Bracket-free invisible orthodontic multi-layer composite film and preparation method thereof

Abstract

The present application relates to a kind of bracketless invisible orthodontic treatment materials, more particularly to a kind of bracketless invisible orthodontic treatment multilayer composite film and preparation method thereof.The multilayer composite film of the present application is formed by including first layer, second layer and third layer arranged from top to bottom hot pressing;First layer and third layer are thermoplastic polyurethane, and second layer is formed by polyurethane and polyethylene terephthalate glycol-1,4-cyclohexane dimethanol extrusion injection molding after blending.The present application improves the material of each layer of composite film to prepare a kind of bracketless invisible orthodontic treatment interlayer composite excellent multilayer composite film.The multilayer composite film prepared by the present application also has the advantages of high transparency, suitable correction force and low stress relaxation rate.

Description

Technical Field

[0001] This invention relates to a bracketless invisible orthodontic material, and more specifically, to a multilayer composite film for bracketless invisible orthodontic treatment and its preparation method. Background Technology

[0002] Orthodontic treatment, also known as dental surgery, involves treating malocclusion through orthodontic methods or surgery to establish a normal bite, resulting in a more aesthetically pleasing facial appearance and restored self-confidence. With continuous advancements in materials science, orthodontic medical devices have evolved from fixed braces to removable braces, and now to the revolutionary clear aligner technology. Compared to the many inconveniences of solid braces, such as inflexibility in removal, difficulty in cleaning, and the aesthetic impact of wire brackets, clear aligners offer the advantages of easy removal for convenient cleaning and transparent aesthetics, allowing patients to feel more confident during treatment.

[0003] Clear aligners are manufactured by molding transparent films onto a mold using heat to soften them and apply mechanical force. Clear aligner materials should possess the following performance requirements: suitable mechanical properties, including a suitable elastic modulus, high yield stress, and low stress relaxation rate; excellent anti-aging properties, including low water absorption and good wear resistance; suitable thickness to balance effective tooth displacement and wearing comfort; excellent transparency to ensure aesthetics during treatment; biocompatibility and chemical stability, as clear aligners are Class II medical devices that come into direct contact with the human body and must not cause rejection reactions, react chemically with complex saliva components, or release toxic or harmful substances; and ideal thermal properties, requiring a specific processing temperature during the hot-press molding process, taking into account the material's decomposition temperature, melting temperature, glass transition temperature, and flow properties at the processing temperature.

[0004] Based on the above performance requirements, commonly used thermoforming polymer materials for clear aligners mainly include polyethylene phthalate (PET), polyethylene terephthalate-1,4-cyclohexanediol (PCTG), thermoplastic polyurethane (TPU), polycarbonate (PC), and ethylene vinyl acetate (EVA). Currently, most commercially available clear aligners are made using these materials alone or in combination. However, most commercial products still have certain defects, such as poor transparency, poor elasticity, excessive or insufficient corrective force, excessive stress relaxation rate, and poor membrane composite properties. Ultimately, these defects prevent the treatment effect from meeting the expected requirements. Summary of the Invention

[0005] To address the shortcomings of existing composite films used in the fabrication of bracketless clear aligners, which suffer from poor interlayer bonding (poor biocompatibility), this invention improves the materials of each layer of the composite film to create a multilayer composite film with excellent interlayer bonding for bracketless clear aligners. The multilayer composite film prepared by this invention also possesses advantages such as high transparency, suitable corrective force, and low stress relaxation rate.

[0006] One of the objectives of this invention is to provide a multi-layer composite film for bracketless invisible orthodontic treatment.

[0007] The bracketless invisible orthodontic multilayer composite film is formed by hot pressing of a first layer, a second layer and a third layer arranged from top to bottom; the first layer and the third layer are thermoplastic polyurethane, and the second layer is formed by extrusion injection molding of polyurethane and polyethylene terephthalate-1,4-cyclohexanediol ester.

[0008] The thickness of the first and third layers is 0.05–0.25 mm, the thickness of the second layer is 0.30–0.50 mm, and the thickness of the multilayer composite film is 0.60–1.00 mm; preferably, the thickness of the first and third layers is 0.15–0.20 mm, the thickness of the second layer is 0.40–0.45 mm, and the thickness of the multilayer composite film is 0.75–0.85 mm.

[0009] The multi-layer composite film for bracketless invisible orthodontic treatment has a first and third layer of thermoplastic polyurethane (hereinafter referred to as TPU layer), with the first layer serving as the surface layer and the third layer serving as the inner layer; and a second layer of polyurethane blended modified polyethylene terephthalate-1,4-cyclohexanediethanol ester (hereinafter referred to as PCTG layer), serving as the intermediate layer.

