Detection method of rubber scorch retarder and conversion product thereof

Abstract

The invention discloses a detection method of a rubber scorch retarder and a conversion product thereof, and belongs to the technical field of chemical detection. According to the technical scheme, the method comprises the following steps: extracting a rubber sample to be detected by using an extracting solution, and then carrying out quantitative analysis on N-cyclohexyl-thiophthalimide and phthalimide by using a liquid chromatograph, a liquid chromatograph adopts a gradient elution mode, a mobile phase in 0-2 min is acetonitrile: water = (30-50): (50-70), a mobile phase in 2-5 min is subjected to gradient from the acetonitrile: water = (30-50): (50-70) to the acetonitrile: water = (80-99): (1-20), and the acetonitrile: water = (80-99): (1-20) is kept in 5-8 min; the ratio of acetonitrile to water is (80-99): (1-20), and the ratio of acetonitrile to water is (30-50): (50-70) in 8-11 min, and the ratio of acetonitrile to water is kept (30-50): (50-70) in 11-13 min. The method has the advantages of simple pretreatment steps, short analysis period and high sensitivity, is suitable for quality control and product inspection in the production process, and can provide a reliable detection means for formula optimization and performance evaluation of rubber products; by adopting the detection method disclosed by the invention, the linear range of CTP and PHT is 0.1-100 mg / L, the detection limit of the method is 0.02-0.04 mg / L, the adding standard recovery rate is 98-103%, the analysis result is accurate, and the sensitivity is high.

Description

Technical Field

[0001] This invention belongs to the field of chemical detection technology, and in particular relates to a method for detecting rubber anti-scorching agents and their conversion products. Background Technology

[0002] N-Cyclohexylthio-phthalimide (CTP), an anti-scorching agent, is an important additive widely used in the processing of natural and synthetic rubber. Its main function is to inhibit premature vulcanization during the mixing and molding stages, thereby extending the scorch time of the rubber compound and ensuring the safety and controllability of the processing. Thanks to its excellent delayed vulcanization properties, CTP has become a common component in the formulations of rubber products such as tires, belts, and seals. However, under the high-temperature environment of rubber processing, or during long-term storage and use of finished products, CTP is prone to pyrolysis or oxidation reactions, generating conversion products such as phthalimide (PHT). These conversion products may alter the activation energy of the vulcanization system, affect the formation process of cross-linked structures, leading to a decrease in the mechanical properties of rubber, weakened aging resistance, and even color changes and surface precipitation. In certain applications, the accumulation of PHT can also have adverse effects on downstream performance or safety indicators. Therefore, monitoring the content of both CTP and PHT is of great significance for raw material quality assessment, production process monitoring, and finished product performance assurance.

[0003] Currently, the analysis and detection of CTP and its conversion products in rubber is lacking. Although existing liquid chromatography methods for detecting CTP in raw materials have certain limitations when applied to the rubber matrix. On the one hand, the rubber matrix has a complex composition, containing various vulcanization accelerators, anti-aging agents, plasticizers, and fillers. These substances easily introduce strong matrix interference in solvents, leading to problems such as overlapping chromatographic peaks and signal suppression. On the other hand, existing pretreatment methods mostly rely on Soxhlet extraction, long-term solvent reflux, and column chromatography purification, which are not only cumbersome, time-consuming, and consume large amounts of solvent, but also have extraction efficiency and repeatability that are easily affected by the operator's experience and condition control. In addition, although there are methods for quantifying CTP in anti-scorching agent raw materials, the simultaneous determination of PHT is still lacking, making the detection results unable to fully reflect the conversion law of anti-scorching agents under actual processing and use conditions.

[0004] As the rubber industry develops towards high-performance, refined, and green production, manufacturers and testing institutions are placing higher demands on the rapid, accurate, and simultaneous analysis of scorch inhibitors and their conversion products. Therefore, there is an urgent need to develop a method that features simple pretreatment, short detection cycle, minimal matrix interference, and high sensitivity and repeatability, capable of simultaneously obtaining CTP and PHT content data in a single test. This would provide a scientific and reliable testing method for formulation optimization, quality control, and product performance evaluation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention aims to solve the problem that the complex matrix of rubber samples leads to problems in the detection of the anti-scorching agent N-cyclohexyl-thiophthalimide (CTP) and its conversion product phthalimide (PHT), including co-extraction of extraction impurities, difficulty in avoiding interference signals, and inaccurate quantitative results. This invention proposes a new method for detecting rubber anti-scorching agents and their conversion products. Unlike conventional methods that rely solely on adjusting a single condition, this invention achieves a synergistic optimization of the injection volume, mobile phase ratio, and detection wavelength, forming a mutually restraining and compensating overall control mechanism. This fundamentally eliminates the influence of overlapping impurity peaks and background interference in rubber samples, effectively avoiding pretreatment interference and ensuring stable and reliable analytical results. This method not only simplifies the sample pretreatment process but also ensures the sensitivity and accuracy of detection, enabling rapid and simultaneous quantitative analysis of CTP and PHT in different types of rubber samples, demonstrating significant inventiveness and practical value.

