Biaxially stretched styrenic resin sheet and molded article of the same

Abstract

To provide a biaxially stretched styrenic resin sheet which is obtained by stretching a composition containing a styrene-methacrylic acid-methyl methacrylate copolymer resin capable of exhibiting satisfactory results with respect to all of the required properties of oil resistance, heat resistance, transparency, impact resistance, blocking resistance, and moldability (e.g. hot plate vacuum pressure moldability).SOLUTION: A biaxially stretched styrenic resin sheet is obtained by biaxially stretching a styrenic resin composition containing a styrene-methacrylic acid-methyl methacrylate copolymer resin (A) obtained by copolymerizing styrene, methacrylic acid and methyl methacrylate, a (meth)acrylic resin (B) having an unsaturated carboxylic acid-based monomer unit, an impact-resistant styrenic resin (C) containing a rubber component containing butadiene, and spherical inorganic granules (D) having a particle size distribution width of 2.5 or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a biaxially oriented styrene-based resin sheet that exhibits excellent results in terms of oil resistance, heat resistance, transparency, impact resistance, blocking resistance, and moldability (hot plate compressed air moldability, etc.), and to a molded product thereof. [Background technology]

[0002] Biaxially oriented styrene resin sheets are widely used for packaging lightweight food packaging containers and other items due to their excellent environmental hygiene, stiffness, transparency, moldability, and recyclability. In recent years, microwave ovens for commercial use at convenience stores and the like have become more widespread, and higher-output appliances are being used to reduce the duration of use of microwave ovens. For this reason, there is a demand for sheets made of resins with better heat resistance.

[0003] Styrene-methacrylic acid copolymer resins have been used for the above-mentioned applications because they have excellent heat resistance and are relatively inexpensive (see, for example, Patent Document 1). In recent years, microwave oven heating has changed from simple "warming" to "cooking," and the heating time has been extended and the contents have become hot, making it difficult to meet the required heat resistance performance in some applications. Furthermore, when molded products made from the biaxially oriented styrene-based resin sheet provided in Patent Document 1 are formed into complex shapes to impart design, etc., mechanical strength such as impact resistance may be insufficient due to relaxation of orientation caused by heating during secondary molding.

[0004] Styrene-methacrylic acid-methyl methacrylate copolymer resin has been provided as a resin that can have even higher heat resistance than styrene-methacrylic acid copolymer resin, and a biaxially oriented styrene-based resin sheet using this has been proposed (see, for example, Patent Document 2).

[0005] Furthermore, in order to further increase the mechanical strength of styrene-methacrylic acid-methyl methacrylate copolymer resin, it has been proposed to biaxially stretch a composition in which a specific resin containing a rubber component is also used (see, for example, Patent Documents 3 and 4). However, due to the insufficient compatibility between the resin containing these rubber components and the copolymer resin, the transparency of the sheet obtained by biaxially stretching the composition is deteriorated or gel is generated. Alternatively, the excellent heat resistance of the styrene-methacrylic acid-methyl methacrylate copolymer resin may be impaired by the combined use of a resin containing a rubber component. Therefore, a styrene-based resin sheet has been proposed, which is obtained by biaxially stretching a composition obtained by mixing a specific amount of impact-resistant styrene-based resin and inorganic granules with a styrene-methacrylic acid-methyl methacrylate copolymer resin at a specific magnification (see, for example, Patent Document 5). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2000-309645 A [Patent Document 2] JP 2011-126996 A [Patent Document 3] JP 2012-031344 A [Patent Document 4] JP 2012-207201 A [Patent Document 5] JP 2015-21074 A Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, microwave oven output has been increasing in order to shorten heating times, and with conventional microwave ovens, the oil from heated food has corroded the lid material, causing whitening and holes in the lid, which has become a prominent problem. In particular, for transparent containers that are intended to come into direct contact with food, conventional styrene-methacrylic acid-methyl methacrylate copolymer resins are corroded by the high-temperature oil from food heated in the microwave, causing the transparent container to crack or whiten, so resistance to edible oil is required. It is also required that a molded article obtained by molding a composition containing a styrene-methacrylic acid-methyl methacrylate copolymer resin is excellent in appearance and mechanical strength. In other words, a biaxially oriented styrene-based resin sheet is required to provide satisfactory results in all evaluation items including heat resistance, transparency, impact resistance, blocking resistance, moldability, and oil resistance. In a composition containing a styrene-methacrylic acid-methyl methacrylate copolymer resin, it is important to determine what components are to be included and in what proportions, or under what conditions (such as stretching conditions) such a composition is to be formed into a sheet in order to obtain the desired sheet performance. For example, if the compatibility of the components is insufficient, if the content ratio of the components is not appropriate, or if the sheet formation conditions are not appropriate, the transparency may be poor, gel may be generated, or heat resistance may be impaired, as described above. At present, there is no biaxially oriented styrene-based resin sheet that can provide well-balanced results in terms of all the requirements for oil resistance, heat resistance, transparency, impact resistance, blocking resistance, and formability (such as hot plate compressed air formability). Therefore, an object of the present invention is to provide a biaxially oriented styrene-based resin sheet obtained by stretching a composition containing a styrene-methacrylic acid-methyl methacrylate copolymer resin, which can show satisfactory results in terms of all requirements such as oil resistance, heat resistance, transparency, impact resistance, blocking resistance, and moldability (hot plate compressed air moldability, etc.), and a molded product thereof. [Means for solving the problem]

[0008] As a result of intensive research into solving the above-mentioned problems, the present inventors have found that a composition comprising a styrene-methacrylic acid-methyl methacrylate copolymer resin, an impact-resistant styrene-based resin containing a rubber component containing butadiene, and inorganic granules, further containing a (meth)acrylic resin having an unsaturated carboxylic acid-based monomer unit, and using the composition containing the respective components constituting the composition in specific ratios, a sheet biaxially stretched under specific stretching conditions can be produced as a sheet capable of solving the above-mentioned problems, thereby solving the present invention.

