A multilayer polymer film with high stress retention, a method of making the same, and articles thereof

Abstract

The present application relates to a kind of multilayer polymer film and its manufacturing method, and the use and method of the multilayer polymer film for manufacturing shaped body and the shaped body obtained by manufacturing.The multilayer polymer film includes hard core layer and soft skin layer, wherein the hard core layer includes one or more polymer layers, the material of each polymer layer of the hard core layer includes at least one transparent polyamide, the glass transition temperature Tg of each transparent polyamide is between 100 DEG C-200 DEG C;Each soft skin layer includes one or more polymer layers, and each polymer layer of each soft skin layer includes at least one thermoplastic polyurethane or polyamide elastomer with Shore hardness of 50D-85D.The multilayer polymer film provided by the present application has excellent transparency, environmental stress cracking resistance, and can keep high stress under warm and moist conditions, and is particularly suitable for making dental film.

Description

Technical Field

[0001] This invention relates to a multilayer polymer film and a method for manufacturing the same. It also relates to the use and method of using the multilayer polymer film to manufacture molded articles, and the resulting molded articles. The multilayer polymer film is particularly suitable for preparing three-dimensional molded articles, such as those prepared by thermoforming processes, for example, for preparing shell-shaped dental instruments. Background Technology

[0002] Clear aligner technology, compared to traditional fixed orthodontic technology, is called clear aligner because it eliminates bands, brackets, and archwires. Clear aligners are made from transparent polymer films, and because they are difficult to detect, they are called "invisible aligners" or "clear braces." Furthermore, due to their shell-like shape, they are often referred to as "shell dental instruments." With the development and integration of modern orthodontics, digital image acquisition and processing technology, rapid prototyping technology, and new materials technology, clear aligner technology has flourished since the 1990s. Notable companies in this field include Align Technology Inc. in the United States and Angelalign in China.

[0003] Typically, clear aligner technology uses a transparent polymer film to be thermoformed into an appliance. After the appliance is worn, it provides stress to slowly move the patient's teeth, thus achieving the orthodontic goal.

[0004] Currently, the most commonly used transparent polymer films are single-layer copolyester films, among which PETG (polyethylene terephthalate-1,4-cyclohexanediol) is a widely used polymer for orthodontic appliances. Typical commercially available PETG films include the German Erkodur dental film. Orthodontic appliances made with single-layer PETG films exhibit good stress retention and show excellent orthodontic results in most cases. However, their drawback is that they are prone to cracking and failure in the oral environment and during repeated removal and wearing.

[0005] To address the aforementioned issues, orthodontic appliances made from a single-layer polymer film of thermoplastic polyurethane (TPU) through thermoforming can solve the cracking problem. A typical commercially available TPU material is Straumann's Zendura A. While TPU materials can have very high initial stress and modulus, which positively contributes to the initial thrust of orthodontic treatment, it also increases patient discomfort and even pain when first wearing the appliance. Furthermore, in the oral environment, the stress in TPU film material relaxes rapidly, resulting in a significant weakening of the orthodontic force during the treatment period, making it difficult to ensure the successful achievement of treatment goals—a drawback of this material.

[0006] In summary, the performance requirements for diaphragms in the field of invisible orthodontic treatment are multifaceted, including transparency, crack resistance, appropriate initial stress, and high and stable stress retention under the warm and moist conditions of the oral cavity. These combined requirements constitute a challenge for the polymer diaphragms used in invisible aligners. In particular, there is currently a more urgent need in the field of invisible aligners for diaphragm products that improve stress relaxation and maintain long-term and effective corrective force during use. At the same time, developing new material solutions to replace / alleviate the market shortage of certain polymer medical materials (such as copolyesters) is also an urgent problem to be solved in the industry. Summary of the Invention

[0007] This invention provides a multilayer polymer film and its manufacturing method, as well as the use and method of using the film in thermoforming bodies and the resulting molded bodies. The multilayer polymer film provided by this invention can be used to prepare films for clear aligners. Clear aligners prepared from this film exhibit excellent balanced performance in terms of transparency, crack resistance, provision of initial stress, and stress retention, and also have high stress retention under simulated oral cavity temperature and humidity conditions.

[0008] According to one aspect of the present invention, a multilayer polymer film is provided, the multilayer polymer film comprising a rigid core layer and soft skin layers respectively located on both sides of the rigid core layer, wherein,

[0009] The rigid core layer includes one or more polymer layers, and the material of each polymer layer of the rigid core layer comprises one or more transparent polyamides, wherein the transparent polyamides are selected from amorphous transparent polyamides or microcrystalline transparent polyamides, and the glass transition temperature Tg of each transparent polyamide layer is between 100℃ and 200℃.

[0010] Each of the soft skin layers comprises one or more polymer layers, and each polymer layer of each of the soft skin layers is made of one or more thermoplastic polyurethanes with a Shore hardness of 50D-85D, or of one or more polyamide elastomers with a Shore hardness of 50D-85D.

[0011] In this invention, "Tg between 100℃ and 200℃" means that it includes the endpoint value, that is, it includes the case where Tg is 100℃ or 200℃, and it also includes the case where Tg is a certain temperature value in the middle of 100℃ and 200℃.

[0012] The present invention sets a lower limit for the Tg of transparent polyamide to ensure stress retention. The upper limit for the Tg is set based on the following considerations: 1. A higher Tg requires a higher appropriate thermoforming temperature, resulting in longer molding time, increased energy consumption, and increased difficulty in precise positioning; 2. High Tg is often accompanied by higher structural rigidity, but it can also easily lead to a decrease in the material's elongation at break; 3. If a high Tg is accompanied by an excessive increase in modulus, it can easily cause excessive pushing force on the teeth, increasing discomfort and making it difficult to maintain the patient's willingness to wear the product.

[0013] The key part of this invention is the soft-hard-soft three-layer structure, and whether or not to set other layers in addition to these three layers is not limited.

