Double-layered dental appliance and material constructions

Abstract

Polymer sheet composition, characterized in that it comprises: at least two outer layers A and C and an intermediate layer B, wherein layers A and C individually comprise a thermoplastic polymer having a tensile modulus of approximately 1000 MPa to 2500 MPa and a glass transition temperature and / or melting point of approximately 80 °C to 180 °C, wherein the intermediate layer B comprises one or more elastomer(s) having a tensile modulus of approximately 50 MPa to approximately 500 MPa and a glass transition temperature and / or melting point of approximately 90 °C to approximately 220 °C, wherein layers A and C comprise a copolyester, and wherein the intermediate layer B preferably comprises a polyurethane, in particular an aromatic polyester polyurethane.

Description

Double-layered dental appliance and material constructions Field of invention Polymer sheet compositions are being disclosed. Polymer sheets are useful, for example, in dental appliances, and are constructed with layers that provide flexibility, mechanical strength, and stain resistance to devices made from the sheets. State of the art There is a need to improve orthodontic and dental appliances capable of facilitating orthodontic tooth movement, stabilizing tooth positions, or protecting teeth from potentially damaging external forces. Existing materials and products are constructed from single-layer, two-layer, or three-layer materials that offer limited functionality and may suffer from performance deficiencies. Aligners are plastic coverings that are placed over the teeth and are designed to apply translational or rotational forces to the teeth. Their ability to move teeth precisely is limited by their effective modulus, elasticity, and ability to resist creep and stress relaxation. Furthermore, they generally need to be resistant to staining and environmental stress cracking. Dental protective devices, such as sports mouthguards and dental splints, have conflicting requirements. On the one hand, they must be able to dissipate impact forces, and on the other hand, they must be thin and not interfere with a person's natural bite or impede speech. Description of the invention In one aspect, a composition comprising at least two outer layers A and C and an intermediate layer B is provided. Layers A and C individually comprise a thermoplastic polymer having a modulus of approximately 1000 MPa to 2500 MPa and a glass transition temperature and / or melting point of approximately 80 °C to 180 °C, and the intermediate layer B comprises at least one elastomer having a modulus of approximately 50 MPa to approximately 500 MPa and one or more glass transition temperatures and / or melting points of approximately 90 °C to approximately 220 °C. In one embodiment, layers A and C are composed of one or more of a copolyester, a polycarbonate, a polyester and carbonate blend, a polyurethane, a polyamide, or a polyolefin. In another embodiment, the intermediate layer B is composed of one or more of a polyurethane elastomer, a polyolefin elastomer, a polyester elastomer, a styrenic elastomer, a polyamide elastomer, a cyclic olefin elastomer, an acrylic elastomer, an aromatic or aliphatic polyether, and a polyester polyurethane. Preferably, the intermediate layer B comprises a polyurethane having a Shore hardness of approximately A80 to D75, A85 to D65, or A90 to D55. In yet another embodiment, the intermediate layer B material exhibits a compressive strain of less than 35%, 30%, 25%, 20% or 10% after 22 hours at 25°C. In yet another embodiment, layers A and C exhibit a lateral restoring force of less than 100 N (Newtons) per cm2, 50 N per cm2, 25 N per cm2, or 10 N per cm2, when displaced from 0.05 mm to 0.1 mm relative to each other. In another embodiment, the interlayer peel strength between layers A and C and layer B is greater than 50 N per 2.5 cm. In one embodiment, the combined thickness of layers A, B, and C is approximately 250 micrometers to approximately 2000 micrometers, and the combined thickness of layers A and C is 25 micrometers to 750 micrometers, 50 micrometers to 1000 micrometers, 100 micrometers to 700 micrometers, 150 micrometers to 650 micrometers, or 200 micrometers to 600 micrometers. Still in other embodiments, one or more of layers A and C comprise a microcrystalline polyamide composed of from 50 to 100 mol% of C6 to C14 aliphatic acid moieties, and approximately 50 to 100 mol% of 4,4-methylenebis(cyclohexylamine) (CAS [1761-71-3]), exhibiting a glass transition of between approximately 100 °C and 180 °C, a heat of fusion of less than 20 J / g and a light transmission of more than 80%. In another embodiment, one or more of layers A and C comprise a copolyester composed of: (a) a dicarboxylic acid component comprising from 70 mol% to 100 mol% of terephthalic acid residues, and (b) a diol component comprising i) from 0 to 95 mol% ethylene glycol, ii) from 5 mol% to 50 mol% of 2,2,4,4-tetramethyl-1,3-cyclobutanediol residues, iii) from 50 mol% to 95 mol% of 1,4-cyclohexanedimethanol residues, and / or iv) from 0 to 1 mol% of a polyol having three or more hydroxyl groups, wherein the sum of the mol% of the diol residues i) and / or ii) and / or iii) and / or iv) amounts to 100% by moles and the copolyester has a glass transition temperature Tg of 80 °C to 150 °C. In another embodiment, the intermediate layer B comprises an aromatic polyether polyurethane having a Shore hardness from approximately A90 to D55 and a compression deformation of less than 35%, wherein the peel strength between layers A and C and layer B is greater than 50 N per 2.5 cm. In one embodiment, one or more of layers A and C comprise a polyurethane composed of (a) a diisocyanate comprising from 80 mol% to 100 mol% of methylenediphenyl diisocyanate residues and / or hydrogenated methylenediphenyl diisocyanate and (b) a diol component comprising i) from 0 to 100 mol% of hexamethylenediol and ii) from 0 to 50 mol% of 1,4-cyclohexanedimethanol, wherein the sum of i) and ii) amounts to more than 90 mol% and the polyurethane exhibits a glass transition temperature Tg from approximately 85 °C to approximately 150 °C. In another aspect, a dental appliance adaptable to one or more teeth is made of a polymeric composition or sheet as described in this document. In one embodiment of the dental appliance, the combined thickness of layers A, B, and C is from approximately 250 micrometers to approximately 2000 micrometers, and the combined thickness of layers A and C is from 25 micrometers to 750 micrometers, from 50 micrometers to 1000 micrometers, from 100 micrometers to 700 micrometers, from 150 micrometers to 650 micrometers, or from 200 micrometers to approximately 600 micrometers. In another aspect, a reversibly deformable dental appliance is provided, composed of a polymeric sheet composition or material as described herein, wherein the elastomeric intermediate layer and the outer layers can be reversibly moved relative to each other and exhibit a lateral restoring force of less than 100 N per cm2, 50 N per cm2, 25 N per cm2, or 10 N per cm2 when displaced from 0.05 mm to 0.1 mm relative to each other. In one embodiment, the elastomeric intermediate layer comprises a polyurethane having a hardness of from approximately A 80 to D 75, from A 85 to D 65, or from A 90 to D 55. In another aspect, a composition, polymer sheet, or dental appliance