A caffeic acid-based epoxy resin and its preparation method

Abstract

The present invention belongs to the field of bio-based thermosetting epoxy resins, and particularly relates to a preparation method of a bio-based epoxy resin with high Tg (heat resistance) and excellent mechanical properties. In the present invention, epichlorohydrin is used to carry out an epoxidation reaction with caffeic acid from a green source to synthesize an epoxy resin with three epoxy functional groups. The obtained epoxy resin has excellent mechanical properties and a high glass transition temperature. This epoxy resin can be applied to adhesives, coatings, electronic device encapsulation, and the aerospace field.

Description

Technical Field

[0001] The present invention belongs to the field of bio-based thermosetting epoxy resins, and particularly relates to a preparation method of a bio-based epoxy resin having a high Tg (heat resistance) and excellent mechanical properties. Background Art

[0002] Thermosetting materials are three-dimensional cross-linked polymer materials that are insoluble in solvents and do not melt, and have been widely used in coatings, circuit components, composite materials, etc. Epoxy resin is one of the most important thermosetting resins, having excellent physical and chemical properties, such as high hardness, superior wettability, oil resistance, water resistance and corrosion resistance, as well as good insulation properties. In addition, its cross-linking and curing volume shrinkage rate is low, and it will not lose adhesion due to internal stress. Due to these excellent properties and long service life, epoxy resins have been widely used. At present, most commercial epoxy resins are bisphenol A (BPA) epoxy resins (DGEBA). BPA is derived from non-renewable fossil resources and is harmful to the human body. Recent studies have shown that the resin precursor bisphenol A may trigger certain receptor responses as effectively as estradiol, which is extremely harmful to infants and young children. Therefore, it has become extremely urgent and important to develop sustainable alternatives to bisphenol A epoxy resins.

[0003] In recent years, under the great pressure of environmental pollution and crude oil shortage, the production of polymers and composites from renewable biomass has attracted increasing attention. Researchers around the world have shown great interest in developing bio-based epoxy resins as partial substitutes for bisphenol A epoxy resins. For example, epoxidized soybean oil-based adhesives were synthesized from epoxidized soybean oil (A. Li and K. Li, ACS Sustainable Chem. Eng., 2014, 2, 2090-2096), but due to the limitation of bonding strength, they can only be applied to ordinary office products. Another example is the epoxy resin synthesized with isosorbide as the matrix (J. Lukaszczyk, B. Janicki and M. Kaczmarek, Eur. Polym. J., 2011, 47, 1601-1606), whose mechanical properties are comparable to those of bisphenol A epoxy resin, but due to its hydrophilic structure, it shows high water absorption performance, which limits its application. In addition, eugenol was electrochemically dehydrogenated and dimerized, and then epoxidized to obtain a bio-based bisphenol epoxy resin with high Tg and mechanical properties (G. H. M. de Kruijff, T. Goschler, N. Beiser, A. Stenglein, O. M. Tuerk and S. R. Waldvogel, Green Chem., 2019, 21, 4815-4823), but its thermal stability is lower than that of bisphenol A epoxy resin. Resveratrol-based epoxy resin has high Tg, excellent mechanical properties and low flammability, enriching the types of bio-based epoxy resins, but it is not convenient for large-scale application due to complex processes and high costs (Y. Tian, Q. Wang, L. Shen, Z. Cui, L. Kou, J. Cheng and J. Zhang, Chem. Eng. J., 2020, 383, 123124). However, high-performance bio-based epoxy resins are still limited, and more new bio-based epoxy resins with simple preparation methods, excellent properties and capable of replacing petroleum-based bisphenol epoxy need to be explored. Summary of the Invention

[0004] In view of the above problems existing in the prior art, the applicant of the present invention provides a caffeic acid-based epoxy resin and a preparation method thereof. The present invention uses epichlorohydrin to carry out an epoxidation reaction with caffeic acid from a green source to synthesize an epoxy resin with three epoxy functional groups. The prepared epoxy resin has excellent mechanical properties and a relatively high glass transition temperature.

[0005] A caffeic acid-based epoxy resin has the following structure:

[0006]

[0007] The present invention also includes a preparation method of the caffeic acid-based epoxy resin, which comprises the following steps:

[0008] Mix caffeic acid with epichlorohydrin, add a phase transfer catalyst, react at 85°C - 100°C for 2 - 3 h, then cool to 0 - 10°C, dropwise add NaOH solution, continue to react for 2 - 3 h, then carry out water washing, add a precipitant for precipitation, and then the product of formula I is obtained. The product of formula I is cured to obtain caffeic acid-based epoxy resin.

