Water-soluble paints based on winyl-acrylic, styrene-acrylic or pure acrylic emulsions contain thickeners in their formulations ensuring appropriate rheological properties at low, medium and high shear rates. Setting the ideal viscosity to ensure no sagging, Stormer-viscosity and ICI-viscosity requires a balancing of different rheological additives and dosages. Often, with selected rheological properties allowing for good application, no sagging and no spattering, there is a problem with leveling, especially when applying with a brush.
First, let’s start with the definition of what leveling is.
leveling of a paint, a measure of its ability to flow out after application so as to obliterate any surface irregularities such as brush marks, orange peel, peaks, or craters that have been produced by the mechanical process of application [ASTM D4062].
As it follows from the definition, leveling concerns not only paint spilling and smoothing brush marks, but also the elimination of other coating defects, such as craters or orange peel. Ensuring a perfect leveling is required especially with gloss paints, because every defect will be visible on such a coating.
The leveling process begins when the application process has been completed and the paint remains wet on the substrate, and physical phenomena allow it to even out the layer before it dries. Thus, the process is also dependent on drying time (also open time) and is related to many raw materials present in the formulation, such as thickeners, coalescing additives, but also surfactants.
The elimination of poor leveling can be done in many ways, depending on what it is caused by. If we are talking about typical brushmarks, they are usually caused by the thickener acting too strongly in the area of mid-shear forces or on the border between mid- and low-shear forces. Often, the leveling adjustment in paints with visible brush marks and short drying times can be performed by adjusting the open time with Open Time Extenders (OTE-additives) or by changing the coalescents. In gloss paints where poor leveling is manifested by wrinkling of the coating or other defects such as craters or orange peel, leveling can be improved with special flow and leveling additives, which are usually polymer surfactants that regulate the surface tension and allow the paint layer to be leveled before it dries.
Leveling test procedure
In order to carry out leveling tests in laboratory conditions, the ASTM D4062 Standard Test Method for Leveling of Paints by Drawdown Method is used, which consists in determining the degree of leveling of the coating after applying it under controlled conditions using a special applicator with spiral grooves.
The test applicator is Leneta Test Blade LTB-2 (groove doctor blade), which differs significantly from typical wire applicators. The applicator allows you to drawdown paint with a spreading rate of 10 m²/L (400 ft²/gal) and create unevenness on the coating that must be leveled during drying. The test paint is applied by drawdown at 0.6 m/s, hence very fast to produce a shear similar to that of brush application. In addition, pre-drawdown paint is pre-sheared by reducing the viscosity by squeezing it out of a syringe and a needle of a certain diameter. The test kit is shown in Figure 1.
The coatings are allowed to dry overnight at 23 °C ± 2 °C (73.5 °F ± 3.5 °F) and 50 % ± 5 % relative humidity and then assessed against levelness standards, expressing a score of 0 (very poor leveling), up to 10 (perfect leveling). Currently, you need to prepare the standards yourself as The Leneta Copmany has withdrawn the sale of levelness standards.
The following is an example of the results of acrylic latex paint case studies for the effect of the tested flow and leveling additive on the leveling rating according to ASTM D4062. Coatings shown are dried specimens obtained according to the leveling test procedure of which the left hand sample is the control paint and the right hand sample is a leveling additive (Fig. 2).
An evenly formed layer of coatings is very important not only for aesthetic reasons, affecting the gloss or its overall appearance, but also affects the protective properties that it carries. Brush marks, craters or unevenness in the coating cause fluctuation in its thickness, which means that the coating is weakened in thinner places, exposed to easier damage or in thicker places to worse coating formation. That is why the importance of properly selected additives and formulation of paints taking into account flow is so important, even in the case of latex paints. A
ppropriate laboratory methods for assessing the performance of flow and leveling additives such as ASTM D4062 are extremely helpful here. We recommend that you familiarize yourself with this testing procedure or contact our laboratory to order leveling and other tests.