exhibiting resistance to environmental stress is provided, comprising at least two outer layers and an inner elastomeric layer, wherein one or more of the outer layers is a polyester or copolyester having a modulus from approximately 1000 MPa to 2,500 MPa, and the inner layer comprises an elastomer having a modulus from approximately 50 MPa to approximately 500 MPa, wherein the interlayer peel strength between at least one outer layer and the elastomer is greater than approximately 50 N / in. In another aspect, a reversibly deformable dental appliance is provided, in which the thickness of the outer layer A is from approximately 175 to approximately 250 micrometers, the thickness of the outer layer C is from approximately 175 to approximately 250 micrometers, and the thickness of the intermediate layer B is from 300 to 500 micrometers, in which the combined thickness of layers A, B and C is 850 to 1000 micrometers. Brief description of the drawings Figure 1A is a schematic representation of a cross-sectional view of a three-layer sheet of the invention with a simple ABC construction. Layer A and layer C may be of the same or different materials, and each layer may be composed of one or more materials, mixtures, or alloys. Layer B may be of a single material, a mixture of materials, or alloys. Figure 1B is a schematic representation of a cross-sectional view of a multi-layered sheet. Each layer A, B, and C may consist of a single layer or multiple layers, and each layer may consist of one or more materials or a mixture of materials. Layer A may consist of more than one layer, for example, layers aya; layer B may consist of more than one layer, for example, layers byb; and layer C may consist of more than one layer, for example, layers cyc, as exemplified in Figure 1B. Figures 2A and 2B are schematic representations of example test specimens for determining the displacement (Figure 2A) and restoring force (translational movement; Figure 2B) of a simple 3-layer sheet composed of two rigid outer layers and an inner elastomeric layer, where A, B, and C are individual layers of the sheet. In this example, layers A and C are reversibly translated relative to each other, and layer B provides a restoring force. More specifically, layers A, B, and C may each be approximately 250 micrometers thick, and each layer may be composed of one or more materials and may individually comprise one or more layers. Figure 3A is a graphical representation of displacement / force curves for elastomers with varying degrees of hardness. The graph demonstrates the restoring force (N / cm²) generated from the translational movement of layer A relative to layer C, which has an intermediate layer B composed of TPU elastomers of different hardnesses. The graph shows that the hardness of the elastomer influences both the displacement and the restoring force. A harder thermoplastic urethane (TPU) will generate a greater restoring force but may limit the amount of movement. Figure 3B is a graphical representation of the restoring force (N / cm²) as a function of time (from 0 to 48 hours) for a given displacement between layer A and layer C, which has an intermediate layer B with TPU elastomers of varying hardness. TPU 75A exhibits low compressive strain and shows the lowest initial force, but the force decreases very little over time. TPU 75D exhibits high compressive strain, and although it shows a much higher initial restoring force, the force decreases rapidly over time. Figure 4 is a graphical representation of the retained strength at 5% tension for different constructions exposed to 37°C and water over a period of 48 hours. It should be appreciated that the constructions and properties illustrated in Figures 1 to 4 are specific examples and are not intended to limit the scope of constructions and tests that may be used. Other materials, constructions, and step sequences may also be realized according to alternative embodiments. For example, alternative embodiments may contain additional layers, including bonding layers, pigments, optical additives, or reinforcing agents, and may be constructed in any manner known in the art, such as by flat sheet extrusion, blown film coextrusion, calendering, lamination, and adhesive bonding. The structures (or polymer sheets) and devices may be realized, in some embodiments, by 3D printing or dip coating. A person skilled in the art will recognize and appreciate many variations, modifications, and alternatives to the constructions. Therefore, the memorandum and drawings should be considered illustrative and not restrictive. However, it will be evident that various modifications and changes may be made without departing from the broader spirit and scope of the disclosure as set forth in the claims. Other variations are within the spirit of this disclosure. Therefore, although the disclosed embodiments are susceptible to various modifications and alternative constructions, certain illustrated embodiments are shown in the drawings and described herein. It should be understood, however, that the disclosure is not intended to be limited to the specific embodiment or embodiments disclosed, but rather to cover all modifications, alternative constructions, and equivalents that fall within the spirit and scope of the disclosure, as defined in the appended claims. Current orthodontic aligners have a very limited elastic range (typically 4% to 7%) and exhibit a rapid decrease in restoring force when deformed. As a result, appliances may need to be changed frequently, increasing manufacturing costs, teeth may not move as desired, and the patient may experience discomfort due to excessively high initial forces. Attempts to improve the elastic range by providing a thin outer layer of elastomer (usually a polyurethane, as described, for example, in US patent 9,655,693 B2) can result in a tooth contact surface that deforms easily, reducing the accuracy of tooth movement and potentially increasing susceptibility to unsightly staining from cigarettes, beverages, or common foods. US patent no. 6,524.Section 101 describes dental appliances that have regions with different elastic moduli and appliances that have added reinforcing elements. Non-staining polyurethanes used to manufacture dental appliances, such as Zendura® A, available from Bay Materials, LLC (Fremont, CA), have excellent properties, but are hygroscopic, requiring rigorous drying before thermoforming, may initially be uncomfortable, are difficult to clean, and may not be ideal for some applications. Many other polyurethanes also require drying before thermoforming, adding time and cost to the manufacturing process. Polyesters or aromatic copolyesters can be used to form the aligners; however, they exhibit poor chemical resistance and low mechanical strength to impact and tearing. Aligners constructed from rigid materials such as polyesters or rigid polyurethanes have a high modulus, for example, greater than approximately 1000 or 1500 MPa, and when deformed, they can exert excessive forces on the teeth, causing discomfort and potential damage to the tooth roots.Highly elastomeric polymers, such as thermoplastic polyurethane (TPU) elastomers and styrenic elastomers (such as SBS, SEBS, SIS, for example), have a low modulus (typically less than 100 or 200 MPa), which may be insufficient to move teeth and stain easily, making them of limited use for producing aligners. This disclosure is based on the