Bio-based epoxy resin curing agent and preparation method thereof

Abstract

The invention relates to a bio-based epoxy resin curing agent and a preparation method thereof. The preparation method comprises the steps of crude glycerol settling separation, amide compound synthesis, imidazole synthesis and the like. The bio-based grease raw material is used for preparing the epoxy resin curing agent, the emission of three wastes in the production process of epichlorohydrin by a glycerol method can be reduced, compared with a curing system obtained by a traditional curing agent, the Tg value is higher, the application field is wider, the use proportion of the bio-based grease can be adjusted according to different application fields, and the application range is wider. And the production cost of the curing agent is greatly reduced. The preparation method is simple in production process and short in flow, and has a very good industrial application prospect.

Description

[Technical Field]

[0001] This invention belongs to the field of polymer resin synthesis technology. More specifically, this invention relates to a bio-based epoxy resin curing agent, and also to a method for preparing the bio-based epoxy resin curing agent. [Background Technology]

[0002] Epoxy resin, as a high-performance polymer material, is widely used in the construction industry. Its excellent bonding strength, outstanding chemical resistance, extremely low curing shrinkage, superior mechanical properties, and electrical insulation make it an ideal choice for many key applications. However, the full realization of its performance is highly dependent on the choice of curing agent, and current mainstream curing agents still have significant limitations. High-performance modified amines, such as polyetheramines, alicyclic amines, and imidazole curing agents, are expensive, significantly increasing the cost of large-scale applications in the construction industry; some curing agents require strict formulation, high-temperature curing, or pre-dehydration treatment, increasing construction complexity. CN116063661B discloses a cashew phenol aldehyde amide curing agent, its preparation method, and its application. Although it attempts to reduce costs by using cashew phenol-modified amine bio-based curing agents, the raw material sources are limited and rely on high-purity chemicals, failing to effectively utilize industrial by-products. Of particular concern is that crude glycerol (containing 20%–40% oil, water, and salt), a by-product of the biodiesel industry, has long been considered a low-value waste due to its high purification costs. If used directly in the synthesis of curing agents, residual grease will severely degrade the water resistance, thermal stability and mechanical strength of epoxy resins. How to efficiently convert crude glycerin into high-value curing agent raw materials and simultaneously solve the problem of grease interference has become a technological gap that the industry urgently needs to overcome.

[0003] To address the aforementioned challenges, this invention innovatively develops a bifunctional epoxy resin curing agent possessing both amide and imidazole groups. This curing agent uses oil from crude glycerol, a byproduct of biodiesel production, as its core raw material. Through a directional transesterification-amine hydrolysis process, impurity oils are converted into long-chain amide groups, significantly reducing raw material costs. The amide group (—CONH—) in its molecular structure enhances the flexibility and adhesion of the cured product, while the imidazole ring provides rapid low-temperature curing capability. The synergistic effect of the bifunctional groups breaks through the performance limits of traditional single-component curing agents, perfectly meeting the needs of on-site construction. This invention not only achieves the high-value utilization of waste crude glycerol for the first time but also innovatively transforms oils from a potentially harmful resource into toughening segments, providing the construction industry with an epoxy curing solution that combines economy, ease of application, and high performance, demonstrating significant environmental and commercial value. [Summary of the Invention]

[0004] [Technical problem to be solved]

[0005] The purpose of this invention is to provide a bio-based epoxy resin curing agent.

[0006] Another object of the present invention is to provide a method for preparing the bio-based epoxy resin curing agent.

[0007] [Technical Solution]

[0008] The present invention is achieved through the following technical solution.

[0009] This invention relates to a method for preparing a bio-based epoxy resin curing agent.

[0010] The preparation steps of this method are as follows:

[0011] A. Sedimentation separation of crude glycerol

[0012] The crude glycerol, a byproduct of biodiesel, is heated to 20–50°C in a storage tank and then allowed to settle at this temperature for 12–72 hours. The crude glycerol is then separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil.

