Novel method for measuring epoxy equivalent of epoxy resin

Abstract

The invention relates to a novel method for measuring the epoxy equivalent of epoxy resin, which comprises the following steps of: 1, preparing a sample, weighing a certain mass of epoxy sample to be measured until the weight is 0.0001 g, and putting the epoxy sample into a dry titration container; step 2, dissolving the sample, adding a strong polar solvent into a titration container containing the sample, then adding excessive HCl standard liquid, and uniformly mixing to obtain a to-be-detected sample solution; step 3, reacting at room temperature in a dark place for a certain time; step 4, adding an indicator, namely adding 4-6 drops of a phenolphthalein indicator into the sample solution; step 5, titration: filling the calibrated NaOH standard solution into a basic burette until the color of the solution is changed from colorless to purple red, namely a titration end point, and recording the volume of the consumed NaOH standard solution; and step 6, carrying out a blank experiment, namely carrying out the blank experiment according to the steps 1-5, namely, not adding the epoxy sample to be measured, then titrating by using a NaOH standard solution, and recording the volume of the consumed NaOH standard solution. The method can effectively improve the accuracy and stability of the epoxy equivalent measurement result.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis and detection technology, specifically to a method for determining the epoxy equivalent in epoxy resin materials based on green, low-toxicity, and non-regulated organic solvents. Background Technology

[0002] Epoxy equivalent weight (EEW) is a key indicator for measuring the epoxy group content in epoxy resins. It is specifically defined as "the mass of epoxy resin containing 1 mole of epoxy groups (-CH(O)CH2-)", typically expressed in grams per equivalent (g / eq). EEW is an important indicator for evaluating the performance of epoxy resins and other epoxy-containing compounds, and its value directly affects the application performance of these compounds in coatings, adhesives, and composite materials. Accurate determination of epoxy equivalent weight is crucial for product quality control, process optimization, and the development of new materials.

[0003] Currently, the most common method for determining epoxy equivalent is chemical titration. This method typically requires dissolving the sample in a suitable solvent and then performing titration analysis through a reaction with a specific reagent. Commonly used chemical titration methods include the perchloric acid-quaternary ammonium salt method (HClO4 method), the hydrochloric acid-acetone method (HCl method), the hydrobromic acid-glacial acetic acid method (HBr method), and the pyridine hydrochloric acid method.

[0004] Existing analytical methods utilize a variety of solvents, such as acetone, toluene, xylene, glacial acetic acid, and pyridine. However, each of these methods has limitations in practical applications. For example, the perchloric acid-quaternary ammonium salt method is costly, and its main reagent, perchloric acid, is a controlled substance and difficult to obtain; other methods involving solvents such as hydrobromic acid, acetone, and toluene face similar problems. Furthermore, in the hydrochloric acid-acetone method, acetone has limited solubility for some high-viscosity epoxy resins at room temperature, often requiring prolonged stirring or heating to achieve complete sample dissolution, a very time-consuming process that may also lead to various side reactions due to heating, affecting the accuracy of the results. Toluene and xylene are also commonly used as solvents in the hydrochloric acid-acetone method. While these aromatic solvents have good solubility, they are highly toxic and have an irritating odor, posing health risks to operators and causing environmental pollution, thus failing to meet the requirements of safe operation and modern green chemistry.

[0005] Therefore, given the limitations of the existing hydrochloric acid-acetone method, such as solvent control, limited solubility, and strong toxicity and irritating odor, finding a non-controlled solvent with strong solubility, non-toxic or low-toxicity, no irritating odor, and the ability to ensure the accuracy and stability of the determination results for epoxy equivalent has become an urgent problem to be solved in this field. Summary of the Invention

[0006] The hydrochloric acid-acetone method for determining epoxy equivalent has several drawbacks, including insufficient acetone solubility, numerous side reactions during titration, poor stability of results, and high toxicity. This invention provides a method for determining epoxy equivalent using tetrahydrofuran (THF), 1,4-dioxane, propylene glycol phenyl ether (PM), and methyltetrahydrofuran (MeTHF) as solvents. Currently, the hydrochloric acid-acetone method (using acetone as solvent) is prone to producing precipitates and oily substances, affecting the accuracy of the determination. This technical solution (using tetrahydrofuran, 1,4-dioxane, propylene glycol phenyl ether, and methyltetrahydrofuran as solvents) effectively avoids these phenomena, thereby improving the accuracy and stability of the epoxy equivalent determination results.

