A multilayer coextruded low density white composite film structure

Abstract

The application discloses a kind of multilayer co-extrusion low-density white composite film structures, it is related to high molecular film technical field, including upper surface layer, core layer and lower surface layer, the upper surface layer, core layer and lower surface layer are fused extrusion after by multilayer co-extrusion process once, and it is integrative three-layer composite film by two-way stretching forming, the core layer is microporous reflective layer, and it is composed of PET resin, incompatible polymer microspheres and titanium dioxide particles.Microporous reflective function layer is encapsulated between two surface layers by A / B / A three-layer co-extrusion structure, and the optical roughness problem caused by the opening of microporous film surface is fundamentally solved.The upper surface layer and the lower surface layer are dense PET layers without incompatible polymer microspheres, the surface is smooth without open micropores, and the arithmetic average surface roughness Ra can be controlled below zero point three microns, avoiding surface diffuse reflection loss in reflective film applications, so that the reflectivity of the film reaches more than 95% while maintaining the advantages of microporous low density.

Description

Technical Field

[0001] This invention relates to the field of polymer film technology, specifically to a multilayer co-extruded low-density white composite film structure. Background Technology

[0002] In the backlight module of an LCD display, the reflective film is a key optical component located below the light guide plate or diffuser plate, reflecting light that leaks towards the back towards the display. The reflectivity of the reflective film directly determines the light utilization rate of the backlight module; every 1% increase in reflectivity can bring a considerable brightness gain to the terminal display module. The requirements for reflective films in modern high-end backlight modules have expanded from simply high reflectivity to a balanced performance of high reflectivity, lightweight, and high mechanical stability. Especially in applications where thickness and weight are extremely sensitive, such as tablet computers and automotive displays, low areal density reflective films can significantly reduce the overall weight of the module, which is beneficial for the thinner and lighter design of portable devices.

[0003] Currently, the white reflective film industry mainly employs two technical routes. The first is a solid-filled reflective film, using polyethylene terephthalate as the substrate and adding high concentrations of inorganic white pigments such as titanium dioxide, barium sulfate, or calcium carbonate to achieve light scattering and reflection. US Patent 6517911B2 discloses a typical solution of this type. The advantages of this type of reflective film are its mature manufacturing process and reflectivity exceeding 95%. However, its fatal flaw is its excessively high density: the addition of a large amount of high-density inorganic filler makes the film's areal density far exceed that of pure PET base film, increasing its weight by 30% to 60% for the same thickness, severely limiting its application in thinner and lighter products. Simultaneously, the high-filling-ratio inorganic particles have poor interfacial compatibility with the PET matrix. During the assembly of the terminal module, after experiencing physical stresses such as hydrostatic adsorption and vibration testing, microcracks easily propagate at the interface between the particles and the matrix, macroscopically manifesting as localized brightness attenuation during illumination testing—what the industry refers to as white spot defects in the reflective film.

[0004] The second type is a microporous reflective film, in which an incompatible polymer forms a dispersed phase within a PET matrix. During biaxial stretching, the debonding at the two-phase interface generates numerous micron to submicron-sized micropores. Light scattering and reflection are achieved by utilizing the refractive index difference between the micropores and the PET matrix. Japanese Patent JP2005041205A discloses a related technology using polypropylene or polymethylpentene as a micropore-forming agent. The greatest advantage of the microporous approach is that it can achieve high reflectivity without adding large amounts of high-density inorganic fillers, resulting in a significant reduction in film density. However, the simple microporous route faces two technical challenges: First, controlling the size and distribution of micropores is difficult, the stretching process window is narrow, and the micropores are prone to over-expansion during production, leading to a sharp decrease in the mechanical strength of the film and irreversible indentation deformation on the film surface during the static pressure test of the final assembly. Second, the surface micropores of a single-layer microporous film form a rough micromorphology after opening on the film surface. Although this is beneficial for the application of release films, for optical reflective films that require a high smooth reflective interface, the open micropores on the surface will cause light diffuse reflection loss. To address this, we propose a multilayer co-extruded low-density white composite film structure. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a multilayer co-extruded low-density white composite film structure, which solves the problems mentioned in the background section.

[0006] To achieve the above objectives, the present invention provides a multilayer co-extruded low-density white composite film structure, comprising an upper surface layer, a core layer, and a lower surface layer. The upper surface layer, core layer, and lower surface layer are melt-extruded in a single process using a multilayer co-extrusion technique and then biaxially stretched to form an integrated three-layer composite film. The core layer is a microporous reflective layer composed of PET resin, incompatible polymer microspheres, and titanium dioxide particles. During biaxial stretching, the incompatible polymer microspheres de-adhere to the PET matrix interface to form a microporous structure. The upper and lower surface layers are microporous closed-cell PET surface layers composed of PET resin and nano-scale inorganic anti-blocking agents, and do not contain incompatible polymer microspheres.

[0007] Furthermore, the incompatible polymer microspheres in the core layer are made of any one of cyclic olefin copolymers, polymethyl methacrylate, or polypropylene, and the amount of microspheres added is three to twelve percent of the total mass of the core layer.

