With the rapid growth of China’s economy and people’s high-level pursuit of life, eco-friendly materials are gradually used in home decoration, furniture and construction industries. Compared to wood, bamboo has its own advantages such as large strength, good toughness, and great abrasion resistance when it is applied as a material to structural engineering or home decor. However, bamboo also has several limitations, for example, a small diameter, a longitudinal difference in diameter, thin and hollow walls, and a disposition to corrosion and cracking. Therefore, there is an urgent need to develop a new efficient and high-value utilization technology of bamboo. “Bamboo flattening” is a kind of high-efficiency utilization technology adopted to construct flattened bamboo boards from bamboo tubes or pre-softened arc-shaped bamboo sheets. Prior to flattening, pretreatments are requested, including softening, outer layer removal, and inner layer removal. For the bamboo tubes, penetrating grooves are requested to be created which are supposed to be widened during the saturated steam heat treatment and are conducive for the tubes to enter into the flattening machine. Moreover, tapered, rectangular or “V”-shaped protrusions nails with uniform dispersions, or diamond-shaped line grooves with a width, depth and spacing of 1.5–2.0 mm, 2.0–4.0 mm and 5.0–8.0 mm are always created to release the generated internal stress during the flattening processes. For the arc-shaped bamboo slices, a non-notched flattening method is introduced with a machine equipped with the unfolding rolling component, the flattening component, and the reverse roller component. There are no grooves, nails or guide lines on the inside surface of the obtained bamboo board. It is worth mentioning that all boards obtained should be treated by cooling, shaping and drying. Bamboo flattening technology can increase the utilization rate of bamboo resource from the original 30–55 %, reduce the amount of adhesive by 30 %, and increase the added value by more than 20 %. Herein, the achievements in bamboo flattening technology based on published papers and patents are described, along with their underlying physicochemical mechanisms.

Bamboo flattening technique: a literature and patent review

Bamboo is now widely used in construction, papermaking, textile, furniture and other fields because of its renewable, fast-growing, high-strength, high-yield and easy processing. However, compared with wood, bamboo and bamboo products are more vulnerable to damage by fungi and pests. An effective and eco-friendly method is urgently needed to improve their physical and chemical properties, decay resistance and anti-mildew properties, and hydrophobic properties. Here, bamboo was heated with methyl silicone oil. The effect of different temperatures (140 °C–200 °C) and different times (2 h–6 h) on the properties of bamboo was studied systematically, including chemical composition, physical and mechanical properties, surface wettability, decay resistance and anti-mildew property. No starch granules were observed inside the parenchymal cell lumen of bamboo specimen heat treated at 200 °C for 6 h. And with the increase of heat treatment temperature and time, the content of cellulose and hemicellulose decreases gradually while relative content of lignin increases due to its better thermal stability. Accordingly, the surface wettability decreases due to the changes of the surface functional groups and micro-morphologies. Under the condition of oil heat treatment at 160 °C for 2 h, the compressive strength parallel to grain of bamboo samples reach the maximum of 109.52 MPa. With further increase of heating temperature, the corresponding compressive strength decreases. The resulted bending strength and MOE both display similar changing trend. However, the optimal parameter is at 180 °C for 2 h, with the highest bending strength and MOE values of 142.42 MPa and 12,373.00 MPa, respectively. Finally, the decay resistance and anti-mildew property are dramatically enhanced with increased heat treatment temperature and time. All the corresponding changing mechanisms are investigated in depth and in detail. Our results provide comprehensive process parameters and micro-mechanism for the performance of oil heat treatment of bamboo, which can be used to guide the actual production.

