Cellulose Aerogel Application in the Built Environment

A thesis presented in partial fulfilment

of the requirements for the degree of

Bachelors of Science in Sustainable Built Environments

at the

University of Arizona

Tucson, Arizona

Dylan Michael Arceneaux

2017

Abstract

A large portion of current architectural design practices utilize insulative materials

that are outdated, unsustainable, and harmful to the environment. There is little

consideration placed in the lifespan of the insulative materials and often lead towards

negative ramifications the environment must face. Continuing in the track of sustainable

development, an emerging material known as cellulose aerogel builds off precedent

aerogel with a green twist. The issue with implementing a new material, especially one

that lacks the research and development of presently used materials, is gathering

enough interest to build research funding. Developing a new material that has the

potential to mitigate the massive energy consumption could aid architects and designers

in designing more sustainable buildings. A cellulose based aerogel system is fabricated

with cellulose, a biomass found in nearly all living organisms, is the answer we may

need to make sustainable building practices a reality. To determine the validity of a

cellulose aerogel system, a rigorous material study and precedence scientific studies

will be analyzed to understand the intrinsic properties. The culmination of this

information is imperative to drive continued development and implementation under the

optimal conditions. Cellulose aerogel will face a multitude of comparisons with each

major used insulative materials such as concrete, wood, and fiberglass. Successfully

completing these studies will help material researchers and designers to prepare for a

greater sustainable future.

Table of Contents

Introduction 4

Defining the Basics 7

Properties of Cellulose Aerogels 10

Architectural Applications 12

Research Methods 14

Data and Analysis

Drying and Curing 16

Thermal Conductivity 21

Hydrophilic and Hydrophobic 23

Acoustic Properties 25

Silica-Cellulose Composite 26

Discussion 29

Limitations 31

Conclusion 33

Annotated Bibliography 34

Introduction

Establishing a physical space that is pleasing for all our senses has remained a

challenge since the birth of architecture. From the very first shelter, a cave that

protected early humans from rain and snowfall, we have sought to seek a comfortable

and safe environment. Only in the recent decades have we developed systems to fully

address these microclimate needs such as lighting, heating, cooling, and acoustics.

When entering a space, an occupant should not have to worry about human thermal

comfort but far too often find themselves in situations that lack proper mechanisms for

achieving such an environment. Common issues include excessive heat, lack of heat,

intrusive noise, and impure air quality. Materials and mechanisms commonly used

consume an abundance of natural resources and can be taxing on the environment as

well as costly for the end user. To alleviate the issue of cost, typical construction

practices increase the amount of cheap, environmentally harmful insulation. Instead of

continuing down the path of applying insulations made of toxic chemicals and

unrenewable resources en mass, research and development of more sustainable

materials could aid in reversing such problematic construction practices.

An emerging insulation material that has proven successful in a multitude of

applications such as star dust collection, space travel, and medical pill casings, is a

cellulose-based aerogel. Aerogels are an ultralight, porous material that were formed in

the early nineteenth century and later implemented as a highly thermal insulative

material. A wide variety of materials can be used to form an aerogel, even agar, the

cellulose derivative from all plant matter. Cellulose aerogels have undergone rigorous

testing since the early twentieth century, but have not been fully implemented in

buildings due to current high costs of production and lack of comprehensive research

for this application. Developing a framework for understanding the intrinsic properties of

the cellulose aerogel is crucial for proving its viability within the built environment

(Kistler, 1931). As the study of cellulose based products gains momentum in scientific

research, the practically of utilizing cellulose aerogels in architecture expands. Through

precedent scientific studies, cellulose aerogel proves to withstand intensive heat far

beyond what the human body encounters, acoustic properties that rival sound

dampening systems, and thermal capacities that exceed nearly all other insulation

materials presently available (Wu et al., 2013). With these proven capabilities of

cellulose aerogels, we can further refine the application possibilities. A specific type of

cellulose is agar, a gelatinous algae-based material that can grow in any environment,

which possesses the capabilities of becoming a primary source for cellulose production

(Wu et al., 2013). Recognizing this unique trait of algae has led scientists to use agar as

the optimal source of cellulose to continue to perpetuate testing of cellulose based

aerogel products.

Implementing a cellulose based aerogel insulation material into buildings has the

potential for achieving optimal human thermal comfort while reducing energy

consumption through clean production techniques. Buildings currently account for

nearly two-fifths of energy consumption in the world and much of that can be attributed

to mechanical systems that regulate a building’s microclimate (Pérez-Lombard, Ortiz,

Pout, 2008). If the barrier protecting the interior microclimate or building envelope’s

resistant to heat loss, heat gain, intrusive noise, and contained clean air, then the

energy consumption of buildings would be drastically reduced. Reaching a point of

major energy consumption reduction could save vital natural resources, begin to

reverse the negative impact on the environment, and help generate a cleaner

atmosphere.

The main goal of this cellulose aerogel research is to determine the viability of

generating a more optimal material to regulate microclimates compared to current

material practices. Another major topic point to be tested is physical space reduction in

wall thickness, which will result in larger habitable spaces and less space given to

mechanical systems. Constructing smaller boundaries between indoor, outdoor, and

adjacent spaces means a reduction in other materials required to support achieving a

neutral microclimate. Other potential testing includes cellulose aerogels fire resistance

capacity. Safety is a major design consideration when developing a building and has

become intrinsically tied in material selection and building practice. Prior scientific

research suggests a salt-based cellulose aerogel system can provide fire resistance

ratings that rivals currently available materials (Wu et al., 2013). If cellulose based

aerogel can withstand haphazard fires, then it will strengthen the case of utilizing the

material as an optimal insulation choice.

