A flexible lignin-based epoxy resin, and a preparation method and application thereof

Abstract

The application relates to the technical field of new materials, and discloses a flexible lignin-based epoxy resin as well as a preparation method and application thereof.The preparation method comprises the following steps: (1) mixing low-molecular-weight eucalyptus kraft lignin with epichlorohydrin, reacting for 2h under the assistance of a phase transfer catalyst, adding sodium hydroxide and reacting for 4h to obtain a lignin-based epoxy resin precursor; (2) mixing the lignin-based epoxy resin precursor obtained in the step (1) with bisphenol A diglycidyl ether in proportion, adding polyetheramine D-400, uniformly mixing, pouring into a polytetrafluoroethylene mold, and curing in an oven to obtain the lignin-based epoxy resin.The application significantly improves the tensile strength and elongation at break of the lignin-based epoxy resin, and can realize a substitution rate of up to 60%, and has stronger tensile strength and elongation at break than pure bisphenol A type epoxy resin.

Description

Technical Field

[0001] This invention relates to the field of new materials technology, specifically to a flexible lignin-based epoxy resin, its preparation method, and its applications. Background Technology

[0002] Epoxy resin is a thermosetting resin with superior heat resistance, chemical resistance, strong adhesion, and excellent mechanical properties. It is widely used in coatings, electrical insulators, electronic components, adhesives, and carbon fiber / glass fiber composites. Currently, the most common epoxy resin on the market is made from bisphenol A diglycidyl ether (DGEBA), a product of the reaction between bisphenol A and epichlorohydrin, accounting for 90% of the world's epoxy precursors. Bisphenol A is an endocrine disruptor that can cause diseases such as thyroid abnormalities, diabetes, liver damage, and cancer, and has been banned from use in food packaging and baby bottles. Finding non-toxic, renewable alternatives to bisphenol A has become a pressing need.

[0003] Lignin is the most abundant aromatic compound in the world, exhibiting excellent properties in terms of thermal stability, antibacterial activity, antioxidant activity, biodegradability, and adhesion. As a byproduct of the pulp and paper industry, lignin is characterized by low cost and high yield. However, the heterogeneity, low reactivity, and high polydispersity of industrial lignin molecules increase the difficulty of its application. The vast majority of lignin is directly burned, resulting in low utilization rates, resource waste, and environmental pollution. Achieving high-value utilization of lignin is an urgent problem to be solved.

[0004] Although epoxy resins possess excellent properties in various aspects, their toxicity and non-renewable nature have forced researchers to seek biomass materials to replace bisphenol A type epoxy resins. Therefore, maximizing the lignin substitution rate while maintaining or even improving the performance of the epoxy resin is key to the preparation of lignin-based epoxy resins.

[0005] The prior art patent publication number CN114591494A discloses a biomass-based modified epoxy resin and its production process, including the following preparation process: (1) Preparation of biomass curing agent: modified polyamine is prepared by using decanediamine, thiourea, carboxylated lignin and succinic anhydride as raw materials; modified polyacid is prepared by using cashew acid, acrylic acid and mercaptopropionic acid as raw materials; the modified polyamine and modified polyacid are mixed and reacted to obtain biomass curing agent; (2) Preparation of modified epoxy resin: epoxy resin and biomass curing agent are mixed and reacted to obtain modified epoxy resin.

[0006] Existing patent publication number CN115449053A discloses a method for preparing lignin epoxy resin and its toughening modification through chemical reaction, comprising the following steps: adding pretreated lignin particles and a mixed solvent to a reaction vessel and heating for reaction; adding epichlorohydrin to the resulting mixture; adding sodium hydroxide at a constant temperature for further reaction; after the reaction is complete, distilling under reduced pressure to recover water and excess epichlorohydrin; adding phthalic anhydride and a catalyst to the recovered water and excess epichlorohydrin for reflux reaction; and after the reaction is complete, removing the water produced by the reaction under reduced pressure to obtain a toughened lignin-based epoxy resin. This patent utilizes lignin solid extracted from papermaking black liquor, which is pulverized to obtain lignin particles with a particle size less than 0.1 μm.

[0007] All of the above-mentioned existing technical solutions require special modification of lignin raw materials. At the same time, the epoxy resins prepared by the above solutions have poor flexibility (elongation at break is about 2.5%-6.9%). Summary of the Invention

[0008] To address the aforementioned problems in the prior art, this invention provides a method for preparing a flexible lignin-based epoxy resin;

[0009] Another object of the present invention is to provide a flexible lignin-based epoxy resin.

