JP2022003122A - Biaxially oriented sheet and molding of the same - Google Patents

Abstract

To provide a biaxially oriented sheet which improves resistance against microwave oven heating property while holding toughness, and is composed of a styrenic resin composition, and a molding of the same.SOLUTION: A biaxially oriented sheet is composed of a styrenic resin composition containing a styrenic polymer (A) and a styrene-butadiene block copolymer (B), in which an average major axis of a butadiene block domain observed from a vertical section of the biaxially oriented sheet is 0.1-1.0 μm, and an average aspect ratio is 3 to 12.SELECTED DRAWING: None

Description

The present invention relates to a biaxially stretched sheet made of a styrene-based resin composition that can be suitably used for food packaging containers, and a molded product thereof.

Stretched sheets made of polystyrene resin, particularly biaxially stretched sheets, are secondary molded and are mainly used in large quantities in molded products such as lightweight containers for foods because they are excellent in transparency and rigidity. Further, attempts have been made to impart heat resistance to polystyrene, which is a raw material, for the purpose of direct contact with boiling water and use for heating in a microwave oven. For example, styrene-based heat-resistant resins such as a styrene-acrylic acid copolymer, a styrene-methacrylic acid copolymer, and a styrene-maleic anhydride copolymer improve heat resistance without impairing transparency and rigidity. ..

In recent years, in the field of chilled delicatessen, where demand is expanding at convenience stores, the diversification of ingredients is progressing, and the number of products containing oily ingredients is increasing. During delivery and display of products, foodstuffs may adhere to the lid material by vibrating or tilting. However, when microwave oven heating is performed with the oily food material attached to the polystyrene resin lid material, there is a problem that the lid material is liable to be perforated or deformed.

Further, in order to shorten the working time at a convenience store or the like, a high-power microwave oven that can be heated in a shorter time is used. Therefore, there is a demand for a food packaging container that is less likely to be perforated or deformed even in a more severe microwave oven heating.

For this reason, the biaxially stretched sheet of styrene-based heat-resistant resin is prevented from being perforated or deformed when oily foodstuffs are attached to the lid material and microwave oven heating is performed at high output. , It is required to improve the heat resistance of the microwave oven.

Conventionally, some attempts to improve the microwave oven heating resistance of the styrene-based heat-resistant resin sheet have been known. As a method for improving the heat resistance, a method for increasing the methacrylic acid content of the styrene-methacrylic acid copolymer can be mentioned. However, if the methacrylic acid content is increased, the mechanical strength becomes insufficient, and there is a concern that the quality of the molded product may be deteriorated due to poor die cutting or generation of dust when the sheet is secondarily molded. Further, Patent Document 1 and Patent Document 2 propose a method of laminating polypropylene or polyethylene terephthalate having excellent oil resistance on a styrene resin sheet to improve oil resistance. However, in the methods of Patent Document 1 and Patent Document 2, since a material incompatible with polystyrene is used, opacity and a decrease in strength are caused at the time of recycling, and it cannot be expected to be used for food containers.

Japanese Unexamined Patent Publication No. 4-145155 Japanese Unexamined Patent Publication No. 6-87191

The present invention has been made in view of the above circumstances. That is, an object of the present invention is to provide a biaxially stretched sheet made of a styrene-based resin composition having improved toughness and microwave oven heating resistance, and a molded product thereof.

In order to solve the above problems, the present inventors have repeatedly studied various configurations capable of improving the microwave oven heating resistance while maintaining the toughness of the biaxially stretched sheet made of a styrene resin composition. rice field. As a result, a styrene-based resin composition composed of the styrene-based polymer (A) and the styrene-butadiene block copolymer (B) is adopted, and by stretching under specific stretching conditions, the styrene-based resin composition is formed. It has been found that the toughness and microwave heat resistance of the biaxially stretched sheet can be appropriately improved. The present invention has been completed based on such findings.

That is, the present invention has the following configuration.
(1) A biaxially stretched sheet comprising a styrene-based resin composition containing a styrene-based polymer (A) and a styrene-butadiene block copolymer (B), wherein the biaxially stretched sheet has a vertical cross section. A biaxially stretched sheet having a more observed average major axis of butadiene block domains of 0.1 to 1.0 μm and an average aspect ratio of 3 to 12.
(2) The above-mentioned (1), wherein the average major axis of the butadiene block domain observed from the cross section in the width direction of the biaxially stretched sheet is 0.1 to 1.0 μm, and the average aspect ratio is 3 to 12. Biaxial stretching sheet.
(3) The biaxially stretched sheet according to the above (1) or (2), wherein the orientation relaxation stress in both the vertical direction and the horizontal direction of the biaxially stretched sheet is 0.1 to 0.6 MPa.
(4) The styrene-based polymer (A) is a styrene-methacrylic acid copolymer and contains 86 to 97% by mass of a styrene monomer unit and 3 to 14% by mass of a methacrylic acid monomer unit. The biaxially stretched sheet according to any one of (1) to (3) above.
(5) The above-mentioned (1) to (4), wherein the styrene-butadiene block copolymer (B) is a styrene-butadiene block copolymer containing 10% by mass or more and less than 40% by mass of a butadiene monomer unit. The biaxially stretched sheet according to any one item.
(6) The biaxially stretched sheet according to any one of (1) to (5) above, wherein the styrene resin composition contains 0.01 to 5% by mass of a butadiene monomer unit.
(7) The styrene-based resin composition contains 91 to 99% by mass of the styrene-based polymer (A) and 1 to 9% by mass of the styrene-butadiene block copolymer (B). ) Is described in any one of the above items.
(8) The biaxially stretched sheet according to any one of (1) to (7) above, wherein the biaxially stretched sheet has a thickness of 0.1 to 0.4 mm.
(9) The biaxially stretched sheet according to any one of (1) to (8) above, wherein the absolute value of the difference between the vertical and horizontal orientation relaxation stresses of the biaxially stretched sheet is less than 0.20 MPa. Molded product consisting of.
(10) A molded product comprising the biaxially stretched sheet according to any one of (1) to (9) above.
(11) The molded product according to (10) above, which is a food packaging container for heating in a microwave oven.

