Heat-shrinkable multilayer film

Abstract

To provide a heat-shrinkable multilayer film with improved blocking suppression function. [Solution] The heat-shrinkable multilayer film comprises a substrate and a surface layer. The substrate has a first surface and a second surface and contains a thermoplastic resin. The surface layer is laminated on at least one side of the first surface and the second surface of the substrate and contains a thermoplastic resin and fine particles held by the thermoplastic resin. The most frequent particle size of the fine particles is 1.2 times or more and 10 times or less the thickness of the thermoplastic resin contained in the surface layer.

Description

Technical Field

[0001] The present disclosure relates to a heat-shrinkable multilayer film.

Background Art

[0002] Patent Document 1 discloses a heat-shrinkable multilayer film in which a front and back layer containing a cyclic olefin resin and organic fine particles and an intermediate layer are laminated. According to Patent Document 1, the front and back layer contains 0.01% by weight or more and 0.3% by weight or less of organic fine particles having an average particle diameter of 0.1 μm or more and 20 μm or less. Thereby, blocking of the heat-shrinkable multilayer film is prevented.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, although the content and average particle diameter of preferable organic fine particles are defined, the dimensional relationship between the resin of the front and back layer and the organic fine particles is not considered. For this reason, even in the case of a heat-shrinkable multilayer film in which the front and back layer contains fine particles as described above, the blocking prevention effect by the fine particles may not be sufficiently exhibited, and blocking may still occur.

[0005] An object of the present disclosure is to provide a heat-shrinkable multilayer film with an improved blocking suppression function.

Means for Solving the Problems

[0006] The heat-shrinkable multilayer film according to the first aspect comprises a substrate and a surface layer. The substrate has a first surface and a second surface and contains a thermoplastic resin. The surface layer is laminated on at least one side of the first surface and the second surface of the substrate and contains a thermoplastic resin and fine particles held by the thermoplastic resin. The most frequent particle size of the fine particles is 1.2 times or more and 10 times or less the thickness of the thermoplastic resin contained in the surface layer.

[0007] The heat-shrinkable multilayer film relating to the second aspect is the heat-shrinkable multilayer film relating to the first aspect, wherein the most frequent particle size of the fine particles is 2 times or more and 8 times or less the thickness of the thermoplastic resin contained in the surface layer.

[0008] A heat-shrinkable multilayer film relating to the third aspect is a heat-shrinkable multilayer film relating to the first or second aspect, wherein the most frequent particle size of the fine particles is 6 μm or less.

[0009] A heat-shrinkable multilayer film relating to the fourth aspect is a heat-shrinkable multilayer film relating to any of the first or third aspects, wherein the thermoplastic resin contained in the surface layer includes a cyclic olefin resin.

[0010] A heat-shrinkable multilayer film according to the fifth viewpoint is a heat-shrinkable multilayer film according to any of the first to fourth viewpoints, further comprising an intermediate layer containing a thermoplastic resin, which is laminated on at least one of the first and second surfaces of a substrate. The surface layer is laminated on the intermediate layer.

[0011] The heat-shrinkable multilayer film according to the sixth aspect is the heat-shrinkable multilayer film according to the fifth aspect, wherein an intermediate layer is laminated on the first and second surfaces of the substrate, and a surface layer is laminated on each intermediate layer.

[0012] A heat-shrinkable multilayer film relating to the seventh aspect is a heat-shrinkable multilayer film relating to the fifth or sixth aspect, wherein the thermoplastic resin contained in the intermediate layer includes a cyclic olefin resin. [Effects of the Invention]

[0013] From the above perspective, a heat-shrinkable multilayer film with improved blocking suppression function is provided. [Brief explanation of the drawing]

[0014] [Figure 1] A cross-sectional view showing an example of a heat-shrinkable multilayer film according to one embodiment. [Figure 2] A cross-sectional view showing an example of a heat-shrinkable multilayer film according to one embodiment. [Figure 3] A cross-sectional view showing an example of a heat-shrinkable multilayer film according to one embodiment. [Modes for carrying out the invention]

