CN113717400B - Lignin fractionation method and preparation method of lignin-based thermosetting resin - Google Patents
Lignin fractionation method and preparation method of lignin-based thermosetting resin Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08H—DERIVATIVES OF NATURAL MACROMOLECULAR COMPOUNDS
- C08H6/00—Macromolecular compounds derived from lignin, e.g. tannins, humic acids
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
- C08G59/00—Polycondensates containing more than one epoxy group per molecule; Macromolecules obtained by polymerising compounds containing more than one epoxy group per molecule using curing agents or catalysts which react with the epoxy groups
- C08G59/02—Polycondensates containing more than one epoxy group per molecule
- C08G59/04—Polycondensates containing more than one epoxy group per molecule of polyhydroxy compounds with epihalohydrins or precursors thereof
- C08G59/06—Polycondensates containing more than one epoxy group per molecule of polyhydroxy compounds with epihalohydrins or precursors thereof of polyhydric phenols
- C08G59/063—Polycondensates containing more than one epoxy group per molecule of polyhydroxy compounds with epihalohydrins or precursors thereof of polyhydric phenols with epihalohydrins
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L63/00—Compositions of epoxy resins; Compositions of derivatives of epoxy resins
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- Chemical Kinetics & Catalysis (AREA)
- Medicinal Chemistry (AREA)
- Polymers & Plastics (AREA)
- Organic Chemistry (AREA)
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Biochemistry (AREA)
- Materials Engineering (AREA)
- Epoxy Resins (AREA)
- Compositions Of Macromolecular Compounds (AREA)
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
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 conventional paper mill or fuel ethanol plant production process, 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 purpose of the embodiments of the present invention is to provide a method for lignin fractionation with green and high efficiency.
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 a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution or a sodium carbonate solution, and the mass volume ratio of the lignin to the alkaline aqueous solution is 10-250; 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; 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; 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 200-500rpm/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 modification 31 P NMR comparison spectrogram;
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:
at room temperature, 10.0g of industrial lignin was added to 200ml of a 0.625M aqueous solution of sodium hydroxide, filtered to remove a small amount of insoluble matter to obtain a lignin dispersion, the pH of the lignin dispersion was adjusted to 6.0 using 6.0M HCl, and the obtained precipitate was freeze-dried to obtain purified lignin, which was designated as L6.
The obtained purified lignin was named as L4 by adjusting the pH of the lignin dispersion to 4.0 by the same method as described above. The pH of the lignin dispersion was adjusted to 2.0, and the obtained purified lignin was designated 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 of the lignin component and 20 equivalents of epichlorohydrin of the active OH value of the lignin component were added to a round-bottomed flask, and magnetic stirring was carried out at 55 ℃ for 5 hours at a stirring speed of 300rpm/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 EL6.
Similarly, a modified lignin obtained by modifying purified lignin L4 is named as EL4; the modified lignin obtained by modifying purified lignin L2 is named as "He 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 respectively 5%, 10%, 15% and 20%, adding 1/5 time of curing crosslinking agent polyether amine D400 with ethylene oxide equivalent into each bottle, adding diluent acetonitrile with the mass of 0.5 time of polyether amine D400, and stirring to obtain uniform viscous liquid. Then introducing the viscous liquid into a polytetrafluoroethylene mold in 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%.
Similarly, the thermosetting resin material obtained by curing 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:
at room temperature, 10.0g of industrial lignin was added to 150ml of a 0.5M aqueous potassium hydroxide solution, filtered to remove a small amount of insoluble matter to obtain a lignin dispersion, the pH of the lignin dispersion was adjusted to 6.0 using 5.0M sulfuric acid, and the resulting precipitate was freeze-dried 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 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 magnetic stirring was carried out at 50 ℃ for 6 hours at a stirring speed of 200rpm/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 EL6.
Similarly, a modified lignin obtained by modifying purified lignin L4 is named as EL4; the modified lignin obtained by modifying the purified lignin L2 is named as 'He' and EL2.
