Dispersion paints on the market vary in terms of quality and performance, which are defined at the very beginning by PVC (Pigment Volume Concentration). It is an indicator not only of the relationship between fillers and the film-forming substance in the coating, but also an indicator of the necessary thickeners to be used to ensure appropriate rheological properties in a system containing a specific amount of binder.
Introduction
The category of medium-high PVC latex paints is divided into two subcategories: PVC 60 to 70% and PVC 70 to 80%. Depending on the binders and functional fillers used, this category produces latex paints classified according to EN 13300 in class 2 wet-scrub resistance and sometimes in class 3 with PVC closer to 80%. Paints of this type are usually standard quality flat interior paints, but depending on the formulation and PVC closer to 60%, they can also be medium quality interior paints.
This type of PVC and the resulting amount of binder in the coating, and more precisely in the liquid paint, require the use of thickeners that build viscosity not only associatively with the binder, but also build viscosity as a gel with other ingredients. In such ranges, medium-high PVC is used to build low-shear forces viscosity and cellulose ethers are used as the basic thickener. Their effectiveness is built by giving the gel structure regardless of the strength of the associative effect with the binder or other fillers. The associative effect can be enhanced by using appropriate types of hydrophobized cellulose ethers, e.g. hydrophobically modified HEC. Cellulose thickeners in medium-high PVC paints can be used as a single thickener, however, it is usually advisable to use an additional one, e.g. HEUR, to increase efficiency in the high-shear forces area, especially in high-quality European formulations. In medium-high PVC and high PVC paints, cellulose thickeners are usually even used as a single rheological modifier (Figure 1).
Case studies
To compare different cellulose ethers, a latex paint formulation PVC 72% was prepared based on a polymer dispersion from the EU market with MFFT approx. 5 °C (styrene-acrylic copolymer, APEO-free, ammonia-free, anionic). For the case studies, 5 cellulose ethers were used, with which paints were prepared in such a way that the dosage level was set at an initial viscosity of approximately 100-110 KU measured with a Stormer viscometer. White latex paint (L-bases) prepared in this way were subjected to tests as follows.
Viscosity
The first step was to measure the viscosity using a Brookfield viscometer. As can be seen in Figure 2, the strongest effect of increasing viscosity due to the strong associative interaction of hydrophobic groups with the binder was demonstrated by the hmHEC thickener (hydrophobically modified HEC). The remaining thickeners showed lower viscosities, with HEC 6,000 thickener showing the lowest viscosity increase. It should be recalled that each of the paints was adjusted with a different dose of each thickener to obtain a similar starting KU-viscosity.
Next, viscosity measurements were made at a high shear rate of 12,000 s-1 using a CAP-viscometer (cone & plate). The results are shown in Figure 3 and show that the samples show some viscosity in the high shear region, but the viscosity level is similar between thickeners. The exceptions with a slightly lower ICI-viscosity are the HEC 30,000 and MEHEC 6,000 thickeners. Here we should stop for a moment about these samples and mention a very dangerous conclusion that is usually drawn when comparing such results. It is often believed that due to low viscosities in the area of high shear rates, samples will have a very high tendency to splash during painting (spattering).
Therefore, in the further part of the secondary rheological properties tests, these myths were verified on the basis of appropriate tests in accordance with ASTM standards. The obtained ICI-viscosity results show that all samples will behave similarly in terms of resistance when applied with brush when subjected to high shear (brush drag).
Spattering resistance
As you can see, the spattering resistance results presented in Figure 4 vary greatly. The ASTM test scale grades results from 0 (most paint drops on catch paper) to 10 (no spattering). The results show that hmHEC 6,000 is the best in terms of spattering resistance and this confirms the idea of using this type of thickeners, where the increased association between the binder and paint particles at all is intended to minimize spattering. The remaining thickeners achieved worse results, with the worst being the MEHEC 6,000 formulation.
And here we should return to the ICI-viscosity results, which would indicate that there will not be such differences between spattering resistance as is customary in the paint industry to interpret viscosity results. These results show something completely different and show that you cannot interpret viscosity results and translate them into properties such as spattering (and as will be seen later, other secondary rheological properties). And it does not matter whether we interpret the results from the CAP viscometer that was used to measure it or from an advanced rheometer, because neither of these devices measures the properties of the paint responsible for spattering, but only the viscosity relationship, which is not entirely responsible for spattering resistance.
The spattering resistance test is performed in accordance with ASTM using a specially standardized notched spool roller to assess only the spattering properties of the paint and not the effect of the type of paint roller.
