4 PIGMENT DISPERSION 4.1 Paint pigments
4.2 Pigment dispersants
The additives used in the grinding process are called pig-ment dispersants. Their primary function is to surround the suspended pigment particles with a barrier enve-lope that by either ionic repulsion or steric hindrance, or both, prevents random contact with other pigments.
In water-borne paints, electrostatic repulsion is usually the most important stabilizing mechanism and the well known DLVO theory that accounts for the balance of
Figure 6.12. Schematic representations of (a) agglomerated, (b) dispersed, and (c) flocculated pigments
Liquid Liquid
attractive van der Waals forces and repulsive electro-static forces in suspensions can be used as a tool to understand the stability characteristics of the system. In media of low dielectric constant, such as white spirit or xylene, steric repulsion is the dominating force. Steric repulsion is achieved by polymers with an anchoring group attached to the particle surface and a segment with high compatibility with the continuous phase extending away from the particle. Steric repulsion can be explained as a loss of entropy when the polymer chains attached to two different pigment particles are forced to pene-trate into each other. In order to maximize this loss in entropy, i.e. to achieve maximum stabilization, the density of polymer chains on the particle surface must be high. Sometimes, a stabilizing system is used that provides both electrostatic and steric stabilization; this is referred to as electrosteric stabilization. Figure 6.13
illustrates electrostatic, steric and electrosteric stabiliza-tion. (Electrosteric stabilization is very common for lat-ices, with Figure 6.4 showing latex particles stabilized by this mechanism.)
Most inorganic pigments are oxides. These exhibit a surface charge in aqueous systems that is characteristic of the oxide and the pH of the water. Each pigment has an isoelectric point which corresponds to the pH at which the positive and negative charges on the oxide surface are balanced. By moving away from this pH, a net charge, positive or negative, starts to develop that leads to pigment particle repulsion. Since the introduction of an ionic dispersant on to the oxide surface alters the balance of charges on this surface, it also acts to alter the isoelectric point of the pigment particle, as shown in Figure 6.14. For any given pH, adding an ionic dispersant may improve or reduce
Figure 6.13. Electrostatic (a), steric (b) and electrosteric (c) stabilization of a dispersion
PH
Figure 6.14. Plot of zeta potential (£) of aqueous dispersions of kaolin pigment particles as a function of pH with and without a phosphate dispersant present. (From T. C. Patton, Paint Flow and Pigment Dispersion, 2nd Edn, John Wiley, New York, 1979, p. 292, Reproduced with permission)
Zeta potential, £(mV)
Increasing acidity Increasing alkalinity
Phosphate
dispersant No dispersant
dispersion stability depending on whether the charge will increase or decrease.
In principle, electrostatic stabilization can be achieved by either anionic or cationic pigment dispersants. In order to be compatible with the other components of a paint formulation, in particular the latex which is almost always negatively charged, anionic dispersants are usually employed. A good dispersant for inorganic pigments for water-borne paints should (i) give a low isoelectric point of the coated particle, i.e. should be the salt of a relatively strong acid, and (ii) have a strong affinity for the pigment surface.
4,2.1 Dispersants for water-borne paints The most common type of dispersants for inorganic pigments for use in water-borne paints are polyelec-trolytes. These may be inorganic, e.g. various types of polymeric phosphates, or organic, such as poly aery lates or styrene-maleate copolymers. Organic polyelectrolyte dispersants are polymers of relatively low molecular weight. For instance, polyacrylate dispersants usually give the best performance when composed of between 12 and 18 monomer units. The optimum in length prob-ably reflects the balance between proper anchoring at the particle surface, which is favoured by a high molec-ular weight, and the rate of adsorption at the surface, which is faster for species of lower molecular weight. In evaluating the dispersion efficiency of added dispersants, one must also take into account the wetting-dispersing properties of the binder used in the formulation. Alkyd resins, in particular, have good wetting properties, and often contribute considerably to the dispersing power of the total coating formulation.
Polyphosphates are versatile inorganic polyelec-trolytes that at relatively low concentrations disperse a variety of inorganic pigments. Table 6.1 shows some common polyphosphates used as pigment dispersants.
