Formation of Macro and Micro Foam

When discussing foam, it is common to distinguish between macro and micro foam. In both cases, air is entrained in the coating material, but at different locations. In the case of macro foam, air forms foam bubbles at the coating surface, stabilized by a surfactant double layer (foam lamella). In the case of micro foam, entrapped air is located in the coating film and is prevented from rising to the surface by high coating viscosity or additional surfactant, which acts as an anchor. Surfactants possess a high tendency toward foam stabilization because of the surfactant structure’s ability to orient itself at the air/liquid interface. By so doing, these foam-stabilizing substances form a stable surface film around the micro or macro bubble.¹

Taking into consideration Stoke’s Law — which states that larger bubbles rise much faster to the surface than smaller bubbles — it becomes clear that micro bubbles rise very slowly due, in part, to their small radii. High coating viscosity further reduces the velocity of rising micro bubbles. And it is common to find high viscosities in energy-cured screen inks or roller-coated wood and furniture coatings.

The Ideal Deaerator

Naturally, an ideal deaerator for energy-cured coatings and inks should be able to eliminate and prevent both micro and macro foam. But this is only one part of the story. Most of the defoamers and deaerators for energy-cured systems currently available are able to destroy foam, but still cause compatibility problems. These compatibility problems manifest themselves either as turbidity or fish eyes in uncured film, or as craters and haziness in cured film. The consequence is poor quality of the applied coating or ink. The ideal deaerator for energy-cured systems, therefore, should strike an optimum balance between foam-destroying effectiveness and system compatibility.

The Search for the Ideal Deaerator Formulations

Several clear and pigmented energy-cured coatings and inks were prepared for the testing and evaluation of different chemical substances — deaerator candidates — and their subsequent modifications. These formulations were based on different types of acrylic oligomers used in the wood/furniture and printing ink industries, such as epoxy/polyester/polyether or oligoether acrylates. Depending on the type of formulation, curing was accomplished with a gallium and/or mercury lamp (120 W/cm) at different line speeds. For initial screening, 0.3% active matter of the chemical substances and their modifications were added. Based on these results, subsequent optimized formulations tested up to 1.0% of active deaerator.

Test Methods

Different test methods, such as foam and compatibility tests, provided evaluations of both the liquid coating or ink material and the cured film. In addition, static surface tension was measured by the duNuoy ring pull-off method and carried out with a Dynometer (BYK-Gardner/Germany). The static surface tension was recorded in mN/m.

Foam Test

The effectiveness of different chemical substances and their modification against micro and macro foam was tested and evaluated with a standardized “foam test.” After incorporation of the different substances into the coating material, each sample was stirred for 3 minutes at 3,000 rpm with a conventional lab dissolver (Getzmann/Germany). Immediately after stirring, a draw down (100 µm) on glass panels was prepared and cured via UV light according to the reactivity of the formulation. The evaluation of effectiveness was conducted both visually and by microscopy. A video camera documented the test results. Differentiation of test results was represented using a number scale from 1 to 6. The number ‘1’ indicated excellent effectiveness against micro or macro foam, and the number ‘6’, no effectiveness.

Compatibility of Liquid Coating Material

To evaluate compatibility, a Turbiscan apparatus measured transmission of a laser light through the liquid coating material, with higher turbidity indicating poor compatibility. Results were recorded on a scale ranging from 0% transmission of light (strong incompatibility) to 100% transmission of light (excellent compatibility).

Compatibility of the Applied Cured Film

In addition to evaluating the turbidity of the liquid coating material, we visually evaluated the applied cured film for turbidity, crater or fish-eye formation, as additional indicators of compatibility. To test the optical appearance, drawdowns (100 µm) on glass were prepared and evaluated after curing. The number scale from 1 to 6 was again used for evaluation, with the number ‘1’ indicating the best result, and ‘6,’ the worst.

Testing of Substance Classes

As a general starting point, different chemical substances were tested for their ability to eliminate micro and macro foam.

