Latex paints dry as a result of the coalescence process of polymer dispersion particles that bind all paint components to form a coating. This process occurs under certain conditions correctly, with appropriate selection of raw materials. To this end, raw materials such as dispersion binder and additives are selected appropriately – in order to reduce the temperature of coating formation (coalescent), as well as to extend the drying time (open time extenders).
How are paint tests carried out for drying? What do laboratory assessment methods look like to ensure reproducibility and reliability of the results in terms of drying stage and drying time?
The process of forming water-borne coatings of dispersion paints, varnishes or plasters is a physical process involving the evaporation of volatile components (water, coalescents) and further physical formation of the coating.
This process occurs smoothly, dividing into three main stages called the three-stage Vanderhoff model.
Laboratory tests of drying time
Dry time and degree laboratory tests consist of coatings under standardized conditions, controlled conditioning and determination of the degree of drying using one of the standardized test methods.
Depending on the type of paint and customer requirements, drying time tests are carried out using test methods, the most important of which are:
- ISO 9117-3 – surface drying test with Ballotini glass beads
- ISO 9117-4 – determination of drying time using automatic recorder
- ISO 9117-5 – modified Bandow-Wolff test
- ASTM D5895 – automatic recorder, but not identical to ISO 9117-4
The drying time of paints depends on many factors, including air temperature, substrate temperature, air humidity and the mechanisms that accompany this process. Physically drying paints dry differently, differently in combination with oxidative drying, and still differently paints drying by reaction with cross-linking agents. The absorbency of the substrate also does not remain indifferent.
Therefore, laboratory drying tests are carried out under standard conditions on certain types of substrate. In the Spektrochem laboratory, there are constant hygrothermic conditions throughout the laboratory. The temperature of 23 °C ± 2 °C and the relative air humidity of 50 ± 5 % is the standard that we maintain 24 hours a day, 7 days a week. A permanent record of recorders located in different parts of the laboratory helps to control the settings and make the necessary adjustments to always ensure these conditions.
Often, however, our customers expect the tests to be carried out in more real conditions, so if necessary, we have the option of carrying out drying tests both at lower and higher temperatures, other humidity conditions, and, very importantly, under certain lighting conditions. This is especially important in the drying tests of alkyd paints, which dry oxidatively with the participation of oxygen, and its binding depends on the intensity of light.
Depending on the type of paint (type of binder), we select test methods to determine the degree and time of drying. Often the choice depends on the initial checking of how the drying takes place and then we propose a specific method, and often it is immediately known which degrees of drying overlap and marking them does not make sense. This is the case, for example, in the case of architectural dispersion paints, which most often may have surface drying marked (using the ISO 9117-3 method with Ballotini beads), and subsequent drying degrees are obtained at the same time (e.g. degrees 3-5 by ISO 9117-5)
The ASTM D5895 (ISO 9117-4) test with the use of an automatic recorder (A method recorder – straight line) is an extremely widely used method for determining the dryness in our laboratory. In accordance with this method, we determine the gradual drying of the coating in one path, marking all the degrees of drying necessary for the practical characterization of the paint.
In the scope of our tests, we use standardized glass substrates, but also absorbent substrates, as well as drying at elevated temperature thanks to the use of a heated recorder plate. With regard to the modified tests, the same precision is ensured, documented by interlaboratory comparisons.
The ASTM D5895 test method is not equivalent to ISO 9117-4, especially regarding the drying test for alkyd paint coatings. It is in ASTM D5895 that the standardized level of illuminance in the laboratory is determined during the test, which is strictly observed in our testing of alkyd paints, both solvent-based and water-borne.
Another test performed to determine the time and degree of dryness is the Modified Bandow-Wolff test according to ISO 9117-5. In this test, the determination of all degrees of drying is used, where the first degree is equivalent to the rule for determining the drying using glass beads, and the subsequent degrees are determined using weights 20g, 200g, 2 kg and 20 kg, with a load of 2 kg and 20 kg is exerted with a piston apparatus, not a weight.
We prefer to use the ISO 9117-5 method for paints with long transitions between individual stages of drying, i.e. long-drying paints. This method allows to determine the degrees of full drying in a given range of a given degree, as well as intermediate degrees – the details of the test are presented on a short video from our YouTube channel.
Open time of latex paints
The drying time is closely related to the open time. Open time, this is the length of time a coating remains wet or open enough to allow for brush-in without the edges of the first coat becoming visible and allowing for repair within the previously painted area [source ASTM D7488].
Open time is particularly important if the painting is performed at elevated temperatures, e.g. on facades, in heated rooms, and therefore where the drying process may be too fast. The ASTM D7488 test method is used by us with the required precision both in open time tests for designed latex paints and during the selection of OTE (open time extenders), i.e. special additives aimed at extending the open time, e.g. Lubranil N20, Lubranil X4000, Eastman Optifilm OT1200. The test method is also used in our laboratory at elevated temperatures.
 Film Formation, Peter Mischke, European Coatings Tech Files, Vincentz 2010
 Coatings Formulation, Bodo Müller, Ulrich Poth, European Coatings Tech Files, 2nd Revised Edition, Vincentz 2011
 Open time modifications of latex paints, 10th Spektrochem Seminar, Poland 2018
 The effect of coalescents on the formation of water-borne coatings of dispersion paints, OXO Conference, Poland 2018