[0009]

[0010] The phase transfer catalyst in the above steps is selected from any one or more of tetrabutylammonium bromide, benzyltriethylammonium chloride, tetrabutylammonium hydrogensulfate, trineopentylmethylammonium chloride, dodecyltrimethylammonium chloride, and tetradecyltrimethylammonium chloride. The molar ratio of caffeic acid or epichlorohydrin to the phase transfer catalyst is 1∶0.03 - 0.5; the molar ratio of caffeic acid to epichlorohydrin is 1∶9 - 12; the precipitant is one or both of anhydrous ether and n-hexane.

[0011] The process of curing the product of formula I is a conventional epoxy resin curing method in the art. For example, it can be: heat the product of formula I to 50 - 60°C, quickly mix it with melted 4,4-diaminodiphenylmethane and stir rapidly. After stirring evenly, cool to room temperature, carry out vacuum degassing, place it in an oven, pre-cure at 70°C - 80°C for 0.5 - 1 h, and then cure in stages at 105 - 110°C for 1 - 2 h, 125 - 130°C for 1 - 2 h, and 145 - 150°C for 1 - 2 h to obtain a thermosetting caffeic acid-based epoxy resin. In addition, the molar ratio of 4,4-diaminodiphenylmethane to the product of formula I in the above steps is 3∶4.

[0012] The present invention also includes an application of the caffeic acid-based epoxy resin, which is mainly applied in the fields of adhesives, coatings, electronic device encapsulation, and aerospace, etc.

[0013] The caffeic acid-based epoxy resin prepared by the present invention has the following characteristics: (1) The reaction is simple and the conditions are mild, which is beneficial to industrial production; (2) The caffeic acid-based epoxy resin obtained by the present invention has a lower curing temperature and more convenient curing compared with bisphenol A-type epoxy resin. It can be shaped by pouring, which broadens the application fields and improves the practicability; (3) The caffeic acid-based epoxy resin obtained by the present invention has excellent mechanical properties and glass transition temperature; (4) The caffeic acid-based epoxy resin obtained by the present invention still has double bonds and still has the possibility of enhancing mechanical properties. Description of the Drawings

[0014] Figure 1 It is the nuclear magnetic resonance hydrogen spectrum of the caffeic acid-based epoxy resin prepared by the present invention.

[0015] Figure 2Tensile curve for comparing the mechanical properties of the caffeic acid-based epoxy resin prepared in the present invention with a commercially available bisphenol A epoxy resin (CFAE).

[0016] Figure 3 Non-isothermal DSC curves of the caffeic acid-based epoxy resin prepared in the present invention and a commercially available bisphenol A epoxy resin.

[0017] Specific implementation method

[0018] The present invention will be further described in detail below with reference to the embodiments, but the implementation manners of the present invention are not limited thereto.

[0019] Example 1

[0020] Step (1) Add 36 g of caffeic acid to a 500 mL three-necked flask, then add 166.5 g of epichlorohydrin and 3.2 g of tetrabutylammonium bromide, and heat up to 85°C. Stir magnetically for 3 h at 200 rpm. Cool the mixed solution to 0°C, add 72 g of sodium hydroxide aqueous solution (30 wt%), and continue to stir for 3 h at 300 rpm.

[0021] Step (2) Directly add the mixed solution in step (1) to 1 L of deionized water, continue magnetic stirring, take the lower-layer oily viscous liquid, wash it 4 times with deionized water, 15 min each time. Add 100 mL of acetone to the collected viscous liquid, stir to dissolve it, perform precipitation, select n-hexane as the precipitant, precipitate one or two times, and collect the viscous oily precipitate as the caffeic acid-based epoxy resin. Add 200 mL of dichloromethane to the caffeic acid-based epoxy resin, stir until it is dissolved, then add 10 g of anhydrous magnesium sulfate and stir, let it stand and then perform solid-liquid separation, take the upper-layer liquid, and remove dichloromethane through a rotary evaporator to obtain yellow caffeic acid-based epoxy resin 1.

[0022] Step (3) Heat the epoxy resin in step (2) to 60°C, melt DDM, quickly mix it with the epoxy resin and stir rapidly, quickly cool the mixture to room temperature, perform vacuum degassing for 30 min, add it to a mold and then place it in an oven for curing, 1 h at 110°C, 2 h at 130°C, and 1 h at 150°C.

