An organic acid ester modified lignin phenol-formaldehyde resin adhesive and a preparation method thereof

Abstract

This invention belongs to the field of wood adhesive technology, specifically relating to an organic ester modified lignin-phenolic resin adhesive and its preparation method. This adhesive uses lignin as the core raw material and innovatively constructs a lignin-organic ester-phenol ternary composite precursor through organic ester bridging activation technology. This method increases the phenolic substitution degree of lignin by 50%, and the resulting adhesive has significant technical advantages: (1) the gel time is shortened to half that of conventional products, significantly improving curing efficiency; (2) the average bonding strength is ≥1.35 MPa, meeting the GB / T 9846 Class I plywood standard; (3) the hot-pressing cycle of the board is shortened by 50%, effectively reducing production energy consumption; (4) formaldehyde release is ≤0.018 mg / m³. 3 To reach E NF The advanced environmental protection standards have established a new approach to the high-value utilization of lignin, resulting in significant environmental and economic benefits in the field of wood-based panel manufacturing.

Description

Technical Field

[0001] This invention relates to an organic ester modified lignin phenolic resin adhesive and its preparation method, which is used in the preparation of wood and bamboo plywood and belongs to the field of wood adhesive technology. Background Technology

[0002] Phenolic resin, as the earliest industrialized and largest-volume thermosetting material among the three major thermosetting resin systems (epoxy, polyurethane, and phenolic), is primarily produced through the phenol-formaldehyde condensation reaction and holds a dominant position in the wood processing industry. However, the non-renewable nature of its main raw material, phenol, and the release of free components during its production process not only limit its sustainable development but also negatively impact the environment. Therefore, developing renewable green raw materials aligns with the sustainable development requirements of today's society, economy, and environmental protection.

[0003] Lignin, as the most abundant natural phenolic polymer in nature, has long been considered a potential raw material to replace phenol in the synthesis of phenolic resin adhesives. Although lignin contains a large number of phenolic ring structures, its physicochemical properties differ significantly from those of phenol. In particular, during reactions, the phenolic hydroxyl groups in lignin mostly exist in the form of phenolic ethers, which cannot form phenol oxide anions under alkaline conditions, thus affecting the hydroxymethylation reaction of ortho- and meta-positions of phenolic hydroxyl groups with formaldehyde. Furthermore, the number of ortho- and para-position sites for phenolic hydroxyl groups in lignin units is limited. During biomass pretreatment or pulping, the C-C coupling reaction between units caused by lignin condensation further consumes the vacant ortho- and meta-position sites. Additionally, the steric hindrance effect brought about by the cross-linking network restricts the accessibility of lignin to formaldehyde or phenol. Therefore, the reactivity of lignin with formaldehyde is much lower than that of phenol with formaldehyde.

[0004] In recent years, many studies have explored methods to improve lignin activity. Currently, chemical activation of lignin is the most researched method, mainly including sulfonation, hydroxymethylation, phenolation, graft copolymerization, and demethoxylation. Sulfonation modification introduces sulfonic acid groups onto the lignin side chain, improving the water solubility of lignin, but it does not increase the active sites of lignin itself. Hydroxymethylation modification reacts the ortho- and posterior positions of phenolic hydroxyl groups with formaldehyde to introduce hydroxymethyl groups, which improves reactivity but occupies the active sites of lignin. Phenolic modification grafts phenol onto the lignin side chain, increasing the active sites of lignin, but only one phenol can be introduced onto a single lignin monomer. Demethoxylation modification removes the active methoxy groups at the C3 and C5 positions on the lignin phenolic ring, but this process is carried out under high temperature and pressure, requiring sophisticated equipment, and only a maximum of two active sites can be added to a single phenolic ring. Graft copolymerization modification is often used to improve the compatibility and dispersibility of lignin, giving it multifunctionality, but it cannot increase the active sites of lignin itself.

