Production of Transparent Wood Using Glycerine Extracted from Transverse and Longitudinal Sections of Poplar Wood

doi:10.21203/rs.3.rs-4274544/v1

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Production of Transparent Wood Using Glycerine Extracted from Transverse and Longitudinal Sections of Poplar Wood

https://doi.org/10.21203/rs.3.rs-4274544/v1

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Innovative and ecologically sustainable transparent wood has garnered notable attention in recent times. Normally, transparency in wood is achieved through petroleum-based polymers, but ongoing studies aim to substitute them with bio-based options to avoid potential harm. In this study, poplar wood was prepared in two distinct sizes for transparency. The wood was longitudinally cut into 10×10×1 mm dimensions and transversely cut into 20×20×1 mm dimensions. Transparency was achieved via glycerol infiltration in the cut wood specimens, which were subsequently coated with transparent epoxy resin for protection against external factors.

The potential applications of transparent wood are diverse. It can be employed in narrow and dark interior spaces, as well as in wall and floor design. Additionally, using transparent wood in attics can create an illusion of spaciousness, thanks to its optical permeability feature. Furthermore, the low thermal conductivity and environmental friendliness of wood make it advantageous for the construction industry.

This study not only highlights the feasibility of transparent wood but also underlines its potential to revolutionise interior design and construction practices. Further research and development in this area are essential to unlocking the full potential of this innovative and sustainable material.

Transparent Wood

Glycerine

Optical Transmittance

To reduce this world's energy resources and to decrease the associated environmental impact, the production of green buildings has become an urgent issue that requires emergency responses (Fumo 2014; Golafshani, Behnood, and Arashpour 2020; Schultz, Nicholas, and Preston 2007; Wang et al. 2018). Sustainable materials are becoming a priority on the list of world interests as part of the solution to replace the non-renewable materials to mitigate the environmental impact. Wood, which has been utilized for tools, fuels, and buildings for millennia, is one of the most abundant biomass materials available. Nevertheless, the intrinsic properties of wood, such as its low corrosion resistance, poor mechanical performance, flammability, non-transparency, and non-thermal conductivity, have restricted its applications in the building industry (Li et al. 2020; Schultz, Nicholas, and Preston 2007; Wang et al. 2018).

Wood is the sole building material with a renewable source. It demands less energy for processing (one-fifth of the amount required for aluminium), is recyclable, and boasts top-notch thermal insulation properties, making it the ideal solution to our era's ecological and energy issues (Hansen et al. 2010; Vermeer and Rahmstorf 2009; Yaddanapudi et al. 2017). One potential method for decreasing energy consumption in the building sector is through the application of semi-transparent building design. These buildings have the potential to reduce the need for artificial lighting by utilizing natural light. Furthermore, they offer privacy without being entirely transparent (Yaddanapudi et al. 2017). The development of new manufacturing technologies has transformed wood into a multifunctional material, expanding its applications to emerging areas such as environmental protection, energy saving, biomedicine, and optical products (Cai et al. 2021; Jiang et al. 2022; Okahisa et al. 2009; Pérez-Lombard, Ortiz, and Pout 2008; Wang et al. 2018). In order to address the issue of energy shortage, it is highly desirable to reduce the energy consumption of residential and commercial buildings, as they account for over 50% of total energy consumption (Pérez-Lombard, Ortiz, and Pout 2008).

Natural wood is not transparent. Wood is a composite material comprising cellulose, hemicellulose, and lignin. Lignin confers the brown colour of wood and binds cellulose and hemicellulose together to form the woody structure. While lignin in the wood structure absorbs light, cellulose and hemicellulose are optically transparent. When comparing wood and reinforced concrete, the tensile strength is provided by cellulosic and hemicellulosic fibres, whereas the compressive strength is contributed to by lignin (Yaddanapudi et al. 2017).

To achieve transparent wood, it is essential to eliminate lignin, which has the capability to absorb light, from the wood. This is accomplished by performing bleaching after the removal of lignin, resulting in a nano-cellulosic porous template. Polymer is then used to fill these gaps, resulting in transparent wood with optical transmittance (Okahisa et al. 2009; Zhu et al. 2016; Liu et al. 2021). There are ongoing studies on innovative transparent wood with diverse chemicals in various countries.

