Transparent wood: new sustainable material

A schematic diagram showing preparation of TWC.

By PRIYA BISHT & KRISHNA K. PANDEY

Wood is one of the oldest raw materials that humans have been using for centuries for construction and housing, agricultural implements, fuel, paper and pulping, etc. It is a renewable material, bio-based with great capabilities to sequester carbon.

Natural wood is opaque due its distinct colour and chemical constituents. The colour and natural grain of wood confers an aesthetic appeal to it. In recent years, wood has been used as a sustainable raw material for developing noble composite materials for specific applications.

Natural wood is composed of three main structural components: cellulose, hemicellulose and lignin. Cellulose and hemicellulose are both colourless; whereas lignin is a phenolic polymer which, along with extractives, impart colour to the wood.

It contains several chromophores groups (carbonyl groups, quinones, conjugated doble bonds, etc.) which lead to maximum light absorption in wood. Lignin alone is responsible for more than 80% of the light absorption in wood (Müller et al., 2003).

Additionally, wood made of several extractives and inorganic compounds which contribute to the natural colouration and light absorption in wood.

One of the recent developments include a see-through, clear and transparent wood with fascinating properties. Transparent wood composite (TWC) is a new wood-based polymeric material prepared by two-step process of de-lignification and/or lignin modification – the bleaching of wood substrate followed by polymer infiltration.

Application areas

Considering the light weight, low density (1.1 gm/cm3) and low thermal conductivity of TWC the material has been considered as a suitable replacement for glass, where it can provide better thermal insulation, uniform and consistent daylight illumination without any glare – as well as without changing the living temperature.

TWC has the potential for use in energy-efficient buildings elements. The material presents light management and heat shielding abilities. Owing to its unique high transmittance and high haze nature, it aims to provide better thermal insulation delivering a calm and uniform indoor temperature without any glare or shadowing effect (Li et al. 2016).

TWC exhibits high optical transmittance and high haze in the visible wavelength range. It also demonstrates favourable mechanical properties and thermal stability. Wood with light colour, low density, high porosity (such as poplar and balsa) is preferred for the easy and smooth fabrication of TWC.

TWC has been conceptualised for thermal energy storage systems, light diffusers, smart windows, flexible devices, packaging, etc.

Due to its attractive features, TWC provides promising scope of utilisation in light transmitting and harvesting devices, energy-efficient elements, smart windows and roofs, display panels, solar cells and other materials.

Play of light

The alignment of vessels / channels and other elements behave as light-guiding agents within the TWC substrate. Due to the unique display of simultaneous high transmittance and high haze, it has been recommended for use in solar cell substrates.

An overall improvement in power efficiency of around 18% was found by placing a TWC sheets over a GaAs (Gallium Arsenide) solar cell (Zhu et al. 2016). In another study, the assembly of TWC with Perovskite solar cell results in enhanced power efficiency of 16.8% (Li et al. 2019).

In order to attain light transmitting characteristics, a material should have low minimum light absorption as well as minimum light scattering within its interface. Therefore, to fabricate a transparent composite originating from wood, the prerequisite step is to reduce light absorption in wood and minimise light scattering due to voids present in wood structure.

The fabrication process is basically a chemical modification of natural wood. In the first step, lignin can either be largely extracted from the wood or can be selectively modified by only targeting the chromophores present in lignin.

Extractives are non-structural elements that can be easily extracted with the help of solvents. Through successive bleaching, natural wood is converted into a cellulose-rich, white wood substrate with improved porosity.

The white wood still causes light attenuation (scattering and absorption) within its interface due to mismatched refractive index among wood polymer (cellulose and hemicellulose) and air trapped in the lumens.

In addition, wood is an anisotropic material and possess multi-scalar dimensions within the wood cell – vessels, fibres, pits and cell wall layers – that cause light scattering.

Therefore, in the subsequent step, a suitable polymer whose refractive index matches closely to that of wood cellulose is filled inside the porous wood to construct a homogenous transparent substrate. These suitable polymers include epoxy resin, poly methyl methacrylate (PMMA) and polyvinyl alcohol (PVA).

 

(a) Natural wood, bleached wood and TWC; (b) Transmittance spectra of 2-mm-thick TWC.

 

Fabrication process

To reduce light absorption in wood, lignin can either be completely removed by de-lignification or selectively modified by targeting only the chromophores present in its matrix by lignin modification bleaching. De-lignification utilises acid chlorite to bleach or de-colourise wood, resulting in drastic reduction in the lignin level.

