Wood foam – a product on the rise?28 March 2018
You wait ages for a fundamentally new wood based product to appear; then two come along at once.
The Mass Plywood Panel described is a new product that may change the way that things are done. Wood Foam is another. Presented at the International Panel Products Symposium in Llandudno, Wales in October, it qualifies as an altogether new class of wood based material and opens new avenues of use and of replacement of petroleum-based products. Frauke Bunzel describes the new material and its potential
Wood Foam is a product being developed by the Fraunhofer Institute for Wood Research in Braunschweig, Germany. It is light; it is porous; it is strong. Potential applications for the product are legion.
As a lightweight, open-structured foam, with low bulk density and highly insulating properties one very obvious use would be to replace the ubiquitous polystyrene foams in packing material (Fig 1). As packaging it is consumer-friendly and easy for the consumer to dispose of via paper recycling outlets.
In the form of tiles it can act as acoustic or thermal insulation. It can take the form of light-weight boards, with structural properties and strengths that offer many possible uses. Some other suggested applications are as lightweight middle layers in sandwich boards for furniture, doors or in complete wall elements. Potential applications for this material are legion and depend only on human ingenuity to find them.
Yet wood foam consists entirely of wood fibres, without additives. Its strength derives solely from the wood fibre’s own bonding forces. No resins, binders or glues are involved. Health concerns from emissions from any such additives can therefore be eliminated.
Its development illustrates once again the remarkable potential of wood and wood fibres. As a sustainable replacement for many products currently made of plastics or other materials its ecological potential would seem enticing in the extreme. Its mechanical and physical qualities would make it a starting-point for a very great number of potential industrial applications.
We can begin with a little history. Attempts to make a foamed or foam-like material from wood fibre date back to the 1940s. Waste liquors containing high concentrations of lignins (these liquors being a by-product of papermaking and fibreboard making) were used as a source of lignin; in one process carbonic acid was added which caused the mix to foam and set. In another, the foaming was created by blown air. Other techniques were also tried. None of them, however, gained commercial acceptance and the know-how became lost.
More recently, around the beginning of the millennium, a wartime idea was revived and adapted. During the shortages of World War Two sawdust had been added to bread dough to save food supplies. In 2003 researchers in Austria adapted the idea, mixing sawdust with wheat flour and water to form a dough, and raising it with the action of yeast. After baking, the product was called ‘Wooden Bread’. Intended as an industrial product rather than a foodstuff, debate over the use of wheat, grown on land that could have produced food, ended the experiment.
The current product has none of those drawbacks. In appearance it is a solid brown or coloured sponge, rigid to the touch, with air spaces varying in size and proportion depending on the density required (Fig 2). Its sole ingredient is wood fibre.
The source can be hardwood or soft – or indeed any source of lignous biological fibres, including agricultural wastes. In the laboratory, beech and pine have been used.
To make it wood-chips are reduced to fibres by thermo-mechanical pulping, being ground in a refiner at 150 C. They are then mixed with water to give a very finely milled wood fibre suspension.
To foam the watery suspension, a protein can be added or air can be blown through it.
The strength of the product is given by the binding forces of the wood fibres themselves. To activate these forces hydrogen peroxide is added. The foamed suspension is then dried by convection at 130 C for half an hour, then held at 70 C overnight. (This of course is the laboratory process. Commercial production is still to come.)
The result is a rigid structure of low density, due to the sponge-like pores. Densities ranging from 40 to 200kg/m3 have so far been produced, varying of course with the amount of air pumped in and hence the size and ratio of the air cavities.
Two mechanisms hold the structure together. One is the natural chemical binding between wood fibres, which here is initiated by the hydrogen peroxide. These chemical forces on their own, however, are not enough to give adequate mechanical strength. The second factor is physical anchorage and entanglement between strands of fibre.
However, untreated fibres in pulp, when examined under a microscope, have very smooth surfaces. To provide anchorages the fibres have to be roughened. It is the grinding in the refiner that has roughens them, disintegrating their surface to a state where they are no longer able to slide past one another.
