Natural adhesive taking coconut pectin biomass as raw material and preparation method thereof

Abstract

The invention belongs to the technical field of natural adhesives, and discloses a natural adhesive with coconut pectin biomass as a raw material and a preparation method thereof.The preparation method comprises the steps that coconut shell fibers, coconut meat residues and other coconut fruit waste are utilized, and active polysaccharide and vegetable protein are obtained through an oxidized saline water extraction technology; the composite adhesive is formed by self-assembly crosslinking of the modified starch, the polyvinyl alcohol, the modified starch and the epoxidized glyceryl polyether. The adhesive is high in bonding strength, the initial strength of 80% or above can still be kept after the adhesive is soaked in water, the content of VOCs is lower than 10 g / L, and the environment-friendly requirement is met. The problem of environmental pollution of a traditional petroleum-based adhesive is solved, meanwhile, the technical defects that an existing bio-based adhesive is insufficient in strength and poor in water resistance are overcome, a green and environment-friendly adhesive substitute is provided for the wood industry, and the production cost is remarkably reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of environmentally friendly adhesives, and in particular to an environmentally friendly adhesive using coconut biomass as a raw material and a preparation method thereof. The adhesive can be applied to bonding processes in the fields of wood processing, furniture manufacturing, packaging, building materials, and the like. Background Art

[0002] With growing global environmental awareness, the development of environmentally friendly adhesives made from renewable resources has become a research hotspot. Traditional adhesives, primarily based on petrochemicals such as polyurethane, epoxy resin, and phenolic resin, offer high bond strength and water resistance. However, these adhesives suffer from high volatile organic compound (VOC) emissions, non-renewable raw materials, and poor biodegradability, resulting in serious environmental pollution. Adhesives used in wood processing, such as urea-formaldehyde and phenolic resins, are particularly prone to formaldehyde emissions, which can have adverse effects on human health.

[0003] As a new type of environmentally friendly adhesive, biomass-based adhesives have developed rapidly in recent years. Common biomass-based adhesives include starch-based, protein-based, cellulose-based, etc., which have the advantages of being renewable, low-toxic, and biodegradable. CN101979452A discloses a method for preparing a water-resistant adhesive for corrugated paper production, but its water resistance is poor and it is only suitable for bonding non-water-resistant materials such as paper. CN104974695A discloses a vinyl acetate-soy protein copolymer emulsion wood adhesive and a preparation method thereof. Although the bonding strength is improved, the preparation process is complicated and the cost is high. CN114292605A discloses a formaldehyde-free antibacterial cellulose-based wood adhesive and a preparation method thereof, but there are problems such as insufficient bonding strength and long drying time. In general, existing biomass-based adhesives generally have the disadvantages of low bonding strength and poor water resistance, making it difficult to replace traditional petroleum-based adhesives.

[0004] Coconut is an important economic crop in tropical regions, with a huge annual output. Its processing generates large amounts of waste, including coconut shells and coconut meat residues. These wastes are rich in bioactive components such as cellulose, hemicellulose, lignin, and plant protein, and are valuable for development and utilization. CN109012602A discloses a method for preparing coconut shell activated carbon. CN103788216B discloses a method for producing microcrystalline cellulose using whole coconut fruit as a raw material. However, research on the use of coconut waste in the preparation of high-performance adhesives is limited.

[0005] Existing processes for extracting active polysaccharides from coconut waste typically require the use of large amounts of organic solvents or strong acids and bases, resulting in low extraction efficiency and secondary environmental pollution. For example, CN109162132B discloses a method for extracting coconut shell fiber that requires the use of reagents such as concentrated sulfuric acid and sodium hydroxide. This method is complex, energy-intensive, and environmentally unfriendly.

[0006] Furthermore, existing adhesives made from coconut extracts have poor bonding strength and water resistance. CN104974695A discloses a vinyl acetate-soy protein copolymer emulsion wood adhesive and its preparation method. While environmentally friendly, the adhesive has a bonding strength of only 2.5 MPa and a strength retention rate of less than 50% after 24 hours of immersion in water, failing to meet the requirements of applications in wood processing and other fields.

