Polyester composition, process for preparing same and film formed using same

Abstract: According to the present invention there is provided a polyester composition comprising an aromatic polyester having copolymerized therein a compound containing at least one sulfonic acid group or sulfonic acid metal salt group, and crosslinked polymer particles, preferably crosslinked polymer particles coated with a water-soluble polymer. The particles are incorporated in a slurried state into the polymer. There is also provided a laminate film containing the polymer with the particles thus incorporated therein. The polyester composition is superior in uniform dispersibility of the particles, re-agglomeratin does not occur in melt-storage for a long time, and films formed using the polyester composition are superior in traveling performance and abrasion resistance.

Description: The present invention relates to a polyester composition containing crosslinked polymer particles which maintain a uniform monodisperse state in the composition even when held in a melted state for a long time, as well as a process for preparing the composition, and film formed from it. To improve dispersibility and packability of magnetic powder in a magnetic layer in a recording medium, it is known to incorporate in the layer, as a binder component, a polyester resin having a metallic salt of a sulphonic acid and a nitrogen containing compound (see JP-A-59-172120). The composition additionally contains cellulose fibre and a bifunctional isocyanate capable of crosslinking with amino groups in the polyester. This document does not, however, relate to polyester compositions containing polymeric particles dispersed therein. Polyesters, particularly polyethylene terephthalate, are widely used as fibers, films and other shaped articles because they have superior physical and chemical properties. However, despite such superior properties it is also known that with such polyesters, problems arise such as deterioration in their working efficiency and in the value of products formed therefrom because of their poor processability when forming them into a film or other shaped article and difficulty in handling such products formed therefrom as a result of their poor slipperiness properties. In an attempt to solve these problems, there have been proposed many methods for improving the surface slipperiness of such products by incorporating fine particles in polyesters to impart moderate concavities and convexities to the surfaces of the shaped articles, and some of the methods have been adopted practically. For example, in methods known from JP-A-55-133431 and JP-A-57-125247, inert inorganic particles such as the particles of silicon oxide, titanium dioxide, calcium carbonate, talc or kaolinite, or fine particles of organic polymers such as benzoguanamine-formaldehyde resin, polytetrafluoroethylene-hexafluoropropylene copolymer or polyphenyl ester resin, are added to a polyester preparing reaction system. Each of US-A-4320207 and JP-A-2-189359 discloses incorporation in a polyester film of fine particles of a crosslinked polymer. According to US-A-4320207, the crosslinked polymer may be post-treated to introduce into it a sulfonic acid group, while according to JP-A-2-189359, the polyester consists principally of an aromatic dicarboxylic acid and at least one glycol component but other components may also participate in the copolymerisation. One example of such other components is 5-sodiumsulfoisophthalic acid. However, in the method of adding inert inorganic particles, the affinity for polyester is generally poor, so when formed into film for example, voids are formed in the vicinity of the particles. Besides, coarse particles are often mixed into the polyester, and even if pulverizing or classifying operations are performed for removing such coarse particles, the incorporation of the coarse particles is unavoidable. If such voids are formed or coarse particles are present, then in the case of films for which transparency is required, such as, for example, films for printing plate and microfilm, the transparency will be markedly deteriorated, and in the case of films for capacitors, a bad influence will be exerted on electrical properties. Further, in the case of a film for magnetic tapes, electro-magnetic conversion properties will be deteriorated, or drop-out will occur, thus resulting in impairment of the film quality. On the other hand, when using the method of adding fine particles of an organic polymer, the affinity for polyester is generally good, but it is difficult to obtain uniform and fine particles, and even if such fine particles are obtained, their dispersibility in the polyester is poor, for example the particles will agglomerate in the polyester; thus, the incorporation of coarse particles is unavoidable. Recently, with respect to films for which transparency is required, films for capacitors and films for magnetic tapes, a higher quality has been required, and a film superior in slipperiness, having a uniform surface and containing particles superior in their affinity for polyester has been desired. The present invention seeks to eliminate the above-mentioned disadvantages of the conventional methods incorporating particles into a polyester. More particularly, the present invention seeks to provide a polyester composition wherein crosslinked polymer particles maintain a uniform monodisperse state even when held in a melted state for a long time, the polyester having specific functional groups, to provide a process for preparing such a composition and to provide a polyester film superior in all of slipperiness, surface uniformity, transparency and abrasion resistance. According to a first aspect of the present invention there is provided a polyester composition comprising (A) an aromatic polyester having copolymerized therein, 0.1 to 10 parts, by weight based on 100 parts by weight of the entire carboxylic acid component, of a compound containing at least one sulfonic acid group or sulfonic acid metal salt group and (B) crosslinked polymer particles. According to a second aspect of the present invention there is provided a process for preparing such a polyester composition characterized in that a slurry of crosslinked polymer particles (B) in water and/or in an organic compound having a boiling point less than 200°C is added to the aromatic polyester (A) or to a synthesis reaction system for preparing the aromatic polyester (A). According to the present invention, by combining a polyester having a sufficient content of a specific functional group with crosslinked polymer particles, the crosslinked polymer particles can be uniformly monodispersed in the polyester and can maintain this state even when held in a melted state for a long time and a polyester film can be obtained which has superior slipperiness, surface uniformity, transparency and abrasion resistance which have been unattainable with known compositions. Thus, according to a third aspect of the present invention there is provided a film, especially a biaxially oriented film formed using the polyester composition. According to a fourth aspect, the invention provides a biaxially oriented polyester laminate film having as at least one