BACKGROUND

[001] Orthodontic treatments involve repositioning misaligned teeth and improving bite configurations for improved cosmetic appearance and dental function. Repositioning teeth is accomplished by applying controlled forces to the teeth of a patient over an extended treatment period.

[002] Teeth may be repositioned by placing a dental appliance such as a polymeric incremental position adjustment appliance, generally referred to as an orthodontic aligner or an orthodontic aligner tray, over the teeth of the patient. The orthodontic alignment tray includes a polymeric shell with a plurality of cavities configured for receiving one or more teeth of the patient. The individual cavities in the polymeric shell are shaped to exert force on one or more teeth to resiliently and incrementally reposition selected teeth or groups of teeth in the upper or lower jaw. A series of orthodontic aligner trays are provided for wear by a patient sequentially and altematingly during each stage of the orthodontic treatment to gradually reposition teeth from misaligned tooth arrangement to a successive more aligned tooth arrangement until a desired tooth alignment condition is ultimately achieved.

Once the desired alignment condition is achieved, an aligner tray, or a series of aligner trays, may be used periodically or continuously in the mouth of the patient to maintain tooth alignment. In addition, orthodontic retainer trays may be used for an extended period to maintain tooth alignment following the initial orthodontic treatment.

[003] A stage of an orthodontic treatment may require that a polymeric orthodontic retainer or aligner tray remain in the mouth of the patient for up to 22 hours a day, over an extended treatment time period of days, weeks or even months.

[004] Multilayer polymer constructions have been proposed for use in forming dental appliances, particularly through thermoforming procedures. For instance, US Patent No. 9,655,693 (Li et. al), features an appliance including a hard polymer layer having a hard polymer layer elastic modulus disposed between a first soft polymer layer having a first soft polymer layer elastic modulus and a second soft polymer layer having a second soft polymer layer elastic modulus.

[005] US 10,549,511 (Stewart et al.) features polymeric sheet compositions useful for making dental appliances having outer layers comprised of a hard material having a modulus of from about 1,000 MPA to 2,500 MPA and an inner core comprised of soft, elastomeric material or materials having a modulus of from about 50 MPa to 500 MPa.

[006] International Publication No. W02020141440 (Liu et al.) features films and appliances including at least 3 alternating polymeric layers ABA, wherein layer A includes a thermoplastic polymer A, layer B includes a thermoplastic polymer B, and the thermoplastic polymer B is different from the thermoplastic polymer A. Each of the thermoplastic polymers A and B have a flexural modulus of about 1.0 GPa to about 4.0 GPa; and each of the thermoplastic polymers A and B have a glass transition temperature (Tg) greater than about 40 °C.

[007] International Publication No. WO2020225657 (Yu et al.) features films and appliances including an interior region with a core layer of a first thermoplastic polymer with a thermal transition temperature of about 70 °C to about 140 °C and a flexural modulus greater than about 1.3 GPa, and first and second interior layers of a second thermoplastic polymer with a glass transition temperature of less than about 0 °C and a flexural modulus less than about 1 GPa; and first and second exterior layers of a third thermoplastic polymer with a thermal transition temperature of about 70 °C to about 140 °C and a flexural modulus greater than about 1.3 GPa.

[008] A need remains for improved polymer films suitable for use in dental and orthodontic applications.

SUMMARY

[009] The present disclosure is directed to orthodontic dental appliances configured to move or retain the position of teeth in an upper or lower jaw of a patient such as, for example, an orthodontic aligner tray or a retainer tray. An orthodontic dental appliance made from a relatively stiff polymeric material selected to effectively exert a stable and consistent repositioning force against the teeth of a patient can cause discomfort when the dental appliance repeatedly contacts oral tissues or the tongue of a patient over an extended treatment time. In addition, the warm and moist environment in the mouth can cause the polymeric materials in the dental appliance to absorb moisture and swell, which can compromise the mechanical tooth-repositioning properties of the dental appliance. These compromised mechanical properties can reduce tooth repositioning efficiency and undesirably extend the treatment time required to active a desired tooth alignment condition. Further, in some cases, repeated contact of the exposed surfaces of the dental appliance against the teeth of the patient can prematurely abrade the exposed surfaces of the dental appliance and cause discomfort.

[0010] Dental appliances such as orthodontic aligner and retainer trays can be manufactured by thermoforming a polymeric film to provide a plurality of tooth-retaining cavities therein. In some cases, the thermoforming process can thin regions of a relatively rigid polymeric film selected to efficiently apply tooth repositioning force over a desired treatment time. This undesirable thinning can cause localized cracking of the thermoformed dental appliance when the patient repeatedly places the dental appliance over the teeth.

[0011] High modulus polymeric materials, such a polyester and copolyester, can have poor stress retention behavior in a hydrated state when used in an oral or other aqueous environment to provide an adequate level of force persistence. Force persistence can be considered in tandem with stress relaxation, with the persistence an inverse of relaxation and defined as 100% minus %stress relaxation (e.g., a stress relaxation of 25% equates to a force persistence of 75%). A rubberier elastomer, such as certain copolyester ethers, can have better stress retention behavior, but in many cases may be too soft to be used alone in a dental appliance to effectively move teeth into a desired alignment condition in a reasonably short treatment time. Multilayer constructions of high modulus polymeric materials and rubberier elastomers can provide improved force persistence relative to the single material compositions but may still suffer from material degradation during wear and defects during manufacturing.

[0012] The present inventors discovered, quite by surprise, that a multilayer film made primarily from certain rigid polyesters demonstrates clinically acceptable force persistence, even comparing favorably to appliance constructions including elastomeric or “soft” layers. The films may lack any layer having flexural modulus lower than about 1.5 GPa, despite such materials commonly thought to be required for adequate force persistence. Moreover, the multilayer polymeric films are uncommonly receptive to laser marking, aiding in the dissemination of treatment information to the patient, the treating professional, and the manufacturer.

[0013] In general, the present disclosure is directed to a multi-layered dental appliance such as, for example, an orthodontic aligner tray or retainer tray, that includes multiple rigid polymeric layers. In one embodiment, the dental appliance includes at least two thermoplastic polymers selected to provide maintain a substantially constant stress profile over an extended treatment time and provide a relatively constant tooth repositioning force over the treatment time to maintain or improve tooth repositioning efficiency, without prematurely cracking from repeated placement on the teeth of a patient. The present disclosure is accordingly directed to dental appliances such as, for example, an orthodontic aligner tray or retainer tray, that include alternating rigid layers of polymers to provide aesthetic optical properties while maintaining an acceptable if not improved level of force persistence. In some embodiments, the multilayered dental appliance is transparent or translucent, and has enhanced crack resistance and force persistence, good staining resistance, improved patient comfort and improved dimensional stability.