[0007] A novel method for determining the epoxy equivalent of epoxy resin includes the following steps:

[0008] Step 1. Sample preparation: Weigh a certain mass of the epoxy sample to be tested, accurate to 0.0001 g, and place it in a dry titration container;

[0009] Step 2. Sample dissolution: Add a strong polar solvent, such as tetrahydrofuran, to the titration container containing the sample. The amount added should be sufficient to completely dissolve the sample, usually 20-50 mL. Shake thoroughly to ensure complete dissolution. Then add an excess of HCl standard solution and mix well to obtain the sample solution to be tested.

[0010] Step 3. React at room temperature (or in a 37°C water bath) in the dark for a certain period of time (60-90 min);

[0011] Step 4. Add indicator: Add 4-6 drops of phenolphthalein indicator to the above sample solution;

[0012] Step 5. Titration: Pour the standardized NaOH standard solution into an alkaline burette and slowly add it to the sample while continuously shaking the conical flask until the solution color changes from colorless to purple-red and does not fade within 30 seconds. This is the titration endpoint. Record the volume of NaOH standard solution consumed.

[0013] Step 6. Blank experiment: Perform a blank experiment according to steps 1-5 above, that is, without adding the epoxy sample to be tested, only add the same amount of tetrahydrofuran solvent, HCl standard solution, and phenolphthalein indicator, and then titrate with NaOH standard solution, and record the volume of NaOH standard solution consumed.

[0014] Furthermore, the highly polar solvent may also be one of 1,4-dioxane, propylene glycol phenyl ether (PM), or methyltetrahydrofuran (MeTHF).

[0015] The beneficial effects of this invention are:

[0016] 1. Excellent solubility and wider applicability: The molecular structure of a strong polar solvent contains oxygen atoms, which can form certain interactions with polar groups such as epoxy groups in epoxy compounds. At the same time, its cyclic structure endows it with moderate non-polarity. This makes it have good solubility for both polar and non-polar epoxy samples, and can quickly and fully dissolve epoxy resin samples of different viscosities.

[0017] 2. Lower toxicity and good safety: Compared to aromatic solvents such as toluene and xylene, tetrahydrofuran has lower acute toxicity, and both oral and dermal toxicity data show that its harm is relatively minor. Although it is volatile, it is lower than acetone, and its irritating odor is weaker than traditional aromatic solvents. Under proper handling, it has less impact on the health of operators and is more in line with environmental protection requirements. In addition, this highly polar solvent is not a controlled reagent, is readily available, and is safer to handle.

[0018] 3. Excellent chemical stability and better compatibility with titration systems with fewer side reactions: Under the titration conditions of this method, the highly polar solvent is unlikely to react chemically with the hydrochloric acid standard solution, phenolphthalein indicator, and the epoxy sample to be tested, thus avoiding the introduction of additional interfering factors and ensuring the specificity and stability of the titration reaction.

[0019] 4. Faster reaction rate and more sensitive titration endpoint: The core reaction in epoxy equivalence determination is the "ring-opening reaction between the epoxy groups of the epoxy resin and the acid." Solvent polarity affects the reaction activity. Acetone requires prolonged stirring to ensure complete reaction. If the reaction is incomplete or the indicator is not chosen appropriately, the titration endpoint may show lag or blurred color transition, resulting in errors in the results. The polarity of strongly polar solvents is better suited to the "nucleophilic-electrophilic synergistic mechanism" of the epoxy ring-opening reaction. Their cyclic ether structure can more effectively solubilize protons (H⁺) in the acid, lowering the proton transfer energy barrier. At the same time, they have good solubility for the generated epoxy ring-opening intermediates (such as hydroxy ethers), preventing intermediate aggregation from hindering the reaction. Therefore, the reaction rate between the epoxy groups and the acid can be effectively increased, the reaction is more complete, the color change at the titration endpoint is more sensitive, and the determination results are more accurate. Attached Figure Description