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 1/2 time of curing crosslinking agent polyether amine D230 of ethylene oxide equivalent into each bottle, adding diluent acetone with the mass of 1 time of polyether amine D230, 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 a 35 ℃ oven for 2.5 hours to remove the solvent, and then continuously curing the viscous liquid for 4.5 hours at 55 ℃; 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%.
Similarly, the thermosetting resin material obtained by curing 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:
at room temperature, 10.0g of industrial lignin was added to 250ml of a 0.7M sodium carbonate solution, filtered to remove a small amount of insoluble matter to obtain a lignin dispersion, the pH of the lignin dispersion was adjusted to 6.0 using 7.0M glacial acetic acid, and the resulting precipitate was freeze-dried 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 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 of the lignin component and 25 equivalents of epichlorohydrin of 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 500rpm/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 EL6.
Similarly, a modified lignin obtained by modifying purified lignin L4 is named as EL4; the modified lignin obtained by modifying purified lignin L2 is named as "He 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 1 time of curing crosslinking agent polyetheramine D2000 of ethylene oxide equivalent into each bottle, adding diluent absolute ethyl alcohol with the mass of 1.0 time of polyetheramine D2000, 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 a 45 ℃ oven for 1.5 hours to remove the solvent, and then continuously curing the viscous liquid for 3.5 hours at 65 ℃; 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%.
Similarly, the thermosetting resin material obtained by curing 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) -1 Strong band of (b) a small amount of condensed structures in PL lignin were observed. In addition, the PL phenolic hydroxyl group content 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 compared with other industrial lignin, and modification and 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 -1 PDI of 3.0) and the molecular weight and distribution range of the purified lignin component are obviously reduced, and the weight average molecular weight value is 940 to 2280g mol -1 In the meantime. L4 shows the highest M w Value (2280 g mol) -1 ) L2 shows the lowest M w Value (940 g 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 w The polydispersity of the lignin is also lowest (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 over a wide temperature range of 120-450 ℃, and the weight loss in this temperature range is due to the release of some gaseous products as a result of the cleavage of the lignin side chains [95] . Internal connecting bond of lignin macromoleculeGradually breaking occurs, and the residue reacts to form coke during further temperature rise until the quality of the coke is basically stable at 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 contained a large amount of condensed structural units [96] . The residue mass of the L2 component was also higher than PL, at 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 infrared spectra of different lignin components and their corresponding structural assignments. 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 show characteristic peaks of lignin, mainly 1800-600cm -1 In between. At 3384cm -1 Stretching vibration of O-H exists; 2936cm -1 Showing stretching vibration of methyl and methylene. Stretching vibrations of the aromatic structure were observed in all samples (1614, 1514 and 1428 cm) -1 ) The basic structure of lignin is not destroyed in the fractionation of lignin. At 1328, 1217cm -1 Characteristic peaks of syringyl and guaiacyl respectively are observed, and 1166cm is not observed -1 The 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 incorporated 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 43.2/2.56ppm. 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 modified lignin samples
By pairs 1 Integration of the peak areas in HNMR allows to accurately calculate the ethylene oxide content grafted onto the lignin backbone. According to HSQC analysis, an overlap signal of the introduced epoxy groups, i.e. a C/H correlation peak of 43.2/2.72ppm, was determined and used 1 H NMR quantitative analysis, wherein p-nitrobenzaldehyde is taken as an internal standard. As shown in Table 3, EL4 contained the highest amount of ethylene oxide, 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 modification 31 P NMR comparison spectra, EPL and EL4 to further evaluate the modification results 31 P NMR analysis to verify the above conclusions (figure 4). As a result, it was found that under alkaline conditionsThe phenolic hydroxyl group and-COOH reaction was complete, while between 146 and 147ppm enhanced the aliphatic hydroxyl signal. 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) w 400gmol -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. TL 4-15% and TL 4-20% graining and wrinkling at the fracture surface indicating that the systems are not fully miscible. The different characteristics of the fracture surface directly affected the mechanical properties of the material, and it was observed by SEM analysis that the fracture surfaces of TPL-5% (a thermosetting resin material containing 5% EPL, which is a product after modification and curing of the industrial prehydrolyzed lignin) and TL-5% (a thermosetting resin material containing 5% EL4) were uniform and dense, demonstrating that EPL, EL4 and BADGE are well miscible and phase separation did not occur.
