Method for determining antioxidants 618 and 626 in polypropylene

Abstract

The invention relates to the technical field of high polymer material analysis, in particular to a method for determining antioxidants 618 and 626 in polypropylene. The method is mainly used for solving the problems of inaccurate quantitative analysis result, unknown peak interference of a spectrogram and the like caused by easy oxidation and hydrolysis of antioxidants 618 and 626 in the sample pretreatment and liquid phase detection process, has the advantages of saving the pretreatment cost, obviously shortening the pretreatment time, thorough target object extraction effect and the like, and has the advantages of high automation degree of operation, high accuracy and high accuracy. The influence of personal errors on quantitative results is reduced, and the method is of great significance to quantitative analysis of efficient bisphosphite antioxidants.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer material analysis, and in particular to a method for determining antioxidants 618 and 626 in polypropylene. Background Art

[0002] Polypropylene is a thermoplastic resin synthesized by homopolymerization of propylene alone or copolymerization with a small amount of ethylene. Due to its excellent physical, chemical and processing properties, polypropylene is widely used in the production fields such as food, drug packaging, clothing fibers, medical devices, automotive parts, pipeline transportation, etc. During the synthesis, storage, processing and application stages of polypropylene, its molecular chain is easily affected by factors such as heat, ultraviolet light, mechanical stress, electric field, radiation and chemical media, resulting in chain breakage, cross-linking or chemical composition changes; the results are often manifested as aging phenomena such as product fading and discoloration, surface cracking, and significant decline in mechanical properties such as strength, stiffness and toughness. Therefore, antioxidants are usually added during the production of polypropylene to improve, inhibit or delay the aging process, thereby extending its service life and increasing its use value.

[0003] At present, the commonly used antioxidants in polypropylene are mainly free radical inhibitors (primary antioxidants) and hydroperoxide decomposer complexes (secondary antioxidants). In phosphite antioxidants, the trivalent phosphorus element can reduce the active oxygen in hydroperoxides to stable alcohols to achieve antioxidant effects. However, due to the influence of electron delocalization, the trivalent phosphorus element is easily oxidized and hydrolyzed in the air. Some studies have shown that tris(2,4-di-tert-butylphenyl) phosphite (hereinafter referred to as antioxidant 168) did not undergo obvious hydrolysis within 150 hours under the conditions of a temperature of 50 °C and a relative air humidity of 75%. Due to the weak steric hindrance effect of the alkyl structure, distearyl pentaerythritol diphosphite (hereinafter referred to as antioxidant 618) and bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite (hereinafter referred to as antioxidant 626) are completely hydrolyzed into mixtures such as phosphite, 2,4-di-tert-butylphenol, isopentaeitol, stearyl alcohol, etc. within a dozen hours. (See: Fu Jianying. Plastic Additives, 2017, (05): 12-17.). In existing standards, HG / T 3712-2010 clearly evaluates the stability of antioxidant 168, while HG / T 3878-2006 and HG / T 3974-2019 do not form chemical industry standards for the stability of antioxidants 618 and 626.

[0004] At present, antioxidants for polymers are mainly obtained through Soxhlet extraction, ultrasonic extraction, microwave extraction and accelerated solvent extraction. GB / T 25277-2010, SNT 1504.1-2014, and CN108120787B mentioned Soxhlet extraction and liquid chromatography to determine antioxidant 168 in resin products, but unfortunately, the standard adopts a heating reflux extraction method, and the pretreatment takes several to more than ten hours. Long-term extraction in high temperature and air atmosphere is prone to oxidation and degradation of antioxidants, and has poor adaptability to highly reactive antioxidants 618 and 626. Secondly, the liquid chromatography ultraviolet detector has poor responsiveness to the long stearyl alcohol chain of antioxidant 618, and the differential detector is not suitable for gradient elution, resulting in poor separation of the analyte and the main antioxidant. Therefore, the existing analysis method is time-consuming and has poor quantitative analysis effect, which is difficult to meet the detection needs of high-efficiency pentaerythritol diphosphite antioxidants. At present, there is no public report on the method of using liquid chromatography to accurately determine the content of antioxidants 618 and 626; in contrast, liquid chromatography-mass spectrometry has extremely strong structural investigation and trace detection capabilities, and is more suitable for accurate quantitative analysis of low-stability antioxidants. In summary, developing a test method with efficient pretreatment process, stable target during extraction, and accurate test results is a technical problem that needs to be urgently solved in the analysis of high-efficiency pentaerythritol bisphosphite antioxidants 618 and 626. Summary of the invention

