Protective film

Abstract

To provide a protective film that allows for the selection of the degree of adhesion to the resin substrate depending on the type of processing applied to the resin substrate. [Solution] The protective film 10 of the present invention is used by attaching it to a resin substrate 21 when the resin substrate 21 is subjected to heat bending. It has a base layer and an adhesive layer located between the base layer and the resin substrate and adhering to the resin substrate. The base layer is composed of a laminate having a first layer located on the opposite side of the adhesive layer and a second layer located on the side of the adhesive layer. The adhesive layer contains an adhesive polyolefin as the main material and has two or more different melting points.

Description

Technical Field

[0001] The present invention relates to a protective film that is attached to a resin substrate and used when the resin substrate is subjected to thermoforming under heating.

Background Art

[0002] A sunglass lens including a resin substrate having a structure in which both surfaces of a polarizer are coated with a coating layer made of a polycarbonate resin, a polyamide resin, or a cellulose resin is, for example, in a state where protective films are attached to both surfaces of the resin substrate having a flat plate shape in plan view, and the resin substrate is punched into a predetermined shape such as a circular shape in plan view, and then the resin substrate is subjected to thermoforming under heating. Then, after peeling the protective film from the thermoformed resin substrate, a polycarbonate layer is injection-molded on the concave surface of the resin substrate to manufacture the lens.

[0003] As this protective film, for example, a structure has been proposed in which a base material mainly made of a polyolefin-based resin is attached to the resin substrate via an adhesive layer mainly made of polyethylene, an ethylene-propylene copolymer, or the like (see, for example, Patent Document 1).

[0004] Here, a plurality of processes such as punching of the resin substrate and thermoforming of the resin substrate as described above are performed on the resin substrate, and during these processes, the protective film is applied in a state where it is attached to the resin substrate, that is, in a state where a laminate is formed.

[0005] During these processing steps, the protective film is required to maintain a stable laminated state without peeling off from the resin substrate. However, since the conditions during each processing step differ, the required adhesion force between the protective film and the resin substrate naturally also differs. In other words, the required adhesion force of the protective film to the resin substrate varies depending on the processing step, but in reality, there is no protective film that offers the flexibility to select the appropriate adhesion force for each processing step, making it highly user-friendly.

[0006] Furthermore, these problems occur not only with the lenses used in sunglasses mentioned above, but also with the resin substrates used in lenses for goggles, visors for helmets, and other similar components. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2003-145616 [Overview of the project] [Problems that the invention aims to solve]

[0008] The object of the present invention is to provide a protective film that allows the degree of adhesion to a resin substrate to be selected according to the type of processing applied to the resin substrate. [Means for solving the problem]

[0009] These objectives are achieved by the present invention as described in (1) to (13) below. (1) A protective film used by attaching it to a resin substrate when the resin substrate is subjected to heat bending under heating, A base layer and It has an adhesive layer positioned between the base material layer and the resin substrate, which adheres to the resin substrate, The substrate layer is composed of a laminate having a first layer located on the opposite side of the adhesive layer and a second layer located on the side of the adhesive layer. The adhesive layer is a protective film characterized by containing an adhesive polyolefin as the main material and having two or more different melting points.

[0010] (2) The protective film according to (1) above, wherein the adhesive layer comprises at least two or more materials having different melting points.

[0011] (3) The protective film according to (1) or (2) above, wherein the difference between the melting point of the constituent material having the lowest melting point and the melting point of the constituent material having the highest melting point is 5°C or more and 80°C or less.

[0012] (4) The protective film according to (3) above, wherein the melting point of the constituent material having the lowest melting point is 60°C or more and 130°C or less.

[0013] (5) The protective film according to (3) or (4) above, wherein the melting point of the constituent material having the highest melting point is 80°C or more and 160°C or less.

[0014] (6) A protective film according to any of (1) to (5) above, wherein the protective film is attached to a polycarbonate substrate in accordance with JIS Z 0273:2009 by pressing it with a rubber roll with a pressure of 5880 N / m at a speed of 2 m / min, then heated at 100°C for 30 minutes, cooled to 23°C, cut to a size of 100 mm in length and 25 mm in width, and then the peel strength measured when one end of the cut protective film is held and peeled off at a speed of 200 mm / min in a 90° direction is 0.05 N / 25 mm or more and 0.6 N / 25 mm or less.

[0015] (7) A protective film according to any of (1) to (6) above, wherein the protective film is attached to a polycarbonate substrate in accordance with JIS Z 0273:2009 by pressing it with a rubber roll with a pressure of 5880 N / m at a speed of 2 m / min, then heated at 145°C for 30 minutes, cooled to 23°C, cut to a size of 100 mm in length and 25 mm in width, and then the peel strength measured when one end of the cut protective film is held and peeled off at a speed of 200 mm / min in a 90° direction is 0.15 N / 25 mm or more and 1.5 N / 25 mm or less.

[0016] (8) The protective film according to any one of (1) to (7) above, wherein the first layer contains a thermoplastic resin as the main material and has a melting point of 150°C or higher, and the second layer contains a thermoplastic resin as the main material and has a melting point of 120°C or higher.

[0017] (9) The protective film according to (8) above, wherein the thermoplastic resin contained in the first layer and the thermoplastic resin contained in the second layer are both polyolefins.

[0018] (10) A protective film according to any one of (1) to (9) above, which is attached to both sides of the resin substrate.

[0019] (11) The protective film according to any one of (1) to (10) above, wherein the resin substrate has a coating layer on both sides, one side or the other side, which is made up of a single layer or a laminate having at least one of a polycarbonate resin layer, a polyamide resin layer and a cellulose resin layer.

[0020] (12) The protective film according to any one of (1) to (11) above, wherein the resin substrate is subjected to the heat bending process by press molding or vacuum forming.

[0021] (13) The heating temperature when performing the thermal bending process on the resin substrate under heating is 110°C or higher and 160°C or lower, and the protective film according to any one of (1) to (12) above.

Advantages of the Invention

[0022] According to the present invention, the adhesive layer provided in the protective film contains a polyolefin having adhesiveness as a main material and is composed of a material having two or more different melting points. Thus, by making the adhesive layer have two or more different melting points, it is possible to make the protective film capable of selecting the magnitude of the adhesive force to the resin substrate according to the type of processing applied to the resin substrate.

Brief Description of the Drawings

[0023] [Figure 1] It is a schematic diagram for explaining a method of manufacturing a lens for sunglasses using a protective film. [Figure 2] It is a longitudinal sectional view showing a preferred embodiment of the protective film of the present invention.

