Biaxially oriented film of polyethylene-2,6-naphthalenedicarboxylate
DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a biaxially oriented film of polyethylene-2,6-naphthalenedicarboxylate. More specifically, it relates to a biaxially oriented film of polyethylene-2,6-naphthalenedicarboxylate which has high strength in the longitudinal direction and which is excellent in flatness and is useful as a base film for magnetic recording media such as a video tape, an audio tape and a computer data cartridge which are capable of recording for a long time. 
     In recent years, magnetic tapes such as a video tape, an audio tape and a computer data cartridge are increasingly required to perform high-density recording for downsizing and high-quality display images, and further, it is also required to decrease the thickness and increase the recordable hours of the magnetic tapes. For complying with the requirement of an increase in the recordable hours of a magnetic tape, it is necessary to decrease the thickness of the base film. The decrease in the thickness may sometimes cause a decrease the stiffness of a tape so that the tape edge is liable to be damaged when the tape is loaded or unloaded, or that a tape undergoes deformation and recorded data is liable to have a strain when high tensile force is instantaneously exerted on the tape. For these reasons, the base film for a magnetic tape having long recordable hours is required to have a high Young&#39;s modulus. 
     Further, VTRs equipped with cameras are recently widely used, and these machines are used outdoors or in automobiles. As a result, the magnetic tape is often exposed to more severe temperature conditions than it was heretofore used, and there are therefore increasing demands for the dimensional stability of the tape at high temperatures and hence, strong demands for the dimensional stability of its base film at high temperatures. 
     A biaxially oriented polyethylene terephthalate film has been conventionally used as a base film for the above magnetic recording tape, and in particular, there is known a so-called supertensilized film of which the Young&#39;s modulus in the longitudinal direction is increased for long recordable hours. In this film, however, the maximum Young&#39;s modulus in the longitudinal direction is 850 kg/mm 2  at most, and the maximum Young&#39;s modulus in the transverse direction is 450 kg/mm 2  at most. Further, an attempt to increase the Young&#39;s modulus in the longitudinal direction inevitably decreases the Young&#39;s modulus in the transverse direction, and the tape is liable to suffer damage on its edge portion when it is running. On the other hand, an attempt to increase the Young&#39;s modulus in width direction (also referred to as &#34;transverse direction&#34; hereinafter) unavoidably results in a failure in obtaining a sufficient Young&#39;s modulus in the longitudinal direction, and the touch (contact) of the magnetic tape to a magnetic head is poor to cause fluctuation in output. Further, a base film imparted with a high Young&#39;s modulus by stretching at a high stretch ratio has a defect in that the strain which has occurred in molding remains to make the dimensional stability poor. The stretching at a high stretch ratio has another problem in that the yield is low. 
     Meanwhile, there are various proposals for magnetic tapes having, as a base film, a film of polyethylene-2,6-naphthalate (to be sometimes referred to as &#34;PEN&#34; hereinafter) which is a polyester having excellent dimensional stability against heat over polyethylene terephthalate (to be sometimes referred to as &#34;PET&#34; hereinafter). 
     Japanese Laid-open Patent Publication No. 60731/1988 discloses a process for producing a PEN film having a high Young&#39;s modulus in the longitudinal direction. However, the PEN film produced by this process has a residual strain which has occurred in molding, and there is a problem in that the film may shrink in the longitudinal direction when the tape is exposed to severe temperature conditions, e.g., in the outdoor use. 
     Japanese Laid-open Patent Publication No. 198328/1992 discloses a polyethylene-2,6-naphthalate film containing (i) 0.02 to 1.0% by weight of particles of at least one member selected from silicon oxide and aluminum oxide each of which is an aggregate of primary particles having an average particle diameter of 0.005 to 0.08 μm and has an average particle diameter of 0.03 to 0.2 μm and (ii) 0.005 to 0.3% by weight of particles of at least one member selected from a crosslinked polymer and calcium carbonate each of which has an average particle diameter of 0.2 to 0.8 μm, and having a plane orientation degree of 0.255 to 0.280. The object of this film is to improve the sliding properties. However, the main orientation direction of this film is not specified, and nothing is described concerning the improvement in the contact of the tape to a magnetic head and improvement in the prevention of the damage of a tape edge by increasing the Young&#39;s modulus in the longitudinal direction. Further, it is described in the above Japanese Laid-open Patent Publication that excellent results are obtained when the tape satisfies the above plane orientation degree and at the same time has an average refractive index in the range of from 1.666 to 1.676, preferably 1.668 to 1.676, and that when the average refractive index is smaller than the above lower limit, the heat shrinkage percentage of the film increases. It is hence easily assumed that when the average refractive index is less than 1.666, the film according to the invention of the above Japanese Laid-open Patent Publication as a base film shows poor dimensional stability under severe temperature conditions. 
