Highly transparent low haze shrink film and method of making same

Abstract

This invention relates to the field of shrink films, specifically disclosing a high-transparency, low-haze shrink film and its preparation method. The shrink film has an A / B / A three-layer structure, with layer A having a thickness of 2–5 μm and layer B having a thickness of 30–60 μm. Layer A, by weight (100 wt%), comprises 90–99 wt% modified PET and 1–10 wt% functional masterbatch; layer B, by weight (100 wt%), comprises 90–100 wt% modified PET and 0–10 wt% PET. This invention utilizes a compounded diol and introduces a branching agent to modify PET, combined with precise proportioning of functional masterbatch and a three-layer co-extrusion structure design, simultaneously achieving a synergistic improvement in anisotropic shrinkage performance, high transparency, and solvent resistance.

Description

Technical Field

[0001] This invention belongs to the field of shrink film, specifically relating to a high-transparency, low-haze shrink film and its preparation method. Background Technology

[0002] Heat-shrinkable polyester film is widely used in beverage bottle labels, battery labels, and irregularly shaped container packaging due to its excellent shrinkage performance, good transparency, and environmentally friendly recyclability. As the packaging industry increasingly demands higher precision in label application, better appearance quality, and adaptability to post-processing, the market is placing higher requirements on heat-shrinkable films: they must not only possess controllable anisotropic shrinkage rates (high transverse, low longitudinal), but also maintain high transparency and low haze after heat shrinking and contact with printing solvents to meet the needs of fine printing, high-speed labeling, and media-resistant applications.

[0003] Ordinary PET heat-shrink film is typically prepared using a biaxial stretching process, with shrinkage rates differentiated by adjusting the longitudinal and transverse stretch ratios and the setting temperature. However, ordinary PET is a semi-crystalline polyester with high molecular chain regularity and a fast crystallization rate. During heat shrinkage, it is prone to light scattering at the interface between crystalline and amorphous regions, resulting in a significant increase in haze after shrinkage (typically >10%), affecting the clarity and visual effect of the label's text and images. To address this issue, some technologies employ copolymer-modified PET, such as replacing part of the terephthalic acid with isophthalic acid (IPA) to reduce crystallinity and improve transparency. However, such modifications often come at the cost of sacrificing heat-shrinkage performance or solvent resistance, making it difficult to achieve a balance between high shrinkage, low haze, and solvent resistance.

[0004] Regarding shrinkage direction control, traditional heat shrink films typically exhibit high shrinkage rates in both the longitudinal and transverse directions (e.g., MD≤3%, TD≥45% in a 90℃-10S water bath test). During labeling, excessive longitudinal shrinkage can easily lead to wrinkling at both ends of the label or deformation of the pattern. To achieve "high transverse, low longitudinal" shrinkage characteristics, existing technologies often employ extremely low longitudinal stretch ratios combined with high-temperature heat setting. However, this method can easily result in insufficient longitudinal strength of the film, leading to film breakage or stretching deformation during winding, slitting, and high-speed labeling. Furthermore, in terms of solvent resistance, labels may come into contact with printing solvents (such as ethyl acetate and ethanol) or organic components in the contents during use. After brief immersion in solvents such as ethyl acetate, ordinary PET film or copolymer-modified PET film is prone to swelling or micro-cracks on the surface, resulting in increased haze and affecting the label's transparency and appearance.