To prove the validity of the cellulose aerogel system, a continued scientific study

of the material properties is imperative. After testing and observing the intrinsic

properties of cellulose aerogel samples is complete, comparisons with current insulation

can take place. If the aerogel proves to meet or exceed the minimum safety values of

rigorous safety standards, then large scale testing can be implemented in trial studies.

To obtain real-world results, an apparatus will be constructed for small-scale

microclimate studies. These tests may include, but are not limited to, acoustic isolation,

heat transfer within a space, ambient heat temperatures, comfort level surveys, and

personal opinions regarding air quality. Depending upon the results, cellulose aerogel

insulation systems could become an essential step forward in environmental

sustainability and optimal human thermal comfort design.

Defining the Basics of Cellulose Aerogel

Cellulose is defined by Klemm (2005) as the most common organic polymer on

the earth. Cellulose is a major component in all plant matter that holds together the

integrity of cell walls. Being the, “most important skeletal component in plants,” cellulose

is a resistant fibrous solid left over from all vegetation’s lifecycle. (Klemm, 2005, p.

3358). Cellulose forms products such as cotton, the purest form of natural cellulose, and

ash-less filter paper, the nearly purest form of man-made cellulose based products

(Senese, 2010). Cellulose has been used in the production of wood, clothing, and

building materials for thousands of years prior to its formal discovery in 1839 (Klemm,

2005). Since 1839, scientists have used cellulose as a basis for natural structural order

by dissolving plant matter in various acids and ammonias, leaving behind natural

cellulose (Klemm, 2005). Subsequent extraction with liquids such as water, alcohol, and

ether, have led to the understanding that cellulose is the, “sugar of the plant wall”

(Klemm, 2005, p. 3359). At a cellular level, cellulose natural polymer that forms a

chemical chain of sugar molecules, providing plants with their natural strength (Senese,

2010). With a large portion of the natural environment containing cellulose, the plant

material makes up for the largest portion of annual biomass production and is a near

inexhaustible source of raw material for our increasing environmentally friendly world

(Klemm, 2005).

Aerogel is a broad term used to classify an extraordinary group of materials

developed since the mid-twentieth century and used across a wide range of industries

(Thomas, 2012). Discovered in 1931 by Kistler, a material scientist, the new type of gel

was accidentally formed by replacing the liquid phase with air during the curing process

(Lee and Gould, 2007). The term aerogel encompasses materials with a very specific

geometric structure that are extremely light and porous (Aerogel 2016). In some cases,

aerogels have weighed in at less than four times the density of sea-level air (Aerogel

2016). Aerogel’s specific structure is, “extremely porous, solid foam, with high

connectivity between branched structures of a few nanometers across” (Thomas, 2012).

Although aerogels are classified in the foam family, the material can take on many

different forms. The most common material composition of aerogels is silica; however,

carbon, iron oxide, gold, copper, and organic polymers such as cellulose can form an

aerogel (Thomas, 2012). Aerogels consist of very minimal solid material with, “up to

99.8% of the structure consisting of nothing but air” (Thomas, 2012). The most notable

property of aerogels is lightweight with extremely low densities. The variation of density

from aerogel to aerogel resides in the solid material used in the polymer formation.

Aerogels are created by supercritical drying a gel in an extremely high temperature

environment. In the initial stages of formation, a gel is heated under intense pressure,

causing the liquid to enter a supercritical state, a state in between liquid and gas, and

lose its surface tension (Aerogel, 2016). Once this physical state is reached, additional

heat is applied to remove the liquid, “without disrupting the porous network formed by

the gel’s solid state” (Aerogel, 2016). This entire process yields a highly porous,

lightweight material that has been nicknamed “solid smoke” or “frozen smoke,” from its

nearly transparent appearance (Aerogel, 2016).

As aerogels have developed with scientific testing, new organic aerogels have

begun surfacing that use cellulose derivatives. As consumer demand for biodegradable

products has risen, so too has the research efforts into developing an organic aerogel or

sol-gel (Seantier, Bendahou, Bendahou, Grohens, and Kaddami, 2016). Furthermore,

the rising scarcity of oil has aided in developing new products of natural origins. When

the original aerogels were being created, little attention was given to organic options

such as cellulose because the pore size of the final product was too large and did not

meet the programmatic criteria. Recently, “macro-porous materials have been prepared

by drying physical gels made from cellulose,” however, they are not considered

traditional aerogels due to their pore size (Seantier et al., 2016). The first attempt in

developing a modern cellulose aerogel was published in 2001; however, it was met with

similar issues or porosity size, where the pore openings were too large and left the

material in a weakened state. Through testing, a new method of supercritical freeze-

drying has shown to yield, “mechanically strong nanomaterials,” in cellulose based

aerogel (Seantier et al., 2016). Combining the rapid renewability, high availability, and

overall sustainability, cellulose based aerogels could become a largely used materials

that pays respect towards the environment.

