Introduction and aim of the thesis
GENERAL INTRODUCTION
Polymers are traditionally reinforced with inorganic fillers to improve their properties, such as
stiffness, toughness, barrier properties, resistance to fire and ignition. The achievement of a
significant improvement in the composite properties often requires incorporation of a large
amount of the filler in the polymer materials which imparts drawbacks to the composite such
as brittleness or opacity. To meet the rising demands of a wide variety of new applications,
functional hybrids combining inorganic fillers and polymers are being continuously
developed so as to better take benefit from their constituent’s properties or to induce new
ones1.
1
Polymer nanocomposites
Polymer nanocomposites are a new class of hybrid materials that are particle-filled polymers
for which at least one dimension of the dispersed component is in the nanometer range. The
reduction of particle size obtained in such materials increases the specific surface area of the
filler, providing larger matrix/filler interface and so more mutual interactions. As a result,
large reinforcing effect may be reached at much lower filler content when compared to
classical microcomposites. Besides, the geometrical shape of the particles plays an important
role in determining the properties of the composites2-4.
One can distinguish three types of nanocomposites depending on how many dimensions of
the dispersed filler are in the nanometer range (Figure 1). When all the three dimensions of
the particles are in the order of nanometers, the inorganic fillers are equidimensional, such as
spheres (like silica nanoparticles) or cubes (like calcium carbonate). Fillers with two
dimensions in the nanometer scale and the third in the range of micrometers, forming an
elongated structure, include carbon nanotubes and whiskers. The third type of nanocomposite
is characterized by only one dimension in the nanometer range. In this case, the filler is
present in the form of sheets of one to a few nanometer thick and hundreds to thousands
nanometers long. Among the different types of sheet-like fillers, those based on layered
silicates (alternatively referred to as clays), have attracted the most attention, notably because
of the easy accessibility and low cost of the starting clay minerals.
1
Introduction and aim of the thesis
Figure 1. Schematic, idealized representation of the three types of nanocomposites (left:
equidimensional nanoparticles, middle: nanotubes, right: nanosheets)
2
Polymer/clay nanocomposites
The preparation of polymer/clay nanocomposites is largely detailed and reviewed in the
scientific literature5-12. The ongoing interest in this field is due to the remarkable properties
improvement of polymer materials triggered by the dispersion of layered silicates
nanofillers5,9,10,12,13. Among those properties, drastic increase in tensile modulus at filler
contents sometimes as low as 1 wt% has drawn a lot of interest13-15. Thermal stability and fire
retardancy through char formation are other interesting and widely searched properties
displayed by nanocomposites16-18. Then, those new materials have also been studied and
applied for their superior barrier properties against gas and vapor transmission19-21.
These cited performances are closely linked to the high length/diameter ratio (aspect ratio),
characteristic of individual layers (clay platelets)2,10,22. Thus, to really utilize all the potential
advantages of polymer/clay nanocomposites, upon dispersion in the polymer matrix, the clay
particles must be exfoliated to a large extent into individual platelets. However, this ideal
structure is rarely achieved23. In fact, the complete and uniform dispersion of silicate layers in
polymer is only one of the three main types of composites that can be obtained when a
layered silicate is associated with a polymer (Figure 2a)5. We speak about intercalated
structure (Figure 2b) when a single (and sometimes more than one) layer of extended polymer
chains is intercalated between the silicate layers resulting in a well-ordered multilayer
morphology. When the polymer is unable to intercalate between the silicate sheets, a phase
separated microcomposite (Figure 2c) is obtained.
Although these categorized structures are largely admitted, it was shown in practice that more
complex structures are frequently obtained. Hence, four morphological entities, i.e. clay
microparticles, intercalated clay stacks, individual layers and silicate network may be present
2
Introduction and aim of the thesis
simultaneously in the composites24. Moreover, depending on the processing story of the
materials and on polymer-clay and clay-clay interactions, specific clay re-organizations such
as clay flocculation25 and exfoliated clay restacking26 can take place, creating original clay
structures.