layer a film formed from the above polyester composition. At least one of the surface layers of the laminate may be a film formed from such a polyester composition. Preferred embodiments of the present invention will now be described in more detail. In such embodiments of the present invention, the aromatic polyester (A) having copolymerized therein a compound containing at least one sulfonic acid group or sulfonic acid metal salt group contains as main monomer components (a) an aromatic dicarboxylic acid or an ester-forming derivative thereof, (b) a glycol and (c) a compound having at least one sulfonic acid group or sulfonic acid metal salt group. Examples of the aromatic dicarboxylic acid (a) are terephthalic acid, 2,6-naphthalenedicarboxylic acid and 1,2-bis (chlorophenoxy) ethane-4,4'-dicarboxylic acid, and examples of the ester-forming derivative thereof (a) are dimethyl terephthalate, dimethyl-2,6-naphthalenedicarboxylate and dimethyl-1,2-bis (chlorophenoxy) ethane-4,4'-dicarboxylate, with terephthalic acid, 2,6-naphthalenedicarboxylic acid, dimethyl terephthalate and dimethyl-2,6-naphthalenedicarboxylate being preferred. Examples of the glycol component (b) include ethylene glycol, butylene glycol, diethylene glycol, propylene glycol, polyethylene glycol and 1,4-cyclohexanedimethanol, with ethylene glycol being particularly preferred. As the compound (c) having at least one sulfonic acid group or sulfonic acid metal salt group, a polyester-forming monomer, especially an aromatic dicarboxylic acid or an ester-forming derivative thereof, having one or two sulfonic acid groups or sulfonic acid metal bases, is preferred. Examples are 5-sodium sulfoisophthalic acid and ester-forming derivatives thereof, 5-lithium sulfoisophthalic acid and ester-forming derivatives thereof, and 5-sodium sulforesorcin. Particularly, 5-sodium sulfoisophthalic acid and ester-forming derivatives thereof, 5-lithium sulfoisophthalic acid and ester-forming derivatives thereof are preferred. The amount of the compound (c) to be copolymerized is 0.1-10 parts by weight, based on 100 parts by weight of the total carboxylic acid components constituting the aromatic polyester (A). This improves the dispersion stability of the crosslinked polymer particles while being held in a melted state. Further, having regard to the ease of handling of a film formed using the resulting composition and the dispersion stability of particles, the amount of the component (c) to be copolymerized is preferably 0.1-8 parts by weight, more preferably 0.1-6 parts by weight, most preferably 0.1-2 parts by weight, based on 100 parts by weight of the total carboxylic acid components constituting the aromatic polyester (A). Other components than the above components (a), (b) and (c) may also participate in the copolymerization. Examples are diols such as neopentyl glycol, polyalkylene glycol and p-xylylene glycol; dicarboxylic acids such as adipic, sebacic, phthalic and isophthalic acids; polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid; and hydroxycarboxylic acids such as p-hydroxyethoxybenzoic acid. In the case where the dicarboxylic acid component is a dicarboxylic acid, it is subjected to an esterification reaction with glycol, while in the case of it being a dicarboxylic acid ester, it is subjected to an ester interchange reaction with glycol, then both reactions are followed by polycondensation at a high temperature under a reduced pressure to obtain polyesters. It is also possible to perform the polycondensation using a prepolymer itself as a starting material. As the crosslinked polymer particles used in compositions embodying the present invention there may be used a copolymer of a monovinyl compound (P) having only one aliphatic unsaturated bond in the molecule with a compound (Q) having two or more aliphatic unsaturated bonds in the molecule as a crosslinking agent, or only the latter, crosslinking agent, (Q). Examples of the compound (P) are aromatic monovinyl compounds such as styrene, α-methylstyrene, fluorostyrene, vinyl pyridine and ethylvinylbenzene; vinyl cyanide compounds such as acrylonitrile and methacrylonitrile; acrylic ester monomers such as butyl acrylate, 2-ethylhexyl acrylate, methyl acrylate, 2-hydroxyethyl acrylate, glycidyl acrylate and N,N'-dimethylaminoethyl acrylate; methacrylic ester monomers such as butyl methacrylate, 2-ethylhexyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, glycidyl methacrylate, N,N'-dimethylaminoethyl methacrylate; mono- or dicarboxylic acids such as acrylic, methacrylic, maleic and itaconic acids, as well as dicarboxylic acid anhydrides; and amide monomers such as acrylamide and methacrylamide. Examples of the compound (Q) are divinylbenzene compounds and polyvalent acrylates and methacrylates such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, 1,3-butylene glycol diacrylate, and 1,3-butylene glycol dimethacrylate. For controlling the particle size, it is desirable to use divinylbenzene, ethylene glycol dimethacrylate or trimethylolpropane trimethacrylate as the compound (Q). The compounds (P) and (Q) exemplified above may each be used in combination of two or more. Preferred examples of the crosslinked polymer particles used in the present invention are crosslinked polymer particles of divinylbenzene polymer, divinylbenzene copolymer (ethylvinylbenzene-divinylbenzene), butyl acrylate-divinylbenzene copolymer, 2-ethylhexyl acrylate-divinylbenzene copolymer and 2-ethylhexyl acrylate-ethylene glycol dimethacrylate copolymer. The particles may be produced using a three- or more multi-component system such as, for example, butyl acrylate-ethylvinylbenzenedivinylbenzene copolymer, styrene-ethylvinylbenzenedivinylbenzene copolymer, or 2- ethylhexyl acrylate-ethylene glycol dimethacrylate-divinylbenzene copolymer. As to the heat resistance of the crosslinked polymer particles used in the present invention, it is preferable for the particles to have a heat resistance of 350°C or higher, more preferably 360°C or higher, particularly preferably 370°C or higher, in terms of a thermal decomposition temperature (10% weight loss temp.) as measured using a thermobalance. If the thermal decomposition temperature is lower than 350°C, the particles will agglomerate during preparation of the polyester composition during melt-molding, or at the time of recovery and re-use of a molded product made of the polyester composition, resulting in impairment of the surface uniformity and abrasion resistance of the molded product. In order for the particles to have the above-mentioned heat resistance, it is necessary that crosslinking is effected to a high degree by the compound (Q) as a crosslinking agent. Although the kind of crosslinking agent to be used is not specially limited, divinylbenzene is preferred and the proportion thereof as a pure divinylbenzene relative to monomer is preferably higher than 5 wt%, more preferably higher than 8 wt%. In the crosslinked polymer particles used in the present invention there may be incorporated an additional component other than the compounds (P) and (Q). An example is diethylbenzene as an impurity of divinylbenzene which is available commercially. In a composition embodying the present invention, an average particle diameter