[0015] An embodiment of the heat-shrinkable multilayer film according to this disclosure will be described below. The heat-shrinkable multilayer film 100 comprises a sheet-like substrate 1 having a first surface and a second surface, an intermediate layer 2 laminated on at least one of the first surface and the second surface of the substrate 1, and a surface layer 3 laminated on the intermediate layer 2. Therefore, the heat-shrinkable multilayer film 100 can take the form of an embodiment in which the intermediate layer 2 is laminated on both sides of the substrate 1, as shown in Figure 1, and a surface layer 3 is laminated on each intermediate layer 2, as shown in Figure 2. Each component will be described in detail below. Furthermore, a film made from each of these materials may be referred to as a film.

[0016] <1. Base material> Substrate 1 contains a thermoplastic resin, such as a propylene resin, a petroleum resin, and an olefin elastomer. Further details are provided below.

[0017] <1-1. Propylene resins> As the propylene-based resin, from the viewpoint of exhibiting heat shrinkability, a binary or ternary random copolymer having propylene as the main component and α-olefin as the copolymerization component is preferable. As the α-olefin, specifically, those composed of ethylene, 1-butene, 1-hexene, 1-octene, etc. are preferable, and it may contain two or more kinds of α-olefins. The ratio of the α-olefin as the copolymerization component is preferably 1 to 10 mol%. Further, the propylene-based resin may be a mixture of different propylene-α-olefin random copolymers.

[0018] Examples of commercially available products of the propylene-based resin as described above include Adsyl (manufactured by Basell), Novatec (manufactured by Japan Polypropylene Corporation), etc.

[0019] The heat deflection temperature (0.45 MPa) of the propylene-based resin is preferably 110°C or lower, and more preferably 90°C or lower. When this propylene-based resin is a mixed resin containing two or more kinds of propylene-based resins having different heat deflection temperatures, the heat deflection temperature of the above propylene-based resin means the apparent heat deflection temperature calculated by summing the products of the heat deflection temperature and the blending ratio (weight ratio) of each propylene-based resin.

[0020] The content of the above propylene-based resin with respect to 100% by mass of the resin component constituting the base material 1 is preferably 50% by mass or more and 75% by mass or less, and more preferably 55% by mass or more and 65% by mass or less.

[0021] <1-2. Petroleum resin> Petroleum resins are resins obtained by polymerizing the remaining C4-C5 fractions or C5-C9 fractions after thermal decomposition of naphtha to obtain ethylene, propylene, butadiene, etc., while keeping them in a mixed state. Examples of such resins include aromatic petroleum resins, aliphatic petroleum resins, aromatic hydrocarbon resin petroleum resins, alicyclic saturated hydrocarbon resin petroleum resins, copolymers of the above petroleum resins, and hydrogenated versions of these petroleum resins. Among these, alicyclic petroleum resins are preferred from the viewpoint of suppressing softening of the film below 100°C and improving transparency. Examples of alicyclic petroleum resins include hydrogenated alicyclic saturated hydrocarbon resin petroleum resins and aromatic petroleum resins.

[0022] Examples of commercially available petroleum resins as described above include iMarb (manufactured by Idemitsu Kosan Co., Ltd.) and Alcon (manufactured by Arakawa Chemical Industries, Ltd.).

[0023] The softening point of petroleum resin is preferably between 80°C and 170°C, and more preferably between 110°C and 155°C. If the softening point is below 80°C, the heat resistance of the film decreases, and petroleum resin components may easily bleed out to the surface in a high-temperature atmosphere. On the other hand, if the softening point exceeds 170°C, the moldability, such as extrusion and stretchability, may deteriorate. A softening point of 110°C or higher is preferable because it can suppress the natural shrinkage of the film. A softening point of 155°C or lower is preferable because it allows for uniform stretching in the stretching process that imparts heat shrinkability to the film. In particular, a softening point of 120°C to 140°C allows for good heat shrinkability. The softening point of petroleum resin can be measured by a method compliant with JIS K2207:2006.

