Solvay: New Dispersant Polymers for the Design of Complex Suspension and Flowable Concentrate Formulations

Developing pesticide formulations with a safer and more environmentally friendly profile is one of the key challenges of the agrochemical industry. In this context, Suspension Concentrates (SC) and Flowable Concentrates for Seed treatment (FS) agrochemical formulations based on aqueous dispersions constitute a major asset. SC formulations ensure a high loading in active ingredients while reducing the exposition to chemical risks compared to solvent-based formulations such as Emulsifiable Concentrates (EC). For optimal efficacy in alignment with market needs, such formulations are more and more complex with high loading of active ingredients and/or combo systems combining ingredients with several modes of action, having different physico-chemical characteristics:  for example, one active can be in the form of a soluble salt and the other one, dispersed in an aqueous phase. Such elaborate systems present high challenges regarding long-term stability and performances at dilution. In addition, strong stability issues are observed with organic actives that are prone to crystal growth via Ostwald ripening mechanism. In this respect, designing modern formulations which often combine two, three or more actives with various physico-chemical characteristics can become a real challenge, with little formulation space for the formulator.

On this basis, the selection of the right wetting/dispersant system is critical to ensure long-term stability of SC/FS formulations, without sedimentation and/or flocculation, as well as adequate dispersion characteristics of the active(s) upon dilution for SC in order to avoid spray nozzle blocking and reduced biological efficacy. Traditional dispersant systems, based for example on the association of acrylic comb polymer paired with a non-ionic ethylene oxide (EO)/propylene oxide (PO) block copolymer are sometimes not effective enough solution especially for organic actives prone to crystal growth or combo system with high electrolyte concentration. Thus there is the need for robust, improved dispersant polymers with enhanced crystal-growth inhibition properties which are highly effective at reduced dosage, and in parallel dispersant polymers that are tolerant to high ionic strength systems, for example in presence of soluble active salts.

Solvay possesses a toolbox of radical polymerisation technologies that affords polymers of controlled architecture, composition and molecular weight. This toolbox including the proprietary controlled radical polymerisation technology MADIX (Macromolecular Design by the Interchange of Xanthates) was employed to synthesize a wide variety of polymeric dispersant candidates. A large screening of polymeric dispersants synthesized with MADIX technology was conducted for coating application to identify promising dispersants for pigment dispersions.

A high throughput robotic methodology was used to prepare more than 2000 of organic & inorganic aqueous based pigment dispersions, through pigment powder distribution, polymer introduction and bead milling step. “Demand curves” with the monitoring of formulation capillary viscosity versus polymer dosage, were derived at different polymer and pigment loadings. Given the structural similarities of organic pigments with some agrochemical active ingredients, the most promising polymeric dispersants among the 80 screened polymers were then tested in challenging SCs formulations. Two promising structures, Geropon HL4 and Geropon ION5, were identified in comparison with Geropon DA1349, a robust and versatile dispersant but with some limitations in presence of high electrolyte concentration or for active ingredients prone to crystal growth.

In this article, studies on three different active ingredients (Folpet, Metribuzin and Terbuthylazine) will be presented to illustrate the versatility of the new Geropon grades. For Folpet formulation, the main challenge is the acidic conditions at storage, which could impact the dispersant performances. For the two polymers, a dose response versus an acrylic comb polymer benchmark (Geropon DA1349) was performed to investigate polymer robustness towards dispersion. For Metribuzin, actual commercial benchmarks exhibit poor performances to control crystal growth, thus Geropon HL4 was tested in comparison with Geropon DA1349 to assess crystal-growth inhibition properties. Finally, for Terbuthylazine active ingredient a combo formulation with the addition of Glyphosate potassium salt after milling was extensively studied. Geropon ION5 was positioned in comparison to Geropon DA1349 and a commercial benchmark specifically promoted for combo formulation with high electrolyte concentration.

SC formulations were produced at the laboratory scale with a vertical bead miller. The compositions of the different SC formulations are detailed in Table 1, the rheological agents were added post milling. For Terbuthylazine SC, 10% w/w of RoundUp Flash Plus at 420 g/L, corresponding to 4.2% w/w of potassium glyphosate salt in the final formulation, were added post milling and before the addition of the rheological agent. For each system, the Brookfield viscosity, particle size distributions, optical microscopy observations, suspensibility and dispersion characteristics were assessed initially and after storage at 45°C (3 months) or 54°C (2 weeks). Particle size distributions were measured by laser diffraction and compared using the parameter D50 corresponding to the particle diameter such as the cumulative undersized volume fraction of particles is equal to 50 %. The suspensibility performances of the different dispersants were evaluated with the CIPAC method MT184. The dispersion test is an in-house test developed to quickly assess the performance of a dispersant system, it involves the preparation of 100 mL of diluted suspension by 10 inversions of the measuring cylinder. After 2 hours at rest in a thermostated bath at 30°C, the height of the sediment formed at the bottom of the graduated cylinder is assessed and expressed as a percentage, with an acceptance criteria to be below than 1%.