[0007] Therefore, how to use coconut waste to extract active ingredients through green processes and prepare adhesives with high bonding strength, good water resistance and environmental protection properties has become a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0008] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and provide an environmentally friendly adhesive using coconut biomass as raw material and a preparation method thereof. The adhesive uses active polysaccharides and plant proteins extracted from coconut waste as the main raw materials, and is cross-linked with components such as polyvinyl alcohol, modified starch and epoxidized glyceryl polyether to form an environmentally friendly adhesive with high bonding strength, excellent water resistance and low VOCs content.

[0009] To achieve the above objectives, the present invention provides the following technical solutions: The invention discloses a natural adhesive using coconut pectin biomass as a raw material. The adhesive comprises the following raw materials, calculated by weight: 30-50 parts of coconut biomass extract; 8-15 parts of polyvinyl alcohol; 5-15 parts of modified starch; 2-8 parts of epoxidized glyceryl polyether; 0.5-2 parts of borax; 1-5 parts of glycerol; 0.5-3 parts of chitosan; 0.2-1 parts of citric acid; and the balance of water.

[0010] Furthermore, the coconut biomass extract is composed of active polysaccharides and plant proteins obtained by extracting coconut shell fiber and coconut meat residue through an oxidizing salt water method, wherein the mass ratio of active polysaccharides to plant proteins is 3:1 to 5:1.

[0011] Furthermore, the modified starch is hydroxypropyl starch or oxidized starch, and its hydroxypropyl substitution degree is 0.05-0.15 or its oxidation degree is 10%-30%.

[0012] Furthermore, the degree of polymerization of the polyvinyl alcohol is 1700-2400, and the degree of hydrolysis is 87%-99%.

[0013] Furthermore, the epoxidized glyceryl polyether has an epoxy equivalent of 200-400 and a molecular weight of 1000-2000.

[0014] Furthermore, the chitosan has a deacetylation degree of 75%-95% and a viscosity of 100-500 mPa·s (1% solution, measured at 25° C.).

[0015] The present invention also provides a method for preparing the natural adhesive using coconut pectin biomass as a raw material, comprising the following steps: Step S1, preparation of coconut biomass extract: pulverize coconut shell fiber and coconut meat residue into 80-120 mesh, add to an oxidizing salt solution with a pH value of 8.5-10.5, control the solid-liquid ratio to be 1:(6-10), stir and extract at 60-90° C. for 2-4 hours, collect the supernatant after centrifugation, adjust the pH to 6.0-7.0, and concentrate under reduced pressure to obtain a coconut biomass extract with a solid content of 30%-50%; Step S2, preparing a polyvinyl alcohol solution: adding polyvinyl alcohol to water, stirring and dissolving at 85-95° C. for 1-2 hours to obtain a transparent and uniform polyvinyl alcohol solution; Step S3, preparation of modified starch solution: dispersing the modified starch in water, heating to 75-85° C., stirring evenly, and cooling to 50-60° C.; Step S4, preparation of borax solution: dissolving borax in water to prepare a 10%-20% aqueous solution; Step S5, compounding the adhesive: add the coconut biomass extract obtained in step S1 to the reactor, heat it to 50-60°C, slowly add the polyvinyl alcohol solution of step S2, stir evenly, add the modified starch solution of step S3, and continue stirring for 30-60 minutes; then add epoxidized glyceryl polyether and chitosan in sequence, stir evenly, add the borax solution of step S4 dropwise, control the reaction temperature at 55-65°C, and stir the reaction for 80-120 minutes; finally, add glycerol and citric acid, adjust the pH to 6.5-7.5, and continue stirring for 30-60 minutes; Step S6, discharging and cooling: the mixture obtained in step S5 is cooled to room temperature, and allowed to stand for 12-24 hours to remove bubbles, thereby obtaining a natural adhesive using coconut pectin biomass as a raw material.

[0016] Furthermore, the oxidizing salt aqueous solution in step S1 is prepared from 0.5%-2% hydrogen peroxide and 1%-3% sodium chloride.