     Further, the invention in the above Japanese laid-open Patent Publication seeks to improve the film in abrasion resistance and lubricity by allowing a PEN film to contain specific proportions of aggregate particles having a specific average particle diameter (particles selected from silicon oxide and aluminum oxide) and particles selected from a crosslinked polymer and calcium carbonate. However, it is difficult to obtain a magnetic tape having highly excellent surface properties for high-density recording, only by incorporating the above particles. For example, since aggregate particles of silicon oxide or aluminum oxide are contained, it is assumed that voids occur in the film, and it is also assumed that these particles cause the formation of coarse projections on the film surface or the deterioration of the film in abrasion resistance. The above Japanese Laid-open Patent Publication does not described any particle size distribution of the particles contained in the film. However, even when the average particle diameter is the same, if the particle size distribution is not sharp, the distribution of projections formed on the film surface is not uniform, and no uniform surface projections having a uniform height are obtained. As a result, the lubricity deteriorates. 
     Further, Japanese Laid-open Patent Publication No. 117421/1993 discloses a technique which specifies the surface roughness of a PEN film. In this technique, however, the base film suffers a load under high tension at a high temperature when a magnetic layer is coated and dried for producing a magnetic tape. In this case, the base film may be elongated to undergo undulation, so that the applied magnetic layer may become nonuniform. 
     U.S. Pat. No 4,804,736 discloses a polymer film for use as a support for high density magnetic recording tapes, which has excellent heat resistance, a Young&#39;s modulus of not less than 800 kg/mm 2  in the machine direction and a Young&#39;s modulus of not less than 600 kg/mm 2  in the transverse direction, said film consisting of polyethylene-2,6-naphthalate. However, the above U.S. Patent discloses nothing concerning the elongation of the polymer film in the longitudinal direction under a high load at a relatively high temperature and the shrinkage percentage of the polymer film under no load at a relatively low temperature. U.S. Pat. No. 4,804.736 therefore fails to disclose any base film for a magnetic recording medium, which is almost free from a dimensional change in the longitudinal direction, has high strength and excellent flatness. 
     Co-pending U.S. patent application Ser. No. 08/181377 discloses a biaxially oriented, unidirectionally long polyethylene-2,6-naphthalate film, 
     (A) which contains 
     (a) 0.05 to 0.4% by weight of first inert solid particles having an average particle diameter of 0.05 to 0.3 μm, and 
     (b) 0.005 to 0.05% by weight of second inert solid particles having an average particle diameter of 0.3 to 1.0 μm, this average particle diameter being greater than the average particle diameter of the first inert solid particles by at least 0.2 μm, wherein: 
     (B) the Young&#39;s modulus in the length direction is at least 650 kg/mm 2 , the Young&#39;s modulus in the width direction is at least 600 kg/mm 2 , the Young&#39;s modulus in the length direction is greater than the Young&#39;s modulus in the width direction, 
     (C) the heat shrinkage factor in the length direction in heat treatment at 70° C. for 1 hour under no load is not more than 0.08%, and 
     (D) the surface roughness, Ra, is 3 to 10 nm. 
     However, the above Application discloses nothing concerning the elongation of the film in the longitudinal direction at a relatively high temperature under a high load. 
     Further, a co-pending application, Ser. No. 08/016259 discloses a biaxially oriented unidirectional long polyethylene-2,6-naphthalenedicarboxylate film having the following characteristic properties; 
     (A) both the Young&#39;s modulus in the longitudinal direction and the Young&#39;s modulus in the transverse direction are at least 600 kg/mm 2 , and the Young&#39;s modulus in the longitudinal direction is greater than the Young&#39;s modulus in the transverse direction, 
     (B) the heat shrinkage factor in the transverse direction when the film has been treated under no load at 105° C. for 30 minutes is at least 1%, and 
     (C) the surface roughness Ra of one surface of the film is up to 5 nm. 
     However, nothing is disclosed concerning the elongation of the film in the longitudinal direction at a relatively high temperature under a high load. 
     As described above, there has been known no polyester film which is almost free from a dimensional change in the longitudinal direction and has high strength and which is excellent in flatness and fully satisfies the requirements of a base film for a magnetic recording medium capable of recording for a long time and of high density recording. 
     It is therefore an object of the present invention to provide a biaxially oriented film of polyethylene-2,6-naphthalenedicarboxylate, which is almost free from a dimensional change in the longitudinal direction and has high strength in the longitudinal direction and which is excellent in flatness and is useful as a base film for a magnetic recording medium capable of recording for a long time with a high density. 
     It is another object of the present invention to provide a biaxially oriented PEN film as a base film for a magnetic recording medium, which overcomes the above-described defects, which has high strength and is excellent in durability when formed into a high-density magnetic recording tape, and which is excellent in dimensional stability against heat and flatness. 
     Other objects and advantages of the present invention will be apparent from the following description. 
     According to the present invention, the above objects and advantages of the present invention are achieved by a biaxially oriented film of a polyethylene-2,6-naphthalenedicarboxylate, which 
     (A) has a Young&#39;s modulus of at least 800 kg/mm 2  in the longitudinal direction and a Young&#39;s modulus of at least 500 kg/mm 2  in the transverse direction, the Young&#39;s modulus in the longitudinal direction being greater than the Young&#39;s modulus in the transverse direction, 
     (B) satisfies the following relationships between an elongation S M  (%) in the longitudinal direction per a load of 1 kg/mm 2  at 120° C. and the Young&#39;s modulus Y M  (kg/mm 2 ) in the longitudinal direction, 
     
