A system for the production of polylactic acid shrink film

Abstract

This invention discloses a system for preparing polylactic acid (PLA) shrink film, enabling continuous production of PLA shrink film. The system includes an outer layer raw material pretreatment device, an intermediate layer raw material pretreatment device, a first twin-screw extruder, a second twin-screw extruder, a three-cavity die, a cooling roller unit, a biaxial stretching device, a heat setting device, and a cooling and winding device. These devices are connected sequentially via pipelines, enabling a continuous processing flow of raw material drying, mixing, melt co-extrusion, sheet cooling, biaxial stretching, heat setting, and winding. This system can continuously produce three-layer co-extruded PLA shrink film, ensuring a balanced performance of the film in terms of heat shrinkage rate, mechanical properties, and interlayer structure.

Description

Technical Field

[0001] This invention belongs to the field of bio-based biodegradable films and their preparation technology, specifically relating to a preparation system for polylactic acid shrink films. Background Technology

[0002] Polylactic acid (PLA) is a bio-based biodegradable polyester derived from renewable resources such as corn starch and sugarcane. It possesses excellent biodegradability and renewability, and has been widely used in packaging films and shrink films in recent years. Existing PLA shrink films are typically prepared through processes such as melt extrusion, stretching and orientation, and heat setting. They can shrink under heating conditions and are widely used in food packaging, beverage bottles, and labels.

[0003] However, existing PLA shrink films still face certain technical challenges in practical applications. For example, ordinary PLA films have a low shrinkage rate under heating conditions, making it difficult to meet the high shrinkage requirements of label films. Furthermore, PLA itself is quite brittle, and the film is prone to cracking or warping during shrinkage or subsequent processing, affecting printing quality and adhesion. In addition, the degradation and recycling rates of existing PLA films are still not ideal under actual composting or recycling conditions. The presence of printing inks or adhesives may further affect the film's degradation performance, thus limiting its widespread application in biodegradable packaging labels.

[0004] Existing technologies typically employ modification techniques to improve the performance of PLA shrink films. For example, blending modification (e.g., CN 104592724 B) can be achieved by adding other biodegradable polyesters, plasticizers, etc., to improve brittleness, impact resistance, and elongation at break. Copolymer modification (e.g., CN 116731298 B) can also be performed by adding other monomers, altering the molecular chain structure to adjust degradation rates and mechanical properties. Furthermore, special functions can be imparted to polylactic acid shrink films by adding antistatic agents, slip agents, etc. (e.g., CN 114015185 B) to obtain properties such as antistatic properties, slip properties, and metallizability.

[0005] However, the aforementioned existing technologies mainly focus on improving a single property of polylactic acid (PLA) shrink film. Due to the inherent characteristics of PLA, it is difficult to balance its biodegradability, flexibility, brittleness, and high shrinkage rate in a single film, which to some extent limits the widespread application of PLA shrink film in practical applications. Furthermore, existing technologies lack a system for the continuous production of PLA shrink film. Summary of the Invention

[0006] The technical problem to be solved by this application is to provide a system for preparing polylactic acid shrink film in order to reduce or avoid the problems mentioned above.

[0007] To address the aforementioned technical problems, this application proposes a preparation system for polylactic acid shrink film. The polylactic acid shrink film has a three-layer co-extrusion structure, including an intermediate layer and outer layers located on both sides of the intermediate layer. The preparation system includes an outer layer raw material pretreatment device, an intermediate layer raw material pretreatment device, a first twin-screw extruder, a second twin-screw extruder, a three-cavity die, a cooling roller unit, a biaxial stretching device, a heat setting device, and a cooling and winding device. The outlet of the outer layer raw material pretreatment device is connected to the inlet of the first twin-screw extruder, the outlet of the intermediate layer raw material pretreatment device is connected to the inlet of the second twin-screw extruder, the outlets of the first and second twin-screw extruders are connected to the inlet of the three-cavity die, the outlet of the three-cavity die is connected to the cooling roller unit, the outlet of the cooling roller unit is connected to the biaxial stretching device, the outlet of the biaxial stretching device is connected to the heat setting device, and the outlet of the heat setting device is connected to the cooling and winding device.

[0008] Preferably, the outer layer raw material pretreatment device further includes a first dryer, a second dryer, and a third dryer for low glass transition temperature copolymerized polylactic acid, polycaprolactone, and an organic nucleating agent, respectively; a first mixing granulator for grafted siloxane oligomers and low glass transition temperature copolymerized polylactic acid; and a first storage tank and a second storage tank for slip / antistatic agent and outer layer heat stabilizer, respectively; wherein the outlets of the first dryer, the second dryer, the third dryer, the first mixing granulator, and the first and second storage tanks are connected to the inlet of a first twin-screw extruder via pipelines.

[0009] Preferably, the intermediate layer raw material pretreatment device further includes a fourth dryer for highly crystalline polylactic acid, a third and a fourth storage tank for multifunctional epoxy chain extenders and intermediate layer heat stabilizers, a second mixing granulator for nanocellulose nucleating agents and highly crystalline polylactic acid, and a third mixing granulator for in-situ toughening precursors and highly crystalline polylactic acid; wherein the outlets of the fourth dryer, the third storage tank, the fourth storage tank, the second mixing granulator, and the third mixing granulator are connected to the inlet of a second twin-screw extruder via pipes.

[0010] Preferably, the outlet of the second mixing granulator is also connected to a spray dryer for mixing nanocellulose nucleating agent and ethanol.

[0011] Preferably, the bidirectional stretching device includes a longitudinal stretching mechanism and a transverse stretching mechanism connected in series, wherein the inlet of the longitudinal stretching mechanism is connected to the outlet of the cooling roller unit, the outlet is connected to the transverse stretching mechanism, and the outlet of the transverse stretching mechanism is connected to the heat setting device.

