Biaxially oriented sheet and method for manufacturing molded product

Abstract

To provide a biaxially stretched sheet capable of suppressing unevenness in thickness of a formed product when formed by vacuum forming. [Solution] A biaxially oriented sheet containing a styrene homopolymer and a styrene-butadiene block copolymer, wherein the butadiene unit content of the styrene-butadiene block copolymer is 5 mass% or more relative to the total mass of the biaxially oriented sheet.

Description

[Technical Field]

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

[0002] Polystyrene has high transparency and rigidity, and is therefore used for transparent food container lids and the like. However, since polystyrene is easily broken as is, biaxially oriented sheets, which are made by biaxially stretching polystyrene to increase its strength, are usually used for the above applications. Biaxially oriented sheets are generally formed into lids and the like by hot plate molding (see Patent Document 1). However, molded products obtained by hot plate molding have a problem in that they do not have a high degree of freedom in shape and are limited to simple, shallow shapes. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6290529 Summary of the Invention [Problem to be solved by the invention]

[0004] To address the above-mentioned problems, it has been considered to mold the biaxially oriented sheet by vacuum forming (also called vacuum compressed air forming), which allows for greater freedom in shape and enables the production of molded products with more complex or deeper shapes. However, according to the studies of the present inventors, when the biaxially oriented sheet is molded by vacuum forming, it has been found that the biaxially oriented sheet is prone to shrinkage during heating before molding, which tends to cause unevenness in the thickness of the resulting molded product.

[0005] Therefore, an object of one aspect of the present invention is to provide a biaxially oriented sheet that can suppress unevenness in thickness of the formed product when formed by vacuum forming. [Means for solving the problem]

[0006] As a result of investigations, the present inventors have found that, in a biaxially oriented sheet containing a styrene homopolymer and a styrene-butadiene block copolymer, if the content of butadiene units in the styrene-butadiene block copolymer in the biaxially oriented sheet is within a specific range, unevenness in the thickness of the molded product can be suppressed when the biaxially oriented sheet is molded by vacuum forming.

[0007] The present invention includes the following aspects. [1] A biaxially oriented sheet containing a styrene homopolymer and a styrene-butadiene block copolymer, wherein the butadiene unit content of the styrene-butadiene block copolymer is 5% by mass or more based on the total mass of the biaxially oriented sheet. [2] The biaxially oriented sheet according to [1], wherein the heat shrinkage stress of the biaxially oriented sheet in MD and TD is 0.1 to 0.3 MPa. [3] The biaxially oriented sheet according to [1] or [2], wherein the haze of the biaxially oriented sheet is 5% or less. [4] The biaxially oriented sheet according to any one of [1] to [3], which is used for vacuum forming. [5] A method for producing a molded article, comprising a step of vacuum-forming the biaxially oriented sheet according to any one of [1] to [3] to obtain a molded article. [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to provide a biaxially oriented sheet that can suppress unevenness in thickness of the formed product when formed by vacuum forming. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

[0010] A biaxially oriented sheet according to one embodiment of the present invention contains a styrene homopolymer and a styrene-butadiene block copolymer.

[0011] Styrene homopolymers are also called general-purpose polystyrenes (GPPS). The weight-average molecular weight (Mw) of the styrene homopolymers may be 150,000 or more, 200,000 or more, or 250,000 or more, and may be 450,000 or less, 400,000 or less, or 350,000 or less. In this specification, the weight-average molecular weight (Mw) is determined by GPC measurement under the following conditions: Apparatus: Tosoh Corporation GPC "HLC-8320GPC" Column: Shodex KF404 x 3 Temperature: 40℃ Solvent: tetrahydrofuran Flow rate: 0.2ml / min Pressure: 10 MPa Detection: RI Sample preparation method: 120 mg of a sample was dissolved in 15 mL of tetrahydrofuran, and then filtered through a syringe filter (Millex (registered trademark) 0.45 μm, manufactured by Merck Millipore). Injection volume: 10μl Calibration curve: Using standard polystyrene (manufactured by Polymer Laboratories), the relationship between elution time and elution amount is converted into molecular weight to determine various average molecular weights.

[0012] The content of the styrene homopolymer in the biaxially oriented sheet may be 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more, and may be 70% by mass or less, 60% by mass or less, or 50% by mass or less, relative to the total mass of the biaxially oriented sheet.