[0010] This invention provides the following technical solution:

[0011] A method for preparing a flexible lignin-based epoxy resin includes the following steps:

[0012] (1) Low molecular weight eucalyptus sulfate lignin was mixed with epichlorohydrin and reacted for 2 hours with the assistance of a phase transfer catalyst. Then sodium hydroxide was added and reacted for 4 hours to obtain a lignin-based epoxy resin precursor.

[0013] (2) The lignin-based epoxy resin precursor obtained in step (1) is mixed with bisphenol A diglycidyl ether in proportion, and then polyetheramine D-400 is added. After mixing evenly, the mixture is poured into a polytetrafluoroethylene mold and cured in an oven to obtain lignin-based epoxy resin.

[0014] Preferably, the low molecular weight eucalyptus sulfate lignin is mainly composed of monomers and dimers, including aldehyde and ketone monomers and eugenol, both of which can improve the flexibility of the resin.

[0015] Preferably, the low molecular weight eucalyptus sulfate lignin has a molecular weight of 500-700 g / mol (M). w The low molecular weight eucalyptus sulfate lignin is prepared by dichloromethane extraction, with the extraction times being 4-8 times.

[0016] Preferably, the phase transfer catalyst is benzyltriethylammonium chloride.

[0017] Preferably, low molecular weight eucalyptus sulfate lignin is mixed with epichlorohydrin and reacted at 60°C for 2 hours with the assistance of a phase transfer catalyst, followed by the addition of sodium hydroxide and reaction at 80°C for 4 hours.

[0018] Preferably, the ratio of lignin-based epoxy resin precursor to bisphenol A diglycidyl ether is (2-6):(8-4).

[0019] Preferably, in step (2), uniform mixing means mixing with ultrasonic assistance or at 120°C.

[0020] Preferably, in step (2), curing in an oven means curing at 120°C for 6 hours.

[0021] A flexible lignin-based epoxy resin prepared by the above preparation method.

[0022] Preferably, the raw materials for synthesizing flexible lignin-based epoxy resins contain aldehyde and ketone monomers and eugenol structures, which can improve the flexibility of the resin.

[0023] The raw material for flexible lignin-based epoxy resin includes low molecular weight eucalyptus sulfate lignin extracted from dichloromethane. This lignin is mainly composed of monomers and dimers, including aldehyde and ketone monomers and eugenol, which play a significant role in improving the flexibility of the epoxy resin.

[0024] An application of the flexible lignin-based epoxy resin in the preparation of coatings and adhesives.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) In this invention, low molecular weight lignin obtained from industrial lignin grading is used as raw material, which is conducive to realizing the high-value utilization of lignin and reducing resource waste and environmental pollution.

[0027] (2) The present invention uses different gradients of lignin substitution rate for reaction. When the substitution rate is as high as 60%, it still has stronger tensile strength and elongation at break than pure bisphenol A type epoxy resin.

[0028] (3) The present invention uses a simple chemical reaction and a simple casting molding method to prepare lignin-based epoxy resin, which significantly improves the tensile strength and elongation at break of lignin-based epoxy resin.

[0029] This invention innovatively obtains low molecular weight lignin raw materials through a simple one-step extraction method using an organic solvent (dichloromethane). Furthermore, epoxy resins made by partially replacing bisphenol A with this raw material can achieve an elongation at break of up to 194.3%-394.6%, and can still maintain a high elongation at break even when the replacement rate is as high as 60%. Attached Figure Description

[0030] Figure 1 Infrared spectra of eugenol in low molecular weight lignin, eugenol epoxy precursor, and eugenol-epoxy resin.

[0031] Figure 2 Infrared spectra of vanillin in low molecular weight lignin, vanillin epoxy precursor, and vanillin-epoxy resin. Detailed Implementation

[0032] The embodiments of the technical solution of the present invention will be described in detail below. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore merely examples and should not be used to limit the scope of protection of the present invention. It should be noted that, unless otherwise stated, the technical or scientific terms used in this application should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0033] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. Raw materials and reagents used without a specified manufacturer can be commercially available conventional products.

[0034] The mechanical properties of the lignin-based epoxy resin and bisphenol A type epoxy resin in the embodiments and comparative examples of this invention were obtained by tensile testing using a universal testing machine. The sample size was [missing information]. The tensile rate was 10 mm / min, the test environment temperature was 25℃, and the relative humidity was 50%.

