The Natural Resources Institute Finland (Luke) is in collaboration with the Universities of Eastern Finland and Oulu developing a wood preservative based on the natural protective mechanisms of trees. The aim is to develop a product suitable for industrial production that would provide wood products used outdoors with a durable protection and to ensure that they are as safe to use as untreated wood.
The natural protective mechanisms of a tree are based on extractives, some of which protect the tree against pests, fungi and bacteria. The concentrations of such substances vary in different parts of the tree, but high levels can be found in heartwood, cones, bark and knots. In the future, bark will also be used as a raw material for a bio-based wood preservative.
Softwood bark as raw material
Extractives protecting trees include tannins, lignans, flavonoids and stilbenes.
“The development of the new wood preservative is based on tannins, as earlier studies have shown that they retard the growth of bacteria and fungi,” says Martti Venäläinen, Senior Research Scientist at the Finnish Natural Resources Institute Finland (Luke).
Another reason for focusing on tannins is that they are easy to extract in high volumes and that their raw material, softwood bark, is cheap. Bark makes up approximately 10% of harvested timber and is so far mostly burned for energy.
“The extraction of tannins from bark residue would be one way to process bioeconomy products bringing added value to industry. Another future-oriented component for wood preservation could perhaps be found in pyrolysis liquids distilled from wood, which are also studied in this project,” says Assistant Professor Antti Haapala of the University of Eastern Finland.
Tannins are extracted from softwood bark using hot water extraction. The method is environmentally friendly, as the extraction is performed using hot water and no organic solvents, acids or alkalis are required for the process.
Nanocellulose improves the fixation of preservatives
Tannins that prevent bacterial and fungal activity have the same weakness as many other wood preservatives: they do not stay in the wood long enough. The aim is to improve binding by fixing the tannins to the wood with nanocellulose, which can form durable bonds with the tree tissue and does not leach out easily.
“As a biomaterial made from wood nanocellulose is environmentally friendly. Preliminary studies indicate that it is nontoxic and can be chemically modified,” says Associate Professor Henrikki Liimatainen of the University of Oulu.
According to Liimatainen, non-toxicity also brings challenges to the prevention of wood-destroying micro-organisms. Therefore, the aim is to modify the surface structure of nanocellulose in such a way that micro-organisms would not be able to degrade it.
Nanocellulose can be prepared from many cellulosic source materials, but perhaps the easiest method is to produce it from commercial bleached chemical pulp. Other interesting raw materials include waste paper, cardboard and rejects from paper mills.
“This project focuses on developing new chemical manufacturing processes for nanocellulose based on deep eutectic solvents (DES). Deep eutectic solvents are solvents based on green chemicals, such as urea and choline chloride, which is used in poultry feed. They are very competitive in terms of costs and commercially well available,” Liimatainen says.
Results in one year
Before the end of the project, the effectiveness of the wood preservatives developed will be tested at Luke’s experimental station in Punkaharju, which has an outdoor test field and heated boxes with soil for testing indoors. The station has a collection of some twenty commercial mould and decay fungal strains that are ready to try to penetrate into bio-preserved wood.
The moisture conditions and temperature of the heated boxes can be adjusted to make them favourable for microorganisms, and thus the first results on the rot resistance of small sticks will be obtained in approximately six months after the start of the test. Results from outdoors can be expected in 5–10 years.
According to Venäläinen, ineffective wood preservatives can be established within three months. If treated wood has retained its quality for a year, the results can be regarded as promising. Testing is usually continued until the strength of the impregnated wood plummets.
“The project aims to combine tannins, wood distillates and nanocellulose into a price-competitive wood preservation product that is so safe that it can be burnt in a masonry oven used for bread baking, for example,” Venäläinen says.
Bio-based wood preservatives are being developed in the Nanotech-driven bio-preservation of wood (SafeWood) project funded by Tekes, the Finnish Funding Agency for Innovation. The project started at the beginning of this year and will end on 31 May 2017. The project is coordinated by the University of Eastern Finland, and the other partners are the University of Oulu and the Natural Resources Institute Finland. In addition, the project involves companies that can make use of the development work.
Areas of responsibility:
- Natural Resources Institute Finland: The extraction of tannins from bark using hot water extraction and the testing of wood preservatives
- University of Eastern Finland: The production of pyrolysis liquids, the development of wood preservatives from nanocellulose and tannins, the test treatment of wood pieces as well as the modification and analysis of chemicals
- University of Oulu: The production of nanocellulose and the fixing of tannins to nanocellulose.