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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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consistent chemical reactions. Initially, the metal ion alkylates and
then the remaining chlorine anion splits off. This generates
catalytically active form of coordination-unsaturated cation of
metallocene containing an active metal-carbon bond. The positive
charge of the inner coordination sphere of the metallocene cation is
compensated by chloride ion eliminated and associated with MAO.
Cl
Zr
Cl
CH
+
Al
Zr
3
CH
O
n
+
AlClO Al
3
Cl
+
Zr
CH
-
Cl
-m
n
AlClO
3
CH
3
O
m
CH
Al
3
O
m
n
-m
Three functions of one substance (metal alkylation,
elimination of the methyl group and the stabilization of the formed
cation) have made MAO a unique co-catalyst and metallocene
activator. A serious drawback of MAO is the complexity of its
synthesis (by hydrolysis of trimethylaluminum) and the formation of
a mixture of molecules of general formula: [-Al(CH3)-O-]n, where
n=4-30. Besides, to obtain highly active catalyst, one gramequivalent of the metallocene compound needs up to ten thousands
of equivalents of MAO, which excess is then required to be removed
from the polymer.
The stabilization of metallocene cation is also possible with
the help of specially designed boron compounds. Metallocenes,
activated by weakly coordinating anions such as B(C6F5)4, exhibit
the same activity as the catalysts activated by MAO. In addition,
there is no need to use an excess of activator, moreover, boron
compounds are not explosive as opposed to MAO. However, when
using this class of activators metallocene catalyst is very sensitive to
different contaminants.
81
Dichlormetallocenes somewhat differ from ferrocenes, in
which cyclopentadienyl ligands are arranged in space in parallel way.
In contrast to the iron ion the tetravalent metal ion of ferrocene in the
dichlormetallocene is connected also with two chloride ions and the
steric hindrance makes cyclopentadienyl rings to be inclined relative
to each other (for example, in zirconocene):
Cl
Fe
Zr
Cl
ferrocene zirconocene
When activating metallocene by MAO cyclopentadienyl rings
slightly slide apart, that creates a space which is sterically needed for
penetration inside of olefin molecules and conveying polymerization
in the coordination sphere of the metallocene. To stabilize the
inclined in the space configuration of the cyclopentadienyl rings they
are connected via bridges. This bridge form is called ansa-
metallocene. The type and structure of the bridges differ
significantly. They could be monoatomic, for example, H2C<,
(CH3)HC, (CH3)2C<, Ph2C<, (CH3)2Si<, and diatomic (-CH2-CH2- ,CHR-CH2-,-CHR-CHR-).
Inside the metallocene cation, there is one vacant
coordination site, which is stabilized by the anion [Cl·MAO]–.
Metastability of the inner coordination sphere of the metallocene
cation is the important reason why the olefin molecule takes a vacant
coordination site with an energy gain for the metallocene. Then the
coordinated olefin molecule penetrates between the transition metal
cation and negatively charged methyl group. As a result of migration
polyaddition, proceeding according to the Kossi-Arlman scheme, the
vacant coordination site interchanges the end of the growing polymer
chain. Although the chain grows within the metallocene cation,
polymerization proceeds according to the anion-coordination
mechanism (as well as in catalytic Ziegler-Natta catalyst systems).
82
+
3
CH
CH
+
2
Zr
3
CH
CH2CH
2
+
CH
CH2CH
Zr
CH
3
2
Zr
CH2CH
2
Cyclopentadienyl metallocenes are used for the
polymerization of ethylene and to receive atactic polypropylene. For
the stereospecific polymerization of propylene there were
synthesized sterically hindered ansa-metallocenes, containing
cyclopentadienyl, as well as indenyl and fluorenyl ligands.
CH
3
Si
CH
3
ZrCl
2
CH
2
2
ZrCl
CH
2
CH
3
C
ZrCl
CH
3
2
These large indenyl and fluorenyl ligands direct incoming
propylene molecules to the inner coordination sphere only by head to
tail type at the same arrangement of methyl groups.
Obtaining the block stereoregular polypropylene is associated
with the creation of so-called "oscillating" zirconocene catalysts. A
typical representative of this class of compounds is zirconium di-(2phenylindenyl) dichloride. This metallocene has no bridge between
two indenyl cycles, so that they can freely rotate around the axis.
Such configuration of the molecule when the cycles are arranged one
above the other is called meso-form, and when the same type of
cycles are directed to different directions – racemic or rac-form. The
cycles continuously rotate relative to each other. As a result, the
compound is in the rac-form for awhile, and after the rotation it takes
the meso-form. When the zirconocene is in racemic form, the
molecules of propylene can approach him with the only orientation one that ensures the formation of isotactic polypropylene. But when
83
the zirconocene goes into the meso-form, propylene molecule can
+
approach the active center with an arbitrary orientation. The result is
a block of atactic polypropylene. During one cycle of rotation four
molecules of propylene attach into isotactic block and four molecules
- into atactic. The model of multiple change of the polymer chain and
formation of stereoblocks was proposed by Coleman and Fox.
