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

Abstract

The present application belongs to the technical field of natural adhesive, and discloses a natural adhesive taking coconut fruit pectin biomass as raw material and a preparation method thereof. The coconut fruit waste such as coconut shell fiber and coconut pulp residue is used to obtain active polysaccharide and plant protein through an oxidative salt water extraction process, and a composite adhesive is formed through self-assembly cross-linking of the active polysaccharide and plant protein, polyvinyl alcohol, modified starch and epoxidized glyceryl polyether. The adhesive has high bonding strength, can maintain more than 80% of the initial strength after being soaked in water, and has VOCs content lower than 10 g / L, meeting the environmental protection requirements. The present application solves the environmental pollution problem of traditional petroleum-based adhesives, overcomes the technical defects of insufficient strength and poor water resistance of existing bio-based adhesives, provides a green and environmentally-friendly adhesive substitute for the wood industry, and significantly reduces the production cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of environment-friendly adhesives, and particularly relates to an environment-friendly adhesive prepared from coconut fruit biomass and a preparation method thereof, which can be applied to bonding processes in the fields of wood processing, furniture manufacturing, packaging, building materials, etc. BACKGROUND

[0002] With the increasing global environmental awareness, the development of environment-friendly adhesives prepared from renewable resources has become a research hotspot. Traditional adhesives are mainly based on petrochemical products, such as polyurethane, epoxy resin, phenolic resin, etc., which have high bonding strength and water resistance, but have the disadvantages of high emission of volatile organic compounds (VOCs), non-renewable raw materials, poor biodegradability, etc., causing serious environmental pollution problems. Especially in the field of wood processing, adhesives such as urea-formaldehyde resin and phenol-formaldehyde resin have high formaldehyde emission, which has adverse effects on human health.

[0003] Biomass-based adhesives as a new type of environment-friendly adhesives have developed rapidly in recent years. Common biomass-based adhesives include starch-based, protein-based, and cellulose-based adhesives, which have the advantages of being renewable, low-toxic, and biodegradable. CN101979452A discloses a preparation method of 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 its preparation method, which improves the bonding strength but has a complex preparation process and high cost. CN114292605A discloses a formaldehyde-free antibacterial cellulose-based wood adhesive and its preparation method, but has the problems of insufficient bonding strength and long drying time. Overall, 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 fruit is an important economic crop in tropical regions, with a huge annual output, and a large amount of waste such as coconut shell and coconut meat residue is generated during its processing. These wastes contain rich bioactive components such as cellulose, hemicellulose, lignin, and plant protein, which have development and utilization value. CN109012602A discloses a preparation method of coconut shell activated carbon. CN103788216B discloses a method for producing microcrystalline cellulose from whole coconut fruit. However, there is little research on using coconut fruit waste to prepare high-performance adhesives.

[0005] In the prior art, the process for extracting active polysaccharide components from coconut waste usually requires the use of a large amount of organic solvents or strong acids and strong bases, which not only has low extraction efficiency but also causes secondary environmental pollution. For example, CN109162132B discloses an extraction method for coconut shell fibers, which requires the use of reagents such as concentrated sulfuric acid and sodium hydroxide, and has complex process, high energy consumption and poor environmental performance.

[0006] In addition, the existing adhesive prepared from coconut extract has poor bonding strength and poor water resistance. CN104974695A discloses a vinyl acetate-soy protein copolymer emulsion wood adhesive and a preparation method thereof, which has good environmental performance, but the bonding strength is only 2.5 MPa, and the strength retention rate after immersion in water for 24 hours is less than 50%, which cannot meet the use requirements in the field of wood processing.

[0007] Therefore, how to extract active ingredients from coconut waste by a green process and prepare an adhesive with high bonding strength, good water resistance and environmental performance has become a technical problem to be solved in the field. SUMMARY

[0008] The present application aims to solve the above problems in the prior art, and provides 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 main raw materials, and is compounded and cross-linked with polyvinyl alcohol, modified starch and epoxidized glyceryl polyether, etc. to form an environmentally friendly adhesive with high bonding strength, excellent water resistance and low VOCs content.

[0009] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0010] A natural adhesive using coconut gum biomass as raw material, the adhesive comprises the following raw materials in mass fraction: coconut biomass extract 30-50 parts; polyvinyl alcohol 8-15 parts; modified starch 5-15 parts; epoxidized glyceryl polyether 2-8 parts; borax 0.5-2 parts; glycerol 1-5 parts; chitosan 0.5-3 parts; citric acid 0.2-1 part; and water in excess.

