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Thermal performance of aerogel-embedded polyester/polyethylene nonwoven fabrics in cross airflow was experimentally studied by using a laboratory-built dynamic heat transfer measuring device in which the fabric could be applied on a heating rod. Experiments were performed with different airflow velocities and heating conditions. The temperature–time histories of different materials were collected and compared. The temperature difference and convective heat transfer coefficient under continuous heating were analyzed and discussed. Results showed that under preheated conditions, the aerogel-embedded nonwoven fabrics had very small decrease in temperature and good ability to prevent against heat loss in cross flow. As for the continuous heating conditions, the heat transfer rate of each material showed an increasing trend with increase in the Reynolds number. The aerogel-treated nonwoven fabric with the least fabric thickness and aerogel content delivered a significantly increased heat transfer rate at higher Reynolds number. Thicker fabrics with higher aerogel content could provide better insulation ability in cross flow.
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Xiaoman Xiong1,3,*, Mohanapriya Venkataraman1, Darina Jašíková2, Tao Yang1,3, Rajesh Mishra1, Jiří Militký1,
Michal Petrů3
1 Technical University of Liberec, Faculty of Textile Engineering, Department of Material Engineering, Liberec 46117, Czech Republic
2 Institute for Nanomaterials, Advanced Technologies and Innovations, Department of Physical Measurements, Liberec 46117, Czech Republic
3 Institute for Nanomaterials, Advanced Technologies and Innovation, Department of Machinery Construction, Technical University of Liberec,
Liberec, 461 17, Czech Republic
*Corresponding author. E-mail:
1. Introduction
High-loft  brous materials are widely used in various elds
for thermal insulating applications due to the large amount
of void space in the  brous structure. The thermal properties
of a  brous material are affected by the physical parameters
of the ber component and the structural parameters of the
brous structure. Silica aerogel, as a coherent with a rigid
three-dimensional network of contiguous particles of colloidal
silica, prepared by the polymerization of silicic acid or by
the aggregation of particles of colloidal silica, demonstrates
superior thermal insulation performance with extremely low
thermal conductivity and thus has been well acknowledged as
one of the most attractive thermal insulating materials [1-2].
However, since silica aerogels generally have poor mechanical
stability, they are usually incorporated with a lightweight  brous
material to deal with various heat transfer problems [3]. Various
aerogel-embedded  brous materials, developed by the sol–gel
method or by incorporating aerogel beads into a nonwoven
brous web by using low-melting  bers or additive binding
materials, have been well studied [4-7]. These materials
exhibit improved thermal insulation and thus have a potential
to be used in buildings, industry facilities, and protective textile
applications such as winter jacket, sleeping bed, and gloves in
extremely cold weather.
Aerogel-embedded nonwoven fabric is basically a highly
porous,  brous material. It is well known that heat transfer in a
brous material is a complex property because the three basic
mechanisms of heat transfer, namely, conduction, convection,
and radiation, play a part [8]. The insulating property, resulting
from the fact that the gas contained in the pores is at rest,
has been well understood and documented. Convective heat
transfer through a  brous material, resulting from a  uid moving
across a surface that carries heat away, involves complex and
diverse ow patterns around the solid particles or bers [9].
Generally, the rate of convective heat transfer is a function of
the  uid and surface temperatures, the surface area, and the
speed of the  ow across the surface. Due to the inherently air-
permeable characteristic of a highly porous material, air intrusion
through the void space usually occurs in the  brous structure,
which strongly affects the convective thermal behavior of a
porous material. In practical use, the in uence of air movement
depends on the structural parameters of the brous material
and the nature of the air ow as well [10]. Forced convection
usually generates highly turbulent, multidirectional  ows as a
function of venting characteristics, material geometries, and
wind speed [11]. Due to the complexity of the geometry and
ow pattern, it is dif cult to obtain a clear relationship between
the individual fabric parameters and the thermal behavior,
as most of them are interrelated to each other and therefore
impossible to separate. In order to observe the convective
heat transfer through textiles, approximate solutions based
on computational uid dynamics and well-established
dimensionless numbers are usually used [12]. The effect of
brous structures on uid ow has been numerically studied
Thermal performance of aerogel-embedded polyester/polyethylene nonwoven fabrics in cross airflow was
experimentally studied by using a laboratory-built dynamic heat transfer measuring device in which the fabric could
beappliedonaheating rod.Experimentswere performedwithdifferentairflowvelocities andheatingconditions.
