Abstract:

Objective In order to address the problems on poor thermal insulation performance of poly-acrylonitrile (PAN) fibers and the difficulty of their large-scale production, carboxylcellulose nanofibers (C-CNF) were introduced in situ during the emulsion polymerization stage to prepare a meltable PAN/C-CNF polymer. This polymer was then blended with SA-PAN filler derived from graft-modified silica aerogel (SA), and PAN/C-CNF/SA-PAN composite thermal-insulating fibers were fabricated by melt spinning. Subsequently, the structure, thermal properties, thermal insulation properties, and mechanical properties of the composite fibers were systematically analyzed and discussed.

Method Using OP-10 as the emulsifier and sodium dodecyl sulfate (SDS) as the surfactant, the PAN/C-CNF polymer was synthesized by emulsion polymerization. SA was hydrolyzed with vinyl triethoxysilane (VTES) in anhydrous ethanol to obtain SA-VTES. Acrylonitrile (AN), anhydrous ethanol, and water were then added to a beaker and thoroughly mixed to prepare the modification solution. SA-VTES was added to the modification solution and rapidly dispersed using a homogenizer, followed by stirring for 2 h to ensure uniform mixing, and then reacted for 2 h in a UV curing machine. After completion of the reaction, SA-PAN was obtained. PAN/C-CNF/SA-PAN blends containing different proportions of SA-PAN were thoroughly mixed in a planetary ball mill, and then extruded and pelletized using a screw extruder to obtain the spinning feedstock.

Results The thermal conductivity of SA was 0.045 1 W/(m·K), while that of SA-PAN was 0.055 2 W/(m·K). Both values remained at relatively low levels, and the thermal conductivity of SA-PAN increased by only 0.010 1 W/(m·K) after modification. Its thermal diffusivity increased from 0.196 1 mm²/s to 0.219 9 mm²/s, indicating that it still possessed good thermal insulation performance. As the SA-PAN content increased, the crystallinity of the fibers gradually decreased from 38.2% to 22.0%. Compared with PAN fibers, the breaking strength of PC fibers containing 0.1% C-CNF increased by 32.0%, and the elongation at break of the as-spun fibers was also significantly improved. Fibers with an SA-PAN content of 1% exhibited the optimum breaking strength of 3.50 cN/dtex, 45.2% higher than that of PAN fibers, indicating that the addition of a small amount of SA-PAN can improve the mechanical properties of PAN-based fibers. The thermal conductivity of PAN fibers was 0.101 9 W/(m·K), while that of PC fibers was 0.119 9 W/(m·K). The slight increase in the thermal conductivity of PC fibers was mainly attributed to the high specific surface area of C-CNF, which enhanced the heat transfer capability of the material to a certain extent. After the addition of 1% SA-PAN, the thermal conductivity of PAN-based fibers decreased by 29.4%. With further increases in SA-PAN content, the thermal conductivity of the fiber fabrics decreased by 32.7%, 37.2%, and 44.8%, respectively. In addition, the thermal diffusivity of the fibers also decreased with increasing SA-PAN content, indicating that the introduction of SA-PAN significantly improved the thermal insulation performance of melt-spun PAN-based fibers.

Conclusion PAN was successfully grafted on the SA surface, and SA-PAN was prepared with a grafting rate of 20.4%. In the emulsion polymerization stage, 0.1% C-CNF was added in situ to obtain meltable PAN/C-CNF polymers, and the PAN/C-CNF polymers were blended with SA-PAN in different proportions, and the PAN/C-CNF polymers were successfully prepared by the melt spinning method to prepare PAN-based fibers with excellent thermal insulation and mechanical properties. The introduction of C-CNF significantly improved the flexibility and tensile strength of the fibers; The tensile strength of the fibers increased by 32%, and the tensile strength increased from 2.41 cN/dtex to 3.18 cN/dtex, and the addition of SA-PAN effectively reduced the thermal conductivity of the fibers and improved the thermal stability and residnal carbon yield, and the thermal conductivity of the fibers increased from 0.119 9 W/(m·K) to 0.075 3 W/(m·K), decreased by 37.2%, and the residnal carbon yield of PAN-based insulation fibers increased from 9.11% to 16.41% and 44.85% after adding 1% SA-PAN and 3% SA-PAN, respectively. However, a small amount of SA-PAN can improve the mechanical properties of the fibers, and the tensile strength of the PAN-based fibers is increased to 3.50 cN/dtex by adding 1% SA-PAN, and the mechanical properties of the fibers reduced due to the addition of too much SA-PAN, but the tensile strength can still reach a good level of 2.38 cN/dtex after adding 5% SA-PAN.

Key words: polyacrylonitrile, cellulose nanofiber, silica aerogel, melt spinning, thermal insulation fiber