The synthesis methods of TPU can be divided into two categories based on the presence or absence of solvents: solvent-free bulk polymerization and solvent-based solution polymerization. Bulk polymerization can be further divided into one-step and prepolymer methods according to the reaction steps. The one-step method involves mixing oligomer diols, diisocyanates, and chain extenders simultaneously to form. The one-step process is simple and easy to operate, but its reaction heat is difficult to eliminate and side reactions are prone to occur. A polyester based thermoplastic polyurethane elastomer was synthesized using a one-step method. Firstly, the formula amount of polyester polyol and chain extender, butanediol, were weighed in a reactor and heated to 120 ℃ for vacuum dehydration. Quickly add the preheated and quickly stir evenly, pour it into the preheated container, vacuum bake at 120 ℃, and then cool to 100 ℃ to obtain a light yellow translucent polyurethane product. Afterwards, press it into test pieces on a flat press, and the prepared TPU has high mechanical and damping properties.
The prepolymer method involves the reaction of oligomer diols and diisocyanates, followed by synthesis with a dry chain extender under a small amount of catalyst conditions. The process of prepolymer method in production is complex, consumes high energy, and the viscosity of the produced prepolymer is high, which increases the difficulty of process operation. However, the prepolymer has fewer side reactions and the resulting product performs better than the one-step method.
According to the continuity of the reaction process, it can be divided into intermittent method and continuous method. The commonly used production equipment for intermittent method includes automated pouring equipment, curing ovens, crushing hammers, extruders, etc., which have low production efficiency, uneven product quality, and are not suitable for large-scale production. Therefore, continuous production processes and equipment have been studied both domestically and internationally. The continuous method equipment is a reaction extrusion production line, which mainly includes raw material storage tanks, pouring machines, parallel twin screw extruders, underwater granulators, separation and drying equipment, and packaging equipment. The twin screw continuous reaction extrusion method is currently the mainstream production process, with high production efficiency and stable product quality, suitable for large-scale production. It can be used for coatings, elastomers, and adhesives.