Rubber composition with improved processability

Abstract

An improved rubber composition has improved extrudate properties and exhibits excellent aging resistance when subjected to a multi-step mixing process and in particular to a mixing process in which the rubber composition is mixed in a first mixer, cooled by passing between a pair of cylinders, and then mixed in a second mixer, the composition is composed of a disulfide silane.

Description

Technical Field

[0001] The subject matter of the present invention relates to a rubber composition having improved raw rubber properties for improved processability. Background Art

[0002] Elastomers have long been used in the manufacture of vehicle tires. Elastomers provide good flexibility, traction, and durability, making elastomer compositions an ideal material for road interface components of vehicles and particularly suitable for tread components of tires. Filler materials added to rubber compositions, such as carbon black, titanium dioxide, and talc, provide additional reinforcement to the elastomer composition. In particular, silica fillers provide excellent abrasion resistance and traction under both dry and wet conditions. Silica-only elastomer blends (defined herein as having 50 parts per hundred parts by weight of rubber (“phr”) of elastomer) are particularly useful in tread rubber applications. Most silica-only blends tend to have significant property variability during aging in their unvulcanized rubber state (also referred to as the “raw rubber” state). These raw rubber properties include varying viscosity, extrusion appearance (or roughness), die swell, and elasticity. This variability in raw rubber properties can limit the use of the product in an extruder.

[0003] Silanes are well-known coupling agents used to increase the bonding of silica fillers to rubber compounds. Silane coupling agents have three basic functions in silica blends: covering the silica and hiding the highly polar silanol groups from the hydrophobic elastomer, providing a chemical link between the silica and the elastomer, and regulating the vulcanization of the silica blend. The use of silanes allows high-silica blends to be used in tires, providing excellent traction and wear properties not achievable with previously carbon black-reinforced rubber compositions.

[0004] Mixing methods utilizing multiple mixers, such as those described in U.S. Patent No. 10,328,608, are used to produce rubber mixtures for new industrial processes. These newer mixing methods enable an efficient rubber mixing process where multiple rubber mixtures are prepared on the same equipment and formed into sheet products and stored until needed later. However, the use of raw rubber sheet products is limited by the quality of the raw rubber. Typically, extruded rubber has undesirable die swell where the rubber does not maintain the shape of the die through which the rubber is extruded, or has an undesirable rough surface when extruded. This surface roughness makes it difficult to drain water used to cool the rubber during intermediate steps by creating holes or generating texture or grooves in the molded product grooves, making the subsequent drying stage more difficult, making further processing more difficult, or affecting tire uniformity or end product weight variation in the final product. Conventional methods of controlling die swell and surface roughness include reducing the extruder screw speed to reduce the extrudate speed, or adding processing aids. These changes, such as reducing the extrusion speed, affect manufacturing costs by slowing production, and the addition of processing aids may affect the physical properties of the final product. SUMMARY OF THE INVENTION

[0005] Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the detailed description, or may be learned by practice of the invention. It has now been found that by replacing currently used polysulfide silanes with disulfide silanes, the disadvantages of die extrusion of high silica mixtures in the current state of the art in the field can be greatly alleviated.

[0006] Accordingly, the present invention relates to the use of disulfide silanes in a rubber mixture that is mixed and extruded as an intermediate raw rubber product having excellent raw rubber properties, where the rubber is composed of silica filler and diene elastomer mixed in a first mixer, then passed through a pair of cylinders to form a rubber sheet, cooled, and mixed in a second mixer before final fabrication.

[0007] In one exemplary embodiment, the disulfide silane described above has at least one alkoxysilyl group. In another exemplary embodiment, the disulfide silane is a bifunctional sulfur-containing organosilane. In at least one exemplary embodiment, the disulfide silane is bis(triethoxysilylpropyl) disulfide.

[0008] While the rubber compositions disclosed herein can be used for a wide range of tire treads, they are particularly useful in specific embodiments of the invention as all-season tires and / or summer tires for passenger cars and light trucks.

[0009] As used herein, "phr" is "parts per hundred parts by weight of rubber" and is a common measure in the art, where the components of a rubber composition are measured relative to the total weight of the rubber in the composition, i.e., the parts by weight of the component per 100 parts by weight of total rubber in the composition.

[0010] In at least one of the above exemplary embodiments, the rubber composition is passed through an extruder to prepare a shaped product. For example, one such second extruder can be a flat nozzle extruder.

[0011] In at least one embodiment, the silica filler is present in an amount of 45 phr to 140 phr. More specifically, at least one embodiment can have a silica content of 60 phr to 90 phr.