The temperature–time histories of different materials were collected and compared. The temperature difference and
convectiveheattransfer coefficientundercontinuous heatingwereanalyzedanddiscussed.Resultsshowedthat
under preheated conditions, the aerogel-embedded nonwoven fabrics had very small decrease in temperature and
fabricwith theleastfabricthicknessand aerogelcontent delivereda significantlyincreasedheattransferrate at
fl o w .
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[13]. A study on the modeling and simulation of convective heat
transfer in aerogel-embedded nonwoven fabric has also been
carried out [14]. Experimental investigations of convective
thermal behavior of aerogel-embedded brous materials in
a controlled climate chamber have been reported [15-16].
However, all these works deal with a simple airow moving
over an absolutely at fabric placed on a heating plate. In some
actual cases, the airow and the state of the fabric are more
complicated, especially in applications involving buildings and
industrial facilities, for example, brous insulators applied on
domestic hot water plumbing lines to improve energy efciency,
on process equipment, piping, steam distribution systems, and
boilers for process control, energy efciency and safety. Such
applications involve a cross airow around a hot cylinder that is
insulated with a brous material.
The aim of the present work is to perform an experimental
study on the thermal behavior of aerogel-embedded polyester/
polyethylene nonwovens when subjected to wind-induced
cross airow. Aerogel-embedded polyester/polyethylene
nonwovens with different structural parameters and aerogel
content were selected to carry out the measurements. A
laboratory-built dynamic heat transfer device was used to gure
out the convective thermal behavior of the selected materials
under different airow velocities and heating conditions. Real-
time temperatures of the fabrics were collected and compared.
The temperature difference and heat transfer coefcient under
continuous heating condition were calculated and investigated.
2. Experimental
2.1. Materials
Three types of polyester/polyethylene nonwoven fabrics
with at 50:50 composition ratios and embedded with aerogel
were selected to assess their thermal performances under
convective heat transfer with air. The aerogel used was
hydrophobic amorphous silica aerogel, which is mesoporous
and has nearly 98% of air and 2% solid. Its specications are
presented in Table 1. Due to the interconnected nanoporous
characteristic, the aerogel can hold air within its structure and
does not allow free ow of air, which enables it to be a superior
thermal insulation material. The aerogel particles were added
during thermal bonding of the nonwoven web. A high-resolution
image of the aerogel/polymer nonwoven fabric B is shown in
Figure 1. It is clear that aerogel particles are dispersed both
on the surface of a single ber and in the void spaces between
bers. The physical properties of the aerogel-embedded
nonwovens are given in Table 2. All these fabrics have high
porosity of >90%.
2.2. Measurement of thermal behavior in cross ow
2.2.1. Experimental setup
The tests were carried out in a laboratory-built device; the
experimental section is presented in Figure 2. The main part of
the measured section consists of a subsonic wind tunnel with
a square cross section of 20 cm x 20 cm, an electric heating
rod with a diameter of 2 cm placed perpendicular to the airow
direction in the wind tunnel, and a fan ventilator connected in
the right corner of the lower duct. The heating rod, composed of
a Ni-Cr 80-20 heating wire with a stainless steel shell, having a
rated power of 500 W and voltage of 230 V, was connected to
a control unit and powered by a Sefram DC power supply. The
input power was adjusted by the control unit. The fan ventilator,
acting as a suction fan, supplies airow at room temperature.