Properties of Cellulose Aerogels

Cellulose aerogels possess a multitude of unique properties which allow them to be

applied to a wide range of possible applications. Organic aerogels, most comprised of

cellulose precursors, are macro-porous, have extremely low densities, and high thermal

conductivity, making them mechanically functional (Nguyen, Feng, Wong, Tan, and

Duong, 2014). High absorption properties of 18 – 20 times the weight of the aerogel are

experiment proven cellulose aerogels qualities, making them ‘super sponges’ (Nguyen

et al., 2014). During the supercritical freeze-drying process of organic aerogels, the final

product can be optimized to either absorb polar or nonpolar liquids such as water and

oil. Further coatings of the aerogels after the drying process can, “improve

hydrophobicity without affecting its absorbency,” allowing the aerogel to absorb non-

polar liquids such as oil in a polar liquid such as water (Nguyen et al., 2014). Allowing

this unique mechanical process to occur can effectively produce a cellulose aerogel

sponge, absorbing oil spillages in large bodies of water.

Aside from extracting cellulose directly from plant matter to create an organic

aerogel, there are other methods of recycling products that contain large amounts of

cellulose. One example of an alternative material for obtaining cellulose is waste

newsprint. Newsprint paper is produced from wood pulp that is rich in cellulose which

can be extracted with organic solvents (Jin, Han, Li, Sun, 2015). An environmentally

friendly freeze-drying method is used to cure the gel-like cellulose and subsequently

produce an organic cellulose aerogel (Jin et al., 2015). Through scanning and x-ray

analysis, the cellulose based aerogels cured through this process prove that the aerogel

possesses high absorption for waste oils and organic solvents such a chloroform (Jin et

al., 2015). A cellulose aerogel of this nature has the potential to become implemented

into a sewer system and aid in purification. The polarity of the aerogel, established

through the freeze-drying process, repels water, allowing for potential building insulation

application (Jin et al., 2015). It is important for a buildings insulation system to repel

water because of the temperature fluctuation. The temperature differences can become

the breeding ground for molds and other harmful contaminants, effectively providing the

opposite desired effect of comfort for the individuals inside of the space. By reversing

the polarity of the aerogel through the supercritical freeze-drying process, the cellulose

aerogel can swell when it encounters water and become rubbery (Jin et al., 2015).

Allowing cellulose aerogel to become a super-absorber of water opens the potential for

storm water mitigation during natural disasters such as tsunamis and hurricanes.

Aerogels produced through the super critical drying method, “exhibit extremely

low density, high surface area, and attractive optical, dielectric, thermal and acoustic

properties," (Lee and Gould 2007). These excellent properties have aided aerogels in

becoming the target for high thermal and acoustic required applications. Through the

curing process for aerogels, solvents can be used to optimize the physical properties of

the aerogel and of the possible solvents is an abundant greenhouse gas, carbon dioxide

(Lee and Gould, 2007). Carbon dioxide is used because it is inexpensive and has a

relatively low critical temperature, optimal for supercritical drying (Lee and Gould, 2007).

With carbon dioxide being a greenhouse gas, utilizing it in the curing process of aerogel

further increases the sustainability of the product.

Densities of various organic aerogels have been tested in Lee and Gould’s

studies against the timeframe that the aerogels underwent supercritical drying. The

conclusion that was found is that after 160 minutes of the drying process, the average

set of aerogels reached a density of 0.025 grams per centimeter squared (Lee and

Gould, 2007). This is an extremely porous sample that would allow heavy permeation of

liquids and contaminants through. Conversely, a sample was created at ten times the

solid structure when dried at only 9 minutes (Lee and Gould, 2007). The shortened

drying time has minimal effect on the functionality of the aerogel because nearly 91% of

the liquid was removed, producing a very low thermal conductivity rating of the sample

(Lee and Gould, 2007). Unfortunately, there is a certain minimal timeframe that causes

the aerogels to deform, producing weak or inadequate results. For the specific organic

aerogel in Lee and Gould’s study, the supercritical time was found to be at exactly two

minutes or less (Lee and Gould, 2007). Therefore, the optimal timing for supercritical

drying is between two and nine minutes. Over or under-drying can lead to rapid

degradation of the overall structure of the aerogel and become too weak for

implementation.

Architectural Applications of Cellulose Aerogels

With the multitude of unique properties possessed by cellulose aerogels, the

realm of application is extensive. An important application of the new aerogel is rooted

in the design and construction of architectural projects. The potential for a cellulose

aerogel to replace existing insulation methods is a highly sustainable option to achieve

human thermal comfort while minimizing energy consumption of buildings. The, “trend

to produce carbon-based materials from biomass materials,” are a relatively cheap

option compared to man-made materials, easy to obtain due to the high abundance of

organic materials, and nontoxic to humans (Wu, et al., 2013). One of the pivotal aspects

of sustainable architecture is that it utilizes environmentally friendly materials without

sacrificing the comfort of the user. A cellulose based aerogel application provides

several applications to achieve the sustainable architecture paradigm such as

insulation, air filtration, and water purification. Additionally, Chen and his team of

material researchers at the Ministry of Science and Technology of China has produced

a carbon-based organic aerogel that mirrors the absorptive properties of cellulose-

based aerogels and with an absorption factor of 310 times the weight of the aerogel

(Wu et al., 2013). In conjunction with the absorption testing, the team discovered that

the electrical conductivity of stretched organic aerogels becomes highly sensitive

towards compressive strain (Wu et al., 2013). This discovery opens the potential for a

pressure-sensitive sensing material and can develop into a smart façade that reacts to

changing climate conditions.

