JP7175744B2 - Sulfur chemical state analysis method - Google Patents

Description

The present invention relates to a method for analyzing sulfur chemical states in sulfur-containing rubber materials.

In a rubber material, a cross-linking reaction (also referred to as a vulcanization reaction) that forms a three-dimensional network structure by introducing cross-linking points connecting rubber molecules is essential for developing rubber elasticity. Conventionally, as a method for examining the degree of progress of the cross-linking reaction, analysis based on mechanical properties, in which changes in torque values are measured using a rheometer, is generally used. However, analysis based on mechanical properties is an indirect method, and a more direct method for analyzing the cross-linking reaction is required.

In Non-Patent Document 1, as a method of dynamically tracking the cross-linking reaction (in-situ measurement), an X-ray absorption spectrum of the K-shell absorption edge of zinc can be obtained while heating the unvulcanized rubber material. Have been described. In this document, attention is paid to zinc oxide blended in the unvulcanized rubber material, and the process in which zinc oxide (ZnO) changes to zinc sulfide (ZnS) as the crosslinking reaction progresses is investigated by XAFS with zinc as the target element. It is analyzed from the change of the spectrum.

By the way, Non-Patent Document 2 describes that the thermal reaction of sulfenamide, which is a vulcanization accelerator, is analyzed by XAFS measurement of the sulfur K-shell absorption edge. However, in this document, an X-ray absorption spectrum is obtained by irradiating X-rays while heating the vulcanization accelerator powder as it is, and analysis using a rubber material is not described.

JP 2017-198548 A

Kenzo Fukumori, "Crosslinking Reaction Analysis of Sulfur Crosslinking Compound Rubber by Time Division XAFS Method", Journal of the Japan Rubber Association, Vol. 87, No. 3, 2014, pp. 83-88 S. Tanaka and 5 others, "A study on thermal reaction with sulfenamide as vulcanization accelerator by means of S K-edge NEXAFS", HiSOR Activity Report 2015, pp. 44-45

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method capable of analyzing changes in the chemical state of sulfur due to heating in rubber materials containing sulfur.

In the sulfur chemical state analysis method according to the present invention, a sulfur-containing rubber material is irradiated with X-rays while being heated to obtain an X-ray absorption spectrum of the sulfur K-shell absorption edge, thereby determining the chemical state of sulfur. It analyzes changes.

In one embodiment, the X-ray absorption spectrum is fitted with at least two components including a sulfur-sulfur component and a sulfur-carbon component, and the peak area of the sulfur-sulfur component and the peak area of the sulfur-carbon component At least one of and may be calculated to see a change in at least one of them. Further, the X-ray absorption spectrum is fitted with at least three components including a sulfur-sulfur component, a sulfur-carbon component and a sulfur-zinc component, and the peak area of the sulfur-zinc component is calculated to calculate the sulfur - You may look at changes in the inter-zinc content.

According to the present invention, it is possible to analyze changes in the chemical state of sulfur due to heating of a sulfur-containing rubber material.

Diagram showing change in X-ray absorption spectrum with heating time Conceptual diagram showing a configuration example around a measurement sample in an X-ray measurement device Conceptual diagram showing another configuration example around the measurement sample in the X-ray measurement device Figure showing an example of X-ray absorption spectrum of sulfur K-shell absorption edge Diagram showing the asymmetric Gaussian function used for the sulfur-sulfur component Graph showing changes in sulfur-sulfur component, sulfur-carbon component, and sulfur-zinc component with heating time Diagram showing changes in ratio of sulfur-sulfur component and sulfur-carbon component with heating time

Matters related to the implementation of the present invention will be described in detail below.

In the present embodiment, the sulfur-containing rubber material to be measured may be an unvulcanized rubber material that is not crosslinked (that is, vulcanized), or a crosslinked vulcanized rubber material, as long as it contains sulfur. A rubber material may also be used. Here, the sulfur contained in the rubber material may be sulfur alone, or may be sulfur in a compound containing a sulfur atom in the molecule (sulfur-containing compound) such as a vulcanization accelerator.