[0010] Compared to existing composite films, the main improvement of this invention lies in using polyurethane-modified polyethylene terephthalate (PET-1,4-cyclohexanediethanol) as the intermediate layer. SEM cross-sectional images show that composite films prepared using PET-1,4-cyclohexanediethanol (hereinafter referred to as PCTG) as the intermediate layer, thermoplastic polyurethane as the surface layer, and inner layer exhibit delamination between layers, indicating poor interfacial compatibility, poor interlayer bonding, and poor interfacial performance. In contrast, composite films prepared using polyurethane-modified PCTG as the intermediate layer, with thermoplastic polyurethane as the surface layer and inner layer, show no obvious interfacial interface, indicating good interfacial compatibility, excellent interlayer bonding, and good interfacial performance. Experiments demonstrate that using polyurethane-modified PCTG as the intermediate layer significantly improves the compatibility between the PCTG layer and the TPU layer compared to using PCTG as the intermediate layer, resulting in excellent interlayer bonding and good interfacial performance in the composite film.

[0011] The multilayer composite membrane of this invention is prepared using a hot-pressing process. Hot pressing causes the layers to melt and fuse together to form a composite interface. Therefore, the contact surfaces of the first and second layers of the multilayer composite membrane of this invention fuse together to form a composite interface, and the contact surfaces of the second and third layers fuse together to form a composite interface. The composite interface is a transitional layer containing components from both the first and second layers, as well as components from the second and third layers, resulting in no clear boundaries between the first, second, and third layers. Therefore, the multilayer composite membrane for bracketless invisible orthodontic treatment of this invention has no clear boundaries between its layers.

[0012] The thermoplastic polyurethane used in the TPU layer has a light transmittance of ≥90% and a Shore hardness of 50D-90D. That is, the thermoplastic polyurethane used in the TPU layer is a transparent, rigid thermoplastic polyurethane. The thermoplastic polyurethane used in the TPU layer can be prepared using methods known in the art, or it can be obtained directly from commercially available sources. Examples include, but are not limited to, Isoplast 2510, 2530, 2540, Estane 2103-65D, 5778-78D, 8091-70D from Lubrizol Corporation (USA); and WHT-M882H, WHT-M880, WHT-1490, WHT-1495, WHT-1172, WHT-1185, WHT-1190, WHT-1195 from Yantai Wanhua Chemical Co., Ltd. (China). It is important to note that the thermoplastic polyurethane used in the TPU layer must be of medical or food grade hygiene rating.

[0013] The weight-average molecular weight of the thermoplastic polyurethane used in the TPU layer should be between 100,000 and 200,000. In this case, the multilayer composite film, as an orthodontic material, can provide suitable initial stress and final maintenance orthodontic force.

[0014] The weight-average molecular weight of polyethylene terephthalate (PET) used in the PCTG layer is 100,000 to 400,000. Composite films obtained by modifying PCTG within this molecular weight range with polyurethane as the intermediate layer possess suitable mechanical strength, initial stress, and extremely low stress relaxation rate. When fabricated into orthodontic appliances, they can provide considerable corrective force without causing discomfort to the wearer. The PET used can be prepared using methods known in the art or obtained directly from commercial sources.

[0015] The polyurethane used in the PCTG layer (polyurethane for blending modification) is a transparent, soft thermoplastic polyurethane. As a compatibilizer, its purpose is to improve the compatibility between the PCTG and TPU layers, enhance interfacial properties, improve the stress relaxation rate and elasticity of the PCTG layer, and promote the synchronization and idealization of the stress relaxation rate of the composite film. Therefore, the molecular weight of the polyurethane for blending modification should not be too high, and the number average molecular weight should be controlled between 10,000 and 50,000 g / mol. The hard segment content of the polyurethane for blending modification in the PCTG layer is 40% to 70%, preferably 50% to 60%. The hard segment content refers to the percentage of the mass of isocyanate and chain extender used in the preparation of the polyurethane relative to the total mass of the synthetic formulation. The polyurethane for blending modification can be prepared using methods known in the art or obtained directly from commercially available sources.

[0016] The multilayer composite membrane, as a therapeutic material, is a single unit. Its overall stress relaxation rate is influenced not only by the stress relaxation rate of each individual layer but also by the differences in stress relaxation rates between layers. Only when the molecular weights of the thermoplastic polyurethane in the TPU layer, the PCTG in the PCTG layer, and the polyurethane used for blending modification in the PCTG layer are within suitable ranges, and when the molecular weights of the thermoplastic polyurethane, the polyurethane used for blending modification, and the PCTG are matched, will the TPU and PCTG layers in the obtained composite membrane exhibit excellent compatibility and matched stress relaxation rates. Therefore, the preferred molecular weight of the thermoplastic polyurethane in the TPU layer is 100,000–150,000; simultaneously, the weight-average molecular weight of the PCTG in the PCTG layer is 150,000–300,000, and the number-average molecular weight of the polyurethane used for blending modification in the PCTG layer is preferably 30,000–45,000 g / mol.

