Optical properties of freeze-dried partial delignified balsa wood

Abstract

Radiative cooling (RC) is natural phenomenon in which an object passively cools itself by highly radiating mid-infrared waves to the cold outer space. The delignified wood (DW), in which lignin molecules are removed from natural wood, shows promising performance of daytime radiative cooling (DRC). However, the DW is usually hot-pressed during oven drying process which results in the collapse of wood cell walls. This problem hinders the application of DW as a scaffold for polymer-wood composite. In this study, we have fabricated a partial delignified wood (PDW) with the combination of the delignification and freeze-drying processes. The delignification process has been performed with boiling hydrogen peroxide (H2O2) solution in renewable system. It is found that the PDW turns to white colour after 30 hours of boiling time. The ATR-FTIR spectroscopy confirms that lignin molecules are removed and altered after the delignification process, and the strong emission peaks due to the molecular vibrations of cellulose in the PDW appear in the atmospheric windows. The averaged solar absorptivity of the PDW, measured by diffuse reflectance UV-Vis-NIR spectroscopy, is equal to 11% which is about three times lower than that of the natural wood. The relative drop of solar radiation absorption has maximum value at the wavelength 529 nm and the full-width-at-half-maximum equal to.