In one embodiment, the rubber material is obtained by blending rubber polymers with various compounding agents including a vulcanizing agent such as sulfur. Examples of rubber polymers include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), nitrile rubber (NBR), chloroprene rubber (CR), butyl rubber (IIR), halogen Diene-based rubbers such as butylated butyl rubber (X-IIR) and styrene isoprene-butadiene rubber (SIBR) can be mentioned, and these can be used alone or in a blend of two or more.

The rubber material may be compounded with sulfur as a vulcanizing agent, and examples thereof include sulfur such as powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, and highly dispersible sulfur. As one embodiment, the amount of sulfur compounded may be 0.1 to 10 parts by mass or 0.5 to 8 parts by mass with respect to 100 parts by mass of the rubber polymer.

The rubber material may be compounded with a vulcanization accelerator comprising a sulfur-containing compound together with or instead of sulfur. Vulcanization accelerators that are not sulfur-containing compounds may also be used. The amount of the vulcanization accelerator compounded may be, for example, 0.1 to 7 parts by mass or 0.5 to 5 parts by mass with respect to 100 parts by mass of the rubber polymer.

Rubber materials also include zinc oxide, fillers, silane coupling agents, softeners such as oils, plasticizers, anti-aging agents, stearic acid, waxes, vulcanization accelerators, and various other substances commonly used in the rubber industry. Compounding agents can be compounded. The amount of zinc oxide compounded may be, for example, 0.1 to 10 parts by mass or 0.5 to 5 parts by mass with respect to 100 parts by mass of the rubber polymer. Fillers include, for example, various inorganic fillers such as carbon black and/or silica. The amount of the filler compounded may be, for example, 10 to 200 parts by mass or 20 to 150 parts by mass with respect to 100 parts by mass of the rubber polymer.

The unvulcanized rubber material can be produced, for example, by kneading each component according to a conventional method using a mixer such as a Banbury mixer. Alternatively, a vulcanized rubber material obtained by heating and vulcanizing the unvulcanized rubber material according to a conventional method may be used as the object to be measured.

The shape of the rubber material to be measured is not particularly limited, and for example, a sheet-like one can be used. In one embodiment, the object to be measured may be an unvulcanized rubber sheet molded into a sheet shape, a vulcanized rubber sheet obtained by vulcanizing the unvulcanized rubber sheet, or a vulcanized tire or the like. A sheet cut out from a rubber product may also be used.

In this embodiment, the rubber material is irradiated with X-rays while being heated to obtain an X-ray absorption spectrum of the sulfur K-shell absorption edge. Specifically, while the rubber material is being heated, the rubber material is irradiated with X-rays at predetermined time intervals to measure the sulfur absorption edge by the XAFS method in order to examine the thermal change of the rubber material over time.

In general, an X-ray absorption spectrum obtained by irradiating an object with X-rays has a steep rise characteristic of elements contained in the object, and this rise is called an absorption edge. The fine structure near this absorption edge is called x-ray absorption fine structure (XAFS). XAFS consists of an X-ray absorption near edge structure (XANES) up to several tens of eV from the absorption edge and an X-ray wide area fine structure (XANES) that appears in a range up to about 1000 eV on the higher energy side. EXAFS: extended x-ray absorption fine structure). Of these, XANES is sensitive to chemical states such as electronic states, and can be applied to analysis of chemical states such as to what kind of atoms a target atom is bonded. According to one aspect of the present embodiment, an X-ray absorption spectrum in the XANES region is obtained for the sulfur K-shell absorption edge, which is the K-shell absorption edge of elemental sulfur.

The XAFS method (particularly the XANES method) can be used as a method for obtaining the X-ray absorption spectrum of the sulfur K-shell absorption edge. Specifically, the rubber material is irradiated with X-rays, and the amount of X-ray absorption (absorption intensity) is measured while changing the energy of the X-rays. X-rays are irradiated at energies corresponding to the K-shell absorption edge of elemental sulfur, which yields an X-ray absorption spectrum in the XANES region for the sulfur K-shell. The X-ray scanning energy range is preferably 2400 to 3000 eV, may be 2450 to 2500 eV, or may be 2460 to 2490 eV.

