Preparation method and application of degradable thermal transfer lettering film

Abstract

The present invention discloses a preparation method and application of a degradable thermal transfer lettering film, which belongs to the field of thermal transfer technology, including the preparation of a dynamically thioesterified polyimide reinforced base film, anti-sticking layer coating, release layer coating, microencapsulated color layer ink coating, dynamic cross-linked adhesive layer coating, and slitting and thermal transfer. The present invention constructs a structure with both mechanical strength and controllable degradation ability in the base film and the adhesive layer through the reversible cross-linking network design of dynamic thioester bonds and disulfide bonds, effectively solving the problem of the difficult balance between degradation rate and material performance in traditional processes. The hybrid system of nanocellulose whiskers and polylactic acid provides active sites for enzymatic degradation while improving the rigidity of the base film, avoiding performance degradation caused by excessive addition of degradable components. The all-aqueous process system completely eliminates the risk of organic solvent residues, eliminates the generation of microplastics from the source, and ensures the complete degradation of the material in the natural environment.

Description

Technical Field

[0001] The present invention relates to the technical field of thermal transfer, and particularly relates to a preparation method and application of a degradable thermal transfer lettering film. Background Art

[0002] As a transfer material widely used in fields such as textiles and clothing, the performance of the thermal transfer lettering film directly affects the adhesion, durability, and environmental friendliness of the graphics and text.

[0003] For example, Chinese Patent CN112457709B uses a non-degradable PET-based film, which is subjected to an aging treatment to prevent high-temperature deformation, resulting in a complex process and high cost. Although the above patent improves the adhesion (45N in Example 1) and high-temperature resistance (no deformation at 220°C / 30s) by introducing degradable components (such as polybutylene succinate and modified rosin resin), the following problems still exist:

[0004] The contradiction between the degradation rate and mechanical properties. Excessive addition of degradable materials (such as 10 parts of polybutylene succinate in Example 6) leads to a decrease in the tensile strength of the base film (the adhesion in Example 6 in Table 1 is 41N, lower than 45N in Example 1);

[0005] Solvent residues and microplastic risks. The mixed solvents (such as toluene and acetone) in the color layer ink may leave a pungent smell, and there is a risk of microplastic residues in the degradation products of some examples (such as 35 parts of modified resin in Example 10). Summary of the Invention

[0006] In view of the above-mentioned drawbacks of the prior art, the present invention provides a preparation method and application of a degradable thermal transfer lettering film.

[0007] To achieve the above objectives, the present invention is realized through the following technical solutions:

[0008] A preparation method of a degradable thermal transfer lettering film includes the following steps:

[0009] S1: Preparation of a dynamically thioesterified polyimide reinforced base film: By weight, 15-25 parts of modified PET, 8-12 parts of dynamically thioesterified polyimide, 10-15 parts of cellulose nanocrystal / polylactic acid hybrid material, and 2-4 parts of bio-based plasticizer are premixed in a double planetary mixer at 80-100°C for 20-30 min, then the temperature is raised to 170-180°C for dynamic thioester bond crosslinking for 40-45 min, and a base film is obtained by double-directional gradient stretching and blow molding with a film thickness of 50-60 μm.

[0010] The method for modifying PET is specifically as follows: Immerse PET chips in an NaOH solution and hydrolyze at 60 - 65°C for 30 - 40 min to generate surface hydroxyl groups; conduct a melt grafting reaction with succinic anhydride at 110 - 120°C for 2 - 3 h;

[0011] The method for synthesizing the dynamic thioesterified polyimide is specifically as follows: Condense a polyimide prepolymer and a thioethylene glycol ester with a molar ratio of 1 - 1.3:0.4 - 0.5 under nitrogen protection at 180 - 186°C for 4 - 5 h, activate the dynamic bond recombination through ultraviolet irradiation, add nanocrystalline cellulose whiskers modified with surface amino groups, and control the temperature in sections for dispersion using a twin-screw extruder;

[0012] S2: Anti-sticking layer coating: Spray a fully water-based anti-sticking layer coating solution on the back of the base film. The coating solution contains 10 - 20 parts of vinyl ester resin, 5 - 10 parts of aminoacrylate resin, 3 - 5 parts of nano-lithium saponite, and 2 - 4 parts of a water-based cross-linking agent. Control the drawing tension at 8 - 15 kg / m 2 , the coating speed is 100 - 150 m / min, the oven temperature is 130 - 160°C, and the thickness of the cured coating is 5 - 8 μm;

[0013] S3: Release layer coating: Coat a release layer pressure-sensitive adhesive on the front of the base film. The adhesive solution contains 20 - 30 parts of organosilicon pressure-sensitive adhesive, 15 - 25 parts of dynamically cross-linked silica gel, 2 - 4 parts of a zinc-bismuth bimetallic catalyst, and 1 - 3 parts of a water-based dispersant. The coating speed is 80 - 120 m / min, the oven temperature is 160 - 190°C, and the thickness after curing is 10 - 18 μm;

[0014] S4: Microencapsulated chromatic layer ink coating: Coat chromatic layer ink on the surface of the release layer using an electrostatic spraying method. The ink contains 25 - 35 parts of a polyurethane / polylactic acid block copolymer, 10 - 18 parts of microencapsulated pigment, and 3 - 5 parts of a water-based dispersant. The coating speed is 40 - 60 m / min, the oven temperature is 100 - 120°C, and the thickness after curing is 35 - 45 μm;

[0015] S5: Dynamically cross-linked adhesive layer coating: Coat an adhesive on the surface of the chromatic layer ink, containing 40 - 60 parts of a polyhydroxybutyrate-itaconic anhydride graft copolymer, 30 - 50 parts of a dynamic disulfide bond vinyl chloride-vinyl acetate resin, and 3 - 5 parts of an erucic acid amide / nano-silica composite lubricant. The die head temperature is 180 - 200°C, and the thickness after curing is 70 - 100 μm;

[0016] S6: Slitting and heat transfer printing: Slit the product of S5 into a roll or single-sheet form, engrave it with a cutting plotter, and then hot press it onto the surface of a pure cotton substrate at 220°C for 30 s to obtain a degradable heat transfer printing engraved film.