[0017] Furthermore, after the borax solution is added in step S5, the stirring speed is 200-400 r / min.

[0018] Furthermore, in step S5, the epoxidized glyceryl polyether and chitosan are first dissolved in a weakly acidic aqueous solution with a pH value of 5.0-6.0, and then added into the reactor.

[0019] Compared with the prior art, the present invention has the following significant advantages and beneficial effects: 1. The natural adhesive with coconut palm biomass as raw material of the present invention extracts active polysaccharides and plant proteins in coconut palm waste by oxidizing salt water method, has high extraction efficiency, is green and environmentally friendly, and saves energy and reduces emissions by more than 30% compared with traditional extraction technology, and has high extraction purity. The present invention adopts coconut palm biomass extract to compound with polyvinyl alcohol, modified starch and epoxidized glyceryl polyether, and introduces chitosan to enhance the cross-linked network structure. Through the synergistic effect of multiple components, a three-dimensional network structure is formed, which significantly improves the bonding strength and water resistance of the adhesive. The bonding strength of the adhesive of the present invention is high, and the strength retention rate exceeds 80% after being immersed in water for 24 hours, which is better than existing biomass-based adhesives. At the same time, the preparation process of the present invention is simple, the reaction is carried out under mild conditions, energy consumption is low, safety is high, and it is suitable for industrial production.

[0020] 2. The adhesive prepared by the present invention has a VOCs content of less than 10g / L, meeting EU and Chinese environmental standards. All components are biodegradable, making it environmentally friendly. Furthermore, the present invention fully utilizes coconut processing waste, achieving resource recycling and significant economic and environmental benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive work, among which: Figure 1 The present invention provides a natural adhesive using coconut pectin biomass as a raw material and a process flow chart for preparing the natural adhesive using coconut pectin biomass as a raw material in a preparation method thereof; Figure 2 This is a comparison chart of the bonding strength of an embodiment and a comparative example of a natural adhesive using coconut pectin biomass as raw material and a preparation method thereof of the present invention; Figure 3 A comparison chart of the strength retention rates of Example 1, Example 3, and a comparative example of a natural adhesive using coconut pectin biomass as a raw material and a preparation method thereof under different immersion times of the present invention; Figure 4 The present invention provides a natural adhesive using coconut pectin biomass as raw material and a schematic diagram of the microstructure of the adhesive in its preparation method. DETAILED DESCRIPTION

[0022] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0023] The present invention mainly utilizes bioactive components such as active polysaccharides and plant proteins in coconut waste (coconut shell fiber and coconut meat residue), efficiently obtains these components through an oxidized salt water extraction process, and compounds them with components such as polyvinyl alcohol, modified starch and epoxidized glyceryl polyether to form a three-dimensional cross-linked network structure to prepare an adhesive with high bonding strength, excellent water resistance and environmental protection properties.

[0024] In the present invention, the active polysaccharides of the coconut biomass extract are mainly composed of polysaccharides such as cellulose, hemicellulose and pectin, which contain a large number of carboxyl and hydroxyl functional groups and can undergo hydrogen bonding and cross-linking reactions with other components; plant protein contains a variety of active amino acids, which undergo ring-opening reactions with the epoxy groups in the epoxidized glyceryl polyether to form covalent bonds. Polyvinyl alcohol contains a large number of hydroxyl groups and can form a hydrogen bond network structure with coconut polysaccharides. Modified starch increases the viscosity and solid content of the system and improves the initial adhesion. Borax, as a cross-linking agent, can promote the formation of borate bonds between polyvinyl alcohol and coconut polysaccharides and strengthen the network structure. Chitosan contains active amino groups and hydroxyl groups, which can enhance the cross-linking density of the network and improve water resistance. Glycerol, as a plasticizer, improves the flexibility of the adhesive. Citric acid is used to adjust the pH value of the system and optimize the reaction conditions.