[0009] That is, the present invention includes the following aspects. [1] A styrene-methacrylic acid-methyl methacrylate copolymer resin (A) obtained by copolymerizing styrene, methacrylic acid, and methyl methacrylate; (B) a (meth)acrylic resin having an unsaturated carboxylic acid monomer unit; an impact-resistant styrene-based resin (C) containing a rubber component containing butadiene; A biaxially oriented styrene-based resin sheet obtained by biaxially stretching a styrene-based resin composition containing spherical inorganic particles (D) having a particle size distribution width of 2.5 or less, a mass ratio (A) / (B) of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) to the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit is in the range of (50 to 95 mass%) / (5 to 50 mass%); the impact resistant styrene-based resin (C) containing a rubber component containing butadiene is contained in an amount of 0.5 to 2.0 parts by mass per 100 parts by mass of the combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid-based monomer unit, The inorganic granules (D) are contained in an amount of 50 to 500 ppm relative to 100 parts by mass of the combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit, The conditions of the biaxial stretching are an index represented by the heat shrinkage stress (ORS), in which the heat shrinkage stress (ORS) in the machine direction (MD: machine direction of the sheet) is 0.4 to 0.9 MPa, and the heat shrinkage stress (ORS) in the transverse direction (CD: transverse direction of the sheet) is 0.4 to 0.9 MPa, and the heat shrinkage stress in the MD and the heat shrinkage stress in the CD have a relationship represented by the following formula (1): |ORS(MD)-ORS(CD)|≦0.2MPa (1) (The above ORS(MD) represents the heat shrinkage stress in the machine direction (MD), and the above ORS(CD) represents the heat shrinkage stress in the cross direction (CD).) [2] The biaxially oriented styrene-based resin sheet according to [1], wherein the biaxially oriented styrene-based resin sheet is thermoformed using a mold (length 94 mm, width 94 mm, depth 30 mm, corners: 2R) in a hot plate compressed air molding machine, the obtained molded product is visually observed, and the range from the temperature at which shape reproducibility is good (lower limit molding temperature) to the temperature at which raindrops occur (upper limit molding temperature) is measured, and the measured value of the secondary molding temperature range is 5°C or more. [3] The biaxially oriented styrene-based resin sheet according to [1] or [2], wherein the biaxially oriented styrene-based resin sheet has a dart impact measurement value according to JIS K7124 of 1 J or more. [4] The biaxially oriented styrene-based resin sheet according to any one of [1] to [3], wherein the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit has an MFR (metroflow rate) of 1.0 g / 10 min or more. [5] The biaxially oriented styrene-based resin sheet according to any one of [1] to [4], wherein the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) has a Vicat softening point according to JIS K7206 of 115°C or higher. [6] The biaxially oriented styrene-based resin sheet according to any one of [1] to [5], wherein the biaxially oriented styrene-based resin sheet is cut into a 10 cm x 10 cm piece, and left in a hot air dryer for 10 minutes to measure the temperature at which the thermal shrinkage rate becomes 2%, and the measured 2% thermal shrinkage temperature is 115°C or higher. [7] A molded article obtained by secondary molding the biaxially oriented styrene-based resin sheet according to any one of [1] to [6]. [8] The molded article according to [7], which is a food packaging material. [9] The molded article according to [8], which is a food packaging material for microwave heating. Effect of the Invention

[0010] The present invention provides a biaxially oriented styrene-based resin sheet and a molded article thereof that can satisfy all requirements, such as oil resistance, heat resistance, transparency, impact resistance, blocking resistance, and moldability (hot plate compressed air moldability, etc.), and can be suitably used as various lightweight packaging containers, particularly packaging containers for heating in a microwave oven. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The present invention will be described in detail below. Note that the following description of the constituent elements is merely an example for explaining the present invention, and the present invention is not limited to these contents.

[0012] (Biaxially oriented styrene resin sheet) The biaxially oriented styrene-based resin sheet of the present invention is formed by biaxially stretching a styrene-based resin composition containing the following specific components (A) to (D) in specific content ratios under specific biaxial stretching conditions.

[0013] That is, the biaxially oriented styrene-based resin sheet of the present invention has the following features: (A): styrene-methacrylic acid-methyl methacrylate copolymer resin, (B): a (meth)acrylic resin having an unsaturated carboxylic acid monomer unit; (C): An impact resistant styrene resin containing a rubber component containing butadiene; (D): A styrene-based resin composition comprising spherical inorganic particles having a particle size distribution width of 2.5 or less, The content ratio of each component constituting the styrene-based resin composition is set within a specific range in the biaxially oriented styrene-based resin sheet.

[0014] More specifically, A styrene-methacrylic acid-methyl methacrylate copolymer resin (A) obtained by copolymerizing styrene, methacrylic acid, and methyl methacrylate; (B) a (meth)acrylic resin having an unsaturated carboxylic acid monomer unit; an impact-resistant styrene-based resin (C) containing a rubber component containing butadiene; A styrene-based resin composition containing spherical inorganic particles (D) having a particle size distribution width of 2.5 or less, a mass ratio (A) / (B) of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) to the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit is in the range of (50 to 95 mass%) / (5 to 50 mass%); the impact resistant styrene-based resin (C) containing a rubber component containing butadiene is contained in an amount of 0.5 to 2.0 parts by mass per 100 parts by mass of the combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid-based monomer unit, The biaxially oriented styrene-based resin sheet is obtained by biaxially stretching a styrene-based resin composition, in which the inorganic particles (D) are contained in an amount of 50 to 500 ppm per 100 parts by mass of the combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid-based monomer unit. First, each of the specific components (A) to (D) constituting the styrene-based resin composition will be described below.

[0015] <Styrene-based resin composition> <<(A): Styrene-methacrylic acid-methyl methacrylate copolymer resin>> In the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) (hereinafter also referred to as copolymer resin (A)) used in the present invention, the proportions of styrene, methacrylic acid, and methyl methacrylate used can be variously set depending on the desired heat resistance, mechanical strength, etc., but in order to easily obtain a resin having an excellent balance of resin fluidity, heat resistance, and mechanical strength, it is preferable for the copolymer resin to be obtained by using styrene in the ranges of 68 to 94 mass %; methacrylic acid in the range of 3 to 16 mass %; and methyl methacrylate in the range of 3 to 16 mass % when the total amount of styrene, methacrylic acid, and methyl methacrylate is taken as 100 mass %.

[0016] In particular, when the content of the styrene-derived portion in the obtained copolymer resin (A) is more preferably 74 to 90 mass %, and further preferably 77 to 86 mass %, the balance between the fluidity and heat resistance of the resin becomes better.

[0017] The content of the methacrylic acid-derived portion in the resulting copolymer resin (A) is more preferably in the range of 7 to 14% by mass, and even more preferably in the range of 9 to 13% by mass. When the content is in this range, the heat resistance of the resulting copolymer resin (A) is improved and the generation of gel can be suppressed.

[0018] Furthermore, the content of the methyl methacrylate-derived portion in the obtained copolymer resin (A) is more preferably in the range of 3 to 12 mass%, and even more preferably in the range of 5 to 10 mass%. A content in this range can suppress the generation of gels, improve the mechanical strength, and also suppress the water absorption of the copolymer resin (A).

[0019] In addition, when methacrylic acid and methyl methacrylate are bonded side by side, a high-temperature, high-vacuum devolatilizer may cause a methanol removal reaction under certain conditions to form a six-membered cyclic acid anhydride. The copolymer resin (A) used in the present invention may contain this six-membered cyclic acid anhydride, but it is preferable to have a small amount of it because it reduces the fluidity.

[0020] The contents of the styrene, methacrylic acid and methyl methacrylate-derived portions in the copolymer resin (A) were determined by proton nuclear magnetic resonance ( 1 It can be calculated from the integral ratio of the spectrum measured by a H-NMR spectrometer.

[0021] The method for producing the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) is not particularly limited, but a radical polymerization method, more specifically a radical polymerization method using a bulk polymerization method or a solution polymerization method, can be preferably adopted. The polymerization method mainly comprises a polymerization step for polymerizing the polymerization raw materials (monomer components) and a devolatilization step for removing volatile matters such as unreacted monomers and polymerization solvent from the polymerization product, and is preferably carried out continuously.

[0022] When the polymerization raw materials are polymerized, a polymerization initiator and a chain transfer agent are contained in the polymerization raw material composition. Examples of the polymerization initiator include organic peroxides, for example, peroxyketals such as 2,2-bis(t-butylperoxy)butane, 2,2-bis(t-butylperoxy)octane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and n-butyl-4,4-bis(t-butylperoxy)valerate, dialkyl peroxides such as di-t-butyl peroxide, t-butylcumyl peroxide, dicumyl peroxide, and α,α'-bis(t-butylperoxyisopropyl)benzene, acetyl peroxide, and isobutyryl peroxide. Examples of the peroxide include diacyl peroxides such as octanoyl peroxide, peroxydicarbonates such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, di-n-propyl peroxydicarbonate, peroxyesters such as t-butyl peroxyacetate, t-butyl peroxyisobutyrate, ketone peroxides such as acetylacetone peroxide, methyl ethyl ketone peroxide, and hydroperoxides such as t-butyl hydroperoxide, cumene hydroperoxide, diisopropyl perbenzene hydroperoxide, etc. Among these, 1,1-bis(t-butylperoxy)cyclohexane is preferred from the viewpoints of decomposition rate and polymerization rate.

