Wood foam – an emerging product (Part 4)

Water absorption rate

Wood foam has a high water absorption rate. Its open-cell structure is similar to that of a sponge. However, pine fibers contain hydrophobic (i.e., water-repellent) components, which reduce the water absorption of pine foam compared to beech. It also has the ability to absorb moisture, regardless of foam density-it's the hydrophobic nature of the fibers, not the size of the pores. A major factor is that both beech and pine foams maintain dimensional stability in water-expanding by less than 1% after 24 hours in cold water, thus effectively preventing swelling. However, water absorption can pose a problem, potentially promoting fungal invasion. A possible solution is to add concrete to the fiber mixture. Adding 10% concrete can reduce the water absorption rate of beech foam from 31 kg/m² to 2 kg/m². Concrete imparts a higher density to the foam. Alternative hydrophobic additives are silanes or waxes. However, both of these negatively impact the strength of the foam.

Water absorption of beech and pine foam

Thermal and acoustic properties

One possible application of this foam is thermal insulation, possibly in the form of floor tiles or panels. The thermal conductivity depends only on the foam's density; the wood species has no effect. For 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 boards have values ​​in the range of 0.029 W/mK and 0.038 W/mK, respectively, making wood foam an excellent alternative to these products.

Similarly, its open structure endows wood foam with excellent sound-absorbing properties. Even high-density wood foam can be used here: for example, a 30 mm thick sample made of pine fiber wood 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 beech foam sample with a density of 70 kg/m³ produces a sound-absorbing effect similar to that of 80 mm thick polystyrene.

Fire resistance is important in both applications. The B2 test of standard EN ISO 9239-1 requires a burning distance of less than 150 mm. Wood foam samples (including both long and short fibers) passed the test with the same burning distance and flame duration. When wood foam is combined with a metal structure, the burning distance and flame duration are significantly reduced in the composite panels.

in conclusion

Wood foam is a novel and effective material with enormous potential, offering both environmental and economic advantages. It can replace petroleum-based products in a wide range of applications. As a product made almost entirely of wood, it is safe. The raw materials are renewable and can be small branches and fragments (of low value, essentially waste from forestry operations). It can also be derived from fibrous agricultural waste and dense vegetation. The finished product is easy to handle, produces no lint, and can be sawed, glued, or drilled like other wood materials, generating very little dust. It is odorless, adhesive-free, and additive-free in its production, thus avoiding concerns about health risks. The demand for lightweight wood-based materials and insulation based on renewable resources is constantly increasing. The Fraunhofer Institute for Wood Research is working on the further development of wood foam with the aim of commercializing this material in the near future.

Sound absorption properties of beech and pine foam