Surface protective film

Abstract

The purpose of this invention is to provide a surface protective film that has appropriate initial adhesion as a masking material for protecting the surface of smooth synthetic resin boards, etc. Even under high temperature environments or long-term transportation, the adhesion is less excessive, making it easy to peel off and resulting in an excellent appearance. The surface protective film of this invention is formed by laminating a substrate layer and an adhesive layer. The substrate layer comprises low-density polyethylene (LDPE), and the adhesive layer comprises styrene-butadiene copolymer hydrogenated (SEBS) and high-density polyethylene (HDPE). The ratio of the thickness of the substrate layer to the thickness of the adhesive layer (thickness of the substrate layer / thickness of the adhesive layer) is 4.0 to 7.0. The mass ratio of the SEBS to the HDPE (SEBS:HDPE) is 70:30 to 55:45. The styrene content of the SEBS is 18 to 22% by mass, and the molecular weight distribution of the HDPE is 4.0 or less.

Description

Technical Field

[0001] This invention relates to a surface protective film. Prior Technology

[0002] Previously, the surfaces of smooth-surfaced synthetic resin sheets such as PMMA (polymethyl methacrylate) and PC (polycarbonate), steel sheets, and glass sheets (hereinafter also referred to as "synthetic resin sheets, etc.") were easily damaged during transportation or storage. Therefore, surface protective films were used to prevent this, and various surface protective films have been proposed. Commonly used surface protective films are laminated films obtained by co-extruding PE (polyethylene), which serves as the substrate layer, and a material that serves as the adhesive layer. In the laminated film, the material that serves as the adhesive layer is appropriately selected according to the required adhesive strength. For example, EVA (ethylene-vinyl acetate copolymer) or POE (ethylene-α-olefin copolymer) can be used as the adhesive layer material.

[0003] The surface protective film is designed to adhere appropriately to the synthetic resin board or similar material in a manner that prevents it from peeling off during handling. On the other hand, the surface protective film must be easily peelable during final stages such as assembly. Therefore, the initial adhesion of the surface protective film is required to be 20-60 g / 25 mm at room temperature, preferably 20-50 g / 25 mm, in a 180° peel test (peeling speed 300 mm / min).

[0004] However, the adhesive strength of protective films applied to the surface of synthetic resin boards tends to increase over time, and this increase is accelerated at higher storage temperatures. Therefore, even if the initial adhesive strength is adequate, over-adhesion occurs, making them difficult to peel off. This results in decreased workability during the final stages of assembly of synthetic resin boards. Furthermore, during sea transport and subsequent land transport of synthetic resin boards, the aforementioned over-adhesion becomes even more pronounced when they are handled directly at high temperatures for extended periods while still covered with the protective film. Therefore, the industry demands a protective film with an adhesive strength no more than twice the initial adhesive strength (approximately 40-120 g / 25 mm, preferably 40-100 g / 25 mm) that is resistant to over-adhesion.

[0005] While the initial adhesion of previous EVA or POE met the requirements, their adhesion became excessive when exposed to high temperatures, thus requiring further improvement.

[0006] On the other hand, SEBS (hydrogenated styrene-butadiene copolymer), a styrene-based thermoplastic elastomer, is a material with a good balance between adhesion and anti-adhesion properties, and has long been used as an adhesive layer material for surface protective films in a wide range of applications. However, due to the high adhesion of its monomers, SEBS tends to adhere excessively to smooth synthetic resin sheets and other materials, making it difficult to peel off. Therefore, the industry has proposed a technique to reduce the adhesion of SEBS by blending it with thermoplastic resins such as PE, thereby adjusting it to a suitable level of adhesion.

[0007] For example, Patent Document 1 discloses a surface protective film that is formed by an adhesive layer on a substrate film containing a thermoplastic resin. The adhesive layer contains 30-70% by mass of a block copolymer represented by the general formula ABA (where A represents a styrene polymer block, B represents a butadiene polymer block or an isoprene polymer block, or an olefin polymer block obtained by hydrogenating such polymers, i.e., butadiene polymers or isoprene polymers), and 70-30% by mass of a polyolefin resin obtained by extraction using the n-pentane extraction method with an extract fraction of less than 1.0% by mass.

[0008] Patent Document 2 discloses a surface protective film formed by forming an adhesive layer on one side of a substrate film containing a polyolefin resin. The adhesive layer is formed in a resin composition containing 50-75% by mass of a block copolymer of the general formula ABA (where A represents an aromatic vinyl polymer block and B represents an olefin polymer block obtained by hydrogenating the carbon-carbon unsaturated double bonds present in an isoprene polymer block or an isoprene / butadiene random copolymer block) and 25-50% by mass of high-density polyethylene resin, with 0-25% by mass of an ethylene-α-olefin copolymer mixed relative to the resin composition.

[0009] Patent document 3 discloses a surface protective film which is integrally formed by laminating an adhesive layer (II) on a substrate layer (I) containing a polyolefin resin. The adhesive layer (II) contains a hydride of a styrene-conjugated diene copolymer (a) and a polyethylene resin (b), and the mixing ratio of the two components is 10 to 50 parts by mass relative to 100 parts by mass of (a) and (b). [Previous Technical Documents] [Patent Literature]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 11-199839 [Patent Document 2] Japanese Patent Application Publication No. 9-143439 [Patent Document 3] Japanese Patent Application Publication No. 2007-161882 Summary of the Invention

[0011] [The problem the invention aims to solve] However, the initial adhesion of the surface protective film in Patent Publication 1 is as low as 12~18 g / 25 mm. Therefore, there is a problem that the surface protective film may float off the surface of the synthetic resin board or the like under environmental changes in the storage location or during storage, thus contaminating the surface of the synthetic resin board or the like and causing poor appearance.

[0012] The surface protective film in Patent Publication 2 has appropriate initial adhesion, but the high-density polyethylene in the block copolymer is not sufficiently dispersed, resulting in an adhesion hyperactivity ratio exceeding 2, making it difficult to peel off from smooth synthetic resin sheets. Furthermore, the block copolymer contains a large amount of aromatic vinyl polymer blocks, leading to insufficient affinity with the PE substrate material. This results in streaks easily forming on the co-extruded surface protective film, causing poor appearance. If streaks appear on the surface protective film, it hinders the inspection of the surface condition of synthetic resin sheets.

[0013] The surface protective film in Patent Publication 3 is designed for adherends with uneven surfaces, and therefore increases the copolymer ratio to achieve high adhesion. Consequently, it will over-adhere to smooth adherends and become difficult to peel off.

