Unoriented polyolefin film

Abstract

The present invention provides an unstretched polyolefin film that exhibits good ease of opening and is less prone to stringing when automatically packaged as a single film. [Solution] An unstretched film for automatic packaging, which is a single-layer film having at least a base layer and a seal layer, wherein the base layer contains 90% by weight or more of homopolypropylene, the seal layer contains 60-80% by weight of propylene random copolymer and 20-40% by weight of high-pressure low-density polyethylene, the homopolypropylene in the base layer has an MFR of 8.5 g / 10 min or less, the propylene random copolymer in the seal layer has a component in which 50% or more is propylene random copolymer with a melting point of 135°C or less, and the high-pressure low-density polyethylene in the seal layer has an MFR of 10.0 g / 10 min or less.

Description

Technical Field

[0001] The present invention relates to an unstretched polyolefin film, and more particularly to an unstretched polyolefin film for automatic packaging which is a single film having at least a base material layer and a seal layer.

Background Art

[0002] For example, when packaging foods such as breads and noodles by automatic packaging such as pillow packaging, an unstretched polyolefin film of a single film without lamination such as a base material film is suitably used as a packaging film. In this type of unstretched polyolefin film for automatic packaging, at the time of automatic packaging such as pillow packaging, bag making by heat sealing and packaging of contents are performed almost simultaneously. In the obtained package, when the heat seal strength is too strong, it becomes difficult to open the package, and excessive force is applied at the time of opening, and the package may be torn more than necessary or the contents may jump out with too much momentum.

[0003] Therefore, an unstretched polyolefin film for packaging is required to have easy-opening properties when formed into a bag as a package. As a method of imparting easy-opening properties to an unstretched polyolefin film, for example, a polyolefin film provided with a heat-fusible layer formed from a specific propylene polymer and an ethylene-based polymer has been proposed (see Patent Document 1). This polyolefin film is formed of 20 to 80% by weight of a propylene polymer having a melting point of 158 ° C or higher and 80 to 20% by weight of an ethylene-based polymer having a density of 0.860 to 0.930 g / cm 3 and by using an ethylene homopolymer or a random copolymer of ethylene and an α-olefin having 4 or more carbon atoms obtained using a single-site catalyst or the like as the ethylene-based polymer, excellent easy-opening properties can be obtained even at the time of heat sealing at a high temperature.

[0004] However, in the above polyolefin film, if an ethylene homopolymer is used as the ethylene-based polymer, poor compatibility with propylene polymers with a melting point of 158°C or higher may result in molding defects. Furthermore, if an ethylene-α-olefin random copolymer obtained using a single-site catalyst is used, increasing the heat seal temperature may result in excessively strong heat seals, making it difficult to achieve easy opening.

[0005] Furthermore, as another method for imparting easy-open properties to unoriented polyolefin films, an easy-open film has been proposed in which the seal layer is composed of a polypropylene resin manufactured using a single-site catalyst and two types of low-density polyethylene resins with different melt viscosities, thereby causing cohesive failure due to a sea-island dispersion structure resulting from the incompatibility of the polypropylene resin and polyethylene resin in the seal layer (see Patent Document 2). In this easy-open film, by using a low-density polyethylene resin with a high melt viscosity and a low melt viscosity as the two types of low-density polyethylene resins, transparency and good cohesive failure and easy-open properties can be obtained when heat-sealed at a relatively low sealing temperature.

[0006] However, the above-mentioned easy-open film is intended to be used as a lid material after lamination with a substrate. If the easy-open film is used as a single film, depending on the type of resin used in the adjacent layer adjacent to the seal layer, delamination may occur without cohesive failure, resulting in film residue remaining on the delaminated surface. Alternatively, even if cohesive failure occurs, stringing may occur, where the resin remains in a thread-like manner near the delaminated surface during or after delamination, potentially preventing proper opening. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2003-127298 [Patent Document 2] Japanese Patent Publication No. 2021-095162 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The present invention has been proposed in view of the above points, and provides an unstretched polyolefin film that, when automatically packaged as a single film, is less prone to stringing when opened and offers good easy-open properties. [Means for solving the problem]

[0009] In other words, the first invention relates to an unstretched polyolefin film for automatic packaging, which is a single-layer film having at least a base layer and a seal layer, wherein the base layer contains 90% by weight or more of homopolypropylene, the seal layer contains 60-80% by weight of propylene random copolymer and 20-40% by weight of high-pressure low-density polyethylene, the homopolypropylene in the base layer has an MFR of 8.5 g / 10 min or less, the propylene random copolymer in the seal layer has a component of 50% or more of which is propylene random copolymer with a melting point of 135°C or less, and the high-pressure low-density polyethylene in the seal layer has an MFR of 10.0 g / 10 min or less.

[0010] The second invention relates to an unstretched polyolefin film in which, in the first invention, a surface layer is laminated on the side of the base layer opposite to the seal layer, and the surface layer contains 70% by weight or more of homopolypropylene. [Effects of the Invention]

[0011] According to the unstretched polyolefin film of the first invention, the unstretched film for automatic packaging is a single-film film having at least a base layer and a seal layer, wherein the base layer contains 90% by weight or more of homopolypropylene, the seal layer contains 60-80% by weight of propylene random copolymer and 20-40% by weight of high-pressure low-density polyethylene, the homopolypropylene in the base layer has an MFR of 8.5 g / 10 min or less, the propylene random copolymer in the seal layer has a component of propylene random copolymer with a melting point of 135°C or less, and the high-pressure low-density polyethylene in the seal layer has an MFR of 10.0 g / 10 min or less. As a result, when the single-film film is used for automatic packaging, stringing and other issues are less likely to occur when opened, and good easy-open properties can be obtained.

