Film

Abstract

To provide a film whose flexural modulus is not too high and whose stress relaxation is relatively gentle.SOLUTION: The film is a film used for an aligner for orthodontic correction. The film comprises at least a first layer and a second layer. The first layer is made of a first resin material containing high impact polystyrene. The second layer is composed of a second resin material containing a polystyrene elastomer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a film, and more particularly to a film used in orthodontic aligners. [Background technology]

[0002] JP 2023-525400 A (Patent Document 1) discloses an orthodontic aligner tray, which is made of, for example, a multilayer polymer film (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2023-525400 Summary of the Invention [Problem to be solved by the invention]

[0004] Generally, films made of PETG (glycol-modified polyethylene terephthalate) are used to manufacture orthodontic aligners. To achieve long-lasting orthodontic effects, it is important that the aligner undergoes gentle stress relaxation. However, the stress relaxation of films made of PETG is not necessarily gentle. Furthermore, if the flexural modulus of the film is too high, patients wearing the aligners may experience severe pain. Patent Document 1 does not disclose a solution to these problems.

[0005] The present invention has been made to solve such problems, and its object is to provide a film that has a flexural modulus that is not too high and that exhibits relatively gentle stress relaxation. [Means for solving the problem]

[0006] The film according to the present invention is a film used for orthodontic aligners. This film comprises at least a first layer and a second layer. The first layer is made of a first resin material containing high-impact polystyrene. The second layer is made of a second resin material containing a polystyrene-based elastomer.

[0007] The present inventors have discovered that films containing high-impact polystyrene exhibit more gradual stress relaxation than films composed of PETG. Because this film contains a first layer composed of a first resin material containing high-impact polystyrene, stress relaxation can be more gradual than in films composed of PETG. Furthermore, because this film contains a second layer composed of a second resin material containing a polystyrene-based elastomer, the flexural modulus can be reduced compared to films composed solely of high-impact polystyrene.

[0008] In the film, the polystyrene elastomer contained in the second resin raw material may have a Shore hardness of A60 or more.

[0009] The film may have a flexural modulus of elasticity of 1500 MPa or less measured with the first layer as the tensile surface. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a film that has a flexural modulus that is not too high and that exhibits relatively gentle stress relaxation. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram schematically illustrating a cross section of a film. [Figure 2] FIG. 1 is a diagram for explaining the effectiveness of HIPS in aligner applications. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment according to one aspect of the present invention (hereinafter also referred to as "the present embodiment") will be described in detail below with reference to the drawings. Note that the same or corresponding parts in the drawings are designated by the same reference numerals, and their description will not be repeated. Furthermore, for ease of understanding, each drawing is drawn schematically with objects appropriately omitted or exaggerated.

[0013] [1. Film composition] FIG. 1 is a diagram schematically illustrating a cross section of a film 10 according to the present embodiment. Referring to FIG. 1, film 10 includes a first layer 11 and a second layer 12 laminated on first layer 11. The thickness of film 10 is, for example, 400 μm or more and 1.6 mm or less, preferably 500 μm or more and 1.2 mm or less, and more preferably 600 μm or more and 900 μm or less. The value obtained by dividing the thickness of first layer 11 by the thickness of second layer 12 is, for example, 0.067 or more and 15 or less, preferably 0.11 or more and 9 or less, and more preferably 0.11 or more and 7 or less.

[0014] The film 10 is used to manufacture orthodontic aligners. For example, the film 10 is processed so that its shape is circular in plan view. When the film 10 is placed over a patient's dental impression, which has been prepared in advance, and heated, the shape of the film 10 changes to match the dental impression. After that, unnecessary portions are cut off from the film 10, completing the aligner.

[0015] The first layer 11 is made of a first resin material containing high impact polystyrene (HIPS (High Impact Polystyrene)). HIPS contains, for example, general purpose polystyrene (GPPS (General Purpose Polystyrene)) and a rubber component. General purpose polystyrene is, for example, a homopolymer of styrene. An example of the rubber component is butadiene rubber particles (for example, styrene-butadiene copolymer). HIPS is made of, for example, a sea-island alloy in which butadiene rubber particles are dispersed in a matrix made of general purpose polystyrene.

[0016] The second layer 12 is made of a second resin material containing a polystyrene-based elastomer. Examples of polystyrene-based elastomers include styrene-ethylene-butylene-styrene block copolymer (SEBS) and styrene-butylene block copolymer (SBC). The Shore hardness of the polystyrene-based elastomer is, for example, A60 or more and D70 or less, and preferably D39 or more and D42 or less. For example, the Shore hardness of SEBS is measured according to a method conforming to ISO 7619, and the Shore hardness of SBC is measured according to a method conforming to ISO 0868.