Through the combination of these two mechanisms wood foams with relatively high mechanical strength can be produced without using any adhesives.
The mechanical strength varies with the density of the foam: the higher the density, the closer the fibres are to one another and the stronger the wood’s own bindings and the entanglement anchorages.
Fibre length also increases mechanical strength. At high densities pine foam, with its longer fibres, outperforms beech foam in both tension and compression.
The increased anchorage of the longer pine fibres reveals itself. At low densities the effect is not so marked, at any rate in compression, since the first 10% of compressive strain is absorbed by the air cavities rather than the wood fibres.
Compressive strengths also vary with density and species. High-density pine of 115kg/m3 attains a compressive strength of over 200kPa at 10% compression; for beech the figure is 145kPa. These figures can be increased by adding binders such as polyurethane. Internal binding strengths can be doubled. But this is not fully reflected in compressive strength: the binder does have an effect but as before the primary mechanism in compression is collapse of the air cavities.
The water absorption of wood foam is, as you would expect, high (Fig 3). Its open pored structure resembles that of a sponge.
However, pine fibres contain hydrophobic - ie water-repelling – constituents, which reduce pine foam water uptake compared to beech. They also have the capacity to absorb water independent of the density of the foam – it is the hydrophobic quality of the fibre rather than the size of the pores that counts.
A major plus factor is that both beech and pine foams remain dimensionally stable in water – swelling after 24 hours in cold water is less that 1%, so effectively they do not swell.
Nevertheless, water absorption can be problematic because it can encourage fungal attack. A possible solution to this is to add concrete to the fibre mix. 10% of added concrete reduces the water absorption of beech wood foam from 31 to 2kg/m2.
Concrete of course imparts higher density to the foam. Alternative hydrophobic additives are silane or wax. Both of these however have a negative influence on the strength of the foam.
Thermal and Acoustic Properties
One possible application of the foam is as thermal insulation, perhaps in the form of tiles or panels.
The thermal conductivity depends only on the density of the foam; the species of wood has no influence. Thermal conductivities can be as low as 0.036W/mK for foam of density 45kg/m3. Values for polystyrene and for wood fibre insulation board are in the region of 0.029W/mK and 0.038W/mK, so wood foam could be eminently suitable as a replacement for these products.
Similarly, its open structure gives wood foam excellent properties as an absorber of sound (Fig 4). Even high density wood foam is useable here: for example a 30mm sample of 150kg/m3 made from pine fibre wood equals expanded polystyrene in performance. At lower densities the advantage is pronounced: 30mm-thick samples of beech foam of density 70kg/ m3 gave absorption that was similar to an 80mm thickness of polystyrene.
In both these applications fire resistance is of course important. A B2 test according to EN ISO 9239-1 requires that the burning distance be less than 150mm.
Wood foam samples, both long-fibre and short-fibre, passed the test, with identical burning distances and flame duration.
When wood foam is combined with a metal structure, in a composite panel, burning distances and flame duration decrease significantly.
Wood foams are therefore a new and interesting material with great potential that offers both environmental and economic advantages. It can replace petroleum-based products in a variety of applications.
As a product that is virtually 100% wood it is safe. Its source material is renewable, and can be from small-sized twigs and brash that is otherwise of low value and indeed that used to be regarded in forest operations as waste. It can also be sourced from fibrous agricultural waste and bushy plants.
The finished product is easy to handle. It does not produce fluff; it can be sawn, glued or drilled like other wooden materials, and produces very little dust.
It is odour-free, and when made without binders is additive-free as well, thus avoiding concerns about health risks. Clearly, then, it would seem a product with a future.
Demand for lightweight wood based materials and insulation based on renewable resources is constantly rising.
The Fraunhofer Institute for Wood Research is working on the further development of wood foam with the aim that this interesting material will be commercially available in the near future.