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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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slurry recycling. Calcium chloride and the required amount of
partially softened water are loaded from tank 9 to the unit 8. Solution
is pumped through the filter 11 into the line of circulating water (see
Fig.), where precipitated calcium stearate is formed when interacting
of potassium hydroxide, potassium stearate and calcium chloride,
which is sent to the degassing system.
2.9 Obtaining Butadiene-Styrene (α-methylstyrene) Rubbers by
Free Radical Initiation of Emulsion Polymerization
Emulsion polymerization is one of the most widespread industrial
methods for synthetic rubber producing. As a result of this process
the synthetic latex is obtained, which is directly used in the rubber
and other industries, or is processed into rubber by coagulation and
separation of the solid polymer.
Emulsion polymerization is characterized by the relative ease
of process control, good heat exchange conditions, the possibility of
obtaining a wide range of rubbers, the modifying of the properties of
the resulting rubbers, latex filling with oil, carbon black, resins, etc.
Styrene butadiene and butadiene-α-methylstyrene rubbers (BSK, or
SKS and SKMS), butadiene (SKN or BNK), chloroprene rubber, etc.
are derived by emulsion polymerization.
The emulsion polymerization processes are always proceed
by a radical mechanism at high (48÷50°C) or cold (5°C) temperature
and are called as "hot" or "cold" polymerization, respectively.
2.9.1 Initiation of Polymerization
Polymerization is initiated by free radicals and the growth of the
macromolecule begins with addition of the initiating radical R• to the
double bond to form new radical.
CH2CHX
R
+ CH2CHX
R CH2CHX
61
R CH2CHX CH2CHX
O
O
O
O
O
O
O
O
In every act of joining a radical regeneration occurs, and the
initiator is involved only in the first act of addition.
Free radicals are received:
- Using compounds that can decompose to form free radicals (real
initiation);
- Photochemical and radiation initiation.
Chemical initiation is used for large polymers production, in
which the special substances (initiators) are loaded into the system,
which decay into free radicals more easily than the monomer.
The initiators can be divided into water-soluble and oil soluble.
The most common group of initiators are peroxides. The decay of
peroxides is as follows:
X O O X' X O
+
O X'.
Water-soluble initiators include hydrogen peroxide (HO-OH).
However, due to its instability during storage it is used rarely for
practical purposes. The most popular among the water-soluble
initiators are inorganic peroxides - persulphates or perborates, the
most accessible and widely spread of which is potassium persulfate
(K2S2O8).
The dissolution of potassium persulfate in the aqueous phase
causes its dissociation with the formation of cation and anion:
K O S
O
O O S
O
O
O
O K
2K
O S
O
O O S
+
+
O
O
-
.
Later, the anion brakes down like any other peroxide
according to the scheme:
O S
O O S
O
-
O S
2
O.
Organic peroxides or hydroperoxides, azo-and diazo
compounds with covalent bonds, which are easy to decompose when
62
heated, can be used as oil-soluble initiators. However, the high
decomposition temperature (70÷140°C) does not allow to use them
as individual initiators for industrial emulsion polymerization.
To lower the decomposition temperature of hydroperoxides to
+5°C the redox systems are used in emulsion polymerization in the
industry. The most common is the iron-Trilon rongalite redox
system. Hydroperoxide is used as an oxidizer, and transition metal
ions in a lower oxidation state are used as a reducing agent among
which iron (Fe+2) is the only practice-relevant, introduced into the
aqueous phase in the form of sulfate FeSO4.
RO–OH + Fe+2 → RO• + Fe+3 + HO–.
Isopropylbenzene hydroperoxide (HPIPB) is often used
among hydroperoxides:
CH
3
C6H
C
CH
O OH
3
.
5
In order to maintain constant concentration of Fe+2 ions for a
long time in the system the complexing agent Trilon B (disodium
ethylenediaminetetraacetate) is used, in which Fe+2 is additionally
linked by coordination bonds with nitrogen atoms.