[0020] Figure 1 , Figure 2 The epoxy equivalent of epoxy resin a was determined using different methods. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] Let's take tetrahydrofuran as an example:

[0023] Tetrahydrofuran is a heterocyclic ether compound. The method proposed in this invention for determining the epoxy equivalent of epoxy resin using tetrahydrofuran (THF) as a solvent (referred to as the hydrochloric acid-THF method) has the following advantages:

[0024] Excellent solubility and wider applicability: Tetrahydrofuran contains oxygen atoms in its molecular structure, which can form certain interactions with polar groups such as epoxy groups in epoxy compounds. At the same time, its cyclic structure endows it with moderate non-polarity. This makes it have good solubility for both polar and non-polar epoxy samples, and can quickly and fully dissolve epoxy resin samples of different viscosities.

[0025] Lower toxicity and good safety profile: Compared to aromatic solvents such as toluene and xylene, tetrahydrofuran has lower acute toxicity, with both oral and dermal toxicity data showing relatively lower levels of harm. While it is volatile, it is lower than acetone, and its irritating odor is weaker than that of traditional aromatic solvents. Under proper handling, it has minimal impact on operator health and is more environmentally friendly. Furthermore, tetrahydrofuran is not a controlled substance, is readily available, and is safer to handle.

[0026] It exhibits excellent chemical stability, better compatibility with titration systems, and fewer side reactions: Under the titration conditions of this assay, tetrahydrofuran is unlikely to react chemically with hydrochloric acid standard solution, phenolphthalein indicator, and the epoxy sample to be tested, thus avoiding the introduction of additional interfering factors and ensuring the specificity and stability of the titration reaction.

[0027] Faster reaction rate and more sensitive titration endpoint: The core reaction in epoxy equivalence determination is the "ring-opening reaction between the epoxy group of the epoxy resin and the acid," and solvent polarity affects the reaction activity. Acetone requires prolonged stirring to ensure complete reaction; if the reaction is incomplete or the indicator is not chosen appropriately, the titration endpoint may show lag or blurred color transition, resulting in errors in the results. Tetrahydrofuran's polarity is better suited to the "nucleophilic-electrophilic synergistic mechanism" of the epoxy ring-opening reaction. Its cyclic ether structure can more effectively solubilize protons (H⁺) in the acid, lowering the proton transfer energy barrier. At the same time, it has good solubility for the generated epoxy ring-opening intermediates (such as hydroxy ethers), preventing intermediate aggregation from hindering the reaction. Therefore, the reaction rate between the epoxy group and the acid can be effectively increased, the reaction is more complete, the color change at the titration endpoint is more sensitive, and the determination results are more accurate.

[0028] The solvents listed above (such as 1,4-dioxane, propylene glycol phenyl ether, methyltetrahydrofuran, etc.) have similar advantages to tetrahydrofuran, such as excellent solubility, low toxicity, good safety, good chemical stability, high efficiency, and non-regulated status. They will not be listed one by one here.

[0029] Taking the determination of epoxy equivalent of epoxy resin using tetrahydrofuran (THF) as a solvent as an example (referred to as the hydrochloric acid-THF method):

[0030] The measurement principle and operating procedures are as follows:

[0031] Determination principle: Tetrahydrofuran is a highly polar solvent that can dissolve epoxy resin well and is miscible with water in any proportion. In tetrahydrofuran solvent, excess hydrochloric acid reacts with the epoxy groups in the epoxy resin under certain conditions to generate chlorohydrin. Then, the excess hydrochloric acid is titrated with sodium hydroxide standard solution, and the epoxy equivalent in the epoxy resin is calculated according to the formula.

[0032] Operating procedures (taking the hydrochloric acid-THF method as an example)

[0033] 1. Preparation of indicators

[0034] (1) Phenolphthalein indicator solution (0.1%): Weigh 1.00 g of phenolphthalein and dissolve it in 100 mL of 95% ethanol. After mixing, transfer it to a brown reagent bottle and store it in the dark.

