Abstract:

Objective Viscose fiber is widely utilized in textiles owing to its excellent moisture absorption and comfort properties. However, its high flammability seriously restricts its application in fields with stringent fire safety requirements, such as military and firefighting protective clothing. Hence, the development of flame-retardant viscose fibers is of significant practical importance. The objective of this work is to synthesize a flame retardant that not only complies with the requirements of the viscose spinning process but also endows the fibers with high flame retardancy while preserving their other essential properties.

Method In this work, an aniline-containing polyphosphamide flame retardant was synthesized via polycondensation of phosphorus oxychloride, piperazine, and aniline. The molecular design incorporated phosphamide group to enhance the stability against hydrolysis. The aniline was introduced to improve the stability of char and flame-retardant efficiency. Then, the aqueous dispersion of aniline-containing polyphosphamide(PPAB) was blended into viscose dope and prepared the flame-retardant viscose fibers via wet spinning. The dispersion stability of PPAB in the spinning dope was investigated, along with the flame retardancy, combustion behavior, mechanical properties, and flame-retardant mechanism of the flame-retardant fibers.

Results Optical microscopy and multiple light scattering analysis discovered the uniform dispersion of PPAB in the viscose spinning dope without agglomeration. The dispersion stability maintained over 24 hours as indicated by an almost unchanged backscattering value. Scanning electron microscope revealed that the surface of the flame-retardant viscose fibers remained uniform and smooth at lower PPAB loadings (10%), while mild wrinkling occurred with higher additive content (15%-25%). No distinct particles were observed on the surface or cross-section of the fibers, suggesting excellent compatibility. Flame retardant test showed that with 20% PPAB incorporation (VF/PPAB₂₀), the limiting oxygen index (LOI) of the fiber reached 29.8%, demonstrating effective flame retardancy. A further increase to 25% PPAB led to an LOI of 30.6%, suggesting a trend toward saturation in flame-retardant efficiency. After 50 laundering cycles, the LOI of VF/PPAB₂₀ decreased only marginally from 29.8% to 29.4%, representing a reduction of 1.3%. Cone calorimetry tests showed a decrease in the peak heat release rate from 136 kW/m² to 83 kW/m² and in the total heat release from 6.0 MJ/m² to 4.5 MJ/m², while total smoke production remained low. Meanwhile, the tensile strength decreased from 1.8 cN/dtex to 1.5 cN/dtex and whiteness decreased from 64.2% to 60.5% with 20% PPAB. The moisture regain of modified fiber was 13.9%, in comparison with 15.6% for pure viscose. Thermogravimetric analysis revealed an increase in residual char yield at 700 ℃ from 15.6% to 30.8% for VF/PPAB₂₀. TG-IR analysis indicated suppression of flammable gas release and detection of phosphorus and nitrogen-containing fragments in the gas phase. Post-combustion residue characterization via SEM and XPS showed expanded char structures with phosphorus-rich domains. This indicates that PPAB possesses both condensed-phase and gas-phase flame-retardant activities, and shows outstanding stability of char.

Conclusion PPAB exhibited excellent dispersibility and stability in the viscose spinning system, meeting the requirements of the viscose spinning process. The flame-retardant fibers showed uniform morphology and good compatibility between the PPAB and cellulose matrix. PPAB significantly enhanced the flame retardancy of fibers, endowing the fibers with high LOI values and remarkable laundry durability. Cone calorimetry results confirmed a substantial reduction in heat release rate and total heat release and remained low smoke production, indicating significantly improved fire safety. Furthermore, the incorporation of PPAB did not substantially compromise the mechanical properties, moisture regain, or whiteness of the fibers, indicating well-preserved practicality and comfort for end-use applications. PPAB functioned in condensed and gas-phase flame-retardant mechanism. It dominantly enhanced the stability of char, leading to a stable and expanded protective barrier in the condensed phase. It released nitrogen-containing species in the gas phase that diluted flammable gases. Given its excellent applicability and high-efficiency flame retardancy in viscose fibers, PPAB is expected to be extended to other regenerated cellulose fibers.

Key words: viscose fiber, flame-retardant, flame-retardant mechanism, fire behavior, functional fiber, wet-spinning