Transparent wood composites are relatively new. Not so many people have heard about wood materials that are 90% transparent and have improved mechanical properties. Transparent woods were discovered in 1992; they are shatterproof and more biodegradable than plastics and glass.
Article by HL North East Architects in the UK
History of Transparent Wood
When the joint research group from the University of Maryland and Swedish KTH University embarked on their scientific exploration, they never knew they were about to discover a unique type of wood. First, they developed a method that strips small wood blocks of their colour and some chemicals. Then they added polymers like epoxy and polymethacrylate at the cellular level to achieve transparency.
Fast forward to about two decades after their discovery, researchers recently discovered a method of manufacturing transparent wood that is more durable and lighter than glass. The best part? The new approach is more efficient, requiring less energy and chemicals. The “solar-assisted chemical brushing” method produces thin wood that is supposedly lighter and 50 times stronger than wood made from the earlier methods.
Manufacturing Process of Transparent Wood
Natural wood is not transparent due to its ability to absorb and scatter light. Instead, it has a tarnish colour that is due to the presence of lignin, hemicellulose, and cellulose. The lignin contributes mostly to the unique colour of wood, and that is why the level of visibility in the wood depends on the amount of lignin. Therefore, the first step in producing a transparent and visible wood material is to reduce its scattering and absorbent properties. That is why the manufacturing process of transparent wood is called de-lignification, i.e., the removal of all the lignin.
Properties of Transparent Wood
As a natural growth material, wood is well-known for its impressive mechanical properties, such as high specific gravity, rich moisture content, excellent durability, and high strength. There are two types of wood – hardwood and softwood. Softwood has shorter longitudinal cells than hardwood. However, both share the same cellular orientation and, consequently, a similar hierarchical structure. With this special anisotropic structure, measuring the properties with special values in different directions leads to the pumping of ions and water for photosynthesis in the wood.
The wood becomes clearer when lignin is removed, and cellulose fibre tubes are retained. Then, it can be soaked in a glue-like epoxy to produce a robust, transparent wood material. This specific type of wood is a valuable raw material with improved mechanical properties and high transmittance capabilities.
Mechanical Properties of Transparent Wood
Two factors contribute to the performance and mechanical properties of transparent wood. The first is the cellulose fibre content, and the second is the geometric arrangement of the fibre tube cells structure. These properties offer a structural base for designing more sophisticated materials applications.
Therefore, we cannot talk about the mechanical property of transparent wood without mentioning its strength. Likewise, transparent wood composite is plastic in nature, making them shatter-resistant, unlike glass and other brittle materials.
Thermal Conductivity and Optical Transmittance of Transparent Wood
The cellulose fibres in transparent wood are arranged tightly and perpendicularly. Therefore, it functions as wide-band waveguides with high light scattering and transmission capabilities, and ultimately a light propagation effect. This is why transparent wood is energy efficient. Such an energy efficient material can help to decrease daytime lighting energy usage by guiding the sunlight into the house. The result is regular and consistent illumination all day.
The thermal conductivity of transparent wood is also due to the alignment of its cellulose fibres, which were retained after removing lignin and adding polymer. According to the findings of the researchers at KTH Royal Institute of Technology, Stockholm, the thermal conductivity of the transparent wood moves from semi-transparent to completely transparent when exposed to high temperatures. Therefore, they could help make buildings more energy efficient by capturing solar energy in the day and releasing it into the interior at night.
Future Potentials of Transparent Wood
Despite being at the prototype and lab-scale levels, transparent wood composites have strong energy efficiency and operational savings potential. As a result, they can play very crucial roles in the building industry. For instance, transparent wood can combine structural and functional performance, and this comes in handy for load-bearing structures with heat-shieling, magnetic, or optical functionalities. They also have excellent potential for use cases on surfaces that are touch-sensitive.
Transparent Wood and Glazing Systems
Using a light transmittance design to replace artificial light with sunlight is one of the most exciting future applications. The research team at the University of Colorado, Boulder, reported that replacing the glass glazing system with transparent wood could achieve a 24.6% to 33.33% reduction in the space condition energy requirement in medium and large offices, respectively.
These are useful potential applications of transparent wood’s functionality, thanks to its higher impact strength and lower thermal conductivity, especially when compared to glass glazing systems.
Transparent wood and its novel applications could champion the transformation of the material sciences and building industries. It also offers enormous potential improvements in energy efficiency, more than glass or other regular materials.
However, more research is required to ensure a complete understanding of the light-wood structure interactions and mastering the mechanical and optical properties. Finally, more advanced transparent wood composite uses can be unveiled and explored with more in-depth studies.