[0017] As described above, in the film 10, the first layer 11 and the second layer 12 are made of different resin materials. Therefore, the load applied to the teeth by the aligner varies depending on whether the film 10 is processed so that the first layer 11 side contacts the teeth or the second layer 12 side contacts the teeth. Depending on the patient's characteristics (such as pain tolerance), for example, the dentist can decide whether to process the film 10 so that the first layer 11 side contacts the teeth or the second layer 12 side contacts the teeth.

[0018] [2. Effectiveness of HIPS in aligner applications] To achieve long-lasting orthodontic effects, it is necessary for the stress relaxation of the aligner to be somewhat gradual. Furthermore, if the flexural modulus of film 10 is too high, patients wearing aligners manufactured using film 10 may experience severe pain. Therefore, it is important that the flexural modulus of film 10 is not too high and that the stress relaxation of film 10 is relatively gradual.

[0019] Figure 2 is a diagram illustrating the effectiveness of HIPS in aligner applications. Referring to Figure 2, the horizontal axis represents time, and the vertical axis represents film stress. Lines L1, L2, and L3 each represent an example of the results of measuring stress over four consecutive hours when a constant strain of 0.25 mm is applied to a sample (film) in a doubly supported beam bending mode using a dynamic mechanical analyzer (DMA; Hitachi High-Tech Science DMA7100). Because the film is heated during aligner manufacturing, each sample was heated (e.g., at 220°C for 30 seconds) and cooled (air-cooled) under identical conditions. Because the aligner is used in a human mouth, stress measurements were performed with the sample placed in water at 37°C. Line L1 represents an example of stress progression in a single-layer PETG film. Line L2 represents an example of stress progression in a single-layer HIPS film. Line L3 represents an example of stress progression in film 10.

[0020] Comparing lines L1 and L2, the stress relaxation of the HIPS monolayer film is slower than that of the PETG monolayer film. Meanwhile, the stress of the HIPS monolayer film is high, and the flexural modulus of the HIPS monolayer film is also high. Comparing lines L2 and L3, the stress relaxation of film 10 is equivalent to or slightly slower than that of the HIPS monolayer film. Furthermore, the stress of film 10 is lower than that of the HIPS monolayer film, and the flexural modulus of film 10 is also lower than that of the HIPS monolayer film. The flexural modulus of film 10, measured with the first layer 11 as the tensile surface, is, for example, 600 MPa or more and 2000 MPa or less, preferably 700 MPa or more and 1600 MPa or less, more preferably 800 MPa or more and 1500 MPa or less, and even more preferably 800 MPa or more and 1100 MPa or less. Furthermore, the flexural modulus of the film 10 measured with the second layer 12 as the tensile surface is, for example, 600 MPa or more and 2000 MPa or less, preferably 700 MPa or more and 1700 MPa or less, more preferably 700 MPa or more and 1600 MPa or less, and even more preferably 700 MPa or more and 1100 MPa or less.

[0021] Thus, the inventor(s) discovered that film 10 containing high-impact polystyrene exhibits more gradual stress relaxation than a film made of PETG. Film 10 contains first layer 11 made of a first resin material containing high-impact polystyrene, allowing for more gradual stress relaxation compared to a film made of PETG. Furthermore, film 10 contains second layer 12 made of a second resin material containing a polystyrene-based elastomer, allowing for a lower flexural modulus compared to a film made solely of high-impact polystyrene. That is, film 10 allows for a flexural modulus within an appropriate range and relatively gradual stress relaxation.

[0022] [3. Features] As described above, film 10 according to the present embodiment includes first layer 11 made of a first resin material containing high-impact polystyrene, which allows for more gradual stress relaxation compared to a film made of PETG. Also, film 10 includes second layer 12 made of a second resin material containing a polystyrene-based elastomer, which allows for a lower flexural modulus compared to a film made only of high-impact polystyrene.

[0023] 4. Other Embodiments The concept of the above embodiment is not limited to the embodiment described above. Below, examples of other embodiments to which the concept of the above embodiment can be applied will be described.