[0005] In order to solve the above problems, the purpose of the present invention is to provide a method for determining antioxidants 618 and 626 in polypropylene. The method of the present invention can accurately detect the content of antioxidants 618 and 626 in polypropylene, and has the advantages of saving pretreatment costs, significantly shortening pretreatment time, thorough extraction effect of the test substance, and high degree of operation automation.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] The present invention provides a method for determining antioxidants 618 and 626 in polypropylene, comprising the following steps:

[0008] (S1) dissolving an antioxidant 618 standard in a solvent and accelerating the solvent extraction by an accelerated solvent extractor, and then preparing standard solutions of different concentrations and testing them by liquid chromatography-triple quadrupole mass spectrometry, using the concentration as the abscissa and the characteristic peak intensity as the ordinate to obtain a standard curve of the antioxidant 618;

[0009] The antioxidant 626 standard product is dissolved in a solvent and accelerated by solvent extraction using an accelerated solvent extractor, and then standard solutions of different concentrations are prepared and tested by liquid chromatography-triple quadrupole mass spectrometry, with the concentration as the abscissa and the characteristic peak intensity as the ordinate to obtain a standard curve of the antioxidant 626;

[0010] (S2) Grind the sample to be tested, dissolve it in a solvent, and perform accelerated solvent extraction using an accelerated solvent extractor to obtain a solution to be tested. Use a liquid chromatography-triple quadrupole mass spectrometer to test the solution to be tested. Substitute the characteristic peak intensity of antioxidant 618 obtained into the standard curve of antioxidant 618 obtained in step (S1) to obtain the content of antioxidant 618 in the sample to be tested. Substitute the characteristic peak intensity of antioxidant 626 obtained into the standard curve of antioxidant 626 obtained in step (S1) to obtain the content of antioxidant 626 in the sample to be tested.

[0011] In one embodiment of the present invention, during the accelerated solvent extraction process, the pressure is 8.27 - 12.4 MPa, heat at 80 - 120 °C for 5 min, static for 3 - 5 min, the rinse volume is 60%, nitrogen blow for 60 s, and cycle 2 - 3 times.

[0012] In one embodiment of the present invention, in step (S2), the sample to be tested is ground to a particle size of less than 0.01 mm.

[0013] In one embodiment of the present invention, in step (S2), the solvent is selected from one or two of methanol, isopropanol, acetonitrile, chloroform or cyclohexane.

[0014] In one embodiment of the present invention, the solvent is isopropanol.

[0015] In one embodiment of the present invention, during the test with the liquid chromatography-triple quadrupole mass spectrometer, the chromatographic column is a Waters BEH C8 column, with a specification of 2.1×100 mm, 1.7 μm; the injection volume is 1 - 10 μL; the column temperature is 40 - 45 °C; the flow rate of the mobile phase is 0.4 - 0.6 mL / min; the mass spectrometry ionization method is the positive ion mode of atmospheric pressure chemical ionization source; the mass spectrometry detection method is the multiple reaction monitoring mode; the ion source temperature is 150 °C, the corona needle current is 2 μA, and the corona needle temperature is 400 °C.

[0016] In one embodiment of the present invention, during the test with the liquid chromatography-triple quadrupole mass spectrometer, mobile phase A is an aqueous solution containing 2 - 5 mmol / L of ammonium salt; the ammonium salt is selected from one or several of ammonium bicarbonate, ammonium formate, ammonium acetate or ammonium fluoride.

[0017] In one embodiment of the present invention, during the test with the liquid chromatography-triple quadrupole mass spectrometer, mobile phase B is a methanol / isopropanol mixed solution containing 2 - 5 mmol / L of ammonium salt; the ammonium salt is selected from one or several of ammonium bicarbonate, ammonium formate, ammonium acetate or ammonium fluoride.

