Thermoplastic polyester series resin foamed material and production process thereof

Abstract: A thermoplastic polyester series foamed material having a thickness of at least 3 mm and an apparent density of from 0.05 to 0.7 g/cm³ is diclosed, wherein the crystallinity of the resin in the surface portion within 0.5 mm from the surface of the foamed material is not more than 30% and at least 1% lower than the crystallinity of the resin in the central portion of the foamed material.

Description: FIELD OF THE INVENTION This invention relates to a structural material composed of a foamed material of a thermoplastic polyester resin obtained by reacting an aromatic dicarboxylic acid with a dihydric alcohol (hereinafter, such a resin will be referred to variously as a "thermoplastic polyester series resin" or as a "PAT") and to a production process thereof. In particular, the invention relates to PAT foamed material capable of being used as building materials such as floor materials, wainscots, etc., furniture such as shelves, desks, tables, etc., and electric housing materials, and to a production process thereof. BACKGROUND OF THE INVENTION As structural materials having a high mechanical strength, iron or timber has been used for a long time. However, iron has a disadvantage of becoming rusty and timber has a disadvantage of being liable to rot. Thus, synthesis resins have been used in place of the aforesaid materials. In the case of making a light weight structural material using a synthetic resin, it has been proposed to form a foamed material by foaming a synthetic resin. As a foamed material for structural materials, a polystyrene resin, a polyethylene resin, or a vinyl chloride series resin has hitherto been used. However, these materials are poor in heat resistance and also the strength thereof is relatively weak. For solving the aforesaid problems, it has been proposed to laminate a layer of a thermosetting resin layer such as a resorcinol series resin, etc., reinforced with fibers on one or both surfaces of a foamed material of a synthetic resin as a base material as described in JP-A-1156051 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Also, in the proposed technique, a foamed material of polystyrene or a soft polyurethane is used as the foamed material of a synthetic resin. However, the foamed material is either soft at room temperature or softens at about 100°C, and also in the case of using a thermosetting resorcinol series resin, it is usually required to heat it to a temperature of at least 100°C while pressing for forming the layer of the resin. As a result, it is impossible to adhere the fiber-reinforced resin layer to the surface(s) of the foamed material while keeping the original form of the foamed material. Accordingly, a tough and lightweight synthetic resin structural material cannot be obtained by the aforesaid proposal. Also, a foamed material of PAT (thermoplastic polyester resin) is known. PAT is known as engineering plastics and has a high toughness, a good shape stability, and an excellent heat resistance capable of enduring 200°C. Furthermore, since PAT is a thermoplastic resin, it may be considered that a foamed material of PAT can be easily obtained only by mixing a foaming agent with PAT and extruding the mixture through an extruding machine. However, it is not easy to foam PAT (in contrast to polystyrene and polyethylene) since PAT does not exhibit a suitable viscosity for foaming at melting. The reason is that, since PAT is a crystalline resin, when it is heated, PAT is suddenly softened to become a liquid having a low viscosity. Thus, PAT has a narrow temperature range suitable for foaming, whereby it is difficult to keep it at a temperature suitable for foaming, and since the viscosity of PAT is low, gases acting as foaming agent are immediately scattered and lost. Thus, various attempts of easily producing foamed materials of PAT have been made. For example, JP-B-56-8858 (the term "JP-B" as used herein means an "examined published Japanese Patent application") or U.S. Patent 4,462,947 proposes that PAT is mixed with a polycarbonate resin and the mixture is heated to a temperature of from 150°C to 250°C to generate carbon dioxide, whereby a foamed material of PAT is formed. However, since the foamed material thus obtained contains a polycarbonate resin, it is soft and pliable, and hence is not suitable as a structural material. Also, JP-B-61-48409 proposes that the PAT is mixed with a diglycidyl ester and the mixture is foamed by extruding. However, the aforesaid patent publication shows only that a foamed material is obtained at a high expansion ratio of 15 times, but the properties of the foamed material obtained thereby are not disclosed. JP-B-61-484410 teaches the foaming of a PAT composition by extruding having regard to the crystallizing speed of PAT. However, the method is limited to the production of strings having a sectional area of from 1 mm² to 200 mm² and also since such patent describes that the strings obtained can be easily drawn and heatworked, it is clear that the invention of the aforesaid patent does not aim at a structural material. Also, JP-B-61-484411 teaches the foaming of a PAT composition by extruding under conditions such that the crystallinity of PAT becomes at least 30%. However, the method is also limited to the production of strings having sectional area of 1 mm² to 200 mm² as in last-mentioned patent, and it is also disclosed that it is required to increase the crystallinity, that the strings obtained can be easily bent and that they can be easily knitted and woven. Thus, the teachings of this patent are not useful for the production of structural materials. As will be understood from the above, it is known that PAT is a crystalline resin and that PAT having different crystallinities can be obtained by appropriate choice of the manner of producing the resin. Also, it is known that, when PAT having a larger crystallinity is used, a foamed material having larger roughness and heat resistance is obtained. Accordingly, it has only been attempted to obtain foamed materials having a large crystallinity. Also, it is known that the crystallinity of PAT can be generally measured by the density, X-ray diffraction patterns, nuclear magnetic resonance spectra, etc., of the PAT resin. However, since a foamed material of PAT contains a large number of small bubbles therein, the crystallinity thereof can not be measured by these methods. Accordingly, although the crystallinity of a foamed material of PAT has been discussed, the method by which the crystallinity of such material could practically be measured was unclear. SUMMARY OF THE INVENTION The inventors have paid special attention to enhancing the rigidity, toughness, mechanical strength, dimensional stability and heat resistance properties of PAT as well as its adhesive property to a thermosetting resin. The inventors have investigated the production of a light-weight structural material which is of enhanced heat resistance and toughness, and is resistant to corrosion, by foaming PAT to form a light-weight foamed material. The present invention has been made as the result of these investigations. That is, an object of this invention is to provide a foamed material of PAT capable of being used as a structural material having excellent mechanical strength such as bending strength, etc., by controlling the crystallinities in the surface portion and the central portion of the foamed material of PAT. Also, other object of this invention is to provide a PAT foamed material which is used as a structural material capable of being nailed by lowering the crystallinity of the surface portion of the PAT foamed material as compared to that of the central portion thereof. A further object of this invention is to provide a process of producing a PAT foamed material which can be used as a structural material having an excellent mechanical strength or a structural material capable of being nailed. Other object of this invention is to provide a PAT foamed material capable of being used as a structural material having an excellent mechanical strength by laminating a thermosetting resin reinforced with fibers on the surface of the PAT foamed material. A still futher object of this invention is to provide a PAT foamed material capable of being used as a lightweight structural material having an excellent mechanical strength by laminating a foamed thermosetting resin reinforced with fibers on the surface of the PAT foamed material. The inventors have confirmed that by a method of utilizing the thermal characteristics of PAT, the crystallinity of PAT can be accurately measured to an order of less than 1%. In this invention, the method of utilizing the thermal characteristics of the PAT foamed material is a method of measuring the heat change upon melting and the heat change upon cooling crystallization