Synthesis Of A Novel Hyperbranched Poly (Phosphorodiamidate) And Its Application To APP/PP Composites

Polypropylene?PP?is widely used in commercial and industrial applications,due to its excellent comprehensive performance.However,PP resin is flammable.Therefore,it is essential to develop the flame-retarded PP composites.The common flame retardants used for PP are intumescent flame retardants?IFRs?.IFRs compose of acid source,blowing agent and carbonization agent.The most widely used acid source is ammonium polyphosphate?APP?,which is not efficient for PP when being used alone.Meanwhile,the conventional charring agents?pentaerythritol,dipentaerythritol,starch,sorbitol?have such drawbacks as low charring efficiency,high addition amount and so on.Hence,more and more researches focus on the development of macromolecule charring agent and its combination with APP to enhance the flame-retarding efficiency of composites.In this article,a novel hyperbranched poly?phosphorodiamidate??HBPPDA?with high expansion rate and carbonation efficiency was successfully synthesized by A₂+B₃ polycondensation method from melamine?B₃ monomer?and phenylphosphonoic dichloride?A₂ monomer?.The chemical structure was characterized by Fourier transform infrared?FTIR?,nuclear magnetic resonance?1H NMR?and X-ray photoelectron spectroscopy?XPS?.The results showed HBPPDA with hyperbranched structure.The thermal degradation process of HBPPDA and the volatile products produced during thermal degradation were analyzed by thermogravimetric analysis/infrared spectrometry?TG-IR?.The scanning electron microscopy?SEM?and FTIR showed that the char layer was smooth,continuous and compact,possessing the P-O-C and P-O-P groups.The synthesized HBPPDA combined with APP was loaded into PP.Under the same loading of 25 wt%,the limiting oxygen index?LOI?for APP/PP and HBPPDA/PP composites were increased from 18.0 vol% to 21.2 vol% and 23.6vol% respectively.However,the HBPPDA/APP/PP?APP:HBPPDA=3:1?composite achieved V-0 rating at vertical burning?UL-94?test,and its LOI value was increased to 30.6 vol%.Cone calorimetry test?CCT?showed that the heat release rate peak?PHRR?of APP/PP composite was 473 kW/m² and that of HBPPDA/PP composite was 263 k W/m²,compared with 902 kW/m² for PP.However,the PHRR of the HBPPDA/APP/PP composite was reduced to 215kW/m²,with the reduction of 76.2% compared with pure PP.Its total heat release?THR?was reduced from 113 MJ/m² of PP to 66 MJ/m²,the smokeproduction release?SPR?was decreased from 0.100 m²/s of PP to 0.036 m²/s,and the total smoke production?TSP?droped from 15.3 m² of PP to 6.6 m².Moreover,the HBPPDA/APP/PP composite had a tensile strength of 28.39 MPa,which is significantly greater than that of APP/PP?24.09 MPa?.The TG curves showed that the char residue of experimental value?12.7wt%?for the HBPPDA/APP/PP composite was higher than the theoretical one?11.4 wt%?at 800 oC due to the synergistic effect of HBPPDA and APP.The Raman results indicated that the HBPPDA/APP/PP composite had the highest degree of graphitization: APP/PP?2.39?> HBPPDA/APP?2.26?>HBPPDA/APP/PP?1.83?.XPS results showed a large number of P-O-P and benzene ring structures existed in the char residue of HBPPDA/APP/PP composite,contributing to the compact and stable char layer.Finally,the reasons for HBPPDA to significantly improve the flame retardancy of APP/PP composites were described in detail,and the synergistic flame retardant mechanism of HBPPDA and APP was further discussed.