle size and narrow particle size distribution
. • free from impurities
. • good dispersibility for easy mixing with water and low water absorption
. • high chemical stability and low solubility in water
. • good compatibility with other coating components
. • good light reflectivity at all wavelengths for high brightness
. • high refractive index for good opacity
. • good glossing properties for eye-pleasing coating gloss and high print gloss
. • low binder demand
. • lood flow properties in an aqueous suspension and low abrasiveness
. • cheapness.
There is of course no pigment that could meet all these requirements, however, different pigments meet different combinations of them. The main coating pig¬ments are clay (kaolin), ground calcium carbonate (GCC), precipitated calcium carbonate (PCC) and talc.
Binders serve as pigment-pigment bonders, and to anchor the pigments to the base paper surface. They are also expected to control the coating color viscosity and water retention, and to prepare the coated papers for printing and converting stages. Today, synthetic binders are preferred in coating kitchens. They are polym¬erized petroleum products that are available in the form of dispersions in very fine distributions, often with 50 % solids.
Some typical starting materials are mono¬meric styrene, butadiene, acrylic acid, and their derivatives from which products such as styrene butadiene latexes, acrylic acid esters and polyvinyl acetate are ob¬tained. Besides synthetic binders, natural binders are also used. They are often multifunctional in that they contribute to bond strength, water retention and im¬proved runnability. Natural binders include starch and derivatives, and with some restriction carboxy methyl cellulose (CMC, see Section 184.108.40.206). These products are normally supplied dry and need to be dissolved during color preparation (see Sec¬tion 220.127.116.11.3).
Among binders, styrene-butadiene latexes dominate beside modified starches. Styrene-acrylate dispersions are specialties and of high importance for impressive prints. Additionally acrylate dispersions have an excellent brightness and ageing resistance and are less odorous. Future requirements of coating process condi¬tions, on paper characteristics and printing technologies forced the production of tailor-made binders with very specific property profiles.
Other Coating Additives
Coating additives are either production additives required for the coating process (see also Section 7.7), or product additives that contribute towards specific paper properties (see also Section 7.8).
7.5 Components and Properties of Coating Colors
18.104.22.168 Production Additives
Coating additives which primarily influence the production process are
. • dispersants for pigments, alkalis for pH control, such as sodium hydroxide solu¬tion (NaOH) or ammonia solution (ammonium hydroxide, NH4OH)
. • products for controlling viscosity and water retention, and for optimizing runn¬ability, (co-binders, thickeners)
. • products that aid lubrication/smoothing of coated paper surfaces, such as cal¬cium stearate or waxes
. • defoamers, deaerators and preservatives (biocides).
The purpose of using dispersants is to prepare a pigment dispersion where neither agglomerates nor aggregates exist and only primary particles are present. Primary particles are to be evenly distributed in water, and the system is to stay stable for a certain period of time (see also Section 22.214.171.124.2).
Synthetic co-binders and thickeners are both used for the same purpose in coat¬ing colors, i. e., adjusting rheological properties (shear viscosity), and water reten¬tion. Most of these products have additional binding power and also act as acceptor chemicals for optical brightening agents (OBA) (see following sections). Products belonging to this group are carboxy methyl cellulose (CMC), polyvinyl alcohol (PVOH, or PVA), acrylic copolymers, and associative thickeners (see Section 126.96.36.199.4).
Thickeners need to be able to interact strongly with water molecules when they are to increase the water retention of coating colors. They also need to interact with other ingredients of coating formulations, especially pigments, in order to display a thickening effect. The nature and strength of these interactions depend on the chemical composition of the polymer.
Coating consolidation is the process during which the liquid coating color on the base paper is transformed into an immobilized coating layer. The runnability of a coating color is determined by the properties of the liquid color, i. e., wet coating structure. Modern coating processes with high solids contents and high coating speeds demand coating colors with pronounced pseudoplasticity, i. e., low viscosity under high shear.
