Polyethylene-based heat shrinkable film and method for manufacturing polyethylene-based heat shrinkable film

Abstract

The present invention provides a polyethylene-based heat-shrinkable film that can effectively suppress natural shrinkage and facilitate specific gravity separation, etc. [Solution] The following are polyethylene-based heat-shrinkable films, etc., that satisfy the characteristics (a) to (d) below. (a) The polyethylene resin composition contains a predetermined amount of polyethylene resin with a melting point of 100°C or higher relative to the total amount of polyethylene resin composition; (b) The natural shrinkage rate after storage at 40°C for 7 days is 1.8% or less; (c) The thermal shrinkage rate in the TD direction when heat-shrinked at 80°C for 10 seconds is in the range of 10 to 20%; and (d) The specific gravity is 0.95 or less.

Description

Technical Field

[0001] The present invention relates to a polyethylene-based heat-shrinkable film (hereinafter, may be simply referred to as a heat-shrinkable film), and a method for producing such a polyethylene-based heat-shrinkable film. More specifically, the present invention relates to a polyethylene-based heat-shrinkable film having a low natural shrinkage rate and being easy to recycle even when stored for a long time, and a method for producing such a polyethylene-based heat-shrinkable film.

Background Art

[0002] Conventionally, heat-shrinkable films are composed of various materials, and are widely used as base films for labels such as PET bottles. In particular, polyethylene-based heat-shrinkable films are inexpensive, have excellent economic efficiency, and have a relatively low specific gravity, so they are easily separated from PET bottles by specific gravity and have excellent recyclability. Therefore, their share as base films for labels is increasing. However, although polyethylene-based heat-shrinkable films have such excellent properties, when heated, their thermal response is rapid, so they shrink unevenly and are likely to break. That is, the storage conditions of polyethylene-based heat-shrinkable films, particularly affected by humidity and the like, cause the heat shrinkage rate at a predetermined temperature as a physical property to change, and thus the problem that the breakage prevention property is likely to decrease has been found.

[0003] Therefore, various polyolefin-based heat-shrinkable films with suppressed natural shrinkage rate after long-term storage have been proposed. For example, a polyolefin-based heat-shrinkable film composite formed by laminating a plurality of polyolefin-based heat-shrinkable films has been proposed (see, for example, Patent Document 1). More specifically, a heat-shrinkable polyolefin film composite has been proposed in which multiple polyolefin heat-shrinkable films are laminated and pressed with a hot plate at 90°C under a pressure of 0.4 MPa for 5 minutes, the peel strength of the bonded joint being 2 N / 15 mm or less, and the solvent adhesion strength using tetrahydrofuran being 2 N / 15 mm or more. Furthermore, the natural shrinkage rate in the main shrinkage direction of such polyolefin-based heat-shrinkable film composite is 2.5% or less under storage conditions of 40°C for 7 days. Furthermore, the thermal shrinkage rate in the main shrinkage direction is 20% or more when immersed in 80°C hot water for 10 seconds and then removed, and then immersed in 25°C water for 10 seconds and then removed.

[0004] Furthermore, a heat-shrinkable polyolefin film has been proposed that is made of a polyolefin resin composition containing a predetermined amount of cyclic polyolefin, wherein the heat shrinkage rate in the main shrinkage direction of the film is 25% or more at 80°C × 10 seconds and 50% or more at 95°C × 10 seconds, and satisfies the following characteristics (1) to (3) (see, for example, Patent Document 2). (1) The thermal shrinkage rate in the main shrinkage direction when exposed to 80°C hot water for 3 seconds is 15% or more and less than 30%. (2) The specific gravity is 0.95 or less. (3) The natural shrinkage rate in the direction perpendicular to the principal shrinkage direction at 40°C for 7 days is less than 0.5%. Furthermore, the polyolefin resin composition is characterized by being composed of a polypropylene resin, a petroleum resin, and a cyclic polyolefin, with the total amount of the polypropylene resin and petroleum resin being in the range of 99 to 90 parts by weight per 1 to 10 parts by weight of the cyclic polyolefin. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2005-193586 (Claims, etc.) [Patent Document 2] Japanese Patent Publication No. 2004-74426 (Claims, etc.) [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, the polyolefin-based heat shrinkage described in Patent Document 1 required laminating multiple polyolefin-based heat shrinkage films to obtain a natural shrinkage rate below a predetermined value, essentially creating a heat shrinkage film laminate. Furthermore, although the heat shrinkage rate obtained under specified heat shrinkage conditions (80°C, 10 seconds) is limited to a value of 20% or more, there was a problem in that the heat shrinkage rate and mechanical properties varied greatly due to the heat shrinkable film laminate, making it difficult to obtain a good appearance after heat shrinkage. Furthermore, there was no intention to use polyethylene resin with a melting point of 100°C or higher as the main component of the polyolefin resin, or to construct it in a multi-layer configuration, in order to obtain a low natural shrinkage rate.

[0007] Furthermore, in order to obtain a low natural shrinkage rate similar to that of conventional polypropylene resins, the polyolefin-based heat-shrinkable film described in Patent Document 2 required, in the examples, to have a multilayer three-layer structure consisting of a base layer mainly composed of polypropylene resin and inner and outer layers containing predetermined amounts of cyclic polyolefins or the like. As a result, manufacturing costs increased, leading to economic problems, and furthermore, it became difficult to make the device thinner. Furthermore, although the thermal shrinkage rate in the main shrinkage direction was limited to values ​​of 25% or more at 80°C for 10 seconds and 50% or more at 95°C for 10 seconds, uniform blending of petroleum-based resins and cyclic polyolefins was not easy. Consequently, there was a large variation in thermal shrinkage rate and mechanical properties, and it was difficult to obtain a good appearance after thermal shrinkage. Furthermore, there was no intention to use polyethylene resin with a melting point of 100°C or higher as the main component in order to obtain a low natural shrinkage rate as a polyolefin resin.