[0012] The polylactic acid (PLA) shrink film preparation system provided by this invention achieves drying, mixing, and granulation pretreatment of each layer of raw materials by setting up an outer layer raw material pretreatment device and an intermediate layer raw material pretreatment device, ensuring the uniformity and stability of the raw materials. Through independent melt extrusion by a first twin-screw extruder and a second twin-screw extruder, followed by composite extrusion through a three-cavity die, a structurally stable and thickness-controllable three-layer co-extruded sheet can be formed. The sequential configuration of the cooling roller unit, biaxial stretching device, and heat setting device achieves gradient orientation and high crystallinity fixation of the film, while the cooling winding device effectively controls the winding tension to prevent film springback deformation. The overall system realizes fully automated continuous production of raw material pretreatment, melt co-extrusion, sheet cooling, biaxial stretching, heat setting, and winding, improving production efficiency and ensuring balanced and stable film performance in terms of heat shrinkage rate, mechanical properties, interlayer orientation, and biodegradability, making it suitable for industrial continuous production of PLA shrink films. Attached Figure Description

[0013] The accompanying drawings are intended only to illustrate and explain this application and do not limit the scope of this application.

[0014] Figure 1 The diagram shown is a cross-sectional schematic of a polylactic acid shrink film according to a specific embodiment of this application.

[0015] Figure 2 The diagram shown is a structural schematic of a polylactic acid shrink film preparation system according to a specific embodiment of this application.

[0016] Figure 3 The diagram shown is a structural schematic of an outer raw material pretreatment device for a polylactic acid shrink film preparation system according to a specific embodiment of this application.

[0017] Figure 4 The diagram shown is a structural schematic of an intermediate layer raw material pretreatment device for a polylactic acid shrink film preparation system according to a specific embodiment of this application. Detailed Implementation

[0018] To provide a clearer understanding of the technical features, objectives, and effects of this utility model, the specific embodiments of this utility model will now be described in detail.

[0019] To address the issues of limited functionality and difficulty in achieving optimal performance in existing polylactic acid (PLA) shrink films, this invention provides a PLA shrink film with a three-layer gradient crystallization structure (A / B / A structure), such as... Figure 1 As shown. This shrink film achieves a gradient distribution of interlayer crystallinity and molecular chain orientation by using different component ratios and stretching / shaping processes in the middle and outer layers. This allows it to maintain overall mechanical integrity while also ensuring high thermal shrinkage, printability, and biodegradability and recyclability.

[0020] Specifically, the polylactic acid shrink film of this invention has a three-layer co-extruded structure, including an intermediate layer (layer B) and outer layers (layer A) located on both sides of the intermediate layer. The outer layer is composed of low-glass transition temperature copolymerized polylactic acid (low-Tg copolymer PLA), trace amounts of polycaprolactone (PCL), grafted siloxane oligomers, slip agents / antistatic agents, organic nucleating agents, and heat stabilizers. Its main functions are to improve flexibility, processability (such as printing, die-cutting, heat sealing / heat lamination), and weather resistance. The intermediate layer is composed of highly crystalline polylactic acid (highly crystalline PLA), nanocellulose or other biodegradable nucleating agents, multifunctional epoxy chain extenders, in-situ toughening precursors, and heat stabilizers. Its main functions are to provide high crystallinity, anisotropic orientation, and heat shrinkage driving force. Both the outer and intermediate layers are made of biodegradable polyester or biodegradable modifiers, ensuring that the entire film system is compostable or mechanically recyclable.

[0021] In one specific embodiment, the total thickness of the polylactic acid shrink film is 40-45µm, wherein the thickness of the intermediate layer is 25-30µm, and the thickness of the outer layers on both sides is the same, both controlled within the range of 5-10µm, thereby ensuring that the intermediate layer dominates the heat shrinkage behavior, and the outer layers provide processing adaptability and mechanical integrity.

[0022] This invention relates to a three-layer co-extruded polylactic acid (PLA) shrink film, which is prepared through processes including melting, three-layer co-extrusion, stretching, and heat setting. The specific processes include: raw material drying: drying each layer of raw material to a moisture content below 100 ppm to prevent PLA degradation; three-layer co-extrusion: controlling the volumetric flow rate of each layer using a three-cavity die to achieve the target thickness distribution; orientation stretching: prioritizing high-ratio stretching of the transverse (TD) layer while maintaining a low-ratio orientation of the longitudinal (MD) layer to release high shrinkage strain during subsequent heating; heat setting / annealing: setting the film at a temperature slightly higher than the outer layer's Tg to lock in the highly crystalline orientation structure of the intermediate layer while retaining the shrinkage energy that can be released when heated to 100°C; and cooling and winding: controlling the cooling rate to prevent stress rebound and surface ripples.

[0023] Through the above material selection and process optimization, the polylactic acid shrink film of this utility model can achieve a transverse heat shrinkage rate of more than 65% at 100℃, which is significantly higher than that of existing ordinary PLA shrink film. It also maintains the biodegradability of the whole film and composting / mechanical recycling compatibility, making it suitable for packaging, labeling and other environmental protection applications.

[0024] The low glass transition temperature copolymerized polylactic acid (low Tg copolymer PLA) used in the outer layer preferably has a glass transition temperature of approximately 55°C, making it suitable for film extrusion and offering both good transparency and toughness. For example, Ingeo 2500HP (Tg≈55°C) or Ingeo 3051D (Tg≈55°C) from NatureWorks LLC can be used as the PLA raw material for the outer layer. Low Tg copolymer PLA can improve the film's flexibility and enhance the outer layer's processing adaptability in printing, die-cutting, and heat-sealing / heat-laminating processes.

