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
2001-09-04
2003-09-16
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...
C524S392000, C524S201000, C524S377000, C524S378000, C525S332600, C525S333100, C525S331800, C525S333200
Reexamination Certificate
active
06620875
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to rubber compositions and a method for increasing the mooney scorch value of the rubber compositions. The rubber compositions are particularly useful for tire tread applications in vehicles, e.g., passenger automobiles and trucks.
2. Description of the Related Art
The tire treads of modern tires must meet performance standards which require a broad range of desirable properties. Generally, three types of performance standards are important in tread compounds. They include good wear resistance, good traction and low rolling resistance. Major tire manufacturers have developed tire tread compounds which provide lower rolling resistance for improved fuel economy and better skid/traction for a safer ride. Thus, rubber compositions suitable for, e.g., tire treads, should exhibit not only desirable strength and elongation, particularly at high temperatures, but also good cracking resistance, good abrasion resistance, desirable skid resistance, low tangent delta values at 60° C. and low frequencies for desirable rolling resistance of the resulting treads. Additionally, a high complex dynamic modulus is necessary for maneuverability and steering control. A high mooney scorch value is further needed for processing safety.
Presently, silica has been added to rubber compositions as a filler to replace some or substantially all of the carbon black filler to improve these properties, e.g., lower rolling resistance. Although more costly than carbon black, the advantages of silica include, for example, improved wet traction, low rolling resistance, etc., with reduced fuel consumption. Indeed, as compared to carbon black, there tends to be a lack of, or at least an insufficient degree of, physical and/or chemical bonding between the silica particles and the rubber to enable the silica to become a reinforcing filler for the rubber thereby giving less strength to the rubber. Therefore, a silica filler system requires the use of coupling agents.
Coupling agents are typically used to enhance the rubber reinforcement characteristics of silica by reacting with both the silica surface and the rubber elastomer molecule. Such coupling agents, for example, may be premixed or pre-reacted with the silica particles or added to the rubber mix during the rubber/silica processing, or mixing, stage. If the coupling agent and silica are added separately to the rubber mix during the rubber/silica processing, or mixing, stage, it is considered that the coupling agent then combines in situ with the silica.
A coupling agent is a bi-functional molecule that will react with the silica at one end thereof and cross-link with the rubber at the other end. In this manner, the reinforcement and strength of the rubber, e.g., the toughness, strength, modulus, tensile and abrasion resistance, are particularly improved. The coupling agent is believed to cover the surface of the silica particle which then hinders the silica from agglomerating with other silica particles. By interfering with the agglomeration process, the dispersion is improved and therefore the wear and fuel consumption are improved.
The use of silica in relatively large proportions for improving various tire properties requires the presence of a sufficient amount of a coupling agent. The coupling agent and silica however retard the cure. Therefore, a silica/coupling agent tread formulation has been found to undesirably slow the cure rate of the rubber. Additionally, by employing high amounts of the coupling agents results in the rubber compositions being more costly since these materials are expensive.
In order to increase the cure rate, secondary accelerators such as, for example, diphenyl guanidine (DPG), have been added to the rubber compositions. However, the use of secondary accelerators, and particularly DPG with polyalkylene oxides, result in the rubber composition having a lower mooney scorch value during its manufacture thereby resulting in decreased processing time. Problems associated with a decreased processing time include, for example, precured compounds and rough surfaces on extruded parts. Additionally, diphenyl guanidine is typically employed in high amounts which result in the rubber compositions being more expensive to manufacture since more material must be used.
It would be desirable to provide a rubber composition which has a decreased cure time and a higher mooney scorch value without sacrificing other physical properties, e.g., tangent delta value. This will allow for better processing of the rubber composition during its manufacture.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a decreased cure time when forming the rubber compositions herein.
It is also an object of the present invention to provide rubber composition possessing a high mooney scorch value.
In keeping with these and other objects of the present invention, the rubber compositions herein comprise (a) a rubber component; (b) a silica filler; (c) a coupling agent; (d) a cure-enhancing amount of a polyalkylene oxide and (e) a thiuram disulfide having a molecular weight of at least about 400.
By employing a cure-enhancing amount of a polyalkylene oxide, lesser amounts of a coupling agent can be used in forming the rubber compositions resulting in the compositions disclosed herein advantageously possessing a higher cure rate. Accordingly, the delay in cure/vulcanization of rubber observed with the use of silica and coupling agent alone as noted above has been lessened, if not substantively overcome, in many cases by the cure-enhancing amount of the polyalkylene oxides of the present invention. Thus, the polyalkylene oxides herein have been found to increase the cure rate and, in some instances, to fully recapture any cure slow down presumed to have resulted from the use of the silica with higher amounts of a coupling agent relative to the present invention which employs lower amounts of a coupling agent with a polyalkylene oxide in this manner, the polyalkylene oxides have enabled achievement of the silica benefits in full without the prior art disadvantage while also achieving a greater economical advantage by using less materials of the more expensive coupling agent.
Additionally, by further employing a high molecular weight thuiram disulfide, i.e., a thiuram disulfide having a weight average molecular weight (M
w
) of at least 400, with the polyalkylene oxides, the mooney scorch value of the rubber compositions are increased thereby allowing for better processing of the compositions without sacrificing other physical properties.
The term “phr” is used herein as its art-recognized sense, i.e., as referring to parts of a respective material per one hundred (100) parts by weight of rubber.
The expression “cure-enhancing amount” as applied to the polyalkylene oxide employed in the rubber compositions of this invention shall be understood to mean an amount when employed with the coupling agent provides a decreased cure time of the rubber composition.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The rubber compositions of this invention contain at least (a) a rubber component; (b) a silica filler; (c) a coupling agent; and (d) a cure-enhancing amount of at least one polyalkylene oxide and (d) a thiuram disulfide having a molecular weight of at least about 400.
The rubber components for use herein are based on highly unsaturated rubbers such as, for example, natural or synthetic rubbers. Representative of the highly unsaturated polymers that can be employed in the practice of this invention are diene rubbers. Such rubbers will ordinarily possess an iodine number of between about 20 to about 450, although highly unsaturated rubbers having a higher or a lower (e.g., of 50-100) iodine number can also be employed. Illustrative of the diene rubbers that can be utilized are polymers based on conjugated dienes such as, for example, 1,3-butadiene; 2-methyl-1,3-butadiene; 1,3-pentadiene; 2,3-dimethyl-1,3-butadiene; and the like, as well as copolymers of such conjugated dienes with
Greene Peter K.
Hannon Martin J.
Hong Sung W.
Carmen Michael E.
Lee Rip A.
Reitenbach Daniel
Uniroyal Chemical Company, Inc.
Wu David W.
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