[0035] The molecular weight of low molecular weight eucalyptus sulfate lignin is 500-700 g / mol (Mw);

[0036] Low molecular weight eucalyptus sulfate lignin was prepared by dichloromethane extraction, with 4-8 extraction cycles.

[0037] Example 1

[0038] (1) 1 g of low molecular weight eucalyptus sulfate lignin (molecular weight 500-700 g / mol), 10.5 mL of epichlorohydrin and 0.61 g of benzyltriethylammonium chloride were added to a three-necked flask and refluxed at 80 °C for 2 h. Then, 20.5 mL of sodium hydroxide solution (8 M) was added and refluxed at 60 °C for 4 h. After the reaction was completed, the mixture was extracted with an appropriate amount of ethyl acetate / water, dried over anhydrous magnesium sulfate, and distilled under reduced pressure to obtain the lignin-based epoxy resin precursor.

[0039] (2) The lignin-based epoxy resin precursor and bisphenol A diglycidyl ether were mixed in a ratio of 1 g total mass to 2:8. Then 0.75 g of polyetheramine D-400 was added. The mixture was thoroughly mixed under ultrasonic assistance or at 120°C. The mixture was poured into a polytetrafluoroethylene mold and cured in an oven at 120°C for 6 h to obtain the lignin-based epoxy resin.

[0040] Example 2

[0041] (1) 1 g of low molecular weight eucalyptus sulfate lignin (molecular weight 500-700 g / mol), 10.5 mL of epichlorohydrin and 0.61 g of benzyltriethylammonium chloride were added to a three-necked flask and refluxed at 80 °C for 2 h. Then, 20.5 mL of sodium hydroxide solution (8 M) was added and refluxed at 60 °C for 4 h. After the reaction was completed, the mixture was extracted with an appropriate amount of ethyl acetate / water, dried over anhydrous magnesium sulfate, and distilled under reduced pressure to obtain the lignin-based epoxy resin precursor.

[0042] (2) The lignin-based epoxy resin precursor and bisphenol A diglycidyl ether were mixed in a ratio of 1g (total mass, mass ratio of lignin-based epoxy resin precursor to bisphenol A diglycidyl ether is 4:6), and then 0.75g of polyetheramine D-400 was added. The mixture was thoroughly mixed under ultrasonic assistance or at 140°C, poured into a polytetrafluoroethylene mold, and cured in an oven at 140°C for 6h to obtain lignin-based epoxy resin.

[0043] Example 3

[0044] (1) 1 g of low molecular weight eucalyptus sulfate lignin (molecular weight 500-700 g / mol), 10.5 mL of epichlorohydrin and 0.61 g of benzyltriethylammonium chloride were placed in a three-necked flask and refluxed at 80 °C for 2 h. Then, 20.5 mL of sodium hydroxide solution (8 M) was added, and the mixture was refluxed at 60 °C for 4 h. After the reaction was completed, the mixture was extracted with an appropriate amount of ethyl acetate / water, dried over anhydrous magnesium sulfate, and distilled under reduced pressure to obtain the lignin-based epoxy resin precursor.

[0045] (2) The lignin-based epoxy resin precursor and bisphenol A diglycidyl ether were mixed in a ratio of 1 g total mass to 6:4. Then 0.78 g of polyetheramine D-400 was added. The mixture was thoroughly mixed under ultrasonic assistance or at 120°C. The mixture was poured into a polytetrafluoroethylene mold and cured in an oven at 120°C for 6 h to obtain the lignin-based epoxy resin.

[0046] Comparative Example 1

[0047] The technical solution of this comparative example is basically the same as that of Example 1, except that:

[0048] Mix 1g of bisphenol A diglycidyl ether and add 0.72g of polyetheramine D-400. Mix thoroughly with ultrasonic assistance or at 120°C. Pour into a polytetrafluoroethylene mold and cure in an oven at 120°C for 6 hours to obtain bisphenol A type epoxy resin.

[0049] Comparative Example 2

[0050] The technical solution of this comparative example is basically the same as that of Example 1, except that:

[0051] (1) 1 g of unfractionated sulfate lignin (molecular weight 1000-1200 g / mol), 5.5 mL of epichlorohydrin, and 1.40 g of benzyltriethylammonium chloride were added to a three-necked flask and refluxed at 80 °C for 2 h. Then, 20.5 mL of sodium hydroxide solution (8 M) was added, and the mixture was refluxed at 60 °C for 4 h. After the reaction was completed, the mixture was extracted with an appropriate amount of ethyl acetate / water, dried over anhydrous magnesium sulfate, and distilled under reduced pressure to obtain the lignin-based epoxy resin precursor.

