Newsletter #3 topic – How to assess the properties of woods and Transparent Wood

Transparent Wood (TW) represents a fascinating development in materials science, transforming the ancient, ubiquitous natural composite that is wood into a material possessing optical clarity while retaining many of its inherent advantageous properties. Historically utilized extensively across numerous sectors from construction to art and furniture, wood is renewable and recyclable, aligning perfectly with contemporary circular economy principles. However, its natural opacity, primarily due to the presence of lignin and light scattering within its complex cellular structure, limits its application where transparency is required. The concept of transparent wood was initially explored by Fink in 1992, but it was rediscovered and rigorously investigated by independent research groups starting about a decade ago.

The fundamental principle behind achieving transparency in wood involves overcoming the factors that impede light transmission: light absorption by chromophores (mainly in lignin) and light scattering caused by the mismatch in refractive indices between the wood's cell wall components (cellulose and hemicellulose) and the air-filled voids (lumens and pores). The fabrication process typically follows a "top-down" approach, beginning with bulk wood and preserving its hierarchical structure. This process primarily involves two key stages: discoloration (either by removing or modifying lignin) and subsequent infiltration with a polymer that is optically compatible with the wood scaffold.

Two main strategies exist for the discoloration step:

1) Complete or Partial Delignification: This involves chemically removing most or all of the lignin from the wood structure. Various chemical treatments are employed, including acidic/alkaline treatments or redox agents. Examples from the sources include using sodium chlorite (NaClO2) with acetate buffer, sodium hypochlorite (NaClO), peracetic acid (PAA), or a combination of sodium hydroxide (NaOH) and sodium sulfite (Na2SO3), sometimes followed by hydrogen peroxide (H2O2) bleaching. This process leaves behind a porous scaffold primarily composed of cellulose and hemicellulose (holocellulose).

2) Lignin Modification: This alternative approach seeks to maintain a significant portion of the lignin content while chemically altering or removing only the chromophoric groups responsible for light absorption and color. This method often involves alkaline/H2O2 systems or UV-assisted H2O2 treatments. A key advantage mentioned is the potential to retain more than 80% of the original lignin content, which can help preserve the wood's structural integrity, as lignin acts as a binder.

Following the discoloration, the porous wood scaffold is infiltrated with a suitable polymer. The polymer serves to fill the now empty cell lumens and other voids, and critically, it must have a refractive index (RI) that closely matches that of the delignified wood cell wall (holocellulose, with an approximate RI of 1.53). Common polymers used include polymethyl methacrylate (PMMA, RI ≈ 1.49) and epoxy resin (RI ≈ 1.50). When the RI of the polymer and the cell wall are similar, light scattering at the interfaces is significantly reduced, leading to transparency. Any remaining air voids, due to incomplete infiltration or polymer shrinkage, create significant RI mismatches (air RI ≈ 1.00) and increase scattering, reducing transparency. Surface modification techniques, such as acetylation, can be employed to improve the compatibility between the wood scaffold and the polymer, reducing the formation of these detrimental interfacial voids.

Comprehensive characterization is essential at every stage of the development process to understand the material's transformation and performance. This involves analyzing its morphology, optical, mechanical, thermal, and other functional properties.

Microstructural and Morphological Analysis: Understanding and visualizing the structural changes in wood during the fabrication process is critical. Scanning Electron Microscopy (SEM) is a primary tool for this purpose. SEM allows researchers to observe:

• The original wood's cellular structure, including cell walls and lumens.
• The effects of delignification, such as the thinning of cell walls and enlargement of lumens.
• The effectiveness of polymer infiltration, verifying that the polymer has successfully filled the internal voids.
• The quality of the interface between the wood scaffold and the infiltrated polymer. Issues like interface debonding or air gaps, which negatively impact transparency and mechanical properties, can be visualized.
• Specific structural features, such as the preserved annual growth rings in aesthetic transparent wood or the porous char structure in flame-retardant variants after fire testing.

Optical Properties Characterization: The most distinctive property of TW is its transparency, which is quantitatively assessed by measuring its optical transmittance and haze.