of the crosslinked polymer particles dispersed in the polyester is preferably in the range of 0.01 to 5µm, more preferably 0.05 to 2µ m, having regard to slipperiness, transparency and abrasion resistance. For superior slipperiness, surface uniformity and abrasion resistance it is desirable that the crosslinked polymer particles are spherical in shape and have a uniform particle size distribution. More specifically, the particles preferably have a volume shape factor of 0.35 to π / 6, more preferably 0.45 or more. The volume shape factor, f, is expressed as f = V/D 3 where V represents the volume of the particles (µm 3 ) and D represents a maximum diameter (µ m) in a projection plane of the particles. The above values V and D are determined in the following manner. From a biaxially oriented film, one of the formed products comprising the polyester composition of the present invention, ultrathin specimens having a thickness of about 0.3µm are prepared using an ultrathin film-preparing apparatus with regard to the cross sections in the MD and TD directions. As to the specimens, the number average particle diameters (a, b and c) are measured with regard to the MD, TD and thickness directions using a transmission electron microscope. The volume of the particles (V) and the maximum diameter in a projection plane of the particles (D) are calculated from the following equations: V = 4 3 π · a · b · c D = max (a, b, c) The crosslinked polymer particles in composition embodying the present invention preferably have a weight average diameter (Dw) to number average diameter (Dn) ratio (Dw/Dn) higher than 1.1 for slurry preservation stability and film abrasion resistance. Those composed of relatively uniform large particles and small particles are more preferable than those composed of particles having irregularly different various diameters. Such a mixed state may be confirmed by taking particles out of a particle-containing slurry or a polyester solution, diluting the particles with pure water or the like and measuring the number-based particle distribution using an apparatus which utilizes the dynamic light scattering method and so forth. By the method mentioned above the presence of a particle group having diameters within a particular range and another particle group having diameters within a range different from the above range can be observed. More preferably, the number-based particle distribution has at least one peak in the range of 0.001-0.2µm (S 1 ) and at least one peak in the range of 0.2-5µm (S 2 ) and the ratio of the number at the highest peak in the range S 1 to that at the highest peak in the range S 2 is preferably 1/1,000-1,000, and more preferably 1/100-100. It is preferred that the range S 2 has two or more peaks having regard to film flatness and rolled film figure. In order to obtain such a particle size distribution, crosslinked polymer particles having some diameters with sharp particle diameter distribution and crosslinked polymer particles having diameters different from the above with sharp particle diameter distribution may be blended. Crosslinked polymer particles which may be used in compositions embodying the invention are those obtained by known methods. The following are mentioned as examples of such known methods: (1) A soap-free polymerization method wherein polymerization is performed without using an emulsifying agent, or using an extremely small amount of an emulsifying agent. (2) A seed polymerization method wherein polymer particles are added into a polymerization system prior to emulsion polymerization. (3) A core-shell polymerization method wherein monomer components are partially emulsion-polymerized and in this polymerization system the remaining monomer portion is polymerized. (4) A polymerization method using Ugelstad, as disclosed in JP-A-54-97582 and JP-A-54-126288. (5) A polymerization method corresponding to method (4), but not using a swelling assistant. In particular, the above methods (3) and (4) are preferred because it is possible to obtain spherical crosslinked polymer particles having a uniform particle size distribution. In a composition embodying the present invention, the crosslinked polymer particles may have functional groups. The kind of such functional group is not specially limited. Examples are carboxyl group, hydroxyl group, sulfonic acid group, ester group, and metal salts of carboxyl group. Although no special limitation is placed on how to introduce the functional group, it is desirable from the standpoint of heat resistance of the particles that particles as a matrix of high crosslinking be once produced and then the functional group be introduced into the surfaces of the matrix particles. For example, when introducing the sodium salt of a carboxyl group, particles are prepared which are highly crosslinked by divinylbenzene, using particles of a styrene-divinylbenzene copolymer as matrix particles, and thereafter the carboxyl group is introduced into the particle surfaces using methacrylic acid. By rendering the interior of the particle preparation system alkaline, the functional group -COONa is introduced into the particle surfaces. The amount of the compound having a functional group to be introduced into the particle surface is usually 0.01-500 parts by weight, preferably 0.1-300 parts by weight based on 100 parts by weight of the matrix particles. The amount of the crosslinked polymer particles to be added relative to the aromatic polyester is preferably in the range of 0.0001 to 20 wt%, more preferably 0.001 to 10 wt%, still more preferably 0.01 to 5 wt%. Although the surface state of the crosslinked polymer particles is not specially limited, if there are used crosslinked polymer particles whose surfaces are partially or wholly coated with a water-soluble high molecular weight compound, the dispersibility of the particles will be improved and there will be obtained an outstanding effect. Such a water-soluble polymer is not especially limited, provided that it has some solubility in water, but those dissolving to at least 0.1 wt% in water at 25°C are preferred. Even in the case where the solubility speed is extremely slow, if an aqueous solution of at least 0.1 wt% of a polymer is obtained by once dissolving the polymer in heated water followed by cooling to 25 °C, such a polymer may also be used. Examples are synthetic polymers such as polyvinyl pyrrolidone, polyvinyl alcohol, polyethylene oxide, polyacrylic acid, polyacrylic ester, polyacrylic acid partial metal salt or partial ammonium salt, polymethacrylic acid, polymethacrylic ester and polyester, semi-synthetic polymers such as partially allylated cellulose and carboxymethyl cellulose, and natural polymers such as sodium alginate and gelatin. These may be homopolymers or copolymers. Polymers having vinylpyrrolidone units are most preferable. It is desirable to use such a water-soluble polymer so as to improve the dispersion stability when added to an aqueous slurry of the crosslinked polymer particles, particularly a water-soluble polymer, polyvinylpyrrolidone for example whose addition results in increase in an absolute value of zeta-potential at the pH range of 5 to 8. As to the molecular weight of the water-soluble polymer, the range