Highly effective flame retardant lignin/polyacrylonitrile composite prepared via solution blending and phosphorylation

Aiming at improving the thermal stability and flame retardancy of polyacrylonitrile (PAN), a novel flame retardant PAN composite was prepared via a facile approach. First, lignin (LIG) and PAN were dissolved in dimethyl sulfoxide (DMSO) to form a uniform blend solution of LIG and PAN, then the solution was poured into the mould to scrape the LIG/PAN composite film. Subsequently, the flame retardant LIG/PAN composite (FR-LIG/PAN) was fabricated via phosphorylation. The structure of pure PAN, LIG, LIG/PAN and FR-LIG/PAN composite was characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The thermal and combustion properties of the samples were evaluated by using thermogravimetric analysis (TGA) and cone calorimeter test (CCT). In addition, the cross section and char residue of FR-LIG/PAN composite after combustion were observed by scanning electron microscopy (SEM). Furthermore, the elemental compositions and contents of the cross section and char residue of FR-LIG/PAN composite after combustion were tested by energy-dispersive X-ray spectroscopy (EDS). The results indicated that hydroxyl group and benzene skeleton were present in LIG/PAN and the phosphorus element was successfully introduced into FR-LIG/PAN composite via phosphorylation. The amount of char residue for FR-LIG/PAN at 800 °C is more than those of pure PAN and LIG/PAN. SEM photos indicate the expansion characteristics of the char residue, and EDS results prove that the uniform distribution of elements in the cross-section of FR-LIG/PAN composite. Moreover, FR-LIG/PAN exhibits the lowest peak heat release rate (pHRR) and smoke production rate (SPR). These results demonstrated that FR-LIG/PAN had excellent flame retardancy and char forming ability. In addition, the composition of the gases evolved from pyrolysis process of FR-LIG/PAN was analyzed by thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR). The main pyrolysis products in gas phase are H₂O, CO₂ and PO•. This work proposes one facile method for preparation of practical flame retardant PAN composite.