Chemical resistance of decorative wood and architectural water-borne coatings

Chemical resistance is commonly associated with industrial coatings for tanks, floors onto which oil or gasoline may spill, or other places that are exposed to constant or intermittent contact with chemicals. With regard to waterborne decorative wall paints or wood coatings, chemical resistance is generally not expected, not to say that a certain group of chemicals cannot periodically act on them and cause damage.

In this article, I have discussed the most important chemical resistance that is often required of high-quality latex paints, both as decorative wall coatings and wood coatings. In this article you will also find a link to my publication in the European Coatings Journal on the resistance of wood coatings to unusual liquid media.

Fig. 1 Some of the chemicals used in chemical resistance tests of high performance latex paints according to MPI criteria

Introduction

One-component dispersion paints are based on acrylic waterborne dispersions, which are emulsified copolymer droplets in the form of particles with a size from ca. 50 to 300 nm, the stability of which is achieved by the use of appropriate surfactants. These, in turn, tend to migrate under the influence of contact with water, which is why you can often encounter the phenomenon of “whitening” of the dispersion film [Figure 2]. In the case of a dispersion-based coating with such a tendency and its contact with a medium more aggressive than water, in addition to migration and partial emulsification of the dispersion film, the coating also penetrates the coating by chemicals to which it has been exposed and, consequently, lifting the coating, swelling and blistering. In the least symptomatic case, there is only a decrease in gloss (permanent or transient) or discoloration at the place of exposure to chemicals.

Fig. 2 Whitening of the polymer dispersion film with various surfactants after exposure to water (on the left a slight whitening, on the right very high migration of surfactants and swelling of the film)

In order to improve the water resistance of the dispersion film during the synthesis of polymer dispersions, surfactants can be used that dissolve in monomer droplets and, unlike conventional surfactants, are not only partially adsorbed on the surface of the polymer particle, but are chemically bound to it. These are non-migrating surfactants (NMS) involved in the polymerization process, hence they are referred to as polymerizable surfactants [Figure 3].

Fig. 3 Scheme of differences in adsorption on the surface of polymer particles of conventional surfactants and chemical bonding of the polymer particle with polymerizable surfactants

The polymer dispersion formed by synthesis with polymerizable surfactants forms a film that is not either completely susceptible to whitening under the influence of water, or this process is very limited and the time of exposure to water can be significantly extended (depending on the proportion of NMS in the formulation of the polymer dispersion and the proportion of conventional surfactants). The diagram of the influence on the lack of migration of surfactants under the influence of contact with water is shown in Figure 4.

Fig. 4 Diagram of migration and non-migration of surfactants in the dispersion film during contact with water

While the resistance to heavier chemicals, such as acids or bases, also depends on other components of the polymer dispersion and other paint components used in the formulation, surfactants in the polymer dispersion play a key role in making the coating resistant to migration, softening and swelling under the influence of water. and chemicals in an aqueous or non-aqueous environment, both with the required wet-scrub resistance, stain resistance or the expected gloss resistance in case of accidental contact with food products, engine oil or detergents.

Chemical resistance of wall paints

With latex paints applied to indoor walls, resistance known as “chemical resistance” is not normally expected, although resistance to a variety of light chemicals is very often required. A special case in the era of the coronavirus is the use of all kinds of disinfectants based on highly concentrated ethyl alcohol, and accidental splashes on the walls of the ubiquitous hand disinfection dispensers are very likely. The action of such disinfectants may cause blistering and discoloration on standard latex paint coatings, both in the premium and low-cost segment, because paints of this type have not usually been designed for resistance to alcohol-containing preparations, especially where most of them are in the form of a gel. and can stay on the coating longer, even in case of accidental splashing.

Fig. 5 ISO 2812-4 test method B (inclined test panel) with an alcohol-based hand sanitizer on wall coatings

Figure 5 shows the coatings during the test according to ISO 2812-4 (method B – inclined test panel) in which the resistance test to the disinfectant flowing down the coating was subjected to. In the case of gel preparations, the test is performed at a point exposure, e.g. according to ISO 2812-3 (with absorbent material) or ISO 2812-4 (method A). As an assessment of the resistance to the action of the disinfectant preparation, the standard assessment of the change in color shade using a spectrophotometer, as well as the assessment of the change in gloss, both of these assessments are performed at different times after exposure to the disinfectant, as the changes may subside over time. In the case of complete lack of resistance and damage to the coating, e.g. in the form of blistering, the assessment is made by determining the degree of blistering, e.g. according to ASTM D714 or corresponding to the scope of ISO 4628-2.