Embodiments for Carrying Out the Invention

[0024] Hereinafter, the protective film of the present invention will be described in detail based on preferred embodiments shown in the accompanying drawings. In this specification, the main material refers to a constituent material contained in each layer at 50% by weight or more. For example, "The first layer 16 contains a thermoplastic resin as its main material." means that when the total weight of the first layer 16 is 100% by weight, the thermoplastic resin contained in the first layer 16 occupies 50% by weight or more in the first layer 16.

[0025] The protective film of the present invention is a protective film used by attaching it to a resin substrate when the resin substrate is subjected to heat bending under heating, and comprises a base layer and an adhesive layer positioned between the base layer and the resin substrate and adhering to the resin substrate, wherein the base layer is composed of a laminate having a first layer located on the opposite side of the adhesive layer and a second layer located on the side of the adhesive layer, and the adhesive layer contains an adhesive polyolefin as the main material and is characterized by having two or more different melting points.

[0026] By providing the protective film with such a configuration, that is, by providing the adhesive layer of the protective film with two or more different melting points, it becomes possible to select the degree of adhesion of the protective film to the resin substrate according to the type of processing applied to the resin substrate.

[0027] Before describing the protective film of the present invention, a method for manufacturing sunglasses lenses using the protective film of the present invention will be described below.

[0028] <Manufacturing method for sunglasses lenses> Figure 1 is a schematic diagram illustrating a method for manufacturing sunglasses lenses using a protective film. For the sake of explanation, the upper part of Figure 1 will be referred to as "top" and the lower part as "bottom" below.

[0029] The following details each step in the manufacturing process for sunglass lenses. [1] First, a flat resin substrate 21 is prepared, and protective film 10 (masking tape) is attached to both sides of this resin substrate 21 to obtain a laminate 100 in which protective film 10 is attached to both sides of the resin substrate 21 (see Figure 1(a)).

[0030] In this embodiment, the resin substrate 21 is provided with a polarizer 23, which functions as an optical element that extracts linearly polarized light having a polarization plane in a predetermined direction from unpolarized natural light, and both sides of the polarizer 23 are covered with a coating layer 24. In this resin substrate 21, the coating layer 24 is composed of a single layer or laminate having at least one layer from a polycarbonate resin layer, a polyamide resin layer, and a cellulose resin layer such as triacetylcellulose. Furthermore, the coating layer 24 may be formed on both sides (both surfaces) of the polarizer 23, or it may be formed on either the upper surface (one surface) or the lower surface (the other surface).

[0031] [2] Next, as shown in Figure 1(b), the prepared laminate 100 is made circular in plan view by punching out the resin substrate 21 in the thickness direction with the protective film 10 attached to both sides of the resin substrate 21.

[0032] [3] Next, as shown in Figure 1(c), the circular laminate 100 is subjected to heat bending.

[0033] This heat bending process is typically carried out by press forming using a die or by vacuum forming (remar forming).

[0034] As mentioned above, in this embodiment, the heating temperature (molding temperature) of the laminate 100 (resin substrate 21) is preferably set to about 110°C to 160°C, more preferably to about 140°C to 150°C, taking into consideration the melting or softening temperature of the coating layer 24. This is because the resin substrate 21 is equipped with a coating layer 24, and the coating layer 24 is composed of a single layer or laminate having at least one layer from a polycarbonate resin layer, a polyamide resin layer, and a cellulose resin layer. By setting the heating temperature within this range, the resin substrate 21 can be softened or molten while preventing alteration or deterioration of the resin substrate 21, thereby reliably allowing the resin substrate 21 to be heat-bent.

[0035] [4] Next, as shown in Figure 1(d), the protective film 10 is peeled off from the heat-bent resin substrate 21, i.e., the laminate 100, and then a polycarbonate layer 30 made of polycarbonate resin is injection molded onto the concave surface of the resin substrate 21. Alternatively, a polyamide layer made of polyamide resin may be formed on the concave surface of the resin substrate 21 instead of the polycarbonate layer 30.

[0036] This allows for the manufacture of sunglasses lenses 200 equipped with a heat-bent resin substrate 21.

[0037] In the above-described method for manufacturing sunglasses lenses, the punching process in the thickness direction of the resin substrate 21 in step [2] and the heat bending process of the resin substrate 21 in step [3] are performed on a laminate 100 in which protective films 10 are attached to both sides of the resin substrate 21.

[0038] Furthermore, during the processing in steps [2] and [3], the protective film 10 is required to stably maintain a state in which the laminate 100 is formed without peeling off from the resin substrate 21. However, since the conditions during each processing step [2] and step [3] are different, the required adhesion force between the protective film 10 and the resin substrate 21 will naturally also be different.

[0039] In contrast, in the present invention, the protective film 10 includes an adhesive layer 11 which contains an adhesive polyolefin as the main material and has two or more different melting points.

[0040] Thus, since the adhesive layer 11 has two or more different melting points, by appropriately selecting the heating temperature of the heat treatment applied during or before the processing in steps [2] and [3], it is possible to select the magnitude of the adhesion force to the resin substrate during each processing step [2] and [3]. The protective film 10 (protective film of the present invention) will be described in detail below.

[0041] <Protective film 10> Figure 2 is a longitudinal cross-sectional view showing a preferred embodiment of the protective film of the present invention. For the sake of explanation, the upper side of Figure 2 will be referred to as "top" and the lower side as "bottom" below.

[0042] The protective film 10 has a base layer 15 and an adhesive layer 11 located between the base layer 15 and the resin substrate 21, which adheres (bonds) to the resin substrate 21. As shown in Figure 2, the base layer 15 has a first layer 16 located on the opposite side of the adhesive layer 11, i.e., the mold side, and a second layer 17 located on the adhesive layer 11 side, i.e., the resin substrate 21 side.

[0043] The following provides a detailed explanation of each of these layers. <<Adhesive layer 11>> The adhesive layer 11 is positioned (interposed) between the base material layer 15 and the resin substrate 21, and adheres to the resin substrate 21 to bond the base material layer 15 to the resin substrate 21.

[0044] Preferably, the adhesive layer 11 is such that it can perform punching and heat bending of the resin substrate 21 in steps [2] and [3] without peeling the protective film 10 from the resin substrate 21, and can also be used to peel off the protective film 10 from the resin substrate 21 in step [4].