of the PAT foamed material. The principle is that when a PAT foamed material is heated and the temperature of the PAT foamed material reaches the crystallizing temperature, the crystal growth takes place. When the PAT foamed material is further heated, the foamed material melts at the melting point of the crystals. In this case, the generation of heat at cystallisation and the absorption of heat at melting are utilized. In practice, the amount of heat generated upon crystallization during temperature elevation and the amount of heat absorbed upon melting are measured, and then the crystallinity is calculated by comparing the values thus obtained with the heat change upon melting of the theoretical completely crystalline material. The inventors have attempted to form a PAT foamed material by mixing PAT with a foaming agent and by extruding the mixture from an extruding machine in a molten state, and it has been confirmed that, in this case, when the mixture is foamed with the inclusion of additives such as a foaming aid, etc., to the mixture by extruding the mixture in a high-temperature molten state, a foamed material having a thickness of at least 3 mm and an apparent density of from about 0.05 to 0.7 g/cm² is obtained. In this case, the foamed material is produced by utilizing the crystalline behavior upon lowering the temperature of PAT during the course of cooling from the molten state to normal (room) temperature. Also, it has been confirmed that such a foamed PAT has an excellent mechanical strength and is suitable as a structural material. Furthermore, the inventors investigated various cooling methods for the PAT foamed material directly after being extruded and also the properties of the PAT foamed materials thus obtained. As a result thereof, it has been discovered that when, in this case, the crystallinity of the surface portion of the PAT foamed material is kept at a low value, a PAT foamed material having a large bending strength and mechanical strength, and which is excellent as a structural material, is obtained in PAT foamed materials obtained by forming at the same expansion ratio. This is utterly astonishing when, conventionally, one would expect to obtain a foamed material having a high rigidity by increasing the crystallinity. Also, the inventors have found that when the resin of a foamed resin material is polystyrene or soft polyurethane, or when the foamed material is heated to cure a resorcinol series resin, the formed material is deformed and cannot keep its original shape; but when a thermoplastic polyester resin is used as the resin of a foamed resin material, even when the foamed material is heated to the temperature of cure of a resorcinol series resin or other thermosetting resin, the foamed material is not deformed. Furthermore, the inventors have found that when an uncured material of not only a resorcinol series resin but also another general thermosetting resin is brought into contact with the surface of a PAT foamed material and the thermosetting resin is cured by heating, the thermosetting resin fills the unevennesses formed by the bubbles on the surface of the foamed material and the thermosetting resin strongly adheres to the PAT foamed material. Still further, it has also been discovered that when reinforcing fibers such as glass fibers, etc., are incorporated in the thermosetting resin, the molding obtained is reinforced by the fibers to provide a tough molding. Even further, it has been discovered that when the fiber-containing thermosetting resin is foamed, synthetic resin-made structural materials which are lighter in weight, excellent in heat insulation, and high in strength are obtained. The present invention has been accomplished based on the aforesaid various knowledges. That is, according to this invention, there is provided a thermoplastic polyester series resin foamed material having a thickness of at least 3 mm and an apparent density of from 0.05 to 0.7 g/cm³, wherein the crystallinity of the resin in the surface portion within 0.5 mm from the surface of the foamed material is not more than 30% and is at least 1% lower than the crystallinity of the resin in the central portion of the foamed material. Also, according to other embodiment of this invention, there is provided a process of producing the aforesaid thermoplastic polyester series resin foamed material, which comprises extruding a thermoplastic polyester series resin containing a foaming agent from an extruding machine in a molten state of at least 200°C and while the extruded resin is foamed and has a surface temperature of at least the crystal melting point, bringing the surface of the foamed material into contact with a light or a solid having a temperature of lower than the glass transition point of the resin to forcibly suddenly cool the resin, so as to produce a thermoplastic polyester series foamed material as defined in the last preceding paragraph. According to a still other embodiment of this invention, there is further provided a synthetic resin structural material formed by bringing at least the surface of a thermoplastic polyester series resin foamed material (PAT foamed material) as defined in the last preceding paragraph but one into contact with an uncured thermosetting resin containing fibers, and then curing the thermosetting resin. According to another embodiment of this invention, there is also provided a synthetic resin structural material formed by bringing at least the surface of a thermoplastic polyester series foamed material as defined in the last preceding paragraph but two into contact with an uncured thermosetting resin containing fibers and a foaming agent, and forming and curing the thermosetting resin to unify them. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is an enlarged sectional view showing an example of the PAT foamed material of this invention. DETAILED DESCRIPTION OF THE INVENTION Now, the invention will be described in detail. In this invention, a thermoplastic polyester series resin (i.e., PAT) is used. PAT is a high molecular weight chain polyester obtained by reacting an aromatic dicarboxylic acid with a dihydric alcohol. As the dicarboxylic acid, terephthalic acid is most frequently used but isophthalic acid or 2,6-naphthalenedicarboxylic acid can be also used. Furthermore, diphenyl ether dicarboxylic acid, diphenylsulfondicarboxylic acid, and diphenoxydicarboxylic acid can be also used. On the other hand, as the dihydric alcohol, ethylene glycol is mainly used but trimethylene glycol, tetramethylene glycol, neopentylene glycol, hexamethylene glycol, cyclohexanedimethylol, trichlorodecanedimethylol, 2,2'bis(4-β-hydroxyethoxyphenyl)propane, 4,4'-bis(β-hydroxyethoxy)diphenylsulfone, or diethylene glycol can be also used. Specific examples of PAT are polyethylene terephthalate, polybutylene terephthalate, a polybutylene terephthalate elastomer, polycyclohexane terephthalate, and polyethylene naphthalate. Such PAT is commercially available and such a commercially available PAT can be used in this invention. Since PAT is a resin which is liable to be hydrolyzed at a high temperature, in the case of foaming the resin, it is desirable to previously dry the resin. For drying the resin, for example, a dehumidifying dryer is preferably used. In the drying condition in the aforesaid case, air having a dew point of, e.g., -30°C is heated to 160°C and PAT can be dried by exposing PAT to the air for about 4 hours. As a foaming agent for foaming PAT, various foaming agents can be used in this invention. They can be generally classified into a solid compound which is decomposed at a temperature higher than the softening point of PAT to generate a gas, a liquid which is vaporized in PAT by heating, and an inert gas capable of dissolving PAT under pressure. The materials classified as above can be all used in this invention. These foaming agents will now be described in more detail. Examples of the aforesaid solid compound are azodicarbonamide, dinitrosopentamethylenetetramine, hydrazocarbonamide, and sodium hydrogencarbonate. Examples of the aforesaid liquid which is vaporized in PAT are saturated aliphatic hydrocarbons such as hexane, n-pentane, n-butane, isobutane, etc" saturated alicyclic hydrocarbons such as cyclohexane etc.; and halogenated hydrocarbons such as methylene chloride, Freon (trade name, made by E.I. du Pont de Nemours & Co., Ltd.), etc. Also, examples of the aforesaid inert gas are carbon dioxide, nitrogen, etc. In the process of this invention, it is required to foam PAT containing a foaming agent by extrusion. For this purpose, PAT containing a foaming agent is previously prepared and the mixture may be extruded by an extruding machine or a foaming agent may be incorporated in PAT in an extruding machine. A mouthpiece having an extrusion hole or slit is fitted to the end of the extruding machine and PAT containing a