Polyester film for magnetic tape

Abstract: A laminated polyester film for a magnetic tape, which film comprises a layer of a crystalline polyester (A) and a layer of a polyester (B) which is laminated on at least one surface of the layer of the polyester (A) and has a melting point lower than that of the polyester (A), and which film has a Young's modulus of at least 600 kg/mm² in both longitudinal and transverse directions, which film is suitable as a base film of a video tape.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a polyester film for a magnetic tape. More particularly, the present invention relates to a polyester film which has high strength in both longitudinal and transverse directions and excellent abrasion resistance and is suitable as a base film of a magnetic tape. Description of the Related Art Hitherto, as a base film of a long-play video tape, a so-called longitudinal tensilized film has been used. Recently, with the increase of needs for a magnetic tape with high image quality and high resolution, a particle size of magnetic powder is decreased and magnetic powder particles are highly orientated in a magnetic layer. As the result, the magnetic layer has decreased strength in the transverse direction. In addition, since a thickness of the magnetic tape is decreased for long time recording and therefore tape stiffness is decreases, a spacing loss arises between the tape and a magnetic head so that output power tends to be deteriorated and also irregularity of an envelope shape tends to appear. Further, a tape surface is smoothened and thereby a coefficient of friction increases, this results in tape damages. To solve the above problems, it is highly desired to increase the strength of the magnetic tape in the transverse (widthwise) direction and to provide a so-called both longitudinal and transverse tensilized film. To produce the both longitudinal and transverse tensilized film, it is necessary to stretch or expand the film in two directions, namely in the longitudinal and transverse directions of the film. As the results, particles tend to fall and abrased powder tends to be generated, so that the fallen particles and the abrased powder adhere onto a calendering roll, whereby productivity of the magnetic tapes is greatly deteriorated. To avoid such problems of the fallen particles and the abrased powder, kinds, particle sizes and amounts of the particles to be added to the film have been examined, but no satisfactory solution has been found. SUMMARY OF THE PRESENT INVENTION One object of the present invention is to provide a base film for a magnetic tape, which does not suffer from the problem of fallen particles and the abrased powder. Another object of the present invention is to provide a base film for a long-play video tape. According to the present invention, there is provided a laminated polyester film for a magnetic tape, which film comprises a layer of a crystalline polyester (A) and a layer of a polyester (B) which is laminated on at least one surface of the layer of the polyester (A), wherein polyester (B) has a melting point lower than that of polyester (A), said film having a Young's modulus of at least 5880 N/mm² (600 kg/mm²) in both longitudinal and transverse directions, and wherein said layer of polyester (B) has a planar orientation of not greater than 0.050. DETAILED DESCRIPTION OF THE INVENTION The polyester (A) used in the polyester film of the present invention may be a polymer prepared through a polycondensation reaction of an aromatic dicarboxylic acid (e.g. terephthalic acid and naphthalene-2,6-dicarboxylic acid) with an aliphatic glycol (e.g. ethylene glycol, diethylene glycol, tetramethylene glycol and neopentyl glycol). Specific examples of the polyester (A) are polyethylene terephthalate (PET) and polyethylene-2,6-naphthalene dicarboxylate (PEN). In addition to a homopolymer, it is possible to use a polymer prepared through a copolymerization with an aromatic or aliphatic dicarboxylic acid or diol in such amount that the crystallinity is not decreased, for example, at most 10 % by mole, preferably at most 5 % by mole. Although it is possible to blend other polymer such as a polyamide, a polyolefin and a polycarbonate in an amount of not larger than 10 % by weight, it is not preferable to use the other polymer in an amount such that the crystallinity is extremely decreased or the film surface is extremely roughened after the film preparation. The polyester (B) used in the polyester film of the present invention has a lower melting point than the polyester (A). The polyester (B) is preferably a copolymeric polyester. The copolymeric polyester may be a copolymerization product having a base polyester component unit