[0023] Examples of the chain transfer agent include α-methylstyrene linear dimer, n-dodecyl mercaptan, t-dodecyl mercaptan, n-octyl mercaptan, 1-phenyl-2-fluorene, and dipentene.

[0024] As the polymerization method, solution polymerization using a polymerization solvent can be adopted as necessary. Examples of the polymerization solvent used include aromatic hydrocarbons, such as toluene, xylene, ethylbenzene, dialkyl ketones, and methyl ethyl ketone. Each of these may be used alone or in combination of two or more. In addition, other polymerization solvents, such as aliphatic hydrocarbons, can be mixed with aromatic hydrocarbons as long as the solubility of the polymerization product is not reduced. These polymerization solvents are preferably used so that the content does not exceed 25% by mass in the total 100% by mass of the monomers (i.e., styrene, methacrylic acid, and methyl methacrylate) and the polymerization solvent, from the viewpoints of good polymerization rate, good impact strength of the resulting resin, and good economic efficiency obtained by reducing the energy required for recovery of the polymerization solvent. The polymerization solvent may be added after the polymerization progresses and the reaction system becomes relatively viscous, or may be added before polymerization. However, it is preferable to add the polymerization solvent at a ratio of 5 to 20% by mass in the total 100% by mass before polymerization, as this makes it easier to uniformize the quality and is also preferable in terms of polymerization temperature control.

[0025] In the present invention, the weight average molecular weight of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) is preferably 120,000 to 250,000, and particularly preferably 150,000 to 220,000. When the weight average molecular weight is within this range, the transparency can be maintained when mixed with the impact-resistant styrene-based resin (C) containing a rubber component containing butadiene, which will be described later, and the fluidity as a resin composition is good, sheet molding can be efficiently performed, the occurrence of bumps can be suppressed, and a molded product having an excellent balance between impact resistance and transparency can be obtained. In addition, this is also the range in which a sheet having excellent secondary moldability can be obtained. The weight average molecular weight is measured by gel permeation chromatography in terms of polystyrene standard.

[0026] The styrene-methacrylic acid-methyl methacrylate copolymer resin (A) used in the present invention may contain an aliphatic primary alcohol having 14 or more carbon atoms. The addition of an alcohol is effective in suppressing a gelling reaction caused by a dehydration reaction of methacrylic acid. The content of the aliphatic primary alcohol having 14 or more carbon atoms in the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) is preferably 0.02 to 1.0 mass%. Among the aliphatic primary alcohols having 14 or more carbon atoms, iso-type aliphatic primary alcohols having a freezing point of -10°C or less are particularly preferred. The content of the aliphatic primary alcohol having 14 or more carbon atoms can be measured by gas chromatography.

[0027] Examples of the aliphatic primary alcohols having 14 or more carbon atoms include n-myristic alcohol, n-palmitic alcohol, n-stearyl alcohol, etc. Furthermore, examples of isoaliphatic primary alcohols having a freezing point of -10°C or less include isotetradecanol having 14 carbon atoms, isohexadecanol having 16 carbon atoms, isooctadecanol having 18 carbon atoms, and isoeicosanol having 20 carbon atoms, and specific examples of the following alcohols can be mentioned. 7-methyl-2-(3-methylbutyl)-1-octanol, 5-methyl-2-(1-methylbutyl)-1-octanol, 5-methyl-2-(3-methylbutyl)-1-octanol, 2-hexyl-1-decanol, 5,7,7-trimethyl-2-(1,3,3-trimethylbutyl)-1-octanol, 8-methyl-2-(4-methylhexyl)-1-decanol, 2-heptyl-1-undecanol, 2-heptyl-4-methyl-1-decanol, 2-(1,5-dimethylhexyl)-(5,9-dimethyl)-1-decanol. Among these, isooctadecanol having 18 carbon atoms is particularly preferred.

[0028] Stabilizers may be added to the styrene-methacrylic acid-methyl methacrylate copolymer resin (A). Examples of common stabilizers include hindered phenol-based antioxidants such as octadecyl-3-(3,5-tertiary butyl-4-hydroxyphenyl)propionate and 4,6-bis(octylthiomethyl)-o-cresol, and phosphorus-based processing heat stabilizers such as tris(2,4-di-tertiary butylphenyl)phosphite. These stabilizers can be used alone or in combination of two or more kinds. The stabilizers can be added during the polymerization process or the devolatilization process. The stabilizers can also be mixed into the resin using mechanical devices such as an extruder or a Banbury mixer.

[0029] The styrene-methacrylic acid-methyl methacrylate copolymer resin (A) has a Vicat softening point according to JIS K7206 of preferably 110° C. or higher, more preferably 115° C. or higher. When the copolymer resin (A) has a Vicat softening point of 115°C or higher, when a sheet obtained by kneading the copolymer resin (A) with the (meth)acrylic resin (B) is used to perform an evaluation test of [2% heat shrinkage temperature of sheet] shown in the following examples, a measurement result of 115°C or higher is likely to be obtained, which is more preferable.

[0030] <<(B): (Meth)acrylic resin having unsaturated carboxylic acid monomer units>> The styrene-based resin composition in this embodiment contains a (meth)acrylic resin (B) having an unsaturated carboxylic acid-based monomer unit (also referred to as (meth)acrylic resin (B) or simply resin (B)). The (meth)acrylic resin (B) has an unsaturated carboxylic acid monomer unit (b1). The inclusion of a predetermined amount of the (meth)acrylic resin (B) in the styrene-based resin composition contributes to improving the oil resistance of the entire styrene-based resin composition, and further enables the transparency and mechanical strength to be maintained. In the present invention, the (meth)acrylic resin (B) is a general term for synthetic resins having an unsaturated carboxylic acid monomer unit (b1) content of 51 mass % or more.

[0031] The unsaturated carboxylic acid monomer unit (b1) constituting the (meth)acrylic resin (B) according to the present invention is preferably at least one repeating unit selected from the group consisting of (meth)acrylic acid monomer units (b1-1) and (meth)acrylic acid ester monomer units (b1-2), and more preferably two or more repeating units selected from the group consisting of (meth)acrylic acid ester monomer units (b1-2).

[0032] -(Meth)acrylic acid monomer (b1-1)- In this embodiment, the (meth)acrylic acid monomer (b1-1) may be acrylic acid or methacrylic acid.

[0033] -(Meth)acrylic acid ester monomer (b1-2)- The (meth)acrylic acid ester monomer unit (b1-2) constituting the (meth)acrylic resin (B) of this embodiment includes a methacrylic acid ester monomer unit and an acrylic acid ester monomer unit. In addition, the (meth)acrylic acid ester monomer (b1-2) includes methyl acrylate, ethyl acrylate, (n-butyl) acrylate, (2-ethylhexyl) acrylate, (n-octyl) acrylate, benzyl acrylate, methyl methacrylate, butyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, cyclohexyl methacrylate, phenyl methacrylate, (2-ethylhexyl) methacrylate, (n-octyl) methacrylate, benzyl methacrylate, etc., and methyl acrylate, (n-butyl) acrylate, and methyl methacrylate are preferred from the viewpoint of industrial availability and low cost. The (meth)acrylic acid ester monomers (b1-2) can be used alone or in combination, and it is preferable to use two kinds of (meth)acrylic acid ester monomers in combination. As a combination of monomer units constituting the (meth)acrylic resin (B) according to the present invention, from the viewpoint of achieving both oil resistance, heat resistance, thermal decomposition property, etc., among the monomer units listed above, a copolymer of a methacrylic acid ester species and an acrylic acid ester species is preferred, and a methyl methacrylate-methyl acrylate copolymer resin is more preferred.

