Preparation And Application Of Phosphorus-containing Siloxane Flame Retardant In Polymers

With the development of economy and science and technology,there are increasingly demanding requirements on the application neighborhood and environmental conditions of polymeric materials,and the development of flame-retardant polymers with high performance has become a key part of the development and application of new materials.Traditional methods improve the flame retardancy by adding halogen compounds to the polymer matrix,however,halogen compounds release toxic and corrosive gases during thermal decomposition and combustion,which can create new hazards,and considering this serious drawback,halogen flame retardants are gradually limiting their use in many fields.Phosphorus（P）system flame retardants as one of the important replacements for halogen-free flame retardants,its low smoke,low toxicity and environmentally friendly features are widely used,but a single phosphorus flame retardants are often difficult to get the desired flame-retardant effect,often need to be compounded with other flame retardants.Compared with a single system,the preparation of multi-element synergistic flame retardants can make up for the shortcomings of a single flame-retardant element and bring higher flame-retardant efficiency.Sol-gel technology can change the chemical composition and arrangement of molecular constituent units through hydrolysis and condensation reactions to achieve nano-and micron structure customization and prepare flame retardants with highly oriented network structures.Trialkoxysilane（-Si（OR）₃）is capable of forming three silanols during hydrolysis,and dehydration occurs between the three polymerizable groups to form highly oriented polymer network structures.To this end,based on such a modulation strategy,phosphorus-containing siloxane monomers were synthesized in this paper,and different flame retardants were prepared from phosphorus-containing siloxane monomers for flame retardant poly（adipic acid-butylene terephthalate）（PBAT）and epoxy resin（EP）,respectively（1）Preparation of phosphorus/silica hybrid microspheres and their flame-retardant properties and mechanism in PBAT.The phosphorus-siloxane-containing precursor Si P was synthesized from terephthalaldehyde and 3-aminopropyltriethoxysilane（KH550）and 9,10-dihydro-9-oxa-10-phosphafi-10-oxide（DOPO）,and its structure was evaluated by nuclear magnetic hydrogen（¹H NMR）and phosphorus spectroscopy(³¹P NMR)as well as infrared spectroscopy（FTIR）.The phosphosiloxane precursor Si P was prepared by sol-gel and emulsion pellet formation techniques,and its structure and composition were characterized by FTIR and scanning electron microscopy（SEM）.The Si P-M was then introduced into PBAT to improve its flame-retardant properties.The results showed that the addition of only 0.1 wt%Si P-M resulted in a UL-94 vertical combustion V0rating for PBAT with a limit oxygen index（LOI）of 26.5 vol%,and with increasing Si P-M content,1 wt%Si P-M increased the LOI of PBAT to 29.8 vol%.The peak heat release rate（p HRR）,total heat release（THR）and total smoke release（TSP）of PBAT decreased by 19.6%,11.7%and 14.7%,respectively,with the introduction of 1 wt%Si P-M compared to pure PBAT.Si P-M in PBAT exhibited a very high flame-retardant efficiency.The high flame-retardant efficiency of Si P-M in PBAT was mainly attributed to three the high flame-retardant efficiency of Si P-M in PBAT is mainly due to three factors.Firstly,during combustion Si P-M releases PO·quenched reactive radicals,which terminate the combustion process of PBAT,and produces non-combustible gases such as CO₂and H₂O,which dilute the combustible gases and extinguish the flame.Secondly,Si P-M maintains a good spherical shape during combustion,which helps to enhance the char layer and exerts a good barrier effect.Finally,Si P-M helps to form large oligomers containing double bonds,which promotes the formation of char and reduces the production of combustible gases and smoke.Overall,this is a mode of action in which the gas phase is the main action and the condensed phase is the secondary action.In addition,the Si P-M content does not exceed 1.0 wt%and has little effect on the mechanical properties of PBAT.The coupling agent KH560 increased the elongation at break of the PBAT composites.（2）Preparation of phosphorus/silicon/boron flame retardants and their flame-retardant properties and mechanism in EP.The phosphorus containing siloxane precursor Si P was reacted with H3BO3 to prepare the phosphorus/silicon/boron hybrid flame retardant BSi P,and BSi P was used in EP to improve its flame-retardant properties.The curing behaviour of BSi P in EP was studied by DSC fitting and BSi P had essentially no effect on the curing process and glass transition temperature of EP.The flame retardant properties of BSi P in EP were investigated and the results showed that the burning time of EP was significantly reduced,with the addition of 3 wt%BSi P（containing 0.19%phosphorus）bringing the EP to UL-94 V0 rating and raising the LOI to 31.5 vol%.When 6 wt%BSi P is added the EP achieves a UL-94 V0 rating and the LOI increases to 35.0 vol%.Cone calorimetry tests showed that p HRR,THR and TSP decreased by 32.1%,12.4%and 13.8%respectively with the addition of 6 wt%BSi P compared to pure EP,and the residual char mass increased from 11.7%to 21.2%after cone calorimetry tests,an improvement of 81.2%,indicating that BSi P effectively promoted the char formation of EP and reduced smoke and heat generation.BSi P mainly acts in the condensed phase,where it promotes the formation of a compact and thick char layer,with cracks and pores gradually decreasing as the BSi P content increases,and where silicon elements from the combustion process form silicon oxides that migrate to the char surface,thereby enhancing the char layer.In the gas phase,the introduction of BSi P significantly reduces the release of combustible small molecule carbonyl compounds during EP degradation,and the significant reduction in carbonyl compound content indicates that BSi P inhibits the dehydrogenation reaction,delays the thermo-oxidative degradation of EP,promotes dehydration and the formation of the char layer,and helps to improve the BSi P had little effect on the bending and tensile properties of EP and improved the impact properties of EP.