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

Abstract

Sandwich compression of wood that can control the density and position of compressed layer(s) in the compressed wood provides a promising pathway for full valorization of low-density plantation wood. This study aims at investigating the effects of preheating temperatures (60-210 °C) on sandwich compression of wood, with respect to density distribution, position and thickness of the compressed layer(s). Poplar (Populus tomentosa) lumbers with moisture content below 10.0% were first soaked in water for 2 h and stored in a sealed plastic bag for 18 h, the surface-wetted lumbers were preheated on hot plates at 60-210 °C and further compressed from 25 to 20 cm under 6.0 MPa at the same temperature on the radial direction. The compressed lumbers were characterized in terms of density distribution, position and thickness of compressed layer(s). It was found that depending on preheating temperatures, sandwich compressed wood with three structural modes, namely, surface compressed wood, internal compressed wood and central compressed wood can be formed. Density of the compressed layer(s) in wood increased gradually as a result of the elevated preheating temperatures. Higher preheating temperatures gave rise to bigger distance between compressed layer(s) and the surface, and preheating temperature elevation from 90 to 120 °C contributed to a maximal distance increase of 2.71 mm. In addition, higher preheating temperatures resulted in bigger thickness of compressed layer(s) over 60-150 °C and temperature elevation from 120 to 150 °C lead to the layers integration from two into one. Further temperature elevation over 150 °C reduced the thickness of the compressed layer in wood. SEM scanning suggested that cell wall bucking rather than cell wall crack occurred in compressed layer(s) and transition layer(s).

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

“…This observation is consistent with Wang's report [21]. However, this finding varies with our previous report wherein higher preheating temperature resulted in higher density of the compressed layer [12], which could be resulting from the smaller specimen size and faster water loss during the preheating. Figure 3c shows that when the preheating temperature is elevated to 210 °C and the preheating time is extended to 8 or 12 min, the surface density increases, and two compressed layers are formed on the surfaces of the resulting compressed wood, and one central compressed layer also appears in the wood center.…”

“…When the pressing time is 17 min, density is uniformly distributed along the wood thickness [21]. In our previous research [12], when wood was preheated at 60-210 °C for 12 min, distance between the compressed layer(s) and the wood surfaces increased from 0 to 7.70 mm when the temperature elevated, and the thickness of the compressed layer(s) ranged from 4.98 to 8.10 mm and the average density of the compressed layer(s) is larger than 0.60 g/m 3 . Compressed wood samples with compressed layers either on the wood surfaces or the inside of wood were obtained after 10, 40, 240, and 420 s of preheating and being compressed at 180 °C [2].…”

“…This sandwich compressed wood is defined as surface compressed wood. By adjusting the original moisture content of wood [12], a moisture gradient can be formed, namely high moisture content on the wood surfaces but low moisture content in the wood center. When moisture content is 20%, T g of lignin is only 80 °C [13].…”

“…Wood sandwich compression can minimize the volume loss of wood during compression, and also the physical and mechanical properties can be adjusted and thus controlled. Unlike wood surface densification where compression only occurs on the softened surfaces [11], wood sandwich compression technology can soften wood at different positions via adjusting preheating time and preheating temperatures [2,12]. During the preheating process, temperature and moisture gradients are formed in the wood thickness direction, forming yield stress gradients inside wood.…”

“…Based on our previous study [2,12], this study systematically examines the effects of the preheating temperature, preheating time, and their interactions on the density distribution in the sandwich compressed wood and the deformation characteristics of the wood cells in the compressed wood. Besides, the functional relationship between the positions of the compressed layer(s) and the compression process parameters was established by applying the multiple nonlinear regression analysis method, which helps to provide an effective pathway for precisely tailoring the position of the compressed layer(s) in the sandwich compressed wood.…”