JP6933438B2 - Thermoforming sheets and food trays - Google Patents

Description

The present invention relates to thermoforming sheets and food trays.

In recent years, frozen foods such as cooked prepared foods have been sold at convenience stores and supermarkets. Polypropylene is generally used as a container for containing this frozen food. Polypropylene is excellent in thermoformability required for thermoforming trays and heat resistance required for cooking in a microwave oven.

For example, Patent Document 1 discloses an olefin-based sheet containing polypropylene, which has excellent heat resistance and cold resistance. In Patent Document 1, the heat resistance when the sheet is heated at 130 ° C. is evaluated.

Japanese Unexamined Patent Publication No. 2000-239458

By the way, if the frozen food contains a lot of oily ingredients, or if it contains something like sticky salty soy sauce, these may be heated to 150 ° C or higher when cooked, causing holes in the tray. There is. When the conventional sheet described in Patent Document 1 and the like is used, it is difficult to sufficiently suppress this hole opening.

Therefore, the present invention provides a thermoforming sheet in which holes in the tray are sufficiently suppressed during cooking even when the frozen food contains a large amount of oily components, and a food tray using the same. The purpose is.

The present invention provides the following [1] to [4].
[1] A thermoforming sheet in which two or more layers are laminated, wherein one outermost layer of the thermoforming sheet contains crystalline polyester and the other outermost layer contains polypropylene. Molding sheet.
[2] The thermoforming sheet according to [1], wherein the outermost layer containing the crystalline polyester has a thickness of 10 μm or more and the thermoforming sheet has a thickness of 200 μm or more.
[3] A food tray for storing food, which has the thermoforming sheet according to [1] or [2] and is located on a surface where the outermost layer containing crystalline polyester comes into contact with food. Food tray.
[4] The ratio (ΔH (crystallization) / ΔH (melting)) of the enthalpy ΔH (crystallization) of the crystallization peak to the enthalpy ΔH (melting) of the melting peak in the crystalline polyester exceeds 0, [3] ] The food tray described in.

According to the present invention, there is provided a thermoforming sheet in which holes in the tray are sufficiently suppressed during cooking even when the frozen food contains a large amount of oily components, and a food tray using the same. can do.

Hereinafter, one embodiment of the present embodiment will be described in detail, but the present invention is not limited to the following embodiments.

The thermoforming sheet of the present embodiment (hereinafter, also simply referred to as "sheet") is formed by laminating two or more layers. In the thermoforming sheet, one outermost layer contains crystalline polyester and the other outermost layer contains polypropylene.

As the crystalline polyester, conventionally known ones can be used. Specific examples thereof include polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polypropylene naphthalate and polybutylene naphthalate. Among these, polyethylene terephthalate is preferable from the viewpoint of further improving the effect of suppressing the opening of holes in the tray.

These crystalline polyesters are obtained by polycondensation of a dicarboxylic acid and a diol, but the following monomers can also be contained in each of the dicarboxylic acids and diols constituting the above polyester.
Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, and diphenylcarboxylic acid. Diphenoxyetanedicarboxylic acid, diphenylsulfoncarboxylic acid, anthracendicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, Maronic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid, trimethyladiponic acid, pimelliic acid, azelaic acid, dimeric acid , Sebacic acid, suberic acid, dodecadicarboxylic acid and the like.
Examples of the diol include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decamethylene glycol, 1,3-propanediol, and 1,4-butane. Examples thereof include diol, 1,5-pentanediol, 1,6-hexadiol, 2,2-bis (4-hydroxyphenyl) propane, and bis (4-hydroxyphenyl) sulfone.

The dicarboxylic acid component and the diol component constituting the polyester resin may be used alone or in combination of two or more. In addition, other acid components such as trimellitic acid and other hydroxyl group components such as trimethylolpropane may be added as appropriate.

Isophthalic acid is used as the dicarboxylic acid, and 1,4-cyclohexanedimethanol is used as the diol, and a copolymerized polyester can also be used.

The content of the crystalline polyester in the outermost layer containing the crystalline polyester is preferably 70% by mass or more, more preferably 85% by mass or more, further preferably 95% by mass or more, and the outermost layer is crystalline. It is particularly preferred that it consists only of sex polyester.

The thickness of the outermost layer containing the crystalline polyester is preferably 5 μm or more, more preferably 10 μm or more, still more preferably 20 μm or more, from the viewpoint of further improving the effect of suppressing the hole opening of the tray. The upper limit of the thickness of the outermost layer is not particularly limited, but can be, for example, 150 μm or less.

