Bio-Based Water-Repellent Coating for Wood

Abstract

Wood is one of the oldest building materials and is currently becoming increasingly popular again due to the present growth in awareness in society regarding environmental issues. Wood is hygroscopic and porous, i.e. it not only absorbs moisture from the atmosphere, but also actively and quickly sucks up large amounts of water through its capillary system particularly via the end grain. This leads to dimensional changes, but above all the wood becomes susceptible to rot. As a result, for many applications wood needs to be protected from moisture. Besides a large number of well-established petrochemical products, there are still some traditional, natural finishes available. The most prominent ones are linseed oil and tung oil. Due to their large content of linoleic acid, both harden in air via autooxidation. However, to achieve a decent degree of protection, both oils need to be applied in multiple layers and the final coating requires at least several days to harden completely. In addition, linseed oil interferes with the food chain and tung oil is said to be toxic in some publications.

We have recently developed an alternative coating for wood based on chitosan, which can be obtained from food-industry waste. The coating is applied from an aqueous solution and adheres nicely to wood surfaces due to the structural similarity of chitosan and cellulose. The polymerised film reliably seals the end-grain of wood against liquid water. Currently, the polymerisation requires the films to be heated to 70 °C in an oven, but a method for UV curing is being worked on.