[0017] Compared to polyether-type polyurethane, polyester-type polyurethane has the following advantages in blending and modifying PCTG as a polyurethane for blending: firstly, it is more compatible with PCTG; secondly, the presence of ester bonds results in a higher degree of hydrogen bonding in the polyester-type polyurethane, better heat resistance, and better suitability for hot pressing processes, thereby leading to lower and more uniform internal stress distribution in the PCTG layer material. Therefore, as a preferred option, polyester-type polyurethane is used for blending and modifying.

[0018] In the PCTG layer, the amount of polyurethane used for blending modification relative to polyethylene terephthalate-1,4-cyclohexanediol ester affects the blending modification effect. When the amount of polyurethane used for blending modification relative to PCTG is less than 5% (mass fraction), it cannot achieve the compatibilizing effect between the PCTG layer and the TPU layer, and a clear interface still exists after lamination, affecting the overall orthodontic performance. When the amount of polyurethane used for blending modification relative to PCTG is greater than 30% (mass fraction), the initial stress is low and the orthodontic force is insufficient, affecting the overall orthodontic performance of the composite membrane. Therefore, as a preferred embodiment, the mass fraction of the polyurethane used for blending modification relative to polyethylene terephthalate-1,4-cyclohexanediol ester is 5-30%, preferably 10-20%.

[0019] To further promote the synchronization and idealization of stress relaxation rates in each layer of the composite membrane, the polyurethane for blend modification is preferably prepared by the following method. The polyurethane for blend modification is prepared from polyester or polyester diol, diisocyanate, and a small molecule diol chain extender; the diisocyanate is selected from one or more of 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, and toluene diisocyanate, preferably 4,4'-diphenylmethane diisocyanate or toluene diisocyanate; the small molecule diol chain extender is selected from one or more of propylene glycol, butanediol, pentanediol, hexanediol, 1,2-propanediol, 1,3-butanediol, 2-methyl-1,3-propanediol, 1,4-butanediol, and 1,6-hexanediol; the mass ratio of polyester or polyester diol, diisocyanate, and small molecule diol chain extender is (15-25):(20-46):(6-21).

[0020] The number-average molecular weight of the polyester diol is 1000–3000 g / mol, preferably 1000–2000 g / mol. The polyester diol is a hydroxyl-terminated polyester diol; preferably, it is a hydroxyl-terminated polyester diol with an acid value ≤1 mgKOH / g and a hydroxyl value of 56–224 KOH / g.

[0021] The polyester glycol should be completely non-crystalline or have extremely low crystallinity to ensure the transparency of the PCTG layer after blending and the overall aesthetics of the orthodontic appliance. The polyester glycol is selected from small-molecule raw materials through random copolymerization, including succinic acid, pentanediol, sebacic acid, 2,3-butanediol, and 1,3-butanediol. The preparation method is as follows: Small-molecule raw materials are added to a reaction vessel, and the air is replaced three times with nitrogen at room temperature. The temperature is then raised to 150–200°C, and the esterification reaction is carried out for 2–6 hours. A catalyst solution is added, and the pressure is reduced to a vacuum of -0.1 to -0.08 MPa, continuing until the acid value drops below 1 mg KOH / g and the hydroxyl value reaches 56–224 KOH / g, at which point the reaction is terminated. The molar ratio of succinic acid to sebacic acid is 0.7:0.3, and the molar ratio of 1,3-propanediol to 2,3-butanediol is 0.8:0.2. The mass of the catalyst tetrabutyl titanate is one ten-thousandth of the total mass of the small-molecule raw materials.

[0022] The second objective of this invention is to provide a method for preparing the multilayer composite film described in the first objective of this invention.

[0023] The preparation method of the multilayer composite film includes: extruding and injection molding a blend of polyurethane and polyethylene terephthalate-1,4-cyclohexanediethanol ester to obtain a second layer; extruding and injection molding a thermoplastic polyurethane to obtain a first layer and a third layer film; stacking the layers in sequence and then hot-pressing them until each layer melts and fuses with each other to form a composite interface; and cooling to obtain the multilayer composite film.

