Method for determining formyl terminal group content in polyacetal resin molecular chain

Abstract

The invention relates to the technical field of analysis and testing of high polymer materials, in particular to a method for measuring the content of formyl terminal groups in a polyacetal resin molecular chain, which comprises the following steps: carrying out digestion reaction on a polyacetal resin sample through a hydrochloric acid and methanol system to quantitatively convert the formyl terminal groups in the molecular chain into methyl formate; a staged temperature control distillation procedure is adopted, and effective separation of methyl formate and a complex matrix is realized by accurately controlling the distillation temperature; collecting distillate, carrying out gas chromatographic analysis, and calculating the concentration of methyl formate in combination with an external standard method. And finally, deriving the formyl terminal group content based on sample mass, distillate mass and methyl formate concentration data. The method effectively solves the technical problem of incomplete target separation caused by azeotropic interference and inaccurate distillation temperature control in a complex digestion matrix, establishes a systematic, accurate and good-reproducibility detection method, and provides a reliable analysis means for polyacetal resin quality control.

Description

Technical Field

[0001] This invention relates to the field of polymer material analysis and testing technology, and in particular to a method for determining the content of formyl end groups in the molecular chain of polyacetal resin. Background Technology

[0002] Polyacetal resin, an engineering plastic polymerized from formaldehyde monomers, has a molecular chain composed of repeating methylene oxygen units. The formyl end groups at the ends of the molecular chains are formyl groups, which usually originate from incomplete reactions or side reactions of the monomers during polymerization. The content of formyl end groups directly affects the thermal stability and processing performance of the resin. Therefore, techniques for determining the content of formyl end groups, such as analytical methods based on infrared spectroscopy or nuclear magnetic resonance, can quantitatively assess the distribution level of end groups in the molecular chain. In the quality control of polyacetal resin, such determinations provide key data for monitoring the polymerization process and resin purity, thereby supporting the optimization of production processes and maintaining the consistency of product performance.

[0003] The determination of formyl end-group content in polyacetal resin molecular chains suffers from several technical challenges, specifically azeotropic interference from complex digestion matrices and inaccurate distillation temperature control leading to incomplete separation of target analytes and low quantitative accuracy. During the quality control of polyacetal resins, the mixture generated after the digestion reaction includes excess hydrochloric acid, methanol, polymer fragments, and byproducts, which readily form azeotropes. For example, during the distillation separation stage, methyl formate may azeotropically react with methanol or water, resulting in the inclusion of high-boiling-point impurities in the distillate, thus interfering with the specificity of gas chromatography analysis. Furthermore, the lack of standardized temperature control procedures, such as inconsistent distillation endpoint temperature settings, can lead to incomplete recovery of low-boiling-point target analytes or the introduction of non-target components, ultimately affecting the reliability of formyl end-group content calculations. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for determining the formyl end group content in the molecular chain of polyacetal resin, solving the technical problems of incomplete separation of target substances and low quantitative accuracy caused by azeotropic interference in complex digestion matrices and inaccurate control of distillation temperature.

[0005] To solve the above-mentioned technical problems, the specific technical solution of the present invention is as follows: This invention provides a method for determining the formyl end group content in a polyacetal resin molecular chain, comprising: Step 1: Collect polyacetal resin samples and obtain sample mass data by weighing. Step 2: Weigh the polyacetal resin sample using the sample mass data from Step 1, and react it with a mixed solution of hydrochloric acid and methanol under heating conditions to convert the formyl end group into methyl formate, generating a digestion solution containing methyl formate. Step 3: Distill the digest from Step 2, control the distillation temperature to distill off methyl formate, collect the distillate containing methyl formate, and record the mass data of the distillate. Step 4: Perform gas chromatography analysis on the distillate from Step 3 to obtain the peak area data of methyl formate, and calculate the concentration data of methyl formate by external standard method. Step 5: Substitute the sample mass data from Step 1, the distillate mass data from Step 3, and the methyl formate concentration data from Step 4 into the calculation formula to obtain the formyl end-group content result.

[0006] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain of the present invention, the digestion reaction in step 2 is carried out under heating and reflux conditions, the round-bottom flask has a capacity of 100 mL to 250 mL, and the condenser has a length of 30 cm to 40 cm; the volume ratio of hydrochloric acid to methanol is 1:5 to 1:10, the concentration of hydrochloric acid is 36% to 38%, and the methanol is anhydrous methanol with a purity of not less than 99.8%; the reaction temperature is controlled at 40°C to 50°C, and the temperature is measured by a digital thermometer with an accuracy of ±0.1°C; the reaction time is maintained at 30 minutes to 60 minutes, and the time is controlled by a timer with an error of no more than ±1 second; the digestion reaction is carried out under stirring conditions, and the stirring speed is 200 rpm to 300 rpm.

