Water-based chitosan / citric acid supramolecular adhesive, and preparation method therefor and use thereof

Abstract

The present invention belongs to the technical field of adhesives. Disclosed are a water-based chitosan / citric acid supramolecular adhesive, and a preparation method therefor and the use thereof. The problem of an existing biological adhesive failing to be used for bonding different wooden units due to having an uncontrollable viscosity, a low bonding strength, poor water resistance, the energy input required during a synthesis process thereof, and a long synthesis time is solved. The water-based chitosan / citric acid supramolecular adhesive is composed of water, chitosan, citric acid and a filler. The method comprises: carrying out mixing and dissolution at a normal temperature. The adhesive is used for the preparation of plywood, laminated veneer lumbers, shaving boards, fiberboards or core-boards. Provided in the present invention are the water-based chitosan / citric acid supramolecular adhesive, and the preparation method therefor and the use thereof.

Description

A water-based chitosan / citric acid supramolecular adhesive and its preparation method and application Technical Field

[0001] The invention belongs to the technical field of adhesives. Background Art

[0002] Sustainable bio-based materials, composed of wood as their basic building block, are closely intertwined with human life. Examples include plywood, particleboard, and fiberboard used in furniture, as well as laminated veneer lumber (LVL) used in structural construction. Most of these materials are composed of both wood and adhesives, but the adhesives used are almost always petroleum-derived resins with formaldehyde as their building block, such as urea-formaldehyde resins, formaldehyde-melamine resins, and phenol-formaldehyde resins. These adhesives continuously release formaldehyde and other volatile organic compounds during use. Formaldehyde has been designated a Class 1 carcinogen and teratogen by the World Health Organization due to its significant harm to the human body. This has prompted the search for safer and more environmentally friendly adhesives. Currently, significant attention is being paid to the development of wood adhesives based on natural, renewable materials, such as soy protein-based adhesives, lignin-based adhesives, and starch-based adhesives, which show great potential as alternatives to formaldehyde resins in wood-based panel applications. However, these bio-based adhesives still have many problems. For example, soy protein, lignin, and starch are all high-molecular-weight biomass raw materials. The viscosity of adhesives prepared from these raw materials is often large, greater than 5000mPa·s, and the viscosity is uncontrollable. They are only suitable for the current commercial sizing method of wood-based panels and plywood (roller sizing), and cannot be applied to the sizing method of particleboard. The commercial sizing method of particleboard is spray sizing, and the viscosity of the required adhesive needs to be below 1000mPa·s. As a result, current bio-based adhesives can only be used for bonding wood veneer biocomposites and cannot be used for bonding other non-veneer materials such as wood shavings. In addition, soy protein and starch have weak adhesion, poor water resistance, and poor cohesion. They often need to be chemically modified or blended with current traditional resins such as epoxy resins and polyurethane resins to improve their adhesion and cohesion, which will undoubtedly increase the cost of bio-based adhesives. At the same time, these adhesives require energy input during the preparation process, and most of the synthesis conditions are high temperature for several hours, which runs counter to the current dual-carbon strategy.

[0003] Citric acid is a low-molecular organic acid that is abundant in nature and widely found in plant fruits, animal bones, muscles, and blood. It has the advantages of being renewable, anti-corrosion and mildew-resistant, and environmentally friendly. At the same time, citric acid has two active functional groups (carboxyl and hydroxyl), and has excellent adhesion when combined with polar materials (such as wood). It has been used in the bonding of fiberboard and particleboard. However, when used as an adhesive, citric acid has low viscosity and easily penetrates into the porous wood voids during the gluing process, resulting in waste and the inability to form a continuous adhesive film on the veneer, resulting in low bonding strength and poor water resistance (not resistant to boiling water).

[0004] Chitosan is a partially deacetylated product of chitin, which is derived from shrimp and crab shells. Similar in structure to cellulose, it is rich in hydroxyl groups, which re-form abundant hydrogen bonds upon dissolution. Furthermore, because chitosan is a high-molecular-weight biomass raw material, its molecules entangle and interact with each other more effectively. This results in adhesives containing chitosan having a low solids content (<10wt%) and a high viscosity, making them prone to cracking when used for bonding single panels.

[0005] Summary of the Invention

[0006] The present invention aims to solve the problems of existing bio-based adhesives, such as uncontrollable viscosity, low bonding strength, poor water resistance, long synthesis time and energy input required in the synthesis process, and inability to be used for gluing different wooden units. The present invention provides a water-based chitosan / citric acid supramolecular adhesive, a preparation method and application thereof.

[0007] A water-based chitosan / citric acid supramolecular adhesive consists of 50 to 70 parts of water, 1.5 to 10 parts of chitosan, 10 to 40 parts of citric acid and 0 to 20 parts of filler in parts by mass.