As the polypropylene, conventionally known polypropylene can be used. It may be a random copolymer with an α-olefin containing ethylene that can be copolymerized with propylene. In the case of a copolymer, although it depends on the type of comonomer, if the amount of the copolymerization component is increased, the melting point of polypropylene is lowered, and there is a concern that the heat resistance when made into a tray is lowered. The polypropylene of the present embodiment preferably has a melting point of 135 ° C. or higher determined by the DSC method, and within this range, the comonomer species and amount can be selected, and similarly, stereoregularity such as isotactic, syndiotactic, or atactic. , And its degree can be selected.

Further, in order to exhibit impact resistance particularly at a low temperature, a block copolymer containing various elastomers such as ethylene-propylene copolymer rubber can also be used as the polypropylene of the present embodiment. The polypropylene of the present embodiment is a block copolymer having an elastomer component, has a melting point of 150 ° C. or higher as a resin, and has an MFR of 2 (g / 10 minutes) or less under a load of 230 ° C. and 2.16 kg. Polypropylene is preferable from the viewpoint of heat resistance, impact resistance, or thermoformability.

As the outermost layer containing polypropylene of the present embodiment, it is preferable to use a film formed of polypropylene containing 90% by weight or more of a constituent unit made of propylene. Here, polypropylene may be a homopolymer of propylene, or may be a copolymer of propylene and another monomer copolymerizable therewith. Moreover, you may use these together. Examples of other monomers copolymerizable with propylene include ethylene and α-olefins. The α-olefin has 4 or more carbon atoms, and is preferably an α-olefin having 4 to 12 carbon atoms. Specific examples of α-olefins having 4 to 12 carbon atoms include linear monoolefins such as 1-butene, 1-pentene, 1-hexene, 1-hexene, 1-octene, and 1-decene; Branched monoolefins such as 3-methyl-1-butene, 3-methyl-1-pentene, 4-methyl-1-pentene; vinylcyclohexane and the like. The copolymer of propylene and another monomer copolymerizable therewith may be a random copolymer or a block copolymer.

When polypropylene is composed of a copolymer, specific examples of the copolymer include a propylene-ethylene random copolymer, a propylene-1-butene random copolymer, and a propylene-ethylene-1-butene random copolymer. Examples thereof include a binary or ternary copolymer of one or more monomers selected from the group consisting of propylene, ethylene, and α-olefin having 4 to 12 carbon atoms.

When polypropylene is made of a copolymer, the propylene-derived structural unit can be selected depending on its properties such as heat resistance. In the case of the present embodiment, it is desirable to have high heat resistance, so it is preferable to contain a large amount of propylene-derived structural units, and specifically, it is preferable to contain 96% by weight or more of propylene-derived structural units. The content of the constituent units derived from the other monomers in the copolymer is determined by infrared rays (IR) according to the method described on page 616 of the "Polymer Analysis Handbook" (published by Kinokuniya Bookstore in 1995). ) It can be obtained by performing spectrum measurement.

Further, the stereoregularity of the propylene-based homopolymer and the propylene-based copolymer may be isotactic, syndiotactic, or atactic, but from the viewpoint of excellent balance of rigidity and transparency after molding into a film, isotactic is used. Polypropylene having high tactical properties is preferable.

In the present embodiment, polypropylene may be a polymer or a copolymer polymerized using a known polymerization catalyst, and examples of the polymerization catalyst include the following.
(A) Ti-Mg-based catalyst composed of solid catalyst components containing magnesium, titanium and halogen as essential components,
(B) A catalyst system in which an organoaluminum compound and, if necessary, a third component such as an electron donating compound are combined with a solid catalyst component containing magnesium, titanium and halogen as essential components.
(C) Metallocene catalyst, etc.

Examples of the solid catalyst component of (A) include catalyst systems described in JP-A-61-218606, JP-A-61-287904, JP-A-7-216017 and the like. Further, preferred examples of the organoaluminum compound in the catalyst system (B) include triethylaluminum, triisobutylaluminum, a mixture of triethylaluminum and diethylaluminum chloride, tetraethyldialmoxane and the like, and electron-donating compounds are preferable. Examples include cyclohexylethyldimethoxysilane, tert-butylpropyldimethoxysilane, tert-butylethyldimethoxysilane, dicyclopentyldimethoxysilane and the like. Examples of the metallocene-based catalyst (C) include the catalyst systems described in Japanese Patent No. 2587251, Japanese Patent No. 2627669, and Japanese Patent No. 2668732.

