Method for evaluating the low-temperature crystallinity of vulcanized rubber

Abstract

To provide a simple and reliable method for evaluating the low-temperature crystalline nature of a vulcanized rubber which progresses under exposure to a low-temperature environment for a long time.SOLUTION: The present invention relates to a method for evaluating a low-temperature crystalline nature of a vulcanized rubber by using a differential scan-type calorimeter (DSC), the method including; a first temperature drop step S2 of cooling a vulcanized rubber to a predetermined temperature (Tc), where the vulcanized rubber can crystallize; a crystallization step S3 of holding the vulcanized rubber for a predetermined time at the predetermined temperature (Tc) and making the crystallization progress; a measurement step S5 of raising the temperature of the vulcanized rubber crystallized in the crystallization step and measuring a DSC curve.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for evaluating the low-temperature crystallinity of vulcanized rubber.

Background Art

[0002] Vulcanized rubber having elasticity is widely used in various fields, for example, as a vibration isolation member that elastically supports a supported body such as an engine or a transmission with respect to a vehicle body in a vehicle such as an automobile, or used in a tire.

[0003] It is known that such vulcanized rubber loses its elasticity in a low-temperature environment. In recent years, products made of vulcanized rubber have been exposed to a low-temperature environment, resulting in problems of reduced function. This is considered to be caused by the crystallization of vulcanized rubber. From such a situation, there is a demand for vulcanized rubber that is difficult to crystallize even when exposed to a low-temperature environment.

[0004] By the way, it is known that natural rubber, which is a raw material of vulcanized rubber, has at least two kinds of crystallization phenomena. Self-reinforcing template crystallization and low-temperature crystallization. Since natural rubber exhibits a particularly unique crystallization behavior among polymers, it is particularly difficult to grasp the behavior of low-temperature crystallization. The generation of crystal nuclei in natural rubber depends on density and fluctuations in microscopic Brownian motion, so it is considered difficult to control crystallization in practice (Non-Patent Documents 1 to 3).

[0005] Conventionally, as a test method for evaluating the properties of vulcanized rubber at low temperatures, there is a method of installing a refrigerator in a static and dynamic spring characteristic tester and measuring the spring constant in a low-temperature environment. Further, Patent Document 1 describes evaluating the low-temperature crystallinity of a rubber composition using the melting enthalpy obtained by a general measurement method of DSC. Patent Document 2 describes discriminating the presence or absence of a peak derived from low-temperature crystallization of a rubber material from a tanδ curve obtained by measuring the temperature dispersion curve of viscoelasticity of a vulcanized rubber composition.

Prior Art Documents

Non-Patent Documents

[0006] [Non-Patent Document 1] "Rubber Science - Its Modern Approach," by Yuko Ikeda et al., published by Asakura Shoten. [Non-Patent Document 2] ▲Koji▼ Shinzo Tani et al., "Rubber Reinforcement - Mechanistic Analysis by Visualization of Nanofillers," Asakura Shoten Publishing. [Non-Patent Document 3] Edited by the Society of Polymer Science, "Basic Polymer Science", Tokyo Kagaku Doujin Publishing [Patent Documents]

[0007] [Patent Document 1] Patent No. 6806786 [Patent Document 2] Patent No. 6141118 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Evaluation using a spring properties tester is not suitable for evaluating the low-temperature crystallinity of vulcanized rubber because it requires test specimens of a specific shape, installs a refrigerator in the static-dynamic spring properties tester, and has limitations in the cooling performance of low-temperature equipment such as refrigerators that can be installed in the static-dynamic spring properties tester, making it impossible to evaluate in extremely low-temperature regions such as below -100°C.

[0009] The evaluation methods described in Patent Documents 1 and 2 predict the crystallinity of a rubber composition by measuring its properties when exposed to low temperatures instantaneously. Crystallization of vulcanized rubber progresses gradually over time, and as mentioned above, it is difficult to grasp the behavior of low-temperature crystallization. However, these conventional methods do not take into account the low-temperature crystallinity unique to natural rubber. Therefore, when these methods are used to evaluate vulcanized rubber with a relatively slow crystallization rate, for example, problems arise such as the lack of significant differences between multiple vulcanized rubbers or the possibility that the evaluation results do not reflect the actual low-temperature crystallinity.

