Note: Descriptions are shown in the official language in which they were submitted.
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Tire for vehicle bearing heavy loads
The field of the present invention is that of tyres for vehicles which are
intended to bear heavy
loads, in particular buses, lorries, agricultural vehicles or civil
engineering vehicles.
These tyres are provided with treads which exhibit, in comparison with the
thicknesses of the
treads of the tyres for light vehicles, in particular for passenger vehicles
or vans, great thicknesses
of rubber material. Typically, the wearing part of the tread of a heavy-duty
vehicle has a thickness
of at least 15 mm and that of a civil engineering vehicle is at least 30 mm,
indeed even up to 120
mm.
During running, a tread is subjected to mechanical stresses and to attacks
resulting from direct
contact with the ground. In the case of a tyre fitted to a vehicle bearing
heavy loads, the
mechanical stresses and the attacks undergone by the tyre are magnified under
the effect of the
weight borne by the tyre. The consequence of this is that the incipient cracks
which are created in
the tread under the effect of these strains and these attacks have a tendency
to further propagate
at the surface of or inside the tread. Crack propagation in the tread can
result in damage to the
tread and can thus reduce the lifetime of the tread or of the tyre.
A tyre running over a stony ground surface is highly exposed to incipient
cracks. The actual
aggressive nature of the stony ground surface exacerbates not only this type
of attack on the tread
but also its consequences with regard to the tread. This is particularly true
for the tyres equipping
civil engineering vehicles which are moving about generally in mines. This is
also true for the tyres
which are fitted to agricultural vehicles, due to the stony ground surface of
arable land. The tyres
which equip heavy-duty vehicles of worksites, which are moving both on stony
ground surfaces and
on bituminous ground surfaces, also experience these same attacks. Due to the
two aggravating
factors, which are the weight borne by the tyre and the aggressive nature of
the running ground
surface, the resistance to crack propagation of a tread of a tyre for a civil
engineering vehicle, an
agricultural vehicle or a worksite heavy-duty vehicle proves to be crucial in
minimizing the impact
of the attacks undergone by the tread.
It is thus important to have available tyres for vehicles bearing heavy loads,
the tread of which
exhibits a resistance to crack propagation which is sufficiently strong to
minimize the effect of an
incipient crack on the lifetime of the tread. In order to solve this problem,
tyre manufacturers use,
for example, natural rubber in the treads due to the properties of resistance
to crack propagation
of natural rubber, as mentioned in Table 3.7, Comparison of elastomers
properties, pp 162-163,
Rubber Technology Handbook, Hofmann, Hanser Publishers (1989).
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The Applicant Companies have discovered that the combined use of a carbon
black in a
predominant amount as reinforcing filler, of a polybutadiene or of a butadiene
copolymer and of a
certain content of a specific thermoplastic elastomer in a tread makes it
possible to improve the
resistance to crack propagation of the tread of a tyre for a vehicle intended
to bear heavy loads,
without substantial damage to the other performances of the tread, which are
the wear and the
rolling resistance.
Thus, a first subject-matter of the invention is a tyre for vehicles which are
intended to bear heavy
loads, the tread of which comprises a composition based on at least:
- an elastomer matrix comprising a first diene elastomer and a
thermoplastic styrene elastomer,
o which thermoplastic styrene elastomer represents at most 50% by weight of
the
elastomer matrix and comprises at least one rigid styrene segment and at least
one
flexible diene segment, which at least one flexible diene segment comprises at
least 20%
by weight of conjugated diene units, it being possible for the conjugated
diene units to be
all or in part hydrogenated,
o which first diene elastomer represents at least 50% by weight of the
elastomer matrix and
is chosen from the group consisting of polybutadienes, butadiene copolymers
and their
mixtures,
- a reinforcing filler which comprises a carbon black which represents
more than 50% by weight
of the reinforcing filler,
- a crosslinking system.
Another subject-matter of the invention is a process for preparing the tyre in
accordance with the
invention.
I. MEASUREMENTS AND TESTS USED
Resistance to crack propagation:
The rate of cracking was measured on test specimens of rubber compositions
using a cyclic fatigue
device (Elastomer Test System) of the 381 type from MTS, as explained below.
The resistance to cracking is measured using repeated tensile actions on a
test specimen initially
accommodated (after a first tensile cycle) and then notched. The tensile test
specimen is composed
of a rubber plaque of parallelepipedal shape, for example with a thickness of
between 1 and 2 mm,
with a length between 130 and 170 mm and with a width between 10 and 15 mm,
the two side
edges each being covered in the direction of the length with a cylindrical
rubber strip (diameter 5
mm) making possible anchoring in the jaws of the tensile testing device. The
test specimens thus
prepared are tested in the fresh state. The test was carried out in air, at a
temperature of 20 C.
