Evonik: Novel Liquid Delivery System for Water-Sensitive Active Ingredients

Pesticides have been used in agriculture for centuries. The protection of nature and the environment is becoming increasingly important. Evonik sees this as one of its most important tasks in the agricultural sector. Therefore, a novel biodegradable carrier fluid based on polyether-technology has been developed at Evonik Operations GmbH (tested by OECD 301 F). The product is water soluble but free of water. The first intention was to use this as a biocompatible carrier fluid to overcome issues with microbial agricultural solutions, which was published in 2022.¹) BREAK-THRU^® BP 787 has the ability to coordinate water and to lower the water activity, which creates optimal conditions for living microbial actives (e.g. Trichoderma spp., Bacillus spp., Clonostachys rosea, and Beauveria bassiana) and thereby enhances their shelf life, even without cooling.

However, BREAK-THRU^® BP 787 can also be used as a carrier for water-sensitive active ingredients like Sulfonylureas that can otherwise only be used in solvents that are not environmentally friendly. BREAK-THRU^® BP 787 enables convenient handling as it is a cold stable liquid with good rheological behaviour. BREAK-THRU^® BP 787 reduces the complexity in the formulation since it is water soluble and does not require an additional emulsifier, making it easier to handle in foliar, soil or seed treatment applications.

Agricultural Formulation Technology

Since agrochemicals are often active at less than 100 grams per hectare, formulation technology is required to apply such small amounts uniformly to the crop. Formulation technology increases the biological efficacy of the active ingredient and allows easy handling and application of the active ingredient. The active ingredient is formulated in a suitable diluent such as water or other solvents with a variety of other additives like surfactants, rheology modifiers, or defoamers. During application, the formulation is further diluted in a spray tank. Adjuvants control the spray droplet spectrum, reduce drift and increase adhesion and retention of the spray droplets on the plants. Adjuvants can also increase the coverage of the actives on the plant and support the penetration into the plant.

The diluent can have a significant positive impact on the biological efficacy. It is known and understood that OD-formulations (Oil Dispersion) are necessary for water sensitive actives but also show a clearly improved biological efficacy compared to SC-formulations (Suspension Concentrate), especially regarding retention and coverage of spray droplets. The carrier oil acts as an adjuvant and OD-formulations are regarded as an adjuvanted formulation type. Common carrier oils vary from paraffinic to aromatic solvent types and vegetable or methylated seed oils. Carrier oils require an emulsifier system to ensure the tank mix stability and reduce negative interactions between formulation ingredients.  

Formulations with BREAK-THRU^® BP 787 can be customized to achieve desired properties by adding Evonik BREAK-THRU^® additives and silica products. Due to the water solubility of the BREAK-THRU^® BP 787, it is easy to prepare OD-formulations without the need for solubilizers or emulsifiers. The typical milling process with a bead mill can be used for the preparation. To evaluate the performance of the BREAK-THRU^® BP 787 we decided to use the chemical pesticide Rimsulfuron (stability on hydrolysis (25°C) DT50 4,6 d (pH 5)). Rimsulfuron is insoluble in BREAK-THRU^® BP 787. Rimsulfuron is usually applied to the tank as a WP-formulation (wettable powder). Since BREAK-THRU^® BP 787 is a water-soluble fluid, we decided to name the formulation type in Table 1 a Dispersible Concentrate (DC) instead of OD.

Table 1. Evonik’s Water-free Guideline Formulation Rimsulfuron 125g/L DC (Dispersible Concentrate).

The DC-formulation in Table 1 is simpler than the usual OD-formulations which generally need a complex emulsifier system to achieve long-term stability. Often the emulsifier system of an OD is a combination of two or even three emulsifiers and co-emulsifiers. A combination of non-ionic and anionic surfactants is typical.

Further reducing the complexity of the BREAK-THRU^® BP 787 formulation is the ability to use a hydrophilic dispersant which will be soluble in the DC-formulation and later in the spray tank. Such a dispersant ensures stability against flocculation, agglomeration, and precipitation not only in the hydrophilic formulation in Table 1, but also in the diluted spray solution. ODs consist of hydrophobic oils and a hydrophobic dispersant is required. Hydrophobic dispersants are typically higher in molecular weight and while they ensure stability in the hydrophobic formulation, they can lead to instability once diluted in water. Aqueous dispersants or additional wetting agents are often needed to prevent tank mix instability. In Table 1, BREAK-THRU^® DA 647 is a polyether-based hydrophilic, non-ionic dispersant which exhibits excellent stability in the DC-formulation but also ensures stability of the active ingredient in the tank mix.

Physical stability results of Evonik’s Water-free Rimsulfuron 125g/L DC

The stability of the formulation in Table 1 was tested after production and after being stored at room temperature and 54°C for 14 days by evaluating particle size distribution and rheology. The particle size distribution after storage for 14 days at room temperature (Figure 1, blue curve) shows no significant difference to the one after storing at 54°C (green curve). This confirms there was no agglomeration or alteration during storage. In addition, the formulation retains its good water solubility/dispersibility since the measurement of particle size distribution is carried out with a highly diluted solution.