[0008] A method for preparing a water-based chitosan / citric acid supramolecular adhesive is carried out according to the following steps:

[0009] 50 to 70 parts of water, 1.5 to 10 parts of chitosan, 10 to 40 parts of citric acid and 0 to 20 parts of filler are weighed in parts by mass, and then mixed and dissolved at room temperature to obtain a water-based chitosan / citric acid supramolecular adhesive.

[0010] The invention discloses an application of a water-based chitosan / citric acid supramolecular adhesive, which is used for preparing plywood, laminated veneer lumber, particleboard, fiberboard or blockboard.

[0011] The beneficial effects of the present invention are:

[0012] (1) The main raw materials of the water-based chitosan / citric acid supramolecular adhesive are chitosan, citric acid, and water. The raw materials are non-toxic, the solvent is water, and it is green and environmentally friendly without any formaldehyde added;

[0013] (2) The preparation process of the water-based chitosan / citric acid supramolecular adhesive is simple. It only has two main components, chitosan and citric acid, which can be prepared by mixing and dissolving. The preparation time is short, the required equipment is simple, the reaction conditions are mild, and no energy input is required. It is very easy to scale up the operation process and realize industrial production.

[0014] (3) The water-based chitosan / citric acid supramolecular adhesive has a very long trial period. The prepared adhesive solution has an applicability period of 1 month or more. The boiling water resistance and shear strength of veneer composite materials (plywood) prepared with the adhesive that has been stored for more than 1 month are almost the same as or stronger than those prepared with the freshly prepared adhesive.

[0015] (4) The water-based chitosan / citric acid supramolecular adhesive for bonding biomass composite materials does not require the addition of any curing agent, and the curing conditions are simple; the water-based chitosan / citric acid supramolecular adhesive for bonding biomass composite materials can fully adopt the traditional liquid sizing process, which is simple, efficient, and particularly easy to scale production;

[0016] (5) Considering the entire production process, the biomass materials bonded by the water-based chitosan / citric acid supramolecular adhesive are non-toxic and harmless to consumers, industrial producers and the environment. It is a truly healthy and environmentally friendly adhesive that complies with the principle of atom economy. The finished plywood of the veneer composite material shows that its performance fully meets the requirements of the latest national standard (GB / T 9846-2015) Class I and II plywood. The water resistance of the veneer laminated wood composited with it and the veneer can meet the requirements of the national standard (GB / T 20241-2021) for impregnation and peeling of structural veneer laminated wood. The finished particleboard of the non-veneer composite material shows that its performance fully meets the requirements of the latest national standard (GB / T 4897-2015) P5 particleboard. In summary, it breaks the technical limitation that the current bio-based adhesive can only be used for bonding veneer composite materials, and breaks through the technical bottleneck of the current organic solvent formaldehyde bonding mode. Provided is a water-based, formaldehyde-free, sustainable, and environmentally friendly supramolecular water-based adhesive with great application prospects.

[0017] The invention is used for a water-based chitosan / citric acid supramolecular adhesive and a preparation method and application thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG1 is a comparison of the insolubility rates of the aqueous chitosan / citric acid supramolecular adhesive film of Example 2 before and after heat treatment;

[0019] FIG2 is a test diagram of the trial period of the aqueous chitosan / citric acid supramolecular adhesive of Example 2;

[0020] FIG3 is a scanning electron microscope image of the dry-state bonding interface of the three-layer plywood in Example 2;

[0021] FIG4 is a scanning electron microscope image of the bonding interface of the three-layer plywood after aging resistance bonding strength test in Example 2;

[0022] FIG5 is a diagram showing the failure mode of the three-layer plywood after dry bonding strength test in Example 2;

[0023] FIG6 is a diagram showing the failure mode of the three-layer plywood after aging resistance bonding strength test in Example 2;

[0024] FIG7 is a graph showing the results of the immersion and peeling performance test of the six-layer LVL according to Example 5;

[0025] FIG8 is a graph showing the results of the immersion and peeling performance test of six-layer LVL in comparative experiment 3;

[0026] FIG9 is a comparison diagram of the static bending strength of the three-layer particleboard of Experiment 4 and Example 6;

[0027] FIG10 is a comparison diagram of the internal bonding strength of the three-layer particleboard of Experiment 4 and Example 6;

[0028] FIG11 is a comparison chart of the 24h water absorption thickness expansion rate of the three-layer particleboard of Experiment 4 and Example 6;

[0029] FIG12 is a diagram showing the failure mode of the three-layer particleboard of Example 6 after the internal bonding strength test. DETAILED DESCRIPTION

[0030] Specific embodiment 1: This embodiment is a water-based chitosan / citric acid supramolecular adhesive, which is composed of 50 to 70 parts of water, 1.5 to 10 parts of chitosan, 10 to 40 parts of citric acid and 0 to 20 parts of filler in parts by mass.

[0031] The principle is as follows: This specific embodiment develops a novel supramolecular adhesive with controllable viscosity and a dynamic three-dimensional network structure based on a supramolecular self-assembly strategy. The adhesive is simple to prepare and is obtained by dissolving chitosan and citric acid in a single step under room temperature aqueous conditions.

