A biodegradable electromagnetic shielding packaging coating and a method for preparing the same

Abstract

The application discloses a kind of biodegradable electromagnetic shielding packaging coatings and preparation method thereof, the coating is by biodegradable water-based polyurethane emulsion, silver-loaded carbon nanotube, MXene nanosheet, C12-C18 long-chain alkyl modified isocyanate crosslinking agent, HDI trimer, polycarbodiimide, trimethylolpropane-triaziridine propionic acid ester, boron-nitrogen coordination small molecule NCBW-X, dopamine hydrochloride, silane coupling agent, functional adjuvant and deionized water balance composition is formed.The application synergistically enhances through "surface hydrophobic barrier+inner layer coordination lock+network densification" triple protection mechanism: long-chain crosslinking agent is turned over to form hydrophobic enrichment layer on the surface of coating during curing process, NCBW-X occupies polyester segment hydrogen bond site by boron-nitrogen coordination bond, PCDI-aziridine double crosslinking system forms interpenetrating network, and is suitable for electromagnetic shielding and waterproof protection of electronic product or jewelry packaging box.

Description

Technical Field

[0001] This invention relates to the field of functional coatings technology, specifically to a biodegradable electromagnetic shielding packaging coating and its preparation method. Background Technology

[0002] With the widespread use of electronic devices, electromagnetic radiation pollution is becoming increasingly serious, not only interfering with electronic signals but also posing a threat to human health. Electromagnetic shielding packaging boxes, as a crucial element in protecting electronic products or jewelry, require coating materials that possess both electromagnetic shielding capabilities and environmental friendliness. However, existing technologies suffer from the following prominent problems: (i) It is difficult to achieve both biodegradability and electromagnetic shielding performance. Most electromagnetic shielding coatings use non-degradable petrochemical-based polymers (such as styrene-acrylic emulsions, waterborne epoxy resins, and hydroxyl acrylic resins), causing a serious environmental burden after disposal. Although biodegradable coatings based on natural bio-based polymers such as starch and cellulose (e.g., CN114479563A) exist, their waterproof performance is severely inadequate—natural polymers are rich in hydrophilic groups (hydroxyl and carbonyl groups), easily swelling or even dissolving upon contact with water, failing to meet the requirements for packaging boxes in humid environments or cold chain transportation. The starch-based electromagnetic shielding coating disclosed in CN114479563A has a contact angle of only 65-75° and a water immersion time of less than 24 hours. The ester bonds (-CO-O-) and terminal hydroxyl groups (-OH) in the molecular chain of biodegradable polyesters (such as PLA / PCL) are intrinsically hydrophilic groups, forming hydrogen bonds with environmental moisture (binding energy 20-30 kJ / mol), resulting in a water absorption rate as high as 8-12%, a mechanical property retention rate (after immersion in water for 72 hours) of less than 60%, and an electromagnetic shielding effectiveness decay of more than 20%.

[0003] (ii) The conductive filler has poor dispersion and unstable shielding effectiveness. Natural polymer systems have limited wetting and dispersing capabilities for conductive fillers (such as carbon nanotubes and MXene nanosheets), leading to easy agglomeration and sedimentation of the fillers, resulting in an incomplete conductive network and large fluctuations in shielding effectiveness. One-dimensional fillers such as carbon nanotubes tend to aggregate in bundles (van der Waals forces). The filler is difficult to disperse evenly. Conventional methods use surfactants or ultrasonic dispersion, but in aqueous systems, the filler re-aggregates rapidly (particle size D90 increases from 500 nm to over 1200 nm within 24 hours). Under wide temperature ranges (-30℃ to 80℃), the conductive network is prone to cracking or displacement due to thermal expansion and contraction, resulting in resistivity changes exceeding 50% and shielding effectiveness decreasing from an initial 42 dB to less than 35 dB.

[0004] (iii) Conflict between the dosage of crosslinking curing agent and VOC regulations To improve crosslinking density and water resistance, water-based coatings require the addition of large amounts of crosslinking agents (such as polyisocyanates). However, when the addition exceeds 8 wt%, the VOC emission exceeds the 50 g / L limit specified in GB 18582-2020. Simultaneously, water-based polyisocyanates undergo partial hydrolysis and deactivation in water-based systems (-NCO groups react with water to form urea bonds and...). The effective crosslinking agent concentration is reduced by 20-30%. CN113502117A proposes adding a water-absorbing agent (molecular sieve) to the formulation to reduce hydrolysis, but this only increases the effective concentration to 85-90%, and introduces inorganic particles that affect the coating transparency. A single crosslinking system is difficult to achieve "VOC compliance (≤50g / L)" and "sufficient crosslinking density (…)". A balance must be struck between the three criteria: "water resistance meets the standard (>72h)".