By fabricating a facile, environmentally friendly, and economic carbon Nano-fiber

based aerogel, the option to develop an insulation for architectural application can

become a reality. Carbon Nano-fiber aerogels have proven to possess excellent fire

resistance properties, meaning that they could potentially restrain the spread of

disasters involving fire (Wu et al., 2013). Along with fire resistance, carbon Nano-fiber

aerogels exhibit extraordinary compressibility, decreasing the size of insulative material

needed to achieve human thermal comfort. Tested absorption rates of CNF aerogels,

“can be as high as 106-312 times its own weight for organic pollutants and oils,” making

them an excellent absorbent for harmful organic materials (Wu et al., 2013). Organic

CNF aerogels possess ultra-lightweight properties and fire resistant abilities that grant

them connection to architectural applications.

Research Methods

For any research question, time, budget, and ethics are critical considerations

within the design paradigm. While experimental design must take part in generalizations

and compromises, researchers must attempt to minimalize these while remaining

realistic. Fore ‘pure’ sciences such as material research, experiments are easily defined

and are strictly quantitative. However, it is nearly impossible to consider all human

factors and conditions with a purely quantitative process.

The primary research method that has taken place are statistical experiments.

These types of methods involve the common practice of statistically manipulating

independent variables to generate analyzable data. The results provided by the studies

are used to test hypotheses, with statistical analysis giving a clear and unambiguous

picture. Before cellulose based aerogels can be put to the test of human interaction, the

quantitative experimental research method must take place. Where the shortcomings of

this research methods come to light is in the highly rigorous design and great expense

required to operate. This is true for experiments that are expected to interact with the

general populace and/or expected to be a larger experiment. It has been argued that

experimental research can be too accurate, causing a taxing drain on time, resources,

and ethical considerations (Shuttleworth, 2008). As with many other fields that do not

have the luxury of definable, quantitative variables, other qualitative research methods

must be utilized.

Looking more through the scope of qualitative research methods, a case study

analysis can provide insight to human thermal comfort. Since cellulose based aerogel is

to become a staple in the built environment, it’s abilities to regulate human comfort as

well as the metabolic rate during occupancy must undergo rigorous testing. These

observational based research methods are arguably the most removed from our

established scientific research methods, and is often looked down upon as ‘quasi-

experimental’ (Shuttleworth, 2008). Defining qualitative research as such is far from the

truth as observational research methods are extremely useful in offering unique insights

and advancing human knowledge. Case studies are often used as a precursor to more

rigorous scientific research methods, helping to avoid unconditional bias (Shuttleworth,

2008).

Drying and Curing

Virtually every physically solid, as well as some gaseous and liquid materials,

can be utilized as a thermal insulate material. However, not every material is suited for

the job. For example, a designer would avoid using metal in hot, arid environments

because metal transfers heat energy with ease. This would cause the spaces on either

side of the metal to fluctuate to meet heat energy equilibrium, leaving an interiorly

conditioned space to become uncomfortably torrid. A properly designed space would

utilize foams or cellulose based products because these materials avoid becoming a

thermal heat bridge. A thermal bridge refers to an area of an object, most commonly a

building, which transfers heat at a higher rate than the surrounding area (Capozzoli,

Alice, and Vincenzo, 2013). Thermal bridging causes a multitude of problems including

reduction in energy efficiency performance, condensation forming, and low human

thermal comfort for those inhabiting the interior space.

There are a multitude of materials that can be used for insulation; however, this

focus of this paper will be on materials that keep our interior spaces at a comfortable

temperature while keeping excessive heat or cold out. One successful material, which

has been used in areas such as star dust collection and pill capsule formation, is

cellulose based aerogels. The light, porous structured material has undergone rigorous

testing and development to yield specific qualities that are believed to be optimal for

becoming an insulation material.

Presently, cellulose aerogels are formed by, “sublimating the water from a

colloidal suspension of cellulose nano-fibers” (Nakagaito, Kondo, and Takagi, 2013).

These cellulose nano-fibers form a complex three-dimensional network, “cross-linked by

hydrogen bonds bridging the surface hydroxyl groups,” and by, “mechanical

entanglements between nano-fibers” (Nakagaito et al., 2013). The individual cell

structures of the cellulose realign with each other to form a complex three-dimensional

voronoi pattern of interconnected links, much like a chain link fence but with less

apparent order. Of course, as with all patterns of nature, there is an underlying order for

the structure and support of the material. The freeze-drying stage of curing in the critical

step that influences the aerogel structuring the most. The speed of freezing and

chemicals used in the process should be maintained at a constant rate throughout the

sample to ensure the formation of evenly sized ice grains (Nakagaito et al., 2013).

Currently, small-batch samples of cellulose based aerogels have been produced

through these freeze-drying methods but with improved cooling techniques, larger scale

production could become a reality (Nakagaito et al., 2013). A few such experiments

have taken place in which cellulose aerogels were impregnated with epoxy resins to

fabricate composites with success (Nakagaito et al., 2013). The high porosity of

cellulose aerogels allowed for, “Complete impregnation of resin and translucent

composites were produced,” although due to the brittleness the overall strength of the

aerogel was decreased (Nakagaito et al., 2013). These failures may have been caused

by the uneven port distribution of the aerogel or the size of the pores but the fact

remains that every trial was more brittle than a pure cellulose aerogel.

