JP2019164047A - Rubber performance evaluation method - Google Patents

Abstract

To provide a method for accurately and simply evaluating chipping-proof of a tire.SOLUTION: The evaluation method comprises: a preparation step for preparing a test piece in a sheet shape, and a tension test machine comprising a pair of chucks; and a measurement step for performing a tension test of the test piece using the tension test machine, to measure breaking extension EB(%). With the breaking extension EB(%) as an index, chipping proof of a tire obtained by using a rubber composition having the same composition as that of the test piece, is evaluated. The test piece comprises: a test part positioned on an almost center in a longitudinal direction; a pair of inclined parts adjacent to the test part; and a pair of grip parts adjacent to the respective inclined parts and form end parts of the test piece. Ratios W0/W1 and W0/W2 between the widths of the test piece and respective grip parts are 0.1 or greater and 0.4 or smaller. Ratios D1/Dt and D2/Dt of distances from a breaking point of the test piece to respective chucks to a distance between chucks, are 0.1 or greater and 0.9 or smaller. The tension speed is 1 m/s or greater and 20 m/s or smaller.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for evaluating rubber performance. Specifically, the present invention relates to a method for evaluating chipping resistance.

  One application of vulcanized rubber is tires. During running, the tread forming the tire surface comes into contact with the road surface. Chipping (defects) occurs on the tread surface due to contact with the road surface. A tire with chipping is inferior in running performance and appearance. Improvement of chipping resistance can contribute to improvement of tire quality. The evaluation method of chipping resistance is an important technique for tire development.

  One of the factors contributing to the tire chipping resistance is the breaking strength of vulcanized rubber. Conventionally, there has been known a method for evaluating the breaking strength of a vulcanized rubber based on the breaking strength and breaking elongation obtained by a test method prescribed in JIS K6251 “vulcanized rubber and thermoplastic rubber—how to obtain tensile properties”. Yes. It is evaluated that the higher the breaking strength (breaking strength and breaking elongation), the better the chipping resistance. However, some of the market evaluation results regarding chipping resistance include those that are reverse to the breaking strength (breaking strength and breaking elongation) that are indicators, and the evaluation accuracy was not fully satisfactory. . There is a need for a method for simply and easily evaluating the chipping resistance of a tire with higher accuracy.

  Japanese Patent Laid-Open No. 2017-129495 proposes a method for evaluating chipping resistance using a test piece having an initial crack. In this evaluation method, the force at the moment when the test piece starts to tear is detected starting from the initial crack, and the tear initiation strength defined by this force is measured. Japanese Patent No. 6216286 discloses a method for evaluating the resistance to chipping of a rubber by causing a striking piece to collide with a rubber test piece in which a hard receiving plate is embedded.

JP 2017-129495 A Patent No. 6216286

  When a vehicle equipped with a tire travels on rough terrain such as a rocky place, a large defect (crack) may occur instantaneously on the tire surface. The evaluation method disclosed in Japanese Patent Application Laid-Open No. 2017-129495 evaluates a fracture phenomenon that gradually progresses from an initial crack. With this evaluation method, it is not possible to obtain an evaluation result reflecting a chipping phenomenon with a large defect that occurs instantaneously. In the evaluation method described in Japanese Patent No. 6216286, it takes time and expense to prepare a special test piece in which a receiving plate is embedded, and the evaluation accuracy is not satisfactory.

  An object of the present invention is to provide a rubber performance evaluation method capable of easily and accurately evaluating the chipping resistance of a tire.

  As a result of diligent study, the present inventors have found that when the tire surface collides with the rock surface or the like at a high speed on rough terrain, the rubber at the collision part expands momentarily and breaks, resulting in a large defect. It has been found that chipping occurs and that the instant elongation and failure of the rubber is reproduced by a high speed tensile test. Furthermore, the present inventors have completed the present invention by finding that the shape of the rubber test piece subjected to the high-speed tensile test greatly affects the correlation with the market evaluation regarding chipping resistance. It is.

  The rubber performance evaluation method according to the present invention includes a preparation step and a measurement step. In the preparation step, a sheet-like test piece formed by vulcanizing the rubber composition and a tensile tester including a pair of chucks for holding the test piece are prepared. In the measurement process, a tensile test of the test piece is performed using a tensile tester to measure the elongation at break EB (%). The test piece includes a rectangular test part located in the center in the longitudinal direction, a pair of inclined parts gradually widening adjacent to the test part, and adjacent to each of the inclined parts. And a pair of gripping portions that are substantially rectangular in plan view and that form ends. When the width of the test part is W0 and the widths of the pair of gripping parts are W1 and W2, respectively, the ratio W0 / W1 is 0.1 or more and 0.4 or less, and the ratio W0 / W2 is 0.1 or more and 0. .4 or less. In the measurement process, when the distance between the chucks when the test piece is broken is Dt, and the distances from the breaking point of the test piece to each chuck are D1 and D2, respectively, the ratio D1 / Dt is 0.1 or more and 0. 0.9 or less, and the ratio D2 / Dt is 0.1 or more and 0.9 or less. The tensile speed in the measurement process is 1 m / s or more and 20 m / s or less. In this evaluation method, the chipping resistance of a tire obtained using a rubber composition having the same composition as that of the test piece is evaluated using the breaking elongation EB (%) as an index.

