Seite/Page: 1 Raising the barriers to dirt In many interior applications, stain resistance and dry burnish resistance are desirable. Yet stains may penetrate the surface of even some "washable" paints and become almost impossible to remove. Some considerations in formulating paints to maximise stain resistance are discussed, with test results. Problems and solutions in formulating waterborne stainresistant paints Oliver Wagner* Chee Seng Yong Jonathan Allen Paint users today expect more and more of their paints. Properties typically only achieved by solventborne paints are now required of waterborne ones. Moreover, waterborne paints and coatings are expected to be ecofriendly and innocuous, as these are the driving forces for the replacement of solventborne paints. Additionally, paint manufacturers may desire either to upgrade their existing interior paint lines and/or offer a USP (unique selling proposition) to their clients. As a consequence, new product lines known as "stain resistant paints" are making their way onto both European and Asia- Pacific markets. These paints require the combination of a tailored polymer dispersion with a specialised paint formulation, in order to deliver stain resistance or cleanability along with other features such as dry burnish resistance, maximum service life and an elegant subdued sheen level (down to dead flat). Key issues in formulating such paints are considered below. Washability does not equal cleanability In a recent market study for a leading European DIY chain, consumers rated "cleanability" (see Figure 1) as the most important feature for an interior paint. Yet most interior paints on offer are marketed instead on the feature of scrub resistance or washability. This is reflected by all sorts of national testing standards for scrub resistance. However, there is no standardised testing procedure (nor even a common understanding) for the desired feature of cleanability. To make matters worse, a great variety of "standard household stains" have to be removable from the paint, which ultimately translates into protection from both hydrophobic and hydrophilic stains. The product should be so resistant that common marks and stains can be removed virtually without trace. With its high scuff-, dry burnish resistance and durability, it will be particularly suited to high-use living areas such as bedrooms, lounges and hallways. Factors influencing cleanability Detergents dissolve or tend to dissolve in water (or other solvents). To enable them to do this, they require distinct chemical characteristics, namely both hydrophilic and hydrophobic groups in their molecular structure. What is known as the HLB value (Hydrophilic-Lipophilic Balance) basically describes the balance of the molecular weight or relative effectiveness of the hydrophobic to the hydrophilic portion. Similarly, it might be useful to devise a term or an approach which indicates the balance of resistance of a paint to hydrophilic and lipophilic stains. Stain resistance could be achieved by a controlled microstructure of the surface, which has to be based on a non-porous continuous polymer matrix. Resistance to hydrophobic stains is then mostly affected by the pigment/filler system, whereas lipophilic resistance is more a function of the choice of polymer dispersion. In practice, of course, a paint must provide resistance to both types of stain. Additives, thickeners, dispersing agents and in general everything water-soluble naturally also contribute to the success or failure of a stain-resistant surface. These, however, can be considered as fine-tuning of the formulation, once the two major points mentioned above are addressed. In general the issue can be summarised by saying that there are two critical factors for good stain performance: Firstly, stopping stains from penetrating into the film; Secondly, release of the stains from the film upon cleaning. This is an important point to make. The polymer must stop liquid stains from penetrating too deeply, or no amount of cleaning will remove them. This is more often than not a problem for standard polymer dispersions. How stain resistance tests were carried out The drawdowns of the paints to be tested were made on black "Leneta" panels with two 120 µm wet paint layers and 24 hours room temperature curing between these individual drawdowns. Stain resistance was tested after final curing for seven days at a constant 23 C. Roughly 1 ml of each selected stain was applied to the paint film at right angles to the length of the panel and left for five minutes. Similarly, solid stains were wiped perpendicularly across the drawn-down film about six times. In these tests, each individual stain needs to be spread until it is approximately 2 cm wide, and 2-3 cm distance must be allowed between stains to prevent cross-contamination. After the five minute exposure period, the excess staining substance was removed with a cloth. This must be done without any pressure, so as not to start the cleaning process prematurely. The stains to be included are not part of any international norm but should make sense either in terms of the type of soiling they represent or in the cultural context of the country where the respective paints are sold. Consequently eight different stains have been chosen over the years to be tested for the European market and seven for Asian markets. In Asia test stains include black tea, coffee, red ink, blue ink, black ink, lipstick and crayon. For Europe black tea, coffee, red wine, beetroot, mustard, ketchup and two different lipsticks are used.
