Recyclable flame retardant paper made from layer-by-layer assembly of zinc coordinated multi-layered coatings

Recyclable flame retardant paper was made by constructing zinc coordinated multi-layered coatings consisting of positively charged polyethylenimine/melamine and negatively charged phytic acid. Herein, with the paper matrix as a carbon source, phytic acid was chosen as the acid source, while polyethylenimine and melamine served as the blowing agent. The successful deposition of zinc ion coordinated coating on the surface of paper fiber was confirmed by scanning electron microscopy and energy-dispersive X-ray. The weight gain of the paper (P) treated with 4 (P–4BL) and 8 (P–8BL) bilayers of coating were 5.5 and 10.1 wt%, respectively. After coordination with zinc ion, P–4BL–Zn and P–8BL–Zn were obtained with weight gains of 6.7 and 13.6 wt%, respectively. The thermal stability and flammability were evaluated by thermogravimetric analysis (TGA), horizontal burning and microscale combustion calorimeter (MCC) tests. TGA results demonstrated that the thermal stability of the treated sample was significantly enhanced at high temperature, as evidenced by the improvement of char residue at 700 °C for P–8BL–Zn to 31.6% as compared with that of untreated paper (1.6%). The P–8BL and P–8BL–Zn samples could achieve self-extinguishing in the horizontal burning test. MCC results showed that the peak heat release rate and the total heat release of P–8BL–Zn were reduced by 43 and 49% as compared to those of the untreated paper, respectively. Furthermore, the zinc ion coordinated coating displayed better in-water durability than the un-coordinated one, which made it possible recyclable flame retardant coating.