Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-05-15
2003-01-28
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S399000, C152S209100, C152S450000
Reexamination Certificate
active
06512035
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the use of processing aids to improve the dispersion of silica reinforcing filler in rubber compounds. More particularly, the invention provides a vulcanizable elastomeric composition containing a filler-dispersing aid, and a pneumatic tire having improved snow and ice traction, wet traction, rolling resistance and hysteresis.
BACKGROUND OF THE INVENTION
When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (T
g
) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, elastic modulus, tan delta (&dgr;), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, better rolling resistance, snow and ice traction, and wet traction, and improved fuel economy for vehicles equipped with such tires. Traditionally, improved dispersion of reinforcing fillers has been accomplished by lengthened mixing times. However, in commercial applications, prolonged mixing times result in decreased production and increased expense.
With the increasing use of silica as a reinforcing filler for rubber, filler dispersion in rubber stocks has become a major concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration of silica particles occurs after compounding, leading to poor silica dispersion, a high compound viscosity and a shorter scorch time. Therefore, it is desirable to improve the dispersion of silica in rubber compounds, especially when used for tire treads, to improve performance characteristics.
Previous attempts at preparing readily processable, vulcanizable silica-filled rubber stocks containing natural rubber or diene polymer and copolymer elastomers have focused on the use, during compounding, of bifunctional silica coupling agents having a moiety (e.g., a silyl group) reactive with the silica surface, and a moiety (e.g., a mercapto, amino, vinyl, epoxy or sulfur group) that binds to the elastomer. Well known examples of such silica coupling agents are mercaptosilanes and bis-(3-trialkoxysilylorgano) polysulfides, such as bis-(3-triethyoxysilylpropyl) tetrasulfide which is known commercially as Si69. With the coupling agent acting as an intermediary, the compound viscosity is reduced and the silica particles are more easily dispersed into the elastomeric matrix. However, such silica coupling agents are expensive. In addition, the reaction of the alkoxy portion of the coupling agent with the rubber can result in the release of a substantial amount of alcohol, resulting in a rubber compound containing undesirable bubbles that may form blisters or surface defects in the resulting formed rubber articles.
To address the expense and other problems related to bifunctional silica coupling agents, recent approaches to providing improved dispersion of silica in rubber compounds have been directed to reducing or replacing the use of such silica coupling agents by employing dispersing agents, such as monofunctional silica shielding agents (e.g., silica hydrophobating agents that chemically react with the surface silanol groups on the silica particles but are not reactive with the elastomer) and agents which physically shield the silanol groups, to prevent reagglomeration of the silica particles after compounding. For example, dispersing agents, such as alkyl alkoxysilanes, glycols (e.g., diethylene glycol or polyethylene glycol), fatty acid esters of hydrogenated and non-hydrogenated C
5
and C
6
sugars (e.g., sorbitan oleates, and the like), polyoxyethylene derivatives of the fatty acid esters, and fillers such as mica, talc, urea, clay, sodium sulfate, and the like, are the subjects of our copending, co-owned U.S. patent applications, Ser. Nos. 08/893,864; 08/893,875; 08/985,859; and 09/203,438. Such silica dispersing agents can be used to replace all or part of expensive bifunctional silica coupling agents, while improving the processability of silica-filled rubber compounds by reducing the compound viscosity, increasing the scorch time, and reducing silica reagglomeration. The use of such dispersing aids includes employing an increased amount of sulfur, to replace sulfur that otherwise would have been supplied by a sulfur-containing silica coupling agent, in order to achieve a satisfactory cure of the rubber compound.
Although the above-described silica dispersing agents provide good silica dispersion in vulcanizable elastomeric compounds, there is still a need for other silica dispersing agents that can be similarly used.
SUMMARY OF THE INVENTION
The present invention provides a processing aid for improving the dispersion of silica in a sulfur-vulcanizable elastomeric composition while eliminating or reducing the use of a bifunctional silica coupling agent. In particular, the processing aid of the invention comprises one or more monofunctional alkyl tin compounds, selected from the group of alkyl tin compounds having the formula
wherein R is independently selected from the group consisting of primary, secondary and tertiary alkyl groups having 1 to about 30 carbon atoms, alkaryl groups having about 6 to about 30 carbon atoms, and cycloaliphatic groups having about 5 to about 30 carbon atoms; A is independently selected from an oxy group and a carboxylate group; X is a halogen; R′ is independently selected from hydrogen, C
1
to about C
20
saturated or unsaturated aliphatics, about C
5
to about C
20
cycloaliphatic, and about C
6
to about Q
0
aromatic; “j” is an integer equal to 1 to 3; “m” is an integer equal to zero to 3; and “k” is an integer equal to zero to 3; and (j+k+m) equals 4; wherein when “k” is 2 or greater, R′ optionally forms a bridge between two A groups.
As used herein, the term “monofunctional” in reference to the subject alkyl tin compound(s) is defined as an alkyl tin compound having a first end comprising at least one constituent group that is chemically reactive with silica (i.e., by covalent bonding), and having a second end comprising a constituent group that is compatible with, but does not form a covalent bond with, the elastomer. For example, the chemically reactive groups may comprise tin in combination with an oxy, carboxylate and/or a halogen group; whereas the R and R′ tail(s) are non-polar groups that may be weakly reactive with the elastomer by hydrogen bonding, van der Waals forces, and the like.
The terms “processing aid” and “dispersing aid” are used interchangeably herein to refer to the dispersion of reinforcing filler, especially silica, in the compositions.
Exemplary monofunctional alkyl tin compounds for use as processing aids in the compositions of the invention include, but are not limited to, butyltin tris-(2-ethylhexanoate), butyltin chloride dihydroxide, butyltin hydroxide oxide hydrate, dibutyltin dilaurate, and dibutyltin dimaleate. Mixtures of monofunctional alkyl tin compounds may also be employed.
The present invention provides a vulcanizable elastomeric composition comprising an elastomer, a reinforcing filler comprising silica or a mixture thereof with carbon black, a processing aid comprising a monofunctional alkyl tin compound, at least one cure agent, and a sufficient amount of sulfur to achieve a satisfactory cure of the composition. The alkyl tin compound is present in an amount of about 0.1% to about 150% by weight based on the weight of the silica. Preferably, the alkyl tin compound is present in an amount of about 0.5% to about 50% by weight and, more preferably, in the amount of about 1% to about 30% by weight based on the weight of the silica.
In another embodiment, the inv
Hergenrother William L.
Lin Chen-Chy
Arndt Barbara
Bridgestone Corporation
Harlan R.
Palmer Meredith E.
Wu David W.
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