Different airow velocities can be achieved by adjusting the fan
The experiments involve measurements of real-time
temperatures of the heating rod, the fabric insulators, and the
upstream airow, as well as determination of the velocity of the
upstream airow. Temperatures of the heating rod and the fabric
were measured with T-type copper–constantan thermocouples.
Two thermocouples were respectively attached on the heating
rod and the fabric surface with the help of high-temperature
RTV silicone. A thermoanemometer sensor FVAD35TH5 was
mounted in the upstream region to monitor the velocity and
temperature of the free stream. All these thermocouples and
sensor were connected through an ALMEMO 2590-2A - 2 data
logger to a personal computer device.
Figure 1. Scanning electron microscope (SEM) image of aerogel/
polymer nonwoven.
Table 1. Amorphous silica aerogel specications
Properties Value range
Particle size range 0.1–0.7 mm
Pore diameter ~20 nm
Density 135±15 kg/m3
Surface chemistry Fully hydrophobic
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2.2.2. Experimental procedures
The selected aerogel-embedded nonwoven fabric was cut into
20 cmx8 cm pieces and wrapped on the heating rod; the lateral
gap was sealed with insulating rubber tape. Any gaps between
the measured fabric and the heating rod were eliminated.
The tests were carried out with two types of heating conditions,
i.e., preheated and continuous heating conditions, to investigate
how the materials prevent against heat loss from the heating rod.
The preheated condition refers to the preheating of the heating
rod to a speci c temperature and subsequently switching off
the heating power to let the system cool down at a selected
air ow velocity. The continuous heating condition involves
nonstop heating during the whole measurement process.
The voltage and current supplied from the direct current (DC)
power for heating were 37 V and 0.33 A, respectively, for each
measurement. Five air ow velocity levels, i.e., 0, 1, 5, 10, and
15 m/s were used to study the effect of air ow velocity on the
heat loss rate of different fabrics during cross  ow.
In this study, the preheated temperature was chosen to be
around 60°C, and the ambient temperature was maintained
at 23°C±2°C. At the beginning of a measurement at the
preheated condition, the heating rod was heated to 60°C, the
power supply was cut off, and the fan ventilator was switched
on to let in air ow with a selected velocity. The sensed data on
temperatures and air ow velocities were collected at intervals
of 3.2 seconds. During the continuous heating measurement,
the heating rod was maintained being powered, and the
air ow was allowed to be delivered when the temperature of
the heating rod reached 60°C. Data were taken as soon as
the inlet air ow reached a selected velocity. The experimental
work includes two parts: the rst dealt with the ow around a
heating rod without fabric, and the second with the  ow around
the porous fabric wrapped on the heating rod. For each part,
measurements were performed under both preheated and
continuous heating conditions at different air velocity levels.
Each measurement was conducted three times, and the results
were averaged.
3. Results and discussion
3.1. Fluid  ow around the heating rod without fabric
For the circular heating rod in cross air ow, as the air ow
approaches the front side of the rod, the  uid pressure rises and
the free stream  uid is brought to rest at the forward stagnation
point [17]. The high pressure forces the  uid to move along the
surface, and boundary layers develop on both sides. The free
stream velocity depends on the angle from the stagnation point.
The pressure force is counteracted by viscous forces, and the
uid separates from both sides of the rod and forms two shear
layers. The innermost part of the shear layers is in contact with
the rod surface and moves slower than the outermost part. As
a result, a highly irregular wake is formed in the downstream
region. The  ow pattern is dependent on the Reynolds number
Re, which is given by the following expression:
= (1)
where V is the velocity of air (in meter/second [m/s]), D is the
diameter (in meters), v is the kinematic viscosity of air at the
lm temperature (in square meter per second [m2/s]), which is
10-6 m2/s at 25°C.