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The effect of oil heat treatment on biological, mechanical and physical properties of bamboo

Bamboo fiber has attracted a lot of interest due to its high specific strength and renewability. Unidirectional long bamboo fiber reinforcing is essential to fully realize its strength. The reinforcing efficiency can also be affected by the physical and chemical structure of bamboo fibers. Therefore, this study reviews the cell wall structure of the bamboo fiber. The alternately arranged structure with thin and thick layers contributes substantially to the mechanical strength of the bamboo fiber. In addition, this study evaluates the mechanical properties, water absorption, and flame retardancy of polymeric and cementitious composites reinforced with long bamboo fibers. Thermoset polymer-based bamboo fiber composites typically show higher mechanical strength than thermoplastic-based fiber composites. Chemical treatment and flame retardant can significantly enhance the hydrophobicity and flame retardancy of fiber composites, respectively. Because of its superior interface performance, the short bamboo fiber has a greater reinforcing effect in cement mortar than in concrete. This study can serve as a reference for further investigations on bamboo fiber/bundle reinforced composites and promote their applications. 

Structural and mechanical properties of bamboo fiber bundle and fiber/bundle reinforced composites: A review

Tunable nano-effect of Cu clusters derived from MOF-on-MOF hybrids for electromagnetic wave absorption

Bamboo is a biomass material, which is a great substitute for wood with the potential to reduce the demand for woody materials. However, there is still a huge challenge to obtain crack-free bamboo boards with an ideal width due to the hollow cylinder and large curvature of bamboo culms. Herein, a method is demonstrated that can flatten bamboo culms by high-pressure saturated steam (180 °C/6 min) under a compression force of 8–10 MPa by two rollers. The macro- and micro-mechanical properties of the flattened bamboo board were also characterized in this paper. Results indicated that the saturated steam had no significant effect on the microstructure of the bamboo, whereas it decreased its hygroscopicity due to the degradation of the hemicellulose. The mechanical properties of the cell walls bamboo samples treated by softening and flattening were elevated compared with those of the untreated ones. During the flattening process, the parenchymal cells became compact. The elastic modulus of bamboo fibers increased from 16.14 to 18.75 GPa, while the hardness increased from 0.58 to 0.77 GPa. The improved mechanical properties of cell walls and increased density had a positive effect on the modulus of elasticity of the bamboo board.

A new approach for fabricating crack-free, flattened bamboo board and the study of its macro-/micro-properties

https://www.techscience.com/jrm/v9n5/41556/pdf

Fabrication of Crack-Free Flattened Bamboo and Its Macro-/MicroMorphological and Mechanical Properties

Bamboo is a woody material that has become a key substitute for wood resources in many fields. This study is aimed to analyze the effects of saturated steam (140, 160, 180 °C) on physical, crystallinity, chemical composition, mechanical properties as well as microstructures at different\n periods (4, 6, 8, 10 min). Expectedly, a reduction of hemicellulose and cellulose and increment of relative content of lignin in bamboo when temperature above 160 °C was positive to reduce the equilibrium moisture content (EMC). Thus, the hygroscopicity improved and the parenchyma cells\n and vascular bundles were shrunk slightly. Both temperature and time positively affected the crystallinity of bamboo samples in comparison with the control. Heat treatment parameters affect the mechanical properties of bamboo. When the treatment was carried out at 140 °C, the modulus of\n rupture (MOR) and modulus of elastic (MOE) increased in comparison to no treatment; Furthermore, The MOR and MOE decreased by 40% and 19% compared with that of the untreated bamboo at 180 °C for 10 min. The temperature and time had a great influence on a*, b*, and L* of the\n bamboo. The results showed that during the heat treatment, the bamboo color changed from light yellow to dark red-brown, and the overall color changed evenly. Among them, a* increased firstly and then decreased, indicating that the treated bamboo was reddish, while b* and\n L* mainly showed a downward trend. The ΔE value positively corresponded to heat treatment severity.

Shouheng Hu

Papers

Fabrication of Crack-Free Flattened Bamboo and Its Macro-/MicroMorphological and Mechanical Properties

A new approach for fabricating crack-free, flattened bamboo board and the study of its macro-/micro-properties

Multi-scale characterization of the effect of saturated steam on the macroscale properties and surface changes of moso bamboo