a new route for preparation of sodium- silicate-based hydrophobic...
TRANSCRIPT
Chapter - 4
A new route for preparation of sodium-
silicate-based hydrophobic silica aerogels
via ambient pressure drying
A new route for preparation … Chapter - 4 59
Chapter - 4
A new route for preparation of sodium silicate based
hydrophobic silica aerogels via ambient pressure drying
4.1 Introduction
Traditionally, silica aerogels are obtained by removing the liquid from
a wet gel by supercritical drying without any shrinkage that are composed of
highly cross-linked network of silica particles [1]. This method of drying of the
gel is expensive, risky to operate, very tedious as well as time consuming.
However, in 1968, Prof. S. J. Teichner at University Claud, Bernard in Lyon,
France developed a method for producing the silica aerogels within a day
using (albeit costly) silicon alkoxide precursors [2]. But, for commercial
production, though, there is a need to produce the silica aerogels using low-
cost precursor such as sodium silicate as well as for drying the wet gels at
ambient pressure. Pure silica aerogels are hydrophilic and became wet with
humid atmosphere and get deteriorated with time due to adsorption of water
molecule from the humid surroundings because they posses polar –OH
groups on their surface that can take part in hydrogen bonding with H2O [3].
Replacement of H from Si-OH groups by hydrolytically stable Si-R groups
through oxygen bond prevents the adsorption of water and hence results in
hydrophobic aerogels [4]. In continuation of research work on the
hydrophobic aerogels, Schwertfeger and co-workers [5] have produced the
silica aerogels using water glass precursor by costly ion exchange resin
(lengthy and time consuming process) to remove sodium salt following
surface modification and an ambient pressure drying method. However, in
this chapter the ion-exchange method for the removal of sodium salt is
replaced by simply washing the gels with water followed by solvent
exchange, surface modification and drying at ambient pressure.
4.2 Experimental procedure 4.2.1 Sample preparation
Preparation of the hydrophobic silica aerogels by ambient pressure
drying using the sodium silicate solution is depicted schematically in fig. 4.1.
A new route for preparation … Chapter - 4 60
The chemicals used were: sodium silicate solution (Na2SiO3, LOBA,
India, Na2SiO3 content 36 wt%, Na2O:SiO2 = 1:3.33) of specific gravity 1.05
diluted from 1.36 specific gravity as a precursor, tartaric acid (C4H6O6)
(Merck Company, Mumbai) as a catalyst and reactant, trimethylchlorosilane
(TMCS) (Fluka, Pursis grade, Switzerland) as a surface modifier, methanol
(MeOH, CH3OH) and hexane (C6H14) (Merck, India) as solvents. Double
distilled water was used to prepare the sodium silicate and tartaric acid
solutions.
Silica hydrosols were prepared by adding 3.6 M tartaric acid dropwise
to a sodium silicate solution of 1.05 specific gravity while stirring for 5
minutes and kept for gelation at 50 oC in a temperature controlled oven. After
gelation, the gels were aged for 3 h at 50 oC to strengthen the gel network.
The gels were then washed four times with water over the course of 24 h.
Next, methanol was exchanged into the gels and surface modification was
carried out by soaking the gels in a mixture of methanol:TMCS:hexane with
a volume ratio of 1:1:1, respectively, for 24 h. The position of gels in water,
methanol and silylating mixture is shown in fig. 4.2. Notably, gels sank in the
water and methanol but floated in the silylating mixture. After decanting the
Fig. 4.1 Schematic preparation of silica aerogels
Na2SiO3 solution +Tartaric acid
(g) Hydrophobic Silica Aerogel
(d) Salt-free gel
(a) Sol (b) Hydrogel
3 h aging at 50
oC
(c) Aged gel
4 times gel washing with water in 24 h
Exchange with methanol once in 24 h
(e) Alcogel
Gelation
Surface modifi-cation
(f) Surface modified gel
50oC
MeOH:TMCS:Hexane 1 : 1 : 1 volume ratio
Drying at R.T. for 24 h and 50, 200
oC
for 1 h each
A new route for preparation … Chapter - 4 61
solvents, the silylated gels were then ambiently dried for 24 h followed by
heating at 50 oC for 1 h and then 200 oC for 1 h. After cooling of oven up to
room temperature, the aerogels were removed from the oven, and the
resulting aerogels were used for characterization.