[0024] The number-average molecular weight of the petroleum resin is preferably between 700 and 1300. If the number-average molecular weight of the petroleum resin is less than 700, the heat resistance of the film will decrease, and the petroleum resin components may easily bleed out to the surface in a high-temperature atmosphere. On the other hand, if the number-average molecular weight of the petroleum resin exceeds 1300, the moldability, such as stretchability, may deteriorate. The number-average molecular weight of the petroleum resin can be confirmed by gel permeation chromatography (GPC).

[0025] The base material 1 preferably contains 10% to 35% by mass of the above-mentioned petroleum resin, and more preferably 15% to 30% by mass, based on 100% by mass of the resin components constituting the base material 1. This content range allows for high shrinkability and high rigidity in the heat-shrinkable multilayer film. Furthermore, keeping the petroleum resin content below the above upper limit suppresses a decrease in elongation at low temperatures and delamination between layers.

[0026] <1-3. Olefin-based elastomers> As the olefin-based elastomer, it is preferable to use a propylene / α-olefin random copolymer elastomer. Other examples of olefin-based elastomers include ethylene / α-olefin random copolymer elastomers. The above-mentioned α-olefin random copolymer elastomer is an elastomer in which the copolymer component of α-olefins having 3 or more carbon atoms is 15 mol% or more. Examples of α-olefins here include propylene, butene-1, pentene-1, hexene-1, octene-1, 4-methylpentene-1, etc.

[0027] Examples of commercially available olefin-based elastomers as described above include Tuffmer (manufactured by Mitsui Chemicals, Inc.).

[0028] The Vicat softening temperature of the olefin-based elastomer is preferably 50°C or higher and 75°C or lower.

[0029] The base material 1 preferably contains 15% by mass or less of the olefin-based elastomer with respect to 100% by mass of the resin components constituting the base material 1.

[0030] <1-4. Thickness of the base material> The thickness of the substrate 1 is preferably, for example, 15 μm or more and 40 μm or less, and more preferably 20 μm or more and 35 μm or less.

[0031] <2. Middle Class> Intermediate layer 2 contains a thermoplastic resin. Intermediate layer 2 may mainly contain a cyclic olefin resin as the thermoplastic resin, and may further contain an ethylene resin and a petroleum resin. These will be explained below.

[0032] <2-1. Cyclic Olefin Resins> The cyclic olefin resin reduces the crystallinity of the thermoplastic resin, increasing the thermal shrinkage rate of the intermediate layer 2 and improving the stretchability during film formation. Furthermore, the surface layer 3, described later, also contains the cyclic olefin resin. This improves the interlayer adhesion strength between the intermediate layer 2 and the surface layer 3.

[0033] As the cyclic olefin resin, cyclic olefin copolymers (COCs) are preferred. Cyclic olefin copolymers can be obtained, for example, by copolymerizing α-olefin and cyclic olefin.

[0034] The cyclic olefin is not particularly limited and includes, for example, norbornene, 6-methylnorbornene, 6-ethylnorbornene, 5-propylnorbornene, 6-n-butylnorbornene, 1-methylnorbornene, 7-methylnorbornene, 5,6-dimethylnorbornene, 5-phenylnorbornene, 5-benzylnorbornene, and norbornene and its derivatives. Also, tetracyclododecene, 8-methyltetracyclo-3-dodecene, 8-ethyltetracyclo-3-dodecene, 5,10-dimethyltetracyclo-3-dodecene, and tetracyclododecene and its derivatives are also included.

[0035] Examples of commercially available cyclic olefin resins as described above include Apel (manufactured by Mitsui Chemicals), TOPAS COC (manufactured by Polyplastics), and ZEONOR (manufactured by Nippon Zeon Corporation).

[0036] The number-average molecular weight of the cyclic olefin resin, as measured by the GPC method, is preferably 1000 or more, and preferably 1,000,000 or less. Keeping it within this range facilitates film formation.