[0024] The preparation method of the multilayer composite film specifically includes: drying the required granules for the three-layer film (polyurethane, polyethylene terephthalate-1,4-cyclohexanediol, and thermoplastic polyurethane) in an oven at 110-150℃; placing the dried thermoplastic polyurethane in the barrel of an injection molding machine, setting an appropriate temperature according to its processing temperature, and obtaining a low-stress TPU film through extrusion injection molding; placing the dried polyurethane and polyethylene terephthalate-1,4-cyclohexanediol in the barrel of an injection molding machine, setting an appropriate temperature according to its processing temperature, and obtaining a low-stress PCTG film through extrusion injection molding; and then processing the TPU film and PCTG film... After wiping with alcohol to eliminate static electricity, TPU film, PCTG film, and TPU film are stacked sequentially on a polyimide film. A suitable sized retaining ring is selected, and the film is then placed in a hot press for preheating. The upper and lower hot plates of the hot press are brought into contact with the polyimide film, ensuring maximum adhesion of the films. Preheating is performed at a temperature near the softening point of the films. After a suitable preheating time, each layer of film reaches a molten state. At this point, the mold is closed and hot-pressed. Under the pressure of the press, the molten material flows, spreads, and fuses together to form a composite interface. After complete mold closure, pressure is maintained for a period of time, followed by cooling and demolding to obtain a three-layer composite film. The thickness and dimensions of the films are controlled by the retaining ring.

[0025] The third objective of this invention is to provide a bracketless invisible oral aligner.

[0026] The bracketless invisible dentistry appliance is manufactured using a multi-layer composite film as described in one of the invention's objectives; the third layer of the multi-layer composite film serves as the inner layer of the appliance, fitting snugly against the teeth during use.

[0027] To improve oral hygiene and comfort during orthodontic treatment, reduce plaque content on the aligner surface, and lower the risk of tooth decay and periodontitis, the clear aligner also includes a polyurethane membrane. The polyurethane membrane is laminated onto the inner surface of the aligner using a wet-film forming process. The thickness of the polyurethane membrane is 0.005–0.01 mm. The polyurethane membrane comprises an antibacterial agent and polyurethane, with the antibacterial agent comprising 0.1–5% of the polyurethane by mass, preferably 0.2–1%, and more preferably 0.6%–1%.

[0028] The polyurethane used to form the polyurethane film can be any type of medical polyurethane. Polyester-type thermoplastic polyurethane, such as silicone polyurethane, can be selected. It can be prepared using methods known in the art or obtained commercially. The antibacterial agent can be a natural antibacterial agent and / or a synthetic antibacterial agent, including inorganic and organic antibacterial agents; preferably a natural antibacterial agent and / or an organic antibacterial agent; specifically, a quaternary ammonium salt antibacterial agent can be selected.

[0029] The wet film formation process includes: mixing an antibacterial agent with polyurethane and dissolving it in a solvent to obtain a homogeneous casting solution; coating the homogeneous casting solution onto the inner surface of the orthodontic appliance to form a coating film; and immersing the coating film in water until a solid film is formed.

[0030] The wet film-forming process specifically includes: preparing a homogeneous casting solution with a solute mass fraction of 20% using an antibacterial agent and polyurethane as solutes and N,N-dimethylformamide as solvent; wherein the antibacterial agent has a mass content of 0.1-5% relative to polyurethane; the homogeneous casting solution is vacuum dried at 60℃ for 2-4 hours until there are no bubbles in the casting solution, and then cooled to room temperature; the cooled casting solution is coated on the inner surface of the orthodontic appliance to form a coating film with a thickness of 0.005-0.01 mm; the orthodontic appliance with the coating film is immersed in deionized water for 48-72 hours until a solid film is formed, and excess solvent is washed away with clean water.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] The multilayer composite membrane provided by this invention comprises a first and third layer of transparent rigid thermoplastic polyurethane, and a second layer of polyurethane-modified polyethylene terephthalate (PET)-1,4-cyclohexanediol ester, manufactured through a hot-pressing process. This material selection and processing method improves the compatibility between the PCTG and TPU layers, enhances interfacial properties, improves the stress relaxation rate and elasticity of the PCTG layer, and promotes the synchronization and idealization of the stress relaxation rate of the composite membrane. This improves patient comfort during the initial 24 hours of wear while ensuring ideal orthodontic performance.

[0033] The multilayer composite film provided by this invention has a fourth layer of polyurethane film with antibacterial properties; that is, the polyurethane film on the tooth lateral layer ensures oral hygiene and comfort for patients during orthodontic treatment, reduces plaque content on the surface of the braces, and lowers the risk of tooth decay and periodontitis.

[0034] The fourth layer is prepared using a wet film-forming process. Compared with chemically grafted antibacterial agents and physically blended antibacterial agent granules, wet film-forming can more effectively and uniformly disperse the antibacterial agent, achieving comprehensive antibacterial effect. Compared with directly coating the antibacterial coating, the polyurethane film can achieve a controlled-release effect, continuously releasing the antibacterial agent in a low amount, ensuring consistent antibacterial effect throughout the wearing period of the orthodontic appliance.