The Reynolds number range for the heating rod was 1259–
18880. The temperature–time histories of the heating rod
are presented in Figure 3. Apparently, under the preheated
condition, there is a rapid decrease in temperature and a high
heat loss rate; this rate subsequently slows down until the
heating rod cools to ambient temperature. The overall heat loss
rate is determined by the air ow velocity. At air velocity 0 m/s,
the rod temperature slowly decreases. As the air ow velocity
increases, the temperature values dramatically decrease,
and the overall heat loss rate signi cantly increases. As for
the continuous heating condition, the same trend is observed.
Remarkably, at air ow velocity 1 m/s, the heating rod quickly
reaches a stable temperature value. The probable reason is
that at this air ow velocity, heat gain due to the heat  ow inlet
into the heating rod is exactly equal to the heat loss, leading
Table 2. Characteristics of aerogel-embedded nonwoven fabrics
Samples Fabric density,
Areal density,
Aerogel content,
A79.6 278.6 3.5 1.5
B 66.7 440.2 6.6 2.0
C 80.4 498.5 6.2 2.5
Figure 2. Schematic diagram of the measurement device.
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to a steady state without any change in temperature of the
heating rod. As expected, an increase in air ow velocity gives
rise to a dramatic rise in the rate of heat transfer and thus a
rapid decrease in heating rod temperature.
3.2. Thermal behavior of the heating rod and fabrics under
preheated condition
In the preheated condition, the heating rod has an initial
temperature around 60°C. It loses heat by conduction due to
the temperature difference between the heating rod and the
brous insulator, and the fabric gains heat. Some of this heat
energy is turned into heat capacity if the fabric temperature
increases, and the remainder is transferred to the fabric
surface and lost as the air  ows over it. The net heat loss from
the system results in the changes of the rod temperature and
the fabric temperature.
The temperature–time histories of different fabrics are
compared in Figure 4. Apparently, a low air ow velocity gives a
gentle downslope of the heating rod and fabric temperatures,
whereas a high velocity causes rapid temperature decline
and higher heat loss rates of the system. Comparing with the
temperature of the naked rod, the heating rod wrapped with
an aerogel-embedded nonwoven fabric has much lower heat
loss rate, indicating an effective insulating function of the
fabrics during cross  ow. Meanwhile, very small temperature
decreases are observed for all the three fabrics, further
improving their good thermal performance during cross ow.
With the increase in air ow velocity, the heating rod temperature
lowers down; the decrease in each temperature–time curve is
more obvious. Generally, the heating rod with aerogel-treated
nonwoven samples B and C demonstrates higher temperature
in comparison with sample A, and the temperature gradient
is more signi cant at higher air ow velocities. This could be
explained by the difference of fabric structural parameters and
aerogel content. Nonwoven fabrics have very close porosity
values > 90%, with pore size ranging from 2 µ to 4.5 µ, which
allows the bulk of air molecules to  ow through [18]. However,
the aerogel particles, with an average pore size around 20 nm,
will retard – or even prevent – air ow stream. As a result,
greater aerogel content gives less permeability to air ow and
thus lowers the heat transfer rate. Meanwhile, thinner fabrics
are easier to be penetrated by the air stream, causing more
heat loss from the heating rod. As a result, aerogel-treated
nonwovens with higher aerogel content and fabric thickness
demonstrate better ability to prevent against heat loss under
air ow-induced convection. Moreover, at higher air ow velocity,
fabrics with more mesopores (diameter: 2 –50 nm) tend to be
less affected by the air ow. This is further proved by the higher
temperature values observed for fabrics with more aerogel
content when the air ow velocity is >5 m/s.
3.3. Thermal behavior under continuous heating
In the continuous heating condition, a speci c electric power
is supplied to the heating rod. For the system composed of
the heating rod and the fabric, the heat ow generated by the
heating rod was partly converted into its heat capacity, and
the remainder was conducted to the fabric and  nally turned
into heat capacity of the fabric or dissipated by air-induced
convection. The whole dynamic process is in uenced by power
supply, air ow nature, physical parameters of the heating rod,
as well as the physical and structural parameters of the fabric.
