Thermoplastic polyester composition and film made therefrom

Abstract: A thermoplastic polyester composition comprises a thermoplastic polyester prepared from an acid component mainly comprising an aromatic dicarboxylic acid and a glycol component mainly comprising an aliphatic glycol and a particulate mixture of two or more aluminium oxide components different from each other in crystal structure types, one of which is the -type crystal structure. Since films formed from the compositions can have excellent slipping property, abrasion resistance and scratch resistance in repeated high-speed running, they are particularly suitable as substrates in magnetic recording media.

Description: The present invention relates to a thermoplastic polyester composition and films made therefrom, and more particularly, relates to a thermoplastic polyester composition suitable for obtaining shaped bodies (films or fibers) excellent in slipping property, abrasion resistance and scratch resistance (extent of scratching during high speed running) by uniformly dispersing fine aluminium oxide particles having specified crystal structures in the thermoplastic polyester composition. Generally, a thermoplastic polyester, for example, polyethylene terephthalate, has excellent mechanical properties, and is widely used in the formation of shaped products, such as films, fibers etc. Usually, inert particles are present in the polyester, thereby forming protrusions on the back surface of a shaped product for the purpose of giving a slipping property to the shaped product. Although there are various kinds of particle such as inert particles, there is a problem in that generally inert particles are poor in affinity with polyester and in abrasion resistance. To solve thus problem, investigations with respect to surface treatment have been carried out. For example, JP-A-SHO 63-221158 and JP-A-SHO 63-280763 propose improvement of the surfaces of colloidal silica particles by a glycol group, JP-A-SHO 63-312345 proposes improvement of the surfaces of colloidal silica particles by a coupling agent, and JP-A-SHO 62-235353 proposes a surface treatment of calcium carbonate particles by a phosphorus compound. However, even if such conventional methods are applied, when subjecting the shaped body to contact running and friction, for example during the use of a magnetic tape, particles are likely to fall off. Therefore, use of particular particles has been recently proposed. Specifically, it has been suggested that aluminum oxide particles have a great advantage for increase of abrasion resistance, for example, in JP-HEI 2-129230 (delta-type aluminum oxide particles), in JP-A-HEI 2-38444 (aluminum oxide particles), and in JP-A-HEI 3-43447 (aluminum oxide particles), which may be selected from ρ, η, γ, χ, κ ,δ and ϑ-type particles and which additionally contains a sulfur compound. However, even if such methods are applied, when a film is brought into contact with a guide pin and repeatedly runs at a high speed, the abrasion resistance and the scratch resistance thereof are insufficient. We have found that the abrasion resistance and scratch resistance of a polyester film depend upon the kind of particles present in the polyester film. Our co-pending EP-A-0577846 discloses, in particular, a thermoplastic polyester composition comprising a thermoplastic polyester (A) prepared from an acid component mainly comprising an aromatic dicarboxylic acid and a glycol component mainly comprising an aliphatic glycol and -type aluminium oxide particles (B) (see claim 1). An object of the present invention is to solve the abovementioned disadvantages in the conventional technologies, and to provide a thermoplastic polyester composition excellent in all of slipping property, abrasion resistance and scratch resistance, particularly a thermoplastic polyester composition from which films and fibers excellent in abrasion resistance in repeated high-speed running can be made. The object of the present invention is accomplished by a thermoplastic polyester composition, which is an aspect within the particular disclosures in EP-A-0577846 as set out above, of the following constitution, namely a thermoplastic polyester composition comprising a thermoplastic polyester (A) prepared from an acid component mainly comprising an aromatic dicarboxylic acid and a particulate mixture of two or more aluminium oxide components different from each other in crystal structure type, one of which crystal structure types is of the -type. The aluminium oxide particles used in the present invention are mixed -type aluminium oxide particles (B), where one component of the particles is -type aluminium oxide, or simply a mixture (C) of two more kinds of aluminum oxide particles different from each other in crystal structure, one of which is the -type. The mixture (C) of two or more kinds of aluminium oxide particles different from each other in crystal structure means a mixture of -type with, for example, α-type aluminium oxide particles well known, γ-type, δ-type, η-type, κ-type and χ-type activated aluminium oxide particles and amorphous aluminium oxide particles. There are various known methods for synthesizing the particles with the above-described crystal structures. For example, it is possible to synthesize δ-type aluminium oxide particles by blowing hydrogen and oxygen into aluminium chloride and hydrolyzing it in a gaseous phase, amorphous aluminium oxide particles by blowing a steam into aluminium oxide and hydrolyzing it in a gaseous phase, γ-type aluminium oxide particles by thermal decomposition of an alum, -type, γ-type and δ-type aluminium oxide particles by thermal decomposition of an alum after neutralizatin in a carbonate, -type, and γ-type aluminium oxide particles by calcining an aluminum hydroxide prepared by hydrolysis of an aluminium alkoxide, and η-type, -type and γ-type aluminium oxide particles by calcining an aluminium hydroxide prepared by spark discharging aluminium metal pellets in water. Further, when such synthesized activated alumina particles are calcined at a high temperature of not less than 1,300°C, α-type aluminium oxide particles can be obtained in any case. The α-type aluminum oxide particles can by synthesized not only by such a method but also by a method usually called "Bayer's process". It is necessary to select a synthesis method and set calcincation temperature and time in order to determine such crystal structures. In the present invention, it is also very important by which synthesis method are selected particles are prepared. Two or more kinds of aluminium oxide particles different from each other in crystal structure, which are selected from activated aluminium oxide particles synthesized by calcining an aluminium hydroxide prepared by hydrolysis of an aluminium alkoxide or α-type aluminium oxide particles calcined at a higher temperature, may be preferred from the following reasons. In more detail, a mixing type of -type and other type crystal strucutres is used. As the mixing type, -type and α-type, -type and δ-type, -type and γ-type, -type and η-type, -type and δ-type and η-type, -type and δ-type and α-type, -type and δ-type and η-type, -type and γ-type and α-type, -type and γ-type and η-type, and -type and η-type and α-type can be employed. Among these types, -type and δ-type, -type and γ-type, -type and α-type, -type and δ-type and α-type, and -type and γ-type and α-type are preferred. Furthermore, a mixed system of two or more components, such as -type and δ-type, -type and γ-type, -type and α-type, -type and δ-type and α-type, and -type and γ-type and α-type is preferred. Irrespective of the above, -type aluminium oxide particles are present, and a mixture containing four or more kinds of aluminium oxide particles different from each other in crystal structure is included in the concept of a mixture of two or more kinds of aluminium oxide particles different from each other in crystal structure according to the present invention. In a mixture of two or more kinds of aluminium oxide particles, the mixing ratio of the weight of -type aluminium oxide particles to the total weight of aluminium oxide particles is preferably more than 0.3, more preferably more than 0.5. The primary particle diameter of the above-described aluminium oxide particles is preferably such as to provide a specific surface area not less than 10 m 2 /g, more preferably not less than 50 m 2 /g, and not more than 600 m 2 /g, particularly preferably not more than 200 m 2 /g. The primary particle diameter of the particles is preferably not less than 3nm and not more than 200nm, more preferably not less than 5nm and not more than 100nm. The primary particle diameter of the particles is calculated from the specific surface area, as described later in the item (1) of particle characteristics. A preferred particle shape is not spherical but is of an acicular or a plate-like shape; in other words the shape of a primary particle is not isometric. Such a shape of a particle can be recognised using a transmission type electron microscope. Namely, when a polymer containing particles is cut to a very thin sample along an appropriate direction and the sample is observed using a transmission type electron microscope, then, for example, in the case of spherical particles, all particles are observed as having the same form, but, in the case of, for example, plate-like particles, because the particles are randomly oriented, the observation is carried out under conditions which allow a single particle to be observed from various directions, and as a result, the particles are observed in any one direction as a mixture of particles having a plate-shape, a rod-shape, etc. Therefore, the shape of the particles can be determined by this method. Similarly, in the case of acicular particles, the particles are observed in any one direction as a mixture of particles having an acicular shape and a spherical shape (when observed from the axial direction), and thereby the shape of the particles can be determined. The definition of "acicular" or "plate-like" according to the present invention will now be explained in more detail with reference to Figs. 1A-1D of the drawings. A circle circumscribing a longitudinal extent of a primary particle observed using a transmission type electron microscope is drawn such that its diameter is a minimum, and the diameter of the circumscribing circle is expressed as "1 1 ". A circle inscribed in a lateral extent of the primary particle is drawn such that the diameter is a maximum, and the diameter of the inscribed circle is expressed as "1 2 ". The acicular or plate-like particle according to the present invention means