Polypropylene is, for example, a solution polymerization method using an inert solvent typified by a hydrocarbon compound such as hexane, heptane, octane, decane, cyclohexane, methylcyclohexane, benzene, toluene, and xylene, and a mass using a liquid monomer as a solvent. It can be produced by a polymerization method, a vapor phase polymerization method in which a gaseous monomer is polymerized as it is, or the like. The polymerization by these methods may be carried out by a batch method or a continuous method.

The polypropylene of the present embodiment forms a part of the multilayer sheet of the present embodiment, is mainly thermoformed, and is used as a tray. Thermoforming includes molding methods such as vacuum molding, compressed air molding, vacuum compressed air molding, and match molding. For example, when vacuum forming is applied, the sheet is heated from above and below with a heater, and then pressed against the mold in a vacuum to form a shape along the mold. In this heater heating process, dripping occurs (drawdown). ) May. In order to prevent this, the polypropylene of the present embodiment has a melt flow rate (MFR) of 10 g / 10 minutes or less measured at a temperature of 230 ° C. and a load of 21.18 N in accordance with JIS K7210. It is preferably in the range of 0.01 to 8 g / 10 minutes, more preferably.

Further, polyethylene can be added to the polypropylene of the present embodiment. By making the composition containing about 5 to 30% by weight of polyethylene, not only can the occurrence of sagging (drawdown) be suppressed in thermoforming such as vacuum forming, but it is also used as a food tray after molding at -20 ° C or lower. It is preferable because the impact resistance at the time can be greatly improved.
As this polyethylene, general polyethylene such as linear low-density polyethylene, low-density polyethylene, and high-density polyethylene can be used. From the viewpoint of suppressing sagging during vacuum forming, polyethylene having an MFR value of 0.02 to 4 g / 10 minutes or less measured at 190 ° C. and a 2.16 kg load in accordance with JIS K7210 is preferable, and more preferable. Is 0.05 to 2 g / 10 minutes or less.

Further, a nucleating agent may be added to polypropylene in order to further enhance the effect of the present embodiment. When the nucleating agent is added, it may be either an inorganic nucleating agent or an organic nucleating agent. Examples of the inorganic nucleating agent include talc, clay, calcium carbonate and the like. Examples of the organic nucleating agent include metal salts of aromatic carboxylic acids, metal salts such as metal salts of aromatic phosphoric acid, poly-3-methylbutene-1, polycyclopentene, polyvinylcyclohexane and the like. Among these, organic nucleating agents are preferable, and the above-mentioned metal salts and high-density polyethylene are more preferable. The amount of the nucleating agent added to polypropylene is preferably 0.01 to 3% by weight, more preferably 0.05 to 1.5% by weight. A plurality of kinds of the above additives may be used in combination.
Further, in order to provide rigidity at high temperature, it is preferable to contain talc in an amount of 5 to 30% by weight, not as a nucleating agent.

Known additives may be blended as long as the effects of the present embodiment are not impaired. Examples of the additive include an antioxidant, an ultraviolet absorber, an antistatic agent, a lubricant, an anti-fog agent, an anti-blocking agent and the like. Examples of the antioxidant include a phenol-based antioxidant, a phosphorus-based antioxidant, a sulfur-based antioxidant, a hindered amine-based photostabilizer, and the like, and for example, a phenol-based antioxidant mechanism in one molecule. A composite type antioxidant having a unit having both a phosphorus-based antioxidant mechanism and a phosphorus-based antioxidant can also be used. Examples of the ultraviolet absorber include known ultraviolet absorbers such as 2-hydroxybenzophenone derivatives and hydroxyphenylbenzotriazole derivatives, such as ultraviolet absorbers and benzoate-based ultraviolet blocking agents. The antistatic agent may be a polymer type, an oligomer type, or a monomer type. Examples of the lubricant include higher fatty acid amides such as erucic acid amide and oleic acid amide, higher fatty acids such as stearic acid, and salts thereof. As the anti-blocking agent, fine particles having a spherical shape or a shape close to the spherical shape can be used regardless of whether it is an inorganic type or an organic type.

The outermost layer containing polypropylene may be a foamed layer. The foaming ratio is not particularly limited, but can be 2 to 10 times. The thickness of the foam layer can be 0.5 to 2 mm. For the foaming agent and the production method, Japanese Patent Application Laid-Open No. 2014-9277 can be referred to.