[0045] In this invention, the adhesive layer 11 contains a polyolefin resin with adhesive properties as its main material. For example, it is preferable that the adhesive layer 11 contains a polyolefin resin alone, or a combination of a polyolefin resin and an elastomer. By having the adhesive layer 11 have such a configuration, the adhesive layer 11 can reliably perform its function.

[0046] Furthermore, the adhesive polyolefin resin is not particularly limited, and examples include polypropylene homopolymers, polyethylene homopolymers, propylene-ethylene block copolymers having an EPR phase (rubber phase), ethylene-vinyl acetate block copolymers, ethylene-ethyl acrylate block copolymers, ethylene-methyl methacrylate block copolymers, etc., and one or more of these can be used in combination, with polyethylene homopolymers being particularly preferred. Polyethylene homopolymers are relatively inexpensive and readily available. Moreover, polyethylene homopolymers can impart transparency to the adhesive layer 11. Therefore, if the base layer 15 is also transparent, the protective film 10 will also be transparent. Consequently, when the protective film 10 is attached to the resin substrate 21 in step [1], it is possible to visually check whether dust or other debris is interposed between the protective film 10 and the resin substrate 21, thereby reliably preventing the laminate 100 containing debris from migrating to step [2] and beyond, and as a result, the yield of the resulting sunglasses lenses 200 can be improved.

[0047] The polyethylene homopolymer is not particularly limited, but examples include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and very low-density polyethylene (VLDPE), and one or more of these can be used in combination. Suitable linear low-density polyethylene resins for use in the present invention include, for example, "Yumerit®" such as Yumerit 2525F and Yumerit 1520F manufactured by Ube Maruzen Polyethylene Co., Ltd., and "Sumikasen®" manufactured by Sumitomo Chemical Co., Ltd.

[0048] Furthermore, while the elastomer is not particularly limited, examples include α-polyolefin resin / polyethylene copolymer elastomer, α-polyolefin resin / polypropylene copolymer elastomer, α-polyolefin resin / polybutene copolymer elastomer, styrene block elastomer, etc., among which styrene block elastomer is preferred, and styrene-olefin-styrene block copolymer elastomer is particularly preferred. In this way, by further including an elastomer in addition to the polyolefin resin, it is possible to accurately suppress or prevent the adhesive layer 11 from remaining on the resin substrate 21 when peeling the protective film 10 from the resin substrate 21 in step [4], that is, the occurrence of adhesive residue on the resin substrate 21, thereby enabling smoother peeling of the protective film 10 from the resin substrate 21. Moreover, by using an elastomer that contains styrene as a monomer component, it is possible to more accurately suppress or prevent the occurrence of adhesive residue on the resin substrate 21 in step [4].

[0049] Examples of α-polyolefin resins include 1-hexene, 4-methyl-1-pentene, 1-octene, 1-butene, 1-pentene, and 1-heptene.

[0050] Furthermore, when using a styrene-olefin-styrene block copolymer elastomer, the styrene content in the elastomer is preferably 25% by weight or less, and more preferably 10% by weight or more and 18% by weight or less. This effectively suppresses or prevents an increase in the hardness of the adhesive layer 11 caused by a high styrene content. As a result, it is possible to reliably maintain the adhesion of the adhesive layer 11 to the resin substrate 21 (coating layer 24) while more effectively suppressing or preventing the occurrence of adhesive residue on the resin substrate 21.

[0051] Furthermore, examples of styrene-olefin-styrene block copolymers include styrene-isobutylene-styrene block copolymer (SIBS), styrene-ethylene-butylene-styrene block copolymer (SEBS), styrene-butadiene-styrene block copolymer (SBS), and styrene-isoprene-styrene copolymer (SIS), but among these, styrene-ethylene-butylene-styrene block copolymer (SEBS) is preferred. By selecting SEBS as the styrene-olefin-styrene block copolymer, the styrene content in the elastomer can be easily set to 25% by weight or less, and the aforementioned effects can be reliably obtained.

[0052] Furthermore, if the adhesive layer 11 contains an elastomer, the elastomer content in the adhesive layer 11 is not particularly limited, but is preferably set to 1% by weight or more and 30% by weight or less, and more preferably 5% by weight or more and 15% by weight or less. This makes the effects obtained by including an elastomer in the adhesive layer 11 more pronounced.

[0053] Furthermore, when the adhesive layer 11 contains an elastomer, the melting point of the elastomer is not particularly limited, but is preferably set to 150°C or lower, more preferably 120°C or lower, and even more preferably 40°C to 100°C. This makes it possible to more clearly demonstrate the effects obtained by including the elastomer in the adhesive layer 11.

[0054] In the adhesive layer 11 having such a configuration, in this specification, the melting point of the adhesive layer 11 is determined according to the melting point of each constituent material contained in the adhesive layer 11. For example, if two constituent materials having different melting points (peak temperatures measured by DSC) are included, the adhesive layer 11 is said to have two different melting points, and if three constituent materials having different melting points are included, the adhesive layer 11 is said to have three different melting points. In other words, if two or more constituent materials having different melting points are included, the adhesive layer 11 can be said to have two or more different melting points.

[0055] The adhesive layer 11 has two or more different melting points, as determined as described above. Therefore, by appropriately selecting the heating temperature of the heat treatment applied during or before the processing in steps [2] and [3], it is possible to select the magnitude of the adhesion force of the adhesive layer 11 (protective film 10) to the resin substrate 21 during each of the processing steps [2] and [3]. Accordingly, the magnitude can be set according to the magnitude of the adhesion force required between the protective film 10 and the resin substrate 21 during each of the processing steps such as the punching process in step [2] and the heat bending process in step [3].

[0056] The adhesive layer 11 may have at least two different melting points by including at least two materials having different melting points as its constituent materials, as described above. In this case, the difference between the melting point of the constituent material with the lowest melting point and the melting point of the constituent material with the highest melting point is preferably 5°C or more and 80°C or less, and more preferably 15°C or more and 60°C or less.

[0057] Furthermore, the melting point of the constituent material having the lowest melting point is preferably 60°C to 130°C, and more preferably 70°C to 110°C. Moreover, the melting point of the constituent material having the highest melting point is preferably 80°C to 160°C, and more preferably 90°C to 135°C.

[0058] By setting the difference between the melting point of the constituent material having the lowest melting point and the melting point of the constituent material having the highest melting point, the melting point of the constituent material having the lowest melting point, and the melting point of the constituent material having the highest melting point to the aforementioned ranges, the effect obtained by having the adhesive layer 11 have at least two different melting points can be made more pronounced.