[0013] A styrene-butadiene block copolymer (hereinafter also simply referred to as a "block copolymer") is a block copolymer containing styrene units and butadiene units. The block copolymer may have a linear molecular structure produced by living anionic polymerization using styrene and butadiene as monomers. Note that high impact polystyrene (HIPS) is a styrene-butadiene graft copolymer and does not fall under the styrene-butadiene block copolymer of this embodiment.

[0014] The block copolymer may have a block A composed of styrene units and a block B composed of butadiene units. The block copolymer may be, for example, an AB copolymer in which the block A and the block B are bonded together, an ABA copolymer in which the first block A, the block B, and the second block B are bonded together, or a BAB copolymer in which the first block B, the block A, and the second block B are bonded together.

[0015] The styrene unit content in the block copolymer 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 block copolymer. The butadiene unit content in the block 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, based on the total mass of the block copolymer.

[0016] Each monomer unit constituting the block copolymer may be a biomass-derived component, or the block 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 block copolymer may be recycled monomer units, or the block copolymer itself may be a recycled block copolymer.

[0017] The content of the block copolymer in the biaxially oriented sheet may be 30% by mass or more, 40% by mass or more, or 50% by mass or more, and may be 95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, or 50% by mass or less, relative to the total mass of the biaxially oriented sheet.

[0018] The content of butadiene units in the block copolymer in the biaxially oriented sheet is 5% by mass or more relative to the total mass of the biaxially oriented sheet from the viewpoint of the effect of suppressing unevenness in thickness of the molded product when molded by vacuum forming, and from the viewpoint of further facilitating the achievement of this effect and improving the toughness (tensile modulus and tensile elongation at break) of the biaxially oriented sheet, it may be 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, 11% by mass or more, or 12% by mass or more. From the viewpoint of further facilitating the achievement of the above effect, the content of butadiene units in the block copolymer in the biaxially oriented sheet may be 15% by mass or less, 14% by mass or less, 13% by mass or less, 12% by mass or less, 11% by mass or less, or 10% by mass or less.

[0019] The butadiene unit content of the block copolymer in the biaxially oriented sheet can be adjusted by adjusting the butadiene unit content in the block copolymer or the block copolymer content in the biaxially oriented sheet. The butadiene unit content of the block copolymer in the biaxially oriented sheet can be measured by dissolving 0.25 g of the biaxially oriented sheet in 50 ml of chloroform, adding iodine monochloride to react with the double bonds in the rubber component, adding potassium iodide to convert the remaining iodine monochloride to iodine, and then back titrating with sodium thiosulfate (iodine monochloride method). The analytical method is described, for example, in "(3) Rubber Content" on page 659 of "New Edition Polymer Analysis Handbook," edited by the Polymer Analysis Research Forum of the Japan Society for Analytical Chemistry, Kinokuniya Shoten (1995 edition), and can be measured using this method.

[0020] The biaxially oriented sheet may further contain additives. Examples of additives include 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. The total content of the additives may be 0.1% by mass or more and 1.0% by mass or less, based on the total mass of the biaxially oriented sheet.

[0021] The heat shrinkage stress of the biaxially oriented sheet may be preferably 0.1 to 0.3 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 more easily achieving the effect 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 0.2 MPa. The heat shrinkage stress in MD and the heat shrinkage stress in TD may be the same or different. The heat shrinkage stress of the biaxially oriented sheet is measured by the method described in the Examples.

[0022] The lower the haze of the biaxially oriented sheet, the better, and may be 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.

[0023] 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.

[0024] The MD and TD stretching ratios of the biaxially oriented sheet may be 1.1 times or more, 1.2 times or more, or 1.3 times or more, and may be 2 times or less, 1.8 times or less, or 1.6 times or less. The MD stretching ratio and the TD stretching ratio may be the same or different. In this specification, the stretching ratio of the biaxially oriented sheet is calculated 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.

[0025] 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.

[0026] The biaxially oriented sheet and multilayer sheet described above can be used, for example, to produce a molded product. As described above, the biaxially oriented sheet according to this embodiment can suppress unevenness in the thickness of the molded product when vacuum formed, and is therefore suitable for use in vacuum forming. The molded product obtained by vacuum forming may be, for example, a container, a whole or part of a food packaging container (food pack), or 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.

[0027] 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]

[0028] 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.

