Process for producing polyester resin foam

Abstract: A process for producing a thermoplastic polyester resin foam comprising melting a thermoplastic polyester resin, mixing the molten resin with a blowing agent and extruding the mixture into a low-pressure zone to carry out expansion molding, characterized in that a compound having two or more acid anhydride groups per molecule, optionally together with a compound of a metal of Group I, II or III elements of the Periodic Table is added to the thermoplastic polyester resin. There is also disclosed a process for producing a thermoplastic polyester resin foam comprising cooling a high-­ temperature foam immediately after expansion and then re-heating the foam. Further, a thermoplastic polyester resin foam sheet and a food container prepared from the foam sheet are also disclosed.

Description: FIELD OF THE INVENTION This invention relates to a process for producing a thermoplastic polyester resin foam by extrusion, which gives fine cells uniformly formed at a high expansion ratio and does not cause the generation of colored contaminants in the foam even in continuous extrusion operation. It also relates to the production of a thermoplastic polyester resin foam sheet excellent in thermoformability. The polyester resin foam of the present invention is high in rigidity, though the foam has a low density and is lightweight. Further, the foam is excellent in heat resistance, chemical resistance, heat insulating properties and shock-absorbing properties and can be widely used. Particularly, the polyester resin foam sheet is suitable for use as a material which can be thermoformed into heat-resistant food containers. BACKGROUND OF THE INVENTION Thermoplastic polyester resins such as polyethylene terephthalate and polybutylene terephthalate are excellent in mechanical characteristics, heat resistance, chemical resistance and dimensional stability and are widely used in the fields of injectionmolded articles, fibers and films. However, it is difficult that the thermoplastic polyester resins during melting are brought into such viscoelastic properties that foams can be obtained. Hence, a blowing agent is easily released during foam extrusion and it is difficult to obtain good foams wherein fine closed cells are uniformly formed. To solve this problem, there was proposed a method wherein diglycidyl esters are incorporated in aromatic polyesters in the foam extrusion of the aromatic polyesters [see, JP-B-61-48409 (the term "JP-B" as used herein means an examined Japanese patent publication)]. In the foam extrusion of thermoplastic polyesters, there was proposed a method wherein polyfunctional diglycidyl esters and polyfunctional carboxylic acid anhydrides are incorporated in thermoplastic polyesters to improve the melt viscosity of the thermoplastic polyesters [see, JP-A-59-210955 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")]. The present inventors have been engaged in the production and study of extruded foams for many years and have had experience in that when the foam extrusion comprising a thermoplastic polyester in admixture with diglycidyl ester compounds is subjected to continuous long run, the resulting foam becomes discolored. The present inventors have continued to make the study and found that when pyromellitic dianhydride is mixed with a thermoplastic polyester without using any diglycidyl ester compound, the resulting foam is not colored and burned even when foam extrusion is continuously conducted over a long period of time. The present inventors have further continued making the study and found that when compounds having two or more acid anhydride groups per molecule such as pyromellitic dianhydride and compounds of Group I, II or III metals of the Periodic Table are added to thermoplastic polyester resins, the viscoelasticity of the molten materials is improved and at the same time, there can be obtained foams having high tensile elongation and more finer cells. JP-A-59-135237 (U.S. Patents 4,462,947 and 4,466,943, European Patent 0115162A) discloses that polyester resin foam sheets can be thermoformed into heat-resistant food containers which can be used in dual ovenable applications. However, the method disclosed therein has disadvantages in that since not only the melt viscosity is low when a polycarbonate is merely mixed, but carbon dioxide liberated from the polycarbonate is used as a blowing agent, expansion ratio is low, heat resistance is poor and the container cannot be removed with the bare hands when used for heating or cooking foods in microwave ovens. The present inventors have found thermoplastic polyester resin foam sheets having a high expansion ratio, good heat insulating properties and excellent thermofoamability in the above-described study. FR-A-2401945 discloses a process for the production of thinwalled, blow-moulded, blown film or foamed PET articles, in which process PET and a polyanhydride such as pyromellitic anhydride, are mixed, melt blended, extruded or melt-shaped and then cooled to produce articles which are amorphous. The polyanhydride is added to increase the melt viscosity. Whilst FR-A-2401945 states that a weak degree of crystallisation may be acceptable and can be achieved by control of the cooling cycle, it is also stated that if crystallisation which is not negligible occurs in the process, the articles produced are opaque and brittle. FR-A-2401945 does not describe in detail how to produce foams. FR-A-2401945 further discloses the possibility of adding anti-oxidants, heat-stabilizers, fillers, pigments and flame retardants. EP-A-26554 discloses a process for producing foamed articles of an aromatic polyester, in which an aromatic polyester, a polyepoxy compound and a compound of a metal of Group 1a, 11a or 111a of the Periodic Table (and optionally a fibrous filler) are mixed in a melt extrusion moulding machine whilst introducing a blowing agent therein under pressure, then extruding the mixture, further expanding the extruded and unsolidified foamed article, and then solidifying the article by cooling under reduced pressure. US-A-4224264 discloses a process for the crystallisation of crystallisable linear polyesters, in which a chemical blowing agent is dispersed in the molten polyester, and the mixture is extruded to form a solid closed cell foam which is then maintained at a temperature above its glass crystallisation temperature until crystallisation occurs, to produce a crystallised polyester having a heat of fusion in the range of about 12 to about 40J/g. The amount of blowing agent is about 0.1 to 1.5 parts by weight per 100 parts by weight of polymer so as to provide a foam density in the range of about 0.4 to about 1.0g/cc. The use of a crystallisation initiator such as TiO₂, talc, MgO, BaSO₄, SiO₂, Al₂O₃, CdO, ZnO, mica, fuller's earth, diatomaceous earth or asbestos is also disclosed. US-A-3170832 discloses a method of making a laminated material, in which a layer of thermoplastic resin is superposed on a layer of resilient, foamed polyester resin consisting of the reaction product of (1) a polyester resin derived from adipic acid and diethylene glycol, (2) toluene diisocyanate, (3) water, and (4) a tertiary amine catalyst; and heat and pressure is applied to the superposed layers at predetermined areas to heat seal the layers together. US-A-4533578 discloses a three-ply trash bag wherein outer and inner layers of one or more high performance polyolefin resin have an interposed foamed thermoplastic layer, e.g., a polyethylene terephthate, of a density at least 25% less than the density of the corresponding unfoamed resin of the layer, and of a thickness of between about 0.25 and about 1.5 mils. US-A-3822332 discloses the production of amorphous transparent copolyester mouldings using catalyst combinations containing one or more of the metals: magnesium, zinc, cobalt and manganese with the warning that metals of the group: lithium, sodium, calcium, barium and strontium should be avoided or present in minute amounts, as they promote crystallisation. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a process for producing a thermoplastic polyester resin foam which is highly expanded by adjusting viscoelastic properties during melting in the extrusion foaming of thermoplastic polyester resins. It is another object of the present invention to provide a process for producing a thermoplastic polyester resin foam which does not cause the generation of colored contaminants therein even when long-term continuous operation is conducted in the extrusion foaming of thermoplastic polyester resins. It is still another object of the present invention to provide a process for producing a thermoplastic polyester resin foam having uniform fine cells in the extrusion foaming of thermoplastic polyester resins. It is another object of the present invention to provide a thermoplastic polyester resin foam excellent in tensile strength and tensile elongation in extrusion foaming. It is still a further object of the present invention to provide a process for producing a thermoplastic polyester resin foam which is highly expanded and has excellent heat resistance by re-heating an extrusion-expanded thermoplastic polyester resin foam to carry out post-expansion. According to the present invention, there is provided a process for producing a thermoplastic polyester resin foam comprising the steps of melting a thermoplastic polyester resin, mixing the molten resin with a a blowing agent, extruding the mixture into a low pressure zone to carry out foaming, and subsequently cooling, characterized in that both an expansion nucleating agent and a viscoelasticity improver are added to the thermoplastic polyester resin, in that the viscoelasticity improver is (a) a compound having two or more acid anhydride groups per molecule is used in an amount of 0.05 to 5 parts by weight per 100 parts by weight of the thermoplastic polyester resin and (b) a compound, not being talc, of a metal of Group I, II or III elements of the Periodic Table, in that the blowing agent is present in an amount of 0.05 to 50 % by weight of the molten blend, and in that the subsequent cooling step is effected to produce a thermoplastic polyester resin foam having a crystallinity of not less than 9% as measured using differential scanning calorimetry and a density of not higher than 0.7 g/cm³. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 to 3 are sectional views of means for carrying out re-heating, Figure 4 shows diagrammatically one embodiment of a means for continuously carrying out re-heating, Figure 5 is a graph illustrating the results of Test Example 2, Figure 6 shows a side view of a food container, Figure 7 shows a planar view of a food container, Figures 8, 9 and 10 are electron microscopic photographs which show the shapes of cells of the foams prepared in Examples 9, 12 and 17, respectively. DETAILED DESCRIPTION OF THE INVENTION In the production of the polyester resin foams of the present invention, extruders are used. Thermoplastic polyester resins are melted under an elevated pressure in the extruders and the molten resins are extruded through die into a low-pressure zone to produce foams. In the production of the polyester resin foams of the present invention, compounds having two or more acid anhydride groups per molecule are added to thermoplastic polyester resins. By adding the compounds having two or more acid anhydride groups per molecule, the viscoelastic properties of the thermoplastic polyester resins during extrusion can be improved, whereby gasified blowing agents can be retained in the interiors of closed cells and uniformly dispersed fine cells can be formed using extruders. It is believed that the compound having two or more acid anhydride groups per molecule is bonded to OH groups in the molecule chain of the thermoplastic polyester resin and crosslinking gently takes place, whereby the viscoelastic properties of the thermoplastic polyester resin during extrusion can be improved. The term "viscoelastic properties during melting" can be confirmed by a phenomenon wherein, the molten resin is swollen or shrunk from the outlet of the die when the molten resin is extruded through the die, and can be generally represented by a die swell ratio. The die swell ratio can be measured when a molten resin is extruded through a round orifice die having a circular section. Die swell ratio can be determined by the following formula. Die swell ratio = (Diameter of extruded melt) (Diameter of outlet of die) Die swell ratio is an important factor in extrusion foaming. It is preferred that die swell ratio is 2 to 5 in order to obtain foamed articles having a large sectional area and uniformly dispersed fine cells in particular. In the present invention, a blend of a thermoplastic polyester resin, a compound having two or more acid anhydride groups and further a compound of a metal of Group I, II or III elements of the Periodic Table is molten in an extruder, a blowing agent is generally injected into the molten blend and the resulting molten blend is extruded through the die of the extruder for foaming into a low-pressure zone to produce a foam. By adding a compound of a metal of Group I, II or III elements of the Periodic Table, there can be obtained a thermoplastic polyester resin foam having finer cells uniformly dispersed therein. Thermoplastic polyester resins used in the present invention are linear polyesters of polycondensates of an aromatic dicarboxylic acid component and a diol component. Examples of dicarboxylic acid components which can be used in the present invention include terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, diphenyl ether carboxylic Acid, diphenyl sulfone dicarboxylic acid and diphenoxyethanedicarboxylic acid. Examples of diol components which can be used in the present invention include ethylene glycol, trimethylene glycol, tetramethylene glycol, neopentyl glycol, hexamethylene glycol, cyclohexanedimethanol, tricyclodecanedimethanol, 2,2-bis(4-β-hydroxyethoxyphenyl)propane, 4,4-bis(β-hydroxyethoxy)diphenyl sulfone, diethylene glycol and 1,4-butanediol. Polyethylene terephthalate, polybutylene terephthalate, polybutylene terephthalate elastomer, amorphous polyesters, polycyclohexane terephthalate, polyethylene naphthalate and mixtures thereof are preferably used as the polyesters comprising these dicarboxylic acid components and these diol components. Modified resins composed of at least 50% of these thermoplastic polyester resins can be used. Any of aromatic acid anhydrides, cyclic aliphatic acid anhydrides, fatty acid anhydrides, halogenated acid anhydrides, etc. can be used as the compounds having two or more acid anhydride groups per molecule, so long as they have at least two acid anhydride groups per molecule. Further, mixtures thereof and modified compounds thereof can be used. Preferred examples of the compounds include pyromellitic dianhydride, benzophenonetetxacarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, diphenyl sulfone tetracarboxylic dianhydride and 5-(2,5-dioxo-tetrahydro-3-furanyl)-3-methyl-3-cyclohexen-1,2-dicarboxylic dianhydride. Among them, pyromellitic dianhydride is more preferred. The compounds having two or more acid anhydride groups per molecule are used in an amount of 0.05 to 5.0 parts by weight per 100 parts by weight of the thermoplastic polyester resin. When the amount of the compound having two or more acid anhydride groups per molecule is less than 0.05 part by weight per 100 parts by weight of the thermoplastic polyester resin, an effect of improving the viscoelastic properties of the thermoplastic polyester resin during extrusion is not sufficient and good foam cannot be formed, while when the amount exceeds 5.0 parts by weight, the gelation of the molten material of the thermoplastic polyester resin proceeds and extrusion foaming cannot be effected. Any of inorganic compounds and organic compounds can be used as the compounds of metals of Group I, II or III elements of the Periodic Table, so long as they have these metals as their constituent atoms. Examples of the inorganic compounds include potassium chloride, sodium chloride, sodium hydrogencarbonate, sodium carbonate, potassium carbonate, zinc carbonate, magnesium carbonate, calcium carbonate, aluminum carbonate, sodium oxide, potassium oxide, zinc oxide, magnesium oxide, calcium oxide, aluminum oxide and the hydroxides of these metals. Examples of the organic compounds include sodium stearate, potassium stearate, zinc stearate, magnesium stearate, calcium stearate, aluminum stearate, sodium montanate, calcium montanate, lithium acetate, sodium acetate, zinc acetate, magnesium acetate, calcium acetate, sodium caprylate, zinc caprylate, magnesium caprylate, calcium caprylate, aluminum caprylate, sodium myristate, zinc myristate, magnesium myristate, calcium myristate, aluminum myristate, calcium benzoate, potassium terephthalate, sodium terephthalate, sodium ethoxide and potassium phenoxide. Among them, the compounds of Group I or II metals of the Periodic Table are preferred and the compounds of Group I metals are more preferred. By using the compounds of Group I, II or III metals, the cells of the resulting thermoplastic polyester resin foam are made finer and at the same time, an effect of increasing the viscoelasticity by the compound having two or more acid anhydride groups per molecule can be increased. The compounds of Group I, II or III metals of the Periodic Table are preferably used in an amount of 0.05 to 5.0 parts by weight per 100 parts by weight of the thermoplastic polyester resin. When the amount of the compound is less than 0.05 part by weight, effects of making the cells of the resulting foam finer and the efficiency of increasing the viscoelasticity by the compound having two or more anhydride groups are not sufficient, while when the amount exceeds 5 parts by weight, the resulting foam is colored and the viscosity of the molten thermoplastic polyester resin is not high enough. Any of blowing agents can be used in the production of the thermoplastic polyester resin foams of the present invention, so long as they are easily vaporizable liquids or thermally decomposable chemicals. Easy vaporizable blowing agents such as inert gases, saturated aliphatic hydrocarbons, saturated alicyclic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, ethers and ketones are preferred. Examples of these easy vaporizable blowing agents include carbon dioxide, nitrogen, methane, ethane, propane, butane, pentane, hexane, methylpentane, dimethylbutane, methylcyclopropane, cyclopentane, cyclohexane, methylcyclopentane, ethylcyclobutane, 1,1,2-trimethylcyclopropane, trichloromonofluoromethane, dichlorodifluoromethane, monochlorodifluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, dichlorotrifluoroetbane, monochlorodifluoroethane, tetrafluoroethane, dimethyl ether, 2-ethoxyethane, acetone, methyl ethyl ketone, acetylacetone, dichlorotetrafluoroethane, monochlorotetrafluoroethane, dichloromonofluoroethane and difluoroethane. Usually, the blowing agent is injected into the molten blend of the thermoplastic polyester resin, the compound having two or more acid anhydride groups per molecule and other additives on the way of an extruder. The amount of the blowing agent to be injected is from 0.05 to 50% by weight based on the amount of the molten blend. When the amount of the blowing agent is less than 0.05% by weight, the resulting foam is not sufficiently expanded, while when the amount is more than 50% by weight, the gas of the blowing agent is not accommodated for foaming, but blows off and the foam cannot be formed into a desired shape. The preferred amount of the blowing agent is 0.1 to 30% by weight based on the amount of the molten blend. In the production of the thermoplastic polyester resin foams of the present invention, stabilizer, pigment, filler, flame retarder and antistatic agent may be optionally added to the resin blend to improve the physical properties of the thermoplastic polyester resin foams and molded articles thereof. In the production of the thermoplastic polyester resin foams of the present invention, foaming can be carried out by any of blow molding process and extrusion process using single screw extruder, multiple screw extruder and tandem extruder. Dies used in the extrusion process or the blow molding process are flat die, circular die and nozzle die according to the shape of the desired foam. In the production of the polyester resin foams of the present invention, the thermoplastic polyester resin can be mixed with the compound having two or more acid anhydride groups per molecule and other additives by any of the following methods. (A) The thermoplastic polyester resin is mixed with the compound having two or more acid anhydride groups per molecule at a low temperature (e.g., a temperature of not higher than 150°C). (For example, the powder of the compound having two or more acid anhydride groups per molecule is stuck on the pellet of the thermoplastic polyester resin). (B) The compound having two or more acid anhydride groups per molecule is previously melt-mixed with a thermoplastic resin, the mixture is pelletized and the pellet is mixed with the thermoplastic polyester resin (this thermoplastic resin may be the same as or different from the thermoplastic polyester resin, but is preferably one compatible with the thermoplastic polyester resin). (C) The thermoplastic polyester resin is previously fed to an extruder hopper to melt it and the compound having two or more acid anhydride groups per molecule is fed through a feed opening provided at the cylinder of the extruder to effect mixing. In any of the above mixing methods, the moisture content of the resin blend should be as small as possible and is reduced to preferably not higher than 200 ppm. It is preferred that the thermoplastic polyester resin is dried at a temperature of 60 to 180°C with hot air having a dew point of not higher than -20°C in a dehumidifying hot-air dryer for about 4 hours. The present invention also relates to a process for producing a thermoplastic polyester resin foam, which comprises cooling a high-temperature thermoplastic polyester resin foam immediately after expansion to a temperature of not higher than the glass transition point of the resin to bring the crystallinity to 30% or below and then heating the foam to higher than 60°C. Pre-expanded (primarily expanded) foam extruded through an extruder has only a low expansion ratio and usually a