[0012] Some embodiments can contain a disulfide silane content between 5 phr and 20 phr. More specifically, at least one embodiment has a disulfide silane content of 6 phr to 10 phr. In the case of, for example, a tire tread of a passenger vehicle, the coupling agent can be less than 20 wt% or even less than 12 wt% or 8 wt% relative to the total weight of the silica filler.

[0013] The present invention includes a rubber mixture containing rubber, a silica filler present in an amount of 45 phr to 140 phr, and a disulfide silane content added in an amount of 6 phr to 20 phr, the rubber mixture being mixed in a first mixer, cooled by passing through a pair of cylinders and sprayed with a cooling solution, then mixed in a second mixer and finally made into a raw rubber sheet product.

[0014] Other components, such as vulcanizing agents, accelerators, antioxidants, waxes, resins or oils, can be added to the composition during the processing of the composition, or can be added in subsequent steps.

[0015] The use of disulfide silanes in the rubber mixture results in a rubber mixture that exhibits favorable extrusion properties, has a smooth outer surface appearance and no agglomerate formation. The disulfide silane rubber mixture provides minimal die swell compared to mixtures prepared with polysulfide silanes.

[0016] The surface roughness appearance of extruded raw rubber (also referred to herein as surface roughness or appearance) improves over time, which is of particular interest because in rubber mixtures using polysulfide silanes the surface roughness or appearance deteriorates over time. The die swell, which already shows favorable values shortly after extrusion, also shows a very small increase over time compared to rubber compositions containing polysulfide silanes. This property indicates that the rubber mixtures containing disulfide silanes according to the invention are suitable for preparation and then storage for an extended period of time before further processing and / or placement in a mold and vulcanization. This aging-resistant rubber mixture allows improved industrial processing techniques, makes machine utilization more efficient by allowing the same machine to be used for multiple mixtures, extends the time for transportation to other locations, and reduces the scrap of raw rubber compositions that would otherwise "age" and be considered too old for further processing into final products.

[0017] Significantly, it has been found that rubber mixtures prepared according to the invention with disulfide silanes do not exhibit any significant disadvantages in their engineering properties compared to comparative reference materials containing more conventional polysulfide silanes.

[0018] The present invention is particularly applicable to rubber compositions with a high silica content. Other fillers can be added to the mixture, including carbon black such as lamp black, furnace black or gas black; and silicates such as silicic acid, synthetic silicates such as aluminum silicate or alkaline earth metal silicates, or natural silicates such as kaolin and other naturally occurring silicic acids and mixtures thereof.

[0019] "Diene" elastomers should be understood to mean elastomers obtained at least in part from diene monomers. The terms "diene elastomer" and "rubber" can be used interchangeably as the two terms are considered synonymous herein. The rubber component of the rubber mixture is not particularly limited and can consist of natural rubber and synthetic rubbers such as polybutadiene (BR), polyisoprene (IR), styrene / butadiene copolymer (SBR), isobutene / isoprene copolymer (IIR), butadiene acrylonitrile copolymer (NBR), partially or fully hydrogenated NBR (HNBR), ethylene / propylene / diene copolymer (EDPM). Each of these diene rubbers can be used alone or in combination of two or more.

[0020] In addition to vulcanization accelerators such as mercaptobenzothiazole, sulfonamides, guanidines, thiurams, dithiocarbamates, thioureas and thiocarbonates, crosslinking agents such as sulfur-containing or peroxide-containing crosslinking agents can also be added.

[0021] The rubber mixture may contain other components, including reaction accelerators, antioxidants, heat stabilizers, light stabilizers, antiozonants, processing aids, plasticizers, tackifiers, dyes, pigments, waxes, extenders, organic acids, retarders, metal oxides, and activators known in the industry.

[0022] A rubber mixture comprising a vulcanizable elastomer, a silica filler, and a disulfide silane is prepared in an internal mixer having a predetermined filling volume. Once mixed, the rubber is dropped or otherwise removed from the mixer and passed through an external mixer, such as through a pair of cylinders, where it is cooled. Cooling sprays, such as water sprays, can be used to improve cooling, and as is known in the art, the cylinders themselves can be cooled by containing a cooling liquid therein. Excess water can be suctioned off, and then the rubber mixture is placed in a second mixer. Additional components, such as one or more vulcanized rubbers, vulcanizing agents, and other components, can be added to the rubber mixture at different stages of the process, such as at the second mixer. The rubber mixture is finally made into a final raw rubber, where it is then extruded into a final form or further processed before being placed in a mold and vulcanized.