[0005] In the prior art, CN117885423A discloses a flexible heat-shrinkable composite film with low longitudinal shrinkage rate and its preparation method, employing an A / B / A three-layer structure. The A layer is made of PETG-modified polyester chips and open slip masterbatch, while the B layer is made of PETG-modified polyester chips and PET-modified polyester chips. It reduces the MD shrinkage rate by cutting off the edges, but the MD shrinkage rate remains positive (0-3%). During labeling, the film still shortens to a certain extent in the longitudinal direction, easily leading to wrinkling at the label ends or pattern deformation. CN115491000B discloses a low-temperature, high-toughness polyester shrink film for heat-shrinkable labels, its preparation method, and applications. It uses physical blending to form a multiphase structure, and the refractive index difference at the phase interface may cause light scattering, increasing the intrinsic haze of the film. Since the existing technology cannot simultaneously meet the requirements of high transverse shrinkage, low longitudinal shrinkage, low haze after shrinkage, and small haze increase after short immersion in organic solvents, this invention develops a heat-shrinkable film with excellent anisotropic shrinkage performance, high transparency, low haze, and good solvent resistance. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention uses compounded diols and introduces branching agents to modify PET, combined with precise proportions of functional masterbatch and a three-layer co-extrusion structure design, to achieve synergistic improvement of multiple performance indicators.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a high-transparency, low-haze shrink film with a three-layer A / B / A structure. The thickness of layer A is 2-5 μm, and the thickness of layer B is 30-60 μm. The raw material of layer A, based on 100 wt%, includes 90-99 wt% modified PET and 1-10 wt% functional masterbatch. The raw material of layer B, based on 100 wt%, includes 90-100 wt% modified PET and 0-10 wt% PET. The preparation steps of the modified PET are as follows: Terephthalic acid, diol, and trifunctional branching agent are melted and mixed, and a catalyst, heat stabilizer, and antioxidant are added. The mixture is stirred at 60-70°C for 30-45 minutes to form a slurry. The slurry is reacted at 210-260°C and 0.01-0.25 MPa until the water content is ≥96%, resulting in an esterification product. The esterification product undergoes three-stage polycondensation, and a terminator is added and stirred for 10-20 minutes to obtain a melt. The melt is extruded through a die, water-cooled and pelletized, and vacuum dried to constant weight to obtain modified PET.

[0008] Preferably, the raw material of layer A, calculated by 100wt%, comprises: 95.8-99.8wt% modified PET and 0.2-4.2wt% functional masterbatch; the raw material of layer B, calculated by 100wt%, comprises: 92-99wt% modified PET and 1-8wt% PET.

[0009] The transparent shrink film provided by this invention has good processing stability and shrinkage uniformity. Layer A is mainly made of modified PET to ensure the shrinkage synergy between layer A and layer B. The addition of an appropriate amount of functional masterbatch can reduce the coefficient of friction and prevent adhesion without affecting the shrinkage of the main body.

[0010] The effect of modified PET in layer B is mainly to ensure overall shrinkage performance and transparency. Adding a small amount of PET can improve film rigidity and adjust costs. Ordinary PET has strong crystallization ability, and adding a small amount can increase modulus. However, its crystallization area will become stress concentration point during heat shrinkage. This invention avoids haze increase and weakens the high transverse shrinkage and low longitudinal shrinkage characteristics given by modified PET by controlling the amount of PET added.

[0011] In some embodiments, the molar ratio of terephthalic acid, diol, and branching agent is 1:(0.9-1.2):(0.002-0.008).

[0012] Preferably, the amounts of catalyst, heat stabilizer, antioxidant, and terminator are 0.03–0.06%, 0.02–0.04%, 0.01–0.03%, and 0.01–0.02% of the molar amount of terephthalic acid, respectively.

[0013] By controlling the amount of diol in the raw materials to increase the esterification reaction rate and compensate for the alcohol loss during the polycondensation stage, and by introducing a small amount of branching agent to form a small number of branching points in the molecular chain without causing cross-linking, the melt can have a moderate strain hardening effect during stretching.

[0014] In some embodiments, the diol comprises at least one of ethylene glycol, neopentyl glycol, or 1,4-cyclohexanediethanol.

[0015] Preferably, the molar ratio of ethylene glycol, neopentyl glycol and 1,4-cyclohexanediethanol is 0.6–1.0: 0.12–0.25: 0.20–0.35.