[0034] <<<Preferable form of (meth)acrylic resin (B)>>> A preferred (meth)acrylic resin (B) of this embodiment is preferably a binary or ternary copolymer resin, and is a methacrylic acid ester-acrylic acid ester copolymer resin obtained by copolymerizing a methacrylic acid ester species (methacrylic acid ester monomer unit) and an acrylic acid ester species (acrylic acid ester monomer unit), and the upper limit of the content of the acrylic acid ester species relative to the total amount of the methacrylic acid ester-acrylic acid ester copolymer resin is preferably 20 mass% or less, 18 mass% or less, 16 mass% or less, 14 mass% or less, 12 mass% or less, 10 mass% or less, 8 mass% or less, 6 mass% or less, 4 mass% or less, 3 mass% or less, and 2.5 mass% or less, and the lower limit is preferably 0.1 mass% or more, 0.3 mass% or more, 0.5 mass% or more, 0.7 mass% or more, 0.9 mass% or more, 1.1 mass% or more, 1.3 mass% or more, 1.5 mass% or more, and 1.7 mass% or more, in that order. The content of the acrylic ester species relative to the total amount of the methacrylic acid ester-acrylic acid ester copolymer resin can be any combination of the upper limit and the lower limit. In particular, by setting the content of the acrylic ester species in the range of 0.5 mass% to 8 mass%, a styrene-based resin composition having a good balance between heat resistance and oil resistance can be obtained. In addition, the (meth)acrylic resin (B) of this embodiment is preferably a methyl methacrylate-methyl acrylate copolymer resin, and is a copolymer resin containing 0.1 to 15 mass% of methyl acrylate relative to the total amount of the copolymer resin. More preferably, it is in the range of 0.2 to 13 mass%, even more preferably 0.3 to 11 mass%, even more preferably 0.4 to 9 mass%, and particularly preferably 0.5 to 7 mass%. This makes it possible to improve the oil resistance of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) while maintaining the effect of heat resistance.

[0035] In the (meth)acrylic resin (B) of this embodiment, the content of the (meth)acrylic acid ester monomer unit (b1-2) in the (meth)acrylic resin (B) is preferably 60.0 to 100 mass%, more preferably 60.0 to 99.5 mass%, further preferably 70.0 to 98.5 mass%, and even more preferably 85 to 98.0 mass%, based on the total amount of the (meth)acrylic resin (B). As the type of (meth)acrylic acid ester monomer (b1-2), methyl acrylate or methyl methacrylate is preferred because of its heat resistance, industrial availability and low cost.

[0036] <<<Other monomers (b2)>>> The (meth)acrylic resin (B) of this embodiment may further have other monomer units (b2) other than the above-mentioned (meth)acrylic acid monomer (b1-1) and (meth)acrylic acid ester monomer units (b1-2). That is, the other monomer units (b2) may be copolymerized with the unsaturated carboxylic acid monomer units (b1) (including the (meth)acrylic acid monomer (b1-1) and / or the (meth)acrylic acid ester monomer (b1-2)) without any particular limitation, as long as the effect of the invention is not impaired. For example, examples of the monomer (b2) other than the above-mentioned monomers include styrene, maleic anhydride, maleic acid, fumaric acid, itaconic acid, dimethyl maleate, dimethyl fumarate, diethyl fumarate, ethyl fumarate, maleimide, and nucleus-substituted maleimide.

[0037] The weight average molecular weight (Mw) of the (meth)acrylic resin (B) is preferably from 50,000 to 1,000,000, more preferably from 600,000 to 900,000, even more preferably from 70,000 to 300,000, and still more preferably from 80,000 to 200,000.

[0038] <<<(Meth)acrylic acid monomer or (meth)acrylic acid ester monomer in styrene-based resin composition>>> In this embodiment, the content of all (meth)acrylic acid monomer units contained in the styrene-based resin composition is preferably 0 to 20 mass %, more preferably 3 to 15 mass %, based on the total amount (100 mass %) of the styrene-based resin composition. When the content of (meth)acrylic acid monomers in the entire composition is within the above range, a sufficient effect of improving heat resistance can be obtained.

[0039] In this embodiment, the content of all (meth)acrylic acid ester monomer units contained in the styrene-based resin composition is preferably 60% by mass or less and 55% by mass or less as the upper limit, and 6.0% by mass or more, 8.0% by mass or more, 10.0% by mass or more, and 12.0% by mass or more as the lower limit, relative to the total amount (100% by mass) of the styrene-based resin composition. The content of the (meth)acrylic acid ester monomer units can be arbitrarily combined with the above upper limit and the above lower limit, relative to the total amount (100% by mass) of the styrene-based resin composition. When the content of the (meth)acrylic acid ester monomer in the entire composition is within the above range, the effect of improving heat resistance, oil resistance, and strength can be sufficiently obtained. In particular, by controlling the content range of the (meth)acrylic acid ester monomer units to 12% by mass or more and 55% by mass or less, a styrene-based resin composition having an excellent balance between heat resistance and oil resistance can be obtained. The total (meth)acrylic acid ester monomer content indicates the total amount of (meth)acrylic acid ester monomer units present in the styrene-based resin composition, and therefore each content of (meth)acrylic acid ester monomer units in resin (A), resin (B) and resin (D) is also converted.

[0040] In a particularly preferred embodiment, all (meth)acrylic acid ester monomer units contained in the styrene-based resin composition contain methyl acrylate, and the content is preferably 3.0 mass% or less, 2.5 mass% or less, 2.0 mass% or less, or 1.5 mass% or less as an upper limit, and 0.01 mass% or more as a lower limit, relative to the total amount (100 mass%) of the styrene-based resin composition. The content of the methyl acrylate monomer unit can be any combination of the above upper limit and the above lower limit, relative to the total amount (100 mass%) of the styrene-based resin composition. In particular, by setting the content range of the methyl acrylate monomer unit to a range of 0.01 mass% or more and 1.5 mass% or less, a styrene-based resin composition having an excellent balance of oil resistance, heat resistance, and mechanical strength and an excellent appearance can be obtained.

[0041] By controlling the content of all (meth)acrylic acid monomer units and all (meth)acrylic acid ester monomer units contained in the styrene-based resin composition within the above range, the heat resistance improving effect of the (meth)acrylic acid monomer units and the oil resistance improving effect of the (meth)acrylic acid ester monomer units can be simultaneously and efficiently obtained, and as a result, a styrene-based resin composition having excellent heat and oil resistance and a sheet molded therefrom can be obtained.

[0042] <<<Production method of (meth)acrylic resin (B)>>> The method for producing the (meth)acrylic resin (B) of the present embodiment is not particularly limited, and the (meth)acrylic resin (B) can be produced by a process such as bulk polymerization in which a (meth)acrylic acid ester monomer and, as necessary, other monomers are polymerized, solution polymerization in which a solvent is added, or suspension polymerization in which an organic layer is dispersed in water using a suspending agent.

[0043] The MFR (metaflow rate) of the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit is preferably 1.0 g / 10 min or more from the viewpoints of preventing poor kneading with the copolymer resin (A) due to flow unevenness, lowering the haze value, and improving transparency. The melt flow rate is a value measured according to the N method (230°C, 3.8 kg) of JIS K7210.

