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

Abstract

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 posttreated 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 hotpressing 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/m 3 , compared with the average & Jian-feng Zhan density of the control specimens being at 350 kg/m 3 . 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.

Effects of preheating temperatures on the formation of sandwich compression and density distribution in the compressed wood

“…For sandwich-compressed wood, the compressed layer is identified as the part with density of 20% higher than the maximal density of the control specimen; the region whose density is less than the maximal density of the control is considered as the uncompressed layer; regions between the compressed layer and uncompressed layer is defined as the transition layer. In this paper, the densities of compressed layer, uncompressed layer and transition layer were in the ranges of > 0.59 g/cm 3 , < 0.49 g/cm 3 , 0.49-0.59 g/cm 3 , respectively. Position of compressed layer(s) is defined as the distance from the edge of compressed layer(s) to the associated surface(s) of the compressed wood.…”

“…In traditional wood compression, high compression ratio is required for the satisfactory properties of compressed wood in terms of surface density and hardness [1]. While wood surface compression is an essential pathway for enhancing the density and hardness of wood surfaces [2][3][4][5][6]. It has been reported that polar wood preheated at 180 °C for 350 s can be softened and thus compressed into the required compressed wood with a relatively high surface density [2], but cell walls inside the compressed wood are also compressed as well.…”

“…It has been reported that polar wood preheated at 180 °C for 350 s can be softened and thus compressed into the required compressed wood with a relatively high surface density [2], but cell walls inside the compressed wood are also compressed as well. Air dried wood with surface sprayed with water can be compressed at 160 °C or 180 °C and compression for 0.4 min can yield surface compressed wood with peak densities of 650-800 kg/m 3 [3]. However, the thickness of the compressed layers is very small and the density distribution is not controllable.…”

Fabrication of densified wood via synergy of chemical pretreatment, hot-pressing and post mechanical fixation

“…In this work, wood hemicellulose and partly lignin were degraded in the pretreatment with NaOH/NaSO 3 solution, which was confirmed by FTIR results, and allowed the wood easier to be compressed in the hot-processing treatment. Meanwhile, the much higher density and strength of the wood could be achieved under sufficient compressibility [38]. This probably mainly attributed to synergistic effect of degradation of hemicellulose and lignin in the chemical pretreatment and recombination of wood cells during the hot-pressing process.…”

Effects of preheating temperature, preheating time and their interaction on the sandwich structure formation and density profile of sandwich compressed wood

“…When the preheated time is further extended to 12 min, the compressed layer becomes the one with only one density peak, but the density peak is lower than that of compressed layer(s) obtained from preheating for 4 or 8 min. During the preheating, water/ moisture move from higher moisture content areas to lower moisture content areas through cell cavities and pit openings, water/moisture, and heat transfer from the wood surfaces to the wood interior, and new temperature and moisture gradients were formed in the compression direction of wood [22]. Once pressure is loaded on the wood, a new yield stress gradient is formed, yielding various positions of compressed layers, and the distance between the wood surface and the associated edge of compressed layer (the compressed layer position) gradually increases, and the compressed layers eventually integrate to form a single layee in the wood center.…”