[0024] In the film 10 according to the above embodiment, only one first layer 11 and one second layer 12 are included. However, the number of first layers 11 and the number of second layers 12 are not limited to this. In the film 10, a plurality of first layers 11 and a plurality of second layers 12 may be provided. The layer structure in the film 10 may be, for example, first layer 11 / second layer 12 / second layer 12, first layer 11 / first layer 11 / second layer 12, first layer 11 / first layer 11 / second layer 12 / second layer 12, or first layer 11 / second layer 12 / second layer 12 / second layer 12. Furthermore, when the film 10 includes a plurality of first layers 11, the first layers 11 may be made of different materials, and when the film 10 includes a plurality of second layers 12, the second layers 12 may be made of different materials.

[0025] The above describes exemplary embodiments of the present invention. That is, the detailed description and the accompanying drawings are disclosed for the purpose of illustrative explanation. Therefore, some of the components described in the detailed description and the accompanying drawings may be non-essential components for solving the problems. Therefore, just because these non-essential components are described in the detailed description and the accompanying drawings, it should not be immediately recognized that these non-essential components are essential.

[0026] Furthermore, the above-described embodiments are merely illustrative of the present invention in all respects. Various improvements and modifications to the above-described embodiments are possible within the scope of the present invention. For example, at least a portion of the configuration of any of the embodiments may be combined with at least a portion of the configuration of any of the other embodiments. In other words, when implementing the present invention, specific configurations can be appropriately adopted depending on the embodiment. [Example]

[0027] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0028] [1. Examples and Comparative Examples] The films of Examples 1-18 and Comparative Examples 1-2 were produced by the following method. The films of Examples 1-18 were obtained by supplying the resin raw materials constituting each layer to an extruder and co-extruding them. The films of Comparative Examples 1-2 were obtained by supplying the resin raw materials to an extruder and extruding them. The total thickness of each film obtained was 800 μm. The films of Examples 1-18 and Comparative Examples 1-2 differed from each other in the combination of the type of resin constituting the first layer, the type of resin constituting the second layer, the thickness ratio of the first and second layers, and the Shore hardness of the elastomer constituting the second layer. These combinations were as shown in Table 1 below.

[0029] [Table 1]

[0030] [2. Various measurements] The films of Examples 1-18 and Comparative Examples 1-2 were subjected to the following measurements.

[0031] <2-1. Stress relaxation> The stress of each film was measured continuously for four hours using a DMA (dynamic viscoelasticity measuring device, Hitachi High-Tech Science DMA7100) in double-support beam bending mode. For each film, the stress was measured both when the first layer side was the tensile surface and when the second layer side was the tensile surface. The displacement of each film was 0.25 mm. The strain rate was 100,000 μm / min. Before stress measurement, each film was heated at 220°C for 30 seconds. The stress measurement was performed with the film placed in water at 37°C. The change in stress every hour was monitored to determine the stress relaxation properties of each film.

[0032] <2-2. Flexural modulus> The flexural modulus of each film was measured using a method essentially conforming to JIS-K-7171. Since each film was 800 μm thick, the distance between supports would be extremely short if the method conformed to JIS-K-7171. Therefore, only the distance between supports was determined using a different standard from JIS-K-7171. Specifically, the distance between supports was set to 20 mm.

[0033] [3. Measurement results] The measurement results for stress relaxation are shown in Table 2 below. The difference between the stress 1 hour after the start of measurement and the stress 4 hours after the start of measurement is shown in Table 3 below. Note that the stress significantly decreased in the first hour after the start of measurement compared to other sections, so the difference between the stress 1 hour after the start of measurement and the stress 4 hours after the start of measurement was checked as the stress relaxation characteristic. Referring to Table 3, it was found that the stress relaxation of each film of Example 1-18 was gentler than that of each film of Comparative Example 1-2.

[0034] [Table 2] [Table 3]

[0035] The measurement results of the flexural modulus are shown in Table 4. Referring to Table 4, it was found that the flexural modulus of each of the films of Examples 1 to 18 was within an appropriate range.

[0036] [Table 4] [Explanation of symbols]

[0037] 10 film, 11 first layer, 12 second layer, L1, L2, L3 lines.

Claims

1. A film used in orthodontic aligners, comprising at least a first layer and a second layer; the first layer is made of a first resin material including high-impact polystyrene; A film, wherein the second layer is made of a second resin material containing a polystyrene-based elastomer.

2. The film according to claim 1 , wherein the polystyrene elastomer contained in the second resin raw material has a Shore hardness of A60 or more.

3. 3. The film according to claim 1, wherein the first layer has a flexural modulus of 1500 MPa or less as measured with the first layer as a tensile surface.

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