[0010] The technology disclosed herein has been made in view of the above, and its objective is to provide a simple and reliable method for evaluating the low-temperature crystallinity of vulcanized rubber that is exposed to low-temperature environments for extended periods. [Means for solving the problem]

[0011] The technologies disclosed herein are A method for evaluating the low-temperature crystallinity of vulcanized rubber using a differential scanning calorimeter (DSC), A cooling step in which the vulcanized rubber is cooled to a predetermined temperature (Tc) in which crystallization of the vulcanized rubber is possible, A crystallization step is performed in which the vulcanized rubber is held at the predetermined temperature (Tc) for a certain period of time to allow crystallization to proceed. The method is characterized by including a measurement step of raising the temperature of the vulcanized rubber, which has undergone crystallization in the crystallization step, and measuring the DSC curve.

[0012] According to this, by using a differential scanning calorimeter (DSC), it becomes possible to easily evaluate various vulcanized rubber samples regardless of their shape, without the need to prepare large-scale low-temperature equipment or samples of a specific shape, as is the case with spring property testers. Since differential scanning calorimeters (DSCs) are commonly available in specifications that can be cooled to below -100°C, even when the vulcanized rubber contains rubber components with a glass transition temperature (Tg) of below -100°C, it is possible to easily maintain a predetermined temperature (Tc) and promote crystallization.

[0013] Furthermore, by holding the vulcanized rubber at a predetermined temperature (Tc) where crystallization is possible for a certain period of time, it is possible to measure vulcanized rubber that approximates the condition in which problems have actually been reported in cold regions. Rather than measuring the instantaneous low-temperature properties of the vulcanized rubber, the measurement results are obtained in a state where crystallization has progressed over time. Therefore, it is easy to compare and evaluate rubbers with relatively slow crystallization rates, and this method of evaluating the low-temperature crystallinity of vulcanized rubber is highly reliable.

[0014] Furthermore, the vulcanized rubber preferably contains natural rubber or isoprene rubber.

[0015] Natural rubber and isoprene rubber are thought to have their arrangement inhibited by steric hindrance of side chains, resulting in slower low-temperature crystallization compared to other rubbers with few or no side chains. Therefore, the low-temperature properties of vulcanized rubber, including natural rubber and isoprene rubber, may differ significantly depending on whether crystallization occurs instantaneously or gradually over time. Evaluation results obtained by instantaneously crystallizing vulcanized rubber, including natural rubber and isoprene rubber, under the same conditions as other rubbers with rapid low-temperature crystallization may deviate from the actual low-temperature properties in cold regions. However, using the method described above, it is possible to obtain evaluation results that approximate the actual low-temperature properties of vulcanized rubber, including natural rubber and isoprene rubber. Therefore, the use of the technology disclosed herein is of great significance, especially as a method for evaluating the low-temperature crystallinity of vulcanized rubber, including natural rubber or isoprene rubber.

[0016] When the vulcanized rubber contains natural rubber or isoprene rubber and does not contain butadiene rubber, the predetermined temperature (Tc) is a temperature 10°C to 70°C higher than the glass transition temperature (Tg) of natural rubber or isoprene rubber, in the cooling step, it is cooled at a constant rate to the predetermined temperature (Tc), After the measurement step, from the peak area of the DSC curve obtained by the measurement step, the heat absorption per unit weight generated when the crystal of the vulcanized rubber melts is calculated, and an evaluation step of evaluating the low-temperature crystallinity of the vulcanized rubber using the change amount of the heat absorption as an evaluation index is preferably included.

[0017] In the vulcanized rubber containing natural rubber or isoprene rubber, by setting the predetermined temperature (Tc) for promoting crystallization to a temperature 10°C to 70°C higher than the glass transition temperature (Tg) of natural rubber or isoprene rubber, the crystallization of the vulcanized rubber can be promoted and it can be made into a state suitable for evaluating low-temperature crystallinity.

[0018] Further, from the peak area of the DSC curve obtained by the measurement step, the heat absorption per unit weight generated when the crystal of the vulcanized rubber melts is calculated, and by using the change amount of the heat absorption as an evaluation index for low-temperature crystallinity, it becomes possible to efficiently and appropriately evaluate vulcanized rubbers with various compositions.