a particle with a ratio of 1 1 /1 2 of not less than 2, preferably not less than 2 and not more than 20, more preferably not less than 2 and not more than 10. Examples of this relationship between 1 1 and 1 2 are illustrated in FIGS. 1A, 1B, 1C and 1D. In the figures, circles 2a, 2b, 2c and 2d circumscribing respective primary particles 1a, 1b, 1c and 1d are depicted such that each diameter is a minimum, circles 3a, 3b, 3c and 3d inscribed within the respective primary particles 1a, 1b, 1c and 1d are depicted such that each diameter is a maximum, and the ratios of the diameters of the circumscribing circles and the inscribed circles are determined as 1 1 /1 2 , respectively. The acicular or plate-like particles used in a composition of the present invention can be selectively obtained by an aforementioned method for calcining an aluminium hydroxide prepared from an aluminium alkoxide by hydrolysis. However, such fine particles are liable to agglomerate, and generally the particles are present, in a slurry or in polymer, as secondary, i.e. agglomerated primary, particles. If such an agglomeration progresses, excessively large particles are formed as the case may be, and when used for films or fibers, the portions including such agglomerated particles are chipped and the chipped portions fall out. This is not desirable. The secondary particle diameter can be freely selected depending upon the purpose of a shaped product made from the thermoplastic polyester composition. When used for films or fibers, if the secondary particle diameter is too large, excessively large protrusions are generated on the shaped product. Therefore, the secondary particle diameter is preferably not more than 1.0µm, more preferably not more than 0.6µm. Furthermore, it is preferably not less than 0.01µm. The secondary particle diameter is defined as follows. When formed as a biaxially stretched film, the interior of the film is observed by a transmission type electron microscope. In the observation, the lengths L (µm) of 500-1,000 secondary particles in a direction parallel to the film surface are measured, and the number average diameter Lav (µm) of the lengths is defined as the secondary particle diameter. Although such a secondary particle diameter has a distribution of particle size in a biaxially stretched film, when the value of L is determined as a number average distribution of particle diameter, the proportion of secondary particles having a diameter of not more than 0.05µm is preferably not less than 20% and not more than 60%. Further, when the lengths is a direction perpendicular to the film surface of such secondary particles is represented as D (µm), the lengths are preferably in the range of the following equation. 0.05≦ D/L ≦ 0.6 The secondary particle preferably contains at least one acicular or plate-like primary particle aforementioned. When the secondary particles are observed using a transmission type electron microscope, the proportion of the agglomerated particles each containing at least one acicular or plate-like primary particle is preferably not less than 50% relative to the total number of agglomerated particles, more preferably not less than 70%. When formed as a film, fine protrusions originating from aluminium oxide particles can be formed on the film surface by such formation and distribution of the secondary particles formed by agglomeraties of primary particles. With respect to the protrusions on the film surface as formed above, the number of protrusions having a height of 10-60 nm is preferably not less than 500,000/mm 2 and not more than 5,000,000/mm, more preferably not less than 700,000/mm 2 and not more than 2,000,000/mm 2 . Such fine protrusions exist between large protrusions formed originating from inert particles additionally present, which are described later, and protect the surface of the film, thereby increasing the scratch resistance in repeated high-speed running. The content of aluminum oxide particles can be selected depending upon the intended purpose of a product formed from the thermoplastic polyester composition. However, because the aluminum oxide particles are necessary to increase the abrasion resistance and scratch resistance in repeated high-speed running, if the content is too small, such advantages cannot be obtained. If the content is too large, excessively large protrusions are present at the surface of a product to be formed. Therefore, the total content of the aluminum oxide particles is preferably in the range of 0.01-10 parts by weight relative to 100 parts by weight of the thermoplastic polyester composition, more preferably in the range of 0.05-1 part by weight. The aluminum oxide particles can be added to the polyester by incorporating them in a polyester reaction system or by blending then in a molten polymer. The former method is preferred from the viewpoint of dispersion of the particles. For addition of aluminum oxide particles having two or more kinds of crystal structure, the respective aluminum oxide particles may be added separately, or they may be added simultaneously. A method for adding two or more kinds of particles simultaneously as a mixture is more preferred from the veiwpoint of dispersion of the particles. Although the timing of the addition of the particles is not particularly restricted, addition before transesterification or from a time before transesterification to a time of the latter half of condensation polymerization is preferred. Furthermore, the particles are preferably added under conditions in which the degree of polymerization of the polyester is such that its intrinsic viscosity is not more than 0.30, determined by dissolving the polyester in o-chloropheol at 25°C. As to the form which the particles take when they are added to the reaction system, the aluminum oxide particles may be added as they are, or alternatively, they may be added in the form of a slurry. The formation of a slurry dispersing the particles in the presence of a solvent is more preferable. When the particles are wet dispersed in the presence of a solvent, because the viscosity increases, the content of the aluminum oxide particles is preferably in the range of 1-50% by weight, more preferably in the range of 3-30% by weight. Although the solvent of such a dispersion is not particularly restricted, it is preferably a glycol component such as a main raw material or a copolymerized component of the polyester in consideration of the fact that the slurry containing the particles is thereafter added to the polyester. More specifically, for example, ethylene glycol, tetramethylene glycol, hexamethylene glycol, diethylene glycol, propylene glycol, butane diol, neopentylglycol, polyoxyalkylene glycol, p-xylylene glycol, 1,4-cyclohexanedimethanolor 5-addiumsulforesordinol, can be used. As the dispersion treatment, the usual agitation, ultrasonic treatment, or a dispersion treatment using a dispersion medium can be employed. Among these treatments, a dispersion treatment using a dispersion medium represented by a treatment such as one using a sand grinder is preferred. In the treatment, a dispersing agent or a surface treatment agent may be used together, as needed. Although there are many kinds of dispersing agent or surface treatment agent which can be used, an agent, which has a good affinity with the polyester when it is added into the polyester, is preferred. In more detail, a compound, which has an acid functional group such as a sulfonic acid, a phosphonic acid or a carboxylic acid or metal salts thereof, and has at least one functional group selected from a carboxylic acid, a carboxylic ester, a hydroxyl group and an epoxy group, is prefered. Compounds which contain such groups and which can be employed are low weight molecular weight compounds such as sulfobenzoic acid, sulfobenzoic ethylene glycol ester or 5-sodium sulfoisophthalic methylester, or polymeric compounds such as polyacrylic acid, poly(acrylic acid/acrylic methylester) copolymer, poly(acrylic acid/methacrylic methylester) copolymer or poly(acrylic acid/methacrylic polyethylene glycol ester) copolymer. However, the compound is not limited to these compounds. The aromatic dicarboxylic acid units in a polyester of a composition according to the present invention can be derived from, for example, terephthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid or 5-sodium sulfoisophthalic acid. The aliphatic glycol units in a polyester of a composition according to the present invention can be derived from, for example ethylene glycol, diethylene glycol, propylene glycol, butane diol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol or polyoxyalkylene glycol. As the thermoplastic polyester comprising the aromatic dicarboxylic acid and the aliphatic glycol, polyethylene terephthalate, polybutylene terephthalate or polyethylene-2,6-naphthalate, can, for example, be used. The polyester may be a homopolyester or may be a copolyester containing a copolymerized component. As the copolymerized component, a dicarboxylic acid component such as adipic acid, sebacic acid, phthalic acid, isophthalic acid and naphthalene-2,6-dicarboxylic acid, 5-sodium sulfoisophthalic acid, polycarboxylic acid such as trimelitic acid and pyromelitic acid, and a diol component such as tetramethylene glycol, hexamethylene glycol, diethylene glycol, propylene glycol, neopentyl glycol, polyoxyalkylene glycol, p-xylylane glycol, 1,4-cyclohexanedimethanol and 5-sodiumsulforesorcinol, can be used. The degree of polymerization of such a thermoplastic polyester composition is preferably such as to provide a limiting viscosity [η] of 0.30, determined by dissolving it in O-chlorophenol. The carboxylic terminal group concentration in the thermoplastic polyester is preferably in the range of 20-100 eq/t, more preferably in the range of 20-70 eq/t. An anti-oxidant may be added as needed. In the polyester composition according to the present invention, inert particles other than the aluminium oxide particles are preferably additionally present, in order to further increase the abrasion resistance and the scratch resistance when formed as a film. The "inert particle" means both an inorganic particle and an organic particle. The primary particle diameter of the inert particles is prefered larger than that of the aluminium oxide particles, and the preferable particle diameter is a diameter of not less than 200nm and not more than 2000nm. As specific examples of the inert particles, inorganic oxide particles such as titanium oxide, silicon oxide and zirconium oxide particles, inorganic carbonate particles such as calcium carbonate and barium carbonate particles, inorganic phosphate particles such as