High quality coatings of high brilliance and color strength are characterized by a perfect pigment dispersion, optimal pigment particle size, and long-term stabilization of the dispersed particle in the formulation.

The dispersion of a pigment in liquid coatings, paints or inks to produce stable suspension, can be divided into the following three processes:

Mechanisms in the dispersion process

• Pigment wetting: All of the air and moisture is displaced from the surface and between the particles of the pigment aggregates and agglomerates (clusters) and is replaced by the resin solution. The solid/gaseous interface ( pigment/air) is transformed into a solid/liquid interface (pigment/resin solution).
  
  
• Grinding stage: Through mechanical energy (impact and shear forces), the pigment agglomerates are broken up and disrupted into smaller units and dispersed (uniformly distributed).
  
  
• Stabilization of pigment suspension: The pigment dispersion is stabilized by dispersing agents in order to prevent the formation of uncontrolled flocculates. The resultant suspension is stabilized due to the adsorption of binder species or molecules at the pigment surface.

The choice of the more efficient dispersing agents is strongly related to the chemical nature of the pigment and the resin solution (for the paint manufacturer, it is essential to distinguish between organic and inorganic types). This topic is discussed further in Formulating an optimum dispersion.

The wetting step consists of replacing the adsorbed materials on the surface of the pigments and inside the agglomerates (water, oxygen, air, and/or processing media) by the resin solution.

The complete wetting out of the primary sized pigments particle helps to enhance the technical performance of a liquid coating that depends very much on interaction between the pigment particles and the binder system. Dispersing additives, which adsorb on the pigment surface, facilitate liquid/solid interfacial interactions and help to replace the air/solid interface by a liquid medium/solid interface.

Replacement of air and water by the resin

The efficiency of the wetting depends primarily on the comparative surface tension properties of the pigment and the vehicle, as well as the viscosity of the resultant mix. The adsorption mechanism depends on the chemical nature of the pigment and the types of dispersing agents used. (visit dispersants families)

Thermodynamic consideration:

The spontaneous wetting process (on wetting solid surfaces) is driven by minimization of the free surface energy. Forced wetting processes (in non-wetting conditions) require the application of external force, and spontaneous de-wetting will take place when the force is removed.

Thermodynamic condition for wetting requires the work of liquid/solid adhesion (Wa) to be as high as possible and, for unlimited wetting, at least more than a half of the work of cohesion (Wk) is required: Wa> Wk.

Velocity of penetration of a liquid into a powder can be explained in terms of the Washburn equation (1921):

Washburn equation

where h is the depth (or height) of penetration during the time t, - is the surface tension of the wetting liquid, - its viscosity, - the wetting angle, r - mean radius of capillaries, C - structural coefficient, associated with parameters of the porous structure, W - energy (heat) of wetting.

The wetting step of dispersing processes can be intensified by the use of wetting agents and/or binders with lower viscosity and surface tension. On the other hand, a resting of pigment/binder premixes prior to their dissolving or grinding helps to accomplish the wetting stage and always eases and accelerates dispersing processes.

After the wetting stage, it is necessary to de-aggregate and deagglomerate the pigment particles. This is usually accomplished by mechanical action provided by high impact mill equipment.

In the grinding stage, the cohesive forces inside the agglomerates have to be overcome. Energy is added to the system and therefore smaller particles (with a larger interface to the resin solution) are formed. This results in loosened inter-particle contact durability, that eases the destruction of pigment clusters under the action of shear stresses, applied in dissolvers, mills etc.

Pigment dispersing

As the pigment powder is broken down to individual particles by mechanical shear, higher surface areas become exposed to the vehicle and larger amounts of additives are required to wet out newly formed surfaces.

Once dispersed, the primary particles have a tendency to re-agglomerate. This process is called flocculation. From a structural standpoint, the flocculates are very similar to the agglomerates; nevertheless, the interstitial spaces between the pigments are now filled with resin solution rather than air.

The grinding process can be regarded as a de-flocculation process. In the absence of stabilizing agents, effects such as reduced color strength, decreased gloss, and altered rheology then may occur.

