Biaxially oriented sheet and method for producing same

Abstract

To improve the toughness of styrene-butadiene copolymer sheets. [Solution] A method for producing a biaxially oriented sheet, comprising a step of biaxially stretching a copolymer containing styrene units and butadiene units, wherein the copolymer has a melt mass-flow rate of 7 g / 10 min or less.

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented sheet and a method for producing the same. [Background technology]

[0002] Beverage containers such as plastic bottles are generally packaged with shrink film. In order to package the containers beautifully, the shrink film is required to have the property of shrinking appropriately when heated and easily adhering to the container. A copolymer containing styrene units and butadiene units (hereinafter also referred to as "styrene-butadiene copolymer") is a polymer that can satisfy these required properties, and is therefore often used for shrink film (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-9515 Summary of the Invention [Problem to be solved by the invention]

[0004] Styrene-butadiene copolymers may be useful for applications other than shrink films. One such application is for sheets that require toughness (excellent tensile modulus and nominal tensile strain at break). Therefore, one aspect of the present invention aims to improve the toughness of styrene-butadiene copolymer sheets. [Means for solving the problem]

[0005] As a result of investigations by the present inventors, it was found that when a sheet of a styrene-butadiene copolymer having a specific melt mass-flow rate (MFR) is biaxially stretched, the toughness of the stretched sheet is significantly improved compared to the sheet before stretching.

[0006] The present invention includes the following aspects. [1] A method for producing a biaxially oriented sheet, comprising a step of biaxially stretching a copolymer containing styrene units and butadiene units, wherein the melt mass-flow rate of the copolymer is 7 g / 10 min or less. [2] The method according to [1], wherein the copolymer is at least one selected from the group consisting of a block copolymer having a block composed of styrene units and a block composed of butadiene units, a copolymer containing styrene units, butadiene units, and methyl (meth)acrylate units, and a copolymer containing styrene units, butadiene units, and acrylonitrile units. [3] The method according to [1] or [2], wherein the stretching ratio when the copolymer is biaxially stretched is 1.2 to 2.0 times. [4] The method according to any one of [1] to [3], wherein the thickness of the biaxially oriented sheet is 0.2 mm or more. [5] A biaxially oriented sheet of a copolymer containing styrene units and butadiene units, wherein the melt mass-flow rate of the biaxially oriented sheet is 7 g / 10 min or less. [6] The biaxially oriented sheet according to [5], wherein the copolymer is at least one selected from the group consisting of a block copolymer having a block consisting of a styrene unit and a block consisting of a butadiene unit, a copolymer containing a styrene unit, a butadiene unit, and a methyl (meth)acrylate unit, and a copolymer containing a styrene unit, a butadiene unit, and an acrylonitrile unit. [7] The biaxially oriented sheet according to [5] or [6], wherein the stretching ratio of the biaxially oriented sheet is 1.2 to 2.0 times. [8] The biaxially oriented sheet according to any one of [5] to [7], wherein the thickness of the biaxially oriented sheet is 0.2 mm or more. [Effects of the Invention]

[0007] According to one aspect of the present invention, the toughness of a styrene-butadiene copolymer sheet can be improved. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

[0009] One embodiment of the present invention is a method for producing a biaxially oriented sheet, comprising a step (stretching step) of biaxially stretching a copolymer containing styrene units and butadiene units (styrene-butadiene copolymer).

[0010] In the stretching step, an unstretched styrene-butadiene copolymer (unstretched sheet) is biaxially stretched. In this specification, "stretching a styrene-butadiene copolymer" means stretching an object to be stretched (unstretched sheet) whose main component is a styrene-butadiene copolymer. The content of the styrene-butadiene copolymer in the object to be stretched (unstretched sheet) may be 90% by mass or more, 91% by mass or more, 92% by mass or more, 93% by mass or more, 94% by mass or more, 95% by mass or more, 96% by mass or more, 97% by mass or more, 98% by mass or more, or 99% by mass or more, based on the total mass of the object to be stretched (unstretched sheet). The object to be stretched (unstretched sheet) may consist solely of a styrene-butadiene copolymer.

[0011] Examples of other components that the stretching target (unstretched sheet) may contain in addition to the styrene-butadiene copolymer include polymers other than styrene-butadiene copolymer, antioxidants, antigelling agents (e.g., polyoxyethylene alkyl ether (POE)), ultraviolet absorbers, light stabilizers, lubricants, plasticizers, colorants, antistatic agents, flame retardants, mineral oil, reinforcing fibers such as glass fiber, carbon fiber, and aramid fiber, and fillers such as talc, silica, mica, and calcium carbonate.

