JP6717599B2 - Styrene resin, styrene resin foam sheet, and food container - Google Patents

Description

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain an extruded foam sheet which has little odor of the extruded foam sheet, is excellent in drawdown resistance at the time of secondary molding and deep drawing moldability, and is excellent in the appearance and waist strength of the secondary molded product. The present invention relates to a styrene resin that can be used.

Extruded foamed sheets of styrene resin are widely used for food trays, bento boxes, instant noodle containers, natto containers, cups, etc. by taking advantage of features such as buffering property and heat insulating property. In these food packaging applications, since the container and the food are in direct contact with each other, it is required to reduce the styrene dimer and styrene trimer contained in the styrene resin for the purpose of suppressing the transfer of odor from the container surface to the contents. However, when oligomer components such as styrene dimer and styrene trimer are reduced, moldability and strength may be deteriorated, and there is a problem that secondary molding into a food container having a deep drawing shape or a complicated shape becomes difficult. It was

As a method of improving moldability and strength when the amount of oligomer components such as styrene dimer and styrene trimer is reduced, Patent Document 1 discloses a method in which a methanol-soluble content is within a specific range. 3 discloses a method of improving low-temperature moldability by setting liquid paraffin within a specific range.

On the other hand, such a container having a deep-drawn shape or a complicated shape usually has a rubber-modified polystyrene resin on one side or both sides of an extruded foam sheet having a basis weight of 200 g/m ² or more from the viewpoint of moldability and strength improvement. A laminated product is often used, but when the extruded foam sheet is heated in a heating furnace during secondary molding to soften it to a temperature suitable for molding, the extruded foam sheet hangs down due to its own weight, a so-called draw. There is a problem in that a down occurs and a creased wrinkle or a dent occurs in the molded container.

Therefore, in order to suppress the occurrence of drawdown, Patent Document 4 discloses a method in which the heat shrinkage ratio of the foamed sheet is within a specific range.

JP, 2002-79622, A JP-A-3-103450 JP-A-2002-80668 JP, 2003-251762, A

However, the related art described in the above literature has room for improvement in the following points.
First, in any of the techniques of Patent Documents 1 to 3, the secondary moldability and drawdown resistance of the foamed sheet were insufficient.
Second, although the technique of Patent Document 4 is improved in drawdown resistance, it is insufficient in secondary molding. Further, in order to adjust the heat shrinkage ratio within a specific range, it is necessary to finely adjust the molding temperature of the foamed sheet, the shape of the die lip, and the like, which causes a problem that the degree of freedom in production is extremely narrowed.
The present invention has been made in view of the above circumstances, and an object thereof is to achieve a problem that the styrene-based resin foam sheet described above has less odor and is excellent in the balance of drawdown resistance and deep drawing formability. To do.

The inventors of the present invention have conducted eager research in order to achieve the problem that the styrene resin foam sheet described above has less odor and is excellent in the balance of drawdown resistance and secondary moldability. It was found that the above-mentioned objects can be achieved by setting the melt mass flow rate, the total amount of styrene dimer and trimer, and the melt tension value (MT) measured at 200° C. within a specific range, and completed the present invention. ..

That is, the present invention is as shown in the following (1) to (7).
(1) Melt mass flow rate (MFR) measured at 200° C. and 49 N load is 1.0 to 2.5 g/10 minutes, the total amount of styrene dimer and trimer is 500 to 2500 μg/g, and at 200° C. A styrene-based resin having a measured melt tension value (MT) of 8 to 20 gf.
(2) The styrene-based resin according to (1), which has a die swell value of 1.5 to 2.5 as measured by a capillary rheometer at 200° C. and a shear rate of 6,080 s ⁻¹ .
(3) The styrene resin according to (1) or (2), wherein the styrene resin has a peak top molecular weight (Mtop) of 200,000 to 280,000.
(4) The styrene resin according to any one of (1) to (3) above, wherein the proportion of the component having a molecular weight of 1,000,000 or more is 2 to 10% by mass.
(5) The styrene resin according to any one of (1) to (4) above, wherein the proportion of the component having a molecular weight of 50,000 or less is 5 to 15% by mass.
(6) A styrene resin foam sheet obtained by foaming and extruding the styrene resin according to any one of (1) to (5).
(7) A food container obtained by molding the styrene resin foam sheet according to (6).

The styrenic resin of the present invention has a small odor due to the low content of styrene dimer and trimer, and it is possible to obtain a foamed sheet having an excellent balance of drawdown resistance and deep drawing moldability. However, it is possible to obtain a secondary molded product having good appearance and waist strength.

Hereinafter, the present invention will be described in detail.