After accommodation, 3 very fine notches with a length of between 15 and 20 mm
are produced
using a razor blade, at mid-width and aligned in the direction of the length
of the test specimen,
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one at each end and one at the centre of the latter, before starting the test.
At each tensile cycle,
the degree of deformation of the test specimen is automatically adjusted so as
to keep the energy
restitution level (amount of energy released during the progression of the
crack) constant at a
value of less than or equal to approximately 500 J/m2. The crack propagation
rate is measured in
nanometres per cycle. The resistance to crack propagation will be expressed in
relative units (r.u.)
by dividing the propagation rate of the control by that of the mixture, the
rates being measured at
the same energy restitution level. A value greater than that of the control,
arbitrarily set at 100,
indicates an improved result, that is to say a greater resistance to crack
propagation.
II- DETAILED DESCRIPTION OF THE INVENTION
In the present description, unless expressly indicated otherwise, all the
percentages (%) shown are
% by weight. The abbreviation "phr" means parts by weight per hundred parts of
elastomers
present in the elastomer matrix, the elastomer matrix denoting all of the
elastomers present in the
rubber composition.
Furthermore, any interval of values denoted by the expression "between a and
b" represents the
range of values greater than "a" and lower than "b" (that is to say, limits a
and b excluded),
whereas any interval of values denoted by the expression "from a to b" means
the range of values
extending from "a" up to "b" (that is to say, including the strict limits a
and b).
The expression "composition based on" should be understood as meaning, in the
present
description, a composition comprising the mixture and/or the in situ reaction
product of the
various constituents used, some of these base constituents (for example the
elastomer, the filler or
other additive conventionally used in a rubber composition intended for the
manufacture of tyres)
being capable of reacting or intended to react with one another, at least in
part, during the various
phases of manufacture of the composition intended for the manufacture of
tyres.
The elastomer matrix of the rubber composition has the essential
characteristic of comprising a
first diene elastomer chosen from the group consisting of polybutadienes
(BRs), butadiene
copolymers and their mixtures.
Suitable in particular as polybutadienes are those having a content of 1,2-
units of between 4% and
80% by weight of the weight of the polybutadiene or those having a content of
cis-1,4- bonds of at
least 90% by weight of the weight of the polybutadiene.
Suitable in particular as butadiene copolymers are the copolymers of butadiene
and styrene (SBR).
The copolymers can be prepared in emulsion (ESBR) or in solution (SSBR).
Mention may be made of
butadiene/styrene copolymers and in particular of those having a glass
transition temperature Tg,
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measured according to ASTM D3418, of between 0 C and -90 C and more
particularly between -
C and -80 C, a styrene content of between 5% and 60% by weight and more
particularly
between 5% and 40%, a content (mol%) of 1,2- bonds of the butadiene part of
between 4% and
75% of the butadiene part and a content (mol%) of trans-1,4- bonds of between
10% and 80% of
5 the butadiene part.
The first diene elastomer, whether it is a polybutadiene or a butadiene
copolymer, can be modified
by a modifying agent, such as, for example, a coupling, star-branching or
functionalizing agent.
Mention may be made, as modifying agent, of compounds comprising a C-Sn bond
or those
10 comprising an amine, silanol or alkoxysilane functional group. Such
elastomers are, for example,
described in Patents EP 0 778 311 B1, EP 0 890 607 B1, EP 0 692 492 B1, EP 1
000 970 B1 and EP 1
457 501 B1 or Patent Applications WO 2009/000750 and WO 2009/133068.
According to a preferred embodiment of the invention, the first diene
elastomer is a
polybutadiene, preferably exhibiting a content of cis-1,4- bonds of greater
than or equal to 90% by
weight of the weight of polybutadiene. This preferred embodiment of the
invention can be
combined with any one of the embodiments of the invention.
The first diene elastomer represents at least 50% by weight of the elastomer
matrix. According to
this embodiment, suitable as elastomer matrix is, for example, a mixture
consisting of 40% by
weight of the thermoplastic styrene elastomer, of 55% by weight of the first
diene elastomer and
of 5% by weight of a second diene elastomer, the percentages being calculated
on the basis of the
total weight of the elastomer matrix.
Second diene elastomer (or without distinction rubber) should be understood,
in a known way, as
meaning an (or several) elastomer composed, at least in part (i.e., a
homopolymer or a copolymer),
of diene monomer units (monomers bearing two conjugated or non-conjugated
carbon-carbon
double bonds), the second diene elastomer being different from the first diene
elastomer and not
being a thermoplastic styrene elastomer.
