Article Type : Review Article
Authors : Acevedo NIA and Rocha MCG
Keywords : Cubati clays; Bentonites; Nanocomposites; Organophilization
The set of
attractive properties is responsible for a wide range of applications for
bentonite clays. In Brazil, about 62% of the reserves of this mineral are found
in the State of Paraíba, mainly in the municipalities of Boa Vista and Cubati.
There are several studies on the Boa Vista clays. However, the clay deposits in
Boa Vista are running out. Therefore, it is interesting to evaluate the
properties and possible applications of Cubati clays. The production of
polymer/clay nanocomposites is one such application. This review aims to
present some studies carried out with Cubati clays, in order to expand its
field of application.
Bentonite clays have a
unique set of properties that combined with low cost and abundance, arouse
great industrial interest. These minerals can be used in a wide range of
applications due to their high swelling capacity and the ability to form gel in
low concentrations, among others interesting features [1-6]. The development of
polymeric nanocomposites is one such applications [4,7-15]. Bentonite is a 2:1
aluminosilicate, [(Mg,Ca)O.Al2O3Si5O10.nH2O)],
formed from the decomposition of volcanic ash, which in its natural form has
the exchangeable cations Na+, Mg2+, Ca2+, Al3+
and Fe3+ [1,16-18]. Montmorillonite is the most important mineral
constituent of these minerals and is responsible by their properties [17,18].
The most important deposits of bentonite clays in Brazil (around 62%) are
located in the state of Paraiba, mainly in the municipalities of Boa Vista and
Cubati. The most common form of this occurrence is as a polycationic bentonite
[1,19]. Although there are many studies on clays from Boa Vista, there are few
works on production of nanocomposites by using clays from Cubati. Boa Vista
deposits are running out. Therefore, our studies are focusing on evaluating the
potential of Cubati clays[17-18].Despite the numerous advantages associated
with the use of bentonite clays in the production of nanocomposites, the
natural hydrophilicity of these silicates hinders the interaction and the
ability to blend them with hydrophobic polymeric matrices[1,18-20]. Therefore,
an organophilization step is necessary to improve the interaction of the clay
with the polymer. Different types of surfactants are often used. In this
process, there is an exchange between the cations present in the clay with the
cations of the surfactants [21-24]. Quaternary ammonium salts containing chains
of different structures are the most widely used surfactants. The size of the
alkyl chains has a significant role on the properties of the nanocomposites
obtained, as the arrangement of the alkyl ammonium ions promotes different
sizes of the basal spacing. Low d-spacing promotes strong interactions between
the clays platelets, providing higher modulus values than those with higher
basal spacing. On the other hand, high basal spacing allows the delamination of
the clay layers during the production of the nanocomposites [21].
Organofilization is not enough to promote good adhesion properties between
clays and hydrophobic polymeric matrices. In order to achieve nanometric
dispersion of clay and a satisfactory interface, the use of compatibilizing
agents is necessary. The nature and composition of the compatibilizers, as well
as the processing conditions, have a significant influence on the properties of
the materials obtained [19,25-34]. The most widely used compatibilizer for
polypropylene/clay nanocomposites is the polypropylene grafted with maleic
anhydride (PP-g-MA) [31-33]. There are many published studies on the evaluation
of the effect of adding PP-g-MA on the properties of the obtained
nanocomposites. The compatibilizing effect of PP-g-MA depends on its content,
molecular weight and overall concentration in the polymer. The miscibility of
this oligomer in the polymer has to be high in order to promote the achievement
of an exfoliated or intercalated clay structure. Therefore, both the
intercalation ability and the miscibility of the PP-g-MA in PP define the
structure of the nanocomposite and its properties [23,25,27,28,32].The
dispersion of clay in the polymer matrix in a melt mixing process is also a
function of the type of clay, the viscosity of the polymer and the operational
conditions[27,31,33]. The reinforcing effect is a function of the surface
contact area between clay and polymer and the interphacial adhesion. Therefore,
an effective dispersion is required to increase both the aspect ratio of the
clay particles and the surface contact area leading to obtaining good mechanical
properties [25].The industrial application of clays requires a complete
identification of its nature and properties. Depending on the deposit from
which the clay was extracted, there may be some difference in properties, even
for a given clay variety. There are some published studies on the
characterization and application of bentonite clays from Cubatí municipality in
Brazil. Granulometric analysis by laser diffraction, wet granulometric
analysis, X-ray diffraction (XRD), thermogravimetric analysis (TGA),
differential thermal analysis (DTA), chemical analysis by X-ray fluorescence
(XRF) and cationic exchange capacity (CEC) are the most often-used
characterization techniques[17,18,35,39]. The clay samples are named according
to their source, such as: Campos Novos clays, specifying that these clays come
from a deposit in Cubati called Campos Novos, or simply, Cubati clays.
Sometimes, the samples are named after their color, such as: white, gray or
green clay. These studies show that Cubati clays are polycationic bentonite
clays, composed mainly of smectite, kaolinite, quartz, and the following
cations: calcium (Ca), magnesium (Mn) and potassium (K) [17,35-40]. The clays
from Boa Vista studied by Aranha show similar composition [41].