[0059] Furthermore, if the adhesive layer 11 contains at least two materials with different melting points as constituent materials, the combination of these constituent materials is not particularly limited, but specifically, examples include a combination of polyolefin resin and an elastomer, and a combination of polyolefin resins. In addition, as for combinations of polyolefin resins, when a polyethylene homopolymer is used as the polyolefin resin, examples include a combination of low-density polyethylene (LDPE) and other low-density polyethylene (LLDPE), a combination of linear low-density polyethylene (LLDPE) and other low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE). When the polyolefin resin contains a single polyethylene homopolymer, examples include a combination of linear low-density polyethylene (LLDPE) and ethylene-methacrylic acid copolymer (EMAA), a combination of linear low-density polyethylene (LLDPE) and ethylene-methyl acrylate copolymer (EMA), and a combination of high-density polyethylene (HDPE) and ethylene-methyl acrylate copolymer (EMA). Furthermore, examples of combinations of polyolefin resins and elastomers include combinations of linear low-density polyethylene (LLDPE) with one of the following: α-polyolefin resin / polyethylene copolymer elastomer, α-polyolefin resin / polypropylene copolymer elastomer, and α-polyolefin resin / polybutene copolymer elastomer.

[0060] Furthermore, in addition to the case where the adhesive layer 11 has at least two different melting points by including at least two constituent materials having different melting points, the adhesive layer 11 can also have at least two different melting points by including at least one constituent material having two different melting points.

[0061] Thus, examples of materials having two different melting points in a single constituent material include, in the case of polyethylene homopolymers, linear low-density polyethylene (LLDPE) with melting points of 99°C and 114°C, linear low-density polyethylene (LLDPE) with melting points of 97°C and 113°C, linear low-density polyethylene (LLDPE) with melting points of 99°C and 114°C, ultra-low-density polyethylene (VLDPE) with melting points of 87°C and 111°C, VLDPE with melting points of 97°C and 113°C, and VLDPE with melting points of 99°C and 114°C.

[0062] The protective film 10 having the adhesive layer 11 having the above configuration is attached to a polycarbonate substrate by pressing the protective film 10 onto it with a rubber roll with a pressure of 5880 N / m at a speed of 2 m / min in accordance with JIS Z 0273:2009, then heated at 100°C for 30 minutes, cooled to 23°C, and then cut to a size of 100 mm in length and 25 mm in width. After that, the peel strength T1 measured when one end of the cut protective film 10 is held and peeled off at a speed of 200 mm / min in a 90° direction is preferably 0.05 N / 25 mm or more and 0.6 N / 25 mm or less, and more preferably 0.1 N / 25 mm or more and 0.5 N / 25 mm or less. Here, in the above step [2], the punching of the resin substrate 21 is usually carried out in a laminated state 100 at a temperature of room temperature to about 100°C. Therefore, by setting the peel strength T1 after heating at 100°C within the range, it is possible to effectively suppress or prevent the protective film 10 from peeling off the resin substrate 21 (laminated body 100) or wrinkles from forming on the protective film 10 in the laminate 100 during step [2].

[0063] Furthermore, the protective film 10 is attached to the polycarbonate substrate by pressing it with a rubber roll with a pressure of 5880 N / m at a speed of 2 m / min in accordance with JIS Z 0273:2009, then heated at 145°C for 30 minutes, cooled to 23°C, and then cut to a size of 100 mm in length and 25 mm in width. After that, the peel strength T2 measured when one end of the cut protective film 10 is held and peeled off at a speed of 200 mm / min in a 90° direction is preferably 0.15 N / 25 mm or more and 1.5 N / 25 mm or less, and more preferably 0.3 N / 25 mm or more and 1.0 N / 25 mm or less. Here, in the above step [3], as described above, the heat bending of the resin substrate 21 is carried out in a laminated state 100 at a temperature of about 110°C to 160°C. Therefore, by setting the peel strength T2 after heating at 145°C within the range, it is possible to effectively suppress or prevent the protective film 10 from peeling off the resin substrate 21 (laminated body 100) or wrinkles from forming on the protective film 10 in the laminate 100 during step [3]. Furthermore, by setting the peel strength to below the upper limit, it is possible to effectively suppress or prevent the adhesive layer 11 from remaining on the surface of the resin substrate 21 during step [4] after step [3], while relatively easily peeling the protective film 10 from the resin substrate 21.

[0064] As described above, by heating the adhesive layer 11 of the protective film 10 at a relatively low heating temperature such as 100°C, the adhesion force of the adhesive layer 11 (protective film 10) to the resin substrate 21 can be set lower compared to when the adhesive layer 11 is heated at a relatively high heating temperature such as 145°C. In other words, by appropriately selecting the heating temperature applied to the adhesive layer 11, the degree of adhesion force of the adhesive layer 11 (protective film 10) to the resin substrate 21 can be selected.

[0065] The difference in the magnitude of this adhesion force can be expressed, for example, as the ratio (T2 / T1) of the peel strength T1, which is the adhesion force of the protective film 10 at a heating temperature of 100°C, to the peel strength T2, which is the adhesion force of the protective film 10 at a heating temperature of 145°C. It is preferable that the magnitude of the ratio (T2 / T1) is between 2.0 and 4.5, and more preferably between 3.0 and 4.5. This makes it possible to relatively easily select the magnitude of the adhesion force of the adhesive layer 11 (protective film 10) to the resin substrate 21 by appropriately selecting the heating temperature applied to the adhesive layer 11.

[0066] Furthermore, the adhesive layer 11 preferably has an average thickness of 5 μm to 40 μm, and more preferably 10 μm to 20 μm. This ensures that the adhesive layer 11 can reliably perform the functions described above.

[0067] <<Base material layer 15>> The base layer 15 is bonded to the resin substrate 21 (coating layer 24) via the adhesive layer 11, and functions as a protective layer (functional layer) that protects (masks) the resin substrate 21 during punching and heat bending of the resin substrate 21 in steps [2] and [3], and peels (detaches) the resin substrate 21 (protective film 10) from the mold used for heat bending after heat bending in step [3].

[0068] Furthermore, in step [4] above, when peeling the protective film 10 from the heat-bent resin substrate 21, peeling does not occur between the base material layer 15 and the adhesive layer 11, and excellent adhesion to the adhesive layer 11 is ensured.

