The bio-polymer chitosan, modified with nitrogen, was used with the PIN FR DOPO, achieving UL 94 V-0 (3.2 mm) in epoxy. Chitosan is the second most abundant bio-polymer, after cellulose, and the most abundant organic compound containing nitrogen, and can be sourced e.g. from seafood wastes. Chitosan was reacted in methanol with nitrobenzaldehyde (C7H5NO3) to introduce NO2. The modified chitosan (CSN) was then cured into epoxy with DOPO (a standard PIN flame retardant, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide). With a total CSN + DOPO loading of 5% (both with 1/3 CSN, 2/3 DOPO and with the converse), UL 94 V-0 was achieved (3.2 mm) and LOI was increased from 22 (neat epoxy) to 32 – 34. Total smoke production and smoke production rate were reduced by more than two thirds. Tensile strength, flexural strength and transparency of the epoxy were maintained. The authors suggest that the fire performance is related to char production (supported by the carbon and nitrogen in the CSN) whilst hydroxyl and amine groups contribute to retaining epoxy mechanical properties.
“Epoxy resin modified with chitosan derivatives and DOPO: Improved flame retardancy, mechanical properties and transparency”, C. Zhou et al., Polymer Degradation and Stability 199 (2022) 109931 https://doi.org/10.1016/j.polymdegradstab.2022.109931