[0014] The present invention was made in view of such problems, and its object is to provide a surface protective film that has appropriate initial adhesion as a masking material for protecting the surface of a smooth synthetic resin board or the like, and exhibits less hyperadhesion even when placed in a high-temperature environment or during long-term transportation, thus making it easy to peel off and having an excellent appearance. [Technical means to solve the problem]

[0015] In order to solve the above-mentioned problems, the inventors have repeatedly and diligently studied and found that the above-mentioned problems can be solved by using the following surface protective film, thereby completing the present invention. The above-mentioned surface protective film is formed by laminating a substrate layer and an adhesive layer. The substrate layer contains low-density polyethylene (LDPE), and the adhesive layer contains styrene-butadiene copolymer hydrogenated (SEBS) with a specific styrene content and high-density polyethylene (HDPE) with a specific molecular weight distribution (Mw / Mn) in a specific mass ratio. The thickness of the substrate layer and the thickness of the adhesive layer are in a specific ratio.

[0016] That is, the present invention is as follows. [1] A surface protective film is formed by laminating a substrate layer and an adhesive layer, wherein the substrate layer comprises low-density polyethylene (LDPE), the adhesive layer comprises styrene-butadiene copolymer (SEBS) and high-density polyethylene (HDPE), the ratio of the thickness of the substrate layer to the thickness of the adhesive layer (thickness of the substrate layer / thickness of the adhesive layer) is 4.0 to 7.0, the mass ratio of the SEBS to the HDPE (SEBS:HDPE) is 70:30 to 55:45, the styrene content of the SEBS is 18 to 22% by mass, and the molecular weight distribution (Mw / Mn) of the HDPE, expressed by the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of standard polystyrene as determined by gel permeation chromatography (GPC), is 4.0 or less.

[0017] [2] As described in [1], the surface protective film has a melt mass flow rate (MFR, 230°C, 2.16 kg load) of 11.0~15.0 g / 10 min for the above-mentioned SEBS.

[0018] [3] As described in [1] or [2], the ratio of the melt viscosity of the SEBS (resin temperature: 210°C, shear rate: 121.6 seconds) to the melt viscosity of the HDPE (resin temperature: 210°C, shear rate: 121.6 seconds) (melt viscosity of the SEBS / melt viscosity of the HDPE) is 0.85 to 1.30.

[0019] [4] The surface protective film described in any of [1] to [3], wherein the melt mass flow rate (MFR, 190°C, 2.16 kg load) of the HDPE is 2.0 to 6.0 g / 10 min. [Effects of the Invention]

[0020] According to the present invention, a surface protective film is provided, which has an appropriate initial adhesion as a protective material for a smooth synthetic resin board or the like, and exhibits less hyperadhesion even when placed in a high-temperature environment or during long-term transportation, thus making it easy to peel off and providing an excellent appearance. Implementation

[0021] The following is a detailed description of the method for implementing the present invention (hereinafter referred to as "this embodiment"). Furthermore, the following embodiments are merely illustrative of the present invention and are not intended to limit the present invention to the following content; the present invention can be implemented with various modifications within its scope.

[0022] [Surface Protective Film] The surface protective film of this embodiment is formed by laminating a substrate layer and an adhesive layer. The substrate layer comprises low-density polyethylene (LDPE), and the adhesive layer comprises styrene-butadiene copolymer hydrogenated (SEBS) and high-density polyethylene (HDPE). The ratio of the thickness of the substrate layer to the thickness of the adhesive layer (substrate layer thickness / adhesive layer thickness) is 4.0 to 7.0. The mass ratio of SEBS to HDPE (SEBS:HDPE) is 70:30 to 55:45. The styrene content of SEBS is 18 to 22% by mass, and the molecular weight distribution (Mw / Mn) of HDPE is 4.0 or less. Furthermore, in this specification, the weight average molecular weight (Mw) and number average molecular weight (Mn) are values ​​converted from standard polystyrene determined by gel permeation chromatography (GPC). Also, the molecular weight distribution (Mw / Mn) is expressed as the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn).

[0023] According to this embodiment, a surface protective film can be provided, which has an appropriate initial adhesion for use as a protective material for smooth synthetic resin boards, etc. Even when placed in a high-temperature environment or during long-term transportation, the adhesion is less excessive, so it is easy to peel off. It is also less likely to produce streaks during the molding of films such as co-extrusion molding, resulting in an excellent appearance. The reason for this is not yet certain, but the inventors presume the following.

[0024] When adjusting the initial tack of PE to SEBS, for example, if LLDPE (linear low-density polyethylene) and LDPE (low-density polyethylene) are used as the PE, it is difficult to adjust to a suitable initial tack because LLDPE and LDPE themselves also have adhesive properties. However, HDPE, due to its relatively low adhesive properties, allows for appropriate adjustment of the initial tack when mixed with SEBS.

[0025] Based on this, HDPE includes Ziegler-catalyzed and metallocene-catalyzed polymers. However, HDPE obtained using a Ziegler catalyst has a wider molecular weight distribution, resulting in a higher proportion of low molecular weight PE, which is presumably the reason for increased adhesion during heat deformation. Furthermore, HDPE obtained using a Ziegler catalyst also has a higher proportion of high molecular weight PE, making it difficult to deform at room temperature. This reduces its adhesion to smooth synthetic resin sheets and makes it difficult to adjust to a suitable initial tack. In contrast, HDPE obtained using a metallocene catalyst has a narrower molecular weight distribution, with relatively fewer low and high molecular weight PE. Therefore, the adhesion caused by heat deformation in this type of HDPE is less likely to deteriorate. Moreover, it deforms easily at room temperature, thus suppressing the reduction in adhesion to smooth synthetic resin sheets. This allows for the attainment of a suitable initial tack. Therefore, by using HDPE, the surface protective film can exhibit excellent resistance to adhesion buildup and initial tack.

[0026] Furthermore, as an adhesive layer, by using HDPE with a narrow molecular weight distribution and SEBS with a styrene content within a specific range in a specific mass ratio, the melt viscosity of HDPE and SEBS can be made similar, improving the dispersibility of HDPE in SEBS. Also, the compatibility between HDPE and SEBS becomes better. Therefore, the surface protective film can have moderate initial adhesion and thus exhibit good anti-adhesion properties. In addition, a surface protective film with excellent appearance can be obtained that is less prone to streaks during film forming such as co-extrusion molding.

