Method for measuring content of free acrylonitrile in acrylonitrile-butadiene rubber

Abstract

The invention relates to a method for determining the content of free acrylonitrile in acrylonitrile-butadiene rubber, which comprises the following steps: weighing a detected (H) NBR sample, putting the sample into a headspace bottle, then transferring a BHT-THN solution, capping and sealing, feeding into a headspace gas chromatograph-mass spectrometer, determining the total ion chromatogram of the BHT-THN solution, integrating the chromatographic peak from acrylonitrile to obtain the peak area, and calculating the content of the free acrylonitrile in the acrylonitrile-butadiene rubber. And then substituting into a standard working curve of free acrylonitrile concentration and peak area to obtain the content of free acrylonitrile in the (H) NBR sample to be detected. Compared with the prior art, the method has the advantages that the problem that the free acrylonitrile in the (H) NBR is difficult to detect is solved, and the accuracy, the detection limit and the like of the content detection of the free acrylonitrile in the (H) NBR can be further improved.

Description

Technical Field

[0001] This invention belongs to the field of acrylonitrile content determination technology, and relates to a method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber. Background Technology

[0002] Hydrogenated nitrile butadiene rubber (HNBR) is a rubber material with a highly saturated molecular backbone, primarily produced by hydrogenating nitrile butadiene rubber (NBR). The acrylonitrile units in the HNBR molecular chain provide excellent oil resistance and high tensile strength, while the hydrogenated butadiene units offer good heat resistance, aging resistance, and low-temperature performance. Simultaneously, the remaining small amount of butadiene provides the unsaturated bonds required for rubber crosslinking, maintaining its elastic characteristics. Therefore, HNBR is widely used in the automotive, petroleum, and machinery industries, particularly in static and dynamic sealing components requiring oil resistance, aging resistance, high-temperature resistance, tear resistance, and compression set resistance.

[0003] Acrylonitrile in HNBR is divided into bound acrylonitrile and free acrylonitrile, and their content determines the various application properties of HNBR. The content of bound acrylonitrile affects many properties of HNBR, such as heat resistance, chemical stability, oil resistance, airtightness, elasticity, and cold resistance. Currently, the main methods for determining bound acrylonitrile include infrared spectroscopy, gas chromatography, and Kjeldahl nitrogen determination (Xu Jiao, Tan Shuaixia, Yao Zhiping, et al. Research progress on the determination of bound acrylonitrile content in NBR [J]. Special Rubber Products, 2014, 35(05):74-80.). Excessive free acrylonitrile content will deactivate the catalyst in the hydrogenation reaction of NBR. Furthermore, since free acrylonitrile is a volatile and toxic substance, its content will affect the final use of the product. Domestic NBR users and NBR products exported from China have clearly required that the content of free acrylonitrile be limited. Therefore, the detection of its content is particularly important.

[0004] Currently, the content of free acrylonitrile is mainly determined by headspace gas chromatography (Petrochemical Technology, 2015, 44(06):758-761.). The spiked recovery rate of this method is 94.8%–98.6%, the relative standard deviation is less than 3.0% (n=6), and the limit of detection is 1.8 mg / kg. It can be seen that there is still room for improvement in the detection limit and accuracy of this method. Therefore, it is of great practical significance to further improve the accuracy and detection limit of free acrylonitrile content in (H)NBR, so as to achieve high-precision detection of free acrylonitrile in the above-mentioned (H)NBR. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber, so as to further improve the accuracy and detection limit of the free acrylonitrile content detection in (H)NBR.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber involves weighing the (H)NBR sample to be tested, placing it in a headspace vial, then transferring BHT-THN solution, sealing the vial, and sending it into a headspace gas chromatography-mass spectrometry (GC-MS) instrument to determine its total ion chromatogram. The peak area is obtained by integrating the chromatographic peaks from acrylonitrile, and then substituting the peak area into a standard working curve of free acrylonitrile concentration versus peak area to obtain the free acrylonitrile content in the (H)NBR sample to be tested.

[0008] Furthermore, the tested (H)NBR sample is an NBR, a partially hydrogenated or fully hydrogenated NBR.

[0009] Furthermore, in the BHT-THN solution, the mass concentration of BHT (2,6-di-tert-butyl-4-methylphenol) is 180-220 mg / L, and the amount of the tested (H)NBR sample added to the BHT-THN (tetrahydronaphthalene) solution is 0.3 g: 4-6 L.