[0032] Based on the supramolecular self-assembly strategy, this specific embodiment utilizes the properties of chitosan and citric acid, cleverly regulating the compounding ratio of the two to prepare a new supramolecular adhesive with controllable viscosity. In the new supramolecular adhesive, the protonated amino groups of chitosan form ionic crosslinks with the carboxylate ions of citric acid. At the same time, the hydroxyl groups in the chitosan and citric acid molecules also form hydrogen bonds to coordinate the supramolecular self-assembly process and form a dynamic three-dimensional network structure. This new supramolecular adhesive has suitable viscosity and good film-forming properties. After heating, the film can be rapidly cured. The dynamic three-dimensional network structure is transformed into a stable three-dimensional network structure and forms an insoluble thermosetting resin. After four hours of boiling water treatment, the insolubility rate can reach more than 80%. The cured resin has high water resistance. Biocomposites (such as plywood, laminated veneer lumber, and particleboard) manufactured using this new supramolecular adhesive also exhibit high water resistance. For example, the water resistance of plywood composited with veneer meets the highest weather resistance requirements (Class I and II plywood) of national standard GB / T 9846-2015; the water resistance of laminated veneer lumber composited with veneer meets the requirements of national standard GB / T 20241-2021 for impregnation and stripping of structural laminated veneer lumber; and the water resistance of three-layer particleboard composited with coarse and fine wood shavings meets the requirements of national standard GB / T 4897-2015 for ordinary particleboard (P5 type) used in wet conditions. This new adhesive addresses the problems of low viscosity, weak bonding strength, and low water resistance of citric acid as an adhesive. It also addresses the problems of low solid content, high viscosity, and explosive board when chitosan is used as an adhesive. This technology also addresses the current difficulty of bio-based adhesives in bonding different wood components, broadening their application in high-performance biomass composites. The adhesive's raw materials are environmentally friendly and renewable, and the sole solvent used is an aqueous solution. The synthesis process is low-carbon and environmentally friendly. Biomass composites manufactured and used with this adhesive completely avoid the hazards of free formaldehyde and VOCs. This represents a new high-performance, environmentally friendly, and highly water-resistant adhesive for biomass composites, offering promising applications.

[0033] The beneficial effects of this specific embodiment are:

[0034] (1) The main raw materials of the water-based chitosan / citric acid supramolecular adhesive are chitosan, citric acid, and water. The raw materials are non-toxic, the solvent is water, and it is green and environmentally friendly without any formaldehyde added;

[0035] (2) The preparation process of the water-based chitosan / citric acid supramolecular adhesive is simple. It only has two main components, chitosan and citric acid, which can be prepared by mixing and dissolving. The preparation time is short, the required equipment is simple, the reaction conditions are mild, and no energy input is required. It is very easy to scale up the operation process and realize industrial production.

[0036] (3) The water-based chitosan / citric acid supramolecular adhesive has a very long trial period. The prepared adhesive solution has an applicability period of 1 month or more. The boiling water resistance and shear strength of veneer composite materials (plywood) prepared with the adhesive that has been stored for more than 1 month are almost the same as or stronger than those prepared with the freshly prepared adhesive.

[0037] (4) The water-based chitosan / citric acid supramolecular adhesive for bonding biomass composite materials does not require the addition of any curing agent, and the curing conditions are simple; the water-based chitosan / citric acid supramolecular adhesive for bonding biomass composite materials can fully adopt the traditional liquid sizing process, which is simple, efficient, and particularly easy to scale production;

[0038] (5) Considering the entire production process, the biomass materials bonded by the water-based chitosan / citric acid supramolecular adhesive are non-toxic and harmless to consumers, industrial producers and the environment. It is a truly healthy and environmentally friendly adhesive that complies with the principle of atom economy. The finished plywood of the veneer composite material shows that its performance fully meets the requirements of the latest national standard (GB / T 9846-2015) Class I and II plywood. The water resistance of the veneer laminated wood composited with it and the veneer can meet the requirements of the national standard (GB / T 20241-2021) for impregnation and peeling of structural veneer laminated wood. The finished particleboard of the non-veneer composite material shows that its performance fully meets the requirements of the latest national standard (GB / T 4897-2015) P5 particleboard. In summary, it breaks the technical limitation that the current bio-based adhesive can only be used for bonding veneer composite materials, and breaks through the technical bottleneck of the current organic solvent formaldehyde bonding mode. Provided is a water-based, formaldehyde-free, sustainable, and environmentally friendly supramolecular water-based adhesive with great application prospects.

[0039] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the water is one or a combination of tap water, mineral water, mountain spring water, distilled water, purified water, deionized water, ultrapure water, and industrial water. Other aspects are the same as specific embodiment 1.

