Note: Descriptions are shown in the official language in which they were submitted.
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Carboxyl group-containing diene rubbers
The present invention provides rubber mixtures which contain a dime rubber
with a
concentration of carboxyl groups of 0.1 to 2 wt.% and a glass transition
temperature
of -120 to -50° and their mixtures with fillers, optionally other
rubbers and rubber
auxiliary substances and vulcanisates prepared therefrom. Rubber mixtures
according
to the invention are suitable for producing highly reinforced, abrasion-
resistant
moulded items, in particular for producing tire treads which have particularly
high
wet skidding resistance and abrasion resistance and a low rolling resistance
and also
for tire sidewalls with especially high fatigue resistance.
Double bond-containing anionic polymerised solution rubbers, such as solution
polybutadiene and solution styrenelbutadiene rubbers, have advantages over the
cor-
1 S responding emulsion rubbers when producing low rolling resistance tire
treads. The
advantages are based, inter alia, on the ability to control the vinyl content
and the
glass transition temperature and molecular branching associated therewith.
Particular
advantages in relation to the wet skidding resistance and rolling resistance
of the tires
result therefrom in a practical application. Thus, US-A 5 227 425 describes
the pro-
duction of tire treads from a solution SBR rubber and silica. To fizrther
improve the
properties, numerous methods for modifying the end groups have been developed,
as
is described in EP-A 334 042, with dimethylaminopropyl-acrylamide or, as
described
in EP-A 447 066, with silyl ethers. However, due to the high molecular weight
of the
rubber, the proportion by weight of the end group is small and can therefore
have
only a small effect on the interaction between filler and rubber molecule.
Inter alia,
the present invention is intended to provide dime rubbers with a much higher
con-
centration of effective groups for interacting with fillers and with a
particularly low
glass transition temperature.
US-A 2 662 874 describes the preparation of elastic materials from metal ion
cross-
linked polymeric carboxylates with a concentration of 0.001 to 0.3 carboxyl
equiva-
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tents per 100 g of rubber. The elastic materials mentioned have a very wide
range of
carboxyl group contents and are unsuitable for the tire application in the
present in-
vention due to the inherent sensitivity to hydrolysis of the metal salts.
A process for preparing carboxyl group-containing (3.9 to 8.9 wt.% of carboxyl
groups) solution polybutadiene rubbers is described, inter alia, in DE-OS 2
653 144.
These rubbers have glass transition temperatures which are too high
(>~50°C) due to
the high concentration of vinyl and carboxyl groups and are associated with
disad-
vantageous damping properties and are therefore not a substitute for 1,4-
polybutadi-
ene rubber in tire treads and tire sidewalk.
Therefore the present invention was intended to provide mixtures of carboxyl
group-
containing solution rubbers from which tires with improved wet skidding
resistance,
lower rolling resistance and high mechanical strength and improved abrasion
behav-
four can be produced.
The present invention therefore provides rubber mixtures containing one or
more
rubbers with a concentration of bonded carboxyl groups or their salts in the
range 0.1
to 2 wt.% and with a glass transition temperature in the range -120 to -
50°C and one
or more fillers in the range 10 to 500 parts by wt. with respect to 100 parts
by wt. of
rubber.
Preferred rubber mixtures according to the invention are those in which the
carboxyl
group-containing rubber has a concentration of bonded carboxyl groups or their
salts
of 0.1 to 1 wt.% and a glass transition temperature in the range -120 to -
SO°C, pref
erably -120 to -70°C and a concentration of 1,2-bonded diolefins (vinyl
content) in
the range 0 to 50 wt.%, in particular 1 to 15 wt.% and a cis-1,4-content in
the range
to 100 wt.%, in particular preferably 90 to 100 wt.% and also has an average
mo-
lecular weight (number average) of 50 000 to 2 000 000, preferably 100 000 to
1 000
30 000 and Mooney viscosities ML 1+4 (100°C) of 10 to 200, preferably
30 to 150.
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The glass transition temperature can be determined using known methods, e.g.
by
using DSC (differential scanning calorimetry, rate of heating 20 K/min). The
con-
centration of carboxyl groups can also be determined using known methods such
as
e.g. titration of the free acid, spectroscopy, elemental analysis, etc.