[0008] Therefore, the inventors of the present invention have found that by using a polyethylene resin composition that contains a considerable amount of polyethylene resin having a predetermined melting point without substantially containing a predetermined amount of cyclic polyolefin, a heat-shrinkable film can be obtained even in a single layer, with a low natural shrinkage rate over a long period of time, and is also easily recyclable, thus completing the present invention. In other words, the present invention aims to provide a polyethylene heat-shrinkable film that is basically a single layer, has a low natural shrinkage rate even when stored for a long period of time, has a low specific gravity, and is easy to recycle, by including a predetermined amount of a predetermined polyethylene resin. [Means for solving the problem]

[0009] According to the present invention, a polyethylene-based heat-shrinkable film is provided that is derived from a polyethylene-based resin composition containing a polyethylene-based resin, and is characterized by satisfying the following properties (a) to (d), thereby solving the above-mentioned problems. (a) The polyethylene resin composition contains polyethylene resin with a melting point of 100°C or higher, measured in accordance with JIS K 7121:2012, in an amount ranging from more than 70% to 100% by weight. (b) The natural shrinkage rate in the principal shrinkage direction after storage at 40°C for 7 days shall be 1.8% or less. (c) When thermal shrinkage occurs in 80°C hot water for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is denoted as A1, and A1 shall be a value within the range of 10 to 30%. (d) The specific gravity measured in accordance with JIS K 7112:1999 shall be a value of 0.95 or less. In other words, such a polyethylene-based heat-shrinkable film, without containing a predetermined amount of cyclic polyolefin, contains a considerable amount of polyethylene-based resin having a predetermined melting point, thereby maintaining excellent usability and suppressing the natural shrinkage rate in the main shrinkage direction to a low level, even when stored in a single layer for a long period of time. Moreover, since such polyethylene heat-shrinkable films have a low specific gravity, their specific gravity separation from PET bottles during recycling can be performed accurately and quickly using a designated cyclone device or the like. In general, the primary shrinkage direction in polyethylene-based heat-shrinkable films is the TD direction, when there is a TD direction and an MD direction perpendicular to it. However, depending on the application, the MD direction can be designated as the primary shrinkage direction.

[0010] In constructing the polyethylene-based heat-shrinkable film of the present invention, the polyethylene-based resin of characteristic (a) includes a plurality of different polyethylene-based resins, and when a polyethylene-based resin with a melting point of 100°C or higher and less than 110°C is designated as the first polyethylene-based resin, and a polyethylene-based resin with a melting point of 110°C or higher is designated as the second polyethylene-based resin, it is preferable that the blending ratio (weight ratio) of the first polyethylene-based resin to the second polyethylene-based resin be within the range of 30:70 to 70:30. By using two or more polyethylene resins with different melting points, with 110°C as the dividing line, it is possible to further suppress the rate of natural shrinkage while maintaining excellent usability, and to further facilitate the adjustment of recyclability.

[0011] In constructing the polyethylene-based heat-shrinkable film of the present invention, it is preferable to use a heat of fusion measured by DSC of 135 mJ / mg or less, in accordance with JIS K 7121:2012. In this way, by setting the heat of fusion, measured by a predetermined method, to a value within a predetermined range, the thermal shrinkage characteristics of the heat-shrinkable film at a predetermined temperature can be made reliable, and the natural shrinkage rate can also be controlled to a predetermined range with even greater precision.

[0012] In constructing the polyethylene-based heat-shrinkable film of the present invention, when heat-shrinking is performed in 90°C hot water for 10 seconds, the heat shrinkage rate in the main shrinkage direction is A2, and it is preferable that A2 be a value within the range of 20 to 40%. Thus, by setting the heat shrinkage rate A2 obtained under predetermined conditions (such as a temperature of 90°C) within a predetermined range, the heat shrinkage characteristics at a predetermined temperature as a heat shrinkable film can be ensured, and the natural shrinkage rate can also be suppressed to a low level.

[0013] In forming the polyethylene-based heat shrinkable film of the present invention, when the heat shrinkage rate in the main shrinkage direction when heat-shrunk under the conditions of hot water (boiling water) at 100°C for 10 seconds is defined as A3, the Set A3 to a value within the range of 40-60%. This is preferable. Thus, by setting the heat shrinkage rate A3 obtained under predetermined conditions (such as a temperature of 100°C) within a predetermined range, the heat shrinkage characteristics at a predetermined temperature as a heat shrinkable film can be ensured, and the natural shrinkage rate can also be suppressed even lower.

[0014] In forming the polyethylene-based heat shrinkable film of the present invention, it is preferable that the haze value measured in accordance with JIS K 7136:2000 is 10% or less. Thus, by setting the haze value within a predetermined range, a heat shrinkable film excellent in transparency and usability can be obtained.

[0015] In forming the polyethylene-based heat shrinkable film of the present invention, it is preferable that the thickness is within the range of 20 to 100 μm. Thus, by setting the film thickness within a predetermined range, the heat shrinkage characteristics at a predetermined temperature as a heat shrinkable film can be ensured, and the natural shrinkage rate and recyclability can be made even better.

[0016] Another aspect of the present invention is a method for producing a polyethylene-based heat shrinkable film derived from a polyethylene-based resin composition containing a polyethylene-based resin, characterized by having the following steps 1 to 2. Step 1: A step to prepare a polyethylene resin composition that satisfies characteristic (a), wherein the polyethylene resin contained in the polyethylene resin composition is considered to be 100% by weight, and the polyethylene resin contained in the polyethylene resin composition is contained in an amount of more than 70% by weight to 100% by weight of polyethylene resin with a melting point of 100°C or higher as measured in accordance with JIS K 7121:2012. Step 2: A step to produce a polyethylene-based heat-shrinkable film that satisfies the following characteristics (b) to (d) by stretching a polyethylene-based resin composition along a predetermined direction. (b) When thermal shrinkage occurs in 80°C hot water for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is denoted as A1, and A1 is set to a value within the range of 10 to 30%. (c) The natural shrinkage rate in the principal shrinkage direction after storage at 40°C for 7 days shall be 1.8% or less. (d) The specific gravity measured in accordance with JIS K 7112:1999 shall be a value of 0.95 or less. In other words, by including such a process, it is possible to efficiently manufacture polyethylene heat-shrinkable films that, even when containing a considerable amount of polyethylene resin, exhibit a low natural shrinkage rate in the main shrinkage direction when stored for a long period under predetermined conditions in a single layer, and are also easily recyclable. [Brief explanation of the drawing]