[0005] (iv) Low surface enrichment efficiency of hydrophobic additives Traditionally, fluorinated silanes or modified cellulose nanocrystals are added as hydrophobic additives, but the surface energy difference between them and waterborne polyurethane emulsions is too small. The insufficient migration driving force leads to surface enrichment concentrations below the critical coverage density. Simultaneously, the unit price of high-performance additives exceeds 300 yuan / kg, and compatibility with the emulsion deteriorates (risk of demulsification) when the addition amount exceeds 5wt%. CN121930713A proposes covalently grafted hydrophobic segments, but this needs to be introduced during the polymerization stage, resulting in complex processes and cost increases exceeding 50%. In physical blending, the concentration difference between the hydrophobic additive in the bulk phase and the surface phase is small (3-5wt% in bulk vs. 8-12wt% on the surface), failing to form a complete hydrophobic barrier, with a contact angle of only 100-110°. Based on these problems, there is an urgent need to develop a biodegradable electromagnetic shielding packaging coating. Summary of the Invention

[0006] This invention aims to solve the technical problems of insufficient waterproof performance, unstable conductive network, and conflict between crosslinking density and VOC regulations in existing biodegradable electromagnetic shielding coatings. It provides a biodegradable electromagnetic shielding packaging coating and its preparation method, which realizes a triple synergistic protection mechanism of surface hydrophobic barrier, inner layer coordination blockade, and network densification. While maintaining biodegradability, it significantly improves waterproof performance, water immersion resistance time, and electromagnetic shielding stability.

[0007] The objective of this invention is achieved through the following technical solution: a biodegradable electromagnetic shielding packaging coating, comprising the following raw materials in parts by weight: 60-80 parts of biodegradable waterborne polyurethane emulsion, 2-4 parts of silver-loaded carbon nanotubes, 1-3 parts of MXene nanosheets, 2-6 parts of C12-C18 long-chain alkyl-modified isocyanate crosslinking agent, 1-3 parts of HDI trimer crosslinking agent, 4-6 parts of polycarbodiimide, 2-3 parts of trimethylolpropane-triazidimide propionate, 2-5 parts of boron-nitrogen coordinated small molecule NCBW-X, 0.3-0.8 parts of dopamine hydrochloride, 0.3-0.8 parts of silane coupling agent, 4-8 parts of functional additives, and 2-10 parts of deionized water.

[0008] Preferably, the functional additives include 0.5-1.5 parts of dispersant, 0.5-1.0 parts of leveling agent, 0.5-1.5 parts of thickener, 0.2-0.5 parts of defoamer, 1-3 parts of film-forming aid, 0.1-0.3 parts of pH buffer and 0.3-0.8 parts of ultraviolet absorber.

[0009] Preferably, the biodegradable aqueous polyurethane emulsion is an aqueous polyurethane emulsion copolymerized from polylactide diol and polycaprolactone diol, with a solid content of 30-35% and a carboxyl content of 2.5-3.5 mmol / g; the silver-loaded carbon nanotubes have a silver loading of 10-15% and an aspect ratio >200. Preferably, the MXene nanosheets are Ti3C2T. x The nanosheets have an interlayer spacing of 1.5-1.8 nm; the C12-C18 long-chain alkyl-modified isocyanate crosslinking agent has an NCO content of 12-15% and a long-chain content of 30-40 wt%; the polycarbodiimide has an amine value of 2.8-3.2 mmol / g; the boron-nitrogen coordinated small molecule NCBW-X contains a rigid eight-membered ring structure and has a molecular weight of 300-500.

[0010] Preferably, the mass ratio of the silver-loaded carbon nanotubes to MXene nanosheets is 1.5-2:1.

[0011] Preferably, the mass ratio of the polycarbodiimide to trimethylolpropane-triazididine propionate is 1.5-2.5:1.

[0012] Preferably, the C12-C18 long-chain alkyl modified isocyanate crosslinking agent is prepared by reacting hexamethylene diisocyanate trimer with a mixture of C12-C18 fatty alcohols, wherein the mass ratio of lauryl alcohol, myristol and stearyl alcohol in the C12-C18 fatty alcohol mixture is 2:3:1.

[0013] Preferably, the particle size of the biodegradable aqueous polyurethane emulsion is ≤100 nm.

[0014] Preferably, the silane coupling agent is KH-570 or KH-550.