<Characteristics of styrene resin>
The styrene resin of the present invention has a melt mass flow rate (MFR) measured under the conditions of 200° C. and a load of 49 N of 1.0 to 2.5 g/10 minutes, preferably 1.1 to 2.4 g/10. Minutes, and more preferably 1.2 to 2.3 g/10 minutes. If it exceeds 2.5 g/10 minutes, the drawdown resistance of the styrenic resin foam sheet may be reduced, which may lead to poor molding of the secondary molded product and lower waist strength. Also, when it is less than 1.0 g/10 minutes, the secondary moldability is deteriorated, and further, the pressure inside the extruder is excessively increased during the production of the styrene resin foam sheet, so that the productivity is deteriorated. The melt mass flow rate (MFR) is 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9. , 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 g/10 min.

The total amount of styrene dimer and trimer of the styrene resin of the present invention is 500 to 2500 μg/g, preferably 500 to 2000 μg/g, and more preferably 500 to 1500 μg/g. When the total amount of styrene dimer and trimer exceeds 2500 μg/g, the resulting container becomes inferior in odor and the heat resistance of the container decreases, which is not preferable. Further, when the total amount of styrene-based dimer and trimer is less than 500 μg/g, moldability is deteriorated. Here, the styrene dimer is the total amount of linear structure 2,4-diphenyl-1-butene, cyclic structure 1,2-diphenylcyclobutane, and 1,3-diphenylcyclobutane, and the styrene trimer is 2,4,6-triphenyl-1-hexene having a linear structure, 2,4,6-triphenylcyclohexane having a cyclic structure, 1,2,3,4-tetrahydro-1-(1'-phenylethyl)- It means the total amount of 4-phenylnaphthalene. The total amount of styrene dimer and trimer is 2500, 2400, 2300, 2200, 2100, 2000, 1900, 1800, 1700, 1600, 1500, 1400, 1300, 1200, 1100, 1000, 900, 800, 700, 600, 500 μg. It may be within the range of any two values of /g.

It is known that styrene dimers and trimers are produced as by-products in the polymerization step for producing a styrene-based resin and are produced due to thermal decomposition in the devolatilization step. The by-product in the polymerization step is generated by the thermal initiation radical, so it can be reduced by using a large amount of the polymerization initiator and polymerizing at a low temperature, and suppressing thermal decomposition in the devolatilization step. In order to reduce the heat history of the devolatilization step, or to suppress the formation of dimers and trimers due to thermal decomposition by adding a hindered phenolic antioxidant described below in the polymerization step, or in the devolatilization step. it can.

The melt tension value of the styrene resin of the present invention measured at 200° C. is 8 to 20 gf, preferably 9 to 18 gf, and more preferably 10 to 17 gf. If the melt tension value is less than 8 gf, the secondary moldability and drawdown resistance of the foamed sheet will be insufficient, and if the melt tension value exceeds 20 gf, the secondary moldability will deteriorate, which is not preferable. The melt tension value may be within the range of any two values among 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 gf.

The styrene resin of the present invention preferably has a die swell value of 1.5 to 2.5, measured by a capillary rheometer at 200° C. and a shear rate of 6,080 s ⁻¹ , and 1.6 to It is more preferably 2.4 and particularly preferably 1.7 to 2.3. If the die swell value is less than 1.5, the drawdown resistance is insufficient, and if the die swell value exceeds 2.5, the secondary formability may deteriorate, which is not preferable. The die swell values are 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5. It may be within the range of any two values.

The styrene resin of the present invention preferably has a peak top molecular weight (Mtop) measured by gel permeation chromatography (GPC) of 200,000 to 280,000, more preferably 210,000 to 270,000, and 22 It is particularly preferable that it is 10,000 to 260,000. If the Mtop is less than 200,000, the drawdown resistance of the foamed sheet will deteriorate. Further, when Mtop exceeds 280,000, the fluidity is lowered and the molding elongation is deteriorated, so that the deep drawing of the styrene resin foam sheet becomes difficult. The Mtop of the styrene resin can be adjusted by the reaction temperature in the polymerization step, the residence time, the type and addition amount of the polymerization initiator, the type and addition amount of the chain transfer agent, and the type and amount of the solvent used during the polymerization. Even if the peak top molecular weight (Mtop) is within the range of any two values among 200,000, 210,000, 220,000, 230,000, 240,000, 250,000, 260,000, 270,000 and 280,000. Good.

In the styrene-based resin of the present invention, the proportion of components having a molecular weight of 1,000,000 or more is preferably 2 to 10% by mass, and the proportion of components having a molecular weight of 1,000,000 or more is more preferably 2 to 9% by mass. It is particularly preferable that the proportion of 1,000,000 or more components is 2 to 8% by mass. The molecular weight ratio of the styrene resin can be adjusted by the reaction temperature in the polymerization step, the residence time, the type and amount of the polymerization initiator, the type and amount of the solvent used during the polymerization, and the type and amount of the chain transfer agent. The ratio of the component having a molecular weight of 1,000,000 or more may be in the range of any two values among 2, 3, 4, 5, 6, 7, 8, 9, and 10.