[0040] Specific embodiment 3: This embodiment differs from specific embodiment 1 or 2 in that the relative molecular weight of the chitosan is ≥600 and the degree of deacetylation is ≥50%. Other aspects are the same as specific embodiment 1 or 2.

[0041] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the citric acid is one of edible citric acid, industrial citric acid, anhydrous citric acid and citric acid monohydrate, or a combination thereof. Other aspects are the same as specific embodiments 1 to 3.

[0042] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the filler is montmorillonite and kaolin, or a combination of the two. Other aspects are the same as specific embodiments 1, 2 to 4.

[0043] Specific embodiment 6: This embodiment is a method for preparing a water-based chitosan / citric acid supramolecular adhesive, which is carried out according to the following steps:

[0044] 50 to 70 parts of water, 1.5 to 10 parts of chitosan, 10 to 40 parts of citric acid and 0 to 20 parts of filler are weighed in parts by mass, and then mixed and dissolved at room temperature to obtain a water-based chitosan / citric acid supramolecular adhesive.

[0045] Specific embodiment 7: This embodiment differs from specific embodiment 6 in that the solid content of the aqueous chitosan / citric acid supramolecular adhesive is 30 wt% to 50 wt%. Other aspects are the same as specific embodiment 6.

[0046] Specific embodiment eight: This embodiment provides an application of a water-based chitosan / citric acid supramolecular adhesive, which is used for the preparation of plywood, laminated veneer lumber, particleboard, fiberboard or blockboard.

[0047] The wood species of the board in this specific embodiment is not limited, and can be any combination of one or more of coniferous wood, broad-leaved wood, bamboo, or other woods; the type of shavings is not limited, and can be any combination of one or more of coniferous wood, broad-leaved wood, bamboo, or other plants such as straw.

[0048] Specific embodiment 9: This embodiment differs from specific embodiment 8 in that it is used for the preparation of plywood, veneer laminated lumber or particle board, and is carried out according to the following steps:

[0049] When preparing plywood or laminated veneer timber, the glue application rate per unit area is 100g / m 2 ~600g / m 2 Under the conditions of , apply glue on one side or both sides, then assemble the blanks, pre-press after assembling, and then hot press for 30s / mm~180s / mm under the conditions of hot pressing temperature of 120℃~230℃ and hot pressing pressure of 0.5MPa~2MPa;

[0050] When preparing particleboard, the particles are assembled after spraying glue, then pre-pressed, and then hot-pressed for 30 to 180 seconds per mm at a temperature of 180° C. to 230° C. and a pressure of 2.2 MPa to 4 MPa. The amount of glue sprayed is such that the percentage of the solid mass of the adhesive to the absolute dry mass of the particles is 4% to 20%. Other aspects are the same as those of the eighth embodiment.

[0051] In this specific embodiment, the hot pressing is 30s / mm to 180s / mm, which specifically means that the hot pressing is 30s to 180s at a unit thickness of 1mm.

[0052] Specific Embodiment 10: This embodiment differs from either Specific Embodiment 8 or 9 in that, when producing plywood or laminated veneer lumber, the pre-pressing is performed at room temperature and a pre-pressing pressure of 0.5 MPa to 1 MPa for 30 minutes or longer; when producing particleboard, the pre-pressing is performed at room temperature and a pre-pressing pressure of 0.5 MPa to 1 MPa for 10 to 60 seconds. Other aspects are the same as Specific Embodiment 8 or 9.

[0053] The following examples are used to verify the beneficial effects of the present invention:

[0054] Example 1:

[0055] A water-based chitosan / citric acid supramolecular adhesive is prepared by weight from 50 parts of water, 10 parts of chitosan and 40 parts of citric acid.

[0056] Described water is distilled water.

[0057] The relative molecular weight of the chitosan is 600,000, and the deacetylation degree is 85%.

[0058] The citric acid is citric acid monohydrate.

[0059] The preparation method of the above-mentioned aqueous chitosan / citric acid supramolecular adhesive is carried out according to the following steps:

[0060] 50 parts of water, 10 parts of chitosan and 40 parts of citric acid were weighed in parts by mass, and then mixed and dissolved at room temperature to obtain a water-based chitosan / citric acid supramolecular adhesive.

[0061] The water-based chitosan / citric acid supramolecular adhesive has a solid content of 50 wt% and a viscosity of 72500 mPa·s.

[0062] Example 2: This example differs from Example 1 in that: a water-based chitosan / citric acid supramolecular adhesive is composed of 70 parts by mass of water, 4 parts by mass of chitosan, and 26 parts by mass of citric acid. Other components are the same as Example 1.

[0063] The water-based chitosan / citric acid supramolecular adhesive has a solid content of 30 wt% and a viscosity of 14700 mPa·s.

[0064] Example 3: This example differs from Example 1 in that: a water-based chitosan / citric acid supramolecular adhesive is composed of 70 parts by mass of water, 2 parts by mass of chitosan, and 28 parts by mass of citric acid. Other components are the same as Example 1.

