CN113717400A - Lignin fractionation method and preparation method of lignin-based thermosetting resin - Google Patents

Abstract

The invention discloses a lignin fractionation method and a preparation method of lignin-based thermosetting resin, wherein the lignin fractionation method comprises the following steps: adding industrial lignin into an alkaline aqueous solution, filtering, and removing insoluble substances to obtain a lignin dispersion liquid; adding an acidic solution into the lignin dispersion liquid, adjusting the pH value of the solution to be acidic, and freeze-drying to obtain a precipitate which is purified lignin. The method separates and purifies the industrial prehydrolysis lignin by an acid gradient precipitation method so as to improve the heterogeneity and the structural complexity of the industrial lignin and realize the fractional purification of the industrial prehydrolysis lignin. The epoxy ethane structure is introduced by carrying out epoxidation modification treatment on the lignin component to improve the reaction activity of the lignin component, and finally the thermosetting resin material with high thermal stability and flexibility is prepared.

Description

Lignin fractionation method and preparation method of lignin-based thermosetting resin

Technical Field

The invention relates to the technical field of resin materials, in particular to a lignin fractionation method and a preparation method of lignin-based thermosetting resin.

Background

Lignin is a renewable aromatic compound abundant in nature and widely regarded as a potential biopolymer substitute for petrochemical products. Lignin is mainly present in woody and herbaceous plants and in all vascular plants, and constitutes plant fiber together with cellulose and hemicellulose. In the production process of a conventional paper mill or fuel ethanol plant, a large amount of industrial lignin is produced as waste. Most of these industrial lignin structures are destroyed to various degrees, and have non-uniform molecular weights and high polydispersities. According to different pulping processes, industrial lignin can be classified into lignosulfonate, kraft lignin, alkali lignin, organic solvent lignin, enzymatic hydrolysis lignin and the like.

The method for fractionation of industrial lignin adopts a solvent method and a membrane filtration method. The fractionation by solvent method is to separate lignin according to different solubility of lignin in different organic solvents. Several different organic solvents are selected to carry out fractional extraction on lignin, and lignin with different fractions can be obtained correspondingly. The membrane separation method is used for separating lignin by utilizing different molecular weights, and is widely applied to the fields of biology, food, medicine and the like.

The separation by the solvent method has the defects of high solvent cost, long separation time, poor separation effect and difficult recovery. The membrane filtration method has the defects of membrane pollution, high cost and long separation time.

Disclosure of Invention

The embodiment of the invention aims to overcome the defects of the prior art and provide a green and efficient lignin fractionation method.

The invention also provides a preparation method of the lignin-based thermosetting resin.

In order to achieve the aim, the invention adopts the technical scheme that:

a method of lignin fractionation comprising the steps of:

adding industrial lignin into an alkaline aqueous solution, filtering, and removing insoluble substances to obtain a lignin dispersion liquid; adding an acidic solution into the lignin dispersion liquid, adjusting the pH value of the solution to be acidic, and freeze-drying to obtain a precipitate which is purified lignin.

The alkaline aqueous solution is sodium hydroxide aqueous solution, potassium hydroxide aqueous solution or sodium carbonate aqueous solution, and the mass volume ratio of the lignin to the alkaline aqueous solution is 10: 150-250; the concentration of the alkaline aqueous solution is 0.5-0.7M.

The acidic aqueous solution is hydrochloric acid, sulfuric acid or glacial acetic acid, and the concentration of the acidic aqueous solution is 5.0-7.0M.

The pH value is 6, 4 or 2.

The industrial lignin is industrial prehydrolysis lignin obtained from prehydrolysis liquid in the pulping process.

The invention also provides application of the purified lignin obtained by the lignin fractionation method in preparation of thermosetting resin.