[0005] While the above-mentioned lignin activation methods improve reactivity to some extent, their effect on increasing active sites is limited. This results in a 20%-30% longer curing time for lignin-based phenolic resin adhesives compared to traditional PF adhesives. Furthermore, it is difficult to increase the lignin substitution rate; when the lignin substitution rate for phenol exceeds 30%, the bond strength decreases significantly, failing to meet the performance standards for Class I plywood. Therefore, there is an urgent need to develop more efficient lignin activation methods to further enhance reactivity, shorten adhesive curing time, and ensure the stability of bond strength at high substitution rates, thereby promoting the widespread application of lignin-based PF adhesives. Summary of the Invention

[0006] Organic acid esters such as methyl formate, ethyl acetate, ethyl butyrate, phenyl acetate, glyceryl triacetate, and propylene carbonate undergo rearrangement under high alkalinity conditions and can react with the ortho-carbon atom on lignin or phenol in a Kolbe-Schmidt-like reaction. Figure 1 This process generates salicylic acid. Since the C3 and C5 reactivity of the phenolic ring in lignin is similar to that of phenol, it can also react with organic acid esters, such as... Figure 2 As shown in the diagram, this invention first reacts lignin and organic acid esters under high alkalinity conditions, replacing the vacant ortho positions on the lignin phenolic ring with organic acid esters to obtain a lignin-organic acid ester binary precursor. The obtained product is then reacted with phenol, grafting one or more phenols onto the lignin molecule via organic acid esters to obtain a lignin-organic acid ester-phenol ternary precursor. This precursor is then reacted with formaldehyde to obtain a modified lignin-phenolic resin adhesive. Since lignin itself has few reactive sites, while the introduced phenolic ring has 1-2 active sites, introducing one or more phenols onto the lignin molecule can significantly increase the number of active sites. Theoretically, phenol groups can be infinitely grafted onto the lignin via organic acid esters, thereby increasing the number of active sites and achieving high lignin activation.

[0007] The technical solution of this invention is to mix phenol, lignin, and organic acid esters in a certain proportion and react them fully under high alkalinity conditions to form phenol-organic acid ester-lignin or (phenol-organic acid ester-). n A ternary lignin precursor is then reacted with a certain proportion of phenol and formaldehyde to form an organic ester-modified lignin-phenolic resin. The advantage of this invention is that one or more phenols can be grafted onto lignin, greatly improving the reactivity of the lignin.

[0008] This invention is mainly achieved through the following technical solutions:

[0009] This invention provides a method for preparing an organic ester-modified lignin-phenolic resin adhesive, comprising the following steps:

[0010] Step S1: Inject lignin particles into the reactor, add caustic soda solution, water, and organic acid esters to react and obtain a lignin-organic acid ester binary precursor solution;

[0011] Step S2: Molten phenol is further injected into the lignin-organic ester binary precursor solution in the reactor to react and obtain a lignin-organic ester-phenol ternary precursor solution;

[0012] Step S3: Add formaldehyde solution dropwise into the lignin-organic acid ester-phenol ternary precursor solution in the reaction vessel and react. Then, inject the remaining caustic soda solution to obtain organic acid ester modified lignin phenolic resin adhesive.

[0013] Preferably, the reaction conditions in step S1 are: stirring at 40-60°C for 30-60 min.

[0014] Preferably, the reaction conditions in step S2 are: stirring at 40-60°C for 90-150 min.

[0015] Preferably, the reaction conditions in step S3 are as follows: first, heat to 60-90℃ and react for 30-60 minutes, then cool to 60-80℃ and continue reacting for 30-90 minutes, then cool to 10-30℃.

[0016] Preferably, the mass ratio of lignin, caustic soda, organic acid ester, and water in step S1 is 1:(0.32-0.61):(0.07-1.43):(1.86-7.86).

[0017] Preferably, the lignin in step S1 includes one or more of alkali lignin, lignin sulfonate, and enzymatically hydrolyzed lignin.

[0018] Preferably, the organic ester in step S1 includes one or more of methyl formate, ethyl acetate, ethyl butyrate, phenyl acetate, glyceryl triacetate, and propylene carbonate.

[0019] Preferably, the mass ratio of phenol, lignin, and organic ester in step S2 is 1:(0.43-1):(0.07-1.43).

[0020] Preferably, the mass ratio of phenol, lignin, formaldehyde, caustic soda, and water in step S3 is 1:(0.43-1):(0.64-1.91):(0.17-1.17):(2.82-8.82).

