Multilayer heat-shrinkable film

Abstract

Provided is a multilayer heat-shrinkable film which exhibits not only a heat shrinkage rate within a desired range but also good recyclability and good interlayer adhesiveness in a well-balanced manner. The multilayer heat-shrinkable film comprises a first layer that is derived from a polyolefin copolymer and a second layer that is derived from another resin, and is configured such that: (1) when immersed in hot water at 100°C for 10 seconds, the heat shrinkage rate of the multilayer heat-shrinkable film in the main shrinkage direction is 60% or more; (2) the crystal melting enthalpy of the polyolefin copolymer that constitutes the first layer is 120 mJ / mg or less as measured by DSC; and (3) a petroleum resin is blended in the first layer, and the blending amount of the petroleum resin is 3-20 wt%.

Description

Multilayer heat shrink film

[0001] The present invention relates to a multilayer heat-shrinkable film (hereinafter sometimes simply referred to as a heat-shrinkable film) mainly composed of an olefin resin. More specifically, the present invention relates to a multilayer heat-shrinkable film having a first layer (hereinafter sometimes referred to as an intermediate layer) derived from an olefin resin containing a specific olefin copolymer, which provides a desired heat shrinkage rate with little variation, and exhibits good interlayer adhesion and recyclability.

[0002] Conventionally, heat-shrinkable films have been derived from styrene-based resins and PET-based resins, and due to their superior mechanical properties and transparency, they have been widely used as base films for labels on PET bottles and other products made from PET-based resins. However, PET-based heat-shrinkable films have the problem of being lightweight and difficult to separate by specific gravity from the PET-based resins that make up PET bottles. Furthermore, during the recycling process, the PET-based heat-shrinkable film itself tends to aggregate, which in turn reduces the recyclability of PET bottles containing the PET-based heat-shrinkable film. On the other hand, in order to improve the appearance of the film or improve printability, it has been proposed to create multilayer heat-shrinkable films by providing a surface layer on the surface of the core layer that can bring about such improvements, but there is a problem that the interlayer adhesion between the surface layer and the core layer tends to decrease.

[0003] Therefore, multilayer heat-shrinkable films using an olefin resin layer as the base layer are attracting attention because they are inexpensive, economically efficient, have a relatively low specific gravity, and are highly recyclable. For example, considering the recyclability after use in shrink packaging, shrink bundling packaging, shrink labels, etc., a heat-shrinkable laminated film has been proposed consisting of two outer layers mainly composed of cyclic olefin compounds (COC) or polystyrene resins, and an intermediate layer mainly composed of polyolefin resins (see, for example, Patent Document 1). More specifically, the overall specific gravity of such a multilayer heat-shrinkable film is less than 0.960 g / cm³. 3 The density of the polyolefin resin constituting the intermediate layer is 0.940 g / cm³. 3The multilayer heat-shrinkable film is characterized in that the heat shrinkage rate Y at a predetermined temperature (70 to 90°C) in the main stretching direction satisfies predetermined equations 1 and 2.

[0004] Furthermore, a heat-shrinkable film has been proposed that suppresses the generation of "eye discharge" caused by the accumulation of resin around the die during extrusion molding, while also having less roughness on the film surface (see, for example, Patent Document 2). More specifically, it is a heat-shrinkable film obtained by stretching a film obtained by extruding a resin composition containing 100 parts by weight of cyclic olefin resin (A), 2 to 400 parts by weight of polyolefin (B), and 0.005 to 0.5 parts by weight of fluororesin (C).

[0005] Furthermore, for the purpose of maintaining rigidity and improving resistance to sebum whitening, a heat-shrinkable multilayer film has been proposed comprising a substrate having a first surface and a second surface and containing a thermoplastic resin, an intermediate layer laminated on at least one of the first surface and the second surface of the substrate, and a surface layer laminated on the intermediate layer and containing a thermoplastic resin (see, for example, Patent Document 3). More specifically, the intermediate layer provided between the substrate and the surface layer contains 50 to 90% by mass of a cyclic olefin resin and 5 to 35% by mass of a petroleum resin, and furthermore, the thickness of the surface layer is limited to 10% or less of the total thickness of the resin constituting the entire heat-shrinkable multilayer film.

[0006] Japanese Patent Publication No. 2003-053912 (Claims, etc.), Japanese Patent Publication No. 2010-116499 (Claims, etc.), Japanese Patent Publication No. 2022-171128 (Claims, etc.)

[0007] However, the multilayer heat-shrinkable film described in Patent Document 1 required the provision of two outer layers, each primarily composed of a cyclic olefin compound (COC) or polystyrene resin, on the surface of the intermediate layer. Therefore, it was necessary to use expensive COC, and the improvement of interlayer adhesion had not been sufficiently verified. Furthermore, although the heat shrinkage rate Y at a predetermined temperature (70-90°C) in the main stretching direction was specified to satisfy the predetermined equations 1 and 2, there was a problem in that specific control factors had not been identified.

[0008] Furthermore, the heat-shrinkable laminated film described in Patent Document 2 also required a resin composition derived from a polyolefin resin composition containing a predetermined amount of cyclic olefin resin (A) or fluororesin (C) in addition to the polyolefin resin. As a result, it was necessary to use a considerable amount of expensive cyclic olefin resin, etc., which increased manufacturing costs and was economically disadvantageous. Moreover, because a predetermined amount of fluororesin (C) was added, problems arose such as reduced or inconsistent interlayer adhesion between the intermediate layer and the surface layer, and furthermore, the printability of the intermediate layer tended to decrease.

[0009] Furthermore, the heat-shrinkable multilayer film described in Patent Document 3 also required a substrate derived from propylene resin, etc., on which an intermediate layer derived from an olefin resin composition containing 50 to 90% by mass of cyclic olefin resin and 5 to 35% by mass or less of petroleum resin was laminated. As a result, the structure of the heat-shrinkable multilayer film became complex, or a considerable amount of expensive cyclic olefin resin had to be used, leading to high manufacturing costs and economic disadvantages.

[0010] Furthermore, these patent documents 1 to 3, which describe olefin-based resins (polyolefin copolymers) used in heat-shrinkable films, did not consider the influence of values ​​such as crystal melting enthalpy measured by DSC. Consequently, in all cases, there was a problem in that the variation in heat shrinkage rate was large (for example, exceeding ±10% of the average value), making it difficult to obtain the desired heat shrinkage rate with high accuracy.

[0011] Therefore, the present inventors have found that in a multilayer heat-shrinkable film comprising a first layer derived from a specific resin and a second layer derived from another specific resin, it is possible to satisfy predetermined characteristics (1) to (3) without substantially incorporating cyclic olefin compounds (COC) or fluororesins, thereby achieving a good balance of not only a desired heat shrinkage rate but also good recyclability and interlayer adhesion, thus completing the present invention. In other words, the present invention aims to provide a multilayer heat-shrinkable film having a first layer derived from an olefin-based resin containing a specific olefin copolymer, which provides a desired heat shrinkage rate with little variation, while also exhibiting good interlayer adhesion and recyclability.

[0012] The present invention provides a multilayer heat-shrinkable film comprising a first layer derived from a polyolefin copolymer (hereinafter sometimes referred to as an intermediate layer) and a second layer derived from a styrene resin (hereinafter sometimes referred to as a surface layer) directly or indirectly laminated on at least one surface of the first layer, characterized in that it satisfies the following characteristics (1) to (3), and can solve the above-mentioned problems. (1) The heat shrinkage rate of the multilayer heat-shrinkable film in the main shrinkage direction when immersed in 100°C hot water for 10 seconds is 60% or more. (2) The crystalline melting enthalpy of the polyolefin copolymer constituting the first layer, as measured by DSC, is 120 mJ / mg or less. (3) Petroleum resin is blended into the first layer, and the amount of petroleum resin blended is in the range of 3 to 20% by weight relative to the total amount (100% by weight) of the first layer. In other words, by satisfying predetermined characteristics (1) to (3) in a multilayer heat-shrinkable film of a predetermined structure, it is possible to obtain a desired heat shrinkage rate with little variation, as well as good interlayer adhesion and recyclability, by having a first layer derived from an olefin resin containing a specific olefin copolymer.

[0013] Furthermore, when constructing the multilayer heat-shrinkable film of the present invention, it is preferable to use at least one petroleum resin selected from C5-based petroleum resin, C9-based petroleum resin, and C5 / C9-based petroleum resin. By using such a petroleum resin, compatibility with the polyolefin copolymer, which is the main component of the first layer, is improved, making it easier to improve the transparency of the multilayer heat-shrinkable film and the resin material discharge performance during the molding of the multilayer heat-shrinkable film.

[0014] Furthermore, when constructing the multilayer heat-shrinkable film of the present invention, it is preferable to use a hydrogenated petroleum resin. By using such a petroleum resin, the compatibility and heat resistance with the polyolefin copolymer, which is the main component of the first layer, are further improved, making it easier to achieve a better balance of good interlayer adhesion, and also making it easier to improve the transparency of the multilayer heat-shrinkable film and the resin material discharge performance during molding.

[0015] Furthermore, in constructing the multilayer heat-shrinkable film of the present invention, it is preferable that the olefin copolymer has at least a first melting point peak and a second melting point peak at a higher temperature in DSC measurement, and that the temperature T1 of the first melting point peak is in the range of 75 to 100°C, and the temperature T2 of the second melting point peak is in the range of 95 to 130°C. By using such an olefin copolymer, the adjustment of the enthalpy of crystalline melting becomes easier and more accurate, making it easier to satisfy characteristics (1) and (2).