used in the polyester (A) such as an ethylene terephthalate unit and an ethylene-2,6-naphthalate unit with at least one other component unit. The copolymerizable component may be at least one dicarboxylic acid such as oxalic acid, malonic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, 1,10-decane dicarboxylic acid, phthalic acid, isophthalic acid, terephthalic acid, naphthalene dicarboxylic acid and diphenylether dicarboxylic acid; or at least one diol such as neopentyl glycol, 1,4-butane diol, trimethylene glycol, propylene glycol, tetramethylene glycol, hexamethylene glycol, diethylene glycol, polyalkylene glycol and 1,4-cyclohexane dimethanol. A hydroxycarboxylic acid such as p-hydroxybenzoic acid and p-hydroxyethoxybenzoic acid; a monofunctional compound such as benzoic acid, benzoylbenzoic acid and methoxypolyalkylene glycol; and a polyfunctional compound such as glycerol and pentaerythritol can be used so long as the product has a substantially straight chain. A content of the copolymerizable component which is other than the base components of the polyester (A) and used in the copolymer (B) is such that a total (T + W) of a ratio [T (% by mole)] of the carboxylic acid in such copolymerizable components to the total amount of all carboxylic acids and a ratio [W (% by mole)] of the diol component in such copolymerizable component to the total amount of all the diols is usually from 6 to 40 % by mole, preferably 10 to 40 % by mole. When the amount of such copolymerizable component is smaller than 6 % by mole, effects on the prevention of particle drop off and the improvement of abrasion resistance are not expected. When the amount is larger than 40 % by mole, the formation of the laminate film is difficult. The polyester used in the present invention may usually contain fine particles such as an organic lubricant and inorganic lubricant to improve a sliding property. If necessary, an additive such as a stabilizer, a pigment, an antioxidant, an antifoamer and an antistatic agent may be added. Specific examples of fine particles giving the sliding property are inert external particles such as kaolin, clay, calcium carbonate, silicon oxide, spherical silica, calcium terephthalate, aluminum oxide, titanium oxide, calcium phosphate, lithium fluoride and carbon black; and internal particles formed in the polymer during the polymer preparation by a high melting point organic compound which is not meltable during the film melt preparation of the polyester resin, a cross-linked polymer and a metal compound catalyst (for example, an alkaline metal compound and an alkaline earth metal compound) used during the polyester preparation. The polyester layer (A) does not necessarily contain the inert particles depending on a form of lamination, for example, when the polyester layer (A) is sandwiched between a pair of the polyester layers (B). If desired, the polyester layer (A) may contain the inert particles having an average particle size of 0.007 to 5 µm, in particular 0.02 to 1.5 µm in an amount of 0.001 to 1.00 % by weight, preferably 0.1 to 0.8 % by weight. An average particle size of the inert particles contained in the polyester layer (B) is not limited and is preferably from 0.007 to 3.5 µm, in particular from 0.02 to 2 µm in view of good rolled form of the film. A content of the particles is usually from 0.001 to 40 % by weight, preferably from 0.005 to 15 % by weight. The polyester film of the present invention has the Young's modulus of at least 600 kg/mm² 5880 N/mm² in both longitudinal and transverse directions. Preferably, the longitudinal Young's modulus is at least 600 Kg/mm² and the transverse Young's modulus is at least 700 kg/mm² 6860 N/mm². More preferably, the Young's modulus both in the longitudinal and transverse directions is at least 700 kg/mm². In particular, the longitudinal Young's modulus is at least 700 kg/mm² and the transverse Young's modulus is at least 800 kg/mm² 7840 N/mm². If the Young's modulus in either of the longitudinal and the transverse direction is less than 600 kg/mm², the film cannot be practically used. Although the film forming method of the film according to the present invention is explained in detail, it is not limited to the following method. In the present invention, the laminated film may be prepared by any one of the conventional methods such as coextrusion, extrusion-lamination, dry lamination and the like. Among them, the coextrusion is advantageous in the preparation of a thin copolymer film and preferable in view of the productivity. Therefore, the following explanation is made by making reference to the coextrusion. The polyester (A) and polyester (B) optionally containing the inorganic particles in a suitable