Adjuvants with Novel Modes of Action for Drift Control and Maintain Biological Efficacy

Left: Dr. Annika Dietrich, Head of Application Technology EMEA at Evonik

Right: Marc McPherson, Agronomist at Evonik

Evonik has undertaken an evaluation of two adjuvant products, BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX, designed to enhance the biological efficacy of agrochemical sprays and also provide a unique mode of action for drift reduction. While the issue of drift from agricultural chemical applications has been a subject of study for a considerable period, there has been a resurgence of interest more recently due to increasing concerns about the unintended movement of pesticides to neighboring crops and native plants.

All modern drift models point to the reduction in driftable droplets (defined as droplets anywhere in the range of 105 µm to 210 µm) as the most effective way to control drift. Typical adjuvants used to reduce drift contain polymers such as polyalkylene glycols, polyacrylates, polyvinyl alcohols and other viscosity modifiers like guar gum. These polymers serve to thicken the liquid (increasing its viscosity) and promote cohesion among spray droplets. However, they also have the drawback of diminishing the spray angle by elevating the elongational viscosity. Furthermore, although these polymers succeed in reducing the number of smaller droplets, they also increase the size of all droplets in the spectrum. This mechanism increases the proportion of larger droplets, which are more prone to bounce off and run-off leading to reduced retention of the overall spray volume on plant surfaces. The consequence is a tradeoff between reducing drift and pesticides efficacy.

Evonik has developed two adjuvants, BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX, with unique chemical properties that reduce drift and improve efficacy of agrochemical sprays. Evonik’s technology hinges on the creation of insoluble droplets with a lower surface tension in comparison to agrichemical in water without either of these two adjuvants.

To assess both adjuvants for drift reduction, Evonik conducted wind tunnel tests at the University of Nebraska, North Platte’s. The adjuvants were tested at four concentrations (0.1%, 0.3%, 0.6% and 1 % v/v) in combination with several commercial pesticide formulation types. The pesticides included azoxystrobin + difenoconazole an SC, mesotrione an SC, propiconazole an EC, unloaded (without adjuvant built into the formulation) glyphosate an SL and glufosinate an SL. The highest label rate of each pesticide was used to provide the most robust test of drift potential and emulate the current grower interest  to reduce carrier volumes – particularly when using drones applications.

In these tests, a TeeJet^® flat fan nozzle XR11004, commonly used for ground and drone application, was utilized at a pressure of 43.5 psi (3 bar). Spray patterns were analyzed at low speed with 15 mph (24 kph) air flow with a Sympatec Helos Vario KR particle size analyzer. With the R7 lens installed, it detects particles in a range from 0.5 to 3500 microns. This system uses laser diffraction to determine particle size distribution. The width of the nozzle plume was analyzed by moving the nozzle across the laser by means of a linear actuator, and each treatment was replicated three times.

Figure 1: Percent of droplets below 150 µm size produced with water and the adjuvants BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX at different use rates. Both adjuvants reduced drift at all rates by ~5% using only water in the spray solution.

Figure 2: Percent of droplets below 150 µm size produced with a commercial SC with azoxystrobin and difenoconazole and the adjuvants BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX at different use rates. Both adjuvants at all rates reduced drift of this fungicide by ~1 to 2% without many differences among the adjuvants at all four rates.

Figure 3: Percent of droplets below 150 µm size produced with a commercial SC with mesotrione and the adjuvants BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX at different use rates. Both adjuvants at all rates reduced drift of the soluble concentrate formulation of the herbicide mesotrione by ~5% without differences among the adjuvants at all four rates.

Figure 4: Percent of droplets below 150 µm size produced with a commercial EC with propiconazole and the adjuvants BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX at different use rates. Both adjuvants at all rates have limited effect on drift with the EC formulation of propiconazole with a general trend that more adjuvant slightly increases drift.

Figure 5: Percent of droplets below 150 µm size produced with a commercial SL with unloaded glyphosate and the adjuvants BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX at different use rates. Both adjuvants at all rates reduced drift of water by >5% without differences among the adjuvants at all four rates.

Figure 6: Percent of droplets below 150 µm size produced with a commercial SL with glufosinate and the adjuvants BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX at different use rates. Both adjuvants are neutral for drift at 0.1 %v/v. Both adjuvants improved drift control with increasing rates of the adjuvant with 5 to 10% reduction in driftable fines at the 1% use rate.

In general, both adjuvants exhibited consistent drift reduction across a range of application rates with water and most of the commercial pesticide formulations. Notably, a minimal application rate of 0.1 %v/v for both adjuvants was sufficient to reduce drift with all the pesticides. Increasing the adjuvant rate did not improve drift control except for two formulations (Figures 1 to 6). The first exception was the EC formulation with propiconazole where both adjuvants were neutral for drift (Figure 4). The second exception was the SL formulation with glufosinate where both adjuvants were neutral at the 0.1 %v/v rate but at each increasing rate significant drift reduction was observed (Figure 6).

Evonik’s technology centers around adjuvants that form insoluble droplets and lower surface tension. The characterization of droplet appearance and the regulation of droplet sizes elucidated via the use of two high speed cameras when placed in the test chambers at the nozzle outlet and further downstream (Figure 7).