Flexible, transparent wood has been successfully produced in India using poplar wood that was cut perpendicular to the trunk axis. A 40×40×1 mm wood sample underwent delignification with an aqueous solution comprising of 3.0% sodium hydroxide and 0.1% Na₂EDTA. This sample was subjected to heat at 70°C for a period of 10 minutes. The bleaching process involved an addition of 0.1% magnesium sulfate and 4.0% H₂O₂, followed by subsequent heating. A dispersion of PVA (at a concentration of 10% w/v) was created in deionised water. Three varying levels of PG were subsequently introduced as plasticisers to the PVA dispersion. Upon completion, the samples were dried by heating in a microwave and characterized following this process (Subba Rao et. Al. 2019).

Researchers in China have successfully created hydrophobic and transparent balsa wood through the process of lignin delignification, using a 2.5% sodium chloride solution with a pH of 3.5, adjusted with acetic acid at room temperature. The balsa wood was left to soak for a day at 70°C to extract lignin, then underwent a three-day washing process, in which the solution was replaced with ethanol each day to remove any remaining chemicals. The polymer utilised as the impregnation solution was obtained through the mixing of epoxy resin and curing agent in a mass ratio of 2:1, for a duration of 10 minutes. The treated wood underwent a vacuum treatment for 30 minutes, allowing the impregnated solution to fully penetrate. Following this, the sample underwent compression between two silica gels and had a weight of 1 kg placed on top of it. Transparent wood was achieved through drying it at 60°C for six hours.

A water-repellent solution was prepared by combining 3% ultrapure water and 95% ethanol, with 2% perfluorodecyltriethoxysilane. The pH of the resulting solution was adjusted to 5 using acetic acid. The solution was magnetically stirred for 24 hours. Thereafter, the hydrophobic solution was applied to the surface of transparent wood for 10 seconds. The sample was then vertically oriented to enable any excess solution to drain. Ultimately, transparent and hydrophobic wood was attained by subjecting the sample to a drying process in an oven at 60°C for a period of 2 hours (Ding et al. 2022).

In a recent Indian study, researchers conducted a life cycle assessment of transparent wood production using emerging technologies and strategic scaling framework. The researchers also explored the life cycle analysis of transparent wood and its potential applications in construction, energy storage, flexible electronics, and packaging. Additionally, they examined its environmental effects during both manufacturing and end-of-life stages.

The approach with the minimal environmental impact entails the use of the epoxy infiltration technique, employing the delignification process with sodium hydroxide, sodium sulfide, and hydrogen peroxide (NaOH + Na₂SO₃ + H₂O₂). This methodology presents a roughly 24% decrease in global warming potential than the polymethyl methacrylate (PMMA) infiltration employing sodium chloride delignification. The researchers observed a decrease of about 15% in terrestrial acidification. Modeled industrial-scale production demonstrates reduced electricity consumption by 98.8% and lowered environmental impact by 28% in global warming potential and almost 97% in human toxicity compared to laboratory-scale methods. The life cycle analysis of transparent wood reveals that it holds commercial potential as a sustainable alternative to conventional petroleum-based materials, in comparison with polyethylene, according to researchers (Rai e al. 2022).

Researchers in China have achieved the production of transparent wood that displays outstanding electrical conductivity and temperature sensing properties. The maximum tensile stress reached 73.9%. To achieve this, they prepared a 3.5% wt sodium chloride solution which was adjusted with the careful addition of acetic acid until the solution reached a pH of 4.6. After three rinses in hot distilled water, the majority of chemicals were eliminated.