amount are dried using a hopper dryer, a paddle dryer, a vacuum dryer or the like, and then molten at 200 to 320°C in separate extruders. Then, the polyesters are flowed together in a pipe or a nozzle, extruded in two layers or three layers, and quenched to prepare an unstretched film. For extrusion, a conventional method such as a T-die method and a tubular method can be used. By adjusting a flow amount in each extruder, a ratio of the thicknesses in a laminate film can be varied. When the T-die method is used to prepare the unstretched film, a film with a uniform thickness can be obtained by using a so-called electrostatic pinning method. Then, the unstretched film is stretched bin longitudinal and transverse directions at a draw ratio of at least 2.0 at a temperature of from (Tg A - 10) to (Tc A - 10) °C [Tg A : a glass transition temperature of the polyester (A), Tc A : a crystallization temperature of the polyester (A)]. Then, the film is again successively biaxially stretched at a temperature of from (Tg A + 10) to (Tm A - 40) °C at a longitudinal draw ratio of from 1.05 to 2.5 and a transverse draw ratio of from 1.05 to 2.5 and then heat-set. In the present invention, the heat set is preferably carried out at a temperature from (Tm B - 10) to Tm A °C, more preferably from Tm B to Tm A °C, in particular from (Tm B + 5) to Tm A °C [Tm B : a melting point of the polyester (B)]. When the heat set temperature is Tm A or higher, the film frequently breaks and when the temperature is lower than (Tm B - 10) °C, the film easily curls. The heat set is usually carried out with maintaining the film in the fixed state. The film can be shrunk or tentered in the longitudinal and/or transverse directions of the film in an amount of not larger than 20 % during the heat treatment or during cooling after the heat treatment. Before, during or after the stretching steps, a corona discharge treatment can be applied to one or both of the film sides so that the adhesion property of the film to a magnetic layer and the like are improved. Before, during or after the stretching steps, a coating can be conducted on one or both of film sides so that an adhesion property, an antistatic property, an easy sliding property and a light shielding property are improved. The film prepared as stated above is wound to give a product. Although the laminate film of the present invention is based on a two layer film consisting of layers (A) and (B), the film of the present invention may have more than two layers. When the polyester layers (B) are laminated on the both surfaces of the polyester layer (A), the kinds, particle sizes and amounts of the particles to be contained in one of the polyester layers (B) may be the same as or different from those in the other in view of the end use of the polyester film. When they are different, one of the polyester layers (B) contains a smaller amount of the particles, for example, 0.007 to 0.5 % by weight, while the other contains a larger amount of the particles, for example, 0.5 to 15 % by weight. Such film can be used as a base film of a nonback-coated magnetic tape. A ratio (t/d) of the thickness (t) of the layer (B) to the average particle size (d) of the inert particles in the layer (B) is preferably from 0.1 to 10, more preferably from 0.2 to 2.0, most preferably from 0.3 to 1.0. When the value of t/d is outside the above range, the rolled form in the rolled product of the film tends to be unsatisfactory. The degree of planar orientation ΔP is not larger than 0.050, preferably not larger than 0.030, more preferably not larger than 0.010. When the ΔP is larger than 0.100, the film has poor abrasive resistance. A thickness of the polyester layer (B) is usually 5 µm or less, preferably from 0.005 to 2 µm, more preferably from 0.01 to 1 µm, in particular from 0.01 to 0.7 µm. When the thickness of the polyester layer (B) exceeds 5 µm, the strength of the polyester film decreases. PREFERRED EMBODIMENT OF THE INVENTION The present invention will be illustrated by the following examples which do not limit the present invention. In Examples, the properties of the film were measured as follows: 1. Melting point (Tm) Using DSC-1 manufactured by Perkin Elmer Corp., an endothermic peak temperature corresponding to crystal melting at a heating rate of 16 °C/min was measured and defined as a melting point. 