[0035] In summary, the composite membrane for invisible orthodontic treatment prepared by this invention has excellent transparency and aesthetics, ideal corrective force, low stress relaxation rate, slow stress decay, blurred interlayer interface, and excellent antibacterial properties and comfort. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the TPU film, PCTG film, and TPU film after lamination and before hot-pressing in Example 2;

[0037] Figure 2 SEM image of the cross-section of the three-layer composite membrane prepared in Example 2;

[0038] Figure 3 SEM image of the cross-section of the PETG / TPU bilayer film prepared in Comparative Example 3;

[0039] Figure 4 SEM image of the cross-section of the TPU / PETG / TPU three-layer film prepared for Comparative Example 4. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0041] The reagents used in the following examples and comparative examples are all commercially available products.

[0042] Succinic acid, purchased from Aladdin;

[0043] Sebacic acid, purchased from Aladdin;

[0044] 1,3-Butanediol, purchased from Aladdin;

[0045] 2,3-Butanediol, purchased from Aladdin;

[0046] 1,4-Butanediol, purchased from Aladdin;

[0047] N,N-Dimethylformamide, purchased from Aladdin;

[0048] 4,4'-Diphenylmethane diisocyanate (MDI), purchased from Aladdin;

[0049] Quaternary ammonium salt antibacterial agent, full name: chitosan quaternary ammonium salt, purchased from Aladdin;

[0050] Polyurethane (TPU), using Isoplast 2530 from Lubrizol Corporation, USA;

[0051] Polyethylene terephthalate-1,4-cyclohexanediol ester (PCTG), using PCTG MP100 from Eastman Chemical Company, USA;

[0052] The silicone polyurethane used is from Shanghai Tangui New Material Technology Co., Ltd., with a number average molecular weight of 3500 g / mol.

[0053] Example 1

[0054] (1) Preparation of polyester glycol: Small molecule raw materials were added to the reaction vessel, and the air was replaced with nitrogen three times at room temperature. The temperature was then raised to 150℃, and the esterification reaction was carried out for 6 hours. Tetrabutyl titanate catalyst was added, and the pressure was reduced to a vacuum of -0.1 MPa for 1 hour. At this time, the acid value decreased to 0.1 mg KOH / g, and the hydroxyl value reached 127 mg KOH / g. The reaction was then terminated. Polyester glycol was obtained, and the number average molecular weight was measured to be 1000 g / mol. The small molecule raw materials were: succinic acid, sebacic acid, 1,3-butanediol, and 2,3-butanediol; the molar ratio of succinic acid to sebacic acid was 0.7:0.3, and the molar ratio of 1,3-propanediol to 2,3-butanediol was 0.8:0.2. The mass of tetrabutyl titanate catalyst was one ten-thousandth of the mass of the small molecule raw materials.

[0055] (2) Preparation of modified polyurethane with a hard segment content of 65%: 20g of polyester diol prepared in step (1) was dehydrated at 110℃ to a vacuum degree of -0.1MPa for 4h, cooled to 60℃, and 27.68g of 4,4'-diphenylmethane diisocyanate (MDI) was added and reacted for 2h. Then 9.32g of small molecule diol chain extender 1,4-butanediol was added and reacted for 2h to obtain modified polyurethane. The number average molecular weight was measured to be 10000g / mol.

[0056] (3) Preparation of composite film: After drying polyurethane (TPU) at 121°C, it is placed in the barrel of an injection molding machine and extruded at 213°C to obtain a TPU film with a thickness of 0.15 mm. The modified polyurethane prepared in step (2) and polyethylene terephthalate-1,4-cyclohexanediethanol ester (PCTG) (the modified polyurethane accounts for 10% of the mass of PCTG) are dried at 130°C and placed in the barrel of an injection molding machine and extruded at 220°C to obtain a PCTG film with a thickness of 0.4 mm. After wiping the TPU film and PCTG film with 95% alcohol to eliminate static electricity, the TPU film, PCTG film and TPU film are stacked on the polyimide film in sequence and placed in a hot press for preheating. The upper and lower hot plates of the hot press are pressed together on the polyimide film to make the hot press film as close as possible. The temperature near the softening point of the film is set for preheating. After preheating, each layer of the film reaches a molten state. At this point, the mold is closed and hot-pressed. Under the pressure of the press, the molten material in each layer flows, spreads, and fuses together to form a composite interface. After the mold is fully closed, pressure is maintained, and then the film is cooled and demolded to obtain a three-layer composite film.

