Lignin-based polyvinyl alcohol composite adhesive as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of adhesives, and discloses a lignin-based polyvinyl alcohol composite adhesive as well as a preparation method and application thereof. After being subjected to oxidation activation treatment, polyvinyl alcohol is compounded with depolymerized lignin and a cross-linking agent, a compact cross-linked network structure is formed, and the lignin and polyvinyl alcohol composite adhesive is obtained. The plywood prepared by using the adhesive disclosed by the invention shows excellent mechanical properties, and the tensile shear strength can be up to 6.28 MPa; the free formaldehyde release amount is only 0.1 g / kg; the adhesive provided by the invention can maintain stable adhesive performance in a wide temperature range of-196-100 DEG C, is significantly improved in solvent resistance, ultraviolet aging resistance and durability, also has the characteristics of environmental friendliness and simple process, and is suitable for adhesion of various materials and processing and repairing of wood products.

Description

Technical Field

[0001] This invention belongs to the field of adhesive technology, and relates to a composite adhesive of depolymerized lignin and lignin-based polyvinyl alcohol, its preparation method and application. Background Technology

[0002] Adhesives are widely used in furniture manufacturing, building decoration, and many other fields, and market demand continues to grow. Traditional commercial adhesives are mainly petroleum-based products such as phenolic resins and urea-formaldehyde resins. These not only rely on non-renewable petroleum resources but also continuously release toxic and harmful substances such as free formaldehyde and phenol, endangering environmental sustainability and human health.

[0003] To address these issues, researchers have turned their attention to renewable biomass resources, leading to the development of various biomass-based adhesives. However, existing biomass-based adhesives generally suffer from drawbacks such as low cross-linking levels, poor water resistance, and the need for high-temperature curing, making them unsuitable for practical applications. Lignin, as the second most abundant natural aromatic polymer after cellulose, is a byproduct of industries such as pulp and paper making, with an annual output of hundreds of millions of tons. Its molecules are rich in aromatic rings and active groups such as hydroxyl groups, possessing reactivity similar to phenol. This allows for the high-value utilization of industrial byproducts and significantly improves the mechanical properties and environmental stability of adhesives. Therefore, lignin-based adhesives have become a key research direction in this field.

[0004] However, natural lignin has a large molecular weight and high steric hindrance, with its active sites encapsulated by a dense three-dimensional network, resulting in extremely low reactivity. Direct application is difficult to achieve the required performance. Depolymerization is the core means to activate its reactivity, but traditional depolymerization processes suffer from problems such as harsh conditions and unstable modification effects. Although there have been attempts to combine polyvinyl alcohol with lignin in existing technologies, key properties of the composite system, such as adhesive strength and water resistance, have not achieved substantial breakthroughs. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, such as low crosslinking degree, the need for high temperature to ensure fluidity in wood, and poor water resistance, stability, and durability, this invention provides a depolymerized lignin. Another objective of this invention is to provide a method for preparing depolymerized lignin. Another objective of this invention is to provide an application of depolymerized lignin. Another objective of this invention is to provide a lignin-polyvinyl alcohol composite adhesive. Another objective of this invention is to provide a method for preparing a lignin-polyvinyl alcohol composite adhesive. Another objective of this invention is to provide an application of a lignin-polyvinyl alcohol composite adhesive.

[0006] The technical solution of this invention: A method for preparing a lignin-based polyvinyl alcohol composite adhesive involves dispersing polyvinyl alcohol in water and heating it to the dissolution temperature while maintaining stirring until the polyvinyl alcohol is completely dissolved; lowering the temperature to the oxidation temperature and adding an oxidant for oxidation activation; raising the temperature to the crosslinking temperature and sequentially adding depolymerized lignin and a crosslinking agent to perform crosslinking; and cooling to room temperature after the reaction is complete to obtain the lignin-based polyvinyl alcohol composite adhesive.

[0007] Polyvinyl alcohol accounts for 8% to 11% of the total mass of the adhesive.

[0008] The amount of depolymerized lignin added is 3.0% to 4.5% of the total mass of the adhesive.

[0009] The mass ratio of water to oxidant is 150 ~ 160:1.

[0010] The mass ratio of oxidant to crosslinking agent is 22 ~ 25:26.

[0011] The mass ratio of polyvinyl alcohol to oxidant is 96 ~ 120:5.

[0012] The dissolution temperature of polyvinyl alcohol is 90~100°C. o C. The stirring speed is 300 rpm, and the stirring time is 3 to 5 hours.

[0013] Oxidation temperature is 50 o C, stirring speed is 150 rpm, oxidation time is 1~4 h.

[0014] The crosslinking temperature is 70°C. o C, the stirring speed is 150 rpm, and the cross-linking time is 2~3 h.