Initial development and testing of nano-fiber cellulose aerogels are attributed to a

group of chemical engineers at the East China University of Science and Technology in

2003. The group of highly skilled

biomaterial scientists began using the

silica aerogel base in branching out

towards biomass materials to develop

a sustainable and rapidly renewable

aerogel material (Hao, Yoshiharu,

Masahisa, and Shigenori, 2004). The

group began developing a highly porous

aerogel consisting of cellulose nano-fibers by regenerating gelatinous cellulose in a

carefully controlled carbon dioxide procedure, much like how silica aerogels are formed

(Hao et al., 2004). Due to the incredibly porous results that proved unusable in practice,

a variety of alternative drying methods were implemented including regular freeze

drying, rapid freeze drying, and solvent exchange drying (Hao et al., 2004). Of these

methods, the solvent exchange methods yielded the highest porous results at 50nm-

wide cellulose microfibrils (Hao et al., 2004). The rapid freezing technique, which

Figure 1 - Hao et al., 2004

utilized a nitrogen-cooled metal plate and copper, yielded asymmetrical porosity on the

metal plate side but a porous structure like the solvent exchange method on the side

that encountered copper (Hao et al., 2004). This alteration in porosity structuring, “are

expected to be useful in materials applications in particle/molecular separation

processes in gas and liquid phases,” with one major drawback being the fragility of the

aerogel sheets (Hao et al., 2004). The solvent exchange drying method should not be

utilized if the cellulose aerogel being produced is to be used in a building insulation

setting due to its fragility.

Cellulose aerogels with nano-sized structuring, such as normal carbon dioxide

drying methods, “are increasingly important in advanced technologies such as

particle/molecules separation and catalytic conversions,” and yield lightweight results

(Hao et al., 2004). Highly porous aerogels are vital to a lightweight construction and a

salt-based aqueous calcium thiocyanate system is an effective solvent for cellulose

aerogel production (Hao et al., 2004). By immersing gelatinous cellulose derivative into

the hot salt solution, a salt-cellulose solution is formed undergoing a reversible sol-gel

transition at approximately 80 degrees Celsius (Hao et al., 2004). This process has

been utilized as an industrial process for manufacturing chromatography packing

materials and could become the process for mass producing a cellulose aerogel

insulation material (Hao et al., 2004).

For testing of the regular freeze-dried and solvent-exchange samples, the Hao,

Yoshiharu, Masahisa, and Shigenori (2014) group performed an electron microscopy

observation, tensile strength, nitrogen absorption, and conductivity test on samples of

the solvent exchange method since the results were nearly identical for the two drying

methods. It was noted that the density of the cellulose concentration is nearly

proportional to the gel’s density used for each specific trial as seen in figure 1. A

discrepancy was noted that gel’s density

was twice that of the starting content of the

solution. “This discrepancy shows the extent

of the shrinkage during regeneration and

freeze drying,” meaning that a significant

amount of initial mass is lost through the

curing process (Hao et al., 2004). The

higher the cellulose concentration in the sample, the greater the overall strength of the

final product, “but the elongation at break was nearly constant (0.2-0.3%), independent

of cellulose starting concentration,

suggesting that the nature of constituent

component is basically the same,” as

displayed in figure 2 and figure 3 (Hao et al.,

2004). This observation points to the

possibility that less cellulose can be used in

the production of insulation material to

decrease the overall production time and cost. Through SEM, scanning electron

Figure 2 - Hao et al., 2004

Figure 3 - Hao et al., 2004

microscopy, testing, the influence of the drying methods on porosity was observed at an

intrinsic level. Freeze dries aerogel samples, figure 4, reveal a highly porous structure,

“but close examination reveals that fibrils severely coagulated to form film-like masses,”

meaning a weaker structure that leads to deterioration of the cellulose aerogel (Hao et

al., 2004). At lower cellulose concentrations, ice crystals form throughout the slow

drying process that lead to squeezing out the cellulose fibrils, but at higher

concentrations of cellulose the density hinders the growth of the ice crystals, “preserving

separation of microfibrils” (Hao et al., 2004).

Thermal Conductivity

To meet the demand of improving energy efficiency in buildings, construction

practices must be altered to meet the demanding need. Aerogels are known for

possessing a very low thermal conductivity rating, resulting from their low solid skeleton

conductivity, low radiative infrared transmission, and low gaseous conductivity

(Jiménez-Saelices, Clara, Bastien, Bernard, and Yves, 2017). To demonstrate their high

conductivities, aerogels are commonly displayed atop a flame with a delicate object

such as a flower or human finger touching the adjacent side of the aerogel. This stunt

demonstrates the lack of heat transfer throughout the material. In a 2014, cellulose

aerogel scientific testing and development of cellulose aerogels for absorption

Figure 4 - Hao et al., 2004

properties, Nguyen et al., (2014) noted that the samples formed displayed, “extremely

low density and thermal conductivities.” Thermal conductivities were measured at room

temperature using a C-Therm TCi Thermal Conductivity Analyzer sensor (Nguyen et al.,

2014). The cellulose used for this study is derived from high-quality paper waste and

prepared in a sodium hydroxide solution through the freeze-drying method (Nguyen et

al., 2014). In the presence of intense heat (upwards of 230 degrees Celsius), a 23%

weight loss was displayed without the mass of the material being effected (Nguyen et

al., 2014). This is probably due to the loss of water through evaporation in the cellulose

structure. At temperatures, higher than 230 degrees Celsius and up to 330 degrees

Celsius, a 42% weight loss is observed, “due to the degradation and burning of the

cellulose aerogel structure” (Nguyen et al., 2014). At temperatures between 550 – 630

degrees Celsius, a miniscule amount of sample weight was lost, “possibly due to the

oxidation of stable local structure of the aerogel” (Nguyen et al., 2014). These tests

demonstrate the feasibility of using cellulose aerogels in building constructions. With

special coatings to keep the moisture out of the cellulose structure and surface, the

ability for cellulose aerogel insulation to become applied to the exterior of buildings is a

possibility. Water repellents were applied to samples of the cellulose aerogels to test

moisture attacks on the recycled cellulose and were then tested in the similar manner

as before (Nguyen et al., 2014). The samples performed more optimally than before,

most likely due to the absence of water in the aerogel from the start (Nguyen et al.,

2014).

