A method for preparing a low-shrinkage transfer film based on multi-stage heat setting

Abstract

The present application relates to the field of high polymer film material manufacturing and precision processing technology, specifically to a low shrinkage transfer film preparation method based on multi-stage heat setting, which comprises preheating treatment, main setting treatment, gradient cooling treatment, activation treatment, buffer adhesive layer coating, curing and rapid quenching steps. This method combines multi-stage temperature gradient and longitudinal tension which is divided into at least three stages of ladder decrease, and uses polyurethane-acrylate interpenetrating network polymer system to construct the buffer adhesive layer; through the cooperation of ladder-like decreasing tension and three-stage temperature field, the non-equilibrium conformation of molecular chain is eliminated to eliminate internal residual stress, and the step of setting the line speed of the production line according to the thickness of the polymer substrate ensures the uniformity of the treatment; the present application effectively solves the problems of irreversible thermal shrinkage of transfer film and uneven heat treatment of different film thickness, and realizes low thermal shrinkage rate and good flexible fitting.

Description

Technical Field

[0001] This invention relates to the field of polymer thin film material manufacturing and precision processing technology, specifically a method for preparing a low-shrinkage transfer film based on multi-stage heat setting. Background Technology

[0002] Transfer film is a functional carrier used for transferring patterns or vacuum-plated metal layers, and it is the main material used in the production of transfer paper. The appropriate type of transfer film must be selected depending on the content and material being transferred. Transfer film is also required in the production of packaging transfer paper: after the holographic effect is photolithographically created, the holographic pattern content needs to be transferred onto the transfer film, and then transferred to the paper substrate using adhesive. At this point, the transfer film is crucial for positioning and bonding.

[0003] With the continuous improvement of precision machining technology, transfer films are developing towards high dimensional stability and high interfacial flexibility and adhesion. In key scenarios such as the production of packaging transfer paper, the mechanical performance and thermal deformation index of the transfer film directly determine the comprehensive performance and service life of the final product. However, in the biaxial stretching film-making process, polymer film materials will inevitably generate macromolecular chain disorientation and residual stress gradients inside. To address the aforementioned underlying defects, traditional methods for modifying transfer films, limited by the material's thermal relaxation time and existing thermal processing technologies, mostly focus on optimizing the surface adhesive formulation or employing single-stage high-temperature rigidification treatment. This single-dimensional approach has extremely limited control, leading to uneven processing of films of different thicknesses and insufficient heat penetration. When polymer substrates are subjected to subsequent complex tensile stress impacts or transient temperature fluctuations under high-temperature processing conditions, non-equilibrium conformation molecular chains are easily induced to relax, resulting in irreversible thermal shrinkage and asymmetric deformation. Existing technical indicators are no longer sufficient to fundamentally meet the extremely demanding requirements of low shrinkage and dimensional stability for high-precision transfer printing. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing low-shrinkage transfer films based on multi-segment heat setting, thereby addressing the problems of existing transfer film preparation and processing technologies in terms of precise temperature gradient control for different thicknesses and macromolecular chain thermal relaxation times, elimination of internal residual stress gradients generated by biaxial stretching, and adaptive heat treatment mechanisms based on film thickness parameters. The specific technical solution is as follows: A method for preparing a low-shrinkage transfer film based on multi-segment heat setting includes the following steps: The polymer substrate is unwound under constant tension and fed into the first temperature zone, where it is preheated to a temperature range 10°C lower than the glass transition temperature of the polymer substrate and 40°C higher than the glass transition temperature of the polymer substrate to obtain a preheated substrate. The preheated substrate enters the second temperature zone, where it is subjected to a longitudinal tension of 0.98-19.6 N / m in at least three stages of progressively decreasing tension to obtain a main shaped substrate, where it is subjected to a temperature range 20°C to 50°C higher than the cold crystallization temperature of the polymer substrate. The preparation process of the coating liquid for preparing the buffer adhesive layer is as follows: under anhydrous or inert gas protection, polyether diol or polyester diol and diisocyanate are mixed in a solvent, a catalyst is added and reacted at 60-80℃ for 2-4 hours to obtain a polyurethane prepolymer; the polyurethane prepolymer, acrylate monomer, crosslinking agent and initiator are mixed and degassed to obtain the coating liquid. The main shaping substrate enters the third temperature zone and is gradually cooled to 20-60℃ to obtain a cooled substrate. The surface of the cooled substrate is activated and coated with the coating liquid. It then enters the fourth temperature zone of 60-140℃ and is cross-linked and cured by hot air with a constant tension of 1.96-3.92 N / m. The coating liquid is cured to obtain a buffer adhesive layer, thus obtaining a composite film. The composite film is laminated with a quenching roller for quenching treatment and then wound up under constant tension to obtain a low-shrinkage transfer film.