According to a preferred embodiment of the invention, only the first diene
elastomer and the
thermoplastic styrene elastomer constitute the elastomer matrix, which means
that the elastomer
matrix does not contain other elastomers than the first diene elastomer and
the thermoplastic
styrene elastomer.
The thermoplastic styrene elastomer comprises at least one rigid styrene
segment and at least one
flexible diene segment comprising at least 20% by weight of conjugated diene
units, it being
possible for the conjugated diene units to be all or in part hydrogenated. The
rigid and flexible
segments can be positioned linearly, or in a star or branched configuration.
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A flexible segment refers to a polymer block of elastomer type and a rigid
segment refers to a
polymer block of thermoplastic type.
According to one embodiment of the invention, the thermoplastic styrene
elastomer is a diblock.
The diblock comprises just one rigid styrene segment connected to just one
flexible diene segment.
According to a preferred embodiment of the invention, the thermoplastic
styrene elastomer
comprises at least two rigid styrene segments. According to this preferred
embodiment of the
invention, generally at least two ends of chains of the thermoplastic styrene
elastomer are each
provided with a rigid styrene segment and the rigid styrene segments are
connected via the flexible
diene segment or segments. According to this preferred embodiment of the
invention, the
thermoplastic styrene elastomer is preferably a triblock. The triblock is then
composed of two rigid
styrene segments and of one flexible diene segment.
In the case where the thermoplastic styrene elastomer is a diblock, the
designation of "the at least
one rigid segment" denotes the rigid segment present in the thermoplastic
styrene elastomer. In
the cases other than a diblock, for example in the case of a triblock, the
designation of "the at least
one rigid segment" denotes the rigid segments present in the thermoplastic
styrene elastomer.
In the case where the thermoplastic styrene elastomer is a diblock or a
triblock, the designation of
"the at least one flexible segment" denotes the flexible segment present in
the thermoplastic
styrene elastomer. In the case where the thermoplastic styrene elastomer is
neither a diblock nor a
triblock, the designation of "the at least one flexible segment" denotes the
flexible segments
present in the thermoplastic styrene elastomer.
The at least one rigid styrene segment is the homopolymer of a styrene monomer
or the block or
random copolymer of several styrene monomers or also the copolymer of one or
more styrene
monomers and of another non-styrene monomer, such as a 1,3-diene.
Styrene monomer should be understood, in the present description, as meaning
styrene or a
substituted styrene. Mention may be made, among substituted styrenes, for
example, of
methylstyrenes (for example, o-methylstyrene, m-methylstyrene or p-
methylstyrene, a-
methylstyrene, a,2-dinnethylstyrene, a,4-dimethylstyrene or diphenylethylene),
para-(tert-
butyl)styrene, chlorostyrenes (for example, o-chlorostyrene, m-chlorostyrene,
p-chlorostyrene, 2,4-
dichlorostyrene, 2,6-dichlorostyrene or 2,4,6-trichlorostyrene), bromostyrenes
(for example, o-
bromostyrene, m-bromostyrene, p-bromostyrene, 2,4-dibromostyrene, 2,6-
dibromostyrene or
2,4,6-tribromostyrene), fluorostyrenes (for example, o-fluorostyrene, m-
fluorostyrene, p-
fluorostyrene, 2,4-difluorostyrene, 2,6-difluorostyrene or 2,4,6-
trifluorostyrene) or also para-
hydroxystyrene.
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According to a preferred embodiment of the invention, the at least one rigid
styrene segment
exhibits a glass transition temperature of greater than 80 C. Preferably, the
at least one rigid
styrene segment is a polystyrene.
The at least one flexible diene segment comprises at least 20% by weight of
conjugated diene
monomer units (also known as conjugated diene units). The at least one
flexible diene segment can
be the homopolymer of a conjugated diene or the block or random copolymer of
several
conjugated dienes or also the copolymer of one or more conjugated dienes and
of at least one
other non-diene monomer, such as a styrene monomer.
The content of conjugated diene units which form the flexible diene segment is
preferably at least
50%, more preferably at least 60% and more preferably still at least 70% by
weight of the weight of
the flexible diene segment. Advantageously, it is at least 80% by weight of
the weight of the flexible
diene segment. These contents, whether or not they are preferred, apply to any
one of the
embodiments of the invention.
Suitable in particular as conjugated diene units are 1,3-butadiene units and
isoprene units. The at
least one flexible diene segment can be a polybutadiene, a polyisoprene or a
copolymer of 1,3-
butadiene and of isoprene. The copolymer of 1,3-butadiene and of isoprene can
be block or
random in nature.
Suitable as thermoplastic styrene elastomer are diblock copolymers, such as
styrene/butadiene
(SB), styrene/isoprene (SI) or styrene/butadiene/isoprene (SBI) block
copolymers, or the mixture of
these copolymers. In this designation, the flexible diene block is a random or
block copolymer.