[0023] Vulcanization of the rubber mixture can be carried out at the temperatures and pressures practiced in the industry. For example, vulcanization can be carried out at a temperature of 100 °C to 200 °C and a pressure of 10 bar to 200 bar. Description of the Drawings

[0024] Referring to the accompanying drawings, a complete and enabling disclosure of the invention for a person of ordinary skill in the art, including its best mode, is set forth in the specification, wherein:

[0025] Figure 1 A cross-sectional view of the extrudate tested is provided, showing the surface roughness. Detailed Description

[0026] Example

[0027] The present invention relates to the use of disulfide silanes in rubber mixtures which are mixed and extruded as intermediate raw rubber products having excellent raw rubber properties such as reduced die swell and surface smoothness, wherein the rubber is a high silica mixture consisting of a silica filler and a diene elastomer mixed in a first mixer, then passed through a pair of cylinders to form a rubber sheet, cooled, and mixed in a second mixer before extrusion. For the purposes of describing the present invention, reference will now be made to examples of embodiments of the present invention. Each example is provided as an illustration of the present invention and not a limitation thereof. Indeed, those skilled in the art will appreciate that various modifications and changes can be made to the present invention without departing from the scope or essence thereof. For example, features or steps described or illustrated as part of one embodiment can be used in conjunction with another embodiment or step to yield yet another embodiment or method. Accordingly, it is intended that the present invention cover such modifications and variations as fall within the scope of the appended claims and their equivalents.

[0028] General methods for preparing rubber mixtures and their vulcanized rubbers are well known in the art and are modified as described herein to conform to the disclosed invention.

[0029] Two rubber formulation compositions were prepared according to the formulations in Table 1. W1 containing polysulfide silane and the test composition T1 containing disulfide silane. The raw rubber properties of the unvulcanized rubber compositions were tested to evaluate the processability and time stability properties of the raw rubber compositions tested.

[0030] The rubber compositions were prepared by placing the elastomer, carbon black, silica, and silane in a closed first mixer and mixing. The mixture was allowed to fall through a pair of cylinders and cooled, and cooled by means of a water spray. Excess water was aspirated from the composition and placed in a second mixer, in which the remaining components were added. The composition was mixed again. The composition was extruded into a flat sheet product. In this example, the samples were subsequently extruded for testing.

[0031] Table 1 .

[0032]

[0033]

[0034] The surface roughness of the raw rubber extrudate was characterized by examining the cross-section of the extrudate at piston speeds corresponding to slow, medium, and fast extrudate speeds at 110 °C. The images were graded, and a higher index was associated with a smoother extrudate surface, while a lower index indicated a rougher surface. A smooth surface was reported as "100". Index values between 100 and 90 were considered acceptable, while values between 80 and 90 were considered slightly rough, and index values less than 80 were considered unacceptable.

[0035] Die swell measures the ratio of the thickness of the extrudate to the thickness of the die through which the extrudate is formed. A value greater than 1 indicates an extrudate larger than the die through which it is formed. Measurements were made on samples extruded at 90 °C at 0.05 mm / s, 0.1 mm / s, 0.25 mm / s, and 0.5 mm / s.

[0036] Table 1

[0037] W1 T1 Mooney (ML 1+4) MU 111 108.4 10% Modulus (MPa) 5.57 5.74 100% Modulus (MPa) 1.84 1.83 300% Modulus (MPa) 2.14 2.09 Tensile Strength (MPa) 23.94 23.26 Elongation at Break (%) 478.6 479.4 Hardness, Shore A 66.16 66.52 Surface Roughness (Low Speed) Index 69 77 Surface Roughness (Medium Speed) 76 81 Surface Roughness (High Speed) 74 76 Die Swell at 37 s-1 (0.05 mm / s)? 1.27 1.24 Die Swell at 37 s-1 (0.1 mm / s)? 1.26 1.22 Die Swell at 37 s-1 (0.25 mm / s)? 1.24 1.20 Die Swell at 37 s-1 (0.5 mm / s)? 1.23 1.20

[0038] Tests have shown that the blend with disulfide silane (T1) exhibits a smoother extruded product surface than the compound containing polysulfide silane (W1), which is particularly evident at low shear rates. Conventional processing techniques used to control surface roughness (also referred to herein as the surface roughness of the extruded rubber product) can result in slower die extrusion rates, which affect processing time, or can result in the introduction of processing aids such as increased oil or resin, which can affect the physical properties of the final product. The use of disulfide silane surprisingly improves the processability of the rubber blend when the rubber compound is mixed in an internal mixer, cooled through an open mixer, and then mixed and extruded again.

[0039] Die swell results show an improvement for the disulfide blend (T1) compared to the polysulfide blend (W1) processed under the same conditions.

[0040] In terms of both surface roughness and die swell tested at the time of extrusion, 14 days after extrusion, and 28 days after extrusion, the properties of the disulfide silane blend (T1) also surprisingly showed less susceptibility to degradation associated with aging compared to the polysulfide silane blend (W1). Table 2 below shows the test results.