[0017] [Figure 1] Figures 1(a) to 1(c) are diagrams illustrating the form and usage of polyethylene heat-shrinkable films, respectively. [Figure 2] Figure 2 is a chart showing the molecular weight distribution of polyethylene-based resins (Type B and Type C) that make up the heat-shrinkable film. [Figure 3] Figure 3 shows the DSC charts of polyethylene-based resins (Type B and Type C) that make up the heat-shrinkable film. [Figure 4] Figure 4 shows the DSC charts of the heat-shrinkable films (corresponding to Example 2 and Comparative Example 1). [Figure 5]Figure 5 shows the relationship between the amount (by weight) of polyethylene resin blended as a property (a) in a heat-shrinkable film and the natural shrinkage rate (%). [Figure 6] Figure 6 shows the relationship between the amount of polyethylene resin blended (weight %) as a property (a) in a heat-shrinkable film and the heat shrinkage rate (A1, %) at 80°C for 10 seconds. [Figure 7] Figure 7 shows the relationship between the amount (weight %) of polyethylene resin blended as a property (a) in a heat-shrinkable film and the haze value (%). [Modes for carrying out the invention]

[0018] [First Embodiment] The present invention relates to a polyethylene heat-shrinkable film 10 derived from a polyethylene resin composition containing a polyethylene resin, as illustrated in Figures 1(a) to (c), and is characterized by satisfying the following properties (a) to (d). (a) The polyethylene resin composition contains polyethylene resin with a melting point of 100°C or higher, measured in accordance with JIS K 7121:2012, in an amount ranging from more than 70% to 100% by weight. (b) The natural shrinkage rate in the principal shrinkage direction after storage at 40°C for 7 days shall be 1.8% or less. (c) When thermal shrinkage occurs in 80°C hot water for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is denoted as A1, and A1 shall be a value within the range of 10 to 30%. (d) The specific gravity measured in accordance with JIS K 7112:1999 shall be a value of 0.95 or less.

[0019] 1. Polyethylene resin composition (1) Types, etc. The polyethylene resin composition is characterized by containing a predetermined amount of polyethylene resin having a melting point of 100°C or higher, as measured in accordance with JIS K 7121:2012. The reason for this is that by incorporating a predetermined amount of polyethylene resin having a melting point above a predetermined temperature, it becomes easier to limit the natural shrinkage rate under predetermined conditions to a value within a predetermined range, and it is also possible to reduce specific gravity separation and the amount of aggregates generated during the recycling of PET bottles. In other words, the polyethylene resin composition is characterized by containing polyethylene resin with a melting point of 100°C or higher in an amount ranging from more than 70% to 100% by weight, with the total amount of polyethylene resin contained in the polyethylene resin composition being 100% by weight. More specifically, if the amount of polyethylene resin with a melting point of 100°C or higher is 70% by weight or less, it becomes difficult to adjust the natural shrinkage rate and thermal shrinkage rate under specified conditions, and it also becomes difficult to separate the specific gravity during recycling. On the other hand, if the amount of polyethylene-based resin with a melting point of 100°C or higher is substantial, the haze value may increase.

[0020] Therefore, in order to achieve a better balance between the adjustment of natural shrinkage rate and thermal shrinkage rate and the haze value, it is more preferable to set the amount of polyethylene resin with a melting point of 100°C or higher to a value in the range of 75 to 100% by weight relative to the total amount of polyethylene resin (100% by weight), even more preferable to a value in the range of 78 to 95% by weight, and even more preferable to a value in the range of 80 to 93% by weight. Furthermore, regarding the amount of polyethylene resin with a melting point of 100°C or higher, if that value is less than 100% by weight, it is preferable to blend at least one of the following in a predetermined amount: polyethylene resin with a melting point of less than 100°C, olefin resin other than polyethylene resin, or other resin other than olefin resin, so that the total amount of the polyethylene resin composition becomes 100% by weight.

[0021] Furthermore, while there are no particular restrictions on the type of polyethylene resin, examples typically include random copolymers such as ethylene-α-olefin random copolymer, propylene-α-olefin random copolymer, and butylene-α-olefin random copolymer, as well as block copolymers such as propylene-ethylene block copolymer. As the α-olefin, α-olefins having 2 to 20 carbon atoms are preferred, more specifically ethylene, propylene, butene-1, hexene-1, octene-1, etc., and ethylene-propylene random copolymer, propylene-butene random copolymer, ethylene-propylene-butene random copolymer, etc. are particularly preferred.

[0022] Furthermore, there are no particular restrictions on the catalyst used when polymerizing polyethylene resins; Ziegler-Natta catalysts, metallocene catalysts, and others can be used. However, since polyethylene resins with excellent stereoregularity and mechanical strength are easily obtained, it is more preferable to use polyethylene resins (such as isotactic or syndiotactic) produced using metallocene catalysts.

[0023] (2) Multiple types Furthermore, it is preferable that the polyethylene resin having a melting point of 100°C or higher (characteristic (a)) is included as a mixture of multiple polyethylene resins having different melting points. More specifically, it is preferable to include at least a polyethylene resin with a melting point of 100°C or higher and less than 110°C as the first polyethylene resin, and a polyethylene resin with a melting point of 110°C or higher as the second polyethylene resin. Furthermore, it is preferable that the blending ratio of the first polyethylene resin to the second polyethylene resin be within the range of 30:70 to 70:30. The reason for this is that with such a blending ratio of the first polyethylene resin to the second polyethylene resin, it becomes easier to limit the natural shrinkage rate under predetermined conditions to a value within a predetermined range, and it also reduces specific gravity separation and the amount of aggregates generated during PET bottle recycling. Therefore, it is more preferable that the blending ratio of the first polyethylene resin to the second polyethylene resin be within the range of 35:65 to 65:35, and even more preferable that it be within the range of 40:60 to 60:40.