[0044] <<(C): Impact-resistant styrene resin containing rubber components including butadiene>> The styrene-based resin composition in this embodiment contains an impact-resistant styrene-based resin (C) (also referred to as impact-resistant styrene-based resin (C) or simply resin (C)) that contains a rubber component containing butadiene. By containing the impact resistant styrene-based resin (C), the impact resistance of the biaxially oriented styrene-based resin sheet of the present invention is further improved.

[0045] The impact-resistant styrene-based resin of component (C) used in the present invention may be a polystyrene-based resin containing a component such as rubber, and a styrene homopolymer containing a rubber component can be suitably used. Examples of the rubber component include polybutadiene, styrene-butadiene copolymer resin, polyisoprene, butadiene-isoprene copolymer resin, etc. The rubber component may be one in which the rubber component is dispersed independently in particulate form in polystyrene, which is the matrix resin, or one in which the rubber component is graft-polymerized onto polystyrene and dispersed in particulate form. The average particle size and shape of the rubber component in the rubber-modified styrene-based resin are not particularly limited, and the morphology of the rubber component is also not particularly limited, and examples thereof include a salami structure, a small particle size salami structure, a core-shell structure, and a structure in which a large particle size salami structure and a small particle size salami structure are mixed.

[0046] The melt flow rate of the impact resistant styrene resin (C) used in the present invention is preferably in the range of 1 to 10 g / 10 min., from the viewpoints of processing characteristics during stretch molding and thickness uniformity during secondary molding. The melt flow rate is a value measured according to JIS K7210 H condition (200°C, 5 kg).

[0047] The content of the rubber component in the impact resistant styrene-based resin (C) is preferably 2.0 to 15.0% by mass from the viewpoint of achieving both impact strength and processing characteristics during stretch molding.

[0048] Examples of commercially available impact-resistant styrene-based resins (B) that contain a rubber component in a styrene homopolymer that satisfies the above conditions include PSJ-Polystyrene 475D, H8117, H0103, and HT478 from PS Japan Co., Ltd.

[0049] <<(D):Inorganic granules>> The styrene-based resin composition in this embodiment contains spherical inorganic particles (D) having a particle size distribution width of 2.5 or less. By containing the spherical inorganic particles (D) having a particle size distribution width of 2.5 or less, the biaxially oriented styrene resin sheet of the present invention can obtain good blocking resistance.

[0050] Examples of inorganic granular material (D) include those having an average particle diameter of 1 to 20 μm. That is, in order to form suitable protrusions on the sheet surface and to exhibit the blocking resistance effect of the sheet and molded article, it is preferable that it is larger than 1 μm, and in order to maintain the transparency of the sheet and molded article, it is preferable that it is 20 μm or less. In terms of excellent balance of these performances and further excellent transparency of the secondary molded article, the range of 2 to 10 μm is more preferable. Here, the average particle diameter is the 50% volume average particle diameter measured by a laser diffraction method.

[0051] The particle size distribution of the inorganic particulate material (D) is a particle size distribution width of 2.5 or less. This allows the uniformity of the protrusions on the sheet surface to be well maintained. Here, the particle size distribution is the ratio of the 90% volume average particle diameter to the 10% volume average particle diameter measured by a laser diffraction method, that is, the value indicated by (90% volume average particle diameter) / (10% volume average particle diameter).

[0052] In addition, the inorganic granules (D) are spherical. When the inorganic granules are spherical in shape, the amount of the inorganic granules added to provide sufficient roughness to the sheet surface can be reduced, and the transparency of the sheet or molded product and the scratch resistance of the sheet are improved. Specifically, such spherical granules preferably have an aspect ratio calculated from the ratio of the major axis to the minor axis of 1 to 1.5.

[0053] Furthermore, in order to generate uniform protrusions on the sheet surface and further improve the appearance, shapes closer to perfect spheres are preferred, and specifically, an aspect ratio of 1 to 1.2 is more preferred.

[0054] In addition, the inorganic granular material (D) has a specific surface area of ​​400 m 2 / g or more is preferable because it has good affinity with the copolymer resin (A), the (meth)acrylic resin (B), and the impact-resistant styrene-based resin (C), can produce uniform protrusions on the sheet surface, and makes it difficult for whitening to occur at the corners of the molded product during deep drawing.

[0055] Also, the inorganic granules (D) are inorganic granules as described above. That is, when the granules are organic, the granules are easily deformed and buried during biaxial stretching and secondary molding, and the effect of blocking resistance is not exhibited. Here, the inorganic granules are not particularly limited, but include silica, glass beads, and those whose surfaces are chemically treated, but spherical silica mainly composed of silicon oxide is preferred because it is chemically stable, does not denature resin by catalytic action, and has good applicability of silicone oil to the sheet.

[0056] <<Content of each ingredient>> The mass ratio (A) / (B) of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) to the (meth)acrylic resin having an unsaturated carboxylic acid monomer unit (B) is in the range of (50 to 95% by mass) / (5 to 50% by mass). By adjusting the content of the (meth)acrylic resin (B) to be from 5% by mass to 50% by mass, the oil resistance of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) can be improved while maintaining the heat resistance effect. The mass ratio (A) / (B) is more preferably in the range of (60 to 80% by mass) / (20 to 40% by mass).

[0057] The impact resistant styrene-based resin (C) containing a rubber component containing butadiene is contained in an amount of 0.5 to 2.0 parts by mass, and preferably 1.2 to 1.8 parts by mass, per 100 parts by mass of the combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit.

[0058] The spherical inorganic particles (D) having a particle size distribution width of 2.5 or less are added to the composition, which is the raw material of the sheet, in a proportion in the range of 50 to 500 ppm. That is, the inorganic particles (D) are contained in an amount of 50 to 500 ppm per 100 parts by mass of the combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit. When the amount of inorganic particulate material (D) added is 50 ppm or more, the number of protrusions on the surface of the obtained biaxially oriented styrene-based resin sheet is at a sufficient level, and the releasability of the molded article is further improved. When the amount is 500 ppm or less, the transparency of the sheet and the molded article thereof is good. From the viewpoint of excellent balance of these performances, 50 to 300 ppm is more preferable, and 50 to 150 ppm is even more preferable.

[0059] <<Other ingredients>> In the present invention, the composition that is the material for producing the biaxially oriented styrene-based resin sheet contains the copolymer resin (A), the (meth)acrylic resin (B), the impact-resistant styrene-based resin (C), and the inorganic particles (D) as essential components as described above, but depending on the purpose, mineral oil, which is added as an internal lubricant, slip agent, antistatic agent, antioxidant, antibacterial agent, ultraviolet absorber, etc. can also be added.

[0060] <Biaxial stretching process> To produce the biaxially oriented styrene-based resin sheet of the present invention from the composition prepared as described above, for example, the composition is melt-kneaded, extruded through a die, and stretched in the machine and cross directions. The stretching method is not particularly limited, but a method in which an unstretched sheet extruded from a die is subjected to successive biaxial stretching using a speed difference between rolls and a tenter is preferred.

[0061] In the present invention, a styrene-based resin composition containing the specific components (A) to (D) in specific content ratios is biaxially stretched under specific stretching conditions to obtain a desired biaxially stretched styrene-based resin sheet.