[0014] In some embodiments, additional optional polymer layers in the dental appliance are also included to improve or maintain other beneficial properties of the dental appliance including, but not limited to, one or any combination of the following: hydration blocking, stain resistance, feel against the oral tissues of the patient, or cosmetic properties such as at least one of transparency and haze. [0015] If the dental appliance is thermoformed from a substantially flat sheet of a multi-layered polymeric film, the multi-layered polymeric film can further optionally include rheological modifying layers with polymeric materials selected to reduce thinning caused by the drawdown during the thermoforming process, which can improve durability of the thermoformed dental appliance over a desired treatment time in the mouth of the patient. The multi-layered polymeric film can also include polymeric layers selected to enhance or maintain release from the mold used during thermoforming. [0016] In one aspect, the present disclosure is directed to a dental appliance for positioning a patient's teeth, which includes a polymeric shell with a plurality of cavities for receiving one or more teeth. The polymeric shell includes at least 3 alternating polymeric layers ABA, wherein layer A includes a thermoplastic polymer A, layer B includes a thermoplastic polymer B, and the thermoplastic polymer B is different from the thermoplastic polymer A. Each of the thermoplastic polymers A and B have a flexural modulus of about 1.5 GPa to about 4.0 GPa; and each of the thermoplastic polymers A and B have a glass transition temperature (T _g ) greater than about 40 °C.

[0017] In another aspect, the present disclosure is directed to method of making a dental appliance, in which a plurality of tooth-retaining cavities are formed in a multilayered polymeric fdm. The multilayered polymeric fdm includes at least 3 alternating polymeric layers ABA, wherein layers A includes a thermoplastic polymer A, layer B includes a thermoplastic polymer B, and the thermoplastic polymer B is different from the thermoplastic polymer A. Each of the thermoplastic polymers A and B have a flexural modulus of about 1.5 GPa to about 4.0 GPa; and each of the thermoplastic polymers A and B have a glass transition temperature (T _g ) greater than about 40 °C. In presently preferred implementations, the polymer A has a higher flexural modulus than the polymer B. [0018] In another aspect, the present disclosure is directed to a method of orthodontic treatment that includes positioning a dental appliance around one or more teeth. The dental appliance includes a polymeric shell with a first major surface having a plurality of cavities for receiving one or more teeth, wherein the cavities are shaped to cover at least some of a patient's teeth and apply a corrective force thereto. The polymeric shell includes at least 3 alternating polymeric layers ABA, wherein layers A includes a thermoplastic polymer A, layer B includes a thermoplastic polymer B, and the thermoplastic polymer B is different from the thermoplastic polymer A. Each of the thermoplastic polymers A and B have a flexural modulus of about 1.5 GPa to about 4.0 GPa; and each of the thermoplastic polymers A and B have a glass transition temperature (T _g ) greater than about 40 °C. [0019] In the above or other aspects, the polymer A is a polycyclohexylenedimethylene terephthalate (PCT) or a poly cyclohexylenedimethylene terephthalate glycol (PCTg). The polymer B is a copolyester comprising terephthalic acid and/or isophthalic acid, cyclohexane dimethanol, and 2, 2, 4, 4- tetramethyl- 1,3 -cyclobutanediol. A suitable polymer B is 2, 2, 4, 4-tetramethyl- 1,3 -cyclobutanediol modified poly(l,4-cyclohexylenedimethylene terephthalate).

[0020] The term “polyester”, as used herein, includes “copolyesters” and means a synthetic polymer prepared by the polycondensation of one or more difimctional carboxylic acids with one or more difimctional hydroxyl compounds.

[0021] The term “polyester elastomer”, as used herein, means a polyester having a modulus of about 1 to 50 megapascals (MPa) (at room temperature). [0022] The term “residue”, as used herein, means any organic structure incorporated into a polymer or plasticizer through a polycondensation reaction involving the corresponding monomer.

[0023] The term “dicarboxylic acid”, as used herein, means dicarboxylic acids and any derivative of a dicarboxylic acid, including its associated acid halides, esters, half-esters, salts, half-salts, anhydrides, mixed anhydrides, or mixtures thereof, useful in a polycondensation process with a diol to make a high molecular weight polyester.

[0024] As used herein, “dental appliance” means any device capable of influencing the position, orientation, or composition of the teeth, including by way of example only, aligners, positioners, night guards, retainers, splints, bleaching trays, and anterior bridges.

[0025] As used herein as a modifier to a property or attribute, the term “generally”, unless otherwise specifically defined, means that the property or attribute would be readily recognizable by a person of ordinary skill but without requiring absolute precision or a perfect match (e.g., within +/- 20 % for quantifiable properties). The term “substantially”, unless otherwise specifically defined, means to a high degree of approximation (e.g., within +/- 10% for quantifiable properties) but again without requiring absolute precision or a perfect match. Terms such as same, equal, uniform, constant, strictly, and the like, are understood to be within the usual tolerances or measuring error applicable to the particular circumstance rather than requiring absolute precision or a perfect match.

[0026] The above summary of the present disclosure is not intended to describe each disclosed embodiment or every implementation of the present invention. The description that follows more particularly exemplifies illustrative embodiments. In several places throughout the application, guidance is provided through lists of examples, which examples can be used in various combinations. In each instance, the recited list serves only as a representative group and should not be interpreted as an exhaustive list.

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a schematic overhead perspective view of an embodiment of a multilayered dental appliance.

[0028] FIG. 2 is a schematic, cross-sectional view of an embodiment of a multilayered dental appliance of FIG. 1.

[0029] FIG. 3 is a schematic, cross-sectional view of an embodiment of a multilayered dental appliance of FIG. 1.

[0030] FIG. 4 is a schematic, cross-sectional view of an embodiment of a multilayered dental appliance of FIG. 1.

[0031] FIG. 5 is a schematic overhead perspective view of a method for using a dental alignment tray by placing the dental alignment tray to overlie teeth. [0032] FIG. 6 is a graphical representation of Normalized Bending Stress over Time for certain Exemplary and Comparative fdms.

[0033] Like symbols in the drawings indicate like elements. While the above-identified figures set forth several embodiments of the disclosure, other embodiments are also contemplated, as noted in the description. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention.

DETAILED DESCRIPTION

[0034] A dental appliance such as an orthodontic appliance 100 shown in FIG. 1, which is also referred to herein as an orthodontic aligner tray, includes a thin polymeric shell 102 having a plurality of cavities 104 shaped to receive one or more teeth in the upper or lower jaw of a patient. In some embodiments, in an orthodontic aligner tray the cavities 104 are shaped and configured to apply force to the teeth of the patient to resiliently reposition one or more teeth from one tooth arrangement to a successive tooth arrangement. In the case of a retainer tray, the cavities 104 are shaped and configured to receive and maintain the position of one or more teeth that have previously been aligned.

