Preparation Of Aromatic Schiff-Based Phosphorus-Nitrogen Co-Effective Flame Retardant And Its Application To Polyamide 6

Polyamide 6（PA6）has excellent mechanical properties,wear resistance,chemical resistance,and electrical insulation,and is widely used in transportation,machinery manufacturing,clothing,and home textiles.PA6,as a flammable material,is accompanied by severe molten droplets and smoke during combustion,which is prone to secondary fires and inconvenience to rescuers.Therefore,the preparation of flame retardant PA6 is of great significance to achieve product versatility and improve safety.PA6 with flame retardants needs to meet the chemical element composition and thermal stability requirements.Therefore,this thesis synthesizes three aromatic Schiff base phosphorus and nitrogen co-effective flame retardants for PA6 through molecular design,using the multi-functional structure,easy modification,and aromatic ring skeletons of aromatic Schiff bases;prepares flame retardant PA6 by copolymerization and blending methods respectively,and studies the effects of different organic functional groups and flame retardant structures on the thermal stability of flame retardants and The effects of different organic functional groups and flame retardant structures on the thermal stability of flame retardants and the flame retardant effect and flame retardant mechanism of PA6 were studied;further,aluminum diethyl hypophosphite（Al Pi）,melamine cyanurate（MCA）or hexagonal boron nitride（h BN）were introduced into some systems to study their synergistic flame retardant effects on PA6.The details of the study are as follows:（1）ThearomaticSchiffbase4,4’-（（1,4-phenylenebis（formylidene）ethylene）bis（azanylidene））dibenzoate（TMA）containing carboxyl groups at both ends were prepared from terephthalaldehyde and p-aminobenzoic acid,and the phosphorus-based flame retardant monomers 9,10-dihydro-9-oxa-10-phosphafi-10-oxide（DOPO）and diphenylphosphine oxide（DPO）containing highly reactive P-H bonds were used for addition reactions with TMA to obtain twophosphorus-nitrogenco-efficientflameretardants4-（（4-（（（4-carboxyphenyl）amino）（6-oxo-4,4a-dihydrodibenzo[c,e][1,2]oxaphosphorin-6-yl）methyl）phenyl）（6-oxo-dibenzo[c,e][1,2]oxaphosphorin-6-yl）amino）benzoic acid（TMA-DOPO）and 4,4’-（（1,4-phenylenebis（（diphenylphosphino）methylene））bis（azoic diyl））dibenzoic acid（TMA-DPO）.The molecular structure and thermal stability of TMA,TMA-DOPO,and TMA-DPO were characterized by infrared,nuclear magnetic,elemental analysis,and thermogravimetry,and the results showed that the molecular structures of TMA,TMA-DOPO,and TMA-DPO were consistent with each other,and both flame retardants had good char formation in a nitrogen atmosphere,and the thermal stability and char formation of TMA-DOPO was better than that of TMA-DPO.The two flame retardants with different contents were introduced into PA6 by copolymerization method,and 3 wt%of MCA or Al Pi was added to 5 wt%of TMA-DOPO system respectively to synergistically flame retard PA6,and the effects of flame retardants on molecular weight,relative viscosity,thermal properties,mechanical properties and flame retardant properties of PA6 were studied.The relative viscosity,number average molecular weight,melting temperature,tensile strength and elongation at break,etc.gradually decreased;when the introduction of TMA-DPO was 7 wt%,the vertical combustion of PA6-7TMA-DPO reached V-0grade and the ultimate oxygen index（LOI）was 29.7%;when the introduction of TMA-DOPO was5 wt%and the introduction of MCA or Al Pi was 3 wt%,the PA6-5TMA-DOPO/3MCA and PA6-5TMA-DOPO/3Al Pi vertical combustion reached V-0 level and LOI of 30.7%and 30.3%,respectively.Analysis by thermogravimetry as well as by scanning electron microscopy photographs,Raman spectroscopy,infrared spectroscopy,and X-ray photoelectron spectroscopy of the residual coals showed that the PO·and PO₂·radicals generated by the decomposition of TMA-DOPO interrupt the combustion chain reaction by trapping the OH·and H·generated by the decomposition of PA6 The TMA-DPO has a strong carbonization ability,and the microcrystalline carbon obtained makes the carbon layer denser,which can be combined with the phosphorus-containing liquid film on the surface to isolate heat sources and combustible gases and play the role of gas-phase and condensed-phase flame retardant.The addition of Al Pi promotes the carbon formation of TMA-DOPO,and MCA can dilute the concentration of surrounding combustible gases to enhance the gas-phase flame retardant effect.（2）The nitrile-containing aromatic Schiff base phosphorus-nitrogen co-efficient flame retardant4,4’-（（1,4-phenylenebis（（6-oxodibenzo[c,e][1,2]oxaphosphin-6-yl）methylene））bis（azo））be nzonitrile（PMAD-DOPO）was synthesized from terephthalaldehyde,4-amino benzonitrile,and DOPO,and its molecular structure and thermal stability were characterized by infrared,nuclear magnetic,mass spectrometry,elemental analysis,and thermogravimetric characterization.The results showed that the molecular structure of PMAD-DOPO was consistent with that expected,its thermal stability and carbon formation were good in the nitrogen atmosphere,and the initial decomposition temperature(T_5%)was higher than the molding processing temperature of PA6（～245℃）.The flame retardant and h BN were blended with PA6 in proportion to obtain flame retardant PA6,and their effects on the thermal properties,crystallization properties,mechanical properties,and flame retardant properties of PA6 were studied,and the results showed that the compatibility of PMAD-DOPO/h BN with PA6 was good,and with the increase of flame retardant addition,the crystallinity of PA6 increased,and the thermal stability,melting temperature,tensile strength and elongation at break gradually decreases;when the addition amount of PMAD-DOPO is 16 wt%,the vertical combustion of PA6/16PMAD-DOPO reaches V-0 level and LOI is 30.3%,and its smoke suppression is better,the peak smoke generation rate（p-SPR）and total smoke generation（TSP）decrease 50%and 68%respectively compared with PA6,while the addition of 4wt%of h BN helps PA6/12PMAD-DOPO vertical combustion to V-0 level,LOI of 27.7%,and p-SPR and TSP decreased by 41.0%and 57.2%,respectively,compared to PA6.The analysis by simultaneous thermal analysis,infrared thermography as well as scanning electron microscopy photos of residual carbon,Raman spectroscopy,infrared spectroscopy,and X-ray photoelectron spectroscopy showed that:because PMAD-DOPO end groups contain nitrile group,the thermal cross-linking reaction can occur at 272～320℃,then it is beneficial to improve the melt-drop resistance of PA6 and does not affect its processability,and h BN forms a thermally conductive pathway during combustion,which can reduce the melt-drop temperature.PMAD-DOPO catalyzes the dehydration of PA6 into carbon,isolates the heat source and combustible gas,and plays the role of cohesive phase flame retardant,and the decomposition produces some phosphorus-containing small molecules to capture the free radicals generated by chain segment breakage,which indicates that it is partly gas phase flame retardant,while h BN effectively improves the density and heat resistance of the carbon layer to enhance the cohesive phase flame retardant effect.