[0011] Further, the coconut biomass extract is composed of active polysaccharides and plant proteins extracted from coconut shell fibers and coconut meat residues by an oxidative salt water method, and the mass ratio of active polysaccharides to plant proteins is 3:1 to 5:1.

[0012] Further, the modified starch is hydroxypropyl starch or oxidized starch, and the hydroxypropyl substitution degree is 0.05-0.15 or the oxidation degree is 10%-30%.

[0013] Further, the polyvinyl alcohol has a polymerization degree of 1700-2400 and a hydrolysis degree of 87%-99%.

[0014] Further, the epoxy equivalent weight of the epoxidized glyceryl polyether is 200-400, and the molecular weight is 1000-2000.

[0015] Further, the deacetylation degree of the chitosan is 75%-95%, and the viscosity is 100-500 mPa·s (1% solution, measured at 25°C).

[0016] The application also provides a preparation method of the natural adhesive with the coconut fruit pectin biomass as the raw material.

[0017] Step S1, preparation of coconut fruit biomass extract: coconut shell fibers and coconut meat residues are crushed to 80-120 mesh, and then added into an oxidizing salt aqueous solution with a pH value of 8.5-10.5, the solid-liquid ratio is controlled to be 1: (6-10), and then stirred and extracted at 60-90°C for 2-4 hours, and then centrifuged to collect the supernatant, and then the pH value is adjusted to 6.0-7.0, and then the coconut fruit biomass extract with a solid content of 30%-50% is obtained by concentration under reduced pressure.

[0018] Step S2, preparation of polyvinyl alcohol solution: polyvinyl alcohol is added into water, and then stirred and dissolved at 85-95°C for 1-2 hours to obtain a transparent and uniform polyvinyl alcohol solution.

[0019] Step S3, preparation of modified starch solution: the modified starch is dispersed in water, heated to 75-85°C, uniformly stirred, and then cooled to 50-60°C.

[0020] Step S4, preparation of borax solution: borax is dissolved in water to prepare a 10%-20% aqueous solution.

[0021] Step S5, compounding of the adhesive: the coconut fruit biomass extract obtained in step S1 is added into a reaction kettle, heated to 50-60°C, and then the polyvinyl alcohol solution in step S2 is slowly added, uniformly stirred, and then the modified starch solution in step S3 is added, and then continuously stirred for 30-60 minutes, and then the epoxidized glyceryl polyether and chitosan are sequentially added, uniformly stirred, and then the borax solution in step S4 is added dropwise, the reaction temperature is controlled to be 55-65°C, and then stirred and reacted for 80-120 minutes, and then glycerol and citric acid are added, the pH value is adjusted to 6.5-7.5, and then continuously stirred for 30-60 minutes.

[0022] Step S6, discharging and cooling: the mixture obtained in step S5 is cooled to room temperature, and then placed to remove bubbles for 12-24 hours to obtain the natural adhesive with the coconut fruit pectin biomass as the raw material.

[0023] Further, the oxidizing salt aqueous solution in step S1 is prepared from 0.5%-2% hydrogen peroxide and 1%-3% sodium chloride.

[0024] Further, the stirring speed is 200-400 r / min after adding the borax solution in the step S5.

[0025] Further, in the step S5, the epoxidized glyceryl polyether and the chitosan are first dissolved in the weak acid aqueous solution with a pH value of 5.0-6.0, and then added into the reaction kettle.

[0026] Compared with the prior art, the present application has the following remarkable advantages and beneficial effects:

[0027] 1. The natural adhesive with coconut fruit pectin biomass as raw material has high extraction efficiency, is green and environmentally friendly, saves energy and reduces emissions by more than 30% compared with traditional extraction technology, and has high extraction purity. The coconut fruit biomass extract, polyvinyl alcohol, modified starch and epoxidized glyceryl polyether are compounded in the present application, and chitosan is introduced to enhance the crosslinking 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 is high, and the strength retention rate is more than 80% after 24 hours of immersion, which is better than existing biomass-based adhesives. At the same time, the preparation process of the present application is simple, the reaction is carried out under mild conditions, the energy consumption is low, the safety is high, and it is suitable for industrial production.

