Note: Descriptions are shown in the official language in which they were submitted.
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Rubber mixtures of dime rubbers containing hydroxyl and carboxyl ~rouns
and sulfur-free crosslinkin~ agents
. The present invention relates to rubber mixtures of dime rubbers containing
hydroxyl
and/or carboxyl groups, fillers and sulfur-free crosslinking agents which can
crosslink the rubber via the hydroxyl or carboxyl groups, and the use thereof
for the
preparation of rubber vulcanization products with improved dynamic damping,
improved mechanical strength, reversion resistance and adhesion to strength
supports. The rubber mixtures and vulcanization products are suitable for the
production of highly reinforced shaped rubber articles, in particular tyres,
which have
a particularly high resistance to thermal and mechanical stresses, a high wet
skidding
resistance, low rolling resistance and high abrasion resistance.
Numerous routes have been investigated for the production of motor vehicle
tyres
with reduced rolling resistance, improved wet skidding resistance, lower
abrasion
and higher heat resistance. The use of anionically polymerized solution
rubbers
containing double bonds, such as solution polybutadiene and solution styrene/-
butadiene rubbers, has proved to be particularly advantageous. The advantages
lie,
inter alia, in the controllability of the vinyl content and of the associated
glass
transition temperature, a favourable cis/trans double bond ratio and the
molecular
branching. When used in practice, particular advantages result from this in
the ratio
of wet skidding resistance and rolling resistance of the tyre. l1S-A 5 227 425
thus
describes the production of tyre treads from a solution SBR rubber and silica.
Emulsion and solution rubbers containing hydroxyl groups are described e.g. in
EP-A 806 452, but here the rubbers are not crosslinked via the hydroxyl groups
but
are vulcanized in the conventional manner by crosslinking with sulfur. Due to
the
process, the hydroxyl contents described there for solution rubbers are in a
particularly lower range (0.009 to 0.061%) and are therefore too low for
effective
crosslinking.
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A process for the preparation of solution polybutadiene rubbers containing
hydroxyl
and/or carboxyl groups (3.9 to 8.9 wt.%) is described, inter alia, in DE-A 2
653 144.
These rubbers are also crosslinked exclusively with sulfur, and because their
strength
is too low and their moduli are too low, they are not suitable as the main
component
in tyres.
A process for the preparation of solution rubbers containing hydroxyl groups
by
reaction of rubbers with particular long-chain hydroxylmercaptans is also
described
in EP-A 464 478. No references are given to the preparation of rubber mixtures
with
sulfur-free crosslinking agents which react with the hydroxyl groups and to
corresponding vulcanization products.
Unsaturated rubbers which have been grafted with metal salts of unsaturated
carboxylic acids are described in US-A S 962 593. Here also, crosslinking of
the
rubbers takes place by sulfur vulcanization. Furthermore, the content of
carboxylic
acid groups is considerably higher.
German Patent Applications no. 198 32 459.6, 198 52 648.2, 199 14 848.1,
199 20 788.7 and 199 20 894.8 describe solution rubbers containing hydroxyl
and/or
carboxyl groups with an advantageous content (0.1 to 3 wt.%) of hydroxyl
and/or
carboxyl groups. However, the patent applications give no indication of
improved
crosslinking systems which react with the functional groups.
The object of the present invention was therefore to provide mixtures of
rubbers,
particularly preferably solution rubbers, containing hydroxyl and/or carboxyl
groups
and specific crosslinking agents which react chemically with the hydroxyl or
carboxyl groups under vulcanization conditions, from which tyres with improved
wet
skidding resistance, lower rolling resistance, high mechanical and heat
resistance and
improved abrasion properties can be produced.
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The present invention therefore provides rubber mixtures comprising at least
one
rubber and 10 to 800 parts by wt. of filler, based on 100 parts by wt. of
rubber,
wherein the rubber has been prepared by polymerization of diolefins and
optionally
further vinylaromatic monomers and introduction of hydroxyl and/or carboxyl
groups, this rubber has a total content of 0.05 to 5 wt.% of bonded hydroxyl
and/or
carboxyl groups or salts thereof, and has a content of polymerized-in
vinylaromatic
monomers of 0 to 50 wt.% and a content of diolefins of 45 to 99.95 wt.%, the
content
of 1,2-bonded diolefins (vinyl content) being 0 to 80 wt.%, and 0.001 to 40
parts by
wt. of a compound which is capable of crosslinking the rubber with the
hydroxyl or
carboxyl groups of the rubber (so-called sulfur-free crosslinking agent), and
optionally further rubbers, rubber auxiliaries and crosslinking agents.