[0024] Furthermore, if the polyethylene resin has different melting points and such melting point is 100°C or higher, it is more preferable to include two or more types of polyethylene resin with a melting point of 100°C or higher. More specifically, it is even more preferable to include at least two types of polyethylene resins, for example, one with a melting point of 100 to less than 110°C and another with a melting point of 110°C or higher. Furthermore, it is preferable to include at least three types of polyethylene resins, for example, a polyethylene resin with a melting point of 100 to less than 110°C, a polyethylene resin with a melting point of 110 to less than 120°C, and a polyethylene resin with a melting point of 120°C or higher.

[0025] (3) Specific gravity The specific gravity of polyethylene resin is measured in accordance with JIS K 7112:1999 (measurement temperature: 25°C, the same applies hereafter). However, as will be described later, it is preferable to select polyethylene heat-shrinkable film containing such polyethylene resin so that its specific gravity is 0.95 or less. The reason for this is that when the specific gravity exceeds 0.95, the separation process can become excessively time-consuming and laborious. However, if the specific gravity becomes excessively low, the types of polyethylene resins that can be used, as well as the average molecular weight, may be excessively limited. Therefore, it is more preferable to determine the specific gravity of the polyethylene resin so that the specific gravity is within the range of 0.91 to 0.945, and even more preferable to determine the specific gravity of the polyethylene resin so that it is within the range of 0.92 to 0.94.

[0026] (4) Average molecular weight Furthermore, it is preferable that the weight-average molecular weight (Mw) of the polyethylene resin be within the range of 150,000 to 250,000. The reason for this is that having such a weight-average molecular weight (Mw) makes it easier to obtain heat-shrinkable films with low haze and controlled heat shrinkage. Furthermore, it is preferable that the number-average molecular weight (Mn) of such polyethylene resin be within the range of (50,000 to 100,000). The reason for this is that polyethylene resins, by having such a number-average molecular weight (Mn), make it easier to obtain heat-shrinkable films with low haze and controlled heat shrinkage. Figure 2 shows an example of the molecular weight distribution of polyethylene resins (types B and C) used in Example 1, etc., as measured by high-temperature GPC.

[0027] Therefore, since polyethylene resins have such weight-average molecular weight (Mw) and number-average molecular weight (Mn), making it easier to control haze, heat shrinkage, etc., it is preferable that the molecular weight distribution (Mw / Mn) be 4.0 or less. The reason for this is that controlling the molecular weight distribution (Mw / Mn) of polyethylene resins further improves transparency, crystallinity (thermal shrinkage rate), and other properties.

[0028] Furthermore, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) of polyethylene resins, as well as the molecular weight distribution (Mw / Mn) based on these, can be measured using high-temperature GPC. More specifically, for example, using HLC-8321GPC / HT (manufactured by Tosoh Corporation), with dichlorobenzene as the solvent and a column temperature of 145°C, the weight-average molecular weight (Mw) of polyethylene resins can be calculated by comparing the results with a calibration curve using standard styrene particles.

[0029] (5) Heat of fusion It is preferable that the enthalpy of fusion (ΔH) of polyethylene resin, measured in accordance with JIS K 7121:2012, be 135 mJ / mg or less. The reason for this is that, as shown in Figure 3, controlling the enthalpy of crystalline melting (ΔH) measured on the DSC chart makes it easier to obtain a desirable thermal shrinkage rate, and also improves natural shrinkage rate and transparency. More specifically, when the enthalpy of crystalline melting (ΔH) exceeds 135 mJ / mg, the thermal shrinkage rate under shrinkage conditions such as 80°C for 10 seconds decreases significantly. Furthermore, when the enthalpy of fusion (ΔH) exceeds 135 mJ / mg, the natural shrinkage rate and transparency tend to decrease.

[0030] However, if the enthalpy of crystalline melting (ΔH) becomes excessively small, the crystallinity decreases, which can conversely make it difficult to control the thermal shrinkage rate. Therefore, it is preferable to set the enthalpy of fusion (ΔH) to a value in the range of 100 to 130 mJ / mg, and more preferably to a value in the range of 110 to 125 mJ / mg. The enthalpy of crystalline melting (ΔH) can be measured in accordance with JIS K 7121:2012, using a DSC (differential scanning calorimeter) under the conditions shown in Example 1 and other examples described later.

[0031] (6) Melt Flow Rate Furthermore, regarding the melt flow rate (MFR) of polyethylene resin, it is preferable that the MFR (190°C, 21.18N load) measured in accordance with JIS K 7210-1:2014 be within the range of 0.1 to 10 g / min. The reason for this is that if the MFR exceeds a predetermined range, it may become difficult to obtain the predetermined heat shrinkage rate or to adjust the natural shrinkage rate when actually using the film as a heat shrink film. Therefore, it is more preferable to set the MFR to a value within the range of 1 to 5 g / min, and even more preferable to set it to a value within the range of 1.5 to 2.5 g / min.

[0032] 2. Polyethylene heat-shrinkable film (1) Thermal shrinkage rate 1 Regarding characteristic (b), with respect to the thermal shrinkage rate, when thermal shrinkage is performed in 80°C hot water for 10 seconds, and A1 is the thermal shrinkage rate in the main shrinkage direction (TD direction), it is preferable that A1 be a value within the range of 10 to 30%. The reason for this is that if the thermal shrinkage rate A1 falls outside this range, the natural shrinkage rate during storage at room temperature may become excessively large, or the types of polyethylene resins and average molecular weights that can be used may be excessively limited. Therefore, when shrinking under the conditions of 80°C for 10 seconds, it is more preferable that the thermal shrinkage rate (A1) in the main shrinkage direction be in the range of 11 to 28%, and even more preferable that it be in the range of 13 to 25%.

[0033] (2) Natural contraction rate Characteristic (c) is that when the heat-shrinkable film is stored for a long period of time under conditions of 40°C for 7 days, the natural shrinkage rate in the direction perpendicular to the main shrinkage direction (TD direction) is 1.8% or less. The reason for this is that if the natural shrinkage rate exceeds 1.8%, the desired thermal shrinkage rate may not be obtained during actual use (e.g., 80°C, 10 seconds). However, if the natural shrinkage rate becomes excessively small, the types of polyethylene resins that can be used, as well as their average molecular weight, may be excessively limited. Therefore, it is more preferable to set the natural shrinkage rate to a value within the range of 0.1 to 1.5%, and even more preferable to set it to a value within the range of 0.5 to 1.2%. The natural shrinkage rate can be measured as the shrinkage rate when left at 40°C for 7 days, in accordance with the method detailed in Example 1, etc.

