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Water absorption
Wood foam has a high water absorption rate. Its open-cell structure is similar to that of a sponge. However, pine fibers contain hydrophobic (water-repellent) components, which reduce the water absorption of pine foam compared to beech. Its ability to absorb water is also independent of foam density-it's the hydrophobic nature of the fibers, not the size of the pores. A key factor is that beech and pine foams remain dimensionally stable in water-swelling is less than 1% after 24 hours in cold water, effectively preventing expansion. However, water absorption can be problematic, potentially promoting fungal attack. A potential solution is to add concrete to the fiber mix. Adding 10% concrete reduced the water absorption of beech foam from 31 kg/m² to 2 kg/m². Concrete imparts a higher density to the foam. Alternative hydrophobic additives include silanes or waxes. However, both have a negative impact on the foam's strength.
Water Absorption of Beech and Pine Foam
Thermal and acoustic properties
One possible application for this foam is thermal insulation, perhaps in the form of floor tiles or panels. The thermal conductivity depends solely on the density of the foam; the wood species has no influence. For a foam with a density of 45 kg/m³, the thermal conductivity can be as low as 0.036 W/mK. Polystyrene and wood fiber insulation panels have values between 0.029 W/mK and 0.038 W/mK, making wood foam a very suitable alternative to these products.
Likewise, its open structure gives wood foam its excellent properties as a sound-absorbing material. Even high-density wood foam can be used here: for example, a 30 mm thick specimen made of pine fiberwood with a density of 150 kg/m³ is equivalent to the performance of expanded polystyrene. At lower densities, the advantages are significant: a 30 mm thick specimen of beechwood foam with a density of 70 kg/m³ produces sound absorption similar to that of an 80 mm thick specimen of polystyrene.
In both applications, fire resistance is crucial. Test B2 of standard EN ISO 9239-1 requires a burning distance of less than 150 mm. Wood foam specimens (both long-fiber and short-fiber) passed this test, with identical burning distances and flame durations. When wood foam is combined with metal structures, the resulting composite panels exhibit significantly reduced burning distances and flame durations.
in conclusion
Wood foam is a novel and cost-effective material with great potential, offering both environmental and economic advantages. It can replace petroleum-based products in a wide range of applications. As a product made from almost 100% wood, it is safe. The raw material is renewable and can be small branches and fragments (which are of low value and are effectively waste from forestry operations). It can also be sourced from fibrous agricultural waste and dense vegetation. The finished product is easy to handle, does not produce lint, and can be sawed, glued, or drilled like other wood materials, generating very little dust. It is odorless, adhesive-free, and manufactured without additives, thus avoiding concerns about health risks. The demand for lightweight wood materials and insulation based on renewable resources is increasing. The Fraunhofer Institute for Wood Research is working on the further development of wood foam with the goal of commercializing it in the near future.
Sound absorption properties of beech and pine foam