Surface Modification of Wood
Modification of wood is a method that is used to improve the material properties of wood by altering its chemical nature and in a broader sense it is also a passive process, where changes in properties also occur, but without an alteration of the chemistry of the material. A typical example of passive modification is, for instance, filling of wood cell lumens with resins. Surface modification of wood has the same purpose as the bulk modification, but it is restricted only to treatments of first few layers of wood surface. This review provides an overview of the development of surface modification of wood in the last 10–15 years. This topic has been extensively studied, especially in relation to research and production of wood based composites. However, in the present review the references dealing with modification of lignocellulosic and wood particulates (for instance, wood dust, flakes, and fibres) are in general excluded and the review is focused on surface modification of solid wood and of veneers. Mainly, modification in a narrow sense was considered, i.e. alternating chemical composition of wood building polymers and other compounds in wood. Nevertheless, some less conventional passive surface treatment techniques, for example sol-gel processes, deposition of nanoparticles or mechanical operations have been considered as well. In the first part, an overview of the most common surface modification methods is presented and in the second part the properties that are affected are illustrated through selected publications in literature. It is shown that the topic of surface modification of wood is currently a very viable area of research in wood science and technology. Wood surfaces can be treated by plasmas from various sources; by chemical or enzymatic grafting of functional molecules; coating by application of sol-gel methods, including deposition of nanoparticles; by surface impregnation and with various mechanical operations. The target properties to be improved or even introduced are mostly surface activation for better gluing and adhesion of surface coatings, wettability and resistance to weathering. However, resistance to wood pests, fire retardancy and mechanical properties, especially hardness and abrasion resistance, can also be effectively improved by wood surface modification.