Rubbers according to the invention for use in rubber mixtures according to the
in-
vention can be prepared preferably by polymerisation using coordination
catalysts or
by anionic solution polymerisation. Coordination catalysts in this connection
are un-
derstood to be Ziegler-Natta catalysts, coordination catalysts and
monometallic
catalyst systems. Coordination catalysts are preferably those based on Ni, Co,
Ti or
Nd. Catalysts for anionic solution polymerisation are based on alkali or
alkaline earth
metals such as e.g. n-butyllithium. In addition, known randomised control
agents for
the microstructure of the polymer may be used. These types of solution
polymerisa-
tions are known and are described e.g. in I. Franta Elastomers and Rubber Com-
1 S pounding Materials; Elsevier 1989, pages 113 - 131 and in Houben-Weyl,
Methoden
der Organischen Chemie, Thieme Verlag, Stuttgart, 1961, vol. XIV/1 pages 645
to
673 or in vol. E20 (1987), pages 114 to 134 and pages 134 to 153.
The diolefms used according to the invention for polymerisation 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 used.
The carboxyl groups may be introduced into the rubber either by adding
carboxyl-
providing compounds, for example CO2, to metalised solution rubbers or by
treating
the final rubber with carboxyl group-containing compounds, for example
carboxyl
group-containing mercaptans, in a subsequent reaction.
'The carboxyl groups are preferably introduced into the rubber after
polymerisation of
the monomers being used in solution by reacting the polymer obtained,
optionally in
the presence of radical starters, with carboxylmercaptans of the formula (I)
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HS-R'-COOX (n,
in which
R' represents a linear, branched or cyclic C1-C36 alkylene group, which may op-
tionally be substituted with up to 3 further carboxyl groups, or which may be
interrupted by nitrogen, oxygen or sulfur atoms or a C6-C12-arylene group,
and
X represents hydrogen or a metal or ammonium ion.
This process is a further subject of the invention.
1 S C,-C36 alkylene groups are understood to be any linear, cyclic or branched
alkylene
groups with 1 to 36 carbon atoms which are known to a person skilled in the
art, such
as methylene, ethylene, n-propylene, i-propylene, n-butylene, i-butylene, t-
butylene,
n-pentylene, i-pentylene, neo-pentylene, n-hexylene, cyclohexylene, i-
hexylene,
heptylene, octylene, nonylene, decylene, undecylene and dodecylene.
Preferred carboxylmercaptans of the formula (I) are thioglycollic acid,
2-mercaptopropionic acid (thiolactic acid), 3-mercaptopropionic acid,
4-mercaptobutyric acid, mercaptoundecanoic acid, mercaptooctadecanoic acid,
2-mercaptosuccinic acid and their alkali and alkaline earth metal or ammonium
salts.
3-mercaptopropionic acid, mercaptobutyric acid, 2-mercaptosuccinic acid 2- and
4-
mercaptobenzoic acid and their lithium, sodium, potassium, magnesium, calcium
or
ammonium salts are particularly preferred. 3-mercaptopropionic acid and its
lithium,
sodium, potassium, magnesium, calcium or ammonium salts are more particularly
preferred.
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In general, reaction of the carboxylmercaptans with the solution rubbers is
performed
in a solvent, for example hydrocarbons such as pentane, hexane, cyclohexane,
ben-
zene and/or toluene, at temperatures of 40 to 150°C in the presence of
radical start-
ers, e.g. peroxides such as dilauroyl peroxide, azo-initiators such as
azobisisobutyro-
nitrite, benzopinacolsilyl ethers or in the presence of photo-initiators and
visible or
L1V light. Preferred radical starters are diacyl peroxides such as dilauroyl
peroxide,
didodecanoyl peroxide, di-(3,5,5-trimethylhexanoyl) peroxide and perketals
such as
1,1-di-(tert.-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di-(tert.-
butylperoxy)-cy-
clohexane and 1,1-di-(tert-.butylperoxy)-butane.
The amount of carboxylmercaptans to be used is governed by the concentration
of
bonded carboxyl groups or their salts required in the solution rubber being
used in
the rubber mixtures.