calcium phosphate and sodium phosphate particles, inorganic sulfate particles such as barium sulfate and calcium sulfate particles, inorganic composite oxide particles such as kaoline and talc, fluoride particles represented by fluorite, and ordinary inorganic particles such as potassium titanate and aluminium hydride particles, can be listed. Among these particles, titanium oxide, silicon oxide, calcium carbonate and calcium phosphate particles are particularly preferred. In the inert particles, it is preferred that particles having a diameter of not less than 5µm are not present within the particle diameter distribution, in order to prevent excessively large protrusions from being generated. To obtain the particles having such a particle diameter distribution, it is effective to pulverize agglomerated particles by dispersion treatment such as a treatment using a sand mill, or to remove agglomerated particles and excessively large particles by a classification operation. The size of the particles is preferred to be made uniform at the stage of the synthesis of the particles. More specifically, a method for obtaining calcite-type calcium carbonate particles with a relatively uniform particle diameter by blowing a carbon dioxide gas into an aqueous solution of calcium hydride and removing excessively large particles remaining in a small amount by dispersion and classification, or a method for obtaining vaterite-type calcium carbonate particles by blowing a carbon dioxide gas into an alcohol solution of calcium hydride and removing excessively large particles remaining in a very small amount by filtration, can be employed. As the examples of the inert organic particles, internal particles due to catalyst residue at the time of synthesis of polyester, low molecular weight organic compound particles such as calcium terephthalate, barium terephtalate and sodium benzoate particles, carbon black, and organic polymeric particles such polytetrafluoroethane resin, polystyrene resin, silicone resin and crosslinked polymeric resin particles, can be listed. Among these particles, internal particles and crosslinked polymeric resin particles are preferred, and in particular, organic polymeric resin particles are preferred. More specifically, a copolymer composed of a monovinyl compound (X)having a single aliphatic unsaturated bond in a molecule and a compound (Y) having two or more aliphatic unsaturated bonds in a molecule as a crosslinking agent, or a compound composed of only the latter crosslinking agent (Y) can be employed. As the examples of the compound (X), an aromatic monovinyl compound such as styrene, α-methylstyrene, fluorostyrene or vinyl pyridine, a vinyl cyanide compound such as acrylonitrile or methacrylonitrile, an acrylate ester monomer such as butylacrylate, 2-ethylhexylacrylate, methylacrylate, 2-hydroxyethylacrylate, glycidylacrylate or N,N'-dimethylaminoethylacrylate, a methacrylate ester monomer such as butylmethacrylate, 2-ethylhexylmethacrylate, methylmethacrylate, 2-hydroxyethylmethacrylate, glycidylmethacrylate or N,N'-dimethylaminoethylmethacrylate, a monomer or dicarboxylic acid and an acid anhydride of a dicarboxylic acid such as acrylic acid, methacrylic acid, maleic acid, or itaconic acid, or an amide monomer such as acrylic amide or methacrylic amide can be listed. Among these compounds, styrene, α-methylstyrene and p-methylstyrene are preferred. As examples of the compound (Y), divinylbenzene, or trimethylolpropanetriacrylate or trimethylolpropanetrimethacrylate, or a polyfunctional acrylate or methacrylate such as polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate trimethylolpropanetriacrylate or trimethylolpropanetrimethacrylate can be listed. Among the above-described compounds, in particular divinylbenzene, butylacrylate, ethylene glycol dimethacrylate or trimethylolpropanetrimethacrylate is preferably used. Two or more compound (X) and (Y) may be employed as a mixture. As examples of the preferable composition for the organic polymeric particles, crosslinked polymeric particles using only divinylbenzene, and crosslinked polymeric particles using styrene-divinylbenzene copolymer, styrene-acrylonitrile-divinylbenzene copolymer, styrene-methylmethacrylate-divinylbenzene copolymer, or butylacrylate-divinylbenzene copolymer can be employed. Among these compound, crosslinked polymeric particles using divinylbenzene, styrene-divinylbenzene copolymer or butylacrylate-divinylbenzene copolymer are preferred from the viewpoint of thermal resistance. The thermal resistance of the crosslinked polymeric particles is preferably not lower than 380°C when determined by the thermal decomposition temperature measured by a thermobalance (10% loss temperature), more preferably not lower than 400°C, and particularly preferably not lower than 410°C. Particles having such a high thermal resistance do not agglomerate when the polyester composition is produced or rendered molten and shaped or when a shaped product is recycled so that the uniformity of the surface of the product and the abrasion resistance thereof do not then deteriorate, and therefor such a condition is preferable. In order to satisfy such a thermal resistance, a high crosslinkage must be achieved by a crosslinking agent of the compound (Y). Although the kind of the crosslinking agent is not particularly limited, divinylbenzene is preferred. The content of the divinylbenzene must not be less than 12% by weight as a pure divinylbenezene relative to the entire monomer component constituting the crosslinked polymeric particles, preferably not less than 35% by weight, more preferably not less than 55% weight. The crosslinked polymeric particles preferably have a spherical shape and a uniform distribution in particle size from the viewpoints of good slipping property, uniformity of the surface and transparency. Namely, the volume shape factor is preferably in the range of 0.35-0.52, and further more preferably not less than 0.45. The volume shape factor is defined as follows. Very thin samples having a thickness of about 0.3µm are prepared from a biaxially oriented film made using a polymer composition according to the present invention on the respective cross sections in the longitudinal and transverse directions, using an apparatus for making a very thin film. Then, the respective samples are observed using a transmission type electron microscope and number average diameter of the observed particles in the longitudinal, transverse and thickness directions are determined (the respective diameters are indicated as "a", "b" and "c"). The volume of the particle (V) is defined by the following equation (2) and the maximum diameter of the particle in a plane of projection (D) is defined by the following equation (3). V= (4/3 π) . a b c D= max (a, b, c) The volume shape factor "f" is calculated from the values of "V" and "D" determined from the equations (2) and (3) by the following equation (1). f=V/D 3 Crosslinked polymeric particles made by known processes can be used. As the known processes, for example, there are the following processes involving emulsion polymerization: (1) a soap free polymerization process: i.e., process for polymerizing without an emulsifier or using a very small amount of an emulsifier; (2) a seed polymerization process for adding polymer particles in a polymerization system prior to emulsion polymerization and thereafter emulsion polymerizing; (3) a core shell polymerization process for emulsion polymerizing a part of a monomer component and polymerizing the residual monomer in the polymerization system; (4) a polymerization process carried out by the "Ugelstad" method disclosed in JP-A-SHO 54-97582 and JP-A-SHO 54-126288; and (5) a polymerization process carried out as in (4) above but without using an expanding agent. In the above processes, particularly the processes (3) and (4) are preferred because spherical crosslinked polymeric particles having a uniform particle diameter distribution can be obtained by these processes. The crosslinked polymeric particles preferably have a functional group from the viewpoint of affinity with the polyester. Although such a functional group is not particularly restricted, for example, a carboxylic group, a hydroxyl group, a sulfonic group, ester group, and a metal salt of a carboxylic group can be employed. Among these groups, a carboxylic group, a metal salt of a carboxylic group and hydroxyl group are preferred, and a metal salt of a carboxylic group is particularly preferred. Although the method for introducing the functional group is not particularly limited, a method for firstly producing particles as the matrix for the high crosslinkage and thereafter introducing the functional group to the surface of the matrix particles is preferred from the viewpoint of the thermal resistance of the particles. For example, in the case of introducing a sodium salt of a carboxylic group, when using styrene-divinylbenzene copolymer as the matrix particles, particles with a high crosslinkage provided by the divinylbenzene are produced, and thereafter, a carboxylic group is introduced into the surface of the particles using a methacrylic acid. A functional -COONa group is then introduced by rendering the particle production system alkaline. As the process for producing the composition containing both the aluminium oxide particles and the inert particles aforementioned, a process for adding both particles to a polyester reaction system may be employed, and a process for preparing master chips in which each of the particles is added to each polymer, blending the master chips and melting and kneading the mixture may be employed. As to the method and timing of addition of the inert particles, a known method, for example adding the particles to a reaction system for a polyester in the form of a powder or glycol slurry or kneading the particles into the polyester in the form of a powder or a slurry using a low boiling point solvent can be employed. In the case of inorganic inert particles, addition of the particles to a reaction system for polyester in the form of a glycol slurry used in the reaction system is preferred, and in the case of organic inert particles, kneading the particles into the polyester in the form of a slurry using a solvent having a low boiling point such as water or in the form of a powder is preferred. During the addition of the particles, it is further preferred to use a vent-type forming apparatus for deaeration. The addition of organic inert particles, for example, crosslinked polymeric particles, will now be explained in more detail. a method for adding a slurry of crosslinked polymeric particles with water and/or organic