Combining Effectiveness and Safety
15 November 2012 Adjuvants are useful for enhancing the performance of nearly all foliar applied herbicides and systemic actives, including plant growth regulators. Typically, adjuvants are used to improve spray retention, spreading, penetration and rainfastness of the active ingredient. Reduction of drift and enhanced compatibility with certain water conditions are also common uses of adjuvants.
Methylated seed oils (MSOs), such as oils derived from soybeans, are popular penetration-enhancement adjuvants. MSOs enable faster sorption of the active ingredient in the waxy leaf cuticle and migration into the leaf tissue. While MSOs are more volatile than most active ingredients, they quickly meld with the leaf’s surface and act mainly as a door opener for the active at the level of the cuticle. Only high-use concentrations can form a transient oil film on the leaf surface that enables interaction with agrochemical particles.
MSOs are produced by reacting methanol with, for example, whole soybean oil in a process called transesterification, which changes soybean triglycerides into soybean fatty acid methyl esters. MSOs work at both ends of the active spectrum: with lipophilic actives such as azole fungicides and also with some hydrophilic actives such as certain sulfonylurea herbicides. However, the performance of an MSO-based adjuvant depends heavily upon the effectiveness of the emulsifier used in the formulation to optimize and stabilize uniform distribution of the adjuvant in a water carrier for application. MSO-based adjuvants often need relatively high threshold use concentrations of 0.25% to 1% in the spray for good efficacy.
Although MSOs are based on renewable resources (seed derived), the choice of emulsifier can negatively impact the environmental product labeling. For example, nonylphenol ethoxylate (NPE) emulsifiers are classified as endocrine disruptors; branched alkyl benzene sulfonates are not very biodegradable, and dialkylsulfosuccinates can cause eye irritation.
Adjuvants based on polyglycerol esters are used for glyphosate, the popular broad-spectrum herbicide, and other hydrophilic agrochemicals. Polyglycerol esters are non-ionic surfactants that have been used for many years in food and personal care products. They are based on renewable glycerol and fatty acids, are non-toxic and non-irritants and have low environmental impact. Due to these advantages, polyglycerol esters have been incorporated into a broad array of applications, including use as adjuvants, emulsifiers and dispersants in agrochemical formulations.
Developing a Sustainable Emulsifier for MSO
Agriculture increasingly demands sustainable chemistry. Polyglycerol esters can replace previously mentioned emulsifiers that negatively affect labeling. How do we develop a chemical derivative based on polyglycerol esters for use as an environmentally acceptable adjuvant, as well as an emulsifier for MSO?
To achieve this new chemistry, key goals in development were identified:
- Enhanced miscibility in MSO with simultaneously optimized emulsification of MSO in water. For that aim, a particular cross-linked polyglycerol ester of fatty acids was developed.
- Suitability for sensitive crops, which is critical because damaged plant tissue precludes uptake of the active. Unsuitable adjuvants destroy biomembranes, preventing osmotically driven translocation in the long distance transport system of the phloem. If cell membrane integrity is lost, the cell is disconnected from the living tissue. This, along with physical sorption of the active with debris, causes quick deactivation. Ability to mobilize the active in the deposited agrochemical is essential.
- Lack of sensitivity to water hardness.
- Versatility for use as tank mix or ability to be formulated with active.
A significant challenge to overcome was combining MSO and polyglycerol esters, as these two chemistries differ in lipophilicity by more than 10 orders of magnitude. The self-emulsifying power of the polyglycerol ester with lipophilic modification needed to be taken into consideration. The new adjuvant would have to be unique: overcoming the challenge of variations in water quality, delivering dose rates within typical use concentrations, yet not requiring a second emulsifier.
Mode of Action
After application of a water-based herbicide, only a small fraction of the agrochemical is taken up during the quick evaporation process. The performance of the herbicide is typically dependent upon what takes place during deposit formation. Therefore, the bioavailability of the active in the deposit is far more important than in the spray droplet (e.g. by a fine dispersion).
Herbicides and systemic agrochemicals typically do not enter the plant as particles. The particle is first dissolved, which then allows it to enter the plant in molecular form. In other words, only the smallest possible part of an agrochemical – the molecule, enters the plant. Therefore, the goal was to provide an emulsifier chemistry using polyglycerol esters that could effectively mobilize the active in the deposit to enhance uptake by the plant.