[0025] The oxidizing salt water extraction method described in the present invention is a green extraction technology. It utilizes the synergistic effect of hydrogen peroxide and sodium chloride to form active hypochlorite in an alkaline environment, selectively oxidizing the ether bonds and carbon-carbon bonds in lignin, destroying its wrapping structure for polysaccharides and proteins, achieving efficient and gentle extraction, while avoiding the environmental pollution problems caused by the use of organic solvents or strong acids and alkalis in traditional extraction methods.

[0026] Example 1: Preparation of a natural adhesive using coconut pectin biomass as raw material Step S1, preparation of coconut biomass extract: a mixture of coconut shell fiber and coconut meat residue (mass ratio 3:2) was taken and crushed to 100 mesh. 500 g was weighed and placed in an extraction tank. 4000 ml of an oxidizing salt solution (containing 1.5% hydrogen peroxide and 2% sodium chloride) with a pH value of 9.5 was added. The solid-liquid ratio was controlled to be 1:8. The mixture was stirred and extracted at 80° C. for 3 hours. After centrifugation (5000 r / min, 20 minutes), the supernatant was collected, the pH was adjusted to 6.5 with citric acid, and the mixture was concentrated under reduced pressure to obtain a coconut biomass extract with a solid content of 40% (the mass ratio of active polysaccharides to plant protein was 4:1).

[0027] Step S2, preparation of polyvinyl alcohol solution: 120 g of polyvinyl alcohol (degree of polymerization 2000, degree of hydrolysis 95%) was weighed, slowly added into 800 ml of hot water, and stirred and dissolved at 90° C. for 1.5 hours to obtain a transparent and uniform polyvinyl alcohol solution.

[0028] Step S3, preparation of modified starch solution: weigh 100 g of hydroxypropyl starch (hydroxypropyl substitution degree 0.1), disperse it in 500 ml of water, heat it to 80° C., stir it for 30 minutes until it is completely dissolved, and then cool it to 55° C.

[0029] Step S4, preparation of borax solution: weigh 20 g of borax and dissolve it in 100 ml of water to prepare a 20% aqueous solution.

[0030] Step S5, compounding of the adhesive: add 400g of the coconut biomass extract obtained in step S1 to the reactor, heat to 55°C, slowly add 600g of the polyvinyl alcohol solution of step S2, stir at a speed of 300r / min, stir evenly, add 400g of the modified starch solution of step S3, and continue stirring for 45 minutes; then dissolve 50g of epoxidized glyceryl polyether (epoxy equivalent 300, molecular weight 1500) and 20g of chitosan (deacetylation degree 85%, viscosity 300mPa·s) in 150ml of acetic acid solution with a pH value of 5.5, add to the reactor, stir evenly, and then add 60g of the borax solution of step S4 dropwise at a rate of 5ml / min, and control the reaction temperature at 60°C. The mixture was stirred at a speed of 300 r / min and reacted for 100 minutes. Finally, 30 g of glycerol and 3 g of citric acid were added, the pH value was adjusted to 7.0, and stirring was continued for 40 minutes. Step S6, discharging and cooling: the mixture obtained in step S5 is cooled to room temperature, and allowed to stand for 18 hours to remove bubbles, thereby obtaining a natural adhesive using coconut pectin biomass as a raw material.

[0031] Example 2: Preparation of a natural adhesive using coconut pectin biomass as raw material Step S1, preparation of coconut biomass extract: a mixture of coconut shell fiber and coconut meat residue (mass ratio 2:1) was taken and crushed to 80 mesh, 600 g was weighed and placed in an extraction tank, 5400 ml of an oxidizing salt solution (containing 0.8% hydrogen peroxide and 1.5% sodium chloride) with a pH value of 10.0 was added, and the material-liquid ratio was controlled to be 1:9. The mixture was stirred and extracted at 70° C. for 4 hours. After centrifugation (4500 r / min, 25 minutes), the supernatant was collected, the pH was adjusted to 6.0 with citric acid, and the mixture was concentrated under reduced pressure to obtain a coconut biomass extract with a solid content of 35% (the mass ratio of active polysaccharides to plant protein was 5:1).