compound having a boiling point of not higher than 200°C to polyester in a vent-type shaping apparatus, removing the water and/or the organic compound with a boiling point of not higher than 200°C under heating and reduced pressure conditions, and melt kneading is preferred because the particles can be dispersed in a more uniform monodispersion condition. The vent-type shaping apparatus is a shaping apparatus having at least one vent hole, and for example, it may be an extrusion shaping apparatus or an injection molding apparatus. At least one vent hole for removing the water and/or the organic compound with a boiling point of not higher than 200°C is preferably maintained at a reduced pressure condition. The reduced pressure at the vent hole is preferably maintained at a pressure of not more than 100 Torr, more preferably not more than 50 Torr, further more preferably not more than 30 Torr. On the other hand, the crosslinked polymeric particles are preferably added to polyester in a form of a slurry of water and/or organic compound having a boiling point or not higher than 200°C, an alcohol such as methanol, ethanol or ethylene glycol, a hydrocarbon compound such as benzene or toluene, an ester, a ketone or an amine can be employed. From these compounds, water is preferred from the viewpoint of handling ability and removability. Of course, water and/or an organic compound can be present in the form of a mixture of two or more kinds of solvent, and in such a case, a water-rich solvent is preferred. In the slurry of crosslinked polymeric particles, an anionic interfacial active agent such as sodium dodecylbenzene sulfonate or sodium lauryl sulfate, a nonionic interfacial active agent such as polyoxyethylenenonilphenylether or polyethylene glycol monostearate, or a protective agent such as polyvinyl alcohol or carboxylmethylcellulose is preferably present from the viewpoint of particle dispersion. Further, although the concentration of the slurry of crosslinked polymeric particles with water and/or organic compound having a boiling point of not higher than 200°C is not particularly restricted, the content of the water and/or the organic compound having a boiling point or not higher than 200°C relative to the polymer is preferably in the range of not less than 2% by weight and not more than 30% by weight, and more preferably in the range of not less than 2% by weight and not more than 20% by weight. Such a condition is preferred because the dispersion of the particles in the polymer is good and the limiting viscosity of the polymer does not decrease. Furthermore, two or more kinds of inert particles different from each other may be used together. The total content of the inert particles in the polyester is preferably in the range of 0.001-20% by weight, more preferably in the range of 0.005-10% by weight, and still more preferably in the range of 0.01-5% by weight, from the viewpoint of balance of slipping property, abrasion resistance and scratch resistance when formed as a film. In the reaction system where internal particles are precipitated by the catalyst residue of a metal or a phosphorus compound at the time of synthesis of polyester, the addition of the aluminium oxide particles to provide a composition according to the present invention enables the formation of aggregate particles having a composite concentration. The master polymer thus obtained and containing the composite particles may be blended again with a master polymer containing other inert particles and melt kneaded. In the thermoplastic polyester composition obtained in the present invention, the resistivity of the polyester when in a molten condition is preferably in the range of 5 x 10 6 to 5 x 10 9 Ω · cm because the property for electrostatic charge casting is improved. The resistivity of the polyester when in a molten condition is determined as follows. The resistivity is measured using a melt resistivity measuring apparatus shown in FIG. 2. Polyester 8 the resistivity of which is to be measured is placed in a container into which a pair of electrodes 9 are inserted. The container is dipped in a heating body 7. The polyester 8 is rendered molten and stored in a N 2 gas atmosphere at a temperature of 280°C, and a voltage is applied from a D.C. high voltage generation apparatus 4. The resistivity is determined from the indicated values of an ammeter 5 and a voltmeter 6, the area of the electrode and the distance between the electrodes by the following equation. P = (V x S)/(I x D) P: melt resistivity (Ω · cm) V: applied voltage (V) S: area of electrode (cm 2 ) I: measured current (a) D: distance between electrodes (cm) The resistivity of the polyester in a molten condition depends upon the kind, existence and amount of the metal compound added in the production of the polyester. Generally, the greater is the amount of the metal in the molten polyester which is not deactivated by a phosphorus compound, or the smaller is the amount of the metal compound which has precipitated in the polyester as particles, the lower tends to be the melt resistivity. Next, the relationship between the melt resistivity and the metal will be explained with reference to the case of polyethylene terephtalate(PET) obtained by transesterification. In transesterification, usually, an alkali compound, alkaline earth metal compound, zinc compound or manganese compound is used as a reaction catalyst, and a phosphorus compound is added at a stage at which the reaction has substantially finished. With this procedure, the metal compound used as the reaction catalyst is partially deactivated by the phosphorus compound. The reaction product thus obtained is condensation polymerized in the presence of an antimony compound, titanium compound or germanium compound which is a catalyst for condensation polymerization to obtain PET. Usually, PET is produced by such a process, and the alkali metal compound, alkaline earth metal compound, zinc compound, manganese compound and phosphorus compound mainly affect the melt resistivity. These metal compounds or phosphorus compound tends to cause the generation of the particles which are insoluble in the polyester. As the metal compounds controlling the melt resistivity of polyester in the present invention, an aliphatic carboxylate of an alkaline earth metal compound, zinc, manganese or alkali metal compound which is added during production of the polyester, or a metal compound which is soluble in glycol such as halide or alcoholate, for example, a methylate, ethylate or ethylene glycolate, can be employed. More specifically, manganese acetate, magnesium acetate, calcium acetate, zinc acetate, lithium acetate, potassium acetate, sodium acetate, magnesium propionate, manganese propionate, potassium propionate, calcium propionate, zinc propionate, magnesium chloride, lithium chloride, manganese bromide, magnesium hydroxide, manganese hydroxide, calcium hydroxide, zinc hydroxide, lithium hydroxide, magnesium glycolate, calcium glycolate, lithium methylate or butylpotassium can be employed. Two or more kinds of these compounds may be together employed. A manganese compound and alkali metal compound are particularly preferable for suppressing the precipitation of the particles. In the transesterification, these metal compounds can be added in an amount dependent upon the amount of the catalyst before the transesterification and can be added again after the transesterification. In the esterification, it is preferred that the compounds are added after the extent of the esterification reaction reaches preferably not less than 90%, more preferably not less than 95%. As the phosphorus compound, at least one selected from phosphates, phosphites and esters thereof can be employed. More specifically, a phosphate, monomethylphospate, dimethylphospate, trimethylphosphate, tributylphosphate, phosphite, trimethylphosphite or tributylphosphite can be employed. The polyester composition is usually extruded in the form of a sheet and cooled on a casting drum to form an amphorus sheet, after which the sheet is stretched and heat set to produce a biaxially stretched film. If the resistivity of the polyester in a molten condition is in the range of 5 x 10 6 to 5 x 10 Ω · cm, the contact property between the sheet and the casting drum can be improved by providing an electrode between a die for extrusion and the casting drum for cooling and applying a high voltage to the sheet, thereby increasing the productivity. A plurality of kinds of polymer may be blended in a range which does not impair the desired characteristics of a shaped product of the present invention, and an organic additive such as oxidation inhibitor, thermal stabilizer, lubricant or ultraviolet absorbent may be added in a volume generally employed. When a film is formed using the thermoplastic polyester according to the present invention, a composite film is preferred, particularly in the case of a film for a magnetic tape, because fine protrusions with a height of 10-60nm can be efficiently formed on a film surface by the aforementioned specified aluminium oxide particles and also having regard to the electromagnetic conversion property of the tape. The "composite film" referred to above means a film having at least two layers in the thickness direction, and includes a film having three or more layers. Furthermore, at least one outermost layer preferably comprises the thermoplastic polyester composition according to the present invention. If the lamination thickness is too small, the aluminium oxide particles approach too closely to the surface and the particles are likely to fall of, and if the lamination thickness is too large, because the amount of the particles is too small near the surface, it is not effective to improve the abrasion resistance in repeated high-speed running. Therefore, when the thickness of the laminated film is referred to as "t", the thickness and the primary particle diameter "D" of the aluminium oxide particles preferably satisfy the following relationship: 0.1D ≦ t ≦ 100D When using two or more kinds of particle, the mean primary particle diameter of the total number of the particles is referred to as "D", and the "D" and the lamination thickness "t" preferably satisfy the above relationship. A process for producing such a laminated film will now be explained in more detail. First, after polyester pellets are blended at a predetermined rate and dried, the pellets are supplied to extruders for melting the pellets and laminating the molten polymers, the polymers are delivered out, from a die with a slit, in the form of a sheet, and the sheet is cooled and solidified on a casting roll to obtain a non-stretched