3 THE BINDER - EMULSION PREPARATION AND FILM
3.1 Latices
The majority of water-borne paints are latex paints, i.e.
aqueous dispersions of water-insoluble polymers made by emulsion polymerization using free-radical initiators.
In the majority of cases, the polymers are based on combinations of monomers, often with a high content of water-insoluble entities such as methyl methacry-late, butyl acrylate and styrene, and a much smaller fraction of water-soluble monomers such as acrylic and methacrylic acid. The water-soluble monomers give oligomeric acid segments at the latex particle surface which improves the colloidal stability of the formula-tion and adhesion and curing characteristics of the film.
Most latices for paints have an average particle diameter in the range 100-500 nm.
The emulsifier used in latex preparation is often a combination of nonionic and anionic surfactants. The nonionic surfactant has traditionally been an alkylphe-nol ethoxylate, but environmental concern has caused a change-over to other ethoxylated surfactants, such as fatty alcohol ethoxylates or fatty acid monoethanolamide ethoxylates. The ethoxylate is the surfactant mainly responsible for dispersion stabilization. The steric sta-bilization provided by surfactants with relatively long polyoxyethylene chains (>10 EO) is needed in order to retain stability at high-solids content in the presence of electrolytes. Steric stablization also gives proper shear stability to the latex. The presence of an anionic surfac-tant, usually an alkyl sulfate or an alkylaryl sulfonate, during the latex synthesis is needed to compensate for the reverse temperature dependence of ethoxylated sur-factants. For nonionics, an increase in temperature leads to a decrease in water solubility and an increase in oil solubility. During the course of the emulsion polymer-ization, there is an increase in temperature which would lead to formation of a water-in-oil emulsion if nonionic surfactants were the sole emulsifier. Upon depletion of the monomer phase, i.e. at high conversion, there would be a phase inversion into an oil-in-water emulsion. Such a phase inversion leads to a very broad particle size dis-tribution, since particle nucleation as well as reaction kinetics will be out of control. A way to circumvent the problem is to use a semi-continuous polymerization pro-cess with the monomer being slowly fed into the reactor during the polymerization.
Figure 6.4 shows a typical surfactant monolayer at the surface of latex droplets. It is important to realize that
the surfactants are not permanently adsorbed at the sur-face but subject to a continuous adsorption-desorption process. As was discussed above, the driving force for adsorption at a hydrophobic latex surface is stronger for nonionic surfactants than for anionics, unless the elec-trolyte concentration is very high. This leads to a higher ratio of nonionic to anionic surfactant at the surface than in the bulk solution.
Anionic surfactants provide electrostatic stabilization and such latices exhibit good storage stability in formu-lations containing low or moderate salt concentrations.
However, latices made with only anionic emulsifier are not stable at high electrolyte concentrations. Evaporation of water during the drying process leads to a continu-ous raise in ionic strength of the formulation. Often, the stability limit is exceeded at a relatively early stage, leading to particle coagulation and consequent loss in film gloss. Steric and electrostatic stabilization of dis-persions is further discussed in Section 4.2 below.
In recent years, there has been a considerable interest in the use of polymerizable surfactants for latices.
Such surfactants contain a polymerizable group which may be an acrylate, a methacrylate, an allyl ether or a maleate double bond. Both anionic and nonionic reactive surfactants are commercially available. The polymerizable surfactant is usually added from the beginning, in essence serving as a comonomer in the emulsion polymerization. The reactive surfactant may
also be added at the end of the polymerization process, in which case conventional surfactants are needed to prepare the latex.
By using surfactants that become covalently bonded to the latex particle, many of the problems encountered with conventional surfactants can be avoided or at least minimized. Positive effects are often obtained both on the dispersion itself and on the dried film.
The surfactant-related problems in latices, as well as in many other dispersions, arise from the fact that surfactants physically adsorbed on the particle surface may desorb into the bulk aqueous phase and that the equilibrium between surface and bulk surfactant concentration is governed by factors such as particle concentration, temperature, ionic strength and pH, all of which may be changed during storage, paint application and film formation. Since a certain surface concentration of surfactant is needed to give proper latex stabilization, a change in the adsorption-desorption equilibrium may severely affect the rheology and stability of the dispersed system.
Formulations containing a latex in combination with another dispersion, such as a pigment slurry, constitute a particular problem from a stability point of view. As discussed above in Section 2, the physically adsorbed latex surfactant may have a higher affinity for the pigment than for the latex, a situation which often leads to latex instability. The surfactants used as pigment Figure 6.4. A mixed monolayer of nonionic and anionic surfactants usually stabilizes latex droplets
dispersants are usually different from those used as emulsifiers in the emulsion polymerization process.
Hence, the different surfactants will compete for both surfaces, i.e. the latex and the pigment, and the surface composition and coverage obtained in the equilibrium situation may be very different from that of the two components before mixing. This type of competitive adsorption may drastically affect the rheology and stability of a formulation.