[0005] Preferably, the mass ratio of the polyether diol or polyester diol, the diisocyanate, the catalyst, the acrylate monomer, the crosslinking agent and the initiator is (30-50):(10-20):(0.1-0.5):(20-40):(1-5):(0.5-2).

[0006] Preferably, the polymer substrate is a polyethylene terephthalate film or a biaxially oriented polypropylene film; The temperature of the first temperature zone is controlled within the range of the glass transition temperature of the polymer substrate to 30°C higher than the glass transition temperature, and the tension of the constant tension unwinding is controlled between 4.9 and 14.7 N / m.

[0007] Preferably, the temperature of the second temperature zone is controlled within a range of 20-40°C higher than the cold crystallization temperature of the polymer substrate.

[0008] Preferably, the third temperature zone has 3-5 sub-temperature zones with successively decreasing temperatures, the cooling rate of the gradient cooling is controlled at 5-15℃ / s, and the target temperature after cooling is 30-50℃.

[0009] Preferably, the activation treatment is online plasma treatment or high-frequency corona treatment; the temperature of the fourth temperature zone is 80-120℃.

[0010] Preferably, the surface temperature of the quenching roller is controlled at 10-15°C, and the quenching time is less than 2 seconds.

[0011] Preferably, the preparation method further includes setting the production line speed according to the thickness of the polymer substrate, and controlling the production line speed corresponding to the polymer substrate with a thickness of 12-100μm between 21.6-54m / min.

[0012] The beneficial effects of this invention are as follows: Through the synergistic effect of multiple temperature fields and stepped decreasing longitudinal tension, the viscoelastic stress relaxation dynamics of polymer materials are conformed to, eliminating the non-equilibrium conformation of molecular chains. Through thermomechanical analysis and testing, the thermal shrinkage rate of the prepared transfer film is reduced to less than 1% under the condition of 150℃ / 30min. Combined with the coating of the buffer adhesive layer and the rapid cooling process, the rigidity of the substrate under tension and high temperature is maintained, while the surface is given a smooth and compliant fit. The buffer adhesive layer effectively dissipates interfacial shear stress, improving the adhesion and dimensional stability of the composite film on uneven interfaces. The adaptive residence time allocation step based on film thickness and the use of PID closed-loop control algorithm for temperature control eliminate the phenomenon of insufficient heat penetration or excessive heat degradation of films of different thicknesses during heat treatment, greatly reducing the difference in shrinkage rate between the transverse and longitudinal directions of the entire roll of transfer film. Differential scanning calorimetry characterization shows that the endothermic peak of internal stress release near the glass transition temperature of the treated transfer film disappears, proving the elimination of residual stress inside the material. Detailed Implementation

[0013] The technical solutions in the embodiments of the present invention will be clearly and completely described below; the described embodiments are some embodiments of the present invention, but not all embodiments; based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] In the step of setting the production line speed based on the thickness of the polymer substrate described in this invention, the linear speed and the thickness of the polymer substrate have a non-linear negative correlation. In actual production, the target heat treatment residence time for different substrate thicknesses is directly determined by a preset mapping rule between substrate thickness and heat treatment residence time. Then, the matching production line speed is calculated and set based on the temperature zone length. The mapping rule is obtained through the formula... The calculation is performed, where V is the production line speed, d is the thickness of the polymer substrate, and k and n are constants set according to the temperature zone length and the thermal conductivity coefficient of the polymer material. The preferred value of constant k is 140-160, and the preferred value of constant n is 0.35-0.45, to ensure that the heat penetration and stress release time are consistent for different film thicknesses.