film. Namely, the polymers are laminated using two or more extruders and a manifold or a feed block for two or more layers (for example, a feed block having a rectangular laminating portion), a sheet having two or more layers is delivered out from a die, and the sheet is cooled to obtain a non-stretched film. In this process, it is effective to provide a static mixer and a gear pump in the polymer path. Furthermore, it is effective to control the melting temperature of the extruder for extruding the polymer of the outermost layer to a temperature lower than that for the base layer by 5-10°C. Next, the non-stretched film is biaxially stretched and biaxially orientated. As the stretching process, a sequential biaxially stretching process or a simultaneous biaxially stretching process can be used. However, a sequential biaxially stretching process for firstly stretching in the longitudinal direction, thereafter stretching in the transverse direction, dividing the longitudinal stretching into three or more stages, and controlling the total longitudinal draw ratio within the range of 3.5 to 6.5 times, is particularly preferred. Although the temperature for the longitudinal stretching varies depending on the kind of polyester and therefore cannot be referred to as a specified temperature, usually it is effective to control the temperature of the first stage in the range of 50 to 130 °C and the temperature of the succeeding stages at a temperature higher than the temperature of the first stage. The stretching speed in the longitudinal direction is preferably in the range of 5000 to 50000 %/min. Generally, a process for using a stenter is employed for the transverse stretching. The draw ratio is preferably in the range of 3.0 to 5.0 times. The stretching speed in the transverse direction is preferably in the range of 1000 to 20000 %/min, and the temperature thereof is preferably in the range of 80 to 160°C. Next, the stretched film is heat treated. The temperature of the heat treatment is preferably in the range of 170 to 220°C, particularly in the range of 180 to 200 °C, and the time is preferably in the range of 0.2 to 20 seconds. Embodiments of the invention will now be described in more detail with reference to the accompanying drawings and the following Examples. In the drawings: Figs. 1A, 1B, 1C and 1D are views showing particles on which are depicted respective circumscribing circles and inscribed circles of respective primary particles, from which the ratios of their diameters are measured. Fig. 2 is a schematic view of an apparatus for measuring the specific resistance of a polyester composition used in the present invention when in a molten state. The reference numerals in the drawings represent the following: 1a, 1b, 1c, 1d: primary particle 2a, 2b, 2c, 2d: circumscribing circle 3a, 3b, 3c, 3d: inscribed circle 4: DC high voltage generator 5: ammeter 6: voltmeter 7: heater 8: polyester 9: electrode Examples The properties of the obtained polyester composition are determined by the following methods: A. Properties of particles: (1) Specific surface area (S) and Primary particle diameter of aluminum oxide particle: The specific surface area S (m 2 /g) is determined by the usual B.E.T method. The primary particle diameter "d" ( µm) is determined by the following equation. When using two or more kinds of aluminum oxide particle having specific surface areas different from each other, the weighted mean of the respective primary particle diameters determined by the following equation is defined as the primary particle diameter. d = 6/ (ρ . S) where, S is the specific surface area (m 2 /g), and ρ is the true specific gravity (g/cm 2 ) of the particle (2) Secondary particle diameter (Lav) of aluminum oxide particles: Aluminum oxide particles are present within a polyester, and after the polyester is formed as a biaxially stretched film, a very thin sample is cut from the film in a direction perpendicular to the thickness direction of the film (0.2 µm in the thickness direction), and the sample is observed by a transmission type electron microscope. The magnification is set to 40,000 times, the secondary particles, 500-1000 in number, are observed, and the length L (µm) in a direction parallel to the film surface and the length D (µm) in a direction perpendicular to the film surface, respectively, are measured. The secondary particle diameter is referred to as "L" and the secondary particle average diameter is a number average diameter referred to as "Lav", which is a number average diameter of the values of the "L". The proportion, by number of particles for which "L" is not more than 0.5µm, is also determined. (3) Crystal structure of aluminum oxide particle: Aluminum oxide particles are determined by X-ray diffraction. To identify the crystal structure of the aluminum oxide particles in a polymer or a shaped body, the aluminum oxide particles taken out from the polymer by dissolving the polymer with a solvent and separating the particles by centrifugation or filtration may be determined in a similar manner. Because the determination by X-ray diffraction is difficult if the polymer adheres to the extracted particles, it is important to repeatedly wash the particles extracted from the polymer with an appropriate solvent capable of dissolving the polymer well and remove the polymer adhered around the particles. As the values of "2" in various crystal structures, the values shown in the following table are used according to the registered numbers in JCPDS (Joint Committee on Powder Diffraction Standards). Crystal structure JCPDS No. α 10-173 γ 10-425, 29-63 δ 4-877, 16-394 ε 21-10 η 4-875  11-517, 23-1009 κ 4-878 χ 4-880, 13-373 For a mixture of a plurality of crystal structures, firstly, the kinds of the mixed crystal structures are determined by X-ray diffraction, and then, the X-ray diffraction is repeatedly carried out by changing the mixing ratio of the particles with the respective crystal structures to respective specified ratios to prepare respective standards of mixtures, and the mixing ratio of the particles to be measured is determined by comparing with the prepared standards. (4) Shape of aluminum oxide particle and Proportion of acicular or plate-like particles: A circle circumscribing a primary particle in a polymer or a shaped body observed using a transmission type electron microscope is drawn such that the diameter in a longitudinal direction is a minimum, and the diameter of the circumscribing circle is expressed as " 1 1 ". A circle inscribed within the primary particle is drawn such that the diameter in a lateral direction is a maximum, and the diameter of the inscribing circle is expressed as "1 2 ". The ratio of 1 1 /1 2 is determined. When secondary agglomerated particles are observed by a transmission type electron microscope, the proportion of the number of the agglomerated particles each containing at least one particle with the above ratio of 1 1 /1 2 of not less than 2 and not more than 20 (acicular or plate-like particle) as a primary particle constituting the agglomerated particle, relative to the total number of agglomerated particles, is determined. The values 1 1 /1 2 in the Examples mean the average values of such determined ratios of 1 1 /1 2 . (5) Content of aluminum oxide particles: After polymer, film or fibers containing aluminum oxide particles are melt reformed, Al element is determined by the usual X-ray fluorescence analysis, and converted to the weight of Al 2 O 3 . B. Properties of polymer: (1) Intrinsic viscosity (IV): The intrinsic viscosity is determined in o-chlorphenol at a temperature of 25°C and a concentration of 1% by weight, using an Ostwald viscometer. (2) Softening point: The softening point is determined by a penetrameter, and represented as a value of SP (°C). In the measurement, a polymer is placed in a heated bath and heated at 6°C/5 min., and the softening point is determined as a temperature at the time when a tip of a load enters into the polymer. (3) Concentration of carboxylic terminal group: This is determined by Maurice's method. A polymer of (2g) is dissolved into 50ml of o-cresol/chloroform (weight ratio: 7/3), and titrated with N/20 NaOH methanol solution, and the concentration of carboxylic terminal group is determined and represented as a value of equivalent weight/polyester of 10 6 g. (4) Thermostability: A polyester rendered is molten in a N 2 gas atmosphere at 300°C, the intrinsic viscosities at the time of 8 minutes and 68 minutes after the starting of the melting are measured, and the thermostability is represented as the different Δ IV. The smaller is the Δ IV, the better is the thermostability. (5) Casting property with electrostatic charge: A D.C. voltage (6 kV) is applied to the film melt extruded using an electrode provided between a die and a casting drum, and when the speed of the casting is gradually increased, the casting speed (m/min.) causing an application irregularly is determined. Accordingly, the faster is this speed, the better is the productivity. C. Properties of film: (1) Surface roughness Ra (µm): The surface roughness is measured based upon JIS-B-0601, and a center-line mean roughness, measured using a Surfcom surface roughness meter using a probe having a diameter of 2µm, a load on the probe of 70mg, a measuring length of 0.25mm and a cut off of 0.08mm, is employed as the surface roughness Ra. The center-line mean roughness is determined by the following equation when a portion with a measuring length "h" is taken out from a roughness curve in a direction along the centre line of the roughness curve, the center line of the taken-out portion is defined as an X-axis, the direction perpendicular to the X-axis is referred to as a Y-axis, and the surface roughness curve is represented by an equation y=f(x). (2) Slipping property; A film is slit to prepare a tape with a width of 1/2 inch. The tape is set in a tape running tester (TBT-300; produced by Yokohama System Kenkvusvo Corporation) and is run around a guide under conditions in which the temperature is 20°C, the relative humidity(RH) is 60%RH, the diameter of the guide is 6 mmØ, the material of the guide is SUS27 (surface roughness: 0.2S), the winding angle is 180 degrees and the running speed is 3.3 cm/sec, and the initial friction coefficient µκ is determined by the equation; µ κ = 0.733 log (T 1 /T 2 ) where T 1 : exit side tension T 2 : entrance side tension The film having a µ κ of not more than 0.35 is good in slipping property. If µ κ is greater than 0.35, the slipping property when the film is processed or a product is made from the film deteriorates considerably. (3) Number of protrusions with a height of 10-60nm on