The biaxially stretched sheet made of the styrene-based resin composition of the present invention and its molded product have improved microwave oven heating resistance while maintaining toughness.

It is a magnified photograph by a transmission electron microscope observed from the vertical cross section of a biaxially stretched sheet. It is a magnified photograph by a transmission electron microscope observed from the cross section in the width direction of a biaxially stretched sheet.

Embodiments of the present invention will be described below. However, the embodiments of the present invention are not limited to the embodiments described below.

The biaxially stretched sheet of the present embodiment comprises a styrene-based resin composition containing a styrene-based polymer (A) and a styrene-butadiene block copolymer (B). Further, the biaxially stretched sheet can be obtained by extruding the styrene resin composition and biaxially stretching the obtained unstretched sheet.

In the biaxially stretched sheet of the present embodiment, the vertical direction (MD direction) is the flow direction at the time of sheet film formation, and the horizontal direction (TD direction) is perpendicular to the flow direction at the time of sheet film formation. The direction.
Hereinafter, each component constituting the styrene-based resin composition will be described.

(Styrene-based polymer (A))
The styrene-based polymer (A) is a polymer containing styrene as a main monomer unit. Examples of the monomer copolymerized with styrene in order to obtain a styrene-based polymer having excellent heat resistance include acrylic acid, methacrylic acid, maleic anhydride and the like. Among these copolymers, a styrene-methacrylic acid copolymer is preferable from the viewpoint of processability and resin performance.

In the styrene-methacrylic acid copolymer, the copolymerization ratio of styrene and methacrylic acid can be variously set according to the desired heat resistance, mechanical strength, and the like. The styrene-methacrylic acid copolymer contains 86 to 97% by mass of styrene monomer unit and methacrylic acid because a resin having an excellent balance of heat resistance, mechanical strength, and transparency when made into a sheet can be easily obtained. It is preferable to contain 3 to 14% by mass of the acid monomer unit. If the content of the methacrylic acid monomer unit is less than 3% by mass, the heat resistance is insufficient, and holes are easily formed and deformation is likely to occur when heating in a microwave oven. The content of the methacrylic acid monomer unit is more preferably 6% by mass or more, still more preferably 8% by mass or more. On the other hand, when the content of the methacrylic acid monomer unit exceeds 14% by mass, the fluidity at the time of film formation and the appearance deterioration due to the generation of gel are likely to occur. The content of the methacrylic acid monomer unit is more preferably 12% by mass or less, still more preferably 10% by mass or less.

Further, the styrene-methacrylic acid copolymer may be appropriately copolymerized with a monomer other than styrene and methacrylic acid, if necessary, as long as the effects of the invention are not impaired. The content of the other monomer is preferably 10% by mass or less, more preferably 5% by mass or less, and further preferably 3% by mass or less. If the content of the other monomer exceeds 10% by mass, the ratio of styrene or methacrylic acid decreases, and sufficient transparency, mechanical strength and heat resistance may not be obtained. Examples of other monomers include methyl methacrylate, maleic anhydride and the like.

The weight average molecular weight (Mw) of the styrene-based polymer (A) is preferably 120,000 to 250,000, more preferably 140,000 to 220,000, and even more preferably 150,000 to 200,000. When the weight average molecular weight is less than 120,000, sheet drawdown, deterioration of film forming property such as neck-in, insufficient stretch orientation, and surface roughness due to hot plate contact during container molding are likely to occur. On the other hand, when the weight average molecular weight exceeds 250,000, unevenness in thickness during film formation due to a decrease in fluidity, deterioration in the appearance of a sheet such as a die line, and poor molding during container molding are likely to occur.

The ratio Mw / Mn of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the styrene-based polymer (A) is preferably 2.0 to 3.0, more preferably 2.2. ~ 2.8. When Mw / Mn exceeds 3.0, surface roughness due to hot plate contact during container molding tends to occur. On the other hand, when Mw / Mn is less than 2.0, uneven thickness during film formation due to a decrease in fluidity and molding defects during container molding are likely to occur. The ratio Mz / Mw of the Z average molecular weight (Mz) to Mw is preferably 1.5 to 2.0, more preferably 1.6 to 1.9. When Mz / Mw is less than 1.5, the drawdown of the sheet, the deterioration of the film-forming property such as neck-in, and the lack of stretch orientation are likely to occur. On the other hand, when Mz / Mw exceeds 2.0, the thickness unevenness at the time of film formation due to the decrease in fluidity and the deterioration of the sheet appearance such as die lines are likely to occur.

