| Epoxy resin(EP)is widely used in various fields such as electrical and aerospace due to its superior dimensional stability and outstanding mechanical behaviour.However,limited by its structure and elemental composition,EP is extremely flammable.Therefore,it is very important to improve the flame retardancy of EP.Although the flame retardants produced by modern industry can inhibit the spread of flames in the condensed phase and the gas phase,some flame retardants are harmful to the environment and ecological development and have been banned.Therefore,people began to try to design and prepare eviromenally friendly green flame retardants with novel molecular structures.Biomass materials are environmentally friendly and renewable,and can promote the formation of char layer.In addition,some biomass materials contain halogen-free flame-retardant elements such as nitrogen and phosphorus,which are expected to play a flame-retardant effect in both the gas phase and the condensed phase.In this paper,using biomass materials such as phytic acid(PA),dopamine,adenine,and tannin acid(TA),combined with nanoscale or microscale materials of different dimensions in a synergistic flame-retardant method,five halogen-free biobased flame retardants containing biobased materials were designed and prepared,and used to prepare of high performance EP composites.The effect of the addition of biobased flame retardants on the flame-retardant properties of EP was studied in depth,and the interaction between the flame retardants and the polymer matrix was explored.The main contents are as follows:(1)Based on self-assembly technology,using silica(Si O2)microspheres modified with silane coupling agent as a template,a nickel phytate shell layer was formed on the surface of nanospheres through electrostatic interaction,and a new green flame retardant Ni@Si O2-PA was prepared.When 5.0 wt%Ni@Si O2-PA was added to the EP matrix,the peak heat release rate(p HRR)of EP composites compared with pure EP was decreased by 51.6%,and the vertical buring test(UL-94)results reached V-1rating.In addition,due to the synergistic flame-retardant effect of the nickel phytate biomass shell and Si O2,a physical barrier could be formed on the surface of the polymer matrix during combustion to avoid further decomposition.PA with high phosphorus content generated P-rich char layer in the condensed phase during combustion,and generated P-containing radicals in the gas phase and produced a quenching effect.In addition,the introduction of nickel phytate shell improved the interfacial compatibility between Si O2 and EP matrix and improved the mechanical properties of the EP.(2)Using polydopamine(PDA)as a biomass bridge,the self-assembly of two 2D materials,hexagonal boron nitride(h-BN)and molybdenum disulfide(Mo S2),was realized through electrostatic interaction,and the hybrid flame h-BN@PDA@Mo S2was prepared.PDA has excellent adhesion and zwitterionic properties,and has different kinds of charges under different p H conditions,which brings favorable conditions for self-assembly between materials.Compared with pure EP,the p HRR of the EP composites added with 2.0 wt%h-BN@PDA@Mo S2 decreased by 32.1%.In addition,the released of toxic gas CO and flammable gases during the combustion process was significantly suppressed.PDA as an adhesive helped to exert the synergistic flame-retardant effect of h-BN and Mo S2 nanosheets.Hybrid flame retardants could form a physical barrier to insulate heat and oxygen transfer during combustion and improved the structural stability of the char layer.In addition,the introduction of h-BN@PDA@Mo S2 could form a strong interfacial interactions with the EP matrix,thereby improving the tensile modulus.(3)Using?-crystalline iron hydroxide oxide(?-Fe OOH)as the core,PDA and TA as the biomass shell,following adsorption of Ni2+on the surface of the shell to prepare bioderived bilayer flame retaradants?-Fe OOH@PTNi with a core-shell structure.The biobased shell had a high carbon content and is rich in hydroxyl groups,which could be dehydrated and carbonized during the combustion process,and finally formed a protective char layer with a robust structure.The study found that when the additive amount of flame retardants reached 5.0 wt%,the UL-94 results of the EP composites reached V-0 rating,and the values of p HRR and total heat release(THR)of the EP composites decreased.The bioderived flame retardants could capture highly reactive free radicals and catalyzed the formation of char layer,which was beneficial to reduce the release of toxic gases to flammable gases.Moreover,the flame retardants was uniformly dispersed in the EP matrix and had good compatibility.Compared with pure EP,the storage modulus of EP composites containing 5.0 wt%Fe OOH@PTNi were improved by 16.0%.(4)Using ammonium polyphosphate(APP)as a template,TA,adenine and Ni2+ions were selected for self-assembly through electrostatic interaction and loaded on the surface of APP particles to prepare bioderived“all-in-one”intumescent flame retardant APP@ATNi.The study found that the limiting oxygen index(LOI)value of the EP/APP@ATNi15 composites reached 33.5 and obtained a V-0 rating in the UL-94 test,showing good flame retardancy.In addition,the incorporation of flame retardants could significantly suppress the amount of smoke released during combustion.Compared with pure EP,the total smoke production(TSP)of EP composites 47.2%.The bioderived assembly could effectively scavenge highly reactive free radicals that promoted the combustion reaction during combustion,generate inflammable gases and catalyze the formation of char layer during combustion to protect the EP matrix from further decomposition.In addition,the introduction of bioderived assembly improved the compatibility of APP particles in the EP matrix,resulting in increased mechanical strength.(5)Based on TA as a carbon source has the excellent carbonization ability,TA,hexachlorocyclotriphosphazene(HCCP)and 4,4'-dihydroxybiphenyl(DOD)were used as monomers to prepare bioderived cross-linked polyphosphazene microspheres PHDT by polycondensation reaction.As a precursor,iron-cobalt layered double hdroxidelayered(Fe Co-LDH)was further formed on the surface of the microspheres,and finally a novel bioderived flame retardant PHDT@Fe Co-LDH was prepared and used to improve the flame retardancy of the EP matrix.Compared with pure EP,EP/PHDT@Fe Co-LDH4.0 composites could achieve LOI value of 29.7 and a UL-94V-0 rating.The PHDT microspheres could be regarded as an“all-in-one”intumescent flame retardant,which could catalyze the formation of a highly graphitized phosphorus-containing cross-linked network char layer in the EP matrix.In addition,the LDH shell could form refractory metal oxides on the surface of EP during combustion to further improve the flame retardancy of EP composites.The excellent flame-retardant effect of PHDT@Fe Co-LDH can be attributed to the catalytic carbonization effect,the scavenging effect of highly reactive radicals,and the physical barrier effect in the condensed phase.Based on the idea of flame-retardant theory,this paper selects phosphorus-containing,nitrogen-containing and carbon-rich polyhydroxy biomass materials as raw materials to prepare five biobased flame retardants and use them to improve the flame retardancy of EP.The flame-retardant mechanism of biobased flame retardants was analyzed from gas phase and condensed phase,which broadened the application of biomass materials in the field of flame-retardant EP,and provided an effective method for the preparation of biobased flame retardants and high-performance EP composites. |
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