4.2.2 Methods of characterization
Bulk density of the aerogels was calculated using a known volume of
the aerogels and dividing by their mass (measured by microbalance, 10-5 g
precision). Volume shrinkage and porosity of aerogels were calculated as
explained in our previous paper [6]. The degree of hydrophobicity was
quantified by measuring the contact angle (θ) of a water droplet placed on
the aerogel surface. It was measured by using a travelling microscope (least
count 0.001 cm) using the formula [7],
where ‘h’ is the height and ‘b’ is the base width of the water droplet on the
aerogel surface. Contact angle was also measured with a contact angle
meter (rame-hart instrument, USA). The surface modification of the aerogels
was confirmed by Fourier Transform Infrared Spectroscopy (FTIR) studies.
Thermal stability of the aerogels was tested by Thermogravimetric Analysis-
Differential Thermal Analysis (TGA-DTA) using a 2960 TA Universal
Instrument, USA.
--- (4.1) θ = 2 tan-1 (2h/b)
Fig. 4.2 Position of gel in (a) water, (b) methanol and (c) silylating mixture
(c) (b) (a)
A new route for preparation … Chapter - 4 62
4.3 Results and discussion
4.3.1 Effect of gel washings with water on optical transmission (%)
Gel washing with water removes trapped salt in the pores of gel
network. The effect of the gel washings with water on the optical
transmission (%) of the aerogels was studied by keeping the Tartaric acid:
Na2SiO3 molar ratio constant at 1.08 and varying it from 1 to 4 in 24 h.
82
84
86
88
90
92
0 1 2 3 4 5
Number of gel washings
Den
sit
y (
g/c
c)
10
20
30
40
50
60
Op
tical
tran
sm
issio
n (
%)
Density (g/cc)
Optical transmission (%)
It was observed that with the increase in number of washings from 1
to 4, the aerogel optical transmission (%) increased from 20 to 50 % while
aerogel density decreased from 0.091 to 0.084 g/cc (see Fig. 4.3). This is
due to the fact that sodium tartarate, which is formed during hydrolysis,
becomes trapped in the pores of the gel network causing a decrease in the
optical transmission and increase in the density of the aerogels. Since the
solubility of sodium tartarate in water is low (29 g/100 ml). Therefore,
multiple washings are required to remove the salt from the pores of the gel to
enhance the transparency of aerogels. The best method of quantitative
Fig. 4.3 Effect of number of gel washings on the optical transmission (%) and density
A new route for preparation … Chapter - 4 63
extraction of solute from one solvent to another is to employ the several
washings instead of one [8]. The quantity of Na+ ions present in the pores of
the aerogels was estimated by Atomic Absorption spectroscopy (AA) and
found to be 1.23 %, while based on stoichiometry the hydrosol is known to
contain 36.5 % Na+ ions. Hence, washing the gel with water after aging
decreases Na+ ions percentage from 36.5 to 1.23 producing the transparent
aerogels.
4.3.2 Effect of hexane (or methanol) percentage in silylating mixture
In silylation process, hexane is used as an inert dilution medium and
MeOH is used to eliminate remained water from the pores of alcogel. The
effect of hexane percentage on physical properties of the silica aerogels was
studied by varying it from 0 to 100 % while keeping the Na2SiO3:H2O:Tartaric
acid:TMCS molar ratio constant at 1:146.67:0.86:9.46 (Table 4.1).
Fig. 4.4 shows the gel position for mixtures containing 0, 50 and 100
% hexane in methanol. In panel (a) the gel did not float for 0 % hexane,
while in panels (b) and (c) gel floated completely in the solution with 50 %
hexane and partially floated in 100 % hexane respectively. This is because,
for complete silylation of the gel, an inert medium (hexane) is one of the
requirements.