[0065] The water-based chitosan / citric acid supramolecular adhesive has a solid content of 30 wt% and a viscosity of 467 mPa·s.

[0066] Comparative Experiment 1: This embodiment differs from the first embodiment in that a citric acid-based adhesive is composed of 70 parts by mass of water and 30 parts by mass of citric acid. Other aspects are the same as those of the first embodiment.

[0067] The citric acid-based adhesive has a solid content of 30 wt % and a viscosity of 2.4 mPa·s.

[0068] Comparative Experiment 2: This embodiment differs from the first embodiment in that a chitosan-based adhesive is prepared by weight consisting of 96 parts water, 3 parts chitosan, and 1 part acetic acid. Other components are the same as those of the first embodiment.

[0069] The acetic acid is glacial acetic acid with a mass percentage of 99.5%;

[0070] The chitosan-based adhesive has a solid content of 4 wt % and a viscosity of 865 mPa·s.

[0071] Example 1:

[0072] The plywood was prepared using the aqueous chitosan / citric acid supramolecular adhesive prepared in Example 2, specifically by the following steps:

[0073] The peeled poplar veneer was dried at room temperature until the moisture content was 7%, and the glue coating amount per unit area was 280g / m 2 Under the conditions of , double-sided glue is applied to obtain a core board with glue applied, the panel and the core board with glue applied are assembled in a cross-grain manner, and after assembly, they are placed in a pre-pressing machine and pre-pressed for 6 hours at room temperature and a pressure of 0.5 MPa to obtain a pre-heated and pressed board, which is placed in a hot press and hot-pressed for 80s / mm at a hot-pressing temperature of 170°C and a hot-pressing pressure of 1 MPa to obtain a three-layer plywood.

[0074] Example 2: This example differs from Example 1 in that plywood was prepared using the aqueous chitosan / citric acid supramolecular adhesive prepared in Example 1. The hot pressing process was performed at a temperature of 160°C and a pressure of 1 MPa for 120 s / mm. All other conditions were the same as in Example 1.

[0075] Example 3: This example differs from Example 1 in that the hot pressing temperature is 180° C. and the hot pressing pressure is 1 MPa, and the hot pressing time is 30 s / mm. Other steps are the same as Example 1.

[0076] Example 4: This example differs from Example 1 in that the hot pressing temperature is 170° C., the hot pressing pressure is 1 MPa, and the hot pressing time is 30 s / mm. Other steps are the same as Example 1.

[0077] Comparative Experiment 1: This comparative experiment differs from Example 1 in that plywood was prepared using the citric acid-based adhesive prepared in Comparative Experiment 1. The hot pressing process was conducted at a temperature of 200°C and a pressure of 1 MPa for 30 s / mm. All other conditions were the same as in Example 1.

[0078] Comparative Experiment 2: This comparative experiment differs from Example 1 in that plywood was prepared using the chitosan-based adhesive prepared in Comparative Experiment 2. The hot pressing was performed at a temperature of 200°C and a pressure of 1 MPa for 30 s / mm. All other conditions were the same as in Example 1.

[0079] Example 5:

[0080] The aqueous chitosan / citric acid supramolecular adhesive prepared in Example 2 was used to prepare laminated veneer lumber, specifically by the following steps:

[0081] The peeled poplar veneer was dried at room temperature until the moisture content was 7%, and the glue coating amount per unit area was 400g / m 2 Under the conditions of single-sided or double-sided glue coating, a core board with glue coating is obtained, the panel and the core board with glue coating are assembled in a manner in which the textures are parallel, and after assembly, they are placed in a pre-pressing machine and pre-pressed for 6 hours at room temperature and a pressure of 0.5 MPa to obtain a pre-heated and pressed board, which is placed in a hot press and hot-pressed for 54 seconds / mm at a hot-pressing temperature of 180°C and a hot-pressing pressure of 1 MPa to obtain a six-layer veneer laminate.

[0082] Comparative Experiment 3: This comparative experiment differs from Example 5 in that the veneer laminate was prepared using the citric acid-based adhesive prepared in Comparative Experiment 1. Other aspects are the same as in Example 5.

[0083] Example 6:

[0084] The particleboard was prepared using the aqueous chitosan / citric acid supramolecular adhesive prepared in Example 3, specifically by the following steps:

[0085] The cut poplar wood shavings are placed in a drying oven at 80°C and dried to a moisture content of 5%. After spraying glue, the shavings are assembled by paving fine shavings on the upper and lower surfaces and coarse shavings in the middle. After assembly, the shavings are placed in a pre-pressing machine and pre-pressed for 30 seconds at room temperature and a pressure of 0.5MPa to obtain a pre-pressed slab. The pre-pressed slab is placed in a hot press and hot-pressed for 180s / mm at a hot-pressing temperature of 180°C and a hot-pressing pressure of 3MPa to obtain a three-layer structure particleboard. The amount of glue applied by spraying glue is such that the percentage of the solid mass in the adhesive to the absolute dry mass of the shavings is 8%; the size of the coarse wood shavings is: length 15mm-45mm, width 3mm-10mm, thickness 0.4mm-0.7mm; the size of the fine wood shavings is: length 3mm-15mm, width 0.5mm-1.5mm, thickness 0.2mm-0.4mm.

