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The sponge-like porosity results in a low-density, rigid structure. Densities currently range from 40 to 200 kg/m³, varying with the amount of air pumped in and, therefore, the size and proportion of the air cavities. Two mechanisms hold the structure together. One is the natural chemical bonds between wood fibers, initiated by hydrogen peroxide. However, these chemical forces alone are insufficient to provide sufficient mechanical strength. The second factor is physical anchoring and entanglement between fiber bundles. When examined under a microscope, untreated fibers in pulp have a very smooth surface. To provide anchoring, the fibers must be rough. Refining in a refiner roughens them, breaking down their surfaces into a state where they can interlock. The combination of these two mechanisms makes it possible to produce wood foam with relatively high mechanical strength without the use of any adhesives.
Mechanical strength varies with foam density: the higher the density, the closer the fibers are, and the stronger the wood's internal bonding and entanglement anchoring. Fiber length also contributes to mechanical strength. At high densities, pine foam, with its longer fibers, outperforms beech foam in both tension and compression. At low densities, compression is less effective, as the first 10% of the compressive strain is absorbed by the air cavities rather than the wood fibers. Compressive strength also varies with density and wood species. High-density pine, at 115 kg/m³, has a compressive strength exceeding 200 kPa at 10% compression; beech has a figure of 145 kPa. This can be increased by adding adhesives such as polyurethane. Internal bond strength can be doubled. However, this is not fully reflected in compressive strength: the adhesive does have an effect, but, as before, the primary mechanism of compression is the collapse of the air cavities.
Composite materials with wood foam as core
To be continued