A soy protein adhesive with multiple network synergistic reinforcement and a preparation method thereof

Abstract

The application discloses a kind of multiple network synergistic enhancement soy protein adhesive and its preparation method, belong to biological base adhesive technical field.The method with soy protein isolate (SPI) as main body, by pH induction realizes network's step-by-step construction: first by citric acid modified bentonite (CAB) and SPI between by hydrogen bond and metal coordination effect form first network;Then by dialdehyde cellulose (DAC) aldehyde group and SPI amino group occur Schiff base reaction and form covalent crosslinking second network;Finally, join the natural cinnamon aldehyde (CA) without chemical modification, by Schiff base reaction and Michael addition reaction form third crosslinking network, to build "hydrogen bond-coordination bond-covalent bond" multiple synergistic enhancement structure.DAC simultaneously as CAB nanosheet layer dispersion carrier and covalent crosslinking agent play dual function.The wet shear strength of the obtained adhesive is 1.31-1.56 MPa, the mildew-proof cycle is 26-32 days, the water absorption is less than or equal to 3.2%, and the residual rate is greater than or equal to 96.1%.The raw material of the application is green and environment-friendly, and the process is simple, without the need for pre-chemical modification of components, and can be widely applied in plywood and wood composite manufacturing field.

Description

Technical Field

[0001] This invention relates to the field of bio-based adhesives, specifically to a soybean protein adhesive based on a multi-network synergistic enhancement constructed stepwise from dialdehyde cellulose, citric acid-modified bentonite, and natural cinnamaldehyde, and its preparation method. Background Technology

[0002] Petroleum-based adhesives such as urea-formaldehyde resin and phenolic resin are widely used in the traditional wood industry. However, these adhesives continuously release volatile harmful substances such as formaldehyde during use, posing a threat to human health and the ecological environment. Therefore, developing green and environmentally friendly bio-based adhesives has become an important development direction for the wood industry.

[0003] Soy protein adhesives have attracted widespread attention due to their advantages such as wide availability, renewability, and biodegradability. However, the large number of hydrophilic groups in soybean protein molecules and their relatively loose natural structure lead to problems such as poor water resistance, low bonding strength, and susceptibility to mold growth, which severely limits their industrial application.

[0004] Existing research mainly employs methods such as crosslinking agent modification, nanofiller reinforcement, and biomimetic structural design to improve the performance of soybean protein adhesives. Among these, constructing a multi-network structure is considered an important strategy for improving the overall performance of soybean protein adhesives. For example, prior patent CN201510304629.X discloses a soybean protein / dioaldehyde polysaccharide / aminated nanoclay composite material, which constructs a crosslinking network through the Schiff base reaction of dialdehyde polysaccharide and the physical reinforcement of nanoclay. However, in this scheme, the nanoclay needs to be pre-aminated and modified, and a one-step mixing and hot pressing molding method is used, without involving a method for controlling the stepwise network construction. In addition, the literature reports research on modifying soybean protein using cinnamaldehyde through the Schiff base reaction, but this usually requires pre-combining cinnamaldehyde with other components (such as the use of cinnamaldehyde / tannic acid / iron ion nanoparticles in CN115948148A) or chemical grafting (such as the phytic acid-cinnamaldehyde graft in CN120025783A), making the process relatively complex. Dialdehyde cellulose has been used as a multialdehyde biomass crosslinking agent in soybean protein systems (e.g., CN201911381511), but in existing reports, dialdehyde cellulose is only used as a single crosslinking agent, and there is no record of it simultaneously serving as a nanofiller dispersion carrier and a covalent crosslinking agent.

[0005] Therefore, developing a soybean protein adhesive that is simple to process, requires no pre-modification, can achieve hierarchical construction of multiple networks, and has excellent water resistance and anti-mold properties remains of great practical significance. Summary of the Invention

[0006] The purpose of this invention is to provide a soybean protein adhesive with multi-network synergistic enhancement and its preparation method, so as to solve the problems of poor water resistance, low bonding strength and easy mold growth of existing soybean protein adhesives, while simplifying the preparation process and avoiding the use of pre-modified components.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing dialdehyde cellulose (DAC), comprising the following steps: (1) Preparation of cellulose suspension: Microcrystalline cellulose is added to deionized water to form a cellulose suspension with a mass concentration of 1 wt% to 3 wt%; the suspension is dispersed by ultrasound for 10 min to 20 min to improve the uniformity of cellulose dispersion.

