JP4971875B2 - Biaxially oriented polyester film - Google Patents

Description

  The present invention relates to a film using an aromatic polyester copolymerized with 6,6 '-(alkylenedioxy) di-2-naphthoic acid.

  Aromatic polyesters typified by polyethylene terephthalate and polyethylene-2,6-naphthalate have excellent mechanical properties, dimensional stability and heat resistance, and thus are widely used for films and the like. In particular, polyethylene-2,6-naphthalate has mechanical properties, dimensional stability, and heat resistance superior to those of polyethylene terephthalate, so that it is used in demanding applications such as a base film for high-density magnetic recording media. in use. However, the demand for dimensional stability in high-density magnetic recording media and the like in recent years is increasing, and further improvement of characteristics is required.

  As described above, the demand for improvement in recording density in recent magnetic recording media and the like is severe, and accordingly, dimensional stability required for the base film is presented to polyethylene-2,6-naphthalate as well as polyethylene terephthalate. It has become a situation that can not be achieved even with a simple film.

  Therefore, Patent Document 1 proposes blending a polyester with a low coefficient of humidity expansion (for example, syndiotactic polystyrene) into polyester. Certainly, according to this method, although the humidity expansion coefficient of the obtained film can be reduced, such a polymer having a low humidity expansion coefficient has poor compatibility with the polyester and causes phase separation in the film. As a result, there was a problem that the surface of the resulting film was rough.

In Patent Document 2, a film using polyethylene-6,6 ′-(ethylenedioxy) di-2-naphthoate has a maximum temperature expansion coefficient of 10 to 35 (ppm / ° C.) and a maximum humidity expansion coefficient. 0 to 8 (ppm /% RH), the difference between the maximum and minimum temperature expansion coefficient is 0 to 6.0 (ppm / ° C.), and the difference between the maximum and minimum humidity expansion coefficient is 0 to 4.0 (ppm /% RH) teaches that a magnetic recording flexible disk with small tracking deviation can be obtained. And, specifically in the embodiment, in the film forming direction of the Young's modulus 485kg ^/ mm 2 (4.8Gpa) and the Young's modulus in the transverse direction is 1100kg ^/ mm 2 (10.9GPa), the maximum temperature expansion coefficient 19 (ppm / ° C.), minimum temperature expansion 16.5 (ppm / ° C.), maximum humidity expansion 6 (ppm /% RH), and minimum humidity expansion 4.5 (ppm / % RH) film is disclosed.

  However, since polyethylene-6,6 ′-(alkylenedioxy) di-2-naphthoate presented in Patent Document 2 has a very high melting point and very high crystallinity, it is intended to form a film or the like. As a result, there are problems such as poor fluidity in the molten state and non-uniform extrusion, and even if trying to stretch after extrusion, crystallization progresses and breaks when stretched at a high magnification. There is also a problem that the temperature expansion coefficient is high although the humidity expansion coefficient is small.

JP 2006-002142 A JP 61-145724 A

  An object of the present invention is to provide a biaxially oriented polyester film excellent in surface flatness while suppressing the temperature and humidity expansion coefficient.

  In a biaxially oriented polyester film, both the humidity expansion coefficient and the temperature expansion coefficient are very closely related to the Young's modulus, and generally lower as the Young's modulus is higher. However, the Young's modulus is not increased as much as possible, and there is a limit in terms of film forming properties and securing the Young's modulus in the orthogonal direction. For this reason, when the same Young's modulus is used, it has been earnestly studied whether a film having an expansion coefficient with respect to a lower temperature and humidity can be obtained. As a result, a film made of the aforementioned polyalkylene-6,6 ′-(alkylenedioxy) di-2-naphthoate. Was considered as a suitable film because it exhibited a low coefficient of humidity expansion even when Young's modulus was low.

However, according to the above-mentioned Patent Document 2, although the film made of polyalkylene-6,6 ′-(alkylenedioxy) di-2-naphthoate can have a very high Young's modulus, the Young There was a problem that the rate was extremely low. Further, as described above, there is a problem that the temperature expansion coefficient is very high although the humidity expansion coefficient is very small. Incidentally, the film disclosed in Example 1 of Patent Document 2 has Young's modulus of 1110 kg / mm ^{2 in the} width direction, but only 485 kg / mm ^{2 in the} film forming direction, and the coefficient of thermal expansion is 19 (ppm / ) And a minimum value of 16.5 (ppm / ° C.), indicating a very high value regardless of the Young's modulus.