In the XAFS method at the sulfur K-shell absorption edge (hereinafter also referred to as the sulfur XAFS method), (1) a transmission method in which the X-ray intensity transmitted through the sample is detected using a photodiode array detector or the like, (2 ) a fluorescence method that detects fluorescent X-rays generated when a sample is irradiated with X-rays using a Lytle detector or a semiconductor detector, and (3) a current that flows when a sample is irradiated with X-rays. There is an electron yield method for detection, and any of them may be used. Preferably, a fluorescence method is used. More specifically, the fluorescence method is a method of measuring fluorescent X-rays generated when a sample is irradiated with X-rays. This is a method of indirectly obtaining the X-ray absorption amount from the intensity of fluorescent X-rays.

X-rays used in the sulfur XAFS method are preferably high-intensity X-rays of, for example, 10 ¹⁰ (photons/s/mrad ² /mm ² /0.1% bw) or more. The number of X-ray photons is preferably 10 ⁷ (photons/s) or more, more preferably 10 ⁹ (photons/s) or more. Synchrotrons that emit such X-rays include SPring-8 of Japan Synchrotron Radiation Research Center and Aichi Synchrotron Light Center of ``Knowledge Hub Aichi''.

In this embodiment, when the X-ray absorption spectrum is obtained by the sulfur XAFS method, the rubber material is irradiated with X-rays at predetermined time intervals while being heated. By irradiating X-rays while the rubber material is heated to a high temperature, the obtained X-ray absorption spectrum changes with time, as shown in FIG. That is, the time-dependent change of the rubber material due to heat can be represented by the X-ray absorption spectrum.

The heating temperature of the rubber material can be appropriately set according to the type of rubber material, the purpose of analysis, etc., and is not particularly limited, but may be, for example, 80 to 250°C or 100 to 200°C. Alternatively, the sulfur XAFS method may be measured at predetermined time intervals while the temperature is raised from room temperature to analyze changes in the chemical state of sulfur during the temperature rise. The heating rate is not particularly limited, but may be, for example, 5 to 20°C/min. The time interval for acquiring the X-ray absorption spectrum is also not particularly limited because it is appropriately set depending on the type of rubber material, the purpose of analysis, etc. For example, it may be acquired every 2 to 20 minutes, or 3 to 10 minutes. The time interval may not necessarily be constant.

The heating device for heating the rubber material is not particularly limited as long as it can irradiate X-rays while heating. For example, as shown in FIG. 2, a sheet-like rubber material as a measurement sample may be fixed to the front surface of a heater such as a ceramic heater, and X-rays may be irradiated from the front surface of the rubber material. A thermocouple is attached to the rubber material, and the rubber material can be heated while monitoring the temperature inside the rubber material with the thermocouple.

Also, in order to heat more uniformly, the sheet-like rubber material may be sandwiched between heaters on both sides. For example, as shown in FIG. 3, a pair of plate-like heaters are provided, and a sheet-like rubber material is sandwiched and fixed between them. The front heater is provided with an opening through which X-rays pass, and the X-rays incident through this opening are applied to the rubber material, and the fluorescent X-rays generated thereby pass through the opening to the outside. released. As in the case of FIG. 2, a thermocouple is attached to the rubber material so as to heat while monitoring the internal temperature with the thermocouple.

In this embodiment, changes in the chemical state of sulfur are analyzed using a plurality of X-ray absorption spectra obtained above. For example, an unvulcanized rubber material may be used as a measurement sample to investigate changes in the chemical state of sulfur during the cross-linking reaction. In this case, since the cross-linking reaction can be dynamically tracked, in-situ vulcanization reaction analysis can be performed. The change from the unvulcanized state to the vulcanized state may be analyzed, and the analysis may also include reversion after proper vulcanization. Alternatively, an unvulcanized rubber material may be used as a measurement sample and heated at a relatively low temperature for analysis of scorch. Alternatively, a vulcanized rubber material may be used as a measurement sample to investigate changes in the chemical state of sulfur during the thermal degradation process (thermal degradation analysis).