of 300 to 1,000,000 is preferable and the range of 1,000 to 500,000 is more preferable. As to the amount of the water-soluble polymers to be added to the crosslinked polymer particles, 0.01-100 parts by weight to 100 parts of the crosslinked particles are preferable, 0.1-30 parts are more preferable. The covering percentage of a crosslinked polymer particle surface by a water-soluble polymer may be determined by observing the particle cross section in an ultrathin specimen by means of a transmission type electron microscope and calculating by the following equation. circumference length of crosslinking polymer particles in contact with water-soluble polymer circumference length of entire particles × 100 The covering percentage is preferably higher than 10% more preferably higher than 30% and most preferably higher than 50% having regard to particle dispersion stability. In order to check the covering state precisely, preferable water-soluble polymers are those recognizable by dyeing in a polyester with osmic acid, ruthenic acid or the like. The treatment of the crosslinked polymer particles with the water-soluble polymer is usually performed in a solvent. Considering the incorporation into the polyester it is preferable that the treatment be conducted in an aqueous system or a glycol system which is to be a structural unit of the polyester. The treatment need not always be carried out by agitation; for example, an ultrasonic wave or a medium type mill such as a sand grinder may be used. In this case, the water-soluble high molecular weight compound may be a single compound, or a plurality of such compounds may be used. In the present invention, it is necessary that the crosslinked polymer particles be monodispersed uniformly in the polyester even when held in a melted state for a long time. The uniformly monodispersed state as referred to herein indicates a state wherein the particles are dispersed as primary particles in the polyester, including few secondary agglomerates. More particularly, when the polymer is observed using a transmission type electron microscope, the number of secondary agglomerate particles is preferably less than 20, more preferably less than 15, still more preferably less than 10, per field of 0.01 mm 2 . The crosslinked polymer particles used in a composition embodying the present invention can be incorporated into the polyester, for example, by a method of adding and mixing it into a polyester preparing reaction system in the form of powder or glycol slurry or by a method of kneading it with the polyester as a powder or a slurry in a low boiling point solvent. According to a method preferred because more uniform monodisperse is attained, a slurry of the crosslinked polymer particles in water and/or an organic compound having a boiling point of lower than 200°C is added to the polyester, followed by melt-kneading. The most preferable method is a method of melt-kneading in a vent-type molding machine while degassing. The vent type molding machine is a melt-molding machine having at least one vent hole. For example, it may be an extruder or an injection molding machine. It is necessary that at least one vent hole for removing water and/or the organic compound having a boiling point of 200°C or lower be held under reduced pressure, preferably 100 Torr or lower, more preferably 50 Torr or lower, still more preferably 30 Torr or lower. On the other hand, it is necessary that the crosslinked polymer particles be added to the polyester or aromatic polyester (A)- preparing system as a slurry thereof in water and/or an organic compound having a boiling not higher than 200°C. Examples of the organic compound are alcohols such as methanol, ethanol and ethylene glycol, hydrocarbons such as benzene and toluene, as well as esters, ketones and amines. Above all, water is preferred from the standpoint of handleability and removability. Of course, water and/or the organic compound may be in the form of a mixed solvent of two or more. In this case, a water-rich mixed solvent is preferred. From the standpoint of particle dispersion, it is desirable that an anionic surfactant such as sodium dodecylbenzenesulfonate or sodium lauryl sulfate or a nonionic surfactant such as polyoxyethylene nonylphenyl ether or polyethylene glycol monostearate which are necessary for the manufacture of the particles be contained in the slurry of the crosslinked polymer particles. Although the concentration of the slurry of the crosslinked polymer particles in water and/or an organic compound boiling not higher than 200 °C is not especially limited, it is preferably in the range of 2 to 30 wt%, more preferably 2 to 20 wt%, having regard to the dispersibility of the particles in the polyester and the intrinsic viscosity of the polyester. According to the process of the present invention, it is possible to incorporate the crosslinked polymer particles at a high concentration into the polyester. Therefore, it is also possible to first prepare a polyester containing a high concentration of the crosslinked polymer particles and then dilute it with a polyester substantially not containing such particles before use. In the above process, the crosslinked polymer particles are preferably mixed with the polyester in the state of a slurry thereof in water and/or an organic compound having a boiling point of lower than 200°C, this method being preferred for the following reasons. When the slurrv is added in the polyester preparing reaction, it is possible to avoid bad influences, e.g. marked delay of the reaction caused by water or a surfactant, and hence the working efficiency is improved. Further, even crosslinked polymer particles relatively poor in heat resistance can be incorporated into the polyester. The aromatic polyester (A) in a composition embodying the present invention may be a mixture of the aromatic polyester having copolymerized therein at least one sulfonic group or sulfonic acid metal salt group (A) and at least one other polyester such as particle-free polyethyleneterephthalate. In a composition embodying the present invention the crosslinked polymer particles are uniformly monodispersed in the polyester, so when stretched film is produced using the resulting polyester composition, a surface having uniform concavities and convexities is obtained. Moreover, since re-agglomeration does not occur when the particles are held in a melted state, it is possible to maintain the particles in a uniformly monodispersed state even in the film forming process. Consequently, it is possible to obtain a film satisfying all of superior slipperiness, transparency and abrasion resistance. The polyester used in a composition embodying the present invention may further contain a metallic compound catalyst such as a lithium, sodium, calcium, magnesium, manganese, zinc, antimony, germanium or titanium compound, usually employed in the manufacture of a polyester, as well as a phosphorus compound as a coloration preventing agent. The polyester composition embodying the present invention may further contain non-incorporated particles containing at least one of lithium and calcium and phosphorus as part of the constituent components. The composition may contain precipitated particles which, as a