The carboxylate salts may also be prepared after introduction of the
carboxylic acids
groups into the rubber, by neutralising them.
Suitable fillers for rubber mixtures according to the invention are any
fillers which
are known to be used in the rubber industry, these including both active and
inactive
fillers.
The following may be mentioned:
- highly disperse silicas prepared e.g. by precipitation from solutions of
sili-
Gates or by flame hydrolysis of silicon halides with specific surface areas of
5
- 1000, preferably 20 - 400, m2/g (BET surface area) and with primary par-
ticle sizes of 10 - 400 nm. The silicas may also optionally be present as
mixed
oxides with other metal oxides such as Al, Mg, Ca, Ba, Zn, Zr or Ti oxides;
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- synthetic silicates such as aluminium silicate or alkaline earth metal
silicates
such as magnesium silicate or calcium silicate, with 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 occurnng silicas;
- glass fibre and glass fibre products (mats, ropes) or glass microbeads;
metal oxides such as zinc oxide, calcium oxide, magnesium oxide, aluminium
oxide;
- metal carbonates such as magnesium carbonate, calcium carbonate, zinc car-
bonate;
- metal hydroxides such as e.g. aluminium hydroxide, magnesium hydroxide;
carbon blacks. The carbon blacks to be used here are prepared by the lamp
black, furnace black or channel black processes and have BET surface areas
of 20 - 200 mz/g, e.g. SAF, ISAF, HAF, FEF or GPF carbon blacks;
- rubber gels, in particular those based on polybutadiene, butadiene/styrene
copolymers, butadiene/acrylonitrile copolymers and polychloroprene.
Highly disperse silicas and/or carbon blacks are preferably used as fillers.
The fillers mentioned may be used individually or as a mixture. In a
particularly pre-
ferred embodiment, the rubber mixtures contain a mixture of pale-coloured
fillers,
such as highly disperse silicas, and carbon blacks, as filler, wherein the
mixing ratio
of pale-coloured fillers to carbon blacks is 0.05 to 20, preferably 0.1 to 10.
The fillers are then used in amounts in the range 10 to S00 parts by wt., with
respect
to 100 parts by wt. of rubber. 20 to 200 parts by wt. are preferably used.
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Rubber mixtures according to the invention may contain, in addition to one,
two,
three or more different carboxyl group-containing solution rubbers according
to the
invention, other rubbers such as natural rubber or also other synthetic
rubbers.
S Preferred synthetic rubbers are described, for example, in W. Hoffinann,
Kautschuk-
technologie, Gentner Verlag, Stuttgart 1980 and I. Franta, Elastomers and
Rubber
Compounding Materials, Elsevier, Amsterdam, 1989. They include, inter alia,
BR polybutadiene
ABR butadiene/C,-Ca-alkyl acrylate copolymers
CR polychloroprene
IR polyisoprene
SBR styrenelbutadiene copolymers with styrene contents of 1-60, prefer-
ably 20-SO wt.%
IIR isobutylene/isoprene copolymers
NBR butadiene/acrylonitrile copolymers with acrylonitrile contents of 5-60,
preferably 10-40 wt.%
HNBR partially hydrogenated or fully hydrogenated NBR rubber
EPDM ethylene/propylene/diene copolymers
and mixtures of these rubbers. For producing vehicle tires, natural rubber,
emulsion
SBR and solution SBR rubbers with a glass transition temperature above -
50°C,
which may optionally be modified with silyl ethers or other functional groups,
e.g.
according to EP-A 447 066, polybutadiene rubber with a high 1,4-cis content
(>90
%), which has been prepared using catalysts based on Ni, Co, Ti or Nd, and
poly-
butadiene rubber with a vinyl content of up to 75 % and their mixtures are of
par-
ticular interest.
Rubber mixtures which are quite specifically preferred according to the
invention
contain, in addition to one or more carboxyl group-containing rubbers with a
glass
transition temperature between -110° and -50°C, additional
rubbers chosen from the
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group consisting of natural rubber, polyisoprene and styreneJbutadiene
copolymers
with styrene contents between 10 and 50 wt.%. The amount of these additional
rub-
bers is usually in the range 0.5 to 95, preferably 40 - 90 wt.%, with respect
to the
entire amount of rubber in the rubber mixture. The amount of these
additionally
added rubbers is again governed by the particular intended use of the rubber
mixtures
according to the invention.