high density. The expansion ratio varies depending on the shapes of foams, but is about 5 times at most when the extruded foam is a sheet. In the present invention, the thus-obtained pre-expanded foam, while its temperature is high immediately after extrusion, is cooled to a temperature of not higher than the glass transition point of the polyester resin. The glass transition point of the polyester resin varies depending on the types of carboxylic acids and alcohols which constitute polyesters, but is generally in the range of 30 to 90°C. Hence, the foam is generally cooled to a temperature of not higher than 60°C. When the pre-expanded foam is cooled, it is settled without having time to crystallize, and hence the crystallinity thereof is low. The crystallinity varies depending on the degree of cooled. For example, the crystallinity varies depending on the type and temperature of cooling media and the contact conditions of the foam with the cooling media. When the pre-expanded foam prepared by extrusion is brought into directly contact with water at room temperature, the crystallinity thereof is several % to ten-odd %, generally not higher than 30%. However, when the preexpanded foam prepared by extrusion is put into a mold to shape it, crystallinity becomes 30% or higher, since the foam is not cooled unless the mold is forcedly cooled. Particularly, the crystallinity of thick-wall pre-expanded foam becomes 30% or higher. Accordingly, when the pre-expanded foam is prepared by means of the extruder, the foam is allowed to proceed along a cooled mold to thereby cool it. In order to conduct effectively the cooling of the pre-expanded foam, it is desirable that the foam has a large surface area in comparison with its volume. Namely, it is desirable that the foam is in the form of a sheet, if possible and its thickness is not more than 10 mm, preferably not more than 3 mm. When the sheet is cylindrical, a mandrel is put into the inside of the cylinder, the sheet is allowed to proceed along the mandrel which is cooled with water and the length of the mandrel should be as long as possible. On the other hand, when the sheet is a flat sheet, the sheet is put between a pair of rollers and allowed to proceed while cooling and at the same time, the rollers are cooled with water and the diameters of rollers should be as large as possible. In this way, the crystallinity of the pre-expanded foam is brought to 30% or below. The foam is then re-heated to carry out post expansion (secondary expansion). For post expansion, the foam is heated to 60°C or higher. Any of heating means can be used. For example, heating may be conducted by conduction in contact with a heating plate. Alternatively, heating may be conducted by radiation, convection or high-frequency power. Any of heating media can be used, so long as polyester resins are not deteriorated by them. A preferred heating method is such that the pre-expanded foam is brought into contact with a heated metal or air or with steam or heated water. The heating time for the post expansion is determined according to the properties of the resins, the shape and the type and temperature of the heating medium. Generally, when the temperature of the heating medium is low, heating time is prolonged, while when the temperature is high, heating time is shortened. Further, when the foam is thick-walled, heating time is prolonged, while when the foam is thin-walled, heating time is shortened. It is preferred that a metal plate is heated to 60 to 200°C and the pre-expanded foam is brought into contact with the metal plate for 5 seconds or longer when the foam is heated by bringing it into contact with the metal plate. When the pre-expanded foam is heated by bringing it into contact with air, it is preferred that the foam is put into an oven, the temperature within the oven is elevated to 100 to 230°C and the foam is heated for 10 seconds to 5 minutes. It is desirable that when the foam is heated by the metal plate or air, the foam is left to stand for at least 24 hours, usually about 3 days after the pre-expansion and then is subjected to the post expansion without conducting post expansion immediately after pre-expansion. On the other hand, when the pre-expanded foam is heated by bringing it into contact with steam or hot water, post expansion can be carried out immediately after pre-expansion. In this case, the temperature of steam or water is 60 to 125°C and contact time is 10 seconds to 5 minutes. The polyester resin foam can be brought into contact with steam or water by various methods. For example, the foam 1 may be immersed in heated water 2 as shown in Fig. 1. In Fig. 1, a numeral 8 means a burner. In another embodiment, a metal gauze 3 is placed above the surface of heated water and the foam 1 is placed on the metal gauze 3 and is brought into contact with steam 4 which is evaporated from water 2 as shown in Fig. 2. In other embodiment, pressurized steam 4 is blown into a container 9 containing the foam 1 as shown in Fig. 3. It is preferred that the foam is placed in a mold and molded into a desirable shape when the foam is to be heated by bringing it into contact with water or steam. When a mold is used, water or steam is introduced into the mold to thereby bring the foam into directly contact with water or steam. When the polyester resin foam is heated to 60°C or higher by bringing it into contact with water or steam in the manner described above, the foam is post-expanded to form a foam having a low density. Generally, highly post expansion can be easily conducted by heating with water or steam rather than air. Further, steam is more preferable than water. When heating is conducted with water or steam, the post expansion ratio is at least 1.3 though it is lower than the pre-expansion ratio, and it is possible that the ratio is 4 or more. In addition thereto, expansion can be uniformly carried out and the resulting post-expanded foam has fine, uniform cells. In this way, a low-density foam of good quality can be obtained. Thus, when the pre-expanded foam is heated, not only a low-density foam can be readily obtained, but the post-expanded foam can be rendered to have a crystallinity of 15% or more. A foam having a crystallinity of 15% or more is a foam which is very excellent in heat resistance in a heating atmosphere and can be used for heat-resistant food containers, heat insulating materials, etc. Further, the melt viscosity, die swell ratio, etc. of the thermoplastic polyester resins are adjusted in the process of the present invention to produce extrusion foam sheets. The extrusion foam sheets of the thermoplastic polyester resins have a density of not higher than 0.7 g/cm³, preferably not higher than 0.5 g/cm³. When the density exceeds 0.7 g/m³, heat insulating properties, lightweight properties and cushioning properties as foam sheet are lost. It has been found that the extrusion foam sheets having a crystallinity of not higher than 20% and a molecular orientation ratio of not higher than 4.5 in the direction of face of foam sheet are preferred from the viewpoint of thermoformability. It is difficult to lower the crystallinity through the thickness, since the extrusion foam sheet immediately after extrusion has heat insulating properties. However, post thermoformability can be improved by lowering the molecular orientation ratio to a specific value or below. The molecular orientation ratio of the extrusion foam sheet looking in a direction from the surface of the foam sheet can be adjusted to 4.5 or below by controlling expansion in the direction of extrusion and in the direction crossing the extrusion direction. As a preferred method therefor, there is generally used a method using a circular die and a cylindrical mandrel. Namely, expansion in the direction of extrusion can be controlled by the ratio of the average flow rate of a foamed resin to a take-off speed in the direction of extrusion at the outlet gap of the circular die, and expansion in the direction crossing the extrusion direction can be controlled by the ratio (hereinafter referred to as blow-up ratio) of the diameter of the outlet of the circular die to the outer diameter of the mandrel. Crystallinity is determined from quantity of heat of cold crystallization and quantity of heat of fusion in heating by heat-flux DSC (differential scanning calorimetry) in the measurement of heat of transition according to JIS-K-71222 (Method for measuring heat of transition of plastics). Namely, crystallinity is determined by the following equation. Crystallinity (%) = Quantity of heat of fusion per mol) - (quantity of heat of cold crystallization per mol) Quantity of heat of fusion per mol of perfect crystallized resin ×100 Crystallinity was measured by using differential scanning calorimeter DSC 200 manufactured by Seiko K.K. For the quantity of heat of perfect crystal fusion of polyethylene terephthalate, there was used 26.9 kJ/mol from Kobunshi Deta Handobukku (published by Baifukan KK). Molecular orientation ratio in the direction of face of foam sheet is the ratio between the maximum value of intensity of microwave transmitted through foam sheet and the minimum value thereof when the surface of foam sheet is perpendicularly irradiated with a polarizer (manufactured by Kanzaki Paper Mfg. Co., Ltd.). Further, food containers can be produced by bonding a non-foam film of a thermoplastic resin to at least one side of the extrusion foam sheet of the thermoplastic polyester resin according to the present invention to form a laminated sheet and thermoforming it into a food container in such a manner that the non-foam sheet is positioned inside. Examples of the thermoplastic resin which constitutes the non-foam film include thermoplastic polyester resins, liquid crystal polyester resins, polyolefin resins, polyamide resins, polyvinyl chloride resins, polyacrylonitrile resins, polyvinylidene chloride resins and ethylene-vinyl alcohol copolymers. Non-foam film to be bonded may be composed of a single-layer film or multi-layer film. The non-foam film may be bonded to both sides of the foam sheet. The thickness of the non-foam film layer is 10 to 500 microns and the thickness of the sheet layer of the expanded polyester resin is thicker, i.e., generally not more than 5 mm. It is preferred that the thickness of the foam sheet layer is 2 to 500 times that of the non-foam film layer. The non-foam film can be laminated onto the foam sheet by using a plurality of extruders. More particularly, a thermoplastic polyester resin is fed to an extruder and an expandable polyester resin is extruded therethrough. Separately, a thermoplastic resin is fed to other extruder and a non-expandable resin is extruded therethrough. These resins are guided to the same one die and combined together in the die to prepare a laminated sheet. In another embodiment, the resins extruded through each extruder are fed to separate dies to prepare a foam sheet and a film, respectively. They are put upon each other and pressed through rollers to prepare a laminated sheet. In a still other embodiment, the foam sheet and the non-foam sheet are separately prepared and separately wound up into rolls. They are laminated onto each other, while unwinding, and passed through a pair of rollers to thereby