A top-down strategy for the preparation of flame retardant, robust, and transparent wood-derived films

Transparent wood is a promising material with many advantages, including high optical transmittance, adjustable optical haze, and excellent mechanical properties. However, the manufacture of transparent wood always requires polymer filling, which affects its sustainability; the flammability of transparent wood also limits its development and application in more fields. Herein, a flame retardant transparent wood film (FR-TWF) was prepared by a top-down chemical treatment. Delignification and TEMPO-mediated oxidation can effectively remove lignin from natural wood and introduce carboxyl groups into the cellulose for further chelation with Al ions. Then, FR-TWF was obtained through self-densification by forming fully collapsed wood cell walls and elastic lumen after drying. The obtained FR-TWF had high optical transmittance (∼76.1%) and haze (∼75.2%); its tensile strength reached 354.3 MPa, exceeding most existing plastic films. Compared with natural wood, the peak of heat release rate and total heat release of FR-TWF decreased by 91.4% and 77.1%, respectively. The excellent flame retardant property of FR-TWF is attributed to the generation of graphitized carbon layers during combustion and the release of more water vapor at high temperatures. Furthermore, the FR-TWF merits being price moderate, sustainable, and biodegradable, which can be widely used in flexible device substrates or optical management devices.