[0027] It is presumed that by using an adhesive layer and a substrate layer of LDPE with a specific thickness, a surface protective film can be obtained. This film has appropriate initial adhesion as a protective material for smooth synthetic resin boards, etc. Even when placed in high-temperature environments or during long-term transportation, there is less adhesion hyperactivity, making it easy to peel off. Furthermore, it is less prone to streaks during film forming such as co-extrusion molding, resulting in an excellent appearance. However, the reasons are not limited to this.

[0028] The following is a description of each layer in the surface protective film.

[0029] [Ratio of substrate layer thickness to adhesive layer thickness] The surface protective film is composed of a substrate layer and an adhesive layer. The ratio of the thickness of the substrate layer to the thickness of the adhesive layer (substrate layer thickness / adhesive layer thickness) is 4.0 to 7.0. A ratio of 4.0 or higher can suppress the reduction in the mechanical strength of the substrate layer, preventing film elongation or breakage during production or use. A ratio of 7.0 or lower can suppress the reduction in adhesion, preventing the surface protective film from lifting and contaminating the surface of the synthetic resin board. The ratio of the thickness of the substrate layer to the thickness of the adhesive layer is preferably 4.5 to 6.0. In this specification, the thicknesses of the substrate layer and the adhesive layer can be measured, for example, by observing the cross-section of the surface protective film using an optical microscope. Specific measurement methods are described in the examples.

[0030] [Thickness of the surface protective film] In terms of a better balance between film strength and adhesion, the overall thickness of the surface protective film is preferably 40~60 μm, and more preferably 43~57 μm.

[0031] [Substrate Layer] The surface protective film contains a substrate layer.

[0032] The substrate layer, in terms of its excellent formability, transparency, and flexibility, includes low-density polyethylene (LDPE). Furthermore, due to its relatively high melt tension, LDPE tends to effectively suppress shrinkage. LDPE's relatively high transparency allows for suitable visual inspection of the surface protective film adhered to the synthetic resin board. LDPE can be used alone or in combination of two or more types.

[0033] Furthermore, without compromising transparency or formability, one or more materials selected from the group consisting of HDPE and LLDPE may be added to the substrate layer. For HDPE, refer to the HDPE included in the adhesive layer. For LLDPE, commercially available products may be used. Examples of commercially available products include: Umerit (registered trademark) manufactured by Ube Maruzen Polyethylene Co., Ltd., L-LDPE resin manufactured by NUC Co., Ltd., and Sumikathene-L (registered trademark), Sumikathene-E (registered trademark), EXCELLEN VL, and EXCELLEN FX (registered trademark) manufactured by Sumitomo Chemical Co., Ltd.

[0034] The density of LDPE is typically 900~930 kg / cm². If the density of LDPE falls within this range, the surface protective film tends to have superior transparency and a better appearance. In this specification, density refers to the value measured according to the density gradient tube method (23°C) as described in JIS K7112 (1999). The specific measurement method can be found in the examples.

[0035] The melt mass flow rate (MFR, 190°C, 2.16 kg load) of LDPE is preferably 0.05~50.0 g / 10 min. If the MFR of LDPE is within the above range, the surface protective film tends to have better transparency and a superior appearance. In this specification, MFR refers to the value measured according to ISO 1133 at a temperature of 190°C or 230°C with a 2.16 kg load. The specific measurement method can be found in the examples.

[0036] The preferred molecular weight distribution (Mw / Mn) of LDPE is 2.0~30.0. If the molecular weight distribution (Mw / Mn) of LDPE is within the above range, the surface protective film tends to have better transparency and a better appearance.

[0037] As for LDPE, high-pressure low-density polyethylene is preferred. The density of high-pressure low-density polyethylene is preferably 910~930 kg / cm2, more preferably 911~929 kg / cm2. The melt mass flow rate (MFR, 190°C, 2.16 kg load) of high-pressure low-density polyethylene is preferably 0.1~5.0 g / 10 min, more preferably 0.2~4.0 g / 10 min. The molecular weight distribution (Mw / Mn) is preferably 2.0~30.0. If the density, MFR, and molecular weight distribution (Mw / Mn) of high-pressure low-density polyethylene are within the above ranges, the surface protective film tends to have better transparency and appearance, and the formability also tends to be further improved.

[0038] As LDPE, commercially available products can be used. Examples of commercially available products include: "Sumikathene L211" manufactured by Sumitomo Chemical Co., Ltd., and "L2340" of the "Suntec-LD" series manufactured by Asahi Kasei Co., Ltd.

[0039] High-pressure low-density polyethylene can usually be manufactured by known methods. For example, high-pressure low-density polyethylene can be manufactured by polymerizing ethylene and α-olefins in an autoclave or tubular reactor under high temperature and pressure of 100~300°C and 100~350 MPa in the presence of free radical generating agents such as peroxides.

[0040] [Adhesive layer] The surface protective film contains an adhesive layer. The adhesive layer comprises styrene-butadiene copolymer hydrogenated (SEBS) and high-density polyethylene (HDPE).

[0041] (SEBS) SEBS is a copolymer obtained by hydrogenating a styrene-based block copolymer. Styrene-based block copolymers are typically block copolymers represented by ABA or ABAB, or mixtures of block copolymers represented by ABA and AB, where A is a styrene polymer block and B is a hydrogenated butadiene polymer block. SEBS can be used alone or in combination with two or more types.

[0042] SEBS is preferably composed of styrene-based polymer blocks. In this specification, "based" means that the polymer blocks of SEBS contain 95% to 100% by mass of styrene units.

[0043] As a SEBS, it is preferably a block copolymer represented by ABA or ABAB. The connecting portion of the styrene polymer block and the butadiene polymer block can be a styrene-butadiene random copolymer.

[0044] The styrene content of SEBS relative to the total amount of SEBS (100% by mass) is 18-22% by mass. If the styrene content is above 18% by mass, the adhesion of SEBS itself becomes better, and the amount of HDPE added can be adjusted to a suitable range. Therefore, the adhesive layer will not become too hard, and the tendency for initial adhesion to decrease can be suppressed. If the styrene content is below 22% by mass, the compatibility between SEBS and HDPE becomes better, streaks are less likely to form in the film, and the appearance of the synthetic resin board with the protective film on is excellent. Furthermore, in this specification, the styrene and butadiene content can be determined, for example, using nuclear magnetic resonance (NMR). The styrene content can be controlled by adjusting the amount and timing of styrene and butadiene added during the polymerization process.

