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INTRODUCTION
The discovery of silica based aerogels in 1930s by
Samuel Stephens Kistler was based on concept of
substituting the liquid phase with the gaseous phase
along with little amount of shrinkage and without
crumpling of gel solid network. Aerogels are smoked
like substrates having resemblance of hologram and
instead of appearing as solid material it looks like
projection. Aerogels are synthesized by Sol-Gel pro-
cess which is simple, cost effective and delivers high
quality substrates [2].
In this method, a chemical reaction was carried out in
a solution at low temperature to produce inorganic
network or creation of an amorphous structure from
the solution. The distinct feature of this reaction was
conversion from colloidal solution to di-or multiphase
gel. Silica based aerogels have 96 % of air and 4 %
of silicon dioxide, making silica based aerogels as
one of the lightest weight solid substrates [1]. A sol is
a colloidal suspension of solid particulates in an
aqueous medium in which range of dispersion phase
is from (1–1000 nm) [3]. Sol can be synthesized
either by condensation or dispersion of particulates.
Condensation occurs when nucleation development
of particulates approaches adequate size. However,
dispersion includes breaking of large particulates to
colloidal sizes. In case of gelatin process, a free flow-
ing sol is transformed into a three dimensional solid
structure encapsulating the solvent media. It was evi-
dent that liquid does not permit the solid structure to
crumple and solid structure does not allow the liquid
to move out. Figure 1 depicts the schematic diagram
of Sol-Gel process [3–4].
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SIO2 aerogels and its application in firefighter protective clothing
JAWAD NAEEM ENGIN AKCAGUN
ADNAN MAZARI ZDENEK KUS
REZUMAT – ABSTRACT
Aerogeluri de SiO2și aplicațiile acestora la temperatură ridicată
Acest articol studiază utilitatea aerogelului pe bază de siliciu ca material de izolație în îmbrăcămintea de protecție
utilizată la temperaturi ridicate, în special pentru pompieri. În trecut, aerogelurile au fost utilizate pentru izolarea clădirilor,
în aplicații aeronautice și aerospațiale. Mai târziu, cercetătorii au determinat utilitatea aerogelilor ca substrat izolator în
îmbrăcămintea de protecție. Mai multe investigații au evidențiat proprietățile de izolare ale aerogelului la temperatură
ridicată. Există mai multe tipuri diferite de aerogel, dar în acest articol, accentul principal este pus pe aerogelul pe bază
de siliciu datorită capacității excelente de izolare și a proprietăților ignifuge, împreună cu stabilitatea termică la
temperaturi ridicate. Prin aplicarea unor tehnici adecvate de acoperire, aerogelul poate fi impregnat în substraturi
nețesute, care pot fi utilizate ca bariere termice, cu rezultate în îmbunătățirea capacității de protecție a îmbrăcămintei
multistratificate la temperaturi ridicate. Toate aceste caracteristici fac din aerogel un potențial candidat pentru a fi utilizat
ca material izolator în îmbrăcămintea de protecție la temperaturi ridicate.
Cuvinte-cheie: aerogeluri, izolație termică, conductivitate termică, îmbrăcăminte de protecție
SIO2aerogels and its application in firefighter protective clothing
This article focuses our attention on utility of silica based aerogel as insulation materials in protective clothing especially
for fire fighters at higher temperature. In past, aerogels were employed for insulation of buildings, aeronautics and
aerospace applications. Later on, researchers determined the utility of aerogels as insulated substrate in protective
clothing. Several investigations revealed insulation properties of aerogel at elevated temperature. There are several
different types of aerogel but in this article, the main emphasis is on silica based aerogel because of its excellent
insulation and outstanding flame proof properties along with thermal stability at raised temperature. By applying suitable
coating techniques, aerogel can be impregnated into nonwoven substrates, which can be employed as thermal barriers
resulting in enhancement of the protective capability of multilayered clothing at higher temperature. All these
characteristics make aerogel a potential candidate to be used as insulation material in protective clothing at elevated
temperature.
Keywords: aerogels, thermal insulation, thermal conductivity, protective clothing
Fig. 1. Process of Sol-Gel [3]
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STEPS INVOLVED IN SOL GEL PROCESS
Three steps required for sol gel process are:
a. Formation of gel
b. Aging
c. Drying
Formation of gel
Mostly precursors utilized for sol-gel process are sili-
con alkoixdes which are available in high degree of
purity. However it is very difficult to purify potassium
silicate [4]. One of the most important precursors of
silica based aerogel is sodium meta-silicates and it
was primarily employed by Kistler for synthesizing sil-
ica aerogels [5].