[0032] Step S2, preparation of polyvinyl alcohol solution: 80 g of polyvinyl alcohol (degree of polymerization 1800, degree of hydrolysis 90%) was weighed, slowly added into 550 ml of hot water, and stirred and dissolved at 85° C. for 2 hours to obtain a transparent and uniform polyvinyl alcohol solution.

[0033] Step S3, preparation of modified starch solution: weigh 150 g of oxidized starch (oxidation degree 20%), disperse it in 600 ml of water, heat it to 85° C., stir it for 30 minutes until it is completely dissolved, and then cool it to 50° C.

[0034] Step S4, preparation of borax solution: weigh 15 g of borax and dissolve it in 100 ml of water to prepare a 15% aqueous solution.

[0035] Step S5, compounding of the adhesive: 500g of the coconut biomass extract obtained in step S1 was added to the reactor, the temperature was raised to 50°C, 450g of the polyvinyl alcohol solution of step S2 was slowly added, the stirring speed was 250r / min, and after stirring, 500g of the modified starch solution of step S3 was added, and stirring was continued for 60 minutes; then 40g of epoxidized glyceryl polyether (epoxy equivalent 250, molecular weight 1200) and 15g of chitosan (deacetylation degree 80%, viscosity 200mPa·s) were dissolved in 120ml of acetic acid solution with a pH value of 5.0, added to the reactor, stirred evenly, and 50g of the borax solution of step S4 was added dropwise at a rate of 4ml / min, the reaction temperature was controlled at 55°C, the stirring speed was 320r / min, and the reaction was carried out for 120 minutes; finally, 40g of glycerol and 2g of citric acid were added, the pH value was adjusted to 7.5, and stirring was continued for 30 minutes; Step S6, discharging and cooling: the mixture obtained in step S5 is cooled to room temperature, and allowed to stand for 24 hours to remove bubbles, thereby obtaining a natural adhesive using coconut pectin biomass as a raw material.

[0036] Example 3: Preparation of a natural adhesive using coconut pectin biomass as raw material Step S1, preparation of coconut biomass extract: a mixture of coconut shell fiber and coconut meat residue (mass ratio 4:1) was taken and crushed to 120 mesh, 450 g was weighed and placed in an extraction tank, 2700 ml of an oxidizing salt solution (containing 2% hydrogen peroxide and 3% sodium chloride) with a pH value of 8.5 was added, and the material-liquid ratio was controlled to be 1:6. The mixture was stirred and extracted at 90° C. for 2 hours. After centrifugation (6000 r / min, 15 minutes), the supernatant was collected, the pH was adjusted to 7.0 with citric acid, and the mixture was concentrated under reduced pressure to obtain a coconut biomass extract with a solid content of 50% (the mass ratio of active polysaccharides to plant protein was 3:1).

[0037] Step S2, preparation of polyvinyl alcohol solution: weigh 150 g of polyvinyl alcohol (degree of polymerization 2400, degree of hydrolysis 99%), slowly add it into 900 ml of hot water, and stir and dissolve it at 95° C. for 1 hour to obtain a transparent and uniform polyvinyl alcohol solution.

[0038] Step S3, preparation of modified starch solution: weigh 80 g of hydroxypropyl starch (hydroxypropyl substitution degree 0.15), disperse it in 400 ml of water, heat it to 75°C, stir it for 40 minutes until it is completely dissolved, and then cool it to 60°C.

[0039] Step S4, preparation of borax solution: weigh 10 g of borax and dissolve it in 100 ml of water to prepare a 10% aqueous solution.

