Note: Descriptions are shown in the official language in which they were submitted.
~52~
Sulphur vulcanizable rubber compositions with improved
degree of vulcanization
~his invention relates to vulcanizable rubber compositions
containing sulphur as vulcanizing agent, a thiuramic (and
optionally a thiazole) accelerator and the salt of a
partial di- or tricarboxylic acid ester residue, prefer-
- ably of a succinic acid semi-ester and/or glutaric acid
semi-ester, as vulcanization promoter, to a process for
the preparation of these rubber compositions by mixing the
components and to-the use of these rubber compositions for
the production of vulcanizates.
Vulcanization accelerators are, as is known, substances
which shorten the vulcanization time or enable vulcaniza-
tion to be carried out at a lower temperature; see
- Ullmanns Encyclopadie der technischen Chemie, 3rd Edition,
Urban & Schwarzenberg, Munich-Berlin 1957, pages 383 et
seq.
Additional accelerators may be used to enable the full
effect of the vulcanization accelerators to be obtained.
So-called "EV-Systems" ("EV" = efficient vulcanization)
are frequently used for sulphur vulcanization, in which
case the vulcanization accelerators used are in most cases
Le A 27 838 - Foreign Countries
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thiuramic compounds such as tetraalkylthiuramic
monosulphides (e.g. tetramethylthiuramic monosulphide),
tetralkylthiuramic tetrasulphides (e.g. tetramethyl-
thiuramic tetrasulphide) and, preferably, tetralkyl-
thiuramic disulphides (e.g. tetramethylthiuramicdisulphide, hereinafter referred to as TMTD). These
thiuramic compounds are also referred to as ultra
accelerators owing to their powerful action; in particular
the vulcanizates produced with the aid of these acceler-
ators have a high resistance to hot air.
Thiuramic compounds have the property that either they orthe reaction products produced from them during vulcaniza-
tion tend to bleed. This has the undesirable effect of
producing a deposit on the vulcanizate. The tendency to
bleed depends inter alia on the nature of the rubber and
on the concentration of the thiuramic compound (see
W. Hofmann in Gummi-Asbest-Kunststoffe 9 (1986), pages 422
et seq).
Vulcanizates of rubbers which have a low CzC double bond
content, such as EPDM, butyl rubber and especially nitrile
rubber, whose C~C double bonds are selectively but not
completely hydrogenated so that they are still available
for sulphur cross-linking, frequently have a low degree of
cross-linking, which i5 recognised by the low tension
values and moderate pressure deformation residue,
generally known as compression set (hereinafter referred
to as DVR =Druckverformungsrest), determined by the
deformation under pressure in the heat, e.g. in hot air or
hot oil. The phenomenon of bleeding mentioned above
limits the amount by which the compression set can be
increased by using larger quantities of thiuramic
compound.
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It has now surprisingly been found that a vulcanizing
system containing a small quantity of sulphur (optionally
in the form of a sulphur donor), thiuramic accelerators,
optionally thiazole accelerators, and the salt of a
partial di- or tricarboxylic acid ester as vulcanization
promoter has an excellent degree of cross-linking with
high tension values and low compression set values, in
particular after ageing at elevated temperatures. The
product is also found to be more easily processible and to
1~ display ;mproved hot-air a~eirlg.
With the aid of the vulcanization promoter the quantity of
sulphur can be reduced without any essential properties of
the vu1canizate being adversely affected. The reduced
quantity of sulphur is advantageous with regard to ageing
and also allows an improvemen~ in the covulcanization of
rubbers having a low content of double bonds with diene
rubbers, such as for example that of partially hydrogenated
NBR wi th normal NBR .
The present invention thus relates to vulcanizable rubber
compositions based on rubber having a low C=C double bond
content and containing
~rom 0.2 to 1% by weight of sulphur,
from 1 to 3.5% by weight of thiuramic accelerator
and optionally up to 2~ by weight, preferably from 0.2 'o
1~ by weight, of thiazole accelerator, and
from 0.1 to 8% by weight, preferably from 0.3 to 6, most
preferably from 0.8 to 4% by weight of the salt of a
partial di- and/or tricarboxylic acid ester,
the percentages being based in each case on the quantity
of rubber to be vulcanized.
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The invention further relates to a process or the
preparation of these rubber compositions by mixing the
components and to their use for the production of
vulcanisates.
Rubbers with a low C=C double bond content for the purpose
of this invention comprise those having iodine numbers of
from 2 to 35, preferably from 3 to 30, in particular from
5 to 25. Determination of the iodine numbers is generally
carried out by the addition of iodochloride in glacial
acetic acid according to Wijs, DIN 53 241, Part 1.
The iodine number defines the quantity of iodine in grams
which is chemically bound by 100 g of substance. Examples
of preferred rubbers include EPDM, butyl rubber and
especially hydrogenated nitrile rubber.
~he rubbers preferably have glass transition temperatures
below O~C, in particular below -lO~C.