[0014] In some embodiments, the rigid core layer is a single layer, which may be made of a single transparent polyamide or a mixture of multiple transparent polyamides. When the material is a single transparent polyamide, the glass transition temperature (Tg) is between 100°C and 200°C. When the material is a mixture of multiple transparent polyamides, the mixture may have one or more Tg values ​​due to differences in component content or compatibility between components, all of which fall within the temperature range of 100°C to 200°C.

[0015] In some alternative embodiments, the rigid core layer can be a multilayer structure, with each layer being one or more of the aforementioned transparent polyamides. When the total thickness of the multilayer polymer film of the present invention is substantially constant, the thickness of each layer in the multilayer core layer is typically less than the thickness of a single-layer core layer. When the rigid core layer is a multilayer structure, the soft skin layers on both sides can each be a single-layer structure, or each can be a multilayer structure, or one side of the soft skin layer can be a single-layer structure while the other side is a multilayer structure.

[0016] In some embodiments, each side of the soft skin layer may be a single layer, the material of which may comprise one or more thermoplastic polyurethanes with a Shore hardness of 50D-85D, or the material of which may also comprise one or more polyamide elastomers with a Shore hardness of 50D-85D. For example, the layer structure on one side may comprise one or more thermoplastic polyurethanes with a Shore hardness of 50D-85D, and the layer structure on the other side may also comprise one or more thermoplastic polyurethanes with a Shore hardness of 50D-85D; alternatively, the layer structure on one side may comprise one or more polyamide elastomers with a Shore hardness of 50D-85D, and the layer structure on the other side may also comprise one or more polyamide elastomers with a Shore hardness of 50D-85D; or alternatively, the layer structure on one side may comprise one or more polyamide elastomers with a Shore hardness of 50D-85D, and the layer structure on the other side may comprise one or more thermoplastic polyurethanes with a Shore hardness of 50D-85D. In cases involving multiple polyurethanes, each polyurethane has a Shore hardness of 50D-85D, and consequently, mixtures of multiple polyurethanes also have a Shore hardness in the range of 50D-85D.

[0017] In some alternative embodiments, the soft skin layers on both sides may be multilayered. Each of the multilayered layers may include one or more polyamide elastomers with a Shore hardness of 50D-85D, or one or more thermoplastic polyurethanes with a Shore hardness of 50D-85D. For the multilayered soft skin layers, some layers may be polyamide elastomers with a Shore hardness of 50D-85D, and other layers may be thermoplastic polyurethanes with a Shore hardness of 50D-85D.

[0018] In some alternative embodiments, the soft skin layer on one side of the rigid core layer may be a single layer, while the soft skin layer on the other side may be multilayered. The specific selection and arrangement of single and multilayer layers are as described above for single-layer and multilayer soft skin layers.

[0019] In this invention, to ensure transparency, in a preferred embodiment, each polymer layer does not contain a large amount of inorganic filler, and is also preferably not compounded with a large amount of other polymers (not shown). For example, for the rigid core layer, each layer can be a mixture of various transparent polyamides, but it is not desirable to be a composite of polyamides with other polymers, nor is it desirable to add a large amount of inorganic filler; for the soft skin layer, each layer can be a mixture of different thermoplastic polyurethanes, or a mixture of different polyamide elastomers, but it is not desirable to be a blend of thermoplastic polyurethanes and polyamide elastomers, nor is it desirable to be a blend of thermoplastic polyurethanes or polyamide elastomers with other polymers, and it is also not desirable to add a large amount of inorganic filler.

[0020] In some embodiments, any or all of the multilayer polymer layers may optionally contain auxiliary substances such as dyes, pigments, heat stabilizers, release agents, and other conventional additives to provide desired color, stability, processability, and performance characteristics. The total amount of auxiliary substances and their compositions is less than 10% by weight and is designed not to significantly impair the overall transparency of the film.

[0021] In a preferred embodiment, the glass transition temperature (Tg) of each layer of the transparent polyamide is between 110°C and 170°C, and more preferably between 125°C and 170°C.

[0022] In a preferred embodiment, each layer of the transparent polyamide has a light transmittance >75%, a tensile yield strength greater than 50 MPa, a yield strain ≥5%, and a tensile modulus greater than 1300 MPa.

[0023] More preferably, the light transmittance of the transparent polyamide is >80%.

[0024] More preferably, the multilayer polymer film can be subjected to single-sided roller-pressing and frosting treatment to further reduce possible light reflection. While frosting treatment reduces light transmittance to some extent, the multilayer polymer film of the present invention can still maintain a light transmittance greater than 80% after frosting treatment.

[0025] In a preferred embodiment, the total thickness of the hard core layer is 300-1200 micrometers, more preferably 350-1000 micrometers, and even more preferably 500-800 micrometers, and the thickness of each soft skin layer is 20-150 micrometers, more preferably 40-90 micrometers, and even more preferably 40-75 micrometers.

[0026] In a preferred embodiment, the total thickness of the layered structure consisting of the hard core layer and the soft skin layers on both sides is 500 micrometers to 1500 micrometers, and more preferably 600 micrometers to 1000 micrometers.

[0027] In one embodiment, the multilayer polymer film is a membrane used to prepare shell-shaped dental instruments.

[0028] According to another aspect of the present invention, the present invention provides a method for manufacturing the above-mentioned multilayer polymer film, which can be obtained by hot lamination, extrusion lamination or multilayer co-extrusion.

[0029] According to another aspect of the invention, the invention provides the use of the above-described multilayer polymer film for hot pressing to form molded articles, said molded articles being, for example but not limited to, shell-shaped dental instruments such as invisible aligners.

[0030] According to another aspect of the invention, the invention provides a molded body manufactured by hot pressing, which is made using a multilayer polymer film provided by the invention. More preferably, the molded body is a shell-shaped dental instrument such as a clear aligner.