In the styrene-based resin of the present invention, the proportion of components having a molecular weight of 50,000 or less is preferably 5 to 15% by mass, and the proportion of components having a molecular weight of 50,000 or less is more preferably 6 to 12% by mass. It is particularly preferable that the proportion of the components of 50,000 or less is 7 to 11% by mass. The molecular weight ratio of the styrene resin can be adjusted by the reaction temperature in the polymerization step, the residence time, the type and amount of the polymerization initiator, the type and amount of the solvent used during the polymerization, and the type and amount of the chain transfer agent. The ratio of the component having a molecular weight of 50,000 or less may be in the range of any two values among 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15.

In the styrene resin of the present invention, the ratio (Mw/Mn) of the weight average molecular weight (Mw) and the number average molecular weight (Mn) is preferably 2.0 to 3.5, and the ratio is 2.3 to 3.3. It is more preferable that it is, and it is particularly preferable that it is 2.5 to 3.1. When the ratio (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is less than 2.0, the moldability and the appearance of the container may be deteriorated, and (Mw/Mn) exceeds 3.5. In some cases, the drawdown resistance may deteriorate. In addition, this (Mw/Mn) is 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, It may be within a range of any two values among 3.0, 3.1, 3.2, 3.3, 3.4, and 3.5.

<Styrene Resin Raw Material and Manufacturing Method>
The styrene resin of the present invention has a styrene monomer as an essential component (an essential content component) as a raw material, but in addition to a homopolymer of styrene, a vinyl monomer copolymerizable with styrene at a ratio of 50% by mass or less. It may be included. Examples of vinyl-based monomers include substituted styrenes such as α-methylstyrene and p-methylstyrene, acrylic-based monomers such as acrylic acid, methacrylic acid, butyl acrylate and methyl methacrylate, and vinyl cyanide-based such as acrylonitrile and methacrylonitrile. Examples thereof include monomers and maleic anhydride.

Examples of the polymerization method of the styrene resin of the present invention include known styrene polymerization methods such as bulk polymerization method, solution polymerization and suspension polymerization method. Further, as the solvent, for example, alkylbenzenes such as benzene, toluene, ethylbenzene, and xylene, ketones such as acetone and methylethylketone, and aliphatic hydrocarbons such as hexane and cyclohexane can be used. As a mode of the reactor, a continuous polymerization system in which a complete mixing type reactor, a plug flow reactor, a loop type reactor and the like are combined is preferably used.

In the case of continuous polymerization, a devolatilization step for removing unreacted styrene monomer and solvent after the completion of polymerization is provided. As an example of the devolatilization step, a vacuum devolatilization tank with a preheater was used in only one stage, a devolatilization extruder was used in only one stage, and a vacuum devolatilization tank with a preheater was connected in two stages. One, a vacuum devolatilization tank with a preheater and a devolatilizing extruder connected in two stages in series, and after reducing the volatile components in the first stage vacuum devolatilizing tank, water was added as a devolatilization aid. After mixing with a mixer, the effective surface area in contact with the gas phase part and the polymer solution is increased by foaming under reduced pressure in the vent part of the devolatilization tank or devolatilization extruder, and the devolatilization efficiency of volatile components is increased. Methods and the like.

When producing the styrene resin of the present invention, from the viewpoint of controlling the polymerization reaction, a polymerization solvent, a polymerization initiator such as an organic peroxide, and a chain transfer agent such as an aliphatic mercaptan may be used if necessary. You can

As the polymerization initiator, a radical polymerization initiator is preferable, and known and conventional ones such as 1,1-di(t-butylperoxy)cyclohexane, 2,2-di(t-butylperoxy)butane, 2,2- Peroxyketals such as di(4,4-di-t-butylperoxycyclohexyl)propane and 1,1-di(t-amylperoxy)cyclohexane, cumene hydroperoxide, t-butyl hydroperoxide and the like Alkyl peroxides such as hydroperoxides, t-butylperoxyacetate, t-amylperoxyisononanoate, etc., t-butylcumylperoxide, di-t-butylperoxide, dicumylperoxide, di-t -Dialkyl peroxides such as hexyl peroxide, peroxyesters such as t-butylperoxyacetate, t-butylperoxybenzoate, t-butylperoxyisopropyl monocarbonate, t-butylperoxyisopropyl carbonate, polyether Peroxycarbonates such as tetrakis(t-butylperoxycarbonate), N,N′-azobis(cyclohexane-1-carbonitrile), N,N′-azobis(2-methylbutyronitrile), N,N′ -Azobis(2,4-dimethylvaleronitrile), N,N'-azobis[2-(hydroxymethyl)propionitrile] and the like can be mentioned, and these can be used alone or in combination of two or more. ..

