Abstract:

Objective Global climate change and extreme weather have increased energy demand for refrigeration. The traditional refrigeration equipment not only consumes a lot of energy in the refrigeration process, but also brings additional heat consumption, further aggravating the urban heat island effect and energy crisis. With the growing concern for energy efficiency, and the key goal of carbon neutrality, much attention has been paid to zero-energy refrigeration technology. Hence, it is imperative to design an environmentally friendly radiation refrigeration material and to devise new refrigeration methods.

Method Poly(L-latic acid)(PLLA) and PLLA/poly(D-latic acid)(PDLA) fiber filaments were prepared by wet spinning technology, and then the fiber filaments were dispersed into uniform fiber dispersion liquid by high-speed shear emulsifier. PLLA and fiber aerogel modified by hydrophilic Al₂O₃ particles were prepared by chemical crosslinking of methyl trimethoxysilane under acidic conditions, modification of hydrophilic Al₂O₃ particles, ice crystal growth and freeze drying. The morphology of fiber and fiber aerogel was analyzed by scanning electron microscopy (SEM). The chemical properties of the samples were analyzed by Fourier infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and differential scanning calorimeter(DSC). The physical properties of aerogel were analyzed by material testing machine and water contact angle analyzer. Finally, the emittance, reflectance and radiation cooling effect were analyzed by infrared spectroscopy with gold integrating sphere, ultraviolet-visible-near infrared spectrophotometer and homemade outdoor device.

Results The optimum parameters for preparing PLLA/PDLA microfibers by wet spinning were obtained by testing the apparent viscosity and structural viscosity index of the spinning solution. The mass ratio of PLLA/PDLA was 10∶0 and 9∶1 and the concentration of the spinning solution was 10%. Under these parameters, smooth and uniform PLLA and PLLA/PDLA microfibers with diameter of (7.914±0.07) μm and (6.39±0.06) μm were successfully prepared. The stereo-polylactic acid fibers have been successfully prepared by wet spinning by FT-IR and DSC analysis. After ice crystal growth and freeze-drying technology, the porous fiber aerogels modified by Al₂O₃ particles were successfully prepared. With the increase of Al₂O₃ particle concentration, the coverage rate of Al₂O₃ on the surface of the aerogels increased. At the same time, the presence of C—O and C—O/H, and C—Si bonds were observed at the peaks of binding energy at 289.0, 286.9 and 284.8 eV. The presence of O—C—O, O—Si and bonds were observed at the peak binding energies at 533.7, 532.6 and 532.2 eV, and the Si—O and Si—C bonds in methyltrimethoxysilane(MTMS) were observed at 103.4 eV and 102.7 eV. At the same time, the presence of Al2p and low energy characteristic peaks were observed at the binding energies of 74.57 eV and 73.68 eV, which prove that the aerogel has formed a stable cross-linked network. Fiber aerogel has excellent hydrophobic and mechanical properties, and the stress of fiber aerogel was increased from 9.84 kPa to 16.71 kPa with the increasing Al₂O₃ contents from 5% to 15% under 60% compression. In the radiation refrigeration performance test, the reflectivity and emissivity of PLLA/PDLA aerogel were higher than that of PLLA aerogel, and when the Al₂O₃ content in the fiber aerogel was increased to 5.0%, the reflectivity reached the highest (81.91%), and the emissivity reached 96.18%.

Conclusion PLLA/PDLA fibers were prepared by wet spinning. The fibers were uniform, smooth and with a diameter of (6.39±0.06) μm. The fiber aerogel with stable structure was obtained by Al₂O₃ modification and freeze-drying. When the Al₂O₃ content in the fiber aerogel was gradually increased from 5.0% to 15.0%, the water contact angle of the fiber aerogel decreased from 153.6° to 128.31°. At the same time, the stress of the fiber aerogel increased from 9.84 kPa to 16.71 kPa under 60% compression. After outdoor radiation refrigeration effect test, it was found that the highest average reflectivity of fiber aerogel was 81.91%, the highest average emissivity was 96.24%, the maximum outdoor daytime (10:30-13:30) cooling temperature was up to 3.6 ℃, and the maximum outdoor night (18:00-21:00) cooling temperature was up to 4.7 ℃, showing strong infrared radiation ability.

Key words: polylactic acid, aerogel, wet spinning, freeze-drying, radiation refrigeration