[0015] Example 1:

[0016] This embodiment provides a method for preparing a low-shrinkage transfer film. The polymer substrate used is a 50 μm thick polyethylene terephthalate film. The glass transition temperature of this polymer substrate is measured to be 78°C, the cold crystallization temperature to be 142°C, and the melting point to be 255°C. The processing time of each temperature zone is controlled using a residence time allocation method based on film thickness. Specifically, the production line speed is adjusted according to the film thickness. In this embodiment, the production line speed is set to 30 m / min, and the effective working length of each temperature zone is fixed. The specific steps are as follows: S1 Preheating Treatment: The polyethylene terephthalate film is unwound with a constant tension of 9.8 N / m and fed into the first temperature zone; the temperature of the first temperature zone is controlled at 100℃, the effective working length is 9m, and the dwell time is controlled at 18s; after preheating, a preheated substrate is obtained. S2 Main Shaping Process: The preheated substrate is sent into the second temperature zone; the temperature of the second temperature zone is controlled at 170℃, the effective working length is 19m, and the dwell time is controlled at 38s; during the process, a stepped decreasing longitudinal tension is applied, which is 11.76N / m, 8.82N / m, 5.88N / m and 2.94N / m respectively; after main shaping, the main shaped substrate is obtained. S3 gradient cooling process: The main shaping substrate is sent into the third temperature zone; the third temperature zone is set with 5 decreasing sub-temperature zones, and the temperatures of each sub-temperature zone are 180℃, 140℃, 100℃, 70℃ and 40℃ respectively. The overall cooling rate is controlled at 10℃ / s, the effective working length is 10m, and the dwell time is controlled at 20s. After cooling, a cooled substrate is obtained. This cooled substrate avoids new thermal stress caused by the temperature difference between the surface layer and the core layer through gradient cooling, and its internal crystal structure is perfect. S4 Activation Treatment, Buffer Adhesive Layer Preparation, Coating and Curing: The surface of the cooled substrate is subjected to high-frequency corona treatment to achieve a surface tension of 44 dyn / cm; the buffer adhesive layer is prepared as follows: under anhydrous conditions, polyester diol, isophorone diisocyanate and dibutyltin dilaurate catalyst are added to a solvent and mixed in a mass ratio of 40:15:0.3. The mixture is kept at 70°C and stirred for 3 hours to obtain a polyurethane prepolymer; The obtained polyurethane prepolymer is then mixed with hydroxyethyl acrylate monomer, trimethylolpropane triacrylate crosslinking agent, and azobisisobutyronitrile initiator. Based on the initial feed, the mass ratio of the polyester diol, hydroxyethyl acrylate monomer, trimethylolpropane triacrylate crosslinking agent, and azobisisobutyronitrile initiator is 40:30:3:1. The mixture is stirred evenly and degassed to obtain a polyurethane-acrylate interpenetrating network buffer layer. The buffer layer is uniformly coated on the surface of the activated substrate and placed in the fourth temperature zone. The effective working length of the fourth temperature zone is 11m. Hot air crosslinking and curing are performed at 100℃ with a micro-tension of 2.94N / m and the residence time is controlled to be 22s to obtain a composite film. At this stage, the composite film has a stable mechanical buffer layer and high interfacial bonding strength. S5 Rapid Cooling and Winding: After the composite film is drawn out, it is tightly bonded to the rapid cooling roller. The roller surface temperature is controlled at 12℃ and the rapid cooling time is controlled at 1.5s, so that the surface temperature of the composite film drops to more than 35℃ below the glass transition temperature of the substrate. It is then wound with a constant tension of 7.84N / m to obtain a low-shrinkage transfer film. In this embodiment, each temperature zone is controlled by PID, and the temperature fluctuation is controlled within ±1℃; a total of 6 infrared temperature sensors are set in the heating zone and cooling zone to monitor the temperature distribution on the film surface.