[0015] This invention also provides a method for preparing a biodegradable electromagnetic shielding packaging coating, comprising the following steps: S1. Synthesis of biodegradable waterborne polyurethane emulsion: Polylactide diol and polycaprolactone diol were mixed at a mass ratio of 1:1 and dehydrated at 120-125℃ and vacuum degree of 0.1 MPa for 2-3 h; isophorone diisocyanate and acetone were added, with an NCO / OH molar ratio of 1.5-1.8, and prepolymerized at 80-85℃ for 3 h; dimethylolpropionic acid and dibutyltin dilaurate were added, and reacted at 80-85℃ for 2 h; 1,4-butanediol was added, and chain extension was performed at 80℃ for 1 h; methanol was added, and end-capping was performed at 60℃ for 1 h; the mixture was cooled to room temperature, neutralized with triethylamine for 10 min, and emulsified with deionized water to obtain a biodegradable waterborne polyurethane emulsion with a solid content of 30-35%. S2, Matrix Coordination Modification: The biodegradable waterborne polyurethane emulsion obtained in step S1 was kept at a temperature of 60-80℃. Boron-nitrogen coordination molecule NCBW-X was added, and the mixture was stirred for 30 minutes. A sodium citrate buffer system was added to adjust the pH to [value missing]. ; S3. Dispersion and surface modification of conductive fillers: Silver-loaded carbon nanotubes and dispersant were added to deionized water and ultrasonically dispersed for 30 min, followed by high-shear dispersion until D90 < 800 nm; MXene nanosheets were added and sheared at 3000-5000 rpm for 30 min; dopamine hydrochloride was added to Tris buffer at pH 8.5 and stirred at room temperature for 2 h, then washed and set aside. S4. Construction of crosslinking agent system: Hexamethylene diisocyanate trimer and a mixture of C12-C18 fatty alcohols were reacted at 80℃ for 4h to prepare a C12-C18 long-chain alkyl modified isocyanate crosslinking agent with NCO content of 12-15% and long chain content of 30-40wt%; polycarbodiimide and trimethylolpropane-triazididine propionate were reacted at 60℃ at a mass ratio of 1.5-2.5:1 for 1h, and then dried and pulverized. S5. Final Mixing and Blending: Mix the coordination-modified polyurethane emulsion of S2 with the conductive filler dispersion of S3, and stir at 800-1200 rpm for 20 min; add the long-chain modified crosslinking agent, HDI trimer crosslinking agent, and PCDI-aziridine compound crosslinking agent prepared in S4 in sequence, and stir until uniform; add the silane coupling agent, leveling agent, thickener, defoamer, film-forming aid, and UV absorber in sequence, and stir at 500-800 rpm for 10 min; degas under vacuum, and adjust to a Forte 4 cup viscosity of 20-30s (25℃) with deionized water, and a solid content of 20-25%; S6. Coating and Curing: The coating prepared in S5 is applied to the substrate and cured in three stages: pre-drying at 60℃ for 3 min, cross-linking reaction at 90℃ for 4 min, and curing at 100℃ for 2 min; then curing at 25℃ and relative humidity of 50-60% for 24 h; and then curing at 50℃ for 2 h.

[0016] Preferably, the biodegradable aqueous polyurethane emulsion is made from the following raw materials in the indicated weight percentages: Poly(lactic acid) diol 29.36%, polycaprolactone diol 29.36%, isophorone diisocyanate 22.95-23.21%, dimethylolpropionic acid 6.03-6.12%, dibutyltin dilaurate 0.132-0.144%, 1,4-butanediol 0.556-0.675%, triethylamine 4.55-4.78%, acetone 6.12-6.18%, methanol 0.665%.

[0017] Preferably, the method for preparing the biodegradable aqueous polyurethane emulsion includes: 1) Dehydrate polylactide diol and polycaprolactone diol at 120°C and 0.1 MPa for 2 h; 2) Add isophorone diisocyanate and acetone, and prepolymerize at 80°C for 3 h; 3) Add dimethylolpropionic acid and dibutyltin dilaurate, and continue the reaction at 80°C for 2 h; 4) Add 1,4-butanediol and perform chain extension reaction at 80℃ for 1 h; 5) Add methanol and cap the reaction at 60°C for 1 hour; 6) Cool to room temperature, add triethylamine to neutralize for 10 min, and obtain a biodegradable waterborne polyurethane emulsion with a solid content of 30-35%.

[0018] Preferably, the silver-loaded carbon nanotubes are prepared by the following method: Step 1: Incubate multi-walled carbon nanotubes at 70°C with a mixed acid ( Acidification treatment for 40 min introduces carboxyl groups; Step 2, The solution was mixed with sodium alginate solution and stirred at 80°C for 15 min. Glucose solution was added and reacted at 80°C for 8 h to obtain nano silver sol. Step 3: Add the acidified carbon nanotubes to the nano-silver sol and stir for 2 hours to allow Ag to react. + Adsorption; Step 4: Centrifuge, wash with deionized water, dry at 60°C to constant weight, and grind to obtain powdered silver-loaded carbon nanotubes.