The growing demand for
oil drilling fluids as well as the depletion of clay deposits traditionally
used have generated incipient studies aiming at the use of Cubati clays in
these applications [18,38,42,43]. To meet the necessary requirements, bentonite
clays must perform some functions, such as acting as thixotropic, lubricant,
permeability reducer and viscosity controlling agents in drilling fluids [44].
However, only the sodium bentonite clays partially meet these requirements.
Therefore, the conversion of polycationic bentonites to sodium bentonites must
be performed. Sodium carbonate is often used in this process. In our research
group, we used sodium chloride [18] for the homoionization of the bentonite.
This procedure, however, is not sufficient to obtain the required properties,
as sodium bentonite is hydrophilic and is not compatible with hydrophobic oil
fluids. As a result, the organofilization of clay becomes necessary.The
surfactant most used in the organophilization process of Cubati clays is the
quaternary ammonium salt, diesteryl dimethyl ammonium chloride [37-40]. This
process involves the following steps: preparation of a clay/surfactant
dispersion, stirring for 20 min at a given temperature, resting at room
temperature for 24 h and washing and vacuum filtration in a study of
organofilization of two samples of Cubaty clays, respectively, gray and green
clays, verified that there was an intercalation of the surfactant between the
clays platelets. The authors also observed that there was no effect of the
following experimental variables time of preparation of the clay/surfactant,
rotor rotation used in the stirring process and resting time on the
organofilization process.The ethoxylated amine surfactant TA50® was also used
in the organophilization of Cubati clays [37,39]. Nonionic surfactants show
greater stability and resistance to degradation than ionic surfactants compared
the two types of surfactants and found that there was greater swelling
capacity, measured by the Foster method, when the clays were organophilized
with the ionic surfactant. The purification of the raw material is a very
important step in the organophilization process. This step aims to eliminate or
reduce the content of minerals that do not contribute to the plasticity of the
clay as well as other impurities, which can interfere with the performance of
the clay [23,43]. Most of the work carried out aiming at using clay in drilling
fluids reports the use of hydrocyclone in the purification process. Studies
aimed at applications of Cubati clays in the cosmetic area have shown promising
results outlined the technological profile of the Cubati and Pedra Lavrada
clays. Both clays have a low and narrow particle size range, appreciable oil
adsorption capacity and good flow properties.There are few studies related to
the incorporation of Cubati clays in polymer composites or nanocomposites
[23,45,46]. Sales studied the effect of incorporating the variety of Cubati
clay, named White clay on the properties of a thermoplastic
acrylonitryle-butadiene-styrene copolymer (ABS) subjected to ionizing
radiation. An acrylonitrile- styrene copolymer (SAN) was used as a
compatibilizer agent. The clays previously activated with sodium carbonate were
organophilized with the quaternary ammonium salt, cetyltrimethyl ammonium
chloride (CCTMA). Initially, a masterbatch SAN / 30% clay was prepared in a
twin-screw extruder. After this stage, the composites were injection-molded.
The ABS/clay composites showed superior values of modulus and resistance to
rupture, both in tensile and in flexural stress. According to the authors, the
clay particles act as barriers to the displacement of the chains. The results
obtained indicated that there was a good dispersion of the clay particles and a
satisfactory interaction between the clay particles and the matrix. The effects
of adding the clay were more intense in the irradiated samples. The stiffness
of the material increased because of the formation of crosslinks promoted by
the irradiation of the polymer matrix. The impact resistance of the composites
decreased with the addition of the clay, but this effect was less pronounced in
the irradiated samples evaluated the efficiency of PP graphted with maleic
anhydride (PP-g-AM) as a compatibilizing agent for PP / clay nanocomposites.
The PP / clay nanocomposites were prepared in a twin-screw extruder. There were
no significant changes in the tensile strength, nor in the Young's
polypropylene modulus with the addition of the clay, even in the presence of
PP-g-MA. These results indicated that there was not the development of a good
interface between polymer and clay. On the other hand, there was a significant
increase in the elongation at break and in the toughness. This effect was
attributed to the tactoids delamination after the yield point and, or a
plasticizing effect of the compatibilizer on the polypropylene chains. However,
a high d-spacing effect caused by the surfactant must be investigated.
Rodriguez had also observed this behavior in PP composites with other Brazilian
clays [47]. Prepared PP/clay nanocomposites using the Green and Gray varieties
of Cubati clay. Ethylene-glycidyl methacrylate copolymer (E-GMA) was used as a
compatibilizer. The clays were organophilized with the non-ionic surfactant
Ultramine TA50, after activation with sodium carbonate. The authors obtained
two different structures. The PP/E-GMA/OGC (organophilized green clay) system
presented the structure of a microcomposite, while the PP/E-MA/OGC (gray
organophilized clay) gave rise to a nanocomposite with intercalated structure.
According to the authors, the higher SiO2 content allowed a greater
adsorption of the compatibilizer, facilitating the intercalation of the
polymer.