Method for producing multilayer films with flexible thickness gradient, multilayer films, film extrusion apparatus

Abstract

The application discloses a preparation method of a multilayer film with a flexible thickness gradient, a multilayer film and a film extrusion device, and belongs to the field of optical films. The application controls the cooling and stretching forming of the two surfaces of the multilayer film melt at different flow rates, the film layers in the multilayer film melt generate different stretching strengths due to the different flow rates of the two surfaces, and then different film layer thicknesses are obtained, and finally the multilayer film with a gradient thickness structure is obtained. Wherein, the film layer thickness decreases from the side with poor fluidity to the side with strong fluidity. The application also sets the cooling area and the heating area in the cooling and forming stage of the film extrusion device, so that the two surfaces of the multilayer film melt have different cooling temperatures, and the flow rates of the two surfaces are different. Compared with the thickness gradient formed by relying on the uneven multiplier, the thickness gradient proportion of the multilayer film obtained by the method of the application can be flexibly adjusted according to the cooling rate.

Description

Technical Field

[0001] This invention relates to the field of optical thin film technology, and in particular to a method for preparing a multilayer film with flexibly adjustable thickness gradient, as well as multilayer film and thin film extrusion equipment. Background Technology

[0002] High-reflectivity films are optical thin films that enhance reflectivity. They possess extremely high reflectivity within a specific wavelength range and are typically composed of alternating layers of two materials with different refractive indices, often categorized as one-dimensional photonic crystal materials. These optical thin films usually have a high-reflectivity band, the center wavelength of which is proportional to the thickness of each layer. The bandwidth of the high-reflectivity band is related to the difference in refractive indices between the two materials; the greater the difference in refractive indices, the wider the high-reflectivity band of the optical thin film.

[0003] Therefore, there are generally two solutions to increase the reflective band width. The first solution is to increase the refractive index difference between the two materials. Since the refractive index difference between different polymer materials is very small, there are usually three options: a high-refractive-index inorganic material and a polymer material, a low-refractive-index inorganic material and a polymer material, or a high-refractive-index inorganic material and a low-refractive-index inorganic material. However, regardless of the choice, one material will inevitably be inorganic. Inorganic material layers are usually processed by magnetron sputtering, vacuum evaporation, etc. High-reflective films produced by this process have high precision and are often used in precision instruments and equipment, but the cost is also high, making them unsuitable for large-area production. The second solution is to stack films of different thicknesses, allowing the reflective band to be extended. Therefore, the second solution can be made using two polymer materials, which is simple, low-cost, and can be fabricated over large areas.

[0004] Multilayer polymer co-extrusion technology, as one of the processing methods for polymer multilayer films, works by melting two polymers separately in two extruders and injecting them into a feed block. Within the feed block, the two polymer melts initially form a layered structure. This layered melt continues to flow through multiple multipliers to reach the desired number of layers. The multilayer polymer melt is then extruded through a die, stretched by rollers, cooled, and wound into the finished material. To achieve gradient thickness films, one or more multipliers are typically replaced with unequal-division multipliers. However, after the machine is installed, it is difficult to change the thickness gradient by adjusting parameters. Summary of the Invention

[0005] To address the aforementioned problems in the background technology, the present invention mainly provides a method for preparing a multilayer film with flexibly adjustable thickness gradient, as well as a multilayer film and thin film extrusion equipment.

[0006] To achieve the above objectives, the present invention provides a method for preparing a multilayer film with flexibly adjustable thickness gradient, comprising the following steps:

[0007] Extrusion: Extruding a multilayer polymer melt to form a multilayer film melt, wherein the multilayer polymer melt comprises two or more different polymers;

[0008] Forming: The two surfaces of the multilayer film melt are cooled and stretched at different flow rates to obtain a multilayer film with a gradient thickness layered structure.

[0009] In some embodiments of this application, a cooling device is provided during the molding step. The cooling device includes a cooling roller and a heater.