[0028] 2. The adhesive prepared in the present application has a VOCs content of less than 10 g / L, which meets the requirements of European and Chinese environmental protection standards, and all components are biodegradable and environmentally friendly. Moreover, the present application makes full use of coconut fruit processing waste to realize resource recycling, which has significant economic and environmental benefits. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor under the premise of not paying creative labor.

[0030] Figure 1 The present application is a natural adhesive with coconut fruit pectin biomass as raw material and a process flow chart for preparing the natural adhesive with coconut fruit pectin biomass as raw material in the preparation method of the natural adhesive.

[0031] Figure 2 The present application is a natural adhesive with coconut fruit pectin biomass as raw material and a process flow chart for preparing the natural adhesive with coconut fruit pectin biomass as raw material in the preparation method of the natural adhesive.

[0032] Figure 3The strength retention rate comparison chart of example 1, example 3 and comparative example in the natural adhesive with coconut fruit pectin biomass as raw material and the preparation method thereof of the present application under different soaking times;

[0033] Figure 4 The microstructure schematic diagram of the adhesive in the natural adhesive with coconut fruit pectin biomass as raw material and the preparation method thereof of the present application. DETAILED DESCRIPTION

[0034] The preferred embodiments of the present application are described below in conjunction with the accompanying drawings, and it should be understood that the embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0035] The present application mainly utilizes the bioactive components such as active polysaccharides and plant proteins in coconut fruit waste (coconut shell fiber and coconut meat residue), efficiently obtains these components through the oxidative salt water extraction process, and forms a three-dimensional cross-linked network structure by compounding with polyvinyl alcohol, modified starch and epoxidized glyceryl polyether, thereby preparing an adhesive with high bonding strength, excellent water resistance and environmental protection characteristics.

[0036] In the present application, the main components of the active polysaccharides in the coconut fruit biomass extract are polysaccharide substances such as cellulose, hemicellulose and pectin, which contain a large number of carboxyl and hydroxyl functional groups and can have hydrogen bonding and cross-linking reactions with other components; the plant proteins contain a variety of active amino acids, which have ring-opening reactions with the epoxy groups in the epoxidized glyceryl polyether to form covalent bonds. The polyvinyl alcohol contains a large number of hydroxyl groups, which can form a hydrogen-bonded network structure with the coconut polysaccharides. The modified starch increases the viscosity and solid content of the system, and improves the initial bonding strength. Borax as a cross-linking agent can promote the formation of borate ester bonds between polyvinyl alcohol and coconut polysaccharides, and enhance the network structure. Chitosan contains active amino and hydroxyl groups, which can enhance the cross-linking density of the network and improve the 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.

[0037] The oxidative salt water extraction method described in the present application is a green extraction technology, which utilizes the synergistic effect of hydrogen peroxide and sodium chloride to form active hypochlorite in an alkaline environment, selectively oxidizes the ether bonds and carbon-carbon bonds in lignin, destroys the wrapping structure of polysaccharides and proteins, realizes efficient and mild extraction, and at the same time avoids the environmental pollution problems caused by the use of organic solvents or strong acids and strong bases in traditional extraction methods.

[0038] Example 1: Preparation of a natural adhesive with coconut fruit pectin biomass as raw material

[0039] Step S1, preparation of coconut fruit biomass extract: Take the mixture of coconut shell fiber and coconut meat residue (mass ratio 3:2), crush to 100 mesh, take 500 g and put it into the extraction tank, add 4000 ml of oxidizing salt aqueous solution (containing 1.5% hydrogen peroxide and 2% sodium chloride) with pH value of 9.5, control the solid-liquid ratio to 1:8, extract at 80℃ for 3 hours, centrifugal separation (5000 r / min, 20 minutes) to collect the supernatant, adjust the pH to 6.5 with citric acid, and concentrate under reduced pressure to obtain a coconut fruit biomass extract with solid content of 40% (the mass ratio of active polysaccharide to plant protein is 4:1).

[0040] Step S2, preparation of polyvinyl alcohol solution: take 120 g of polyvinyl alcohol (polymerization degree 2000, hydrolysis degree 95%), slowly add 800 ml of hot water, stir and dissolve at 90℃ for 1.5 hours to obtain a transparent and uniform polyvinyl alcohol solution.

[0041] Step S3, preparation of modified starch solution: take 100 g of hydroxypropyl starch (hydroxypropyl substitution degree 0.1), disperse in 500 ml of water, heat to 80℃, stir for 30 minutes until completely dissolved, then cool to 55℃.

