Transparent wood without plastic additives
A potassium hydroxide treatment for delignified wood makes it transparent without any polymer additives.
Lignin and microscopic air cavities called lumens in wood scatter light, typically making it brown and opaque. Removing lignin can turn it translucent and white. This effect is usually enhanced by adding a transparent polymer, but achieving true transparency is challenging.
Researchers at the University of Osaka, Japan, have discovered that an alkali treatment removes any remaining hemicellulose in delignified wood and changes the chemical state of carboxyl groups in wood’s cell walls. These changes soften the internal cellulose microfibril skeleton.
When the treated wood is dried, the softened cell walls collapse more completely, reducing internal air gaps and decreasing light scattering. As a result, the material becomes highly transparent.
The team’s results have been published in the paper Anisotropic transparency of alkali-treated wood in Macro-Molecular Materials and Engineering.
The paper explains further, 'In the delignified wood, the residual hemicellulose and carboxyl group with H+ counterions rendered the cell walls rigid…even after drying, the delignified wood maintained the lumens in the wood cells and became white and translucent owing to the scattering of light from the air voids.'
The KOH treatment removed more hemicellulose, so the 'counterion of the carboxyl group was exchanged with K+, resulting in the wood cell walls being softer and easier to deform.
'The lumens of the fibres and ray cells were deformed into flattened cavities, and the vessel elements were deformed into elliptical cavities, rendering the dried, delignified KOH-treated wood transparent.'
By removing the need for polymer additives, the researchers suggest the KOH treatment could create a more sustainable and natural alternative to transparent glass and plastic materials.
The team identifies that transparency also depends on the wood’s anisotropic structure. This is based on the arrangement of cellulose microfibrils inside the cell walls – the optical anisotropy can be longitudinal, radial or tangential.
Following treatment and drying, the paper describes how the tangentially oriented samples are more transparent, transmitting about 69% of light at a wavelength of 550nm compared to 59% for radial samples. This is because 'the tangential and radial sections with lignin and hemicellulose exhibited different water-swelling ratios, but the same light transmittance.
'When the lignin and hemicellulose were removed, they exhibited considerable swelling and optical anisotropy, with the swelling ratio being 4.1% of the thickness of the tangential section and 13.3% of the thickness of the radial section.'
This causes the vessel elements in the tangential section to deform into flattened cavities, while those in the radial section deform into elliptical cavities. The tangential sections essentially undergo more complete collapse of internal lumens during drying, leading to a denser structure and higher transparency.
For comparison, when reinforced with a transparent polymer, the tangential composites have been reported to achieve over 90% transmittance.
Until now, the reason why delignified wood sometimes remains cloudy was not fully understood. Professor Masaya Nogi at the university says, 'Understanding this directional effect gives us new freedom to design sustainable transparent materials from natural resources.'
The study lays out design principles for bio-based transparent materials, which could be used in architectural panels, lightweight optical components and flexible, wood-based electronics.
