Diffusion and Transport Behaviors of Aliphatic Probe Molecules Through Untreated and Treated
Metakaolin Filled Natural Rubber Composites
Diffusion and Transport Behaviors of
Aliphatic Probe Molecules Through
Untreated and Treated Metakaolin
Filled Natural Rubber Composites
S. Rohini Thimmaiah1,2, Siddaramaiah*1
1Department
of Polymer Science and Technology, Sri Jayachamarajendra College of
Engineering, Mysore - 570 006, Karnataka, India
2Department of Chemistry, Coorg Institute of Technology, Ponnampet - 571216,
Karnataka, India
Received: 30 April 2013, Accepted: 17 August 2013
Summary
Molecular transport of a series of aliphatic probe molecules (hexane, heptane and
decane) through untreated and silane treated metakaolin (MK) filled cured natural
rubber (NR) composite membranes has been studied in the temperature range
25-60°C using the sorption-gravimetric method. The coefficients of sorption
(S), diffusion (D), and permeation (P) have been calculated for elastomeric
membrane-aliphatic penetrant systems. Significant increases in the diffusion
and permeation coefficients were observed with increase in the temperature
of sorption experiments. In all of the elastomeric-probe molecule systems, the
transport phenomenon was found to follow the non-Fickian mode of transport.
From the temperature dependence of diffusion and permeation coefficients, the
Arrhenius activation parameters, such as the activation energy for diffusion (ED)
and permeation (EP) processes, have been estimated. Furthermore, the sorption
results have been interpreted in terms of thermodynamic parameters such as
change in enthalpy (ΔH) and change in entropy (ΔS).
Keywords: NR, Untreated and treated metakaolin, Molecular transport, Aliphatic
penetrants, Thermodynamic parameters
INTRODUCTION
Naturally occurring mineral reinforced composites have applications in
automobile industries [1, 2]. The diffusion and transport behavior of filled rubber
*e-mail [email protected]
©Smithers
Rapra Technology, 2013
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S. Rohini Thimmaiah, Siddaramaiah
composites depends upon the nature of fillers, volume fraction of fillers, degree
of adhesion and the compatibility between polymer matrix and filler [3, 4].
Liquid swelling experiments of rubber composites are important for analyzing
their service performance in contact with solvents [5, 6]. Swelling of rubber
vulcanizates in a number of solvents have been studied [7, 8]. The transport
properties of rubber components play an important role in the manufacture
of oil seals, gaskets, hoses and protective apparels.
Lovely et al. [9] observed swelling behaviour and reported that the uptake
of aromatic solvents is higher than aliphatic solvents for NR composites.
Thomas and Sunny reported that the diffusion and transport behaviors of
organic solvents through lignin filled natural rubber (NR) composites [10]
followed the Fickian’s law of diffusion. Unnikrishnan and Thomas [11] studied
the molecular transport of benzene and methyl-substituted benzenes through
filled NR sheets. Boonstra and Dannenberg [12] study on the equilibrium
swelling data of filled NR and a number of synthetic rubbers in a variety of
solvents indicates that fillers like carbon black cause a reduction in swelling
of the membranes depending on the volume loading of the filler; however,
the non-carbon black filler causes a reduction in rubber swelling that is not
dependent on the filler content. Studies of Stickney and Mueller [13] on the
kinetics of swelling of carbon black-filled styrene-butadiene rubber (SBR)
vulcanizates in isooctane indicate that for rubber vulcanizates, diffusivity
increases with the concentration of the penetrant.
The transport of liquids through untreated and silane treated metakaolin (MK)
filled natural rubber has not been reported in the scientific literature; and this
work reports the effect of MK filler content without and with silane treatment,
nature of penetrants and temperature on the transport behaviors of n-alkanes
through vulcanized natural rubber/MK composites.
EXPERIMENTAL
Materials
Natural rubber conforming to the standard African Rubber grade was obtained
from the Rubber Research Institute of Nigeria, Benin City, Nigeria. Meta kaolin
(MK) was obtained from Speciality Minerals, Baroda, India. The average
particle size and specific gravity of MK filler is 1.5 µm and 2.5 respectively. It
was dried before compounding. The coupling agent [(Si-69): bis(3-triethoxy
silyl propyl) tetrasulfide] was imported from Degussa-Huls, Germany. Hexane,
heptane and decane all are of analytical reagent grade, which were supplied
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Diffusion and Transport Behaviors of Aliphatic Probe Molecules Through Untreated and Treated
Metakaolin Filled Natural Rubber Composites
from Sd. fine chemicals, Mumbai. Some of the typical properties of the probe
molecules are listed in Table 1.
