Ultrasonic-assisted double crosslinking modified konjac starch formaldehyde-free adhesive and preparation method thereof

Abstract

This invention discloses a formaldehyde-free adhesive for engineered wood products based on modified konjac starch and its preparation method. Using modified konjac starch, epichlorohydrin, polyvinyl alcohol, and polyacrylate as main materials, a chemical cross-linking network is constructed through composite modification to solve the problem of insufficient water resistance in natural starch-based adhesives. The invention achieves the following technical objectives: formaldehyde release ≤0.01mg / m³ (superior to national standard GB / T 18883-2022); 72-hour water immersion strength retention ≥90%, meeting GB / T 9846 Class I water resistance standard; room temperature bonding strength ≥2.0MPa, suitable for mainstream engineered wood product manufacturing processes such as particleboard and plywood.

Description

Technical Field

[0001] This invention relates to an ultrasound-assisted double crosslinked modified konjac starch formaldehyde-free adhesive and its preparation method, belonging to the field of polymer materials. Background Technology

[0002] Currently, adhesives such as urea-formaldehyde resin and phenolic resin, widely used in the engineered wood-based panel industry, have serious formaldehyde release problems. According to data from the State Administration for Quality Supervision, Inspection and Quarantine in 2024, the formaldehyde exceedance rate in my country's engineered wood-based panels still reached 18.7%. Insufficient water resistance is the core technical bottleneck restricting the replacement of traditional adhesives with natural starch-based adhesives. Traditional starch-based adhesives, because their molecules form a physical network only through hydrogen bonds, are prone to swelling and dissociation in aqueous media, leading to a sharp drop in bonding strength. For example, in the boiling water test of GB / T 17657-2013, commercially available ordinary starch adhesives showed a delamination rate exceeding 70%, failing to meet the water resistance requirements for Class I plywood (GB / T 9846-2015).

[0003] Although existing studies have improved performance through methods such as polyvinyl alcohol (PVA) thickening and polyacrylate emulsion to enhance cohesive strength, it is difficult to construct a long-lasting water-resistant network through simple physical blending. Therefore, developing a chemically cross-linked starch-based adhesive that combines environmental friendliness and high water resistance has become a pressing technical challenge in this field. Summary of the Invention

[0004] (a) Purpose of the invention To address the shortcomings of existing technologies, this invention introduces epoxy-based crosslinking agents to construct a chemical crosslinking network, thereby solving the problem of insufficient water resistance in natural starch-based adhesives and achieving the following technical objectives: The formaldehyde release of the adhesive is ≤0.01mg / m³ (superior to the national standard GB / T 18883-2022). The adhesive strength retention rate after immersion in water for 72 hours is ≥90%, meeting the Class I water resistance standard of GB / T 9846-2015; Room temperature bonding strength ≥10MPa, suitable for mainstream engineered wood products such as particleboard and plywood.

[0005] (II) Technical Solution 1. Raw material composition and proportions (parts by weight).

[0006] Components Basic proportion range Optimal formulation (water-resistant reinforced type) Konjac starch 30-50 40 Polyvinyl alcohol (PVA) 5-15 10 Polyacrylate emulsion 10-20 15 epichlorohydrin 0.5-2.0 1.2 (Optimized value for crosslinking density) preservative 0.5-1.5 1.0 viscosity stabilizer 0.5-1.0 0.8 Detailed Implementation

[0007] Example 1 (Basic Water-Resistant Type) Raw material composition (parts by weight): 40 parts konjac starch, 10 parts PVA, 15 parts polyacrylate emulsion, 1.0 part epichlorohydrin, 1.0 part preservative, and 0.8 parts viscosity stabilizer.

[0008] Preparation points: pre-crosslinking temperature 50℃, reaction time 20 minutes; ultrasonic frequency 25kHz, power 150W, mixing time 30 minutes.