[0086] Comparative Experiment 4: This comparative experiment differs from Example 6 in that the particleboard was prepared using the citric acid-based adhesive prepared in Comparative Experiment 1. Other aspects were the same as in Example 6.

[0087] The solid content test of the adhesive in Examples 1 to 3 and Comparative Experiments 1 and 2 was conducted with reference to the national standard GBT14074-2006; the viscosity test was conducted with reference to the national standard GB / T2794-1995.

[0088] The aqueous chitosan / citric acid supramolecular adhesive of Example 2 was spread on a plastic culture dish by casting and dried at room temperature to form a film, i.e., an unheated film. The unheated film was dried in a drying oven at 170° C. until the mass was constant, to obtain a heat-treated film.

[0089] The insolubility rate of the film is determined as follows:

[0090] Record the initial mass of the film as m0. Then, place the film in boiling water at 100°C for 4 hours. Then, dry the boiled film in a 63°C oven until the mass stabilizes. Record the final mass as m1. Test each film three times and take the average. The insolubility ratio is calculated as follows:

[0091] Figure 1 compares the insolubility of the water-based chitosan / citric acid supramolecular adhesive film from Example 2 before and after heat treatment. The figure shows that the film solidifies upon heating, transforming its dynamic three-dimensional network structure into a stable one. After four hours of boiling water treatment, the insolubility reaches over 80%. The insolubility of the film before heat treatment is 0%, while the insolubility after heat treatment is 87.03 ± 0.75%.

[0092] The performance indicators of plywood are determined as follows:

[0093] Glue shear strength test: Tested in accordance with the national standard GB / T17657-2013. Specimens were prepared in accordance with this standard. The wood grain of the core board between the glue layers was perpendicular to the length of the specimen. The slot width and depth of the specimens were designed to comply with the dimensions and requirements outlined in the national standard. Prepared in accordance with GB / T17657-2013, the standard specimens were stored in a desiccator for two days. Testing was performed using a universal mechanical testing machine (RGT-20), with a 50 kN force transducer and a tensile speed of 5 mm / min.

[0094] Dry bonding strength: The prepared standard specimens are directly subjected to tensile shear tests without any treatment.

[0095] Wet bonding strength: According to GB / T17657-2013, the prepared standard specimens were immersed in hot water at 63°C for 3 hours, cooled at room temperature for 10 minutes, and then subjected to tensile shear tests.

[0096] Aging resistance bonding strength: According to GB / T17657-2013 standard, the prepared standard specimens are immersed in boiling water for 4 hours, then dried in a forced air drying oven at (60±3)℃ for 16 hours to 20 hours, immersed in boiling water for another 4 hours, and then placed in cold water below 30℃ for at least 1 hour before conducting tensile shear tests.

[0097] As shown in Examples 1 and 2 in Table 1, the finished plywood made from this adhesive fully meets the requirements of the latest national standard (GB / T 9846-2015) for Class I and Class II plywood. Class II plywood requires a wet bond strength of ≥ 0.7 MPa, while Class I plywood requires an aging-resistant bond strength of ≥ 0.7 MPa.

[0098] Table 1. Dry, wet and aging resistance bond strength of three-layer plywood

[0099] The aqueous chitosan / citric acid supramolecular adhesive from Example 2 was stored at room temperature for one month. Plywood was then prepared using the one-month-stored adhesive as in Example 1, and its aging-resistant bond strength was tested, as shown in Figure 2. Figure 2 shows a trial period test of the aqueous chitosan / citric acid supramolecular adhesive from Example 2. As can be seen from the figure, the aqueous chitosan / citric acid supramolecular adhesive has a very long trial period. The plywood prepared using the one-month-stored adhesive as in Example 1 exhibited stronger aging-resistant bond strength than plywood prepared using freshly prepared adhesive.

[0100] Figure 3 is a scanning electron micrograph of the dry-bonded interface of the three-layer plywood of Example 2. As can be seen from the figure, the adhesive forms a continuous film at the interface between the two wood units, indicating that the adhesive film has film-forming properties. At the same time, after heating, it quickly solidifies to form a stable resin and forms a mechanical interlocking structure with the hierarchical porous structure of the wood.

[0101] Figure 4 is a scanning electron micrograph of the bonding interface of the three-layer plywood of Example 2 after aging resistance bonding strength testing. As can be seen from the figure, the continuous adhesive film did not dissolve after aging treatment and still maintained its original mechanical interlocking structure. It was also found that even under conditions where multiple parts of the wood cell wall were damaged, the bonding interface remained in a closed state, indicating that the adhesive formed an effective bond with the wood unit.

