Preparation Of Novel Phosphorus-containing Flame Retardant Curing Agents And Their Application In Flame Retarded Epoxy Resins

Epoxy resins are widely used in coating, casting, adhesives, composites, encapsulants for semiconductors and insulating materials for electric devices due to their excellent mechanical properties and resistance ability to acid and alkali. But the epoxy resin thermosets are easily flammable and their applications are limited. Traditional methods of imparting flame retardancy to epoxy resins are adding halogen-containing flame retardants into materials. Although halogen-containing flame retardants played an excellent effect in flame retardancy, toxic and corrosive gases released during combustion. The phosphorus-containing flame retardant, as an alternative, is considered to be a type of important compound due to its properties of "enveriment-friendly" and high efficient. In recent years, the phosphorus-containing flame retardants have become a hot topic of the flame retardants developed filed.In this paper, Diphenylphosphaneoxide as the raw material, three novel flame retardant curing agents, diphenyl-(2,5-dihydroxyphenyl)-phosphine oxide (DPDHPPO), diphenyl-(l,2-dicarboxyl-ethyl)-phosphine oxide (DPDCEPO) and diphenyl-(2,3-dicarboxyl-propyl)-phosphine oxide (DPDCPPO) were designed and synthesized with. Chemical structure of three flame retardant curing agents were characterized and analyzed by Fourier transform infrared spectroscopy (FTIR) and:H,13C,31P nuclear magnetic resonance (NMR) spectroscopy. The ratio of reactants, reaction temperature, reaction time, solvent amount and solvent type on yield effect were studied and optimized the reaction conditions. The thermal degradation of three flame retardant curing agents was assessed by thermogravimetric analysis (TGA). The test results revealed that three flame retardant curing agents had low thermal stability and a bad propensity for char-forming.The flame retardant epoxy resins were prepared using DPDHPPO and DPDCEPO (DPDCPPO) as flame retardant curing agents, and the flame retardant epoxy resins were studied and analyzed by vertical burning test (UL-94), limiting oxygen index (LOI), cone calorimeter test (CONE), thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and TGA-FTIR. The results of LOI and UL-94 indicated that EP/60 wt% PDA/40 wt% DPDHPPO, EP/80 wt% PA/20 wt% DPDCEPO and EP/80 wt% PA/20 wt% DPDCPPO systems achieved UL-94 V-0 rating, and the LOI values of EP/60 wt% PDA/40 wt% DPDHPPO, EP/80 wt% PA/20 wt% DPDCEPO and EP/80 wt% PA/20 wt% DPDCPPO systems were 31.9%,33.2% and 33.2%, respectively. The results of CONE showed that the main parameters (PHRR, THR, av-EHC) reduced and char residue had a slight increase compared to non-flame retardant epoxy resins. The results of thermogravimetric analysis (TGA) revealed that the initial degradation temperature of EP/60 wt% PDA/40 wt% DPDHPPO, EP/80 wt% PA/20 wt% DPDCEPO and EP/80 wt% PA/20 wt% DPDCPPO systems decreased gradually, but the char residual yield was much higher than non-flame retardant system. The results of SEM revealed that the char-forming ability of epoxy resins was improved and then a protective layer formed with the addition of flame retardant curing agents. The results of XPS reflected that phosphoric acid compounds were produced during the degradation process, and these acid compounds stimulated the formation of char layer. The results of TGA-FTIR showed that phosphorus-containing compounds were produced in the gas phase during the degradation process. The results of mechanical properties test indicated that the tensile, flexural strength and izod impact strength of EP/60 wt% PDA/40 wt% DPDHPPO, EP/80 wt% PA/20 wt% DPDCEPO and EP/80 wt% PA/20 wt% DPDCPPO systems decreased compared to non-flame retardant system. The results of water resistance showed that moisture adsorption behavior of EP/60 wt% PDA/40 wt% DPDHPPO, EP/80 wt% PA/20 wt% DPDCEPO and EP/80 wt% PA/20 wt% DPDCPPO systems reduced with the incorporation of flame retardant curing agents into epoxy resins, and the effect of the test on flame retardancy, thermal stability, combustion behavior and mechanical properties was low.Lignin is a good char-forming agent, and contain phenolic hydroxyl group, alcoholic hydroxyl and other active groups in the molecular structure. Lignin replaced o-Phthalic anhydride in different formulations, and prepared a series of epoxy resin thermosets. The flame retardant epoxy resins were studied and analyzed by vertical burning test, limiting oxygen index, cone calorimeter test, thermogravimetric analysis, scanning electron microscopy and TGA-FTIR. The results of LOI and UL-94 indicated that the value of LOI and Flame retardant grade were improved with the addition of lignin. The results of CONE showed that the main parameters (PHRR, THR) of EP/65 wt% PA/20 wt% DPDCEPO/15 wt% Lignin system had a decrese and char residue had a increase compared to EP/80 wt% PA/20 wt% DPDCEPO system. The results of thermogravimetric analysis (TGA) revealed that the initial degradation temperature decreased gradually, but the char residual yield was much higher than EP/80 wt% PA/20 wt% DPDCEPO system. The results of SEM revealed that the char-forming ability of epoxy resin was improved with the addition of lignin. The results of TGA-FTIR showed that phosphorus-containing compounds were still produced in the gas phase during the degradation process at high temperatures. The results of mechanical properties test indicated that the tensile, flexural strength and izod impact strength decreased with the addition of lignin.