discovery that many of the deficiencies in prior art materials and dental appliances constructed from them can be reduced or eliminated with a sheet or device having outer layers composed of a material having a modulus of more than approximately 1000 MPa up to 2500 MPa and an inner elastomeric layer or core composed of elastomeric material or materials having a modulus of approximately 50 MPa to 500 MPa, which may not stain, has a lower cost than rigid urethanes, has improved elastic properties, and exhibits surprisingly greater resistance to environmental stress cracking. A polymer sheet or device may consist of more than two rigid layers; for example, a third rigid layer may be sandwiched between two or more elastomeric layers. This multilayer construction provides a double-layered dental appliance that can be adapted to move teeth, retain teeth in an existing position, or protect teeth from impact. As disclosed herein, the outer layer material that comes into contact with the teeth may be substantially rigid to precisely conform to the teeth, providing accurate forces while maintaining the ability to exert a nearly constant force over longer distances. By selecting an appropriate outer and inner material module and thickness, two or more substantially rigid shells can be reversibly displaced relative to one another to a greater extent than a rigid material of comparable thickness and shape. This provides a dental appliance that can apply the desired forces to the teeth with a greater range of movement, while avoiding excessive forces or excessive stress relaxation during deformation. Although not limiting disclosure to specific constructions, a dental appliance may be referred to herein as a "double-shell" appliance. A "double-shell" appliance may comprise two or more shells or layers. The shells or layers may be of the same or different thicknesses.A series of dental appliances composed of this "double-layer" construction can be used to move teeth in incremental stages, where two or more appliances can be constructed from the same or different materials. The appliances can be fabricated by thermoforming a double-layer material onto a model of one or more teeth, or they can be constructed by sequentially thermoforming rigid and elastomeric precursor sheets, or by sequentially dipping a model with polymer solutions or polymer-forming monomers or oligomers that can be optionally cured or further processed. The inventors have found that this unique construction can significantly reduce the amount of stress cracking in a layer or material, thereby expanding the range of materials that can be used in the sheets or the appliance. Definitions The use of the terms "a" and "an" and similar referents in the context of describing the disclosed embodiments (especially in the context of the following claims) should be construed as covering both the singular and the plural, unless otherwise indicated herein or clearly contradicted by the context. The terms "comprising," "features," "includes," and "containing" should be construed as open terms (i.e., meaning "including, but not limited to") unless otherwise indicated. The term "connected" should be construed as containing partly or completely within, joined to, or attached to something, even if there is something in between. The expression "based on" should be construed as open and not limiting in any way and should be interpreted or in any case read as "based at least in part on," as appropriate.The mention of value ranges in this document is intended merely as a shorthand method of referring individually to each independent value within the range, unless otherwise stated herein, and each independent value is incorporated in the document as if it were mentioned individually. All methods described herein may be performed in any appropriate order unless otherwise stated herein or clearly contradicted by the context. The use of any and all examples, or example expressions (e.g., "such as") provided herein is intended merely to better illustrate the ways in which the disclosure may be made and does not impose a limitation on the extent of the disclosure, unless otherwise claimed.It should be construed that no statement in the report indicates that an unclaimed element is essential to the implementation of the disclosure. The term "dental appliance" is used in this dissertation to refer to any device placed on or over a subject's teeth. Dental appliances include, but are not limited to, orthodontic, prosthetic, retention, anti-snoring / airway, cosmetic, therapeutic, protective (e.g., mouthguards), and habit-modifying devices. The expression "ASTM D638 standard" is used in this document with reference to the tensile strength test of plastics. The expression "ASTM D1364 standard" is used in this document with reference to the peel strength test between layers. The term "compressive deformation" is used in this document with reference to the permanent deformation of a material when a force is applied and removed. The term "flexural modulus" is used in this document with reference to the stiffness of a material and / or its resistance to bending deformation. The higher the flexural modulus of the material, the more resistant it is to bending. For an isotropic material, the elastic modulus measured in any direction is the same. The term "hardness" is used in this document with reference to the Shore hardness scale. Shore hardness and modulus are generally correlated and can be converted approximately if only one value is known. The term "modulus" or "tensile modulus" is used in this document with reference to the stiffness of a material and / or its resistance to stretching. The higher the modulus of the material, the stiffer it is. The flexural modulus and the tensile modulus of a material may be the same or different. For isotropic materials such as A, B, and C, the flexural modulus and the modulus (which may also be called the tensile modulus) are substantially equal, and either one or the other can be measured depending on the circumstances. The term "polymer sheet" is used interchangeably in this document with the term "plastic sheet". The expression "lateral restoring force" with respect to layers A and C of a polymer sheet refers to the force that a displaced layer can exert on another layer that is fixed in position. If layers A and C are made to move independently of each other, they will subsequently return to their original positions if not restrained. The term "shear force", as used in this thesis, means the translational force applied to two surfaces that are connected by an elastic material. The term "cover" is used in this document with reference to polymer covers that fit over the teeth and can be removably placed over the teeth. The expression "stain-resistant" is used in this document with reference to a material designed to be resistant to staining. The term "thermoplastic polymer" is used in this document with reference to a polymer that becomes flexible or moldable above a specific temperature and solidifies upon cooling, provided that the heat and pressure do not chemically decompose the polymer. The terms "tooth" and "teeth" include natural teeth, including natural teeth that have been modified with fillings or crowns, implanted teeth, artificial teeth that are part of a bridge or other attachment fixed to one or more natural or implanted teeth, and artificial teeth that are part of a removable