[0017] Example 2:

[0018] The polymer substrate used in this embodiment is a 12μm thick polyethylene terephthalate film. The glass transition temperature of this polymer substrate is measured to be 78℃, the cold crystallization temperature to be 142℃, and the melting point to be 255℃. The residence time in each temperature zone is shortened to accommodate a smaller film thickness, specifically by increasing the production line speed. In this embodiment, the production line speed is set to 54m / min. The specific steps are as follows: S1 Preheating Treatment: The substrate is unwound with a constant tension of 4.9 N / m and fed into the first temperature zone. The effective working length of the first temperature zone is 9 m, the temperature is controlled at 105 ℃, and the dwell time is controlled at 10 s. After preheating, a preheated substrate is obtained. At this time, the surface chain segments of the substrate have been effectively activated and show a preliminary relaxed state at the nanoscale. S2 Main Shaping Process: The preheated substrate is sent into the second temperature zone, with an effective working length of 19.8m, a temperature control of 175℃, and a dwell time control of 22s. The longitudinal tension is applied in a step-decreasing manner, at 7.84N / m, 5.88N / m, 3.92N / m, and 1.96N / m respectively. After the main shaping process, the main shaped substrate is obtained, and its internal residual stress is gradually released under the step tension, forming a relatively stable heat-shaped structure. S3 gradient cooling process: The third temperature zone is set with three decreasing sub-temperature zones, with temperatures of 150℃, 95℃ and 35℃ respectively. The overall cooling rate is controlled at 15℃ / s, the effective working length of the third temperature zone is 10.8m, and the dwell time is controlled at 12s. After cooling, the cooled substrate is obtained, and its internal structure is stably frozen under gradient cooling, effectively suppressing the re-accumulation of local stress. S4 activation treatment, buffer layer preparation, coating and curing: Online plasma treatment is used to make the surface tension of the substrate reach 42dyn / cm; the polyether diol, diisocyanate and catalyst in the buffer layer are mixed in a solvent with a mass ratio of 30:10:0.1, and reacted at 65℃ for 2h to obtain polyurethane prepolymer. The obtained polyurethane prepolymer was then mixed with hydroxyethyl acrylate monomer, trimethylolpropane triacrylate crosslinking agent, and azobisisobutyronitrile initiator. Based on the initial feed, the mass ratio of the polyether diol, hydroxyethyl acrylate monomer, trimethylolpropane triacrylate crosslinking agent, and azobisisobutyronitrile initiator was 30:40:1:0.5. After stirring and degassing, a buffer adhesive layer was obtained. The buffer adhesive layer was then coated and sent to the fourth temperature zone, with an effective working length of 13.5m. It was cured at 80℃ with a micro-tension hot air of 1.96N / m and a residence time of 15s to obtain a composite film. This composite film still achieved good interfacial crosslinking at a lower temperature. S5 rapid cooling and winding: The composite film is cooled by a 10℃ rapid cooling roller, the rapid cooling time is controlled at 1.0s, and the winding tension is controlled at 5.88N / m; In this embodiment, the temperature fluctuation in each temperature zone is controlled within ±1℃.

[0019] Example 3:

