JP7464943B2 - Method for measuring elastic modulus of vulcanized rubber - Google Patents

Description

The present invention relates to a method for measuring the elastic modulus of vulcanized rubber, and more specifically, to a method for measuring the elastic modulus of vulcanized rubber containing a filler using an atomic force microscope.

There is a demand for a method to measure and evaluate the elastic modulus of vulcanized rubber at the nano level. For example, Non-Patent Document 1 discloses observing the elastic modulus of a rubber material containing carbon black as a filler at the nano scale by measuring the force curve with an atomic force microscope (AFM).

On the other hand, in a rebound test to measure the resilience of vulcanized rubber, it is known that a preliminary impact is given to a test piece of vulcanized rubber and the resilience is measured after the rebound height has stabilized (Non-Patent Document 2).

Ken Nakajima et al., “NANOMECHANICS OF THERUBBER-FILLER INTERACTION”, Rubber Chemistry and Technology Vol.90, No.2, pp.272-284, 2017. JIS K6255:2013, "Vulcanized rubber and thermoplastic rubber - Determination of rebound elasticity"

According to the above-mentioned non-patent document 1, the elastic modulus of vulcanized rubber containing fillers can be measured at the nano level. However, internal strain exists in the rubber sample after vulcanization. That is, in vulcanized rubber containing fillers, there are areas where the rubber polymer has expanded and areas where it has contracted, and there is internal strain caused by vulcanization. Due to the influence of this internal strain, it may not be possible to accurately measure the elastic modulus of vulcanized rubber when measuring with an atomic force microscope.

The object of the present invention is to provide a method for measuring the elastic modulus of vulcanized rubber that can measure the elastic modulus while suppressing the effects of internal strain.

The method for measuring the elastic modulus of vulcanized rubber according to an embodiment of the present invention is a method for measuring the elastic modulus of vulcanized rubber using an atomic force microscope, and includes obtaining a sample in which the internal strain in the vulcanized rubber containing a filler is reduced by applying elastic deformation to the vulcanized rubber, and measuring the elastic modulus of the obtained sample by measuring the force curve with the atomic force microscope.

According to an embodiment of the present invention, when measuring the elastic modulus of vulcanized rubber using an atomic force microscope, the effect of internal strain can be suppressed and the elastic modulus can be measured.

Graph showing the results of measuring the elastic modulus by AFM for Examples 1 to 4 and Comparative Example 1 Graph showing the relationship between elastic modulus and macro elastic modulus measured by AFM for Example 5 and Comparative Example 2 Graph showing the relationship between elastic modulus and macro elastic modulus measured by AFM for Example 6 and Comparative Example 3

The following provides a detailed explanation of the issues involved in implementing the present invention.

The method for measuring the elastic modulus of vulcanized rubber according to the embodiment is a method for measuring the elastic modulus of vulcanized rubber using an atomic force microscope (AFM), and includes a step of obtaining a sample in which the internal strain in the vulcanized rubber containing a filler is reduced by applying elastic deformation to the vulcanized rubber (hereinafter referred to as step 1), and a step of measuring the elastic modulus of the obtained sample by measuring the force curve with the atomic force microscope (hereinafter referred to as step 2).

The vulcanized rubber to be measured contains a filler. Examples of the filler include reinforcing fillers such as carbon black and/or silica.

Carbon black is not particularly limited, and examples thereof include furnace black, acetylene black, thermal black, channel black, etc. More specifically, various furnace blacks such as SAF grade (N100 series), ISAF grade (N200 series), HAF grade (N300 series), FEF grade (N500 series), and GPF grade (N600 series) (all ASTM grades) may be used.

The silica is not particularly limited, and wet silica such as wet precipitation silica or wet gel silica may be used.

Vulcanized rubber is made by vulcanizing a rubber composition that contains a rubber polymer and a filler.

The rubber polymer is not particularly limited, and examples thereof include diene rubbers such as natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), butyl rubber (IIR), halogenated butyl rubber (X-IIR), and styrene isoprene butadiene rubber (SIBR), which may be used alone or in a blend of two or more types.

