INJECTION MOLDED CONTAINER FOR FOOD AND MANUFACTURING METHOD OF THE SAME

Abstract: The present invention provides an injection molded food container which is produced by injection molding a poly­ arylene sulfide resin composition containing 20-70% by weight of glass fiber or glass fiber and other inorganic filler and is excellent in heat resistance, oil resistance and mechanical strength and is suitable as a food container used in microwave oven and convection oven having a heater. The injection molded food container can be manufactured by injection molding a polyarylene sulfide resin composition containing 20-70% by weight of glass fiber or glass fiber and other inorganic filler, using metal mold of which sur­ face roughness is not greater than 5 µm, under condition of mold temperature 130-180°C.

Description: The present invention relates to an injection molded food container (claim 1) and a method of manufacturing the same (claim 8), and more particularly to an injection molded food container which is excellent in heat resistance, oil resistance and mechanical strength and which can be suitably used as a food container for a microwave oven and convection oven having a heater. Frozen foods and delicatessens have become very popular with change of life style. Heating devices such as a microwave oven, convection oven having a heater and a combined microwave and convection oven are widely used for thawing or heating of frozen foods or heating of delecatessens as their use is convenient. Furthermore, with change of eating habits, it is common to cook bread by heating devices such as a convection oven having a heater or a combined microwave and convection oven. On the other hand, synthetic resins, for example, polyesters such as polyethylene terephthalate, polycarbonate, polyolefins such as polypropylene, polystyrene and polyvinyl chloride resins have been used for various containers. When foods are heated by the above-mentioned heating devices, there are used containers made of, for example, polycarbonate, polypropylene, polyester inorganic filler-containing polypropylene. However, these conventional containers for heating foods can stand heating by microwave oven, but have the problem that when they are heated at about 250°C by a convection oven having a heater which becomes popular from the points of feel in eating of the heated food or appearance such as formation of broiled marks, they undergo heat distortion and thus cannot be used for this purpose. Therefore, it is required to distinguish between using plastic containers for a microwave oven and using containers made by other materials for a convection oven having a heater. This distinction is too inconvenient. For this reason, plastic containers are not widely used in convection ovens having a heater. Furthermore, even when heating is carried out by a microwave oven, if the content to be enclosed or filled therein contains a specific oil, the inside surface is often attacked thereby. As combined microwave and convection ovens are spread recently, there is a demand for plastic containers which can be used for combined microwave and convection ovens. The present invention has been accomplished under the above circumstances. That is, the object of the present invention is to provide an injection molded food container which is excellent in the heat resistance, oil resistance and mechanical strength and can be suitably used for heating by a microwave oven, a convection oven having a heater and a combined microwave and convection oven and which can be manufactured with high productivity. As a result of intensive research conducted by the inventor in order to solve the above problems, it has been found that food containers made by injection molding a specific resin composition are excellent in heat resistance, oil resistance and mechanical strength and can have optional molded shapes and can be suitably used in heating by any of a microwave oven, convection oven having a heater and a combined microwave and convection oven, and besides can be manufactured with high productivity. Furthermore, food containers made by injection molding a specific resin composition and further heat treated under specific condition possess the above merits and in addition are non-odorous at using after heating and have excellent characteristics and stability after heating. The adhesion of a protective film to the food containers made in this manner is further improved. That is, the present invention provides an injection molded container for food which is made by injection molding a polyarylene sulfide resin composition containing 20-70% by weight of glass fiber or glass fiber and other inorganic filler. Further, the invention provides an injection molded container for food which is made by injection molding a polyarylene sulfide resin composition containing 20-70% by weight of glass fiber or glass fiber and other inorganic filler and then subjected to a heat treatment at 150-280°C for 10 seconds - 300 minutes. Any of the above containers may have a protective film on at least an inner surface and can be used for a convection oven