Examples of chain transfer agents include monofunctional chain transfer agents such as aliphatic mercaptans, aromatic mercaptans, pentaphenylethane, α-methylstyrene dimer and terpinolene, and ethylene glycol, tetraethylene glycol, neopentyl glycol, trimethylolpropane. , Polypentaerythritol, dipentaerythritol, tripentaerythritol, sorbitol, and other polyhydric alcohol hydroxyl groups are esterified with thioglycolic acid or mercaptopropionic acid. One kind or a combination of two or more kinds can be used.

The styrene-based resin of the present invention preferably contains 0.01 to 0.5 parts by mass of a hindered phenolic antioxidant with respect to 100 parts by mass of the resin. By including a hindered phenol-based oxidizer in the styrene resin, it is possible to suppress the amount of dimer and trimer generated due to heat history when molding the styrene resin into a foamed sheet, and reduce the odor of the molded product. Can be suppressed.

The hindered phenolic antioxidant is an antioxidant having a phenolic hydroxyl group in the basic skeleton, and examples thereof include octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and 3,9. -Bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5. 5] Undecane, ethylenebis(oxyethylene)bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate], pentaerythritol tetrakis[3-(3,5-di-t-butyl-4) -Hydroxyphenyl) propionate] 4,6-bis(octylthiomethyl)-o-cresol, 4,6-bis[(dodecylthio)methyl]-o-cresol, 2,4-dimethyl-6-(1-methylpenta Decyl)phenol, tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, DL-α-tocopherol, 2-t-butyl-6-(3-t-butyl) 2-Hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, 2-[1-(2-hydroxy-3,5-di-t-pentylphenyl)ethyl]-4,6-di-t-pentyl Phenyl acrylate, 6-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propoxy]-2,4,8,10-tetra-t-butyldibenzo[d,f][1,3 , 2] Dioxaphosphepine, 4,4′-thiobis(6-t-butyl-3-methylphenol), dimyristyl-3,3′-thiodipropionate, 1,1,3-tris(2- Methyl-4-hydroxy-5-t-butylphenyl)butane, 4,4′-butylidenebis(3-methyl-6-t-butylphenol), bis-[3,3-bis-(4′-hydroxy-3′) -Tert-butylphenyl)-butanoic acid]-glycol ester and the like. Preferably, octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 3,9-bis[2-[3-[3-(3-tert-butyl-4-hydroxy-5-methyl) Phenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane, ethylenebis(oxyethylene)bis[3-(5-tert-butyl-4) -Hydroxy-m-tolyl)propionate], and these may be used alone or in combination of two or more kinds.

The styrene-based resin of the present invention contains rubber-reinforced aromatic vinyl-based resin such as HI-PS resin and MBS resin, or aromatic vinyl-based thermoplastic elastomer such as SBS as several% as a component containing rubber if necessary. It may be contained to some extent. Further, higher fatty acids such as stearic acid, zinc stearate, calcium stearate and magnesium stearate and their salts, lubricants such as ethylenebisstearylamide, plasticizers such as liquid paraffin, phenolic antioxidants, phosphorus antioxidants, etc. Antioxidants, ultraviolet absorbers, antistatic agents, flame retardants, colorants, pigments, deodorants, etc. may be included.

Further, the raw material for producing the styrene resin of the present invention may contain 0 to 50% by mass of a recycled raw material composed of at least one of a polystyrene resin foam sheet and a polystyrene resin non-foam sheet. As the polystyrene resin foam sheet, a foam sheet made of the styrene resin of the present invention may be used, or a foam sheet made of other polystyrene resin may be used. As the polystyrene resin non-foamed sheet, a biaxially stretched polystyrene resin sheet or the like can be used. The content rate of this recycled material is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. , 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45. , 46, 47, 48, 49, 50% by mass may be in the range of any two values.

Recycled raw materials include not only the polystyrene-based resin foamed sheets and polystyrene-based resin non-foamed sheets described above, but also thermoformed products of these, scrap materials called skeletons generated when the thermoformed products are punched from the sheet, and the like. Be done. As the recycled raw material, a crushed product such as a sheet, a thermoformed product, or a skeleton may be directly mixed with the virgin material, and recycled pellets may be prepared from the crushed product and used as a virgin material before being introduced into the foam extruder. You may use it after dry blending. The molecular weight of the recycled raw material is not particularly limited, but the weight average molecular weight (Mw) is preferably 200,000 or more.
When mixing the recycled raw materials, the characteristics after mixing the recycled raw materials should be within the scope of the present invention.

