JP2019206664A - Biaxially stretched sheet and molded article thereof - Google Patents

Abstract

To provide a stretched sheet comprising a styrene-based resin composition that can reduce a discharge amount of carbon dioxide and is excellent in heat resistance because the sheet has good film formability, transparency, sheet strength and moldability even when the wall thickness is thinned and to provide a molded article of the stretched sheet.SOLUTION: A biaxially stretched sheet comprises a styrene-based resin composition which contains (A) a styrene/methacrylic acid copolymer resin containing a styrene monomer unit and a methacrylic acid monomer unit in a mass ratio of 84/16 to 94/6 and having a weight average molecular weight of 120,000 to 250,000, (B) a multi-branched type styrene-based copolymer resin having a weight average molecular weight of 350,000 to 550,000 and (C) an impact-resistant styrene-based resin containing a rubber component. In the biaxially stretched sheet, the mass ratio (A)/(B) of the styrene/methacrylic acid copolymer resin (A) and the multi-branched type styrene-based copolymer resin (B) is 70/30 to 95/5; the content of the rubber component in the biaxially stretched sheet is 0.05 to 0.3 mass% and a rubber particle formed from the rubber component in the biaxially stretched sheet has an average particle size of 1.2 to 12 μm.SELECTED DRAWING: None

Description

  The present invention relates to a biaxially stretched sheet comprising a styrenic resin composition that can be suitably used for food packaging containers, and a molded product thereof.

  Polystyrene stretched sheets, particularly biaxially stretched sheets, are excellent in transparency and rigidity, so they are secondarily molded and used in large quantities mainly for molded products such as lightweight containers for food. In addition, attempts have been made to impart heat resistance to polystyrene as a raw material for the purpose of use in direct contact with boiling water or use in heating in a microwave oven. For example, styrene copolymers such as styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, and styrene-maleic anhydride copolymer have improved heat resistance without impairing transparency and rigidity. .

  Most of the sheets used for these food containers are recycled in consideration of environmental properties, recyclability, and cost reduction of the containers. In particular, food containers and lids are punched out and used as containers alone after the sheet is formed into a container. Therefore, an unnecessary part becomes punching scraps (skeleton), and depending on the shape of the container, punching scraps of 30% or more may be generated.

  For this reason, in most molding processing sites, this punching waste is used alone or mixed with punching waste from impact-resistant styrenic resin (high impact polystyrene, HIPS) sheets to reduce waste of raw materials. However, the manufacturing cost is reduced.

  However, the styrene-based heat-resistant resin sheet made of a copolymer such as the above-mentioned styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-maleic anhydride copolymer is composed of styrene monomer units. The compatibility with styrene-based resins (hereinafter sometimes referred to as general-purpose polystyrene) is poor, and both cannot be substantially mixed and reused. For this reason, these styrene-based heat-resistant resin sheet scraps can only be discarded or returned to the same material, resulting in increased manufacturing costs and increased waste. There was a problem that the amount of emissions increased.

  On the other hand, in recent years, with consideration for the environment, a sheet that can reduce the amount of energy required for manufacturing and transportation and reduce carbon dioxide emissions by reducing the thickness compared to conventional products to reduce environmental impact. It is desired.

  In order to achieve a thinner wall than conventional products, it is necessary to increase the strength of the sheet. Therefore, some attempts have been made to increase the strength of the sheet by adding various ideas to the processing conditions and rubber components of the sheet (Patent Documents 1 and 2).

JP 2004-51680 A JP 2007-237732 A

  However, by increasing the strength of the sheet, the moldability of the container is lowered, and there is a problem that the moldability is deteriorated particularly when a container having a large drawing ratio is formed.

  The problem of the present invention is that since the film forming property, transparency, sheet strength and moldability are good even if the thickness is reduced, the amount of carbon dioxide emission can be reduced, and the styrenic resin composition also has excellent heat resistance. It is providing the extending | stretching sheet | seat and its molded article which become.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies on a styrene resin composition and a stretched sheet or film using the styrene resin composition. As a result, styrene-methacrylic acid copolymer resin, multi-branched styrene system By using a resin composition containing a copolymer resin and an impact-resistant styrenic resin, it was found that the object was achieved, and the present invention was completed.

That is, the present invention has the following configuration.
(1) A styrene-methacrylic acid copolymer resin (A) containing a styrene monomer unit and a methacrylic acid monomer unit in a mass ratio of 84/16 to 94/6 and having a weight average molecular weight of 120,000 to 250,000. ), A styrene resin composition containing a multi-branched styrene copolymer resin (B) having a weight average molecular weight of 350,000 to 550,000 and an impact-resistant styrene resin (C) containing a rubber component. An axially stretched sheet, wherein the mass ratio (A) / (B) of the styrene-methacrylic acid copolymer resin (A) and the multi-branched styrene copolymer resin (B) is 70/30 to 95/5. Yes, the content of the rubber component in the biaxially stretched sheet is 0.05 to 0.3% by mass, and the average particle diameter of the rubber particles formed by the rubber component in the biaxially stretched sheet is 1.2 to A biaxially oriented sheet that is 12 μm.
(2) In the above (1), the orientation relaxation stress in the MD direction and the TD direction are both in the range of 0.8 to 2.0 MPa, and the tensile elastic modulus in the MD direction and the TD direction are both 2800 to 3400 MPa. The biaxially stretched sheet as described.
(3) The biaxially stretched sheet according to (1) or (2) above, wherein both the MD and TD stretch ratios are 2.0 to 4.5 times.
(4) The biaxially stretched sheet according to any one of (1) to (3), wherein the thickness is 0.01 mm to 0.7 mm.
(5) The biaxially stretched sheet according to any one of (1) to (4), wherein the gel content is 1.0% by mass or less.
(6) The biaxially stretched sheet according to any one of (1) to (5), wherein the content of the styrene monomer is 1000 ppm or less and the total content of the styrene oligomer is 10,000 ppm or less.
(7) A molded article comprising the biaxially stretched sheet according to any one of (1) to (6).
(8) The molded product according to (7), which is a food packaging container.