[0031] According to another aspect of the present invention, the present invention provides a method for manufacturing the above-described molded article, the basic steps of which are as follows:

[0032] i) The above-mentioned multilayer polymer film is prepared by multilayer co-extrusion or extrusion composite process;

[0033] ii) Vacuum hot pressing and / or positive pressure hot pressing of the multilayer polymer film to obtain a molded body;

[0034] iii) Trim and grind the shaped body.

[0035] In one embodiment, the molded body is a shell-shaped dental instrument, such as a clear aligner or retainer.

[0036] The shell-shaped dental instruments mentioned in the technical solution of this invention typically include clear aligners and retainers.

[0037] The present invention has the following beneficial effects:

[0038] The multilayer polymer film provided by this invention can be thermoformed under negative or positive pressure to obtain the desired molded body. The finished product is transparent and possesses the following excellent performance characteristics: resistance to environmental stress cracking under warm and humid conditions, ability to provide suitable initial stress, and ability to stably maintain high stress under warm and humid conditions. Therefore, the polymer film of this invention is particularly suitable for dental films and is applicable to the manufacture of shell-shaped dental instruments such as invisible orthodontic aligners. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the structure of the multilayer polymer film with high stress retention of the present invention;

[0040] Figure 2 This is a schematic diagram of a diaphragm with obvious microcracks, which is a common feature in stress cracking tests.

[0041] Figure 3 These are the stress retention curves for Comparative Example 3, Comparative Example 5, and Example 2. Detailed Implementation

[0042] This invention provides a multilayer polymer film with high stress retention, the structure of which is as follows: Figure 1 As shown, it includes:

[0043] 1) A rigid core layer with a total thickness of 300-1200 micrometers, more preferably 350-1000 micrometers; the rigid core layer may include one or more polymer layers, each polymer layer comprising at least one transparent polyamide, wherein the transparent polyamide is an amorphous transparent polyamide or a microcrystalline transparent polyamide, and the glass transition temperature Tg of the transparent polyamide is between 100°C and 200°C (inclusive); here "at least one" means that each polymer layer may comprise one or more transparent polyamides, and when multiple are included, it may include both amorphous transparent polyamide and microcrystalline transparent polyamide, or multiple amorphous transparent polyamides, or multiple microcrystalline transparent polyamides;

[0044] 2) Soft skin layer: Located on both sides of the core layer, the thickness of each skin layer is 20-150 micrometers, each skin layer contains one or more polymer layers, each polymer layer contains at least one thermoplastic polyurethane with a Shore hardness of 50D to 85D, or contains at least one polyamide elastomer with a Shore hardness of 50D to 85D.

[0045] The terms "hard" and "soft" used above are relative concepts, referring to the core layer being harder than the outer layer and the outer layer being softer than the core layer. The related properties defined by "hard" and "soft" are hardness and stiffness.

[0046] Among them, the amorphous or microcrystalline transparent polyamides suitable for use in rigid core layers include aromatic / semi-aromatic polyamides or linear aliphatic polyamides, more preferably transparent polyamides formed by random copolymerization or block copolymerization of diamines and diacids with side chains and cyclic structures and linear diacids and diamines as comonomers. In one example, a typical alicyclic diamine is, for example, 3,3′-dimethyl-4,4′-diaminodicyclohexylmethane (MACM).

[0047] The total thickness of the multilayer polymer film provided by the present invention is 500 micrometers to 1500 micrometers, preferably 600 micrometers to 1000 micrometers.

[0048] The multilayer polymer film provided by this invention is particularly suitable for use as a membrane in the manufacture of shell-shaped dental instruments, such as clear aligners.

[0049] In a preferred embodiment of the present invention, the multilayer polymer film is:

[0050] 1) The rigid core layer is a single layer, mainly containing at least one amorphous transparent polyamide, the glass transition temperature Tg of which is between 125℃ and 170℃; here "at least one" refers to one or more types.

[0051] 2) A rigid core layer is located between two soft skin layers. Each soft skin layer is a single layer, and each soft skin layer contains at least one thermoplastic polyurethane with a Shore hardness of 50D to 85D. Here, "at least one" means one or more.

[0052] According to a preferred embodiment of the present invention, the rigid core layer comprises an amorphous transparent polyamide having a light transmittance >85% (ASTM D1003), a glass transition temperature Tg between 125°C and 170°C (ISO 11357-1 / -2), a tensile strength >50 MPa, a yield strain ≥5%, and a tensile modulus between 1400 MPa and 2200 MPa (ISO 527). In alternative embodiments, the rigid core layer may comprise a variety of amorphous transparent polyamides, and mixtures of such polyamides may meet the above-mentioned parameter requirements.

[0053] According to a preferred embodiment of the invention, each side of the soft skin layer comprises a thermoplastic polyurethane having a Shore hardness of 50D to 85D (ISO 7619-1). Alternatively, one or both sides of the soft skin layer may comprise multiple thermoplastic polyurethanes, each having a Shore hardness between 50D and 85D, and consequently, the hardness of a mixture of multiple thermoplastic polyurethanes may also be between 50D and 85D (ISO 7619-1).

[0054] The thermoplastic polyurethane can be prepared by selecting a difunctional diisocyanate and an oligomeric diol in combination with a small-molecule chain extender. The oligomeric diol is typically a polyester diol or a polyether diol. Polyether diols are particularly preferred. The use of polyethers provides better hydrolysis resistance and is more suitable for manufacturing shell-shaped dental instruments for use in the oral cavity, such as clear aligners.

[0055] In yet another preferred embodiment of the present invention, the multilayer polymer film is:

[0056] 1) The rigid core layer is a single layer, the material of which comprises at least one microcrystalline transparent polyamide, the glass transition temperature Tg of which is between 125°C and 170°C; specifically, when the material of the rigid core layer is a single microcrystalline transparent polyamide, the glass transition temperature Tg of which is between 125°C and 170°C; when the material of the rigid core layer is a mixture of multiple microcrystalline transparent polyamides, the glass transition temperature Tg of which is between 125°C and 170°C.

