Ecological coating based on a living fungus

Dr. Michael Sailer, Saxion University of Applied Sciences; Xylotrade B.V.

The concept of a protective biological surface treatment is based on the fungus Aureobasidium pullulans. The potential of this fungus to create a wood protecting coating using living organisms was discovered in in the late 90s during experiments for an EU project and developed in the following years.

Wood impregnated with natural oils turned out to be discolored outdoors. The causes of these color changes, usually associated with irregular discoloration and staining, were investigated. Samples treated with linseed oil showed a more homogeneous color and fungal growth. Aureobasidium pullulans was on these surfaces recognized as the dominant fungus. Microscopic analysis showed that the fungus is able to create different types of cells which fulfill the requirements of a coating such as pigmentation and UV-protection, the creation of binders to achieve adhesion and the creation of a coating ”body”.

Interesting in this context is that Aureobasidium is able to survive on exposed surfaces at different conditions. It is considered a polymorphic fungus which can survive conditions like a various range of temperatures (-20° and above 60°C), low water availability or UV-radiation.

The goal of this treatment was to develop a natural, environmentally friendly wood protection concept. In contrast to conventional concepts, the selective growth of desired naturally occurring microorganisms is stimulated and controlled.

This idea has been developed into an industrially applicable process in the Netherlands at TNO (a Dutch research institute) and Xylotrade B.V. over the past 15 years to the so-called Biofinish treatment. The combination of the hydrophobizing effect of linseed oil and the surface treatment creates an aesthetically appealing dark surface, which significantly prolongs the service life of wood outdoors and significantly reduces maintenance costs.

By the combination of several factors, e.g. water repellence and selected nutrients, the fungal protection system can survive and last long. Since the linseed oil repels liquid water, but is permeable to water vapor, it creates a system which is dimensional stable; -it shrinks and swells less due to the delayed water uptake. In combination with linseed oil exudation, small cracks can be filled and small damage can be overgrown by the fungus. This effect allows damaged surfaces to be closed again under certain conditions and can be considered as a self-healing principle.

Despite its robustness, Aureobasidium pullulans responds to small changes, so that the desired results are influenced by many small details. Since the fermentation requires few resources, a fungal suspension can be produced inexpensively with sugar, water and some trace elements. The fungus Aureobasidium pullulans in combination with linseed oil forms a durable protective coating on wood. This living layer prevents many other organisms from growing into the wood, significantly extending the service life of e.g. a façade element. This is also supported by the selectivity of linseed oil, which fulfills several functions in this system. In addition, the dark fungal layer reduces the UV degradation of linseed oil, which prolongs the life of the entire system.

Two main functionalities of a timber based façade elements should be considered separately. The protection of the carrier material itself and the function of the surface are different aspects with different influence on the rating. The protective function is relevant for the safety and a requirement whereas the aesthetics must also satisfy the appearance. At the moment, an aesthetic lifetime of 6-8 years seems realistic before the fungi should be provided with new nutrients. These can be easily applied e.g. by spraying. No grinding or other work is needed.

Recent research has suggested that the Biofinish may play a role as a fire retardant surface treatment. Thermographic analyses (TGA) of Biofinish treatments suggest that a Biofinish treatment has positive effects on flame-retardance. In a recent SBI test according to EN 13823 the combination of Biofinish and a phosphate-based fire-retardant achieved a classification B1 according to EN 13501-1 (low flammability).

In order to be able to recycle building materials in a circular economy, the materials should be homogeneous and contain no harmful substances. Conventional coatings often contain harmful substances, which makes re-use difficult.

Since biobased materials treated with a Biofinish do not contain harmful compounds several re-use options can be considered and are under investigation at present.

Experiments have shown that the fungus no longer grows significantly at 35 ° C - 37 ° C and therefore is a less risk of growth in the human body. Also, Aureobasidium pullulans is approved as a biological plant protection. There is no evidence in the reviewed literature that Aureobasidium pullulans is toxic to humans but, as with other substances, it may cause allergies in susceptible individuals. On the other hand, numerous research results are known which examined the substances that Aureobasidium produces as potential remedies. For example, for β-glucans anti-allergenic effects and anti-inflammatory were found.

The Biofinish concept is developed in such a way, that the fungal coating can be applied with common industrial machines as well as with common brushes or coating rollers. Since it is consisting of mainly water it can be diluted to a certain degree with water. Although the Biofinish works on all wood species and even modified wood, recent tests showed that Biofinish can be applied on existing (conventional) coatings or even on old grey wood, leading to an excellent adhesion. This suggests that Biofinish could be used for retrofitting, as long as the required colors fit.

At present, the practical application of the Biofinish concept is being tested in several pilot projects. Biofinish was first applied on a wooden façade in combination with pine wood in Putten (Netherlands). The building was opened in November 2016. In the meantime, more than 25 further projects have been successfully completed at different locations.