[0069] In order to allow the base layer 15 to exhibit these functions, the present invention, as shown in Figure 2, is composed of a laminate having a first layer 16 located on the opposite side of the adhesive layer 11, i.e., the mold side, and a second layer 17 located on the adhesive layer 11 side, i.e., the resin substrate 21 side.

[0070] The following describes the first layer 16 and the second layer 17. <<Layer 16>> The first layer 16 is located on the opposite side of the adhesive layer 11 and functions as the outermost layer for protecting (masking) the resin substrate 21 during punching and heat bending of the resin substrate 21 in steps [2] and [3].

[0071] The first layer 16 is designed to maintain excellent detachability from the mold after the heat bending in step [3], that is, to prevent the first layer 16 from adhering to the mold (die). Therefore, the melting point of the first layer 16 is preferably 150°C or higher, and more preferably 155°C to 170°C. As mentioned above, the heating temperature of the coating layer 24 (resin substrate 21) during heat bending in step [3] is preferably set to approximately 110°C to 160°C. By setting the melting point of the first layer 16 as described above, it is possible to reliably prevent the first layer 16 from melting or softening during heat bending in step [3], thereby ensuring that the laminate 100 can be reliably detached from the mold after the heat bending in step [3].

[0072] Such a first layer 16 contains, as its main material, a thermoplastic resin having a melting point preferably of 150°C or higher, more preferably of 155°C to 170°C. This makes it relatively easy to set the melting point of the first layer 16 to 150°C or higher. Therefore, after the heat bending in step [3], excellent release properties of the laminate 100 from the mold can be maintained.

[0073] In this specification, the melting points of the first layer 16 and the second layer 17 constituting the base layer 15 are defined as the sum of the values ​​obtained by multiplying the melting point (peak temperature measured by DSC) of each constituent material contained in each layer by the ratio in which each constituent material is contained.

[0074] Furthermore, if the second layer 17, described later, also contains a thermoplastic resin with a temperature of 150°C or higher, the first layer 16 and the second layer 17 can be made to have better adhesion. Therefore, in step [4], when peeling the protective film 10 from the heat-bent resin substrate 21, it is possible to effectively suppress or prevent peeling between the first layer 16 and the second layer 17.

[0075] Furthermore, it is preferable that the first layer 16 contains, as a thermoplastic resin with a temperature of 150°C or higher, a polyolefin resin with a temperature of 150°C or higher, as the first layer 16 alone. Polyolefin resins with a temperature of 150°C or higher can be obtained relatively easily and inexpensively as thermoplastic resins with a temperature of 150°C or higher.

[0076] Furthermore, the polyolefin resin having a melting point of 150°C or higher is not particularly limited. Examples include polypropylene homopolymers, polyethylene homopolymers, propylene-ethylene copolymers, ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers, ethylene-methyl methacrylate copolymers, etc., all of which have a melting point of 150°C or higher. One or more of these can be used in combination, and among them, polypropylene homopolymers having a melting point of 150°C or higher are preferred. This makes it possible to easily and inexpensively obtain a polyolefin resin having a melting point of 150°C or higher. In addition, transparency can be imparted to the first layer 16. Therefore, if the second layer 17 and the adhesive layer 11 also have transparency, the protective film 10 will be transparent. Therefore, when the protective film 10 is attached to the resin substrate 21 in step [1], and when the protective film 10 is reattached to the resin substrate 21 prior to step [2], it is possible to visually check whether or not dust or other debris is interposed between the protective film 10 and the resin substrate 21. As a result, it is possible to reliably prevent the laminate 100 containing debris from migrating to step [2] and beyond, thereby improving the yield of the resulting sunglasses lenses 200.

[0077] The copolymer may be either a block copolymer or a random copolymer.

[0078] Furthermore, the first layer 16 preferably has an average thickness of 2 μm to 40 μm, and more preferably 5 μm to 25 μm. This ensures that the first layer 16 can reliably perform the functions described above.

[0079] <<Second Layer 17>> The second layer 17 is located on the adhesive layer 11 side, that is, on the resin substrate 21 side, and is positioned between the adhesive layer 11 and the first layer 16, functioning as an intermediate layer that joins them together.

[0080] The second layer 17 contains a thermoplastic resin with adhesive properties as its main material in order to perform the aforementioned function, thereby enabling the adhesive layer 11 and the first layer 16 to be joined with excellent adhesion via the second layer 17. Therefore, when peeling the protective film 10 from the resin substrate 21 in step [4], delamination between the adhesive layer 11 and the second layer 17, and between the first layer 16 and the second layer 17 is effectively suppressed or prevented. Consequently, even if a "whiskers" of the adhesive layer 11 extend toward the resin substrate 21 at the cut surface of the resin substrate 21 formed by punching in step [2], the protective film 10 (adhesive layer 11) can be peeled from the resin substrate 21 while effectively suppressing or preventing these whiskers from remaining on the cut surface of the resin substrate 21.

[0081] The adhesive thermoplastic resin (adhesive resin) is not particularly limited, but examples include polyolefin resins, elastomers, acrylic resins, polyurethane resins, etc., and one or more of these can be used in combination, but among them, adhesive polyolefin resins and elastomers are preferred. Furthermore, if the adhesive layer 11 contains an elastomer, a combination of a polyolefin resin and an elastomer is particularly preferred. This improves the adhesion between the adhesive layer 11 and the second layer 17, and effectively suppresses or prevents delamination between the adhesive layer 11 and the second layer 17.

[0082] Furthermore, examples of polyolefin resins include ethylene-vinyl acetate copolymer (EVA), ethylene-maleic anhydride copolymer, ethylene-methyl methacrylate copolymer (EMMA), ethylene-methyl acrylate copolymer (EMA), ethylene-methacrylic acid copolymer (EMAA), ethylene-acrylic acid copolymer (EAA), ethylene-ethylacrylic acid copolymer (EEA), ethylene-methacrylate-glycidyl acrylate terpolymer, as well as grafted polyolefin resins with monobasic unsaturated fatty acids such as acrylic acid and methacrylic acid, dibasic unsaturated fatty acids such as maleic acid, fumaric acid, and itaconic acid, or their anhydrides, and functional group-introduced polyolefin resins into which functional groups such as carboxylic acid groups, hydroxyl groups, amino groups, acid anhydride groups, oxazoline groups, and epoxy groups are introduced. One or more of these can be used in combination. Examples of the grafted material include maleic acid-grafted EVA and maleic acid-grafted ethylene-α-polyolefin resin copolymer.