The content of polypropylene in the outermost layer containing polypropylene is preferably 70% by mass or more, more preferably 85% by mass or more, further preferably 95% by mass or more, and the outermost layer is made of polypropylene only. Is particularly preferable.

The thickness of the outermost layer containing polypropylene is not particularly limited, but can be, for example, 100 μm or more, and 200 μm or more and 2000 μm or less.

The outermost layer containing crystalline polyester and the outermost layer containing polypropylene may be directly laminated, but are preferably bonded via an intermediate layer for adhering them. As the intermediate layer, a commercially available adhesive or a layer made of an adhesive resin can be used. As the adhesive used for the intermediate layer, conventionally known adhesives can be used, and specific examples thereof include aliphatic polyester adhesives, aromatic polyester adhesives, aliphatic polyether adhesives, and aromatic adhesives. Examples thereof include polyester adhesives and polyethylene imines. Of these, aliphatic polyester adhesives are most preferable because of their high adhesiveness and ease of use in food applications. The thickness of the intermediate layer when the adhesive is used as the intermediate layer is not particularly limited, but can be, for example, 0.5 μm or more and 4 μm or less as the thickness in the sheet until thermoforming.

When the intermediate layer is a layer made of an adhesive resin, a conventionally known adhesive resin can be used. Specific examples thereof include a copolymer of ethylene and (meth) acrylic acid, (meth) acrylic acid ester, vinyl acetate, glycidyl (meth) acrylate, and the like, or a copolymer of three or more elements, or maleic anhydride-modified polyethylene. , Maleic anhydride-modified polyolefin such as polypropylene maleic anhydride can be used. When the adhesive resin is used as the intermediate layer, the thickness of the intermediate layer is not particularly limited, but can be, for example, 3 μm or more and 100 μm or less as the thickness in the sheet until thermoforming.

The sheet of the present embodiment includes layers other than the above-mentioned two outermost layers and intermediate layers, for example, a gas barrier resin layer such as an ethylene-vinyl alcohol copolymer and a polyamide, and if necessary, a gas barrier resin layer and a re-outer layer. The same as the above-mentioned intermediate layer for adhesion of the above, or another intermediate layer may be used.

The thickness of the sheet of the present embodiment can be 200 μm or more, preferably 300 μm or more, more preferably 400 μm or more, and more preferably 500 μm or more, from the viewpoint of rigidity when made into a tray. Is even more preferable. The upper limit of the thickness of the sheet is not particularly limited, but can be, for example, 2100 μm or less.

The polypropylene or other resin constituting the sheet of the present embodiment consists of parts other than both ends of the sheet that are cut and removed for width adjustment during sheet manufacturing and parts that are formed into a tray by thermoforming. It is also possible to include a so-called loss portion sheet-like material that has been crushed or remelted (recycled material) as long as the effect of the present invention is not impaired. The addition ratio of the recycled material is preferably about 40 wt% or less, and the resin to which the recycled material is added is a resin that forms a polypropylene layer, or when an adhesive resin is used for the intermediate layer, an intermediate layer is formed. It is preferable to add it to the resin. Since this recycled material is a composition composed of a large number of resins, the breaking strength and impact strength of the resin may be low. In that case, a resin called a compatibilizer is added and remelted. It can also be pelletized. Further, an extruder for a recycled material may be installed at the time of sheet molding, and a recycled layer made of the recycled material may be provided. In this case, the content of the recycled material is preferably 10 wt% or more.

When the sheet of the present embodiment is produced, the resin or recycled material that forms the outermost layer containing crystalline polyester is dried to reduce the water content in the resin to 3000 ppm or less, preferably 1000 ppm or less. More preferably, it can be used at 300 ppm or less. This is a viewpoint capable of suppressing hydrolysis of crystalline polyester due to moisture. The means of resin drying is not limited, and a general dryer, a dehumidifying dryer, a vacuum dryer, an N ₂ reflux type dehumidifying dryer, or the like can be appropriately used.

The sheet of the present embodiment can be produced by a known laminated molding method such as a coextrusion T-die melt extrusion method, an extrusion lamination method, or a dry lamination method. The intermediate layer using the adhesive resin described above can be applied to a coextrusion T-die melt extrusion method or an extrusion lamination method, and the intermediate layer using the adhesive described above can be applied to dry lamination. .. Among these, molding by the coextrusion T-die melt extrusion method is preferable from the viewpoint of high productivity and cost. This co-extrusion T-die melt extrusion method is equipped with a required number of extruders, each extruder is connected to one T-die, and the resin is extruded from the T-die into a film in a laminated state, and a cooling roll is used. This is a method of manufacturing a multi-layer sheet by cooling and solidifying with.