This is not favourable because lignin also acts as a binding agent within cellulose microfibrils to impart mechanical strength. The bleached wood obtained through de-lignification was found to be structurally unstable and fragile. Moreover, harmful chemicals and toxic effluents from de-lignification had the potential to cause health hazards.

As an alternative, a relatively green, less time-consuming, alkaline-based lignin modification bleaching approach was evolved (Li et al. 2017). This method leads to large preservation of lignin within the bleached wood substrate by specifically targeting the chromophores units and retaining 80% of lignin (Rao et al. 2019).

This method leads to a structurally stable wood substrate, which is essential to provide mechanical strength to the TWC. After obtaining white wood substrate, the lignin modified samples were dehydrated using acetone to prepare hydrophilic wood substrate compatible with hydrophobic polymers such as epoxy and PMMA.

If hydrophilic polymer such as PVA is used, the step of acetone dehydration is not required. Subsequently, bleached wood samples were infiltrated with a suitable polymer (epoxy) using the vacuum impregnation method.

Under the influence of vacuum, the air inside the lumens is forced out and replaced with epoxy. The epoxy-saturated wood substrates were then placed in between transparent polyester sheets and cured for 24 hours at room temperature to obtain solid TWC sheets.

When obtained in this manner, TWC exhibits high optical properties with transmittance of ~83% within the visible wavelength range, in contrast to opaque wood, where the optical transmittance was as low as 3%.

Economically viable

Through the simple two-step process, a structurally stable, strong, clear and transparent wood composite is obtained with superior optical characteristics, favorable mechanical and good thermal properties (Li et al. 2016; Bisht et al 2022b).

Owing to its shatter-proof nature and low thermal conductivity (~0.36 W/mK), it has potential as a sustainable alternative to glass and plastics. Low density, light coloured, highly porous wood such as balsa, beech, poplar, etc., are suitable for fabrication of TWC.

Such wood species allow fast and easy processing, which can be economically viable. TWC prepared from 2-mm-thick, low-density Poplar infiltrated with epoxy resin exhibits high optical transmittance of around 83% at 550 nm wavelength. The TWC material also exhibits high optical haze of around 85% due to the anisotropic ultrastructure of wood.

While using moderately heavy wood species for instance – such as Melia dubia and Silver oak – the optical transmittance is slightly reduced to 76% and 72% respectively, for 2-mm-thick wood sections (Bisht et al. 2022b).

In comparison, a clear epoxy sheet shows a transmittance value of around 91% and almost 0% haze at 550 nm wavelength due to its homogeneous interface.

The optical transmittance of TWC decreases with increasing wood thickness. In thicker sections light needs to travel longer distances and suffers from scattering due to the anisotropy caused by hierarchical wood ultrastructure.

The optical transmittance values for 4.5-mm and 10-mm-thick TWC were found to be 65.5% and 48% respectively at 550 nm (Bisht et al. 2022a). However, efforts are being made to improve the optical properties of thicker sections.

 

(a) TWC prepared from longitudinal poplar wood veneer section and epoxy resin; (b) TWC produced from silver oak wood with retained intrinsic grain pattern of the wood; (c) thick multilayered TWC from Melia dubia veneer; (d) Flexible TWC prepared from Melia dubia wood veneer and polyvinyl alcohol.

 

Flexible material

TWC can be made highly flexible and foldable by changing the infiltrating polymer. The infiltration of a water-soluble dispersion of PVA and polyethylene glycol in the bleached wood substrate results in a flexible TWC with 80% light transmittance (Rao et al. 2019).

Similarly, flexible TWC can be achieved with the use of flexible epoxy resin, polyurethane as the infiltrating polymer. These can be considered flexible devices, photonics, light diffusers, etc.

Wood being a porous material allows easy accommodation of external materials such as nanoparticles, quantum dots, dye, and other functional compounds to produce multi-functional properties. In TWC, the addition of these materials leads to the development of magnetic transparent wood, luminescent transparent wood, photo-chromic/ thermo-chromic transparent woods, etc.

For intended outdoor applications, weathering-resistant TWC is needed. Recently, a UV-resistant, photo-stable TWC functionalised with an UV absorber was fabricated which can be effectively used outdoors.

 

– Priya Bisht is a Ph.D. scholar and DST Inspire Fellow at the Institute of Wood Science and Technology, Bengaluru. Dr Krishna Pandey retired as a scientist from the IWST, and is now Editor-in-Chief of the journal of the Indian Academy of Wood Science. He can be contacted at kkpandey77@gmail.com.

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