When analyzing the change in the chemical state of sulfur from the X-ray absorption spectrum, for example, for each of the plurality of X-ray absorption spectra, the following fitting is performed to calculate parameters that serve as indicators for analysis, Just look at the change over time.

For example, in one embodiment, an X-ray absorption spectrum is obtained from at least two components including a sulfur-sulfur component (hereinafter referred to as SS component) and a sulfur-carbon component (hereinafter referred to as SC component). may be fitted to calculate at least one of the peak area of sulfur-sulfur components and the peak area of sulfur-carbon components to see changes in at least one of them.

Fitting is more preferably performed using at least three components including an SS component, an SC component and a sulfur-zinc component (hereinafter referred to as S-Zn component), and more preferably, As an example is shown in FIG. 4, fitting is performed using multiple scattering components and step function components together with SS, SC and S-Zn components.

Since the X-ray absorption at the sulfur K-shell absorption edge is mainly due to the SS component and the SC component, it is preferable to fit the X-ray absorption spectrum using these two components, and calculate the area of each of these peaks. can do. In addition to the SS component and the SC component, in addition to the SS component and the SC component, S The -Zn component may be used for fitting, whereby the peak area of the S-Zn component can be calculated. In addition to the S—S, SC, and S—Zn components, the X-ray absorption at the sulfur K-shell absorption edge is also affected by multiple scattering components and step function components. By fitting the X-ray absorption spectrum with the components, each peak area can be calculated with higher accuracy.

Here, the SS component is an X-ray absorbing component based on the SS bond, and the SC component is an X-ray absorbing component based on the SC bond. The sulfur cross-linked structure in rubber materials is a structure in which carbon (C) in the rubber polymer chain is bonded through sulfur (S), the bond between the sulfur atoms in the cross-linked portion is an S—S bond, and the rubber polymer chain is the SC bond.

The S-Zn component is an X-ray absorbing component based on the S-Zn bond, taking into account the X-ray absorption by zinc sulfide (ZnS) generated by the reaction of zinc oxide (ZnO) blended in the rubber material. It is.

A multiple scattering component is an X-ray absorbing component based on multiple scattering by photoelectrons in the XANES region. The step function component is the x-ray absorption component based on electron transitions to the continuum.

As the function used when fitting the X-ray absorption spectrum, various functions can be used as long as they can express each of the above components.

For example, Gaussian functions may be used for SC components, S-Zn components, and multiple scattering components. As the Gaussian function, for example, one represented by the following formula (1) can be used.

In formula (1), a is the peak height (peak intensity), b is the X-ray energy (eV) at the peak top, c is the peak half-width (eV), and x is the irradiation X-ray energy (eV). .

A sigmoid function may be used as the step function component. As the sigmoid function, for example, one represented by the following formula (2) can be used.

In equation (2), d is the edge jump height, e is a constant, and f is an ionization potential (eV).

The SS component can be expressed using a Gaussian function with a bilaterally symmetrical distribution, but considering the fluctuation of the SS bond length due to thermal vibration of the bridged sulfur chain, it has a bilaterally asymmetrical distribution. Preferably, an asymmetric Gaussian function is used. An asymmetric Gaussian function can be expressed by adding a plurality of Gaussian functions represented by the above equation (1). As shown in FIG. 5, the reference Gaussian function (C1 function) represented by the above equation ( ₁ ) is defined, and the peak _top is shifted to the high energy side of the C1 function at equal intervals and the peak height is equal Define a plurality of Gaussian functions (C _m functions: C ₁ , C ₂ , . In the C ₁ function, the above a, b and c are constants, and for the C _m function (m = 2 ~) after the C ₁ function, the equal difference reduction value between the shift width of the peak top and the peak height is determined, Define _{m Cm} -functions. At that time, the product of the half width of the _Cm function and the peak height is assumed to be constant. Summing _m Cm-functions yields an asymmetric Gaussian function. For the obtained asymmetric Gaussian function, the above fitting is preferably performed with the X-ray energy (eV) at the peak top as a constant and the peak height as a variable.