[0009] Performance metrics: Bond strength: 10.2 MPa (GB / T 9846-2015 standard ≥8 MPa) Strength retention rate after 72 hours of immersion in water: 90% Pot life: 3.5 hours (at 25℃) Example 2 (High Humidity Environment Special Type) Raw material composition (parts by weight): 35 parts konjac starch, 12 parts PVA, 18 parts polyacrylate emulsion, 1.8 parts epichlorohydrin, 1.2 parts preservative, and 0.9 parts viscosity stabilizer.

[0010] Preparation points: pre-crosslinking temperature 60℃, reaction time 30 minutes; ultrasonic frequency 40kHz, power 300W, mixing time 60 minutes.

[0011] Performance metrics: Bond strength: 11.8 MPa Boiling water test (3h): No delamination of the adhesive layer (GB / T 17657-2013 judgment standard) Degree of crosslinking: 72% as calculated by the area of ​​the -OH peak in infrared spectroscopy. Comparative example (reference group without cross-linking agent) Raw material composition (parts by weight): 40 parts konjac starch, 10 parts PVA, 15 parts polyacrylate emulsion, 1.0 part preservative, and 0.8 parts viscosity stabilizer.

[0012] Performance comparison: Strength retention rate after 72 hours of immersion: 58% (a decrease of 32 percentage points compared to Example 1) Stability at room temperature: stratification occurred after 7 days (Example 1, storage period ≥ 30 days). Innovation points and technological advantages Double cross-linking enhancement technology: Physical gelatinization forms a primary network (starch granules swell into a gel). Chemical crosslinking constructs a secondary network (epoxy groups react with hydroxyl groups to form a dual network structure), which upgrades the adhesive's water resistance rating from Class II (indoor moisture resistance) to Class I (outdoor water resistance).

[0013] Green crosslinking system: Bio-based compatible epichlorohydrin (EU REACH certified) is selected. There are no harmful byproducts after the reaction. The crosslinking efficiency is 3 times higher than that of traditional aldehyde crosslinking agents, and the production cost only increases by 8%-12%.

[0014] Industrial Adaptability Design: Applicable application temperature: 15-40℃ (wide temperature range) Coating amount: 150-200g / m² (compatible with urea-formaldehyde resin adhesive) Curing time: Hot pressing 120-150℃ × 3-5 min (matching existing production lines).

Claims

1. An ultrasonic-assisted double crosslinking modified konjac starch formaldehyde-free adhesive, characterized in that, The adhesive is composed of the following raw materials by weight: konjac starch 30-50 parts, polyvinyl alcohol 5-15 parts, polyacrylate emulsion 10-20 parts, epichlorohydrin 0.5-2.0 parts, water 150-300 parts, preservative 0.3-1.5 parts, and viscosity stabilizer 0.5-1.0 parts; the konjac starch is gelatinized to form a physical primary network after pretreatment, the pretreatment is: selecting konjac starch with a moisture content of 8-12%, adding 0.05% of the mass of the starch β-mannanase, and enzymatically hydrolyzing at a temperature of 50°C and a pH of 6.0 for 30 minutes, then drying to a moisture content of 8-10% after enzyme inactivation treatment; the epichlorohydrin reacts with the hydroxyl groups in the pretreated konjac starch and polyvinyl alcohol molecules to form a chemical secondary network, and the physical primary network and the chemical secondary network are interpenetrated and wound to form a double synergistic crosslinked structure; the adhesive has a room temperature bonding strength ≥10MPa, a bonding strength retention rate ≥90% after 72 hours of water immersion, a formaldehyde release amount ≤0.01mg / m³, a room temperature storage period ≥30 days, and no harmful by-products.

2. The ultrasonic-assisted double-crosslinking modified konjac starch formaldehyde-free adhesive according to claim 1, characterized in that, The preservative is selected from one or more of the following: casiton, BIT, Nisin, and a lemon acid compound system, wherein the Nisin and lemon acid compound system is a natural and environmentally friendly preservative that can effectively prevent mold growth during storage of the adhesive; the viscosity stabilizer is a composite system selected from two or more combinations of the following: hydroxypropyl methylcellulose (HPMC), hydroxyethyl cellulose (HEC), polyacrylic acid sodium (PAAS), montmorillonite (MMT), and bentonite, preferably a composite system of HPMC and montmorillonite, to significantly improve the storage stability of the adhesive.