         -0.001Y.sub.M +0.89&lt;S.sub.M &lt;-0.001Y.sub.M +1.57 
    
     when Y M  is 1,200 kg/mm 2  or less, 
     and 
     
         -0.31&lt;S.sub.M &lt;0.37 
    
     when Y M  is greater than 1,200 kg/mm 2 , 
     and 
     (C) has a heat shrinkage percentage of 0.1% or less in the longitudinal direction when the biaxially oriented film has been heat-treated under no load at 70° C. for 1 hour. 
     For the polyethylene-2,6-naphthalenedicarboxylate for forming the film of the present invention, 2,6-naphthalenedicarboxylic acid is a main acid component, while a small amount other dicarboxylic acid may be used as a comonomer. Further, ethylene glycol is a main glycol component, while a small amount of other glycol component may be used as a comonomer. The dicarboxylic acid component other than 2,6-naphthalenedicarboxylic acid includes aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, diphenylsulfonedicarboxylic acid, benzophenonedlcarboxylic acid and isomers of 2,6-naphthalenedicarboxylic acid. e.g., 2,7-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as succinic acid, adipic acid, sebacic acid and dodecanedicarboxylic acid; and alicyclic dicarboxylic acids such as hexahydroterephthalic acid and 1,3-adamantanedicarboxylic acid. The glycol component other than ethylene glycol includes 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, p-xylylene glycol and diethylene glycol. Further, the polymer (polyethylene-2,6-naphthalenedicarboxylate) may contain additives such as a stabilizer and a colorant. 
     The above PEN polymer is generally produced by a known method according to a melt polymerization method. At this time, additives such as a catalyst can be optionally used as required. The intrinsic viscosity of PEN is preferably in the range of from 0.45 to 0.90. 
     The biaxially oriented film of the present invention has a Young&#39;s modulus in the longitudinal direction, of at least 800 kg/mm 2 . When the biaxially oriented film has the Young&#39;s modulus in the longitudinal direction of less than 800 kg/mm 2 , a magnetic tape having a tape thickness of 16 μm or less and a film thickness of 12 μm or less, and having the capacity of recording and reproducing for a long time can have no sufficient Young&#39;s modulus as a tape. When the magnetic tape is allowed to run repeatedly, the tape edge may bend or may be elongated, so that the pressing of the tape to a magnetic head is not sufficient and that the electromagnetic conversion characteristics is poor. Thus, undesirably, no high-density, high-sensitivity magnetic recording tape can be obtained. 
     The Young&#39;s modulus in the longitudinal direction is preferably at least 900 kg/mm 2 , more preferably at least 1,000 kg/mm 2 . 
     Further, the biaxially oriented film of the present invention has a Young&#39;s modulus in the transverse direction, of at least 500 kg/mm 2 , which is a factor that prevents the damage of a side edge of the tape while it is running. That is, when the Young&#39;s modulus in the transverse direction is less than 500 kg/mm 2 , even if a magnetic binder having high strength is used, a tape edge may be deformed or brought into a crinkly form when the tape is running, and the damage to the tape side edge cannot be prevented. 
     The Young&#39;s modulus in the transverse direction is preferably more than 550 kg/mm 2 , more preferably more than 600 kg/mm 2 . 
     In the biaxially oriented film of the present invention, the Young&#39;s modulus in the longitudinal direction is required to be greater than the Young&#39;s modulus in the transverse direction. Preferably, the Young&#39;s modulus in the longitudinal direction is greater than the Young&#39;s modulus in the transverse direction by at least 250 kg/mm 2 . 
     The biaxially oriented film of the present invention can be imparted with the above Young&#39;s moduli by a method in which an unstretched film is simultaneously stretched, or consecutively stretched, in the transverse and longitudinal directions. Further, the unstretched film may be stretched in the longitudinal and transverse directions once in each direction, or may be stretched at least twice in the longitudinal and/or transverse direction directions, although the number of stretching in each direction is not specially limited. 
     For example, when the stretching is carried out in a two-stage stretching manner, PEN is melt-extruded at a temperature of between Tm and (Tm+70°) (in which Tm refers to the melting point of PEN) to prepare an unstretched film having an intrinsic viscosity of 0.45 to 0.9 dl/g, the unstretched film is monoaxially stretched (i.e., in the transverse or longitudinal direction) at a temperature between (Tg-10)° and (Tg+70)° C. (in which Tg refers to the glass transition temperature of PEN) at a stretch ratio of 2.5 to 7.0, and then the monoaxially stretched film is stretched at right angles with the above stretching direction (e.g., in the transverse direction in the second stage when the stretching in the first stage is carried out in the longitudinal direction) at a temperature between Tg (°C.) and (Tg+70)° C. at a stretch ratio of 2.5 to 7.0. In this case, the area stretch ratio is preferably 15 to 35, more preferably 20 to 35. 
     When the stretching is carried out in an at least three-stage stretching manner, preferably, the above biaxially oriented film is heat-set at a heat-set temperature between (Tg+20)° and (Tg+70)° C., and then, the film is stretched in the longitudinal or transverse direction at a temperature higher than the above heat-set temperature by 10° to 40° C. and then stretched in the transverse or longitudinal direction at a temperature higher than the above stretching temperature by 20° to 50° C., so that the total stretch ratio in the longitudinal direction is 5.0 to 10.0 and that the total stretch ratio in the transverse direction is 4.0 to 8.0. In this case, the area stretch ratio is preferably 20 to 50, more preferably 25 to 45. 
     The characteristic performance of the biaxially oriented film of the present invention is that the biaxially oriented film satisfies the following relationships (A) and (B) between an elongation S M  (%) in the longitudinal direction per a load of 1 kg/mm 2  at 120° C. and the Young&#39;s modulus Y M  (kg/mm 2 ) In the longitudinal direction, 
     
         -0.001Y.sub.M +0.89&lt;S.sub.M &lt;-0.001Y.sub.M +1.57           (A) 
    
     when Y M  is 1,200 kg/mm 2  or less, 
     and 
     
         -0.31&lt;S.sub.M &lt;0.37                                        (B) 
    
     when Y M  is greater than 1,200 kg/mm 2 . 
     As defined by the above relationship (A), the elongation S M  (%) decreases with an increase in the Young&#39;s modulus Y M  (kg/mm 2 ) in the longitudinal direction when the Y M  is in the range of from 800 to 1,200 kg/mm 2 . When Y M  exceeds 1,200 kg/mm 2 , S M  is a constant value regardless of Y M . A negative value of S M  (%) represents a shrinkage percentage (%). 
     When a biaxially oriented film fails to satisfy the above relationships (A) and (B), the base film is elongated to cause undulation when a magnetic coating composition is applied and dried in a tape production step, and a magnetic layer suffers nonuniformity. 
     Between S M  and Y M , preferably, the following relationship (A)-1 is satisfied when Y M  is 1,200 kg/mm 2  or less, 
     
         -0.001Y.sub.M +1.05&lt;S.sub.M &lt;-0.001Y.sub.M +1.48           (A)-1 
    
     and 
     the following relationship (B)-1 is satisfied when Y M  is greater than 1,200 kg/mm 2 . 
     
         -0.15&lt;S.sub.M &lt;0.28                                        (B)-1 
    
     Further, between S M  and Y M , more preferably, the following relationship (A)-2 is satisfied when Y M  is 1,200 kg/mm 2  or less, 
     
         -0.001Y.sub.M +1.14&lt;S.sub.M &lt;-0.001Y.sub.M +1.40           (A)-2 
    
     and 
     the following relationship (B)-2 is satisfied when Y M  is greater than 1,200 kg/mm 2 . 
     
