Shrink film and method for producing the same

Abstract

The application discloses a shrink film and a preparation method thereof, and relates to the technical field of shrink films. The shrink film comprises a plurality of first polyethylene layers, a first ethylene / vinyl acetate layer arranged on the polyethylene layer, a polyvinylidene chloride layer arranged on the first ethylene / vinyl acetate layer away from the polyethylene layer, a second ethylene / vinyl acetate layer arranged on the polyvinylidene chloride layer away from the first ethylene / vinyl acetate layer, and a mixed functional layer arranged on the second ethylene / vinyl acetate layer away from the polyvinylidene chloride layer. The application solves the problem that the blue compensation scheme in the prior art is mostly added by manual experience, cannot be dynamically adjusted in real time according to the color change of the film body, is prone to cause local color difference, transparency fluctuation and haze increase, and is difficult to meet the requirements of high-end transparent packaging on color consistency and optical stability.

Description

Technical Field

[0001] This invention relates to the field of shrink film technology, and in particular to a shrink film and its preparation method. Background Technology

[0002] Shrink films are widely used in food packaging, daily chemical packaging, pharmaceutical packaging, and electronic product packaging due to their excellent heat shrinkage, transparent packaging, and barrier properties. Among them, shrink films with polyvinylidene chloride (PVDC) as the core barrier layer have broad application value in the high-end packaging field due to their excellent oxygen barrier, water vapor barrier, and freshness preservation properties.

[0003] However, existing PVDC shrink films still have the following problems in actual production: First, PVDC materials are prone to slight thermal degradation during high-temperature melt extrusion and hot stretching, which can cause a yellow hue shift in the film, reduce the film's visual transparency, and in severe cases affect the packaging's appearance quality.

[0004] Secondly, traditional transparent shrink films mainly improve transparency by increasing resin purity or adding transparent additives, but it is difficult to actively adjust the yellowing problem from the perspective of spectral compensation, resulting in limited improvement in transparency.

[0005] In addition, existing blue compensation schemes mostly rely on manual experience to add blue, which cannot be dynamically adjusted in real time according to changes in film color. This can easily cause local color differences, transparency fluctuations, and increased haze, making it difficult to meet the requirements of high-end transparent packaging for color consistency and optical stability.

[0006] Meanwhile, during the stretching and heat setting process, multilayer co-extruded shrink films are prone to interfacial stress concentration due to interlayer stress differences, which further leads to problems such as decreased film transparency, uneven shrinkage, and insufficient dimensional stability.

[0007] Therefore, how to provide a highly transparent shrink film with high transparency, high optical stability, high shrinkage uniformity, and online intelligent hue adjustment capability, as well as its preparation method, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] Objective of the invention: To provide a shrink film and its preparation method, so as to at least solve one of the problems existing in the prior art.

[0009] Technical solution: A shrink film, comprising: Several first polyethylene layers; A first ethylene / vinyl acetate layer is disposed on the polyethylene layer; A polyvinylidene chloride layer is disposed on the first ethylene / vinyl acetate layer on the side away from the polyethylene layer; A second ethylene / vinyl acetate layer is disposed on the polyvinylidene chloride layer on the side away from the first ethylene / vinyl acetate layer; and A hybrid functional layer is disposed on the second ethylene / vinyl acetate layer on the side away from the polyvinylidene chloride layer; The polyvinylidene chloride layer contains a blue hue modifier, which is used to absorb the yellow light band of the polyvinylidene chloride material and compensate for the blue light band, so as to improve the yellowing phenomenon of the shrink film and increase its transparency. The polyvinylidene chloride core layer is also provided with an optical stabilizing agent, which includes one or more of heat stabilizers, antioxidants and dispersants, to improve the dispersion uniformity and long-term optical stability of the blue hue modifier in the polyvinylidene chloride core layer. The CIE color values ​​a* and b* of the shrink film satisfy -3≤a*≤0 and -5≤b*≤0, respectively. The average transmittance in the visible light range of 400nm-760nm is ≥90%, and the haze is ≤1.5%.

[0010] Preferably, the blue hue modifier is selected from at least one of inorganic blue pigments and organic blue pigments; The inorganic blue pigment includes at least one of ultramarine, cobalt blue, cobalt silicon blue, and cobalt zinc silicon blue; The organic blue pigment includes at least one of the phthalocyanine series blue pigments and the indanone series blue pigments; The blue hue modifier has a particle size of 0.01μm-2μm to reduce the increase in haze caused by particle scattering.

[0011] Preferably, the blue hue modifier in the polyvinylidene chloride core layer has a mass fraction of 0.001wt%-0.1wt%, more preferably 0.005wt%-0.05wt%. The blue hue modifier is distributed in a gradient or uniform manner along the thickness direction of the polyvinylidene chloride core layer to reduce color difference and improve the uniformity of light transmission of the film.

[0012] Preferably, the total thickness of the shrink film is 12μm-45μm; The thickness of the polyvinylidene chloride core layer accounts for 35%-55% of the total thickness; The first ethylene / vinyl acetate layer and the second ethylene / vinyl acetate layer have the same thickness, each accounting for 10%-20% of the total thickness; The first polyethylene layer and the mixed functional layer have the same thickness, each accounting for 5%-15% of the total thickness; The first ethylene / vinyl acetate layer and the second ethylene / vinyl acetate layer are used to improve the interlayer bonding strength and reduce the interfacial stress concentration during the multilayer co-extrusion process.

[0013] Preferably, the first polyethylene layer comprises one or more of linear low-density polyethylene, low-density polyethylene, and high-density polyethylene. The hybrid functional layer is a blend of polyethylene and ethylene / vinyl acetate copolymer; The hybrid functional layer also contains one or more of the following: slip agent, anti-blocking agent, antistatic agent, toughening agent, and wear-resistant modifier, to improve the surface slip, anti-blocking properties, and packaging adaptability of the shrink film.

[0014] Preferably, the shrink film has a longitudinal heat shrinkage rate of 40%-60% and a transverse heat shrinkage rate of 45%-65% at 120°C, and a heat shrinkage stress ≤2.0MPa.