[0034] (3) Thermal shrinkage rate 2 Regarding the thermal shrinkage rate, when thermal shrinkage is performed in 90°C hot water for 10 seconds, and the thermal shrinkage rate in the main shrinkage direction (TD direction) is defined as A2, it is preferable that A2 be a value within the range of 20% to less than 40%. The reason for this is that if the heat shrinkage rate A2 obtained under predetermined conditions (such as a temperature of 90°C) falls outside this range, the natural shrinkage rate during storage at room temperature may become excessively large, or the types of polyethylene resins and average molecular weights that can be used may be excessively limited. Therefore, when shrinking under the conditions of 90°C for 10 seconds, it is more preferable that the thermal shrinkage rate (A2) in the main shrinkage direction be in the range of 21-35%, and even more preferable that it be in the range of 22-30%.

[0035] (4) Thermal shrinkage ratio 3 Regarding the thermal shrinkage rate, when thermal shrinkage is performed under the conditions of being immersed in boiling water at 100°C for 10 seconds, and the thermal shrinkage rate in the main shrinkage direction (TD direction) is defined as A3, it is preferable that A3 be a value within the range of 40 to 60%. The reason for this is that if the heat shrinkage rate A3 obtained under predetermined conditions (such as a temperature of 100°C) falls outside this range, the natural shrinkage rate during storage at room temperature may become excessively large, or the types of polyethylene resins and average molecular weights that can be used may be excessively limited. Therefore, when shrinking under the conditions of 100°C for 10 seconds, it is more preferable that the thermal shrinkage rate (A3) in the main shrinkage direction be in the range of 42-55%, and even more preferable that it be in the range of 45-53%.

[0036] (5) Specific gravity Furthermore, the specific gravity of the heat-shrinkable film, measured in accordance with JIS K 7112:1999 (measurement temperature: 25°C, the same applies hereafter), should be the same as that of the polyethylene resin raw material, but preferably a value of 0.95 or less. The reason for this is that by setting the specific gravity of the heat-shrinkable film below a predetermined value, it becomes easier to separate the specific gravity during the recycling of PET bottles, and the overall weight reduction is also facilitated. However, if the specific gravity becomes excessively low, the types of polyethylene-based resin compositions that can be used may be excessively limited, the mechanical strength of the polyethylene-based heat-shrinkable film may decrease, or the natural shrinkage rate may become excessively high. Therefore, it is more preferable to set the specific gravity to a value within the range of 0.9 to 0.920, and even more preferable to set it to a value within the range of 0.91 to 0.915.

[0037] (6) Haze value Furthermore, the haze value of the heat-shrinkable film (equivalent to a normal thickness of 40 μm), measured in accordance with JIS K 7136:2000, is the same as that of the polyethylene resin raw material, but is characterized by being 10% or less. The reason for this is that by specifically limiting the haze value in such a film to a predetermined range, the transparency of the heat-shrinkable film can be controlled quantitatively, and its versatility can be further enhanced due to its good transparency. More specifically, if the haze value of the film before heat shrinkage exceeds 10%, its transparency decreases, which may make it difficult to apply to decorative purposes. On the other hand, if the haze value of the film before heat shrinkage becomes excessively small, it becomes difficult to control stably, which can lead to a significant decrease in production yield. Therefore, it is more preferable to set the haze value of the heat-shrinkable film to a value in the range of 0.1 to 5%, and even more preferable to set it to a value in the range of 0.5 to 3%.

[0038] (7) Heat of fusion Furthermore, regarding the enthalpy of fusion (ΔH) of the heat-shrinkable film derived from the heat of fusion, it is preferable that the value measured by DSC be 135 mJ / mg or less, in accordance with JIS K 7121:2012. The reason for this is that by controlling the enthalpy of crystalline melting in such a heat-shrinkable film, it becomes easier to obtain a desirable thermal shrinkage rate, and transparency is also improved. More specifically, when such crystalline melting enthalpy exceeds 135 mJ / mg, the thermal shrinkage rate under shrinkage conditions of, for example, 100°C for 10 seconds decreases significantly. Furthermore, when the enthalpy of crystalline melting in such heat-shrinkable films exceeds 135 mJ / mg, transparency tends to decrease. However, if the enthalpy of crystalline melting in such a heat-shrinkable film becomes excessively small, the crystallinity decreases, which can conversely make it difficult to control the rate of thermal shrinkage. Therefore, it is preferable to set the crystalline melting enthalpy in such a heat-shrinkable film to a value in the range of 100 to 130 mJ / mg, and more preferably to a value in the range of 110 to 125 mJ / mg. As shown in Figure 4, the enthalpy of crystalline melting in the heat-shrinkable film (Example 2 and Comparative Example 1) can be measured using a DSC (differential scanning calorimetry). As shown in Figure 5, the enthalpy of crystalline melting in a heat-shrinkable film can be measured using a DSC (differential scanning calorimetry).

[0039] (8) Thickness Furthermore, it is preferable that the thickness of the 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 usability improved, but the specific gravity is controlled to a value below the desired value, and excellent thermal shrinkage and transparency can be obtained. More specifically, when the thickness of the heat-shrinkable film is less than 20 μm, it can become difficult to adjust the specific gravity, and handling properties may be significantly reduced. On the other hand, when the thickness of the heat-shrinkable film exceeds 100 μm, transparency may decrease, and it may even become difficult to achieve a uniform thickness. Therefore, it is more preferable to set the thickness of the heat-shrinkable film to a value in the range of 30 to 80 μm, and even more preferable to set it to a value in the range of 40 to 60 μm.