[0035] The shell 102 of the orthodontic appliance 100 is an elastic polymeric material that generally conforms to a patient's teeth, and may be transparent, translucent, or opaque. In the embodiment of FIG. 1, the shell 102 includes at least three alternating polymeric layers in an ABA arrangement. The polymeric layers include polymers A and B, wherein A and B are different thermoplastic polymeric materials. The thermoplastic polymers A and B are selected to provide maintain a sufficient and substantially constant stress profile during a desired treatment time, and to provide a relatively constant tooth repositioning force over the treatment time to maintain or improve the tooth repositioning efficiency of the shell 102.

[0036] In the embodiment of FIG. 1, a polymeric layer 110 forms an external surface 106 of the shell 102, a polymeric layer 112 forms an internal surface 108 of the shell 102, and a polymeric layer 114 resides between the polymeric layers 110 and 112. The polymeric layers 110, 112, 114 each include layers of a thermoplastic polymeric material A or B. The thermoplastic polymeric materials in the layers 110, 112, 114 are arranged to alternate such as, for example, in the arrangement ABA or BAB, with the letter A or B designating the identity of the thermoplastic polymer making up the layer. For example, in the embodiment of FIG. 1, the layer 110 can include polymer A, the layer 114 can include polymer B, and the layer 112 can include polymer A. Or, the layer 110 can include polymer B, the layer 114 can include polymer A, and the layer 112 can include polymer B. In presently preferred implementations, the outermost polymeric layers 110, 112 includes polymer A, while the interior (i.e., core) layer 114 includes a polymer B. Any or all of the layers 110, 112, 114 may include a number of sublayers, with each sublayer of the greater layer 110, 112, 114 including the same thermoplastic polymer A or B.

[0037] Each of the thermoplastic polymers A and B have a flexural modulus of about 1.5 GPa to about 4.0 GPa, and a glass transition temperature (Tg) greater than about 40 °C. In addition, the thermoplastic polymers A and B can be selected to provide particular properties to the shell 102 including, but not limited to, impressive force persistence, resistance to moisture absorption, resistance to staining, desired optical properties such as, for example, color, visible light transmission, and haze, ease of release from a thermoforming mold used to form the cavities 104, and resistance to cracking following repeated placement over the teeth of the patient.

[0038] In some embodiments, the polymeric shell 102 has an overall flexural modulus necessary to move the teeth of a patient. In some embodiments, the polymeric shell 102 has an overall flexural modulus of greater than about 1.5 GPa, or about 1.6 GPa, or about 1.7 GPa to about 1.9 GPa, or about 1.6 GPa to about 1.8 GPa.

[0039] In various embodiments, each of the polymers A and B have a flexural modulus of about 1.5 GPa to about 3 GPa, or about 2.0 GPa to about 2.5 GPa. In some embodiments, the flexural modulus of a polymer A, B in a layer 110, 112, 114 in the dental appliance 100 is no greater than twice the flexural modulus of the polymer present in an adjacent layer. The flexural modulus can be tested according to ASTM D790-17, as in the Examples below.

[0040] In various embodiments, each of the polymers A and B have a Tg of about 50 °C to about 200 °C, or about 70 °C to about 170 °C, or about 75 °C to about 150 °C. In some embodiments, the difference between the Tg of the polymers A, B in any two adjacent layers 110, 112, 114 in the dental appliance 100 is not greater than about 70 °C. Glass transition temperature can be determined by either standard differential scanning calorimetry or modulated differential scanning calorimetry (DSC and/or MDSC™).

[0041] In some embodiments, the layer 110 on the outer major surface 106 of the dental appliance 100 and the layer 112 on the inner surface 108 include the same polymeric layer A or B. In other embodiments, the layer 110 on the outer major surface 106 of the dental appliance 100 and the layer 112 on the inner surface 108 include different polymeric layers A or B.

[0042] In some embodiments, the polymers A and B in the each of the layers 110, 112, 114 of the polymeric shell 102 are polyesters or co-polyesters. In some embodiments, the polymer A is a polyester and the polymer B is a co-polyester.

[0043] In various embodiments, the polymer A is chosen from polycyclohexylenedimethylene terephthalate (PCT), polycyclohexylenedimethylene terephthalate glycol (PCTg), polyethylene terephthalate glycol (PETg), poly(l,4 cyclohexylenedimethylene) terephthalate (PCTa), and mixtures and combinations thereof. Suitable PETg, PCT, PCTa, and PCTg resins can be obtained from various commercial suppliers such as, for example, Eastman Chemical, Kingsport, TN; SK Chemicals, Irvine, CA; DowDuPont, Midland, MI; Pacur, Oshkosh, WI; and Scheu Dental Tech, Iserlohn, Germany. For example, EASTAR VM318 and MN006 PCTg resins from Eastman Chemical have been found to be particularly suitable.

[0044] In various embodiments, the polymer A is a polyester blend having at least 50% by weight of PCT or PCTg (i.e., the primary polyester in the blend). The term “polyester blend,” as used herein, means a physical blend of at least 2 different polyesters. Typically, polyester blends are formed by blending the polyester components in the melt phase. Polyester blends may be synthesized via condensation polymerization, melt polymerization, solid-state polymerization, or combinations thereof.

[0045] The PCT or PCTg (or combination thereof) is typically present in the blend at least 60%weight, based on the total weight of the blend. In the same or other embodiments, the PCT or PCTg is present in the blend at least 65 %weight, at least 75 %weight, at least 80 %weight, at least 85%weight, and at least 90%weight, based on the total weight of the polyesters in the blend. In blends for use in typical dental applications, the range of PCTg or PCT in the blend is about 50 to 95 % weight, based on the total weight of the blend. Suitable polyester for blending with the first polyester include PCTT copolyesters as described below. The polyester blend preferably maintains the rigid and non-elastomeric nature of the primary polyester.

