Aiming to produce bioinspired impact and puncture resistant materials, the mesocarp of the Brazil nut (Bertholletia excelsa) was characterized. The mesocarp composition was investigated by chemical extraction and its microstructure was analyzed by optical microscopy and microtomography (microCT). A compression test evaluated the force needed to open the mesocarp shell. Shore D hardness testing and nanoindentation measured the local mechanical properties at different length scales. Brazil nut mesocarp has a higher content of lignin (56%) than other nutshells and is mainly composed of sclereids and fibers cells arranged together and not in separated layers as usually found in nature. The mesocarp has an internal and external layer with fibers oriented from peduncle to opercular opening and a middle layer where entangled fibers are latitudinally oriented. To open a Brazil nut mesocarp, compression forces of 10 079 ± 1460 N (parallel to latitudinal section) and 14 785 ± 4050 N (perpendicular to latitudinal section) are needed. Such forces are higher than the forces needed to open most nutshells, if fracture force is normalized by shell thickness. The Shore D hardness test showed that hardness is uniform in the mesocarp, although it is higher in the center of the thickness than close to the inner or outer surface. The cell wall of fibers has a higher reduced modulus than the cell wall of sclereids although they have a similar hardness. These microstructural and mechanical results indicate that Brazil nutshell has great potential as a source for bioinspiration and motivates further studies.
Recycled polyvinyl butyral (PVB) from the automotive industry was chemically modified by melt mixing with a vinyl trimethoxysilane (VTMS) silanation reagent, which tends to react with the hydroxyls present in the PVB structure to generate crosslinked bonds between the chains. This chemical modification resulted in the improvement of the solvent resistance to organic solvents without deeply impairing the mechanical properties of the polymer. The mixing of PVB with VTMS was carried out in an internal mixer equipped with roller type rotors and the mixture was subsequently compression molded. Soxhlet extraction confirmed a 70% increase in the modified polymer gel content. Depending on the mixing time, the dynamic crosslinking reactions occurring during this time did not prevent the compression molding of the polymer. Furthermore, static crosslinking was observed during compression molding, which resulted in the maximum crosslinking degree and solvent resistance. DMA analysis indicated different molecular structures produced by different mixing times and varying static and dynamic crosslink ratios. Infrared spectroscopy (FTIR) indicated that dibutyl sebacate was the main plasticizer of the recycled PVB. Thermogravimetric analysis suggested that the molecular structure of the modified polymer affected the decomposition of PVB. POLYM. ENG. SCI. 56:971–979, 2016. © 2016 Society of Plastics Engineers
Aiming to understand Nature´s strategies that inspire new composite materials, the hierarchical levels of organization of the Brazil nut (Bertholletia excelsa) mesocarp were investigated. Optical microscopy, scanning electron microscopy (SEM), microtomography (MicroCT) and small-angle X-ray scattering (SAXS) were used to deeply describe the cellular and fibrillary levels of organization. The mesocarp is the middle layer of the fruit which has developed several strategies to avoid its opening and protect its seed. Fibers have a different orientation in the three layers of the mesocarp, what reduces the anisotropy of the structure. Sclereids cells with thick cell walls fill the spaces between the fibers resembling a foam-filled structural composite. The mesocarp has several tubular channels and fractured surfaces which may work as sites for crack trapping and increase toughness. The thick and lignified cell wall of sclereids and fibers and the weak interface between cells can promote a longer and tortuous intercellular crack path. Additionally, fibers with high strength and stiffness due to microfibrils oriented along the main cell axis (µ = 0° to 17°) were identified in the innermost layer of the mesocarp. Such an understanding of each hierarchical level can inspire the development of new cellular composites with improved mechanical behaviorThe mesocarp layer of Bertholletia excelsa fruit, the seed of which is known as brazil nut, is an impressive structure. It is a shell capable to resist falls as high as 50 m and compression forces higher than 10 kN 1 . Such a strong natural structure has great potential as a source for bioinspiration to produce new high performance composites 1,2 . The impressive properties of the mesocarp arise from its hierarchical structure, which emerged from millions of years of evolution.This hierarchy is the main factor to explain how relatively weak components organized in a complex way can result in a system with outstanding properties 3 . Four hierarchical levels of structuring have been described for plants: macroscopic, cellular, fibrillar, and molecular 3 . Each hierarchical level has its own contribution to the overall properties of the natural composite.On the largest length-scale, the "macroscopic" level, trunk, leaves, roots, flowers, and fruit are distinguished, and it is analyzed how the association of different tissues leads to different geometries and functions in the plant. At this level, the mesocarp is described as a spherical or elliptical shell of 10-12 cm diameter with a wall thickness of approximately 1 cm. Its function is to protect the seeds against predators and impact on the ground when the fruit falls from the tree 1,4,5 . The mesocarp has a rough surface with a peduncle and an opercular opening on opposite sides of the fruit. The opening is a hole with a diameter of approximately 2 cm, which is, however, not large enough to allow the dispersion of the seeds, as it was millions of years ago.On the cellular hierarchical level, the cells and vegetable tissues are ...
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