[0015] To achieve the above objectives, according to another aspect of this application, a method for preparing a shrink film is also provided.

[0016] The method for preparing the shrink film according to this application includes the following steps: S1. Mix polyvinylidene chloride resin and blue hue modifier evenly according to a preset ratio to prepare modified polyvinylidene chloride mixture, and at the same time prepare the first polyethylene layer, the first ethylene / vinyl acetate layer, the second ethylene / vinyl acetate layer and the raw materials for the mixed functional layer respectively. S2. Each layer of raw material is fed into an independent extruder for melting and plasticizing, and then extruded through a five-layer co-extrusion die to form a multi-layer primary film. S3. The primary film is continuously and in real time tested by an online color mark detector to obtain CIE a value, b value and transmittance parameters. The control system automatically closes the loop to adjust the blue hue modifier addition rate, extrusion temperature and traction speed according to the detection deviation. S4. The qualified primary film is subjected to longitudinal stretching, transverse stretching, heat setting and cooling treatment in sequence to form a highly transparent directional shrink film. S5. Perform online haze and shrinkage rate detection on the finished film, and feed the detection results back to the control system to form a coordinated control mode for transparency and heat shrinkage performance.

[0017] Preferably, in step S2, the barrel temperature of the polyvinylidene chloride extruder is 165℃-195℃, and the die temperature is 175℃-185℃. The barrel temperature of polyethylene and ethylene / vinyl acetate extruders is 140℃-170℃; The five-layer co-extrusion die head adopts a flow channel pressure equalization structure to improve the consistency of melt flow rate in each layer and reduce film thickness deviation.

[0018] Preferably, in step S3, the online color mark detector has a detection frequency of 1 time / 2 seconds to 1 time / 5 seconds and a detection accuracy of ±0.01 color coordinate units. When the detection b value > 0, increase the amount of blue hue modifier added in a gradient of 0.0002wt%-0.0008wt%. When the b-value is less than -5, reduce the amount of blue hue modifier added at the same gradient. The control system establishes a color deviation prediction model to compensate for the yellowing trend of the membrane in advance.

[0019] Preferably, in step S4, the longitudinal stretching temperature is 75℃-85℃, and the longitudinal stretching ratio is 3.5-4.5. The transverse stretching temperature is 85℃-95℃, and the transverse stretching ratio is 3.5-4.5; The heat setting temperature is 100℃-115℃, and the heat setting time is 10s-25s; The heat setting stage employs a zoned gradient temperature control method to reduce residual stress within the film and improve the dimensional stability and transparency stability of the shrink film.

[0020] Beneficial Effects: In this embodiment, a blue hue modifier is added to the polyvinylidene chloride (PVDC) layer. This blue hue modifier absorbs the yellow light band of the PVDC material and compensates for the blue light band, thereby improving the yellowing phenomenon of the shrink film and increasing its transparency. An optical stabilizing agent, including one or more of heat stabilizers, antioxidants, and dispersants, is also added to the PVDC core layer to improve the uniformity of the blue hue modifier's dispersion and its long-term optical stability. The CIE color values ​​a* and b* of the shrink film satisfy -3≤a*≤0 and -5≤b*≤0, respectively, within the visible light range of 400nm-760nm. With an average light transmittance of ≥90% and a haze of ≤1.5%, the film achieves blue light compensation for the yellow hue produced by PVDC materials, thereby significantly improving the transparency and visual whiteness of the shrink film. This solves the problem that existing blue compensation schemes often rely on manual experience to add blue light, which cannot be dynamically adjusted in real time according to changes in film color. This can easily lead to local color differences, transparency fluctuations, and increased haze, making it difficult to meet the requirements of high-end transparent packaging for color consistency and optical stability. At the same time, multi-layer co-extruded shrink films are prone to interlayer stress concentration due to stress differences during stretching and heat setting, which can further lead to decreased film transparency, uneven shrinkage, and insufficient dimensional stability. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the shrink film structure of the present invention; and Figure 2 This is a schematic diagram of the shrink film preparation method of the present invention.

[0022] The attached figures are labeled as follows: 10. First polyethylene layer; 20. First ethylene / vinyl acetate layer; 30. Polyvinylidene chloride layer; 40. Second ethylene / vinyl acetate layer; 50. Hybrid Functional Layer; 60. Blue hue adjuster. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0024] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0025] Furthermore, the terms "installation," "setup," "equipped with," "connection," "linking," and "socketing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0027] like Figure 1-2As shown, this application relates to a shrink film and a method for preparing the same. Figure 1 As shown, the shrink film includes: a plurality of first polyethylene layers 10; a first ethylene / vinyl acetate layer 20 disposed on the polyethylene layers; a polyvinylidene chloride layer 30 disposed on the first ethylene / vinyl acetate layer 20 on the side away from the polyethylene layers; a second ethylene / vinyl acetate layer 40 disposed on the polyvinylidene chloride layer 30 on the side away from the first ethylene / vinyl acetate layer 20; and a hybrid functional layer 50 disposed on the second ethylene / vinyl acetate layer 40 on the side away from the polyvinylidene chloride layer 30; wherein, the polyvinylidene chloride layer 30 contains a blue hue modifier 60, which is used to absorb the yellow light band of the polyvinylidene chloride material and compensate for the blue light band, so as to improve the yellowing phenomenon of the shrink film and improve its transparency. The polyvinylidene chloride core layer is further provided with an optical stabilizing agent, which includes one or more of heat stabilizers, antioxidants and dispersants, to improve the dispersion uniformity and long-term optical stability of the blue hue modifier 60 in the polyvinylidene chloride core layer. The CIE color values ​​a* and b* of the shrink film satisfy -3≤a*≤0 and -5≤b*≤0, respectively. The average transmittance in the visible light range of 400nm-760nm is ≥90%, and the haze is ≤1.5%.

[0028] Specifically, the shrink film employs several first polyethylene layers 10 (PE) as a basic support layer structure. Through the lamination and composite of multiple layers of polyethylene material, a base system with flexibility, tensile strength, and heat shrinkage stability is formed. The first polyethylene layers 10 can be one or a combination of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), or metallocene polyethylene to balance the film's mechanical strength and heat-sealing performance. A gradient structure is formed through multi-layer co-extrusion, further optimizing the stress balance of the film in the longitudinal and transverse directions, avoiding problems such as localized stress concentration, warping deformation, or uneven shrinkage that occur in single-layer polyethylene structures during heat shrinkage. The first polyethylene layers 10 can be two layers.