The term "EPDM" stands ~or ethylene/propylene/diene
terpolymers. EPDMs comprise rubbers in which the ratio by
weight o~ ethylene to propylene groups is in the range o~
~rom 40:60 to 65:35 and which may contain from 1 to 20 C-C
double bonds per 1000 carbon atoms. The following are
examples o~ suitable dlene monomers in the EPDM:
Conjugated dienes, e.g. isoprene and butadiene-(1,3), and
non-conjugated dienes having 5 to 25 carbon atoms, e.g.
1,4-pentadiene, 1,4-hexadiene, 1,5-hexadiene, 2,5-
dimethyl-1,5-hexadiene and 1,4-octadiene; cyclic dienes,
e.g. cyclopentadiene, cyclohexadiene, cyclooctadiene and
dicyclopentadiene; alkylidene and alkenyl norbornenes,
e.g. 5-ethylidene-2-norborne, 5-butylidene-2-norbornene,
2-methallyl-5-norbornene, 2-isopropenyl-5-norbornene and
tricyclodienes.
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The non-conjugated dienes, hexadiene-(1,5), ethylidene
norbornene and dicyclopentadiene, are preferred. The diene
content of the EPDN is preferably from 0.5 to 10% by
weight, based on the EPDM.
s
EPDM rubbers of this type are described e.g. în DE-OS
2 808 709.
The term "butyl rubber" used in the context of this
invention covers isobutene copolymers of from 95 to 99.5%
by weight, preferably from 97.5 to 99.5% by weight, of
isobutene and from 0.5 to 5% by weight, preferably from
0.5 to 2.5% by weight, of copolymerisable diene, e.g.
butadiene, dimethylbutadiene, pentadiene-(1,3) and
especially isoprene. Butyl rubber is produced almost
exclusively as an isobutene/isoprene copolymer on a large
technical scale by cationic solution polymerisation at a
low temperature; see e.g. Kirk-Othmer, Encyclopedia of
Chemical Technology, 2nd Edition, Volume 7, page 688,
Interscience Publisher, New York-London-Sydney 1965 and
Winnacker-Kuchler, Chemische Technologie, 4th Edition,
Volume 6, pages 550-555, publishers Carl Hanser Verlag,
Munich-Vienna 1962.
~he preferred hydrogenated nitrile rubbers are based on
butadiene/acrylonitrile copolymers having a copolymerised
acrylonitrile content of from 5 to 60% by weight,
preferably from 10 to 50% by weight. "Hydrogenated" means
in this context that from 90 to 99%, preferably from 95 to
98.5%, and in particular from 96 to g8.5 % of the C=C-
double bonds which are capable of being hydrogenated are
hydrogenated. The degree of hydrogenation may be determined
IR spectroscopically.
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The hydrogenation of nitrile rubber is known; US-PS
3 700 637, DE-OS 25 39 132, 30 46 008, 30 46 251,
32 27 650 and 33 29 974, EP-A 111 412 and FR-PS 2 540 503.
Hydrogenated nitrile rubber is particularly distinguished
by its comparatively high resistance to oxidation.
s
Rubbers suitable for the process according to
the invention generally have Mooney viscosities
(DIN 53 523) of from 10 to 150, preferably from 25 to 80
(ML 1~4)/lOO~C.
;''
The preferred thiuramic accelerators include, for example,
the above-mentioned tetraalkylthuramic mono- and poly-
sulphides in which the alkyl groups generally have 1 to 4,
preferably 1 or 2 carbon atoms, but the substituents may
also be cycloaliphatic, aromatic or araliphatic.
Preferred thiazole accelerators which may also be used
according to the invention include in particular
2-mercaptobenzothiazole,
dibenzothiazyl-disulphide,
benzothiazyl-2-cyclohexylsulphenamide (CBS),
benzothiazyl-2-tert.-butylsulphenamide (TBBS),
~-morpholinothio-2-benzothiazole (~BS~,
benzothiazyl-2-diisopropylsulphenamide (DIBS),
benzothiazyl-2-tert.-amylsulphenamide ~AMZ),
benzothiazyl-dicyclohexylsulphenamide (DCBS) and
morpholino-thiocarbonyl-sulphenomorpholide ~OTOS).
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The preferred partial di- and tricarboxylic acid esters
(the semiesters in the case of dicarboxylic acid esters;
the diesters in the case of tricarboxylic acid esters)
whose salts are used according to the invention as
vulcanization promoters include the esterification
products of aliphatic C4-C10-dicarboxylic acids (preferab-
ly adipic acid, in particular succinic acid, glutaric
acid), of cycloaliphatic C8-C12-dicarboxylic acids
(preferably tetrahydrophthalic acid, hexahydro-phthalic
acid) of aliphatic C6-C12-tricarboxylic acids (preferably
citric acid) and of C8-C14-benzene di- and tri-carboxylic
acids (preferably phthalic acid, isophthalic acid,
terephthalic acid, trimesic acid, trimellitic acid) with
C1-C18, preferably C4-C18 alcohols. The alcohol components
may be aliphatic, cycloaliphatic, araliphatic or aromatic;
they may contain (cyclo)olefinic C=C-double bonds as well
as halogen substituents, e.g. chlorine substituents, and
they may be linear or branched. Preferred alcohol
components include methanol, isopropanol, n-, iso- and
tert.-butanol, hexanol, octanol, decanol, dodecanol,
stearyl alcohol, cyclohexanol, benzyl alcohol and phenol,
Succinic acid monooctylesters and glutaric acid monobutyl-
esters are particularly preferred.
