Wood used outdoors is exposed to many sources of degradation. Each of these sources (UV, moisture, decay fungi, wind, dust, etc.) bring their own contribution to the degradation, but they can also interact together [1,2]. Moisture is one of the most important agents of wood degradation, as it provokes dimensional changes, allows the growth of decay fungi and molds, promotes the leaching of wood treatments and water-soluble wood compounds, and so forth. After an extended period, the dimensional changes will make the wood warp, crack, and lose its coating, exposing unprotected wood to environmental hazards and hastening its degradation [3,4].

Unlike wood used indoor, where humidity is usually lower and more effectively controlled, wood exposed outdoor is subject to a vast range of moisture conditions (dry periods, wet periods, rain, snow) [5]. These variations in humidity are the source of the dimensional changes, as they influence the moisture content of the wood, which shrinks and swells accordingly [3]. In order to reduce its degradation, wood must therefore be treated with preservatives and good building designs must be encouraged [5]. Impregnation treatments with different resins (phenolic, amino, silane, acrylic) can mitigate the intake of liquid water, but protection against air moisture is mainly achieved through wood modification (acetylation, esterification, thermal treatments, etc.) [6,7,8,9,10]. When a moisture content above 20% is reached, decay fungi can develop and feast on the wood cell materials, changing its mechanical and chemical properties [11]. A great diversity of organic and inorganic biocides can be impregnated to prevent the biodegradation from decay fungi and mold, such as triazoles, copper oxides and carbonates, borates, and quaternary compounds [12,13,14]. Impregnation treatments are often carried out in autoclaves, where methods employing vacuum and/or pressure can be used to ease the treatments into the wood structure [15,16].

To effectively protect the surface of wood, diverse coatings can be used. They can be waterborne or solvent borne, usually contain an alkyd or acrylic resin, and have different levels of transparency [5,17,18]. They can also contain various nanocomponents, such as nanoparticles, nanoclays and nanoxides, to improve their properties (resistance to weathering and decay, hydrophobicity, UV absorption, fire-proofing, etc.) [19,20,21]. Because of the increasingly strict legislations surrounding vocs (volatile organic compounds), the use of solvent-based coatings is decreasing to prioritize waterborne formulations [22]. The latter are much more permeable, but have better aging properties and a higher flexibility [23,24,25]. This last point is crucial, as it allows them to follow the dimensional changes of wood without failing [26]. Alkyd resins lead to a better wetting and a deeper penetration into the wood substrate than acrylic resins, which increases their mechanical anchoring and improves their adhesion. However, acrylics are more flexible, lose less flexibility while aging, and allow better exchanges of moisture, which makes them overall more durable [27,28].

This entry is adapted from the peer-reviewed paper 10.3390/coatings10040366