[0029] It is important to understand that the multiple first polyethylene layers 10 can be composited using polyethylene materials with different melt flow rates and density parameters, giving the inner layer high toughness and the outer layer high wear resistance and impact resistance, thus forming a composite mechanical system of "flexible buffer + structural support". This structure not only enhances the shrink film's tensile strength during high-speed packaging but also improves the film's resistance to brittle fracture in low-temperature environments.

[0030] Its beneficial effects are as follows: by setting several first polyethylene layers 10, the overall mechanical strength, flexibility and heat shrinkage uniformity of the shrink film can be effectively improved, and the stability and adaptability of the film material in the packaging process can be improved; at the same time, the multi-layer polyethylene structure can form a good stress dispersion channel, reduce wrinkles, curling and edge lifting caused by uneven local heating during heat shrinkage, and improve the flatness of the packaging appearance and the product adhesion effect.

[0031] Specifically, by setting a first ethylene / vinyl acetate (EVA) layer on the first polyethylene layer 10, the good polar compatibility and flexible buffering properties of EVA material are used to construct a transition bonding layer between the polyethylene base layer and the subsequent functional layers. Since polyethylene material itself has low polarity, while the subsequent polyvinylidene chloride layer has high polarity, the interface transition through the EVA layer can effectively improve the interlayer bonding force between different materials and avoid delamination, peeling, or interface separation problems.

[0032] Furthermore, the vinyl acetate content in the first EVA layer can be controlled within the range of 8%-20% to balance flexibility and thermal bonding performance. A higher vinyl acetate content enhances interlayer adhesion and heat-sealing properties, while a lower content improves structural strength and heat resistance. By adjusting the polarity parameters of the EVA, synergistic matching between different functional layers can be achieved.

[0033] Its beneficial effects are as follows: the first EVA layer can effectively improve the interfacial bonding stability between the polyethylene layer and the polyvinylidene chloride layer, reducing the risk of interlayer delamination in the multilayer composite structure during heat shrinkage, high-temperature encapsulation and long-term storage; at the same time, the EVA layer has high flexibility, which can buffer and absorb heat shrinkage stress, and improve the overall shrinkage stability and impact resistance of the film material.

[0034] Specifically, a polyvinylidene chloride (PVDC) layer is placed on top of the first EVA layer as the core barrier layer of the shrink film. PVDC's excellent gas barrier, water vapor barrier, and transparency properties enhance the shrink film's ability to block oxygen, moisture, and odor gases. PVDC's high density significantly reduces oxygen and water vapor permeability, thereby extending the shelf life of packaged products.

[0035] Furthermore, the PVDC layer can be formed into a highly oriented, dense structure using a biaxial stretching process, allowing the molecular chains to be arranged in an orderly manner along the longitudinal and transverse directions, thereby further improving both barrier and transparency properties. Simultaneously, the thickness of the PVDC layer can be controlled within the range of 5μm-30μm to achieve a balance between barrier performance and flexibility.

[0036] Its beneficial effects are as follows: by setting a PVDC layer, the shrink film's ability to block oxygen, moisture and retain aroma can be effectively improved, reducing the impact of external oxygen, water vapor and pollutants on the contents of the packaging; at the same time, the PVDC layer has high transparency, which can enhance the visual display effect of the packaged products.

[0037] Specifically, a second EVA layer is placed on the outside of the PVDC layer to form a "double EVA clamping interface structure," providing double-sided buffering and bonding protection for the PVDC layer. Since PVDC material itself is somewhat brittle, it is prone to microcracks during thermal shrinkage or bending. The second EVA layer can provide flexible support for the PVDC layer, reducing the direct impact of external mechanical stress on the PVDC layer.

[0038] Furthermore, the second EVA layer can also function as a heat-sealing layer, improving packaging sealing strength and sealing stability by adjusting the melt index and vinyl acetate ratio. Simultaneously, the second EVA layer can also serve as an adhesion interface for the subsequent mixed functional layer 50, enhancing the composite stability of the functional layers.

[0039] Its beneficial effects are as follows: by flexibly wrapping the PVDC layer on both sides with the second EVA layer, the risk of cracking of the PVDC layer can be effectively reduced, and the overall bending resistance and heat shrinkage stability of the film material can be improved; at the same time, it can enhance the heat sealing performance of the film material and improve the reliability of packaging sealing.

[0040] Specifically, by adding a blue hue modifier 60 to the PVDC layer, the yellowing phenomenon that occurs in PVDC materials during processing, thermal shrinkage, and long-term storage is offset by utilizing the blue hue modifier 60 to absorb the yellow light band and compensate for the blue light band. Since PVDC materials are prone to slight dechlorination reactions under thermal history, leading to a yellowish-brown tendency in the film, the introduction of the blue hue modifier 60 forms a comprehensive color compensation mechanism, which can significantly improve the visual transparency of the film.

[0041] Furthermore, the blue hue modifier 60 can be one or more of ultramarine blue, phthalocyanine blue, organic blue fluorescent whitening system or inorganic nano blue light compensation system, and its uniformity in the PVDC layer can be improved by micro-nano-level dispersion process to avoid color spot or light scattering problems caused by the addition of traditional color masterbatch.

[0042] Its beneficial effects are as follows: by using blue hue modifier 60 to spectrally compensate for the yellow light region, the yellowing degree of PVDC film can be significantly reduced, and the transparency and visual whiteness of the film can be improved; at the same time, it can improve the display effect of packaged products and enhance the appearance quality of high-end food, cosmetic and pharmaceutical packaging.

[0043] Specifically, by further setting optical stabilizing agents within the PVDC layer, including one or more of heat stabilizers, antioxidants, and dispersants, the long-term stability of the blue hue modifier 60 and the PVDC matrix is ​​improved by inhibiting the thermo-oxidative degradation reaction during PVDC processing.

