Facile Approach for Preparing Chitosan/Phosphaphenanthrene-Containing Epoxy Resins with Superior Flame Retardancy and Mechanical Properties

Flame-retardant epoxy (EP) composites with excellent mechanical properties that can be produced via facile, cost-effective, and environmentally friendly fabrication are highly desirable. A variety of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivatives are used as flame retardants for EP resin due to their high efficiency, but their synthesis is often complicated. In this work, a biomass chitosan (CS) was chosen as a synergist for DOPO to obtain high-performance, fire-safe EPs through simple blending. The total loading of CS and DOPO was limited to 10 wt % in EP, with mass ratios of CS/DOPO being 1:1, 1:2, 1:3, 2:1, or 3:1. At CS/DOPO ratios of 1:2 and 2:1, the obtained EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 achieved a UL-94 V-0 rating, and their limiting oxygen index increased to 33.7 and 32.5%, respectively. Compared with virgin EP, the heat release and smoke generation of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 decreased, especially the total smoke production decreasing by 61.9 and 71.0%. Furthermore, the flexural strength of EP/10% CS1/DOPO2 and EP/10% CS2/DOPO1 was 36.0 and 38.4% higher than that of EP, respectively. The results indicated the synergistic effects of DOPO and CS in enhancing both flame retardancy and mechanical properties of EPs. This work offers a simple and practical strategy for preparing flame-retardant EPs with satisfactory mechanical properties.