Even if the biaxially stretched sheet and the molded product of the present invention are thin, the film forming property, transparency, sheet strength, and moldability are good, so that the amount of carbon dioxide emission can be reduced and the heat resistance is also improved. Are better. Therefore, it can be suitably used for food packaging containers and the like for use in heating with a microwave oven.

  Embodiments of the present invention will be described below. However, the embodiment of the present invention is not limited to the following embodiment.

  The biaxially stretched sheet of this embodiment comprises a styrene-methacrylic acid copolymer resin (A), a multi-branched styrene copolymer resin (B), and an impact resistant styrene resin (C) containing a rubber component. It consists of a styrene resin composition containing. The biaxially stretched sheet can be obtained by extruding the styrene resin composition and biaxially stretching the resulting unstretched sheet.

  In the biaxially stretched sheet of the present embodiment, the MD direction (Machine Direction direction) is a flow direction during film formation and is also referred to as a longitudinal direction. Further, the TD direction (Transverse Direction direction) is a method perpendicular to the flow direction during sheet deposition, and is also referred to as a lateral direction.

Hereinafter, each component which comprises a styrene resin composition is demonstrated.
(Styrene-methacrylic acid copolymer resin (A))
In the styrene-methacrylic acid copolymer resin (A), the copolymerization ratio of styrene and methacrylic acid can be variously set according to desired heat resistance, mechanical strength, and the like. When the total amount of styrene monomer units and methacrylic acid monomer units is 100% by mass, a resin excellent in balance of heat resistance, mechanical strength, and transparency when formed into a sheet can be easily obtained. In addition, it is necessary to contain a styrene monomer unit and a methacrylic acid monomer unit in a mass ratio of 84/16 to 94/6. When the content of the methacrylic acid monomer unit is less than 6% by mass, the heat resistance is insufficient, and holes are formed during the microwave heating, and deformation is likely to occur. The content of the methacrylic acid monomer unit is preferably 8% by mass or more, more preferably 9% by mass or more. On the other hand, when the content of the methacrylic acid monomer unit exceeds 16% by mass, in addition to a decrease in film formability due to a decrease in fluidity and a decrease in workability such as a decrease in moldability during secondary molding, the gel The appearance of the sheet is likely to deteriorate due to the occurrence. The content of the methacrylic acid monomer unit is preferably 14% by mass or less, more preferably 13% by mass or less. The styrene-methacrylic acid copolymer resin (A) may be appropriately copolymerized with other monomers other than styrene and methacrylic acid as needed, as long as the effects of the invention are not impaired. The content of other monomers is preferably 10% by mass or less, more preferably 5% by mass or less, and further preferably 3% by mass or less. When the content of other monomers exceeds 10% by mass, the ratio of styrene or methacrylic acid decreases, and sufficient transparency, mechanical strength, and heat resistance may not be obtained.

  The weight average molecular weight (Mw) of the styrene-methacrylic acid copolymer resin (A) is 120,000 to 250,000, preferably 140,000 to 220,000, and more preferably 150,000 to 200,000. When the weight average molecular weight is less than 120,000, the fluidity is excessive, so that the film-forming property such as the drawdown of the sheet and the occurrence of neck-in and the appearance of the sheet are liable to occur. On the other hand, if the weight average molecular weight exceeds 250,000, the fluidity is insufficient, resulting in uneven thickness at the time of film formation, film forming properties such as die lines, and sheet appearance.

  The ratio Mw / Mn between the weight average molecular weight (Mw) and the number average molecular weight (Mn) of the styrene-methacrylic acid copolymer resin (A) is preferably 2.0 to 3.0, more preferably 2. 2 to 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, unevenness in thickness at the time of film formation due to a decrease in fluidity and molding failure at the time of container molding tend to occur. Moreover, it is preferable that ratio Mz / Mw of Z average molecular weight (Mz) and Mw is 1.5-2.0, More preferably, it is 1.6-1.9. When Mz / Mw is less than 1.5, the sheet is likely to be drawn down, necking-in and the like, and the film-forming property is lowered, and the stretch orientation is insufficient. On the other hand, when Mz / Mw exceeds 2.0, sheet appearance deterioration such as unevenness of thickness at the time of film formation and die line due to decrease in fluidity tends to occur.

The number average molecular weight (Mn), the weight average molecular weight (Mw), and the Z average molecular weight (Mz) described above are calculated by calculating the molecular weight at each elution time from the elution curve of monodisperse polystyrene in the GPC measurement method described below. The molecular weight calculated in terms of polystyrene.
Model: Shodex GPC-101 manufactured by Showa Denko KK
Column: Polymer Laboratories PLgel 10 μm MIXED-B
Mobile phase: Tetrahydrofuran Sample concentration: 0.2% by mass
Temperature: 40 ° C oven, 35 ° C inlet, 35 ° C detector
Detector: Differential refractometer

  Examples of the polymerization method of the styrene-methacrylic acid copolymer resin (A) include known polymerization methods such as a bulk polymerization method, a solution polymerization method, and a suspension polymerization method that are industrialized with polystyrene and the like. In terms of quality and productivity, bulk polymerization and solution polymerization 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.