[0008] Furthermore, the amount of titanium dioxide particles added to the core layer is 0.5% to 5% of the total mass of the core layer, and the titanium dioxide particles are uniformly dispersed in the PET matrix after surface organic coating treatment.

[0009] Furthermore, the nano-scale inorganic anti-adhesion agent is nano-silica or nano-calcium carbonate, and the amount added is 0.05% to 0.5% of the total mass of the corresponding surface layer.

[0010] Furthermore, the thickness of the upper and lower surface layers is independently three to fifteen micrometers, the thickness of the core layer is thirty to two hundred micrometers, and the total thickness of the three-layer composite film is thirty-six to two hundred and thirty micrometers.

[0011] Furthermore, the core layer, after biaxial stretching, has a micropore volume porosity of 15% to 45%, an average pore diameter of 0.8 micrometers to 10 micrometers, and an apparent density of 0.7 grams to 1.2 grams per cubic centimeter.

[0012] Furthermore, the upper and lower surfaces do not contain open micropores with a porosity exceeding one percent, and the arithmetic mean surface roughness Ra of both the upper and lower surfaces is between 0.05 micrometers and 0.3 micrometers.

[0013] Furthermore, the three-layer composite film has a light reflectance of 93% to 98% at a wavelength of 550 nanometers and a total light transmittance of less than 5%.

[0014] Furthermore, the absolute value of the refractive index difference between the incompatible polymer microspheres and PET resin in the core layer is 0.05 to 0.20. The micropores are filled with air, and the refractive index difference between air and PET resin is about 0.58. The interfacial light scattering of the micropores and the bulk light scattering of the titanium dioxide particles are synergistically superimposed in the core layer.

[0015] This invention provides a multilayer co-extruded low-density white composite film structure, which has the following beneficial effects: The multilayer co-extruded low-density white composite film structure: First, by encapsulating the microporous reflective functional layer between the two surface layers using an A / B / A three-layer co-extrusion structure, the optical roughness problem caused by open micropores on the surface of a single-layer microporous film is fundamentally solved. The upper and lower surface layers are dense PET layers without incompatible polymer microspheres, with smooth surfaces and no open micropores. The arithmetic average surface roughness Ra can be controlled below 0.3 micrometers, avoiding surface diffuse reflection loss in reflective film applications. This allows the film to maintain the advantages of low density due to microporous structure while achieving a reflectivity of over 95%.

[0016] Secondly, the core layer achieves high reflectivity through a synergistic effect of light scattering between the micropores formed by the debonding of incompatible polymer microspheres and the titanium dioxide particles. The refractive index difference between air and PET in the micropores is as high as 0.58, generating strong interfacial light scattering; the titanium dioxide particles have a refractive index as high as 2.7, further enhancing the scattering ability of visible light. The spatial superposition of these two scattering mechanisms allows the core layer to achieve or even exceed the reflectivity levels of traditional high-filler reflective films with only a relatively low amount of titanium dioxide added, avoiding the problems of increased density and deteriorated interfacial compatibility caused by excessive inorganic fillers.

[0017] Third, the micropore volume porosity of the core layer can reach 15% to 45%, with a large amount of air replacing the mass of PET resin and inorganic fillers, reducing the apparent density of the core layer to 0.7 to 1.2 grams per cubic centimeter, which is only 50% to 85% of the density of pure PET. With the same total thickness, the film surface density is reduced by 25% to 45% compared to solid-filled reflective films, significantly meeting the rigid requirements for lightweight portable electronic devices.

[0018] Fourth, the three-layer co-extrusion process creates a continuous transition layer between the core and surface layers, where molten molecular chains are entangled, eliminating the risk of adhesive interfaces or physical delamination. During static pressure, vibration, and drop tests in final assembly, no separation or peeling occurs between the layers, providing structural assurance for the long-term reliability of the reflective film. Simultaneously, the addition of nano-inorganic anti-blocking agents in the surface layer endows the film with excellent winding and subsequent cutting properties, preventing adhesion problems between films. Detailed Implementation

[0019] Example 1 This embodiment prepares a multilayer co-extruded low-density white composite film with a total thickness of 100 micrometers. The thickness of the upper and lower surface layers is 8 micrometers each, and the core layer thickness is 84 micrometers.

[0020] The formulation for the upper surface layer is: 100 parts PET resin and 0.2 parts nano-silica with an average particle size of 80 nanometers. The formulation for the lower surface layer is the same as that for the upper surface layer.

[0021] The core layer formulation consists of 100 parts PET resin, 6 parts COC microspheres with an average particle size of 4 micrometers, and 3 parts titanium dioxide particles with an average particle size of 0.3 micrometers.

[0022] Preparation process: The raw materials for the upper surface layer, core layer, and lower surface layer are melt-plasticized separately in three twin-screw extruders. The extrusion temperature for the upper surface layer is 275 degrees Celsius, for the core layer 280 degrees Celsius, and for the lower surface layer 275 degrees Celsius. The three melt streams converge through a three-layer co-extrusion die and are extruded from a T-die lip. The extruded film is then rapidly cooled on a cooling drum to obtain an unstretched sheet. The sheet is then subjected to a 3.5-fold longitudinal stretch at 95 degrees Celsius, followed by a 4.0-fold transverse stretch at 120 degrees Celsius, and finally heat-set at 190 degrees Celsius. The finished film is then obtained by winding.