[0018] In one embodiment of the present invention, the flow rate of the mobile phase is 0.4 to 0.6 mL / min; and the gradient elution condition is 0 to 2 minutes.

[0019] The volume ratio of mobile phase B is increased from 80% to 100%, the volume ratio of mobile phase B is maintained at 100% for 2 to 8 minutes, and the volume ratio of mobile phase B is reduced from 100% to 80% for 8 to 10 minutes.

[0020] In one embodiment of the present invention, during the liquid chromatography-triple quadrupole mass spectrometry test, the mass spectrometry ion mode is the APCI positive ion mode.

[0021] One of the main sources of unknown peaks in the spectrum is the decomposition products of two antioxidants. Liquid chromatography or liquid triple quadrupole chromatography does not have qualitative analysis capabilities and can only indirectly determine the ownership of the unknown peaks based on the retention time of the standard sample of the decomposition product on the spectrum. The unknown peak may be close to the analyte, resulting in poor separation and inaccurate quantitative results. However, under the dual effects of optimizing the extraction method to reduce the pretreatment time and adding an ammonium salt stabilizer to the liquid mobile phase to inhibit the acid-catalyzed hydrolysis of the antioxidant, after stability testing, the organic solvent heating reflux method requires about 6 hours of pretreatment before the test is completed on the machine, and the decomposition rate of the antioxidant 618 accounts for about 22% of the total. The method of the present invention can complete the test on the machine within 30 to 40 minutes, and the problem of unknown peak interference is significantly solved.

[0022] Another source of unknown peaks is other chemical auxiliary components in the extraction matrix. The present invention adopts ultra-high performance liquid chromatography UPLC, which can save single-needle sampling time and reduce the diffusion efficiency of the test solution in the mobile phase, optimize the peak shape, and improve the spectral peak separation, thereby solving the problem of unknown peak interference from another angle.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The invention discloses a method for accurately quantifying the addition amount of antioxidants 618 and 626 in polypropylene resin, which is mainly used to solve the problems that the antioxidants 618 and 626 are easily oxidized and hydrolyzed during sample pretreatment and liquid phase detection, resulting in inaccurate quantitative analysis results, and the interference of unknown peaks in the spectrum. The method not only has the advantages of saving pretreatment costs, significantly shortening pretreatment time, and thoroughly extracting the target object, but also reduces the influence of human errors on the quantitative results due to the high degree of operation automation, and is of great significance for the quantitative analysis of high-efficiency bisphosphite antioxidants. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is the total ion current chromatogram of antioxidants 618 and 626 in Example 1;

[0026] Figure 2 is the secondary mass spectrum of the antioxidant 618 in Example 1;

[0027] Figure 3 It is the secondary mass spectrum of antioxidant 626 in Example 1;

[0028] Figure 4 It is the standard curve of antioxidant 618 in Example 1;

[0029] Figure 5 It is the standard curve of antioxidant 626 in Example 1. Detailed implementation manners

[0030] The present invention provides a method for determining antioxidants 618 and 626 in polypropylene, comprising the following steps:

[0031] (S1) Dissolve the standard product of antioxidant 618 in a solvent and perform accelerated solvent extraction by an accelerated solvent extractor, then prepare standard product solutions with different concentrations and test them by a liquid chromatography-triple quadrupole mass spectrometer. Use its concentration as the abscissa and its characteristic peak intensity as the ordinate to obtain the standard curve of antioxidant 618;

[0032] Dissolve the standard product of antioxidant 626 in a solvent and perform accelerated solvent extraction by an accelerated solvent extractor, then prepare standard product solutions with different concentrations and test them by a liquid chromatography-triple quadrupole mass spectrometer. Use its concentration as the abscissa and its characteristic peak intensity as the ordinate to obtain the standard curve of antioxidant 626;

[0033] (S2) Grind the sample to be tested and dissolve it in a solvent, then perform accelerated solvent extraction by an accelerated solvent extractor to obtain a solution to be tested. Use a liquid chromatography-triple quadrupole mass spectrometer to test the solution to be tested. Substitute the characteristic peak intensity of antioxidant 618 obtained into the standard curve of antioxidant 618 obtained in step (S1) to obtain the content of antioxidant 618 in the sample to be tested. Substitute the characteristic peak intensity of antioxidant 626 obtained into the standard curve of antioxidant 626 obtained in step (S1) to obtain the content of antioxidant 626 in the sample to be tested.