In the spray solution the solid active particles are evenly dispersed. Following application and the evaporation of the water, these particles are more or less evenly distributed in the deposit and coated with the emulsified MSO/polyglycerol ester adjuvant.
This novel adjuvant remains in a liquid state regardless of the ambient humidity. The adjuvant suppresses or even prevents the formation of crystalline particles while in the tank with the carrier (water) and following evaporation on the plant leaf. While the novel adjuvant cannot dissolve actives completely, it forms an effective interface between the active and the leaf surface, acting like a shuttle for the active molecules. A fraction of the active ingredient available for foliar uptake is increased, and this efficiency gain helps to exploit its full biological potential. The mobilization of the active is a particular strength of this new adjuvant.
Test Results
Numerous agrochemicals from all indications have been tested to determine the impact of the MSO/polyglycerol ester adjuvant on foliar penetration of actives from commercial formulations. A typical example is shown in Figure 1 (p. 28), where metazachlor is used as a suspension concentrate formulation. With the MSO/polyglycerol adjuvant, the level of metazachlor penetration within four hours is as great as the penetration after a day without the adjuvant. This increased penetration speed impacts factors including dose optimization, biological performance and rainfastness. After 24 hours, penetration is more than two times higher with the MSO/polyglycerol ester adjuvant than without.
Evaluation of the spray deposit under the microscope shows that the commercial suspension concentrate formulation produced crystalline particles in the deposit.
In contrast, in the presence of the MSO/polyglycerol ester adjuvant, there is no evidence of crystalline particles. As previously mentioned, this means a higher fraction of active ingredient is available for penetration with the use of this novel adjuvant. By comparison, the addition of MSO alone at the same concentration shows visible crystalline particles. However, the role of the MSO is still important as it interacts with the leaf surface such that the active moves faster into the leaf interior, which is a prerequisite for systemic distribution within the crop or weed, respectively.
The resulting chemistry is a homogeneous blend of more than 50% polyglycerol ester adjuvant with MSO that can be used as a building block in formulations, as a ready-to-use tank mix adjuvant or as a modified tank mix adjuvant for specific crops.
The emulsion quality is excellent and does not depend on water quality. Compatibility with tank mix partners of all formulation types is provided. Typical use concentration range in the spray is between 0.1% and 0.5%. The product has excellent plant compatibility at all use rates.
Sustainability and Performance Outline
The novel MSO/polyglycerol ester blend offers many properties that are rare for tank mix adjuvants. Because both the polyglycerol esters and the MSO are derived from non-toxic, renewable resources, this unique blend is environmentally responsible and likely suitable for organic crops. In fact, over 95% of this blend is renewable.
Due to its composition of various fatty methyl acid esters and polyglycerol esters, the adjuvant blend is reasonably biodegradable and has a favorable ecological profile with no environmental hazard labeling. Of all comparable emulsifiers traditionally used, only sorbitan ester ethoxylates have a similar favorable profile.
Enhancing its environmental attributes, the novel adjuvant is suitable for low water volume applications. Growers globally favor this approach as they strive for improved efficiency and lower eco impact. Low water volume applications save water and fuel and reduce time and equipment utilization.
Additionally, the product can be handled safely and does not have hazardous labeling as the product will not cause any skin or eye irritations, making the product more favorable than many alkyl or aryl alkoxylates which are commonly used as emulsifiers.
Product manufacturing is waste efficient and ISO 9001 and ISO 14001 certified. Stable emulsions can be obtained without an additional emulsifier and have a potential use for water-based and oil-based systems.
The two major substances have strong adjuvant properties that cover the whole range of agrochemicals with respect to active lipophilicity. The mode of action is different from existing adjuvants in that the delivery step affected most is the mobilization of the agrochemical on the plant surface. Mobilization is often a greater limiting factor to product performance as compared to the product not reaching the target and/or wetting behavior. Therefore, this adjuvant has great potential to increase the efficiency of agrochemicals.
Combined, all of these attributes make this new adjuvant blend a uniquely effective, flexible and sustainable solution.
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