[0042] Step S4, preparation of borax solution: take 20 g of borax, dissolve in 100 ml of water to prepare a 20% aqueous solution.

[0043] Step S5, compounding of adhesive: add 400 g of coconut fruit biomass extract obtained in step S1 to the reaction kettle, heat to 55℃, slowly add 600 g of polyvinyl alcohol solution in step S2, stir at 300 r / min, after uniform stirring, add 400 g of modified starch solution in step S3, continue to stir for 45 minutes; then dissolve 50 g of epoxidized glyceryl polyether (epoxy equivalent weight 300, molecular weight 1500) and 20 g of chitosan (degree of deacetylation 85%, viscosity 300 mPa·s) in 150 ml of acetic acid solution with pH value of 5.5, add to the reaction kettle, stir uniformly, then add 60 g of borax solution in step S4 at a speed of 5 ml / min, control the reaction temperature at 60℃,

[0044] stir at 300 r / min for 100 minutes; finally add 30 g of glycerol and 3 g of citric acid to adjust the pH to 7.0, continue to stir for 40 minutes;

[0045] Step S6, discharging and cooling: cool the mixture obtained in step S5 to room temperature, stand for 18 hours to remove bubbles, and obtain a natural adhesive with coconut fruit gum biomass as raw material.

[0046] Example 2: Preparation of a natural adhesive with coconut fruit gum biomass as raw material

[0047] Step S1, preparation of coconut fruit biomass extract: Take the mixture of coconut shell fiber and coconut meat residue (mass ratio 2:1), crush to 80 mesh, take 600 g and put into the extraction tank, add 5400 ml of aqueous oxidizing salt solution (containing 0.8% hydrogen peroxide and 1.5% sodium chloride) with pH value of 10.0, control the solid-liquid ratio to 1:9, extract at 70℃ for 4 hours, centrifugal separation (4500 r / min, 25 minutes) to collect the supernatant, adjust the pH to 6.0 with citric acid, and concentrate under reduced pressure to obtain a coconut fruit biomass extract with solid content of 35% (mass ratio of active polysaccharide to plant protein is 5:1).

[0048] Step S2, preparation of polyvinyl alcohol solution: take 80 g of polyvinyl alcohol (polymerization degree 1800, hydrolysis degree 90%) and slowly add it into 550 ml of hot water, stir and dissolve at 85℃ for 2 hours to obtain a transparent and uniform polyvinyl alcohol solution.

[0049] Step S3, preparation of modified starch solution: take 150 g of oxidized starch (oxidation degree 20%) and disperse it in 600 ml of water, heat to 85℃ and stir for 30 minutes until completely dissolved, then cool to 50℃.

[0050] Step S4, preparation of borax solution: take 15 g of borax and dissolve it in 100 ml of water to prepare a 15% aqueous solution.

[0051] Step S5, compounding of adhesive: in a reaction kettle, add 500 g of coconut fruit biomass extract obtained in step S1, heat to 50℃, slowly add 450 g of polyvinyl alcohol solution in step S2, stir at 250 r / min, after uniform stirring, add 500 g of modified starch solution in step S3, continue to stir for 60 minutes; then dissolve 40 g of epoxidized glyceryl polyether (epoxy equivalent weight 250, molecular weight 1200) and 15 g of chitosan (degree of deacetylation 80%, viscosity 200 mPa·s) in 120 ml of acetic acid solution with pH value of 5.0, add to the reaction kettle, stir uniformly, then add 50 g of borax solution in step S4 at a speed of 4 ml / min, control the reaction temperature at 55℃, stir at 320 r / min, react for 120 minutes; finally, add 40 g of glycerol and 2 g of citric acid, adjust the pH to 7.5, continue to stir for 30 minutes;

[0052] Step S6, discharging and cooling: cool the mixture obtained in step S5 to room temperature, stand for 24 hours to remove bubbles, and obtain a natural adhesive with coconut fruit gum biomass as raw material.