[0034] Further, during the accelerated solvent extraction process, the pressure is 8.27 - 12.4 MPa, heat at 80 - 120 °C for 5 min, static for 3 - 5 min, the rinse volume is 60%, nitrogen blow for 60 s, and cycle 2 - 3 times.

[0035] Further, in step (S2), grind the sample to be tested to a particle size of less than 0.01 mm.

[0036] Further, in step (S2), the solvent is selected from one or two of methanol, isopropanol, acetonitrile, chloroform or cyclohexane;

[0037] Preferably, the solvent is isopropanol.

[0038] Further, during the test of the liquid chromatography-triple quadrupole mass spectrometer, the chromatographic column is a Waters BEH C8 column with a specification of 2.1×100 mm, 1.7 μm; the injection volume is 1-10 μL; the column temperature is 40-45 °C; the flow rate of the mobile phase is 0.4-0.6 mL / min; the mass spectrometry ionization mode is the positive ion mode of atmospheric pressure chemical ionization source; the mass spectrometry detection mode is the multiple reaction monitoring mode; the ion source temperature is 150 °C, the corona needle current is 2 μA, and the corona needle temperature is 400 °C.

[0039] Further, during the test of the liquid chromatography-triple quadrupole mass spectrometer, mobile phase A is an aqueous solution containing 2-5 mmol / L of ammonium salt; the ammonium salt is selected from one or more of ammonium bicarbonate, ammonium formate, ammonium acetate, or ammonium fluoride.

[0040] Further, during the test of the liquid chromatography-triple quadrupole mass spectrometer, mobile phase B is a methanol / isopropanol mixed solution containing 2-5 mmol / L of ammonium salt; the ammonium salt is selected from one or more of ammonium bicarbonate, ammonium formate, ammonium acetate, or ammonium fluoride.

[0041] Further, the flow rate of the mobile phase is 0.4-0.6 mL / min; the gradient elution condition is 0-2 minutes.

[0042] The volume ratio of mobile phase B increases from 80% to 100%, the volume ratio of mobile phase B remains 100% from 2-8 minutes, and the volume ratio of mobile phase B decreases from 100% to 80% from 8-10 minutes.

[0043] Further, during the test of the liquid chromatography-triple quadrupole mass spectrometer, the mass spectrometry ion mode is the APCI positive ion mode.

[0044] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] In the following embodiments, unless otherwise specified, the reagents used are commercially available reagents; the detection means and methods used are conventional detection means and methods in the art.

[0046] Example 1

[0047] (1) Preparation of prefabricated test samples: Take pure isotactic polypropylene powder without additives. Mix one group with 0.1 wt% of antioxidant 618; mix another group with 0.1 wt% of antioxidant 626. Keep the temperature at 170 °C and the die pressure at 1.2 MPa, hot melt press for 10 minutes and then cool to room temperature. Put the pressed sheet into a cryogenic ball mill for crushing to obtain polypropylene pellets containing 0.1% of antioxidant 618 and polypropylene pellets containing 0.1% of antioxidant 626 respectively. Accurately weigh 1.000 g of polypropylene pellets containing 0.1 wt% of antioxidant 618 and polypropylene pellets containing 0.1 wt% of antioxidant 626 as samples for standby.

[0048] (2) Preparation of standard solutions: Dissolve the samples obtained in step (1) (polypropylene pellets containing 0.1 wt% of antioxidant 618 and polypropylene pellets containing 0.1 wt% of antioxidant 626) in isopropanol (processed by an accelerated solvent extractor: pressure is 10.3 MPa, heated at 80 °C for 5 min, static for 3 min, rinse volume is 60%, nitrogen blow for 60 s, cycle 2 times), and successively dilute to obtain standard solutions with gradient concentrations of 100.00, 50.00, 10.00, 5.00, 1.00, 0.50, 0.10 μg / L (prepared for antioxidant 618 and antioxidant 626 respectively) for standby.