[0044] Among them, heat stabilizers can be used to suppress the dechlorination reaction during PVDC processing; antioxidants are used to reduce molecular chain breakage caused by high-temperature oxidation; and dispersing agents are used to improve the microscopic distribution uniformity of blue hue modifier 60 in PVDC and avoid local aggregation that causes optical defects.

[0045] Furthermore, optical stabilizing agents may also include light stabilizers and ultraviolet absorbers to improve the shrink film's resistance to yellowing and its ability to maintain transparency under long-term light exposure.

[0046] Its beneficial effects are as follows: by forming a synergistic stabilization mechanism between the optical stabilizing agent and the blue hue modifier 60, the dispersion uniformity and long-term stability of the hue modifier in the PVDC layer can be effectively improved, reducing yellowing, fogging and decreased light transmittance after long-term storage, thereby improving the long-term optical quality and service life of the shrink film.

[0047] Specifically, the CIE color value of the shrink film satisfies -3≤a*≤0 and -5≤b*≤0, where an a value close to 0 indicates a small red-green deviation, and a negative b value indicates that the film material has blue compensation characteristics, which can effectively counteract the yellow tendency of traditional PVDC films. By precisely controlling the overall hue, the shrink film exhibits high transparency, low yellowing, and high whiteness.

[0048] Meanwhile, the shrink film has an average light transmittance of ≥90% in the visible light range of 400nm-760nm, indicating that the film material has excellent light transmittance performance and can improve the visibility of the contents of the package; the haze is ≤1.5%, which indicates that the degree of light scattering inside the film material is low and can maintain a high-definition visual effect.

[0049] Furthermore, by combining a comprehensive hue adjustment system, optical stabilizing agents, and a multi-layer interface structure, high transparency and low haze can be achieved while ensuring high barrier performance, thus solving the problems of yellowing, haze, and insufficient transparency of traditional high-barrier PVDC shrink films.

[0050] Its beneficial effects are as follows: While maintaining the high barrier performance of PVDC, this solution achieves comprehensive optical performance of high transparency, high whiteness and low haze, which can significantly improve the display quality of packaged products; at the same time, the excellent optical stability performance can meet the long-term appearance stability requirements of high-end food, pharmaceutical, fresh food and cosmetic packaging fields.

[0051] As can be seen from the above description, this application achieves the following technical effects: In this embodiment, a blue hue modifier 60 is added. This blue hue modifier 60 is incorporated into the polyvinylidene chloride (PVDC) layer 30. The blue hue modifier 60 absorbs the yellow light band of the PVDC material and compensates for the blue light band, thereby improving the yellowing phenomenon of the shrink film and increasing its transparency. An optical stabilizing agent, including one or more of heat stabilizers, antioxidants, and dispersants, is also incorporated into the PVDC core layer to improve the uniformity of the blue hue modifier 60's dispersion and its long-term optical stability. The CIE color values ​​a* and b* of the shrink film satisfy -3≤a*≤0 and -5≤b*≤0, with a visible light range of 400nm-760nm. With an average light transmittance of ≥90% and haze of ≤1.5% within the specified range, the blue light compensation for the yellow hue produced by PVDC materials is achieved, thus significantly improving the transparency and visual whiteness of the shrink film. This solves the problem that existing blue compensation schemes often rely on manual experience to add blue light, which cannot be dynamically adjusted in real time according to changes in film color. This can easily lead to local color differences, transparency fluctuations, and increased haze, making it difficult to meet the requirements of high-end transparent packaging for color consistency and optical stability. At the same time, multi-layer co-extruded shrink films are prone to interlayer stress concentration due to stress differences during stretching and heat setting, which can further lead to decreased film transparency, uneven shrinkage, and insufficient dimensional stability.

[0052] Furthermore, the blue hue modifier 60 is selected from at least one of inorganic blue pigments and organic blue pigments; The inorganic blue pigment includes at least one of ultramarine, cobalt blue, cobalt silicon blue, and cobalt zinc silicon blue; The organic blue pigment includes at least one of the phthalocyanine series blue pigments and the indanone series blue pigments; The blue hue modifier 60 has a particle size of 0.01μm-2μm to reduce the increase in haze caused by particle scattering.

[0053] Specifically, the blue hue modifier 60 can be selected from at least one of inorganic and organic blue pigments. Through the combined hue compensation capabilities of different types of blue pigments, it corrects the yellowing phenomenon that occurs during the processing of polyvinylidene chloride (PVDC) materials. Inorganic blue pigments are characterized by high heat resistance, strong chemical stability, and good migration resistance, while organic blue pigments have the advantages of high tinting strength, good transparency, and high color purity. The combination of the two can balance the requirements of long-term stability and high transparency.

[0054] Among them, inorganic blue pigments include at least one of ultramarine, cobalt blue, cobalt silicon blue, and cobalt zinc silicon blue. Ultramarine has good yellow light absorption capacity, which can effectively reduce the yellowish tint of PVDC materials; cobalt blue and cobalt silicon blue have high temperature resistance and can adapt to the high-temperature extrusion processing environment of PVDC; cobalt zinc silicon blue has both high stability and low migration, which can reduce pigment precipitation or color deviation during long-term storage.

[0055] Organic blue pigments include at least one of the phthalocyanine series and the indanone series. Among them, phthalocyanine blue has high coloring strength and excellent transparency, and can achieve a significant blue light compensation effect at extremely low addition levels; while indanone blue has excellent lightfastness and heat stability, which can further improve the long-term optical stability of the film material.

[0056] Furthermore, the particle size of the blue hue modifier 60 is controlled to be 0.01 μm-2 μm. By controlling the pigment particle size within the micro-nano scale range, the scattering effect of particles on visible light can be reduced, avoiding the problems of increased haze and decreased light transmittance caused by traditional coarse-particle pigments. At the same time, small-particle pigments can form a more uniform dispersion in the PVDC matrix, improving the uniformity of hue compensation.

[0057] Its beneficial effects are as follows: by selecting different types of blue pigments and controlling the particle size range, precise yellowing compensation can be achieved while ensuring high transparency, avoiding light scattering, fogging and color spot defects caused by excessively large pigment particles, thereby improving the optical quality and visual clarity of the shrink film.