[0007] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain described in this invention, the distillation temperature program in step 3 is divided into two stages: the first stage controls the heating power to make the system gently boil, with the power set to 100 watts to 150 watts, the distillation temperature measurement range being 0°C to 100°C, the scale division being 0.1°C, and the reading not exceeding 41°C, lasting for 10 to 15 minutes; the second stage increases the heating power to 200 watts to 250 watts, the distillation rate is controlled at a distillation volume of 1% to 2% of the total liquid per minute, and the total liquid volume being 50 ml to 100 ml, and heating is immediately terminated when the distillation temperature reading rises to 61°C.

[0008] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain described in this invention, the distillation separation in step 3 is carried out under ice-water bath cooling conditions. The ice-water bath container has dimensions of 20 cm × 15 cm × 10 cm, and the ratio of crushed ice to water inside is 1:1. The condensation temperature is maintained at 0°C to 5°C, and the temperature is monitored by thermocouples with an accuracy of ±0.5°C. The receiving bottle is made of borosilicate glass with a capacity of 50 ml to 100 ml, and the ratio of the ice-water bath volume to the receiving bottle volume is 3:1.

[0009] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain described in this invention, the gas chromatography analysis in step 4 employs a flame ionization detector, with a hydrogen flow rate of 30 mL / min, an air flow rate of 300 mL / min, and a make-up gas flow rate of 25 mL / min; the chromatographic column is a polar capillary column, the stationary phase is polyethylene glycol, the column length is 30 m, the inner diameter is 0.25 mm, and the film thickness is 0.25 μm; the column temperature program is an initial temperature of 40 °C, held for 2 minutes, then increased to 100 °C at a rate of 5 °C / min, and held for 5 minutes; the carrier gas is high-purity nitrogen with a purity of not less than 99.999%, and the flow rate is controlled at 1.0 mL / min; the injection port temperature is 200 °C, the injection volume is 1 μL, and the split ratio is 10:1.

[0010] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain described in this invention, the external standard method in step 4 is implemented by preparing a series of methyl formate standard solutions. The standard solution series includes 5 concentration gradients, ranging from 10 ppm to 100 ppm, with an interval of 20 ppm. The standard solutions use methanol as solvent, with a preparation accuracy of ±0.05 mL. The injection volume is 1 μL, and the transfer accuracy is ±0.1 μL. The linear calibration curve is plotted using the least squares method, with no less than 5 data points and a goodness-of-fit coefficient of no less than 0.995.

[0011] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain of the present invention, when calculating the formyl end group content in step 5, the molar ratio of formyl end group to methyl formate is 1:1, the molecular weight of formyl end group is 29 g / mol, and the molecular weight of methyl formate is 60 g / mol; the calculation is performed by the formula: formyl end group content = (methyl formate mass × 29) / (sample mass × 60) × 10^6, where the mass of methyl formate is obtained by multiplying the net weight of the distillate in step 3 and the concentration data in step 4, and the sample mass is imported from step 1.

[0012] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain described in this invention, the duration of the first stage distillation in step 3 is controlled with an accuracy of ±0.5 minutes, and the set value is 10 to 15 minutes; the second stage distillation is terminated by temperature monitoring, with a measurement error not exceeding ±0.5℃, and the heating power drops to zero within 1 second after the termination signal is triggered.

[0013] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain described in this invention, the ice-water bath cooling in step 3 is carried out under circulating conditions with a flow rate of 5 to 10 liters per minute; the water temperature fluctuation range is controlled within ±1°C; and the vertical distance between the receiving bottle and the outlet of the condenser is 5 to 10 centimeters.

[0014] Furthermore, in the method for determining the formyl end group content in the polyacetal resin molecular chain described in this invention, steps 1 to 5 are carried out under ventilated conditions with a wind speed of 0.5 m / s to 1.0 m / s; relative humidity below 30%; weighing accuracy of 0.1 mg; and sample mass data recorded to four decimal places; all glassware is pretreated at 105°C for 1 hour with a temperature fluctuation of ±2°C.

[0015] The beneficial effects of this invention are: The method for determining the formyl end group content in the molecular chain of polyacetal resin provided by this invention effectively solves the technical problems of incomplete separation of target substances and low quantitative accuracy caused by azeotropic interference in complex digestion matrices and inaccurate distillation temperature control through a systematic operation process and precise parameter control. The method uses a hydrochloric acid and methanol hydrolysis system to promote the specific conversion of formyl end groups to methyl formate, and combines a staged distillation temperature program to achieve separation based on boiling point differences, reducing the entrainment of high-boiling-point impurities. High-sensitivity detection is then achieved by gas chromatography with external standard method. This invention has the advantages of clear derivatization pathway, controllable reaction conditions, high separation efficiency and good reproducibility, providing a reliable and practical analytical means for quality monitoring, process optimization and product consistency evaluation in the production process of polyacetal resin. Attached Figure Description

[0016] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on the accompanying drawings without creative effort.