Specifically, as an example of the method for producing the multilayer sheet by the coextrusion T-die melt extrusion method, polypropylene / maleic anhydride-modified polyolefin / crystalline polyester constituent product will be described below as an example.

Each of the three extruders is heated to about 180 to 330 ° C., and a powder or pellet of polypropylene, maleic anhydride-modified polyolefin, and crystalline polyester is supplied to each extruder. It is melt-kneaded by the screws of each extruder and supplied to the feed block or converting adapter installed on the upstream side of the T-die. Then, after being melted and co-extruded into a sheet from the slit at the tip of the T-die, it is brought into contact with a cooling roll by various means and cooled to produce a multilayer sheet.

The temperature of the molten sheet-like material containing co-extruded polypropylene is preferably about 180 to 300 ° C. If the temperature of the molten sheet-like material at this time is lower than 180 ° C., the ductility is not sufficient, the thickness of the obtained film becomes non-uniform, and the film may have uneven phase difference. Further, if the temperature exceeds 300 ° C., the resin is likely to be deteriorated or decomposed, and bubbles may be generated in the sheet or carbides may be contained in the sheet.

The three extruders may be single-screw extruders or twin-screw extruders, and the extruders may not be unified. Further, it may be preferable to select the size of the extruder according to the composition ratio of the multilayer sheets of each layer because the extrusion amount can be produced with the most equipment-stable extrusion amount. Each resin melt-kneaded by each extruder passes through a temperature-controlled single tube called an adapter and is supplied to a feed block or a converting adapter. These are selected according to the type of T-die, and those having one flow path in the T-die are supplied to the T-die after arranging the resins in the feed block in advance according to the resin composition of the multilayer film. NS. On the other hand, when there are a plurality of flow paths in the T-die called a multi-manifold and each resin layer is laminated immediately before the T-die lip portion, the flow paths from each extruder are set in each of the T-dies according to the film configuration. The flow path is connected by a converting adapter.

From the viewpoint of suppressing extrusion fluctuations of polypropylene, crystalline polyester, maleic anhydride-modified polyolefin, etc., a gear pump is installed between each extruder and the T-die to stabilize the pressure and supply the resin to the T-die. can do. The pressure at this time is preferably within 0.3 MPa as a fluctuation value. By doing so, it is possible to reduce the variation in the thickness of the obtained sheet.

Furthermore, in order to remove foreign substances contained in polypropylene, crystalline polyester, and maleic anhydride-modified polyolefin, a metal mesh held by a breaker plate or a boiled mesh made of metal fibers can be attached to the tip of each extruder. preferable. In the case of a metal mesh, a metal mesh of 400 mesh or less can be used, and in the case of a metal fiber fired mesh, a mesh having a filtration accuracy of 200 μm or less can be used.

The molten sheet-like multilayer sheet extruded from the T-die is continuously in contact with a metal cooling roll (also referred to as a chill roll or a casting roll) and is cooled by being in close contact with the cooling roll. Adhesion to the cooling roll affects the thickness accuracy of the sheet. As a means of adhering the cooling roll, for example,
a) A method in which static electricity is applied to a molten sheet-like multilayer sheet, and the surface condition is brought into close contact with a mirror-surfaced cooling roll to cool the material.
b) A method of sandwiching a molten sheet-like multilayer sheet-like material between a cooling roll having a surface state and a rubber roll (also referred to as a touch roll) having a surface state, and bringing them into close contact with the cooling roll to cool the material.
c) When the molten sheet-like multilayer sheet is brought into contact with the cooling roll, it can be carried out by a known method such as a method of bringing the molten sheet-like multilayer sheet into close contact with the cooling roll by air blown from an air chamber or an air knife to cool the material.
Of the above, in the case of the method b), a general rubber having a hardness of 60 to 100, such as silicon rubber or neoprene rubber, can be used.

The cooling rolls used in the above three types of methods preferably have, for example, a surface temperature adjusted in the range of 20 to 80 ° C. If the surface temperature of the cooling roll exceeds 80 ° C., it takes time to cool and solidify the molten multilayer sheet-like material, so that the polypropylene in the multilayer sheet-like material is not sufficiently cooled, and the sheet is manufactured by losing the take-back tension of the sheet. May cause deformation in the direction. On the other hand, when the surface temperature of the cooling roll is lower than 20 ° C., the surface of the cooling roll tends to be dewed and water droplets adhere to it, which deteriorates the appearance of the obtained film.

