Façade paint can leach when water condenses on freshly dried painted walls, leaving behind shiny vertical “snail trails” that alter and deteriorate the façade’s appearance.
To date, predicting a paint formulation’s tendency towards leaching is difficult, because the phenomenon is poorly understood, and no standardised analytical methods exist. A newlydeveloped test method can mimic leaching conditions and help in studying the influence of paint ingredients on the occurrence of snail trails. In addition, we devised a new polymeric dispersant that is resistant to leaching to aid formulators.
Freshly painted façades are open to weather influences, particularly in spring or autumn, when temperatures during painting more easily fall below the dew point . Latex paint’s film curing process becomes slower under these conditions, making the paint susceptible to humidity. If the binder film has not sealed the paint surface completely, condensed water can mobilise water-soluble paint ingredients, which then migrate to the surface. As the condensed water evaporates, the concentrated residues appear as stains, gloss patterns or snail trails. It is still not fully understood which ingredients are extracted, but surfactants are suspected to be a main contributor, due to their water solubility and mobility. They are introduced into the initial paint formulation via binders, pigment paste, as wetting agents and as emulsifiers. Formulating a surfactant-free paint is a futile endeavour though, since surfactants serve essential functions in the paint manufacturing and application process.
Snail trails are a particular problem for dark shades as the tinting colorants can introduce high amounts of water soluble ingredients. Even if a white paint has been optimised, all the benefits can be extinguished by the tinting. The shiny “snail trails” are in strong contrast to the rest of the façade finish. The consequences are customer complaints and even legal claims, which can lead to the façade having to be repainted and compensation costs on the side of the paint manufacturer. With white paint, “snail trails” can become visible when microorganisms or dirt stick to them, making them look dark. This is termed dirt pick-up.
Novel test methods help to track down the source of the snail trails
To gain deeper insight into the source of the snail trails rather than speculating, we set out to develop methods that can investigate the phenomenon in a qualitative and semi-quantitative manner. The methods that have been published to date mainly focus on the binder itself  or on the environmental impact . ASTM standard D7190-10 (2015) on how to evaluate surfactant leaching only gives an optical rating when water droplets are sitting on the paint film.
A qualitative test should allow the screening of paint formulations and quick exclusion of those that leach. A semi-quantitative test should, with comparison to standards, identify the ingredient responsible. Using head-to-head comparison, two types of ingredients can be quantified.
Qualitative test – does the paint formulation develop snail trails?
For the qualitative analysis, a conventional Kesternich cabinet is easy to use. The paint of interest was applied to black Leneta Scrub Test panels (polyvinyl chloride/acetate copolymer; according to DIN 53778). The paint was applied at 400 μm wet film thickness (corresponding to approximately two layers of paint film on a façade), leaving 3–4 cm blank at both short ends of the panel. After drying, one panel was cut into two to run the test in duplicate. The remaining blank end of each panel was used to form “pockets” using sticky tape, so that the leached eluate that runs down the painted surface can be collected (Figure 1). The flexibility of the foil also allowed for punching holes. The punched holes served as a means to hang the foil in the Kesternich cabinet vertically to mimic the orientation of a façade. Tailor-made hooks also ensured that the painted surface pointed outwards towards the glass walls of the Kesternich chamber, so that they were exposed to plenty of water.
After being subjected to warm water vapour according to the “Schwitzwassertest” (DIN 50017) and a thorough drying time, the paint surface was evaluated. The presence of snail trails can be determined quickly on tinted paint. They can be seen in higher magnification with a handheld microscope (Figure 2). In addition to the paint surface, the pockets in which condensed water may collect, served as an indicator for leaching. After drying, the presence of residue here hinted at highly water-soluble leached ingredients that may not be visible on the small panel, yet may cause a problem on a tall façade. In our standard test, we hung a set of duplicate foils in the Kesternich chamber and studied three paint samples at a time. This Kesternich cabinet test was a reliable screening method for various paint formulations.
Illustrating the practical use of the test: various binders in a paint formulation
As proof-of-concept, we investigated the impact of various binders on a highly simplified white paint formulation. The paint formulation was fabricated as one batch of slurry that was then split into batches mixed with different binders, to avoid weighing errors for the other ingredients. Some ingredients that are also suspected to leach were deliberately left out. The paint formulation therefore did not include defoamer, nor additional rheology modifiers after the grinding step (hydrophobically modified alkali swellable emulsion (HASE) thickener). The wetting agent was reduced to the minimum amount necessary for grinding titanium dioxide and fillers.