[0037] The glass transition temperature of the cyclic olefin resin is preferably 20°C or higher and 130°C or lower, and more preferably 50°C or higher and 100°C or lower. When the glass transition temperature is 20°C or higher, the heat resistance of the film surface is good, which can suppress the occurrence of blocking between containers on the mounting line and allow the natural shrinkage rate to be within a good range. When the glass transition temperature is 130°C or lower, the lateral thermal shrinkage rate can be sufficiently large.

[0038] The density of cyclic olefin resin is 1000 kg / m³. 3 More than 1050kg / m 3 Preferably, it is 1010 kg / m 3 More than 1040kg / m 3 The following is more preferable:

[0039] Intermediate layer 2 preferably contains 55% to 85% by mass of the above-mentioned cyclic olefin resin, more preferably 60% to 80% by mass, and even more preferably 65% ​​to 75% by mass, based on 100% by mass of the resin components constituting intermediate layer 2. When the content of the cyclic olefin resin is within the above range, the rigidity, thermal shrinkability, and transparency of the heat-shrinkable multilayer film 100 can be improved.

[0040] <2-2. Ethylene-based resins> Examples of ethylene-based resins include branched low-density polyethylene resins, linear low-density polyethylene resins, ethylene-vinyl acetate copolymers, ionomer resins, or mixtures thereof. Also, copolymers of ethylene and α-olefins are used. Examples of α-olefins include 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The copolymers may be random copolymers or block copolymers. In particular, it is preferable that the intermediate layer 2 contains a linear low-density polyethylene resin.

[0041] Commercially available linear low-density polyethylene resins as described above include Evolu (manufactured by Prime Polymer Co., Ltd.), Yumerit (manufactured by Ube Maruzen Polyethylene Co., Ltd.), and Novatec (manufactured by Nippon Polyethylene Co., Ltd.). Commercially available low-density polyethylene resins include Sumikasen (manufactured by Sumitomo Chemical Co., Ltd.) and Novatec (manufactured by Nippon Polyethylene Co., Ltd.).

[0042] The density of ethylene resin is 880 kg / m³. 3 More than 950kg / m 3 The following is preferable:

[0043] Intermediate layer 2 preferably contains 5% to 25% by mass of linear low-density polyethylene resin, and more preferably 10% to 20% by mass of linear low-density polyethylene resin, relative to 100% by mass of the resin components constituting intermediate layer 2.

[0044] <2-3. Petroleum Resins> The intermediate layer 2 may contain petroleum resin as already described in the description of the base material 1. The intermediate layer 2 may contain the same petroleum resin as the base material 1, or it may contain a different petroleum resin than the base material 1.

[0045] The intermediate layer 2 preferably contains 15% to 35% by mass of petroleum resin, and more preferably 20% to 30% by mass of petroleum resin, relative to 100% by mass of the resin components constituting the intermediate layer 2.

[0046] <2-4. Thickness> The thickness of the intermediate layer 2 is preferably 2 μm or more and 5.5 μm or less, and more preferably 3 μm or more and 4.5 μm or less. Furthermore, when an intermediate layer is provided, the thickness ratio of the base material 1 to the intermediate layer 2 is preferably in the range of 9:1 to 5:1, more preferably 8:1 to 6:1. By setting it within the above range, excellent shrinkage finish can be achieved as a heat-shrinkable multilayer film.

[0047] <3.Surface layer> The surface layer 3 contains a thermoplastic resin and fine particles 4 held therein. A cyclic olefin resin is preferred as the thermoplastic resin, and a cyclic olefin copolymer (COC) is more preferred. The thickness of the thermoplastic resin in the surface layer 3 is preferably 0.2 μm or more and 5 μm or less, and more preferably 0.4 μm or more and 1 μm or less. In particular, when a cyclic olefin copolymer is used as the thermoplastic resin, the thickness of the thermoplastic resin in the surface layer 3 is preferably 1 μm or less in order to maintain gloss and transparency and to reduce sebum whitening when it comes into contact with sebum.

[0048] <3-1. Cyclic Olefin Resins> As the cyclic olefin copolymer, the cyclic olefin copolymer already described in the description of intermediate layer 2 can be used. Surface layer 3 may contain the same cyclic olefin copolymer as intermediate layer 2, or it may contain a different cyclic olefin copolymer than that of intermediate layer 2.

