Study On The Influence Of Morphological Structure Of Polyphosphazene Micro/nano-materials On PET Flame Retardancy

PET,the most produced and used thermoplastic polyester,is easily ignited and has a low viscosity due to its linear structure,resulting in a large number of molten droplets with fire,leading to catastrophic consequences.Therefore,the development of high-performance flame-retardant PET materials has become a research trend.As people's awareness of environmental protection increases,the most used halogen flame retardants are gradually being restricted.Polyphosphazene micro-nano materials have become a promising flame retardant system as an environmentally friendly flame retardant because of their high phosphorus and nitrogen content and easy to prepare controlled morphology structure.Many researchers have developed polyphosphazene flame retardants with various morphologies and structures,but there is no systematic study on the effects of polyphosphazene micro-nano materials with different morphologies,sizes and functional groups on the flame retardancy and mechanical properties of PET.Meanwhile,there are weak interactions between flame retardants and substrates such as hydrogen bonding and?-?stacking,and the effects of such weak interactions on the flame retardancy and mechanical properties of the materials have not been reported.In this paper,polyphosphazene micro-nano materials with different morphologies,sizes and functional groups were prepared in a controlled manner using hexachlorocyclotriphosphazene(HCCP),bisphenol S(BPS)and 4,4'-diaminodiphenyl sulfone(DDS)as reaction monomers,respectively.The different polyphosphazene micro-nano materials were added into PET,and different PET flame retardant composites were prepared after melt blending to investigate the effects of morphology,size and functional groups of flame retardants on the flame retardant properties and mechanical properties of the composites.The effect of the weak interaction between different sizes and functional groups of polyphosphazene micro-nano materials and PET molecules was investigated in combination with molecular simulation.And the effect of the weak interaction on the flame retardant properties and mechanical properties of the flame retardant PET composites was studied to enrich the flame retardant mechanism of polyphosphazene micro-nano materials on PET and provide theoretical guidance for the molecular structure design of polyphosphazene micro-nano materials.(1)Preparation of polyphosphazene micro-nano materials with different morphologies and their flame retardancyFour kinds of PZS micro-nano materials with different morphologies,namely,spherical(PZS?SP),tubular(PZS?NT),capsicum-like(PZS?CLNT)and branched(PZS?BNT),were successfully prepared by controlling the reaction conditions using HCCP and BPS as monomers and tetrahydrofuran as solvent.Prepared PZS micro-nano materials were characterized by SEM,TEM,FTIR and XRD.The PZS nanotubes can be prepared by the reaction of HCCP and BPS at room temperature.At80?,both tubular and spherical PZS nanomaterials could be obtained.When the reaction temperature was increased to 120?,PZS nanomicrospheres were completely prepared.The capsicum-like and branched PZS nanotubes could be prepared by changing the dropwise addition of the reactants,respectively.The PZS nanomaterials with 5 wt%different morphologies were added into PET and melt blended to form PET/PZSs composites,respectively.The TGA test results showed that all the PZS micro-nano materials with different morphologies could catalyze the degradation of PET,while the residual carbon content of the composites were all enhanced at 800?.And the order of the residual carbon content was PZS?CLNT>PZS?SP>PET/PZS?BNT>PET/PZS?NT.The flame retardancy tests showed that the flame retardancy of PET was significantly improved after adding several flame retardants with different morphologies,and the UL-94 rating of several composites was improved from V-2 to V-0 for PET,and the melt drop phenomenon was reduced during combustion.The LOI value of PET was only 25.2 vol%,and the LOI values??of PET/PZS?CLNT,PET/PZS?SP,PET/PZS?BNT,and PET/PZS?NT increased to 34.4 vol%,33.1 vol%,32.8 vol%,and 32.5 vol%,respectively.The combustion behavior of cone calorimetry test is consistent with the LOI results,the flame retardancy of PET/PZS?CLNT is the best,and the PHRR and THR values??are reduced by 29.3%and 29.8%,respectively.This was followed by PET/PZS?SP,with only a 25.9%reduction in PHRR,but a 22.7%reduction in final THR value.PET/PZS?NT had the worst flame retardancy with 26.6%and 15.8%reduction in PHRR and THR values,respectively.PET/PZS?CLNT and PET/PZS?SP not only showed better flame retardancy,but also had obvious smoke suppression effect.The TSP values of PET/PZS?CLNT and PET/PZS?SP were reduced by 25.2%and26.4%,respectively,compared with PET,indicating that the flame retardant safety could be significantly improved during the combustion process.The results of Raman spectroscopy,XPS,TG-IR and thermal pyrolysis analysis show that the addition of flame retardants can catalyze the decomposition of more noncombustible CO₂gas and reduce the content of combustible aromatic compounds from cracking.The addition of flame retardants with different morphologies enhances the compactness of the carbon layer formed after combustion,and the dense and continuous carbon layer can inhibit the energy and heat transfer,in which the order of the compactness of the carbon layer is PET/PZS?CLNT>PET/PZS?SP>PET/PZS?BNT>PET/PZS?NT.Meanwhile,the residual carbon still contains N and P elements,which can form a stable phosphate carbon layer and play a flame retardant role in the condensed phase.The flame retardants of different morphologies can play the flame retardant role in the condensed phase and the gas phase synergistically,thus obtaining excellent flame retardant effect.The breaking strengths of several different morphological flame retardant composites were improved compared with PET,and the breaking strengths of nanotubular flame retardant composites with higher aspect ratios were improved more significantly.