JP7648876B2 - Rubber composition and studless tire using same - Google Patents

Description

The present invention relates to a rubber composition and a studless tire using the same, and more specifically, to a rubber composition that has low heat generation and excellent performance on ice, and a studless tire using the same.

On snowy and icy roads, the coefficient of friction is lower than on normal roads, making the roads more slippery. As a result, numerous methods have been proposed to improve the performance of studless tires on ice (braking performance on ice). For example, one known method is to compound hard foreign matter or hollow polymers into the rubber, which creates microscopic irregularities on the rubber surface, thereby removing the water film that forms on the ice surface and improving friction on ice. Another method involves using rubber that has flexibility at low temperatures.

However, the use of rubber having low-temperature flexibility causes a problem of deterioration in heat build-up, and the industry is demanding further improvement in on-ice performance.
Meanwhile, the following Patent Document 1 discloses a rubber composition in which 1 to 20 parts by mass of a silicone-modified polyolefin is blended with 100 parts by mass of a diene rubber. Here, the following Patent Document 1 describes the finding that when an unmodified polyolefin or a maleic acid-modified polyolefin is blended, performance on ice is reduced.

JP 2018-135454 A

The object of the present invention is to provide a rubber composition that has low heat generation and excellent performance on ice, and a studless tire using the same.

As a result of extensive research, the inventors discovered that a rubber composition in which diene rubber is blended with carbon black and/or a white filler and a specific amount of polyolefin elastomer having a specific hardness and compression set can solve the above problems, and thus completed the present invention.

That is, the present invention provides a rubber composition characterized by blending (I) 100 parts by mass of diene rubber with (II) 30 to 100 parts by mass of carbon black and/or white filler, and (III) 0.5 to 20 parts by mass of a polyolefin elastomer having a JIS A hardness of 0 to 50 and a compression set of 10 to 30%.

The rubber composition of the present invention is characterized by blending (I) 100 parts by mass of diene rubber with (II) 30 to 100 parts by mass of carbon black and/or white filler, and (III) 0.5 to 20 parts by mass of a polyolefin elastomer having a JIS A hardness of 0 to 50 and a compression set of 10 to 30%, and therefore can provide a rubber composition with low heat buildup and excellent performance on ice, and a studless tire using the same.

The polyolefin-based elastomer (III) of the present invention has a JIS A hardness of 0 to 50 and a compression set of 10 to 30%, which gives low-temperature flexibility to studless tires and improves their performance on ice. The polyolefin-based elastomer (III) of the present invention also improves the dispersibility of the carbon black and/or white filler (II) in the diene rubber (I), which gives the tires low heat generation.

The present invention will now be described in further detail.
(I) Diene Rubber The diene rubber used in the present invention can be any diene rubber that can be blended in a rubber composition, and examples thereof include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), and ethylene-propylene-diene terpolymer (EPDM). These may be used alone or in combination of two or more. The molecular weight and microstructure are not particularly limited, and the diene rubber may be terminally modified with an amine, amide, silyl, alkoxysilyl, carboxyl, or hydroxyl group, or may be epoxidized.
From the viewpoint of improving performance on ice, it is preferred that polybutadiene accounts for 30 parts by mass or more, and preferably 40 parts by mass or more, per 100 parts by mass of the diene rubber (I).
The diene rubber (I) preferably has a glass transition temperature (Tg) of not more than −50° C. By specifying the Tg in this manner, performance on ice is improved.
In the case where a plurality of diene rubbers are contained, the Tg referred to in this specification is the sum of the products of the glass transition temperatures of each rubber multiplied by the weight fraction of each rubber, i.e., a value calculated based on a weighted average. In the calculation, the sum of the weight fractions of each component is set to 1.0. The glass transition temperature (Tg) referred to in the present invention refers to the temperature at the midpoint of the transition region measured by differential scanning calorimetry (DSC) at a heating rate of 20°C/min.
More preferably, the average Tg is −60° C. or less.

(II) Carbon Black and/or White Filler Specific examples of the carbon black used in the present invention include furnace carbon blacks such as SAF, ISAF, HAF, FEF, GPE, and SRF. These may be used alone or in combination of two or more.
From the viewpoint of improving performance on ice, the carbon black preferably has a nitrogen adsorption specific surface area (N ₂ SA) of 10 to 300 m ² /g, and more preferably 50 to 150 m ² /g.
The nitrogen adsorption specific surface area (N ₂ SA) is a value measured in accordance with JIS K 6217-2:2001 "Part 2: Determination of specific surface area - Nitrogen adsorption method - Single point method".

Specific examples of the white filler used in the present invention include silica, calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium oxide, calcium sulfate, and the like. These may be used alone or in combination of two or more.
Of these, silica is preferred because of its better performance on ice.

Specific examples of silica include wet silica (hydrated silicic acid), dry silica (anhydrous silicic acid), calcium silicate, and aluminum silicate. These may be used alone or in combination of two or more.