The increase in the color viscosity by an increase in the shear rate is called dilatancy or shear thickening (Fig. 7.10). The final quality of the coated paper depends on the dry coating structure. The transfer from wet to dry coating structure is a result of the aqueous phase penetration from the coating color into the base paper (influence of water retention; Fig. 7.11) and, later, due to evaporation during drying.
Air/gas has a negative effect on the rheology of coating colors. If the coating color contains large amounts of air/gas bubbles, the extreme consequence can be skip coating, i. e. occurrence of uncoated spots, so physical and chemical aids have to be applied to reduce the air content (see Section 188.8.131.52.4.5). Preservatives, re¬spectively biocides or microbiocides, are chemicals used in the pigment slurry and/or in the finished coating color to prevent the growth of microbes (see Section 3.7.4).
184.108.40.206 Product Additives
Shading dyes, optical brightening agents (OBA) and their carriers, cross-linking agents for natural binders and wet strength agents for offset papers, belong to the group of product additives which influence primarily the properties of the coated paper surface. The last two product classes are also called insolubilizers (see Sec¬tion 220.127.116.11.5).
The brightness of pulp, fillers and coating pigments is often not high enough to reach today’s brightness targets. Therefore there is a need to use blue/violet shad¬ing dyes and especially so-called optical brightening agents (OBA) as coating additives (see Sections 18.104.22.168.5 and 3.6.2).
Optical brighteners work only when they are fixed to a carrier. A good carrier is linear and contains OH- or other hydrophilic groups. Linearity increases the contact between the carrier and OBA so physical bonds (such as hydrogen bonds and van der Waals forces) can be formed between the OBA and the hydrophilic groups of the carrier. Carriers in a coating color are, among others, starch, CMC (carboxy methyl cellulose), PVA (polyvinyl alcohol), or PVFA (polyvinyl formamide) derivatives.
There are many chemically different types of insolubilizers or crosslinkers. They all have the same function – to increase water resistance (see also Section 22.214.171.124.5.2). Water resistance is particularly important in offset printing, but also for wallpaper and in the storage of board packages. In double-coated boards, crosslinkers are used in the precoating to impart water resistance against the topcoat. The water resistance can be measured as wet rub and wet pick or is seen as less pick, print mottle, or binder migration.
The water sensitivity of paper and board coating origi¬nates from the fact that water-soluble binders tend to lose their binding power in contact with water and dissolve. This water sensitivity of binders can be described as the amount of O-atoms in the molecule (in hydroxy and carboxy groups). The water sensitivity can be decreased by crosslinking the soluble binders together with insolubilizers or by building an insoluble net around the binders.
Traditional insolubilizers in paper and board coating are based on formaldehyde and its amino compounds (melamine, urea) or on glyoxal. Imidazoline derivatives are also used as crosslinkers. Another class of insolubilizers are based on zirconium; the prod¬uct most widely used is ammonium zirconium carbonate (AZC).
Lubricants improve the runnability of the coating color by reducing the friction between the machine part and the coating color. This can be seen for example as fewer scratches in the coating and longer lifetime of coater blades. Lubricants enhance the plastic deformation of the dry coating in the supercalender by pre¬venting the cracking of the soluble binder film that would lead to dusting.
It also improves gloss. During calendering, lubricants migrate from the coating onto the hot calender rolls and form a monolayer on the rolls, thus preventing sticking of the coating to the rolls. Dusting at printing machines has been a problem in LWC, especially when delaminated kaolins are used. Different kaolins have different dusting tendencies, dusting can often be reduced or eliminated by using a lu¬bricant.
The most commonly used lubricant is calcium stearate. Wax emulsions (mostly emulsions of paraffin waxes, microcrystalline waxes, or polyethylene waxes) are the oldest group of lubricants in paper and board coating. These emul¬sions give good runnability but have less effect as antidusting agents than has stearate.
A new group of substances used as lubricants are soy lecithin/oleic acid blends. Polyethylene and polypropylene glycols are used in blends with calcium stearate or alone as a lubricant with additional influence on rheology and flow properties of the coating color.