U.S. patent application number 14/468947 was filed with the patent office on 2015-01-29 for rubber composition for tire including a novel anti-oxidant system.
The applicant listed for this patent is Michelin Recherche et Technique S.A., Societe de Technologie Michelin. Invention is credited to Jose Carlos Araujo Da Silva, Arnaud Remy.
Application Number | 20150031810 14/468947 |
Document ID | / |
Family ID | 40364254 |
Filed Date | 2015-01-29 |
United States Patent
Application |
20150031810 |
Kind Code |
A1 |
Araujo Da Silva; Jose Carlos ;
et al. |
January 29, 2015 |
Rubber Composition for Tire Including a Novel Anti-oxidant
System
Abstract
Rubber composition, especially for a tyre, based on at least one
diene elastomer, a reinforcing filler, a crosslinking system and an
antioxidant system, wherein the antioxidant system has at least two
antioxidants "A" and "B": A. an
N-alkyl-N'-phenyl-para-phenylenediamine corresponding to the
formula (I): ##STR00001## B. a
4,4',4''-tris(1,3-dimethylbutylamino)triphenylamine corresponding
to the specific formula: ##STR00002## wherein R.sup.1 represents a
linear or branched alkyl group having from 1 to 12 carbon atoms or
a cycloalkyl group having from 5 to 8 carbon atoms, the A/B weight
ratio being between 1/10 and 5/1.
Inventors: |
Araujo Da Silva; Jose Carlos;
(Pont Du Chateau, FR) ; Remy; Arnaud; (Orleat,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Societe de Technologie Michelin
Michelin Recherche et Technique S.A. |
Clearmont-Ferrand
Granges-Paccot |
|
FR
CH |
|
|
Family ID: |
40364254 |
Appl. No.: |
14/468947 |
Filed: |
August 26, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12992830 |
Feb 9, 2011 |
8833417 |
|
|
PCT/EP2009/055842 |
May 14, 2009 |
|
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14468947 |
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Current U.S.
Class: |
524/258 ;
106/499 |
Current CPC
Class: |
C08K 3/04 20130101; C08K
5/18 20130101; Y10T 152/1081 20150115; C08L 21/00 20130101; C08K
5/18 20130101; C08K 5/0008 20130101; C08K 3/013 20180101 |
Class at
Publication: |
524/258 ;
106/499 |
International
Class: |
C08K 5/18 20060101
C08K005/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2008 |
FR |
08/53145 |
Claims
1. A rubber composition based on at least one diene elastomer, a
reinforcing filler, a crosslinking system and an antioxidant
system, wherein said antioxidant system comprises at least two
antioxidants "A" and "B": A. an
N-alkyl-N'-phenyl-para-phenylenediamine corresponding to the
formula (I): ##STR00017## B. a
4,4',4''-tris(1,3-dimethylbutylamino)triphenylamine corresponding
to the specific formula: ##STR00018## wherein R.sup.1 represents a
linear or branched alkyl group having from 1 to 12 carbon atoms or
a cycloalkyl group having from 5 to 8 carbon atoms, the A/B weight
ratio being between 1/10 and 5/1.
2. The composition of claim 1, wherein R.sup.1 represents an alkyl
group having from 2 to 8 carbon atoms or a cyclohexyl group.
3. The composition of claim 2, wherein R.sup.1 represents an alkyl
group chosen from the group composed of isopropyl,
1,3-dimethylbutyl and 1,4-dimethylpentyl.
4. The composition of claim 3, wherein antioxidant A is
N-isopropyl-N'-phenyl-para-phenylenediamine corresponding to the
formula (III-a): ##STR00019##
5. The composition of claim 3, wherein antioxidant A is
N-1,3-dimethylbutyl-N'-phenyl-para-phenylenediamine corresponding
to the formula (III-b): ##STR00020##
6. (canceled)
7. (canceled)
8. The composition of claim 1, wherein the content of each
antioxidant A and B is between 0.5 and 5 phr.
9. The composition of claim 1, wherein the A/B weight ratio is
between 1/7 and 4/1.
10. The composition of claim 1, wherein the diene elastomer is
chosen from the group formed by polybutadienes, natural rubber,
synthetic polyisoprenes, butadiene copolymers, isoprene copolymers
and blends of these elastomers.
11. The composition of claim 10, wherein the diene elastomer is an
isoprene elastomer.
12. The composition of claim 11, wherein the isoprene elastomer is
chosen from the group composed of natural rubber, synthetic
cis-1,4-polyisoprenes, isoprene copolymers and blends of these
elastomers.
13. The composition of claim 12, wherein the isoprene elastomer is
natural rubber.
14. (canceled)
15. An antioxidant system that can be used for the anti-ageing
protection of a rubber composition for a tire, wherein said system
comprises at least two antioxidants "A" and "B": A. an
N-alkyl-N'-phenyl-para-phenylenediamine corresponding to the
formula (I): ##STR00021## B. a
4,4',4''-tris(1,3-dimethylbutylamino)triphenylamine corresponding
to the specific formula: ##STR00022## wherein R.sup.1 represents a
linear or branched alkyl group having from 1 to 12 carbon atoms or
a cycloalkyl group having from 5 to 8 carbon atoms, the A/B weight
ratio being between 0.1 and 5.0.
16. (canceled)
17. A tire comprising the rubber composition of claim 1.
18. The tire of claim 17, said tire being a heavy vehicle tire.
19. The tire of claim 17, wherein the rubber composition is in the
belt of said tire.
20. A process for preparing a rubber composition based on at least
one diene elastomer, a reinforcing filler, a crosslinking system
and an antioxidant system, said process comprising the steps of: a.
incorporating into a diene elastomer, in a mixer: (i) a reinforcing
filler; and (ii) an antioxidant system, by thermomechanically
kneading the entire mixture, in one or more stages, until a maximum
temperature between 110.degree. C. and 190.degree. C. is reached;
b. cooling the entire mixture to a temperature below 100.degree.
C.; c. then incorporating a crosslinking system; d. kneading the
entire mixture up to a maximum temperature below 110.degree. C.,
and being characterized in that said antioxidant system comprises
at least two antioxidants "A" and "B": A. an
N-alkyl-N'-phenyl-para-phenylenediamine corresponding to the
formula (I): ##STR00023## B. a
4,4',4''-tris(1,3-dimethylbutylamino)triphenylamine corresponding
to the specific formula: ##STR00024## wherein R.sup.1 represents a
linear or branched alkyl group having from 1 to 12 carbon atoms or
a cycloalkyl group having from 5 to 8 carbon atoms, the A/B weight
ratio being between 0.1 and 5.0.
21. The composition of claim 8, wherein the content of each
antioxidant A and B is between 0.5 and 3 phr.
Description
RELATED APPLICATIONS
[0001] This is a continuation of U.S. patent application Ser. No.
12/992,830, filed on Nov. 15, 2010, which is a U.S. National stage
under 35 U.S.C. .sctn.371 of International application No.