[0058] Furthermore, the blue hue modifier 60 has a mass fraction of 0.001wt%-0.1wt% in the polyvinylidene chloride core layer, preferably 0.005wt%-0.05wt%. The blue hue modifier 60 is distributed in a gradient or uniform manner along the thickness direction of the polyvinylidene chloride core layer to reduce color difference and improve the uniformity of light transmission of the film.

[0059] Specifically, the mass fraction of blue hue modifier 60 in the PVDC core layer is 0.001wt%-0.1wt%, preferably 0.005wt%-0.05wt%. By controlling the addition amount within an ultra-low proportion range, yellowing compensation can be achieved while avoiding problems such as excessive blue pigment causing the film to appear bluish, reduced light transmittance, or increased haze.

[0060] When the addition amount is below 0.001wt%, the blue light compensation effect is insufficient, making it difficult to effectively counteract the yellowing tendency of PVDC after processing; while when the addition amount exceeds 0.1wt%, pigment aggregation may lead to a decrease in light transmittance and an increase in color deviation. Therefore, by optimizing the range, an optimal balance can be achieved between overall hue, transparency, and visual naturalness.

[0061] Furthermore, the blue hue modifier 60 can be distributed in a gradient or uniform manner along the thickness direction of the PVDC core layer. A uniform distribution improves the overall color consistency of the film; while a gradient distribution allows for differentiated blue light compensation in different areas based on the light propagation path. For example, the concentration of the blue hue modifier 60 can be appropriately increased near the light incident side to improve overall visual whiteness, while simultaneously reducing the pigment concentration in internal areas to minimize light scattering loss.

[0062] Its beneficial effects are as follows: by precisely controlling the amount and distribution pattern of blue hue modifier 60, the color difference of the film material can be effectively reduced, the uniformity of light transmission and the overall color stability can be improved, and the problems of local color deviation, blue spots or uneven transparency that occur in the traditional color masterbatch addition method can be avoided.

[0063] Furthermore, the total thickness of the shrink film is 12μm-45μm; The thickness of the polyvinylidene chloride core layer accounts for 35%-55% of the total thickness; The first ethylene / vinyl acetate layer 20 and the second ethylene / vinyl acetate layer 40 have the same thickness, each accounting for 10%-20% of the total thickness; The first polyethylene layer 10 and the hybrid functional layer 50 have the same thickness, each accounting for 5%-15% of the total thickness; The first ethylene / vinyl acetate layer 20 and the second ethylene / vinyl acetate layer 40 are used to improve the interlayer bonding strength and reduce the interfacial stress concentration during the multilayer co-extrusion process.

[0064] Specifically, the total thickness of the shrink film is controlled between 12μm and 45μm to balance the requirements of packaging flexibility, mechanical strength, and heat shrinkability. A thinner structure improves shrink fit and transparency, while a thicker structure enhances barrier properties and puncture resistance.

[0065] The PVDC core layer accounts for 35%-55% of the total thickness, allowing it to play a core role as the main barrier layer. Because PVDC itself has excellent oxygen and moisture barrier properties, increasing its thickness proportion can effectively extend the shelf life of packaged products.

[0066] The first and second EVA layers are of equal thickness, each accounting for 10%-20% of the total thickness. This dual-EVA symmetrical buffer structure effectively balances the thermal stress differences between layers during heat shrinkage, reducing the risk of interfacial delamination and film warping. Simultaneously, the symmetrical structure helps improve co-extrusion flow stability.

[0067] The first polyethylene layer 10 and the hybrid functional layer 50 have the same thickness, each accounting for 5%-15% of the total thickness. The polyethylene layer is used to provide flexible support properties, while the hybrid functional layer 50 is used to impart functional properties to the surface. The coordinated thickness ratio of the two can improve the overall structural stability.

[0068] Furthermore, the first and second EVA layers are used to improve interlayer bonding strength and reduce interfacial stress concentration during multilayer co-extrusion. Since different materials have different coefficients of thermal expansion and melt rheological properties, the flexible EVA transition layer can effectively buffer melt flow shear stress and improve co-extrusion stability.

[0069] Its beneficial effects are: by optimizing the multilayer thickness ratio and symmetrical structural layout, a comprehensive balance can be achieved between barrier performance, transparency performance, heat shrinkage performance and mechanical properties, thereby improving the long-term stability and processing consistency of the membrane material.

[0070] Furthermore, the first polyethylene layer 10 includes one or more of linear low-density polyethylene, low-density polyethylene, and high-density polyethylene. The hybrid functional layer 50 is a blend layer of polyethylene and ethylene / vinyl acetate copolymer; The hybrid functional layer 50 is further provided with one or more of the following: slip agent, anti-blocking agent, antistatic agent, toughening agent, and wear-resistant modifier, in order to improve the surface slipness, anti-blocking performance, and packaging adaptability of the shrink film.

[0071] Specifically, the first polyethylene layer 10 includes one or more of linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), and high-density polyethylene (HDPE). LLDPE has excellent puncture resistance and flexibility; LDPE has high transparency and heat-sealing properties; and HDPE has high rigidity and dimensional stability. By combining different polyethylene materials, a base layer system that balances flexibility and strength can be formed.

[0072] The hybrid functional layer 50 is a blend of polyethylene and EVA copolymer. Polyethylene provides mechanical stability, while EVA provides flexibility and surface adhesion properties, thus forming a composite functional layer with good processability and surface properties.

[0073] Furthermore, the hybrid functional layer 50 also contains one or more of the following: a slip agent, an anti-blocking agent, an antistatic agent, a toughening agent, and a wear-resistant modifier. The slip agent reduces the coefficient of friction during packaging, improving film feeding smoothness; the anti-blocking agent prevents film adhesion during winding; the antistatic agent reduces the problem of static electricity attracting dust; the toughening agent improves low-temperature crack resistance; and the wear-resistant modifier improves surface scratch resistance.

[0074] Its beneficial effects are as follows: through the multifunctional modification system, the operational stability, automatic packaging adaptability and surface durability of the shrink film packaging process can be effectively improved, and the problems of film jamming, adhesion and surface wear during high-speed packaging can be reduced.

