Transparent Wood Could Be the Window of the Future

Transparent wood composites are novel wood materials which have up to 90% transparency. Some have better mechanical properties than wood itself. They were made for the first time in 1992. These materials are significantly more biodegradable than glass and plastics. Transparent wood is also shatterproof.

History
A research group from Swedish KTH University and a University of Maryland research group developed a method to remove the color and some chemicals from small blocks of wood, followed by adding polymers, such as polymethacrylate and epoxy, at the cellular level, thereby rendering them transparent. More recently, in 2021, researchers reported a way to manufacture transparent wood lighter and stronger than glass that requires substantially smaller amounts of chemicals and energy than methods used before. The thin wood produced with “solar-assisted chemical brushing” is claimed to be lighter and about 50 times stronger than wood treated with previous processes.

Process
In its natural state, wood is not a transparent material because of its scattering and absorption of light. The tannish color in wood is due to its chemical polymer composition of cellulose, hemicellulose, and lignin. The wood’s lignin is mostly responsible for the wood’s distinctive color. Consequently, the amount of lignin determines the levels of visibility in the wood, around 80–95%. To make wood a visible and transparent material, both absorption and scattering need to be reduced in its production. The manufacturing process of transparent wood is based on removing all of the lignin called the de-lignification process.

Properties
Wood is a natural growth material that possesses excellent mechanical properties, including high strength, good durability, high moisture content, and high specific gravity. Wood can be classified in two types of wood, softwood and hardwood. While each type is different—e.g., the longitudinal cells in softwood are shorter in length when compared to hardwood—both types have a similar hierarchical structure, meaning the orientation of the cells is identical in the wood. This unique anisotropic structure, the properties with distinctive values when measured in several directions, allows it to pump ions and water for photosynthesis in the wood. Similarly, in transparent wood composites, removing the lignin and maintaining the cellulose fiber tubes it allows it to become a clear wood that can get soaked in a glue-like epoxy that makes it a robust and transparent material. An excellent raw material with high transmittance and enhanced mechanical properties.

Mechanical Properties
Transparent wood derives its mechanical properties and performance primarily from its cellulose fiber content and the geometric orientation of the fiber tube cells (radial and tangential) structure, providing the structural base for the design of advanced materials applications. One aspect of the transparent wood mechanical property is therefor the very high strength of the material.
Also, the plastic nature of transparent wood composite provides advantages compare to other brittle materials like glass, meaning it does not shatter upon impact.

Optical Transmittance and Thermal Conductivity
The transparent wood, tightly packed and perpendicularly aligned cellulose fibers operate as wide-band wave-guides with high transmission scattering losses for light. This unique light management capacity results in a light propagation effect. As a result, transparent wood as an energy efficient material could be used to decrease the daytime lighting energy usage by efficiently guiding the sunlight into the house while providing uniform and consistent illumination throughout the day.

Similarly, the transparent wood’s thermal conductivity is attributed to the alignment of the wood cellulose fibers, which has been preserved after lignin removal and polymer infiltration. Based on the study done by KTH Royal Institute of Technology in Stockholm, the transparent wood’s thermal conductivity, which transforms from semi-transparent to transparent when heated, could be used to make buildings more energy-efficient by capturing the sun’s energy during the day and releasing it later at night into the interior.

Future Application
Although the development of transparent wood composites is still at a lab-scale and prototype level, their potential for energy efficiency and operational savings in the building industry are very promising. An essential advantage with transparent wood is its combination of structural and functional performance for load-bearing structures that combine optical, heat-shielding, or magnetic functionalities. Transparent wood is also researched for potential use for touch-sensitive surfaces.

Glazing System Potentials
Such is the case in building applications where artificial light can be replaced by sunlight through a light transmittance design. Based on research and simulation performed at the University of Colorado Boulder, transparent wood as a glass glazing system replacement could reduce the space conditioning energy consumption by 24.6% to 33.3% in medium and large office spaces respectably. These are relevant insights in transparent wood’s potential functionality because it shows lower thermal conductivity and better impact strength compared to popular glass glazing systems.

Transparent wood could transform the material sciences and building industries by enabling new applications such as load-bearing windows. These components could also generate improvements in energy savings and efficiency over glass or other traditional materials. A lot of work and research is needed to understand the interaction between light and the wood structure further, to tune the optical and mechanical properties, and to take advantage of advanced transparent wood composite applications.

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