[0021] The present invention also provides an organic ester modified lignin phenolic resin adhesive, which is prepared by the above preparation method.

[0022] The beneficial effects achieved by this invention are as follows:

[0023] This invention uses organic esters (such as propylene carbonate) to graft phenol onto lignin molecules (such as... Figure 2 As shown), it significantly increased the active sites of lignin. Infrared spectroscopy analysis (as shown) Figure 3 It can be observed that, compared with ordinary lignin PF glue (Comparative Example 1), the absorption peak of tert-butanol at 1383 cm⁻¹ in the present invention (Example 1) is significantly enhanced, which is attributed to the products generated by the reaction of propylene carbonate and phenol with lignin. Furthermore, the ¹³C NMR spectrum ( Figure 4 , Figure 5 This further confirms the successful grafting of phenol: compared to ordinary lignin PF (Comparative Example 1), the present invention (Example 1) detected a peak of tert-butanol CH2-OH (C7) at 61 ppm, and simultaneously detected phenolic ring carbon peaks linked to tert-butanol at 119 ppm (C6, C8, C14). These data fully demonstrate that phenol was effectively grafted onto the lignin molecule via propylene carbonate.

[0024] DSC spectrum ( Figure 6 The results showed that the peak reaction temperature of the modified lignin adhesive decreased significantly from 156℃ to 120-130℃, indicating a substantial reduction in its curing temperature. The basic performance indicators of the samples showed that the gelation time of the organic ester-modified lignin-phenolic resin adhesive was shortened from a minimum of 6.5 minutes to 2.2 minutes, further verifying the significant improvement in its reactivity.

[0025] This invention increases the phenolic substitution degree of lignin by 50%, resulting in an adhesive with significant technical advantages: (1) the gel time is shortened to half that of conventional products, significantly improving curing efficiency; (2) the average bonding strength is ≥1.35 MPa, meeting the GB / T 9846 Class I plywood standard; (3) the hot pressing cycle of the board is shortened by 50%, effectively reducing production energy consumption; (4) formaldehyde emission is ≤0.018 mg / m³. 3 To reach E NF The advanced environmental protection standards have established a new approach to the high-value utilization of lignin, resulting in significant environmental and economic benefits in the field of wood-based panel manufacturing. Attached Figure Description

[0026] Figure 1 A schematic diagram of the Kolbe-Schmidt reaction;

[0027] Figure 2 The reaction mechanism is a ternary precursor reaction of lignin-organic ester-phenol.

[0028] Figure 3 The adhesive spectra before and after the introduction of organic esters;

[0029] Figure 4It is a ternary precursor of lignin-organic acid ester-phenol. 13 C spectrum;

[0030] Figure 5 Conventional lignin adhesive 13 C spectrum;

[0031] Figure 6 The DSC spectra are for both the example and comparative examples. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments. These embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0033] The sources of the products used in the embodiments and comparative examples of this invention are shown in Table 1 below.

[0034] Table 1. Raw materials used in the examples and comparative examples.

[0035]

[0036] Example 1

[0037] A method for preparing an organic ester-modified lignin-phenolic resin adhesive includes the following steps:

[0038] Step S1: Add 100g of lignin to the reaction vessel, add 249g of water and stir, add 73g of 50% sodium hydroxide solution and 40g of propylene carbonate, stir at 60℃ for 60min to obtain lignin-propylene carbonate binary precursor solution.

[0039] Step S2: Inject 100g of molten phenol into the binary precursor solution from step S1, and continue stirring at 60℃ for 120min to obtain a lignin-propylene carbonate-phenol precursor solution.

[0040] Step S3: Slowly add 192g of 50% formaldehyde solution to the lignin-propylene carbonate-phenol precursor solution from step S2, react at 75°C for 30min, cool to 70°C, continue reacting for 70min, cool down to 25°C, and add 80g of 50% caustic soda to obtain propylene carbonate modified lignin phenolic resin adhesive.

[0041] Example 2

[0042] A method for preparing an organic ester-modified lignin-phenolic resin adhesive includes the following steps:

[0043] Step S1: Add 80g of lignin to the reaction vessel, add 249g of water and stir, add 73g of 50% sodium hydroxide solution and 100g of propylene carbonate, stir at 60℃ for 60min to obtain lignin-propylene carbonate binary precursor solution.