[0083] Furthermore, the elastomer can be the same as that described for the elastomer included in the adhesive layer 11, and among these, a styrene block elastomer is preferred, and a styrene-olefin-styrene block copolymer elastomer is particularly preferred. By using an elastomer that contains styrene as a monomer component in this way, the adhesion of the second layer 17 to the adhesive layer 11 is improved, and thus, in step [4], the occurrence of adhesive residue on the resin substrate 21 can be more effectively suppressed or prevented.

[0084] Furthermore, the second layer 17 may contain, as a constituent material, a non-adhesive thermoplastic resin (non-adhesive resin) in addition to an adhesive thermoplastic resin. Since the non-adhesive thermoplastic resin shows excellent affinity to the polyolefin resin with a melting point of 150°C or higher contained in the first layer 16, the adhesion between the first layer 16 and the second layer 17 can be improved. Therefore, when peeling the protective film 10 from the resin substrate 21 in step [4], delamination between the first layer 16 and the second layer 17 can be effectively suppressed or prevented.

[0085] Examples of non-adhesive thermoplastic resins include polyolefin resins, polyesters, polyurethanes, silicone resins, polyamides, polyimides, polyvinyl chlorides, and polycarbonates. One or more of these can be used in combination, but polyolefin resins are preferred, and polyolefin resins with a melting point of 150°C or higher are more preferred. This allows the aforementioned effects to be exhibited more significantly.

[0086] Furthermore, the polyolefin resin is not particularly limited and includes, for example, polypropylene homopolymers, polyethylene homopolymers, propylene-ethylene copolymers, ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers, ethylene-methyl methacrylate copolymers, etc. One or more of these can be used in combination, and among them, polypropylene homopolymers are preferred. This makes it possible to obtain the polyolefin resin easily and inexpensively. In addition, transparency can be imparted to the second layer 17. Therefore, if the first layer 16 and the adhesive layer 11 also have transparency, the protective film 10 will also have transparency. Therefore, when the protective film 10 is attached to the resin substrate 21 in step [1], and when the protective film 10 is reattached to the resin substrate 21 prior to step [2], it is possible to visually check whether or not dust or other debris is interposed between the protective film 10 and the resin substrate 21. As a result, it is possible to reliably prevent the laminate 100 containing debris from migrating to step [2] and beyond, thereby improving the yield of the resulting sunglasses lenses 200.

[0087] Furthermore, for polyolefin resins with a melting point of 150°C or higher, the melting point is preferably between 155°C and 170°C.

[0088] The copolymer may be either a block copolymer or a random copolymer. However, polyolefin resins with a melting point of 150°C or higher are generally not adhesive on their own and are therefore used as non-adhesive thermoplastic resins. For example, propylene-ethylene random copolymer (melting point: 151°C) is adhesive and is therefore used as an adhesive thermoplastic resin.

[0089] Based on the above, when the second layer 17 contains a non-adhesive thermoplastic resin, the adhesive thermoplastic resin and the non-adhesive thermoplastic resin are preferably an elastomer and a polyolefin resin with a melting point of 150°C or higher. By using such a combination, it is possible to effectively suppress or prevent delamination between the adhesive layer 11 and the second layer 17, and between the first layer 16 and the second layer 17.

[0090] The second layer 17, having the above-described structure, preferably has a melting point of 120°C or higher. As mentioned above, the heating temperature of the coating layer 24 (resin substrate 21) during the heat bending in step [3] is preferably set to approximately 110°C to 160°C. Therefore, when the melting point of the second layer 17 is set to 120°C or higher and less than 150°C, the second layer 17 can be relatively easily melted or softened during the heat bending in step [3]. Consequently, in step [3], the second layer 17 is made to function as a molten or softened intermediate layer, causing the first layer 16 to be misaligned with respect to the surface direction of the resin substrate 21, thereby forming a gripping margin formed by the first layer 16 at the edge of the laminate 100. Therefore, the peeling of the protective film 10 in step [4] can be easily performed by gripping the gripping margin. Furthermore, in step [3], the second layer 17 is made to function as a molten or softened intermediate layer, thereby improving the cushioning effect of the second layer 17 during molding with a mold. As a result, it is possible to effectively absorb the irregularities of the mold or the irregularities of contaminants that have unintentionally entered between the mold and the protective film 10, thereby obtaining a heat-bent resin substrate 21 with an excellent appearance.

[0091] Furthermore, when the melting point of the second layer 17 is set to 150°C or higher, it is possible to suppress or prevent the second layer 17 from melting or softening during the heat bending in step [3]. Therefore, it is possible to effectively suppress or prevent a decrease in the adhesion force between the first layer 16 and the second layer 17. Consequently, when peeling the protective film 10 from the heat-bent resin substrate 21 in step [4], the occurrence of peeling between the first layer 16 and the second layer 17 is reliably prevented.

[0092] Furthermore, the second layer 17 preferably has an average thickness of 10 μm or more and 60 μm or less, and more preferably 10 μm or more and 40 μm or less. This ensures that the second layer 17 can reliably perform the functions described above.

[0093] Furthermore, each layer of the protective film 10 described above, including the adhesive layer 11 and the base layer 15 (first layer 16 and second layer 17), may contain various additives in addition to the constituent materials described above, such as antiblocking agents, antioxidants, light stabilizers, and antistatic agents. The content of these additives may be 10% by weight or less, 8% by weight or less, 0.001% by weight or more, or 0.1% by weight or more.

[0094] In particular, it is preferable that the first layer 16 contains an antiblocking agent. This allows the first layer 16 to more reliably exhibit its function of maintaining excellent release properties from the mold.

[0095] Furthermore, as antiblocking agents, inorganic particles and organic particles can be used, and one or more of these can be used in combination. While not particularly limited, inorganic particles include, for example, silica, zeolite, smectite, mica, vermiculite, and talc. Organic particles, while not particularly limited, include, for example, acrylic resins, polyolefin resins, polyester resins, polyurethane resins, polystyrene resins, silicone resins, and fluororesin.

[0096] If the first layer 16 contains an antiblocking agent, the content of the antiblocking agent in the first layer 16 is not particularly limited, but is preferably set to 0.5% by weight or more and 10% by weight or less, and more preferably to 1% by weight or more and 9% by weight or less. This makes the function of the first layer 16 more pronounced. Furthermore, an intermediate layer containing the above-mentioned additives may be formed between each of these layers.

[0097] Furthermore, the protective film 10 described above may be manufactured by any method, but for example, it can be manufactured using a co-extrusion method.