[0010] Furthermore, in the headspace gas chromatography-mass spectrometry system, the headspace gas chromatography conditions are as follows:

[0011] The chromatographic column was a polyethylene glycol capillary column modified with nitroterephthalic acid; detection was performed using a single quadrupole mass spectrometer.

[0012] The headspace heating temperature is 90–120℃, the headspace equilibration time is 30–300 min, the injection needle temperature is 110℃, the transfer line temperature is 120℃, the injection pressure is 15 psi, the injection pressure time is 1.0 min, and the injection time is 0.1 min; the gas chromatography method uses a programmed temperature ramp method.

[0013] The temperature of the gas chromatograph inlet is 200℃, the split ratio is 50:1, and the carrier gas is helium.

[0014] Furthermore, the temperature program is as follows: the column temperature is 50℃ and held for 5 minutes; the temperature is increased to 150℃ at a rate of 5℃ / min and held for 20 minutes.

[0015] The volume fraction purity of the helium gas is higher than 99.999%.

[0016] Furthermore, the chromatographic column is an HP-FFAP polyethylene glycol nitrobenzene modified capillary column (i.e., HP-FFAP type capillary column), with column dimensions of 50m × 0.32mm × 0.5μm.

[0017] Furthermore, in the headspace gas chromatography-mass spectrometry system, the mass spectrometry detection conditions are as follows:

[0018] The temperature of the chromatography-mass spectrometry interface is 150℃;

[0019] The ion source for the mass spectrometer is an electron ionization source (EI) at a temperature of 200 °C and an electron energy of 70 eV.

[0020] The mass analyzer is a quadrupole mass analyzer;

[0021] The scanning mode is ion monitoring mode for quantification;

[0022] The mass spectrometry scanning range is 33–300 m / z;

[0023] The solvent delay time is 1 to 5 minutes.

[0024] Furthermore, the intensity of the characteristic ion with m / z = 53 was extracted in the scanning mode.

[0025] Furthermore, when establishing the standard working curve, NBR without free acrylonitrile is added to eliminate the matrix effect.

[0026] Furthermore, the process conditions for establishing the standard working curve are the same as those for determining the peak area of ​​the (H)NBR sample being tested.

[0027] Compared with existing technologies, the method of the present invention has a lower detection limit (0.1 mg / kg), a higher spiked recovery rate (96.0% to 98.0%), and good precision (relative standard deviation less than 2.5%). The established method and standard are beneficial to the quality control of free acrylonitrile products in (H)NBR and have great application value in protecting human health and safety and preventing potential hazards. Attached Figure Description

[0028] Figure 1 This is the curve showing the change of acrylonitrile peak area with headspace temperature in this invention;

[0029] Figure 2 This is the curve showing the change in acrylonitrile peak area with equilibrium time in this invention;

[0030] Figure 3 The total ion chromatogram of acrylonitrile in this invention (top) and the selected ion chromatogram of m / z = 53 (bottom);

[0031] Figure 4 This is the calibration standard working curve for acrylonitrile in this invention;

[0032] Figure 5 This is the calibration standard working curve (without matrix) of acrylonitrile used as a comparison in this invention;

[0033] Figure 6 This is the calibration standard working curve of acrylonitrile used as a comparison after the chromatographic column was replaced with HP-INNOWax in this invention;

[0034] Figure 7 This is the calibration standard working curve (FID detector) of acrylonitrile used as a comparison in this invention. Detailed Implementation

[0035] This invention employs headspace sampling technology to analyze the content of free acrylonitrile in (H)NBR. Headspace sampling is a gas-liquid equilibrium process that uses gas above the sample matrix to determine the content of unknown components in the sample. Factors affecting headspace analysis results mainly come from three aspects: firstly, parameters related to the GC itself; secondly, the selection of headspace parameters; and thirdly, the influence of sample components.

[0036] Therefore, to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and do not constitute any limitation on the scope of protection of the present invention. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other, and the resulting technical solutions are also considered to fall within the content disclosed in the embodiments of this application.

[0037] The commercially available NBR and HNBR used in this invention are 3306G from Lanzhou Petrochemical Company of China National Petroleum Corporation and Therban 3446 from ARLANXEO.