Table 1. Some physical properties of n-alkane penetrants at 25°C
Penetrants
Mol. vol. Density
(cm3/mol) (g/cm3)
Viscosity
(µPa)
ε
Solubility
parameter
(cal/cm3)1/2
Boiling point
(°C)
Hexane
115.2
0.658
294
1.88
7.3
69
Heptane
131.6
0.700
386
1.93
7.4
98
Decane
195.9
0.728
920
1.99
7.7
174
Treatment of MK by Coupling Agent
The silane coupling agent (2 g) was mixed with ethyl alcohol (100 ml) to
make a solution for applying to the filler (100 g) [14, 15]. The filler was mixed
with the solution of coupling agent in ethanol with stirring to ensure uniform
distribution of the coupling agent; mixing was continued for 30 min. The treated
MK filler was then dried at 90°C in an oven for about 5 h to allow complete
evaporation of the ethanol.
Compounding of NR with MK Filler
Compounding of rubber formulations was carried out in a miniature two rollmixing mill as per ASTM D 3182. The typical recipe used to make the NR/MK
composites is listed in Table 2. Rubber was first masticated to form a sheet
Table 2. Typical formulations of NR/MK composites
Formulation, phr
1
2
3
4
5
NR
100
100
100
100
100
Zinc oxide (ZnO)
5.0
5.0
5.0
5.0
5.0
Stearic acid
1.0
1.0
1.0
1.0
1.0
Antioxidant
1.5
1.5
1.5
1.5
1.5
Zinc dimethyl
dithiocarbamate (ZMDC)
0.5
0.5
0.5
0.5
0.5
Sulphur
1.5
1.5
1.5
1.5
1.5
Mercapto benzothiazole
disulphide (MBTS)
1.0
1.0
1.0
1.0
1.0
Carbon black
50
45
40
35
30
Metakaolin
0
5
10
15
20
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S. Rohini Thimmaiah, Siddaramaiah
on the front roll of the mill and the compounding ingredients were sequentially
added in the order: ZnO, stearic acid, fillers (carbon black/MK), antioxidant,
MBTS and sulfur. Sheets were processed by passing through a small nip of
the rolls. The compounded material was kept in a compression mold, and
was cured under pressure and temperature (dimensions of 150 mm x 150 mm
x 3 mm). The temperature of the compression mold was kep at 170°C and
the curing was carried out for 8 min. The molded sheets were subjected to
conditioning for 24 hr before characterization.
Sorption Measurements
The circularly cut and dried samples weighing initially WO, were placed in
screw-tight test bottles containing 15-20 ml of the respective solvents. These
were periodically removed; the surface adhered liquid drops were dried by
carefully pressing the samples in between soft filter papers and weighed on
a digital Mettler balance, model AE 240 (Switzerland) within the precision of
± 0.01 mg. Samples reach equilibrium saturation within 30-36 hr and retained
the same for next 48 hours. The percent weight gain Q (%) during solvent
sorption was calculated by the following equation:
Q(%) =
(Wt − W0 )
W0
x100 (1)
Results and Discussion
Sorption
The sorption curves for n-hexane to n-decane for untreated and silane treated
MK filled NR membranes are presented in Figure 1. It shows the maximum
solvent uptake in decane and minimum solvent uptake for the hexane for all
membranes at all temperatures. The mol% sorption coefficient (S) obtained
from the true equilibrium values for the sorption process (calculated as moles
of solvent sorbed per 100 g of the polymer) in the investigated temperature
intervals i.e., from 25°C to 60°C are summarized in Table 3. The figure
demonstrates that the equilibrium sorption data follows the trend; hexane <
heptane < decane at 25°C and this data increases systematically with the
increasing temperature. The increasing order of the molecular volume, dipole
moment and solubility parameter of these solvents are; hexane < heptane <
decane (Table 1). This result clearly indicates that the sorption data strongly
depends on the solubility parameter of the solvent.