[0057] Example 2

[0058] (1) Preparation of modified polyurethane with 50% hard segment content: 35g of polyester diol prepared in Example 1 was dehydrated at 110°C to a vacuum degree of -0.1MPa for 4h, cooled to 60°C, and 27.9g of 4,4'-diphenylmethane diisocyanate (MDI) was added and reacted for 2h. Then 7.1g of small molecule diol chain extender 1,4-butanediol was added and reacted for 2h to obtain modified polyurethane. The number average molecular weight was measured to be 15000g / mol.

[0059] (2) Preparation of composite film: Polyurethane (TPU) was dried at 121°C and placed in the barrel of an injection molding machine. A TPU film with a thickness of 0.15 mm was obtained by extrusion injection molding at 213°C. The modified polyurethane prepared in step (1) and polyethylene terephthalate-1,4-cyclohexanediol ester (PCTG) (the modified polyurethane accounts for 10% of the mass of PCTG) were dried at 130°C and placed in the barrel of an injection molding machine. A PCTG film with a thickness of 0.4 mm was obtained by extrusion injection molding at 220°C. The TPU film and PCTG film were wiped with 95% alcohol to eliminate static electricity, and then the TPU film, PCTG film, and TPU film were stacked sequentially (e.g., ...). Figure 1 (As shown) The film is placed on a polyimide film and preheated in a hot press, so that the upper and lower hot plates of the hot press are in contact with the polyimide film, making the hot-pressed film as close together as possible. The temperature is set near the softening point of the film for preheating. After preheating, each layer of the film has reached a molten state. At this time, the mold is closed and hot-pressed. Under the pressure of the press, the molten material of each layer flows and spreads, and fuses together to form a composite interface. After the mold is completely closed, pressure is held, and then the film is cooled and demolded to obtain a three-layer composite film.

[0060] The SEM image of the cross-section of the three-layer composite film obtained in this embodiment is shown below. Figure 2 As shown. Figure 2 The three layers show no obvious interface, indicating excellent interlayer composite and good interface performance.

[0061] Example 3

[0062] A composite membrane includes a first layer, a second layer, and a third layer arranged from top to bottom. The first and third layers are thermoplastic polyurethane, and the second layer is formed by extrusion injection molding of polyurethane and polyethylene terephthalate-1,4-cyclohexanediethanol ester. The thicknesses of the first and third layers are 0.20 mm and the thickness of the second layer is 0.35 mm, respectively.

[0063] The preparation method is the same as in Example 1.

[0064] Example 4

[0065] A composite membrane includes a first layer, a second layer, and a third layer arranged from top to bottom. The first and third layers are thermoplastic polyurethane, and the second layer is formed by extrusion injection molding of polyurethane and polyethylene terephthalate-1,4-cyclohexanediethanol ester. The thicknesses of the first and third layers are 0.10 mm and the thickness of the second layer is 0.55 mm, respectively.

[0066] The preparation method is the same as in Example 1.

[0067] Example 5

[0068] The composite membrane prepared in Example 1 was processed into a bracketless invisible orthodontic appliance. During processing, the third layer of the composite membrane was used as the inner layer that fits against the teeth when the appliance is in use.

[0069] Using N,N-dimethylformamide as a solvent, a homogeneous casting solution was prepared by dissolving silicone polyurethane and a quaternary ammonium salt antibacterial agent (0.2% by mass relative to silicone polyurethane) in a 20% solute mass ratio. The homogeneous casting solution was placed in a drying oven and vacuum-dried at 60°C until no bubbles remained. It was then removed and cooled to room temperature. This solution was then coated onto the third layer of the aligner, forming a 0.008 mm thick coating. The aligner with the coating was immersed in deionized water until the coating became a solid film, and excess solvent was washed away with water. This yielded a clear aligner with an antibacterial polyurethane film on its inner surface. This antibacterial polyurethane film on the inner layer of the clear aligner is the fourth layer.

[0070] Comparative Example 1

[0071] The single-layer film is composed of polyethylene terephthalate-1,4-cyclohexanediethanol ester (PCTG) with a thickness of 0.75 mm.

[0072] Comparative Example 2

[0073] A PETG / TPU bilayer film includes a first layer and a second layer disposed from bottom to top. The first layer is polyurethane (TPU), and the second layer is polyethylene terephthalate-1,4-cyclohexanediol (PCTG). The thickness of the first layer is 0.15 mm, and the thickness of the second layer is 0.40 mm. It is formed by hot pressing using the same process steps and parameters as in Example 1.

[0074] The SEM image of the cross-section of the PETG / TPU bilayer film obtained in this comparative example is shown below. Figure 3 As shown.

[0075] Figure 3 In the middle, the upper layer is the second layer (PCTG layer), and the lower layer is the first layer (TPU layer); the obvious interface between the upper PCTG layer and the lower TPU layer can be clearly observed, indicating that the two layers have extremely poor compatibility.