[0049] <3-2. Fine particles> The fine particles 4 held in the thermoplastic resin of the surface layer 3 primarily serve to prevent blocking, which occurs when the heat-shrinkable multilayer films 100 fuse together and become difficult to peel off. These fine particles 4 can be either organic or inorganic. Examples of organic fine particles include acrylic resin fine particles, styrene resin fine particles, styrene-acrylic resin fine particles, urethane resin fine particles, and silicone resin fine particles. These may or may not be crosslinked, but crosslinking is desirable to enhance the heat resistance of the fine particles 4. The fine particles 4 are preferably acrylic resin fine particles, particularly from the viewpoint of compatibility with the cyclic olefin resin and improved transparency of appearance, and even more preferably polymethyl methacrylate crosslinked fine particles.

[0050] Examples of commercially available organic microparticles as described above include Techpolymer (manufactured by Sekisui Chemical Co., Ltd.), Finesphere (manufactured by Nippon Paint Co., Ltd.), Gantzpearl (manufactured by Aica Kogyo Co., Ltd.), and Artpearl (manufactured by Negami Kogyo Co., Ltd.).

[0051] Examples of inorganic nanoparticles that can be used include silica, zeolite, and alumina.

[0052] The most frequent particle size of the fine particles 4 is preferably 1.2 times or more and 10 times or less the thickness of the thermoplastic resin of the surface layer 3, and more preferably 1.2 times or more and 8 times or less. That is, as shown in Figures 1 and 2, the fine particles 4 are mainly held in the thermoplastic resin of the surface layer 3, but some of them may extend outside the thermoplastic resin of the surface layer 3, or may penetrate into the intermediate layer 2. By setting the relationship between the thickness of the thermoplastic resin of the surface layer 3 and the most frequent particle size of the fine particles in this way, the blocking suppression function of the heat-shrinkable multilayer film 100 can be suitably enhanced. If the most frequent particle size of the fine particles 4 exceeds 10 times the thickness of the thermoplastic resin of the surface layer 3, the fine particles 4 are more likely to fall off, which can easily lead to printing defects during printing. It is preferable that the most frequent particle size of the fine particles 4 is 8 times or less the thickness of the thermoplastic resin of the surface layer 3, as this problem is less likely to occur. Note that Figures 1 and 2 are schematic diagrams for illustrative purposes and do not necessarily reflect the actual dimensions of the substrate 1, intermediate layer 2, surface layer 3 and fine particles 4, or the distribution of fine particles 4.

[0053] The most frequent particle size of the fine particles 4 is preferably 6 μm or less, more preferably 5.5 μm or less, and even more preferably 5 μm or less. Furthermore, the most frequent particle size of the fine particles 4 is preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 3 μm or more. If the most frequent particle size exceeds 6 μm, transparency decreases and the particles become more likely to detach from the thermoplastic resin of the surface layer 3. The most frequent particle size can be measured by known laser diffraction / scattering methods, etc. Also, from the viewpoint of maintaining the transparency of the heat-shrinkable multilayer film 100, it is preferable that the refractive index of the fine particles is close to the refractive index of the thermoplastic resin constituting the surface layer 3.

[0054] The content of the fine particles 4 is preferably 0.05 parts by mass or more, and more preferably 0.1 parts by mass or more, per 100 parts by mass of the resin component constituting the surface layer 3. Furthermore, the content of the fine particles 4 is preferably 0.5 parts by mass or less, and more preferably 0.4 parts by mass or less. If the above content is above the lower limit, irregularities are formed on the surface of the heat-shrinkable multilayer film 100, which can improve the blocking suppression function of the heat-shrinkable multilayer film 100. On the other hand, if the above content is below the upper limit, the transparency of the appearance can be sufficiently maintained.