attachment. Detailed description of some ways of implementing the invention The following description outlines various embodiments. Specific configurations and details are provided for explanatory purposes to ensure a comprehensive understanding of these embodiments. However, it will also be evident to those skilled in the art that these embodiments can be implemented without such specific details. Furthermore, well-known features may be omitted or simplified to avoid complicating the embodiment being described. In some embodiments (referred to herein as embodiment no. 1), a thermoformable polymer sheet comprises at least two outer layers A and C, and an intermediate layer B, wherein layers A and C are individually composed of a thermoplastic polymer having a modulus greater than approximately 1000 MPa, for example from 1000 MPa to 1500 MPa; from 1100 MPa to 1600 MPa; from 1200 MPa to 1700 MPa; from 1300 MPa to 1800 MPa; from 1400 MPa to 1900 MPa; from 1500 MPa to 2000 MPa; 1100 MPa; 1200 MPa; 1300 MPa; 1400 MPa; 1500 MPa; 1600 MPa; 1700 MPa; 1800 MPA, 1900 MPA; 2000 MPA; or up to 2500 MPA; and a glass transition temperature (Tg) and / or a melting point from approximately 80 °C to 180 °C; from 90 °C to 170 °C; from 100 °C to 160 °C; from 110 °C to 150 °C; from 120 °C to 150 °C; from 130 °C to 170 °C; from 140 °C to 180 °C; 80 °C; 90 °C; 100 °C; 110 °C; 120 °C; 130 °C; 140 °C; 150 °C; 160 °C; 170 °C; or 180 °C. In such embodiments, the intermediate layer B is composed of at least one elastomer having a modulus from approximately 50 MPa to approximately 500 MPa; from 60 MPa to 470 MPa; from 70 MPa to 440 MPa; from 80 MPa to 400 MPa; from 100 MPa to 350 MPa; from 150 MPa to 300 MPa; from 200 MPa to 400 MPa; 60 MPa, 70 MPa; 80 MPa, 90 MPa; 100 MPa; 110 MPa; 120 MPa; 130 MPa; 140 MPa; 150 MPa, 160 MPa; 170 MPa; 180 MPa; 190 MPa; 200 MPa, 250 MPa, 300 MPa, 350 MPa, 400 MPa, 450 MPa, or up to 500 MPa, and one or more of (a) a glass transition temperature, or (b) a melting point of approximately 90 °C to approximately 220 °C; 100 °C to approximately 200 °C; 120 °C to approximately 180 °C; 140 °C to 220 °C; or 160 °C to approximately 220 °C. In some embodiments, the intermediate layer B is an elastomeric layer or coating, which may include one or more materials and one or more layers. In embodiment No. 1, layers A and C may comprise a polyester or copolyester, a polyurethane, a polyamide, a polyolefin, an (met)acrylic polymer, a polycarbonate, a vinyl polymer such as a poly(vinyl chloride) or a fluoropolymer. In embodiment No. 1, layer B may comprise a polyurethane elastomer, a polyester elastomer, a styrenic elastomer, a polyamide elastomer, a siloxane elastomer, a polyether elastomer, a polyolefin elastomer, an olefin copolymer, an acrylic elastomer, or a fluoroelastomer. In embodiment No. 1, the material of layer B exhibits a compressive strain at 22 hours at 25 °C of less than approximately 35%, 30%, 25%, 20%, 10%, less than 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, or 10%. Contrary to the findings of document US 9,655,693 B2 where an elastomer is used as an outer layer, a lower compression strain has been found to be more effective than a higher compression strain. In certain aspects of embodiment no. 1, the sheet has a total thickness from approximately 250 micrometers to approximately 2000 micrometers. In certain aspects of embodiment No. 1, the combined thickness of layers A and C is from approximately 25 micrometers to approximately 1000 micrometers, from 50 micrometers to 750 micrometers, from 100 to 750 micrometers, from 250 micrometers to 750 micrometers, or from 250 micrometers to approximately 600 micrometers. In certain aspects of embodiment No. 1, the thermoformable sheet has a flexural modulus from approximately 100 MPa to approximately 2000 MPa, from approximately 250 MPa to approximately 2000 MPa, from approximately 500 MPa to 1500 MPa, from approximately 750 MPa to approximately 2000 MPa, or from approximately 750 micrometers to approximately 1500 MPa. In certain aspects of embodiment No. 1, layers A and C have a Tg of between approximately 80 °C and 150 °C, and layer B has a Tg or melting point of between approximately 180 °C and 220 °C and a heat of fusion from approximately 5 joules / g to approximately 20 joules / g, or from 5 joules / g to 15 joules / g. In certain aspects of embodiment No. 1, the peel strength between layers of a layer A is greater than approximately 50 N / inch, greater than approximately 60 N / inch, greater than approximately 70 N / inch. In certain aspects of embodiment No. 1, layers A and C each have a thickness of 25 micrometers to approximately 1000 micrometers, 50 micrometers to 750 micrometers, 100 to 750 micrometers, 125 to 300 micrometers, 250 micrometers to 750 micrometers, or 250 micrometers to approximately 600 micrometers, and may have a combined thickness of approximately 250 micrometers to approximately 600 micrometers, being composed of a rigid copolyester or polyurethane having a modulus from 1000 MPa to 2,500 MPa with a Tg of between 95 °C and 150 °C, the elastomeric layer B having a thickness from approximately 200 micrometers to approximately 1000 micrometers. micrometers or from 200 to 500 micrometers, which is composed of a polyester or polyester polyurethane having a hardness from approximately D 35 to approximately D 65, and a compression deformation at 22 hours at 25 °C of less than approximately 35%, 30%,25%, 20%, 10%, less than 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11% or 10%, and a layer A exhibiting interlayer peel strength greater than approximately 50 N / in, greater than approximately 60 N / in or greater than approximately 70 N / in, wherein the polymer sheet exhibits a flexural modulus from approximately 750 MPa to approximately 1500 MPa; from approximately 100 MPa to approximately 2000 MPa; from approximately 250 MPa to approximately 2000 MPa; from approximately 500 MPa to 1500 MPa; or from approximately 750 MPa to approximately 2000 MPa. In some aspects of embodiment No. 1, additional thin polymer layers (bonding layers) may be present to improve the adhesion of polymer layers that are not naturally adhesive to each other; for example, a layer of polypropylene grafted with maleitic anhydride may be used. a layer A of polypropylene and a layer B of polyester or polyamide. In some embodiments (referred to herein as embodiment no. 2), the A and C layers of the sheet or device can be reversibly moved relative to each other (e.g., in translation) from approximately 0.05 mm to approximately 0.1 mm with a force of less than 100 N per cm2, 50 N per cm2, 25 N per cm2, or 10 N per cm2. In some aspects of embodiment No. 2, layers A and C of the sheet or device have a total thickness from approximately 500 micrometers to 1000 micrometers and can be reversibly moved relative to each other by a distance of 0.05 mm to 0.1 mm with a force of less than 100 N per cm2, 50 N per cm2, 25 N per cm2, or 10 N per cm2. In some aspects of embodiment No. 2, the material of layer B exhibits a compressive strain at 22 hours at 25 °C of less than approximately 35%, 30%, 25%, 20%, 10%, less than 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, or 10%. In some embodiments (referred to herein as embodiment No. 3), one or more of layers A and C comprise a microcrystalline polyamide composed of 50 to 100, 50 to 90, 50 to 80, 50 to 70, 60 to 90, 60 to 80, or 70 to 90 mol% of aliphatic diacid moieties from C6 to C14, and approximately 50 to 100, 50 to 90, 50 to 80, 50 to 70, 60 to 90, 60 to 80, or 70 to 90 mol% of 4,4-methylene-bis(cyclohexylamine) (CAS [1761-71-3]), having a glass transition temperature between approximately 100 °C and 180 °C, a heat of fusion of less than 20 J / g, for example, approximately 20 joules / g, or 15 joules / g. See, for example, application DE no. 4310970 (embodiment 3). In some aspects of embodiment no.