[0006] To solve the aforementioned technical problem, the technical solution adopted by the present invention is as follows: This invention provides a method for detecting rubber anti-scorching agents and their conversion products, comprising the following steps: The rubber sample to be tested was extracted with the extraction solution and then N-cyclohexyl-thiophthalimide and phthalimide were quantitatively analyzed by liquid chromatography. The liquid chromatograph uses a gradient elution mode: for 0-2 min, the mobile phase is acetonitrile:water = 30-50:50-70; for 2-5 min, the mobile phase gradients from acetonitrile:water = 30-50:50-70 to acetonitrile:water = 80-99:1-20; for 5-8 min, the mobile phase is maintained at acetonitrile:water = 80-99:1-20; for 8-11 min, the mobile phase gradients back from acetonitrile:water = 80-99:1-20 to acetonitrile:water = 30-50:50-70; and for 11-13 min, the mobile phase is maintained at acetonitrile:water = 30-50:50-70.

[0007] Preferably, the flow rate of the mobile phase is 0.8-1.2 mL / min.

[0008] Preferably, the liquid chromatograph uses automatic injection with an injection volume of 8-12 μL.

[0009] Preferably, the detector of the liquid chromatograph is a diode array ultraviolet detector with a detection wavelength of 285-295 nm and a reference wavelength of 350-370 nm.

[0010] Preferably, the extractant is acetonitrile.

[0011] Preferably, the ratio of the amount of rubber sample to be tested to the amount of extract is 5-15 mg: 1-10 mL.

[0012] Preferably, the detection method for the rubber anti-scorching agent and its conversion products includes the following steps: The rubber sample to be tested was cut into small pieces, added to the extraction solution, and subjected to ultrasonic-assisted extraction. The extraction solution was collected and filtered to obtain the filtrate. Then, liquid chromatography was used to quantitatively analyze N-cyclohexyl-thiophthalimide and phthalimide in the filtrate.

[0013] Preferably, a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane is used for filtration.

[0014] Preferably, the temperature for ultrasound-assisted extraction is 30-50℃ and the time is 5-25 min.

[0015] Preferably, the liquid chromatograph uses a liquid chromatograph column with a column temperature of 30-50℃, EC-C18 packing material with a length of 150 mm, an inner diameter of 4.6 mm, and a packing particle diameter of 4 µm.

[0016] As a preferred method, the external standard method is used to plot the standard curve. The concentrations of the target compounds N-cyclohexyl-thiophthalimide and phthalimide are used as the abscissa, and the peak areas of the target compounds are used as the ordinate. The standard curves of N-cyclohexyl-thiophthalimide and phthalimide are obtained by linear fitting.

[0017] In another aspect, the present invention provides the application of the detection method of the above-mentioned rubber anti-scorching agent and its conversion products in rubber testing.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a method for detecting rubber anti-scorching agents and their conversion products. Addressing the practical challenges of significant polarity differences between CTP and PHT, strong interference from the rubber matrix, and the inapplicability of national standard conditions, it proposes an overall optimization strategy combining mobile phase ratio and detection wavelength. This strategy works simultaneously across multiple dimensions: kinetics (solvent focusing and peak shape control), thermodynamics (selective distribution and retention regulation), and spectroscopy (absorption band misalignment avoidance). This enables, for the first time, rapid, accurate, and simultaneous quantitative analysis of CTP and PHT in complex rubber matrices. It can serve as a technical support for controlling the usage of anti-scorching agents, optimizing formulations, and monitoring quality in rubber production processes, possessing broad application prospects and industrial value. Using the detection method of this invention, the correlation coefficient R between CTP and PHT in the concentration range of 0.1-100 mg / L is [value missing]. 2 The range of analytical parameters was 0.9996-0.9999, the limit of detection was 0.02-0.04 mg / L, the intra-day precision was 0.45-0.78%, the inter-day precision was 1.1-1.9%, and the recovery rate was 98-103% (n=3). The analytical results were accurate and reproducible. Attached Figure Description

[0019] Figure 1 This is a flowchart of the testing method provided in an embodiment of the present invention; Figure 2 The chromatograms provided in Comparative Examples 1-3 of this invention; Figure 3 Chromatograms of different mobile phases provided in Example 1 and Comparative Examples 4-6; Figure 4 The graph shows the effect of different extraction solution types provided in Example 1 and Comparative Examples 7-8 on the extraction recovery rates of CTP and PHT. Figure 5 Chromatograms of different detection wavelengths provided for Comparative Examples 9-11. Detailed Implementation

[0020] The technical solutions in specific embodiments of the present invention will now be described in detail and completely with reference to the accompanying drawings. Obviously, the described embodiments are merely some specific implementations of the overall technical solution of the present invention, and not all implementations. Based on the overall concept of the present invention, all other embodiments obtained by those skilled in the art fall within the protection scope of the present invention.