The invention also provides a preparation method of the lignin-based thermosetting resin, which comprises the following steps:

modification of purified lignin: adding the purified lignin, acetone, NaOH and epichlorohydrin obtained by the lignin fractionation method into a container, magnetically stirring, adding water to terminate the reaction, adjusting the pH value of the solution to 3.5, centrifugally separating the obtained solid component, washing with deionized water, and freeze-drying to obtain light brown modified lignin;

and (3) curing: adding modified lignin, bisphenol A diglycidyl ether BADGE, a curing crosslinking agent and a diluent into a container, stirring to obtain a uniform viscous liquid mixture, pouring the mixture into a mold, putting the mold into an oven to remove the solvent, and curing to obtain the lignin-based thermosetting resin.

The mass volume ratio of the purified lignin to the acetone solution is 2: 200-400; the concentration of the acetone solution is 45-55% v/v; the amount of NaOH is 2-4 times equivalent of purified lignin; the using amount of the epichlorohydrin is 15-25 times of the active OH value of the purified lignin; the volume mass ratio of the water added for terminating the reaction to the purified lignin is 150-250: 2; the pH of the solution was adjusted using 0.1-0.2M HCl.

The temperature of the magnetic stirring is 50-60 ℃, the stirring time is 4-6 hours, and the stirring speed is 200-500 rpm/min.

The mass ratio of the modified lignin to the BADGE is 2-25%, preferably 5-20%, and more preferably 5-15%; the curing crosslinking agent is added in an amount of 0.2 to 1 ethylene oxide equivalent, and the mass ratio of the diluent to the curing crosslinking agent is 0.5 to 1.5.

The curing crosslinking agent is selected from polyether amine D230, D400 or D2000; the diluent is selected from one or more of acetonitrile, acetone, ethyl acetate and absolute ethyl alcohol, and the mold is a polytetrafluoroethylene mold; the temperature when the solvent is removed in the oven is 35-45 ℃ and the time is 1.5-2.5 hours; the curing temperature is 55-65 ℃, and the curing time is 3.5-4.5 hours.

The technical scheme provided by the embodiment of the invention has the following beneficial effects:

aiming at the difficulties of large molecular weight, complex and variable structure and low activity of the industrial prehydrolysis lignin, the invention separates and purifies the industrial prehydrolysis lignin by an acid gradient precipitation method so as to improve the heterogeneity and the structural complexity of the industrial lignin and realize the fractional purification of the industrial prehydrolysis lignin. The epoxy ethane structure is introduced by carrying out epoxidation modification treatment on the lignin component to improve the reaction activity of the lignin component, and finally the thermosetting resin material with high thermal stability and flexibility is prepared.

Drawings

FIG. 1 is a TGA and DTG curve for different lignin compositions provided by an embodiment of the present invention;

FIG. 2 is an infrared spectrum of various lignin components provided by an example of the present invention;

FIG. 3 is a comparison chart of 2D HSQC NMR spectra before and after lignin modification;

FIG. 4 shows lignin before and after modification³¹P NMR comparison spectrum;

fig. 5 is a cross-sectional view of a thermosetting epoxy resin analyzed by SEM as provided by an embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Example 1

A method of lignin fractionation comprising the steps of:

the lignin dispersion was obtained by adding 10.0g of industrial lignin to 200mL of a 0.625M aqueous solution of sodium hydroxide at room temperature, filtering the mixture to remove a small amount of insoluble matter, adjusting the pH of the lignin dispersion to 6.0 with 6.0M HCl, and freeze-drying the resulting precipitate to obtain purified lignin, which was designated L6.

The pH of the lignin dispersion was adjusted to 4.0 by the same method as described above, and the obtained purified lignin was named L4. The pH of the lignin dispersion was adjusted to 2.0, and the obtained purified lignin was designated as L2.