The real-time temperature curves of the heating rod and
different fabrics under continuous heating are presented
in Figure 5. Generally, the fabrics maintain quite stable
temperature values under continuous heating, while the
heating rod temperature curves have slight uctuations at
low air ow speed (1 m/s and 5 m/s), which gently increase at
a higher air ow speed (10 m/s). These uctuations are well
consistent with the current switch. It is notable that the heating
rod with Fabric A has rapid temperature decrease from 58°C to
48.8°C when the air ow velocity is 15 m/s. This implies that the
aerogel content and fabric thickness are very important factors
in protecting against heat loss at high air ow velocity. Similar
to the results from the preheated condition, samples B and C,
with higher fabric thickness and aerogel content, demonstrate
better ability to prevent against heat loss from the heating rod.
Especially at higher air ow speed (10 m/s and 15 m/s), the
heating rod is able to achieve quite high temperature values,
indicating better thermal protection given by these fabrics. For
Figure 3. Temperature-time histories of the heating rod under preheated
condition and continuous heating condition.
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Figure 4. Temperature-time histories of the heating rod and fabric under preheated condition.
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Figure 4 continued. Temperature-time histories of the heating rod and fabric under preheated condition.
the reason given with reference to the preheated condition,
the real-time temperature curves of different fabrics vary with
air ow velocity.
3.4. Heat transfer coeffi cient under continuous heating
The continuous heating of the system under air ow-induced
convection is a dynamic process involving complex ow
patterns around solid particles or bers. Due to the random
orientations of the solid phase, exact solutions to the detailed
local  ow  eld are impossible. In order to compare the thermal
performance of different  brous materials, the system can
be considered to reach steady state if the temperature of the
heating rod and the fabric are  uctuating within a narrow range
or showing a very gentle upslope. In this case, the heat gain
by the system is roughly equal to the heat loss from the  brous
material by convection, as shown in Figure 6.
Heat  ow was measured by noting the current and voltage
input to the heating rod. The average heat transfer coef cient
is determined by the following expression:
where Qinlet is the heat ow into the heating rod (in Watts); Tf
is the temperature of the fabric surface (in Kelvin); Tair is the
temperature of ambient air (in Kelvin); and S is the surface area
of the fabric exposed to the air ow (in square meters [m2]).
Since the Reynolds number is important in predicting  ow
patterns in different uid  ow situations for convective heat
transfer problems, the heat transfer coef cient is plotted
for different Reynolds number in Figure 7. The heat transfer
coef cient increases with the increase in Reynolds number.
The data fall in distinct groups depending on the fabric density
and permeability of the material. For lower-density materials,
dispersion dominates any molecular conduction, resulting
in heat transfer independent of solid conductivity but more
affected by the air ow. The heat transfer coef cient of each
material shows an increasing trend with the increasing of
Reynolds number. A at upstream trend is observed for aerogel-
treated nonwoven fabrics B and C. Aerogel-treated nonwoven
fabric A, having the least fabric thickness and aerogel content,
delivers slightly lower heat transfer rate at small Reynolds
number. However, the coef cients signi cantly increase with
the increase in air ow velocity and Reynolds number. For
Reynolds number <10000 (air ow velocity <10 m/s), the heat
transfer rates of different fabrics are quite close because of free
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Figure 5. Temperature–time histories under continuous heating.
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brous materials for thermal insulation. Additional fundamental
and numerical works are necessary for better understanding
of convective heat transfer through aerogel-embedded  brous
This work is supported by the Ministry of Education, Youth
and Sports of the Czech Republic and the European Union
(European Structural and Investment Funds-Operational
Programme Research, Development and Education) in the
frames of the project “Modular platform for autonomous chassis
of specialized electric vehicles for freight and equipment
transportation”, Reg. No. CZ.02.1.01/0.0/0.0/16_025/0007293.
convection and radiation [19]. With the increase in Reynolds
number (air ow velocity >10 m/s), the forced convective heat
transfer – which strongly depends on the fabric permeable
property – overwhelms free convection and radiation [19]. As a
result, the heat transfer rate for Fabric A remarkably increases,
while fabrics with higher thickness and aerogel content
(samples B and C) could maintain relatively stable values.