[0021] This invention provides a method for detecting rubber anti-scorching agents and their conversion products, comprising the following steps: The rubber sample to be tested was extracted with the extraction solution and then N-cyclohexyl-thiophthalimide and phthalimide were quantitatively analyzed by liquid chromatography. The liquid chromatograph uses gradient elution mode: 0-2 min mobile phase is acetonitrile:water = 30-50:50-70; 2-5 min mobile phase gradient from acetonitrile:water = 30-50:50-70 to acetonitrile:water = 80-99:1-20; 5-8 min maintain acetonitrile:water = 80-99:1-20; 8-11 min gradient from acetonitrile:water = 80-99:1-20 back to acetonitrile:water = 30-50:50-70; 11-13 min maintain acetonitrile:water = 30-50:50-70.

[0022] It should be noted that currently, regarding mobile phase ratios, the existing national standard GB / T24801-2019, "Anti-scorching agent N-cyclohexylthiophthalimide (CTP)," uses an acetonitrile / water isocratic mobile phase with an 80:20 ratio for raw material CTP detection. While this condition is adequate for raw material detection, it has significant limitations when applied to complex rubber matrices: although PHT can be eluted at low acetonitrile ratios, it is easily co-eluted with polar impurities, making accurate quantification impossible; while high acetonitrile ratios, although beneficial for CTP elution, cause the PHT signal to be submerged in the matrix background, resulting in severely insufficient resolution. Since CTP is a hydrophobic thiophthalimide derivative, while PHT is a more polar phthalimide, their chromatographic retention characteristics differ fundamentally, making simultaneous detection difficult under a single isocratic condition.

[0023] To address the above issues, this invention first employs a high aqueous phase ratio starting condition (acetonitrile / water = 30-50:50-70) to achieve "solvent focusing" at the column head after sample loading. Since the sample extraction solvent is acetonitrile, direct large-volume injection can cause solvent strength mismatch, leading to peak forward tilting. With an aqueous phase-dominant starting condition, the sample band is narrowed again and fixed at the column head after entering the chromatographic column, suppressing the broadening and forward tilting caused by the solvent effect from the source. Subsequently, a gradient elution (acetonitrile / water) of 30-50:50-70 to 80-99:1-20 is implemented: a strong polar separation force is maintained in the early stage, widening the retention difference between PHT and the polar matrix; in the middle and later stages, the organic phase ratio is rapidly increased to effectively elute the more retained CTP, while simultaneously carrying out the hydrophobic matrix in a "wash-remove" mode, avoiding baseline interference caused by broad peaks / tailing in the PHT and CTP elution regions. Experimental results show that under isocratic conditions, a 40:60 ratio can only detect PHT while CTP is difficult to elute. Changing the isocratic ratio to 70:30 or 90:10 can make both PHT and adjacent impurities elute, but baseline separation of PHT and adjacent impurities is still difficult, and quantification is impossible at low concentrations. Only the above gradient program can simultaneously meet the requirements of separation and quantitative stability of both. Figure 2 ).

[0024] In a preferred embodiment, the flow rate of the mobile phase is 0.8-1.2 mL / min.

[0025] In a preferred embodiment, the liquid chromatograph uses automatic injection with an injection volume of 8-12 μL.

[0026] In a preferred embodiment, the detector of the liquid chromatograph is a diode array ultraviolet detector with a detection wavelength of 285-295 nm and a reference wavelength of 350-370 nm.

[0027] It should be noted that, regarding the detection wavelength, national standards and conventional methods mostly choose 254 nm to pursue universal sensitivity. The results of testing various types of rubber samples using the national standard "GB / T24801-2019 Anti-scorching agent N-cyclohexylthiophthalimide (CTP)" are as follows: Figure 5 As shown, a large number of matrix impurity peaks are present simultaneously in the expected peak regions of the target compounds PHT and CTP, resulting in severe signal overlap and making baseline separation impossible. This makes it difficult to qualitatively identify the target peaks, let alone accurately quantify them. This is because 254 nm is precisely the strong absorption band of common antioxidants, anti-aging agents, and oligomers in rubber, causing significant baseline rise and interference. While increasing the wavelength to 285-295 nm reduces impurity interference, the sensitivity decreases significantly under normal conditions, resulting in unsatisfactory detection performance.

[0028] This invention fully utilizes the selective absorption of the target analyte at 285-295 nm by synergistically optimizing the detection wavelength and the aforementioned mobile phase procedure, and combines a 350-370 nm reference channel to offset baseline fluctuations, ultimately obtaining highly sensitive, highly selective and repeatable detection results.

