Preparation method of low-formaldehyde finger joint plate based on bio-based modified adhesive

Abstract

This invention belongs to the field of engineered wood products manufacturing, specifically a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives. The method includes preparing an adhesive composed of modified soybean protein, non-formaldehyde crosslinking agents, micro / nano cellulose, nano-silica, and epoxidized vegetable oil elastomers; pre-treating the wood and cutting the finger joints; applying a sealing liquid via atomization for end-face damping treatment; performing quantitative adhesive application and stepped butt joint molding; and employing a multi-stage hot-pressing process involving preheating penetration, high-temperature curing, and cooling and pressure holding. Through the above technical solution, this application can achieve the formation of a dense adhesive film at the finger joint interface, significantly improving bonding strength and stability while reducing formaldehyde release.

Description

Technical Field

[0001] This invention belongs to the field of engineered wood products manufacturing, specifically a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives. Background Technology

[0002] Solid wood finger-jointed boards have broad application prospects in many fields. The research and application of bio-based modified adhesives such as soybean protein and lignin to replace formaldehyde-based synthetic resins has become the core development direction of the wood adhesive industry. This can not only eliminate the hidden danger of formaldehyde release, but also is a key path to realize the high-value utilization of forestry resources.

[0003] In the current preparation of finger-jointed boards, in order to achieve wetting and penetration of bio-based adhesives on the surface of wood, the apparent viscosity is usually reduced by means of molecular weight control, etc. The flowability of low viscosity fluid is used to fill the micro-concavities of the tenon teeth and form mechanical anchoring. However, this design idea of ​​simply pursuing high flowability has an irreconcilable technical contradiction with the complex porous anisotropic biological structure of wood.

[0004] The tracheids and vessels at the finger joint interface of wood form a strong capillary system. Low-viscosity bio-based adhesives will migrate excessively under pressure and siphon action, resulting in poor adhesive at the interface and the formation of hollow bonds. Furthermore, the moisture gradient of the adhesive causes the wood fibers to absorb moisture and expand and dry and shrink, generating microscopic internal stress, which in turn leads to unstable finger joint strength, brittle fracture or cracking. The core technical bottleneck is to solve the problems of poor adhesive at the interface, insufficient load-bearing capacity and curing shrinkage internal stress while ensuring low formaldehyde emissions.

[0005] Therefore, the present invention provides a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, at least one technical problem raised in the background art is solved.

[0007] The technical solution adopted by this invention to solve its technical problem is as follows: A method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive, comprising the preparation of bio-based modified adhesive, pretreatment of wood units, precision finger joint cutting, end-face damping treatment, quantitative adhesive application, stepped butt joint forming, and multi-stage hot-pressing curing process.

[0008] Preferably, in the method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive provided by the present invention, the specific composition and weight ratio of the bio-based modified adhesive are as follows: 150 to 200 parts deionized water; 60 to 80 parts modified soy protein isolate; 15 to 25 parts non-aldehyde multifunctional crosslinking agent; 10 to 15 parts micro / nano cellulose suspension; 3 to 8 parts hydrophobically modified nano silica; 5 to 12 parts epoxidized vegetable oil elastomer; 2 to 4 parts pH adjuster; and 0.1 to 0.3 parts defoamer.

[0009] Preferably, the preparation process of the modified soy protein isolate strictly follows the following steps: First, soy protein isolate powder with a protein content greater than or equal to 90% is selected and slowly added to a reaction vessel containing deionized water. The mixture is stirred for 40 to 60 minutes at a rate of 300 to 500 rpm in a constant temperature water bath at 40 to 45 degrees Celsius to ensure complete dispersion of the protein powder without agglomeration. Then, a pH adjuster is added to adjust the pH of the system to 10.5 to 11.0, utilizing the alkaline environment to induce the unfolding of protein molecular chains and the exposure of polar groups. Based on this, sodium sulfite, at a weight percentage of 3% to 5% of the soy protein isolate, is added as a reducing agent to break the disulfide bonds within the protein molecules, further reducing the dispersion of molecular weight distribution and improving its chemical reactivity. After the modification reaction continues for 2 to 3 hours, the system temperature is lowered to room temperature for later use.

[0010] Preferably, the non-aldehyde multifunctional crosslinking agent is a polyamide polyamine epichlorohydrin resin, with a solid content controlled at 12.5% ​​to 15.0% and a viscosity between 50 mPa·s and 100 mPa·s. The nitrogen-containing heterocyclic butyl groups in the crosslinking agent's molecular structure can covalently bond with the amino and carboxyl groups on the soybean protein molecular chain and the hydroxyl groups on the surface of wood cellulose, forming a highly crosslinked three-dimensional network structure during curing. This fundamentally solves the problem of insufficient water resistance in bio-based adhesives.

[0011] Preferably, the mass percentage concentration of the micro / nanocellulose suspension is 1.5% to 2.5%, and the diameter of the micro / nanocellulose is distributed between 20 nanometers and 80 nanometers, while the length is distributed between 1 micrometer and 10 micrometers. In this invention, the role of micro / nanocellulose is to construct a physically entangled three-dimensional scaffold within the adhesive system through its extremely high radial ratio and abundant surface hydroxyl groups. This scaffold structure endows the adhesive with significant thixotropy, exhibiting good flowability under shear conditions to facilitate coating, while generating extremely high static yield stress when static or under capillary suction. This characteristic is key to suppressing the end-face siphon effect, ensuring that the active adhesive components remain at the finger joint interface rather than penetrating deep into the wood vessels.

[0012] Preferably, the epoxidized vegetable oil elastomer is prepolymerized from epoxidized soybean oil and maleic anhydride under the action of a catalyst. The introduction of this component aims to provide internal stress buffering for the hard and brittle soybean protein solidified layer through its flexible long-chain structure. When the finger joint is subjected to lateral stress or deformed due to changes in temperature and humidity, the epoxidized vegetable oil elastomer can absorb and dissipate energy through the deformation of the molecular chain, thus preventing the early initiation of cracks at the root of the finger joint.