  Preferably, the ratio t / W0 between the thickness t of the test piece and the width W0 of the test portion is 0.2 or more and 1.0 or less.

  Preferably, in the measurement step, the time from the start of elongation of the test portion to the break is 0.5 milliseconds or more and 500 milliseconds or less.

  Preferably, when the length of the test portion is L0 and the length of the pair of inclined portions is L1 and L2, respectively, the ratio L0 / L1 is 0.5 or more and 3.0 or less. The ratio L0 / L2 is 0.5 or more and 3.0 or less.

  The evaluation result obtained by the evaluation method according to the present invention correlates with high market accuracy with respect to the tire chipping resistance.

FIG. 1 is a plan view showing a test piece for an evaluation method according to an embodiment of the present invention. FIG. 2 is a conceptual diagram showing a state where a chuck is mounted on the test piece of FIG. FIG. 3 is a conceptual diagram showing the state of the chuck and the test piece when the test piece is broken.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

  The evaluation method according to the present invention includes a preparation process and a measurement process. In the preparation step, a test piece and a tensile tester are prepared. In the measurement step, the breaking elongation EB (%) of the test piece is measured by a tensile test using this tensile tester.

  The test piece is formed by vulcanizing the rubber composition. FIG. 1 is a plan view showing a test piece 10 prepared by an evaluation method according to an embodiment of the present invention. The test piece 10 has a sheet shape. As shown in the drawing, the test piece 10 includes a test portion 12 having a rectangular shape in a plan view located at a substantially central portion in the longitudinal direction, a pair of inclined portions 14 that are gradually widened adjacent to the test portion 12, and respective inclined portions. 14 and a pair of gripping portions 16 having a substantially rectangular shape in plan view and forming an end portion of the test piece 10. The test piece 10 has a so-called dumbbell shape.

  A double-headed arrow W <b> 0 shown in FIG. 1 is the width of the test unit 12. The width W0 of the test unit 12 is constant in the longitudinal direction. The widths of the pair of gripping portions 16 are shown in FIG. 1 as double arrows W1 and W2, respectively. In this embodiment, the widths W1 and W2 of each gripping portion 16 are both constant in the longitudinal direction. In this evaluation method, a test piece 10 having a ratio W0 / W1 of 0.1 to 0.4 and a ratio W0 / W2 of 0.1 to 0.4 is prepared.

  The tensile tester includes a pair of chucks 18 for holding the test piece 10. In the measurement process, first, the test piece 10 is installed in a tensile tester. FIG. 2 is a conceptual diagram showing a state of the test piece 10 held by the pair of chucks 18. The vertical direction in FIG. 2 is the vertical direction (length direction), and the horizontal direction is the horizontal direction (width direction).

  As shown in the figure, in this embodiment, the chuck 18 is composed of a first chuck 18a and a second chuck 18b that are opposed to each other with a space therebetween. The first chuck 18a and the second chuck 18b are mounted on the pair of gripping portions 16 of the test piece 10, respectively. Hereinafter, for convenience, the grip 16 held by the first chuck 18a may be referred to as a first grip 16a, and the grip 16 held by the second chuck 18b may be referred to as a second grip 16b. Usually, the width of the first chuck 18a is larger than the width W1 of the first gripping portion 16a, and the width of the second chuck 18b is larger than the width W2 of the second gripping portion 16b. Accordingly, in this tensile testing machine, the pair of gripping portions 16 can be held by the pair of chucks 18 in the width direction.

  Although not shown, the tensile tester includes a drive unit for moving the chuck 18, a detector for detecting the displacement of the test piece 10 during the tensile test, and an imaging device for observing the state of the test piece 10. Is further provided. In this embodiment, a drive unit is connected to the second chuck 18b. The arrow F shown in FIG. 2 is the moving direction of the second chuck 18b. The first chuck 18a is fixed. In the measurement process, when the second chuck 18b connected to the drive unit moves in the direction of the arrow F, the test piece 10 is extended in the vertical direction. In this embodiment, the moving speed of the second chuck 18b is the tensile speed in the tensile test.

  In other embodiments, other driving units may be coupled to the first chuck 18a. In this case, the second chuck 18b moves in the direction of arrow F, and the first chuck 18a moves in the direction opposite to the arrow F, whereby the test piece 10 is extended. The pulling speed in this embodiment is the sum of the moving speed of the first chuck 18a and the moving speed of the second chuck 18b.