[0029] In summary, the technical breakthrough of this invention lies in its approach: it does not rely on the conventional adjustment of a single parameter, but rather addresses the practical challenges of significant polarity differences between CTP and PHT, strong interference from the rubber matrix, and the inapplicability of national standard conditions by proposing a holistic optimization strategy that couples the mobile phase ratio with the detection wavelength. This strategy works simultaneously across multiple dimensions—kinetics (solvent focusing and peak shape control), thermodynamics (selective distribution and retention regulation), and spectroscopy (absorption band misalignment avoidance)—thus achieving, for the first time, rapid, accurate, and simultaneous quantitative analysis of CTP and PHT in complex rubber matrices. The improved separation selectivity and baseline cleanliness resulting from this holistic strategy represent a systematic solution to the challenge of simultaneous quantitative analysis in complex matrices.

[0030] In a preferred embodiment, the extractant is acetonitrile.

[0031] In a preferred embodiment, the ratio of the amount of rubber sample to be tested to the amount of extract is 5-15 mg: 1-10 mL.

[0032] In a preferred embodiment, the method for detecting rubber anti-scorching agents and their conversion products includes the following steps: The rubber sample to be tested was cut into small pieces, added to the extraction solution, and subjected to ultrasonic-assisted extraction. The extraction solution was collected and filtered to obtain the filtrate. Then, liquid chromatography was used to quantitatively analyze N-cyclohexyl-thiophthalimide and phthalimide in the filtrate.

[0033] In a preferred embodiment, filtration is performed using a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane.

[0034] In a preferred embodiment, the temperature of ultrasound-assisted extraction is 30-50°C and the time is 5-25 min.

[0035] In a preferred embodiment, the liquid chromatograph column of the liquid chromatograph has a column temperature of 30-50°C, the packing material is EC-C18, the length is 150 mm, the inner diameter is 4.6 mm, and the packing particle diameter is 4 µm.

[0036] In a preferred embodiment, a standard curve is plotted using the external standard method, with the concentrations of the target N-cyclohexyl-thiophthalimide and phthalimide as the abscissa and the peak area of ​​the target substance as the ordinate. The standard curves of N-cyclohexyl-thiophthalimide and phthalimide are obtained by linear fitting.

[0037] In another aspect, the present invention provides the application of the detection method of the above-mentioned rubber anti-scorching agent and its conversion products in rubber testing.

[0038] To provide a clearer and more detailed description of the detection method for a rubber anti-scorching agent and its conversion products provided by the embodiments of the present invention, specific embodiments will be described below.

[0039] The composition and content of the rubber samples used in the following embodiments and comparative examples of the present invention are as follows: Rubber Sample 1 (parts by weight): 100 parts masterbatch, 0.3 parts scorch inhibitor (CTP), 60 parts carbon black, 10 parts plasticizer, 5 parts zinc oxide, 2 parts stearic acid, 0.7 parts accelerator, 1.8 parts sulfur, 2 parts antioxidant; Rubber Sample 2 (parts by weight): 100 parts masterbatch, 0.45 parts scorch inhibitor (CTP), 70 parts carbon black, 12 parts plasticizer, 7 parts zinc oxide, 2 parts stearic acid, 0.8 parts accelerator, 2.5 parts sulfur, 3 parts antioxidant; Rubber Sample 3 (parts by weight): 100 parts masterbatch, 0.35 parts scorch inhibitor (CTP), 65 parts carbon black, 15 parts plasticizer, 5 parts zinc oxide, 2 parts stearic acid, 1.0 part accelerator, 1.8 parts sulfur, and 3 parts resin.

[0040] Example 1 Detection methods for CTP, an anti-scorching agent in rubber, and its conversion product PHT, such as... Figure 1 As shown, the specific steps include: Weigh 10 mg of the rubber sample to be tested, cut it into small particles of about 1-2 mm, place it in a glass container, and add 5 mL of acetonitrile as the extraction solution; The mixed rubber particles and the extraction solution were subjected to ultrasonic-assisted extraction at 40°C for 10 min. After extraction, the supernatant was collected and filtered through a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane to obtain the filtrate. The resulting filtrate was used for liquid chromatography analysis; The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, the detection wavelength is 290 nm, and the reference wavelength is 360 nm; The injection method was automatic injection, and the injection volume was 10 μL. The mobile phase gradient condition is: 0-2 min acetonitrile:water = 40:60, 2-5 min gradient from 40:60 to 95:5, 5-8 min maintain 95:5, 8-11 min gradient back from 95:5 to 40:60, 11-13 min maintain 40:60; mobile phase flow rate is 1 mL / min; The column temperature is 40℃; the column packing material is EC-C18, with a length of 150 mm, an inner diameter of 4.6 mm, and a particle diameter of 4 μm. Standard curves were plotted using the external standard method, with the concentrations of CTP and PHT as the x-axis and the peak area of ​​the target substance as the y-axis. Standard curves for CTP and PHT were obtained by linear fitting.

