Biaxially oriented polyester film

Abstract: A biaxially oriented polyester film formed from an intimate mixture comprising (1) an aromatic polyester, and (2) 0.01 to 4 % by weight, based on the aromatic polyester, of spherical fine particles of silica having (a) an average particle diameter of 0.05 to 4 µm, (b) a particle diameter ratio, defined by the ratio of maximum diameter to minimum diameter, of from 1.0 to 1.2, and (c) a relative standard deviation (σ) of particle size, of up to 0.5. A biaxially oriented polyester film formed from an intimate mixture comprising (1) an aromatic polyester, (2) 0.005 to 3 % by weight, based on the aromatic polyester, of spherical fine particles of silica having the same properties as the above-mentioned (a), (b) and (c), and (3) 0.005 to 3 % by weight, based on the aromatic polyester, of inert fine particles having an average particle diameter of 0.01 to 3 µm and not exceed­ ing the average particle diameter of the spherical fine particles of silica. The biaxially oriented films are useful for a magnetic recording medium.

Description: This invention relates to a biaxially oriented polyester film. More specifically, it relates to a biaxially oriented polyester film containing substantially spherical fine particles of silica having a very sharp particle size distribution. Polyester films typified by a polyethylene terephthalate film are used in a wide range of applications such as magnetic tapes, capacitances, photography, packaging and overhead projectors because of their excellent physical and chemical properties. The slipperiness and abrasion resistance of polyester films are two great factors which affect not only workability in film production and processing steps in various uses but also product quality. If a polyester film has insufficient slipperiness or abrasion resistance, various troubles occur. For example, when a polyester film is to be used as a magnetic tape by coating a magnetic layer on its surface, friction between the film surface and a coating roll is great during the coating of the magnetic layer. The film surface is heavily worn by the friction, and in an extreme case, creases, scratches, etc. occur on the film surface. An audio or video tape or a computer tape obtained by slitting the film coated with the magnetic layer also markedly undergoes friction with many guide portions, reproduction heads, etc. in unwinding or winding from and on reels or cassette and in other operations, and scratches and distortions will occur. Furthermore, a white powdery substance is deposited as a result of abrasion of the film surface. Frequently, these troubles become a great cause of dropout which is dropping of recorded magnetic signals. Generally, the slipperiness of a film is improved by imparting protrusions and depressions to the film surface and thereby reducing the area of contact between the film surface and guide rollers or the like. Roughly, this is achieved by (i) a method in which inert fine particles are deposited from the catalyst residue during production of polymer used as a raw material for film formation, and (ii) a method in which inert inorganic fine particles are added to the starting polymer. Generally, with larger particle sizes, the fine particles in the starting polymer have a greater slipperiness improving effect. However, in precision applications as in magnetic tapes, particularly video magnetic tapes, the large particle size of the fine particles itself might cause occurrence of dropout, and to prevent dropout, the protrusions and depressions on the film surface should be as fine as possible. It is desired therefore to satisfy these contradictory properties at the same time. A film composed of a polyester containing inert fine particles usually has voids around the fine particles because peeling occurs between the fine particles and the polyester as a result of biaxial stretching. The voids become greater as the fine particles are larger in size, their shape is closer to a spherical shape rather than a plate-like shape, the fine particles are single particles which are more difficult of deformation, the area stretch ratio is higher in stretching the unstretched film, and the stretching is carried out at higher temperatures. Since the shape of the protrusions is gentler with incresing size of the voids, the film has a high coefficient of friction and scratches (small injuries) generated on the surface during repeated use facilitate dropping of the particles. Consequently, the durability of the film is decreased, and dust is generated from abrasion. It has been the previous practice to add calcium carbonate, titanium dioxide, kaolin, etc. as the inert fine particles. Since these fine particles lead to the formation of large voids, they inherently have the aforesaid problem. Japanese Patent Publication No. 34088/1982 discloses an oriented polyester film containing (I) 0.03 to 1 % by weight of an inert substance (A) having an average particle diameter of not more than 0.8 µm and a volume shape factor of less than 0.08, and (II) 0.002 to 0.1 % by weight of an inert substance (B) having an average particle diameter larger than that of the inert substance (A) but not more than 1.8 µm and a volume shape factor of 0.08 to π/6, provided that the volume shape factor f is defined by f=V/D 3 wherein D is the maximum diameter (µm) of the particles of each said inert substance and V is the volume (µm 3 ) of the particles. This patent publication states that the volume shape factor f of the inert substance (A) and (B) varies depending upon the method of manufacturing these substances, the place of their occurrence, etc., but is generally in the following ranges according to the types of the inert substances, namely 0.003 to 0.40 for calcium terephthalate and manganese terephthalate, 0.005 to 0.12 for silica, kaolin, potter's clay, diatomaceous earth and aluminosilicates, 0.005 to 0.40 for calcium carbonate, 0.40 to 0.52 for glass beads, 0.08 to 