The aim of stabilization is to keep the pigment particles separated as achieved in the last step, and to control the degree of pigment particle size through the let-down and filling phase, storage and and later in coating films during film formation.

Flocculated pigment suspensions are characterized by the non-uniform spatial distribution of particles, which are allowed for immediate interparticle contacts. This results in worse rheology (structural viscosity, blob-flow), low storage stability (in paints) and poor optical and color properties (in coatings).

It is well known, that even well grinded but not stabilized, fine particle size pigment suspension can easily be destroyed by the letting down into a non-suitable paint base: flocculation typically breaks down when shear is applied and will form again, when the shear is removed.
Therefore, immediately after grinding pigment suspensions must be stabilized by the addition of additives, whether they are intended to be used immediately in let-down or as pigment preparations (colorants).

Dispersing agents avoid flocculation

Stabilization is achieved through absorption of stabilizing molecules on the pigment surface, so that repulsive forces prevent other particles from approaching close enough for the attractive van der Waals forces to cause agglomeration. To know more about the factors influencing the stability, take a look at the colloidal stabilization.

There are two principal mechanisms for the stabilization of pigmented dispersions:

• Electrostatic stabilization: electrostatic stabilization occurs when equally-charged local sites on the pigment surface come into contact with one another. Two particles having the same charges give a repelling effect. The resulting Coulomb-repulsion of the charged particles allows the system to remain stable.
  
  
• Steric stabilization A pigment is said to be sterically stabilized when the surface of the solid particles are completely covered by polymers, making particle-to-particle contact impossible. Strong interactions between polymers and solvents (organic solvent or water) prevent the polymers from coming too closely into contact with one another (flocculation).

Stability of dispersion is a result of interpay of the heat (kinetic) energy of particles, attractive interparticle forces and repulsive forces, all act persistently between any neighboring particles.

Possessing kinetic (thermal) energy and being subjected to Brownian movement, colloid particles persistently approach one another and collide . Without restricting factors their approachment may occur so close that even relatively short-range van der Waals forces will be capable to joint particles irreversible, thereby destroying the dispersion. Alternatively, having a certain source for interparticle repulsion, capable to prevent particles from immediate contacts,a dispersion can persist indefinitely without significant changes in particle size and properties.

Attraction/ repulsion between two particles

The existence of sufficient repulsive forces between neighboring colloid-size particles is therefore a matter of life or death for any dispersion. Those repulsive forces arise when lyophilized near-particle layers interfere : the persistent exchange of the molecules of water between the near-surface layer and the non-changed water outside the layer creates the repulsive force, similar to osmotic pressure (Figure 1). The repulsive forces can be of different nature:

• Compression of electrical double layers, which surround the particles,
• Osmotic pressure in non-ionically stabilized system (Derjagin, Fischer),
• Chain elasticity in the dispersions stabilized with polymer surfactants, entropic repulsion (Mackor),
• Barrier-type stabilization with polymeric dispersants.

Practically, three systems of coating stabilization can be used in aqueous medium:

1. the use of ionic surfactants or chemical lyophilizers which produce carboxy, ammonium etc. ions, see electrostatic stabilization
2. non-ionic stabilization, owed to the adsorption of non-ionic surfactants or corresponding chemical modification of the polymer phase (or surface only) with non-ionic lypophilic fragments, see steric stabilization
3. combined ionic non-ionic stabilization, which is widely used in technologies of latexes, emulsions and paints, allowing to achieve prominently high disperse stability toward the action of various destabilizing factors.

As we are able to calculate of interparticle forces, we can characterize dispersion stability via "potential curves" (Figure), where potential energies of attractive and repulsive forces and total interaction are expressed as a function of distance between particles "Potential barrier" is the energy particles shouldn't be able to overcome on expense of their heat energy in order to maintain the dispersion steady.

Potential energy curves between two particles

The pigment particles in the liquid paint carry electrical charges on their surfaces. Through the use of additives, it is possible to increase the charges and, furthermore, to make all pigment particles equally charged.