constituent part thereof, contain at least one of lithium and calcium and also contain phosphorus. Also, the polyester composition of the present invention may contain inorganic particles having an average particle size of 0.001-5µm. Examples of the inorganic particles are inorganic oxides such as titanium oxide, silicon oxide, zirconium oxide and aluminum oxide, inorganic carbonates such as calcium carbonate and barium carbonate, inorganic phosphates such as calcium phosphate and sodium phosphate, inorganic sulfates such as barium sulfate and calcium sulfate, inorganic mixed oxides such as kaolin and talc, fluorides, such as fluorite and other conventional inorganic particles such as potassium titanate, and aluminum hydroxide. Among them, titanium oxide, silicon oxide, calcium carbonate, zirconium oxide and aluminum oxide are most preferred. They may be substituted by any compound containing other element(s) up to 50% based on the total weight. For preparing a composition containing both the above inorganic particles or non-incorporated particles and polymer particles a method may be adopted wherein both are added to the polyester reaction system or the molten polyester, or wherein master chips obtained by blendings thereof with the polymer independently are again blended together followed by melt-kneading. The film formed according to the present invention may be a single layer film, of course. But if a polyester composition containing the crosslinked polymer particles and a polyester containing or substantially not containing particles are co-extruded for at least one-side lamination, followed by biaxial orientation, there will be obtained a film having a surface superior in uniformity, slipperiness and abrasion resistance. The lamination or laminated film may be any film having two or more layers in the thickness direction. so at least one surface layer of the laminate film may be formed from a polyester composition of the invention. If the film thickness of the above laminate is set at 0.1 to 10 times, particuiary 0.1 to 5 times as large as the average particle diameter of the crosslinked polymer particles added, an improvement of slipperiness and abrasion resistance will be attained. Having regard to slipperiness and abrasion resistance, it is preferable that the content of the crosslinked polymer particles in the laminate portion be in the range of 0.01 to 20 wt%, particularly 0.05 to 10 wt%. How to prepare such biaxially oriented film will now be described more specifically. Pellets of the polyester composition are mixed at a predetermined ratio, then dried, thereafter fed to a known extruder, whereby the mixture is extruded in the form of sheet through a silt-like die, then the sheet is cooled to solidify on a casting roll to obtain unoriented film. When a laminated film is prepared, lamination is effected using two or more extruders and a manifold or a junction block (e.g. a junction block having a square joining portion) of two or more layers, and a sheet of two or more layers is extruded through the die, then cooled on the casting roll to obtain an unoriented film. In this case, it is effective to provide a static mixer and a gear pump in the polymer flow path. It is also effective to set the melting temperature in the extruder for the polymer on the top layer side of the laminate at a level 5°C to 10°C lower than that on the base layer side. Next, the unoriented film is biaxially stretched and oriented. As the stretching method a successive biaxial stretching method or a simultaneous biaxial stretching method may be used. But it is particularly preferred to use a successive biaxial stretching method wherein stretching is performed first in the longitudinal direction and then in the transverse direction, and to divide the longitudinal stretching into three or more stages so that the total longitudinal stretch ratio is in the range of 3.5 to 6.5. The longitudinal stretching temperature differs, depending on the kind of the polyester used, but usually it is effective to set the first-stage temperature at 50 °C to 130°C and the second and the following stage temperature at a higher level. A suitable longitudinal stretching rate is 5,000 to 50,000%/min. Commonly adopted for the transverse stretching is a method using a stenter, and a suitable stretch ratio is in the range of 3.0 to 5.0. The stretching rate and temperature in the transverse direction are suitably in the ranges of 1,000 to 20,000 %/min and 80° to 160°C, respectively. The thus-oriented film is then heat-treated. A suitable heat treatment temperature is in the range of 170° to 220°C, more suitably 180° to 200°C, and a suitable heat treatment time is in the range of 0.2 to 20 seconds. Since the polyester composition according to the present invention contains a specific polyester and crosslinked polymer particles, preferably crosslinked polymer particles treated with a water-soluble high molecular weight compound, the particles are uniformly monodispersed in the polyester and maintain a dispersion stability even when held in a melted state. Consequently, the following excellent effects are also exhibited. For example, when film is to be formed from the polyester composition, there will be no clogging of the filter caused by agglomeration in the course of melt-molding, nor will there occur breakage of the film caused by coarse agglomerated particles. In the case of a stretched film obtained from the polyester compositon, a surface havinq uniform concavities and convexities is obtained. Therefore, the film is superior in slipperiness, transparency and abrasion resistance, so is suitable for a magnetic tape, photograph, printing plate or capacitor. In the case where a laminate film is formed using the composition of the present invention, the film has few coarse projections of agglomerated particles and is superior in abrasion resistance. Examples The following Examples are given to illustrate embodiments of the present invention in more detail. Characteristic values of the polyesters and the particles obtained in those examples were measured in the following manner. (1) Particle Diameter An average particle diameter was measured from a particle equivalent sphere diameter at a point of 50 vol%, using an electron photomicrograph of particles. The equivalent sphere diameter represents the diameter of a sphere having the same volume as the particle volume. Volume Shape Factor of Particles From a biaxially oriented polyester film prepared by using a polymer composition of the present invention, ultrathin specimens having a thickness of about 0.3µm are prepared using an ultrathin film-preparing apparatus with regard to the cross sections in the MD and TD directions. As to the specimens, the number average particle diameters (a, b and c) are measured with regard to the MD, TD and thickness directions using a transmission electron microscope. The volume of the particles (V) and the maximum diameter in a projection plane of the particles (D) are calculated from the following equations: V = 4 3 π · a · b · c D = max (a, b, c) The volume shape factor f is calculated from the following equation: f = V/D 3 (2) Intrinsic Viscosity of Polymer Determined at 25°C using o-chlorophenol as a solvent. (3) Thermal