Polyphosphates are suitable as pigment dispersants on two accounts. First, the polyphosphoric acids are rel-atively strong acids, i.e. the first p ^as are low, and
Table 6.1. Examples of alkali phosphate dispersing agents Common name Ratio of Number of Representative
basic oxide P atoms formula to acidic oxidea
Orthophosphate 3.00 1 K3PO4
Pyrophosphate 2.00 2 K4P2O7
Tripolyphosphate 1.67 3 K5P3O10
Metaphosphate 1.00 n Nan(POs)n
^Basic oxide: Na2O, K2O; acidic oxide: P2O5.
thus the polyphosphate-coated particles become strongly negatively charged at all relevant pH values. Secondly, the phosphates bind remarkably strongly to many inor-ganic surfaces. The anchoring can occur by either of two mechanisms, i.e. acid-base interactions or chemisorp-tion. Acid-base interactions usually occur with oxide or hydroxide pigments, and the polyphosphate molecule with its regular pattern of P-O ~, P = O and P-OH units can act both as the base, being an electron donor, or acid, being an electron acceptor, in the interaction with the pigment surface. Chemisorption may occur at the surface of pigments such as CaCO3, BaSO4 and ZnO, since phosphates form insoluble precipitates with cal-cium, barium and zinc ions (as well as with many other metal ions). Regardless of the mechanism of anchoring, the binding of polyphosphates to an inorganic pigment can often be regarded as irreversible. For this reason, polyphosphates are very efficient dispersants for many inorganic pigments in water-based formulations.
However, a number of problems are associated with the use of polyphosphates. They contribute to eutrophication, thus leading to an acceleration of the metabolism and reproduction of some fungi on paint films and at runoff areas. Polyphosphates may also give rise to blooming, although this occurs much less with potassium than with sodium salts, and less with polymeric phosphates than with monomeric species.
The bloom is a frosty, crystalline deposit that may appear on the surface of aged paint films. The bloom-ing problem is the reason why the more expensive potassium polyphosphates are preferred to the corre-sponding sodium salts. Furthermore, polyphosphates may degrade by hydrolysis, particularly when the paints are kept at an alkaline pH (and most latex paints are formulated at a pH of 8-9), or when certain transition metals, which form strong com-plexes with phosphates, are present. The degradation may lead to unexpected rheological changes in the formulation.
Other types of polyelectrolytes that are now being used as pigment dispersants include polysilicates, polya-luminates and polyborates. These polyelectrolytes func-tion in the same way as the polyphosphates. Blooming is not usually a problem with these other inorganic dis-persants as it is with the polyphosphates. The anchoring to the pigment surface is often not as good with other polyelectrolytes when compared to the polyphosphates.
However, the pigment-polyelectrolyte interaction can be very specific, leading to a chemisorption type of bind-ing. In practice, it is not uncommon to use mixtures of inorganic dispersants, for instance, various ratios of soluble alumina and silica. The isoelectric point of the
pigment particles can then be adjusted by varying the Al2O3:SiO2 ratio.
In addition organic dispersants are used with inor-ganic pigments for water-borne coatings. For instance, polyamines impart excellent dispersion properties to many inorganic pigments. In order to achieve optimum properties, the spacer length between the amino groups must be such that it matches the distance between the adsorption sites on the pigment surface. As an example, 1,3-propylene diamine, with three carbons between the amino nitrogen atoms, is a better dispersant for clay par-ticles than either ethylene diamine (with a two-carbon spacer) or 1,4-butylene diamine (with a four-carbon spacer).
Various types of amino alcohols are excellent disper-sants for some inorganic pigments used in water-based formulations. The amino group anchors the molecule to the pigment surface and the hydrophilic hydroxyl group extends out into the aqueous phase. Amino diols are often preferred to simple amino alcohols.
Organic pigments for water-borne paints are often more hydrophobic in character than inorganic pigments.
Table 6.2 gives a list of the HLB values for common paint pigments, both organic and inorganic. (HLB stands for hydrophilic-lipophilic balance, on a numerical scale from O (extremely hydrophobic) to 20 (extremely hydrophilic).) Inorganic poly electrolytes often do not bind strongly to the surface of these hydrophobic parti-cles. Instead, surfactants of various types are commonly used. Anionic surfactants may be employed to pro-vide electrostatic stabilization, nonionics to give steric
Table 6.2. HL B values of various organic and inorganic pigments. (From R. H. Pascal and F. L. Reig, Off. Dig., 36, 839 (1964))
Pigment HLB Organic pigments
Phthalocyanide blue (green shade) 14-16 Azo yellow 13-15 Quinacridone red 12-14 Phthalocyanide green (yellow shade) 12-14 Nickel azo yellow 11-13 Quinacridone violet 11-13 Phthalocyanide blue (red shade) 11-13 Phthalocyanide green (blue shade) 10-12 Toluidine yellow 9-11 Toluidine red (medium) 8-10 BON red (dark) 6-8 Inorganic pigments
Iron oxide (yellow) 20 Chrome yellow 18-20 Titanium dioxide 17-20 Molybdate orange 16-18 Iron oxide (red) 13-15
stabilization, or a mixture of the two used to impart electrosteric stabilization, as discussed above and illus-trated in Figure 6.13. Alkylbenzene sulphonates are the most commonly used anionic surfactants for this pur-pose, while alkylphenol ethoxylates with relatively long polyoxyethylene chains, typically 20-30 oxyethylene units, have been the nonionic surfactants of choice.