[0045] The butadiene content of SEBS relative to the total amount of SEBS (100% by mass) is preferably 78-82% by mass.

[0046] The vinyl bond content of the butadiene portion is preferably 27-30% by mass relative to the total amount of SEBS (100% by mass). In this specification, the vinyl bond content refers to the total content of 1,2-vinyl bonds (butadiene incorporated into the polymer as 1,2-bonds) and 3,4-vinyl bonds (butadiene incorporated into the polymer as 3,4-bonds) relative to the total butadiene content (here, when 1,3-butadiene is used as the conjugated diene, it is the 1,2-vinyl bond content; when isoprene is used as the conjugated diene, it is the 3,4-vinyl bond content). The vinyl bond content can be determined using nuclear magnetic resonance (NMR). The butadiene-derived microstructure (cis / trans ratio, vinyl bond content) in SEBS can be arbitrarily controlled by using known polar compounds, etc.

[0047] The hydrogenation rate of the unsaturated double bonds derived from butadiene in SEBS is preferably 70% by mass or higher, more preferably 85% by mass or higher, and even more preferably 95% by mass or higher. The upper limit of the hydrogenation rate is typically below 100% by mass, and can also be below 99% by mass. If the hydrogenation rate is within the above range, gelation during SEBS film formation can be further suppressed, resulting in better paste residue and thus a tendency to obtain a superior appearance. The hydrogenation rate can be controlled, for example, by adjusting the amount of catalyst during hydrogenation. Furthermore, the hydrogenation rate during polymerization can be controlled, for example, by adjusting the amount of catalyst, hydrogen feed rate, pressure, and temperature during hydrogenation. The hydrogenation rate can be measured using nuclear magnetic resonance (NMR).

[0048] The melt mass flow rate (MFR, 230°C, 2.16 kg load) of SEBS is preferably 11.0~15.0 g / min, more preferably 12.0~14.0 g / min. If the MFR of SEBS is within the above range, the dispersibility of HDPE in SEBS tends to be better, and the balance between initial tack and anti-adhesion tends to be more favorable. Furthermore, there is a tendency to obtain suitable anti-adhesion. There is also a tendency to reduce the likelihood of film streaking and poor appearance of the surface protective film.

[0049] The melt viscosity of SEBS (resin temperature: 210°C, shear rate: 121.6 s⁻¹) is preferably 700~1000 mPa·s, more preferably 800~900 mPa·s. If the melt viscosity of SEBS is within the above range, the dispersibility of HDPE in SEBS tends to be better, and the balance between initial tack and resistance to tackiness tends to be more favorable. There is a tendency to obtain suitable resistance to tackiness. In this specification, regarding melt viscosity, it can be continuously measured using a capillary rheometer at a resin temperature of 210°C and a shear rate of 12.2 s⁻¹ to 12160 s⁻¹, and the value is obtained at a shear rate of 121.6 s⁻¹. The specific measurement method can be found in the examples.

[0050] The number average molecular weight (Mn) of SEBS is preferably 50,000~150,000, more preferably 60,000~130,000, and even more preferably 70,000~120,000. If the number average molecular weight of SEBS is within the above range, the dispersibility of HDPE in SEBS tends to be better, and the balance between initial tack and anti-adhesion properties tends to be more excellent.

[0051] The molecular weight distribution (Mw / Mn) of SEBS is preferably below 1.20, more preferably below 1.15, and even more preferably below 1.13. If the molecular weight distribution (Mw / Mn) of SEBS is within the above range, there is a tendency for the mechanical strength of SEBS to increase. SEBS does not have a branched structure and tends to be easily manufactured through successive polymerization.

[0052] As a SEBS, commercially available products can be used. Examples of commercially available products include: "H1052 (product name)", "H1062 (product name)", "H1521 (product name)" and "P1083 (product name)" of the "Tuftec (registered trademark)" series manufactured by Asahi Kasei Corporation.

[0053] SEBS can be manufactured using known methods. For example, Japanese Patent Publication Nos. 36-19286, 43-17979, 46-32415, 49-36957, 48-2423, 48-4106, 51-49567, and 59-166518 can be referenced.

[0054] (HDPE) High-density polyethylene (HDPE) can be exemplified by ethylene homopolymers or copolymers of ethylene and α-olefins. Because HDPE has relatively low tackiness, when mixed with SEBS, the initial tack can be appropriately adjusted. HDPE can be used alone or in combination with two or more other types.

[0055] Examples of α-olefins include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1-tetracene, and 1-tetradecene, which are α-olefins with 3 to 20 carbon atoms. An α-olefin can be used alone or in combination of two or more.

[0056] HDPE is preferably polyethylene with a comonomer content of less than 0.1% by mass, such as α-olefin, and more preferably ethylene homopolymer without comonomers. In this specification, "high density" is generally 930~970 kg / cm3, and preferably 940~960 kg / cm3.

[0057] HDPE has a molecular weight distribution (Mw / Mn) of less than 4.0, resulting in a narrow molecular weight distribution. When the molecular weight distribution of HDPE is less than 4.0, the low molecular weight components are relatively fewer, making it less prone to deformation even at high temperatures, thus achieving excellent adhesion resistance. Furthermore, the high molecular weight components are also relatively fewer, making it easier to deform at room temperature and suppressing the reduction in adhesion to smooth synthetic resin sheets. Therefore, suitable initial tack can be obtained. By using HDPE, the surface protective film can exhibit excellent adhesion resistance and initial tack. The preferred molecular weight distribution of HEPD is below 3.8. The lower limit of the molecular weight distribution is, for example, above 3.2.

[0058] The melt mass flow rate (MFR, 190°C, 2.16 kg load) of HDPE is preferably 2.0~6.0 g / 10 min, more preferably 3.0~5.5 g / 10 min. If the MFR of HDPE is within the above range, the dispersibility of HDPE in SEBS tends to be better, and the balance between the required tack and resistance to excessive tack tends to be more favorable. Furthermore, there is a tendency to obtain suitable resistance to excessive tack.

[0059] The melt viscosity of HDPE (resin temperature: 210℃, shear rate: 121.6 s⁻¹) is preferably 700~1000 mPa·s, more preferably 800~900 mPa·s. If the melt viscosity of HDPE is within the above range, there is a tendency for HDPE to exhibit better dispersibility in SEBS, and a more favorable balance between initial tack and resistance to tackiness. Furthermore, there is a tendency to obtain suitable resistance to tackiness.

