Dr. Li Lei, Deputy Dean of Research Institute of Lianhong New Materials Technology Co., Ltd.

First, Dr. Li Lei introduced the company. Lianhong New Materials Technology Co., Ltd. is a high-tech enterprise engaged in research, development, production and sales of advanced polymer materials and special chemicals. As a subsidiary company of Lianhong New Materials, Jiangsu Soviet Hong Technology Co., Ltd. is mainly engaged in the development, sale and service of ethylene oxide derivative surfactants, polyethers and functional products. Relying on Lianhong New Material, the company is located in the production base of Lunan High-tech Chemical Industrial Park, Tengzhou, Shandong Province. It uses its own ethylene oxide and adopts advanced PRESS 5th generation alkane oxidation production equipment. The company has strong product development ability, application service ability and marketing ability in the application fields related to surfactant and polyether products. Relying on the strong strength of Lenovo Holding Co., Ltd. and China Academy of Sciences Holding Co., Ltd., the company has helped Lenovo grow rapidly into a leading new material enterprise in the industry in terms of brand, innovation and talent.

Then, Dr. Li Lei introduced the four aspects: market analysis of suspension agent, selection of dispersant in suspension agent, case analysis of formulation, and selection and summary of dispersant.

Market Analysis of Suspension Agent

Pesticide registration is a means of pesticide management in various countries, an important gateway for the safe entry of pesticides into the market, and an umbrella for users, crops and the environment. Therefore, pesticide registration data have attracted wide public attention. Dr. Li Lei analyzed the registration of product dosage forms.

Judging from the cumulative registration data of product dosage forms, although the emulsifiable oil has been keeping ahead, the proportion has been declining gradually. The cumulative proportion of environmental protection formulations such as suspension agent, water agent and water dispersible granule increased gradually in that year, especially the rapid growth of suspension agent.

From the data of dosage form registration in that year, the data of environmental protection dosage form increased rapidly. In the year of suspension registration, the registration data have firmly occupied the first place and steadily increased. The proportion of water and oil suspension formulations increased gradually, while that of water dispersible granules increased steadily. The proportion of registered emulsifiable oils continued to decline, and the registration data of wettable powders tended to decline.

In the registration of new pesticides in recent years, the registration ratio of environmentally friendly formulations has been at a high level, and the proportion of environmentally friendly formulations is expected to continue to rise in the future. Since 2013, the proportion of new suspension pesticides registered has remained stable. Except for a few years, the registration of new suspension pesticides has been very few.

It can be seen that environmental friendly formulations are increasing rapidly, especially suspension agent is rising continuously. Suspension agent, as a new formulation with great prospects and rapid development, can not be underestimated in the future.

Brief Introduction of Suspension Agent and Relevant Difficulties

Suspension agent is a kind of stable liquid-solid system with fine particles and high suspension, which can flow by grinding and grinding with some additives and dispersing insoluble or slightly soluble solid raw materials evenly in water.

Suspension agent has developed rapidly in recent years, mainly because of its good dispersion, good surface spreading and adhesion, environmental protection, easy use and low cost. However, there are some problems in the research and development of this formulation, such as easy stratification, pasting and sensitivity to formulation system. The storage stability of suspension has always been a major problem restricting the development and production of suspension.

The unstable storage of pesticide suspension is due to the fact that suspension is a highly dispersed and complex multiphase system, and its composition includes not only effective components and dispersing media, but also many additives. It always exists in the form of a high concentration suspension. Chemical instability may occur during long-term storage, while physical stability is more often encountered.

The physical stability of pesticide suspensions usually involves three aspects: flocculation and aggregation caused by the interaction of particles; Austenite ripening; and stratification and particle settling caused by gravity. The causes of the problems can be explained by DLVO theory, Austenite ripening and Stokes formula.

Most of the poor physical stability of suspension is due to the flocculation and coagulation of particles in dispersed phase caused by collision of particles during Brownian motion, and even aggregation, as well as irreversible coagulation and aggregation caused by van der Waals force between particles (fig. 4). In order to improve this situation, enough protective layer can be provided to prevent the attraction and aggregation of particles, i.e. ionic dispersants with electrostatic repulsion, non-ionic dispersants with steric hindrance effect, or polymer surfactant dispersants with better performance.

The suspension system belongs to a thermodynamically unstable system. With the passage of time, the size and distribution of particles move towards larger particles, that is, crystallization growth. The process of forming large particles by consuming small particles is called Austenite ripening, which is caused by the difference of particle size and solubility. This problem can be solved by changing the processing technology to obtain narrow particle size distribution. Generally speaking, when the solubility of pesticide active ingredients in water is less than 100 mg/L, this phenomenon of crystallization and growth will rarely occur.

Under the action of gravity, the suspension tends to be stratified and precipitated. According to Stokes formula, the settling velocity (V) of spherical particles is proportional to the square of particle diameter (d2), the density difference of dispersed phase and the density difference of dispersed medium, and inversely proportional to the viscosity of dispersed medium. At present, any two changes have little effect on the settling velocity of particles. It is easy to prevent and delay the settling velocity of particles and improve the stratification and precipitation of suspensions by increasing the viscosity of dispersing media. However, it should be noted that the use of thickeners is improper and easier to cure.