PCT/EP2009/055842, filed on May 14, 2009, and which claims the
priority of French application No. 08/53145, filed on May 15, 2008,
the entire content of which is hereby incorporated by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to diene elastomer
compositions and also to the antioxidant agents used for the
anti-ageing protection of such compositions.
[0003] More particularly, it relates to the diene elastomer
compositions used to form all or part of the rubbery matrix of
tyres, especially to the crown reinforcements of these tyres, also
known as "belts".
BACKGROUND OF THE INVENTION
[0004] It will briefly be recalled that a tyre having a radial
carcass reinforcement comprises, in a known manner, a tread, two
inextensible beads, two sidewalls joining the beads to the tread
and a belt placed circumferentially between the carcass
reinforcement and the tread, this belt consisting of various plies
(or "layers") of rubber which may or may not be reinforced by
reinforcing elements (or "reinforcements") such as cords or
monofilaments, of the metal or textile type.
[0005] The tyre belt generally consists of at least two superposed
belt layers or plies, sometimes known as "working" plies or
"crossed" plies, the reinforcements of which are in practice
arranged parallel to one another within a layer, but crossed from
one layer to the other, that is to say inclined, whether
symmetrically or not, to the median circumferential plane, by an
angle which is generally between 10.degree. and 45.degree.
depending on the type of tyre in question. Each of these two
crossed layers consists of a rubber matrix generally based on
isoprene, sometimes known as "calendering gum" that coats the
reinforcements. The crossed layers may be finished off by various
other auxiliary rubber plies or layers, having widths that vary
depending on the case, and which may or may not contain
reinforcements; mention will be made by way of example of simple
rubber pads, of layers known as "protective" layers, the role of
which is to protect the rest of the belt from external attack,
perforations, or else layers known as "hoop reinforcement" layers
comprising reinforcements oriented substantially along the
circumferential direction (layers known as "zero degree" layers),
whether they are radially external or internal compared to the
crossed layers.
[0006] This tyre belt must meet, in a known manner, numerous,
sometimes contradictory, requirements, in particular: [0007] (i) to
be as rigid as possible at low deformation, as it substantially
contributes to stiffening the tyre crown; [0008] (ii) to have as
low a hysteresis as possible, to on the one hand, minimise
overheating of the internal zone of the crown when rolling and, on
the other hand, to reduce the rolling resistance of the tyre, which
is synonymous with fuel economy; and [0009] (iii) finally to have a
high endurance, in particular with respect to the phenomenon of
separation or cracking of the ends of the crossed layers in the
"shoulder" zone of the tyre, a problem which is known by the term
"cleavage".
[0010] The third condition especially requires that rubber
compositions incorporated into the formation of tyre belts have a
very high resistance to crack propagation and to thermal oxidation,
in particular achieved due to the use of antioxidant agents that
offer an effective anti-ageing protection.
[0011] This requirement is particularly high for the tyre covers of
heavy vehicles, which are designed to be able to be retreaded one
or more times when the treads that they comprise reach a critical
degree of wear after prolonged rolling.
[0012] The antioxidants used for a very long time as anti-ageing
protective agents in rubber compositions for tyres, especially in
the belts of such tyres, belong to the family of the derivatives of
para-phenylenediamine (PPD) such as for example
N-isopropyl-N'-phenyl-para-phenylenediamine (I-PPD) or
N-1,3-dimethylbutyl-N'-phenyl-para-phenylenediamine (6-PPD) which
are simultaneously excellent antioxidants and antiozonants (see,
for example, Applications WO 2004/033548, WO 2005/063510, WO
2005/133666).
SUMMARY OF THE INVENTION
[0013] The inventors discovered during their research that the
replacement of a part of such a para-phenylenediamine-derived
antioxidant by another specific antioxidant, which to date has not
been used in tyres, makes it possible to increase the long-term
resistance to fatigue and to crack propagation of these rubber
compositions for tyres.
[0014] Consequently, a first aspect of the invention relates to a
rubber composition based on at least one diene elastomer, a
reinforcing filler, a crosslinking system and an antioxidant
system, characterized in that said antioxidant system comprises at
least two antioxidants "A" and "B": [0015] A. an
N-alkyl-N'-phenyl-para-phenylenediamine corresponding to the
formula (I):
[0015] ##STR00003## [0016] B. a
4,4',4''-tris(alkylamino)triphenylamine corresponding to the
formula (II):
[0016] ##STR00004## [0017] in which formulae, R.sup.1, R.sup.2,
R.sup.3 and R.sup.4, which are identical or different, each
represent a linear or branched alkyl group having from 1 to 12
carbon atoms or a cycloalkyl group having from 5 to 8 carbon atoms,
the A/B weight ratio being between 0.1 and 5.0.
[0018] The invention thus offers the tyre belts, and also these
tyres themselves, a novel and particularly advantageous overall
compromise of properties.
[0019] Another aspect of the invention is the use of a composition
according to the invention for manufacturing new tyres or for
retreading worn tyres, most particularly in the case of heavy
vehicle tyres.
[0020] The rubber compositions are prepared by a process which
constitutes another aspect of the present invention, said process
comprising the following steps: [0021] incorporating into a diene
elastomer, in a mixer: [0022] a reinforcing filler; and [0023] an
antioxidant system, [0024] by thermomechanically kneading the
entire mixture, in one or more stages, until a maximum temperature
between 110.degree. C. and 190.degree. C. is reached; [0025]
cooling the entire mixture to a temperature below 100.degree. C.;
[0026] then incorporating a crosslinking system; [0027] kneading
the entire mixture up to a maximum temperature below 110.degree.
C., said process being characterized in that said antioxidant
system comprises at least the two aforementioned antioxidants "A"
and "B", the A/B weight ratio being between 0.1 and 5.0.
[0028] Another aspect of the invention relates to any tyre
comprising a composition according to an embodiment of the
invention, in particular whether this tyre is of radial or
non-radial type.
[0029] The tyres made according to an embodiment of the invention
are particularly intended to equip motor vehicles of the following
types: passenger vehicles, SUVs (sport utility vehicles), two-wheel
vehicles (especially motorcycles), aircraft, for instance
industrial vehicles chosen from vans, "heavy" vehicles--i.e.
underground trains, buses, heavy road transport vehicles (lorries,
tractors, trailers), off-road vehicles such as agricultural or
civil engineering machines--, and other transport or handling
vehicles.
[0030] Another aspect of the invention relates to an antioxidant
system that can be used in particular for the anti-ageing
protection of a rubber composition for a tyre, characterized in
that said system comprises at least the two aforementioned
antioxidants "A" and "B", the A/B weight ratio being between 0.1
and 5.0.
BRIEF DESCRIPTION OF THE SINGLE DRAWING
[0031] The drawing is a schematic FIGURE relating to exemplary
embodiments of the invention which represents a radial cross
section of a heavy vehicle tyre with radial carcass
reinforcement.
MEASUREMENTS AND TESTS USED
[0032] The rubber compositions are characterized before and after
curing, as indicated below.