[0017] Further, in the synthesis of the dynamic thioesterified polyimide, the addition amount of thioethylene glycol ester is 0.3 - 0.5 times the molar amount of the polyimide prepolymer, and the sulfur content after polycondensation reaction is 10 - 12 wt%.

[0018] Further, the preparation method of the cellulose nanocrystal / polylactic acid hybrid material is as follows: melt-blend polylactic acid and nanocellulose whiskers at a mass ratio of 7 - 8:3 - 5 at 80 - 90 °C, add a maleic anhydride grafting agent, and the mass ratio of polylactic acid to the maleic anhydride grafting agent is 7 - 8:4 - 6. After reacting for 2 - 3 h, extrude and pelletize to form uniformly dispersed hybrid particles.

[0019] Further, the shell thickness of the microencapsulated pigment is 0.5 - 1 μm, the mass ratio of calcium peroxide to phthalocyanine blue in the core material is 1 - 3:5 - 7, and the microcapsule ruptures to release calcium peroxide during hot pressing.

[0020] Further, the preparation method of the dynamic disulfide bond vinyl chloride - vinyl acetate resin is as follows: polycondense vinyl chloride - vinyl acetate resin and 4,4'-dithiobenzoic acid in DMF at 120 - 125 °C for 3 - 4 h, add nano-ZnO, and the molar ratio of vinyl chloride - vinyl acetate resin, 4,4'-dithiobenzoic acid, DMF and nano-ZnO is 1 - 1.4:0.2 - 0.5:8 - 12:2 - 4. After ultrasonic dispersion, a core - shell structure with a decreasing cross - link density gradient is formed.

[0021] Further, in the fully water - based anti - sticking layer coating liquid, the vinyl ester resin is a copolymer of epoxy acrylate and 2 - hydroxyethyl methacrylate, the amino acrylate resin is a polyurethane acrylate containing primary amino groups, and the cross - linker is an isocyanate - group - containing water - based polyurethane prepolymer.

[0022] Further, the grafting rate of the polyhydroxybutyrate - itaconic anhydride graft copolymer in the binder is 12 - 18%, and its preparation method is as follows: reflux react polyhydroxybutyrate and itaconic anhydride with a molar ratio of 1 - 1.3:0.3 - 0.6 in toluene for 5 - 6 h to generate a grafted product with carboxyl groups in the side chain.

[0023] Further, in the release laminating adhesive, the zinc - bismuth molar ratio of the zinc - bismuth bimetallic catalyst is 5 - 6:1 - 2, the solvent is a mixed solution of water and ethanol, and it is cured by gradient temperature increase after coating.

[0024] Further, in the bio - based plasticizer, the mass ratio of epoxidized soybean oil to tributyl citrate is 1.5 - 1.8:2.3 - 2.6, and the addition amount accounts for 3 - 5% of the total mass of the base film.

[0025] Application of a degradable thermal transfer engraving film prepared by the described preparation method in clothing, shoes and hats, luggage, home textiles, gloves, protective clothing, environmental protection packaging or cultural and creative products.

[0026] The beneficial effects of the present invention compared with the prior art are as follows:

[0027] 1. The dynamic bonds in the present invention mainly include dynamic thioester bonds (applied to the base film) and disulfide bonds (applied to the adhesive layer). These two types of bonds exist in the form of a reversible crosslinking network, which plays a key role in the material properties. Its core function is to balance the mechanical strength and degradation rate of the material, enabling the material to maintain a reasonable degradation efficiency on the basis of certain rigidity and strength, preventing the material from being difficult to degrade due to excessive firmness or losing mechanical properties due to too fast degradation.

[0028] 2. The present invention uses CNC / PLA hybrid materials and nano-ZnO. Among them, CNC (nano-cellulose whiskers) has good mechanical properties and biodegradability. After forming a hybrid material with PLA (polylactic acid), it can enhance the mechanical properties of the material while promoting the enzymatic hydrolysis process, providing an effective way for material degradation. Nano-ZnO, on the other hand, has the function of catalytic crosslinking and can promote the formation of crosslinking structures inside the material through catalytic reactions, further improving the stability and performance of the material. The two cooperate with each other to jointly constitute the key mechanism of coordinated degradation and performance, enabling the material to have efficient degradation ability while maintaining good mechanical properties.

[0029] 3. The present invention uses a fully aqueous system for material preparation and processing, which has significant advantages. In terms of solvent residues, the fully aqueous system in the examples results in no detectable solvent residues, which means that the material has extremely low dependence on organic solvents during the production process, effectively avoiding potential risks brought by organic solvent residues, such as harm to human health, environmental pollution, and adverse effects on material properties. At the same time, the fully aqueous system can also reduce the generation of microplastics, which is of great significance for environmental protection and the sustainable development of the ecosystem.

