Mineralised solid wood boards

Fire protection is a key factor in the use of wood as a construction material. Whereas load-bearing elements must bear the load of the structure for a certain period of time in a fire, the protective goal for construction elements without a load-bearing function is essentially to prevent and restrict the spread of fire. The fire behaviour of combustible construction materials such as wood can be significantly improved through the use of fire retardants. Fire retardants include a wide range of products which aim to improve the fire performance of highly combustible materials. Their chemical composition, mode of action and method of application may also differ. The most commercially successful fire retardants for wood are inorganic salts, which are usually impregnated into the wood. Many of these inorganic salts are made of heavy metals and/or halogen compounds such as bromides. These components present a high level of risk to the environment as well as health risks for humans and animals. This is why the ‘Mineralised solid wood boards’ project is evaluating the application of mineral organic salts – which do not present any environmental or health risks – as a fire retardant for wood.

Procedure

The mineralisation of solid wood boards aims to improve the fire-retardant properties of solid wood. A mineral organic salt is precipitated in situ in the wood. The wood is consecutively impregnated with salts dissolved in water in a two-stage process. They then react to produce a highly fixed, water-insoluble mineral salt. The impregnation and implementation of the salts used to produce a water-insoluble target mineral makes up the first part of the research project. The interaction in the form of colour change with the wood components, for example tannins in the wood, is also evaluated. The fire behaviour of the mineralised wood is then tested. The impact of the mineralisation on other properties is also assessed. For example, the humidity absorption behaviour of the mineralised wood is monitored.

Results

It was shown that the two-stage process can be used for the mineralisation of solid wood. Various types of wood (e.g. beech, oak and fir) were mineralised. The mineralisation of thin oak boards (thickness of around 4 mm) for use as a material for decorative surface layers of multilayered parquet was particularly successful. Oak is a difficult wood to impregnate (EN 350:2016 treatability class 4, difficult to treat). A dark colouring of the oak is a positive side-effect of mineralisation (image 1).

Various tests to evaluate the fire behaviour of mineralised woods show the potential of mineralisation as a fire retardant. A significant improvement compared to untreated wood was proven (image 2).

Owing to the hygroscopicity of the salts, the wood equilibrium moisture only increases slightly compared with untreated wood. A significant increase in wood equilibrium moisture of mineralised wood can only be expected in high levels of humidity (image 3).

Outlook

Further organic minerals and formulations will be analysed in follow-up research projects. They will continue to focus on application as a fire retardant. Modifying the formulations of the mineral salts and the specific properties created should extend the areas of application of mineralised wood. For example, wood is to be reinforced long-term for use as a façade material. The material not only has to meet stringent fire protection requirements, but must also be resistant to weathering.