º 3, the combined thickness of layers A and C is less than approximately 500 micrometers, less than approximately 400 micrometers, less than approximately 300 micrometers. In some forms of realization (referred to in this document as form of realization n.4) A dental appliance adaptable to one or more teeth comprises at least two outer layers A and C, and an intermediate layer B, wherein layers A and C are individually composed of a thermoplastic polymer having a modulus greater than approximately 1000 MPA, for example from 1000 MPA to 1500 MPA; from 1100 MPA to 1600 MPA; from 1200 MPA to 1700 MPA; from 1300 MPA to 1800 MPA; from 1400 MPA to 1900 MPA; from 1500 MPA to 2000 MPA; 1100 MPA; 1200 MPA; 1300 MPA; 1400 MPA; 1500 MPA; 1600 MPA; 1700 MPA; 1800 MPA; 1900 MPA; 2000 MPa; up to 2500 MPa, in certain aspects greater than 1500 MPa, and a glass transition temperature and / or melting point from approximately 80 °C to 180 °C; from 90 °C to 170 °C; from 100 °C to 160 °C; from 110 °C to 150 °C; from 120 °C to 150 °C; from 130 °C to 170 °C; from 140 °C to 180 °C; 80 °C; 90 °C; 100 °C; 110 °C; 120 °C; 130 °C; 140 °C; 150 °C; 160 °C; 170 °C; or 180 °C, in certain aspects from 80 to 150 °C or from 95 to 150 °C.In such embodiments, the intermediate layer B is composed of at least one elastomer having a modulus from approximately 50 MPa to 500 MPa; from 70 MPa to 450 MPa; from 80 MPa to 400 MPa; from 100 MPa to 350 MPa; from 150 MPa to 300 MPa; from 200 MPa to 400 MPa; 60 MPa, 70 MPa; 80 MPa, 90 MPa; 100 MPa; 110 MPa; 120 MPa; 130 MPa; 140 MPa; 150 MPa, 160 MPa; 170 MPa; 180 MPa; 190 MPa; 200 MPa, up to 250 MPa, and one or more of a glass transition temperature or melting point from approximately 90 °C to approximately 220 °C. In some aspects of the form of realization n.4. Layers A and C have a combined thickness, in certain aspects, of approximately 25 micrometers to approximately 600 micrometers, for example, 250 micrometers, 300 micrometers, 350 micrometers, 400 micrometers, 450 micrometers, 500 micrometers, 550 micrometers or 600 micrometers, being composed of a rigid copolyester or polyurethane having a modulus greater than 1000 MPa, for example, from 1000 MPa to 1500 MPa; from 1100 MPa to 1600 MPa; from 1200 MPa to 1700 MPa; from 1300 MPa to 1800 MPa; from 1400 MPa to 1900 MPa; from 1500 MPA to 2000 MPA; 1100MPA; 1200MPA; 1300MPA; 1400MPA; 1500MPA; 1600MPA; 1700MPA; 1800MPA, 1900MPA; 2000 MPA; or up to 2,500 MPA with a Tg of from 80 °C to 180 °C; 90°C to 170°C; from 100°C to 160°C; 110°C to 150°C; 120°C to 150°C; 130°C to 170°C; 140°C to 180°C; 80°C; 90°C; 100°C; 110°C; 120 °C; 130 °C; 140 °C; 150 °C; 160 °C; 170 °C; or 180 °C, for example, from 80 to 150 °C or from 95 to 150 °C. In some aspects of embodiment No. 4, the elastomeric layer B has a thickness from approximately 200 micrometers to approximately 1000 micrometers, for example, 100 micrometers, 200 micrometers, 250 micrometers, 300 micrometers, 350 micrometers, 375 micrometers, 400 micrometers, 500 micrometers, 750 micrometers, or 1000 micrometers, and is composed of a polyether or polyester polyurethane having a hardness from approximately D 35 to approximately D 65 and a compressive strain at 22 hours at 25 °C of less than approximately 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, or 25%. %, 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11% or 10%, and a layer A with an interlayer peel strength greater than approximately 50 N / in, greater than approximately 55 N / in, greater than approximately 60 N,greater than approximately 70 N, wherein the polymer sheet exhibits a flexural modulus from approximately 100 MPa to approximately 2000 MPa, from approximately 250 MPa to approximately 2000 MPa, from approximately 500 MPa to 1500 MPa, from approximately 750 MPa to approximately 2000 MPa, for example, from approximately 750 MPa to approximately 1500 MPa. In some aspects of embodiment no. 4, layers A and C exhibit a lateral restoring force of 0.05 mm to 0.1 mm with a force of less than 100 N per cm2, 50 N per cm2, 25 N per cm2, or 10 N per cm2. In some embodiments (referred to in this document as embodiment no. 5), a dental appliance is formed by thermoforming a multilayer sheet onto a tooth model, wherein the thermoforming is carried out at a temperature that is at least higher than the glass transition temperature and / or melting point of the outer layers and lower than the glass transition temperature and / or upper melting point of at least one elastomeric material of the inner layer. In one embodiment of embodiment No. 5, a dental appliance is prepared by thermoforming a multilayer sheet having at least one layer A and C having a Tg of from approximately 80 °C to 180 °C; from 90 °C to 170 °C; from 100 °C to 160 °C; from 110 °C to 150 °C; from 120 °C to 150 °C; from 130 °C to 170 °C; from 140 °C to 180 °C; 80 °C; 90 °C; 100 °C; 110 °C; 120 °C; 130 °C; 140 °C; 150 °C; 160 °C; 170 °C; or 180 °C, and layer B has a glass transition temperature and / or melting point from approximately 90 °C to 220 °C, for example, from 180 °C to 220 °C and a heat of fusion from approximately 5 J / g to approximately 20 J / g, for example, from approximately 5 J / g to approximately 20 joules / g, or from 5 joules / g to 15 joules / g. In one aspect of embodiment No. 5, layers A and C comprise a copolyester or polyurethane having a Tg of approximately 90 °C to approximately 120 °C, layer B is composed of a polyurethane having a modulus of approximately 50 MPa to 500 MPa and a glass transition temperature and / or melting point of approximately 170 °C to approximately 220 °C and is thermoformed at a temperature of between approximately 150 °C and 200 °C. It should be understood that elements from two or more forms of realization can be combined. In some embodiments, the thermoformable polymer sheet is composed of at least two outer layers A and C, and an intermediate layer B, wherein one or more of the layers A and C comprise a microcrystalline polyamide composed of from 50 to 100 mol% of aliphatic diacid moieties from C6 to C14, and approximately 50 to 100 mol% of 4,4-methylene-bis(cyclohexylamine) (CAS [1761-71-3]), exhibiting a glass transition of between approximately 100 °C and 180 °C, a heat of fusion of less than 20 J / g and a light transmission of more than 80%. In some embodiments, the thermoformable polymer sheet comprises at least two outer layers A and C, and an intermediate layer B, wherein one or more of layers A and C comprises a copolyester composed of a dicarboxylic acid component comprising 70 mol% to 100 mol% terephthalic acid residues, and a diol component comprising (i) 0 to 95 mol% ethylene glycol, (ii) 5 mol% to 50 mol% 2,2,4,4-tetramethyl-1,3-cyclobutanediol residues, (iii) 50 mol% to 95 mol% 1,4-cyclohexanedimethanol residues, and / or (iv) 0 to 1 mol% a polyol having three or more hydroxyl groups, wherein the sum of the mol% of the diol residues (i), (ii), (iii) and / or (iv) amounts to 100% by moles and the