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Figure 1(a). The mol% uptake of different probe molecules in 10% MK filled NR
membrane as function of square root of time at (a) 25°C and (b) 60°C
Figure 1(b). The mol% uptake of different probe molecules in 10% silane treated MK
filled NR membrane as function of square root of time at (a) 25
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Figure 1(b) cont'd. The mol% uptake of different probe molecules in 10% silane
treated MK filled NR membrane as function of square root of time at (b) 60°C
The effect of MK filler content with and without silane treatment on the sorption
behaviors of the NR/MK membranes is shown in Figure 2. The solvent uptake
values of the NR/MK formulations increase with increase in MK filler content.
Sorption is a surface phenomenon. From the figure it is also clear that the
treated filler loaded NR composites showed less sorptivity as compared to
untreated MK loaded NR composites. This is because silane treated filler
increases the rubber-filler interaction, which leads to reduction in void content,
and hence leads to reduction in solvent uptake behavior.
Figure 3 represents typical plots for 10% untreated and treated MK filled NR
composites in decane at different temperatures. As the temperature increases
the extent of sorption of solvents by the membrane also increases. This
shows the temperature dependence of sorption. This confirms that at higher
temperatures increase in free volume occurs due to increased movement of
the chain segments of the elastomers [16]. Various parameters are critical in
the interpretation of sorption results; these include temperature, penetrant
size, shape, solubility parameters and polarity in addition to sample history
[17]. Johnson and Thomas [18] and Igwe et al. [19] in their sorption studies
reported that the sorption coefficient increases with increase in sorption
temperature. From these figures it is also clear that treated filler shows less
sorptivity values at all temperatures under investigation, as compared to
untreated MK loaded NR systems.
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MKC20
MKC10
MK20
MK10
MK0
Filler content
(phr)
3.5
4.7
4.9
25
40
60
3.3
4.1
40
60
4.32
60
3.2
4.17
40
25
2.97
2.47
60
25
1.62
2.20
25
2.68
2.69
40
60
40
1.54
Dx
cm2/s
107
25
Temp.
(°C)
1.38
1.35
1.31
1.07
1.05
1.03
1.59
1.56
1.51
1.49
1.46
1.43
1.44
1.41
1.39
S (g/g)
Hexane
6.98
6.61
4.99
4.45
3.51
3.21
6.87
6.51
4.48
3.68
3.66
2.31
3.87
3.77
2.14
Px
(cm2/s)
107
4.6
4.3
3.2
3.8
3.2
2.9
3.66
3.58
2.69
2.47
2.18
1.35
2.54
2.35
1.39
Dx
cm2/s
107
1.38
1.37
1.33
1.12
1.09
1.06
1.59
1.57
1.54
1.49
1.48
1.46
1.46
1.44
1.41
S (g/g)
Heptane
6.43
6.03
4.62
4.25
3.48
3.07
5.82
5.62
4.14
3.68
3.43
1.97
3.72
3.38
1.96
Px
(cm2/s)
107
4.13
3.85
2.37
3.81
2.64
2.33
4.14
3.32
2.23
2.39
2.16
1.14
2.43
2.07
1.21
Dx
cm2/s
107
1.39
1.37
1.35
1.17
1.15
1.12
1.64
1.61
1.58
1.54
1.52
1.49
1.53
1.51
1.48
S
(g/g)
Decane
5.93
5.50
3.73
4.15
3.03
2.60
5.79
5.31
3.52
3.53
3.28
1.69
3.71
3.12
1.79
P x 107
(cm2/s)
Table 3. Diffusion, sorption and permeation coefficients of silane treated and untreated MK filled NR in aliphatic
penetrates at different temperatures
Diffusion and Transport Behaviors of Aliphatic Probe Molecules Through Untreated and Treated
Metakaolin Filled Natural Rubber Composites
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S. Rohini Thimmaiah, Siddaramaiah
Figure 2. The percentage mass uptake versus square root of time for10% untreated
and treated MK in NR with hexane penetrant at different temperatures
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Figure 3. The percentage mass uptake vs square root of time for 10% untreated and
treated MK in NR composite with decane at different temperatures (a) and (b)
Diffusion
The Fickian diffusion theory is used to calculate the diffusion coefficient (D)
of liquid in NR membranes [20, 21]. According to Fick’s law of diffusion,
mass transport occurs in the direction of concentration gradient and this
generates concentration profiles in the polymer membrane. The expression
for the concentration independent D from a solution of one-dimensional Fick’s
equation, is as follows:
D = π (h θ/4M∞)(2)
where, h is the sample thickness; θ is the slope of the initial line of sorption
curves, i.e., before the attainment of 50% of equilibrium uptake; and M∞ is
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S. Rohini Thimmaiah, Siddaramaiah
equilibrium mole uptake. The calculated values of diffusion coefficient (D)
for NR/MK composite membranes are tabulated in Table 3. A systematic
decrease in D values from n-hexane to n-decane has been observed at all
temperatures suggesting a dependence of diffusivity on molecular size/solubility
parameter of n-alkanes beside the structural characteristics of the elastomer.