From the viewpoint of improving performance on ice, the silica preferably has a CTAB adsorption specific surface area of 50 to 300 m ² /g, and more preferably 90 to 200 m ² /g.
The CTAB adsorption specific surface area is a value obtained by measuring the amount of n-hexadecyltrimethylammonium bromide adsorbed on the silica surface in accordance with JIS K6217-3:2001 "Part 3: Determination of specific surface area - CTAB adsorption method."

(III) Polyolefin-based elastomer The (III) polyolefin-based elastomer used in the present invention has a JIS A hardness of 0 to 50 and a compression set of 10 to 30%. The JIS A hardness in the present invention is the JIS A hardness measured at 25°C in accordance with the durometer hardness test (type A) of JIS K6253. The compression set was measured in accordance with JIS K6262 (issued in 2013) by hot pressing the polyolefin-based elastomer at 200°C for 10 minutes to prepare a sheet having a thickness of about 2 mm, punching out the obtained sheet into a disk shape having a diameter of 29 mm, stacking seven sheets, and preparing a sample having a height (thickness) of 12.5±0.5 mm, compressing the obtained sample by 25% with a dedicated jig, and leaving it at 70°C for 22 hours, and then measuring the compression set (unit: %) in accordance with JIS K6262 (issued in 2013).

The polyolefin elastomer (III) used in the present invention is not particularly limited as long as it satisfies the JIS A hardness and compression set, but examples thereof include homopolymers or copolymers of modified or unmodified olefin monomers, specifically ethylene-propylene copolymers, ethylene-propylene-diene copolymers, ethylene-vinyl acetate copolymers, polybutene, polyisobutylene, etc. These polyolefin elastomers may be crosslinked. The JIS A hardness and compression set can be adjusted by adjusting the amount of oil impregnated into the polyolefin elastomer (III).

On the other hand, a publicly known example of a polyolefin-based elastomer that satisfies the JIS A hardness and compression set is a polyolefin-based elastomer that is impregnated with oil so as to satisfy the JIS A hardness and compression set, as disclosed in JP 2017-57393 A, which is "at least one selected from the group consisting of elastomeric polymer (A) having a side chain (a) containing a hydrogen-bond crosslinkable moiety having a carbonyl-containing group and/or a nitrogen-containing heterocycle and having a glass transition point of 25°C or less, and elastomeric polymer (B) having a hydrogen-bond crosslinkable moiety and a covalent crosslinkable moiety in the side chain and having a glass transition point of 25°C or less."

In addition, the polyolefin-based elastomer (III) used in the present invention may be a commercially available product, such as the Gelatic series manufactured by ENEOS Corporation.

(Rubber composition blending ratio)
The rubber composition is characterized in that it comprises 100 parts by mass of (I) diene rubber, 30 to 100 parts by mass of (II) carbon black and/or white filler, and 0.5 to 20 parts by mass of (III) polyolefin elastomer having a JIS A hardness of 0 to 50 and a compression set of 10 to 30%.
If the amount of (II) carbon black and/or white filler is less than 30 parts by mass per 100 parts by mass of (I) diene rubber, the mechanical properties and abrasion resistance of the rubber composition will deteriorate. Conversely, if the amount exceeds 100 parts by mass, the low-temperature flexibility of the rubber composition will decrease, resulting in poor performance on ice.
If the amount of (III) polyolefin-based elastomer is less than 0.5 parts by mass per 100 parts by mass of (I) diene-based rubber, the amount added is too small to achieve the effects of the present invention, whereas if the amount exceeds 20 parts by mass, the breaking properties deteriorate.

The amount of carbon black to be mixed is preferably 5 to 80 parts by mass based on 100 parts by mass of the diene rubber.
The amount of the white filler to be mixed is preferably 15 to 80 parts by mass based on 100 parts by mass of the diene rubber.
The blending amount of the polyolefin elastomer (III) is preferably 1 to 20 parts by mass based on 100 parts by mass of the diene rubber.

(Other ingredients)
In addition to the above-mentioned components, the rubber composition of the present invention may contain various additives that are generally incorporated in rubber compositions, such as vulcanizing or crosslinking agents, vulcanizing or crosslinking accelerators, zinc oxide, antioxidants, plasticizers, silane coupling agents, and thermally expandable microcapsules, and these additives can be kneaded in a general manner to form a composition, which can be used for vulcanization or crosslinking. The amounts of these additives may be conventional amounts, provided that they do not violate the object of the present invention.

The tire of the present invention can be prepared using the rubber composition of the present invention, and is preferably a pneumatic tire, which can be filled with air, an inert gas such as nitrogen, or other gases. The tire of the present invention is preferably applied to a tread, particularly a cap tread, to make a studless tire.

The present invention will be further explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

Standard Example, Examples 1 to 5, Comparative Examples 1 to 3
In the formulation (parts by mass) shown in Table 1, the components other than the vulcanization system (vulcanization accelerator, sulfur) and the curing agent were mixed in a 1.7-liter closed Banbury mixer for 5 minutes, then discharged from the mixer and cooled at room temperature. The composition was then placed in the same Banbury mixer again, and the vulcanization system was added and mixed to obtain a rubber composition. The rubber composition obtained was press-vulcanized at 170°C for 10 minutes, and the physical properties were measured using the test methods shown below.