[0012] According to the unoriented polyolefin film of the second invention, in the first invention, a surface layer is laminated on the side of the base layer opposite to the seal layer, and the surface layer contains 70% by weight or more of homopolypropylene, so that the unoriented polyolefin film can be properly protected. [Modes for carrying out the invention]

[0013] An unoriented polyolefin film according to one embodiment of the present invention is a single-layer film having at least a base layer and a sealing layer. A single-layer film is a film that is used alone as a packaging film or the like without laminating it with other films such as a base film. As the unoriented polyolefin film, a polypropylene film mainly composed of polypropylene resin is preferably used from the viewpoint of the rigidity and strength of the film.

[0014] This unoriented polyolefin film is used as a packaging film for packaging food products such as bread and noodles, and is particularly suitable as a film for automated packaging such as pillow packaging. With the unoriented polyolefin film, a package containing the contents can be formed by heat sealing together with the packaging during automated packaging. This package can be opened by peeling off the heat-sealed portion.

[0015] In the unoriented polyolefin film of the present invention, the resin material used in each layer described later is appropriately selected from resins produced from appropriate starting materials such as petroleum-derived, biomass-derived, material-recycled, and chemical-recycled materials.

[0016] The unoriented polyolefin film of the present invention is obtained by melting the resins constituting each layer and forming them to a predetermined thickness using known film forming methods such as the T-die method or the inflation method. In particular, forming by the T-die method is preferred. Film forming by the T-die method is advantageous in that it can obtain the high thickness-to-thinness accuracy required for a film. In this invention, cases in which the film is subjected to unavoidable stretching during film formation are also included in the definition of unoriented.

[0017] The base layer is the main layer of the unoriented polyolefin film, formed with a thicker layer thickness compared to other layers, and contains 90% by weight or more of homopolypropylene. Homopolypropylene is a resin with excellent heat resistance, chemical resistance, strength, and rigidity, and is suitable as the constituent resin of the base layer, which defines the basic film properties of the unoriented polyolefin film. As homopolypropylene, materials generally recognized as homopolypropylene can be used. Specifically, in addition to propylene homopolymers, propylene-ethylene random copolymers with an ethylene content of 2% by weight or less can be used. If the proportion of homopolypropylene in the base layer is too low, the film performance, such as heat resistance and rigidity, may be insufficient.

[0018] For this homopolypropylene, the melt flow rate (MFR), measured under conditions of 230°C and a load of 2.16 kg in accordance with JIS K 7210, is 8.5 g / 10 min or less, preferably 8.0 g / 10 min or less. If the MFR of the homopolypropylene is too high, the degree of stringing upon opening may worsen when automatically packaged as a single film. While there is no particular lower limit to the MFR of the homopolypropylene, from the viewpoint of moldability and thickness accuracy, it is preferable that it be 2.3 or higher when the molding method is the T-die method. If the MFR is too low, moldability may worsen and thickness accuracy may deteriorate.

[0019] Stringing is a phenomenon in which, when a heat-sealed portion formed during bag manufacturing is peeled off, the resin remains in a thread-like form on the peeled surface or seal edge (the boundary between the heat-sealed and unheat-sealed portions). Stringing at the seal edge is thought to be a defect that occurs when cohesive failure occurs in the seal layer during opening, causing a portion of the peeled seal layer to extend into the unheat-sealed portion. In other words, it is thought that the seal layer resin is not properly cut at the seal edge, leaving it as a thread-like resin. Furthermore, stringing that occurs at the heat-seal peeled surface is more likely to occur when peeling involves not only cohesive failure of the seal layer but also interlayer delamination between the base layer and the seal layer. Note that when the resin remains in a film-like form over a wider area than the thread-like form of stringing, it is called film residue. Film residue is often a problem when peeling is not cohesive failure of the seal layer but interlayer delamination between the base layer and the seal layer, and is thought to be a defect that occurs when the seal layer resin is not properly cut at the seal edge, similar to stringing at the seal edge.

[0020] Although the detailed mechanism regarding the relationship between the MFR of the homopolypropylene in the base material layer and the degree of thread drawing is unknown, if the MFR of the homopolypropylene in the base material layer is too large, the interfacial strength between the base material layer and the seal layer may become weak, and it is conceivable that the cutting of the seal layer resin may not be smoothly performed after the peeling of the heat-sealed portion. Further, if the MFR of the base material layer is too large, the cohesive force of the base material layer decreases, and it is also possible that the cutting of the resin of the seal layer is less likely to occur because the cohesive force of the seal layer becomes relatively large. By providing the homopolypropylene in the base material layer with an appropriate MFR, thread drawing can be suppressed.

[0021] The seal layer is the layer on the inner side of the bag formed by bag-making and has properties such as low-temperature sealability and easy-opening property. The seal layer is composed of a resin material containing a propylene random copolymer and a high-pressure low-density polyethylene.

[0022] Examples of the propylene random copolymer used for the seal layer include propylene-α-olefin random copolymers such as propylene-ethylene random copolymers and propylene-ethylene-butene random copolymers. In the propylene random copolymer, the melt flow rate (MFR) measured under the conditions of 230 °C and a load of 2.16 kg in accordance with JIS K 7210 is preferably 2.3 to 15.0 g / 10 min, and more preferably 4.0 to 10.0 g / 10 min. If the value of the MFR of the propylene random copolymer is too small or too large, the moldability may deteriorate and the thickness accuracy may worsen.

[0023] The high-pressure low-density polyethylene used for the seal layer is polyethylene produced by the high-pressure method. In the case of high-pressure low-density polyethylene, the melt flow rate (MFR) measured under the conditions of 190 °C and a load of 2.16 kg in accordance with JIS K 7210 is preferably 0.7 to 13.0 g / 10 min, and more preferably 1.0 to 10.0 g / 10 min. If the MFR of the high-pressure low-density polyethylene is too small, the moldability deteriorates, and there is a risk of occurrence of thickness non-uniformity and appearance defects. If the MFR of the high-pressure low-density polyethylene is too large, when automatically packaged with a single-layer film, the degree of thread drawing during opening may deteriorate.