[0098] Specifically, three extruders are prepared, and the constituent materials for the adhesive layer 11, the first layer 16, and the second layer 17 are placed in each of them. These materials are then extruded in a molten or softened state, and the resulting laminate, in which these materials are stacked in layers, is supplied from the co-extrusion T-die to a sheet molding section composed of multiple cooling rolls, etc. After that, the laminate is cooled in this sheet supply section to produce the protective film 10.

[0099] Although the protective film of the present invention has been described above, the present invention is not limited thereto, and each layer constituting the protective film can be replaced with any configuration that can perform similar functions.

[0100] Furthermore, in the above embodiment, Figure 2 describes a case in which the protective film is reattached to the resin substrate after a portion of the protective film has been peeled off the resin substrate. However, the invention is not limited to this, and the protective film can also be reattached after the entire protective film has been peeled off the resin substrate.

[0101] Furthermore, while the above embodiment described the case in which the protective film of the present invention is used by attaching it to a resin substrate when heat-bending a resin substrate that has a lens for sunglasses, the protective film of the present invention can be used not only for heat-bending the resin substrate of such a sunglass lens, but also when heat-bending resin substrates such as lenses for goggles or visors for helmets. [Examples]

[0102] The present invention will be described in more detail below based on the examples. However, the present invention is not limited in any way by these examples.

[0103] 1. Preparation of raw materials First, the raw materials used to prepare the protective films for each example and comparative example are as follows.

[0104] <<Non-adhesive thermoplastic resin>> <Polyolefin resins with a melting point of 150°C or higher> Homopolypropylene (h-PP) with a melting point of 167°C, manufactured by Nippon Polypropylene Co., Ltd., "Novatec EA9FTD", MFR = 0.4g / 10min

[0105] <Polyolefin resins with a melting point of 120°C or higher and less than 150°C> Linear low-density polyethylene (LLDPE, manufactured by Ube Maruzen Polyethylene Co., Ltd., "Yumerit 2525F", MFR = 2.5g / 10min) with a melting point of 121°C. High-density polyethylene (HDPE, manufactured by Prime Polymer, "Hyzex 3300F", MFR = 1.1g / 10min) with a melting point of 132°C.

[0106] <Polyolefin resins with a melting point of less than 120°C> Linear low-density polyethylene (LLDPE, manufactured by Tosoh Corporation, "Nipolon-Z HF211R", MFR = 2.0g / 10min) with a melting point of 106°C. Linear low-density polyethylene (LLDPE, manufactured by Tosoh Corporation, "Nipolon-Z HF212R", MFR = 2.0g / 10min) with a melting point of 93°C. Linear low-density polyethylene (LLDPE, manufactured by Ube Maruzen Polyethylene Co., Ltd., "Yumerit 165HK", MFR = 2.0g / 10min) with melting points of 114°C and 99°C.

[0107] <<Adhesive thermoplastic resin>> <Elastomer> Styrene-ethylene-butylene-styrene block copolymer (SEBS, manufactured by Asahi Kasei Corporation, "ToughTec H1221") An α-polyolefin resin / polyethylene copolymer elastomer with a melting point of 66°C (Elastomer A, manufactured by Mitsui Chemicals, "Tafmer A-1085S"). α-polyolefin resin / polyethylene copolymer elastomer with a melting point of 77°C (Elastomer B, manufactured by Mitsui Chemicals, "Toughmer A-4090S") An α-polyolefin resin / polypropylene copolymer elastomer with a melting point of 83°C (Elastomer C, manufactured by Mitsui Chemicals, "Toughmer XM-7080"). α-polyolefin resin / polypropylene copolymer elastomer with a melting point of 98°C (Elastomer D, manufactured by Mitsui Chemicals, "Toughmer XM-7090") An α-polyolefin resin / polybutene copolymer elastomer with a melting point of 110°C (Elastomer E, manufactured by Mitsui Chemicals, "Tafmer BL-311M"). An α-polyolefin resin / polypropylene copolymer elastomer with a melting point of 140°C (Elastomer F, manufactured by Mitsui Chemicals, "Tafmer PN-2070").

[0108] <Polyolefin resins> Ethylene-methacrylic acid copolymer with a melting point of 106°C (EMAA, manufactured by Mitsui DuPont Polychemicals, "Nucrel AN4214C") Ethylene-methyl acrylate copolymer (EMA, manufactured by Nippon Polyethylene Co., Ltd., "Rexpearl EB330H") with a melting point of 90°C.

[0109] <Polyolefin resins with a melting point of 120°C or higher and less than 150°C> Random polypropylene copolymer with a melting point of 131°C (r-PP, manufactured by Sumitomo Chemical Co., Ltd., "Noblen S131", MFR = 1.3g / 10min)

[0110] <Antiblocking agent> Antiblocking agent (Kinoplus FPP-AB05A, manufactured by Sumika Color Co., Ltd.)

[0111] 2. Manufacturing of protective film (Example 1) [1] First, in order to form the adhesive layer (innermost layer), an adhesive layer forming material (resin composition) was prepared by kneading LLDPE, which has a melting point of 121°C as a polyolefin resin, and elastomer A as an elastomer, so that the elastomer A content was 10% by weight.

[0112] [2] Next, in order to form the second layer (intermediate layer) of the base layer, a second layer-forming material (resin composition) was prepared by kneading SEBS as an adhesive thermoplastic resin and h-PP with a melting point of 167°C as a non-adhesive thermoplastic resin, such that the SEBS content was 20% by weight.

[0113] [3] Next, the prepared adhesive layer-forming material, the prepared second layer-forming material, and h-PP with a melting point of 167°C, which is a polyolefin resin with a melting point of 150°C or higher for forming the first layer (outermost layer), were each placed in three extruders.

[0114] [4] Next, the molten material was extruded from three extruders to obtain a laminate of the molten material in layers from a co-extrusion T-die. After cooling this laminate, the protective film of Example 1 was obtained, in which the average thicknesses of the first layer, second layer, and adhesive layer were 10 μm, 30 μm, and 10 μm, respectively.

[0115] (Examples 2-12, Comparative Example 1, Comparative Example 2) Protective films for Examples 2 to 12, Comparative Example 1, and Comparative Example 2 were obtained in the same manner as in Example 1, except that the type of polyolefin resin, the type of elastomer, the presence or absence of an antiblocking agent used in step [1], the elastomer content in the adhesive layer-forming material to be prepared, the type of adhesive thermoplastic resin, the non-adhesive thermoplastic resin used in step [2], the adhesive thermoplastic resin content in the second layer-forming material to be prepared, and at least one of the types of polyolefin resin with a melting point of 150°C or higher used in step [3] were changed as shown in Table 1.

