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Artur Palasz, Ph.D., Spektrochem, analyses the use of the filler barium sulphate in architectural coatings exposed to harsh weather conditions, such as acid rain
Barium sulphate in its natural form, baryte, has been a filler present in the paint industry for an extremely long time. Historically, it was one of the first fillers in wall paints, used as a ground powder, but also in a mixture with zinc sulphide and used as an extending pigment known as lithopone. Barium sulphate is a heavy filler with a density of 35.9lbs/US gal (4.3g/cm3) and its use in paints decreased dramatically when the coatings industry moved to volume sales of paints because it increased the density of paints much more than calcium carbonate.
Barium sulphate is chemically inert, and its practically zero solubility in water and no tendency to form salts means that it is used, but mainly in industrial paints, e.g. anti-corrosion, automotive or chemical-resistant coatings. Barium sulphate can be extracted and ground as natural baryte or obtained by precipitation as blanc fixe. Barium sulphate used in paints has basic requirements defined in the standard ISO 3262-2 for natural barium sulphate (baryte), ISO 3262-3 for blanc fixe and ASTM D602 for both, baryte and blanc fixe.
Exterior architectural coatings
The formulations of typical architectural paints usually contain a standard filler in the form of calcium carbonate, which is readily used due to its low price and lower density compared to barium sulphate (22.5 lbs/US gal, 2.7 g/cm3). Functional fillers are also used in architectural paints, such as quartz, nepheline syenite or talc, but their density also oscillates in a similar range to that of calcium carbonate, which does not affect changes in the density of the paints. The share of functional fillers is usually additional to calcium carbonate and depends on the desired properties of the coatings.
Architectural paint coatings, e.g. facade paints or roof coatings (elastomeric liquid acrylic roof membranes) are exposed to atmospheric factors that vary depending on the location in the world where they are used. In addition to weathering caused by solar radiation, moisture or changing temperatures, architectural paint coatings are also exposed to, for example, acid rain. In many locations around the world, air pollution causes rainfall to take on an etching form, causing acid etch, including on architectural coatings. Coatings of elastomeric roof membranes based on acrylic polymer dispersions are also applied on the roofs of production halls, from which exhaust gases and condensate containing corrosive substances are often emitted, especially when appropriate environmental requirements are not observed or when the installations for discharging such gasses fail.
As a consequence of acid etch caused by acid rain and gas fumes, architectural coatings undergo destruction in the form of leaching, erosion, blistering and/or discolouration, which leads to increased porosity and further increased dirt retention, reduced protective properties, etc. The main reason for the lack of resistance to such aggressive factors is the lack of chemical resistance of calcium carbonate as the basic filler in such coatings. As a result of the reaction of the coating containing calcium carbonate as a filler, a chemical reaction occurs, for example with sulphuric acid:
CaCO3 + H2SO4 → CaSO4 + CO2↑ + H2O
The binder is also responsible for the lack of resistance to acid etch, as well as the susceptibility to chemical reactions with the acidic environment of other ingredients present in the coating. However, from the point of view of reactivity, the carbonate filler seems to be most responsible for the potential lack of resistance to acidic factors causing coating destruction. Such application areas often show visible discoloration, erosion caused by etching, and also greater absorption of dirt, or finally blistering, softening of the coatings, etc.
Calcium carbonates GCC – ground calcium carbonate and PCC – precipitated calcium carbonate were used for the tests as control fillers. Barium sulphate test sample as natural barite was used Paint Barium Sulphate 200 – 5000 mesh from Majiang JGL Barite Mine, China (see specification in Table 1).
|Table 1. Baryte specification|
|BaSO4||92 – 98%|
|FeO3||≤ 1000 ppm|
|Al2O3||≤ 500 ppm|
|Fineness||200 – 5000 mesh|
|Specific gravity||4.2 – 4.3 g/cm3|
|pH in water||7 – 8|
|Soluble alkaline earth metals (as calcium)||≤ 250 ppm|
|Whiteness||80 – 93%|
For use in formulation and calculation of constants, the oil absorption of the tested barium sulphate was determined. The test according to ASTM D281 by spatula rub-out showed the result of 18 lbs/100 lbs and this amount was taken into account in the calculations in the formulations.
Two base formulations were prepared for the case studies: an acrylic facade paint for application on external mineral surfaces, e.g. concrete, and the second, elastomeric liquid acrylic roof membrane for application on new and renovated roof surfaces in industrial facilities.