Classic colloidal science explains electrostatic stabilization in terms of an electrical double-layer. A charge is generated on the pigment surface, and a more diffuse cloud of oppositely charged ions develops around it. As two particles approach each other the charge effectively provides a barrier to closer particle interactions. Stabilization increases along with the thickness of this layer.

Electrostatic stabilization

Chemically speaking, the additives used for dispersion in such systems are polyelectrolytes - high molecular weight products which contain a multitude of electrical charges in the side chains.

In addition to polyphosphates, many polycarboxylic acid derivatives are utilized as polyelectrolytes in the coatings industry. The polyelectrolytes adsorb onto the pigment surface and consequently transfer their charge to the pigment particle. Through electrostatic repulsion between equally charged pigments, the flocculation tendency is dramatically reduced so that the deflocculated state is stabilized.

Electrostatic stabilization is effective in media of reasonably high dielectric constant, principally water; although even in water-based coatings systems, steric stabilization, or a combination of steric and charge stabilization will often provide better overall performance.

Charge stabilization is not be effective in media of low dielectric constant (the vast majority of organic solvents and plasticisers), and steric stabilization is required to maintain dispersed particles in a stable non-flocculated state.

Steric stabilization relies on the adsorption of a layer of resin or polymer chains on the surface of the pigment. As pigment particles approach each other these adsorbed polymeric chains intermingle and in so doing they lose a degree of freedom which they would otherwise possess. This loss of freedom is expressed, in thermodynamic terms, as a reduction in entropy, which is unfavorable and provides the necessary barrier to prevent further attraction. Alternatively one can consider that, as the chains intermingle, solvent is forced out from between particles. This leads to an imbalance in solvent concentration which is resisted by osmotic pressure tending to force solvent back between the particles, thus maintaining their separation.

Steric stabilization

One fundamental requirement of steric stabilization is that the chains are fully solvated by the medium. This is important because it means the chains will be free to extend into the medium, and possess the above mentioned freedom. This requirement is usually expressed by saying that the medium needs to be a better-than-theta solvent (i.e. a relatively good solvent) for the polymer chain. In systems where the chains are not so well solvated they will prefer to lie next to each other on the surface of the pigment, providing a very much smaller barrier to inter-particulate attraction.

The greater steric repulsion generated by the addition of polymeric dispersants  moves the minimum in the Potential Energy Curve, and thus reduces the overall viscosity.

This stabilization mechanism occurs in solvent- based systems and in water-reducible systems which contain solvated resins. Through specific structural elements composed of pigment affinic groups (polar) and resin-compatible chains (nonpolar), these dispersing agents exhibit definitive surface active properties. In other words, they not only stabilize the pigment dispersion, but they also function as wetting additives.

Dispersing agents are not just additives to conventional millbases. The choice of the most suitable dispersing agents is sometimes difficult and their usage require sometimes specific guidelines.

In this part of the Dispersion Center we will discuss the main items that have to be taken into account when formulating a pigment solution. To learn more about how to formulate an optimum dispersion, click on the links below:

• Dispersant Choice
• Grinding medium
• Pigments**/Fillers
• Order of addition
• Surface Area

With any effective polymeric dispersant, the two-component structure is made up from a polymeric chain and a pigment affinic anchor group. The nature of the polymeric chain is critical to the performance of the dispersant. If the chains are not sufficiently solvated, then they will collapse on to the pigment surface, allowing the particles to aggregate or flocculate. This need for compatibility with the medium extends throughout the final drying stages of any applied coating. If it ceases to be compatible, flocculation may occur leading to reduced gloss and tinctorial strength.

In order to meet the need for good compatibility, several different polymer chains can be utilised effectively covering the variety of solvents encountered. The structure of some dispersing additives can be described as one or more spatially close anchor groups with a number of polymer chains. Other dispersing additives are designed to improve the flocculation resistance of pigments (particularly non-polar, organic pigments) and have higher molecular weights through the attainment of more complex polymer like structures.

The molecular weight of the dispersant is sufficient to provide polymer chains of optimum length to overcome Van der Waals forces of attraction between pigment particles. If the chains are too short, then they will not provide a sufficient thick barrier to prevent flocculation. This will lead to an increase in viscosity and a loss of tinctorial properties.

