Abstract:

Objective Polylactic acid (PLA) fiber is an eco-friendly thermoplastic degradable synthetic fiber, which has biocompatibility and easy biodegradability but poor flame retardancy and easy to produce droplets. Biomass flame retardant has the advantages of innocuity, harmless and easy biodegradation. In order to solve the problem of flame retardancy and droplet resistance of PLA fiber products, it is urgent to develop flame retardancy technology for PLA fiber biomass to improve its flame retardancy and droplet resistance.

Method The subject design uses biomass chitosan (CS), biomass phytic acid (PA), and inorganic boric acid (BA) as the main raw materials, and uses the one bath method to finish the polylactic acid nonwovens with flame retardancy and anti-dripping, so as to improve their flame retardancy and anti-dripping performance. FT-IR, SEM, EDS and TG were used to characterize the chemical structure, surface morphology, surface element distribution, and thermal stability of flame retardant polylactic acid nonwovens. The flammability of flame retardant polylactic acid nonwovens was evaluated by vertical flammability tester and limiting oxygen index tester.

Results After the one bath method treatment of polylactic acid nonwovens, many granular or flaky substances co-deposited on the surface of the polylactic acid fiber, and the fiber surface became rough(Fig. 3), indicating that chitosan, phytic acid, boric acid and composites co-deposited on the fiber. After the one bath method treatment of polylactic acid nonwovens, the continuous burning and smoldering time of the flame retardant treated samples are zero, only a small amount of melting shrinkage without dripping, the carbon length is only 11.3 cm, and the limiting oxygen index of the original sample is 21.0% increased to 34.6% of the treated samples(Fig. 2 and Tab. 1), which improves the flame retardant and anti-dripping effect of polylactic acid nonwovens. After the one bath method treatment of polylactic acid nonwovens, the flame retardant treated samples contain not only carbon and oxygen, but also phosphorus (5.26%), boron (12.91%), nitrogen (4.26%) (Fig. 3 and Tab. 2). The reactive P—OH and B—OH in phytic acid and boric acid may form a complex with the —NH₂ of chitosan(Fig. 4). It is beneficial to promote the catalytic dehydration of PLA fiber into carbon and improve its flame retardancy and anti-dripping performance. After the "one bath method" treatment of polylactic acid nonwovens, the thermal cracking of flame retardant-treated samples in a nitrogen atmosphere includes three processes: decomposition, thermal degradation, and carbonization. The initial decomposition temperature of flame retardant treated samples is 139.3 ℃ earlier (from 299.1 ℃ of the original sample to 159.8 ℃), and the carbon residue at 800 ℃ reaches 14.57% (Fig. 5 and Tab. 3).It is shown that the flame-retardant system is helpful to promote the catalytic dehydration of PLA fiber to form carbon, and the dense carbon layer has a blocking effect, which can inhibit the transfer of combustible gas and the diffusion of oxygen, and conforms to the condensed phase flame retardant mechanism (Fig. 6).

Conclusion Biomass chitosan (CS), biomass phytic acid (PA), and inorganic boric acid (BA) were used as the main raw materials to finish the polylactic acid nonwovens in a one bath method. The continuous burning time and smoldering time of the treated samples were zero, with no melt drops. The carbon length was only 11.3 cm, and the limiting oxygen index reached 34.6%. The thermal cracking in the nitrogen atmosphere was roughly divided into three stages: initial decomposition, thermal degradation, and carbonization. The carbon residue reached 14.57% at 800 ℃. The improvement of flame retardancy, droplet resistance, and thermal stability of PLA fiber conforms to the mechanism of condensed phase flame retardancy, and the influence of wearing properties (whiteness, breaking strength, etc.) is small. The one bath method flame retardant system of PA/CS/BA improves the flame retardancy, anti-dripping effect, and thermal stability of PLA nonwovens. It conforms to the flame retardant mechanism of the condensed phase. This flame-retardant system provides a new environment-friendly flameretardant and anti-droplet strategy for PLA fiber and has good market application potential.

Key words: polylactic acid fiber, nonwoven, phytic acid, chitosan, flame retardant finishing, one bath method