Wetting Agents for Coatings: Chemistries, Considerations, and Applications

What are wetting agents?

A wetting agent is a surfactant with a hydrophilic and a hydrophobic part. This specific structure self-orientates the additive at the surface. Thus, it reduces the surface tension of the liquid paint.

The polar parts stay in the aqueous phase usually based on polyether chains.
The non-polar parts orientate at the interface based on hydrocarbon.

Structure of Wetting Agent
(Source: The LibreTexts libraries)¹

The key benefits of wetting agents in paints and coatings include:

✓  Improves substrate wetting
✓  Prevents surface defects
✓  Improves flow and leveling
✓  Improves spreading of droplets in spray application
✓ Possesses defoaming ability
✓  Reduces bubbling in the backing application
✓  Improves polymer film formation

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Chemistries of Wetting Agents

Wetting agents for paints and coatings are available in various chemical types. Each chemistry offers specific properties and advantages. Here are some of the common chemistries of wetting agents:

Simple organic-based agents

Organic-based agents are primarily hydrocarbon-based. They offer acceptable surface tension reduction. However, they are not as efficient as silicone or fluoro-based agents.

Silicone-based agents

Silicone-based agents enable good static and dynamic surface tension reduction. They have a wide range of uses including excellent wetting penetration on wood substrates. They are very efficient on difficult substrates. The silicone chemistry is well-known and is very flexible. Polysiloxanes surfactants is an example of silicone-based wetting agents.

Chemical Model of a Silicone-based Wetting Agent (Here, R_x are Hydrophilic Polyether Chains)

Silicone-free agents

Silicone-free agents are acetylenic and alkoxylate derivatives. Their key properties are:

excellent reduction in dynamic surface tension
no foam tendency

They are a good alternative to formulations where silicone structures are not accepted. Green products and environmentally friendly alternatives are emerging in this category.

Polymeric silicone-free agents

Polymeric silicone-free agents are mainly acrylate or maleate derivatives. This chemistry offers a good alternative to silicone-based products. However, their surface tension reduction is not as high as silicones or fluorosurfactants. They can also be used to fine-tune the formulation.

Fluoro-based agents

Fluoro-based agents offer excellent static surface tension reduction and poor spreading performances. They can prevent craters on contaminated substrates. They are more expensive than silicone-based agents.

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Factors to Consider When Selecting Wetting Agents

The additive market proposes many variations of each chemistry, but there is no miracle product. Many parameters must be taken into consideration while selecting the right wetting agent.

Substrate wetting

The substrate wetting of a liquid on a solid substrate is defined by the surface tension and 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.

Lower the Contact Angle Better the Substrate Wetting
(Source: SAN NOPCO LIMITED)²

The liquid coating must wet the substrate well during application to ensure optimal paint quality in the end. Incorrect substrate wetting may result in premature degradation of coating film and substrate. A liquid coating with poor substrate wetting can provoke various defects, such as:

Apparition of craters
Ghosting
Bubbling due to entrapment of air during the application, and
Weak adhesion and crawling/de-wetting

These issues not only degrade the aesthetic of the film but also its physical properties. This happens due to the creation of many weak points, corrosion, and peeling. Understanding the wetting concept and selecting the correct wetting agents can improve your paint quality.

Substrate wetting is a surface tension theory. Hence, it is ideal to know how to measure surface tension before moving further. The two methods to measure surface tension include:

Static surface tension

Du Noüy Ring method tensiometer is one of the easiest techniques for measuring the surface tension of liquids. In this method:

A platinum-iridium ring is dipped into the liquid
It is then slowly pulled out so that a lamella is formed at the air interface

The force needed to pull this lamella is a direct measure of the surface tension of the liquid. The Wilhelmy Plate method is similar, using a plate.

Du Noüy Ring Method Tensiometer
(Source: ResearchGate)³

This method compares the surface tensions of surfactants in waterborne and solvent-borne coatings. However, it is not suitable for pigmented systems. This is because of the presence of pigments that hinder lamella stability.

Dynamic surface tension

The wetting must be fast enough to obtain the required film quality while using a dynamic application process. For example, printing. Some surfactants can offer the required results using the Du Noüy Ring method but fail during the application.

During static conditions — The surfactant has time to equilibrate at the interfaces
During dynamic conditions — The surfactant becomes mobile. It must orientate rapidly to be efficient

The “Maximum Bubble Pressure Method” measures the ability of the surfactant to adsorb rapidly at the air/liquid interface. Pressurized air flows continuously through the capillary. When the radius of the capillary tip and the radius of curvature of the bubble are equal, the pressure becomes maximum. At that point, the surface tension is calculated. After the maximum pressure, the pressure decreases rapidly by expansion of the bubble. This happens until the bubble is detached from the capillary.

