Preparation,Structure And Properties Of Graphene-Based Flame Retardant Copolyester

In order to overcome the problem of inflammability of PET,the design and synthesis of multiple composite flame retardants for flame retardant modification of PET has become a research hot topic in recent years.Based on the structural characteristics of carbon materials,flame-retardant properties as well as PET processing conditions,this thesis focused on the using the graphene as the matrix to synthesize graphene oxide（GO）by modified Hummer’s method,and further designing and synthesis of two graphene-based multi-element integrated flame retardants.In addition,the multi-element integrated flame retardant（HGO,HPGO）with the traditional flame retardant 2-carboxyethylphenylphosphinic acid（CEPPA）,were employed in preparation of polyester and finally a series of low heat release,Smoke suppression,anti-melt dripping flame-retardant copolyesters were successfully prepared.Finally,the synergistic flame-retardant mechanism of the graphene-based multi-element integrated composite flame retardant and CEPPA in the matrix was proposed and discussed in detail by using a variety of characterization.The main research contents and conclusions of this paper are as follows:1.The results showed that GO has a high degree of oxidation,and the lateral dimension of the lamella is about 1μm.PET-GO composites containing different amounts of GO were prepared by in situ polymerization and the results showed that the graphene oxide in the resultant PET was in two forms:copolymerization and doping.The incorporation of GO will increase the thermal conductivity of the polyester and thus lead to a slight decrease in the thermal stability of the polyester;but the good insulation of the GO sheet can also promotes the formation of carbon in the matrix and delays the release of heat at high temperature at the same time.Considering the influences of different amount GO on the overall performance of PET,the optimized GO content is 0.4 wt%.2.Two types of new integrated composite flame retardant were successfully prepared and the results clearly indicated that both HCCP and NH₂-POSS were covalently grafted on the surface of the GO sheet,and their grafting amounts on the surface of GO increased with the increase of grafting reactants amounts.HGO and HPGO were respectively applied to the conventional PET polymerization system by in-situ polymerization to prepare graphene-based phosphorus-nitrogen flame-retardantcopolyester（PET-HGO）andgraphene-based phosphorus-nitrogen-silicon flame-retardant copolyester（PET-HPGO）.Based on this,in order to further improve the limiting oxygen index LOI of the retardant polyester,these two new flame retardants were composited with CEPPA and applied to the polyester synthesis,finally the graphene-based phosphorus-nitrogen composite flame-retardant copolyester（CPPET-HGO）and graphene-based phosphorus-nitrogen-silicon composite flame-retardant copolyester（CPPET-HPGO）were successfully prepared with excellent properties such as low heat release,smoke suppression,and anti-smoke.3.The crystalline and mechanical properties of the obtained flame-retardant polyester were studied.It is found that the composite flame retardant HGO and HPGO,which are not involved in the copolymerization reaction,have a nucleation effect in the matrix,which accelerates the crystallization rate of PET-HGO and PET-HPGO,increases the crystallinity,and forms a larger number of smaller-sized crystals in the matrix.The number of grains increases exponentially.When CEPPA is further added,the regular structure and symmetry of the PET molecular chain are destroyed and it is difficult for the polymer chain to fill the crystal lattice.Thus the crystallization ability is reduced and the crystal defects are also larger,and the crystallinity is reduced.The results showed that HGO and HPGO increased the tensile strength and elastic modulus of PET,but reduced the elongation at break.4.HGO can effectively reduce the release of heat and smoke during the combustion of PET.When the amount of HCCP grafted in the graphene-based phosphorus-nitrogen composite flame retardant is 29.07%,the LOI of PET-HGO can reach 26%,and the peak heat release rate（PHRR）and peak smoke release rate（PSPR）were reduced by 47.92%and 39.28%respectively when comparing with the results by PET.Compared with PET-HGO,when the grafting amount of NH₂-POSS on the surface of HPGO is 4.22%,the total heat release of PET-HPGO（THR）and total smoke production（TSP）were reduced by 7.2%and 10.9%,respectively.Adding CEPPA with a phosphorus content of 1.0 wt%can significantly increase the LOI of CPPET-HGO to 32%and make the UL-94 grade reach V-0.Compared with PET-HGO without CEPPA,the THR of CPPET-HGO is reduced by 29.44%,but because CEPPA promotes the degradation of PET in the early stage of combustion,the smoke release rate of CPPET-HGO and total smoke emission have increased.In order to solve the problem of increased smoke production in CPPET-HGO,this study combined HPGO with CEPPA,and the TSP of CPPET-HPGO is reduced by 27.51%when comparing with CPPET-HGO.In a word,Graphene-based phosphorus-nitrogen-silicon integrated composite flame retardant and CEPPA can effectively improve the overall flame retardant performance of PET and improve fire safety5.In this part,the flame-retardant mechanism of the obtained polyester is analyzed by the kinetics of thermal weight loss degradation,carbon layer morphology,carbon layer structure,carbon layer composition,thermogravimetric infrared technology（TG-IR）and thermal pyrolysis-gas chromatography-mass spectrometry（Py-GC-MS）.The proposed mechanism is as follows:In the initial stage of heating,the good thermal conductivity of GO promotes the degradation of grafted HCCP and NH₂-POSS,in which the PO·and PO₂·radicals generated by the degradation of HCCP can terminate the free radical and decrease the concentration of free radicals in the gas phase;while the cage structure of NH₂-POSS transforms into a network structure at high temperature and located the carbon layer obtained from the catalytic carbonization of the middle P element in the condensed phase;in addition,the shielding effect and the tortuous path effect of the GO sheet also reduce the mass and heat transfer between the combustion zone and the substrate;These three mechanism synergistically suppresses the release of heat and smoke in PET-HPGO and PET-HGO.Based on this result,after adding two flame retardants containing CEPPA to PET,the premature degradation of CEPPA in the initial heating stage will release a large amount of PO·and PO₂·free radicals that will terminate the free radicals in the gas phase,thus the LOI and UL-94 grades of the flame retardancy CPPET-HGO and CPPET-HPGO are increased.At the same time,CEPPA can also promote the dehydration of the substrate into carbon during the degradation process,and produce phosphoric acid,polymetaphosphoric acid and other substances to cover the surface of the substrate,further enhancing the insulation performance of the carbon layer.Therefore,this study believes that the shielding effect and tortuous path of the graphene sheet,promoting carbon formation and solidification in the condensed phase,and the free radical quenching mechanism in the gas phase synergistically inhibit the combustion of flame-retardant copolyesters,and endow the flame-retardant copolyesters with high LOI,low heat release,smoke suppression,anti-melt dripping and other good comprehensive flame retardant properties.