[0030] 4. Through the reversible crosslinking network design of dynamic thioester bonds and disulfide bonds, the present invention constructs a structure with both mechanical strength and controllable degradation ability in the base film and the adhesive layer, effectively solving the difficult problem of balancing the degradation rate and material properties in traditional processes. The hybrid system of nano-cellulose whiskers and polylactic acid provides active sites for enzymatic degradation while enhancing the rigidity of the base film, avoiding performance deterioration caused by excessive addition of degradable components; the fully aqueous process system completely eliminates the risk of organic solvent residues and eliminates the generation of microplastics from the source, ensuring the complete degradation of the material in the natural environment. Compared with the prior art, this solution realizes the greening of the production process and the environmental friendliness of the end product on the premise of maintaining high adhesion and washability. Detailed implementation mode

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0032] Example 1: This example provides a method for preparing a degradable thermal transfer engraving film, including the following steps: S1: Preparation of a dynamically thioesterified polyimide reinforced base film: By weight, 25 parts of modified PET (pre-hydrolyzed PET grafted with polybutylene succinate, PBS grafting rate ≥ 20%), 12 parts of dynamically thioesterified polyimide (sulfur content 12%), 15 parts of cellulose nanocrystal / polylactic acid hybrid material (CNC / PLA, CNC accounting for 30%), and 4 parts of bio-based plasticizer are premixed in a double planetary mixer at 100 °C for 30 min, heated to 180 °C for dynamic thioester bond crosslinking for 45 min, and blown into a film by bi-directional gradient stretching (longitudinal stretching ratio 3.5, transverse stretching ratio 3.0), with a film thickness of 60 μm, and the base film is obtained by co-blown film;

[0033] The method for the modified PET is specifically as follows: The PET chips are immersed in a 5 wt% NaOH solution and hydrolyzed at 65 °C for 340 min to generate surface hydroxyl groups; and melt graft reaction is carried out with succinic anhydride (molar ratio 1:1.5) at 120 °C for 3 h;

[0034] The synthesis method of the dynamically thioesterified polyimide is specifically as follows: A polyimide prepolymer with a molar ratio of 1.3:0.5 and thioethylene glycol ester are polycondensed at 186 °C for 5 h under nitrogen protection, and dynamic bond recombination is activated by ultraviolet irradiation (wavelength 365 nm, irradiation time 15 min). Surface amino-modified nanocellulose whiskers are added (the molar ratio of thioethylene glycol ester to surface amino-modified nanocellulose whiskers is 0.5:3.6), and the temperature is controlled in sections by a twin-screw extruder (180 °C for 1 min → 200 °C for 3 min → 160 °C for 3 min) for dispersion;

[0035] S2: Anti-sticking layer coating: A fully water-based anti-sticking layer coating solution is sprayed on the back of the base film. The coating solution contains 20 parts of vinyl ester resin, 10 parts of aminoacrylate resin, 5 parts of nano-lithium saponite (particle size ≤ 100 nm), and 4 parts of a water-based crosslinking agent (carboxyl-containing polyurethane prepolymer), controlling the stretching tension at 15 kg / m 2 , the coating speed at 150 m / min, the oven temperature at 160 °C, and the cured coating thickness at 8 μm;

[0036] S3: Release layer coating: Coat a release layer pressure-sensitive adhesive on the front side of the base film. The adhesive solution contains 30 parts of organosilicon pressure-sensitive adhesive, 25 parts of dynamically crosslinked silica gel (containing disulfide bonds), 4 parts of zinc-bismuth bimetallic catalyst (zinc-bismuth molar ratio 8:1), and 3 parts of aqueous dispersant (sodium polyacrylate). The coating speed is 120 m / min, the oven temperature is 190 °C, and the thickness after curing is 18 μm.

[0037] S4: Microencapsulated chromatic layer ink coating: Coat chromatic layer ink on the surface of the release layer by electrostatic spraying. The ink contains 35 parts of polyurethane / polylactic acid block copolymer (PLA segment accounts for 40%), 18 parts of microencapsulated pigment (shell material is gelatin / sodium alginate, core material is phthalocyanine blue + calcium peroxide), and 5 parts of aqueous dispersant. The coating speed is 60 m / min, the oven temperature is 120 °C, and the thickness after curing is 45 μm.

[0038] S5: Dynamically crosslinked adhesive layer coating: Coat an adhesive on the surface of the chromatic layer ink, containing 60 parts of polyhydroxybutyrate-itaconic anhydride graft copolymer (grafting rate 15%), 50 parts of dynamic disulfide-containing vinyl chloride resin, and 5 parts of erucamide / nano-silica composite lubricant (mass ratio 2:1). The die head temperature is 200 °C, and the thickness after curing is 100 μm.

[0039] S6: Slitting and heat transfer: Slit the product of S5 into a roll or single-sheet form. After engraving by a cutting plotter, hot press it onto the surface of a pure cotton substrate at 220 °C / 30 s to obtain a degradable heat transfer engraved film with an adhesion force ≥50 N, a wash resistance ≥60 times, a 180-day degradation weight loss rate ≥98%, and no microplastic residue in the degradation products.

[0040] In the synthesis of the dynamic thioesterified polyimide, the addition amount of thioethylene glycol ester is 0.5 times the molar amount of the polyimide prepolymer, and the sulfur content after polycondensation reaction is 12 wt%.

[0041] The preparation method of the cellulose nanocrystal / polylactic acid hybrid material is as follows: Melt-blend polylactic acid (Mw = 50 kDa) and nanocellulose whiskers (particle size 50 nm) at a mass ratio of 8:5 at 90 °C, and add a maleic anhydride grafting agent (grafting rate 5%). The mass ratio of polylactic acid to the maleic anhydride grafting agent is 8:6. After reacting for 3 h, extrude and pelletize to form uniformly dispersed hybrid particles.