[0016] Furthermore, in constructing the multilayer heat-shrinkable film of the present invention, the density of the olefin copolymer, measured in accordance with JIS K 7112-1:2023 (equivalent to ISO 1183-1:2019), is set to 0.95 g / cm³. 3 The following values ​​are preferable. By using such an olefin copolymer, the adjustment of the enthalpy of crystalline melting becomes easier and more precise, making it easier to satisfy characteristics (1) and (2).

[0017] Furthermore, when constructing the multilayer heat-shrinkable film of the present invention, it is preferable to set the haze, measured in accordance with JIS K 7136:2000 (corresponding to ISO 14782:1999), to a value of 15% or less. By controlling the haze in this way, it becomes easier to quantitatively manage the transparency of the multilayer heat-shrinkable film.

[0018] Furthermore, when constructing the multilayer heat-shrinkable film of the present invention, it is preferable to set the thickness to a value within the range of 20 to 100 μm. By controlling the thickness in this way, it becomes easier to satisfy the predetermined characteristics (1) to (3), which in turn increases the yield during manufacturing and facilitates the stable production of multilayer heat-shrinkable films.

[0019] Furthermore, when constructing the multilayer heat-shrinkable film of the present invention, it is preferable that the natural shrinkage rate when stored naturally under conditions of 30°C for 30 days be 2% or less. By further controlling the natural shrinkage rate in this way, it becomes easier to quantitatively manage the storability and usability of the multilayer heat-shrinkable film.

[0020] Figure 1(a) is a schematic cross-sectional view of an example of the multilayer heat-shrinkable film of the present invention, and Figure 1(b) is a diagram provided to illustrate an example of how to use the multilayer heat-shrinkable film of the present invention. Figure 2 is a diagram provided to illustrate the relationship between the amount of petroleum resin blended in the olefin resin (weight %) and the seal strength of the heat-shrinkable film. Figure 3 is a diagram provided to illustrate the relationship between the amount of petroleum resin blended in the olefin resin (weight %) and the peelability evaluation (relative value) of the heat-shrinkable film. Figure 4 is a diagram provided to illustrate the relationship between the amount of petroleum resin blended in the olefin resin (weight %) and the natural shrinkage rate (%) of the heat-shrinkable film. Figure 5 is a diagram provided to illustrate the relationship between the amount of petroleum resin blended in the olefin resin (weight %) and the heat shrinkage rate (%) in the TD direction of the heat-shrinkable film. Figure 6 is a diagram provided to illustrate the relationship between the enthalpy of crystalline melting of the polyolefin copolymer and the heat shrinkage rate of the heat-shrinkable film. Figure 7 is a diagram provided to illustrate the method of calculating the enthalpy of crystalline melting in an example of a DSC chart for illustrating two melting point peaks. Figure 8 shows examples of DSC charts measured for various polyolefin copolymers (types A to D). Figures 9(a) and 9(b) are diagrams used to explain the method for measuring solvent seal strength and the method for measuring continuous peelability (interlayer adhesion), respectively. Figures 10(a) and 10(b) are diagrams (photographs) used to explain the peeling modes in Example 1 and Comparative Example 1, respectively.

[0021] [First Embodiment] The first embodiment is a multilayer heat shrinkable film 10, as illustrated in Figure 1(a), comprising a first layer 10a derived from a polyolefin copolymer and second layers 10b and 10c derived from a styrene resin directly or indirectly laminated on at least one surface of the first layer 10a, characterized in that it satisfies the following characteristics (1) to (3): (1) The heat shrinkage rate of the multilayer heat shrinkable film in the main shrinkage direction when immersed in 100°C hot water for 10 seconds is 60% or more. (2) The crystalline melting enthalpy of the polyolefin copolymer constituting the first layer, as measured by DSC, is 120 mJ / mg or less. (3) Petroleum resin is blended into the first layer, and the amount of petroleum resin blended is within the range of 3 to 20% by weight relative to the total amount (100% by weight) of the first layer. The multilayer heat-shrinkable film of the first embodiment, and the first and second layers constituting the multilayer heat-shrinkable film, will be described in detail below.

[0022] 1. The first layer is an intermediate layer (in the case of three or more layers) or a base material (in the case of two layers) derived from a predetermined polyolefin copolymer, and is a resin layer derived from an olefin resin composition containing at least a predetermined amount of petroleum resin and a polyolefin copolymer (particularly a polyethylene copolymer).

[0023] (1) Petroleum resin (1)-1 Amount The olefin resin composition constituting the first layer contains petroleum resin, and is characterized in that the amount of petroleum resin is within the range of 3 to 20% by weight relative to the total amount (100% by weight) of the first layer. The reason for this is that if the amount of petroleum resin is less than 3% by weight, the interlayer adhesion (interlayer delamination mode) between the first layer and the second layer changes significantly, the adhesion between the first layer and the second layer decreases significantly, and interlayer delamination is more likely to occur at this interface. On the other hand, if the amount of petroleum resin exceeds 20% by weight, it is presumed that the cohesive force of the first layer decreases, or that the amount of low molecular weight substances at the interface increases, but the interlayer adhesion between the first layer and the second layer may decrease. Furthermore, if the amount of petroleum resin exceeds 20% by weight, the uniformity of mixing between the petroleum resin and the olefin copolymer decreases, which can lead to a decrease in the transparency of the heat-shrinkable film or an increase in the specific gravity of the entire film, thus reducing its recyclability. Therefore, it is more preferable to set the amount of petroleum resin within the range of 4 to 15% by weight, and even more preferable to set it within the range of 5 to 10% by weight.

[0024] Here, referring to Figure 2, the relationship between the amount of petroleum resin and the seal strength will be explained. Specifically, the horizontal axis of Figure 2 shows the amount of petroleum resin (weight %) in the olefin resin composition constituting the first layer, and the vertical axis shows the seal strength value (N / 15mm) as detailed in Example 1, etc. As shown in the characteristic curve in Figure 2, although there is some variation, when the amount of petroleum resin is 0 to less than 3% by weight, the seal strength value is approximately 0.8 N / 15mm. Furthermore, when the amount of petroleum resin is about 3 to 10% by weight, the seal strength value tends to increase slightly, and is in the range of 0.8 to 1.9 N / 15mm, with an average value of approximately 1.2 N / 15mm. Moreover, when the amount of petroleum resin exceeds 10% by weight and increases to the range of 20% by weight, the seal strength value clearly tends to increase, and is in the range of approximately 1.5 to 2.5 N / 15mm. Therefore, it can be said that the seal strength value tends to gradually increase as the amount of petroleum resin added increases. Thus, by setting the amount of petroleum resin added to a value within the range of 3 to 20% by weight relative to the total amount (100% by weight) of the first layer, it can be said that a seal strength value of at least 0.5 N / 15 mm or higher can be stably obtained.

[0025] Next, referring to Figure 3, the relationship between the amount of petroleum resin blended and interlayer adhesion will be explained. Specifically, the horizontal axis of Figure 3 shows the amount of petroleum resin blended (weight %) in the olefin resin composition constituting the first layer, and the vertical axis shows the evaluation score (relative value) of interlayer adhesion. As shown in Example 1, etc., the evaluation score of interlayer adhesion is calculated and recorded as relative data, with ◎ being 5 points, ○ being 3 points, △ being 1 point, and × being 0 points. As shown in the characteristic curve in Figure 3, it can be seen that when the amount of petroleum resin blended is 0 to less than 3% by weight, the evaluation score of interlayer adhesion is extremely low, ranging from 0 to 1. However, when the amount of petroleum resin blended is 3% or more, around 5%, the evaluation score of interlayer adhesion improves to 5 points, and when it increases further to around 10% to 20%, an evaluation score of 5 points can be stably obtained. Therefore, by setting the amount of petroleum resin to a value within the range of 3 to 20% by weight relative to the total amount (100% by weight) of the first layer, good interlayer adhesion can be obtained.

[0026] Next, referring to Figure 4, the relationship between the amount of petroleum resin blended and the natural shrinkage rate will be explained. Specifically, the horizontal axis of Figure 4 shows the amount of petroleum resin blended in the olefin resin composition constituting the first layer (by weight), and the vertical axis shows the natural shrinkage rate (%) measured under the conditions of 30°C for 30 days as a heat-shrinkable film. As shown in the characteristic curve in Figure 4, although there is some variation, when the amount of petroleum resin blended is less than 0-3% by weight, the natural shrinkage rate is in the range of approximately 1-4%, with an average value of about 3%. Furthermore, when the amount of petroleum resin blended is about 3-10% by weight, there is a tendency for the value of the natural shrinkage rate to decrease significantly, and it is in the range of approximately 1-1.5%, with an average value of about 1.1%. Furthermore, when the amount of petroleum resin exceeds 10% by weight and increases to the range of 20% by weight, the value of the natural shrinkage rate tends to decrease further, and is in the range of approximately 0.5 to 1.0%, with an average value of about 0.7%. Therefore, by setting the amount of petroleum resin to a value in the range of 3 to 20% by weight relative to the total amount (100% by weight) of the first layer, it can be said that a low value of about 0.5 to 1.5% as the natural shrinkage rate can be stably obtained.

