Processing bulk natural wood into a high-performance structural material

Abstract

A process is described for the transformation of bulk wood into a low-cost, strong, tough, lightweight structural material, by the partial removal of lignin and hemicellulose followed by hot-pressing to densify the natural wood. Densification is a common processing route for wood, but defects often remain and wood can be susceptible to weakening in the presence of humidity. This paper reports a method for treating wood before densification in order to substantially enhance its strength and resilience. The authors show that partially removing lignin and hemicellulose from the wood makes it around three times denser, with an 80% reduction in thickness. However, complete removal of lignin and hemicellulose results in a poor-quality material, leading the authors to suggest that some lignin must be retained to bind the wood. The resulting materials and their laminates show impressive strength, toughness and ballistic resistance while being more lightweight, simple to process and less environmentally damaging than many other structural materials. Synthetic structural materials with exceptional mechanical performance suffer from either large weight and adverse environmental impact (for example, steels and alloys) or complex manufacturing processes and thus high cost (for example, polymer-based and biomimetic composites)1,2,3,4,5,6,7,8. Natural wood is a low-cost and abundant material and has been used for millennia as a structural material for building and furniture construction9. However, the mechanical performance of natural wood (its strength and toughness) is unsatisfactory for many advanced engineering structures and applications. Pre-treatment with steam, heat, ammonia or cold rolling10,11,12,13,14,15,16,17,18,19,20,21 followed by densification has led to the enhanced mechanical performance of natural wood. However, the existing methods result in incomplete densification and lack dimensional stability, particularly in response to humid environments14, and wood treated in these ways can expand and weaken. Here we report a simple and effective strategy to transform bulk natural wood directly into a high-performance structural material with a more than tenfold increase in strength, toughness and ballistic resistance and with greater dimensional stability. Our two-step process involves the partial removal of lignin and hemicellulose from the natural wood via a boiling process in an aqueous mixture of NaOH and Na2SO3 followed by hot-pressing, leading to the total collapse of cell walls and the complete densification of the natural wood with highly aligned cellulose nanofibres. This strategy is shown to be universally effective for various species of wood. Our processed wood has a specific strength higher than that of most structural metals and alloys, making it a low-cost, high-performance, lightweight alternative.