What is the difference between a wetting agent and a levelling agent?

Quite often one sees the terms wetting agent and levelling agent used interchangeably. In many descriptions they would appear to do the same thing: reduce the surface tension of the coating, thus improving surface wetting and eliminating defects resulting from a mismatch in surface tensions of the coating/substrate. So are they the same thing? Do they have a similar mechanism and are they used for the same function?

At this point it is worth mentioning that there are some excellent articles available online that give an overview of surface tension, the importance of surface tension considerations in coatings and an introduction to how additives can alter this in coatings. This article isn’t intended to address these points and I would encourage the reader to refer to the “Additives for Surface and Substrate” brochure for more information on these topics.  What this article aims to tackle is the difference between a wetting agent and a levelling agent.

So why are levelling agents and wetting agents often regarded in the same light? Well, in some cases the use of a “levelling agent” gives the same perceived benefit that a wetting agent would: it reduces cratering. In this case the “levelling agent” is interacting with incompatible coating ingredients (such as a defoamer) in a way akin to that of a surfactant. It is addressed as a levelling agent but is actually more similar to a wetting agent. The interfacial tension of the liquid is changed when using such products i.e. you can measure a difference with and without “levelling agent” when using a drop shape analyser or bubble pressure device. EDAPLAN® LA 410 would be an example of such a product.

True levelling agents have little to no influence on the surface tension at the substrate-liquid interface, but instead form a thin layer on top of the film to give a uniform surface tension at the air-liquid interface. This prevents an uneven evaporation of diluents and results in a smoother surface, which improves the gloss and “slip” properties of the coating. Reducing the surface roughness also improves the scratch resistance due to a reduction in frictional resistance. In this case the interfacial tension of the liquid is not changed i.e. you cannot measure any difference with and without levelling agent when using a drop shape analyser or bubble pressure device. This type of additive is something which is frequently employed in the formulation of inks and OPV’s.

EDAPLAN® LA 414 and Edaplan® LA 415 are two of the new generation levelling agents from MÜNZING CHEMIE for enhancing slip, levelling and scrat­­­­ch resistance in both aqueous and non-aqueous lacquers. Recoatability is not negatively influenced due to the extensive side chain modifications and they don’t tend to stabilise foam. Both grades highlighted also have indirect food contact approval. For more information on these two products please see the product presentation.

We also see a difference in terms of the chemical make-up of wetting agents and levelling agents. Wetting agents are generally based on ionic surfactants, non-ionic surfactants, or highly ethoxylated/propoxylated (EO/PO) siloxanes. Levelling agents are more typically based on acrylic copolymers or more heavily modified EO/PO siloxanes.

In most cases the reason behind a surface defect is the wrong choice of defoamer. We would encourage you to first get in touch with us to see if switching defoamer can improve the quality of your coating. Should a suitable alternative not be found then evaluation of a wetting agent would be the next logical step.

If you think that a wetting or levelling agent might be beneficial to your project then please give us a call on 01827 314151, to speak with one of our technical sales team. They will be able to advise on which are the most suitable grades for your application. More information is also available on our product page and the MÜNZING CHEMIE website.