[0027] Next, referring to Figure 5, the relationship between the amount of petroleum resin blended and the thermal shrinkage rate (%) in the TD direction will be explained. Specifically, the horizontal axis of Figure 5 shows the amount of petroleum resin blended in the olefin resin composition constituting the first layer (by weight), and the vertical axis shows the thermal shrinkage rate (%) in the TD direction when the heat-shrinkable film is immersed in hot water at 100°C for 10 minutes. As shown by the characteristic curve in Figure 5, it can be seen that this thermal shrinkage rate (%) is approximately 68%, regardless of the amount of petroleum resin blended. Therefore, by setting the amount of petroleum resin blended to a predetermined range (3 to 20% by weight) relative to the total amount of the first layer (100% by weight), it can be said that a stable thermal shrinkage rate (%) of at least 68% can be obtained with good accuracy under the conditions of 100°C for 10 minutes.

[0028] (1)-2 Type Regarding the type of petroleum resin, it may be a partially hydrogenated petroleum resin, but it is more preferably a fully hydrogenated petroleum resin for further improvement in transparency, heat resistance and the like. Further, such a petroleum resin is preferably at least one fully hydrogenated petroleum resin selected from C5-based petroleum resins, C9-based petroleum resins, and C5 / C9-based petroleum resins. The reason for this is that the use of such a fully hydrogenated petroleum resin can improve the transparency of the film. This is also because such a fully hydrogenated petroleum resin has good compatibility with the polyolefin copolymer that is the main component of the first layer. Examples of such C5-based petroleum resins include petroleum resins obtained by polymerizing C5 fractions such as isoprene, 1,3-pentadiene, cyclopentadiene, methylbutene, and pentene, and particularly aliphatic petroleum resins. Further, examples of C9-based petroleum resins include petroleum resins obtained by polymerizing C9 fractions such as α-methylstyrene, o-vinyltoluene, m-vinyltoluene, and p-vinyltoluene, and particularly aromatic petroleum resins. Further, examples of C5 / C9-based resins include petroleum resins obtained by mixing the aforementioned C5 fraction and C9 fraction at a predetermined weight ratio (for example, 10 / 90 to 90 / 10) and polymerizing the mixture.

[0029] (1)-3 Softening point The softening point of such a petroleum resin is measured in accordance with JIS K 2207, and it is usually preferably a value within the range of 100°C to 160°C. The reason for this is that if a petroleum resin has such a softening point, even when blended in a relatively small amount, the interlayer adhesion between the first layer mainly composed of a polyolefin copolymer and the second layer can be efficiently improved. Moreover, a petroleum resin having such a softening point is usually solid at room temperature and easy to handle. Therefore, in consideration of more efficient improvement of interlayer adhesion and handling properties, the softening point of the petroleum resin is more preferably set to a value within the range of 110°C to 145°C, and even more preferably a value within the range of 120°C to 150°C.

[0030] (1)-4 Weight-average molecular weight The weight-average molecular weight (Mw) of such petroleum resin is usually preferably in the range of 500 to 5000. The reason for this is that by setting the weight-average molecular weight in this way, even with a relatively small amount of formulation, the interlayer adhesion between the first layer, which mainly consists of polyolefin copolymer, and the second layer can be efficiently improved. Moreover, petroleum resins with such softening points are usually solid at room temperature and are therefore easier to handle. Accordingly, it is more preferable to set the weight-average molecular weight in the range of 1000 to 4500, and even more preferable to set it in the range of 1500 to 4000. The weight-average molecular weight (Mw) of such petroleum resin can be measured using GPC or the like, as shown in Example 1, etc.

[0031] (2) Polyolefin copolymer The olefin resin composition constituting the first layer is characterized by containing a polyolefin copolymer as a main component. More specifically, at least one of polyethylene copolymers, polypropylene copolymers, and α-olefin copolymers is preferred, with at least one of ethylene-1-hexene copolymer, ethylene-1-hexene copolymer, ethylene-1-octene copolymer, ethylene-1-hexene copolymer, etc. being typical. Furthermore, due to the low density of polyolefin copolymers, one or more combinations of LLDPE and MDPE are preferred. That is, LLDPE and MDPE can be obtained by copolymerizing ethylene and α-olefin using a single-site catalyst such as a Ziegler catalyst or a metallocene catalyst, and are preferred in that the density range can be controlled by adjusting the type and amount of α-olefin. Here, the amount of such polyolefin copolymer blended is preferably in the range of 80 to 97% by weight of the total amount (100% by weight) of the resin composition. The reason for this is that by adjusting the amount of polyolefin copolymer in this way, it becomes easier to control the specific gravity and enthalpy of fusion of the entire heat-shrinkable film within a predetermined range, even when a predetermined amount of petroleum resin is added. Therefore, it is possible to obtain a desired heat shrinkage rate with little variation, as well as good interlayer adhesion. Accordingly, it is more preferable to set the amount of such polyolefin copolymer to a value in the range of 85 to 95% by weight of the total amount, and even more preferable to set it to a value in the range of 91.5 to 94% by weight. Considering costs and other factors, it is preferable to substantially omit the addition of cyclic olefin compounds and fluororesins to the main component, the polyolefin copolymer, etc. However, as long as the objective of the present invention is not deviated from, it is possible to add them in amounts less than the predetermined amount, for example, less than 10% by weight for cyclic olefin compounds and less than 0.001% by weight for fluororesins.

[0032] (3) Crystal melting enthalpy As feature (2) of the present invention, the polyolefin copolymer contained in the first layer is characterized in that the crystal melting enthalpy (ΔH) measured by DSC is 120 mJ / mg or less. The reason for this is that by controlling the crystal melting enthalpy measured on a DSC chart to fall within the above numerical range, a desired heat shrinkage rate can be stably obtained when a heat shrinkable film is formed.

[0033] Here, FIG. 6 shows the relationship between the crystal melting enthalpy measured by DSC and the heat shrinkage rate of a multilayer heat shrinkable film for a polyolefin-based copolymer. The X axis represents the crystal melting enthalpy (mJ / mg) of the polyolefin-based copolymer, and the Y axis represents the heat shrinkage rate (%) at 100° C. for 10 seconds of the polyolefin-based heat shrinkable film. The data correspond to Example 1 and Comparative Example 1, which will be described later, respectively. As shown in the characteristic curve in FIG. 6, by controlling the crystal melting enthalpy of the polyolefin-based copolymer measured by DSC to a predetermined numerical range, that is, 120 mJ / mg or less, a preferable heat shrinkage rate of 50% or more can be obtained in the polyolefin-based heat shrinkable film.

[0034] More specifically, by setting the crystal melting enthalpy to 120 mJ / mg or less, for example, the heat shrinkage rate at 100° C. for 10 seconds can be increased. Conversely, when the crystal melting enthalpy exceeds 120 mJ / mg, the heat shrinkage rate is significantly reduced, and furthermore, the film may break during heat shrinkage. That is, it is considered that controlling the crystal melting enthalpy to a predetermined value or lower reduces the crystallinity, which in turn increases the number of molecular chains contributing to orientation during stretching.

[0035] However, if the value of the enthalpy of fusion becomes excessively small, the crystallinity becomes too low, which reduces the mechanical properties of the film and causes it to tear during stretching, making manufacturing difficult. Therefore, it is more preferable to set the enthalpy of fusion to a value in the range of 70 to 120 mJ / mg, even more preferable to set it to a value in the range of 80 to 110 mJ / mg, and even more preferable to set it to a value in the range of 90 to 100 mJ / mg.

[0036] The enthalpy of crystalline melting can be measured in accordance with JIS K 7121:2012 (equivalent to ISO 3146) using a DSC (Differential Scanning Calorimeter) under the conditions shown in Example 1, which will be described later. Here, a specific method for measuring the enthalpy of crystalline melting (ΔH) will be explained with reference to Figure 7. Figure 7 shows a schematic example of a DSC curve obtained in the second scan of the DSC curve. In the section of the DSC curve obtained in the second scan of the DSC curve obtained when evaluating the melting point peak as described above, the section from 140°C to 180°C is a straight line because the phase transition is completely completed. The point where this straight line intersects the DSC curve again when extended to 20°C is defined as the melting start point. The point where the baseline becomes a straight line again is defined as the melting end point. A straight line is then drawn from this melting start point to this melting end point. Therefore, as shown in Figure 7, the enthalpy of crystalline melting (ΔH) is automatically calculated by the accompanying analysis software using the area of ​​the region enclosed by the straight line from the melting start point to the melting end point and the DCS curve. This enthalpy of crystalline melting can then be adjusted to a desired range by appropriately adjusting, for example, the composition ratio of the monomer components constituting the polyolefin copolymer, or the manufacturing conditions (such as the degree of elongation). For example, if the polyolefin copolymer is a polyethylene-α-olefin copolymer, the value of the enthalpy of crystalline melting obtained can be adjusted by adjusting the ethylene content and / or α-olefin content.

[0037] (4) The monomer components used as raw materials for the polyolefin copolymer are not particularly limited in type, but typically include random copolymers such as ethylene-α-olefin random copolymers, propylene-α-olefin random copolymers, and butylene-α-olefin random copolymers, as well as block copolymers such as propylene-ethylene block copolymers. Particularly preferred is that the polyolefin copolymer is a polyethylene copolymer, and more specifically, an ethylene-α-olefin copolymer, in order to achieve a preferred density. These polyolefin copolymers have at least two crystalline parts with different melting temperatures, and the enthalpy of crystalline melting can be controlled by controlling the melting points of these crystalline parts, and it is believed that this control contributes to achieving a preferred thermal shrinkage rate. Therefore, the control of the melting point described later is also considered to contribute to achieving a preferred thermal shrinkage rate.

