Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PREPARATION OF POLYTHIOPOLYCARBOXYLIC
ACIDS
FIELD OF THE INVENTION
The present invention relates to a process for the preparation of polythio-
polycarboxylic acids which is based on the reaction of
rnercaptoalkylcarboxylic acids
with sulfur at elevated temperature. The polythiocarboxylic acids prepared in
this
manner have a high content of bonded sulfur in the form of a polysulfidic
distribution
and are suitable, for example, for the preparation of rubber mixtures and
rubber
vulcanization products.
BACKGROUND OF THE INVENTION
Polythiodicarboxylic acids of the structure R1-(S)n-R2 are known from U.S.
Patent
No. 5,130,363 and are suitable e.g. for the production of rubber vulcanization
products with improved hysteresis properties. These compounds were prepared by
the reaction of mercaptoalkylcarboxylic acids with sulfur chlorides, such as
sulfur
dichloride or disulfur chloride. A disadvantage here is the handling of the
corrosive
sulfur chlorides and of the by-product hydrogen chloride.
EP-A 0,780,429 describes a preparation process for a mixture of di-, tri- and
tetra-
thiodipropionic acid starting from 3-chloropropionic acid and sodium
polysulfide in
water. In addition to the formation of amounts of NaCI as a waste product, a
further
disadvantage of this process is that the content of bonded sulfur is narrowly
limited
and a mixture which comprises dithiodipropionic acid to the extent of approx.
70%
and tri- and tetrathiodipropionic acid only to the extent of approx. 30% is
therefore
formed.
U.S. Patent No. 4,119,549 discloses a process for the preparation of
sulfurized
compounds by reaction of olefins with hydrogen sulfide and sulfur. The process
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leads to non-uniform products if the olefin components employed are those with
double bonds which readily undergo polymerization, in particular a,(3-
unsaturated
carboxylic acids.
SUMMARY OF THE INVENTION
Therefore, the object of the present invention is a preparation process for
particularly
pure polythiopolycarboxylic acids which allows the preparation of relatively
high
contents of bonded sulfur and gives no hydrochloric acid and sodium chloride
as
undesirable waste products.
It has now been found that polythiopolycarboxylic acids can be obtained in a
high
purity and with a high content of bonded sulfur and without sodium chloride or
hydrogen chloride as a waste product by a simple route by heating mercapto
alkylcarboxylic acids with sulfur.
DETAILED DESCRIPTION OF THE INVENTION
Therefore, the present invention provides a process for the preparation of
polythiopolycarboxylic acids of the formula
R'-(S)n R2 (I)~
wherein
Rl and R2 are identical or different and represent a linear or branched C1-C36-
alkyl
radical, preferably C~-C12-alkyl radical, or a CS-C18-, preferably CS-C12-
cycloalkyl radical, wherein the alkyl radical and the cycloalkyl radical are
substituted by 1 to 4, preferably 1 to 2, carboxyl or carboxylate groups and
wherein, in the case of a carboxylate group, an ammonium group, a C1-C18-
mono-, -di- or -trialkylammonium group, a C6-C1g-mono-, -di- or
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-triarylammonium group, a C7-C21-mono-, -di- or -trialkylarylammonium
group or a 1- to 4-valent metal ion is present as the cation, and
n represents an integer from 2 to 14,
wherein mercaptoalkylcarboxylic acids of the formulae
Rl-S-H (IIa),
R2-S-H (IIb),
wherein
R' and R2 have the above meaning,
or mixtures of corresponding mercaptoallcylcarboxylic acids are reacted with
sulfur
in a molar ratio of 1:0.25 to 1:5, preferably 1:0.5 to 1:3, more preferably
1:1 to 1:2.5,
at temperatures of 70 to 220°C, preferably 100 to 170°C,
optionally in the presence of
acid or alkaline catalysts and optionally in the presence of an inert solvent.
Possible radicals R1 and R2 are, for example:
-CH2COOH, -CH2CH2COOH, -CH(CH3)COOH, -CH(COOH)CH2COOH,
-cyclohexyl-COOH.
Examples which may be mentioned of ammonium groups serving as cations are:
ammonium, N-butylammonium, N-cyclohexylammonium, N-octadecylammonium,
trimethylammonium, triethylammonium and tributylammonium.
Possible 1- to 4-valent metal ions are e.g.:
Li, Na, K, Mg, Ca, Zn, A1 and Ti.
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Possible mercaptoalkylcarboxylic acids of the above formulae (IIa) and (IIb)
are e.g.
thioglycollic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, 4-
mercaptobutyric acid, u~-mercaptoundecanoic acid and 2-mercaptosuccinic acid,
more preferably 2-mercaptopropionic acid, 3-mercaptopropionic acid and 2-
mercaptosuccinic acid.
