One hour wood board becomes “glass”, which can be realized by patterning transparent wood

  This is the “transmutation” of a wooden board. What “magic” can make it change from brown to transparent?
  Looking at the flowers below through this board, the light transmittance is so good that people think it is a glass board, but it is actually a 100% wood board-resin composite.
  After lignin modification treatment and resin filling, the wood board can have glass-like transparency, it can penetrate more than 90% of the light in nature, and has better mechanical properties and heat insulation properties than glass, and its strength is comparable to other transparent wood similar.
  If the wall of the room is made of this kind of wood, the blue sky can be seen through the wall, so that the weather can be seen without opening the window.
  This is the “king of wood”-the team of Professor Liangbing Hu from the School of Materials Science and Engineering of the University of Maryland has developed a new generation of patterned transparent wood with solar-assisted synthesis after the last time they developed a new type of transparent wood with aesthetic patterns.
  Related papers are published in the journal Science with the title of “Large Patternable Transparent Wood for Solar-Assisted Manufacturing”. The corresponding author of the paper is Hu Liangbing, the first is a post-ninety Chinese girl Xia Qinqin, and Dr. Chen Chaoji, a postdoctoral fellow in Hu Liangbing’s research group .
Preparation: Why does the board become transparent?

  In the specific operation, take a 1 meter long and 1 mm thick balsa wood board into a transparent wood board as an example, first brush the surface of the wood board with hydrogen peroxide.

Apply hydrogen peroxide

  Then it is exposed to ultraviolet light or sunlight for one hour. At this time, hydrogen peroxide will bleach off the chromophore of lignin and keep most of the lignin.
  Then the wood is immersed in ethanol to remove lignin and other residues. Finally, a transparent epoxy resin is filled in the pores of the wood. This epoxy resin is designed for marine use and matches the refractive index of the wood cell wall. After the wood is filled and hardened, a wood board with 90% transparency is born. , Which is much higher than the transparency of natural wood by 6% to 36%. Since brushes and chemical solutions are used, this method can be understood as a “chemical brush” method.


Preparation steps of transparent wood (illustration in English)

The birth of decolorized wood

  The wood itself is not transparent, which is related to its two basic components: cellulose and lignin. Cellulose, in fact, is not far away from us. When you tear the paper towel in half, you can see a small amount of cellulose fibers along the edge of the tear. Lignin, on the other hand, contains chromophore molecules, which make the wood brown and prevent light from passing through. The cellulose fiber has a hollow tubular structure, and the air in the hollow tube scatters light, which further reduces the transparency of the wood.
  In the early days of making transparent wood, people mainly used the “sodium chloride solution delignification” method, which involves the use of hazardous chemicals to remove lignin, which not only requires high temperature conditions, but also consumes a lot of time, is expensive and compared with the prepared wood brittle. In order to avoid the above situation, Hu Liangbing’s team chose to modify lignin instead of removing lignin, and the new technology used this time is cheap and easy, so simple that it can be done in their own yard.
  The principle of this technology is that the wood hair color group is mainly produced by lignin and can absorb ultraviolet rays. The excitation of ultraviolet rays can also make lignin form free radicals. Benefiting from the inspiration of the properties of lignin, the team used ultraviolet light excitation and the oxidation of hydrogen peroxide to remove the chromophoric groups in lignin.
  Wood has a three-dimensional interconnected porous structure, and has microchannels with a diameter between 15 and 300 μm. This unique porous structure can promote the rapid penetration of hydrogen peroxide and the capture of ultraviolet light, and ultimately effectively remove light absorption and color Group.

Transparent wood structure

“Tai Chi Bagua” transparent wooden board

  Moreover, solar energy is the most abundant energy source on earth. Part of solar energy is composed of ultraviolet (UV) radiation, so sunlight can decolor wood in a short time and obtain transparent wood.
  This process is not difficult to understand. After immersing a paper towel in water, the transparency of the paper towel will increase. This is because the water filled with the holes of the paper has a refractive index closer to cellulose than the air originally filled, thereby reducing the scattering of incident light. , So that the paper towel becomes translucent.
  Talking about the advanced nature of the technology, Professor Hu Liangbing said that this method of using sunlight and chemical scrubbing can selectively process designated areas of wood samples to prepare transparent boards with different patterns.
  The team made a variety of transparent wooden boards with different patterns. Since most of the team members are Chinese, they specially made a wooden board with Tai Chi and Bagua patterns. The method is to dip a brush in hydrogen peroxide, brush the “A” shape on the wooden board, place it under ultraviolet light for one hour, and fill it with epoxy resin to obtain a hollow “A”-shaped wooden board.
  They also showed transparent wood samples with more complex geometric shapes, such as two transparent circles minus an opaque diamond, which shows that transparent wood with arbitrary patterns can be achieved using this method.