[0013] Preferably, the pretreatment process of the wood unit is the foundation for ensuring the accuracy of subsequent processing. First, select wood boards without knots, decay, or straight grain, and place them in a vacuum drying kiln for drying. The drying process is controlled as follows: the initial temperature is set at 45 to 50 degrees Celsius, and the relative humidity is maintained above 85% for 12 to 24 hours to achieve a preliminary balance of internal moisture in the wood. Subsequently, the temperature is increased to 75 degrees Celsius at a rate of 2 to 3 degrees Celsius per hour, while the humidity is gradually reduced, ultimately controlling the moisture content of the wood precisely within the range of 8% to 10%. After drying, the wood needs to be stored in a constant temperature and humidity curing room for more than 48 hours to eliminate the growth stress and moisture gradient stress generated during the drying process.

[0014] Preferably, the precision finger joint cutting process is performed using a high-precision finger joint milling cutter. The cutting parameters are set as follows: tenon length of 12 mm to 15 mm, tooth root width of 3.5 mm to 4.5 mm, tooth tip clearance controlled at 0.1 mm to 0.2 mm, and finger joint bevel angle set between 12 degrees and 15 degrees. In this process, it is essential to ensure that the milling cutter speed is not less than 6000 rpm and the feed rate is controlled at 8 m to 12 m / min to obtain a cutting surface with extremely high smoothness and complete tracheid opening.

[0015] Preferably, an end-face damping treatment process is added before adhesive application. A low-solids-content sealing liquid composed of modified starch, nano-clay, and water is applied to the finger joint surface via high-frequency ultrasonic atomization. This sealing liquid has extremely low viscosity, which can quickly fill the tracheid openings and form an extremely thin semi-permeable membrane within 5 to 10 seconds. The function of this membrane is to selectively prevent the loss of macromolecular protein components in the adhesive while allowing a small amount of water migration, thereby constructing a damping barrier at the physical level to prevent dehydration and adhesive deficiency.

[0016] Preferably, in the method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive provided by the present invention, the quantitative adhesive application process adopts automatic induction double-sided roller coating technology, and the amount of adhesive applied is strictly controlled to be between 180 grams and 220 grams per square meter. Due to the aforementioned thixotropic properties of the bio-based modified adhesive, its viscosity decreases rapidly under shear force during the roller coating process, allowing it to be evenly spread on the bevel and bottom of the finger joints.

[0017] Preferably, the stepped butt joint forming process is achieved using a hydraulic finger jointing machine. The butt jointing process is divided into three stages: the first stage is the pre-butt jointing stage, where the end pressure is set to 0.5 MPa to 1.0 MPa for 2 seconds, aiming to achieve initial engagement of the tenons and the microscopic redistribution of the adhesive; the second stage is the pressurization stage, where the end pressure increases to 8 MPa to 12 MPa at a rate of 2 MPa per second; the third stage is the pressure holding stage, where high pressure is maintained for 5 to 8 seconds, forcing the adhesive to penetrate the microscopic texture of the finger joint surface, forming a strong mechanical anchor.

[0018] Preferably, the multi-stage hot-pressing curing process is the core step that determines the quality of finger joints. The finger-jointed engineered wood is fed into a multi-layer hot press, and the hot-pressing process curve is as follows: The first stage is a preheating and penetration stage, where the surface pressure of the board is set at 1.5 MPa to 2.0 MPa, the temperature of the hot press plate is controlled at 85°C to 95°C, and the time is 180 to 240 seconds. In this stage, the lower temperature is beneficial for the active components in the adhesive to further wet the wood fibers before the cross-linking reaction occurs, while simultaneously achieving controlled moisture migration. The second stage is a high-temperature curing stage, where the pressure remains constant. The temperature of the hot press plate is raised to 125 to 140 degrees Celsius for a time calculated based on the plate thickness, set at 45 to 60 seconds per millimeter of thickness. During this stage, the non-aldehyde multifunctional cross-linking agent undergoes vigorous chemical cross-linking with soybean protein, forming a high-strength three-dimensional network. The third stage is a cooling, pressure holding, and stress relief stage. Pressure is maintained, the heating system is shut off, and circulating cooling water is introduced to lower the plate temperature to below 60 degrees Celsius within 5 minutes before depressurization. This effectively prevents the sudden vaporization of moisture caused by instantaneous depressurization, thus avoiding root cracking caused by internal stress concentration.

[0019] Preferably, the hydrophobically modified nano-silica plays a role in regulating the moisture migration path in the adhesive system. The nanoparticles, treated with a silane coupling agent, possess some hydrophobic groups. During adhesive curing, these groups tend to align near the interface between the adhesive layer and the wood, forming a microscopic hydrophobic barrier. This barrier slows down the rate of water loss from the adhesive layer into the wood during curing, resulting in a more uniform shrinkage process and significantly reducing residual internal stress caused by uneven moisture gradients.

[0020] Preferably, after hot-press curing, the finger-jointed boards need to be placed in a humidity-balanced chamber for final curing. The environmental conditions are set as follows: temperature 25 degrees Celsius, relative humidity 55% to 65%, and curing time of no less than 7 days. During this process, the residual stress inside the wood is further released, and the moisture content of the board eventually stabilizes at a state suitable for its service environment.

[0021] Preferably, in the method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives provided by this invention, strict requirements are placed on the stability control of the bio-based modified adhesives. The prepared adhesive must be stored in a sealed environment at 20°C to 25°C, and its pot life is controlled within 4 to 6 hours. In continuous production lines, the adhesive delivery pipeline must be equipped with a circulating temperature control system to ensure that the viscosity fluctuation of the adhesive solution upon reaching the roller coating head does not exceed ±5%. This is a prerequisite for ensuring precise quantitative coating accuracy.

