Methods and Applications for the Fabrication of Transparent Wood Composites

Methods and Applications for the Fabrication of Transparent Wood

Composites

Christopher Perez

Laurent Pilon's Research Group

Mechanical and Aerospace Engineering Department

MSD Scholars, Fall 2016

December 10, 2016

2 | Christopher Perez

Abstract

To mitigate the energy consumption of buildings and the environmental impact associated with

fossil fuel-dependent energy sources, transparent wood composites offer a promising solution for

their potential use in energy efficient building applications. Their effective use of sunlight for

natural indoor lighting rivals that of glass as they offer the additional advantages of high-impact

absorption and low thermal conductivity. The anisotropic structure of wood consists of vertically

aligned channels comprised of voids and colorless wood cell walls made of cellulose and

hemicellulose.[6] The constituent that binds the wood cells and lends wood its hard, rigid properties

and dark color, is known as lignin.[7] To generate these novel composites, basswood was used as

the wood template and its lignin was removed in a delignification process. This involved sample

immersion in a boiling solution of NaOH and Na2SO3, as well as immersion in boiling and room

temperature H2O2. By replacing the lignin with optically clear epoxy resin, hardener, and a

resin/hardener mix, polymeric infiltration was carried out. Two experiments were conducted, the

first of which consisting of three samples with the following dimensions: 1x1x1 cm, 1x1x0.5 cm,

3.8x2.85x0.325 cm. The second experiment was comprised of eight 1x3x0.325 cm samples. The

first experiment, using boiling H2O2, revealed that a more refined process was needed due to the

dissociation of H2O2 at high temperatures. The second experiment, using the resin/hardener mix,

indicated that the delignification process was not sufficient as the samples were subjected to epoxy

encapsulation.

Word Count: 243


1. Introduction

The heating, ventilation, and air conditioning of commercial and residential buildings accounts for

approximately 14% of the total amount of primary energy consumed in the United States.[3] This

is expected to be intensified with the general trend of increased building size among residential

buildings, requiring more energy to maintain comfortable indoor temperatures.[1] Coupled with

this energy use, direct greenhouse gas emissions (GHGs) from these buildings account for roughly

12% of the total GHG emissions in the United States, largely due to the combustion of fossil fuels

for space heating.[2] In order to mitigate the effects of unsustainable energy consumption and

climate change, the U.S. Department of Energy (DOE) requires a 20% reduction in the energy

consumption of buildings by 2020 and a 50% reduction for the long-term.[3] Since the energy used

for lighting and thermal comfort comprises about half of the total energy consumption in

residential and commercial buildings, their conservation has the potential to yield substantial

savings to meet these goals, especially during the daytime.[4]

The interface between the interior of a building and the surrounding environment, the building

envelop is where heat transfer must be minimized to reduce the need for space heating or cooling.

Windows, despite successfully using the sun to provide indoor lighting during the day, are one of

the most ineffective components of this envelope for preventing heat losses. This is due to the

relatively high thermal conductivity of glass through which one-third of the energy used to heat or

cool a building can be lost with single pane windows.[5] Even so, single pane windows remain

widely used in the U.S despite the exceptional thermal and optical performance of their double

pane counterparts.


The modification of wood templates for achieving optical transparency shows promise in ensuring

sustainable energy use. By partially replacing artificial light with natural sunlight in a building,

their potential use in constructing light-transmitting structures can enable reduced energy demand

by being more thermally insulating, and thus replacing, traditional glass. The aim of the study

presented here is to establish a comprehensive method of fabricating transparent wood composites.

Such a method will provide insight on the mechanisms that dictate the thermophysical and optical

properties of the generated building material. The study is performed in the hopes that this material

fabrication will provide a basis for the added-value applications of future transparent wood

composite materials.

2. Background

2.1 Wood Growth and the Nature of Lignin

Used as a structural material for thousands of years, wood demonstrates excellent mechanical

strength and thermal properties in contrast to glass. Its low density and competitive cost are also

key contributors to its widespread and traditional use in buildings. The most notable feature of

wood however, lies in the unique growth of trees and their directionally dependent structure, or

anisotropy.[6] Situated within tree trunks exist numerous aligned vertical channels, used by the tree

to transport water, ions, and other nutrients through the trunk to accommodate the photosynthetic

process.[7] In their mature state, the wood cells comprising these channels are dead and hollow,

essentially only cell walls and voids. The cell walls are made up of colorless cellulose and

combinations of carbohydrates collectively known as hemicellulose. The low thermal conductivity

of wood is attributed to the voids that reside within these cell walls. The major constituent that


permeates the cell walls and renders wood a hard, rigid material is known as lignin. A highly

branched molecule of complex structure with dark color and high molecular weight, lignin is also

found in the intercellular regions between the channels, binding all wood cells together.[6] It is the

removal and replacement of lignin in processes known as delignification and polymer infiltration

respectively, as well as the existing cellulosic channel structure, that is utilized for the generation

of transparent wood composites that allow the transmission of light.

