Preparation methods and principles of microcapsules
Traditional microcapsule preparation methods can be roughly divided into three categories: physical methods, physical and chemical methods, and chemical methods. The physical method uses physical and mechanical methods to prepare microcapsules, and its pioneer is D.E. Wurster in the United States. In the late 1940s, he first used the air suspension method to prepare microcapsules and successfully applied it to the field of drug coating. The air suspension method is still called the Wurster method. B.K. Green of NCR Company in the United States is a pioneer in the preparation of microcapsules using physical and chemical principles. In the early 1950s, he invented the phase separation complex coacervation method to prepare oil-containing gelatin microcapsules, obtained a patent, and used it to prepare carbonless copy paper, which was a great commercial success, thus opening up the physical chemistry based on phase separation. A new field in the preparation of microcapsules. From the late 1950s to the 1960s, people began to study the application of polymerization methods of synthetic polymers to the preparation of microcapsules, and published many patents for the preparation of microcapsules using chemical methods based on polymer polymerization reactions. Among the specific methods, interfacial polymerization, in-situ polymerization and condensation methods are the most widely used.
The technical characteristics of the interfacial polymerization method are: the two reaction monomers exist in the immiscible dispersed phase and the continuous phase in the emulsion, and the polymerization reaction proceeds on the phase interface, as shown in Figure 1 . This process for preparing microcapsules is convenient, simple, has fast reaction speed and good effect. It does not require expensive and complicated equipment and can be carried out at normal temperature. Although this method is simple, it has higher requirements on coating materials. The coating monomer must have high reactivity and can carry out polycondensation reaction.
In-situ polymerization method: all monomers and initiators are placed outside the capsule core, and the monomer is required to be soluble, while the resulting polymer is insoluble, and the polymer is deposited in the capsule. The surface of the core material is coated to form microcapsules. Many polymer reactions, such as homopolymerization, polymerization and condensation polymerization, can be used to prepare microcapsules by in-situ polymerization. Both the in-situ polymerization method and the interfacial polymerization method use monomers as raw materials and use synthetic polymer materials as shell materials. The difference between the two methods is that during the interfacial polymerization microencapsulation process, both the dispersed phase and the continuous phase must be able to provide monomers, and two or more incompatible monomers are dissolved in incompatible monomers. In two phases; for in-situ polymerization, the monomers are only provided by one of the dispersed phase or the continuous phase. When comparing the two methods, it can be seen that the interfacial polymerization method produces microcapsule shells at a much faster rate than the in situ polymerization method.
The characteristics of the condensation method are to use methods such as changing the temperature or adding inorganic salt electrolytes and non-solvents of film-forming materials to the solution, or creating conditions to induce the mutual bonding of two film-forming materials, so that the wall material solution Phase separation occurs and two new phases are formed, one is a polymer-rich phase with a high concentration of wall material, also known as the encapsulation material phase, and the other is a polymer-deficient phase with very little wall material. The formed polymer-rich phase is fully mobile and can stably and gradually surround the core particles. The phase separation step is key to the preparation of microcapsules. The condensed phase separation method can be divided into water phase separation method and oil phase separation method according to the dispersion medium. The water phase separation method is divided into compound coagulation method and simple coagulation method according to the different film-forming materials.
The composite coagulation method is the most commonly used method among the coagulation methods, as shown in Figure 2. This method is to use two or more linear random polymer materials with opposite charges as the wall material, and disperse the capsule core material in the aqueous solution of the wall material. Under appropriate conditions (changes in pH value, temperature changes , dilution, addition of inorganic salt electrolyte), causing electrostatic interaction between oppositely charged polymer materials. The solubility of the polymer material decreases and phase separation occurs, and the colloid condenses out of the solution. Microencapsulation of gelatin and gum arabic in the aqueous phase through complex coacervation can coat many hydrophobic liquids. The core materials that can be coated using this method mainly include pressure-sensitive copy paper materials, adhesives, foaming agents, heat-sensitive recording materials, liquid crystals, photo developers, epoxy resin curing, magnetic materials, etc.