[0052] (2) The lignin-based epoxy resin precursor and bisphenol A diglycidyl ether were mixed in a ratio of 1 g total mass to 2:8. Then 0.71 g of polyetheramine D-400 was added. The mixture was thoroughly mixed under ultrasonic assistance or at 120°C. The mixture was poured into a polytetrafluoroethylene mold and cured in an oven at 120°C for 6 h to obtain the lignin-based epoxy resin.

[0053] Comparative Example 3

[0054] The technical solution of this comparative example is basically the same as that of Example 1, except that:

[0055] (1) 1 g of high molecular weight sulfate lignin (molecular weight 1600-1800 g / mol), 2.4 mL of epichlorohydrin and 0.61 g of benzyltriethylammonium chloride were added to a three-necked flask and refluxed at 80 °C for 2 h. Then, 20.5 mL of sodium hydroxide solution (8 M) was added and refluxed at 60 °C for 4 h. After the reaction was completed, the mixture was extracted with an appropriate amount of ethyl acetate / water, dried over anhydrous magnesium sulfate, and distilled under reduced pressure to obtain the lignin-based epoxy resin precursor.

[0056] (2) The lignin-based epoxy resin precursor and bisphenol A diglycidyl ether were mixed in a ratio of 1 g total mass to 2:8. Then 0.70 g of polyetheramine D-400 was added. The mixture was thoroughly mixed under ultrasonic assistance or at 120°C. The mixture was poured into a polytetrafluoroethylene mold and cured in an oven at 120°C for 6 h to obtain the lignin-based epoxy resin.

[0057] Comparative Example 4

[0058] The technical solution of this comparative example is basically the same as that of Example 1, except that:

[0059] (1) 1 g of low molecular weight eucalyptus sulfate lignin (molecular weight 500-700 g / mol), 21.0 mL of epichlorohydrin and 0.61 g of benzyltriethylammonium chloride were added to a three-necked flask and refluxed at 80 °C for 2 h. Then, 20.5 mL of sodium hydroxide solution (8 M) was added and refluxed at 60 °C for 4 h. After the reaction was completed, the mixture was extracted with an appropriate amount of ethyl acetate / water, dried over anhydrous magnesium sulfate, and distilled under reduced pressure to obtain the lignin-based epoxy resin precursor.

[0060] (2) The lignin-based epoxy resin precursor and bisphenol A diglycidyl ether were mixed in a ratio of 1 g total mass to 2:8. Then 0.73 g of polyetheramine D-400 was added. The mixture was thoroughly mixed under ultrasonic assistance or at 120°C. The mixture was poured into a polytetrafluoroethylene mold and cured in an oven at 120°C for 6 h to obtain the lignin-based epoxy resin.

[0061] Comparative Example 5

[0062] The technical solution of this comparative example is basically the same as that of Example 1, except that:

[0063] (1) 1 g of low molecular weight eucalyptus sulfate lignin (molecular weight 500-700 g / mol), 10.5 mL of epichlorohydrin and 0.61 g of benzyltriethylammonium chloride were added to a three-necked flask and refluxed at 80 °C for 2 h. Then, 12.8 mL of sodium hydroxide solution (5 M) was added and refluxed at 60 °C for 4 h. After the reaction was completed, the mixture was extracted with an appropriate amount of ethyl acetate / water, dried over anhydrous magnesium sulfate, and distilled under reduced pressure to obtain the lignin-based epoxy resin precursor.

[0064] (2) The lignin-based epoxy resin precursor and bisphenol A diglycidyl ether were mixed in a ratio of 1 g total mass to 2:8. Then 0.74 g of polyetheramine D-400 was added. The mixture was thoroughly mixed under ultrasonic assistance or at 120°C. The mixture was poured into a polytetrafluoroethylene mold and cured in an oven at 120°C for 6 h to obtain the lignin-based epoxy resin.

[0065] Performance testing of the flexible lignin-based epoxy resin of this invention

[0066] The epoxy resin samples obtained in Examples 1-3 and Comparative Examples 1-5 were subjected to mechanical property tests. The mechanical property results of the flexible lignin epoxy resin are shown in Table 1.