foaming agent is extruded through the extruding hole or slit. In this case, the extrusion temperature of PAT is as high as 200°C or higher so that PAT exists in a molten state. That is, PAT is extruded at the temperature of melt-kneading PAT and a foaming agent or at a high temperature which is slightly lower than the aforesaid temperature. Practically speaking, when polyethylene terephthalate (hereinafter referred to as PET) is used as PAT, it is melt-kneaded with a foaming agent at 280°C and the mixture is extruded from a extruding machine at 280°C as it is, and a foamed material having a density of from 0.05 to 0.7 g/cm³ can be obtained. When, polybutylene terephthalate (hereinafter referred to as PBT) is used as PAT, it is melt-kneaded with a foaming agent at 260°C, and the restultant mixture is extruded from an extruding machine at 240°C, i.e., 20°C lower than the melt-kneading temperature, to provide a foamed material having the aforesaid density. It is unexpected to extrude PAT at such a high temperature. This is because such a high temperature is not a temperature at which PAT shows a particularly suitable viscosity for foaming PAT and such a high temperature is not employed for other resins. That is, in the case of other resin such as, for example, polyethylene, the resin is melt-kneaded with a foaming agent at 220°C and it is required to lower the temperature by 110°C so that the resin is foamed by extruding at about 110°C. Also, in the case of polypropylene, the resin is melt-kneaded with a foaming agent at 250°C and it is required to lower the temperature by 80°C so that the resin is foamed by extruding at about 170°C. In this invention, when extruded PAT is foamed and the surface temperature of the foamed resin is at a temperature higher than the crystal melting point, the foamed resin is suddenly cooled. For sudden cooling the foamed resin is brought into contact with a liquid or a solid which is at a temperature lower than the glass transition point of PAT. The crystal melting point and the glass transition point of PAT differ according to the kinds of carboxylic acid and alcohol constituting the PAT, but the crystal melting point of PAT is generally in the range of from 220°C to 290°C and the glass transition point thereof is generally in the range of from 30°C to 90°C. Accordingly, for practising sudden cooling, the foamed resin is usually brought into contact with a liquid or a solid which is at a temperature of lower than 60°C. As a liquid used for sudden cooling, water is suitable. Also, as a solid used for sudden cooling, a metal having a good heat conductivity, such as, in particular, aluminum, stainless steel, copper, etc., is suitable. It is desirable that the solid for sudden cooling has a surface shape such that it can be brought into good contact with the surface of the PAT foamed material. For example, when PAT is extruded as a cylindrical sheet and a mandrel is used as the solid for sudden cooling, the foamed PAT cylindrical sheet is advanced along the mandrel and the mandrel is cooled by water. In this case, it is better for the mandrel to be as long as possible. Also, when PAT is extruded as a plate-like sheet and a pair of rolls is used as the solid for sudden cooling, the foamed PAT sheet is advanced while it is in a close contact state with the roll surfaces and the rolls are cooled with water. In this case, it is preferred for the diameter of each roll to be as large as possible. Thus, the crystallinity of the surface portion of the PAT foamed material is kept low. When the extruded PAT foamed material is suddenly cooled, the foamed material is solidified without a time for crystallization to take place, and the crystallinity of the foamed material is low. In general, when a material is being extruded is a foamed material and the foamed material is cooled or is introduced into a mold to adjust the shape thereof, the foamed material is usually not suddenly cooled. This is because, if the foamed material is suddenly cooled, the bubbles are shrunk and so reduce the surface quality of the foamed material. Thus, if in the case of PAT, the PAT foamed material is not suddenly cooled as in the conventional technique, crystallization occurs at the surface to increase the crystallinity to usually about 30%. Also, in a foamed material having a large thickness, the central portion thereof cannot be suddenly cooled, which results in increasing the crystallinity thereof. On the other hand, in this invention, when the surface of the PAT foamed material is at a temperature above the crystal melting point, and the surface is brought into contact with a liquid or a solid which is at a temperature lower than the glass transition point of the PAT foamed material to effect sudden cooling, the crystallinity can be kept low. In this invention, it is necessary that the surface portion of the PAT foamed material has a crystallinity of not more than 30% is the surface portion of the PAT foamed material. The term "surface portion" in this invention means the portion within 0.5 mm (perpendicular to the surface) from the surface of the foamed material. Also, the term "central portion" means the portion excluding the portion within 1.0 mm (perpendicular to the surface) from the surface of the foamed material. The crystallinity of a PAT foamed material is practically measured as follows. For example, to measure the crystallinity of the surface portion of a PAT foamed material, a portion 0.5 mm in depth from the surface of the PAT foamed material is torn off and then the heat change upon cooling crystallization and the heat change upon melting of the surface portion are measured. It is desirable that the measurement is made by a differential scanning calorimetry measuring method. In the differential scanning calorimetry measuring method, heaters for the sample being measured and a standard sample function independently and since if a temperature difference occurs between both the samples during heating at a constant rate, the heat quantity increasing or decreasing mechanism of either of them acts to negate the temperature difference, and the difference of the heat flow rates is directly recorded. The crystallinity is theoretically calculated by the following formula: Crystallinity(%) = (A)/(B) × 100 (A): Heat change upon melting per mol - heat change upon cooling crystallization per mol. (B): Heat change upon melting per mol of completely crystalline PAT. In this case, according to Koobunshi (Polymer) Data Handbook , (published by Baifuukan), the heat change upon melting of completely crystalline polyethylene phthalate is 26.9 KJ and the value is used in this invention. In this invention, the surface of the PAT foamed material extruded is suddenly cooled to hold the crystallinity of the surface portion thereof not more than 30%. In this case, the inside of the foamed material is not suddenly cooled because of the existence of bubbles and is gradually cooled, whereby the crystallinity of the inside is usually higher than that of the surface portion. In this invention, it is necessary that the crystallinity of the surface portion of the PAT foamed material is at least 1% lower than the crystallinity of the central portion. Furthermore, it is more preferred that the crystallinity of the surface portion is at least 1.5% lower than that of the central portion. In the PAT foamed materials of this invention, the whole mean crystallinity of preferred foamed materials is from 15% to 50%, and more preferably from 20% to 40%. Also, it is preferable that the crystallinity of the surface portion of the PAT foamed materials is from 14% to 30%. An example of the PAT foamed material of this invention is shown in the accompanying drawing. Fig. 1 shows an enlarged sectional view of a part of an example of the PAT foamed sheet of this invention. In Fig. 1 A denotes the surface portion within 0.5 mm (perpendicular to the surface) from the surface of the foamed material. Also, B denotes a the shell portion existing within 1 mm (perpendicular to the surface) from the surface of the foamed material. C denotes the central portion of the foamed material. D denotes the portion for calculating the whole mean crystallinity of the foamed material. E denotes an inner portion of shell portion B . The surface portion A has a structure in which very fine bubbles are densely distributed, the inner portion E succeeding the surface portion A has a structure in which bubbles coarser than the bubbles in the surface portion A are roughly distributed, and the central portion C has a structure in which even coarser bubbles are more roughly distributed. In this invention, various additives can be added to PAT. For example, small amounts of a talc powder as a nucleating agent; epoxy compounds such as diglycidyl terephthalate, etc., acid dianhydrides such as pyromellitic dianhydride, benzophenonetetracarboxylic acid