[0055] <4. Thickness of heat-shrinkable multilayer film> The overall thickness of the heat-shrinkable multilayer film 100, excluding the fine particles 4, is preferably, for example, 20 μm or more and 60 μm or less, and more preferably 25 μm or more and 45 μm or less. In particular, the upper limit of the thickness is even more preferably 30 μm or less. When the overall thickness of the heat-shrinkable multilayer film 100 is within the above range, excellent heat shrinkability can be obtained.

[0056] <5. Blocking suppression function of heat-shrinkable multilayer films> The blocking strength of a heat-shrinkable multilayer film can be evaluated by measuring the peel adhesion strength at which two samples cut from the heat-shrinkable multilayer film are separated after being stacked, pressure is applied, and then pulled to 180°. A lower peel adhesion strength indicates a higher blocking suppression function, while a higher peel adhesion strength indicates that the heat-shrinkable multilayer films are more likely to fuse together and blockage is more likely to occur. The peel adhesion strength of the heat-shrinkable multilayer film 100 is preferably 1300 g / cm or less, more preferably 1100 g / cm or less, and even more preferably 1000 g / cm or less.

[0057] <6. Other ingredients> The base material 1, intermediate layer 2, and surface layer 3 may contain additives such as antioxidants, heat stabilizers, ultraviolet absorbers, light stabilizers, lubricants, antistatic agents, flame retardants, antibacterial agents, fluorescent whitening agents, and colorants, as needed.

[0058] <7. Thermal shrinkage performance of heat-shrinkable multilayer films> When the heat-shrinkable multilayer film 100 is immersed in 100°C hot water for 10 seconds, then immersed in 20°C water for 10 seconds, and removed, the heat shrinkage rate in the main shrinkage direction (TD direction) is preferably 64% or more, and preferably 76% or less. Furthermore, when the heat-shrinkable multilayer film 100 is immersed in 100°C hot water for 10 seconds, then immersed in 20°C water for 10 seconds, the heat shrinkage rate in the direction perpendicular to the main shrinkage direction (MD direction) is preferably 5% or more, and preferably 20% or less. When the heat shrinkage rate is within the above range, problems such as poor shrinkage will not occur, and it can be suitably used as a heat-shrinkable multilayer film, especially for attachment to containers.

[0059] <8. Method for manufacturing heat-shrinkable multilayer film> The method for manufacturing the heat-shrinkable multilayer film 100 is not particularly limited, but a method of simultaneously forming each layer by co-extrusion is preferred. When the co-extrusion method is co-extrusion using a T-die, the lamination method may be a feed block method, a multi-manifold method, or a method combining these.

[0060] A specific method for manufacturing the heat-shrinkable multilayer film 100 is to feed the raw materials constituting the base material, adjacent layers, and surface layer into an extruder, extrude them into a sheet using a die, cool and solidify them using a take-up roll, and then stretch them uniaxially or biaxially. For example, the stretching method can be a roll stretching method, a tenter stretching method, or a combination thereof. The stretching temperature is adjusted according to the softening temperature of the resin constituting the heat-shrinkable multilayer film 100, the shrinkage characteristics required for the heat-shrinkable multilayer film 100, etc., but is preferably 65°C or higher, more preferably 70°C or higher, preferably 120°C or lower, and more preferably 115°C or lower.

[0061] The stretch ratio in the main shrinkage direction is changed according to the resin constituting the heat-shrinkable multilayer film 100, the stretching means, the stretching temperature, etc., but is preferably 3 times or more, more preferably 4 times or more, preferably 7 times or less, and more preferably 6 times or less.

[0062] <9. Other embodiments of heat-shrinkable multilayer films> In the above description, the heat-shrinkable multilayer film 100 is composed of a base material 1, an intermediate layer 2, and a surface layer 3. However, as shown in the heat-shrinkable multilayer film 101 in Figure 3, the surface layer 3 can be omitted, and a surface layer 3A can be constructed by laminating it on at least one of the first and second surfaces of the base material 1 with the same configuration as the intermediate layer 2. In this case, fine particles 4 may be added to the surface layer 3A. It is also possible to form the surface layer 3A on only one surface of the base material 1.

[0063] The thickness of the surface layer 3A can be, for example, 1 to 10 μm. In such a case, it will be observed as three layers in a cross-sectional photograph of the heat-shrinkable multilayer film 101.