[0010] One surface of the multilayer film melt is attached to the surface of the cooling roller, and the heater heats the surface of the multilayer film melt away from the cooling roller, so that the two surfaces of the multilayer film melt are cooled and stretched at different flow rates.

[0011] In some embodiments of this application,

[0012] Prior to the extrusion step, the following steps are also included:

[0013] Melting: Melting two or more different dry polymers separately to obtain two or more different polymer melts;

[0014] Layering: The two or more different polymer melts initially form a layered polymer melt within the feed block;

[0015] Multiplication: The layered polymer melt is multiplied to form a multilayer polymer melt.

[0016] In some embodiments of this application,

[0017] The layered polymer melt is multiplied in a multiplication system including a multiplier to form the multilayer polymer melt, wherein...

[0018] The types of cascade multipliers include "one-to-two", "one-to-three", "one-to-four", and "one-to-five".

[0019] And / or, the stack multiplier includes an equal-layer stack multiplier or an unequal-layer stack multiplier;

[0020] And / or, multiple of the said stacked multipliers are connected in series to form the multiplication system;

[0021] And / or, the number of the stacked multipliers ranges from 1 to 10.

[0022] In some embodiments of this application, the polymer includes thermoplastic polymers;

[0023] And / or, the refractive index ratio between any two of the polymers is greater than 1.0;

[0024] And / or, the multilayer film includes a thickest layer and a thinnest layer, wherein the thickness ratio of the thickest layer to the thinnest layer is 1.0-1.2.

[0025] In some embodiments of this application, the polymer includes at least one of polymethyl methacrylate (PMMA), polymethylpentene (TPX), polycarbonate (PC), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl alcohol (PVA), acrylonitrile-butadiene-styrene copolymer (ABS), and polyethylene (PE).

[0026] To achieve the above objectives, this application also provides a film extrusion apparatus, which includes a die and a cooling device. The cooling device includes a cooling zone and a heating zone. The two surfaces of the multilayer film melt extruded by the die face the cooling zone and the heating zone, respectively. The film is cooled and stretched at different flow rates to obtain a multilayer film with a gradient thickness layered structure.

[0027] In some embodiments of this application, the cooling region includes a cooling roller, the heating region includes a heater, one surface of the multilayer film melt is attached to the surface of the cooling roller, and the heater heats the surface of the multilayer film melt away from the cooling roller, so that the two surfaces of the multilayer film melt are cooled and stretched at different flow rates.

[0028] In some embodiments of this application, the heater is fixed by a bracket, the bracket including a rotatable bracket, and the heater can rotate with the rotatable bracket;

[0029] And / or, the die opening width of the die head is in the range of 20μm-3mm;

[0030] And / or, the temperature range of the die head is 20℃-500℃;

[0031] And / or, the heater includes an infrared heater, a hot air blower, and a heating roller;

[0032] And / or, the heater is provided with an insulation cover.

[0033] In some embodiments of this application, the size of the cooling roller is replaceable;

[0034] And / or, the position of the cooling roller is adjustable;

[0035] And / or, the cooling method of the cooling roller includes oil cooling and water cooling.

[0036] To achieve the above objectives, this application also provides a multilayer film prepared by the multilayer film preparation method of the present invention as described above.

[0037] The beneficial effects that this invention can achieve are:

[0038] In the multilayer film forming stage, by controlling the cooling and stretching of the two surfaces of the multilayer melt at different flow rates, the film layers of the multilayer melt exhibit different tensile strengths due to the different flow rates on the two surfaces, thus resulting in different film thicknesses and ultimately a multilayer film with a gradient thickness. Specifically, the surface with a high flow rate (i.e., strong fluidity) has stronger tensile strength, while the surface with a low flow rate (i.e., weak fluidity) has weaker tensile strength. Therefore, the film thickness of the multilayer film decreases from the side with poor fluidity to the side with strong fluidity, thereby obtaining a multilayer film with a layered structure of gradient thickness.