The cations of the salts of partial esters to be used
according to the invention are preferably derived from
alkali metals and alkaline earth metals and zinc, the zinc
salts being particularly preferred.
Vulcanization auxiliaries and, if required, activators,
fillers such as carbon black, plasticizers, age resisters
and/or processing aids may be added to the rubbers in the
usual quantities before vulcanization.
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The most important inorganic activators are metal oxides,
in particular zinc oxide. Magnesium oxide or calcium
hydroxide is also used in individual cases.
The processing auxiliaries used may be, for example, fatty
acids, e.g. stearic acid.
Mixing of the components may be carried out in convention-
al mixing apparatus.
The mixing apparatus used are preferably those convention-
ally used in the rubber industry, such as kneaders,
rollers, internal mixers and mixing extruders, which
generally operate with shear rates of from 1 to 1000
sec~1, preferably from 1 to 200 sec~1.
Vulcanization may be carried out at temperatures from 100
to 200~C, preferably at 130 to 180QC, optionally under a
pressure of from 10 to 200 bar.
The excellent vulcanization properties are generally
obtained even without tempering but may often be further
improved by tempering.
The vulcanizates obtainable according to the invention are
excellent materials for transmission belts or toothed
belts, for which their high recovery capacity is a great
advantage. They also have excellent properties as sealing
materials of all kinds and may be used inter alia for the
inner or outer layers of tubes. They may also be used for
rubber-izing textiles, for lining cavities and as
insulating materials and sheaths for cables. The vulcani~ation
systems can also be used for the production of friction
linings.
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Exammles
The rubber used for the following Examples was a hydrogen-
ated acrylonitrile/butadiene copolymer having an acrylo-
nitrile content of 33.7% by weight, a degree of hydrogena-
tion of 96.4~, based on the C=C-double bonds originally
present, and a Mooney viscosity of 67 (ML 1 ~ 4~ lOO~C
((R)Therban 1707 S of Bayer AG).
100 Parts of rubber were masticated in a laboratory
kneader at 50~C for O.S minutes and 0.51 parts of sulphur,
1 part of stearic acid, 2 parts of zinc oxide, l part of
octylated diphenylamine ((R)Vulkanox OCD of Bayer AG),
0.4 parts of zinc methylmercaptobenzimidazole ((R)Vulcanox
ZMB2 of Bayer AG), 45 parts of carbon black (Corax N550 of
Degussa/Wesseling) and varying quantities (see Table 1) of
the zinc salt of succinic acid octyl semiester were then
added and the mixture was kneaded until homogeneous (4.5
minutes).
After the rubber mass had been cooled to about 100CC on a
roller, an accelerator system consisting of 2 parts of
tetramethylthiuramic disulphide (~R)Vulkacit Thiuram C of
Bayer AG) and 0.5 parts of benzothiazyl-2-cyclohexyl-
sulphenamide (~R)Vulkacit CZ of Bayer AG) were added.
The properties of the resulting mixtures are listed below:
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Therban 1707 S lOo 100 100
Sulphur 0.51 0.51 0.51
5 Vulkanox OCD 1 1 1
Vulkanox ZMB2 0.4 0.4 0.4
10 Carbon black N 550 45 45 45
Zinc oxide 5 5 5
15 Stearic acid 0.5 0.5 0.5
Vulkacit Thiuram C 2 2 2
Vulkacit CZ 0.5 0.5 0.5
Zinc succinic acid
octyl semiester _ l.S 3
======z====================== ======= ====== =========
Mooney viscosity MLl+4/120C 78 76 76
25 Mooney-Scorch at 130C (min) 18.5 18.6 17.3
Vulkameter 160C
tlo ~min) 4,5 4.8 4.8
t8n ~min) 7.9 8.2 9.0
Fm~n ~N) 2.1 2.1 2.0
Fmax ~N) 5~4 58.6 57.6
35 Vulcanization 30 min./1609C ~ 2 rods, DIN 53 02~53504)
Tensile strength ~MPa) 29.7 29.8 30.1
Elongation at break ~%) 490 480 480
40 Tension S100 ~MPa) 3.8 4.2 4.3
Shore hardness 72 72 73
45 C.S.*,Probek. II 70 h/100C 59.2 47.7 44.4
C.S.*, Probek. I, 70 h/100C 44.8 30.3 25.2
with temp. 6 h/150C ~%)
relative elongation~) after
50 hot air ageing(l0 days/1509C) 41 54 52
* Compression set
Le A 27 838 11
