Overview of pesticide suspensions

Most pesticide agents have low solubility in water, so they cannot be used directly. Because of the waxy layer on the surface of various crops, pests and weeds, the common solution is difficult to penetrate, and the vast majority of pesticide agents do not have adhesion properties, and the retention time after spraying is short, if the pesticide agents are used directly, not only can not play the efficacy, control diseases and pests, but also easy to pollute the environment and harm human health, so the original agents must be processed before they can be used. By adding a certain proportion of auxiliary agents to the pesticide and using a certain process to prepare and process into various forms of pesticide processing. The pesticide after processing has a specific form, composition and specification, which is called the pesticide dosage form.

Through processing, the storage stability of pesticide preparations can be improved to prevent the decomposition or failure of active ingredients; After processing, some dosage forms can control the release rate of active ingredients, prolong the duration, reduce the application times and dosage; After processing and diluting the high concentration of the original drug, the suspension performance is enhanced, the effective components can be distributed evenly, and the dispersion effect can be achieved in application. The high concentration of the original drug is made into low-toxicity preparations, which can be used safely and reduce environmental pollution. It is beneficial for the effective ingredients to penetrate and be absorbed by the target organism to improve the drug effect.

The development of modern agriculture not only requires the safe production of agricultural products, but also the construction of a good agricultural ecological environment. Therefore, the agricultural production in the 21st century has continuously strict requirements for the dosage form and use of pesticide varieties. Formulations such as wettable powders and cream-based products are being improved or replaced by water-based and environmentally friendly formulations. Suspension agent is the representative of water-based dosage form, which refers to the polyphase dispersion system formed by the solid drug uniformly dispersed in water with a certain dispersion degree. Its main components include pesticide agents, various fillers and media (water). The suspension agent has many advantages: compared with the wettable powder, in the process of processing and use by farmers, it avoids the pollution of the environment caused by dust flying and personal injury; However, emulsifiable oil requires a large amount of organic solvents, which not only pollutes the environment, but also wastes petroleum resources. The suspension agent has the advantages of wettable powder and emulsion at the same time, such as improving dilution suspension rate, convenient packaging, no dust, easy mixing, safety for production and use personnel, environmental friendliness and low production cost, and is significantly superior to traditional dosage forms such as wettable powder and emulsion in terms of comprehensive performance such as efficacy, safety and economy.

In recent years, the number of pesticide environmental protection dosage forms registered in China continues to rise, from the number of registered dosage forms, the number of emulsion, wettable powder product registration is decreasing year by year, and the number of environmentally friendly product registration such as suspension agent and water dispersion granule is increasing. The suspended agent is the representative of water-based dosage form, and it is also a mainstream direction of future pesticide dosage form research and development.

China began to develop pesticide suspensions in the 1970s, and the number of suspensions has been increasing. In recent years, China's suspension products are mainly developing in the direction of high content, and great breakthroughs have been made in pesticide active agents with low melting point and high water solubility, and it is difficult to prepare high concentration suspension agents, such as dimethylpentyl-imidacloprid suspension agent and dinotefuran suspension agent. However, compared with Germany, the United States and other developed countries, China's suspension agent in preparation research and development, auxiliary screening and innovation, equipment, processing technology and other aspects of the gap. Suspended agent is an unstable thermodynamic dispersion system, some suspended agent products will appear in the long-term storage process of water thickening, flocculation, stratification and precipitation, which seriously affects the shelf life and use of products.

1 Pesticide suspension processing technology

There are two main methods to prepare the suspension agent: grinding wet crushing method and hot melt condensation method. The method commonly used in our country is grinding wet crushing method, the processing process is to mix the active ingredients of pesticides, various additives and water evenly, the emulsifier is pre-dispersed for about 30 min, and then crushed with a sand mill to a multiphase suspension system with a particle size of ≤5μm, of which more than 85% of the pesticide particles are greater than 1μm, and the coarse dispersion system accounts for a larger proportion.

FIG. 1 Process flow chart of grinding wet crushing method

The hot melt condensation method is to add the original drug solution in the molten state to a cooler liquid (water or oil), and form crystal particles under high-speed agitation, while adding a variety of surfactants with different functions to make a suspension agent.

2 Formulation and composition of pesticide suspension agent

Active ingredients: During the processing, the grinding temperature is high, if the melting point of the original drug is low, it will melt, so the melting point of the original drug is required to be not less than 60 ° C, and the solubility in water is low, the nature is stable, and the light does not decompose.

