Viscosity is an important indicator that macroscopically reflects the degree of polymerization of materials, and it is also a concrete manifestation of the degree of reaction in the polymerization process.In the synthesis process of polyurethane prepromers, when the oligomer polyol is polymerized with polyisocyanate, as the reaction time continues to extend, the degree of reaction gradually deepens, and the viscosity of the reaction material, the polyurethane prepromer, will increase accordingly, which will bring difficulties to production operations.
The main reasons for the increase in viscosity of polyurethane prepomer materials are:
1.The choice or process of raw materials must be clear, because the fluidity of different raw materials is different, and the fluidity of polyester in polyurethane prepolders is much worse than that of polyether, so when synthesizing polyester prepolders, their operability is also relatively poor; if you must choose polyester prepolders for reaction, you can choose unsaturated polyester, the effect will be better; of course, you can also use the semi-prepolerization process to change the disadvantages brought about by polyester prepolders.
2.The determination of the ratio of raw materials, in the ratio of raw materials, the increase of the content of the hard section will accelerate the degree of reaction and the viscosity of the material will become larger. Therefore, the mass ratio of the hard section to the soft section is adjusted, and then their ratio is determined through experiments.
3.The choice of reaction temperature and time, the higher the reaction temperature or the longer the reaction time, the stronger the degree of polymerization of the material and the greater the viscosity of the material. The equilibrium point of reaction temperature and time must be adjusted through tests.
In the course of many tests, a method of how to adjust the viscosity was drawn. Specifically, when the material reacts to a certain extent, the NCO value of the reaction material is sampled to determine its NCO value, and compared with the pre-designed NCO value to see if it is close. If the NCO value is close to the design value, cool down and stop heating, and prepare for discharge and storage.The best experimental method is to make a diagram of the relationship between the NCO value and the viscosity of the reaction material to facilitate the experiment.This method is suitable for the synthesis test of polyurethane prepromers in the small or pilot stage.
In addition, the relationship between the viscosity of the polyurethane prepomer and the NCO value was determined from the pilot and pilot tests. According to their relationship ratio, and under several specific ratios, the speed of the agitator was measured, and the viscosity/NCO ratio and the speed of the agitator were made. The diagram of the relationship between the viscosity/NCO ratio and the speed of the agitator is used to determine whether to enter the next step.This method is suitable for large-scale production of polyurethane prepromers.