[0013] B. Synthesis of Amide Compounds

[0014] According to the weight ratio of bio-based oil, dimer acid and ethylene amine 1:0.5-2.70:0.65-1.00, the bio-based oil, dimer acid and ethylene amine obtained in step A are added to a reactor, heated to 120-160°C with stirring, and then reacted at this temperature for 1.0-3.0 hours to obtain an amide compound;

[0015] C. Imidazole synthesis

[0016] The amide compound obtained in step B is slowly heated to a temperature of 180–200°C, and then reacted at this temperature, atmospheric pressure and with stirring for 1.0–3.0 hours. Next, the pressure is reduced to 7–15 kPa, and the compound is dehydrated under reduced pressure at this pressure and temperature of 180–200°C for 1.0–4.0 hours, thus obtaining a bio-based epoxy resin curing agent.

[0017] According to a preferred embodiment of the present invention, in step A, the crude glycerol, a biodiesel byproduct, contains 60-85% glycerol, 10-15% oil and fat and 10-16% sodium chloride by weight.

[0018] According to another preferred embodiment of the present invention, in step A, the bio-based oil contains 0.1 to 1.0% glycerol and 99.0 to 99.9% oil by weight.

[0019] According to another preferred embodiment of the present invention, in step B, the ethylene amine is one or more amine compounds selected from ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine or pentaethylenehexamine; and the viscosity of the dimer acid is 5000-9000 mPa·s.

[0020] According to another preferred embodiment of the present invention, in step B, the amide compound is a long-chain aliphatic compound containing an amide group.

[0021] According to another preferred embodiment of the present invention, in step B, the stirring speed of the stirrer is 200 to 800 rpm during stirring.

[0022] According to another preferred embodiment of the present invention, in step C, the heating rate of the amide compound is 1 to 5 °C / min.

[0023] According to another preferred embodiment of the present invention, in step C, the dehydration under reduced pressure is carried out until the water content is 0.01 to 0.10% by weight.

[0024] The present invention also relates to a bio-based epoxy resin curing agent obtained by the preparation method described above.

[0025] According to another preferred embodiment of the present invention, the bio-based epoxy resin curing agent is composed of 40-60% by weight of a compound containing an amide group and 40-60% by weight of a compound containing an imidazole group.

[0026] The invention will now be described in more detail.

[0027] The epoxy resin curing agent of this invention is an epoxy resin curing agent containing imidazole groups and amide groups, and it has the following chemical structural formula:

[0028]

[0029] This invention relates to a method for preparing a bio-based epoxy resin curing agent.

[0030] The preparation steps of this method are as follows:

[0031] A. Sedimentation separation of crude glycerol

[0032] The crude glycerol, a byproduct of biodiesel, is heated to 20-50°C in a storage tank and then allowed to settle at this temperature for 12-72 hours. The crude glycerol is then separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil.

[0033] The main function of this crude glycerol sedimentation and separation step in the epoxy resin curing agent preparation method of the present invention is to separate the glycerol and inorganic salts in the crude glycerol.

[0034] The crude glycerol used in this invention is a biodiesel byproduct produced during the transesterification reaction of oils (vegetable oils / waste oils) and methanol. It is a lower byproduct after separating biodiesel (fatty acid methyl esters). For details, please refer to the literature "Comprehensive Utilization of Crude Glycerol, a Biodiesel Byproduct" (Chemical Industry Progress, 2018).

[0035] According to the analytical methods of GB / T 13206-2022, GB / T 21496-2008, and SN / T 5298-2021, the crude glycerol of the biodiesel byproduct contains 60-85% glycerol, 10-15% oil and fat and 10-16% sodium chloride by weight.

[0036] In this step, since the physicochemical properties of the components contained in the crude glycerol by-product of biodiesel are significantly different, the crude glycerol can be separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil by static sedimentation separation method.

[0037] The described static sedimentation separation method involves allowing crude glycerol, a byproduct of biodiesel, to settle at a temperature of 20–50°C for 12–72 hours. Within this range, if the settling temperature is below 20°C, the crude glycerol viscosity becomes too high, hindering phase separation and resulting in a high glycerol content in the separated oil. Conversely, if the settling temperature exceeds 50°C, the phase separation rate and efficiency are not significantly improved, increasing energy consumption. Prolonged high temperatures can also lead to partial polymerization of glycerol and increased impurity content. Therefore, a settling temperature of 20–50°C is reasonable, preferably 26–44°C, and more preferably 30–40°C.