         -0.06&lt;S.sub.M &lt;0.20                                        (B)-2 
    
     For producing the biaxially oriented film of the present invention which satisfies the above relationships between S M  and Y M , it is necessary to take into consideration means of decreasing S M  when the above means of increasing the Young&#39;s modulus Y M  is carried out. For this purpose, for example, it is preferred to make the stretch ratio in the longitudinal direction greater than the stretch ratio in the transverse direction, to select heat-setting conditions suitable for keeping the elongation at a low level, and optionally to adjust the heat-relaxation conditions for decreasing the heat shrinkage percentage. That is, S M  greatly differs depending upon the stretch ratio and heat-set temperature, and it is therefore advantageous to carry out the following procedures. Films are preliminarily prepared at various stretch ratios and at various heat-set temperatures under a range of conditions proper to obtain Young&#39;s moduli of a biaxially oriented film of the present invention in the longitudinal and transverse directions, and the films are measured for elongations in the longitudinal direction and for Young&#39;s moduli in the longitudinal and transverse directions at the same time. While the found measurement values are compared, an optimum stretch ratio and an optimum heat-set temperature are selected. Further, the shrinkage percentage is brought into a preferred range by heat relaxation, and conditions for the heat relaxation are selected such that the Young&#39;s modulus decreased by the heat relaxation is still within the range of the Young&#39;s modulus in the longitudinal direction, specified in the present invention, whereby the biaxially oriented film of the present invention is obtained. 
     Further, after the biaxially oriented film of the present invention is heat-treated under no load at 70° C. for 1 hour, the heat shrinkage percentage thereof in the longitudinal direction is 0.1% or less. When the heat shrinkage percentage is greater than 0.1%, undesirably, the film undergoes a thermal irreversible change even after it is formed into a magnetic tape, and the tape sometimes causes a skew on a screen when the temperatures for recording and reproducing with a VTR are different. Further, when the heat shrinkage percentage is large, the base film may be imprinted on a magnetic surface, and the surface roughness of the magnetic surface is liable to increase. The heat shrinkage percentage is preferably 0.08% or less, more preferably 0.05% or less. 
     For decreasing the heat shrinkage percentage, it is general practice to increase the temperature for heat-treating the stretched film. However, when the heat treatment temperature is increased to excess, the mechanical properties of the film consequently deteriorate, and scratches often occurs when the film is processed into a magnetic tape. And, scratch dust adheres to the magnetic surface of the magnetic tape to cause drop-out. The heat shrinkage percentage is therefore decreased by heating the heat-treated film under low tension and relaxing the film in the longitudinal direction. The film Is relaxed in the longitudinal direction according to an air-floating treatment method in which the film is relaxed in a non-contact state while heated under low tension, a method in which the film is relaxed by passing the film through nips of heating rolls and cooling rolls each of which has a mutually different speed, or a method in which the proceeding rates of clips holding the film in a tenter are consecutively moderated. Any one method may be used if the film can be relaxed in the longitudinal direction. 
     In any one of the above methods, the biaxially oriented film may be heat-set at a temperature between (Tg+70)° C. and Tm (°C.). For example, the heat-setting is preferably carried out at a temperature between 190° and 240° C. for 1 to 60 seconds. Further, the heat relaxation is carried out, whereby the film of the present invention having a desirable heat shrinkage percentage can be obtained. 
     The plane orientation coefficient NS of the biaxially oriented film of this invention preferably has the following relationships, represented by the formulae (C) and (D), with an average refractive index nA. 
     
         NS≧1.607nA-2.434                                    (C) 
    
     
         1.658≦nA≦1.665                               (D) 
    
     in which nA is determined by the following equation, ##EQU1## in which nx is the refractive index of a biaxially oriented film in the machine direction, ny is the refractive index at right angles with the machine direction, and nz is the refractive index in the film thickness direction. 
     More preferably, the biaxially oriented film has the plane orientation coefficient NS of at least 0.281 and the average refractive index nA satisfies the above relationship (D). The plane orientation coefficient NS is determined by the following equation. ##EQU2## wherein nx is the refractive index of a biaxially oriented film in the machine direction, ny is the refractive index at right angles with the machine direction, and nz is the refractive index in the film thickness direction. 
     A base film having the above plane orientation coefficient has sufficient nerve, has excellent thickness uniformity and serves to obtain a magnetic layer capable of high-density recording in the production of a magnetic tape. Therefore, the so-obtained magnetic layer is excellent in tracing contact to a magnetic head, and gives a magnetic recording medium stable in output. 
     The above formula (C) is preferably represented by the following formula (C)-1, 
     
         NS≧1.607nA-2.430, 
    
     more preferably by the following formula (C)-2, 
     
         NS≧1.607nA-2.427 
    
     The base film which satisfies both the relationships represented by the above formulae (C) and (D) has sufficient nerve, has a higher orientation degree in the longitudinal direction than that in the transverse direction, has excellent thickness uniformity and serves to give a uniform magnetic layer capable of high-density recording in the production of a magnetic tape. Therefore, the so-formed magnetic layer is excellent in tracing contact to a magnetic head, and gives a magnetic recording medium stable in output. 
     When the above formula (C) is not satisfied, both the Young&#39;s modulus in the longitudinal direction and the Young&#39;s modulus in the transverse direction are low, and when the magnetic tape is running, it is liable to undergo elongation and damage to its edge, and it is difficult to keep contact to a magnetic head under predetermined conditions, so that errors are liable to occur in recording and reproducing. Further, the flatness is poor, or the thickness uniformity is poor and slacking occurs, so that nonuniformity of a magnetic layer and magnetic omission are liable to occur, and the output is liable to fluctuate in reproducing. Further, the magnetic tape is poor in resistance to heat and deterioration. 
     When the above formula (D) is not satisfied and when the average refractive index nA&gt;1.665, the strength and Young&#39;s modulus in the longitudinal direction are insufficient to some extent, and a magnetic tape is poor in contact to a magnetic head to cause errors in recording and reproducing in some cases. When the average refractive index nA&lt;1,658, the mechanical properties such as strength and Young&#39;s modulus in one or both directions of the longitudinal and transverse directions are insufficient to some extent, a magnetic tape is liable to undergo elongation and damage to its edge, so that errors are liable to occur in recording and reproducing. In particular, a polyethylene terephthalate film comes under the above range, and shows a poor balance in mechanical properties in the longitudinal direction and transverse direction and poor dimensional stability against heat. 
     Studies by the present inventors have revealed that the biaxially oriented film of the present invention are excellent in take-up properties as a base film and running properties and electromagnetic conversion characteristics as a magnetic tape when the height of projections formed on the film surface and the number of the projections are in specific ranges. In the biaxially oriented film of the present invention, the height [h (unit: nm)] and the number of projections formed on the film surface are preferably in the following range, 
     
         ______________________________________                                    
 1 ≦ h &lt; 50                                                        
                 1,000-10,000                                             
                            pieces/mm.sup.2                               
 50 ≦ h &lt; 100                                                      
                 10-200     pieces/mm.sup.2                               
100 ≦ h &lt; 150                                                      
                 10-100     pieces/mm.sup.2                               
150 ≦ h   0          piece/mm.sup.2                                
______________________________________                                    
 
    
     more preferably in the following range, 
     
         ______________________________________                                    
 1 ≦ h &lt; 50                                                        
                 1,500-8,000                                              
                           pieces/mm.sup.2                                
 50 ≦ h &lt; 100                                                      
                 10-200    pieces/mm.sup.2                                
100 ≦ h &lt; 150                                                      
                 10-100    pieces/mm.sup.2                                
150 ≦ h   0         piece/mm.sup.2                                 
______________________________________                                    
 
    
     particularly preferably in the following range. 
     