0.25 for barium sulfate, and 0.003 to 0.20 for calcium phosphate. Thus, the above Japanese patent publication does not disclose spherical silica having an f value of about 0.52. Japanese Laid-Open Patent Publication No. 171623/1984 discloses a biaxially stretched polyester film containing 0.01 to 5 % by weight of substantially spherical particles derived from colloidal silica having an average particle diameter (primary particle diameter) of 0.1 to 1 µm. This patent document totally fails to describe the particle size distribution of colloidal silica. This Japanese Laid-Open Patent Publication states that the colloidal silica having an average particle diameter of 0.1 to 1 µm is formed during removing of the alkali from sodium silicate as a raw material. However, it does not at all disclose a specific method of its production. According to the state of art in 1983 when the application of Japanese Laid-Open Patent Publication No. 171623/1984 was filed, colloidal silica obtained by using sodium silicate as a raw material and removing the alkali from it contained a considerable amount of secondary agglomerates of primary spherical particles, as described, for example, in Journal of Colloid and Interface Science, Vol. 26, pg 62-69, (1968) and "Powder and Powder Metallurgy", Vol. 23, No. 4,pg 19-24, (1976). It is believed therefore that the colloidal silica used in the method described in Japanese Laid-Open Patent Publication No. 171623/1984 contained a considerable amount of secondary agglomerates of primary spherical particles and was incoporated as such in polyesters. It is an object of this invention to provide a biaxially oriented polyester film having excellent slipperiness and abrasion resistance. Another object of this invention is to provide a biaxially oriented polyester film having many protrusions of a relatively uniform size on its surface. Still another object of this invention is to provide a biaxially oriented polyester film in which slipperiness equivalent to those of conventional biaxially oriented polyester films can be achieved by a smaller content of a slip agent than in the conventional films. A further object of this invention is to provide a magnetic tape having slipperiness and abrasion resistance as well as excellent magnetic recording characteristics. Other objects of this invention along with its advantages will become apparent from the following description. According to this invention, these objects and advantages of the invention are achieved by a biaxially oriented polyester film formed from an intimate mixture consisting of (1) an aromatic polyester, and (2) 0.01 to 4 % by weight, based on the aromatic polyester, of spherical fine particles of silica having (a) an average particle diameter of 0.05 to 4 µm, (b) a particle diameter ratio, defined by the ratio of maximum diameter to minimum diameter, of from 1.0 to 1.2, and (c) a relative standard deviation (σ) of particle size, defined by the following equation wherein D i is the diameter (µm) of the equivalent circular area of each of the particles, D is an average value (µm) of the diameter of the equivalent circular area defined by the following equation and n is the number of the particles, with the proviso that the diameter of the equivalent circular area denotes the diameter of each particle calculated when it is assumed that each particle is of a true spherical shape, of up to 0.3. The aromatic polyesters (1) in the present invention is a polyester derived from an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. Examples of the aromatic dicarboxylic acid include terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenylethanedicarboxylic acid, diphenyldicarboxylic acid, diphenylether dicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenylketonedicarboxylic acid and anthracenedicarboxylic acid. Examples of the aliphatic glycol include alkylene glycols having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and decamethylene glycol, and alicyclic diols such as cyclohexane dimethanol. Preferably, the aromatic polyester used in the invention comprises an alkylene terephthalate and/or an alkylene naphthalate as a main component. Polyethylene terephthalate, polyethylene 2,6-naphthalate, and copolymers composed of a dicarboxylic acid component at least 80 mole % of which consists of terephthalic acid and/or 2,6-naphthalenedicarboxylic acid and a glycol component at least 80 mole % of which consists of ethylene glycol are especially preferred among the above aromatic polyesters. In these preferred polyesters, not more than 20 mole % of the acid component may consist of any of the above aromatic dicarboxylic acids other than terephthalic acid and/or 2,6-naphthalenedicarboxylic acid, or an aliphatic dicarboxylic acid such as adipic acid and sebacic acid, or an alicyclic dicarboxylic acid such as cyclohexane-1,4-dicarboxylic acid. Moreover, not more than 20 mole % of the glycol component may consists of any of the above-mentioned glycol other than ethylene glycol, or an aromatic diol such as hydroquinone, resorcinol or 2,2-bis(4-hydroxyphenyl)propane, or an aliphatic diol containing an aromatic ring such as 1,4-dihydroxymethylbenzene, or a polyalkylene glycol (polyoxyalkylene glycol) such as polyethylene glycol, polypropylene glycol or polytetramethylene glycol. The aromatic polyester used in this invention may also have a component derived from a hydroxycarboxylic acid, for example an aromatic hydroxycarboxylic acid such as hydroxybenzoic acid or an aliphatic hydroxycarboxylic acid such as omega-hydroxycaproic acid copolymerized or bonded in an amount of not more than 20 mole % based on the total amount of the dicarboxylic acid component and the hydroxycarboxylic acid component. The aromatic polyester used in this invention may also contain a trifunctional or higher polycarboxylic acid or polyhydroxy compound such as trimellitic acid or pentaerythritol copolymerized in an amount which maintains the polyester