Dispersant choice involves a number of criteria, and the nature of the pigment, the resin involved and the solvent can all affect the performance of the polymeric additive.

Anchoring of the polymeric dispersant to the pigment surface can be affected by competition between the resin and dispersant for the surface of the particle. Once the anchor group of the dispersant is attached to the pigment surface it will remain firmly attached. Molecules of resin, however, are transiently adsorbed on the surface of the pigment, and even though not firmly anchored they can hinder the dispersant anchoring process.

Minimal-resin solids (or resin-free systems) can be used in the dispersion, as long as good let-down stability is sufficiently available. Dispersant technology is now more advanced, and in some systems dispersing without resin is possible.

As an example Avecia SOLSPERSE 43000 polymeric dispersant would be recommended for dispersing various pigments in resin-free dispersions for general industrial and decorative water-based paints. A key benefit of using this kind of product is the wide resin compatibility it offers in various resin systems, including polyester, acrylic, polyurethane, alkyd and epoxy.

The choice of dispersant is also related to the surface nature of the pigment. The polarity of the surface of the pigment differs from organic (non-polar) to inorganic (more polar), and this means that the nature of the dispersant anchor group is critical for optimum adsorbtion. The choice of anionic anchor group should allow for better performance with inorganic pigments and a cationic anchor group should be more appropriate for organic pigments.

The surface area of the pigment also affects the level of dispersant used, and in general, if too little is used then the full benefits will not be realized. If too much is used, it can be shown that the thickness of the protective barrier is actually reduced as a result of overcrowding on the pigment surface. Therefore the use of an excess level of dispersant actually leads to final coating properties which are inferior to those obtained with an optimized dosage. Furthermore, film properties such as adhesion or hardness can be adversely affected by the use of an excess of dispersant because of the free molecules in the drying film.

The Table below show a range of typical starting point formulations for inorganic and carbon black based decorative universal tinters using polymeric dispersant (Avecia Solsperse).

The usual way of incorporation of dispersant in a coating formulation would be in three stages:

1. Mix the dispersant in the millbase solvent or in the resin/solvent mixture,
2. Add any other additives,
3. Add the pigment, in stages, and disperse in the normal manner.

In case you want to optimize a Polymeric dispersant millbase, 4 stages are involved. The following example explaining using Avecia Solsperse products (referred to as a ladder series).

Using the % AOWP of Polymeric agent (calculated above) + synergist (if required) prepare a series of millbases with increasing pigment contents in a GRINDING MEDIUM containing APPROXIMATELY 10% SOLID RESIN. Note: the ratio of [hyperdispersant:pigment] must be maintained.

The pigment concentration giving the same viscosity as the control should now be used in Stage 3.

Using the higher pigment content established in Stage 2: Carry out a series of Polymeric agent dosages around the theoretical % AOWP (+ any synergist required) to optimize the required agent dosage. Determine the best hyperdispersant dosage by measuring desired property.

Using the % agent on weight of pigment established in Stage 3: Prepare final ladder series of pigment contents - maintaining [agent:pigment] ratio determined in Stage 3 to determine optimum amount which gives best final product.

As a general rule, 2-2.5mg of polymeric dispersant, per square meter of pigment surface area will be close to the optimum amount required.


A ladder series of polymeric dosage levels should be evaluated based around this 2-2.5mg/m2 level. Measurement of dispersion viscosity will show a minimum at the optimum dosage; although it is also possible to measure gloss or color strength of the coating which will show a maximum at the same optimum dosage.

Typically, the surface area of phthalocyanine blue pigment is 50 m²/g:

So a typical dosage would be :

The choice of the dispersing agents is a key issue in the coating and ink industry. Formulators have to find the most suitable products for their formulation taking into account the final application of their coating, the coating system (water based, solvent based, etc.) and the other additives.

The role of the dispersing agents is to enhance the dispersion process and ensure a fine particle size in order to stabilize pigments in the resin solution. As explained earlier, an efficient dispersant has to perform the three main functions : pigment wetting, dispersing, and stabilizing. Dispersing agents generally differ for aqueous and solvent-based coatings.