[0038] Furthermore, as the α-olefin, α-olefins having 2 to 20 carbon atoms are preferred, α-olefins having 2 to 10 carbon atoms are more preferred, and α-olefins having 4 to 8 carbon atoms are even more preferred. More specifically, the monomer component is preferably a polyolefin copolymer derived from ethylene, propylene, 1-butene, 1-hexene, 1-octene, etc. Therefore, the polyolefin copolymer is more preferably linear low-density polyethylene (LLDPE), and as the monomer component, in addition to the ethylene component, the LLDPE is preferably derived from at least one of 1-butene, 1-hexene, or 1-octene as the α-olefin component. The reason for this is that such LLDPE allows for more reliable control of desirable thermal shrinkage rate, density, and even crystal melting enthalpy to values ​​within the desired range.

[0039] Furthermore, while there are no particular restrictions on the catalyst used when obtaining polyolefin copolymers by polymerization, Ziegler-Natta catalysts and metallocene catalysts can be used because they offer good handling during polymerization. Moreover, since polyethylene resins with excellent stereoregularity and mechanical strength can be easily obtained, it is even more preferable to use a predetermined amount of metallocene catalyst to obtain a stereoregular polyethylene copolymer (such as isotactic or syndiotactic).

[0040] (5) Density or density of the polyolefin copolymer contained in the first layer is 0.912 g / cm³ 3 The following values ​​are preferable. This is because, by setting the density to a predetermined value, the desired heat shrinkage rate can be stably obtained when constructing a heat-shrinkable film. Furthermore, such densities reduce the time and effort required for specific gravity separation of the heat-shrinkable film, and also facilitate weight reduction of the heat-shrinkable film as a whole or the container packaged with the heat-shrinkable film. Moreover, if the density is too high, the possibility of breakage during stretching in film manufacturing increases.

[0041] Here, the density of the polyolefin copolymer is set to a value within a predetermined numerical range, for example, 0.912 g / cm³. 3 It has been found that by controlling the following, a high thermal shrinkage rate of 60% or more can be stably obtained when constructing a heat-shrinkable film under the thermal shrinkage conditions of 100°C for 10 seconds. Conversely, this means that the density is 0.912 g / cm³. 3 By using the following polyolefin copolymer as the main component of the first layer resin composition, a relatively high thermal shrinkage rate can be obtained in a heat-shrinkable film under predetermined conditions. However, if the density of the ethylene polyolefin copolymer becomes excessively low, the mechanical strength, heat resistance, and punchability of the film may decrease, and the enthalpy of fusion may also decrease accordingly, making it difficult to adjust the thermal shrinkage rate within the predetermined range. Therefore, the lower limit of the density of the polyolefin copolymer should be 0.885 g / cm³. 3 It is preferable to use 0.895 g / cm³.3 more preferably 0.898 g / cm 3 or less, and even more preferably 3 or less. Therefore, in consideration of values such as the heat shrinkage rate and the melting enthalpy, the density of the polyolefin copolymer is preferably set to a value within the range of 0.885 to 0.912 g / cm 3 , more preferably a value within the range of 0.895 to 0.910 g / cm 3 , and even more preferably a value within the range of 0.898 to 0.908 g / cm . Such density can be measured in accordance with JIS K 7112-1:2023 (equivalent to ISO 1183-1:2019), and more specifically, can be measured according to the method shown in Example 1 described later.

[0042] Next, the relationship between the density of the polyolefin copolymer and the crystal melting enthalpy will be described. That is, as shown in FIG. 8, DSC charts corresponding to a plurality of polyolefin copolymers (types A to D) used in Examples 1 and the like and Comparative Examples 1 and the like are obtained, and data of crystal melting enthalpy has been acquired. It has been separately found that there is a strong correlation (a linear functional relationship) between the density of the polyolefin copolymer and the crystal melting enthalpy. Therefore, as the density increases, the crystal melting enthalpy increases in a linear functional manner, so by controlling the density of the polyolefin copolymer, the value of the crystal melting enthalpy can also be controlled within a desired range. Accordingly, under certain conditions, for example, by controlling the density of the polyolefin copolymer to 0.912 g / cm 3 or less, a value of 120 mJ / mg or less for the crystal melting enthalpy of the polyolefin copolymer can be obtained with high accuracy.

[0043] (6) Melt Flow Rate (MFR) Furthermore, regarding the melt flow rate (MFR) of the polyolefin copolymer contained in the first layer, it is preferable that the MFR (190°C, 2.16 kg / min) measured in accordance with JIS K 7210-1:2014 (corresponding to ISO 1133-1:2011) be 3.8 g / 10 min or less. The reason for this is that if such an MFR exceeds a predetermined range, the predetermined heat shrinkage rate may not be obtained when actually using it as a heat shrinkable film. Also, having such an MFR within a predetermined range makes it easier to extrude during film manufacturing. However, if such an MFR becomes excessively small, although there is an effect from density, the values ​​of enthalpy of fusion and heat shrinkage rate also tend to be low. Therefore, it is more preferable to set the MFR to a value within the range of 0.1 to 3.7 g / 10 min, even more preferable to set it to a value within the range of 0.5 to 3.5 g / 10 min, and even more preferable to set it to a value within the range of 0.8 to 3.0 g / 10 min.

[0044] (7) Melting point 1 The polyolefin copolymer contained in the first layer may have one melting point peak in the DSC chart obtained in the DSC measurement, but it is preferable to have multiple melting point peaks (at least a first melting point peak and a second melting point peak with a higher temperature).

[0045] Here, the significance of these two melting point peaks will be explained again with reference to Figure 7. Specifically, Figure 7 shows a schematic example of a DSC chart obtained by performing DSC measurement (dynamic scan) on a polyolefin copolymer constituting a heat-shrinkable film. Therefore, it is preferable that at least a first melting point peak appears in a predetermined temperature range on such a DSC chart, and a second melting point peak appears at a higher temperature, and that these are identified. The reason for this is that having such two melting point peaks makes it easier to adjust the value and distribution of the crystal melting enthalpy, which is the sum of the melting enthalpy of the first and second melting point peaks. Therefore, it becomes easier to satisfy at least properties (1) to (3), and consequently, the heat-shrinkable film can accurately exhibit the desired heat shrinkability.

[0046] Furthermore, as shown in Figure 8, the DSC charts of the type A to D polyolefin copolymers used in Examples 1 to 3 and Comparative Example 5, obtained by DSC measurement, confirm that each polyolefin copolymer has at least two melting point peaks. It is understood that by controlling the temperatures (T1 and T2) and distribution of these multiple melting point peaks, the value of the enthalpy of crystalline melting can be adjusted with high precision, and consequently, the value of the thermal shrinkage rate can be stably controlled within a desired range. Therefore, as described above, it is preferable that the polyolefin copolymer contained in the first layer has multiple melting point peaks (at least a first melting point peak and a second melting point peak with a higher temperature) in the DSC chart obtained by DSC measurement.

[0047] (8) Melting Point 2 Furthermore, if the polyolefin copolymer contained in the first layer has multiple melting point peaks, it is preferable that the temperature of the first melting point peak (T1) is in the range of 75 to 100°C, and the temperature of the second melting point peak (T2) is in the range of 95 to 130°C. Here, the temperature T1 of the first melting point peak means the temperature of the lowest melting point peak among the multiple melting point peaks. That is, the temperature T1 of the first melting point peak is lower than the temperature T2 of the second melting point peak, so T1 < T2. Also, the temperature T2 of the second melting point peak means the temperature of the highest melting point peak among the multiple melting point peaks. It has been found that by limiting the temperature range of these melting point peaks, it becomes easier to adjust the enthalpy of crystal melting in the polyolefin copolymer, and it becomes easier to satisfy characteristics (1) to (3). It can be said that the lower the temperature of the first melting point peak, the higher the thermal shrinkage rate can be. This is thought to be because the partial melting and recrystallization of the film during stretching significantly affects the thermal shrinkage rate. Therefore, by using a polyolefin copolymer with a first melting point peak temperature within a predetermined range as the main component of the olefin resin composition of the first layer, a desirable thermal shrinkage rate can be achieved.

[0048] (9) Melting point 3 Furthermore, if the polyolefin copolymer contained in the first layer has multiple melting point peaks, it is preferable that the difference (T2-T1) between the temperature of the first melting point peak (T1) and the temperature of the second melting point peak (T2), which is at a higher temperature, be within the range of 13 to 40°C. The reason for this is that by controlling the temperature difference (T2-T1) of these melting point peaks to a predetermined range, it becomes easier to adjust the enthalpy of crystal melting, and it becomes easier to satisfy characteristics (1) to (3). Accordingly, it is more preferable that the temperature difference (T2-T1) of such melting point peaks be within the range of 14 to 35°C, and even more preferable that it be within the range of 15 to 30°C.

[0049] (10) The average molecular weight or the weight-average molecular weight (Mw) of the polyolefin copolymer is preferably in the range of 50,000 to 500,000. The reason for this is that having such a weight-average molecular weight (Mw) makes it easier to obtain a heat-shrinkable film with low haze and controlled heat shrinkage. Therefore, it is more preferable that the weight-average molecular weight (Mw) be in the range of 70,000 to 400,000, even more preferable that it be in the range of 100,000 to 300,000, and even more preferable that it be in the range of 150,000 to 280,000.