For the above-mentioned number average molecular weight (Mn), weight average molecular weight (Mw), and Z average molecular weight (Mz), the molecular weight at each elution time is calculated from the elution curve of monodisperse polystyrene by the following method by GPC measurement. However, it is calculated as a polystyrene-equivalent molecular weight.
Model: Tosoh HLC-8320GPC
Column: Showa Denko Co., Ltd. Shodex GPCKF-404 4 connected in series Mobile phase: Tetrahydrofuran Sample concentration: 0.2% by mass
Temperature: Oven 40 ° C, injection port 35 ° C, detector 35 ° C
Detector: Differential refractometer

Examples of the polymerization method of the styrene-based polymer (A) include known polymerization methods such as a massive polymerization method, a solution polymerization method, and a suspension polymerization method, which are industrialized with polystyrene or the like. In terms of quality and productivity, the bulk polymerization method and the solution polymerization method are preferable, and continuous polymerization is preferable. As the solvent, for example, alkylbenzenes such as benzene, toluene, ethylbenzene and xylene, ketones such as acetone and methyl ethyl ketone, and aliphatic hydrocarbons such as hexane and cyclohexane can be used.

When the styrene-based polymer (A) is polymerized, a polymerization initiator and a chain transfer agent can be used, if necessary. An organic peroxide can be used as the polymerization initiator. Specific examples of the organic peroxide include benzoyl peroxide, t-butylperoxybenzonate, 1,1-di (t-butylperoxy) cyclohexane, and 1,1-bis (t-butylperoxy) -3. , 3,5-trimethylcyclohexane, 2,2-bis (4,4-di-t-butylperoxycyclohexyl) propane, t-butylperoxyisopropylcarbonate, dicumyl peroxide, t-butylcumyl peroxide, t -Butylperoxyacetate, t-butylperoxy-2-ethylhexanoate, polyethertetrakis (t-butylperoxycarbonate), ethyl-3,3-di (t-butylperoxy) butyrate, t-butyl Peroxyisobutyrate and the like can be mentioned. Specific examples of the chain transfer agent include aliphatic mercaptans, aromatic mercaptans, pentaphenylethanes, α-methylstyrene dimers, terpinolene and the like.

(Styrene-butadiene block copolymer (B))
The styrene-butadiene block copolymer (B) is a copolymer containing styrene and butadiene as a main monomer unit. The present inventors have found that the toughness of the styrene-based polymer (A) can be appropriately improved by blending a predetermined amount of the styrene-butadiene block copolymer (B) with the styrene-based polymer (A). The copolymerization ratio of styrene and butadiene can be variously set according to the desired heat resistance, toughness and the like. The styrene-butadiene block copolymer (B) preferably contains 10% by mass or more and less than 40% by mass of a butadiene monomer unit from the viewpoint of obtaining a resin having an excellent balance between heat resistance and toughness. When the content of the butadiene monomer unit is within the above range, the styrene-butadiene block copolymer (B) has excellent compatibility with the styrene-based polymer (A), and the styrene-based resin composition is transparent. It is possible to improve the toughness without impairing the sex. When the content of the butadiene monomer unit of the styrene-butadiene block copolymer (B) is less than 10% by mass, the effect of improving the toughness becomes small. The content of the butadiene monomer unit is more preferably 15% by mass or more, still more preferably 20% by mass or more. On the other hand, when the content of the butadiene monomer unit is 40% by mass or more, the compatibility with the styrene-based polymer (A) is lowered, and the transparency of the sheet is lowered. The content of the butadiene monomer unit is more preferably 30% by mass or less.

The styrene-butadiene block copolymer (B) is a block copolymer of styrene and butadiene. As the block composition of the block copolymer, when the polystyrene block is S and the polybutadiene block is B, SB, SBS, and SBSB are preferable, and SBS is more preferable. preferable. Further, when the polystyrene block is S and the tapered block of styrene and butadiene is SB, S-SB and S-SB-S are preferable.

When the styrene-butadiene block copolymer (B) is a block copolymer of styrene and butadiene, the polymerization method is a living anion polymerization method using an organic lithium compound as a polymerization initiator in an organic solvent, or the living. An anionic polymerization method followed by a coupling reaction using a polyfunctional coupling agent can be used. As the organic solvent used in these polymerization methods, low molecular weight organic compounds composed of carbon and hydrogen are used, and specifically, aliphatic hydrocarbons such as butane, pentane, hexane, isopentan, heptane, octane, and isooctane; Examples thereof include alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane and ethylcyclohexane, aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylene, and tetrahydrofuran and the like. The organic lithium compound is a compound in which one or more lithium atoms are bonded in a molecule, and specifically, ethyl lithium, n-propyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, tert. -An example is butyllithium.

In the case of polymerizing a block having a so-called tapered structure (tapered block) in which styrene and butadiene continuously change the ratio in a block copolymer, styrene and butadiene may be added to the reaction system at the same time. ..

Further, in the case of polymerizing a block having a random structure of styrene and butadiene (random block) in a block copolymer, styrene and butadiene may be charged into the reaction system little by little at the same time. Further, when polymerizing the random block, it is preferable to keep the temperature of the reaction system constant at 40 ° C. to 120 ° C. in order to make the reaction as uniform as possible. Further, when forming a tapered structure or a random structure, a randomizing agent can be appropriately added to the reaction system in order to further enhance the randomness of styrene and butadiene.