2. Young's modulus Using Tensilon OTM-II (manufactured by Toyo Boldwin), the Young's modulus of the film was measured at 25°C, 50 %RH under the following conditions: Sample shape: Rectangular (length of 15 cm, width of 1 cm) Chuck distance: 10 cm Pulling rate: 100 %/sec. 3. Amount of generated white powder White powder adhered to a feed roller in a rolling instrument for a stretched film was visually observed and evaluated as follows: Good: No generation of white powder Fair: Slight generation of white powder Bad: Much generation of white powder 4. Contamination of a calendering roller A contamination degree of a roller surface contacting to a base film was evaluated using a five step miniature supercalender. The supercalender was five-step calender consisting of mirror finished metal rollers and polyester composite resin rollers. In each roller, a temperature was fixed at 95 °C, a line pressure was fixed at 250 kg/cm and a travel velocity was fixed at 80 m/min. The magnetic tape of 5000 m was repeatedly traveled seven times, and white powder adhered to the resin roller was visually evaluated as follows: Good: Substantially no adhesion of white powder to the resin roller Fair: Slight adhesion of white powder Bad: Apparent adhesion of white powder 5. Surface roughness (Ra) By the use of a surface roughness tester (SE-3FK manufactured by Kosaka Kenkyusho, Ltd.), the center line average roughness was measured according to JIS B-0601-1976, with necessary modifications. The measuring conditions were the use of a contact needle having having a tip radius of 2 µm, 30 mg of probe contact pressure, 80 µm of cutoff, and 2.5 mm of a measuring length. The measurement was performed at 10 points on the film and the measured values were averaged. The average was reported in the Table. 6. Coefficient of dynamic friction against metal pin Around a fixed hard chromium plated metal roll having a diameter of 6 mm, a film was contacted at a winding angle of 135° (ϑ). With applying a load of 53 g (T₂) to one end of the film, the film was moved at a rate of 1 m/min. and a resistance (T₁ g) at the other end was measured. A coefficient of friction during moving (µd) was calculated according to the following formula: µd = 1 ϑ ln (T₁/T₂) = 0.424 ln (T₁/53) Comparative Example 1 (Preparation of polyester) Terephthalic acid (87 parts by weight) was esterified with ethylene glycol (42 parts by weight) under an atmospheric pressure at 260 °C in the presence of bis-(β-hydroxyethyl) terephthalate oligomer (100 parts by weight) to prepare a polyester oligomer having an esterification rate of 97 %. Then, an ethylene glycol slurry of spherical silica particles having an average particle size of 0.3 µm was added in an amount of 0.35 % by weight based on the polyester. Then, ethyl acid phosphate (0.014 part by weight), antimony trioxide (0.022 part by weight) and magnesium acetate (0.086 part by weight) were added to perform a polycondensation reaction so as to prepare a polyester having an intrinsic viscosity of 0.660 (polyester (I)). (Preparation of film) The polyester (I) was dried and extruded at 295 °C by an extruder to prepare a sheet and an amorphous sheet was prepared by the electrostatic pinning cooling method. The resultant amorphous sheet was firstly 2.5 times stretched at 100 °C in a longitudinal direction, and then 1.8 times at 90 °C in the same direction. Then, the sheet was 3.62 times stretched at 105 °C in a transverse direction, and again 1.06 times at 125 °C in the longitudinal direction and 1.32 times at 190 °C in the transverse direction, followed by heat setting at 220 °C to obtain a film having a thickness of 10 µm. Examples 1 and 2 As the polyester (B), was used a copolymeric polyester (II) in which the dicarboxylic acid component consists of 80 % by mole of a terephthalic acid units and 20 % by mole of isophthalic acid units and the diol component consists of 100 % by mole of ethylene glycol units, and which contains the same spherical silica particles as used in Comparative Example 1 and has an intrinsic viscosity of 0.70. The polyesters (I) and (II) were dried and molten in separate extruders at 287 °C, flowed together in a pipe and coextruded in the form of a laminate consisting of an inner layer of the polyester (I) and outer layers of the polyester (II), followed by quenching to obtain an unstretched film. The unstretched film was stretched in the manner as in Comparative Example 1 to obtain a film having a thickness of 10 µm. The film consisted of three layers having thicknesses of 0.1 µm, 9.8 µm and 0.1 µm (Example 1) or 0.05 µm, 9.9 µm and 0.05 µm (Example 2). On one surface of the obtained film, a magnetic paint was coated to fabricate a magnetic tape. In the calendering step, no white powder was generated and the magnetic tape had good properties. Comparative Example 2 (Preparation of polyethylene naphthalate) In a reactor, dimethyl naphthalene-2,6-dicarboxylate (100 parts by weight), ethylene glycol (60 parts by weight) and calcium acetate monohydrate (0.1 part by weight) were charged to carry out transesterification. That is, a