+
Zr
Zr
isotactic fragment atactic fragment
Economically and technologically important was the fact that
the "oscillating" catalysts belong to the group of metallocenes, which
can be effectively activated not only by MAO but also
triisobutylaluminum. At the same its molar excess with respect to
zirconocene is low ([TIBA]/[Zr]=300).
Among synthetic rubbers based on the olefins the highest
production volume with the use of metallocene catalysts falls at the
triple ethylene-propylene rubber (EPDR) and elastomeric stereoblock
polypropylene (SBPP). In addition, the use of metallocenes allowed
implementing a number of new processes: the production of
ethylene-octene and ethylene-norbornene elastomers, copolymers of
ethylene with styrene, norbornene, and carbon oxide.
2.12.3 Obtaining Ethylene-Propylene Rubber
In 1968 the production of polyethylene started with gas-phase
polymerization at low pressure using Ziegler-Natta catalysts, which
significantly simplified the technology and reduced the cost of the
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polymer. In 1985, this technology has been
with
takes
on
norbornen
psuedoliquid
gas
polymerizer.
gglomerator,
entered
gas phase process for the production of
polypropylene, and in 1992 semiwork-scale plant
operation to obtain ethylene-propylene rubbers
500 t/year.
The new gas-phase polymerization process
inert carrier particles coated with catalytic complex
Ethylene, propylene, and (if applied) ethylidene
into the reactor in gaseous form and create a
particles (Figure 13). The polymer is formed in the
form of solid granules and is derived from the
prevent adhesion of the rubber particles the anti-a
as the general purpose carbon black, is also
polymerizer. At the moment of polymer granules
particles of carbon deposit on their surface.
extended for
was put into
a capacity of
place in an
their surface.
e are fed
layer of
phase in the
To
such
into the
formation the
Fig.13 Union Carbide
polyethylene.
85
Copolymerization of ethylene and propylene is accompanied
by significant heat removal taken away with the gaseous monomers,
circulating in the system polymerizer - water-cooled heat exchanger
(similar to gas-phase polymerization of butadiene). Unreacted
monomers after an appropriate treatment are returned to the
polymerization process.
Since the gas-phase process does not use solvents or diluents,
does not require the stripping of the volatile products out of the
copolymer, therefore there is no need to dry rubber and press it into
briquettes.
Gas-phase EPDR is supplied in a free-flowing granular form
with an average particle size from 0.6 to 0.8 mm and does not raise
the dust during transportation and dosing. Rubber contains up to 25
wt parts of carbon black, which specifically interact with the polymer
coating the granules and effectively preventing from sticking
together. It provides long-term maintaining free-flowing condition of
the material, which greatly facilitates the dosing of rubber while
processing. In addition, part of the filler, adsorbed on the surface of
the particles, are found to be already soaked with rubber, so the
energy expended on the preparation of mixtures from this rubber is
about 5% less than using solution rubbers (in the form of briquettes).
A new gas-phase technology produces copolymers with a broad
range of molecular weight and composition, which creates the
possibility of producing different types and purposes rubbers.
2.12.4 Producing Stereoblock Polypropylene Rubber
The industrial synthesis of stereoblock polypropylene is carried out
in the gas phase using fluidized-bed reactor. The process is carried
out at the pressure in the range 2-4 MPa and at the temperature of 70115°C. The difficulties occuring during the gas-phase polymerization
processes are mainly associated with the problems of static electricity
and heat regulation from the polymer particles.
86
To carry out the polymerization in the gas phase the
metallocene catalyst should be fixed to the carrier without loss of
properties of homogeneous complexes. The nature of carrier and the
method of metallocene coating have a significant effect on the
properties of the catalyst. As the carrier there can be used silica gel
and other materials with high surface area - aluminum oxide,
magnesium chloride, zeolites, silica minerals, cyclodextrins,
polystyrene, polysiloxane derivatives. The most suitable carrier for
the metallocenes is silica gel. The surface of the carrier is premodified by calcination, silylation or aluminum alkyls treatment.
In heterogeneous polymerization processes the carrier serves
as a matrix for polymer. Thus, the spherical polymer particles with
desired size and porosity are formed as well on the spherical particles
of silica gel due to the replication (reproduction of the catalytic
particle form by the growing polymer on it). The size, shape, size
distribution, porosity and bulk density of the particles of applied
catalysts mean their morphology. To ensure high stability and
polymerization processing speed the catalyst morphology is
optimized. It is important to have narrow size distribution of catalyst
particles, where the optimal size is between 20-30 microns, the
absence of dust fraction and low porosity and, consequently, high
bulk density of powder (460-500 g/dm3).
The particles of applied catalyst do not stick together, so there
is no contamination of the reactor, and optimal morphological
characteristics of the polymers are achieved. In addition, at the
activation stage of metallocenes the number of co-catalyst MAO can
be markedly reduced (from several thousand to several hundred
equivalents in relation to metallocene). The way of the application of
metallocenes is the top secret of any manufacturer, as this is the basic
"know-how" in the synthesis of catalysts.
An important point at all stages of the synthesis of applied
catalytic systems is the homogeneous distribution of the catalytically
active sites on the particles. The number of undesirable residue of the
catalyst in the polymers depends on the homogeneity of the
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distribution. Note that in the polymerization process the carrier can
break under the influence of the growing polymer chains.
