phosphaphenanthrene 10 oxide to undergo condensation and polymerization the phenol compound used in the preparation method of the present disclosure may be phenol o cresol m cresol p cresol o phenylphenol m phenylphenol p phenylphenol 2 6 dimethylphenol 2 4 dimethylphenol or a combination thereof the bisphenol compound may be biphenol bisphenol f bisphenol a p thiobisphenol bisphenol s dicyclopentadiene phenol resin or a combination thereof the phenol based compounds and or the bisphenol based compounds used in embodiments 1 10 and comparative examples 1 2 are listed in table 1 using different phosphor containing hardeners p 1 to p 10 as hardeners for bisphenol a novolac epoxy resin bne the bisphenol a novolac epoxy resin bne is mixed with the hardeners p 1 to p 10 homogeneously the equivalent ratio the epoxy group and the phenol group is 1 1 and 0 5 phr of 2 phenylimidazole of the total weight of the hardener and epoxy resin was added as a curing accelerator grounded into a powder in a mortar and mixed uniformly and then the mold was filled with this powder heated at a temperature of 150 u00b0 c and at a pressure of 50 kg cm 2 for 1 hour then heated at 170 u00b0 c for 2 hours and then heated at 200 u00b0 c for 3 hours to obtain the cured product using the phosphorous containing hardener in comparative example 1 p 11 as the hardener for bisphenol a novolac epoxy resin bne the bisphenol a novolac epoxy resin bne is mixed with the hardener p 11 homogeneously.