[0102] FIG5 is a diagram showing the failure mode of the three-layer plywood in Example 2 after dry bonding strength testing; as can be seen from the figure, the strength of the cured resin of the adhesive is higher than that of the wood.

[0103] FIG6 is a diagram showing the failure mode of the three-layer plywood after aging resistance bonding strength test in Example 2; as can be seen from the figure, the bulk strength of the resin cured by the adhesive is still higher than that of the wood after aging treatment.

[0104] The performance indicators of laminated veneer lumber are determined as follows:

[0105] Immersion stripping performance test: The test is carried out in accordance with GB / T20241-2021 standard.

[0106] (1) Cold water immersion stripping:

[0107] Immerse the entire specimen in 25°C water for 24 hours. Remove and place in a drying oven at (70±3)°C until the specimen mass is within 105% of its pre-test mass. Observe and measure the ratio of the peeled adhesive line on all four sides of the adhesive layer to the total length of all adhesive layers, as well as the ratio of the peeled adhesive line length of any adhesive layer to the sum of the four sides of that adhesive layer.

[0108] (2) Boiling and dipping peeling:

[0109] Immerse the entire specimen in boiling water for 4 hours. Remove and immerse in 25°C water for 1 hour. Then, dry in a drying oven at (70±3)°C until the specimen mass is within 105% of its pre-test mass. Observe and measure the ratio of the peeled adhesive line on all four sides of the adhesive layer to the total length of all adhesive layers, as well as the ratio of the peeled adhesive line length of any adhesive layer to the sum of the four sides of that adhesive layer.

[0110] Figure 7 shows the results of the immersion stripping performance test of the six-layer veneer lumber in Example 5. As can be seen from the figure, after cold water immersion stripping, the ratio of the peeled adhesive lines on all four sides of the six-layer veneer lumber to the total length of all the adhesive layers, as well as the ratio of the peeled adhesive length of any one adhesive layer to the sum of its four sides, were both 0%. Similarly, after boiling water immersion stripping, the ratio of the peeled adhesive lines on all four sides of the six-layer veneer lumber to the total length of all the adhesive layers, as well as the ratio of the peeled adhesive length of any one adhesive layer to the sum of its four sides, were also 0%. This meets the requirements of the national standard GB / T 20241-2021 for immersion stripping of structural veneer lumber.

[0111] Figure 8 is a graph showing the test results of the immersion and peeling performance of the six-layer veneer laminated lumber in comparative experiment 3. As can be seen from the figure, after the cold water immersion and peeling, the ratios of the peeling lines on the four sides of all the adhesive layers to the total length of all the adhesive layers, as well as the ratio of the peeling length of any one adhesive layer to the sum of the four sides of the adhesive layer, are both 100%. At the same time, after the boiling water immersion and peeling, the ratios of the peeling lines on the four sides of all the adhesive layers to the total length of all the adhesive layers, as well as the ratio of the peeling length of any one adhesive layer to the sum of the four sides of the adhesive layer, are also both 100%, which does not meet the requirements of the national standard GB / T 20241-2021 for immersion and peeling of structural veneer laminated lumber.

[0112] The performance indicators of particleboard are determined as follows:

[0113] (1) Static flexural strength test: Specimens were prepared and tested according to the national standard GB / T17657-2013. Specimen dimensions were 150 mm × 50 mm, with a span of 100 mm. The force sensor was 50 kN, and the loading speed was 5 mm / min.

[0114] (2) Internal bonding strength test: The specimens were prepared and tested according to the national standard GB / T17657-2013. The specimen size was 50 mm × 50 mm. The adhesive used between the specimen and the fixture was hot melt adhesive. According to the GB / T17657-2013 standard, the prepared bonded specimens were placed in a balancing chamber with a temperature of (20 ± 2) °C and a relative humidity of (65 ± 5)%. The test should be completed within 1 hour after the specimens were removed from the balancing chamber. The force sensor was 50 kN and the tensile speed was 5 mm / min.

[0115] (3) 24h water absorption thickness expansion rate: Specimens were prepared and tested according to the national standard GB / T17657-2013. Specimen size was 50 mm × 50 mm. Tap water was used with a pH of 7 ± 1. The temperature was 20 ± 1°C and the specimens were immersed in a water tank, which was kept constant during the test. The specimen surface was perpendicular to the water surface. The specimens were separated by at least 15 mm from each other and from the bottom and walls of the water tank. The upper portion of the specimen was (25 ± 5) mm below the water surface.

[0116] Figure 9 compares the static bending strength of the three-layer particleboards from Comparative Experiment 4 and Example 6. The figure shows that the static bending strength of the three-layer particleboard from Comparative Experiment 4 is 8.84±2.50 MPa, which is lower than the national standard GB / T 4897-2015 requirement of ≥13 MPa for ordinary particleboard (P5 type) used in wet conditions. The static bending strength of the three-layer particleboard from Example 6 is 15.71±0.83 MPa, which is higher than the national standard GB / T 4897-2015 requirement of ≥13 MPa for ordinary particleboard (P5 type) used in wet conditions.