Polythiocarboxylic acids which are preferably prepared by the process
according to
the present invention are of the following formulae:
HOOC~ ~COOH
HOOC~Sn~ COOH Sn
COOH ~ ~.COOH
HOOC Sn~ HOOC Sn
HOOC~Sn~COOH
HOOC Sn COOH HOOC COOH
~Sn~
HOOC~Sn~ COOH
HOOC Sn,~COOH
HOOC~ ~COOH HOOC COOH
Sn ~Sn~
HOOC COOH HOOC
Sn
HOOC Sn COOH
HOOC COOH HOOC
C
OOH
HOOC Sn COOH HOOC Sn~
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The preferred compounds, of course, . also include their corresponding
ammonium or
metal salts. In the formulae, the number n has the above-mentioned meaning
with
the preferred range described.
Because of the incorporation of varying amounts of sulfur during the process
according to the present invention, the compounds (I) are as a rule mixtures
of
polysulfide compounds, so that n in the empirical composition of the compound
(I)
can also assume non-integral arithmetic mean values, so that the mean value n
is
preferably in the range from 2.5 to 7, more preferably 2.5 to S.
To carry out the process according to the present invention, the
mercaptocarboxylic
acids in question, either individually or as a mixture with one another, are
mixed with
sulfur in the above-mentioned molar ratio and the mixture is heated to the
temperatures also stated. Hydrogen sulfide escapes during the reaction. The
reaction
times for the reaction are in the range from a few minutes up to several
hours,
depending on the reaction conditions and the mercaptocarboxylic acid employed.
The reaction according to the present invention can be accelerated by
catalysts.
Suitable catalysts are e.g. water and basic compounds, such as amines, e.g.
water
and cyclohexylamine or primary C12- or C24-alkylamines, such as dodecylamine,
oleylamine or stearylamine, or acid compounds, such as disulfur dichloride.
The catalysts are preferably employed in amounts of 0.01 to 1 wt.%, in
particular
0.05 to 0.5 wt.%, based on the total amount of the reaction mixture.
The reaction according to the present invention can optionally be carned out
in a
suitable inert solvent, such as an ether, e.g. tetrahydrofuran and dioxane, or
an
aliphatic hydrocarbon, such as cyclohexane. However, the process according to
the
present invention is preferably carried out without a solvent.
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After the reaction, the reaction mixture can be after-treated by known
measures for
fiurther purification and removal of the dissolved hydrogen sulfide, e.g. by
degassing
in vacuo or blowing out with nitrogen or air, washing with water or extraction
with
organic solvents, or by treatment with appropriate oxidizing agents, e.g.
hydrogen
peroxide or organic peroxides. In the normal case, the purity of the resulting
polythiopolycarboxylic acid after the degassing is already sufficient, so that
further
washing with water or extraction with organic solvents can be omitted.
Both natural and synthetic rubbers are suitable for the production of the
rubber
mixtures according to the invention containing the polythiocarboxylic acids
prepared
according to the invention.
Preferred synthetic rubbers are described for example in W. Hofinann,
Kautschuk-
technologie, Gentner Verlag, Stuttgart 1980, and I. Franta, Elastomers and
Rubber
Compounding Materials, Elsevier, Amsterdam 1989. They include, inter alias
BR polybutadiene
ABR butadiene/acrylic acid C1-4 alkyl ester copolymers
CR polychloroprene
IR polyisoprene
SBR styrene/butadiene copolymers having styrene contents of 1-60,
preferably 20-50 wt.
IIR isobutylene/isoprene copolymers having isoprene contents of 0.01 to
5 wt. % (butyl rubber)
BR-IIR brominated isobutylene copolymers having bromine contents of
between 0.01 and 4 wt. % (bromobutyl rubber)
Cl-IIR chlorinated isobutylene copolymers having chlorine contents of
between 0.01 and 4 wt. % (chlorobutyl rubber)
BIMS brominated copolymers containing 0.05 to 3 mol% of benzyl- bound
bromine
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NBR butadiene/acrylonitrile copolymers having acrylonitrile contents of S-
60, preferably 10-40 wt.
HNBR partially or completely hydrogenated NBR rubber
EPDM ethylene/propylene/diene copolymers having dime contents of
between 0.1 and 20 wt. %, preferably 0.5 to 10 wt.
CO polyepichlorohydrin
ECO copolymers of epichlorohydrin and ethylene oxide
EAM ethylene/vinyl acetate copolymers having vinyl acetate contents of 20
to 90 wt. %, preferably 40 to 80 wt.
and mixtures of these rubbers. Of interest for the production of motor vehicle
tyres
are in particular natural rubber, polyisoprene, emulsion SBR and solution SBR
rubbers having a glass transition temperature of higher than -50°C,
which can
optionally be modified with silyl ethers or other functional groups, in
accordance
with EP-A 447,066, polybutadiene rubber having a high 1,4-cis content (>90%)
which has been produced with catalysts based on Ni, Co, Ti or Nd, as well as
polybutadiene rubber having a vinyl content of up to 75%, butyl rubber,
bromobutyl
rubber, chlorobutyl rubber and mixtures thereof.
