The standard adopted since 2001 to classify the scrub resistance of wall paint coatings in Europe is EN 13300. It defines 5 classes depending on the thickness loss of coatings after 40 or 200 cycles, determined in accordance with ISO 11998. Thanks to this method, many paints on the European market can be well characterized in terms of scrub strength, which is the loss in µm (micrometers) after applying a certain number of feeds under normalized conditions. This number (40 cycles or 200 cycles) depends on the strength of the coating and therefore the class that the paint can achieve.
More and more often you can meet the situation that high-quality paint coatings have 1 st class (loss less than 5 µm, usually at the level of 1-3 µm), and yet they still differ significantly in the quality of the coating strength, even if they have different resistance for taking stains. The EN 13300 classification was created in the years when the durability of latex paints available on the European markets was not as high as it is today. The constantly growing demands of consumers make latex paints come closer to the standards prevailing in the USA with their properties, such as abrasion resistance or stain resistance.
So how can we better show the differences in the scrub resistance of the paints that are today in the “one bag” of the first class EN 13300? Please see the article.
Formulations of latex paints
The quality of wall latex paints in the world depends on many factors. Starting from legal regulations, through market habits, to the basis on which they are used. In most cases, the quality of the paints is also determined by the price consumers are willing to pay. Consumers’ expectations of paints, as well as the willingness to pay more for quality, have been observed for some time in western and central Europe. This is the reason why more and more often you can find paints with high resistance to scrubbing, shining, scratching or everyday stains.
Still, balancing the price and quality is a challenge for the R&D departments of paint manufacturers, however, opportunities to improve the quality in the area of scrub resistance are available not only by increasing the share of polymer dispersion (lowering PVC, i.e. pigment volume concentration). Fillers with high hardness and appropriate grain size, waxes and self-cross-linking dispersions enable obtaining high resistance to scrubbing. Also, the idea of a high volume of non-volatile constituents, taken from the USA, makes the coating with greater thickness resist rubbing longer. It is thanks to such formulation procedures that more and more often in Europe, paints of premium quality from the NAFTA market (USA, Canada and Mexico) are obtained. For such paints, the resistance to scrubbing is practically always below 5 µm loss after 200 scrubbing cycles according to ISO 11998. Marketing departments of manufacturers of such paints can use additional testing methods that originate in the USA – test methods developed by ASTM International, of which Spektrochem is a member and takes an active part in standards development testing as well as in US meetings where draft standards are discussed and voted on.
Test methods complementary to ISO 11998 and EN 13300
ASTM standards are dedicated to the scrub resistance tests for paints with very high coating strength. Hence, the satisfaction of the premium segment paint differentiation needs is based on tests conducted in accordance with ASTM standards based on much more severe coating test conditions than those described in ISO 11998.
ASTM D2486 is the primary method for testing scrub resistance in accordance with ASTM International standards. This method consists in scrubbing the coatings after 7 days of conditioning to determine the number of cycles until the coating is wiped. There would be nothing unique about this method, if not for the details of the test that make it unique.
In the ASTM D2486 method, the coating is applied to Leneta P121-10N panels with a 7 mil (175 µm) gap height applicator providing a wet film thickness of approximately 3 mil (90 µm), and then conditioned in a standardized atmosphere at 23 °C ± 2 °C and a relative air humidity of 50 % ± 5 %. After this time, the coatings are scrubbed with brushes weighing 454 g with nylon bristles. The scrubbing medium used is the composition indicated in the standard containing acetic acid, detergent and silica, which makes the medium abrasive.
It is a much stricter type of brush than the non-woven cover with the handle, which in the ISO 11998 method weighs about 140 g, which is 3 times less. In addition, the detergent solution used in ISO 11998 is many times weaker in terms of aggressiveness towards coatings and is not abrasive.
Scrubbing is performed until the coating is rubbed in a specific area, which further aggravates the test. This place is obtained by placing a special 0.25 mm shim under the coating, the use of which causes a bulge in the coating. The rubbing brush performs aggressive abrasion of the coating at this point.
The photo below shows a typical wear pattern where a shim is applied.
A second test method to complement the ISO 11998 Scrub Resistance Test is ASTM D4213. It is based on a wet scrub of the test paint against the calibration paint, and the end result is comprised of three components:
• RM – milligrams of calibration coating erosion for every 100 mg of test coating erosion
• RDV – milligrams of calibration coating erosion per 100 µL of test coating erosion (coating volume without air)
• Rwv – the number of milligrams of calibration coating erosion per 100 µL of liquid paint erosion equivalent
The coatings for the test are prepared identically to those for the ASTM D2486 test: 7 mil auto applicator advance application. The coatings are also conditioned 7 days prior to the test.
The test method uses non-woven overlays (identical to ISO 11998), mounted in a special holder with a polyurethane sponge. The holder with the covers weighs 450 g and the sponge is saturated to a total weight of 20 g.
A very important factor influencing the test results are the coatings of the calibration inks. These are coatings with a known scrub resistance value determined in accordance with ASTM D2486 and prepared to show interdependencies: paint coating A (poor) has a scrub resistance of 35 % of paint C (good), and paint C has 50 % of paint resistance. for scrubbing paint D (very good). Calibration coatings must be conditioned for 6 months prior to testing in accordance with ASTM D4213. In the Spektrochem laboratory, we rely on paints prepared according to recipes that ensure such scrubbing values and we constantly maintain the condition of the coatings ensuring the possibility of performing tests with reference to 6-month conditioned calibration panels.
The ASTM D4213 test method performs a maximum of 800 cycles on the test coating and simultaneously on the calibration coating until the first signs of abrasion are noticed. If the test coating does not rub, the test is completed with 800 cycles. For the test, it is very important to select the calibration panels to relate the result to the appropriate paint, which in this case is the basis for calculating the test results.
But is it not enough to provide the result of classification according to EN 13300? It all depends on the needs, expectations and requirements of the market. Sometimes it is sufficient to state in the technical data sheet the result that the paint is in 1st class of the EN 13300 classification with a result of <5 µm after 200 cycles. Often, however, there is a problem of distinguishing the quality of several paints that have 1st class scrub resistance, and the ISO 11998 method does not allow for their deeper analysis in terms of wet abrasive strength.
This is where methods such as ASTM D2486 and ASTM D4213 come in handy, allowing to distinguish between many paints that seem to have the same strength (1 st class according to EN 13300).
Below we present a comparison of the results of two paint samples for which the scrub resistance was determined with all the test methods presented in the publication.
The inks used in the tests were designated “sample 001” and “sample 004”. These are samples of latex paints from the Polish market with EN 13300 classification in terms of resistance to scrubbing as 1st class. The tests were carried out in the Spektrochem laboratory in 2020 (tests in accordance with ASTM D4213: paint 001 tested for calibration paint C, paint 004 tested with reference to the calibration paint D).
As you can see the results of the scrub resistance test according to ISO 11998 for both paints look identical. Both paints have the same classification according to EN 13300. Only the scrub tests according to ASTM D2486 show a very significant difference between these paints.
Such a distinction is necessary many times when testing various paints with different price ranges. ASTM test methods, also thanks to the Spektrochem Laboratory, are becoming more and more popular also in Europe and are the standard for assessing high-quality premium class paints.
The Spektrochem laboratory uses all test methods for the scrub resistance of latex paints that are valid in various countries around the world. We use ISO, DIN, ASTM as well as BS, JIS, ANSI, SAE, AATCC and other test methods. Having so much experience in laboratory tests of coatings in various world markets, we can always offer an appropriate solution for the application of the test method for our clients.