Oxidized regenerated celluloses to fabricate high fire safety for epoxy resin with super expansion char layer

Recently, oxidized regenerated celluloses has exhibited superior performance as a carbonization agent for polymers. However, it is a major challenge to fabricate fire safety and intumescent flame-retardant polymer composites with high-efficiency, and “heterogeneous char-forming agent (HCA)” is worth pursuing. Herein, regenerated cellulose (RC) was oxidized by H2O2 to achieve oxidized RC (ORC) with high carboxyl content. Interestingly, ORC, the potential HCA, achieved higher catalytic charring effect and lower content of the toxic gas release compared with the presented carbonization agent. 5 wt% ORC and 3.75 wt% microencapsulated ammonium polyphosphate (MFAPP) as intumescent flame retardants (IFR) were utilized to fabricate IFR epoxy resin (EP) (noted as EP/MFAPP/ORC). Amazingly, a super expansion ratio (41.5-fold) intumescent char layer was formed in EP/MFAPP/ORC27 (ORC with 27% carboxyl content) after the cone calorimetry test. Moreover, the peak heat release rate, total heat release and total smoke production of EP/MFAPP/ORC27 largely decreased by 55.6%, 61.8% and 62.2%, respectively. Furthermore, the residual char yield (41.8%) significantly enhanced by 9.7-fold, compared with those of EP. Meanwhile, it achieved 30.3% limiting oxygen index and V-0 burning test, and the flame retardant index (FRI) showed "good" flame-retardant performances. The concept of HCA and flame-retardant mechanism were elaborated by analyzing the evolution process of IFR-EP. These results demonstrate that ORC27, acts as HCA, is a novel and efficient strategy for fabricating fire safety EP with excellent flame-retardant efficiency.Graphic abstract