\”Wood King\” Hu Liangbing \”ACS Nano\”: bamboo how to \”refining\” into a high-performance structural materials?
Structural materials play a crucial role in modern society, providing load-bearing and other functions in the construction, automotive, electronics substrates, packaging and many other structures. Mechanical strength and material weight (or density) are two important factors engineering applications, particularly in the case of energy efficiency must be considered, such as light vehicles, aircraft and high-rise buildings. In recent centuries, the traditional, such as engineering steel, concrete, brick and petroleum-based composite materials in the construction and automobile has played a leading role. However, a recent United Nations sustainable development goals to increase the development of renewable biological resources for material needs and interests. In addition, the need to improve the energy efficiency requirements of both light and strong structural material, it is a challenge to the traditional structure of the material. Recently, University of Maryland Professor on \”ACS Nano\” published an article entitled \”Rapid Processing of Whole Bamboo with Exposed, Aligned Nanofibrils toward a High-Performance Structural Material\” article presents a simple, fast and scalable the method of production by partial delignification and microwave heating natural processed into structural materials. Compared with natural bamboo, bamboo delignification microwave heating to improve the tensile strength of nearly 2-fold, 3.2-fold increase toughness, flexural strength increased by 2-fold, with excellent mechanical properties. Further, the tensile strength than modified bamboo structure reaches 560 MPa cm3 g-1, in view of its low density (1.0g cm3), its performance is superior to the conventional structural materials, such as steel, metal alloys, composite materials, and petroleum . These excellent mechanical properties combined with abundant bamboo resources, renewable and sustainable features, and fast, the manufacturing process can scale production, making this powerful microwave processing bamboo structure for lightweight, energy-efficient engineering applications attractive.
1. Bamboo is how to become a high-performance structural materials?
Preparation method consists of two steps, namely partial delignification and microwave heating. First, chemically treated natural bamboo and Na2SO3 in NaOH solution, hemicellulose and lignin in the cell wall is partially removed, thereby making them more multipleHoles and soft. Subsequent microwave heating can quickly remove water bamboo structure, and results in severe shrinkage and uniform, so as to form a tight and complete material.
natural bamboo has a layered cell structure, cell wall is mainly composed of long, stiff cellulosic fibers, cellulosic fibers embedded in a polymer matrix of lignin and hemicellulose thereof. Delignification process, some of the cellulose, hemicellulose and lignin is removed from the cell walls and dispersed into the bulk solution. After delignification, the microstructure of bamboo has also undergone substantial changes. Due to the removal of hemicellulose and lignin, bamboo structure after partial delignification becomes more porous (FIGS. 2D-K). Cells showed significantly smaller cell wall separation, and cell wall thinning larger cells, indicated that both removal of certain components lead to a more porous structure (FIG. 2F, G, L, M). Further, after the chemical treatment, the ultrafine fibers maintain a long and straight, as shown in FIG 2H, I, N, O shown in FIG.
may provide a rapid, uniform heating, and therefore, they are fast drying using microwave heating wet, partially delignified bamboo, to induce a vigorous, but uniform shrinkage of bamboo structure (i.e. densification). During the drying process, when bamboo subjected to an electromagnetic field of microwave radiation, the water dipole molecules bamboo begins oscillating and generates significant heat, so the sample in a short timeUniform temperature rise. As shown in FIG. 3A-C, part of the temperature rise of delignification bamboo rapidly to 300 seconds in 70 ℃, and maintained at ~ 78 ℃ about 30-60 minutes (depending on the sample size). As a result, the porous structure becomes highly dense bamboo, such as scanning electron microscopy (FIG. 3D-G) FIG. Further, small-angle X-ray scattering (the SAXS) and SEM observation proved enlarged, partially after delignification and microwave heating, arranged in the cell wall cellulose nanofiber was well preserved (FIG. 3H, I). This highly compact structure is an important reason for its high mechanical properties.