[0117] Figure 10 compares the internal bond strength of the three-layer particleboards from Comparative Experiment 4 and Example 6. The figure shows that the internal bond strength of the three-layer particleboard from Comparative Experiment 4 is 0.19±0.90 MPa, which is lower than the national standard GB / T 4897-2015 requirement of ≥0.30 MPa for ordinary particleboard (P5 type) used in wet conditions. The internal bond strength of the three-layer particleboard from Example 6 is 1.16±0.27 MPa, which is higher than the national standard GB / T 4897-2015 requirement of ≥0.3 MPa for ordinary particleboard (P5 type) used in wet conditions.

[0118] Figure 11 compares the 24-hour water absorption thickness expansion rate of the three-layer particleboards from Comparative Experiment 4 and Example 6. As can be seen from the figure, the 24-hour water absorption thickness expansion rate of the three-layer particleboard from Comparative Experiment 4 is 29.43±6.04%, which does not meet the national standard GB / T 4897-2015 requirement of ≤23% for ordinary particleboard (P5 type) used in wet conditions. The 24-hour water absorption thickness expansion rate of the three-layer particleboard from Example 6 is 19.90±2.42%, exceeding the national standard GB / T 4897-2015 requirement of ≤23% for ordinary particleboard (P5 type) used in wet conditions.

[0119] FIG12 is a diagram showing the failure mode of the three-layer particleboard of Example 6 after the internal bonding strength test; as can be seen from the figure, the failure mode of the particleboard is multiple wood failures, indicating that the resin strength of the adhesive after curing is higher than the bulk strength of the wood particles.

Claims

1. A water-based chitosan / citric acid supramolecular adhesive, characterized in that It consists of 50 to 70 parts of water, 1.5 to 10 parts of chitosan, 10 to 40 parts of citric acid and 0 to 20 parts of filler by weight.

2. The aqueous chitosan / citric acid supramolecular adhesive according to claim 1, characterized in that The water is one of tap water, mineral water, spring water, distilled water, purified water, deionized water, ultrapure water and industrial water, or a combination of several of them.

3. The aqueous chitosan / citric acid supramolecular adhesive according to claim 1, characterized in that The relative molecular weight of the chitosan is ≥600, and the deacetylation degree is ≥50%.

4. The aqueous chitosan / citric acid supramolecular adhesive according to claim 1, characterized in that The citric acid is one of edible citric acid, industrial citric acid, anhydrous citric acid and citric acid monohydrate, or a combination of several of them.

5. The aqueous chitosan / citric acid supramolecular adhesive according to claim 1, characterized in that The filler is one of montmorillonite and kaolin or a combination of the two.

6. The method for preparing an aqueous chitosan / citric acid supramolecular adhesive as claimed in claim 1, characterized in that It is carried out in the following steps: Weigh 50 to 70 parts of water, 1.5 to 10 parts of chitosan, 10 to 40 parts of citric acid and 0 to 20 parts of filler by mass, and then mix and dissolve them at room temperature to obtain a water-based chitosan / citric acid supramolecular adhesive.

7. The method for preparing an aqueous chitosan / citric acid supramolecular adhesive according to claim 6, characterized in that The solid content of the aqueous chitosan / citric acid supramolecular adhesive is 30wt% to 50wt%.

8. The use of an aqueous chitosan / citric acid supramolecular adhesive as claimed in claim 1, characterized in that It is used in the preparation of plywood, laminated veneer lumber, particleboard, fiberboard or blockboard.

9. The use of an aqueous chitosan / citric acid supramolecular adhesive according to claim 8, characterized in that It is used in the preparation of plywood, laminated veneer lumber or particle board in the following steps: When preparing plywood or veneer laminated timber, the glue application rate per unit area is 100g / m 2 ~600g / m 2 Under the conditions of single-sided or double-sided glue coating, and then assembling, pre-pressing after assembling, and then hot pressing for 30s / mm~180s / mm at a hot pressing temperature of 120℃~230℃ and a hot pressing pressure of 0.5MPa~2MPa; When preparing particleboard, the particles are assembled after spraying glue, and then pre-pressed, and then hot pressed for 30s / mm to 180s / mm at a hot pressing temperature of 180°C to 230°C and a hot pressing pressure of 2.2MPa to 4MPa; the amount of glue applied by spraying glue is 4% to 20% of the solid mass of the adhesive to the absolute dry mass of the particles.

10. The use of an aqueous chitosan / citric acid supramolecular adhesive according to claim 9, characterized in that When preparing plywood or veneer laminated timber, the pre-pressing is performed at room temperature and a pre-pressing pressure of 0.5MPa to 1MPa for ≥30min; when preparing particleboard, the pre-pressing is performed at room temperature and a pre-pressing pressure of 0.5MPa to 1MPa for 10s to 60s.

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