Highly efficient phosphorous-containing flame retardant for transparent epoxy resin with good mechanical properties

To develop transparent flame-retardant epoxy resin (EP) having good mechanical properties, a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derived flame retardant (PAHDOPO) was prepared by a neutralization reaction between 10-hydroxy-9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-OH) and piperazine. When 7% PAHDOPO was added to EP, the obtained flame-retardant EP (EP7) can pass the UL-94 V-0 rating test and exhibit a high limiting oxygen index (LOI) value of 35.8%. Microscale combustion calorimetry investigation results proved that, compared with EP, the peak heat release rate (PHRR) and heat release capacity (HRC) of EP7 decreased by 34.6% and 33.6%, respectively. The observation of the char residue morphology indicated that the addition of PAHDOPO can lead to the formation of the continuous and dense char residue layer, which can improve EP flame retardancy. Meanwhile, compared with EP, EP7 exhibited similar transparency with less than 8% reduction in the average transmittance. Moreover, the mechanical properties of EP7 hardly decreased in comparison with EP. All these results indicated that PAHDOPO can be applied as a high-performance flame retardant of EP with good mechanical properties, transparency, and flame retardancy simultaneously.