copolyester has a glass transition temperature Tg of 80 °C to 150 °C.In some aspects of this embodiment, the thermoformable polymer sheet includes an intermediate layer B comprising an aromatic polyether polyurethane having a Shore hardness from approximately A90 to D55 and a compression deformation of less than 35%, wherein the peel strength between layers A and C and layer B is greater than 50 N per 2.5 cm. In some embodiments, a dental appliance adaptable to one or more teeth is made of the microcrystalline polyamide or copolyester described above. Construction methods Multilayer sheets can be prepared by various means, including but not limited to hot or cold rolling, adhesive lamination, fusion lamination, multilayer coextrusion, or other known methods. The sheets can be prepared entirely before being shaped to form an orthodontic appliance, or an appliance can be produced using a sequence of individual thermoforming stages to create multiple layers. Thermoforming of sheets to produce test specimens or dental appliances can be performed using a Biostar pressure former available at Great Lakes Orthodontics, employing procedures commonly used in the industry. Alternatively, thermoforming can be performed using a roll-fed thermoformer, a vacuum former, or other known thermoforming techniques. Thermoforming can be carried out using different conditions, shapes, or patterns to vary the stretch ratio and thickness of the part. Multilayer appliances can be manufactured using one or more 3D printing processes or by sequential dip coating, spray coating, powder coating, or similar processes known to produce 3D films, sheets, and structures. The temperature of the sheet during thermoforming can be measured using an infrared thermometer or a surface thermocouple. Utility The sheets and materials described herein are useful as thermoformable materials that exhibit superior dimensional stability, impact damping, and restorative strength. The sheets can be fabricated into various types of oral appliances, for example, for tooth movement, for use as a sports mouthguard with enhanced impact resistance, and for use as an orthodontic retainer. The improved properties of the materials and appliances described herein, compared to currently available materials and appliances, include, but are not limited to, greater flexibility resulting in improved end-user comfort, better results in tooth movement, greater resistance to staining and stress cracking, and excellent esthetics, all of which promote more consistent use by patients. Testing methods Tensile properties were measured using an Instron universal materials testing machine. Procedures were followed according to ASTM D638 unless otherwise noted. Color and transparency were measured using a BYK Gardner Spin colorimeter. Impact resistance was measured using a Gardner impact tester. Tear resistance was measured using a materials testing machine at a speed of 250 mm per minute. The stress relaxation of the samples at 37 °C in water was measured using the method described in US patent no. 8,716,425 B2. Stain resistance was measured by exposing the test items to a staining medium such as mustard or coffee for 24 hours at 37°C and measuring the color on a white tile before and after exposure. Translational recovery force was measured by constructing a three-layer structure (or polymer sheet) as shown in Figures 1 and 2. Samples were displaced from 0 to 0.5 mm and the force was reported in N / cm2. Peel strength between layers is measured at a speed of 50 mm / min and can be reported as Newtons (N) per inch or per 2.54 cm (N). Details can be found in the test method according to ASTM D3164. Thermal tests to determine glass transition temperatures, melting and freezing points were measured using a differential scanning colorimeter at a heating and cooling rate of 10 °C per minute, unless otherwise stated. Environmental stress cracking resistance can be determined by clamping a sheet sample around a cylindrical mandrel to induce a specific stress on the outer surface, for example, 3% or 5%, and exposing the samples to a specific environment for a specific time, for example, a solution mimicking saliva, mouthwash, or another solution of interest. The response can be measured semi-quantitatively by visually observing the type and number of cracks, or quantitatively by subsequently measuring a mechanical property such as tear strength. Materials and methods. Construction Materials. A wide variety of commercially available materials can be used to produce the sheets and apparatus described herein. Table 1 provides an example list of materials for use in component A or C. Table 2 provides an example list of materials for use in component B. Similar or related materials can be obtained from other manufacturers or produced using known methods. Table 1. Useful example materials as primary components of materials A or C Table 2. Useful illustrative materials as primary components of B materials Additional suitable materials for layers A, B, or C may include compatible or incompatible blends, for example, blends of two or more copolyesters, blends of polypropylene and polyethylene and ethylene-propylene elastomers, fluoropolymers such as poly(vinylidene fluoride) or its copolymers, styrene-acrylonitrile resins, acrylonitrile-styrene-butadiene (ABS) resins, polyurethanes containing polycarbonate soft blocks, siloxane soft blocks, silicone elastomers such as Geniomer™, a siloxane-urea copolymer, and cyclic olefin copolymers and cyclic olefin elastomers. Examples The disclosure is further illustrated by the following examples. The examples are provided for illustrative purposes only. They should not be construed as limiting the scope or content of the disclosure in any way. Example 1 A series of single-layer and multi-layer sheets with a nominal total thickness of 0.76 mm were prepared as shown in Table 3. Test samples 1 to 4 were prepared by compression molding and thermal lamination of individual films or by extrusion lamination. The prior art material examples P1, P2, and P3 were prepared by compression molding films and optionally by thermal lamination. Pressure lamination was performed at between 200 and 220 °C, extrusion lamination was performed using a polyurethane melting temperature of 210 to 240 °C, and co-extrusion was performed with a polyester melting temperature of 240 °C to 260 °C, and a polyurethane melting temperature of 210 to 240 °C. The time, temperature, and pressure conditions were varied to maximize the quality, thickness, and adhesion of the structure (polymer sheet). Mechanical properties, optical properties, stress relaxation, and shape recovery were measured to compare the suitability of the resulting structures (polymer sheets). Table 3. Single-layer and multi-layer sheets Prior art material P1 is a commercial thermoformable aligner material supplied by Bay Materials, LLC, Fremont, CA. Prior art material P2 is a polyester exhibiting a glass transition temperature of approximately 90°C, manufactured by Eastman Chemical and sold under the trade name Eastar 6763. Prior art material P3 is described in US Patent 9,655,693 B2. Test specimens 1-4 are multilayer laminated materials (as described herein) demonstrating improved stress relaxation properties, increased tear strength, and excellent stain resistance. Compared to prior art materials, test specimens 1-4 exhibited several unexpected properties. Comparing