Influence of Surfactants on the Dynamic Surface Tension

Using these tests, we have prepared a summary table regrouping the surface tension of different materials.

Surface tension of various materials (mN/m at 20°C)
Liquids - Solvents
Hexamethyl disiloxane	16
Isopropanol	22
n-Butyl Acetate	28
Butyl Glycol	30
Xylene	32
Ethylene Glycol	48
Water	72
Resins
Long Oil Alkyd	26
Polyacrylate	35
Polyester	41
Epoxy	47
Melamine (HMMM)	58
Substrates
PTFE	19
Paraffin wax	27
Polypropylene	30
Polyethylene	36
PVC	40
Steel, pre-treated	45
Glass	70

Solvent-borne coatings have surface tensions similar to or lower than the common substrates. They are less exposed to the substrate-wetting problem. However, severe application defects can occur leading to ghosting, cratering, or crawling. This can be due to oil contamination and non-homogenous substrates with strong differences in surface tension. A correct wetting agent can overcome these issues.

Waterborne coatings have quite high surface tensions. The surface tension of the water itself (72 mN/m) is even higher than most of the substrates. Here, a wetting agent is a real need. Selecting the right one can also have some positive effects such as leveling improvement or crater prevention.

How to improve substrate wetting?

The substrate wetting is a crucial aspect of waterborne coatings than solvent-borne coatings. The surface tension of the water is higher than other substrates. As a consequence, a wetting agent becomes an obligation.

In solvent-borne coatings, surface tension of paints/solvents are closer to the one of the substrate. Thus, the phenomenon is not so strong. However, here the wetting agent can overcome many other problems. For example, the craters caused by oil-contaminated areas.

There are 2 solutions to improve the substrate wetting:

Increase the surface tension of the substrate — This can be done by cleaning, removal of dust, oils, contaminants, and surface treatment (corona pre-treatment, flaming, acid, or basic treatment). This is sometimes hard and complex.
Decrease the surface tension of the coating — The most common method is to add a component, substrate-wetting agents, into the liquid paint.

Compatibility

The wetting agent must be compatible with other ingredients in the formulation.

Resin Compatibility
- Wetting agents must be compatible with the resin system used in the coating formulation.
- Incompatibility can lead to issues like haze, separation, or poor film formation.

Pigment compatibility
- Wetting agents should be compatible with the pigments and fillers used in the coating.
- Incompatibility can result in poor dispersion, color separation, or loss of tint strength.

Solvent compatibility
- Wetting agents need to be soluble and stable in the solvent system used in the coating.
- Incompatibility can cause precipitation, haze, or other stability issues.

Select wetting agents based on their compatibility with various resins from our database:

Foam formation

Some wetting agents also have defoaming properties. They can help control foam generation during:

the coating manufacturing process
the application process

Coatings film defects

Poor substrate wetting in the liquid coating can cause film defects like craters, leveling, intercoat adhesion, etc. Hence, resolving coating defects with suitable wetting agents is compulsory.

Like other coatings additives, wetting agents must be effective at the lowest dosage. They must not have any negative effects. While working at the interface, such an additive must not increase foam or trouble the intercoat adhesion.

Solving coating defects with suitable wetting agents

Like other coatings additives, wetting agents must be effective at the lowest dosage. They must not have any negative effects. While working at the interface, such an additive must not increase foam or trouble the intercoat adhesion.

Crawling Due to Oil Contamination in Waterborne Acrylic Coating

Crawling in Waterborne Acrylic Coating Can be Cured by Silicone-based Agents

Craters in 2K Epoxy Can be Cured by Silicone-free Agent

Substrate adhesion of the dry film, haze and turbidity of the applied film are other properties to consider while selecting wetting agents.

Property Assessement for the Right Wetting Agent Selection

A formulator has a large role to play in the selection of the correct wetting agent. Various chemistries can be tested. The table below lists the types of wetting agents available in market. Let's have a look at their properties:

Wetting Agent Chemistry	Static Surface Tension	Dynamic Surface Tension	Compatibility	Foaming Tendency	Crater Prevention
Organic	Good	Excellent	Excellent	Excellent	Low
Silicone-free	Acceptable	Excellent	Very good	Low	Acceptable
Polymeric silicone-free	Good	Good	Acceptable	Acceptable	Good
Low molecular weight silicones	Very good	Good	Very good	Acceptable	Good
High molecular weight silicones	Good	Poor	Good	Low	Very good
Fluorosurfactants	Excellent	Poor	Good	Poor	Excellent

In some cases, a silicone base agent can be compared with a silicone-free one, and proceed with all the tests to select the right one. Some examples are:

Silicone-free agents — Used to achieve dynamic surface tension reduction. For example, in the printing field.
Silicone-based agents — Used to achieve strong surface tension reduction in wood coatings or industrial fields.
Fluorosurfactants — Solves strong cratering issue.

References:

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