[0057] 2) A rigid core layer is located between two soft skin layers, and each soft skin layer comprises at least one thermoplastic polyurethane with a Shore hardness of 50D to 85D. Specifically, when the material of the soft skin layer is a thermoplastic polyurethane, the Shore hardness of the thermoplastic polyurethane is between 50D and 85D; when the material of the rigid core layer is multiple thermoplastic polyurethanes, the Shore hardness of each polyurethane is between 50D and 85D, and furthermore, the Shore hardness of a mixture of multiple thermoplastic polyurethanes is between 50D and 85D.

[0058] According to a preferred embodiment of the present invention, the rigid core layer comprises a microcrystalline transparent polyamide, particularly preferably, the monomer of which contains an alicyclic diamine, 4,4′-diaminodicyclohexylmethane (PACM), and the grain size of this preferred polyamide does not scatter visible light. This preferred microcrystalline transparent polyamide has a light transmittance of not less than 85%, a corresponding glass transition temperature (Tg) between 125°C and 170°C, a tensile strength > 50 MPa, a yield strain ≥ 5%, and a tensile modulus > 1300 MPa. In alternative embodiments, the rigid core layer may comprise a variety of microcrystalline transparent polyamides, and the properties of a mixture of these microcrystalline transparent polyamides may meet the above-mentioned parameter requirements.

[0059] In yet another preferred embodiment of the present invention, the multilayer polymer film is:

[0060] 1) The rigid core layer is a single layer, the material of which includes at least one amorphous transparent polyamide, and the glass transition temperature Tg is between 125°C and 170°C; specifically, when the material of the rigid core layer is an amorphous transparent polyamide, the glass transition temperature Tg of the amorphous transparent polyamide is between 125°C and 170°C; when the material of the rigid core layer is a mixture of multiple amorphous transparent polyamides, the glass transition temperature Tg of the mixture of the multiple amorphous transparent polyamides is between 125°C and 170°C.

[0061] 2) A rigid core layer is located between two soft skin layers, and each soft skin layer contains at least one polyamide elastomer with a Shore hardness of 50D to 85D. Specifically, when the material of the soft skin layer is a polyamide elastomer, the Shore hardness of the thermoplastic polyurethane polyamide elastomer is between 50D and 85D; when the material of the rigid core layer is multiple polyamide elastomers, the Shore hardness of each polyamide elastomer is between 50D and 85D, and furthermore, the Shore hardness of the mixture of the multiple polyamide elastomers is also between 50D and 85D.

[0062] Suitable polyamide elastomers for use in the skin layer are block copolymers polymerized from hard segments of polyamide and soft segments of polyester or polyether with low glass transition temperatures. The soft segments are typically hydroxyl-terminated polytetramethylene ether (PTMG), polyethylene glycol (PETG), or polypropylene glycol (PPG). The hard segments are typically PA6, PA66, PA11, PA12, and PA612, as well as aromatic polyamides. This invention preferably uses polyether soft segments, with a Shore hardness of 50D to 85D.

[0063] In some embodiments, the transparent polyamide used in this invention can be commercially available, for example, under the trade name... From EMS, or product name From Arkema. The microcrystalline polyamides used in this invention are also commercially available, for example, under the trade name... From Evonik. The thermoplastic polyurethane used in this invention is also commercially available, for example, under the trade name... From Lubrizol. The polyamide elastomers used in this invention are also commercially available, for example, under the trade name... From Arkema, or product name From Evonik.

[0064] In a preferred embodiment, the multilayer polymer film according to the present invention comprises a core layer and a skin layer, wherein the core layer is located in the middle of the skin layer.

[0065] - The core layer thickness is 300 micrometers to 1200 micrometers, preferably 350-1000 micrometers, and more preferably 500-800 micrometers;

[0066] - The thickness of a single dermal layer is 20 micrometers to 150 micrometers, preferably 40 micrometers to 90 micrometers, and more preferably 40 micrometers to 75 micrometers;

[0067] - The total thickness of the three-layer structure consisting of the two outer skin layers and the core layer is 500 micrometers to 1500 micrometers, preferably 600 micrometers to 1000 micrometers.

[0068] Based on any of the above preferred embodiments, the core layer and / or skin layer can also be selected as multilayer polymer layers. In order to keep the total thickness of the core layer and skin layer controllable, it is necessary to select the specific thickness of each layer of the multilayer polymer layers in the core layer and / or skin layer so that their total thickness is preferably still within the range defined above.

[0069] The present invention also provides a method for manufacturing such a multilayer polymer film, as well as the use and method of using the multilayer polymer film in a hot-pressed molded body and the resulting molded body.

[0070] The multilayer polymer film according to the present invention can be obtained by hot lamination, extrusion lamination, and multilayer co-extrusion. Among these methods,

[0071] Thermal lamination involves using multiple pre-prepared single-layer films to fabricate multilayer films of corresponding thicknesses under given temperature and pressure.

[0072] Extrusion lamination involves using a single-layer flat extrusion die (T-die) to extrude the core material, which is then rapidly laminated with the pre-prepared single-layer skin layer via roll forming.

[0073] Multilayer co-extrusion is a process in which polymer particles in the core and skin layers are melted and transported using multiple screws, and then extruded, rolled, cooled, and shaped through a multi-layer flat die.

[0074] The above three preparation methods are known to those skilled in the art, and the specific parameters in the preparation process can be selected through experiments, taking into account the properties of the raw materials.

[0075] In a preferred embodiment, the multilayer polymer film is prepared using a multilayer co-extrusion process.

[0076] The multilayer polymer film of the present invention can be used to manufacture molded articles by hot pressing. Specifically, for example,

[0077] The multilayer polymer film is thermoformed into sheets to produce test samples and final orthodontic appliances. Many existing processes are suitable for thermoforming, including but not limited to vacuum forming, positive pressure thermoforming, or a combination of both, as well as a combination of manual and automated conveyor systems.