[0116] 3. Evaluation The protective films of each example and each comparative example were evaluated by the following method.

[0117] <1> Evaluation of the adhesion strength of protective films First, for each example and each comparative example, the protective film was attached to the polycarbonate substrate by pressing it with a rubber roll at a speed of 2 m / min under a pressure of 5880 N / m, in accordance with JIS Z 0273:2009.

[0118] Next, the protective film was cut to a size of 100 mm in length and 25 mm in width. Then, under conditions of 23°C, the peel strength T0 [N / 25 mm] was measured when one end of the cut protective film was held and peeled off at a speed of 200 mm / min in a 90° direction.

[0119] <2> Evaluation of the adhesion strength of protective film at a heating temperature of 100°C. First, for each example and comparative example, the protective film was attached to a polycarbonate substrate by pressing it with a rubber roll at a speed of 2 m / min under a pressure of 5880 N / m, in accordance with JIS Z 0273:2009. Then, it was heated at 100°C for 30 minutes and cooled to 23°C.

[0120] Next, the protective film was cut to a size of 100 mm in length and 25 mm in width. Then, the peel strength T1 [N / 25 mm] was measured when one end of the cut protective film was held and peeled off at a speed of 200 mm / min in a 90° direction.

[0121] <3> Evaluation of the adhesion strength of protective film at a heating temperature of 145°C. First, for each example and comparative example, the protective film was attached to a polycarbonate substrate by pressing it with a rubber roll at a pressure of 5880 N / m at a speed of 2 m / min, in accordance with JIS Z 0273:2009. Then, it was heated at 145°C for 30 minutes and cooled to 23°C.

[0122] Next, the protective film was cut to a size of 100 mm in length and 25 mm in width. Then, the peel strength T2 [N / 25 mm] was measured when one end of the cut protective film was held and peeled off at a speed of 200 mm / min in a 90° direction.

[0123] The evaluation results for the protective films of each example and comparative example obtained as described above are shown in Table 1 below.

[0124] [Table 1]

[0125] As shown in Table 1, the protective film in each embodiment has two different melting points in the adhesive layer. As a result, the ratio (T2 / T1) of the peel strength T1, which is the adhesion force of the protective film at a heating temperature of 100°C, to the peel strength T2, which is the adhesion force of the protective film at a heating temperature of 145°C, is 2.0 or greater. This shows that the degree of adhesion to the polycarbonate substrate (resin substrate) can be selected by appropriately setting the heating temperature for the protective film.

[0126] In contrast, the protective films in each comparative example had only one melting point in the adhesive layer, and it is presumed that this resulted in the ratio of the peel strength T1 to the peel strength T2 (T2 / T1) being less than 2.0. Therefore, it became clear that the degree of adhesion to the polycarbonate substrate (resin substrate) cannot be selected by appropriately setting the heating temperature for the protective film. [Explanation of Symbols]

[0127] 10 Protective Films 11 Adhesive layer 15 Base material layer 16. The first layer 17. The second layer 21 Resin substrate 23 Polarizer 24 Covering layer 30 Polycarbonate layer 100-layer structure 200 Sunglasses Lenses

Claims

1. A protective film used by attaching it to a resin substrate when the resin substrate is subjected to heat bending under heating conditions, A base layer and It has an adhesive layer positioned between the base material layer and the resin substrate, which adheres to the resin substrate, The substrate layer is composed of a laminate having a first layer located on the opposite side of the adhesive layer and a second layer located on the side of the adhesive layer. The adhesive layer is a protective film characterized by containing an adhesive polyolefin as the main material and having two or more different melting points.

2. The protective film according to claim 1, wherein the adhesive layer comprises at least two or more materials having different melting points.

3. The protective film according to claim 1 or 2, wherein the difference between the melting point of the constituent material having the lowest melting point and the melting point of the constituent material having the highest melting point is 5°C or more and 80°C or less.

4. The protective film according to claim 3, wherein the melting point of the constituent material having the lowest melting point is 60°C or more and 130°C or less.

5. The protective film according to claim 3 or 4, wherein the melting point of the constituent material having the highest melting point is 80°C or higher and 160°C or lower.

6. A protective film according to any one of claims 1 to 5, wherein the protective film is attached to a polycarbonate substrate in accordance with JIS Z 0273:2009 by pressing it with a rubber roll applied at a pressure of 5880 N / m at a speed of 2 m / min, then heated at 100°C for 30 minutes, cooled to 23°C, cut the protective film to a size of 100 mm in length and 25 mm in width, and then, by holding one end of the cut protective film and peeling it off at a speed of 200 mm / min in a 90° direction, the peel strength measured is 0.05 N / 25 mm or more and 0.6 N / 25 mm or less.

7. A protective film according to any one of claims 1 to 6, wherein the protective film is attached to a polycarbonate substrate in accordance with JIS Z 0273:2009 by pressing it with a rubber roll applied at a pressure of 5880 N / m at a speed of 2 m / min, then heated at 145°C for 30 minutes, cooled to 23°C, cut the protective film to a size of 100 mm in length and 25 mm in width, and then, by holding one end of the cut protective film and peeling it off at a speed of 200 mm / min in a 90° direction, the peel strength measured is 0.15 N / 25 mm or more and 1.5 N / 25 mm or less.

8. The protective film according to any one of claims 1 to 7, wherein the first layer contains a thermoplastic resin as the main material and has a melting point of 150°C or higher, and the second layer contains a thermoplastic resin as the main material and has a melting point of 120°C or higher.

9. The protective film according to claim 8, wherein the thermoplastic resin contained in the first layer and the thermoplastic resin contained in the second layer are both polyolefins.

10. A protective film according to any one of claims 1 to 9, which is attached to both sides of the resin substrate.

11. The protective film according to any one of claims 1 to 10, wherein the resin substrate comprises a coating layer on both sides, one side or the other side, which is composed of a single layer or a laminate having at least one of a polycarbonate resin layer, a polyamide resin layer, and a cellulose resin layer.

12. The protective film according to any one of claims 1 to 11, wherein the resin substrate is subjected to the heat bending process by press molding or vacuum forming.

13. The protective film according to any one of claims 1 to 12, wherein the heating temperature when the resin substrate is subjected to the heat bending process under heating is 110°C or more and 160°C or less.

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