Self-assembly And Flame Retardant Synergistic Mechanism Of NCC/APP Polyelectrolyte Complex

Polyelectrolyte was kind of high polymer with ionizable groups in the molecular chain,which was characterized by hydrophilicity and chargeability.The self-assembly of a suitable anionic and cationic polyelectrolyte into a polyelectrolyte complex(PEC)could effectively control the flame retardant properties of the material and show great potential in the field of flame retardancy.In this paper,nanocrystalline cellulose(NCC)and ammonium polyphosphate(APP)doped nanocomposites were used as anionic polyelectrolytes,and polyethyleneimine(PEI)was cationic polyelectrolyte could formed anionic and cationic polyelectrolyte complex.The synthesis mechanism of the polyelectrolyte complex was systematically studied.The polyelectrolyte complex was applied to wood-plastic composites(WPC)to further study its effect on the thermal properties,flame retardancy and mechanical properties of wood-plastic composites.The flame retardant mechanism of wood-plastic composites was demonstrated.The details were as follows:(1)The NCC used as template to self-assemble APP to form an anionic polyelectrolyte.APP and NCC form intermolecular hydrogen bonds.The thermal analysis results show that the initial decomposition temperature of the anionic polyelectrolyte was 298°C.Compared with APP,the initial decomposition temperature shifts to the low temperature direction by 21°C.NCC could promoted the early pyrolysis of the anionic polyelectrolyte.When the amount of NCC was added9%of APP,the anion polyelectrolyte formed by self-assembly had a carbon residual ratio of 52.8%at 700°C and increased by 28.5%compared with that of APP.The anionic polyelectrolyte could released non-combustible gas to form dense char layer,which was characterized by excellent flame retardant properties.Further introducing the anionic polyelectrolyte into the wood-plastic composite material as flame retardant,the results showed that the limit oxygen index(LOI)was27.1%,which was significantly higher than that of APP(26.1%).The Average heat release rate,Peak heat release rate and total heat release(THR)of decreased by 5.1%,3.3%and 5.4%compared with those of WPC/APP,respectively.Compared with those of WPC/APP,The tensile strength and elongation at break increased by 68.2%and 117%,respectively.Howerer,the impact strength decreased by 5.4%.There were rich hydroxyl groups in NCC,which could form hydrogen bonds with wood fibers and improved the interfacial bonding between anionic polyelectrolyte and wood-plastic composites.However,the introduction of anionic polyelectrolytes as rigid materials led to the decrease in the impact strength of wood-plastic composites.(2)The NCC and APP doped nanocomposites as anionic polyelectrolytes,polyethyleneimine(PEI)as cationic polyelectrolyte,respectively.The primary amine carried on the PEI molecular chain undergoes ionic bonding reaction with the surface hydroxyl group of the NCC and APP.At the same time,the PEI adsorbs the anionic polyelectrolyte NCC and APP by electrostatic force to prepare polyelectrolyte complex having crosslinked structure.When the ratio between NCC,PEI,and APP was 9:9:100,the initial decomposition temperature of the polyelectrolyte complex formed by self-assembly was 262°C,Compared with APP,the initial decomposition temperature shifts to the low temperature direction by 57°C.A large amount of non-combustible gas released by the polyelectrolyte complex formed uniform and dense char layer,which has strong adsorption effect on the smoke generated during the combustion process,improved the flame retardant property of the wood-plastic composite.The polyelectrolyte complex was further introduced into the wood-plastic composite,showing the LOI was 24.4%,which was significantly improved compared with that of WPC/APP(23.9%).The peak heat release rate,THR and total smoke release(TSR)decreased by 25.2%,6.8%and 27.8%,respectively,compared with those of WPC/APP.In addition,good dispersibility in PEI could improved the interfacial compatibility between polyelectrolyte complex and wood-plastic composites.The tensile strength,impact strength and elongation at break of wood-plastic composites treated by flame retardant have been obviously improved.When the ratio of NCC,PEI and APP was 9:9:100,self-assembly forms polyelectrolyte complex,and when it was added at 15%introduced into WPC,its tensile strength,elongation at break and impact strength were22.28MPa and 10.30%,6.77KJ/m~2 increased by 8.4%,44.6%and 21.3%compared with WPC/APP,respectively.