Suitable in particular as thermoplastic styrene elastomer are copolymers, such
as
styrene/butadiene/styrene (SBS), styrene/isoprene/styrene (SIS) or
styrene/butadiene/isoprene/styrene (SBIS) block copolymers, or the mixture of
these copolymers.
In this designation, the flexible diene block is a random or block copolymer.
Very particularly
suitable is a styrene/butadiene/isoprene/styrene (SBIS) block copolymer.
According to a first alternative form of the invention, a fraction of the
conjugated diene units of the
at least one flexible diene segment is hydrogenated. A person skilled in the
art will understand that
he can equivalently use a thermoplastic styrene elastomer, the double bonds of
a fraction of the
conjugated diene units of the flexible diene segment of which will have been
reduced to a single
bond by a process other than a hydrogenation. Mention may be made, among the
processes which
make it possible to reduce the double bonds of the diene units to a single
bond, of reductions with
an aluminium hydride or with diimine, for example.
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According to a second alternative form of the invention, all of the conjugated
diene units of the at
least one flexible diene segment are hydrogenated. A person skilled in the art
will understand that
he can equivalently use a thermoplastic styrene elastomer, the double bonds of
all of the
conjugated diene units of the flexible diene segment of which will have been
reduced to a single
bond by a process other than a hydrogenation.
According to this second alternative form of the invention, suitable as
thermoplastic elastomer are
styrene/ethylene/butylene (SEB), styrene/ethylene/propylene
(SEP) or
styrene/ethylene/ethylene/propylene (SEEP) block copolymers or the mixtures of
these
copolymers. In this designation, the hydrogenated flexible diene block is a
random or block
copolymer.
According to this second alternative form of the invention, also suitable as
thermoplastic elastomer
are styrene/ethylene/butylene/styrene (SEBS),
styrene/ethylene/propylene/styrene (SEPS) or
styrene/ethylene/ethylene/propylene/styrene (SEEPS) block copolymers or the
mixtures of these
copolymers. In this designation, the hydrogenated flexible diene block is a
random or block
copolymer.
Any one of the embodiments of the invention applies to the first alternative
form of the invention
or to the second alternative form of the invention.
Also suitable as thermoplastic styrene elastomer are the mixtures of an
abovementioned triblock
copolymer and of an abovementioned diblock copolymer. This is because the
triblock copolymer
can comprise a minor fraction by weight of diblock copolymer consisting of a
rigid styrene segment
and of a flexible diene segment, the rigid block and the flexible block being
respectively of the
same chemical nature, in particular of the same microstructure, as the rigid
and flexible blocks of
the triblock. The presence of the diblock copolymer in the triblock copolymer
generally results from
the process of synthesis of the triblock copolymer, which can result in the
formation of byproduct,
such as the diblock copolymer. Generally, the percentage of diblock copolymer
in the triblock
copolymer does not exceed 40% by weight of triblock copolymer.
According to a preferred embodiment of the invention, the content by weight of
the at least one
rigid styrene segment is between 5 and 40% of the weight of the thermoplastic
styrene elastomer.
Below the minimum indicated, there is a risk of the thermoplastic nature of
the thermoplastic
styrene elastomer being substantially reduced while, above the recommended
maximum, the
elasticity of the composition can be affected. For these reasons, the content
by weight of the at
least one rigid styrene segment in the thermoplastic styrene elastomer is
preferably within a range
extending from 10 to 35%, more preferably from 10 to 20%, of the weight of the
thermoplastic
styrene elastomer. These contents, whether or not they are preferred, apply to
any one of the
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embodiments of the invention, very particularly when the polystyrene forms the
at least one rigid
styrene segment of the thermoplastic styrene elastomer.
The number-average molar mass (denoted Mn) of the thermoplastic styrene
elastomer is
preferably between 50 000 and 500 000 g/mol, more preferably between 60 000
and 450 000
g/mol and more preferably still between 80 000 and 300 000 g/mol.
Advantageously, it is between
100 000 and 200 000 g/mol. These preferred ranges of number-average molar mass
values apply
whatever the embodiment of the invention.
The molar mass is determined, in a known way, by size exclusion chromatography
(SEC). The
sample is dissolved beforehand in tetrahydrofuran at a concentration of
approximately 1 g/I and
then the solution is filtered through a filter with a porosity of 0.45 p.m
before injection. The
apparatus used is a Waters Alliance chromatographic line. The elution solvent
is tetrahydrofuran,
the flow rate is 0.7 ml/min, the temperature of the system is 35 C and the
analytical time is 90 min.
