More and more products nowadays are made from renewable sources such as trees. From jeans made out of wood fibers to wooden batteries for solar panels, trees are an attractive feedstock because they are biodegradable and easy to harvest. While we used to build our houses and buildings from natural wood, we’ve since moved on to using steel and other metal alloys due to their strengths. These alloys are much heavier than wood, however, and are quite expensive. Dr. Lianbing Hu and Dr. Teng Li from the University of Maryland led a team that created a process to treat wood to make it as strong as metal alloys.
What’s inside a piece of wood?
Wood is made up of three components: cellulose fibers, hemicellulose, and lignin. The cellulose fibers are what makes up paper, while the hemicellulose and lignin bind together the fibers and give wood its rigid structure. In the papermaking process, the hemicellulose and lignin are removed by adding a white liquor which consists of sodium sulfite and sodium hydroxide. This leaves behind the cellulose fibers as pulp, which goes on to be pressed into paper.
Wood is a porous solid, with empty spaces around its cell walls on the inside of the wood. To increase the strength of wood, its density needs to increase (called densification) which either means collapsing the empty spaces inside the wood or stitching together incredibly thin slices of wood using a thermosetting synthetic resin.
How the treatment method works:
The process Dr. Hu and Dr. Li designed starts by treating wood with white liquor, much like in papermaking. This breaks down some of the lignin in the wood, but not all of it. If the lignin is not removed the cell walls will be too rigid, but if too much of the lignin is removed the wood will fall apart under pressure.
After enough lignin has been removed, the wood is then mechanically hot-pressed to compact it together. The hot-pressing compresses the wood until it’s one-fifth the thickness of the original, and this makes sure that there are no remaining air bubbles or cell walls that did not collapse properly which could seriously compromise the strength of the wood. This is where the partial removal of lignin comes into play: too much lignin left in the wood and the cell walls won’t all collapse properly, but not enough lignin left and the wood falls apart when pressed. Afterwards, a layer of paint is applied on the surface of the wood for moisture.
What can we use the wood for?
After going through the two-step treatment method, the wood we get is about as strong as steel but six times lighter. It is 12 times as strong and 10 times as tough as natural wood, and it is also comparable to carbon fiber despite being much cheaper. The researchers shot a bullet-like projectile at a wood sample with the thickness of 5 pieces of plywood, and the sample was able to stop the projectile before it penetrated the sample.
The potential applications for this wood are numerous. Because of its moisture resistance when painted, the wood can be used outdoors as well as indoors. It has comparable strength to steel despite being much lighter, so it can be used to replace steel in applications such as car bodies or building materials.
The biggest challenge moving forward is scaling up this process so that it will be feasible on an industrial scale. Right now, it takes over one day to treat wood samples the size of a coffee table book. A scaled-up process must not only be able to produce high volumes of wood but do it in a short amount of time to remain economical.