         ______________________________________                                    
 1 ≦ h &lt; 50                                                        
                 2,000-5,000                                              
                           pieces/mm.sup.2                                
 50 ≦ h &lt; 100                                                      
                 10-80     pieces/mm.sup.2                                
100 ≦ h &lt; 150                                                      
                 10-50     pieces/mm.sup.2                                
150 ≦ h   0         piece/mm.sup.2                                 
______________________________________                                    
 
    
     A base film which satisfies the above rang is excellent in handling, and is excellent in running properties and electromagnetic conversion characteristics as a magnetic tape. A film having more than 10,000 pieces/mm 2  of projections having a height h (unit: nm) of 1≦h&lt;50 is liable to cause film abrasion with a roll when calender-treated. A film having more than 200 pieces/mm 2  of projections having a height of 50≦h&lt;100 is liable to show a decrease in electromagnetic conversion characteristics as a magnetic tape. A film having more than 100 pieces/mm 2  of projections having a height h of 100≦h&lt;150 is excellent in running properties as a magnetic tape, while it is liable to show a decrease in electromagnetic conversion characteristics and cause drop-out as a magnetic tape. Further, when a film has projections having a height of 150≦h, the electromagnetic conversion characteristics is liable to decrease and the drop-out is liable to occur frequently, since the frequency in the presence of coarse particles is large. Further, course projections having a height of 150≦h are abraded when the tape is running, and consequently promotes the occurrence of drop-out. When a film has less than 1,000 pieces/mm 2  of projections having a height of 1≦h&lt;50, less than 10 pieces/mm 2  of projections having a height of 50≦h&lt;100 and less than 10 pieces/mm 2  of projections having a height of 100≦h&lt;150, the coefficient of friction increases, and it is very difficult to handle the film and take up the film in the form of a roll. Further, it shows poor running properties as a tape. The number of projections having a height of h&lt;1 is not specially limited. 
     A film having the above-described surface properties can be obtained, for example, by incorporating into PEN several kinds of inert solid fine particles having different particle size distributions. Examples of the inert solid fine particles preferably include (1) silicon dioxide (including hydrate, diatomaceous earth, silica sand and quartz); (2) alumina; (3) silicates containing at least 30% by weight of an SiO 2  component (e.g., amorphous or crystalline clay mineral and aluminosilicate (including calcined materials and hydrates), chrysotile, zircon and fly ash)); (4) oxides of Mg, Zn, Zr and Ti; (5) sulfides of Ca and Ba; (6) phosphates of Li, Na and Ca (including monohydrates and dihydrates); (7) benzoates of Li, Na and K; (8) terephthalates of Ca, Ba, Zn and Mn; (9) titanates of Mg, Ca, Ba, Zn, Cd, Pb, St, Mn, Fe, Co and Ni; (10) chromates of Ba and Pb; (11) carbon (e.g., carbon black and graphite); (12) glass (e.g., glass powder and glass beads); (13) carbonates of Ca and Mg; (14) fluorite; (15) ZnS and (15) fine particles of polymers having high heat resistance such as a silicone resin and crosslinked polystyrene. More preferred are fine particles of any one of silicon dioxide, silicic acid anhydride, hydrous silicic acid, aluminum oxide, aluminum silicate (including calcined material and hydrates), monolithium phosphate, trilithium phosphate, sodium phosphate, calcium phosphate, barium sulfate, titanium oxide, lithium benzoate, double salts of these compounds (including hydrates), glass powder, clay (including kaolin, bentonite and china clay), talc, diatomaceous earth, calcium carbonate, a silicone resin and crosslinked polystyrene. Particularly preferred are silicon dioxide, titanium oxide, calcium carbonate, fine particles of a silicone resin and fine particles of crosslinked polystyrene. The film is allowed to contain at least one kind of the above inert solid fine particles. 
     The above inert solid fine particles may be added to PEN before the polymerization of PEN or during the polymerization. Further, the inert solid fine particles may be mixed with PEN in an extruder when the polymerized PEN is pelletized, or may be dispersed in PEN in an extruder before PEN is extruded in the form of a sheet. In view of dispersibility, it is preferred to add them before the polymerization. However, the method of obtaining a biaxially oriented film having the surface properties according to the present invention is not limited to the method of adding inert solid fine particles to PEN. There may be preferably employed a method in which a phosphorus component or other necessary additive is added during the polymerization to generate a particle source to form particles to be contained in the film, or a method in which PEN polymerized in the presence of a phosphorus component and PEN polymerized in the presence of inert solid fine particles are blended. 
     The film surface properties according to the present invention can be adjusted by effecting other surface treatment, e.g., coating treatment, on the surface of a film containing no inert solid fine particles or a film containing the inert solid fine particles. 
     The thickness of the biaxially oriented film of PEN of the present invention is not specially limited, while it is preferably 12 μm or less, particularly preferably 6 μm or less for using the biaxially oriented film for a video tape used for long-hour recording and reproducing as long as 180 minutes or longer or a DCC tape. The lower limit of the film thickness is preferably 3 μm in view of film stiffness. 
     The present invention will be further explained hereinafter with reference to Examples. Various physical property values and characteristics in the present invention are measured and defined as follows. 
     (1) Number of surface projections 
     Measured with a non-contact, three-dimensional roughness tester (TOPO-3D) supplied by WYKO at a measurement magnification of 40 times at a measurement area of 242 μm×239 μm (0.058 mm 2 ). A histogram of the heights of surface projections and the number of the projections was obtained on the basis of an average roughness of the film surface by projection analysis, and the numbers of projections in the specific ranges were determined on the basis of the histogram. The same film was measured in five places, and the numbers of projections were added up and converted to the number of projections per a unit area (1 mm 2 ). 
     (2) Young&#39;s modulus 
     A film was cut to prepare a sample having a width of 10 mm and a length of 150 mm, and the sample was tensioned with an Instron type universal tensile tester at an interchuck distance of 100 mm, at a tension rate of 10 mm/minute and at a charting rate of 500 mm/minute. The Young&#39;s modulus was calculated on the basis of a tangent in a rising portion of the resultant load-elongation curve. 
     (3) Refractive index 
     A film was measured for a refractive index with an Abbe refractometer using sodium D ray (589 nm) as a light source. At the same time, the film was measured for an orientation degree with a molecular orientation meter (MOA-2001A) supplied by Kanzakl Paper Mfg. Co., Ltd. A refractive index of a large value unmeasurable with the Abbe refractometer was determined on the basis of a graph for a correlation between the orientation degree and the refractive index. 
     (4) Elongation of film 
     Film samples having a width of 5 mm and a length of 15 mm were set in TMA (TM-3000L) supplied by Shinkurlko K.K., and temperature-increased from 20° C. to 150° C. at a temperature elevation rate of 5° C./minute under loading conditions: 0 g (no load), 25 g, 50 g, 75 g and 100 g. An elongation percentage of each film sample at 120° C. was determined, a graph for a correlation between the elongation percentage and a load per the cross section of the film (kg/mm 2 ) was prepared. The elongation (%) per a load of 1 kg/mm 2  was determined on the basis of the graph. 
     (5) Electromagnetic conversion characteristics 
     A magnetic tape for VTR was measured for an S/N ratio with a noise meter supplied by Shibasoku Co., Ltd. Further, a difference between the so-obtained S/N ratio and the S/N ratio of a tape of Comparative Example 3 in Table 1 was determined. A VTR, EV-S700, supplied by Sony Co., Ltd., was used. 
     (6) Durability of magnetic tape 
     While a magnetic tape was allowed to run and stop repeatedly with EV-S700 supplied by Sony Co., Ltd., for 100 hours, the tape was examined on its running state and measured for an output. The running durability of the tape was determined as follows. 
     &lt;Evaluation based on 3 ratings&gt; 
     O: Tape edge neither elongates nor causes any crinkly form. The output does not fluctuate. 
     Δ: Tape edge elongates and becomes crinkly to some extent. The output fluctuates to some extent. 
     X: Tape edge extraordinarily elongates and becomes crinkly. The tape unstably runs, and the output greatly fluctuates. 
     (7) Heat shrinkage percentage 
     A film having a length of about 30 cm and a width of 1 cm, which had been accurately measured for its length in advance, was placed in an oven at 70° C. under no load, and heat-treated for 1 hour. Then, the film was taken out of the oven, and allowed to stand until it had a room temperature. Then, the film was measured for a length to determine a change in the length. The heat shrinkage percentage was determined by the following equation, ##EQU3## in which L 0  is the length before the heat treatment and ΔL is a dimensional change amount due to heat treatment. 
     (8) Skew 
     A video tape used for recording at an ordinary temperature (20° C.) at an ordinary humidity was heat-treated at 70° C. for 1 hour, and the tape was played back at an ordinary temperature at an ordinary humidity. And, a skew deviation at a VTR head switching point was measured. 
     (9) Average Particle Diameter of Inert Fine Particles 
     Measured with a CP-50 model centrifugal particle size analyzer supplied by Shimadzu Corporation. An accumulative curve of particles having predetermined sizes and amounts thereof was prepared on the basis of a centrifugal sedimentation curve obtained, and a particle diameter corresponding to 50 mass percent was determined. This value was taken as the above average particle diameter. 
    
