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Coatings Ingredients

Substrate Wetting: Wetting Agents Selection

What would be a paint without any correct substrate wetting ? When it is easy to explain that a paint should flow and wet properly the substrate, the real case application of this concept is sometimes complex, especially in waterborne coatings. This guide will remind you the generalities of paint substrate wetting topic and the way to select the best substrate wetting agent for your formulation.

Wetting Agent : A Surface Tension Story


The additive market propose many variations of each chemistry. But there is no miracle product. Many parameters must be taken in consideration when selecting the right Wetting Agent. Each chemistry, each product has its own benefits, and the formulator must of course consider the most important point to solve but also all the positive and negative properties of the product after validation tests in order to achieve the best compromise.


As an evidence, the perfect wetting of the liquid coating on the substrate during the application phase is needed and required to offer the best final paint properties. Incorrect substrate wetting may result in premature coating film degradation as well as substrate degradation.

A liquid coating with a poor substrate wetting can provoke various defects, such as the apparition of craters, ghosting, bubbling due to air entrapped during the application, weak adhesion and crawling / de-wetting. These issues not only degrade the aesthetic of the film, but also its physical properties by creating many weakness points source of corrosion and peeling.

Understanding this wetting concept and selecting the correct raw materials, especially wetting agents, will improve drastically your paint quality.

The substrate wetting of a liquid on a solid substrate is defined by the surface tension, function of the contact angle. The contact angle θ is the angle between the intersection of the liquid-solid interface and the liquid-vapor interface at the three phase contact line.

Regarding coatings field, we can define the substrate wetting as the replacement of the air at the substrate by the paint. Wetting will be difficult on low energy surface, or contaminated with oil or dusts.

contact angle
Courtesy : Houston University / Y. Yuan and T.R. Lee

Fig 1 : Contact Angle


θ < 90° : low contact angle

θ > 90° : high contact angle

Favorable wetting

Unfavorable wetting

The paint will spread over the surface

The paint will minimize its contact with the substrate

Good wetting of the paint

Good wetting of the paint

Poor wetting of the paint

Poor wetting of the paint


Ideal theory, a perfect coating would have a contact angle θ = 0° regarding the substrate, which is not realistic.

 

Nevertheless, we can consider reaching a good wetting when the contact angle θ < 90° , in other words, when the liquid has a lower surface energy than the substrate : γ (L)  < γ (S)

θ < 90° → γ (Liquid)  < γ (Substrate)



A liquid with a low surface tension will have a good wetting.

A substrate with a high surface tension will be easy to wet.

> The substrate preparation / treatment prior to the application is determinant for a perfect substrate wetting !

> Using surfactants ie. wetting agents will help controlling the surface tension of the liquid !



Solving Surface Tension

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