A set of four Waters columns in series, with the Styragel tradenames (HMW7,
HMW6E and two
HT6E), is used. The injected volume of the solution of the polymer sample is
100 pl. The detector is
a Waters 2410 differential refractometer and its associated software, for
making use of the
chromatographic data, is the Waters Millennium system. The calculated number-
average molar
masses are relative to a calibration curve produced with polystyrene
standards.
The thermoplastic styrene elastomer is present in a proportion by weight of at
most 50% of the
weight of the elastomer matrix of the rubber composition of the tread. Above
the maximum value
indicated, there is no longer a benefit with regard to the resistance to crack
propagation of the
rubber composition forming the tread of a tyre intended to bear heavy loads.
The content of
thermoplastic styrene elastomer varies within a range extending preferably
from 5 to 50%, more
preferably from 10 to 45% and more preferably still from 20 to 45% by weight
of the weight of the
elastomer matrix. Advantageously, it varies from 25 to 45% by weight of the
weight of the
elastomer matrix. When the thermoplastic styrene elastomer is a mixture of
unsaturated
thermoplastic styrene elastomers in accordance with the invention, the
contents shown apply to
the mixture and not to each of the thermoplastic styrene elastomers. These
contents, whether or
not they are preferred, apply to any one of the embodiments of the invention.
According to a specific embodiment of the invention, the thermoplastic styrene
elastomer exhibits
a glass transition temperature of less than -20 C. This glass transition
temperature is generally
attributed to the glass temperature of the flexible diene segment of the
thermoplastic styrene
elastomer. The glass transition temperature is measured by means of a
differential calorimeter
(differential scanning calorimeter) according to Standard ASTM D3418 (1999).
According to this
specific embodiment of the invention, the thermoplastic styrene elastomer
exhibits a Tg preferably
of less than -30 C, more preferably of less than -40 C and more preferably
still of less than -50 C.
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The reinforcing filler can be any type of "reinforcing" filler known for its
abilities to reinforce a
rubber composition which can be used for the manufacture of tyres, for example
an organic filler,
such as carbon black, a reinforcing inorganic filler, such as silica, with
which is combined, in a
known way, a coupling agent, or also a mixture of these two types of fillers.
A reinforcing filler typically consists of nanoparticles, the (weight-)average
size of which is less than
a micrometre, generally less than 500 nm, generally between 20 and 200 nm, in
particular and
more preferably between 20 and 150 nm.
According to the present invention, the reinforcing filler comprises a carbon
black which represents
more than 50% by weight of the reinforcing filler. Carbon black is understood
to mean one or more
carbon blacks. The carbon black is then regarded as the predominant
reinforcing filler.
The carbon black exhibits a BET specific surface preferably of at least 90
m2/g, more preferably of
at least 100 m2/g. The blacks conventionally used in tyres or their treads
("tyre-grade" blacks) are
suitable as such. Mention will more particularly be made, among the latter, of
the reinforcing
carbon blacks of the 100, 200 or 300 series (ASTM grade), such as, for
example, the N115, N134,
N234 or N375 blacks. The carbon blacks can be used in the isolated state, as
available
commercially, or in any other form, for example as support for some of the
rubber additives used.
The carbon blacks might, for example, be already incorporated in an isoprene
elastomer in the
form of a masterbatch (see, for example, Application WO 97/36724 or WO
99/16600). The BET
specific surface of the carbon blacks is measured according to Standard D6556-
10 [multipoint (at a
minimum 5 points) method ¨ gas: nitrogen ¨ relative pressure p/po range: 0.1
to 0.3].
According to one embodiment of the invention, the reinforcing filler also
comprises a reinforcing
inorganic filler. The term "reinforcing inorganic filler" should be understood
here as meaning any
inorganic or mineral filler, whatever its colour and its origin (natural or
synthetic), also known as
"white filler", "clear filler" or even "non-black filler", in contrast to
carbon black, capable of
reinforcing, by itself alone, without means other than an intermediate
coupling agent, a rubber
composition intended for the manufacture of pneumatic tyres, in other words
capable of replacing,
in its reinforcing role, a conventional tyre-grade carbon black; such a filler
is generally
characterized, in a known way, by the presence of hydroxyl (¨OH) groups at its
surface.
Mineral fillers of the siliceous type, preferably silica (S102), are suitable
in particular as reinforcing
inorganic fillers. The silica used can be any reinforcing silica known to a
person skilled in the art, in
particular any precipitated or fumed silica exhibiting a BET specific surface
and a CTAB specific
surface both of less than 450 m2/g, preferably from 30 to 400 m2/g, in
particular between 60 and
300 m2/g. Mention may be made, as example of silica of use for the
requirements of the invention,
of the Ultrasil VN3 silica sold by Evonik. Mention will be made, as highly
dispersible precipitated
silicas ("HDSs"), for example, of the Ultrasil 7000 and Ultrasil 7005 silicas
from Degussa, the Zeosil
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1165MP, 1135MP and 1115MP silicas from Rhodia, the Hi-Sil EZ150G silica from
PPG, the Zeopol
8715, 8745 and 8755 silicas from Huber or the silicas having a high specific
surface as described in
Application WO 03/016387.
