Civil Engineering Reference
In-Depth Information
by their ion (e.g., cation) exchange capacities, which can vary widely. A
characteristic number of these types of clay is the cation exchange capacity
(CEC), which is a number for the amount of cations between the surfaces.
The CEC of montmorillonite ranges from 80 to 120 meq/100 g (milli-
equivalents per 100 grams), whereas kaolinite has CEC values ranging from
3 to 5.
The expansion pressure of montmorillonite in which sodium ions consti-
tute the majority of the adsorbed cations (called Na-montmorillonite) is
very high, leading to the exfoliation and dispersion of the crystal in the
manner of fi ne particles or even single layers. When Ca
2+
, Mg
2+
and ammo-
nium are the dominant exchangeable cations, the dispersion is relatively
low and the size of the particle is relatively large. Separation of the clay
discs from each other will result in a nanoclay with an enormous large active
surface area (it can be as high as 700-800 m
2
per gram). This helps to have
an intensive interaction between the nanoclay and its environment (bitumen
in our case). The process to realize the separation (surface treatment) is
dependent on the type of material to be mixed (Lan
et al.
, 1995).
A necessary prerequisite for successful formation of polymer-clay nano-
composite is therefore alteration of the clay polarity to make the clay
'organophilic'. To achieve fi ne dispersion, mechanical forces alone are not
enough; there should be a thermodynamic driving force as well to separate
the layers into the primary silicate sheets. This thermodynamic driving force
is introduced by inserting a certain coating of surfactants (an agent such as
detergent which reduces surface tension) on each individual layer (Theng,
2012). These surfactant molecules increase the layer distance, improve the
compatibility with the polymer and can give an increase in entropy because
they can mix with the polymer. Organophilic clay can be produced from
normally hydrophilic clay by ion exchange with an organic cation. The
organic reagents are quaternary ammonium salt with alkyl chains such as
12-aminododecanoic acid (ADA), octadecanoic alkyl trimethyl quaternary
ammonium salt. The reaction process is described as:
(
)
(
)
M-CLAY
+
CH
CH
NR X
→
CH
CH
NR -Clay
+
MX
3
2
3
3
2
n
3
n
(
)
R-H-CHX-Cl-
=
,
;
=
, Br -I M
,;
=
Na
+
,
Ca
2
+
,
Mg
2
+
[6.1]
3
Addition of a positively loaded surface active material, a kind of ADA,
will in this case form an ADA layer around each clay disc. The clay disc in
this case changes from a hydrophilic disc into a hydrophobic disc. These
modifi ed clay discs will separate automatically in water and can be used as
nanoparticles. The correct selection of modifi ed clay is essential to ensure
effective penetration of the polymer into the interlayer spacing of the clay
and result in the desired exfoliated or intercalated product. In intercalate
structure, the organic component is inserted between the layers of the clay
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