laminate them. The temperature of the roller on the side of the foam sheet is preferably low, for example, room temperature to prevent polyester resin from being crystallized. On the other hand, the roller on the side of the non-foam film is kept preferably at a high temperature so as to soften the surface of the film. The laminated sheet is re-heated and thermoformed into a container. The thermoforming is carried out by using a molding die. The die may be composed of a male mold and a female mold, but may be composed of either one of them. When a die composed of both molds is used, molding can be carried out merely by putting the laminated sheet between both molds and pressing it. However, when either one of molds is used, air present between the sheet and the mold is removed, or the sheet is pressurized from the upper side thereof and pressed. The non-foam film is so arranged that the film is positioned inside the container. The heating temperature at which the sheet is molded into a container is determined according to the thermoplastic polyester resin constituting the foam sheet and the thermoplastic resin constituting the non-foam film. The expansion ratio of the foam sheet is increased or decreased by the heating temperature. Accordingly, even when a foam sheet having the same thickness is used, a difference in the thickness of the laminated sheet is caused. Further, the crystallinity of the foam sheet is accelerated depending on the heating temperature and the heating time. If the foam sheet by extrusion, which is composed of a thermoplastic polyester resin, has a crystallinity of 15% or more, it is excellent in heat resistance in a heating atmosphere in air. However, in the case that it is exposed to heated steam of 100°C or higher, since the film wall thickness of first layer cells on the surface is thin, the first layer cells are re-swollen. Further, in the case that it is used for a food container to heat foodstuffs such as soup and gratin, the first layer cells are re-swollen, whereby juices from the foodstuffs were likely penetrated into the cells. On the other hand, since in the present invention a non-foam film is bonded such that it is positioned inside the container, the re-swelling by heated steam or hot water and the penetration of juices from the foodstuffs into the cells can be prevented. The following examples 1 to 39 are provided to illustrate the present invention. Examples 40 to 43 illustrate uses of extruded foam sheet prepared by the process of the invention. The reference examples are provided to illustrate how some on the steps of the process of the present invention can be performed. TEST EXAMPLE 1 An experiment was carried out to examine the effects of the addition of the compound having two or more acid anhydride groups per molecule to the thermoplastic polyester resin on viscoelastic properties during melting. (1) Test-1 Polyethylene terephthalate (PET 9902, a product of Eastman Kodak Company) was dried at 160°C with hot air having a dew point of -30°C in a dehumidifying dryer for 4 hours. 1 kg of the resulting dried material was mixed with 5 g of pyromellitic dianhydride (a product of Chemische Werke Hüls AG). The blend was fed to the hopper of a single screw extruder (diameter of screw: 40 mm, L/D: 30, manufactured by Sekisui Koki K.K.) equipped with a cylindrical channel die (diameter: 5 mm, L/D: 1.6). The molten blend was extruded through the cylindrical channel die at an extrusion rate of 7 kg/Hr under the following production conditions to carry out molding. Production conditions by the single screw extruder Temperature of feed zone of extruder : 270°C Temperature of compression zone of extruder : 280°C Temperature of melting zone of extruder : 270°C Temperature of die : 270°C Number of revolutions of screw of extruder : 32 rpm (2) Test-2 (control) The extrusion of polyethylene terephthalate was carried out in the same manner as in Test-1 except that pyromellitic dianhydride was omitted. (3) Test results Die swell ratio of extrudate and the melt viscosity of the molten resin in the single screw extruder are shown in Table 1. It is clear from Table 1 that when pyromellitic dianhydride is added to polyethylene terephthalate, die swell ratio of extrudate is increased and the melt viscosity of the molten resin in extrusion is also increased. REFERENCE EXAMPLE 1 10 kg of polyethylene terephthalate (PET 9902, a product of Eastman Kodak Company) was dried in a dehumidifying dryer (160°C with hot air having a dew point of -30°C) for 4 hours and then mixed with 20 g of pyromellitic dianhydride (a product of Chemische Werke Hüls AG) and 60 g of talc (expansion nucleating agent) in a tumbling mixer. The mixture was fed to the hopper of a single screw extruder (diameter of screw: 40 mm, L/D: 30, bore of nozzle die: 5 mm) and melt-mixed. Isopentane (blowing agent) was injected into the molten mixture in an amount of 2.2% by weight. The molten mixture was extruded through the nozzle die under following production conditions into air to produce a rod-shaped foam. Production conditions with single screw extruder Temperature of feed zone of extruder : 273 to 282°C Temperature of compression zone of extruder : 280 to 290°C Temperature of melting zone of extruder : 271 to 290°C Temperature of extruder head : 280 to 290°C Number of revolutions of screw of extruder : 32 rpm Extrusion rate : 7 to 8 kg/hr The expansion ratio and diameter of the foam are shown in Table 2. REFERENCE EXAMPLE 2 The procedure of Reference Example 1 was repeated except that the amount of pyromellitic dianhydride was 30 g and isopentane was injected into the molten mixture in an amount of 2.5% by weight to produce a rod-shaped foam. The expansion ratio and diameter of the foam are shown in Table 2. REFERENCE EXAMPLE 3 The procedure of Reference Example 1 was repeated except that the amount of pyromellitic dianhydride was 40 g and isopentane was injected into the molten mixture in an amount of 2.6% by weight to produce a rod-shaped foam. The expansion ratio and diameter of the foam are shown in Table 2. COMPARATIVE EXAMPLE 1 Effect (1) of the use of pyromellitic dianhydride The procedure of Reference Example 1 was repeated except that pyromellitic dianhydride was omitted and isopentane was injected into the molten mixture in an amount of 2.7% by weight to produce a rod-shaped foam. The expansion ratio and diameter of the foam are shown in Table 2. COMPARATIVE EXAMPLE 2 Effect (2) of the use of pyromellitic dianhydride The procedure of Reference Example 1 was repeated except that 50 g of phthalic anhydride was used in place of 20 g of pyromellitic dianhydride and isopentane was injected into the molten mixture in an amount of 2.7% by weight to produce a rod-shaped foam. The expansion ratio and diameter of the foam are shown in Table 2. Effect of use of pyromellitic dianhydride Rod-shaped Foam Example No. Amount of Pyromellitic Dianhydride Amount of Isopentane Density Expansion Ratio Diameter (g) (%) (g/cm³) (times) (mm) Reference Example 1 20 2.2 0.35 4.0 21 Reference Example 2 30 2.5 0.18 7.8 27 Reference Example 3 40 2.6 0.13 10.8 30 Comp. Ex. 1 - 2.7 0.76 1.8 9 Comp. Ex. 2 - 2.7 0.70 2.0 11 Discussion: It is clear from Table 2 that in Reference Examples 1 to 3 wherein pyromellitic dianhydride is used, expansion ratios are 4 to 10, the diameters of the rod-shaped foams are increased to 4 to 6 times the size of the nozzle die and the increasing ratio thereof is in proportion to the increase in the amount of isopentane injected. In Comparative Examples 1 and 2 wherein pyromellitic dianbydride is omitted, expansion ratios are about 2, though isopentane (blowing agent) is used in an amount larger than that in Reference Examples 1 to 3, and the diameters of the rod-shaped foams are about twice as large as the bore of the nozzle die. Accordingly, both the expansion ratio and the diameter are smaller than those of Reference Examples 1 to 3. These effects are thought to be due to the fact that when pyromellitic dianhydride is not used, the blowing agent isopentane is released from the foam in expansion molding, while when pyromellitic dianhydride is used in expansion molding, the melt viscosity of the thermoplastic polyester is increased, whereby gaseous isopentane (blowing agent) is not allowed to be released from the foam. REFERENCE EXAMPLE 4 In the procedure of Reference Example 3, the expansion molding operation was continuously carried out for 24 hours. No change for the rod-shaped foam was observed. REFERENCE EXAMPLE 5 In the procedure of Reference Example 3, the expansion molding operation was carried out for 5 hours and the operation was then stopped for 19 hours. Thereafter, operation and stopping were repeated five times in a 24 hour cycle. In any case, there could be obtained a rod-shaped foam, which was neither colored nor contaminated within 30 minutes from the commencement of the operation. COMPARATIVE EXAMPLE 3 In the procedure of Reference Example 3, diglycidyl terephthalate (Blemmer ® , a product of Nippon Oil & Fats Co., Ltd) was used in place of pyromellitic dianhydride and isopentane was injected into the molten mixture in such an amount as to give a 4 g/100 g mixture. In the same manner as in Reference Example 5, the operation was carried out. In the operation after the third operation, it took 90 minutes or longer untill there could be obtained a rod-shaped foam which was neither colored nor contaminated. In the fifth operation, the foam was discolored yellow and contaminated with black matters after 4 hours from re-operation. REFERENCE EXAMPLE 6 The procedure of Reference Example 1 was repeated except that 50 g of cyclopentanetetracarboxylic acid dianhydride was used in place of 20 g of pyromellitic dianhydride to produce a rod-shaped foam. The expansion ratio of the foam was 4 and the diameter thereof was 20 mm. REFERENCE EXAMPLE 7 The procedure of Reference Example 1 was repeated except that 50 g of benzophenonetetracarboxylic acid dianhydride was used in place of 20 g of pyromellitic dianhydride to produce a rod-shaped foam. The expansion ratio of the foam was 4.5 and the diameter thereof was 21 mm. REFERENCE EXAMPLE 8 In Reference Examples 6 and 7, the operation was carried out in the same manner as in Reference Example 5. Within 30 minutes from the commencement of the operation, there could be obtained a rod-shaped foam which was neither colored nor contaminated in any case. EXAMPLE 1 TO 8 AND REFERENCE EXAMPLE 9 Polyethylene terephthalate given in Table 3 was dried with hot air having a dew point of -30°C at 160°C in a dehumidifying dryer for 4 hours. 100 parts by weight of the dried polyethylene terephthalate, pyromellitic dianhydride in an amount given in Table 3, a metallic compound given in Table 3 and 0.6 part by weight of talc (expansion nucleating agent) were mixed in a tumbling mixer. The mixture was fed to the hopper of a single screw extruder (diameter of screw: 40 mm, L/D: 30) equipped with a nozzle die (bore: 5 mm). 