[0024] Although the detailed mechanism regarding the relationship between the MFR of the high-pressure low-density polyethylene in the seal layer and the degree of thread drawing is unclear, if the MFR of the high-pressure low-density polyethylene in the seal layer is too large, the dispersion shape of the high-pressure low-density polyethylene contained in the seal layer may become too thin and elongated in the machine (MD) direction. As a result, the interlayer strength between the base material layer and the seal layer may become weak, and it is conceivable that the cutting of the seal layer resin may not be smoothly performed after peeling of the heat-sealed portion.

[0025] In the case of high-pressure low-density polyethylene, the density is preferably 0.915 to 0.935 g / cm 3 If it is within this density range, good peelability can be obtained.

[0026] In the case of high-pressure low-density polyethylene, the melting point determined from the DSC curve obtained in accordance with the differential scanning calorimetry (DSC) of JIS K 7121 (2012) is preferably 105 to 120 °C. If it is within this melting point range, good peelability can be obtained.

[0027] In this sealing layer, the constituent materials, propylene random copolymer and high-pressure low-density polyethylene, are immiscible, resulting in the formation of a sea-island structure within the resin layer. This causes cohesive failure during peeling of the heat-sealed portion, leading to easy opening. However, even if the constituent materials of the sealing layer are immiscible resins, good film performance such as easy opening may not be obtained depending on the combination.

[0028] For example, when homopolypropylene is combined with high-pressure low-density polyethylene, the heat seal strength may become too weak. This can lead to insufficient strength in the sealed portion of the packaging, potentially causing the packaging to rupture due to contact with the contents during transport. Furthermore, poor compatibility between homopolypropylene and high-pressure low-density polyethylene can result in molding defects. Similarly, when high-melting-point propylene random copolymer is combined with high-pressure low-density polyethylene, the heat seal strength may also become too weak. This can lead to insufficient strength in the sealed portion of the packaging, potentially causing the packaging to rupture due to contact with the contents during transport. Additionally, the moldability may be poor, potentially reducing thickness accuracy. In the case of unoriented polyolefin films, poor thickness accuracy during automatic single-film packaging can easily lead to unexpected tearing of the film from the thin portion upon opening; therefore, thickness accuracy is a crucial requirement for film performance. Furthermore, in the case of a combination of propylene random copolymer and linear low-density polyethylene, the compatibility is relatively good, which tends to result in high seal strength, and there is a risk that easy opening may not be achieved.

[0029] The combination of propylene random copolymer and high-pressure low-density polyethylene is preferable because it is moderately immiscible, making it easier to adjust the strength of cohesive failure. Therefore, the preferred blending ratio of propylene random copolymer to high-pressure low-density polyethylene is 60-80% by weight of propylene random copolymer and 20-40% by weight of high-pressure low-density polyethylene. If there is too little propylene random copolymer, i.e., too much high-pressure low-density polyethylene, delamination is likely to occur, potentially leading to film residue and stringing. If there is too much propylene random copolymer, i.e., too little high-pressure low-density polyethylene, a proper sea-island structure may not be formed, and good easy-open properties may not be obtained.

[0030] Furthermore, in the propylene random copolymer incorporated into the seal layer, at least 50% of the components must be propylene random copolymer with a melting point of 135°C or lower. If the proportion of propylene random copolymer with a melting point of 135°C or lower is too small, the compatibility with high-pressure low-density polyethylene will be poor, potentially resulting in insufficient moldability and thickness-to-thinness accuracy. By ensuring that at least 50% of the components of the propylene random copolymer have a melting point of 135°C or lower, the compatibility between the propylene random copolymer and high-pressure low-density polyethylene is improved, resulting in improved moldability and better thickness-to-thinness accuracy.

[0031] The proportion of propylene random copolymer with a melting point of 135°C or lower in the seal layer also affects the heat seal temperature range in which easy opening is achieved. If the proportion of propylene random copolymer with a melting point of 135°C or lower is too small, the difference in melting point between it and the substrate layer or surface layer becomes small, which may narrow the heat seal temperature range in which easy opening is achieved. By making 50% or more of the propylene random copolymer component have a melting point of 135°C or lower, the heat seal temperature range in which easy opening is achieved becomes wider.

[0032] If the heat sealing temperature range is too narrow, errors in the set temperature may result in an inability to obtain a good heat seal strength, potentially leading to an unstable heat sealing process. A wider heat sealing temperature range makes it easier to consistently obtain a good heat seal strength even if there are errors in the set temperature. The preferred heat sealing temperature range is 120 to 155°C, and it is desirable to obtain a good heat seal strength over as wide a range as possible within this range. If a good heat seal strength is obtained within this range, it becomes easier to consistently obtain a good heat seal strength even if there is an error between the set temperature of the packaging machine and the actual temperature when using automatic packaging with a single film.

[0033] In the unoriented polyolefin film of the present invention, a surface layer may be laminated on the side of the base layer opposite to the seal layer, if necessary. The surface layer is the outer layer of the package after bag formation and protects the unoriented polyolefin film. Surface processing such as printing can be applied to this surface layer as needed. In this case, surface treatment such as corona treatment may be applied to the surface to improve the printability of the film surface.

[0034] The surface layer is composed of a resin material containing 70% by weight or more of homopolypropylene. Homopolypropylene is a resin with excellent heat resistance, chemical resistance, strength, and rigidity, and is suitable as a constituent resin for the surface layer from the viewpoint of protecting the unoriented polyolefin film. If the proportion of homopolypropylene in the surface layer is too low, the film performance, such as heat resistance and rigidity, may be insufficient.

[0035] The homopolypropylene contained in the surface layer can be any material generally recognized as homopolypropylene. Specifically, in addition to propylene homopolymers, propylene-ethylene random copolymers with an ethylene content of 2% by weight or less can be used. Furthermore, the homopolypropylene contained in the surface layer may also be the homopolypropylene contained in propylene block copolymers. Generally, propylene block copolymers produce homopolypropylene in the first polymerization stage and copolymers of propylene with ethylene or α-olefins in the subsequent second polymerization stage. Therefore, the proportion of homopolypropylene contained in the propylene block copolymer can be used to determine which propylene block copolymer can be used.

