By Ian Randall
A fire-retardant structural material can be made by chemically softening and compressing wood to remove the spaces between cell walls. When burnt, the resulting material forms a protective char layer on its outside which helps preserve its internal strength.
The use of wood in structural applications is limited by both its inherent flammability and susceptibility to rapid collapse on burning. Wood can be made more fire-proof by chemical treatments – such as through injections of halogenated flame retardants, or coatings of inorganic nanoparticles – but these approaches are typically either prohibitively expensive, fail environmental and health standards, or result in insufficient structural strength.
Liangbing Hu and colleagues of the University of Maryland in the US show that their process to create bullet-proof wood through densification also confers fire-resistant properties without recourse to potentially toxic or environmentally-unfriendly materials.
The densified material – which Hu dubs ‘super wood’ – is created by first chemically treating timber with sodium hydroxide and sodium sulfite to partially remove its lignin, the organic polymer which makes cell walls rigid. Subsequent hot pressing creates a dense, laminated material free of lumina – the tiny channels that create a porous structure, supplying oxygen and increasing flammability.