The preparation methods of coated red phosphorus flame retardants are diverse. The following are some common preparation methods:
Inorganic coating
The inorganic substances are used as the coating materials, and the chemical deposition method and sol-gel method are adopted. For instance, taking the coating of aluminium hydroxide as an example, the chemical deposition method involves suspending red phosphorus in a water medium containing aluminium hydroxide, and then adding sodium hydroxide to adjust the pH value of the solution to between 6 and 8, so that the generated aluminium hydroxide deposits on the surface of the red phosphorus to form a uniform and dense coating layer.
Organic coating
The coating is carried out by using organic matter as the coating material through interfacial polymerization or in-situ polymerization. Thermosetting resins are mostly used, such as phenolic resin, melamine resin, furfuryl alcohol resin, aniline resin, epoxy resin and alkyd resin, etc. For instance, a certain proportion of melamine and formaldehyde are placed in a reaction flask, heated to 70℃, and a 10% sodium carbonate solution is added to keep the system weakly alkaline with a pH value of 8 to 9 all the time. Thirty minutes later, melamine was completely dissolved to obtain a transparent melamine resin prepolymer solution. Add a certain proportion of dispersant, red phosphorus and distilled water to the reaction flask and stir thoroughly. Then add a certain amount of prepolymer and stir and disperse at 60℃. Cool to room temperature, adjust the pH value of the material to 3-6 with acetic acid, slowly increase the temperature to 60℃, keep it warm and stir for 2 hours, cool, and adjust the pH value to 8-9 with 10% sodium carbonate solution. After filtration, washing and drying, red phosphorus coated with melamine resin is obtained.
Inorganic-organic composite coating
First, coat the red phosphorus with an inorganic layer, such as aluminium hydroxide, zinc hydroxide, etc., and then coat it with an organic layer, such as phenolic resin, melamine-formaldehyde resin or epoxy resin, etc. This method is currently a relatively ideal process for the surface modification of red phosphorus.
Multi-layer coating
A study has prepared microencapsulated red phosphorus with four coating layers, from the inside out, they are metal hydroxide, liquid compound containing phosphate esters, metal hydroxide, and resin and initiator that can undergo polymerization reactions. This method can further enhance the thermal stability of red phosphorus and simultaneously improve the mechanical properties of the engineering plastics modified with it.
Hydrothermal synthesis method
A dispersant and inner wall material are added to the red phosphorus solution, which is processed by grinding equipment to obtain a mixed solution. Then, it is transferred to a hydrothermal reactor and reacted at 150 to 200℃ for 20 to 24 hours. After cooling and pouring out, waterborne polyurethane resin is added and mixed evenly. The final product is obtained by drying with a spray dryer. The coated red phosphorus obtained by this method has a smaller particle size and its performance has been improved.
Bubble liquid film method
The aqueous solution of raw materials, the aqueous solution of sedimentation agent, ultrafine red phosphorus particles and surfactants were added to the bubble liquid film reactor to prepare the foam of ultrafine red phosphorus composite flame retardant. After filtration, washing and drying, the ultrafine red phosphorus composite flame retardant was obtained. The coated red phosphorus obtained by this method is a whiteness product with a wide range of applications and is safe. The process is simple and concise, which is conducive to industrial production.