[0008] (2) Oxidation reaction of dialdehyde cellulose: Sodium periodate is added at a molar ratio of cellulose to sodium periodate of 2:1; the pH of the system is adjusted to 3.5 to 4.5 using hydrochloric acid or acetic acid; the reaction is carried out at 40℃ to 50℃ in the dark for 3 to 6 hours, so that the vicinal diol structure at the C2-C3 position of the glucose unit of cellulose undergoes selective oxidative cleavage to generate dialdehyde cellulose.

[0009] Purification and drying of dialdehyde cellulose: After the reaction was completed, ethylene glycol was added to terminate the oxidation reaction; the obtained product was dialyzed for 24 h to 48 h to remove unreacted small molecules and byproducts; finally, dialdehyde cellulose (DAC) powder was obtained by freeze drying. The DAC had an oxidation degree of 40% to 50%, an aldehyde density of 4.2 to 5.1 mmol / g, a degree of polymerization (DP) range of 180 to 220, a polydispersity index of 1.4 to 1.8, and retained part of the cellulose backbone structure to maintain its chain rigidity and multi-point cross-linking ability.

[0010] Secondly, the present invention provides a method for preparing citric acid modified bentonite (CAB), comprising the following steps: (1) Acidification modification treatment of bentonite: Add bentonite to 1 mol / L citric acid solution, with a mass-volume ratio of bentonite to citric acid solution of 1:20 to 1:25 (g / mL), and mechanically stir at 400 to 600 rpm for 3 to 5 h at room temperature to allow citric acid to fully enter the interlayer structure of bentonite and undergo modification.

[0011] (2) Product separation and washing: After the reaction is completed, the solid product is separated by centrifugation and washed repeatedly with deionized water until neutral to remove residual citric acid and soluble impurities.

[0012] (3) Drying and grinding: The washed product was vacuum dried at 60℃ for 10 h to 12 h; after drying, it was ground and passed through a 200-mesh sieve to obtain citric acid modified bentonite (CAB). The interlayer spacing of the obtained CAB increased from about 1.26 nm in the unmodified bentonite to 1.52 to 1.58 nm (the 2θ angle corresponding to the (001) plane diffraction peak in the XRD pattern shifted from about 7.0° to 6.2° to 6.5°), and the citric acid loading was 15 to 25 mmol / g CAB.

[0013] Thirdly, the present invention provides a method for preparing a soybean protein adhesive with multi-network synergistic enhancement, comprising the following steps: (1) Preparation of CAB / DAC composite suspension: CAB is added to deionized water to form a suspension; then DAC is added to the suspension and ultrasonically treated at 200 W to 400 W for 15 min to 30 min. During this process, the aldehyde groups on the DAC molecular chain form hydrogen bonds and physical adsorption with the surface of the CAB nanosheets. DAC plays a dual role in dispersing and stabilizing CAB and subsequent cross-linking.

[0014] (2) SPI dispersion and system alkalization: Soy protein isolate (SPI) was added in batches to the CAB / DAC composite system and mechanically stirred at 400-600 rpm for 20-40 min at room temperature; then the pH of the system was adjusted to 9-10 with 1 mol / L NaOH. Under alkaline conditions, the SPI molecular chains fully unfolded and exposed the internal active amino and hydroxyl groups; at the same time, the system first formed a first-layer network between CAB and SPI through hydrogen bonding between carboxyl and amino groups and coordination with metal ions.

[0015] (3) pH-induced hierarchical network construction and cinnamaldehyde crosslinking: Maintain pH 9-10 and continue stirring for 20-30 min to allow the aldehyde groups of DAC to undergo Schiff base reaction with the amino groups of SPI, forming a second covalently crosslinked network. Then, natural cinnamaldehyde (CA) is added to the system without any chemical modification (distinguishing it from phytic acid-grafted or nanoparticle-composite cinnamaldehyde), and stirring is continued for 20-40 min. The α,β-unsaturated aldehyde groups in CA and the free amino groups in SPI that did not participate in the aforementioned reaction undergo Schiff base reaction and Michael addition reaction simultaneously to form a third crosslinked network, ultimately yielding a soybean protein adhesive synergistically enhanced by a multi-network of "hydrogen bonds-coordination bonds-covalent bonds".