In term of chemical structure one can divide dispersing agents into the two following classes:

• Polymeric dispersants
  
• Surfactants

The main differences of those two types of dispersants being the molecular weight, the stabilization mechanism and the resulting let down stability.

Polymeric dispersants stabilize paints, coatings and ink systems via a steric stabilization mechanism previously described. They have a two-component structure which combines the following two very different requirements:

1. It must be capable of being strongly adsorbed into the particle surface and thereby possess specific anchoring groups
   
2. The molecule must contain polymeric chains that give steric stabilization in the required solvent or resin solution system.

There are many copolymer/functional polymer configurations that might be expected to give effective polymeric dispersants. Six possible arrangements are illustrated in Figure 1:

Figure 1: These anchor onto the particle surface either through functional groups (b and c) or through polymeric- blocks (a and d-f). The steric stabilization polymer chain is either anchored to the particle surface at one end (b, d, and f) or at both ends (a, c, and e).

Polymeric dispersants differentiate themselves from the other types of dispersing agents through considerably higher molecular weights. Because of its structural features, a polymeric dispersant is bound to numerous sites at the same time, forming durable adsorption layers upon many pigment particles. Optimal steric stabilization is achieved when the polymer chains are well solvated and properly unfurled, therefore they must be highly compatible with the surrounding resin solution. If this compatibility is obstructed, the polymer chains collapse causing the steric hindrance and the resulting stabilization to be lost.

In order for additives to be effective, the adsorption into the pigment surface must be durable and permanent. The surface properties of the pigment particles are therefore crucial to the additive's effectiveness:

• With pigments possessing high surface polarities, such as inorganic pigments that are ionically constructed, the adsorption of any dispersing additive is relatively easy.
  
• However, for pigments with nonpolar surfaces, such as organic pigments whose crystals are composed of nonpolar individual molecules, a proper adsorption is rather difficult to obtain with conventional additives. The wide range of anchor groups that polymeric dispersants provide make them very efficient to anchor on pigments with nonpolar surfaces.

In the traditional method of stabilizing pigments in water, the stabilizing charges used are often disturbed by impurities, such as other ions, or the presence of other pigments with different zeta-potentials. This leads to a destabilizing effect, caused by the reduction of the repulsive forces. Steric stabilization can avoid this issue, making polymeric dispersants very useful for dispersing all types of pigments, even the organic ones, that are very difficult to be deflocculated by traditional wetting and dispersing additives.

The level of the polymeric dispersant used is very important, since performance depends on the optimum amount of saturation by the dispersant of the pigment surface.

It does not matter whether the previously discussed polymer chains are provided by polymeric dispersants containing either single chains or up to many hundreds of chains. The essential requirement is that the chains are successfully anchored to the pigment surface, and that the surface of the particles are covered with a sufficient density of chains to ensure minimum particle-particle interaction.

As shown in the picture below the anchoring function of a polymeric dispersant may be a single functional group, or an oligomeric or polymeric chain:

Schematic molecular structure of dispersants

Studies have shown that steric stabilization chains anchored at only one end are most efficient. Given that steric stabilization is entropically driven in non aqueous media, this conclusion is not surprising. Anchoring both ends of the polymer chain will clearly inhibit its freedom of movement, even before it starts to intermingle with the steric stabilization chains of an adjacent particle.

Anchoring Mechanisms

As the nature of the surface of pigments differ, according to their chemical type, many different chemical groups can be found as anchor groups for polymeric dispersants. This wide range of anchoring possibility enables polymeric dispersants to disperse inorganic pigments as well as pigments with polar surfaces. The actual anchoring can then take place through a variety of mechanisms:

Anchoring Through Ionic or Acidic/Basic Groups.
When a pigment particle has a relatively reactive surface (eg: inorganic pigments) it is possible to form an ion-pair bond between a charged site on the particle surface and an oppositely charged atom or functional group on the dispersant. This situation is illustrated in Figure 1a and is effective because organic solvents normally have a relatively low dielectric constant, so charge separation is not favored.