The randomizing agent is a molecule having polarity, and amines, ethers, thioethers, phosphoramides, alkylbenzene sulfonates, potassium or sodium alkoxides and the like can be used. Examples of amines include tertiary amines such as trimethylamine, triethylamine, tetramethylethylenediamine, and cyclic tertiary amines. Examples of ethers include dimethyl ether, diethyl ether, diphenyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, and tetrahydrofuran. Other randomizing agents include triphenylphosphine, hexamethylphosphoramide, potassium or sodium alkylbenzene sulfonate, butoxide of potassium or sodium, and the like.

The weight average molecular weight (Mw) of the styrene-butadiene block copolymer (B) is preferably 100,000 to 200,000, more preferably 120,000 to 180,000. The method for measuring the weight average molecular weight (Mw) of the styrene-butadiene block copolymer (B) can be measured according to the above-mentioned method for measuring the weight average molecular weight (Mw) of the styrene-based polymer (A).

(Styrene-based resin composition)
The styrene-based resin composition of the present embodiment preferably has a butadiene monomer unit content of 0.01 to 5% by mass, more preferably 0.1 to 2.5% by mass, and 0. .2 to 1.5% by mass is more preferable. The butadiene monomer unit is mainly derived from the butadiene monomer unit of the styrene-butadiene block copolymer (B). When the content of the butadiene monomer unit in the styrene resin composition is in this range, the toughness can be improved while maintaining the transparency of the biaxially stretched sheet of the styrene resin composition. Here, the content of the butadiene monomer unit in the styrene resin composition can be measured by the following iodine monochloride method.
The styrene resin composition is dissolved in chloroform, iodine monochloride is added to react with the double bond in the rubber component, then potassium iodide is added to change the remaining iodine monochloride to iodine, and sodium thiosulfate is used. Back titrate.

The styrene-based resin composition of the present embodiment preferably contains 91 to 99% by mass of the styrene-based polymer (A) and 1 to 9% by mass of the styrene-butadiene block copolymer (B). If the styrene-butadiene block copolymer (B) exceeds 9% by mass, the transparency of the sheet may be impaired and the contents may not be clearly visible when used as a lid material. On the other hand, if the amount of the styrene-butadiene block copolymer (B) is less than 1% by mass, the effect of improving the toughness is insufficient, and there is a risk that breakage during sheet film formation or die cutting failure during secondary molding may occur. There is. The styrene-based resin composition is more preferably 92 to 99% by mass of the styrene-based polymer (A) and 1 to 8% by mass of the styrene-butadiene block copolymer (B), and the styrene-based polymer (A). ) Is 94 to 99% by mass, and the styrene-butadiene block copolymer (B) is more preferably 1 to 6% by mass.

The styrene-based resin composition preferably has an unreacted styrene monomer content of 1000 ppm or less, and an unreacted methacrylic acid monomer content of 150 ppm or less. If the content of these unreacted monomers is more than the specified amount, the surface is liable to bleed out to the surface of the sheet, or the surface is liable to be roughened or soiled when in contact with the roll of the extruder or the drawing machine. In addition, there is a concern that the sheet may adhere to the mold or the like of the molding machine when the sheet is molded, which may impair the appearance of the molded product or cause stains on the mold, thereby impairing the appearance of the subsequent molded product.
The quantification of the unreacted styrene monomer and the unreacted methacrylic acid monomer was measured by the internal standard method using the gas chromatography described below.
Device name: GC-12A (manufactured by Shimadzu Corporation)
Column: Glass column φ3 [mm] x 3 [m]
Quantitative method: Internal standard method (cyclopentanol)

Two adjacent methacrylic acid monomer units contained in the styrene-methacrylic acid copolymer may form a six-membered cyclic acid anhydride in a high temperature, high vacuum extrusion step. This styrene-based resin composition containing a large amount of 6-membered cyclic anhydride may manifest as a transparent gel-like foreign substance when formed into a sheet, and may impair the appearance of the sheet. Therefore, the content of the 6-membered cyclic acid anhydride in the styrene resin composition is preferably 1.0% by mass or less.
The content of the 6-membered cyclic acid anhydride was determined from the integral ratio of the spectra measured by a carbon nuclear magnetic resonance (13C-NMR) measuring device.

The styrene-based resin composition preferably has a Vicat softening temperature in the range of 106 to 132 ° C. If the Vicat softening temperature is less than 106 ° C., the heat resistance of the sheet is insufficient, and deformation is likely to occur when heating in a microwave oven. The Vicat softening temperature is preferably 112 ° C. or higher, more preferably 116 ° C. or higher. On the other hand, if the Vicat softening temperature exceeds 132 ° C., the workability during film formation and container molding may deteriorate. The Vicat softening temperature is preferably 128 ° C. or lower, more preferably 126 ° C. or lower. The Vicat softening temperature was measured in accordance with JIS K-7206 under the conditions of a heating rate of 50 ° C./hr and a test load of 50N.

The melt flow index (MFI) of the styrene resin composition is preferably in the range of 0.5 to 4.5 g / 10 minutes, more preferably 0, from the viewpoint of drawdown during film formation and thickness uniformity. It is 9.9 to 3.6 g / 10 minutes, more preferably 1.0 to 3.0 g / 10 minutes. The melt flow index (MFI) was measured according to the H condition (200 ° C., 5 kg) of JIS K7210.