a skin surface of a film: A film is placed on a sample table, and a platinum is deposited on the film surface. The platinum-deposited film is set at an angle of 82.5 degrees, ten fields of the film surface are observed using a scanning type electron microscope at a magnification of 10,000-30,000 times, and the number with a height of 10-60nm on the surface is calculated. (4) Abrasion resistance 1 ○ : A film is slit to prepare a tape with a width of 1/2 inch. The tape is set in a tape running tester (TBT-300; produced by Yokohama System Kenkyusyo Corporation) and is run repeatedly 1000 times under conditions of 20°C and 60%RH, and thereafter, the abrasion resistance 1 ○ is ranked by the amount of chipped white powder generated on the surface of a guide. The diameter of the guide is 8mm , the material of the guide is SUS27 (surface roughness: 0.2S), the winding angle is 180 degrees and the running speed is 3.3 cm/sec. The stands of ranking are as follows. Rank 1: The amount of the generated white powder is extremely small. Rank 2: The amount of the generated white powder is small. Rank 3: The amount of the white powder is fairly large. Rank 4. The amount of the generated white powder is very large. The smaller is the amount of the generated white powder, the better is the abrasion resistance. (5) Abrasion resistance 2 ○: A tape applied with a magnetic layer is calendering treated at a temperature of 70°C and a line pressure of 200 kg/cm using a small-sized test calendering apparatus (five stage type having steel rolls and nylon rolls, the nylon rolls come into contact with the base film surface). After the treatment is carried out over a length of 10000m, the white powder generated by the treatment and adhered to the nylon rolls is observed, the abrasion resistance 2 ○ is ranked as follows. The smaller is the amount of the generated white powder, the better is the abrasion resistance. Rank 1: There is almost no adherence of white powder. Rank 2: The amount of adhered white powder is small. Rank 3: The amount of adhered white powder is fairly large. Rank 4: The amount of adhered white powder is very large, and it causes problems during processing and adversely affects the properties of products, and the film cannot be used. (6) Scratch resistance (in repeated high-speed running): A film is slit to prepare a tape with a width of 1/2 inch. The tape is set in a tape running tester and is run on a guide pin SUS304 with a surface roughness Rt of 0.2µm and a diameter of 8mm (running speed: 250 m/min., running length: 100m, running times 10 paths, winding angle: 60 degrees). At that time, the scratches generated on the film are observed in the width direction of the film by a microscope for ten appropriate fields, and the number of the scratches per one width is determined and ranked as follows: Rank A: The number of the scratches is in the range of 0-3. Rank B: The number of the scratches is in the range of 4-10. Rank C: The number of the scratches is not less than 11. The fewer are the scratches, the better is the scratch resistance. (7) Electromagnetic conversion property: A magnetic coating solution with the following composition is coated on the film using a gravure roll and the coated magnetic layer is magnetically orientated and dried. The coated film is calendered by a small-sized test calendering apparatus (steel roll/nylon roll; 5 stages) at a temperature of 70°C and a line pressure of 200 kg/cm, then cured at a temperature of 70°C for 48 hours. The film is slit to a tape with a width of 1/2 inch and a pancake is made from the tape. The tape is incorporated into a VTR cassette from the pancake by a length of 250m to make a VTR cassette tape. (Composition of magnetic coating solution)(All parts are by weight). Iron oxide : 100 parts Vinyl chloride/vinyl acetate copolymer : 10 parts Polyurethane elastomer : 10 parts Polyisocyanate : 5 parts Lecitin : 1 part Methylethylketone : 75 parts Methylisobutylketon : 75 parts Toluene : 75 parts Carbon black : 2 parts Lauric acid : 1.5 parts 100% chromatic signal generated by a television testing wave generator is recorded on the above tape using a domestic VTR, and chromatic S/N is determined from the regenerated signal using a colour video noise measuring apparatus. In the following Examples, Examples 1-9 are Reference Examples showing compositions disclosed in our corresponding EP-A-0577846 and retained by way of explanation. Example 1 (Table 1): Aluminium oxide particles (a) were prepared by calcining an aluminium hydroxide (boehmite) obtained by hydrolysis of an aluminium isopropylalkoxide. The specific surface area of the aluminium oxide particles (a) was 84m 2 /g, and when the crystal structure thereof was analyzed by X-ray diffraction, the main structure was -type. The -type aluminium oxide particles (a) (10 parts by weight) and ethylene glycol (90 parts by weight) were mixed to prepare a slurry, and the mixture was treated for dispersion using a sand grinder. Then, the slurry was filtered using a filter of glass fibers with an absolute cut size of 3 µm, and ethylene glycol slurry (b) of -type aluminium oxide particles (100 parts by weight) was prepared. The "50% equivalent diameter in conversion to sphere" of agglomerated particles was 0.24µm. Separately, dimethyl terephthalate (100 parts by weight) and ethylene glycol (64 parts by weight), to which magnesium acetate (0.05 part by weight) was added as a catalyst, were transesterified by a regular method. Thereafter, the previously prepared slurry (b), in an amount of 20 parts by weight, was added to it at 230°C, and the ethylene glycol was distilled for ten minutes maintaining the temperature at 230°C. Antimony oxide (0.03 by weight) as a catalyst and trimethylphosphate (0.03 part by weight) as a thermal stabilizer were then added, pressure reduction was started after ten minutes from the addition, and a condensation polymerization was carried out by a regular method. The temperature at the end of the condensation polymerization was 278°C, and the pressure was 0.1 mmHg. The intrinsic viscosity of the polyethylene terephthalate composition (c) thus obtained was 0.618, the Δ IV was 0.016, the softening point was 259.8°C, and the concentration of carboxylic terminal group was 38 eq/10 6 g. Next, calcite-type calcium carbonate particles (15 parts by weight), ethylene glycol (83 parts by weight), and poly(acrylic acid/acrylic methylester) copolymer (molecular weight: 4600), in an amount of 2 parts by weight, as a surface treatment agent for the calcite-type calcium carbonate particles, were mixed. The mixture was then dispersed and surface treated using a sand grinder, then classified in a wet system, and thereafter, the slurry was filtered by a polypropylene filter with an absolute cut size of 5 µm. After the filtration, the concentration of the calcium carbonate particles in the ethylene glycol slurry (d) of calcium carbonate particles was 10% by weight, and the "50% equivalent diameter in conversion to sphere" of the particles was 0.51 µm. Separately, dimethyl terephthalate (100 parts by weight) and ethylene glycol (64 parts by weight), to which magnesium acetate (0.05 part by weight) was added as a catalyst, were transesterified by a regular method. Thereafter the previously prepared slurry (d) in an amount of 20 parts by weight was added to it at 230°C, and the ethylene glycol was distilled for ten minutes maintaining the temperature at 230°C. Antimony oxide (0.03 part by weight) as a catalyst and trimethylphosphate (0.03 part by weight) as a thermal stabilizer were then added, pressure reduction was started after ten minutes from the addition, and a condensation polymerization was carried out by a regular method. The temperature at the end of the condensation polymerization was 278°C, and the pressure was 0.1 mmHg. The intrinsic viscosity of the polyethylene terephthalate composition (e) thus obtained was 0.616, the Δ IV was 0.014, the softening point was 260.2°C, and the concentration of carboxylic terminal group was 36 eq/10 6 g. Next, polyethylene terephthalate (f) with an intrinsic viscosity of 0.615, a Δ IV of 0.010, a softening point of 260.1°C and a concentration of carboxylic terminal group of 26 eq/10 6 g, which did not contain particles, was used, and the previously prepared polyethylene terephthalate composition (c), (e) and the polyethylene terephthalate (f) were blended to form pellets such that the mixing ratio thereof was 15/15/70 in weight ratio. The mixture was vacuum dried at a temperature of 140°C for three hours, then melt kneaded using a twin-screw extruder at a temperature of 285°C, the molten polymer was delivered out from a die with slit, and the delivered polymer was cooled on a casting drum to form an amorphous polyethylene terephthalate composition. Thereafter, the composition was stretched at a draw ratio of 3.5 times in longitudinal and transverse directions, respectively, and then it was heat treated at a temperature of 220°C for five seconds to form a biaxially stretched polyethylene terephthalate flim with a thickness of 14 µm. When the state of the particles present in the film was observed using an electron microscope as aforementioned, the shape of the primary particles of -type aluminium oxide particles (a) was I 1 /I 2 =5.2, the diameter of the agglomerated, i.e. secondary, particles was Lav=0.18 µm (D=0.04 µm), and the proportion of the number of the agglomerated particles each containing at least one acicular or plate-like particle relative to the total number of agglomerated particles was 75%. Furthermore, the proportion of the particles of L ≦ 0.05 µm was 53%. When the state of the film surface was observed in a manner as aforementioned, the surface roughness Ra was 0.020 µm, and very fine protrusions with a height of not less than 10nm and not more than 60nm originating from -type aluminium oxide particles (a) were observed to 1,300,000/mm 2 in number. When the properties of the film were estimated, the running ability µ κ was 0.18, the abrasion resistance 1 ○ was rank 1, the abrasion resistance 2 ○ was rank 1, and particularly the scratch resistance in repeated high-speed running was rank A, and thus, the properties of the film were excellent. Examples 2-4 (Table 1): Example 1 was repeated except that the proportion of particles of aluminum oxide having an acicular or plate-like shape was changed by changing the calcination conditions during the synthesis of the aluminum oxide from those adopted when preparing particles (a), and the kind and particle diameter of the inert particles were changed. Other conditions were the same as those of Example 1. Thus, biaxially stretched polyethylene terephthalate films having a