Na O C(O)
Na O C(O)
CH2N CH2C(O)
CH
2
CH
2
CH2N CH2C(O)
Fe
O
O
To reduce the number of iron ions in the rubber, getting in
while separating from latex, there is used an additional reducing
agent - a product of the interaction of formaldehyde with sodium
sulfite (rongalite):
HO-CH2-S(O)-O-Na.
The rongalite action is shown in Figure:
63
+ RO
Рекомбинаци
я
rongalite
+2
Fe
+ ронгалит
ОН
Fe
+3
RO
.
+ HO
-
2.9.2 Reactions of Polymer Chain Growth
The radical formed at the initiation stage (e.g., RO•) is only once
involved in the reaction with the monomer, originating the chain
growth reaction, and this completes its role:
RO• + M → RO-M•.
Further the growth of the polymer chain occurs only as the
addition of monomer to the active macroradical:
~~M• + M → ~~M-M•.
2.9.3 Chain Termination and Transfer
The main reactions of chain termination are recombination, and
disproportionation, occurring during the interaction of two growing
macroradicals:
CH2CH CH CH
+
X X
Recombination
2
Диспропор-
Disproportionation
ционирование
~~
CH2CH
~~
CH2CH
CH CH
X
X
+
2
X
HC
2
CH
X
In principle, the transfer of the active center can occur to any
molecule present in the reaction mass.
Chain transfer to monomer occurs according to the scheme:
~~M• + M → ~~M-H + •M-H,
where M-N is the molecule of the monomer from which a hydrogen
atom splits off homolytically.
When transferring the polymer chain in the macromolecule
hydrogen atom can be split off from any part of the polymer chain:
64
~~M• + ~~MM~~ → ~~MH + ~~M-•M-H~~
Formed as the part of one of the links, the free radical gives
rise to the growth of new chain which leads to the branching of the
macromolecule.
In addition, chain transfer can occur to any molecule of
another substance (solvent, special additives, etc.). In general, the
reaction can be written as:
~~M• + A-H → ~~M-H + A•.
If the radical A• is active enough to bind the molecule of the
monomer, a new polymer chain is formed:
A • + M → M • A, etc.
Substances of such action are called as chain-terminating
agents. One of the most effective industry chain-terminating agents is
tert-dodecyl mercaptan (S12N25SH).
If the radical A• is insufficiently active and unable to bind the
molecule of the monomer, it will only enter into the recombination
reactions with the growing polymer chains, stopping their growth:
~~M• + A• → ~~M-A.
Therefore, these substances are the inhibitors of radical
reactions.
2.9.4 Components of Emulsion Polymerization
In emulsion polymerization in a heterogeneous system there are
always: the dispersed phase (monomer or monomers mixture in the
copolymerization), a dispersion medium (aqueous phase), an
emulsifier and polymerization initiator.
Monomers. In the manufacture of synthetic rubber there are used:
butadiene, chloroprene, styrene, α-methyl styrene, acrylonitrile, 2methyl-5-vinyl pyridine, vinylidene chloride, methacrylic acid, etc.
Emulsifiers. In the production of the majority of emulsion rubbers
and latexes there are used such anionic surfactants as: alkaline or
ammonium salts of carboxylic acids RC(O)OMt (where Mt = Na, K),
65
sulfonic acids (RSO3Mt - sulfonates) and sulfoesters (ROSO3Mt alkylsulphates).
One of the major industrial emulsifiers are soaps based on
rosin, containing about 90% resin acids, the basic of which is abietic
acid and its derivatives.
СН
СОО Н
3
СН
СН
СН
Abietic acid
Аби ети нова я ки сло та
3
3
Н С
3
Abietic acid usually goes through disproportionation.