(2)Preparation of polyphosphazene microspheres of different sizes and their flame retardancyBy adjusting the concentrations of reactants HCCP and BPS,monodisperse PZS microspheres of different sizes were controllably prepared:PZS600(600 nm),PZS800(800 nm),PZS1600(1600 nm),and surface acetyls-capped PZS600?no H was prepared using acetic anhydride and PZS600.The results of TGA tests showed that the larger the size of the individual PZS microspheres,the higher the residual carbon amount of the microspheres at 800?;after adding different sizes of PZS microspheres into PET,the smaller PZS sizes led to an increase in the residual carbon amount of the composites instead,and the experimental residual carbon amount was opposite to the trend of the calculated value.The same size of PZS600?no H added into PET resulted in higher residual carbon than PZS600.The averaged noncovalent interaction(a NCI)analysis showed that there were strong hydrogen bonds and?-?interactions between PZS and PET,and the thermal stability in the molten state was also good.Energy decomposition analysis based on molecular force field(EDA-FF)analysis showed that the smaller the particle size of PZS microspheres,the stronger the hydrogen bonds and?-?interactions with PET.The rheological test results show that the smaller the size of PZS,the greater the viscosity increase of the composite due to the stronger hydrogen bonding and?-?interaction between the flame retardant and PET,which is beneficial to suppress the melt drop phenomenon.The results of flame retardancy tests showed that the flame retardancy of PET was significantly improved by different sizes of PZS.The smaller the size of PZS added to PET,the better the flame retardancy of the composites.when the size of PZS was reduced to600 nm,the LOI value reached 32.7 vol%with UL-94 grade V-0,and the THR and TSP decreased by 25.5%and 26.2%,respectively.PZS600?no H had more residual carbon than PZS600 at 800?,but LOI values were lower,indicating that increasing the carbon residue alone was not enough to improve the flame retardancy,and the weak intermolecular interactions played an important role.The flame retardants of different sizes all increased the release of noncombustible CO₂gas and reduced the content of combustible aromatic compounds produced by cracking,while increasing the graphitization of PET and promoting the formation of dense carbon layers.The weak interaction between the smaller size of PZS flame retardant and PET is stronger,and a denser carbon layer is formed after combustion,resulting in better flame retardancy.The mechanical property test results show that the rigidity and breaking strength of the composites are improved to some extent after adding smaller size of PZS flame retardant,which greatly increases the spinning performance of PET.(3)Preparation of polyphosphazene microspheres with different functional groups and their flame retardancyTwo kinds of monodisperse microspheres with the same particle size but different functional groups were prepared by polymerization of HCCP with BPS and DDS,respectively:PZS microspheres containing hydroxyl groups and PDS microspheres containing amino groups.The morphology and structure of the micro-nano materials were characterized by SEM,TEM,FTIR and XRD.The TGA results showed that the residual carbon amount of PET/PZS?no H at 800?was higher than that of PET/PZS,while the residual carbon amount of PET/PDS was also higher than that of PET/PZS.The IGM analysis showed that there were strong hydrogen bonds and?-?interactions between PDS and PET,and the thermal stability was also good in the molten state.And the EDA-FF analysis showed that the weak intermolecular interaction between PZS and PET containing hydroxyl groups was stronger than that of PDS.PDS exhibited better flame retardancy for the same size and addition amount,with a LOI value of 33.1 vol%for PET/PDS and a UL-94 rating of V-0.Although the melt drop phenomenon increased during combustion,the resulting melt drops did not cause secondary ignition.The stronger hydrogen bonding and?-?interactions between the PZS and PET molecules play an important role.Compared with PET,the PHRR,THR and TSP of PET/PZS and PET/PDS composites are greatly reduced,which proves that PZS and PDS improve fire safety.PET/PDS releases more heat and smoke than PET/PZS by cracking ammonia etc.in the initial stage of combustion,and the rate of heat release decreases after a period of combustion,and finally PET/PDS does not release as much heat as PET/PZS instead.The results of dynamic rheology,thermal cracking and TG-IR analysis show that because the weak intermolecular interaction between PDS and PET is weaker than that of PZS,it will decompose and produce ammonia and more carbon dioxide when the temperature reaches above 270?,making the viscosity of PET/PDS always lower than that of PET/PZS,and it is easy to produce more molten droplets.The non-combustible gas produced dilutes the surrounding oxygen concentration and carries away a large amount of heat along with the non-combustible molten droplets.PET/PDS has a higher degree of graphitization than PET/PZS and can form a more dense carbon layer to inhibit heat transfer after burning.PDS flame retardant obtains better flame retardancy through the synergistic flame retardant of gas phase and condensed phase.The results of mechanical property tests showed that the incorporation of PDS in PET had higher rigidity than PZS,however,the breaking strength of PET/PDS was lower than that of PET/PZS.PET/PDS composite has high rigidity and lower breaking strength,which makes PET/PDS composite more brittle than PET/PZS.