[0060] As HDPE, commercially available products can be used. Examples of commercially available products include Asahi Kasei Corporation's "Creolex" series, specifically "T4750" and "T701A".

[0061] HDPE can be manufactured using known methods employing metallocene catalysts. For example, Japanese Patent Application Publication Nos. 2007-106857, 2015-89937, and 2013-249094 can be consulted as methods for manufacturing HDPE.

[0062] [SSEBS to HDPE mass ratio] The mass ratio of SEBS to HDPE (SEBS:HDPE) is 70:30 to 55:45. If the mass ratio of HDPE is 30 or higher and the mass ratio of SEBS is 70 or lower, the initial adhesion is 60 g / 25 mm or lower, and the adhesion after adhesion enhancement is 120 g / 25 mm or lower. Preferably, the initial adhesion is 50 g / 25 mm or lower, and the adhesion after adhesion enhancement is 100 g / 25 mm or lower. Therefore, the surface protective film is easily peeled off from the synthetic resin board, etc. If the mass ratio of HDPE is 45 or lower and the mass ratio of SEBS is 55 or higher, the initial adhesion is 20 g / 25 mm or higher. The surface protective film is less likely to float off the synthetic resin board, etc., and is less likely to contaminate the surface of the synthetic resin board, etc. Furthermore, if the mass ratio of HDPE is below 45 and the mass ratio of SEBS is above 55, the adhesive force after the adhesion is enhanced will also be above 40 g / 25 mm. Therefore, even when placed in a high-temperature environment or during long-term transportation, the adhesion enhancement is less and the peeling is easier. In this specification, initial adhesion and adhesion strength can be evaluated based on a "180° peel test" conducted using a universal tensile and compression testing machine at a temperature of 23°C and 50% relative humidity at a peel speed of 300 mm / min. Specific evaluation methods can be found in the examples provided.

[0063] [Melt viscosity ratio] The ratio of the melt viscosity of SEBS (resin temperature: 210°C, shear rate: 121.6 s⁻¹) to the melt viscosity of HDPE (resin temperature: 210°C, shear rate: 121.6 s⁻¹) (SEBS melt viscosity / HDPE melt viscosity, hereinafter referred to as the "melt viscosity ratio") is preferably 0.85~1.30, more preferably 0.90~1.25, and even more preferably 0.95~1.20. If the melt viscosity ratio is within the above range, the viscosities of SEBS and HDPE are similar, resulting in better dispersibility of HDPE in SEBS during compounding in the film extruder. Therefore, the surface protective film can possess moderate initial adhesion and thus exhibit good anti-adhesion properties.

[0064] (Other ingredients) Without impairing the performance of the surface protective film, other thermoplastic resins, thermoplastic elastomers, UV absorbers, anti-aging agents, antioxidants, fillers, anti-blocking agents, lubricants, antistatic agents, plasticizers, and colorants may also be added to the adhesive layer. Other components may be used alone or in combination of two or more.

[0065] [Method for manufacturing the adhesive layer] The adhesive layer can be manufactured using known methods. As a method for manufacturing the adhesive layer, for example, a method using adhesive layer materials containing SEBS, HDPE, and other components as needed can be cited.

[0066] Regarding the adhesive layer material, it can be manufactured, for example, by dry mixing of SEBS, HDPE and other components as needed, or by adjusting it using conventional equipment for mixing polymeric substances. There are no particular limitations on the mixing equipment; examples include: Bambly mixers, Labo Plastomills, single-spindle extruders, and twin-spindle extruders. From the point of view of productivity and good mixing properties, it is preferable to manufacture by melt mixing using an extruder.

[0067] Furthermore, regarding the adhesive layer material, SEBS granules and HDPE granules can be pre-made and mixed together to produce composite granules, or SEBS granules and HDPE granules can be melt-mixed using a mixing device to obtain composite granules.

[0068] The melting temperature of the adhesive layer material during mixing can be set appropriately, usually in the range of 130~300℃, preferably in the range of 150~250℃.

[0069] [Manufacturing Method of Surface Protective Film] The surface protective film forms an adhesive layer on one side of the substrate layer. There are no particular limitations on the manufacturing method of the surface protective film; for example, common film manufacturing methods include T-die extrusion, blow molding, and calendering. Among these, T-die extrusion is preferred for manufacturing the surface protective film.

[0070] As a method for obtaining a surface protective film by T-die extrusion, an example is as follows: components that can form an adhesive layer, such as adhesive layer material, and components that can form a substrate layer, such as LDPE particles, are separated into two fluids, an adhesive layer forming fluid and a substrate layer forming fluid, using a melt co-extruder. These two fluids are then combined in the die orifice to form a single fluid and extruded, thereby combining the adhesive layer and the substrate layer to manufacture the film.

[0071] In the case of obtaining a surface protective film using an extruder, the material forming the adhesive layer can also be manufactured by pre-dry mixing the various components for the adhesive layer, thus making it a highly productive method. Furthermore, when using an extruder to extrude and form an adhesive film, there is a trend towards producing a surface protective film with superior adhesion and bonding strength.

[0072] [use] The surface protective film possesses both moderate initial adhesion and resistance to excessive adhesion. Furthermore, even during film forming processes such as co-extrusion, the surface protective film is less prone to streaks and other appearance defects. Therefore, the surface protective film can be temporarily adhered to the surfaces of optical components such as light guide plates or corner plates, synthetic resin boards, metal plates, decorative plywood, coated steel plates, and various nameplates, and is suitable as a surface protective film to prevent damage or contamination during processing, handling, and storage of these adhered bodies. The surface protective film is even more suitable for protecting the surfaces of smooth-surfaced synthetic resin boards such as PMMA or PC boards used in IT information terminal housings, smooth-surfaced steel plates used in building materials, and various smooth-surfaced plate-like bodies such as glass plates. To further enhance the effectiveness of the surface protective film, it can also be suitable for use on synthetic resin boards. [Example]

[0073] The present invention will be described in more detail below with reference to the embodiments, but the present invention is not limited to these embodiments. The measurement methods and physical property measurement methods applied to the embodiments and comparative examples are shown below.

[0074] <Determination Method> [MFR] The MFR (melt mass flow rate, g / 10 min) of SEBS (hydrogenated styrene-butadiene copolymer) was determined according to ISO 1133 at a temperature of 230°C and a load of 2.16 kg. The MFR (melt mass flow rate, g / 10 min) of HDPE (high-density polyethylene) is determined according to ISO 1133 at a temperature of 190°C and a load of 2.16 kg.

