In recent times, pesticide suspension concentrate has widely been used in agriculture as a formulation with good comprehensive performance.
However, pesticide suspension concentrate is a highly dispersed multiphase complex system, and its processing difficulty and stability of the finished product are related to many factors, including technical with different synthesis processes or different contents, different processing technology, different recipes and so on.
For a certain product, due to the different production processes from different technical factories, the same recipe will produce many problems in suspension concentrate processing. Some batches of technical cannot be used and have to be returned. This has become a big problem for the importers and exporters who must purchase technical to process pesticide suspension concentrate.
In the pesticide management system globally, the recipe is strictly limited given the principle of safe use and environmental protection. Therefore, it is not realistic to adopt a variety of recipes to deal with a variety of technical sources. Only some control can be made in the early screening of technicals and detection before shipment.
The production process of Chinese pesticide manufacturers is often adjusted, so the technical purchased is not necessarily consistent with the pre-screened products in the purchase process. For polycrystalline technicals, if each batch is tested by X-ray diffraction or other crystal forms, it is only to determine the crystal structure, and cannot completely determine its processing performance.
At present, the export inspection of technical is mostly limited to the control of the content and the physical and chemical criterion, while the detection of technical processing performance is still blank. To solve the problems faced by importers and exporters, Pilarcise Laboratory Co., LTD. has established a laboratory for processing the performance test of pesticides for export by making use of self-superiorities, which can make a comprehensive test of the processing performance for the technical. Such steps should be taken to avoid the awkward situation that the technical cannot be processed into a suspension concentrate after exports.
For the sake of technical confidentiality, the testing of the technical processing performance can be carried out from the following three aspects:
1. Pliarcise sets up three to five general recipes, and the samples are tested with them.
2. Testing according to the specified recipe from the customer.
3. Customers provide mixed adjuvants samples, and Pliarcise tests according to customer requirements;
In addition, Pilarcise can also provide preliminary screening services for the technical processing performance, and provide technical support to suppliers.
The following is the experimental equipment related to the processing performance of pesticides, including Mastersizer 3000, Surface tension meter, Brookfield rheometer, Dynamic contact Angle meter, horizontal sand mill, Cooling water recirculation apparatus etc.
The processing performance control of pesticide technical involves many factors such as the molecular structure, impurity, content of active ingredient, polycrystalline state, multielement complex and so on. Here are some examples:
1. Effects of functional groups:
Take Pymetrozine for example. As a heterocyclic pesticide, it is easy for Pymetrozine to form hydrogen bonds or water clusters with water to form hydrates. Therefore, in the process of grinding, Pymetrozine will absorb water, and the finished product will be sticky or creaming.
The molecular structure of Flumorph contains a strong polar group and contains fluorine element. So, it has a strong hydrophobicity. In the process of grinding, the oil impurities, which were originally tightly bound to the active substance, will separate. Creaming may occur.
2. Influence of Impurities
Take Imidacloprid as an example. In the production of Imidacloprid 60% SC, though the technical contents from different suppliers all exceeding 97%, different production processes bring different unknown impurities, which directly affect the charge neutralization of the adsorption layer of dispersant, resulting in instability of the product.
The content of Emamectin Benzoate is generally not higher than 80%. In the process of grinding, more than 20% of organic impurities will be free from the technical, forming a free state. The melting point of these impurities is often lower than 54℃. So, it is easy for creaming to happen in the process of grinding.
3. High-content preparations
High-content formulation usually refer to suspensions with a content of 45% to 60%. The more active ingredients added to the suspension concentrate, the less water added to the system, so various problems will arise during processing. In the production of Imidacloprid 60% SC, for some technicals, as the material is stirred faster, the viscosity will also increase, and sometimes it will become more viscous as it is ground. In the production process of high-content Buprofezin and Iprodione suspension concentrate, the probability of flocculation or coalescence of the technical from different sources is different. The particle size of some finished products produced by some technicals will increase during storage, which will cause the aggregates between particles to merge and become larger, leading to problems such as precipitation, and agglomeration.
4. Polymorphic technical
The dissolved technical is affected by various factors during crystallization, and the intra-molecular or intermolecular bonding method may change, resulting in different arrangements of molecules or atoms in the lattice space, forming different crystal structures. This directly leads to different physical and chemical properties (such as solubility, melting point, hardness, crystal shape) of the technical.
For example, Azoxystrobin has three forms: crystal form A, crystal form B, and the amorphous form; Pyraclostrobin has five forms: amorphous form and 4 different interchanging crystal forms I-IV, among which crystal form IV is the most thermodynamically stable crystal form that can be used to process stable suspension concentrates. There are two crystal forms of Imazethapyr, crystalline form I and crystalline form II; the technical of Metazachlor is also polymorphic technical, and it is easy to convert between the various crystal forms. Pesticides such as Mesotrione, Boscalid and S-indoxacarb also exist in two crystal forms. For polymorphic technical processing suspension concentrate products, if the purity of the technical is not enough, the mixture of polymorphs will inevitably contain low melting point crystal forms, which will lead to instability such as precipitation, and agglomeration.
5. Compounding of multiple technical ingredients
In most cases, the compound material of multi-component technical compound is more susceptible to damage due to the intermolecular force, causing the melting point of the entire system to decrease. At this time, the technical with a higher melting point will be processed into a low-melting technical in the process of a suspension concentrate.
For example, Thiamethoxam + Bifenthromycin 19% SC. Thiamethoxam has a melting point of 139.1°C (high melting point pesticide) and Bifenthrin has a melting point of 68～70.6°C. The compound melting point of the two technicals is 57～64°C (the lowest eutectic temperature), which is equivalent to processing low melting point pesticides.
The melting point of Difenoconazole is 76°C, and the melting point of Thiophanate-methyl is 178°C. When the compound of Difenoconazole +Thiophanate-methyl 40% SC is processed, the lowest eutectic temperature is 61.4°C. For technicals from different sources, the degree to which the particle changes from solid to liquid (melted) at high temperatures is different, and the processing performance is also different. Some technicals will be pasted or solidified during the grinding process, the viscosity will increase before and after heat storage, the particle size will increase, and crystals will be easily precipitated.
In the process of processing the ternary suspension products of Napropamide, Metazachlor and Metamitron, the melting points of the mixture materials are also different.
Note: Part of the above background information is cited from "Development, Characteristics, Processing Difficulties and Analysis of Pesticide Suspended Concentrate" published by Hua Naizhen in "World Pesticide" in 2019. We would express our thanks to him.