[0016] This invention utilizes a compound of three diols to synergistically regulate the crystallization behavior and heat shrinkage properties of polyester. Ethylene glycol provides main chain flexibility, while the side methyl groups of neopentyl glycol and the cyclohexane structure of 1,4-cyclohexenedimethanol jointly disrupt the regularity of the molecular chain, significantly reducing the crystallization ability of the copolyester. This makes the film more inclined to release stress in the amorphous region during heat shrinkage. The orientation stress stored in the amorphous region during stretching is uniformly released when heated, avoiding local light scattering or uneven shrinkage caused by the shrinkage difference between the crystalline and amorphous regions. This lays the molecular foundation for high transparency and anisotropic shrinkage.

[0017] In some embodiments, the trifunctional branching agent is at least one of glycerol, trimellitic anhydride, and trimethylolpropane.

[0018] Taking ethyl acetate as a printing solvent as an example, the setting temperature must be above 78℃ to ensure the printing immersion effect of ethyl acetate. However, the transverse shrinkage rate is negatively correlated with the heat setting temperature: that is, the lower the setting temperature, the higher the transverse shrinkage rate (usually 70-75℃). This means that the printing immersion effect directly conflicts with the high transverse shrinkage rate required by this invention. Accordingly, the process needs to increase the stretch ratio (usually 4.3-4.6) to improve the transverse shrinkage rate, but this will lead to a certain increase in haze.

[0019] This invention introduces a trifunctional branching agent into the copolymer system to form a microbranched ternary copolyester. The branching agent introduces a small number of long branches into the ternary copolymer backbone, causing strain hardening of the molecular chain during melt stretching. This facilitates the orientation of the molecular chain along the TD direction and maintains the oriented structure during transverse stretching. The cyclohexane structure of 1,4-cyclohexenedimethanol and the side methyl groups of neopentyl glycol jointly disrupt the regularity of the molecular chain, and the branching points further hinder the orderly arrangement of chain segments, thus inhibiting crystallization. The synergistic effect of these three factors significantly reduces the crystallinity of the copolyester, resulting in very few light scattering centers during heat shrinkage, and the haze after shrinkage can be controlled below 5.0%.

[0020] Modified PET stores high elastic deformation after stretching and orientation, and releases strong lateral shrinkage force when heated; the branching points restrict the longitudinal creep and slippage of the molecular chains, reducing the longitudinal shrinkage rate or even causing negative shrinkage (elongation). The two work together to further improve the TD / MD shrinkage ratio, thereby achieving high anisotropy.

[0021] The cyclic structure of 1,4-cyclohexanediethanol increases molecular chain rigidity, and the branching points form a physical cross-linking network that extends the solvent molecule penetration path. Both factors synergistically reduce the solvent penetration rate. In terms of processing stability, the branched structure improves melt strength, reducing the risk of edge necking and film breakage during longitudinal stretching. The reduced crystallization rate of 1,4-cyclohexanediethanol expands the stretching temperature range, making it less prone to crystallization haze even at lower stretching temperatures. These two factors synergistically improve production stability and yield.

[0022] In some embodiments, the conditions for the three-stage polycondensation are as follows: reaction at 260–270°C and 1000–5000 Pa for 60–90 min; reaction at 270–280°C and 100–500 Pa for 60–90 min; and reaction at 280–290°C and 50–100 Pa until the intrinsic viscosity is 0.65–0.75 dL / g.

[0023] In the preparation of modified PET, the vacuum and temperature are gradually increased through three stages of polycondensation. The first stage of pre-polymerization removes small molecules under a lower vacuum to prevent boiling. The second stage of intermediate polycondensation further increases the vacuum and promotes molecular chain growth. The third stage reacts under high vacuum to the target intrinsic viscosity. This viscosity range ensures sufficient melt strength during film forming and avoids excessive processing load due to excessively high molecular weight. After reaching the target viscosity, a terminator is added to capture active centers in time to prevent excessive branching reaction that could lead to gelation or crosslinking.

[0024] In some embodiments, the functional masterbatch is composed of inorganic powder and erucamide in a mass ratio of (1-4):1.

[0025] In some embodiments, the particle size of the inorganic powder is 2 to 4 μm.