[0050] 2. The second and / or third layer is a layer derived from a styrene resin, formed on one or both sides of the first layer to improve adhesion, etc., and may be formed from a styrene resin composition containing a styrene resin. Therefore, such a styrene resin composition is preferably a styrene block copolymer resin composition. This is because the second layer derived from such a styrene resin composition can achieve a desirable haze of the heat shrinkable film, and can also achieve a desirable low-temperature heat shrinkage rate and a desirable natural shrinkage rate of the heat shrinkable film. More specifically, such a second and / or third layer, depending on its thickness, can achieve a haze of 15% or less of the heat shrinkable film even when laminated to the first layer described above, and as a result, the heat shrinkable film can be used in a wide range of applications. Therefore, it is more preferable to set the haze of such a heat shrinkable film to a value of 12% or less, and even more preferable to set it to a value of 10% or less.

[0051] The resin composition constituting the second and / or third layer is preferably a styrene-based block copolymer resin composition (hereinafter also referred to as "SBC resin composition") mainly composed of the same or different styrene-butadiene copolymer (SBC). This is because SBC can impart solvent sealing properties to the film surface, and furthermore, it can provide low-temperature shrinkage and / or stiffness. Furthermore, SBC can also contribute to improved transparency. Therefore, the SBC content in such an SBC resin composition is preferably 70% by weight or more, more preferably 80% by weight or more, and more preferably 85% by weight or more, based on the total amount of the SBC resin composition.

[0052] The second and / or third layers may be made of a resin consisting solely of SBC, but it is preferable that other components are included in the resin composition in addition to SBC to improve film properties such as impact resistance and / or antiblocking properties. Therefore, the SBC content in the SBC resin composition is preferably 99% by weight or less, more preferably 95% by weight or less, and even more preferably 90% by weight or less, based on the total amount of the SBC resin composition. The density of the SBC is usually 1 to 1.05 g / cm³. 3 The value is preferably within the range of 1.01 to 1.04 g / cm³. 3 It is more preferable that the value be within this range. The reason for this is that if SBC has such a predetermined density, even when it constitutes part of an olefin-based heat shrink film, it becomes easier to control its specific gravity, or an equivalent density, to be below the desired value.

[0053] Furthermore, the second and third layers preferably further contain hydrogenated styrene-based thermoplastic elastomer (SEBS). This is because SEBS has high compatibility with the olefin copolymer forming the first layer, and SEBS can improve the interlayer adhesion of the heat-shrinkable film. In other words, if the SEBS content is too low, the interlayer adhesion may decrease. On the other hand, if the SEBS content is too high, the cost will increase, which may be economically disadvantageous. Therefore, the SEBS content in such an SBC resin composition is preferably in the range of 1 to 20% by weight, and more preferably in the range of 5 to 15% by weight, relative to the total amount (100% by weight) of the SBC resin composition. Furthermore, the density of such SEBS is usually 0.95 to 1.05 g / cm³. 3 It is preferable that the value be within the range of 1 to 1.04 g / cm³. 3 It is more preferable to set the value within this range. The reason for this is that if SEBS has such a predetermined density, it becomes easier to control its specific gravity (which is virtually identical to density) to a desired relatively small value when it is used to construct a heat-shrinkable film.

[0054] Furthermore, the same or different styrene-based block copolymer resin compositions constituting the second and third layers preferably further contain an antiblocking agent to prevent film blocking. This is because the antiblocking agent effectively prevents blocking without significantly altering the properties of the heat-shrinkable film. Such an antiblocking agent may be an organic or inorganic antiblocking agent, but is preferably a styrene-based antiblocking agent, for example, high-impact polystyrene (HIPS). The density of such an antiblocking agent is, for example, 1 to 1.05 g / cm³. 3 It is preferable to set the value within this range. This is because such a density makes it easier to adjust the specific gravity of the entire film to a lower level. Such an antiblocking agent may be melted in the film manufacturing process of the present invention, that is, its shape may be such that it cannot be recognized as a predetermined blocking agent within the film.

[0055] Furthermore, by adjusting the content ratio of the antiblocking agent, it is possible to achieve both desirable antiblocking properties and good heat shrinkage. However, if the content ratio of the antiblocking agent is too high, it tends to be disadvantageous in terms of cost, and in addition, the stretchability and heat shrinkage rate of the film may decrease. Moreover, if the content ratio of the antiblocking agent is too high, the haze of the film may increase. Therefore, it is preferable that the content ratio of the antiblocking agent in such an SBC resin composition is within the range of 0.1 to 5% by weight relative to the total amount (100% by weight) of the SBC resin composition, and more preferably within the range of 1 to 3% by weight. In addition, it is preferable that the SBC resin composition constituting such a second layer does not substantially contain cyclic polyolefin copolymers or fluororesins, but as long as it does not deviate from the purpose of the invention, it is possible to blend them in amounts below a predetermined amount, for example, less than 10% by weight for cyclic olefin compounds and less than 0.001% by weight for fluororesins.

[0056] 3. Multilayer Heat Shrinkable Film (1) Heat Shrinkage Rate One of the characteristics of the present invention is that the multilayer heat shrinkable film has a heat shrinkage rate (hereinafter sometimes referred to as heat shrinkage rate A1) of 60% or more in the main shrinkage direction (usually the TD direction) when immersed in 100°C hot water for 10 seconds. The reason for this is that by controlling the heat shrinkage rate to a value above a predetermined value, the range of applications for the multilayer heat shrinkable film is expanded, the thickness can be controlled within a predetermined range during manufacturing, and the usability is improved. However, if the heat shrinkage rate is excessively large, the types of monomers that can be applied and the manufacturing methods may be excessively limited, or the yield during manufacturing may be excessively small. Therefore, it is more preferable to set the heat shrinkage rate to a value in the range of 61 to 85%, even more preferable to set it to a value in the range of 62 to 80%, and even more preferable to set it to a value in the range of 65 to 75%.

[0057] (2) Natural shrinkage rate Furthermore, as a further characteristic of the present invention, it is preferable that the natural shrinkage rate of the multilayer heat-shrinkable film, when left at 30°C for 30 days, is 2% or less in the direction perpendicular to the main shrinkage direction. The reason for this is that by setting the natural shrinkage rate to 2% or less, the storage conditions are not excessively restricted, and the predetermined heat shrinkage rate can be obtained more reliably and reproducibly in actual use. However, if the natural shrinkage rate is excessively small, the types of usable olefin monomer species and average molecular weight may be excessively restricted. Therefore, it is more preferable that the natural shrinkage rate be in the range of 0.1 to 1.8%, even more preferable that it be in the range of 0.2 to 1.5%, and even more preferable that it be in the range of 0.3 to 1.2%. The natural shrinkage rate can be measured as the heat shrinkage rate when left at 30°C for 30 days, according to the method described in detail in Example 1, etc.

[0058] (3) Specific gravity, or the specific gravity of the heat-shrinkable film measured in accordance with JIS K 7112-1:2023 (equivalent to ISO 1183-1:2019) (measurement temperature: 23°C, which can be considered almost identical to density), is preferably set to a value of 0.95 or less. The reason for this is that by setting the specific gravity of the heat-shrinkable film to a value below a predetermined value, it becomes easier to separate the specific gravity during the recycling of PET bottles, as well as to adjust the value of the enthalpy of fusion and the value of the heat shrinkage rate. However, if such a specific gravity becomes excessively small, the types of polyethylene resin compositions that can be used may be excessively limited, the mechanical strength as a multilayer heat-shrinkable film may decrease, or the natural shrinkage rate may become excessively large. Therefore, it is more preferable to set such a specific gravity to a value in the range of 0.85 to 0.95, and even more preferable to set it to a value in the range of 0.90 to 0.94. Note that such specific gravity is the weight ratio to water, and in the case of the present invention, it is a characteristic that can be considered virtually identical to density.

[0059] (4) Haze Furthermore, it is preferable that the haze of the multilayer heat shrinkable film (typically for a thickness of 40 μm) measured in accordance with JIS K 7136:2000 (corresponding to ISO 14782:1999) be 15% or less. The reason for this is that by specifically limiting the haze of such a film to a predetermined range, the transparency of the heat shrinkable film can be controlled quantitatively, and the good transparency further enhances its versatility. More specifically, if the haze of the film before heat shrinkage exceeds 15%, the transparency decreases, and it may become difficult to apply it to decorative purposes, etc. On the other hand, if the haze of the film before heat shrinkage becomes excessively small, it becomes difficult to control it stably, and the production yield may decrease significantly. Therefore, it is more preferable to set the haze of the heat-shrinkable film to a value in the range of 0.5 to 15%, even more preferable to set it to a value in the range of 0.7 to 10%, particularly preferable to set it to a value in the range of 0.8 to 5.0%, even more preferable to set it to a value in the range of 1.0 to 4.0%, and even more preferable to set it to a value in the range of 1.1 to 3.5%.

[0060] (5) Thickness It is preferable that the thickness of the multilayer heat shrinkable film be within the range of 20 to 100 μm. The reason for this is that by controlling the thickness to a predetermined range, not only is the usability improved, but the specific gravity is controlled to a value below the desired value, and excellent heat shrinkability and transparency can be obtained. More specifically, if the thickness of the multilayer heat shrinkable film is less than 20 μm, it may become difficult to adjust the specific gravity, or the handling may be significantly reduced. On the other hand, if the thickness of the multilayer heat shrinkable film exceeds 100 μm, the transparency may decrease, or it may become difficult to achieve a uniform thickness. Therefore, it is more preferable that the thickness of the multilayer heat shrinkable film be within the range of 25 to 60 μm, and even more preferable that it be within the range of 30 to 50 μm.

