Sorption and Surface Energy Properties of Thermally Modified Spruce Wood Components

Abstract

The objective of this work is to study the water vapor sorption and surface energy properties of thermally modified wood (TMW) components, ie wood processing residuals in the form of sawdust. The thermal modification was performed on spruce wood components using a steam-pressurized laboratoryscale reactor at two different temperature (T ) and relative humidity (RH) conditions, T ¼ 150°C and RH ¼ 100% (TMW150), and T ¼ 180°C and RH ¼ 46% (TMW180). A dynamic vapor sorption (DVS) technique was used to determine water vapor sorption isotherms of the samples for three adsorption-desorption cycles at varying RH between 0% and 95%. Inverse gas chromatography (IGC) was used to study the surface energy properties of the samples, including dispersive and polar characteristics. The DVS results showed that the EMC was reduced by 30-50% for the TMW samples compared with control samples of unmodified wood (UW) components. A lower reduction was, however, observed for the second and third adsorption cycles compared with that of the first cycle. Ratios between EMC of TMW and that of UW samples were lower for the TMW180 compared with the TMW150 samples, and an overall decrease in such EMC ratios was observed at higher RH for both TMW samples. The IGC results showed that the dispersive contribution * Corresponding author † SWST member to the surface energy was higher at lower surface coverages, ie representing the higher energy sites, for the TMW compared with the UW samples. In addition, an analysis of the acid-base properties indicated a higher K B than K A number, ie a higher basic than acidic contribution to the surface energy, for all the samples. A higher K B number was also observed for the TMW compared with the UW samples, suggested to relate to the presence of ether bonds from increased lignin and/or extractives content at the surface. The K B was lower for TMW180 compared with TMW150, as a result of higher modification temperature of the first, leading to cleavage of these ether bonds.

Water vapour sorption characteristics and surface chemical composition of thermally modified spruce (Picea abieskarst).

“…Sorption tests performed at higher temperature conditions will lead to a lower EMC at a given RH. However, it can be concluded that the EMC at 75% RH for the sorption isotherm is in accordance with the moisture content that was obtained from the IGC measurements on the same samples, at 30°C, 75% RH for both TM and UM wood (Källbom et al 2015).…”

“…It is also evident that the wettability, surface chemical composition and surface energetics of TM wood is significantly different compared with conventional wood as well as other types of modification approaches, e.g. acetylation and furfurylation of wood Källbom et al 2015). In a parallel study to this work, Källbom et al (2015) used inverse gas chromatography (IGC) to study the surface energetics of ground TM spruce components indicating significantly higher dispersive surface energy heterogeneity of the modified samples compared with UM samples.…”

“…For the DVS analysis, ground wood component samples were prepared in a two-step grinding procedure, described in Segerholm, Vellekoop and Wålinder (2012), sifted through a 120 mesh (<0.125 mm), and then stored and aged for several weeks in indoor room environment (20°C and 20-40% RH). Two replicates of both the UM and TM wood components were exposed to three drymoist cycles at 0 and 75% RH conditions at 30°C, following procedures as described in a parallel IGC study (Källbom et al 2015). The TM wood components were also exposed to an additional three cycles at 0 and 25% RH conditions at 30°C.…”

“…In particular, it may be of great benefit from an environmental consequences perspective to reuse byproducts or wood residuals from the construction or wood processing sectors to manufacture biocomposites (Erlandsson and Sundquist 2014). Moreover, the surface energetics of thermally modified (TM) wood residual components (Källbom, Wålinder, Segerholm and Jones 2015), in combination with the influencing effects from water sorption can lead to an increased understanding in how to design and develop efficient material combinations in terms of novel composite materials from recycled sources.…”

Sorption and diffusion properties of untreated and thermally modified beech wood dust

“…Little is known about the hygroscopic properties of wood dust from thermally modified wood. Källbom et al (2018) is the only study we are aware of that analyzed hygroscopicity of sawdust, in that case for heat-treated Norway spruce wood, with sorption isotherms evaluated over three adsorption-desorption cycles. They reported that equilibrium moisture content reduction is partially reversible.…”