The processing speed at the time of producing the multilayer sheet of the present embodiment can be carried out in a general range, but is generally preferably 2 to 20 m / min. Further, at the time of production, either side of the outer layers made of polypropylene and crystalline polyester may be arranged on the cooling roll side, but arranging the crystalline polyester side on the cooling roll side can improve crystallinity. It is preferable in that sense. This is from the viewpoint that when a large amount of crystal components having a high melting point are generated in the molding of the tray in the subsequent step, the crystalline polyester layer lacks elongation at the polypropylene molding temperature, which may impair the thermoforming property.

In the sheet or food tray of the present embodiment, the ratio of the enthalpy ΔH (crystallization) of the crystallization peak to the enthalpy ΔH (melting) of the melting peak in the crystalline polyester (ΔH (crystallization) / ΔH (melting)). : Hereinafter, it may be referred to as an enthalpy ratio) is preferably more than 0, more preferably 0.10 or more, further preferably 0.2 or more, and more preferably 0.30 or more. More preferred. If this ratio exceeds 0, it means that there is crystalline polyester that has not been crystallized in the state of the sheet or food tray, and that this ratio is 0.20 or more or 0.30 or more. , Indicates a large amount of uncrystallized crystalline polyester in the state of a sheet or food tray. An example of such a crystalline polyester is A-PET.

As a method for increasing the amount of uncrystallized components in the polyester layer of the sheet and the food tray immediately after thermoforming of the present invention, for example, the conditions for producing a multi-layered sheet and the thermoforming of the sheet are used. There is a method of controlling each condition when making a tray. In the former method, when the molten sheet-like material extruded from the die is cooled in the production of the multilayer sheet, the surface temperature of the cooling roll for cooling and solidifying the sheet is set to 70 ° C. or lower, and the polyester layer is rapidly cooled. The amount of uncrystallized components can be increased. The surface temperature of this cooling roll is affected by the total thickness of the sheet and the line speed.

In the latter method, in the thermoforming of the sheet, the temperature (near the melting point) at which the crystals of the layer made of crystalline polyester in the sheet melt is set, and the conditions such as the position (distance), temperature, and time of the heater are set. By heating the sheet to form a tray, the amount of uncrystallized components in the polyester layer can be increased.
By manufacturing a tray by applying the above two methods alone or both, there are many uncrystallized components, so a tray with less orientation can be manufactured, and there is no problem even if food is put in and cooked in a microwave oven. A tray with less deformation and less deformation can be obtained.

The crystalline polyester in the food tray of the present embodiment may be crystallized by, for example, aging at 100 to 160 ° C. after obtaining a liquid crystal polyester layer containing an uncrystallized component by heat molding. Further, the food may be contained in the presence of uncrystallized crystalline polyester and crystallized when the food is cooked. Thereby, the heat resistance can be improved. After obtaining a liquid crystal polyester layer containing uncrystallized components by thermoforming, the tray is fitted into a mold that can retain the shape of the tray, or sandwiched between two upper and lower molds, for example, 120 molds. The temperature may be adjusted to ° C. or higher and 230 ° C. or lower and held for about 2 seconds or longer and 30 seconds, that is, the crystalline polyester in the tray may be crystallized by heat treatment in a subsequent step. Further, the crystalline polyester may be crystallized by holding the tray in a heating furnace such as a temperature-controlled gear oven.

When sealing food trays and lids with heat seals, it is easier to heat seal the trays before accelerating crystallization and then heat the trays to increase the crystalline components in the crystalline polyester. It is preferable because the selection of the lid material becomes relatively easy.
By crystallizing the crystalline polyester contained in the outermost layer located on the surface in contact with food, it is possible to more effectively suppress the perforation of the tray. This is because the upper limit of the heat resistance of the tray becomes the temperature near the melting point of the crystal by crystallization.

The food tray for accommodating the food of the present embodiment is formed by molding the above-mentioned sheet by a conventionally known thermoforming method so that the outermost layer containing the above-mentioned crystalline polyester is located on the surface in contact with the food. Obtained by

The shape of the tray can be a conventionally known one, and is not particularly limited.

Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is not limited to these Examples.