  The tensile speed in the evaluation method according to the present invention is 1 m / s or more and 20 m / s or less. This tensile speed is considerably faster than, for example, the speed of 100 to 500 mm / min specified in JIS K6251. Due to the high tensile speed, tensile strain is applied to the test piece 10 at a high speed. The stress is concentrated on a specific portion of the test piece 10 due to the tensile strain applied at a high speed, thereby breaking the test piece 10.

  FIG. 3 shows the state of the test piece 10 at the time of breaking by the tensile test. The vertical direction in FIG. 3 is the vertical direction, and the horizontal direction is the horizontal direction. A double-headed arrow Dt shown in FIG. 3 is a distance between chucks when the test piece 10 is broken.

  A symbol X shown in FIG. 3 is a breaking point of the test piece 10. In FIG. 3, the distance from the breaking point to the upper surface of the first chuck 18a is shown as a double arrow D1. The distance from the breaking point to the lower surface of the second chuck 18b is shown as a double arrow D2. The distances Dt, D1, and D2 are all measured in the vertical direction. In this evaluation method, the ratio D1 / Dt is 0.1 or more and 0.9 or less, and the ratio D2 / Dt is 0.1 or more and 0.9 or less.

  In this evaluation method, the displacement of the test piece 10 at the time of fracture is detected by a detector, and the elongation at break EB is measured. The breaking elongation EB means the elongation percentage (%) of the test piece 10 at the time of breaking. Usually, it is calculated | required as a percentage with respect to the distance between gauge points before a test of the distance between gauge points at the time of a fracture | rupture. In this evaluation method, the chipping resistance of a tire obtained using a rubber composition having the same composition as the test piece 10 is evaluated using the breaking elongation EB (%) of the test piece 10 as an index.

  In the evaluation method according to the present invention, when a test piece 10 having a ratio W0 / W1 and a ratio W0 / W2 of 0.1 to 0.4 is subjected to a tensile test at a tensile speed of 1 m / s to 20 m / s. The test piece 10 breaks at a position where the ratio D1 / Dt is 0.1 or more and 0.9 or less and the ratio D2 / Dt is 0.1 or more and 0.9 or less. In other words, the test piece 10 does not break in the vicinity of the mounted chuck 18. The fracture in the vicinity of the chuck 18 is generally caused by stress concentration by the chuck 18 regardless of the mechanical characteristics of the vulcanized rubber forming the test piece 10. The mechanical properties of the vulcanized rubber forming the test piece 10 are not sufficiently reflected in the breaking elongation EB (%) obtained at the time of breaking in the vicinity of the chuck 18.

  According to the evaluation method according to the present invention, in the tensile test, the test piece 10 is broken at a position sufficiently away from the chuck 18. The position of this breaking point is appropriate. The mechanical properties of the vulcanized rubber forming the test piece 10 are reflected in the breaking elongation EB (%) obtained for the test piece 10 broken at an appropriate position. This breaking elongation EB (%) correlates with high accuracy with a market evaluation regarding the chipping property with a momentary large breakage of a tire obtained by using a rubber composition having the same composition as the test piece 10.

  Preferably, the rubber composition of the test piece 10 includes a base rubber and various additives usually used in the rubber field. Preferred base rubbers include natural rubber (NR), styrene butadiene rubber (SBR), butadiene rubber (BR), epoxidized natural rubber (ENR), isoprene rubber (IR), ethylene propylene diene rubber (EPDM), chloroprene rubber ( CR), acrylonitrile butadiene rubber (NBR), acrylonitrile butadiene styrene rubber (ABS) and the like. Two or more kinds of base rubbers may be used in combination.

  Examples of additives blended with the base rubber include fillers such as carbon black and silica, silane coupling agents, oils, waxes, zinc oxide, anti-aging agents, processing aids, resins, vulcanizing agents, vulcanizing Accelerators and vulcanization accelerators are mentioned. As long as the effect of the present invention is not inhibited, it is possible to use other additives not specified in the present specification.

  In the evaluation method according to the present invention, the blending of the rubber composition forming the test piece 10 is not particularly limited. Preferably, a rubber member constituting the tire surface is used. Typically, the compounding of the rubber composition for treads is illustrated.

  In the evaluation method according to the present invention, the method for preparing the test piece 10 is not particularly limited. For example, a rubber composition is prepared by putting a base rubber and various additives exemplified above into an open roll, a Banbury mixer or the like according to a predetermined composition and kneading, and this rubber composition is press vulcanized. After forming the rubber sheet, the test piece 10 may be prepared by appropriately processing the rubber sheet.

  As another embodiment, after the prepared rubber composition is extruded in accordance with the shape of the tread, a tire is manufactured by heating and pressing in a vulcanizer together with other tire members. You may prepare the test piece 10 by processing the extract | collected vulcanized rubber piece.

