The new methods for enhancing wood hardness have been developed at SGGW
A new method of densifying wood and consequently enhancing its hardness has been developed at the Warsaw University of Life Sciences. The process may be applied in modifying wood materials used in construction products, e.g. floors, as well as in furniture components, musical instruments, and wood products.
This densification process for modifying wood by compression is the original idea of dr inż. Marek Grześkiewicz, Department of Technology and Entrepreneurship in the Wood Industry, SGGW. The method was initially applied to beech wood. Afterward, with his colleagues, business partners, and students, dr inż. Marek Grześkiewicz carried out long-term experiments to better explore the properties of the new materials, in various combinations and applications.
The technology developed by Dr Grześkiewicz involves treating wood with high temperatures at a humidity several percent higher than the final product. Under appropriate conditions, the moist lignin incorporated in the wood softens. Then, the wood undergoes the pressing process using high pressure of the heated shelf press. During the process, the moisture content of the wood is reduced. Depending on the materials, the process may last several minutes to more than an hour. The process can be applied to natural or modified wood, i.e., any wood that has undergone a chemical, physical, or thermal process.
Domestic woods as hard and dense as exotic woods
The wood structure changes after the woodworking processes beyond the lignin softening temperature. The degree of densification can be regulated as desired. The natural limit for the densification of wood using this method is to achieve a wood density close to that of the wood substance. The obtained material is of a high hardness value (it is difficult to remove even a piece of wood during the processing).
The densification method reduces the volume of the wood. “After the process, the refined piece can be up to 50% thinner compared to its initial thickness, in the case of wood of domestic species such as alder, pine, beech, and oak. However, it is worth pointing out that such extreme action to achieve the new desired properties of the wood due to densification is unnecessary. Reducing the thickness of the component by 20 to 30 percent is enough to achieve satisfactory results, such as nearly two times the hardness of the material,” says dr Grześkiewicz.
The researcher emphasizes that the method can be used for the densification of single materials, such as dehorning, which can be used for the top layers of tabletop boards or floor elements. It is possible to densify the wood veneer used to produce laminated materials such as plywood or LVL. Hardwood elements such as parquet friezes can also be densified to make them ideal for use as flooring blocks with increased hardness and suitable for underfloor heating systems. It is possible to compact planks, which can then be used to form structural elements, e.g. mechanically improved beams. Densified wood can be used to produce furniture for public facilities with high mechanical resistance.
The advantages of densified wood
Wood that has been densified using the method developed by dr inż. Marek Grześkiewicz demonstrates many properties that can be of key importance to the following businesses: the interior finishing and construction industries, manufacturers of specialized tools, woodwork, and even producers of musical instruments.
The main advantage of densified wood is its increased hardness “We have confirmed that oak and ash wood are twice as hard when pressed, leading to a 30% reduction in the thickness of the components. It is possible to densify wood that has previously undergone thermal modification. Thermal modification of the wood reduces the equivalent moisture content of the wood, and makes the material less ‘working’, i.e. it reduces its dimensions with changes in the humidity, as is the case with ordinary wood. This is important when installing wooden floors and other elements used in interior finishing,” adds dr Grześkiewicz. A solid dense wood will not spread the fire as fast as other woods and is characterized by improved fire resistance but when it burns, it gives off more heat. Furthermore, densified wood has a higher thermal conductivity compared to natural wood. Therefore, modified wood flooring elements, parquet, and solid floorboards can be used in rooms where underfloor heating is installed, improving energy efficiency. When the wood is compressed, densified, and hardened, it contains fewer pores, hence it requires far less finishing material to protect its surface, e.g. by varnishing. When bonding wooden parts, it requires less glue. When using a heated press, a smooth surface of the processed element may be obtained. However, it is possible to implement any design onto the surface during pressing, giving the wood additional aesthetic qualities.
The boom in poplar prices?
The studies conducted at SGGW have shown that the developed method of wood modification works very well for low-density wood species with poor technical parameters. In Poland, this type of wood includes poplar, which is very low-priced even in the firewood category. The physical and mechanical properties of poplar wood can be improved by the densification process. Furthermore, poplars grow relatively quickly, and this species may become more widely used by the wood processing industry. However, it all depends on the manufacturers – whether they are interested in the new wood modification technology and the market availability of the material.
The key role of lignin in wood densification
Wood is a composite; it consists mainly of lignin and cellulose. When heating, the lignin softens, thus allowing the material to reshape. Pressing the material results in densification. “Thermally modified wood resembles polyurethane foam as far as its physical properties are concerned; when it is reshaped by pressing, it tends to return to its original shape. This is because the wood contains a shape-memory cellulose. When heated, lignin behaves like a plasticizer, it decomposes when heated. Thus, when the wood is exposed to significant loads, such as pressure at high temperatures, the viscosity of the lignin will partially stop the elastic cellulose from trying to return to its original shape. That is a way of achieving a permanent densification of the wood,” says dr Grześkiewicz.
The method of wood densification developed by Dr Grześkiewicz has been granted patent protection. The university is currently looking for partners among manufacturers of wood products who would like to use the advantages of modified wood in the industry. In terms of the scale of production of wood products, Poland is the undisputed leader in Europe. Therefore, entering the market with domestic producers seems to be the most appropriate strategy. “We believe that the physical properties exhibited by densified wood will attract the interest not only of manufacturers but, above all, of consumers who demand durability, aesthetics, and energy efficiency”, summarizes dr Maciej Paszewski, Head of the Center for Innovation and Technology Transfer, SGGW.