  A polymerization initiator and a chain transfer agent can be used as necessary during the polymerization of the styrene-methacrylic acid copolymer resin (A). An organic peroxide can be used as the polymerization initiator. Specific examples of the organic peroxide include benzoyl peroxide, t-butylperoxybenzoate, 1,1-di (t-butylperoxy) cyclohexane, 1,1-bis (t-butylperoxy) -3. , 3,5-trimethylcyclohexane, 2,2-bis (4,4-di-t-butylperoxycyclohexyl) propane, t-butylperoxyisopropyl carbonate, dicumyl peroxide, t-butylcumyl peroxide, t -Butyl peroxyacetate, t-butylperoxy-2-ethylhexanoate, polyether tetrakis (t-butylperoxycarbonate), ethyl-3,3-di (t-butylperoxy) butyrate, t-butyl Examples include peroxyisobutyrate. Specific examples of the chain transfer agent include aliphatic mercaptans, aromatic mercaptans, pentaphenylethane, α-methylstyrene dimer, terpinolene, and the like.

(Multi-branched styrene copolymer resin (B))
The multi-branched styrene copolymer resin (B) is a co-polymer of a vinyl monomer containing styrene and a solvent-soluble polyfunctional vinyl compound copolymer (hereinafter referred to as “polyfunctional vinyl copolymer”). Polymerized resin. The polyfunctional vinyl copolymer is a copolymer of a polyfunctional vinyl monomer and a monofunctional vinyl monomer. By including the multi-branched styrene copolymer resin (B) in the styrene resin composition, the entanglement effect of molecules can be improved in the styrenic resin composition when forming a biaxially stretched sheet or container. And strain-hardening properties can be developed. As a result, the strength of the sheet can be improved, the thickness unevenness at the time of molding can be reduced, and the strength as a container can be improved.

  The polyfunctional vinyl monomer constituting the polyfunctional vinyl copolymer includes divinyl aromatic compounds represented by divinylbenzene and aliphatic and alicyclic (meth) acrylates represented by ethylene glycol di (meth) acrylate. Etc. are used. Of these polyfunctional vinyl monomers, divinylbenzene is preferred. On the other hand, as the monofunctional vinyl monomer constituting the polyfunctional vinyl copolymer, styrene monomers such as styrene and ethyl vinyl benzene, and acrylic monomers such as 2-phenoxyethyl methacrylate are used. The copolymerization molar ratio of the polyfunctional vinyl monomer and the monofunctional vinyl monomer is preferably 10/90 to 90/10, and more preferably 30/70 to 90/10.

Since the polyfunctional vinyl copolymer has a polymerizable functional group derived from the polyfunctional vinyl monomer, it can be copolymerized with a vinyl monomer containing styrene.
The polyfunctional vinyl copolymer is solvent-soluble. Here, the solvent is a solvent used when polymerizing the multi-branched styrene copolymer resin (B), and specifically, toluene, xylene, ethylbenzene and the like. The weight average molecular weight of the polyfunctional vinyl copolymer is preferably 5000 to 70,000. When the weight average molecular weight of the polyfunctional vinyl copolymer is within the above numerical range, the polyfunctional vinyl copolymer is soluble in the solvent.

  The vinyl monomer containing styrene may be 100% styrene or a mixture containing styrene and another vinyl monomer. Other vinyl monomers may be those having an olefinic double bond copolymerizable with styrene, aromatic vinyl monomers such as paramethylstyrene, acrylic acid monomers such as acrylic acid and methacrylic acid, Vinyl cyanide monomers such as acrylonitrile and methacrylonitrile, (meth) acrylic acid ester monomers such as butyl acrylate and methyl methacrylate, α, β-ethylenically unsaturated carboxylic acids such as maleic anhydride and fumaric acid, phenylmaleimide And imide monomers such as cyclohexylmaleimide. These other vinyl monomers can be used alone or in combination of two or more. The mixing ratio of styrene and other vinyl monomers is preferably 50 to 100 mol% of styrene and 0 to 50 mol% of other vinyl monomers.

As a raw material for producing the multi-branched styrene copolymer resin (B), a vinyl monomer containing styrene, a polyfunctional vinyl copolymer, and, if necessary, a solvent, a polymerization catalyst, a chain transfer agent, and the like are used. As a polymerization method for the multi-branched styrene copolymer resin (B), a so-called continuous bulk polymerization method is preferably used. As a polymerization apparatus, after the raw materials are uniformly mixed, one or more reactors arranged in series and / or in parallel and a volatile matter removing step for removing unreacted monomers and the like are continuously added to the monomers. Is used, and an apparatus for proceeding polymerization in stages is used. Examples of the reactor type include a fully mixed tank reactor, a column reactor having plug flow properties, a loop reactor in which a part of the polymerization solution is withdrawn while the polymerization proceeds. There are no particular restrictions on the order of arrangement of these reactors, but in order to suppress the formation of gel-like substances in continuous production, the polyfunctional vinyl copolymer is unreacted and is not present in the reactor wall film. It is important not to develop a state of staying at a high concentration, and it is preferable to select a fully mixed tank reactor as the first reactor.
The polyfunctional vinyl copolymer can be added from the middle of the reactor as necessary in a state dissolved in a polymerization solvent or the like.

  The weight average molecular weight (Mw) of the multi-branched styrenic copolymer resin (B) is 350,000 to 550,000, preferably 400,000 to 500,000. If the weight average molecular weight of the multi-branched styrene copolymer resin (B) is less than 350,000, the moldability at the time of container molding may be reduced, and the container strength may also be reduced. In addition, when the weight average molecular weight of the multi-branched styrene copolymer resin (B) exceeds 550,000, unevenness in thickness during film formation, die line, etc. are likely to occur, and the film-forming property is deteriorated or the sheet appearance is deteriorated. descend.