[0017] Figure 1 This is a schematic flowchart of a method for determining the formyl end group content in a polyacetal resin molecular chain according to the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. The technical solutions provided by various embodiments of this invention will be described in detail below with reference to the accompanying drawings.

[0019] To better understand the purpose of this invention, the invention will now be described in further detail.

[0020] This invention provides a method for determining the formyl end group content in a polyacetal resin molecular chain, comprising: Step 1: Collect polyacetal resin samples and obtain sample mass data by weighing. Step 2: Weigh the polyacetal resin sample using the sample mass data from Step 1, and react it with a mixed solution of hydrochloric acid and methanol under heating conditions to convert the formyl end group into methyl formate, generating a digestion solution containing methyl formate. Step 3: Distill the digest from Step 2, control the distillation temperature to distill off methyl formate, collect the distillate containing methyl formate, and record the mass data of the distillate. Step 4: Perform gas chromatography analysis on the distillate from Step 3 to obtain the peak area data of methyl formate, and calculate the concentration data of methyl formate by external standard method. Step 5: Substitute the sample mass data from Step 1, the distillate mass data from Step 3, and the methyl formate concentration data from Step 4 into the calculation formula to obtain the formyl end-group content result.

[0021] This invention provides a method for determining the formyl end-group content in the molecular chain of polyacetal resin, belonging to the field of polymer material analysis and testing technology. It aims to achieve reliable quantification of end-group content through a systematic operational process. The core of the method lies in converting the formyl end-group into detectable methyl formate, and then determining it through distillation separation and gas chromatography analysis. This method is suitable for quality monitoring scenarios in the production process of polyacetal resin.

[0022] When collecting polyacetal resin samples, an analytical balance must be used for weighing to obtain sample mass data. The weighing process is conducted under controlled environmental conditions to avoid introducing moisture that could affect the accuracy of subsequent reactions. The sample mass data serves as the initial input for the entire analytical chain, providing a baseline value for subsequent calculations.

[0023] After weighing, the sample was reacted with a mixed solution of hydrochloric acid and methanol under heating conditions. The reaction was carried out in a reflux apparatus, with a round-bottom flask and a condenser forming the reflux system. Hydrochloric acid, acting as a protic acid catalyst, formed a suitable reaction environment with methanol, promoting a nucleophilic addition-elimination reaction of the formyl end groups in the polyacetal resin molecular chain, specifically converting them to methyl formate. Simultaneously, the polyacetal resin backbone dissolved, generating a complex digestion solution including the target product, excess reagent, and polymer fragments. The reaction temperature and time were monitored using a digital thermometer and timer to maintain system stability and achieve quantitative conversion of the formyl end groups.

[0024] The digested solution was then subjected to distillation. The distillation apparatus included a distillation flask, a condenser, and a receiving flask, which was placed in an ice-water bath to maintain a low-temperature condensation environment. The distillation temperature program consisted of two stages. In the first stage, the heating power was controlled to maintain a gentle boil, keeping the distillation thermometer reading low to ensure complete sample dissolution and prevent premature loss of volatile components. In the second stage, the heating power was increased, accelerating the distillation rate. Heating was immediately stopped when the distillation thermometer reading reached a specific endpoint. Methyl formate, due to its low boiling point, preferentially distilled out, while high-boiling impurities remained in the distillation flask, thus overcoming azeotropic entrainment interference and obtaining a pure distillate dominated by methyl formate. The mass data of the distillate was recorded by weighing the receiving flask, serving as a key input for subsequent calculations.

[0025] The distillate was analyzed by gas chromatography using a flame ionization detector. A polar capillary column was used, and the column temperature program started at the initial temperature and increased at a constant rate to the required separation temperature. The external standard method was implemented by preparing a series of methyl formate standard solutions using methanol as the solvent. Analysis was performed under identical chromatographic conditions to obtain a linear calibration curve of peak area versus concentration. After injection of the distillate, the methyl formate peak area data was recorded, and the methyl formate concentration was calculated using the calibration curve. Optimization of chromatographic analysis conditions aimed to improve separation efficiency and detection sensitivity.

[0026] The final step integrates all data to calculate the formyl terminal group content. Sample mass data is imported from the initial weighing, distillate mass data from distillation separation, and methyl formate concentration data from chromatographic analysis. The calculation is based on a stoichiometric relationship of a 1:1 molar ratio of formyl terminal group to methyl formate. The formyl terminal group content is derived using a mass conversion formula, and the results are expressed in ppm. This invention employs a data flow-driven analysis workflow, with each step's output serving as subsequent input, forming a closed-loop logic chain. This effectively solves the quantitative deviation problem caused by complex matrix interference and incomplete separation.