Substance A: polyacrylates Substance B: fluoro-modified polysiloxanes Substance C: polyethers Substance D: polar-modified polysiloxanes Substance E: polysiloxanes

Figures 2–6, taken by way of 2X magnification microscopy, corresponding to Substances A–E show the results of the foam test and the effectiveness of each substance as a foam eliminator. (Figure 1 is a control without additive.)

Comparing results displayed in Figures 1-6, it became obvious that the most promising results were obtained from Substance Class A (Figure 2). Therefore, Substance Class A was selected as the basic chemistry for further work toward optimization and increased efficacy.

Modification of Substance Class A

Different chemical structures are possible with Substance Class A. First, chain length and molecular weight of the backbone structure were varied. The variations obtained were then tested for effectiveness and compatibility in energy-cured coatings and inks.

Variation of the Molecular Weight

Figures 7, 8 and 9 show the foam elimination effectiveness of three chemical structures based on Substance Class A (2X magnification). These products differ in molecular weight of their backbone.

In addition to the foam test, the compatibility of these three substances was evaluated as well. Table 1 summarizes the test results.

The most promising foam elimination performance is obtained from the substance with a medium molecular weight backbone (Substance A.2). Shorter (Substance A.1) or longer (Substance A.3) backbone structures lead to less effectiveness against micro and macro foam, greater turbidity and lower compatibility. Substance A.2 was selected for further modifications, focused mainly on improving compatibility with the cured coating material.

Variation of the Modification

In general, varying the organic modification of the substance backbone influences compatibility with the coating material. This modification can be more polar/hydrophilic or more non-polar/hydrophobic in character. For further testing, different substances were synthesized based on Substance A.2, but linked with different organic modifications being either more or less polar. These substances were tested for foam elimination and compatibility. Figures 10, 11, 12, and 13 display the results of the foam test (2X magnification). Table 2 gives an overview, with additional test results on compatibility.

Test Results

Figure 13 and Table 2 demonstrate that the best combination of effectiveness and compatibility was obtained with Substance A.2.4. This substance provides excellent defoaming and deaeration (foam test: 1), while at the same time achieving high compatibility (light transmission value: 96%; optical appearance: 1). Substances A.2.1 and A.2.3 have more polar modifications, but this did not lead to an improvement in compatibility (optical appearance: 5).

Instead, the opposite is true, as can be seen with Substances A.2.2 and A.2.4. These substances were characterized by less polar modifications leading to improvement in compatibility (optical appearance: 1 and 2; transmission: 100% and 96%). However, polarity of the modification is again only one side of the story. The type of modification is important as well. A non-polar polyether modification (Substance A.2.2) improved compatibility but, at the same time, tremendously reduced effectiveness against micro and macro foam. In contrast, Substance A.2.4, characterized by a different kind of organic modification, provides an excellent balance of high effectiveness and compatibility.

Based on the described test results the ideal chemical substance to reduce and eliminate micro and macro foam in energy-cured coatings and inks, and maintain compatibility with the coating or ink system, is based on a polyacrylate of medium molecular weight, modified with relatively non-polar organic groups.

Conclusion

This work has shown that designing a specific chemical structure leads to an additive capable of reducing and eliminating micro and macro foam in energy-cured coatings and inks. The vital parameter of this product is its balance of high effectiveness against micro and macro foam and the highest possible compatibility with the coating or ink formulation. Modified polyacrylates are able to reach this balance. However, this additive class enables the formulator to eliminate micro or macro foam in energy-cured coatings and inks without such negative side effects as turbidity, crater or fish-eye formation. In addition, the additive will not negatively influence other properties such as recoatability, reprintability or glueability. Promising results have been obtained in a variety of energy-cured coating and ink formulations using different application techniques: spray, roller, curtain coating and silk screen.

For more information on eliminating foam, contact Tego Chemie Service USA, PO Box 1299, Hopewell, VA 23860; phone 800/446.1809; fax 804/541.2783; e-mail frances.eggleston@us.goldschmidt.com.