For environmental reasons, the slowly biodegradable alkylphenol ethoxylates are gradually being replaced by fatty alcohol ethoxylates of similar HLB values. The switch away from alkylphenol ethoxylates is not, how-ever, without problems. Anchoring of the fatty alcohol chain to the pigment surface is often not as good as for alkylphenols. It is probable that alkylphenol derivatives can form electron donor-acceptor (EDA) complexes with pigment surfaces that contain electron-deficient groups, as is illustrated in Figure 6.15. Such EDA com-plexes may also form with alkylbenzene sulfonates. No
Pigment
Figure 6.15. Schematic representation of an electron donor-acceptor complex formed between an alkylphenol ethoxylate and unsaturated moieties on a pigmented surface. (From K. Holmberg, Surf. Coatings Int., 76, 481 (1993))
X
such contribution is, of course, possible in the case of alcohol ethoxylates which are always based on sat-urated alkyl chains. Monoethanolamide ethoxylates of polyunsaturated fatty acids, which were discussed above in Section 3 are an interesting class of surfactants in this context. This type of nonionic surfactant seems to work well as a pigment dispersant and it is likely that these compounds, which contain n -electrons in the acyl chain, can form EDA complexes with some organic pig-ment surfaces, although these complexes may not be as strong as for alkylphenol ethoxylates. Figure 6.16 shows the structures of an alkylbenzene sulfonate, an alkylphenol ethoxylate, an alcohol ethoxylate and a monoethanolamide ethoxylate of a di-unsaturated fatty acid, i.e. linoleic acid. The latter is the main fatty acid found in common drying oils such as soy been oil and sunflower oil. Figure 6.16 also shows the structure of lecithin, a zwitterionic surfactant which has a long tra-dition as a efficient of organic pigments in paints.
Dispersion of pigments in aqueous formulations is in reality a more complex issue than has been described above. First, many pigments, and in partic-ular the inorganic pigments, have been surface treated by the pigment producer and relevant details about the composition of the applied coating is often kept as pro-prietary information. The common white pigment, rutile titanium dioxide, for instance, can be obtained with an anionic or a cationic surface charge and its HLB value can vary from very hydrophilic to extremely hydropho-bic. The paint formulator may not possess the detailed knowledge of the pigment surface characteristics needed to perform the dispersion experiments in a scientific manner. For this reason, the process of pigment dis-persion in the paint industry is still more of an art rather a science.
Another complicating factor is that optimized electrostatic stabilization of pigment particles in water-based formulations may lead to sub-optimization when
Figure 6.16. Structures of (a) dodecylbenzene sulfonate, (b) nonylphenol ethoxylate, (c) dodecyl alcohol ethoxylate, (d) the monoethanolamide ethoxylate of linoleic acid, and (e) lecithin (phosphatidylcholine)
it comes to binder-pigment compatibility. If, at the formulation pH, the pigment particles carry a very strong negative charge, the interaction with the binder, which is usually slightly negatively charged, will be strongly repulsive in character. Such incompatibility may lead to phase separation in the film forming step with adverse effects on the optical properties of the dried film. For this reason, a compromise may be needed between the efficiency in pigment dispersion and the compatibility between the pigment and the binder. As a general rule, the best pigment-binder compatibility is achieved if the two surfaces do not carry the same charge and if the degree of hydrophobicity of the binder and the coated pigment, for instance, expressed in terms of the HLB, is approximately the same.
4.2.2 Dispersants for solvent-borne paints Pigment dispersions in organic solvent formulations are based on steric stabilization, since electrostatic inter-actions in media of low dielectric constant are weak.
In order to give proper stabilization, the lyophilic part of the dispersant molecule, i.e. the part that stretches out into the continuous liquid phase, must be relatively large; thus, pigment dispersants for solvent-borne paints are polymeric in nature. It can be shown from calcula-tions using Hamaker constants of the particles across the liquid medium that the thickness of the stabilizing layer needs to be in the range of 5 - 2 0 nm for most pigments of particle size 0.1-1.0 urn. An extension of 5 - 2 0 nm is not very large for a polymer; the molecular weight of polymeric dispersants, therefore, need not be high. In practice, polymeric dispersants are seldom above 40 000 in molecular weight.