    
     EXAMPLE 1 
     Pellets of a PEN containing 0.25% by weight of spherical silica particles having an average particle diameter of 0.2 μm and 0.03% by weight of calcium carbonate particles having an average particle diameter of 0.5 μm and having an intrinsic viscosity of 0.62 dl/g (measured in o-chlorophenol at 25° C.) were dried at 170° C. and then melt-extruded at 300° C., and the extrudate was rapidly cooled on a casting drum kept at 60° C. to give an unstretched film having a thickness of about 170 μm. 
     The above unstretched film was consecutively biaxially stretched 5.6 times in the longitudinal direction at 130° C. and subsequently 3.8 times in the transverse direction at 135° C., and further heat-set at 215° C. This heat-set film was heated with a hot roll at 110° C., and then relaxation-treated by adjusting the tension of the film with a cooling roll, so that the film had a shrinkage percentage of 0.05% in the longitudinal direction. The so-obtained biaxially oriented PEN film had a thickness of 8 μm. 
     Separately, a composition of the following components was placed in a ball mill, and kneaded and dispersed for 16 hours, and 5 parts by weight of an isocyanate compound (Desmodur L, supplied by Bayer AG). The mixture was shear-dispersed at a high speed for 1 hour to give a magnetic coating composition. 
     Composition for magnetic coating composition 
     
         ______________________________________                                    
                  Part by weight                                          
______________________________________                                    
Acicular Fe particles                                                     
                    100                                                   
Vinyl chloride-vinyl acetate                                              
                    15                                                    
copolymer (S-LEC 7A, supplied                                             
by Sekisui Chemical Co., Ltd.)                                            
Thermoplastic polyurethane resin                                          
                    5                                                     
Chromium oxide      5                                                     
Carbon black        5                                                     
Lecithin            2                                                     
Fatty acid ester    1                                                     
Toluene             50                                                    
Methyl ethyl ketone 50                                                    
Cyclohexanone       50                                                    
______________________________________                                    
 
    
     The above-prepared magnetic coating composition was applied to one surface of the above biaxially oriented PEN film to form a coating having a thickness of 3 μm, and the coating was subjected to orientation treatment in a direct current magnetic field at 2,500 Gausses, dried under heat at 100° C., and treated with a super calender (linear pressure 200 kg/cm, temperature 80° C.). Then, the film was taken up. The so-obtained roll of the film was allowed to stand In an oven at 55° C. for 3 days. 
     Further, a back coating composition of the following components was applied to form a back coating having a thickness of 1 μm and dried, and the back-coated film was cut to a width of 8 mm to give a magnetic tape. 
     Back coating composition 
     
         ______________________________________                                    
                    Part by weight                                        
______________________________________                                    
Carbon black          100                                                 
Thermoplastic polyurethane resin                                          
                      60                                                  
Isocyanate compound (&#34;Coronate L,                                         
                      18                                                  
supplied by Nippon Polyurethane                                           
Industry Co., Ltd.)                                                       
Silicone oil          0.5                                                 
Methyl ethyl ketone   250                                                 
Toluene               50                                                  
______________________________________                                    
 