[0038] When the settling temperature is within the specified range, if the settling time is less than 12 hours, the separated oil will have a higher glycerol content, which is detrimental to subsequent reactions. If the settling time is longer than 72 hours, it will significantly increase production time and negatively impact the separation effect. Therefore, a settling time of 12–72 hours is appropriate, preferably 18–65 hours, and more preferably 24–58 hours.

[0039] According to the standard methods of GB / T 13206-2022 and SN / T 5298-2021, the bio-based oils obtained by static sedimentation separation contain 0.1-1.0% glycerol and 99.0-99.9% oil by weight.

[0040] The glycerol salt solution obtained by static sedimentation separation is used to produce refined glycerol, while the bio-based oils undergo further processing.

[0041] B. Synthesis of Amide Compounds

[0042] According to the weight ratio of bio-based oil, dimer acid and ethylene amine 1:0.5-2.70:0.65-1.00, the bio-based oil, dimer acid and ethylene amine obtained in step A are added to a reactor, heated to 120-160°C with stirring, and then reacted at this temperature for 1.0-3.0 hours to obtain an amide compound;

[0043] The main role of this amide compound synthesis step in the preparation method of epoxy resin curing agent of the present invention is to react dimer acid, oil and ethylene amine under given conditions to generate amide.

[0044] In this step, the ethylene amine is one or more amine compounds selected from ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or pentaethylenehexamine; the ethylene amines used in this invention are all products currently sold on the market, such as those sold by Shanghai Maclean Biochemical Technology Co., Ltd. under the trade name Polyethylene Polyamine.

[0045] Dimeric acid is a dimer fatty acid obtained by polymerizing unsaturated fatty acids, such as oleic acid and linoleic acid. It has excellent thermal stability and can maintain fluidity over a wide temperature range. The dimeric acid used in this invention has a viscosity of 5000–9000 mPa·s. If the viscosity of the dimeric acid is lower than 5000 mPa·s, the viscosity is too low, resulting in too few polymers and affecting the product's application performance; if the viscosity is higher than 9000 mPa·s, the viscosity is too high, resulting in too many polymers, affecting the reaction rate and hindering product application. Therefore, a viscosity of 5000–9000 mPa·s is suitable, preferably 5600–8400 mPa·s, and more preferably 6000–7900 mPa·s. The dimeric acid used in this invention is a product currently sold on the market, such as the product sold by Shanghai Dibai Biotechnology Co., Ltd. under the trade name "Dimeric Acid".

[0046] In this step, the weight ratio of bio-based oil, dimer acid, and ethylene amine is 1:0.5–2.70:0.65–1.00. When the amount of bio-based oil and ethylene is within the aforementioned range, if the amount of dimer acid is less than 0.5, the amount of dimer acid is too low, resulting in poor flexibility after product application; if the amount of dimer acid is higher than 2.70, the amount of oil is too low, resulting in excessive viscosity of the product, which is not conducive to application. Therefore, an amount of dimer acid of 0.5–2.70 is preferable, preferably 1.0–2.20, and more preferably 1.4–1.80.

[0047] When the amount of bio-based oil and dimer acid is within the specified range, if the amount of ethylene amine is less than 0.65, the amine value of the product is too low and the viscosity is too high; if the amount of ethylene amine is greater than 1.00, the amine value of the product is too high, which is not conducive to product application. Therefore, the amount of ethylene amine is suitable at 0.65 to 1.00, preferably 0.70 to 0.95, and more preferably 0.75 to 0.90.

[0048] Preferably, the weight ratio of bio-based oil, dimer acid and ethylene amine is 1:1.0-2.20:0.70-0.95.

[0049] More preferably, the weight ratio of bio-based oil, dimer acid and ethylene amine is 1:1.4-1.80:0.75-0.90.