thickness of 14 µm were formed. The properties of the films obtained were excellent as shown in Table 1. Example 5 (Table 2): Aluminum oxide particles (g) were made by calcining aluminum dorsonite synthesized by neutralization and synthesis of an alum with a carbonate. The specific surface area of the aluminum oxide particles (g) was 99 m 2 /g, and when the crystal structure of the particles was analyzed by X-ray diffraction, it was of the -type. A biaxially stretched polyethylene terephthalate film having a thickness of 14µm was formed under the same conditions as those of Example 1, except for the use of the above aluminum oxide particles and change in the kind and particle diameter of the inert particles. The properties of the film obtained were good as shown in Table 2. Example 6 (Table 2): A biaxially stretched polyethylene terephthalate film having a thickness of 14 µm was formed under the same conditions as those of Example 5, except for a change in the specific surface area of the above aluminum oxide particles (g) and in the kind and particle diameter of the inert particles. The properties of the film obtained were good as shown in Table 2. Examples 7-10 (Table 3): Example 1 was repeated except that the proportion of particles of aluminum oxide having an acicular or plate-like shape (and in the case of Example 10 the kind of crystal structure) was changed by changing the calcination conditions during the synthesis of the aluminum oxide from those adopted when preparing particles (a), and the kind and particle diameter of the inert particles were changed. Other conditions were the same as those of Example 1. Thus, biaxially stretched polyethylene terephthalate films having a thickness of 14 µm were formed. The properties of the films obtained were good as shown in Table 3. Example 11 (Table 4): Aluminum oxide particles (h) with a specific surface area of 83 m 2 /g and 100%  -type crystal structure in X-ray diffraction and aluminum oxide particles (i) with a specific surface area of 24 m 2 /g and 100% α -type crystal structure in x-ray diffraction were prepared in a manner similar to that of Example 1. The aluminum oxide particles (h) in an amount of 9.5 parts by weight, the aluminum oxide particles (i) in an amount of 0.5 part by weight, and ethylene glycol in an amount of 90 parts by weight were mixed to form a slurry, and the slurry was dispersion treated using a sand grinder. Thereafter, the slurry was filtered in a manner similar to that of Example 1, and ethylene glycol slurry (j) of  / α mixture-type aluminium oxide particulars of (100 parts by weight) was prepared. The "50% equivalent diameter in conversion to sphere" of agglomerated particles was 0.24 µm. Polyethylene terephthalate composition (k) was prepared in a manner similar to that of Example 1 except for addition of the slurry (j) instead of slurry (b). The intrinsic viscosity of the polyethylene terephthalate composition (k) thus obtained was 0.617, the Δ IV was 0.016, the softening point was 259.9°C, and the concentration of carboxylic terminal group was 36 eq/10 6 g. Next, non-dried polyethylene terephthalate pellets with a water content of 0.4% by weight and an intrinsic viscosity of 0.621 were rendered molten using a vent-type twin screw extruder, and a water slurry of polydivinylbenzene particles [divinylbenzene/ethylvinylbenzen(weight ratio: 80/20) copolymer] having sodium carboxylate as a functional group and having a mean particle diameter of 0.43 µm was added to the polymer so that the final content of the particles was 2.0% by weight. The vent port was maintained to be 10 torr in vacuum degree, and the polymer was melt extruded at a temperature of 280°C to obtain a polyethylene terephthlate composition (1) containing polydivinylbenzene particles. The intrinsic viscosity of the polyethylene terephthalate composition (1) thus obtained was 0.616, the Δ IV was 0.013, the softening point was 260.0°C, and the concentration of carboxylic terminal group was 28 eq/10 6 g. Next, polyethylene terephthalate (f) prepared in Example 1, which did not contain particles, was used, and the previously prepared polyethylene terephthalate compositions (k) and (l) and the polyethylene terephthalate (f) were blended in the form of pellets such that the mixing ratio thereof was 15/10/75 in weight ratio. Using this mixture, a biaxially stretched polyethylene terephthalate film having a thickness of 14 µm was formed in a manner similar to that of Example 1. When the state of the particles present in the film was observed using an electron microscope as aforementioned, the shape of the primary particles of  / α mixture-type aluminium oxide particles (h,i) was I 1 /I 2 =3.8, the diameter of the agglomerated, i.e. secondary, particles was Lav 0.16 µm (D=0.04 µm), and the preparation of the number of the agglomerated particles each containing at least one acicular or plate-like particle relative to the total number of the agglomerated particles was 76%. Furthermore, the preparation of the particles of L≦ 0.05 µm was 46%. When the state of the film surface was observed in a manner as aforementioned, the surface roughness Ra was 0.019 µm, and very fine protrusions with a height of not less than 10 nm and not more than 60 nm originating from  / α mixture-type aluminium oxide particles (h, i) were observed to be 1,220,000/mm 2 in number. When the properties of the film were estimated, the running ability µ κ was 0.17, the abrasion resistance 1 ○ was rank 1, the abrasion resistance 2 ○ was rank 1, and particularly the scratch resistance in repeated high-speed running was rank A, and thus, the properties of the film were excellent. In Table 4, "DVB · EVB" represents a divinylbenzene/ethylvinylbenzene copolymer. Examples 12, 13 (Table 4): Example 1 was repeated except that the proportion of particles of aluminum oxide having an acicular or plate-like shape and the kind of crystal structure were changed by changing the calcination conditions during the synthesis of the aluminum oxide from those adopted when preparing particles (a), and the kind and particle diameter of the inert particles were changed. Other conditions were the same as those of Example 1. Thus, biaxially stretched polyethylene terephthalate films having a thickness of 14 µm were formed. The properties of the films obtained were good as shown in Table 4. Examples 14-16 (Table 5): Biaxially stretched polyethylene terephthalate films having a thickness of 14 µm were formed under the same conditions as those of Example 11, except for a change in the mixing ratio of the aluminum oxide particles (h) and the aluminum oxide particles (i) and in the kind and particle diameter of inert particles. The properties of the films obtained were good as shown in Table 5. Example 17 (Table 6): A slurry prepared by kneading terephthalic acid (86.5 parts by weight) and ethylene glycol (37.1 parts by weight) (mole ratio: 1.15) was continuously supplied for 3.5 hours to an esterification reactor in which a molten reaction intermediate product (bishydroxyethylterephthalate) (110 parts by weight) prepared from ethylene glycol/terephthalic acid (mole ratio: 1.15) was stored. The temperature of the reaction system was maintained at 250°C from the beginning of the supply, and the water generated due to the esterification reaction was distilled from the top of a fractioning column. A bishydroxyethylterephthalate with a reaction ratio of 98.0% was obtained by continuing the esterification reaction of the slurry. The bishydroxyethylterephthalate of 104.8 parts by weight (corresponding to polyethylene terephthalate of 100 parts by weight) was transferred to a condensation polymerization reactor, lithium acetate of 0.25 part by weight was then added, and after maintaining the temperature at 250°C for ten minutes, the slurry of aluminium oxide particles used in Example 10 was added in an amount of 5 parts by weight. Then, calcium acetate (0.13 part by weight) and anitomy trioxide (0.03 part by weight) were added. After maintaining the temperature at 250°C for ten minutes, trimethylphospate (0.20 part by weight) and phosphorous acid (0.02 part by weight) were added. Thereafter, condensation polymerization was carried out in a regular manner, and a polyethylene terephthalate (m) containing particles, in which internal particles having a structure of Ca-Li-P (0.8% by weight) (mean particle diameter: 1.6 µm) and  / α mixture-type aluminium oxide particles were mixed and dispersed, was obtained. The intrinsic viscosity of this polymer was 0.616, the Δ IV was 0.013, the softening point was 261.2°C, and the concentration of carboxylic terminal group was 31 eq/10 6 g. Next, polyethylene terephthalate composition (1) prepared in Example 11 and the polyethylene terephthalate composition (m) prepared above were blended in the form of pellets such that the mixing ratio of (1)/(m) was 5/95 in weight ratio. Using this mixture, a biaxially stretched polyethylene terephthalate film having a thickness of 14 µm was formed in a manner similar to that of Example 1. When the state of the particles present in the film was observed using an electron microscope as aforementioned, the shape of the primary particles of  / α mixture-type aluminium oxide particles was I 1 /I 2 = 5.1, the diameter of the agglomerated, i.e. secondary, particles was Lav=0.16 µm (D=0.04 µm), and the proportion of the number of the agglomerated particles each containing at least one acicular or plate-like particle relative to the total number of agglomerated particles was 87%. Further, the proportion of the particles of L≦ 0.05 µm was 28%. When the state of the film surface was observed in a manner as aforementioned, the surface roughness Ra was 0.023 µm, and very fine protrusions with a height of not less than 10 nm and not more than 60 nm originating from  / α mixture-type aluminium oxide particles (p) were observed to be 1,100,000/mm 2 in number. When the properties of the film were estimated, the running ability µ κ was 0.16, the abrasion resistance 1 ○ was rank 1, the abrasion resistance 2 ○ was rank 1, and particularly the scratch resistance in repeated high-speed running was rank A, and thus, the properties of the film were excellent. Example 18 (Table 6): A biaxially stretched polyethylene terephthalate film having a thickness of 14µm was formed under the same conditions as those of Example 10 except that aluminum oxide particles having two or more kinds of crystal structures prepared by changing the calcination conditions during the synthesis of the aluminum oxide from those adopted when preparing particles (a) in Example 1. Although the properties of the film: obtained were good as shown in Table 6, the scratch resistance