[0075] Furthermore, the shrink film has a longitudinal heat shrinkage rate of 40%-60% and a transverse heat shrinkage rate of 45%-65% at 120°C, and a heat shrinkage stress ≤2.0MPa.

[0076] Specifically, the shrink film exhibits a longitudinal heat shrinkage rate of 40%-60% and a transverse heat shrinkage rate of 45%-65% at 120℃, with a heat shrinkage stress ≤2.0MPa. By controlling the longitudinal and transverse shrinkage ratios, the film material can achieve a balanced wrapping effect during heat shrink packaging, improving its ability to adhere to irregularly shaped products.

[0077] The transverse shrinkage rate is slightly higher than the longitudinal shrinkage rate, which improves the film's ability to cover the packaging edges and reduces corner wrinkles. Controlling the heat shrinkage stress below 2.0 MPa reduces the compression damage to the packaging contents during heat shrinkage.

[0078] Its beneficial effects are: by optimizing the heat shrinkage rate and heat shrinkage stress parameters, the uniformity of shrink film coating and the flatness of the packaging appearance can be improved, while reducing the risk of deformation caused by high shrinkage stress to fragile products, flexible packaging products or precision products.

[0079] like Figure 2 As shown, this application also relates to a method for preparing a shrink film, comprising the following steps: S1. Mix polyvinylidene chloride resin and blue hue modifier 60 evenly according to a preset ratio to prepare modified polyvinylidene chloride mixture, and at the same time prepare raw materials for the first polyethylene layer 10, the first ethylene / vinyl acetate layer 20, the second ethylene / vinyl acetate layer 40 and the mixed functional layer 50 respectively. Specifically, PVDC resin and blue hue modifier 60 are first mixed uniformly according to a preset ratio to prepare a modified PVDC mixture. High-shear mixing or twin-screw pre-dispersion processes can improve the dispersion uniformity of blue hue modifier 60 in PVDC.

[0080] Simultaneously, raw materials for the first polyethylene layer 10, the first EVA layer, the second EVA layer, and the mixed functional layer 50 are prepared separately to achieve independent formulation control of each layer material.

[0081] Its beneficial effects are: by pre-dispersing the blue hue modifier 60, the hue stability and transparency uniformity in the subsequent extrusion process can be improved.

[0082] S2. Each layer of raw material is fed into an independent extruder for melting and plasticizing, and then extruded through a five-layer co-extrusion die to form a multi-layer primary film. Specifically, each layer of raw material is fed into an independent extruder for melt plasticization, and then combined through a five-layer co-extrusion die to form a multi-layer primary film. The independent extrusion structure allows for independent control of the temperature, pressure, and flow rate of different materials, improving the stability of the multi-layer structure.

[0083] Furthermore, the barrel temperature of the PVDC extruder is controlled at 165℃-195℃, and the die temperature is controlled at 175℃-185℃ to ensure that the PVDC is fully plasticized while avoiding thermal degradation; the extrusion temperature of polyethylene and EVA is controlled at 140℃-170℃ to improve the melt flow stability.

[0084] The five-layer co-extrusion die head adopts a flow channel pressure equalization structure, which makes the melt flow rate of each layer tend to be consistent, reducing layer thickness fluctuations and interface flow marks.

[0085] Its beneficial effects are: through precise temperature control and pressure equalization channel design, the uniformity of film thickness and the stability of multilayer interfaces can be improved.

[0086] S3. The primary film is continuously and in real time tested by an online color mark detector to obtain CIE a value, b value and transmittance parameters. The control system automatically closes the loop to adjust the addition rate of blue hue modifier 60, extrusion temperature and traction speed according to the detection deviation. Specifically, an online color mark detector is used to detect the primary film in real time to obtain the CIE a value, b value and transmittance parameters, and the control system automatically adjusts the addition rate of blue hue modifier 60, extrusion temperature and traction speed.

[0087] The detection frequency is 1 time / 2 seconds to 1 time / 5 seconds, and the detection accuracy is ±0.01 color coordinate units, enabling high-precision real-time monitoring. When the b value > 0, it indicates that the membrane is yellowish, so blue hue adjuster 60 is automatically added; when the b value < -5, the amount added is reduced to avoid excessive blueness.

[0088] Furthermore, the control system establishes a color deviation prediction model to provide advance compensation control for yellowing trends.

[0089] Its beneficial effects are: through online closed-loop control, stable control of membrane color can be achieved, improving batch consistency and transparency stability of products.

[0090] S4. The qualified primary film is subjected to longitudinal stretching, transverse stretching, heat setting and cooling treatment in sequence to form a highly transparent directional shrink film. Specifically, the primary film undergoes longitudinal stretching, transverse stretching, heat setting, and cooling treatment sequentially. The longitudinal stretching temperature is 75℃-85℃, with a stretching ratio of 3.5-4.5; the transverse stretching temperature is 85℃-95℃, with a stretching ratio of 3.5-4.5. Through biaxial stretching, the molecular chain orientation can be improved, thereby enhancing transparency, strength, and heat shrinkage properties.

[0091] The heat setting temperature is 100℃-115℃, the heat setting time is 10s-25s, and a zoned gradient temperature control method is adopted to gradually release the residual stress inside the membrane.

[0092] Its beneficial effects are as follows: through biaxial stretching and gradient heat setting processes, the dimensional stability, transparency stability and heat shrinkage uniformity of the shrink film can be improved, and the problems of curling, deformation and decreased transparency during subsequent storage can be reduced.

[0093] S5. Perform online haze and shrinkage rate detection on the finished film, and feed the detection results back to the control system to form a coordinated control mode for transparency and heat shrinkage performance.