[0050] Bio-based fats, dimer acids, and ethylene amines are reacted at 120–160°C with a stirrer speed of 200–800 rpm for 1.0–3.0 hours to yield a long-chain aliphatic compound containing an amide group. This compound has the following chemical formula:

[0051]

[0052] In this step, the stirrer speed and reaction time are within the aforementioned range. If the reaction temperature is below 120°C, a large amount of reactants will not participate in the reaction, thus affecting the product performance. If the reaction temperature is above 160°C, a series of side reactions may occur, which will affect the product purity. Therefore, a reaction temperature of 120–160°C is preferable, more preferably 128–154°C, and even more preferably 132–148°C.

[0053] Within the specified range of stirrer speed and reaction temperature, if the reaction time is less than 1 hour, a large amount of reactants will not participate in the reaction, thus affecting the subsequent synthesis of imidazole. If the reaction time is longer than 3 hours, the extended time does not bring significant beneficial effects to the reaction, while excessively long synthesis time increases production costs. Therefore, a reaction time of 1 to 3 hours is appropriate, preferably 1.3 to 2.6 hours, and more preferably 1.6 to 2.2 hours.

[0054] Within the range of reaction temperature and reaction time, if the speed of the stirrer is less than 200 rpm, the mixing will be uneven during the reaction and it will be difficult to react completely; if the speed of the stirrer is greater than 800 rpm, the excessive stirring speed will easily cause the reactants to splash. Therefore, a stirrer speed of 200 to 800 rpm is feasible, preferably 280 to 720 rpm, and more preferably 350 to 660 rpm.

[0055] Preferably, the bio-based oil, dimer acid, and ethylene amine are reacted at a temperature of 128–154°C and a stirrer speed of 280–720 rpm for 1.3–2.6 hours.

[0056] More preferably, the bio-based oil, dimer acid and ethylene amine are reacted at a temperature of 132-148°C and a stirrer speed of 350-660 rpm for 1.6-2.2 hours.

[0057] According to the standard test method in GB / T 6040-2019, the amide compound obtained in this step is a long-chain aliphatic compound containing an amide group and having the following chemical formula:

[0058]

[0059] C. Imidazole synthesis

[0060] The amide compound obtained in step B is slowly heated to a temperature of 180–200°C, and then reacted at this temperature, atmospheric pressure and with stirring for 1.0–3.0 hours. Next, the pressure is reduced to 7–15 kPa, and the compound is dehydrated under reduced pressure at this pressure and temperature of 180–200°C for 1.0–4.0 hours, thus obtaining a bio-based epoxy resin curing agent.

[0061] The main function of this imidazole synthesis step in the preparation method of this invention is to partially dehydrate the amide compound obtained in step B to form a compound with a cyclic imidazole structure.

[0062] The amide compound obtained in step B is heated to a temperature of 180-200°C at a heating rate of 1-5°C / min. Its main function is to dehydrate the functional groups of the amide structure at high temperature to form a cyclic imidazole structure compound.

[0063] The amide compound underwent an imidazole group synthesis reaction at 180–200 °C, atmospheric pressure, and stirring for 1.0–3.0 hours. The imidazole group synthesis reaction is as follows:

[0064]

[0065] Under normal pressure and stirring conditions, when the reaction time is within the aforementioned range, if the reaction temperature is below 180°C, it will be difficult to synthesize the imidazole group; if the reaction temperature is above 200°C, a series of side reactions will occur, and excessively high temperatures will cause some of the product to coke, affecting product performance. Therefore, a reaction temperature of 180–200°C is feasible, preferably 184–196°C, and more preferably 188–192°C.

[0066] Under normal pressure and stirring conditions, when the reaction temperature is within the aforementioned range, if the reaction time is less than 1 hour, the reaction will be incomplete, resulting in an excessively low imidazole content in the reaction system, which will affect product quality. If the reaction time is longer than 3 hours, it will not significantly affect the effectiveness of this step and will also affect product quality. Therefore, a reaction time of 1 to 3 hours is appropriate, preferably 1.2 to 2.8 hours, and more preferably 1.4 to 2.6 hours.

[0067] Next, the reaction product is dehydrated under reduced pressure at a pressure of 7–15 kPa and a temperature of 180–200 °C for 1.0–4.0 hours, and the water content of the reaction product reaches 0.01–0.10% by weight, thus obtaining a bio-based epoxy resin curing agent.

