PEER-REVIEWED REVIEW ARTICLE bioresources.com Rudak et al. (2021). “Densification of wood surface,” BioResources 16(4), 8379-8393. 8379 Densification of Wood – Chemical and Structural Changes Due to Ultrasonic and Mechanical Treatment Oksana Rudak, a, * Stefan Barcik, b Pavel Rudak, c Vadzim Chayeuski, d and Peter Koleda b This paper presents the state of the art of wood surface densification method by pressing with ultrasound. The properties of ultrasound and its effects on the structure and properties of wood, as well as ultrasound- induced chemical changes in wood material, are described. The following research results were analyzed: the effects of acoustic cavitation in wood material, plasticization of wood lignin by processing with ultrasound, the influence of ultrasound on the wood anatomical structure, the combined effect of ultrasound and wood pressing, and the sterilization of wood using ultrasonic action. Ultrasound causes conversion of lignin from glassy into a quasi-rubbery state, which facilitates compaction of the workpiece surface. Additionally, under ultrasound, growth and collapse of gas bubbles (cavitation phenomena) occur within a liquid medium of wooden substance accompanied by high local temperatures and production of chemically active radicals. This contributes to the destruction of the former and the formation of new bonds in the wood substance, which is important for increasing the stability of the workpiece size after densification. The conclusions made about the ultrasound can be effectively used for the wood plasticization and about prospects of joint use of wood pressing and ultrasound for wood surface densification. Keywords: Surface densification; Wood plasticization; Softening lignin; Pressure with ultrasound; Hardness of wood; Surface quality Contact information: a: Department of Technology and Design of Wooden Articles, BSTU, Sverdlova 13a, Minsk, Republic of Belarus; b: Department of Manufacturing and Automation Technology, Technical University in Zvolen, Studentska 26, Zvolen, Slovak Republic; c: Limited Liability Company "BalanceContact", Smolenskaya 15, room 303b, Minsk, Republic of Belarus; d: Department of Physics, BSTU, Sverdlova 13a, Minsk, Republic of Belarus; *Corresponding author: oksrudak@mail.ru INTRODUCTION In recent decades, the issue of using different types of wood as non-metallic structural materials capable of replacing ferrous and non-ferrous metals, textolite, and some plastics has become topical. The production of other structural materials (steel, alloys, plastics, etc.) is associated with the consumption of a lot of raw materials, whose reserves are not renewed, but are continually depleted. Additionally, the creation of most construction materials requires a large expenditure of energy, the deficit of which, especially in recent years, is particularly acute. The production of number wooden details requires wood with high physical and mechanical characteristics, mainly the wood density. Wood of higher density has higher additional mechanical characteristics and overall wood quality. For example, density of wood strongly correlates with its hardness and strength (Wangaard 1950; Fang et al. 2012). Wood surface hardness determines its resistance to abrasion. This is especially important for parts such as flooring, parquet, countertops, and facial surfaces of furniture.