3.5. Difference in temperature between heating rod and
air under continuous heating
Figure 8 shows the difference in temperature between heating
rod and air ow under continuous heating. The temperature
difference is fairly invariant over a range of Reynolds numbers.
For the heating rod without fabric, temperature difference lies
in the range of 32°Cto 8°C, showing considerable decrease
with increasing Reynolds number, as expected. The present
fabric maintains the heating rod at a higher temperature
because of its ability to prevent against heat loss. Comparing
the temperature differences of different fabrics, it is seen that
the heat losses from fabrics B and C are very close because
of the close proximity of their thickness and greater aerogel
content. Meanwhile, Fabric A gives much greater heat loss.
It can be concluded that fabrics with higher thickness and
aerogel content provide better insulation ability during cross
ow, especially for high Reynolds numbers.
4. Conclusions
In this work, three aerogel-embedded nonwoven fabrics were
selected to investigate thermal behavior in cross air ow by using
a laboratory-made device. It was found that under preheated
conditions, all the three fabrics showed very small temperature
decrease, indicating that these fabrics are effective in preventing
against heat loss during cross  ow. The fabric with higher
thickness and aerogel content had better ability to retain heat
during cross  ow. The results from continuous heating showed
that the thinner fabric with lower aerogel content had limited
ability to protect against heat loss at high air ow velocity, while
the thicker fabrics with higher aerogel content demonstrated
lower heat transfer rate, especially at high Reynolds numbers
or air ow velocity >10 m/s. The  ndings can be used for further
study in the areas of aerogel-embedded high-performance
Figure 6. Heat transfer through the system in the steady state.
Figure 7. Heat transfer coef cients of the materials.
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... For effective or efficient heating capability, the core principle is to prevent the loss of body heat or the heat that is being generated, towards the environment (Lian et al., 2020). For that, raw materials that are being used, should have good thermal insulation properties (Xiong et al., 2021). It could be achieved by retaining the heat or reflecting back the heat towards the body by using materials that reflects back the infrared heat radiations (Lian et al., 2020). ...
Multilayer needle-punched nonwoven electric heating pads were developed, and their heating behavior was studied by changing the fiber type and needle punch density. Samples were tested by applying the power of 2.9 V over 10 cm² of area for 10 s. IR thermography was used to investigate the electrical heating behavior. Thermographs revealed that heat spreads more significantly in wool samples as compared to cotton. For optimization, scoring of samples was done. Wool samples score is higher than that of cotton. The score increases by increasing the punch density of nonwoven. Heat retains more significantly in wool as compared to cotton. Heating behavior also changes by changing the punch density as by increasing the fiber entanglement heat holds more ominously within the structure. These attributes proposed that the needle-punched nonwoven heating pads are suitable for better electrical heating.
INTRODUCTION Deacetylation of cellulose acetate restores hydroxyl groups on the surface of fibers and improves hydrophilicity. From an environmental point of view, the conventional deacetylation process involves alkalinity and large effluent volume. The goal of this work is to introduce a new eco-friendly bio-treatment process. METHOD In this study, cellulose acetate fabrics were bio-treated with laccase enzyme. Then, the untreated and bio-treated fabrics were dyed with direct and dispersed dyes. Laccase pretreatment improved color strength (16%) and crocking durability. After bio-treatment, the bending rigidity decreased for the warp (17.8) and weft (10.8) directions. The Freundlich model was the best model to describe the adsorption of direct dye onto the untreated fabric. In contrast, the Langmuir model better described the adsorption behavior of bio-treated fabric. RESULT Nernst model was suitable for dispersed dye adsorption. The partition coefficient was increased after laccase treatment. Thermodynamic analysis showed that the dye sorption was endothermic and nonspontaneous. CONCLUSION It was also found that bio-treated fabrics require less external energy. All performed experiments approved the efficiency of the deacetylation process, which led to an improvement in dyeing properties.
