JP2010032427A - Fatigue testing method of rubber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fatigue testing method of rubber which enables the estimation of the fatigue life of rubber in a short time. <P>SOLUTION: Strain is repeatedly applied to a rubber test piece in an arbitrary extension, a breaking extension ratio λb and tensile stress M100 are measured with respect to the rubber test piece to which strain is applied an arbitrary number of times, the difference Y between imaginary tensile stress M100* calculated from the breaking extension ratio λb and the actually measured tensile stress M100 is calculated to make a change with the elapse of time in the tensile stress difference Y serve as an index of fatigue resistance. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fatigue test method for predicting the fatigue life of rubber, and more particularly to a rubber fatigue test method that enables prediction of the fatigue life of rubber in a short time.

  As a fatigue test method for predicting the fatigue life of rubber, there is a method in which strain is repeatedly applied to a rubber test piece and evaluation is performed based on the number of repetitions and time until the rubber test piece breaks (for example, Patent Documents 1 to 3). reference).

Such a rubber fatigue test method is used, for example, to predict the occurrence of cracks called groove cracks that occur in the main grooves and lug grooves on the tire tread surface due to repeated deformation, and to suppress the occurrence of cracks. Widely used in the development of rubber compositions. However, this type of rubber fatigue test method has a problem that the test efficiency is poor because it takes a long time for the rubber test piece to break.
JP-A-8-193933 JP-A-8-304250 Japanese Patent Laid-Open No. 3-245035

  An object of the present invention is to provide a rubber fatigue test method capable of predicting the fatigue life of rubber in a short time.

  In order to achieve the above object, the rubber fatigue test method of the present invention is such that the rubber test piece is repeatedly subjected to strain at an arbitrary elongation rate, and the elongation at break ratio of the rubber test piece given an arbitrary number of strains. λb and tensile stress M100 are measured, and the difference Y between the virtual tensile stress M100 * calculated from the breaking elongation ratio λb and the actually measured tensile stress M100 is obtained. It is characterized by being used as an index of fatigue resistance.

  In the Japanese Patent Application No. 2008-9463 filed earlier, the present inventor repeatedly applied strain to a rubber test piece at an arbitrary elongation rate, and applied the tensile stress to the rubber test piece to which an arbitrary number of strains were applied. It was proposed that the change in tensile stress over time was used as an index of fatigue resistance of rubber. According to such a fatigue test method, it is possible to predict the fatigue life of rubber in a shorter time than when the fatigue test is continued until the rubber test piece breaks. However, when the change in tensile stress over time is used as an index of rubber fatigue resistance, it has been found that if the strain applied to the rubber specimen is increased, an accurate prediction result cannot always be obtained.

  Therefore, in the present invention, the difference Y between the virtual tensile stress M100 * calculated from the breaking elongation ratio λb and the actually measured tensile stress M100 is obtained, and the change over time in the tensile stress difference Y is determined as the fatigue resistance of rubber. This makes it possible to predict the fatigue life of rubber in a short time even when the strain applied to the rubber specimen is increased.

In the present invention, the virtual tensile stress M100 * can be calculated from the following equation (1).
log λb = −0.75 log M100 * + C (1)
However, C is an arbitrary constant.

Further, in the present invention, the following equation (2) obtained by the least square method from the elapsed time T from the start of applying strain to the rubber test piece and a plurality of values of the tensile stress difference Y obtained at different times of applying strain to the rubber test piece The slope A in 2) can be used as an index of the fatigue resistance of rubber by using the deterioration rate.
Y = A × T ^1/2 + B (2)
However, A and B are arbitrary constants.

  The strain elongation rate is preferably 30% to 90%. With such an elongation rate, the fatigue life of the rubber can be accurately predicted based on the change with time of the tensile stress difference Y. Moreover, it is good to use a dumbbell shape, a strip shape, or a cylindrical thing as a rubber test piece.