A) Rheometry
[0033] The measurements are carried out at 150.degree. C. with an
oscillating-chamber rheometer according to the standard DIN
53529--part 3 (June 1983). The change in the rheometric torque as a
function of time describes the change in the stiffening of the
composition following the vulcanization reaction. The measurements
are processed according to the standard DIN 53529--part 2 (March
1983): t.sub.i is the induction time, that is to say the time
necessary for the start of the vulcanization reaction;
t.sub..alpha. (for example, t.sub.99) is the time necessary to
attain a conversion of .alpha. %, that is to say .alpha. % (for
example, 99%) of the difference between the minimum and maximum
torques. Also measured is the 1.sup.st order conversion rate
constant denoted by K (expressed in min.sup.-1), calculated between
30% and 80% conversion, which makes it possible to assess the
vulcanization kinetics.
B) Tensile Tests
[0034] These tests make it possible to determine the elasticity
stresses and the properties at break. Unless indicated otherwise,
they are carried out in accordance with French standard NF T 46-002
of September 1988. The "nominal" secant moduli (or apparent
stresses, in MPa) or "true" secant moduli (relative in this case to
the actual cross section of the test piece) at 10% elongation
(denoted respectively by MA10 and E10), 100% elongation
(respectively MA100 and E100) and 300% elongation (respectively
MA300 and E300) are measured in a second elongation (i.e. after an
accommodation cycle). All these tensile measurements are carried
out under normal temperature (23.+-.2.degree. C.) and hygrometry
(50.+-.5% relative humidity) conditions, according to the French
standard NF T 40-101 (December 1979). Also measured are the tensile
strength (in MPa) and the elongations at break (in %), at a
temperature of 23.degree. C.
C) "MFTRA" Test
[0035] The resistance to fatigue and to the propagation of notches
(with prior initiation), expressed as the number of cycles or in
relative units (u.r.) is measured in a known manner on a test piece
comprising a 1 mm notch and subjected to repeated tensile tests of
low frequency up to an elongation of 20%, using a Monsanto ("MFTR"
type) machine, until the test piece breaks, according to the French
standard NF T 46-021.
[0036] The above test is carried out on the one hand in the initial
state and on the other hand after an accelerated thermal oxidative
ageing of 24 days, the composition sample tested then being placed
in a ventilated oven kept at a temperature of 77.degree. C. and
under an ambient humidity of 40%. A value greater than that of the
control, arbitrarily set at 100, indicates an improved result, that
is to say a greater resistance.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0037] The elastomeric compositions of the invention are based on
at least the following constituents: (i) a (at least one) diene
elastomer; (ii) a (at least one) reinforcing filler; (iii) a
crosslinking system; and (iv) an antioxidant system as described in
detail below.
[0038] Of course, the expression "composition based on" should be
understood to mean a composition comprising the reaction product
and/or mixture of the various constituents used, some of these base
constituents being capable of, or intended to, react together, at
least partly, during the various manufacturing phases of the rubber
compositions, belts and tyres, in particular during their
vulcanization.
[0039] In the present description, unless expressly indicated
otherwise, all the percentages (%) indicated are % by weight.
II-1. Diene Elastomer
[0040] The term "diene" elastomer (or rubber, the two being
considered to be synonymous) is understood to mean, generally, an
elastomer derived at least in part (i.e., a homopolymer or a
copolymer) from diene monomers, that is to say from monomers
bearing two (conjugated or unconjugated) carbon-carbon double
bonds.
[0041] Diene elastomers may be classified, in a known manner, in
two categories: those said to be "essentially unsaturated" and
those said to be "essentially saturated". Generally, the expression
"essentially unsaturated diene elastomer" is understood here to
mean a diene elastomer resulting at least partly from conjugated
diene monomers, having a number of diene units or units of diene
origin (conjugated dienes) that is greater than 15% (mol %). Thus,
for example, diene elastomers such as butyl rubbers or
diene/.alpha.-olefin copolymers of the EPDM type do not fall within
the above definition and may in particular be termed "essentially
saturated diene elastomers" (small or very small number of units of
diene origin, always less than 15%). Within the "essentially
unsaturated" diene elastomer category, the expression "highly
unsaturated diene elastomer" is understood in particular to mean a
diene elastomer having a number of units of diene origin
(conjugated dienes) that is greater than 50%.
[0042] Having given these definitions, it will be understood more
particularly that a diene elastomer that can be used in the
compositions according to the invention means: [0043] (a)--any
homopolymer obtained by polymerizing a conjugated diene monomer
preferably having 4 to 12 carbon atoms; [0044] (b)--any copolymer
obtained by copolymerizing one or more conjugated dienes with one
another or with one or more vinylaromatic compounds preferably
having 8 to 20 carbon atoms; [0045] (c)--a ternary copolymer
obtained by copolymerizing ethylene, an .alpha.-olefin preferably
having 3 to 6 carbon atoms with an unconjugated diene monomer
preferably having 6 to 12 carbon atoms, such as for example the
elastomers obtained from ethylene, propylene and an unconjugated
diene monomer of the aforementioned type such as in particular
1,4-hexadiene, ethylidene norbornene and dicyclo-pentadiene; and
[0046] (d)--a copolymer of isobutene and isoprene (butyl rubber),
and also the halogenated, in particular chlorinated or brominated,
versions of this type of copolymer.
[0047] Although the present invention applies to any type of diene
elastomer, a person skilled in the art of tyres will understand
that it is firstly used with essentially unsaturated diene
elastomers, in particular of the (a) or (b) type above.
[0048] More preferably, the diene elastomer is chosen from the
group formed by polybutadienes (BR), natural rubber (NR), synthetic
polyisoprenes (IR), various butadiene copolymers, various isoprene
copolymers, and blends of these elastomers. Such copolymers are
more preferably chosen from the group formed by butadiene-styrene
(SBR) copolymers, whether the latter are prepared by polymerization
in emulsion (ESBR) or in solution (SSBR), isoprene-butadiene (BIR)
copolymers, isoprene-styrene (SIR) copolymers and
isoprene-butadiene-styrene (SBIR) copolymers.
[0049] Among the polybutadienes, those that are suitable are in
particular those having a (mol %) content of -1,2 units between 4%
and 80% or those having a (mol %) content of cis-1,4 units greater
than 80%. Among the synthetic polyisoprenes, those that are
particularly suitable are cis-1,4-polyisoprenes, preferably those
having a (mol %) content of cis-1,4 bonds greater than 90%. Among
the butadiene or isoprene copolymers, these are understood to mean
in particular the copolymers obtained by copolymerizing at least
one of these two monomers with one or more vinylaromatic compounds
having 8 to 20 carbon atoms. Suitable vinylaromatic compounds are,
for example, styrene, ortho-, meta- and para-methylstyrene, the
commercial "vinyl-toluene" mixture, para-tert-butylstyrene,
methoxystyrenes, chlorostyrenes, vinylmesitylene, divinylbenzene
and vinylnaphthalene. The copolymers may contain between 99% and
20% of diene units and between 1% and 80% of vinylaromatic
units.