Figure 1: Anchoring through Ionic or Acidic/Basic Groups

In fact, many inorganic pigment particle surfaces are quite heterogeneous, with both positive and negative sites. It is therefore quite common to find that a pigment can be dispersed by using polymeric dispersants with either negatively or positively charged anchor groups, as illustrated in Figures 1b and 1c .

Examples of functional groups that can be used to anchor polymeric chains to charged or acidic/basic surfaces include amines; ammonium and quaternary ammonium groups; carboxylic, sulfonic, and phosphoric, acid groups and their salts ; and acid sulfate and phosphate ester groups.

Anchoring Through Hydrogen-Bonding Groups.
Although most organic pigment particles and some relatively inert inorganic particles such as quartz do not have charged sites on their surface, they may have hydrogen-bond donor or acceptor groups, such as esters, ketones, and ethers It is therefore possible for a hydrogen bond between the particle and an anchor group on the polymeric dispersant to form. Even individual hydrogen bonds will be weak. A strong interaction may be developed between the pigment particle and a polymeric dispersant containing many hydrogen-bond donors and acceptors in its anchor chain, see figure 2

Figure 2: Anchoring by hydrogen bonding to a polymeric group.

Polyamines and polyols are used to anchor via hydrogen bonding, either donor or acceptor. Polyethers can be used to anchor via hydrogen-bond acceptance.

Anchoring Through Polarizing Groups.

An interaction can also take place between polarized or polarizable groups on an organic pigment particle surface, and similarly polarized or polarizable groups on the anchoring function of the polymeric dispersant. Again, these interactions will often be relatively weak, but strong interaction may be developed with a polymeric dispersant possessing an anchor chain composed of several of these groups.

Figure 3 : Anchoring through polarizing groups

Polyurethanes are commonly used as polarizable anchor groups.

Anchoring Through Solvent-Insoluble Polymer Blocks.

It is possible to anchor a polymeric dispersant onto a pigment particle surface simply via van der Waals interactions and without recourse to ionic, hydrogen-bonding, or polarizing effects. The polymeric block within the dispersant must simply be insoluble in the medium, see figure 4.
It is possible, for example, to disperse a pigment in an aliphatic hydrocarbon using a polymeric dispersant based on poly(tert-butylstyrene) chains, which are solvent-soluble, and polystyrene chains, which are not solvent-soluble.

Figure 4 : Anchoring through solvent insoluble polymer blocks.

Polyurethane anchor groups are said to operate via this mechanism. In fact, it is very difficult in practice to distinguish between this and the previous two adsorption mechanisms. Most polymeric anchor chains probably anchor via a mixture of electrostatic forces (hydrogen bonding and/or polarization) and van der Waals forces. One of the mechanisms may be dominant, but the most effective polymeric dispersants probably maximize the effect from all three mechanisms.

Derivatives of the Dispersed Particle.

Some organic pigments (phthalocyanine blues and dioxazine violet are good examples) are not very responsive to any of the anchoring mechanisms just described . In such systems it can be very difficult to obtain anything other than dispersions of relatively low pigment concentration, and these dispersions are prone to flocculation on letdown. Then the only way to solve the issue is by modifying the chemical structure of the particle itself in order to make it act as the anchor group. This system works most effectively on higher molecular weight pigments with large planar structures, because the anchor group can pack very closely onto the pigment particle surface and maximize the van der Waals attractive forces between particle and anchor groups.

The copper phthalocyanine molecule has been modified by the addition of polymeric chains to give a particularly effective dispersing agent for copper phthalocyanine pigments. Alternatively, derivatives with substituted ionic groups can be used to activate the surface of a pigment and make it receptive to the charged anchor group of a polymeric dispersant. This mechanism is illustrated in the figure 5 below.

Figure 5 : synergists

The nature of the polymeric chain is critical to the performance of polymeric dispersants. If the chains are not sufficiently solvated, then they will collapse on to the pigment surface allowing the particles to aggregate or flocculate. The need for compatibility with the medium extends throughout the final drying stages of any applied coating. If it ceases to be compatible, flocculation may occur leading to a decrease of surface properties such as losses in gloss and tinctorial strength, etc.