Pigment dispersants have for a long time been the most important application for polymeric surfactants and both graft and block copolymers are used for this purpose. Graft copolymers, often referred to as
"comb polymers", have now become more important than block copolymers. It is important that the seg-ments that extend away from the pigment surface are very compatible with the binder-solvent mixture, or, expressed in other words, that the binder-solvent mix-ture is a very good solvent for those segments. Only then do the dispersants keep the particles well sepa-rated, which is a necessary condition in order to achieve a low viscosity with a minimal amount of solvent in the formulation. Different lyophilic segments, with different solubility characteristics, are needed for different types of binder-solvent mixtures. One approach to this prob-lem has been to use the solubility parameter concept of
Hildebrand. Maximum dispersion efficiency is obtained when the solubility parameters of the binder-solvent mixture match the solubility parameters of the polymer segments that stretch out in to the solution.
The polymeric dispersants need to contain groups that provide strong anchoring of the molecule to the pigment surface. As was discussed above for disper-sants for pigments in water-borne paints, the choice of an anchoring group depends on the type of pigment used in the formulation. The anchoring mechanism is usually based on acid-base interaction, with the pigment sur-face constituting either the acid or the base moiety. For instance, acidic pigments are properly dispersed by graft copolymers with a poly(alkylene imine) backbone that forms a multiple of acid-base interactions with the pig-ment surface. Pigpig-ments with a basic character, on the other hand, may be dispersed by copolymers containing pendant carboxylic acid groups. PoIy(12-hydroxystearic acid) is a well known example of such a polymeric dispersant. Hydrogen bonding should be regarded as one example, and indeed a very important example, of acid-base interactions, and the anchoring of disper-sant to pigments in solvent-based paints can often be viewed as hydrogen-bonding interactions. For instance, phthalimine groups, which are typical hydrogen-bond acceptors, can be used as anchoring groups of poly-meric dipersants to hydroxyl-containing pigments. Such an interaction is illustrated in Figure 6.17.
5 WETTING OF THE SUBSTRATE Proper wetting of the surface by the coating is an absolute requirement in order to achieve good film properties. Wetting is normally measured by determining the contact angle obtained when a drop of the liquid formulation is put on a planar solid surface. Whether or not a given liquid spreads on a given substrate depends on both the liquid and the solid. In a scientific approach to wetting, it is important to be able to determine the wetting characteristics of the substrate. Determination of surface tension or surface free energy for a solid surface is not as straightforward as it is for a liquid.
The most common approach is that devised by Zisman who introduced a useful scheme for classifying low-energy surfaces with respect to their wettability. For many series of liquids on solids - including plastics, metals and metal oxide - it was shown that the contact angle decreases with decreasing surface tension of the liquid. For a homologous series of non-polar liquids, the increase in cos 0 with decreasing liquid surface tension is linear for a given solid, as illustrated in Figure 6.18.
Figure 6.17. Stabilization of a pigment with hydroxyl groups at the surface by a graft copolymer containing phthalimine anchoring groups
The critical liquid surface tension, yc, is defined as the point where the plotted line intersects the cos 6 line, i.e. the line representing complete wetting. In theory, all liquids with a liquid-gas surface tension (YLG) equal to or lower than the yc will spread on that surface. In practice, however, yc is not a constant for any given solid, but varies somewhat with the liquid type.
These regularities in the wetting properties of low-energy surfaces, such as polymers, and adsorbed ori-ented monolayers of organic materials on high-energy surfaces are significant. Even for non-homologous liq-uids, a plot of YLG against cos 6 shows points lying in a narrow rectilinear band. However, the line may exhibit curvature if hydrogen bonding can take place between the liquid molecules and the molecules in the solid surface.
The use of a "Zisman plot" to determine the critical surface tension is relatively straightforward and has become a widely used method to characterize a low-energy solid with respect to the surface free low-energy.
Table 6.3 gives the critical surface tension values for a number of common polymers, while Table 6.4 shows such values for various surface functional groups. The
Surface tension (mN/m)
Figure 6.18. A Zisman plot for n-alkanes on polytetrafluo-roethylene
latter values have been calculated from a large collection of experimentally determined yc values for polymers.
