Synthetic Design Of Linear Polyphosphazene And Study On Performance And Mechanism Of Its Flame Retardant And Toughened Epoxy Resin

With the accelerated development of global pillar industries such as aerospace,transportation,construction and building materials,and energy and power,the global market demand for epoxy resin(EP)for infrastructure is growing rapidly.However,EP is generally flammable,smoky and brittle in nature,which makes it difficult to meet the current requirements of fire safety and mechanical properties in their applications.Therefore,flame retardant,smoke suppressing and toughening modification has become a key technology for the development and application of EP.Based on the unique inorganic backbone with alternating single and double bonds of phosphorus and nitrogen atoms and adjustable functional side groups,the linear polyphosphazene is act as a platform for the design of flame retardant and toughened structures.A series of functional derivatives such as polyphosphazene-based nanohybrids and linear polymers are designed and synthesized for flame retardancy,smoke suppression and toughening by combining the theories and methods of synergistic toughening,synergistic flame retardancy and nano-reinforcement.The relationship between structure and property is discussed,and the flame-retardant,toughening and reinforcing mechanisms are clarified.The specific researches are as follows.1.Linear polyphosphazene was used as the surface modifier of nano-materials and the nanoplatform,and transition metals and silicon/sulfur elements were introduced on the basis of phosphorus/nitrogen synergistic flame retardant system to achieve multielement synergistic flame retardant.Two types of polyphosphazene-based nanohybrids,including linear polyphosphazene hybrid nanotubes(LPP/MoSe2)and nanospheres(SiPPz),were incorporated separately as additive flame retardants into bisphenol-A epoxy resin cured with 4,4-diaminodiphenylmethane(DGEBA/DDM)by solution blending.The results show that at lower addition levels(<3 wt%),LPP-MoSe2 and SiPPz not only show excellent performance in catalytic char formation,flame retardancy,toughening and reinforcement,but also achieve smoke suppression effects that ordinary phosphorus-containing flame retardants do not have.At an addition level of 2 wt%,LPP-MoSe2 and SiPPz can reduce the total smoke release(TSP)of EP by 31.3%and 57.8%,respectively.The effective smoke suppression is due to the synergistic multi-element catalytic char formation,where the barrier effect of the char layer inhibits the release of smoke.2.A furfuryl alcohol-based linear polyphosphazene(PFMP)containing rigid short-chain side groups and a cardanol-based linear polyphosphazene(PCDP)containing flexible long-chain side groups were designed and synthesized,in which phosphaphenanthrene groups(DOPO)were also introduced into the side chain of PCDP for synergistic flame retardancy.They were incorporated separately as additive flame retardants into DGEBA/DDM system by mechanical compounding to obtain flameretardant and toughened EP.Differential scanning calorimetry(DSC)analysis showed that both PFMP and PCDP could achieve good compatibility with EP and had a slight effect on the glass transition temperature(Tg)of EP.In terms of flame retardant properties,PFMP showed good catalytic char formation(condensed phase flame retardant effect),reducing the peak heat release rate(pHRR)and total heat release(THR)of EP by 50.2%and 49.6%,respectively,when added at 15 wt%.PCDP with a DOPO structure in the side chain shows a significant gas phase flame retardant effect and enables EP/PCDP composites to achieve rapid self-extinguishing after ignition.In terms of mechanical properties,both PFMP and PCDP can improve the toughness and strength of EP,with PCDP performing better.When 3 wt%PCDP was added,the impact strength,tensile strength and elongation at break of EP were increased by 188.9%,63.0%and 76.7%,respectively.The effective toughening and strengthening effect is due to the interaction and balance between the flexible phosphazene backbone and the long alkyl side chains,as well as the rigid aromatic structure and the phosphaphenanthrene groups.3.Monosubstituted linear polyphosphazene(PEP)containing epoxidized eugenol side groups and co-substituted linear polyphosphazene(PEDP)containing both DOPOOH and epoxidized eugenol side groups were designed and synthesized.They were added separately to the DGEBA/DDM system by mechanical blending to obtain flameretardant and toughened EP.DSC analysis showed that both PEP and PEDP can achieve good compatibility with EP,while also reducing the Tg of EP slightly(within 12℃).The addition of both PEP and PEDP improved the flame retardancy,toughness and tensile strength of the EP composites,with PEDP performing better.The introduction of DOPO-OH can achieve better flame retardancy and mechanical properties of EP with a reduced amount of additives,demonstrating that the introduction of phosphoruscontaining co-effective flame retardant groups and bulky side groups on the side groups can improve the flame retardancy efficiency and toughening and strengthening effects of linear polyphosphazene,respectively.4.Two novel imidazolium-based polyionic liquids(PIDPP and PIDOP)were synthesized and designed with a linear polyphosphazene as the cationic backbone and diphenylphosphonic acid and DOPO acid as the anions.They were added separately to the DGEBA/DDM system by solution blending,thereby meeting the transparency requirements of flame retardant EP in various application fields.Both PIDPP and PIDOP can significantly reduce the pyrolysis rate of EP and promote EP pyrolytic charring to produce char with higher thermal and oxygen stability,while inhibiting the escape of gas phase pyrolysis products,thus achieving good flame retardancy.The anionic structure has different effects on the flame retardant properties of EP,with PIDOP showing a better flame retardant effect than PIDPP.The impact and tensile tests showed that PIDPP and PIDOP had significant toughening effects,with PIDPP increasing the impact strength,tensile strength and elongation at break of EP by 275%,74.4%and 88.3%,respectively,and PIDOP increasing the impact strength,tensile strength and elongation at break of EP by 212.5%,72.0%and 76.7%,respectively,when added at 12 wt%.PIDPP and PIDOP undergo in-situ toughening in EP in the form of energy absorption.5.In order to meet the demand for flame retardant toughening of functional epoxy resins with higher crosslinking density and aromatic ring content,PCDP with good overall performance was selected from the linear polyphosphazene derivatives synthesized in the previous chapters based on a comprehensive consideration of the amount of flame retardant,flame retardant effect and toughening enhancement effect.The effect of PCDP on the performance of tetrafunctional aromatic epoxy resins(MF)and its mechanism of action in MF was investigated.At low addition levels(≤5 wt%),PCDP achieved good flame retardant effects on MF,effectively increasing the flame retardancy of MF(resulting in a V-0 class for both 3.2 mm and 1.6 mm vertical combustion samples)and reducing the thermal hazard of combustion(44.3%reduction in pHRR and 42.2%reduction in THR).In addition,PCDP achieved a strengthening and toughening modification of MF,where a 3 wt%addition increased the impact strength,tensile strength and elongation at break of MF by 75.0%,52.9%and 130.8%,respectively.