[0026] Preferably, the inorganic powder is one or more of silica, talc, clay, and kaolin.

[0027] Inorganic powder provides rigid micro-protrusions to prevent interlayer adhesion, while organic erucic acid amide forms a low surface energy lubricating layer on the surface. The combination of the two can achieve a dynamic and static friction coefficient of ≤0.45, solving the adhesion problem during film winding and use, and meeting the requirements of high-speed winding and post-processing. At the same time, by strictly controlling the particle size of inorganic powder (2~4μm), it is ensured that the haze will not increase significantly after shrinkage.

[0028] Another aspect of the present invention provides a method for preparing the above-mentioned high-transparency, low-haze shrink film, the specific steps of which are as follows: S1. The modified PET and PET are dried to a moisture content of ≤50 ppm. Then, the feeding ratio of the A layer raw material and the B layer raw material is controlled by the metering feeding device. After the materials are initially mixed by the mixer, they are plasticized and melted in the twin-screw exhaust extruder. They are then fed into the three-layer co-extrusion die through the coarse filter, metering pump, fine filter and static mixer. S2. The melt is extruded through a three-layer co-extrusion die, cooled into a cast sheet, and then stretched longitudinally and laterally, heat-set, cooled, and measured online before being wound up to obtain a transparent shrink film.

[0029] In some embodiments, in step S2, the longitudinal stretching ratio is 1.0 to 1.5, the stretching temperature is 75 to 95°C, the transverse stretching ratio is 4.0 to 5.2, the stretching temperature is 75 to 95°C, the heat setting temperature is 70 to 82°C, and a 2 to 5% transverse relaxation is provided at the end of the setting section.

[0030] The low longitudinal stretch ratio combined with the high transverse stretch ratio creates an asymmetric orientation, allowing the film to store a large amount of high elastic deformation in the TD direction and a weak orientation in the MD direction. Heat setting can partially fix the orientation structure to avoid excessive shrinkage without excessively eliminating shrinkage potential. Combined with 2-5% transverse relaxation, the final shrinkage rate can be precisely controlled.

[0031] Compared with the prior art, the present invention has the following beneficial effects: 1. This transparent shrink film achieves a balance between processing stability and shrinkage uniformity through a three-layer structure. Both the surface layer and the core layer are mainly made of modified PET. The surface layer also adds functional masterbatch to reduce the coefficient of friction and prevent sticking without affecting the shrinkage of the main body. The core layer adds PET to improve the rigidity of the film and adjust the cost.

[0032] 2. Modified PET utilizes the synergistic combination of three diols to regulate the crystallization behavior and heat shrinkage characteristics of polyester. Ethylene glycol provides main chain flexibility, while the side methyl groups of neopentyl glycol and the cyclohexane structure of 1,4-cyclohexenedimethanol jointly disrupt the regularity of the molecular chain, thereby reducing the crystallization ability of the copolyester. This avoids local light scattering or uneven shrinkage caused by the shrinkage difference between crystalline and amorphous regions, laying the molecular foundation for high transparency and anisotropic shrinkage.

[0033] 3. Modified PET introduces a small number of long branches into the ternary copolymer backbone by adding a trifunctional branching agent. This causes strain hardening of the molecular chain during melt stretching, which is beneficial for the molecular chain to orient along the TD direction and maintain the oriented structure during transverse stretching. Furthermore, the branching points further hinder the orderly arrangement of chain segments, thereby inhibiting crystallization, reducing light scattering centers, and reducing haze after shrinkage. In addition, the branching points can restrict the longitudinal creep and slip of the molecular chain, reducing the longitudinal shrinkage rate or even causing negative shrinkage (elongation), thus achieving high anisotropy.

[0034] 4. In the production of transparent shrink film, the branched structure in modified PET can improve melt strength and reduce the risk of edge necking and film breakage during longitudinal stretching. The addition of 1,4-cyclohexanediethanol reduces the crystallization rate and expands the stretching temperature range. The two work together to improve production stability and yield.