The physical state under which the reinforcing inorganic filler is provided is
not important, whether
it is in the form of a powder, microbeads, granules or also beads. Of course,
reinforcing inorganic
filler is also understood to mean mixtures of different reinforcing inorganic
fillers, in particular of
highly dispersible silicas as described above.
A person skilled in the art will understand that use might be made, as filler
equivalent to the
reinforcing inorganic filler described in the present section, of a
reinforcing filler of another nature,
in particular organic nature, such as carbon black, provided that this
reinforcing filler is covered
with an inorganic layer, such as silica, or else comprises, at its surface,
functional sites, in particular
hydroxyl sites, requiring the use of a coupling agent in order to establish
the bond between the
filler and the elastomer. Mention may be made, by way of example, for example,
of carbon blacks
for tyres, such as described, for example, in patent documents WO 96/37547 and
WO 99/28380.
In the present account, as regards the silica, the BET specific surface is
determined in a known way
by gas adsorption using the Brunauer-Emmett-Teller method described in The
Journal of the
American Chemical Society, Vol. 60, page 309, February 1938, more specifically
according to French
Standard NF ISO 9277 of December 1996 (multipoint (5 point) volumetric method -
gas: nitrogen -
degassing: 1 hour at 160 C - relative pressure p/po range: 0.05 to 0.17). The
CTAB specific surface is
the external surface determined according to French Standard NF T 45-007 of
November 1987
(method B).
In order to couple the reinforcing inorganic filler to the diene elastomer,
use is made, in a well-
known way, of an at least bifunctional coupling agent, in particular a silane,
(or bonding agent)
intended to provide a satisfactory connection, of chemical and/or physical
nature, between the
inorganic filler (surface of its particles) and the diene elastomer. Use is
made in particular of at
least bifunctional organosilanes or polyorganosiloxa nes.
Use is made in particular of silane polysulphides, referred to as
"symmetrical" or "unsymmetrical"
depending on their specific structure, such as described, for example, in
Applications WO
03/002648 (or US 2005/016651) and WO 03/002649 (or US 2005/016650).
Particularly suitable, without the definition below being limiting, are silane
polysulphides
corresponding to the general formula (V):
Z - A - Sx - A - Z (V)
in which:
- x is an integer from 2 to 8 (preferably from 2 to 5);
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- the A symbols, which are identical or different, represent a divalent
hydrocarbon radical
(preferably a C1-C18 alkylene group or a C6-C12 arylene group, more
particularly a C1-C10, in
particular Ci-C4, alkylene, especially propylene);
- the Z symbols, which are identical or different, correspond to one of the
three formulae
below:
R1 R1 R2
=
¨ = Si¨R1 ¨Si¨R2 ¨Si¨R2 ,
R2 R2 R2
in which:
- the R2 radicals, which are substituted or unsubstituted and identical to or
different from
one another, represent a C1-C18 alkyl, C5-C18 cycloalkyl or C6-C18 aryl group
(preferably C1-C6
alkyl, cyclohexyl or phenyl groups, in particular C1-C4 alkyl groups, more
particularly methyl
and/or ethyl);
- the R2 radicals, which are substituted or unsubstituted and identical to or
different from
one another, represent a C1-C18 alkoxyl or C5-C18 cycloalkoxyl group
(preferably a group
chosen from Ci-C8 alkoxyls and C5-C8 cycloalkoxyls, more preferably still a
group chosen from
C1-C4alkoxyls, in particular methoxyl and ethoxyl).
In the case of a mixture of alkoxysilane polysulphides corresponding to the
above formula (I), in
particular normal commercially available mixtures, the mean value of the "x"
indices is a fractional
number preferably of between 2 and 5, more preferably of approximately 4.
However, the
invention can also advantageously be carried out, for example, with
alkoxysilane disulphides (x =
2).
Mention will more particularly be made, as examples of silane polysulphides,
of bis((C1-
C4)alkoxyl(C1-C4)alkylsilyl(C1-C4)alkyl) polysulphides (in particular
disulphides, trisulphides or
tetrasulphides), such as, for example, bis(3-trimethoxysilylpropyl) or bis(3-
triethoxysilylpropyl)
polysulphides. Use is made in particular, among these compounds, of bis(3-
triethoxysilylpropyl)
tetrasulphide, abbreviated to TESPT, of formula ((C2H50)3Si(CH2)3S2h, or bis(3-
triethoxysilylpropyl)
disulphide, abbreviated to TESPD, of formula [(C2H50)3Si(CH2)3Sk.
