Two-component paints (or multiple-component paints) require the addition of a cross-linking agent prior to application. However, it is very important to know about the time the paint should be used up after mixing with the hardener or other recommended crosslinking ingredient. Often, in two-component paints, the determinant is an increase in viscosity that prevents further application, but not in water-based epoxy paints.
This type of paint requires a slightly different approach, which is described in today’s blog article.
The most common pot-life indicator is the change in viscosity in the form of a (usually) 50% increase in the viscosity of the original mixture with the hardener. For some, the measure of pot-life is the change in adhesion of coatings, however, in the case of water-borne epoxy paints, the test to determine pot-life is the change in gloss. This test is carried out according to ISO 9514 “Paints and varnishes – Determination of the pot life of multicomponent coating systems – Preparation and conditioning of samples and guidelines for testing”
In pot-life tests of multicomponent systems, the term adiabatic transformation should be introduced. Adiabatic transformation (adiabatic process) is a thermodynamic process in which the insulated system does not exchange heat, but all energy is supplied or received from it as work. When many multi-component systems react, heat is generated, which is released to the environment by a can containing a mixture of ingredients and hardener. Creating adiabatic conditions is to prevent this exchange, thanks to which the heat is retained and takes part in the reactions in the can. Top, therefore, often when using 2K systems, it is recommended to wrap the can with heat-insulating material for the reaction time.
In 2019, the ISO 9514 standard was changed, which abolished the use of adiabatic conditions.
In the case study, the sample was a water-borne two-component epoxy paint prepared in the Spektrochem laboratory. The tests were performed as initial – immediately after mixing with the hardener and at intervals as shown in the charts.
Consistency tests are not listed in ISO 9514, however, in the Spektrochem laboratory they are obligatory in such cases as they provide a lot of important data for subsequent pot-life analysis and provide additional results for the final pot-life determination. The consistency is determined according to ASTM D562 (method B) using a digital Stormer viscometer and expressed in KU (Krebs Units).
The graph below shows the changes in viscosity for the samples under adiabatic conditions. For samples of the same paint with the hardener, but with non-adiabatic conditions, the changes in consistency look very similar. The limit value was assumed to be 110 KU, which is such a significant increase in viscosity that it makes it impossible to easily apply the paint and its good flow. An interesting fact is that in the case of the tested sample and most water based 2K epoxy systems, it is not possible to determine pot-life only by changing the viscosity, because the samples, even after a few days from mixing with the hardener, still remain liquid, of course with high viscosity.
On the basis of the assumptions, it was found that the pot-life was between 3 and 6 hours for adiabatic and non-adiabatic conditions (consistency ~ 99 KU after 3 hrs from mixing with the hardener and ~ 111 KU after 6 hrs).
During the consistency determination, the temperature of the samples was also measured in order to check how the temperature of the mixture increases during the reaction and how the adiabatic variation affects the heat exchange.
As can be seen in the graph below of the temperature change of the mixture of component A and B during the chemical reaction, large differences between the adiabatic non-adiabatic conditions are noticeable. The sample with the hardener stored in a special XPS container (adiabatic conditions) allowed the temperature to rise to max. 28.1 °C after 6 h and maintaining the temperature of the reaction mixture at 27.4 °C to 27.9 °C for 2 h to 3 h.
On the other hand, under non-adiabatic conditions, the maximum temperature that was reached by the reaction mixture was 26.4 °C after 1 hour 30 minutes from mixing with the hardener and remained at the level of 26.3 °C for up to 3 hours after mixing.
One can clearly see the effect of the XPS container which, due to the adiabatic conditions created, allowed the temperature to stay longer in the container with the reaction mixture. The verification of these results is finally achieved by determining the changes in the gloss of the coating from samples taken at intervals, analogous to the consistency and temperature measurements.
The gloss of the coatings was assessed in accordance with ISO 2813 on samples prepared by applying the mixtures with the hardener to Leneta WDX cards using an applicator with a 6 mils gap with 14 mm/s drawdown speed. The gloss was assessed at an angle of 60°, 24 hours after application of the coatings.
The gloss results are shown in the diagram below. The assumption of epoxy paint in this case is to create a glossy coating, so the value of the marked gloss at 60° should be as high as possible. The samples of liquid mixtures were taken after various times counting from mixing with the hardener.
Determining the limit value of acceptable gloss in this case is crucial and it was at least 60 GU at 60°. Of course, you need to be aware of the need to extend the tests to check that the coating properties remain intact, such as resistance to water, abrasion, aggressive media, etc. Basing results on gloss alone can be deceptive.
In this case, however, let’s only focus on the gloss assessment. The results show that the minimum gloss is fulfilled only up to a maximum of 1.5 hours after mixing with the hardener. These results are very similar for adiabatic and non-adiabatic conditions (slight predominance of non-adiabatic conditions).
Coatings obtained from samples taken 6 hours after mixing with the hardener show complete cracking, both from adiabatic and non-adiabatic conditions.
Markings prepared on the basis of the three types of tests performed (consistency measurement, temperature measurement and coating gloss measurement) indicate changes in the properties of the mixtures, which later have a key impact on the application of paints (viscosity) and the gloss of the coatings. Gloss determination as a determinant of pot-life for water-borne epoxy two-component paints is the most important method of assessing whether, after exceeding pot-life, there will be a risk of obtaining a matt coating, which does not only have a matt effect, but completely different coating parameters from the assumed ones.
Below is a short video of our lab while performing the tests discussed here. Details of the results of the presented tests are available on request