[0064] <10. Features> The heat-shrinkable multilayer films 100 and 101 provide a heat-shrinkable multilayer film that is less prone to blocking. Furthermore, by using a cyclic olefin copolymer as the thermoplastic resin of the surface layer 3 and setting its thickness to 1 μm or less, a heat-shrinkable multilayer film with high surface gloss and transparency and less susceptibility to sebum whitening is provided. This improves the quality of printing on the heat-shrinkable multilayer film. The heat-shrinkable multilayer films 100 and 101 are not limited to these, but can be suitably used as packaging films and base films for shrink labels attached to metal cans, plastic containers, etc. [Examples]

[0065] The embodiments of this disclosure will be described in detail below. However, this disclosure is not limited to these embodiments.

[0066] <1. Preparation of Examples and Comparative Examples> Heat-shrinkable multilayer films were prepared according to Examples 1-8 and Comparative Examples 1 and 2 as described below. Examples 1-7 and Comparative Examples 1 and 2 had a five-layer structure as shown in Figure 1. Example 8 had a three-layer structure as shown in Figure 3.

[0067] The raw material compositions for the base material, intermediate layer, and surface layer of Examples 1 to 7 and Comparative Examples 1 and 2 were obtained by using the components shown in Table 1 as raw materials for the base material, intermediate layer, and surface layer, and mixing them in the proportions shown in Table 1. For the cyclic olefin resin of the surface layer and intermediate layer, Apel APL6509T (manufactured by Mitsui Chemicals, Inc.) was used. For the linear low-density ethylene resin (LLDPE) of the intermediate layer, Evolu SP1020 (manufactured by Prime Polymer Co., Ltd.) was used, and for the petroleum resin, Alcon P125 (manufactured by Arakawa Chemical Industries, Ltd.) was used. For the olefin elastomer of the base material, Tuffmer A4070S (manufactured by Mitsui Chemicals, Inc.) was used, for the propylene resin, Novatec FW3GT (manufactured by Nippon Polypropylene Co., Ltd.) was used, and for the petroleum resin, Alcon P125 (manufactured by Arakawa Chemical Industries, Ltd.) was used. As the fine particles, Artpearl J-4PY (manufactured by Negami Kogyo Co., Ltd.) was used in Examples 1-5 and Comparative Examples 1 and 2, while Artpearl J-6PF (manufactured by Negami Kogyo Co., Ltd.) was used in Examples 6 and 7. For reference, the refractive index of the cyclic olefin resin was 1.54, and the refractive index of the fine particles was 1.5.

[0068] In Example 8, the surface layer used the same cyclic olefin resin, linear low-density ethylene resin, and petroleum resin as in the other examples and comparative examples, and Novatec (manufactured by Nippon Polyethylene Co., Ltd.) was also used as the low-density ethylene resin (LDPE). In Example 8, an olefin elastomer was not used as the base material. In addition, Artpearl SE-006T was used as the fine particles.

[0069] Next, the raw material compositions constituting the base material, intermediate layer, and surface layer were melted using a separate extruder at a barrel temperature of 180°C for the base material, 210°C for the intermediate layer, and 210°C for the surface layer. These were extruded from a T-die and cooled and solidified on a roll cooled to 30°C to produce an unstretched sheet. This was then stretched five times in the TD direction using a tenter-type stretcher at 90°C to produce a heat-shrinkable multilayer film. Table 2 shows the thickness (μm) of each layer, the amount of fine particles added (parts by mass), and the most frequent particle size (μm) of the added fine particles. Note that the layer thickness is the thickness of the thermoplastic resin constituting that layer.

[0070] [Table 1] The units for each material constituting the surface layer, intermediate layer, and base material are expressed in mass percent.

[0071] [Table 2]

[0072] <2. Evaluation> The following evaluations were performed on Examples 1-8 and Comparative Examples 1 and 2 described above.