[0053] Example 3: Preparation of a natural adhesive with coconut fruit gum biomass as raw material

[0054] Step S1, preparation of coconut fruit biomass extract: Take the mixture of coconut shell fiber and coconut meat residue (mass ratio 4:1), crush to 120 mesh, take 450 g and put into the extraction tank, add 2700 ml of oxidized salt aqueous solution (containing 2% hydrogen peroxide and 3% sodium chloride) with pH value of 8.5, control the solid-liquid ratio to 1:6, extract at 90℃ for 2 hours, centrifugal separation (6000 r / min, 15 minutes) to collect the supernatant, adjust the pH to 7.0 with citric acid, and concentrate under reduced pressure to obtain a coconut fruit biomass extract with a solid content of 50% (the mass ratio of active polysaccharide to plant protein is 3:1).

[0055] Step S2, preparation of polyvinyl alcohol solution: take 150 g of polyvinyl alcohol (polymerization degree 2400, hydrolysis degree 99%), slowly add 900 ml of hot water, and stir to dissolve at 95℃ for 1 hour to obtain a transparent and uniform polyvinyl alcohol solution.

[0056] Step S3, preparation of modified starch solution: take 80 g of hydroxypropyl starch (hydroxypropyl substitution degree 0.15), disperse in 400 ml of water, heat to 75℃, and stir for 40 minutes until completely dissolved, then cool to 60℃.

[0057] Step S4, preparation of borax solution: take 10 g of borax, dissolve in 100 ml of water to prepare a 10% aqueous solution.

[0058] Step S5, compounding of adhesive: add 300 g of coconut fruit biomass extract obtained in step S1 to the reaction kettle, heat to 60℃, slowly add 800 g of polyvinyl alcohol solution of step S2, stir at 350 r / min, after uniform stirring, add 300 g of modified starch solution of step S3, continue to stir for 45 minutes; then dissolve 70 g of epoxidized glyceryl polyether (epoxy equivalent weight 400, molecular weight 2000) and 25 g of chitosan (degree of deacetylation 95%, viscosity 450 mPa·s) in 200 ml of acetic acid solution with pH value of 6.0, add to the reaction kettle, stir uniformly, then add 40 g of borax solution of step S4 at a speed of 6 ml / min, control the reaction temperature at 65℃, and stir at 400 r / min for 80 minutes; finally add 20 g of glycerol and 5 g of citric acid, adjust the pH to 6.5, and continue to stir for 60 minutes;

[0059] Step S6, discharging and cooling: cool the mixture obtained in step S5 to room temperature, stand for 12 hours to remove bubbles, and obtain a natural adhesive with coconut fruit gum biomass as raw material.

[0060] Comparative Example 1: Preparation of adhesive with coconut fruit gum biomass as raw material without epoxidized glyceryl polyether and chitosan

[0061] Prepared by the method of Example 1, but without adding epoxidized glyceryl polyether and chitosan, and the other components and steps remain unchanged.

[0062] Preparation of a coconut fruit biomass adhesive without using the oxidized brine method

[0063] Step S1, preparation of a coconut fruit biomass extract: Take a mixture of coconut shell fibers and coconut meat residues (mass ratio 3:2), crush to 100 mesh, weigh 500 g into an extraction tank, add 4000 ml of a 5% sodium hydroxide solution, control the solid-liquid ratio to 1:8, stir and extract at 80°C for 3 hours, centrifuge (5000 r / min, 20 minutes) to collect the supernatant, adjust the pH to 6.5 with hydrochloric acid, and concentrate under reduced pressure to a coconut fruit biomass extract with a solid content of 40%;

[0064] Steps S2 to S6 are the same as in Example 1.

[0065] Comparative Example 3: Commercial urea-formaldehyde resin adhesive.

[0066] Comparative Example 4: Commercial polyvinyl alcohol adhesive.

[0067] Performance testing

[0068] The adhesives prepared in Examples 1 to 3 and Comparative Examples 1 to 4 above were tested for performance, and the results are shown in Table 1

[0069] Bonding strength test: tested according to GB / T 17657-2013 "Test methods of physical and chemical properties of wood-based panels and veneered wood-based panels". The adhesive was evenly applied to the surface of 120 mm x 50 mm x 10 mm beech test pieces at a coating amount of 200 g / m², two test pieces were butted together, and the assembly was pressed at a pressure of 0.8 MPa for 24 hours at room temperature, then placed in standard conditions (temperature 23 ± 2°C, relative humidity 50 ± 5%) for 7 days. A universal material testing machine was used to test the shear strength of the bonded assembly at a tensile speed of 2 mm / min. Ten parallel samples were tested for each adhesive, and the average value was taken.