According to the calculation results of Table 4, T of the lignin-based cured material s T 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 atmosphere 600 The values are higher than the cured DGEBA resin (table 4). TL 4-5%, indicating that the thermosetting resin material has good thermal properties. TL4_10%, and TL4_15% residualThe amount is respectively increased by 5.1%,0.5% and 1.5% compared with 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 w And 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 poor mechanical properties of TL4 — 15%, which is consistent with 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 highest thermal stability.
TABLE 5 analysis of the main mechanical properties of the different thermosetting epoxy resins of example 1 by DMA tests and tensile tests
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 TL 4-10% also has improved tensile stress and elongation at break 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% (4237.1 MPa) was significantly higher than the E' value of BADGE (3427.4 MPa), 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 heat resistance index temperature
T max : temperature of maximum decomposition rate
Char 600 : 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: 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 -1 And 940g mol -1 Are 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%, 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 (6)
1. A preparation method of lignin-based thermosetting resin is characterized by comprising 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 aqueous 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 pH value is 6, 4 or 2;
the industrial lignin is industrial prehydrolysis lignin obtained from prehydrolysis liquid in the pulping process;
modification of purified lignin: adding the obtained purified lignin, acetone, naOH and epichlorohydrin 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;
the using amount of the epichlorohydrin is 15-25 times of the active OH value of the purified 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 lignin-based thermosetting resin;
when the pH value is 6, the mass ratio of the modified lignin to the BADGE is 5%;
when the pH value is 4 or 2, the mass ratio of the modified lignin to the BADGE is 5-15%;
the curing crosslinking agent is added in an amount of 0.2-1 ethylene oxide equivalent, and is selected from polyetheramine D230, D400 or D2000.
2. The method for preparing the lignin-based thermosetting resin according to claim 1, wherein the alkaline aqueous solution is an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution or an aqueous sodium carbonate solution, and the mass-to-volume ratio of the lignin to the alkaline aqueous solution is 10 to 250 g/ml; the concentration of the alkaline aqueous solution is 0.5-0.7M.
3. The method for preparing a lignin-based thermosetting resin according to claim 1, wherein the aqueous acidic solution is hydrochloric acid, sulfuric acid or glacial acetic acid, and the concentration of the aqueous acidic solution is 5.0-7.0M.
4. The method according to any one of claims 1 to 3, wherein the mass to volume ratio of the purified lignin to the acetone solution is from 2 to 400 g/ml; the concentration of the acetone solution is 45-55% v/v; the amount of NaOH is 2-4 times equivalent of purified lignin; the volume mass ratio of the water added for terminating the reaction to the purified lignin is 150-250 ml/g; the pH of the solution was adjusted using 0.1-0.2M HCl.
5. The method according to claim 4, wherein the temperature of the magnetic stirring is 50 to 60 ℃, the stirring time is 4 to 6 hours, and the stirring speed is 200 to 500 rpm.
6. The method according to claim 5, wherein the mass ratio of the diluent to the curing crosslinking agent is 0.5 to 1.5;
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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| "Synthesis and Characterization of Epoxy Thermosetting Polymers from Glycidylated Organosolv Lignin and Bisphenol A";Lena Charlotte Over等;《Macromolecular Chemistry and Physics》;20171231;第218卷(第4期);第1-11页 * |
| "工业木质素分级分离的基础研究";刘曌;《中国优秀硕士学位论文全文数据库 工程科技Ⅰ辑》;20120315(第3期);B016-228 * |
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