[0041] Example 2 The detection method for CTP, an anti-scorching agent in rubber, and its conversion product PHT includes the following steps: Weigh 5 mg of the rubber sample to be tested, cut it into small particles of about 1-2 mm, place it in a glass container, and add 1 mL of acetonitrile as the extraction solution; The mixed rubber particles and the extraction solution were subjected to ultrasonic-assisted extraction at 30°C for 25 min. After extraction, the supernatant was collected and filtered through a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane to obtain the filtrate. The resulting filtrate was used for liquid chromatography analysis; The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, the detection wavelength is 285 nm, and the reference wavelength is 350 nm; The injection method was automatic injection, and the injection volume was 8 μL. The mobile phase gradient condition is: The mobile phase was acetonitrile:water = 30:70 for 0-2 min; the mobile phase was gradiented from acetonitrile:water = 30:70 to acetonitrile:water = 80:20 for 2-5 min; the mobile phase was maintained at acetonitrile:water = 80:20 for 5-8 min; the mobile phase was gradiented back from acetonitrile:water = 80:20 to acetonitrile:water = 30:70 for 8-11 min; and the mobile phase was maintained at acetonitrile:water = 30:70 for 11-13 min.

[0042] The column temperature is 30℃; the column packing material is EC-C18, with a length of 150 mm, an inner diameter of 4.6 mm, and a particle diameter of 4 μm. Standard curves were plotted using the external standard method, with the concentrations of CTP and PHT as the x-axis and the peak area of ​​the target substance as the y-axis. Standard curves for CTP and PHT were obtained by linear fitting.

[0043] Example 3 The detection method for CTP, an anti-scorching agent in rubber, and its conversion product PHT includes the following steps: Weigh 15 mg of the rubber sample to be tested, cut it into small particles of about 1-2 mm, place it in a glass container, and add 10 mL of acetonitrile as the extraction solution; The mixed rubber particles and the extract were subjected to ultrasonic-assisted extraction at 50°C for 5 min. After extraction, the supernatant was collected and filtered through a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane to obtain the filtrate. The resulting filtrate was used for liquid chromatography analysis; The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, the detection wavelength is 300 nm, and the reference wavelength is 370 nm; The injection method was automatic injection, and the injection volume was 12 μL. The mobile phase gradient condition is: From 0 to 2 min, the mobile phase was acetonitrile:water = 50:50. From 2 to 5 min, the mobile phase gradient changed from acetonitrile:water = 50:50 to acetonitrile:water = 99:1. From 5 to 8 min, the mobile phase was maintained at acetonitrile:water = 99:1. From 8 to 11 min, the mobile phase gradient changed back from acetonitrile:water = 99:1 to acetonitrile:water = 50:50. From 11 to 13 min, the mobile phase was maintained at acetonitrile:water = 50:50. The column temperature is 50℃; the column packing material is EC-C18, with a length of 150 mm, an inner diameter of 4.6 mm, and a particle diameter of 4 μm. Standard curves were plotted using the external standard method, with the concentrations of CTP and PHT as the x-axis and the peak area of ​​the target substance as the y-axis. Standard curves for CTP and PHT were obtained by linear fitting.

[0044] Example 4 The relevant parameters of the working curves of CTP and PHT, as well as the method detection limit and precision, were determined according to the method in Example 1: 1. Detection limit test: CTP and PHT at concentrations of 0.1 mg / L, 0.2 mg / L, 0.5 mg / L, 1 mg / L, 5 mg / L, 10 mg / L, 20 mg / L, 50 mg / L, and 100 mg / L were added dropwise to the masterbatch rubber compound (i.e., raw rubber + filler + oil + antioxidant, excluding CTP and PHT) to obtain rubber compound samples. The linear range, method curve regression equation, and correlation coefficient (R²) of the determination of the content of anti-scorching agent CTP and its conversion product PHT in rubber samples were performed according to the method in Example 1. 2 The method detection limit is shown in Table 1 below; 2. Precision testing: CTP and PHT at a concentration of 5 mg / L were added dropwise to the masterbatch rubber compound (i.e., raw rubber + filler + oil + antioxidant, excluding CTP and PHT) to obtain rubber samples. The rubber sample was measured according to the method of Example 1. The intraday precision was assessed by six parallel measurements within one day and the interday precision was assessed by one measurement per day for six parallel days (expressed as relative standard deviation, and the specific data are shown in Table 2).

[0045] Table 1. Relevant parameters and method detection limits for the working curves of CTP and PHT.

[0046] As shown in Table 1, when the content of anti-scorching agent CTP and its conversion product PHT in rubber was determined according to the method in Example 1 of this application, the method detection limit for CTP was 0.02 mg / L and the method quantitation limit was 0.06 mg / L; the method detection limit for PHT was 0.04 mg / L and the method quantitation limit was 0.13 mg / L.