substantially linear, for example not more than 2 mole % based on the entire acid components. The above polyesters are known per se , and can be produced by methods known per se. The aromatic polyester used in this invention is substantially linear and has film-formability, particularly by melt-shaping. Preferably, the aromatic polyesters has an inherent viscosity, determined at 35 °C for its ortho-chlorophenol solution, of about 0.4 to about 0.9. The spherical fine particles of silica (2) used in this invention are characterized by having (a) a small particle diameter, (b) a shape close to a true sphere and (c) a sharp particle size distribution. Specifically, the spherical fine particles of silia used in this invention have an average particle diameter of 0.05 to 4 µm (a). The average particle diameter is preferably 0.1 to 2 µm, more preferably 0.1 to 1 µm. The spherical silica particles has a particle size ratio, defined by the ratio of maximum diameter to minimum diameter, of from 1.0 to 1.2 (b), preferably from 1.0 to 1.15, more preferably from 1.0 to 1.12. The spherical silica particles used in the invention further have a relative standard deviation (σ) of particle size, defined by the following equation wherein D i is the diameter (µm) of the equivalent circular area of each of the particles, D is an average value (µm) of the diameter of the equivalent circular area defined by the following equation and n is the number of the particles, with the proviso that the diameter of the equivalent circular area denotes the diameter of each particle when it is assumed that each particle is of a true spherical shape, of up to 0.3, more preferably up to 0.12, (c). The spherical fine particles of silica used in this invention are each in the shape of a sphere very close to a true sphere, and are markedly different from silica particles previously known as a slip agent, which are ultrafine block-like particles having a particle diameter of about 10 mµ or agglomerated paticles having a particle diameter of about 0.5 µm resulting from agglomeration of the block-like particles. Since the spherical fine particles used in this invention have a relative standard deviation (σ) of up to 0.3, a spherical shape close to a true sphere and a very sharp particle size distribution, they give protrusions having a very uniform height. Since these protrusions give only relatively small voids around the sherical fine particles, they have a very sharp shape and give a film having much better slipperiness than protrusions having a gentle shape if they are equal in number. So long as the spherical silic particles meet the aforesaid requirements, there is no limitation on the method of their production and their other properties. For example, the spherical silica particles may be produced by hydrolyzing ethyl ortho-silicate [Si(OC 2 H 5 ) 4 ] to prepare monodisperse spheres, and subjecting the monodisperse spheres of hydrous silica [Si(OH) 4 ], subjecting the monodisperse spheres of the hydrous silica to a dehydration treatment to grow the silica bond (≡Si-O-Si≡) three-dimensionally (see, Journal of the Japanese Chemical Society, '81, No. 9, p. 1503). This reaction is shown as follows:- Si(OC 2 H 5 ) 4 +4H 2 O → Si(OH) 4 +4C 2 H 5 OH → ≡Si-OH+HO-Si≡ →≡Si-O-Si≡+H 2 O In the aromatic polyester of this invention, the spherical fine particles of silica are included in an amount of 0.01 to 4 % by weight, preferably 0.05 to 2 % by weight, more preferably 0.1 to 1 % by weight, based on the aromatic polyester. If the content of the spherical silica particles is less than 0.01 % by weight, their effect of improving the slipperiness and shaving resistance of the film is insufficient. If, on the other hand, it exceeds 4 % by weight, the surface flatness of the film is reduced undesirably. The aromatic polyester film of this invention can be produced by melt-shaping an intimate mixture comprising the aromatic polyester and the spherical fine particles of silica into a film, and stretching the resulting unstretched film biaxially. The intimate mixture of the aromatic polyester and the spherical fine particles of silica may be produced usually by adding the spherical silica particles (preferably as a slurry in a glycol) to the reaction system during the reaction of forming a polyester, for example during ester interchange reaction in the ester interchange method, at any desired time of polycondensation reaction or any desired time during the performance of the direct polymerization method. Preferably, the spherical silica particles are added to the reaction system in the initial stage of the polycondensation, for example before the inherent viscosity of a reaction mixture reaches about 0.3. The biaxially oriented polyester film of this invention may be produced in accordance with conventional methods of producing biaxially stretched films. For example, it may be produced by melt-shaping an intimate mixture of the spherical silica particles and the aromatic polyester into an amorphous unstretched film, stretching the unstretched film biaxially, and heat-setting the stretched film, and as required, heat-treating it under relaxation. The stretching may be carried out in two directions either consecutively or simultaneously. In the consecutive stretching, the film may, for example, be stretched first in the machine direction at a stretching temperature (T 1 ) of (Tg - 10) to (Tg + 45) °C (where Tg is the glass transition temperature of the polyester), and then in the transverse direction at a stretching temperature (T 2 ) of from (T 1 + 15) to (T 1 + 40) °C. The stretch ratio may be at least 2.5 times, particularly at least 3 times, in one direction and selected such that the area ratio becomes at least 8, particularly at least 10. The heat-setting temperature may be selected from the range of 180 to 250 °C, particularly 200 to 230 °C. When the stretchings in two directions are carried out simultaneously, the stretching temperature may be selected