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The main aim of this research is to improve the protective thermal performance of fabrics. Flame-resistant fabrics characterizing comparable thermal properties were chosen, cotton fabric with a flame-retardant finish and Nomex® fabric. To improve thermal parameters the coating mixture, based on silica aerogel, was applied on one side of the sample surface. Parameters such as the thermal conductivity, resistance to contact, and radiant heat were determined based on the standards, which set high expectations for the protective clothing. Analysis of the coated fabrics surfaces was conducted based on confocal microscopy. It was found that the coating mixture caused a decrease in thermal conductivity. All the modified fabrics reached 1st efficiency level of protection against contact and radiant heat. The best sample from the point of view of protection against contact and radiant heat was modified cotton fabric with a flame-retardant finish. The coating mixture contained 45 wt% of silica aerogel. Moreover, better adhesion of the coating mixture to the cotton fabric compared with Nomex® fabric was observed.
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Silica aerogel blankets made from silica aerogel integrated into nonwoven fabrics are superinsulative thin materials that can be also used in technical textiles and clothing. Textile technological properties of silica aerogel blanket laminated with a water vapour permeable membrane and polyester warp knitted fabric were studied. The five layers laminate had good mechanical properties, and was resistant to rubbing, was water vapour permeable, hydrophobic and oleophilic material with good thermal insulation. Laminated material is a little too heavy to be used as clothing and also a little rigid. Silica aerogel is prone to crushing during use. The laminate will be softer and more flexible after being used for a certain time. Analysed laminated silica aerogel blanket is suitable for technical textiles, such as sleeping bags, flexible protective covers such as outdoor pillows, wheelchairs pillows for winter conditions, for personal protective garments, etc.
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Silica aerogels are lightweight and highly porous materials, with a three-dimensional network of silica particles, which are obtained by extracting the liquid phase of silica gels under supercritical conditions. Due to their outstanding characteristics, such as extremely low thermal conductivity, low density, high porosity and high specific surface area, they have found excellent potential application for thermal insulation systems in aeronautical/aerospace and earthly domains, for environment clean up and protection, heat storage devices, transparent windows systems, thickening agents in paints, etc. However, native silica aerogels are fragile and sensitive at relatively low stresses, which limit their application. More durable aerogels, with higher strength and stiffness, can be obtained by proper selection of the silane precursors, and constructing the silica inorganic networks by compounding them with different organic polymers or different fiber networks. Recent studies showed that adding flexible organic polymers to the hydroxyl groups on the silica gel surface would be an effective mechanical reinforcing method of silica aerogels. More versatile polymer reinforcement approach can be readily achieved if proper functional groups are introduced on the surface of silica aerogels and then co-polymerized with appropriate organic monomers. The mechanical reinforced silica aerogels, with their very open texture, can be an outstanding thermal insulator material for different industrial and aerospace applications. This paper presents a review of the literature on the methods for mechanical reinforcing of silica aerogels and discusses the recent achievements in improving the strength and elastic response of native silica aerogels along with cost effectiveness of each methodology.