[0039] This invention can generate a temperature difference between the two surfaces of a multilayer film melt by cooling them, thereby creating a difference in flow rate between the two surfaces. That is, the surface of the multilayer film melt with a higher cooling temperature has strong flowability, while the surface with a lower cooling temperature has poor flowability. The thickness of the multilayer film decreases from the side with a lower cooling temperature to the side with a higher cooling temperature, thereby obtaining a multilayer film with a gradient thickness layered structure.

[0040] Compared to existing technologies that rely on non-uniform multipliers to create thickness gradients, the thickness gradient ratio obtained by the method of this invention can be adjusted by controlling the formation of the two surfaces of the multilayer film melt at different flow rates based on factors such as cooling temperature. Therefore, the thickness gradient of the multilayer film can be adjusted according to production needs to regulate the bandwidth of the reflective band, achieving excellent reflective properties. It can be used as a high-reflectivity film, and a single set of equipment can produce multiple multilayer films with different bandwidths. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0042] Figure 1 This is a partial structural schematic diagram of a film extrusion device according to the present invention.

[0043] Figure 2 The diagram shows the reflection bandwidth results of the multilayer films obtained in Embodiment 1 and Comparative Example 1 of the present invention.

[0044] Among them, 1: die head; 2: support; 3: heater; 4: multi-layer film melt; 5: cooling roller.

[0045] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0046] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0048] In this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. Furthermore, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. If the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed by this invention.

[0049] This invention provides a method for preparing a multilayer film with flexibly adjustable thickness gradient, comprising the following steps:

[0050] Extrusion: Extruding a multilayer polymer melt to form a multilayer film melt, wherein the multilayer polymer melt includes two or more different polymers;

[0051] Molding: The two surfaces of the extruded multilayer film melt are cooled and stretched at different flow rates to obtain a multilayer film with a gradient thickness layered structure.

[0052] The multilayer film of the present invention is composed of film layers of different thicknesses stacked together, and has a gradient thickness layered structure. Its reflective band has a wide bandwidth due to the superposition of film layers of different thicknesses, and has excellent reflective function, and can be used as a high reflective film.

[0053] Furthermore, by adjusting the flow rate on both surfaces of the multilayer film melt during the cooling and forming stage, the film layers of the multilayer film can obtain different thicknesses. Stacking film layers of different thicknesses is beneficial to obtaining a wide reflective bandwidth. The reflective bandwidth of the multilayer film can be flexibly adjusted by controlling the flow rate as needed.

[0054] In the molding process, the present invention does not limit the way in which the two surfaces of the multilayer film melt obtain different flow rates. In some embodiments, the cooling temperature of the two surfaces of the multilayer film melt can be controlled to produce a temperature difference to obtain a flow rate difference. Finally, the film is cooled and stretched to obtain a multilayer film with a layered structure of gradient thickness.

[0055] In some embodiments, a cooling device is provided in the molding step. This cooling device includes a cooling roller and a heater. One surface of the multilayer film melt is attached to the surface of the cooling roller. The heater heats the surface of the multilayer film melt facing away from the cooling roller, creating a temperature difference between the two surfaces of the multilayer film melt. This allows for cooling and stretching at different flow rates to obtain a gradient thickness layered structure. In this embodiment, the two surfaces of the multilayer film melt have different cooling temperature differences due to their different cooling temperatures, resulting in a flow rate difference. Specifically, the surface with a higher cooling temperature has stronger flowability, while the surface with a lower cooling temperature has weaker flowability. The film thickness decreases from the side with a lower cooling temperature to the side with a higher cooling temperature. Compared to the thickness gradient formed by an uneven multiplier in the prior art, the thickness gradient ratio of the multilayer film can be flexibly adjusted according to the cooling temperature. The size of the multilayer film thickness gradient can be adjusted according to production needs to regulate the bandwidth of the reflective band of the multilayer film.