3. The ultrasonic-assisted double-crosslinking modified konjac starch formaldehyde-free adhesive according to claim 1, characterized in that, The enzyme inactivation treatment uses a heating enzyme inactivation method, with a heating temperature of 85-90°C and a holding time of 15 minutes to ensure complete inactivation of the β-mannanase and avoid subsequent effects on the crosslinking reaction efficiency and storage stability of the adhesive.

4. A method for preparing an ultrasonic-assisted double crosslinking modified konjac starch formaldehyde-free adhesive, characterized in that, The method comprises the following steps: ① Preparation of polyvinyl alcohol solution: Polyvinyl alcohol and a portion of water are mixed at a weight ratio of 1:

9. The degree of polymerization of the polyvinyl alcohol is 1700-1800 and the degree of alcoholysis is 87%-89%. The mixture is heated to 80-85℃ and stirred for 30 minutes to dissolve, ensuring no particle residue. The mixture is then cooled to 55-60℃ and kept warm for later use. The portion of water accounts for 45-55% of the total water volume. ② Construction of pre-crosslinking system: Polyacrylate emulsion, preservative, and viscosity stabilizer are added sequentially to the polyvinyl alcohol solution obtained in step ①. The solid content of the polyacrylate emulsion is 40%-45%, and the glass transition temperature is -10℃ to 5℃. After stirring evenly, epichlorohydrin is added. The epichlorohydrin conforms to the EU REACH certification standard. The mixture is stirred at 200 rpm at 55±5℃ for 25±5 minutes. The epoxy groups at 910 cm⁻¹ are monitored by infrared spectroscopy. The intensity of the characteristic peaks of the group decreases, and the degree of pre-crosslinking is controlled to be 30%-50%; ③ Preparation of starch paste: Konjac starch pretreated according to claim 3 is mixed with the remaining water to prepare a starch paste with a mass concentration of 20%, and gelatinized at 75-80℃ for 10-15 minutes, during which it is continuously stirred at 150rpm to prevent local over-gelatinization, and then cooled to 60℃ for later use; ④ Ultrasonic-assisted composite crosslinking: The starch paste obtained in step ③ is injected into the pre-crosslinked system obtained in step ② at a rate of 5mL / min, and intermittent ultrasonic treatment is applied simultaneously. The ultrasonic frequency is 20-40kHz, the power is 200W, the sound energy density is 0.5-1.0W / cm³, and the treatment time is 45±15 minutes; ⑤ Preparation of finished product: It is naturally cooled to room temperature, allowed to stand for degassing for 30 minutes, and the viscosity at 25℃ is measured to be 8000-12000mPa·s, which is the finished product.

5. The production method according to claim 4, characterized by, The ultrasonic treatment described in step ④ adopts an intermittent process, working for 3 minutes and then stopping for 1 minute, repeating the cycle until the set treatment time is reached, in order to avoid excessive local temperature damaging the cross-linked network structure.

6. The preparation method according to claim 4, characterized in that, The pre-crosslinking degree mentioned in step ② is calculated by the attenuation rate of the characteristic peak intensity of the epoxy group. The attenuation rate = (initial peak intensity - peak intensity after reaction) / initial peak intensity × 100%. The attenuation rate range corresponding to a pre-crosslinking degree of 30%-50% is 28%-48%. This pre-crosslinking degree can balance the initial tack and final crosslinking strength of the adhesive, and is suitable for the glue application and hot pressing process requirements of artificial boards.

7. The ultrasonic-assisted dual-crosslinking modified konjac starch formaldehyde-free adhesive according to claim 1, characterized in that, The polyvinyl alcohol has a degree of polymerization of 1700-1800 and a degree of hydrolysis of 87%-89%, which can improve the initial tack and cohesive strength of the adhesive; the polyacrylate emulsion has a solid content of 40%-45% and a glass transition temperature of -10℃ to 5℃, which can enhance the flexibility and water resistance of the adhesive film and optimize the interfacial bonding force between the adhesive and the engineered wood substrate.