[0020] The polymer substrate used in this embodiment is a 100μm thick polyethylene terephthalate film. The glass transition temperature of this polymer substrate is measured to be 78℃, the cold crystallization temperature to be 142℃, and the melting point to be 255℃. The residence time in each temperature zone is extended for a larger film thickness, specifically by reducing the production line speed. In this embodiment, the production line speed is set to 21.6m / min. The specific steps are as follows: S1 Preheating Treatment: The substrate is unwound with a constant tension of 14.7 N / m and fed into the first temperature zone. The effective working length of the first temperature zone is 9 m, the temperature is controlled at 110 ℃, and the dwell time is controlled at 25 s. After preheating, a preheated substrate is obtained. The amorphous region of the thick substrate achieves deep heat penetration and chain segment activation under this long preheating time. S2 Main Shaping Process: The preheated substrate is sent into the second temperature zone, with an effective working length of 19.8m, a temperature control of 185℃, and a dwell time control of 55s. The longitudinal tension is applied in a stepped decreasing manner, at 14.7N / m, 11.76N / m, 7.84N / m, and 3.92N / m respectively. After main shaping, the main shaped substrate is obtained, and the residual orientation in the thickness direction is uniformly and fully eliminated under high temperature and decreasing tension. S3 gradient cooling process: The third temperature zone is set with 5 decreasing sub-temperature zones, with temperatures of 190℃, 150℃, 110℃, 80℃ and 50℃ respectively. The overall cooling rate is controlled at 5℃ / s. The effective working length of the third temperature zone is 10.8m and the dwell time is controlled at 30s. After cooling, a cooled substrate is obtained. The slow gradient cooling allows the crystals inside the thick film to develop completely and the nanoscale stress distribution to be more uniform. S4 activation treatment, buffer layer preparation, coating and curing: High-frequency corona treatment is used to make the surface tension of the substrate reach 46 dyn / cm; the polyester diol, diisocyanate and catalyst in the buffer layer are mixed in a solvent with a mass ratio of 50:20:0.5, and reacted at 80℃ for 4h to obtain polyurethane prepolymer. The obtained polyurethane prepolymer is then mixed with hydroxyethyl acrylate monomer, trimethylolpropane triacrylate crosslinking agent and azobisisobutyronitrile initiator. The mass ratio of the polyester diol, hydroxyethyl acrylate monomer, trimethylolpropane triacrylate crosslinking agent and azobisisobutyronitrile initiator is 50:20:5:2 based on the initial feed. After stirring and degassing, a buffer adhesive layer is obtained; the buffer adhesive layer is coated and sent to the fourth temperature zone, the effective working length of the fourth temperature zone is 10.08m, and cured by applying 3.92N / m micro-tension hot air at 120℃, with the residence time controlled at 28s, to obtain a composite film. S5 rapid cooling and winding: The composite film is cooled by a 15℃ rapid cooling roller, with the rapid cooling time controlled at 1.8s and the winding tension controlled at 9.8N / m; In this embodiment, the temperature fluctuation in each temperature zone is controlled within ±1℃.

[0021] Example 4:

[0022] The polymer substrate used in this embodiment is a 25μm thick polyethylene terephthalate film. The production line speed is set to 38.5m / min. The specific steps are as follows: S1 preheating treatment: The substrate is unwound with a constant tension of 7.84 N / m and fed into the first temperature zone, where the temperature is controlled at 90°C and the dwell time is controlled at 14 s; S2 Main Shaping Process: The preheated substrate is sent into the second temperature zone, the temperature is controlled at 180℃, and the dwell time is controlled at 30s; the longitudinal tension is applied in a step-decreasing manner, at 9.8N / m, 6.86N / m, 4.9N / m and 1.96N / m respectively; S3 gradient cooling process: The third temperature zone is set with 4 decreasing sub-temperature zones, with temperatures of 165℃, 120℃, 80℃ and 30℃ respectively. The overall cooling rate is controlled at 12℃ / s and the dwell time is controlled at 16s. S4 Activation Treatment, Buffer Layer Preparation, Coating and Curing: Online plasma treatment is used to achieve a substrate surface tension of 43 dyn / cm; the polyester diol, diisocyanate and catalyst in the buffer layer are mixed in a solvent at a mass ratio of 35:12:0.2, and reacted at 68℃ for 2.5h to obtain a polyurethane prepolymer; the polyurethane prepolymer is then mixed with acrylate monomers, crosslinking agents and initiators at a mass ratio of 35:35:2:0.8 based on the initial feed; after stirring and degassing, a buffer layer is obtained; the buffer layer is coated and sent to the fourth temperature zone, where it is cured at 90℃ with 2.45 N / m micro-tension hot air, and the residence time is controlled to be 18s; S5 quenching and winding: The composite film is cooled by a 13℃ quenching roller, the quenching time is controlled at 1.2s, and the winding tension is controlled at 6.86N / m; In this embodiment, the temperature fluctuation in each temperature zone is controlled within ±1℃.