  Hereinafter, the configuration of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows an example of an apparatus for carrying out the rubber fatigue test method of the present invention. In FIG. 1, 1 is a rubber test piece, 2 is an upper chuck, and 3 is a lower chuck. After the rubber test piece 1 is gripped by the upper chuck 2 and the lower chuck 3, strain is repeatedly applied in the longitudinal direction at a constant elongation rate. Although the shape of the rubber test piece 1 is not particularly limited, a dumbbell shape, a strip shape, or a column shape can be used.

  Using the apparatus as described above, strain was repeatedly applied to the rubber test piece 1 at an arbitrary elongation rate, and a rubber test piece given an arbitrary number of strains was stretched in the longitudinal direction. The breaking elongation ratio λb and the tensile stress M100 are measured. More specifically, a plurality of rubber test pieces 1 made of the same rubber composition are prepared, strain is applied to the rubber test pieces 1 under the same conditions, and at a plurality of time points where the number of strains applied is different. At least one rubber test piece 1 is removed, and the physical property value of each rubber test piece 1 is measured. If the measurement is performed on a plurality of rubber test pieces 1 at the same number of times of strain application, the test accuracy can be increased. The breaking elongation ratio λb means a value obtained by dividing the elongation at break (%) specified in JIS K6251 by 100, and the tensile stress M100 means the tensile stress at 100% extension specified in JIS K6251. To do.

  The main causes of rubber degradation are oxidative crosslinking and main chain breakage. The elongation at break λb decreases with increasing oxidative cross-linking, but is assumed not to change as main chain scission increases. This is because the breaking elongation ratio λb is determined by the molecular weight between the crosslinking points. On the other hand, the actually measured tensile stress M100 increases with an increase in oxidative crosslinking and decreases with an increase in main chain breakage. Therefore, when the virtual tensile stress M100 * is calculated from the breaking elongation ratio λb as a substitute for the crosslinking density and the actual tensile stress M100 is subtracted from the virtual tensile stress M100 *, the tensile stress difference Y is obtained. The difference Y is a parameter representing the amount of main chain cleavage.

  According to the knowledge of the present inventor, main chain breakage is a phenomenon that always occurs regardless of the amount of strain applied to the rubber test piece. By using as an index of fatigue resistance, the fatigue life of rubber can be accurately predicted even when the strain applied to the rubber specimen is increased. Moreover, according to such a fatigue test method, it is possible to predict the fatigue life of rubber in a shorter time than when the fatigue test is continued until the rubber test piece breaks.

  The strain elongation applied to the rubber test piece is preferably set in the range of 30% to 90%, more preferably 40% to 80%. If the strain elongation is less than 30%, the change in the main chain will not appear in a short time, so it takes a long time to predict the fatigue life. Conversely, if it exceeds 90%, the measured value will vary and the fatigue life will be It becomes difficult to accurately predict the above. Further, it is desirable that the speed for applying the distortion is 0.8 Hz to 8.5 Hz.

The virtual tensile stress M100 * can be calculated from the following equation (1).
log λb = −0.75 log M100 * + C (1)
However, C is an arbitrary constant.

  This equation (1) shows a general relationship between the elongation at break λb and the tensile stress M100 * when the rubber is oxidized and deteriorated. In other words, as the crosslinking progresses due to oxidation, the tensile stress M100 * increases along the straight line of −0.75 and the breaking elongation ratio λb decreases (Awane Asahiro, “Oxidative degradation of carbon compounded rubber due to tensile properties” Analysis of cross-linking and main-chain breakage by "Nippon Rubber Association Journal, Vol. 62, No. 2 (1989), P107-114".