In summary, to maintain the physical stability of pesticide suspension during storage, suspension accumulation, deposition and crystal growth can be controlled by formulation and processing technology.

How to choose dispersant in suspension agent?

Dispersant is used to inhibit the flocculation and coagulation of particles in the dispersed phase. It plays the role of stabilizing the suspension dosage form and promoting the dilution of dispersant into uniform suspension in water. It is beneficial for users to spray the target. At present, the main dispersants are anionic, non-ionic and polymer.

The anionic dispersant dissociates in water and forms a charge ion. The hydrophilic group adsorbs on the surface of the pesticide active ingredient particles. The hydrophilic group extends into the water phase and stabilizes the dispersed suspension through the electrostatic repulsion between the particles. At present, the commonly used dispersants in suspensions are lignin sulfonate, naphthalene sulfonate, alkyl naphthalene sulfonate formaldehyde condensate, sulfate and phosphate, etc. The efficiency of nonionic dispersants is related to their adsorption capacity. The hydrophilic group adsorbs on the surface of the pesticide particles and the hydrophilic chain extends into the water phase, forming a hydrate layer around the pesticide particles, which acts as a steric hindrance to prevent flocculation and coagulation caused by collision between particles. The non-ionic dispersant can effectively disperse in the suspension by changing the chain length of EO or PO and designing customized molecules. It is easy to correct the balance between hydrophilicity and lipophilicity. The non-ionic dispersants available in suspension agent are alkylphenol polyoxyethylene ether, aliphatic alcohol polyoxyethylene ether, polyarylphenol polyoxyethylene ether, EO-PO block copolymer non-ionic dispersant, etc. The larger the molecular weight of the dispersant, the stronger its adsorption capacity, not easy to desorb, and conducive to stability.

In recent years, a comb copolymer (polymer polycarboxylic acid dispersant) with poly (methacrylic acid) / methacrylate copolymer as its framework, its adsorption chain anchored on particles and suitable length of polyoxyethylene chain as hydrophilic chain extended into water has attracted special attention. The comb copolymer has high molecular weight, high steric resistance, no increase in viscosity when dissolved in water, and is very effective in stabilizing high concentration suspension agent.

Case Analysis of Formulation

How to choose a suitable dispersant in a formulation system? There are three main factors to be considered: 1) good wetting effect on the surface of dispersed pesticide particles; 2) effective reduction of particle size and system viscosity during grinding; 3) formation of stable suspension dispersion.

The molecular weight, charge and configuration of dispersants determine whether dispersants can be effectively adsorbed on the surface of particles, which directly affects the dispersing and stabilization of dispersants. Therefore, in the development of suspension agent, it is particularly important to select the appropriate dispersant. The structure and type of the selected dispersant determine the stability mechanism and properties of suspension agent.

Dr. Li Lei illustrated with three typical dispersant cases.

First case: high content. Taking 720 g/L chlorothalonil SC as an example, the combination of polycarboxylic acid dispersant and anionic dispersant can solve the problems of easy pasting and caking. From the appearance, it can be seen that after two weeks of hot storage at 70 C, the incoming water is well dispersed, no pasting, obvious stratification, and the growth trend of particle size is small.

Second case: low melting point. Taking 25% pyrazole ether ester SC as an example, choosing block polyether dispersant and anionic dispersant together can solve the problems of easy pasting, caking and stratification. After heat storage at 54 C, there was no obvious stratification and water analysis, no pasting phenomenon, and the particle size of the sample increased within 2 micron, with a small growth trend.

The third case: high water solubility. Taking 600 g/L imidacloprid SC as an example, we choose block polyether dispersant and polycarboxylic acid dispersant to use together, which can solve the problems of easy crystallization and pasting of the formula. After heat storage at 54 C, the incoming water has good dispersion, no pasting, obvious stratification, and the growth trend of particle size is small.

Summary of Dispersant Selection

1. Polycarboxylic acid, block polyether, phosphate and sulfonate dispersants have better dispersing and suspension effect in suspension.

2. Polycarboxylic acids and phosphate esters have good viscosity reduction effect in suspension.

3. For the high content suspension agent, the combination of polycarboxylic acid and phosphate ester will have a good dispersion and suspension effect.

4. Active ingredients with low melting point can be used as suspension agent and polymer block polyether and sulfonate can effectively prevent the excessive growth of particle size and pasting.

5. For products with high water solubility of active ingredients, the combination of polycarboxylic acid and block polyether can effectively prevent pasting and particle size growth due to Austenitic ripening.

Finally, Dr. Li Lei said that pesticides will remain one of the main means to combat diseases, insects and weeds for a long time to come. Suspension agent as a new formulation will have great potential for development. The demand of pesticide water suspension agent for surfactant will certainly promote the development of surfactant industry. We sincerely hope that the synthesis and production of surfactant in China and the research and development of pesticide water suspension agent will work hand in hand to create a better future.