test specimens 1-4 with prior art materials P1 and P2, it can be observed that test specimens 1-4 exhibit substantially lower initial forces in the stress relaxation test (believed to translate to greater user comfort), yet surprisingly, they maintain these forces for longer periods. This contradicts the teachings of US patent 9,655,693 B2, which states that an outer elastomer layer is required to protect the inner hard layer. The ability of multilayer sheets to maintain appropriate force levels for extended periods under demanding conditions can be seen in Figure 4. Specimens A and B in Figure 4 are single-layer sheets, while specimens 1 and 2 are multilayer sheets, as described in Table 3. Tear resistance is an important property of dental appliances. Materials with low tear resistance exhibit poor durability and may crack in areas of high stress. Comparison of the tear resistance of prior art materials P1, P2, and P3 with test samples 1-4 shows that such multilayer structures (or polymer sheets) with an elastomeric B layer exhibit significantly higher tear resistance than comparable single-layer structures or prior art multilayer structures. To further investigate the effect of construction on tear strength, another laminate (No. 5) was prepared with 0.25 mm thick layers A and C composed of Eastar 6763, a copolyester available from Eastman Chemical with a glass transition temperature (Tg) of 86 °C, and a 0.2 mm thick layer B of 50 Shore D urethane elastomer, resulting in a total thickness of 0.7 mm. The tear strength of this sample was compared to that of prior art materials P1, P2, and P3. Sample No. 5 exhibited a tear strength of 120 N, more than 200% of the value for prior art material P3, while showing similar strengths to polyurethane and polyester. EXAMPLE 2 (Measurement of translational force) A three-layer sheet was prepared as described in Example 1 for test material 2. A 2.54 cm x 1 cm strip of the sheet was sandwiched between two 2.54 cm wide strips of rigid polyester to create a 0.5 cm overlap ("multilayer specimen A2"). A control test specimen was prepared using the same size and thickness of polyester A (prior art) sandwiched between two strips of rigid polyester. The displacement / force response was measured at a rate of 0.04 MPa / min, and the results are reported in Table 4. The multilayer construction allows the two outer layers (or the two covers) of an apparatus to accommodate greater elastic movement with appropriate forces than prior art constructions. Table 4. Controlled elastic movement of the multilayer material Orthodontic appliances were fabricated using the materials and methods described herein and compared with appliances of the same shape and thickness made from Zendura A and Essix Plus. The reported appliances were substantially more elastic and more comfortable to wear. Because the inner and outer shells can deform independently of each other, they can accommodate greater movement between the actual teeth and the appliance without causing undue discomfort to the patient and exert a nearly constant force over extended periods to move the teeth precisely. Example 3 A 0.25 mm thick clarified polypropylene film (BFI 257) supplied by Blue Ridge Films (Petersburg, Virginia) was laminated onto both sides of a 0.25 mm thick film prepared from Kraton GF (maleated SEBS, available from Kraton Polymers) using a hot press at 180°F. The laminate was cooled and cut to a 125 mm circle. The polypropylene modulus is reported to be 1100 MPa. The SEBS elastomer has a reported hardness of 71 A and a modulus of 25 MPa. The multilayer film exhibited minimal staining and was thermoformable on a dental model to produce a retainer with excellent elastic recovery properties. Example 4 The durability of sheet materials in the presence of mouthwash was investigated, as dental appliances are known to be easily damaged by alcohols and / or surfactants. Test sheets were prepared with a thickness of 0.75 mm and a width of 2.54 cm and a length of 12 cm. A mandrel of sufficient diameter to produce a 5% deformation was wrapped with prior art materials P1, P2, and P3, and multilayer sheet (test material) No. 2. The samples were immersed in mouthwash and maintained at 37 °C. This environment is known to promote environmental stress cracking and induce deformation, causing the materials to adopt a ring shape rather than being flat. After 24 hours, the samples were rinsed with deionized water, and the amount of recovery was measured immediately and again after 24 and 48 hours at room temperature.Subsequently, the samples were examined under a microscope to determine the amount of tension cracking on the extended side. A sample that returned to a completely flat state was considered to have 100% recovery. Tension cracking was rated from 1 to 5, with 5 being no visible cracking and 1 being severe cracking. Table 5 provides information on the shape recovery of the samples. The multilayer sheet (No. 2) recovered more quickly and completely than the prior art materials P1, P2, and P3. Table 5. Shape recovery of samples Example 5 Three laminated materials were prepared as in Example 1, Sample 2, and designated as Samples No. 6, No. 7, and No. 8. Sample No. 6 was extruded using an untreated polyester film at a roll temperature of 40 °C, Sample No. 7 was extruded using a corona-treated polyester film at a roll temperature of 60 °C, and Sample No. 8 was extruded using a corona-treated film at a roll temperature of 80 °C. Corona treatment is commonly used to activate film surfaces to increase their polarity. A control sample of polyester A was designated Sample No. 9. The mechanical properties and resistance to environmental stress cracking of the three samples are provided in Table 6. Table 6. Effect of interlayer peel resistance on the ESC resistance of layer A The dramatic improvement in environmental resistance observed for samples #7 and #8 compared to samples #6 and #9 is unexpected and surprising. In each case, the material exposed to the environment is chemically identical and subjected to equal amounts of stress. While not wishing to be confined to theory, it is hypothesized that some of the strain-induced concentrated stress present in the outer polyester layer may be transferred to the elastomeric material and that force transfer is more efficient in materials with greater interlayer bond strength. However, no precedent for this result is known. It is well known that non-crystalline thermoplastic copolyesters (PETG and PCTG) exhibit poor resistance to environmental stress cracking and are prone to rapid degradation when used as dental appliances. US patent 9,655,691 shows that coating both sides of such a copolyester with a thermoplastic polyurethane elastomer exhibiting a hardness from approximately 60A to approximately 85D dramatically increased the durability of dental aligners made from such materials (described as a "hard polymer layer sandwiched between two soft polymer layers"). Presumably, the outer material provides a physical and / or chemical protective layer. One disadvantage of such materials is that polyurethane and other elastomers exhibit poor stain resistance, and the disclosed multilayer structure exhibits poor tear resistance. The inventors have unexpectedly discovered that the stress cracking resistance of amorphous polyester films, sheets, or thermoformed parts made from them can be dramatically improved by bonding an