Na2SiO3+ 2HCl + (x–1)H2O → SiO2· xH2O + 2NaCl
(1)
The preparatory materials for the sol-gel process
must be soluble in reaction medium and sufficiently
reactive to have contribution in the gel-creating pro-
cess. All of these preparatory substrates are
described by the existence of Si – O polar covalent
bonds [6]. The covalent nature of the Si – O bond is
adequate to allow distribution of ≡ Si – O – Si ≡ angle
values creating a three dimensional structure in ran-
dom manner which resembles structure of silica
glass. Hydrolysis can be catalyzed via acid catalysis
or base catalysis. Acids like HCl, HNO3, H2SO4, HF,
oxalic, formic and acetic acid are employed for acid
catalysis. When pH of solution is less, the time peri-
od for gelatin formation is normally elongated [4].
According to Dieudonne et al. [7] base catalysis is a
simple procedure in which there is an easy formation
of network of uniform particles in the solution and the
subsequent pore volume is significantly enhanced.
Aging
During aging, a neck growth due to reprecipitation of
silica deliquesces from the surface of particle upon
necks between particles and smaller particles disso-
lute and conversion into bigger ones by precipitation
[8]. The purpose of this step is to mechanically rein-
force the weak solid skeleton generated during sol-
gel process [4].
Drying
In case of drying, the most commonly used method is
Super Critical Drying method. This method is gov-
erned by evacuation of pore liquid above the critical
temperature (Tc) and critical pressure (Pc). At this
instant, there is no liquid-vapor boundary and conse-
quently there is no capillary pressure. This procedure
is completed in three stages [9]: In first step, wet gel
along with appropriate quantity of solvent is set in an
autoclave by steadily elevating the temperature. This
will result in escalation of pressure. Temperature and
pressure of the concerned solvent are attuned to
acquire values above critical points. In second step,
the fluid is gradually expelled at constant temperature
causing decrease in pressure. In third step, the tem-
perature of vessel drops to room temperature once
ambient pressure is acquired. Figure 2 depicts auto-
clave for performing supercritical drying. Drying can
also takes place via ambient pressure drying and
freeze drying [10].
PROPERTIES AND UTILITIES OF SILICA BASED
AEROGELS
Structure of pore
Silica based aerogels are mostly mesoporous having
interlocked pore size with range from 5 to 100 nm.
The diameter of average pore is between 20 to
40 nm. Micropores having pore size less than 2 nm
becomes pertinent in aerogels produced under acidic
catalysis conditions [11]. The specific surface range
from 250 to 800 m2g–1 and can surpass 1000 m2g–1.
Thermal insulation, flame proof property
Silica based aerogels have very small portion of solid
silica (nearly 1–10%) due to which they have lesser
solid conductivity and thus exchange lesser thermal
energy [4]. At ambient pressure, temperature and rel-
ative humidity, silica based aerogels have very low
thermal conductivity of the order 0.015 W/mK which
is expressively lesser than thermal conductivity of air
(0.025 W/mK) under same circumstances [12]. Apart
from having thermal insulation property, silica aerogel
has remarkable flame proof property [13]. By means
of mass, Aerogel is 99.8% air making it least dense
man-made substrate [14]. Aerogel can abrogate all
three modes of heat transfer. Conductive heat trans-
fer is blocked because of gaseous structure of aero-
gel and thermal conductivity of gas is very low. The
gaseous structure of aerogel is derived from its porous
construction. Convective heat transfer is averted
because structure of aerogel does not allow circula-
tion of air. Infrared radiation that plays role in trans-
ference of heat can also be absorbed by aerogel. As
consequence, aerogel can function as outstanding
thermal insulator [15].
Sorption and entrapment properties
Aerogels can be utilized to adsorb some chemical
compounds i.e. waste water treatment for restricting
Fig. 2. Schematic representation of supercritical drying
autoclave [6]
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radioactive waste or for filtration of gases. Silica
aerogels soaked with CaCl2, LiBr and MgCl2salts
have also been confirmed to absorb/adsorb water for
retention of heat at low temperature [16]. It was sug-
gested hollow silica aerogel droplets for inertial
entrapment of fluids, specifically blends of liquid deu-
terium and tritium, as the aim in fusion experimenta-
tion under very strong laser. Bacteria can also be
successfully entrapped in aerogels while remaining
alive.
HEAT EXCHANGE PROCESS
Heat exchange equation illustrates transfer of heat in
porous substrates like aerogels [17].
T
q + F= r· c · (2)
t
This equation depicts law of conservation of energy
by balancing heat fluxes across interfaces of infinite
volume. In the above equation qis the heat flux den-
sity, r– the density and is the specific heat, F– the
heat source and T– local temperature.
q= – k T(3)
kis three dimensional tensor of the thermal conduc-
tivity. This equation is the Fourier law which narrates
that heat flux density is proportional to local temper-
ature difference and describes the thermal conductiv-
ity k. The heat source Fexplains the impact of phase
changes or sorption phenomenon within an aerogel
associated with the discharge or intake of reaction
enthalpies and increase or decrease of thermal radi-
ation. Thermal radiation is explained by heat source
term in following way:
F= qr (4)
Here qris radiative heat flux density. In case of
isotropic aerogel and when heat exchange is depen-
dent only on local temperature difference:
1 T
DT= · (5)
at
Where ais the thermal diffusivity which is equal
to k /(r·c). In this situation, experimentally evalu-
ated thermal conductivity is characteristics of the
substrate. Where r·cis termed as volumetric
specific heat and is suitable for the situation of
nonstationary heat exchange where tempera-
ture and heat fluxes varies with time [12, 18].