[0094] Specifically, after biaxial stretching and heat setting, the finished film sequentially passes through an online optical inspection area and a heat shrinkage performance inspection area. A continuous automatic inspection system monitors the film's transparency, haze parameters, heat shrinkage performance, and dimensional stability in real time. The online optical inspection area is equipped with a high-precision haze meter, a transmittance meter, and a comprehensive color analysis module to acquire the film's average transmittance, haze value, and CIE color parameters a* and b* in the visible light spectrum. The heat shrinkage performance inspection area uses a constant-temperature hot air simulation device or an infrared rapid heating device to conduct instantaneous heat shrinkage simulation tests on the film to obtain the longitudinal heat shrinkage rate, transverse heat shrinkage rate, and heat shrinkage stress parameters.

[0095] Furthermore, the online haze detector adopts a multi-angle scattered light detection method. By simultaneously detecting the incident light intensity, transmitted light intensity, and scattered light intensity, a film light scattering analysis model is established to improve the identification accuracy of microscopic haze defects, local crystallization anomalies, and uneven pigment dispersion. The transmittance detector adopts a 400nm-760nm full-band scanning mode to continuously collect the transmittance performance in different wavelength regions to analyze the compensation effect of blue hue modifier 60 on the yellow light band and the overall optical uniformity.

[0096] Furthermore, during the heat shrinkage performance testing process, the control system rapidly heats the membrane material according to the preset temperature curve and uses a high-precision visual measurement system to collect the dimensional changes of the membrane material before and after heating in real time. The longitudinal and transverse heat shrinkage rates are automatically calculated through image recognition algorithms. At the same time, combined with tension sensors and stress analysis modules, the instantaneous shrinkage stress generated during the heat shrinkage of the membrane material is dynamically monitored to avoid uneven heat shrinkage, curling, or edge warping of the membrane material due to excessive local orientation.

[0097] Furthermore, the control system establishes a coupling relationship model between transparency parameters, hue parameters, and heat shrinkage parameters based on online detection results, and uses a closed-loop feedback method to coordinately adjust extrusion temperature, traction speed, stretch ratio, heat setting temperature, and the amount of blue hue modifier 60 added. For example, when an increase in haze is detected, the control system can automatically reduce the extrusion shear rate of the PVDC layer or improve the heat setting uniformity to reduce micro-grain aggregation; when an excessive deviation in longitudinal and transverse shrinkage rates is detected, the longitudinal and transverse stretch ratio and heat setting tension are automatically adjusted to improve shrinkage balance; when a gradually positive b-value is detected, the amount of blue hue modifier 60 added is compensated in advance to suppress subsequent yellowing trends.

[0098] Furthermore, the control system establishes a transparency stability prediction model and a heat shrinkage behavior database. By performing correlation analysis on historical process parameters, environmental temperature and humidity parameters, and online detection data, it can predict the trend of transparency stability and heat shrinkage stability of the film material during subsequent storage. At the same time, the system can automatically switch between high transparency mode, high shrinkage mode, or balanced mode according to the needs of different packaging products, so as to achieve adaptive adjustment of the process under different application scenarios.

[0099] Furthermore, the finished membrane after online inspection can undergo surface quality analysis through a defect identification module. The industrial camera performs high-speed scanning of the membrane surface to identify crystal points, color spots, flow marks, scratches, bubbles, and localized hazy areas. Combined with an automatic defect location marking system, abnormal areas are located and recorded for subsequent automatic cutting or quality traceability.

[0100] Its beneficial effects are as follows: By constructing a multi-parameter collaborative control system that combines online haze detection, light transmittance detection, heat shrinkage rate detection, and comprehensive hue detection, the transparency and heat shrinkage performance of shrink film can be optimized simultaneously, avoiding the problem of "difficulty in achieving both high transparency and high shrinkage performance" in traditional processes; at the same time, through a closed-loop feedback adjustment mechanism, the batch stability of products can be effectively improved, the risk of film yellowing and haze fluctuation can be reduced, and the dimensional stability and packaging consistency of film materials during high-speed automatic packaging processes can be improved, thereby enhancing the overall quality and market applicability of high-end shrink packaging products.

[0101] Furthermore, in step S2, the barrel temperature of the polyvinylidene chloride extruder is 165℃-195℃, and the die temperature is 175℃-185℃. The barrel temperature of polyethylene and ethylene / vinyl acetate extruders is 140℃-170℃; The five-layer co-extrusion die head adopts a flow channel pressure equalization structure to improve the consistency of melt flow rate in each layer and reduce film thickness deviation.

[0102] Furthermore, in step S3, the online color mark detector has a detection frequency of 1 time / 2 seconds to 1 time / 5 seconds, and a detection accuracy of ±0.01 color coordinate units; When the b-value is greater than 0, increase the amount of blue hue modifier 60 in a gradient of 0.0002wt%-0.0008wt%. When the b-value is less than -5, reduce the amount of blue hue modifier 60 by the same gradient. The control system establishes a color deviation prediction model to compensate for the yellowing trend of the membrane in advance.

[0103] 10. The preparation method according to claim 7, characterized in that, in step S4, the longitudinal stretching temperature is 75℃-85℃, and the longitudinal stretching ratio is 3.5-4.5; The transverse stretching temperature is 85℃-95℃, and the transverse stretching ratio is 3.5-4.5; The heat setting temperature is 100℃-115℃, and the heat setting time is 10s-25s; The heat setting stage employs a zoned gradient temperature control method to reduce residual stress within the film and improve the dimensional stability and transparency stability of the shrink film.

[0104] This application also has the following beneficial effects: 1. This application improves the dispersion uniformity of blue hue modifier in PVDC layer by setting optical stabilizing agents, reduces the haze increase problem caused by pigment agglomeration, and improves long-term optical stability.

[0105] 2. This application adopts online color mark detection and closed-loop control to realize real-time detection and dynamic compensation control of membrane color parameters, thereby improving the color consistency and automation control level of the product.

[0106] 3. This application effectively reduces stress concentration at the multi-layer co-extrusion interface and improves interlayer bonding strength and heat shrinkage uniformity through a five-layer composite structure design and EVA buffer layer.

[0107] 4. This application effectively reduces residual stress within the membrane and improves the dimensional stability and transparency stability of the shrink membrane through a zoned gradient thermal setting control method.

[0108] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and all such equivalent transformations fall within the protection scope of the present invention.