[0035] 5. By combining inorganic powder and organic erucic acid amide, the coefficients of dynamic and static friction are both ≤0.45, solving the adhesion problem during film winding and use, and meeting the requirements of high-speed winding and post-processing. Detailed Implementation

[0036] The present invention will be described below with reference to specific implementation schemes. It should be noted that the following embodiments are examples of the present invention and are used only to illustrate the invention, not to limit it. Other combinations and various modifications within the scope of the present invention can be made without departing from its spirit or scope. It is worth noting that, unless otherwise specified, the raw materials used in the following preparation examples and embodiments are all from any commercially available manufacturer: The PET model is CR-8816 and can be purchased from any manufacturer.

[0037] Preparation Example 1 The preparation steps for modified PET-A are as follows: 1 mol of terephthalic acid, 0.7 mol of ethylene glycol, 0.16 mol of neopentyl glycol, 0.24 mol of 1,4-cyclohexanediol, and 0.005 mol of trimethylolpropane were mixed, and 0.0004 mol of antimony acetate catalyst, 0.0003 mol of trimethyl phosphate heat stabilizer, and 0.0002 mol of triphenyl phosphite antioxidant were added. The mixture was stirred at 65°C for 40 min to form a slurry. The slurry was reacted at 240°C and -0.01 MPa until the water content was ≥96%, yielding the esterified product. The esterified product was then subjected to three-stage polycondensation under the following conditions: 265°C and 3000 Pa for 80 min; 275°C and 400 Pa for 80 min; and 285°C and 50 Pa for 285°C until the intrinsic viscosity reached 0.7. Add 0.00015 mol of phosphoric acid as a terminator to dL / g and stir for 20 min to obtain a melt; extrude the melt through a die, water-cool and pelletize, and vacuum dry to constant weight to obtain modified PET-A.

[0038] Preparation Example 2 The preparation steps for modified PET-B are as follows: 1 mol terephthalic acid, 0.7 mol ethylene glycol, 0.16 mol neopentyl glycol, 0.24 mol 1,4-cyclohexanediol, and 0.005 mol trimethylolpropane were mixed, and 0.0004 mol antimony acetate catalyst, 0.0003 mol trimethyl phosphate heat stabilizer, and 0.0002 mol triphenyl phosphite antioxidant were added. The mixture was stirred at 65°C for 40 min to form a slurry. The slurry was reacted at 240°C and -0.01 MPa until the water content was ≥96%, yielding the esterified product. The product was then reacted at 280°C and 100 Pa until the intrinsic viscosity was 0.7 dL / g. Phosphoric acid was added as a terminator and stirred for 20 min to obtain a melt. The melt was extruded through a die, water-cooled and pelletized, and vacuum dried to constant weight to obtain modified PET-B.

[0039] Preparation Example 3 The preparation steps for modified PET-C are as follows: Mix 1 mol terephthalic acid, 0.7 mol ethylene glycol, 0.16 mol neopentyl glycol, and 0.24 mol 1,4-cyclohexanediethanol, and follow the same steps as in Preparation Example 1.

[0040] Preparation Example 4 The preparation steps for modified PET-D are as follows: Mix 1 mol terephthalic acid, 0.8 mol ethylene glycol, 0.3 mol neopentyl glycol, and 0.005 mol trimethylolpropane, and follow the same steps as in Preparation Example 1.

[0041] Preparation Example 5 The preparation steps for modified PET-E are as follows: Mix 1 mol terephthalic acid, 0.8 mol ethylene glycol, 0.3 mol 1,4-cyclohexanediethanol and 0.005 mol trimethylolpropane, and follow the same steps as in Preparation Example 1.