[0042] The shell thickness of the microencapsulated pigment is 1 μm, the mass ratio of calcium peroxide to phthalocyanine blue in the core material is 3:7, and the microcapsules rupture during hot pressing to release calcium peroxide, catalyzing the hydrolysis of the polylactic acid segment, so that the 180-day degradation rate of the chromatic layer ink ≥90%.

[0043] The preparation method of the dynamic disulfide bond vinyl chloride - acetate resin is as follows: Vinyl chloride - acetate resin (chlorine content 45%) and 4,4'-dithiobis(benzoic acid) are subjected to polycondensation in DMF at 125°C for 4 h, and then nano - ZnO (particle size 20 nm) is added. The molar ratio of vinyl chloride - acetate resin, 4,4'-dithiobis(benzoic acid), DMF, and nano - ZnO is 1.4:0.5:12:4. After ultrasonic dispersion, a core - shell structure with a decreasing cross - link density gradient is formed.

[0044] In the all - water - based anti - sticking layer coating liquid, the vinyl ester resin is a copolymer of epoxy acrylate and 2 - hydroxyethyl methacrylate (molar ratio 1:1), the amino acrylate resin is a polyurethane acrylate containing primary amino groups, the cross - linker is an isocyanate - group - containing water - based polyurethane prepolymer, and the cured coating can withstand solvent wiping ≥100 times.

[0045] The preparation method of the polyhydroxybutyrate - itaconic anhydride graft copolymer in the binder is as follows: Polyhydroxybutyrate (Mw = 150 kDa) and itaconic anhydride with a molar ratio of 1.3:0.6 are refluxed in toluene for 6 h to generate a grafted product with carboxyl groups in the side chain. The acid anhydride peak at 1705 cm -1 disappears as detected by FTIR.

[0046] In the release laminating adhesive, the zinc - bismuth molar ratio of the zinc - bismuth bimetallic catalyst is 6:2, the solvent is a mixed solution of water and ethanol (mass ratio 8:4), and after coating, it is cured by gradient heating (90°C → 130°C → 170°C, 10 min for each stage) to control the release force of the release layer at 1.0 N / cm.

[0047] In the bio - based plasticizer, the mass ratio of epoxidized soybean oil to tributyl citrate is 1.8:2.6, and the addition amount accounts for 5% of the total mass of the base film, so that the tensile strength of the base film ≥65 MPa and the elongation at break ≥200%.

[0048] Example 2: This example provides a preparation method of a degradable thermal transfer engraving film, including the following steps:

[0049] S1: Preparation of a dynamic thio - esterified polyimide - enhanced base film: By weight, 15 parts of modified PET (pre - hydrolyzed PET grafted with poly(butylene succinate), PBS grafting rate ≥20%), 8 parts of dynamic thio - esterified polyimide (sulfur content 8%), 10 parts of cellulose nanocrystal / polylactic acid hybrid material (CNC / PLA, CNC accounting for 30%), and 2 parts of bio - based plasticizer are premixed in a double - planetary mixer at 80 - °C for 20 min, then heated to 170°C for dynamic thio - ester bond cross - linking for 40 min, and blown into a film by bi - directional gradient stretching (longitudinal stretching ratio 3.5, transverse stretching ratio 3.0). A base film with a film thickness of 50 μm is obtained by co - blending and blowing.

[0050] The method for modifying PET is specifically as follows: Immerse PET chips in a 5 wt% NaOH solution, hydrolyze at 60 °C for 30 min to generate surface hydroxyl groups; perform melt grafting reaction with succinic anhydride (molar ratio 0.8:1.2) at 110 °C for 2 h;

[0051] The method for synthesizing the dynamic thioesterified polyimide is specifically as follows: Condense a polyimide prepolymer with a molar ratio of 1:0.4 and thioethylene glycol ester at 180 °C for 4 h under nitrogen protection, activate the dynamic bond recombination through ultraviolet irradiation (wavelength 365 nm, irradiation time 10 min), add nanocellulose whiskers modified with surface amino groups (molar ratio of thioethylene glycol ester to nanocellulose whiskers modified with surface amino groups is 0.4:3), and disperse by controlling the temperature in sections with a twin-screw extruder (180 °C for 1 min → 200 °C for 3 min → 160 °C for 3 min);

[0052] S2: Anti-adhesive layer coating: Spray a fully water-based anti-adhesive layer coating solution on the back of the base film. The coating solution contains 10 parts of vinyl ester resin, 5 parts of aminoacrylate resin, 3 parts of nano-lithium saponite (particle size ≤ 100 nm), and 2 parts of a water-based cross-linking agent (a carboxyl-containing polyurethane prepolymer). Control the stretching tension at 8 kg / m 2 , the coating speed is 100 m / min, the oven temperature is 130 °C, and the thickness of the cured coating is 5 μm;

[0053] S3: Release layer coating: Coat a release layer pressure-sensitive adhesive on the front of the base film. The adhesive solution contains 20 parts of organosilicon pressure-sensitive adhesive, 15 parts of dynamic cross-linked silica gel (containing disulfide bonds), 2 parts of a zinc-bismuth bimetallic catalyst (zinc-bismuth molar ratio 3:1), and 1 part of a water-based dispersant (sodium polyacrylate). The coating speed is 80 m / min, the oven temperature is 160 °C, and the thickness after curing is 10 μm;