[0047] Table 2 Precision determination of CTP and PHT

[0048] As shown in Table 2, the intra-day precision of CTP and its conversion product PHT in rubber was determined by the method in Example 1 of this application. The intra-day precision of CTP and PHT was 0.45-0.78%. The inter-day precision was assessed by six parallel determinations over six days. The intra-day precision of CTP and PHT was 1.1-1.9%, which met the analytical requirements.

[0049] Application Example 1 The CTP and PHT contents in three rubber samples (rubber samples 1-3) were determined using the method described in Example 1: Three rubber samples were collected, and the CTP and PHT contents in the three rubber samples were determined using the method described in Example 1. Then, solutions of CTP and PHT at 1 mg / L were prepared and added dropwise to the rubber samples for spiked recovery testing. The spiked recovery rate was calculated as: [(spike sample measurement value - background sample measurement value) / spiked amount] × 100%. Generally, when the analyte concentration in the sample is less than 1 mg / L, the required spiked recovery rate is 80-120%; when the analyte concentration in the sample is greater than or equal to 1 mg / L, the required spiked recovery rate is 90-110%. The concentration point was measured three times, and the average value, relative standard deviation, and spiked recovery rate of the three measurements were calculated. CTP and PHT were detected in all three rubber samples. The experimental results are shown in Table 3.

[0050] Table 33 shows the precision and accuracy results of CTP and PHT in 33 rubber samples.

[0051] As can be seen from Table 3, the recovery rate of spiked samples using the method of Example 1 was 98-103%, and the relative standard deviation was 0.2-3.1%. Therefore, the analytical method for CTP and PHT content in rubber samples of Example 1 has high accuracy and good reproducibility.

[0052] Comparative Example 1 The national standard GB / T24801-2019, "Anti-scorching agent N-cyclohexylthiophthalimide (CTP)" (standard method: C18 stationary phase, 5 μm particle size, 250 mm length, 4.6 mm inner diameter; mobile phase: acetonitrile:water = 80:20; flow rate: 1 mL / min; column temperature: 40℃; injection volume: 10 μL; detection wavelength: 254 nm), was used to detect the anti-scorching agent CTP and its conversion product PHT in rubber. The specific steps included: Weigh 10 mg of the rubber sample to be tested, cut it into small particles of about 1-2 mm, place it in a glass container, and add 5 mL of acetonitrile as the extraction solution; The mixed rubber particles and the extraction solution were subjected to ultrasonic-assisted extraction at 40°C for 10 min. After extraction, the supernatant was collected and filtered through a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane to obtain the filtrate. The resulting filtrate was used for liquid chromatography analysis; The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, and the detection wavelength is 254 nm; The injection method was automatic injection, and the injection volume was 10 μL. The mobile phase is: acetonitrile:water = 80:20; The column temperature is 40℃; the column packing material is EC-C18, with a length of 150 mm, an inner diameter of 4.6 mm, and a particle diameter of 4 μm. Standard curves were plotted using the external standard method, with the concentrations of CTP and PHT as the x-axis and the peak area of ​​the target substance as the y-axis. Standard curves for CTP and PHT were obtained by linear fitting.

[0053] The liquid chromatogram of CTP and PHT in rubber sample 1 tested using the method of Comparative Example 1 is shown below. Figure 2 As shown, based on the chromatographic performance of actual samples, the national standard method, under the conditions of "80% acetonitrile + 254 nm isocratic chromatography," lacks sufficient selectivity and resolution for CTP and PHT in the rubber matrix. Furthermore, it suffers from high baseline background, broad hump coverage, and retention time drift and inter-peak interference due to weak retention, resulting in unreliable peak identification and integration, and excessively high detection / quantitation limits. Therefore, the national standard method is not suitable for the direct quantification of CTP and PHT in complex rubber samples.

[0054] Comparative Example 2 The difference between this comparative example and Comparative Example 1 is: Change the mobile phase to gradient elution mode: The mobile phase gradient condition is: 0-2 min Acetonitrile:water = 40:60, 2-5 min Gradient from 40:60 to 95:5, 5-8 min Maintain 95:5, 8-11 min Gradient from 95:5 back to 40:60, 11-13 min Maintain 40:60.

[0055] Comparative Example 3 The difference between this comparative example and Comparative Example 1 is: The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, the detection wavelength is 290 nm, and the reference wavelength is 360 nm.

[0056] The liquid chromatograms of CTP and PHT in rubber sample 1 tested using the methods of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 are shown below. Figure 2 and Figure 3As shown, in Comparative Example 1, when detected strictly according to national standard conditions (80% acetonitrile isocratic, 254 nm), PHT and CTP co-eluted with a large amount of rubber matrix components in the same time period, resulting in small and asymmetrical peaks with broad hump and long tails. The resolution and repeatability were insufficient, making reliable quantification difficult. Furthermore, simply changing the isocratic method to a gradient (Comparative Example 2) or changing the detection wavelength to 290 nm (Comparative Example 3) did not eliminate matrix interference due to only changing a single variable; the identification and integration of PHT and CTP remained unstable. In contrast, Example 1, using an optimized gradient program and detection wavelength, produced narrow and symmetrical peaks for PHT and CTP that were separated from the matrix baseline. The baseline was stable, and the signal-to-noise ratio was significantly improved, enabling accurate and low-detection-limited levels of CTP and PHT in complex rubber samples.