As shown in fig. 4.5, it was observed that volume shrinkage (%) and
density of the silica aerogels decreased with an increase in hexane
concentration to 50 % in silylation mixture, and then increased with a further
Fig. 4.4 Position of gel in (a) 0%, (b) 50% and (c) 100% hexane (or methanol)
(c) (b)(a)
A new route for preparation … Chapter - 4 64
increase in hexane concentration up to 100 %. The reason for this is that at
0 % hexane (100 % MeOH) due to absence of inert medium, which reduces
the reaction rate of TMCS with pore water, the silylation of the surface does
not occur systematically. Also, the low surface tension of hexane helps to
reduce the capillary pressure which is associated with drying shrinkage.
Hence due to incomplete surface modification more shrinkage occurs in the
gels producing the dense aerogels. And at 100 % hexane (0 % MeOH) due
to absence of MeOH, which facilitates polar intermediates in silylation,
silylation does not occur as effectively and again the density of the aerogels
increases. The effects of presence of hexane and MeOH in mixture are
dependent on each other. On the other hand, at 50 % hexane and 50 %
MeOH, sufficient surface modification occurs resulting in low shrinkage and
low density (0.084 g/cc) aerogels.
70
100
130
160
190
220
0 25 50 75 100Hexane percentage
Den
sit
y (
g/c
c)
20
30
40
50
60
70
Vo
lum
e s
hri
nkag
e (
%)
Density (g/cc)
Volume shrinkage (%)
Fig. 4.5 Effect of hexane percentage in silylating mixture on density and volume shrinkage (%) of the aerogels
A new route for preparation … Chapter - 4 65
4.3.3 Influence of Tartaric acid: Na2SiO3 molar ratio (A)
The influence of Tartaric acid: Na2SiO3 molar ratio (A) on the physical
properties of the silica aerogels was studied by varying it from 0.27 to 1.2
(Table 4.2). The gel aging period and Na2SiO3:H2O:TMCS molar ratio were
kept constant at 3 h and 1:146.67:9.46, respectively.
During the gel formation the hydrolysis and condensation reactions
take place as follows,
Sr. No.
Variation Porosity
(%)
Pore volume (cc/g)
Contact angle
(θ, deg.)
Thermal conductivity
(W/m.K)
A Effect of percentage of hexane (or methanol) (%)
1 0 89.8 4.7 132 0.124
2 25 94.8 9.8 144 0.098
3 50 95.5 11.4 146 0.090
4 75 94.8 9.9 145 0.098
5 100 94.5 9 142 0.102
B Effect of aging period (hours)
1 0 92.4 6.4 134 0.118
2 1 92.8 6.8 136 0.113
3 2 93.1 7.2 137 0.112
4 3 94.7 9.5 143 0.099
5 4 92.3 6.3 135 0.117
C Effect of weight % of silica
1 1.5 89.5 4.5 130 0.125
2 2.3 89.8 4.7 132 0.124
3 3 94.7 9.5 143 0.099
4 4 95.5 11.4 146 0.090
5 5 95.7 11.8 146 0.090
6 6 91.7 5.8 134 0.120
7 8 89.4 4.5 130 0.126
Table 4.1 Porosity, pore volume, contact angle and thermal conductivity of silica aerogels with variation of the sol-gel parameters
A new route for preparation … Chapter - 4 66
80
110
140
170
200
0.18 0.43 0.68 0.93 1.18
Den
sit
y (
g/c
c)
0.2
1
1.8
2.6
3.4
Lo
g [
Gela
tio
n t
ime
(m
in)]
Density (g/cc)Log [Gelation time (min)]
The amount of catalyst added strongly affects the gelation time and
density of the silica aerogels. As shown in fig. 4.6, it was observed that with
Hydrolysis: OH
OH
Si HO OH Na2SiO3 + H2O
Tartaric acid
C4H6O6
Sodium silicate solution Silicic acid Sodium tartarate
+ C C NaOOC COONa
HO
OH H
H
---(4.2)
Condensation:
---(4.3)
Silicic acid
+
OH
Si
OH
HO OH
OH
Si
OH
HO OH + H2O Si Si
OH
OH
O OH
OH
HO
OH
Fig. 4.6 Effect of Tartaric acid:Na2SiO3 molar ratio on the gelation time and density
Tartaric acid:Na2SiO3 molar ratio
A new route for preparation … Chapter - 4 67
an increase in A to 0.51, the gelation time decreased and density increased.