having a heater or a combined microwave and convection oven. The injection molded container for food can be manufactured by injection molding a polyarylene sulfide resin composition containing 20-70% by weight of glass fiber or glass fiber and other inorganic filler, using a metal mold having a surface roughness of not greater than 5 µm, under the condition of a mold temperature of 130-180°C. The above polyarylene sulfide resin composition contains at least a polyarylene sulfide resin and glass fiber. The polyarylene sulfide resin has no special limitation and there may be used, for example, those which are obtained by the known method which comprises carrying out a polymerization reaction of a halogenated aromatic compound with a sulfur source such as an alkali metal sulfide in a polar solvent. Specific examples are those which have a p- or m-phenylene group, alkyl-substituted phenylene group, naphthalene groupp, p,p′-diphenylenesulfone group, diphenylene ether group or p,p′-biphenylene group as arylene group and those which are mainly composed of them. Furthermore, as suitable ones, mention may be made of block copolymers mainly composed of a p-phenylene group such as those which contain at least 70 mol%, preferably at least 85% of a recurring unit having a p-phenylene group and at most 30 mol%, preferably at most 15 mol% of a recurring unit having an other group such as a m-phenylene group in the form of a block in the chain. Furthermore, the polyarylene sulfide resin may be either a straight or branched chain polymer or it may be one partially cured with heat or mixtures thereof. The melt viscosity of the polyarylene sulfide resin used in the present invention is usually 300-30000 poises, preferably 400-20000 poises (measured at 300°C and at a shear rate of 100 sec⁻¹). With regard to the Melt Index (MI) measured by way of ASTM D 1238 (316.5°C, load 5 kg), the MI of a linear polyarylene sulfide resin used in the present invention is 10-2,000 g/10 min. and the MI of a heat cured or branched chain polyarylene sulfide resin used in the present invention is 20-1,000 g/10 min. The glass fiber may be any of alkali-containing glass, low alkali glass and non-alkali glass. The shape of this glass fiber has no limitation and is, for example, a milled fiber and chopped strand, the chopped strand being preferred. The glass fiber may be subjected to a surface treatment such as silane treatment, Volan treatment or chromium treatment. The diameter of the glass fiber is generally 5-20 µm, preferably 7-15 µm. The length of the glass fiber is usually 0.1-20 mm, preferably 1-20 mm. If the fiber length is less than 0.1 mm, mechanical strength and heat resistance may be insufficient. If it is more than 20 mm, kneadability and dispersibility are inferior and appearance of the resulting container may become poor. The aspect ratio of the glass fiber whose length and diameter are within the range mentioned above is normally 10-10000, preferably 15-3000. In the present invention, the polyarylene sulfide resin composition may contain other inorganic fillers in addition to the glass fiber. Examples of the inorganic fillers are silica, calcium carbonate, talc, clay, titanium white, calcium sulfate, calcium sulfite, mica, zeolite, carbon black, carbon fiber, boron fiber, glass powder and whisker such as potassium titanate which are 0.1-30 µm, preferably 0.5-15 µm in particle diameter or fiber diameter. The content of the glass fiber or the glass fiber and the inorganic filler in the polyarylene sulfide resin composition is usually 20-70% by weight, preferably 30-60% by weight. If this content is less than 20% by weight, the heat resistance and strength of injection molded food container made therefrom are inferior and if it is more than 70% by weight, the flowability is decreased to cause deterioration of the moldability. When the polyarylene sulfide resin composition contains glass fiber and inorganic filler, the content of the inorganic filler other than the glass fiber is 10-90% by weight, preferably 20-70% by weight of the glass fiber (20-70% by weight) and the content of the glass fiber is desirably at least 20% by weight of the polyarylene sulfide resin composition. In particular, when only glass fiber is contained in the polyarylene sulfide resin composition, the content of it is 25-60% by weight, preferably 30-50% by weight. When both of glass fiber and inorganic filler are contained in the polyarylene sulfide resin composition, the content of polyarylene sulfide is 25-60% by weight, preferably 25-50% by weight, the content of glass fiber is 15-50% by weight, preferably 20-40% by weight and the content of an inorganic filler is 10-50% by weight, preferably 15-40% by weight. In case that the content of glass fiber is not less than 50% by weight, the moldability of the polyarylene sulfide resin composition tends to be lower. In case that the content of inorganic filler is not less than 40% by weight, the polyarylene sulfide resin composition tends to be brittle and a container made therefrom tends to be crushed when it is fallen. The polyarylene sulfide resin composition containing