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Coalescing Aid

Last updated: December 15, 2018

What Does Coalescing Aid Mean?

A coalescent aid is a small amount of solvent contained in latex coatings. It is not a true solvent because it does not actually dissolve the latex resins. The coalescent aid helps the latex resins flow together, aiding in film formation.

A coalescing aid is often referred to as a temporary plasticizer for a film-forming polymer. The function of the coalescent is to soften the polymer during the crucial period of fusion so that the individual particles combine to form a continuous film.


Corrosionpedia Explains Coalescing Aid

The coalescing aid acts as a temporary plasticizer in latex emulsions. It lowers the glass transition temperature (Tg) of the latex polymer. As the paint dries, the polymers that have been softened by the coalescing aid are allowed to flow together and form a film after the water has left the system. Since coalescing aids are volatile, they evaporate out of the film. This allows the polymer to return to the original Tg, therefore giving harder films for better block and print resistant coatings.

An ideal coalescing aid must fulfill three criteria:

  • The coalescent must be an active solvent for the base polymer of the system.
  • The coalescing aid must efficiently lower the minimum film-formation temperature (MFFT) of the system.
  • A coalescent must have an evaporation rate much lower than that of water.

In addition to these three properties, it is highly desirable that the coalescing aid has very low solubility in water.

There are many different coalescing aids commercially available, all with their specific affinity for different polymers, water solubility and evaporation rate.

Plasticizers are sometimes used in latex paints, but they do not allow the polymer to return to its original Tg. Since they do not evaporate out of the coating, the polymers remain soft and tacky, which can cause poor block and print resistance. Therefore, the coalescing aid must slowly evaporate, allowing the paint film to develop its maximum durability and appearance properties.


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