[0056] In some embodiments, the flow rate of the two surfaces of the multilayer film melt can be controlled by adjusting parameters such as the working temperature and size of the die head of the film extrusion equipment, and the position and size of the rollers through which the film passes after extrusion, thereby creating a flow rate difference between the two surfaces and forming a thickness gradient structure. For example, by increasing the size of the rollers, the surface of the multilayer film melt close to the rollers receives a slower stretching flow rate, while the surface away from the rollers receives a faster stretching flow rate, thus creating a flow rate difference. The resulting multilayer film is thicker on the side close to the rollers and thinner on the side away from the rollers, thereby forming a gradient thickness structure.

[0057] In some embodiments, prior to the extrusion step, the following step is also included:

[0058] Melting: Melting two or more different dry polymers separately to obtain two or more different polymer melts;

[0059] Layering: The two or more different polymer melts initially form a layered polymer melt within the feed block;

[0060] Multiplication: The layered polymer melt is multiplied to form a multilayer polymer melt.

[0061] In the melting step of this embodiment, the polymer can be placed in a drying device for later use. The drying device includes an atmospheric pressure dryer and a vacuum dryer, preferably a vacuum dryer. The drying temperature and time are determined according to the properties of the polymer, and the drying temperature should not exceed the decomposition temperature of the polymer. Polymers with a higher degree of drying are more likely to produce a uniform polymer melt with better flowability in the melting stage.

[0062] In some embodiments, the polymer is PMMA, and the PMMA is dried at a temperature of 90°C for 12 hours.

[0063] In some embodiments, the polymer is PC, and the drying temperature of PC is 110°C and the drying time is 12 hours.

[0064] In the layering step of this embodiment, the polymer melt flows into the feed block through the flow channel, forming a layered polymer melt within the feed block. It is understood that different polymer melts are stacked in this step to form a layered polymer melt, and subsequent process steps allow each polymer film layer to achieve different thicknesses. The resulting multilayer film has a wide reflective band, meeting the requirements of a high-reflectivity film. Compared to the preparation process using inorganic materials as the substrate for high-reflectivity films, the preparation method of this invention is simpler, lower in cost, and more conducive to large-area production.

[0065] In some embodiments, the layered polymer melt is multiplied in a multiplication system including a multiplier to form a multilayer polymer melt.

[0066] The present invention does not limit the type of cascade multiplier. In some embodiments, the type of cascade multiplier includes, but is not limited to, "one-to-two", "one-to-three", "one-to-four", and "one-to-five".

[0067] In this embodiment, the stack multiplier includes an equal-layer stack multiplier or an unequal-layer stack multiplier.

[0068] In some embodiments, a plurality of said multipliers are connected in series to form the multiplication system. It is understood that the multiplication system can be formed by multiple multipliers of the same type connected in series, or by multiple multipliers of different types connected in series to form the multiplication system.

[0069] The present invention does not limit the number of multipliers in the multiplication system. In some embodiments, the number of multipliers ranges from 1 to 10, and can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc.

[0070] This invention does not limit the type of polymer. As long as the polymer has fluidity within a certain temperature range and does not decompose, and can be extruded into shape by an extrusion device, it meets the requirements of this invention.

[0071] In some embodiments, the polymer includes, but is not limited to, thermoplastic polymers, which have fluidity and plasticity within a certain temperature range and can be extruded and molded by an extrusion device. Moreover, because the polymer has fluidity and plasticity, the flow rate on different surfaces can be controlled, so that different surfaces obtain flow rate differences, thereby obtaining a gradient thickness structure.

[0072] In some embodiments, the thermoplastic polymer includes, but is not limited to, thermoplastic resins and thermoplastic polyesters. The aforementioned thermoplastic resins and thermoplastic polyesters can be used to prepare film-like products via extrusion processes, suitable for the film industry.

[0073] In some embodiments, the polymer has high transparency. The high transparency polymer, when extruded, can achieve better light transmittance. Moreover, under the action of the gradient thickness structure, it is beneficial to increase the reflectivity of its reflective band and reduce the absorption rate.