The polymer utilised in creating transparent wood is composed largely of choline chloride and acrylic acid. Once choline chloride was thoroughly dried, it was combined with acrylic acid in a 1:2 ratio. The ensuing mixture was stirred continuously and heated to 90°C for around four hours until it became a uniform, colourless solution. PEG(200)DA and photoinitiator 2959 were introduced to commence the polymerisation reaction of the solution. The polymer was created by combining crosslinkers (PEG(200)DA and photoinitiator 2959) in a ratio of 100:3.5:1 with the assistance of ultrasound for 5 minutes. The delignified wood samples were then soaked in the polymer and put in a vacuum oven at 80°C for 3 hours to guarantee full infiltration of the wood template by the polymer. Ultimately, the samples were taken out and subjected to ultraviolet light for 5 minutes to solidify the polymer and accomplish fully transparent wood.

SEM images were used to observe the microstructure properties of wood, showing successful polymerisation and good compatibility with the wood cell matrix. The study indicated that the transparent wood produced has high light transmittance and excellent flexibility. Due to choline chloride's ability to facilitate the circulation of positive and negative ions, the transparent wood exhibits precise electrical conductivity and temperature-sensing properties. This suggests promising potential for utilisation as a temperature sensor (Yang et al. 2021).

In a separate study carried out in China, scientists created shape memory transparent wood that could be programmed by introducing transparent, refractive index-compatible, and intelligent epoxy-based vitrimers into delignified wood. The team made a significant breakthrough by producing programmable, shape memory clear wood through the in-situ infiltration of transparent and refractive index-matched epoxy vitrimers onto a drilled wood template. A permeable transparent wood with a 60% permeability was obtained, utilising a simple, two-step process that involved delignification and vitrimer infiltration. Balsa wood with a 2 mm thickness was used to produce the wood. The delignification process relied on sodium chlorite (NaClO₂, 80%) and glacial acetic acid (CH₃COOH), while Tri-SH (trimethylolpropane tris(3-mercaptopropionate), 85%) and Sn(Oct)₂ (tinned octoate, 95%) were used in the infiltration process. The researchers utilized the distinct characteristics of epoxy vitrimers and dynamic covalent transesterification networks.

The transparent wood obtained exhibits cooling-induced shape fixation and heating-induced shape recovery behavior. It is responsive to programmable stimuli, displaying features such as shape memory, shape programmability, shape recovery, shape deletion, and reprogramming. No visible modifications were observed in the multi-shape fixation after recovery operations, and the achieved shape-recovery rate is 85% (Wang et al. 2021).

Wheat straw fibres were utilised in producing value-added, transparent composites that are appropriate for use in buildings designed for light transmission. After removing the lignin, transparent composites employing wheat straw were successfully manufactured by impregnating pre-polymerised methyl methacrylate. The removal of lignin from wheat straw was accomplished utilising sodium hydroxide (NaOH), sodium sulphide (Na₂SO₃), and hydrogen peroxide (H₂O₂, 30% by weight). The impregnation procedure employed chemically pure methyl methacrylate. Microscopic morphology analysis confirmed reliable bonding efficacy between wheat straw fibres and polymethyl methacrylate (PMMA). Consequently, the bio-based composites demonstrated elevated optical clarity and mechanical strength. Transparent composites, fashioned from 3 mm thick wheat straw, evinced 74.63% light transmittance, 54.87% haze, 58.19 MPa tensile strength, and 4.26 kJ/m² impact strength. Thermal testing of the transparent composites using wheat straw indicated a thermal conductivity of 0.07 Wm − 1k − 1. Additionally, the transparent composites exhibited outstanding thermal insulation performance, thermal dimensional stability, and UV resistance. These properties have promising potential for transparent building applications (Tong et al. 2021).

In a study carried out in China, researchers examined the influence of hemicellulose on the microstructure and macroscopic properties of transparent wood, a recently developed structural material. They used a peracetic acid process to prepare delignified linden wood with a high hemicellulose content (approximately 23% by weight) and a well-preserved cellulose supramolecular structure. Transparent wood is typically prepared using two necessary steps. (1) To decolorise the wood, the elimination of light-absorbing components, primarily lignin, is necessary. Subsequently, the polymer needs to be leached with a refractive index matching that of petroleum-based and wood substrates.