[0023] Example 2

[0024] Step (1) Add 36 g of caffeic acid to a 500 mL three-necked flask, then add 166.5 g of epichlorohydrin and 1.9 g of tetrabutylammonium bromide, and heat up to 100°C. Stir magnetically for 2 h at 200 rpm. Cool the mixed solution to 10°C, add 72 g of sodium hydroxide aqueous solution (30 wt%), and continue to stir for 2 h at 300 rpm.

[0025] Step (2): Directly add the mixed solution from step (1) into 1 L of deionized water, continue magnetic stirring, take the lower-layer oily viscous liquid, wash it 4 times with deionized water, 15 minutes each time. Add 100 mL of acetone to the collected viscous liquid, stir to mix and dissolve it, conduct precipitation, select n-hexane as the precipitant, and precipitate one or two times. Collect the viscous oily precipitate as caffeic acid-based epoxy resin. Add 200 mL of dichloromethane to the caffeic acid-based epoxy resin, stir until it is mixed and dissolved, then add 30 g of anhydrous magnesium sulfate and stir. After standing, perform solid-liquid separation, take the upper-layer liquid, and remove dichloromethane through a rotary evaporator to obtain yellow caffeic acid-based epoxy resin 2.

[0026] Step (3): Heat the epoxy resin from step (2) to 60 °C, melt DDM, quickly mix it with the epoxy resin and stir rapidly. Quickly cool the mixture to room temperature, perform vacuum degassing for 30 minutes, add it to a mold and place it in an oven for curing, 2 hours at 105 °C, 1 hour at 130 °C, and 2 hours at 145 °C.

[0027] Example 3

[0028] Step (1): Add 36 g of caffeic acid to a 500 mL three-necked flask, then add 222 g of epichlorohydrin and 1.9 g of tetrabutylammonium bromide, and heat to 100 °C. Stir magnetically for 2 hours at 200 revolutions per minute. Cool the mixed solution to 10 °C, add 72 g of sodium hydroxide aqueous solution (30 wt%), and continue stirring for 2 hours at 300 revolutions per minute.

[0029] Step (2): Directly add the mixed solution from step (1) into 1 L of deionized water, continue magnetic stirring, take the lower-layer oily viscous liquid, wash it 4 times with deionized water, 15 minutes each time. Add 100 mL of acetone to the collected viscous liquid, stir to mix and dissolve it, conduct precipitation, select n-hexane as the precipitant, and precipitate one or two times. Collect the viscous oily precipitate as caffeic acid-based epoxy resin. Add 200 mL of dichloromethane to the caffeic acid-based epoxy resin, stir until it is mixed and dissolved, then add 10 g of anhydrous magnesium sulfate and stir. After standing, perform solid-liquid separation, take the upper-layer liquid, and remove dichloromethane through a rotary evaporator to obtain yellow caffeic acid-based epoxy resin 3.

[0030] Step (3): Heat the epoxy resin from step (2) to 60 °C, melt DDM, quickly mix it with the epoxy resin and stir rapidly. Quickly cool the mixture to room temperature, perform vacuum degassing for 30 minutes, add it to a mold and place it in an oven for curing, 1 hour at 110 °C, 2 hours at 125 °C, and 2 hours at 150 °C.

[0031] Example 4

[0032] Step (1) Add 36 g of caffeic acid into a 500 mL three-necked flask, then add 222 g of epichlorohydrin and 3.2 g of tetrabutylammonium bromide, and heat up to 100 °C. Stir magnetically for 3 h at 200 rpm. Cool the mixed solution to 0 °C, add 72 g of sodium hydroxide aqueous solution (30 wt%), and continue to stir for 2 h at 300 rpm.

[0033] Step (2) Directly add the mixed solution from step (1) into 1 L of deionized water, continue magnetic stirring, take the lower-layer oily viscous liquid, wash it with deionized water 6 times, 15 min each time. Add 100 mL of acetone into the collected viscous liquid, stir to dissolve it, perform precipitation, select n-hexane as the precipitant, precipitate one or two times, and collect the viscous oily precipitate as caffeic acid-based epoxy resin. Add 200 mL of dichloromethane into the caffeic acid-based epoxy resin, stir until it is completely dissolved, then add 20 g of anhydrous magnesium sulfate and stir. After standing, perform solid-liquid separation, take the upper-layer liquid, and remove dichloromethane through a rotary evaporator to obtain yellow caffeic acid-based epoxy resin epoxy resin 4.

[0034] Step (3) Heat the epoxy resin from step (2) to 60 °C, melt DDM, quickly mix it with the epoxy resin and stir rapidly. Quickly cool the mixture to room temperature, perform vacuum degassing for 30 min, add it into a mold and cure it in an oven at 110 °C for 2 h, 130 °C for 1 h, and 145 °C for 1 h.