[0075] [Styrene content in SEBS] The styrene content (mass %) of SEBS was determined by dissolving a certain amount of SEBS in chloroform and measuring it using a UV spectrophotometer (Shimadzu Corporation, UV-2450 (trade name)). Subsequently, based on the peak intensity of the absorption wavelength (262 nm) derived from styrene, a calibration curve was used to calculate the content of styrene monomer units in SEBS (styrene content of SEBS).

[0076] [Melt viscosity ratio] The melt viscosity (mPa·s) of SEBS or SEPS was determined by continuously measuring SEBS or SEPS using a capillary rheometer (manufactured by Toyo Seiki Co., Ltd., trade name: Capillograph 1D, model: PM-C) at a resin temperature of 210°C and a shear rate of 12.2 s⁻¹ to 12160 s⁻¹, and the value was obtained as the value at a shear rate of 121.6 s⁻¹. The melt viscosity (mPa·s) of HDPE or LDPE was determined using a capillary rheometer in the same manner as above, continuously measuring HDPE at a resin temperature of 210°C and a shear rate of 12.2 s⁻¹ to 12160 s⁻¹, and the result was obtained as the value at a shear rate of 121.6 s⁻¹. The ratio of the melt viscosity of SEBS (resin temperature: 210℃, shear rate: 121.6 s⁻¹) to that of HDPE (resin temperature: 210℃, shear rate: 121.6 s⁻¹) (melt viscosity ratio, melt viscosity of SEBS / melt viscosity of HDPE) is calculated using this equivalent value. Similarly, the ratio of the melt viscosity of SEPS (resin temperature: 210°C, shear rate: 121.6 s⁻¹) to that of HDPE (resin temperature: 210°C, shear rate: 121.6 s⁻¹) (melt viscosity of SEPS / melt viscosity of HDPE), and the ratio of the melt viscosity of SEBS (resin temperature: 210°C, shear rate: 121.6 s⁻¹) to that of LDPE (resin temperature: 210°C, shear rate: 121.6 s⁻¹) (melt viscosity of SEBS / melt viscosity of LDPE) are also calculated using these equivalent values.

[0077] [Molecular weight distribution (Mw / Mn)] The molecular weight distribution (Mw / Mn) of HDPE is determined by the ratio (Mw / Mn) of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of standard polystyrene, as measured by GPC. The GPC measurement was performed using a Waters GPCV2000 (trade name), with the column consisting of a Showa Denko UT-807 (trade name, 1 column) and a Tosoh GMHHR-H(S)HT (trade name, 2 columns) connected in series. The measurement conditions were as follows: trichlorobenzene (TCB) was used as the mobile phase, and the measurements were performed at a column temperature of 140°C, a flow rate of 1.0 mL / min, a sample concentration of 20 mg / 15 mL (TCB), a sample dissolution temperature of 140°C, and a sample dissolution time of 2 hours. Regarding the molecular weight correction, 12 points were used with the standard polystyrene manufactured by Tosoh (stock) with a molecular weight (Mw) ranging from 10.5 million to 2.06 million. The Mw of each standard polystyrene was multiplied by a coefficient of 0.43 to obtain the converted molecular weight of polyethylene. A correction straight line was constructed based on the graph of dissolution time versus the converted molecular weight of polyethylene to determine the weight average molecular weight (Mw) and the number average molecular weight (Mn). Similarly, the weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular weight distribution (Mw / Mn) of LDPE were calculated.

[0078] [density] The densities (kg / m3) of HDPE and LDPE were determined according to JIS K7112:1999, density gradient tube method (23℃).

[0079] <Evaluation Methods and Properties of Surface Protective Films> [Preparation of Surface Protective Film] The surface protective film was fabricated using a three-layer film forming machine manufactured by Research Laboratory of Plastics Technology (T-mode). Specifically, LDPE granules were fed into the hopper of the substrate extruder as the substrate layer material, while granules made from a mixture of dry-mixed SEBS and HDPE granules, dry-mixed SEPS and HDPE granules, or dry-mixed SEBS and LDPE granules were fed into the hopper of the extruder as the adhesive layer material. The process involved casting and co-extrusion to create the surface protective film. The forming temperature was 210°C, and the draw speed was set to 7 m / min.

[0080] [The overall thickness of the membrane, and the ratio of the thickness of the substrate layer to the thickness of the adhesive layer] The protective film was cut cross-section using a razor. The cross-section was then observed using an optical microscope, and the overall film thickness (μm), substrate layer thickness (μm), and adhesive layer thickness (μm) were measured. Using the substrate layer thickness and adhesive layer thickness values, the ratio of substrate layer thickness to adhesive layer thickness (substrate layer thickness / adhesive layer thickness) was calculated.

[0081] [PMMA adhesion] The adhesion of the surface protective film was determined. The test was conducted using a universal tensile and compression testing machine (Technograph TGE-500N, manufactured by Minebea Mitsumi). The evaluation was performed as follows, based on a 180° peel test at 23°C and 50% relative humidity, assessing both initial adhesion and adhesion resistance.

[0082] (Initial adhesion) At 23°C and 50% relative humidity, a 25 mm wide protective film was applied to a PMMA board (polymethyl methacrylate, arithmetic mean surface roughness: 0.1 μm), and then rolled with a 2 kg rubber roller (10 cm in diameter) to ensure tight adhesion. The application was left for 30 minutes. Subsequently, the initial adhesion (g / 25 mm) of the protective film was measured at a peeling speed of 300 mm / min. A surface protective film with an initial adhesion of 20~50 g / 25 mm is considered qualified.

[0083] (Adhesion) A 25 mm wide protective film was adhered to a PMMA board (polymethyl methacrylate, arithmetic mean surface roughness: 0.1 μm) at 23°C and 50% relative humidity. A 2 kg rubber roller (10 cm in diameter) was then rolled to ensure tight adhesion, and the adhesion was left for 30 minutes. Afterward, the board was heated in an 80°C GIL aging oven for 1 hour, and then returned to 23°C and 50% relative humidity. The adhesion (g / 25 mm) of the protective film was then measured at a peel speed of 300 mm / min.