dianhydride, cyclopentatetracarboxylic acid dianhydride, diphenylsulfontetracarboxylic acid dianhydride, etc., and compounds of metals belonging to groups Ia and IIa of the periodic table or sodium carbonate for improving the melting characteristics of PAT can be added singly or as a mixture thereof. The amount of the additives is selected in the range of from 0.1 to 5 parts by weight to 100 parts by weight of PAT. Also, in this invention, if the foamed material obtained by extrusion-foaming the resin has an expansion ratio which is too low, the foamed material can be further heated to cause secondary foaming. In this case, there is no specific restriction on the heating means. That is, heating by conduction, radiation or by high frequency electric power can be used. Also, any heating medium in which PAT can be immersed can be used in this case. In a preferred heating method in the aforesaid methods, the PAT foamed material obtained by extrusion-foaming is brought into contact with steam or heated water. In the case of causing secondary foaming by bringing the foamed material into contact with steam or heated water, the foamed material can be immediately subjected to secondary foaming following extrusion-foaming. In this case, the temperature of steam or water is from 60 to 125°C and the contact time is from 10 seconds to 5 minutes. If the temperature of the heating medium is over 125°C, there is a possibility that steam or water having such a high temperature hydrolyzes the PAT foamed material. Thus, it is better to avoid the employment of such a high temperature. Also, in the structural material of this invention, as a thermosetting resin which is laminated on the PAT foamed material obtained as described above or other PAT foamed material, various kinds of resins can be used. For example, there are unsaturated polyester resins, epoxy resins, and phenol resins. These resins are in uncured states and used as a powder or a liquid. The unsaturated polyester resin is a thermosetting resin formed by dissolving a vinylic monomer in a resin (base resin) obtained by the condensation reaction of an unsaturated or saturated dicarboxylic acid and a dihydric alcohol. The main chain of the base resin contain an unsaturated group and the unsaturated group causes a copolymerization reaction with the vinylic monomer to cause crosslinking and curing. As the dicarboxylic acid, maleic anhydride or phthalic anhydride is frequently used. As a dihydric alcohol, ethylene glycol or propylene glycol is used, and as a vinylic monomer, styrene is used. The uncured resin is obtained as a powdery material or a liquid having a low viscosity. When an organic peroxide such as benzoyl peroxide is added to the unsaturated polyester resin and the mixture is heated, the resin is cured to form a solid resin. In this case, when a curing accelerator such as dimethylaniline and cobalt naphthenate is added to the system, the resin is cured without need of heating. Also, the resin forms a layer without pressing. The aforesaid epoxy resin is a resin having two or more epoxy groups in the molecule. The resin is classified into various types such as bisphenol A type resins, alicyclic resins, and resins induced from polybutadiene but in these resins, bisphenol A type resins are frequently used. The bisphenol A type resins are generally shown by the following formulae; wherein n represents an integer of from 1 to 12. According to the number of n, the resins take various states from liquid to solid. They are uncured epoxy resins. When a curing agent is added to the uncured epoxy resin, the resin is cured to form a solid resin. As the curing agent, an aliphatic diamine, an aromatic diamine, an aromatic acid anhydride, an aliphatic polyamine, etc., can be used. When an aliphatic polyamine is added to the uncured epoxy resin, the resin can be cured at normal temperature without need of heating, while when an aromatic diamine or an aromatic acid anhydride is added to the uncured epoxy resin, the resin is cured by heating. The resin may be cured by applying pressure, or it may be cured at normal pressure i.e., without applying pressure. The phenol resin is a resin formed by the reaction of phenol and formaldehyde. According to the kind of catalyst being used for the reaction, two kinds of resin each having a different chemical structure are obtained. That is, the phenol resin obtained by using an alkali as the catalyst is called "resol" and the phenol resin obtained by using an acid as the catalyst is called "novolak". Resol is initially in a liquid state but when resol is heated, it becomes solid. Novolak is a brittle solid and when hexamethylenetetramine is added thereto as a curing agent and the mixture is heated, novolak is cured to form a solid resin. In other embodiments of this invention, fibers are used for reinforcing the thermosetting resin. As the fibers, there are glass fibers, carbon fibers, aromatic polyamide fibers, metal fibers, potassium titanate fibers, silicon carbide fibers, etc. The diameters of the fibers are from several microns to several tens of microns and fibers having various lengths can be used according to requirements. Also, the state of the fibers may be single fiber or may be yarn, strand, roving, chopped strand, mat, cloth, etc. The uncured thermosetting resin containing fibers can be easily prepared by mixing the fibers and the resin. When an unsaturated polyester resin is used as the thermosetting resin, the resin generally called a "sheet molding compound" (generally abbreviated to "SMC") can be used as it is. The structural material of this invention can be prepared by bringing the uncured thermosetting resin containing fibers into contact with the surface of a PAT foamed material and thereafter curing the resin. It is unnecessary to use a specific adhesive on the foamed material for laminating the thermosetting resin. In this case, however, when heating is required for curing the thermosetting resin, the resin is heated and also when pressing is required, the laminate formed is pressed in a press. However, since the PAT foamed material is not usually softened at a temperature lower than 200°C and hence when the laminate is heated for curing the thermosetting resin, the foamed material is not deformed. Also, since the PAT foamed material is scarcely deformed even by applying thereto a slight pressure, the foamed material is scarcely deformed even when the laminated is pressed for curing the thermosetting resin. If the laminate of the foamed material and the thermosetting resin is strongly pressed, the PAT foamed material causes an elastic deformation only and does not lose the foamed structure. Thus, a structural material composed of the fiber containing thermosetting resin and the PAT foamed material in a unitary body can be easily and surely prepared. The fiber-containing thermosetting resin can be formed at one surface of the PAT foamed material but may be formed on both the surfaces of the PAT foamed material to provide a sandwich structure in which the PAT foamed material is interposed between two sheets of the fiber containing thermoplastic resin. In a still further embodiment of this invention, for foaming the thermosetting resin, a foaming agent is added to the resin while the resin is in an uncured state and the resin is foamed and cured. In this case, various kinds of materials can be used as the foaming agent. The foaming agent being used for the thermosetting resin is generally classified into a solid compound which is decomposed at a temperature of higher than the curing temperature of the resin to generate a gas and a liquid or a gas which is dissolved in the resin at a temperature of lower than the curing temperature of the resin but is vaporized in the resin when the temperature becomes higher than the curing temperature of the resin. Examples of the aforesaid solid compound are 2,2'-azoisobutyronitrile, diazoaminobenzene, and tetraphthalazide and also examples of the aforesaid liquid or gas are petroleum ether and acetone. The uncured thermosetting resin containing fibers and a foaming agent can be easily prepared by mixing the resin with the fibers and the foaming agent. When an unsaturated polyester resin is used as the thermosetting resin, a mixture of a sheet molding compound (SMC) and a foaming agent impregnated therein can be used as it is. The aforesaid structural material of this invention can be prepared by bringing the thermosetting resin containing fibers and a foaming agent into contact with the surface of the PAT foamed material, forming a space capable of foaming the resin in a direction perpendicular to the contact surface between the PAT foamed material and the thermosetting resin, and only heating the resin. In this case, as the crystallinity of the surface portion of the PAT foamed material is lower, the adhesion between the foamed material and the thermosetting resin is