[0078] Taking the thermoforming of shell-shaped dental instruments as an example, the mold used for molding is an orthodontic dental model. This orthodontic dental model can be obtained through traditional plaster molding or modern methods such as obtaining tooth data through oral scanning, adjusting the data in software to obtain the target data, and then 3D printing. The multi-layer polymer film is thermoformed on this orthodontic dental model, and the thermoformed body is then trimmed and polished to produce the final product.

[0079] Compared to traditional fixed orthodontic treatment, bracketless invisible orthodontic treatment can make the treatment process more discreet through the selection of materials and processes. The transparency of the multilayer polymer film of this invention meets the requirement of "invisibility".

[0080] The light transmittance (or light transmittance) (ASTM D1003) of the multilayer polymer film according to the present invention is greater than 75%, preferably greater than 80%.

[0081] According to a preferred embodiment of the present invention, the multilayer polymer film can be subjected to single-sided roll-pressing and sanding treatment to further reduce possible light reflection, while maintaining a light transmittance of more than 80%.

[0082] use

[0083] The multilayer polymer film provided by this invention has a flexible outer layer and a rigid core layer, and features transparency, resistance to environmental stress cracking, low stress relaxation in warm and humid environments, and high retention. This multilayer polymer film or sheet, and its molded products, are suitable for dental applications such as invisible orthodontic appliances and retainers. Dental instruments, such as shell-shaped dental instruments, can be manufactured through positive or negative pressure thermoforming. The resulting dental instruments exhibit good interlayer adhesion, excellent light transmittance (greater than 75%), resistance to environmental stress cracking, and high stress retention in warm and humid environments.

[0084] In this document, the phrase "between one value and another value" is a general way of indicating a range to avoid listing all the values ​​in that range in the specification. Therefore, the description of a particular value range includes the endpoints of the range and any values ​​within those endpoints, as well as the smaller range of values ​​defined by any value in the range, just as if the arbitrary value and the smaller range of values ​​were explicitly stated in the specification.

[0085] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Improvements and adjustments made by those skilled in the art based on the present invention in practical applications still fall within the scope of protection of the present invention.

[0086] Example

[0087] The present invention is further illustrated by the following embodiments, but they should not be construed as limiting the scope or content of the invention in any way.

[0088] (I) Raw Materials

[0089] 2530 ETPU: Purchased from Lubrizol, it is a rigid thermoplastic polyurethane engineering plastic with a glass transition temperature of 93°C, a tensile yield strength of 63 MPa, a yield strain of 6%, and a tensile modulus of 1900 MPa.

[0090] Eastar TM Copolyester 6763 was purchased from Eastman. It has a glass transition temperature of 80°C, a tensile yield strength of 52 MPa, a yield strain of 4%, and a tensile modulus of 1900 MPa.

[0091] Tritan TM The copolyester TX1000 was purchased from Eastman and has a glass transition temperature of 110°C, a tensile yield strength of 47 MPa, a yield strain of 7%, and a tensile modulus of 1500 MPa.

[0092] TR90 NAT was purchased from EMS. It is an amorphous transparent polyamide with a glass transition temperature of 150℃, a tensile yield strength of 60MPa, a yield strain of 6%, a tensile modulus of 1600MPa, and a light transmittance of approximately 93%.

[0093] CX7323 was purchased from EMS. It is a microcrystalline transparent polyamide with a glass transition temperature of 140℃, a tensile yield strength of 60MPa, a yield strain of 6%, a tensile modulus of 1600MPa, and a light transmittance of 91.7%.

[0094] ClearG170, purchased from Arkema, is an amorphous transparent polyamide with a glass transition temperature of 168°C, a tensile yield strength of 74 MPa, a yield strain of 8%, a tensile modulus of 2020 MPa, and a measured light transmittance of 92%.

[0095] Borealis PP HC205CF, purchased from Borealis AG, is a polypropylene homopolymer with added crystallizing nucleating agent. It has a tensile yield strength of 35 MPa, a yield strain of 8%, and a tensile modulus of 1750 MPa.

[0096] 2363 65D was purchased from Lubrizol. It is a thermoplastic polyurethane elastomer, polyether type, with a Shore hardness of 62D.

[0097] 2363 75D was purchased from Lubrizol. It is a thermoplastic polyurethane elastomer, polyether type, with a Shore hardness of 76D.

[0098] Utechllan UE-64DU10 was purchased from Covesro. It is a thermoplastic polyurethane elastomer, polyether type, with a Shore hardness of 66D.

[0099] 7233sa01med was purchased from Arkema. It is a thermoplastic polyamide elastomer, polyether type, with a Shore hardness of 61D.

[0100] Ecdel TM The 9966 copolyester elastomer was purchased from Eastman and has a Shore hardness of 55D.

[0101] (II) Preparation of Extruded Films

[0102] Extrusion preparation of single-layer or multi-layer films

[0103] A production line for extruding films includes:

[0104] Main extruder, 65mm in diameter, 38D length-to-diameter ratio;

[0105] Auxiliary extruder (auxiliary machine), diameter 45mm, length-to-diameter ratio 33D;

[0106] Melt pump;

[0107] A three-layer co-extrusion flat die head with a width of 500mm;

[0108] Three-roll calendering unit;

[0109] Cooling roller assembly;

[0110] Edge trimming and waste edge winding device;

[0111] Traction machine;

[0112] Winding machine;

[0113] Electrical control system.

[0114] After being dehumidified and dried, the granules are conveyed to the extruder, plasticized and melted, and extruded through the die head. The granules are then passed through a three-roll calendering unit (mirror steel rolls and / or frosted steel rolls) to obtain films of the corresponding thickness, which are then cooled and shaped. Finally, the films are traction-cut and wound to obtain the film products.

[0115] (III) Preparation of the molded body

[0116] The prepared single-layer or multi-layer polymer films are cut into circular slices using a GT-7016-AR3 fully automated pneumatic slicer with a 120mm diameter circular cutter. Then, they are processed in ERKODENT Germany. Using the prepared dental mold on the company's ERKOFORM-3d+ molding machine, the invisible aligner is prepared by baking and hot pressing, and then trimmed and polished to make the final product.

