Synthesis Of DOPO-based Multi-flame Retardant And Its Study On The Flame Retardant Performance And Mechanism Of Polyamide 6

Polyamide 6（PA6）is widely used in automobile manufacturing,electronic appliances,building materials and other fields because of its excellent wear resistance,electrical insulation and mechanical properties.Due to its own structure,PA6 is flammable,and it is accompanied by serious droplets in the combustion process,which makes it have great security risks when applied in the above fields.In order to improve this shortcoming,people have developed many flame retardants suitable for PA6,such as red phosphorus,melamine cyanurate,Aluminum diethylphosphinate（ADP）and so on,among which ADP is widely used because of its environmental protection and high efficiency.However,the poor compatibility between ADP and PA6 makes it difficult to disperse evenly in PA6,and ADP has certain acidity,which will promote the decomposition of PA6,which will damage the mechanical properties of PA6.On the other hand,the research shows that the flame retardant with multiple flame retardant elements can play a role in both gas phase and condensed phase,improve the flame retardant efficiency,and effectively avoid the deterioration of mechanical properties of PA6 caused by the addition of a large number of single flame retardant elements.10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide（DOPO）is a phosphorus-based flame retardant intermediate,and its P-H bond in the structure is highly active,which can react with unsaturated bonds such as asymmetric double bonds,Schiff bases and carbonyl groups to prepare a series of flame retardants based on DOPO.Moreover,changing the structure of flame retardant can also adjust the proportion of DOPO in condensed phase and gas phase,so as to adjust the flame retardant efficiency and improve the mechanical properties.Moreover,when the flame retardant efficiency is sufficient,the material can be extinguished before the droplet is generated,so as to achieve the effect of inhibiting the droplet.Among many flame retardants based on DOPO,boron（B）,nitrogen（N）and sulfur（S）can be combined with DOPO.Because of its structural diversity,its properties can be adjusted,so it has good application potential in many polymers.In this paper,in order to solve the problems of poor mechanical properties of PA6 caused by ADP and serious droplets in the combustion process,two kinds of multi-structure flame retardants,P/N/B and P/N/S,based on DOPO were designed and synthesized,and ADP was surface modified by the reaction of the former with epoxy monomer to improve the compatibility between ADP and PA6 and the droplet phenomenon during the combustion of PA6.The latter was compounded with PA6,and two chain extenders,epoxy and oxazoline,were introduced to adjust the mechanical properties of PA6 composites,in order to prepare high-performance flame retardant PA6 composites.The specific research contents and results are as follows:（1）A P/N/B multi-flame retardant BHD was synthesized from 4-formylphenylboronic acid,1,6-ethylenediamine and DOPO.An epoxy monomer DE with excellent char formation was synthesized from daidzein and epichlorohydrin.The structures of BHD and DE were characterized by nuclear magnetic resonance（NMR）.The surface of ADP was modified by the curing reaction of BHD and DE and named BD-ADP.The structure of BD-ADP was characterized by Fourier transform infrared spectroscopy（FTIR）,thermogravimetric analysis（TGA）,water contact angle test,scanning electron microscope（SEM）and energy spectrum analyzer（EDS）.The flame retardant PA6 composites were prepared by melt blending BD-ADP and PA6.Through UL-94 vertical combustion and LOI test,it was found that PA6 composites with 12 wt%BD-ADP could reach UL-94 V-0 grade,and the LOI was increased to 30.0 vol%.PA6 composites with 11 wt%BD-ADP and 1 wt%DE can not only pass the V-0 grade,but also produce no droplets.The cone calorimeter test results show that BD-ADP can significantly reduce the peak heat release rate（PHHR）and total heat release（THR）during the combustion of PA6,and the addition of DE can effectively delay the ignition time（TTI）of PA6.The flame retardant mechanism of PA6 composites was studied by TG-IR and the microstructure of carbon layer.The results showed that BD-ADP released PO free radicals（PO·）during the combustion process to capture the active free radicals generated by PA6 combustion and interrupt the chain reaction of combustion.The addition of BD-ADP and DE is beneficial to the formation of dense carbon layer in the combustion process of PA6.High-quality carbon layer can effectively isolate heat and combustible gas and inhibit combustion.（2）A P/N/S multi-flame retardant SNDP was synthesized from p-aminobenzonitrile,2-thiophene formaldehyde and DOPO.The structure of SNDP was characterized by NMR and FTIR.The flame retardant PA6 composite was prepared by melt blending SNDP and PA6.The test results of UL-94 and LOI show that the PA6 composite with 10 wt%SNDP can reach UL-94 V-0 grade,and the LOI is increased to 27.4 vol%.The cone calorimeter test results show that the TTI of PA6 can be reduced from 128 s to 96 s,and the time to PHHR(t _PHHR)can also be reduced from 267 s to 194 s with the addition of SNDP,which indicates that SNDP can change the heat release process of PA6,leading to the peak of heat release rate of PA6 at the initial stage of combustion.The results of TG-IR show that the addition of SNDP will not change the types of degradation products,but will catalyze the early degradation of PA6 and reach the peak of thermal degradation rate at a lower temperature.The quadrupole-time-of-flight mass spectrometry（Q-TOF）and pyrolysis gas chromatography/mass spectrometry（Py-GC/MS）test results show that SNDP will generate phosphorus and sulfur compounds during combustion,thus promoting the formation of carbon and exerting the flame retardant effect of condensed and gas phase.Two chain extenders,EDPDO and PBO,were added to adjust the mechanical properties of PA6/SNDP composites.The results showed that PBO had a better enhancement effect on the mechanical properties than EDPDO.The crystallization behavior of PA6 composites shows that the addition of SNDP will slightly reduce the melting point and crystallization temperature of PA6,but it will increase the crystallinity of PA6,while the addition of EDPDO and PBO will further increase the crystallinity of PA6/SNDP composites.