  FIG. 2 is a graph showing the relationship between the rubber elongation at break λb and the tensile stress M100. In FIG. 2, the vertical axis represents log λb, and the horizontal axis represents log M100 (MPa). In FIG. 2, equation (1) is displayed. The constant C plots the breaking elongation ratio λb and tensile stress M100 of the rubber test piece in the initial state at a point P0, and the slope −0 through the point P0. .75 is obtained by drawing a straight line. Here, when the breaking elongation ratio λb and the tensile stress M100 measured from a rubber specimen subjected to an arbitrary number of strains are plotted at a point P1, this point P1 is located below the straight line of the equation (1). It will be. On the other hand, when the measured value of the breaking elongation ratio λb is substituted into Equation (1), a virtual tensile stress M100 * is obtained as the horizontal coordinate value of the point P2. The value obtained by subtracting the actual tensile stress M100 from the virtual tensile stress M100 * obtained in this way is the tensile stress difference Y (M100 * -M100).

When the time-dependent change of the tensile stress difference Y is used as an index of the fatigue resistance of the rubber, the elapsed time T from the start of applying strain to the rubber test piece and the tensile stress difference Y obtained at different times of applying strain to the rubber test piece The slope A in the following equation (2) obtained from the plurality of values by the least square method is defined as the deterioration rate.
Y = A × T ^1/2 + B (2)
However, A and B are arbitrary constants.

FIG. 3 is a graph showing the relationship between the tensile stress difference Y and the elapsed time T. In FIG. 3, the vertical axis represents the tensile stress difference Y (MPa), and the horizontal axis represents T ^1/2 (min ^1/2 ). As shown in FIG. 3, the tensile stress difference Y (M100 * −M100) increases as the elapsed time T increases. This means that main chain scission increases as the fatigue test progresses. Here, the constants A and B are arbitrary constants given by the method of least squares. However, the larger the slope A, the faster the main chain scission increase rate, in other words, the faster the rubber degradation rate. Therefore, the inclination A is set as the deterioration rate, and the fatigue resistance of the rubber can be determined based on the deterioration rate.

  As described above, the rubber fatigue test method according to the present invention can predict the fatigue life of rubber in a short time compared to the case where the fatigue test is continued until the rubber test piece breaks. It can be widely used for predicting the life of various rubber products and developing rubber compositions that suppress deterioration.

  In a heavy duty tire having a tire size of 275 / 70R22.5 and a traction pattern formed on the tread surface, three types of rubber compositions X, Y, and Z having the composition shown in Table 1 were used as cap tread rubbers, respectively. Three types of tires were manufactured. However, the specifications excluding the cap tread rubber were common to each tire.

  After mounting these three types of tires on a vehicle with a total weight of 25 tons and traveling 100,000 km on a general road, the situation (number and size) of the occurrence of groove cracks was evaluated, and the result was capped with rubber composition X The tire used for the tread rubber is shown in Table 1 with an index of 100. A smaller index value means less occurrence of groove cracks.

＊１：ＳＴＲ２０
＊２：日本ゼオン社製 Ｎｉｐｏｌ ＢＲ１２２０
＊３：キャボットジャパン社製 ショウブラックＮ２３４
＊４：正同化学工業社製 酸化亜鉛３種
＊５：日油社製 ビーズステアリン酸
＊６：住友化学社製 アンチゲン６Ｃ
＊７：ＦＬＥＸＳＹＳ社製 ＳＡＮＴＯＣＵＲＥ ＴＢＢＳ
＊８：鶴見化学工業社製 金華印油入微粉硫黄

* 1: STR20
* 2: Nipol BR1220 manufactured by Nippon Zeon
* 3: Show Black N234 manufactured by Cabot Japan
* 4: 3 types of zinc oxide manufactured by Shodo Chemical Industry Co., Ltd. * 5: Bead stearic acid manufactured by NOF Corporation * 6: Antigen 6C manufactured by Sumitomo Chemical Co., Ltd.
* 7: SANTOCURE TBBS manufactured by FLEXSYS
* 8: Fine powder sulfur with Jinhua seal oil manufactured by Tsurumi Chemical Co., Ltd.