(Impact resistant styrene resin (C))
The impact-resistant styrene resin (C) is so-called high impact polystyrene (HIPS). The impact-resistant styrenic resin (C) may be any styrene resin containing a rubber component, which includes a rubber component in a styrene homopolymer, and polystyrene grafted onto the rubber component. Any of these can be suitably used. The rubber component may be dispersed in the form of particles independently in polystyrene serving as the matrix resin, or may be one in which polystyrene is graft-polymerized and dispersed in the rubber component. . 5-30 mass% is preferable and, as for content of the rubber component in impact-resistant styrene-type resin (C), 8-15 mass% is more preferable.

  Examples of the rubber component include polybutadiene, styrene-butadiene copolymer, polyisoprene, butadiene-isoprene copolymer, and the like. In particular, it is preferably contained as a polybutadiene or styrene-butadiene copolymer.

  The rubber component in the biaxially stretched sheet is mainly derived from the rubber component contained in the impact-resistant styrene resin (C) in the styrene resin composition. Content of the rubber component in a biaxially stretched sheet is 0.05-0.3 mass%. If the content of the rubber component is less than 0.05% by mass, the effect of improving the rigidity and folding resistance of the sheet may not be exhibited sufficiently. On the other hand, when the content of the rubber component exceeds 0.3% by mass, the transparency of the sheet may be lowered. As for content of the rubber component in a biaxially stretched sheet, it is more preferable that it is 0.1-0.2 mass%.

  Here, the content of the rubber component in the biaxially stretched sheet can be quantified by the following iodine monochloride method. Dissolve the biaxially stretched sheet in chloroform, add iodine monochloride to react with the double bond in the rubber component, add potassium iodide, change the remaining iodine monochloride to iodine, and reverse with sodium thiosulfate Titrate.

  The average particle diameter of the rubber particles formed by the rubber component in the biaxially stretched sheet is 1.2 to 12 μm, and preferably 2.0 to 8.0 μm. If the average particle diameter is less than 1.2 μm, the effect of improving the rigidity and folding resistance of the sheet may not be sufficiently exhibited. On the other hand, when the average particle diameter exceeds 12 μm, the sheet appearance may be deteriorated or the transparency may be deteriorated.

The average particle size of the rubber component in the biaxially stretched sheet is cut by an ultrathin section method so that the observation surface is parallel to the sheet plane, and after dyeing the rubber component with osmium tetroxide (OsO ₄ ), The particle diameter of 100 particles is measured with a transmission microscope, and is a value calculated by the following equation.
Average particle size = Σni (Di) ⁴ / Σni (Di) ³
Here, ni represents the number of measured particles, and Di represents the measured particle size.

(Styrenic resin composition)
In the styrene resin composition, the mass ratio (A) / (B) of the styrene-methacrylic acid copolymer resin (A) to the multi-branched styrene copolymer resin (B) is 70/30 to 95/5. It is preferable that it is 80 / 20-90 / 10. When the mass ratio of the multi-branched styrenic copolymer resin (B) exceeds 30, it is easy to cause film thickness deterioration such as unevenness in film formation, die line, etc. May decrease. On the other hand, if the mass ratio of the multi-branched styrenic copolymer resin (B) is less than 5, the moldability at the time of container molding may be reduced, and the container strength may also be reduced. The content of the impact resistant styrene resin (C) in the styrene resin composition is such that the content of the rubber component derived from the impact resistant styrene resin (C) is the rubber component in the biaxially stretched sheet. The content is adjusted appropriately so as to be 0.05 to 0.3% by mass. The content of the impact-resistant styrene resin (C) is usually 0.5 to 3.0% by mass with respect to the styrene resin composition.

You may mix | blend various additives with a styrene resin composition according to a use. Examples of additives include antioxidants, anti-gelling agents, ultraviolet absorbers, light stabilizers, lubricants, plasticizers, colorants, antistatic agents, flame retardants, mineral oils, glass fibers, and carbon fibers. And reinforcing fibers such as aramid fibers, and fillers such as talc, silica, mica and calcium carbonate. Moreover, it is preferable to mix | blend antioxidant and an antigelling agent individually or in combination of 2 or more types from a viewpoint of the external appearance when the said styrene-type resin composition is sheeted. These additives may be added in the polymerization process or devolatilization process or granulation process of each component resin, or may be added when producing a styrene resin composition.
Although there is no restriction | limiting in the addition amount of the said additive, It is preferable to add in the range which does not impair the transparency of the sheet | seat of a styrene-type resin composition.

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

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

  The thickness of the biaxially stretched sheet is preferably 0.01 mm or more, more preferably 0.1 mm or more, and still more preferably 0.2 mm or more in order to ensure the strength and particularly rigidity of the sheet and the container. On the other hand, from the viewpoints of formability and economy, the thickness of the biaxially stretched sheet is preferably 0.7 mm or less, more preferably 0.6 mm or less, and even more preferably 0.5 mm or less.

  It is preferable that both the stretching ratios in the machine direction and the transverse direction of the biaxially stretched sheet are in the range of 2.0 to 4.5 times. If the draw ratio is less than 2.0 times, a sheet having sufficiently high strength may not be obtained. On the other hand, when the draw ratio exceeds 4.5 times, there is a possibility that the formability is impaired due to the excessive shrinkage during thermoforming. It is more preferable that the stretching ratios in the longitudinal direction and the transverse direction of the biaxially stretched sheet are both in the range of 2.1 to 3.0 times.