in repeated high-speed running and the abrasion resistances 1 ○ and 2 ○ were slightly poor as compared with those of Example 10. Example 19 (Table 7): Aluminum oxide particles (n) were made by calcining aluminum hydride (Bayerite) prepared from a metal aluminium as a main raw material. The specific surface area of the aluminum oxide particles (n) was 42 m 2 /g, and when the crystal structure of the particles was analyzed by X-ray diffraction, it was found to be a mixture type of -type and η-type. α-type was not included. A biaxially stretched polyethylene terephthalate film having thickness of 14 µm was formed under the same conditions as those of Example 11, except for use of the above aluminum oxide particles (n). Although the properties of the films obtained were good as shown in Table 7, the the scratch resistance in repeated high-speed running and the abrasion resistance 2 ○ were slightly poor as compared with those of Example 11. Example 20 (Table 7): Example 1 was repeated except that the proportion of particles of aluminum oxide having an acicular or plate-like shape and the kind of crystal structure was changed by changing the calcination conditions during the synthesis of the aluminum oxide from those adopted when preparing particles (a), and the kind and particle diameter of the inert particles were changed. Other conditions were the same as those of Example 1. Thus, a biaxially stretched polyethylene terephthalate film. having a thickness of 14 µm was formed. The properties of the film obtained were good as shown in Table 7. Example 21 (Table 8): The polyethylene terephthalate composition containing  + α type aluminum oxide particles used in Example 10 and polyethylene terephthalate compositions (e) and (f) used in Example 1 were blended so that the mixing ratio was 15/30/55 in weight ratio, the mixture was dried under a reduced pressure condition (3torr) at 180°C for three hours, and the mixture was supplied to a vent-type twin-screw extruder No. 1 and rendered molten at 280°C (polymer I). Furthermore, only the polyethylene terephthalate composition (f) was dried under a reduced pressure condition (3torr) at 180°C for three hours, and the composition (f) was supplied to another twin-screw extruder No. 2 and rendered molten at 290°C (polymer II). These two polymers were then each filtered to a high accuracy filtration degree, respectively, the polymers were laminated using a three-layer feed block with a rectangular lamination portion such that the polymer II was arranged to form a base layer and the polymer I was arranged to form both surface portions. The laminated polymer was delivered out from a fish-tale type die in the form of a sheet, and the delivered sheet was cooled and solidified on a casting drum using an electrostatic casting method to form a non-stretched film having a thickness of about 160 µm. At that time, the draft ratio was 6.5. The non-stretched film was stretched in the longitudinal direction in three stretching stages wherein the first stage was at 123°C and 1.2 times, the second stage was at 126°C and 1.45 times and the third stage was at 114°C and 2.3 times. The uniaxially stretched film obtained was stretched in the transverse direction in two stretching stages of a first stage at 111°C and 3.7 times and a second stage at 113°C and 1.2 times using a stenter, and the film was then heat treated under conditions of a constant length at 200°C for five seconds to obtain a film having a thickness of 14 µm. The thickness of the outermost layer "t" of the film obtained was 1.0 µm, and the relationship between the thickness "t" and the mean diameter "d" (=0.51 µm) of calcite-type calcium carbonate particles was t=1.96d. The properties of the film are shown in Table 8. The abrasion resistance 1 ○ and 2 ○ and the scratch resistance in repeated high-speed running were all good, similarly to Example 10. Furthermore, when the electromagnetic conversion property was estimated, it was +2.0dB when the data of Example 10 was set as the standard, and it was very good film. Examples 22, 23 (Table 8), Examples 24-26 (Table 9): Biaxially stretched polyethylene terephthalate films having a thickness of 14 µm were formed in the same manner as that of Example 21 except for a change in the crystal structure and content of the aluminium oxide particles, in the kind and particle diameter of the inert particles and in the thickness of the laminate. The properties of the films obtained are shown in Tables 8 and 9, and not only the abrasion resistance and the scratch resistance in repeated high-speed running but also the electromagnetic conversion property were good. Comparative Example 1(Table 10): Aluminium oxide particles (o) were obtained by sublimation of aluminium chloride and flame hydrolysis thereof in a gaseous phase of hydrogen and oxygen atmosphere. The specific surface area of the aluminium oxide particles (o) was 97 m 2 /g, and when the crystal structure of the particles was analyzed by X-ray diffraction, it was 100% δ-type. A biaxially stretched polyethylene terephthalate film having a thickness of 14 µm was formed under the same conditions as those of Example 1 except for the use of the above aluminium oxide particles (o). The properties of the film obtained are shown in Table 10, and the scratch resistance in repeated high-speed running was not at a satisfactory level. Comparative Examples 2-4 (Table 10): Biaxially stretched polyethylene terephthalate films having a thickness of 14 µm were formed in the same manner as that of Comparative Example 1, except for a change in the crystal structure of the aluminium oxide particles by changing the synthesis method and a change in the kind and particle diameter of the inert particles. The properties of the films obtained are shown in Table 10, and the abrasion resistance and the scratch resistance in repeated high-speed running were insufficient. Comparative Examples 5,6 (Table 11): Biaxially stretched polyethylene terepathalate films having a thickness of 14 µm were formed in the same manner as that of Example 1, except for the use of oxide particles with a large specific surface area instead of aluminium oxide particles. The properties of the films obtained are shown in Table 11, and the abrasion resistance and the scratch resistance in repeated high-speed running were all poor. Comparative Examples 7,8 (Table 12): Biaxially stretched polyethylene terephthalate films having a thickness of 14 µm were formed in the same manner as that of Example 21, except for a change in the crystal structure of the aluminium oxide particles. The properties of the films obtained are shown in Table 12, and the abrasion resistance and the scratch resistance in repeated high-speed running were insufficient. Furthermore, the electromagnetic conversion property was not improved as compared with that in Example 10. In a thermoplastic polyester composition according to the present invention, the films formed from the compositions can have particularly excellent slipping property, abrasion resistance and scratch resistance, especially excellent scratch resistance in repeated high-speed running. Therefore, the thermoplastic polyester composition according to the present invention can be applied to various industrial fields requiring these properties, and it is particularly useful as a base film in magnetic recording media.

Claims: A thermoplastic polyester composition comprising a thermoplastic polyester prepared from an acid component mainly comprising an aromatic dicarboxylic acid, a glycol component mainly comprising an aliphatic glycol and a particulate mixture (C) of two or more aluminum oxide components different from each other in crystal structure type, one of which crystal structures is the -type. A thermoplastic polyester composition according to claim 1, wherein the particulate mixture (C) is a mixture of two or more kinds of aluminum oxide particles, at least one kind of particle in the said mixture being -type, and at least one other kind of particle being selected from α-type, η-type, κ-type, χ-type aluminum oxide particles. A thermoplastic polyester composition according to claim 1 or claim 2, wherein the aluminum oxide particles have a primary particle diameter d not less than 3nm and not more than 200nm, the primary particle diameter d being that given by the equation d = 6/(ρ.s) where s is the specific area (m 2 /g), as determined by the B.E.T. method, and ρ is the true specific gravity, of the particles. A thermoplastic polyester composition according to any preceding claim, wherein inert particles having a primary particle diameter more than the primary particle diameter of said aluminum oxide particles are additionally present. A thermoplastic polyester composition according to claim 4, wherein said inert particles are inorganic particles. A thermoplastic polyester composition according to claim 5, wherein said inert particles are selected from calcium carbonate, silica, titanium oxide, kaolin and calcium phosphate particles. A thermoplastic polyester composition according to claim 4, wherein said inert particles are organic particles. A thermoplastic polyester composition according to claim 7, wherein said inert particles are selected from crosslinked polymeric particles and internal particles precipitated in a polyester reaction system. A polyester film composed of a thermoplastic polyester composition according to any preceding claim. A polyester film having a laminate structure with at least one layer composed of a thermoplastic polyester composition according to any of claims 1 to 8. A polyester film according to claim 9 or claim 10, which contains secondary aluminum oxide particles, which said particles have respective particle diameters, determined by measuring respective secondary particle lengths in a direction parallel to a film surface, such that the proportion of the number of secondary aluminum oxide particles having a diameter not more than 0.05µm to the total number of secondary aluminum oxide particles is not less than 20% and not more than 60%. A polyester film according to claim 9 or claim 10, wherein the number of protrusions having a height of not less than 10nm and not more than 60nm on a surface of the polyester film is not less than 500,000/mm 2 and not more than 5000,000/mm 2 . Use of a polyester film according to any one of claims 8 to 12 as a base film in a magnetic recording medium.

Inventors: AOYAMA MASATOSHI, SASAKI TOSHIHIRO, SUZUKI MASARU, UEDA TOSHIAKI, YOSHIDA MINORU, AOYAMA, MASATOSHI, SASAKI, TOSHIHIRO, SUZUKI, MASARU, UEDA, TOSHIAKI, YOSHIDA, MINORU
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1993
Main classification: C08L67
IPC codes: C08J5, C08K3