[0036] In the unoriented polyolefin film of the present invention, various additives such as antiblocking agents, slip agents, antistatic agents, antifogging agents, heat stabilizers, antioxidants, light stabilizers, and crystal nucleating agents, as well as scraps, can be added to each layer as needed, within a range that does not impair the properties of each layer. The various additives may be added directly to the powder after polymerization of each resin, or they may be mixed in any step from preparing a high-concentration masterbatch to obtaining the film. When using a masterbatch, a small amount of resin may be unintentionally incorporated, but it can be used within a range that does not impair the properties of each layer.

[0037] In the unstretched polyolefin film of the present invention, as described above, it is preferable that the heat seal strength be approximately 1.5 to 4.0 N / 15 mm, from the viewpoint of achieving both easy opening and tear resistance after bag making. If the heat seal strength is too weak, the sealed portion of the packaging will lack sufficient strength, and the packaging may tear due to contact with the contents during transportation, etc. If the heat seal strength is too strong, the sealed portion of the packaging will be excessively strong, making it difficult to obtain good easy opening properties, and requiring excessive force to open may cause the packaging to tear more than necessary.

[0038] As described above, the unoriented polyolefin film of the present invention has a base layer containing 90% by weight or more of homopolypropylene, and a seal layer containing 60-80% by weight of propylene random copolymer and 20-40% by weight of high-pressure low-density polyethylene. The MFR of the homopolypropylene in the base layer is 8.5 g / 10 min or less, and 50% or more of the propylene random copolymer component in the seal layer is propylene random copolymer with a melting point of 135°C or less. The MFR of the high-pressure low-density polyethylene in the seal layer is 10.0 g / 10 min or less. Therefore, when automatically packaged as a single film, the seal layer appropriately causes cohesive failure when the heat-sealed portion is opened, suppressing the occurrence of unexpected film tearing and stringing. Accordingly, the unoriented polyolefin film of the present invention provides good easy-openability, as stringing and other issues are less likely to occur when automatically packaged as a single film. [Examples]

[0039] [Preparation of unoriented polyolefin film] In the preparation of the unoriented polypropylene films for prototypes 1 to 16, the materials described later were supplied to an extruder, melted and kneaded, and extruded into three layers—a surface layer, a base layer, and a sealing layer—by the T-die method, resulting in a film with a total thickness of 28 μm. At that time, the layer thickness (layer ratio) of each layer to the total thickness was adjusted to 5:19:4. In prototypes 1 to 16, the resin blending ratio for each layer was 100% by weight, and additives such as antiblocking agents were omitted. The resin composition of each layer in prototypes 1 to 16 is shown in Tables 1 to 3 described later.

[0040] [Materials used] The following resins were used as the resin material for each layer. For the properties of each resin, the melt flow rate (MFR) was measured in accordance with JIS K 7210 (2014), with propylene-based resins measured at 230°C and 2.16 kg, and ethylene-based resins measured at 190°C and 2.16 kg. The melting point was determined in accordance with differential scanning calorimetry (DSC) measurement in JIS K 7121 (2012), using a differential scanning calorimeter (NETC Japan Co., Ltd.; "DSC 214 Polyma"), and the melting peak temperature was obtained from the DSC curve obtained when the temperature was increased at a heating rate of 10°C / min. If multiple peaks were observed, the highest melting peak temperature was used as the melting point.

[0041] <Propylene resin> PP1: Homopolypropylene, MFR (230℃ / 2.16kg) 7.5g / 10min, melting point 163℃ PP2: Homopolypropylene, MFR (230℃ / 2.16kg) 4.2g / 10min, melting point 168℃ PP3: Homopolypropylene, MFR (230℃ / 2.16kg) 8.0g / 10min, melting point 165℃ PP4: Propylene random copolymer, MFR (230℃ / 2.16kg) 7.0g / 10min, melting point 125℃ PP5: Homopolypropylene, MFR (230℃ / 2.16kg) 13.0g / 10min, melting point 165℃ PP6: Propylene block copolymer (containing 87% by weight of homopolypropylene component), MFR (230℃ / 2.16kg) 8.5g / 10min, melting point 165℃ PP7: Propylene random copolymer, MFR (230℃ / 2.16kg) 7.0g / 10min, melting point 147℃

[0042] <Ethylene-based resin> PE1: High-pressure low-density polyethylene, MFR (190℃ / 2.16kg) 2.5g / 10min, melting point 111℃, density 0.922g / cm³ 3 PE2: Linear low-density polyethylene, MFR (190℃ / 2.16kg) 3.8g / 10min, melting point 117℃, density 0.918g / cm³ 3 metallocene catalytic polymerization • PE3: High-pressure low-density polyethylene, MFR (190℃ / 2.16kg) 7.5g / 10min, melting point 108℃, density 0.918g / cm³ 3 • PE4: High-pressure low-density polyethylene, MFR (190℃ / 2.16kg) 20.0g / 10min, melting point 109℃, density 0.919g / cm³ 3

[0043] [Prototype Example 1] Prototype Example 1 is an unstretched polyolefin film in which the surface layer is composed of 100% by weight of PP3, the base layer is composed of 100% by weight of PP1, and the sealing layer is composed of 70% by weight of PP4 and 30% by weight of PE1.

[0044] [Prototype Example 2] Prototype Example 2 is an unoriented polyolefin film that is identical to Prototype Example 1 except that the base layer PP1 is changed to PP2.

[0045] [Prototype Example 3] Prototype Example 3 is an unoriented polyolefin film in which the constituent materials of the sealing layer have been changed from Prototype Example 1 to 30% by weight of PP4 and 70% by weight of PE2, while all other components remain the same.