Exterior facade paint
The facade paint was prepared by combining slurries prepared as mill-base with an acrylic binder and other ingredients in the let-down process. Mill-base slurries were prepared according to the formulations shown in Table 2.
|Table 2. Mill-base formulations|
|Raw material||GCC slurry||PCC slurry||Baryte slurry|
|Water||45.9 lbs||49.5 lbs||54.0 lbs|
|In-can preservative||0.1 lbs||0.7 lbs||0.2 lbs|
|Defoamer||0.4 lbs||0.7 lbs||0.5 lbs|
|Dispersing agent A||2.8 lbs||—||—|
|Dispersing agent B||—||2.4 lbs||—|
|Dispersing agent C||—||—||3.3 lbs|
|Ground calcium carbonate filler||92.2 lbs||—||—|
|Precipitated calcium carbonate filler||—||80.1 lbs||—|
|Maijang Paint Barium Sulfate||—||—||108.6 lbs|
|Hydroxyethylcellulose thickener||0.4 lbs||—||0.5 lbs|
|Neutralizing agent||—||0.3 lbs||—|
|HEUR thickener / grinding aid||—||0.3 lbs||—|
|Grind for 20 minutes by cowles dissolver|
|Total||141.9 lbs||133.5 lbs||167.0 lbs|
|Density||14.19 lbs/US gal||13.35 lbs/US gal||16.70 lbs/US gal|
|Solid content:||66 wt%||61 wt%||66 wt%|
The facade paint was prepared as an ultra-deep base for dark (deep) colours. Tinting was performed during let-down to obtain a dark coating colour to better observe changes in coating discoloration during testing. The control paint was prepared using GCC and PCC, and the test paint was prepared with barium sulphate (baryte). The substitution of calcium carbonates by barite was performed 1:1 by weight in the formulation, as shown in Table 3.
|Table 3. Let-down formulations for acrylic exterior paints|
|Raw material||Control paint||Barium sulfate paint|
|Acrylic polymer latex||180.9 lbs||180.9 lbs|
|Mill-base – GCC slurry||159.2 lbs||—|
|Mill-base – PCC slurry||86.8 lbs||—|
|Mill-base – Baryte slurry||—||246.0 lbs|
|Film preservative||0.9 lbs||0.9 lbs|
|Coalescing agent||12.7 lbs||12.7 lbs|
|Pigment concentrate (blue PB 15:3)||21.7 lbs||21.7 lbs|
|Flash rust inhibitor||0.9 lbs||0.9 lbs|
|Attapulgite 20% suspension in water||34.3 lbs||34.3 lbs|
|HEUR thickener||2.6 lbs||2.6 lbs|
|Total:||500.0 lbs||500.0 lbs|
|Pigment Volume Concentration (PVC):||45%||35%|
|Solid content:||51 wt%||51 wt%|
|Density:||10.7 lbs/US gal||11.4 lbs/US gal|
After formulating, it can be seen that the use of the same amount of barium sulphate by weight (calculated for the entire recipe resulting from the filler concentration in the mill-base) resulted in a decrease in PVC from 45% (control sample) to 35% (test sample). This is due to the higher density of the filler, therefore the CPVC and Q-value (PVC to CPVC ratio) also change automatically. The density of the control paint was 10.7 lbs/US gal (1.28 g/cm3), and after changing the filler to barium sulphate, the density increased to 11.4 lbs/US gal (1.37 g/cm3), i.e. from 7%. With relatively low PVC used in the facade paint, the increase in density is visible, but it is not a significant increase.
Elastomeric liquid acrylic roof membrane
The elastomeric liquid roof coating formulation is shown in Table 4. The formulation also uses a 1:1 weight substitution of calcium carbonate with barium sulfate. The remaining ingredients did not require adjustment.