In addition, the measuring method of a draw ratio is as follows. A straight line having a length of 100 mm is drawn in the MD direction and the TD direction 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 left to shrink for 60 minutes in an oven having a temperature 30 ° C. higher than the Vicat softening temperature of the sheet measured according to JIS K7206: 2016. The draw ratio (times) in the MD direction and TD direction is a numerical value calculated by the following equation.
Stretch ratio (times) = 100 / L

  The orientation relaxation stress in the MD direction and the TD direction of the biaxially stretched sheet is preferably in the range of 0.8 to 2.0 MPa, more preferably 0.9 to 1.1 MPa. If the orientation relaxation stress is less than 0.8 MPa, when the sheet is made thinner than the conventional product, the strength is insufficient and there is a possibility that it cannot be used practically. On the other hand, if the orientation relaxation stress exceeds 2.0 MPa, the shrinkage stress at the time of thermoforming is too large, and the formability may be impaired. Further, from the viewpoint of the balance between the folding resistance and the formability of the sheet, the difference in the orientation relaxation stress in the MD direction and the TD direction is preferably 0.2 MPa or less. The orientation relaxation stress of the biaxially stretched sheet is a value measured as a peak stress value in silicone oil at a temperature 30 ° C. higher than the Vicat softening temperature of the resin composition constituting the sheet according to ASTM D1504.

  The tensile modulus in the MD direction and the TD direction of the biaxially stretched sheet are both preferably 2800 to 3400 MPa, and more preferably 2900 to 3100 MPa. The tensile elastic modulus can be measured according to JIS K 7161. When the tensile modulus in the MD direction and the TD direction are both within the above ranges, good container strength and moldability can be exhibited.

  The gel content in the biaxially stretched sheet is preferably small from the viewpoint of workability during secondary molding and transparency of appearance. The gel content in the biaxially stretched sheet is preferably 1.0% by mass or less, and more preferably 0.5% by mass or less. The gel content in the biaxially stretched sheet is obtained by dissolving the biaxially stretched sheet in a MEK (2-butanone) solvent, centrifuging, allowing the solvent-insoluble matter to settle, removing the supernatant, and drying and weighing. Can be sought.

The biaxially stretched sheet preferably has a styrene monomer content of 1000 ppm or less. If the content of styrene monomer is more than 1000 ppm, it will adhere to the mold of the molding machine when molding the sheet, impairing the appearance of the molded product, and causing mold contamination and subsequent molding containers There are concerns that damage the appearance of. In addition, the quantification of the styrene monomer can be measured by an 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)

  Moreover, the total content of the styrene oligomer in the biaxially stretched sheet is 10000 ppm or less, and preferably 5000 ppm or less. When the amount of styrene oligomer is more than 10,000 ppm, there is a concern that the appearance of the molded product is impaired or mold contamination is caused when the sheet is molded. The styrene oligomer refers to a dimer or trimer of a styrene monomer, and includes structural isomers thereof.

The amount of styrene oligomer was determined by dissolving 200 mg of sample in 2 mL of 1,2-dichloromethane (containing internal standard substance), adding 2 mL of methanol to precipitate the resin, and using the supernatant after standing, gas chromatograph Was measured under the following conditions.
Gas chromatograph: HP-5890 (manufactured by Hewlett-Packard Company)
Column: DB-1 (ht) 0.25 mm × 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 reference material: n-eicosane Carrier gas: Nitrogen

  For the biaxially stretched sheet, known release agents / release agents (for example, silicone oil), antifogging agents (for example, nonionic surfactants such as sucrose fatty acid ester and polyglycerin fatty acid ester, polyether, etc.) Modified silicone oil, silicon dioxide, etc.), antistatic agents (for example, various nonionic surfactants, cationic surfactants, anionic surfactants, etc.) are mixed in one or more kinds, and a stretched sheet You may apply | coat to one side or both sides of.

  The method for coating the above compound on the surface of the stretched sheet is not particularly limited, and a known method can be used. Specifically, a coating method using a roll coater, a knife coater, a gravure roll coater or the like can be mentioned. Moreover, methods, such as spraying and immersion, can also be employ | adopted.

  There is no restriction | limiting in particular as a method of obtaining a molded article from a biaxially stretched sheet, The method currently used in the secondary secondary molding method of the conventional stretched sheet can be used. For example, the secondary molding can be performed by a thermoforming method such as a vacuum forming method or a pressure forming method. These methods are described in, for example, “Plastic Processing Technology Handbook” edited by the Society of Polymer Science, Nikkan Kogyo Shimbun (1995).

  As the use of the molded product of the biaxially stretched sheet of the present invention, a food packaging container or the like is particularly preferable because the features of the present invention are sufficiently exhibited.

Hereinafter, the biaxially stretched sheet of the present invention will be described more specifically using examples, but the present invention is not limited to these examples.
Resin used as a raw material used for manufacture of a biaxially stretched sheet was prepared by the method described below.
(Experimental example 1)

[Production of styrene-methacrylic acid copolymer resin (A)]
100 kg of pure water and 100 g of polyvinyl alcohol were added to an autoclave with an internal volume of 200 L and a stirrer, and the mixture was stirred at 130 rpm. Subsequently, 72.0 kg of styrene, 4.0 kg of methacrylic acid and 20 g of t-butyl peroxide were charged, the autoclave was sealed, the temperature was raised to 110 ° C., and polymerization was performed for 5 hours (Step 1). Further, 4.0 kg of methacrylic acid was uniformly added over 2 hours from the time when the polymerization temperature reached 110 ° C. (Step 2). Further, the temperature was maintained at 140 ° C. for 3 hours to complete the polymerization (Step 3). The obtained beads were washed, dehydrated and dried, and then extruded to obtain pellet-shaped styrene-methacrylic acid copolymer resin (A) shown in Table 1. As a result of analysis using pyrolysis gas chromatography, the mass ratio of styrene monomer unit / methacrylic acid monomer unit was 90/10. The number average molecular weight (Mn), weight average molecular weight (Mw), and Z average molecular weight (Mz) determined by GPC measurement were 70,000, 180,000 and 350,000, respectively.