[0015] The oxidant is at least one of sodium persulfate, ammonium persulfate, and potassium persulfate.

[0016] The crosslinking agent is at least one of glutaraldehyde, formaldehyde, benzaldehyde, trioxymethylene, and glyoxal.

[0017] A method for preparing depolymerized lignin involves a depolymerization reaction of lignin and a methanol-alkali hydrothermal system; after the reaction is completed, the mixture is cooled and depressurized to obtain depolymerized lignin; wherein lignin accounts for 18% to 21% of the total mass of the depolymerization reaction system; the methanol-alkali hydrothermal system includes methanol, water, and sodium hydroxide, with a volume ratio of methanol to water of 1:4 and a solid-liquid ratio of sodium hydroxide to solvent (the sum of methanol and water) of 0.9 to 1:2 (g: mL).

[0018] Lignin is pre-hydrolyzed lignin from eucalyptus trees.

[0019] The reaction temperature is 120~180°C. oC, reaction time 1 h.

[0020] The stirring speed is 800 rpm.

[0021] An application of depolymerized lignin in the preparation of composite adhesives.

[0022] A lignin-polyvinyl alcohol composite adhesive, prepared using depolymerized lignin.

[0023] An application of the lignin-based polyvinyl alcohol composite adhesive is provided for bonding non-porous materials, porous materials, or biological tissues, and is suitable for bonding and repairing wood products. In application, the adhesive is coated onto the surface of the wood product and hot-pressed. The hot-pressing conditions are: temperature 25~80°C. o C. Time: 10 min.

[0024] This invention uses lignin as raw material, alkaline water as solvent, and methanol as end-capping agent to selectively break down β-elements in lignin through a hydrothermal depolymerization reaction. O -4 ether bonds reduce molecular weight and maximize the exposure of phenolic hydroxyl active sites. After polyvinyl alcohol is oxidized and activated with potassium sulfate, new active sites such as carbonyl and hydroxyl groups are formed on the molecular chain. Under the bridging effect of crosslinking agents, depolymerized lignin undergoes a condensation reaction with oxidized polyvinyl alcohol, and simultaneously forms covalent bonds such as Schiff base bonds and acetal bonds with crosslinking agent molecules. In addition, a large number of intermolecular hydrogen bonds are formed between lignin and polyvinyl alcohol molecules due to the interaction of polar groups. Finally, a dense three-dimensional network structure with covalent bonds as the backbone and hydrogen bonds as the auxiliary is formed. This structure significantly improves the cohesive energy and interfacial adhesion energy of the adhesive, enabling the adhesive to form a strong interfacial bond with the substrate surface. At the same time, the bulk structure has no obvious pores and defects, laying the core structural foundation for various excellent properties.

[0025] The beneficial effects of this invention are: (1) Significantly improves bonding strength and water resistance. This invention forms a dense cross-linked network between depolymerized lignin and oxidized activated polyvinyl alcohol under the action of glutaraldehyde, which makes the prepared plywood exhibit excellent mechanical properties. The tensile shear strength can reach up to 6.28 MPa, and the shear strength still reaches 3.26 MPa after soaking in water for 24 h.

[0026] (2) Excellent environmental performance. This invention uses renewable lignin and polyvinyl alcohol as the main raw materials, which is in line with the green and low-carbon development trend; the free formaldehyde release of the product is only 0.1 g / kg, which is far below the national standard limit, and the volatile organic compounds do not meet the quantitative analysis standards, making it safe for the environment and human body.

[0027] (3) The process is simple and controllable, and energy-saving and efficient. The lignin depolymerization conditions are mild, and there is no need for complex catalysts and aeration operations. The adhesive preparation process is simple to operate. The curing method is flexible, and it can be cured at room temperature or hot-pressed at low temperature. Compared with traditional high-temperature hot-pressing adhesives, it significantly reduces energy consumption, is suitable for large-scale production, and has a production cost of only 2,000 yuan / ton, which has a significant cost advantage.

[0028] (3) Strong adaptability to a wide temperature range and outstanding environmental stability. The adhesive of this invention can be used in temperatures ranging from -196°C to 100°C. o It maintains stable adhesive properties within the extreme temperature range of C, -196 o It retains 77.7% of its room temperature strength at C, 100 o It retains 85.4% of its room temperature strength at temperature C. Attached Figure Description

[0029] Figure 1 These are two-dimensional NMR spectra and lignin-derived monomer structures (2D HSQC NMR) of eucalyptus pre-hydrolyzed lignin and depolymerized lignin; where a is the two-dimensional NMR spectrum of the lignin raw material, and b is the two-dimensional NMR spectrum of the depolymerized lignin; A γ It is a β-aryl ether substructure (β- O -4' type substructure), G series (G2, G5, G6) are guaiac wood-based units, S series (S 2,6 S' 2,6 ) is a syringyl unit, Me-H α It is a methylphenol derivative, Et-S β It is an ethyl eugenol derivative, Pr-G / Pr-S γ It is a derivative of propyl guaiacol and propyl eugenol. ArOMe is an aromatic cyclomethoxyl group, and DMSO- d 6 is the solvent deuterated dimethyl sulfoxide.

