Hydrophobization of up-conversion luminescent films based on nanocellulose/MF$_2$:HO particles (M = Sr, Ca) by acrylic resin

Luminescent hydrophobic composite films based on nanocrystalline (CNC) and nanofibrillated (CNF) cellulose matrix with up-conversion MF$_2$:Ho (M = Ca, Sr) particles and acrylic resin (ACR) as a coating have been synthesized. Flexible, translucent composite films were obtained by molding from the CNC/CNF suspensions with up-conversion particles. ACR coating was applied to the composite film by spraying. Studies have shown that ACR coating with a layer thickness of 7–10 $\mu$m provides hydrophobic properties for the films, increasing the water contact angle up to 100$\pm$2$^\circ$ with a simultaneous improvement in the luminescent properties. Transparency of CNC/CNF/MF$_2$:Ho-ACR films in the visible and near IR region improves by 20–25% without compromising the flexibility and thermal stability. The manufactured water-resistant composite films can be utilized as potential photonics materials, in particular for visualization of near-IR laser radiation and luminescent labels.