[0041] Table 2

[0042]

[0043] Tests have shown that the polysulfide silane composition (W1) exhibits an increasing surface roughness over time, while the surface roughness of the disulfide silane mixture (T1) remains constant and even improves over time. Similarly, the tested embossing properties of the materials extruded at low and high speeds both show that the disulfide composition is consistent even after 28 days of aging, while the embossing value of the polysulfide mixture (W1) increases over time, doubling in the case of the low-speed extrudate and increasing by 50% in the case of the high-speed extrudate W1 mixture. The results of die swell measurements after 14 days and 28 days show the excellent dimensional stability of the extruded cross-section of the disulfide silane mixture (T1), while the extruded cross-section of the polysulfide silane mixture (W1) increases over time.

[0044] When processing rubber by mixing in a first mixer, cooling through a pair of cylinders, mixing in a second mixer, and extruding, the excellent aging properties resulting from the use of disulfide silanes are surprising. This finding is particularly useful in improving rubber processing by allowing rubber extrudates to be prepared, stored, and subsequently further processed or incorporated into mixtures in an economical manner while minimizing the adverse effects of aging on the rubber composition.

[0045] Selected combinations of aspects of the disclosed technology correspond to various different embodiments of the invention. It should be noted that each of the exemplary embodiments presented and discussed herein should not be construed as a limitation on the subject matter. Additionally, certain features may be interchanged with similar devices or features not explicitly mentioned that perform the same or similar functions.

[0046] The terms "a," "an," and the singular form of a word shall be considered to include the plural form of the same word, such that these terms mean to provide one or more of something. The terms "at least one" and "one or more" are used interchangeably. A range described as "between a and b" includes the values of "a" and "b."

[0047] The citation of any document does not admit that the document is prior art with respect to any invention disclosed or claimed herein, nor does it admit that the document teaches, suggests, or discloses any such invention, either alone or in combination with any other one or more reference documents. Additionally, in the case where any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition given to the term in this document shall prevail.

Claims

1. A tread for a tire, the tread comprising a rubber composition based on a crosslinkable rubber composition, the crosslinkable rubber composition comprising per hundred parts by weight of rubber (phr): 100 phr of rubber selected from the group consisting of natural rubber between 0 phr and 20 phr, functionalized styrene-butadiene rubber between 0 phr and 100 phr, non-functionalized styrene-butadiene rubber between 0 phr and 100 phr, polybutadiene rubber between 0 phr and 50 phr, and combinations thereof; A plasticizer system comprising a plasticizing resin between 10 phr and 30 phr and 0 phr to 40 phr of a plasticizing liquid; Silica filler between 60 phr and 125 phr; A disulfide silane; and A sulfur curing system.

2. The rubber composition according to claim 1, wherein the disulfide silane is present between 6 wt% and 20 wt% relative to the total weight of the silica filler.

3. The rubber composition according to claim 2, wherein the disulfide silane is present between 6 wt% and 12 wt% relative to the total weight of the silica filler.

4. The rubber composition according to any one of the preceding claims, wherein the disulfide silane is bis(triethoxysilylpropyl)disulfide.

5. The tread according to claim 4, wherein the bis(triethoxysilylpropyl)disulfide is present between 6 wt% and 10 wt% relative to the total weight of the silica filler.

6. The rubber composition according to any one of the preceding claims, wherein the silica filler accounts for 10% to 40% of the composition by weight of the entire composition.

7. The rubber composition according to claim 5, wherein the silica filler accounts for 30% to 35% of the composition by weight of the entire composition.

8. The rubber according to any one of the preceding claims, wherein the rubber is prepared by a method comprising the following process: Mixing a composition comprising the rubber, the silica filler, and the disulfide silane in a first mixer; Cooling the rubber mixture by the following process: Passing the rubber mixture through a pair of cylinders to form the mixture into a rubber sheet; and Spraying the rubber with a cooling liquid; Mixing the rubber mixture again in a second mixer; and Extruding the rubber mixture through an extruder.

9. The rubber composition according to claim 8, further comprising: Passing the rubber composition through a second extruder to form a shaped product.

10. The rubber composition according to claim 9, wherein the second extruder is a flat nozzle extruder.

11. The rubber composition according to claim 10, wherein the second extruder is a cold-feed barrel rubber processing extruder.

12. The rubber composition according to any one of claims 8 to 11, wherein the cooling liquid is water.

13. The rubber composition according to any one of the above claims, wherein the rubber composition is vulcanized into a final product.

14. The rubber composition according to claim 11, wherein the final product is a tire.

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