The molecular weight of the polymeric dispersants must be sufficient to provide polymer chains of optimum length to overcome Van der Waals forces of attraction between pigment particles:

• If the chains are too short, then they will not provide a sufficiently thick barrier to prevent flocculation. It means that too low a molecular weight will cause dispersion instability and will lead to an increase in viscosity and a loss of tinctorial properties.
• When the chains are too long, they have a tendency to "fold back" on to themselves. Too high a molecular weight will also give reduced performance.

Ideally the chains should be free to move in the dispersing medium. As previously said: chains with anchor groups at one end only, have shown to be the most effective in providing steric stabilization.

Finally, for good surface coating properties and performances, the polymer must be fully compatible with the coating resin after the solvent has evaporated off and the resin has been cross-linked.

Chemistry of the Steric Stabilization Chain

In order to meet the need for good compatibility, several different polymer chain types are utilized in the polymeric dispersant range, effectively covering the variety of solvents encountered.

Examples, spanning the range of solvent from nonpolar aliphatic hydrocarbons to alcohol/water includes:

• Polyisobutylene
• Polyesters
• Poly methyl methacrylate
• Polyethylene oxides

The amount of polymeric dispersant used is also an important parameter to consider. Many surface coating systems will tolerate a polymeric dispersant at low levels of addition, but problems will be caused at higher loading. Some systems are particularly tolerant to the presence of polymeric dispersants. Long-oil alkyd resins for air-drying paints and resins used in publication gravure inks and offset lithography inks are all good in this respect. Similarly, paper or wooden substrates tend not to give major adhesion problems. Higher quality stoving or two-pack paint systems and many packaging ink systems pose much more severe requirements.

It is therefore vital that after an initial screening of polymeric dispersants for rheological and color/gloss changes, their effect on the performance of the surface coating be checked by the appropriate tests.

Surfactants are conventional low molecular weight dispersing agents. Surfactant molecules are able to modify the properties and, in particular, they lower the interfacial tension between the pigment and the resin solution.

This surface activity arises because the surfactants' structure consists of two groups of contrasting solubility or polarity. In aqueous systems, the polar group is known as a hydrophilic group and the non-polar group as hydrophobic or lipophilic. In non-aqueous systems, the polar group is known as the oleophobic group and the non-polar group as oleophilic. Surfactants are classified according to their chemical structure and, more specifically, their polar group: anionic, cationic, electroneutral and non-ionic (see figure 1).

As with the polymeric dispersing agents, their effectiveness is determined by:

• The absorption of the polar group onto the pigment surface. The anchoring groups can be amino, carboxylic, sulfonic, phosphoric acids or their salts.
• The behavior of the nonpolar chain in the medium surrounding the particle. This part of the molecule (aliphatic or aliphatic-aromatic segments) must be highly compatible with the binder system.

The stabilization mechanism of surfactant-like dispersing agents is electrostatic: the polar groups forming an electrical double layer around the pigments particles. Due to the Brownian movement the pigment particles frequently encounter each other in the liquid medium thus having a strong tendency to re-flocculate on the let down stage.

Because of their chemical structure (eg: low molecular weight) and the electrostatic method of stabilization, surfactants may cause the following defects:

_ Water sensitivity: Surfactants generally have a tendency to provide water sensitivity to the final coating, thus making them inappropriate for use in outdoor applications.
_ Foam formation: Many surfactants generate foams which lead to surface defects (e.g.. fish eyes, craters) on the final coating. If foaming occurs at the milling stage it can also cause a loss of production capacity.
_ Interference with intercoat adhesion.

Over the past years specific surfactants have been developed to minimize these defects, and some provide other advantages to the final paints such as defoaming/dearation or difficult substrate wetting.

The most widely used surfactants for pigment dispersion in coating formulations are:

• Fatty acid derivatives
  
• Phosphate esters
  
• Sodium polyacrylates / polyacrylic acid
  
• Acetylene diols
  
• Soya lecithin

For more information, click on the links above.