[0016] In the preparation method, the mass ratio of SPI, CAB, DAC to CA is 9:1:0.5 to 2.0:0.3; preferably, the overall performance is best when the mass ratio of DAC to SPI is 1.5:9.

[0017] Fourthly, the present invention provides a soybean protein adhesive with multi-network synergistic enhancement obtained by the above preparation method, wherein a first CAB-SPI hydrogen bond / coordination network, a second DAC-SPI Schiff base covalent network, and a third CA-SPI Schiff base / Michael addition network are sequentially constructed therein; the adhesive has a wet shear strength of not less than 1.43 MPa and a mildew resistance period of not less than 26 days.

[0018] Fifthly, the present invention provides the application of the above-mentioned soybean protein adhesive in the field of engineered wood processing. Beneficial effects

[0019] (1) Stepwise construction of multiple networks significantly improves water resistance: Unlike existing one-step mixing methods (such as CN201510304629.X), this invention achieves stepwise network construction through pH control. First, CAB and SPI form a non-covalent network under alkaline conditions, then DAC and SPI form a Schiff base covalent network, and finally CA further crosslinks. This stepwise construction method ensures that the three networks do not interfere with each other, maximizing the crosslinking density. The resulting adhesive has a wet shear strength of up to 1.56 MPa (Example 3), which is significantly better than the control group without DAC (1.20 MPa), demonstrating excellent resistance to water erosion.

[0020] (2) Dual function of DAC: The dialdehyde cellulose (DAC) in this invention not only acts as a multialdehyde crosslinking agent to form covalent bonds with SPI, but also serves as a dispersion carrier for CAB nanosheets during the preparation process, thus avoiding the aggregation of CAB. In the prior art, DAC is only used as a single crosslinking agent (such as CN201911381511), and its dual function has not been reported.

[0021] (3) Direct use of natural cinnamaldehyde: This invention directly uses unmodified natural cinnamaldehyde, crosslinking it with SPI through a dual reaction mechanism of Schiff base and Michael addition, while utilizing its natural antibacterial properties to improve antifungal performance. Unlike schemes that require pre-grafting cinnamaldehyde with phytic acid (CN120025783A) or forming nanoparticles with tannic acid / iron ions (CN115948148A), the process of this invention is simpler and less costly.

[0022] (4) Excellent anti-mildew performance: By reducing the free water content of the adhesive layer through a dense cross-linked network, combined with the natural antibacterial effect of cinnamaldehyde, the anti-mildew cycle of the adhesive of the present invention is extended from 16 days in the control group to 32 days (Example 3), which significantly improves the stability of use in humid and hot environments.

[0023] (5) Green and environmentally friendly: All raw materials are derived from natural renewable resources and do not contain harmful substances such as formaldehyde, which is in line with the development direction of bio-based adhesives. Attached Figure Description

[0024] Figure 1 Cross-sectional scanning electron microscopy (SEM) images of soybean protein adhesives modified with different DAC addition amounts.

[0025] Figure 2 Photographs of wood breakage rate tested with adhesives from different embodiments: a) dry shear strength, b) wet shear strength.

[0026] Figure 3 Water absorption rate of modified soybean protein adhesives with different DAC addition amounts.

[0027] Figure 4 Test results of the anti-mildew performance of DAC modified soybean protein adhesive. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited to the embodiments described. The raw materials used in the embodiments are: soy protein isolate (SPI, protein content ≥90%), microcrystalline cellulose (degree of polymerization approximately 220), sodium periodate (analytical grade), cinnamaldehyde (CA, purity ≥98%), bentonite (sodium-based, cation exchange capacity ≥90 mmol / 100g), and citric acid (analytical grade). Unless otherwise specified, all experimental conditions were at room temperature (25±2℃). Example 1

[0029] Raw materials (by weight): 9 g soy protein isolate (SPI), 90 g deionized water, 1 g citric acid modified bentonite (CAB), 0.5 g dialdehyde cellulose (DAC), and 0.3 g cinnamaldehyde (CA).

