Transparent wood-bulding material of the future?
You 100% know what wood is. It was one of the first materials, which people tamed. People make furniture, toys, houses, sport equipment and more of wood. But have you ever thought about what is inside of this material?
Compostion of traditional wood
There are three main components in wood: cellulose, hemicellulose, and lignin. In short, cellulose is the core of wood fibers, lignin makes the fibers strong and colorful, and hemicellulose adds flexibility and fills out the rest of the volume.
Cellulose
Cellulose is a polysaccharide composed of a linear chain of β-1,4 linked d-glucose units. It is the most abundant organic polymer on the Earth. Cellulose accounts for approximately 40–50% of wood fiber and is the main component of it. The main function of cellulose is wood strength.
Hemicellulose
Hemicellulose is a polysaccharide, whose monomers include the six-carbon sugars mannose, galactose, glucose, and 4-O-methyl-d-glucuronic acid and the five-carbon sugars xylose and arabinose. It forms some of the “flesh” that helps fill out the fiber. The main function of hemicellulose is wood flexibility.
Lignin
This generic term “lignin” describes a large group of aromatic biopolymers, i.e. the second most abundant class of biopolymers on Earth. In contrast with cellulose and hemicellulose (which are polysaccharides), lignin is a phenolic polymer. Its monomers include paracoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Lignin acts as a binder for cellulose fibers, adding strength and stiffness to wood cell wall, as well as the color to the wood fibers.
As you see, these are 3 main components of traditional wood. But what if we remove one of these components from our wood fibers?
Transparent wood
It might sound ridiculous from the first glance, but yes, wood can be made transparent. If you read the first part of the article carefully, you might have noticed that lignin gives color to wood fibers. Cellulose and hemicellulose are pretty much transparent by themselves, so scientists decided to remove lignin from wood and try to make the fibers transparent. In the next part of the article I’ll describe how wood can be made transparent, and what its applications are.
How wood can be made transparent
First of all, a thin (around 1,5 mm) piece of wood is taken.
Next, we should eliminate the components of wood those absorb light. As you remember, the biggest light absorbent is lignin, which is responsible for 95% of the absorbed light. This light absorption can be drastically reduced by chemical treatment of the wood: either removing “all” of the lignin (this process is called delignification) or deactivating the chromophores within lignin. In the first method the structure of wood is damaged a lot, and the final product is very weak. The second method helps to solve this problem.
To implement the first method, the wood can be treated with a 5% aqueous solution of sodium hypochlorite (NaClO) or with sodium chlorite (NaClO2). The second method can be realized by using alkaline H2O2 treatment of wood.
Microscale and nanoscale voids appear in the cell walls after removal of lignin. Because of them light scatters inside the wood piece, stopping the material from becoming fully transparent. To solve this problem, the wood piece is infiltered with a polymer, where the refractive index (n) is matched to the wood template, for example polymethyl methacrylate (PMMA, n ≈1.49) or epoxy resin (n ≈1.5). If the mismatch between refractive indices of wood and the infiltrated polymer is small, this can reduce the scattering drastically.
And that’s it. We generally refer to this composite material, a wood template infiltrated with a suitable polymer, as “transparent wood.”
Applications of transparent wood
The resulting wood composite is much stronger than natural wood and allows up to 90% of light to pass through it. The material has the practical utilisation potential to replace glass and plastic materials in a lot of places. Some examples are:
- Replacing glass windows on buildings. Composite wood is immensely better in terms of thermal insulation when compared to glass, so it will likely have a hugely positive effect on the energy efficiency of buildings in the future. In addition, it’s a lot stronger than glass, which reduces the use of materials.
- Solar panels and solar cells. Composite wood has a high light transmittance and good durability against environmental damage such as hail and the capacity to implement “light trapping” properties that reflect light back to the cells. This will reduce the rate of escaping rays and increase the energy efficiency of the panels.
And that’s not everything. This material has emerged recently, and a lot of research is still required, but due to its outstanding properties it has a potential to change our world immenesly.