[0019] Preferably, the mass ratio of carbon nanotubes to silver nitrate in the silver-loaded carbon nanotubes is 2:2.5, and the silver loading is 10-15%.

[0020] 1. Surface flipping mechanism of long-chain hydrophobic groups In the 90℃ cross-linking reaction zone, isocyanate undergoes a cross-linking reaction with hydroxyl groups, resulting in a 2-3% volume shrinkage. According to rubber elasticity theory, the shrinkage stress... (Macroscopic stress), considering a stress relaxation factor of 0.15, effective stress This stress field acts on long-chain hydrophobic groups (a single long chain is subjected to force). ), driving it to overcome van der Waals forces ( The gas-liquid interface flips to a low-constraint state, forming a hydrophobic long-chain enriched layer with a thickness of 10-20 nm and a local concentration of over 15 wt% on the surface.

[0021] 2. Boron-nitrogen coordination bond blocking mechanism The NCBW-X molecule contains a rigid eight-membered ring structure (BN eight-membered ring), in which the boron atom acts as a Lewis acid center, forming a BO coordination bond (binding energy 80-120 kJ / mol) with the carbonyl oxygen (Lewis base) in the PLA / PCL polyester segment, while the nitrogen atom forms an NH coordination bond with the hydroxyl group. Each NCBW-X molecule can occupy 2-3 hydrogen bond sites that would normally be bound to water molecules (total hydrogen bond sites approximately 6-8 mmol / g), achieving an occupancy rate of 70-85%. The rigid eight-membered ring (approximately 0.6 nm in diameter) forms a steric barrier around the coordination sites, effectively preventing water molecules (0.28 nm in diameter) from approaching, reducing the water absorption rate from 8-12% to 2-4% (a reduction of 75%).

[0022] 3. Formation mechanism of PCDI-aziridine interpenetrating network The carbodiimide group (-N=C=N-) in PCDI reacts with the carboxyl group of the polyurethane soft segment to form an N-acylurea bond. The hydrolysis constant of this bond is... It exhibits stability in water exceeding 6 months. The aziridine three-membered ring undergoes ring-opening at room temperature, reacting with hydroxyl / amino groups to form covalent bonds. Curing degree is ≥85% after 24 hours, and it does not release small molecule byproducts (avoiding...). (Bubble defects). Two crosslinking mechanisms spatially form a complementary "rigid-flexible" interpenetrating network structure: PCDI crosslinking nodes (rigid, bond energy 350 kJ / mol) restrict long-range motion, while aziridine crosslinking points (flexible, bond energy 280 kJ / mol) fill network voids, resulting in a total crosslinking node density from... Upgraded to (Increased by 52%), the moisture diffusion coefficient D from Down to (Reduced by 40%).

[0023] 4. Triple protection synergy mechanism This invention is the first in China to propose a triple protection mechanism consisting of "surface hydrophobic barrier + inner coordination blockade + network densification": The first line of defense (surface hydrophobic barrier): C12-C18 long chains are driven to flip to the surface through cross-linking shrinkage stress, forming a 10-20nm thick hydrophobic enrichment layer (local concentration >15wt%) with a contact angle of 120-125°, and the water contact interface is repelled. The second line of defense (inner layer coordination blockade): The boron-nitrogen coordinated small molecule NCBW-X competes to occupy the carbonyl / hydroxyl sites of the polyester chain segment through BO coordination bonds (occupancy rate 70-85%), and the rigid eight-membered ring forms a steric barrier to block the hydrogen bonding channels between residual moisture and the matrix. The third line of defense (network densification): PCDI-aziridine dual crosslinking agent forms an interpenetrating network structure, restricting the movement of molecular chain segments and making the water diffusion path tortuous.

[0024] The triple mechanism forms a gradient protection in space (top-middle-inner layer), achieves full life cycle protection in time (rapid surface sealing 8-10 min + medium-speed cross-linking curing 24 h + long-term coordination stability for more than 6 months), and constructs multiple barriers chemically (physical hydrophobicity + chemical blockade + dense network), ultimately achieving a breakthrough effect of reducing water absorption rate by 75% and increasing water immersion time by 5 times.

[0025] The beneficial effects of this invention are as follows: (1) Synergistic effect of triple protection mechanism to overcome the limitations of single protection: The present invention achieves a breakthrough effect of contact angle of 120-125°, water absorption rate reduced from 8.12% to 24%, and water immersion time extended from 48 hours to more than 240 hours through the triple synergy of surface hydrophobic barrier, inner coordination blockade and network densification. None of the three mechanisms can be missing: without surface hydrophobicity, water will directly contact the matrix; without inner coordination, hydrogen bond water absorption channels will be unobstructed; without network densification, diffusion path will be loose.