better. Anyhow, in this case, it is unnecessary to use a specific adhesive on the foamed material. If it is required to press the laminate for adjusting the shape when curing the thermosetting resin, the laminate is pressed in a press. In this case, however, since the PAT foamed material is not usually softened at a temperature of lower than 200°C, the foamed material is not deformed at heating for curing the thermosetting resin. Also, since the PAT foamed material is scarcely deformed by the application of a slight pressure, the foamed material is scarcely deformed when a pressure is applied whilst curing the thermosetting resin. Also, even when the laminate is strongly pressed, the PAT foamed material causes an elastic deformation only and does not lose the foamed structure. Thus, a structural material wherein the fiber-containing foamed thermosetting resin and the PAT foamed material are laminated in a unitary body can be easily and surely prepared. In this embodiment, the fiber-containing foamed thermosetting resin may be formed on one surface of the PAT foamed material but may be formed on both surfaces of the PAT foamed material to form a sandwich structure in which the PAT foamed material is interposed between two sheets of the fiber-containing foamed resin. Now, since the PAT foamed material of this invention has a thickness of at least 3 mm, preferably from 3 to 50 mm, and an apparent density of from 0.05 to 0.7 g/cm³, the PAT foamed material has sufficient strength and heat resistance and further is light-weight. Furthermore, since in the PAT foamed material, the crystallinity of the surface portion within 0.5 mm from the surface of the foamed material is held low so as not to be more than 30% and also is at least 1% lower than the crystallinity of the central portion of the foamed material, the PAT foamed material has a high bending strength and a high nail pulling out strength. The nail pulling out strength is the resisting power to pulling out of a nail after nailing. A high nail pulling out strength means that the nail is reluctant to be pulled out, which is suitable for a construction work. Thus, the PAT foamed material of this invention is suitable as a structural material. Also, according to the process of this invention, PAT containing a foaming agent is extruded from an extruding machine in a molten state of a temperature of at least 200°C, PAT is foamed at a low expansion ratio and a PAT foamed material having an apparent density of from 0.05 to 0.7 g/cm³ can be easily obtained. Also, since while extruded PAT is foamed and has a surface temperature of higher than the crystal melting point, the surface of the PAT foamed material is brought into contact with a liquid or a solid having a temperature of lower than the glass transition point of PAT to forcibly suddenly cool the surface, the surface of the PAT foamed material is solidified without time for crystallization, whereby the crystallinity of the surface is lowered. Thus, a PAT foamed material having the crystallinity of the surface portion of at least 1% lower than the crystallinity of the central portion of the foamed material can be easily obtained. As the result thereof, a foamed material having a high bending strength (e.g., 50 to 100 kg.f/cm²) and a nail pulling out strength (e.g., 9.5 to 25.0 kg.f) as well as having sufficient strength and heat resistance and being light in weight is obtained. Accordingly, the foamed material is suitable for use as a structural material. The process of this invention gives large industrial advantages in relation to the ease of manufacture of the aforesaid structural material. Also, in this invention, PAT is used as the material for forming the foamed material and PAT has a high heat resistance such that PAT is not softened at a temperature lower than 200°C, thereby even when a thermosetting resin is cured on the surface of the foamed material, the foamed material does not lose the foamed structure and hence the layer of the thermosetting resin can be directly formed on the foamed material. Furthermore, in this case, the surface of the foamed material is uneven because of the existence of bubbles, and hence the thermosetting resin strongly adheres to the foamed material, whereby the product obtained has a resistance to delamination. Also, in the case where the thermoplastic resin contains fibers, the product is reinforced by the fibers and has a sufficient mechanical strength. Furthermore, for making the aforesaid product or laminate, a specific adhesive is not required and hence the production thereof is easy. Moreover, since the product contains the PAT foamed material, the product is also light in weight and has a high heat resistance. Furthermore, since the PAT foamed material and the fiber-containing thermosetting resin have sufficient heat resistance and mechanical strength, the whole product is light-weight and has high heat resistance and high mechanical strength. Accordingly, the product is suitably for use as a structural material for motor cars, aeroplanes, ships, etc. The present invention will be described more practically by the following examples, wherein parts are by weight. In the examples, the bending strength and bend-flexed amount were measured according to JIS K 7203. The measurement conditions are as follows. In Examples 1 to 5 and Comparison Examples 1 and 2, the bending rate is 10 mm/min, the distance between supporting points is 70 mm, the width of the sample is 15 mm, and the thickness of the sample is the thickness of the original sample. In Examples 6 to 12 and Comparison Examples 3 to 6, the bending rate is 2.5 m/min., the distance between the supporting points is 80 mm, the width of the sample is 30 mm, and the thickness of the sample is the thickness of the original sample. Also, the nail pulling off strength is the value measured as follows. As a nail, a wire nail having a length below the head of 49 mm and an outside diameter of 2.5 mm was used, the nail was held perpendicularly on the surface of a PAT foamed sheet, and was nailed to a depth of 15 mm from the surface by an oil pressure press. Thereafter, the nail was pulled off from the PAT foamed sheet by using a tension test machine, and the value of maximum load (kg·f) under which the nail was pulled off at a pulling off rate of 10 mm/min. was employed as the nail pulling off strength. Example 1 Polyethylene terephthalate (hereinafter referred to as "PET") (PET 9902, trade name, made by Eastman Kodak Co.) was used as PAT. First, the PET was placed in a dehumidifying dryer and was dried for 4 hours at 160°C while circulating air having a dew point of -30°C. Then, the following mixture was prepared using the thus dried PET. PET 100 parts Talc (nucleating agent) 0.6 part Pyromellitic Anhydride 0.5 part Sodium Carbonate 0.1 part The mixture was placed in an extruding machine having an aperture of 65 mm and L/D of 35, mixed well at a screw rotation rate of 25 r.p.m. and at a barrel temperature of from 270°C to 290°C. Butane was introduced under pressure into the mixture as a foaming agent at a position intermediate the ends of the barrel in an amount of 1% by weight to the mixture, and the extrusion pressure was 55 kg/cm². Thus, PET containing the foaming agent was extruded into air at 30°C in the form of a board from a flat die. The die had a slit width of 75 mm and depth of 1.5 mm, and was kept at 265°C. PET extruded into the air was immediately foamed. The PET foamed sheet was immediately inserted between metal plates for cooling and advanced while being closely contacted by the cooling metal plates. The cooling metal plates were cooled by passing water at 20°C through the inside of each plate. Thus, a PET foamed board having a width of 180 mm and a thickness of 35 mm was obtained. The foamed board was cut perpendicularly to the extruding direction and the section was observed. As the result thereof, a shell having a different foamed state was observed within the range of 2 mm (perpendicular to the surface) from the surface. When the properties of the foamed board were measured, the mean density was 0.35 g/cm³, the density of the surface portion (thickness of 0.5 mm) was 0.38 g/cm³, the density of the central portion was 0.35 g/cm³, the whole mean crystallinity was 30.7%, the crystallinity of the surface portion was 26.7%, and the crystallinity of the central portion was 30.8%. Also, the bending strength thereof was 95.3 kg·f/cm³, the bend-flexed amount was 2 mm, and the nailing off strength was 20 kg·f. Comparison Example 1 In the comparison example, a PET foamed board having almost the same density as that in Example 1 was prepared for comparing with the foamed board formed in Example 1 and the properties of both the foamed boards were compared. In the comparison example, PET was foamed as in Example 1 except that the cooling metal plates were not used for forcibly cooling the extruded PET foamed