[0012] The melt mass-flow rate (MFR) of the styrene-butadiene copolymer of this embodiment is 7 g / 10 min or less. This allows the styrene-butadiene copolymer to be suitably biaxially stretched, thereby improving the toughness of the styrene-butadiene copolymer sheet. The MFR of the styrene-butadiene copolymer may be 6 g / 10 min or less, 5 g / 10 min or less, 4 g / 10 min or less, or 3 g / 10 min or less, or may be 1 g / 10 min or more or 2 g / 10 min or more. In this specification, MFR refers to a value measured under conditions of 200°C and 49 N in accordance with JIS K7210-1:2014 (ISO 1133-1:2011).

[0013] The MFR of a styrene-butadiene copolymer can be adjusted by its molecular weight (Mw). For example, the lower the molecular weight (Mw) of the styrene-butadiene copolymer, the higher the MFR tends to be, and vice versa.

[0014] The content of styrene units in the styrene-butadiene copolymer may be 50% by mass or more, 60% by mass or more, 70% by mass or more, or 80% by mass or more, and 95% by mass or less, 92% by mass or less, or 90% by mass or less, relative to the total mass of the styrene-butadiene copolymer. The content of butadiene units in the styrene-butadiene copolymer may be 5% by mass or more, 8% by mass or more, or 10% by mass or more, and 30% by mass or less, 25% by mass or less, or 20% by mass or less, relative to the total mass of the styrene-butadiene copolymer.

[0015] The styrene-butadiene copolymer may consist of only styrene units and butadiene units, or may further contain other units in addition to styrene units and butadiene units.

[0016] When the styrene-butadiene copolymer is composed only of styrene units and butadiene units, the styrene-butadiene copolymer may be a block copolymer (hereinafter also referred to as "SBC") having a block composed of styrene units and a block composed of butadiene units. When the styrene-butadiene copolymer further contains other units, the styrene-butadiene copolymer may be a copolymer (hereinafter also referred to as "MBS") containing styrene units, butadiene units, and methyl (meth)acrylate units, or a copolymer (hereinafter also referred to as "ABS") containing styrene units, butadiene units, and acrylonitrile units. In one embodiment, the styrene-butadiene copolymer may be at least one selected from the group consisting of SBC, MBS, and ABS.

[0017] The SBC may have a linear molecular structure produced by living anionic polymerization using, for example, styrene and butadiene as monomers. Note that high impact polystyrene (HIPS) is a styrene-butadiene graft copolymer and does not fall under the category of SBC in this embodiment.

[0018] The SBC may have a block A composed of styrene units and a block B composed of butadiene units. The SBC may be, for example, an AB configuration in which the block A and the block B are bonded together, an ABA configuration in which the first block A, the block B, and the second block B are bonded together, or a BAB configuration in which the first block B, the block A, and the second block B are bonded together.

[0019] The styrene unit content in the SBC may be 70% by mass or more, 75% by mass or more, or 80% by mass or more, and 95% by mass or less, 92% by mass or less, or 90% by mass or less, based on the total mass of the SBC. The butadiene unit content in the SBC may be 5% by mass or more, 8% by mass or more, or 10% by mass or more, and 30% by mass or less, 25% by mass or less, or 20% by mass or less, based on the total mass of the SBC.

[0020] The styrene unit content in MBS may be 30% by mass or more, 35% by mass or more, or 40% by mass or more, and 60% by mass or less, 55% by mass or less, or 50% by mass or less, based on the total mass of MBS. The butadiene unit content in MBS may be 3% by mass or more, 5% by mass or more, or 7% by mass or more, and 20% by mass or less, 15% by mass or less, or 10% by mass or less, based on the total mass of MBS. The methyl (meth)acrylate unit content in MBS may be 15% by mass or more, 20% by mass or more, or 25% by mass or more, and 50% by mass or less, 45% by mass or less, or 40% by mass or less, based on the total mass of MBS.

[0021] The styrene unit content in the ABS may be 10% by mass or more and 50% by mass or less, based on the total mass of the ABS. The butadiene unit content in the ABS may be 5% by mass or more and 30% by mass or less, based on the total mass of the ABS. The acrylonitrile unit content in the ABS may be 5% by mass or more and 30% by mass or less, based on the total mass of the ABS.