Wetting dispersant: the suspended agent is prone to water dissolution, thickening, caking and other problems in storage, so it is necessary to develop special and environmentally safe high-quality wetting dispersant. The main function of the wetting agent is to reduce the surface tension of the system and play a wetting role by forming nano-polymer microspheres. The water solubility of wetting agent is good if the balance value of hydrophilic oil is high. Low hydrophilic lipophilic balance value means high activity. Dispersant plays a good dispersion role through the steric hindrance formed by functional polymers. The dispersants used in the market have undergone the upgrading process, mainly polyacrylates, acrylate copolymers, linear macromolecular ionic or non-ionic compounds, hydrophobic copolymer dispersants, etc.

Thickening agent: The density of lipid compounds in organic solvents is lower than that of water, and the density difference between them and the pesticides themselves is larger, which accelerates the settlement of particles to a certain extent. Adjusting the viscosity of the preparation can slow down the particle settlement, and the general dosage of thickener is 0.5% to 5%. Thickeners can be divided into inorganic, natural products, polyacrylates, cellulose derivatives, polyester derivatives, etc. The thickener achieves the thickening effect mainly through its own expansion, and the associated thickener forms a stable network structure mainly through the hydrogen bond between the thickener molecule and the dispersed phase.

Antifreeze: The preparation will encounter low temperature during transportation and storage, so a small amount of antifreeze should be added in the preparation processing to enhance the freeze-thaw ability and low temperature stability, and the general dosage is 5%. Antifreeze should be selected with good performance, non-volatile, and difficult to decompose. Commonly used antifreeze agents for pesticide suspension are ethylene glycol, propylene glycol and propylene glycol.

Defoamer: When processing the preparation, if an inappropriate surfactant is selected, the slurry will produce many bubbles, which will affect the viscosity change of the preparation, weight calculation, product quality and packaging. Defoamer can be selected with a small amount and good effect, and no adverse effects with other auxiliaries, silicone defoamer is a commonly used class of defoamer.

Preservatives: Preservatives are substances that can prevent the growth of microorganisms, or prevent the growth of microorganisms that react to a prepared product. When the suspension agent is processed, a certain amount of preservatives can be added to avoid the decay and fermentation of the product in storage and transportation, and the commonly used preservatives are casson and so on.

3. Evaluation index of pesticide suspension quality control

The quality control index of suspension agent mainly includes dispersibility, fluidity, particle size, suspension rate, viscosity, lasting foaming, pH value and storage stability.

Dispersity of pesticide refers to the ability of the original drug particles to disperse into fine individual particles suspended in water. If the original drug particles are too large, it is easy to poly settling phenomenon. Adding dispersant can solve the problem of particle settling well.

Fluidity can reflect the quality of the dumping of the preparation, if the fluidity is good, the user is easy to pour the sample out of the packaging bottle, reduce waste and save the preparation.

Particle size, in the process of suspension processing, processing time mostly depends on the particle size and its distribution, China's suspension particle size is generally controlled within 5μm, usually using laser particle size analyzer to determine the D90 of the sample. In order to enhance the stability of the suspension agent and reduce the Austenite ripening, the particle size range should be kept small during the processing of the suspension agent.

As an important evaluation index of suspension agent, the suspension rate can reflect the dispersion suspension time of the original drug particles in the suspension. Generally, the product has good stability and higher efficacy, the better the suspension rate.

The viscosity of the suspension agent is moderate, and the storage stability is good. If the viscosity is large and the fluidity of the agent is poor, it will bring inconvenience both in the processing process and in the later sample measurement and use.

By measuring the amount of bubbles in the product and selecting the appropriate type and dosage of defoamer, the adverse effects of foam on the product can be reduced.

The pH value will affect the particle size of the preparation, so the pH value should be controlled within a certain range to reduce the impact on the particle size distribution.

Storage stability refers to the change of physical and chemical properties of the suspended agent after storage for a period of time. The method of accelerated experiment is generally adopted, that is, heat storage at (54±2) ℃ for 14 days and cold storage at (0±2) ℃ for 7 days, to determine whether the deposition, water evolution, particle size, etc., meet the standards. But accelerated storage does not mean normal temperature storage.

(Selected from Liu Yunxia, Northwest A&F University, "Development of 20% Pyrazolium and Phyllobacterin Suspension based on Wheat Scab Prevention and Control")