Figure 7: Technical drawing of flat fan nozzle spray experiment.

Figure 8 shows the images captured by the high-speed camera systems for pure water (Left) and water combined with an anti-drift adjuvant (Right). In the case of pure water, the film emerging from the nozzle displays a broad lamella characterized by an optimal initial angle (spray angle). This broad lamella leads to a reduction in film thickness, thereby instigating the onset of local instability. Subsequently, this local instability triggers the puncturing and fragmentation of the film upon reaching a specific threshold of thickness. Consequently, the resultant droplet spectrum exhibits a proliferation of smaller droplets.

Figure 8: High speed camera images of initial spray pattern break-up; above: Spray behavior of pure water; below: Spray behavior of water combined with adjuvant BREAK-THRU^® SP 133. With the adjuvant the spray angle is not decreased, but the spray sheet breaks up earlier and the early droplets tend to be larger.

In contrast, the BREAK-THRU^® adjuvants form insoluble droplets in the spray liquid. These insoluble droplets migrate to the air / water interface of the lamella and cause its break-up earlier than water alone. Because the lamella breaks up while it is thicker, the child droplets formed are larger in size. This behavior can be explained by the thermodynamic requirements (Figure 9; Equation 1). The insoluble droplets of the anti-drift adjuvant need a lower surface energy (σo) than the spray liquid (σl). As a result, the entering coefficient remains positive, and the droplets migrate to the surface. To spread on the surface, it is important that surface energy (σo) again is lower than the spray liquid (σl), so that the spreading coefficient also remains positive, and the drop has the power to increase its surface area.

Equation 1: Thermodynamic Requirement for Surface Activity.

Figure 9: Diagram of thermodynamic effect of somewhat insoluble adjuvant and equations used to describe this behavior.

Evonik’s BREAK-THRU^® adjuvants, such as BREAK-THRU^® MSO MAX and BREAK-THRU^® SP 133, have the right balance of water solubility to reduce drift, because they have low solubility but are not completely immiscible. Both adjuvants do not decrease the spray angle as typical viscosity modifiers tend to do. In addition, both products show excellent adjuvant properties once on a leaf surface, whereas many anti-drift adjuvants may not provide other benefits aside from drift reduction. BREAK-THRU^® SP 133 is a great biobased sticker and penetrant, and BREAK-THRU^® MSO MAX is a known cuticle swelling agent and humectant for systemic actives. The drift reduction and efficacy benefit for both of Evonik’s adjuvants is applicable to conventional and drones tank mix applications.

Based on the thermodynamic requirements, which is described above, Evonik’s BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX low water solubility and lower surface energy causing an earlier break-up of the water film. This lower surface energy also causes the spray solution to adhere and spread on plant surfaces. This effect can be achieved by lowering the surface tension or spreading (Table 1 and Figure 10). A pure water droplet has a high surface tension (~72mN/m, blue curve in Figure 10) and a spherical shape with a high contact angle. However, the water droplets when mixed with Evonik’s BREAK-THRU^® adjuvants has a lower surface tension and smaller contact angle. The static surface tension of water with 0.1% BREAK-THRU^® SP 133 (green curve in Figure 10) and 0.1% BREAK-THRU^® MSO MAX (red curve in Figure 10) is reduced to 54mN/m and 27mN/m, respectively.

Table 1: Spreading of pure water compared to Evonik’s adjuvants BREAK-THRU^® MSO MAX and BREAK-THRU^® SP 133 (0.1% in water); data is the mean from two replications.

Figure 10: Surface Tension of pure water (blue curve), water combined with 0.1% BREAK-THRU^® MSO MAX (red curve) and water combined with 0.1% BREAK-THRU^® SP 133 (green curve).

Conclusions:

Evonik has developed two adjuvants, BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX, with unique mode of action for drift reduction and improved efficacy of agrochemicals. Unlike most conventional antidrift adjuvants, these products do not reduce the spray angle and improve retention, spread and penetration. These adjuvants provide a benefit for both conventional and drone applications with a novel mode of action to reduce drift with a flat fan nozzle. Low rate of 0.1 %v/v of both adjuvants reduced drift with each of the different formulation types and active ingredients with only two exceptions. Both adjuvants were neutral at all rates with the EC formulation of propiconazole and neutral with the SL formulation of glufosinate at low rates significantly decreased drift at higher rates. The highest concentration of each formulated agriculture pesticide was used in this study to emulate drift with a low volume application, which is a trend for conventional ground applications but is also applicable to new applications with drones. In some agriculture applications, the use of a flat fan nozzle is preferred for the droplet spectrum and working pressure. In this situation the use of BREAK-THRU^® SP 133 and BREAK-THRU^® MSO MAX would provide a great benefit because they maintain the spray angle and overall efficacy (lower droplet size spectrum) and provide significant drift reduction. In summary, these adjuvants provide excellent drift control and biological efficacy.  

Evonik is in the process of conducting field trials with these adjuvants to evaluate field efficacy when applying agrichemical pest control with drones and these novel adjuvants.