[0117] (iv) Examples and Comparative Examples

[0118] Comparative Example 1

[0119] Using the above extrusion line, a single-component rigid thermoplastic polyurethane, Isoplast, is employed. 2530, host temperature set 220℃-245℃, extruded 0.76mm film.

[0120] Comparative Example 2

[0121] Using the above extrusion line, a 0.8 mm film was extruded using a single-component copolyester (Eastar 6763) and the main machine temperature was set to 240℃-260℃.

[0122] Comparative Example 3

[0123] Using the above extrusion line, the main extruder temperature is set to 240℃-260℃, using Eastar 6763 copolyester; the auxiliary extruder temperature is set to 205℃-235℃, using Pellethane. 2363 65D, extruded to obtain a 0.8 mm thick film with a TPU / PETG / TPU structure, wherein the TPU thickness on each side is 75 micrometers.

[0124] Comparative Example 4

[0125] Using the above extrusion line, the auxiliary machine temperature is set to 240℃-260℃, using Eastar 6763 copolyester; the main machine temperature is set to 205℃-235℃, using Pellethane. 2363 65D, extrusion produced a 0.8 mm thick film with a PETG / TPU / PETG structure, wherein the core TPU layer is 150 micrometers thick.

[0126] Comparative Example 5

[0127] Using the above extrusion line, the main extruder temperature is set to 220℃-260℃, using the copolyester Tritan TX1000; the auxiliary extruder temperature is set to 205℃-240℃, using thermoplastic polyurethane. 236365D; A 0.8 mm thick film with a TPU / Tritan / TPU structure was obtained by extrusion, wherein the TPU on both sides is 75 micrometers thick.

[0128] Comparative Example 6

[0129] Using the above extrusion line, A 0.8 mm thick single-layer transparent polyamide film was prepared using TR90, and then a 100 μm thick Ecdel film was obtained using a GT-7014-P hot pressing mechanism with high-speed rail testing. TM 9966 copolyester elastomer film. A multilayer film with a transparent polyamide core layer of 0.62 mm and a copolyester elastomer skin layer of 90 micrometers was prepared by laminating the two materials at 210℃ and 0.2 MPa.

[0130] Comparative Example 7

[0131] Using the aforementioned extrusion line, the main machine temperature is set to 200℃-230℃, and the auxiliary machine temperature is set to 200℃-230℃. The core layer uses Borealis PP HC205CF, a polypropylene homopolymer with added crystallizing nucleating agents, and the skin layer uses thermoplastic polyurethane. 2363 75D. A multilayer composite film with a total thickness of 0.8 mm was prepared, with a core layer thickness of 0.68 mm and a skin layer thickness of 60 micrometers.

[0132] Example 1

[0133] Using the above extrusion line, the core layer is made of amorphous transparent polyamide. TR90, with a skin made of thermoplastic polyurethane. 2363 65D. The main machine temperature is set to 220℃-260℃, and the auxiliary machine temperature is set to 205℃-240℃. A multilayer composite film with a total thickness of 0.8mm is obtained by extrusion, with a core layer thickness of 0.64mm and skin layer thicknesses of 60 micrometers and 80 micrometers, respectively.

[0134] Example 2

[0135] Using the above extrusion line, the core layer is made of amorphous transparent polyamide. TR90, with a skin made of thermoplastic polyurethane. 2363 75D. The main machine temperature is set to 220℃-260℃, and the auxiliary machine temperature is set to 205℃-240℃. A multilayer composite film with a total thickness of 0.8mm is obtained by extrusion, with a core layer thickness of 0.66mm and a skin layer thickness of 70 micrometers.

[0136] Example 3

[0137] Using the aforementioned extrusion line, the second roller is replaced with a frosted metal roller. The main machine temperature is set to 220℃-260℃, and the auxiliary machine temperature is set to 205℃-240℃. The core layer is made of amorphous transparent polyamide. TR90, with a skin made of thermoplastic polyurethane. 2363 75D. A multilayer frosted composite film with a total thickness of 0.8 mm was obtained by extrusion, wherein the core layer thickness was 0.66 mm and the skin layer thickness was 70 micrometers.

[0138] Example 4

[0139] Using the above extrusion line, the core layer is made of amorphous transparent polyamide. TR90, with a symmetrical skin layer made of polyether-type thermoplastic polyamide elastomer. 7233. The main unit temperature is set to 220℃ to 260℃, and the auxiliary unit temperature is set to 220℃ to 250℃. A multilayer composite film with a total thickness of 0.76mm is obtained, with a core layer thickness of 0.58mm and a skin layer thickness of 90 micrometers.

[0140] Example 5

[0141] Using the above extrusion line, the core layer is made of microcrystalline transparent polyamide. CX7323, the skin is made of thermoplastic polyurethane. 2363 75D. The main unit temperature was set to 230℃ to 265℃, and the auxiliary unit temperature was set to 205℃ to 240℃. A multilayer composite film with a total thickness of 0.8mm was obtained, with a core layer thickness of 0.65mm and a skin layer thickness of 75 micrometers.

[0142] Example 6

[0143] Using the above extrusion line, the core layer is made of amorphous transparent polyamide. TR90, with the skin layer made of thermoplastic polyurethane Utechllan UE-64DU10. The main unit temperature was set to 220℃-260℃, and the auxiliary unit temperature was set to 205℃-235℃. A multilayer composite film with a total thickness of 0.75mm was obtained, with a core layer of transparent polyamide of 0.62mm and skin layers of 65μm each.

[0144] Example 7

[0145] Using the above extrusion line, amorphous transparent polyamide is employed. Using the Clear G170 line with a temperature setting of 220℃-260℃, a 0.76mm single-layer transparent polyamide film was produced. The same line was also used to produce thermoplastic polyurethane... 2363 75D, with the temperature set at 200℃-235℃, a film with a thickness of 0.1mm was prepared. Then, a multilayer film with a thickness of 0.73mm was prepared by using a high-speed rail testing GT-7014-P hot press at 215℃ and 0.2MPa. The core layer thickness was 0.6mm and the skin layer thickness was 65 micrometers.

