Phytic acid (PA), as a natural source of phosphorus, was immobilized on cotton (CO) in a layer-by-layer (LbL) approach with polyvinylamine (PVAm) as the oppositely charged electrolyte to create a partly bio-based flame-retardant finish. PVAm was employed as a synthetic nitrogen source with the highest density of amine groups of all polymers. Vertical flame tests revealed a flame-retardant behavior with no afterflame and afterglow time for a coating of 15 bilayers (BL) containing 2% phosphorus and 1.4% nitrogen. The coating achieved a molar P:N ratio of 3:5. Microscale combustion calorimetry (MCC) analyses affirmed the flame test findings by a decrease in peak heat release rate (pkHRR) by more than 60% relative to unfinished CO. Thermogravimetric analyses (TGA) and MCC measurements exhibited a shifted CO peak to lower temperatures indicating proceeding reactions to form an isolating char on the surface. Fourier transform infrared spectroscopy (FTIR) coupled online with a TGA system, allowed the identification of a decreased amount of acrolein, methanol, carbon monoxide and formaldehyde during sample pyrolysis and a higher amount of released water. Thereby the toxicity of released volatiles was reduced. Our results prove that PA enables a different reaction by catalyzing cellulosic dehydration, which results in the formation of a protective char on the surface of the burned fabric.

In the present study, DOPO-based alkoxysilane (DOPO-ETES) and amido alkoxysilane (DOPO-AmdPTES) were synthesized by one-step and without by-products as halogen-free flame retardants. The flame retardants were applied on cotton fabric utilizing sol–gel method and pad-dry-cure finishing process. The flame retardancy, the thermal stability and the combustion ehaviour of treated cotton were evaluated by surface and bottom edge ignition flame test (according to EN ISO 15025), thermogravimetric analysis (TGA) and micro-scale combustion calorimeter (MCC). Unlike CO/DOPO-ETES sample, cotton treated with DOPO-AmdPTES nanosols exhibits self-extinguishing ehaviour with high char residue, an improvement of the LOI value and a significant reduction of the PHRR, HRC and THR compared to pristine cotton. Cotton finished with DOPO-AmdPTES reveals a semi-durability after ten laundering cycles keeping the flame-retardant properties unchanged. According to the results obtained from TGA-FTIR, Py-GC/MS and XPS, the major activity of flame retardant occurs in the condensed phase via catalytic induced char formation as physical barrier along with the activity in the gas phase derived mainly from the dilution effect. The early degradation of CO/DOPO-AmdPTES compared to CO/DOPO-ETES, triggered by the cleavage of the weak bond between P and C=O, as the DFT study indicated, provides the beneficial effect of this flame retardant on the fire resistance of cellulose. Graphical abstract

Nylon-cotton textile blends (Nyco) are known to be challenging substrates regarding their flame-retardant functionalization and its durability. In this study, two different water-soluble flame retardants based on cyclophosphazene together with glycerol (GlyCPZ) and thioglycerol (ThioGlyCPZ) were synthesized enabling a waterborne finishing for Nyco. An extensive investigation of the flame-retardant performance and mechanism as well as the washing fastness was conducted. Thermogravimetric analysis (TGA) indicated changed degradation mechanisms for treated samples, which resulted in a reduced heat release capacity in microscale combustion calorimetry (MCC) measurements and an increased char yield. As a consequence, the limiting oxygen index (LOI) was enhanced from 20% for pure Nyco to 23% and 27.5% for GlyCPZ and ThioGlyCPZ, respectively. Also, the standardized flammability test according to ISO 15025 was passed. In addition, self-extinguishing characteristics were observed even after 10 washing cycles at 80 °C confirming the durability of the finishing. Sulfur was found to be gas phase active, as recombined sulfur compounds were identified in pyrolysis gas chromatography mass spectrometry (Py-GC/MS). Phosphorus was mainly active in the condensed phase, which was verified by analyses of digested burned textile samples using inductively coupled optical emission spectroscopy (ICP-OES).