Decomposition Temperature of Particles A thermobalance weight-loss curve was measured at a heat-up rate of 20°C/min in a nitrogen atmosphere, using TAS-100 (a device of Rigaku Denki), and the temperature at which 10% decomposition occurred was defined as the thermal decomposition temperature. (4) State of Particle Dispersion in Polymer The polymers obtained were each cut into ultra-thin pieces of 0.3µm or so by means of an ultra-thin film preparing device, then the state of particle dispersion in the polymer was observed, and the number of secondary agglomerated particles per field of 0.01mm 2 was counted. (5) Dispersibility of Particles in Molten Polymer The polymers obtained were each held in a nitrogen gas atmosphere at 280°C for 10 minutes when a water soluble polymer is not used or at 290°C for 30 minutes when a water soluble polymer is used, then cooled to solidify, and analysis was made by the above method (4). (6) Characteristics of Film (6-1) Surface Roughness, Ra (µ m) Measured using a feeler type surface roughness tester (cut-off value: 0.08 mm, measurement length: 4 mm), according to JIS-B-0601. (6-2) Slipperiness (µ k) Film was slit to 1/2 inch and was allowed to travel in an atmosphere of 20°C, 60% RH, using a tape traveling tester TBT-300 (a device of Yokohama System Kenkyu-Sho K.K.), then initial µk was determined using the following equation: µk = 0.733 log (T 1 /T 2 ) where T 2 and T 1 represent an incoming-side tension and an outgoing-side tension, respectively. Guide diameter is 6 mm, guide material is SUS27 (surface roughness: 0.2S), winding angle is 180°C, and traveling speed is 3.3 cm/sec. When the µk value is not larger than 0.35, good slipperiness is obtained. The µk value of 0.35 is a critical value of whether the slipperiness will be considerably deteriorated or not during the formation of film or as a product. (6-3) Film Haze Measured according to ASTM-D1003-52. A film haze of 0.55% or less is a value at which a film attains the objects of the present invention. (6-4) Abrasion Resistance Using a tape traveling tester TBT-300 (a device of Yokohama System Kenkyu-Sho K.K.), film is allowed to travel repeatedly 1,000 times in an atmosphere of 25°C, 50% RH, and then white shaving powder (white powder) deposited on the guide portion is evaluated visually. Guide diameter is 8 mm, guide material is SUS27 (surface roughness: 0.2S), winding angle is 180°C, and tape traveling speed is 3.3 cm/sec. The following are criteria: o ○ : The amount of white powder is very small; desirable. ○ : The amount of white powder is small; desirable. Δ : The amount of white powder is somewhat large; not desirable. × : The amount of white powder is very large; not desirable. In the tables which will appear later the following abbreviations and amounts in connection with copolymerization, addition and treatment are used: SSIA : 5-sodium dimethyl sulfoisophthalate LSIA : 5-lithium dimethyl sulfoisophthalate DMI : dimethyl isophthalate DVB : divinylbenzene copolymer particles DVB-1 : commercially available divinylvenzene copolymer particles (divinylbenzene 55% ethylvinylbenzene 40%) DVB-2 : divinylbenzene copolymer particles (divinylbenzene 70%, ethylvinylbenzene 25%) DVB-3 : divinylbenzene copolymer particles (divinylbenzene 50%, ethylvinylbenzene 20%) BA : butyl acrylate 2EHA : : 2-ethylhexyl acrylate ST : styrene PVP : polyvinyl pyrrolidone P(VP/MA): poly(vinyl pyrrolidone/methyl acrylate) copylymer Copolymerization Amount: Part by weight based on 100 parts by weight of an aromatic dicarboxylic acid or an ester-forming derivative thereof Amount of Particles added: Part by weight based on 100 parts by weight of copolymerized polyester Amount of particle surface treatment: Part by weight based on 100 parts by weight of crosslinked polymer particles Example 1 100 parts by weight of dimethyl terephthalate, 2 parts by weight of 5-sodium dimethyl sulfoisophthalate and 70 parts by weight of ethylene glycol were fed into a flask equipped with a rectification column and melted at 160°C, then 0.04 part by weight of manganese acetate was added as catalyst. Thereafter, the temperature was raised up to 240°C over a period of 3 hours while methanol produced in the reaction was removed from the rectification column. After making sure that a predetermined amount of methanol had been distilled off, 0.03 part by weight of antimony trioxide and 0.03 part by weight of trimethyl phosphate were added and polycondensation was conducted by a conventional method to afford a copolyester having an intrinsic viscosity of 0.712. Then, using a vent type twin-screw extruder, chips of the polymer were melted and an aqueous slurry of divinylbenzene copolymer particles (DVB-1) having an average particle diameter of 0.40 µm and Dw/Dn = 1.5 was added so as to provide a final particles content in the polymer of 0.5 wt%. Melt extrusion was performed at a resin temperature of 280°C while holding the vent hole at a vacuum degree of 10 Torr (1.3kPa), to obtain polyethylene terephthalate (I) containing the divinylbenzene copolymer particles. The intrinsic viscosity of the polymer was 0.613. As a result of observation of the polymer using a transmission type electron microscope, the number of secondary agglomerates per field of 0.01 mm 2 was 3 and the particles were present substantially in a uniformly monodispersed state. The polymer was then held in a nitrogen gas atmosphere at 280°C for 10 minutes, then cooled to solidify and evaluated in the same manner as above. As a result, the number of secondary agglomerates per field of 0.01 mm 2 was four, and it turned out that the dispersion stability of the particles was maintained even when stored in a melted state. On the other hand, polyethylene terephthalate (II) having an intrinsic viscosity of 0.620 was prepared by a conventional method, using 100 parts by weight of dimethyl terephthalate, 70 parts by weight of ethylene glycol, 0.06 part by weight of magnesium acetate as an ester interchange reaction catalyst, 0.03 part by weight of antimony trioxide as a polymerization catalyst and 0.03 part by weight of trimethyl phosphate as a heat stabilizer. Then, this polymer (II) and the foregoing crosslinked polymer particles-containing polymer (I) were melt co-extruded at 290°C and the extrudate was wound round a casting drum having a surface temperature of 30°C, using an electrostatic casting method, then cooled to solidify to obtain unstretched laminate film having about 120 µm and 30 µm thick layers. This unstretched film was then stretched 3.4 times in the longitudinal direction at 90°C and 3.6 times in the transverse direction at 100°C, with respect to the original size thereof. Subsequent heat-setting at 210°C afforded a biaxially oriented laminate film having a thickness of 15µm (polymer (I) 3µm/polymer (II) 12µm). A particles-containing surface of the film was evaluated for characteristics; as a result, slipperiness surface uniformity, transparency and abrasion resistance were all satisfactory (Table 1). Examples 2-11 Polyethylene terephthalates (I') were prepared using a vent type extruder in the same way as in Example 1 except that the amount of 5-sodium dimethyl sulfoisophthalate to be copolymerized, and the composition, volume shape factor and average