This inverse dependence of D on molecular volume of alkanes proves the
conjecture that larger molecules in a related series of liquids occupy larger
free volumes leading to hindered diffusivities through the elastomer matrix.
Table 3 also reveals that the D values generally increase with increase in
sorption temperature for any particular solvent and filler nature considered.
This is supported by literature data published elsewhere [18]. Johnson and
Thomas [18] have reported an increase in the diffusion coefficient with increase
in sorption temperature. The table shows higher D values for silane treated
MK filler. This is because silane treated filler enhances the interfacial adhesion
between filler and rubber.
The transport behaviour of small molecules through elastomers generally
occurs through a solution diffusion mechanism, i.e., the solvent molecules
are first sorbed by the polymer followed by diffusion through the polymer
membrane. The net diffusion through polymer depends on the difference in
the amount of penetrant molecules between the two successive layers. Hence,
the permeability [22]: P = D x S, where, D is diffusivity and S is sorptivity. The
obtained P data are also included in Table 3. It was generally observed that
the results of P followed the same pattern as those of D in the investigated
temperature range. Johnson and Thomas [18] also made similar kind of
observation for epoxidized natural rubber with n-alkanes. Igwe et al. [19] also
reported that the aromatic solvents uptake by polypropylene (PP) films found
that the permeability increased with increase in dipole movement, molecular
mass volume, and molecular mass of the aromatic solvents. The table shows
that the higher P values for silane treated MK filler loaded formulations.
Transport Behavior
The initial sorption results before 50-55% equilibrium sorption have been
fitted to the empirical equation [23, 24];
log Mt/M∞ = log K + n log t (3)
The slope of the plots log Mt/M∞ vs. log t gives the value of n, indicating the
mechanism of transport and its y-intercept is the value of K. According to the
n values obtained from the above equation, three basic modes of transport
are distinguished. If n = 1/2 the diffusion mechanism is Fickian, in that case
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Metakaolin Filled Natural Rubber Composites
the rate of diffusion of permeant molecules is much less than the polymer
segment mobility. If n = 1, the mechanism is non-Fickian, and this may be
considered in systems in which permeant diffusion rates are much faster
than polymer relaxation process. If n lies between 1/2 < n < 1 the diffusion
mechanism is non-Fickian and is anomalous, it occurs when the permeant
mobility and polymer segment relaxation rates are similar. The value of K
implies the structural characteristics of the elastomer and gives an idea about
the nature of the interaction between elastomer and solvent. The estimated
values of K and n are given in Table 4 and are accurate to ± 0.0005 and ± 0.01
respectively. The values of n lie between 0.40-0.72, which indicate that the
sorption process of NR/MK with alkanes is non-Fickian and is anomalous. The
influence of MK content and its silane treatment on n values is insignificant.
The values of n strongly depend on the molecular size of the penetrants.
The values of n and K are not dependent on temperature but depend on the
molecular volume of the penetrant molecules.
Table 4. Sorption data n- and K- values of silane treated and untreated
MK filled NR in aliphatic penetrants at different temperatures
Filler
loading
(phr)
MK0
MK10
MK20
MKC10
MKC20
Temp.