A preparation method of lignin-based thermosetting resin comprises the following steps:

modification: 2.0g of purified lignin L6, 300mL of acetone/water (50% v/v), 3 equivalents of NaOH for the lignin component and 20 equivalents of epichlorohydrin for the active OH value of the lignin component were added to a round-bottomed flask and stirred magnetically at 55 ℃ for 5 hours at a stirring speed of 300 rpm/min. The reaction was stopped by introducing 200mL of water and the pH of the solution was adjusted to 3.5 with 0.1M HCl. The obtained solid component was centrifuged and washed with deionized water. The mixture was freeze-dried to obtain a light brown modified lignin, designated EL 6.

Similarly, a modified lignin obtained by modifying purified lignin L4 was named EL 4; the modified lignin obtained by modifying purified lignin L2 was named "Hehe EL 2".

And (3) curing: taking four small glass sample bottles, respectively adding modified lignin EL4 and BADGE, wherein the mass ratio of the modified lignin EL4 to the BADGE is 5%, 10%, 15% and 20%, respectively, adding a curing crosslinking agent polyetheramine D400 with the weight of 1/5 times of that of ethylene oxide into each bottle, adding a diluent acetonitrile with the weight of 0.5 time of that of the polyetheramine D400, and stirring to obtain uniform viscous liquid. Then introducing the viscous liquid into a polytetrafluoroethylene mold of GB/528-2009 standard, firstly keeping the viscous liquid in an oven at 40 ℃ for 2 hours to remove the solvent, and then continuously curing the viscous liquid at 60 ℃ for 4 hours; the resulting thermoset resin material was named TPL/L4_ A, with A representing the percent incorporation of modified lignin. Obtaining the thermosetting resin materials TPL/L4-5%, TPL/L4-10%, TPL/L4-15% and TPL/L4-20%, respectively.

Similarly, the thermosetting resin material obtained by curing with the modified lignin EL6 was named TPL/L6_ a%; the thermosetting resin material obtained by curing the modified lignin EL2 was named TPL/L2_ A%.

Example 2

A method of lignin fractionation comprising the steps of:

the lignin dispersion was obtained by adding 10.0g of industrial lignin to 150mL of a 0.5M aqueous solution of potassium hydroxide at room temperature, filtering the mixture to remove a small amount of insoluble matter, adjusting the pH of the lignin dispersion to 6.0 with 5.0M sulfuric acid, and freeze-drying the resulting precipitate to obtain purified lignin, which was designated as L6.

The pH of the lignin dispersion was adjusted to 4.0 by the same method as described above, and the obtained purified lignin was named L4. The pH of the lignin dispersion was adjusted to 2.0, and the obtained purified lignin was designated as L2.

A preparation method of lignin-based thermosetting resin comprises the following steps:

2.0g of purified lignin L6, 200mL of acetone/water (45% v/v), 2 equivalents of NaOH of the lignin component and 15 equivalents of epichlorohydrin of the active OH value of the lignin component were added to a round-bottomed flask and stirred magnetically at 50 ℃ for 6 hours at a stirring speed of 200 rpm/min. The reaction was stopped by introducing 150mL of water and the pH of the solution was adjusted to 3.5 with 0.1M HCl. The obtained solid component was centrifuged and washed with deionized water. The mixture was freeze-dried to obtain a light brown modified lignin, designated EL 6.

Similarly, a modified lignin obtained by modifying purified lignin L4 was named EL 4; the modified lignin obtained by modifying purified lignin L2 was named "Hehe EL 2".

And (3) curing: taking four small glass sample bottles, respectively adding modified lignin EL4 and BADGE in different proportions, wherein the mass ratio of the modified lignin EL4 to the BADGE is respectively 5%, 10%, 15% and 20%, adding a curing crosslinking agent polyether amine D230 in an amount which is 1/2 times of the equivalent weight of ethylene oxide into each bottle, adding diluent acetone in an amount which is 1 time of the amount of the polyether amine D230, and stirring to obtain uniform viscous liquid. Then introducing the viscous liquid into a polytetrafluoroethylene mold of GB/528-2009 standard, keeping the temperature in an oven at 35 ℃ for 2.5h to remove the solvent, and then continuously curing the viscous liquid at 55 ℃ for 4.5 h; the resulting thermoset resin material was named TPL/L4_ A, with A representing the percent incorporation of modified lignin. Obtaining the thermosetting resin materials TPL/L4-5%, TPL/L4-10%, TPL/L4-15% and TPL/L4-20%, respectively.