  As long as the effects of the present invention can be obtained, the method of processing into a predetermined shape is not particularly limited. Typically, a punching blade or a razor blade is used. From the viewpoint of reproducibility of the tensile test and easy handling, a punching blade that conforms to the provisions of JIS K6250 is preferably used.

  A double arrow Lt shown in FIG. 1 is the length of the test piece 10. As long as the object of the present invention is achieved, the length Lt of the test piece 10 is not particularly limited. From the viewpoint of easy processing and reproducibility, the length Lt of the test piece 10 is preferably 20 mm or more, more preferably 30.0 mm or more, and particularly preferably 50.0 mm or more. From the viewpoint of avoiding enlarging the tensile tester, it is preferably 200 mm or less, more preferably 150 mm or less, and particularly preferably 120 mm or less.

  In the evaluation method according to the present invention, the ratio W0 / W1 of the test piece 10 is 0.1 or more and 0.4 or less, and the ratio W0 / W2 is 0.1 or more and 0.4 or less. As long as the effect of the present invention is not inhibited, the ratio W0 / W1 and the ratio W0 / W2 may be the same or different. From the viewpoint of measurement accuracy and easy molding, the ratio W0 / W1 and the ratio W0 / W2 are preferably the same.

  From the viewpoint of measurement accuracy, the ratio W0 / W1 is more preferably 0.2 or more. From the viewpoint that the position of the breaking point is appropriate, the preferred ratio W0 / W1 is 0.3 or less.

  From the viewpoint of measurement accuracy, the ratio W0 / W2 is preferably 0.2 or more. From the viewpoint that the position of the breaking point is appropriate, the preferred ratio W0 / W2 is 0.3 or less.

  The width W0 of the test unit 12 is appropriately adjusted so that the ratio W0 / W1 and the ratio W0 / W2 satisfy the numerical range described above. In light of obtaining an appropriate breaking point, the width W0 is preferably equal to or greater than 1.0 mm, more preferably equal to or greater than 2.0 mm, and particularly preferably equal to or greater than 4.0 mm. From the viewpoint of avoiding the enlargement of the test apparatus, the preferred width W0 is 10 mm or less. From the viewpoint of measurement accuracy, the width W0 is preferably constant over the entire length in the longitudinal direction of the test unit 12.

  The width W1 of the first gripping portion 16a and the width W2 of the second gripping portion 16b are not particularly limited, and are appropriately adjusted so that the ratio W0 / W1 and the ratio W0 / W2 satisfy the above-described numerical ranges, respectively. As long as the effect of the present invention is not hindered, the first gripping portion 16a and the second gripping portion 16b may each have a width that varies in the longitudinal direction. In this case, the width of the first gripping portion 16a at the position where the first chuck 18a is mounted is the width W1, and the width of the second gripping portion 16b when the second chuck 18b is mounted is the width W2. The

  From the viewpoint of avoiding stress concentration on the gripping portion 16 during the tensile test, the width W1 of the first gripping portion 16a is preferably 5.0 mm or more, more preferably 8.0 mm or more, and particularly preferably 10.0 mm or more. From the viewpoint of avoiding the enlargement of the test apparatus, the preferred width W1 is 40.0 mm or less.

  From the viewpoint of avoiding stress concentration on the gripping portion 16 during the tensile test, the width W2 of the second gripping portion 16b is preferably 5.0 mm or more, more preferably 8.0 mm or more, and particularly preferably 10.0 mm or more. From the viewpoint of avoiding the enlargement of the test apparatus, the preferable width W2 is 40.0 mm or less.

  The width W1 of the first gripping portion 16a and the width W2 of the second gripping portion 16b may be the same or different. From the viewpoint of measurement accuracy and ease of molding, the width W1 and the width W2 are preferably the same.

  From the viewpoint of measurement accuracy, the ratio t / W0 between the thickness t of the test piece 10 and the width W0 of the test portion 12 is preferably 0.2 or more, more preferably 0.3 or more, and particularly preferably 0.4 or more. From the viewpoint that the position of the breaking point is appropriate, the preferred ratio t / W0 is 1.0 or less.

  In this evaluation method, the thickness t of the test piece 10 is not particularly limited. Preferably, the thickness t is appropriately set so that the ratio t / W0 satisfies the numerical range described above. From the viewpoint of measurement accuracy and easy molding, the thickness t of the test piece 10 is preferably 0.5 mm or more, and more preferably 1.0 mm or more. From the viewpoint that the position of the breaking point is appropriate, the preferable thickness t is 3.0 mm or less. From the viewpoint of measurement accuracy, the test piece 10 having a uniform thickness t is preferable.

  A double arrow L <b> 0 shown in FIG. 1 is the length of the test unit 12. The lengths of the pair of inclined portions 14 are shown in FIG. 1 as double arrows L1 and L2, respectively. The lengths L0, L1 and L2 are each measured in the vertical direction.

