9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-Based Diamine Flame Retardant for Polyurea with Superior Mechanical Properties
Jing Yang, Qinyong Liu, Ximei Wen, Xiaowen Liu, Xiaojun Di, Yujie Kang, Jun Sun, Hongfei Li, Xiaoyu Gu, Sheng Zhang
Abstract
A dual-amino reactive flame retardant (DOPO-A) was synthesized via a condensation reaction of 1,3-diaminopropane, 1,4-phthalaldehyde, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-derived diamine (DOPO). The flame retardancy of the polyurea (PUA) was significantly enhanced by incorporation of 14 wt % DOPO-A, achieving a limiting oxygen index (LOI) of 25.5% and a UL-94 V-0 rating. Compared to the control PUA, the peak heat release rate (pHRR), total heat release (THR), and average effective heat of combustion (av-EHC) reduced by 24, 29, and 26%, respectively. The residual char yield increased to 15.9%, which was 3.3 times higher than that of the control PUA (3.7%). DOPO-A promoted the formation of a dense protective char layer and reduced the combustible gas release during combustion. Notably, 14 wt % DOPO caused severe mechanical degradation (61.5% loss in tensile strength and 84.3% loss in elongation at break); by using 14 wt % DOPO-A, the losses were only 3.6 and 13.2%, respectively. This mechanical maintaining should be ascribed to the covalent bonding of DOPO-A with the PUA network during curing. This research provides valuable insights for designing reactive flame retardants that preserve the elasticity of PUA.