Paper Coating Functional Chemicals
Reagglomeration is avoided if particles are kept far enough from each other. Two general principles are considered as stabilization mechanisms: electrostatic stabili¬zation and steric stabilization. When both occur simultaneously, it is called “elec¬trosteric stabilization”. In electrostatic stabilization, the charges of particle surfaces are made to be the same sign. Dispersant is adsorbed onto the particle surface, and thus the surface gets a highly localized charge with the same sign as that of the dispersant. A negative charge on the surface, when anionic dispersant is used, creates a cloud of positive counterions around the particle (the so-called “electric” or “ionic” double layer).
The closer to the surface, the more localized are the coun¬terions. At a greater distance from the particle, in a continuous water phase, neg¬ative and positive ions are in balance. The counterion cloud acts as a stabilizer, creating repulsive forces between particles. Electrostatic stabilization is the most commonly used stabilization method in paper pigment dispersions. To act as a good stabilizer, the dispersant must be highly charged. Examples of these kinds of dispersants are salts of polyacrylic acid and polyphosphates.
In steric stabilization, particle surfaces are covered with uncharged polymer, the chain of which extends into the water. When two particles with polymers on them approach each other, they cannot approach too closely because the polymer chains would overlap, and this is not entropically favored. Thus polymers create a steric hindrance against particle interaction. Dispersants, which work as steric stabilizers, are also called “protective colloids”. Examples of these are starches and polyvinylalcohols. A good example of a dispersant, which works as an electrosteric stabilizer, is carboxy methyl cellulose (CMC).
The viscosity minimum is the optimum dosage of dispersant. After the opti¬mum the viscosity is slowly increased.Viscosity decreases when dispersant is added, due to the increased stability of the dispersion. The stability is a maximum at the viscosity minimum. The viscosity starts to increase after the optimum be¬cause additional dispersant can no longer be adsorbed onto the pigment surface, and thus stays in the water increasing its electrolyte concentration and decreasing the stability. Overdosing of dispersant therefore must be avoided. Pigment type, dispersant type, pH, and additional components all affect the dispersant dosage required. The smaller the particle size the larger the total surface area of pigment, and the more dispersant is needed for stabilization. Generally it can be said that the required dispersant dosage is in the range 0.1 to 0.5 % of dry pigment.
Today, the most commonly used pigment dispersants are polyacrylate salts. Usu¬ally they are low molecular weight polymerization products of acrylic acid, which have been neutralized with sodium or ammonium hydroxide. They are very resis¬tant towards different types of attack like high pH, high temperature, or high shear forces. Polyphosphates were previously used as dispersants. They are effective, but lack hydrolytic stability during storage of the dispersion; they easily hydrolyze to orthophosphates, which have no deflocculation power. This causes an increase in the viscosity of the dispersion. The longer the polyphosphate chain, the more effective it is as a dispersant. Tetrasodium pyrophosphate, Na4P2O7, is the simplest polyphosphate that can be used as a dispersant.
Sodium tripolyphosphate, Na5P3O10, is widely used as a dispersant and as a builder in detergents. Other anionic polymeric dispersants are lignin sulfonic acid salts and formaldehyde con¬densation products with aromatic sulfonic acids. Because of the nature of sulfonic acid, these types of dispersants bear the maximum anionicity over the whole pH range typically used in pigment dispersions. Lignin sulfonates are made from native lignin. Their effectiveness as a dispersant depends on their purity and de¬gree of sulfonation. Lignin sulfonates can be used as a dispersant with hydro¬phobic surfaces. Their disadvantages are poor thermal stability, tendency to foam and they bring a high load of detrimental substances to the papermaking process. Formaldehyde condensation products with aromatic sulfonic acids have an aro¬matic backbone and are also effective dispersants with hydrophobic surfaces.
Nonionic polymers, which can be used as stabilizers are, e. g., starches, polyvinyl alcohols, and polyacrylamides. Nonionic polymers work as protective colloids; their mechanism of stabilization is steric stabilization. Carboxy methyl cellulose bears a small anionic charge along the chain. However, it is often considered to act as a protective colloid. Actually, carboxy methyl cellulose can be considered to use both its protective colloid properties and its charge in stabilizing, thus acting as an electrosteric stabilizer.
The charge of pigment dispersants is usually anionic but, in some applications, cationic dispersants are preferred. They are seldom needed in coating color pig¬ments, but they are beneficial in pigment dispersions meant for filler applications or for specialty coating applications. Cationic dispersants are typically cationically charged polymeric compounds e. g. modified polyethylenimines, polyvinylamines. Usually they do not act as effectively as anionic dispersants.