from the range of 70 to 120 °C. The stretch ratio and heat-setting temperature may be the same as those described above. Thus, the present invention provides a biaxially oriented polyester film characterized by having uniform raised and depressed surface characteristics, excellent slipperiness and abrasion resistance and generating only very small amounts of scratches, white powders, etc. The biaxially stretched polyester film of this invention is obtained by using the spherical fine particles of silica having the characteristics (a), (b) and (c). Thus, when the surface of the film is observed under an electron microscope, protrusions attributed to the spherical fine particles of silica are seen. When the surface of the film is ion-etched and then observed under an electron microscope, the spherical silica particles are directly seen. Electron microscopic observation also shows that the adjoining spherical silica particles in one protrusion are in direct contact with each other or interrupted by a void space. The film of this invention is also characterized in that only up to 10 protrusions among 100 arbitrarily selected protrusions on the film surface before ion etching have a group of spherical silica fine particles having the aforesaid relations. The distribution of large protrusions formed on the surface of the polyester film by the addition of fine silica particles having a very sharp particle size distribution is highly uniform and the heights of these protrusions are substantially equal. The biaxially oriented polyester film of this invention has uniform raised and depressed surface characteristics, excellent slipperiness and abrasion resistance and generates only very small amounts of scratches, white powder, etc. By utilizing these properties, the film of the invention can be used in various applications. For example, when it is used as a base film for magnetic recording, for example in video, audio and computer applications, excellent electromagnetic converting characteristics, slipperiness and travelling durability can be obtained. Accordingly, the present invention also provides a magnetic recording medium comprising the biaxially oriented film of the invention and a magnetic layer on one or both surfaces of the film. When the film of the invention is used in a capacitance, a low coefficient of friction, excellent windability, a low collapsing load and high transparency can be obtained. Preferably, the biaxially oriented polyester film of the invention is used as a base film of a magnetic recording medium, particularly a magnetic tape. It may also be widely applied to other fields, for example in electrical and packaging applications and as a film for vapor deposition. The film may also be subjected to a treatment for obtaining easy adhesion, for example coating of an easily adherent layer and a surface treatment such as corona discharge treatment. Fine particles of a slip agent may be separated from the biaxially oriented polyester film or the intimate mixture before film formation in the following manner. A sample (the polyester film or the intimate mixture) is fully washed with water to remove matter adhering to its surface, further washed with water, and dried. Five hundred grams of the sample is taken, and 4.5 kg of o-chlorophenol is added. With stirring, the mixture is heated to 100 °C, and then left to stand at this temperature for 1 hour to dissolve the polyester portion. If the polyester portion is difficult to dissolve because of its high crystallization, it is first melted and rapidly cooled before submitting it to the above dissolving operation. To remove dust or coarse insoluble materials other than the fine particles of the slip agent, such as a reinforcing agent contained in the polyester, the solution is filtered through a C-1 glass filter. The weight of the dust or the coarse materials is subtracted from the weight of the sample. The solution after filtration is introduced into an ultracentrifuge equipped with a 40P-type rotor RP30 (made by Hitachi Limited) at a rate of 30 cc per cell. The rotor is then rotated at 4500 rpm. After confirming that there is no trouble in rotation, the inside of the rotor was evacuated, and the rotating speed is increased to 30000 rpm. At this rotating speed, the particles are separated centrifugally. The separation is completely in about 40 minutes. If required this is confirmed by determining that the light transmittance at 375 mµ of the solution after the separation is a fixed value higher than that of the solution before separation. After the separation, the supernatant is removed by decantation to obtain the separated particles. Since the separated particles might include the polyester as a result of insufficient separation, o-chlorophenol at room temperature is added to the separated particles and the particles are nearly uniformly suspended. The suspension is again treated in the ultracentrifuge. This operation should be repeated until a fusion peak assigned to the polymer can no longer be detected by scanning differential calorimetric analysis on the dried particles. The resulting separated particles are dried in vacuum at 120 °C for 16 hours and then weighed. The following examples illustrate the present invention in greater detail. The various properties and characteristics in this invention are measured as described below. (1) Particle diameter of particles of spherical silica There are two cases of measuring the particle diameter of particles. 1) Determination of the average particle diameter, particle diameter ratio, etc. of a powder itself. 