Breaking strength (TB): Tested at room temperature according to JIS K 6251. The results are expressed as an index, with the standard value being 100. The larger the index, the better the breaking strength. An index of 80 or more can be considered sufficient for practical use.
Storage modulus (E'): According to JIS K6394, the storage modulus (E') at -10°C was determined using a viscoelasticity spectrometer manufactured by Toyo Seiki Seisakusho under the conditions of an initial strain of 10%, an amplitude of ±0.5%, and a frequency of 20 Hz. The results are expressed as an index, with the value of the control example being 100. The smaller the index, the more excellent the low-temperature flexibility.
Heat generation: Using a viscoelasticity spectrometer manufactured by Toyo Seiki Seisakusho Co., Ltd., tan δ (60°C) was measured under the conditions of initial strain = 10%, amplitude = ±2%, and frequency = 20 Hz, and the heat generation was evaluated based on this value. The results were expressed as an index, with the value of the standard example being 100. The smaller the index, the lower the heat generation.
Performance on ice: The obtained vulcanized rubber test pieces were attached to a flat cylindrical rubber base and the coefficient of friction on ice was measured using an inside drum type ice friction tester under the following conditions: temperature -3.0°C, load 5.5 kg/ ^cm2 , drum rotation speed 25 km/h. The obtained coefficient of friction on ice was expressed as an index, with the standard example value set at 100. A higher index indicates greater frictional force on ice and better performance on ice.
The results are shown in Table 1.

*1:NR (RSS#3)
*2: BR (Nipol BR1220 manufactured by Nippon Zeon Co., Ltd.)
*3: Carbon black (Seest KHA manufactured by Tokai Carbon Co., Ltd.)
*4: Silica (Zeosil 1165MP manufactured by Rhodia, CTAB specific surface area = 159 ^m2 /g)
*5: Silane coupling agent (Si69, bis(3-triethoxysilylpropyl)tetrasulfide, manufactured by Evonik Degussa)
*6: Oil (Extract No. 4 S manufactured by Showa Shell Sekiyu K.K.)
*7: TPE (styrene-based thermoplastic elastomer, AR-SC-30, manufactured by Aronkasei Co., Ltd.)
*8: Polyolefin elastomer 1 (ENEOS Corporation Gelatic Hs0, JIS A hardness = 0, compression set = 18%)
*9: Polyolefin elastomer 2 (ENEOS Corporation Gelatic Hs5, JIS A hardness = 5, compression set = 27%)
*10: Polyolefin elastomer 3 (ENEOS Corporation Gelatic Hs20, JIS A hardness = 20, compression set = 25%)
*11: Sulfur (Kinka oil-filled fine sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.)
*12: Vulcanization accelerator (Noccela CZ-G manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)

As can be seen from the results in Table 1, the rubber composition of each Example is composed of 100 parts by mass of (I) diene rubber, 30 to 100 parts by mass of (II) carbon black and/or white filler, and 0.5 to 20 parts by mass of (III) polyolefin elastomer having a JIS A hardness of 0 to 50 and a compression set of 10 to 30%, and therefore has lower heat buildup and better performance on ice compared to the standard example.
In contrast, Comparative Example 1 is an example in which (III) a styrene-based elastomer was used instead of a polyolefin-based elastomer, and therefore showed little improvement in performance on ice and heat build-up compared to the standard example.
In Comparative Example 2, the blending amount of (III) polyolefin-based elastomer was less than the lower limit specified in the present invention, and therefore a styrene-based elastomer was used instead, and no improvement was observed in any of the properties compared to the standard example.
In Comparative Example 3, the blend amount of (III) polyolefin-based elastomer exceeded the upper limit specified in the present invention, and therefore the rubber strength was reduced compared to the standard example.

Claims (3)

(I) per 100 parts by mass of diene rubber,
(II) 30 to 100 parts by mass of carbon black and/or a white filler; and (III) 0.5 to 20 parts by mass of a polyolefin-based elastomer having a JIS A hardness of 0 to 50 and a compression set of 10 to 30% as measured by the following measurement method .
Measurement method for the compression set: The polyolefin elastomer is heat-pressed at 200°C for 10 minutes to prepare a sheet having a thickness of about 2 mm. The obtained sheet is punched out into a disk shape having a diameter of 29 mm, and seven sheets are stacked together to prepare a sample having a height (thickness) of 12.5±0.5 mm. The obtained sample is compressed by 25% using a dedicated jig, and the compression set (unit: %) after leaving it at 70°C for 22 hours is measured in accordance with JIS K6262 (issued in 2013). The rubber composition according to claim 1, characterized in that the butadiene rubber accounts for 30 parts by mass or more of the diene rubber (I) per 100 parts by mass. A studless tire using the rubber composition according to claim 1 or 2.