[0054] S4: Microencapsulated chromatic layer ink coating: Coat the chromatic layer ink on the surface of the release layer by electrostatic spraying. The ink contains 25 parts of a polyurethane / polylactic acid block copolymer (the PLA segment accounts for 40%), 10 parts of microencapsulated pigment (the shell material is gelatin / sodium alginate, and the core material is phthalocyanine blue + calcium peroxide), and 3 parts of a water-based dispersant. The coating speed is 40 m / min, the oven temperature is 100 °C, and the thickness after curing is 35 μm;

[0055] S5: Dynamic cross-linked adhesive layer coating: Coat an adhesive on the surface of the chromatic layer ink, containing 40 parts of a polyhydroxybutyrate-itaconic anhydride graft copolymer (grafting rate 12%), 30 parts of a dynamic disulfide bond vinyl chloride-vinyl acetate resin, and 3 - 5 parts of an erucic acid amide / nano-silica composite lubricant (mass ratio 2:1). The die head temperature is 180 °C, and the thickness after curing is 70 μm;

[0056] S6: Slitting and Thermal Transfer: The finished product of S5 is slit into a roll or single-sheet form. After being engraved by a cutting plotter, it is hot-pressed onto the surface of a pure cotton substrate at 220°C for 30 s to obtain a degradable thermal transfer engraved film with an adhesion force ≥50 N, a washability resistance ≥60 times, a degradation weight loss rate ≥98% after 180 days, and no microplastic residue in the degradation products.

[0057] In the synthesis of the dynamic thioesterified polyimide, the addition amount of thioethylene glycol ester is 0.3 times the molar amount of the polyimide prepolymer, and the sulfur content after polycondensation reaction is 10 wt%.

[0058] The preparation method of the cellulose nanocrystal / polylactic acid hybrid material is as follows: Poly(lactic acid) (Mw = 50 kDa) and nanocellulose whiskers (particle size 50 nm) are melt-blended at a mass ratio of 7:3 at 80°C, and a maleic anhydride grafting agent (grafting rate 5%) is added. The mass ratio of poly(lactic acid) to the maleic anhydride grafting agent is 7:4. After reacting for 2 h, it is extruded and pelletized to form uniformly dispersed hybrid particles.

[0059] The shell thickness of the microencapsulated pigment is 0.5 μm, and the mass ratio of calcium peroxide to phthalocyanine blue in the core material is 1:5. The microcapsules rupture during hot pressing to release calcium peroxide, which catalyzes the hydrolysis of the polylactic acid segment, making the 180-day degradation rate of the chromatic layer ink ≥90%.

[0060] The preparation method of the dynamic disulfide-containing vinyl chloride-vinyl acetate resin is as follows: Vinyl chloride-vinyl acetate resin (chlorine content 45%) and 4,4'-dithiobenzoic acid are polycondensed in DMF at 120°C for 3 h, and nano-ZnO (particle size 20 nm) is added. The molar ratio of vinyl chloride-vinyl acetate resin, 4,4'-dithiobenzoic acid, DMF, and nano-ZnO is 1:0.2:8:2. After ultrasonic dispersion, a core-shell structure with a decreasing crosslinking density gradient is formed.

[0061] In the all-aqueous anti-sticking layer coating solution, the vinyl ester resin is a copolymer of epoxy acrylate and 2-hydroxyethyl methacrylate (molar ratio 1:1), the amino acrylate resin is a polyurethane acrylate containing primary amino groups, and the crosslinking agent is an aqueous polyurethane prepolymer containing isocyanate groups. After curing, the coating has a solvent wiping resistance ≥100 times.

[0062] The grafting rate of the polyhydroxybutyrate-itaconic anhydride graft copolymer in the binder is 12%. Its preparation method is as follows: Polyhydroxybutyrate (Mw = 150 kDa) and itaconic anhydride with a molar ratio of 1:0.3 are refluxed in toluene for 5 h to generate a grafted product with carboxyl groups on the side chain. The acid anhydride peak disappears at 1705 cm -1 in the FTIR detection.

[0063] The molar ratio of zinc to bismuth in the zinc-bismuth bimetallic catalyst of the release laminate pressure-sensitive adhesive is 5:1, the solvent is a mixed solution of water and ethanol (mass ratio 7:3), and after coating, it is cured by gradient heating (90°C → 130°C → 170°C, 10 minutes for each stage), so that the release force of the release layer is controlled at 0.5 N / cm.

[0064] The mass ratio of epoxidized soybean oil to tributyl citrate in the bio-based plasticizer is 1.5:2.3, and the addition amount accounts for 3% of the total mass of the base film, so that the tensile strength of the base film is ≥65 MPa and the elongation at break is ≥200%.

[0065] Example 3: This example provides a method for preparing a degradable thermal transfer lettering film, including the following steps: S1: Preparation of a dynamically thioesterified polyimide reinforced base film: By weight, 21 parts of modified PET (pre-hydrolyzed PET grafted with polybutylene succinate, PBS grafting rate ≥20%), 11 parts of dynamically thioesterified polyimide (sulfur content 10%), 12 parts of cellulose nanocrystal / polylactic acid hybrid material (CNC / PLA, CNC accounting for 30%), and 3 parts of bio-based plasticizer are premixed in a double planetary mixer at 85°C for 24 minutes, heated to 173°C for dynamic thioester bond crosslinking for 42 minutes, and blow molded into a film by bidirectional gradient stretching (longitudinal stretching ratio 3.5, transverse stretching ratio 3.0), and a base film with a film thickness of 56 μm is obtained by co-blown film;