This is because, with increase in catalyst concentration, the rate of
hydrolysis and condensation reactions increases, and as a result, silica
clusters aggregate at a relatively faster rates to form a three dimensional,
dense silica network in short time [9]. Further, the gelation time increased
with increase in A (>0.51), however, possibly since the silica particles are
negatively charged and, therefore, particles crosslinking is slowed down by
charge repulsion. At lower A (<0.51) the gelation time may be high because
silica particles are positively charged, and hence, repel each other [9]. The
density of aerogels decreased with increase in A up to 1.08 due to the
presence of excess tartaric acid which enhances the rate of hydrolysis and
condensation reactions that lead to cluster formation, in turn resulting in
denser aerogels [10]. At A~1.08, this is believed to occur because of
complete hydrolysis and condensation of particles, formation of uniform
network takes place, which led to low density (0.100 g/cc) aerogels.
Sr. No.
Variation
Volume shrinkage
(%)
Porosity (%)
Pore volume (cc/g)
Contact angle (deg.)
Thermal conductivity
(W/m.K)
D Effect of Tartaric acid/Na2SiO3 molar ratio
1 0.27 70 94 8.6 140 0.105
2 0.51 79 89 4.6 132 0.124
3 0.72 59 93 7.5 137 0.110
4 0.90 50 94.5 9.2 142 0.102
5 1.08 45 94.7 9.5 143 0.099
6 1.20 72 89.5 4.5 130 0.125
E Effect of TMCS percentage (%)
1 20 59 92 6.1 133 0.118
2 26.67 41 94.5 9.2 140 0.100
3 33.33 24 95.6 11.4 146 0.090
4 40 9 95.9 12.3 146 0.089
Table 4.2 Effect of Tartaric acid:Na2SiO3 molar ratio and TMCS percentage on physical properties of silica aerogels
A new route for preparation … Chapter - 4 68
4.3.4 Effect of gel aging period
Aging a gel before drying helps to strengthen the network and thereby
reduces the risk of the fracture [11]. The effect of gel aging period on the
volume shrinkage (%) and density of aerogels was studied with variation
from 0 to 4 h (Table 4.1) by keeping Tartaric acid:Na2SiO3 molar ratio
constant at 1.08. At lower and higher gel aging periods (<3h<) the volume
shrinkage (%) and density increased while at 3 h aging period volume
shrinkage and density decreased as shown in fig. 4.7. This is because
during the gel aging, a number of chemical and physical changes take place,
such as condensation of surface –OH groups, syneresis, coarsening and
segregation, all of which strongly affects the properties of the aerogels [12].
The lower volume shrinkage (%) and bulk density of the aged gels indicates
that they were coarse, means the dissolution and reprecipitation driven by
differences in solubility between surfaces with different radii of curvature
occurs. This causes growth of necks between particles, so the capillary
pressure was lower and the aerogels were probably stiffer and stronger.
95
110
125
140
155
0 1 2 3 4Gel aging period (hours)
De
nsit
y (
g/c
c)
40
46
52
58
64
Vo
lum
e s
hri
nkag
e (
%)
Density (g/cc)Volume shrinkage (%)
Fig. 4.7 Effect of gel aging period on density and volume shrinkage
A new route for preparation … Chapter - 4 69
4.3.5 Influence of weight % of silica (B)
The influence of the weight % of silica (B) in the hydrosol on the
physical properties of the silica aerogels was studied by varying it from 1.5 to
8 wt % (Table 4.1). The aerogels were aged for 3 h keeping Tartaric
acid:Na2SiO3 molar ratio constant at 1.08. From fig. 4.8, it can be seen that
as value of B was increased to 5, gelation time decreased. As B was further
increased (to a value of > 5), the gelation time remained constant. The
volume shrinkage (%) of aerogels decreased and then increased with
increase in B value from 5 to 8 wt %. This is likely since, at lower B value,
lower silica content in the hydrosol slows the rate of hydrolysis and
condensation reactions, resulting in longer gelation time and a weaker silica
network. Shrinkage during the drying process in turn increases due to weak
silica network. At higher B values, the rates of hydrolysis and condensation
increase and cluster formation takes place, leading to shorter gelation time
and higher silica content per unit volume (i.e., a denser aerogel).