[0038] Unless otherwise specified, the reagents, methods, instruments and equipment used in this invention are conventional reagents, methods, instruments and equipment in the art.

[0039] Unless otherwise specified, all reagents and materials used in the following examples are commercially available and of analytical grade.

[0040] To further improve the accuracy and detection limit of free acrylonitrile content in (H)NBR, in some specific embodiments, a method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber is provided. The (H)NBR sample to be tested is weighed and placed in a headspace vial. Then, BHT-THN solution is transferred, the vial is sealed, and the vial is sent to a headspace gas chromatography-mass spectrometry (GC-MS) instrument to determine its total ion chromatogram. The peak area is obtained by integrating the chromatographic peaks from acrylonitrile, and then substituted into a standard working curve of free acrylonitrile concentration versus peak area to obtain the free acrylonitrile content in the (H)NBR sample to be tested.

[0041] This invention, through research, focuses on the characteristics of free acrylonitrile in (H)NBR and examines the analytical methods from three aspects: the main factors affecting the analytical results, namely, chromatographic separation conditions, headspace parameter selection, and the influence of sample components. A method for determining the content of free acrylonitrile in (H)NBR has been established. The specific optimization process of each factor is shown in Example 1 below.

[0042] Example 1:

[0043] (1) Selection of headspace temperature

[0044] The headspace equilibrium temperature is usually selected based on a combination of the boiling point of the analyte in the sample and the thermal decomposition behavior of the sample. Acrylonitrile (AN) has a boiling point of 77.3℃, and 90℃ is often used as the headspace equilibrium temperature in the literature. This experiment uses a commercially available NBR sample as an example to investigate the experimental results at headspace temperatures of 80, 90, 100, 110, and 120℃. Figure 1 ).from Figure 1 It can be seen that the sensitivity of acrylonitrile increases significantly with increasing headspace temperature. When the headspace temperature is increased from 100℃ to 120℃, due to the instability of acrylonitrile, the peak area of ​​acrylonitrile decreases significantly with prolonged heating time. Therefore, a headspace temperature of 100℃ is more suitable.

[0045] (2) Selection of balancing time

[0046] Taking a commercially available NBR sample as an example, the change in acrylonitrile peak area was investigated within a heating equilibrium time of 30–300 min at a headspace temperature of 100℃. The experimental results are shown in [Figure number missing]. Figure 2 .Depend on Figure 2 As can be seen, the curve remained essentially unchanged after 150 minutes of heating, indicating that the system in the headspace vial had reached phase equilibrium. Therefore, an equilibrium time of 150 minutes is appropriate.

[0047] The remaining headspace gas chromatography conditions are:

[0048] The chromatographic column used was an HP-FFAP polyethylene glycol nitrobenzene modified capillary column with dimensions of 50 m × 0.32 mm × 0.5 μm; detection was performed using a single quadrupole mass spectrometer detector.

[0049] The injection needle temperature was 110℃, the transfer line temperature was 120℃, the injection pressure was 15psi, the injection pressure time was 1.0min, and the injection time was 0.1min. The gas chromatography method used the temperature programmed method, with the column temperature at 50℃ for 5min, then increased to 150℃ at a rate of 5℃ / min and held for 20min.

[0050] The temperature of the gas chromatograph inlet is 200℃, the split ratio is 50:1, and the carrier gas is helium with a volume fraction purity higher than 99.999%.

[0051] Mass spectrometry conditions are:

[0052] The temperature of the chromatography-mass spectrometry interface is 150℃;

[0053] The ion source for the mass spectrometer is an electron ionization source (EI) at a temperature of 200 °C and an electron energy of 70 eV.

[0054] The mass analyzer is a quadrupole mass analyzer;

[0055] The scanning mode is ion monitoring mode for quantification;

[0056] The mass spectrometry scanning range is 33–300 m / z;

[0057] The solvent delay time is 1 to 5 minutes.

[0058] (3) Plotting the standard working curve

[0059] Preparation of standard solutions

[0060] Weigh appropriate amounts of 2,6-di-tert-butyl-4-methylphenol (BHT) and acrylonitrile to prepare a THN solution with an acrylonitrile mass concentration of 1000 mg / L (the mass concentration of BHT in the solution is approximately 200 mg / L). Using this solution as the mother liquor, dilute stepwise to prepare a series of THN standard solutions with an acrylonitrile mass concentration of 0.2–20.0 mg / L.