[0027] In the method for determining the formyl end-group content in the polyacetal resin molecular chain, the digestion reaction in step two is carried out using a heating reflux apparatus. The capacity of the round-bottom flask is adjusted according to the sample volume to accommodate the reaction mixture and avoid excessive cavity leading to heat loss. The condenser length is designed to fully condense the reflux vapor and maintain the closed reaction system. The optimized volume ratio of hydrochloric acid to methanol provides a suitable protonation environment to promote the conversion of formyl end-groups; the hydrochloric acid concentration is controlled to ensure catalytic activity, and the methanol purity reduces moisture interference. The reaction temperature is monitored by a digital thermometer to maintain stable thermal conditions and avoid side reactions; the reaction time is set to ensure complete conversion, and the stirring speed homogenizes the mixture and enhances mass transfer efficiency. This setup, in practical applications such as rapid testing in production lines, can efficiently process batches of samples and reduce operational variability through standardized parameters.

[0028] In the distillation separation operation, the temperature program is divided into two stages. In the first stage, the heating power is adjusted to induce a gentle boil, limiting the distillation thermometer reading to a low range to prevent premature evaporation of volatile components while ensuring complete sample dissolution. In the second stage, the heating power is increased to accelerate the distillation process. Heating is immediately terminated when the distillation thermometer reading reaches a specific endpoint, separating methyl formate based on boiling point differences. The distillation rate is controlled based on the total liquid volume ratio to avoid the distillate carrying high-boiling-point impurities. Under laboratory conditions, this staged temperature management supports reproducibility and is suitable for quality comparisons between different batches of resin, reducing azeotropic interference through real-time monitoring.

[0029] Ice-water bath cooling is implemented during distillation. The ice-water bath container is sized to match the receiving flask capacity, and the ratio of crushed ice to water inside maintains low-temperature stability. The condensation temperature is monitored by thermocouples to ensure efficient condensation of the distillate. The receiving flask is made of borosilicate glass, which is resistant to chemical corrosion, and the ice-water bath volume ratio enhances the cooling effect. In terms of operational details, such as in research laboratories, this cooling method effectively prevents the loss of methyl formate, especially when processing highly volatile samples, as precise temperature control improves collection efficiency.

[0030] Gas chromatography analysis employs a flame ionization detector, with optimized detection sensitivity achieved through settings for hydrogen, air, and make-up gas flow rates. A polar capillary column is used, with polyethylene glycol selected as the stationary phase. Column length and inner diameter are balanced to optimize separation efficiency and analysis time. The column temperature program starts at an initial temperature and gradually increases to the required separation temperature, while high-purity carrier gas ensures baseline stability. Injector temperature settings minimize sample decomposition, and the split ratio controls the injection volume. In practical applications such as product certification, these chromatographic conditions clearly distinguish the methyl formate peak, enabling accurate quantification through comparison with standard solutions.

[0031] The external standard method is implemented by preparing a series of methyl formate standard solutions, with the concentration gradient covering the expected range. The standard solutions are prepared using methanol as the solvent, and precise control of transfer reduces errors. The linear calibration curve is plotted using the least squares method, with the number of data points ensuring goodness of fit, and consistent injection volumes ensuring comparability. In quality control scenarios, such as factory laboratories, this method simplifies the calibration process, establishes a reliable quantitative relationship through a series of concentration points, and supports rapid sample analysis.

[0032] When calculating the formyl end-group content, the molar ratio of formyl end-group to methyl formate is based on stoichiometry, and molecular weight data is used for mass conversion. The calculation formula integrates sample mass, net weight of distillate, and methyl formate concentration to derive the end-group content. In data streams, such as process optimization projects, this calculation method directly reflects molecular chain stability, reducing the influence of complex matrices through simple arithmetic operations.

[0033] The duration of the first-stage distillation is controlled by a timer, minimizing time deviation. The second-stage distillation termination relies on temperature monitoring; heating is stopped when the measurement error is within acceptable limits. In operation, such as pilot-scale testing, this control strategy avoids over-distillation and maintains separation purity through rapid response.

[0034] Ice-water bath cooling is performed under circulating conditions, with flow rate regulation maintaining a stable water temperature. The vertical distance of the receiving bottle placement optimizes condensation efficiency. In continuous production monitoring, the circulating cooling system supports long-term operation and adapts to environmental changes through flow rate monitoring.

[0035] This invention is implemented under ventilated conditions, with wind speed control reducing the accumulation of volatiles and relative humidity limitation preventing moisture interference. It offers high weighing accuracy, detailed sample quality data recording, and glassware pretreatment to eliminate contamination. In safety and compliance scenarios, such as industry standard testing, this environmental management enhances operational reliability, and standardized procedures ensure consistent results.