2.2 The Types of Wood

Stemming from its prevalent use in building applications, much is known about the different types

of wood with regard to mechanical properties. From this, various wood classifications have been

identified but are generally divided between hardwoods and softwoods.[6] Chief among their

differences is the mesostructure present within the woods. Due to their fast growth, softwoods

typically have a more porous internal structure with a fibrous form.[7] Conversely, hardwoods are

composed of large fibers and wider cell walls that form more pronounced tubes or channels

mentioned earlier. Additionally, hardwoods are normally more dense and grow larger when

compared to softwoods, making them ideal candidates for wood template modification.

2.3 Transparent Wood as a Building Material

Rigorous research on transparent wood composites and their characterization is sparse and

generally lacking due to their relatively novel nature. Commercially, there exists no businesses for

producing, developing, and marketing these transparent building materials. The state of the art is

concerned with altering the infiltrating substance from traditional polymeric epoxy, or making

more efficient the process of delignification.


Initial attempts at cellulose-based modification involved realizing transparent paper preparation.

To this end, cellulose fibers extracted from wood have been subjected to cellulose dissolution and

regeneration, cellulose fibrous diameter decrease to nanoscale, and polymer infiltration. Iwamoto

et al.[8] created composite paper structures using a range of cellulose nanofiber volumes.

Additionally, all-cellulose transparent paper was prepared using TEMPO-oxidized nanocellulose,

achieving an optical transparency of over 90%. It was seen that although the increased

transmittance was a result of fiber diameter decrease, the oriented structure of the wood cell walls

was no longer present.

Li T. et al.[9] set out to demonstrate the application of transparent wood as an energy efficient

building material by using basswood and AeroMarine #300/21 Epoxy. Li reported a high

transmittance comparable to glass (90%) with a directional forward scattering effect, as well as a

lower thermal conductivity than glass and even a solid block of the infiltrating epoxy. Additionally,

the wood microstructure of the transparent wood exhibited high impact absorption and ductility.

The delignification process involved immersing the wood into a solution of diluted boiling NaOH

and Na2SO3 for 3 hours, followed by immersion in boiling H2O2 until the wood was bereft of its

color, about 2–3 hours. The resulting material was then immersed in liquid epoxy resin for

thorough infiltration, the solidification of which would result in the final transparent wood

composite.

Zhu et al.[10] examined anisotropic wood composites by removing the colored lignin and filling

index-matching polymers to achieve high optical transparency. Using basswood, two types of

transparent wood were fabricated, where the channels in the wood aligned either along or

perpendicular to the wood plane. It was found that for the radially cut wood, where the channels


were perpendicular to the wood plane, the delignification and infiltration processes were much

faster with a higher transmittance. The longitudinally cut wood, where the channels aligned with

the wood plane, exhibited twice the fracture strength and modulus of elasticity of radially cut

wood, as well as a higher degree of optical anisotropy. Although the delignification and infiltration

processes used essentially the same methods and materials as Li et al., the immersion of wood into

a solution of diluted boiling NaOH and Na2SO3 was conducted for a much longer period, 12 hours.

The infiltration process also used a cycling degassing method under 200 Pa of pressure to remove

the gases and solvents from the wood, followed by a depressurization to atmospheric conditions,

allowing the resin to infiltrate the wood.

Li Y. et al.[11] fabricated optically transparent wood while noting a relationship between the

cellulose volume fraction and optical transmittance, as well as the mechanical properties. Using

balsa wood, the delignification process was markedly different than others in that the wood

samples were dried at high temperatures for 24 hours before immersing in a NaClO2 and acetate

buffer solution at elevated temperatures for 6–12 hours depending on the sample thickness. After

washing with deionized water, ethanol and acetone were used in a mixture for dehydration. The

infiltration process was also unique since the samples were infiltrated using a prepolymerized

methyl methacrylate (MMA) solution under a vacuum and heated in an oven for 4 hours.