2.0% by weight of n-pentane was injected into the molten mixture. The molten mixture was extruded under the following conditions to obtain a rod-shaped foam. The expansion ratios of the foams and the state of cells are shown in Table 3. Fig. 8 is an electron microscopic photograph showing the state of cells of the foam prepared in Example 1, which is enlarged about 25 times. The cells shown in Fig. 8 were small. Further, Figs. 9 and 10 are electron microscopic photographs showing the states of cells of the foams prepared in Example 4 and Reference Example 9 respectively, which are enlarged about 25 times. The cells of the foam prepared in Reference Example 9 were slightly larger than those in Example 4. COMPARATIVE EXAMPLE 4 The procedure of Example 1 was repeated except that pyromellitic dianhydride and the metallic compound were omitted to produce a rod-shaped foam. The expansion ratio of the foam and the state of cells are shown in Table 3. EXAMPLES 9 TO 11 The procedure of Example 1 was repeated except that polyester resins, acid anhydrides and metallic compound given in Table 4 were used to obtain foams. The expansion ratios of the foams and the state of cells are shown in Table 4. COMPARATIVE EXAMPLE 5 The procedure of Example 1 was repeated except that polyethylene terephthalate and diglycidyl terephthalate given in Table 4 were used and the metallic compound was omitted to obtain a foam. The expansion ratio of the foam and the state of cells are shown in Table 4. EXAMPLE 12 Polyethylene terephthalate given in Table 5 was dried with hot air having a dew point of -30°C at 160°C in a dehumidifying dryer for 4 hours. 100 parts by weight of the dried polyethylene terephthalate, 0.5 part by weight of pyromellitic dianhydride, 0.1 part by weight of sodium carbonate and 0.6 part by weight of talc (expansion nucleating agent) were mixed in a tumbling mixer. The mixture was fed to the hopper of a single screw extruder (diameter of screw: 65 mm, L/D: 35) equipped with a cylindrical mandrel (bore: 205 mm, L/D: 1.5) and a circular die (bore: 60 mm, gap: 0.65 mm), 1.7% by weight of butane was injected into the molten mixture. The molten mixture was extruded through the circular die under the following conditions. The extrudate was taken off by a cylindrical mandrel and molded. A part of the resulting cylindrical foam was cut open and the resulting foam sheet was wound up. Temperature of feed zone of extruder : 280°C Temperature of compression zone of extruder : 290°C Temperature of melting zone of extruder : 280°C Temperature of extruder head : 280°C Temperature of extruder die : 270°C Injection pressure of blowing agent : 80 kg/cm² Extrusion pressure (head) : 115 kg/cm² Number of revolutions of screw : 30 rpm Extrusion rate : 24 kg/hr The expansion ratio of the foam sheet, the state of cells, tensile strength at 23°C and tensile elongation at break at 23°C are shown in Table 5. COMPARATIVE EXAMPLE 6 The procedure of Example 12 was repeated except that diglycidyl terephthalate and sodium montanate in amounts given in Table 5 were used and pyromellitic dianhydride and sodium carbonate were omitted. The resulting foam sheet was wound up. The expansion ratio of the foam sheet, the state of cells, tensile strength at 23°C and tensile elongation at break at 23°C are shown in Table 5. COMPARATIVE EXAMPLE 7 The procedure of Example 12 was repeated except that diglycidyl terephthalate given in Table 5 was used, 0.9% by weight of butane was injected, extrusion was carried out under the following conditions and sodium carbonate was omitted. The resulting foam sheet was wound up. Temperature of feed zone of extruder : 280°C Temperature of compression zone of extruder : 290°C Temperature of melting zone of extruder : 280°C Temperature of extruder head : 285°C Temperature of extruder die : 275°C Injection pressure of blowing agent : 40 kg/cm² Extrusion pressure (head) : 90 kg/cm² Number of revolutions of screw : 25 rpm Extrusion rate : 24 kg/hr The extrusion ratio of the foam sheet, the state of cells, tensile strength at 23°C and tensile elongation at break at 23°C are shown in Table 5. TEST EXAMPLE 2 Test Sample: Foam sheets prepared in Examples 12 and Comparative Example 6 Test method: Dynamic visco-elastometer (manufactured by Toyo Seiki Seisakusho, Ltd.) was used. Complex elastic modulus of test samples [5 mm (width) x 21 mm (length) x 1.5 mm (thickness)] was measured under the following conditions. Frequency for measurement : 10 Hz Heating rate : 3°C/min Distance between clamps: 15 mm The results are shown in Fig. 5. Discussion: Complex elastic modulus of the foam sheet of Example 12 is higher than that of the foam sheet of Comparative Example 6. Hence, it can be understood that the heat resistance of the foam sheet obtained by using pyromellitic dianhydride and sodium carbonate is high. The process for producing the thermoplastic polyester resin foam by re-heating is illustrated by means of the following Examples and Comparative Examples. Parts and percent given below are by weight unless otherwise stated. EXAMPLE 13 Pre-expansion (primary expansion): TR8580 (trade name, a product of Teijin Limited) was used as polyethylene terephthalate (hereinafter referred to as PET). PET was placed in a dehumidifying dryer and dried at 160°C for 4 hours while circulating hot air having a dew point of -30°C. 100 parts of PET, 0.6 part of talc, 0.35 part of pyromellitic dianhydride and 0.1 part of sodium carbonate were thoroughly mixed in a tumbling mixer. The mixture was fed to an extruder (diameter of screw: 65 mm, L/D: 35) and thoroughly mixed at a screw revolution number of 25 rpm and at a barrel temperature of 270 to 290°C. 1.3 parts of butane as a blowing agent per 100 parts of the mixture was introduced into the mixture under pressure on the way of the barrel. PET containing the blowing agent was extruded through the circular die into air to produce a tube. The die had a circular die gap of 0.4 mm and a bore of 60 mm and was kept at 270 to 285°C. PET extruded into air was expanded and the tube as extruded was taken off while bringing it into contact with the outer surface of a cylindrical mandrel. The mandrel had an outer diameter of 205 mm and cooling water at 30°C was circulated inside the mandrel so that the PET foam was quenched. The quenched PET foam was cut open and the resulting flat foam sheet was wound up and referred to as pre-expanded foam sheet (primarily expanded foam sheet). The foam sheet was 643 mm in width and had an apparent density (hereinafter referred to simply as density) of 0.26 g/cm³, a thickness of 1.5 mm and a crystallinity of 9%. Post-expansion (secondary expansion): A piece of 100 mm x 100 mm was cut off from the above pre-expanded foam sheet and subjected to post-expansion. The post expansion was carried out by immersing the piece in warm water at 63°C for 5 minutes as shown in Fig. 1. The thickness was expanded from 1.5 mm to 2.1 mm. The ratio V₂/V₁ of the volume (V₂) of the post-expanded foam sheet to the volume (V₁) of the pre-expanded foam sheet was 1.37. The post-expanded foam sheet had a density of 0.19 g/cm³ and a crystallinity of 9%. The post-expanded foam sheet was finely expanded and found to be a good foam. EXAMPLE 14 The same pre-expanded foam sheet as that obtained in Example 13 was used and post expansion was carried in the same way as in Example 13 except that the temperature of warm water was 83°C and immersion was conducted for 5 minutes. There was obtained a post-expanded foam sheet which had a thickness of 3.02 mm, a density of 0.13 g/cm³ and a crystallinity of 10%. The ratio V₂/V₁ was 2.00. The foam sheet was finely, uniformly expanded, had a low density and was found to be a good foam. EXAMPLE 15 The same pre-expanded foam sheet as that obtained in Example 13 was used and post expansion was carried out by bringing the sheet into contact with steam as shown in Fig. 2. Namely, post expansion was carried out by bringing the sheet into contact with steam at 62°C for 5 minutes. There was obtained a post-expanded foam sheet which had a thickness of 2.51 mm and a density of 0.16 g/cm³. The ratio V₂/V₁ was 1.63. EXAMPLE 16 The procedure of Example 15 was repeated except that the temperature of steam for post expansion was 75°C. There was obtained a post-expanded foam sheet which had a thickness of 2.73 mm and a density of 0.14 g/cm³. The ratio V₂/V₁ was 1.86. EXAMPLE 17 The procedure of Example 15 was repeated except that the temperature of steam was 100°C and the contact time was 0.5 minute in the post expansion. There was obtained a post-expanded foam sheet which had a thickness of 2.78 mm, a density of 0.14 g/cm³ and a crystallinity of 10%. The ratio V₂/V₁ was 1.86. EXAMPLE 18 The procedure of Example 17 was repeated except that post expansion was carried out by bringing the sheet into contact with steam at 100°C for 2 minutes as shown in Fig. 2. There was obtained a post-expanded foam sheet which had a thickness of 3.92 mm, a density of 0.10 g/cm³ and a crystallinity of 16%. The ratio V₂/V₁ was 2.60. EXAMPLE 19 The procedure of Example 17 was repeated except that post expansion was carried out by bringing the sheet into contact with steam at 100°C for 5 minutes. There was obtained a post-expanded foam sheet which had a thickness of 5.63 mm, a density of 0.065 g/cm³ and a crystallinity of 26%. The ratio v₂/v₁ was 3.77. EXAMPLE 20 The procedure of Example 17 was repeated except that post expansion was carried out by bringing the sheet into contact with steam at 100°C for 7 minutes. There was obtained a post-expanded foam sheet which had a thickness of 5.96 mm and a density of 0.065 g/cm³. The ratio V₂/V₁ was 4.00. EXAMPLE 21 Post expansion was carried out in the same manner as in Example 20 by bringing the sheet into contact with steam at 100°C for 7 minutes except that post expansion was carried out by placing a pre-expanded foam of 200 mm x 280 mm in an aluminum mold of 210 mm x 290 mm x 5 mm. There was obtained a post-expanded foam sheet which had a thickness of 5.00 mm and a density of 0.078 g/cm³. The ratio v₂/v₁ was 3.33. The sheet obtained was a flat foam sheet. EXAMPLE 22 The same pre-expanded foam sheet as that obtained in Example 13 was used and post expansion was carried out by blowing pressurized steam as shown in Fig. 3. Namely, post expansion was carried by bringing the sheet into contact with steam at 110°C for 3 minutes. There was obtained a post-expanded foam sheet which had a thickness of 3.41 mm and a density of 0.11 g/cm³. The ratio v₂/v₁ was 2.36. EXAMPLE 23 The procedure of Example 22 was repeated except that post expansion was carried out by bringing the sheet into contact with steam at 120°C for 0.5 minute. There was obtained a post-expanded foam sheet which had a thickness of 3.00 mm and a density of 0.13 g/cm³. The ratio V₂/V₁ was 2.00. EXAMPLE 24 Pre-expansion was carried out in the same manner as in Example 13 except that carbon dioxide was used as the blowing agent in place of butane and the amount of carbon dioxide was 1.1 parts. The resulting pre-expanded foam sheet was 643 mm in width and had a density of 0.26 g/cm³, a thickness of 1.5 mm and a crystallinity of 9%. Post expansion was carried out in the same manner as in Example 19 to obtain a post-expanded foam sheet which had a thickness of 3.00 mm and a density of 0.13 g/cm³. The ratio V₂/V₁ was 2.00. EXAMPLE 25 The procedure of Example 13 was repeated except that post expansion was carried out by using hot air at 80°C in place of warm water at 63°C and bringing the sheet into contact with hot air for 5 minutes to obtain