However, the popular acid chloride-based method employed in the first step results in the degradation of most lignin, as well as a portion of hemicellulose, crucial structural components in wood. This degradation weakens the interconnected cell networks, thereby causing a reduction in the stiffness of cell walls. To overcome this challenge, we systematically examined the microstructure and properties of transparent wood abundant in hemicellulose. This was accomplished through a combined process of delignification with peracetic acid (PAA) and treatment with polymethyl methacrylate (PMMA), a low-cost petroleum-based polymer.

Delignified lime (Tilia) and pine wood (Pinus), each measuring 20 mm x 20 mm x 0.37 mm, and enriched in selectively extracted hemicellulose with a 9% moisture content by weight, were acquired by using peracetic acid (PAA) delignification process on pure wood. Although the majority of lignin was eliminated by this method, most hemicellulose was conserved. For comparison, reference samples were created using a NaClO-based method for delignified wood, while H₂O₂ treatment was used to produce decolorized wood retaining lignin, which may result in significant degradation of lignin chromophoric structures.

By emphasising the vital function of hemicellulose, a fresh approach is presented to bolster the optical and mechanical properties of transparent wood. Through the use of a PAA-based technique, delignified wood with significant hemicellulose content and a well-maintained cellulose supramolecular structure was generated. The wooden cell wall which is enriched with hemicellulose displayed a positive mechanical strength using the hemicellulose that was retained and worked as a binding agent for the cellulose fibrils (Jiang et al. 2022).

A reversible thermochromic transparent bamboo with dynamically temperature-matched functions was developed by incorporating thermochromic microcapsule powder in a separate study. The natural bamboo was converted to thin veneers using the rotary cutting process. This method allows all bamboo plants to be utilised with a certain level of flexibility and controllability in size, thereby eliminating the need for bamboo spreading pretreatment. The bamboo veneers were cut into square samples measuring 50 x 50 x 1 mm (length, width and thickness).

A procedure described in the literature (Yano, 2001) was followed, whereby a 2% weight solution of sodium chlorite (NaClO₂) was used at 50°C to eliminate lignin and whiten the bamboo naturally. Maximum whiteness was achieved by the bamboo within 96 hours of delignification time. A reversible thermochromic transparent bamboo was prepared for use in temperature-compatible intelligent windows by impregnating delignified bamboo with epoxy resin. The resin contained colorless thermochromic microcapsule powders which turn purple at low temperatures. The thermochromic clear bamboo has potential for intelligent window applications, and its notable haze and light absorption properties also make it suitable as a solar energy storage material (Ji et al. 2023).

2.1. Materials

Poplar wood samples, cut in the longitudinal direction with dimensions of 10×10×1 mm, and cut in the transverse direction with dimensions of 20×20×2 mm, were examined. For lignin delignification, NaClO₂ (sodium chloride 31%) and pure water (supplied by TEKKIM) were used. In the bleaching process, H₂O₂ (hydrogen peroxide, TEKKIM 35%) and pure water (supplied by TEKKIM) were employed. After bleaching, taxol (TEKKIM) was used as the next-stage polymer to purify the sample from chemicals. Vegetable glycerin was employed in the infiltration process, and transparent epoxy resin chemicals were applied to coat the final stage of transparent wood.

2.2. Method

For lignin delignification of the poplar tree sample cut in the 10×10×1mm longitudinal direction, a solution was prepared using 36 g of H₂O and 144 g of NaClO₂ (sodium chloride), and sample 1 was boiled. The sample was then boiled in a 6% hydrogen peroxide solution obtained with distilled water at boiling temperature for bleaching and left for one day. The samples taken from the solution the next day were immersed in taxol to remove the remaining chemicals and left for one day. For the subsequent stage of polymer infiltration, three cycles of vacuum operation were conducted using glycerin, with each cycle lasting 15 minutes. To enhance the strength of the sample obtained in the last step, a coating was applied using transparent epoxy resin obtained from the mixture of hardener (1) and epoxy (2) at a ratio of 1:2, and it was left to harden.

