Synergistic effect of flame retardants and carbon nanotubes on flame retarding and electromagnetic shielding properties of thermoplastic polyurethane

Thermoplastic polyurethane (TPU)-based composites filled with carbon nanotubes (CNTs) and intumescent flame retardants (IFRs) were fabricated through melt compounding method. For comparison, TPU/IFR and TPU/CNT composites with different filling proportions were prepared, respectively. The results presented that the addition of 1 wt% CNTs and 10 wt% IFRs into TPU could achieve good flame retarding and electromagnetic interference (EMI) shielding properties simultaneously. With regard to flame retardancy, the compounding of CNTs and IFRs effectively suppressed the heat release rate and the UL-94 rating was upgraded to V-0 accompanied with prompt self-extinguishment and disappeared melt-dripping, which was equivalent to incorporate 20 wt% IFRs alone. However, the flame retardancy became deteriorated by loading more CNTs. It was revealed that the addition of CNTs could strengthen the carbonization effect of IFRs, but suppress their intumescent effect. Hence, a right amount of CNT loading was of critical importance to achieve synergistic effect with IFRs. Moreover, compared with the TPU/CNT system, the TPU/IFR/CNT system possessing the same CNT contents had a lower percolation threshold. When 1 wt% CNTs were introduced, the electrical conductivity was increased about two orders of magnitude and EMI shielding effectiveness was doubled in the presence of IFRs. The morphological observation and rheological analysis demonstrated that mixing with IFRs not only improved the dispersion of CNTs through particle collisions, but also benefited for establishing the continuous conductive pathways among the IFR particles. Accordingly, this work provided a promising and facile route to fabricate EMI shielding polymeric composites with excellent flame retardancy.