[0050] The elastomers can have any microstructure which depends on
the polymerization conditions used, in particular on the presence
or absence of a modifying and/or randomizing agent and on the
amounts of modifying and/or randomizing agent employed. The
elastomers can, for example, be block, random, sequential or
microsequential elastomers and can be prepared in dispersion or in
solution; they can be coupled and/or star-branched or also
functionalized with a coupling and/or star-branching or
functionalization agent. Mention may be made, for coupling to
carbon black, for example, of functional groups comprising a C--Sn
bond or aminated functional groups, such as benzophenone, for
example; mention may be made, for coupling to a reinforcing
inorganic filler, such as silica, of, for example, silanol or
polysiloxane functional groups having a silanol end (such as
described, for example, in FR 2 740 778 or U.S. Pat. No.
6,013,718), alkoxysilane groups (such as described, for example, in
FR 2 765 882 or U.S. Pat. No. 5,977,238), carboxyl groups (such as
described, for example, in WO 01/92402 or U.S. Pat. No. 6,815,473,
WO 2004/096865 or US 2006/0089445) or polyether groups (such as
described, for example, in EP 1 127 909 or U.S. Pat. No.
6,503,973). Mention may also be made, as other examples of
functionalized elastomers, of elastomers (such as SBR, BR, NR or
IR) of the epoxidized type.
[0051] The compositions according to the invention are preferably
intended for tyres, in particular for the carcass reinforcements
for tyres for industrial vehicles such as vans or heavy vehicles,
and also for crown reinforcements of tyres intended both for
passenger vehicles and for industrial vehicles.
[0052] Use is then preferably made of at least one isoprene
elastomer, that is to say an isoprene homopolymer or copolymer, in
other words a diene elastomer chosen from the group composed of
natural rubber (NR), synthetic polyisoprenes (IRs), various
isoprene copolymers and blends of these elastomers. Among the
isoprene copolymers, mention will in particular be made of the
isobutene/isoprene (butyl rubber--IIR), isoprene/styrene (SIR),
isoprene/butadiene (BIR) or isoprene/butadiene/styrene (SBIR)
copolymers.
[0053] The isoprene elastomer is preferably natural rubber or a
synthetic cis-1,4-polyisoprene. Among these synthetic
polyisoprenes, preferably use is made of polyisoprenes having a
(mol %) content of cis-1,4 bonds greater than 90%, more preferably
still greater than 98%.
[0054] By blending (i.e. mixing) with the isoprene elastomer above,
the compositions of the invention may contain diene elastomers
other than isoprene elastomers, preferably in a minority (i.e. less
than 50 phr). The isoprene elastomer more preferably represents 75
to 100% by weight of all the diene elastomers, i.e. 75 to 100 phr
(phr=parts by weight per hundred parts of rubber).
[0055] As such diene elastomers other than isoprene elastomers,
mention will especially be made of any diene elastomer of the
unsaturated type chosen, in particular, from the group composed of
polybutadienes (BRs), in particular cis-1,4 or 1,2-syndiotactic
polybutadienes and those having a (mol %) content of 1,2-units
between 4% and 80%, and butadiene copolymers, especially
styrene/butadiene (SBR) copolymers, and in particular those having
a styrene content between 5 and 50% by weight and more particularly
between 20% and 40% by weight, a (mol %) content of 1,2-bonds of
the butadiene part between 4% and 65%, a (mol %) content of
trans-1,4 bonds between 30% and 80%, styrene/butadiene/isoprene
(SBIR) copolymers, and blends of these various elastomers (BR, SBR
and SBIR).
[0056] By way of example, when the composition is intended for a
tyre of the passenger vehicle type, if such a blend is used, it is
preferably a mixture of SBR and of BR which is used as a blend with
natural rubber, preferably to a limit of less than 25% by weight
(or less than 25 phr) of SBR and BR mixture.
[0057] The composition of the invention can be particularly used in
a heavy vehicle tyre belt, whether this is a new tyre or a worn
tyre (in the case of retreading). In such a case, the isoprene
elastomer is preferably used alone, that is to say without blending
with another diene elastomer or polymer. More preferably still,
this isoprene elastomer is exclusively natural rubber.
II-2. Reinforcing Filler
[0058] It is possible to use any type of reinforcing filler known
for its abilities to reinforce a rubber composition that can be
used for manufacturing tyres, for example an organic filler such as
carbon black, or else an inorganic reinforcing filler such as
silica, with which a coupling agent must be associated.
[0059] As carbon blacks, all the carbon blacks are suitable,
especially the blacks of the HAF, ISAF, SAF type conventionally
used in tyres (known as tyre-grade blacks). Among the latter,
mention will more particularly be made of the reinforcing carbon
blacks of the 100, 200 or 300 series (ASTM grades), such as for
example the blacks N115, N134, N234, N326, N330, N339, N347, N375,
or else, depending on the targeted applications, the blacks of
higher series (for example, N660, N683, N772, N900). The carbon
blacks could be, for example, already incorporated into the
isoprene elastomer in the form of a masterbatch (see, for example,
Applications WO 97/36724 or WO 99/16600).
[0060] Mention may be made, as examples of organic fillers other
than carbon blacks, of the functionalized polyvinylaromatic organic
fillers as described in Applications WO-A-2006/069792 and
WO-A-2006/069793.
[0061] The expression "inorganic reinforcing filler" should be
understood, in the present application, to mean, by definition, any
inorganic or mineral filler (regardless of its colour and its
origin (natural or synthetic)), also known as a "white" filler,
"clear" filler or even a "non-black filler" as opposed to carbon
black, capable of reinforcing, by itself without any means other
than an intermediate coupling agent, a rubber composition intended
for manufacturing 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 manner, by the
presence of hydroxyl (--OH) groups at its surface.
[0062] The physical state under which the reinforcing inorganic
filler is provided is not important, whether it is in the form of a
powder, of micropearls, of granules, of beads or any other
appropriate densified form. Of course, the term reinforcing
inorganic filler is also understood to mean mixtures of different
reinforcing inorganic fillers, in particular of highly dispersible
siliceous and/or aluminous fillers as described below.
[0063] Suitable inorganic reinforcing fillers are especially
mineral fillers of the siliceous type, in particular silica
(SiO.sub.2) or of the aluminous type, in particular alumina
(Al.sub.2O.sub.2). The silica used may be any reinforcing silica
known to a person skilled in the art, especially any precipitated
or fumed silica having a BET surface area and also a CTAB specific
surface area that are both below 450 m.sup.2/g, preferably from 30
to 400 m.sup.2/g. As highly dispersible (known as "HD")
precipitated silicas, mention will be made, for example, of the
silicas Ultrasil 7000 and Ultrasil 7005 from Degussa, the silicas
Zeosil 1165MP, 1135MP and 1115MP from Rhodia, the silica Hi-Sil
EZ150G from PPG, the silicas Zeopol 8715, 8745 and 8755 from Huber,
and the silicas having a high specific surface area such as
described in Application WO 03/16837.