Mention will in particular be made, as coupling agent other than alkoxysilane
polysulphide, of
bifunctional POSs (polyorganosiloxanes), or else of hydroxysilane
polysulphides, such as described
in Patent Applications WO 02/30939 (or US 6 774 255) and WO 02/31041 (or US
2004/051210), or
else of silanes or POSs bearing azodicarbonyl functional groups, such as
described, for example, in
Patent Applications WO 2006/125532, WO 2006/125533 and WO 2006/125534.
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The content of coupling agent is advantageously less than 20 phr, it being
understood that it is
generally desirable to use as little as possible of it. Typically, the content
of coupling agent
represents from 0.5% to 15% by weight, with respect to the amount of inorganic
filler. Its content is
preferably between 0.5 and 12 phr, more preferably within a range extending
from 3 to 10 phr.
This content is easily adjusted by a person skilled in the art depending on
the content of inorganic
filler used in the composition.
The rubber composition in accordance with the invention can also comprise, in
addition to the
coupling agents, coupling activators, agents for covering the inorganic
fillers or more generally
processing aids capable, in a known way, by virtue of an improvement in the
dispersion of the filler
in the rubber matrix and of a lowering in the viscosity of the compositions,
of improving their ease
of processing in the raw state, these processing aids being, for example,
hydrolysable silanes, such
as alkylalkoxysilanes (in particular alkyltriethoxysilanes), polyols,
polyethers (for example,
polyethylene glycols), primary, secondary or tertiary amines (for example,
trialkanolamines),
hydroxylated or hydrolysable POSs, for example a,w-
dihydroxypolyorganosiloxanes (in particular
a,w-dihydroxypolydimethylsiloxanes), or fatty acids, such as, for example,
stearic acid.
According to a specific embodiment of the invention, the silica can be used at
contents ranging
from 2 to 35 phr, preferably from 3 to 25 phr and in particular from 5 to 20
phr. According to this
embodiment, preferably, the rubber composition comprises from 0 to less than 2
phr of a coupling
agent, more preferably from 0 to less than 1 phr of a coupling agent; more
preferably still, it does
not comprise a coupling agent. In the case where the rubber composition does
not comprise a
coupling agent, the silica is not regarded as a reinforcing filler and the
rubber composition
preferably comprises a covering agent which is preferably a polyethylene
glycol. This specific
embodiment of the invention, in or not in its preferred forms, can be combined
with any one of the
embodiments of the invention.
The content of reinforcing filler is preferably within a range extending from
10 to 90 phr. Below 10
phr, the reinforcement of the rubber composition can be insufficient to
contribute an appropriate
level of cohesion or wear resistance of the rubber component of the tyre
comprising this
composition. Above 90 phr, there exists a risk of increasing the hysteresis of
the rubber
composition and thus a risk of heating the tread and the tyre. The content of
total reinforcing filler
is more preferably from 25 to 70 phr, more preferably still from 35 to 60 phr.
These contents of
reinforcing filler, whether or not they are preferred, apply to any one of the
embodiments of the
invention.
The rubber composition can also comprise all or a portion of the usual
additives customarily used
in elastomer compositions, such as, for example, plasticizers, pigments,
protective agents, such as
antiozone waxes, chemical antiozonants or antioxidants, antifatigue agents, a
crosslinking system,
vulcanization accelerators or retardants, or vulcanization activators.
According to any one
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embodiment of the invention, the crosslinking system is preferably based on
sulphur but it can also
be based on sulphur donors, on peroxide, on bismaleimides or on their
mixtures.
The rubber composition can be manufactured in appropriate mixers, using two
successive phases
of preparation well known to a person skilled in the art: a first phase of
thermomechanical working
or kneading ("non-productive" phase) at high temperature, up to a maximum
temperature of
between 130 C and 200 C, followed by a second phase of mechanical working
("productive" phase)
down to a lower temperature, typically below 110 C, for example between 40 C
and 100 C, during
which finishing phase the crosslinking system is incorporated.
The process for preparing the tyre in accordance with the invention comprises,
for example, the
following stages:
- adding, during a first "non-productive" stage, to the first diene
elastomer, the thermoplastic
styrene elastomer and the reinforcing filler, by kneading thermomechanically
until a maximum
temperature of between 130 C and 200 C is reached,
- cooling the combined mixture to a temperature of less than 70 C,
- subsequently incorporating the crosslinking system,
- kneading everything up to a maximum temperature of less than 90 C in
order to obtain a mixture,
- then calendering or extruding the mixture obtained in order to form a tread.