Various raw material charging amounts of Experimental Example 1 were adjusted to obtain a plurality of various styrene-methacrylic acid copolymer resins (A) listed in Tables 1 to 3.
(Experimental example 2)

[Production of multi-branched styrene copolymer resin (B)]
160 g of divinylbenzene, 94 g of ethylvinylbenzene, 223 g of styrene, 633 g of 2-phenoxyethyl methacrylate, and 1080 g of toluene are added to a 3 L reaction vessel, and 57 g of diethyl ether complex of boron trifluoride is added at 50 ° C. for 6 hours. I let you. After the polymerization solution was stopped with an aqueous sodium hydrogen carbonate solution, the oil layer was washed three times with pure water, and the reaction mixture was poured into a large amount of methanol at room temperature to precipitate a polymer. The obtained polymer was washed with methanol, filtered, dried and weighed to obtain a polyfunctional vinyl copolymer (α).

  On the other hand, two tank reactors having an internal volume of 40 L connected in series and having a complete mixing property, a 20 L internal reactor having a built-in static mixer having a plug flow property, a preheater, A continuous bulk polymerization equipment having a flash chamber type devolatilization equipment having a vacuum chamber was prepared. To the continuous bulk polymerization equipment, 85 parts by mass of styrene, 15 parts by mass of ethylbenzene, and 0.5 parts by mass of the polyfunctional vinyl copolymer (α) were uniformly mixed and then continuously fed at 15 L / hr. . The temperature of the first tank reactor is 135 ° C., the second tank reactor is 135 ° C., the third tower reactor is 145 ° C. at the inlet, and 165 ° C. at the outlet. Raised. Then, it transferred to the preheater heated to 225 degreeC, and the unreacted monomer and solvent were removed by throwing into the vacuum chamber right under the preheater which adjusted the pressure to 665 Pa (5 Torr). Then, a multi-branched styrene copolymer resin was obtained by cutting the resin from the vacuum chamber while drawing it in a strand shape with a gear pump. As a result of measuring the weight average molecular weight Mw by GPC, it was 450,000.

A total of five types of multi-branched styrene copolymer resins (B) having different weight average molecular weights listed in Tables 1 to 3 were prepared by changing the amount of the polyfunctional vinyl compound copolymer (α). .
(Experimental example 3)

[Production of impact-resistant styrene resin (C)]
Using 3.4% by mass of low-cis polybutadiene rubber (trade name: Diene 55AS, manufactured by Asahi Kasei) as a rubbery polymer, dissolved in 91.6% by mass of styrene and 5.0% by mass of ethylbenzene as a solvent. A polymerization raw material was used. Moreover, 0.1 mass part of antioxidant (made by Ciba Geigy, trade name Irganox 1076) of rubber was added. This polymerization raw material was supplied at 12.5 kg / hr to a 14-liter jacketed reactor (R-01) equipped with a vertical stirring blade having a blade diameter of 0.285 m. The reaction temperature was 140 ° C., and the rotation speed was 2.17 sec ⁻¹ . The obtained resin solution was introduced into two jacketed plug flow reactors having an internal volume of 21 liters arranged in series. In the first plug flow reactor (R-02), the reaction temperature is 120 to 140 ° C. in the flow direction of the resin liquid. In the second plug flow reactor (R-03), the reaction temperature is resin. The jacket temperature was adjusted to have a gradient of 130 to 160 ° C. in the liquid flow direction. The obtained resin liquid was heated to 230 ° C. and then sent to a devolatilization tank having a vacuum degree of 5 torr to separate and recover unreacted monomers and solvents. Then, after extracting with a gear pump from the devolatilization tank and making it a strand through a die plate, it pelletized through the water tank and obtained impact-resistant styrene resin (C). The resin ratio of the obtained resin was 70%. Here, the resin rate is calculated by the following formula.
Resin ratio (%) = 100 × (polymer amount) / {(amount of monomer charged) + (amount of solvent)}
The rubber component content in the obtained resin was 10.0% by mass, and the average particle size of the rubber particles was 5 μm.

By adjusting the amount of raw materials charged in the same manner as described above, the rubber component content shown in Tables 1 to 3 is 10.0% by mass, and the rubber particles have different average particle diameters. An impact styrene resin (C) was produced.
Example 1

Below, the preparation example of a biaxially stretched sheet is described.
90 parts by mass of a styrene-methacrylic acid copolymer resin (A) (Mw = 180,000), 10 parts by mass of a multi-branched styrene copolymer resin (B) (Mw = 450,000), and an impact-resistant styrene resin ( C) Each pellet of 1 part by mass (rubber component content 10.0 mass%, average particle diameter of rubber particles 5 μm) was hand blended. Using a pellet extruder (a biaxial co-directional extruder TEM35B with a vacuum vent TEM35B (manufactured by Toshiba Machine)), a strand was passed through a die plate at an extrusion temperature of 230 ° C., a rotation speed of 250 rpm, and a vent devolatilization pressure of −760 mmHg. Then, after cooling in a water tank, it was pelletized through a pelletizer to obtain a styrene resin composition. In addition, the vent devolatilization pressure was shown as a differential pressure value with respect to normal pressure. Using a sheet extruder (T-die width 500 mm, lip opening 1.5 mm, φ40 mm extruder (manufactured by Tanabe Plastic Machinery Co., Ltd.)), the styrene resin composition was extruded at an extrusion temperature of 230 ° C. and a discharge rate of 20 kg / h. An unstretched sheet was obtained. This sheet was preheated using a batch type biaxial stretching machine (manufactured by Toyo Seiki Co., Ltd.), stretched to MD 3.5 times and TD 3.5 times at a stretching temperature of 128 ° C. and a strain rate of 0.1 / sec. A 25 mm biaxially oriented sheet was obtained. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet was 1.00 MPa / 1.00 MPa, and the tensile modulus (MD / TD) was 3100 MPa / 3100 MPa. In addition, the rubber component content in the biaxially stretched sheet is 0.1% by mass, the average particle size of the rubber particles is 5 μm, the gel content is 0.8% by mass, the content of the styrene monomer is 500 ppm, The total content of styrene oligomers was 5000 ppm.
(Examples 2-13, Comparative Examples 1-12)