[0040] Step S5, compounding of the adhesive: 300g of the coconut biomass extract obtained in step S1 was added to the reactor, the temperature was raised to 60°C, 800g of the polyvinyl alcohol solution of step S2 was slowly added, the stirring speed was 350r / min, and after stirring, 300g of the modified starch solution of step S3 was added, and stirring was continued for 45 minutes; then 70g of epoxidized glyceryl polyether (epoxy equivalent 400, molecular weight 2000) and 25g of chitosan (deacetylation degree 95%, viscosity 450mPa·s) were dissolved in 200ml of acetic acid solution with a pH value of 6.0, added to the reactor, stirred evenly, and 40g of the borax solution of step S4 was added dropwise at a rate of 6ml / min, the reaction temperature was controlled at 65°C, the stirring speed was 400r / min, and the reaction was carried out for 80 minutes; finally, 20g of glycerol and 5g of citric acid were added, the pH value was adjusted to 6.5, and stirring was continued for 60 minutes; Step S6, discharging and cooling: the mixture obtained in step S5 is cooled to room temperature, and allowed to stand for 12 hours to remove bubbles, thereby obtaining a natural adhesive using coconut pectin biomass as a raw material.

[0041] Comparative Example 1: Preparation of an adhesive using coconut pectin biomass as raw material without epoxidized glyceryl polyether and chitosan The preparation method was similar to that in Example 1, except that epoxidized glyceryl polyether and chitosan were not added, and other components and steps remained unchanged.

[0042] Comparative Example 2: Preparation of Coconut Pectin Biomass Adhesive without Extraction by Oxidized Salt Water Method Step S1, preparation of coconut biomass extract: a mixture of coconut shell fiber and coconut meat residue (mass ratio 3:2) was taken, crushed to 100 mesh, 500 g was weighed and placed in an extraction tank, 4000 ml of 5% sodium hydroxide solution was added, the material-liquid ratio was controlled to be 1:8, and the mixture was stirred and extracted at 80° C. for 3 hours. After centrifugation (5000 r / min, 20 minutes), the supernatant was collected, the pH was adjusted to 6.5 with hydrochloric acid, and the mixture was concentrated under reduced pressure to obtain a coconut biomass extract with a solid content of 40%; Steps S2 to S6 are the same as those in Example 1.

[0043] Comparative Example 3: Commercially available urea-formaldehyde resin adhesive.

[0044] Comparative Example 4: Commercially available polyvinyl alcohol adhesive.

[0045] Performance Testing The adhesives prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were tested for performance, and the results are shown in Table 1. Adhesive strength testing: Tests were conducted in accordance with GB / T17657-2013, "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels." Adhesive was evenly applied to the surface of beech wood specimens measuring 120 mm × 50 mm × 10 mm at a coating weight of 200 g / m². The two specimens were butt-jointed and glued at room temperature under a pressure of 0.8 MPa for 24 hours. The specimens were then allowed to stand for 7 days under standard conditions (23 ± 2°C, 50 ± 5% relative humidity). Shear strength of the bonded components was tested using a universal testing machine at a tensile speed of 2 mm / min. Ten replicates were tested for each adhesive, and the average value was calculated.

[0046] Water resistance test: The prepared adhesive parts were immersed in water at 23±2°C for 24 hours, 72 hours, and 168 hours. After removal, the surface moisture was wiped off and the adhesive parts were placed under standard conditions for 2 hours. The shear strength was then tested as described above, and the strength retention rate was calculated (strength after immersion / initial strength × 100%).

[0047] VOCs content test: Test in accordance with GB / T33372-2016 "Determination of Volatile Organic Compound (VOC) Content of Adhesives".

[0048] Table 1 Comparison of adhesive properties of various examples and comparative examples From the test results in Table 1 we can see that: In terms of bonding strength, the natural adhesives prepared using coconut pectin biomass in Examples 1-3 achieved bonding strengths of 4.8-5.7 MPa, significantly higher than the adhesives in Comparative Example 1 (which lacks epoxidized glyceryl polyether and chitosan) and Comparative Example 2 (which does not use the oxidized brine extraction method), as well as the commercially available polyvinyl alcohol adhesive (Comparative Example 4). While slightly lower than the urea-formaldehyde resin adhesive (Comparative Example 3), they still meet the strength requirements for adhesives used in the wood industry.

[0049] Water resistance: The adhesives prepared in Examples 1-3 of the present invention showed strength retention of 82%-88% after 24 hours of immersion in water, significantly higher than those in Comparative Examples 1-4. This, in particular, clearly demonstrates the significant effect of epoxidized glyceryl polyether and chitosan on improving the adhesive's water resistance compared to Comparative Example 1. Even after 168 hours of immersion in water, the adhesives of the present invention retained 65%-74% of their initial strength, demonstrating excellent long-term water stability.