Layered silicate
a) Exfoliated
(nanocomposite)
Polymer
c) Phase separated
(microcomposite)
b) Intercalated
(nanocomposite)
Figure 2. Schematic representation of different types of composites arising from interaction
of layered silicate and polymer
One of the main reasons for the non trivial preparation of exfoliated polymer/clay
nanocomposites is the strong hydrophilic nature of clay. Therefore, the modification of clay
surface is very widely used to render it more organophilic and hence more prone to induce
clay delamination with non-polar polymers27. The most common method is the ionic
exchange between the inorganic cations originally lying in the interlayer space of the clay and
cationic surfactants such as ammonium cations bearing at least one long hydrocarbon chain
(Figure 3)28. As a result of this ionic exchange, the surface energy of the clay platelets is
decreased and the interlayer distance is increased (d’>d). Such organomodified clays are
referred to as “organoclays” and their preparation is the object of the literature review (Part A)
presented in this thesis.
d
Na+ Na+ Na+
Na+ Na+ Na+
+ Br
+
+
d’
Cationic surfactant
Hydrophilic clay
+
+
+
+
+ NaBr
+
Organoclay
Figure 3. Schematic representation of ionic exchange in layered silicate
3
Introduction and aim of the thesis
3
Potential impact of polymer/clay nanocomposites
Nanocomposites technology is among the most widely watched technology areas within the
plastics arena. A market research published by Frost & Sullivan in July 2007 details the key
developments with respect to the usage of nanomaterials, namely nanoclays, carbon
nanotubes and carbon nanofibers, in plastics29 (Table 1). They relate that the last few years
have seen frantic activity in the world nanocomposites market with important technological
breakthroughs. However, some major challenges faced by suppliers were identified,
including: moving the industry away from the labs and into the market, timing capital
expenditure and ensuring a good return on investment, achieving key technological
breakthroughs through the nano-supply chain, creating testing and measurement standards for
the industry and generating favorable public opinion towards nanotechnology.
Table 1. Polymer nanocomposites market: market engineering measurements, base year
2006, world coverage29
Measurement name
Measurement
Trend
Market size in revenues (nanoparticles)
$33.7 million
increasing
Market size in volumes (nanocompounds)
33.0 kt
increasing
Base year market growth rate (revenue terms)
16.1%
increasing
Forecast period growth rate (revenue terms)
23.1%
increasing
Maximum future market size
$144.6 million
increasing
Price sensitivity
low/medium
increasing
Number of competitors in 2006
> 50
increasing
Market saturation (current/potential users)
very low
increasing
Market age/product life cycle
introduction/growth
increasing
The main application segments of polymer/clay nanocomposites include: automotive30, sports
goods, electric cable and packaging industry31. The automotive industry has been an early
adaptor of the nanoclay technology and they are already being extensively used in polyolefin
molded components. For example, the cargo bed of the 2005 General Motors Hummer has
parts containing nanoclay composites. A new wave runner commercialized by Yamaha in
2008 incorporates nanoclay resin instead of heavy fillers, resulting in a 25 wt% reduction of
weight.
4
Introduction and aim of the thesis
The packaging sector is also emerging as a key end-user segment for nanoclay composites,
where they provide increased gas barrier properties. Currently much of the applications for
nanoclay composites have been based on nylon as the resin matrix. Examples include beer
bottles commercialized by Honeywell and Durethan food packaging films from Bayer AG.
Nevertheless, technical advancements are still needed with clay nanocomposites, especially
with respect to their dispersion in polyolefins. Solutions are also searched for in other nonpolar matrices such as silicone-based and fluorinated polymers as well as in polyethylene
terephthalate and in elastomers.
4
Supercritical carbon dioxide
Supercritical carbon dioxide (scCO2) is an abundant, inexpensive, nontoxic and
nonflammable solvent that has attracted extensive interest as a polymerization and processing
medium since the 90’s, primarily driven by the need to replace conventional organic solvents
with more environmentally benign and economically viable procedures32. The supercritical
state of CO2 is reached above its critical temperature of 31.1°C and its critical pressure of 73.8
bar. In this state, the fluid has both liquid-like properties such as high density and gas-like
properties, such as low viscosity and high diffusivity (Table 2).
Table 2. Physical properties of supercritical fluids compared to liquids and gases33
Liquid
Supercritical fluid
Gas
Density (g/cm³)
1
0.1-1
10-3
Viscosity (Pa.s)
10-3
10-4-10-5
10-5
Diffusivity (cm²/s)
10-5
10-3
10-1
The low viscosity, near-zero surface tension, relative chemical inertness and high diffusivity
of supercritical carbon dioxide (scCO2) results in negligible competitive adsorption with guest
molecules on the host surface and therefore facilitates solute transfer relative to classical
solvents34. Furthermore, since CO2 is a gas at ambient conditions, the costly drying procedure
associated with conventional liquid solvents is circumvented, and the product is free of
residual solvent upon depressurization.