10
25
40
55
70
85
100
1 2.5 4 5.5 7 8.5Weight % of silica
Vo
lum
e s
hri
nkag
e (
%)
0.3
0.9
1.5
2.1
2.7
3.3
Lo
g [
Gela
tio
n t
ime (
min
)]
Volume shrinkage (%)
Log [Gelation time (min)]
Fig. 4.8 Effect of weight % of silica on gelation and volume shrinkage
A new route for preparation … Chapter - 4 70
During the drying process, evaporation of a liquid from the gel creates
a capillary tension (P) in the liquid. This tension is balanced by the
compressive stresses on the solid network, causing shrinkage of the dried
gel. The stresses during the drying depend on the interfacial energies
(surface tension of pore liquid), the bulk modulus of the network and the
pressure gradient in the liquid. According to Darcy’s law, the liquid flow (J)
through gel is given by
where D is the permeability of the gel, ∇P is the pressure gradient, and ηL is
the viscosity of the liquid. During liquid evaporation, the pressure (P) in the
liquid phase of the gel is related to the volumetric strain rate of the gel (έ) by
The resulting stress in the solid phase of a gel plate of thickness L is given
by [13].
where CN ≡ (1-2N)(1-N), N is Poisson’s ratio, and is the liquid evaporation
rate. Equation (4.6) indicates that the stress is proportional to the thickness
of the gel plate and the liquid evaporation rate. At the same time, if the
permeability is high, then the stress is small. Hence at B~4, due to high
permeability and low stress, the shrinkage of aerogel decreased resulting in
low density silica aerogels.
As shown in fig. 4.9, at both lower and higher B value (<4<) the
thermal conductivity of the aerogel is more because of greater shrinkage and
thus higher density of the aerogel. At B~4, higher pertinent hydrolysis and
condensation reactions result in less shrinkage and thus low density (0.084
g/cc) and low thermal conductivity (0.09 W/m.K) of the
aerogel
J = (D/ηL)∇P, ---(4.4)
---(4.6) σx ≈ CN(LηLVE/3D), .
.
VE
---(4.5) (D/ηL)∇2P = - έ
A new route for preparation … Chapter - 4 71
50
90
130
170
210
250
1 2.5 4 5.5 7 8.5Weight % of silica
De
ns
ity
(g
/cc
)
0.07
0.085
0.1
0.115
0.13
Th
erm
al c
on
du
cti
vit
y (
W/m
.K)
Density (g/cc)
Thermal conductivity
(W/m.K)
4.3.6 Effect of TMCS percentage on silylation
Drying of wet gels without surface modification causes the shrinkage
of the gel due to continuous condensation of end –OH groups leading to
dense aerogels. This is because of capillary pressure exerted by pore fluid
evaporation causes irreversible shrinkage in the aerogels. Capillary collapse
in wet gel can be prevented by replacing hydrophilic –OH groups on surface
of gel backbone with non-reactive Si–CH3 species by means of surface
modification with silane coupling agents such as TMCS. The capillary
pressure generated during drying is given by Laplace equation [14].
where γLV is the liquid-vapor surface tension, θ is the contact angle of the
liquid with a pore wall and rp is the pore radius. The negative sign is due to
--- (4.7) rp
γLV cos θ P = -2
Fig. 4.9 Effect of weight % of silica on density and thermal conductivity
A new route for preparation … Chapter - 4 72
the negative radius of curvature of the meniscus at the liquid-vapor interface.