[0035] (2) Methyl red-bromocresol green mixed indicator:

[0036] a. Methyl red ethanol solution (0.1%): Weigh 0.10 g of methyl red solid, place it in a beaker, add 100 mL of 95% ethanol, stir until completely dissolved, transfer to a brown reagent bottle, seal and store in the dark.

[0037] b. Bromocresol green ethanol solution (0.1%): Weigh 0.10 g of bromocresol green solid, place it in a beaker, add 100 mL of 95% ethanol (if dissolution is slow, you can heat it slightly to help dissolve), stir until completely dissolved, transfer it to a brown reagent bottle, seal it and store it away from light.

[0038] c. Preparation of mixed indicator (mixed at a volume ratio of 3:1): Take 3 volumes of the above "0.1% methyl red ethanol solution", mix it with 1 volume of "0.1% bromocresol green ethanol solution", and transfer it to a brown reagent bottle for storage in the dark.

[0039] 2. Preparation of standard solutions

[0040] (1) Preparation and standardization of 0.2 mol / L HCl standard solution

[0041] a. Preparation of HCl standard solution: Pipette 16.7 mL of concentrated hydrochloric acid (12 mol / L) into a 1 L volumetric flask, and add distilled water to bring the volume to 1 L.

[0042] b. Standardization of HCl standard solution: Accurately weigh three portions (approximately 0.2 g each) of anhydrous sodium carbonate (Na₂CO₃) into Erlenmeyer flasks. Add 30-50 ml of distilled water to each flask, shake until dissolved, and then add 4-6 drops of methyl red-bromocresol green mixed indicator (which turns green). Titrate the Na₂CO₃ solution with the prepared HCl standard solution until the solution turns light purple. Then boil the solution for 2 minutes (to remove the influence of CO₂ on the titration endpoint). After cooling to room temperature, continue titrating until a dark red color is achieved (the endpoint is reached if the color does not fade for 30 seconds). Calculate the precise concentration of the HCl standard solution using the following formula:

[0043]

[0044] Note:

[0045] c(HCl): Concentration of HCl standard solution (unit: mol / L);

[0046] m(Na2CO3): The mass of anhydrous sodium carbonate weighed (unit: g);

[0047] M(Na2CO3): Molar mass of anhydrous sodium carbonate (unit: g / mol), theoretical value is 105.99 g / mol;

[0048] V(HCl): Volume of standard hydrochloric acid solution consumed in the titration (unit: mL).

[0049] (2) Preparation and standardization of 0.2 mol / L NaOH standard solution

[0050] a. Preparation of NaOH standard solution: Accurately weigh 8.0000 g NaOH into a beaker using a 0.001 g balance, add a certain amount (100-200 ml) of distilled water, and slowly shake until completely dissolved. Transfer to a 1 L volumetric flask, continue to wash the inner wall of the beaker with distilled water and transfer all the liquid to the volumetric flask (repeat the operation 3 times), and finally add water to the mark of the volumetric flask to make up to 1 L.

[0051] b. Standardization of NaOH standard solution: Accurately weigh three portions (approximately 1 g each) of anhydrous potassium hydrogen phthalate (KHP) into Erlenmeyer flasks. Add 30-50 ml of distilled water to each flask, shake until dissolved, then add 4-6 drops of phenolphthalein indicator (which should be colorless). Titrate the KHP solution with the prepared NaOH standard solution until the solution turns a light pink (the endpoint is reached when the color does not fade for 30 seconds). Calculate the precise concentration of the NaOH standard solution using the following formula:

[0052]

[0053] Note:

[0054] c(NaOH): Concentration of sodium hydroxide standard solution (unit: mol / L);

[0055] m(KHP): The mass of anhydrous potassium hydrogen phthalate weighed (unit: g);

[0056] M(KHP): Molar mass of potassium hydrogen phthalate (unit: g / mol), with a theoretical value of 204.23 g / mol;

[0057] V(NaOH): Volume of sodium hydroxide standard solution consumed in the titration (unit: mL).