[0026] (2) Overcoming the traditional limitations of hydrophobic additive migration and achieving efficient surface enrichment: Traditional methods rely on the surface energy difference between hydrophobic additives and the matrix to generate migration driving force, but the surface energy of waterborne polyurethane is about 30 mN / m, and that of fluorosilanes is about 18 mN / m. Only 12 mN / m, migration driving force (Insufficient to overcome the interfacial tension of emulsion particles), resulting in low surface enrichment concentration (5 wt% in bulk phase vs. 812 wt% on surface). This invention utilizes the volume shrinkage generated by the crosslinking reaction to produce effective stress. The driving force acting on the end of a long chain This method achieves a surface hydrophobic group concentration exceeding 15wt%. It eliminates the need for expensive hydrophobic additives, carries no risk of demulsification, and completes surface enrichment in one step.

[0027] (3) Resolving the conflict between crosslinking density and VOC regulations: This invention cleverly balances performance and compliance through the PCDI aziridine dual crosslinking agent system: the total amount of crosslinking agent is 78wt%, the effective crosslinking sites are equivalent to traditional 1012wt% isocyanate, the water immersion resistance is increased from 48 hours to 240 hours, and the tensile strength is increased by 30%.

[0028] (4) Wide temperature range stability of conductive network: The conductive network is stable in the temperature range of 30℃ to 80℃ through "one-dimensional and two-dimensional hybridization + interpenetrating network locking". The resistivity change is <10% after 100 temperature cycles, the shielding effectiveness retention rate is >95% at 30℃, and the shielding effectiveness decay is <5% after 240 hours of immersion in water.

[0029] (5) Maintaining biodegradability: The degradation half-life of PLA / PCL-based waterborne polyurethane is 1824 months (composting conditions). The degradation half-life of urethane bonds in long-chain modified crosslinking agents is about 20 months. Boron-nitrogen coordination bonds can be hydrolyzed under the action of microbial enzymes. Although the N-acylurea bonds of PCDI are water-resistant, they can be degraded by esterases. The overall degradation rate is synchronous. Under composting conditions, the degradation rate is >92% after 6 months (meeting the ≥90% standard). Detailed Implementation

[0030] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments. The content mentioned in the embodiments is not intended to limit the present invention.

[0031] Example 1 A biodegradable electromagnetic shielding packaging coating, characterized in that it comprises the following raw materials in parts by weight: A method for preparing a biodegradable electromagnetic shielding packaging coating, characterized by comprising the following steps: S1. Synthesis of biodegradable waterborne polyurethane emulsion: In a 500 mL three-necked flask equipped with a mechanical stirrer, thermometer, and condenser, 20 g of PLA-diol (molecular weight 2000, hydroxyl value 56 mgKOH / g) and 20 g of PCL-diol (molecular weight 2000, hydroxyl value 56 mgKOH / g) were added; the mixture was dehydrated at 120 °C under a vacuum of 0.1 MPa for 2 h; the moisture content was measured using a Karl Fischer moisture analyzer and found to be <0.05%. Add 6.67 g IPDI (NCO / OH molar ratio = 1.65:1) and an appropriate amount of acetone, and add 0.027 g dibutyltin dilaurate (0.1 wt% of the total reactants) as a catalyst; prepolymerize at 80 °C for 3 h; monitor the reaction by FTIR. The intensity of the -NCO characteristic peak is stable; Add 1.5g DMPA and react at 80℃ for 2 h; add 0.12g 1,4-butanediol and extend the chain at 80℃ for 1 h; add methanol (about 0.14g) and end-cap at 60℃ for 1 h; cool to room temperature and add 1.2g triethylamine to neutralize for 10 min; under vigorous stirring (1000rpm), slowly add deionized water to emulsify and obtain a biodegradable waterborne polyurethane emulsion with a solid content of 35%.

[0032] S2, matrix coordination modification: The temperature of the biodegradable waterborne polyurethane emulsion obtained in step one is controlled at 70°C. (Using a PID temperature controller); add 0.9g NCBW-X (pre-dissolved in 3g DMF, concentration 30wt%); stir for 30 minutes (300rpm). Take a sample for FTIR analysis, and the following characteristic peaks appeared: (BO coordinate bond stretching vibration) (BN coordinate bond vibration) The intensity of the free -OH peak decreased by 75% (indicating that hydrogen bond sites were occupied). Add 0.2g of sodium citrate (pH buffer) to adjust the pH to... .

