Abstract:

Objective Polyurethane (PU)-coated polyamide 6 (PA6) fabrics are widely used in various applications such as parachutes, luggage fabrics, tent covers, and other canopy materials. However, the PU-coated PA6 fabrics are highly flammable and cannot meet the fire-proof regulations. Besides, the generation of droplets during combustion can further increase the risk of fire. Therefore, it is essential to develop an environmentally friendly and efficient flame-retardant system to enhance the flame retardancy of PU-coated PA6 fabrics. Phytic acid (PA) is primarily sourced from plant seeds, roots, and stems, making it a valuable resource, the advantages of which include natural degradability, eco-friendliness, and high phosphorus content (28%). This study aims to investigate the effectiveness of biomass phytate salt in modifying water-based PU coatings to improve the flame retardancy and reduce the dripping behavior of coated PA6 fabrics.

Method Waterproof PA6 fabrics with grade 4 waterproof performance were developed to prevent permeation during the PU coating process. The phytate salt flame-retardant was prepared by combining PA and sodium phytate to a pH of 7.5 with a mass ratio of 5∶17. The phytate salt was then incorporated into the PU resin along with the capping polyisocyanate crosslinker 903. The resulting flame-retardant PU was applied to the PA6 fabrics using a laboratory small-scale scraper, and the coated PA6 fabrics were pre-dried at 110 ℃ for 3 min and baked at 150 ℃ for 3 min. The coating add-on to the fabrics was controlled to 50, 100, and 150 g/m² to achieve specific performance characteristics.

Results The flame retardancy, washing resistance, static water pressure resistance, thermal stability, and flame-retardant mechanism of coated PA6 fabrics were analyzed. The results suggested that the phytate salt was highly compatible with PU and did not significantly affect its film-forming performance. PA6 fabrics coated with phytate salt-modified PU exhibited the self-extinguishing properties during the vertical burning test without producing melting drips. The damaged length decreased from 30.0 cm to 12.4 cm, and the limiting oxygen index was increased to 29.0% from 20.8% of the pristine PA6 fabric. As a result, the coated PA6 fabrics met the B1 classification according to standard GB/T 17591. Even after 10 washing cycles, the coated PA6 fabrics retained their self-extinguishing properties, demonstrating good flame retardancy and washing resistance. The introduction of phytate salt-based PU coating had a significant impact on the thermal degradation of the PA6 fabric. The initial degradation temperature of the coated PA6 fabric shifted to lower temperatures, indicating that the phytate salt decomposed at a lower temperature and promoted the dehydration of the PA6 fabric, thereby accelerating its thermal degradation. When the coated PA6 fabrics were calcinated at varying temperatures, significant dimensional changes at the beginning were observed, and bubbles on the surface of the residue were formed. These observations could be attributed to the thermal degradation of the phytate salt, which caused the dehydration of the coated PA6 fabric. Additionally, the phosphorus content of the char residues of coated PA6 fabric showed a slight increase below 400 ℃, followed by a significant increase at higher temperatures. This behavior was linked to the thermal degradation of the PU-coated PA6 fabric, which released gases such as acetaldehyde, methane, and carbon monoxide after reaching 400 ℃. These findings were consistent with the thermogravimetric analysis, confirming that phosphorus primarily operated in the condensed phase during the combustion process.

Conclusion Phytate salt demonstrated the high flame-retardant efficiency for PA6 fabrics coated with PU. Even after undergoing 10 washing cycles, the coated PA6 fabrics were able to pass the vertical burning test and achieve a B1 classification. Analyses conducted on thermal and char residue revealed that the phytate salt system primarily improved flame retardancy through a solid-phase flame-retardant mechanism in these PU-coated PA6 fabrics. Overall, the modification of phytate salt into water-based PU coatings presents a promising and environmentally friendly solution for enhancing the fire safety of outdoor PA6 fabrics coated with PU and holds significant potential for commercial applications.

Key words: polyamide 6 fabric, polyurethane, phytic acid, flame-retardant coating, fame-retardant fabric, functional textile