[0029] [Example 1] Using a pellet extruder (φ40mm single-screw extruder (Tabata Machinery Co., Ltd.)), a mixture of 40% by mass of pellets of styrene homopolymer (GPPS, manufactured by Toyo Styrene Co., Ltd., model number: HRM63, weight average molecular weight (Mw): 260,000) and 60% by mass of pellets of styrene-butadiene block copolymer (manufactured by Denka Co., Ltd., model number: Clearene 440L, butadiene unit content: 16% by mass) was 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 into a sheet extruder (T-die width 350mm, lip opening 0.5mm, φ40mm 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 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 118°C and a stretch ratio of 1.45 in both the longitudinal and transverse directions to obtain a biaxially stretched sheet with a thickness of 0.26 mm.

[0030] [Example 2] A biaxially oriented sheet having a thickness of 0.26 mm was obtained in the same manner as in Example 1, except that the mixing ratio of styrene homopolymer / styrene-butadiene block copolymer was changed to 20% by mass / 80% by mass, the stretching temperature during biaxial stretching was changed to 116°C, and the stretching ratio was changed to 1.5 times.

[0031] [Example 3] A biaxially oriented sheet having a thickness of 0.27 mm was obtained in the same manner as in Example 1, except that the mixing ratio of styrene homopolymer / styrene-butadiene block copolymer was changed to 60% by mass / 40% by mass and the stretching ratio during biaxial stretching was changed to 1.35 times.

[0032] [Comparative Example 1] A biaxially oriented sheet having a thickness of 0.26 mm was obtained in the same manner as in Example 1, except that the mixing ratio of styrene homopolymer / styrene-butadiene block copolymer was changed to 80% by mass / 20% by mass, the stretching temperature during biaxial stretching was changed to 119°C, and the stretching ratio was changed to 1.35 times.

[0033] Comparative Example 2 Except for changing the screw rotation speed of the sheet extruder to 40 rpm, an unstretched sheet having a thickness of 0.28 mm was obtained in the same manner as in Example 1. This unstretched sheet was used as is (without being biaxially stretched).

[0034] Comparative Example 3 A biaxially oriented sheet having a thickness of 0.25 mm was obtained in the same manner as in Example 1, except that high impact polystyrene (HIPS, styrene-butadiene graft copolymer, butadiene unit content: 9% by mass) was used instead of the mixture of styrene homopolymer and styrene-butadiene block copolymer, the stretching temperature during biaxial stretching was changed to 125°C, and the stretching ratio was changed to 1.4 times.

[0035] The following evaluations were carried out for each sheet of the Examples and Comparative Examples. The results are shown in Table 1. Table 1 also shows the content of butadiene units in each sheet.

[0036] (Measurement of thermal shrinkage stress) The sheet was cut into strips measuring 20 mm wide x 150 mm long to prepare samples for measurement. Both ends of the sample were held with chucks and submerged in an oil bath at 130°C for heating. The maximum load (unit: N) applied to the chuck due to thermal shrinkage of the sheet (due to the relaxation of the molecular orientation in the sheet due to thermal motion) was measured using a thermal shrinkage stress measuring device (TP-501, manufactured by Tester Sangyo Co., Ltd.). The maximum load was calculated as a function of the cross-sectional area of ​​the sample (unit: mm 2 The heat shrinkage stress (unit: MPa) was calculated by dividing the calculated value by the heat shrinkage stress (unit: MPa). The heat shrinkage stress is an index of the degree of molecular orientation of the biaxially stretched sheet.

[0037] (Evaluation of thickness unevenness of vacuum formed products) The sheet was molded into a cylindrical container (diameter: 80 mm, height: 50 mm) using a vacuum pressure molding machine (FVS-500, manufactured by Wakisaka Engineering Co., Ltd.). The thickness of the vacuum molded product was measured at 49 points every 20 mm, and the standard deviation σ of the thickness was calculated. The smaller the standard deviation σ, the more the thickness unevenness of the vacuum molded product was suppressed.

[0038] (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.

[0039] (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.

[0040] [Table 1]

Claims

1. A biaxially oriented sheet containing a styrene homopolymer and a styrene-butadiene block copolymer, The biaxially oriented sheet has a butadiene unit content of the styrene-butadiene block copolymer of 5% by mass or more relative to the total mass of the biaxially oriented sheet.

2. 2. The biaxially oriented sheet according to claim 1, wherein the heat shrinkage stress of the biaxially oriented sheet in MD and TD is 0.1 to 0.3 MPa.

3. 10. The biaxially oriented sheet of claim 1, wherein said biaxially oriented sheet has a haze of 5% or less.

4. The biaxially oriented sheet according to any one of claims 1 to 3, which is used for vacuum forming.

5. A method for producing a molded article, comprising a step of vacuum forming the biaxially oriented sheet according to any one of claims 1 to 3 to obtain a molded article.

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