Use of oligomeric lignin in the preparation of thermosetting phenol-formaldehyde resins

Abstract

The application of oligomeric lignin in the preparation of thermosetting phenolic resin. The preparation steps are as follows: ultrasonic depolymerization of oligomeric lignin replaces 55% of phenol, and thermosetting phenolic resin is prepared from formaldehyde and urea under alkaline conditions. The depolymerization reaction of the application depolymerizes lignin at room temperature, and the release amount of formaldehyde in the preparation of phenolic resin is reduced from 0.147 mg / L to 0.07 mg / L after replacing 55% of phenol with oligomeric lignin with relatively narrow molecular weight distribution (<2), the gel temperature is also greatly reduced to 70 DEG C, and the wet shear strength is increased from 1.17 MPa to 1.31 MPa. The advantages of the application lie in that the oligomeric lignin is obtained by the method with mild reaction conditions, low price of required reagents, low requirement for equipment and good repeatability, and the environmental protection performance, curing performance and gluing performance of the phenolic resin adhesive prepared from the oligomeric lignin are all stronger than those of the commercial formula phenolic resin adhesive.

Description

Technical Field

[0001] This invention relates to the fields of agricultural and forestry waste utilization and phenolic resin preparation, specifically to the application of oligomeric lignin in the preparation of thermosetting phenolic resin. Background Technology

[0002] Phenolic resin (PF) was the first synthetic polymer with excellent water and weather resistance, making it an ideal adhesive for outdoor engineered wood products. However, phenolic resin production relies on phenol and formaldehyde from the petrochemical industry, both of which are highly toxic and non-renewable. Furthermore, thermosetting phenolic resins present several challenges as adhesives, such as excessively high curing temperatures (higher curing temperatures lead to higher energy consumption in plywood production, increasing production costs) and slow curing speeds, limiting their application. Moreover, indoor use releases large amounts of volatile organic compounds (VOCs), negatively impacting indoor air quality and human health. Therefore, utilizing low-toxicity or non-toxic biomass raw materials to replace phenol and formaldehyde is a key strategy for developing more environmentally friendly and efficient adhesives.

[0003] Lignin is one of the most abundant natural aromatic polymers and a non-petroleum source of natural renewable aromatic compounds, making it an excellent substitute for phenol. This is because its phenylpropane unit and abundant phenolic hydroxyl groups give it a structure similar to phenol. However, while directly replacing phenol with lignin in the preparation of phenolic resins can reduce the use of harmful substances like phenol and improve waste utilization, the presence of numerous methoxy and ether bonds in the lignin structure increases steric hindrance and reduces its reactivity. This results in a higher initial curing temperature for lignin-based phenolic resins, leading to persistently high formaldehyde release when used in wood adhesives, and poor bonding performance that fails to meet national standards. Summary of the Invention

[0004] Technical problem solved: This invention provides an application of oligomeric lignin in the preparation of thermosetting phenolic resins. By directly obtaining oligomeric lignin with a narrow molecular weight distribution (<2) without high temperature and expensive reagents, it can replace most phenols to prepare lignin-based phenolic resins with low pollution, low temperature curing and strong bonding strength.

[0005] Technical solution: Application of oligomeric lignin in the preparation of thermosetting phenolic resins under conditions of partial phenol substitution.

[0006] The application method is as follows: replace 55% of phenol with oligomeric lignin obtained by ultrasonic depolymerization, and prepare thermosetting phenolic resin with formaldehyde and urea under alkaline conditions.

[0007] The specific steps are as follows: Phenol, water, and sodium hydroxide solution are added to the reactor at a mass ratio of 1:1.25:1.25. Lignin / oligomeric lignin and the first batch of formaldehyde solution are added according to a substitution rate of 55% and a phenol-formaldehyde ratio of 0.4:1. The temperature is raised to start the reaction. After the reaction is completed, the temperature is lowered and the second batch of formaldehyde solution and sodium hydroxide solution are added. The temperature is raised to react. The third batch of formaldehyde solution is added and the temperature is raised to react. Urea is then added, the temperature is lowered, and the product is discharged to obtain lignin-modified phenolic resin (LPF) / oligomeric lignin-modified phenolic resin (DLPF).

[0008] Preferably, the mass ratio of the two additions of the above-mentioned sodium hydroxide solution is 3:2.

[0009] Preferably, the mass ratio of the three additions of the above formaldehyde solution is 2:2:1.

[0010] Preferably, the mass fraction of the sodium hydroxide solution is 10%-30%.

[0011] Preferably, the lignin is enzymatically hydrolyzed lignin, sulfate lignin, alkali lignin, or biomass butanol lignin.