[0058] 3. Sample preparation for testing

[0059] (1) Experimental group: Accurately weigh a certain amount (0.3-0.6 g) of epoxy resin, transfer it into a 250 mL conical flask, add 20-25 mL of tetrahydrofuran and shake until completely dissolved, then add 15-20 mL of HCl standard solution (to ensure that excess HCl reacts with the sample) and mix well.

[0060] (2) Blank control group: Add 20-25 mL of tetrahydrofuran and 15-20 mL of HCl standard solution (without epoxy resin) to a 250 mL conical flask as a blank control.

[0061] (3) Place the experimental group and the blank control group in a 37℃ water bath and react in the dark for 65-90 min.

[0062] (4) After the reaction is complete, take out the sample and wait for it to cool to room temperature before titrating.

[0063] 4. Titration

[0064] (1) Preparation before titration: Rinse the burette with NaOH standard solution at least 3 times, then add NaOH standard solution to the burette, remove air bubbles and read the value.

[0065] (2) Titration process: Add 4-6 drops of phenolphthalein indicator to each of the experimental group and the blank control group, shake well, start titration, and record the reading of the NaOH standard solution at the titration endpoint (the solution changes from colorless to light red and remains colorless for 15 s).

[0066] 5. Calculate the epoxy equivalent (EEW)

[0067]

[0068] Note:

[0069] m: The mass of the epoxy resin sample weighed, in grams (g).

[0070] V0: The volume of sodium hydroxide standard titration solution consumed in the blank test, in milliliters (mL);

[0071] V1: The volume of sodium hydroxide standard titration solution consumed in the sample test, in milliliters (mL).

[0072] c NaOH : The accurate concentration of the sodium hydroxide standard titration solution, in moles per liter (mol / L).

[0073] n: refers to the number of epoxy groups contained in one molecule of epoxy resin.

[0074] Measurement data analysis

[0075] 1. Example 1

[0076] In this embodiment, tetrahydrofuran was used as a solvent to determine the epoxy equivalent of epoxy resin a and excess hydrochloric acid under different reaction time conditions, as shown in Table 1.

[0077] Table 1. Epoxy equivalent of epoxy resin a determined using tetrahydrofuran as a solvent.

[0078]

[0079] The data above show that the results are stable when the reaction time is between 65 and 90 minutes. To verify the stability of the results within this reaction time range, based on the data in Table 1, a reaction time of 70 minutes was selected, and the epoxy equivalent of the samples was measured repeatedly. The results are shown in Table 2.

[0080] Table 2. Epoxy equivalent of epoxy resin a at the same reaction time

[0081]

[0082] 2. Example 2

[0083] In this embodiment, 1,4-dioxane was used as a solvent to determine the epoxy equivalent of epoxy resin a with excess hydrochloric acid under different reaction time conditions, as shown in Table 3.

[0084] Table 3. Epoxy equivalent of epoxy resin a determined using 1,4-dioxane as solvent.

[0085]

[0086] The data above show that the results are stable when the reaction time is controlled between 65 and 90 minutes. To verify the stability of the results, based on the above, a reaction time of 80 minutes was selected, and the epoxy equivalent of the samples was measured again. The results are shown in Table 4.

[0087] Table 4. Epoxy equivalent of epoxy resin a determined using 1,4-dioxane as solvent.

[0088]

[0089] 3. Example 3

[0090] In this embodiment, propylene glycol phenyl ether (PM) was used as a solvent to determine the epoxy equivalent of epoxy resin a and excess hydrochloric acid under different reaction time conditions, as shown in Table 5.

[0091] Table 5. Epoxy equivalent of epoxy resin a determined using propylene glycol phenyl ether as a solvent.

[0092]

[0093] The data above show that the results are stable when the reaction time is controlled between 70 and 90 minutes. To verify the stability of the results, based on the above data, a reaction time of 75 minutes was selected, and the epoxy equivalent of the sample was measured again. The results are shown in Table 6.

[0094] Table 6. Epoxy equivalent of epoxy resin a determined using propylene glycol phenyl ether as a solvent.

[0095]

[0096] 4. Comparative Example

[0097] This comparative example uses the conventional hydrochloric acid-acetone method to determine the epoxy equivalent of epoxy resin a under the same experimental conditions as the present invention, and can be used as a comparative example of the method of the present invention, as shown in Tables 7 and 8.