The other transparent sample is the wood obtained by cutting in the transverse direction from 20×20×1 mm poplar wood. In the lignin delignification of the sample, a 3.5% by weight NaClO₂ (sodium chloride) solution was used. The sample, boiled in the solution for 40 minutes, was rinsed with distilled water twice to purify the chemicals left in the wood from the boiled solution. 6% hydrogen peroxide solution was prepared using distilled water as the third solution, and the wood sample was boiled in the bleaching solution and left for one day. The following day, the wood sample was immersed in taxol and kept for 1–2 hours, after which the polymer infiltration stage commenced. For the next step, polymer infiltration, vacuuming was performed with glycerin for three cycles of 15 minutes each. To increase its strength, the wood sample was coated with transparent epoxy resin. A 1:2 mixture of hardener (1) and epoxy (2) was used, and transparent epoxy resin was applied to both surfaces and left to harden for two days (Fig. 1).

Transparency was achieved in the poplar tree sample obtained by cutting in the longitudinal direction, measuring 10×10×1 mm (Fig. 2).

Transparent wood with the desired transparency and optical transmittance was obtained due to the operations performed on the cross-cut poplar tree sample measuring 20×20×2 mm (Fig. 3, Fig. 4).

3.1. Scanning Electron Microscope (SEM) Images

SEM was utilized to observe the morphology and structure of the sample. In Fig. 5, high-magnification SEM images of the transparent wood obtained from the cross-section wood sample were captured before the application of transparent epoxy following glycerin infiltration. The wood's microstructure can be clearly seen in these SEM images.

3.2. Energy Dispersive X-Ray Microanalysis (EDX)

The structure of the transparent wood's elements resulting from the EDX analysis after glycerine infiltration is shown in Fig. 6. EDX analyses show that the sample contains 44.58% Carbon, 51.95% Oxygen, and 3.47% sodium by weight. These elements were observed due to the glycerine impregnated after the lignin in the wood was removed.

In this study, attempts were made to increase the transparency of poplar wood samples by cutting them both transversely and longitudinally. The results showed that the transversely cut specimens were more transparent than the longitudinally cut ones, due to the vertical cutting of the wood grain. As shown in Fig. 3, the transversely cut specimens display the inscriptions on the back more clearly. Similarly, the longitudinally cut specimens (Fig. 2) also reveal the writings on the back. Nevertheless, it could be argued that the transverse sample appears to be clearer than the current one. Furthermore, it is worth noting that the glycerin impregnated after the lignin part of the samples is a natural material, which could be interpreted as an indication of the ecologically sound nature of the transparent wood produced.

The transparent wood created has excellent optical permeability, making it an ideal material for roofing. It also allows for the creation of bright and spacious attics in narrow and dimly lit indoor spaces. Furthermore, the use of glycerin, an organic substance derived from wood, in the production process enhances the material's safety and suitability for a range of household applications, including food and cosmetic products. This study highlights the potential of transparent wood as a sustainable and innovative solution in various fields. The life cycle analysis of transparent wood reveals that it holds commercial potential as a sustainable alternative to conventional petroleum-based materials, in comparison with polyethylene, according to researchers (Rai e al. 2022). Additionally, the transparent composites exhibited outstanding thermal insulation performance, thermal dimensional stability, and UV resistance. These properties have promising potential for transparent building applications (Tong et al. 2021).

In the next stages of the project, we plan to manufacture large transparent planks that can be used in dimly lit and confined areas of buildings. Additionally, we aim to develop transparent wood moulds that can be used on construction sites to guarantee the accurate placement of concrete within the moulds.

Author Contribution

All authors reviewed the manuscript.

ACKNOWLEDGEMENT

This study was supported by Tokat Gaziosmanpaşa University Scientific Research Projects as the Startup Program Project under project number 2021/79.

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No competing interests reported.

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Production of Transparent Wood Using Glycerine Extracted from Transverse and Longitudinal Sections of Poplar Wood

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Abstract

Figures

1. Introduction

2. Materials and Method

2.1. Materials

2.2. Method

3. Results

3.1. Scanning Electron Microscope (SEM) Images

3.2. Energy Dispersive X-Ray Microanalysis (EDX)

4. Conclusion

Declarations

Author Contribution

ACKNOWLEDGEMENT

References

Additional Declarations

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