[0061] (6) Solvent seal strength It is preferable that the solvent seal strength of the multilayer heat shrink film be 0.5 N / 15 mm or higher. Solvent seal strength refers to the peel strength value between two multilayer heat shrink films sealed with a solvent, measured by the method described in the examples below. That is, after the multilayer heat shrink film is wrapped around a container, the overlapping portion is sealed with a solvent and used as a tubular structure. Therefore, by controlling the solvent seal strength to a value within a predetermined range, the surfaces of the multilayer heat shrink films used after sealing are bonded to a value of a predetermined value or higher, thereby reducing the possibility of problems such as peeling of the sealed portion. On the other hand, if the solvent seal strength is excessively high, it may become difficult to easily and quickly separate two multilayer heat shrink films that are wrapped around a PET bottle or the like. Therefore, it is generally preferable that the solvent seal strength be in the range of 0.5 to 10 N / 15 mm, more preferably in the range of 0.6 to 8 N / 15 mm, and even more preferably in the range of 0.7 to 5 N / 15 mm.

[0062] (7) Layer structure The multilayer heat shrinkable film basically includes a first layer 10a derived from a polyolefin copolymer and second layers 10b and 10c derived from a styrene resin laminated on at least one side of the first layer, as shown in Figure 1(a). The first layer 10a is the base layer of the multilayer heat shrinkable film and is preferably a base layer (corresponding to an intermediate layer in the case of three or more layers) that constitutes 50% by weight or more, preferably 60% by weight or more, more preferably 70% by weight or more, and even more preferably 80% by weight or more of the total amount of the heat shrinkable film. The reason for this is that by having the base layer constitute each layer containing the polyolefin copolymer in such a weight ratio, it becomes easier to adjust the specific gravity of the entire heat shrinkable film, and consequently, the adjustment of the enthalpy of fusion and the heat shrinkage rate can be adjusted accurately and stably.

[0063] Furthermore, in order to form a two-layer structure, it is preferable that one of the second layers 10b or 10c derived from a styrene-based resin is laminated on one side of the first layer 10a. That is, the multilayer heat shrinkable film may be a two-layer heat shrinkable film consisting of the first layer and the second layer laminated on one side of the first layer. In this case, of the two surfaces of the multilayer heat shrinkable film, one surface will have the first layer exposed, and the other surface will have the second layer laminated on it and exposed.

[0064] Furthermore, in order to form a three-layer structure, it is preferable that, in addition to the second layer 10b laminated on one side of the first layer 10a, a second layer 10c derived from a styrene resin is also laminated on the other side of the first layer 10a. That is, the multilayer heat-shrinkable film is preferably a three-layer heat-shrinkable film consisting of the first layer 10a, the second layer 10b laminated on one side of the first layer, and the second layer 10c laminated on the other side of the first layer 10a. In this case, the styrene resin composition forming the second layer laminated on one side may be the same as the styrene resin composition forming the second layer laminated on the other side, or it may be formed from mutually different styrene resin compositions.

[0065] Furthermore, although not shown in the figures, one or more additional layers may exist between the first layer and the second layer laminated on one of its surfaces. That is, it is preferable that one or more additional layers exist between the first layer and the second layer laminated on the other surface. Examples of such additional layers include adhesive layers derived from various known adhesives and decorative layers derived from decorative materials. In other words, the second layer, which is directly or indirectly laminated on one or both surfaces of the first layer, can impart solvent sealing properties and / or printability to the film surface. Moreover, such a second layer can easily improve the thermal shrinkage rate at low temperatures and reduce the natural shrinkage rate. Furthermore, such a second layer can also improve the decorative properties, informational properties, rigidity, and ease of use of the multilayer heat-shrinkable film.

[0066] As described above, since such a multilayer heat-shrinkable film includes other layers, the first layer is preferably 99% by weight or less of the total amount of the heat-shrinkable film, more preferably 95% by weight or less, even more preferably 90% by weight or less, and even more preferably 85% by weight or less.

[0067] (8) Manufacturing method / Usage method The manufacturing method of the multilayer heat shrink film will be described in detail in the second embodiment described later. Furthermore, the usage method of the multilayer heat shrink film 10, as illustrated in Figure 1(b), will be described in detail in the third embodiment described later.

[0068] [Second Embodiment] The second embodiment is an invention relating to a method for manufacturing a multilayer heat-shrinkable film as described in the first embodiment.

[0069] 1. Preparation of raw materials and melting process A polyolefin copolymer and petroleum resin are prepared as raw materials for the resin composition that forms the first layer. A styrene resin and, if necessary, a hydrogenated styrene thermoplastic elastomer and / or an antiblocking agent are prepared as raw materials for the resin composition that forms the second layer. These raw materials may be as described in the first embodiment above. Next, the raw materials for the resin composition that forms the first layer are weighed and added to a stirring container, and it is preferable to heat each in a stirring container until it becomes uniform and melts. Alternatively, the raw materials for the resin composition that forms the second layer are weighed and added to another mixing container, and it is preferable to heat each in a stirring container until it becomes uniform and melts.

[0070] 2. Process for creating the raw material sheet Next, the uniformly mixed raw materials are dried to an oven-dry state, and then, typically, extrusion molding is performed to create a raw material sheet of a predetermined thickness. More specifically, the extrusion temperature is usually preferably in the range of 190 to 230°C, and more preferably in the range of 200 to 220°C. The screw diameter (D) of the extruder is preferably in the range of 15 to 35 mm, and more preferably in the range of 20 to 30 mm. Considering the length (L) and screw diameter (D) of the extruder, the L / D ratio is preferably in the range of 20 to 40, and more preferably in the range of 25 to 35. Then, by extrusion molding using such an extruder, the width of the raw material sheet is usually preferably in the range of 30 to 1000 μm, more preferably in the range of 50 to 300 μm, and even more preferably in the range of 100 to 200 μm.

[0071] Here, it is preferable to use, for example, a three-layer extruder. When manufacturing a multilayer heat-shrinkable film using such a three-layer extruder, it is preferable to feed the olefin resin composition into the raw material inlet for forming the first layer (base layer), one of the three raw material inlets of the three-layer extruder. Similarly, it is preferable to feed, for example, a styrene resin composition (particularly a styrene block copolymer resin composition) into the raw material inlet for forming the second layer (a surface layer laminated on one or both sides of the base layer). In other words, a two-type, three-layer raw material sheet can be formed using a three-layer extruder in this way.

[0072] 3. Process for creating multilayer heat shrinkable film Next, it is preferable to stretch the obtained raw sheet by moving it on and between rolls while heating and pressing using a shrink film manufacturing apparatus. More specifically, it is preferable that the stretching ratio in the TD direction be in the range of 300 to 700%, more preferably in the range of 350 to 660%, and even more preferably in the range of 300 to 600%. On the other hand, the stretching ratio in the MD direction is usually preferable to be in the range of 80 to 120%, more preferably in the range of 85 to 115%, and even more preferably in the range of 90 to 110%. In other words, a multilayer heat shrinkable film can be created by stretching in this way.

[0073] Furthermore, in such stretching processes, it is preferable to combine the preheating process and the fixing process. Therefore, it is generally preferable to set the preheating temperature to a value in the range of 80 to 100°C, and more preferably to a value in the range of 85 to 95°C. The stretching temperature is also generally preferable to a value in the range of 80 to 100°C, and more preferably to a value in the range of 85 to 95°C. It is also preferable to perform a relaxation operation after stretching. The heat-fixing temperature in such a relaxation operation is generally preferable to a value in the range of 80 to 100°C, and more preferably to a value in the range of 80 to 90°C. As a result of such a relaxation operation, it is generally preferable to set the stretching ratio in the TD direction to a value in the range of 250 to 650%, and more preferably to a value in the range of 350 to 550%. On the other hand, the stretching ratio in the MD direction is generally preferable to a value in the range of 80 to 120%, more preferably to a value in the range of 90 to 110%, and even more preferably to a value in the range of 95 to 105%. In addition, during such relaxation operations, only the stretching ratio reported in the TD direction may be changed, and the stretching ratio in the MD direction may not be changed.

[0074] 4. Inspection Process for Multilayer Heat Shrinkable Film (Optional Process) It is preferable to continuously or intermittently measure the following characteristics of the prepared multilayer heat shrinkable film and to provide a predetermined inspection process (optional process). That is, by measuring the following characteristics through the predetermined inspection process and confirming that they fall within a predetermined range, it is possible to obtain a multilayer heat shrinkable film with more uniform specific gravity separation properties and heat shrinkage properties. 1) Visual inspection of the appearance of the multilayer heat shrinkable film 2) Measurement of thickness variation 3) Measurement of tensile modulus 4) Measurement of tear strength 5) Measurement of viscoelastic properties using SS curve

[0075] [Third Embodiment] The third embodiment is an embodiment of a method for using a multilayer heat shrink film. Therefore, any known method for using shrink film can be suitably applied. For example, when implementing a method for using a multilayer heat shrink film, first, the multilayer heat shrink film is cut to an appropriate length and width and formed into a long cylindrical object. Next, the long cylindrical object is supplied to an automatic labeling device (shrink labeler) and further cut to the required length. Then, it is fitted onto a container filled with contents. Examples of such containers include, but are not limited to, resin containers (e.g., PET resin containers, particularly PET bottles, and polyolefin resin containers, particularly PP bottles) or glass containers (e.g., glass bottles).