[0068] In this step, when the dehydration temperature and time are within the specified range, if the dehydration pressure is below 2 kPa, the low pressure may cause the reaction system to boil violently, which is detrimental to the smooth implementation of the preparation method. If the dehydration pressure is above 15 kPa, the water generated in the reaction cannot be discharged in time, which is detrimental to the synthesis of imidazole groups. Therefore, a dehydration pressure of 2–15 kPa is reasonable, preferably 4–12 kPa, and more preferably 6–10 kPa.

[0069] When the pressure and time of vacuum dehydration are within the aforementioned range, if the vacuum dehydration temperature is below 180°C, it will be difficult to generate imidazole groups; if the vacuum dehydration temperature is above 200°C, a series of side reactions may occur, and some products may coke, affecting their performance. Therefore, a vacuum dehydration temperature of 180–200°C is suitable, preferably 184–196°C, and more preferably 188–192°C.

[0070] When the pressure and temperature during vacuum dehydration are within the aforementioned range, if the vacuum dehydration time is less than 1.0 hour, the reaction will be incomplete, resulting in an excessively low imidazole content and affecting product quality. If the vacuum dehydration time is longer than 4.0 hours, a series of side reactions may occur, and some products may coke, further impacting product performance. Therefore, a vacuum dehydration time of 1.0 to 4.0 hours is appropriate, preferably 1.6 to 3.4 hours, and more preferably 2.0 to 3.0 hours.

[0071] The water content of the reaction product was determined according to the analytical method of GB / T 11133-2015 standard.

[0072] The amine value of the reaction product was determined according to the following method;

[0073] Weigh 0.29 g (accurate to 0.0002 g) of the sample and place it in a 250 mL Erlenmeyer flask. Then add 50 mL of anhydrous ethanol and boil on a hot plate until dissolved. Cool to room temperature and add 3–4 drops of 0.49 g / L bromophenol blue indicator solution. Titrate with 0.1 mol / L hydrochloric acid standard solution until the solution changes from blue to yellow.

[0074] The amine value X is calculated using the following formula:

[0075] X=(V×c×M) / m

[0076] In the formula:

[0077] X represents the amine value, mgKOH / g;

[0078] V represents the volume of the standard hydrochloric acid titration solution, in mL;

[0079] c represents the concentration of the standard hydrochloric acid titration solution, in mol / L;

[0080] m represents the mass of the sample, in grams;

[0081] M represents the molar mass of potassium hydroxide, in g / mol.

[0082] The viscosity of the reaction product was determined according to the GB / T 22314-2008 standard;

[0083] The infrared spectrum (FTIR) of the reaction product was obtained according to the standard GB / T 6040-2019.

[0084] The present invention also relates to a bio-based epoxy resin curing agent prepared by the aforementioned preparation method.

[0085] The epoxy resin curing agent is composed of 40-60% by weight of a compound containing amide groups and 40-60% by weight of a compound containing imidazole groups.

[0086] The composition of the epoxy resin curing agent was determined by the above-mentioned infrared spectroscopy (FTIR) data, and details are provided in the subsequent embodiments section.

[0087] [Beneficial Effects]

[0088] The beneficial effects of this invention are: the synthesis method of this invention can obtain a bio-based epoxy resin curing agent, and the raw material for synthesis is the waste oil in crude glycerin by-product of biodiesel. This invention not only makes full use of bio-based materials, but also solves the problem of the inability to process the oil in crude glycerin. In addition, the reaction process is simple, stable and reliable. [Attached Image Description]

[0089] Figure 1This is the infrared spectrum of the bio-based epoxy resin curing agent of this invention.

Detailed Implementation Methods

[0090] The invention will be better understood through the following examples.

[0091] Example 1: Preparation method of bio-based epoxy resin curing agent

[0092] The preparation steps of this method are as follows:

[0093] A. Sedimentation separation of crude glycerol

[0094] The crude glycerol, a byproduct of biodiesel, is heated to 30°C in a storage tank and then allowed to settle at this temperature for 72 hours. The crude glycerol is separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil.