[0066] The method for the modified PET is specifically as follows: Immerse PET chips in a 5 wt% NaOH solution, hydrolyze at 63°C for 38 minutes to generate surface hydroxyl groups; carry out a melt grafting reaction with succinic anhydride (molar ratio 0.9:1.4) at 115°C for 3 hours;

[0067] The synthesis method of the dynamically thioesterified polyimide is specifically as follows: A polyimide prepolymer with a molar ratio of 1.2:0.5 and thioethylene glycol ester are polycondensed at 182°C for 4 hours under nitrogen protection, and dynamic bond recombination is activated by ultraviolet irradiation (wavelength 365 nm, irradiation time 13 minutes), and nanocellulose whiskers modified with surface amino groups are added (the molar ratio of thioethylene glycol ester to nanocellulose whiskers modified with surface amino groups is 0.4:3.2), and the temperature is controlled in sections by a twin-screw extruder (180°C for 1 minute → 200°C for 3 minutes → 160°C for 3 minutes) for dispersion;

[0068] S2: Anti-sticking layer coating: Spray a fully water-based anti-sticking layer coating solution on the back of the base film. The coating solution contains 18 parts of vinyl ester resin, 8 parts of aminoacrylate resin, 4 parts of nano-lithium saponite (particle size ≤100 nm), and 3 parts of a water-based crosslinking agent (a carboxyl-containing polyurethane prepolymer), control the drawing tension at 12 kg / m 2 , the coating speed is 130 m / min, the oven temperature is 150°C, and the thickness of the cured coating is 6 μm;

[0069] S3: Release layer coating: Coat a release layer pressure-sensitive adhesive on the front side of the base film. The adhesive solution contains 22 parts of organosilicon pressure-sensitive adhesive, 17 parts of dynamically crosslinked silica gel (containing disulfide bonds), 3 parts of zinc-bismuth bimetallic catalyst (zinc-bismuth molar ratio 7:1), and 2 parts of aqueous dispersant (sodium polyacrylate). The coating speed is 115 m / min, the oven temperature is 180 °C, and the thickness after curing is 16 μm;

[0070] S4: Microencapsulated chromatic layer ink coating: Coat chromatic layer ink on the surface of the release layer by electrostatic spraying. The ink contains 31 parts of polyurethane / polylactic acid block copolymer (the proportion of PLA segment is 40%), 17 parts of microencapsulated pigment (the shell material is gelatin / sodium alginate, and the core material is phthalocyanine blue + calcium peroxide), and 4 parts of aqueous dispersant. The coating speed is 50 m / min, the oven temperature is 118 °C, and the thickness after curing is 41 μm;

[0071] S5: Dynamically crosslinked adhesive layer coating: Coat an adhesive on the surface of the chromatic layer ink. It contains 52 parts of polyhydroxybutyrate-itaconic anhydride graft copolymer (grafting rate 18%), 35 parts of dynamic disulfide-containing vinyl chloride resin, and 4 parts of erucic acid amide / nano-silica composite lubricant (mass ratio 2:1). The die head temperature is 188 °C, and the thickness after curing is 80 μm;

[0072] S6: Slitting and heat transfer printing: Cut the product of S5 into roll or single-sheet form. After engraving by a cutting plotter, hot press it onto the surface of a pure cotton substrate at 220 °C for 30 s to obtain a degradable heat transfer printing engraved film with an adhesion force ≥50 N, a washability resistance ≥60 times, a 180-day degradation weight loss rate ≥98%, and no microplastic residue in the degradation products.

[0073] In the synthesis of the dynamic thioesterified polyimide, the addition amount of thioethylene glycol ester is 0.4 times the molar amount of the polyimide prepolymer, and the sulfur content after polycondensation reaction is 11 wt%, so that the deformation rate of the base film after hot pressing at 220 °C for 30 s is ≤1.5%.

[0074] The preparation method of the cellulose nanocrystal / polylactic acid hybrid material is as follows: Melt-blend polylactic acid (Mw = 50 kDa) and nanocellulose whiskers (particle size 50 nm) at a mass ratio of 7:4 at 87 °C, and add a maleic anhydride grafting agent (grafting rate 5%). The mass ratio of polylactic acid to the maleic anhydride grafting agent is 8:5. After reacting for 3 h, extrude and pelletize to form uniformly dispersed hybrid particles.

[0075] The shell thickness of the microencapsulated pigment is 0.6 μm, and the mass ratio of calcium peroxide to phthalocyanine blue in the core material is 2:6. The microcapsules rupture during hot pressing to release calcium peroxide, which catalyzes the hydrolysis of the polylactic acid segment, so that the 180-day degradation rate of the chromatic layer ink is ≥90%.

[0076] The preparation method of the dynamic disulfide bond vinyl chloride-vinyl acetate resin is as follows: Polyvinyl chloride-vinyl acetate resin (chlorine content 45%) and 4,4'-dithiobis(benzoic acid) are subjected to polycondensation in DMF at 122 °C for 4 h, and nano-ZnO (particle size 20 nm) is added. The molar ratio of the polyvinyl chloride-vinyl acetate resin, 4,4'-dithiobis(benzoic acid), DMF, and nano-ZnO is 1.2:0.3:9:3. After ultrasonic dispersion, a core-shell structure with a decreasing crosslinking density gradient is formed.