  As shown in Table 1, in the tire using the rubber composition X as the cap tread rubber, a relatively large number of groove cracks occurred, and in the tire using the rubber composition Y as the cap tread rubber, the occurrence of the groove crack was compared. In the tire using the rubber composition Z as the cap tread rubber, no occurrence of groove cracks was observed. From this result, it was confirmed that deterioration of the rubber was suppressed by blending butadiene rubber with natural rubber and increasing the blending ratio.

  Next, using the three types of rubber compositions X, Y, and Z used in the above-described actual vehicle test, three types of dumbbell-shaped No. 3 type rubber test pieces were prepared. And using these rubber test pieces, the physical properties of the rubber compositions X, Y and Z were evaluated by the fatigue test methods of Comparative Examples 1 to 6 and Examples 1 and 2, and the results are shown in Table 2.

Comparative Example 1:
About each rubber test piece, the initial breaking elongation was measured based on JISK6251. The evaluation results are indicated by an index with the measured value of the rubber composition X being 100. A larger index value means a greater elongation at break.

  Comparative example 2: About each rubber test piece, the breaking elongation after aging (100 degreeC x 48 h) was measured based on JISK6251. The evaluation results are indicated by an index with the measured value of the rubber composition X being 100. A larger index value means a greater elongation at break.

  Comparative Example 3: A 60% strain was repeatedly given to each rubber test piece in accordance with JIS K6270, and the fatigue test was continued until the rubber test piece broke. As the evaluation results, the number of strains applied at break (number of breaks) and the test time are shown.

  Comparative Example 4: 100% strain was repeatedly applied to each rubber test piece in accordance with JIS K6270, and the fatigue test was continued until the rubber test piece broke. As the evaluation results, the number of strains applied at break (number of breaks) and the test time are shown.

  Comparative Example 5: 60% strain was repeatedly given to each rubber test piece in accordance with JIS K6270, and when the number of applied strains (fatigue number) was 400,000 times, 600,000 times, 1.2 million times, rubber test pieces The tensile stress M100 of each was measured, and the tensile stress M100 was plotted against the ½ power of the elapsed time T from the start of strain application, and the slope of the equation obtained by the least square method was taken as the deterioration rate. The evaluation results are indicated by an index with the measured value of the rubber composition X being 100. A larger index value means a faster deterioration rate.

  Comparative Example 6: When each rubber test piece was repeatedly given 80% strain in accordance with JIS K6270 and the number of strains applied (fatigue number) was 50,000 times and 100,000 times, the tensile stress M100 of the rubber test piece The tensile stress M100 was plotted against the 1/2 power of the elapsed time T from the start of strain application, and the slope of the equation obtained by the least square method was taken as the deterioration rate. The evaluation results are indicated by an index with the measured value of the rubber composition X being 100. A larger index value means a faster deterioration rate.

  Example 1: 60% strain was repeatedly given to each rubber test piece in accordance with JIS K6270, and when the number of applied strains (fatigue number) was 400,000 times, 600,000 times, 1.2 million times, rubber test pieces Measure the elongation at break λb and the tensile stress M100, and obtain the difference Y between the hypothetical tensile stress M100 * calculated from the elongation at break λb based on the formula (1) and the actually measured tensile stress M100. The tensile stress difference Y was plotted against the half power of the elapsed time T from the start of strain application, and the slope A of the equation (2) obtained by the least square method was defined as the deterioration rate. The evaluation results are indicated by an index with the measured value of the rubber composition X being 100. A larger index value means a faster deterioration rate.

  Example 2: When each rubber test piece was repeatedly subjected to 80% strain in accordance with JIS K6270 and the number of applied strains (fatigue number) was 50,000 times and 100,000 times, the elongation at break of the rubber test piece was λb and tensile stress M100 are measured, respectively, and a difference Y between the virtual tensile stress M100 * calculated from the elongation at break λb based on the formula (1) and the actually measured tensile stress M100 is obtained. The difference Y was plotted against the ½ power of the elapsed time T from the start of strain application, and the slope A of the equation (2) obtained by the least square method was taken as the deterioration rate. The evaluation results are indicated by an index with the measured value of the rubber composition X being 100. A larger index value means a faster deterioration rate.