Overall, the few studies in the literature for transparent wood composites delve into preliminary

detail and are beginning to explore alternatives in fabrication. It remains clear that more research

is needed to explore the potential of these special composites if they are to be considered

dependable materials that rival and mitigate the use of highly treated, opaque lumber.


3. Methods and Materials

3.1 Wood Selection and Preparation

Two experimental investigations were carried with the selection of basswood as the starting

material for its low cost and proven history. Figure 1 shows the three basswood samples created

for the first experiment with the following dimensions: (1) 1 x 1 x 1 cm, (2) 1 x 1 x 0.5 cm, (3) 3.8

x 2.85 x 0.325 cm. The second experiment used eight basswood samples, all 1 x 3 x 0.325 cm.

After the samples were cut to their desired sizes, they were cleaned with de-ionized water and left

to dry for 48 hours days at room temperature (~20℃). The basswood was purchased from Waddell

Manufacturing®, a company based in Ohio, United States.

3.2 The Delignification Process

The two part delignification process was started with the preparation of the NaOH and Na2SO3

solution. The NaOH was diluted with de-ionized water using a target molarity of 2.5 moles per

liter, requiring 100 grams of NaOH per liter. Similarly, the Na2SO3 was also diluted with de-

ionized water using a target molarity of 0.4 moles per liter, warranting 50.4 grams of Na2SO3 per

liter. These two solutions were then mixed to form a single solution which was then dispensed into

a 250 mL beaker. The volume of the solution varied depending on the experiment, 150 mL and

200 mL for experiments 1 and 2 respectively. A hot plate was then used to heat the solution to a

boil. Due to the contact resistance between the hot plate and the glass beaker, the hot plate was set

too 500 ℃ to ensure a boil. Figure 2 illustrates the hot plate heating the solution. The samples

were then immersed in the solution and left to boil for 3 hours. As the de-ionized water was the

solvent in the solution, it would evaporate with the passing of time. More de-ionized water was

then added intermittently to return the solution to the original solution volume, depending on the


experiment. After subjecting the samples to this treatment, they were then taken out of the

NaOH/Na2SO3 solution, washed with de-ionized water, and left to dry for 24 hours at room

temperature.

In the first experiment, the second step of delignification was carried out as an H2O2 solution was

prepared by diluting 30%w/v H2O2 with de-ionized water using a desired molarity of 2.5 moles

per liter. This required 250 mL of 30%w/v H2O2 per liter of de-ionized water. The samples were

then immersed in this H2O2 solution for 2–3 hours until the yellow color of the wood samples

disappeared. Again, more de-ionized water was added as the solution began to evaporate. The

second experiment was similar but did not boil the H2O2. Both concentrated 2.5M H2O2 and

30%w/v were used to immerse the samples at room temperature for 48 hours until, again, the

yellow color of the wood vanished. It is important to note that the larger samples required more

time to rid themselves of their color with the solution treatment. After they were colorless and

white, the samples were left to dry at room temperature for 48 hours.

3.3 Polymeric Infiltration

The last portion of the fabrication transparent wood involved infiltrating the now delignified

samples with a polymeric substitute for lignin, in this case Loctite Poxy Pak Fast Cure Epoxy®.

The first experiment entailed separately immersing the samples in both constituent parts of the

epoxy, the resin and hardener. This was performed by filling petri dishes with the resin and the

hardener and immersing the samples in each. The second experiment involved mixing both

components of the epoxy into a petri dish and immersing the dry sample inside. To prevent the

epoxy from encrusting the samples, they were removed 30 seconds before the advertised setting

time of 5 minutes. Figure 3 shows the petri dishes containing the resin, hardener, and

resin/hardener mix with a sample situated in each configuration.


3.4 Measuring Thermal Properties

To measure one of the most important thermal properties of interest, thermal conductivity, a

guarded hot plate apparatus will be used. The device was designed to comply with ASTM C177-

13.[12] The guarded hot plate approach for measuring thermal conductivity requires two identical

samples to be secured between hot and cold plates. By strategically inserting thermocouples in the

sample at known distances and observing the linear function of voltage for each thermocouple, the

steady-state thermal energy dissipated in the hot plate is then ensured to be evenly distributed

between the two identical samples. Modifying Fourier’s law, the thermal conductivity may be

evaluated using the following expression:

k𝑖𝑖(T�) =q𝑖𝑖L𝑖𝑖

A�T2,𝑖𝑖 − T1,𝑖𝑖� with 𝑖𝑖 = A or B

Where qi is the heat transfer rate �Wm� through the sample 𝑖𝑖 and L𝑖𝑖 is the distance the thermocouples

measuring T1,𝑖𝑖 and T2,𝑖𝑖.[13] Figure 4 demonstrates a schematic for this guarded hot plate setup.