<Styrenic resin foam sheet and method for producing the same>
The styrene resin of the present invention is suitably used as an extruded foam sheet. As a method for manufacturing the extruded foam sheet, a known extruded foam sheet manufacturing apparatus can be used. Specifically, two single-screw extruders or two-screw extruders are arranged in series, and the first extruder is used. In this method, the foaming agent is melt-kneaded together with the foaming nucleating agent, the resin temperature is adjusted to 120° C. to 180° C. by cooling with the second extruder, and then the resulting mixture is discharged into the atmosphere by a circular die and foamed under reduced pressure.

The foaming agent, propane, normal butane, isobutane, pentane, aliphatic hydrocarbons such as hexane, cyclobutane, cycloaliphatic hydrocarbons such as cyclopentane, trichlorofluoromethane, dichlorodifluoromethane, 1,1-difluoroethane, Physical foaming agents such as halogenated hydrocarbons such as 1,1-difluoro-chloride and methylene chloride can be used. Further, decomposing type foaming agents such as azodicarbonamide, dinitrosopentamethylenetetramine, azobisisobutyronitrile, sodium bicarbonate and citric acid, and inorganic gas such as carbon dioxide and nitrogen and water can also be used. These foaming agents can be appropriately mixed and used, but industrially, butane is often used, and from the viewpoint of foam extrudability, secondary moldability of foam sheet, and foaming agent, a mixture of isobutane and normal butane is used. Preference is given to using butane. Since butane has a low permeation rate through polystyrene resin, it usually remains in the foamed sheet in an amount of about 0.5 to 3% by mass immediately after foaming and extrusion. Since this residual amount affects the secondary foaming thickness and thermoformability in the secondary molding, it is appropriately adjusted by providing a certain aging period.

Examples of the foam nucleating agent include inorganic powders such as talc, calcium carbonate and clay, and these can be used alone or as a mixture. Among them, talc is most preferable because it has a large effect of reducing the bubble diameter and is inexpensive. The method for adding the foam nucleating agent is not particularly limited, and it may be added directly to the feed holes of the extruder, or may be added together with the styrene resin. It is also possible to prepare a masterbatch using a styrene homopolymer or polystyrene as a base material, and supply the masterbatch. The amount of the foam nucleating agent added is usually 0.1 to 5 mass %. Further, higher fatty acid or a metal salt of higher fatty acid may be mixed in advance with the masterbatch. Further, even if a lubricant such as ethylenebisstearylamide, a spreading agent such as liquid paraffin or silicone oil, other surfactants, an antistatic agent, an antioxidant, a plasticizer, a weathering agent, a pigment, etc. are contained. good.

The thickness of the extruded foam sheet of the present invention is preferably 0.5 to 4.0 mm, more preferably 1.0 to 3.0 mm. If the thickness of the extruded foam sheet is less than 0.5 mm, the strength and heat insulation of the container after the secondary molding are deteriorated. When the thickness of the extruded foam sheet exceeds 4.0 mm, temperature unevenness of the sheet is likely to occur during secondary molding, resulting in deterioration of moldability.

Density of the extruded foam sheet of the present invention is preferably from 70~300kg / ^{m 3,} more preferably 90~250kg / ^{m 3.} If the density of the extruded foam sheet is less than 70 kg/m ³ , deep drawing becomes difficult. If the density exceeds 300 kg/m ³ , the heat insulation of the container will be insufficient. The density D (kg/m ³ ) can be calculated by D=S/T from the basis weight S (g/m ² ) of the foamed sheet and the sheet thickness T (mm).

In the extruded foam sheet of the present invention, the average cell diameter X in the thickness direction of the sheet is preferably 0.10 to 0.40 mm. When the average cell diameter X in the thickness direction of the sheet is less than 0.10 mm, the formability in the secondary forming is reduced. When the average cell diameter X in the thickness direction of the sheet exceeds 0.40 mm, the appearance of the foamed sheet deteriorates and the strength also decreases.

Further, the ratio of the average cell diameter Y in the extrusion direction and the average cell diameter X in the thickness direction (Y/X), and the ratio of the average cell diameter Z in the width direction and the average cell diameter X in the thickness direction (Z/X) are respectively It is preferably 1.0 to 3.0. When Y/X and Z/X are less than 1.0, drawdown resistance of the foamed sheet deteriorates, which is not desirable. Further, when Y/X and Z/X are more than 3.0, the flatness of the cells is large and the secondary formability of the foamed sheet is deteriorated.