the polyarylene sulfide resin and glass fiber or glass fiber and inorganic filler may contain additional components such as coloring agents such as titanium oxide, carbon black, red oxide, cadmium pigments, azo pigments, phthalocyanine pigments, quinacridone pigmenta, and dioxazine pigments; and various additives such as flowability improver; stabilizer; releasing agent, crystallization accelerator. It may further contain, as far as the attainment of the object is not affected, resin components, e.g., general-purpose resins such as polyolefin resin, vinyl chloride resin and their copolymer resin, vinylidene chloride resin, vinyl acetate resin, polystyrene and copolymer resin thereof, polyamide resin, polyacetal, polycarbonate, thermoplastic polyester resin, polyphenylene oxide and Noryl resin, polysulfone, fluoro resin and resin modified by unsaturated carboxilic acid. When the polyarylene sulfide resin composition contains the other components mentioned above, the content of these components is generally at most 40% by weight in total. It may be necessary to include the other components in the polyarylene sulfide resin composition so as not to deteriorate the heat resistance of it. For preventing the deterioration of heat resistance, it is required to select a suitable resin which does not change the heat distortion temperature of the polyarylene sulfide resin composition from the other components and to determine the content of it. It is important for the injection molded food container of the invention to have high heat resistance. The heat distortion temperature of it (measured by ASTM D648 264 PSI) is not less than 260°C, preferably not less than 265°C and more preferably not less than 270°C. It is also important for the injection molded food container of the invention to have good high-impact properties. The Izod impact strength measured with notch is not less than 3 kg.cm/cm, preferably 5 kg.cm/cm. The injection molded food container of the first invention is obtained by injection modling the above mentioned polyarylene sulfide resin composition. The heating temperature (cylinder temperature), injection pressure and mold temperature in injection molding vary depending on the composition of the polyarylene sulfide resin composition and, therefore, cannot be simply specified. Usually, the heating temperature (cylinder temperature) is about 280-350 °C, the injection pressure is about 29.4-147 MPa (300-1500 kg/cm²) and the mold temperature is about 130-180°C, preferably 135-170°C. The surface roughness of the metal mold is not greater than 5 µm, preferably not greater than 4 µm. In case that the surface roughness is greater than 5 µm, glossiness of the container of the invention. In case that the mold temperature is below 130°C, the surface of the container tends to have flow-marks caused by the relief of the glass fiber so that the visual surface of the container becomes bad and the value of it as a commodity becomes lower. The shape of the food container made by injection molding has no limitation as long as it has a portion for containing food and it may have any shape of tray, cup, tank, box and other shapes. The surface of the container may have a ground pattern such as satin pattern, cut pattern, and rugged pattern. Such shape and pattern can be optionally obtained by injection molding the polyarylene sulfide resin composition. Although the above-mentioned shape and pattern can be obtained by methods other than injection molding such as compression molding, extrusion molding and stretch blow molding, it is impossible by these molding methods to produce containers of high strength and high heat resistance in optional shapes from the composition containing glass fiber in an amount of at least 20% by weight. It is preferred in the present invention to further subject the injection molded container to a heat treatment at 150-280°C, preferably 170-260°C for 10 seconds - 300 minutes, preferably 1-200 minutes. By this heat treatment, the property of having no odor at heating and adhesion of protective film if this is provided can be further improved without damaging the excellent heat resistance, oil resistance and strength of the injection molded container. Furthermore, the mechanical strength and heat resistance of the polyarylene sulfide resin composition become higher and more stable because the wetting property of glass fiber or inorganic filler and crystallizability of polyarylene sulfide are improved by this heat treatment. If the heat treating temperature is lower than 150°C or the heat treating time is shorter than 10 seconds, the above effects cannot be sufficiently obtained. On the other hand, even if the heat treating temperature is raised to higher then 280°C or the time is prolonged to more than 300 minutes, improvements of said effects cannot be observed, and rather a heat deformation of the container results. The heat treatment may be conducted under atmospheric pressure or reduced pressure. Furthermore, it may be conducted in water vapor or hot water. By this heat treatment the property of having no odor at heating and adhesion of