[0036] The method for determining the formyl end-group content in polyacetal resin molecular chains requires a systematic operational plan to address industry challenges such as azeotropic interference from complex digestion matrices and incomplete separation due to inaccurate distillation temperature control. In polyacetal resin production quality monitoring scenarios, such as rapid testing at the end of the production line, this method ensures the reliability of test results through standardized procedures.

[0037] The process begins with sample pretreatment. Polyacetal resin particles are weighed using an analytical balance, and the mass is recorded. Ambient humidity must be controlled during weighing to prevent moisture from affecting the stability of the subsequent reaction system. Next, the sample is placed in a round-bottom flask, and a mixture of hydrochloric acid and methanol in a specific ratio is added. The digestion reaction is then carried out in a reflux reflux apparatus. Hydrochloric acid, acting as a protonic acid catalyst, forms a suitable reaction environment with methanol, promoting a nucleophilic addition-elimination reaction of the formyl end group, specifically converting it to methyl formate. The reaction temperature is monitored using a digital thermometer to maintain system stability, while the stirrer operates at a constant speed to ensure sufficient contact between the reactants.

[0038] Distillation separation was performed immediately after digestion. The distillation apparatus consisted of a distillation flask, a condenser, and a receiving flask, which was placed in an ice-water bath to maintain a low-temperature environment. A staged temperature control strategy was employed during the distillation process. In the first stage, the heating power was controlled to maintain a gentle boil, keeping the distillation thermometer reading low, primarily ensuring complete sample dissolution and system stabilization. In the second stage, the heating power was increased, and the distillation thermometer reading was closely monitored. Heating was immediately terminated when the temperature reached a specific endpoint. This temperature program design effectively separates methyl formate from complex matrices, utilizing boiling point differences to achieve selective distillation of the target analyte.

[0039] The distillate was collected and then subjected to gas chromatography analysis. The chromatographic system was equipped with a flame ionization detector and used a polar capillary column for separation. The column temperature program started at the initial temperature and increased at a constant rate to the required separation temperature. An external standard method was used to establish a quantitative benchmark by preparing a series of methyl formate standard solutions. The standard solutions were prepared with methanol solvent, and the concentration gradient covered the expected detection range. The injection volume was kept consistent during injection, and a linear calibration curve was plotted using the least squares method.

[0040] The content calculation stage integrates all the aforementioned data, and based on the molar ratio of formyl end groups to methyl formate, substitutes sample mass data, distillate mass data, and methyl formate concentration data into the conversion formula. This invention is performed under ventilated conditions, glassware must be pre-treated to eliminate contaminants, and the relative humidity of the experimental environment is controlled below a specific threshold. In continuous production quality monitoring applications, this standardized method can effectively assess the stability of end group content in different batches of resin products, providing data support for process adjustments.

[0041] Special attention was paid to the stability of temperature control during the experiment. For example, the condensation temperature was monitored in real time using thermocouples during the distillation stage, and the ice-water bath system maintained efficient circulating cooling. For the handling of volatile components, the receiving bottle was placed at a specific vertical distance from the condenser outlet to optimize the condensation path. In gas chromatography analysis, the coordinated setting of carrier gas flow rate and injection port temperature helped obtain sharp chromatographic peaks and improve quantitative accuracy.

[0042] This invention demonstrates excellent adaptability in practice, applicable not only to routine laboratory testing but also integrating into online monitoring systems for production lines. Through modular operating unit design, the three core processes of sample digestion, distillation separation, and chromatographic analysis maintain relative independence while forming an organic whole, meeting the technical requirements of the polyacetal resin industry for rapid and accurate quality monitoring.

[0043] Example 1 of this invention: Data collection of polyacetal resin sample mass was performed using an analytical balance to weigh the sample with an accuracy of 0.1 mg, recording the sample mass to four decimal places. The digestion reaction was carried out in a round-bottom flask with a capacity of 100 mL and a condenser length of 30 cm. The volume ratio of hydrochloric acid to methanol was 1:5, the hydrochloric acid concentration was 36%, and the methanol was anhydrous methanol with a purity of 99.8%. The reaction temperature was controlled at 40°C, monitored with a digital thermometer with an accuracy of ±0.1°C. The reaction time was 30 minutes, and the stirring speed was 200 rpm. The first stage of distillation separation involved heating at 100 W, with the distillation thermometer reading not exceeding 41°C, lasting for 10 minutes. In the second stage, the power was increased to 200 W, and the distillation rate was controlled at 1% of the total liquid volume per minute, with a total liquid volume of 50 mL. Heating was terminated when the distillation thermometer reading rose to 61°C. The distillate was collected in a borosilicate glass receiving flask with a capacity of 50 mL, and cooled in an ice-water bath at 0°C. Gas chromatography analysis was performed using a flame ionization detector with a hydrogen flow rate of 30 mL / min. The column was a polar capillary column, and the column temperature program was initially 40 °C for 2 minutes, then increased to 100 °C at a rate of 5 °C / min and held for 5 minutes. The external standard method used methyl formate standard solutions with concentrations ranging from 10 ppm to 100 ppm, and the linear calibration curve had a goodness-of-fit coefficient of 0.995. When calculating the formyl terminal group content, the molar ratio of formyl terminal group to methyl formate was 1:1, derived based on sample mass, distillate mass, and concentration data.