board and the foamed board was allowed to naturally cool in the air at 30°C. When the foamed board was cut perpendicularly to the extruding direction and the section was observed, no shell portion was observed on the surface. When the properties of the foamed board were measured, the mean density was 0.35 g/cm³, the density of the portion corresponding the surface portion within 0.5 mm from the surface and the density of the central portion were all 0.35 g/cm³, the whole mean crystallinity, the crystallinity of the portion corresponding to the shell portion, and the crystallinity of the central portion were all 30.8%. Also, the bending strength was 80.0 kg·f/cm³, the bend-flexed amount was 1.2 mm, and the nail pulling off strength was 9.5 kg·f. On comparing the foamed board obtained in Comparison Example 1 with the foamed board obtained in Example 1, it can be seen that the foamed board of this invention (Example 1) are excellent in the bending strength and the nail pulling off strength as compared with the foamed board in the comparison example although the expansion ratio was almost same in both cases. Example 2 By following the same procedure as Example 1 except that the content of butane as a foaming agent was 1.8% by weight of the resin mixture and the extrusion pressure was 70 kg/cm², a PET foamed sheet was prepared. The PET foamed sheet obtained had a width of 180 mm and a thickness of 5 mm. When the foamed sheet was cut perpendicularly to the extruding direction and the section was observed, a shell portion having a different foamed state to the central portion was observed within 1 mm (perpendicular to the surface) from the surface. When the properties of the foamed sheet were measured, the mean density was 0.22 g/cm³, the density of the surface portion was 0.24 g/cm³, the density of the central portion was 0.21 g/cm³, the whole mean crystallinity was 22.3%, the crystallinity of the surface portion (thickness of 0.5 mm) was 18.7%, the crystallinity of the central portion was 22.7%, the bending strength of 53.9 kg·f/cm², the bend-flexed amount was 12 mm, and the nail pulling off strength was 9.8 kg·f. Comparison Example 2 In the comparison example, a PET foamed sheet having almost the same density as that in Example 2 was prepared for comparing with the foamed sheet in Example 2 and the properties of the foamed sheet were compared. In the comparison example, the PET foamed sheet was prepared by the same manner as Example 2 except that the cooling metal plates were not used and the extruded PET foamed sheet was allowed to naturally cool without forcibly cooling the foamed sheet. When the foamed sheet was cut perpendicularly to the extruding direction and the section was observed, no shell portion was observed at the surface thereof. When the properties of the foamed sheet were measured, the mean density was 0.22 g/cm³, the density of the portion corresponding to the surface portion within 0.5 mm from the surface and the density of the central portion were all 0.22 g/cm³ and the whole mean crystallinity, the crystallinity of the portion corresponding to the surface portion, and the crystallinity of the central portion were all 22.5%. Also, the bending strength was 41.6 kg·f/cm², the bend-flexed amount of 4.6 mm, and nail pulling off strength was 4.6 kg·f. On comparing the foamed sheet obtained in Comparison Example 2 with the foamed sheet obtained in Example 2, it can be seen that the foamed sheet of this invention in Example 2 was excellent in the bending strength, the nail pulling off strength, and the bend-flexed amount as compared with the comparison foamed sheet although the expansion ratios (densities) of them were almost same. Example 3 By following the same procedure as Example 2 except that the extruded foamed sheet was cooled by immersing in water of 35°C in place of using the cooling metal plates, a PET foamed sheet was prepared. The foamed sheet obtained had a width of 180 mm and a thickness of 5 mm. When the foamed sheet was cut perpendicularly to the extruding direction and the section was observed, a shell portion having a different foamed state to that of the central portion was observed within 1 mm (perpendicular to the surface) from the surface. When the properties of the foamed sheet were measured, the mean density was 0.22 g/cm³, the density of the surface portion was 0.24 g/cm³, the density of the central portion was 0.21 g/cm³, the whole mean crystallinity was 22.1%, the crystallinity of the surface portion was 18.5%, the crystallinity of the central portion was 22.6%, the bending strength was 50.2 kg·f/cm², and the nail pulling off strength was 9.3 kg·f. Example 4 By following the same procedure as Example 1 except that the screw rotation rate was changed to 45 r.p.m., n-pentane was used as a foaming agent in an amount of 4% by weight of the resin mixture, and the extrusion pressure was changed to 120 kg/cm³, a PET foamed plate was prepared. The foamed plate obtained had a width of 185 mm, a thickness of 20 mm, and a mean density of 12 g/cm³. The foamed plate was contact-heated with steam of 100°C for 5 minutes to cause secondary foaming and immediately cooled by placing the foamed plate between two cooling metal plates kept at 20°C. The thickness of the foamed board obtained by the secondary foaming was 52 mm. When the foamed board was cut perpendicularly to the extruding direction and the section was observed, a shell portion having a different foamed state to that of the central portion was observed within 1.5 mm (perpendicular to the surface) from the surface. When the properties of the foamed board were measured, the mean density was 0.05 g/cm³, the density of the surface portion (thickness of 0.5 mm) was 0.056 g/cm³, the density of the central portion was 0.049 g/cm³, the whole mean crystallinity was 26.3%, the crystallinity of the surface portion was 23.4%, and the crystallinity of the central portion was 26.7%. Also, the bending strength was 15.6 kg·f/cm² and the nail pulling off strength was 5.2 kg·f. Example 5 Polybutylene terephthalate (hereinafter referred to as PBT) (XD478, trade name, made by Polyplastic Co.) was used as PAT. First, PBT was placed in a dehumidifying dryer and dried for 4 hours at 130°C while circulating air having a dew point of -30°C. Then, the following mixture was prepared using the thus dried PBT. PBT 100 parts Talc (nucleating agent) 0.6 part T6-5 (trade name of higher fatty acid ester, made by Kao Corporation) 0.5 part The mixture was placed in an extruding machine having an aperture of 65 mm and L/D of 35, mixed well at a screw rotation rate of 25 r.p.m., and barrel temperature of from 235 to 265°C, butane was pressed in as a foaming agent from a middle portion of the barrel in an amount of 1% by weight of the mixture and the mixture was extruded at an extrusion pressure of 50 kg/cm². Thus, PBT containing the foaming agent was extruded from a flat die into the air of 30°C in a tabular form. The die had a slit width of 75 mm and a depth of 1.5 mm, and was kept at 225°C. PBT extruded into the air foamed immediately. The foamed PBT sheet was immediately placed between cooling metal plates and advanced while contacting the cooling metal plates. The cooling metal plates were cooled by passing water at 15°C through the inside of each metal plate. Thus, a PBT foamed sheet having a width of 130 mm and a thickness of 5 mm was obtained. When the foamed sheet was cut perpendicularly to the extruding direction and the section was observed, a shell portion having a different foamed state to that in the central portion was observed within 1 mm (perpendicular to the surface) from the surface. When the properties of the foamed sheet were measured, the mean density was 0.17 g/cm³, the density of the surface portion (thickness of 0.5 mm) was 0.19 g/cm³, the density of the central portion was 0.17 g/cm³, the whole mean crystallinity was 24.3%, the crystallinity of the surface portion was 22.7%, the crystallinity of the central portion was 24.8%. Also, the bending strength was 57 kg·f/cm² and the nail pulling off strength was 16 kg·f. Example 6 As PET, 100 parts of polyethylene terephthalate pellets having an intrinsic viscosity of 0.81 (TR 8580, trade name, made by Teijin Limited) were used. After drying the pellets for 5 hours with a hot blast at 160°C having a dew pint of -20°C, 0.3 part of pyromellitic anhydride, 0.1 part of sodium carbonate (both components were viscosity controlling agent of PET), and 0.6 part of a talc powder (bubble controlling agent) were added to the pellets, and after mixing well the resultant mixture in a tumbler, the mixture was placed in an extruding machine. The cylinder temperature of the extruding machine was from 274°C to 287°C and the mouthpiece temperature thereof was 277°C. Also, butane was introduced under pressure into an intermediate portion of the cylinder as a foaming agent in an amount of about 