a post-expanded foam sheet. The post-expanded foam sheet was 2.1 mm in thickness and had a density of 0.19 g/cm³ and a crystallinity of 10%. The ratio V₂/V₁ was 1.37. EXAMPLE 26 The procedure of Example 25 was repeated except that the temperature of hot air in the post expansion was 100°C to obtain a post-expanded foam sheet. The post-expanded foam sheet had a thickness of 2.6 mm, a density of 0.15 g/cm³ and a crystallinity of 10%. The ratio V₂/V₁ was 1.73. EXAMPLE 27 The procedure of Example 25 was repeated except that the temperature of hot air in the post expansion was 110°C to obtain a post-expanded foam sheet. The post-expanded foam sheet had a thickness of 2.8 mm, a density of 0.14 g/cm³ and a crystallinity of 12%. The ratio V₂/V₁ was 1.86. EXAMPLE 28 The procedure of Example 25 was repeated except that the temperature of hot air in the post expansion was 140°C to obtain a post-expanded foam sheet. The post-expanded foam sheet had a thickness of 3.01 mm, a density of 0.13 g/cm³ and a crystallinity of 25%. The ratio V₂/V₁ was 2.00. EXAMPLE 29 The procedure of Example 25 was repeated except that the temperature of hot air in the post expansion was 230°C to obtain a post-expanded foam sheet. The post-expanded foam sheet had a thickness of 4.04 mm, a density of 0.097 g/cm³ and a crystallinity of 26%. The ratio V₂/V₁ was 2.68. EXAMPLE 30 Pre-expansion: Pre-expansion was carried out in the same manner as in Example 13 except that the die provided at the extruder head was changed from the circular die to a flat die and a flat plate was used in place of the mandrel. The flat die had a straight line-form extrusion opening having a width of 150 mm and a gap of 0.7 mm. The flat plate was an aluminum plate of 500x500 mm which was cooled with water at 30°C. The foam sheet was extruded between the aluminum plates, whereby the extruded foam sheet was quenched. In this way, a pre-expanded foam sheet was obtained. The foam sheet had a width of 200 mm, a thickness of 5 mm, a density of 0.52 g/cm³ and a crystallinity of 12%. Post expansion: Post expansion was carried out in the same manner as in Example 20 by bringing the above pre-expanded foam sheet into contact with steam at 100°C for 7 minutes. There was obtained a post-expanded foam sheet which had a thickness of 12.5 mm and a density of 0.204 g/cm³. The ratio V₂/V₁ was 2.55. EXAMPLE 31 Pre-expansion was carried out in the same manner as in Example 30 except that the temperature of the aluminum plates was slightly elevated and the cooling rate of the foam sheet was slightly smaller than that in Example 30 to obtain a pre-expanded foam sheet. The width, thickness and density of the foam sheet were the same as those of the sheet of Example 30. However, the crystallinity was 25%. Post expansion: Post expansion was carried out in the same manner as in Example 30 to obtain a post-expanded foam sheet which had a thickness of 11.0 mm and a density of 0.232 g/cm³. The ratio V₂/V₁ was 2.24. COMPARATIVE EXAMPLE 8 The procedure of Example 13 was repeated except that post expansion was carried out by using hot air at 60°C in place of warm water at 63°C and bringing the sheet into contact with hot air for 5 minutes to obtain a post-expanded foam sheet. The post-expanded foam sheet had a thickness of 1.5 mm and a density of 0.26 g/cm³. The ratio V₂/V₁ was 1.00. Accordingly, the post expansion did substantially not take place. COMPARATIVE EXAMPLE 9 The procedure of Example 13 was repeated except that the temperature of water in the post expansion was lowered to 53°C to obtain a post-expanded foam sheet. The post-expanded foam sheet had a thickness of 1.5 mm and a density of 0.26 g/cm³. The ratio V₂/V₁ was 1.00 as in Comparative Example 8. Accordingly, the post expansion did substantially not take place. COMPARATIVE EXAMPLE 10 The procedure of Example 13 was repeated except that the temperature of steam in the post expansion was lowered to 58°C to obtain a post-expanded foam sheet. The post-expanded foam sheet had a thickness of 1.5 mm and a density of 0.26 g/cm³. The ratio V₂/V₁ was 1.00. Accordingly, the post expansion did substantially take place. COMPARATIVE EXAMPLE 11 Pre-expansion was carried out in the same manner as in Example 31 except that the temperature of the aluminum plates was raised to a temperature higher than that of Example 31 and the cooling rate of the extruded sheet was smaller than that of Example 40 to obtain a pre-expanded foam sheet. The width, thickness and density of the foam sheet were the same as those of the foam sheet of Example 31, but the crystallinity thereof was 32%. Post expansion was carried out by bringing the pre-expanded foam sheet into contact with steam at 100°C for 7 minutes to obtain a post-expanded foam sheet which had a thickness of 5 mm and a density of 0.52 g/cm³. The ratio V₂/V₁ was 1.00. Accordingly, the post expansion did substantially not take place. EXAMPLE 32 In this Example, the pre-expansion and the post expansion were continuously carried out as shown in Fig. 4. In Fig. 4, an extruder 5 worked in the same manner as in the pre-expansion of Example 13 and continuously delivered a pre-expanded foam sheet. Without winding up the pre-expanded foam sheet, the sheet was successively introduced into a steam tank 6. The temperature of the surface of the sheet was lowered to 30°C before it was introduced into the steam tank 6. In Fig. 4, a numeral 10 means a winding machine. The pre-expanded foam sheet was brought into contact with steam at 100°C in the steam tank 6 for 5 minutes to carry out post expansion. The sheet was then cooled. The resulting post-expanded foam sheet had a width of 645 mm, a density of 0.07 g/cm³ and a thickness of 5.5 mm and was a low-density fine sheet which was expanded at a high expansion ratio and had fine, uniform cells. EXAMPLE 33 100 parts of polyethylene terephthalate pellets (trade name: TR8580, manufactured by Teijin Limited) was dried with hot air having a dew point of -20°C at 160°C for 5 hours. 0.3 part of pyromellitic dianhydride, 0.1 part of sodium carbonate and 0.6 part of talc as an expansion nucleating agent were uniformly mixed with the pellets in a tumbling mixer. The mixture was fed to the hopper of an extruder (screw diameter: 65 mm, L/D: 35). Cylinder temperature was 265 to 290°C, the temperature of the extruder head was 265°C, die temperature was 265°C and number of revolutions of screw was 25 rpm. 2.4% by weight of butane as a blowing agent was introduced into the mixture under pressure on the way of the cylinder. Die used was a circular die having a diameter of 60 mm and a circular die gap of 0.45 mm. A tube was extruded through the port of the die into air and molded into a cylindrical form by means of a cylindrical mandrel while expanding the molten resin and taking off the tube. Part of the resulting cylindrical foam was cut open and the resulting sheet was wound up. In this case, surface temperature was kept at 20°C while circulating cooling water in the cylindrical mandrel. The resulting foam sheet had a density (D1) of 0.225 g/cm³, a width of 640 mm and a thickness of 1.6 mm. The sheet had a crystallinity of 9.7% and a glass transition temperature of 75°C. A re-heating treatment was carried out by contacting the sheet with heating for 30 seconds by using a hot plate having a surface temperature of 160°C. There was obtained a post-expanded foam sheet having a density (D2) of 0.133 g/cm³ and a thickness of 2.7 mm. The ratio of D1/D2 by the heat treatment was 1.69. The crystallinity of the sheet was 24.3%. A sample of 100x100 mm was cut off from the sheet and heated at 200°C in a constant temperature bath for 30 minutes. The ratio V a /V b of the volume (V a ) after heating to the volume (V b ) before heating was 1.02. It was found that the sheet was excellent in heat resistance. EXAMPLE 34 100 parts of polyethylene terephthalate pellets (trade name: PET10368, manufactured by Eastman Kodak Company) was dried with hot air having a dew point of - 20°C at 160°C. The dried pellets, 0.25 part of diglycidyl terephthalate (Blemmer ® MT manufactured by Nippon Oil & Fats Co., Ltd.), 0.1 part of sodium montanate and 0.6 part of talc as an expansion nucleating agent were uniformly mixed in a tumbling mixer. The mixture was fed to the hopper of the same extruder as that used in Example 33. The cylinder temperature was 280 to 290°C, the temperature of the extruder head was 290°C, the die temperature was 290°C and the number of revolutions of screw was 25 rpm. 2.2% by weight of pentane as a blowing agent was pressure-fed to the mixture on the way of the cylinder. The resulting foam sheet had a density (D1) of 0.242 g/cm³, a width of 640 mm and a thickness of 17 mm. The crystallinity of the sheet was 10.6% and the glass transition temperature thereof was 76°C. A re-heating treatment was carried out by contacting the sheet with heating for 30 seconds by using a hot plate having a surface temperature of 160°C. The resulting sheet had a density (D2) of 0.147 g/cm³ and a thickness of 2.8 mm. The ratio D1/D2 by the heat treatment was 1.65. The crystallinity of the post-expanded foam sheet was 24.4%. A sample of 100x100 mm was cut off from the sheet and heated at 200°C in a constant temperature bath for 30 minutes. The ratio V₂/V₁ was 1.02. EXAMPLE 35 The extruded foam sheet obtained in Example 33 was contacted with heating for 6 seconds by using a hot plate having a surface temperature of 170°C to obtain a foam sheet having a density (D2) of 0.106 g/cm³ and a thickness of 3.4 mm. The ratio of D1/D2 by the heat treatment was 2.12. The crystallinity of the post-expanded foam sheet was 16.7%. A sample of 100x100 mm was cut off from the sheet and heated at 200°C in a constant temperature bath for 30 minutes. The ratio V a /V b was 1.06. EXAMPLE 36 The extruded foam sheet obtained in Example 33 was heated with steam having a vapor pressure of 4 atm for 30 seconds to obtain a foam sheet having a density (D2) of 0.157 g/cm³ and a thickness of 2.3 mm. The ratio D1/D2 by this heating was 1.43. The crystallinity of the post-expanded foam sheet was 24.3%. A sample of 100x100 mm was cut off from the sheet and heated at 200°C in a constant temperature bath for 30 minutes. The ratio V₂/V₁ was 0.99. COMPARATIVE EXAMPLE 12 A sample of 100x100 mm was cut off from the foam sheet which was extruded in Example 33. The crystallinity was 9.7%. The sample was heated at 200°C in a constant temperature bath for 30 minutes. The ratio V a /V b was 1.74. Heat treatment after extrusion was not made. Hence, the sheet was poor in heat resistance and greatly deformed. COMPARATIVE EXAMPLE 13 The extruded foam sheet obtained in Example 33 was heated with a hot plate having a surface temperature of 140°C for 10 seconds to obtain a post-expanded foam sheet. The crystallinity was 13.1%. The sheet had a density of 0,114 g/mol and a thickness of 3.15 mm. The ratio D1/D2 by this heating was 1.97. A sample of 100x100 mm was cut off from the sheet and heated at 200°C in a constant temperature bath for 30 minutes. The ratio Va/Vb was 1.11. The results of Examples 33 to 36 and Comparative Examples 12 and 13 are shown in Tables 6 and 7. It is clear from Tables 6 and 7 that the volumes of the sheets which were not heat-treated are greatly changed and the sheets are poor in heat resistance. Further, even when the sheets were heat-treated, the sheet having a crystallinity of less than 15% are inferior in heat resistance. The following Examples 37 to 39 and Comparative Examples 14 to 16 illustrates the extrusion foam sheets of thermoplastic polyester resins and the thermoformability thereof. EXAMPLES 37 TO 39 AND COMPARATIVE EXAMPLES 14 TO 16 The production unit of extrusion foam sheets, which was used in these examples and comparative examples was a single screw extruder (screw diameter: 65 mm, L/D: 35). The extrusion die was a circular die (bore: 60 mm), and the circular die gap of the extrusion die was changed as shown in Table 9. The cylindrical mandrel was a water-cooled mandrel (outer diameter: 205 mm, L/D: 1.5). In the compositions of extrusion foam sheets used in these examples and comparative examples, 100 parts by weight of polyethylene terephthalate (PET) was used as the thermoplastic polyester resin. The resin grade was changed as shown in Table 8. 