For years, architectural painters preferred the use of solvent-based paints due to their ability to maintain a wet edge much longer than new water-based paints. This ability may otherwise be referred to as open-time. It corresponds to the working time of a paint, such as when a painter is working on a door frame or a wall. In solvent-based paints, it becomes possible to repair any drips, sags, or marks in the wet paint even after 15 to 20 minutes; in typical water-based paints, however, wet paint starts to “close” within 5 to 10 minutes.
In literature, an open-time is also described as the moment when the coating’s viscosity reaches a critical value. It is possible to alter the open time through the addition of co-solvents that slow down the evaporation of water, thereby extending the open-time. This process usually entail a higher VOC level that could leave a messy film at the end.
The common test method for evaluating the open-time of paints involves casting a uniform film of paint and then immediately creating a mark in the paint. As the paint dries, the operator brushes onto the finish and evaluates the effectivity of the technique in altering the initial mark. This process is done over several periods of timed intervals. The open-time corresponds to the time until which the mark could still be removed.
This method is not recommended because of its subjectivity and tediousness. Moreover, the method fails to provide relevant information on other paint properties during the process, including tackiness.
Rheolaser Coating® Method
The Rheolaser Coating® is able to directly measure open-time because it is linked to the viscosity of the coatings. The measurement begins at the first drop of the signal. To show the effectiveness of the device, two waterborne paints were analyzed on glass 120 μm (wet thickness) using the Rheolaser Coating® and the common method. Through altering the co-solvent used, it became possible to extend open-time; however, this was projected to result in a tackier film. Results are presented in the table below.
Figure 1. Drying kinetics of two waterborne paints
The use of Rheolaser Coating® in paints enables a quick, reliable, and objective evaluation of open-time in comparison to other common methods. Additionally, the device provides a complete outline of the drying mechanism.
This information has been sourced, reviewed and adapted from materials provided by Formulaction.
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