reaction temperature was gradually raised from the initial reaction temperature of 180 °C to 230 °C over 4 hours with evaporating off methanol to complete the transesterification. After adding phosphoric acid (0.04 part by weight), titanium oxide having an average particle size of 0.3 µm (0.40 part by weight) and antimony trioxide (0.04 part by weight) were added to initiate the polycondensation reaction. That is, the temperature was gradually raised to 290°C while the pressure was gradually decreased from the atmospheric pressure to 0.3 mmHg. After 4 hours from the start of the reaction, the polycondensation reaction was terminated, and polyethylene naphthalate was discharged in a nitrogen atmosphere. The polymer melt as such was filtered through a filter corresponding 2500 mesh directly connected to an extruder and then extruded in a strand form and cut to obtain chips (polymer raw material (III)), which had the intrinsic viscosity of 0.68. (Preparation of polyethylene naphthalate film) The above raw material (III) was dried and extruded at 295 °C by an extruder to prepare a sheet and an amorphous sheet was prepared by the electrostatic pinning cooling method. In the above steps, the polymer was filtered through a two-step filter corresponding to 2000 mesh and 2500 mesh. The amorphous film was 3.0 times stretched at 135 °C in the longitudinal direction and then 3.95 times at 135 °C in the transverse direction. The biaxially stretched film was again 1.40 times stretched at 138 °C the longitudinal direction and then 1.22 times at 209 °C in the transverse direction. The stretched film was heat set at 239 °C to obtain a film having a thickness of 7.0 µm. Example 3 In the same manner as in Example 1 except using, as the polyester (B) (polyester (IV)), the polyester containing titanium oxide having an average particle size of 0.3 µm in place of the spherical silica particles, the film was prepared. The polyester of Comparative Example 2 (polyester (III)) and the above polyester (IV) were dried and molten in the separate extruders at 295 °C and 287 °C, respectively. Then, they were flowed together in the pipe and coextruded and quenched to obtain an unstretched film. The unstretched film was stretched in the same manner as in Comparative Example 3 to obtain a film having a thickness of 7.0 µm. The thicknesses of the layers of the polyester (III) and the polyester (IV) were 0.05 µm and 6.95 µm, respectively. In the stretching steps, the surface which contacted to the stretching rolls was that of the layer of the polyester (IV). After forming a magnetic layer on the polyester layer (IV), the film was calendered. No white powder adhered to the calendering roll, and the magnetic tape had good properties. The results are summarized in the Table.

Claims: A laminated polyester film for a magnetic tape, which film comprises a layer of a crystalline polyester (A) and a layer of a polyester (B) which is laminated on at least one surface of the layer of polyester (A), wherein polyester (B) has a melting point lower than that of polyester (A), said film having a Young's modulus of at least 5880 N/mm² (600 kg/mm²) in both longitudinal and transverse directions, and wherein said layer of polyester (B) has a planar orientation of not greater than 0.050. The laminated polyester film according to claim 1, wherein the polyester (A) is a polymer prepared through a polycondensation reaction of an aromatic dicarboxylic acid with an aliphatic glycol. The laminated polyester film according to claim 2, wherein the polyester (A) is one selected from the group consisting of polyethylene terephthalate (PET) and polyethylene-2,6-naphthalene dicarboxylate (PEN). The laminated polyester film according to claim 1, wherein the polyester (B) is a copolymerized polyester which is a copolymerization product of a base polyester component unit used in the polyester (A) with at least one other component. The laminated polyester film according to claim 1, wherein the Young's modulus in the longitudinal directions is at least 5880 N/mm² (600 Kg/mm²) and the Young's modulus in the transverse direction is at least 6860 N/mm² (700 kg/mm²). The laminated polyester film according to claim 1, wherein the Young's modulus both in the longitudinal and transverse directions is at least 6860 N/mm² (700 kg/mm²). The laminated polyester film according to claim 1, wherein the Young's modulus in the longitudinal direction is at least 6860 N/mm² (700 kg/mm²) and the Young's modulus in the transverse direction is at least 7840 N/mm² (800 kg/mm²).

Inventors: UTSUMI SHIGEO, UTSUMI, SHIGEO
Applicants: DIAFOIL HOECHST CO LTD, DIAFOIL HOECHST CO., LTD.
Application year: 1992
Main classification: G11B5
IPC codes: C08L67, G11B5, B32B27