1.0 part. The end of the extruding machine was equipped a mouthpiece having an annular slit, PAT containing butane was extruded through the annular slit as a cylindrical form, and cylindrical sheet thus extruded was advanced over a cylindrical mandrel while foaming took place to form a cylindrical foamed sheet. Thereafter, the cylindrical foamed sheet was cut open to form a flat sheet and the sheet was wound into a roll. The foamed sheet obtained had a density of 0.262 g/cm³, a thickness of 1.45 mm, and a width of 640 mm. (Fiber-containing thermosetting resin) As a fiber-containing thermosetting resin, SMC (DIC MAT 2454A, trade name, made by Dainippon Ink and Chemicals, Inc.) was used. (Production of structural material) A 200 mm square piece of the PET foamed sheet was placed between the 150 mm square sheets of the aforesaid SMC and the assembly was placed in a mold frame. The mold frame was prepared by punching a regular square hole having a side length of 200 mm through a regular square metal plate having a thickness of 2 mm and a side length of 250 mm, and thereby was to produce a square frame having a frame width of 25 mm. The aforesaid assembly was placed in a press together with the mold frame and heated to 150°C for 5 minutes while applying a pressure of 30 kg/cm². Thus, a structural material was obtained. The structural material had a thickness of 2.7 mm and a density of 1.17 g/cm². The reason that the thickness of the structural material became greater than the thickness of the mold frame was that the PAT foamed material was further foamed when it was taken out from the press. The bending strength of the structural material measured according to JIS K 7203 was 9.64 kg·f in bending load and 14.6 mm in the flexed amount. Also, when it was tried to peel off SMC from the PAT foamed material at the interface, SMC could not be easily peeled off and when SMC was forcibly peeled off, the PET foamed material was broken. Example 7 On both surfaces of the PET foamed material prepared by the same manner as Example 6 were placed 150 mm square of a glass fiber mat (FEM-300-04, trade name, made by Fuji Fiber Glass Co.) having a weight of 300g/M², (basis weight), 52 g of a phenol resin (PF-0015m trade name, made by Asahi Yuuki Zai K,K., viscosity 5300 cp/35°C) and 4 g of a curing agent (AD-14, trade name, made by the aforesaid company), and the laminate was placed in a mold frame. The mold frame was made by punching a regular square hole having a side length of 150 mm in a regular square metal plate having a thickness of 5 mm and a side length of 180 mm, so as to produce a square frame having a frame width of 15 mm. The aforesaid laminate (assembly) was placed in a press together with the mold frame and heated to 80°C for 30 minutes while applying a pressure of 30 kg/cm² to provide a structural material. The structural material had a thickness of 5.8 mm and a density of 0.88 g/cm³. When the bending load was measured as in Example 6, the bending load was 27.7 kg·f and the flexed amount was 6.9 mm. When it was tried to peel off the laminate at the interface, the laminate could not be delaminated by peeling until the PET foamed material was broken, which demonstrated a very good adhesion. Comparison Example 3 When the same procedure as Example 6 was followed except that a heat-resisting polystyrene foamed material having a basis weight of 200 g and a thickness of 1.85 mm was used in place of the PET foamed material, the polystyrene foamed material was melted and had no trace of the original form. Comparison Example 4 When the same procedure as Example 6 was followed except that a polypropylene resin foamed sheet having a basis weight of 567 g and a thickness of 0.7 mm was used in place of the PET foamed material, no deformation of the polypropylene resin foamed sheet was observed but the adhesion at the interface was poor and the laminate could be easily delaminated by peeling. Comparison Example 5 When the same procedure as Example 7 was followed except that a polystyrene foamed sheet having a basic weight of 200 g and a thickness of 1.85 mm was used in place of the PET foamed material, any deformation of the polystyrene resin foamed layer was not observed but the adhesion at the interface was poor and the laminate could be easily delaminated by peeling. Example 8 As PET, 100 parts of polyethylene terephthalate pellets having an intrinsic viscosity of 0.81 (TR 8580, trade name, made by Teijin Limited) were used. After drying the aforesaid pellets for 5 hours with a hot blast having a temperature of 160°C and a dew point of -20°C, 0.33 part of pyromellitic anhydride 0.1 part of sodium carbonate (both being viscosity controlling agents for PET), and 0.6 part of a talc powder (bubble controlling agent) were added to the pellets, and after mixing well the mixture in a tumbler, the mixture was placed in an extruding machine. The cylinder temperature of the extruding machine was from 275°C to 287°C and the die temperature thereof was 277°C. Also, about 1.0 part of butane was introduced under pressure into the system at an intermediate portion of the cylinder as a foaming agent. The end of the extruding machine was equipped a die having an annular fine slit, PET containing butane was extruded through the annular fine slit as a cylindrical form, the cylindrical sheet was advanced along the surface of a cylindrical mandrel while foaming took place to provide a foamed cylindrical sheet. The cylindrical sheet was cut open to form a flat sheet and the sheet was wound into a roll form. The foamed sheet obtained had a density of 0.2 g/cm³ and a thickness of 3 mm. From the foamed sheet thus obtained, a regular square sheet having a side length of 150 mm was cut to provide a PET foamed sheet. (Fiber and foaming agent-containing thermosetting resin) By uniformly impregnating a 150 mm square piece of a glass fiber mat having a basis weight of 300 g (FEM-300-04, trade name, made by Fuji Fiber Glass K.K.) with a mixture of 30 g of a phenol resin (PF-0015, trade name, made by Asahi Yuuki Zairyo K.K, viscosity 5100 c.p./25°C), 5 g of a curing agent (CA-645, trade name, made by the aforesaid company), and 3.6 g of a foaming agent (AA-30, trade name, made by the aforesaid company), an uncured thermosetting resin sheet containing glass fibers and a foaming agent was prepared. Two such thermosetting resin sheets were prepared. (Production of structural material) The aforesaid PET foamed sheet was placed between the aforesaid uncured thermosetting resin sheets containing the fibers and the foaming agent and the assembly was placed in a mold frame. The mold frame was a frame formed by punching a regular square hole having a side length of 150 mm at the center of a regular square metal plate having a thickness of 5 mm and a side length of 180 mm, to produce a square frame having a frame width of 15 mm. The laminated assembly placed in the mold frame had a space in the thickness direction of the sheet for allowing the expansion. The mold frame containing the assembly was placed in a press and heated to 90°C for 20 minutes while applying a pressure of 40 kg/cm² to provide a structural material. The structural material had a thickness of 5 mm and a density of 0.75 g/cm³. When the bending strength of the structural material was measured according to JIS K 7203, the bending strength of 292 kg·f/cm² and the flexed amount was 12.4 mm. Also, the elasticity was 19400 kg·f/cm². In addition, it was tried to peel off the phenol resin layer from the PAT foamed sheet at the interface but the layer could not easily be peeled off. Thus, it was confirmed that the structural material could be sufficiently used as the structural materials for motor cars, aeroplanes, and ships. Example 9 By following the same procedure as Example 8 except that the foaming agent mixed with the phenol resin was not used, a structural material was prepared. The structural material obtained had a thickness of 5 mm and the density of 0.75 g/cm³. The bending strength of the structural material was 254 kg·f/cm², the flexed amount thereof was 6.93 mm, and the elasticity was 22800 kg·f/cm². Thus, the aforesaid material could be used as a structural material but the flexibility was poor as compared with the product in Example 8. Example 10 In this example, the compounding ratio of the fibers and foaming agent-containing thermosetting resin in Example 8 was changed, the same PET foamed material as in Example 8 was used, and the sticking condition of the PET foamed material and the thermosetting resin sheet was changed. That is, the glass fiber mat (FEM-300-04) as used in Example 8 was impregnated with a mixture of 19 g of the phenol resin (PF-0015), 2.8 g of the curing agent (CA-645), and 1 g of the foaming agent (AA-30) each being used in Example 8 to provide an uncured thermosetting resin sheet. Thus, two such thermosetting resin sheets were prepared. The PET foamed