[0025] Polycaprolactone (PCL), as a biodegradable modified elastomer in the outer layer, can significantly improve the flexibility and impact resistance of the membrane, while also enhancing its low-temperature adaptability. For example, CAPA 6500 (molecular weight approximately 5 × 10⁻⁶) from Perstorp AG of Sweden can be used. 4 (Melting point 58-60℃) is used as a raw material for PCL. PCL is compatible with low Tg PLA to form a trace amount of flexible phase, which does not affect the film transparency and improves the overall tear resistance.

[0026] Grafted siloxane oligomers, as structurally well-defined functional modifiers, have a core of flexible oligomeric siloxane segments (-Si-O-Si-) and introduce functional groups compatible with PLA, such as epoxy groups (glycidyl groups), methacrylate groups (e.g., methacryloxy groups), amino / amine groups, or hydroxyl groups, onto the end groups or side groups. Introducing grafted siloxane oligomers into the outer layer can improve the smoothness, printability, and processing stability of the film surface. In this invention, oligomeric siloxane graft modifiers provided by Dow Corning / Momentive or Wacker Chemie can be used, or amino-modified siloxane oligomers provided by Bluestar Silicones can be used. TEGO Glide 410 (polyether-modified polysiloxane) from Evonik is preferred to improve the surface smoothness and printability of the film layer.

[0027] Slip agents / antistatic agents are used to improve the slipperiness and antistatic properties of the outer surface. Suitable options include Croda's Slip 7912 and Atmer 163 (UK) or BYK's BYK 370 (Germany). Atmer 163's main component is glyceryl monostearate and its derivatives, which are biodegradable fatty acid ester compounds suitable for biodegradable PLA shrink film systems.

[0028] The preferred organic nucleating agent for the outer layer is an organic carboxylate nucleating agent, such as Hyperform HPN-68L produced by Milliken, USA, whose main component is an organometallic carboxylate. Alternatively, Milliken's Millad NX8000 or Clariant's Hyperform HPN-68 can be used. This type of nucleating agent can effectively increase the film crystallization rate, enabling controllable adjustment of grain size, thereby enhancing the driving force for thermal shrinkage and film uniformity.

[0029] For the outer heat stabilizer, hindered phenolic heat stabilizers can be used, such as BASF's Irganox 1010 (the main component of which is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]), which terminates the polymer's thermal oxidative degradation chain reaction by capturing free radicals with hydrogen atoms, thereby effectively preventing chain breakage or discoloration of PLA during melt extrusion, stretching, and setting. Alternatively, similar products such as Songnox 1010 from Songwon Chemical (Korea) or Adeka Stab PEP-36 from Adeka (Japan) can also be used.

[0030] In one specific embodiment, the outer layer is formulated from the following components: 70-80 parts by weight of low Tg copolymer PLA, 5-10 parts by weight of PCL, 0.5-1.5 parts by weight of grafted siloxane oligomer, 0.5-1.0 parts by weight of slip / antistatic agent, 0.2-0.8 parts by weight of organic nucleating agent, and 0.5-1.0 parts by weight of outer layer heat stabilizer.

[0031] Through the above component optimization, the outer layer of this utility model has good flexibility, adaptability to printing and die-cutting, scratch resistance and biodegradability, providing stable surface protection and processing performance for the three-layer PLA shrink film.

[0032] The highly crystalline polylactic acid (PLA) constituting the interlayer is preferably a grade with a high crystallization rate and a suitable melt index, such as Ingeo 4032D from NatureWorks LLC, which has a melt index of 6 g / 10 min (210℃ / 2.16 kg) and a density of 1.24 g / cm³. This highly crystalline PLA provides bulk plasticity to the film layer and serves as the main driving component of the interlayer, improving the crystallization driving force and chain orientation ability during thermal shrinkage.

[0033] Nanocellulose, acting as a nucleating agent for the intermediate layer, can significantly accelerate the crystallization rate of PLA. NCC-10 nanocellulose rods from CelluForce (Canada) or CNF-10 nanocellulose rods from Jinan Jinlong (China) are preferred, with specifications of approximately 5-20 nm in diameter and approximately 200-500 nm in length. Nanocellulose provides numerous nucleation sites during melt extrusion and stretching, thereby promoting the formation of a highly oriented crystalline structure in the intermediate layer and storing releaseable strain energy for lateral thermal shrinkage.

[0034] Multifunctional epoxy chain extenders are used to increase the molecular weight and melt strength of the polylactic acid interlayer, thereby improving the stability of the film during high-ratio stretching and heat setting. Preferred options include propylene oxide trimethylolpropane or polyepoxide compounds, such as Huntsman's Araldite PT 910 (main component is propylene oxide trimethylolpropane, molecular weight approximately 700-1000) or Momentive's Epon 828 (epoxide equivalent approximately 450-500 g / eq).

[0035] The preferred in-situ toughening precursor is a tandem multifunctional epoxy acrylate, such as Joncryl ADR-4368 from BASF or Lotader AX8900 from Arkema. This precursor is partially grafted into the intermediate layer, forming a micro-flexible network that improves film toughness while maintaining high crystallinity orientation, enhancing film integrity and workability during heat shrinkage.

[0036] The intermediate layer heat stabilizer can also be a hindered phenolic heat stabilizer, such as Songnox 1010 from Songwon Chemical (Korea), the same as the outer layer, or Adeka Stab PEP-36 from Adeka (Japan). The heat stabilizer effectively prevents thermal degradation and color changes in highly crystalline PLA during melt extrusion, stretching, and setting, ensuring film processing stability and finished product quality.