[0046] In various embodiments, the polymer B comprises a co-polyester comprising terephthalic acid and/or isophthalic acid, cyclohexane dimethanol, and 2, 2, 4, 4-tetramethyl- 1,3 -cyclobutanediol. Such a copolyester can include a dicarboxylic acid component comprising 70 mole % to 100 mole % of terephthalic acid residues, and a diol component comprising, (i) 0 to 95% ethylene glycol, (ii) 5 mole % to 50 mole % of 2,2,4, 4-tetramethyl-l,3-cyclobutanediol residues, and (ii) 50 mole % to 95 mole % 1,4-cyclohexanedimethanol residues, and (iii) 0 to 1% of a polyol having three or more hydroxyl groups, wherein the sum of the mole % of diol residues (i) and (ii) and (iii) amounts to 100 mole % and the copolyester exhibits a glass transition temperature Tg from 80° C. to 150° C. A suitable copolyester for use as the polymer B is 2,2,4,4-tetramethyl-l,3-cyclobutanediol modified poly(l,4- cyclohexylenedimethylene terephthalate) (PCTT) as further explored in US 9,2373,206 (Neill et al.), and is commercially available under the TRITAN brand from Eastman Chemical, Kingsport, TN. [0047] In various embodiments, the polymer B is a polyester blend having at least 50% by weight of PCTT (the primary polyester in polymer B). The PCTT is typically present in the blend at least 60%weight, based on the total weight of the blend. In the same or other embodiments, the PCTT is present in the blend at least 65 %weight, at least 75 %weight, at least 80 %weight, at least 85%weight, and at least 90%weight, based on the total weight of the polyesters in the blend. In blends for use in typical dental applications, the range of PCTT in the blend is about 50 to 95 % weight, based on the total weight of the blend. Suitable polyester for blending with the primary polyester include the polyesters (e.g., PCTg, PCTa, PCT) of polymer A as described above. [0048] PCTT may also be blended with semi-crystalline, copolyester ether elastomers (e.g., poly(l,4- cyclohexanedimethylene 1,4-cyclohexanedicarboxylate) (PCCE), as well as PCCE modified with polytetramethylene ether glycol. In such embodiments, the PCTT is typically present in the blend at a concentration of at least 85%weight, and at least 90%weight, based on the total weight of the polyesters in the blend, such that the blend retains a modulus above 1.5 GPa. Techniques for blending polyesters, along with exemplary polyester blends can be found, for example, in commonly owned, co-pending International Application No., PCT/IB2023/052933, entitled “Miscible Polyester Blends Suitable For Dental Appliances and Methods for Forming The Same”.

[0049] In one presently preferred embodiment, the polymer A is PCTg and the polymer B is PCTT. In the same or other embodiments, the polymer A is miscible with the polymer B. The term “miscible,” as used herein, is synonymous with the term “homogeneous” and means that a hypothetical blend of the two polymers A and B has a single, homogeneous phase as indicated by a single, composition-dependent glass transition temperature (abbreviated herein as “Tg”) as determined by either standard differential scanning calorimetry or modulated differential scanning calorimetry (DSC and/or MDSC™). Suitable polymers A and B individually possess sufficiently different glass transition temperatures such that the presence of a single Tg in the blend is a reasonable proxy for miscibility. Without wishing to be bound by theory, the miscibility of the polymers enhances interfacial adhesion between adjacent polymer layers and which in turn can enhance Force Persistence.

[0050] In exemplary and presently preferred embodiments of a film with an ABA or BAB layering configuration, the outer layers each have a thickness of about 15 microns to 100 microns, with the core layer having a thickness of about 300 microns to about 800 microns. Accordingly, the core layer typically constitutes greater than 60% of the total thickness of the film. The outer layers, when arranged in multilayer sheets of the present disclosure, can have a combined thickness of 25 microns to about 1000 microns, 50 microns to 750 microns, 100 to 750 microns, 250 microns to 750 microns, or 250 microns to about 600 microns. The interior layer, in the same or other embodiments, can have a thickness of from about 100 microns to about 1000 microns or 200 to 900 microns, or 300 to 800. [0051] An exemplary construction consistent with the above includes an ABA layer construction, where the outer layers (i.e., first and third layers) each include a thermoplastic polymer A having a flexural modulus of about 1.5 GPa to about 3 GPa, or about 2.0 GPa to about 2.5 GPa, a Tg of about 50 °C to about 200 °C, or about 70 °C to about 170 °C, or about 75 °C to about 150 °C, and a thickness of about 15 microns to 100 microns. The core layer includes a thermoplastic polymer B having a flexural modulus of about 1.5 GPa to about 3 GPa, or about 2.0 GPa to about 2.5 GPa, a Tg of about 50 °C to about 200 °C, or about 70 °C to about 170 °C, or about 75 °C to about 150 °C, and a thickness of about 300 microns to 800 microns. [0052] The multilayer polymer materials of this disclosure are useful in creating shaped articles for multiple applications. The shaped article can be produced by any method known in the art including, but not limited to, extrusion, calendering, thermoforming, blow-molding, extrusion blow-molding, injection stretch blow-molding, injection molding, injection blow-molding, compression molding, profde extrusion, cast extrusion, melt-spinning, drafting, tentering, or blowing.

[0053] A schematic cross-sectional view of another embodiment of a dental appliance 200 is shown in FIG. 2, which includes a polymeric shell 202 with a multilayered polymeric structure. The polymeric shell 202 includes alternating packets of layers in an ABA layering configuration and includes the same layer A proximal a first major surface 220 and a second major surface 222, with the letter A or B designating the identity of the thermoplastic polymer making up the layer. The layers A and B can be selected from any of the thermoplastic polymers A and B discussed with respect to FIG. 1, which maintain a substantially constant stress profile during a treatment time, provide a relatively constant tooth repositioning force over the treatment time to maintain or improve tooth repositioning efficiency (e.g., enhanced force persistence), resist staining, resist moisture absorption, resist cracking, provide desired optical properties, and/or provide ease of release from a thermoforming mold.

[0054] In the embodiment of FIG. 2, the polymeric shell 202 further includes additional optional performance enhancing layers that can be included to improve properties of the shell 202. In various embodiments, which are not intended to be limiting, the performance enhancing layers can be, for example, barrier layers that are resistant to staining and moisture absorption; abrasion-resistant layers; cosmetic layers that may optionally include a colorant, or may include a polymeric material selected to adjust the optical haze or visible light transparency of the polymeric shell 202; tie layers that enhance compatibility or adhesion between packet of layers ABA or between any given layer A and B in each packet, elastic layers to provide a softer mouth feel for the patient; thermal forming assistant layers between packets of layers ABA or between layers A and B in each packet to enhance thermoforming, layers to enhance mold release during thermoforming, and the like.

[0055] The performance enhancing layers may include a wide variety of polymers selected to provide a particular performance benefit, but the polymers in the performance enhancing layers are generally selected from materials that are softer and more elastic that the polymers A and B. In various embodiments, which are not intended to be limiting, the performance enhancing layers include thermoplastic polyurethanes (TPU) and olefins.