[0084] (Adhesion resistance (adhesion resistance ratio)) Using the adhesion value of the surface protective film obtained from the above measurements and the initial adhesion value of the surface protective film obtained from the above measurements, the adhesion enhancement ratio (adhesion value of surface protective film / initial adhesion value of surface protective film) is calculated. When the adhesion hyperactivity ratio is within 2.5 times, the surface protective film is evaluated as having good adhesion hyperactivity resistance; when the adhesion hyperactivity ratio is within 2.0 times, the surface protective film is evaluated as having the best adhesion hyperactivity resistance.

[0085] [Membrane Formability] Film formability is assessed by visually observing the appearance of the surface protective film formed by co-extrusion, according to the following criteria. (Benchmark) ○: No defects such as stripes were found. ×: Stripes or other defects indicate an appearance defect.

[0086] [Paste Residue (Appearance Assessment)] The paste residue was assessed by visually observing the adhesion of the above-mentioned surface protective film (80°C, 1 hour) on the surface of the PMMA board, according to the following criteria. (Benchmark) ○: No paste residue (no trace of pasting was detected). ×: There is paste residue (traces of pasting film were confirmed).

[0087] [Example 1] As the base material for the surface protective film, LDPE (Suntec-LD L2340, a high-pressure low-density polyethylene manufactured by Asahi Kasei Corporation) is used. As an adhesive layer material for the surface protective film, a resin composition (granules) is made by using 66% by mass of SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a trade name, with a styrene content of 20% by mass and a hydride of styrene-butadiene copolymer) and 34% by mass of HDPE particles (Asahi Kasei Corporation's Creolex T4750, a trade name, with a molecular weight distribution of 3.7 and a high density polyethylene). Following the above-described method for fabricating a surface protective film, the substrate layer material and the adhesive layer material are formed into a surface protective film with a thickness of approximately 50 μm using a cast co-extrusion method. The resulting surface protective film has a transparent appearance and a good finish. The physical properties of the obtained surface protective film were evaluated according to the above evaluation method. The evaluation results, together with the test results of SEBS and HDPE, are shown in Table 1.

[0088] [Example 2] The SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a registered trademark) were changed from 66% by mass to 60% by mass, and the HDPE particles (Asahi Kasei Corporation's Creolex T4750, a registered trademark) were changed from 34% by mass to 40% by mass. Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0089] [Example 3] The SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a registered trademark) were changed from 66% by mass to 58% by mass, and the HDPE particles (Asahi Kasei Corporation's Creolex T4750, a registered trademark) were changed from 34% by mass to 42% by mass. Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0090] [Example 4] Instead of SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a registered trademark), SEBS particles (Asahi Kasei Corporation's Tuftec H1062, a styrene-butadiene copolymer with a styrene content of 18% by mass) were used, and instead of HDPE particles (Asahi Kasei Corporation's Creolex T4750), HDPE particles (Asahi Kasei Corporation's Creolex T701A, a high-density polyethylene with a molecular weight distribution of 3.5) were used, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0091] [Example 5] Instead of HDPE granules (Asahi Kasei Corporation Creolex T4750, a registered trademark), HDPE granules (Asahi Kasei Corporation Creolex T701A, a registered trademark) were used. Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0092] [Example 6] Instead of SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a registered trademark), SEBS particles (Asahi Kasei Corporation's Tuftec H1521, a styrene-butadiene copolymer with a styrene content of 18% by mass) were used, and instead of HDPE particles (Asahi Kasei Corporation's Creolex T4750), HDPE particles (Asahi Kasei Corporation's Creolex T701A) were used. Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0093] [Example 7] Instead of SEBS particles (Asahi Kasei Corporation's Tuftec H1052), the granules were replaced with SEBS particles (Asahi Kasei Corporation's Tuftec P1083, a 20% by weight styrene-butadiene copolymer hydride), and instead of HDPE particles (Asahi Kasei Corporation's Creolex T4750), the granules were replaced with HDPE particles (Asahi Kasei Corporation's Creolex T701A). Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0094] [Example 8] Instead of SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a registered trademark), SEBS particles (Asahi Kasei Corporation's Tuftec H1062, a registered trademark) were used. Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0095] [Comparative Example 1] Instead of HDPE granules (Asahi Kasei Corporation Creolex T4750), HDPE granules (Asahi Kasei Corporation Suntec HD J240 HD J240, molecular weight distribution: 7.3, high-density polyethylene) were used. Otherwise, a surface protective film was formed in the same manner as in Example 2, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0096] [Comparative Example 2] Instead of HDPE granules (Asahi Kasei Corporation Creolex T4750), HDPE granules (Asahi Kasei Corporation Suntec HD J320, molecular weight distribution: 8.0, high-density polyethylene) were used. Otherwise, a surface protective film was formed in the same manner as in Example 2, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0097] [Comparative Example 3] Instead of HDPE granules (Asahi Kasei Corporation Creolex T4750), HDPE granules (Asahi Kasei Corporation Suntec HD B161, molecular weight distribution: 10.0, high-density polyethylene) were used. Otherwise, a surface protective film was formed in the same manner as in Example 2, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0098] [Comparative Example 4] Instead of SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a registered trademark), SEBS particles (Asahi Kasei Corporation's SOES1613, a styrene-butadiene copolymer with a styrene content of 33% by mass) were used as a substitute for SEBS particles. A surface protective film was formed in the same manner as in Example 2, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0099] [Comparative Example 5] Instead of SEBS particles (Asahi Kasei Corporation's Tuftec H1052, a registered trademark), SEBS particles (KRATON Corporation's G1657, a styrene-butadiene copolymer with a styrene content of 13% by mass) were used as a substitute for SEBS particles. Otherwise, a surface protective film was formed in the same manner as in Example 2, and the film was evaluated. These evaluation results, along with the respective measurement results for SEBS and HDPE, are shown in Table 1.

[0100] [Comparative Example 6] Instead of SEBS particles (Asahi Kasei Corporation's Tuftec H1052), SEPS particles (Kuraray Corporation's Septon 2002), MFR (230°C, 2.16 kg load): 70 g / 10 min, melt viscosity (resin temperature: 210°C, shear rate: 121.6 s⁻¹): 230 mPa·s, styrene content: 30% by mass, styrene-ethylene-propylene-styrene hydride) were used, and instead of HDPE particles (Asahi Kasei Corporation's Creolex T4750), HDPE particles (Asahi Kasei Corporation's Suntec-HD B161), molecular weight distribution: 10.0, high-density polyethylene) were used. Otherwise, a surface protective film was formed in the same manner as in Example 2, and the film was evaluated. These evaluation results, along with the measurement results for SEPS and HDPE, are shown in Table 1.