Rubber mixtures according to the invention which are preferred are those in
which
the rubber constituent has a content of bonded hydroxyl and/or carboxyl groups
or
salts thereof of 0.1 to 3 wt.% in total, and has a content of polymerized-in
vinylaromatic monomers of 0 to 40 wt.%, particularly preferably 10 to 40 wt.%,
and
a content of diolefins of 99.9 to 60 wt.%, the content of 1,2-bonded diolefins
(vinyl
content) being in the range from 5 to 55 wt.%, and which have a content of 0.1
to 20
parts by wt. of sulfur-free crosslinking agent of the abovementioned type.
The amount of fillers is preferably 20 to 200 parts by wt.
Diolefins which are used according to the invention for the polymerization are
1,3-
butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 1-vinyl-1,3-
butadiene
and/or 1,3-hexadiene. 1,3-Butadiene and isoprene are particularly preferably
employed.
Examples which may be mentioned of vinylaromatic monomers which can be
employed for the polymerization are styrene, o-, m- and p-methylstyrene, p-
tert-
butylstyrene, a-methylstyrene, vinylnaphthalene, divinylbenzene,
trivinylbenzene
and/or divinylnaphthalene. Styrene is particularly preferably employed.
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The rubbers based on diolefins and optionally further vinylaromatic monomers
which
are to be employed according to the invention in the rubber mixtures have
average
molecular weights (number-average) of 50,000 to 2,000,000, preferably 100,000
to
1,000,000, and glass transition temperatures of -110°C to +20°C,
preferably -100°C
to 0°C, and Mooney viscosities ML 1+4 (100°C) of 10 to 200,
preferably 30 to 150.
In addition to the hydroxyl and carboxylic acid groups or salts thereof, the
rubbers
according to the invention can also have further known functional groups, such
as
carboxylic acid ester, carboxylic acid amide or sulfonic acid groups.
Solution rubbers containing hydroxyl and carboxyl groups are particularly
preferred,
in particular such as are described in the abovementioned German patent
applications
and in DE-A 2 653 144 and EP-A 464 478.
Possible sulfur-free crosslinking agents in the sense according to the
invention are all
the polyfunctional compounds which are capable of bringing about vulcanization
of
the rubber mixture, i.e. crosslinking to form covalent chemical bonds,
exclusively via
the hydroxyl or carboxyl groups. In this connection, the term "sulfur-free" is
understood as meaning that the compounds contain no vulcanization-active
sulfur-
containing groups. Crosslinking agents which have sulfur-containing groups
which
are stable under the vulcanization conditions, such as e.g. sulfone or
monothioether
groups, can also be employed in the context according to the invention.