Similarly, the thermosetting resin material obtained by curing with the modified lignin EL6 was named TPL/L6_ a%; the thermosetting resin material obtained by curing the modified lignin EL2 was named TPL/L2_ A%.

Example 3

A method of lignin fractionation comprising the steps of:

the lignin dispersion was obtained by adding 10.0g of industrial lignin to 250mL of 0.7M sodium carbonate solution at room temperature, filtering, removing a small amount of insoluble matter, adjusting the pH of the lignin dispersion to 6.0 using 7.0M glacial acetic acid, and freeze-drying the resulting precipitate to obtain purified lignin, which was designated as L6.

The pH of the lignin dispersion was adjusted to 4.0 by the same method as described above, and the obtained purified lignin was named L4. The pH of the lignin dispersion was adjusted to 2.0, and the obtained purified lignin was designated as L2.

A preparation method of lignin-based thermosetting resin comprises the following steps:

2.0g of purified lignin L6, 400mL of acetone/water (55% v/v), 4 equivalents of NaOH for the lignin component and 25 equivalents of epichlorohydrin for the active OH value of the lignin component were added to a round-bottomed flask and stirred magnetically at 60 ℃ for 4 hours at a stirring speed of 500 rpm/min. The reaction was stopped by introducing 250mL of water and the pH of the solution was adjusted to 3.5 with 0.2M HCl. The obtained solid component was centrifuged and washed with deionized water. The mixture was freeze-dried to obtain a light brown modified lignin, designated EL 6.

Similarly, a modified lignin obtained by modifying purified lignin L4 was named EL 4; the modified lignin obtained by modifying purified lignin L2 was named "Hehe EL 2".

And (3) curing: taking four small glass sample bottles, respectively adding modified lignin EL4 and BADGE in different proportions, wherein the mass ratio of the modified lignin EL4 to the BADGE is respectively 5%, 10%, 15% and 20%, adding a curing crosslinking agent polyetheramine D2000 with the equivalent of 1 time of ethylene oxide into each bottle, adding a diluent absolute ethyl alcohol with the mass of 1.0 time of the amount of the polyetheramine D2000, and stirring to obtain a uniform viscous liquid. Then introducing the viscous liquid into a polytetrafluoroethylene mold of GB/528-2009 standard, firstly keeping the viscous liquid in an oven at 45 ℃ for 1.5h to remove the solvent, and then continuously curing the viscous liquid at 65 ℃ for 3.5 h; the resulting thermoset resin material was named TPL/L4_ A, with A representing the percent incorporation of modified lignin. Obtaining the thermosetting resin materials TPL/L4-5%, TPL/L4-10%, TPL/L4-15% and TPL/L4-20%, respectively.

Similarly, the thermosetting resin material obtained by curing with the modified lignin EL6 was named TPL/L6_ a%; the thermosetting resin material obtained by curing the modified lignin EL2 was named TPL/L2_ A%.

Embodiments of the present invention preferably use industrial Prehydrolyzed Lignin (PL) which is obtained from prehydrolyzed liquid during pulping. The industrial prehydrolyzed lignin has a lower molecular weight and less carbohydrate content than other industrial lignin. In the FT-IR spectrum (914 cm)^-1Strong band at (b) a small amount of condensed structures in PL lignin were observed. In addition, PL phenolic hydroxyl groupThe content of the base group is relatively high, which is caused by the breakage of aryl-ether bond in the prehydrolysis process, so that the industrial prehydrolysis lignin has relatively high reaction activity relative to other industrial lignin, and the modification and the subsequent preparation of thermosetting resin materials are facilitated.

