The influence of the initial moisture content on densification process of D. asper bamboo: Physical-chemical and bending characterization

Abstract Densification process aims to improve the physical and mechanical properties of wood and bamboo products. However, its processing parameters were not yet thoroughly investigated for bamboo. In this study, Dendrocalamus asper bamboo was densified in its radial direction in an open thermal press with different starting moisture content (MC), from 0 to 20%, to evaluate its effect on bending and physical-chemical properties. A maximum densification degree of 31.2% was achieved. Physical characterization and three-point bending tests showed that densification process increases density and all related bending properties (modulus of rupture (MOR), modulus of elasticity (MOE), the limit of proportionality (LOP), and specific energy (SE)) of bamboo, producing a more homogeneous material. The densified samples with 10% MC presented the best bending properties, with an average MOR, MOE and dynamic MOE of 318, 27,754 and 34,120 MPa respectively, with an increase of 56% for MOR and 41% for MOE in comparison with un-densified samples. SEM analysis of fractured samples showed an improvement of the fibers-parenchyma interface after thermo-mechanical modification, confirmed by the presence of unitary fiber failure. XRD analysis revealed that although densified bamboo had higher cellulose crystallinity compared to un-densified samples, the starting moisture content did not affect on the cellulose structure. FTIR showed that there are no significant changes in the chemical composition in all the analyzed conditions. However, the samples with moisture content below 5% presented cracks during the thermal-mechanical process, which resulted in higher thickness swelling and water absorption. Additionally, when samples with 20% MC are densified, an excess of water entrapped in the middle of the samples causes heterogeneous densification. The control of the initial moisture content of bamboo is a strategic parameter to improve the efficiency of the densification process. An initial moisture content around 10% is recommended for bamboo, which can guarantee enough plasticization and at the same time homogeneous properties in the final product.