[0046] [Prototype Example 4] Prototype Example 4 is an unoriented polyolefin film that is identical to Prototype Example 1 except that the constituent material of the sealing layer is changed to PE1 at 100% by weight.

[0047] [Table 1]

[0048] [Prototype Example 5] Prototype Example 5 is an unoriented polyolefin film that is identical to Prototype Example 1 except that the base layer PP1 is changed to PP5.

[0049] [Prototype Example 6] Prototype Example 6 is an unoriented polyolefin film in which the constituent materials of the sealing layer are changed from Prototype Example 1 to 50% by weight of PP4 and 50% by weight of PE1, while all other components remain the same.

[0050] [Prototype Example 7] Prototype Example 7 is an unoriented polyolefin film in which the constituent materials of the sealing layer have been changed from Prototype Example 1 to 60% by weight of PP4 and 40% by weight of PE1, while all other components remain the same.

[0051] [Prototype Example 8] Prototype Example 8 is an unoriented polyolefin film in which the constituent materials of the sealing layer are changed from Prototype Example 1 to 80% by weight of PP4 and 20% by weight of PE1, while all other components remain the same.

[0052] [Prototype Example 9] Prototype Example 9 is an unoriented polyolefin film in which the constituent materials of the sealing layer have been changed from Prototype Example 1 to 90% by weight of PP4 and 10% by weight of PE1, while all other components remain the same.

[0053] [Table 2]

[0054] [Prototype Example 10] Prototype 10 is an unoriented polyolefin film that is identical to Prototype 1, except that the constituent materials of the sealing layer are changed to 40% by weight of PP4, 30% by weight of PP7, and 30% by weight of PE1.

[0055] [Prototype Example 11] Prototype 11 is an unoriented polyolefin film in which the constituent materials of the sealing layer are changed from those of Prototype 1 to 70% by weight of PP7 and 30% by weight of PE1, while all other components remain the same.

[0056] [Prototype Example 12] Prototype 12 is an unoriented polyolefin film in which the constituent materials of the sealing layer are changed from those of Prototype 1 to 70% by weight of PP1 and 30% by weight of PE1, while all other components remain the same.

[0057] [Prototype Example 13] Prototype 13 is an unoriented polyolefin film that is identical to Prototype 1 except that the PE1 in the sealing layer has been changed to PE2.

[0058] [Prototype Example 14] Prototype 14 is an unoriented polyolefin film that is identical to Prototype 1 except that the PE1 in the sealing layer has been changed to PE3.

[0059] [Prototype Example 15] Prototype 15 is an unoriented polyolefin film that is identical to Prototype 1 except that the PE1 in the sealing layer has been changed to PE4.

[0060] [Prototype Example 16] Prototype 16 is an unoriented polyolefin film that is identical to Prototype 1 except that the surface layer's PP3 is changed to PP6.

[0061] [Table 3]

[0062] [Evaluation of unstretched polyolefin films] For prototypes 1-16 of the unoriented polypropylene film, we performed heat seal strength measurements, evaluated stringing and film residue, observed the peel surface, evaluated moldability, and evaluated tearing upon opening. An overall evaluation was then conducted by combining the evaluations of each item. The results of each test are shown in Tables 4-6 below. In Tables 4 and 6, items where the test was not performed are indicated as "Not performed (-)".

[0063] [Measurement of heat seal strength] The heat seal strength (N / 15mm) was measured as follows. For each of the 16 prototype films, the sealing layers of two films were stacked and heat-sealed using a heat seal tester (manufactured by Toyo Seiki Seisakusho Co., Ltd.; "Thermal Gradient Tester") with a sealing bar shape of 10mm x 25mm, a sealing pressure of 0.4MPa, and a sealing time of 1 second, under multiple heat seal temperature conditions. The heat seal temperature conditions ranged from 120°C to 155°C in 5°C increments. After heat-sealing each of the 16 prototype films, a 15mm wide test piece was cut out, opened to 180°, and the sealed portion was peeled off at a tensile speed of 200mm / min using a tensile tester (manufactured by Shimadzu Corporation; "Small Tabletop Tester EZ-SX") to measure the peel strength. The maximum strength during peeling at each heat seal temperature was defined as the heat seal strength. In the prototype where thickness discrepancies were observed, the peel strength was measured at the portion where the film thickness (total thickness) was approximately 28 μm. The target heat seal strength for achieving both easy opening and tear resistance was set at 1.5 to 4.0 N / 15 mm.

[0064] [Evaluation of stringiness and membrane residue] For each test specimen from prototype examples 1 to 16 at each heat-seal temperature after the heat-seal strength measurement described above, the delamination area (heat-seal delamination interface) was visually observed to evaluate the presence or absence of stringing and film residue. The evaluation criteria were as follows: "Excellent (◎)" if neither stringing nor film residue was observed; "Good (○)" if slight stringing was observed but no film residue was observed; "Poor (△)" if severe stringing was observed but no film residue was observed; and "Unacceptable (×)" if film residue was observed.

[0065] [Observation of the delamination surface] For each test specimen from prototype examples 1 to 16 at each heat-seal temperature after the heat-seal strength measurement described above, the delamination area (heat-seal delamination interface) was visually observed to determine whether it was cohesive failure or interlayer delamination. In Tables 4 to 6, since the same results were obtained for each heat-seal temperature in prototype examples 1 to 12 and 14 to 16, the results for each prototype example are listed together. In prototype example 13, cohesive failure occurred at 120°C to 125°C, and "film breakage" occurred at 130°C to 155°C, where the film was cut before the heat-sealed portion peeled off. Therefore, the results are listed along with the temperature in the column for prototype example 13 in Table 6.

[0066] [Evaluation of moldability] The film manufacturing process for each prototype example (1-16) was examined, and the moldability was evaluated. "Good" was defined as having no significant variation in thickness, good thickness-to-thinness accuracy, and no problems with film moldability. "Ear wobble" was defined as the pulsating edge (ear) of the resin coming out of the T-die, and "Poor thickness-to-thinness" was defined as the uneven thickness resulting from the ear wobble and a decrease in thickness-to-thinness accuracy.

