Dow: Using Encapsulation Technology to Address the Agrochemical Formulation Trend
Oct. 31, 2018
The use of microcapsules is well known in chemical art, including pharmaceutical, dye, ink, fragrance, food and agriculture application. In agriculture, encapsulation technology is seen as the next generation capability to address trends in formulated products, specifically designed to enhance performance. The technology is useful to improve the efficiency of herbicides, insecticides, fungicides, bactericides, seed coatings and fertilizers by improving the stability of active ingredients and compatibility, reducing toxicity and environmental influence, enabling slow and/or controlled release and delivery, and prolonging drug efficacy etc.
Various processes of microencapsulating material have been previously developed in the art, including typical categories of physical, phase separation, in-situ and interfacial reaction methods. Interfacial polymerization reaction methods have been proven to be an effective process for use in the agricultural industry for the microencapsulation of pesticides. In the interfacial reaction, the core material is emulsified or dispersed in an immiscible continuous phase and then an interfacial polymerization reaction takes place at the surface of the core particles, thereby forming microcapsules, as illustrated in Figure 1. In general, such reactions involve the condensation of an isocyanate moiety as one monomer with a second moiety such as an amine as a second monomer.
After a reaction method and polymerized monomers are confirmed, several key parameters will be considered for the preparation of microcapsules: a) solvents are used to dissolve active ingredients to form an oil phase; high solvency and low water solubility are preferred to facilitate microcapsule formation; b) emulsifiers are critical to form stable and uniform oil-in-water emulsion; and the size distribution of emulsion droplets and final microcapsules can be adjusted by changing emulsifier structures and dosage; c) dispersants help disperse microcapsules well, and avoid microcapsule aggregates during polymerization; it is also useful to stabilize the microcapsules in suspension formulation during storage in an ambient condition. Encapsulation technology is a complex process, and each parameter mentioned above could have an impact on others, and thus, high R&D investment is often needed to develop good microcapsule formulation.
By leveraging Dow’s encapsulation capability, technical solutions have been built up for agrochemical formulation application by providing solvents, emulsifiers and dispersants, and polymeric isocyanate and amine monomers. Herein, 3.5% abamectin aqueous Capsule Suspension (CS) formulation is taken as an example to illustrate the process.
Abamectin is widely used to control insects and mite pests on a range of agronomic, fruit, vegetable and ornamental crops. However, the active ingredient is sensitive to UV radiation, resulting in quick degradation in ambient conditions. In addition, it is relatively highly toxic by acute ingestion and inhalation. The capsule formulation is thus speculated to keep the active ingredient stable and reduce toxicity by providing a polymer shell barrier.
The low solubility of abamectin in water 1.21 mg/L (20-25ºC), makes it suitable for encapsulation by interfacial polymerization. Polymeric MDI PAPI™ and ethylenediamine from Dow, as typical monomers in interfacial reaction, are used here as oil soluble and water soluble monomers, respectively. As described above, proper solvent(s) are critical to form a concentrated abamectin solution as the oil phase. However, the solubility of abamectin in typical hydrophobic solvents for the agrochemical area is very low, even lower than 1%. In addition, abamectin has hydroxyl groups (1 secondary OH, 2 tertiary OH) in the molecule, as shown in Figure 2, which may react with -NCO group in isocyanate molecules, and potentially result in the degradation of abamectin during the capsule formation process. A solvent, named POWERBLOX™ SV-17 Solvent, was identified with >14% solubility for abamectin. The solvent has –OH group, which can protect the abamectin molecule from reaction with –NCO groups. The emulsifier and dispersant package was also investigated based on the selected solvent. ECOSURF™ EH Surfactant is a branched alcohol alkoxylate surfactant, which is widely used as an emulsifier in agrochemical formulation. It was found that the surfactant has good emulsification performance on SV-17 and the emulsion droplet size can be adjusted by the dosage of the EH product. POWERBLOX™ D-305 Dispersant is a polycarboxylate type of dispersant. The product has been well recognized to improve dispersion and formulation stability in broad Suspension Concentrate formulation practices. D-305 was also proven to be effective in the abamectin CS preparation and formulation stability during aging tests.
Several measurements were used to prove the formation of microcapsules, and typical morphology from Scanning Electron Microscope (SEM) is shown in Figure 2. Microcapsules in the range of 2-10 µm were observed. The key performance results are also listed in Table 1. The results indicate that by the said interfacial polymerization process, >95 wt% of abamectin ingredient can be successfully encapsulated. The prepared CS formulation showed qualified physical and chemical stability during storage at a wide temperature range. In addition, as mentioned above, one of the purposes of encapsulating abamectin is to reduce its degradation rate. From the data, it can be seen that the degradation rate during both the preparation process and storage can be controlled within 0.5 wt%. In contrast, the degradation in normal Emulsifiable Concentrate (EC) formulation could be 5-10 wt% under the same conditions. Besides abamectin, the encapsulation technology has also been successfully implemented on other active ingredients, e.g. lambda-cyhalothrin, chlorpyrifos, pendimethalin and fosthiazate.
Technology is always seen as one of the key advantages of Dow, both in terms of wide product portfolios and strong R&D capabilities. The encapsulation technology is a good example of Dow’s R&D capability that is applied to serve the agrochemical industry. We will continue to offer technical solutions that would meet the development trend and lead the technology transformation, as a preferred solution provider for pesticide formulators globally.
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