The epoxidation modification method and steps for purifying lignin can adopt the existing method.

Performance analysis of the purified lignin prepared in example 1:

as shown in table 1, when the pH of the solution was 4.0, almost all lignin had precipitated, with a yield of 65.5%. The yields of lignin were reduced at pH 6.0 and 2.0, 22.5% and 10.7%, respectively. Some re-dissolution of lignin may occur at pH 2, and unstable colloids at pH 4 become stable at pH 2.

All lignin samples showed negative zeta potential values (| zeta | >25), indicating that these lignin samples dispersed well and had strong electrostatic repulsion between the particles.

The GPC analysis results in Table 1 showed that the molecular weight of the polymer was 2640g mol in comparison with PL^-1PDI of 3.0) was significantly reduced in molecular weight and distribution of the lignin component after purification, with weight average molecular weight values ranging from 940 to 2280g mol^-1In the meantime. L4 shows the highest M_wValue (2280g mol)^-1) L2 shows the lowest M_wValue (940g mol)^-1). As can be seen from Table 1, the dispersion of PL is maximal (3.0), while the distribution of polydispersity of the three samples after purification is similar, remaining between 1.8 and 2.4. Minimum M at pH 2_wThe polydispersity of the lignin is also minimal (1.8).

TABLE 1 yield, zeta-potential value, molecular weight and polydispersity factor of solutions of different pH for precipitating lignin

The method uses acid method gradient precipitation to continuously refine PL so as to obtain lignin components with lower molecular weight and low polydispersity, which is very beneficial to the subsequent preparation of lignin-based thermosetting epoxy resin.

The thermal stability of lignin affects the performance of thermosetting epoxy resins. TGA analysis of example 1 purified lignin was performed for this purpose. As can be seen in fig. 1, the curve trends for the four lignin samples are approximately the same. The weight loss curve of lignin can be divided into three stages. First, before 120 ℃, a slight weight loss of lignin is a result of moisture loss. The degradation of lignin mainly occurs in a broad temperature range of 120-450 ℃ where the weight loss is due to the cleavage of the lignin side chains resulting in the release of some gaseous products^[95]. The internal connecting bonds of the lignin macromolecules are gradually broken, residues react to form coke in the process of further increasing the temperature, and the quality of the coke is basically stable and does not change until the temperature is 600 ℃. The residual mass of PL was 44.1% of the initial lignin mass, and the residual mass of L4 was the highest and 54.1% of the initial lignin mass, because L4 contains a large amount of condensation structural units^[96]. The residue mass of the L2 component was also higher than PL, and was 49.8%; the residual mass of L6 was the lowest, 39.3%.

Therefore, the purification process can obviously improve the thermal stability of the lignin and lay a good foundation for the subsequent preparation of the thermosetting epoxy resin.

Referring to fig. 2, the type, structure and characteristics of the functional groups of the lignin feedstock can be determined by infrared spectroscopic analysis. FIG. 2 is a FT-IR spectrum comprising feedstock PL and various lignin components after purification. Table 2 shows the characteristic absorption peaks of the infrared spectra of different lignin components and the assignment of their corresponding structures. As can be seen from fig. 2, all lignin samples have characteristic absorption peaks of lignin and show similar spectral profiles, with little variation in the peaks. All spectra showed characteristic peaks of lignin, mainly at 1800-600cm^-1In the meantime. At 3384cm^-1Stretching vibration of O-H exists; 2936cm^-1Showing stretching vibration of methyl and methylene. Stretching vibrations of the aromatic structure were observed in all samples (1614, 1514 and 1428 cm)^-1) The fractionation of lignin is describedWithout destroying the basic structure of the lignin. At 1328, 1217cm^-1Characteristic peaks of syringyl and guaiacyl respectively are observed, and 1166cm is not observed^-1The characteristic peak of p-hydroxyphenyl group appears nearby, indicating that it is typical SG type broad-leaf raw material. By combining the analysis, the framework structure of the lignin is completely reserved in the continuous refining process of the acid gradient precipitation of the lignin.