[0074] In some embodiments, the polymer may also include modified polymers obtained by modifying the polymer thereon, the modification including but not limited to blending modification, filler modification, chemical modification and composite materials.

[0075] In some embodiments, the polymers include, but are not limited to, polymethyl methacrylate (PMMA), polymethylpentene (TPX), polycarbonate (PC), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl alcohol (PVA), acrylonitrile-butadiene-styrene copolymer (ABS), and polyethylene (PE).

[0076] The above-mentioned polymers have fluidity and plasticity within a certain temperature range, and can be extruded and molded by extrusion equipment. Moreover, their fluidity and plasticity allow for control of the flow rate on different surfaces, resulting in different flow rate differences and thus a gradient thickness structure.

[0077] Furthermore, an interface can be formed between any two of the above polymers, which is not easily peeled off. Moreover, the refractive index between any two of the above polymers is relatively large, and the large difference in refractive index makes it easier to meet the bandwidth requirements of high-reflection films.

[0078] In some embodiments, during the process of polymer melt flowing into the feed block for layering, a metering pump can be used to adjust the outflow of polymer melt. By controlling the outflow of polymer melt to control the layered structure of the layered polymer melt and the thickness of each film, two or more polymers can form a layered structure before multiplication and ensure the thickness ratio between them, which is beneficial to adjusting the formation of a gradient thickness structure of multilayer films.

[0079] In some embodiments, the polymer melt is a PMMA melt and a PC melt, and the outflow ratio of the PMMA melt and the PC melt is adjusted to 16:15 by a metering pump, so that the melt flows into the feed block and stacks to form a layered polymer melt with a double-layer structure.

[0080] The thickness ratio of each film layer in the layered polymer melt in the feed block is adjustable. In some embodiments, the thickness of the corresponding film layer can be determined according to the refractive index of the polymer. Preferably, the thickness ratio between the corresponding film layers of different polymers in the layered polymer melt is the inverse ratio of the refractive indices of the polymers.

[0081] This invention does not limit the refractive index ratio between any two polymers in the multilayer film. A larger refractive index ratio between any two polymers makes it easier to meet the bandwidth requirements of high-reflectivity films. In some embodiments, the refractive index ratio between any two polymers is greater than 1.0, and can be 1.01, 1.02, 1.05, 1.07, 1.1, etc. Such refractive index ratios are more likely to meet the bandwidth requirements of high-reflectivity films.

[0082] This invention does not limit the thickness ratio of the thickest and thinnest layers in the multilayer film. In some embodiments, the thickness ratio of the thickest and thinnest layers is 1.0-1.2, for example, it can be any ratio within the range of 1.0-1.2, such as 1.01, 1.02, 1.05, 1.07, 1.1, 1.15, 1.2, etc. Within the above ratio range, it is easy to control the thickness gradient of the multilayer film to obtain excellent reflective function and meet the requirements for the use of high-reflectivity films.

[0083] The present invention also provides a film extrusion apparatus that can produce not only single-layer films but also multilayer films. The film extrusion apparatus includes at least a die and a cooling device. The cooling device includes a cooling zone and a heating zone. The two surfaces of the multilayer film-like melt extruded from the die face the cooling zone and the heating zone, respectively. Cooling and stretching are performed at different flow rates to obtain a multilayer film with a gradient thickness layered structure.

[0084] The film extrusion equipment of this invention, in addition to the die head and cooling device, also includes other components commonly found in film extrusion equipment in this technical field, including but not limited to feeding devices, barrels, screws, multipliers, etc.

[0085] It should be noted that when a film extrusion device is equipped with a multiplier, the film preparation process also includes multiplying the film using the multiplier in the film extrusion device, resulting in a multilayer film.

[0086] It is understandable that the two surfaces of the multilayer film melt face the cooling region and the heating region respectively, and the two surfaces generate a temperature difference and have different cooling rates. Therefore, it can be cooled and stretched at different flow rates to obtain a multilayer film with a layered structure of gradient thickness.