They were compared to manufacturing process of bamboo obtained. First, without chemical pretreatment of the wet natural bamboo were direct microwave heating, results show that shrinkage bamboo structure is limited (only 1-2%), good structural integrity is maintained (FIG. 3J) . Then, using partial delignification of bamboo as a raw material, but the air-dried to prepare a final product. While reaching sufficient volume shrinkage, but the shrinkage is uneven, prone to cracking bamboo structure (FIG. 3K). In addition, the time required for air drying process is 25 times (<60分钟vs 1500分钟），以达到最终产品中相同的含水量（图3M）。通过对比，可以得出部分脱木素和微波加热这两步工艺对于实现均匀致密的竹层结构的重要性。
2 microwave heating process. Tensile Properties of Bamboo based structural material
Mechanical properties, such as tensile strength, flexural strength and toughness is essential for engineering applications they microwave heating bamboo delignification of mechanical tensile tests performed, and the natural bamboo, microwave andAnd hot air dried bamboo natural bamboo delignification three control samples were compared. 4A, the microwave heating delignification bamboo shows the highest tensile strength of 560MPa, Young\’s modulus of 36.1 GPa and a fracture work 6.8MJ m-3 (i.e., tensile toughness), 1.9 times higher than the natural bamboo , 2.0-fold and 3.2-fold (Fig. 4A, B). Microwave heating delignification bamboo tensile toughness than some bio-based (e.g., natural basswood, basswood dense), polymer-based (e.g., high density polyethylene, polylactic acid, polymethyl methacrylate) and carbon (e.g., the graphene oxide / polyvinyl alcohol) in engineering materials. Since the configuration of the microwave heating density bamboo delignification up 1.0 g cm3, so the ratio of the tensile strength of up to 560MPa cm3 g-1, a 41.4% increase over natural bamboo, much higher than the most common structural materials, such as wood, stainless steel, aluminum alloy 7075 and some common polymers (Figure 4C). Microwave heating delignification bamboo excellent tensile properties attributed to its dense layer structure (FIG. 4E). On a macro scale, due to the intense uniform shrinkage during microwave heating compared to natural bamboo, the dense and porous material is poor. On the scale, since the majority of the open cell collapse shrinkage, cell wall dense layered structure. Further, in the micro and nano-scale, cell walls of cellulose fibers arranged preserved long after treatment. In addition, some hemicellulose and lignin is removed to expose more surface area of the cellulose fibers, which may enhance the interaction between the fibers, thereby enhancing the hydrogen bonding density (by a number of hydroxyl groups on the cellulose molecular chain) and the collective van der Waals force. Reducing portion delignification and the microwave induced densification defects also contributes to better mechanical properties.
3. Bamboo bending properties based structural material
Another group of bending property is characteristic of engineering materials. Thus, they were three-point bending test (FIG. 5A). FIG. 5B, C as shown, a microwave heating flexural strength (248 MPa) and Young\’s modulus (16.5GPa) maximum delignification bamboo, respectively 2 times and 4 times the natural bamboo. Critical role similar to the microwave heating delignification bamboo multi-scale structure in the stretching behavior played, such a layered structure is dense it significantly enhanced bending performance reasons.
In summary, the authors demonstrated that the use of microwave heating can be partially delignified natural bamboo processed to a layer having a dense structure high-performance structural materials. High tensile strength 560MPa structural material had a toughness 6.8MJ m-3 and a bending strength of 248MPa, 1.9 times higher than the natural bamboo, 2 and 3.2 times. Due to its relatively low density of 1 g cm3, so microwave induced densification bamboo having a high 560 MPa cm3 g-1 ratio of the tensile strength, than the most common structural material, such as steel, metal alloys, oil matrix composites, and some natural fibers. Microwave dense bamboo light weight, high mechanical strength, abundant raw materials, renewable, carbon footprint, etc. is almost zero, is expected to become the next generation of energy efficient buildings, cars and other high performance structural materials sustainable, and other engineering applications. The full text link: https: //pubs.acs.org/doi/10.1021/acsnano.9b08747