No more undulations - leveling control for waterborne coatings

By Anne Odoux, R&D Engineer, Synthron, France, Indre et Loire

Whether decorative or industrial, water-based finishes require perfect appearance and optimum mechanical properties

During the drying process of water-based paints, especially after high-speed application (such as spraying), some irregularities or obvious texture may appear on the surface paint online coatingol.

Mastering the phenomenon of surface tension

Surface defects

Surface defects are usually caused by inhomogeneities in the surface tension of the liquid-gas interface, where the liquid tends to reduce its surface area and thus energy

The most common surface defects are craters or fish eyes

leveling

Our surface additives are surfactant-like polymers

The chemical structure of the additive must be slightly incompatible with the medium so that it can migrate towards the interface and act to reduce short waves

Substrate wetting

Proper substrate wetting is critical to achieving good flow and leveling

Water is a liquid with a high surface tension (73 mN.

The development of surface additives requires better integration: if the surface tension provided by the additive is too low, substrate wettability is improved, but surface fluctuation effects are difficult to avoid

Other Effects of Leveling Agents

Depending on their chemical structure, certain surfactants may also have other positive or negative effects on formulations

two chemical families

Two different chemicals are commonly used to produce leveling agents for waterborne coatings

A range of new additives

Synthron's Modarez® PW line includes several highly effective leveling and substrate wetting agents

Modarez PW 336 is an acrylic based additive containing free hydroxyl functionality that is particularly effective in improving flow and leveling

In terms of regulations, all these additives are VOC-free and compliant with the EU Ecolabel (2014/312/UE), which may help reduce other solvent-containing ingredients in the formulation

Experimental Study

formula

We evaluated the leveling agent with a traditional white waterborne architectural paint, and Tables 2 and 3 detail these base paint formulations to test their performance
.
Finally add the leveling agent and mix it well under an automatic mixer
.
The addition amount used in these experiments was 2.
5%
.
For each paint, keep a sample of the base paint without additives as a reference
.

Testing process

Leveling and Substrate Wetting Testing

To get a better view of the coating surface, we applied the paint to a smooth glass panel that had been degreased with ethanol
.
Coating samples were applied using a K-coat bar with a wet film thickness of 100 µm
.
To complete the test, a second application was also performed on the aluminum Q model A-35
.
The drying process was carried out at ambient temperature (23°C)
.
In each test, coatings with and without leveling agents were used
.

To test the wettability of the substrates, the coatings were applied on polypropylene and polyethylene panels
.
These materials have a very low surface energy of 30 dynes/cm and are difficult to recoat with traditional paints and even more difficult with water-based paints
.
To emphasize the open time and drying time of the paint samples tested in Figure 4
.
• Differences in substrate wettability in substrates that have not been surface-treated (eg plasma technology) prior to coating
.
The plastic samples were cleaned briefly with ethanol before use
.

Apply paint samples in thin layers using wet and squeeze brushes
.
In the next few seconds, use a small wooden stick to draw a mark on the wet film, which helps to observe the final dewetting effect
.
In fact, when the coating fails to properly wet the substrate, it shrinks, allowing plastic pores to appear and sometimes droplets (the creep effect)
.
The wetting ability of the paint is visible immediately after application
.

open time measurement

We measured open time and drying time using a drying time recorder according to the Beck Koller method (ISO9117-4:2012)
.
Coating samples were applied to 300 x 25mm glass strips with a wet film thickness of 120 µm using a cube applicator
.
The coated sample was placed on the apparatus and the hemispherical needle carried by the mobile rack passed the length of the template within 6 hours
.
After the experiment is complete, read the drying time stage as shown in Figure 4
.
In practice, all these stages are not always obvious
.
Usually, we can easily observe the dry time (we call it dry time) and the tack time (we call it open time)
.

Experimental results

The addition of Modarez PW additive significantly improved the flow and leveling of the tested coatings
.
Table 4 summarizes their evaluation results
.
By acting on the surface tension of the liquid coating, these levelling agents for waterborne systems can help eliminate many surface defects such as pinholes, craters or orange peel
.

Among different experiments conducted in the laboratory, we highlight some examples of improved leveling below
.
Coating A containing Modarez PW 677 was applied to a glass panel and the results showed that the additive could significantly improve the appearance of the dried paint film by eliminating pinholes on the surface of the coating (Figure 5)
.

In addition, Modarez PW 677 improved the flow and leveling properties of Paint B on aluminum Q-panel A-35
.
As shown in Figure 6, coating B without the additive showed some marks caused by coating (in this case, the K-coater), while the surface of the coating with the additive was improved
.

Experiments also demonstrated that Modarez PW 677 improved substrate wettability
.
In fact, Coating A shrinks immediately after application and shows many dewetting areas on the polypropylene and polyethylene panels
.
However, when the coating contains additives, the surface tension of the liquid coating is sufficiently modified to properly spread on the substrate and obtain better wetting
.
This is evident in Figure 7
.
In addition, we tested the coating for intercoat adhesion and the surface Modarez PW 677 did not negatively affect this parameter
.

Regarding the measurement of the open time, experiments showed that the additive Modarez PW363 had a positive effect (Fig.
8)
.
When the additive was incorporated into Acrylic Waterborne Paint C at a dose of 2.
5%, the open time of the paint with the additive was 17 minutes, compared to 8 minutes without the addition of the paint, an increase of 9 minutes
.
The total drying time (30 minutes) did not increase and the total drying time results were the same for both samples
.

in conclusion

Modarez PW products are important additives for waterborne coatings
.
Since this is their primary use, they provide very good flow and leveling properties in liquid systems
.
They are fully compatible with many water-based adhesives, prevent haze, and eliminate surface defects such as craters, fisheyes, or pinholes
.
They also reduce the cellulite effect, which helps improve gloss and DOI
.

Due to their chemical structure, they have no negative effect on the water resistance of multilayer systems or the adhesion between coatings
.
In addition, they are all VOC-free additives
.

Finally, the Modarez PW range of additives has its own characteristics, such as Modarez PW 363 for improved open time, Modarez PW336 for enhanced flow and leveling properties, and Modarez PW 677 for excellent substrate wetting
.