Synthesis of multifunctional flame retardant with toughening and transparency and its application in epoxy resin

The transparency and mechanical properties of EP are inevitable influenced by the addition of most conventional flame retardants. A multifunctional flame retardant, phosphorus-containing pyrazolyl derivative (DPAP) was prepared, which may provide exceptional flame retardancy, toughness, and transparency to EP materials. With only 10% DPAP added, the EP/DPAP composite can achieve a UL-94 V-0 rating and LOI of 35.2%. Compared to pure EP, the PHR and THR of EP/DPAP-10 (containing 10%wt DPAP) are dramatically reduced by 40.9% and 23.2%, respectively. Meanwhile, the elongation at break of the sample surprisingly increased by 30.2% and the transmittance only decreased from 82% to 72%. The combination of a large number of hydrogen bonds and low crosslink density enhances the toughness of EP/DPAP composites. Although the flame retardant function is added, the EP/DPAP composite still maintains a high transparency. The results show that the addition of flame retardant function has little effect on the transmittance of EP. All of these findings revealed that the EP/DPAP composite with exceptional overall performance has the potential to be used in a broader range of applications.