Hexane
Heptane
Decane
(oC) n + 0.01 K x 102 n + 0.01 K x 102 n + 0.01 K x 102
(g/gminn)
(g/gminn)
(g/gminn)
25
0.60
2.45
0.66
3.03
0.69
3.52
40
0.46
2.77
0.60
2.89
0.61
3.18
60
0.40
3.03
0.54
3.66
0.56
3.73
25
0.58
2.59
0.59
3.07
0.59
3.40
40
0.46
1.84
0.48
2.02
0.51
3.03
60
0.42
1.59
0.50
1.69
0.54
2.50
25
0.49
2.54
0.54
2.98
0.56
3.38
40
0.44
1.68
0.47
2.25
0.49
3.09
60
0.46
1.73
0.54
1.75
0.53
2.70
25
0.53
2.71
0.60
3.12
0.62
3.67
40
0.63
3.08
0.69
3.57
0.72
3.92
60
0.44
1.68
0.48
2.12
0.54
2.56
25
0.51
2.63
0.56
2.91
0.62
3.60
40
0.59
2.87
0.63
3.33
0.71
3.89
60
0.49
1.90
0.52
2.37
0.54
2.59
Activation Parameters
The activation energy for diffusion (ED) and permeation (EP) are calculated
using Arrhenius equation [25]:
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S. Rohini Thimmaiah, Siddaramaiah
X = Xo exp (-Ea/RT) (4)
where, X is P or D; Ea is the activation energy; R is the universal gas constant
and T is the absolute temperature. Figures 4 and 5 displayed the dependence
of log D and log P versus 1/T wherein linearity was observed in the investigated
temperature range. The activation energies are obtained by least square analysis
and are given in Table 5. From the table it is also clear that silane treated filler
loaded NR formulations showed higher activation energies as compared to
untreated MK loaded NR systems. This is because as the filler is treated with
silane the void content decreases which increases the activation energy of
diffusion of the NR membrane. It is also clear from the experiment results
that, as the size of the penetrant increases the activation energy of diffusion
increases. Similar observation was made by Johnson and Thomas [18].
Figure 4. Arrhenius plots of diffusivity for 10% untreated and treated MK in NR
composite (a) and (b)
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Figure 5. Arrhenius plots of permeability for 10% treated and untreated MK in NR
composite (a) and (b)
In the same manner, the sorption coefficient was expressed in terms of van’t
Hoff relationship:
log Ks = ΔS/2.303R - ΔH/2.303 RT (5)
where, Ks is equilibrium sorption constant which is given by Ks = No. of
moles of solvent sorbed at equilibrium mass of the polymer. The values of ΔS
and ΔH are obtained by regression analysis of the plots of log Ks versus 1/T
(Figure 6). The calculated ΔS and ΔH values for all solvents were positive,
and did not show any relationship to the MK content, particle size, penetrant
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S. Rohini Thimmaiah, Siddaramaiah
(solvent) size, or solvent dipole moment. The positive values of ΔH suggest
that sorption is dominated by Henry’s law mode sorption with an endothermic
contribution. The values of ΔS and ΔH are higher in case of treated MK filler
filled formulations, which may be due to the decrease in void content in the
silane treated MK filler filled systems.
Figure 6. van’t Hoff plots of ln Ks for 10% treated and untreated MK in NR composite
Swelling
The interaction between polymer and solvent can be established by knowing
the amount of polymer in the swollen polymer. The swelling index, which is
the measure of degree of swelling of the rubber compound, is calculated
using the equation:
Swelling index (%) = As x 100/W (6)
where, As = amount of solvent sorbed by the sample and W = initial weight
of the sample before swelling.