[0056] In some non-limiting examples, the olefins in the performance enhancing layers are chosen from polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), cyclic olefins (COP), copolyolefins with moieties chosen from ethylene, propylene, butene, pentene, hexene, octene, C2- C20 hydrocarbon monomers with polymerizable double bonds, and mixtures and combinations thereof; and olefin hybrids chosen from olefin/anhydride, olefin/acid, olefin/styrene, olefin/acrylate, and mixtures and combinations thereof. [0057] For example, in the embodiment of FIG. 2, the polymeric shell 202 includes an optional moisture barrier layer 240 on each external surface, which can prevent moisture intrusion into the polymeric layers A and B, as well as maintain for the shell 202 a substantially constant stress profde during a treatment time. The polymeric shell 202 further includes tie or thermoforming assist layers 250, which can be the same or different, between individual layers A and B in each packet of alternating layers ABA. In some embodiments, the tie/thermoforming assist layers 250 can improve compatibility between the polymers in the layers A and B as the polymeric shell 202 is formed from a multilayered polymeric fdm or reduce delamination between layers A and B and improve the durability and crack resistance of the polymeric shell 202 over an extended treatment time. The polymeric shell 202 in FIG. 2 further includes elastic layers 260, which can be the same or different, and can be included to improve the softness or mouth feel of the shell 202. In the embodiment of FIG. 2, the elastic layers 260 are located proximal the major surfaces 220, 222 of the shell 202. [0058] In other embodiments a polymeric shell may be formed from a film that includes at least 5 alternating layers of thermoplastic polymers A and B. Such a polymeric shell can include an interior region including a core layer with a first major surface and a second major surface. The interior region further includes interior layers arranged on the first major surface and the second major surface, respectively, of the core layer. The polymeric shell further includes exterior regions on opposed sides of the interior region. The exterior regions, which may also be referred to herein as skin layers, include first and second external surface layers, which face outwardly on the exposed surfaces of the polymeric shell. In one embodiment, the first and second external surface layers, which may be the same or different, each include one or more layers of the thermoplastic polymer A utilized in the core layer. In another embodiment, the first and the second external surface layers may include at one or more layers of a thermoplastic polymer C, different from the thermoplastic polymer A, but otherwise meeting the specifications for thermoplastic polymer A above. The interior layers, which may be the same or different, each include one or more layers of a thermoplastic polymer B. The layers AB can be selected from any of the thermoplastic polymers A and B discussed above with respect to FIGS. 1- 2. Further details of ABABA, CBABC, and ABCBA constructions are found in International Publication No. WO2020225657 (Yu et al.).

[0059] A schematic cross-sectional view of another embodiment of a dental appliance 300 is shown in FIG. 4, which includes a polymeric shell 302 with a multilayered polymeric structure. The polymeric shell includes alternating layers of thermoplastic polymers AB, and includes a different layer identity proximal a first major surface 320 and a second major surface 322, with the letter A or B designating the identity of the thermoplastic polymer making up the layer. The layers AB can be selected from any of the thermoplastic polymers A and B discussed above with respect to FIGS. 1-2. [0060] In the embodiment of FIG. 3, the polymeric shell 302 includes a moisture barrier and stain resistant layer 440 on each external (i.e., outer) surface, which can prevent intrusion of moisture into the polymeric layers AB and reduce damage to the shell 302 from colored foods (for example, tea, coffee, red wine and the like). The polymeric shell 302 further includes tie or thermoforming assistant layers 350, which can be the same or different, between each packet of alternating layers AB. In some embodiments, the layers 350 can improve compatibility between the polymers in the layers AB as the polymeric shell 302 is formed from a multilayered polymeric film or reduce delamination between layers AB during the treatment time.

[0061] A schematic cross-sectional view of another embodiment of a dental appliance 400 is shown in FIG. 4, which includes a polymeric shell 402 with a multilayered polymeric structure (AB) _n , wherein n = 2 to about 500, or about 5 to about 200, or about 10 to about 100. The layers AB, which include different polymers, can be selected from any of the thermoplastic polymers A and B discussed above with respect to FIGS. 1-3, with the letter A or B designating the identity of the thermoplastic polymer making up the layer. In some non-limiting embodiments, the polymers in layers A and B are chosen from polycyclohexylenedimethylene terephthalate (PCT), polycyclohexylenedimethylene terephthalate glycol (PCTg), 2,2,4,4-tetramethyl-l,3-cyclobutanediol modified poly(l,4- cyclohexylenedimethylene terephthalate) (PCTT), and mixtures and combinations thereof.

[0062] The multilayered polymeric structure (ABA) of Fig. 2 may similarly be repeated “n” number of times, wherein n = 2 to about 500, or about 5 to about 200, or about 10 to about 100. The layers ABA, which include different polymers, can be selected from any of the thermoplastic polymers A and B discussed above with respect to FIGS. 1-3, with the letter A or B designating the identity of the thermoplastic polymer making up the layer. In some non-limiting embodiments, the polymers in layers A and B are chosen from polycyclohexylenedimethylene terephthalate (PCT), polycyclohexylenedimethylene terephthalate glycol (PCTg), 2, 2, 4, 4-tetramethyl- 1,3 -cyclobutanediol modified poly(l,4-cyclohexylenedimethylene terephthalate) (PCTT), and mixtures and combinations thereof.

[0063] Referring again to FIG. 1, in some embodiments, the polymeric shell 102 is formed from substantially transparent polymeric materials. In this application the term substantially transparent refers to materials that pass light in the wavelength region sensitive to the human eye (about 400 nm to about 750 nm) while rejecting light in other regions of the electromagnetic spectrum. In some embodiments, the reflective edge of the polymeric materials selected for the shell 102 should be above about 750 nm, just out of the sensitivity of the human eye.

[0064] In some embodiments, any or all of the layers of the polymeric shell 102 can optionally include dyes or pigments to provide a desired color that may be, for example, decorative or selected to improve the appearance of the teeth of the patient.

[0065] The orthodontic appliance 100 may be made using a wide variety of techniques. In one embodiment, a suitable configuration of tooth (orteeth)-retaining cavities are formed in a substantially flat sheet of a multilayered polymeric film that includes layers of polymeric material arranged like the configurations discussed above with respect to FIGS. 1-4. In some embodiments, the multilayered polymeric film may be formed in a dispersion and cast into a film or applied on a mold with tooth-receiving cavities. In some embodiments, the multilayered polymeric film may be prepared by extrusion of multiple polymeric layer materials through an appropriate die to form the film. In some embodiments, a reactive extrusion process may be used in which one or more polymeric reaction products are loaded into the extruder to form one or more layers during the extrusion procedure.