APPLICATIONS OF SILICA AEROGELS AT HIGH
TEMPERATURE
At present the most common utility of aerogel prod-
ucts are in oil and gas pipelines, building insulations
along with aeronautics/aerospace and high tempera-
ture applications [19]. On commercial bases aerogel
is available asNanogelTM, which can be employed as
superinsulating filling substrates; however CABOT is
also supplying NanogelTM based components like
Thermal wrapTM and Compression packTM for special
applications like pipe in pipe and cryogenic insulation
systems. Apart from that, ASPEN company is provid-
ing blanket-based products for insulation of building
on commercial scale and also developing products
for utility in acute hot and cold climates (pyrogel and
cryogel) [18–19]. Novel advancements of aerogel sci-
ence have made it viable to create more flexible
aerogels in simplified ways. All these features make
aerogel favorable prospects to be utilized for enhanc-
ing thermal protection and thermal insulation in fire
fighter protective clothing.
Fire fighters are subjected to several threats with
respect to their working atmosphere. In addition to
numerous toxic ingredients in the surrounding atmo-
sphere, extreme radiant heat fluxes and hot flames
are probable hazards in fire extinguishing activity.
Thermal protective performance of fire fighter protec-
tive clothing is of huge significance to the lives of fire-
fighters [20].
The key purpose of fire fighter clothing is to decline
the rate of heat accumulation in human skin so as to
give time for the firefighter to respond and avert or
reduce skin burn injury [21]. Mostly fire fighter pro-
tective clothing comprised of three layers such as
exterior shell, middle layer and interior layer or two
layers like outer shell and inner layer with assembly
of moisture barrier and a thermal barrier as shown in
figure 3 [22]. The exterior layer averts body skin from
hazards of heat radiation or flame and intermediate
layer delivers execution of waterproof and heat insu-
lation. Mostly the aramid fibers are utilized for insula-
tion layer and PTFE membranes are employed as
breathable waterproofing layer. Jin et al. investigated
thermal protective behavior of nonwovens employed
with aerogels [21]. It can be noticed that specimen
coated with aerogels had greater LOI values than the
untreated specimen. This might be due the fact that
inorganic aerogel particles being attached on the sur-
face of specimen might enhance the flame retardant
characteristics [21]. For evaluation of effect of aero-
gel on thermal protective performance (TPP) of the
whole fire fighter garment, aerogel treated firefighter
clothing utilizing thermal liner in fighter protective gar-
ment was developed by Jin et al. [22]. Instrumented
manikin system under heat flux density of 84 kW/m2
with 8 second of exposure time was employed. For
aerogel treated fire fighter clothing, total burn injury
was 12.7% which was lesser than that of existing
Fig. 3. Configuration of fire fighter clothing assemblies [22]
garment which was 25.1% [22]. It was also witnessed
that aerogel impregnated sample when utilized next
to skin can absorb moisture and discharge it ambient
surrounding with great ease. Moreover there was
increase in the rate of moisture absorption when
aerogel impregnated layer was employed next to skin
[22]. Thus it was deduced that aerogel when coated
on textile substrate can enhance thermal resistance
of the fabric and delivers better thermal insulation
properties.
CONC L U S ION
Aerogels are invented long time ago but its applica-
tion in daily life started a decade ago. The application
of aerogels for insulation at higher temperature is still
an open field and needs to be researched deeply. In
this report the application of aerogels specially for
insulation properties are discussed and will be a
basis for continuing research related to use for aero-
gels in firefighter clothing, fire protection buildings
and facade insulation. With extremely low thermal
conductivity and high porosity the aerogels have a
great future for the application for insulation at higher
temperature. Another open field of aerogels is also
comfort properties and could be a great application
for clothing for protection with comfort requirement.
The breathe ability of aerogels can play an important
role for the comfort and insulation application.
ACKNOWLEDGMENT
This study is supported under student grant scheme of
SGS-21200.
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Authors:
JAWAD NAEEM1
ADNAN MAZARI1
ENGIN AKCAGUN 2
ZDENEK KUS 1
1 Technical University of Liberec, Faculty of Textile Engineering, Department of Clothing
Studentska 2, Husova, 1402/2, Liberec, Czech Republic
e-mail: jawadnaeem.qau@gmail.com, adnanmazari86@gmail.com, zdenek.kus@tul.cz
2 Mimar Sinan Fine Arts University, Istanbul, Turkey
e-mail: enginakcagun@gmail.com
Corresponding author:
JAWAD NAEEM
e-mail: jawadnaeem.qau@gmail.com