The crosslink density and the swelling ratio of the NR composites were
determined by equilibrium swelling method for all penetrants. The crosslink
density was calculated using the equation:
ν=
260
1
2Mc (7)
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Metakaolin Filled Natural Rubber Composites
Table 5. Activation parameters for diffusion ED (kJ/mol), permeation
EP (kJ/mol), enthalpy of sorption ∆H (kJ/mol) and entropy of sorption
∆S (J/mol/K) of silane treated and untreated MK filled NR in aliphatic
penetrants
Filler loading (phr)
MK0
MK10
MK20
MKC10
MKC20
Activation
parameters
Hexane
Heptane
Decane
ED
43.54
43.76
44.16
EP
47.53
47.83
48.26
∆H
9.09
8.51
10.01
∆S
0.12
0.04
0.20
ED
43.48
43.54
44.14
EP
47.46
47.60
48.10
∆H
10.77
9.09
9.42
∆S
0.25
0.08
0.62
ED
43.11
43.14
42.82
EP
47.32
47.92
48.98
∆H
10.31
9.50
10.20
∆S
0.17
0.17
0.29
ED
44.24
44.38
44.90
EP
48.16
49.40
49.79
∆H
11.86
12.22
11.81
∆S
0.08
0.58
0.25
ED
44.15
44.19
44.41
EP
47.86
48.16
49.53
∆H
12.08
13.39
13.33
∆S
0.29
0.79
0.33
where, Mc is the molecular weight of polymer between crosslinks:
Mc =
-ρr VS φr1/3
ln 1-φr  χ φr + φr 2
(8)
where, rr is the density of polymer, Vs is the molar volume of solvent and fr is
the volume fraction of rubber in the solvent-swollen sample and is given by:
φr =
d/ρp
d/ρp +As/χ s
(9)
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S. Rohini Thimmaiah, Siddaramaiah
where, d is the deswollen weight, rp is the density of polymer and As is the
amount of solvent absorbed. In equation (10), c is the interaction parameter
and is given by Hildebrand equation:
χ=β+
(
Vs δs -δp
)
RT
(10)
where, b is the lattice constant, R is the universal gas constant, T is the
absolute temperature and ds and dp are the solubility parameters of solvent
and polymer respectively. The penetration velocity (v) of solvents in each
polymer membrane was determined by weight gain method as described by
Peppas and coworkers [26, 27].
Table 6 shows the swelling index of NR/MK formulations for different aliphatic
hydrocarbon penetrants. Swelling index increases with increase in filler content
and it also increases with an increase in the molecular size of the penetrants.
Table 6. Swelling index of NR formulations with silane treated and
untreated MK in different aliphatic hydrocarbon solvents
Filler loading (phr)
MK0
Swelling index
Hexane
Heptane
Decane
96
100
108
MK10
104
105
111
MK20
110
117
122
MKC10
102
102
106
MKC20
108
115
116
Figure 7 shows the results of measurements of crosslink density of NR/MK
composites. As the filler loading increases there is a decrease in crosslink density.
This is because there is less contribution from rubber-filler networks towards
the crosslink density of the filled systems. The decrease in crosslink density
with increase in MK filler content is an indication of less reinforcement of MK
filler in NR matrix and decrease in stiffness of the material. It is also observed
that the silane treated MK filler showed an increase in the crosslink density
which is due to the increase in reinforcing behaviors between filler and rubber.
Conclusions
The transport behaviors of three aliphatic solvents (hexane, heptane and
decane) through MK filled natural rubber composites has been reported. The
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Metakaolin Filled Natural Rubber Composites
Figure 7. Crosslink density as a function of 10% treated and untreated MK in NR
composites in aliphatic solvent
molar percentage solvent uptake (%) in the filled NR was found to show initial
increase in the mass of the solvents sorbed, until the maximum absorption
was reached. The sorption coefficient obtained for the aliphatic solvents in NR/
MK composites increased with increase in the sorption temperature. Treated
MK filler reinforced NR composites showed less sorptivity as compared to
untreated MK filled NR composites. This is because silane treated filler shows
more interfacial adhesion with rubber matrix, hence, reduces the solvent uptake
of the NR/MK systems. As the size of the penetrant molecules increases the
diffusion and permeation coefficient decrease. The mode of transport of the
aliphatic solvents into filled natural rubber has been found to be non-Fickian
and anomalous. Kinetic results have been analyzed using first order kinetics.
The transport parameters presented in this study have not only provided
additional characterization of NR/MK but gave an insight into the behavior of
MK filled NR in external liquid environment which is essential for their successful
applications. The data obtained could be of importance in problem solving
like designing a barrier material or tubes for transporting liquids.
Acknowledgment
The authors would like to acknowledge the financial support from the
Visvesvaraya Technological University, (No.VTU/Aca/2009-10/A-9/11472,
dated 02/01/2010) Belgaum.
Applied Polymer Composites, Vol. 1, No. 4, 2013
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S. Rohini Thimmaiah, Siddaramaiah
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