Suitable
crosslinking agents in the context of the invention are, above all,
polyisocyanates,
polyuretdiones, blocked polyisocyanates and/or polyepoxides. Particularly
preferred
classes of crosslinking agents are:
(A) Polyisocyanates, such as e.g. hexamethylene-diisocyanate (HDI), isophorone-
diisocyanate (IPDI), toluene-diisocyanate (TDI), 4,4'-diisocyanatodiphenyl-
methane (MDI), 1,6-diisocyanato-2,2,4-trimethylhexane (IPDI), tris-(4-
isocyanatophenyl)-methane, phosphoric acid tris-(4-isocyanato-phenyl ester),
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thiophosphoric acid tris-(4-isocyanato-phenyl ester) and oligomerization
products thereof which have been obtained by reaction of the low molecular
weight diisocyanates mentioned with diols or polyalcohols, in particular
ethylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane or penta-
erythritol, and have a residual content of free isocyanate groups, furthermore
oligomerization products which have been obtained by reaction of the low
molecular weight diisocyanates mentioned with polyesters containing
hydroxyl groups, such as e.g. polyesters based on adipic acid and butanediol
and hexanediol and having molecular weights of between 400 and 3,000, or
by reaction with polyethers containing hydroxyl groups, such as polyethylene
glycols or polypropylene glycols, or polytetrahydrofurans with molecular
weights of 150 to 3,000 and have a residual content of free isocyanate groups,
furthermore oligomerization products which have been obtained by reaction
of the low molecular weight diisocyanates mentioned with amines or
1 S polyamines, in particular ammonia, diaminobutane or diaminohexane, and
have a residual content of free isocyanate groups, or oligomerization products
which have been obtained by reaction of the low molecular weight
diisocyanates mentioned with water or by dimerization or trimerization, such
as e.g. dimerized toluene-diisocyanate (Desmodur TT) and trimerized
toluene-diisocyanate, or aliphatic uretdiones and polyuretdiones containing
isocyanate groups, e.g. based on isophorone-diisocyanate, and have a residual
content of free isocyanate groups. Preferred contents of free isocyanate
groups are in the range from 2.5 to 25 wt.%. Such polyisocyanates are known
and are commercially obtainable, in this context see Houben-Weyl, Methoden
der Organischen Chemie [Methods of Organic Chemistry], volume XIV,
pages 56-98, Georg Thieme
Verlag Stuttgart 1963, and the commercial products of the Desmodur and
Crelan series (Bayer AG);
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(B) Blocked polyisocyanates which react with the hydroxyl and/or carboxyl
groups of the rubber under the vulcanization conditions. These include all the
polyisocyanates mentioned under (A), the isocyanate groups in each case
being blocked with suitable splitting-off groups which split off again at a
higher temperature and thereby liberate the isocyanate groups. Suitable
splitting-off groups are, in particular, caprolactam, malonic acid esters,
phenol and alkylphenols, such as e.g. nonylphenol, as well as imidazole and
sodium hydrogen sulfite. Polyisocyanates blocked with caprolactam, malonic
esters and alkylphenol, in particular those based on toluene-diisocyanate or
trimerized toluene-diisocyanate and isophorone-diisocyanate, are particularly
preferred. Preferred blocked polyisocyanates are also obtained by
dimerization of the isocyanate groups to uretdiones and polyuretdiones.
Preferred contents of blocked isocyanate groups are in the range from 2.5 to
wt.%. Such blocked polyisocyanates are known and are commercially
15 obtainable, in this context see Houben-Weyl, Methoden der Organischen
Chemie [Methods of Organic Chemistry], volume XIV, pages 56-98, Georg
Thieme Verlag Stuttgart 1963, and the commercial products of the Desmodur
and Crelan series (Bayer AG);
20 (C) Polyepoxides which react with the hydroxyl or carboxyl groups of the
rubber
under the vulcanization conditions. Preferred polyepoxides are obtained by
reaction of epichlorohydrin with di- and polyphenols, in particular glycidyl
ethers of bisphenol A or tris-4-hydroxyphenylmethane, and of phenolic
resins, such as phenol/formaldehyde novolaks, phenol/xylene/formaldehyde
resins and phenol/methylolurea resins with molecular weights of between 350
and 5,000. Further preferred polyepoxides are obtained by reaction of
epichlorohydrin with di- and polyamines, such as e.g. diepoxides based on
cyclohexylamine or aniline, or tetraepoxides based on bis-(4-amino-phenyl)-
methane or epoxides based on N-containing heterocyclic compounds, in
particular triglycidyl isocyanurate (TGIC) and triglycidylurazole. Further
preferred polyepoxides are epoxidation products of unsaturated hydrocarbons
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or unsaturated esters, such as e.g. epoxidized Soya oil and epoxidized
cyclohexene-3-carboxylic acid esters, as well as epoxidized rubbers, such as
e.g. epoxidized natural rubber, in which 10 to 70 mol% of the double bonds
are epoxidized. Preferred polyepoxides have epoxide equivalent weights, i.e.
amounts of resin in grams which contain 16 g epoxidically bonded oxygen
(DIN 16945), of 100 g to 1,000 g. Such polyepoxides are known and are
commercially obtainable, in this context see Houben-Weyl, Methoden der
Organischen Chemie [Methods of Organic Chemistry], volume XIV, pages
56-98, Georg Thieme Verlag Stuttgart 1963.