[0044] Example 2 of this invention: Optimized digestion reaction conditions: 250 mL round-bottom flask, 40 cm condenser, hydrochloric acid to methanol volume ratio 1:10, hydrochloric acid concentration 38%, reaction temperature 50°C, reaction time 60 minutes, stirring speed 300 rpm. The first stage of distillation separation uses a heating power of 150 W, with the distillation thermometer reading not exceeding 41°C, lasting 15 minutes. The second stage uses a power of 250 W, with a distillation rate of 2% of the total liquid volume per minute, a total liquid volume of 100 mL, and a termination temperature of 61°C. The ice-water bath container dimensions are 20 cm x 15 cm x 10 cm, with a crushed ice to water ratio of 1:1 and a condensation temperature of 5°C. The gas chromatograph uses high-purity nitrogen (99.999%) as the carrier gas, with a flow rate of 1.0 mL / min, an injection port temperature of 200°C, and a split ratio of 10:1. The external standard solution is prepared with an accuracy of ±0.05 mL and an injection volume of 1 μL. Environmental conditions: ventilation speed 0.5 m / s, relative humidity below 30%, glassware pretreatment at 105℃ for 1 hour.

[0045] Embodiment 3 of this invention: Focused distillation temperature program control. The digestion reaction is carried out at 40°C for 30 minutes. The first stage of distillation is a micro-boiling state with a power of 100 watts and a duration of 10 minutes with an accuracy of ±0.5 minutes. The second stage has a power of 200 watts, and the termination temperature is 61°C with a measurement error of ±0.5°C. The heating power drops to zero within 1 second. An ice-water bath cooling circulation flow rate of 5 liters / minute is used, with water temperature fluctuations of ±1°C. The vertical distance between the receiving flask and the condenser outlet is 5 centimeters. The chromatographic analysis column is 30 meters long, with an inner diameter of 0.25 millimeters and a film thickness of 0.25 micrometers. The air flow rate is 300 ml / minute, and the make-up gas flow rate is 25 ml / minute. This method demonstrates the advantages of precise temperature management in quality control scenarios, reducing azeotropic interference.

[0046] Example 4 of this invention: The ratio of hydrochloric acid to methanol was varied to 1:7 (volume ratio), with other parameters the same as in Example 1. The reaction temperature was 45°C, and the reaction time was 45 minutes. The total liquid volume after distillation was 75 mL. The distillate was collected and weighed with high accuracy. External standard solutions were used at 20 ppm intervals, with 5 data points. The molecular weight of the formyl end group was 29 g / mol, and the molecular weight of methyl formate was 60 g / mol. The results reflect the effect of the ratio change on the conversion efficiency, supporting the scope of reagent optimization in the claims.

[0047] Example 5 of this invention: Environmental conditions were adjusted as follows: ventilation speed 1.0 m / s, relative humidity 25%, sample weighing performed under a high-precision balance, and digestion reaction stirring speed 250 rpm. The ratio of the distillation ice-water bath volume to the receiving flask volume was 3:1, and the condensation temperature was monitored using thermocouples with an accuracy of ±0.5℃. The gas chromatography injection volume was 1 μL, with a transfer accuracy of ±0.1 μL. This example highlights the contribution of environmental control to reproducibility and is suitable for batch-to-batch comparisons.

[0048] Example 6 of this invention: The distillation termination temperature was precisely controlled. The digestion reaction was carried out at 50°C for 60 minutes. The first stage of distillation had a power of 120 watts and a duration of 12 minutes. The second stage had a power of 220 watts, and a rapid stop was triggered at a termination temperature of 61°C. The receiving flask was placed vertically at a distance of 10 cm, and the ice-water bath circulation flow rate was 10 L / min. The chromatographic column temperature program was optimized, with an initial temperature of 40°C held for 2 minutes and a heating rate of 5°C / min, resulting in good separation. The goodness of fit of the external standard method linear calibration curve was not less than 0.995, ensuring quantitative accuracy.

[0049] Example 7 of this invention: Sample pretreatment details. The polyacetal resin particle sample came from batch 1 of manufacturer A. The digestion reaction was carried out in a 150 mL round-bottom flask with a hydrochloric acid concentration of 37% and a methanol purity of 99.8%. The reaction temperature was 42 °C and the reaction time was 35 minutes. The total liquid volume of the distillate was 60 mL. The mass data of the distillate was recorded completely, and the peak area of ​​methyl formate was clearly visible in the gas chromatography.

