How does a coating film formation take place?
Role of coalescing agents and major types
Role of coalescing agents and major types
Coalescing agents play an important role in optimizing film formation to ensure a uniform and smooth surface finish. They typically demonstrate one or more of the following effects:
- Lowering Tg of the polymer
- Reduction of polymer particle surface area
- Increase in capillary forces by controlled evaporation of water
- Reduction of repulsive forces between polymer particles
If a dispersion paint film dries below the MFFT of its polymer, the film will be relatively brittle. It is unlikely to form a coherent film as full film coalescence will not occur.
The MFFT can be reduced by the addition of coalescing agent. Coalescing agents function as temporary plasticizers for the polymer particle, reducing the MFFT. Coalescing agents allow the formation of polymeric films at ambient temperatures of film application. They also allow film formation for polymers demonstrating an MFFT above the ambient temperatures.
Types of coalescing agents
The selection of a wide range of coalescing agents is made based on specific product features and benefits. Coalescing agents are classified according to their distribution in the polymer dispersion system. The following groups are considered:
Hydrophobic coalescing agents
A part of this category includes hydrocarbon solvents. These coalescing agents tend to be concentrated in the dispersed polymeric particle. Generally, these products demonstrate low efficiency as coalescing agents. They result in poor shelf stability and an increase in the viscosity of the paint. This happens due to swollen and flocculated particles.
The representatives of this group are difficult to incorporate properly in a paint formulation. They may cause flocculation, reducing:
- gloss,
- film transparency, and
- opacity
They contribute to the VOC level of the paint formulation. Although these products soften the polymer, they are less effective in influencing film formation. A major advantage of this group of coalescing agents is their low price.
Hydrophilic/water-soluble coalescing agents
Hydrophilic coalescents include glycols or water-soluble glycol ethers. For example, propylene glycol butyl ether (PB) and dipropylene glycol butyl ether (DPB). The benefits and limitations of these agents are as follows:
| Benefits |
Limitations |
| Medium to low volatility |
Risk of losing material due to diffusion into the porous substrate |
| Partially dissolves the polymer after water evaporation |
Tendency to contribute to blocking or affecting early water resistance |
| Good performance in increasing open time |
Generally poor in efficiency of reduction MFFT |
| Controls water evaporation and freeze-thaw stability |
Contributing to VOC |
Partially water-soluble coalescing agents
The partially water-soluble coalescing agents have the highest efficiency. They show the most favorable properties as a coalescing agent. This group of coalescing agents is concentrated at the boundary region of the polymeric particle.
The water-solubility of the main representatives is low, in the range of 0.5 to 5%. Their hydrophobic/hydrophilic balance allows them to be concentrated at the surface area of the polymeric particle in the dispersed stage. During the concentration stage of the film-formation process, the polymer is fully surrounded by the coalescing agent.
Their high efficiency is explained by the low rate of evaporation of these products. The optimal dosage is dependent on formulation and application characteristics like:
- the Tg of the polymer and
- the application temperature conditions of the final paint formulation.
The applied dosage has to be optimized in order to minimize the risk of side effects such as blocking. Some of the examples include 2,2,4-trimethyl-1,3-pentane diol mono-isobutyrate; "Texanol®" (TPiB) and propylene glycol mono esters of C6/C10-aliphatic acids.
MFFT Reduction as a Function of Coalescing Agent Concentration
Related Read: Select coalescents using HSP to improve film formation.
What are the properties of coalescing agents?
What are the properties of coalescing agents?
Ease of addition
Coalescing agents are added during the let-down stage of paint manufacturing. This takes place after the addition of a polymer dispersion binder. The addition should be carried out slowly, applying efficient mixing conditions. This avoids coagulation of the polymer binder. The risk for coagulation in the liquid system is highest in the case of using water-insoluble coalescing agents.
Efficiency
The coalescing efficiency is determined by checking its capability of reducing MFFT. This method works by starting from the (slightly diluted) polymer dispersion using a ladder range of coalescing agent dosages. It then applies the polymer dispersion as a film onto a temperature gradient plate (e.g., variation from 0 °C to 40 °C).
After drying, the film's appearance is visually evaluated. The MFFT reaches a point where the dried film is totally transparent. The film below the MFFT stays milky white and cloudy.
Other test methods are paint system-related and include checking the effect on paint film properties, such as:
The main indicator for determining the efficiency of the coalescing agent is the determination of the effect on wet scrub resistance. This is true for the application in high PVC paints (PVC >60).
The table below discusses some other properties of coalescing agents:
| Property
& Description |
Evaporation rate
A general characteristic of coalescing agents is that the evaporation rate is slower than water and high enough to ensure proper film formation under a wide range of temperature and humidity conditions. However, it should be fast enough to leave the paint film after application and not cause film softness.
|
VOC
For legislative reasons, a coalescing agent should preferably have no effect on the VOC of the paint. Unfortunately, there is no global uniformity on VOC definition, or uniform method to determine VOC.
However, the main definition is related to the boiling point, threshold for VOC being >250 °C. A method to determine VOC is described in ISO 11890-1:2007.
|
Odor
The effect on the odor of the paint formulation should be minimal, especially in case of applications for indoor paints. Products like TPiB and main mineral spirits demonstrate sharp and characteristic odor, whereas main glycol ethers are odorless. |
Color
Preferably, the coalescing agent should be colorless, to prevent discoloration. The dosage in high Tg-based polymer dispersion systems can be as high as 3% on the final system. |
Water solubility
Low water solubility is preferred over good solubility. Low solubility favors the coalescing agent to be concentrated at the polymer particle.
Additionally, the risk of losing the coalescing agent, being penetrated into porous substrates, or affecting early water-resistance of the paint film is reduced. |
Hydrolytic stability
Main application of coalescing agents is in dispersion paints to show a weakly alkaline pH. Ester-based coalescing agents may be hydrolyzed and lose efficiency on storage. A high degree of resistance to hydrolysis is desirable.
|
Cold temperature touch-up
The cold temperature touch-up test is performed to determine the effect of a coalescing agent on the film-forming process at low temperatures.
Improper coalescence results in poor mechanical properties, scrub, and touch-up performance (clear color differences between the touched part of the paint layer and the non-corrected part). This test is typically performed at 5 °C. The test image on the left shows the
top region with poor coalescence and the bottom region with good
coalescence. |
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