TMCS minimizes the shrinkage of the gel through the reduction in surface
tension of the solvent and contact angle between the solvent and surface of
silica network [15]. Hence the hydrophobic aerogels are obtained by
replacing the Hs from end capped silanol groups with non-polar hydrolytically
stable –Si-(CH3)3 groups [16] using TMCS as follows,
Percentage of TMCS in silylating mixture was found to be a
dominating parameter that affects the silylation and hence physical
properties of silica aerogels (Table 4.2). The effect of TMCS percentage on
silylation was studied by varying concentration of TMCS from 20 to 40 %
while keeping Na2SiO3:H2O:Tartaric acid molar ratio constant at
1:146.67:0.86. Fig. 4.10 shows the decrease in the density and % of optical
transmission of the aerogels with increase in TMCS percentage. This is likely
due to the fact that at lower percentages of TMCS (< 33 %), incomplete
silylation occurs and unsilylated –OH groups can undergo condensation in
turn causing more shrinkage and thus denser aerogels. Furthermore,
because of smaller particle and pore sizes caused by increased shrinkage,
the % of optical transmission of these aerogels was higher. At higher
percentages of TMCS (> 26.67 %), complete modification of silanol groups
to non-polar, hydrolytically stable –Si(CH3)3 groups occur and causes
repulsion between end capped – Si(CH3)3 groups. Because of this, spring
back of the gels solid network occurs, facilitating an increase in the aerogel
volume with big pores and thus low-density and semitransparent aerogels.
At higher percentage of TMCS (> 33 %), the excess TMCS deposited in the
pores causing opacity of the aerogels. So, for further studies 33 % TMCS
was used. The hydrophobicity of aerogels increased with TMCS percentage,
which is quantified by contact angle measurement as shown in fig. 4.11.
Surface modification
+
Silica surface Trimethylchlorosilane
OH Si
OH Si
O
Si (CH3)3 Cl
Si (CH3)3 Cl
Modified silica surface
(CH3)3
(CH3)3 Si
Si O Si
O Si
O + 2HCl ---(4.8)
A new route for preparation … Chapter - 4 73
70
95
120
145
170
18 22 26 30 34 38 42TMCS percentage
Den
sit
y (
g/c
c)
15
30
45
60
75
Op
tical
tra
nsm
issio
n (
%)
Density (g/cc)
Optical transmission (%)
The sphericity of a water drop on a solid surface is characterized by
the contact angle (θ). Greater is the hydrophobicity of the solid surface,
higher is the contact angle and larger would be the sphericity of the water
drop. Under equilibrium conditions, the relation between the solid-vapour
(γSV), solid-liquid (γSL) and liquid-vapour (γLV) interactions at the intersection
of the three phases, is given by the Young’s equation [17]:
133 o 146
o
Fig. 4.11 Water droplets on the aerogel surfaces for (a) 20 %, (b) 33 % TMCS
(a) (b)
Fig. 4.10 Effect of TMCS percentage on density and optical transmission
A new route for preparation … Chapter - 4 74
For a hydrophobic surface, θ > 90o and therefore, from the above
equation it follows that solid-liquid (γSL) interaction is greater than solid-
vapour (γSV) interaction. Fig. 4.11 (a & b) show the water droplets placed on
the hydrophobic silica aerogel surfaces modified with 20 % and 33 % TMCS
with contact angle (θ) is 133 o and 146 o, respectively. It is observed that the
contact angle decreases with decrease in TMCS percentage.
4.3.7 Effect of silylation period
Silylation period plays a significant role in the surface modification of
gels. The effect of the silylation period on the physical properties of silica
aerogels was studied by varying the silylation period from 6 to 24 h by
keeping Na2SiO3:H2O:Tartaric acid:TMCS molar ratio constant at
1:146.67:0.86:9.46.
60
110
160
210
260
310
3 8 13 18 23 28silylation period (hours)
Den
sit
y (
g/c
c)
137
139
141
143
145
147
Co
nta
ct
an
gle
(d
eg
ree
)
Density (g/cc)Contact angle (degree)
Fig. 4.12 shows with increase of the silylation period, density of the
aerogels decreased and hydrophobicity increased. This is believed to be
Fig. 4.12 Effect of Silylation period on density and contact angle
γSV = γSL + γLV cos θ --- (4.9)
A new route for preparation … Chapter - 4 75
because, for shorter periods of silylation, incomplete surface modification of
wet gels occurs leading to dense and less hydrophobic aerogels. For the
longer silylation periods the bulk density decreases because of complete
surface modification of the gel, increases the hydrophobicity of the aerogel.