[0040] [Second Embodiment] The second embodiment is a method for producing a polyethylene heat-shrinkable film derived from a polyethylene resin composition containing a polyethylene resin, characterized in that it includes the following steps (1) to (2). Step 1: A step to prepare a polyethylene resin composition that satisfies characteristic (a), which contains more than 70% to 100% by weight of a polyethylene resin having a melting point of 100°C or higher as measured in accordance with JIS K 7121:2012. Step 2: A step to produce a polyethylene-based heat-shrinkable film that satisfies the following characteristics (b) to (d) by stretching a polyethylene-based resin composition along a predetermined direction. (b) When thermal shrinkage occurs in 80°C hot water for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is denoted as A1, and A1 is set to a value within the range of 10 to 30%. (c) The natural shrinkage rate in the principal shrinkage direction after storage at 40°C for 7 days shall be 1.8% or less. (d) The specific gravity measured in accordance with JIS K 7112:1999 shall be a value of 0.95 or less.

[0041] 1. Preparation of raw materials and melting process First, prepare polyethylene resin as the raw material. Therefore, it is preferable to prepare linear low-density polyethylene (LLDPE), which is suitable as an olefin resin, as the raw material. Next, it is preferable to weigh the raw materials and add them to the stirring container, then heat them using the stirring container until they become uniform and melted.

[0042] 2. Process for creating the raw material sheet Next, the uniformly mixed raw materials are dried to an extremely dry state, and then, typically, extrusion molding is performed to create a raw material sheet of a predetermined thickness. More specifically, for example, under conditions of an extrusion temperature of 245°C, extrusion molding can be performed using an extruder (manufactured by Tanabe Plastic Machinery Co., Ltd.) with an L / D ratio of 24 and an extrusion screw diameter of 50 mm, and a raw material sheet of a predetermined thickness (usually 30 to 1000 μm) can be obtained while melt-kneading at 160 to 220°C. In other words, it is generally preferable to perform extrusion molding after preheating the material to a temperature (stretching temperature) of 80 to 100°C and preheating for 10 to 60 seconds.

[0043] 3. Process for creating polyethylene heat-shrinkable film Next, the obtained raw material sheet was moved using a shrink film manufacturing apparatus while being heated and pressed on and between rolls, and stretched to, for example, a stretch ratio of 200-800% in the TD direction and 90-120% in the MD direction, thereby creating a polyethylene heat-shrinkable film.

[0044] 4. Inspection process for polyethylene heat-shrinkable film (optional process) It is preferable to continuously or intermittently measure the following characteristics of the prepared polyethylene-based heat-shrinkable film and to include a predetermined inspection process (optional process). In other words, by measuring the following characteristics through a predetermined inspection process and confirming that they fall within a predetermined range, a polyethylene-based heat-shrinkable film with more uniform specific gravity separation properties and heat shrinkage characteristics can be produced. 1) Visual inspection of the appearance of polyethylene 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 curves

[0045] [Third Embodiment] The third embodiment is an embodiment relating to a method for using polyethylene heat-shrinkable film. Therefore, known methods of using shrink film can be suitably applied to any of them. For example, when implementing a method for using polyethylene heat-shrinkable film, first, the polyethylene heat-shrinkable film is cut to an appropriate length and width, and a long cylindrical object is formed. Next, the long cylindrical object is fed into an automatic labeling device (shrink labeler) and cut to the required length. Next, it is fitted onto the PET bottle or similar container that has been filled with its contents.

[0046] Next, the polyethylene heat-shrinkable film fitted onto PET bottles, etc., is subjected to heat treatment by passing it through a hot air tunnel or steam tunnel at a predetermined temperature. Then, by using radiant heat such as infrared rays provided in these tunnels, and by blowing heated steam at around 90°C from the surroundings, the polyethylene heat-shrinkable film is uniformly heated and thermally shrunk. Therefore, by adhering the label to the outer surface of a PET bottle or similar container, a labeled container can be quickly obtained.

[0047] In other words, as detailed in the first embodiment, the polyethylene-based heat-shrinkable film of the present invention contains a predetermined amount of polyethylene-based resin with a melting point of 100°C or higher, thereby suppressing the decrease in natural shrinkage rate during storage, and consequently, when a PET bottle or the like is attached and heat-shrinked, a good heat shrinkage rate can be obtained. Therefore, as a heat-shrinkable film, it can provide a good appearance after being attached to a bottle, and it can also improve the ability to prevent the label from tearing during transportation and storage. [Examples]

[0048] Next, the present invention will be described in more detail by reference to examples. However, the present invention is not limited by the following examples, and can be implemented with appropriate modifications, all of which are included within the technical scope of the present invention.

[0049] The polyethylene resins used in Example 1 and other examples are as follows. In addition, in the present invention, if multiple melting peaks exist, the melting peak temperature on the lowest side is considered to be the melting point. 1) Type A Ethylene-1-hexene copolymer produced using a metallocene catalyst Density: 0.910g / cm 3 MFR: 1.7g / 10min, Melt Tension: 104mN Melting point: 98°C, Enthalpy of fusion (ΔH): 115 mJ / mg Mn: 8.2 × 10 4 Mw: 21.6 × 10 4 Mw / Mn: 2.6

[0050] 2) Type B Ethylene-1-hexene copolymer produced using a metallocene catalyst Density: 0.908g / cm 3 MFR: 2.0g / 10 minutes Melting point: 100°C, Enthalpy of fusion (ΔH): 110 mJ / mg Mn: 7.0 × 10 4 Mw: 23.8 × 10 4 Mw / Mn: 3.4

[0051] 3) Type C Ethylene-1-hexene copolymer produced using a metallocene catalyst Density: 0.919g / cm 3 MFR: 2.0g / 10 minutes Melting point: 114℃

[0052] 4) Type D Ethylene-1-butene copolymer produced using Ziegler-Natta catalysts Density: 0.893g / cm 3 MFR: 3.6g / 10 minutes Melting point: 66℃

[0053] [Example 1] 1. Preparation of polyethylene heat-shrinkable film In a stirring vessel, 30 parts by weight (same as parts by mass; the same applies hereinafter) of ethylene-1-hexene copolymer produced with a type B metallocene catalyst and 70 parts by weight of ethylene-1-hexene copolymer produced with a type C metallocene catalyst were placed, and the mixture was stirred while heating at 190°C to obtain a homogeneous solution. In other words, the total content of the two types of polyethylene resins with a melting point of 100°C or higher was set to 100% by weight relative to the total amount of polyethylene resin.