  However, the present inventors surprisingly found that when a 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component was used as a copolymerization component, polyalkylene-6,6 ′-(alkylenedioxy). It was found that a film having excellent characteristics of both di-2-naphthoate and an aromatic polyester which is a copolymerization partner thereof can be obtained. Moreover, since it is not necessary to add a polymer having low compatibility with polyester that forms a phase separation structure as in Patent Document 1, it has been found that the flatness of the surface is not impaired.

（上記構造式（Ｉ）中のＲは、炭素数１〜１０のアルキレン基を示す。）
で表される６，６’−（アルキレンジオキシ）ジ−２−ナフトエ酸成分であって、
少なくとも一方の表面は、表面粗さが１０ｎｍ以下である二軸配向ポリエステルフィルムが提供される。 Thus, according to the present invention, a biaxially oriented polyester film comprising an aromatic polyester of an aromatic dicarboxylic acid component and a glycol component,
5 to 80 mol% of the aromatic dicarboxylic acid component is represented by the following formula (I)

(R in the structural formula (I) represents an alkylene group having 1 to 10 carbon atoms.)
A 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component represented by:
At least one surface is provided with a biaxially oriented polyester film having a surface roughness of 10 nm or less.

Moreover, according to this invention, as a preferable aspect of this invention, at least one direction in a film surface direction is the following formula | equation (1).
αh <−1.2Y + 17 (1)
(Wherein αh in the general formula (1) is a humidity expansion coefficient (ppm /% RH), Y is a Young's modulus (GPa)), and the melting point of the aromatic polyester is 200 to 260 ° C. There is also provided a biaxially oriented polyester film having at least one of being in the range and being used for a base film of a magnetic recording medium.

  According to the present invention, while maintaining the excellent characteristics of polyalkylene-6,6 ′-(alkylenedioxy) di-2-naphthoate, the film forming property can be enhanced to a high degree, which is surprising as a result. In addition, a particularly remarkable effect is obtained in that a biaxially oriented polyester film made of an aromatic polyester having simultaneously excellent dimensional stability and surface flatness that cannot be predicted from the prior art is obtained.

  Therefore, according to the present invention, in applications where high surface flatness is required from the viewpoint of high dimensional stability and electromagnetic conversion characteristics including the influence of humidity and temperature, particularly for base films of high-density magnetic recording media. A suitable film is provided, and if the film of the present invention is used, a high-density magnetic recording medium having excellent dimensional stability can be provided.

<Aromatic polyester>
In the present invention, the aromatic polyester forming the biaxially oriented polyester film is composed of an aromatic dicarboxylic acid component and a glycol component. Specific examples of the aromatic dicarboxylic acid component other than the aromatic dicarboxylic acid component represented by the formula (I) include terephthalic acid component, isophthalic acid component, 2,6-naphthalenedicarboxylic acid component, and 2,7-naphthalenedicarboxylic acid. An acid component etc. are mentioned. Examples of the glycol component include an ethylene glycol component, a trimethylene glycol component, a tetramethylene glycol component, and a cyclohexane dimethanol component.

  Specific aromatic polyesters formed from aromatic dicarboxylic acid components other than the aromatic dicarboxylic acid component represented by the aforementioned formula (I) include polyalkylene terephthalates such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate. Polyalkylene-2,6-naphthalate such as polyethylene-2,6-naphthalate, polytrimethylene-2,6-naphthalate, polybutylene-2,6-naphthalate and the like are preferable. To polyethylene terephthalate and polyethylene-2,6-naphthalate are preferable, and polyethylene-2,6-naphthalate is particularly preferable. From such a viewpoint, 90 mol% or more of the glycolic acid component is preferably an ethylene glycol component. The proportion of the ethylene glycol component is preferably in the range of 90 to 100 mol%, more preferably 95 to 100 mol%.