The amount of filler contained is not particularly limited, but is preferably 40 to 150 parts by mass, and more preferably 40 to 100 parts by mass, per 100 parts by mass of rubber polymer. In general, the greater the filler content, the greater the internal strain of the vulcanized rubber. The method of this embodiment is more effective for vulcanized rubber with greater internal strain due to vulcanization, so the greater the filler content, the more preferable it is.

In addition to the rubber polymer and filler, the rubber composition may contain various optional compounding agents that are commonly used in the rubber industry. Examples of such compounding agents include vulcanizing agents, vulcanization accelerators, softeners, plasticizers, antioxidants, zinc oxide, stearic acid, wax, and silane coupling agents.

For example, sulfur is used as a vulcanizing agent. The amount of vulcanizing agent is not particularly limited, and may be, for example, 0.1 to 10 parts by mass, 0.3 to 5 parts by mass, or 0.5 to 3 parts by mass per 100 parts by mass of rubber polymer.

The vulcanized rubber is obtained by preparing a rubber composition by kneading each component in a mixer such as a Banbury mixer in a conventional manner, and then heating and vulcanizing the rubber composition in a conventional manner. The shape of the vulcanized rubber to be measured is not particularly limited, and a sheet-like shape may be used, for example.

The vulcanized rubber to be measured preferably has a ratio of the rebound resilience of the fourth hit to the rebound resilience of the first hit in the Lupke rebound resilience test (hereinafter referred to as the rebound ratio) of 1.04 or more. As described above, the method of this embodiment is more effective for vulcanized rubber with a larger internal strain. In the Lupke rebound resilience test, the greater the internal strain of the vulcanized rubber, the greater the change in the rebound resilience due to the number of hits. In other words, vulcanized rubber with a ratio (rebound ratio) of the rebound resilience of the fourth hit to the rebound resilience of the first hit of 1.04 or more has a large internal strain, so if the vulcanized rubber has such a large rebound ratio, the effect of this embodiment of reducing the internal strain and measuring the correct elastic modulus can be enhanced. It is more preferable that the rebound ratio is 1.05 or more. There is no particular limit to the upper limit of the rebound ratio, but it is usually 1.30 or less.

In this embodiment, in step 1, elastic deformation is applied to the vulcanized rubber. Internal strain exists in the vulcanized rubber. By applying macroscopic elastic deformation to the vulcanized rubber, the internal strain can be reduced. Therefore, a sample with reduced internal strain in the vulcanized rubber can be obtained.

The elastic deformation may be, for example, elongation or compression, but is preferably elongated to the vulcanized rubber. The elongation rate when elongating the vulcanized rubber is not particularly limited, but is preferably 10 to 150%, and more preferably 30 to 100%. An elongation rate of 10% or more can enhance the effect of reducing internal strain. Also, an elongation rate of 150% or less can reduce the influence of residual strain due to elongation. The elongation rate refers to the ratio of the elongation at maximum deformation to the length of the sample before deformation.

When elastic deformation is applied to vulcanized rubber, it may be applied only once, but it may also be applied repeatedly multiple times. In this case, the number of times that elastic deformation is repeated is not particularly limited, and may be, for example, 2 to 10 times, or 3 to 5 times.

After the vulcanized rubber is thus subjected to preliminary elastic deformation to remove the internal strain, in step 2, the elastic modulus of the vulcanized rubber sample is measured by measuring the force curve with an atomic force microscope. The sample used for measurement with an atomic force microscope may be the elastically deformed vulcanized rubber as is, or it may be vulcanized rubber cut into a predetermined shape from the elastically deformed vulcanized rubber so that measurement with an atomic force microscope is possible.

