Thermoplastic polyurethane with good mechanical and processing performances via blocking and deblocking of isocyanates

Increasing the molecular weight of polyurethanes (PU) is an effective way to improve its mechanical properties, while, it is at the expense of sacrificing processability. Congruent modification in both mechanical and processing performances is still a challenge. Here, bis-phenol-A (BPA) derivatives, including tetrachlorobisphenol-A and tetrabromobisphenol-A, were chosen as the extender to synthesize thermoplastic PUs (TPU) containing blocked isocyanate units. This blocked isocyanate has a temperature-responsive reversible reaction due to phenol urethane bonds, so as to ensure the good mechanical properties at the service temperature and better processing performance via deblocking into smaller molecular weight at the processing temperature; in addition, negative induction effect of the halogen substituents endow BPA derivatives with deblocking reaction at lower temperature. The experimental results confirmed that the blocking reaction carried out completely at 70 degrees C, and accompanying by the dramatical increase of molecular weight. The obvious deblocking reaction happened at 140 degrees C, where the average molecular weight of TPUs decreased from 22,600 g/mol to 13,690 g/mol. Meanwhile, the melt flow index increased significantly with the extension of heating time, indicating that the PU has good processing properties. After the deblocking and re-blocking reactions were repeated alternatively, there is almost no change in its tensile properties, molecular weight and melt outflow index within two cycles under air atmosphere. This article proposes a strategy of congruent modification in both mechanical and processing performances of TPU.

Automated summary

The article discusses a method to improve the mechanical and processing properties of thermoplastic PUs (TPU) by using blocked isocyanate units.