  By the way, one of the characteristics of the present invention is that the acid component of the above-mentioned aromatic polyester is in the range of 5 to 80 mol%, and 6,6 ′-(alkylenedioxy) di-2 represented by the above formula (I). -It is a naphthoic acid component. When the ratio of the 6,6 '-(alkylenedioxy) di-2-naphthoic acid component is less than the lower limit, the effect of reducing the humidity expansion coefficient of the present invention due to copolymerization is hardly exhibited. On the other hand, if it exceeds the upper limit, it tends to be difficult from the viewpoint of moldability and the like, and particularly preferably less than 50 mol%. Surprisingly, the effect of reducing the coefficient of humidity expansion by the 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component is expressed very efficiently even in a small amount, and already in the portion below the upper limit. A humidity expansion coefficient equivalent to or lower than that of the film described in the example of Patent Document 2 has been achieved, and it can be said that the effect from the viewpoint of the humidity expansion coefficient is saturated even if it is added above the upper limit. A preferable upper limit of the copolymerization amount of the 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component is 45 mol% or less, further 40 mol% or less, more 35 mol% or less, particularly 30 mol% or less. On the other hand, the lower limit is 5 mol% or more, further 7 mol% or more, further 10 mol% or more, particularly 15 mol% or more.

  By using an aromatic polyester copolymerized with such a specific amount of 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component, a molded product having both a low temperature expansion coefficient and a low humidity expansion coefficient, For example, a film etc. can be manufactured.

  In addition, as the 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component represented by the structural formula (I), the R portion is an alkylene group having 1 to 10 carbon atoms, Preferably, 6,6 ′-(ethylenedioxy) di-2-naphthoic acid component, 6,6 ′-(trimethylenedioxy) di-2-naphthoic acid component and 6,6 ′-(butylenedioxy) di -2-naphthoic acid component, and the like. Among these, from the viewpoint of the effect of the present invention, those having an even number of carbon atoms of R in the general formula (I) are preferred, and in particular, 6,6 ′-(ethylene An oxy) di-2-naphthoic acid component is preferred. In addition, the aromatic polyester in this invention may copolymerize other well-known copolymerization components in the range which does not inhibit the effect of this invention.

  Next, the aromatic polyester in the present invention has a melting point measured by DSC in the range of 200 to 260 ° C., further in the range of 210 to 255 ° C., particularly in the range of 220 to 253 ° C. preferable. When the melting point exceeds the above upper limit, when melt extrusion is performed, the fluidity is inferior, and the discharge is likely to be non-uniform. In addition, the effect of reducing the temperature expansion coefficient is poor due to poor film-forming properties. On the other hand, when it is less than the above lower limit, although the film-forming property is excellent, the mechanical properties of the aromatic polyester are easily impaired. That is, if other acid components are usually copolymerized and the melting point is lowered, the mechanical properties and the like are also lowered at the same time. It was found that the same mechanical properties as those of the polymer having the main repeating unit of the ester of 6,6 ′-(alkylenedioxy) di-2-naphthoic acid described in 2 can be developed.

  Further, the aromatic polyester in the present invention has a glass transition temperature (hereinafter sometimes referred to as Tg) measured by DSC in the range of 90 to 118 ° C., more preferably in the range of 95 to 115 ° C., particularly 100 to 115 ° C. It is preferable from the viewpoint of heat resistance and dimensional stability. Such a melting point and glass transition temperature can be adjusted by controlling the type and amount of copolymerization component, dialalkylene glycol as a byproduct.

<Film>
The biaxially oriented polyester film of the present invention can be obtained by melt-forming the aforementioned aromatic polyester and extruding it into a sheet. And as above-mentioned, since the fluidity | liquidity at the time of a fusion | melting and subsequent crystallinity are improved, it becomes a uniform film excellent in film forming property, for example, without a thickness spot.

  The biaxially oriented polyester film of the present invention has a surface roughness (Ra) of 10 nm or less from the viewpoint of exhibiting excellent electromagnetic conversion characteristics when used as a base film such as a magnetic tape. It is necessary. If any surface has a Ra exceeding 10 nm, the surface properties of the magnetic layer formed are impaired when used as a base film of a magnetic recording medium, particularly a linear recording high-density magnetic recording tape. Will be poor. The preferred surface roughness is preferably at least one surface in the range of 1 to 5 nm, and more preferably in the range of 2 to 4 nm, particularly when improving electromagnetic conversion characteristics while maintaining appropriate winding properties. From the viewpoint of developing a high degree of winding property while sufficiently expressing electromagnetic conversion characteristics, at least one surface is preferably in the range of 5 to 10 nm, and more preferably in the range of 6 to 8 nm.

  In order to make such a film having a surface roughness of 10 nm or less, it is necessary to reduce the content of the roughening substance that forms irregularities on the surface while reducing the size. Examples of the roughening substance include polymers that are incompatible with polyester such as polyimide, liquid crystal resin, and polyolefin, and particles for forming protrusions such as inorganic particles and organic particles.