[0076] Comparative Example 3

[0077] A TPU / PETG / TPU film comprises a first layer, a second layer, and a third layer arranged from top to bottom. The first and third layers are polyurethane, and the second layer is polyethylene terephthalate-1,4-cyclohexanediol ester. The thicknesses of the first and third layers are 0.15 mm and the thickness of the second layer is 0.40 mm, respectively. It is formed by hot pressing using the same process steps and parameters as in Example 1.

[0078] The SEM image of the cross-section of the TPU / PETG / TPU film obtained in this comparative example is shown below. Figure 4 As shown.

[0079] from Figure 4 It can be clearly observed in the lower right of the middle of the figure that there is interlayer delamination, that is, there is delamination between the second and third layers, indicating that the two layers have poor compatibility.

[0080] Compare Figure 2 , Figure 3 , Figure 4 It can be seen that, compared with Comparative Examples 2 and 3, the compatibility of the composite membrane prepared in Example 2 is significantly improved. Therefore, it can be concluded that, compared with existing composite membranes, the compatibility of the composite membrane of the present invention is significantly improved.

[0081] Performance testing

[0082] The transmittance, yield stress, tensile modulus, hardness, and stress relaxation rate of the composite films prepared in Examples 1-4, the clear aligner prepared in Example 5, and the films prepared in Comparative Examples 1-3 were tested. The test results are shown in Table 1. Specifically, transmittance was measured according to ISO 13468-2:1999; yield stress was measured according to GB / T 9341-2008; tensile modulus was measured according to GB / T 1040.3-2006; hardness was measured according to ASTM D2240; and stress relaxation rate was measured according to ASTM E328-2013.

[0083] Table 1

[0084] Light transmittance Yield stress tensile modulus hardness Stress relaxation rate Example 1 93.17％ 52.9MPa 1451.3MPa 70D 0.23MPa / h Example 2 92.87％ 50.1MPa 1470.5MPa 68D 0.25MPa / h Example 3 93.23％ 48.8MPa 1465.4MPa 67D 0.21MPa / h Example 4 91.27％ 54.6MPa 1400.6MPa 72D 0.29MPa / h Example 5 89.21％ 52.9MPa 1451.3MPa 70D 0.23MPa / h Comparative Example 1 91.1％ 41.1MPa 1860.3MPa 74D 0.77MPa / h Comparative Example 2 90.2％ 39.3MPa 1204.3MPa 72D 0.26MPa / h Comparative Example 3 89.35％ 54.7MPa 1638.2 MPa 75D 0.71MPa / h

[0085] Example 5 differs from Example 1 only in that the inner layer of the composite membrane in Example 5 has an additional antibacterial polyurethane film formed using a wet film-forming process. Table 1 shows that the light transmittance, yield stress, tensile modulus, hardness, and stress relaxation rate of Example 5 are the same as those of Example 1. Therefore, it can be concluded that forming an antibacterial polyurethane film in the inner layer of the composite membrane using a wet film-forming process does not adversely affect the performance of the composite membrane.

[0086] Compared with Examples 1 and 2, Comparative Example 3 differs only in that the intermediate layer of Comparative Example 3 uses PCTG, while the intermediate layer of Examples 1 and 2 uses polyurethane blended modified PCTG. Table 1 shows that, compared with Comparative Example 3, the light transmittance, yield stress, and stress relaxation rate of Examples 1 and 2 are significantly improved. This indicates that, compared with PCTG, polyurethane blended modified PCTG can improve the light transmittance, yield stress, and stress relaxation rate of the composite film.

[0087] The antibacterial properties of the fourth layer of the clear aligner prepared in Example 5 and the third layer of Example 1 were tested according to QB / T 2591-2003. The test results showed that after 24 hours of contact with the test bacteria (Staphylococcus aureus), the antibacterial rate of the fourth layer surface of Example 5 was 99%, while the antibacterial rate of the third layer surface of Example 1 was 87%. This indicates that the antibacterial performance of the clear aligner is significantly improved after the fourth layer is added.

Claims

1. A multi-layer composite diaphragm for bracketless invisible orthodontic treatment, characterized in that, The composite film is formed by hot pressing together a first layer, a second layer, and a third layer arranged from top to bottom; the first and third layers are thermoplastic polyurethane, and the second layer is formed by extrusion injection molding of polyurethane and polyethylene terephthalate-1,4-cyclohexanediol ester. The thickness of the first and third layers is 0.05-0.25 mm, the thickness of the second layer is 0.30-0.50 mm, and the thickness of the multilayer composite film is 0.60-1.00 mm. The thermoplastic polyurethane has a weight-average molecular weight of 100,000 to 200,000. The weight-average molecular weight of the polyethylene terephthalate-1,4-cyclohexanediethanol ester is 100,000 to 400,000. The number-average molecular weight of the polyurethane is 10,000 to 50,000 g / mol.

