Thixatrol PM 8058: Alcohol-Based Solvents Are Not the Answer Either

Marine coatings and protective coatings generally have very high requirements in terms of sag stability. Typical application in these systems involves spray application in a single coat at layer thickness of 1000 μm or higher. Although these high demands from systems limit the selection of suitable rheology modifiers, organic thixotropes possess the quality to meet this special need. However, these technologies, particularly older types, have complex requirements in terms of mixing and activation. To achieve optimum performance, depending on the system, a composition of various solvents and binder manifolds is required over a certain period at high temperature range and high shear rate. [caption id="attachment_133441" align="aligncenter"] Figure 1[/caption] As a process, the batch-by-batch technique is used here. Since external heating/cooling equipment is generally not available in production facilities, active temperature control during the process is not possible, or is only available at laboratory scale. The energy required to achieve and adjust activation temperature is normally generated by friction during dispersion in such cases. The final temperature value, which has a strong effect throughout the entire process, depends on parameters such as disperser speed and pigment/extender loading level. For this reason, rheology agents that can be used over wider temperature ranges have significant advantages.

Comparison with Other Thickener Classes

Organic rheological additives provide quite good low shear viscosity together with a strong thixotropic flow character in non-water-based systems. As shown in Figure 2, when organic thixotropes are compared with fumed silica, the low shear viscosity of thixotropes is much higher. According to this practical relationship, organic thixotropes potentially provide the highest film formation properties. On the other hand, fumed silica provides higher viscosities at high shear rates. This effect generally adversely affects spraying during application, particularly atomization. Organoklays, less preferred in this market segment, are more useful in terms of spread performance along with balanced anti-sag performance. [caption id="attachment_133442" align="aligncenter"] Figure 2[/caption] Diamide-based rheology modifiers need to be exposed to mechanical forces applied over a period with a specific temperature for activation. The activation temperature mentioned must be appropriate to the relevant solubility power.