An atomic force microscope (AFM) is a type of scanning probe microscope that detects the force acting between atoms in a sample and a probe. The probe is attached to the end of a cantilever (cantilever spring), and the force (deflection) acting on the cantilever is measured while changing the distance between the sample and the probe to obtain a force curve, which is a curve plotting the relationship between the two. By analyzing this force curve, the elastic modulus (hardness) of the sample surface can be determined, and the elastic modulus of rubber materials can be measured at the nano level. The method of determining the elastic modulus of a sample surface by force curve measurement in this way is itself publicly known, and can be carried out using such publicly known methods.

As a method for calculating the elastic modulus from a force curve, for example, there is a method for calculating the elastic modulus by fitting the force curve according to the Johnson-Kendall-Roberts (JKR) theory. In the JKR theory, the force F applied to the cantilever and the sample deformation amount δ are expressed by the following formulas (1) and (2) with adhesion energy w.

Here, a is the radius of the contact line between the probe and the sample, R is the radius of curvature of the probe tip, and K is the elastic modulus. The elastic modulus can be calculated by fitting the F-δ curve obtained by force curve measurement with equations (1) and (2).

In one embodiment, the elastic modulus of the sample may be obtained by scanning a specified range of the sample surface to obtain force curves at multiple points within the specified range, determining the elastic modulus from each force curve, and calculating the average value of the elastic modulus.

In one embodiment, the filler component, which is a filler, may be identified from the indentation depth, elastic modulus, or adhesion energy obtained from the force curve measurement, and the portion excluding the filler component may be regarded as the rubber component, and the elastic modulus of the sample may be obtained by calculating the average elastic modulus of the rubber component. When identifying the filler component, the volume fraction of the filler may be calculated from the content of the filler blended in the vulcanized rubber, and the portion with a small indentation depth, a high elastic modulus, or a low adhesion energy corresponding to the filler may be identified as the filler component based on the volume fraction. Alternatively, the threshold value of the indentation depth, elastic modulus, or adhesion energy corresponding to the filler component may be specified in advance, and the portion with the indentation depth equal to or less than the specified value, the elastic modulus equal to or more than the specified value, or the adhesion energy equal to or less than the specified value may be identified as the filler component.

According to this embodiment, by applying preliminary elastic deformation to the vulcanized rubber prior to measurement with an atomic force microscope, the internal strain of the vulcanized rubber can be reduced, thereby improving the accuracy of measuring the elastic modulus with an atomic force microscope. Therefore, for example, this can be useful in determining the type and amount of filler to be used in the development of rubber materials.

The following are examples of the present invention, but the present invention is not limited to these examples.

[First embodiment]
Using a Banbury mixer, in accordance with the formulation (parts by mass) shown in Table 1 below, first, in the first mixing stage, the compounding ingredients except for sulfur and vulcanization accelerator were added to the rubber polymer and kneaded (discharge temperature = 160 ° C.). Next, in the final mixing stage, sulfur and vulcanization accelerator were added to the obtained kneaded product and kneaded (discharge temperature = 90 ° C.). In this way, an unvulcanized rubber composition was prepared. Details of each compounding ingredient are as follows.

SBR: Styrene butadiene rubber, "Tufden 2000" manufactured by Asahi Kasei Corporation
SAF: Carbon black SAF, "Seat 9" manufactured by Tokai Carbon Co., Ltd.
・Zinc oxide: "Zinc oxide No. 3" manufactured by Mitsui Mining & Smelting Co., Ltd.
Stearic acid: "Lunac S-20" manufactured by Kao Corporation
・Sulfur: "Powdered sulfur" manufactured by Tsurumi Chemical Industry Co., Ltd.
Vulcanization accelerator: "Soxinol CZ" manufactured by Sumitomo Chemical Co., Ltd.

The resulting unvulcanized rubber composition was press-vulcanized at 160°C for 20 minutes to produce a vulcanized rubber with a thickness of 1 mm. The resulting vulcanized rubber was subjected to preliminary elongation deformation in order to remove internal strain. The method for applying preliminary elongation deformation is as follows.