Obviously, rubber mixtures according to the invention may also contain other
rubber
auxiliary substances which are used, for example, to cross-link the
vulcanisates pro-
duced from the rubber mixtures or which improve the physical properties of the
vul-
canisates produced from rubber mixtures according to the invention for a
specific
ultimate use.
Sulfur or sulfur-providing compounds, and also radical-providing cross-linking
agents such as organic peroxides may be used, for example, as cross-linking
agents.
Sulfur is preferably used as a cross-linking agent. In addition, as mentioned
above,
rubber mixtures according to the invention may contain further auxiliary
substances
such as known reaction accelerators, antioxidants, thermal stabilisers, light
stabi-
users, anti-ozonants, processing aids, plasticisers, tackifiers, blowing
agents, color-
ants, pigments, waxes, extenders, organic acids, delaying agents, metal oxides
and
activators.
The rubber auxiliary substances according to the invention are used in
conventionally
disclosed amounts, wherein the amount used is governed by the later ultimate
use of
the rubber mixtures. Amounts of rubber auxiliary substances in the range 2 to
70
parts by wt., with respect to 100 parts by wt. of rubber, are, for example,
convention-
ally used.
In the case of rubber mixtures according to the invention which are filled
with highly
active silicas, the use of additional filler activators is particularly
advantageous. Pre-
ferred filler activators are sulfur-containing silyl ethers, in particular bis-
(tri-
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alkyloxysilyl-alkyl)-polysulfides, as are described in DE 2 141 159 and DE 2
255
577. In addition, oligomers and/or polymers of sulfi~r-containing silyl ethers
corre-
sponding to the description in DE 4 435 311 and EP 670 347 are also suitable.
Mer-
captoalkyltrialkoxysilanes, in particular mercaptopropyltriethoxysilane and
thio-
cyanatoalkly silyl ethers (see DE 19 544 469) may also be used. The filler
activators
are used in conventional amounts, i.e. in amounts of 0.1 to 15 parts by wt.,
with re-
spect to 100 parts by wt. of rubber.
Rubber mixtures according to the invention may be prepared, for example, by
mixing
the carboxyl group-containing solution rubber with the corresponding fillers
and op-
tionally further rubbers and further rubber auxiliary substances in
appropriate mixing
equipment such as compounders, rollers or extruders.
Rubber mixtures according to the invention are preferably prepared by first
poly-
1 S merising the monomers mentioned in solution, introducing the carboxyl
groups into
the solution rubber and, after completing polymerisation and introduction of
the car-
boxyl groups, mixing the solution rubber in the corresponding solvent with the
ap-
propriate fillers and optionally further rubbers and further rubber auxiliary
sub-
stances, in particular plasticisers, in appropriate amounts, and then removing
the sol-
vent with hot water and/or steam at temperatures of 50 to 200°C,
preferably under
vacuum.
The invention also provides use of the rubber mixtures according to the
invention for
producing moulded items of all types, in particular for producing tires,
especially tire
treads and tire sidewalk.
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Examples
Example 1
6.25 of 3-mercaptopropionic acid and 1 g of dilauroylperoxide were added to a
solu-
tion of S00 g solution polybutadiene rubber, CB 45 NF (Bayer AG, Li-type, cis-
1,4-
content about 40%) in 41 cyclohexane at 80°C. The mixture was then
stirred for 7
hours at 80°C. Then 2,5 antioxidant Vulkanox 4020 (Bayer AG) and 101.3
g aro-
matic mineral oil Enerthene 1849-1 (BP) were added and the solvent was
distilled off
using steam (100-110°C). After drying at 70°C under vacuum, 593
g of rubber ex-
tended with 20 phr mineral oil were obtained.Sulfur content 0.3 wt%, carboxyl
group
content 0.5 wt% (based on rubber), viscosity ML 1+4 (100°C) 59, cis-1,4-
content
40%, glass transition temperature: -88°C.
