Tire sidewall compositions

Details Tire sidewall compositions Tire sidewall compositions

1998-05-01

2002-04-23

Ball, Michael W. (Department: 1733)

Resilient tires and wheels

Tires, resilient

Pneumatic tire or inner tube

C525S209000, C525S241000

Reexamination Certificate

active

06374889

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to compositions for making tire sidewalls and other rubber constructions which exhibit improved ozone resistance and fatigue crack propagation resistance, as well as a reduction in staining and discoloration. The composition comprises a blend of halogenated copolymer of isoolefin and para-alkylstyrene of relatively high aromatic comonomer content and relatively low halogen content with general purpose rubbers (GPR) such as butadiene rubber (BR), natural rubber (NR) and/or isoprene rubber (IR). The tire sidewall may comprise a single layer or a veneer construction wherein an outer layer comprises the blend of the halogenated copolymer with one or more general purpose rubbers, and an inner layer can comprise a blend of general purpose rubbers.
BACKGROUND OF THE INVENTION
Rubber tires, such as pneumatic tires, include many components, such as, for example, sidewalls. Sidewalls are continuously subjected to distortion under normal road operating conditions. The sidewalls are subjected to extensive continuous flexing and can crack under such conditions. In addition to flex cracking, sidewalls are also subjected to atmospheric chemical action such as ozone attack. The overall effect is that the sidewalls may erode and degrade. The sidewall may even separate from the tire carcass during use, leading to tire failure.
To reduce the problems caused by ozone attack and flex cracking, tire manufacturers add chemical protectants to the general purpose rubbers used in tire sidewalls. The problem with these protectants is that they tend to be fugitive and can cause staining when in contact with white sidewalls. In some cases, particularly in white sidewalls, polymer blends have been used to effect improvements in ozone and flex resistance.
Published International application PCT/US91/05666 filed Aug. 9, 1991 discloses a tire sidewall composition comprising a single layer prepared from a blend of a halogenated copolymer of isoolefin and para-alkylstyrene with one or more general purpose rubbers.
U.S. Pat. No. 5,376,438 to Costemalle et al. discloses multi-layer tire sidewalls having an outer layer prepared from a blend of halogenated isomonoolefin/para-alkylstyrene copolymer and general purpose rubbers. The inner layer comprises general purpose rubber. The sidewalls are said to exhibit good ozone resistance and fatigue crack resistance as well as a reduction in staining and discoloration. This patent is hereby incorporated herein by reference in its entirety.
In the prior art, the halogen content of the halogenated isoolefin/para-alkylstyrene copolymers generally varied proportionately with the para-alkylstyrene content, e.g. higher para-methylstyrene content was used to get a higher halogen content. If the copolymer was excessively halogenated, some undesirable dihalo benzylic substitution would occur. Thus, excess para-methylstyrene was usually used. In tire sidewall compositions of the prior art, a relatively high bromine level was needed to achieve adhesion and ozone resistance, but such high bromination levels led to relatively tight cures and crack propagation. It would be desirable to be able to use a rubber blend of the halogenated isoolefin/para-alkylstyrene copolymer that has both good adhesion and ozone resistance, as well as flex crack resistance and cure properties.
SUMMARY OF THE INVENTION
It has been found that the use of blends of halogenated copolymers of isoolefin and para-alkylstyrene having specific contents of aromatic monomers and halogenation, with general purpose rubbers yield tire sidewall compositions having significantly improved ozone resistance and fatigue crack propagation resistance. It has now been discovered that excellent curing and crack propagation resistance can be achieved simultaneously with good adhesion and ozone resistance by using a relatively low halogenation content in combination with a relatively high aromatic or para-alkylstyrene content.
In one aspect, the invention is a blend of rubber and a brominated copolymer of an isoolefin and a para-alkylstyrene. The copolymer comprises at least 9.5 but less than 20 weight percent of aromatic monomers (styrenics), and at least 0.2 but less than 1 mole percent para-bromoalkylstyrene. The blend preferably comprises from 35 to 65 phr of the copolymer, and from 35 to 65 phr of the general purpose rubber. The styrenics can include para-alkylstyrene, brominated para-alkylstyrene, alpha-methylstyrene, brominated alpha-methylstyrene or combinations thereof. The isoolefin is preferably isobutylene, whereas the para-alkylstyrene is preferably para-methylstyrene and the para-bromoalkylstyrene is preferably para-bromomethylstyrene. The general purpose rubber can be a natural rubber, styrene-butadiene rubber, polybutadiene rubber and combinations thereof. The copolymer preferably comprises from 12 to 17 weight percent unbrominated styrenics, and from 0.4 to 0.8 mole percent para-bromoalkylstyrene.
In another aspect, the invention provides a tire sidewall comprising a layer of the blend described above. The blend can also be used in a veneered tire sidewall wherein an outer layer comprises the blend described above, and an inner layer comprises a highly unsaturated rubber or blend of unsaturated rubbers.
In a further aspect, the invention provides an improvement in a tire sidewall comprising at least an outer layer comprising a blend of a copolymer of an isoolefin and a para-alkylstyrene and an unsaturated rubber, and an inner layer optionally comprising a highly saturated rubber or blend of unsaturated rubbers. The improvement is that the copolymer comprises from 9.5 to 20 weight percent aromatic monomers and from 0.2 to 1.0 mole percent para-bromoalkylstyrene.
Preferably, the blend contains quantities of bromoalkylstyrene and para-alkylstyrene which satisfy the following formula:
X=[1.91−(0.094×Y)]±0.1
where “X” is mole percent bromoalkylstyrene and “Y” is weight percent para-alkylstyrene (between the proposed limits of 9.5 to 20 weight percent). ±0.1 as used herein indicates that the value X may be any number within the range of 0.1 above to 0.1 below [1.91−(0.094×Y)]. The bromoalkylstyrene is preferably para-bromomethylstyrene and the para-alkylstyrene is preferably para-methylstyrene. More preferably, the blend of the present invention has 0.96 mole percent para-bromomethylstyrene and 10 weight percent para-methylstyrene. Alternatively, the blend of the present invention preferably has 0.84 mole percent para-bromomethylstyrene and 12.5 weight percent para-methylstyrene.
In the preferred embodiment, the polymerized para-alkylstyrene comonomer unit is characterized by the formula:
wherein R and R
1
are independently selected from the group consisting of hydrogen, alkyl groups having from 1 to about 5 carbon atoms, and primary and secondary alkyl halides having from 1 to about 5 carbon atoms, and A is selected from the halogen group consisting primarily of bromine, chlorine and mixtures thereof. Furthermore, the halogenated para-alkylstyrene unit is shown as being pendant from the isoolefin polymer chain, represented by the wavy lines in the formula.
The tire sidewall composition and layers of the present invention can be compounded by methods generally known in the art, such as by mixing with the uncured polymers various fillers such as titanium dioxide; carbon black, when black sidewalls are desired, or non-black fillers and pigments; extenders such as rubber process oils; curing aids such as zinc oxide, sulfur; accelerators or retarders and other additives such as antioxidants and antiozonants.
DETAILED DESCRIPTION OF THE INVENTION
The principal advantages realized by practice of the present invention primarily stem from the fact that by employing a blend of copolymer of an isoolefin and para-alkylstyrene with general purpose rubbers (GPR), wherein the copolymer has a low halogen content but high aromatics content, superior ozone resistance and flex fatigue crack resistance and adhesion can be

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