An integrated hydrothermal process of bamboo flattening, densification and drying: Mechanical properties and strengthening mechanisms

Bamboo is widely recognized as an engineering material with great potential. Conventional rectangular bamboo strip lumber has low utilization rate from milling arc-shaped strips flat and variable strength properties from natural bamboo’s hierarchical tissue structure. In this work an energy-efficient hydrothermal compression process to flatten and densify arc-shaped bamboo is presented. The green (70 % moisture content) bamboo strips were hot compressed in the radial direction at 3.0 MPa pressure, and at 170 °C or 190 °C heat for 20 min. After compression, arcs became rectangular with low (<10 %) moisture content and significant enhancement of mechanical properties. The bending strength, bending modulus, compressive strength, and shear strength of bamboo were improved by 128.1 %, 91.6 %, 58.2 %, and 74.5 %, respectively. Increased density and fiber bundle fraction, altered failure modes and chemical characteristics, increased cellulose crystallinity and crystalline size, and increased nanoscale mechanical properties were observed as contributing factors to tissue strength enhancement. The process could potentially eliminate the conventional strip milling and lengthy drying processes currently used to produce laminate bamboo lumber.