In-situ penetration of ionic liquids during surface densification of Scots pine

Abstract

The moisture-induced recovery of compressed wood is one of the major problems of wood densification technology. Achieving a cost-efficient surface densification process without the need for additional resins to eliminate the set-recovery may lead to an increase in value of low-density wood species. A previous study has shown that a pre-treatment with ionic liquids (ILs) can nearly eliminate the set-recovery. It was however observed that during the pre-treatment process the IL did not penetrate sufficiently deep into the wood to explain the achieved reduction in set-recovery. Based on these findings, the hypothesis was posed that further penetration of the IL into the wood occurs during the densification stage as a consequence of the applied heat and pressure. Thermo-gravimetric analysis (TGA) and gas-chromatography mass-selective-detection (GC-MSD) showed that the depth of penetration of the IL was greater after the densification process than before. Digital image correlation (DIC) showed that in regions with a high IL concentration, there was almost no set-recovery, and it gradually increased with a decrease in the IL concentration, as observed with TGA and GC-MSD analysis.

Effect of Ionic Liquid [emim][OAc] on the Set Recovery Behavior of Densified Wood

“…The samples impregnated with IL and subjected to compression for a duration of 0.5 h (DW_0.5w) and 1 h (DW_1w) exhibited negligible SR (Table 1), highlighting the effectiveness of IL in mitigating the undesired deformation of DW associated with internal stress and water exposure. The sustained negligible SR of the IL treated samples, extending to repeated wet-dry cycles, implies that plastic deformation of crystalline cellulose occurred during the densification process due to the reaction with IL (Neyses et al 2020(Neyses et al , 2021. (Table 1).…”

“…Moreover, impregnating wood with resins raises environmental concerns and poses challenges for wood recycling and cascading (Cabral et al 2022;Neyses et al 2020). According to the proposed SR mechanism, it is hypothesized that if plastic deformation of crystalline cellulose can be achieved prior to or during the densification process, the SR can be reduced (Neyses et al 2021). This hypothesis presents an intriguing opportunity to develop efficient and environmentally friendly techniques for plasticizing crystalline cellulose without compromising its mechanical strength.…”

Chemical surface densification of sugar maple through Michael addition reaction

“…The mechanical compression process requires the application of heat and steam, but this procedure often leads to issues like spring-back, color alteration, and reduced wettability [10]. Alternatively, chemical methods, such as chemical modification or impregnation, involve saturating the cells with polymeric substances (Neyses, Peeters, Buck, Rautkari, & Sandberg, 2021). Chemical wood densification can enhance material strength, hardness, and dimensional stability.…”

Modification of wood via biochar particle impregnation

“…In the last decades, the wood modification industry, especially in Europe, has developed new technologies to improve wood properties and preserve them over time (Sandberg et al 2017). Modifications can be divided into four main groups: chemical processes (acetylation, furfurylation, impregnation) (Rowell 2014;Kocaefe et al 2015;Mantanis 2017); thermal-based modification (TBM) processes (thermal-hydro (TH); thermal-mechanical (TM): thermal-hydro-mechanical (THM) processes (Navi and Sandberg 2012); physical processes (using microwaves, plasma, laser light) (Haller et al 2004;Wust 2006;Tendero et al 2006;Leonelli et al 2010); and other processes (e.g., biological treatments, minerals, supercritical fluids, and ionic liquids processes) (Sandberg et al 2021). Thermally modified timber (TMT) is a product of a pyrolysis process that reaches temperatures between 160 and 260 °C and changes the chemical composition of wood cell wall due to the degradation of wood polymers (Esteves and Pereira 2009); the result is an improvement of durability and hygroscopicity (Kesik et al 2014).…”

“…Schwarzkopf (2021) successfully impregnated wood with phenol resin in poplar and beech wood. Steam treatments have also been used before densification (Sandberg et al 2021). Kutnar and Kamke (2012) performed mechanical compression under saturated steam, superheated steam, and transient conditions.…”