[0064] When the compositions of the invention are intended for tyre
treads having low rolling resistance, the inorganic reinforcing
filler used, in particular when this is silica, preferably has a
BET surface area between 45 and 400 m.sup.2/g, more preferably
between 60 and 300 m.sup.2/g.
[0065] Preferably, the total reinforcing filler content (carbon
black, inorganic reinforcing filler or mixture of these two types
of filler) is between 20 and 200 phr, more preferably between 30
and 150 phr, the optimum being, in a known manner, different
depending on the particular applications targeted: the level of
reinforcement expected with regard to a bicycle tyre, for example,
is, of course, less than that required with regard to a tyre
capable of running at high speed in a sustained manner, for example
a motorcycle tyre, a tyre for a passenger vehicle or a tyre for a
utility vehicle, such as a heavy vehicle.
[0066] According to one preferred embodiment of the invention, a
reinforcing filler having between 30 and 150 phr, more preferably
between 50 and 120 phr, of inorganic filler, particularly of
silica, and optionally of carbon black, is used; the carbon black,
when present, is preferably used at a content below 20 phr, more
preferably below 10 phr (for example between 0.1 and 10 phr).
[0067] In order to couple the inorganic reinforcing filler to the
diene elastomer, use is made, in a known manner, of an at least
bifunctional coupling agent (or bonding agent) intended to provide
a sufficient connection, of chemical and/or physical nature,
between the inorganic filler (surface of its particles) and the
diene elastomer, in particular bifunctional organosilanes or
polyorganosiloxanes.
[0068] Use is especially made of polysulphide silanes, said to be
"symmetrical" or "asymmetrical" depending on their particular
structure, such as described, for example, in Applications WO
03/002648 and WO 03/002649.
[0069] Particularly suitable, without the definition below being
limiting, are polysulphide silanes said to be "symmetrical"
corresponding to the following general formula:
Z-A-S.sub.n-A-Z, in which [0070] n is an integer from 2 to 8
(preferably from 2 to 5); [0071] A is a divalent hydrocarbon-based
radical (preferably C.sub.1-C.sub.18 alkylene groups or
C.sub.6-C.sub.12 arylene groups, more particularly
C.sub.1-C.sub.10, especially C.sub.1-C.sub.4, alkylenes, in
particular propylene); and [0072] Z corresponds to one of the
formulae below:
[0072] ##STR00005## [0073] in which: [0074] the R.sup.1' radicals,
which are substituted or unsubstituted, and identical to or
different from one another, represent a C.sub.1-C.sub.18 alkyl,
C.sub.5-C.sub.18 cycloalkyl or C.sub.6-C.sub.18 aryl group
(preferably C.sub.1-C.sub.6 alkyl, cyclohexyl or phenyl groups,
especially C.sub.1-C.sub.4 alkyl groups, more particularly methyl
and/or ethyl); and [0075] the R.sup.2' radicals, which are
substituted or unsubstituted and identical to or different from one
another, represent a C.sub.1-C.sub.18 alkoxy or C.sub.5-C.sub.18
cycloalkoxy group (preferably a group chosen from C.sub.1-C.sub.8
alkoxy and C.sub.5-C.sub.8 cycloalkoxy groups, more preferably
still a group chosen from C.sub.1-C.sub.4 alkoxy groups, in
particular methoxy and ethoxy groups).
[0076] In the case of a mixture of polysulphide alkoxysilanes
corresponding to the formula above, especially the usual
commercially available mixtures, the average value of "n" is a
fraction preferably between 2 and 5, more preferably close to 4.
But the invention may also advantageously be carried out, for
example, with disulphide alkoxysilanes (n=2).
[0077] As examples of polysulphide silanes, mention will more
particularly be made of polysulphides (especially disulphides,
trisulphides or tetrasulphides) of
bis((C.sub.1-C.sub.4)alkoxy(C.sub.1-C.sub.4)alkylsilyl(C.sub.1-C.sub.4)al-
kyl) such as for example the polysulphides of
bis(3-trimethoxy-silylpropyl) or of bis(3-triethoxysilylpropyl).
Among these compounds, use is made, in particular, of
bis(3-triethoxysilylpropyl)tetrasulphide, abbreviated to TESPT, of
formula [(C.sub.2H.sub.5O).sub.3Si(CH.sub.2).sub.3S.sub.2].sub.2 or
bis(triethoxysilylpropyl)disulphide, abbreviated to TESPD, of
formula [(C.sub.2H.sub.5O).sub.3Si(CH.sub.2).sub.3S].sub.2. Mention
will also be made, as preferred examples, of the polysulphides
(especially disulphides, trisulphides or tetrasulphides) of
bis(mono(C.sub.1-C.sub.4)alkoxydi(C.sub.1-C.sub.4)alkyl-silylpropyl),
more particularly bis(monoethoxydimethyl-silylpropyl)tetrasulphide
as described in Patent Application WO 02/083782.
[0078] As a coupling agent other than a polysulphide alkoxy silane,
mention will especially be made of bifunctional POSs
(polyorganosiloxanes) or else of hydroxysilane polysulphides
(R.sup.2'=OH in the formula above) as described in Patent
Applications WO 02/30939 and WO 02/31041, 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.
[0079] In the rubber compositions according to the invention, the
content of coupling agent is preferably between 4 and 12 phr, more
preferably between 3 and 8 phr.
[0080] The coupling agent could be pregrafted to the diene
elastomer or to the inorganic reinforcing filler. It is preferred
however, especially for reasons of better processing of the
compositions in the green state, to use the coupling agent either
grafted to the inorganic reinforcing filler, or in the free state
(i.e. ungrafted).
[0081] Finally, a person skilled in the art will understand that,
as a filler equivalent to the inorganic reinforcing filler
described in the present section, a reinforcing filler of another
nature, especially of organic nature, could be used as long as this
reinforcing filler is covered with an inorganic layer such as
silica, or else comprises, at its surface, functional sites,
especially hydroxyl sites, requiring the use of a coupling agent to
establish the bond between the filler and the elastomer.
II-3. Antioxidant System
[0082] The antioxidant system of the invention has the main feature
of comprising, in combination, two antioxidant agents denoted by
"A" and "B": [0083] A. an N-alkyl-N'-phenyl-para-phenylenediamine
corresponding to the formula (I):
[0083] ##STR00006## [0084] B. a
4,4',4''-tris(alkylamino)triphenylamine corresponding to the
formula (II):
[0084] ##STR00007## [0085] in which formulae, R.sup.1, R.sup.2,
R.sup.3 and R.sup.4, which are identical or different, each
represent a linear or branched alkyl group having from 1 to 12
carbon atoms or a cycloalkyl group having from 5 to 8 carbon atoms,
the A/B weight ratio being between 1/10 and 5/1.