  From the viewpoint that the position of the break point is appropriate, the ratio L0 / L1 between the length L0 and the length L1 is preferably 0.5 or more, and more preferably 0.6 or more. From the viewpoint of avoiding enlargement of the test apparatus, the preferable ratio L0 / L1 is 3.0 or less.

  From the viewpoint that the position of the break point is appropriate, the ratio L0 / L2 between the length L0 and the length L2 is preferably 0.5 or more, and more preferably 0.6 or more. From the viewpoint of avoiding enlargement of the test apparatus, the preferable ratio L0 / L2 is 3.0 or less.

  As long as the effect of the present invention is not inhibited, the ratio L0 / L1 and the ratio L0 / L2 may be the same or different. From the viewpoint of measurement accuracy and easy molding, the ratio L0 / L1 and the ratio L0 / L2 are preferably the same.

  In this evaluation method, the length L0 of the test unit 12 is not particularly limited. Preferably, the length L0 is appropriately adjusted so that the ratio L0 / L1 and the ratio L0 / L2 satisfy the numerical range described above. In light of obtaining an appropriate breaking point, the length L0 is preferably equal to or greater than 6.0 mm, and more preferably equal to or greater than 10.0 mm. From the viewpoint of avoiding enlarging the test apparatus, the preferable length L0 is 60.0 mm or less. When the test piece 10 is subjected to a tensile test, the length L0 of the test portion 12 may be a so-called inter-score distance.

  In this evaluation method, the lengths L1 and L2 are not particularly limited. Preferably, the lengths L1 and L2 are appropriately adjusted so that the ratio L0 / L1 and the ratio L0 / L2 satisfy the numerical ranges described above, respectively. As long as the effect of the present invention is not hindered, the length L1 and the length L2 may be the same or different. From the viewpoint of measurement accuracy and ease of molding, the length L1 and the length L2 are preferably the same.

  From the viewpoint that the position of the breaking point is appropriate, the length L1 is preferably 3.0 mm or more, more preferably 5.0 mm or more, and particularly preferably 10.0 mm or more. From the viewpoint of avoiding enlarging the test apparatus, the length L1 is preferably 70.0 mm or less, more preferably 50.0 mm or less, and particularly preferably 30.0 mm or less.

  From the viewpoint that the position of the breaking point is appropriate, the length L2 is preferably 3.0 mm or more, more preferably 5.0 mm or more, and particularly preferably 10.0 mm or more. From the viewpoint of avoiding enlargement of the test apparatus, the length L2 is preferably 70.0 mm or less, more preferably 50.0 mm or less, and particularly preferably 30.0 mm or less.

  In this evaluation method, in the measurement process, the first chuck 18a is mounted on the first gripping portion 16a, and the second chuck 18b is mounted on the second gripping portion 16b. The mounting positions of the first chuck 18a and the second chuck 18b may be the same or different. From the viewpoint of measurement accuracy and reproducibility, the mounting positions of the first chuck 18a and the second chuck 18b are preferably the same.

  From the viewpoint of measurement accuracy, the first chuck 18a is preferably mounted at a position away from the end of the first gripping portion 16a by 15.0 mm or more, more preferably 16.0 mm or more. From the viewpoint of avoiding stress concentration during the tensile test, the first chuck 18a is preferably mounted at a position where the distance from the end of the first gripping portion 16a is 25.0 mm or less.

  From the viewpoint of measurement accuracy, the second chuck 18b is mounted at a position that is preferably 15.0 mm or more, more preferably 16.0 mm or more away from the end of the second gripping portion 16b. From the viewpoint of avoiding stress concentration during the tensile test, the second chuck 18b is preferably mounted at a position where the distance from the end of the second gripping portion 16b is 25.0 mm or less.

  As described above, in the evaluation method according to the present invention, in the measurement process, a tensile test is performed at a tensile speed of 1 m / s or more and 20 m / s or less, and the elongation EB (%) at the time of breaking the test piece 10 is measured. From the viewpoint that it is easy to reproduce the chipping phenomenon accompanied by an instantaneous large fracture, the tensile speed is preferably 2 m / s or more, and more preferably 5 m / s or more. From the viewpoint that it is easy to determine whether the chipping resistance is superior or inferior, the tensile speed is preferably 15 m / s or less, and more preferably 12 m / s or less. As long as the effect of the present invention is not inhibited, the tensile speed during the tensile test may be constant or may vary. When it fluctuates, it is preferable to increase or decrease the speed gradually or stepwise within the aforementioned speed range.

  The type and configuration of the tensile tester are not particularly limited as long as the tensile test at the above-described tensile speed is possible and the object of the present invention is achieved. From the viewpoint of measurement accuracy and reproducibility, a tensile tester equipped with a thermostatic bath is preferable. The test piece 10 during the tensile test can be set to a desired temperature or temperature range by the thermostatic bath.