Product Chemistry

As explained in Figure 3, due to the nature of amide functionality, after activation the rheological structure consists of fibril structures of molecules oriented toward each other. The driving forces of the process are intermolecular interactions such as hydrogen bonds and Van-der-Waals forces. Castor oil waxes, the oldest class of organic thixotropes, require a similar process, but the system is much more sensitive to dissolution forces and changes in process temperature. Furthermore, they do not cover all solvent ranges. Additionally, they have much higher risk side effects such as 'seeding' and 'false body'. The latest diamide rheology modifiers, Thixatrol PM and Thixatrol AS series, allow the formulator a robust, predictable process and appropriate activation with very wide application temperature ranges, in real-life conditions where fluctuations such as winter-summer temperature changes frequently occur. Furthermore, these products can be applied at considerably lower temperatures compared to conventional products with similar chemistry. This property is particularly important because activation temperatures in solvent-free systems are normally high. The greatest advantage of such low temperature activation is energy and cost savings in the production process. The process can be carried out without external heating and therefore faster. Use of Renewable Resources In terms of sustainable sources, such rheology modifiers are also quite attractive. The new rheology modifiers in the Thixatrol series are produced from renewable sources (>75 percent bio-based content). However, despite the major advances defined by recent developments, not all formulations can be covered. Particularly when high polarity systems formulated with various alcohols such as benzyl alcohol and butanol are compared with low polarity systems, generally a decrease in efficiency is observed. Recently, alcohols have become increasingly preferred and are frequently formulated at high concentrations to reduce the amount of aromatic solvents. Additionally, further improvements are needed to stabilize the system and increase coating adhesion to the substrate layer. The new Thixatrol PM 8058 was developed to provide clear solutions for these types of formulations using higher amounts of high polar solvents. Additionally, Thixatrol PM 8058 is significantly more effective than other rheology modifiers in the Thixatrol PM and Thixatrol AS series. These improvements with Thixatrol PM 8058 are clearly demonstrated in the following model study, where a high solids epoxy-based primer with 83 percent solid content was formulated with varying solvent ratios compared to a commercially available reference organic thixotrope. In all cases, the concentration level was selected at 0.5 percent. This model was prepared to emphasize the effect of the organic thixotrope at lower viscosities and to show the effect of various solvent compositions on viscosity structure. As shown in Figure 4, xylene dominates the solvent portion in the standard formulation. In both other versions, isobutanol or benzyl alcohol is used at the highest concentration. The activation process was performed with a gear-tooth blade mixer at 16 m/s speed. The activation temperature was set at 66°C for 25 minutes. Comparison of rheological properties was conducted 24 hours after production. The data shown in Figure 5 demonstrate the performance of both organic thixotropes in systems with various solvent compositions and equipped with special low shear viscosity. The small graph in the upper right corner of Figure 5 shows viscosity changes of alcohol-rich samples compared to xylene-rich samples. In both alcohol-rich systems, the reference thickener experienced significant viscosity loss at low shear rate. Different results can be seen when Thixatrol PM 8058 is used in the same system range. In contrast to the reference product on the market, viscosity increase was observed. This is a condition particularly seen in formulations dominated by benzyl alcohol. In cases dominated by isobutanol, the difference was observed to be slightly lower at approximately 10 percent positive increase. In a recovery test of a structure, the viscoelastic properties of the tested coating also underline the benefits of Thixatrol PM 8058. To simulate post-application behavior, the coating structure was disrupted at high shear rate of 1000 s-1 in the rotational measurement step. Secondly, in the oscillation step, the behavior after the initially applied shear stops, analyzing the coating properties after affecting the substrate, is shown in Figure 6. If the damping factor, also called tan delta value, is above 1, it shows that the sample's fluid behavior is dominant and has no structure whatsoever in practice, the system flows. If the damping factor moves below 1, the sample's elastic behavior is dominant and this indicates the presence of an internal structure. Strong structure exhibits excellent sag resistance and this should be connected with results produced in application. However, time behavior also plays a quite important role in this test. The faster the structure recovers, the earlier it intersects tan delta value of 1 coming from a higher value, and the shorter the time interval for final sag. The dominance of flow character of all samples can be seen in tan delta values above 1 immediately after the shear force is removed. The strong effect of the reference thixotrope's different solvent compositions on viscoelasticity can be observed. Immediately after the shear process is stopped, both alcohols strongly shift their properties toward more fluid-dominant behavior. In relation to the time scale, a re-dominance of elasticity could not be determined. These effects are strongest in the mixture with high benzyl alcohol concentration. The effect of various alcohols on the sample with Thixatrol PM 8058 is different. The absolute damping factor of the system with high xylene shows very little difference with the reference sample's absolute damping factors. The fundamental difference is the time point at which the tan delta value crosses the 1 line. This structure recovery time is somewhat shorter in systems dominated by both alcohols. Consequently, based on this data, coating applications made with Thixatrol PM 8058 are expected to potentially have higher layer thicknesses compared to the reference rheology modifier. Systems with high xylene content were selected to define the optimum activation temperature range (Figure 7). In this study, samples were prepared either with Thixatrol PM 8058 or with a reference thickener activated at individual temperatures. As a result of tests conducted at different temperatures between 35°C and 75°C at shear rate of 0.1 s-1, it was observed that Thixatrol PM 8058 provided significantly higher viscosity compared to the reference product. With Thixatrol PM 8058, a stable and predictable viscosity structure can be obtained in a temperature range between 45°C and 75°C. The effect of Thixatrol PM 8058's activation times at different temperatures on viscosity structure and sag control is shown in Figure 8. The results confirm the wide activation temperature range. At the lowest activation temperature of 35°C with a 30-minute dispersion period and low shear of 0.1 s-1, excellent viscosity formation can be achieved. The most stable values were obtained in a repeatable temperature range from 45°C to 75°C. In this range, a 30-minute activation period is sufficient for appropriate viscosities. Extending the activation period to 45 minutes provides only minor performance improvement. Thixatrol® PM 8058 provides excellent stability results even after 4-week storage at high temperature of 50°C. Figure 9 shows that only visible changes in viscosity or sag stability occur when activated at 35°C. From an activation temperature of 45°C onward, only minor changes in viscosity were observed and no change in sag control was seen. In Figure 10, the comparison of maximum applicable layer thicknesses achievable after application with airless spray clearly shows that Thixatrol PM 8058 performs significantly better compared to reference rheology modifiers. The market reference organic thixotrope permits a maximum layer thickness of 300 μm. Thixatrol® PM 8058 formulated under equal concentration and activation conditions guarantees sag control up to the range of 500-700 μm. Investigation of various process conditions has shown that Thixatrol PM 8058 activation can be successfully accomplished with various methods (Figure 11). Best results are obtained when Thixatrol PM 8058 is added to the grinding process from the beginning of production or in an easily formulated but un-thickened coating system. In the latter case, the shear applied to prevent material splashing from the kettle should not be equally as high as grinding activation. In this case, a dispersion speed of 12 m/s should be used. The relevant activation temperature must be controlled externally. In the case of direct addition, speeds of 16 m/s or higher can be used. Additionally, since friction is sufficiently high to generate the required energy, artificial temperature control is not necessary. Addition of organic thixotrope during grinding has been carried out more in factory-based production. Post-activation provides additional options such as time-efficient laboratory screening. Due to Thixatrol PM 8058's low shear requirements for activation, it is also possible to form pre-gels in specific solvents or solvent/resin mixtures in systems that can subsequently be used in coating processes.

Conclusion

The results and properties addressed demonstrate that Thixatrol PM 8058 perfectly expands the organic thixotrope series with vision. Thixatrol PM 8058 uses the latest technology of highly sustainable diamide waxes based on more than 75 percent renewable raw materials. Thixatrol PM 8058 can withstand higher amounts of various alcohols and other high polar solvents and clearly demonstrates better performance than other thickener classes. Compared to other amide-based rheology modifiers, significantly higher viscosities and sag stabilities can be achieved when compared under similar formulation conditions. Thixatrol PM 8058 provides a wide activation temperature range for robust production in real-life conditions. When processed at lower temperatures for limited dispersion periods, it behaves stably in storage. From 30 minutes onward, short activation periods provide a coating system with stable viscosity. Furthermore, the potential activation temperature starts from low levels. From 45°C onward, stable viscosity formation as well as excellent sag stability has been noted as important. Further increasing activation temperatures up to 75°C in an equal dispersion period resulted only in minor changes. Compared to commercially available reference organic thixotropes, Thixatrol PM 8058 demonstrates significantly better efficiency. This allows the formulator to significantly reduce loading level while maintaining viscosity and sag control. In conclusion, Thixatrol PM 8058 is a cost-effective and sustainable ideal rheology modifier that can be used in the formulation of industrial, marine and protective coatings as well as other non-aqueous systems. Acknowledgment We thank the entire Cologne laboratory team Anja Wingerath, Wolfgang Fuchs, Jörg Bungarten, Parwis Adli and Thibault Leseur. Literature Rheology Handbook, 2013, Elementis Specialties, Inc., East Windsor, NJ Mathy, G.C., J. Phys. Sci., 2007, 11, 156-171 Van Esch, J.H. Schoonbeek, F. de Loos, M. Koijman, H. Spek, A.L. Kellogg, R.M. Feringa, B.L. Chem. Eur. J., 1999, 5 (3), 937-950.