The average cell diameter X in the thickness direction of the sheet, the average cell diameter Y in the extrusion direction, and the average cell diameter Z in the width direction were observed by using a scanning electron microscope to observe a vertical cross section in the extrusion direction and a vertical cross section in the width direction of the foamed sheet. , ASTM D2842-06 based on the average chord length can be calculated using the following formula.
Average chord length=straight line length/number of bubbles Average bubble diameter=average chord length/0.616

In addition, the extruded foam sheet of the present invention can be provided with a surface layer, which is so-called a skin layer, having a higher density than the central portion in the thickness direction on the front and back surfaces of the sheet. By providing the skin layer, the strength of the sheet can be increased and the appearance can be finished beautifully. The skin layer can be adjusted by air-cooling the surface of the foamed sheet immediately after leaving the circular die.

The extruded foam sheet of the present invention can have improved moldability, strength and rigidity by laminating a thermoplastic resin sheet or film on one or both sides thereof. As the thermoplastic resin forming the sheet or film, polystyrene, polystyrene-based resin such as high-impact polystyrene, polypropylene-based resin, polyester-based resin, high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate Examples thereof include a copolymer, but a polystyrene-based resin that can be laminated without using an adhesive layer and has good recyclability is preferable.

The thickness of the thermoplastic resin sheet or film laminated above is not particularly limited, but is preferably 10 to 300 μm, more preferably 50 to 250 μm, and particularly preferably 70 to 200 μm. A thicker sheet or film is advantageous for deep drawing, but if it is too thick, the weight of the container increases, which is not desirable.

The extruded foamed sheet of the present invention can be subjected to thermoforming such as vacuum forming or pressure forming to be secondarily formed into a container such as a tray, an instant noodle container, a natto container, or a cup, and is particularly suitable for deep drawing applications. ing.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.

<Production of styrene resins PS-1 to PS-10>
The polymerization step was constituted by connecting the first reactor, which is a complete mixing type stirring tank, the second reactor, and the third reactor, which is a plug flow type reactor with a static mixer, in series. The volume of each reactor was 39 liters for the first reactor, 39 liters for the second reactor, and 16 liters for the third reactor. A raw material solution was prepared with the raw material composition shown in Table 1, and the raw material solution was continuously supplied to the first reactor at the flow rate shown in Table 1. The polymerization initiator was added to the raw material solution at the inlet of the first reactor so as to have the addition concentration shown in Table 1 (concentration on a mass basis with respect to the raw material styrene), and uniformly mixed. The following polymerization initiators were used as shown in Table 1.
Polymerization initiator 1: 2,2-di(4,4-t-butylperoxycyclohexyl)propane (Pertetra A manufactured by NOF CORPORATION was used.)
Polymerization initiator 2: t-amyl peroxyisononanoate (Ruperox 570 manufactured by Arkema Yoshitomi Co., Ltd. was used.)
Polymerization initiator 3: 1,1-di(t-butylperoxy)cyclohexane (Perhexa C manufactured by NOF CORPORATION was used.)
In the third reactor, a temperature gradient was applied along the flow direction, and the temperature was adjusted so that the temperatures in Table 1 were obtained at the intermediate portion and the outlet portion.
Then, the solution containing the polymer continuously taken out from the third reactor was introduced into a vacuum devolatilization tank with a preheater, which was composed of two stages in series, and the preheater was adjusted so that the resin temperatures shown in Table 1 were obtained. The unreacted styrene and ethylbenzene are separated by adjusting the temperature of the above and adjusting the pressures shown in Table 1, and then extruded in a strand form from a porous die, and the strand is cooled and cut by a cold cut method. Pelletized. PS-1 to 5 are examples, and PS-6 to 10 are comparative examples. At the outlet of the third reactor, the following antioxidants were added to PS-1 to 9 at the concentrations shown in Table 1 (concentrations based on the mass of the polymer to be produced), and the mixture was uniformly mixed and then devolatilized. Transferred to the process.
Antioxidant-1: 2-[1-(2-hydroxy-3,5-di-t-pentylphenyl)ethyl]-4,6-di-t-pentylphenyl acrylate (Sumitomo Chemical Co., Ltd. Sumilizer GS used.)
Antioxidant-2: 4,4′-thiobis(6-t-butyl-3-methylphenol) (Sumitomo Chemical Co., Ltd. Sumilizer WX-R was used.)
Antioxidant-3: Dimyristyl-3,3'-thiodipropionate (using Sumitizer Chemical Co., Ltd. Sumilizer TPM).