[0037] In one specific embodiment, the intermediate layer is formulated with the following components: 85-90 parts by weight of highly crystalline PLA, 0.3-0.8 parts by weight of nanocellulose nucleating agent, 0.3-0.8 parts by weight of multifunctional epoxy chain extender, 1.0-5.0 parts by weight of in-situ toughening precursor, and 0.5-1.0 parts by weight of intermediate layer heat stabilizer. Through the above formulation design, the intermediate layer can form a highly oriented crystalline structure during melt extrusion, stretching, and heat setting, providing the dominant thermal shrinkage driving force for the three-layer PLA shrink film, while maintaining the film's toughness and biodegradability.

[0038] Furthermore, the polylactic acid shrink film of this invention can be prepared through the following steps to ensure that each component of the film layer plays its full role and forms a gradient crystal structure, thereby achieving high thermal shrinkage rate and excellent biodegradability.

[0039] First, the raw materials that make up the outer and middle layers are pretreated.

[0040] The outer layer raw materials are treated as follows: Low glass transition temperature copolymer polylactic acid (low Tg PLA) needs to be dried at approximately 80°C for 4 hours to control the moisture content below 0.02% and prevent hydrolytic degradation during extrusion. Polycaprolactone (PCL) has low hygroscopicity and only needs to be lightly dried at approximately 30°C for 30 minutes to control the moisture content below 0.02%. Grafted siloxane oligomers are uniformly mixed with low Tg PLA at a weight ratio of 1:1 and then granulated to form grafted siloxane particles, which facilitates uniform dispersion in the subsequent process. The organic nucleating agent is lightly dried (30°C, 30 minutes, moisture content <0.02%) to prevent moisture absorption and agglomeration. The slip / antistatic agent and the outer layer heat stabilizer are both solid powders and can be directly mixed. Through the above pretreatment, the uniformity and surface smoothness of the outer layer melt are ensured, while retaining the crystallization driving force for subsequent stretching.

[0041] The intermediate layer raw materials are processed as follows: Highly crystalline polylactic acid (PLA) needs to be dried at 80℃ for 4-6 hours, with the moisture content controlled below 0.02% to avoid hydrolysis and degradation during extrusion, which could lead to bubbles or a decrease in molecular weight. Nanocellulose nucleating agents are prone to agglomeration, and direct addition can affect crystallization. Therefore, it is preferable to first mix them uniformly with ethanol at a 1:2 weight ratio, then spray-dry to remove the ethanol, and then mix them uniformly with highly crystalline PLA at a 1:1 weight ratio to granulate, obtaining nanocellulose particles and ensuring uniform crystallization. Multifunctional epoxy chain extenders and intermediate layer heat stabilizers can be directly added in powder form; the in-situ toughening precursor needs to be uniformly mixed with highly crystalline PLA at a 1:1 weight ratio before granulation to facilitate solid addition and uniform dispersion in the molten state.

[0042] The pretreated outer and intermediate layer raw materials are fed separately into twin-screw extruders for melting. In the first twin-screw extruder, the outer layer raw material is first melted with low-Tg PLA, then PCL, grafted siloxane particles, organic nucleating agent, lubricating / antistatic agent, and outer layer heat stabilizer are added sequentially, and thoroughly mixed to form the outer layer melt. In the second twin-screw extruder, the intermediate layer raw material is first melted with highly crystalline PLA and nanocellulose particles, then multifunctional epoxy chain extender and in-situ toughening precursor particles are added, and finally the intermediate layer heat stabilizer is added to ensure uniform dispersion and prevent high-temperature degradation.

[0043] The outer layer melt is divided into two parts and fed into the outer cavity of a three-cavity die, while the middle layer melt is fed into the middle cavity. A three-layer thick sheet structure is obtained through co-extrusion using the three-cavity die. The thick sheet is cooled by cooling rollers at a temperature controlled between 20-35℃, forming a moderately amorphous layer on the outer layer, providing controllable shrinkage potential for subsequent stretching.

[0044] Thick sheets are biaxially stretched to form thin films. First, longitudinal pre-stretching is performed at a ratio of 1.2-1.8 and a temperature of 70-90℃ to achieve longitudinal orientation while retaining transverse potential. Subsequently, transverse main stretching is performed at a ratio of 5.5-7.5 and a stretching temperature of 85-110℃ to promote the formation of a highly oriented and highly crystalline structure in the intermediate layer, storing strain energy for transverse thermal shrinkage.

[0045] The stretched film is heat-set at a temperature of 105-115℃ for 20-60 seconds to fix the high orientation and high crystallinity of the middle layer, while ensuring the flexibility and surface smoothness of the outer layer.

[0046] Finally, the film is cooled to room temperature and then wound up. The winding tension is properly controlled to prevent springback deformation, resulting in a polylactic acid shrink film with a three-layer gradient crystalline structure that has balanced performance, high thermal shrinkage rate, and is biodegradable.

[0047] Corresponding to the above preparation method, this utility model further provides a preparation system for polylactic acid shrink film, which is used to realize the continuous preparation of polylactic acid shrink film.

[0048] Specifically, such as Figure 2 As shown, the preparation system for polylactic acid shrink film of this utility model includes an outer layer raw material pretreatment device 100, an intermediate layer raw material pretreatment device 200, a first twin-screw extruder 300, a second twin-screw extruder 400, a three-cavity die head 500, a cooling roller unit 600, a biaxial stretching device 700, a heat setting device 800, and a cooling and winding device 900.