Stereoblock elastomeric polypropylene has a molecular
weight from 120000 to 200000, and the coefficient of polydispersity
(Mw/Mn) varies from 1.7 to 6.0, which largely depends on the
preparing method of the catalyst. The broadening of the molecular
weight distribution of the SBPP, obtained using the "oscillating"
catalyst systems, is unusual for metallocene catalysis and is due to
the chain transfer to the organoaluminum compound by means of
equilibrium exchange reactions of alkyl - polymer chain. The
samples, which combine high molecular weight and moderate
crystallinity (30-40%), show good strain properties, and the
elongation at break can reach 2000%.
Elastic polypropylene has such important properties as the
thermoplasticity combined with high melting point (from 140 to
160°C). Frost resistance of the rubber, depending on the degree of
crystallinity, is in the range from -10°C to -35°C. The polymer has a
low density (about 0.9 g/sm3) and typical for polyolefins high
chemical inertness, but low oil resistance. Stereoblock polypropylene
is durable, has excellent abrasion resistance, the colored objects can
be obtained on the base of it.
Owing to the low cost of propylene and the possibility of gasphase polymerization technology SBPP is available polymeric
material. The latter fact in combination with a set of service
properties suggests its use in many industries. It is used in the auto
industry for the manufacturing of the special bumpers, decorative
materials, insulating pads. The widespread use of the elastic
polypropylene has found in construction as the different types of
finishing materials and thermoplastic foam. Its use is promising in
military equipment, electronics and electrical equipment. This
polymer is used for the manufacture of knitted fabrics, elastic shoe
parts, medicine and hygiene products.
88
A large number of studies are directed to developing
materials based on the SBPP, which can replace the plasticized
polyvinyl chloride, as well as some expensive elastomers.
3. MAIN TYPES OF RUBBERS, PROPERTIES, PRACTICAL
APPLICATION
3.1 Natural Rubber
Natural rubber (NR), also called India rubber or caoutchouc, as
initially produced, consists of suitable polymers of the organic
compound isoprene with minor impurities of other organic
compounds plus water. Forms of polyisoprene that are useful as
natural rubbers are classified as elastomers. Currently the rubber is
harvested mainly in the form of the latex from certain trees. The latex
is a sticky, milky colloid drawn off by making incisions into the bark
and collecting the fluid in vessels. This process is called "tapping".
The latex then is refined into rubber ready for commercial
processing.
History
The para rubber tree is indigenous to South America. The first
use of rubber was by the Olmecs, who centuries later passed on the
knowledge of natural latex from the Hevea tree in 1600 BC to the
ancient Mayans. They boiled the harvested latex to make a ball for a
Mesoamerican ballgame.
Charles Marie de La Condamine introduced samples of
rubber to the Académie Royale des Sciences of France in 1736. Then
he presented a paper to the Académie (eventually published in 1755)
which described many of the properties of rubber. This has been
referred to as the first scientific paper on rubber. In England, it was
observed by Joseph Priestley, in 1770, that a piece of the material
was extremely good for rubbing off pencil marks on paper, hence the
name rubber.
South America remained the main source of the limited
amounts of latex rubber that were used during much of the 19th
89
century. Later the seedlings were then sent to India, Ceylon (Sri
Lanka), Indonesia, Singapore and British Malaya. Malaya (now
Malaysia) was later to become the biggest producer of rubber. In the
early 1900s, the Congo Free State in Africa was also a significant
source of natural rubber latex, mostly gathered by forced labor.
Liberia and Nigeria also started production of rubber.
In the 19th and early 20th century, it was often called "India
rubber". In 2010, India's natural rubber consumption stood at 978
thousand tons per year, with production at 893 thousand tons; the rest
was imported with an import duty of 20%.
Varieties
The major commercial source of natural rubber latex is the
Pará rubber tree (Hevea brasiliensis). This species is widely used
because it grows well under cultivation and a properly managed tree
responds to wounding by producing more latex for several years.
Many other plants produce forms of latex rich in isoprene
polymers, though not all produce usable forms of polymer as easily
as the Pará rubber latex does; some of them require more elaborate
processing to produce anything like usable rubber, and most are more
difficult to tap. Some produce other desirable materials, for example
gutta-percha (Palaquium gutta) and chicle from Manilkara species.
Others that have been commercially exploited, or at least have shown
promise as sources of rubber, include the rubber fig (Ficus elastica),
Panama rubber tree (Castilla elastica), various spurges (Euphorbia
spp.), lettuce (Lactuca species), the related Scorzonera tau-saghyz,
various Taraxacum species, including common dandelion
(Taraxacum officinale) and Russian dandelion (Taraxacum kok-
saghyz), and guayule (Parthenium argentatum). To distinguish the
tree-obtained version of natural rubber from the synthetic version,
the term gum rubber is sometimes used.
Chemistry
Latex is the polymer – cis-1,4-polyisoprene – with a
molecular weight of 100000 to 1000000. Typically, a small
percentage (up to 5% of dry mass) of other materials, such as
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