The unique chemical structures of the alkanolamine additives presented in Figure 1 can be used the illustrate the wide spectrum of chemical equilibrium or boosting effects these chemistries can provide in modern waterborne coating formulations (Figure 2).
The presence of both amine and alcohol functionality allow for the respective ionic and hydrogen bonding,1 while the compact molecular structure leads to fast diffusion in liquid media. Additionally, the combination of the high-purity and amine-bearing quaternary carbon, which provides color stability, makes these an excellent choice as dispersing agents (Figure 2, equilibrium 1 – 4). Beyond this well-documented feature, it has not been widely understood that these alkanolamines can also be used as remediation for volatile organic compounds (VOCs)2 due to chemical scavenging reactions (i.e., with aldehydes, Figure 2, equilibrium 5), as well as having a boosting effect with isothiazolinone (i.e., benzisothiazolinone) to help extend the shelf life of waterborne formulations (Figure 2, synergistic effect 6).
This investigation highlights the latest understanding around these alkanolamines, and the growing interest in their use in formulation for color stability and pigment dispersion. The first part discloses the delta E difference of architectural paints stabilized by alkanolamines (see Table 1 for physico-chemical properties) versus a selection of common ethanolamines, including monoethanolamine (MEA) and triethanolamine (TEA). The second section is focused on sharing how the adsorption and the pigment dispersion effect differs from fillers to pigments.
The durability of dry-film color is linked to the chemical stability of all selected ingredients. The unique composition of these molecules containing a quaternary carbon covalently bonded to the amine can support a longer pure white rendering.
A commercial satin waterborne paint was purchased in a do-it-yourself (DIY) retail shop in France, with the Ecolabel, with “A+” level of the indoor air quality label, 5g/L of VOC. By using a design of experiment methodology (DoE), the impact of the presence of several stabilizers at 0.5% wt has been assessed (AEPD, AMP, DMAMP, MEA, and TEA). The paint was drawn down on a Leneta panel at 150 μm with an automatic applicator (Elcometer 4340) and dried in controlled climate chambers at 23 °C, 50% humidity for 24 hours. Dry films were exposed to indirect natural sunlight for 9 months in a laboratory located in the Paris area.
The evolution of the color was measured with a Konica Minolta CM-5 spectrophotometer. Results were statistically analyzed by JMB software in dE*, da*, and db* (Figure 3 and Figure 4).
Paint formulations containing AMP, AEPD and DMAMP performed significantly better compared to ethanolamine-based paints. Both da* and db* are affected by MEA and TEA solely after 9 months, showing the much lower color stability despite the mild condition of this test (indirect natural sunlight exposure).
Selective Adsorptions on Pigment
As shown in Table 2, the adsorption ratio of AMP at the surface of several pigments was measured. The protocol used in this investigation is based on the preparation of each individual pigment as a slurry (both CaCO3 and TiO2 with 60% wt, PB 15:3 at 20% wt) with a disperser at high shear (VMA Dispermat AE01 C1, 20 min at 1700 rpm), in the presence of several different concentrations of 95% AMP and 5% water. After this dispersing step, slurries were centrifuged (Sorvall ST8, minimum 2.5 h at 4500 rpm) until full settlement of dispersed pigments, and the unabsorbed amount of AMP, free in the filtrate, was determined by a straightforward titration with a strong acid (Mettler T5 equipped with an DGi 115-SC electrode). All raw materials were used as received.
As seen in this dataset, AMP adsorption depends on the nature of a pigment’s particle surface. With calcium carbonate filler, alkanolamines are not attracted by the surface even at a high dosage (1% wt of AMP per filler weight in the slurry). This is potentially the result of the slight alkaline nature of the surface (isoelectric point at 8 to 9.53). Conversly, titanium dioxide and copper phthalocyanine surfaces display a higher affinity for AMP, leading to quantitative adsorption at very low dosages, 0.12% wt and 0.35% wt respectively. By adding more AMP, a saturation point is attained and the increase in dosage does not lead to any additional significant adsorption. It can be stressed that the saturation point depends on the particle size (i.e., the particle area).