  Considering the surface temperature of the tire during running, the preferred test temperature is -10 ° C or higher and 150 ° C or lower. From the viewpoint of tire temperature during running on rough terrain, the test temperature is more preferably 0 ° C. or higher, further preferably 20 ° C. or higher, and particularly preferably 50 ° C. or higher. From the viewpoint that it is easy to determine superiority or inferiority regarding chipping resistance, the test temperature is more preferably 120 ° C. or less, and particularly preferably 100 ° C. or less.

  In the evaluation method according to the present invention, the test piece 10 subjected to the tensile test has a ratio D1 / Dt of 0.1 to 0.9 and a ratio D2 / Dt of 0.1 to 0.9. Break at a certain position. The sum of the ratio D1 / Dt and the ratio D2 / Dt is approximately 1 although it may vary depending on the composition of the vulcanized rubber forming the test piece 10 and the measurement conditions of the tensile test.

  The ratio D1 / Dt is preferably 0.2 or more and 0.8 or less, and preferably 0.3 or more and 0.00 from the viewpoint that the position of the breaking point is appropriate and the correlation with the market evaluation regarding chipping resistance is high. 7 or less is more preferable, and 0.4 or more and 0.6 or less is particularly preferable. The ideal ratio D1 / Dt is 0.5.

  The ratio D2 / Dt is preferably 0.2 or more and 0.8 or less, and preferably 0.3 or more and 0.00 from the viewpoint that the position of the break point is appropriate and the correlation with the market evaluation regarding chipping resistance is high. 7 or less is more preferable, and 0.4 or more and 0.6 or less is particularly preferable. The ideal ratio D2 / Dt is 0.5.

  From the viewpoint that it is easy to reproduce the chipping phenomenon with a large break that occurs instantaneously, in the measurement process, the time from the start of elongation of the test section 12 to the break is preferably 500 milliseconds or less, and more preferably 450 milliseconds or less. 400 milliseconds or less is particularly preferable. From the viewpoint that it is easy to determine superiority or inferiority with respect to chipping resistance, a preferable breaking time is 0.5 milliseconds or more.

  In this specification, the normal rim means a rim defined in a standard on which a tire depends. “Standard rim” in the JATMA standard, “Design Rim” in the TRA standard, and “Measuring Rim” in the ETRTO standard are regular rims. In this specification, the normal internal pressure means an internal pressure defined in a standard on which the tire depends. “Maximum air pressure” in JATMA standard, “maximum value” published in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in TRA standard, and “INFLATION PRESSURE” in ETRTO standard are normal internal pressures. In the present specification, the normal load means a load defined in a standard on which the tire 2 depends. “Maximum value” published in “Maximum load capacity” in JATMA standard, “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in TRA standard, and “LOAD CAPACITY” in ETRTO standard are normal loads.

  Hereinafter, the effects of the present invention will be clarified by examples. However, the present invention should not be construed in a limited manner based on the description of the examples.

(Manufacture of vulcanized rubber)
[Production Example A]
According to the formulation shown as A in Table 1, styrene butadiene rubber (manufactured by JSR, SBR1502), carbon black (manufactured by Cabot Japan, show black N220), silica (manufactured by Solvay Japan, ZEOSIL 115GR), coupling agent ( Evonik, trade name “Si266”), oil (made by Idemitsu Kosan Co., Ltd., Diana Process NH-60), zinc oxide (made by Toho Zinc Co., Ltd., Ginseng R), anti-aging agent (made by Ouchi Shinsei Chemical Industry Co., Ltd. ) And wax (manufactured by Nippon Seiwa Co., Ltd., Ozoace 0355), and charged into a 1.7-liter Banbury mixer (manufactured by Kobe Steel) with a filling rate of 58%. The mixture was kneaded while heating at a rotational speed of 80 rpm until the temperature of the charged material reached 140 ° C. To the kneaded product taken out, 1.4 parts by mass of sulfur (manufactured by Tsurumi Chemical Co., Ltd., 5% oil-containing powder sulfur) and 1.6 parts by mass of vulcanization accelerator NS (manufactured by Ouchi Shinsei Chemical Industry Co., Ltd. And an unrolled rubber was obtained by mixing at 80 ° C. for 5 minutes using an open roll. The obtained unvulcanized rubber was press vulcanized at 160 ° C. for 20 minutes to obtain a vulcanized rubber of Production Example A. The vulcanized rubber of Production Example A was a sheet having a thickness of 2.0 mm.

[Production Example B-H]
A vulcanized rubber of Production Example BH was obtained in the same manner as Production Example A except that the formulation shown as BH in Table 1 was used. All of the vulcanized rubbers of Production Examples B-H were in the form of a sheet having a thickness of 2.0 mm.