[0067] [Evaluation of tearing upon opening] Each of the prototype examples 1-16 was placed in a single-film horizontal pillow packaging machine (Fuji Kikai Co., Ltd.; "FUJI αwrapper II FW3400") with the sealing layer of the film facing inward, and pillow packaging bags were created under the following conditions. Note that the sealing bar setting temperature below differs from the heat sealing temperature used to measure the heat seal strength above, so the numerical correlation between temperature and heat seal strength may not necessarily match. Seal bar temperature settings: Bottom top / 110℃, Bottom bottom / 110℃, Center (back adhesive area) / 110℃ Bag making speed: 50 pieces / min Bag size: 250mm (height) x 150mm (width)

[0068] Ten pillow packaging bags corresponding to each prototype example 1-16 were prepared, and the bags were opened at the heat-sealed portion to evaluate whether or not they tore. If fewer than three bags tore, it was marked as "Good (○)," and if three or more bags tore, it was marked as "Unacceptable (×)." Prototype examples 11 and 12 were not successfully produced as pillow packaging bodies could not be created due to inconsistent thickness and shape on the horizontal pillow packaging machine. Therefore, prototype examples 11 and 12 were marked as "Not performed (-)."

[0069] [Overall rating] In each prototype, those that showed a heat seal strength of 1.5 to 4.0 N / 15 mm at five or more measurement temperatures, had a stringing and film residue evaluation of good or better, exhibited cohesive failure at the peeling surface, had good moldability, and had a good evaluation of tearing upon opening were classified as "Good (○)," while all others were classified as "Unacceptable (×)."

[0070] [Table 4]

[0071] [Table 5]

[0072] [Table 6]

[0073] [Results and Discussion] As shown in Table 1, prototypes 1 and 2 are unoriented polyolefin films of the present invention, using propylene random copolymer and high-pressure low-density polyethylene as constituent materials for the seal layer, and each using homopolypropylene with different MFRs for the base layer. On the other hand, prototype 3 is an example in which propylene random copolymer and linear low-density polyethylene are used as constituent materials for the seal layer, and prototype 4 is an example in which only high-pressure low-density polyethylene is used as the constituent material for the seal layer.

[0074] As shown in Table 2, prototypes 7 and 8 are unoriented polyolefin films of the present invention. Prototypes 7 and 8 use homopolypropylene as the base layer, similar to prototype 1, and use propylene random copolymer and high-pressure low-density polyethylene as the constituent materials of the seal layer. In prototypes 1, 7, and 8, the types of propylene random copolymer and high-pressure low-density polyethylene constituting the seal layer are the same, but the blending ratios are different. Prototypes 6 and 9 also consist of the same constituent materials as prototypes 7 and 8, but the blending ratios of propylene random copolymer and high-pressure low-density polyethylene constituting the seal layer are different. Prototype 5 uses homopolypropylene as the base layer and uses propylene random copolymer and high-pressure low-density polyethylene as the constituent materials of the seal layer. The homopolypropylene constituting the base layer of prototype 5 has a different MFR than that of prototype 1.

[0075] As shown in Table 3, prototypes 10, 14, and 16 are unoriented polyolefin films of the present invention. Prototype 10 uses homopolypropylene as the base layer and uses two propylene random copolymers with different melting points and high-pressure low-density polyethylene as the constituent materials of the seal layer. Prototypes 14 and 16 also use homopolypropylene as the base layer and use propylene random copolymer and high-pressure low-density polyethylene as the constituent materials of the seal layer. The high-pressure low-density polyethylene constituting the seal layer of prototype 14 has a different MFR than that of prototypes 1, 7, 8, and 10. Prototype 16 has the same base layer and seal layer structure as prototype 1, but differs in that the constituent material of the surface layer is propylene block copolymer. On the other hand, prototype 11 uses homopolypropylene as the base layer and uses propylene random copolymer with a different melting point than that of prototype 1 and high-pressure low-density polyethylene as the constituent materials of the seal layer. Prototype Example 12 uses homopolypropylene as the base layer and homopolypropylene and high-pressure low-density polyethylene as the constituent materials for the seal layer. Prototype Example 13 uses homopolypropylene as the base layer and propylene random copolymer and linear low-density polyethylene as the constituent materials for the seal layer. Prototype Example 15 uses homopolypropylene as the base layer and propylene random copolymer and high-pressure low-density polyethylene with a different MFR than that used in Prototype Example 1 as the constituent materials for the seal layer.

[0076] As can be seen from Table 4, the films of prototypes 1 and 2 underwent cohesive failure, and all film performance aspects, including stringing, film residue, moldability, and tearing upon opening, were good (overall evaluation: "Good (○)"). On the other hand, prototype 3 exhibited delamination, and although stringing, film residue, and moldability were good, sufficient performance was not obtained in terms of tearing upon opening. Prototype 4 also exhibited delamination, and although tearing upon opening was good, sufficient performance was not obtained in terms of stringing, film residue, and moldability.

[0077] Furthermore, in prototypes 1 and 2, where the film performance was good, good heat seal strength was obtained that balanced easy opening and tear resistance over a wide heat seal temperature range of 130 to 155°C. On the other hand, in prototype 3, good heat seal strength was obtained at a heat seal temperature of 120°C, but at heat seal temperatures of 125°C or higher, the heat seal strength became too strong, and good heat seal strength could not be obtained. In prototype 4, good heat seal strength could not be obtained at any heat seal temperature between 120 and 155°C.

