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)
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
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.
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.
Stabilization of pigment suspension - Overview
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
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
|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
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
- 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).
Stabilization of pigment suspension - Colloidal stabilization
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
- Barrier-type stabilization with polymeric dispersants.
Practically, three systems of coating stabilization can be used in aqueous medium:
- the use of ionic surfactants or chemical lyophilizers which produce carboxy, ammonium
etc. ions, see electrostatic stabilization
- 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
- 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
Stabilization of pigment suspension - Electrostatic stabilization
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.
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
Stabilization of pigment suspension - Steric stabilization
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
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
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.
Formulating an optimum dispersion
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:
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
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
Pigments / Fillers
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).
|Iron Oxide - Yellow
Order of Addition
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).
Stage 1 - Calculation of % AOWP of Polymeric hyperdispersant
The theoretical amount of Polymeric dispersants (e.g. Avecia Solsperse) agent required in
a millbase is 2mg Polymeric agent per sq. meter pigment surface area.
Pigment surface area - 70m2/g
Therefore 140mg Polymeric agent/1g pigment are required = 14g agent/100g pigment i.e. 14%
Synergists (if required) are used with the Polymeric in ratios between 4:1 and 9:1 [Polymeric:synergist].
|Stage 2 - Determines the higher pigment content required
(Can be performed on lab. shaker e.g. Red Devil)
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.
|Stage 3 - Determines optimum amount of Solsperse Hyperdispersant
(Can be performed on lab. shaker e.g. Red Devil)
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
|Stage 4 - Optimizes final pigment concentration
(Should be done in equipment representative of bulk production)
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 :
| Phtalocyanine blue pigment
||30.0 (i.e: 10% active dispersant on the weight of pigment)
| Polymeric dispersing agents
Dispersants families - Introduction
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
The main differences of those two types of dispersants being the molecular
weight, the stabilization mechanism and the resulting let down stability.
Polymeric disperants - Description
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:
- It must be capable of being strongly adsorbed into the particle surface and thereby
possess specific anchoring groups
- The molecule must contain polymeric chains
that give steric stabilization in the required solvent or resin solution
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
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.
Polymeric disperants - Anchor groups
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.
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
Polymeric disperants - Polymeric chains
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
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
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:
- 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
- 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
_ 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
For more information, click on the links above.
Fatty Acid Derivatives
Nonionic fatty acid derivatives such as the alkyl
phenol ethoxylates (APEs) and fatty alcohol ethoxylates (FAEs) are one of the
main types of surfactant used in coating applications as wetting and dispersing
agents for pigment particles, particularly in decorative emulsion paints and water
Typical structures of fatty alcohol ethoxylates (FAE) and alkyl phenol athoxylates
These type of surfactants help stabilizes the aqueous dispersions
of organic pigment particles by the steric stabilization mechanism. Most of the
time, they are used in combination with an anionic surfactant which provides stabilization
of the dispersion by the electrostatic stabilization mechanism, but concerns over
the APEs nonionic surfactants has led to the recent apparition of new product
blends advertised as APE free.
Coating formulated with these types of nonionic dispersing agents are sometimes
subjected to foaming, water sensitivity, intercoat adhesion and blistering
Due to the anionic structure of the phosphate group,
phosphate esters dispersing agents provide steric stabilization to the pigment
solution. It offers the following benefits:
- Efficient dispersing agents in waterborne coatings
- Good wetting properties on difficult surfaces
- Anti-rusting properties
- Effective with polymeric rheology modifiers
The chemical structure of various phosphate esters are shown in
the figure below.
Structures of various phosphate esters
Phosphate esters surfactants are used in waterborne coatings for
their wetting and dispersing properties and the economical alternative to other
dispersing system that it provides. Phosphate esters are sometimes used in combination
with non-ionic surfactants to enhance dispersion stability, especially reduce
Polyacrylic acid/ Sodium polyacrylate
Polyacrylic acid (PAC) and salts of polyacrylates
are anionic surfactants used as dispersants in water based coating and ink formulations.
They generally consist of low molecular weight polymers that are able to keep
the pigment particles suspended in the resin solution by imparting a negative
charge to the particles (electrostatic stabilization).
Polyacrylic Acid structure and conversion to sodium polyacrylate
To reduce the side effects of standard surfactant
types of dispersing agent such as foaming, oligomeric acetylenic ethoxylate glycols
have been developed with multi-functional properties and especially defoaming
property which benefit water based coatings:
- defoaming/low foam
- excellent wetting
- reduced millbase viscosity/allows higher pigment loading
- improved color strength development
- enhanced flow and levelling
- reduced water sensitivity
The multi-functional properties of the gemini surfactant are related
to its unique chemical structure (Carbon-Carbon triple bond, two symmetrical oxygen
atoms, ethoxylate branched chains) containing two hydrophobic
groups and two hydrophilic groups attached by a short chain coupler.
Typical structure of ethoxylated acetylene diols