Coatings R&D Notebook

Minimize Additives in Waterborne Coatings

What are some of the problems of waterborne technology, other than the obvious flash-rusting issue?

Waterborne resins are generally made with surfactants. These are nonvolatile, water associating and remain in the coating. Unlike the resin, these emulsifier surfactants are mobile and detract from the mechanical properties of the film. In addition, they hold water and can decrease weathering performance. Often, additional surfactants are added to increase the wetting of pigments and to wet the surface during application. These also decrease performance of the dry film. Many of the thickeners will also decrease the tensile strength of the film. The defoamer and anti-foam additives, which are used to combat the effects of the surfactant, will decrease the strength and overall durability of the coating.

Other additives, such as biocides, flash-rust inhibitors and similar oligomeric or small-molecule additives, may reduce long-term performance. In general, most of the additives used in waterborne coatings to overcome the problems caused by the use of water in manufacturing and application will degrade the performance of the dry film.

Try to minimize the use of additives, not just because of their cost, but because of the effect they have on overall performance of the dry film. Additives are necessary to aid the formulator, but they should not be used to shortcut development to meet minimum application performance specifications or solve one problem without regard to the other factors affected by the additive. Waterborne coatings use a large number of additives, thus the problems are greater here for OEM coatings application.

Drawdown Bars' Influence on Thickness

Do drawdown bars really lay down a consistent film thickness independent of formulation?>/b>

The shear of the blade can influence the amount of paint deposited. The thinner the layer being applied, the more shear is applied to the paint and the more the paint can be shear-thinned. The result is a different thickness for different paints. The thickness will vary nonlinearly with gap.

A Bird blade will show a larger difference than a standard blade of the same gap due to the area of the blade in contact with the paint. The Bird bar has a larger area in contact with the paint, thus imparting more shear to the paint.

If a color database is to be prepared using different gap drawdown bars, the actual dry-film thickness must be measured to ensure that the real thickness is known. Different thickeners will have different responses to shear, as indicated in a rheology curve. The shear thinning of a thixotropic paint will result in a significantly different film deposition from that of a pseudoplastic coating.

Thickeners Reduce Performance Properties

Do thickeners improve or degrade the strength of the paint film?

The thickener usually will degrade the strength of the film. Many thickeners have structural elements that attract water or produce a weak interaction, which results in a decrease in film strength. For example, clay thickeners require more resin to bind to them since they are, in effect, a pigment particle that will not form a film without a resin.

Associative thickeners, acrylic acid salt type and cellulosic have an affinity for water that may degrade coating performance in some manner, such as corrosion resistance. The scrub, abrasion and adhesion, as well as flexibility and impact and chip resistance, can be affected. If the thickener is a resin-like thickener of sufficient molecular weight and relatively hydrophobic, it may not be detrimental. But this would be a rare additive and usually not result in thixotropic behavior.

To gain the rheological behavior desired, some of the performance properties of the coating must be sacrificed. The object is to minimize these sacrifices without loss of the desired application and storage rheological properties.