The presence of surfactant in the dried latex film may also impair the film properties. During drying, the surfactant is adsorbed on the latex particles. As the particles coalesce during the annealing process, the surfactant migrates out of the bulk phase and concen-trates at the interface. It has been shown that surfactant molecules preferably go to the film-air interface, where they align with their hydrophobic tails pointing towards the air. Calculations from ESCA (Electron Spectroscopy for Chemical Analysis) spectra show that a lacquer film containing 1% surfactant may have an average surface surfactant concentration of around 50%. Such a high concentration of a non-chemically incorporated, water-soluble component at the film surface will adversely affect the adhesion properties and create problems in terms of repaintability. It may also impair the water resistance of the film.
Furthermore, AFM (Atomic Force Microscopy) studies have shown that during the film-forming process many conventional surfactants phase separate from the binder. When the surfactant has phase separated, the water flux may carry it to the film surface. Alter-natively, it may accumulate in the interstices between
the particles from where it will migrate to the film-air or film-substrate interfaces through a long-term exudation process. Eventually, the surfactant will be present in aggregates of considerable size, seen by AFM as "hills".
After treatment with water, the surfactant aggregates are washed away and the hills are replaced by distinct
"valleys". The rough surface will give rise to poor gloss.
It has been shown that many of the surfactant-related problems in latex paints can be minimized by the use of a polymerizable surfactant as emulsifier in the emul-sion polymerization process. Several types of reactive surfactants are used for this purpose, some of which are shown in Figure 6.5. Block copolymers of ethy-lene oxide and propyethy-lene or butyethy-lene oxide with a polymerizable group at the far end of the hydrophobic segment (compounds I and II) have become popular, partly due to ease of preparation. Of particular inter-est from the performance point of view are surfactants which preferably undergo copolymerization rather than homopolymerization. A good example of such surfac-tants are the maleate half-esters of fatty alcohols, such as compound III in Figure 6.5. However, even surfac-tants based on highly reactive groups such as maleate do not become quantitatively copolymerized during the emulsion polymerization.
As mentioned above, AFM is an excellent method to evaluate the influence of the latex surfactant on the film topography. A smooth film with as little as possible of hills and craters is needed in order to obtain high gloss. Figures 6.6 and 6.7 illustrate the difference in topography, as seen during the film formation process,
Figure 6.5. Examples of polymerizable surfactants for latices
Figure 6.6. AFM images of films cast from a butyl acrylate-styrene- acrylic acid latex stabilized by sodium dodecyl sulfate: (a) before rinsing with water; (b) after rinsing with water: x- and y-axes, 0.500 |im/di vision; z-asis, 25.000 nm/division. (From A.-C. Hellgren et aL, Prog. Org.
Coatings, 903, 1 (1999) Reprinted with permission from Elsevier Science)
that can be obtained with a conventional anionic surfactant, sodium dodecyl sulfate (SDS), and with a polymerizable anionic surfactant, i.e. compound III in Figure 6.5. The same scale is used in all of the figures, but one should note that in each picture the z-dimension is plotted at 20 times larger magnification than the x-and y-dimensions.
Figure 6.6(a) shows a film formed from the SDS-stabilized latex. The film has a smooth, wavy surface, as would be expected after annealing for 48 h far above the glass transition temperature of the film. After rinsing with water, large pits were created, as shown in Figure 6.6(b). This change in the film morphology is an effect of migrating surfactant. During the drying stage, SDS moves with the evaporating water towards the
Figure 6.7. AFM images of films cast from a butyl acry-late-styrene-acrylic acid latex stabilized by the maleate surfactant, compound III in Figure 6.5: (a) before rinsing with water; (b) after rinsing with water: x- and y-axes, 0.500 jam/division; z-asis, 25.000 nm/division. (From A.-C.
Hellgren et aL, Prog. Org. Coatings, 903, 1 (1999) Reprinted with permission from Elsevier Science)
film surface where it crystallizes to form a continuous separate phase, thus covering the total surface area.
Upon rinsing with water, the highly water-soluble SDS is washed away. The roughness of the remaining film surface is caused by the migrating surfactant phase preventing orderly packing of the latex particles.
Figures 6.7(a) and (b) show the topographies of films cast from maleate-stabilized latex, before and after rins-ing, respectively. The film before rinsing showed "hills", indicative of surfactant aggregates at the surface. After the film was rinsed with water, holes appeared in a regular pattern. This is indicative of removal of surfac-tant from the surface. The situation is much improved compared to the appearance of the films from the SDS-based latex but, evidently, even with the reactive maleate surfactant, a substantial portion is not anchored to the latex particle. Chemical analysis showed that about 1/3 of the surfactant was not chemically incorporated into the film.