[0022] Each monomer unit constituting the styrene-butadiene copolymer may be a biomass-derived component, or the styrene-butadiene copolymer itself may be a biomass-derived component. For example, some or all of the styrene units may be biomass-derived components. Furthermore, some or all of the monomer units contained in the styrene-butadiene copolymer may be recycled monomer units, or the styrene-butadiene copolymer itself may be a recycled block copolymer.

[0023] The stretching ratio when biaxially stretching in the stretching step may be 1.2 to 2.0 times in both biaxial directions (longitudinal and transverse directions). For a styrene-butadiene copolymer having an MFR of 7 g / 10 min or less, even when biaxially stretching at such a relatively low stretching ratio, biaxial stretching can achieve a significant improvement in toughness. The lower limit of the stretching ratio may be 1.25 times, 1.3 times, or 1.35 times. The upper limit of the stretching ratio may be 1.9 times, 1.8 times, 1.7 times, 1.6 times, or 1.5 times. The stretching ratio in the longitudinal direction and the stretching ratio in the transverse direction may be the same or different.

[0024] The temperature at which the biaxial stretching is performed in the stretching step can be appropriately set depending on the type of styrene-butadiene copolymer, and may be, for example, 90°C or higher or 100°C or higher, and 150°C or lower or 140°C or lower.

[0025] The biaxially oriented sheet obtained by the above-described manufacturing method is a biaxially oriented sheet of a styrene-butadiene copolymer. In this specification, "biaxially oriented sheet of a styrene-butadiene copolymer" refers to a biaxially oriented sheet whose main component is a styrene-butadiene copolymer. The content of the styrene-butadiene copolymer in the biaxially oriented sheet may be 90% by mass or more, 91% by mass or more, 92% by mass or more, 93% by mass or more, 94% by mass or more, 95% by mass or more, 96% by mass or more, 97% by mass or more, 98% by mass or more, or 99% by mass or more, based on the total mass of the biaxially oriented sheet. The biaxially oriented sheet may consist solely of a styrene-butadiene copolymer. Components that the biaxially oriented sheet may contain other than the styrene-butadiene copolymer are the same as the components that the above-described stretching target (unstretched sheet) may contain other than the styrene-butadiene copolymer.

[0026] This biaxially stretched sheet has a melt mass-flow rate of 7 g / 10 min or less. The melt mass-flow rate remains almost unchanged before and after biaxial stretching. The MFR of the biaxially stretched sheet may be 6 g / 10 min or less, 5 g / 10 min or less, 4 g / 10 min or less, or 3 g / 10 min or less, or may be 1 g / 10 min or more, or 2 g / 10 min or more.

[0027] The heat shrinkage stress of the biaxially oriented sheet may be preferably 0.10 to 0.30 MPa in each of the MD (machine direction; sheet flow direction) and TD (transverse direction; direction perpendicular to the sheet flow direction), from the viewpoint of suppressing unevenness in thickness of the formed product when formed by vacuum forming. The lower limit of the heat shrinkage stress may be 0.12 MPa, 0.14 MPa, or 0.16 MPa. The upper limit of the heat shrinkage stress may be 0.28 MPa, 0.26 MPa, 0.24 MPa, 0.22 MPa, or 0.20 MPa. The heat shrinkage stress in MD and the heat shrinkage stress in TD may be the same or different.

[0028] The lower the haze of the biaxially oriented sheet, the better, and may be 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less, or may be 0.1% or more. The haze of the biaxially oriented sheet is measured by the method described in the examples.

[0029] The thickness of the biaxially oriented sheet may be 0.1 mm or more, 0.15 mm or more, or 0.2 mm or more from the viewpoint of improving the strength of the molded product, and may be 0.7 mm or less, 0.5 mm or less, 0.4 mm or less, or 0.3 mm or less from the viewpoint of excellent transparency.