[0057] Comparative Example 4 The difference between this comparative example and Example 1 is as follows: Elution was performed under isocratic conditions, with the following mobile phase: Maintain an acetonitrile:water ratio of 40:60 for 0-13 minutes.

[0058] Comparative Example 5 The difference between this comparative example and Example 1 is as follows: Elution was performed under isocratic conditions, with the following mobile phase: Maintain an acetonitrile:water ratio of 70:30 for 0-13 minutes.

[0059] Comparative Example 6 The difference between this comparative example and Example 1 is as follows: Elution was performed under isocratic conditions, with the following mobile phase: Maintain an acetonitrile:water ratio of 90:10 for 0-13 minutes.

[0060] The liquid chromatograms of CTP and PHT in rubber sample 1 tested using the methods of Example 1, Comparative Example 4, Comparative Example 5, and Comparative Example 6 are shown below. Figure 3 As shown, under isocratic conditions, only PHT can be detected at 40:60, while CTP is difficult to precipitate. Although changing the isocratic ratio to 70:30 or 90:10 can make both precipitate, PHT and adjacent impurities are still difficult to achieve baseline separation, and quantification is not possible at low concentrations. Only the gradient program of Example 1 can simultaneously meet the requirements of separation and quantitative stability of both.

[0061] Comparative Example 7 The difference between this comparative example and Example 1 is as follows: The extract was methanol.

[0062] Comparative Example 8 The difference between this comparative example and Example 1 is as follows: The extract was ethanol.

[0063] The CTP and PHT in rubber sample 1 were tested using the methods of Comparative Examples 9, 10, and 11. The recovery rate test results of Example 1, Comparative Examples 8, and 9 are as follows: Figure 4 As shown, although methanol and ethanol have good polarity, they readily dissolve a large number of small molecule impurities in the rubber matrix, resulting in co-extraction with the target analytes. This leads to recovery rates of only 78.47% / 67.30% and 62.77% / 66.49% for CTP and PHT, respectively, which are insufficient for accurate quantification. In contrast, Example 1 uses acetonitrile as the extraction solvent, which combines strong aromatic compound solubility with low affinity for non-polar matrix components. This ensures efficient extraction of CTP and PHT while significantly reducing the dissolution of impurities in the rubber matrix, thereby greatly improving the selectivity and recovery rate of the target analytes. Experimental results show that when using acetonitrile, the recoveries of PHT and CTP reach 94.04% and 94.28%, respectively, far exceeding those of methanol and ethanol.

[0064] Comparative Example 9 The difference between this comparative example and Example 1 is as follows: The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, the detection wavelength is 254 nm, and the reference wavelength is 360 nm.

[0065] Comparative Example 10 The difference between this comparative example and Example 1 is as follows: The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, the detection wavelength is 270 nm, and the reference wavelength is 360 nm.

[0066] Comparative Example 11 The difference between this comparative example and Example 1 is as follows: The liquid chromatography analysis conditions are as follows: the detector is a diode array ultraviolet detector, the detection wavelength is 280 nm, and the reference wavelength is 360 nm.

[0067] The liquid chromatograms of CTP and PHT in rubber sample 1, tested using the methods of Comparative Examples 9, 10, and 11, are shown below. Figure 5 As shown, by Figure 3It is evident that the responses of PHT and CTP at 290 nm (peak areas of 34.8 and 17.6, respectively) are significantly higher than those at 270 nm (17.1 / 12.2) and 280 nm (24.3 / 12.6). While 254 nm produces strong general absorption, it also introduces a large amount of broad peaks and baseline elevation from the rubber matrix, causing the true signal to be "submerged." The essence of choosing 290 nm is to utilize the relative absorption advantage of phthalimide / thiophthalimide chromophores in a longer wavelength range, avoiding the co-absorption band of the matrix in the 254–280 nm range, thereby improving the signal-to-mass ratio from a spectral perspective. Combined with diode array detection and using 360 nm as a reference channel, the baseline fluctuations caused by the gradient can be further weakened, ensuring the repeatability of integration and quantification.