Except for the types and blending amounts of the styrene-methacrylic acid copolymer resins (A), multi-branched styrene copolymer resins (B), and impact-resistant styrene resins (C) listed in Tables 1 and 3, A biaxially stretched sheet having a thickness of 0.25 mm was obtained in the same manner as in Example 1. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet, the tensile elastic modulus, the rubber component content in the biaxially stretched sheet, the average particle diameter of the rubber particles, the gel content, the styrenic monomer Tables 1 and 3 show the contents and the total contents of styrene oligomers.
(Examples 14 to 17)

The types and blending amounts of the styrene-methacrylic acid copolymer resin (A), the multi-branched styrene copolymer resin (B), and the impact-resistant styrene resin (C) described in Tables 1 and 2 were used. Is the same as Example 1, except that the stretching temperature is 132 ° C, Example 15 is 130 ° C, Example 16 is 126 ° C, and Example 17 is 124 ° C. A biaxially stretched sheet having a thickness of 0.25 mm was obtained. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet, the tensile elastic modulus, the rubber component content in the biaxially stretched sheet, the average particle diameter of the rubber particles, the gel content, the styrenic monomer Tables 1 and 2 show the contents and the total contents of styrene oligomers.
(Examples 18 to 21)

The types and amounts of the styrene-methacrylic acid copolymer resin (A), the multi-branched styrene copolymer resin (B), and the impact-resistant styrene resin (C) shown in Table 2 were used. 124 ° C., draw ratio (MD / TD) 1.8 times, Example 19 has a draw temperature 126 ° C., draw ratio (MD / TD) 2.5 times, and Example 20 has a draw temperature 130 ° C. Thickness is the same as in Example 1 except that the draw ratio (MD / TD) is 4.0 times and Example 21 is drawn at a draw temperature of 132 ° C. and the draw ratio (MD / TD) is 4.7 times. A 0.25 mm biaxially stretched sheet was obtained. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet, the tensile elastic modulus, the rubber component content in the biaxially stretched sheet, the average particle diameter of the rubber particles, the gel content, the styrenic monomer Table 2 shows the content and the total content of styrene oligomers.
(Examples 22 to 25)

The types and amounts of the styrene-methacrylic acid copolymer resin (A), the multi-branched styrene copolymer resin (B), and the impact-resistant styrene resin (C) shown in Table 2 were used. Biaxial stretching in the same manner as in Example 1 except that 0.008 mm, Example 23 has a thickness of 0.02 mm, Example 24 has a thickness of 0.60 mm, and Example 25 has a thickness of 0.80 mm. A sheet was obtained. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet, the tensile elastic modulus, the rubber component content in the biaxially stretched sheet, the average particle diameter of the rubber particles, the gel content, the styrenic monomer Table 2 shows the content and the total content of styrene oligomers.
(Example 26)

The types and amounts of the styrene-methacrylic acid copolymer resin (A), the multi-branched styrene copolymer resin (B), and the impact-resistant styrene resin (C) shown in Table 2 were used. A biaxially stretched sheet was obtained in the same manner as in Example 1 except that pelletization was performed twice through an extruder under the same conditions as in Example 1. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet, the tensile elastic modulus, the rubber component content in the biaxially stretched sheet, the average particle diameter of the rubber particles, the gel content, the styrenic monomer Table 2 shows the content and the total content of styrene oligomers.
(Example 27)

The types and amounts of the styrene-methacrylic acid copolymer resin (A), the multi-branched styrene copolymer resin (B), and the impact-resistant styrene resin (C) listed in Table 2 were used. Except for using a extruder (a twin-screw co-directional extruder with a vacuum vent TEM35B (manufactured by TOSHIBA MACHINERY)), extruding temperature 230 ° C., rotating speed 250 rpm, vent devolatilization pressure −250 mmHg. A biaxially stretched sheet was obtained in the same manner as in Example 1. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet, the tensile elastic modulus, the rubber component content in the biaxially stretched sheet, the average particle diameter of the rubber particles, the gel content, the styrenic monomer Table 2 shows the content and the total content of styrene oligomers.
(Example 28)

  The types and blending amounts of the styrene-methacrylic acid copolymer resin (A), the multi-branched styrene copolymer resin (B), and the impact-resistant styrene resin (C) shown in Table 2 were used. Except using an extruder (TEM35B twin screw co-axial extruder with vacuum vent TEM35B (manufactured by TOSHIBA MACHINE)), extruding temperature 230 ° C, rotating speed 250rpm, and without venting and removing the strand through the die plate. A biaxially stretched sheet was obtained in the same manner as in Example 1. The orientation relaxation stress (MD / TD) of the obtained biaxially stretched sheet, the tensile elastic modulus, the rubber component content in the biaxially stretched sheet, the average particle diameter of the rubber particles, the gel content, the styrenic monomer Table 2 shows the content and the total content of styrene oligomers.

  About the obtained biaxially stretched sheet, various performances were measured and evaluated by the method described below. In the relative evaluation of ◎, ○, △, and ×, ◎, ○, and △ were judged as acceptable. The results are shown in Tables 1-3.