[0078] Therefore, comparing prototypes 1 and 2 with prototype 3, in prototypes 1 and 2, good heat seal strength was obtained because the seal layer was composed of an incompatible propylene random copolymer and high-pressure low-density polyethylene. However, in prototype 3, although the seal layer was composed of an incompatible propylene random copolymer and linear low-density polyethylene, the heat seal strength was too strong and good heat seal strength could not be obtained. Thus, it was shown that even if the seal layer is composed of incompatible resins, appropriate heat seal strength may not be obtained depending on the type of resin. Furthermore, it was shown that appropriate heat seal strength could not be obtained when the seal layer was composed only of high-pressure low-density polyethylene, as in prototype 4.

[0079] Next, comparing prototype example 1 and prototype example 2, a difference in the degree of stringing was observed. In prototype example 1, no stringing or film residue was observed in the test specimens with a heat sealing temperature of 120-130°C (evaluation: "Excellent (◎)"), while slight stringing was observed in the test specimens with a heat sealing temperature of 135-155°C (evaluation: "Good (○)"). In particular, the evaluation in the range of 130-155°C, where the heat sealing strength was good, was generally "Good (○)". On the other hand, in prototype example 2, no stringing or film residue was observed at any heat sealing temperature between 120-155°C (evaluation: "Excellent (◎)").

[0080] Therefore, examining the differences between prototype example 1 and prototype example 2, it can be found that the MFR of the homopolypropylene constituting the base layer in prototype example 2 is smaller than that of the homopolypropylene constituting the base layer in prototype example 1. From this, it was shown that reducing the MFR of the homopolypropylene constituting the base layer improves the degree of stringing on the heat-sealed peel surface.

[0081] As can be seen from Table 5, the films of prototypes 7 and 8 underwent cohesive failure, and all film performance aspects, including moldability, stringing and film residue, and tearing upon opening, were good (overall evaluation: "Good (○)"). On the other hand, prototypes 5 and 6 underwent cohesive failure, and although moldability and tearing upon opening were good, sufficient performance was not obtained in terms of stringing and film residue. Prototype 9 underwent cohesive failure, and although stringing, film residue, and moldability were good, sufficient performance was not obtained in terms of tearing upon opening.

[0082] Furthermore, regarding prototypes 7 and 8, where the film performance was good, prototype 7 achieved good heat seal strength that balanced easy opening and tear resistance over a wide heat seal temperature range of 130 to 155°C. Prototype 8 achieved good heat seal strength that balanced easy opening and tear resistance over a wide heat seal temperature range of 120 to 145°C. Regarding prototypes 5, 6, and 9, where the film performance was insufficient, prototype 5 achieved good heat seal strength over a wide heat seal temperature range of 130 to 155°C. Prototype 6 achieved good heat seal strength over a wide heat seal temperature range of 135 to 155°C. Prototype 9 achieved good heat seal strength at 120 to 130°C, but at heat seal temperatures above 135°C, the heat seal strength became too strong, and good heat seal strength could not be obtained.

[0083] First, we compare prototypes 1, 2, and 5. In prototypes 1 and 2, the MFR of the homopolypropylene constituting the base layer was 7.5 g / min and 4.2 g / min, respectively, resulting in good stringing and film retention performance. However, in prototype 5, the MFR of the homopolypropylene constituting the base layer was 13.0 g / min, and sufficient performance in terms of stringing and film retention was not obtained. Thus, it was shown that if the MFR of the homopolypropylene constituting the base layer is too high, the degree of stringing upon opening may worsen.

[0084] Next, we compare prototypes 1, 6-9. In prototypes 1, 6-9, the blending ratio of propylene random copolymer and high-pressure low-density polyethylene that constitute the seal layer differs. In prototypes 1, 7, and 8, the seal layer contained 60-80% by weight of propylene random copolymer and 20-40% by weight of high-pressure low-density polyethylene, resulting in good ease of opening, and good stringing and film residue. In prototype 6, the seal layer contained 50% by weight of propylene random copolymer and 50% by weight of high-pressure low-density polyethylene, and sufficient performance in terms of stringing and film residue was not obtained. Thus, it was shown that if the amount of propylene random copolymer constituting the seal layer is too small, that is, if the amount of high-pressure low-density polyethylene is too large, film residue and stringing may occur. Furthermore, in prototype 9, the seal layer contained 90% by weight of propylene random copolymer, resulting in a narrow temperature range in which good heat seal strength could be obtained, and sufficient performance was not obtained in the evaluation of tearing upon opening. Thus, it has been shown that if there is too much propylene random copolymer in the sealing layer, that is, too little high-pressure low-density polyethylene, good ease of opening may not be achieved.

[0085] As can be seen from Table 6, the films of prototypes 10, 14, and 16 underwent cohesive failure, and all film performance aspects, including moldability, stringing and film residue, and tearing upon opening, were good (overall evaluation: "Good (○)"). On the other hand, prototypes 11 and 12 underwent cohesive failure, and although stringing and film residue performance were good, sufficient moldability was not obtained, and tearing upon opening could not be evaluated. Prototype 13 had good moldability, but when the heat sealing temperature was 130°C or higher, the film tore before peeling. Prototype 13 did not achieve sufficient performance in the evaluation of tearing upon opening. Prototype 15 underwent cohesive failure, and although moldability and tearing upon opening were good, sufficient stringing and film residue performance was not obtained.

[0086] Furthermore, regarding prototypes 10, 14, and 16, which exhibited good film performance, prototype 10 achieved good heat seal strength that balanced easy opening and tear resistance over a wide heat seal temperature range of 135 to 155°C. Prototypes 14 and 16 also achieved good heat seal strength that balanced easy opening and tear resistance over a wide heat seal temperature range of 130 to 155°C. On the other hand, while prototypes 11 and 12 achieved good heat seal strength at 155°C, the heat seal strength was too weak at temperatures below 150°C, resulting in unsatisfactory heat seal strength. In prototype 13, good heat seal strength was achieved at 120°C, but the heat seal strength was too strong at temperatures above 125°C, resulting in unsatisfactory heat seal strength. In prototype 15, good heat seal strength was achieved over a wide heat seal temperature range of 130 to 155°C.