Whatever the embodiment of the invention, the tyre for vehicles intended to
bear heavy loads in
accordance with the invention is preferably an off-road tyre, tyre for
vehicles running over non-
bituminous ground surfaces, such as civil engineering vehicles, worksite heavy-
duty vehicles or
agricultural vehicles. The tyre is preferably a tyre for a civil engineering
vehicle, whatever the
embodiment of the invention.
The invention relates to the tyres described above, both in the raw state
(that is to say, before
curing) and in the cured state (that is to say, after crosslinking or
vulcanization).
The abovementioned characteristics of the present invention, and also others,
will be better
understood on reading the following description of several implementational
examples of the
invention, given by way of illustration and without limitation.
III. IMPLEMENTATIONAL EXAMPLES OF THE INVENTION
The formulation of compositions Ti, A and B is described in Table I and that
of compositions 12 and
C is described in Table II.
Compositions A to C are in accordance with the invention in that the elastomer
matrix comprises a
polybutadiene or a butadiene copolymer and at most 50% by weight of a
thermoplastic styrene
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=
elastomer in accordance with the invention and in that the reinforcing filler
comprises more than
50% by weight of a carbon black. A and B differ from one another in the nature
of the
thermoplastic styrene elastomer. C differs from A and B in that it comprises
an SBR instead of a BR
as a first diene elastomer.
Composition Ti, devoid of thermoplastic styrene elastomer, is the control
composition for
compositions A and B; composition T2, devoid of thermoplastic styrene
elastomer, is the control
composition for C.
Compositions Ti, T2, A, B and C are prepared in accordance with the process
described above.
The compositions thus obtained are subsequently calendered, either in the form
of plaques (with a
thickness ranging from 2 to 3 mm) or thin sheets of rubber, for the
measurement of their physical
or mechanical properties, or in the form of profiled elements which can be
used directly, after
cutting and/or assembling to the desired dimensions, as tyre tread.
The results are recorded in Table III for A and B and the control Ti and in
Table IV for C and the
control T2.
The results which appear in Tables III and IV show a very strong improvement
in the resistance to
crack propagation for A, B and C, in comparison with their respective
controls. The improvement is
outstanding when the first diene elastomer is a polybutadiene and even more
outstanding when
the thermoplastic styrene elastomer is an SBIS.
The invention makes it possible to significantly improve the lifetime of tyres
bearing heavy loads, in
particular moving off-road, such as tyres equipping heavy-duty vehicles, in
particular agricultural
vehicles, civil engineering vehicles and worksite heavy-duty vehicles, since
these tyres become
much less sensitive to crack propagation at their treads.
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Table I
Ti A
BR (1) 100 70 70
SBS (2) 30
SBIS (3) 30
Silica (4) 15 15 15
Carbon black (5) 40 40 40
Antioxidant 2.5 2.5 2.5
Paraffin 1 1 1
PEG (6) 2.5 2.5 2.5
Stearic acid 1 1 1
ZnO 2.7 2.7 2.7
CBS (7) 1 1 1
Sulphur 1.7 1.7 1.7
(1) BR comprising 4.3% of 1,2-; 2.7% of trans-1,4-; 93% of cis-1,4- (Tg -106
C)
(2) SBS, D1101, sold by Kraton
(3) SBIS, D1170, sold by Kraton
(4) Ultrasil VN3, sold by Evonik
(5) N115
(6) polyethylene glycol with an Mn of 6000-20 000 g/mol, from Sasol Marl
(7) N-cyclohexy1-2-benzothiazolesulphenamide, Santocure CBS, sold by Flexsys
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Table II
12 C
SBR (1) 100 70
SBS (2) 30
Silica (3) 15 15
Carbon black (7) 40 40
Antioxidant 2.5 2.5
Paraffin 1 1
PEG (8) 2.5 2.5
Stearic acid 1 1
ZnO 2.7 2.7
CBS (9) 1 1
Sulphur 1.7 1.7
(1) SBR with 25% of styrene (% by weight relative to the weight of SBR) and
40% of 1,2-
butadiene units (% by weight of the butadiene part)
(2) SBS, D1101, sold by Kraton
(3) Ultrasil VN3, sold by Evonik
(4) Zeosil 1165 MP, from Rhodia (HDS type)
(5) TESPT, Si69, from Evonik
(6) diphenylguanidine, Perkacit DPG, from Flexsys
(7) N115
(8) polyethylene glycol with an Mn of 6000-20 000 g/mol, from Sasol Marl
(9) N-cyclohexy1-2-benzothiazolesulphenamide, Santocure CBS, sold by Flexsys
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Table III
Resistance to crack
Ti A
propagation
at 20 C 100 430 778
Table IV
Resistance to crack T2
propagation
at 20 C 100 312
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