Chemical resistance of water-borne paints according to MPI criteria

Water-based acrylic paints for painting commercial and light industrial surfaces, exposed to moderate abrasion and incidental contact with chemicals where strong cleaning agents are used, are described in the MPI (Master Painters Institute) requirements, which define, among others, chemical resistance. One of such standards is the series dedicated to water-based acrylic paints with different gloss levels. One of the sample criteria for MPI gloss level 3 (eggshell) are defined in the MPI #161.

These criteria, including MPI #161, are used to assess whether the paint meets the guidelines imposed therein and can be certified and entered on the list of paints approved by MPI for a given application.

Fig. 6 Coating tests at the Spektrochem laboratory in accordance with MPI #161 (to assess whether the formulation meets the criteria, however, tests performed by Spektrochem do not constitute MPI certification)

Table 1 lists the required media (chemicals) that are used to assess the chemical resistance of coatings in accordance with MPI #161 along with the exposure time used.

ChemicalExposure time
50% sulfuric acid1 hour
10% hydrochloric acid1 hour
5% phosphoric acid1 hour
25% sodium hydroxide1 hour
Methyl alcohol1 hour
Aliphatic hydrocarbon solvent2 hour
Motor oil (Pennzoil 10W-30)2 hour
Vegetable oil (Crisco Vegetable Oil)2 hour
Table 1. Chemicals for chemical resistance testing in accordance with MPI #161

The chemicals specified in MPI #161 are quite aggressive chemical substances, especially highly concentrated for the tests of water-based coatings, e.g. sulfuric acid or sodium hydroxide, and through their reactivity select coatings that meet the given criteria. In turn, methyl alcohol or aliphatic hydrocarbon solvent show their aggressiveness by emulsifying their action on many water-based coatings, which makes the overall requirements specified in MPI quite demanding. The test of resistance to engine oil or vegetable oil allows you to verify whether the coating will resist the action of a greasy substance, which may reduce the gloss or leave a permanent stain.

Chemical resistance to unusual liquid media of wood coatings

Wood coatings are used in a variety of locations, the exposure of which determines the conditions in which the wood component is used. Wooden benches, fences, saunas, facades of wooden buildings, etc., are also covered (or even mainly) with water-based paints, clear varnishes or transparent stains. When such wooden elements are used outside buildings, such as benches or wooden gazebos, they are exposed, apart from the action of weather conditions and changing temperatures, to unusual media, such as in parks. We are talking about flowing tree resin or bird droppings. Usually, such coatings are not cleaned of dirt accidentally appearing on them in the form of the above-mentioned and are exposed to contact with them for a long time. The proper resistance of the coatings, as well as the ease of removing such dirt, e.g. by rainfall, is very important, while maintaining the properties of the coating (e.g. not softening due to the action of bird droppings or tree resin). In turn, in the case of varnishes for painting benches, saunas or stair railings, functional resistance to human sweat is required. For this purpose, artificial perspiration tests are conducted according to standardized standards and the performance is assessed for effects on gloss, softening, discoloration etc. I wrote an article on this which was published in the European Coatings Journal. It contains examples from case studies made in our laboratory based on the comparison of three polymer dispersions in terms of resistance to unusual liquid media of wood coatings.

If you want to read more about resistance to bird droppings, perspiration and tree resin, please find my technical article here (ECJ, April 2022 issue)

Fig. 7 An example of… a masterpiece maded by pigeons on a bench painted with water-borne paints

Summary

Of course, the liquid media for chemical resistance testing discussed in this article does not exhaust all the cases of chemical resistance tests of architectural paint coatings or wood coatings, however, it shows that the topic of chemical resistance does not only apply to paints for garages, basements, tanks or other heavy industrial applications. The examples presented in the article raise an important topic of the necessity to test resistance to chemicals that are relatively difficult to meet high criteria for resistance to unusual chemicals by water-based paints.

Both in the case of water-based paints for walls and wood, in the age of ubiquitous disinfection liquids, as well as in the case of unusual required resistance to bird droppings, sweat or resin, the requirements created on the market can be met by the appropriate selection of raw materials for the formulation of water-based paints. For this purpose, it is worth using extensive start point formulations and case studies showing how the resistance of coatings changes depending on the raw materials used.

Every day in the laboratory of the Spektrochem paint technical center, we prepare such case studies for raw material producers, so that they can then be sent in the form of marketing materials and technical bulletins to paint producers, both through raw material distributors and at various events in the coating industry around the world.

Published by Artur Palasz

Scientist, paint formulator and testing expert.

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