An intumescent flame retardant polypropylene system with simultaneously improved flame retardancy and water resistance

Abstract A traditional intumescent flame retardant (IFR) has very poor water resistance due to the existence of ammonium polyphosphate (APP). How to improve the water resistance of APP or IFR without sacrificing its flame retardancy is an important issue for the wide application of IFR. In this study, the coated APP with polysiloxane shell (Si-APP) was prepared by in situ polymerization, and was used to flame retard polypropylene (PP) together with charring agent (CA). The resulting Si-APP was characterized by Fourier transform infrared spectra (FTIR), transmission electron micrographs (TEM) and thermogravimetric (TG) analysis. The water solubility of Si-APP was also investigated. The thermal stability and combustion behaviors of PP/Si-APP/CA and PP/APP/CA composites were also investigated through TG, limiting oxygen index (LOI), vertical burning test (UL-94), and cone calorimeter (CC) test. The results showed that the coated APP with polysiloxane shell could significantly improve the flame retardancy of IFR PP systems. At a total flame retardant loading of 25 wt%, the LOI value of PP/Si-APP/CA was 34.0%, which was higher than 30.6% of PP/APP/CA, and the UL-94 rating of the former was V-0 in the case of the specimen thickness of 1.6 mm, while the later was V-2 rating. CC test results showed that the average value of heat release rate (HRR), the total heat release (THR), and the peak of smoke production rate (SPR) of PP/Si-APP/CA decreased in comparison with PP/APP/CA system, especially, THR decreased by 50.0%. Further, the char residue of the former increased significantly compared with the latter, greatly increased by 238.9%. In addition, the thermal stability and water resistance of IFR PP composites were also improved due to the modification of APP. The mechanism for the improvement of flame retardancy was also discussed based on the experimental results. All these results illustrate that the coating of APP with polysiloxane shell is an efficient method to improve the flame retardancy and water resistance of APP-containing IFR PP systems.