[0098] Table 7. Determination of epoxy equivalent of epoxy resin a using the conventional hydrochloric acid-acetone method.

[0099]

[0100] The data above show that the epoxy equivalent of epoxy resin a determined by the traditional hydrochloric acid-acetone method fluctuates significantly at different reaction times and deviates considerably from the benchmark value (official epoxy equivalent reference value, as shown in Table 9). Therefore, to verify whether this method has a certain degree of accuracy and stability, the reaction time point of 80 min, which is closest to the epoxy equivalent reference value, was selected to determine the epoxy equivalent of the sample. The results are shown in Table 8.

[0101] Table 8. Epoxy equivalent of epoxy resin a determined by the hydrochloric acid-acetone method at the same reaction time.

[0102]

[0103] The following is a summary and comparison of the epoxy equivalent (EEW) data of epoxy resin a determined by the method of this invention (using tetrahydrofuran, 1,4-dioxane, and propylene glycol phenyl ether as solvents) and the traditional hydrochloric acid-acetone method, as shown in Tables 9 and 10.

[0104] Table 9. Epoxy equivalent of epoxy resin a determined by four different methods at different reaction times.

[0105]

[0106] Table 10. Determination of epoxy equivalent of epoxy resin a using four different methods at the optimal reaction time.

[0107]

[0108] Figure 1 In the study, the relative standard errors of the epoxy equivalent of epoxy resin a were determined by four different methods at different reaction times: hydrochloric acid-tetrahydrofuran method -0.41%; hydrochloric acid-1,4-dioxane -0.34%; hydrochloric acid-propylene glycol phenyl ether method -0.31%; and hydrochloric acid-acetone method -3.68%.

[0109] Figure 2 In the study, the relative standard errors of the epoxy equivalent of epoxy resin a were determined by four different methods at the optimal reaction time: hydrochloric acid-tetrahydrofuran method -0.16%; hydrochloric acid-1,4-dioxane method -0.11%; hydrochloric acid-propylene glycol phenyl ether (PM) method -0.07%; and hydrochloric acid-acetone method -2.98%.

[0110] 1. The above data demonstrates that the method of this invention, using tetrahydrofuran (THF), 1,4-dioxane, and propylene glycol methyl ether (PM) as solvents to determine the epoxy equivalent of epoxy resins, exhibits excellent stability. However, the specific implementation requires controlling the reaction time based on the specific sample. The results from the above examples show that if the reaction time is controlled within 70-80 minutes, the results are quite stable, and the measured epoxy equivalent values ​​are all within the official reference range. This demonstrates the effectiveness of this method. Figure 1 As shown.

[0111] 2. The above data demonstrates that, compared to the hydrochloric acid-acetone method, the method of this invention, using tetrahydrofuran (THF), 1,4-dioxane, and propylene glycol methyl ether (PM) as solvents to determine the epoxy equivalent of epoxy resins, exhibits superior stability and accuracy within the same reaction time range when determining the epoxy equivalent in epoxy resin materials. Figure 2 As shown.

[0112] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A novel method for determining the epoxy equivalent weight of an epoxy resin, characterized in that, Comprising the following steps: Step 1, sample preparation, weigh a certain mass of the epoxy sample to be determined, accurate to 0.0001g, and place it in a dry titration container; Step 2, sample dissolution, add a strong polar solvent to the titration container containing the sample, the strong polar solvent is tetrahydrofuran, the amount of addition is appropriate to make the sample completely dissolved, then shake well to make the sample fully dissolved, then add excess HCl standard solution and mix well, which is the sample solution to be measured; Step 3, room temperature or 37℃ water bath reaction for 60-90 min in the dark; Step 4, indicator addition, add 4-6 drops of phenolphthalein indicator to the above sample solution; Step 5, titration, load the calibrated NaOH standard solution into the alkaline burette, slowly add it to the sample, and constantly shake the conical flask at the same time, until the solution color changes from colorless to purple red, and does not fade within 30s, which is the titration endpoint, record the volume of NaOH standard solution consumed; Step 6, blank experiment, according to the above steps 1-5, do the blank experiment, that is, do not add the epoxy sample to be determined, only add the same amount of tetrahydrofuran solvent, HCl standard solution, phenolphthalein indicator, then titrate with NaOH standard solution, record the volume of NaOH standard solution consumed, to calculate the epoxy equivalent.