Figure 1. Particle Size Distribution of Evonik’s Water-free Rimsulfuron 125g/L DC from Table 1 in water.

The rheological behaviour of the DC-formulation was also tested under the same storage conditions. In Figure 2, the non-Newtonian behaviour of the DC-formulation is shown after storage for 14 days at room temperature (blue curve) and at 54°C (red curve). There is no significant difference from the initial rheological behavior. The rheological control is created by the hydrophilic fumed silica, AEROSIL^® 200. It forms a ″flocculated network″ and thus strengthens the formulation’s long-term stability. BREAK-THRU^® BP 787 provides the same performance as vegetable oils and paraffinic oils when combined with rheology modifiers.

Figure 2. Rheology characteristic of Evonik’s Water-free Rimsulfuron 125g/L DC from Table 1.

Evonik’s Guideline Formulation Rimsulfuron 125g/L OD based on sunflower oil as a comparison sample

Table 2 shows our Rimsulfuron 125g/L OD-formulation, which was developed as common standard formulation with sunflower oil as carrier oil and can be compared to our Rimsulfuron 125g/L DC-formulation with BREAK-THRU^® BP 787. Both formulations were tested in two glasshouse trials which allows us to compare both carrier oils regarding biological efficacy.

Table 2. Evonik’s Guideline Formulation Rimsulfuron 125 g/L OD in Sunflower oil.

Evonik’s Rimsulfuron OD in Table 2 contains an emulsifier system containing BREAK-THRU^® EM V20, a modified ethoxylated castor oil (HLB 8,3), and TEGIN^® OV, a co-emulsifier based on glycerol mono-dioleate (HLB 3,3). The total HLB value of this system is approximately 8,0 which is optimal for vegetable oils. BREAK-THRU^® DA 646 is a non-ionic dispersant which is soluble in oil but forms good emulsions in water. It is an optimized dispersant for vegetable oil OD-formulations. AEROSIL^® 200 (hydrophilic fumed silica) is utilized as a thixotropic rheology modifier to prevent precipitation of the particles. A hydrophilic silica is of advantage as it provides an excellent increase of the thixotropic effect in oils and in water. Hydrophilic silicas will not separate and swim on the top of the tank mix or cause nozzle blockage.

A commercially available benchmark was also part of this evaluation to distinguish the biological efficacy performance of liquid and solid formulations. The commercial benchmark is a solid formulation, a Rimsulfuron 250 g/kg WG-formulation (also WDG, water dispersible granulate).

Trial description: Efficacy of the formulations against Poa Pratensis

Poa pratensis was grown for four weeks in Fruhstorfer Erde (special soil mixture ″fine″). The formulations were applied at the 3-4-leaf stage of Poa pratensis. The application was calculated for 200 l/ha and was conducted with a mini-compressor. Each treatment was replicated five times.

Figure 3. Percent Control of Poa pratensis with different formulations.

Trial description: Efficacy of the formulations against Galium aparine
                                                                
Galium aparine was grown for four weeks in Fruhsdorfer Erde (Special soil mixture ″fine″). The formulations were applied at the 3-4-leaf stage of Galium aparine. The application was calculated for 200 l/ha and was conducted with a mini-compressor. Each treatment was replicated four times.

Figure 4. Percent Control of Galium aparine with different formulations.

The biological efficacy of the formulations against Poa pratensis in Figure 3 and against Galium aparine in Figure 4 are shown on the vertical-axis as percent control [%].

The results show that OD-formulations in general can outperform the WG/WP formulation with the same active ingredient concentration. The oil provides better contact to the cuticle which is the limiting barrier of the leaf for the transport of the active ingredients. This barrier can be significantly lowered by oils, such as the carrier system of the shown OD-formulation. Because the transport of the molecules through the plant cuticle is based on diffusion, one can conclude that surface activity, but also swelling of cuticle waxes increases the transport and uptake. This leads to better biological efficacy compared to solid formulations like WG and WP. Evonik’s Rimsulfuron 125g/L DC based on the novel carrier liquid BREAK-THRU^® BP 787 outperforms the OD-formulation (Table 2) in both glasshouse trials. These examples suggest that the BREAK-THRU^® BP 787 is similar to the sunflower oil regarding interaction with the cuticle and has therefore an additional adjuvant effect.

Conclusion

In conclusion, BREAK-THRU^® BP 787 is an excellent addition to the formulator’s toolbox as a water-soluble carrier oil for chemical actives, especially for water sensitive actives. The performance is equivalent or superior to conventional OD-formulations consisting of mineral oils or vegetable oils. Since BREAK-THRU^® BP 787 is water soluble, no additional emulsifier is needed significantly simplifying the development of such formulations. BREAK-THRU^® BP 787 is biodegradable as tested by OECD 301 F and fulfills requirements for sustainable environmentally friendly formulations.

¹ Agro Pages ″Formulation & Adjuvant Technology″ magazine published in May 2022; Evonik: How to prepare a liquid microbe formulation with extended shelf life and excellent biological efficacy

This article was published in AgroPages '2023 Formulation & Adjuvant Technology ' magazine published this May.

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