[0054] Next, the material was supplied to an extruder (manufactured by Tanabe Plastic Machinery Co., Ltd.) with an L / D ratio of 24 and an extrusion screw diameter of 50 mm, under conditions of an extrusion temperature of 245°C, and extrusion molding was performed to obtain a raw material sheet with a thickness of 160 μm. Specifically, the preheating temperature (stretching temperature) for extrusion molding was set to 90°C and the preheating time to 30 seconds before extrusion molding was performed.

[0055] Next, the obtained raw material sheet was subjected to a shrink film manufacturing apparatus, moving it on and between rolls while heating and pressing, and stretched to a stretch ratio of 500% in the TD direction and 105% in the MD direction, thereby creating a heat-shrinkable film with a thickness of 50 μm.

[0056] 2. Evaluation of polyethylene heat-shrinkable films (1) Evaluation 1: Natural contraction rate A sample of heat-shrinkable film with a length of 500 mm in the main shrinkage direction was left at 40°C for 7 days. After that, the length of the sample (mm) was measured, and the natural shrinkage rate was calculated using the following formula (1).

[0057]

number

[0058] Figure 5 shows the relationship between the amount of polyethylene (weight %) as characteristic (a) in the heat-shrinkable film of Example 1, etc., and the above-mentioned natural shrinkage rate.

[0059] (2) Evaluation 2: Thermal shrinkage coefficient in the TD direction A1 (80°C, 10 seconds) The stretched film was cut into 10cm x 10cm squares with one side parallel to the film's direction of flow, and these were immersed in a water bath heated to 80°C ± 0.5°C for 10 seconds.

[0060] After immersing the film in the hot water in the tank, and after 10 seconds, it was immediately immersed for 10 seconds in a separate tank containing water maintained at 25°C. Next, the lengths of the film in the main shrinkage direction (TD direction) and the direction perpendicular to it (MD direction) were measured, and the thermal shrinkage rate (A1) in the TD direction was calculated using the following formula (2). The obtained results are shown in Table 1 as Evaluation 2.

[0061]

number

[0062] Figure 6 shows the relationship between the amount of polyethylene (weight %) as characteristic (a) in the heat-shrinkable film of Example 1, etc., and the heat shrinkage rate (A1, %) at 80°C for 10 seconds.

[0063] (3) Evaluation 3: Thermal shrinkage coefficient in the TD direction A2 (90°C, 10 seconds) In Evaluation 3, the thermal shrinkage rate A2 in the TD direction was measured in the same manner as in Evaluation 2, except that the temperature of the hot water used for thermal shrinkage was changed to 90°C ± 0.5°C. The results obtained are shown in Table 1.

[0064] (4) Evaluation 4: Thermal shrinkage coefficient in the TD direction A3 (100℃, 10 seconds) In Evaluation 4, the thermal shrinkage rate A3 in the TD direction was measured in the same manner as in Evaluation 2, except that the temperature of the hot water used for thermal shrinkage (boiling water temperature) was changed to 100℃ ± 0.5℃. The obtained results are shown in Table 1.

[0065] (5) Rating 5: Specific gravity In accordance with JIS K 7112:1999, the specific gravity of the obtained heat-shrinkable film was determined from the ratio of the density measured by the density gradient tube method to the density of water at 23°C. The results are shown in Table 1.

[0066] (6) Rating 6: Haze value The haze value of the obtained heat-shrinkable film was measured in accordance with JIS K 7136:2000. The results are shown in Table 1. Figure 7 shows the relationship between the amount of polyethylene (weight %) as characteristic (a) and the haze value in the heat-shrinkable film of Example 1, etc.

[0067] (7) Rating 7: Melting point In accordance with JIS K 7121:2012, the melting point (°C) of the obtained heat-shrinkable film was measured using DSC. As shown by line A in Figure 4, at least two melting points were observed on the DSC chart, and their respective melting points (high melting point and low melting point) were measured.

[0068] (8) Rating 8: Enthalpy of fusion (ΔH) In accordance with JIS K 7121:2012, the enthalpy of crystalline melting (ΔH) of the obtained heat-shrinkable film was measured using DSC.

[0069] [Example 2] In Example 2, a heat-shrinkable film was prepared and evaluated in the same manner as in Example 1, except that 70 parts by weight of ethylene-1-hexene copolymer produced with a type B metallocene catalyst and 30 parts by weight of ethylene-1-hexene copolymer produced with a type C metallocene catalyst were used as the polyethylene resin. Specifically, the total content of the two types of polyethylene resins with a melting point of 100°C or higher was evaluated as 100% by weight relative to the total amount of the polyethylene resin composition.

[0070] [Example 3] In Example 3, a heat-shrinkable film was prepared and evaluated in the same manner as in Example 1, except that 20 parts by weight of ethylene-1-hexene copolymer produced with a type A metallocene catalyst and 80 parts by weight of ethylene-1-hexene copolymer produced with a type B metallocene catalyst were used as the polyethylene resin. Specifically, for characteristic (a), the evaluation was performed with a polyethylene resin content of 80% by weight (melting point of 100°C or higher) and a polyethylene resin content of 20% by weight (melting point of less than 100°C).

[0071] [Example 4] In Example 4, a heat-shrinkable film was prepared and evaluated in the same manner as in Example 1, except that 25 parts by weight of ethylene-1-hexene copolymer produced with a type A metallocene catalyst and 75 parts by weight of ethylene-1-hexene copolymer produced with a type B metallocene catalyst were used as the polyethylene resin. Specifically, for characteristic (a), the evaluation was performed with a polyethylene resin content of 75% by weight (melting point of 100°C or higher) and a polyethylene resin content of 25% by weight (melting point of less than 100°C).

[0072] [Comparative Example 1] In Comparative Example 1, a heat-shrinkable film was prepared and evaluated in the same manner as in Example 1, except that 30 parts by weight of ethylene-1-hexene copolymer produced with a type A metallocene catalyst and 70 parts by weight of ethylene-1-hexene copolymer produced with a type C metallocene catalyst were used as the polyethylene resin. Specifically, for characteristic (a), the evaluation was performed with a polyethylene resin content of 70% by weight (melting point of 100°C or higher) and a polyethylene resin content of 30% by weight (melting point of less than 100°C).