Leveling
The leveling test is extremely important from the point of view of obtaining a well-flowing coating, especially when painting with a brush, where it is undesirable to leave brush marks on the coating after drying. The test is carried out by applying paint appropriately prepared using a leveling test blade under high shear to obtain parallel ridges and valleys, and then, after drying, the obtained brush mark simulations are compared with the leveling reference standards. The results are shown in Figure 5.
Another “anomaly” may be noted in relation to the belief that high Brookfield viscosity equals low leveling. And here the hmHEC result of 6,000 shows that the flow result can be achieved at a decent level for this type of latex paint. This is another proof that other rheological properties of the paint cannot be interpreted based only on viscosity measurements. The resulting leveling 8 result may be acceptable for medium and low quality latex paints or can be improved by using appropriate additional rheological modifiers in a paint aspiring to higher quality. The next result obtained was a rating of 6 for the MEHEC 6,000 thickener, however, due to the previously obtained spattering and now leveling results, it is clear that it is necessary to use this thickener in the formulation with an additional rheological modifier improving both properties. The remaining thickeners achieved very poor leveling results.
Sagging resistance and tintability
Sagging resistance is a test that allows you to obtain a result that paint flows down from vertical surfaces when applied in a thick layer. The test is performed in accordance with ASTM under strictly defined test conditions using a drawdown using an anti-sag meter, which is a drawdown blade with a series of notches of successively higher clearance. Wet strips of paint after the drawdown are placed vertically to dry, and then the Anti-Sag Index is determined, which is an expression of the gap in the drawdown blade from which the wet layer did not flow, and added to it a mathematical calculation of how the next strip of paint merged with the following ones.
The test was performed in the range of 0-24 mils (0-600 µm) for L-base (white base for tinting) and after tinting with a pigment concentrate dosed into the paint at 5 US fl.oz/gal. The results are shown in Figure 6. White L-base showed the highest sagging resistance result for samples with cellulose ethers hmHEC 6,000 and HEMC 6,000 (no sagging at a gap of 24 mils / 600 µm). The remaining samples showed slightly lower sagging resistance values for the white base (before tinting). Upon tinting, a drastic drop in sagging resistance can be observed for hmHEC 6,000, which was the sample with the worst result in this case. Other drops for samples upon tinting can be seen for HEMC 6,000, but this drop is acceptable. Similarly, in the case of the MEHEC 6,000 sample, this decrease is visible, but not drastic. In the case of HEC 6,000 and HEC 30,000 samples, the pre-tinting and after-tinting bases showed no changes in sagging resistance. This type of comparison also provides valuable knowledge about the effectiveness of thickeners in tinting bases, necessary for further possible modification of viscosity stability upon tinting using additional thickeners.
Storage stability
Syneresis evaluation was performed after accelerated stability testing by storing the paints in containers at 125 °F (52 °C) for 14 days in accordance with ASTM. After removal and cooling to ambient temperature, the appearance of a water layer on the surface of the paints in the packaging was assessed and rated on a scale from 1 (thick water layer) to 5 (no syneresis). The results are shown in Figure 7.
The complete absence of syneresis was demonstrated by the MEHEC 6,000 sample. Subsequently, samples HEMC 6,000 and at the same level HEC 30,000 and hmHEC 6,000 showed a visible water layer after the storage stability test. The thickest water layer appeared on the HEC 30,000 sample. These results show that not every cellulose thickener necessarily causes syneresis to occur in high PVC, as is also often heard.
Summary
Case studies carried out in the medium-high PVC 72% formulation show that cellulose thickeners, depending on the substituent and their modifications, can introduce very different rheological properties and cannot be classified according to one key. Each thickener interacts with the binder, surfactants and even fillers in the paint and therefore it is necessary to test them through application studies in specific formulations. Such comparisons prepared in our laboratory for thickener manufacturers help in making accurate recommendations for formulators – producers of latex paints and facilitate their research and development work on specific formulations.
The results presented as part of the completed project are also helpful in debunking certain myths that circulate in the paint industry, often misleading and causing the abandonment of further tests when the obtained viscosity is too low. Please remember that all results should be verified in your own formulations, but based on good case studies such as those presented in this article, you can save a lot of time, work and stress on your samples. Good luck in choosing thickeners, and if you want such recommendations for your thickeners, please contact me and my laboratory.
Disclaimer
The presented results are part of a project in which specific types of thickeners were used at a specific dosage level and a specific formulation was prepared. The translation of the results to other formulations and other thickeners, even with a similar or the same chemical base, may differ from the results obtained. If you have any doubts or would like to consult your case or want to know the details of the project, please contact me directly.
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