Silica aerogels are highly porous and open-cell materials made of amorphous silica nanoparticles, interconnected in a 3D random network. Silica aerogel-based materials have a great potential as thermal insulation in building thanks to their very low thermal conductivity. However, pure silica aerogels are fragile with low mechanical moduli. Making aerogel composite materials by combining fibers with a pre-gel mixture of a gel precursor or by impregnating a fiber network by such a mixture seems to be a promising way to enhance the mechanical properties of such materials. After drying, the resulting composite is called aerogel blanket. The aerogel blanket is mechanically strengthened, flexible and still has a very low thermal conductivity. Aerogel blankets are usually dried using supercritical process but it is considered as a main drawback for large scale industrialization. The present study uses an innovative micro wave drying. The purpose of this work is to analyze and characterize a handy, light, super-insulating aerogel blanket dried in ambient conditions and see if it could be suitable for building thermal insulation. Two types of blankets have been investigated: the first one with a glass fiber web and the second one with a PET (polyethylene terephthalate) fiber web. Hygro-thermal characterizations were done and show that the aerogel blankets have an excellent thermal conductivity (0.015 W m⁻¹ K⁻¹) and a hydrophobic behavior. The studied aerogel blankets obtained using a new ambient drying process show practically the same characteristics as their counterpart dried with a supercritical process and mark a step forward in the aerogel blanket industrialization.
Simulation and numerical modeling are becoming increasingly popular due to the ability to seek solutions for a problem without undertaking real-life experiments. For the problems of heat transfer, these techniques to generate relevant data by incorporating different changes to the input parameters. Heat transfer property of textile materials is a major concern since it influences comfort properties of clothing. In this paper, numerical simulation was applied to evaluate the heat flux, temperature distributions, and convective heat transfer coefficients of the fibrous insulating materials treated with aerogel. The computational model simulated the insulation behavior of nonwoven fabrics without and with aerogel. Ansys and Comsol were used to model and simulate heat transfer. The simulation was performed assuming laminar flow and since the Mach number was < 0.3, the compressible flow model with Mach number < 0.3 was used. The results of simulation were correlated to experimental measurements for validation. Furthermore, aerogel-treated fabric samples showed better thermal performance. Using this model, the heat transfer properties of the nonwoven fabrics treated with aerogel can be optimized further.
Odour formation in the textile is a serious and embarrassing problem for an individual. The axilla born bacterial species are noted as the main reason for odour formation in axilla. In this research an attempt has been made to identify the odour generating compounds on the textile material after wear trial using gas chromatography and mass spectrum (GC-MS). The result indicates that the worn textile material consisted steroidal fractions of 5a-androst-16-ene-3-one and cholesterol, the major odour forming source from axilla. The results also identified the other important odour forming fatty acids and alcohols like lauric acids, diethyl esters of 1,2-benzenedicarboxylic acid, methyl esters of tetradecanoic acid, 3- methylhexanoic acid, Tetradecanol and acetic acid in axilla worn textile. These components were the derivatives of axilla specific odourous components like phthalic acid, myristic acid, isobutric acid and alcohols. The effect of Terminalia chebula extract finish on the odour formation also analysed and the results shows a considerable reduction in odour causing short chain volatile fatty acids (VFAs) in the worn textile compare to the untreated textile. The analysis also identified more amounts of active components of Terminalia chebula on the fabric surface instead of the odourous components from axilla.
The validity of an improved theoretical model for radiative heat transfer through high-porosity fiber insulation is examined by comparison of experimental measurements with theoretical predictions of heat transfer. Radiative thermal conductivity of an optically thick medium is modeled by a diffusion approximation in which the spectral extinction properties are calculated by utilizing a rigorous treatment of the fiber medium scattering phase function and the composition of the fiber material. A semiempirical model is used to calculate the fiber-matrix conduction. The accuracy of the model is tested by comparison with experimental heat transfer data measured in vacuum for temperatures from 400 to 1500 K for three types of bonded silica fiber thermal insulation materials having different fiber size distributions. The validity of the modeling approach is demonstrated by the excellent agreement between the theoretical predictions and the experimental results. Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
The effects of air movement on the thermal performance of permeable insulation systems are reviewed. The paper addresses air convection in and around porous insulation exclusively. Air infiltration through the insulation, from uncondi tioned to conditioned space is not covered The results reported indicate that the thermal insulation actually provided by a porous material may differ substantially from that which is calculated by simple application of R-values (as reported in the lit erature), due to air convection effects The application of an airtight membrane can reduce this R-value degradation.