Suitable fillers for the rubber mixtures according to the invention are all
known
fillers used in the rubber industry, including both active and inactive
fillers.
The following fillers may be mentioned:
- highly dispersed silicas produced for example by the precipitation of
solutions
of silicates or by the flame hydrolysis of silicon halides having specific
surface areas of 5 - 1000, preferably 20-400 m2/g (BET surface area) and
primary particle sizes of 10-400 nm. The silicas can optionally also be
present in the form of mixed oxides with other metal oxides, such as for
example Al, Mg, Ca, Ba, Zn, Zr or Ti oxides;
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- synthetic silicates, such as for example aluminium silicate, alkaline earth
silicate, such as magnesium silicate or calcium silicate, having BET surface
areas of 20-400 m2/g and primary particle diameters of 10-400 nm;
S - natural silicates, such as kaolin and other naturally occurring silicas;
glass
fibres and glass fibre products (mats, strands) or microglass beads;
- metal oxides, such as zinc oxide, calcium oxide, magnesium oxide and
aluminium oxide;
- metal carbonates, such as magnesium carbonate, calcium carbonate and zinc
carbonate;
- metal hydroxides, such as for example aluminium hydroxide and magnesium
hydroxide; and
- carbon blacks. The carbon blacks to be used are produced by the flame black,
furnace or gas black processes and have BET surface areas of 20 - 200 m2/g,
e.g. SAF, ISAF, HAF, FEF or GBF blacks.
Highly dispersed silicas and/or carbon blacks are preferably used as the
fillers, the
mixing ratio of carbon black to silica being 0.05 to 30, more preferably 0.1
to 10.
The rubber mixtures according to the invention can also contain other rubber
auxiliaries which serve, for example, to further crosslink the rubber
mixtures, or
which improve the physical properties of the vulcanizates produced from the
rubber
mixtures according to the invention for their specific type of application.
The cross-linking agents used are sulphur or sulphur-donating compounds or
peroxides. Sulphur or sulphur-donating compounds are particularly preferably
used
in quantities of 0.01 to 3 parts by weight, based on the rubber. In addition,
as
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mentioned, the rubber mixtures according to the invention can contain other
auxiliaries, such as the known reaction accelerators, antioxidants, heat
stabilizers,
light-stability agents, antiozonants, processing aids, reinforcing resins,
such as for
example phenolic resins, steel cord bonding agents, such as for example
silica/resorcinol/hexamethylene tetramine or cobalt naphthenate, plasticizers,
tackifiers, blowing agents, dyestuffs, pigments, waxes, extenders, organic
acids,
retardants, metal oxides and filler activators, and in particular
polysulphidic silanes,
such as bis-(triethoxysilylpropyl)-tetrasulphide.
The rubber auxiliaries according to the invention are used in the customary,
known
quantities, the quantity used depending on the subsequent type of application
of the
rubber mixtures. Quantities of rubber auxiliaries in the range from 2 to 70
parts by
weight, based on 100 parts by weight of rubber, are for example customary.
The rubber mixtures according to the invention can for example be produced by
mixing the rubbers with the polythiocarboxylic acids prepared according to the
invention, fillers, rubber auxiliaries and crosslinking agents, in suitable
mixing
apparatuses, such as kneaders, rollers or extruders.
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EXAMPLE
Preparation of HOOC-CH2CH2-Sh CH2CH2-COOH
A mixture of 53 g (0.5 mol) 3-mercaptopropionic acid, 24 g (0.75 mol) sulfur
and
0.3 g oleylamine was heated to 130°C, during which evolution of
hydrogen sulfide
started. The mixture was subsequently stirred at 130°C for 2.5 hours
and a vacuum
was then applied for 5 minutes in order to remove residual hydrogen sulfide.
After
cooling, 69.1 g polythiodipropionic acid were obtained as pale yellow
crystals.
Melting range: 95 to 130°C. The HPLC chromatogram shows that it is a
mixture of
homologous polysulfides where n = 3-11. (Mean value: 4).
TABLE 1
Elemental Analysis: C H S
Calculated for -S4- 26.3 3.7 46.7%
Found 26.3 3.6 47.5%
Although the invention has been described in detail in the foregoing for the
purpose of
illustration, it is to be understood that such detail is solely for that
purpose and that
variations can be made therein by those skilled in the art without departing
from the
spirit and scope of the invention except as it may be limited by the claims.
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