[0087] In some embodiments, the cooling zone includes a cooling roller, and the heating zone includes a heater. One surface of the multilayer film melt is attached to the surface of the cooling roller. The heater heats the surface of the multilayer film melt away from the cooling roller, creating a temperature difference between the two surfaces of the multilayer film melt, allowing for cooling and stretching at different flow rates. It is understood that the surface of the multilayer film melt attached to the cooling roller has a lower temperature and a faster cooling rate, resulting in a slower flow rate and thus poorer tensile strength. Conversely, the surface of the multilayer film melt heated by the heater has a higher temperature and a slower cooling rate, resulting in a faster flow rate and thus stronger tensile strength. Therefore, the two surfaces of the multilayer film melt have a difference in tensile strength due to the different flow rates, and the film thickness decreases from the side with the lower cooling temperature to the side with the higher cooling temperature. Compared to the thickness gradient formed by an uneven multiplier in the prior art, the thickness gradient ratio of the multilayer film can be flexibly adjusted according to the cooling temperature. The size of the multilayer film thickness gradient can be adjusted according to production needs to regulate the bandwidth of the reflective band of the multilayer film.

[0088] In some embodiments, the heater is fixed by a bracket, which includes a rotatable bracket. The heater can rotate with the rotatable bracket to the desired angle to ensure that the multilayer film melt is heated more evenly.

[0089] The present invention does not limit the die opening width of the die head, and can be designed according to requirements. In some embodiments, the die opening width of the die head can be in the range of 20μm-3mm.

[0090] This invention does not limit the working temperature of the die head. The working temperature of the die head needs to be adjusted according to the type of polymer. The temperature needs to be adjusted so that the polymer is cooled and stretched in a molten state, while ensuring that the polymer does not decompose due to excessive temperature.

[0091] For example, PMMA has a melting point of around 150°C, but it is prone to decomposition at around 300°C. Therefore, when the polymer contains PMMA, the die temperature can be adjusted between 150°C and 300°C to keep the PMMA in a molten state, making it easy to stretch and mold, while ensuring that it is not decomposed by high temperature.

[0092] For example, PC has a melting point of around 240°C and a decomposition temperature of around 350°C. When the polymer contains PC, the die temperature can be adjusted between 240°C and 350°C to keep the PC in a molten state, making it easy to stretch and mold, while ensuring that it is not decomposed by high temperature.

[0093] For example, the temperature range of the die head can be 20℃-500℃. This temperature range allows the polymers of the above types in this invention to enter a molten state, making them easy to stretch and mold, while also ensuring that the polymers do not decompose due to high temperatures.

[0094] In some embodiments, the heater includes an infrared heater, a hot air blower, and a heating roller, preferably an infrared heater, which can make the multilayer film melt heated more evenly.

[0095] In some embodiments, the heater is provided with an insulation cover, which helps to ensure that the multilayer film melt is heated more evenly.

[0096] The size and position of the cooling roller in the film extrusion equipment of the present invention are adjustable. In some embodiments, the position of the cooling roller is adjustable in four directions, including vertical and horizontal movement. Vertical movement moves the roller closer to the die head and further away from the die head, respectively, adjusting the length of the section from the die head to the cooling roller. Horizontal and horizontal movement adjust the angle and position of the contact point between the melt film and the cooling roller.

[0097] In some embodiments, the cooling rollers are cooled by oil cooling and water cooling, which are beneficial for uniformly cooling the multilayer film melt.

[0098] In some embodiments, reference Figure 1 , Figure 1 This is a partial structural diagram of a film extrusion apparatus according to the present invention. The film extrusion apparatus includes a die head 1 and a cooling device. The cooling device includes a heater 3 and a cooling roller 5. The heater 3 is fixed to the extrusion port of the die head 1 by a bracket 2. The multilayer film-like melt 4 extruded through the die head 1 has one surface facing the heater 3 and the other surface facing the cooling roller 5, and is cooled and stretched into shape under the action of the cooling roller. Because the heater 3 heats one surface of the multilayer film-like melt 4 and the cooling roller 5 cools the other surface of the multilayer film-like melt 4, a temperature difference is generated between the two surfaces of the multilayer film-like melt 4, resulting in different cooling rates. Therefore, cooling and stretching can be performed at different flow rates to obtain a multilayer film with a gradient thickness layered structure.