2) Determination of the average particle diameter, particle diameter ratio, etc. of particles in the film. 1) Determination on the powder The powder was spread on a sample stand of an electron microscope in such a manner as to avoid superposition of the individual particles as much as possible, and a thin vapor-deposited gold film layer having a thickness of 200 to 300 Å was formed on the surface of the spread powder by means of a gold sputtering device. The coated powder was then observed under a scanning electron microscope at a magnification of 10,000 to 30,000 X. The long diameters (D 1i ), short diameters (D si ) and circular area equivalent diameters (D i ) of at least 100 particles were measured by means of Luzex 500 (made by Japan Regulator Co., Ltd.). The long diameter (D 1 ), short diameter (D s ) and average particle diameter ( D ) of the particles are expressed by number average values represented by the following equations. 2) Determination on the particles in the film A small sample piece of the film was fixed to a sample stand for a scanning electron microscope, and ion etching was performed on the surface of the sample film under the following conditions by means of a sputtering device (JFC-1100 type ion etching device made by Nippon Denshi Co., Ltd.). Specifically, the sample was set in a bell jar, and the degree of vacuum was raised to about 10 -3 torr. Ion etching was carried out at a voltage of 0.25 KV and a current of 12.5 mA for about 10 minutes. Furthermore, in the same device gold sputtering was carried out on the film surface. The coated film was then observed with a scanning electron microscope at a magnification of 10,000 to 30,000 X. The long diameters (D 1i ), short diameters (D si ) and diameters (D i ) of the equivalent circular area of at least 100 particles were measured by means of Luzex 500 (made by Japan Regulator Co., Ltd.). Thereafter, the same procedure as in 1) above was taken. (2) Film surface roughness (Ra) This is a centerline average roughness (Ra) defined in JIS-B 0601. In this invention, it is measured by a needle-contacting type surface roughness tester (SURFCORDER SE-30C mabe by Kosaka Kenkusho Co., Ltd.). The measuring conditions are as follows:- (a) Radius of the tip of the contacting needle: 2 µm (b) Measuring pressure: 30 mg (c) Cutoff: 0.25 mm (d) Measuring length: 0.5 mm (e) Method of rearranging the data The measuring was repeated five times on the same sample. One largest value was removed. The average value- of the remaining four data was calculated. The resulting value was rounded off at the fourth place down the decimal point, and expressed down to the third place from the decimal point. (3) Void ratio In accordance with the method described in (1), 2) above, the slip agent particles were exposed on the film surface, and the long diameters of at least 50 solid fine particles and the long diameters of voids around them were measured. The void ratio is calculated in accordance with the following equation, and expressed as a number average value. Void ratio = Long diameter of a void Long diameter of a solid fine particle (4) Coefficient of friction (µk) of the film A film sample cut to a width of 1/2 inch was brought into contact with a fixed rod (surface roughness 0.3 µm) at an angle  of 152/180π radian (152°) and moved at a rate of 200 cm per minute in an environment kept at a temperature of 20 °C and a humidity of 60 %. A tension controller was adjusted so that the inlet tension T 1 became 35 g. The outlet tension (T 2 ,g) at this time was detected by an outlet tension detector after the film travelled 90 m. The coefficient of travelling friction, µk, was calculated in accordance with the following equation. µk = (2.303/) log (T 2 /T 1 ) = 0.868 log (T 2 /35) (5) Abrasion resistance The abrasion resistance of the travelling surface of the base film was evaluated by using a 5-stage mini-supercalender. The calender was a 5-stage calender composed of nylon rolls and steel rolls. The film was allowed to travel at a speed of 50 m/min. with a linear pressure on the film of 200 kg/cm at a treating temperature of 80 °C. When the travelling film was allowed to travel over 2,000 m, the abrasion resistance of the base film was evaluated by soiling adhering to the top roller of the calender on a scale of the following four grades. o ○ : No soiling on the nylon roll ○ : Hardly any soiling on the nylon roll X : Much soiling on the nylon roll XX : Heavy soiling on the nylon roll (6) Haze The haze of the film was measured by an integral sphere type HTR meter (made by Nippon Seimitsu Kogaku Co., Ltd.) in accordance with JIS-K 674. (7) Coefficient of static friction (µs) A fixed glass plate was placed under two superimposed films. The lower film contacting the glass plate was taken up by fixed speed rolls at a rate of 15 cm/min. A detector was fixed to one end of the upper film which was located opposite to the direction of taking up the lower film. The pulling force of the film/film was detected. At this time, a thread having a weight of 1 kg and an underside area of 70 cm was used. (8) Evaluation of scratching By using the same device for measuring the coefficient of friction as used in paragraph (4), a magnetic tape (1/2 inch wide) was stretched over the device so that the base film surface of the tape contacted the fixed rod at an angle of 152°. It was then allowed to travel 20 meters at a speed of 5 cm/sec. This travelling was repeated 30 times, and then the size, depth and number of the scratches occurring on the surface of the base film were comprehensively considered, and the scratches were evaluated on a scale of the following five grades. o ○ : No scratch was observed on the base film ○ : Hardly any scratch was observed on the base film Δ : Some scratches were observed on the base film X : Some thick scratches were observed on the base film XX : Many thick and deep scratches were observed throughout the base film. (9) Windability In the process of manufacturing a biaxially oriented polyester film, the film was wound up in a roll form having a width of 500 mm and a length of 4000 m. The appearance of this roll was examined in detail, and the number of nodular protrusions having a long diameter of at least 2 mm was counted, and rated on the following scale. 