Mineral nitrogen FR flame retardants in wood function as indoor air purifier, removing oxygen ions. The natural mineral tourmaline (an acyclic silicate containing metals such as Na, Ca, K, Mg, Fe, Al, Li) and KH550 (a silane: 3-aminopropyl-triethoxysilane) were vacuum impregnated into balsa wood. The balsa had been partially delignified by soaking in acetate then in sodium hydroxide; to increase specific surface area and improve impregnation. The PIN FR increased LOI (Limiting Oxygen Index) from 21 (untreated balsa wood) to 25, reduced peak heat release rate by nearly 50% and smoke production rate by over 80%, achieving UL 94 V-1. The treated balsa showed a capacity to purify indoor air by releasing negative ions (1000 pcs/cm3/20s). This was also demonstrated visually by reduction of cigarette smoke from air.
“Wooden “Air Purifiers” with Fire-Retardancy and Smoke-Suppression Properties”, X. Sun et al., Adv. Sustainable Syst. 2022, 2200346 https://doi.org/10.1002/adsu.202200346