[0030] Figure 2 These are phosphorus spectra of lignin and depolymerized lignin prepared at different temperatures. 31 P NMR).

[0031] Figure 3 The thermogravimetric analysis (TG) curves of lignin-based polyvinyl alcohol composite adhesives prepared under the depolymerization temperature gradient are shown.

[0032] Figure 4 This is the differential scanning calorimetry (DSC) curve of the lignin-based polyvinyl alcohol composite adhesive prepared under the depolymerization temperature gradient.

[0033] Figure 5These are stress-strain curves and tensile shear strength histograms of lignin-based polyvinyl alcohol composite adhesives with gradients of depolymerization temperature, polyvinyl alcohol content, and lignin content; where a is the stress-strain curve and b is the tensile shear strength histogram.

[0034] Figure 6 It is a bar chart of tensile shear strength under different curing methods. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0036] Example 1 (1) Preparation of depolymerized lignin; First, 3.0 g of lignin, 0.9 g of sodium hydroxide, and a methanol-water mixture (5 ml methanol, 20 ml water) were added to the reactor. After leak testing, the temperature was raised to 120°C. o C, the reaction was stirred at 800 rpm for 1 hour. The resulting solution was then cooled to room temperature, depressurized, and the liquid components were separated through a sand core funnel. After concentration under reduced pressure, depolymerized lignin was obtained.

[0037] (2) Preparation of adhesives; First, disperse 13 g of polyvinyl alcohol in 100 mL of deionized water and heat to 95°C. o C is stirred at 300 rpm until completely dissolved, and then cooled to 50°C. o After step C, add 0.625 g of potassium persulfate and allow the oxidation activation reaction to proceed for 2 hours. Then, heat the activated polyvinyl alcohol solution to 70°C. o C. Add 3.0% depolymerized lignin and 600 μL of glutaraldehyde, stir evenly, and cool to room temperature to obtain lignin-polyvinyl alcohol composite adhesive.

[0038] (3) Preparation of plywood Poplar veneer was selected to prepare three-layer plywood. The bonded three-layer plywood was then hot-pressed at a pressure of 0.5 MPa and a temperature of 70°C. o C. The hot-pressing time is 10 minutes. After hot pressing, the plywood is left at room temperature for 24 hours.

[0039] Example 2 Same as Example 1, except that the temperature is raised to 140°C in step (1). o C.

[0040] Example 3 Same as Example 1, except that the temperature is raised to 160°C in step (1). o C.

[0041] Example 4 Same as Example 1, except that the temperature is raised to 180°C in step (1). o C.

[0042] The effects of lignin depolymerization temperature on the adhesive and mechanical properties of the prepared lignin-based polyvinyl alcohol composite adhesive are shown in Table 1.

[0043] Table 1. Main performance parameters of the lignin-based polyvinyl alcohol composite adhesives prepared in Examples 1-4

[0044] Example 5 Same as Example 3, except that the mass of polyvinyl alcohol added in step (2) is replaced with 14 g.

[0045] Example 6 Same as Example 3, except that the mass of polyvinyl alcohol added in step (2) is replaced with 15 g.

[0046] Example 7 Same as Example 3, except that the mass of polyvinyl alcohol added in step (2) is replaced with 16 g.

[0047] The effects of polyvinyl alcohol content on the adhesive and mechanical properties of the prepared lignin-based polyvinyl alcohol composite adhesive are shown in Table 2.

[0048] Table 2. Main performance parameters of the lignin-based polyvinyl alcohol composite adhesives prepared in Examples 3 and 5-7

[0049] Example 8 Same as Example 3, except that the oxidation time in step (2) is replaced with 1 h.

[0050] Example 9 Same as Example 3, except that the oxidation time in step (2) is replaced with 3 h.

[0051] Example 10 Same as Example 3, except that the oxidation time in step (2) is replaced with 4 h.

[0052] The effects of oxidation time on the adhesive and mechanical properties of the prepared lignin-based polyvinyl alcohol composite adhesive are shown in Table 3.

[0053] Table 3. Main performance parameters of the lignin-based polyvinyl alcohol composite adhesives prepared in Examples 3 and 8-10

[0054] Example 11 Same as Example 10, except that the crosslinking agent in step (2) is replaced with formaldehyde.

[0055] Example 12 Same as Example 10, except that the crosslinking agent in step (3) is replaced with trioxymethylene.