[0062] <<Specific conditions for biaxial stretching>> In the present invention, the specific biaxial stretching conditions are as follows. This is an index expressed by heat shrinkage stress (ORS), and the heat shrinkage stress (ORS) in the machine direction (MD: machine direction of the sheet) is 0.4 to 0.9 MPa, and the heat shrinkage stress (ORS) in the cross direction (CD: cross direction of the sheet) is 0.4 to 0.9 MPa. The sheet is biaxially stretched so that the heat shrinkage stress in MD and the heat shrinkage stress in CD have the relationship expressed by the following formula (1). |ORS(MD)-ORS(CD)|≦0.2MPa (1) (The above ORS(MD) represents the heat shrinkage stress in the machine direction (MD), and the above ORS(CD) represents the heat shrinkage stress in the cross direction (CD).)

[0063] A biaxially oriented styrene-based resin sheet obtained by biaxially stretching a styrene-based resin composition containing the specific components (A) to (D) above in specific content ratios under specific stretching conditions can exhibit satisfactory results in terms of all requirements regarding oil resistance, heat resistance, transparency, impact resistance, blocking resistance, and moldability (hot plate compressed air moldability, etc.).

[0064] In the above biaxial stretching conditions, if the heat shrinkage stress (ORS) is too low, the impact resistance is poor, whereas if it is too high, problems such as breakage during stretching, inability to form a sheet, failure to obtain suitable molded products, and whitening occur. In particular, in the present invention, by incorporating the above-mentioned (B) component, the oil resistance is increased but the heat resistance is decreased. Therefore, if the heat shrinkage stress (ORS) is too high, the stretched sheet may whiten or crack during molding. For example, as shown in the comparative examples below, if the ORS in the transverse and longitudinal directions is less than 0.4, the impact resistance (dart impact) is poor, and if it is more than 0.9, the sheet shape cannot be maintained and a suitable molded product cannot be obtained. When the heat shrinkage stress (ORS) is within the above range, a good sheet can be formed, and the formability and impact resistance of the biaxially oriented sheet can be optimized.

[0065] The heat shrinkage stress (ORS) can be determined by measurement in accordance with ASTM D-1504. In addition, as shown in the above formula (1), the difference between the heat shrinkage stress (ORS) in the longitudinal direction and the heat shrinkage stress (ORS) in the transverse direction must be within 0.2 MPa. More preferably, |ORS(MD)-ORS(CD)|≦0.1 MPa; More preferably, |ORS(MD)-ORS(CD)|=0. In other words, it is preferable that the difference between the ORS in the transverse direction and the ORS in the longitudinal direction is small. In particular, it is more preferable to stretch the film at a longitudinal heat shrinkage stress (ORS) of 0.4 when the transverse heat shrinkage stress (ORS) is 0.4, or at a longitudinal heat shrinkage stress (ORS) of 0.6 when the transverse heat shrinkage stress (ORS) is 0.6, so that the transverse heat shrinkage stress (ORS) and the longitudinal heat shrinkage stress (ORS) have the same numerical value.

[0066] The heat shrinkage stress (ORS) is more preferably from 0.6 to 0.9 MPa, and further preferably from 0.7 to 0.9 MPa.

[0067] <Method of manufacturing biaxially oriented styrene-based resin sheet> The biaxially oriented styrene-based resin sheet of the present invention is A styrene-methacrylic acid-methyl methacrylate copolymer resin (A) obtained by copolymerizing styrene, methacrylic acid, and methyl methacrylate; (B) a (meth)acrylic resin having an unsaturated carboxylic acid monomer unit; an impact-resistant styrene-based resin (C) containing a rubber component containing butadiene; The styrene resin composition contains (D) and spherical inorganic particles having a particle size distribution width of 2.5 or less in the specific ratio described above, and the styrene resin composition is subjected to a biaxial stretching treatment under biaxial stretching conditions in which the index, expressed as the heat shrinkage stress (ORS), is in the range of 0.4 to 0.9 MPa in the longitudinal direction and in the range of 0.4 to 0.9 MPa in the transverse direction.

[0068] <Characteristics of biaxially oriented styrene resin sheet> [Sheet transparency (HAZE)] The transparency (haze) of the sheet is determined by a haze measurement value conforming to JIS K7136. The sheet according to the present invention preferably has a haze value of less than 3.5%, more preferably less than 2.5%.

[0069] [Sheet impact resistance (dirt impact)] The impact resistance (dirt impact) of the sheet is determined by measuring the dirt impact strength in accordance with JIS K7124. The seat according to the present invention preferably has a dart impact value of 1.0 J or more.

[0070] [Secondary forming temperature range for sheets] The secondary forming temperature range of the molded product (drawing ratio: 0.3) formed from the sheet is preferably 5° C. or higher. The secondary forming temperature range is obtained by the following measurement. The drawing ratio is the value obtained by dividing the depth by the short side. -Measurement method- The sheet is thermoformed using a mold (length 94 mm, width 94 mm, depth 30 mm, corners: 2R) in a hot plate compressed air molding machine, and the resulting molded product is visually observed to measure the range from the temperature at which shape reproducibility is good (lower molding temperature limit) to the temperature at which raindrops begin to appear (upper molding temperature limit).

[0071] [Oil resistance of sheet] The oil resistance of the sheet can be evaluated by the whitening temperature or whitening time, and the whitening temperature obtained by the following measurement is preferably 100° C. or higher, and more preferably 105° C. or higher. The whitening time obtained by the following measurement is preferably 1 hour or higher, and more preferably 4 hours or higher. -Whitening temperature measurement method- The sheet is molded into a container of a specified shape, and MCT (medium-chain triglyceride) oil is added and heated for a certain period of time (5 minutes), at which the temperature at which it turns white is measured. -How to measure bleaching time- The sheet is molded into a container of a specified shape, and MCT (medium-chain fatty acid) oil is added and kept at a constant temperature (85°C), and the time it takes for the sheet to whiten is measured.

[0072] [2% heat shrinkage temperature of sheet] The 2% heat shrinkage temperature of the sheet, at which the heat shrinkage rate of the sheet reaches 2%, is preferably 110° C. or higher, and more preferably 115° C. or higher, as determined by the following measurement. -Measurement method- A sheet cut to 10 cm x 10 cm is left in a hot air dryer for 10 minutes, and the temperature at which the thermal shrinkage rate reaches 2% is measured.

[0073] [Sheet blocking resistance] The unwinding property (blocking property) from a rolled sheet is measured, and it is preferable that the sheet according to the present invention can be unwound without any resistance.

[0074] <Biaxially oriented styrene resin sheet processing> The surface of the biaxially oriented styrene resin sheet of the present invention can be appropriately coated with an anti-fog agent, an antistatic agent, and silicone oil, but in the present invention, it is preferable to coat the silicone oil, since the effects of the present invention are particularly prominent, that is, the effects of improving the blocking property of the sheet, the peelability of the secondary molded product, and the improvement of scratch resistance are particularly prominent. In particular, in the present invention, even if the silicone oil is coated on the surface of the sheet, the appearance of the secondary molded product is excellent.

[0075] The amount of silicone oil to be applied is not particularly limited, but in order to achieve the above-mentioned sheet performance, it is necessary to apply 5 to 100 mg / m on at least one side. 2 It is preferable to apply the silicone oil in the range of 100 to 200 mm. In addition, as the silicone oil, polydimethylsiloxane is preferable because it is unlikely to attack the sheet surface during high temperature molding.