( Reference example 1)
(Laminated sheet manufacturing: Dry Lami)
An unstretched polyethylene terephthalate film (manufactured by Towa Kako Co., Ltd .: thickness 20 μm, width = 500 mm) is attached to a dry laminator feeding machine, and a two-component mixed aliphatic polyester adhesive is diluted with an organic solvent to prepare an adhesive. The containing solution was gravure-coated so that the film thickness after drying was 2 μm. Then, it was passed through an oven whose temperature was controlled to 120 ° C. to volatilize and remove the organic solvent. Further, a polypropylene sheet having a thickness of 550 μm (melting point = 164 ° C.) was laminated on the adhesive surface, and laminated with a nip roll. Laminated sheet 1 was obtained by aging at 40 ° C. for 2 days.

(Tray vacuum forming)
The laminated sheet 1 is attached to a batch type vacuum forming machine, heated for 12 seconds from the vertical direction with a heater set at 508 ° C., and when the sheet surface temperature reaches 190 ° C., the heater is released to form a tray (50 mm × 120 mm ×). A tray for evaluation was obtained by vacuum forming to a depth of 40 mm) with the polyester side inside. The sheet surface temperature was measured by using a plurality of commercially available thermolabels having a temperature display of 10 ° C. and different measurement temperature ranges.

(Evaluation of crystallinity after vacuum forming)
The polyester layer inside the tray was peeled off and measured using a DSC at a heating rate of 10 ° C./min and a nitrogen flow of 50 ml / min to determine peaks associated with crystallization and peaks associated with melting, respectively.

(Cooking and subsequent crystallinity evaluation)
Then, a commercially available salad oil was poured into a molded tray to a height of 10 mm from the bottom, allowed to stand in a microwave oven, and then heated at 700 W for 180 seconds. The presence or absence of holes was visually confirmed, then the salad oil was discarded, washed with water, the polyester layer was peeled off, and DSC measurement was performed again.

(High temperature durability test)
Commercially available salad oil was poured into another molded tray to a height of 10 mm from the bottom, allowed to stand in a microwave oven, and then heated at 1800 W until the temperature of the salad oil reached 160 ° C. After that, it was visually observed whether or not the tray was deformed.

( Reference example 2: Crystallization by dry lami + heat treatment)
Using the laminated sheet 1, a tray was produced in the same manner as in Reference Example 1 except that the heater temperature was 550 ° C., the heating time was 10 seconds, and the sheet surface temperature was 190 ° C. The enthalpy ratio measured by DSC of the obtained tray was 0.50. After that, the molded tray was placed in a gear oven heated to 150 ° C., and the evaluation was carried out in the same manner as in Reference Example 1 except that the tray was crystallized by heat treatment for 15 minutes. The DSC measurement of the sheet after the heat treatment was also performed. Since the enthalpy ratio after the heat treatment is 0, it can be seen that the tray is crystallized.

( Reference example 3: Extruded lami)
Using the same unstretched polyethylene terephthalate and polypropylene sheet as the laminate 1, a modified polyolefin resin (Admer SF731 manufactured by Mitsui Chemicals, Inc.) was put into a 50 mmφ extruder whose temperature was adjusted to 280 ° C. and extruded to a thickness of 30 μm from the T die. Extruded by amount between the unstretched polyethylene terephthalate and the polypropylene sheet. Then, the laminated sheet 2 was obtained by nipating between the metal roll and the rubber roll whose temperature was adjusted to 40 ° C. A tray was prepared by vacuum forming in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 11 seconds, and the sheet surface temperature was 190 ° C., and the tray was evaluated in the same manner as in Reference Example 1.

