Thermally Modified Wood Exposed to Different Weathering Conditions: A Review

Abstract

Outdoor wood applications are exposed to several different biotic and abiotic factors, and for that reason, they require protection to increase their service life. Several technologies of wood protection are already commercialized. One of these technologies is thermal modification, which refers to the structural, mechanical, and chemical transformations occurring in the lignocellulosic material when gradually heated up to specific temperature ranges. In the past few years, several researchers have undertaken weathering resistance evaluations on different wood species. Some cases have considered natural exposure in different countries with different climatic conditions, while others focused on artificial exposure under UV and xenon radiation tests. Most works evaluated the weathering effects on the chemical, mechanical and physical, and anatomical shifts compared to the original characteristics of the material. This review has established a considerable lack of studies in the bibliography focusing on abiotic factors, such as the industrial and maritime environment, or even isolated climatic factors such as salt spray (simulating maritime environments) or pollutant gases (simulating industrial environments). This lack of information can be an opportunity for future work. It could help to understand if thermally modified wood is or is not sensitive to pollutant gases or salinity, or to a combination of both. By knowing the degradation mechanisms caused by these factors, it will be possible to study other forms of protection.

The behavior of thermally modified wood after exposure in maritime/industrial and urban environments

“…The loss of extractives can also contribute to color change because of the erosion caused by rain, wind, and photodegradation. Extractive loss on weathered wood has also been reported in the literature [ 9 , 11 , 20 , 40 , 49 ]. For instance, in recent studies, authors reported this phenomenon on thermally modified woods of Pinus sylvestris , Picea abies , and Fraxinus excelsior [ 50 ] and in Cunninghamia lanceolate [ 51 ].…”

“…During the natural exposure, samples suffered changes in color since wood is susceptible to photodegradation [ 6 , [9] , [10] , [11] , 19 , 20 , 29 ]. Because both locations present high irradiation values, it was expected that the color change would occur in all samples caused by the environmental conditions and chemical changes on the wood surface caused by weathering, namely the reactions in lignin [ 21 ].…”

“…Weathering describes the slow degradation that occurs by combining different biotic and abiotic agents [ [9] , [10] , [11] ]. Understanding the mechanisms of natural weathering, the role of the altering factors, and their effect on the anatomical, chemical, and physical properties of wood is fundamental to assessing the actual conditions of timber artefacts.…”

“…It is also known that salt water (seawater and other salinity sources) can be corrosive to wood. These phenomena happen when the wood is exposed to wetting cycles that promote tracheid defibration or “fuzzy wood,” which can sometimes be confused with wood decay [ 9 , 14 , 15 ].…”

Mobile technology of thermal modification of wood

“…Wood used outdoors is exposed to several biotic and abiotic factors, and for this reason, it needs protection to extend its life [4]. Some wood protection technologies are already being used commercially.…”