2. The method of claim 1, wherein: The strong polar solvent can also be one of 1,4-dioxane, propylene glycol phenyl ether PM, methyl tetrahydrofuran MeTHF.

3. The method of claim 1, wherein: The calibration of the HCl standard solution: accurately weigh 3 portions of about 0.2 g of anhydrous sodium carbonate Na2CO3 into a conical flask, and add 30-50 ml of distilled water to each conical flask, shake until dissolved, then add 4-6 drops of methyl red-bromocresol green mixed indicator, then titrate the Na2CO3 solution with the prepared HCl standard solution until the solution is light purple, then boil the solution for 2 min, cool to room temperature, and continue to titrate until dark red. The accurate concentration of the HCl standard solution is calculated by the following formula: Wherein, c(HCl): the concentration of HCl standard solution, unit: mol / L; m(Na2CO3): the mass of anhydrous sodium carbonate weighed, unit: g; M(Na2CO3): the molar mass of anhydrous sodium carbonate, unit: g / mol, the theoretical value is 105.99 g / mol; V(HCl): the volume of HCl standard solution consumed in titration, unit: mL.

4. The method of claim 1, wherein: The phenolphthalein indicator: weigh 1.00 g of phenolphthalein and dissolve it in 100 mL of 95% ethanol, mix well, then transfer to a brown reagent bottle and store in the dark.

5. The method of claim 1, wherein: Calibration of NaOH standard solution, accurately weigh 3 portions of about 1 g of anhydrous potassium hydrogen phthalate KHP into a conical flask, and add 30-50 ml of distilled water to each conical flask, shake until dissolved, then add 4-6 drops of phenolphthalein indicator, then titrate the KHP solution with the prepared NaOH standard solution until the solution is light pink. The accurate concentration of the NaOH standard solution is calculated by the following formula: Wherein, c(NaOH): the concentration of sodium hydroxide standard solution, unit: mol / L; m(KHP): the mass of the weighed anhydrous potassium hydrogen phthalate, unit: g; M(KHP): the molar mass of potassium hydrogen phthalate, unit: g / mol, the theoretical value of which is 204.23 g / mol; V(NaOH): the volume of the sodium hydroxide standard solution consumed in titration, unit: mL.

6. The method of claim 1, wherein: In step 1, the sample: (1) Experimental group: accurately weigh a certain amount (0.3-0.6 g) of epoxy resin into a 250 mL conical flask, add 20-25 mL of tetrahydrofuran and shake until completely dissolved, then add 15-20 mL of HCl standard solution and mix well; (2) Blank control group: add 20-25 mL of tetrahydrofuran and 15-20 mL of HCl standard solution into a 250 mL conical flask as a blank control; (3) Put the experimental group and the blank control group into a 37°C water bath and react in the dark for 65-90 min; (4) After the reaction is completed, take out the sample and cool it to room temperature before titration.

7. The method of claim 1, wherein: The titration: (1) Preparation before titration: first rinse the burette with NaOH standard solution at least 3 times, then add NaOH standard solution to the burette, and read the number after the bubbles are discharged; (2) Titration process: add 4-6 drops of phenolphthalein indicator to the experimental group and the blank control group, shake well, then start titration, and record the NaOH standard solution reading at the end point of titration; Calculate the epoxy equivalent weight EEW wherein, m: the mass of the weighed epoxy resin sample, unit: grams (g); V0: the volume of the sodium hydroxide standard titration solution consumed in the blank test, unit: milliliters (mL); V1: the volume of the sodium hydroxide standard titration solution consumed in the sample test, unit: milliliters (mL); c NaOH : accurate concentration of the sodium hydroxide standard titrant in moles per liter (mol / L); n: refers to the number of epoxy groups contained in one molecule of epoxy resin.

Images