The Effects of Surface Treatment at the Interface

Published: July 21, 2015, By Mark Miller

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There are a variety of methods for encouraging fluids to stick to substrates in coating applications—corona, plasma or flame treating the substrate; adding a primer prior to coating; and utilizing a vacuum system at the coating head interface. The addition of surfactants is one method of adjusting the fluid itself that should be considered.

Especially with the use of aqueous fluids, surfactants provide the ability of the solution chemistry to adsorb at the substrate interface. This encouragement does not require a large dilution of the fluid with the surfactant additive, but definitely provides a strong anchor for the water-based fluid to the substrate if nothing else is working.

Surfactants even out the flow of the fluid as it travels across the substrate. Especially when there is a quick expansion or contraction (like what is experienced in pre-metered coating methods), the fluid surface deformation may vary across the substrate. This deformation results in concentration gradients and surface tension gradients. These surface tension gradients lead to variability in the coating.  Surfactants help overcome these gradients and variations.

But how does a surfactant provide this encouragement? The easiest way to think of a surfactant is as a collection of active monomers that bridge the fluid and the substrate on a moving web. Initially, the fluid is loaded with a pre-determined amount of surfactant that has free radical end groups. What this means to you and me is that the chemical additive (surfactant) has more energy to attach to the substrate surface than the polymer solution has on its own. Because the surfactant is free to move about in the solution, the surfactant tends to migrate to the interface point where the fluid is in contact with the substrate, instead of staying in the bulk of the fluid. The diffusion of the surfactant monomer to the substrate, in essence “grabs” the substrate surface and the solution has an extended residence time on the substrate prior to drying and curing. This extra open time on the substrate allows the fluid to coat better and may reduce coating defects. This can be seen dramatically if you place a drop of fluid onto the substrate and it beads up without surfactant and place a drop of the fluid with surfactant on the substrate and it wets out and flows. The dramatic change of surface tension by a small volume of surfactant additive may be just what is required for improved coating performance.

In pre-metered coating, the use of surfactants can also allow for a wider gap between the coating head and the substrate. This can be considered a macro-sized bridge for the fluid and the substrate beyond the microscopic view we have already discussed. The advantages of this increased distance include coating a thinner fluid coating than would be normally allowed and reduction in the use of a vacuum system.

In addition to providing the connection between the liquid coating and the solid substrate that is required to make a viable coated product, surfactants can sometimes provide ancillary benefits like improved solubility for the solution or altering the surface roughness of the final coated product. Even though surfactants provide many benefits, their application should not be applied in a slapdash manner. Surfactants can provide many benefits, but have also been recognized as creating coating defects, most noticeably hydrophobic spots. Proper use and application of surfactants can provide advantages to coating that cannot be accomplished otherwise.