[0099] In some embodiments, after the molding step, the following step is further included:

[0100] Biaxial stretching: Heating and biaxially stretching a multilayer film;

[0101] Rewinding: After the biaxially stretched multilayer film has cooled, it is cut and rewound by a rewinding device to form a finished roll.

[0102] By biaxial stretching, the center wavelength of the multilayer film's reflective band can be made to meet application requirements.

[0103] The present invention also provides a multilayer film prepared by the method for preparing a multilayer film with adjustable thickness gradient as described above, and a multilayer film prepared by the film extrusion equipment as described above. Both of the above multilayer films have a multilayer structure with gradient thickness and can be used as high-reflectivity films.

[0104] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following specific embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0105] Example 1

[0106] refer to Figure 1 The film extrusion equipment of this embodiment includes a die head 1 and a cooling device. The cooling device includes a heater 3 and a cooling roller 5. The heater 3 is fixed at the extrusion port of the die head 1 by a bracket 2. The heater 3 is an infrared heater.

[0107] The preparation method of the multilayer film in this embodiment is as follows:

[0108] Drying: Polymethyl methacrylate (PMMA) and polycarbonate (PC) were dried separately in a low-pressure dryer. The PMMA was dried at 90°C for 12 hours, and the PC was dried at 110°C for 12 hours.

[0109] Feeding: The two dried polymers were added to the feed ports of two extruders that had been preheated for 4 hours.

[0110] Melting: PMMA is melted at 285°C to obtain PMMA melt, and PC is melted at 270°C to obtain PC melt.

[0111] Layering: PMMA polymer melt and PC polymer melt are regulated by metering pump to flow into the feed block at a ratio of 16:15, stacking to form a layered polymer melt with a double-layer structure.

[0112] Multiplication: The layered polymer melt with the above-mentioned double-layer structure is multiplied by three "one-to-four" and one "one-to-two" equal-multiplier to form a multilayer polymer melt with 256 layers.

[0113] Extrusion: The length of the extrusion die of the extrusion equipment is set to 40cm, the temperature is set to 210℃, and a multilayer film melt of 256 layers is obtained by extrusion at a speed of 17kg / h.

[0114] Forming: The heating temperature of heater 3 is set to 200℃, the diameter of cooling roller 5 is 30cm, the cooling temperature is set to 110℃, and the rotation speed of cooling roller 5 is 1rpm. The multilayer film melt 4 with 256 layers is cooled under the dual action of heater 3 and cooling roller 5, and after 2×2 biaxial stretching, a multilayer film with a gradient thickness layered structure is formed. The thickness ratio of the thickest layer to the thinnest layer of the multilayer film is 1.1.

[0115] Biaxial stretching: The multilayer film is heated and biaxially stretched.

[0116] Rewinding: After the biaxially stretched multilayer film has cooled, it is rewound using a rewinding machine.

[0117] The total thickness of the multilayer film finally formed in Example 1 is 48 μm.

[0118] Comparative Example 1

[0119] Comparative Example 1 prepared a multilayer film according to Example 1, but the difference was that, in the forming stage, no heater was used for heating, and the two surfaces of the multilayer film melt were kept at the same cooling temperature for cooling and stretching to obtain a multilayer film with no gradient thickness.

[0120] Performance testing

[0121] The theoretical reflection bands of the multilayer films obtained in Example 1 and Comparative Example 1 are compared. Figure 2 As shown. By Figure 2 It can be seen that multilayer films with gradient thickness have a wider reflection bandwidth than multilayer films without gradient thickness, which can meet the requirements of high-reflection films.