elastomeric material such as polyurethane between two layers of polyester. The resulting structure, which features a soft polymer layer sandwiched between two hard polymer layers, exhibits excellent chemical resistance, high transparency, and excellent stain resistance. Additionally, the tear strength of the multilayer structure is greater than that of either the polyester or the elastomer alone. The inventors have also found that the improved properties require high bond strength between the layers and that a material with poorly bonded layers exhibits lower stress cracking and tear strength. In the art, it is known that rigid polyurethane sheets exhibit good resistance to stress cracking on their own. Unexpectedly, it was observed that a three-layer ABA structure, consisting of rigid polyurethane A layers (outer) and an elastomeric B layer (inner) with excellent adhesion, exhibited WORSE resistance to environmental stress cracking than rigid polyurethane alone—the opposite effect to that observed with an outer polyester layer. Example 6 Tests were conducted to investigate the effect of heat treatment and thermoforming conditions on the performance of devices made from the sheets. Three sheets (2A, 2B, and 2C) of test material 2 (three layers: polyester, polyurethane, polyester) were dried at 60 °C under vacuum for 12 hours. The three samples were placed in moisture-barrier bags and subjected to the heat treatment and thermoforming conditions shown in Table 7. Sample 2A was held at 22 °C, and samples 2B and 2C were annealed at 100 °C for 24 hours. The samples were then thermoformed to produce flat sheets using different thermoforming temperatures. Samples 2A and 2B were thermoformed at a temperature below the upper end of the polyurethane melting range, while 2C was thermoformed at a temperature above the polyurethane melting point. Table 7. Effect of heat treatment / thermoforming conditions on retained stress Test samples were cut from the thermoformed samples, analyzed by DSC, and subjected to stress relaxation tests at 37 °C in water. The DSC showed that the melting point and heat of fusion of the samples increased with annealing to 100 °C, and that thermoforming reduced both the amount of heat of fusion and the melting range. However, the sample thermoformed below the upper melting range of polyurethane retained more crystallinity and performed better in the stress relaxation test. The conditions for sample 2B in Table 7 were used to fabricate a dental appliance. Example 7 Additional compositions can be obtained by selecting suitable layer materials that exhibit differences in modulus and elasticity, as shown in Table 8. Table 8. Example multilayer sheet materials Example 8 A 2 mm thick sheet was prepared by laminating two outer films of 0.250 mm thick polypropylene homopolymer (Blue Ridge Films BFI 3270, modulus 1, 200 MPa) and a 1.50 mm thick inner layer of ethylene-propylene microcrystalline elastomer (Noito PN 2070, Mitsui Chemical), modulus 150 MPa. The sheet was cut to produce a 125 mm diameter disc, thermoformed onto a model of an individual's maxillary teeth, and trimmed to create a high-impact resistant sports mouthguard. Surprisingly, the mouthguard provides better impact protection and comfort than a conventional device made from 4 mm thick ethylene-vinyl acetate copolymer marketed by Dreve under the trade name Drufosoft. Example 9 An aligner was fabricated by thermoforming a three-layer sheet onto a tooth model. The two outer layers consisted of a rigid polyurethane with a glass transition temperature (Tg) of approximately 120 °C, and the inner layer (B) consisted of a Shore A 85 aromatic polyether polyurethane with a hard-block melting point of 160–195 °C and a heat of fusion of 8 J / gram. The appliance was annealed at 100 °C for 24 hours, which is below the Tg of the outer layer. No deformation was observed. Tests showed that this appliance was more elastic and exhibited less creep under load than the one prior to annealing at 100 °C. This improvement is believed to be due to an enhanced microstructure of the polyurethane elastomer. In a second test, a comparison was made between a multilayer device and a single-layer device, where Zendura A materials were used as the A / C material or the A / B / C material, respectively. The devices were annealed at 90 °C for 24 hours. It was observed that the single-layer device deformed extensively, while the multilayer device maintained its shape. It was hypothesized that in the multilayer device, the elastomer maintains a stabilizing force on the more rigid material during annealing to prevent unwanted dimensional changes. Additional references 1001: Displacement force 1002: Restoration Force 1003: Translational force of layers A and B for TPU B layers of three different hardnesses 1004: Restoration force between layers A and C with elastomer layer B of different hardness, over time 1005: Force retained at 5% tension

Claims

1. A polymer sheet composition, characterized in that it comprises: at least two outer layers A and C and an intermediate layer B, wherein layers A and C individually comprise a thermoplastic polymer having a tensile modulus of approximately 1000 MPa to 2500 MPa and a glass transition temperature and / or melting point of approximately 80°C to 180°C, wherein the intermediate layer B comprises one or more elastomer(s) having a tensile modulus of approximately 50 MPa to approximately 500 MPa and a glass transition temperature and / or melting point of approximately 90°C to approximately 220°C, wherein layers A and C comprise a copolyester, and wherein the intermediate layer B preferably comprises a polyurethane, in particular an aromatic polyester polyurethane. 2.A composition according to claim 1, characterized in that the intermediate layer B comprises a polyurethane having a Shore hardness of approximately A80 to D75, A85 to D65, or A90 to D55.

3. A composition according to claim 1, characterized in that the combined thickness of layers A, B, and C is from approximately 250 micrometers to approximately 2000 micrometers.

4. A composition according to claim 3, characterized in that the combined thickness of layers A and C is from 200 micrometers to 600 micrometers.

5. A composition according to claim 1, characterized in that layers A, B, and C are each approximately 250 micrometers thick.

6. Composition according to claim 1, characterized in that it has a flexural modulus of approximately 250 MPa to approximately 2000 MPa, of approximately 750 MPa to approximately 2000 MPa, or of approximately 750 MPa to approximately 1500 MPa. 7.Composition according to claim 1, wherein layers A and C exhibit a lateral restoring force of less than 100 N per cm², 50 N per cm², 25 N per cm², or 10 N per cm² when offset from each other by 0.05 mm to 0.1 mm.

8. Composition according to claim 1, characterized in that the interlayer detachment strength between layers A and C and layer B is greater than 50 N per 2.5 cm.

9. Dental appliance adaptable to one or more teeth, characterized in that it comprises a polymer sheet composition according to any one of claims 1 to 8.

10. Dental appliance according to claim 9 for moving teeth, characterized in that it is for or adapted for moving teeth or for retaining teeth in an existing position.

11. Dental appliance according to claim 9, characterized in that it is an orthodontic, prosthetic, retention, therapeutic, or habit-modifying appliance.