Flame Retardancy And Synergistic Effects Of Microencapsulated Ammonium Polyphosphate In Thermoplastic Polyurethane

Thermoplastic polyurethane (TPU) is one of the most versatile engineering thermoplastics with elastomeric properties. Due to its excellent physical properties, chemical resistance, abrasion resistance and good adhesion, TPU has been widely used incoatings and films. However, its flammability and melt dripping nature restricts its applications, it is therefore important to improve flame retardancy of TPU. The intumescent flame retardant (IFR) system is an effective strategy to impart polymeric materials with improved flame retardancy.In this paper, the development of flame retardants were reviewed as well as the reserch in the flame retardant thermoplastic polyurethane and the improvement of intumescent flame retardant (IFR) systems based on many literatures and the relative works in our group firstly. The Microencapsulated ammonium polyphosphate (MAPP) with polyurethane resin and boron phosphate (BP)/ ferric pyrophosphate(FePP) were chosen to be flame retardant and synergist in the thermoplastic polyurethane, respectively. Results showed that a suitable substitution of MAPP by BP or FePP could improve flame retardancy of the TPU/MAPP composites, and TPU composites with lower MAPP/BP (or MAPP/FePP) achieving UL-94 V-0 rating. The CONE and MCC data also showed synergistic effects between BP or FePP with MAPP in the composites. Besides, the thermal degradation and the mechanism about the synergetic effects were studied.The main research results are as following:(1) Microencapsulated ammonium polyphosphate (MAPP) with polyurethane resin has been prepared by in situ polymerization. The combination of MAPP or APP on the flammability properties of thermoplastic polyurethane (TPU) was studied by vertical burning (UL-94) tests and limiting oxygen index (LOI) tests. UL-94 results showed that the optimum mass ratio of MAPP or APP was 25% in the thermoplastic polyurethane composites.(2) Both the boron phosphate (BP) and ferric pyrophosphate(FePP)were chosen as synergist in the flame retardant system. The combination of MAPP and synergist on the flammability properties of thermoplastic polyurethane (TPU) was studied by vertical burning (UL-94) tests, limiting oxygen index (LOI) tests, cone calorimetry (CONE) and microscale combustion calorimeter (MCC); the morphology for char residual was also observed by photographs and scanning electron microscope (SEM). Results showed that a suitable substitution of MAPP by synergist could improve the flame retardancy of the TPU/MAPP composites, and TPU composites with lower MAPP/synergist could achieving UL-94 V-0 rating. LOI of the composites also increased a little after adding synergist. The heat release rates (HRR) and weight loss rates of TPU/MAPP composites were also considerably decreased by the synergist. The CONE and MCC data showed synergistic effects between synergist and MAPP in the composites.(3) Thermal stability was investigated by thermogravimetric analysis (TGA) and real time Fourier transform infrared (RTFTIR). Boron phosphate and ferric pyrophosphate can improve the thermal stability of the composite at higher temperature from the TGA and the RTFTIR data. They also make the char residual of the composites more compact and continuous from their photographs and SEM, but had different synergistic effects: in BP system, during heating, polyphosphoric acid produced by the elimination of ammonia from APP could attack B-O bonds. The formation of boric acid appeared in the surface of residue, and adsorb combustible gases to make a deoxidizel atmosphere, made the char structures more stable. The (PO4)3- could promoted the release of ammonia from APP. Also APP could react with [B(PO4)x]n+, which took as bridges, the formation would brought about a stabilization of the composites. In FePP system, there is also a [Fe(PO4)x]n+, brought about a stabilization char layers, ferric also can capture the free radical in the combustion reaction.(4) The thermal stability and the degradation kinetics of the BP/MAPP/TPU and the FePP/MAPP/TPU systems were studied. Results showed that BP/MAPP/TPU system has a higher activation energy.