Effects of thermo-hygro-mechanical (THM) treatment on the viscoelasticity of in-situ lignin

Abstract

For producing wood products without fractures based on thermo-hygro-mechanical (THM) treatments, it is essential to understand how steaming and compression change the wood softening and cell wall components. In this paper, the effects of compression combined with steam treatment (CS) on the viscoelasticity of the in-situ lignin of Chinese fir has been investigated through dynamic mechanical analysis (DMA) under fully saturated conditions. Several variations were studied, such as the softening temperature (T g ) and apparent activation energy (ΔH a ) of the softening process in response to CS treatment conditions (such as steam temperature and compression ratio) under separate consideration of earlywood (EW) and latewood (LW). No difference between EW and LW with respect to the viscoelasticity was noted. T g and ΔH a of the lignin softening were nearly unaffected by the compression ratio, but were highly influenced by the steam temperature. The T g decreased significantly with CS treatments at or above 160 o C, but showed no appreciable change, compared to the native wood, at the lower steaming temperature of 140 o C. ΔH a increased at higher steam temperatures, while ΔH a showed a decreasing tendency with decreasing T g . This indicates that lignin undergoes a simultaneous depolymerization as well as a condensation during CS treatment.

Investigation of changes in compressed moso bamboo (Phyllostachys pubescens) after hot-press molding

“…8). The general feature of E′ variation with temperature for bamboo samples is similar to those observed in previous studies of wood [20,21]. The decrease of the E′ value of all samples with increasing temperature was attributed to the higher chain mobility of cell wall components at elevated temperature.…”

Frequency-dependent viscoelastic properties of Chinese fir (Cunninghamia lanceolata) under hygrothermal conditions. Part 1: moisture adsorption

“…During the temperature-scan process, several relaxation peaks are observed, in which a major relaxation peak occurs over a range of 70-200°C. This relaxation peak is attributed to the glass transition of lignin in wet or water-saturated state of wood (Salmén 1984;Olsson and Salmén 1997;Laborie et al 2004;Placet et al 2007;Guo et al 2017). The glass transition temperature (T g ) of lignin moves to lower temperature ranges at higher wood MCs and is related to the testing frequency and the lignin structure (Olsson and Salmén 1997).…”

Effects of superheated steam treatment on the physical and mechanical properties of light red meranti and kedondong wood

“…Increasing environmental awareness prompts the development of heat treatment in enhancing the properties of wood, especially dimensional stability and durability. Heat treatment, or thermal treatment, is an environmentally friendly approach where wood properties can be improved by the rearrangement of hemicelluloses, lignin and cellulose during the treatment without application of toxic chemicals (Guo et al 2017). Heat treatment of wood is gaining popularity worldwide and is currently the most investigated treatment approach for wood owing to the stringent regulations in the application of toxic wood preservatives (Salman et al 2016).…”