[0122] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A method for producing a multilayer film with a flexible thickness gradient, characterized by, Includes the following steps: Extrusion: Extruding a multilayer polymer melt to form a multilayer film melt, wherein the multilayer polymer melt comprises two or more different polymers; Molding: A cooling device is provided, the cooling device including a cooling roller and a heater, one surface of the multilayer film melt is attached to the surface of the cooling roller, and the heater heats the surface of the multilayer film melt away from the cooling roller, so that the two surfaces of the multilayer film melt are cooled and stretched at different flow rates to obtain a multilayer film with a gradient thickness layered structure.

2. The method for preparing a multilayer film with flexibly adjustable thickness gradient according to claim 1, characterized in that, Prior to the extrusion step, the following steps are also included: Melting: Melting two or more different dry polymers separately to obtain two or more different polymer melts; Layering: The two or more different polymer melts initially form a layered polymer melt within the feed block; Multiplication: The layered polymer melt is multiplied to form a multilayer polymer melt.

3. The method for preparing a multilayer film with flexibly adjustable thickness gradient according to claim 2, characterized in that, The layered polymer melt is multiplied in a multiplication system including a multiplier to form the multilayer polymer melt, wherein... The types of cascade multipliers include "one-to-two", "one-to-three", "one-to-four", and "one-to-five"; And / or, the stack multiplier includes an equal-layer stack multiplier or an unequal-layer stack multiplier; And / or, multiple of the said stacked multipliers are connected in series to form the multiplication system; And / or, the number of the stacked multipliers ranges from 1 to 10.

4. The method for preparing a multilayer film with flexibly adjustable thickness gradient according to claim 1, characterized in that, The polymer includes thermoplastic polymers; And / or, the refractive index ratio between any two of the polymers is greater than 1.0; And / or, the multilayer film includes a thickest layer and a thinnest layer, wherein the thickness ratio of the thickest layer to the thinnest layer is 1.0-1.

2.

5. The method for preparing a multilayer film with flexibly adjustable thickness gradient according to claim 4, characterized in that, The polymer includes at least one of polymethyl methacrylate (PMMA), polymethylpentene (TPX), polycarbonate (PC), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl alcohol (PVA), acrylonitrile-butadiene-styrene copolymer (ABS), and polyethylene (PE).

6. A film extrusion apparatus characterized by, The film extrusion equipment includes a die head and a cooling device. The cooling device includes a cooling zone and a heating zone. The two surfaces of the multilayer film melt extruded by the die head face the cooling zone and the heating zone, respectively. It is cooled and stretched at different flow rates to obtain a multilayer film with a gradient thickness layered structure. The cooling zone includes a cooling roller, and the heating zone includes a heater. One surface of the multilayer film melt is attached to the surface of the cooling roller, and the heater heats the surface of the multilayer film melt away from the cooling roller, so that the two surfaces of the multilayer film melt are cooled and stretched at different flow rates.

7. The film extrusion apparatus of claim 6, wherein The cooling zone includes a cooling roller, and the heating zone includes a heater. One surface of the multilayer film melt is attached to the surface of the cooling roller, and the heater heats the surface of the multilayer film melt away from the cooling roller, so that the two surfaces of the multilayer film melt are cooled and stretched at different flow rates.

8. The film extrusion apparatus of claim 7, wherein, The heater is fixed by a bracket, which includes a rotatable bracket, and the heater can rotate with the rotatable bracket; And / or, the die opening width of the die head is in the range of 20μm-3mm; And / or, the temperature range of the die head is 20℃-500℃; And / or, the heater includes an infrared heater, a hot air blower, and a heating roller; And / or, the heater is provided with an insulation cover.

9. The film extrusion apparatus of claim 8, wherein, The size of the cooling roller is replaceable; And / or, the position of the cooling roller is adjustable; And / or, the cooling method of the cooling roller includes oil cooling and water cooling.

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