[0070] Water resistance test: the bonded assembly prepared above was immersed in water at 23 ± 2°C for 24 hours, 72 hours and 168 hours, then removed, wiped off the surface moisture, and placed in standard conditions for 2 hours, then tested for shear strength according to the above method, and the strength retention rate was calculated (strength after immersion / initial strength x 100%).

[0071] VOC content test: tested according to GB / T 33372-2016 "Determination of volatile organic compounds (VOC) content of adhesives".

[0072] Table 1 Comparison of properties of adhesives of various examples and comparative examples

[0073]

[0074] From the test results of Table 1, it can be seen that:

[0075] In terms of bonding strength: the natural adhesive prepared by the coconut fruit pectin biomass as raw material in the embodiments 1-3 of the present application has a bonding strength of 4.8-5.7 MPa, which is significantly higher than that of the comparative example 1 (not containing epoxy glyceryl polyether and chitosan) and the comparative example 2 (not using the oxidation salt water method for extraction), as well as the commercially available polyvinyl alcohol adhesive (comparative example 4). Although it is slightly lower than that of the urea-formaldehyde resin adhesive (comparative example 3), it has reached the strength requirement of the adhesive for wood industry.

[0076] In terms of water resistance: the strength retention rate of the adhesive prepared by the embodiments 1-3 of the present application is 82%-88% after being immersed in water for 24 hours, which is much higher than that of the comparative examples 1-4. Especially compared with the comparative example 1, it is evident that the epoxy glyceryl polyether and chitosan have a significant effect on improving the water resistance of the adhesive. After being immersed in water for 168 hours, the adhesive of the present application still maintains 65%-74% of the initial strength, indicating that it has excellent long-term water resistance stability.

[0077] In terms of VOCs content: the VOCs content of the adhesive prepared by the embodiments 1-3 of the present application is less than 10 g / L, which is much lower than that of the urea-formaldehyde resin adhesive (180 g / L), meeting the requirement of the environmentally friendly adhesive, and embodying the environmental protection characteristics of the present application.

[0078] In summary, the natural adhesive provided by the present application, which uses coconut fruit pectin biomass as raw material, efficiently extracts the active ingredients in the coconut fruit waste by the oxidation salt water method, and is compounded with polyvinyl alcohol, modified starch, epoxy glyceryl polyether and chitosan, etc., to form a three-dimensional cross-linked network structure, realizing the organic unity of high bonding strength, excellent water resistance and low VOCs emission, successfully solving the technical problems of low bonding strength and poor water resistance of the existing biomass-based adhesives, while avoiding the environmental pollution problem of traditional petroleum-based adhesives, and having a broad application prospect.

[0079] Microstructure and mechanism of the adhesive:

[0080] The three-dimensional cross-linked network structure of the natural adhesive prepared by the coconut fruit pectin biomass as raw material in the present application is shown in Figure 4 The excellent performance of the adhesive is mainly due to the following synergistic mechanisms:

[0081] 1. The active polysaccharides in the coconut fruit biomass extract contain a large number of hydroxyl and carboxyl groups, which can form a strong hydrogen bond network with the hydroxyl groups in the polyvinyl alcohol molecules, providing the initial bonding strength.

[0082] 2. Borax as a crosslinking agent, promotes the formation of borate ester bonds (-B-O-) between the hydroxyl groups of polyvinyl alcohol and the hydroxyl groups in the polysaccharides in the coconut fruit extract, building a chemical crosslinking network, enhancing the cohesion and cohesive strength of the adhesive.

[0083] 3. The epoxy groups in the epoxidized glyceryl polyether can undergo ring-opening reactions with the amino groups of the plant proteins and the amino groups of chitosan in the coconut fruit extract, forming stable covalent bonds, further enhancing the crosslinking degree and stability of the network structure.

[0084] 4. Chitosan not only participates in the crosslinking reaction, but also enhances the water resistance and bonding durability of the adhesive due to its special molecular structure (containing amino and hydroxyl groups).

[0085] 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.

[0086] 6. Glycerol as a plasticizer improves the flexibility of the adhesive, reduces internal stress, and prevents cracks in the adhesive layer during drying.

[0087] The synergistic effect between the above-mentioned multiple components forms a stable three-dimensional network structure, making the adhesive not only have good bonding strength, but also exhibit excellent water resistance. In particular, the introduction of epoxidized glyceryl polyether and chitosan significantly improves the water stability of the adhesive by forming stable covalent bond crosslinking structures, solving the problem of poor water resistance commonly existing in traditional biomass-based adhesives.