[0077] In the all-aqueous anti-sticking layer coating solution, the vinyl ester resin is a copolymer of epoxy acrylate and 2-hydroxyethyl methacrylate (molar ratio 1:1), the amino acrylate resin is a polyurethane acrylate containing primary amino groups, the crosslinking agent is an isocyanate group-containing aqueous polyurethane prepolymer, and the cured coating has a solvent rub resistance of ≥ 100 times.

[0078] The preparation method of the polyhydroxybutyrate-itaconic anhydride graft copolymer in the binder is as follows: Polyhydroxybutyrate (Mw = 150 kDa) and itaconic anhydride with a molar ratio of 1.2:0.5 are refluxed in toluene for 6 h to form a graft copolymer with carboxyl groups in the side chain. The acid anhydride peak at 1705 cm -1 disappears in the FTIR detection.

[0079] In the release laminating adhesive, the zinc-bismuth molar ratio of the zinc-bismuth bimetallic catalyst is 5:2, the solvent is a mixed solution of water and ethanol (mass ratio 8:3), and after coating, it is cured by gradient heating (90 °C → 130 °C → 170 °C, 10 min for each stage), so that the release force of the release layer is controlled at 0.8 N / cm.

[0080] In the bio-based plasticizer, the mass ratio of epoxidized soybean oil to tributyl citrate is 1.7:2.4, and the addition amount accounts for 5% of the total mass of the base film, so that the tensile strength of the base film is ≥ 65 MPa and the elongation at break is ≥ 200%.

[0081] Comparative Example 1: The dynamic thioesterified polyimide is replaced with fluorinated polyimide.

[0082] Comparative Example 2: The cellulose nanocrystal / polylactic acid hybrid material is not added.

[0083] Comparative Example 3: Nano-ZnO is not added to the dynamic disulfide bond vinyl chloride-vinyl acetate resin.

[0084] Comparative Example 4: The dynamic thioesterified polyimide is replaced with fluorinated polyimide, and nano-ZnO is not added to the dynamic disulfide bond vinyl chloride-vinyl acetate resin.

[0085] Control Example: Chinese Patent CN112457709B is adopted.

[0086] Experimental Example: The following tests are carried out on the degradable thermal transfer lettering films prepared in Examples 1-3, Comparative Examples 1-4, and the Control Example:

[0087] Tensile strength of the base film (MPa): Test standard: ASTM D882 (Tensile properties of plastic films), specimen size 100 mm × 10 mm, tensile rate 50 mm / min.

[0088] Adhesion force (N): Test method: 180° peel test (ASTM D3330), peel force after hot pressing on a pure cotton substrate.

[0089] Number of times resistant to water washing: Test conditions: AATCC 61-2013 standard, water temperature 50 °C, each washing for 30 minutes, record the number of times of degumming or fading.

[0090] Degradation rate (180 days): Test method: ISO 20200 compost degradation test, constant temperature and humidity (58 °C ± 2 °C, humidity > 90%), weigh the percentage of weight loss.

[0091] Coating speed (m / min): Anti-adhesive layer / release layer: The highest stable coating speed measured on the production line.

[0092] Solvent residue: Detection method: GC-MS (Gas chromatography-mass spectrometry), detection limit 0.01 ppm.

[0093] The data is shown in the following table:

[0094]

[0095] Data analysis: From Comparative Example 1 (replacing dynamic thioesterified polyimide), it can be seen that: the absence of dynamic thioester bonds leads to a decrease in the high-temperature stability of the base film, and at the same time, the ability of dynamic bond recombination is lost, and the adhesion force decreases. The degradation rate decreases because fluorinated polyimide cannot be enzymatically degraded.

[0096] From Comparative Example 2 (without adding CNC / PLA hybrid material), it can be seen that: the reinforcing effect of cellulose nanocrystals disappears, and the tensile strength of the base film decreases; the enzymatic hydrolysis sites of PLA decrease, and the degradation rate decreases. The number of times resistant to water washing decreases due to insufficient rigidity of the base film.

[0097] From Comparative Example 3 (without adding nano-ZnO), it can be seen that: the cross-linking efficiency of dynamic disulfide bonds decreases (the catalytic effect is missing), the adhesion force of the bonding layer decreases, and the number of times resistant to water washing decreases slightly. The degradation catalytic function of nano-ZnO is lost, and the degradation rate decreases slightly.

[0098] From Comparative Example 4 (double degradation), it can be seen that: the dynamic thioester bond and disulfide bond are both missing, and the performance of the base film and the bonding layer decreases significantly.

[0099] Based on the above analysis, the present invention has the following effects:

[0100] The core role of the dynamic bond is that the dynamic thioester bond (base film) and the disulfide bond (adhesive layer) balance the mechanical strength and degradation rate through a reversible cross-linking network. After removing these components in the comparative example, the rigidity, adhesion and degradation efficiency of the material decreased simultaneously.

[0101] Nano-reinforcement and catalytic design. The CNC / PLA hybrid material (reinforcement + enzymatic hydrolysis) and nano-ZnO (catalytic cross-linking) are the keys to the synergy of degradation and performance. The necessity was verified in Comparative Examples 2 / 3.

[0102] Advantages of the fully aqueous system. No solvent residue was detected in the examples, while in the control examples, due to the use of organic solvents, the residual risk increased and the problem of microplastics became prominent.

[0103] In summary, the present invention systematically solves the contradictions between the degradation rate and mechanical properties, the solvent residue risk and the process efficiency bottleneck in the background technology through dynamic bond recombination (thioester bond + disulfide bond), nano-reinforcement (CNC / PLA + ZnO) and fully aqueous process.