(1) Film-forming property The thickness is measured using a micro gauge at 25 points of intersection when 5 straight lines are drawn in a grid at 20 mm intervals in the MD and TD directions on an unstretched sheet. Evaluation was made according to the following criteria.
◎: σ is less than 0.03 mm ○: σ is 0.03 mm or more and less than 0.05 mm △: σ is 0.05 mm or more and less than 0.07 mm ×: σ is 0.07 mm or more

(2) Sheet appearance Regarding the range of biaxially stretched sheet 350 mm × 350 mm, 1) area of roll 100 mm ² or more, 2) area of 10 mm ² or more bubbles, 3) transparent and opaque foreign matter, 4) adhesion defect, 5) A die line having a width of 3 mm or more (a defect running in the sheet flow direction generated at the T-die outlet during film formation) was regarded as a defect, and the number of defects was evaluated according to the following criteria.
◎: 0 ○: 0 (However, there are bubbles with an area of less than 10 mm ² and die lines with a width of less than 3 mm)
Δ: 1-2 pieces ×: 3 or more pieces

(3) Transparency (haze)
According to JIS K-7361-1, the haze of the biaxially stretched sheet was measured using a haze meter NDH5000 (Nippon Denshoku).
◎: Haze less than 2.0% ○: Haze 2.0% or more, less than 3.0% Δ: Haze 3.0% or more, less than 5.0% ×: Haze 5.0% or more

(4) Rigidity A 500g weight is placed in the main body of the lunch box (dimensions: length 163 x width 223 x depth 30mm), and the lunch box fitted with a lunch box lid, which will be described later, is stacked in five stages and left for 24 hours. The deformation state of the lid was confirmed.
A: No change in shape.
○: It bends but returns to its original state when the load is removed.
Δ: Deformed.
X: There is a crack.

(5) Folding resistance According to ASTM D2176, the bending strength in the sheet extrusion direction (MD direction) and the direction perpendicular thereto (TD direction) was measured, the minimum value was determined, and evaluated as follows.
◎: 7 times or more ○: 5 times or more, less than 7 times △: 2 times or more, less than 5 times ×: Less than 2 times

(6) Formability With a hot plate molding machine HPT? 400A (Wakisaka Engineering Co., Ltd.), under the conditions of a hot plate temperature of 150 ° C and a heating time of 2.0 seconds (dimension lid: length 165 x width 225 x 26 mm in height) was formed, and formability was evaluated according to the following criteria.
◎: Good ○: Slightly poor shape at the corner, but inconspicuous level △: Slightly poor shape at the corner ×: Remarkably poor shape at the corner or corner

(7) Mold fouling property When molding the lunch box lid, the transfer of dirt on the mold and the like was evaluated according to the following criteria.
A: No transfer (clear, no cloudiness)
○: Slightly transferred (opaque, slightly cloudy surface)
Δ: Transfer in part (opaque, cloudy surface)
×: Transferred throughout (opaque, surface cloudy)

(8) Heat resistance The lunch box lid obtained under the above molding conditions was placed in a hot air dryer set at 110 ° C for 60 minutes, and then the deformation of the container was visually observed.
◎: No deformation ○: Minor deformation,
Δ: Outside dimension change less than 5% ×: Large deformation, outside dimension change over 5%

(9) Container strength The lunch box lid obtained under the above molding conditions was fitted into the main body container, applied with a surface load uniformly from the top and pushed in for 10 mm, and then the state of the lid after removing the load was evaluated.
A: No deformation.
○: Slight deformation.
Δ: Deformation with change rate of outer dimension less than 5%.
X: Large deformation, change rate of outside dimension is 5% or more.

  From the results of Tables 1 to 3, any of the biaxially stretched sheets that satisfy the provisions of the present invention obtained in Examples 1 to 28 has film forming properties, transparency, sheet strength, moldability, container strength, and heat resistance. In any case, it had excellent performance.

  On the other hand, any of the biaxially stretched sheets obtained in Comparative Examples 1 to 12 that do not satisfy the provisions of the present invention is any of film forming properties, transparency, sheet strength, moldability, container strength, and heat resistance. Was an undesirable result.

Claims (8)

A styrene-methacrylic acid copolymer resin (A) containing a styrene monomer unit and a methacrylic acid monomer unit in a mass ratio of 84/16 to 94/6 and having a weight average molecular weight of 120,000 to 250,000,
Multi-branched styrene copolymer resin (B) having a weight average molecular weight of 350,000 to 550,000 and impact-resistant styrene resin (C) containing a rubber component
A biaxially stretched sheet comprising a styrene-based resin composition containing
The mass ratio (A) / (B) of the styrene-methacrylic acid copolymer resin (A) and the multi-branched styrene copolymer resin (B) is 70/30 to 95/5,
The content of the rubber component in the biaxially stretched sheet is 0.05 to 0.3% by mass,
The biaxially stretched sheet wherein the rubber particles formed by the rubber component in the biaxially stretched sheet have an average particle diameter of 1.2 to 12 μm.   The biaxial structure according to claim 1, wherein the orientation relaxation stress in the MD direction and the TD direction are both in the range of 0.8 to 2.0 MPa, and the tensile elastic modulus in the MD direction and the TD direction is both 2800 to 3400 MPa. Stretched sheet.   The biaxially stretched sheet according to claim 1 or 2, wherein both the MD and TD stretch ratios are 2.0 to 4.5 times.   The biaxially stretched sheet according to any one of claims 1 to 3, wherein the thickness is 0.01 mm to 0.7 mm.   The biaxially stretched sheet according to any one of claims 1 to 4, wherein the gel content is 1.0 mass% or less.   The biaxially stretched sheet according to any one of claims 1 to 5, wherein a content of the styrene monomer is 1000 ppm or less and a total content of the styrene oligomer is 10,000 ppm or less.   The molded article which consists of a biaxially stretched sheet of any one of Claims 1-6.   The molded article according to claim 7, which is a food packaging container.