In Europe, it has been a standard for many years to determine the wet scrub resistance of latex paint coatings using the ISO 11998 method and to classify the test results in accordance with EN 13300. But what is this test and how does it affect the quality of wall paints? The prepared article discusses the rules for performing the test in accordance with ISO 11998, the EN 13300 classification and some tips that we want to share with the technicians performing this test to improve their daily work in the laboratory.
The wet scrub resistance according to ISO 11998 is the ability of the dry coating to show less thickness loss than determined, calculated as the average over a given area, after subjecting the coating to 200 wet scrubbing cycles. In addition to determining scrub resistance, the method also determines cleanability, which is defined as the ability of a dry coating to resist penetration and release from soils during the cleaning process, without reducing the thickness by more than stated.
The principle of the method consists in scrubbing the coating after conditioning in standardized temperature and humidity conditions, and performing operations related to the determination of the dry coating density. Scrubbing is carried out in accordance with ISO 11998 with 200 cycles with a standardized non-woven pad mounted on the handle, however, according to EN 13300 (European classification), scrubbing is carried out with lower quality paints also with 40 cycles. Cleanability is performed using the same conditions as for wet scrub resistance, but with the removal of staining media (soilants), the type and method of application of which is determined by the user.
The European classification EN 13300 for interior wall and ceiling paints defines a number of parameters divided into different classes. One of them is wet scrub resistance. It classifies 5 classes differing in increasing loss of coating thickness after scrubbing according to ISO 11998. The requirements for each class are presented in the table.
|EN 13300 class||Requirements|
|< 5 µm after 200 cycles|
≥ 5 µm and < 20 µm after 200 cycles
≥ 20 µm and < 70 µm after 200 cycles
< 70 µm after 40 cycles
≥ 70 µm after 40 cycles
This is the basic scale for evaluating dispersion paints in Europe, which makes paint manufacturers aspire to class 1 and the lowest possible coating loss after 200 cycles. Usually, in technical sheets and marketing data, you can find the indicated classifications posted by manufacturers on the basis of internal tests or performed outside in other laboratories. In some European countries, there are additional classification extensions based on these classifications, e.g. in Poland (PN-C-81914:2002) or in Germany (DIN 53778-1:1983, withdrawn but still used historically in many technical data of paints on the market). On the European market you can find all kinds of classes in every country. The most common are 1st and 2nd class paints, however, due to the necessity to produce cheap paints, the other classes are also quite popular. The requirements for 1st and 2nd classes also define the criteria for obtaining the EU Ecolabel certificate for interior latex paints. Classes 4th and 5th are usually low-cost ceiling paints.
The abrasive pad is a cut-to-size 3M Schotch Brite non-woven pad. A new abrasive pad is used for each test. The length of the abrasive pad is included in the stroke length in accordance with ISO 11998. The abrasive pad is supported by a 135 g scrub pad holder.
The surfactant used in the test method is sodium n-dodecylbenzenesulfonate as a 2.5 g/L solution in demineralized water. It is a non-aggressive surfactant that does not cause additional abrasion of the coating, which saturates the non-woven pad to a certain constant weight and which is spread over the coating with a brush immediately before the test is performed. The surfactant solution is left for 60 seconds before starting the scrub test.
Scrub resistance tester
The machine – scrub tester is an automatic device that reciprocates the handle to move the handle with the abrasive pad. The scrub tester performs 37 cycles per minute driving the handle with the scrub pad along the length 300 mm and recording the number of scrub cycles performed on the counter. There are many models of scrub testers available on the market. In the Spektrochem laboratory, we use a tester from the Dutch company TQC Sheen.
Performing the test and key steps
Performing a wet scrub test according to ISO 11998 has several steps, each step being essential to obtain the correct test result and the required repeatability and reproducibility. Below we present the most important steps in performing the test and general information about the activity that requires special attention.
Drawdown is a paint application procedure for a test. According to ISO 11998, drawdown is performed on PVC panels, e.g. Leneta P121-10N black panels. They provide proper adhesion and are flexible, which is important for the next stages of the test. ISO 11998 indicates that it is advantageous to perform a drawdown using the automatic film applicator, however, due to the large impact of the application method on the repeatability of the coatings, the use of the automatic film applicator is even necessary.
This ensures that the paint is not only applied at a constant speed, but also at a constant pressure. Drawdown is performed at a very slow speed from 10 mm/s to 15 mm/s. It is preferable to use a vacuum bed which makes the substrate perfectly flat during the drawdown.