As can be seen from Table 6.4, the value of yc for a solid is indicative of the molecules that make up its surface. The surfaces having the lowest value of yc, and hence the lowest surface energy, consist of closely packed CF3 groups. Replacing one fluorine atom by hydrogen considerably raises the value of yc. This low
Table 6.3. Critical surface tension, yc, for various polymers at 20°C
Polymer yc (mN/m)
Poly(l,l-dihydroperfluorooctyl methacrylate) 10.6 Polyhexafluoropropylene 16.2 Polytetrafluoroethylene 18.5 Polytrifluoroethylene 22 Poly(vinylidene fluoride) 25 Poly(vinyl fluoride) 28 Polyethylene 31 Polytrifluorochloroethylene 31 Polystyrene 33 Poly(vinyl alcohol) 37 Poly(methyl methacrylate) 39 Poly(vinyl chloride) 39 Poly(vinylidene chloride) 40 Poly(ethylene terephthalate) 43 Cellulose 45 Poly(hexamethylene adipamide) 46
0 (degrees)
Cos6>
segment tyophilic
Polymer backbone
Table 6.4. Critical surface tension, yc, in relation to the surface constitution at 20° C Surface group yc (mN/m) Fluorocarbon surfaces
-CF3 6
-CF2H 15
-CF3 and - C F2- 17
- C F2- C F2- 18
-CF2-CFH 22
- C F2- C H2- 25
- C F H - C H2- 28
Hydrocarbon surfaces
-CH3 (crystal) 20-22
-CH3 (monolayer) 22-24
- C H2- C H2- 31
-CH-(phenyl ring edge) 35 Chlorocarbon surfaces
-CClH-CH2- 39
- C C l2- C H2- 40
=CC12 43
value of the critical surface tension indicates that the adhesion between liquids and surfaces containing triflu-oromethyl groups is very low. (Sometimes, however, the introduction of a terminal CF3 group does not decrease the wettability very much since the introduction of the dipole associated with the CF3-CH2 group gives an effect in the other direction.) Another observation that can be made from Table 6.4 is that CH3 groups have very low values when compared with CH2 groups. This implies that surfaces rich in methyl groups should have low surface tensions, which indeed is true. The most common type of silicone oil, i.e. polydimethylsiloxane, is probably the best example of such a methyl-rich surface.
The concept of the critical surface tension of a solid is useful for many practical applications, e.g.
surface coatings. In order for a coating to spread on a substrate, the surface tension of the liquid coating must be lower than the critical surface tension of the substrate.
(In addition, since a liquid is easier to break up and atomize when the surface tension is low, a lower surface tension means a better sprayability of the coating.) The well known coatings defect,"cratering", is also surface-tension related. Contaminants on the surface, such as fingerprints and oil spots, usually have lower critical surface tension values than those of the surrounding areas, thus putting extra demand on the coating with regard to low surface tension.
The surface tension of the coating is largely deter-mined by the polymer and the solvent. The polymers usually have relatively high surface tensions, with val-ues between 35 and 45 mN/m being typical. The organic
Table 6.5. Surface tension, y, for selected solvents at 20° C.
Solvent type y (mN/m) Alcohols 21-35 Esters 21-29 Ketones 23-27 Glycol ethers 27-35 Glycol ether esters 28-32 Aliphatic hydrocarbons 18-28 Aromatic hydrocarbons 28-30 Water 73
solvents have surface tension values between 20 and 30 mN/m, as can be seen from Table 6.5.
In general, when comparing solvents within the same solvent class it has been found that faster evaporat-ing solvents usually have lower surface tension values than their slower evaporating counterparts. Furthermore, increased branching of the solvent molecule leads to a lowering of the surface tension. Isopropyl alcohol, which fulfils the two above-mentioned requirements, has an extremely low surface tension, i.e. 21.4 mN/m. Evi-dently, the higher the solvent content of the coating, than the lower the surface tension. Conventional coat-ings normally lie in the range of 25-32 mN/m. This is low enough to give proper wetting on most surfaces, i.e.
to move below the values of the critical surface tension of the substrates (see Table 6.3).
As the resin becomes the major component of the coating, problems related to surface tension are frequently encountered. So-called high-solids coatings are, therefore, extremely sensitive to dirt and fingerprints (the surface tension of which is around 24 mN/m). Paint defects, such as "cratering" and "picture framing" occur more frequently with high-solids coatings than with conventional systems.
Water is a liquid of high surface tension and is obviously not suitable for wetting of surfaces. Use of water-borne paints would have been very limited had it not been possible to use surfactants in the formulation.
A good surfactant reduces the surface tension of water down to 28-30 mN/m, i.e. to the same range as that of the organic solvents used in paints and lacquers.