|Table 4. Elastomeric liquid roof membrane formulations|
|Raw material||Control liquid roof membrane||Barium sulfate liquid
|Water||67.5 lbs||67.5 lbs|
|Hydroxyethylcellulose||1.5 lbs||1.5 lbs|
|Propylene glycol||9.0 lbs||9.0 lbs|
|Dispersing agent||2.0 lbs||2.0 lbs|
|In-can preservative||1.5 lbs||1.5 lbs|
|Defoamer||1.0 lbs||1.0 lbs|
|Ground calcium carbonate filler||182.0 lbs||—|
|Maijang Paint Barium Sulfate||—||182.0 lbs|
|Titanium dioxide pigment||40.0 lbs||40.0 lbs|
|Acrylic polymer latex (Tg –35 °C)||177.5 lbs||177.5 lbs|
|Defoamer||0.5 lbs||0.5 lbs|
|Coalescing agent||2.5 lbs||2.5 lbs|
|Film preservative||5.0 lbs||5.0 lbs|
|Water||10.0 lbs||10.0 lbs|
|Total:||500.0 lbs||500.0 lbs|
|Pigment Volume Concentration (PVC):||44%||34%|
|Solid content:||67 wt%||67 wt%|
|Density:||12.1 lbs/US gal||13.0 lbs/US gal|
In the case of elastomeric roof coating, a decrease in PVC can also be observed (control sample 44%, barium sulphate 34%) and a very significant decrease in Q, which is also caused by the use of a high-density filler. The density of liquid membranes increased from 12.1 lbs/US gal (1.45 g/cm3) to 13.0 lbs/US gal (1.56 g/cm3), i.e. by 7.6%. This is not a drastic increase, however, during R&D work it could be considered to further modify the formulation (e.g. PVC regulation) to reduce the density of the liquid product and thus the density of the coating, which in the case of roof coatings is important to avoid loading the roof with a coating heavier than the coating before modification.
In the experimental part, the prepared samples were tested to demonstrate the differences between the use of calcium carbonate as a standard filler and barium sulphate in paints intended for application in environments exposed to various acidic pollutants.
The test was performed on elastomeric roof membrane coatings obtained on Leneta release paper, after 7 days of conditioning at 23 °C ± 2 °C and relative humidity 50% ± 5%. After separation from the substrate, the coatings were cut out and immersed in 50% sulfuric acid to observe the reaction as previously described. The test result is shown in Figure 1.
Figure 1. Behaviour of elastomeric roof coatings in 50% sulphuric acid
After immersing the coatings in sulphuric acid, the reaction begins in the beaker with the coating where a control sample containing calcium carbonate as a filler was placed. Carbon dioxide begins to escape from the coating, which carries it to the surface of the acid solution. In the test sample with barium sulphate, the reaction does not occur. After one hour of exposure to 50% sulphuric acid, the control coating releases bubbles, there is still no reaction in the beaker with the test sample, and the coating remains at the bottom.
In this case, the exposure was performed using quite an acid and at quite a high concentration, but the use of such a solution was dictated by the desire to obtain the most extreme results for comparison.
The fact of the reaction in the control sample with the release of carbon dioxide and the decomposition of calcium carbonate into calcium sulfate is one thing, but what effect did it have on the properties of the coatings? For this purpose, dirt pick-up resistance (DPUR) was performed in accordance with UNI 10792 for coatings before exposure to acid solution and after 1 hour of immersion in acid. The results are presented in Table 5. Figure 2 shows a photo of the coatings after DPUR tests for the control and test samples.
|Table 5. Changes in DPUR coatings of elastomeric roof membranes|
|Parameter||Control sample||Barium sulfate sample|
|Before acid immersion||After 1h immersion
in 50% H2SO4
|Before acid immersion||After 1h immersion
in 50% H2SO4
|UNI 10792 rating||Very low||High||Very low||Very low|
Figure 2. Results of the DPUR test of coatings after immersion in sulphuric acid for 1 hour control coating on the left, barium sulphate coating on the right.
The results of the DPUR determinations clearly show how significantly the porosity of the coating and the tendency to accumulate dirt increased. The chemical reaction of the decomposition of calcium carbonate resulted in an increase in dirt retention, which in the case of roof coatings means a decrease in the ability to reflect solar radiation (reducing the cool-roof effect), the accumulation of pollutants causing the accumulation of microbiological pollutants, and, above all, a violation of the tightness of the coating, which results in weathering convection and further exposure. acidic factors may damage the coating.
Elastomeric roof coating based on barium sulphate as a filler and the prepared formulation withstood the extremely aggressive action of sulphuric acid at a concentration of 50%, which shows that changing the filler to barium sulphate is a valid concept when designing formulations aimed at application in areas exposed to acidic solutions.