[0023] Comparative Example 1: The difference between this comparative example and Example 1 is that the temperature of the first temperature zone is changed to 60°C, while the other operating steps and process parameters are exactly the same as those in Example 1.

[0024] Comparative Example 2: The difference between this comparative example and Example 1 is that the longitudinal tension in the second temperature zone is not reduced in a stepwise manner, but is kept constant at 9.8 N / m throughout. Other operating steps and process parameters are exactly the same as in Example 1.

[0025] Comparative Example 3: The difference between this comparative example and Example 1 is that the temperature of the second temperature zone is changed to 130°C, while the other operating steps and process parameters are exactly the same as in Example 1.

[0026] Comparative Example 4: The difference between this comparative example and Example 1 is that: no decreasing sub-temperature zone is set in the third temperature zone, and the substrate after main shaping is directly reduced from 220°C to 40°C, with the average cooling rate controlled at 25°C / s. Other operating steps and process parameters are exactly the same as in Example 1.

[0027] Comparative Example 5: The difference between this comparative example and Example 1 is that the buffer adhesive layer does not use a polyurethane-acrylate interpenetrating polymer network system, but is instead a polyacrylate monolayer formed only by acrylate monomers, crosslinking agents and initiators. Other operating steps and process parameters are exactly the same as in Example 1.

[0028] Comparative Example 6: The difference between this comparative example and Example 1 is that the activation treatment step is omitted, and a buffer adhesive layer is directly coated on the surface of the cooling substrate. Other operation steps and process parameters are exactly the same as in Example 1.

[0029] Comparative Example 7: The difference between this comparative example and Example 1 is that the rapid cooling step of the quenching roller is omitted, and the composite film is directly wound up after conventional cooling. Other operating steps and process parameters are exactly the same as in Example 1.

[0030] Performance testing and data sheets: The heat shrinkage rate test method is as follows: Take a sample and cut 100mm×100mm specimens along the longitudinal and transverse directions. Treat them at 150℃ for 30min, measure the dimensional changes before and after treatment, and calculate the longitudinal heat shrinkage rate and transverse heat shrinkage rate respectively; the endothermic peak of internal stress release is measured by DSC, the heating rate is 10℃ / min, and the area of ​​the endothermic peak near the glass transition temperature is recorded; the surface tension is measured by a dyne pen according to GB / T14216 standard; the haze is measured by a haze meter according to GB / T2410 standard; the 180° peel strength is measured by a tensile testing machine according to GB / T2792 standard at a peel speed of 300mm / min. Table 1 Performance test results of each embodiment and comparative example

[0031]

[0032]

[0033] As can be seen from the comparison of the test results of Example 1 and Comparative Example 1 in Table 1, the lower temperature in the first temperature zone leads to an increase in thermal shrinkage rate, an increase in the difference between longitudinal and transverse shrinkage rates, and an obvious endothermic peak near the glass transition temperature. When the preheating temperature is lower than the range required for full activation of the chain segments, the oriented chain segments in the amorphous region cannot enter a sufficiently relaxed state. Although the second temperature zone has high-temperature treatment, the stress release starting point is insufficient, and a lot of biaxial tensile residual stress is still retained inside the film. Therefore, it is more likely to shrink under the condition of 150°C, and an obvious endothermic peak of internal stress release can still be detected in DSC. As can be seen from the comparison of the test results of Example 1 and Comparative Example 2 in Table 1, the failure to use a stepped decreasing tension in the second temperature zone will lead to an increase in the thermal shrinkage rate and an increase in the difference between the longitudinal and transverse shrinkage rates. Under high temperature conditions, the chain segments already have high mobility. If the external tension remains constant, the stretched molecular chains will continue to be stretched and restricted during the relaxation process, making it difficult to complete sufficient retraction and random rearrangement. As a result, the residual orientation in the film layer cannot be uniformly eliminated, and therefore the shrinkage rate and directional differences are significantly increased. As can be seen from the comparison of the test results of Example 1 and Comparative Example 3 in Table 1, a lower temperature in the second temperature zone will lead to an increase in the thermal shrinkage rate and retain a higher endothermic peak for internal stress release. When the main setting temperature is insufficient, the heat penetration of the film core is insufficient, and some amorphous chain segments and crystalline boundary areas are still in a restricted state, which cannot form a relatively stable thermal setting structure, resulting in the inability to effectively eliminate internal residual stress.