4 Results and Discussion

4.1 Composite Wood Fabrication

In fabricating these composite woods, the delignification process in these experiments was closely

modeled by the protocol outlined in the work by Li T. et al. mentioned previously. As stated earlier,

the NaOH/N2SO3 solution was prepared in a 150 mL volume for the first experiment. When heated

to a boil, the de-ionized water evaporated and required refilling by external means. Figure 5

reveals the chronological order of the first three hour process. Although the heavy discoloration of

the solution shows that delignification is indeed occurring, the persistent dark color of the samples


is concerning. The color may indicate an incomplete reaction that is suspected to be due to the

relatively small volume of the solution used. Figure 6 shows how the wood samples still possess

their original color even after the NaOH/Na2SO3 treatment. This was part of the motivation behind

the second experiment as it used more solution (200 mL) to carry out the delignification reaction.

Although more solution seemed to aid the reaction, the persisting color demonstrates the need for

either more solution and boiling time, or boiling under reflux.

The second step of delignification involved boiling the wood samples in a solution of H2O2.

Carrying out this task for 3 hours proved to be difficult due to the decomposition of H2O2 as it

bubbled and seemingly reacted with the wood for a short time before subsiding almost completely.

It is known that under high temperatures, H2O2 may dissociate into oxygen and water, as seen in

the following reaction:

2H2O2 → H2O + O2(g)

In order to confirm this decomposition, drops of H2O2 were added to the solution once the original

bubbling subsided and the result was violent bubbling followed by another standstill. Figure 7

shows the chronological series of events once the additional H2O2 drops were added. This

phenomenon was another motivation behind the second experiment; to somehow treat the wood

samples with H2O2 and remove its color without the dissociation of H2O2. This prompted the

investigation of using either highly concentrated or diluted H2O2 without boiling. The extended

duration (24 hours compared to 3 hours) of immersion was performed in an attempt to consolidate

the missing element of boiling. Figure 8 shows wood samples being immersed in both the 2.5M

and 30%w/v H2O2 solution. The outcome of both of these treatments yielded similar results to the

original boiling method and it was revealed that there was no observable difference between using

the highly concentrated or diluted H2O2 solution. Still, the color of the samples after H2O2


treatment were not as white as detailed by the accounts in literature reviews. Similar to the

NaOH/Na2SO3 treatment, this encourages the boiling of the H2O2 solution under reflux.

4.2 Methods for Polymeric Infiltration

The polymeric infiltration was again, emulated using the process by Li T. et al. as a framework.

Due to the ambiguous nature of the literature concerning the mixing of the hardener and resin, both

of these constituent parts were separately used to immerse the samples as mentioned earlier. Due

to the inability of these components to solidify, the mixing of both the hardener and the resin served

as yet another motivation for the second experiment. Once mixed, the epoxy was allowed to react

for 30 seconds less than its curing time of five minutes at room temperature, and the sample was

removed from the epoxy. Unfortunately, infiltration did not occur as the epoxy did not penetrate

the samples and merely surrounded them, effectively encapsulating the samples. Figure 9

illustrates a wood sample removed from epoxy encapsulation. This was not a welcome result as it

indicated that the delignification process had not taken place to the desired degree, blocking the

epoxy from accessing the hollow channels of the wood mesostructure. This realization encourages

the use of more sophisticated methods for polymer infiltration such as those mentioned in the work

by Zhu et al. involving the cyclic degassing of the sample and epoxy. The type of epoxy is also of

interest due to its fast cure time. A slower curing alternative may aid in the infiltration of polymer

and prevent the encapsulation of the wood samples.

5 Future Work

Thus far, a superficial investigation of transparent wood composites has been conducted. Although

there has been much trial and error, especially with polymer infiltration, the availability of

literature on the creation of these composites has proven invaluable. As experienced however, the


incomplete and imperfect delignification process is an issue that must be resolved. To this end,

future experiments are likely to implement a condenser for recycling the liquid in the solution in

order to maintain the reaction indefinitely. Doing so would prevent the dissociation of H2O2 into

water and oxygen. This would be performed with the addition of more solution to ensure the

reaction is carried out completely.

