Improving the environmental performance of resins in wood treatment by using renewable chemicals has been a topic of interest for a long time. At the same time, lignin, the second most abundant biomass on earth, is produced in large scale as a side product and mainly used energetically. The use of lignin in wood adhesives or for wood modification has received a lot of scientific attention. Despite this, there are only few lignin-derived wood products commercially available. This review provides a summary of the research on lignin application in wood adhesives, as well as for wood modification. The research on the use of uncleaved lignin and of cleavage products of lignin is reviewed. Finally, the current state of the art of commercialization of lignin-derived wood products is presented.

In this study, modification of European beech wood (Fagus sylvatica) through in situ esterification of sorbitol and citric acid (SorCA) was investigated at two curing temperatures, 140 and 160 °C. In order to determine the optimum condition for modification, an aqueous solution of SorCA at varying concentrations was evaluated (10, 20, 30, and 55% w/w). Physical, mechanical, chemical, and durability properties of the modified wood were examined. Results have disclosed that among different SorCA concentrations, treatment with SorCA at 30% w/w was considered as the optimum concentration independent of the curing temperature. Under these conditions, anti-swelling efficiency increased to approx. 55% and modulus of elasticity increased slightly with approx. 9% in comparison with untreated wood. Decay resistance against white-rot (Trametes versicolor), brown-rot (Coniphora puteana), and soft-rotting microfungi classified the SorCA-30%-modified wood as very durable. In addition, thermogravimetric study evidenced that SorCA 30%-modified wood has a lower thermal decomposition temperature in comparison with untreated wood/wood control by 20 °C difference, indicating that the SorCA-modified wood possibly has fire-retardant properties. However, due to the acidic properties of the SorCA solution, modulus of rupture (MOR) and mainly work to maximum load in bending decreased considerably.

This study reviews previous and recent findings on wood modification via wood-polymer-integration with cyclic N-methylol compounds and puts them into a common context. Originally used for the finishing of cellulose-based textiles, the treatment of wood with low-molecular-weight N-methylol compounds of ethylene urea has been studied in the late 1950s for the first time. An improved dimensional stability was found and confirmed in the 1990s coming along with improved durability. Since such reagents can penetrate in wood cell walls and react with hydroxyl groups of cell wall polymers, a good fixation and permanent enhancement of some wood properties are achieved. Since 1990 this promising approach was followed by intensive fundamental and applied research, focused on treatment of Scots pine (Pinus sylvestris L.) and European beech wood (Fagus sylvatica L.) with 1.3-dimethylol-4.5-dihydroxyethyleneurea. In the early 2000s, a modification process was developed up to pilot scale, optimized for Scots pine and close to the market launch (Belmadur^®Technology, BASF). However, market penetration was not reached yet. Therefore, investigations were resumed in 2016. To forward this modification technique, recent trials focus on formaldehyde-reduced and formaldehyde-free cross-linking agents from textile and paper applications. Different permeable fast-growing wood species are subject of further research.

During the past decade, researchers worldwide have developed and investigated new treatment techniques to improve intrinsic wood properties. Because of environmental concerns, the pressure on the wood industry is higher than ever before to find alternatives to tropical hardwoods and preservative-treated wood. This is the reason why some new modification technologies, e.g. furfurylation, heat treatments, acetylation and resin treatments, have been introduced to the market. Researchers from universities and research institutes, the wood modification industry, wood producers and consumers face a lot of new challenges. The combination of several research areas, such as chemistry, biology and material technology, unknown and partly new material properties, new production technology, non-existent or inappropriate testing and analytical methods, changed aesthetic properties, etc., are just some of the many bottlenecks to overcome to introduce successfully a new technology to the market.

Citric acid together with sorbitol (SorCA) have been used to modify wood and improve its properties, such as dimensional stability and biological durability, which partly result from its swelling and sorption behavior. However, the underlying mechanism of water interaction with SorCA-treated wood is very complex and not fully understood. Previous research confirmed cell wall bulking and suggested cross-linking, however the extent of their contribution to moisture-induced changes has not been researched. This study investigated the effect of SorCA treatment on sorption properties of wood in the hygroscopic range (0–95% RH). Scots pine sapwood (Pinus sylvestris L.) was chemically modified with an aqueous SorCA solution at different treatment levels and measured by dynamic vapor sorption (DVS). The observed permanent increase in oven-dry dimensions did not result in a decreased swelling compared to untreated specimens. It was ascribed to the excessive expansion of cell wall matrix caused by a degradation of cell wall constituents by the acidic impregnation solution. However, a reduction in moisture content in comparison to untreated reference was detected. Present findings suggest that the SorCA polyester structure is altered after impregnation inside the wood and affects its sorption behavior by covalent bonding and, presumably, cross-linking with wood polymer constituents.