[0146] (V) Performance Test

[0147] 1. Interlayer adhesion test of multilayer films with a core layer of rigid polymer

[0148] The testing scope includes Comparative Example 3, Comparative Examples 5-7, and Examples 1-7.

[0149] Test method: Following the ISO 2409 cross-cut adhesion test, an Elcometer 1542 cross-cut adhesion tester was used to cut through the epidermis, and then the corresponding adhesive tape was used for peeling. The peeling was graded from 0 (no separation at all) to 5 (more than 65% of the cut area was affected) according to the severity of the peeling.

[0150] Test results:

[0151] The results for Comparative Examples 3 and 5, and Examples 1-7 were 0, while the results for Comparative Examples 6 and 7 were 5.

[0152] These results demonstrate that the three-layer film with a core layer containing transparent polyamide can achieve excellent interlayer adhesion with thermoplastic polyurethane and polyamide elastomers (Examples 1 to 7). However, the results of Comparative Examples 6 and 7 show that if the polymer types of the core and skin layers are not properly matched, they are prone to peeling off, posing a risk of delamination in subsequent processes and use, such as grinding and immersion environments. Therefore, for polymer multilayer film products, the selection of materials for the core and skin layers must consider the interlayer bonding strength between the two components, and cannot be simply based on the required performance of each core and skin layer individually. This is because fundamental material properties, such as polarity, can lead to poor interlayer bonding, resulting in unqualified final products or unexpected failures.

[0153] The test results above show that thermoplastic polyurethane has excellent adhesion to copolyester and transparent polyamide. However, if the material combination of the core layer and the surface elastomer is not suitable (as in Comparative Example 6), or for example, if the skin layer is made of thermoplastic polyurethane and the core layer is made of highly transparent polypropylene (Comparative Example 7), even if lamination is used, it is still difficult to avoid the risk of delamination during use, resulting in product defects or unexpected failures.

[0154] 2. Light transmittance of the multilayer films prepared in Examples 1 to 7

[0155] Test method: ASTM D1003, test equipment: BYK Haze-gard plus.

[0156] The test results are shown in Table 1 below:

[0157] Table 1

[0158] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 pass rate % 92.3 92.1 90.8 88.4 91.5 92.2 91.3

[0159] As can be seen from the test results in Table 1, even when using single-sided frosting treatment (Example 3) or using a semi-permeable polyamide elastomer for the skin layer (Example 4), the multilayer films prepared in these examples all have very high light transmittance and are suitable for the manufacture of invisible dental films.

[0160] 3. Stress cracking resistance test

[0161] Test method:

[0162] Cut an 80mm x 20mm rectangular strip along the MD direction of the sample and fix it to a 60mm diameter stainless steel semi-circular tile. Place the fixed device into distilled water and artificial saliva (Phygene). Incubate the sample at 37°C for 24 hours in either pH 1843 or Listerine Ice Blue Refreshing Mouthwash. After releasing the sample, rinse with distilled water, dry with absorbent paper, and observe on the lab bench. (See attached image) Figure 2 This is a schematic diagram of a diaphragm with relatively obvious microcracks. The test results of the stress cracking resistance test are represented by 0 to 5. 0 represents that it is intact after bending stress treatment, and 5 represents that there are severe and dense cracks after treatment.

[0163] The test results are shown in Table 2 below.

[0164] Table 2

[0165]

[0166] As shown in Table 2 above, the multilayer systems with a transparent polyamide core (Examples 1 to 7) exhibited excellent stress cracking resistance. Comparative Examples 1-5 showed varying performance, with Comparative Example 1, using TPU, showing no issues with stress cracking resistance. Comparative Examples 2 to 5, using polyester copolymers, showed different performance. In Comparative Example 2, the single-layer PETG exhibited significant microcracks under conditions of distilled water, artificial saliva, or mouthwash. Comparative Example 4, with its PETG / TPU / PETG structure and a flexible TPU buffer, showed improved stress cracking resistance, but microcracks still appeared. Comparative Examples 3 and 5, using TPU as a protective outer layer and polyester copolymer as the core, significantly improved the stress cracking resistance of polyester copolymers in warm solutions.

[0167] 4. Initial stress test and stress holding experiment

[0168] Test scope: Comparative Examples 1, 3, and 5, and Examples 1-7. Comparative Examples 2 and 4 had poor stress cracking resistance, and Comparative Examples 6 and 7 had insufficient interlayer bonding strength, therefore these were not tested.

[0169] Experimental Method: The diaphragm was cut into Type 5 strips corresponding to GB / T 1040.1-2006. The strips were then immersed in water at 37℃ for 24 hours. A stress relaxation / retention test with a water bath was conducted using a Zwick Z010 tensile testing machine. The 37℃ constant temperature water bath device was based on patent CN202092909U. After the tensile displacement reached 2 mm, the corresponding stresses (MPa) at 0H, 1H, 3H, 6H, 9H, and 12H were recorded and converted into stress / load retention percentages. The calculation results are shown in Table 3 below.

[0170] Table 3

[0171]

[0172] From the stress / load retention percentage calculation results in Table 3 above, the multilayer films with transparent polyamide as the core layer (Examples 1 to 7) exhibited significantly better stress retention rates after 12 hours compared to single-layer TPU (Comparative Example 1) and TPU / PETG / TPU structures (Comparative Example 3).

[0173] Comparative Example 1 is a TPU monolayer film. Although it has good stress cracking resistance, the stress dissipates rapidly under warm and humid conditions, and the actual stress retention rate is low over a long period of time, only a little over ten percent.

[0174] Comparative Example 3 and Comparative Example 5 used different polyester copolymers, with corresponding core glass transition temperatures (Tg) of 80℃ and 110℃, respectively.