0 - 2: ○ 3 - 5: Δ 6 or more: X (10) Electromagnetic converting characteristics of a magnetic tape By using a commercial home VTR, a signal obtained by superimposing a 100 % chroma level signal on a 50 % white level signal (a 100 % white level signal has a peak-to-peak voltage of 0.714 V) was recorded. It was reproduced and measured by a Shibasoku noise meter (Type 925 R). The chroma S/N is defined by the following equation in accordance with the definition of Shibasoku. Chroma S/N (dB)=20 log ES (p-p) EN (rms) wherein ES (p-p) is the voltage potential (p-p) of the peak-to-peak of the reproduced signal of the white level signal. ES (p-p)=0.714 V (p-p) EN (rms) is the root mean square value of the voltage of the peak of the reproduced signal of the chroma level signal. EXAMPLE 1 Dimethyl terephthalate and ethylene glycol were polymerized in a customary manner using manganese acetate (ester-interchange catalyst), antimony trioxide (polymerization catalyst), phosphorous acid (stabilizer) and silica particles having the characteristics shown in Table 1 to give polyethylene terephthalate having an inherent viscosity, measured at 35 °C in ortho-chlorophenol, of 0.62. Pellets of the polyethylene terephthalate were dried at 170 °C for 3 hours, and melted in an extruder at a temperature of 280 to 300 °C. The molten polymer was extruded through a slit die (1 mm) onto a rotating cooled rum having a surface finish of about 0.3 S and a surface temperature of 20 °C to form an unstretched film having a thickness of 200 µm. The unstretched film was pre-heated to 75 °C, and stretched to 3.6 times between a low-speed roll and a high-speed roll while it was heated by an IR heater having a surface temperature of 900 °C positioned 15 mm above the film. It was then rapidly cooled, fed to a stenter, and stretched transversely to 3.7 times at 105 °C. The resulting biaxially oriented film was heat-set for 5 seconds at a temperature of 205 °C to give a heat-set biaxially oriented film having a thickness of 15 µm. The properties of the stretched film are shown in Table 1. COMPARATIVE EXAMPLE 1 A heat-set biaxially oriented polyester film was prepared in the same way as in Example 1 except that kaolin having an average particle diameter of 1.2 µm and a particle diameter ratio of 10.0 as a slip agent was used instead of the silica particles. The resulting film was unsatisfactory because it had a void ratio of 1.7 and in calendering, a white powder adhered to it. The properties of the film are shown in Table 1. COMPARATIVE EXAMPLE 2 Polyethylene terephthalate pellets were obtained in the same way as in Example 1 except that calcium carbonate having an average particle diameter of 0.8 µm and a particle diameter ratio of 1.5 was used instead of silica particles. A biaxially oriented film having a thickness of 15 µm was prepared as in Example 1 using the pellets. The resulting film had a void ratio of 2.5. Although it had good travelling property, a white powder formed in the calendering step. The properties of the film are shown in Table 1. COMPARATIVE EXAMPLE 3 Polyethylene terephthalate pellets were obtained as in Example 1 except that titanium oxide having an average particle diameter of 0.4 µm and a particle diameter ratio of 2.0 was used instead of silica particles. A biaxially oriented film having a thickness of 15 µm was prepared from the pellets in the same way as in Example 1. The film had a void ratio of 2.0. Although the surface of the film could be designed in a flat shape, it had poor travelling property and a white powder formed in the calendering step or in the travelling system. Since it could not be directly utilized as a tape, an easily slippable layer had to be formed on the surface of the film. The properties of the film are shown in Table 1. EXAMPLES 2-4 AND COMPARATIVE EXAMPLES 4-5 In each run, polyethylene terephthalate pellets were prepared in the same way as in Example 1 except that fine silica particles having the average particle diameter and particle diameter ratio indicated in Table 1 were used. Thereafter, a heat-set biaxially oriented polyester film having a thickness of 15 µm was prepared in the same way as in Example 1 except that the resulting pellets were used. The properties of the film are shown in Table 1. The results show that the biaxially oriented films obtained in Examples 1 to 4 had excellent slipperiness in spite of their flat surfaces, and had excellent abrasion resistance in the calendering step, indicating excellent quality. EXAMPLE 5 Biaxially oriented polyester films were prepared in the same way as in Examples 3 and 4 except that the stretch ratios were changed to 4.5 times in the machine direction and to 3.6 times in the transverse direction. The properties of the film are shown in Table 2. EXAMPLES 6-10 AND COMPARATIVE EXAMPLE 6 In each run, polyethylene terephthalate pellets were prepared in the same way as in Example 1 except that fine silica particles having the average particle diameter and particle diameter ratio indicated in Table 3 were used. A heat-set biaxially oriented polyester film having a thickness of 30 µm was prepared in the same way as in Example 1 except that the results pellets were used. The properties of the film are shown in Table 3. The biaxially oriented films obtained in Exampels 6 to 10 showed excellent slipperiness and scratch resistance and very good abrasion resistance in the D/F treating step. Furthermore, they had a low friction between films and good processability. All of them thus showed excellent quality. When the spherical silica particles had a particle diameter of more than 4 µm as in Comparative Example 6, a white powder formed in the dust fabric treatment, and the film had poor abrasion resistance. EXAMPLES 11-12 Biaxially oriented polyester films were obtained in the same way as in Examples 6 and 7 except that the stretch ratios were changed to 4.5 times in the machine direction and to 3.6 times in the transverse directions. The properties of the films are shown in Table 3. COMPARATIVE EXAMPLE 7 A heat-set biaxially