The type of crevice applicator used for the drawdown depends on the paint type. In the Spektrochem laboratory, we use a “dual” applicator that allows the drawdown of two pathes at once, the clearance we use is 300 µm or 400 µm.
Conditioning of the coatings is critical to proper film formation and includes conditions ranging from drawdown to drying and further conditioning. Maintaining a controlled temperature and relative humidity is standard procedure in the paint and coating testing laboratory. The use of devices that ensure constant temperature and humidity during both drawdown and drying should be a priority. In our laboratory, we use a central heating system that constantly regulates and maintains the temperature at the level of 23 °C ± 2 °C and the relative air humidity of 50% ± 5%. Thanks to this, during all operations, the conditions in which the wet paint layer stays and continues to dry are constant and controlled. The conditions in the laboratory are recorded on automatic loggers.
After drying, the coatings are transferred to a climatic chamber, which is also subject to the above conditions, in accordance with ISO 3270. Depending on the requirements agreed with the client, the coatings are conditioned within a specified time, usually 28 days conditioning for the ISO 11998 test. 24 hours before the end of the conditioning period, the coatings are subjected to further application of staining agents (if we test the cleanability) or the density of the dry coating is determined (if the test concerns wet scrub resistance).
Dry coating density
In order to determine the final result, it is necessary to determine the thickness of the dry coating. The determination is carried out in accordance with Annex A of the ISO 11998 method and consists in weighing the cut coatings and calculating the area weight, and then calculating the coating density after measuring the coating thickness. At this point, all operations are essential to obtain the proper dry film density, which will be used to determine film thickness loss after wet scrubbing.
To determine the density of a dry coating, you need an analytical balance, a micrometer or other device for determining the thickness of the coatings, a calibrated ruler with an appropriate accuracy and a guillotine for cutting out the coatings.. This section also explains why an automatic drawdown is necessary since the dry film thickness is determined from a separate coating than the one being scrubbed. Hence, it is necessary to maintain a certain repeatability of the application. The type of substrate used for the test is also important, hence the standardized panels for the scrubbability test must be of appropriate quality and have a repeatable basis weight and thickness.
Wet-scrub resistance test
The scrub resistance test is performed with the abrasive pad described above, mounted on a handle, by performing 40 or 200 scrub cycles. After scrubbing, the coatings are rinsed under running water and dried. The trace left by the overlay is then assessed, which is usually seen in the case of dispersion paints as strong or less burnish (gloss increase on the scrub path). The length of the path is marked and adding abrasive pad length, which is entire length of the stroke. Marking the length of the path requires a lot of experience, especially with paints that are themselves high gloss or are matt but have high resistance to burnish.
Before and after scrubbing, the coatings are rolled up and weighed on an analytical balance. From these data and the dry film density, the loss in thickness is calculated and then classified according to EN 13300. The ISO 11998 standard requires that the result be reported not only as coating thickness loss, but also as loss per unit area of coating weight.
Cleanability is performed on coatings prepared in the same way. The use of the dual applicator is important when testing cleanability because on one panel you can compare two coatings – subject to cleaning with staining agents and without staining agents. The ISO 11998 method does not indicate which stains to use, however, each manufacturer of paints and the market determines their type similarly (foodstuffs, e.g. wine, mustard, chocolate, coffee; cosmetics, e.g. lipstick, sunscreen cream, office supplies, e.g. felt-tip pens, crayons, etc.)
Applied stains are also washed using 200 scrub cycles with the same equipment as for the wet scrub tests. The method of applying staining media and the time of their action on the coating are determined individually.
The ISO 11998 test method and the EN 13300 classification form an integral whole in Europe for characterizing paints in terms of their resistance to wet scrubbing. Correct test execution is essential to obtain a meaningful result. The multitude of operations related to the preparation of coatings for the test (cutting, weighing, thickness measurement, determination of dry coating density) as well as conditioning, drying and drawdown conditions affect the possibility of errors that must be eliminated to zero to obtain comparable results. It is very important to follow the guidelines of ISO 11998 and perform the test strictly according to the standard. For the characteristics of higher-quality paints, it is also worth remembering about the test methods that allow for a broader characterization of scrubbing loss, such as ASTM D4213 or to distinguish low PVC paints by performing wet scrubbing according to ASTM D2486.