TABLE 2 assignment of absorption peaks in the infrared spectrum of lignin

Referring to fig. 3, for comparison of 2D HSQC NMR spectra before and after lignin modification, the epoxidation modification of lignin can greatly improve the reactivity of lignin. Through 2D HSQC NMR analysis, the change of the lignin structure and the connecting bond before and after modification can be judged more accurately and intuitively. As shown in FIG. 3, HSQC NMR analysis of EPL and EL4 determined the peak of the introduced functional oxirane moiety, i.e., δ_C/δ_H: 68.9/3.89, 69.1/4.26, 50.0/3.27, 43.2/2.72 and 43.2/2.56 ppm. Except for epoxidation modification, no other obvious structural change exists, which shows that the chemical modification of lignin introduces an ethylene oxide structure into lignin macromolecules and simultaneously maintains the integrity of a structural framework. After the successful introduction of the ethylene oxide structure into the lignin framework is determined, quantitative analysis of ethylene oxide is required, and detailed results are shown in table 3.

TABLE 3 ethylene oxide content in each lignin sample after modification

By pairs¹Integration of the peak areas in HNMR allows to accurately calculate the ethylene oxide content grafted onto the lignin backbone. According to HSQCAnalysis determined the overlap signal of the introduced epoxy groups, i.e., the C/H correlation peak was 43.2/2.72ppm, which was used for¹H NMR quantitative analysis, wherein p-nitrobenzaldehyde is taken as an internal standard. As shown in Table 3, EL4 contained the highest ethylene oxide content, 2.33mmol g^-1. The ethylene oxide content trend is similar to the concentration of functional groups (mainly phenolic hydroxyl groups and carboxylic acids), and the higher the content of phenolic hydroxyl groups and carboxylic acids, the more grafted ethylene oxide structures in the molecule.

See FIG. 4, before and after lignin modification³¹P NMR comparison spectra, EPL and EL4 to further evaluate modification results³¹P NMR analysis to verify the above conclusions (figure 4). As a result, it was found that the reaction of phenolic hydroxyl group with-COOH was completed under alkaline conditions, while the aliphatic hydroxyl group signal was enhanced between 146 and 147 ppm. Studies have shown that this signal may be derived from the NMR-phosphite of a phenol-linked epoxide or that the introduction of a hydroxyl group after epichlorohydrin addition results in the formation of an incomplete oxirane structure.

The invention exerts the advantages of the lignin component after the fractionation and purification, improves the activity of the modified lignin, and prepares the epoxy thermosetting resin material by taking the modified lignin as a suitable substitute of a petroleum-based chemical bisphenol A diglycidyl ether (BADGE). Mixing modified lignin with BADGE at different ratio, and processing with flexible polyether diamine (M)_w400gmol^-1) And the epoxy thermosetting resin is successfully obtained after crosslinking and curing.

Referring to fig. 5, the fracture surfaces of BADGE and lignin-based cured epoxy were analyzed by SEM, and at ultra-low temperatures, samples with high EL content could be automatically fractured, while materials with lower modified lignin content (< 5% EL) were destroyed by external force. The fracture surface of BADGE was smooth indicating brittle fracture. Bright textures were produced on the surface of BADGE and thermoset TPL — 5%, showing significant ductile fracture. TL4_ 15% and TL4_ 20% appeared grainy and wrinkled at the fracture surface, indicating that the systems were not completely miscible. The mechanical properties of the material are directly influenced by different characteristics of fracture surfaces, and the fracture surfaces of TPL-5% (thermosetting resin material containing 5% EPL which is a product obtained by modifying and curing industrial prehydrolyzed lignin) and TL 4-5% (thermosetting resin material containing 5% EL 4) are uniform and compact through SEM analysis, which proves that EPL, EL4 and BADGE are well miscible and do not undergo phase separation.