Example 2:
The following rubber mixtures were prepared in a 1.5 1 kneader (mixing time 5
min-
utes, ejection temperature 150°C). Sulfur and accelerator were admixed
afterwards
on a mill (50°C):
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Comparison 2.A Example 2.1
in the kneader mixed:
Buna VSL 5025-1 (37,5 phr mineral oil ex-
tended L-SBR, Bayer AG)
61,9 61,9
natural rubber 10 10
polybutadiene rubber Buna CB 45 (Bayer)
45 0
carboxylic group containing BR according ex-
ample 1 (20 phr oil content)
0 54
mineral oil Enerthene 1849-1 (BP) 20 11
silica Vulkasil S (Bayer AG) 70 70
silane Si (69 (Degussa Hiils) 6 6
carbon black Corax N121 (Degussa 10 10
Huls)
zinc oxide 3 3
stearic acid 1 1
protective wax Antilux 654 (Rheinchemie1,5 1,5
antioxidant Vulkanox HS (Bayer 1 1
AG)
antioxidant Vulkanox 4020 (Bayer 1 1
AG)
On the mill admixed
N-cyclohexylmercaptobenzthiazolsulfenamide
Vulkacit CZ (Bayer AG) 1,8 1,8
diphenylguanidine Vulkacit D (Bayer2 2
AG)
sulfur 1,5 1,5
The rubber mixtures were subsequently vulcanized at 170°C for 15
minutes. The
following vulcanisate properties were obtained.
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Comparison 2.A Example 2.1
tensile strength (Mpa) 16,8 18,2
elongation at break (%) 450 330
modulus at 100% 2,4 2,9
modulus at 300% elongation (Mpa) 9,5 16,3
rebound elasticity at 70°C (%) 54 63
hardness (shore A) 66 66
tan delta at 70°C 0,138 0,108
The experimental data confirm the significant lower dynamic damping at
70°C,
measured by rebound elasticity and tan delta, which in practice correlates
with a sig-
nificantly reduced rolling resistance in tires.
Example 3:
Preparing a masterbatch from precipitated silica and carboxyl group-contain-
ing BR rubber:
Using the method in example 1, 500 g of BR rubber Buna CB 65 in 4 1 of
cyclohex-
ane at 80°C were reacted with 12.5 g of 3-mercaptopropionic acid and
0,5 g of dilau-
royl peroxide. Reaction time: 5 hours. At this point 36% of the 3-
mercaptopropionic
acid had reacted. The carboxyl group content of the polymer was 0.38 wt.%.
Then,
with stirring at 75°C, 2.5 g of stabiliser Vulkanox~ 4020 (Bayer AG),
189.5 g of
aromatic mineral oil Renopal~ 450 (Fuchs Mineralolwerke) and 405 g of highly
ac-
tive precipitated silica Vulkasil~ S (N2 surface area about 180 m2/g, Bayer
AG) were
added and the mixture was stirred for about 30 minutes at this temperature
until these
components were uniformly distributed. The solvent was then removed by passing
steam (100-110°C) through the mixture. The reaction vessel was heated
to 75-80°C
from outside during this procedure. Finally, the moist solid was removed,
finely di-
vided silica was filtered off through a sieve, and the product was dried at
65°C under
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vacuum. 1090 g of a brown silica/rubber masterbatch (99 % of theoretical) were
ob-
tamed. The waste water contained no silica.
Comparison example 3.A:
S
The same procedure was used as described in example 3, wherein a solution of
S00 g
of BR rubber Buna CB 65 and 2.5 g of Vulkanox~ 4020 in 4 1 of cyclohexane were
mixed at 75°C with 400 g of highly active silica Vulkasil~ S. The
solvent was then
removed by passing steam {100-110°C) through the mixture, wherein the
reaction
vessel was heated to 75-80°C from outside. Finally, the moist solid was
removed,
finely divided silica was filtered off through a sieve and the product was
then dried at
65°C under vacuum. 597 g (66 % of theoretical) of an inhomogeneous
silica/rubber
masterbatch were obtained. The waste water contained large amounts (about 75 %
of
the amount used) of silica.
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