<Examples 1 to 5, Comparative Examples 1 to 5>
Next, a foamed sheet was manufactured using a tandem type extruder having screw diameters of 40 mmφ and 50 mmφ. First, 2.3 parts by mass of talc masterbatch consisting of 60% by mass of polystyrene and 40% by mass of talc was uniformly mixed with 100 parts by mass of the above-mentioned styrene-based resins PS-1 to PS-10, and a mixture having a screw diameter of 40 mmφ was prepared. It was fed to the extruder. Further, a mixed butane of isobutane and normal butane of 60/40 (mass ratio) was injected as a foaming agent from the tip of the extruder at a ratio of 1.5 parts by mass with respect to 100 parts by mass of the resin, and melt-mixed. At this time, the cylinder temperature was 160 to 210°C, the resin temperature was 190 to 210°C, and the pressure was 12 to 18 MPa.
Then, transferred to an extruder with a screw diameter of 50 mmφ through a connecting pipe set to 210° C., adjusted to a cylinder temperature of 150 to 170° C., a resin temperature of 148 to 160° C., and 15 to 17 MPa, and a lip opening of 0.6 mm, A circular die having a diameter of 40 mm was extruded at a discharge rate of 10 kg/hr along with a cooled cylinder having a diameter of 152 mm, and was taken out. The foamed sheet was obtained by cutting with a cutter at one lower portion of the circumference. The thickness of the foamed sheet was 1.8 mm, the density was 150 kg/m ³ , and the average cell diameter X in the thickness direction of the sheet was 0.15 mm. The properties of the foamed sheet are shown in Table 2.

The characteristics of the styrene resin were evaluated by the following methods.
(1) Molecular weight The number average molecular weight (Mn), the weight average molecular weight (Mw), the Z average molecular weight (Mz), and the peak top molecular weight (Mtop) are determined under the following conditions using gel permeation chromatography (GPC). It was measured.
GPC model: Shodex GPC-101 manufactured by Showa Denko KK
Column: PL Laboratories PLgel 10 μm MIXED-B
Mobile phase: Tetrahydrofuran Sample concentration: 0.2% by mass
Temperature: oven 40°C, inlet 35°C, detector 35°C
Detector: Differential Refractometer The molecular weight of the present invention is measured by calculating the molecular weight at each elution time from the elution curve of monodisperse polystyrene and calculating the molecular weight in terms of polystyrene. The peak top molecular weight (Mtop) represents the polystyrene-equivalent molecular weight corresponding to the peak of the elution curve obtained by the above measurement.
(2) Quantification of styrene dimer and trimer 200 mg of styrene-based resin was dissolved in 2 mL of 1,2-dichloromethane, 2 mL of methanol was added to precipitate styrene-based resin, and the mixture was allowed to stand. It measured using the gas chromatography HP-5890 made by a company. The detailed conditions will be described below.
(A) Column: DB-1(ht) 0.25 mm×30 m, film thickness 0.1 μm
(B) Injection temperature: 250°C
(C) Column temperature: 100-300°C
(2) Detector temperature: 300℃
(E) Split ratio: 50/1
(F) Internal standard substance: n-eicosane (3) Melt mass flow rate It was determined under the conditions of 200°C and 49N load based on JIS K7210.
(4) Melt tension As for the melt tension value, using Capillograph 1D type (manufactured by Toyo Seiki Co., Ltd.), barrel temperature 200° C., barrel diameter 9.55 mm, capillary length: L=10 mm, capillary diameter: D=1 mm (L /D=10), the piston descending speed is 10 mm/min, the resin is extruded in a room temperature environment (25±2° C.), the load measuring unit is set 60 cm below the die, and the strand-shaped resin flowing out from the capillary is removed. It is set in a winder, the winding linear velocity is gradually increased from 4 m/min, and the load until the strand breaks is measured. Since the load stabilizes at a constant value as the winding linear velocity is increased, the range in which the load is stable was averaged to obtain the melt tension value.
(5) Die swell value As for the die swell value, using a Capillograph 1D type (manufactured by Toyo Seiki Co., Ltd.), a barrel temperature of 200° C., a barrel diameter of 9.55 mm, a capillary length: L=40 mm, a capillary diameter: D1=1 mm( L/D=40), piston descent rate 500 mm/min (shear rate 6,080 s ⁻¹ ), room temperature environment (25±2° C.), the resin was extruded in a strand shape, and the tip of the strand was extruded by 200 mm from the die. The strand diameter D2 20 mm below the die at that time was measured and determined by the following formula.
Die swell value = Strand diameter D2 (mm) / Capillary diameter D1 (mm)