protective film if provided of the container can be improved further. However, the heat treating temperature, when the treatment is conducted in air under atmospheric pressure or reduced pressure, is preferably 200-280°C, and the heat treating temperature, when it is conducted in water vapor or hot water, is preferably 150-200°C. The resulting injection molded food container with or without the heat treatment may have a protective film on at least an inner surface. This protective film has the function or action to improve the sticking resistance, oil resistance, scuff resistance, property of having no odor and appearance of the injection molded food container of the present invention. The protective film having such function or action can be formed by coating an epoxy coating agent, acrylic coating agent, fluorine-contained resin coating agent and the like or by plating, vapor deposition or sputtering of, for example, nickel and copper. Among these methods for the formation of a protective film, preferred is a method of coating a fluorine-containing resin coating agent such as polytetrafluoroethylene, polyvinyl fluoride, or polyvinylidene fluoride. In the formation of the protective film, it is preferred to carry out a surface treatment of the surface on which the protective film is to be formed such as the inner wall surface, to enhance adhesion of the protective film thereto. Such surface treatment includes, for example, sand-blasting, solvent treatment, strong acid etching, corona discharge treatment, chromic acid treatment, flame treatment, hot air treatment, and treatments with treating agents such as hydrogen peroxide, acetic acid or mixtures thereof. The thickness of the protective film is not critical, but ordinarily is about 0.1-20 µm. The injection molded food container of the present invention can be suitably used for heating foods such as frozen foods and delecatessens contained therein by any of a microwave oven, convection oven, and combined microwave and convection oven, especially even when it is heated by convection oven. The present invention will be explained further specifically by the following examples and comparative examples. Example 1 A box-shaped injection molded food container of 150 mm x 100 mm x 50 mm with a thickness of each portion of 3 mm was prepared by injection molding a polyphenylene sulfide resin composition having the following composition ("PPS-200SG" manufactured by Idemitsu Petrochemical Co., Ltd.) by an injection molding machine [Clamping Pressure: 14.7x10³ MPa (150 tons), molding pressure: 78.4 MPa (800 kg/cm²), cylinder temperature: 330°C, mold temperature: 135°C and surface roughness of mold; 2.0 µm]. Polyphenylene sulfide resin composition Polyphenylene sulfide resin 35% by weight [Heat cured type, Melt Viscosity; 400 poise (temperature: 300°C, shear rate; 200 sec⁻¹), Melt Index; 70 g/10 min (temperature; 316.5°C, load; 5 kg)] Glass fiber (length: 3 mm, average diameter: 13 µm) 30% by weight Calcium carbonate (average particle diameter: 10 µm) 35% by weight The thus obtained food container was evaluated on heat resistance and a test piece molded under the same condition was evaluated on heat distortion temperature, flexural strength and odor by the following methods. The results are shown in Table 1. Heat resistance: The container was left to stand at 260°C for 1 hour and thereafter appearance of this container was evaluated by the following three grades. 3 No change occurred. 2 Slight discoloration occurred, but this was practically acceptable. 1 Much defermation occurred, so that the container was not able to be used. Heat distortion temperature: According to 264 PSI (conforming to ASTM D648) Flexural strength: According to ASTM D790 Odor: A frozen uncooked hamburger steak placed in the container was put in the center of a convection type electric oven for home use (output: 1200 W) and cooked at a heating temperature of 260°C. The odor in the oven in this case was evaluated by organoleptic examination. Example 2 A box-shaped injection molded food container of 150 mm x 100 mm x 25 mm with thickness of each portion of 2 mm was prepared by injection molding a polyphenylene sulfide resin composition having the following composition ("PPS-100SG" manufactured by Idemitsu Petrochemical Co., Ltd.) under the same conditions as in Example 1. Polyphenylene sulfide resin composition: Polyphenylene sulfide resin 60% by weight Glass fiber (length: 3 mm, average diameter: 13 µm) 40% by weight The thus obtained food container and test piece were evaluated in the same manner as in Example 1 and the results are shown in Table 1. Example 3 The injection molded container of Example 1 was placed in an oven and heat treated at 250°C for 60 minutes and then the container was cooled by leaving it at room temperature. Then, the inner wall surface of the portion of the container where food is to be contained was subjected to a Teflon coating to obtain an injection molded food container of the present invention. This container was evaluated on heat resistance, heat distortion temperature, flexural strength, odor, and adhesion of the protective