[0066] In some embodiments, the multilayer polymeric film may later be thermoformed into a dental appliance with tooth-retaining cavities or injected into a mold including tooth-retaining cavities. The tooth-retaining cavities may be formed by any suitable technique, including thermoforming, laser processing, chemical or physical etching, and combinations thereof, but thermoforming has been found to provide good results and excellent efficiency. In some embodiments, the multilayered polymeric film is heated prior to forming the tooth-retaining cavities, or a surface thereof may optionally be chemically treated such as, for example, by etching, or mechanically embossed by contacting the surface with a tool, prior to or after forming the cavities.

[0067] A general process for thermoforming an appliance using the polyester blend containing films of the present disclosure can share similarities with common thermoforming techniques. One, some, or all of the steps of method may be performed in a temperature and pressure-controlled chamber. At the outset, a physical, dental model of the patient’s teeth in a target or current arrangement is provided. A sheet of material including at least one layer comprised of a semi -crystalline polymer is provided and placed over the dental model. The model and the sheet of material are placed under a first pressure and heated to a first temperature. The combination of heat and pressure/or vacuum causes the material to soften. The model and sheet are maintained at the first temperature and pressure until such time as the sheet has conformed to the shape and orientation of the dental model. The temperature is subsequently decreased (preferably isobarically) to create a shell appliance in a configuration having a geometry corresponding to the dental arrangement of the first model.

[0068] In some embodiments, the polymeric film is heated to a temperature above the T _g , for example, above 120°C, about 130°C, about 140°C, during the forming process. However, various temperatures and times may be utilized. In some embodiments, the pressure applied is greater than lOkPa, e.g., greater than 50 kPa, 75 kPa, 100 kPa, 125 kPa, or greater than 150 kPa. In some embodiments, the pressure is maintained for greater than 30 seconds, e.g., greater than 45 seconds, 60 seconds, 2.5 minutes, 5.0 minutes, 10 minutes, 20 minutes, 30 minutes, 60 minutes, greater than 90 minutes, or even greater than 120 minutes, before release of pressure back to nominal atmospheric pressure. The pressure may be applied by direct force on the polymeric film material and/or vacuum. [0069] In some or all embodiments, the temperature is gradually reduced. In other embodiments, appliance may be quenched by rapid reduction in temperature. In any event, it is presently preferred that the parameters selected remain consistent for each appliance. For example, the rate of temperature reduction could be in the range of about 0.5°C to about 10°C per minute, but is typically held at the same rate within the range for each temperature reduction step in the process.

[0070] The multilayered polymeric fdm, the formed dental appliance, or both, may optionally be crosslinked with radiation chosen from ebeam, gamma, UV, and mixtures and combinations thereof. [0071] Irradiation, if used to crosslink the material, can be done at room temperature or at elevated temperatures typically below the first molding temperature. Irradiation can be performed in air, in vacuum, or in oxygen-free environment, including inert gases such as nitrogen or noble gases. Irradiation can be performed by using electron-beam, gamma irradiation, or x-ray irradiation. In some embodiments, an ionizing radiation (e.g., an electron beam, x-ray radiation or gamma radiation) is employed to crosslink the non-segmented, polymeric material. In specific embodiments, gamma radiation is employed to crosslink the substantially non-crosslinked polymeric material. In some embodiments, the irradiating (with any radiation source) is performed until the sample receives a dose of at least 0.25 Mrad (2.5 kGy), e.g., at least 1.0 Mrad (10 kGy), at least 2.5 Mrad (25 kGy), at least 5.0 Mrad (50 kGy), or at least 10.0 Mrad (100 kGy). In some embodiments, the irradiating is performed until the sample receives a dose of between 1.0 Mrad and 6.0 Mrad, e.g., between 1.5 Mrad and 4.0 Mrad.

[0072] In other embodiments, the appliance is treated to create chemical crosslinks using methods known in the art. For example, peroxides can be added to the polymer, and the polymer can be maintained at an elevated temperature after forming into the first stored geometry to allow the peroxides to react. In addition, silanes can be grafted to a polymer backbone, such as polyethylene, and the polymer can be crosslinked upon exposure to a hot, humid environment.

[0073] The thickness of the multilayer polymer film is chosen to provide a clinically appropriate thickness of the material in the resultant appliance. The thickness of the material should typically be selected such that the appliance is stiff enough to apply sufficient force to the teeth but remains thin enough to be comfortably worn. In various embodiments, the multilayered polymeric film used to form the dental appliance has a thickness of less than about 1 mm, or less than about 0.8 mm, or less than about 0.5 mm. The thickness of the walls of the resulting appliance may be between 0.05 mm and 2 mm, or between 0.1 mm and 1 mm.

[0074] In various embodiments, the dental appliance is substantially optically clear. The Expected light transmission can be determined by ISO 13468-1:2019 or ASTM D1003-13 using CIE illuminate C and the Expected haze can be determined using ISO 14782-1: 1999 or ASTM DI 003- 13 using CIE illuminate C. The term “Expected” is used herein to indirectly represent the transmission and haze of a formed appliance, as the geometry (e.g., size and surface features) of the appliance is not conducive to direct testing. Instead, a representative polymeric film is subjected to the same temperature and processing conditions as would normally be used to create the appliance but without drawing the film down on a mold, allowing the film to remain sufficiently planar for subsequent testing.

[0075] Some embodiments have an Expected light transmission of at least about 50%. Some embodiments have an expected light transmission of at least about 75%. Some embodiments have an Expected haze of no greater than 15 or no greater than 10%. Some embodiments have an Expected haze of no greater than 5%. Some embodiments have an Expected haze of no greater than 2.5%. The Expected haze of dental appliance of certain presently preferred embodiments is less than 10% and the Expected light transmission of dental appliance is greater than 80%.

[0076] In some embodiments, the multilayered polymeric film may be manufactured in a roll-to- roll manufacturing process and may optionally be wound into a roll until further converting operations are required to form one or more dental appliances.

[0077] The orthodontic articles made from polymeric films of the present disclosure can exhibit a percent loss of relaxation modulus of 50% or less as determined by Dynamic Mechanical Analysis (DMA) of either the film used to create the appliance or the appliance itself, which equate to a Force Persistence of 50% or greater. While not wishing to be bound by theory, it is believed that a dental appliance thermoformed from a film having a Force Persistence of 50% or greater will also demonstrate a Persistence of 50% or greater. The DMA procedure for film samples is described in detail in the Examples below. The loss is determined by comparing the initial relaxation modulus to the (e.g., 4 hour) relaxation modulus at 37 °C and 1% strain. It was discovered that films according to at least certain embodiments of the present disclosure exhibit a smaller loss in relaxation modulus (and thus greater Force Persistence) than articles made of different materials that purport to offer enhanced orthodontic performance. In presently preferred embodiment, a film or orthodontic article exhibits loss of relaxation modulus after hydration of 50% or less, 40% or less, 38% or less, 36% or less, 34% or even 32% or less. In some embodiments, the loss of relaxation modulus is at least 15%, 20%, or 25% or greater.