The method of fitting (also referred to as curve fitting) to the X-ray absorption spectrum using each of the above components is not particularly limited, and a general method can be used. For example, fitting may be performed so that the function obtained by adding the functions of each component and the residual sum of squares of the X-ray absorption spectrum approaches zero. Thereby, the X-ray absorption spectrum can be peak-separated into each component. That is, a curve after fitting processing is obtained for each component. FIG. 4 shows each curve of the above five components after fitting processing and a synthetic curve (approximate curve by fitting) of these five components.

The peak area of each component can be calculated from the obtained curve after the fitting process of each component. Here, the peak area is the area of the portion surrounded by each fitting curve within the measurement range.

For example, from the fitting curves of the SS component and the SC component, it is possible to calculate the peak area of the SS component and the peak area of the SC component, respectively, so changes in at least one of these peak areas By observing, for example, the cross-linking reaction can be dynamically tracked, and the deterioration process due to heat can be analyzed. As one embodiment, the peak area of the SS component and the peak area of the SC component are calculated, and the respective changes are examined, and the ratio of the peak area of the SS component and the peak area of the SC component can be obtained to examine its change.

Alternatively, the peak area of the S--Zn component may be calculated from the fitting curve of the S--Zn component and its change may be observed. The larger the peak area, the more the reaction from zinc oxide to zinc sulfide progressed, so the degree of progress of the cross-linking reaction can be grasped.

Examples of the present invention are shown below, but the present invention is not limited to these examples.

Using a Banbury mixer, 100 parts by mass of SBR, 2 parts by mass of zinc oxide, 1 part by mass of stearic acid, 3 parts by mass of sulfur, and 1 part by mass of a vulcanization accelerator are kneaded, and then pressed at a low temperature of 100 ° C. to obtain a thickness. An unvulcanized rubber sheet of 1.0 mm was obtained.

Details of each component are as follows.
・SBR: “JSR1502” manufactured by JSR Corporation
・Zinc oxide: Mitsui Mining & Smelting Co., Ltd. “zinc white 1”
・ Stearic acid: “Lunac S-20” manufactured by Kao Corporation
・ Sulfur: “Rubber powder sulfur 150 mesh” manufactured by Hosoi Chemical Industry Co., Ltd.
- Vulcanization accelerator: "Noccellar CZ" manufactured by Ouchi Shinko Kagaku Kogyo Co., Ltd.

The obtained unvulcanized rubber sheet was used as a measurement sample, and was fixed to the front surface of a ceramic heater as shown in FIG. C./min to 140.degree. C. After reaching 140.degree. C., the temperature was maintained at 140.degree. The XANES measurement at the sulfur K-shell absorption edge was performed with the point of time when the temperature reached 140° C. being 0 minutes, and thereafter, the XANES measurement at the sulfur K-shell absorption edge was performed every 3 to 5 minutes. The XANES measurement was carried out under the following measurement conditions at the Aichi Synchrotron Light Center of "Knowledge Hub Aichi".
・X-ray brightness: 2.0×10 ¹² photons/s/mrad ² /mm ² /0.1%bw
・Number of X-ray photons: ~3.0×10 ¹⁰ photons/s
Spectrometer: crystal spectrometer X-ray detector: silicon drift detector Measurement method: fluorescence method X-ray energy range: 2400 to 2500 eV.