[0050] In terms of VOCs content: the VOCs content of the adhesives prepared in Examples 1-3 of the present invention is all lower than 10 g / L, which is much lower than urea-formaldehyde resin adhesive (180 g / L), meeting the requirements of environmentally friendly adhesives and reflecting the environmental protection characteristics of the present invention.

[0051] In summary, the natural adhesive provided by the present invention uses coconut pectin biomass as raw material, efficiently extracts active ingredients from coconut waste through an oxidized salt water method, and is compounded with components such as polyvinyl alcohol, modified starch, epoxidized glyceryl polyether and chitosan to form a three-dimensional cross-linked network structure, thereby achieving an organic unity of high bonding strength, excellent water resistance and low VOCs emissions, successfully solving the technical problems of low bonding strength and poor water resistance of existing biomass-based adhesives, while avoiding the environmental pollution problems of traditional petroleum-based adhesives, and has broad application prospects.

[0052] Microstructure and action mechanism of adhesive: The three-dimensional cross-linked network structure formed by the natural adhesive using coconut pectin biomass as raw material is as follows: Figure 4 The excellent performance of this adhesive is mainly due to the following synergistic mechanisms: 1. The active polysaccharides in coconut biomass extract contain a large number of hydroxyl and carboxyl groups, which can form a strong hydrogen bond network with the hydroxyl groups in polyvinyl alcohol molecules, providing initial bonding strength.

[0053] 2. Borax acts as a cross-linking agent to promote the formation of borate bonds (-BO-) between polyvinyl alcohol and the hydroxyl groups in coconut polysaccharides, thereby building a chemical cross-linking network and enhancing the cohesion and cohesive strength of the adhesive.

[0054] 3. The epoxy groups in epoxidized glyceryl polyether can undergo ring-opening reaction with the amino groups of plant proteins in coconut extract and chitosan to form stable covalent bonds, further enhancing the cross-linking degree and stability of the network structure.

[0055] 4. Chitosan not only participates in the cross-linking reaction, but also enhances the water resistance and bonding durability of the adhesive due to its special molecular structure (containing amino and hydroxyl groups).

[0056] 5. Modified starch (hydroxypropyl starch or oxidized starch) increases the solid content and initial viscosity of the system, and its modified functional groups also participate in the formation of the network structure.

[0057] 6. Glycerol acts as a plasticizer, which improves the flexibility of the adhesive, reduces internal stress, and prevents cracks in the adhesive layer during the drying process.

[0058] The synergistic interaction between these components forms a stable three-dimensional network structure, giving the adhesive both good bonding strength and excellent water resistance. In particular, the introduction of epoxidized glyceryl polyether and chitosan significantly improves the adhesive's water stability by forming a stable covalent cross-linked structure, resolving the poor water resistance commonly found in traditional biomass-based adhesives.

[0059] In addition, the oxidizing salt water extraction process adopted in the present invention can selectively destroy the structure of lignin in coconut waste, efficiently release active polysaccharides and plant proteins, improve the extraction rate and activity of these active ingredients, and provide a good foundation for subsequent reactions with other components. It is one of the key factors for the excellent performance of the adhesive of the present invention.

[0060] Although specific embodiments of the present invention have been described above, those skilled in the art will appreciate that these specific embodiments are merely illustrative, and that those skilled in the art may omit, substitute, and modify the details of the methods and systems described above without departing from the principles and spirit of the present invention. For example, combining the above method steps to perform substantially the same functions in substantially the same manner to achieve substantially the same results falls within the scope of the present invention. Accordingly, the scope of the present invention is limited solely by the appended claims.

Claims

1. A natural adhesive using coconut pectin biomass as raw material, characterized in that, The adhesive comprises the following raw materials, calculated by weight: 30-50 parts of coconut biomass extract; 8-15 parts of polyvinyl alcohol; 5-15 parts of modified starch; 2-8 parts of epoxidized glyceryl polyether; 0.5-2 parts of borax; 1-5 parts of glycerol; 0.5-3 parts of chitosan; 0.2-1 parts of citric acid; and the balance of water.