[0030] 5 g of microcrystalline cellulose was added to 250 mL of deionized water and ultrasonically dispersed for 10 min. Sodium periodate was added at a molar ratio of cellulose to sodium periodate of 2:1, and the pH was adjusted to 4.0 with 0.1 mol / L hydrochloric acid. The reaction was magnetically stirred at 45 °C in the dark for 4 h. After the reaction was completed, 20 mL of ethylene glycol was added to terminate the reaction. The mixture was dialyzed for 24 h using a dialysis bag (molecular weight cutoff 8000-14000 Da), and then freeze-dried for 48 h to obtain a pale yellow DAC powder. The degree of oxidation was 45.3%, the aldehyde density was 4.7 mmol / g, the degree of polymerization (DP) was 205, and the polydispersity index was 1.6, as determined by the hydroxylamine hydrochloride method.

[0031] 10 g of bentonite was added to 200 mL of 1 mol / L citric acid solution and mechanically stirred at 500 rpm for 3 h at room temperature. After the reaction was complete, the mixture was centrifuged at 8000 rpm for 5 min and washed with deionized water until neutral. The mixture was then vacuum dried at 60℃ for 12 h, ground, and passed through a 200-mesh sieve to obtain CAB. XRD analysis showed that the interlayer spacing of unmodified bentonite was 1.26 nm (2θ≈7.0°), the interlayer spacing of CAB was 1.55 nm (2θ≈6.4°), and the citric acid loading was 18.6 mmol / g CAB.

[0032] 1 g of CAB was added to 90 g of deionized water to form a suspension. 0.5 g of DAC was added, and the mixture was sonicated at 300 W for 20 min. 9 g of SPI was added in three portions, and the mixture was mechanically stirred at 500 rpm for 30 min each time. The pH was adjusted to 9–10 with 1 mol / L NaOH, and stirring was continued for 20 min. 0.3 g of CA was added, and stirring was continued for 30 min to obtain the adhesive. Example 2

[0033] The preparation method is the same as in Example 1, except that the amount of DAC is 1.0 g, while the amounts of SPI, CAB, and CA remain unchanged. Example 3

[0034] The preparation method is the same as in Example 1, except that the amount of DAC is 1.5 g, while the amounts of SPI, CAB, and CA remain unchanged. Example 4

[0035] The preparation method is the same as in Example 1, except that the amount of DAC is 2.0 g, while the amounts of SPI, CAB, and CA remain unchanged. control group

[0036] The preparation method is the same as in Example 1, except that DAC is not added, while the other raw material composition and process conditions remain the same.

[0037] Bond strength test: According to the testing requirements for Class II plywood in Section 4.17 of GB / T 17657-2022 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels", plywood specimens with dimensions of 25 mm × 100 mm were prepared, and grooves were cut along the thickness direction of the core layer, with the groove depth not exceeding 2 / 3 of the core board thickness. Half of the specimen was placed in a constant temperature water bath at 63±2℃ for 3 h, and then removed and allowed to stand in a room temperature environment for 10 min to complete thermal equilibrium. The dry / wet bond strength of 6 parallel specimens was measured using a universal testing machine, and the average value was taken.

[0038] Water absorption test: After 10 g of adhesive is cured, weigh it (m1) and place it in a constant temperature and humidity chamber at 28±0.5℃ and 80±2% RH until constant weight (m2). Calculate the water absorption rate according to the formula (m2-m1) / m1×100%.

[0039] Residual rate test: Wrap 1 g of cured adhesive powder (M1) in filter paper and weigh it (M2). Soak it in a water bath at 60±2℃ for 6 h, dry it to constant weight and weigh it (M3). Calculate the residual rate according to the formula (M3-M2) / M1×100%.

[0040] Anti-mold test: Place the uncured adhesive in a disposable petri dish and incubate it in a constant temperature and humidity incubator at 28°C and 80% relative humidity. Observe and record the mold growth daily (the appearance of visible mold spots is the standard).

