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The sponge-like pores result in a low-density, rigid structure. The density has so far ranged from 40 to 200 kg/m³, varying with the amount of air pumped in, thus affecting the size and proportion of the air chambers. Two mechanisms hold the structure together. One is the natural chemical bond between wood fibers, initiated by hydrogen peroxide. However, these chemical forces alone are insufficient to provide adequate mechanical strength. The second factor is physical anchoring and entanglement between fiber bundles. Under a microscope, untreated fibers in pulp have very smooth surfaces. To provide anchoring, the fibers must be roughened. Grinding in a fine mill roughens them, breaking down their surfaces into a state where they can interlock. Through the combination of these two mechanisms, wood foam with relatively high mechanical strength can be produced without the use of any adhesives.
Mechanical strength varies with foam density: higher density means closer fiber proximity, resulting in stronger self-bonding and entanglement anchoring of the wood. Fiber length also increases mechanical strength; at high densities, pine foam, with its longer fibers, outperforms beech foam in both tension and compression. At low densities, compression is minimal regardless, as the first 10% of compressive strain is absorbed by the air cavities rather than the wood fibers. Compressive strength also varies with density and tree species. High-density pine (115 kg/m³) has a compressive strength exceeding 200 kPa at 10% compression; beech data shows 145 kPa. These can be increased by adding adhesives such as polyurethane. Internal bond strength can be doubled. However, this is not fully reflected in compressive strength: adhesives do have an effect, but as before, the primary mechanism of compression is air cavity collapse.
Composite materials with wood foam as the core layer
To be continued