[0022] Preferably, the pH adjuster is a buffer system prepared by mixing a 20% sodium hydroxide solution and a 10% ammonia solution at a volume ratio of 1:2. This system provides a stable, strongly alkaline environment, promoting the full dissociation of the secondary structure of protein molecules. Simultaneously, the introduction of ammonia helps to improve the initial tack of the adhesive layer in the early stages of curing and accelerates the cross-linking reaction rate in the subsequent hot-pressing stage.

[0023] Preferably, the defoamer is a polyether-modified silicone defoamer. Since soybean protein readily generates bubbles during stirring, if these bubbles remain in the adhesive layer, they will form micropores after curing, becoming stress concentration points. The defoamer selected in this invention, by reducing surface tension, can quickly eliminate kinetic bubbles generated during preparation and roller coating, ensuring the density of the adhesive film.

[0024] Preferably, during the preparation of the finger-jointed board, the deviation in the width and thickness directions of the board is controlled within ±0.2 mm. Under the action of the lateral pressure mechanism of the finger-jointing machine, the alignment deviation of the board edge shall not exceed 0.1 mm. This high-precision geometric control, combined with the adhesive characteristics of the present invention, ensures the uniform distribution of pressure across the entire finger-jointed cross section, avoiding excessive local pressure leading to over-extrusion of adhesive or insufficient pressure leading to under-penetration.

[0025] The beneficial effects of this invention are as follows:

[0026] 1. The present invention discloses a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives. By constructing a bio-based modified adhesive system with thixotropic properties, and utilizing a three-dimensional physical network formed by micro- and nano-cellulose to impart high static yield stress to the adhesive, it effectively resists the capillary action generated by open tracheids at the finger joint end. Combined with the semi-permeable membrane physical barrier formed by the end-face damping treatment process, this method precisely inhibits the uncontrollable excessive migration of active adhesive components into the wood interior through a dual mechanism of rheological regulation and physical barrier. As a result, the adhesive is enriched at the finger joint interface to form a continuous, complete, and dense polymer film, avoiding the thin, broken, or even void defects in the interfacial adhesive layer caused by dehydration-induced glue deficiency in traditional processes. This lays the microstructural foundation for achieving stable high bonding strength.

[0027] 2. The method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives described in this invention effectively prevents the adhesive from migrating deep along the longitudinal cavities of the wood through the synergistic effect of end-face damping treatment and thixotropic adhesives. This ensures that the main components of the adhesive remain within the effective bonding interface of the tenon joint, allowing the adhesive layer to form sufficient physical anchorage and chemical bonding with the surfaces of the tenons on both sides, rather than forming a hollow bonding morphology far from the interface. Under mechanical loading, the adhesive layer can act as a continuous stress transfer medium, achieving uniform stress distribution on the finger-joint interface. This significantly improves the bending strength, elastic modulus, and overall structural load-bearing capacity of the finger-joint, avoiding premature failure caused by insufficient effective bonding area.

[0028] 3. The present invention discloses a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives. This method introduces hydrophobically modified nano-silica and epoxidized vegetable oil elastomers into the adhesive system. During the adhesive curing process, the hydrophobically modified nano-silica forms a microscopic hydrophobic barrier at the interface, slowing the migration rate of moisture from the adhesive layer to the interior of the wood. This makes the curing shrinkage process of the adhesive layer more uniform and reduces residual internal stress caused by uneven moisture gradients. Simultaneously, the epoxidized vegetable oil elastomer, through its flexible molecular chains forming an interpenetrating structure within the soybean protein curing network, buffers and dissipates stress concentration. Combined with the cooling and pressure-holding stress-relieving stage in the multi-stage hot-pressing curing process, the internal stress of the cured board is fully relaxed, significantly inhibiting the initiation of microcracks at the root of the joints. This improves the water absorption thickness expansion rate of the board under humid and hot conditions and its dimensional stability during long-term service.

[0029] 4. The present invention discloses a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives. The bio-based modified adhesive used in this invention is primarily composed of modified soy protein isolate, combined with a non-aldehyde multifunctional crosslinking agent. After curing, a highly crosslinked three-dimensional network structure is formed. The system does not contain formaldehyde or other harmful substances. Through adhesive component optimization and process synergy, excellent mechanical properties are achieved while eliminating the potential threat of formaldehyde release to the indoor environment at the source. This enables the prepared finger-jointed boards to meet the most stringent global environmental standards, aligning with the industrial orientation of green, low-carbon, and sustainable development.

[0030] 5. The present invention discloses a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives. By regulating the rheological properties of the adhesive through micro- and nano-cellulose, the adhesive combines shear-thinning fluidity during coating with anti-migration properties after application. Combined with a quantitative coating process and adhesive pot life control (including circulating temperature control in the delivery pipeline), the coating accuracy and adhesive layer uniformity are significantly improved. The end-face damping treatment process has a wide operating window and strong applicability. The process parameters for the stepped butt joint forming and multi-stage hot-pressing curing processes can be flexibly adjusted for different wood species. The entire method exhibits excellent process repeatability and production stability, making it suitable for continuous industrial production. It effectively solves the problem of product quality fluctuations caused by the difficulty in controlling the rheological properties of traditional bio-based adhesives.

[0031] 6. The method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives described in this invention forms a complete technology chain from material design to process control. This chain involves protein chemical modification at the molecular level, construction of rheological physical networks at the microscopic level, end-face physical sealing at the mesoscopic level, and precision machining and multi-stage hot pressing process optimization at the macroscopic level. This multi-scale synergistic effect not only solves the common technical bottlenecks of bio-based adhesives in finger-jointing applications, but also achieves a systematic improvement in the integrity and reliability of the adhesive interface, enabling the finger-jointed boards to achieve a highly unified comprehensive performance level in terms of environmental protection, mechanical strength, water resistance, and long-term durability. Attached Figure Description

[0032] The invention will now be further described with reference to the accompanying drawings.

