A novel strategy to fabricate nylon 6 based flame retardant microfiber nonwoven fabric with durability

A fundamental challenge in nylon-6-based microfiber non-woven fabrics (NWFs) are their flammability, which limits their industrial application in different fields. Inspired by chrome tanned leather with excellent flame retardancy, we developed a photo-grafting method to introduce carboxyl groups (–COOH) onto polyurethane-impregnated nylon-6-based microfiber nonwoven fabric (PNWF). By simulating the chrome tanning process of natural leather, we utilized a coordination reaction between chromium ion (Cr3+) and -COOH to fabricate the flame retardant PNWF. Compared with the control sample, the modified PNWF show an increased the limited oxygen index (LOI) value from 19.9% to 27.1%, and their peak heat release rate (pk-HRR) and peak smoke production rate (pk-SPR) were reduced by 32.1% and 54.2% respectively, suggesting a significant improvement in flame retardancy and the amelioration of smoke inhibition. In addition, we employed scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) to evaluate the flame retardant mechanisms of modified PNWF. EDS and XPS analysis of the chars of PNWFs after cone calorimetry tests showed the formation of a Cr2O3 protective layer for Cr3+-crosslinked PNWF and the coordination between Cr3+ and –COOH enhances the degree of graphitization of char residue, both promoting the flame retardancy of PNWF. Due to the chemical bonding between Cr3+ and –COOH, the resultant PNWF retained 95.01% of the original LOI after undergoing the laundering procedure 50 times, revealing durable flame retardant characteristics.