If desired, the crosslinking, i.e. the reaction between hydroxyl or carboxyl
groups and
the crosslinking agents, can be accelerated with known catalysts, for example
with
the aid of amines, such as diazabicyclooctane (DABCO), or Lewis acids, such as
dibutyltin dilaurate (DBTL). The amounts of catalyst depend on the
vulcanization
kinetics sought, the vulcanization temperature and the amount of crosslinking
agent
and hydroxyl or carboxyl groups, and can easily be determined in preliminary
experiments. Conventional amounts lie in the range from 0 to 3 wt.% catalyst,
based
on the crosslinking agent.
The vulcanization temperatures are between room temperature and 220°C,
preferably
100°C to 180°C. The vulcanization time is a few minutes to
several hours. The
vulcanization can also be carried out in two stages in the presence of
conventional
crosslinking agents, since in this case it is a matter of separate
crosslinking processes,
that is to say, for example, by pre-crosslinking via the sulfur-free
crosslinking agent
and post-crosslinking with the aid of known vulcanizing agents, in particular
sulfur,
sulfur donors or peroxides.
Possible fillers for the rubber mixtures according to the invention are all
the known
fillers used in the rubber industry, these comprising both active and inactive
fillers.
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There may be mentioned:
- highly disperse silicas prepared e.g. by precipitation of solutions of
silicates
or flame hydrolysis of silicon halides, with specific surface areas of 5-
1,000,
preferably 20-400 m2/g (BET surface area) and with primary particle sizes of
10-400 nm. The silicas can optionally also be in the form of mixed oxides
with other metal oxides, such as oxides of A1, Mg, Ca, Ba, Zn, Zr or Ti;
- synthetic silicates, such as aluminium silicate or alkaline earth metal
silicate,
such as magnesium silicate or calcium silicate, with BET surface areas of 20-
400 m2/g and primary particle diameters of 10-400 nm;
- naturally occurring silicates, such as kaolin and other naturally occurring
silica;
- glass fibres and glass fibre products (mats, strands) or glass microbeads;
- metal oxides, such as zinc oxide, calcium oxide, magnesium oxide or
aluminium oxide;
- metal carbonates, such as magnesium carbonate, calcium carbonate or zinc
carbonate;
- metal hydroxides, such as e.g. aluminium hydroxide or magnesium
hydroxide;
- carbon blacks. The carbon blacks to be used here are prepared by the flame
black or furnace or gas black process and have BET surface areas of 20-
200 m2/g, e.g. SAF, ISAF, HAF, FEF or GPF carbon blacks;
- rubber gels
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- rubber powder, which has been obtained, for example, by grinding rubber
vulcanization products.
Highly disperse silicas and/or carbon blacks are preferably employed as
fillers.
The fillers mentioned can be employed by themselves or as a mixture. In a
particularly preferred embodiment, the rubber mixtures comprise as fillers a
mixture
of pale fillers, such as highly disperse silicas, and carbon blacks, the
mixing ratio of
pale fillers to carbon blacks being 0.05 to 20, preferably 0.1 to 10.
In addition to the solution rubbers mentioned containing hydroxyl and carboxyl
groups, the rubber mixtures according to the invention can also comprise other
rubbers, such as natural rubber and also other synthetic rubbers.
Preferred synthetic rubbers are described, for example, by W. Hofinann,
Kautschuktechnologie [Rubber Technology], Gentner Verlag, Stuttgart 1980 and
I.
Franta, Elastomers and Rubber Compounding Materials, Elsevier, Amsterdam 1989.
