Examples of prepolymer preparation
1. Dehydration of polyester polyol (in dehydration kettle): heat and melt polyester with mn2000 (hydroxyl value 53-59, mgkoh/g, melting point 40-50 C) in a stirred stainless steel or enamel kettle. The dehydration temperature is 100-140 C (polyether 100-110 C). The polyester polyol is vacuum dehydrated for 30-60 minutes, the residual pressure is 5mmHg, and the moisture content is less than 0.05% (moisture content). Dehydration can be seen with a mirror. In order to fully degas the liquid in the tank, the mixing device can be started and stopped intermittently. After vacuuming for a period of time, the heating of the reaction tank can be stopped to lower the temperature of the material to 70-80°C.
2. Prepolymer synthesis (in the reactor); the performance of the product depends on the raw materials used, the ratio and the processing technology. First, select the appropriate raw material according to the performance requirements such as temperature, stress, contact medium and service life, and then select the appropriate ratio and process conditions (such as mixing temperature, pouring and demoulding time, vulcanization conditions, etc.). Hardness upon request. In order to ensure the quality of the prepolymer, the dehydration of the polyester and the synthesis of the prepolymer should not be carried out in the same kettle, and the synthesis of the prepolymer should be carried out under the protection of dry nitrogen. In order to prevent violent reaction and rapid temperature rise, a cooling device must be provided. Suggested feeding order: first add TDI, then add polyester. When the reaction is violent, polyester can be added step by step to make the reaction process stable and the reaction temperature easy to control. The content of free isocyanate monomer in the synthesized prepolymer is low and the structure is more regular. When the isocyanate content is kept at 80±5°C and the reaction time is 1≤2 h, determine the isocyanate content. Domestic and foreign brand prepolymers that meet the requirements can also be purchased under possible conditions.
3. Desorption of the prepolymer (can be carried out in the polymer synthesis reactor or the storage tank of the filling machine), defoaming at 85±5°C and 5 mmHg residual pressure for 30 minutes.