[0076] In addition, when the secondary molded product is used as a food packaging material, it is preferable to coat one side with silicone oil and the other side with an anti-fog agent. In the present invention, even if one surface is coated with silicone oil and the other surface is coated with an anti-fog agent, various effects such as improvement of blocking property of the sheet, peelability of the secondary molded product, improvement of scratch resistance, etc. are maintained, and the appearance of the secondary molded product is excellent. The amount of anti-fog agent applied is not particularly limited, but in order to achieve the above-mentioned sheet performance, it is necessary to apply 10 to 100 mg / m on at least one side. 2 It is preferable to apply the antifogging agent in the range of 100 to 200°. Known surfactants can be used as the antifogging agent, and polyhydric alcohol fatty acid esters (sucrose fatty acid esters, polyglycerin fatty acid esters, etc.) are particularly preferable, and mixtures of two or more of polyhydric alcohol fatty acid esters and water-soluble polymers (polyvinyl alcohol, polyoxyethylene polyoxypropylene block copolymer resins, etc.) or polysaccharides (methylcellulose, cyclodextrin, etc.) are particularly preferable. When applying these, it is preferable to perform corona treatment so that the contact angle of the sheet surface with water is 20° to 80°.

[0077] The biaxially oriented styrene-based resin sheet of the present invention is preferably one having a thickness of 0.1 to 0.5 mm after stretching in terms of ease of secondary molding, and more preferably has a thickness of 0.2 to 0.4 mm. If necessary, the sheet may be laminated with the same or different thermoplastic resins by coextrusion, dry lamination, or the like.

[0078] The biaxially oriented styrene-based resin sheet of the present invention is not particularly limited in its applications, but from the viewpoint of excellent oil resistance, heat resistance, transparency, impact resistance, blocking resistance, and moldability (hot plate compressed air moldability, etc.) of the molded product, the biaxially oriented styrene-based resin sheet of the present invention can be suitably used as a food packaging material by secondary molding into a desired container shape by a hot plate compressed air vacuum molding method (direct heating method), a compressed air molding method, or a vacuum compressed air molding method (indirect heating method). In particular, it is suitable for foods to be heated in a microwave oven, such as lunch boxes and side dishes, and foods containing ingredients for steamed dishes, etc., to be heated and cooked in a microwave oven. EXAMPLES

[0079] The present invention will be described in more detail below using examples. Of course, the present invention should not be limited to the scope of these examples. In the following, "parts" and "%" are by mass unless otherwise specified.

[0080] [GPC measurement] The molecular weight of the resin was measured by the following method. Sample preparation: Approximately 0.05% of the resin was dissolved in tetrahydrofuran. (Measurement conditions) Equipment: TOSHOH HLC-8220GPC (gel permeation chromatography) Column: superHZM-H Temperature: 40℃ Carrier: THF 0.35ml / min Detector: RI, UV: 254nm Calibration curve: Standard PS manufactured by TOSOH

[0081] [Vicat softening point] The Vicat softening point of the copolymer resin (A) was measured in accordance with JIS K7206.

[0082] [Sheet transparency (HAZE)] The transparency of the sheet was evaluated by measuring the haze in accordance with JIS K7136. -Evaluation criteria- Less than 2.5% △ 2.5% or more and less than 3.5% × 3.5% or more

[0083] [Sheet ORS] The heat shrinkage stress (ORS) of the sheets was measured according to ASTM D-1504.

[0084] [Sheet impact resistance (dirt impact)] The impact resistance of the sheet was evaluated by measuring the dart impact strength in accordance with JIS K7124. -Evaluation criteria- ◎ 1.6 or higher ○ 1.0 or more and less than 1.6 × Less than 1.0

[0085] [Secondary forming temperature range for sheets] The sheet was thermoformed using a mold (length 94 mm, width 94 mm, depth 30 mm, corners: 2R) in a hot plate compressed air molding machine, and the resulting molded product was visually observed to measure the range from the temperature at which good mold reproducibility was achieved (lower molding temperature limit) to the temperature at which raindrops began to appear (upper molding temperature limit). -Evaluation criteria- 〇 5℃ or higher × Less than 5℃

[0086] [Sheet oil resistance: whitening temperature] The sheet was molded into a container of a specified shape, and MCT (medium chain triglyceride) oil was added and heated for a certain period of time (5 minutes), and the temperature at which it turned white was measured. -Evaluation criteria- ◎ 105℃ or higher ○ 100℃ or higher but lower than 105℃ × Less than 100℃

[0087] [Oil resistance of sheet: whitening time] The sheet was molded into a container of a specified shape, and MCT (medium chain triglyceride) oil was added and kept at a constant temperature (85°C), and the time it took for the sheet to whiten was measured. -Evaluation criteria- ◎ 4 hours or more ○ 1 hour or more but less than 4 hours × Less than 1 hour

[0088] [2% heat shrinkage temperature of sheet] A sheet cut to a size of 10 cm x 10 cm was left in a hot air dryer for 10 minutes, and the temperature at which the thermal shrinkage of the sheet reached 2% was measured. -Evaluation criteria- 〇 115℃ or higher △ 110℃ or higher but lower than 115℃ × Less than 110℃

[0089] [Sheet blocking resistance] The unwinding property (blocking property) from the rolled sheet was measured. -Evaluation criteria- ○ It can be unwound without any resistance × There is resistance when unwinding, or the films do not separate when unwinding

[0090] (Synthesis Example 1) Synthesis of styrene-methacrylic acid-methyl methacrylate copolymer resin (A-1) A polymerization raw material composition liquid consisting of 77 parts of styrene, 5 parts of methacrylic acid, 3 parts of methyl methacrylate, 15 parts of ethylbenzene, and 0.025 parts of 1,1-bis(t-butylperoxy)cyclohexane was continuously fed at a rate of 1.1 liters / hour to a polymerization apparatus consisting of a 4-liter complete mixing reactor, then a 2-liter laminar flow reactor, and then to a devolatilizer connected to a single-screw extruder for removing volatile matters such as unreacted monomers and polymerization solvents, in sequence to prepare a resin. The polymerization reaction conditions in the polymerization process were a polymerization temperature of 118 to 128 ° C for the complete mixing reactor and a temperature of 121 to 143 ° C for the laminar flow reactor. The polymer content in the final polymerization liquid was obtained by drying the polymerization liquid at 215 ° C and a reduced pressure of 2.5 kPa for 30 minutes to obtain a copolymer resin (A-1). The resin composition, weight average molecular weight, and Vicat softening point of A-1 are shown in Table 1.

[0091] (Synthesis Examples 2 to 4) Synthesis of styrene-methacrylic acid-methyl methacrylate copolymer resins (A-2 to A-4) The amounts and conditions of styrene, methacrylic acid, and methyl methacrylate were adjusted to obtain the resin compositions A-2 to A-4 in Table 1. The obtained resin pellets were fed into a single-screw extruder, and the isoaliphatic primary alcohol having 18 carbon atoms (manufactured by Nissan Chemical Industries, Ltd., trade name Fine Oxocol 180) of the type shown in Table 1 was kneaded in the amount shown in Table 1 to obtain copolymer resins A-2 to A-4.

