Transversal mechanical properties of surface-densified and hydrothermally modified needle fir wood

Hydrothermal–mechanical (HTM) treatment is a combined action of temperature, moisture, and mechanical loads. Various kinds of HTM processes have been developed to produce eco-friendly modified wood products. Among these modification technologies, a combination of surface densification and hydrothermal post-treatment is a promising option that can be used to enhance plantation wood properties and generate value-added softwood products. This paper aimed to investigate the transversal dynamic transversal modulus of elasticity (dtMOE) profile and transversal density profile of surface-densified and hydrothermally post-treated needle fir (Abies nephrolepis) wood, and provide a practical guide for the processing optimization of surface-densified and thermally treated low-grade plantation softwood. Needle fir lumbers were surface-densified in an open hot-pressing system, at a compression ratio of 16.1% and two hot-pressing temperatures (160 and 180 °C) for a pressure holding duration of 10 min with a subsequent hydrothermal post-treatment at two temperatures (180 and 200 °C) for 60 min. The transversal dtMOE profiles for six types of specimens (moisture-saturated, conditioned control, and four types of surface-densified specimens) were measured and analyzed by a dynamic thermo-mechanical analysis (DMA) apparatus. A dtMOE gradient variable (G _M) was defined herein to quantitatively formulate the dtMOE distribution of the surface-densified and hydrothermally post-treated wood specimens. Upon hot pressing, compressed cell tissues were observed within a 1.0- to 2.0-mm layer beneath the surface of the densified wood. The density peaks of compressed sections were increased to 650–800 kg/m³, compared with the average density of the control specimens being at 350 kg/m³. The dtMOEs of the compressed sections were increased from 650 MPa of the control specimens, to 750–900 MPa, achieving the objective of surface densification. As the temperature of hydrothermal post-treatment increased from 180 to 200 °C, the differences of the dtMOEs between the compressed surface sections and their inner adjacent ones decreased significantly, indicating the effective fixation of the compression deformations. Based on the test results obtained herein, it was concluded that the impact of the hot-pressing temperature and thermal post-treatment temperature over the dtMOE was insignificant. Nevertheless, it was also found that the hydrothermal treatment temperature showed significant influences over the dtMOE difference between the densified surface and its adjacent layer; the possible explanations for these complicated behaviors were also proposed herein.