[0095] B. Synthesis of Amide Compounds

[0096] According to the weight ratio of bio-based oil, dimer acid and triethylenetetramine 1:2.0:1.00, the bio-based oil, dimer acid and triethylenetetramine obtained in step A were added to the reactor, heated to 132°C with stirring, and then reacted at this temperature for 2.0 hours to obtain an amide compound.

[0097] C. Imidazole synthesis

[0098] The amide compound obtained in step B was slowly heated to 188°C, and then reacted at this temperature, atmospheric pressure, and with stirring for 2.4 hours. The pressure was then reduced to 7 kPa, and the mixture was dehydrated under reduced pressure at this pressure and temperature of 190°C for 3.0 hours, thus yielding a bio-based epoxy resin curing agent. Its infrared spectrum is shown in the appendix. Figure 1 .

[0099] The product has a brownish-brown appearance.

[0100] The infrared spectrum of this embodiment was obtained by testing according to the standard method of GB / T 6040-2019. The epoxy resin curing agent prepared in this embodiment is composed of 40% amide group compound and 60% imidazole group compound by weight.

[0101] According to the method described in this application, the viscosity of the product obtained by the preparation method in this embodiment is 956 mPa·s (40℃), and the amine value is 290 mg KOH / g.

[0102] Example 2: Preparation method of bio-based epoxy resin curing agent

[0103] The preparation steps of this method are as follows:

[0104] A. Sedimentation separation of crude glycerol

[0105] The crude glycerol, a byproduct of biodiesel, is heated to 20°C in a storage tank and then allowed to settle at this temperature for 32 hours. The crude glycerol is separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil.

[0106] B. Synthesis of Amide Compounds

[0107] According to the weight ratio of bio-based oil, dimer acid and ethylenediamine 1:0.5:0.65, the bio-based oil, dimer acid and ethylenediamine obtained in step A were added to the reactor, heated to 146°C with stirring, and then reacted at this temperature for 1.0 hour to obtain an amide compound;

[0108] C. Imidazole synthesis

[0109] The amide compound obtained in step B is slowly heated to 180°C, and then reacted at this temperature, atmospheric pressure and stirring for 3.0 hours. Then the pressure is reduced to 15 kPa, and the compound is dehydrated under reduced pressure at this pressure and temperature of 180°C for 1.0 hour, thus obtaining a bio-based epoxy resin curing agent.

[0110] The product has a brownish-brown appearance.

[0111] According to the infrared spectrum determination method of GB / T 6040-2019, the epoxy resin curing agent prepared in this embodiment is composed of 48% amide group compound and 52% imidazole group compound by weight.

[0112] According to the method described in this application, the viscosity of the product prepared by this embodiment is 1947 mPa·s (40℃), and the amine value is 268 mgKOH / g.

[0113] Example 3: Preparation method of bio-based epoxy resin curing agent

[0114] The preparation steps of this method are as follows:

[0115] A. Sedimentation separation of crude glycerol

[0116] The crude glycerol, a byproduct of biodiesel, is heated to 50°C in a storage tank and then allowed to settle at this temperature for 12 hours. The crude glycerol is separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil.

[0117] B. Synthesis of Amide Compounds

[0118] According to the weight ratio of bio-based oil, dimer acid and diethylenetriamine 1:2.7:0.76, the bio-based oil, dimer acid and diethylenetriamine obtained in step A were added to the reactor, heated to 160°C with stirring, and then reacted at this temperature for 3.0 hours to obtain an amide compound.

[0119] C. Imidazole synthesis

[0120] The amide compound obtained in step B is slowly heated to 200°C, and then reacted at this temperature, atmospheric pressure and stirring for 1.0 hour. Then the pressure is reduced to 12 kPa, and dehydrated under reduced pressure at this pressure and temperature of 200°C for 4.0 hours, thus obtaining a bio-based epoxy resin curing agent.

[0121] The product has a brownish-brown appearance.

[0122] The infrared spectrum of this embodiment was detected using the standard determination method of GB / T 6040-2019. The epoxy resin curing agent prepared in this embodiment is composed of 52% amide group compound and 48% imidazole group compound by weight.

[0123] According to the method described in this application, the viscosity of the product prepared by this embodiment is 5132 mPa·s (40℃), and the amine value is 288 mg KOH / g.