Claims

1. A method for preparing a soybean protein adhesive with multi-network synergistic enhancement, characterized in that, Includes the following steps: (1) Prepare citric acid modified bentonite (CAB), wherein the interlayer spacing of the citric acid modified bentonite (CAB) is 1.52-1.58 nm and the citric acid loading is 15-25 mmol / g CAB; (2) Preparation of dialdehyde cellulose (DAC), wherein the dialdehyde cellulose (DAC) is prepared by selective oxidation of microcrystalline cellulose with sodium periodate, the oxidation degree is 40% to 50%, the aldehyde density is 4.2 to 5.1 mmol / g, and some of the cellulose main chain structure is retained to maintain chain rigidity; (3) Disperse CAB in deionized water to form a CAB suspension, add DAC to the CAB suspension and perform ultrasonic dispersion treatment, wherein DAC and CAB work together, and the aldehyde group on the DAC molecular chain plays a dual role in dispersing and stabilizing the CAB nanosheets and participating in subsequent covalent cross-linking. (4) Add soy protein isolate (SPI) to the suspension obtained in step (3), and adjust the pH of the system to 9-10 with 1 mol / L NaOH to allow the SPI molecular chains to unfold and expose the internal active amino and hydroxyl groups. At the same time, the alkaline pH induces the stepwise construction of the network, including: firstly, the first network is formed between CAB and SPI through hydrogen bonding between carboxyl and amino groups and coordination with metal ions; then, the second network is formed by Schiff base reaction between the aldehyde group of DAC and the amino group of SPI. (5) Add unmodified natural cinnamaldehyde (CA) and continue stirring the reaction so that CA and the amino groups in SPI that did not participate in the above reaction can form a third cross-linking network through Schiff base reaction and Michael addition reaction, thereby obtaining a soybean protein adhesive that is synergistically enhanced by multiple networks of "hydrogen bond-coordination bond-covalent bond". In the preparation method, the mass ratio of SPI, CAB, DAC to CA is 9:1:0.5 to 2.0:0.

3.

2. The preparation method according to claim 1, characterized in that: The method for preparing the dialdehyde cellulose (DAC) includes: Microcrystalline cellulose was added to deionized water to form a cellulose suspension with a mass concentration of 1 wt% to 3 wt%, and ultrasonically dispersed for 10 to 20 min. Sodium periodate was added at a molar ratio of cellulose to sodium periodate of 2:1, and the pH was adjusted to 3.5 to 4.5 with hydrochloric acid. The reaction was carried out at 40 to 50 °C in the dark for 4 h. After the reaction was completed, ethylene glycol was added to terminate the reaction. After dialyzing for 24 to 48 h, the product was freeze-dried to obtain dialdehyde cellulose (DAC) powder. The degree of polymerization (DP) of the DAC ranged from 180 to 220, and the polydispersity index was 1.4 to 1.

8.

3. The preparation method according to claim 1, characterized in that: The preparation method of the citric acid modified bentonite (CAB) includes: Bentonite was dispersed in a 1 mol / L citric acid solution at a mass-to-volume ratio of 1:20 to 1:25 (g / mL). The mixture was mechanically stirred at 500 rpm for 3 h at room temperature. After the reaction, the mixture was centrifuged, washed with deionized water until neutral, dried under vacuum at 60℃ for 12 h, and then ground through a 200-mesh sieve. In the XRD pattern of the CAB, the 2θ angle corresponding to the (001) plane diffraction peak shifted from approximately 7.0° in unmodified bentonite to 6.2°–6.5°, corresponding to an increase in interlayer spacing from approximately 1.26 nm to 1.52–1.58 nm.

4. The preparation method according to claim 1, characterized in that: In step (3), the ultrasonic dispersion is treated at a power of 200-400W for 15-30 min; in step (4), the mechanical stirring speed is 400-600 rpm and the stirring time is 20-40 min; in step (5), after adding CA, stirring is continued for 20-40 min.

5. The preparation method according to claim 1, characterized in that: The optimal amount of DAC to be added is when the quality ratio of SPI to DAC is 9:1.

5.

6. The soybean protein adhesive with multi-network synergistic enhancement prepared by the method according to any one of claims 1 to 5, characterized in that: The adhesive is internally constructed with a first network consisting of hydrogen bonds and coordination bonds between CAB and SPI, a second network consisting of Schiff base covalent bonds between DAC and SPI, and a third network consisting of Schiff base bonds and Michael addition bonds between CA and SPI. The adhesive has a wet shear strength ≥1.43 MPa, a mildew resistance period ≥26 days, a water absorption rate ≤3.2%, and a residual rate ≥96.1%.

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