[0092] [Table 1]

[0093] Preparation of methyl methacrylate-methyl acrylate copolymer resin (B) As the methyl methacrylate-methyl acrylate copolymer resin (B), the following commercially available resin was used. B-1: Asahi Kasei Corporation, product name "Delpet 80N", MFR 2.0 B-2: Asahi Kasei Corporation, product name "Delpet 80NB", MFR 0.5

[0094] Preparation of impact-resistant styrene resin (C) As the impact resistant styrene-based resin (C), the following commercially available resin was used. C-1: PS Japan, product name "PSJ Polystyrene H0103", MFR 2.6 C-2: PS Japan, product name "PSJ Polystyrene H8117", MFR 1.3

[0095] Preparation of inorganic granules (D) As the inorganic particulate material (D), the following spherical silica was used. D-1: Fuji Silysia Chemical Co., Ltd., product name "Silosphere C-1504" (average particle diameter 4.5 μm, particle size distribution width 1.9, specific surface area 520 m 2 / g, spherical silica with an aspect ratio of 1 to 1.1) D-2: Fuji Silysia Chemical Co., Ltd., product name "Silosphere C-1510" (average particle diameter 10.0 μm, particle size distribution width 2.3, specific surface area 520 m 2 / g, spherical silica with an aspect ratio of 1 to 1.1) D-3: DSL. Japan, product name "Carplex CS-7" (average particle size 6.0 μm, amorphous silica)

[0096] Example 1 90 parts by mass of copolymer resin (A-3), 10 parts by mass of copolymer resin (B-1), impact-resistant styrene-based resin (C-1) were melt-mixed in an extruder so that 1.0 part by mass per 100 parts by mass of copolymer resin (A-1) and copolymer resin (B-1) was added, and inorganic granular material (D-1) was melt-mixed in an extruder so that 500 ppm per 100 parts by mass of copolymer resin (A-1) and copolymer resin (B-1) was added. This was extruded from a T-die to prepare a sheet with a thickness of about 1.5 mm. Next, the sheet extruded from the T-die was continuously stretched using a sequential biaxial stretching device by appropriately changing the roll temperature and the transverse stretching machine temperature so that the heat shrinkage stress (ORS) in the transverse and longitudinal directions was 0.8 MPa, respectively, to obtain a sheet with a thickness of 0.25 mm. Next, corona treatment was performed on both surfaces of the sheet, and 20 to 30 mg / m was applied to one surface of the sheet. 2 Silicone oil (Shin-Etsu Chemical, product name "KM-9373A") is applied to the other surface at 30 to 40 mg / m 2 Sucrose laurate (Rikemal A, product name, manufactured by Riken Vitamin) was applied to the sheet. The corona treatment intensity was adjusted by adjusting the output of the high frequency power source so that the contact angle between the silicone oil-coated surface and the water was 60-70°, and the opposite surface was 35-45°. The evaluation results of the sheet are shown in Table 2-1. In Table 2-1 (Tables 2-2, 3-1, and 3-2 as well), St-MAA-MMA copolymer resin refers to styrene-methacrylic acid-methyl methacrylate copolymer resin, MA / MMA copolymer resin refers to methyl methacrylate-methyl acrylate copolymer resin, and HIPS resin refers to an impact-resistant styrene-based resin that contains a rubber component including butadiene.

[0097] (Examples 2 to 14) The sheets of Examples 2 to 14 were produced in the same manner as in Example 1, except that the content ratio of each component and the heat shrinkage stress (ORS) in the transverse and longitudinal directions were changed as shown in Table 2-1 or Table 2-2. The sheets of Examples 2 to 14 were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 2-1 or Table 2-2.

[0098] (Comparative Examples 1 to 14) The sheets of Comparative Examples 1 to 14 were produced in the same manner as in Example 1, except that the content ratio of each component and the heat shrinkage stress (ORS) in the transverse and longitudinal directions were changed as shown in Table 3-1 or Table 3-2. The sheets of Comparative Examples 1 to 14 were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 3-1 or Table 3-2. In Table 3-1 or Table 3-2, "-" indicates that the sheet could not be formed well and an appropriate molded product could not be obtained, so that the evaluation test for oil resistance could not be performed.

[0099] [Table 2-1]

[0100] [Table 2-2]

[0101] [Table 3-1]

[0102] [Table 3-2] [Industrial Applicability]

[0103] The biaxially oriented styrene-based resin sheet of the present invention can show satisfactory results in all requirements, such as oil resistance, heat resistance, transparency, impact resistance, and moldability (hot plate compressed air moldability, etc.). Therefore, it can be suitably used as a food packaging material by secondary molding into a desired container shape. In particular, it can be suitably used for foods to be heated in a microwave oven, such as lunch boxes and side dishes, and foods containing ingredients for steamed dishes, etc., to be heated and cooked in a microwave oven, and it plays a major role in the industrial world.

Claims

1. A styrene-methacrylic acid-methyl methacrylate copolymer resin (A) obtained by copolymerizing styrene, methacrylic acid, and methyl methacrylate; (B) a (meth)acrylic resin having an unsaturated carboxylic acid monomer unit; an impact-resistant styrene-based resin (C) containing a rubber component containing butadiene; A biaxially oriented styrene-based resin sheet obtained by biaxially stretching a styrene-based resin composition containing spherical inorganic particles (D) having a particle size distribution width of 2.5 or less, a mass ratio (A) / (B) of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) to the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit is in the range of (50 to 95 mass%) / (5 to 50 mass%), the impact resistant styrene-based resin (C) containing a rubber component containing butadiene is contained in an amount of 0.5 to 2.0 parts by mass per 100 parts by mass of a combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid-based monomer unit, the inorganic granular material (D) is contained in an amount of 50 to 500 ppm relative to 100 parts by mass of a combined resin of the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) and the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit; The conditions of the biaxial stretching are expressed as an index represented by heat shrinkage stress (ORS), the heat shrinkage stress (ORS) in the machine direction (MD: machine direction of the sheet) is 0.4 to 0.9 MPa, the heat shrinkage stress (ORS) in the transverse direction (CD: transverse direction of the sheet) is 0.4 to 0.9 MPa, and the heat shrinkage stress in the MD and the heat shrinkage stress in the CD have a relationship represented by the following formula (1): |O. R. S(MD)-O. R. S(CD)|≦0.2MPa (1) (The above ORS (MD) represents the heat shrinkage stress in the machine direction (MD), and the above ORS (CD) represents the heat shrinkage stress in the cross direction (CD).)

2. The biaxially oriented styrene-based resin sheet according to claim 1, wherein the biaxially oriented styrene-based resin sheet is thermoformed using a mold (length 94 mm, width 94 mm, depth 30 mm, corners: 2R) by a hot plate compressed air molding machine, the obtained molded product is visually observed, and the range from the temperature at which the mold reproducibility becomes good (lower limit molding temperature) to the temperature at which raindrops occur (upper limit molding temperature) is measured, and the measured value of the secondary molding temperature range is 5°C or more.

3. The biaxially oriented styrene-based resin sheet according to claim 1, wherein the biaxially oriented styrene-based resin sheet has a dart impact measurement value according to JIS K7124 of 1 J or more.

4. 2. The biaxially oriented styrene-based resin sheet according to claim 1, wherein the (meth)acrylic resin (B) having an unsaturated carboxylic acid monomer unit has an MFR (metroflow rate) of 1.0 g / 10 min or more.

5. The biaxially oriented styrene-based resin sheet according to claim 1, wherein the styrene-methacrylic acid-methyl methacrylate copolymer resin (A) has a Vicat softening point according to JIS K7206 of 115°C or higher.

6. The biaxially oriented styrene-based resin sheet according to claim 1, wherein the biaxially oriented styrene-based resin sheet is cut into a size of 10 cm x 10 cm, and left in a hot air dryer for 10 minutes to measure the temperature at which the heat shrinkage rate becomes 2%, and the measured value of the 2% heat shrinkage temperature is 115°C or higher.

7. A molded article obtained by secondary molding the biaxially oriented styrene-based resin sheet according to claim 1.

8. The molded article according to claim 7, which is a food packaging material.

9. The molded article according to claim 8, which is a food packaging material for heating in a microwave oven.

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