Abstract
Effects of preheating times (4, 8, and 12 min), preheating temperatures (75–210 °C, with 15 °C interval), and their interactions on structures and density profiles of sandwich compressed poplar wood (Populus × euramericana cv. ‘Neva’) were studied to achieve better control of the position(s) of compressed layer(s), with the aim of better utilization of the low-density wood resources. Our findings revealed that, as a result of preheating temperature elevation or preheating time extension, compressed layers move gradually from wood surfaces to wood interior center, forming three types of sandwich compressed wood, namely, surface compressed wood, internal compressed wood, and central compressed wood. The characteristics of wood cell deformation in the sandwich compressed wood match well with the density distribution, and no obvious cell-wall cracks were observed. Effects of preheating temperature, preheating time, and preheating temperature–time interaction on the density of the compressed layer(s) are statistically insignificant. But their effects on the position and thickness of the compressed layer(s) in the sandwich compressed wood were statistically highly significant (p < 0.001), and the preheating temperature and compressed layer(s) positions were significantly related to the functions and fitted the polynomial of the fourth order.
Highlights
Wood densification improves the physical and mechanical properties of solid wood by increasing the wood density via reducing the void volume of the lumens [1–3]
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
In this study, three structure modes of sandwich compressed wood were achieved by controlling the preheating temperature and the preheating time
Summary
Wood densification improves the physical and mechanical properties of solid wood by increasing the wood density via reducing the void volume of the lumens [1–3]. Wood softening is highly related to moisture and temperature [13], and complete softening of wood avoids fissure and fracture of wood cells in the compressed wood [14], and exerts no negative effects on the physical and mechanical properties of the compressed wood [15]. Moisture content in compressed wood depends on the pressing variables, but highly dependent of temperature and time [16]. It has been reported that pressing/preheating temperature is the key parameter affecting moisture diffusion and transferring rate in wood [17], while pressing/preheating time mainly affects the heat-transfer scope and vapor pressure. Higher pressing/preheating temperatures accelerate moisture immigration and can reduce the required time for compression and drying [19, 20]. Temperature, time, and their interaction are the key factors affecting the position, thickness, and density, and the density peaks of the compressed wood
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