0.6 part by weight of talc was used as the expansion nucleating agent per 100 parts by weight of PET. Melt property modifiers and metallic compounds used together with the modifiers were changed as shown in Table 8. Liquefied butane was used as the blowing agent in an amount given in Table 8. Extrusion foam sheets used in Examples 37 to 39 and Comparative Examples 14 to 16 were produced in the following manner. Polyethylene terephthalate was dried in a dehumidifying drier (160°C, dew point of -30°C) for 4 hours. Predetermined amounts of polyethylene terephthalate, modifier, metallic compound and talc were mixed in a tumbling mixer. The mixture was fed to the extruder hopper and melt-mixed. Liquefied butane as the blowing agent was injected into the mixture on the way of the extruder. The mixture was extruded through the circular bore of the circular die into air in the form of a tube. The extrudate was taken while expanding the molten resin, and the foam was cooled by bringing it into contact with the outer surface of the cylindrical mandrel to shape it into a cylinder. Part of the cylindrical foam was cut open and wound up as the foam sheet. The manufacturing conditions of the extrusion foam sheets used in these examples and comparative examples were as follows. Temperature of feed zone of extruder: 275 to 285°C, temperature of compression zone of extruder: 285 to 295°C, temperature of melting zone of extruder: 265 to 285°C, temperature of extruder head: 265 to 285°C, temperature of circular die: 260 to 285°C, injection pressure of blowing agent: 40 to 140 kg/cm³, and extrusion pressure (head pressure): 50 to 120 kg/cm³. The number of revolutions of screw and take-off speed are shown in Table 9. The resulting foam sheets were 640 to 643 mm in width. The apparent density, thickness, crystallinity and molecular orientation ratio thereof are shown in Table 10. The post thermoformer and thermoforming conditions of the extrusion foam sheets which were used for evaluation in these examples and comparative examples, were as follows. The post thermoformer was a one-shot molding machine for expanded polystyrene, which had a heating zone with infrared radiation and a press part with air cylinder. The molding tool was a plag-assist press tool (bore: 180 mm x 155 mm, depth: 95 mm) for container. Molding conditions were such that 360x360 mm foam sheets were heated at 175°C in the heating zone for 15 seconds and immediately thereafter, contacted with the thermoformer for 25 seconds to effect the molding. The resulting molded articles were evaluated by the following criteria. Appearance O: The whole of sheet was uniformly extended, could be molded into the same shape as that of press tool and not broken. Δ: Sheet which could be molded into the same shape as that of press tool, but part of surface was broken and cracks were formed. X: Sheet which was greatly broken and could not be molded. Thickness Ratio The ratio of the thickness of the bottom of the molded article to that of sidewall thereof. The mark "-" shows that the article is greatly broken and measurement cannot be made. Surface Profile O : The surface of the molded article was smooth. Δ: The surface was partly uneven. X: The surface was considerably uneven. Overall Evaluation Evaluation was made as a whole by taking all of the surface profile of the extrusion foam sheet and the appearance and thickness of the post thermoformed article into consideration. o ○ Particularly superior O: Superior X: Bad The results are shown in Table 11. Apparent Density Thickness Crystallinity Molecular Orientation Ratio (g/cm³) (mm) (%) Example 37 0.19 1.5 10 1.82 Example 38 0.18 1.7 11 1.73 Example 39 0.16 2.6 18 1.37 Comp. Ex. 14 0.19 2.0 11 4.88 Comp. Ex. 15 0.23 2.0 10 5.36 Comp. Ex. 16 0.18 1.7 22 1.70 Apparence of Molded Article Thickness Ratio of Molded Article Surface Profile of Molded Article Overall Evaluation Example 37 ○ 1.2 ○ o ○ Example 38 ○ 1.3 ○ ○ Example 39 ○ 1.5 Δ ○ Comp. Ex. 14 × - ○ × Comp. Ex. 15 × - Δ × Comp. Ex. 16 Δ 1.5 × × Molecular orientation ratio can be adjusted by take-off speed and blow-up ratio, since when the take-off speed of the sheet is increased, the sheet is orientated in the MD direction, while when the blow-up ratio is increased (cooling mandrel diameter is increased), the sheet is orientated in the TD direction. However, when the width and thickness of the sheet and expansion ratio are fixed, molecular orientation ratio cannot be properly adjusted only by the controlling of take-off speed and blow-up ratio. In this case, die temperature is elevated (Example 38, Comparative Example 15). Molecular orientation ratio can be lowered by reducing the amount of the melt property modifier (Example 38, Comparative Example 14). Crystallinity can be lowered by lowering the temperature of cooling water for cylindrical mandrel. A food container produced from an extruded foam sheet prepared by the process of the present invention is illustrated by means of the following examples and comparative examples. Parts given below are by weight unless otherwise stated. EXAMPLE 40 In this Example, the same thermoplastic polyester resin was used for both foam sheet and non-foam sheet. 100 parts of polyethylene terephthalate pellets (trade name: TR8580 manufactured by Teijin Limited) was used as the polyester resin and dried with hot air having a dew point of -20°C at 160°C for 5 hours. 0.3 part of pyromellitic dianhydride, 0.1 part of sodium carbonate and 0.6 part of talc were added thereto. The mixture was uniformly mixed in a tumbling mixer and then fed to an extruder. The cylinder temperature of the extruder was set to 274 to 287°C and the die temperature thereof was 277°C. About 1.0% by weight of butane as a blowing agent was pressure-fed to the mixture on the way of the cylinder. A die having a circular gap was provided at the head of the extruder. The polyester resin containing butane was extruded through the circular gap into a cylindrical shape. The extrudate was allowed to proceed on a cylindrical mandrel, while expanding the resin, to obtain a foam sheet. The cylindrical foam sheet was cut open and the resulting flat sheet was wound up into a roll. The resulting foam sheet had a density of 0.262 g/cm³, a thickness of 1.45 mm and a width of 640 mm. A polyethylene terephthalate resin film (FFL manufactured by Teijin Limited) of 50 µm in thickness was used as the non-foam thermoplastic resin film. This film and the above-described sheet were placed upon each other in layers and put between a pair of rollers to laminate them onto each other. The temperature of the roller on the side of the foam sheet was room temperature, while the temperature of the roller on the non-foam sheet was set to 135°C. In this way, there was obtained a laminated sheet in which the non-foam sheet was bonded to only one side of the foam sheet. A sample having a size of 250×250 mm was cut off from the laminated sheet and preheated by bringing the sample into contact with a hot plate having a surface temperature of 140°C for 6 seconds. Successively, the preheated sample was put between a male die heated to 180°C and a female die heated to 180°C for 6 seconds to thereby mold it into a container and at the same time to accelerate crystallization. Immediately thereafter, the molded article was put between a male die and a female die at room temperature for 6 seconds to cool it, said dies having the same shapes as those of the dies heated to 180°C. The molding was conducted in such a manner that the non-foam film layer was positioned inside the container. The resulting container had a shape as shown in Fig. 6 and 7. Fig. 6 shows a side view of the resulting container 7. Fig. 7 shows a plane view of the container 7. In this molding, the composite sheet increased in expansion ratio and the thickness of the bottom of the container 7 became 3.80 mm. 150 cc of water was put into the container 7 and a lid was put thereon. The container was sealed so that water did not leak therefrom. The container was placed in a 500 W microwave oven and heated for 3 minutes to cause water to boil. Immediately after heating, the container could be taken out from the microwave oven with bare hands. The container did not cause any change by the heating. To make sure, the thickness of the bottom of the container was measured. It was found that the thickness of the bottom was 3.95 mm. The container in the heating within the microwave oven increased in thickness only by 4%. Accordingly, it could be considered that no deformation was substantially caused. EXAMPLE 41 The foam sheet obtained in Example 40 was used, a polypropylene resin film of 100 µm in thickness was used as the non-foam film and an ethylene-vinyl acetate copolymer resin was used as the adhesive to laminate the films. One side of the polypropylene resin film was coated with the ethylene-vinyl acetate copolymer resin. The coated side of the film was placed on the polyester resin foam sheet obtained in Example 40. They were passed through a pair of rollers to laminate them. The temperature of the roller on the side of the foam sheet was room temperature, while the temperature of the roller on the side of the non-foam film was 125°C. A sample of 250×250 mm was cut off from the laminated sheet obtained above and preheated by bringing the foam sheet side into contact with a hot plate having a surface temperature of 140°C and simultaneously bringing the non-foam film side into contact with a hot plate having a surface temperature of 100°C for 6 seconds. Successively, the preheated sheet was put between a male die and a female die for 8 seconds in such a manner that the foam sheet side was contacted with the female die heated to 140°C and the non-foam film side was contacted with the male die heated to 100°C, whereby the composite sheet could be molded into a container which was the same as that of Example 40 and at the same time, crystallization was accelerated. Immediately thereafter, the container was put between two dies at room temperature for 6 seconds to cool it, said two dies having the same shapes as those of the dies heated. The molding was conducted so that the non-foam film was positioned inside the container. The foam sheet was swollen by this molding and the thickness of the bottom of the