[0175] Taking the transparent polyamide in Example 1 / 2 as an example, the corresponding core layer Tg is 150°C. Further saturation water absorption treatment was performed on the transparent polyamide (ISO 62, using a method of placing it at 23°C for several days), and the corresponding Tg remained as high as 128°C.

[0176] It is evident that the transparent polyamide and the transparent polyamides in the other examples can all mitigate the effects of stress relaxation caused by molecular chain movement under warm and humid conditions. Examples 1-7 all exhibited excellent stress / load retention percentages, with Examples 1-6 outperforming the best-performing comparative example—Comparative Example 5. While Example 7's result was slightly lower than that of Comparative Example 5, the results were still comparable. The data from Example 7 still showed a significant advantage compared to Comparative Example 1 (PETG) and Comparative Example 3 (TPU), which are commonly used as films.

[0177] Besides the percentage of stress / load retained, the amount of residual stress / load after prolonged wear is also an important indicator in clear aligner treatment, affecting the efficiency of orthodontic treatment. (See attached diagram) Figure 3 The stress retention curves are for Comparative Example 3 (TPU / PETG / TPU multilayer film), Comparative Example 5 (TPU / Tritan / TPU), and Example 2.

[0178] from Figure 3 It can be seen that, compared with Comparative Examples 3 and 5, the transparent polyamide three-layer composite film used in Example 2 maintains higher stress over a long period of time and has a comparable initial stress to Comparative Example 5. Although Comparative Example 3 has a higher initial stress, its stress decays faster within the same time period, resulting in lower stress retention.

[0179] Therefore, it can be seen that the multilayer polymer film provided by the present invention has suitable initial stress and high and stable stress retention under the warm and moist conditions of the oral cavity.

[0180] In summary, the multilayer polymer film provided by this invention comprehensively considers high transparency, resistance to environmental stress cracking, sufficient initial stress and low stress relaxation in a warm and humid environment, and high holding power in the selection and matching of interlayer polymers, making it ultimately suitable for the preparation of dental films and dental invisible aligners.

[0181] Under the guidance of the present invention and the above embodiments, those skilled in the art will readily foresee that all the raw materials or their equivalents, processing methods or their equivalents listed or exemplified in the present invention can achieve the present invention, and that the upper and lower limits and range values ​​of the parameters of each raw material and processing method can also achieve the present invention. Examples are not listed one by one here.

Claims

1. A multilayer polymer film, characterized in that, The multilayer polymer film comprises a rigid core layer and soft skin layers located on both sides of the rigid core layer, wherein, The rigid core layer includes one or more polymer layers, and the material of each polymer layer of the rigid core layer comprises one or more transparent polyamides, wherein the transparent polyamides are selected from amorphous transparent polyamides or microcrystalline transparent polyamides, and the glass transition temperature Tg of each transparent polyamide layer is between 100℃ and 200℃. Each of the soft skin layers comprises one or more polymer layers, and each polymer layer of each of the soft skin layers is made of one or more thermoplastic polyurethanes with a Shore hardness of 50D-85D, or of one or more polyamide elastomers with a Shore hardness of 50D-85D.

2. The multilayer polymer film as described in claim 1, characterized in that, The glass transition temperature (Tg) of each layer of the transparent polyamide is between 110°C and 170°C.

3. The multilayer polymer film as described in claim 1, characterized in that, The glass transition temperature (Tg) of each layer of the transparent polyamide is between 125°C and 170°C.

4. The multilayer polymer film as described in claim 1, characterized in that, The transparent polyamide has a light transmittance of >75%, a tensile yield strength of >50MPa, a yield strain of ≥5%, and a tensile modulus of >1300MPa.

5. The multilayer polymer film as described in claim 1, characterized in that, The transparent polyamide has a light transmittance of >80%.

6. The multilayer polymer film as described in claim 1, characterized in that, The multilayer polymer film is subjected to single-sided roller pressing and sanding treatment.

7. The multilayer polymer film as described in claim 1, characterized in that, The total thickness of the hard core layer is 300-1200 micrometers, and the thickness of each soft skin layer is 20-150 micrometers.

8. The multilayer polymer film as described in claim 7, characterized in that, The total thickness of the hard core layer is 350-1000 micrometers, and the thickness of each soft skin layer is 40-90 micrometers.

9. The multilayer polymer film as described in claim 8, characterized in that, The total thickness of the hard core layer is 500-800 micrometers, and the thickness of each soft skin layer is 40-75 micrometers.

10. The multilayer polymer film according to claim 1, characterized in that, The total thickness of the layered structure consisting of the hard core layer and the soft skin layers on both sides is 500 micrometers to 1500 micrometers.

11. The multilayer polymer film according to claim 1, characterized in that, The total thickness of the layered structure consisting of the hard core layer and the soft skin layers on both sides is 600 micrometers to 1000 micrometers.

12. The multilayer polymer film according to claim 1, characterized in that, The multilayer polymer membrane is a membrane used to manufacture shell-shaped dental instruments.

13. A method for manufacturing a multilayer polymer film according to any one of claims 1-12, characterized in that, The multilayer polymer film is prepared by hot lamination, extrusion lamination, or multilayer co-extrusion.

14. Use of a multilayer polymer film according to any one of claims 1-12 for hot pressing to form a molded article.

15. The use of the multilayer polymer film as described in claim 14 for hot pressing to form molded articles, characterized in that, The molded body is a shell-shaped dental instrument.

16. A molded body manufactured by hot pressing, characterized in that, Prepared using the multilayer polymer film as described in any one of claims 1-12.

17. The molded article as claimed in claim 16, characterized in that, The molded body is a shell-shaped dental instrument.

18. A method for manufacturing the molded article according to claim 16 or 17, characterized in that, The basic steps are as follows: i) The above-mentioned multilayer polymer film is prepared by multilayer co-extrusion or extrusion composite process; ii) Vacuum hot pressing and / or positive pressure hot pressing of the multilayer polymer film to obtain a molded body; iii) Trim and grind the above-mentioned molded body.

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