2. A natural adhesive using coconut pectin biomass as raw material according to claim 1, characterized in that The coconut biomass extract is composed of active polysaccharides and plant proteins obtained by extracting coconut shell fibers and coconut meat residues through an oxidizing salt water method, wherein the mass ratio of active polysaccharides to plant proteins is 3:1 to 5:

1.

3. A natural adhesive using coconut pectin biomass as raw material according to claim 1, characterized in that: The modified starch is hydroxypropyl starch or oxidized starch, and its hydroxypropyl substitution degree is 0.05-0.15 or its oxidation degree is 10%-30%.

4. A natural adhesive using coconut pectin biomass as raw material according to claim 1, characterized in that: The degree of polymerization of the polyvinyl alcohol is 1700-2400, and the degree of hydrolysis is 87%-99%.

5. A natural adhesive using coconut pectin biomass as raw material according to claim 1, characterized in that: The epoxy equivalent of the epoxidized glyceryl polyether is 200-400 and the molecular weight is 1000-2000.

6. A natural adhesive using coconut pectin biomass as raw material according to claim 1, characterized in that: The chitosan has a deacetylation degree of 75%-95% and a viscosity of 100-500 mPa·s (1% solution, measured at 25° C.).

7. A method for preparing a natural adhesive using coconut pectin biomass as raw material as claimed in any one of claims 1 to 6, characterized in that: The following steps are involved: Step S1, preparation of coconut biomass extract: pulverize coconut shell fiber and coconut meat residue into 80-120 mesh, add to an oxidizing salt solution with a pH value of 8.5-10.5, control the solid-liquid ratio to be 1:(6-10), stir and extract at 60-90° C. for 2-4 hours, collect the supernatant after centrifugation, adjust the pH to 6.0-7.0, and concentrate under reduced pressure to obtain a coconut biomass extract with a solid content of 30%-50%; Step S2, preparing a polyvinyl alcohol solution: adding polyvinyl alcohol to water, stirring and dissolving at 85-95° C. for 1-2 hours to obtain a transparent and uniform polyvinyl alcohol solution; Step S3, preparation of modified starch solution: dispersing the modified starch in water, heating to 75-85° C., stirring evenly, and then cooling to 50-60° C.; Step S4, preparation of borax solution: dissolving borax in water to prepare a 10%-20% aqueous solution; Step S5, compounding the adhesive: add the coconut biomass extract obtained in step S1 to the reactor, heat it to 50-60°C, slowly add the polyvinyl alcohol solution of step S2, stir evenly, add the modified starch solution of step S3, and continue stirring for 30-60 minutes; then add epoxidized glyceryl polyether and chitosan in sequence, stir evenly, add the borax solution of step S4 dropwise, control the reaction temperature at 55-65°C, and stir the reaction for 80-120 minutes; finally, add glycerol and citric acid, adjust the pH to 6.5-7.5, and continue stirring for 30-60 minutes; Step S6, discharging and cooling: the mixture obtained in step S5 is cooled to room temperature, and allowed to stand for 12-24 hours to remove bubbles, thereby obtaining a natural adhesive using coconut pectin biomass as a raw material.

8. A method for preparing a natural adhesive using coconut pectin biomass as raw material according to claim 7, characterized in that: The oxidizing salt solution in step S5 is prepared from 0.5%-2% hydrogen peroxide and 1%-3% sodium chloride.

9. A method for preparing a natural adhesive using coconut pectin biomass as raw material according to claim 7, characterized in that: After the borax solution is added in step S5, the stirring speed is 200-400 r / min.

10. A method for preparing a natural adhesive using coconut pectin biomass as raw material according to claim 7, characterized in that: In the step S5, the epoxidized glyceryl polyether and chitosan are first dissolved in a weakly acidic aqueous solution with a pH value of 5.0-6.0, and then added into the reaction kettle.

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