Water Based Coatings
Additives -Coalescing Agents
Mr Veeramani shares his knowledge in this Column devoted exclusively to Water Based Coatings
NOW let us turn our attention to another important additive known as “COALESCING AGENTS” which is incorporated in the formulation. Let's examine now why this additive is very important and what is its function.
Coalescing Agents are liquids having high boiling points when added in a formulation, aids the film formation by temporarily softening the vehicle. These aids facilitate the transition from liquid to solid state during the film formation process. The Emulsion Polymer consists of tiny solid polymer particles dispersed in water and when the drying starts, the particles come closer and closer. Though the repulsive forces keep them separated, finally the particles come together to form a continuous solid film, under the forces of the particles that have come into play between the particles. The particle will fuse together, when the latex collapses, if they are soft. But, if they are hard they will not fuse, the coating will crack, flake or pulverize. It is the job of the coalescent to soften these particles so that they fuse together to form a continuous film. Eventually the coalescent also should evaporate so that the paint film can be formed. A good coalescent agent must do: (1) be an active solvent for the polymer in the coating (2) should effectively lower the minimum film formation temperature (MFFT) and (3) have a rate of evaporation lower than water and also have a very low solubility in water.
The ability of a Polymer to form a film is temperature related. Each polymer has a characteristic temperature below which it will not form a film. This is called the Minimum Film Formation Temperature (MFFT) of the polymer. Some polymers have low MFFT values and easily form films but they are not suitable for paint applications. The polymers generally used in coatings have MFFT values in the range of 15 – 45 Deg.C. To form a good film at temperatures below the MFFT of the polymer, it is necessary to add a Coalescing Aid to the system. Formulators often consider the most severe conditions of temperature and humidity for a coating. Varying solvent mixtures and blends can vary the film hardness. Levels of coalescent affect drying time and ultimate film properties.
Typical example of Coalescing Aids are Ethylene Glycol monobutyl Ether (Butyl CELLOSOLVE), Diethylene Glycol Monobutyl (Butyl CARBITOL), and 2,2,4– trimethyl 1,3- pentanediol monoisobutyrate (TEXANOL). The Formulator should take care to choose the right Coalescing Aid for the formulation. As would be expected, the nature of the polymer - its solubility and affinity for various compounds, will affect the particular Coalescing Agent chosen.
Coalescents promote film formation by softening (like a plasticizer) the emulsion particle to allow viscous flow and fusion of the emulsion particles at application temperatures. Evaporation rate is very important because the Coalescent must do its job as just described and also be evaporative enough to leave the film. Some are very fast (Glycol Ethers) and some are very slow. The formulator needs to also consider the evaporation rate of water and especially the humidity conditions that may be present during coating application, which will affect the formation of the film. As humidity increases, the evaporation rate of water will decrease. However, humidity has no effect on coalescing solvents. Under controlled conditions with low humidity, a faster Coalescing Agent can be used. Most commonly used has been Ethylene Glycol Butyl Ether (EB). A very slow material needs to be used for high humidity situations. It is not uncommon for formulators to have a blend of Coalescing Agents in the formula that gives good Coalescing Activity and fast film – hardening rates.
As said earlier, Polymer dispersions, used as binders in dispersion paints, consist of particles finely distributed in water. The layers of surfactant and/ or colloid, present on the surface of the particles, act as stabilizers, preventing coagulation and provide stability in the dispersed stage. The particles have diameters of about 30 to 500 nm, or even higher in case of coarse, opaque dispersions. The film formation process, after application, involves evaporation of the solvent and formation of a continuous polymer film by purely physical means.
The coalescent should be the last component to evaporate from the latex coating. Since the coalescent is an organic compound, humidity will have little effect upon its evaporation rate. However, at the same time, humidity will have a drastic effect upon the evaporation rate of water. Thus, application of a latex paint under high humidity conditions can have a dramatic effect upon the final film properties of a coating. To compensate for the effect of high humidity, at least part of the coalescent package should contain a slow evaporating coalescent such as glycol ethers. Evaporation rates of the Coalescents will also have a direct effect upon the hardness development rate of a latex coating, since the coalescent should be the last component to evaporate from a latex coating.
Coalesced Vs. Un-coalesced Films
Therefore, in order for a formulator to select the most efficient coalescent package, two of the most important properties to consider are evaporation rate and coalescent efficiency. All of this becomes even more important when working with industrial latex coatings, which often require large amounts of Coalescents to ensure proper film formation and must have the fastest possible hardness development rate.
It is important for the Formulator to consider the activity of the coalescing material and it is partitioning tendency in the emulsion. Because an aqueous emulsion is a two – phase system, ie: a water and poly phase, the coalescing agent will partition itself between the two phases. The concentration in each phase depends on the hydrophobic and hydrophilic balance of both the solvent and the polymer. The way this distribution occurs is the most important factor in determining the activity of a coalescing material. Materials that are hydrophobic will tend to partition themselves mainly in the hydrophobic polymer phase with very little of the coalescent found in the aqueous phases. In contrast, hydrophilic coalescing aids will tend to partition themselves mainly in the aqueous phase and little will be found in the polymer phase.
Choice and amount of coalescing solvent may also affect thickener performance. The partitioning of the solvent between the continuous water phase and the polymer phase of the paint may impact the thickening efficiency and other properties. The coalescing solvent is intimately involved in film formation, and may also impact properties such as gloss, corrosion resistance, water resistance, adhesion and drying time.
Before I proceed to the next topic, I ran through all the topics written and published so far, I found that Co-dispersant was left out. I thought I can give some insight on this Additive here.
In Water-Based paints, adjustment of pH is generally required in order to stabilize the latex. This adjustment is called neutralization. Moreover, when a pigment concentrate is diluted in a white paint or white base, in water-based system, it is also necessary to neutralize these pigment concentrates. The development of color in a white base always requires optimization of the desired color intensity, to keep down the cost. The use of a polymeric primary dispersant in order to disperse large amounts of pigments in the pigment concentrates (or pastes), may necessitate having recourse to additives with a considerable co-dispersion effect in order to reduce the amount of primary dispersant, which is generally expensive. An anionic polymer dispersant for water-based paints. It's also used with solvent based systems. It works well with organic and inorganic pigments, reduces mill-base viscosity and improves gloss of the paint film. It has anti- flooding effect. A tested, proven and preferred amine used as Co-dispersant is 2-Amino-2-Methyl-1-Propanol. It does not exhibit any odour having a high boiling point.
It reduces dispersant demand when used in conjunction with conventional dispersants.
- Optimizes pigment dispersion
- Reduces foam (through dispersant reduction)
- Provides effective pH control
- Lowers raw material cost
- Improves thickener performance and Minimal contribution to system VOC
- Eliminates need for ammonia, resulting in a lower odour paint
- Improves color acceptance of shading pastes
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