[0086] Preferably, the A/B weight ratio is between 1/10 and 5/1,
and more preferably between 1/7 and 4/1 (i.e. between 0.14 and
4.0).
[0087] Preferably, R.sup.1, R.sup.2, R.sup.3 and R.sup.4, which are
identical or different, each represent an alkyl group having from 2
to 8 carbon atoms, preferably chosen from the group composed of
ethyl, propyl, (i.e. n-propyl, iso-propyl), butyl (i.e. n-butyl,
sec-butyl and tert-butyl), pentyl, hexyl, heptyl and octyl groups,
or a cycloalkyl group having from 5 to 8 carbon atoms (a
cyclopentyl, cyclohexyl, cycloheptyl or cyclooctyl group), more
preferably a cyclohexyl group.
[0088] Among the compounds of formula (I) and (II) above, use is
more preferably made of compounds for which the R.sup.1, R.sup.2,
R.sup.3 and R.sup.4 groups are branched, of respective formulae
(I-bis) and (II-bis) below:
##STR00008##
##STR00009##
in which R.sup.5, R.sup.6, R.sup.7, R.sup.8, R.sup.9, R.sup.10,
R.sup.11 and R.sup.12 which are identical or different from one
another, each represent an alkyl group for which the number of
carbon atoms conforms to the preferred definitions given above for
R.sup.1, R.sup.2, R.sup.3 and R.sup.4.
[0089] The compounds of formula (I-bis) above are well known to a
person skilled in the art, as recalled in the introduction of the
present document.
[0090] Compounds corresponding to the generic formulae (II) and
(II-bis) above are known as antiozonants for rubber; some have, for
example, been described, and also their synthesis, in Patent FR
1,354,536. But no application in diene elastomer compositions for
tyres, a fortiori in the belts of said tyres, has been envisaged
therein.
[0091] As more preferred examples of branched R.sup.1, R.sup.2,
R.sup.3 and R.sup.4 radicals, mention will, in particular, be made
of isopropyl (a), 1,3-dimethylbutyl (b) and 1,4-dimethylpentyl (c)
radicals of formulae below:
##STR00010##
[0092] Thus, according to one particularly preferred embodiment of
the invention, use is made, as antioxidant A, of one of the
following two compounds: [0093]
N-isopropyl-N'-phenyl-para-phenylenediamine ("I-PPD") corresponding
to the formula:
[0093] ##STR00011## [0094]
N-1,3-dimethylbutyl-N'-phenyl-para-phenylenediamine ("6-PPD")
corresponding to the formula:
##STR00012##
[0095] According to another particularly preferred embodiment, that
may or may not be combined with the previous one, use is made, as
antioxidant B, of one of the following two compounds: [0096]
4,4',4''-tris(1,3-dimethylbutylamino)triphenylamine corresponding
to the specific formula:
[0096] ##STR00013## [0097]
4,4',4''-tris(1,4-dimethylpentylamino)triphenylamine corresponding
to the specific formula:
##STR00014##
[0098] In the rubber composition of the invention, the content of
each antioxidant A and B is preferably between 0.5 and 5 phr, that
of the total antioxidant system itself preferably being between 1
and 10 phr. Below the minimum indicated, the endurance may prove to
be insufficient whereas above the maximum recommended, there is a
risk of degradation of the mechanical properties due to an unwanted
plasticizing effect. For all these reasons, the content of each
antioxidant A and B is more preferably between 0.5 and 3.0 phr,
that of the total antioxidant system itself more preferably being
between 1.5 and 4 phr.
II-4. Various Additives
[0099] The rubber compositions according to the invention may also
comprise all or some of the customary additives used in rubber
compositions for tyres, especially for tyre belts, such as for
example extending oils, plasticizers, anti-ageing protective agents
other than those mentioned above, such as anti-ozone agents, other
chemical antiozonants or antioxidants, anti-fatigue agents,
acceptors and donors of methylene, bismaleimides or other
reinforcing resins, a crosslinking system based either on sulphur,
or on sulphur and/or peroxide donors, vulcanization accelerators,
vulcanization activators or retarders, anti-reversion agents such
as, for example, sodium hexathiosulphonate or
N,N'-m-phenylenebiscitraconimide, systems for promoting the
adhesion of the rubber to metal such as, for example, metallic
complexes or salts (for example containing cobalt, boron,
phosphorus, or else lanthanide salts such as described in the
aforementioned Application WO 2005/133666).
[0100] The isoprene matrices may also contain, in addition to
optional coupling agents for inorganic reinforcing fillers, agents
for covering these inorganic fillers, or more generally processing
aids capable of, in a known manner, owing to an improvement in the
dispersion of the filler in the rubber matrix and to a reduction in
the viscosity of the compositions, improving their
processability.
II-5. Rubber Compositions and Tyres According to the Invention
[0101] The diene compositions described previously can especially
be used to form all or part of the rubbery matrix of the belt of a
tyre, in particular of a tyre for a heavy vehicle or a passenger
vehicle.
[0102] They can, for example, be used as a rubber gum for
calendering a belt layer or ply of cord fabric, whether this is a
"crossed" layer, a protective layer or a layer for hoop
reinforcement (at zero degrees), or for example they are intended
to form a simple pad, band or strip of rubber gum, free of
reinforcements, positioned radially on top of or underneath the
various aforementioned belt layers, or even inserted between the
latter, for example to form a sublayer of the tread, or else placed
at the lateral ends of these belt layers, in the "shoulder" zones
of the tyre, for example to form decoupling rubber gums.
[0103] By way of example, the appended single FIGURE schematically
represents a radial cross section of a heavy vehicle tyre 1 having
radial carcass reinforcement which may or may not conform to the
invention, in this general representation. This tyre 1 comprises a
crown 2, two sidewalls 3, two beads 4, a radial carcass
reinforcement 7 extending from one bead to the other. The crown 2,
topped with a tread (not represented in this very schematic FIGURE,
for simplification) is, in a known manner, reinforced by a belt 6
composed of at least two "crossed" crown layers, covered with at
least one crown protection layer, all these layers being reinforced
by metallic cords made of carbon steel. The carcass reinforcement 7
is wound around two bead wires 5 in each bead 4, the upturn 8 of
this reinforcement 7 being, for example, directed towards the
outside of the tyre 1 which is shown here mounted on its rim 9. The
carcass reinforcement 7 is made up of at least one ply or layer
reinforced with metallic cords known as "radial" cords, that is to
say that these cords are arranged practically parallel to one
another and extend from one bead to the other so as to form an
angle of between 80.degree. and 90.degree. with the median
circumferential plane (the plane perpendicular to the axis of
rotation of the tyre which is located half-way between the two
beads 4 and passes through the middle of the belt 6).
[0104] The tyre according to the invention from the above example
has the main feature of comprising, in its crown 2, a belt 6 for
which the calendering gum of the belt layers 6 (two crossed layers
and one protective layer, in this example) comprises the
antioxidant system of the invention.