[0033] Figure 1 This is a flowchart of a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives according to the present invention. Detailed Implementation

[0034] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0035] like Figure 1As shown in the embodiment of the present invention, a method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives is centered on the systematic construction of modified soy protein isolate. In specific production practice, firstly, 150 to 200 parts of deionized water are injected into a reactor equipped with a constant temperature jacket and a variable frequency stirring system. The stirrer is then started, and the speed is set between 300 and 500 revolutions per minute. Subsequently, 60 to 80 parts of soy protein isolate powder are slowly and evenly added. To ensure that the protein powder achieves molecular-level dispersion in the aqueous phase, the stirring process is typically carried out at 40 to 45 degrees Celsius. The mixture is kept in a water bath at a certain temperature for 40 to 60 minutes. Within this temperature range, the thermal motion of protein molecules is enhanced, which is conducive to the entry and swelling of water molecules, while avoiding premature denaturation and inactivation of proteins due to excessively high temperatures. After the system presents a uniform, non-agglomerated emulsion state, a pH adjuster is added to precisely adjust the alkalinity of the system to between 10.5 and 11.0. This strongly alkaline environment can effectively break the hydrogen bonds and hydrophobic interactions within the soybean protein molecules, causing the tightly folded globular protein structure to unfold, thereby exposing more reactive groups, such as amino, carboxyl, and hydroxyl groups.

[0036] After the protein molecular chains are fully extended, sodium sulfite, at a weight ratio of 3% to 5% of soy protein isolate, is added as a reducing agent. Under alkaline conditions, sodium sulfite specifically cleaves the disulfide bonds between protein molecular chains, breaking down large molecular weight proteins into polypeptide fragments with lower molecular weight and higher motility. This degradation process significantly reduces the base viscosity of the adhesive, providing greater rheological control in subsequent formulation adjustments. This modification reaction is carried out at a constant temperature for 2 to 3 hours until the rheological curve of the system stabilizes. After modification, the system temperature is adjusted to room temperature using a cooling system, serving as the base resin for subsequent compounding.

[0037] To achieve the transformation of bio-based adhesives from low water resistance to high-performance woodworking adhesives, this invention introduces 15 to 25 parts of a non-aldehyde multifunctional crosslinking agent. In one specific embodiment, the crosslinking agent is selected as polyamide polyamine epichlorohydrin resin, with a solid content strictly controlled between 12.5% ​​and 15.0%. The numerous nitrogen-containing heterocyclic butyl groups carried in its molecular chain exhibit extremely high chemical activity. During hot-press curing, these active groups can cross the protein molecular chain and wood cellulose interface, forming stable covalent bonds with nucleophilic groups on the protein and hydroxyl groups on the wood surface, respectively. This cross-interface covalent crosslinking network not only greatly enhances the cohesive strength of the adhesive layer but also blocks the erosion path of water molecules through chemical bonding.

[0038] To address the common end-face siphon effect in finger bonding of bio-based adhesives, this invention modifies the rheological properties of the system by adding 10 to 15 parts of a micro / nanocellulose suspension. The mass percentage concentration of the micro / nanocellulose suspension is 1.5% to 2.5%, and the micro / nanocellulose has a diameter of 20 to 80 nanometers and a length of 1 to 10 micrometers. This high aspect ratio nanomaterial establishes an extremely sensitive three-dimensional network within the adhesive system through hydrogen bonds and physical entanglement. Specifically, when the adhesive... When the adhesive is subjected to shear force by the roller coating machine, the network structure rapidly and reversibly breaks down, causing a sharp drop in viscosity and ensuring that the adhesive can smoothly spread on the finger joint surface. Once the shear force disappears, the network structure rebuilds within milliseconds, giving the adhesive extremely high static yield stress. This thixotropic property allows the adhesive to stubbornly remain at the tracheid openings on the bevel of the finger joint, preventing it from penetrating too quickly into the wood even under strong capillary suction from the wood vessels. This retains sufficient active components at the interface to form a dense adhesive film.

[0039] Furthermore, to address the common brittleness issue after curing soybean protein, this invention incorporates 5 to 12 parts of epoxidized vegetable oil elastomer into the formulation. This elastic system is prepolymerized from epoxidized soybean oil and maleic anhydride under the action of a specific catalyst, possessing a long-chain flexible molecular structure. During the curing process of the adhesive layer, the epoxidized vegetable oil elastomer forms an interpenetrating or semi-interpenetrating network structure with the soybean protein network, embedding soft segments into the hard and brittle protein backbone. This microscopic phase-separated structure can absorb and dissipate interfacial stress through the micro-movement and deformation of chain segments when the finger joint is subjected to bending loads or drastic fluctuations in ambient temperature and humidity, preventing the initiation and propagation of cracks at the root of the tenon teeth.

[0040] Regarding the functional enhancement of adhesives, this invention also incorporates 3 to 8 parts of hydrophobically modified nano-silica. These nanoparticles are pre-treated with a silane coupling agent, enabling them to possess both inorganic rigidity and good organic affinity. During the dehydration and curing process of the adhesive layer, the nano-silica, due to its surface energy difference, tends to migrate and accumulate in a directional manner at the interface between the adhesive layer and the wood, forming a microscopic hydrophobic barrier. This barrier macroscopically reduces the water absorption and swelling rate at the finger joint interface, and microscopically regulates the kinetic rate of water migration from the adhesive layer to the wood, preventing the adhesive film from cracking due to excessively rapid local dehydration.