Details of the compounds described in Table 1 are as follows.
SBR: Styrene butadiene rubber from JSR (bonded styrene content 23.5% by mass), trade name “SBR1502”
Carbon black: trade name “Show Black N220” manufactured by Cabot Japan (N ₂ SA = 119 m ² / g)
Silica: Product name “ZEOSIL 115GR” of Solvay Japan
Coupling agent: Evonik silane coupling agent, trade name “Si266”
Oil: Process oil manufactured by Idemitsu Kosan Co., Ltd., trade name “Diana Process NH-60”
Zinc oxide: Toho Zinc Co., Ltd. trade name “Ginbao R”
Anti-aging agent: Phenylenediamine-based anti-aging agent from Ouchi Shinsei Chemical Industry Co., Ltd., trade name “NOCRACK 6C” (N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine)
Wax: Nippon Seiwa Co., Ltd. trade name “Ozoace 0355”
Sulfur: Soluble powder sulfur from Tsurumi Chemical Co., Ltd. (containing 5% oil)
Vulcanization accelerator NS: “Noxeller NS” (N-tert-butyl-2-benzothiazylsulfenamide), trade name of Ouchi Shinsei Chemical Co., Ltd.
Vulcanization accelerator DPG: Trade name “Noxeller D” (diphenylguanidine) manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.

[Experiment 1]
[Example 1]
(Creation of specimen)
From the vulcanized rubber of Production Example AH, five test pieces AH each having the shape shown in FIG. 1 were prepared. A punching blade was used to prepare the test pieces AH. All the test pieces had the same shape, the ratio W0 / W1 and the ratio W0 / W2 were 0.2, the ratio L0 / L1 and the ratio L0 / L2 were 0.8, and the ratio t / W0 was 0.4. The thickness t of each test piece was 2.0 mm, the length Lt of the test piece was 100 mm, the length L0 of the test part was 20 mm, and the width W0 of the test part was 5 mm.

(High speed tensile test)
In Example 1, the elongation at break of the test piece AH was measured using a high-speed tensile testing machine (“HITS-T10” manufactured by Shimadzu Corporation). At the time of measurement, a chuck was mounted at a position 15 mm away from the end of the grip portion of each test piece, and a high-speed tensile test was conducted at a tensile speed of 8.3 m / s, a temperature of 75 ° C., and a distance between gauge points of 20 mm. The median value when measuring 5 sheets each is shown in Table 2 as HS-EB (%). All the test pieces were broken at positions where the ratio D1 / Dt and the ratio D2 / Dt were in the range of 0.1 to 0.9. The time from the start of elongation of the test portion of each test piece to the break was 10 milliseconds.

[Comparative Example 1]
Five comparative test pieces AH were prepared from the vulcanized rubber of Production Example AH. The shape and size of the comparative specimen AH are the same as the specimen AH described above as Example 1. In Comparative Example 1, the elongation at break of the comparative specimen AH was measured using a conventional tensile tester (“AG / IS” manufactured by Shimadzu Corporation). The measurement conditions were based on JIS K6251 and the tensile speed was 0.5 m / min, the temperature was 23 ° C., and the distance between the gauge points was 20 mm. The median value when each of the five sheets is measured is shown as EB (%) in Table 2. All the test pieces were broken at positions where the ratio D1 / Dt and the ratio D2 / Dt were in the range of 0.1 to 0.9.

[Reference example]
In the reference example, an actual vehicle running test was performed to evaluate chipping resistance. First, an unvulcanized rubber obtained according to the formulation shown as AH in Table 1 is extruded as a tread rubber, put into a tire molding machine together with other tire members, and molded by a commonly used method. Thus, an unvulcanized tire was formed. The obtained unvulcanized tire was heated and pressurized in a vulcanizer to produce a size 285 / 60R18 pneumatic tire A-H. The specifications of the tires A-H are the same except that the tread rubber composition is different.

  Each of the obtained tires A-H was incorporated into a regular rim, filled with air to a regular internal pressure, and then mounted on the vehicle to run on uneven terrain for 4 hours at a speed of 30 km / hour. After running, the circumferential length of all cracks generated on the tire surface was measured, and the maximum circumferential length was determined for each tire. The index when the value obtained for the tire C is set to 100 is shown in Table 2 as Index (C). A larger index (C) value means smaller cracks and better chipping resistance.

  As Table 2 shows, the larger the elongation at break HS-EB (%) obtained in Example 1, the higher the Index (C) obtained in the Reference Example, and the higher the correlation coefficient. On the other hand, in Comparative Example 1, the magnitude relationship between the breaking elongation EB (%) and Index (C) was reversed, and the correlation coefficient showed a negative value.

  From the evaluation results shown in Table 2, the running test results related to chipping with a somewhat large defect are not correctly reflected in the breaking elongation of Comparative Example 1 obtained in the conventional tensile test, and the evaluation method of Example 1 It can be seen that the obtained elongation at break is correctly reflected. Therefore, it can be seen that by using the elongation at break obtained by the evaluation method of Example 1 as an index, the chipping resistance of the tire can be predicted with high accuracy, including the case with a large defect.