As emulsifiers there can be used carboxylic acetates (for
example, laurates, stearates, oleates); synthetic fatty acids (SFA) of
C10÷C13-, C10÷C18-fractions (paraffinates), C12÷C14-fractions; tall oil
soaps; potassium alkilbenzylbenzoates of C14÷C18-fraction (ABBP);
fluorinated aliphatic acids of general formula H(SF2)nCOOH, where
n = 4 ÷ 10.
Another type of anion-active emulsifiers in the synthetic
rubber industry are alkylarylsulfonates among which is successfully
used the nekal emulsifier, a mixture of sodium salts of mono-, di-and
tributylnaphthalenesulfonic acid.
Among the group of alkylarylsulfonates is also leikanol
(dispersing agent NF), a sodium salt of the β-naphthalenesulfonic
acid and formaldehyde condensation product, corresponding to the
formula:
Н
,
n
Na
3
where n = 1÷9.
SO
С Н
2
Na
3
SO
Electrolytes and addition agents. Electrolytes are used to lower the
surface tension and the viscosity of latex. Potassium chloride or
sodium sulfate are the most commonly used electrolytes. Buffering
agent is used to create a constant pH during the synthesis of rubber.
66
Sodium carbonate (soda ash) and trisodium phosphate are the most
commonly used buffer additives, which support the pH in the range
10,2±1,0 and 12,4 ± 1,0, respectively. Their content is 2-4% of the
polymer weight.
Initiators. In the SR industry potassium persulfate is used as a
water-soluble initiator, while hydroperoxides in combination with the
oxidation-reduction systems are used as oil-soluble initiators.
Polymers chain-terminating agents. In the SR industry mercaptans
are usually used as chain-terminating agents, particularly, dodecyl
mercaptan (S12N25SH), mostly tertiary; as well as organic disulfides,
thioethers (for example, diisopropylxanthogen disulfide (diproxyd)
and bis-ethylxanthogen disulfide). The amount of agent in the
emulsion is 2-5% by weight of the polymer.
Stoppers (short-stopping agent). Emulsion polymerization is rather
high speed process up to 60% conversion of monomers. At
conversion more than 60% such secondary processes as cross-linking
and isomerization start playing a significant role. Using active
peroxides the monomer conversion can reach 70%.
The time required to achieve 60-62% conversion depends on
the mass ratio of the aqueous and hydrocarbon phases. The reaction
time for standard system of butadiene-α-methylstyrene at ratios of
200:100 and 133:100 is 16 and 20 hours, respectively.
Hydroquinone and sodium dimethyldithiocarbamate are used
as stopper in the industry.
Stabilizers (other terms are also used: antioxidants, antiaging agents,
oxidation preventives). Stabilizer is a substance that protects the
polymer in the latex and commercial rubber from oxidation and
thereby ensures the latex and rubber properties retention during
storage and processing. As stabilizers there are used secondary
aromatic amines (naphtham-2 and diphenyl-para-phenylendiamine
(DPPD)), VTS-60 and VTS-61, DPPD derivatives; secondary amines
VTS-120, VTS-150, VTS-200; VS-1, a condensation product of
alkyl phenols with urotropin. There are also used phenolic
compounds such as VS-30 (dioktylbutylphenol), different grades of
67
Agidols; resorcinformaldehyde, alkylresorcinformaldehyde,
aminophenol oligomers (resins) as stabilizers.
2.9.5 Polymerization Rate
For the systems with water-soluble initiator the polymerization rate
(W) is proportional to the concentration of emulsifier raised to the
0,5-th power:
0,5
W = k2⋅[M]⋅[I]
⋅[E]
0,5
,
where k2 - an effective polymerization rate constant, [M], [I] and [E]
the concentration of monomer, initiator and emulsifier, respectively.
For the systems with water-insoluble initiators the
polymerization rate is proportional to the concentration of emulsifier
raised to the first power:
W = k2⋅[M]⋅[I]
0,5
⋅[E].
Thus, an emulsifier does not only stabilize the drops of the
monomer and polymer-monomeric particles, but it also plays an
important role in the kinetics of the polymerization process.
