Fire Performance of Intumescent Flame Retardant/Nanosilica/Thermoplastic Polyurethane Composite: A Study on Synergism

Hypothesis: 

The aim of this research is to improve the fire behavior of thermoplastic polyurethane (TPU) using an intumescent flame retardant system (IFR) and nanosilica as a synergistic agent 

A three-component IFR system consisting of ammonium polyphosphate (APP), melamine polyphosphate (MPP), and pentaerythritol (PER) was added to TPU by melt mixing. The flammability of the TPU-IFR was evaluated by UL94 vertical burning test and the efficiency of the IFR was investigated by cone calorimeter test (CCT). A nanosilica as a synergist with low loading was added to the TPU-IFR composite, and fire properties were investigated. Thermal stability and char morphology were investigated by thermal analysis and scanning electron microscopy, respectively.

The results show that the IFR system is effective with a significant decrease of 62.6% in peak heat release rate (PHRR) and 58.3% in peak smoke production (pSPR). By incorporation of 0.5% (by wt) nanosilica into TPU-IFR, there are decreases in PHRR and pSPR by 75.0% and 79.2%, respectively, compared to the original TPU, while the dripping is removed. This has confirmed the effective synergism of nanosilica in enhancing the flame retardancy of TPU-IFR. Further, the amount of residual char has reached 31.2% and 58.9% for TPU-IFR and TPU-IFR-nanosilica, respectively, compared to 6.2% in a neat TPU. Both TPU composites have reached V0 grade in UL-94 test. FESEM shows an integrated compact char in IFR-TPU-nanosilica, while there are small holes in the char structure of IFR-TPU. Thermal analysis (TGA) has shown enhanced thermal stability in the two TPU composites by formation of a carbon layer as a thermal barrier during burning. This work introduces an efficient intumescent flame retardant system for improving the fire behavior of TPU which can significantly enhance the fire safety of TPU by a low loading of nanosilica as a synergist.