Claims

1. A shrink film, characterized in that, include: Several first polyethylene layers (10); A first ethylene / vinyl acetate layer (20) is disposed on the polyethylene layer; A polyvinylidene chloride layer (30) is disposed on the first ethylene / vinyl acetate layer (20) on the side away from the polyethylene layer; A second ethylene / vinyl acetate layer (40) is disposed on the polyvinylidene chloride layer (30) on the side away from the first ethylene / vinyl acetate layer (20); and A hybrid functional layer (50) is disposed on the second ethylene / vinyl acetate layer (40) on the side away from the polyvinylidene chloride layer (30); The polyvinylidene chloride layer (30) contains a blue hue modifier (60), which is used to absorb the yellow light band of the polyvinylidene chloride material and compensate for the blue light band, so as to improve the yellowing phenomenon of the shrink film and increase its transparency. The polyvinylidene chloride core layer is also provided with an optical stabilizing agent, which includes one or more of heat stabilizers, antioxidants and dispersants, in order to improve the dispersion uniformity and long-term optical stability of the blue hue modifier (60) in the polyvinylidene chloride core layer. The CIE color values ​​a* and b* of the shrink film satisfy -3≤a*≤0 and -5≤b*≤0, respectively. The average transmittance in the visible light range of 400nm-760nm is ≥90%, and the haze is ≤1.5%.

2. The shrink film according to claim 1, characterized in that, The blue hue modifier (60) is selected from at least one of inorganic blue pigments and organic blue pigments; The inorganic blue pigment includes at least one of ultramarine, cobalt blue, cobalt silicon blue, and cobalt zinc silicon blue; The organic blue pigment includes at least one of the phthalocyanine series blue pigments and the indanone series blue pigments; The blue hue modifier (60) has a particle size of 0.01 μm-2 μm to reduce the increase in haze caused by particle scattering.

3. The shrink film according to claim 2, characterized in that, The blue hue modifier (60) in the polyvinylidene chloride core layer has a mass fraction of 0.001wt%-0.1wt%, preferably 0.005wt%-0.05wt%. The blue hue modifier (60) is distributed in a gradient or uniform manner along the thickness direction of the polyvinylidene chloride core layer to reduce color difference and improve the uniformity of light transmission of the film.

4. The shrink film according to claim 1, characterized in that, The total thickness of the shrink film is 12μm-45μm; The thickness of the polyvinylidene chloride core layer accounts for 35%-55% of the total thickness; The first ethylene / vinyl acetate layer (20) and the second ethylene / vinyl acetate layer (40) have the same thickness, each accounting for 10%-20% of the total thickness; The first polyethylene layer (10) and the hybrid functional layer (50) have the same thickness, each accounting for 5%-15% of the total thickness; The first ethylene / vinyl acetate layer (20) and the second ethylene / vinyl acetate layer (40) are used to improve the interlayer bonding strength and reduce the interfacial stress concentration during the multilayer co-extrusion process.

5. The shrink film according to claim 1, characterized in that, The first polyethylene layer (10) includes one or more of linear low-density polyethylene, low-density polyethylene and high-density polyethylene; The hybrid functional layer (50) is a blend layer of polyethylene and ethylene / vinyl acetate copolymer; The hybrid functional layer (50) is further provided with one or more of the following: slip agent, anti-blocking agent, antistatic agent, toughening agent and wear-resistant modifier, in order to improve the surface slip, anti-blocking performance and packaging adaptability of the shrink film.

6. The shrink film according to claim 1, characterized in that, The shrink film has a longitudinal heat shrinkage rate of 40%-60% and a transverse heat shrinkage rate of 45%-65% at 120℃, and a heat shrinkage stress ≤2.0MPa.

7. A method for preparing a shrink film, characterized in that, Includes the following steps: S1. Mix polyvinylidene chloride resin and blue hue modifier (60) evenly according to the preset ratio to prepare modified polyvinylidene chloride mixture, and at the same time prepare the raw materials for the first polyethylene layer (10), the first ethylene / vinyl acetate layer (20), the second ethylene / vinyl acetate layer (40) and the mixed functional layer (50). S2. Each layer of raw material is fed into an independent extruder for melting and plasticizing, and then extruded through a five-layer co-extrusion die to form a multi-layer primary film. S3. The primary film is continuously and in real time tested by an online color mark detector to obtain CIE a value, b value and transmittance parameters. The control system automatically closes the loop to adjust the addition rate of blue hue modifier (60), extrusion temperature and traction speed according to the detection deviation. S4. The qualified primary film is subjected to longitudinal stretching, transverse stretching, heat setting and cooling treatment in sequence to form a highly transparent directional shrink film. S5. Perform online haze and shrinkage rate detection on the finished film, and feed the detection results back to the control system to form a coordinated control mode for transparency and heat shrinkage performance.

8. The preparation method according to claim 7, characterized in that, In step S2, the barrel temperature of the polyvinylidene chloride extruder is 165℃-195℃, and the die temperature is 175℃-185℃. The barrel temperature of polyethylene and ethylene / vinyl acetate extruders is 140℃-170℃; The five-layer co-extrusion die head adopts a flow channel pressure equalization structure to improve the consistency of melt flow rate in each layer and reduce film thickness deviation.

9. The preparation method according to claim 7, characterized in that, In step S3, the online color mark detector has a detection frequency of 1 time / 2 seconds to 1 time / 5 seconds and a detection accuracy of ±0.01 color coordinate units. When the detection b value > 0, the amount of blue hue modifier (60) added is increased in a gradient of 0.0002wt%-0.0008wt%. When the b value is less than -5, the amount of blue hue modifier (60) added is reduced by the same gradient; The control system establishes a color deviation prediction model to compensate for the yellowing trend of the membrane in advance.

10. The preparation method according to claim 7, characterized in that, In step S4, the longitudinal stretching temperature is 75℃-85℃, and the longitudinal stretching ratio is 3.5-4.5; The transverse stretching temperature is 85℃-95℃, and the transverse stretching ratio is 3.5-4.5; The heat setting temperature is 100℃-115℃, and the heat setting time is 10s-25s; The heat setting stage employs a zoned gradient temperature control method to reduce residual stress within the film and improve the dimensional stability and transparency stability of the shrink film.

Images