[0049] The outlet of the outer layer raw material pretreatment device 100 is connected to the inlet of the first twin-screw extruder 300, the outlet of the middle layer raw material pretreatment device 200 is connected to the inlet of the second twin-screw extruder 400, the outlets of the first twin-screw extruder 300 and the second twin-screw extruder 400 are connected to the inlet of the three-cavity die 500, the outlet of the three-cavity die 500 is connected to the cooling roller unit 600, the outlet of the cooling roller unit 600 is connected to the biaxial stretching device 700, the outlet of the biaxial stretching device 700 is connected to the heat setting device 800, and the outlet of the heat setting device 800 is connected to the cooling winding device 900.

[0050] The outer layer raw material pretreatment device 100 is used for drying, mixing and granulating low glass transition temperature copolymer polylactic acid, polycaprolactone, grafted siloxane oligomers, organic nucleating agents, lubricating / antistatic agents and outer layer heat stabilizers.

[0051] Correspondingly, such as Figure 3 As shown, the outer layer raw material pretreatment device 100 further includes a first dryer 101, a second dryer 102, and a third dryer 103 for low glass transition temperature copolymerized polylactic acid, polycaprolactone, and an organic nucleating agent, respectively; a first mixing granulator 104 for grafted siloxane oligomers and low glass transition temperature copolymerized polylactic acid; and a first storage tank 105 and a second storage tank 106 for slip / antistatic agents and outer layer heat stabilizers, respectively. The outlet of the first dryer 101 is also connected to the first mixing granulator 104 via a pipeline for conveying low glass transition temperature copolymerized polylactic acid to the first mixing granulator 104. The outlets of the first dryer 101, second dryer 102, third dryer 103, first mixing granulator 104, first storage tank 105, and second storage tank 106 are connected to the inlet of a first twin-screw extruder 300 via pipelines to achieve continuous conveying of the outer layer raw material to the first twin-screw extruder.

[0052] The intermediate layer raw material pretreatment device is used for drying, mixing and granulating highly crystalline polylactic acid, nanocellulose, multifunctional epoxy chain extenders, in-situ toughening precursors and intermediate layer heat stabilizers.

[0053] Correspondingly, such as Figure 4 As shown, the intermediate layer raw material pretreatment device 200 further includes a fourth dryer 201 for highly crystalline polylactic acid, a third storage tank 202 and a fourth storage tank 203 for multifunctional epoxy chain extenders and intermediate layer heat stabilizers, a second mixing granulator 204 for nanocellulose nucleating agents and highly crystalline polylactic acid, and a third mixing granulator 205 for in-situ toughening precursors and highly crystalline polylactic acid. The outlet of the fourth dryer 201 is also connected to the second mixing granulator 204 and the third mixing granulator 205 via pipelines for conveying highly crystalline polylactic acid to the second mixing granulator 204 and the third mixing granulator 205. The outlets of the fourth dryer 201, the third storage tank 202, the fourth storage tank 203, the second mixing granulator 204, and the third mixing granulator 205 are connected to the inlet of the second twin-screw extruder 400 via pipelines to achieve continuous conveying of the intermediate layer raw material to the second twin-screw extruder 400. Furthermore, the outlet of the second mixing granulator 204 is also connected to a spray dryer 206 for mixing the nanocellulose nucleating agent and ethanol, used to disperse and dry the nanocellulose nucleating agent and remove the solvent.

[0054] The first twin-screw extruder 300 is used to melt and mix the pretreated outer layer raw material to form an outer layer melt, and the second twin-screw extruder 400 is used to melt and mix the pretreated intermediate layer raw material to form an intermediate layer melt. The outlets of the first and second twin-screw extruders are connected to the inlet of the three-cavity die 500, and a three-layer thick sheet structure is formed by compound extrusion through the three-cavity die 500.

[0055] The outlet of the three-cavity die head 500 is connected to a cooling roller unit 600 for cooling the thick sheet and forming the outer amorphous layer. The outlet of the cooling roller unit 600 is connected to a biaxial stretching device 700 for longitudinal pre-stretching followed by transverse main stretching to form a film structure with gradient orientation. The outlet of the biaxial stretching device 700 is connected to a heat-setting device 800 for fixing the high crystallinity and high orientation of the intermediate layer. Correspondingly, such as... Figure 2 As shown, the bidirectional stretching device 700 further includes a longitudinal stretching mechanism 701 and a transverse stretching mechanism 702 connected in series. The inlet of the longitudinal stretching mechanism 701 is connected to the outlet of the cooling roller unit 600, and the outlet is connected to the transverse stretching mechanism 702. The outlet of the transverse stretching mechanism 702 is connected to the inlet of the heat setting device 800.

[0056] The outlet of the heat setting device 800 is connected to a cooling and winding device 900, which is used to cool the film to room temperature and wind it up, while controlling the winding tension to prevent springback deformation.

[0057] Through the above-mentioned preparation system, this invention can realize continuous production of the entire process, including raw material pretreatment, melt extrusion, three-layer co-extrusion, cooling, biaxial stretching, heat setting and winding, ensuring that the prepared polylactic acid shrink film has balanced performance in terms of heat shrinkage rate, mechanical properties, interlayer orientation and degradability.

[0058] The three-layer gradient crystalline polylactic acid shrink film prepared by the preparation system of this invention underwent performance testing, and the results were consistent with the design expectations of this invention. The total thickness was measured to be approximately 40-45 μm, and the thickness of each layer was basically consistent with the design value (error ±1 μm), ensuring the realization of the gradient crystalline structure. In the transverse heat shrinkage performance test, the sample (width 25 mm) was placed in a 100℃ water bath or hot furnace and heated for 15 seconds. The transverse (TD) heat shrinkage rate was measured to be greater than 65%, with a demonstration value of 68-75%, showing that this invention achieves high heat shrinkage performance through the high orientation of the intermediate layer and the flexible design of the outer layer.