Once the delignification process is perfected, more complex post-processing would be of interest.

As mentioned in the beginning of this paper, the production of added-value products is a pursuit

that aligns well with the theme of energy conservation and sustainability. By infiltrating the

delignified wood with an organic or less prolific epoxy with desirable optical properties, the

transparent wood composite will truly be sustainable. In addition, by sealing micro-encapsulated

phase change materials (PCMs) inside the channels and wood cell wall voids, the latent heat

storage of the PCM can be utilized. Since the PCMs are optically clear, the resulting material has

the potential to replace glass for windows, at the same time stabilizing building temperatures with

the addition of a new thermal storage mechanism.


Figures

Figure 1: The basswood samples used for the first experiment. Dimensions: (1) 1x1x1 cm, (2) 1x1x0.5 xm, (3) 3.8x2.85x0.325 cm

Figure 2: The hotplate warming up to a target temerature of 500 ℃ as it is used to heat the solutions to a boil.


Figure 3: (Bottom left) The Loctite Poxy Pak Fast Cure Epoxy® used for both experiments. (Top) The samples for the first

experiment immersed in resin and hardener separately.(Bottom right) A sample from the second experiment immersed in the

hardener/resin mix.

Figure 4: The setup used to measure the effective thermal conductivity of composite based materials using the guarded hot

plate method.[12]


Figure 5: A chronological view of the delignification process; the wood samples are boiled in a diluted NaOH/Na2SO3 solution

Figure 6: The wood samples for the first experiment after boiling in a diluted NaOH/Na2SO3 solution. Notice the persisting dark

color.

Figure 7: A chronological view of the delignification process; the wood samples are boiled in a H2O2 solution.


Figure 8:(Left) A wood sample immersed in the 2.5M H2O2 solution. (Right) Several wood samples immersed in the 30%w/v

H2O2 solution. Both are at room temperature.

Figure 9:A wood sample removed from epoxy in the polymer infiltration phase, Notice the void left from the wood indicative of

encapsulatio and infiltration failure.


References

[1] DOE, Energy Efficiency Trends in Residential and Commercial Buildings, August 2010.

[2] Buildings and Climate Change: A Summary for Decision-Makers (2009): n. pag. United Nations Environment Programme, accessed: October 2016.

[3] Buildings, Department of Energy, http://energy.gov/eere/efficiency/buildings, accessed: October, 2016.

[4] US Department of Energy, Buildings Energy Data Book, http://buildingsdatabook.eren.doe.gov/, accessed: October, 2016.

[5] High R Value Windows, http://www.gsa.gov/portal/content/180891, accessed: October 2016.

[6] A. P. Russell; L. G. Richard, Formation and Structure of Wood, American Chemical Society, Washington, DC. USA 1984.

[7] E. Sjöström, Wood Chemistry: Fundamentals and Applications, Academic Press, San Diego, CA USA 1993.

[8] Iwamoto, S.; Nakagaito, A. N.; Yano, H.; Nogi, M. Applied Physics A: Material Science Process. 2005, 81, 1109 – 1112.

[9] Li T., Zhu M., Yang Z., Song J., Dai J., Yao Y., Luo W., Pastel G., Yang B., Hu L. (2016). Wood Composite as an Energy Efficient Building Material: Guided Sunlight Transmittance and Effective Thermal Insulation. Adv. Energy Mater., 6: 1601122. DOI: 10.1002/aenm.201601122

[10] Zhu, M., Song, J., Li, T., Gong, A., Wang, Y., Dai, J., Yao, Y., Luo, W., Henderson, D. and Hu, L. (2016), Highly Anisotropic, Highly Transparent Wood Composites. Adv. Mater., 28: 5181–5187. DOI: 10.1002/adma.201600427

[11] Yuanyuan Li, Qiliang Fu, Shun Yu, Min Yan, and Lars Berglund (2016), Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance. Biomacromolecules 17 (4), 1358-1364. DOI:10.1021/acs.biomac.6b00145

[12] Rickleffs, A., Thiele, A., Falzone, G., Sant., G., Pilon, L. (2016) Thermal Conductivity of Cementitious Composites Containing Microencapsulated Phase Change Materials. University of California, Los Angeles.

Methods and Applications for the Fabrication of Transparent Wood Composites

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