material as used in Example 8 was placed between the thermosetting resin sheets and the laminated assembly was pressed in a press for 30 minutes at 80°C and at a pressing pressure of 35 kg/cm² to provide a structural material. The structural material had a thickness of 5 mm and the density of 0.59 g/cm³. Also, the bending load was 21.2 kg·f, the bending strength was 302 kg·f/cm², the flexed amount was 10.9, and the elasticity was 14900 kg·f/cm². Example 11 By following the same procedure as Example 10 except that the foaming agent mixed with the phenol resin was not used, a structural material was prepared. The structural material obtained had a thickness of 5 mm and a density of 0.59 g/cm³. Also, the bending load was 15.3 kg·f, the bending strength was 222 kg·f/cm², the flexed amount was 7.64 mm, and the elasticity was 21400 kg·f/cm². Example 12 (Production of PET foamed material) From the foamed sheet (having the density of 0.2 g/cm³ and the thickness of 3 mm) obtained in Example 8, five regular square sheets each having a side length of 250 mm were cut. Three such sheets were superposed to each other, a regular square polyester series hot melt film (PS-2810, trade name, made by Daicel Chemical Industries, Ltd.) having a side length of 250 mm and a thickness of 50 µm was inserted between each two sheets and the assembly thus obtained was placed in a press. The assembly was heated to 180°C for 2 minutes while applying a pressure of 0.2 kg/cm² to provide a primary PET laminate. Furthermore, the remaining two uncured thermosetting resin sheets were applied to opposite surface of the primary PET laminate, a regular square polyester series hot melt film (PS-2810, trade name, made by Daicel Chemical Industries, Ltd.) having a side length of 250 mm and a thickness of 50 µm was inserted between the primary PET laminate and each sheet as described above, the assembly was placed in a press, and heated to 180°C for 2 minutes while applying thereto a pressure of 0.2 kg/cm² to provide a secondary PET laminate. The reason for laminating twice as described above is that since the PET sheet itself is a heat-insulating material, if large sheets are laminated in one operation, the inside layers do not adhere to each other. (Thermosetting resin containing fibers and foaming agent) By uniformly impregnating a 250 mm square of a glass fiber mat (FEM-300-04, trade name, made by Fuji Fiber Glass K.K.) having a basis weight of 300 g/m² with a mixture of 192 g of a phenol resin (PF-0015, trade name, made by Asahi Yuuki Zai K.K., viscosity 5100 cp/25°C), 29 g of a curing agent (CA- 645, trade name, made by the aforesaid company), and 12 g of a foaming agent (AA-30, trade name, made by the aforesaid company), an uncured thermosetting resin sheet containing the fibers and the foaming agent was prepared. Two such sheets were prepared. (Production of structural material) The PET foamed sheet was placed between the aforesaid uncured thermosetting resin sheets containing the fibers and the foaming agent and the laminated assembly was placed in a mold frame. The mold frame was a frame prepared by punching a regular square hole having a side length of 250 mm at the center of a regular square metal plate having a thickness of 28 mm and a side length of 280 mm so as to form a square frame having a frame width of 15 mm. The laminate assembly placed in the mold frame had a space capable of expanding in the thickness direction of the sheet. The mold frame containing the assembly was placed in a press and heated to 80°C for 30 minutes while applying thereto a pressure of 45 kg/cm² to provide a structural material. The structural material had a thickness of 28 mm and a density of 0.41 g/cm³. Also, the heat conductivity thereof was 0.0339 Kcal/m·hr·°C at 20°C. Comparison Example 6 In the comparison example, a PET foamed material was not used. (Thermosetting resin containing fibers and foaming agent) By uniformly impregnating a 250 mm square of a glass fiber mat having a basis weight of 300 g/m² (FEM-30004, trade name, made by Fuji Fiber Glass K.K.) with a mixture of 330 g of a phenol resin (PF-0015, trade name, made by Asahi Yuuki Zai K.K., viscosity 5100 cp/25°C), 50 g of a curing agent (CA- 645, trade name, made by the aforesaid company), and 17.5 g of a foaming agent (AA30, trade name, made by the aforesaid company), an uncured thermosetting resin sheet containing the fibers and the foaming agent was obtained. Such two sheets were prepared. (Production of structural material) The aforesaid two uncured thermosetting resin sheets containing the fibers and the foaming agent were superposed to each other and placed in the mold frame as used in Example 12. The laminate placed in the mold frame had a space capable of expanding in the thickness direction of the sheet. The mold frame containing the laminate was placed in a press and heated to 80°C for 30 minutes while applying thereto a pressure of 45 kg/cm² to provide a structural material. The structural material obtained had a thickness of 28 mm and a density of 0.42 g/cm³. Also, the heat conductivity thereof was 0.0494 Kcal/m.hr.°C at 20°C.

Claims: A thermoplastic polyester series resin foamed material having a thickness of at least 3 mm and an apparent density of from 0.05 to 0.7 g/cm³, wherein the crystallinity of the resin in the surface portion within 0.5 mm from the surface of the foamed material is not more than 30% and at least 1% lower than the crystallinity of the resin in the central portion of the foamed material. A thermoplastic polyester series resin foamed material as claimed in claim 1, wherein the whole mean crystallinity thereof is from 15 to 50%. A thermoplastic polyester series resin foamed material as claimed in claim 1 or 2, wherein the crystallinity in the surface portion is from 14 to 30%. A thermoplastic polyester series resin foamed material as claimed in any preceding claim, wherein the crystallinity in the surface portion is at least 1.5% lower than the crystallinity in the central portion. A thermoplastic polyester series resin foamed material as claimed in any preceding claim, wherein the foamed material has a thickness of from 3 to 50 mm. A thermoplastic polyester series resin foamed material as claimed in any preceding claim, wherein the bending strength thereof is from 50 to 100 kg·f/cm². A thermoplastic polyester series resin foamed material as claimed in any preceding claim, wherein the nailing strength is from 9.5 to 25.0 kg·f. A thermoplastic polyester series resin foamed material as claimed in any preceding claim, wherein the thermoplastic polyester series resin is polyethylene terephthalate, polybutylene terephthalate, a polybutylene terephthalate elastomer, polycyclohexane terephthalate, or polyethylene naphthalate. A process of producing a thermoplastic polyester series resin foamed material as claimed in any preceding claim, which comprises extruding a thermoplastic polyester series resin containing a foaming agent from an extruding machine in a molten state of a temperature of at least 200°C and bringing the surface of the extruded resin into contact with a liquid or a solid having a temperature lower than the glass transition point of the resin while the extruded resin is foamed and has a surface temperature of higher than the crystal melting point to forcibly suddenly cool the surface, so as to produce a thermoplastic polyester series foamed material as claimed in any preceding claim. A process as claimed in claim 9, wherein the thermoplastic polyester series resin is mixed with pyromellitic dianhydride, benzophenonetetracarboxylic acid dianhydride, cyclopentanetetracarboxylic acid dianhydride, diphenylsulfonetetracarboxylic acid dianhydride, diglycidyl terephthalate, or a compound of a metal belonging to group Ia or IIa of the periodic table. A thermoplastic polyester series resin structural material obtained by bringing the surface of a thermoplastic polyester series foamed material as claimed in any one of claims 1 to 8 into contact with an uncured thermosetting resin containing fibers, and curing the thermosetting resin to form a laminate. A structural material as claimed in claim 11, wherein the thermosetting resin is an unsaturated polyester resin, an epoxy resin, or a phenol resin. A thermoplastic polyester series structural material obtained by bringing the surface of a thermoplastic polyester series resin foamed material as claimed in any one of claims 1 to 8 into contact with a thermosetting resin containing fibers and a foaming agent, and foaming and curing the thermosetting resin to form a laminate.

Inventors: AMANO NORIO, HAYASHI MOTOSHIGE, HIRAI TAKAAKI, ISHIBASHI MASATOSHI, TAKI TAKESHI, AMANO, NORIO, HAYASHI, MOTOSHIGE, HIRAI, TAKAAKI, ISHIBASHI, MASATOSHI, TAKI, TAKESHI
Applicants: SEKISUI PLASTICS, SEKISUI KASEIHIN KOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: B29C44
IPC codes: B29C70, C08J9, B29C47, B29C44