(Example 4: Coextrusion)
^{A polyester resin (PET: SHINPET TM} 5522W manufactured by SHINKONG) dried at 150 ° C. for 4 hours in a 50 mmΦ extruder (extruder A) set at 270 ° C. was put into a 50 mmΦ extruder (extruder B) also set at 260 ° C. ) With an adhesive resin (Admer SF731 manufactured by Mitsui Chemicals Co., Ltd.), and a polypropylene resin composition (Noblen WF836DG3 manufactured by Sumitomo Chemical Co., Ltd./Excellen manufactured by Sumitomo Chemical Co., Ltd.) in a 40 mmΦ extruder (extruder C) set at 260 ° C. GH030 = 70/30) was added, and the layer ratio of each extruder was set to 270 ° C. with a 300 mm wide multi-manifold die so that the layer ratio was extruder A / extruder B / extruder C = 50/20/230 μm. The laminated sheet 3 was obtained by extruding after adjusting the screw rotation speed, sandwiching it between a rubber roll whose temperature was adjusted at 15 ° C. and a metal roll whose temperature was adjusted at 20 ° C., and cooling. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 17 seconds, and the sheet surface temperature was 210 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Example 5: Coextrusion)
^{A polyester resin (PET: SHINPET TM} 5522W manufactured by SHINKONG) dried at 150 ° C. for 4 hours in a 50 mmΦ extruder (extruder A) set at 270 ° C. was put into a 50 mmΦ extruder (extruder C) also set at 260 ° C. ) With an adhesive resin (Admer SF731 manufactured by Mitsui Kagaku Co., Ltd.), and a polypropylene resin composition (Noblen AH1311 manufactured by Sumitomo Chemical Co., Ltd./Nipolon Hard 6300 manufactured by Toso Co., Ltd.) in a 65 mmΦ extruder (extruder B) set at 280 ° C. = 85/15) was charged, and the extruder A / extruder C / extruder B were laminated and extruded in this order from a 1000 mm wide multi-manifold die set at 280 ° C. The layer ratio is extruded after adjusting the screw rotation speed of each extruder so that the extruder A / extruder C / extruder B = 10/20/270 μm, and the temperature is adjusted with a rubber roll adjusted to 15 ° C and a temperature adjusted to 50 ° C. The laminated sheet 3 was obtained by sandwiching it between the metal rolls and cooling it. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 14 seconds, and the sheet surface temperature was 220 ° C., and the evaluation was performed in the same manner as in Reference Example 1. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 14 seconds, and the sheet surface temperature was 220 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Example 6: Coextrusion)
A laminated sheet was obtained in the same manner as in Example 5 except that the layer ratio was set to extruder A / extruder B / extruder C = 20/20/260 μm. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 10 seconds, and the sheet surface temperature was 210 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Example 7: Coextrusion)
A laminated sheet was obtained in the same manner as in Example 5 except that the layer ratio was set to extruder A / extruder B / extruder C = 100/20/180 μm. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 17 seconds, and the sheet surface temperature was 200 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Example 8: Coextrusion)
A laminated sheet was obtained in the same manner as in Example 5 except that the layer ratio was set to extruder A / extruder B / extruder C = 20/20/160 μm. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 17 seconds, and the sheet surface temperature was 210 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Example 9: coextrusion)
A laminated sheet was obtained in the same manner as in Example 5 except that the layer ratio was set to extruder A / extruder B / extruder C = 20/20/760 μm. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 14 seconds, and the sheet surface temperature was 210 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Example 10: Coextrusion)
A laminated sheet was obtained in the same manner as in Example 5 except that the layer ratio was set to extruder A / extruder B / extruder C = 30/50/1120 μm. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 530 ° C., the heating time was 46 seconds, and the sheet surface temperature was 220 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Example 11: Coextrusion)
A laminated sheet was obtained in the same manner as in Example 5 except that the layer ratio was set to extruder A / extruder B / extruder C = 20/20/260 μm. A tray was prepared in the same manner as in Reference Example 1 except that the heater temperature was 380 ° C., the heating time was 63 seconds, and the sheet surface temperature was 160 ° C., and the evaluation was performed in the same manner as in Reference Example 1.

(Comparative Example 1)
For comparison, by using a polypropylene sheet used in the manufacture of the laminated sheet 1, by molding the tray in the same manner as in Reference Example 1, and evaluated by the microwave oven in the same manner using the salad oil in the same manner as in Reference Example 1 rice field.

Table 1 shows the composition and manufacturing conditions of the sheets obtained in Examples and Comparative Examples, and Table 2 shows the test results.

Claims (2)

A thermoforming sheet having a laminate of two or more layers.
One outermost layer of the thermoforming sheet contains crystalline polyester, and the other outermost layer contains 70% by mass or more of polypropylene and 5 to 30% by weight of polyethylene.
And enthalpy of crystallization peak [Delta] H (crystallized) in the crystalline polyester, the ratio of the enthalpy of melting peak [Delta] H (melting) ([Delta] H (crystallization) / [Delta] H (melting)) is Ri der 0.20 or more,
The outermost layer containing the crystalline polyester has a thickness of 10 to 100 μm.
A thermoforming sheet having a thickness of the outermost layer containing polypropylene of 160 to 2000 μm (however, the outermost layer is heated at a rate of 99.9 ° C./min by the DSC method and reaches 140 ° C. When the temperature is maintained at that temperature, the time until the crystallization peak appears is 1 minute or more). A food tray for storing food
A food tray having the thermoforming sheet according to claim 1 and having an outermost layer containing crystalline polyester located on a surface in contact with food.