Alternate cellulose hydrogels have been prepared with chemical additives such

as sodium oxide, thiourea, and water solvent systems as an alternate to cellulose

aerogels. These hydrogels are finer-tuned solvent system and immersion methods that

could, “reduce the thermal conductivity of cellulose aerogel,” making it a more effective

insulation (Shi, Lu, Guo, Zhang, and Cao, 2013). These composite SiO2 aerogels

remain close to the similar composition of cellulose aerogels and reduce,

“hydrophobicity of cellulose aerogel,” but environmental humidity, “had a significant

influence on heat insulation performance” (Shi et al., 2013). These derivative aerogels

are vastly under researched but with more testing and development could become a

positive bridge between the aerogel and hydrogel substitutes.

Hydrophilic and Hydrophobic

In a recycled cellulose material for aerogel study conducted by Nuygen et al.

(2104), recycled cellulose fibers were immersed in a sodium hydroxide formula and

urea solution to develop a hydrophilic aerogel. Hydrophilic materials attract liquids with

chemical bonding of the surface tension. The mixture was then placed in freezing drying

temperatures to then be thawed in an ethanol solution (Nuygen et al., 2014). The

sample was then refrozen over the course of a few days to achieve the desired effect.

After multiple testing of this cellulose aerogel in water submersions, it was noted that

the desired effect of water absorption was a resounding success. The samples

absorbed as high as 19.8 times the weight of the original sample, proving that the

chemical integration during the critical drying stage provoked the porous cells of the

aerogel to accept and hold water (Nuygen et al., 2014). The absorbance capacities of

this developed cellulose aerogel is more than five times higher than sand or sawdust,

“and almost equal to those of commercial polymer sorbents (Nuygen et al., 2014). The

aerogel also preserved its overall shape throughout the submersion and drying trials,

indicating that the aerogel has a stable structure, “due to the cellulose to cellulose

hydrogen bonding” (Nuygen et al., 2014). Further demonstrations for the flexibility and

absorbance rates of the cellulose aerogels resulted in, “7.4 times its dry weight of water

for 20 minutes in the first test,” and how water can only be removed from absorbent

polymers by drying (Nuygen et al., 2014). In contrast, this cellulose aerogel could be

squeezed to release nearly

all of it’s absorbed water

content as seen in figure 5.

Using the same

aerogels as the hydrophilic

testing, specialized

hydrophobic physical and

chemical coatings were

applied to test the

hydrophobic properties of

cellulose aerogels. A commercial grade water repellent was spray applied to the exterior

surface of two samples while the other two samples received chemical baths in

methltrimethoxysilane (MTMS), a cheap chemical commonly used for hydrophobic

aerogels (Nuygen et al., 2014). All samples were then placed in an over for a few hours

to bake the coating into the structure of the cellulose aerogel and increase the results.

With the successful repelling of water, cellulose aerogels become a promising material

to use in building construction (Nuygen et al., 2014). After testing the water angles on

Figure 5 - (a) absorption test apparatus (b) Aerogel pre-test (c) Wet sample after first test (d) Squeezed sample after first test (e) Squeezed sample in water (f) Wet sample after second test (g) Wet sample after third test

Nuygen et al., 2014

the surfaces of the cellulose samples, it was determined that the MTMS samples, the

non-commercial agent, is more water repellent and cost effective than its counterpart

(Nuygen et al., 2014). All four samples were then placed in the air and sunlight for

multiple days for retesting. After the retesting, water droplets angles were re-measured

and there was little to no change, “indicating their excellent water repellent durability”

(Nuygen et al., 2014). With cellulose aerogels displaying good flexibility, mechanical

properties, and hydrophobic properties, the possibility of becoming the next sustainable

insulation material is one step closer to becoming a reality.

Acoustic Properties

An undeveloped research area of cellulose aerogels is the intrinsic acoustic

insulation properties. In the silica-cellulose aerogel trials, however, acoustic testing was

performed on samples ranging from pure cellulose to hybrids of silica aerogel with

infused cellulose. The coefficient of sound absorption through the cellulose matrix,

“decreases (0.399x0.303) with an increase in density (0.039x0.059 g/cm^3” (Feng et al.,

2016). The sound absorption coefficient of the aerogel samples decreases with an

Table 1 - Feng et al., 2016

increase in the density because of an increase in sound velocity (Feng et al., 2016). It

has been discovered that when the, “sound wave propagates to a depth of cellulose

aerogel, the air in the pores begin to vibrate, possibly leading to the vibration of the

cellulose fibers” (Feng et al., 2016). Throughout this process, due to the acoustic energy

being partially absorbed, the amplitude and velocity of acoustic waves are decreased.

As displayed in table 1, the sound absorption coefficients of the silica-cellulose samples

range from 0.39 to 0.50, making the material’s acoustic properties comparable to other

insulation material. Further testing revealed that adding silica particles to the samples,

the sound absorption coefficients generally increased (Feng et al., 2016). This increase

can be explained by, “acoustic energy partially being partially absorbed by interface

between the silica particles and cellulose fibers” (Feng et al., 2016). The unique

composition of silica particles and cellulose fibers allows for a higher sound absorption

coefficient then their pure cellulose counterparts, making it an effective sound barrier

material.