[0049] (3) Preparation of mobile phase: Accurately weigh a certain amount of ammonium acetate and ammonium fluoride, and prepare an aqueous solution with an ammonium acetate concentration of 450 mM and an ammonium fluoride concentration of 50 mM and an isopropanol / methanol mixed solution (volume ratio of isopropanol / methanol mixed solution is 1:3), mix well for standby.

[0050] (4) Use a liquid chromatography-triple quadrupole mass spectrometer to detect the standard solutions obtained in step (2) (the total ion chromatograms of antioxidant 618 and 626 are as shown in Figure 1 , and the secondary mass spectra are as shown in Figure 2 , Figure 3 ), and establish standard curves for antioxidant 618 ( Figure 4 ) and antioxidant 626 ( Figure 5 ) respectively;

[0051] Among them, during the detection process of the liquid chromatography-triple quadrupole mass spectrometer, the column temperature of the ultra-high performance liquid chromatography-triple quadrupole mass spectrometer is 45 °C; the ion source temperature is 150 °C; the transfer line temperature is 250 °C; the chromatographic column is Waters BEH-C8 (2.1×100 mm, 1.7 μm); the injection volume is 1 μL; mobile phase A is an aqueous solution containing 2 mmol / L ammonium bicarbonate, and mobile phase B is a mixed solution of isopropanol / methanol containing 2 mmol / L ammonium bicarbonate; the flow rate of the mobile phase is 0.4 - 0.6 mL / min; the gradient elution conditions are that the volume ratio of mobile phase B increases from 80% to 100% in 0 - 2 minutes, the volume ratio of mobile phase B remains 100% in 2 - 8 minutes, and the volume ratio of mobile phase B decreases from 100% to 80% in 8 - 10 minutes. The mass spectrometry ion mode is APCI positive ion mode; mass analyzer: triple quadrupole mass analyzer; scanning mode: multiple reaction monitoring mode (MRM); mass spectrometry scanning range: 90 - 760 (m / z); solvent delay: 3 min; gain coefficient: 1.00.

[0052] (5) Grind the sample to be tested (containing 0.090 wt% antioxidant 618 and 0.090 wt% antioxidant 626) by the method of step (1), dissolve it in isopropanol, and perform accelerated solvent extraction by an accelerated solvent extractor (the same as step (2)), test it using a liquid chromatography-triple quadrupole mass spectrometer, and compare it with the standard curve obtained in step (4) to obtain the contents of antioxidant 618 and 626; through quantitative analysis, the mass ratios of antioxidant 618 and 626 in the polypropylene sheet detected in this example are 0.087 wt% and 0.091 wt%.

[0053] Example 2

[0054] Take an appropriate amount of commercially available random copolymer polypropylene pipe containing 0.070 wt% antioxidant 618 for accelerated solvent extraction liquid chromatography mass spectrometry analysis, adopt the method in Example 1, and through quantitative analysis, the content of antioxidant 618 in the polypropylene pellets detected in this example is 0.062 wt%.

[0055] Example 3

[0056] Take an appropriate amount of commercially available biaxially oriented polypropylene material containing 0.150 wt% antioxidant 626 for accelerated solvent extraction liquid chromatography mass spectrometry analysis, adopt the method in Example 1, and through quantitative analysis, the content of antioxidant 626 in the polypropylene pellets detected in this example is 0.128 wt%.

[0057] Example 4

[0058] An appropriate amount of commercially available injection-molded block copolymer polypropylene containing 0.500 wt% antioxidant 626 was subjected to accelerated solvent extraction liquid chromatography-mass spectrometry analysis. Using the method in Example 1, after quantitative analysis, the content of antioxidant 626 in the polypropylene pellets detected in this example was 0.454 wt%.

[0059] In summary, the method of the present invention is simple, fast, accurate and reliable. It can effectively separate and simultaneously detect antioxidant 618 and antioxidant 626 in polypropylene products within 30 minutes, greatly reducing the quantitative deviation caused by the oxidation and hydrolysis of high-efficiency phosphite antioxidants.