The test was carried out using an autograph manufactured by Shimadzu Corporation. A 1 mm thick vulcanized rubber sheet was punched into the shape of a No. 3 dumbbell, and a tensile test was carried out at a speed of 500 mm/min to give the sample elongations of 50% (Example 1), 100% (Example 2), 150% (Example 3), and 200% (Example 4). The tensile test was carried out for four cycles. The elongation rate is the ratio of the elongation to the test length before the test, with the gauge length of the dumbbell-shaped sample being the test length.

For the samples of Examples 1 to 4 that had been subjected to preliminary elongation deformation, the elastic modulus was measured by AFM using the portion between the gauge lines. For comparison, the elastic modulus was also measured by AFM for a vulcanized rubber sheet that had not been subjected to preliminary elongation deformation (elongation rate: 0%) as Comparative Example 1. The method for measuring the elastic modulus by AFM is as follows.

[Elasticity measurement by AFM]
The atomic force microscope used was a Bruker "Dimension Icon," and the cantilever used was an Olympus "OMCL-AC240TS-R3" (spring constant: 1.7 N/m). The measurement range was 3 μm x 3 μm square, and the number of measurement points was 128 x 128, totaling 16,384 points, and the force curve was measured at each point (measurement frequency: 10 Hz).

The F-δ curve obtained from the force curve was fitted using the JKR theory to calculate the elastic modulus. In addition, the components with low adhesion energy were determined from the adhesion energy obtained from the force curve as filler components, and the average elastic modulus was calculated by subtracting these components, and this was used as the elastic modulus of the vulcanized rubber measured by AFM (AFM elastic modulus). In this case, the filler components were identified by calculating the volume fraction of carbon black as a filler using the specific gravity of each compounding agent from the compounding amounts of the rubber composition, and the parts with high elastic modulus were identified as the filler components based on this volume fraction.

[Macro elastic modulus]
The macro elastic modulus of the vulcanized rubber sheet of the above formulation was measured. In detail, a dynamic viscoelasticity test was carried out in accordance with JIS K6394 using a dynamic viscoelasticity device manufactured by Ueshima Seisakusho. The test conditions were as follows: sample shape: 2 mm x 4 mm x 40 mm strip, measurement mode: tension, measurement temperature: 25°C, frequency: 10 Hz, static strain: 1%, dynamic strain: 0.05%. From the test results, the storage elastic modulus was calculated, and the value was used as the macro elastic modulus (mechanical property).

The results are shown in Table 1 and Figure 1. In Examples 1 to 4, in which the vulcanized rubber was subjected to preliminary elongation deformation by a tensile test, the AFM elastic modulus was lower than that of Comparative Example 1, in which preliminary elongation deformation was not applied, and was closer to the macro elastic modulus. Therefore, it can be seen that the internal strain caused by vulcanization was removed by preliminary elastic deformation, allowing for more accurate measurement of the elastic modulus. In Example 4, the AFM elastic modulus was higher than that of Examples 1 to 3, and tended to approach that of Comparative Example 1. This is thought to be due to the influence of residual strain caused by tension. For this reason, in order to remove internal strain caused by vulcanization while avoiding the influence of residual strain, it is preferable that the elongation rate is 150% or less.

[Second embodiment]
According to the composition (parts by mass) shown in Table 2 below, an unvulcanized rubber composition was prepared in the same manner as in Example 1 (details of each compounding agent were the same as in Example 1). The obtained unvulcanized rubber composition was press-vulcanized at 160°C for 20 minutes to prepare a vulcanized rubber sheet with a thickness of 1 mm. The obtained vulcanized rubber sheet was subjected to preliminary elongation deformation in order to remove internal strain. The method of applying preliminary elongation deformation was the same as in Example 1, and all elongation rates were set to 50%.

The elastic modulus of the sample that had been subjected to the above-mentioned pre-stretch deformation (with pre-stretching) was measured by AFM using the portion between the gauge lines (Example 5). For comparison, the elastic modulus of a vulcanized rubber sheet that had not been subjected to pre-stretching deformation (without pre-stretching) was also measured by AFM (Comparative Example 2). The method for measuring the elastic modulus by AFM was the same as in Example 1.