Silica-Cellulose Aerogel Composite

Of the numerous chemicals and

additives utilized in cellulose aerogel

formation, the combination of silica and

cellulose stands out as an optimal

match. Standard silica aerogel was

prepared using ammonium hydroxide

while the cellulose aerogel prepared was

Figure 6 - Feng et. al., 2016

derived from recycled hydrophobic cellulose (Feng et al., 2016). To form the

silicacellulose aerogel, the silica aerogel base was formed and then the hydrophobic

cellulose was immersed into the silica sol (Feng et al., 2016). The structure of this newly

formed material underwent the standard testing

with field-emission SEM, the same technique

used for all previous aerogels. From the

testing, it was determined that the pore

dimensions were similar in average size with

their pure silica or cellulose aerogel

counterparts (Feng et al., 2016). Interestingly

however, the cellulose bonded tighter via hydrogen bonding, constricting the silica

particles to fill the pores within the aerogel samples (Feng et al., 2016). These

interconnected silica particles, “stiffen the silica-cellulose aerogels by supporting the

cellulose fiber matrices,” increasing the overall compressive strength (Feng et al.,

2016). As more cellulose content is added to the samples, the cellulose structure

became more refined and silica content more evenly distributed within the porous

structure (Feng et al., 2016).

The hydrophobicity of the silica-cellulose aerogel composite, “exhibited an

inherent super-hydrophobicity,” far surpassing its pure cellulose counterparts (Feng et

al., 2016). The contact angle of 151.4 degrees as observed and recorded in figure 6

where the water droplet can be observed completely separated from the surface of the

aerogel (Feng et al., 2016). The samples were then sliced in half and water contact

angle measurements were taken on interior portions of the samples. No significant

difference was noted between he exterior and interior surfaces, confirming that super

hydrophobicity was distributed evenly amongst the silica-cellulose aerogel samples

(Feng et al., 2016).

To understand the thermal insulation abilities of this composite aerogel, standard

testing practices with a C-Therm TCi Thermal Conductivity Analyzer System were

carried out. The silica-cellulose samples underperformed compared to pure cellulose

samples, particularly those pure samples formed with sodium hydroxide substitutes

(Feng et al., 2016). However, the composite aerogels matrices are found to more stable

than those samples prepared with the sodium hydroxide (Feng et al., 2016). At higher

temperatures measuring 300 degrees Celsius, silica-cellulose samples displayed a loss

of only 40% weight compared to 85% by the pure cellulose samples and a stronger

thermal stability (Feng et al., 2016). In figure 6, the underperformance of the pure

cellulose aerogel is noted with the colored lines, displaying the 85% weight loss around

300 degrees Celsius. With these results of weak thermal stability, industrial applications

of pure cellulose aerogels are impeded due to safety concerns. The thermal

performance of the silica-cellulose aerogels yielded results of 0.15 W/mK, making it,

“competitive to those of conventional insulation materials, such as polyurethane foams

and insulation boards” (Feng et al., 2016). The thermal conductivities of the samples are

labeled as a resounding success and only increase with the density of the sample (Feng

et al., 2016).

Discussion

Aerogels have become the highlight of light, airy material research without an

investment into organic substitutes for the major component. Cellulose, being the most

widely available and rapidly renewable resource on the plant, possesses the ability to

become an organic staple for suture aerogel production. Current cellulose aerogel

research has proven that the organic substitute embodies similar properties to its

counterparts including lightweight structuring, high potential thermal capacity, acoustic

deafening, and unique porosity. While researching the abilities of cellulose aerogels to

surpass traditional silica aerogels, no current significant findings presented themselves.

Cellulose and silica composite aerogels, however, have documented research findings

describing a more optimal mixture than a singular major component of either cellulose

or silica. These composite aerogels provide promising results of higher isolative values,

more noise absorption, and lighter-weight structuring.

Despite the single component cellulose aerogel studies yielding insignificant

findings, further testing and development is required to truly determine the intrinsic

properties of the material. Much of testing has been conducted to validity the use of

cellulose aerogels for collecting stardust and consumption as the main component of pill

capsules. These areas of conducted research prove that cellulose aerogels, prepared

similarly to silica aerogels, possess a high tolerance to heat absorption and safety with

human interaction over prolonged periods of time. These successes of cellulose

aerogels could be the beginning for developing a construction material as an insulator in

buildings. Also, cellulose silica aerogel composites have, throughout multiple tests,

yielded the results of a highly successful insulation material. Further testing and

development of this composite aerogel is required to validate it’s use in the build

environment. Inevitably, a more sustainable, rapidly renewable material has become the

forefront of architectural research and cellulose aerogels present an optimal potential

source.

The thermal absorption properties of cellulose based aerogels have equaled and

in some cases rivaled those of silica based aerogels, especially in the case of star dust

collection. Silica aerogels have undergone rigorous testing to become the major

insulation material on spacecraft. These successes were the baseline for using

cellulose aerogels in star dust collection paired with the greater porosity size to collect

the floating particles. With cellulose aerogel being able to withstand the trip back

through the intensive heat generated by the stratosphere, we can infer that cellulose

aerogels provide similar insulation properties as silica aerogels. Further testing and

development is required to determine if cellulose aerogels can maintain a human

thermal comfort level of an interior space. The acoustics of cellulose aerogels have

undergone testing as well, providing similar sound absorption properties as silica

aerogels with slight deviations. During the composite testing, it was found that the

combination of silica and cellulose absorbed more sound frequencies than its pure

cellulose counterparts due to formed structure of the cellulose. There are more tests

required to prove the validity of cellulose aerogels but the prior testing has shown that

cellulose aerogels could be the answer for our sustainable material. The validity of

cellulose aerogels has yet to be resolved and need to strenuously test in areas such as

thermal absorption, acoustic dampening, and moisture attraction.