[0101] [Comparative Example 7] The SEBS granules (Asahi Kasei Corporation's Tuftec H1052, a registered trademark) were changed from 66% by mass to 50% by mass, and the HDPE granules (Asahi Kasei Corporation's Creolex T4750, a registered trademark) were changed from 34% by mass to 50% by mass of LDPE (Asahi Kasei Corporation's Suntec-LD L2340, a registered trademark) with MFR (190°C, 2.16 kg load): 3.8 g / 10 min, melt viscosity (resin temperature: 210°C, shear rate: 121.6 s⁻¹): 537 mPa·s, molecular weight distribution (Mw / Mn): 9.8, high-pressure low-density polyethylene). Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEPS and HDPE, are shown in Table 1.

[0102] [Comparative Example 8] The SEBS particles (Tuftec H1052, manufactured by Asahi Kasei Corporation, a registered trademark) were changed from 66% by mass to 75% by mass, and the HDPE particles (Creolex T4750, manufactured by Asahi Kasei Corporation, a registered trademark) were changed from 34% by mass to 25% by mass. Otherwise, a surface protective film was formed in the same manner as in Example 1, and the film was evaluated. These evaluation results, along with the measurement results for SEBS and HDPE, are shown in Table 1.

[0103] [Table 1] Surface protective film Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Comparative Example 7 Comparative Example 8 Substrate layer LDPE (mass %) Suntec-LD L2340 (MFR: 3.8 g / 10 minutes) 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 Adhesive layer SEBS (mass %) Tuftec H1052 (melt viscosity: 860 mPa·s) 66 60 58 66 60 60 60 50 75 Tuftec H1062 (melt viscosity: 4000 mPa·s) 66 66 Tuftec H1521 (melt viscosity: 400 mPa·s) 66 Tuftec P1083 (melt viscosity: 580 mPa·s) 66 SOES1613 (melt viscosity: 880 mPa·s) 60 KRATON G1657 (melt viscosity: 1280 mPa·s) 60 SEPS (quality%) Septon 2002 (melt viscosity: 100 mPa·s) 60 HOPE (mass %) Creolex T4750 (melt viscosity: 820 mPa·s) 34 40 42 34 40 40 25 Creolex T701A (melt viscosity: 450 mPa·s) 34 34 34 34 Suntec-HD J240 (melt viscosity: 570 mPa·s) 40 Suntec-HD J320 (melt viscosity: 300 mPa·s) 40 Suntec-HD B161 (melt viscosity: 1130 mPa·s) 40 40 LDPE (mass %) Suntec-LD L2340 (melt viscosity: 570 mPa·s) 50 Overall membrane thickness (μm) 53 51 45 51 52 51 52 51 48 51 53 54 55 47 53 53 The ratio of the thickness of the substrate layer to the thickness of the adhesive layer 5.1 6.6 4.6 5.2 5.2 5.1 5.2 5.1 5.2 5.1 4.6 5.0 6.6 4.6 4.8 5.1 Styrene content (mass %) of SEBS or SEPS in the adhesive layer 20 20 20 18 20 18 20 18 20 20 20 33 13 30 20 20 Molecular weight distribution of HDPE or LDPE in the adhesive layer 3.7 3.7 3.7 3.5 3.5 3.5 3.5 3.7 7.3 8.0 10.0 3.7 3.7 10.0 7.3 3.7 MFR of SEBS or SEPS in the adhesive layer (g / 10 min, 230°C, 2.16 kg) 13.0 13.0 13.0 4.1 13.0 11.0 16.0 4.1 13.0 13.0 13.0 14.0 8.0 70.0 13.0 13.0 melt viscosity ratio 1.05 1.05 1.05 8.88 1.91 0.88 1.29 4.87 1.52 2.87 0.76 1.07 1.57 0.09 1.50 1.05 MFR (g / 10 min, 190℃, 2.16 kg) of HDPE or LDPE for the adhesive layer 5.0 5.0 5.0 12.0 12.0 12.0 12.0 5.0 5.0 12.0 1.4 5.0 5.0 1.4 3.8 5.0 Density (kg / cm3) of HDPE or LDPE in the adhesive layer 947 947 947 966 966 966 966 947 966 959 963 947 947 963 923 947 physical properties PMMA adhesion Initial adhesion (g / 25 mm, 23°C, 30 minutes) 50 twenty two 20 45 47 50 48 46 14 13 7 9 9 55 13 275 Adhesion (g / 25 mm, 80℃, 1 hour) 100 42 40 104 108 110 110 101 39 34 twenty one 56 34 160 38 622 Adhesion hyperactivity ratio (adhesion hyperactivity tolerance) 2.0 2.0 2.0 2.3 2.3 2.2 2.3 2.2 2.8 2.6 3.0 6.2 3.8 2.9 2.9 2.3 Membrane Formability ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ × ○ ○ Paste residue ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○

Claims

1. A surface protective film comprising a substrate layer and an adhesive layer, wherein the substrate layer comprises low-density polyethylene (LDPE), the adhesive layer comprises styrene-butadiene copolymer (SEBS) and high-density polyethylene (HDPE), the ratio of the thickness of the substrate layer to the thickness of the adhesive layer (thickness of the substrate layer / thickness of the adhesive layer) is 4.0 to 7.0, the mass ratio of the SEBS to the HDPE (SEBS:HDPE) is 70:30 to 55:45, the styrene content of the SEBS is 18 to 22% by mass, and the molecular weight distribution (Mw / Mn) of the HDPE, expressed by the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of standard polystyrene as determined by gel permeation chromatography (GPC), is 4.0 or less.

2. The surface protective film as requested in item 1, wherein the melt mass flow rate (MFR, 230°C, 2.16 kg load) of the aforementioned SEBS is 11.0 to 15.0 g / 10 min.

3. The surface protective film of claim 1, wherein the ratio of the melt viscosity of the SEBS (resin temperature: 210°C, shear rate: 121.6 s⁻¹) to the melt viscosity of the HDPE (resin temperature: 210°C, shear rate: 121.6 s⁻¹) (melt viscosity of the SEBS / melt viscosity of the HDPE) is 0.85 to 1.

30.

4. The surface protective film as requested in item 1, wherein the melt mass flow rate (MFR, 190°C, 2.16 kg load) of the aforementioned HDPE is 2.0 to 6.0 g / 10 min.