According to the calculation results of Table 4, T of the lignin-based cured material_sT lower than BADGE cured material_s. The more rapid thermal degradation of lignin-based systems is caused by the cleavage of ether linkages in the polymer backbone (TL 4-5%: 10.1%>BADGE: 5.0%). Char of lignin-based resin material under nitrogen atmosphere₆₀₀The values are higher than the cured DGEBA resin (table 4). TL 4-5%, indicating that the thermoset resin material has good thermal properties. The residual amounts of TL 4-10% and TL 4-15% were increased by 5.1%, 0.5%, 1.5%, respectively, over the BADGE material.

TABLE 4 thermal Property data of the cured epoxy obtained according to TGA analysis

As shown in Table 4, the Young's modulus of the material is related to the molecular weight of lignin, and the modified lignin component M is different_wAnd the Young's modulus are in positive correlation. As the EL content increases, the stiffness and elongation at break of the material decrease slightly. The increased concentration of EL decreased the miscibility of the system, resulting in poorer mechanical properties of TL4 — 15%, which is consistent with the SEM analysis described above. TGA and DSC analysis showed that the lignin-based thermoset resin material had excellent thermal properties, with TL4 — 5% being the most thermally stable.

Table 5 the different thermosetting epoxy resins of example 1 were analysed by DMA tests and tensile tests for their main mechanical properties

Table 6 the different thermosetting epoxy resins of example 2 were analysed by DMA tests and tensile tests for their main mechanical properties

Table 7 the different thermosetting epoxy resins of example 3 were analysed by DMA tests and tensile tests for their main mechanical properties

In view of the results of combining tensile strength and elongation at break of tables 5, 6 and 7, tensile test results found that TL4 — 5% has better mechanical properties than pure BADGE thermoset materials, with tensile stress and elongation at break increased by about 29.7 and 26.8%, respectively. The tensile stress and elongation at break of TL 4-10% are also improved compared to pure BADGE thermoset resin material by about 6.3 and 13.8%, respectively. The results show that 5-10% of the thermosetting resin material added with the modified lignin has mechanical properties superior to those of the commercial DADGE epoxy resin, and the tensile strength and flexibility of the material are obviously improved. Can be widely applied to the field of high-performance composite materials such as electronic element sealing. DMA analysis showed that E 'in the glassy region of TL 4-5% (4237.1MPa) was significantly higher than the E' value of BADGE (3427.4MPa), a result consistent with tensile testing analysis. A series of characterization results prove that the thermal property and the mechanical property of the lignin-based thermosetting material are superior to those of a pure BADGE material, and the modified lignin has great potential for replacing the BADGE.

The meaning of each English in the invention is as follows:

tg: glass transition temperature

T_5％: temperature at 5% weight loss

T_s％: statistical temperature of heat resistance index

T_max: temperature of maximum decomposition rate

Char₆₀₀: percentage of residue at 600 ℃

E': storage modulus

σ_break: tensile stress

ε_break: elongation at break

E_young: young's modulus

Weight%: weight percent (TGA, thermogravimetric analysis)

DTG: WeChat thermogram (first derivative of TGA)

Temperature: temperature of

Absorbance: absorbance of the solution

Wavenumber: wavelength of light

DMSO, DMSO: deuterium-containing dimethyl sulfoxide

Methoxyl: methoxy radical

Aliphatic OH: aliphatic hydroxy group

Phenolic OH: phenolic hydroxyl group

COOH: carboxyl group

IS: an internal standard.

