Synthesis Of Novel Phosphonate Flame Rtardant And Its Application In Flame Retardant Epoxy Resins

Epoxy resins have been widely used in many areas due to its excellent performance, such as the adhesive of the metal materials and the nonmetal materials, the corrosion resistance of coatings, the electrical insulation materials and the FRP/Composites, etc. However, the flammability of epoxy resins lead to fire disaster inevitably, the epoxy resins are subject to various mandatory controls for safety reasons. Halogenated flame retardants played a very important role in the field of epoxy resins due to its excellent flame retardance, however they were limited with the consideration of environmental protection and security issues. The phosphorus-containing flame retardants have the advantages of high efficiency, environmental protection and other advantages, however there are some problems of low thermal stability and easy to hydrolysis. In recent years, phosphonate flame retardants have become a hot topic due to its excellent flame retardance and hydrolytic resistance.In this paper, a novel phosphonate flame retardant additive bis[4-(methylene-hydroxy-DOPO)phenoxy]phenyl phosphine oxide defined as DOPO-PPO was successfully synthesized by phenylphosphonic dichloride, p-hydroxybenzaldehyde and DOPO. Its chemical structure was characterized by Fourier transform infrared (FTIR) spectroscopy,1H NMR and 31P NMR spectroscopy. The results of the thermogravimetric analysis(TGA) demonstrated that the initial thermal decomposition temperature of DOPO-PPO was 252℃ and the char residue was 30.0 wt% at 700℃, which demonstrated that the synthesized flame retardant possessed a relatived high thermal stability and excellent charring ability. The prepared flame retardant additive was incorporated into epoxy resins and cured with diaminodiphenyl methane (DDS) to prepare flame retardant epoxy resins thermosets. The flame retardancy of thermosets were investigated by limiting oxygen index (LOI) and vertical burning (UL-94) tests, the thermal degradation and combustion behavior were measured by TGA and cone calorimeter (CONE), the water resistance was investigated by water resistant test and the morphologies of the char layer after cone test were measured by scanning electron microscopy (SEM). The results demonstrated that the thermosets passed UL-94 V-0 flamability rating and its LOI value reached 34.0% when the loading amount of DOPO-PPO was 12 wt% and the phosphorus content was only 1.1 wt% in the thermosets, which indicated that the prepared flame retardant possessed excellent flame retardant efficiency for epoxy resins. TGA test results revealed that the incorporation of DOPO-PPO into epoxy resins stimulated the degradation of thermosets ahead of time and charring formation, the char yield of thermosets increased from 14.1 wt% to 27.0 wt% at 700℃. According to the cone test, HRR, THR and TSP of the flame retardant epoxy resin decreased significantly compared to that of pure epoxy resin. Water resistance tests shown the introduction of DOPO-PPO reduced the moisture absorption of thermosets and the materials maintained excellent flame retardancy after moisture resistance tests. The incorporation of DOPO-PPO promoted the epoxy resins thermosets forming more homogeneous and compact char layer during combustion by SEM tests, which prevented the underlying matrix from further decomposition and combustion, consequently enhanced the flame retardancy of epoxy resins thermosets.Another novel phosphonate flame retardant bis[4-(parahydroxyaniline-DOPO-methylene)phenoxy]phenyl phosphine oxide(DOPO-PPAO) was also synthesized by phenylphosphonic dichloride, p-hydroxybenzaldehyde, p-aminophenol and DOPO and the structure was confirmed. The TGA results demonstrated that the initial thermal decomposition temperature of DOPO-PPAO was 305℃ and the char residue was 44.2 wt% at 700℃ which demonstrated that the synthesized flame retardant possessed a relative high thermal stability and charring ability. The prepared flame retardant was incorporated into epoxy resins and cured with DDS to prepare flame retardant epoxy resins thermosets. The properties of the flame retardant epoxy resins thermosets were also investigated. The results demonstrated that when the thermosets passed UL-94 V-0 flamability rating and its LOI value reached 33.5% when the loading amount of DOPO-PPAO was 14 wt% and the phosphorus content was only 1.1 wt% in the thermosets, which indicated that the prepared flame retardant possessed excellent flame retardant efficiency for epoxy resins. TGA results revealed that the incorporation of DOPO-PPAO into epoxy resins stimulated the degradation of thermosets ahead of time and charring formation, the char yield of thermosets increased from 14.1% to 27.8% at 700℃. According to the cone test, HRR, THR, SPR and TSP of the flame retardant epoxy resin decreased significantly compared to that of pure epoxy resin. Water resistance tests shown the introduction of DOPO-PPAO reduced the water absorption of thermosets and the materials maintained excellent flame retardancy after water resistance tests. The results of water resistance test indicated that the water uptake of flame retarded resin was low and the samples kept good flame redartancy after the test.