[0044] Step S2: Inject 120g of molten phenol into the binary precursor solution from step S1, and continue stirring at 60℃ for 120min to obtain a lignin-propylene carbonate-phenol precursor solution.

[0045] Step S3: Slowly add 192g of 50% formaldehyde solution to the lignin-propylene carbonate-phenol precursor solution from step S2, react at 75°C for 30min, cool to 70°C, continue reacting for 70min, cool down to 25°C, and add 80g of 50% caustic soda to obtain propylene carbonate modified lignin phenolic resin adhesive.

[0046] Example 3

[0047] A method for preparing an organic ester-modified lignin-phenolic resin adhesive includes the following steps:

[0048] Step S1: Add 60g of lignin to the reaction vessel, add 249g of water and stir, add 73g of 50% sodium hydroxide solution and 40g of triacetin, stir at 60℃ for 60min to obtain lignin-triacetin binary precursor solution.

[0049] Step S2: Inject 139.8g of molten phenol into the binary precursor solution from step S1, and continue stirring at 60℃ for 120min to obtain a lignin-triacetylglycerol-phenol precursor solution.

[0050] Step S3: Slowly add 192g of 50% formaldehyde solution to the lignin-triacetylglycerol-phenol precursor solution from step S2, react at 75°C for 30min, cool to 70°C, continue reacting for 70min, cool down to 25°C, and add 80g of 50% caustic soda to obtain triacetylglycerol-modified lignin phenolic resin adhesive.

[0051] Comparative Example 1

[0052] A method for preparing an unmodified lignin-phenolic resin adhesive includes the following steps:

[0053] Step S1: Add 150g of water and 39g of lignin to the reactor, add 70g of 50% formaldehyde solution, add 80g of 50% caustic soda solution, and inject 91g of molten phenol into the reactor. Heat to 80℃.

[0054] Step S2: Add 55g of 50% formaldehyde solution, heat to 90℃, continue the reaction for 60-80min, cool down to 75℃, react for 100-120min, and then cool down to 25℃.

[0055] Comparative Example 2

[0056] A method for preparing a conventional phenolic resin adhesive includes the following steps:

[0057] Step S1: Add 150g of water to the reactor, inject 130g of molten phenol into the reactor, add 80g of 50% sodium hydroxide solution, add 70g of 50% formaldehyde solution, and heat to 80℃.

[0058] Step S2: Add 55g of 50% formaldehyde solution, heat to 90℃, continue the reaction for 60-80min, cool down to 75℃, react for 100-120min, and then cool down to 25℃.

[0059] Basic indicator test:

[0060] The adhesives prepared according to the above embodiments and comparative examples were tested for basic performance indicators. The relevant test data are shown in Table 2 below.

[0061] Table 2 Performance test data of adhesives prepared in different embodiments and comparative examples

[0062]

[0063] Application performance testing:

[0064] According to the requirements of the national standard GB / T 9846-2015 Class I plywood, the performance of the plywood prepared in the examples and comparative examples was tested (the board preparation process is detailed in Table 3).

[0065] Table 3 Hot pressing process

[0066]

[0067] Table 4 Performance test data of plywood prepared in different embodiments and comparative examples

[0068]

[0069] Test results (see Table 4) show that, under the condition of halving the hot-pressing time, the plywood prepared by this invention has performance comparable to that of plywood under conventional hot-pressing time, significantly improving hot-pressing efficiency and reducing production energy consumption. In contrast, conventional lignin adhesives and PF adhesives both exhibited board bursting and blistering after shortening the hot-pressing time, failing to guarantee board quality. Under conventional hot-pressing time, the bonding performance of the lignin adhesive of this invention is comparable to that of PF adhesive, fully meeting the requirements of Class I plywood in the national standard, and showing a significant advantage in formaldehyde emission, which is reduced by approximately 20%-50% compared to PF adhesive; compared with ordinary lignin adhesive, under the same substitution rate, the bonding strength is increased by approximately 10%, the wood breakage rate is increased by 50%, and the formaldehyde emission is reduced by 20%. More importantly, the substitution rate of lignin for phenol in this invention can reach up to 50%, which is superior to the highest substitution rate of conventional lignin adhesives. These data fully demonstrate the triple advantages of this invention in bonding performance, environmental performance, and hot-pressing efficiency, providing strong technical support for the industrial application of lignin-based adhesives.