[0042] Example 1 A transparent shrink film has a three-layer structure (A / B / A), with layer A having a thickness of 4 μm and layer B having a thickness of 45 μm. Layer A, by weight (100 wt%), comprises: 97.5 wt% modified PET-A and 2.5 wt% functional masterbatch (2.0 wt% 3 μm silica and 0.5 wt% erucamide). Layer B, by weight (100 wt%), comprises: 95 wt% modified PET-A and 5 wt% PET. The method for preparing the transparent shrink film in this embodiment is as follows: S1. The modified PET-A and PET are dried to a moisture content of ≤50 ppm. Then, the A layer raw material and the B layer raw material are fed in proportion controlled by a metering device. After the materials are initially mixed by a mixer, they are fed into a twin-screw exhaust extruder for plasticizing and melting. The extruder feed section temperature is 230℃, the compression section temperature is 250℃, the speed is 200rpm, and the material residence time is 3min. The material passes through a coarse filter, a metering pump, a fine filter, and a static mixer before entering the three-layer co-extrusion die. S2. The melt is extruded through a three-layer co-extrusion die and cooled into a casting. The casting is then stretched longitudinally and transversely in sequence. The longitudinal stretching ratio is 1.0 and the stretching temperature is 85℃. The transverse stretching ratio is 5.0 and the stretching temperature is 90℃. The casting is then heat-set at 80℃, with a 4% transverse relaxation at the end of the setting section. After cooling, the thickness is measured online and the casting is wound up to obtain a transparent shrink film.

[0043] Example 2 A transparent shrink film has a three-layer structure (A / B / A), with layer A having a thickness of 4 μm and layer B having a thickness of 45 μm. Layer A, by weight (100 wt%), comprises: 95.8 wt% modified PET-A and 4.2 wt% functional masterbatch (3.2 wt% 2 μm silica and 1 wt% erucamide). Layer B, by weight (100 wt%), comprises: 92 wt% modified PET-A and 8 wt% PET. The method for preparing the transparent shrink film in this embodiment is as follows: S1. The modified PET-A and PET are dried to a moisture content of ≤50 ppm. Then, the A layer raw material and the B layer raw material are fed in proportion controlled by a metering device. After the materials are initially mixed by a mixer, they are fed into a twin-screw exhaust extruder for plasticizing and melting. The extruder feed section temperature is 230℃, the compression section temperature is 250℃, the speed is 200rpm, and the material residence time is 3min. The material passes through a coarse filter, a metering pump, a fine filter, and a static mixer before entering the three-layer co-extrusion die. S2. The melt is extruded through a three-layer co-extrusion die and cooled into a casting. The casting is then stretched longitudinally and transversely in sequence. The longitudinal stretching ratio is 1.2 and the stretching temperature is 80℃. The transverse stretching ratio is 4.7 and the stretching temperature is 90℃. The casting is then heat-set at 80℃, with a 2% transverse relaxation at the end of the setting section. After cooling, the thickness is measured online and the casting is wound up to obtain a transparent shrink film.

[0044] Example 3 A transparent shrink film has a three-layer structure (A / B / A), with layer A having a thickness of 4μm and layer B having a thickness of 45μm. Layer A, by weight (100wt%), comprises: 99.8wt% modified PET-A and 0.2wt% functional masterbatch (0.1wt% 4μm silica and 0.1wt% erucamide). Layer B, by weight (100wt%), comprises: 97wt% modified PET-A and 3wt% PET. The method for preparing the transparent shrink film in this embodiment is as follows: S1. The modified PET-A and PET are dried to a moisture content of ≤50 ppm. Then, the A layer raw material and the B layer raw material are fed in proportion controlled by a metering device. After the materials are initially mixed by a mixer, they are fed into a twin-screw exhaust extruder for plasticizing and melting. The extruder feed section temperature is 230℃, the compression section temperature is 250℃, the speed is 200rpm, and the material residence time is 3min. The material passes through a coarse filter, a metering pump, a fine filter, and a static mixer before entering the three-layer co-extrusion die. S2. The melt is extruded through a three-layer co-extrusion die and cooled into a casting. The casting is then stretched longitudinally and transversely in sequence. The longitudinal stretching ratio is 1.5 and the stretching temperature is 95℃. The transverse stretching ratio is 4.5 and the stretching temperature is 95℃. The casting is then heat-set at 80℃, with a 5% transverse relaxation at the end of the setting section. After cooling, the thickness is measured online and the casting is wound up to obtain a transparent shrink film.