Various additives may be added to the styrene resin composition, if necessary. Examples of the additives include antioxidants, antioxidants, ultraviolet absorbers, light stabilizers, lubricants, plasticizers, colorants, antioxidants, flame retardants, mineral oil and other additives, glass fibers, and carbon fibers. Examples thereof include reinforcing fibers such as aramid fibers and fillers such as talc, silica, titanium oxide and calcium carbonate. Further, from the viewpoint of the appearance when the styrene resin composition is made into a sheet, it is preferable to combine the antioxidant and the antigelling agent alone or in combination of two or more. These additives may be added in the polymerization step, the devolatilization step or the granulation step of the styrene-based polymer (A) and the styrene-butadiene block copolymer (B), or the styrene-based resin composition may be added. It may be added at the time of manufacture. The amount of the above additive is not limited, but it is preferably added within a range that does not impair the transparency of the sheet of the styrene resin composition.

(Biaxial stretching sheet)
The biaxially stretched sheet of the present embodiment is obtained by biaxially stretching the above-mentioned styrene resin composition. The biaxial stretching may be either sequential biaxial stretching or simultaneous biaxial stretching.

The biaxially stretched sheet can be manufactured, for example, by the following method. First, the styrene-based resin composition is melt-kneaded by an extruder and extruded from a die (particularly a T-die) to produce an unstretched sheet. Next, the biaxially stretched sheet is manufactured by sequentially or simultaneously stretching the unstretched sheet in the biaxial directions in the vertical direction and the horizontal direction.

The thickness of the biaxially stretched sheet is preferably 0.1 to 0.4 mm, more preferably 0.20 to 0.35 mm in order to secure the strength and shapeability of the sheet and the container.

The stretching ratios of the biaxially stretched sheet in both the vertical direction and the horizontal direction are preferably in the range of 1.8 to 3.2 times. If the draw ratio is less than 1.8 times, the rigidity and folding resistance of the sheet may decrease. On the other hand, if the draw ratio exceeds 3.2 times, the shrinkage rate during thermoforming may be too large and the shapeability may be impaired. It is more preferable that the stretching ratios of the biaxially stretched sheet in both the vertical direction and the horizontal direction are in the range of 2.0 to 3.0 times.

The method for measuring the draw ratio is as follows. Draw a straight line 100 mm long in the vertical and horizontal directions with respect to the test piece of the biaxially stretched sheet. The length L [mm] of the straight line is measured after the test piece is allowed to stand for 60 minutes and contracted in an oven having a temperature 30 ° C. higher than the Vicat softening temperature of the sheet measured according to JIS K7206: 2016. The stretching ratios (times) in the vertical direction and the horizontal direction are numerical values calculated by the following equations, respectively.
Stretching ratio (times) = 100 / L

When the styrene-butadiene block copolymer (B) is blended with the styrene-based polymer (A), the biaxially stretched sheet made of the styrene-based resin composition has a special morphology.
FIG. 1 is a magnified photograph (6000 times) taken by a transmission electron microscope observed from a vertical cross section of a biaxially stretched sheet. Further, FIG. 2 is a magnified photograph (6000 times) taken by a transmission electron microscope observed from a lateral cross section of the biaxially stretched sheet. Butadiene block domain was stained with ruthenium tetroxide. A scale with a length of 1 μm is shown in the lower right of the figure.
As can be seen from FIGS. 1 and 2, there is a butadiene block domain having a shape elongated in the stretching direction. As can be seen from Tables 1 and 2 of the examples described later, the average major axis of the butadiene block domain observed from the longitudinal cross section of the biaxially stretched sheet is 0.1 to 1.0 μm, and the average aspect ratio is 3 to 3. When the ratio is 12, it is a biaxially stretched sheet that satisfies toughness, transparency, and heat resistance in a well-balanced manner. On the other hand, when the average major axis of the butadiene block domain is less than 0.1 μm, the toughness and heat resistance tend to decrease, and when it exceeds 1.0 μm, the transparency tends to decrease. Further, when the average aspect ratio of the butadiene block domain is less than 3, the toughness tends to decrease. It is preferable that the average major axis and the average aspect ratio of the butadiene block domain observed from the lateral cross section of the biaxially stretched sheet are the same as described above.
The average major axis of the butadiene block domain in the longitudinal cross section of the biaxially stretched sheet is preferably 0.2 to 0.6 μm, and the average aspect ratio is preferably 4 to 10. The average major axis of the butadiene block domain in the lateral cross section of the biaxially stretched sheet is more preferably 0.2 to 0.4 μm, and the average aspect ratio is more preferably 4 to 10.
Further, when HIPS, which is a conventional rubber-containing resin, was used instead of the styrene-butadiene block copolymer (B), the average major axis of the butadiene block domain exceeded 3 μm, which was inferior in toughness. ..
As described above, by using the styrene-butadiene block copolymer (B) as the rubber-containing resin, it has become possible to satisfy the toughness, transparency, and heat resistance of the biaxially stretched sheet in a well-balanced manner.

The present inventors have found that by controlling the orientation relaxation stress to a considerably lower numerical range than in the past, the occurrence of holes and deformation during microwave oven heating is greatly reduced. If the orientation relaxation stress is small, even if the part is locally heated to a temperature close to the glass transition point by microwave heating or the like, it is difficult for that part to be significantly deformed or a hole is generated due to the contraction stress. I think it is.

