Investigation of bisphenol-substituted spirocyclic phosphazenes as cotton textile-based flame retardants

Authors: Easson, Michael; Jordan, Jacobs; Chang, Sechin; Bland, John; Condon, Brian

Submitted to: Journal of Engineered Fibers and Fabrics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2020
Publication Date: 5/12/2020
Citation: Easson, M.W., Jordan, J.H., Chang, S., Bland, J.M., Condon, B.D. 2020. Investigation of bisphenol-substituted spirocyclic phosphazenes as cotton textile-based flame retardants. Journal of Engineered Fibers and Fabrics. 15:1-10. https://doi.org/10.1177/1558925020920887.
DOI: https://doi.org/10.1177/1558925020920887

Interpretive Summary: The latest generation of flame retardants contain phosphorous and nitrogen and stop cotton fabric from burning by reaction with the cellulose polymer of the fabric in a process called phosphorylation. Instead of burning, a protective layer of char is formed on the surface of the fabric which provides a barrier to further decomposition. The formation of protective char is aided by nitrogen which acts as a blowing agent, expanding the char layer with gaseous bubbles of nitrogen, keeping the destructive heat away from the fabric. Two novel flame retardants were prepared and applied to cotton fabrics. These treated fabrics then underwent a battery of tests to determine how effective the flame retardants were. Based upon testing, these flame retardants performed well.

Technical Abstract: Industrial, governmental, and academic research groups have sought for years to synthesize effective flame retardant compounds for application to cotton textile and many reviews have been written on this subject [1-3]. In recent years legislative action in the European community has restricted the use of flame retardants (FR), causing the focus of research to shift to more environmentally friendly nitrogen and phosphorus-based compounds. Phosphorus-nitrogen based flame retardant compounds retard in the condensed phase of combustion through a mechanism of protective phosphorylation of the cellulosic polymer and the formation of protective char [4-6]. Given the pressing need to find evermore effective and environmentally friendly flame retardants, this research group began investigating phosphazene derivatives which have high percentages of phosphorus and nitrogen in their molecular structures. There are many journal articles written and patents issued concerning polymeric phosphazene blends [7-13], but this research focused on monomeric spirocyclic phophazene derivative forms that are covalently linked to cotton. Specifically, this research searched for easily-synthesized, high yielding phosphazene derivatives, which offer promise as flame retardants and are covalently linked to cotton fabrics, unlike the aforementioned blends which are extruded as polymers or coated onto fabric. Additionally, this investigation explored the thermal decomposition of the FR using micro-scale combustion calorimetry (MCC), differential scanning calorimetry (DSC), Fourier transform infrared thermogravimetic analysis (FTIR-TGA) as well as basic TGA. Limiting Oxygen Index (LOI) analyses determined the minimum concentration of oxygen that would support combustion. Scanning electron microscopy (SEM) provided images of the textile fibers after application of the flame retardant and burning. Compounds TSDBC 3 and DBDBC 4 were synthesized in yields of 94% and 85%, respectively. Chromatography was not required, and multi-gram quantities were readily obtained by precipitation of the final product from the reaction solution. The high yields and simple work-up make these compounds ideal candidates for investigation as flame retardants. These two compounds were chosen because they each possess favorable structural characteristics that are beneficial in flame retardants. Specifically, each compound has a phosphorous-nitrogen core that: 1) provides an acid source for phosphorylation of the decomposing cellulose polymer, thus preventing levogucosan formation; 2) provides gaseous nitrogen during thermal decomposition for dilution of oxygen and as a char blowing agent. Additionally, TSDBC 3 and DBDBC 4 possess a bisphenol ring system that 3) provides a carbon source for char formation. Neither TSDBC 3 nor DBDBC 4 was soluble in the most common laboratory organic solvents with the exception of chloroform and both were insoluble in water. When the textile fabrics were treated with each flame retardant, they maintained a high degree of whiteness, although some stiffening of fabric hand was noted. Cotton twill treated with low concentrated solutions of TSDBC 3 and DBDBC 4 were analyzed for flame retardant properties using LOI, TGA-FTIR, TGA, DSC, MCC and SEM. The results are reported herein.