[0056] Example 13 Same as Example 10, except that the crosslinking agent in step (2) is replaced with benzaldehyde.

[0057] Example 14 Same as Example 10, except that the crosslinking agent in step (2) is replaced with glyoxal.

[0058] The effects of crosslinking agents on the adhesive and mechanical properties of the prepared lignin-based polyvinyl alcohol composite adhesives are shown in Table 4.

[0059] Table 4. Main performance parameters of the lignin-based polyvinyl alcohol composite adhesives prepared in Examples 10 and 11-14

[0060] Example 15 Same as Example 1, except that the hot pressing temperature in step (3) is replaced with 25°C. o C.

[0061] Example 16 Same as Example 1, except that the hot pressing temperature in step (3) is replaced with 50°C. o C.

[0062] Example 17 Same as Example 1, except that the hot pressing temperature in step (3) is replaced with 60°C. o C.

[0063] The effects of hot pressing temperature on the adhesive and mechanical properties of the prepared lignin-based polyvinyl alcohol composite adhesive are shown in Table 5.

[0064] Table 5. Main performance parameters of the lignin-based polyvinyl alcohol composite adhesives prepared in Examples 1 and 15-17

Claims

1. A method for preparing a lignin-based polyvinyl alcohol composite adhesive, characterized in that, Disperse polyvinyl alcohol in water and heat to the dissolution temperature, stirring continuously until the polyvinyl alcohol is completely dissolved. The temperature was lowered to the oxidation temperature, and an oxidant was added for oxidation activation. The temperature was then raised to the crosslinking temperature, and depolymerized lignin and a crosslinking agent were added sequentially for crosslinking. After the reaction was completed, the mixture was cooled to room temperature to obtain a lignin-based polyvinyl alcohol composite adhesive.

2. The method for preparing the lignin-based polyvinyl alcohol composite adhesive according to claim 1, characterized in that, The polyvinyl alcohol accounts for 8% to 11% of the total mass of the lignin-based polyvinyl alcohol composite adhesive; The amount of depolymerized lignin added is 3.0% to 4.5% of the total mass of the lignin-based polyvinyl alcohol composite adhesive.

3. The method for preparing the lignin-based polyvinyl alcohol composite adhesive according to claim 1, characterized in that, The oxidant is at least one of sodium persulfate, ammonium persulfate, and potassium persulfate; The crosslinking agent is at least one of glutaraldehyde, formaldehyde, benzaldehyde, trioxymethylene, and glyoxal; The mass ratio of water to oxidant is 150-160:1; The mass ratio of the oxidant to the crosslinking agent is 22 to 25 to 26. The mass ratio of polyvinyl alcohol to oxidant is 96-120:

5.

4. The method for preparing the lignin-based polyvinyl alcohol composite adhesive according to claim 1, characterized in that, The polyvinyl alcohol has a dissolution temperature of 90~100°C. o C. The stirring speed is 300 rpm, and the stirring time is 3 to 5 hours.

5. The method for preparing the lignin-based polyvinyl alcohol composite adhesive according to claim 1, characterized in that, The oxidation temperature is 50°C. o C, stirring speed is 150 rpm, oxidation time is 1~4 h.

6. The method for preparing the lignin-based polyvinyl alcohol composite adhesive according to claim 1, characterized in that, The crosslinking temperature is 70°C. o C, the stirring speed is 150 rpm, and the cross-linking time is 2~3 h.

7. The method for preparing the lignin-based polyvinyl alcohol composite adhesive according to claim 1, characterized in that, The method for preparing the depolymerized lignin comprises the following steps: Lignin and a methanol-alkali hydrothermal system are subjected to a depolymerization reaction; after the reaction is completed, the system is cooled and depressurized to obtain depolymerized lignin; wherein, lignin accounts for 18% to 21% of the total mass of the depolymerization reaction system; the methanol-alkali hydrothermal system includes a solvent and sodium hydroxide, wherein the solvent is a mixture of methanol and water in a volume ratio of 1:4, and the solid-liquid ratio of sodium hydroxide to solvent is 0.9 to 1:2, g: mL.

8. The method for preparing the lignin-based polyvinyl alcohol composite adhesive according to claim 7, characterized in that, Lignin is pre-hydrolyzed lignin from eucalyptus trees; The reaction temperature is 120~180°C. o C, reaction time 1 h; The stirring speed is 800 rpm.

9. An application of a lignin-based polyvinyl alcohol composite adhesive, characterized in that, This adhesive is used for bonding non-porous materials, porous materials, or biological tissues, and is suitable for bonding and repairing wood products. In application, the adhesive is coated onto the surface of the wood product and then hot-pressed. The hot-pressing conditions are: temperature 25~80°C. o C. Time: 10 min.

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