[0105] In the case of a tyre comprising, for example, one or more
"zero degree" layers, it is preferred that the rubber gum for
calendering the corded fabric, whether it is in the form of a layer
having a certain width, close to the width of the crossed layers,
in the form of narrower strips, or even in the form of a single
rubber-sheathed wire, is also based on a diene composition,
especially an isoprene composition, comprising the antioxidant
system of the invention.
[0106] According to one preferred embodiment of the invention, the
rubber composition based on the diene elastomer (especially the
isoprene elastomer), the reinforcing filler and the antioxidant
system of the invention has, in the vulcanized state (i.e. after
curing), a secant modulus in extension (MA10) which is greater than
3 MPa, more preferably between 5 and 18 MPa. It is in the moduli
ranges indicated above that the best endurance compromise is
recorded.
II-6. Preparation of the Rubber Compositions
[0107] The rubber compositions are manufactured in suitable mixers,
using two successive preparation stages well known to a person
skilled in the art: a first stage of thermomechanical working or
kneading (sometimes referred to as a "non-productive" stage) at
high temperature, up to a maximum temperature (denoted by
T.sub.max) between 110.degree. C. and 190.degree. C., preferably
between 130.degree. C. and 180.degree. C., followed by a second
stage of mechanical working (sometimes referred to as a
"productive" stage) at a lower temperature, typically below
110.degree. C., for example between 40.degree. C. and 100.degree.
C., during which finishing stage the crosslinking or vulcanization
system is incorporated.
[0108] The process for manufacturing the compositions of the
invention is characterized in that at least the reinforcing filler
and the antioxidant system of the invention are incorporated by
kneading into the diene elastomer (especially the isoprene
elastomer) during the first non-productive stage, that is to say
that at least these various base constituents are introduced into
the mixer and are thermomechanically kneaded, in one or more steps,
until a maximum temperature between 110.degree. C. and 190.degree.
C., preferably between 130.degree. C. and 180.degree. C., is
reached.
[0109] By way of example, the first (non-productive) stage is
carried out in a single thermomechanical step during which the
following are introduced into a suitable mixer such as a
conventional internal mixer: firstly all the necessary base
constituents (diene elastomer, reinforcing filler and antioxidant
system), then secondly, for example after kneading for one to two
minutes, the optional complementary processing aids and other
various additives, apart from the crosslinking or vulcanization
system. The total kneading time, in this non-productive stage, is
preferably between 2 and 10 minutes.
[0110] After cooling the mixture thus obtained, the vulcanization
system is then incorporated at low temperature, generally in an
external mixer such as a roll mill; the whole mixture is then
blended (productive stage) for a few minutes, for example between 5
and 15 minutes.
[0111] The final composition thus obtained is then calendered, for
example in the form of sheets (thickness of 2 to 3 mm) or of thin
films of rubber in order to measure its physical or mechanical
properties, especially for laboratory characterization, or else
extruded to form a rubber profile that can for example be used
directly, after cutting or assembling to the targeted dimensions,
and after incorporating the desired textile or metallic
reinforcements, such as a tyre belt layer.
[0112] In summary, the process according to the invention, for
preparing a rubber composition based on at least one diene
elastomer, a reinforcing filler, a crosslinking system and an
antioxidant agent, comprises the following steps: [0113]
incorporating into a diene elastomer, especially an isoprene
elastomer, in a mixer: [0114] a reinforcing filler; and [0115] an
antioxidant system, [0116] by thermomechanically kneading the
entire mixture, in one or more stages, until a maximum temperature
between 110.degree. C. and 190.degree. C. is reached; [0117]
cooling the entire mixture to a temperature below 100.degree. C.;
[0118] then incorporating a crosslinking system; [0119] kneading
the entire mixture up to a maximum temperature below 110.degree.
C., and it is characterized in that said antioxidant system
comprises at least the antioxidants A and B described in detail
previously, in the recommended respective proportions.
[0120] The vulcanization or curing is carried out in a known manner
at a temperature preferably between 130.degree. C. and 200.degree.
C. and under pressure, for a sufficient time which may vary, for
example, between 5 and 90 min depending, in particular, on the
curing temperature, the vulcanization system used, the
vulcanization kinetics and for example the size of the tyre in
question.
[0121] The crosslinking system itself is preferably based on
sulphur and on a primary vulcanization accelerator. Generally added
to this vulcanization system, incorporated during the first
non-productive stage and/or during the productive stage, are
various known secondary accelerators or vulcanization activators
such as zinc oxide, stearic acid, guanidine derivatives (in
particular, diphenylguanidine), vulcanization retarders, etc.
Sulphur is used in an amount preferably between 1 and 10 phr, more
preferably between 2 and 8 phr, especially when the invention is
applied to a heavy vehicle type tyre. The primary vulcanization
accelerator is used in an amount preferably between 0.5 and 5 phr,
more preferably between 0.5 and 2 phr.
[0122] It is possible to use, as a (primary or secondary)
accelerator any compound capable of acting as a vulcanization
accelerator for diene elastomers in the presence of sulphur,
especially accelerators of the thiazole type and also derivatives
thereof, accelerators of the thiuram or zinc dithiocarbamate type.
These accelerators are more preferably chosen from the group
composed of 2-mercaptobenzothiazyl disulphide (abbreviated to
METS), N-cyclohexyl-2-benzothiazyl sulphenamide (abbreviated to
CBS), N,N-dicyclohexyl-2-benzothiazyl sulphenamide (abbreviated to
DCBS), N-tert-butyl-2-benzothiazyl sulphenamide (abbreviated to
TBBS), N-tert-butyl-2-benzothiazyl sulphenamide (abbreviated to
TBSI), zinc dibenzyldithiocarbamate (abbreviated to ZBEC) and
mixtures of these compounds. Preferably, a primary accelerator of
the sulphenamide type is used.
[0123] It goes without saying that the invention relates to the
rubber compositions and tyres described previously both in the
"green" state (i.e. before curing) and in the "cured" or vulcanized
state (i.e. after crosslinking or vulcanization).
III. Exemplary Embodiments
III-1. Synthesis of
4,4',4''-tris(1,3-dimethylbutyl-amino)triphenylamine
[0124] The compound of formula (IV-a) was prepared according to
synthesis as follows.
[0125] Added to a 0.6 litre autoclave equipped with a stirrer were
11.0 g of tris(p-aminophenyl)amine from Organica Feinchemie GmbH,
300 ml of methyl isobutyl ketone and 0.8 g of 5% platinum on
carbon. While being stirred, the medium was placed under a hydrogen
pressure between 2.1 and 3.5 MPa for 60 minutes at 160-165.degree.
C. The medium was then filtered in order to remove the catalyst,
then evaporated under reduced pressure. The
4,4',4''-tris(1,3-dimethylbutylamino)triphenylamine residue
obtained in an amount of 20.2 g was 97.6% pure according to gas
chromatography and was in the form of a deep violet solid.