5
Introduction and aim of the thesis
These unique properties of scCO2 have been exploited in various processes including
extraction35, chromatography36, nanoparticles preparation37,38, polymer synthesis39 and
foaming40. In the field of polymer/clay nanocomposites, three types of applications emerged
recently in the scientific literature. One method is the use of scCO2 as a polymerization
medium of various monomers in presence of organomodified montmorillonite clay for the in
situ synthesis of nanocomposites41-48. A second technique is the injection of CO2 during the
extrusion of a polymer/nanoclay mixture; it then acts as plasticizer to promote
macromolecular chain diffusion between the clay galleries49-51. The third report is a pretreatment of organoclays in scCO2, followed by rapid gas depressurization52,53. After this
process, the organoclay stacks are found to be disorganized, which favors their subsequent
dispersion in polymer.
5
Solubility in supercritical carbon dioxide
Solubility in scCO2 is comparable to hydrocarbons54. Indeed, small organic molecules and
many monomers soluble in hexane are also soluble in compressed CO2. On the contrary, polar
molecules and organometallic compounds are poorly soluble in this medium.
As a rule, high molecular weight polymers are insoluble in scCO2 except for three families of
polymers that can readily be dissolved under mild conditions of pressure and temperature
(T°<100°C, P<1000 bar). These polymers are amorphous (or poorly crystalline) fluorinated
polymers55, silicones56 and aliphatic poly(ether-co-carbonate)s57. Their solubility can be
explained by specific interactions existing between CO2 quadrupolar moment and some polar
functions (carbonyl, ether, siloxane or C-F bonds) of these polymers58. In the case of silicone
and poly(ether-co-carbonate), the high chain flexibility also contributes to their solubility in
scCO2.
6
Introduction and aim of the thesis
SCOPE AND AIM OF THE THESIS
The present research focuses on the preparation of new organoclays for polymer
nanocomposites application. The organomodification of natural clays has been used for more
than fifteen years to facilitate the dispersion of individual silicate platelets. The process
generally employed for clay surface modification is an ionic exchange between the sodium
ions of the clay and a cationic surfactant. This exchange has been largely studied in water due
to the capacity of this solvent to swell the clay. However, it can only be applied to watersoluble surfactants, which strongly limits the range of organoclays available. This limitation is
reflected in the structures of organomodifiers in commercially available organoclays: all are
alkylammonium salts of similar structure. Now, the intense development of polymer/clay
nanocomposites has raised two major needs for organoclays: the first is linked to the high
processing temperature of several polymers. Indeed, commonly used temperatures in
extrusion and injection molding (up to 250°C) can exceed the onset degradation temperature
of alkylammonium salts (~180°C). As a consequence, there exists a strong demand for high
temperature stable organoclays. Secondly, in parallel to the melt blending process, the
development of in situ polymerization from the clay surface requires organomodifiers with
specific functional groups. This method is suggested to facilitate the formation of an
exfoliated structure, with functional organomodifiers increasing the compatibility between
clay surface and polymer or even initiating the polymerization.
To meet these new requirements, we propose the use of scCO2 as a medium for the ionic
exchange. This solvent is chosen for its numerous advantages compared to classical solvents,
like low environmental impact, non toxicity and tunable polarity and density. Compared to
the water process, the two main advantages are the possibility to use non water-soluble
surfactants and the recovery of a dry powder upon depressurization, which represents time
and energy savings. The main objective of this thesis is to demonstrate the versatility of the
scCO2 process, through the preparation of thermally stable organoclays and functional
organoclays, and their evaluation as nanofillers in polymer matrices. To attain this objective,
the adopted strategy involves four steps: 1) modification of natural clay in scCO2; 2) study of
the organoclays properties in comparison with reference products; 3) qualitative and
quantitative evaluation of organoclay platelets dispersion in a host polymer matrix; 4)
characterization of the polymer/clay nanocomposites properties.