Previous testing and developing of composite aerogels have brought to light the

cellulose-silica blend which encompasses the sustainability of cellulose but the

structuring of silica. Through acoustic testing, unexpected results of sound dampening

occurred where pure samples of cellulose amplified certain sound waves while

absorbing others. Introducing silica into newer samples of composite aerogels yielded

greater sound dampening results and inevitably reaching a mixture where the difference

between composite and silica aerogel results were negligible. These findings strengthen

the case for formulating a silica-cellulose hybrid, backed by the engineer test-approved

traditional silica aerogel. Our current scientific studies of pure cellulose aerogels fall

short in engineered testing and development and require strenuous civil engineering

tests.

Limitations

Throughout the course of this scientific research study, many limitations were

noted from funding issues to lack of facility. Developing and studying new materials can

prove to be costly, especially with one that lacks a substantial precedence. Although

aerogels have undergone strenuous testing and development, an organic derivative

yields drastically different issues that must be addressed. To successfully complete

developing an organic aerogel, similar testing such as thermal capacity, acoustic

isolation, compression, tensile, and flexural properties much be researched. To

complete this research specific civil engineering apparatus’ are utilized and require

funding to complete. For this research, I could document the currently found intrinsic

properties of cellulose aerogels but lacks the specific testing to become a feasible

construction material. Cellulose aerogels have been utilized in pill capsulation and star

dust collection, so certain properties have been discovered. There was no funding

provided for this study, therefore paying for testing equipment, materials form a

cellulose aerogel, or renting a proper facility were not possible. Within the institution

where this research was conducted there was a materials lab but proper testing

machinery were not available. The materials lab possessed rudimentary testing

apparatus’ such as tensile, compression, and flexural testing but not the machinery

required to test the thermal capacity, acoustic isolation, or hydrophobicity testing. Make-

shift devices could be locally produced in the shop but would not have the engineering

backing to viable used for ASTM testing. ASTM testing is the engineering authority that

documents and verifies the intrinsic properties of all materials used in construction. To

be ASTM certified is to say that a material has undergone strenuous testing and has

been proven to be safely used for human habitation.

The timeframe used to research prior knowledge of cellulose aerogels was

adequate for compiling and understanding precedence of the organic material, but not

enough time for conducting further testing and development. Organic materials typically

degrade at a more rapid rate than inorganic materials and therefore need additional

testing to develop means of safely preserving the material beyond its degradation point.

Cellulose, derived from plant material, requires such testing and development,

especially when it becomes a material that comes in close contact with human thermal

comfort levels over prolonged periods. The length of this study was inadequate for

producing and testing a cellulose aerogel. Further development in a controlled lab

setting over a prolonged period is required for a cellulose based aerogel material.

Being that cellulose aerogels are derived from organic components, many issues

facing the health and wellbeing of inhabitants as well as the material become a major

question. This limitation of the organic aerogel may lead to an elongated development

process and inevitably the impediment of research. To combat that from happening,

continuing development of a silica-cellulose composite may be able to combat the

discontinuation of development by using the resounding successes of silica aerogels.

The composite aerogel could also reduce the total research and development time,

allowing it to be implemented into the built environment sooner and inevitably begin

saving energy costs.

Conclusion

In conclusion, a cellulose based derivative of the traditional silica aerogel

provides a promising solution for a sustainable alternative to building insulation.

Although the material has only been utilized in small-scale situations and requires

substantial more development to be used in a building context, the rapidly renewability

and resounding success of similar aerogels tell a story of potential success. As the

world continues to change and population continues to rise, more sustainable solutions

to our growing natural resource consumption for energy production are imperative.

Much of our produced energies are used in building operations to provide a human

thermally comfortable environment for both residential and commercial.

There are many limitations that face the development of cellulose based

aerogels, such as cost of production, success rate in various biomes, and viability in the

built environment. Continuing to test and develop cellulose aerogel from current known

properties will find the optimal place within the built environment. With currently known

thermal properties of aerogels, it can be inferred that cellulose aerogels have a

promising rate in keeping our buildings at a thermally acceptable level without using

grotesque amounts of produced energy. Continuing research on cellulose aerogels is

imperative to the successful implementation into the built environment.

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Reflection

When the capstone course began, I had high hopes that my career path would

be directed towards material research in an architectural setting. Over the course of the

year while researching two separate materials I’ve come to learn this is not what I wish

to pursue. Although, I wholeheartedly admit that this enlightening experience was

intriguing enough to provoke the thought of applying my skills in a more challenging

means. Through the content of this course, the instructors, and my mentor, I felt

empowered enough to push the boundaries of my post-undergraduate career. I have

now applied and been admitted to a graduate program that focuses on ecology and

architecture combined. This degree combination was only made possible by this

capstone and through the articles I engaged with. I began noticing that a large portion of

scientific research in architecture was being conducted at three major institutions, one

of which I will now be attending. This capstone was a challenging endeavor, not only in

the content but also in the maturity to continue creating in a methodical manner.

Speaking to peers in other majors, I’m learning that many other degrees on campus do

not require a final thesis, capstone, or dissertation out of their graduates. I’m glad to

have taken part in the experience and have grown as a professional thanks to it.