[Experiment 2]
According to the specification shown as S1-S7 in Table 3, five test pieces A (S1) -A (S7) were prepared from the vulcanized rubber of Production Example A. A punching blade was used to create the test piece. Subsequently, the elongation at break of the test pieces A (S1) -A (S7) was measured using a high-speed tensile tester (“HITS-T10” manufactured by Shimadzu Corporation). At the time of measurement, a chuck was mounted at a position 15 mm away from the end of the grip portion of the test piece. The distance between the gauge points was the length L0 (mm) of the test part of each test piece. A high-speed tensile test was conducted at a tensile speed of 8.3 m / s and a temperature of 75 ° C., and the ratio D1 / Dt and ratio D2 / Dt at break and elongation at break were recorded.

  Subsequently, a high-speed tensile test was performed in the same manner except that the vulcanized rubber of Production Example BH was used, and the ratio D1 / Dt and ratio D2 / Dt at break and the elongation at break were recorded.

[Example 2]
From the high-speed tensile test results obtained for each of the five test pieces A (S1) -A (S7), the elongation at break HS-EB (%) when the ratio D1 / Dt and the ratio D2 / Dt are 0.1 or more. The average value was calculated. Similarly, for the test pieces prepared from the vulcanized rubber of Production Example BH, the average values of the elongation at break HS-EB (%) when the ratio D1 / Dt and the ratio D2 / Dt are 0.1 or more, respectively. Was calculated.

  The larger the average value of the elongation at break HS-EB (%) obtained for Production Examples AH, the higher the Index (C) obtained in the Reference Example of [Experiment 1], and the correlation coefficient R is 0. 78.

[Comparative Example 2]
From the high-speed tensile test results obtained for each of the five test pieces A (S1) -A (S7), the elongation at break HS-EB when either the ratio D1 / Dt or the ratio D2 / Dt is less than 0.1. (%) Was selected and the average value was calculated. Similarly, about the test piece created from the vulcanized rubber of Production Example BH, the elongation at break HS-EB (%) when either the ratio D1 / Dt or the ratio D2 / Dt is less than 0.1, respectively. The average value of was calculated.

  The average value of the elongation at break HS-EB (%) obtained for Production Example AH has no clear correlation with the Index (C) obtained in Reference Example of [Experiment 1], and its correlation coefficient R was 0.13.

  The elongation at break HS-EB (%) obtained in Example 2 was significantly higher in correlation with Index (C) obtained in Reference Example than in Comparative Example 2. From this evaluation result, the superiority of the present invention is clear.

  The method described above can be applied as a method for evaluating physical properties of various members formed by vulcanizing a rubber composition.

DESCRIPTION OF SYMBOLS 10 ... Test piece 12 ... Test part 14 ... Inclined part 16 ... Gripping part 16a ... First holding part 16b ... Second holding part 18 ... Chuck 18a ... First One chuck 18b ... Second chuck

Claims (4)

A preparation step of preparing a sheet-like test piece formed by vulcanizing the rubber composition and a tensile tester including a pair of chucks for holding the test piece;
A measurement step of measuring the elongation at break EB (%) by performing a tensile test of the test piece using the tensile tester;
Contains
The test piece is a rectangular test part located in the center in the longitudinal direction, a pair of inclined parts gradually widening adjacent to the test part, and adjacent to each inclined part. A pair of gripping portions that are substantially rectangular in plan view and that form ends,
When the width of the test part is W0 and the width of the pair of gripping parts is W1 and W2, respectively, the ratio W0 / W1 is 0.1 or more and 0.4 or less, and the ratio W0 / W2 is 0.1. Is 0.4 or less,
In the measurement step, when the distance between the chuck when the test piece is broken is Dt, and when the distance from the break point of the test piece to each chuck is D1 and D2, the ratio D1 / Dt is 0.1. Is 0.9 or less and the ratio D2 / Dt is 0.1 or more and 0.9 or less,
The tensile speed in the measurement step is 1 m / s or more and 20 m / s or less,
An evaluation method for evaluating the chipping resistance of a tire obtained using the rubber composition having the same composition as that of the test piece, using the breaking elongation EB (%) as an index.   The evaluation method according to claim 1, wherein a ratio t / W0 between a thickness t of the test piece and a width W0 of the test portion is 0.2 or more and 1.0 or less.   3. The evaluation method according to claim 1, wherein in the measurement step, the time from the start of elongation of the test portion to the break is 0.5 milliseconds or more and 500 milliseconds or less.   When the length of the test part is L0 and the length of the pair of inclined parts is L1 and L2, respectively, the ratio L0 / L1 is 0.5 or more and 3.0 or less, and the ratio L0 / L2 is 0. The evaluation method according to claim 1, wherein the evaluation method is 5 or more and 3.0 or less.

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