[0034] The above are merely specific embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any conventional modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a low-shrinkage transfer film based on multi-segment heat setting, characterized in that, Including the following steps: The polymer substrate is unwound under constant tension and fed into the first temperature zone, where it is preheated to a temperature range 10°C lower than the glass transition temperature of the polymer substrate and 40°C higher than the glass transition temperature of the polymer substrate to obtain a preheated substrate. The preheated substrate enters the second temperature zone, where it is subjected to a longitudinal tension of 0.98-19.6 N / m in at least three stages of progressively decreasing tension to obtain a main shaped substrate, where it is subjected to a temperature range 20°C to 50°C higher than the cold crystallization temperature of the polymer substrate. The preparation process of the coating liquid for preparing the buffer adhesive layer is as follows: under anhydrous or inert gas protection, polyether diol or polyester diol and diisocyanate are mixed in a solvent, a catalyst is added and reacted at 60-80℃ for 2-4 hours to obtain a polyurethane prepolymer; the polyurethane prepolymer, acrylate monomer, crosslinking agent and initiator are mixed and degassed to obtain the coating liquid. The main shaping substrate enters the third temperature zone and is gradually cooled to 20-60℃ to obtain a cooled substrate. The surface of the cooled substrate is activated and coated with the coating liquid. It then enters the fourth temperature zone of 60-140℃ and is cross-linked and cured by hot air with a constant tension of 1.96-3.92 N / m. The coating liquid is cured to obtain a buffer adhesive layer, thus obtaining a composite film. The composite film is laminated with a quenching roller for quenching treatment and then wound up under constant tension to obtain a low-shrinkage transfer film.

2. The method for preparing a low-shrinkage transfer film based on multi-segment heat setting according to claim 1, characterized in that, The mass ratio of the polyether diol or polyester diol, the diisocyanate, the catalyst, the acrylate monomer, the crosslinking agent and the initiator is (30-50):(10-20):(0.1-0.5):(20-40):(1-5):(0.5-2).

3. The method for preparing a low-shrinkage transfer film based on multi-segment heat setting according to claim 1, characterized in that, The polymer substrate is a polyethylene terephthalate film or a biaxially oriented polypropylene film. The temperature of the first temperature zone is controlled within the range of the glass transition temperature of the polymer substrate to 30°C higher than the glass transition temperature, and the tension of the constant tension unwinding is controlled between 4.9 and 14.7 N / m.

4. The method for preparing a low-shrinkage transfer film based on multi-segment heat setting according to claim 1, characterized in that, The temperature of the second temperature zone is controlled within a range of 20-40°C higher than the cold crystallization temperature of the polymer substrate.

5. The method for preparing a low-shrinkage transfer film based on multi-segment heat setting according to claim 1, characterized in that, The third temperature zone has 3-5 sub-temperature zones with successively decreasing temperatures. The cooling rate of the gradient cooling is controlled at 5-15℃ / s, and the target temperature after cooling is 30-50℃.

6. The method for preparing a low-shrinkage transfer film based on multi-segment heat setting according to claim 1, characterized in that, The activation treatment is an online plasma treatment or a high-frequency corona treatment; the temperature of the fourth temperature zone is 80-120℃.

7. The method for preparing a low-shrinkage transfer film based on multi-segment heat setting according to claim 1, characterized in that, The surface temperature of the quenching roller is controlled at 10-15℃, and the quenching time is less than 2 seconds.

8. The method for preparing a low-shrinkage transfer film based on multi-segment heat setting according to claim 1, characterized in that, The preparation method further includes setting the production line speed according to the thickness of the polymer substrate, and controlling the production line speed corresponding to the polymer substrate with a thickness of 12-100μm between 21.6-54m / min.

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