[0033] S3, Conductive filler dispersion and dopamine surface modification: Add 50g of deionized water and 1.0g of BYK-190 (dispersant) to a 250mL beaker; add 3.0g of Ag@CNT and stir for 5 minutes to pre-wet; use an ultrasonic disperser (frequency 20 kHz, power 400W) to intermittently sonicate for 30 minutes (5 seconds on, 2 seconds off cycle); cool to room temperature; measure D90 < 800nm ​​using a laser particle size analyzer; 2.0 g of MXene nanosheets were added to the Ag@CNT dispersion above; a high-shear disperser was used to shear at 5000 rpm for 30 minutes; XRD diffraction peak analysis showed that (002) the crystal plane peak position shifted from 9.8° to 6.5°, and the interlayer spacing increased from 0.9 nm to 1.36 nm (indicating successful exfoliation); Take 50 mL of Tris buffer (pH 8.5, 50 mM), add 0.5 g of dopamine hydrochloride, and stir until completely dissolved; add the conductive filler dispersion and stir at room temperature for 2 hours. FTIR analysis: 1510 cm⁻¹ -1(Catechol C=C skeletal vibration) and The (phenolic hydroxyl -OH) peak appears; the Zeta potential increases from -35mV to -15mV.

[0034] S4. Construction of the crosslinking agent system: (1) Synthesis of long-chain modified crosslinking agent: In a 250 mL three-necked flask, 10 g of HDI trimer (NCO content 21.8%) was added; 4.5 g of a C12-C18 fatty alcohol mixture (1.8 g lauryl alcohol + 2.7 g myristol + 0.9 g stearyl alcohol) was dissolved in 10 g of anhydrous toluene; under nitrogen protection, 0.007 g of dibutyltin dilaurate catalyst was added dropwise (dropping rate 0.8 mL / min); the temperature was raised to 80°C and the reaction was stirred for 4 hours. FTIR monitoring: The NCO peak intensity decreased to 55-60% of its initial value; the NCO content, as determined by titration, decreased to 13.5%. Verification showed a long-chain content of 35 wt%; (2) PCDI-aziridine compound: Add 5.0 g PCDI (amine value 3.0 mmol / g) to a 100 mL Erlenmeyer flask; add 2.5 g trimethylolpropane-aziridine propionate (pre-dissolved in 5 g NMP, concentration 20 wt%); stir the reaction in a 60°C water bath for 1 hour (200 rpm); take FTIR samples every 20 minutes for analysis. The intensity of the -N=C=N- characteristic peak remained stable; after the reaction was completed, the mixture was transferred to an evaporating dish and dried under vacuum at 60°C (-0.08 MPa) for 2 hours to remove the NMP solvent; after cooling to room temperature, it was ground through a 60-mesh sieve.

[0035] S5, Final Mixing and Blending Add the coordination-modified polyurethane emulsion (containing 70g of emulsion and NCBW-X) obtained in step 2 to a 500mL beaker; maintain the temperature at 25°C and the stirring speed at 500 rpm. Slowly add the conductive filler dispersion obtained in step 3, increase the stirring speed to 1000 rpm, and stir at medium speed for 20 minutes. Add the following ingredients in sequence: 3.5g of the long-chain modified crosslinking agent obtained in step four, 1.5g of HDI trimer, and 7.5g of the PCDI-aziridine composite crosslinking agent obtained in step four (added in three portions, 2.5g each time, with a 5-minute interval), and stir until homogeneous. Add the following ingredients in sequence: 0.5g KH-570 (pre-diluted with 2g ethanol), 0.8g BYK-333 (leveling agent), 1.0g thickener (pre-diluted with 5g deionized water to a 10% slurry), 0.3g BYK-024 (defoamer), 2.0g dipropylene glycol butyl ether (film-forming aid), and 0.5g UV-531 (ultraviolet absorber, pre-dissolved in 1g dipropylene glycol butyl ether); stir at 500-800 rpm for 10 minutes until homogeneous. Transfer the mixture to a vacuum degassing tank, slowly evacuate to -0.09 MPa, and allow it to stand at room temperature for 20 minutes to degas. Adjust the viscosity to 24 seconds (25°C) using deionized water, with a solid content of 23.2%.

[0036] S6. Coating and Curing: Using wooden boards (quantity 250 g / m²) 2 The substrate (0.8mm thick) was used as the base material and subjected to corona treatment (0.5 W / cm²). 2 The surface tension was increased to 42 dyn / cm by applying a wire bar coater (#50 size) at a speed of 30 m / min. 2 .

[0037] Three-stage curing: Pre-dry at 60°C for 3 minutes; cross-linking reaction at 90°C for 4 minutes (a non-contact infrared thermometer is installed inside the oven to monitor the coating surface temperature in real time, and the temperature deviation is monitored). ); cure at 100°C for 2 minutes.

[0038] Room temperature ripening: Relative humidity 50-60%, 24 hours. Post-curing: 50°C, 2 hours.

