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Substrate Wetting: Wetting Agents Selection

What would be paint without any correct substrate wetting? When is it easy to explain that paint could flow properly and wet the substrate? The real case application of this concept is sometimes complex, especially in waterborne coatings.

Trim your difficulties!

 » Understand the correlation between substrate wetting and surface tension
 » Learn how to improve substrate wetting of Coatings
 » Select the right wetting agent for your coating system

Wetting Agents for Coating Formulations


Substrate WettingThe 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:

  • Apparition of craters
  • Ghosting
  • Bubbling due to air entrapped 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 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.

Substrate wetting is a surface tension theory. Hence, it is ideal to have good knowledge on surface tension before moving further.

 » Brief Yourself on Surface Tention & Function of Contact Angle in Here! 

Solving Surface Tention Issues


Measuring Surface Tension


Static surface tension


Du Noüy Ring Method tensiometer is one of the easiest technique for measuring the surface tension of liquids. A platinum-iridium ring is dipped into the liquid, and 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
Du Noüy Ring Method Tensiometer

When this method works perfectly to compare the surface tensions of surfactants in waterborne, or of clear solventborne coatings, it is not suitable for pigmented systems due to the presence of pigment hinders lamella stability.

Dynamic surface tension


When using dynamic application process, such as printing, the wetting must be fast enough to obtain the required film quality.

Some surfactants can offer required results using the Du Noüy Ring method, but fail during the application.

Under the static conditions, the surfactant has time to equilibrate at the interfaces, but during the dynamic process, the surfactant becomes mobile and 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 decrease rapidly by expansion of the bubble until the bubble is detached from the capillary.

Influence of surfactants on the dynamic surface tension
Influence of surfactants on the dynamic surface tension


Using these tests, we have dressed 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



Solventborne coatings have surface tensions similar or lower than the common substrates. They are less exposed to the substrate wetting problem. Nevertheless, oil contamination, not homogenous substrate with strong differences in the surface tension, can provoke some severe application defects such as ghosting, cratering or crawling. 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. But selecting the right one can also have some positive side effect such as leveling improvement or craters prevention.

 » Improve Leveling of Your Coating Further! 


Improving Substrate Wetting of Coatings


The substrate wetting topic is a crucial point in the waterborne coatings. Indeed, the surface tension of the water is higher than other substrates. As a consequence, a wetting agent becomes an obligation. In solventborne coatings, surface tensions of paints / solvents are closer to the one of the substrate, so the phenomena is not so strong, but here the wetting agent can overcome many other problems, like craters caused by oil contaminated areas for instance.

In order to improve the substrate wetting, there are 2 solutions :

  1. Increase the surface tension of the substrate through (cleaning, removal of dust, oils and contaminants, surface treatment (corona pre-treatment, flaming, acid or basic treatment…). Sometimes hard and complex.

  2. Decrease the surface tension of the coating. The most common method is to add a component, a substrate wetting agents, into the liquid paint.

Role of Wetting Agents


A wetting agent is a surfactant, having both a hydrophilic and a hydrophobic part. This specific structure self-orientates the additive at the surface, reducing the surface tension of the liquid paint, the polar parts stay in the aqueous phase when the non-polar parts orientates at the interface. Hydrophobic parts are often based on hydrocarbon. Usually, polar molecules are based on polyether chains. Specific properties can be achieved by polymer surfactants based on a fluorinated or polisiloxane backbone.

Polymerization of Surfactant Molecules
Surfactant molecules can be polymerized to form polymers surfactants (case of polisiloxanes surfactants)

Silicone-based Wetting Agent
Chemical model of a silicone based substrate wetting agent (Rx are hydrophilic Polyether chains)

Wetting Agent Chemistries


  • Simple organic based agents (mainly hydrocarbon based): Acceptable surface tension reduction, not as efficient as Silicone or Fluor based agents.
  • Silicone based agents: Good static and dynamic surface tension reduction. Broad range of use, excellent wetting penetration on wood substrates, very efficient on difficult substrates. The silicone chemistry is well known and very flexible.
  • Silicone Free agents (Acetylenic and alkoxylates derivatives): Excellent dynamic surface tension reduction, usually no foam tendency, good alternative to formulations where silicone structures are not accepted. Green products and environmental friendly are emerging in this category.
  • Polymeric Silicone Free agents (mainly acrylates or maleates derivatives): This chemistry offer a good alternative to silicone based products, but their surface tension reduction is not as high as silicones or fluoro surfactants. Can also be used to fine tune the formulation.
  • Fluor based agents: Excellent static surface tension reduction, but poor spreading performances. Very good to prevent craters on contaminated substrates. Generally more expensive than Silicone based agents.

Defects Cured by the Right Wetting Agent


  • Crawling due to oil contamination in a waterborne acrylic coating

    Crawling Due to Oil Contamination in Waterborne Acrylic Coating

  • Craters in 2K Epoxy

    Craters in 2K Epoxy

Like many coatings additives, the wetting agent must be effective at the lowest dosage, without any negative side effects. As a working at the interface, such an additive must not increase the foam or trouble the intercoat adhesion.

 » Improve Adhesion in your Coating System by Choosing Right Adhesion Promoter 

Waterborne coatings require the use of wetting agents in particular than the solventborne coatings. As each coatings market uses specific evaluations and focus on different properties, it is necessary to lead some pre-tests in laboratory to select the correct substrate wetting agent :

  • Compatibility of the wetting agent with the liquid coating
  • Wetting of the substrate
  • Foam formation during the coating manufacturing process
  • Foam formation during the application
  • Haze and turbidity of the applied film
  • Substrate adhesion of the dry film
  • Coatings film defects : craters, leveling, intercoat adhesion


Select the Right Wetting Agent for your Coating System


Table below lists the types of wetting agents available in market; let's have a look at their properties:

Type of Agent

Static Surface Tension Reduction

Dynamic Surface Tension Reduction

Compatibility

Tendency to Foaming

Crater Prevention

Organic

Silicone-free

Polymeric Silicone-free

Low Molecular Weight Silicone

High Molecular Weight Silicone

X

Fluoro-surfactant

X

X



- Excellent

- Very Good

- Good

- Acceptable

- Low

X - Poor



The formulator has a large choice in the selection of the correct wetting agent. He can test various chemistry and, in some cases, combine a silicone base agent with a silicone-free one, and proceed with all the tests to select the right one.

Some examples
:

  • When dynamic surface tension reduction is required, like in printing field, silicone-free agents would be the first choice.
  • Due to their strong surface tension reduction effects, silicone based agents may be the first approach in wood coatings or industrial field.
  • Where strong cratering is an issue, fluro-surfactants should be tested

Over the substrate wetting improvement, these agents can have various benefits:

  1. Prevent surface defects (such as craters)
  2. Improve the flow and leveling
  3. Improve the droplets spreading in spray application
  4. Have defoaming ability
  5. Reduce the bubbling in backing application
  6. Improve the polymer film formation

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