They include, inter alia,
BR - polybutadiene
ABR - butadiene/acrylic acid C1-4-alkyl ester copolymers
CR polychloroprene
IR - polyisoprene
SBR - styrene/butadiene copolymers with styrene contents
of 1-60, preferably 20-
50 wt.%
IIR - isobutylene/isoprene copolymers
NBR - butadiene/acrylonitrile copolymers with acrylonitrile
contents of 5-60,
preferably 10-40 wt./>
HNBR - partly hydrogenated or completely hydrogenated NBR
rubber
EPDM - ethylene/propylene/diene copolymers
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and mixtures of these rubbers. For the production of motor vehicle tyres,
natural
rubber, emulsion SBR and solution SBR rubbers with a glass transition
temperature
above -50°C, which can optionally be modified with silyl ethers or
other functional
S groups in accordance with EP-A 447.066, polybutadiene rubber with a high 1,4-
cis
content (> 90%), which has been prepared with catalysts based on Ni, Co, Ti or
Nd,
and polybutadiene rubber with a vinyl content of up to 75% and mixtures
thereof are
of particular interest.
The rubber mixtures according to the invention can of course also comprise
other
rubber auxiliaries which serve, for example, for further crosslinking of the
vulcanization products prepared from the rubber mixtures, or which improve the
physical properties of the vulcanization products prepared from the rubber
mixtures
according to the invention for their specific intended use.
Sulfur or sulfur-donating compounds or peroxides are employed as additional
crosslinking agents. Sulfur or sulfur-donating compounds in amounts of 0.01 to
3
parts by wt., based on the rubber, are particularly preferred. Furthermore, as
mentioned, the rubber mixtures according to the invention can comprise further
auxiliaries, such as the known reaction accelerators, anti-ageing agents, heat
stabilizers, light stabilizers, ozone stabilizers, processing auxiliaries,
reinforcing
resins, e.g. phenolic resins, steel cord adhesives, such as e.g.
silicalresorcinol/-
hexamethylenetetramine or cobalt naphthenate, plasticizers, tackifiers,
blowing
agents, dyestuffs, pigments, waxes, extenders, organic acids, retardants,
metal oxides
and activators.
The rubber auxiliaries according to the invention are employed in the
conventional
known amounts, the amount employed depending on the later intended use of the
rubber mixtures. For example, amounts of rubber auxiliaries in the range from
2 to
70 parts by wt., based on 100 parts by wt. of rubber, are conventional.
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As mentioned above, in addition to the solution rubber containing hydroxyl and
carboxyl groups, additional rubbers can also be admixed to the rubber mixtures
according to the invention. The amount thereof is conventionally in the range
from
0.5 to 70, preferably 10 to 50 wt.%, based on the total amount of rubber in
the rubber
mixture. The amount of rubbers additionally added again depends on the
particular
intended use of the rubber mixtures according to the invention.
For the rubber mixtures according to the invention with a filler content of
highly
active silicas, the use of additional filler activators is particularly
advantageous.
Preferred filler activators are sulfur-containing silyl ethers, in particular
bis-
(trialkoxysilyl-alkyl) polysulfides, such as are described in DE-A 2 141 159
and
DE-A 2 255 577. Oligomeric and/or polymeric sulfur-containing silyl ethers
corre-
sponding to the description in DE-A 4 435 311 and EP-A 670 347 are moreover
possible. Mercaptoalkyltrialkoxysilanes, in particular
mercaptopropyltriethoxysilane
and thiocyanatoalkylsilyl ethers (see DE-A 19 544 469) and silyl ethers
containing
amino groups, such as e.g. 3-aminopropyltriethoxysilane and N-oleyl-N-
propyltri-
methoxysilane are furthermore to be employed. The filler activators are
employed in
conventional amounts, i.e. in amounts of 0.1 to 15 parts by wt., based on 100
parts by
wt. of rubber.
The rubber mixtures according to the invention can be prepared e.g. by mixing
the
solution rubbers containing hydroxyl and carboxyl groups with the
corresponding
fillers and sulfur-free crosslinking agents in suitable mixing apparatuses,
such as
kneaders, mills or extruders.
The present invention also provides the use of the rubber mixtures according
to the
invention for the preparation of vulcanization products, which in turn are
used for the
production of highly reinforced shaped rubber articles, in particular for the
production of tyres.