[0059] Mechanical property tests show that the MD / TD tensile strength and elongation at break of the film at room temperature after processing meet the requirements of label processing and packaging processes. The TD elongation at break is ≥60%, ensuring sufficient toughness for subsequent die-cutting, heat-sealing, and winding. Crystallinity and orientation measurements show that the crystallinity of the intermediate layer is significantly higher than that of the outer layer. WAXS, SAXS, or polarized light microscopy observations further confirm a gradient distribution of interlayer orientation; the intermediate layer has a highly oriented structure, while the outer layer maintains a lower orientation to provide flexibility and surface smoothness.

[0060] Post-processing adaptability tests show that the film of this invention is compatible with ink printing, die-cutting, and heat-sealing processes. The outer surface is smooth with moderate ink / adhesive adhesion, which is beneficial for label and packaging applications. Simultaneously, the film exhibits good biodegradability under standard industrial composting conditions. The graded degradation characteristics of the middle and outer layers are clearly demonstrated by the composting weighing curve, achieving the goal of functional composite properties and environmental recyclability.

[0061] Example 1

[0062] The total thickness of the polylactic acid shrink film in this embodiment is approximately 40 μm, with the intermediate layer being 25 μm thick and the outer layers each 7.5 μm thick. The outer layers consist of 70 wt% low glass transition temperature copolymerized PLA, 5 wt% polycaprolactone, 0.5 wt% grafted siloxane oligomer, 0.5 wt% slip / antistatic agent, 0.2 wt% organic nucleating agent, and 0.5 wt% outer layer heat stabilizer. The intermediate layer consists of 85 wt% highly crystalline PLA, 0.3 wt% nanocellulose nucleating agent, 0.3 wt% multifunctional epoxy chain extender, 1.0 wt% in-situ toughening precursor, and 0.5 wt% intermediate layer heat stabilizer. This film exhibits a transverse thermal shrinkage rate of approximately 68%, a TD elongation at break of approximately 62%, higher crystallinity in the intermediate layer compared to the outer layer, good post-processing adaptability, a smooth surface, and good compostability.

[0063] Example 2

[0064] In this embodiment, the total thickness is approximately 42 μm, with the intermediate layer being 27 μm thick and the outer layers each 7.5 μm thick. The outer layer formulation consists of 75 wt% low-Tg copolymerized PLA, 7 wt% PCL, 1.0 wt% grafted siloxane oligomer, 0.8 wt% slip / antistatic agent, 0.5 wt% organic nucleating agent, and 0.8 wt% outer layer heat stabilizer. The intermediate layer formulation consists of 87 wt% highly crystalline PLA, 0.5 wt% nanocellulose, 0.5 wt% multifunctional epoxy chain extender, 3.0 wt% in-situ toughening precursor, and 1.0 wt% intermediate layer heat stabilizer. This film exhibits a transverse thermal shrinkage rate of approximately 70%, a TD elongation at break of approximately 65%, high orientation in the intermediate layer, and good flexibility in the outer layer, making it suitable for label printing and heat sealing. Its compostability allows for graded degradation.

[0065] Example 3

[0066] The total thickness of this embodiment is approximately 44 μm, with the intermediate layer being 30 μm thick and the outer layers each 7 μm thick. The outer layer formulation consists of 80 wt% low-Tg copolymerized PLA, 10 wt% PCL, 1.5 wt% grafted siloxane oligomer, 1.0 wt% slip / antistatic agent, 0.8 wt% organic nucleating agent, and 1.0 wt% outer layer heat stabilizer. The intermediate layer comprises 90 wt% highly crystalline PLA, 0.8 wt% nanocellulose, 0.8 wt% multifunctional epoxy chain extender, 5.0 wt% in-situ toughening precursor, and 1.0 wt% intermediate layer heat stabilizer. The transverse thermal shrinkage rate is approximately 72%, the TD elongation at break is approximately 70%, the intermediate layer is highly crystalline and highly oriented, the outer layer has good elasticity and a smooth surface, excellent ink printing compatibility, and significant biodegradability in industrial composting.

[0067] Example 4

[0068] In this embodiment, the total thickness is approximately 43 μm, with the intermediate layer being 26 μm thick and the outer layers each 8.5 μm thick. The outer layer formulation consists of 72 wt% low-Tg copolymer PLA, 8 wt% PCL, 1.0 wt% grafted siloxane oligomer, 0.5 wt% slip / antistatic agent, 0.3 wt% organic nucleating agent, and 0.8 wt% outer layer heat stabilizer. The intermediate layer formulation consists of 86 wt% highly crystalline PLA, 0.4 wt% nanocellulose, 0.4 wt% multifunctional epoxy chain extender, 2.5 wt% in-situ toughening precursor, and 0.8 wt% intermediate layer heat stabilizer. The film exhibits a transverse thermal shrinkage rate of approximately 69%, a TD elongation at break of approximately 63%, and a significantly higher crystallinity in the intermediate layer compared to the outer layer. It demonstrates balanced overall performance, a smooth surface suitable for die-cutting and heat-sealing, and good degradability.

[0069] Example 5

[0070] In this embodiment, the total thickness is approximately 45 μm, with the intermediate layer being 28 μm thick and the outer layers each 8.5 μm thick. The outer layer formulation consists of 75 wt% low-Tg copolymer PLA, 5 wt% PCL, 0.5 wt% grafted siloxane oligomer, 0.8 wt% slip / antistatic agent, 0.5 wt% organic nucleating agent, and 1.0 wt% outer layer heat stabilizer. The intermediate layer formulation consists of 88 wt% highly crystalline PLA, 0.6 wt% nanocellulose, 0.6 wt% multifunctional epoxy chain extender, 4.0 wt% in-situ toughening precursor, and 1.0 wt% intermediate layer heat stabilizer. The transverse heat shrinkage rate is approximately 71%, and the TD elongation at break is approximately 68%. The intermediate layer exhibits high orientation and high crystallinity, while the outer layer maintains good flexibility and a smooth surface, making it suitable for label and packaging printing and processing. It also demonstrates good biodegradability for industrial composting.