[0073] [Comparative Example 2] In Comparative Example 2, a heat-shrinkable film was prepared and evaluated in the same manner as in Example 1, except that 70 parts by weight of ethylene-1-hexene copolymer produced with a type C metallocene catalyst and 30 parts by weight of ethylene-1-hexene copolymer produced with a type D Ziegler-Natta catalyst were used as the polyethylene resin. Specifically, for characteristic (a), the evaluation was performed with a polyethylene resin content of 70% by weight (melting point of 100°C or higher) and a polyethylene resin content of 30% by weight (melting point of less than 100°C). However, the heat-shrinkable film melted, making it impossible to form a film and stretch it, so further evaluation was discontinued.

[0074] [Table 1] Rating 1: Natural contraction rate Evaluation 2: Thermal shrinkage coefficient in the TD direction A1 (80°C, 10 seconds) Evaluation 3: Thermal shrinkage coefficient in the TD direction A2 (90°C, 10 seconds) Evaluation 4: Thermal shrinkage coefficient in the TD direction A3 (100°C, 10 seconds) Rating 5: Specific Gravity Rating 6: Haze Value Evaluation 7: Melting point (°C) Evaluation 8: Enthalpy of fusion (ΔH) [Industrial applicability]

[0075] The present invention provides a polyethylene heat-shrinkable film containing a predetermined amount of polyethylene resin, wherein the polyethylene resin has a predetermined composition (characteristic a) of polyethylene resin with a melting point of 100°C or higher, a heat shrinkage rate in the TD direction when heat-shrinked at 80°C for 10 seconds (characteristic b), a natural shrinkage rate under predetermined conditions (characteristic c), and a specific gravity (characteristic d) that satisfies predetermined conditions, thereby solving the problems of the conventional invention.

[0076] In other words, with such polyethylene-based heat-shrinkable films, even if they are basically single-layer films and do not contain a predetermined amount of cyclic polyolefin (COC), the natural shrinkage rate in the main shrinkage direction can be kept low for a long period of time. Moreover, with this type of heat-shrinkable film, the separation of PET bottles (crushed material) by specific gravity during recycling can now be done accurately, quickly, and easily.

[0077] Furthermore, according to the method for manufacturing polyethylene-based heat-shrinkable films of the present invention, it is now possible to stably manufacture polyethylene-based heat-shrinkable films that satisfy the following characteristics: a polyethylene-based resin having a melting point of 100°C or higher (characteristic a), a heat shrinkage rate in the TD direction when heat-shrinked at 80°C for 10 seconds (characteristic b), a natural shrinkage rate under predetermined conditions (characteristic c), and specific gravity (characteristic d). Therefore, the polyethylene-based heat-shrinkable film of the present invention can be suitably applied to various PET bottles, outer covering materials for lunch boxes, etc., significantly expanding its versatility, and is also easily recyclable, making it extremely promising for industrial use. [Explanation of symbols]

[0078] 10, 10': Polyethylene heat-shrinkable film 10a: Other resin layers 20: PET bottles

Claims

1. A polyethylene heat-shrinkable film derived from a polyethylene resin composition containing a polyethylene resin, characterized in that it satisfies the following properties (a) to (d). (a) The polyethylene resin contains, in an amount of more than 70% to 100% by weight, a polyethylene resin having a melting point of 100°C or higher as measured in accordance with JIS K 7121:2012, relative to 100% by weight of the polyethylene resin. (b) The natural shrinkage rate in the principal shrinkage direction after storage at 40°C for 7 days shall be 1.8% or less. (c) When the material is heat-shrunk in 80°C hot water for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is A1, and A1 is a value within the range of 10 to 30%. (d) The specific gravity measured in accordance with JIS K 7112:1999 shall be 0.95 or less.

2. The polyethylene heat-shrinkable film according to claim 1, characterized in that the polyethylene resin having a melting point of 100°C or higher according to characteristic (a) comprises a plurality of polyethylene resins having different melting points, wherein the polyethylene resin having a melting point of 100°C or higher and less than 110°C is designated as the first polyethylene resin, and the polyethylene resin having a melting point of 110°C or higher is designated as the second polyethylene resin, and the blending ratio (weight ratio) of the first polyethylene resin to the second polyethylene resin is within the range of 30:70 to 70:

30.

3. A polyethylene-based heat-shrinkable film according to claim 1 or 2, characterized in that the heat of fusion measured by DSC is 135 mJ / mg or less, in accordance with JIS K 7121:2012.

4. The polyethylene heat-shrinkable film according to claim 1 or 2, characterized in that when heat-shrinkable in 90°C hot water for 10 seconds, the heat shrinkage rate in the main shrinkage direction is A2, and A2 is a value within the range of 20 to 40%.

5. The polyethylene heat-shrinkable film according to claim 1 or 2, characterized in that when heat-shrinkable under the conditions of being immersed in boiling water at 100°C for 10 seconds, the heat shrinkage rate in the main shrinkage direction is A3, and A3 is a value within the range of 40 to 60%.

6. The polyethylene heat-shrinkable film according to claim 1 or 2, characterized in that the haze value measured in accordance with JIS K 7136:2000 is 10% or less.

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

8. A method for producing a polyethylene heat-shrinkable film derived from a polyethylene resin composition containing a polyethylene resin, characterized by comprising the following steps (1) and (2). Step 1: (a) A step of preparing the polyethylene resin composition, wherein the polyethylene resin is 100% by weight, and the polyethylene resin is contained in an amount of more than 70% by weight to 100% by weight of a polyethylene resin having a melting point of 100°C or higher as measured in accordance with JIS K 7121:2012. Step 2: A step to produce a polyethylene-based heat-shrinkable film that satisfies the following characteristics (b) to (d) by stretching a polyethylene-based resin composition along a predetermined direction. (b) When the material is heat-shrunk in 80°C hot water for 10 seconds, the thermal shrinkage rate in the main shrinkage direction is A1, and A1 is a value within the range of 10 to 30%. (c) The natural shrinkage rate in the principal shrinkage direction after storage at 40°C for 7 days shall be 1.8% or less. (d) The specific gravity measured in accordance with JIS K 7112:1999 shall be 0.95 or less.

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