We evaluated several different monomer compositions in the binder and minimum filmforming temperatures (MFT) (Tables 1 and 2). In a challenging high PVC paint, different results were obtained. Binders 3 and 4 showed almost no snail trails and clean pockets, while Binders 1 and 2 clearly exhibited shiny marks on the paint surface and residues in the pocket (Figure 3). This means that amounts of Binders 1 and 2 should be increased to seal the paint surface better and deliver similar results to Binders 3 and 4, which already function even at high PVC formulation. Due to the higher MFT for Binders 1 and 2, a coalescing agent may be necessary for better performance.
Quantitative test: finding the root cause of leaching in a paint
While the Kesternich cabinet test helps to judge whether a paint formulation is prone to leaching, it does not reveal which ingredient is the culprit for the snail trails in a more complex paint formulation. To answer this question we needed a more sophisticated test. For a known paint formulation, those ingredients suspected to be a contributor were analysed for their retention time via liquid chromatography (high performance liquid chromatography (HPLC) for small molecules and gel permeation chromatography (GPC) for polymers). The paint formulation was applied to the back side of ceramic tiles (bathroom tiles) as mineral substrate. The paint was applied with a foam roller brush and the weight of paint was recorded. About 11 g wet paint per tile (ca. 14 x 14 cm) was desirable (corresponds roughly to the layer thickness of 400 μm on the panels that were used for the Kesternich test).
The Kesternich cabinet was exchanged for a modified aquarium with a cold-water vapour condenser attached to it (Figure 4). The cold water vapour served as a comparison for real leaching conditions in a spring or autumn morning dew.
In the modified aquarium, four tiles can be studied at a time. We worked with duplicates, so we investigated two paint formulations simultaneously. Each tile was erected using a metal holder that stood in a flat glass basin. The eluate was collected in the glass basin. At the end of the “leaching cycle”, the water from the glass basin was weighed and then concentrated to compensate for HPLC detection limits. The retention time of any peak in the chromatogram can be compared to known standards. This makes it possible to identify the leached paint ingredient or ingredients. The test itself is more cumbersome than the mere visual evaluation by the Kesternich test. Nevertheless, it is very valuable should the root cause of the leaching need to be identified. In a semiquantitative fashion, amounts of leached components can be compared relative to each other. Weighing errors can occur during the eluate transfers or sample concentration. In addition, the amount of the paint on the tile can vary slightly and additional weighing errors occur when determining wet and dry paint. All these slight variations lead us to the conclusion that a completely quantitative evaluation is not possible. It also may not be required for the time being, as the study of the root cause of snail trails in various paint formulations has only just begun.
Case study: dispersant
A dispersant is present in (tinted) paint in an amount that can lead to optical deterioration of a façade when it leaches out. We therefore focused our attention on developing a novel polymeric dispersant that could overcome this challenge. In theory, a polymeric dispersant is less mobile in the paint formulation due to slower Brownian motion and entanglement of the polymer chains with other paint ingredients. To study this effect, a pigment paste featuring the new polymeric dispersant and a comparative paste featuring a conventional surfactant-type dispersant were made. Both pastes were used to tint the same white base paint. As a pigment, we chose an iron oxide red (PR 101) because snail trails are more visible on darker shades and because inorganic pigments are frequently used on exterior walls. This formulation is shown in Table 3. Again, it was simplified as much as possible to avoid the influence of other ingredients. The pigment paste was added to the base paint at 8 %.
The optical evaluation with the Kesternich cabinet test revealed shiny snail trails on the smooth pigment surface for the paint that had been formulated with the surfactant-type dispersant. The paint formulated with the novel polymer, however, remained unaltered even after being subjected to the test. To quantify the amount of leached ingredient, we also ran the aquarium test with tiles as a substrate.
Standards of the two dispersants were used for the method development via liquid chromatography. Figure 5 shows that the surfactant has a retention time of 15 min on the HPLC chromatogram. The polymer, on the other hand, needs to be detected via gel permeation chromatography, due to its higher molecular weight. Its retention time was about 20 min. When injecting the eluate of the corresponding glass basin into a HPLC device, a large peak with retention time 15 min was observed for the surfactant that leached off the tile. The eluate from the tile featuring the polymeric dispersant, in contrast, only had a small peak. Using the area under the curves and comparing it to a calibration curve, we quantified the leached dispersants very roughly. For the surfactant-type dispersant, roughly 2.1 g/L were found, while for the polymer, only 0.4 g/L were detected. These numbers should not be taken as absolute numbers, but can be seen relative to each other. As a ratio, we see that five times less polymeric dispersant was washed out than the surfactant-type dispersant.
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ASTM D7190-10 (2015)