[0073] <2-1. Hayes> Samples of the same size were cut from the heat-shrinkable multilayer films of Examples 1-8 and Comparative Examples 1 and 2, and the haze (%) was measured according to JIS K7136. The evaluation was as follows: a haze of 4% or less was judged as "1" indicating good appearance, and a haze greater than 4% was judged as "0" indicating a problem with appearance.

[0074] <2-2. Blocking> Two measurement samples measuring 100 mm in length and 30 mm in width (with the TD direction of the film as the vertical direction and the MD direction as the horizontal direction) were cut from any point on each of the heat-shrinkable multilayer films according to Examples 1-8 and Comparative Examples 1 and 2. Next, the two measurement samples were placed so that the same sides overlapped by an area of ​​40 mm in length and 30 mm in width. Then, the overlapping measurement samples were sandwiched between two glass plates, and a 5 kg weight was placed on top of the overlapping portion of the samples. The sample set up in this way was placed in a 40°C constant temperature bath and left for 48 hours. After that, the sample was removed from the constant temperature bath and placed in a peel tester (Peeling TESTER HEIDON-17, manufactured by Shinto Kagaku Co., Ltd.), and pulled 180° at a tensile speed of 200 mm / min. The peel adhesion strength at which the two samples separated was defined as the blocking strength.

[0075] The strength of blocking was evaluated as follows: a value of "1" for blocking suppression function being within an acceptable range if it was 1300 g / cm or less, a value of "2" for good range if it was 1100 g / cm or less, and a value of "3" for even better range if it was 1000 g / cm or less. A value of "0" for blocking exceeding 1300 g / cm was used to indicate that blocking of a problematic degree is likely to occur.

[0076] <3. Evaluation Results> The evaluation results are as follows: [Table 3]

[0077] Based on the results above, the blocking strength of Examples 1 to 8 was 1100 g / cm or less in all cases, indicating that good blocking suppression function was exhibited (rating "2"). In particular, Examples 1 to 4 and 6 to 8 exhibited even better blocking suppression function (rating "3"). On the other hand, Comparative Examples 1 and 2 did not exhibit blocking suppression function (rating "0"). This confirmed that the blocking suppression function was improved by increasing the most frequent particle size of the fine particles to the thickness of the thermoplastic resin in the surface layer from 1.2 to 10 times.

[0078] In Examples 1-7, the haze was kept low (rating "1"). On the other hand, in Example 8, the haze was high (rating "0"), which is thought to be due to the relatively large thickness of the thermoplastic resin in the surface layer, similar to Comparative Example 2. [Explanation of symbols]

[0079] 1 Base material 2. Middle Class 3, 3A surface layer 4 Fine particles

Claims

1. A substrate having a first surface and a second surface and containing a thermoplastic resin, A surface layer is laminated on at least one side of the first and second surfaces of the substrate, and contains a thermoplastic resin and fine particles held by the thermoplastic resin. Equipped with, The most frequent particle size of the fine particles is 1.2 times or more and 10 times or less the thickness of the thermoplastic resin contained in the surface layer. Heat-shrinkable multilayer film.

2. The most frequent particle size of the fine particles is between 2 and 8 times the thickness of the thermoplastic resin contained in the surface layer. The heat-shrinkable multilayer film according to claim 1.

3. The most frequent particle size of the aforementioned fine particles is 6 μm or less. A heat-shrinkable multilayer film according to claim 1 or 2.

4. The heat-shrinkable multilayer film according to any one of claims 1 to 3, wherein the thermoplastic resin contained in the surface layer includes a cyclic olefin resin.

5. An intermediate layer containing a thermoplastic resin is laminated on at least one of the first and second surfaces of the substrate. Furthermore, The surface layer is laminated on the intermediate layer. A heat-shrinkable multilayer film according to any one of claims 1 to 4.

6. The intermediate layer is laminated on the first and second surfaces of the substrate, and the surface layer is laminated on each of the intermediate layers. The heat-shrinkable multilayer film according to claim 5.

7. The thermoplastic resin contained in the intermediate layer includes a cyclic olefin resin. The heat-shrinkable multilayer film according to claim 5 or 6.

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