[0012] The above-mentioned phenol, oligomeric lignin and formaldehyde are mixed and subjected to aldehyde condensation under the action of sodium hydroxide catalyst. The reaction time for each phenol-aldehyde condensation is 60-100 min, the reaction temperature is 90-95℃, and the amount of catalyst used is 0.75 times the mass of phenol.

[0013] Beneficial Effects 1. Currently, there are no reports of using oligomeric lignin obtained under room temperature ultrasonic conditions to prepare thermosetting phenolic resins. Oligomeric lignin has a higher hydroxyl content and more reaction sites than raw material lignin (see details). Figure 1 1. The lignin oligomers exhibit a narrower molecular weight distribution and better uniformity, which helps improve the performance of lignin-based phenolic resins. 2. The entire lignin depolymerization process is characterized by mild conditions, simple operation, and readily available and inexpensive reagents, resulting in significant depolymerization effects. This provides a positive reference for the industrial production of lignin-based phenolic resins. 3. Oligomeric lignin has a high substitution rate in the synthesis of lignin-based phenolic resins, which can significantly reduce the production cost of lignin-based phenolic resins. 4. The formaldehyde release in oligomeric lignin-based phenolic resins synthesized from oligomeric lignin is extremely low, which can effectively improve the low-pollution performance of lignin-based phenolic resin adhesives. 5. The oligomeric lignin-based phenolic resins synthesized by the method of this invention have excellent curing performance, curing at 70℃, which is far lower than the gel temperature of other lignin-based phenolic resins and industrial phenolic resins. 6. The oligomeric lignin-based phenolic resins synthesized by the method of this invention have good properties such as bonding strength. Attached Figure Description

[0014] Figure 1 This is the quantitative 1H NMR spectrum of lignin oligomers.

[0015] After acetylation of oligomeric lignin, and using p-nitrobenzaldehyde as an internal standard, quantitative nuclear magnetic resonance (NMR) spectroscopy was performed. Calculations based on peak area showed that the hydroxyl content of the oligomeric lignin was 36.9% higher than that of the undepolymerized lignin raw material. This is the main reason why adding oligomeric lignin to phenolic resins can significantly improve their performance.

[0016] Figure 2 This is a temperature-dependent rheological test diagram of lignin-modified phenolic resin.

[0017] As shown in the figure, the addition of lignin or oligolignin to phenolic resin can reduce the curing temperature of phenolic resin. Specifically, after adding oligolignin, the curing temperature of phenolic resin decreased from 200℃ to 70℃, a decrease of 65%.

[0018] Figure 3 This is a graph showing the relationship between the degree of curing and the curing rate of lignin-modified phenolic resin and temperature.

[0019] The curing rate and degree of curing of lignin-modified phenolic resin can be calculated by performing first and second derivatives based on the storage modulus in the temperature-varying rheological test. The figure clearly shows that the addition of oligomeric lignin increases the curing rate of the phenolic resin by an order of magnitude, significantly improving both the curing rate and the degree of curing. Figure 2 Consistent.

[0020] Figure 4 This is a comparison chart of the bonding performance and formaldehyde release of lignin-modified phenolic resin.

[0021] Figure 5 A radar comparison chart of six key indicators for lignin-modified phenolic resin. Detailed Implementation

[0022] The preparation method of oligomeric lignin is described in reference to CN114957704A. The present invention will now be further described with reference to the following examples:

[0023] Example 1

[0024] Lignin-based phenolic resin was prepared by replacing phenol with lignin as the raw material. The lignin substitution rate (based on the total mass of lignin and phenol) was fixed at 55%, and the molar ratio of formaldehyde to phenol was 2.5. 2.0 kg of phenol, 2.5 kg of water, and 1.5 kg of sodium hydroxide solution (30% by mass, 60% of the total) were added to a 20 L reactor. Then, 2.5 kg of raw lignin and the first batch of formaldehyde solution (40% of the total) were added, and the mixture was heated to 90 °C to start the reaction for 1 hour. The second batch of formaldehyde solution (40% of the total) and 1.0 kg of sodium hydroxide solution (40% of the total) were added, and the reaction was maintained at 90 °C for 60 minutes. The third batch of formaldehyde solution (20% of the total) was added, and the mixture was cooled to 85 °C and reacted for 30 minutes. Finally, urea was added, and the mixture was cooled to room temperature before being discharged to obtain lignin-modified phenolic resin (LPF).

[0025] Analysis showed that the lignin-modified phenolic resin adhesive had a wet strength of 0.9 MPa, a formaldehyde release of 1.29 mg / L, and a curing temperature of 180℃.