The coating leaching and colour change simulation test was performed according to ASTM D7190, but instead of demineralised water, an artificially prepared acid rain solution from ASTM D7356 (Jacksonville Acid Rain) was used. This solution is used in the ASTM D7356 acid etch test method in a weathering chamber, however here it was used for the ASTM D7190 standard test method which is used to evaluate the leaching of surfactants from facade paint coatings. The acid rain solution was prepared in the laboratory according to the guidelines of pH 3.30 (Figure 3).
Figure 3. Artificial acid rain solution for ASTM D7356
Figure 4. Results of acid leaching according to the ASTM D7190 method and artificial acid rain according to ASTM D7356
Variations in resistance to acid leaching are noticeable on facade paint coatings (Figure 4). The control paint coating (bottom) shows significant discoloration after 10 minutes of exposure to artificial acid rain drops. The top coat of facade paint with barium sulphate shows barely noticeable changes in discolouration (only noticeable from the right angle). The difference in the shade of blue of the control and barium sulphate coatings results from the different tintability after changing the filler.
Leaching results after 10 minutes showed that the effect of artificial acid rain was also visible. It was also decided to carry out a test in which the tested facade paint coatings were exposed to acid rain flowing over the coating for a longer period of time. For this purpose, the ISO 2812-4 method B (inclined panel) was used, in which the coatings were subjected to spot exposure using the same artificial acid rain as above, but with 1-2 drops per second flowing for 30 minutes (Figure 5).
Figure 5. Coatings during the ISO 2812-4 method B spot test with artificial acid rain
The results are shown in Figure 6 and Table 6 (colour change calculation).
Figure 6. Facade paint coatings after exposure to spotting inclined panel ISO 2812-4 method B
|Table 6. Changes in color after exposure to acid rain 30 min ISO 2812-4 method B (spotting inclined test)|
|Parameter||Control sample||Barium sulfate sample|
|Acid rain spotting test
As shown in Figure 6, the difference is very noticeable. The stain left on the control paint is much more visible than the stain left on the barium sulphate paint. It should be mentioned, however, that the most likely result obtained for the barium sulphate paint depends on the polymer dispersion used for the prepared formulation, which should be taken into account in the case of further formulation work in order to select a binder with greater acid rain resistance properties. However, the results obtained with barium sulfate are much better and promise to provide good acid rain resistance properties.
Long contact with acid solutions
To make the test more stringent and to observe the results in even more extreme exposure conditions, it was decided to carry out a spot exposure test of 10% and 50% sulfuric acid solution in accordance with ISO 2812-4 method A – spotting method (Figure 7).
Figure 7. Test of facade paint coatings against 10% and 50% sulfuric acid ISO 2812-4 method A
The test was carried out during 1 hour of exposure to sulfuric acid of the coatings, covered with a Petri dish. After this time, the coatings were rinsed under running water and assessed for any changes to the coating.
Figure 8. Coatings after spot action with acid at a concentration of 10% and 50%
The changes shown in Figure 8 clearly demonstrate the positive effect of the barium sulphate filler in making the coating resistant to two concentrations of sulphuric acid. The control coating with calcium carbonate after 1 hour of acid exposure shows very significant changes, both for the concentration of 10% (greater discolouration) and 50%. The coating with the tested barium sulphate shows virtually no colour change when exposed to 10% acid and slight blistering of the coating when exposed to 50% acid. The presence of blistering at such a high acid concentration is the result of the lack of resistance of the polymer dispersion used in the formulation, but the positive results are still extremely surprising, as it should be remembered that we are dealing here with architectural coatings, not chemically resistant protective coatings.
After carrying out case studies involving the replacement of calcium carbonate with barium sulphate in the form of natural baryte, it was shown that this way it was possible to obtain surprisingly good resistance properties of architectural coatings to acidic solutions, the presence of which in heavily polluted environments causes the destruction of coatings. The barium sulphate used in the tests showed very good prospects for further work on the formulations of both facade paints and elastomeric roof coatings, where, together with appropriately selected acrylic binders with surfactants ensuring greater chemical resistance, it can be an extremely important component of architectural paints for heavy duty applications, where usually was not taken into account due to its higher density. However, ensuring much higher performance of coatings that can be used in polluted environments compensates for the significantly higher density, which seems to be negligible when ensuring such good protective properties.
Author: Artur Palasz, Ph.D., SPEKTROCHEM – Technical Center of Raw Materials for Architectural Paints, Poland
E-mail: [email protected]