Abstract:

Objective Cotton stalk bast microcrystalline cellulose (MCC) is microcrystalline cellulose extracted from waste cotton stalks. MCC fiber prepared from MCC has outstanding performances in coloration and moisture absoprtion. However, the flame retardant performance of MCC fiber is unsatisfactory, which limits the continued development of MCC fiber. The surface grafting method was adopted to modify graphene oxide (GO) by adding phosphorus. The modified GO was mixed with MCC to prepare composite fibers to improve the flame retardant performance of MCC fibers.

Methods In this paper, 9,10-dihydro-9-oxa-10-phosphophenanthrene-10-oxide (DOPO) was used as the modifier to modify graphene oxide (GO) by electrophilic substitution. In the process of GO modification, P—H in DOPO molecule opens the epoxy ring on the surface of GO, so that P forms a covalent bond with C on the epoxy ring on the surface of GO, and was grafted with GO. The phosphorus-containing flame retardant DOPO-GO prepared in the previous process was added to the MCC spinning liquid by physical blending, and DOPO-GO was uniformly distributed in the spinning liquid by ultrasonic dispersion, and MCC/DOPO-GO fiber with good flame retardant performance was prepared by wet spinning. The mechanical properties, thermal properties and flame retardant properties were analyzed.

Results DOPO was successfully grafted onto graphene oxide, decreases the DOPO-GO particle diameter,and destroys the arrangement regularity of GO lamellar, which was conducive to reducing GO agglomeration and improving the dispersion uniformity of GO in the spinning solution, and increases the break strength of MCC/DOPO-GO fibers by 450%. The enthalpy values of MCC/GO and MCC/DOPO-GO fibers are increased by 916.2 and 1 280.2 J/g, respectively, compared with MCC fibers. It shows that the thermal stability of the fiber is improved, indirectly showing that GO and DOPO-GO can improve the flame retardant properties of the fibers. The flame retardant DOPO-GO contains the green flame retardant element P, and the P element is evenly distributed in the fiber cross section, which can greatly improve the flame retardant performance. The intensity D-peak/intensity-G-peak (I_D/I_G) of the residual carbon after combustion of MCC/GO and MCC/DOPO-GO fiber decreased by 4.0% and 34.2%, respectively, compared with that of MCC fiber. In other words, DOPO-GO flame retardant can form a denser carbon layer, which can effectively prevent high-temperature ablation and further improve the flame retardant property of the fiber. When the dosage of flame retardant DOPO-GO was 7% of MCC, the limiting oxygen index LOI value of MCC/DOPO-GO fiber reached 27.3%, which was 66.5% higher than that of MCC fiber 16.4%, and the fiber changed from flammable fiber to refractory fiber.

Conclusion The thermal stability and mechanical properties of the MCC/DOPO-GO fibers modified by DOPO-GO are greatly improved, and the flame retardant performance of MCC/ DOPO-GO fibers changes from flammable to refractory. This study provides a new idea for the study of flame retardant performance of cotton straw husk microcrystalline cellulose.

Key words: cotton stalk bast microcrystalline cellulose, 9,10-dihydro-9-oxa-10-phosphoheterofi-10-oxide, graphene oxide, flame retardant performance, mechanical property