[0026] Example 2

[0027] Lignin-based phenolic resin was prepared by replacing phenol with oligomeric lignin, with the oligomeric lignin substitution rate (based on the total mass of oligomeric lignin and phenol) fixed at 55%, and the molar ratio of formaldehyde to phenol at 2.5. 2.0 kg of phenol, 2.5 kg of water, and 1.5 kg of sodium hydroxide solution (30% by mass, 60% of the total) were added to a 20 L reactor. Then, 2.5 kg of oligomeric lignin and the first batch of formaldehyde solution (40% of the total) were added, and the mixture was heated to 90 °C to start the reaction for 1 hour. The second batch of formaldehyde solution (40% of the total) and 1.0 kg of sodium hydroxide solution (40% of the total) were added, and the reaction was maintained at 90 °C for 60 minutes. The third batch of formaldehyde solution (20% of the total) was added, and the reaction was cooled to 85 °C for 30 minutes. Finally, urea was added, and the mixture was cooled to room temperature before being discharged to obtain oligomeric lignin-modified phenolic resin (DLPF).

[0028] Analysis showed that the wet strength of the oligomeric lignin-modified phenolic resin adhesive was 1.31 MPa, its formaldehyde release was 0.07 mg / L, and its gelation temperature was 70℃.

[0029] Example 3

[0030] Lignin-based phenolic resin was prepared by replacing phenol with lignin as the raw material. The lignin substitution rate (based on the total mass of lignin and phenol) was fixed at 33%, and the molar ratio of formaldehyde to phenol was 2.5. 2.0 kg of phenol, 2.5 kg of water, and 1.5 kg of sodium hydroxide solution (mass fraction 30%, 60% of the total) were added to a 20 L reactor. Then, 2.5 kg of raw lignin and the first batch of formaldehyde solution (40% of the total) were added, and the mixture was heated to 90 °C to start the reaction for 1 hour. The second batch of formaldehyde solution (40% of the total) and 1.0 kg of sodium hydroxide solution (40% of the total) were added, and the reaction was maintained at 90 °C for 60 minutes. The third batch of formaldehyde solution (20% of the total) was added, and the mixture was cooled to 85 °C and reacted for 30 minutes. Finally, urea was added, and the mixture was cooled to room temperature before being discharged to obtain lignin-modified phenolic resin (LPF).

[0031] Analysis showed that the wet strength of the lignin-modified phenolic resin adhesive was 1.22 MPa, and its formaldehyde release was 0.25 mg / L.

[0032] Example 4

[0033] Lignin-based phenolic resin was prepared by replacing phenol with lignin as the raw material. The lignin substitution rate (based on the total mass of lignin and phenol) was fixed at 43%, and the molar ratio of formaldehyde to phenol was 2.5. 2.0 kg of phenol, 2.5 kg of water, and 1.5 kg of sodium hydroxide solution (mass fraction 30%, 60% of the total) were added to a 20 L reactor. Then, 2.5 kg of raw lignin and the first batch of formaldehyde solution (40% of the total) were added, and the mixture was heated to 90 °C to start the reaction for 1 hour. Next, the second batch of formaldehyde solution (40% of the total) and 1.0 kg of sodium hydroxide solution (40% of the total) were added, and the reaction was maintained at 90 °C for 60 minutes. Finally, the third batch of formaldehyde solution (20% of the total) was added, and the mixture was cooled to 85 °C and reacted for 30 minutes. Urea was then added, and the mixture was cooled to room temperature before being discharged to obtain lignin-modified phenolic resin (LPF).

[0034] Analysis showed that the wet strength of the lignin-modified phenolic resin adhesive was 0.79 MPa, and its formaldehyde release was 0.08 mg / L.

[0035] Comparative Example 1

[0036] Lignin-based phenolic resin was prepared by replacing phenol with lignin and the lignin / oligomeric lignin obtained in Example 2. The lignin / oligomeric lignin substitution rate (based on the total mass of lignin / oligomeric lignin and phenol) was fixed at 55%, and the molar ratio of formaldehyde to phenol was 2.5. 2.0 kg of phenol, 2.5 kg of water, and 1.5 kg of sodium hydroxide solution (30% by mass, 60% of the total) were added to a 20 L reactor. Then, 2.5 kg of raw lignin / oligomeric lignin and the first batch of formaldehyde solution (40% of the total) were added, and the mixture was heated to 90 °C to begin the reaction for 1 hour. Next, the second batch of formaldehyde solution (40% of the total) and 1.0 kg of sodium hydroxide solution (40% of the total) were added, and the reaction was maintained at 90 °C for 60 minutes. Finally, the third batch of formaldehyde solution (20% of the total) was added, and the mixture was cooled to 85 °C to begin the reaction for 30 minutes. Urea was then added, and the mixture was cooled to room temperature before being discharged to obtain lignin-modified phenolic resin (LPF) / oligomeric lignin-modified phenolic resin (DLPF). For detailed performance comparisons, please refer to [link to relevant documentation]. Figure 4