    
     Table 1 shows the physical properties of the above-obtained film and magnetic tape. As is clearly shown in Table 1, the film of the present invention is without any problem in processability, and excellent in dimensional stability against heat. It has a high Young&#39;s modulus in the longitudinal direction, and the tape obtained therefrom is excellent in electromagnetic conversion characteristics, running durability and skew, and is also excellent in use under high-temperature atmosphere. 
     EXAMPLE 2 
     An unstretched film was obtained in the same manner as in Example 1. The unstretched film was stretched 2.3 times in the longitudinal direction at 130° C., then stretched 3.9 times in the transverse direction at 130° C., and subjected to intermediate heat treatment at 160° C. Further, this biaxially oriented film was stretched 2.6 times in the longitudinal direction at 170° C., stretched 1.3 times in the transverse direction under the heat-setting at 215° C. Then, the so-obtained film was treated in the same manner as in Example 1 to obtain a film having a heat shrinkage percentage of 0.04% In the longitudinal direction. As a result, a biaxially oriented film having a thickness of 8 μm was obtained. Then, a tape was obtained in the same manner as in Example 1. Table 1 shows the results. The film and tape showed results as excellent as those in Example 1. 
     EXAMPLE 3 
     An unstretched film was obtained in the same manner as in Example 1. The unstretched film was stretched 2.5 times in the longitudinal direction at 130° C., then stretched 3.9 times in the transverse direction at 130° C., and subjected to intermediate heat treatment at 160° C. Further, this biaxially oriented film was stretched 2.8 times in the longitudinal direction at 170° C., stretched 1.2 times in the transverse direction under the heat-setting at 215° C. Then, the so-obtained film was treated in the same manner as in Example 1 to obtain a film having a heat shrinkage percentage of 0.05% in the longitudinal direction. As a result, a biaxially oriented film having a thickness of 8 μm was obtained. Then, a tape was obtained in the same manner as in Example 1. Table 1 shows the results. The film and tape showed results as excellent as those in Example 1. 
     EXAMPLE 4 
     An unstretched film was obtained in the same manner as in Example 1. The unstretched film was stretched 2.4 times in the longitudinal direction at 130° C., then stretched 3.8 times in the transverse direction at 130° C., and subjected to intermediate heat treatment at 160° C. Further, this biaxially oriented film was stretched 2.8 times in the longitudinal direction at 170° C., stretched 1.2 times in the transverse direction under the heat-setting at 215° C. Then, this heat-set film was heated to 110° C. with a hot roll, and then relaxation-treated by adjusting the tension of the film with a cooling roll, so that the film had a shrinkage percentage of 0.05% in the longitudinal direction. The so-obtained biaxially oriented PEN film had a thickness of 8 μm. Then, a tape was obtained in the same manner as in Example 1. Table 1 shows the results. As is clearly shown in Table 1, the film of the present invention is without any problem in processability, and the tape obtained therefrom is excellent in running durability, electromagnetic conversion characteristics and skew, and is also excellent in use under high-temperature atmosphere. 
     EXAMPLE 5 
     An unstretched film was obtained in the same manner as in Example 1. The unstretched film was stretched 2.5 times in the longitudinal direction at 120° C., then stretched 3.8 times in the transverse direction at 130° C., and subjected to intermediate heat treatment at 160° C. Further, this biaxially oriented film was stretched 3.0 times in the longitudinal direction at 170° C., stretched 1.2 times in the transverse direction under the heat-setting at 215° C. Then, the so-obtained film was treated in the same manner as in Example 1 to obtain a film having a heat shrinkage percentage of 0.05% in the longitudinal direction. As a result, a biaxially oriented PEN film having a thickness of 8 μm was obtained. Then, a tape was obtained in the same manner as in Example 1. Table 1 shows the results. The film and tape showed results as excellent as those in Example 1. 
     Comparative Example 1 
     An unstretched film was obtained in the same manner as in Example 1. The unstretched film was consecutively biaxially stretched 3.5 times in the longitudinal direction at 130° C., then stretched 3.5 times in the transverse direction at 135° C., and heat-set at 215° C. Then, the so-obtained film was treated in the same manner as in Example 1 to obtain a film having a heat shrinkage percentage of 0.05% in the longitudinal direction. As a result, a biaxially oriented film having a thickness of 8 μm was obtained. Then, a tape was obtained in the same manner as in Example 1. Table 1 shows the results. The film elongation per a load of 1 kg/mm 2  at 120° C. was large, so that the base film was elongated at the step of forming the tape and caused undulation to cause a nonuniformity of the magnetic layer. Further, the film had a high average refractive index, and that it had a low Young&#39;s modulus in the transverse direction. As a result, the contact of the magnetic tape to a magnetic head was defective, and the magnetic tape was considerably poor in electromagnetic conversion characteristics to cause errors in recording and reproducing. 
     Comparative Example 2 
     Pellets of PET containing particles of the same two kinds as those in Example 1 and having an intrinsic viscosity of 0.65 dl/g (measured in o-chlorophenol at 25° C.) were dried at 160° C. and then melt-extruded at 280° C., and the extrudate was rapidly cooled on a casting drum kept at 40° C. to give an unstretched film having a thickness of about 110 μm. The so-obtained unstretched film was stretched 1.8 times in the longitudinal direction by passing it through nips of two rolls having different rotation rates, stretched 3.1 times in the transverse direction with a tenter, and further subjected to intermediate heat treatment at 150° C. This film was further stretched 3.6 times in the longitudinal direction at 155° C., and then heat-set at 230° C. This heat-set, biaxially oriented PET film was heated with a hot roll at 90° C., and then subjected to a treatment for decreasing the heat shrinkage percentage in the longitudinal direction by adjusting the tension of the film with a cooling roll, so that a biaxially oriented PET film having a thickness of 8 μm was obtained. Thereafter, a tape was obtained therefrom in the same manner as in Example 1. Table 1 shows the results. Although the heat shrinkage percentage was decreased by the relation heat treatment, the film had a high heat shrinkage percentage and was very poor with regard to the skew due to PET as a raw material. Further, a film having a Young&#39;s modulus increased according to the present invention was obtained by increasing the stretch ratio in the longitudinal direction, while the Young&#39;s modulus in the transverse direction was considerably low since it was difficult to increase the stretch ratio in the transverse direction. As a result, when the tape was running, the tape suffered edge damage and showed poor running durability. Further, since the elongation in the longitudinal direction per a load of 1 kg/mm 2  at 120° C. was very large, the film had an undulation due to the elongation of the film at the step of forming a tape, to cause a nonconformity of the magnetic layer. As a result, the tape was very poor in electromagnetic conversion characteristics. These data show that a PET material is inferior to a PEN material as far as the total balance of the properties is concerned. 
     Comparative Example 3 
     An unstretched film was obtained in the same manner as in Example 1. The unstretched film was stretched 2.2 times in the longitudinal direction at 130° C., then stretched 3.9 times in the transverse direction at 130° C., and subjected to intermediate heat treatment at 160° C. This film was further stretched 2.2 times in the longitudinal direction at 170° C., stretched 1.8 times in the transverse direction under the heat-setting at 215° C. The relaxation treatment for decreasing the heat shrinkage percentage in Example 1 was not carried out. As a result, a biaxially oriented film having a thickness of 7 μm was obtained, and a tape was obtained therefrom in the same manner as in Example 1. Table 1 shows the results. In particular, since the heat shrinkage percentage was high, the tape was very poor with regard to the skew. Further, since the elongation in the longitudinal direction per a load of 1 kg/mm 2  was large to some extent, the magnetic layer had a nonuniformity as a coating, and the tape was therefore poor in electromagnetic conversion characteristics. Further, the Young&#39;s modulus in the longitudinal direction was low, and the contact of the tape to a magnetic head was poor to cause errors in recording and producing. 
     
