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Structure and Behavior of Wetting Agents

Posted on March 12, 2021 by Jochum Beetsma — Leave a Comment

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woman painting a chair with yellow paint - Structure and Behavior of Wetting AgentsIntroduction

Wetting is the adsorption of liquid at a solid surface. The process is important in coatings and inks, for example when solid particles must be dispersed in a liquid medium or when a liquid paint or ink is applied on a substrate.

Pre-condition for wetting

In the following, we concentrate on wetting of a flat solid substrate by a liquid paint or ink. The two properties that govern whether or not complete wetting will take place are the surface tension of the liquid γlg and the surface energy of the substrate γsg1. Complete wetting of a flat solid surface will occur when the surface tension of the liquid is lower than the surface energy of the solid: γlg < γsg. This precondition is often called the wetting condition for flat surfaces.

surface energy and surface tension diagram - Structure and Behavior of Wetting Agents
Wetting of a substrate is governed by surface energy and surface tension.

Wetting agents

Wetting agents are additives that are used to improve wetting behavior of paints and inks. To be more precise, wetting agents lower the surface tension of a liquid so that the complete system, consisting of solid and liquid, complies with the wetting condition. Wetting agents have a surfactant structure, implying that the molecules have a hydrophilic part and a hydrophobic part2. Nonionic wetting agents do not carry a charge. The hydrophilic part of anionic wetting agents carries a negative charge in water-based systems.

diagram of nonionic and anionic example - Structure and Behavior of Wetting Agents
Example of a nonionic (left) and an anionic (right) surfactant.

Wetting agents lower the surface tension of a liquid because the surfactant molecules adsorb and orient at the liquid-air interface in such a way that the hydrophobic tails point towards the air.

diagram of surfactant structure - Structure and Behavior of Wetting Agents
Behavior of molecules that have a surfactant structure.

Wetting agents are especially important in water-based systems. The reason for this is that water is a liquid with high surface tension: γlg of water is 73 mJ/m2 at room temperature1.

Challenges and innovations

A range of challenges, related to the use of wetting agents in water-based systems, are well known. I address two of them. First, many wetting agents stabilize foam bubbles. This problem can be solved by using defoamers. However, defoamers themselves often cause problems like cratering and orange peel. Secondly, the wetting agent molecules must move to the liquid-air interface as fast as possible when application of the system has stopped. It is said that the equilibrium surface tension (EST) must be obtained in a short time. It will come as no surprise that the industry is looking for innovative wetting agents that obtain EST as fast as possible and do not give foam problems.

An example

A concept used to make wetting agents with low foam sensitivity is the Gemini structure3. A molecule that has Gemini structure contains, for example, 2 hydrophilic hydroxyl (-OH) groups and two hydrophobic tails (R2 and R3).

nonionic-surfactant - Structure and Behavior of Wetting Agents
Nonionic surfactant molecule having Gemini structure.

Surfynol® 107 L is a nonionic alkane diol surfactant having Gemini structure. The additive is particularly suitable as wetting agent for water-based systems. The molecules adsorb fast at the water-air interface and they have a tendency to destabilize foam bubbles.

References

  1. Article Surface Tension & Surface Energy, Jochum Beetsma, 27 September 2019.
  2. Article Lowering surface tension – Surfactants in coating materials, Marc Hirsch, 25 February 2021.
  3. Article Sticking to the Subject about superwetters for waterbased systems, Roger Reinartz et al., European Coatings Journal, issue 7/8 of 2015, page 20-25.

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Filed Under: Paint & Coatings

About Jochum Beetsma

With over 30 years of experience in the coating and ink industry, Drs. Jochum Beetsma brings expertise in physical chemistry, raw materials, product development, innovation implementation and problem solving to his role as a paint and coatings industry expert at Prospector. As graduated physical chemist and PDMA-certified New Product Development Professional, Jochum Beetsma authors in-depth technical articles for Knowledge.ULProspector.com.

Since 2004, Jochum is independent consultant, advising companies from around the world in coatings, inks and related industries. Jochum develops and executes in-company training programs on coatings and he works as interim project manager.

Prior to becoming consultant, he spent 13 years at DSM Coating Resins. While at DSM, he was project manager of New Product Development projects, focusing on water-based, solvent-based, high-solids and powder coatings.

He also served as chairman of the Project Management Platform within DSM. He coached project teams and prepared and facilitated complex project sessions like Project Start-Ups.

Previous to his time at DSM, Jochum was a coating chemist at PPG.

Jochum is widely published in various international scientific magazines and has delivered lectures worldwide. He is a current lecturer and member of the Expert Team of SpecialChem. He is also a coatings technology lecturer for, amongst others, Polymer Technology Netherlands.

He currently resides in Zwolle, The Netherlands.

Contact Jochum via email ([email protected]) to learn more about his consulting services.

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