The saturation point with Kronos 2190 is around 0.12% wt (particle diameter of 300 nm); in comparison, the saturation point with PB15:3 is 0.65% wt.
The adsorption can also be linked to the acidic nature of the particle itself. As an illustration of this effect, Figure 5 shows the particle saturation by plotting adsorptions of amino alcohols that have been standardized by dividing the molar adsorption by TiO2 surface areas. Four different titanium dioxides were selected with increased amounts of SiO2 surface treatment (K2043 10.3%; K2160 3.8%; K2056 2.1%; K2190 0.1%). A higher SiO2 surface treatment providing higher acid value requires more alkanolamines at the surface. Despite a low SiO2 level, K2190 can interact significantly, which may be a result of the 0.4% ZrO2 surface treatment (this treatment is not present on other examples).
Consequence on the Selectivity of the Enhancement of Dispersing Agents
By considering the selective adsorption of alkanolamine on TiO2 and organic pigments, dispersant demand curves of corresponding combinations were obtained. Pigment dispersions were carried out in a VMA Dispermat AE01 C1, and viscosity measured with a Krebs viscosimeter. This conventional procedure consists in the determination of the minimum required concentration of dispersing agents leading to the optimum wetting point (71.0% wt for CaCO3, 69.0% wt for TiO2, 26.5% wt for PB 15:3). This point corresponds to the concentration where the viscosity no longer decreases. Durcal 5 (Figure 6), Kronos 2190 (Figure 7), and Hostaperm Blue B2G (Figure 8) were used and a regular polymeric dispersing agent Orotan 731 AER was then combined with either AMP or AEPD.
As expected, neither AMP or AEPD are suitable for calcium carbonate dispersion as all dispersant demand curves, with or without co-dispersant, are similar (Figure 6). The poor interaction, leading to low adsorption, correlates with this weak dispersing performance.
The synergistic effect between conventional dispersing agent and alkanolamines becomes highly relevant with titanium dioxide and organic pigment PB15:3. Even at the extremely low dosage of 0.05% wt, AMP can drastically improve the efficiency of a common anionic polyacrylic dispersing agent (about 70% reduction of the dispersant demand), and can disperse TiO2 without the support of another dispersing agent at a higher dosage (Figure 7) as reported earlier.4 In the case of PB15:3, the much higher surface area of the pigment requires higher concentration, but the synergistic effect is shown at 0.3% wt. The optimum boosting effect occurred around the adsorption saturation point of 0.7% wt (see Table 2 and Figure 8). This means that the use of AEPD enables the reduction in primary dispersing agents in water-based formulated products, minimizing the drawbacks of these hydrophilic anionic species.5,6
Alkanolamines, such as 2-amino-2-methyl-1-propanol (AMP) and 2-amino-2-ethyl-1,3-propanediol (AEPD), are commonly used as key stabilizing agents in a wide range of waterborne paint formulations. Due to the strong interaction between specific amino alcohols and both pigments and dispersing agents, the level of agglomerates and the overall particle size distribution of pigments can be improved in paint formulations. This chemical link seems to be stronger with acid containing particle surfaces. This selectivity leads to a much better boosting effect on challenging pigments, such as titanium dioxides or organic pigments, and minimizes the consumption by the adsorption mechanism on a conventional filler such as calcium carbonate. AMP and AEPD support the development of higher whiteness and colour strength and provide sustainable tints due to a high stability in the dry film in comparison to ethanolamines.
The authors would like to thank all raw material suppliers, in particular Kronos for providing both materials and valuable feedback along this investigation. In addition, thanks go to Farah Shaik Dawood and Marianne Riffault who performed laboratory works during their student internship at ANGUS Chemical Company.
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