  This preparation method will not completely destroy the aromatic structure of lignin, and the obtained transparent wood board has a high tensile strength of 31.4-46.2MPa, which is 1.8 to 44.8 times higher than the tensile strength of natural wood, and has less than 4%. % Low optical absorption, and 60%-80% high optical turbidity.
  At the same time, the modified lignin can be retained. According to the test, the lignin content of the transparent wood board is 19.9%, which is only a few percentage points lower than the 23.5% lignin content of natural wood, so that the lignin can still be effective Combine and wrap cellulose fibers to maintain the mechanical properties of the board. The internal microchannel diameter of the transparent wood board is between 10 and 270 μm, which means that after treatment, it still has the porous structure inside the natural wood.

Transparent wooden boards with different patterns (illustration in English)

  According to infrared spectroscopy, the “chemical brushing” method can not only degrade lignin chromophores, but also retain the aromatic skeleton of lignin, which can provide strong support for polymer penetration and make transparent wood more tough. . The electron microscope image of the transparent wooden board shows that due to the injection of transparent epoxy resin, the transparent wooden board can form a dense structure, which can increase the light transmittance and inhibit light scattering.
  After measuring the optical transmittance of natural wood and transparent wood between 200 nanometers and 2000 nanometers, it was found that compared with the transmittance of natural wood (L<6%, T<36%), the transparency along the L and T directions Wood has a high transmittance of about 90% in the visible wavelength range (400nm ~ 800nm). The absorption rate of transparent wood is close to 0%, and it is far lower than natural wood in the wavelength of visible light. This is due to the removal of the light-absorbing chromophore group of lignin, so that almost all visible light can pass through the transparent wood.

Optical and mechanical properties of transparent wood (illustration in English)
Possesses large-scale preparation capabilities: it can be used to make transparent walls or as a protective covering

  According to the team, transparent wood panels can already be manufactured quickly on a large scale. As shown in the figure below, under 1 hour of sunlight, three lignin-modified wood boards with a length of 1m can be produced. After being infiltrated by epoxy resin, the final 400 mm × 110 mm × 1 mm transparent wood board can be reached. .
  The waste liquid produced in this preparation method is 30 ml, and no toxic gas is generated, while the "sodium chloride solution delignification" method requires 960 ml of waste liquid. Therefore, the "chemical brush" method has greater environmental advantages.

Transparent wood panels have large-scale preparation capabilities (English icon)

  Hu Liangbing said: "This kind of transparent wood is not only lighter than glass, it is stronger than glass, and more heat-insulating. It can be used as load-bearing windows and roofs, and can also be used to build transparent houses." For example, it can be used in public places such as airports and museums. Compared with the brittleness of glass, the wall material will not cause damage by throwing stones on the transparent wooden wall. Of course, not everyone wants to live in a transparent house.
  In winter, glass windows are the main source of indoor heat loss, and transparent wood has better heat insulation performance, and does not require high temperatures during the preparation process, and is more environmentally friendly than glass preparation.
  Wood is also a renewable energy source. During the growth of wood, trees release oxygen by breathing carbon dioxide, which is very beneficial to humans. In the harsh environment of sand and dust, it can also be used as a cover for solar panels, allowing it to continue to generate electricity in bad weather with sunlight. Moreover, the annual output of cellulose, the main component of wood, is 20 times that of steel. The choice of wood for transparent boards is also relatively wide, including oak and cedar.
  In terms of cost, compared with the "solution delignification" method, chemical brushes have higher production efficiency, lower costs, and are more sustainable and controllable. This is a cost-effective manufacturing process in which natural solar energy can also be used.
  Talking about the future application of the material, Hu Liangbing said that due to its unique hierarchical structure, high resistivity and good light management characteristics, transparent wood will also become an emerging material. Therefore, it may be widely used in optoelectronic devices, energy-saving buildings, and solar cells and touch panels.