[0041] In the method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives provided by this invention, the pretreatment of wood units is a prerequisite for ensuring the overall processing quality. Wood boards without knots, decay, or straight grain are selected and first placed in a high-precision vacuum drying kiln. The drying process employs a stepped heating and humidity control strategy: the initial temperature is set at 45°C to 50°C, and the relative humidity is maintained above 85% by introducing high-pressure steam for 12 to 24 hours. The purpose of this stage is to allow the wood to achieve internal [residual stress] without generating severe stress. After initial equilibration of external moisture, the temperature is slowly increased to 75 degrees Celsius at a rate of 2 to 3 degrees Celsius per hour. Meanwhile, the ambient humidity is gradually reduced based on real-time monitoring data of wood moisture release, ultimately precisely controlling the wood's moisture content within the golden range of 8% to 10%. The dried wood is not immediately used in production but is instead stored in a curing room with constant temperature (20-25 degrees Celsius) and constant humidity (50%-60% RH) for at least 48 hours. This allows the wood's viscoelastic properties to automatically release residual growth stress and moisture distribution stress accumulated during the drying process.

[0042] The precision finger joint cutting process is the physical foundation for determining mechanical strength and adhesive uniformity. High-precision finger joint milling cutters are used, and the cutting parameters reflect a deep consideration for protecting the wood cellular structure. The tenon length is set to 12-15 mm, the tooth root width to 3.5-4.5 mm, the tooth tip clearance to 0.1-0.2 mm, and the finger joint bevel angle to 12-15 degrees. The milling cutter speed is no less than 6000 rpm, coupled with a feed rate of 8-12 meters per minute. The aim is to obtain a mirror-like smooth finger joint bevel through high-speed shearing, ensuring that the wood tracheids are fully opened on the cutting surface without tearing. This high-quality cutting surface provides the optimal physical interface for the subsequent microscopic anchoring of the adhesive.

[0043] A key technological innovation of this invention is the addition of an end-face damping treatment process before adhesive application. A low-solids-content (3%-5%) sealing liquid, composed of modified starch and nano-clay, is uniformly sprayed onto the finger joint surface using a high-frequency ultrasonic atomization device (40kHz-60kHz). Upon contact with the wood surface, this sealing liquid rapidly fills the openings of large vascular bundles due to its strong wettability, forming a film in situ within 5 to 10 seconds. This semi-permeable membrane possesses a unique pore size screening function, effectively preventing the loss of high-molecular-weight proteins and micro / nano-cellulose in the adhesive while allowing moderate penetration of small-molecule water and cross-linking agents. This characteristic of blocking large molecules and allowing small molecules to pass through fundamentally eliminates the phenomenon of weak welds caused by dehydrated, low-adhesion adhesive at the finger joint interface.

[0044] The quantitative adhesive application process employs automated induction double-sided roller coating technology, with the adhesive amount strictly controlled at 180 to 220 grams per square meter. Because the bio-based modified adhesive has already constructed an excellent thixotropic network in the aforementioned formulation, the shear force during the roller coating process reduces its viscosity to a level that completely covers the peaks and troughs of the tenon and finger joint. The subsequent stepped jointing process is completed by a high-precision hydraulic finger jointing machine. The first stage of the jointing is pre-jointing, with the end pressure set at 0.5 to 1.0 MPa for 2 seconds. During this time, the adhesive undergoes microscopic positional adjustment under slight pressure, filling the geometric gaps between the finger joints. The second stage is a rapid pressurization stage, where the end pressure increases from 2 MPa per second to 8 to 12 MPa. This immense pressure forces the thixotropic adhesive to overcome static friction and forcefully embed itself into the micropores of the wood surface. The third stage is a pressure-holding stage, maintained under high pressure for 5 to 8 seconds, allowing the adhesive and wood fibers to form a tight mechanical interlock and eliminating any trace air that may remain between the finger joints.

[0045] The multi-stage hot-pressing curing process is the core technological step that determines the final performance of finger-jointed boards. The process curve designed in this invention consists of three distinct functional stages. The first stage is the preheating and penetration stage, where the surface pressure of the board is set at 1.5 MPa to 2.0 MPa, the temperature of the hot-pressing plate is controlled at 85°C to 95°C, and the time is 180 to 240 seconds. In this stage, the relatively low temperature avoids the instantaneous hardening of protein molecules, maintains the fluidity of the adhesive layer, and allows the active components in the adhesive sufficient time to further diffuse and wet the wood fibers. The second stage is the high-temperature curing stage, where the temperature is rapidly increased to 125°C while maintaining constant pressure. The temperature is increased from 100°C to 140°C, with a time of 45 to 60 seconds per millimeter of board thickness. During this stage, the non-aldehyde multifunctional cross-linking agent undergoes a vigorous chemical cross-linking reaction with soybean protein, rapidly transforming the system from a viscous flow state to a highly cross-linked elastic solid state, forming a high-strength three-dimensional network. The third stage is the cooling and pressure-holding stress-relieving stage, which is crucial to prevent later deformation of the board. While maintaining pressure, the heat source is turned off and circulating water is introduced for rapid cooling until the board temperature drops below 60°C. This cold-pressing and reset process greatly alleviates the internal stress caused by the inconsistency in the thermal expansion coefficients of the wood and the adhesive layer, ensuring the structural integrity of the finger joint.

[0046] After hot pressing, the finger-jointed boards need to be cured in a final balanced humidity environment. The environmental parameters are set at a temperature of 25 degrees Celsius and a relative humidity of 55% to 65%, with a curing period of no less than 7 days. This process allows the thermal stress and moisture gradient generated by hot pressing inside the wood to completely relax and balance, ensuring that the finished boards do not warp or crack during subsequent mechanical processing (such as sanding and grooving).

[0047] In a series of experimental verifications of this invention, in order to scientifically and objectively evaluate the superiority of the method, researchers designed and executed detailed comparative experiments. The following are examples and comparative demonstrations based on actual engineering production data.