The invention has the following advantages:

(1) the method takes the prehydrolysis lignin with relatively high activity in the industrial lignin as a raw material, and overcomes the defects of high molecular weight and poor uniformity of the industrial prehydrolysis lignin by a green and efficient acid gradient precipitation strategy. The molecular weights of the three fractions after purification, analyzed by GPC, were 2280g mol each^-1，1720g mol^-1And 940g mol^-1Are all lower than 2640g mol of the lignin of the raw material^-1。

(2) The grafted and modified lignin is used as a raw material to replace a petroleum-based chemical BADGE with the highest substitution rate of 20 percent, and the epoxy thermosetting material with higher thermal stability and flexibility than pure BADGE is successfully prepared. The research expands the application of lignin in the field of fine polymer production and has higher theoretical application value.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims (10)

1. A method for fractionation of lignin, comprising the steps of:

adding industrial lignin into an alkaline aqueous solution, filtering, and removing insoluble substances to obtain a lignin dispersion liquid; adding an acidic solution into the lignin dispersion liquid, adjusting the pH value of the solution to be acidic, and freeze-drying to obtain a precipitate which is purified lignin.

2. The lignin fractionation method according to claim 1, wherein the alkaline aqueous solution is sodium hydroxide aqueous solution, potassium hydroxide aqueous solution or sodium carbonate solution, and the mass volume ratio of the lignin to the alkaline aqueous solution is 10: 150-250; the concentration of the alkaline aqueous solution is 0.5-0.7M.

3. The method of lignin fractionation according to claim 1 or 2, wherein the acidic aqueous solution is hydrochloric acid, sulfuric acid or glacial acetic acid, and the concentration of the acidic aqueous solution is 5.0-7.0M.

4. A method of lignin fractionation according to claim 3, characterized in that said pH is 6, 4 or 2.

5. The method for fractionation of lignin according to claim 4, characterized in that the industrial lignin is industrial prehydrolyzed lignin obtained from prehydrolyzed liquid in the pulping process.

6. Use of the purified lignin obtained by the lignin fractionation method according to any one of claims 1 to 5 for the preparation of thermosetting resins.

7. A preparation method of lignin-based thermosetting resin is characterized by comprising the following steps:

modification of purified lignin: adding purified lignin, acetone, NaOH and epichlorohydrin obtained by the lignin fractionation method according to any one of claims 1 to 5 into a container, magnetically stirring, adding water to terminate the reaction, adjusting the pH value of the solution to 3.5, centrifugally separating the obtained solid component, washing with deionized water, and freeze-drying to obtain light brown modified lignin;

and (3) curing: adding modified lignin, bisphenol A diglycidyl ether BADGE, a curing crosslinking agent and a diluent into a container, stirring to obtain a uniform viscous liquid mixture, pouring the mixture into a mold, putting the mold into an oven to remove the solvent, and curing to obtain the lignin-based thermosetting resin.

8. The method as claimed in claim 7, wherein the mass-to-volume ratio of the purified lignin to the acetone solution is 2: 200-400; the concentration of the acetone solution is 45-55% v/v; the amount of NaOH is 2-4 times equivalent of purified lignin; the using amount of the epichlorohydrin is 15-25 times of the active OH value of the purified lignin; the volume mass ratio of the water added for terminating the reaction to the purified lignin is 150-250: 2; the pH of the solution was adjusted using 0.1-0.2M HCl.

9. The method as claimed in claim 8, wherein the temperature of the magnetic stirring is 50-60 ℃, the stirring time is 4-6 hours, and the stirring speed is 200-500 rpm/min.

10. The preparation method according to claim 9, wherein the mass ratio of the modified lignin to the BADGE is 2-20%, the amount of the added curing crosslinking agent is 0.2-1 ethylene oxide equivalent, the mass ratio of the used diluent to the curing crosslinking agent is 0.5-1.5,

the curing crosslinking agent is selected from polyether amine D230, D400 or D2000; the diluent is selected from one or more of acetonitrile, acetone, ethyl acetate and absolute ethyl alcohol, and the mold is a polytetrafluoroethylene mold; the temperature when the solvent is removed in the oven is 35-45 ℃ and the time is 1.5-2.5 hours; the curing temperature is 55-65 ℃, and the curing time is 3.5-4.5 hours.

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