• Transmittance measures the total percentage of incident light that passes through the material.
• Haze quantifies the amount of transmitted light that is scattered or diffused. A material with high transmittance and low haze appears clear, while high transmittance with high haze results in a translucent appearance, useful for applications requiring privacy. These properties are typically measured using a UV-Vis spectrophotometer, recording the transmission spectrum in the visible light range (approx. 400-700 nm). Measurements can also reveal the optical anisotropy of the material, which is inherited from the directional structure of the wood. The achieved transparency levels can be significant, with studies reporting transmittances ranging from around 70% to over 90% depending on the wood species, thickness, and fabrication method. Thickness is a significant factor, as transmittance generally decreases with increasing thickness.

Mechanical Properties Evaluation: Maintaining adequate mechanical strength is crucial for many potential applications, especially in structural roles. Complete delignification can weaken the wood scaffold. Mechanical tests are conducted to measure properties such as tensile strength, flexural strength/modulus, and toughness (measured as work to fracture). Studies have shown that retaining a substantial amount of lignin can result in improved mechanical properties compared to completely delignified TW. The interface quality between the polymer and the wood cell wall, often observed via SEM on fracture surfaces, also significantly impacts mechanical performance; poor adhesion or voids can act as stress concentration points, reducing strength.

Thermal and Other Functional Properties: Depending on the desired functionality, other properties are rigorously characterized:

• Thermal Conductivity: Important for building applications where thermal insulation is a factor. Measurements show TW can have lower thermal conductivity than glass and exhibits anisotropy.
• Flame Retardancy: Measured using standardized fire tests that determine properties like Heat Release Rate (HRR), Time to Ignition (TTI), and Limiting Oxygen Index (LOI). SEM can be used to examine the resulting char structure.
• Aesthetic Properties: Evaluated by visualizing the preservation of natural wood patterns like growth rings and assessing light distribution effects (light guiding, anti-glare).
• Chromic Properties: Tested by measuring changes in transmittance or color in response to temperature or light stimuli.
• Electrical Conductivity: Assessed by measuring sheet resistance and observing performance in applications like flexible electronics or sensors.

Chemical Composition Analysis: While not always detailed regarding specific instruments in the sources, methods like FTIR spectroscopy are commonly used to analyze the chemical structure of the wood before and after treatments. These analyses help confirm the removal or modification of lignin and assess changes to other components like cellulose and hemicellulose. Techniques like the Klason lignin method are used to quantify the remaining lignin content.

Linking Process, Structure, and Properties: The suite of analytical tools and techniques described above is indispensable for understanding the complex relationships between the TW fabrication process, the resulting material structure, and its final properties. Researchers use these measurements to optimize parameters such as the choice of wood species, the delignification chemistry and duration, the type and infiltration method of the polymer, and any post-treatments like interface modification. For example, SEM reveals how a process change affects the microstructure, while UV-Vis and mechanical tests quantify the resulting performance impact.

Challenges and Future Outlook: Despite significant progress, challenges remain. The intrinsic variability of natural wood makes achieving uniform properties across large samples difficult. Many current fabrication methods, while effective in research settings, involve the use of toxic chemicals and can be time-consuming and energy-intensive, presenting obstacles to large-scale industrial production. Developing more environmentally friendly ("green") processes is a key focus area. Furthermore, producing transparent wood with greater thickness (e.g., beyond 3.5-4 mm) with high transparency has been challenging, often limited by interface issues. Analytical methods for quality control and rapid characterization on a larger scale will also be necessary for industrial adoption. The overall sustainability of TW compared to conventional materials like glass or PMMA can be assessed using analytical tools like Life Cycle Assessment.

In conclusion, Transparent Wood is a promising, sustainable material leveraging the natural architecture of wood. Its transformation into a transparent yet strong composite relies heavily on precise chemical processing and thorough material characterization using a range of analytical techniques, including SEM for microstructure, UV-Vis spectrophotometry for optical clarity, and various mechanical and thermal tests for structural and functional performance. Continued research, guided by these analytical insights, is driving the development of TW towards scalable, multi-functional applications as a potential sustainable alternative to conventional transparent materials.