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Examples
Example 1: Solution SBR rubber containing hydroxyl groups
2.25 kg mercaptoethanol and 0.18 kg dilauroyl peroxide were added to a
solution of
45 kg Buna VSL 5020-0 (solution SBR rubber with a styrene content of 20 wt.%
and
a vinyl content of 50 wt.%, manufacturer: Bayer AG) in 275 kg cyclohexane at
80°C
and the mixture was stirred at 80°C for 2 hours. 0.23 kg Vulkanox 4020
(stabilizer of
the 6-PPD type, manufacturer: Bayer AG) and 17.72 kg Enerthene 1849 (aromatic
mineral oil, manufacturer BP) were then added and the solvent was removed by
stripping with steam. After drying at 70°C in vacuo, a solution SBR
rubber extended
with 37.5 phr aromatic mineral oil and with a content of hydroxyl groups of 1
wt.%
(based on the oil-free rubber), an OH number of 24 (oil-extended rubber) and a
glass
transition temperature of -24°C with a viscosity ML 1+4 (100°C)
of SS was obtained.
Example 2: Solution SBR rubber containing carboxyl groups
0.563 kg 3-mercaptopropionic acid and 0.045 kg dilauroyl peroxide were added
to a
solution of 45 kg Buna VSL 5020-0 (solution SBR rubber with a styrene content
of
20 wt.% and a vinyl content of 50 wt.%, manufacturer: Bayer AG) in 275 kg
cyclohexane at 80°C and the mixture was stirred at 80°C for 2
hours. 0.23 kg
Vulkanox 4020 (stabilizer of the 6-PPD type, manufacturer: Bayer AG) and 17.12
kg
Enerthene 1849 (aromatic mineral oil, manufacturer BP) were then added and the
solvent was removed by stripping with steam. After drying at 70°C in
vacuo, a
solution SBR rubber extended with 37.5 phr aromatic mineral oil and with a
content
of carboxyl groups of 0.5 wt.% (based on the oil-free rubber), an acid number
of 5
(oil-extended rubber), a glass transition temperature of -29°C and a
viscosity ML 1+4
(100°C) of 38 was obtained.
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Example 3: Sulfur-free crosslinking agent
Cycloaliphatic polyisocyanate blocked with caprolactam and with a total
content of
NCO of 11.5%. Trade name: Crelan UI, manufacturer: Bayer AG.
S
Example 4: Sulfur-free crosslinking agent
Cycloaliphatic polyuretdione free from blocking agents and with a total
content of
NCO of 13.5%. Trade name: Crelan VP LS 2147, manufacturer Bayer AG.
Example 5: Sulfur-free crosslinking agent triglycidyl isocyanurate
(CAS-RN 2451-62-9)
Example 6: Rubber mixtures
The following rubber mixtures were prepared in a 1.5 1 internal mixer at 130-
140°C.
The accelerator and crosslinking agent were finally admixed on a mill at
50°C.
Component Example Comparison
6.1 Exam le
6.A
mixed in the internal mixer:
hydroxylgroup-containing solution-SBR-rubber137.5 0
accordin to exam lel
oil-extended solution-SBR-rubber 0 137.5
Buna VSL 5025-1 Ba er AG
Carbon black Coraxx N 339 (Degussa-Hiils50 50
AG)
zincoxide 2.5 2.5
antioxidant ulkanox 4020 1 1
ozone-protective wax Antilux 654 (Rheinchemie)I .5 I .5
admixed on a mill:
sulfur-free crosslinking agent according15 15
to example 3
(sulfur 0.5 0.5
i
i diphenylguanidine Vulkacit D (Bayer1 1
AG)
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The rubber mixtures were subsequently vulcanized at 170°C for 30
minutes. The
following vulcanisate properties were obtained:
Example Comparison
6.1 Example
6.A
tensile strength (MPa) 18.1 5.4
modulus at 300% elongation (MPa) 8.5 1.4
elongation at break (%) 519 975
rebound elasticity at 23C (%) 21 29
rebound elasticity at 70C (%) 54 45
difference of rebound elasticity at 33 16
23 and 70C
hardness at 23C (Shore A) 54 40
hardness at 70C (Shore A) 50 24
The results demonstate the strong reeinforcing effect (measured by means of
hardness, modulus, tensile strength) of the rubber mixture of hydroxylmodified
SBR-
rubber and sulfur-free crosslinking agent (blocked isocynate) and the
advantageous
dynamic damping properties in comparison to the combination of unmodified SBR-
rubber and sulfur-free crosslinking agent.