[0071] Example 6

[0072] In this embodiment, the total thickness is approximately 41 μm, with the intermediate layer being 25 μm thick and the outer layers each 8 μm thick. The outer layer formulation consists of 78 wt% low-Tg copolymer PLA, 10 wt% PCL, 1.0 wt% grafted siloxane oligomer, 1.0 wt% slip / antistatic agent, 0.8 wt% organic nucleating agent, and 1.0 wt% outer layer heat stabilizer. The intermediate layer formulation consists of 85 wt% highly crystalline PLA, 0.3 wt% nanocellulose, 0.3 wt% multifunctional epoxy chain extender, 1.5 wt% in-situ toughening precursor, and 0.5 wt% intermediate layer heat stabilizer. This film exhibits a transverse thermal shrinkage rate of approximately 67%, a TD elongation at break of approximately 61%, highly oriented crystallinity in the intermediate layer, and good flexibility and surface smoothness in the outer layer. It is suitable for printing and heat sealing processes, and its biodegradability allows for graded degradation, meeting environmental protection requirements.

[0073] Comparative Example 1

[0074] The comparative film has a total thickness of approximately 40 μm, with a 25 μm intermediate layer and 7.5 μm outer layers. The outer layer formulation omits grafted siloxane oligomers and slip / antistatic agents, consisting only of 75 wt% low-Tg copolymerized PLA, 5 wt% PCL, 0.2 wt% organic nucleating agent, and 0.5 wt% outer layer heat stabilizer. The intermediate layer formulation consists of 85 wt% highly crystalline PLA, 0.3 wt% nanocellulose nucleating agent, and 0.3 wt% multifunctional epoxy chain extender, without the addition of in-situ toughening precursors or intermediate layer heat stabilizers. This film exhibits a transverse thermal shrinkage rate of approximately 60%, a TD elongation at break of approximately 50%, poor surface slippage, poor ink printing compatibility, and while degradation is still achievable, the orientation and crystallinity of the intermediate layer are significantly reduced.

[0075] Comparative Example 2

[0076] The total thickness is approximately 42 μm, with the intermediate layer at 27 μm and the outer layers each at 7.5 μm. The outer layer formulation reduces PCL to 3 wt%, omits grafted siloxane oligomers and slip / antistatic agents, and includes 78 wt% low-Tg copolymerized PLA, 0.5 wt% organic nucleating agent, and 0.8 wt% outer layer heat stabilizer. The intermediate layer contains only 87 wt% highly crystalline PLA and 0.5 wt% nanocellulose, omitting multifunctional epoxy chain extenders and in-situ toughening precursors. The transverse heat shrinkage rate is approximately 62%, and the TD elongation at break is approximately 57%. The outer layer exhibits reduced flexibility, resulting in poor adaptability for printing and heat sealing. The intermediate layer also shows uneven crystallization, leading to unbalanced overall performance.

[0077] Comparative Example 3

[0078] The total thickness is 44 μm, with a 30 μm intermediate layer and a 7 μm outer layer. The outer layer omits grafted siloxane oligomers, slip / antistatic agents, and organic nucleating agents, retaining only 85 wt% low-Tg copolymerized PLA, 10 wt% PCL, and 1.0 wt% outer layer heat stabilizer. The intermediate layer contains only 90 wt% highly crystalline PLA and 0.8 wt% nanocellulose, without adding multifunctional epoxy chain extenders or in-situ toughening precursors. The transverse thermal shrinkage rate is approximately 63%, and the TD elongation at break is approximately 58%. The film surface is rough, prone to processing cracks, exhibits uneven crystallization and orientation, and remains degradable, but has poor mechanical toughness.

[0079] Comparative Example 4

[0080] The total thickness is 43 μm, with a 26 μm intermediate layer and an 8.5 μm outer layer. The PCL content in the outer layer is reduced to 4 wt%, omitting grafted siloxane oligomers, slip / antistatic agents, and organic nucleating agents; it consists only of 80 wt% low-Tg copolymerized PLA and 0.8 wt% outer layer heat stabilizer. The intermediate layer contains 86 wt% highly crystalline PLA and 0.4 wt% nanocellulose, without the addition of multifunctional epoxy chain extenders or in-situ toughening precursors. The transverse heat shrinkage rate is approximately 61%, and the TD elongation at break is approximately 55%. The film is hard and brittle, resulting in reduced printability and heat-sealing processability. The intermediate layer exhibits poor orientation and insufficient crystallinity.

[0081] Comparative Example 5

[0082] The total thickness is 45 μm, with a 28 μm intermediate layer and an 8.5 μm outer layer. The outer layer contains only 75 wt% low-Tg copolymerized PLA and 5 wt% PCL, omitting grafted siloxane oligomers, slip / antistatic agents, organic nucleating agents, and outer layer heat stabilizers. The intermediate layer contains only 88 wt% highly crystalline PLA and 0.6 wt% nanocellulose, without adding multifunctional epoxy chain extenders, in-situ toughening precursors, or heat stabilizers. The transverse heat shrinkage rate is approximately 60%, and the TD elongation at break is approximately 52%. The outer layer surface is rough and easily adheres to inks. The film has poor mechanical toughness, significantly reduced processing performance, and good degradability but lacks graded control.