2. The multilayer composite film as described in claim 1, characterized in that, The thickness of the first and third layers is 0.15~0.20mm; the thickness of the second layer is 0.40~0.45mm; and the thickness of the multilayer composite film is 0.75~0.85mm.

3. The multilayer composite film as described in claim 1, characterized in that, The thermoplastic polyurethane has a light transmittance of ≥90% and a Shore hardness of 50D~90D; or / and, The thermoplastic polyurethane has a weight-average molecular weight of 100,000 to 150,000.

4. The multilayer composite film as described in claim 1, characterized in that, The weight-average molecular weight of the polyethylene terephthalate-1,4-cyclohexanediethanol ester is 150,000 to 300,000.

5. The multilayer composite film as described in claim 1, characterized in that, The polyurethane has a number-average molecular weight of 30,000 to 45,000 g / mol.

6. The multilayer composite film as described in claim 1, characterized in that, The hard segment content of the polyurethane is 40% to 70%; or / and, The polyurethane has a mass fraction of 5-30% relative to polyethylene terephthalate-1,4-cyclohexanediethanol ester.

7. The multilayer composite film as described in claim 1, characterized in that, The hard segment content of the polyurethane is 50%–60%; or / and, The polyurethane has a mass fraction of 10-20% relative to polyethylene terephthalate-1,4-cyclohexanediol ester.

8. The multilayer composite film as described in claim 1, characterized in that, The polyurethane is prepared from polyester glycol, diisocyanate and small molecule glycol chain extender; The diisocyanate is selected from one or more of 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, and toluene diisocyanate; The small molecule diol chain extender is selected from one or more of propylene glycol, butanediol, pentanediol, hexanediol, 1,2-propanediol, 1,3-butanediol, 2-methyl-1,3-propanediol, 1,4-butanediol and 1,6-hexanediol; The mass ratio of polyester diol, diisocyanate and small molecule diol chain extender is (15~25):(20~46):(6~21).

9. The multilayer composite film as described in claim 8, characterized in that, The diisocyanate is 4,4'-diphenylmethane diisocyanate or toluene diisocyanate.

10. The multilayer composite film as described in claim 8, characterized in that, The polyester diol is a hydroxyl-terminated polyester diol; The polyester diol is formed by random copolymerization of small molecule raw materials; wherein, the small molecule raw materials include succinic acid, pentanediol, sebacic acid, 2,3-butanediol and 1,3-butanediol.

11. The multilayer composite film as described in claim 8, characterized in that, The polyester diol has a number-average molecular weight of 1000–3000 g / mol; or / and, The polyester diol has an acid value ≤1mgKOH / g and a hydroxyl value of 56~224KOH / g.

12. The multilayer composite film as described in claim 8, characterized in that, The number-average molecular weight of the polyester diol is 1000–2000 g / mol.

13. A method for preparing a multilayer composite film as described in any one of claims 1-12, characterized in that, The preparation method includes: extruding and injection molding a blend of polyurethane and polyethylene terephthalate-1,4-cyclohexanediethanol ester to obtain a second layer; extruding and injection molding a thermoplastic polyurethane to obtain a first layer and a third layer film; stacking the layers in sequence and then hot-pressing them until each layer melts and fuses with each other to form a composite interface; and cooling to obtain the multilayer composite film.

14. A bracketless invisible oral aligner, characterized in that, The bracketless invisible dentistry appliance is made of a multi-layer composite film as described in any one of claims 1-12 or a multi-layer composite film prepared by the preparation method described in claim 13; the third layer of the multi-layer composite film serves as the inner layer of the dentistry appliance and fits against the teeth during use.

15. The clear aligner as described in claim 14, characterized in that, It also includes a polyurethane film; the polyurethane film is laminated onto the inner surface of the orthodontic appliance using a wet film-forming method; the thickness of the polyurethane film is 0.005 to 0.01 mm; the polyurethane film includes an antibacterial agent and polyurethane, and the mass content of the antibacterial agent relative to the polyurethane is 0.1 to 5%.

16. The clear aligner as described in claim 15, characterized in that, The antibacterial agent has a mass content of 0.2-1% relative to the polyurethane.

17. The clear aligner as described in claim 15, characterized in that, The antibacterial agent has a mass content of 0.6-1% relative to the polyurethane.

18. The clear aligner as described in any one of claims 15-17, characterized in that, The wet film formation process includes: mixing an antibacterial agent with polyurethane and dissolving it in a solvent to obtain a homogeneous casting solution; coating the homogeneous casting solution onto the inner surface of the orthodontic appliance to form a coating film; and immersing the coating film in water until a solid film is formed.

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