oriented film having a thickness of 30 µm was prepared in the same way as in Example 1 except that kaolin having the characteristics shown in Table 4 was used. The resulting film was unsatisfactory because it had a void ratio of 1.7, and a white powder adhered to the film in the calendering step. The properties of the film are shown in Table 4. COMPARATIVE EXAMPLE 8 Polyethylene terephthalate pellets were obtained in the same way as in Comparative Example 7 except that calcium carbonate having an average particle diameter of 0.8 µm and a particle diameter ratio of 1.5 was used instead of kaolin. A biaxially oriented film having a thickness of 30 µm was prepared in the same way as in Comparative Example 7 except that the resulting pellets were used. The film had a void ratio of 2.0. Although it had good travelling property, a white powder formed in the dust fabric treatment. The properties of the film are shown in Table 4. COMPARATIVE EXAMPLE 9 Polyethylene terephthalate pellets were prepared in the same way as in Comparative Example 7 except that titanium oxide having an average particle diameter of 0.4 µm and a particle diameter ratio of 2.0 was used instead of kaolin. A biaxially oriented film having a thickness of 30 µm was prepared in the same way as in Comparative Example 7 except that the resulting pellets were used. The film had a void ratio of 1.8. The film had a high film-to-film friction and poor processability and travelling property, and a white powder formed in the calendering step or in the travelling system. Since it could not be directly utilized as a tape, an easily slippable layer had to be formed on the back surface of the film. The properties of the film are shown in Table 4. EXAMPLE 13 Dimethyl terephthalate and ethylene glycol were polymerized in a customary manner using manganese acetate as an ester-interchange catalyst, antimony trioxide as a polymerization catalyst, phosphorus acid as a stabilizer and spherical silica particles having an average particle diameter of 0.27 µm and a particle diameter ratio of 1.05 as a slip agent to give polyethylene terephthalate having an intrinsic viscosity, measured at 35 °C in o-chlorophenol, of 0.62. Pellets of the polyethylene terephthalate were dried at 170 °C for 3 hours, and melted in an extruder at a temperature of 280 to 300 °C. The molten polymer was extruded through a slit die (1 mm) onto a rotating cooled drum having a surface finish of about 0.3 S and a surface temperature of 20 °C to form an unstretched film having a thickness of 200 µm. The unstretched film was pre-heated to 75 °C, and stretched to 3.6 times between a low-speed roll and a high-speed roll while it was heated by an IR heater having a surface temperature of 900 °C positioned 15 mm above the film. It was then rapidly cooled, fed to a stenter, and stretched transversely to 3.7 times at 105 °C. The resulting biaxially oriented film was heat-set for 5 seconds at a temperature of 205 °C to give a heat-set biaxially oriented film having a thickness of 15 µm. Separately, acicular alpha-FeOOH containing 5 % of cobalt was reduced with hydrogen at an elevated temperature to obtain Fe 2 O 3 . It was heated in air to obtain a ferromagnetic iron powder having an average needle length of 0.2 µm. One hundred parts by weight (to be referred to simply as "parts" hereinafter) was kneaded with the following ingredients for 12 hours. Polyester-polyurethane 12 parts Vinyl chloride/vinyl acetate/maleic anhydride copolymer 10 parts alpha-Alumina 5 parts Carbon black 1 part Butyl acetate 70 parts Methyl ethyl ketone 35 parts Cyclohexanone 100 parts After dispersion, 1 part of oleic acid and 1 part of palmitic acid as fatty acids and 1 part of amyl stearate as a fatty acid ester were further added. The mixture was kneaded for 15 to 30 minutes, and 7 parts of a 25 % ethyl acetate solution of a triisocyanate compound was added. The mixture was dispersed for 1 hour under high-speed shear to prepare a magnetic coating composition. The magnetic coating solution was coated on the above heat-set biaxially oriented film (thickness 15 µm) to a dry thickness of 3.5 µm. The coated film was subjected to an orientation treatment in a dc magnetic field, dried at 100 °C, and thereafter calendered. The film was then slit to a width of 1/2 inch to obtain a video magnetic tape having a thickness of 18.5 µm. The resulting film had very good windability, a low coefficient of friction after 50 passes, good scratch resistance and good electromagnetic conversion characteristics. The properties of the resulting film are shown in Table 5. EXAMPLE 14 A biaxially oriented film and a magnetic tape were prepared in the same way as in Example 13 except that the average particle diameter of the spherical silica particles was changed as shown in Table 5. The film and the tape had very good quality as shown in Table 5. COMPARATIVE EXAMPLE 10 A biaxially oriented film and a magnetic tape were prepared in the same way as in Example 13 except that the average particle diameter of the spherical silica particles was changed as shown in Table 5. The results are shown in Table 5. The magnetic tape had poor scratch resistance and windability. COMPARATIVE EXAMPLES 11-12 A biaxially oriented film and a magnetic tape were prepared in the same way as in Example 13 except that calcium carbonate (Comparative Example 11) or kaolin (Comparative Example 12) was used as the slip agent. The results are shown in Table 5. The films and magnetic tapes obtained had a high coefficient of friction, very poor scratch resistance, and insufficient windability or electromagnetic conversion properties. EXAMPLE 15 A biaxially oriented film and a magnetic tape were prepared in the same way as in Example 13 except that in film formation, the stretch ratios were changed to 4.5 times in the machine direction and to 3.6 times in the transverse direction. The properties of the films and tapes are shown in Table 5. EXAMPLE 16 Dimethyl terephthalate and