[0070] This invention uses organic esters (such as propylene carbonate) to graft phenol onto lignin molecules (such as... Figure 2 As shown), it significantly increased the active sites of lignin. Infrared spectroscopy analysis (as shown) Figure 3 It can be observed that, compared with ordinary lignin PF glue (Comparative Example 1), the absorption peak of tert-butanol at 1383 cm⁻¹ in the present invention (Example 1) is significantly enhanced, which is attributed to the products generated by the reaction of propylene carbonate and phenol with lignin. Furthermore, the ¹³C NMR spectrum ( Figure 4 , Figure 5 This further confirms the successful grafting of phenol: compared to ordinary lignin PF (Comparative Example 1), the present invention (Example 1) detected a peak of tert-butanol CH2-OH (C7) at 61 ppm, and simultaneously detected phenolic ring carbon peaks linked to tert-butanol at 119 ppm (C6, C8, C14). These data fully demonstrate that phenol was effectively grafted onto the lignin molecule via propylene carbonate.

[0071] DSC spectrum ( Figure 6 The results showed that the peak reaction temperature of the modified lignin adhesive decreased significantly from 156℃ to 120-130℃, indicating a substantial reduction in its curing temperature. The basic performance indicators of the samples showed that the gelation time of the organic ester-modified lignin-phenolic resin adhesive was shortened from a minimum of 6.5 minutes to 2.2 minutes, further verifying the significant improvement in its reactivity.

[0072] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing an organic ester-modified lignin-phenolic resin adhesive, characterized in that, Includes the following steps: Step S1: Inject lignin particles into the reactor, add caustic soda solution, water, and organic acid esters to react and obtain a lignin-organic acid ester binary precursor solution; Step S2: Molten phenol is further injected into the lignin-organic ester binary precursor solution in the reactor to react and obtain a lignin-organic ester-phenol ternary precursor solution; Step S3: Add formaldehyde solution dropwise into the lignin-organic acid ester-phenol ternary precursor solution in the reaction vessel and react, then inject the remaining caustic soda solution to obtain organic acid ester modified lignin phenolic resin adhesive. The lignin includes one or more of alkali lignin, lignin sulfonate, and enzymatically hydrolyzed lignin. The organic esters include one or more of methyl formate, ethyl acetate, ethyl butyrate, phenyl acetate, glyceryl triacetate, and propylene carbonate; The mass ratio of phenol, lignin, and organic acid esters is 1:(0.43-1):(0.07-1.43).

2. The method for preparing an organic ester-modified lignin-phenolic resin adhesive according to claim 1, characterized in that, The reaction conditions in step S1 are: stirring at 40-60℃ for 30-60 min.

3. The method for preparing an organic ester-modified lignin-phenolic resin adhesive according to claim 1, characterized in that, The reaction conditions in step S2 are: stirring at 40-60℃ for 90-150 min.

4. The method for preparing an organic ester-modified lignin-phenolic resin adhesive according to claim 1, characterized in that, The reaction conditions in step S3 are as follows: first, heat to 60-90℃ and react for 30-60 minutes, then cool to 60-80℃ and continue reacting for 30-90 minutes, then cool to 10-30℃.

5. The method for preparing an organic ester-modified lignin-phenolic resin adhesive according to claim 1, characterized in that, The mass ratio of lignin, caustic soda, organic acid ester, and water in step S1 is 1:(0.32-0.61):(0.07-1.43):(1.86-7.86).

6. The method for preparing an organic ester-modified lignin-phenolic resin adhesive according to claim 1, characterized in that, The mass ratio of phenol, lignin, formaldehyde, caustic soda, and water in step S3 is 1:(0.43-1):(0.64-1.91):(0.17-1.17):(2.82-8.82).

7. An organic ester-modified lignin-phenolic resin adhesive, characterized in that, It is prepared by the preparation method described in any one of claims 1-6.

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