  As can be seen from Table 2, each of Comparative Examples 1 and 2 uses the initial or post-aging elongation at break as an index of the fatigue resistance of rubber, and the evaluation results based on these elongation at break are the results of an actual vehicle test. It was not a match.

  Comparative Example 3 uses the number of strains applied at the time of fracture as an index of the fatigue resistance of rubber, and the evaluation result is almost the same as the evaluation result by the actual vehicle test, but it takes 120 hours to complete the test. there were. Moreover, although the comparative example 4 shortens the test time to a fracture | rupture by making distortion larger than the comparative example 3, in this case, the evaluation result based on the distortion | straining frequency | count at the time of a fracture | rupture becomes an evaluation result by a real vehicle test. There was a disagreement.

  In Comparative Example 5, the change in tensile stress with time was used as an index of the fatigue resistance of rubber, and the evaluation result was generally in agreement with the evaluation result by the actual vehicle test. However, when the test time to break is shortened by making the strain larger than that of Comparative Example 5 (Comparative Example 6), the evaluation result based on the temporal change of the tensile stress does not agree with the evaluation result of the actual vehicle test. It was. Therefore, the fatigue test method using the change in tensile stress over time as an index of the fatigue resistance of rubber is not effective when the strain applied to the rubber test piece is increased.

  In Example 1, the temporal change of the tensile stress difference Y obtained from the virtual tensile stress M100 * and the actual tensile stress M100 is used as an index of the fatigue resistance of rubber, and the evaluation result is an evaluation result by an actual vehicle test. And generally agreed. In addition, even when the test time until breakage is shortened by making the strain larger than that in Example 1 (Example 2), the evaluation result based on the temporal change of the tensile stress difference Y is almost the same as the evaluation result by the actual vehicle test. I did it.

It is a schematic side view which shows an example of the apparatus for enforcing the fatigue test method of the rubber | gum of this invention. It is a graph which shows the relationship between the breaking elongation ratio (lambda) b of rubber | gum, and the tensile stress M100. 4 is a graph showing a relationship between a tensile stress difference Y and an elapsed time T.

Explanation of symbols

1 Rubber test piece 2 Upper chuck 3 Lower chuck

Claims (5)

  The rubber test piece was repeatedly subjected to strain at an arbitrary elongation rate, and the rubber test piece subjected to arbitrary number of strains was measured for the breaking elongation ratio λb and the tensile stress M100, and calculated from the breaking elongation ratio λb. Fatigue test of rubber characterized in that the difference Y between the virtual tensile stress M100 * and the actually measured tensile stress M100 is obtained, and the change in the tensile stress Y over time is used as an index of the fatigue resistance of rubber. Method. 2. The rubber fatigue test method according to claim 1, wherein the virtual tensile stress M100 * is calculated from the following equation (1).
log λb = −0.75 log M100 * + C (1)
However, C is an arbitrary constant. In the following formula (2) obtained by the least square method from an elapsed time T from the start of applying strain to the rubber test piece and a plurality of values of tensile stress difference Y obtained at different times of applying strain to the rubber test piece. The rubber fatigue test method according to claim 1 or 2, wherein the slope A is used as a deterioration rate as an index of the fatigue resistance of the rubber.
Y = A × T ^1/2 + B (2)
However, A and B are arbitrary constants.   The method for fatigue testing of rubber according to any one of claims 1 to 3, wherein an elongation rate of the strain is set to 30% to 90%.   The rubber test piece according to claim 1, wherein the rubber test piece has a dumbbell shape, a strip shape, or a column shape.

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