The characteristics of the foamed sheet were evaluated by the following methods.
(6) Sheet Impact Strength A film impact tester (manufactured by Toyo Seiki Co., Ltd.) was used to measure the impact spherical surface 10R. The measurement was performed 20 times for each of the front surface and the back surface of the foamed sheet, and the average value of all was taken as the sheet impact strength.
(7) Drawdown resistance When the foamed sheet is fixed to a clamp frame (500 mm x 500 mm) of a single-shot vacuum forming machine, the heater temperature is kept constant at 280°C, and the heating time is changed from 1 to 15 seconds in steps of 1 second. The maximum drawdown width was measured. Drawdown resistance was evaluated by setting the maximum drawdown width to 5 mm or less as ⊚, 5 to 7 mm as ◯, 7 to 10 mm as Δ, and 10 mm or more as x.
(8) Formability A foam sheet was thermoformed into a deep-drawing bowl-shaped container having a diameter of 100 mm and a depth of 60 mm using a single-shot molding machine. The heater temperature is kept constant at 280°C, the heating time is changed in 0.5 second increments, and the heating time width that does not cause perforation or pimples in the container is confirmed. When the moldable time width is 10 seconds or more, ◎, 8 The deep drawing formability was evaluated by ◯ for 10 seconds, Δ for 5 to 8 seconds, and x for 5 seconds or less.
(9) Odor The container obtained under the above moldable conditions is covered with aluminum foil, heated at 40° C. for 30 minutes, and then smelled when the cover is opened, and the container having no odor is selected. ⊚, those with odor were marked with x.
(10) Appearance of container Regarding the container obtained under the above moldable conditions, the surface appearance is visually confirmed, and the surface of the container is not wrinkled or dented or rough, and the surface of the container is wrinkled or dented. The appearance of the container was evaluated by assigning a slight roughness to O, a wrinkle or dent, and a rough roughness to X.
(11) Compressive Strength of Container With respect to the container obtained under the above-mentioned moldable conditions, a small tabletop tester Ez-test (manufactured by Shimadzu Corporation, model: Ez-SX) was used, and both ends in the mouth TD direction of the container. With one part sandwiched between two plates, one end was compressed at a speed of 100 m/mm, and the load when displaced by 10 mm was measured. The measurement was carried out on 30 molded containers, and the average value was taken as the compressive strength of the containers.

The foamed sheets of the examples are superior in drawdown resistance, deep drawability, compressive strength of the container, and have less odor than the comparative examples.

In Comparative Example 1, since the melt mass flow rate was low and the melt tension (MT) was too high, the moldability, the appearance of the container, and the strength deteriorated.
In Comparative Example 2, since the melt tension (MT) was too low, the drawdown resistance, the appearance of the container, and the strength deteriorated.
In Comparative Example 3, since the melt mass flow rate was high and the melt tension (MT) was too low, the drawdown resistance, the appearance of the container, and the strength deteriorated.
In Comparative Example 4, since the total amount of styrene dimer and trimer was too low, the moldability, the appearance of the container, and the strength deteriorated.
In Comparative Example 5, since the total amount of the styrene dimer and the trimer was too high, the odor was deteriorated, and the drawdown resistance, the container appearance, and the strength were deteriorated.

From the above results, extrusion with excellent odor reduction and drawdown resistance during secondary molding and moldability is obtained only when the melt mass flow rate, the total amount of styrene dimer and trimer, and the melt tension (MT) are within the specified ranges. It was found that a foamed sheet was obtained and the appearance and waist strength of the container were improved.

By using the styrene resin of the present invention, it is possible to obtain a foamed sheet having an excellent balance of drawdown resistance and deep drawability during secondary molding, and a container obtained by secondary molding of the foamed sheet has a good appearance. In addition, since the waist strength is excellent, it is possible to reduce the thickness and weight of the container. Further, since it has little odor, it can be suitably used as a food packaging container.

Claims (7)

Melt mass flow rate (MFR) measured at 200° C. and 49 N load was 1.0 to 2.5 g/10 minutes, total amount of styrene dimer and trimer was 500 to 1500 μg/g, and measured at 200° C. melt tension value (MT) is Ri 8~20gf der, weight average molecular weight (Mw) and the ratio of the number average molecular weight (Mn) (Mw / Mn) is Ru der 2.0 to 3.3, a styrene resin. The styrene-based resin according to claim 1, which has a die swell value of 1.5 to 2.5 measured with a capillary rheometer at 200° C. and a shear rate of 6,080 s−1. The styrene resin according to claim 1 or 2, which has a peak top molecular weight (Mtop) of 200,000 to 280,000. The styrene resin according to any one of claims 1 to 3, wherein the proportion of the component having a molecular weight of 1,000,000 or more is 2 to 10% by mass. The styrene resin according to any one of claims 1 to 4, wherein the proportion of the component having a molecular weight of 50,000 or less is 5 to 15% by mass. A styrene-based resin foam sheet obtained by foaming and extruding the styrene-based resin according to claim 1. A food container formed by molding the styrene resin foam sheet according to claim 6.