Seite/Page: 2 The use of lipstick should be explained in more detail since it may not be obvious at first sight. On the one hand, kids find it a great entertainment to decorate the walls of the bathroom with mum s lipstick. On the other hand, lipsticks of different composition are fine representatives of lipophilic stains, with excellent reproducibility in their composition. Defined colour shades (mainly red, achieved by use of water-insoluble dye) assist colorimetric measurement. The two lipsticks used in Europe differ greatly in their oil content (beeswax and castor oil) and thus their hydrophobicity. Scrub testing determines stain resistance The panels were then placed in a standard scrub machine with 4 ml of a 0.25 % solution of "Marlon A350" (alkylbenzene sulfonate, sodium salt) spread on the surface of the panel. The abrasive pad was saturated with the solution and the panel was subjected to 100 scrub cycles according to DIN EN ISO 11998. Subsequently the panels were removed from the wet abrasion tester, rinsed with water, and dried at room temperature for 24 hours. CIELAB measurements are performed on areas exposed to stains and subjected to the abrasive pad as well as on unstained, but washed sections of the panel. The difference between exposed and unexposed areas is then determined as?e values: the smaller the?e, the better the stain resistance. It has been established in project work that the level of reproducibility for liquid stains is ± 10% of?e. When comparing different paints/formulations, sometimes averaged?e values (overall staining) are considered; sometimes the averages of hydrophilic or hydrophobic?e values are considered separately. Figure 2 shows three test specimens after stain removal. Burnish resistance test has practical significance Burnishing will occur with people walking in narrow hallways, standing against walls or when shopping bags rub on walls. This is a dry rub that polishes the film. For the test, a Leneta panel draw down is placed in a standard scrub machine and subjected to 500 scrub cycles. Instead of a brush, a PU block wrapped with a dry cotton cloth is used. Gloss measurements are then used to define a value for the so called "dry burnish resistance" by calculating the difference in 85 gloss before and after polishing, with a good value being below 1.0. Initial comparison of some premium paint types Three different paint systems were compared for various properties, with the main emphasis being of course on stain resistance. What these systems have in common is that they are of premium quality and price. They are: Superhydrophobic silicone effect paint; Semi-gloss interior paint; Stain-resistant paint. Comparing the staining performance of the three different paints gives a picture of great contrast (see Figures 2 and 3). The semi-gloss paint performs very badly with hydrophilic stains, e.g. tea, coffee and red wine. The silicone effect paint in contrast is very bad on lipsticks and ketchup. On the hydrophilic stains it is still poor. Only the stain-resistant paint is able to perform well on all stains tested. The silicone paint is simply too porous to prevent initial stain penetration, making subsequent stain removal useless. The semi-gloss paint in contrast does exhibit a "closed" surface, but is apparently too sensitive towards hydrophilic stains. That stems either from water soluble/sensitive materials in the paint film or a "non-optimised" microstructure of the surface. Nine binders compared for stain resistance The comparison of binders and subsequent selection of the most useful polymeric vehicle was rather more difficult than is implied below. In fact it was the outcome of an iterative process. This is because only an optimum formulation lets one see all the advantages of the correct binder, but by the same token only the optimum binder lets one develop the correct formulation. In an optimised stain-resistant formula at a PVC of 45 %, nine different polymers were compared for their stain removal properties with the European stains (see Figure 4). The newly developed specialised stain-resistant straight acrylic binder (No.9) exhibits the lowest averaged?e value for all eight stains. However, there are binders with lower?e values in averages of hydrophilic stains alone (No.1), but they do not perform well on the hydrophobic stains. It comes as no surprise that the protective colloid based VAc/E copolymer (No.8) performs exceptionally poorly on the averaged hydrophilic stains. Overall, for the European stain testing the averaged?e value for all eight stains should be below 1.0 to provide a well-balanced stain resistant formula with good performance. It appears at first sight that the underlying principle for good stain resistance stems from the choice of main monomers, where either styrene or methyl methacrylate in combination with butyl acrylate would be suitable. The strongest influence with polymer dispersions lies, however, with their respective stabilising system, e.g. emulsifier and/ or protective colloids, their nature and amount. Optimising PVC for cost and performance The most basic formulation parameter is the PVC. To avoid any stain penetration, the surface must be non-porous, ie the PVC must be below the CPVC. Depending on the binder and fillers employed this allows a range of 0 % to 55 %. To restrict this, it must be considered that the paint should be of premium quality, e.g. class 1 according to DIN EN ISO 11998 (abrasion loss less than 5 µm), as well as resistant to dry burnish and completely flat. Yet the raw material cost per unit volume should be as low as possible. These factors lead to a practical working PVC somewhere between 40 % and 50 %. For a modern interior formulation it goes without saying that the paint should not just be "VOC-free", it should rather be "emission free", ie, free of solvents, coalescing agents, plasticisers, ammonia, formaldehyde, etc. This imposes some demands on the binder, and also on the choice of alkali, defoamer, in-can preservative, thickener and dispersing/wetting agent. Effects of dispersants and thickeners Four different dispersing agent chemistries were compared for their stain removal properties with the Asian stains. As could be expected, hydrophobic types such as a maleic acid diisobutylene copolymer or a polycarboxylate significantly outperformed hydrophilic ones such as TKPP or polyacrylate. (In fact, the addition of 0.1 % TKPP negated