container became 2.94 mm. In the same way as in Example 40, water was put into the resulting container. The container was heated in the microwave oven. Immediately after heating, the container could be taken out with bare hands. It was found that no deformation was caused after heating. To make sure, the thickness of the bottom of the container was measured. The bottom was 3.05 mm in thickness. An increase in thickness was only 4%. Accordingly, it could be considered that the container could withstand heating by microwave oven. EXAMPLE 42 The foam sheet obtained in Example 40 was used and a polyethylene terephthalate resin film of 150 µm in thickness was used as the non-foam film. They were laminated onto each other without using any adhesive to obtain a composite sheet. The laminating of the film was carried out by using a pair of rollers in which the temperature of the roller on the side of the foam sheet was room temperature and the temperature of the roller on the side of the non-foam film was 155°C. A sample of 250×250 mm was cut off from the laminated sheet. In the same way as in Example 40, a container was prepared from the sample. The thickness of the bottom of the container was swollen to 4.39 mm. Water was placed in the container. The container was heated in the microwave oven in the same way as in Example 40. Immediately after heating, the container could be taken out from the oven with bare hands. It was found that the container taken out did not cause any deformation. To make sure, the thickness of the bottom of the container was measured and the thickness was 4.44 mm. Deformation ratio was as small as only 1%. EXAMPLE 43 The procedure of Example 42 was repeated except that a polybutylene terephthalate resin film of 30 µm in thickness was used in place of the polyethylene terephthalate resin film. The thickness of the bottom of the molded container was 4.20 mm. After heating, the container could be taken out from the microwave oven with bare hands. It was found that the container taken out was not deformed as compared with that before heating. The thickness of the bottom of the container after heating was measured. The thickness of the bottom was 4.34 mm. The deformation ratio of the thickness was only 3%. COMPARATIVE EXAMPLE 17 A container was prepared only from the polyester resin foam sheet obtained in Example 40 without laminating the non-foam film onto the foam sheet. Namely, the polyester resin foam sheet obtained in Example 40 was preheated by bringing it into contact with a hot plate heated to 140°C for 6 seconds. Successively, the preheated sheet was put between a male die heated to 180°C and a female die heated to 180°C for 8 seconds, whereby the molding was carried out and at the same time, crystallization was accelerated. Immediately thereafter, the molded article was put between a male die and a female die at room temperature for 6 seconds to cool it, said dies having the same shapes as those of the dies heated to 180°C. In this way, the same container as that of Example 40 was obtained. The thickness of the bottom thereof was 3.39 mm. In the same way as in Example 40, water was placed in the container and the container was heated in the microwave oven. Immediately after heating, the container could be taken out from the microwave oven with bare hands. The container taken out was highly expanded and recessed parts and protruded parts were formed on the inner surface thereof. The thickness of the bottom of the container after heating was measured. The thickness thereof was 4.04 mm. Namely, this showed an increase of as large as 19% in thickness. Accordingly, it was considered that deformation was large and the container could not withstand heating in the microwave oven. COMPARATIVE EXAMPLE 18 In this Comparative Example, the foam sheet of polystyrene was used and the non-foam film of the thermoplastic polyester resin was used. They were laminated onto each other by using an ethylene-vinyl acetate copolymer resin to obtain a laminated sheet corresponding to one described in JP-A-62-70037. Namely, one side of the non-foam polyethylene terephthalate film having a thickness of 50 µm was coated with the copolymer resin. The coated side of the film was placed on the expanded polyethylene sheet having a thickness of 2.4 mm and a base weight of 200 g/m² to laminate them. The laminating of them was carried out by using a pair of rollers heated to 150°C. A sample of 250×250 mm was cut off from the laminated sheet and preheated by bringing it into contact with a hot plate heated to 140°C for 8 seconds. Immediately thereafter, the sheet was put between a male die and a female die at room temperature for 6 seconds to cool it. There was obtained a container having the same shape as that of the container of Example 40. The container was prepared in such a manner that the non-foam film was positioned inside the container. The thickness of the bottom of the container was increased to 4.2 mm. In the same way as in Example 40, water was placed in the container, and the container was heated in the microwave oven. Though the container could be taken out from the microwave oven with bare hands immediately after heating, the container was considerably deformed by heating. Particularly, the expanded polystyrene layer on the outer side of the container was deformed by swelling. As a result, protrusions and recesses were formed on the non-foam film on the inner side of the container. The thickness of the bottom of the container after heating was measured. The thickness of the bottom was 4.88 mm. This showed an increase of 16% in thickness. Hence, it was considered that the container was not suited for use in the microwave oven. As stated above, the present invention has the following effects. The foams of the thermoplastic polyester resins are those having fine cells uniformly dispersed therein. The foams of the present invention are excellent in rigidity and heat resistance, though they are lightweight. Further, the foams have high tensile strength and high tensile elongation at break. The foams give molded foams having a higher expansion ratio and much fine cells uniformly dispersed therein when re-heated. The extrusion foam sheets of the present invention are excellent in thermoformability. Food containers thermoformed can be used in dual ovenable applications.

Claims: A process for producing a thermoplastic polyester resin foam comprising the steps of melting a thermoplastic polyester resin, mixing the molten resin with a blowing agent, extruding the mixture into a low-pressure zone to carry out foaming, and subsequently cooling, characterized in that both an expansion nucleating agent and a viscoelasticity improver are added to the thermoplastic polyester resin, in that the viscoelasticity improver is (a) a compound having two or more acid anhydride groups per molecule is used an amount of 0.05 to 5 parts by weight per 100 parts by weight of the thermoplastic polyester resin and (b) a compound, not being talc, of a metal of Group I, II or III elements of the Periodic Table, in that the blowing agent is present in an amount of 0.05 to 50 % by weight of the molten blend, and in that the subsequent cooling step is effected to produce a thermoplastic polyester resin foam having a crystallinity of not less than 9% as measured using differential scanning calorimetry and a density of not higher than 0.7 g/cm³. A process as claimed in claim 1, wherein said compound having two or more acid anhydride groups per molecule is pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, diphenyl sulfone tetracarboxylic dianhydride or 5-(2,5-dioxotetrahydro-3-furanyl)-3-methyl-3-cyclohexen-1,2-dicarboxylic dianhydride. A process as claimed in any preceding claim, wherein said foam is an extruded foam sheet having a density of not higher than 0.5 g/cm³. A process as claimed in any preceding claim, wherein said thermoplastic polyester resin is polyethylene terephthalate, polybutylene terephthalate, polybutylene terephthalate elastomer, polycyclohexane terephthalate, polyethylene naphthalate or a mixture thereof. A process as claimed in claim 1, wherein said compound of a metal of Group I, II or III elements of the Periodic Table is used in an amount of 0.05 to 5 parts by weight per 100 parts by weight of the thermoplastic polyester resin. A process as claimed in any preceding claim, wherein said metallic compound is a compound of a metal of Group I elements of the Periodic Table. A process as claimed in claim 1, wherein said foam has a density of not higher than 0.5 g/cm³. A process as claimed in any preceding claim, further including the steps of cooling a high-temperature thermoplastic polyester resin foam immediately after expansion to a temperature of not higher than the glass transition point of the resin to bring the crystallinity to 9 to 30% as measured using differential scanning calorimetry, and then heating the foam to 62°C or higher so as to effect further expansion of the foam. A process as claimed in claim 8, wherein said heating step is carried out by bringing the polyester resin foam into contact with steam or hot water. A process as claimed in claim 8, wherein said heating step is carried out by bringing the polyester resin foam into contact with a hot plate. A process as claimed in claim 8 wherein said foam is re-expanded 1.3 times or more by heating. A process as claimed in claim 1, wherein the extruding step is effected to produce an extruded foam sheet having a molecular orientation ratio of 4.5 or lower looking in a direction from the surface of the foam sheet, and then the extruded foam sheet is cooled so that is has a crystallinity of 9 to 20% as measured using differential scanning calorimetry. A process as claimed in claim 12, wherein the extruding step is effected using a circular die and a cylindrical mandrel. A process as claimed in claim 13, wherein the sheet has a thickness of not more than 5 mm. A process as claimed in any preceding claim, further comprising the steps of laminating a non-foam film of a thermoplastic resin on at least one side of a sheet of said thermoplastic polyester resin foam having a crystallinity of not less than 15% as measured using differential scanning calorimetry to obtain a laminated sheet, and molding the laminated sheet into the shape of a food container in such a manner that the non-foam film is positioned inside the container. A process as claimed in claim 15, wherein said non-foam film is prepared from thermoplastic polyester resins, liquid crystal polyester resins, polyolefin resins, polyamide resins, polyvinyl chloride resins, polyacrylonitrile resins, polyvinylidene chloride resins or ethylene-vinyl alcohol copolymers. A process as claimed in claim 15 or 16 wherein said molding step is effected by thermoforming a laminated sheet having a thickness of 0.5 to 5 mm. A process as claimed in claim 15, 16 or 17, wherein the density of the foam is brought to not higher than 0.5 g/cm³ by thermoforming the laminated sheet.

Inventors: AMANO NORIO, HAYASHI MOTOSHIGE, HIRAI TAKAAKI, TAKI TAKESHI, AMANO, NORIO, HAYASHI, MOTOSHIGE, HIRAI, TAKAAKI, TAKI, TAKESHI
Applicants: SEKISUI PLASTICS, SEKISUI KASEIHIN KOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: C08G63
IPC codes: C08J9, B65D1, C08K5