[0061] Using NBR samples without free acrylonitrile as the matrix (obtained by placing NBR in a vacuum drying oven and drying at 80℃ for 72 hours), the NBR samples were cut into particles with a particle size of about 0.3 cm. 0.3 g of the cut NBR particles were accurately weighed and placed in a 20 mL headspace vial. 5 mL of the above series of standard solutions were transferred to the headspace vial, and the vial was sealed to prepare a series of standard solutions.

[0062] Plotting standard working curves

[0063] Under the optimal analytical conditions described above, the total ion chromatograms (TICs) of the prepared series of standard solutions were determined. The main elution positions and quantitative peaks of acrylonitrile (retention time approximately 7.15 min) were determined by matching with the standard NIST chromatogram library and extracting the ion chromatogram at m / z = 53. The results are shown in [Figure number missing]. Figure 3 .

[0064] The peak area (A) of the acrylonitrile chromatographic peak at 7.15 min was obtained by integrating the peak area, and a quantitative calibration curve was established between the free acrylonitrile concentration C and the peak area A (see...). Figure 4The standard working curve of acrylonitrile was obtained by linear fitting as A = 151229.7C - 37582.6, with a linear correlation coefficient of 0.99995.

[0065] (4) Spike recovery rate

[0066] Spiking recovery experiments were conducted on acrylonitrile using a standard solution (concentration of 6.0 mg / kg), commercially available NBR, and HNBR as samples. The results are shown in Table 1. As can be seen from Table 1, the spiked recovery rate of acrylonitrile was 96.0%–98.0%, indicating that the method has high accuracy.

[0067] Table 1

[0068]

[0069] (5) Precision test

[0070] Using a standard solution (concentration of 6.0 mg / kg), commercially available NBR, and HNBR as samples, the three samples were measured six times in parallel to examine the precision of the method. The experimental results are shown in Table 2. As can be seen from Table 2, the relative standard deviation of the method is less than 2.5%, indicating that the method has good repeatability.

[0071] Table 2

[0072]

[0073]

[0074] (6) Limit of detection

[0075] Using a 6.0 mg / kg acrylonitrile solution as the spiking solution, the assay was performed 10 times in parallel under the above analytical conditions. The peak height and baseline noise of acrylonitrile were recorded. The acrylonitrile content corresponding to a signal-to-noise ratio of 3 was taken as the limit of detection of this method. The limit of detection calculated from the standard working curve was 0.1 mg / kg.

[0076] (7) Influence of sample matrix

[0077] Taking a 6.0 mg / kg acrylonitrile solution as an example, the standard working curves obtained from NBR samples with and without added free acrylonitrile were examined respectively. The results are shown in [Figure number missing]. Figure 5 .from Figure 5 It can be observed that the linear correlation coefficient of the working curve without the added matrix is ​​only 0.99121, which is significantly lower than that of the standard working curve with the matrix present. Figure 4 This indicates that adding a matrix to the standard solution can significantly improve the impact of the matrix effect on the accuracy of the analytical method.

[0078] Taking commercially available NBR as an example, two samples were prepared, one of which was an NBR sample without added free acrylonitrile. Each sample was measured in parallel six times, and the results are shown in Table 3. Table 3 shows that the relative standard deviation (RSD) was 6.7% for the sample without matrix, while it decreased to 1.5% after adding matrix. This indicates that adding matrix significantly improves the matrix effect on the precision of the analytical method.

[0079] Table 3

[0080]

[0081] (8) The influence of column type

[0082] Using an HP-INNOWax column (stationary phase: polyethylene glycol) with dimensions of 60m × 0.25mm × 0.5μm as a comparative example, and with all other conditions consistent with the above, the obtained standard working curve was examined. Figure 6 As shown. The standard working curve is compared with that of a chromatographic column using an HP-FFAP polyethylene glycol nitrobenzene modified capillary column with dimensions of 50m × 0.32mm × 0.5μm. Figure 4 Compared to this, the linear correlation coefficient of the comparative proportion is 0.99978, which is lower. Figure 4 The standard working curve (0.99995) yielded a detection limit of 0.6 mg / kg. The above comparative examples show that different types of chromatographic columns can affect the accuracy and detection limit of this method. This method preferably uses an HP-FFAP polyethylene glycol nitrobenzene modified capillary column with a column size of 50 m × 0.32 mm × 0.5 μm for the determination of free acrylonitrile.