[0037] Comparative Example 2

[0038] The oligomeric lignin obtained in Example 2, used to prepare lignin-based phenolic resin (LPF), was compared with a laboratory-made phenolic resin (PF). The molar ratio of formaldehyde to phenol was 2.5. 4.4 kg of phenol, 2.5 kg of water, and 1.5 kg of sodium hydroxide solution (30% by mass, 60% of the total) were added to a 20 L reactor. The first batch of formaldehyde solution (40% of the total) was added, and the mixture was heated to 90°C and reacted for 1 hour. The second batch of formaldehyde solution (40% of the total) and 1.0 kg of sodium hydroxide solution (40% of the total) were added, and the reaction was maintained at 90°C for 60 minutes. The third batch of formaldehyde solution (20% of the total) was added, and the mixture was cooled to 85°C and reacted for 30 minutes. Urea was then added, and the mixture was cooled to room temperature before being discharged to obtain the laboratory-made phenolic resin (PF). For detailed performance comparisons, please refer to [link to relevant documentation]. Figure 4 .

[0039] Comparative Example 3

[0040] The lignin-based phenolic resin (LPF) prepared by replacing phenol with oligomeric lignin obtained in Example 2 was compared with the laboratory-made urea-formaldehyde resin (UF). For detailed performance comparison, please refer to [link to relevant documentation]. Figure 4

[0041] Comparative Example 4

[0042] The wood oligolignin-based phenolic resin (DLPF) prepared by replacing phenol with oligolignin obtained in Example 2 was compared with the laboratory-made urea-formaldehyde resin (PF). For detailed performance comparison, please refer to [link to relevant documentation]. Figure 4 .

[0043] In summary, this invention improves the reactivity of lignin by ultrasonically assisted depolymerization at room temperature, and then applies this process to the field of phenolic resins to prepare green and efficient lignin-based phenolic resins. Compared with other lignin-based phenolic resins, the lignin depolymerization conditions of this invention are mild, the equipment is simple, and it is environmentally friendly. Furthermore, the oligomeric lignin-based phenolic resin prepared from this invention not only outperforms undepolymerized lignin-based phenolic resins but also surpasses the performance of commercially available phenolic resins. Specifically, compared to commercially available phenolic resins, the formaldehyde release of the oligomeric lignin-based phenolic resin is reduced by approximately 50%, the gel temperature is significantly reduced from 200℃ to 70℃, and the wet shear strength is increased from 1.1 MPa to 1.31 MPa, resulting in an environmentally friendly, low-pollution, and high-performance phenolic resin adhesive.

Claims

1. Use of oligomeric lignin for the preparation of a thermosetting phenol-formaldehyde resin in partial substitution of phenol, characterized in that, A thermosetting phenolic resin was prepared by replacing 55% of phenol with oligomeric lignin obtained by ultrasonic depolymerization and then reacting it with formaldehyde and urea under alkaline conditions.

2. The application according to claim 1, characterized in that, Phenol, water, and sodium hydroxide solution were added to a reaction vessel at a mass ratio of 1:1.25:1.

25. Oligomeric lignin and the first batch of formaldehyde solution were added with a substitution rate of 55% and a phenol-formaldehyde ratio of 0.4:

1. The reaction was started by heating. After the reaction was completed, the temperature was lowered and the second batch of formaldehyde solution and sodium hydroxide solution were added, and the temperature was raised to react. The third batch of formaldehyde solution was added and the temperature was raised to react. Finally, urea was added, the temperature was lowered, and the product was discharged to obtain oligomeric lignin-modified phenolic resin.

3. The application according to claim 2, characterized in that, The mass ratio of the two additions of the sodium hydroxide solution is 3:

2.

4. The application according to claim 2, characterized in that, The formaldehyde solution was added three times in a mass ratio of 2:2:

1.

5. The application according to claim 2, characterized in that, The sodium hydroxide solution has a mass fraction of 10%-30%.

6. The application according to claim 2, characterized in that, The phenol, oligomeric lignin, and formaldehyde are mixed and undergo phenolic condensation under the action of sodium hydroxide catalyst. The reaction time for each phenolic condensation is 60-100 min, the reaction temperature is 90-95 ℃, and the amount of catalyst used is 0.75 times the mass of phenol.

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