Flame-retardant wood-based materials thanks to melon enzymes: sawdust becomes a fire protection material

Sawdust is produced with every tree trunk that is sawn. Millions of tons of sawdust are produced worldwide every year, most of which is incinerated. In the process, the carbon dioxide stored in the wood is released back into the atmosphere - which would be better avoided for climate protection reasons. A research team at the Chair of Wood-Based Materials at ETH Zurich and Empa has now developed a process with which sawdust can be processed into an environmentally friendly and recyclable composite. This allows the sawdust to be kept in the material cycle for longer. The researchers use the mineral struvite, a crystalline, colorless ammonium magnesium phosphate.

It was already known that struvite has interesting properties for fire protection. However, it was difficult to combine the mineral with sawdust particles due to its crystallization behaviour. The researchers are now using an enzyme obtained from the seeds of watermelons to control the crystallization of struvite in an aqueous suspension with sawdust. This produces large crystals that fill the cavities between the sawdust particles and bind the particles firmly together. The material, which is pressed for two days, is then removed from the mold and dried at room temperature.

Wooden elements that protect themselves

«The material is more resistant to pressure than the original spruce wood perpendicular to the direction of the wood fibers,» explains Ronny Kürsteiner, who developed the process as part of his doctoral thesis under the supervision of Ingo Bungert, Professor of Wood-based Materials. Due to its mechanical properties and high fire resistance, it is particularly suitable for interior fittings. This is because struvite is not only non-combustible, but also actively contributes to increasing fire resistance. The mineral decomposes under heat, releasing water vapor and ammonia. This is a process that absorbs heat from the environment and thus has a cooling effect. In addition, the non-combustible gases released displace the air, which the fire lacks to spread further, so that the material chars more quickly.

The team collaborated with researchers at the Polytechnic of Turin, who tested the material in a so-called cone calorimeter. This is a standardized test method that simulates the behavior of the material when exposed to external heat. While untreated spruce wood catches fire after around 15 seconds, the struvite-sawdust composite takes more than three times as long. Once it has caught fire, a protective layer of inorganic material and carbon quickly forms, which protects the material from the fire spreading further. «The struvite-sawdust panels therefore protect themselves, so to speak,» says Kürsteiner.

Initial estimates have shown that the material could achieve the same fire protection class as conventional cement-bonded particleboard. However, this still needs to be confirmed with larger flame retardant experiments. Such chipboards are widely used today in interior construction for flame protection applications. They consist of 60 to 70 percent cement by weight, are correspondingly heavy and have a poor carbon footprint due to the high energy consumption in cement production. Struvite sawdust boards, on the other hand, consist of only 40 percent binder and are therefore significantly lighter.

Simple recycling process

The innovative composite has another decisive advantage over other composite building materials: cement-bonded chipboard usually ends up in a hazardous waste landfill after demolition. The struvite sawdust boards, on the other hand, can be broken down into their individual components. To do this, the material is mechanically broken up in a mill and heated to just over 100 degrees Celsius, releasing the ammonia and allowing the sawdust to be sieved off. The mineral source material for struvite, known as newberyite, is then precipitated again as a solid.

Newberyite can then be reprocessed with sawdust to make composites. This means that the new material could one day make an important contribution to the circular economy. It can also be used as a natural fertilizer. This is interesting for agriculture because it releases the bound phosphorus that plants need for growth slowly and in a controlled manner.

In the next step, the researchers want to further optimize and scale up the production process. Whether the material will become established in the construction industry depends above all on the cost of the binder, says Kürsteiner. Compared to polymer binders or cement, struvite is relatively expensive. However, this could change by tapping into another cycle: struvite accumulates in large quantities in sewage treatment plants and clogs the wastewater pipes there. «We could use these deposits as a base material for our building material,» says Kürsteiner.

Source: Empa