film. The results are shown in Table 1. Evaluation of the protective film was conducted in the following manner. Adhesion of protective film: The container was put in the center of a convection type electric oven for home use (output: 1200 W) and was left to stand at 260°C for 1 hour. The degree of peeling of the protective film was visually evaluated. Example 4 Example 3 was repeated except that the Teflon coating was not carried out. The results are shown in Table 1. Example 5 Example 3 was repeated except that the heat treatment was not carried out. The results are shown in Table 1. Example 6 Example 4 was repeated except that the heat treating temperature was changed from 250°C to 180°C. The results are shown in Table 1. Comparative Example 1 An injection molded container was prepared in the same manner as in Example 2 except that a polyethylene sulfide resin composition containing 10% by weight of glass fiber was used. The results of evaluation are shown in Table 1. Comparative Example 2 An injection molded container was prepared in the same manner as in Example 1 except that a polyethylene terephthalate resin composition having the following composition ("TOUGHEIGHT A300" manufactured by Idemitsu Petrochemical Co., Ltd.) was used in place of the polyphenylene sulfide resin composition ("PPS-200SG" manufactured by Idemitsu Petrochemical Co., Ltd.). Then, the inner wall surface of the portion of the container in which food is to be contained was subjected to a Teflon coating. Polyethylene terephthalate resin composition: Polyethylene terephthalate resin 70% by weight Glass fiber (length: 3 mm, average diameter: 13 µm) 30% by weight Example 7 Example 1 was repeated except that talc (average diameter; 3 µm) was used in place of using calcium carbonate. The results are shown in Table 1. Example 8 Example 1 was repeated except that a linear polyphenylene sulfide having a melt viscosity of 1,000 poise and a melt index of 0.24 g/10 minutes was used in place of the polyphenylene sulfide in example 1. The results are shown in Table 1. In case that example 1 was repeated except that a metal mold having a roughness of 7 µm was used, the glossiness of the container was bad. In case that example 1 was repeated except that the molding temperature was 120°C, flow marks occured on a surface of the container caused by the relief of the glass fiber. According to the present invention, an injection molded food container having various advantages as mentioned below can be provided. (1) Since the container is made by injection molding a polyarylene sulfide resin composition containing a polyarylene sulfide resin excellent in heat resistance, oil resistance and mechanical strength and besides a specific amount of glass fiber or glass fiber and other inorganic filler, the container can be suitably used not only for heating of foods such as frozen foods or delicatessens by a microwave oven, but also for heating of them by a convection oven or a combined microwave and convection oven which are higher in heating temperature. Accordingly it is not required to consider which oven should be used for heating food in a container of the invection. (2) The container can be easily produced by injection molding and hence productivity thereof is high. (3) The container is further improved in odorless property and adhesion of protective film thereto if this is provided, by subjecting it to a specific heat treatment. (4) The container is further improved in scuff resistance and strength by providing the protective film.

Claims: An injection molded food container which is produced by injection molding a polyarylene sulfide resin composition containing 20-70% by weight of glass fiber or glass fiber and other inorganic filler. An injection molded food container according to claim 1, which is further heat treated at 150-280°C for 10 seconds - 300 minutes. An injection molded food container according to claim 1, wherein the glass fiber has a length of 0.1-20 mm and an aspect ratio of 10-10,000. An injection molded food container according to claim 1, wherein the glass fiber has a length of 1-20 mm and an aspect ratio of 15-3,000. An injection molded food container according to claim 1, wherein the inorganic filler is calcium carbonate, talc, titanium white, or calcium sulfate. An injection molded food container according to claim 1 which has a protective film on at least the inner wall surface thereof. An injection molded food container according to claim 1, which is used for heating in a convection oven having a heater or combined microwave and convection oven. A method of manufacturing an injection molded food container wherein a polyarylene sulfide resin composition containing 20-70% by weight of glass fiber or glass fiber and other inorganic filler is molded by injection molding using a metal mold of which the surface roughness is not greater than 5 µm, under the condition of a mold temperature of 130-180°C.

Inventors: NAKAMICHI, MASAHIRO, IDEMITSU PETROCHEM. CO., LTD.
Applicants: IDEMITSU PETROCHEMICAL CO. LTD.
Application year: 1989
Main classification: B65D81
IPC codes: A47J36, B29C45