[0023] The composite film obtained in Example 1 has the following properties: total thickness of 100 micrometers, areal density of 82 grams per square meter, and density of approximately 0.82 grams per cubic centimeter. It has a reflectivity of 96% at a wavelength of 550 nanometers and an arithmetic mean roughness Ra of 0.2 micrometers on its upper surface. After undergoing standard static pressure, ball drop, and vibration tests during module assembly, no white spots or defects were found during inspection, and cross-sectional scanning electron microscopy showed continuous interfaces between the three layers without delamination.

[0024] Example 2 This embodiment prepares an ultrathin multilayer co-extruded low-density white composite film with a total thickness of 60 micrometers. The thickness of the upper and lower surface layers is 5 micrometers each, and the core layer thickness is 50 micrometers.

[0025] The core layer formulation consists of 100 parts PET resin, 8 parts PMMA microspheres with an average particle size of 2 micrometers, and 1.5 parts titanium dioxide particles with an average particle size of 0.5 micrometers. The formulations of the upper and lower surface layers are the same as in Example 1. The lateral stretching ratio is increased to 4.5 times to enhance micropore formation.

[0026] The composite film obtained in Example 2 has an areal density of 51 grams per square meter, a reflectivity of 93% at a wavelength of 550 nanometers, and an arithmetic mean roughness Ra of 0.25 micrometers on its upper surface. After reliability testing, it showed no white spots and no delamination. This example is suitable for ultra-thin tablet backlight modules with extremely stringent requirements for thickness and weight.

[0027] Example 3 In this embodiment, a high-reflectivity enhanced composite film with a total thickness of 180 micrometers was prepared. The amount of titanium dioxide particles added in the core layer formulation was increased to five parts, and the amount of COC microspheres added was adjusted to four parts.

[0028] The composite film obtained in Example 3 has an areal density of 162 grams per square meter, a reflectivity of 98% at a wavelength of 550 nanometers, and an arithmetic mean roughness Ra of 0.18 micrometers on its upper surface. This example is suitable for large-size television backlight modules with extremely high brightness requirements.

[0029] The above embodiments demonstrate that by adjusting the ratio of incompatible polymer microspheres and titanium dioxide particles in the core layer, as well as the thickness ratio of the three-layer structure, the present invention can flexibly control the reflectivity, areal density, and thickness of the thin film within a wide range to adapt to the performance requirements of different end-application scenarios.

Claims

1. A multilayer co-extruded low-density white composite film structure, characterized in that, The film comprises an upper surface layer, a core layer, and a lower surface layer. The upper surface layer, core layer, and lower surface layer are melt-extruded in one step through a multi-layer co-extrusion process and then biaxially stretched to form an integrated three-layer composite film. The core layer is a microporous reflective layer composed of PET resin, incompatible polymer microspheres, and titanium dioxide particles. During biaxial stretching, the incompatible polymer microspheres de-adhere to the PET matrix interface to form a microporous structure. The upper and lower surface layers are microporous closed-cell PET surface layers composed of PET resin and nano-scale inorganic anti-blocking agents, and do not contain incompatible polymer microspheres.

2. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The incompatible polymer microspheres in the core layer are made of any one of cyclic olefin copolymers, polymethyl methacrylate, or polypropylene, and the amount of microspheres added is three percent to twelve percent of the total mass of the core layer.

3. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The amount of titanium dioxide particles added to the core layer is 0.5% to 5% of the total mass of the core layer. The titanium dioxide particles are uniformly dispersed in the PET matrix after surface organic coating treatment.

4. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The nano-scale inorganic anti-adhesion agent is nano-silica or nano-calcium carbonate, and the amount added is 0.05% to 0.5% of the total mass of the corresponding surface layer.

5. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The thickness of the upper and lower layers is independently three to fifteen micrometers, the thickness of the core layer is thirty to two hundred micrometers, and the total thickness of the three-layer composite film is thirty-six to two hundred and thirty micrometers.

6. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The core layer has a micropore volume porosity of 15% to 45% after biaxial stretching, an average pore diameter of 0.8 micrometers to 10 micrometers, and an apparent density of 0.7 grams to 1.2 grams per cubic centimeter.

7. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The upper and lower surfaces do not contain open micropores with a porosity exceeding one percent, and the arithmetic mean surface roughness Ra of the upper and lower surfaces is between 0.05 micrometers and 0.3 micrometers.

8. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The three-layer composite film has a light reflectance of 93% to 98% at a wavelength of 550 nanometers and a total light transmittance of less than 5%.

9. The multilayer co-extruded low-density white composite film structure according to claim 1, characterized in that, The absolute value of the refractive index difference between the incompatible polymer microspheres and PET resin in the core layer is 0.05 to 0.

20. The micropores are filled with air, and the refractive index difference between air and PET resin is about 0.

58. The interfacial light scattering of the micropores and the bulk light scattering of the titanium dioxide particles are synergistically superimposed in the core layer.