Wall paints in terms of stain resistance are experiencing their extraordinary growth. However, not all companies have properly prepared formulations yet, because the preparation of paints that will be resistant to everyday stains and enable their easy removal is extremely difficult and requires extensive knowledge in the field of raw materials and tests to verify such resistance.
Of course it is possible to paint the wall, apply the stains and wash it off, however professional testing should first be based on laboratory standard methods that ensure fully reproducible and reproducible testing procedures. In today’s article, I will discuss examples of tests that are carried out in an external independent paint laboratory, Spektrochem, both in the development of raw materials for the formulation of stain-resistant paints, and in the tests of paints from our customers who want to develop such formulations.
The Spektrochem architectural paints laboratory already in 2014 at its Spektrochem Seminar conferences presented test methods and the beginnings of developing paints resistant to stains in the form of pens, markers or foodstuffs. Over the years, we have been improving the knowledge base in the field of selection of raw materials, formulation of recipes and tests in accordance with standardized methods allowing for testing stain resistance of paints at a level verified in interlaboratory comparisons.
Today, we are a leader in the development of raw materials and the formulation of stain-resistant paints, offering product development for raw material producers, laboratory tests and formulation development for the introduction of architectural paints in the world with proven resistance to stains, such as dirt from mud, scuff-traces, difficult stains like wine, ketchup, mustard, lipstick, permanent markers, etc.
As a member of ASTM International, we use ASTM standards for tests, which are the result of many years of validation and introducing them to our SOP’s used in projects implemented for our clients.
First step – before stain resistance tests
The first step is to find out if the paint can even be tested for stain resistance. The point is not to remove paint and stains when using a load and washing medium. If the paint is too weak, it is not subjected to stain resistance tests. For initial verification, it is necessary to perform tests according to ASTM D2486 – number of cycles to rub for general strength assessment, as well as according to ASTM D4213 to determine the weight loss of the coating relative to calibration paints (RM, RDV, RWV).
This check is basic, as too low strength of the coating may result in unnecessary testing of further tests, but what is more dangerous, it may turn out that the abrasive strength of the coating is so strong that it will not be rubbed off in a stain test, but weak enough that a few µm will wear with the stain and give a good result. As a result, it will not be due to the stain resistance, but the surface abrasion of the coating together with the stains. The photo below shows examples of paints that were unsatisfactory with ASTM D2486 scrub performance after the specified number of cycles and were not approved for further stain resistance testing in accordance with ASTM D4828
If the test results according to ASTM D4213 and ASTM D2486 are good, the coatings can be subjected to further testing and cleanability can be determined. In laboratory practice, several standards are used to assess the stain resistance of coatings, of which the two most important methods are:
- ASTM D4828 Standard Test Methods for Practical Washability of Organic Coatings
- ASTM D3450 Standard Test Method for Washability Properties of Interior Architectural Coatings
The test to determine practical stain resistance is ASTM D4828. The method consists in applying a test paint to the panel and staining media is applied after a specified conditioning time. According to the standard, the following stains are used as standard: pencil, crayon, mercurochrome, ball-point pen, waterborne felt-tip markers, lipstick and mineral-oil-borne soilant as outlined in Test Method D3450, however, the standard is not limited to these. Usually, the tests use staining media agreed with the customer.
Solid media, e.g. lipstick, are applied with a special applicator (as in the photo below) that allows the stain to be applied with a specific pressure. In practice, when other media are used, the method of their application is agreed with the client individually.
According to the ASTM D4828 standard, the staining media is left on the coating for 1 hour, however, here too, specific staining times are usually agreed with the customer. In R&D works, when we develop stain-resistant paint formulations, we perform tests on a number of coatings to determine not only the resistance to a given type of stain, but also the time after which it must be removed. Such tests are important in the selection of raw materials and recipes that are to guarantee stain resistance.
The ASTM D4828 method allows manual washing of stains, however, our laboratory uses only mechanical method with the use of apparatus for wet-scrub resistance tests with a special cellulose sponge with weight, and the cleaning medium used in the tests is either a cleaning powder, cleaning media with ASTM D3450 (abrasive or non-abrasive) or other agreed with the client. We often use tested surfactants for washing, which are also checked at the request of their manufacturers who want to dedicate their cleaning agents also as recommended for washing stains from the walls. In addition to the effectiveness in removing stains, they are then checked in the laboratory for the impact on discoloration of colored coatings, change of sheen/gloss, etc.
The following case study of a stain-tested paint was based on the determination of practical stain removal from: 1 – espresso coffee (15 min), 2 – Merlot California red wine (15 min), 3 – Heinz Yellow Honey mustard (1 h), 4 – Heinz mild ketchup (1 h), 5 – Coca-Cola (15 min), 6 – Lipstick (1 h). In this case, the stain removal was carried out with a cleaning powder. Below are the test results in the form of a panel photo and the results in the table.
In the presented case, the coating did not achieve satisfactory results with regard to difficult stains. In addition, the coating showed an increase in sheen (initial sheen @85° was 1.2) after soil removal, which affects the final result. Such a sample cannot be called a stain-resistant paint.
Below, as a curiosity, we present the appearance of the coating tested in the same set of paints, but with unsatisfactory scrub resistance ASTM D4213 and ASTM D2486. As you can see, the stains were removed along with the coating, and additionally penetrated so strongly into the coating that the place of their occurrence is clearly visible even after 100 strokes during the test.
Below are pictures of other coatings that were tested with the same stain set as above but with significantly different results. On the left side is a paint coating with excellent stain resistance, from which all stains have been removed for up to 100 cycles. The coating also showed no increase in sheen and erosive changes. On the right, the same paint, but with a slightly modified raw material composition in terms of thickeners, the deterioration of resistance to coffee and wine is already visible.
Removal of standardized soilant
Another test to determine the stain resistance of coatings is ASTM D3450. This method determines the ease of removing a standardized soil from a coating with cellulose sponge and an abrasive or non-abrasive medium. The handle and sponge in this test weigh 1500 g, and four series of 25 washes are made, each with rinsing the sponge and using a new batch of cleaning medium.
The soiling medium consists of carbon black, mineral oil and odorless mineral spirit, while the cleaning media consists of water, surfactant, acetic acid (glacial), trisodium phosphate, thickener – in a non-abrasive medium, and in the abrasive medium there is additionally silica.
Washing of the soilant is carried out 16-24 hours after its application, after which the coatings are rinsed under tap water and allowed to dry.
Prior to the application of soilant, the reflectance of the coating is measured and the measurement is repeated where the soilant has been removed. The calculated reflectance ratio represents percentage reflectance recovery and is the result of the test in accordance with ASTM D3450. However, this method is limited to coatings with a reflectance of 60% or more.
Depending on the type of paint, one or two cleaning media (abrasive or non-abrasive) are used. This method allows you to determine under standardized conditions how the coating accepts the dirt and how easily it can be removed from it.
Below are pictures of the two coatings with different test results according to ASTM D3450. This method is used in our laboratory both for testing stain-resistant paints on the market, and when formulating paint recipes from new raw materials, in order to check their suitability for use in this type of paint.
 Formulating recipes for stain-resistant paints for interior painting, 6th Spektrochem Seminar, Poland 2014
 Stain resistance latex paints formulations and case studies 2017-2020, Spektrochem Paint Laboratory