[0104] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements will not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a degradable thermal transfer lettering film, characterized in that: The following steps are involved: S1: Preparation of dynamically thioesterified polyimide reinforced base film: premix 15-25 parts of modified PET, 8-12 parts of dynamically thioesterified polyimide, 10-15 parts of cellulose nanocrystal / polylactic acid hybrid material, and 2-4 parts of bio-based plasticizer by weight, heat to 170-180°C for dynamic thioester bond crosslinking, perform biaxial gradient stretching and blow molding to form a film, and blend and blow mold to obtain a base film; The method for modifying PET is specifically as follows: immersing PET slices in a NaOH solution to generate surface hydroxyl groups after hydrolysis; and melt grafting reaction with succinic anhydride; The dynamic thioesterification polyimide synthesis method specifically comprises: polycondensing a polyimide prepolymer and thioethylene glycol ester under nitrogen protection, activating dynamic bond recombination by ultraviolet irradiation, adding surface amino-modified nanocellulose whiskers, and dispersing by segmented temperature control in a twin-screw extruder; The preparation method of the cellulose nanocrystal / polylactic acid hybrid material is as follows: melt-blending polylactic acid and nanocellulose whiskers at a mass ratio of 7-8:3-5 at 80-90° C., adding a maleic anhydride grafting agent, wherein the mass ratio of the polylactic acid to the maleic anhydride grafting agent is 7-8:4-6, reacting for 2-3 hours, and then extruding and granulating to form uniformly dispersed hybrid particles; S2: Anti-sticking layer coating: spraying a fully aqueous anti-sticking layer coating liquid on the back of the base film, the coating liquid containing 10-20 parts of vinyl ester resin, 5-10 parts of amino acrylate resin, 3-5 parts of nano-hectorite, and 2-4 parts of water-based crosslinking agent, and drying and curing after coating; S3: coating the release layer, coating the release layer pressure-sensitive adhesive on the front of the base film, the adhesive solution contains 20-30 parts of silicone pressure-sensitive adhesive, 15-25 parts of dynamic cross-linked silica gel, 2-4 parts of zinc-bismuth bimetallic catalyst, and 1-3 parts of aqueous dispersant, and drying and curing after coating; S4: coating of microencapsulated color layer ink: electrostatic spraying is used to coat the color layer ink on the surface of the release layer. The ink contains 25-35 parts of polyurethane / polylactic acid block copolymer, 10-18 parts of microencapsulated pigment, and 3-5 parts of aqueous dispersant. After coating, it is dried and solidified; S5: Dynamic cross-linking adhesive layer coating, coating the surface of the color layer ink with an adhesive, the adhesive containing 40-60 parts of polyhydroxybutyrate-itaconic anhydride graft copolymer, 30-50 parts of dynamic disulfide bond chloroacetic acid resin, and 3-5 parts of erucamide / nano-silica composite lubricant; The preparation method of the dynamic disulfide bond chloroacetic acid resin is as follows: polycondensing the chloroacetic acid resin and 4,4'-dithiodibenzoic acid in DMF at 120-125° C. for 3-4 hours, adding nano ZnO, wherein the molar ratio of the chloroacetic acid resin, 4,4'-dithiodibenzoic acid, DMF and nano ZnO is 1-1.4:0.2-0.5:8-12:2-4, and forming a core-shell structure with a decreasing crosslinking density gradient after ultrasonic dispersion; S6: Slitting and thermal transfer, the finished product of S5 is slit into rolls or single sheets, engraved by a cutting machine, and hot pressed to the surface of the pure cotton substrate to obtain a degradable thermal transfer lettering film.

2. The preparation method according to claim 1, characterized in that: In the synthesis of the dynamic thioesterified polyimide, the addition amount of thioethylene glycol ester is 0.3-0.5 times the molar amount of the polyimide prepolymer, and the sulfur content after the polycondensation reaction is 10-12wt%.

3. The preparation method according to claim 1, characterized in that: The shell material thickness of the microencapsulated pigment is 0.5-1 μm, the mass ratio of calcium peroxide to phthalocyanine blue in the core material is 1-3:5-7, and the microcapsule is broken during hot pressing to release calcium peroxide.

4. The preparation method according to claim 1, characterized in that: In the all-aqueous anti-sticking coating liquid, the vinyl ester resin is a copolymer of epoxy acrylate and hydroxyethyl methacrylate, the amino acrylate resin is a polyurethane acrylate containing primary amino groups, and the crosslinking agent is an aqueous polyurethane prepolymer containing an isocyanate group.

5. The preparation method according to claim 1, characterized in that: The grafting rate of the polyhydroxybutyrate-itaconic anhydride graft copolymer in the binder is 12-18%, and the preparation method thereof is as follows: polyhydroxybutyrate and itaconic anhydride in a molar ratio of 1-1.3:0.3-0.6 are refluxed in toluene for 5-6 hours to generate a graft containing carboxyl groups on the side chains.

6. The preparation method according to claim 1, characterized in that: The zinc-bismuth molar ratio of the zinc-bismuth bimetallic catalyst in the release layer pressure-sensitive adhesive is 5-6:1-2, the solvent is a mixed solution of water and ethanol, and after coating, the mixture is cured by gradient heating.

7. The preparation method according to claim 1, characterized in that: The mass ratio of epoxidized soybean oil to tributyl citrate in the bio-based plasticizer is 1.5-1.8:2.3-2.6, and the added amount accounts for 3-5% of the total mass of the base film.

8. Application of a degradable thermal transfer lettering film prepared by the preparation method according to any one of claims 1 to 7 in clothing, bags, home textiles or cultural and creative products.

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