7
Introduction and aim of the thesis
To study the versatility of the scCO2 process for clay organomodification, a large number of
commercially available surfactants including ammonium, phosphonium and imidazolium
salts, are first tested. The mechanism of ionic exchange in this medium is then analyzed,
followed by the screening of some key experimental conditions. The possibility of preparation
of organoclays in a pilot-scale reactor is investigated and the process is further extended to
new functional organomodifiers.
After the preparative step, organoclays characteristics are detailed. In particular, the interlayer
distance is measured by X-ray diffraction while the organic content and exchange yield are
determined by thermogravimetry. The latter technique also serves to assess the thermal
stability of organoclays. For this purpose, phosphonium organoclays are compared with
ammonium organoclays of exactly the same structure, both prepared in scCO2. The effect of
the nature of the clay is also studied, by comparing two natural clays: montmorillonite and
hectorite. In addition, surface coverage, mean particle size and surface energy of a selected
phosphonium-montmorillonite prepared in scCO2 pilot reactor are calculated and compared
with pristine (natural, purified) clay and a commonly used commercial organoclay.
In the next step, the most promising organoclays are dispersed in selected matrices by two
distinct methods. Thermally stable organoclays are melt blended with polymers at high
temperature either by internal mixing or (mini)-extrusion. For this method, polyamide-6 (PA6) is chosen as reference matrix because it is processed at a temperature above the onset
degradation of ammonium salts (230°C) and the largest extent of exfoliation, as depicted in
the literature, was obtained in this polymer. Then, a second polyamide is used:
poly(metaxylylene adipamide) (PA mXD-6), as specialty polymer processed at even higher
temperature (250°). The second method is the in situ polymerization in scCO2 and is applied
with functional organoclays. It involves mixing of the nanofiller with a liquid monomer in
suitable conditions for polymerization. Methyl methacrylate is chosen as monomer because its
polymerization in scCO2 is already mastered by our laboratory.
In both cases, the extent of clay exfoliation is evaluated qualitatively by X-ray diffraction and
transmission electron microscopy. Polyamide-6 nanocomposites are further analyzed by
rheology and two distinct quantitative methods are used to assess the degree of exfoliation.
8
Introduction and aim of the thesis
The last, but not least, step to validate the process is the evaluation of some key properties of
nanocomposites prepared with scCO2 organoclays. Hence, the fire properties of PA-6
nanocomposites are evaluated quantitatively by cone calorimetry and the implication of
onium stability on the results is discussed. Mechanical properties (Young modulus and yield
stress) are also measured for the same polymer, with different types of organoclays and
different extents of exfoliation, while oxygen barrier properties are determined for PA mXD-6
nanocomposites.
THESIS OUTLINE
The present thesis is divided in five chapters of experimental results. Preceding these
chapters, a literature review is presented. It provides an outline of the patenting activity in the
field of manufacturing organoclays through ionic exchange (Part A). The variety of organic
modifiers and the diverse processing techniques are detailed, with the aim to extract the most
relevant organoclays for successful nanocomposite formation at industrial scale. In Part B of
the review, a survey of the state of the art concerning the use of thermally-stable organoclays
is presented. Chapter 1 is dedicated to the process of clay organomodification in supercritical
carbon dioxide. The preparation of ammonium-, phosphonium- and imidazolium-based
organoclays is reported. In the second chapter, the thermal stability of phosphonium-modified
clays (montmorillonite and hectorite) is compared with that of ammonium-modified clays.
Nanocomposites are prepared by melt blending organoclays containing onium cations of
identical structure with polyamide-6 as host matrix. Then, the morphology as well as the fire
properties of the phosphonium and ammonium clay-based compounds are evaluated and
compared. Chapter 3 relates the preparation of nanocomposites with the same matrix, PA-6,
and focuses on the effect of clay modification on structure and mechanical properties. In this
case, a phosphonium clay prepared in scCO2 is compared with pristine clay and a commercial
organoclay. The same fillers are used in Chapter 4 but with a semi-aromatic polyamide (PA
mXD-6). The study of the obtained nanocomposites is centered on the oxygen barrier
properties. Chapter 5 is devoted to the preparation of functional organoclays and their
application for the in situ polymerization of methyl methacrylate in scCO2. Finally, general
conclusions are drawn and some comments are made on the potential of commercialization of
organoclays prepared with the scCO2 process.
9
Introduction and aim of the thesis
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12