Influence of Silicon-Containing Compounds on Adhesives for and Adhesion to Wood and Lignocellulosic Materials: A Critical Review

Silicon (Si) is the second most abundant element in the earth's crust and it is no surprise that various contents of this element may be found in plants, and also in different lignocellulosic materials. This may have some undesired effects when products from wood or other lignocellulosic materials are produced and utilised, typically with regards to adhesion of adhesives or surface coatings. However, there are plenty of positive effects of silicon-containing compounds in wood-and lignocellulose-based products, so very often silicon-containing compounds are added into the products on purpose. Sometimes, they might decrease adhesion properties of adhesives and coatings, but their advantages are a major priority and care has to be taken to make sure the drawbacks do not become predominant. Moreover, sometimes the silicon-containing compounds in lignocellulosic products are a necessity. A typical example is silicon-containing compounds to function as coupling agents in wood-polymer composites. The number of publications on Si in lignocellulosics, including wood, is enormous. Even if we narrow our search on this topic only to reports related to adhesion, the number of papers is still astonishing. Therefore, this review does not discuss all recent publications about this topic and it must be rather regarded as a critical review, exhibiting the most important issues that must be taken into account when considering the influence of Si in lignocellulosic materials with regards to adhesion. On the basis of selected papers, it is hoped that the reader will gain a satisfactory insight into the topic. In the first part, after Introduction, there is a brief overview of the most important compounds of Si that are relevant for the field of wood science and technology. In the next section, the influence of the presence of silicon in adhesives or in coatings or substrates on adhesion is discussed. In order to understand these effects, it is essential to know about the interactions/reactions of Si-compounds with cellulose, lignin and wood, which is the topic of the subsequent section. Reactions with the components of lignocellulosics are essential for proper functioning of coupling agents in wood-polymer composites, so these agents are presented in this review as well. Finally, an insight into the vast field of lignocellulosic composites is given, with a focus on a variety of different compounds of silicon and their function in the composites.