[0124] Example 4: Preparation method of bio-based epoxy resin curing agent

[0125] The preparation steps of this method are as follows:

[0126] A. Sedimentation separation of crude glycerol

[0127] The crude glycerol, a byproduct of biodiesel, is heated to 40°C in a storage tank and then allowed to settle at this temperature for 52 hours. The crude glycerol is separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil.

[0128] B. Synthesis of Amide Compounds

[0129] According to the weight ratio of bio-based oil, dimer acid and tetraethylenepentamine 1:1.2:0.88, the bio-based oil, dimer acid and tetraethylenepentamine obtained in step A were added to the reactor, heated to 120°C with stirring, and then reacted at this temperature for 2.0 hours to obtain an amide compound.

[0130] C. Imidazole synthesis

[0131] The amide compound obtained in step B was slowly heated to 192°C, and then reacted at this temperature, atmospheric pressure and with stirring for 1.6 hours. Then the pressure was reduced to 10 kPa, and the compound was dehydrated under reduced pressure at this pressure and temperature of 190°C for 2.0 hours, thus obtaining a bio-based epoxy resin curing agent.

[0132] The product has a brownish-brown appearance.

[0133] According to the infrared spectrum determination method of GB / T 6040-2019, the epoxy resin curing agent prepared in this embodiment is composed of 60% amide group compound and 40% imidazole group compound by weight.

[0134] According to the method described in this application, the viscosity of the product prepared by this embodiment is 756 mPa·s (40℃), and the amine value is 282 mg KOH / g.

Claims

1. A method for preparing a bio-based epoxy resin curing agent, characterized in that... The preparation steps of this method are as follows: A. Sedimentation separation of crude glycerol The crude glycerol, a byproduct of biodiesel, is heated to 20–50°C in a storage tank and then allowed to settle at this temperature for 12–72 hours. The crude glycerol is then separated into a lower layer of glycerol salt solution and an upper layer of bio-based oil. B. Synthesis of Amide Compounds According to the weight ratio of bio-based oil, dimer acid and ethylene amine 1:0.5-2.70:0.65-1.00, the bio-based oil, dimer acid and ethylene amine obtained in step A are added to a reactor, heated to 120-160°C with stirring, and then reacted at this temperature for 1.0-3.0 hours to obtain an amide compound; C. Imidazole synthesis The amide compound obtained in step B is slowly heated to a temperature of 180–200°C, and then reacted at this temperature, atmospheric pressure and with stirring for 1.0–3.0 hours. Next, the pressure is reduced to 7–15 kPa, and the compound is dehydrated under reduced pressure at this pressure and temperature of 180–200°C for 1.0–4.0 hours, thus obtaining a bio-based epoxy resin curing agent.

2. The preparation method according to claim 1, characterized in that... In step A, the crude glycerol, a biodiesel byproduct, contains 60-85% glycerol, 10-15% oil and fat, and 10-16% sodium chloride by weight.

3. The preparation method according to claim 1, characterized in that... In step A, the bio-based oil contains 0.1 to 1.0% glycerol and 99.0 to 99.9% oil by weight.

4. The preparation method according to claim 1, characterized in that... In step B, the ethylene amine is one or more amine compounds selected from ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or pentaethylenehexamine; the viscosity of the dimer acid is 5000–9000 mPa·s.

5. The preparation method according to claim 1, characterized in that... In step B, the amide compound is a long-chain aliphatic compound containing an amide group.

6. The preparation method according to claim 1, characterized in that... In step B, the stirring speed of the stirrer is 200-800 rpm during stirring.

7. The preparation method according to claim 1, characterized in that... In step C, the heating rate of the amide compound is 1–5 °C / min.

8. The preparation method according to claim 1, characterized in that... In step C, the dehydration under reduced pressure is carried out until the water content is 0.01-0.10% by weight.

9. The bio-based epoxy resin curing agent obtained by the preparation method according to any one of claims 1-8.

10. The epoxy resin curing agent according to claim 9, characterized in that... It consists of 40-60% by weight of compounds containing amide groups and 40-60% by weight of compounds containing imidazole groups.

Images