The orientation relaxation stress in the longitudinal direction and the lateral direction of the biaxially stretched sheet is preferably in the range of 0.1 to 0.6 MPa, more preferably 0.2 to 0.4 MPa. If the orientation relaxation stress is less than 0.1 MPa, the toughness is lowered, and fracture during sheet film formation and die cutting failure during secondary molding are likely to occur, which may make it impossible to use practically. On the other hand, if the orientation relaxation stress exceeds 0.6 MPa, holes and deformations are likely to occur during microwave heating.

In order to control the orientation relaxation stress in the longitudinal and lateral directions of the biaxially stretched sheet within the above numerical ranges, the stretching ratio, the temperature at the time of biaxial stretching, the stretching speed, and the thickness of the unstretched sheet are set within the predetermined ranges. It is preferable to manage it. As described above, the draw ratio is preferably in the range of 1.8 to 3.2 times in both the vertical direction and the horizontal direction. The temperature during biaxial stretching is preferably in the range of 145 to 155 ° C. The stretching speed is preferably in the range of 10 to 50 mm / sec. The thickness of the unstretched sheet is preferably in the range of 1.0 to 3.0 mm.

Further, from the viewpoint of the balance between the folding resistance and the shapeability of the sheet, the difference in the orientation relaxation stress in the vertical direction and the horizontal direction is preferably less than 0.20 MPa in absolute value, and preferably less than 0.10 MPa. More preferred. The orientation relaxation stress of the biaxially stretched sheet is a value measured as a peak stress value in silicone oil having a temperature 30 ° C. higher than the Vicat softening temperature of the resin composition constituting the sheet according to ASTM D1504.

The tensile elastic modulus in both the longitudinal direction and the transverse direction of the biaxially stretched sheet is preferably 2500 to 3400 MPa, more preferably 2700 to 3100 MPa. The tensile modulus can be measured according to JIS K 7161. When the tensile elastic modulus in both the vertical direction and the horizontal direction is within the above range, good container strength and shapeability can be exhibited.

The gel content in the biaxially stretched sheet is preferably small from the viewpoint of processability at the time of secondary molding and transparency in appearance. The gel content in the biaxially stretched sheet is preferably 1.0% by mass or less, more preferably 0.5% by mass or less. The gel content in the biaxially stretched sheet is determined by dissolving the biaxially stretched sheet in MEK (2-butanone) solvent, centrifuging, precipitating the solvent-insoluble component, removing the supernatant, drying and weighing. You can ask.

The biaxially stretched sheet preferably has a styrene monomer content of 1000 ppm or less. If the content of the styrene monomer is more than 1000 ppm, it adheres to the mold of the molding machine when the sheet is molded, which spoils the appearance of the molded product or causes stains on the mold, resulting in the subsequent molding container. There is a concern that it will spoil the appearance of. The quantification of the styrene monomer can be measured by the internal standard method using the gas chromatography described below.
Device name: GC-12A (manufactured by Shimadzu Corporation)
Column: Glass column φ3 [mm] x 3 [m]
Quantitative method: Internal standard method (cyclopentanol)

The total content of the styrene oligomer in the biaxially stretched sheet is preferably 10,000 ppm or less, and more preferably 5000 ppm or less. If the amount of styrene oligomer is more than 10,000 ppm, there is a concern that the appearance of the molded product may be impaired or the mold may be contaminated when the sheet is molded. The styrene oligomer refers to a dimer or trimmer of a styrene monomer, and structural isomers thereof are also included.

To quantify the styrene oligomer, dissolve 200 mg of the sample in 2 mL of 1,2- dichloromethane (containing an internal standard substance), add 2 mL of methanol to precipitate the resin, and use the supernatant liquid after standing to form a gas chromatograph. It can be measured under the following conditions.
Gas chromatograph: HP-5890 (manufactured by Hewlett-Packard)
Column: DB-1 (ht) 0.25 mm x 30 m Film thickness 0.1 μm
Injection temperature: 250 ° C
Column temperature: 100-300 ° C
Detector temperature: 300 ° C
Split ratio: 50/1
Internal standard substance: n-icosane Carrier gas: Nitrogen

The biaxially stretched sheet may contain, if necessary, a known release agent / release agent (for example, silicone oil), an antistatic agent (for example, sucrose fatty acid ester, polyglycerin fatty acid ester, or other nonionic surfactant, polyether). One or more of the modified silicone oil, silicon dioxide, etc.) and antistatic agents (for example, various nonionic surfactants, cationic surfactants, anionic surfactants, etc.) are mixed and stretched sheet. It may be applied to one side or both sides of.

The method of applying the above compound to the surface of the stretched sheet is not particularly limited, and a known method can be used. Specifically, a method of coating using a roll coater, a knife coater, a gravure roll coater, or the like can be mentioned. Further, a method such as spraying or dipping can also be adopted.

(Molding)
The method for obtaining a molded product from the biaxially stretched sheet is not particularly limited, and a method commonly used in a conventional secondary molding method for a stretched sheet can be used. For example, secondary molding can be performed by a thermoforming method such as a vacuum forming method or a compressed air forming method. These methods are described, for example, in "Handbook of Plastic Processing Technology" edited by the Society of Polymer Science, Nikkan Kogyo Shimbun (1995).

The molded product of the biaxially stretched sheet of the present invention has various types of containers, and can be widely used as packaging containers for various articles. Above all, a food packaging container for heating in a microwave oven or the like is preferable because the features of the present invention are fully exhibited.