[0076] Next, the multilayer heat-shrinkable film fitted onto the container is subjected to a heat treatment by passing it through a hot air tunnel or steam tunnel at a predetermined temperature. Then, by blowing radiant heat such as infrared rays, or heated steam at about 90°C, from the surroundings into these tunnels, the multilayer heat-shrinkable film is uniformly heated and heat-shrinked. As a result, the film can be tightly attached to the outer surface of the container, and a labeled container can be quickly obtained.

[0077] In other words, as detailed in the first embodiment, the multilayer heat-shrinkable film of the present invention includes a first layer (intermediate layer) containing a specific polyolefin copolymer, and by satisfying various properties, not only can good continuous peelability and sealing properties be obtained, but a desired heat shrinkage rate can also be obtained with high precision, and furthermore, recyclability can be improved by lower density (specific gravity), etc. Moreover, the multilayer heat-shrinkable film of the present invention can also reduce the natural shrinkage rate during storage before heat shrinking, and improve transparency (haze).

[0078] Next, the present invention will be described in more detail by reference to examples. However, the present invention is not limited to the following examples without particular reason, and it is possible to implement it with appropriate modifications, all of which are included within the technical scope of the present invention.

[0079] (Preparation of Polyolefin Copolymers) In carrying out each example and comparative example, various polyolefin copolymers (Types A to C) exhibiting the following properties were prepared. A predetermined amount (for example, 3 to 20% by weight) of petroleum resin is added to each polyolefin copolymer. It has been found that with such a blending amount, almost the same properties (density, crystal melting enthalpy (ΔH), number of peaks, melting point peak temperature, melting point peak temperature difference, average weight molecular weight, molecular weight distribution, MFR, haze, etc.) can be obtained whether or not petroleum resin is added.

[0080] 1) Type A (used in Examples 1, 2, 5 and Comparative Examples 1, 4) Ethylene-1-hexene copolymer produced by Ziegler-Natta catalyst Density: 0.900 g / cm³ 3 MFR (190°C, 2.16 kgf, the same applies hereafter): 0.8 g / 10 min. Temperature of the first melting peak T1: 86.7°C, temperature of the second melting peak T2: 110.6°C. Melting difference: 23.9°C. Crystal melting enthalpy (ΔH): 92 mJ / mg. Mw: 26.8 × 10⁻⁶ 4 Mn: 6.4 × 10 4 , Mw / Mn: 4.2

[0081] 2) Type B (used in Example 3 and Comparative Example 2) Density of ethylene-1-hexene copolymer produced by a metallocene catalyst: 0.901 g / cm³ 3 MFR: 2.0 g / 10 min. Temperature of the first melting point peak T1: 81.0°C, Temperature of the second melting point peak T2: 97.2°C. Melting point difference: 16.2°C. Crystal melting enthalpy (ΔH): 96 mJ / mg. Mw: 17.2 × 10⁻⁶ 4 Mn: 6.8 × 10 4 , Mw / Mn: 2.5

[0082] 3) Type C (used in Example 4 and Comparative Example 3) Ethylene-1-octene copolymer produced by a metallocene catalyst: Density: 0.903 g / cm³ 3 MFR: 1.2 g / 10 min. Temperature of the first melting point peak T1: 93.4°C, Temperature of the second melting point peak T2: 114.7°C. Melting point difference: 21.3°C. Crystal melting enthalpy (ΔH): 94.7 mJ / mg. Mw: 20.9 × 10⁻⁶4 Mn: 6.6 × 10 4 , Mw / Mn: 3.2

[0083] 4) Type D (used in Comparative Example 6) Density of ethylene-1-hexene copolymer produced by a metallocene catalyst: 0.913 g / cm³ 3 MFR: 2.0 g / 10 min, temperature of the first melting point peak T1: 101°C, temperature of the second melting point peak T2: 114°C, melting point difference: 13°C, enthalpy of fusion (ΔH): 122 mJ / mg, Mw: 20.1 × 10⁻⁶ 4 Mn: 8.7 × 10 4 , Mw / Mn: 2.3

[0084] (Preparation of petroleum resins) On the other hand, as petroleum resins, the following fully hydrogenated petroleum resins, C5 and C9 (types E to F), were prepared, respectively.

[0085] 5) Type E (used in Examples 1-3 and Comparative Example 4) C5 fully hydrogenated petroleum resin: iMarb P140 (manufactured by Idemitsu Kosan Co., Ltd.)

[0086] 6) Type F (used in Example 4) C9 fully hydrogenated petroleum resin: Alcon P140 (manufactured by Arakawa Chemical Industries, Ltd.)

[0087] [Example 1] 1. Evaluation of Polyolefin Copolymer As Example 1, the above-described polyolefin copolymer was evaluated as follows. Table 1 shows the evaluation results obtained. In this invention, if multiple melting peaks exist, the temperature of the lowest-temperature melting peak was considered as the melting point.

[0088] (1) Evaluation 1: Density The density of the polyolefin copolymer was measured by the density gradient tube method in accordance with JIS K 7112-1:2023 (corresponding to ISO 1183-1:2019).

[0089] (2) Evaluation 2: Enthalpy of Melt (ΔH) The enthalpy of melt (ΔH) of the polyolefin copolymer was measured using a DSC (DSC7000X manufactured by Hitachi High-Tech Corporation) in accordance with JIS K 7121:2012 (corresponding to ISO 11357-2:2013) at a heating rate of 10 seconds / min.

[0090] (3) Evaluation 3: Melting Point Peak The melting point (°C) of polyolefin copolymers was measured using a DSC in accordance with JIS K 7121:2012 (corresponding to ISO 11357-2:2013). Specifically, the analysis of the melting point peak was performed using the analysis software "Software for NEXTA" attached to the DSC "DSC7000X" manufactured by Hitachi High-Tech Corporation. That is, as shown in Figure 3, at least two melting points were measured on the DSC chart for each polyolefin copolymer used, and the lowest melting point was taken as the temperature of the first melting point peak (T1), and the highest melting point was taken as the temperature of the second melting point peak (T2). In addition, the difference in temperatures between these melting point peaks (T2 - T1) was calculated.

[0091] (4) Evaluation 4: Using a high-temperature GPC (HLC-8321 GPC / HT (manufactured by Tosoh Corporation)) with dichlorobenzene as the solvent, the elution time was measured at a column temperature of 145°C. The weight-average molecular weight (Mw) of the polyolefin copolymer was calculated by comparing the elution time with a calibration curve using standard styrene particles.

[0092] (5) Evaluation 5: MFR The MFR of the polyolefin copolymer was measured in accordance with JIS K 7210-1:2014 (measurement temperature: 190°C, load: 210 kgf).

[0093]

[0094] 2. Preparation of Heat Shrinkable Film (1) As Example 1, an olefin resin composition containing the above-mentioned Type A polyolefin copolymer and C5-type fully hydrogenated petroleum resin (iMarb P140, manufactured by Idemitsu Kosan Co., Ltd.) was prepared as the composition constituting the first layer (intermediate layer) of the heat shrinkable film. A styrene block copolymer (SBC) resin composition was prepared as the composition constituting the second layer (surface layer) of the film. More specifically, the SBC resin composition was prepared having a composition of 87.7% by weight of SBC, 10% by weight of hydrogenated elastomer, and 2.3% by weight of antiblocking agent. Such SBC is a styrene-butadiene copolymer (styrene / butadiene weight ratio is 82 / 18). The hydrogenated elastomer is a copolymer obtained by hydrogenating the butadiene units of the styrene-butadiene block copolymer, and the ratio of styrene units to butadiene units is 70 / 30. Furthermore, the antiblocking agent is HIPS (High Impact Polystyrene).

[0095] (2) Next, the olefin resin composition and the SBC resin composition were supplied to an extruder (manufactured by Labotec Co., Ltd.) with an L / D ratio of 30 and an extrusion screw diameter of 25 mm at an extrusion temperature of 210°C, and extrusion molding was performed in two types and three layers to obtain a raw material sheet with a thickness of 160 μm. That is, in this extrusion molding, the olefin resin composition was fed into the raw material supply port for forming the base layer (first layer), and the SBC resin composition was fed into two raw material supply ports for forming two surface layers (second layer) laminated on each of the two surfaces of the base layer, thereby forming a raw material sheet with a two-type, three-layer structure. Thus, a raw material sheet with a three-layer structure was formed, comprising a base layer formed from the olefin resin composition, an SBC resin composition surface layer laminated on one side of the base layer, and an SBC resin composition surface layer laminated on the other side of the base layer.

[0096] Next, using a shrink film manufacturing apparatus, the obtained three-layer raw material sheet was stretched under conditions of a preheating temperature of 90°C and a stretching temperature of 90°C, resulting in a stretch ratio of 100% in the MD direction and a stretch ratio of 500% in the TD direction. Then, immediately afterward, a relaxation operation was performed using the shrink film manufacturing apparatus at a heat-fixing temperature of 90°C to change the stretch ratio in the MD direction to 100% and the stretch ratio in the TD direction to 450%, thereby creating a heat-shrinkable film with a thickness of 40 μm (layer ratio (%): surface layer on both sides 30 (= 15 + 15) / intermediate layer 70) (hereinafter, this may be referred to as the heat-shrinkable film of Example 1).