[0078] Referring now to FIG. 5, a shell 502 of an orthodontic appliance 500 includes an outer surface 508 and an inner surface 508 with cavities 504 that generally conform to one or more of a patient's teeth 600, and an external surface 506. In some embodiments, the cavities 504 are slightly out of alignment with the patient's initial tooth configuration, and in other embodiments the cavities 504 conform to the teeth of the patient to maintain a desired tooth configuration. In some embodiments, the shell 502 may be one of a group or a series of shells having substantially the same shape or mold, but which are formed from different materials to provide a different stiffness or resilience as need to move the teeth of the patient. In this manner, in one embodiment, a patient or a user may alternately use one of the orthodontic appliances during each treatment stage depending upon the patient's preferred usage time or desired treatment time period for each treatment stage. [0079] No wires or other means may be provided for holding the shell 502 over the teeth 600, but in some embodiments, it may be desirable or necessary to provide individual anchors on teeth with corresponding receptacles or apertures in the shell 502 so that the shell 502 can apply a retentive or other directional orthodontic force on the tooth which would not be possible in the absence of such an anchor.

[0080] The shells 502 may be customized, for example, for day time use and night time use, during function or non-fiinction (chewing vs. non-chewing), during social settings (where appearance may be more important) and nonsocial settings (where the aesthetic appearance may not be a significant factor), or based on the patient's desire to accelerate the teeth movement (by optionally using the more stiff appliance for a longer period of time as opposed to the less stiff appliance for each treatment stage).

[0081] For example, in one aspect, the patient may be provided with a clear orthodontic appliance that may be primarily used to retain the position of the teeth, and an opaque orthodontic appliance that may be primarily used to move the teeth for each treatment stage. Accordingly, during the daytime, in social settings, or otherwise in an environment where the patient is more acutely aware of the physical appearance, the patient may use the clear appliance. Moreover, during the evening or nighttime, in non-social settings, or otherwise when in an environment where physical appearance is less important, the patient may use the opaque appliance that is configured to apply a different amount of force or otherwise has a stiffer configuration to accelerate the teeth movement during each treatment stage. This approach may be repeated so that each of the pair of appliances are alternately used during each treatment stage.

[0082] Referring again to FIG. 5, an orthodontic treatment system and method includes a plurality of incremental position adjustment appliances, each formed from the same or a different material, for each treatment stage of orthodontic treatment. The orthodontic appliances may be configured to incrementally reposition individual or multiple teeth 600 in an upper or lower jaw 602 of a patient. In some embodiments, the cavities 504 are configured such that selected teeth will be repositioned, while other teeth will be designated as a base or anchor region for holding the repositioning appliance in place as the appliance applies the resilient repositioning force against the tooth or teeth intended to be repositioned.

[0083] Placement of the elastic positioner 502 over the teeth 600 applies controlled forces in specific locations to gradually move the teeth into the new configuration. Repetition of this process with successive appliances having different configurations eventually moves the teeth of a patient through a series of intermediate configurations to a final desired configuration.

[0084] The devices of the present disclosure will now be further described in the following nonlimiting examples. EXAMPLES

[0085] The following Examples are merely for illustrative purposes and are not meant to be overly limiting on the scope of the appended claims. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the present disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0086] Unless otherwise noted, all parts, percentages, ratios, and the like in the Examples and the rest of the specification are provided on the basis of weight. Solvents and other reagents used may be obtained from Sigma-Aldrich Chemical Company (Milwaukee, WI) unless otherwise noted.

Materials

PETg: from Eastman Chemicals, Kingsport, TN, grade: EASTAR GN071

PCTg: from Eastman Chemicals, grade: VM318

PCTT: copolyester from Eastman Chemicals, grade: TRITAN MX710

TPE: polyester elastomer from Celanese, grade: HYTREL 8238 PCTT*: copolyester from Eastman Chemicals, grade: TRITAN FX150 TPU: thermoplastic polyurethane from Covestro AG, grade: TEXIN 970DU

Properties of Selected Polyesters for Lavers A and B

[0087] Properties of some of the polymeric materials used in the examples below are shown in Table 1.

Table 1

Summary of Test Procedures

[0088] The following test procedures were used in the examples below.

Flexural Modulus and Elongation at Break

[0089] The flexural modulus can be tested according to ASTM D790-17 and tensile properties by ASTM D638-14. The specimen made by die cutting was placed in the grips of a universal testing machine. The stress-strain curve was then utilized to determine the modulus and elongation at break.

Procedure 1 Stress Relaxation by Dynamic Mechanical Analyzer (DMA)

[0090] A rectangular sample was tested by single cantilever bending in a TA Instruments DMA 850 machine (New Castle, DE) enclosed with an environmental chamber kept at 37 °C and 95% relative humidity. Stress relaxation was monitored after applying 1% strain. The testing time was about 8 hours. The stress relaxation is determined by comparing the initial relaxation modulus or stress to the 8 hour relaxation modulus or stress respectively (e.g., a stress relaxation of 25% equals to a Force Persistence of 75%).

Procedure 2 Flexural Modulus Measurement by Dynamic Mechanical Analyzer (DMA)

[0091] Flexural modulus was measured by single cantilever bending in a TA Instruments DMA 850 machine (New Castle, DE) at room temperature with a dynamic displacement amplitude of 30 microns under a constant frequency of 1 Hz. Procedure 3 Laser Marking Experiment

[0092] Laser marking experiment was performed using a laser marking machine with an 8 Watt, 355 nm UV laser. Focal point depth and laser power were varied to generate laser marked samples for visual rating of laser marking performance. The visual rating can be premised on a 1-5 scale, with a 5 demonstrating superior resolution and contrast, and a 1 exhibiting poor resolution and contrast.

Example 1

[0093] A 3 -layer ABA (PCTg/PCTT/PCTg) film was extruded using a pilot scale coextrusion line equipped with a feedblock and film die. The skin layer (A) extruder was fed with the first rigid resin, PCTg VM318. The skin layer (A) extrusion temperature was controlled at 516 °F. The throughput was 3.52 Ibs/hr. The core layer (B) extruder was fed with a second rigid resin, Tritan MX710 and the extrusion temperature was controlled at 585 °F. The core layer extrusion throughput was 21.97 Ibs/hr. The extruded sheet was chilled on a cast roll. The overall sheet thickness was controlled at 30 mils. This sample has a flexural modulus of 1.685 GPa by Procedure 2 and the 8 hour stress relaxation of 28.8% (Force Persistence - 71.2%) by Procedure 1. The film exhibited a 5 rated laser marking appearance.