[0068] Experimental Example 1 The CTP and PHT contents in rubber samples 1-3 were determined using the method of Example 1 and Comparative Examples 1-3. The results are shown in Table 4. Then, solutions of CTP and PHT at 1 mg / L were prepared and added dropwise to the rubber samples for spiked recovery testing. The spiked recovery rate was calculated as: [(spike sample measurement value - background sample measurement value) / spiked amount] × 100%. Generally, when the analyte concentration in the sample is less than 1 mg / L, the required spiked recovery rate is 80-120%; when the analyte concentration in the sample is greater than or equal to 1 mg / L, the required spiked recovery rate is 90-110%. The concentration point was measured three times, and the average value, relative standard deviation, and spiked recovery rate of the three measurements were calculated. CTP and PHT were detected in all three rubber samples.

[0069] Table 4. Precision and accuracy results of CTP and PHT in rubber samples tested in Example 1 and Comparative Examples 1-3.

[0070] Combination Figure 2 The chromatographic morphology and the spiked results in Table 4 show that Example 1 achieved recoveries of 98–103% for both PHT and CTP in the three rubber samples, with RSD ≤ 3.1%, achieving both accuracy and precision (90–110% is considered acceptable for 1 mg / L concentration points). However, Comparative Examples 1–3 still suffered from matrix co-elution, broad hump, and peak asymmetry, resulting in unstable integration boundaries, low signal-to-noise ratios, and significant fluctuations in recovery rates, with RSDs mostly between 5–10%. For example, the recovery rate of PHT in Comparative Example 1 was only 89% in Sample 2, below the acceptable limit; although the recovery rates of Comparative Examples 2 and 3 fell within the 90–110% range, repeatability remained poor (e.g., the RSD for CTP could reach 8–9%). This is consistent with… Figure 2The poor separation and consistent baseline elevation shown indicate that changing the gradient or wavelength alone is insufficient to eliminate matrix interference. Only the optimized procedure in Example 1 enabled PHT and CTP to achieve baseline separation and a high signal-to-noise ratio, thus providing significantly better accuracy and precision in complex rubber samples.

Claims

1. A method for detecting a rubber anti-scorching agent and its conversion products, characterized in that, Includes the following steps: The rubber sample to be tested was extracted with the extraction solution and then N-cyclohexyl-thiophthalimide and phthalimide were quantitatively analyzed by liquid chromatography. The liquid chromatograph uses a gradient elution mode: for 0-2 min, the mobile phase is acetonitrile:water = 30-50:50-70; for 2-5 min, the mobile phase gradients from acetonitrile:water = 30-50:50-70 to acetonitrile:water = 80-99:1-20; for 5-8 min, the mobile phase is maintained at acetonitrile:water = 80-99:1-20; for 8-11 min, the mobile phase gradients back from acetonitrile:water = 80-99:1-20 to acetonitrile:water = 30-50:50-70; and for 11-13 min, the mobile phase is maintained at acetonitrile:water = 30-50:50-70.

2. The detection method for the rubber anti-scorching agent and its conversion products according to claim 1, characterized in that, The liquid chromatograph uses automatic injection with an injection volume of 8-12 μL.

3. The detection method for the rubber anti-scorching agent and its conversion products according to claim 2, characterized in that, The detector of the liquid chromatograph is a diode array ultraviolet detector with a detection wavelength of 285-295 nm and a reference wavelength of 350-370 nm.

4. The detection method for the rubber anti-scorching agent and its conversion products according to claim 1, characterized in that, The extractant was acetonitrile.

5. The detection method for the rubber anti-scorching agent and its conversion products according to claim 4, characterized in that, The ratio of the amount of rubber sample to be tested to the amount of extract added is 5-15 mg: 1-10 mL.

6. The method for detecting the rubber anti-scorching agent and its conversion products according to any one of claims 1-5, characterized in that, Includes the following steps: The rubber sample to be tested was cut into small pieces, added to the extraction solution, and subjected to ultrasonic-assisted extraction. The extraction solution was collected and filtered to obtain the filtrate. Then, liquid chromatography was used to quantitatively analyze N-cyclohexyl-thiophthalimide and phthalimide in the filtrate.

7. The method for detecting the rubber anti-scorching agent and its conversion products according to claim 6, characterized in that, The temperature for ultrasound-assisted extraction is 30-50℃, and the time is 5-25 min.

8. The method for detecting the rubber anti-scorching agent and its conversion products according to claim 5, characterized in that, The liquid chromatograph uses a liquid chromatograph column with a column temperature of 30-50℃, EC-C18 packing material with a length of 150 mm, an inner diameter of 4.6 mm, and a packing particle diameter of 4 µm.

9. The method for detecting the rubber anti-scorching agent and its conversion products according to claim 5, characterized in that, The external standard method was used to plot the standard curves. The concentrations of the target compounds N-cyclohexyl-thiophthalimide and phthalimide were plotted on the x-axis and the peak areas of the target compounds were plotted on the y-axis. The standard curves of N-cyclohexyl-thiophthalimide and phthalimide were obtained by linear fitting.

10. The application of the detection method of the rubber anti-scorching agent and its conversion products according to any one of claims 1-9 in rubber testing.

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