[0039] Performance testing Example 2 A biodegradable electromagnetic shielding packaging coating, characterized in that it comprises the following raw materials in parts by weight: Preparation method: Same as in Example 1.

[0040] Performance test results: Example 3 A biodegradable electromagnetic shielding packaging coating, characterized in that it comprises the following raw materials in parts by weight: Preparation method: Same as in Example 1.

[0041] Performance test results: Example 4 A biodegradable electromagnetic shielding packaging coating, characterized in that it comprises the following raw materials in parts by weight: Preparation method: Same as in Example 1.

[0042] Performance test results: Comparative Example 1 The difference between this comparative example and Example 1 above is that: no C12C18 long-chain modified crosslinking agent is added in this comparative example (replaced by an equal amount of HDI trimer), no NCBWX coordination modifier is added, and no PCDI aziridine double crosslinking system is used (only 5 wt% of waterborne polyisocyanate is used). Preparation method: The matrix and conductive filler were prepared according to the steps of Example 1, but the PCDI aziridine compounding in steps two (matrix coordination modification) and four (construction of crosslinking agent system) was omitted. Only 5 wt% of aqueous polyisocyanate (Bayhydur 3100) was added in step five. The rest of the contents of this comparative example are the same as those of Example 1, and will not be repeated here.

[0043] Performance test results: Conclusion: Lacking a triple protection mechanism, although the initial shielding effectiveness meets the standards, the waterproof performance and long-term stability are seriously insufficient, failing to meet the needs of practical applications.

[0044] Comparative Example 2 The difference between this comparative example and Example 1 above is that: 3.5 parts of a C12C18 long-chain modified crosslinking agent are added to this comparative example, NCBWX is not added, PCDI aziridine double crosslinking is not used, and only a conventional crosslinking agent is used. The rest of the contents of this comparative example are the same as those of Example 1, and will not be repeated here.

[0045] Performance test results: Conclusion: The surface hydrophobic layer effectively improves the contact angle, but without the inner layer coordination blockade and network densification, water can still penetrate through the substrate, resulting in insufficient long-term waterproofing.

[0046] Comparative Example 3 The difference between this comparative example and Example 1 above is that 3.0 parts of NCBW-X are added, no long-chain modified crosslinking agent is added, and PCDI-aziridine dual crosslinking is not used. The rest of the contents of this comparative example are the same as in Example 1, and will not be repeated here.

[0047] Performance test results: Conclusion: Coordination bonds effectively reduce water absorption, but the surface is still easily wetted by water, and the lack of the first line of defense limits the overall waterproof performance.

[0048] Comparative Example 4 The difference between this comparative example and Example 1 above is that this comparative example uses a PCDI aziridine dual crosslinking agent system, without adding a long-chain modified crosslinking agent or NCBWX. The rest of the contents of this comparative example are the same as in Example 1, and will not be repeated here.

[0049] Performance test results Conclusion: Interpenetrating networks improve crosslinking density and water resistance, but do not solve the problems of surface wetting and internal hydrogen bond water absorption, resulting in limited performance improvement.

[0050] Comparative summary Comparative conclusion: 1. No single solution can meet all performance requirements. Although Comparative Example 24 has some improvements, it has obvious shortcomings. 2. Example 1 achieves comprehensive performance optimization through a triple protection mechanism, with a comprehensive score of 95 points, which is significantly better than the comparative example. 3. All three mechanisms are indispensable: without a hydrophobic surface, water will directly contact the matrix; without inner coordination, hydrogen bond water absorption channels will be unobstructed; without network densification, diffusion paths will be loose.

[0051] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present invention are within the protection scope of the present invention.

Claims

1. A biodegradable electromagnetic shielding packaging coating characterized by: The raw materials include the following parts by weight: 60-80 parts of biodegradable waterborne polyurethane emulsion, 2-4 parts of silver-loaded carbon nanotubes, 1-3 parts of MXene nanosheets, 2-6 parts of C12-C18 long-chain alkyl-modified isocyanate crosslinking agent, 1-3 parts of HDI trimer crosslinking agent, 4-6 parts of polycarbodiimide, 2-3 parts of trimethylolpropane-triazidimide propionate, 2-5 parts of boron-nitrogen coordinated small molecule NCBW-X, 0.3-0.8 parts of dopamine hydrochloride, 0.3-0.8 parts of silane coupling agent, 4-8 parts of functional additives, and 2-10 parts of deionized water.

2. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The functional additives include 0.5-1.5 parts of dispersant, 0.5-1.0 parts of leveling agent, 0.5-1.5 parts of thickener, 0.2-0.5 parts of defoamer, 1-3 parts of film-forming aid, 0.1-0.3 parts of pH buffer and 0.3-0.8 parts of ultraviolet absorber.

3. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The biodegradable waterborne polyurethane emulsion is a waterborne polyurethane emulsion copolymerized from polylactide diol and polycaprolactone diol, with a solid content of 30-35% and a carboxyl content of 2.5-3.5 mmol / g; the silver-loaded carbon nanotubes have a silver loading of 10-15% and an aspect ratio >200.

4. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The MXene nanosheets are The nanosheets have an interlayer spacing of 1.5-1.8 nm; the C12-C18 long-chain alkyl modified isocyanate crosslinking agent has an NCO content of 12-15% and a long-chain content of 30-40 wt%; the polycarbodiimide has an amine value of 2.8-3.2 mmol / g.

5. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The mass ratio of the silver-loaded carbon nanotubes to MXene nanosheets is 1.5-2:

1.

6. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The mass ratio of the polycarbodiimide to trimethylolpropane-triazididine propionate is 1.5-2.5:

1.

7. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The C12-C18 long-chain alkyl modified isocyanate crosslinking agent is prepared by reacting hexamethylene diisocyanate trimer with a mixture of C12-C18 fatty alcohols, wherein the mass ratio of lauryl alcohol, myristol and stearyl alcohol in the C12-C18 fatty alcohol mixture is 2:3:

1.

8. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The particle size of the biodegradable waterborne polyurethane emulsion is ≤100 nm.

9. The biodegradable electromagnetic shielding packaging coating according to claim 1, characterized in that: The silane coupling agent is KH-570 or KH-550.

10. A method for preparing a biodegradable electromagnetic shielding packaging coating as described in any one of claims 2-9, characterized in that: Includes the following steps: S1. Synthesis of biodegradable waterborne polyurethane emulsion: Polylactide diol and polycaprolactone diol were mixed at a mass ratio of 1:1 and dehydrated at 120-125℃ and vacuum degree of 0.1 MPa for 2-3 h; isophorone diisocyanate and acetone were added, with an NCO / OH molar ratio of 1.5-1.8, and prepolymerized at 80-85℃ for 3 h; dimethylolpropionic acid and dibutyltin dilaurate were added, and reacted at 80-85℃ for 2 h; 1,4-butanediol was added, and chain extension was performed at 80℃ for 1 h; methanol was added, and end-capping was performed at 60℃ for 1 h; the mixture was cooled to room temperature, neutralized with triethylamine for 10 min, and emulsified with deionized water to obtain a biodegradable waterborne polyurethane emulsion with a solid content of 30-35%. S2, Matrix Coordination Modification: The biodegradable waterborne polyurethane emulsion obtained in step S1 was kept at a temperature of 60-80℃. Boron-nitrogen coordination molecule NCBW-X was added, and the mixture was stirred for 30 minutes. A sodium citrate buffer system was added to adjust the pH to [value missing]. ; S3. Dispersion and surface modification of conductive fillers: Silver-loaded carbon nanotubes and dispersant were added to deionized water and ultrasonically dispersed for 30 min, followed by high-shear dispersion until D90 < 800 nm; MXene nanosheets were added and sheared at 3000-5000 rpm for 30 min; dopamine hydrochloride was added to Tris buffer at pH 8.5 and stirred at room temperature for 2 h, then washed and set aside. S4. Construction of crosslinking agent system: Hexamethylene diisocyanate trimer and a mixture of C12-C18 fatty alcohols were reacted at 80℃ for 4h to prepare a C12-C18 long-chain alkyl modified isocyanate crosslinking agent with NCO content of 12-15% and long chain content of 30-40 wt%; polycarbodiimide and trimethylolpropane-triazididine propionate were reacted at 60℃ at a mass ratio of 1.5-2.5:1 for 1h, and then dried and pulverized. S5. Final Mixing and Blending: Mix the coordination-modified polyurethane emulsion of S2 with the conductive filler dispersion of S3, and stir at 800-1200 rpm for 20 min; add the long-chain modified crosslinking agent, HDI trimer crosslinking agent, and PCDI-aziridine compound crosslinking agent prepared in S4 in sequence, and stir until uniform; add the silane coupling agent, leveling agent, thickener, defoamer, film-forming aid, and UV absorber in sequence, and stir at 500-800 rpm for 10 min; degas under vacuum, and adjust to a Forte 4 cup viscosity of 20-30s (25℃) with deionized water, and a solid content of 20-25%; S6. Coating and Curing: The coating prepared in S5 is applied to the substrate and cured in three stages: pre-drying at 60℃ for 3 min, cross-linking reaction at 90℃ for 4 min, and curing at 100℃ for 2 min; then it is cured at 25℃ and relative humidity of 50-60% for 24 h. Then cure at 50°C for 2 hours.

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