[0035] Example 5

[0036] Step (1) Add 18 g of caffeic acid into a 500 mL three-necked flask, then add 93 g of epichlorohydrin and 1.6 g of tetrabutylammonium bromide, and heat up to 100 °C. Stir magnetically for 3 h at 300 rpm. Cool the mixed solution to 0 °C, add 36 g of sodium hydroxide aqueous solution (30 wt%), and continue to stir for 3 h at 300 rpm.

[0037] Step (2) Directly add the mixed solution from step (1) into 1 L of deionized water, continue magnetic stirring, take the lower-layer oily viscous liquid, wash it with deionized water 3 times, 15 min each time. Add 60 mL of acetone into the collected viscous liquid, stir to dissolve it, perform precipitation, select n-hexane as the precipitant, precipitate one or two times, and collect the viscous oily precipitate as caffeic acid-based epoxy resin. Add 100 mL of dichloromethane into the caffeic acid-based epoxy resin, stir until it is completely dissolved, then add 10 g of anhydrous magnesium sulfate and stir. After standing, perform solid-liquid separation, take the upper-layer liquid, and remove dichloromethane through a rotary evaporator to obtain yellow caffeic acid-based epoxy resin epoxy resin 5.

[0038] Step (3): Heat the epoxy resin in step (2) to 60°C, melt DDM, quickly mix it with the epoxy resin and stir rapidly. Then quickly cool the mixture to room temperature, perform vacuum degassing for 30 min, add it to a mold and cure it in an oven at 105°C for 2 h, 130°C for 2 h, and 145°C for 1 h.

[0039] Effect Example 1

[0040] The room-temperature tensile properties of the caffeic acid-based epoxy resin of Example 1 of the present invention and commercially available bisphenol A (Beijing Innochem Science & Technology Co., Ltd.) were tested on a 5967X universal testing machine. The dimensions of the epoxy resin were all 30 mm (length) × 5 mm (width) × 2 mm (thickness), and the tensile rate was 10 mm / min. The tensile strength of the caffeic acid epoxy resin of Example 1 of the present invention reached 75 MPa, and the mechanical properties were significantly better than those of bisphenol A epoxy resin, which was 58 MPa.

[0041] Effect Example 2

[0042] The non-isothermal DSC test was carried out on the caffeic acid-based epoxy resin prepared in Example 2 of the present invention after curing with commercially available bisphenol A epoxy resin. The glass transition temperature of the caffeic acid-based epoxy resin was 175°C, which was much higher than 140°C of bisphenol A epoxy resin.

[0043] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited thereto. Any form of evolution and improvement based on the inventive concept of the present invention belongs to the scope of protection of the present invention.

Claims

1. A caffeic acid-based epoxy resin, characterized in that, The epoxy resin contains a compound with a structural formula as shown in formula (I): The preparation method of the epoxy resin comprises the following steps: Mix caffeic acid with epichlorohydrin, add a phase transfer catalyst, react at 85 - 100 °C for 2 - 3 h, then cool to 0 - 10 °C, dropwise add NaOH solution, continue to react for 2 - 3 h, then perform water washing, add a precipitating agent for precipitation, and thus obtain the product of formula I. Cure the product of formula I to obtain caffeic acid-based epoxy resin; The curing method is: pre-cure at 70 °C - 80 °C for 0.5 - 1 h, and then cure in stages at 105 - 110 °C for 1 - 2 h, 125 - 130 °C for 1 - 2 h, and 145 - 150 °C for 1 - 2 h to prepare a thermosetting caffeic acid-based epoxy resin.

2. The preparation method of the caffeic acid-based epoxy resin according to claim 1, characterized in that: In the above steps, the molar ratio of caffeic acid to epichlorohydrin is 1∶9 - 12.

3. The preparation method of the caffeic acid-based epoxy resin according to claim 1, wherein The phase transfer catalyst is selected from one or more of benzyltriethylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium hydrogensulfate, trioctylmethylammonium chloride, dodecyltrimethylammonium chloride, and tetradecyltrimethylammonium chloride.

4. The preparation method of the caffeic acid-based epoxy resin according to claim 3, characterized in that, The molar ratio of the product of formula I to the phase transfer catalyst is 1∶0.03 - 0.

05.

5. The preparation method of the caffeic acid-based epoxy resin according to claim 1, wherein The precipitating agent is one or both of n-hexane and ether.

6. Use of the caffeic acid-based epoxy resin according to claim 1, characterized in that: This epoxy resin is applied to adhesives, coatings, electronic device encapsulation, and the aerospace field.

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