[0030] The stretching ratios in MD and TD of the biaxially oriented sheet may each be 1.2 to 2.0. The lower limit of the stretching ratio may be 1.25, 1.3, or 1.35. The upper limit of the stretching ratio may be 1.9, 1.8, 1.7, 1.6, or 1.5. The stretching ratios in MD and TD may be the same or different. In this specification, the stretching ratio of the biaxially oriented sheet is determined by the following formula: Stretching ratio = Y / Z (unit [times]) In this formula, Y represents the length [mm] of a straight line drawn in MD or TD on a test piece of a biaxially oriented sheet before heating, and Z represents the length [mm] of the straight line after the test piece is allowed to stand for 60 minutes in an oven at a temperature 30°C higher than the Vicat softening point temperature of the biaxially oriented sheet measured in accordance with JIS K7206 and allowed to shrink. If a straight line is drawn in MD, the MD stretch ratio can be calculated, and if a straight line is drawn in TD, the TD stretch ratio can be calculated.

[0031] At least one surface of the biaxially oriented sheet may be coated with a coating agent containing one or more selected from known release agents (peeling agents), antifogging agents, and antistatic agents. The method for applying the coating agent to the biaxially oriented sheet is not particularly limited, and may be a coating method using a roll coater, knife coater, gravure roll coater, or the like, or may be spraying, immersion, or the like. That is, one embodiment of the present invention is a multilayer sheet comprising the above-mentioned biaxially oriented sheet and a coating agent layer formed on at least one surface of the biaxially oriented sheet and comprising the above-mentioned coating agent.

[0032] The biaxially oriented sheet and multilayer sheet described above can be used, for example, to produce a molded product. In one embodiment, the biaxially oriented sheet according to this embodiment is suitable for vacuum forming because it can suppress unevenness in the thickness of the molded product when formed by vacuum forming. The molded product obtained by vacuum forming may be, for example, a container, or may be the entire or partial food packaging container (food pack), or may be a deep-draw container. A deep-draw container may be a container in which h > 2 / 3dmax, where dmax is the maximum diameter of the opening of the container and h is the height of the container. Here, the container has a bottom surface facing the opening and a sidewall portion surrounding the periphery of the bottom surface, and the height of the container refers to the distance from the bottom surface to the opening. The container may also be a container in which h > 4 / 5dmax or h ≥ dmax is satisfied.

[0033] One embodiment of the present invention may be a method for producing a molded article, comprising a step of vacuum-forming the biaxially oriented sheet (or the multilayer sheet) to obtain a molded article. The specific vacuum-forming method may be a known method (for example, the method described in "Plastic Processing Technology Handbook" edited by the Society of Polymer Science, Nikkan Kogyo Shimbun, Ltd. (1995)). [Example]

[0034] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0035] [Example 1] Using a pellet extruder (φ40 mm single-screw extruder (manufactured by Tabata Machinery Co., Ltd.)), pellets of a styrene-butadiene block copolymer (MFR: 6 g / 10 min, butadiene unit content: 16% by mass, hereinafter also referred to as "SBC(1)") were extruded into strands through a die plate at an extrusion temperature of 200°C and a screw rotation speed of 80 rpm. The strands were then cooled in a water bath and pelletized through a pelletizer to obtain pellets of the resin composition. The pellets of the resin composition were extruded using a sheet extruder (T-die width 350 mm, lip opening 0.5 mm, φ40 mm extruder (manufactured by Tanabe Plastic Machinery Co., Ltd.)) at an extrusion temperature of 200°C and a screw rotation speed of 50 rpm to obtain an unstretched sheet of SBC(1) with a thickness of 0.4 mm. Using a batch-type biaxial stretching machine (EVER SOKKI, SDR-507D), this unstretched sheet was preheated and biaxially stretched at a stretching temperature of 110°C and a stretch ratio of 1.4 times in both the longitudinal and transverse directions to obtain a biaxially stretched sheet with a thickness of 0.24 mm.

[0036] [Example 2] A styrene-butadiene block copolymer (MFR: 3 g / 10 min, butadiene unit content: 8 mass%, hereinafter also referred to as "SBC(2)") was used instead of SBC(1), and the stretching temperature during biaxial stretching was changed to 100°C. A biaxially stretched sheet of SBC(2) having a thickness of 0.26 mm was obtained in the same manner as in Example 1.

[0037] [Comparative Example 1] An attempt was made to obtain a biaxially oriented sheet in the same manner as in Example 1, except that a styrene-butadiene block copolymer (MFR: 8 g / 10 min, butadiene unit content: 24 mass%, hereinafter also referred to as "SBC(3)") was used instead of SBC(1) and the stretching temperature during biaxial stretching was changed to 100°C. However, the unstretched sheet of SBC(3) was damaged during biaxial stretching, and biaxial stretching was not possible.

