Flame Retardant Application Of Phosphaphenanthrene-based Salt Derivative And Its Hybrid-modified Ammonium Polyphosphate In Epoxy Resin

Epoxy resin（EP）is one of the most commonly used thermosetting polymers.Due to its excellent mechanical strength,relatively low dielectric constant and curing shrinkage,good adhesion to the substrate and good resistance to heat,chemical and corrosion,EP has been widely used in aerospace,coatings,adhesives,electronic equipment,laminates and packaging.However,epoxy resin is very flammability and during combustion it would release large smoke,which limits its application in many fields such as transportation,construction,electrical and electronic industries.As an important industrial flame retardant,halogen flame retardant has been widely used because of its high flame-retardant efficiency;however,a large number of toxic products and smoke produced during the combustion process are the main drawbacks of this type of flame retardant.Therefore,the researches on the design of halogen-free flame retardants,especially organophosphorus flame retardants,have attracted more and more attention.Among them,9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide（DOPO）and its derivatives have attracted much attention because of its highly flame-retardant efficiency to epoxy resins.While,phosphaphenanthrene-based salt derivatives are rarely reported and their study on the comprehensive properties of epoxy resins are still not clear.In this work,we have synthesized two kinds of phosphaphenanthrene-based salt derivatives,namely DOPO-based phosphonic acid organic ammonium salt（DOA）and DOPO-based triazole zinc salt（Zn-D-ATA）,and then further investigated the effect of them on the properties of EP.At the same time,considering that the DOPO-based derivatives always exhibited a weak effect on reducing the smoke release for epoxy resin,Zn-D-ATA was further used to modify ammonium polyphosphate（APP）so as to prepare Zn-D-ATA@APP hybrid material,and finally the low toxicity and light smoke,highly flame-retardant epoxy resin was obtained.Firstly,DOPO-OH was synthesized by the oxidization of DOPO using H₂O₂ as the oxidant,and then reacted with dimethylolmethylaminomethane（AMPD）through acid-base neutralization reaction to obtain DOPO-based phosphonate ammonium salt（DOA）.Fourier transform infrared spectroscopy（FT-IR）,nuclear magnetic resonance(~1H NMR,³¹P NMR)and other characterization methods were used to analyze the chemical structure of DOA,indicating that DOA was successfully synthesized.Then DOA was used to modify epoxy resin to obtain flame-retardant modified epoxy resin（EP-DOA）,and the comprehensive performance of EP-DOA was studied.The results show that compared with pure epoxy resin,the thermal stability of flame-retardant modified epoxy resin decreased,but when the amount of DOA was 5%,the limiting oxygen index（LOI）of flame-retardant modified epoxy resin was promted from 25.0%of pure epoxy resin to35.5%,and it passed the UL-94 V-1 level.At the same time,the cone calorimetric test（CC）results showed that DOA inhibited the heat release of epoxy resin,as the peak heat release rate（PHRR）and total heat release（THR）was dropped.Thermogravimetric-infrared（TG-IR）,scanning electron microscopy（SEM）and Raman test（Raman）were used to characterize the pyrolysis gas phase products of DOA and the microscopic morphology of the char residue after combustion.The results showed that the flame-retardant mechanism of DOA was that the inert gas and phosphorus-containing free radicals derived from thermal decomposition of DOA exhibited the dilution effect and quenching free radical behavior respectively.In addition,its decomposition production such as phosphoric/pyrophosphoric acid etc.promoted epoxy resin to form a denser char layer,acting as a heat and oxygen barrier effect.Apart from the above,the mechanical performance test results showed that the tensile strength and flexural strength of the epoxy resin modified by DOA was improved to a certain extent,which as caused by the increase of the macromolecular cohesion of the epoxy resin.Secondly,Zn-D-ATA was prepared by coordination reaction using zinc cation as the coordination centre and DOPO-triazole（D-ATA）as the organic ligand made by our laboratory.The chemical structure,composition and physical morphology of Zn-D-ATA were analyzed by means of FTIR,~1H NMR,¹³C NMR,X-ray powder diffraction（XRD）,inductively coupled plasma emission spectrum element analyzer（ICP-AES）and scanning electron microscope-X-ray energy spectroscopy（SEM-EDS）.The results showed that Zn-D-ATA with the plate-like morphology was successfully prepared.Then Zn-D-ATA was used to modify epoxy resin to prepare flame-retardant epoxy resin（EP-Zn-D-ATA）,and the comprehensive performance of EP-Zn-D-ATA was studied.The results showed that when the addition amount of Zn-D-ATA was 3%,the LOI of flame-retardant epoxy resin was increased to 28.3%,and it passed the UL-94 V-1 level,and compared with EP-DOA system,it exhibited higher thermal stability.Cone calorimetric test results showed that the heat release rate（HRR）and carbon monoxide production（COP）during the combustion process of EP-Zn-D-ATA was reduced to a certain extent.Using TG-IR to analyze the gas phase products of Zn-D-ATA,it was found that Zn-D-ATA released inert gases such as NH₃ during the thermal decomposition process,which could dilute oxygen and fuel;SEM-EDS was used to observe the morphology and constituention of the char residue after the combustion of materials.Compared with the pure sample,the char residue of the epoxy resin modified by Zn-D-ATA contained zinc and phosphorus elements,and the inner char layer had fewer cracks and the outer char layer was much denser;Raman was used to characterize the degree of graphitization of char residue,and the degree of graphitization of char residue in epoxy resin modified by Zn-D-ATA was higher.Surprisingly,the mechanical performance test results showed that the epoxy resin modified by Zn-D-ATA had a significant increase in mechanical strength,especially flexural strength,due to that lamellar Zn-D-ATA was uniformly dispersed in the epoxy resin,and there was a strong interface interaction between them,thereby improving the mechanical strength of the epoxy resin.Finally,Zn（NO₃）₂·6H₂O and ammonium polyphosphate（APP）were used as raw materials to prepare Zn²⁺modified APP(APP-Zn²⁺)through the cation exchange reaction;The Zn²⁺on the surface of APP-Zn²⁺underwent a coordination reaction with D-ATA to obtain Zn-D-ATA@APP hybrid material.The chemical structure and physical morphology of Zn-D-ATA@APP were analyzed by FTIR,~1H NMR,XRD and SEM-EDS and the results showed that flower-like Zn-D-ATA@APP was successfully prepared.Then the Zn-D-ATA@APP hybrid material was used to modify the epoxy resin to obtain the flame-retardant epoxy resin（EP-Zn-D-ATA@APP）,and the EP-Zn-D-ATA@APP comprehensive performance research were further studied.The results showed that when the addition amount of Zn-D-ATA@APP was 7%,the flame-retardant modified epoxy resin owned a LOI value of 31.1%,passed the UL-94 V-0 level,and tis residual mass at high temperature was significantly improved.Meanwhile,the cone calorimetric test results showed that Zn-D-ATA@APP effectively inhibited the heat release of epoxy resin,and PHRR and THR were significantly decreased.SEM and Raman were used to characterize the morphology and graphitization degree of char residue of materials after burning.The results showed that compared with APP modified EP,the inner and outer char layers of Zn-D-ATA@APP modified epoxy resin were more complete and had a higher degree of graphitization,so that they exhibited more excellent flame retardant properties.In addition,the mechanical performance test results showed that the epoxy resin modified by Zn-D-ATA@APP basically maintained the mechanical strength and even the flexural strength was enchaned to a certain extent.This was owing to that compared with APP,the flower-like Zn-D-ATA@APP had a larger specific surface area and stronger interface interaction with the epoxy matrix.