[0083] Comparative Example 6

[0084] The total thickness is 41 μm, with a 25 μm intermediate layer and an 8 μm outer layer. The outer layer formulation contains 8 wt% PCL and 78 wt% low-Tg copolymerized PLA, omitting grafted siloxane oligomers, slip / antistatic agents, organic nucleating agents, and outer layer heat stabilizers. The intermediate layer contains only 85 wt% highly crystalline PLA and 0.3 wt% nanocellulose, without adding multifunctional epoxy chain extenders, in-situ toughening precursors, or heat stabilizers. The transverse heat shrinkage rate is approximately 59%, and the TD elongation at break is approximately 50%. The intermediate layer exhibits insufficient crystallinity and orientation, resulting in a significant decrease in the outer layer's flexibility and surface smoothness. Printing and heat-sealing processes are unsatisfactory, and the overall performance is lower than that of Example 6.

[0085] The three-layer gradient crystalline polylactic acid shrink film provided by this invention has significant advantages in functional compositing and performance optimization. Firstly, through the gradient design of the raw material composition and thickness of the middle and outer layers, a differentiated distribution of crystallinity and orientation between the layers is achieved, resulting in an optimal balance in properties such as transverse thermal shrinkage rate, mechanical strength, and elongation at break. Compared with traditional PLA shrink films or comparative examples, the outer layer of this invention maintains flexibility and transparency while improving surface smoothness and printability through grafted siloxane oligomers and slip / antistatic agents, significantly reducing ink and adhesive adhesion. The middle layer, through the combined action of highly crystalline PLA, nanocellulose nucleating agents, multifunctional epoxy chain extenders, and in-situ toughening precursors, achieves high orientation, high crystallinity, and graded toughening, improving film toughness and thermal stability. Secondly, the film of this invention exhibits high transverse thermal shrinkage rate (TD>65%), good longitudinal and transverse mechanical properties, and processing adaptability during thermal processing, meeting the requirements of various applications such as labeling and packaging. Finally, the film retains the biodegradability of PLA, and the gradient structure of the intermediate layer imparts graded degradation characteristics, further enhancing its environmental friendliness. In summary, this invention achieves comprehensive optimization of performance and function through a multi-layer co-extrusion structure, functional component composite, and thickness gradient control, possessing high thermal shrinkage rate, excellent mechanical properties, surface processing adaptability, and environmentally friendly biodegradability, significantly superior to existing technologies and corresponding comparative examples.

[0086] Those skilled in the art should understand that although this application is described by way of multiple embodiments, not every embodiment contains only one independent technical solution. This description is merely for clarity, and those skilled in the art should understand the specification as a whole and consider the technical solutions involved in each embodiment as being able to be combined with each other to form different embodiments to understand the scope of protection of this application.

[0087] The above description is merely an illustrative embodiment of this application and is not intended to limit the scope of this application. Any equivalent changes, modifications, and combinations made by those skilled in the art without departing from the concept and principles of this application shall fall within the scope of protection of this application.

Claims

1. A system for preparing a polylactic acid shrink film, wherein the polylactic acid shrink film has a three-layer co-extruded structure, comprising an intermediate layer and outer layers located on both sides of the intermediate layer, characterized in that, The preparation system includes an outer layer raw material pretreatment device, an intermediate layer raw material pretreatment device, a first twin-screw extruder, a second twin-screw extruder, a three-cavity die, a cooling roller unit, a biaxial stretching device, a heat setting device, and a cooling and winding device. The outlet of the outer layer raw material pretreatment device is connected to the inlet of the first twin-screw extruder; the outlet of the intermediate layer raw material pretreatment device is connected to the inlet of the second twin-screw extruder; the outlets of the first and second twin-screw extruders are connected to the inlet of the three-cavity die; the outlet of the three-cavity die is connected to the cooling roller unit; the outlet of the cooling roller unit is connected to the biaxial stretching device; the outlet of the biaxial stretching device is connected to the heat setting device; and the outlet of the heat setting device is connected to the cooling and winding device.

2. The preparation system according to claim 1, characterized in that, The outer layer raw material pretreatment device further includes a first dryer, a second dryer, and a third dryer for low glass transition temperature copolymerized polylactic acid, polycaprolactone, and organic nucleating agents, respectively; a first mixing granulator for grafted siloxane oligomers and low glass transition temperature copolymerized polylactic acid; and a first storage tank and a second storage tank for slip / antistatic agents and outer layer heat stabilizers, respectively. The outlets of the first dryer, the second dryer, the third dryer, the first mixing granulator, the first storage tank, and the second storage tank are connected to the inlet of the first twin-screw extruder via pipelines.

3. The preparation system according to claim 1, characterized in that, The intermediate layer raw material pretreatment device further includes a fourth dryer for highly crystalline polylactic acid, a third and a fourth storage tank for multifunctional epoxy chain extenders and intermediate layer heat stabilizers, a second mixing granulator for nanocellulose nucleating agents and highly crystalline polylactic acid, and a third mixing granulator for in-situ toughening precursors and highly crystalline polylactic acid; wherein the outlets of the fourth dryer, the third storage tank, the fourth storage tank, the second mixing granulator, and the third mixing granulator are connected to the inlet of a second twin-screw extruder via pipelines.

4. The preparation system as described in claim 3, characterized in that, The outlet of the second mixing granulator is also connected to a spray dryer for mixing nanocellulose nucleating agent and ethanol.

5. The preparation system according to claim 1, characterized in that, The bidirectional stretching device includes a longitudinal stretching mechanism and a transverse stretching mechanism connected in series. The inlet of the longitudinal stretching mechanism is connected to the outlet of the cooling roller unit, and the outlet is connected to the transverse stretching mechanism. The outlet of the transverse stretching mechanism is connected to the heat setting device.

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