Hereinafter, the biaxially stretched sheet of the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
The resin used as a raw material for producing the styrene-based resin composition of the biaxially stretched sheet was prepared by the method described below.

[Styrene-based polymer (A)]
A copolymer of styrene and methacrylic acid was produced by the following operations (1) to (4).
(1) Using an autoclave having an internal volume of 210 L, 100 g of polyvinyl alcohol was added to 90 kg of pure water and stirred.
(2) Styrene 45.0 kg, methacrylic acid 5.0 kg and t-butylperoxy-2-ethylhexanoate 55 g, ethyl-3,3-di (t-butylperoxy) butyrate 10 g, α-methylstyrene dimer 45 g was charged, the temperature was raised to 112 ° C., and polymerization was carried out for 6 hours. The conversion was 98% when the polymerization was carried out at a temperature of 112 ° C. for 6 hours.
(3) The polymerization was completed by holding at a temperature of 132 ° C. for 4.5 hours.
(4) The beads obtained by polymerization were washed, dehydrated and dried, and then a pellet-shaped copolymer was obtained using an extruder.
(5) The obtained copolymer was used as the styrene-based polymer (A) of Example 1.
The obtained styrene-methacrylic acid copolymer (A) had a Mw / Mn of 2.5 and an Mz / Mw of 1.8.
Further, by adjusting the charging amounts of styrene and methacrylic acid, the copolymers having different contents of various styrene monomer units and methacrylic acid monomer units shown in Tables 1 and 2 described later are similarly prepared. Got

[Styrene-butadiene block copolymer (B)]
Block copolymers of styrene and 1,3-butadiene were produced by the following operations (1) to (7).
(1) 500.0 kg of cyclohexane and 75.0 g of tetrahydrofuran (THF) were placed in the reaction vessel.
(2) 1,000 mL of a 10 mass% cyclohexane solution of n-butyllithium was added thereto as a polymerization initiator solution, and the temperature was maintained at 30 ° C.
(3) 20.0 kg of styrene was added to carry out anionic polymerization of styrene. The internal temperature rose to 35 ° C.
(4) After the styrene was completely consumed, 48.0 kg of 1,3-butadiene and 72.0 kg of styrene were added at the same time.
(5) After the styrene and 1,3-butadiene were completely consumed, the internal temperature of the reaction system was lowered to 75 ° C., and 60.0 kg of styrene was added all at once to complete the polymerization.
(6) Finally, all the polymerization active ends were inactivated with water to obtain a polymerization solution containing a polystyrene block and a polymer having a tapered block of styrene and butadiene.
(7) This polymer solution was devolatile to obtain a pellet-shaped block copolymer using an extruder. The conjugated diene-containing mass ratio was 24.0%.
(8) The obtained block copolymer was used as the styrene-butadiene block copolymer (B) of Example 1.
The Mw of the obtained styrene-butadiene block copolymer (B) was 151,000.
Further, by adjusting the charging amounts of styrene and butadiene, copolymers having different contents of various styrene monomer units and butadiene monomer units shown in Tables 1 and 2 described later can be obtained in the same manner. rice field.

An example of manufacturing a biaxially stretched sheet is described below.
(Example 1)
The obtained styrene-based polymer (A) is kneaded at 99.0% by mass and 1.0% by mass of the styrene-butadiene block copolymer (B) at 230 ° C. and 70 rpm using a 40 mm single-screw extruder. Later, it was pelletized. The obtained pellets were melted at 230 ° C. and 70 rpm using a 40 mm single-screw extruder, and passed through a T-die to obtain a 1.5 mm-thick unstretched sheet. The obtained unstretched sheet was cut out and sequentially stretched using a biaxial stretching machine at a temperature of 150 ° C. at the time of biaxial stretching, a stretching speed of 30 mm / sec, a stretching ratio of 2.5 times in the vertical direction, and 2.5 times in the horizontal direction. By doing so, the biaxially stretched sheet described in Example 1 was obtained.

(Examples 2 to 16, Comparative Examples 1 to 5)
As shown in Tables 1 and 2, the types and amounts of the styrene-based polymer (A) and the styrene-butadiene block copolymer (B), the thickness of the unstretched sheet, the stretching ratios in the vertical and horizontal directions, and 2. A biaxially stretched sheet was obtained in the same manner as in Example 1 except that the temperature at the time of axial stretching was changed. Tables 1 and 2 show the orientation relaxation stress (longitudinal direction, horizontal direction) of the obtained biaxially stretched sheet, the difference between the vertical direction and the horizontal direction of the orientation relaxation stress, and the sheet thickness.
The Mw / Mn of the styrene-methacrylic acid copolymer (A) of Example 4 was 2.8, and the Mz / Mw was 1.8. The styrene-methacrylic acid copolymer (A) of Example 5 had a Mw / Mn of 2.5 and an Mz / Mw of 1.7. The Mw of the styrene-butadiene block copolymer (B) of Example 6 was 163,000. The Mw of the styrene-butadiene block copolymer (B) of Example 7 was 159000. The Mw of the styrene-butadiene block copolymer (B) of Example 8 was 148,000. The Mw of the styrene-butadiene block copolymer (B) of Example 9 was 145,000.
As Comparative Example 5, HIPS (manufactured by Toyo Styrene Co., Ltd., H850N) was used.