Example 2

[0094] A 5 -layer ABABA (PCTg/PCTT/PCTg/PCTT/PCTg) film was extruded using a pilot scale coextrusion line equipped with a feedblock and film die. The skin layer (A) extruder was fed with the first rigid resin, PCTg VM318. The skin layer (A) extrusion melt temperature was controlled at 525 °F (273.9 °C). The throughput was 5.89 Ibs/hr. The core layer (C) extruder was also fed with the first rigid resin, PCTg VM318, and the extrusion melt temperature was controlled at 516 °F (268.9 °C). The core layer extrusion throughput was 4.58 Ibs/hr. The middle layer (B) extruder was fed with a second rigid resin, Tritan MX710, and the extrusion temperature was controlled at 511 °F (266.1 °C). The middle layer extrusion throughput was 8.79 Ibs/hr. The extruded sheet was chilled on a cast roll. The overall sheet thickness was controlled at 30 mils. The 8 hour stress relaxation of this sample is determined 51.9% (Force Persistence - 48.1%) by Procedure 1 and a flexural modulus of 1.69 GPa by Procedure 2.

Comparative Example 1

[0095] A single-layer polymeric film with 100% PCTT resin was extruded through a film die using a pilot scale extruder at a throughput of 15 Ibs/hr. The extrusion melt temperature was controlled to be 521 oF. The extruded sheet thickness was controlled at 30 mils. This sample has a flexural modulus of 1.559 GPa by Procedure 2 and the 8 hour stress relaxation is determined 42.1% (Force Persistence - 57.9%) by Procedure 1. Comparative Example 2

[0096] A single-layer polymeric film with 100% PCTg resin was extruded through a film die using a pilot scale extruder at a throughput of 15 Ibs/hr. The extrusion melt temperature was controlled to be 520 °F. The extruded sheet thickness was controlled at 30 mils. This sample has a flexural modulus of 1.742 GPa by Procedure 2 and the 8 hour stress relaxation is determined 57.8% (Force Persistence - 42.2%) by Procedure 1.

Comparative Example 3

[0097] A single-layer polymeric film with 100% PETg resin was extruded through a film die using a pilot scale extruder at a throughput of 15 Ibs/hr. The extrusion melt temperature was controlled to be 520 oF. The extruded sheet thickness was controlled at 30 mils. This sample has a flexural modulus of 1.97 GPa by Procedure 2 and the 8 hour stress relaxation is determined 65.3% (Force Persistence - 34.7%) by Procedure 1.

Comparative Example 4

[0098] A 3-layer ABA (TPE/PCTT*/TPE) film was extruded using a pilot scale coextrusion line equipped with a feedblock and film die. The skin layer (A) extruder was fed with the first elastomer resin, Hytrel 8238. The skin layer (A) extrusion temperature was controlled at 507 oF. The throughput was 6.7 Ibs/hr. The core layer (B) extruder was fed with a second rigid resin, Tritan FX150 and the extrusion temperature was controlled at 547 oF. The core layer extrusion throughput was 17.39 Ibs/hr. The extruded sheet was chilled on a cast roll. The overall sheet thickness was controlled at 30 mils. This sample has a flexural modulus of 1.53 GPa by Procedure 2 and the 8 hour stress relaxation of 43.1% (Force Persistence - 66.9%) by Procedure 1.

Comparative Example 5

[0099] A 3-layer ABA (TPU/PCTT*/TPU) film was extruded using a pilot scale coextrusion line equipped with a feedblock and film die. The skin layer (A) extruder was fed with the first rigid resin, Texin 970DU. The skin layer (A) extrusion temperature was controlled at 440 oF. The throughput was 5.5 Ibs/hr. The core layer (B) extruder was fed with a second rigid resin, Tritan FX150 and the extrusion temperature was controlled at 559 oF. The core layer extrusion throughput was 17.68 Ibs/hr. The extruded sheet was chilled on a cast roll. The overall sheet thickness was controlled at 30 mils. This sample has a flexural modulus of 1.19 GPa by Procedure 2 and the 8 hour stress relaxation is determined 32.1% (Force Persistence - 67.9%) by Procedure 1. Comparative Example 6

[00100] Zendura FLX is a 3-layer film and supplied from Bay Materials. Fremont, CA. The outer skins of Zendura FLX are made from copolyester and the core layer from a thermoplastic polyurethane. This sample has a flexural modulus of 1.35 GPa by Procedure 2 and the 8 hour stress relaxation of 31.2% (Force Persistence - 68.8%) by Procedure 1.

Comparative Example 7

[00101] A 5-layer ABABA (PETg/PCTg/PETg/PCTg/PETg) film was extruded using a pilot scale coextrusion line equipped with a feedblock and film die. The skin layer (A) extruder was fed with the first rigid resin, PETg GN071. The skin layer (A) extrusion melt temperature was controlled at 524 °F (273.3 °C). The throughput was 5.85 Ibs/hr. The core layer (C) extruder was also fed with the first rigid resin, PETg GN071, and the extrusion melt temperature was controlled at 524 °F (273.3 °C). The core layer extrusion throughput was 5.85 Ibs/hr. The middle layer (B) extruder was fed with a second rigid resin, PCTg VM318, and the extrusion temperature was controlled at 506 °F (263.3 °C). The middle layer extrusion throughput was 8.81 Ibs/hr. The extruded sheet was chilled on a cast roll. The overall sheet thickness was controlled at 30 mils. The 8 hour stress relaxation of this sample is determined 69.8% (Force Persistence - 70.2%) by Procedure 1. The extruded sheet demonstrated inferior laser marking performance, at a 3 rating, comparing to Example 1 by Procedure 3.

[00102] FIG. 6 depicts the stress relaxation over 8 hours for each of the Examples and Comparative Examples identified above. As can be seen from the direct comparison, the rigid films of the present disclosure demonstrate enhanced Force Persistence compared to previous rigid single and multilayer constructions, as well as reasonably equivalence to films containing elastomeric layers.

[00103] The patents, patent documents, and patent applications cited herein are incorporated by reference in their entirety as if each were individually incorporated by reference. Although specific embodiments of the present disclosure have been shown and described herein, it is understood that these embodiments are merely illustrative of the many possible specific arrangements that can be devised in application of the principles of the present disclosure. Numerous and varied other arrangements can be devised in accordance with these principles by those of ordinary skill in the art without departing from the spirit and scope of the present disclosure. Thus, the scope of the present disclosure should not be limited to the structures described in this application, but only by the structures described by the language of the claims and the equivalents of those structures.