[0050] Example 8 of this invention: chromatographic conditions were fine-tuned. The flame ionization detector parameters remained the same as before, but the column temperature program was initially set at 40°C and held for 3 minutes, then increased to 100°C and held for 4 minutes. The carrier gas flow rate was 1.2 mL / min. The concentration range of the external standard solution was expanded, while the injection volume remained consistent. The proportion of crushed ice in the ice-water bath was optimized for distillation separation, improving condensation efficiency.

[0051] Example 9: Digestion reaction time variation, set at 30 minutes and 60 minutes for comparison, other conditions same as in Example 2, with cross-validation at reaction temperatures of 40℃ and 50℃. The termination temperature of the distillation stage was strictly controlled at 61℃, and the ice-water bath temperature fluctuation was within ±1℃. When calculating the formyl end-group content, the data integration showed the effect of time on the conversion rate.

[0052] Example 10: Implementation with comprehensive parameters: typical sample mass 1.0000 g; hydrochloric acid to methanol volume ratio 1:8; reaction temperature 48℃; reaction time 50 minutes; stirring speed 280 rpm. Distillation: first stage power 130 W, duration 13 minutes; second stage power 230 W; termination temperature 61℃. Gas chromatography analysis: injection port temperature 200℃; split ratio 10:1; external standard calibration curve data points 6. Ambient ventilation velocity 0.8 m / s; glassware pretreatment temperature 105℃ with fluctuation ±2℃.

[0053] Example 11: Focusing on ice-water bath cooling conditions, the container dimensions were 20 cm x 15 cm x 10 cm, the ratio of crushed ice to water was 1:1, the circulation flow rate was 7 L / min, and the water temperature monitoring accuracy was high. The distillation receiving flask had a capacity of 75 mL and was 7 cm away from the condenser. The digestion reaction was carried out at 45 °C for 40 minutes, and the column temperature program for chromatographic analysis was stable.

[0054] Example 12: Details of the external standard method. Methyl formate standard solution was prepared using methanol as solvent, with concentration gradients of 10 ppm, 30 ppm, 50 ppm, 70 ppm, and 100 ppm. The transfer accuracy was ±0.05 mL, and the injection volume was 1 μL with an accuracy of ±0.1 μL. Linear calibration was performed using the least squares method, with a goodness-of-fit coefficient of 0.996. Distillation separation was carried out at a total liquid volume of 80 mL, with a termination temperature of 61°C.

[0055] Example 13: Sample batch variation was observed. Polyacetal resin samples from manufacturers B and C were used. Digestion reaction conditions were the same as in Example 1, and the distillation temperature program strictly followed two-stage control. Gas chromatography analysis showed differences in the peak area of ​​methyl formate between different batches, but the consistency was high when calculated using the external standard method. The ambient relative humidity was 28%, and the weighing data were accurate.

[0056] Example 14: The reaction temperature was precisely controlled using a digital thermometer with an accuracy of ±0.1℃. Three temperature levels were set: 40℃, 45℃, and 50℃. The digestion time was fixed at 40 minutes. The distillation termination temperature was 61℃, and the ice-water bath temperature was 0-5℃. The chromatographic conditions were the same as before. The results show the effect of temperature on the conversion rate of formyl terminal groups.

[0057] Example 15: Distillation rate optimization was performed, controlling the distillate volume at 1.5% of the total liquid volume per minute (total liquid volume 70 mL). The first stage power was 140 W, lasting 14 minutes. The hydrochloric acid to methanol volume ratio was 1:6, and the reaction temperature was 47°C. Consistent external standard solution injections were used, and the contribution of the distillation rate to the separation purity was calculated based on the formyl end-group content.

Claims

1. A method for determining the formyl end group content in a polyacetal resin molecular chain, characterized in that, include: Step 1: Collect polyacetal resin samples and obtain sample mass data by weighing. Step 2: Weigh the polyacetal resin sample using the sample mass data from Step 1, and react it with a mixed solution of hydrochloric acid and methanol under heating conditions to convert the formyl end group into methyl formate, generating a digestion solution containing methyl formate. Step 3: Distill the digest from Step 2, control the distillation temperature to distill off methyl formate, collect the distillate containing methyl formate, and record the mass data of the distillate. Step 4: Perform gas chromatography analysis on the distillate from Step 3 to obtain the peak area data of methyl formate, and calculate the concentration data of methyl formate by external standard method. Step 5: Substitute the sample mass data from Step 1, the distillate mass data from Step 3, and the methyl formate concentration data from Step 4 into the calculation formula to obtain the formyl end-group content result.

2. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 1, characterized in that, The digestion reaction described in step 2 is carried out under reflux heating conditions. The round-bottom flask has a capacity of 100 mL to 250 mL and a condenser length of 30 cm to 40 cm. The volume ratio of hydrochloric acid to methanol is 1:5 to 1:10, the concentration of hydrochloric acid is 36% to 38%, and the methanol is anhydrous methanol with a purity of not less than 99.8%. The reaction temperature is controlled at 40°C to 50°C, and the temperature is measured using a digital thermometer with an accuracy of ±0.1°C. The reaction time is maintained at 30 to 60 minutes, and the time is controlled using a timer with an error of no more than ±1 second. The digestion reaction is carried out under stirring conditions at a stirring speed of 200 rpm to 300 rpm.

3. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 2, characterized in that, The distillation temperature program described in step 3 is divided into two stages: In the first stage, the heating power is controlled to make the system boil gently, with the power set to 100 watts to 150 watts, the distillation temperature meter range being 0°C to 100°C, the scale division being 0.1°C, and the reading not exceeding 41°C, for a period of 10 to 15 minutes; In the second stage, the heating power is increased to 200 watts to 250 watts, the distillation rate is controlled at a distillate volume of 1% to 2% of the total liquid volume per minute, and the total liquid volume being 50 ml to 100 ml, and heating is immediately stopped when the distillation temperature meter reading rises to 61°C.

4. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 3, characterized in that, The distillation separation described in step 3 is carried out under ice-water bath cooling conditions. The ice-water bath container has dimensions of 20 cm × 15 cm × 10 cm, and the ratio of crushed ice to water inside is 1:

1. The condensation temperature is maintained between 0°C and 5°C, and the temperature is monitored by thermocouples with an accuracy of ±0.5°C. The receiving bottle is made of borosilicate glass with a capacity of 50 ml to 100 ml, and the ratio of the ice-water bath volume to the receiving bottle volume is 3:

1.

5. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 4, characterized in that, The gas chromatography analysis described in step 4 uses a flame ionization detector, with a hydrogen flow rate of 30 mL / min, an air flow rate of 300 mL / min, and a make-up gas flow rate of 25 mL / min. The chromatographic column is a polar capillary column with polyethylene glycol as the stationary phase. The column is 30 m long, 0.25 mm in inner diameter, and 0.25 μm thick. The column temperature program is set at 40 °C for 2 minutes, then increased to 100 °C at a rate of 5 °C / min and held for 5 minutes. The carrier gas is high-purity nitrogen with a purity of not less than 99.999%, and the flow rate is controlled at 1.0 mL / min. The injection port temperature is 200 °C, the injection volume is 1 μL, and the split ratio is 10:

1.

6. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 5, characterized in that, The external standard method described in step 4 is implemented by preparing a series of methyl formate standard solutions. The standard solution series includes 5 concentration gradients, ranging from 10 ppm to 100 ppm, with an interval of 20 ppm. Methanol is used as the solvent for the standard solutions, and the preparation accuracy is ±0.05 mL. The injection volume is 1 μL, and the transfer accuracy is ±0.1 μL. The linear calibration curve is plotted using the least squares method, with no less than 5 data points and a goodness-of-fit coefficient of no less than 0.

995.

7. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 6, characterized in that, In step 5, when calculating the formyl end group content, the molar ratio of formyl end group to methyl formate is 1:1, the molecular weight of formyl end group is 29 g / mol, and the molecular weight of methyl formate is 60 g / mol. The calculation is performed using the formula: Formyl end group content = (methyl formate mass × 29) / (sample mass × 60) × 10^6, where the mass of methyl formate is obtained by multiplying the net weight of the distillate in step 3 and the concentration data in step 4, and the sample mass is imported from step 1.

8. The method for determining the content of formyl end groups in the polyacetal resin molecular chain according to claim 7, characterized in that, In step 3, the duration of the first stage of distillation is controlled with an accuracy of ±0.5 minutes, and the setting value is 10 to 15 minutes. The second stage of distillation is terminated by temperature monitoring, with a measurement error of no more than ±0.5℃. After the termination signal is triggered, the heating power drops to zero within 1 second.

9. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 8, characterized in that, The ice-water bath cooling described in step 3 is carried out under circulating conditions, with a flow rate of 5 to 10 liters per minute; the water temperature fluctuation range is controlled within ±1℃; and the vertical distance between the receiving bottle and the outlet of the condenser tube is 5 to 10 centimeters.

10. The method for determining the formyl end group content in the polyacetal resin molecular chain according to claim 9, characterized in that, Steps 1 to 5 are carried out under ventilated conditions with a wind speed of 0.5 m / s to 1.0 m / s; relative humidity below 30%; weighing accuracy of 0.1 mg; and sample mass data recorded to four decimal places. All glassware is pre-treated at 105℃ for 1 hour with a temperature fluctuation of ±2℃.

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