A simple synthesis of a guaiacol based reactive flame retardant and its application in epoxy resins

Abstract

Under the premise of ensuring the flame retardancy and heat resistance of flame-retardant epoxy resin (FREP), there are broad application prospects for the development of alternative flame retardants which are safer and more environmentally friendly than halogen-based flame retardants. A phosphorus-containing bio-based flame retardant (PGD) with reactive groups was synthesized in this study by a one-step reaction of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO), 1,4-phthalaldehyde and guaiacol. When synthesizing PGD, we chose the bio-based raw material guaiacol and the green solvent ethanol to meet the environmental requirements. We performed a series of experiments to verify the successful synthesis of PGD and to explore the properties of PGD-modified FREP. The test results indicated that the curing temperature of FREP was significantly reduced after adding a certain amount of PGD. Meanwhile, the FREP has superior flame-retardant properties at low phosphorus content, with EP/PGD-5.0 (phosphorus content: 0.39 wt%) obtaining a V-0 rating in a UL-94 test, and a limiting oxygen index (LOI) value of 32.6%. In addition, the peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP) of the EP/PGD-7.5 sample were 38.8%, 23.1% and 23.0% lower than those of EP, meanwhile the char residue content was improved by 24.1%. It is worth noting that the glass transition temperature (T_g) of EP/PGD-2.5 was increased by 4.7% compared to EP, and the T_g of EP/PGD-7.5 was not much lower than that of EP. In addition the bending tests and tensile tests showed that the mechanical properties of FREP were better than those of EP, with the flexural strength and flexural modulus of EP/PGD-7.5 increased by 12.0% and 29.3% compared to those of EP, and the tensile strength and modulus of EP/PGD-5.0 improved by 13.3% and 14.0%, respectively. In summary, EP after addition of PGD showed excellent flame retardancy and heat resistance while having relatively good mechanical properties.