[0048] Example 1: This example uses rubberwood as the substrate. The adhesive composition is as follows: 180 parts deionized water, 70 parts modified soy protein isolate, 20 parts polyamide polyamine epichlorohydrin resin, 12 parts micro / nano cellulose suspension (2% by mass), 5 parts hydrophobically modified nano silica, 8 parts epoxidized vegetable oil elastomer, 3 parts pH adjuster (sodium hydroxide and ammonia buffer system), and 0.2 parts polyether modified silicone defoamer. Process parameters: wood dried to 9% moisture content, tenon cutting bevel angle 14 degrees, end-face damping treatment sealing liquid-solid content 4%, adhesive application rate 200 g / m², maximum butt joint pressure 10 MPa, hot-pressing segmented process: first segment 90°C, 210 seconds; second segment 135°C, 50 seconds / mm; third segment cooled to 55°C.

[0049] Comparative Example 1: A conventional low-viscosity soybean protein adhesive was used, without the addition of micro-nano cellulose, hydrophobically modified nano silica and epoxidized vegetable oil elastomer, and without end-face damping treatment. The amount of adhesive and hot-pressing parameters were kept as consistent as possible with Example 1, but a traditional one-stage hot-pressing process (directly heating to 135 degrees Celsius and maintaining it) was used.

[0050] Comparative Example 2: The product was manufactured using traditional urea-formaldehyde resin adhesive (UF) and following conventional finger-jointed board production processes.

[0051] For the finger-jointed boards prepared in the above embodiments and comparative examples, various performance tests were conducted in accordance with national standards and high industry standards. The test results are summarized in the table below:

[0052] Table 1: Summary of performance comparison data of finger-jointed boards based on different preparation methods

[0053] Performance indicators Example 1 Comparative Example 1 (Traditional Bio-glue) Comparative Example 2 (Urea-Formaldehyde Resin) Flexural strength (MOR) / MPa 48.6 32.1 42.5 Elastic modulus (MOE) / GPa 12.8 9.2 11.5 Internal bond strength / MPa 1.35 0.82 1.15 24-hour water absorption thickness expansion rate / % 2.8 8.5 4.2 Formaldehyde release (mg / L) Not detected (<0.05) Not detected (<0.05) 0.45 (E1 level) Cracking rate at finger joints under wet heat cycling / % 1.5 18.6 5.8 Shear failure surface wood coverage rate / % 92 58 85

[0054] As can be clearly observed from the data in Table 1, the method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives provided by this invention exhibits significant technical advantages in key performance indicators.

[0055] In terms of mechanical strength, the bending strength (MOR) of Example 1 reached 48.6 MPa, which is not only far higher than the 32.1 MPa of traditional bio-based adhesives, but also surpasses that of traditional urea-formaldehyde resin adhesives. This improvement is mainly attributed to the thixotropic network constructed by micro-nano cellulose in this invention, which solves the problem of poor adhesive at the interface caused by end-face siphoning, ensuring the continuity of the adhesive layer at the finger joint interface. At the same time, the strong chemical bonds formed between the non-aldehyde crosslinking agent and soybean protein, as well as the covalent bonds with wood fibers, provide extremely high cohesion and adhesion. The wood breakage rate of up to 92% directly proves that the strength of the finger joint interface has exceeded the strength of the wood matrix itself, achieving an ideal bonding effect.

[0056] In terms of dimensional stability and water resistance, Example 1 exhibited a 24-hour thickness swelling rate of only 2.8%, significantly superior to Comparative Examples 1 and 2. This strongly demonstrates that the hydrophobic barrier formed at the interface by the hydrophobic modified nano-silica, and the effective buffering of curing internal stress by the epoxidized vegetable oil elastomer, can greatly resist moisture erosion and internal stress caused by drying shrinkage and wetting expansion. In the stringent damp heat cycling test, the cracking rate of Example 1 was as low as 1.5%, while the traditional bio-based adhesive, due to its brittleness and high water absorption, had a cracking rate as high as 18.6%. The performance difference between the two represents a generational leap.

[0057] In terms of environmental performance, this invention completely eliminates formaldehyde-containing components. The formaldehyde emission level of Example 1 is below the detection limit, which is far superior to the urea-formaldehyde resin product of Comparative Example 2. This allows the finger-jointed board prepared by this invention to enter the international market with extremely high environmental protection requirements without any obstacles, which is in line with the current development trend of green home furnishings.

[0058] The reason this invention achieves the aforementioned superior technical effects lies in its underlying logic: a high degree of coupling between the microscopic molecular design of the adhesive and the macroscopic production process of finger-jointed boards through a systems engineering methodology. This logic is also reflected in the selection of the pH adjuster during the adhesive preparation stage. The preferred pH adjuster is a mixture of 20% sodium hydroxide solution and 10% ammonia water at a volume ratio of 1:2. Sodium hydroxide, as a strong alkali, provides the kinetic environment required for protein denaturation. The introduction of ammonia water allows it to rapidly diffuse into the micropores of the wood surface during the initial stage of hot-pressing curing, playing a role in drainage and pre-wetting. Simultaneously, it evaporates at subsequent high temperatures, carrying away some moisture, which helps the adhesive layer quickly establish initial adhesion and accelerates the overall hot-pressing efficiency.

[0059] In the defoaming process, the polyether-modified silicone defoamer selected in this invention exhibits excellent kinetic defoaming ability. Because soybean protein contains a large number of surface-active groups, it easily introduces microbubbles under high-speed stirring. If these bubbles are not eliminated before coating, they will form micropores with diameters of 10 to 50 micrometers at the finger joint interface, becoming crack sources under stress. This invention, by adding 0.1 to 0.3 parts of defoamer, combined with a vacuum degassing process (maintaining a pressure of -0.08 MPa in the reactor for 15 minutes), ensures the density of the adhesive solution, thereby guaranteeing the integrity of the cured film.

