Preparation And Properties Of MDI-based Polyurethane Prepolymer With Low Viscosity

Polyurethane prepolymer is widely used in adhesives, coatings, elastomers and foam fields.4,4-Diphenyl methane diisocyanate (MDI) and polypropylene glycol (PPG) are used as raw materials, the effects of bulk polymerization conditions on the synthesis kinetics, viscosity and storage stability of prepolymer are studied. Firstly, the prepolymer properties under different polymerization temperatures are researched to investigate the kinetics and molecular weight characteristics. Secondly, in order to improve the trimer content of the prepolymer, dropwise method to undertake the prepolymer synthesis and performance was envisaged. The lower polymerization temperature is, the lower viscosity of the prepolymer will be during the reaction. It’s interesting to study the length of the low temperature polymerization during the experiment and the temperature at different stages of prepolymer synthesis. The effects of acid concentration on the polymerization kinetics, and prepolymer molecular weight, viscosity and storage stability were investigated. The results can be regarded as the guidance for the industrial production of prepolymer.Polymerization between MDI and PPG at different temperatures follow the second-order reaction. It showed that polymerization rate increased with increasing temperature, the activation energy Ea=62.38kJ-mol-1. The prepolymer is pseudoplastic fluid, its viscosity decreases with the shear rate increases. Excellent storage stability of prepolymers was got at T+20℃and T+30℃. Different feeding methods at T+10℃, dropping PPG to MDI within0,2and4hours respectively, indicated that the degree of reaction of the prepolymer synthesized from dropping method will continue to rise during storage, which is different from the one-time feeding method. As for the newly synthesized prepolymer, the viscosity was lower than the one-time feeding, and the longer the dropping was, the lower viscosity was got. However, during storage, prepolymer dropping the longer, the greater it changed in viscosity, cooling intermediate dropping gave the prepolymer the lowest viscosity. Increased the length of cooling time was good for reducing viscosity of the prepolymer, lowering the temperature in the later synthesis stage can also reduce the viscosity of the prepolymer. Changing the acidity will significantly accelerate the prepolymer reaction, and greatly increase the viscosity of the prepolymer.