III-2. Preparation of the Rubber Compositions
[0126] The tests below were carried out in the following manner:
introduced into an internal mixer, filled to 70% and of which the
initial chamber temperature was around 60.degree. C., were an
isoprene elastomer, a reinforcing filler (carbon black), then,
after kneading for one to two minutes, the various other
ingredients, including the antioxidant, or antioxidants, depending
on the case but excluding the vulcanization system. Next a
thermomechanical working stage (non-productive stage) was carried
out in one or two steps (total kneading time equal, for example, to
around 7 min), until a maximum "dropping" temperature of around
165-170.degree. C. was reached. The mixture thus obtained was
recovered, cooled, then the vulcanization system (sulphur and
primary sulphenamide accelerator) was added in an external mixer
(homo-finisher) at 30.degree. C., by mixing everything (productive
phase) for example for 3 to 10 min.
[0127] The compositions thus obtained were then either extruded in
the form of sheets (thickness of 2 to 3 mm) in order to measure
their physical or mechanical properties, or calendered to produce a
metallic cord fabric forming a belt layer ("working" layer) of a
heavy vehicle tyre.
III-3. Characterization Tests
[0128] The objective of this test was to demonstrate the improved
endurance properties of an isoprene composition for a tyre belt,
when it comprises an antioxidant system according to the invention,
in comparison to a first control rubber composition using no
antioxidant and a second control rubber composition using only the
conventional antioxidant (6-PPD) of formula (III-b).
[0129] For this, three compositions based on natural rubber were
prepared: [0130] composition denoted by C-1 (control with
conventional antioxidant A); [0131] composition denoted by C-2
(according to the invention, with combined antioxidants A and B,
having the same content of active principle with respect to
composition C-1). [0132] composition denoted by C-3 (according to
the invention, with combined antioxidants A and B, having the same
content of active principle with respect to composition C-1 but
with different proportions of antioxidants A and B with respect to
composition C-2). [0133] composition denoted by C-4 (according to
the invention, with combined antioxidants A and B, having the same
content of the conventional antioxidant A with respect to
composition C-1).
[0134] These four compositions, having formulations that were
vigorously identical apart from the nature and the concentration of
antioxidant, were intended to constitute the "calendering gum" of
working layers of a belt for a heavy vehicle tyre.
[0135] Tables 1 and 2 give the formulation of the four compositions
(Table 1--contents of the various products expressed in phr), their
properties before and after curing (60 min at 140.degree. C.) and
also the results of the MFTRA endurance test, in relative units
(r.u.) (base 100 set for the control composition C-1, both in the
initial state and after ageing).
[0136] The vulcanization system was composed of sulphur and
sulphenamide. In composition C-3 of the invention, half of the
conventional antioxidant (6-PPD of formula III-b) was replaced by
the antioxidant of formula (IV-a).
[0137] Remember that the compound 6-PPD, reference antioxidant in
the rubber compositions for tyres, especially in the belts of the
latter, has the expanded formula (III-b):
##STR00015##
[0138] For comparison, the
4,4',4''-tris(1,3-dimethylbutyl-amino)triphenylamine used in
compositions C-2, C-3 and C-4 as a complement, has the formula
(IV-a):
##STR00016##
[0139] It can immediately be noted that the second molecule has a
significantly greater steric hindrance and therefore a reduced
ability to diffuse, which, for a person skilled in the art, is, a
priori, unfavourable to an effective antifatigue protection.
[0140] On examining the results from Table 2, it is first noted
that the three compositions C-2, C-3 and C-4 according to the
invention have rubbery properties similar to those for the control
composition C-1, both before curing (rheology properties) and after
curing (mechanical properties in extension).
[0141] As now regards the endurance (MFTRA fatigue test according
to paragraph I-D), it is necessary to distinguish between the
properties in the initial state and those after ageing.
[0142] In the non-aged (initial state) compositions, it may be
noted that the endurance is substantially the same for compositions
C-2, C-3 and C-4 according to the invention in comparison to the
control composition C-1.
[0143] It is therefore important to note that the replacement of
part of the (conventional) antioxidant A by the antioxidant B
therefore does not have, before ageing of the compositions, any
negative effect on the endurance result.
[0144] It has however been noted that the use of only antioxidant
B, in other words the pure and simple replacement of antioxidant A
by antioxidant B under conditions that are otherwise
identical--leads to a degraded performance.
[0145] On the other hand, unexpectedly, an endurance is observed
that is considerably improved (since it is multiplied by three for
composition C-2 compared to the control C-1, multiplied by nineteen
for composition C-3 compared to the control and by thirteen for
composition C-4 compared to C-1) for the compositions according to
the invention C-2, C-3 and C-4, an improvement which can only be
attributed to the replacement of some of the antioxidant A by the
antioxidant B.
[0146] Consequently, the plasticizing system of the invention gives
the rubber compositions of the invention an overall improved
fatigue resistance, with a compromise between resistance in the
initial state and after heat ageing which is very markedly
improved, in comparison with the compositions of the prior art.
[0147] This result allows a person skilled in the art to anticipate
a high endurance of the belts and tyres according to the invention,
in particular with regard to the problem of separation of the ends
of the crown plies ("cleavage") described in the introduction to
the present application.
TABLE-US-00001 TABLE 1 Composition No. C-1 C-2 C-3 C-4 NR (1) 100
100 100 100 Carbon black (2) 50 50 50 50 Antioxidant (3) 2.0 1 0.5
1 Antioxidant (4) -- 1.35 2.03 1 ZnO (5) 4 4 4 4 Stearic acid (6)
2.5 2.5 2.5 2.5 Sulphur 4 4 4 4 Accelerator (7) 1 1 1 1 (1) Natural
rubber; (2) Carbon black N330 (ASTM grade); (3)
N-1,3-dimethylbutyl-N-phenyl-para-phenylene-diamine (SANTOFLEX
6-PPD from Flexsys); (4)
4,4',4''-tris(1,3-dimethylbutylamino)tri-phenylamine; (5) Zinc
oxide (industrial grade - Umicore); (6) Stearin (PRISTERENE 4931 -
Uniqema); and (7) N-dicyclohexyl-2-benzothiazyl sulphenamide
(SANTOCURE DCBS from Flexsys).
TABLE-US-00002 TABLE 2 Composition No. C-1 C-2 C-3 C-4 Properties
before curing t.sub.i (min) 3.3 2.9 2.8 3.3 t.sub.99 (min) 18.1
17.9 17.9 18.2 t.sub.99-t.sub.i (min) 14.8 15.0 15.1 14.9 K
(min.sup.-1) 0.310 0.307 0.305 0.302 Properties after curing MA10
(MPa) 6.84 6.85 6.98 6.95 MA100 (MPa) 3.9 3.8 3.8 3.8 MA300 (MPa)
4.5 4.4 4.3 4.5 Tensile strength (MPa) 26 27 25 26 Elongation at
break (%) 364 382 363 362 Endurance (METRA fatigue) In the initial
state (r.u.) 100 102 108 100 After ageing (r.u.) 100 360 1962
1300
* * * * *