Wood moisture content during the thermal modification process affects the improvement in hygroscopicity of Scots pine sapwood

Abstract

Elevated wood moisture contents during the thermal modification process have been shown to adversely affect the improvement in dimensional stability and hygroscopicity. This study tested the hypothesis that the effect of elevated wood moisture content is based on the impact of water on chemical reactions which determine the cell wall matrix stiffness. Samples of Scots pine sapwood (Pinus sylvestris L.) were thermally modified in saturated water vapor at different peak temperatures and durations starting either in oven-dry or in water-saturated state. For a given mass loss caused by the modification process, the improvement in maximum swelling and equilibrium moisture content was stronger for oven-dry samples. After removal of water-soluble degradation products, which caused a cell wall bulking effect, the maximum swelling even increased after modification in water-saturated state. Based on dynamic vapor sorption measurements, it was evidenced that the modification in oven-dry state increased the cell wall matrix stiffness which improved dimensional stability and hygroscopicity. Enhanced bond formation in the polymeric network, i.e., via condensation and cross-linking reactions during the treatment of oven-dry wood, is suggested as a cause for this increase in matrix stiffness. In contrast, the modification in water-saturated state enhanced the flexibility of the cell wall matrix, which increased the cell wall swelling and limited the improvement of hygroscopicity to the reduction in OH groups by removal of hemicelluloses. This enhanced matrix flexibility was potentially caused by predominant hydrolytic cleavage of bonds in case of water-

Sorption and Surface Energy Properties of Thermally Modified Spruce Wood Components

“…A similar behavior has been observed for wood that is modified with cross-linking agents (Himmel and Mai 2015). For thermally modified Scots pine sapwood, the decrease in EMC ratio with increasing RH has been assigned to the formation of covalent bonds and cross-links by repolymerisation reactions under dry heat-conditions that hinder the creation of sorption sites by expansion of nanopores (Altgen et al 2016b).…”

Application of time–temperature–humidity superposition to the mass loss of wood through hygrothermally accelerated ageing at 95–140 °C and different relative humidity levels

“…Although the effects of oven-heating (0% RHh) have been studied well, few systematic experiments have been conducted on the ML values at intermediate and high RHh: the t, T, and RHh ranges are often limited. In addition, unusual ML values are sometimes reported; the heating of water-saturated wood yielded smaller ML values than oven-heating did [1], despite the accelerated degradation of wood in the presence of moisture. Such unusual ML values are excluded for a convincing discussion.…”

Multi-Scale Evaluation of the Effect of Phenol Formaldehyde Resin Impregnation on the Dimensional Stability and Mechanical Properties of Pinus Massoniana Lamb.

“…However, as one of the common fast-growing tree species, Masson pine wood also presents drawbacks which limit its practical application, such as low dimensional stability, softness, and low bio-durability [4,5]. In the past few decades, a number of modification methods have been proposed to enhance the quality and high value-added utilization of plantation wood, including thermal or densification treatment, surface coating, and chemical impregnation [6][7][8][9][10]. Above all, chemical impregnation under vacuum or pressure has been proven to be an effective method to improve the properties of the wood.…”