The effect of the silylation period on the porosity and thermal
conductivity of silica aerogels is depicted in fig. 4.13. Increasing silylation
period to 24 h led to an increase in porosity and decrease in the thermal
conductivity. This may be because, after higher silylation periods, more
complete surface modification facilitates better spring back. The thermal
conductivity, which depends on the porosity of aerogels, is lower because
there is less solid content per unit volume of the aerogel [18]. Spring back
implies that the gels densify and then undensify upon drying [19].
83
86
89
92
95
98
4 9 14 19 24Silylation period (hours)
Po
rosit
y (
%)
0.08
0.095
0.11
0.125
0.14
Th
erm
al
co
nd
ucti
vit
y (
W/m
.K)
Porosity (%)Thermal conductivity (W/m.K)
Fig. 4.14 shows the variation in % of volume change with drying
temperature. It has been found that the % of volume change is more up to
100 oC and then decreased above 100 oC and remained constant above 150
oC, clearly indicating the effect of spring back.
Fig. 4.13 Effect of Silylation period on density and contact angle
A new route for preparation … Chapter - 4 76
0
20
40
60
80
100
0 50 100 150 200 250
Temperature (oC)
% o
f v
olu
me c
ha
ng
e
Further, the surface modification of the aerogel with the variation of
silylation period from 6 to 24 h is confirmed by FTIR spectra of the aerogels
as shown in fig. 4.15.
Fig. 4.15 Infrared spectra of aerogels with the variation of the Silylation period, (a) 6 h, (b) 12 h, (c) 18 h, and (d) 24 h.
Fig. 4.14 Effect of drying temperature on % of volume change
Temperature (oC)
-OH
Wave number (cm-1)
% o
f o
pti
cal
tran
sm
issio
n (
A.U
.)
-OH
A new route for preparation … Chapter - 4 77
It was observed that with increase in the silylation period, the intensity of –
OH bond peaks at 1600 and 3400 cm-1 [20] decreased and the peaks related
to C-H at 2960, 1450 cm-1and Si-C at 840 and 1260 cm-1 increased [21].
Thus, effect of surface modification with silylation period was clearly
observed.
Thermal stability of the hydrophobic silica aerogels silylated with 33%
TMCS for 24 h was tested using TGA-DTA as shown in fig. 4.16. It shows
that, the weight loss with an exothermic peak at 435 oC, was considered to
correspond to the decomposition of surface -CH3 groups [22]. The gradual
weight loss at higher temperatures could be attributed to the dehydration
and condensation of silanols. Thus, it clears that the silica aerogels have
high heat-resistance up to around 435 oC.
Fig. 4.16 TGA-DTA of hydrophobic silica aerogel
100
Weig
ht
(%)
435oC
Tem
pera
ture
dif
fere
nce (
oC
/mg
)
Temperature (oC)
TGA
DTA
200 300 400 500 600
100
88
90
92
94
96
98
-1
0
1
2
3
0
A new route for preparation … Chapter - 4 78
4.4 Conclusions
Superhydrophobic, low density and semitransparent silica aerogels
were obtained using a sodium silicate precursor by an ambient pressure
drying method. Both, the gel washings with water and sol-gel parameters
have striking effects on the physical properties of the silica aerogels
produced by this technique. It was observed that for more gel washing times,
the optical transmission (%) of the aerogel improved. Increasing the silylation
period and TMCS percentage reduces the density of the aerogels. Also, the
50 % hexane (or methanol) in the silylating mixture produced the lowest
density aerogels. From FTIR spectra of the aerogels, it was observed that
the intensity of –OH bond at 1600 and 3400 cm-1 decreased and C-H bond
at 2960, 1450 cm-1, Si-C bond at 840 and 1260 cm-1 increased with increase
in the silylation period. The TGA-DTA showed that the silica aerogels were
thermally stable up to 435 oC. The semitransparent aerogels with density
~0.084 g/cc, porosity ~95 %, thermal conductivity ~0.090 W/m.K and
hydrophobicity ~146o were obtained for the molar ratio of
Na2SiO3:H2O:Tartaric acid:TMCS at 1:146.67:0.86:9.46 respectively, with 4
times gel washing with water in 24 h, 3 h aging, 24 h silylation period and 50
% hexane (or methanol) in silylating mixture by ambient pressure drying
method.
A new route for preparation … Chapter - 4 79
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