Table 1: Detailed compositions of the SC formulations studied.
* Rhodasurf 860/P; ** Rheozan; *** Rhodopol 23

Figure 1 presents the dispersion and suspensibility performances of Geropon HL4 and Geropon ION5 compared to the benchmark Geropon DA1349 in Folpet SC formulation for different polymer dose rates after 3 months of storage at 45°C. Superior dispersion performances are observed for the new dispersant grades compared to the commercial reference, especially at lower polymer dose rate. At a dose rate of 14 g/L of active polymer, a dispersion value equals to 1.3% is measured for the commercial reference, whereas this parameter is inferior to 1% for both Geropon HL4 and Geropon ION5. In addition, formulations with Geropon HL4 and Geropon ION5 displayed low viscosity after milling, as well as excellent storage stability characteristics, such as viscosity stability and no phase separation after 3 months at 45°C. 

Figure 1 a). Dispersion and b). suspensibility performances of Geropon HL4, Geropon ION5 and the commercial benchmark Geropon DA1349 in Folpet SC formulation after 3 months of storage at 45°C.

In Figure 2 are presented the performances of Geropon HL4 compared to Geropon DA1349 in Metribuzin SC formulation in terms of crystal growth inhibition and suspensibility performances initially and after 2 weeks of storage at 54°C. With Geropon DA1349, a large increase of D50 parameters was observed after ageing (+187% of relative increase), whereas the dispersant Geropon HL4 permits to control the crystal growth with a limited relative increase of D50 parameter. This effect is also clearly visible in the microscopy pictures presented in Figure 2. A significant decrease of suspensibility is also observed with Geropon DA1349 after storage, whereas Geropon HL4 maintains excellent suspensibility performance.

Figure 2. Comparison of the performances of Geropon HL4 and the commercial benchmark Geropon DA1349 in Metribuzin SC formulation:  a). microscopy images of the SC formulations, b). particle size distribution initially and after 2 weeks at 54°C, c). suspensibility performances initially and after 2 weeks at 54°C.

Figure 3 summarizes the suspensibility performances in the combo formulation Terbuthylazine SC / Round Up Flash Plus of Geropon ION5 in comparison to Geropon DA1349 and a commercial graft copolymer especially promoted for application in high ionic strength system (Benchmark 1). Initial suspensibility and values measured after two weeks of storage at 54°C are reported. The two commercial benchmarks had poor performances with suspensibility values inferior to 70%. In comparison, excellent suspending performances are observed with Geropon ION5, highlighting the interest of Geropon ION5 for such systems containing soluble active salts like glyphosate potassium. In addition, the characteristics and storage stability performance of the formulation with Geropon ION5 were found to be excellent.

Figure 3. Suspensibility performances of Geropon ION5, Geropon DA1349 and a commercial benchmark in a combo formulation Terbuthylazine SC / Round Up Flash Plus initially and after 2 weeks of storage at 54°C.

Geropon HL4 and Geropon ION5 were confirmed as very efficient dispersants, interestingly, high loadings of actives at reduced dispersant polymer dosage can be achieved for a large range of phytosanitary actives tested. Geropon HL4 prevented crystal growth of challenging actives such as metribuzin, enabling a sharp control on particle size distribution. In addition, in comparison with the benchmark, a better tolerance in the presence of water-soluble electrolytes such as salts of glyphosate was obtained for Geropon ION5, enabling the formulation of complex combos of high ionic strength. The two polymers were also successfully tested in various SC systems (with metazachlor, chlorothalonil...) and FS formulations (with imidacloprid and thiamethoxam) confirming their superior dispersion performance and high versatility. In addition, the two dispersant grades present high solid content compared to benchmark, ranging from 40 to 45%wt, and represent therefore perfect solutions for systems with very limited formulation space, with the need to introduce low levels of dispersants.

This article will be published in AgroPages '2021 Formulation & Adjuvant Technology' magazine to be published this May.

Welcome you to join this magazine to promote your products and solutions. Please contact Grace Yuan via: grace@agropages.com