                                           TABLE 1                                 
__________________________________________________________________________
            Unit Ex. 1                                                    
                      Ex. 2                                               
                           Ex. 3                                          
                                Ex. 4                                     
__________________________________________________________________________
Properties of based                                                       
film                                                                      
Number of surface                                                         
projections                                                               
 1 &lt; h &lt; 50 piece/                                                        
                 5,050                                                    
                      4,850                                               
                           4,800                                          
                                4,800                                     
            mm.sup.2                                                      
 50 &lt; h &lt; 100                                                             
            piece/                                                        
                 53   58   47   40                                        
            mm.sup.2                                                      
100 &lt; h &lt; 150                                                             
            piece/                                                        
                 24   18   20   15                                        
            mm.sup.2                                                      
150 &lt; h     piece/                                                        
                 0    0    0    0                                         
            mm.sup.2                                                      
Refractive index:                                                         
nx          --   1.768                                                    
                      1.777                                               
                           1.789                                          
                                1.813                                     
ny          --   1.731                                                    
                      1.732                                               
                           1.719                                          
                                1.710                                     
nz          --   1.482                                                    
                      1.482                                               
                           1.480                                          
                                1.476                                     
Average refractive                                                        
            --   1.660                                                    
                      1.664                                               
                           1.663                                          
                                --                                        
index: nA                                                                 
Plane orientation                                                         
            --   0.268                                                    
                      0.272                                               
                           0.273                                          
                                0.286                                     
coefficient: NS                                                           
1.607 nA - 2.434                                                          
            --   0.234                                                    
                      0.240                                               
                           0.238                                          
                                --                                        
Young&#39;s modulus:                                                          
EM          kg/mm.sup.2                                                   
                 1,020                                                    
                      970  1,200                                          
                                1,250                                     
ET          &#34;    530  650  550  550                                       
Elongation in                                                             
            %    0.30 0.35 0.08 0.06                                      
longitudinal                                                              
direction per load                                                        
of 1 kg/mm.sup.2 at 120° C.                                        
Heat Shrinkage                                                            
            %    0.05 0.04 0.05 0.05                                      
percentage                                                                
(longitudinal                                                             
direction)                                                                
70° C. × 1 hr.                                               
Properties of tape                                                        
            dB   -1.5 -1.5 -2.5 -2.5                                      
Electromagnetic                                                           
conversion                                                                
characteristics                                                           
Y-S/N                                                                     
Running durability                                                        
            --   Excel.                                                   
                      Excel.                                              
                           Excel.                                         
                                Excel.                                    
Skew        μsec                                                       
                 4    3    3    3                                         
__________________________________________________________________________
            Unit Ex. 5                                                    
                      CEx. 1                                              
                           CEx. 2                                         
                                CEx. 3                                    
__________________________________________________________________________
Properties of based                                                       
film                                                                      
Number of surface                                                         
projections                                                               
 1 &lt; h &lt; 50 piece/                                                        
                 4,600                                                    
                      5,650                                               
                           3,950                                          
                                5,000                                     
            mm.sup.2                                                      
 50 &lt; h &lt; 100                                                             
            piece/                                                        
                 38   65   154  54                                        
            mm.sup.2                                                      
100 &lt; h &lt; 150                                                             
            piece/                                                        
                 15   30   12   14                                        
            mm.sup.2                                                      
150 &lt; h     piece/                                                        
                 0    0    0    0                                         
            mm.sup.2                                                      
Refractive index:                                                         
nx          --   1.820                                                    
                      1.752                                               
                           1.641                                          
                                1.750                                     
ny          --   1.695                                                    
                      1.756                                               
                           1.678                                          
                                1.778                                     
nz          --   1.473                                                    
                      1.502                                               
                           1.487                                          
                                1.482                                     
Average refractive                                                        
            --   --   1.670                                               
                           1.602                                          
                                1.670                                     
index: nA                                                                 
Plane orientation                                                         
            --   0.285                                                    
                      0.252                                               
                           0.173                                          
                                0.282                                     
coefficient: NS                                                           
1.607 nA - 2.434                                                          
            --   --   0.250                                               
                           0.140                                          
                                0.250                                     
Young&#39;s modulus:                                                          
EM          kg/mm.sup.2                                                   
                 1,350                                                    
                      610  930  620                                       
ET          &#34;    520  630  440  950                                       
Elongation in                                                             
            %    0.04 0.85 1.90 0.72                                      
longitudinal                                                              
direction per load                                                        
of 1 kg/mm.sup.2 at 120° C.                                        
Heat Shrinkage                                                            
            %    0.05 0.05 0.17 0.15                                      
percentage                                                                
(longitudinal                                                             
direction)                                                                
70° C. × 1 hr.                                               
Properties of tape                                                        
            dB   -2.5 +2.0 +2.0 0                                         
Electromagnetic                                                           
conversion                                                                
characteristics                                                           
Y-S/N                                                                     
Running durability                                                        
            --   Excel.                                                   
                      Poor Poor Poor                                      
Skew        μsec                                                       
                 3    6    10   10                                        
__________________________________________________________________________
 Notes: Ex. = Example,                                                    
 CEx. = Comparative Example                                               
 Excel. = Excellent                                                       
 
    
     The magnetic recording tape produced from the biaxially oriented film of the polyethylene-2,6-naphthalenedicarboxylate of the present invention is free from the occurrence of tape elongation and edge bending during the running of the tape. Therefore, the intimacy (head touch) of the magnetic tape with a magnetic head is excellent so that it shows electromagnetic conversion characteristics performance necessary for high-density magnetic recording, and further, it is excellent in dimensional stability against heat. Therefore, the tape is almost free of a skew strain. In particular, the biaxially oriented film of the polyethylene-2,6-naphthalenedicarboxylate is useful as a base film for a high-density magnetic recording tape capable of recording for a long time.