[0067] Table 1 Mechanical properties of lignin-based epoxy resins

[0068]

[0069]

[0070] As shown in Table 1, the lignin-based epoxy resin prepared in Example 1 exhibits the best mechanical properties (tensile strength and elongation at break) among Examples 1-3. Furthermore, the mechanical properties of Examples 1-3 are superior to those of the bisphenol A-type epoxy resin in Comparative Example 1, indicating the feasibility of using lignin to replace bisphenol A in epoxy resin preparation. Moreover, excellent mechanical properties are maintained even with a lignin substitution rate as high as 60%, thus suggesting the potential to prepare lignin-based epoxy resins with a lignin substitution rate approaching 100%, significantly expanding its application range. In addition, Examples 1-3 demonstrate superior performance compared to Comparative Examples 2-3, indicating that low molecular weight lignin is more suitable as a raw material for epoxy resins than ungraded or high molecular weight lignin.

[0071] Example 1 demonstrates superior performance compared to Comparative Examples 4-5, indicating that the reaction conditions (amounts of epichlorohydrin and sodium hydroxide) used in the invention affect the properties of the prepared epoxy resin. Increasing the amount of sodium hydroxide enhances the glycidyl etherification reaction of lignin, which is beneficial for improving mechanical properties. Therefore, Example 1 outperforms Comparative Example 5. Conversely, increasing the amount of epichlorohydrin leads to an increased degree of polymerization of lignin in the glycidyl etherification reaction, resulting in a decrease in the content of reactive groups, which is detrimental to improving mechanical properties. Therefore, Comparative Example 4 is inferior to Example 1.

[0072] Low molecular weight lignin contains eugenol and aldehyde / ketone monomers. The furan ring of eugenol undergoes a ring-opening reaction with polyetheramine D-400, such as... Figure 1 As shown, 910cm -1 The absorption peak disappears after the reaction, and the reaction mechanism may be due to steric hindrance at the γ-position by NH2 attack. Taking vanillin as an example, aldehyde and ketone monomers react to form a Schiff base (C=N). Figure 2 It is evident that CHO is converted to C=N after the reaction. All of the above reactions can enhance the flexibility of the resulting lignin-based epoxy resin.

[0073] In addition, the lignin-based epoxy resin obtained can be used in coatings and adhesives.

[0074] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention 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 or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for preparing a flexible lignin-based epoxy resin, characterized in that, Includes the following steps: (1) Low molecular weight eucalyptus sulfate lignin was mixed with epichlorohydrin and reacted for 2 hours with the assistance of a phase transfer catalyst. Then sodium hydroxide was added and reacted for 4 hours to obtain a lignin-based epoxy resin precursor. (2) The lignin-based epoxy resin precursor obtained in step (1) is mixed with bisphenol A diglycidyl ether in proportion, and then polyetheramine D-400 is added. The mixture is mixed evenly, poured into a polytetrafluoroethylene mold, and cured in an oven to obtain lignin-based epoxy resin. The molecular weight of the low molecular weight eucalyptus sulfate lignin is 500-700 g / mol; In step (1), for every 1g of low molecular weight eucalyptus sulfate lignin, 10.5mL of epichlorohydrin, 0.61g of phase transfer catalyst, and 20.5mL of 8mol / L sodium hydroxide solution are used. The phase transfer catalyst is benzyltriethylammonium chloride; The ratio of lignin-based epoxy resin precursor to bisphenol A diglycidyl ether is (2-6):(8-4).

2. The method for preparing the flexible lignin-based epoxy resin according to claim 1, characterized in that, The low molecular weight eucalyptus sulfate lignin was prepared by dichloromethane extraction, with the extraction cycle being 4-8 times.

3. The method for preparing the flexible lignin-based epoxy resin according to claim 1, characterized in that, Low molecular weight eucalyptus sulfate lignin was mixed with epichlorohydrin and reacted at 60°C for 1-4 hours with the aid of a phase transfer catalyst. Sodium hydroxide was then added and the reaction was carried out at 80°C for 2-6 hours.

4. The method for preparing the flexible lignin-based epoxy resin according to claim 1, characterized in that, In step (2), uniform mixing means mixing with ultrasonic assistance or at 100-140°C.

5. The method for preparing the flexible lignin-based epoxy resin according to claim 4, characterized in that, In step (2), curing in an oven means curing at 100-140℃ for 4-8 hours.

6. A flexible lignin-based epoxy resin prepared by the preparation method according to any one of claims 1-5.

7. The flexible lignin-based epoxy resin according to claim 6, characterized in that, The raw materials for flexible lignin-based epoxy resins include low molecular weight eucalyptus sulfate lignin extracted from dichloromethane. This lignin is mainly composed of monomers and dimers, including aldehyde and ketone monomers and eugenol.

8. The use of the flexible lignin-based epoxy resin of claim 6 in the preparation of coatings and adhesives.

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