[0060] The method of this invention exhibits exceptional process adaptability to different tree species. For example, when processing poplar wood with its coarse texture and well-developed vessels, increasing the content of micro- and nano-cellulose to the upper limit of 15 parts and appropriately increasing the amount of end-face spraying with ultrasonic atomized sealing liquid can perfectly counteract the strong water absorption of poplar wood, maintaining a bonding strength above 40 MPa. For pine wood, which is rich in oils and difficult to penetrate, a stepped butt joint stage is used to increase the end pressure to 12 MPa, and the time of the first stage of hot pressing is extended to 240 seconds. Relying on the synergistic effect of pressure and time, the active components are forced to break through the oil barrier on the wood surface, achieving deep anchoring.

[0061] In industrial continuous production, this invention also imposes stringent requirements on process control. To ensure the accuracy of quantitative adhesive application, the production line is equipped with a thickness detection system based on a laser displacement sensor. This system monitors the thickness of the sheet material entering the roller coater in real time at a sampling frequency of 100Hz and dynamically adjusts the roller gap. Through this closed-loop control, it ensures that even with a thickness deviation of 0.2 mm, the surface adhesive pressure fluctuation can be controlled within ±3%. In addition, the temperature difference between each layer of hot-pressed plates is controlled within ±2 degrees Celsius. By using data feedback from K-type thermocouples embedded inside the test sample, the hot-pressing process curve is automatically corrected, ensuring that every finger-jointed plate undergoes a completely consistent chemical cross-linking process.

[0062] In the precision tenon cutting process involved in this invention, the maintenance of the milling cutter's cutting edge is also part of the technical system. The surface of the milling cutter is treated with a DLC (diamond-like carbon) film, which has extremely high hardness and an extremely low coefficient of friction. This treatment ensures that when processing hard broadleaf materials, the cutting edge can maintain a surface roughness level of Ra below 0.4 micrometers for a long time, thereby cutting a tenon tooth surface with extremely high smoothness. This microscopic flatness is the basis for achieving uniform wetting of the adhesive and effective exertion of intermolecular forces.

[0063] In summary, this invention, through chemical modification, physical network construction, and rheological behavior control of bio-based adhesives, coupled with precise end-face damping pretreatment, stepped butt jointing, multi-stage hot-pressing curing, and balanced curing processes, successfully constructs a technical system that resolves the inherent contradictions of strength, environmental friendliness, and stability in bio-based finger-jointed boards. This method not only performs excellently in laboratory environments but also demonstrates extremely high stability and reproducibility in industrial mass production. Through precise control of every technical detail, this invention achieves end-to-end quality assurance from raw materials to the final product, providing a definitive technical path for the large-scale application of low-formaldehyde environmentally friendly finger-jointed boards.

[0064] In specific engineering implementations, those skilled in the art can make targeted optimizations within the range of technical parameters provided by this invention, based on the specific density, grain direction, and temperature and humidity conditions of the wood to be treated and the expected usage environment. For example, for finger-jointed boards that need to be used outdoors, the upper limit of the ratio of hydrophobic modified nano-silica and epoxidized vegetable oil elastomer can be appropriately increased to enhance their tolerance to harsh environments. Regardless of the adjustment, as long as the core technical logic described in this invention is followed—suppressing siphoning through micro-nano thixotropic networks, balancing stress through multi-segment hot pressing, and strengthening the interface through end-face damping—significantly superior finger-jointing performance to existing technologies can be obtained. The embodiments of this invention cover all parameter fine-tuning and equipment upgrades based on the above core concepts.

[0065] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing low-formaldehyde finger-jointed boards based on bio-based modified adhesives, characterized in that, Includes the following steps: Step 1: Preparation of bio-based modified adhesive: Using modified soy protein isolate as the main resin, a bio-based adhesive system with thixotropic properties and internal stress buffering function is constructed by compounding non-aldehyde multifunctional crosslinking agent, micro-nano cellulose suspension, hydrophobically modified nano silica and epoxidized vegetable oil elastomer. Step 2, Pre-treatment of wood units: Select wood boards and place them in a drying kiln. Use a stepped heating and humidity control process to dry the wood units so that the moisture content reaches 8% to 10%. Then, carry out constant temperature and humidity curing to eliminate growth stress and moisture gradient stress. Step 3, Precision finger joint cutting: Use a high-precision finger joint milling cutter to cut the end face of the wood unit to process finger joints with specific geometric parameters, and ensure that the tracheid openings on the cut surface are complete. The finger joint includes tooth top and tooth root. Step 4, End-face damping treatment: Before applying adhesive, a sealing liquid is sprayed onto the surface of the tenon using a high-frequency ultrasonic atomizing device, forming a semi-permeable membrane in situ within 5 to 10 seconds to selectively prevent the loss of macromolecular components in the adhesive. Step 5, quantitative adhesive application: Using an automatic induction double-sided roller coating technology, the bio-based modified adhesive prepared in Step 1 is uniformly coated onto the damped tenon surface to form a continuous adhesive film. Step 6, Stepped joint forming: The glued wood units are joined together using a hydraulic finger jointing machine. The jointing process goes through a pre-jointing stage, a pressurizing stage and a pressure holding stage in sequence, so that the finger joints mesh with each other, and the glue is microscopically redistributed and mechanically anchored at the tooth tip gap and meshing interface formed by the interlocking of the finger joints. Step 7, Multi-stage hot pressing curing: The butt-jointed slab is fed into a hot press and hot-pressed in three stages: preheating penetration, high-temperature curing, and cooling and pressure holding to relieve stress, inducing the active components in the adhesive film to form a covalent bond network with the wood fibers.

2. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 1, characterized in that, The composition and weight ratio of the bio-based modified adhesive are as follows: 150-200 parts deionized water; 60-80 parts modified soy protein isolate; 15-25 parts non-aldehyde multifunctional crosslinking agent; 10-15 parts micro-nano cellulose suspension; 3-8 parts hydrophobically modified nano silica. Epoxidized vegetable oil elastomer 5-12 parts; pH adjuster 2-4 parts; defoamer 0.1-0.3 parts; The pH adjuster is a buffer system prepared by mixing a 20% sodium hydroxide solution and a 10% ammonia solution at a volume ratio of 1:2; the defoamer is a polyether-modified silicone defoamer.

3. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 2, characterized in that, The preparation steps of the modified soy protein isolate include: Add soy protein isolate powder with a protein content of ≥90% to deionized water at 40℃-45℃ and stir at a rate of 300-500r / min for 40-60min until no agglomeration occurs. The pH of the system is adjusted to 10.5-11.0 by adding the pH adjuster to induce the protein molecular chains to unfold; sodium sulfite, with a weight ratio of 3%-5% of soy protein isolate, is added as a reducing agent, and the modification reaction is carried out at a constant temperature for 2-3 hours to break the disulfide bonds between protein molecules, thereby reducing the dispersion of molecular weight distribution and improving chemical reactivity. After the reaction is complete, lower the system temperature to room temperature for later use.

4. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 2, characterized in that, The non-aldehyde multifunctional crosslinking agent is a polyamide polyamine epichlorohydrin resin with a solid content of 12.5%-15.0% and a viscosity of 50-100 mPa·s; the mass percentage concentration of the micro-nano cellulose suspension is 1.5%-2.5%, wherein the micro-nano cellulose has a diameter of 20-80 nm and a length of 1-10 μm; The micro- and nano-cellulose structures form a physically entangled three-dimensional scaffold structure within the adhesive system through their radial ratio and surface hydroxyl groups, thereby generating static yield stress in the adhesive and inhibiting the siphon penetration of the adhesive film into the wood tracheids when the adhesive is at rest or under capillary suction.

5. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 2, characterized in that, The epoxidized vegetable oil elastomer is a flexible long-chain prepolymer formed by prepolymerization of epoxidized soybean oil and maleic anhydride under the action of a catalyst; the surface of the hydrophobically modified nano silica is treated with a silane coupling agent. During the curing process of the adhesive layer, the epoxidized vegetable oil elastomer and the modified soy protein isolate form an interpenetrating network structure to buffer the internal stress at the root of the tooth. The hydrophobic modified nano silica accumulates at the interface between the adhesive layer and the wood to form a hydrophobic barrier, thereby slowing down the rate of water migration during the curing process and reducing the residual stress generated by the moisture gradient.

6. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 1, characterized in that, The specific process parameters for the pretreatment of the wood units in step two are as follows: First, initial treatment is carried out in a vacuum drying kiln, with the temperature set at 45℃-50℃ and the relative humidity maintained above 85% for 12-24 hours to achieve initial moisture balance. The temperature was then increased to 75°C at a rate of 2°C-3°C / h, and the ambient humidity was gradually reduced until the moisture content of the wood dropped to 8%-10%. After drying, the wood should be stored in a curing room at a temperature of 20℃-25℃ and a relative humidity of 50%-60% for more than 48 hours. During the drying process, radio frequency drying technology is used as an auxiliary means to induce a uniform temperature rise in the internal moisture of the wood through electromagnetic waves.

7. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 1, characterized in that, The specific parameters for the precision tenon cutting described in step three are as follows: The processed finger tenon has a tooth length of 12-15mm, a tooth base width of 3.5-4.5mm, a tooth tip gap controlled at 0.1-0.2mm, and a finger tenon bevel angle set between 12° and 15°. During cutting, the milling cutter speed should not be less than 6000 r / min, and the feed rate should be 8-12 m / min; The high-precision finger-joint end mill has a diamond-like carbon (DLC) film coated on its blade surface to maintain the sharpness of the cutting edge and ensure that the tracheid wall edges on the cutting surface are free from breakage and tearing.

8. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 1, characterized in that, The process details of the end-face damping treatment described in step four are as follows: The sealing liquid is composed of modified starch, nano-clay and water, with a solid content of 3%-5%. A high-frequency ultrasonic atomizing device with a frequency of 40kHz-60kHz is used to spray the sealing liquid onto the surface of the finger joint. The sealing liquid fills the openings of the tracheids and forms a semi-permeable membrane in situ, which produces a pore size screening effect, preventing the loss of macromolecular protein components in the adhesive into the wood, while allowing moisture and crosslinking agent molecules to penetrate.

9. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 1, characterized in that, The specific stage parameters for the stepped docking forming described in step six are as follows: The first stage is the pre-interlocking stage: the end pressure is set to 0.5-1.0MPa for 2 seconds to achieve the initial engagement of the tenons; The second stage is the pressurization stage: the terminal pressure rises to 8-12 MPa at a rate of 2 MPa / s; The third stage is the pressure holding stage: maintaining a high pressure of 8-12MPa for 5-8 seconds forces the thixotropic adhesive to embed into the micro-texture of the wood surface, forming a mechanically interlocking structure.

10. The method for preparing low-formaldehyde finger-jointed board based on bio-based modified adhesive according to claim 1, characterized in that, The process curve for the multi-stage hot-press curing described in step seven is as follows: The first stage is the preheating and penetration stage: the surface pressure of the board is set to 1.5-2.0MPa, the temperature of the hot press plate is controlled at 85℃-95℃, and the time is 180-240s, to maintain the fluidity of the adhesive layer to fully wet the wood fibers. The second stage is the high-temperature curing stage: the pressure is maintained at 1.5-2.0MPa, the temperature of the hot press plate is raised to 125℃-140℃, and the curing time is set at 45-60s per millimeter of board thickness. The cross-linking agent induces a chemical cross-linking reaction with soybean protein to form a three-dimensional network. The third stage is the cooling, pressure holding, and stress relief stage: maintain a pressure of 1.5-2.0MPa, shut off the heating system and introduce circulating cooling water, and release the pressure after the temperature of the board material drops below 60℃ within 5 minutes to prevent moisture vaporization and tooth root cracking caused by instantaneous pressure release. After hot pressing, the boards need to be cured in a balanced humidity control room at 25℃ and 55%-65% relative humidity for no less than 7 days.

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