In addition, for the vulcanized rubber sheets of each compound, the macro elastic modulus was measured, and the rebound ratio was measured to evaluate the magnitude of internal strain. The method for measuring the macro elastic modulus was the same as in Example 1, and the method for measuring the rebound ratio was as follows.

[Rebound ratio]
Using a Lübke rebound resilience test, the rebound resilience of the first hit (the percentage of the height after rebound relative to the drop height) and the rebound resilience of the fourth hit were measured, and the rebound ratio, which is the ratio of the two (fourth rebound resilience/first rebound resilience), was calculated.

The results are shown in Table 2 and Figure 2. In Example 5, in which the vulcanized rubber was given a preliminary elongation deformation prior to AFM measurement, the AFM elastic modulus and the macroelastic modulus were in a nearly linear proportional relationship. In contrast, in Comparative Example 2, in which the vulcanized rubber was not given a preliminary elongation deformation prior to AFM measurement, the linear proportional relationship between the AFM elastic modulus and the macroelastic modulus was broken as the amount of carbon black blended increased and the internal strain (rebound ratio) increased. This shows that the internal strain is removed by giving a preliminary elongation deformation, allowing the elastic modulus to be measured accurately by AFM. It is also clear that the greater the amount of filler blended, the greater the effect of removing the internal strain.

[Third Example]
An unvulcanized rubber composition was prepared in the same manner as in Example 2 according to the composition (parts by mass) shown in Table 3 below. Regarding the details of each compounding agent, HAF is carbon black HAF, N339 is "Seest KH" manufactured by Tokai Carbon Co., Ltd., and the other compounding agents are the same as those in Example 1.

Using the obtained unvulcanized rubber composition, a vulcanized rubber sheet was prepared in the same manner as in Example 2, and a pre-elongation deformation (elongation rate: 50%) was given to the vulcanized rubber sheet in order to remove internal strain. The elastic modulus of the sample that had been given the pre-elongation deformation (with pre-elongation) was measured by AFM in the same manner as in Example 2 (Example 6), and for comparison, the elastic modulus of the vulcanized rubber sheet that had not been given the pre-elongation deformation (without pre-elongation) was also measured by AFM (Comparative Example 3). Furthermore, the macro elastic modulus and rebound ratio were measured for the vulcanized rubber sheets of each formulation in the same manner as in Example 2.

The results are shown in Table 3 and Figure 3. Even when the carbon black was changed from SAF to HAF, which has a smaller specific surface area, the same tendency as in Example 2 was observed. That is, in Example 6, in which the vulcanized rubber was given a preliminary elongation deformation prior to AFM measurement, the AFM elastic modulus and the macro elastic modulus were in a nearly linear proportional relationship. In contrast, in Comparative Example 3, in which the vulcanized rubber was not given a preliminary elongation deformation prior to AFM measurement, the linear proportional relationship between the AFM elastic modulus and the macro elastic modulus was broken as the amount of carbon black added increased and the internal strain (rebound ratio) increased.

Claims (3)

A method for measuring the elastic modulus of vulcanized rubber using an atomic force microscope, comprising:
The method includes: obtaining a sample in which internal strain in the vulcanized rubber is reduced by applying elastic deformation to a vulcanized rubber containing a filler; and measuring the elastic modulus of the obtained sample by force curve measurement using an atomic force microscope ;
The vulcanized rubber to be measured has a ratio of the rebound resilience of the fourth impact to the rebound resilience of the first impact in a Lupke rebound resilience test of 1.04 or more.
Method for measuring elastic modulus of vulcanized rubber. The method for measuring the elastic modulus of vulcanized rubber according to claim 1, wherein the elastic deformation is elongation. The method for measuring the elastic modulus of vulcanized rubber according to claim 2, wherein the elongation rate of the elongation applied to the vulcanized rubber is 10 to 150%.