[0088] In addition, the oxidative salt water extraction process used in the present application can selectively destroy the structure of lignin in coconut fruit waste, efficiently release active polysaccharides and plant proteins, and improve the extraction rate and activity of these active ingredients, providing a good foundation for subsequent reactions with other components, which is one of the key factors for the excellent performance of the adhesive of the present application.

[0089] Although the specific embodiments of the present application are described above, those skilled in the art should understand that these specific embodiments are only illustrative, and those skilled in the art can make various omissions, substitutions and changes to the details of the above-mentioned methods and systems without departing from the principles and essence of the present application. For example, combining the above-mentioned method steps, performing substantially the same function in substantially the same way to achieve substantially the same result according to the same method is within the scope of the present application. Therefore, the scope of the present application is only limited by the appended claims.

Claims

1. A method for preparing a natural adhesive from a coconut fruit pectin biomass, characterized by, The method comprises the following steps: Step S1, preparation of coconut fruit biomass extract: coconut shell fibers and coconut meat residues are crushed to 80-120 mesh, and then added into an oxidized salt aqueous solution with a pH value of 8.5-10.5, which is prepared from 0.5%-2% hydrogen peroxide and 1%-3% sodium chloride, and the solid-liquid ratio is controlled to be 1: (6-10), and then stirred and extracted at 60-90 ℃ for 2-4 hours, and then centrifuged to collect the supernatant, and then the pH value is adjusted to 6.0-7.0, and then concentrated under reduced pressure to obtain a coconut fruit biomass extract with a solid content of 30%-50%; the coconut fruit biomass extract is composed of active polysaccharides and plant proteins, and the mass ratio of the active polysaccharides to the plant proteins is 3:1 to 5:1; Step S2, preparation of polyvinyl alcohol solution: polyvinyl alcohol is added into water, and then dissolved by stirring at 85-95 ℃ for 1-2 hours to obtain a transparent and uniform polyvinyl alcohol solution; Step S3, preparation of modified starch solution: the modified starch is dispersed in water, heated to 75-85 ℃, uniformly stirred, and then cooled to 50-60 ℃; Step S4, preparation of borax solution: borax is dissolved in water to prepare a 10%-20% aqueous solution; Step S5, compounding of adhesive: the coconut fruit biomass extract obtained in step S1 is added into a reaction kettle, heated to 50-60 ℃, and then the polyvinyl alcohol solution in step S2 is slowly added, and then the modified starch solution in step S3 is added after uniform stirring, and then stirred for 30-60 minutes; then 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, and then uniformly stirred, and then the borax solution in step S4 is added dropwise, and the reaction temperature is controlled to be 55-65 ℃, and the stirring speed is controlled to be 200-400 r / min, and the stirring reaction is performed for 80-120 minutes; finally, glycerol and citric acid are added, the pH value is adjusted to 6.5-7.5, and then the stirring is continued for 30-60 minutes; Step S6, discharging and cooling: the mixture obtained in step S5 is cooled to room temperature, and then placed for 12-24 hours to remove bubbles to obtain a natural adhesive with coconut fruit gum biomass as a raw material; The raw materials of the adhesive include, by mass fraction, 30-50 parts of coconut fruit 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 part of citric acid, and the balance of water.

2. The method according to claim 1, wherein the natural adhesive is prepared from a coconut fruit pectin biomass, characterized in that, The modified starch is hydroxypropyl starch or oxidized starch, the hydroxypropyl substitution degree of which is 0.05-0.15 or the oxidation degree of which is 10%-30%.

3. The method according to claim 1, wherein the natural adhesive is prepared from a coconut fruit pectin biomass, characterized in that, The polyvinyl alcohol has a polymerization degree of 1700-2400 and a hydrolysis degree of 87%-99%.

4. The method according to claim 1, wherein the natural adhesive is prepared from a coconut fruit pectin biomass. The epoxidized glyceryl polyether has an epoxy equivalent weight of 200-400 and a molecular weight of 1000-2000.

5. The method according to claim 1, wherein the natural adhesive is prepared from a coconut fruit pectin biomass, characterized in that, The chitosan has a deacetylation degree of 75%-95%, and a viscosity of 100-500 mPa·s measured at 1% solution and 25 ℃.

Images