[0019] In the crystallization step, it is preferable to hold the predetermined temperature (Tc) for at least 48 hours, and the predetermined temperature (Tc) is preferably -60°C to 0°C.

[0020] By holding the predetermined temperature (Tc) for at least 48 hours, it is possible to sufficiently grow the crystals in the vulcanized rubber, and it also becomes easy to find a significant difference between vulcanized rubbers when evaluating a plurality of vulcanized rubbers with a slow crystallization rate.

[0021] Furthermore, when the vulcanized rubber contains butadiene rubber, the predetermined temperature (Tc) is preferably a temperature 10°C to 130°C higher than the glass transition temperature (Tg) of butadiene rubber.

[0022] Vulcanized rubber containing butadiene rubber is considered to be prone to crystallization. In addition to natural rubber or isoprene rubber, in vulcanized rubber containing butadiene rubber, a predetermined temperature (Tc) for promoting crystallization can be set to a temperature 10°C to 130°C higher than the glass transition temperature (Tg) of butadiene rubber. In this temperature range, the crystallization of vulcanized rubber can be promoted, and a state suitable for evaluating low-temperature crystallinity can be achieved.

[0023] Also in this case, it is preferable to hold the predetermined temperature (Tc) for at least 48 hours, and the predetermined temperature (Tc) can be set to -100°C to 20°C.

Advantages of the Invention

[0024] As described above, according to the technology disclosed in this specification, it is possible to provide a simple and highly reliable method for evaluating the low-temperature crystallinity of vulcanized rubber that progresses when exposed to a low-temperature environment for a long time.

Brief Description of the Drawings

[0025] [Figure 1] It is a flowchart showing an example of a method for evaluating the low-temperature crystallinity of vulcanized rubber of the present disclosure.

Modes for Carrying Out the Invention

[0026] Hereinafter, modes for carrying out the present disclosure will be described. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit the present disclosure, its applications, or its uses.

[0027] The technology disclosed in this specification uses a differential scanning calorimeter (DSC) to cool the vulcanized rubber to a predetermined temperature (Tc) at which crystallization of the vulcanized rubber is possible, hold the vulcanized rubber at the predetermined temperature (Tc) for a certain period of time to promote crystallization, heat up the vulcanized rubber after promoting crystallization to measure the DSC curve, and evaluate the low-temperature crystallinity based on the change in the heat absorption per unit weight obtained from the DSC curve.

[0028] [Vulcanized rubber] The vulcanized rubber being evaluated is made by vulcanizing a rubber component (polymer) with fillers and additives.

[0029] The rubber component is not particularly limited, but may include, for example, natural rubber (NR) or isoprene rubber (IR). It may contain natural rubber (NR) or isoprene rubber (IR) alone, or it may be a mixture of natural rubber (NR) or isoprene rubber (IR) with one or more other rubber components, such as butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), ethylene propylene rubber (EPDM), or chloroprene rubber (CR). The rubber component to be mixed with natural rubber (NR) or isoprene rubber (IR) is not limited to the above, and other rubber components can be used as long as they enable the implementation of the technology of this disclosure. When mixing multiple rubber components, the vulcanized rubber preferably contains natural rubber (NR) or isoprene rubber (IR) as 20% or more of the total rubber component, and more preferably contains natural rubber (NR) or isoprene rubber (IR) as the main component. "Main component" refers to the proportion of natural rubber (NR) or isoprene rubber (IR) among the various rubber components that make up vulcanized rubber.

[0030] The fillers may include, for example, reinforcing fillers such as silica-based inorganic fillers like SiO2, kaolin, talc, clay, mica, and diatomaceous earth, nanofillers such as cellulose nanofiber and graphite, and carbon black, as well as non-reinforcing fillers such as metal oxides and metal hydroxides like calcium carbonate, barium sulfate, and titanium dioxide. The strength of the vulcanized rubber can be increased by incorporating reinforcing fillers. In addition to fillers, other compounding agents commonly used in the manufacture of vulcanized rubber, such as vulcanization accelerators like zinc oxide and stearic acid, antioxidants, processing aids, softeners, and vulcanizing agents like sulfur, can be appropriately added. The amounts of each of these components are arbitrary.

[0031] Vulcanized rubber can be obtained by general rubber manufacturing methods.

[0032] [Method for evaluating low-temperature crystallinity] The resulting vulcanized rubber sample is placed in a differential scanning calorimeter (DSC) to measure the crystallization progress of the vulcanized rubber and the DSC curve. The shape of the vulcanized rubber used as the sample is not particularly limited.

[0033] By setting a cooling step in the automatic measurement program of a differential scanning calorimeter (DSC) to cool the vulcanized rubber to a predetermined temperature (Tc) at which crystallization is possible, a crystallization step in which the vulcanized rubber is held at the predetermined temperature (Tc) for a certain period of time to allow crystallization to proceed, and a measurement step in which the vulcanized rubber that has undergone crystallization in the crystallization step is heated and the DSC curve is measured, the series of steps from crystallization to melting can be performed automatically, making it possible to implement the technology of this disclosure more efficiently. Specifically, crystallization, measurement, and evaluation are performed in the order shown in Figure 1.

[0034] (1) Preparation process S1 First, a sample of vulcanized rubber is placed in a differential scanning calorimeter (DSC), and the vulcanized rubber is heated to melt it, creating a blank state in which no crystals are present. The purpose of this preparation step S1 is to clearly distinguish it from vulcanized rubber after crystallization has progressed by melting the vulcanized rubber before cooling it to create a blank state. The heating temperature in the preparation step should be any temperature at which the vulcanized rubber can melt, and it is preferable to heat it to approximately 70°C, for example.

[0035] (2) First temperature lowering step S2 Next, in the first cooling step S2, the vulcanized rubber in this blank state is cooled at a constant rate to a temperature 10°C to 70°C higher than the glass transition temperature (Tg) of natural rubber or isoprene rubber. The "constant rate" is any rate, but for example, it is 1°C to 5°C / min. The "cooling step" described in the claims is the first cooling step S2.

[0036] Generally, the glass transition temperature (Tg) of vulcanized rubber varies depending on changes in composition and manufacturing conditions, resulting in a temperature range for the glass transition temperature (Tg). In the evaluation method of this disclosure, when the vulcanized rubber to be evaluated contains natural rubber or isoprene rubber, the crystallization of the vulcanized rubber can be promoted in this temperature range by setting the predetermined temperature (Tc) to 10°C to 70°C higher than the glass transition temperature (Tg) of the natural rubber or isoprene rubber and holding it for a certain period of time, thereby facilitating the evaluation of low-temperature crystallinity. Generally, the glass transition temperature (Tg) of natural rubber and isoprene rubber is known to be approximately -70°C to -50°C. For example, if the glass transition temperature (Tg) of natural rubber is -70°C, the preferred predetermined temperature (Tc) for promoting crystallization is -60°C to 0°C.

[0037] If the vulcanized rubber to be evaluated contains butadiene rubber in addition to natural rubber or isoprene rubber, the predetermined temperature (Tc) is preferably 10°C to 130°C higher than the glass transition temperature (Tg) of butadiene rubber. Generally, the glass transition temperature (Tg) of butadiene rubber is known to be approximately -120°C to -100°C. For example, if the glass transition temperature (Tg) of butadiene rubber is -110°C, the preferred predetermined temperature (Tc) for promoting crystallization of the vulcanized rubber containing butadiene rubber is -100°C to 20°C.

[0038] (3) Crystallization step S3 In the crystallization step S3, the vulcanized rubber, cooled to a predetermined temperature (Tc), is held at the predetermined temperature (Tc) for a certain period of time to allow crystallization to proceed. Since the rate of crystallization varies depending on the rubber components contained in the vulcanized rubber, the time for holding at the predetermined temperature (Tc) in the crystallization step S3 is not particularly limited as long as it is long enough for the crystallization of the vulcanized rubber to proceed, but it is preferable to hold it for at least 48 hours, for example. If vulcanized rubber that is difficult to crystallize is included in the evaluation, it may be set to a slightly longer time, for example, 72 hours.

[0039] (4) Second temperature lowering step S4 This process involves lowering the temperature to below the temperature at which the micro-Brownian motion of the polymer completely stops. Before raising the temperature of the crystallized vulcanized rubber, it is first lowered to a temperature at which melting for crystallization does not begin. In this case, it is preferable to lower the temperature to below the glass transition temperature (Tg) of the vulcanized rubber. For example, in the case of vulcanized rubber containing natural rubber or isoprene rubber, it is cooled to -90°C.

[0040] (5) Measurement process S5 In measurement step S5, the temperature is increased at 1-5°C / min, and the DSC curve is measured.

[0041] (6) Evaluation process S6 After the measurement step S5, in the evaluation step S6, the amount of heat absorbed per unit weight (J / mg) generated when the vulcanized rubber crystals melt is calculated from the peak area of ​​the obtained DSC curve. In addition, a measurement is performed without the crystallization step S3, and the low-temperature crystallinity of the vulcanized rubber is evaluated using the obtained amount of heat absorbed (J / mg) and the change in the amount of heat absorbed obtained from the measurement including the crystallization step S3 (the difference caused by low-temperature crystallization) as evaluation indicators.

[0042] When comparing and evaluating the low-temperature crystallinity of multiple types of vulcanized rubber, it is desirable to calculate the heat absorbance per unit weight (J / mg) for each after ensuring that all conditions from preparation step S1 to measurement step S5 are the same. [Examples]

[0043] [Composition of vulcanized rubber] The following describes specific examples of the technology of this disclosure based on examples and comparative examples, but the technology of this disclosure is not limited thereto.

[0044] Compositions 1-3 and 4-6 consist of the compositions shown in Table 1. The three vulcanized rubbers of compositions 1-3 were evaluated under the measurement conditions of Examples 1-12 and Comparative Examples 1 and 4. The three vulcanized rubbers of compositions 4-6 were evaluated under the measurement conditions of Examples 13-16 and Comparative Examples 2, 3, and 5.

[0045] Compositions 1-3 contain 80 parts by weight of natural rubber (NR) and 20 parts by weight of polybutadiene rubber (BR) out of a total of 100 parts by weight of rubber components. Compositions 4-6 contain 100 parts by weight of natural rubber (NR) out of a total of 100 parts by weight of rubber components, and do not contain polybutadiene rubber (BR). The differences in the low-temperature crystallinity of vulcanized rubber were evaluated by changing the amount of sulfur in compositions 1-3 and 4-6.

[0046] [Table 1]

[0047] The types of raw materials used in Table 1 are shown below. • Natural rubber (NR): RSS#3 • Polybutadiene rubber (BR): Manufactured by Ube Industries, Ltd. Ubepol BR150 • Carbon Black: Tokai Carbon Co., Ltd. Seast S • Zinc oxide: Two types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. • Stearic acid: NOF Corporation's Beads Stearic Acid (Camellia) • Wax: Suntight S manufactured by Seiko Chemical Co., Ltd. • Anti-aging agent 1 (N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine): Nocrack 6C, manufactured by Ouchi Shinko Chemical Co., Ltd. • Anti-aging agent 2 (2-mercaptobenzimidazole): Nocrack MB, manufactured by Ouchi Shinko Chemical Industry Co., Ltd. • Vulcanization accelerator 1 (N-oxydiethylene-2-benzothiazolyl sulfenamide): Noxellar MSA-G, manufactured by Ouchi Shinko Chemical Co., Ltd. • Vulcanization accelerator 2 (tetramethylthiuram disulfide): Noxellar TT, manufactured by Ouchi Shinko Chemical Industry Co., Ltd. • Sulfur: Finely powdered sulfur, 325 mesh, manufactured by Hosoi Chemical Industry Co., Ltd. [Manufacturing of vulcanized rubber] While vulcanized rubber can generally be manufactured using methods commonly employed in the production of vulcanized rubber, in this embodiment, it was manufactured using the following method.

[0048] Compositions 1-3 and 4-6 were prepared by kneading all materials except sulfur and vulcanization accelerator using a Banbury mixer, according to the compositions shown in Table 1. Next, sulfur and vulcanization accelerator were added to the resulting mixture and kneaded using an open roll to obtain an unvulcanized rubber mixture. Furthermore, the obtained unvulcanized rubber mixture was injected into a 2 mm thick mold and press-vulcanized at 170°C for 5 minutes to obtain vulcanized rubber.

[0049] [Evaluation of Low-Temperature Crystallinity of Vulcanized Rubber] The obtained vulcanized rubber was subjected to crystallization and heat endothermic measurements using a differential scanning calorimeter (Hitachi High-Tech Science Corporation, DSC7000X). The crystallization, measurement, and evaluation of vulcanized rubber compositions 1 to 3 in Example 1 will be described in detail. The crystallization, measurement, and evaluation methods for the other examples and comparative examples are substantially the same as those in Example 1, so their descriptions will be omitted.

[0050] An aluminum pan containing a 10 mg sample of vulcanized rubber was placed in a differential scanning calorimeter (DSC), and the temperature was increased from room temperature to 70°C at a rate of 2°C / min to melt the vulcanized rubber, creating a blank state in which no crystals were present (preparation step S1). Next, the temperature was decreased at a rate of 2°C / min to -30°C (predetermined temperature Tc), which is approximately 80°C higher than the glass transition temperature (Tg) of butadiene rubber (first cooling step S2). The sample was held at -30°C (predetermined temperature Tc) for 48 hours to allow crystallization to proceed (crystallization step S3). The temperature was further decreased at a rate of 2°C / min to -90°C (second cooling step S4), and after holding at -90°C for 5 minutes, the temperature was increased from -90°C to 70°C at a rate of 2°C / min, and the DSC curve was measured (measurement step S5).

[0051] In the curve obtained by drawing a blank line from the DSC curve obtained in measurement step S5, the amount of heat absorbed when the vulcanized rubber crystals melt was found as a peak area around -40°C to 0°C. The integral value of this peak area was converted to heat quantity and divided by the weight of the vulcanized rubber sample used for measurement to calculate the amount of heat absorbed per unit weight (J / mg) when the vulcanized rubber crystals melt. Under the same measurement conditions as in Example 1, each vulcanized rubber sample with composition 1 to 3 was measured, and the change in heat absorbed (J / mg) was used as an evaluation index to evaluate the low-temperature crystallinity of the vulcanized rubber.

[0052] Tables 2 to 4 show the crystallization conditions for crystallization step S3 and their evaluation results. For Examples 1 to 16 and Comparative Examples 1 to 5, the low-temperature crystallinity of multiple vulcanized rubbers was relatively evaluated using the change in endothermic energy (J / mg) as the evaluation index. The sample with the largest change in endothermic energy (J / mg) was marked with "×" as having high low-temperature crystallinity. Samples with a smaller change in endothermic energy (J / mg) than those marked with "×" were marked with "〇". If no significant difference was observed in the change in endothermic energy (J / mg), it was marked as "difficult to evaluate".

[0053] The evaluation method is not limited to such relative evaluation. For example, the value of the change in endothermic capacity (J / mg) used as an indicator can be arbitrarily set, and the low-temperature crystallinity can be evaluated by absolute evaluation based on that value.

[0054] [Table 2]

[0055] [Examples 1-12, Comparative Example 1] From the results of Examples 1 to 3 shown in Table 2, the preferred holding time in the crystallization step S3 is at least 48 hours. In Example 1, where the holding time was 48 hours, the change in endothermic energy (J / mg) of compositions 1 to 3 showed a sufficient difference to allow for comparative evaluation. In Example 2, where the holding time was 72 hours, the difference in the change in endothermic energy (J / mg) of compositions 1 to 3 was slightly larger. In Example 3, where the holding time was 120 hours, the difference in the change in endothermic energy (J / mg) of compositions 1 to 3 was even larger, but it was still possible to evaluate it sufficiently with a holding time of 48 hours. Therefore, considering the efficiency of the measurement, it is preferable to set the holding time at least 48 hours.

[0056] From the results of Examples 1 to 12 and Comparative Example 1 shown in Table 2, when the vulcanized rubber being evaluated contains butadiene rubber, the preferred predetermined temperature (Tc) in the crystallization step S3 is 10°C to 130°C higher than the glass transition temperature (Tg) of butadiene rubber. As shown in Example 9, when the predetermined temperature (Tc) was set to 130°C higher than the glass transition temperature (Tg) of butadiene rubber, a detailed evaluation of the change in endothermic amount between composition 1 and composition 2 was not possible. However, a slight difference in the change in endothermic amount was observed between composition 1 and composition 2 and composition 3, and it was possible to evaluate that composition 3 had higher low-temperature crystallinity compared to compositions 1 and 2. As shown in Example 12, when the predetermined temperature (Tc) was set to 10°C higher than the glass transition temperature (Tg) of butadiene rubber, the difference in the change in endothermic amount between the samples of composition 1 to 3 was small. However, similar to Example 9, it was possible to evaluate that composition 3 had higher low-temperature crystallinity compared to compositions 1 and 2. In contrast, as shown in Comparative Example 1, when the glass transition temperature (Tg) of butadiene rubber was set to a predetermined temperature (Tc), no difference was observed in the change in heat absorption among samples of composition 1 to 3, making it difficult to evaluate low-temperature crystallinity.

[0057] [Table 3]

[0058] [Examples 13-16, Comparative Examples 2,3] From the results of Examples 13-16 and Comparative Examples 2 and 3 shown in Table 3, when the vulcanized rubber being evaluated contains natural rubber or isoprene rubber but does not contain butadiene rubber, the preferred predetermined temperature (Tc) in the crystallization step S3 is 10°C to 70°C higher than the glass transition temperature (Tg) of natural rubber or isoprene rubber. In Examples 14 and 15, a detailed evaluation between composition 4 and composition 5 was not possible, but a slight difference was observed in the change in heat absorption between composition 4 and composition 5 and composition 6, making it possible to evaluate that composition 6 has higher low-temperature crystallinity compared to compositions 4 and 5. In contrast, as shown in Comparative Example 2, when the predetermined temperature (Tc) was set to 10°C lower than the glass transition temperature (Tg) of natural rubber, no difference was observed in the change in heat absorption between the samples of composition 4 to 6, making evaluation difficult. As shown in Comparative Example 3, when the predetermined temperature (Tc) was set to 80°C higher than the glass transition temperature (Tg) of natural rubber, no difference was observed in the change in endothermic amount among the samples with compositions 4 to 6, making evaluation difficult, similar to Comparative Example 2. From the results of Example 16, evaluation was possible even when the holding time in the crystallization step S3 was 48 hours.

[0059] [Table 4]

[0060] [Comparative Examples 4 and 5] Comparative Examples 4 and 5 shown in Table 4 are the results of evaluating the low-temperature crystallinity of compositions 1-3 and 4-6 when the holding temperature in crystallization step S3 is set to 0 hours, in other words, when measured using a method similar to the conventional measurement method without crystallization step S3.

[0061] In Comparative Example 4, differences in the change in endothermic amount were observed among samples with compositions 1 to 3, making evaluation possible. However, in Comparative Example 5, no differences in the change in endothermic amount were observed among samples with compositions 4 to 6, making evaluation difficult. From these results, it was found that while evaluation of vulcanized rubber containing butadiene rubber is possible when the crystallization step S3 is not performed, evaluation of vulcanized rubber without butadiene rubber is difficult.

[0062] Generally, it is known that polymer crystals are formed by the alignment of polymer molecular chains. Natural rubber and isoprene rubber are thought to undergo slow crystallization because the methyl groups in their side chains act as steric hindrance, inhibiting the alignment of the molecular chains. On the other hand, butadiene rubber does not have such side chains, so its molecular chains align easily, and crystallization is thought to proceed quickly. The evaluation technique of this disclosure makes it possible to find significant differences between samples even with natural rubber alone, which has low low-temperature crystallinity, and thus makes evaluation easier. Therefore, it can be applied to vulcanized rubbers of various compositions, including natural rubber and isoprene rubber.

[0063] [Comparison with dynamic spring test results] Vulcanized rubber exposed to the same predetermined temperature (Tc) and holding time as in Examples 1 to 16 was subjected to a dynamic spring test at the same predetermined temperature (Tc), and the results were compared with the results obtained using the technology of this disclosure.

[0064] Specifically, a cylindrical test shape of vulcanized rubber with a diameter of φ30 mm and a thickness of 30 mm was held at a predetermined temperature (Tc) for a predetermined time to promote crystallization. Then, under the predetermined temperature (Tc), the vulcanized rubber was subjected to a test using a dynamic properties testing machine (Sagimiya Seisakusho, KCH701) that applied constant displacement harmonic compression vibration with an amplitude of ±0.1 mm at a frequency of 20 Hz, and the dynamic spring constant (Kd1) at 20 Hz was determined in accordance with JIS K6385. Separately, the same measurement was performed on a sample at room temperature that had not undergone crystallization, and its dynamic spring constant (Kd2) was determined. The following equation is then obtained. (Kd1-Kd2) / Kd2×100 The value derived from this was used as the rate of change and served as an evaluation index for low-temperature crystallinity. When a rate of change of 500% or less was judged as "○" and a rate of change greater than 500% was judged as "×", this judgment result correlated with the evaluation results using the technology of this disclosure in Examples 1 to 16. This confirmed that the low-temperature crystallinity evaluation method using the technology of this disclosure is a reliable method for evaluating the low-temperature crystallinity of vulcanized rubber.

Claims

1. A method for evaluating the low-temperature crystallinity of vulcanized rubber using a differential scanning calorimeter (DSC), The vulcanized rubber includes natural rubber or isoprene rubber, but does not include butadiene rubber. A cooling step in which the vulcanized rubber is cooled to a predetermined temperature (Tc) in which crystallization of the vulcanized rubber is possible, A crystallization step is performed in which the vulcanized rubber is held at the predetermined temperature (Tc) for a certain period of time to allow crystallization to proceed. A measurement step is to heat the vulcanized rubber, which has undergone crystallization in the crystallization step, and measure the DSC curve. The evaluation step includes, after the measurement step, calculating the amount of heat absorbed per unit weight when the crystals of the vulcanized rubber melt from the peak area of ​​the DSC curve obtained in the measurement step, and evaluating the low-temperature crystallinity of the vulcanized rubber using the amount of heat absorbed as an evaluation index. The predetermined temperature (Tc) is 10°C to 70°C higher than the glass transition temperature (Tg) of natural rubber or isoprene rubber. The method for evaluating the low-temperature crystallinity of vulcanized rubber is characterized in that the cooling step is performed by cooling to a predetermined temperature (Tc) at a constant rate.

2. The method for evaluating the low-temperature crystallinity of vulcanized rubber according to claim 1, characterized in that the crystallization step is to maintain the predetermined temperature (Tc) for at least 48 hours.

3. The low-temperature crystallinity evaluation method for vulcanized rubber according to claim 2, characterized in that the predetermined temperature (Tc) is -60°C to 0°C.

4. A method for evaluating the low-temperature crystallinity of vulcanized rubber using a differential scanning calorimeter (DSC), The vulcanized rubber comprises natural rubber or isoprene rubber and butadiene rubber. A cooling step in which the vulcanized rubber is cooled to a predetermined temperature (Tc) in which crystallization of the vulcanized rubber is possible, A crystallization step is performed in which the vulcanized rubber is held at the predetermined temperature (Tc) for a certain period of time to allow crystallization to proceed. A measurement step is to heat the vulcanized rubber, which has undergone crystallization in the crystallization step, and measure the DSC curve. The evaluation step includes, after the measurement step, calculating the amount of heat absorbed per unit weight when the crystals of the vulcanized rubber melt from the peak area of ​​the DSC curve obtained in the measurement step, and evaluating the low-temperature crystallinity of the vulcanized rubber using the amount of heat absorbed as an evaluation index. The predetermined temperature (Tc) is 10°C to 130°C higher than the glass transition temperature (Tg) of butadiene rubber. The method for evaluating the low-temperature crystallinity of vulcanized rubber is characterized in that the cooling step is performed by cooling to a predetermined temperature (Tc) at a constant rate.

5. The method for evaluating the low-temperature crystallinity of vulcanized rubber according to claim 4, characterized in that the crystallization step is maintained at the predetermined temperature (Tc) for at least 48 hours.

6. The low-temperature crystallinity evaluation method for vulcanized rubber according to claim 5, characterized in that the predetermined temperature (Tc) is -100°C to 20°C.

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