Regarding the latex paint tests, the wet-scrub test is one of the most popular tests performed for wall, ceiling and facade paint coatings worldwide. One of the test methods is ASTM D2486, which is the most frequently performed scrub test in our laboratory and is a useful method both for comparing the quality of various paints and for determining the effect of different raw materials on the strength of latex paint coatings.
The test starts with a drawdown. The paint application is made on Leneta P121-10N black panels using the Dow Latex 7 mil gap applicator. The drawdown is performed using an automatic film applicator and the coatings are conditioned under standardized conditions of 23 °C ± 2 °C (73.5 °F ± 3.5 °F) and 50% ± 5% humidity for 7 days. The coatings are conditioned in a climatic chamber with 24/7 registration of air temperature and humidity.
Constant conditioning conditions are particularly important in latex paint coatings, both after the coating has dried in the required 7 days, and in the drying process itself. That is why our laboratory has the required temperature and humidity in the entire laboratory room, and we also have climate chambers. We wrote about the proper conditioning of the coatings before testing on the blog here: https://paintlaboratory.wordpress.com/2021/06/23/the-importance-of-the-right-environment-for-the-application-drying-and-conditioning-of-coatings-prior-to-testing/
The test is performed using nylon brushes of the specified weight with a load of 454 g ± 10 g and a bristle length of not less than 15 mm. Before use, brushes are prepared by soaking in water and performing 400 pre-scrubbing cycles on the coatings prior to the actual test. This is important because an untreated brush could result in too aggressive test conditions when scrubbed for the first time. Preparation by proper softening of the bristles makes the test series reproducible.
The abrasive medium Leneta SC-2 is used as a scouring medium, prepared as an aqueous suspension of silica and additives (including trisodium phosphate, ammonia) allowing to obtain a suspension with a pH 9.5 to 10 (adjustment with acetic acid) and Stormer viscosity 110 to 120 KU (thanks to hydroxyethyl cellulose). The thoroughly mixed abrasive medium is applied to the brushes in the amount of 10 g.
The coatings are placed in an apparatus that performs a reciprocating motion, performing 37 cycles per minute. 12.7 mm wide shims are placed under the coatings to cause abrasion in a specific place, where a slight fold is formed and the brush with the abrasive medium rubs it. 5 mL of water is poured onto the coating in front of the brushes. The operation of adding the abrasive medium to the brushes and adding 5 mL of water is repeated by you every 400 scrubbing cycles until you get two lines on the shim with a distance between them 12.7 mm, resulting from the width of the shim.
The test in accordance with ASTM D2486 is performed in two methods of conducting the A test – test cycles to failure on a single coating or the B – cycles to failure method, where the test is performed on the test coating and the reference coating applied simultaneously. In method A, the result is the number of cycles to failure, and in method B, the result is the percentage scrub resistance of the test coating relative to the reference coating.
But why is this test method so demanding, and what can it be used for? It is the only test method that uses an abrasive medium as an additional abrasion factor. The load in the form of an extremely heavy brush, unheard of in other methods of determining wet-scrub resistance, and the shim under the coating causing abrasion in a specific place, makes the ASTM D2486 test method one of the most aggressive wet-scrub resistance tests for latex paints. For this reason, it is used to determine the scrubbability of medium and low PVC paints, because for cheap and high PVC paints, abrasion occurs after several to several dozen cycles.
In practice, the use of this test method is not limited to testing latex paints from the market and comparing them with each other. The ASTM D2486 test method is an excellent tool to verify latex paint formulations and determine the effect of raw materials on scrubbability. In the Spektrochem laboratory dealing with examining raw materials, developing case studies and start point formulations, we use this method to test raw materials, including determining:
• differences in polymer dispersions and the quality of film formation with different coalescing agents, including the determination of the scrubbability of the coating after forming at different temperatures
• binder demand for formulations with various fillers, especially with high oil absorption by determining the scrubbability resulting from binding pigments and fillers by a latex binder
• sensitivity to re-emulsification of the coating when using different doses (ladder tests) and types of dispersing additives in formulations
• the effectiveness of additives aimed at increasing the resistance to scrubbing, e.g. waxes, fibers, slip additives, surfactants, etc.
All this makes the scrubbability test in accordance with ASTM D2486 the most comprehensive and demanding, especially for paints and formulations that approve to higher quality. The use of a demanding method is necessary because some test methods are not able to show the differences with paints formulated on different raw materials and low PVC.
Also see our blog article on the ASTM D4213 Test Method, which complements and extends the scrubbability testing: