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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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number of components used in obtaining high-temperature rubbers,
the lack of built-in surfaces or additional heat removal, the use of
industrial water as a coolant for removing the copolymerization heat.
To initiate the polymerization the hot water is fed to jacket of the
first polymerizer, and the batch in the device is heated to 50°C; the
cooling devices of the following apparatuses are supplied by cold
water.
2.10 Technology of Butyl Rubber Obtaining with Cationic
Initiation in Methyl Chloride
Cationic (electrophilic) polymerization is a process of
macromolecules formation, where the active growing chains are
positively charged, and cationic polymerization initiator is a source
of positive charge.
In the synthetic rubber industry cationic initiation is used in
the production of butyl rubber, polyisobutylene, and silicone rubber.
Cationic polymerization catalysts are acidic substances.
When obtaining butyl rubber and polyisobutylene the aprotic Lewis
acids are used (AlCl3, BF3, Al(C2H5)
1,5Cl1,5
). When using aprotic
acids to form the active site it is necessary to use cocatalysts
(promoters). They are the substances of proton-donor nature - proton
acids and aprotic bases.
As a result of the interaction of Lewis acids with proton acids
and aprotic bases the complexes are formed:
AlCl3 HOH
H+[AlCl3 OH]
-
C Cl+AlCl
3
Protons or carbocations as a part of complexes initiate the
polyaddition reaction:
CH
3
H2C C
+ H+[AlCl3OH]
CH
3
+
C
[AlCl4]
-
CH
CH
3
+
3
[AlCl3OH]
-
-
H3C C
71
Regardless of the nature of the catalytic complex the active
center of polymerization after the interaction with the unsaturated
monomer is carbonium ion:
H3C C
CH
CH
3
+
3
[AlCl3OH]
-
+
H2C C
CH
CH
3
3
H3C C
CH
CH
3
CH2C
3
CH
CH
3
+
3
[AlCl3OH]
Because of the numerous reactions of chain transfer in
cationic isobutylene polymerization the lifetime of active centers is
small. The common feature of the active centers of ∼MnM+ type is
the tendency to chain termination and transfer:
CH
3
H2C C
~ OH
CH
3
CH2C
CH
CH
3
AlCl
+
3
3
-
~
H2C C
CH
+
CH
3
[AlCl3OH]
3
CH
3
H2C C
~
CH
3
-
CH
3
H2C C
~
CH
3
CH
3
H2C C
~
CH
3
CH2C
CH
CH2C
CH2C
CH
CH
CH
CH
H2C C
CH
C
3
CH
+
H
CH
3
+
[AlCl3OH]
CH
3
3
+
[AlCl3OH]
3
+
[AlCl3OH]
-
-
-
3
+
H
3
3
+
CH
2
3
CH
2
The activation energy of chain transfer reaction is superior to
the activation energy of the chain growth reaction. Therefore,
lowering the temperature to a certain critical point leads to the fact
that the chain growth reaction rate constant starts to dominate the rate
constants of chain transfer reactions (Figure 10.):
72
>
K
E
E
T
transfer
K
growth
K
E
A
growth
growth
K
transfer
K
> K
transfer
A transfer
> E
growth
A
growth
Fig.10 Rate constants of growth and chain transfer reactions changes
behavior depending on the reaction temperature
Receiving butyl rubber and polyisobutylene, this temperature
is quite low. To achieve the required values of molecular weights of
polyisobutylene and butyl rubber the polymerization is carried out at
-95°C. In modern production processes the temperature of synthesis
could be raised up to -70°C by increasing the activity of the catalyst.
The process of butyl rubber obtaining consists of the following mane
different stages: the preparation of the catalyst solution or the
catalytic complex; batch preparation; copolymerization; water
degassing; separation, drying and packing of rubber.
Additional operations are preparing the stopper solution and
stabilizer suspension; anti-agglomerator preparing; cooling,
compression, drying and rectification of returnable products;
rectification of the washing solvent.
Catalyst solution is prepared from anhydrous aluminum
chloride and purified methyl chloride in the jacketed apparatus
(Figure 11.) at 20~-30°C. The apparatus is blown off with nitrogen
before loading. Aluminum chloride is loaded from a hopper 1 in the
form of granules or powder, then the apparatus 2 is filled with
purified methyl chloride. The first portion of catalyst solution is
drained into the tank 9, whence the substandard catalyst solution is
73
removed from the system. After that the apparatus 2 is filled again
with methyl chloride and is put into normal operation.
Fig.11 Catalyst solution preparation scheme for the butyl rubber
production.
1 – hopper; 2 - apparatus for concentrated catalyst solution
preparing; 3,7 – separators; 4 – tank; 5 – pump; 6 – cooler; 8 – filter;
9 - receiving tank of substandard catalyst solution.
I - Aluminum chloride; II - nitrogen; III - methyl chloride; IV - liquid
propane; V - substandard catalyst solution; VI - liquid ethylene; VII catalyst solution towards polymerization.
To maintain a constant temperature of the solution of
aluminum chloride the liquid propane is supplied to a jacket 2 from
the separator 3 at a boiling point. The saturated catalyst solution
containing 1.0% (wt.) of aluminum chloride, diluted with methyl
chloride to the concentration of about 0.1% (wt.) is piped to the tank
4, and then working catalyst solution with the pump 5 is fed into the
cooler 6, cooled by liquid ethylene from the separator 7. Catalyst
74
solution with a temperature below -90°C is fed to the polymerization
through the filter 8.
2.11 Batch Preparation, Copolymerization and Rubber
Separation.
The batch is prepared in the tank 1 by mixing isobutylene, purified
from contamination by rectification, isoprene, dried by azeotropic
dehydration, which are fed from the store, and return methyl chloride
by circulation pump 2 (Figure 12). On the line of the final product
the chromatographs are installed, according to chromatographic
analysis, depending on the obtained rubber grade the composition of
the batch is adjusted. The approximate composition of the batch
when obtaining the most common grades of butyl rubber, % (wt.):
Table 5.
The batch composition of butyl rubber
BK-2045 BK-1675
Isobutylene 17,6 17,4
Methyl chloride 81,8 82,1
Isoprene 0,6 0,5
Finished batch with the pump 3 is supplied to the propane 4
and ethylene 5 refrigerators, where it is cooled down to operating
temperature (about -95°C) under the evaporation of propane (at 41°C) and ethylene (at -110°C). The copolymerization of isobutylene
with isoprene is carried out in a tube type reactor 6, where the liquid
ethylene maintains the temperature at about -100°C. The batch and
the catalyst solution are fed into the bottom of the reactor, mixed
with the help of the axial circulation pump and the reaction mass is
moved through the central overflow pipe to the upper part of the
polymerizer, and then it returns to the bottom of the reactor by means
of the peripheral circulation tubes flown over by liquid ethylene.
75
Fig.12 The scheme of the copolymerization of isobutylene with
isoprene
I – isobutylene, II – isoprene; III - return methyl chloride; IV - liquid
propane; V - liquid ethylene; VI - stopper; VII – anti-agglomerator;
VIII - hydrocarbons towards compression; IX - cooled catalyst
solution; X - vapour; XI - the air to the atmosphere; XII - to the
vacuum pump; XIII - rubber for packing.
The resulting copolymer is insoluble in methyl chloride. The
dispersion of butyl rubber, containing 8-12% polymer, 6-10%
monomer and methyl chloride, is supplanted by supplied batch and is
fed out of the polymerizer by pipe to the water degasser 7. For
deactivation of the catalyst isopropyl alcohol is fed to the discharge
pipe.
Polymerizer works in a cyclic mode: the system is in a duty
cycle for about 24 h and bleeds for 16 h, and then it is washed out
with hexane fraction and prepares for operation. The necessity to
wash polymerizer with hydrocarbons, dissolving butyl rubber, is
determined by declining the heat removal due to the deposition of the
polymer on the heat transfer surfaces.
76
The constant water level is kept in the degasser 7 at
temperature 70°C, meanwhile the most part of the monomer and
methyl chloride is vaporized, and are directed then to the
compression and recycling from the condensers 20 and 21. To
prevent adhesion of the rubber crumb the degasser is served by antiagglomerator - calcium stearate (1.5% of the rubber weight). The
water suspension of rubber is moved from degasser 7 by the pump 8
to the vacuum degasser 9, where the monomer and the methyl
chloride residues are removed. The rubber with water is directed
from the vacuum degasser 9 to the separation, drying and packaging
which follow two ways.
According to the first way the rubber drying is carried out in
an air dryer 14 with hot air at 110-120°C. Before drying the butyl
rubber is separated from the water on the drum vacuum filter 10. The
detachable water from the vacuum receiver 11 with the pump 12 is
returned through the heater 23 to the degasser 7.The vacuum in the
system is created by vacuum pump 13. The humidity of butyl rubber
at the dryer 14 inlet is 40-50%, after drying it is 0.5%. Rubber is
homogenized in a screw extrusion machine 15, from which it comes
in the form of tape, 180 mm wide and 9 thick. With the help of
conveyor 16 rubber is fed to the rollers 17, where water and other
volatile products residues are removed at 110-120°C. Then the
rubber is cooled by air on the conveyor 18, is briquetted by machine
19, packed into polyethylene film and put into containers. The
weight of briquette is 30 kg.
According to the second way butyl rubber is dried in a screwdrying press along the schemes adopted in the manufacture of
solution rubbers (LK-4 aggregates, the "Andersen" company, USA
and others) at 210-230°C in the expander.
To stabilize the butyl rubber there are used staining
(naphtham-2, etc.) or nonstaining [2,5-di-tert-butylhydroquinone,
2,2-methylenebis (4-methyl-6-tert-butylphenol), three (nnonylphenyl)phosphite] antioxidants (up to 0.3% of the rubber
weight), which are loaded together with the anti-agglomerator into
77
the polymerizate. Preparation of stopper solution, stabilizer
suspension and anti-agglomerator suspension is performed in the
usual way.
The rubber derived from the polymerizer when washing with
hexane fraction is separated from the solution in a special unit,
executed in the usual way of the solution rubbers separation, is dried
in the screw-drying press and released as the second rate.
2.12 Producing Ethylene-Propylene and Stereoblock Propylene
Rubbers by Gas-Phase Polymerization with Metallocene
Catalysts
2.12.1 Chemical Structure of Elastomeric Polyolefins
Polyolefins of regular structure are inclined to crystallization,
therefore they do not exhibit elastic properties under normal
conditions (despite high flexibility of the chains and weak
intermolecular interactions). Reducing the crystallinity degree or
complete suppressing of the crystallization make such polymers and
copolymers typical elastomers.
Among synthetic rubbers based on olefins the highest
production volume falls at statistic copolymers of ethylene with
propylene (EPR) or tercopolymers with any monomer containing two
double bonds (EPDR). In statistical copolymerization of these
monomers the regularity of the polymer chains structure gets broken,
the crystallinity degree decreases, and at 30-50% mol associated
propylene content copolymers become typical uncrystallizable
rubbers.
Stereoblock polypropylene, in which small isotactic blocks
alternate with atactic blocks, is also low crystallized. For example, if
the number of units in the crystallizable isotactic blocks amounts of
not more than 10, polypropylene will have low crystallinity degree
(5-30%) and at high molecular weight it will exhibit the properties
which are typical for elastomers.
78
Polypropylene with branched macromolecules also exhibits
the properties of elastomer, in which the main chains are atactic, and
the side are stereoregular (isotactic or syndiotactic) and relatively
short. These side chains in a mass of atactic polypropylene form
well-dispersed crystalline domains connected by flexible chains of
the amorphous polymer. Domains act as multifunctional network
junctions, and since they have only the physical interacting forces,
such polypropylene is a thermoplastic elastomer.
2.12.2 Metallocene Catalysts
Until recently, the main method of producing polyolefin rubbers is
solution polymerization (less suspension) in the presence of modified
heterogeneous Ziegler-Natta catalysts (titanium, magnesium,
vanadium, zirconium). Such catalyst systems are described in more
details in section 3.1. Thus synthesized elastomeric polyolefins have
broad MWD, which is a consequence of multicentric nature of the
active centers formed in heterogeneous catalysts. In addition, for this
reason it is difficult to purposefully synthesize polymers with
predetermined characteristics.
The breakthrough in metal complex catalysis is connected
with the discovery of new highly active and stereospecific
homogeneous metallocene systems (one-center). This means that the
metallocenes are single-site catalysts. In metallocenes the
catalytically active metal atom is usually closed and the access of
monomer molecules to it follows the only way. The advantages of
metallocene catalysts for olefin polymerization are the ability to
synthesize polymer with given and uniform microstructure, narrow
molecular weight distribution and ultra-high molecular weight by
changing the structure of the catalyst. Thus obtained polyolefins have
no any oligomers and easily extractable fractions.
The first reports on the ethylene polymerization with the
titanium metallocene dichlordicyclopentadienyl complex were
published in 1957 (Breslow and G. Natta). In these works there were
79
used organoaluminum compounds as a co-catalyst, but such systems
H
exhibit low catalytic activity and stability during the polymerization
of ethylene and do not lead to the formation of high molecular
polypropylene. A significant increase in the catalytic activity of
metallocene catalysts is achieved using polymethylaluminoxane as a
cocatalyst (Sinn and Kaminsky, 1980). The maximum activity of
these catalysts is achieved at very low concentrations (about 10-4-10-7
mol/L, on the basis of monomer). It was found that the catalyst based
on zirconium has the greater thermal resistance.
The synthesis of metallocenes is a multi-stage process in
which one of the main initial components are cyclopentadiene and its
derivatives. Cyclopentadiene molecule contains five carbon atoms,
one of which is completely saturated and is connected to two
hydrogen atoms, which display acid properties. Therefore, the
interaction of cyclopentadiene with metallic sodium is a redox
reaction, which results in a negatively charged cyclopentadienyl
anion with the aromatic cycle structure, and the counter ion is Na+.
H
+
2
2
Na
-
H
2
-
2
Na
+
For the polymerization of olefins, the most widely used are
sandwich and semisandwich metallocenes of IV group transition
metals: zirconium (Zr), hafnium (Hf) or titanium (Ti). For example,
to obtain dichlorzirconocene the reaction is conducted between ZrCl4
and sodium cyclopentadienyl in the boiling toluene environment:
+
-
2
Na
+
ZrCl
4
Zr
Cl
Cl
NaCl
2
+
Interacting of polymethylaluminoxane (MAO), which is an
activator of catalyst, with dichlormetallocene causes several
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