ethylene glycol were polymerized in a customary manner using manganese acetate as an ester-interchange catalyst, antimony trioxide as a polymerization catalyst, phosphorus acid as a stabilizer and spherical silica particles having an average particle diameter of 0.22 µm and a particle diameter ratio of 1.04 as a slip agent to give polyethylene terephthalate having an intrinsic viscosity, measured at 35 °C in o-chlorophenol, of 0.62. Pellets of the polyethylene terephthalate were dried at 170 °C for 3 hours, and melted in an extruder at a temperature of 280 to 300 °C. The molten polymer was extruded through a slit die (1 mm) onto a rotating cooled drum having a surface finish of about 0.3 S and a surface temperature of 20 °C to form an unstretched film. The unstretched film was pre-heated to 75 °C, and stretchd to 3.6 times between a low-speed roll and a high-speed roll while it was heated by an IR heater having a surface temperature of 900 °C positioned 15 mm above the film. It was then rapidly cooled, fed to a stenter, and stretched transversely to 3.7 times at 105 °C. The resulting biaxially oriented film was heat-set for 5 seconds at a temperature of 205 °C to give a heat-set biaxially oriented film having a thickness of 15 µm. The properties of the film are shown in Table 6. EXAMPLE 17 A heat-set biaxially oriented polyester film was prepared in the same way as in Example 16 except that the average particle diameter and amount added of the spherical silica particles were changed as in Table 6. The properties of the film were good as shown in Table 6. EXAMPLE 18 A heat-set biaxially oriented polyester film was prepared in the same way as in Example 16 except that the stretch ratios in film formation were changed to 4.5 times (pre-heating temperature 70 °C) in the machine direction and to 3.5 times in the transverse direction (stretching temperature 105 °C). The properties of the resulting film are shown in Table 6. COMPARATIVE EXAMPLES 13-18 In each run, Example 1 was repeated except that particles having the characteristics described in Table 7 were used. Table 7 shows the properties of the resulting heat-set biaxially oriented films having a thickness of 15 µm. COMPARATIVE EXAMPLE 19 A biaxially oriented polyester film was prepared in the same way as in Comparative Example 13 except that bentonite shown in Table 8 was used instead of kaolin. The properties of the resulting film are shown in Table 8. COMPARATIVE EXAMPLES 20-24 In each run, a heat-set biaxially oriented polyester film having a thickness of 15 µm was prepared in the same way as in Example 1 except that the fine particles shown in Table 9 were used as a slip agent. The properties of the films are shown in Table 9. The calcium terephthalate particles indicated in Table 9 were prepared by the following procedure. A 5 % by weight aqueous solution of sodium terephthalate was added to a 10 % by weight aqueous solution of calcium chloride to form a white precipitate of calcium terephthalate. The calcium terephthalate precipitate was separated, washed with water and then heated at 200 °C to form an anhydrous salt. The anhydrous calcium terephthalate was pulverized in a ball mill and dispersed in ethylene glycol to form a slurry. Subsequent classification gave a glycol slurry of calcium terephthalate having a predetermined particle diameter.

Claims: A biaxially oriented polyester film formed from an intimate mixture consisting of (1) an aromatic polyester, and (2) 0.01 to 4 % by weight, based on the aromatic polyester, of spherical fine particles of silica having (a) an average particle diameter of 0.05 to 4 µm, (b) a particle diameter ratio, defined by the ratio of maximum diameter to minimum diameter, of from 1.0 to 1.2, and (c) a relative standard deviation (σ) of particle size, defined by the following equation wherein D i is the diameter (µm) of the equivalent circular area of each of the particles, D is an average value (µm) of the diameter of the equivalent circular area defined by the following equation and n is the number of the particles, with the proviso that the diameter of the equivalent circular area denotes the diameter of each particle calculated when it is assumed that each particle is of a true spherical shape, of up to 0.3, wherein when the surface of the film is observed under an electron microscope, protrusions attributed to the spherical fine particles of silica are seen; when the surface of the film is ion-etched and then observed under an electron microscope, the spherical silica particles are directly seen, and in this observation, it is seen that the adjoining spherical silica particles in one protrusion have either a relation of being in direct contact with each other, or a relation of being interrupted by void space, and that only up to 10 protrusions among 100 arbitrarily selected protrusions on the film surface before ion etching have a group of spherical fine silica particles having the aforesaid relations. The film of claim 1 wherein the aromatic polyester is derived from an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. The film of claim 1 wherein the spherical fine particles of silica have an average particle diameter of 0.1 to 2 µm. The film of claim 1 wherein the spherical fine particles of silica have a particle diameter ratio of from 1.0 to 1.15. The film of claim 1 wherein the spherical fine particles of silica have a relative standard deviation (σ) of particle size of up to 0.12. The film of claim 1 wherein the amount of the spherical fine particles of silica is 0.05 to 2 % by weight based on the aromatic polyester. A magnetic recording medium comprising the biaxially oriented film of claim 1 and a magnetic layer on one or both surfaces of the film.

Inventors: HAMANO HISASHI, HASEGAWA KINJI, KATOH HIDEO, NOUMI YOSHIHIRO, HAMANO, HISASHI, HASEGAWA, KINJI, KATOH, HIDEO, NOUMI, YOSHIHIRO
Applicants: TEIJIN LTD, TEIJIN LIMITED
Application year: 1987
Main classification: C08K3
IPC codes: C08J5, C08K3