2.9.6 Process of Obtaining Styrene-Butadiene Rubbers
The manufacture of styrene-butadiene rubber is a continuous process
and consists of the following stages: preparation of the hydrocarbon
and aqueous phases; the preparation of solutions of the initiator,
activator, regulator, stopper, and the antioxidant dispersion;
polymerization and its termination; distillation of unpolymerized
monomers from latex; rubber separation and drying.
The hook-up for emulsion continuous low-temperature
styrene-butadiene rubbers is shown in Figure 9. The aqueous phase
including solution of the main emulsifier, electrolyte and second
emulsifier (leikanol) is prepared by mixing these components in the
apparatus 1 dosed in accordance with the given formulation and has
pH 10-3.28. Finished aqueous phase by the pump 2 through the
68
cooler 3, chilled by brine, is fed to the diaphragm mixer 6 for mixing
with the hydrocarbon phase.
Fig.9 The polymerization scheme for obtaining emulsion lowtemperature styrene-butadiene rubbers:
1 - tank for the aqueous phase preparing; 2, 7, 9, 11, 13, 15 - pumps;
3, 5 - coolers; 4, 6 - diaphragm mixers; 8, 10, 12, 14 - apparatuses for
the preparation of components; 161-1612 - polymerizers; 17 - filter.
I - butadiene; II - styrene; III - softened water; IV - emulsifiers; V -
initiator; VI - iron complex; VII - rongalite; VIII - chain-terminating
agent; IX - stopper; X - brine; XI - latex for degassing.
The hydrocarbon phase is prepared by continuous mixing of
butadiene and styrene, pumping in a given ratio to the diaphragm
mixer 4, is cooled in the brine refrigerator 5 and mixed with an
aqueous phase in the diaphragm mixer 6, after which is supplied by
the use of pump 7 to the first unit of the polymerizers battery, as a
rule consisting of 12 standard polymerizers of 12 or 20 m3.
The initiator emulsion is prepared in the apparatus 8 out of
softened water, initiator and emulsifier, dosed from the
corresponding measuring tanks, and is fed to the mixing with the
69
emulsion of hydrocarbons by pump 9 into the batch line before the
first polymerizer 16.
The rubber chain-terminating agent (tert-dodecylmercaptan)
is used as a solution in styrene. Iron complex is prepared in the
absence of air in the form of fine suspension in the water by heating
the mixture of solutions of potassium pyrophosphate and iron (II)
sulfate with stirring or in the form of solution by saponification of
ethylenediaminetetraacetic acid by caustic potash followed by
reaction of the formed salt with the calculated amount of iron (II)
sulphate. Rongalite dissolves in water with stirring. Solutions
prepared according to the formulation of polymerization are fed to
the batch line before the first curing for mixing. All solutions of the
initial components are prepared and stored under nitrogen.
Polymerizers in the battery are connected so that the
polymerizable batch goes to the bottom of the apparatus through the
siphon and goes to the next unit from the top. Polymerizer is an
autoclave with a jacket and built-in tube bundles which via the brine
eliminate the heat released during polymerization. The apparatus has
a gate type agitator. All polymerizers (161-162 units) are connected
by three lines: one of them - the main one - is for the transferring the
product from one apparatus to another, the second - shunt line - is
intended for the deactivating of any apparatus from the battery for
repairing and cleaning, third - compensation - is used to discharge
switched off polymerizer. Usually there are 10-11 polymerizers in
use.
Upon reaching 60-70% conversion of monomers
(polymerization time is usually 10-11 h) latex is injected with 1%
aqueous solution of stopper - sodium dimethyldithiocarbamate.
Stopper is fed to the latex line after the last polymerizer, and then the
latex passes through the filter 17 where shots are separated and goes
to degassing. If necessary, antioxidant is injected in latex
simultaneously with the stopper.
The high-temperature rubbers are obtained according the
same scheme. The main differences in this case related to a smaller
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