Some of the multiple X-ray absorption spectra obtained by XANES measurement are shown in FIG. Each of the obtained multiple X-ray absorption spectra was fitted with five components: SS component, SC component, S-Zn component, multiple scattering component and step function component. At that time, the Gaussian function of equation (1) was used for the SC component, the S-Zn component, and the multiple scattering component. The parameters in formula (1) are, for the SC component, a (peak height) as a variable, b (energy at the peak top) as 2473 eV (constant), and c (peak half width) as 1.8 eV. For the S-Zn component, a and b are variables and c is 1.8 eV (constant), and for the multiple scattering component, a and b are variables and c is 4 eV (constant). For the step function component, the sigmoid function of Equation (2) is used. The parameters in equation (2) were set such that d (edge jump height) is a variable, e (constant)=0.7, and f (ionization potential)=2476 eV (constant).

An asymmetrical Gaussian function was used for the SS component. The asymmetric Gaussian function is represented by adding multiple Gaussian functions in Equation (1). Specifically, using formula ( ₁ ), the C1 function with a of 2 (constant) and b of 2471.1 eV (constant) is determined, and the peak _top is on the high energy side in order from the C1 function. 100 C _m -functions (m=1-100) were defined that shift in intervals (0.015 eV) and decrease in peak heights by equal amounts (0.003). At that time, the _Cm function was defined so that the product of the peak height and the half width was a constant value (2.8). By summing these 100 C _m -functions, an asymmetric Gaussian function of the SS component was obtained. The peak top energy (eV) of the asymmetric Gaussian function was set to 2472 eV (constant), and the peak height was used as a variable.

A function obtained by summing the five components defined in this way: the SS component, the SC component, the S-Zn component, the multiple scattering component, and the step function component, and the residual sum of squares of the measured spectrum approaches 0. Fitting was performed as described above, and the peak areas of the SS, SC and S-Zn components were calculated. In addition, the ratio of the peak area of the SS component and the peak area of the SC component (bonding ratio of SS and SC: the ratio of the respective peak areas with the sum of the peak areas of both as 100%) asked. The results are shown in FIGS. 6 and 7. FIG.

FIG. 6 also shows changes in torque values measured using a rheometer for unvulcanized rubber prepared in the same manner as the unvulcanized rubber sheet. The rheometer was measured at 140°C for 90 minutes in accordance with JIS K6300.

As shown in FIGS. 6 and 7, the peak area of the SC component slightly decreased, but the peak area of the SS component increased slightly until about 10 minutes after the start of measurement. This is considered to be due to the reaction of the vulcanization accelerator contained in the unvulcanized rubber sheet.

About 10 minutes after the start of the measurement, the peak area of the SC component decreased and then tended to be almost constant or slightly increasing, while the peak area of the SS component decreased significantly. The ratio of SC component peak areas increased. From the results of rheometer measurements, it was found that cross-linking started at this stage, and as the cross-linking progressed, the S--S bonds decreased and the proportion of S--C bonds relatively increased. This is presumably because the 8-membered ring of sulfur, which is a vulcanizing agent, is cut when cross-linking begins.

In addition, after about 30 minutes from the start of measurement, the peak area of the SS component decreased significantly and became almost constant once, but after about 45 minutes, it started to decrease gradually. It was suggested that reversion (reversion to vulcanization) occurred and cross-linking was broken.

On the other hand, the amount of change in the S--Zn component was smaller than that of the S--S component, but increased with the rise of the torque of the rheometer, suggesting the formation of zinc sulfide (ZnS).

As described above, according to this example, the changes in the chemical state of sulfur during the cross-linking reaction could be analyzed by measuring the sulfur XAFS in-situ while heating the unvulcanized rubber sheet.

Claims (2)

A method for analyzing changes in the chemical state of sulfur by irradiating an unvulcanized rubber material containing sulfur with X-rays while heating to obtain an X-ray absorption spectrum of the sulfur K-shell absorption edge. ,
The X-ray absorption spectrum is fitted with at least three components including a sulfur-sulfur component, a sulfur-carbon component and a sulfur-zinc component, and the peak area of the sulfur-zinc component is calculated to calculate the sulfur-zinc Sulfur chemical state analysis method to see changes in intermetallic components . 2. The sulfur chemical state analysis method according to claim 1, wherein at least one of a sulfur -sulfur component peak area and a sulfur-carbon component peak area is calculated and changes in at least one are observed.

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