Phosphorus- and Nitrogen-Bridged Chitooligosaccharide as an Effective Flame-Retardant Coating of Fiber Cardboard

Fiber cardboard as one type of plant fiberboard is often used as a package and building material; however, its intrinsic flammability throws up big obstacles to its wide application. In this work, a phosphorus- and nitrogen-bridged chitooligosaccharide (CPDF) is synthesized via a two-step substitution method. The CPDF can be well dissolved in alcohol solvents and coated on the surface of the fiber cardboard. With increasing coating rate, the flame retardation efficiency and bending resistance of fiber cardboard increase. Because the coating rate is 21.7 wt %, the fiber cardboard exhibits outstanding fire safety, of which its limiting oxygen index is 28% and its UL-94 rating is B-1 grade. The peak heat release rate (PHRR), total heat release (THR), and average effective heat release (Av-EHR) decreased correspondingly. The inflaming retarding mechanism was put forward. A seed layer is produced initially to retard heat exchange and lower the damage to the internal polymer matrix; subsequently, phosphorus–oxygen radical dequenching as well as noncombustible gas dilution and a ripe carbon layer barrier cross each other, “braking” the “fire-car” completely. CPDF coating enhances the bending resistance of fiber cardboard, and stiffness climbed from 1124.8 to 1198.5 mN·m. Even after undergoing durability tests, CPDF retains its good inflammable retarding ability.