diameter of crosslinked polymer particles, the particles used together and lamination thickness were changed. The particles in the polymers were substantially in a monodispersed state, with few secondary agglomerates. The polymers were subjected to a melt test in the same manner as in Example 1 and it turned out that the dispersion stability of the particles was maintained without any marked re-agglomeration. Using the polymers prepared above and the foregoing polymer (II), biaxially oriented laminate films were formed in the same way as in Example 1. All of the films were superior in slipperiness, surface uniformity, transparency and abrasion resistance. Comparative Example 1 Using a vent type twin-screw extruder, chips of the polymer (II) were melted and an aqueous slurry (2.5 wt% based on the polymer weight) of divinylbenzene copolymer particles having an average particle diameter of 0.40 µm [obtained by the polymerization of a commercially available divinylbenzene (divinylbenzene 55%, ethylvinylbenzene 40%)] (volume shape factor 0.51) was added so as to provide a final particles content in the polymer of 0.5 wt%. Melt extrusion was performed at a resin temperature of 280°C while holding the vent hole at a vacuum degree of 10 Torr (1.3 kPa) to afford polyethylene terephthalate (III) containing the divinylbenzene copolymer particles. The intrinsic viscosity of the polymer was 0.610. As a result of observation of the polymer using a transmission type electron microscope, the number of secondary agglomerates per field of 0.01 mm 2 was 3 and the particles were present substantially in a uniformly monodispersed state. The polymer was then held in a nitrogen gas atmosphere at 280°C for 10 minutes, then cooled to solidify and evaluated in the same manner as above. As a result, the number of secondary agglomerates was twenty-four and thus it was impossible to maintain the dispersion stability at melt. Then, the polymer (II) and the crosslinked polymer particles-containing polymer were melt co-extruded at 290°C and the extrudate was wound round a casting drum having a surface temperature of 30°C, using an electrostatic casting method, then cooled to solidify to obtain unstretched laminate film having about 120µm and 30µm thick layers. This unstretched film was then stretched 3.4 times in the longitudinal direction at 90°C and 3.6 times in the transverse direction at 100 °C . Subsequent heat-setting at 210°C afforded a biaxially oriented laminate film having a thickness of 15µm. A particles-containing surface of the film was evaluated for characteristics; as a result, slipperiness, surface uniformity, transparency and abrasion resistance were all not satisfactory. Comparative Examples 2-5 Polyethylene terephthalates (III') were prepared using a vent type extruder in the same way as in Comparative Example 1 except that the crosslinked polymer particles were changed. The particles in the polymers were substantially in a monodispersed state with few secondary agglomerates. When the polymers were subjected to a melt test in the same manner as in Comparative Example 1, re-agglomeration occurred and the dispersion stability was not maintained. Using the polymers (III') thus prepared and the foregoing polymer (II), biaxially oriented laminate films were formed in the same way as in Comparative Example 1. All of the films were unsatisfactory in slipperiness, surface uniformity, transparency and abrasion resistance. The resuts obtained in Examples 1-11 and Comparative Examples 1-5 are as shown in Tables 1 and 2 below. Example 12 100 parts by weight of dimethyl terephthalate, 1 part by weight of 5-sodium dimethyl sulfoisophthalate and 70 parts by weight of ethylene glycol were fed into a flask equipped with a rectification column and melted at 160°C, then 0.06 part by weight of magnesium acetate was added as catalyst. Thereafter, the temperature was raised up to 240°C over a period of 3 hours while methanol produced in the reaction was removed from the rectification column. After making sure that a predetermined amount of such methanol had been distilled off, 0.03 part by weight of antimony trioxide and 0.03 part by weight of trimethyl phosphate were added and polycondensation was conducted by a conventional method to afford an aromatic copolyester (IV) having an intrinsic viscosity of 0.680. 10 parts by weight of divinylbenzene copolymer particles DVB-1 having an average diameter 0.3µm, 0.1 part by weight of polyvinyl pyrrolidone (molecular weight 10,000) and 90 parts by weight of water were mixed together and then agitated by means of turbine blades at room temperature for 1 hour to afford an aqueous slurry of the divinylbenzene copolymer particles each having a surface coated with the polyvinyl pyrrolidone. Then, using a vent type twin-screw extruder, the aromatic copolyester (IV) was melted and the aqueous slurry of divinylbenzene copolymer particles mentioned above was added in amount of 3.0 parts by weight based on 100 parts by weight of the polyester (IV). Melt extrusion was performed at a resin temperature of 280 °C while holding the vent hole at a vacuum degree of 10 Torr to afford a polyethylene terephthalate composition (V) containing the divinylbenzene copolymer particles. The intrinsic viscosity of the polymer obtained was 0.610. As a result of observation of the polymer using a transmission type electron microscope, the number of secondary agglomerates per field of 0.01 mm 2 was 3 and the particles were present substantially in a uniformly monodispersed state. The polymer was then held in a nitrogen gas atmosphere at 290°C for 30 minutes, thereafter cooled to solidify and then evaluated in the same manner as above. As a result, the number of secondary agglomerates per field of 0.01 mm 2 was four and it turned out that the dispersion stability was retained even when stored in a melted state. 0.06 part by weight of magnesium acetate as catalyst was added to 100 parts by weight of dimethyl terephthalate and 64 parts by weight of ethylene glycol, and an ester interchange reaction was conducted. Then, 0.03 part by weight of antimony trioxide and 0.03 part by weight of trimethyl phosphate were added and a polycondensation reaction was performed. As a result, there was obtained a polyethylene terephthalate (VI) having an intrinsic viscosity of 0.620. The polyethylene terephthalate composition (V ) was melt co-extruded onto the polyethylene terephthalate (VI) at 290°C to obtain unstretched laminate film. Thereafter, the film was stretched to three times its original size in both longitudinal and transverse directions at 90°C, followed by heat-treatment at 220°C for 15 seconds, to obtain a biaxially oriented film having an 8 µm thick layer of the polyethylene terephthalate (VI) and a 0.3µm thick layer of the polyethylene terephthalate composition (V) laminated onto the (VI) layer. As a result of evaluation, this film proved to be very superior in abrasion resistance, Ra = 0.014 µm, µk = 0.27, as shown in Table 3. Examples 13-18 Biaxially oriented films were produced in the same way as in Example 12 12 except that the amount of comonomer copolymerized in the aromatic copolyester (IV), the composition, average diameter, addition amount and volume shape factor of crosslinked polymer