Various performances of the obtained biaxially stretched sheet were measured and evaluated by the methods described below. In the relative evaluation of ◎, ○, △, ×, ◎, ○, △ were judged to be acceptable, and × was judged to be unacceptable. The results are shown in Tables 1-2.

(1) Average major axis and average aspect ratio The average major axis of the obtained biaxially stretched sheet was measured by the following method. First, a microtome was used to cut a cross section of the biaxially stretched sheet parallel to the vertical direction. The obtained cross section was stained with ruthenium tetroxide, and then photographed at 6000 times using a transmission electron microscope (manufactured by Hitachi High-Tech, Inc., model number H-7500). Next, 100 arbitrary butadiene block domains were selected using the image analysis software Mac-View, the average major axis and the average minor axis of the butadiene block domains were measured, and the average aspect ratio was calculated from the obtained results. The average major axis in the width direction was also determined by the same method after cutting a cross section parallel to the width direction of the biaxially stretched sheet.

(2) Toughness (breaking resistance)
According to ASTM D2176, the bending resistance in the sheet extrusion direction (vertical direction) and the direction perpendicular to it (horizontal direction) was measured under an atmosphere of 23 ° C., and the average value was obtained and evaluated as follows.
Using the MIT folding resistance tester BE-202 manufactured by Tester Sangyo Co., Ltd., the bending resistance was measured at a test speed of 175 cpm, a bending angle of 90 °, and a load of 1 kgf, and the bending resistance was measured in the sheet extrusion direction (vertical direction) and perpendicular to it. The average value in the direction (horizontal direction) was calculated and evaluated as follows.
⊚: 30 times or more ○: 20 times or more, less than 30 times Δ: 10 times or more, less than 20 times ×: less than 10 times

(3) Transparency (haze)
According to JIS K-7631-1, the haze of the biaxially stretched sheet was measured using a haze meter NDH5000 (Nippon Denshoku Co., Ltd.).
⊚: Haze less than 5.0% ○: Haze 5.0% or more and less than 10.0% Δ: Haze 10.0% or more and less than 20.0% ×: Haze 20.0% or more

(4) After applying Seven-Eleven's Mapo tofu to the surface of the lid material on which the microwave-resistant heat-resistant biaxially stretched sheet is molded, replace it with the lid material at the time of purchase and fit the container. I let you. Then, the microwave oven heating was carried out at the recommended output and 1.5 times the recommended output under the recommended microwave oven heating condition output of 1500 W and the heating time of 80 seconds. The heated container was taken out, the number of holes in the lid material and the degree of deformation were visually confirmed, and evaluated as follows.
◎: No holes and almost no deformation ○: There are 3 or less holes and the deformation is small.
Δ: There are more than 3 holes and less than 10 and the deformation is small. ×: There are 10 or more holes or the deformation is large.

From the results in Tables 1 and 2, the biaxially stretched sheets satisfying the provisions of the present invention obtained in Examples 1 to 16 all have excellent performance in terms of toughness, transparency, and microwave oven heating resistance. I had.

On the other hand, all of the biaxially stretched sheets obtained in Comparative Examples 1 to 5 that do not satisfy the provisions of the present invention had undesired results in terms of toughness, transparency, and microwave oven heating resistance.

Claims (11)

A biaxially stretched sheet comprising a styrene-based resin composition containing a styrene-based polymer (A) and a styrene-butadiene block copolymer (B).
A biaxially stretched sheet having an average major axis of butadiene block domain of 0.1 to 1.0 μm and an average aspect ratio of 3 to 12 observed from a vertical cross section of the biaxially stretched sheet. The biaxial stretching according to claim 1, wherein the butadiene block domain observed from the cross section in the width direction of the biaxially stretched sheet has an average major axis of 0.1 to 1.0 μm and an average aspect ratio of 3 to 12. Sheet. The biaxially stretched sheet according to claim 1 or 2, wherein the orientation relaxation stress in both the longitudinal direction and the lateral direction of the biaxially stretched sheet is 0.1 to 0.6 MPa. Claim 1 in which the styrene-based polymer (A) is a styrene-methacrylic acid copolymer and contains 86 to 97% by mass of a styrene monomer unit and 3 to 14% by mass of a methacrylic acid monomer unit. The biaxially stretched sheet according to any one of 3 to 3. The invention according to any one of claims 1 to 4, wherein the styrene-butadiene block copolymer (B) is a styrene-butadiene block copolymer containing 10% by mass or more and less than 40% by mass of a butadiene monomer unit. Biaxially stretched sheet. The biaxially stretched sheet according to any one of claims 1 to 5, wherein the styrene resin composition contains 0.01 to 5% by mass of a butadiene monomer unit. One of claims 1 to 6, wherein the styrene-based resin composition contains 91 to 99% by mass of the styrene-based polymer (A) and 1 to 9% by mass of the styrene-butadiene block copolymer (B). The biaxially stretched sheet according to. The biaxially stretched sheet according to any one of claims 1 to 7, wherein the biaxially stretched sheet has a thickness of 0.1 to 0.4 mm. The biaxially stretched sheet according to any one of claims 1 to 8, wherein the absolute value of the difference between the vertical and horizontal orientation relaxation stresses of the biaxially stretched sheet is less than 0.20 MPa. A molded product comprising the biaxially stretched sheet according to any one of claims 1 to 9. The molded product according to claim 10, which is a food packaging container for heating in a microwave oven.

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