[0038] Comparative Example 2 Except for changing the screw rotation speed of the sheet extruder to 40 rpm, an unstretched sheet of SBC(1) having a thickness of 0.25 mm was obtained in the same manner as in Example 1. This unstretched sheet of SBC(1) was used as is (without biaxial stretching).

[0039] [Example 3] A biaxially stretched sheet of MBS(1) having a thickness of 0.26 mm was obtained in the same manner as in Example 1, except that a methyl methacrylate-butadiene-styrene copolymer (MFR: 3 g / 10 min, butadiene unit content: 8 mass%, hereinafter also referred to as "MBS(1)") was used instead of SBC(1) and the stretching temperature during biaxial stretching was changed to 122°C.

[0040] Comparative Example 3 An unstretched sheet of MBS(1) having a thickness of 0.28 mm was obtained in the same manner as in Example 3, except that the screw rotation speed of the sheet extruder was changed to 40 rpm. This unstretched sheet of MBS(1) was used as is (without biaxial stretching).

[0041] [Example 4] A biaxially stretched sheet of ABS(1) having a thickness of 0.23 mm was obtained in the same manner as in Example 1, except that an acrylonitrile-butadiene-styrene copolymer (MFR: 2 g / 10 min, butadiene unit content: 15% by mass, hereinafter also referred to as "ABS(1)") was used instead of SBC(1) and the stretching temperature during biaxial stretching was changed to 135°C.

[0042] Comparative Example 4 An unstretched sheet of ABS (1) having a thickness of 0.28 mm was obtained in the same manner as in Example 3, except that the screw rotation speed of the sheet extruder was changed to 40 rpm. This unstretched sheet of ABS (1) was used as is (without biaxial stretching).

[0043] The following evaluations were carried out on each sheet of the Examples and Comparative Examples. The results are shown in Tables 1 and 2.

[0044] (Evaluation of toughness) The biaxially stretched sheet was cut into a dumbbell shape No. 1 as specified in JIS K6251:2017 so that the machine direction (MD) of the sheet was the longitudinal direction of the sheet, to prepare a measurement sample. A tensile test was performed using an Autograph AGS-X (Shimadzu Corporation) at a temperature of 23°C, a humidity of 50% RH, and a tension speed of 5 mm / min, and the tensile modulus (MPa) and nominal tensile strain at break (%) were determined in accordance with JIS K7161-1:2014.

[0045] (Transparency Assessment) The haze (%) of the sheet was measured using an NDH5000 (manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7136:2000. The smaller the haze, the higher the transparency. The haze represents the degree of cloudiness of both the surface and the interior of the sheet.

[0046] [Table 1] (Note) For Comparative Example 2, the evaluation results for an unstretched sheet are shown.

[0047] [Table 2] (Note) For Comparative Examples 3 and 4, the evaluation results are shown for unstretched sheets.

Claims

1. A method for producing a biaxially oriented sheet, comprising a step of biaxially stretching a copolymer containing styrene units and butadiene units, The melt mass flow rate of the copolymer is 7 g / 10 min or less.

2. 2. The production method according to claim 1, wherein the copolymer is at least one selected from the group consisting of a block copolymer having a block composed of the styrene units and a block composed of the butadiene units, a copolymer containing the styrene units, the butadiene units, and methyl (meth)acrylate units, and a copolymer containing the styrene units, the butadiene units, and acrylonitrile units.

3. 3. The method according to claim 1, wherein the copolymer is biaxially stretched at a stretching ratio of 1.2 to 2.

0.

4. The method according to claim 1 or 2, wherein the biaxially oriented sheet has a thickness of 0.2 mm or more.

5. A biaxially oriented sheet of a copolymer containing styrene units and butadiene units, The biaxially oriented sheet has a melt mass flow rate of 7 g / 10 min or less.

6. 6. The biaxially oriented sheet according to claim 5, wherein the copolymer is at least one selected from the group consisting of a block copolymer having a block composed of the styrene unit and a block composed of the butadiene unit, a copolymer containing the styrene unit, the butadiene unit, and a methyl (meth)acrylate unit, and a copolymer containing the styrene unit, the butadiene unit, and an acrylonitrile unit.

7. 7. The biaxially oriented sheet according to claim 5, wherein the stretching ratio of the biaxially oriented sheet is 1.2 to 2.0 times.

8. 7. The biaxially oriented sheet according to claim 5, wherein the thickness of the biaxially oriented sheet is 0.2 mm or more.

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