Synthesis Of A Functional Polyphosphazene-containing Microsphere And Its Flame Retardancy On Epoxy Resins

Epoxy resins（EP）is widely used in many industrial field due to its outstanding properties including chemical resistance,mechanical properties and so on.However,its flammability is one of the main drawbacks.Therefore,it is essential to improve the flame retardancy of EP composites to extend its application.To date,although most of the flame retardants can enhance the flame retardancy of the epoxy resin,the mechanical properties will be decimated.In this paper,a new type of flame retardant is designed which can improve both the flame retardancy and mechanical properties of epoxy resin at the same time.Polyphosphazene has numbers of advantages including low toxicity,environmental protection and high flame retarded efficiency.When combined with other flame retardant element such as sulfur（S）,which would generate a synergistic effect on the improvement of the flame retardancy.In addition,flame retardants containing hydroxyl active groups on the surface can enhance intermolecular interaction force,then solve the deterioration of mechanical properties.In this paper,poly（cyclotriphosphazene-co-4,4’-sulfonyldiphenol）microspheres（PZS）with highly cross-linked structures were synthesized via an in situ template method under mild conditions and obtained PZS microsphere is insoluble or meltable under high temperature.The chemical structure was characterized by FTIR and the morphology and chemical composition of PZS microspheres were characterized by SEM,TEM,EDS and element Mapping.The thermal stability of PZS microspheres was studied through Thermogravimetric analysis（TGA）.TGA data indicated that the initial decomposition temperature of PZS microspheres is 423^oC and the char yield is 46.7%（800^oC）,revealing PZS microspheres have outstanding thermal stability and charring ability.The TEM and XRD analysis show that the residual char is composed of 20³0 lamellar graphene-like structures and also have higher degree of graphitization.Dynamic infrared spectroscopy studies show that phosphorus in PZS microspheres can generate phosphoric acid and promote the char generation during heating degradation.the nitrogen elements tend to be gasified and produce noncombustible gases（N₂,NH₃）,which makes the carbon layer expand.Sulfur will consist in solid carbon residue in the form of S-S bond.The dense carbon layer is formed to effectively protect the matrix and prevent the further decomposition of EP.PZS microspheres were then put into use on epoxy resin（EP）as a novel flame retardant.The thermal-stability of PZS/EP formulations were studied through TG,as compared to that of EP;the flammability was surveyed by LOI tests and cone calorimeter（CONE）tests.Thermogravimetric data suggested that the incorporation of PZS significantly enhanced the char yield（42.2%）.When 3 wt%PZS were added into the epoxy resin,the LOI value was increased to 29.8%from 26.6%.Compared with EP,the peak-heat release rate（PHRR）,total heat release（THR）and peak-smoke production release（SPR）of PZS/EP-5.0 were reduced by46.0%,13.6%and 23.5%,respectively.The mechanical properties including tensile strength and tensile modulus were both advanced with the addition of PZS microspheres.When the addition of the flame retardant is 1 wt%and 3 wt%,the tensile strength is increased by 16.7%and 9.1%,respectively.It is reasonable to assume that the PZS microspheres catalyze the early degradation of EP,and form phosphoric and polyphosphoric acids during the heat process.And then the produced acids accelerated the decomposition（dehydration and carbonization）of the EP to form a stable char or aromatic precursors,retarding the mass loss rate of the EP at high temperatures.In conclusion,PZS microspheres can not only be used as an enhancer of EP at room temperature,but also act as a flame retardant at high temperature.It has dual functions in mechanical properties,flame retardancy and smoke suppression.