[0060] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. Obviously, those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention as interpreted by the present invention should be within the protection scope of the present invention.

Claims

1. A method for determining antioxidant 618 and 626 in polypropylene, characterized in that, it comprises the following steps: (S1) Dissolve the antioxidant 618 standard in a solvent and perform accelerated solvent extraction using an accelerated solvent extractor. Then, prepare standard solutions with different concentrations and test them using a liquid chromatography-triple quadrupole mass spectrometer. Use its concentration as the abscissa and its characteristic peak intensity as the ordinate to obtain the standard curve of antioxidant 618; Dissolve the antioxidant 626 standard in a solvent and perform accelerated solvent extraction using an accelerated solvent extractor. Then, prepare standard solutions with different concentrations and test them using a liquid chromatography-triple quadrupole mass spectrometer. Use its concentration as the abscissa and its characteristic peak intensity as the ordinate to obtain the standard curve of antioxidant 626; (S2) Grind the sample to be tested and dissolve it in a solvent, and perform accelerated solvent extraction using an accelerated solvent extractor to obtain a test solution. Use a liquid chromatography-triple quadrupole mass spectrometer to test the test solution. Substitute the characteristic peak intensity of antioxidant 618 obtained into the standard curve of antioxidant 618 obtained in step (S1) to obtain the content of antioxidant 618 in the sample to be tested. Substitute the characteristic peak intensity of antioxidant 626 obtained into the standard curve of antioxidant 626 obtained in step (S1) to obtain the content of antioxidant 626 in the sample to be tested.

2. The method for determining antioxidant 618 and 626 in polypropylene according to claim 1, characterized in that, during the accelerated solvent extraction process, the pressure is 8.27 - 12.4 MPa, heat at 80 - 120 °C for 5 min, static for 3 - 5 min, the rinse volume is 60%, nitrogen blow for 60 s, and cycle 2 - 3 times.

3. The method for determining antioxidant 618 and 626 in polypropylene according to claim 1, characterized in that, in step (S2), the sample to be tested is ground to a particle size of less than 0.01 mm.

4. The method for determining antioxidant 618 and 626 in polypropylene according to claim 1, characterized in that, in step (S2), the solvent is selected from one or two of methanol, isopropanol, acetonitrile, chloroform or cyclohexane.

5. The method for determining antioxidant 618 and 626 in polypropylene according to claim 4, characterized in that, the solvent is isopropanol.

6. The method for determining antioxidant 618 and 626 in polypropylene according to claim 1, characterized in that, during the test using a liquid chromatography-triple quadrupole mass spectrometer, the column temperature is 40 - 45 °C; the flow rate of the mobile phase is 0.4 - 0.6 mL / min; the mass spectrometry ionization mode is the positive ion mode of atmospheric pressure chemical ionization source; the mass spectrometry detection mode is the multiple reaction monitoring mode; the ion source temperature is 150 °C, the corona needle current is 2 μA, and the corona needle temperature is 400 °C.

7. The method for determining antioxidant 618 and 626 in polypropylene according to claim 1, characterized in that, during the test using a liquid chromatography-triple quadrupole mass spectrometer, mobile phase A is an aqueous solution containing 2 - 5 mmol / L ammonium salt.

8. A method for determining antioxidants 618 and 626 in polypropylene according to claim 7, characterized in that, during the test with a liquid chromatography-triple quadrupole mass spectrometer, mobile phase B is a methanol / isopropanol mixed solution containing 2 - 5 mmol / L ammonium salt.

9. A method for determining antioxidants 618 and 626 in polypropylene according to claim 8, characterized in that, the flow rate of the mobile phase is 0.4 - 0.6 mL / min; the gradient elution condition is 0 - 2 minutes. The volume ratio of mobile phase B increases from 80% to 100%, the volume ratio of mobile phase B remains 100% for 2 - 8 minutes, and the volume ratio of mobile phase B decreases from 100% to 80% for 8 - 10 minutes.

10. A method for determining antioxidants 618 and 626 in polypropylene according to claim 1, characterized in that, during the test with a liquid chromatography-triple quadrupole mass spectrometer, the mass spectrometry ion mode is APCI positive ion mode.

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