[0083] (9) The impact of the detector

[0084] Using the headspace and gas chromatography conditions described in Example 1, a quantitative calibration curve was established using an FID (flame ionization detector) to determine the relationship between the free acrylonitrile concentration C and the peak area A (see Example 1). Figure 7 The standard working curve for acrylonitrile, obtained through linear fitting, was A = 243.2C + 4336.7, with a linear correlation coefficient of 0.99162. This is significantly lower than the standard working curve obtained using headspace gas chromatography-mass spectrometry (HGC-MS). Figure 4 This indicates that the working curve obtained using the mass spectrometer detector of Example 1 has significantly better accuracy than that obtained using the FID detector.

[0085] Using a 6.0 mg / kg acrylonitrile solution as the spiking solution, ten parallel determinations were performed under the analytical conditions described above. The peak height and baseline noise of acrylonitrile were recorded. The acrylonitrile content corresponding to a signal-to-noise ratio of 3 was taken as the limit of detection (LOD) of this method. The LOD calculated from the standard working curve was 1.1 mg / kg. This is significantly lower than the LOD obtained by headspace gas chromatography-mass spectrometry (0.1 mg / kg), indicating that the LOD obtained using the mass spectrometry detector of Example 1 is significantly better than that obtained by the FID detector.

[0086] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber, characterized in that, Weigh the (H)NBR sample to be tested and place it in a headspace vial. Then, transfer the BHT-THN solution, seal the vial, and send it into a headspace gas chromatography-mass spectrometry instrument to determine its total ion chromatogram. Integrate the chromatographic peak from acrylonitrile to obtain the peak area, and then substitute it into the standard working curve of free acrylonitrile concentration versus peak area to obtain the free acrylonitrile content in the (H)NBR sample to be tested.

2. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 1, characterized in that, The tested (H)NBR sample is an NBR, a partially hydrogenated or fully hydrogenated NBR.

3. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 1, characterized in that, The BHT-THN solution contains BHT at a concentration of 180-220 mg / L, and the ratio of the tested (H)NBR sample to the BHT-THN solution is 0.3 g: 4-6 L.

4. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 1, characterized in that, The headspace gas chromatography-mass spectrometry (HCGS) system used in the above system has the following headspace gas chromatography conditions: The chromatographic column was a polyethylene glycol capillary column modified with nitroterephthalic acid; detection was performed using a single quadrupole mass spectrometer. The headspace heating temperature is 90–120℃, the headspace equilibration time is 30–300 min, the injection needle temperature is 110℃, the transfer line temperature is 120℃, the injection pressure is 15 psi, the injection pressure time is 1.0 min, and the injection time is 0.1 min; the gas chromatography method uses a programmed temperature ramp method. The temperature of the gas chromatograph inlet is 200℃, the split ratio is 50:1, and the carrier gas is helium.

5. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 4, characterized in that, The heating program is as follows: Column temperature is 50℃ and held for 5 min; then the temperature is increased to 150℃ at a rate of 5℃ / min and held for 20 min. The volume fraction purity of the helium gas is higher than 99.999%.

6. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 4, characterized in that, The chromatographic column is an HP-FFAP polyethylene glycol nitrobenzene modified capillary column with a column size of 50m × 0.32mm × 0.5μm.

7. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 1, characterized in that, The mass spectrometry detection conditions in the headspace gas chromatography-mass spectrometry system are as follows: The temperature of the chromatography-mass spectrometry interface is 150℃; The ion source for the mass spectrometer is an electron ionization source (EI) at a temperature of 200 °C and an electron energy of 70 eV. The mass analyzer is a quadrupole mass analyzer; The scanning mode is ion monitoring mode for quantification; The mass spectrometry scanning range is 33–300 m / z; The solvent delay time is 1 to 5 minutes.

8. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 7, characterized in that, The intensity of the characteristic ion m / z = 53 was extracted in the scanning mode.

9. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 1, characterized in that, When establishing the standard working curve, NBR without free acrylonitrile is added to eliminate the matrix effect.

10. The method for determining the free acrylonitrile content in acrylonitrile-butadiene rubber according to claim 1, characterized in that, The process conditions for establishing the standard working curve are the same as those for determining the peak area of ​​the (H)NBR sample being tested.

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