[0097] 3. Evaluation of Multilayer Heat Shrink Film (1) Evaluation 6: Heat Shrinkage Rate in the TD Direction (100°C, 10 seconds) The obtained multilayer heat shrink film was cut into a predetermined shape of 10 cm x 10 cm squares to be used as measurement samples. Next, the measurement samples were immersed for 10 seconds in a water bath containing hot water maintained at 100°C ± 0.5°C. After removing them from the water bath, they were immediately immersed for 10 seconds in a separate water bath containing water maintained at 25°C, and then removed from the outside. The length of the obtained measurement samples in the main shrinkage direction (TD direction) was measured, and the heat shrinkage rate in the TD direction was calculated using the following formula (Evaluation 1). Heat shrinkage rate (%) = 100 × (Length before shrinkage - Length after shrinkage) / (Length before shrinkage)

[0098] (2) Evaluation 7: Haze In accordance with JIS K 7136:2000 (equivalent to ISO 14782:1999), the haze of multilayer heat shrinkable film cut to a predetermined shape was measured, similar to the sample for which the heat shrinkage rate in the TD direction was measured. The obtained evaluation results are shown in Table 2.

[0099] (3) Evaluation 8: Natural Shrinkage A roll of multilayer heat shrinkable film was prepared. Two markings were then made at 300 mm intervals (S1) along the main shrinkage direction (TD direction) of the multilayer heat shrinkable film as an initial value, and the film was left to stand naturally at 30°C for 30 days. After standing, the dimension between the markings that had shortened due to natural shrinkage (mm, S2) was measured using calipers or an optical microscope, and the natural shrinkage rate was calculated using the following formula (1). The obtained evaluation results are shown in Table 2.

[0100]

[0101] (4) Evaluation 9: Specific Gravity The specific gravity of the multilayer heat shrink film (at a temperature of 23°C) was measured in accordance with JIS K 7112-1:2023 (corresponding to ISO 1183-1:2023). The obtained evaluation results are shown in Table 2.

[0102] (5) Evaluation 10: Recyclability Test (Floating Test) Ten multilayer heat shrinkable films (length in the TD direction: 10 mm, length in the MD direction: 10 mm) were floated on the surface of water (23°C) contained in a container, and a recyclability test (floating test) was conducted to see if the multilayer heat shrinkable films remained floating on the water for 60 seconds or more, and were evaluated according to the following criteria. ○: All 10 multilayer heat shrinkable films floated on the water. ×: Nine or fewer multilayer heat shrinkable films floated on the water.

[0103] (6) Evaluation 11: Seal Strength Test The obtained multilayer heat shrink film was cut to a size of 15 mm in width and 100 mm in length in the TD direction to create strip-shaped samples. Next, a solvent (ethyl acetate / cyclohexane = 70 / 30) was applied 10 mm from the tip of the strip-shaped sample, and two strip-shaped samples were overlapped and air-dried at room temperature for 12 hours in a solvent-sealed state to obtain the measurement sample shown on the left side of Figure 9(a). Next, the T-shaped peel force was measured on the measurement sample using a tensile testing machine (AGX-10kNVD, manufactured by Shimadzu Corporation) at a peeling speed of 200 mm / min, and this was evaluated as the seal strength according to the following criteria. ◎: Seal strength is 1 N / 15 mm or more ○: Seal strength is 0.5 N / 15 mm or more △: Seal strength is 0.2 N / 15 mm or more ×: Seal strength is less than 0.2 N / 15 mm.

[0104] (7) Evaluation 12: Continuous peelability (interlayer adhesion) The obtained multilayer heat shrink film was cut to create strip-shaped samples with a width of 15 mm and a length of 100 mm in the TD direction. Next, a solvent (ethyl acetate / cyclohexane = 70 / 30) was applied to a length of 10 mm from the tip of the strip-shaped sample. Two strip-shaped samples were then stacked on the solvent-coated area and allowed to air dry at room temperature for 12 hours while remaining in a solvent-sealed state to obtain the measurement sample. Next, with the measurement sample in a horizontal position, one of the strip-shaped samples was forcibly peeled continuously in the 180° direction. The interlayer delamination state between the surface layer and the intermediate layer was then visually observed and the continuous peelability was evaluated according to the following criteria. When one of the strip-shaped samples is continuously peeled with the measurement sample in a horizontal position, as shown in Figure 10(a), it is preferable that substantially no interlayer delamination is observed between the surface layer and the intermediate layer on the stacked surface of the two strip-shaped samples, and that a continuous peel mode occurs in the intermediate layer. On the other hand, in the case of Comparative Example 1, which will be described later, as shown in Figure 10(b), delamination is observed between the surface layer and the intermediate layer, and the problem is that a continuous delamination mode does not occur in the intermediate layer. ○: Substantially no delamination is observed between the surface layer and the intermediate layer. ×: Substantially delamination is observed between the surface layer and the intermediate layer.

[0105]

[0106] [Examples 2-6] In Examples 2-6, as shown in Table 2, multilayer heat-shrinkable films were prepared and evaluated using the same method as in Example 1, except that types A to C were used as polyolefin copolymers, and the type and amount of petroleum resin were changed. The evaluation results obtained are shown in Table 2.

[0107] [Comparative Examples 1-5] In Comparative Examples 1-5, as shown in Table 2, types A-D were used as polyolefin copolymers, and each was either free of petroleum resin or the amount of petroleum resin added was changed to less than a predetermined amount. A multilayer heat-shrinkable film was then prepared and evaluated using the same method as in Example 1.

[0108] According to the present invention, a multilayer heat-shrinkable film satisfying at least characteristics (1) to (3) exhibits low variability (for example, within ±5% of the average value), a desired heat shrinkage rate, and good interlayer adhesion and recyclability. Furthermore, such a multilayer heat-shrinkable film can achieve a low natural shrinkage rate of 2% or less even under conditions of being left at 30°C for 7 days. Therefore, even when an olefin resin, particularly a polyethylene resin, is the main component, it is now possible to stably provide a multilayer heat-shrinkable film that satisfies all the required characteristics.

[0109] Therefore, with such a multilayer heat-shrinkable film, even under heat shrinkage conditions of, for example, 100°C for 10 minutes, a high heat shrinkage rate of at least 60% can be obtained with little variation and high accuracy. Consequently, it can be said that it is suitable as a packaging film and the like where a high shrinkage rate is required at a relatively low heating temperature. Moreover, in such a multilayer heat-shrinkable film, 0.912 g / cm² 3 Even at the following low specific gravities, it is now possible to easily and stably achieve this, and consequently, it has become easier to adjust the values ​​of the enthalpy of fusion and the thermal shrinkage rate to the desired range. Therefore, the thermal shrinkage rate can be obtained with less variation and with high accuracy, and when recycling PET bottles containing multilayer thermal shrinkage films, it is now possible to accurately and quickly separate the specific gravity of the multilayer thermal shrinkage film using a predetermined cyclone device or the like.

[0110] Furthermore, according to the manufacturing method of the present invention, it is possible to stably and with good yield produce the multilayer heat shrinkable film of the present invention that satisfies predetermined characteristics (1) to (3), and consequently, it is possible to stably and with good yield produce multilayer heat shrinkable films that have a low specific gravity and whose melting enthalpy value and heat shrinkage rate are adjusted to a desired range. Therefore, the multilayer heat shrinkable film of the present invention can be suitably applied to various PET bottles, outer covering materials for bento boxes, etc., greatly expanding its versatility, and since it is also easily recyclable, its industrial applicability is extremely high.

[0111] 10: Multilayer heat shrink film 10a: First layer 10b, 10c: Second layer 20: PET bottle

Claims

1. A multilayer heat shrinkable film comprising a first layer derived from a polyolefin copolymer and a second layer derived from a styrene resin directly or indirectly laminated to at least one surface of the first layer, characterized in that it satisfies the following characteristics (1) to (3): (1) The heat shrinkage rate of the multilayer heat shrinkable film in the main shrinkage direction when immersed in 100°C water for 10 seconds is 60% or more. (2) The crystalline melting enthalpy of the polyolefin copolymer constituting the first layer, as measured by DSC, is 120 mJ / mg or less. (3) Petroleum resin is blended into the first layer, and the amount of petroleum resin blended is within the range of 3 to 20% by weight relative to the total amount of the first layer.

2. The multilayer heat-shrinkable film according to claim 1, characterized in that the petroleum resin is at least one selected from C5 petroleum resin, C9 petroleum resin, and C5 / C9 petroleum resin.

3. The multilayer heat-shrinkable film according to claim 1 or 2, characterized in that the petroleum resin is a hydrogenated resin.

4. The multilayer heat-shrinkable film according to claim 1 or 2, wherein the olefin copolymer has, in DSC measurement, at least a first melting point peak and a second melting point peak at a higher temperature, and the temperature of the first melting point peak is in the range of 75 to 100°C, and the temperature of the second melting point peak is in the range of 95 to 130°C.

5. The density of the olefin copolymer, measured in accordance with JIS K 7112-1:2023, is 0.95 g / cm³. 3 The multilayer heat-shrinkable film according to claim 1 or 2, characterized in that the following values ​​are used.

6. The multilayer heat-shrinkable film according to claim 1 or 2, characterized in that the haze measured in accordance with JIS K 7136:2000 is 15% or less.

7. The multilayer heat-shrinkable film according to claim 1 or 2, characterized in that the thickness is within the range of 20 to 100 μm.

8. The multilayer heat-shrinkable film according to claim 1 or 2, characterized in that the natural shrinkage rate when left at 30°C for 30 days is 2% or less.

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