Seite/Page: 3 all the good stain removal properties of the hydrophobic types.) Thickeners also significantly affect stain resistance. Cellulosic thickeners are very hydrophilic and so reduce stain resistance on hydrophilic stains (usually severely), and so are not useful here. Different thickener combinations were compared for their stain removal properties with the European stains (see Figure 5). For hydrophilic stains only, HASE (hydrophobically modified alkali-soluble emulsion) thickeners are the best choice. For overall balanced stain resistance modified bentonite clay/ HASE (No.5) delivered the best values followed by No.1, an HEUR/HEUR combination. But a premium paint must offer other good properties, which further limits the choice of thickeners. Thickening with HASE alone leads to syneresis as well as an unacceptable increase in 85 gloss after the stain removal scrub. Formulations containing the clay thickener also showed a borderline dry burnish gloss increase, and the rheology profile was unsatisfactory. In terms of scrub resistance, all paints were class 1 according to DIN EN ISO 11998. Thus, ideally one should work with HEUR/HEUR or HASE/ HEUR combinations for the best overall paint property profile. Extender choice must be checked experimentally Extenders have a significant influence not only on the overall properties of any interior paint but also on the key properties of stain- and dry burnish-resistance. The choice of extenders at first sight is limitless. However, the need to meet standard properties, such as hiding power, low gloss, freedom from cracking, availability, cost competitiveness etc narrows down the choice. Suitable individual extenders (along with a standard matting agent) were rated for their stain removal properties as shown in Figure 6. However, for a better rating of the individual extender performance, the 85 gloss and the dry burnish should also be considered, as in Figure 7. The specific clay tested gave the best performance against tea, inks and coffee stains, but had some deficiency in red wine and crayon resistance. The two talc products tested, however, come with very similar technical specifications but with huge differences in performance, which leads to the conclusion that one cannot just generalise from all the data presented on extender classes. Strictly speaking, the results therefore only hold true for the specific selected fillers, all from leading suppliers. Naturally, two or three of the above extenders should then be combined in such way that their "weak spots" are levelled out. The most successful combinations turned out to be mica in a blend with clay or talc in a blend with clay. The addition of nepheline syenite also proved to be beneficial. Finally it should be mentioned that the system is still a waterbased paint and not some kind of ceramic wall cladding, and marketing as well as consumers expectations should be adapted accordingly. Exposure times, e.g. how long the stain is allowed to sit on the surface of the coating, are of course crucial especially when attempting to clean hydrophilic stains after complete evaporation of the liquid. Of equal importance is the drying time of the paint (especially since it is coalescent-free) before soiling occurs. For example, if freshly brewed coffee was used to stain a 1 m² test panel after 1 day s drying of the paint and the same testing was repeated after 7 days drying, then the first one yielded a?e of 1.96 and the latter 0.75. If all of the above mentioned points are taken into account, a new premium, high value paint emerges to address so far unmet needs of consumers. Results at a glance Waterborne paints are increasingly being required to offer properties which match those of solventborne ones. In many interior applications, stain resistance and dry scuff resistance are desirable but may not be easily achievable. Stain resistance is not the same as washability : a paint which is not damaged by washing may allow stains to penetrate the surface so that they become almost impossible to remove. Tests were carried out to evaluate and optimise formulations for stain-resistant paints in terms of binder type, extender type and major additives (as hydrophilic thickeners or dispersants may significantly reduce performance). An optimised starting point formulation is presented which uses a pure acrylic dispersion, HEUR thickeners, mica and clay as the variable ingredients. * Corresponding author: Oliver Wagner BASF SE T +49 621 60-99854 oliver.a.wagner@basf.com Starting point formulation and some practical issues From this work a starting point formulation emerges (Table 1) based on the findings above. At a PVC somewhere between 40 % and 46 % with a TiO2 loading of 20 % (plus 3 % of an organic hollow sphere), a filler content of 16 % to 18 %, the "emission free" paint has the specialised stain resistant straight acrylic binder as a backbone. The dispersing agent is hydrophobic in nature, the rheology package HEUR/HEUR and the filler combination is mica with clay.
Seite/Page: 4 Figure 1: All pictures left section, standard interior paint; right section, stainresistant paint
Seite/Page: 5 Figure 2: Stain removal properties of three commercial paints: top, superhydrophobic silicone effect paint; middle, semi-gloss paint; bottom, stainresistant paint
Seite/Page: 6 Figure 3: Comparison of premium paint systems where?e corresponds to stain removal rating (the lower the better)
Seite/Page: 7 Figure 4: Comparison of polymer dispersions with?e denoting stain removal performance (letters beneath numbering indicate monomer composition)
Seite/Page: 8 Figure 5: Comparison of thickener systems with?e denoting stain removal performance (letters beneath numbering indicate chemical type)
Seite/Page: 9 Figure 6: Comparison of extenders, with?e denoting stain removal performance (letters indicate chemical nature of the extender)
Seite/Page: 10 Figure 7: Comparison of the same extenders as in Figure 6, 85 gloss and dry burnish
Seite/Page: 11 Figure from Raising the barriers to dirt