Prof Dr Frédéric Pichelin and Prof Dr Thomas Volkmer from the Bern University of Applied Sciences, explain the reasons behind why the research group on wood modification are trying to improve the fire resistance of wood.
Wood is a natural organic material with a very specific property profile, and therefore can be used in numerous technical applications. Wood can be used as construction material for furniture, flooring, windows or instruments. As well as being utilised for fields such as civil engineering (e.g. for building houses, bridges or towers), or as an insulation material and provide an extraordinary living standard during both summer and winter. Moreover because of its perfect ratio between strength and density, wood becomes an ideal lightweight material.
The wide range of possibilities to process wood, opens the door for both extraordinary buildings and constructions. In addition to this, the high degree of pre-fabrication and the outstanding precision of pre-fabricated elements offers the possibility to reduce construction times and guarantee a high-quality standard. Also because of woods natural origin, wood materials provide a wide spectrum of esthetical features variations, which is a key factor for many applications, as well as the fact that most wooden products can be recycled and processed into new materials. Continually, wood becomes increasingly more important in the field of biorefinery as a renewable source for glue or coating systems.
Limitations of wood as a construction material
Beside the large number of advantages of using wood, we tend to face some draw backs when implementing wood as construction material. Due to the chemical composition wood has a high affinity to humidity; a high moisture level supports the biological degradation and humidity changes induce significant changes of the dimensions. So, in every application an appropriate moisture protection is necessary. Another challenge is the UV-protection of wooden surfaces. The matrix substance lignin, which provides the stiffness, gets degraded by the absorption of (UV) light, which is highly relevant for building facades. Several UV-protections systems and concepts have proofed their efficiency during the last decades.
By using wood as a construction material, the combustibility is a crucial point. This becomes even more challenging when used to build multi-story buildings. In comparison to steel, glass or concrete, wood needs a special protection or treatment to improve its fire resistance. Beside the pure material properties, the dimensions of the elements and the construction of the building, influence the combustibility a lot.
Improving the fire resistance of wood
Components with a small ratio between surface and volume are highly fire resistant, because the initial energy to ignite the beam is high. From this point of view, wood constructions are safer than steel ones. Steel transmits heat very quickly and loses strength significantly when the temperate increases. On the contrary, wood creates a charcoal layer which isolates the core and prevents any further heating; as a result, the strength remains at a high level. This provides the possibility of a save evacuation of buildings in case of fire.
There are several strategies to improve the fire resistance of wood based on the application of fire-retardant systems. Depending on the materials used, the following effects will influence the reaction of when fire comes into contact with wood: delayed ignition, promotion of char formation at lower temperature than wood usually degrades, free-radicals trapping in the flame, dilution of combustible gases coming from wood with non-combustible gases, reduction of heat content of the volatile gases, and foaming coatings creating a heat insulation layer. All the above effects are based on different chemicals which are partly leachable und toxic. That’s why the field of application, outdoor or indoor, is limited.
No-toxic system: improving woods resistance against fire
Over the last 6 years, within a research program, we have been working on a sustainable no-toxic system to improve the resistance of wood against fire using different minerals. The invention focuses on an in-situ process to produce insoluble inorganic minerals, e.g. siliconates, oxalates, carbonates or sorbates, and others in wood.
The modification follows a two-step process. Highly water-soluble minerals get impregnated one after the other into the wood. The two components react with each other to insoluble minerals and get deposited into the cells or cell walls. The minerals are non-leachable and remain stable in the wood structure. This way a hybrid material is created which combines the properties of wood and minerals.
The reaction to fire will especially be influenced by this treatment. The inflammability will be delayed, as well as the spread of smoke and fire will get reduced. This increases the safety and offers new possibilities for wooden buildings (size and construction details). The creation of insoluble minerals in the wood also influences the optical properties. Due to the mineralisation, the substrate gets completely coloured which creates a depth effect, a high colour and light stability.
However, because of the complexity and novelty of the project, the research is still active. There are still several open questions, which need to be solved in order the provide a reliable system. Some future challenges include: the up scaling of the process, the recovery of the substances during the process, the influence of the wood specie and the dimensions of the samples.
Head of Institute for Materials and Wood Technology and deputy head of research
Architecture, Wood and
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Please note, this article will appear in issue 32 of Scitech Europa Quarterly, which is available to read now.