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Coalescing Agents in Coatings

Coalescing agents/ film formers are used in dispersion paints for optimizing the film formation process of the polymeric binder particles. Coalescing agents typically reduce the minimal formation temperature and as a consequence, optimize film coherence and properties such as scrub resistance, mechanical properties as well as appearance.

Understand the process of coalescence and role of coalescing agents in coating film formation along with their major types and property profile.

Role of Coalescing Agents


Uniform Film
Coalescing agents play an important role in optimizing film formation to ensure uniform and smooth surface finish.

To understand their functioning better, let's first start off with the chemistry behind film formation.


How does a Coating Film Formation Takes Place?


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 course, 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 film formation of polymer dispersions can be divided into 3 steps:

  1. Concentration
  2. Compaction
  3. Coalescence

Film Formation Scheme Polymeric Dispersion System
Film formation scheme in polymeric dispersion system


After application of the polymer dispersion or the dispersion paint, containing polymer dispersion, a volume reduction occurs, as the result of evaporation of water and solvent as well as absorption by appropriate substrates. The decrease in volume forces the particles to move closer together and finally the repulsive forces -as a result of adsorbed stabilizers- is overcome and there is a direct contact between the particles.

The total solids content of the film at this stage is typically around 70 to 80%. After the stabilizing system has collapsed, film formation is irreversible. Next, the particles are forced to deform, mainly by capillary forces, until ultimately there is a homogeneous, coherent film. The total solids content at this stage is typically around 90% and slowly increasing, approaching 100%.


Film formationHowever, in order to accomplish full coalescence, deformation resistance forces, associated with the polymer hardness (Tg: glass-transition temperature), have to be overcome. Hard particles are unable to take part in coalescence.


For any polymer dispersion there is a typical limiting temperature above which the particles are so soft enabling capillary forces to overcome deformation resistance and allow full coalescence. This temperature is termed the minimum film-formation temperature: MFFT and can be determined by a MFFT bar (temperature gradient plate).


Role of Coalescing Agents / Film Formers


If a dispersion paint film dries below the MFFT of its polymer, the film will be relatively brittle and will be 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 and thereby reduce the MFFT. Coalescing agents allow the formation of polymeric films at ambient resp. temperature conditions of film application, even of polymers demonstrating a MFFT of above these temperature conditions.

Coalescing agents typically demonstrate one or more of following effects:

  1. Lowering Tg of the polymer
  2. Reduction of polymer particle surface area
  3. Increase in capillary forces by controlled evaporation of water
  4. Reduction repulsive forces between polymer particles


Types of Coalescing Agent


A wide range of coalescing agents is available; selection is made based on specific product features and benefits. Coalescing agents are classified according to the preferred distribution in the polymer dispersion system.

Following groups are considered:


Hydrophobic Coalescing Agents


Part of this category are hydrocarbon solvents. These coalescing agents tend to be concentrated in the dispersed polymeric particle. Generally these products demonstrate low efficiency as coalescing agent and typically result in poor shelf stability, viscosity increase of the paint, explained by swollen, flocculated particles.

Furthermore, representatives of this group are difficult to incorporate properly in a paint formulation and may cause flocculation, reducing gloss, film transparency, opacity and contributing to the VOC level of the paint formulation. Although these products soften the polymer, they are less effective in influencing the film formation. A major advantage of this group of coalescing agents is the low price.

Hydrophilic Coalescing Agents: Water Soluble


Hydrophilic coalescents include glycols or water-soluble glycol ethers, such as PB (propylene glycol butyl ether) and DPB (dipropylene glycol butyl ether). The benefits and limitations of these agents are as follows:

Benefits Limitations
Medium to low volatility Risk of losing material due to diffusion into porous substrate
Partially dissolve the polymer after water evaporation Tendency to contribute to blocking or effecting early water resistance
Good performance in increasing open time Generally poor in efficiency of reduction MFFT
Control water evaporation and freeze-thaw stability Contributing to VOC

Partially Water Soluble Coalescing Agents


Partially Water Soluble Coalescing AgentsRepresentatives of this class, partially water soluble coalescing agents, typically show highest efficiency and most favorable properties as a coalescing agent. Typically this group of coalescing agents is concentrated at the boundary region of the polymeric particle.

Although the water-solubility of main representatives is low and in the range of just 0.5 to 5%, the hydrophobic/ hydrophilic balance provides excellent conditions for being 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 coalescing agent.

Additionally the high efficiency is explained by it the low rate of evaporation of these products. The optimal dosage is dependent on formulation and application characteristics such as the Tg of the polymer and 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 examples: TPiB (2,2,4-trimethyl-1,3-pentane diol mono-isobutyrate; "Texanol®") and propylene glycol mono esters of C6/ C10-aliphatic acids.

MFFT Reduction as Function of Coalescing agent Concentration

MFFT reduction as function of coalescing agent concentration



Properties of Coalescing Agent


Ease of Addition

Easy Addition of Coalescing Agent
Coalescing agents are preferably added during the let down stage of paint manufacturing, after the polymer dispersion binder. In order to avoid coagulation of the polymer binder the addition should be carried out slowly, applying efficient mixing conditions. The risk for coagulation in the liquid system is highest in case of using water insoluble coalescing agents.

Efficiency


Efficiency of Coalescing AgentThe coalescing efficiency is determined 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 and applying the polymer dispersion as a film onto a temperature gradient plate (e.g: variation from 0°C to 40°C).

After drying, the film appearance is visually evaluated. The MFFT reaches a point where the dried film is totally transparent. The film below the MFFT stays milky white, cloudy.

Other test methods are paint system related and include checking the effect on paint film properties, such as:
  • Mechanical properties
  • Film impermeability
  • Cracking
  • Corrosion resistance
  • Film appearance, gloss

In case of application in high PVC-paints (PVC >60), the determination of the effect on wet scrub resistance is a main indicator for determining the efficiency of the coalescing agent.

The table below discusses some other properties of coalescing agents:

 Property  Description
Evaporation Rate 
Evaporation Rate of Coalescing Agents
A general characteristic of coalescing agents is that the evaporation rate must be slower than water and should be 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 causing film softness.
VOC 
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, a 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
Odor Properties of Coalescing Agents
The effect on the odor of the paint formulation should be minimal, special in case of application for indoor paints. Products like TPiB and main mineral spirits demonstrate sharp and characteristic odor, whereas main glycol ethers are odorless.
Color 
Color Properties of Coalescing Agents
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
Solubility of Coalescing Agents
Low water solubility is preferred over to good solubility. As stated before, low solubility will favor the coalescing agent being concentrated at or into the polymer particle.

Additionally, risk of losing the coalescing agent, being penetrated into porous substrates or effecting early water resistance of the paint film is reduced.
Hydrolytic Stability 
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 
Cold Temperature test with Coalescing Agents

Cold Temp Touch up Test.: Poor coalescence (top); Good coalescence (bottom)
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 not only results in poor mechanical properties and poor scrub but also poor 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.


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3 Comments on "Coalescing Agents in Coatings"
MUNIR A Aug 27, 2018
Our company is interested in bulk import of coalescing agent . Pls. contact only manufacturer. threestarchemicals@gmail.com
MUNIR A May 15, 2018
Our company is interested in bulk import of coalescing agent . Pls. contact only manufacturer. threestarchemicals@gmail.com
Jack G Feb 5, 2018
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