particles in the polyethylene terephthalate composition (V ), the amount and kind of surfactant, and the method for the addition of slurry were changed. In Example 13, after the ester interchange reaction in the preparation of the aromatic copolyester (IV), a slurry of the divinylbenzene copolymer particles coated with polyvinyl pyrrolidone in water was added to obtain the polymer. In Example 16, a single layer film was obtained without using polyethylene terephthalate (VI). Table 3 shows the results of evaluation of these films, from which it is seen that the films are very superior in both particle dispersibility after melting and abrasion resistance. Comparative Example 6 10 parts by weight of divinylbenzene copolymer particles DVB-1 having an average diameter 0.3µm and 90 parts by weight of water were mixed together and agitated by means of turbine blades at room temperature for 1 hour to afford an aqueous slurry of the divinylbenzene copolymer particles. Next, the polyethylene terephthalate (VI) was melted using a vent type twin-screw extruder and the aqueous slurry of the divinylbenzene copolymer particles was added in an amount of 3.0 parts by weight based on 100 parts of the polyester (VI). Melt extrusion was performed at a resin temperature of 280°C while holding the vent hole at a vacuum degree of 10 Torr to obtain a polyethylene terephthalate composition (VII) containing the divinylbenzene copolymer particles. The intrinsic viscosity of the polymer obtained was 0.612. As a result of observation of the polymer using a transmission type electron microscope, the number of secondary agglomerates per field of 0.01 mm 2 was 4 and the particles were present substantially in a uniformly monodispersed state. The polymer was then held in a nitrogen gas atmosphere at 290°C for 30 minutes, thereafter cooled to solidify and evaluated in the same manner as above. As a result, the number of secondary agglomerates per field of 0.01 mm 2 was 44 and it was impossible to maintain the dispersion stability in melt storage. The polyethylene terephthalate composition (VII) was melt co-extruded onto the polyethylene terephthalate (VI) at 290°C to obtain unstretched laminate film. The film was then stretched to three times its original size in both longitudinal and transverse directions at 90°C, followed by heat treatment at 220°C for 15 seconds to afford a biaxially oriented laminate film comprising an 8µm thick layer of the polyethylene terephthalate (VI) and a 0.3µm thick layer of the polyethylene terephthalate composition (VII). As a result of evaluation, this film proved to be inferior in abrasion resistance, Ra = 0.016µm, µk = 0.25, as shown in Table 5. Comparative Examples 7 and 8 Biaxially oriented films were obtained in the same way as in Comparative Example 1 except that the amount of comonomer copolymerized in the aromatic copolyester (IV), the composition and average particle diameter of crosslinked polymer particles in the polyethylene terephthalate compositions (V ) and (VII), and the amount and kind of surfactant, were changed. Table 5 shows the results of evaluation of these films, from which it is seen that the films are inferior in particle dispersibility after re-melting and also in abrasion resistance because they are outside the scope of the present invention. The results obtained in Examples 6-12 and Comparative Examples 6-8 are as shown in Table 3 below.

Claims: A polyester composition comprising (A) an aromatic polyester having copolymerized therein, 0.1 to 10 parts, by weight based on 100 parts by weight of the entire carboxylic acid component, of a compound containing at least one sulfonic acid group or sulfonic acid metal salt group and (B) crosslinked polymer particles. A polyester composition according to claim 1, wherein the compound containing at least one sulfonic acid group or sulfonic acid metal salt group is a compound selected from 5-sodium sulfoisophthalic acid, ester-forming derivatives thereof, 5-lithium sulfoisophthalic acid and ester-forminc derivatives thereof. A polyester composition according to claim 1 or claim 2, wherein a part or the whole of the surface of the crosslinked polymer particles (B) is treated with a water-soluble polymer. A polyester composition according to any preceding claim, wherein the water-soluble polymer has at least a vinylpyrrolidone unit as a repeating unit. A polyester composition according to any preceding claim, wherein the amount of the crosslinked polymer particles (B) is 0.0001 to 20 wt% based on the weight of the aromatic polyester (A). A polyester composition according to any preceding claim, wherein the crosslinked polymer particles (B) have an average particle diameter of 0.01 to 5µm. A polyester composition according to any preceding claim, wherein the crosslinked polymer particles (B) have a volume shape factor not smaller than 0.35. A polyester composition according to any preceding claim, wherein the crosslinked polymer particles (B) have a weight average diameter (Dw) to number average diameter (Dn) ratio, Dw/Dn, of not lower than 1.1. A polyester composition according to any preceding claim, wherein the crosslinked polymer particles (B) are divinylbenzene copolymer particles. A polyester composition according to any preceding claim, further containing precipitated particles which, as a constituent part thereof, contain at least one of lithium and calcium and also contain phosphorus. A polyester composition according to any preceding claim, containing at least one kind of inorganic particle having an average particle diameter of 0.001 to 5µm. A process for preparing a polyester composition according to any preceding claim, characterized in that a slurry of the crosslinked polymer particles (B) in water and/or an organic compound having a boiling point of not higher than 200°C is added to the aromatic polyester (A) or to a reaction system of the aromatic polyester (A). A biaxially oriented polyester film formed from a polyester composition according to any of claims 1 to 11. A biaxially oriented polyester laminate film having as at least one layer a film formed from a polyester composition according to any of claims 1 to 11. A biaxially oriented polyester laminate film according to claim 14, at least one surface layer of which is a said film formed from a polyester composition according to any one of claims 1 to 11. A biaxially oriented polyester laminate film according to claim 14 or claim 15, wherein the film thickness of the laminate is 0.1 to 10 times as large as the average particle diameter of the crosslinked polymer particles (B) present therein and the content of the said particles is in the range of 0.01 to 20 parts by weight based on 100 parts by weight of the polyester composition of the said laminate. Use of a polyester film according to any one of claims 13 to 16, as a base film for a magnetic recording medium. A magnetic recording medium having, as a base film, a film according to any one of claims 13 to 16.

Inventors: KIMURA MASAHIRO, SASAKI TOSHIHIRO, SUZUKI MASARU, YOSHIDA MINORU, KIMURA, MASAHIRO, SASAKI, TOSHIHIRO, SUZUKI, MASARU, YOSHIDA, MINORU
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1992
Main classification: C08L67
IPC codes: C08J5, G11B5, B32B27, C08L67