[0087] First, we compare prototypes 1, 10, and 11. In prototypes 1, 10, and 11, the seal layer contains 70% by weight of propylene random copolymer and 30% by weight of high-pressure low-density polyethylene, which is the same, but the composition of the propylene random copolymer differs. The seal layer of prototype 1 contains 70% by weight of propylene random copolymer with a melting point of 125°C, the seal layer of prototype 10 contains 40% by weight of propylene random copolymer with a melting point of 125°C and 30% by weight of propylene random copolymer with a melting point of 147°C, and the seal layer of prototype 11 contains 70% by weight of propylene random copolymer with a melting point of 147°C. While prototypes 1 and 10 showed good moldability, prototype 11 exhibited edge wobble during film molding and reduced thickness accuracy, indicating that the physical properties of the propylene random copolymer contained in the seal layer affect moldability. From a comparison of prototype examples 1 and 11, it is considered that the seal layer exhibits better moldability when it contains a propylene random copolymer with a lower melting point. From prototype example 10, it is considered that when propylene random copolymers with different melting points are included, better moldability is achieved by including a larger amount of propylene random copolymer with a lower melting point. From these findings, it is considered important that the propylene random copolymer contained in the seal layer contains propylene random copolymers with melting points below a certain level in an appropriate blending ratio.

[0088] Next, comparing prototype example 1 and prototype example 12, both prototype examples contain 30% by weight of high-pressure low-density polyethylene in the sealing layer. However, prototype example 1 differs in that the remaining 70% by weight is propylene random copolymer, while prototype example 12 contains homopolypropylene. Prototype example 1 showed good moldability, while prototype example 12 exhibited edge wobble during film molding and reduced thickness accuracy. This indicates that good moldability is achieved by combining propylene random copolymer and high-pressure low-density polyethylene in the sealing layer. It is thought that homopolypropylene's poor compatibility with high-pressure low-density polyethylene leads to poor moldability.

[0089] Comparing prototype example 1 and prototype example 13, both contain 70% by weight of propylene random copolymer in the seal layer. However, prototype example 1 differs in that the remaining 30% by weight is high-pressure low-density polyethylene, while prototype example 13 uses linear low-density polyethylene. In prototype example 1, the seal layer, composed of incompatible propylene random copolymer and high-pressure low-density polyethylene, yielded good heat seal strength and performance against stringing and film residue. However, in prototype example 13, although the seal layer is composed of incompatible propylene random copolymer and linear low-density polyethylene, the heat seal strength became too strong, resulting in poor heat seal strength and film tearing. Similar to the comparison of prototype examples 1-3 described above, this demonstrates that even if the seal layer is composed of incompatible resins, appropriate heat seal strength may not be obtained depending on the type of resin.

[0090] Comparing prototype example 3 and prototype example 13, both consist of a seal layer composed of propylene random copolymer and linear low-density polyethylene. However, prototype example 3 has a higher proportion of linear low-density polyethylene, while prototype example 13 has a higher proportion of propylene random copolymer. When the seal layer is composed of propylene random copolymer and linear low-density polyethylene, it was shown that increasing the proportion of propylene random copolymer results in excessively high heat seal strength and poor peel performance. Thus, it was shown that not only the type of resin but also its proportion significantly affects the performance of the film.

[0091] This is compared with prototypes 1, 14, and 15. In prototypes 1, 14, and 15, the seal layer contains 70% by weight of propylene random copolymer and 30% by weight of high-pressure low-density polyethylene, but the physical properties of the high-pressure low-density polyethylene differ in each case. In prototype 1, 14, the MFR of the high-pressure low-density polyethylene contained in the seal layer was 2.5 g / min and 7.5 g / min, respectively, and good film retention and stringing performance were obtained. In prototype 15, the MFR of the high-pressure low-density polyethylene contained in the seal layer was 20.0 g / min, and sufficient performance was not obtained in the evaluation of film retention and stringing. Thus, it was shown that if the MFR of the high-pressure low-density polyethylene contained in the seal layer is too high, stringing and film retention are more likely to occur when opening the package.

[0092] Comparing prototype example 1 and prototype example 16, prototype example 1 has a surface layer composed of homopolypropylene, while prototype example 16 has a surface layer composed of propylene block copolymer. The propylene block copolymer constituting the surface layer of prototype example 16 contains 87% by weight of homopolypropylene. Homopolypropylene is considered suitable as a constituent resin for the surface layer from the viewpoint of protecting the film. If the proportion of homopolypropylene in the surface layer is too low, the film performance such as heat resistance and rigidity may be insufficient. Therefore, it is thought that the film performance such as heat resistance and rigidity will be good if the surface layer contains a certain amount or more of homopolypropylene. [Industrial applicability]

[0093] The unoriented polyolefin film of the present invention achieves good heat-seal strength that balances easy opening and tear resistance. When the heat-sealed portion is peeled off, appropriate cohesive failure occurs in the heat-seal layer, suppressing stringing, film residue, and tearing of the film upon opening, resulting in good easy opening. Therefore, it is promising as a substitute for unoriented polyolefin films used in automated packaging with single films.

Claims

1. An unstretched film for automatic packaging, which is a single-layer film having at least a base layer and a sealing layer, The aforementioned base layer contains 90% by weight or more of homopolypropylene, The sealing layer contains 60 to 80% by weight of propylene random copolymer and 20 to 40% by weight of high-pressure low-density polyethylene. The homopolypropylene in the substrate layer has an MFR of 8.5 g / 10 min or less. The propylene random copolymer in the sealing layer is composed of propylene random copolymer with a melting point of 135°C or lower, with 50% or more of its components being propylene random copolymer. The high-pressure low-density polyethylene in the sealing layer has an MFR of 10.0 g / 10 min or less. An unstretched polyolefin film characterized by the following features.

2. The unstretched polyolefin film according to claim 1, wherein a surface layer is laminated on the side of the base layer opposite to the seal layer, and the surface layer contains 70% by weight or more of homopolypropylene.

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