[0045] Example 4 This embodiment provides a transparent shrink film and its preparation method. The specific implementation method is the same as that in Embodiment 1, except that the modified PET-A is replaced by an equal amount of modified PET-B.

[0046] Example 5 This embodiment provides a transparent shrink film and its preparation method. The specific implementation method is the same as that in Embodiment 1, except that the modified PET-A is replaced by an equal amount of modified PET-C.

[0047] Example 6 This embodiment provides a transparent shrink film and its preparation method. The specific implementation method is the same as that in Embodiment 1, except that the modified PET-A is replaced by an equal amount of modified PET-D.

[0048] Example 7 This embodiment provides a transparent shrink film and its preparation method. The specific implementation method is the same as that in Embodiment 1, except that the modified PET-A is replaced by an equal amount of modified PET-E.

[0049] Comparative Example 1 A transparent shrink film has a three-layer structure of A / B / A, with layer A having a thickness of 4μm and layer B having a thickness of 45μm; the raw material of layer A is 100wt% modified PET-A; the raw material of layer B, based on 100wt%, includes: 95wt% modified PET-A and 5wt% PET. The preparation method of the transparent shrink film in this embodiment is the same as that in Example 1.

[0050] Comparative Example 2 A transparent shrink film has a three-layer structure (A / B / A), with layer A having a thickness of 4 μm and layer B having a thickness of 45 μm. Layer A, by weight (100 wt%), comprises: 97.5 wt% modified PET-A and 2.5 wt% functional masterbatch (2.0 wt% 3 μm silica and 0.5 wt% erucamide). Layer B is composed of 100 wt% modified PET-A. The preparation method of the transparent shrink film in this embodiment is the same as that in Example 1.

[0051] Performance testing The transparent shrink films provided in Examples 1-7 and Comparative Examples 1-2 were tested as follows, and the results are shown in Table 1: 1. Heat shrinkage rate test: Cut the film into 10cm×0cm square samples, place them at 80℃ for 10 seconds, remove them and cool them to room temperature, measure the dimensional changes of the samples in the TD and MD directions, and calculate the shrinkage rate.

[0052] 2. Haze test: Take a film sample and test it according to GB / T 2410 using a haze meter (such as BYK-Gardner Haze-Gard); treat the sample according to the above heat shrinkage test conditions (80℃ / 10s), and test the haze at the same position after cooling; immerse the sample in ethyl acetate at room temperature for 5 seconds, take it out and let it air dry naturally, and then test the haze.

[0053] 3. Friction coefficient test: The two A-layer surfaces (i.e., the outer surface of the film) are brought into contact, and the static friction coefficient is tested at a speed of 100 mm / min on a friction coefficient meter.

[0054] 4. Mechanical property testing: Cut dumbbell-shaped specimens along the MD direction and test the elongation at break at a tensile speed of 100 mm / min on a universal testing machine.

[0055] Table 1

[0056] In Table 1, Examples 1-3 achieved excellent shrinkage anisotropy and low haze, and also exhibited good solvent resistance. Example 1 showed the best overall performance and process stability. Compared to Example 1, Example 4 used a one-step polycondensation process, which may have resulted in a more intense and uneven branching reaction, leading to excessively high branching density or even microgels in some areas, thus decreasing the MD and TD shrinkage rates. Example 5, without the addition of a branching agent, showed reduced anisotropy, and the increased haze after ethyl acetate immersion indicated that the branched network contributes to solvent resistance.

[0057] As demonstrated in Examples 1, 6, and 7, the side methyl group of neopentyl glycol and the cyclohexane structure of 1,4-cyclohexenedimethanol synergistically disrupt the regularity of the molecular chain, increasing the amorphization degree of the copolyester and avoiding scattering haze caused by the shrinkage difference between the crystalline and amorphous regions. When used alone, crystallization suppression may be incomplete, leading to residual microcrystals forming light scattering centers during shrinkage, thus increasing haze. Simultaneously, the rigidity of the cyclohexane structure significantly improves solvent resistance.

[0058] Compared to Example 1, Comparative Example 1 has a static friction coefficient as high as 0.68, making it prone to sticking and unable to achieve high-speed winding and labeling, and also has high initial haze. The B layer material of Comparative Example 2 is a single modified PET-A, which significantly reduces the elongation at break. This may be because the two types of PET can form microcrystalline physical cross-linking points in the core layer, improving the film modulus and rigidity.

[0059] The embodiments and comparative examples described above do not limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A high-transparency, low-haze shrink film, characterized in that, It has a three-layer structure (A / B / A), with layer A having a thickness of 2–5 μm and layer B having a thickness of 30–60 μm. The raw materials of layer A, based on 100 wt%, include: 90–99 wt% modified PET and 1–10 wt% functional masterbatch. The raw materials of layer B, based on 100 wt%, include: 90–100 wt% modified PET and 0–10 wt% PET. The preparation steps of the modified PET are as follows: Terephthalic acid, diol, and trifunctional branching agent are melted and mixed, and a catalyst, heat stabilizer, and antioxidant are added. The mixture is stirred at 60-70°C for 30-45 minutes to form a slurry. The slurry is reacted at 210-260°C and 0.01-0.25 MPa until the water content is ≥96%, resulting in an esterification product. The esterification product undergoes three-stage polycondensation, and a terminator is added and stirred for 10-20 minutes to obtain a melt. The melt is extruded through a die, water-cooled and pelletized, and vacuum dried to constant weight to obtain modified PET.

2. The high-transparency, low-haze shrink film according to claim 1, characterized in that, The molar ratio of terephthalic acid, diol, and branching agent is 1:(0.9-1.2):(0.002-0.008).

3. The high-transparency, low-haze shrink film according to claim 1, characterized in that, The diol comprises at least one of ethylene glycol, neopentyl glycol, or 1,4-cyclohexanediethanol.

4. The high-transparency, low-haze shrink film according to claim 1, characterized in that, The trifunctional branching agent is at least one of glycerol, trimellitic anhydride, and trimethylolpropane.

5. The high-transparency, low-haze shrink film according to claim 1, characterized in that, The conditions for the three-stage polycondensation are as follows: reaction at 260–270℃ and 1000–5000 Pa for 60–90 min; reaction at 270–280℃ and 100–500 Pa for 60–90 min; and reaction at 280–290℃ and 50–100 Pa until the intrinsic viscosity is 0.65–0.75 dL / g.

6. The high-transparency, low-haze shrink film according to claim 1, characterized in that, The functional masterbatch is composed of inorganic powder and erucamide in a mass ratio of (1-4):

1.

7. The high-transparency, low-haze shrink film according to claim 6, characterized in that, The particle size of the inorganic powder is 2–4 μm.

8. A method for preparing a high-transparency, low-haze shrink film according to any one of claims 1-7, comprising the following specific steps: S1. The modified PET and PET are dried to a moisture content of ≤50 ppm. Then, the feeding ratio of the A layer raw material and the B layer raw material is controlled by the metering feeding device. After the materials are initially mixed by the mixer, they are plasticized and melted in the twin-screw exhaust extruder. They are then fed into the three-layer co-extrusion die through the coarse filter, metering pump, fine filter and static mixer. S2. The melt is extruded through a three-layer co-extrusion die, cooled into a cast sheet, and then stretched longitudinally and laterally, heat-set, cooled, measured online, and wound up to obtain a high-transparency, low-haze shrink film.

9. The method for preparing a high-transparency, low-haze shrink film according to claim 8, characterized in that, In step S2, the longitudinal stretching ratio is 1.0 to 1.5, and the stretching temperature is 75 to 95°C; the transverse stretching ratio is 4.0 to 5.2, and the stretching temperature is 75 to 95°C; the heat setting temperature is 70 to 82°C, and 2 to 5% transverse relaxation is set at the end of the setting section.

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