  By the way, the protrusion forming particles such as inorganic particles and organic particles may be reduced in size or contained in accordance with the target surface roughness. Therefore, the upper limit of the average particle diameter of the projection forming particles contained is preferably 1 μm or less, more preferably 0.8 μm or less. .5 weight or less is preferable. On the other hand, in the method described in Patent Document 1, the ratio of the polymer incompatible with polyester cannot be reduced in order to reduce the humidity expansion coefficient of the obtained film. Therefore, a film having a flat surface property such as a small coefficient of humidity expansion and a surface roughness of 10 nm or less has not been obtained so far. In other words, the feature of the present invention is that in the above-mentioned Patent Document 1 and the like, humidity is not used without using a blend of incompatible polymers with polyester, which must be added in a large amount in order to reduce the coefficient of humidity expansion. In the above-mentioned Patent Document 1, the problem of the contradictory relationship between the reduction of the humidity expansion coefficient and the maintenance of the flatness of the surface of the film was solved at once. is there.

  The biaxially oriented polyester film of the present invention has a Young's modulus of 6.0 GPa or more in at least one direction in the film surface direction so that the base film does not stretch due to stress applied to the film when used as a base film of a magnetic tape. It is preferable to have a high Young's modulus. Moreover, by having such a high Young's modulus, the humidity expansion coefficient can be further reduced. The upper limit of the Young's modulus is not limited, but is usually 11 GPa. The preferred Young's modulus is 5.1 to 11 GPa in the longitudinal direction of the film, more preferably in the range of 5.2 to 10 GPa, particularly 5.5 to 9 GPa, and 5.0 to 11 GPa in the width direction of the film, further 6 to 10 GPa. In particular, it is in the range of 7-10 GPa.

In the biaxially oriented polyester film of the present invention, the humidity expansion coefficient (αh) and Young's modulus (Y) of the film in at least one direction, preferably the width direction of the film, satisfy the relationship of the following formula (1). It is preferable to do.
αh <−1.2Y + 17 (1)
(Where αh is the humidity expansion coefficient (ppm /% RH) and Y is the Young's modulus (GPa).)

When the obtained biaxially oriented polyester film does not satisfy the relationship of the above formula (1), it becomes only αh for Young's modulus equivalent to a film made of conventional polyethylene terephthalate or polyethylene-2,6-naphthalate, and polyalkylene- The effect of reducing humidity expansion due to the copolymerization of 6,6 ′-(alkylenedioxy) di-2-naphthoate is not sufficiently exhibited. The coefficient “−1.2” in the above relational expression is derived from the relationship between the Young's modulus and αh of the polyethylene-2,6-naphthalate film described in Comparative Examples 1 to 3 of the present specification. is there. Moreover, as an aromatic polyester copolymerized with 6,6 ′-(alkylenedioxy) di-2-naphthoic acid, polyethylene-2,6-naphthalate is particularly preferable because Young's modulus is easily increased. The relationship between the preferred humidity expansion coefficient (αh) and Young's modulus (Y) is
αh <−1.2Y + 16.5 (1 ′)
Αh <−1.2Y + 16.0 (1 ″)
It is.

The lower limit of the relational expression between the Young's modulus and αh is not particularly limited.
αh> −1.2Y + 12.0 (1 ′ ″)
Degree. The Young's modulus, αh, and αt described later can be adjusted by the copolymerization composition described above and the stretching described later.

Furthermore, the preferable aspect of the biaxially-oriented polyester film of this invention is explained in full detail.
The biaxially oriented polyester film of the present invention preferably has a temperature expansion coefficient (αt) in at least one direction, preferably in the width direction of the film, of 10 ppm / ° C. or less from the viewpoint of exhibiting excellent dimensional stability. When the temperature expansion coefficient in at least one direction of the film is 10 ppm / ° C. or less, excellent dimensional stability against environmental changes can be exhibited. From the results of Patent Document 2, it is expected that when polyalkylene-6,6 ′-(alkylenedioxy) di-2-naphthalate is copolymerized, the temperature expansion coefficient is expected to increase. By making it in the above-mentioned range and stretching it, the temperature expansion coefficient can be surprisingly reduced. The lower limit of the temperature expansion coefficient is not limited, but is usually −15 ppm / ° C. The preferred temperature expansion coefficient (αt) is in the range of −10 ppm / ° C. to 10 ppm / ° C., more preferably −7 ppm / ° C. to 7 ppm / ° C., particularly −5 ppm / ° C. to 5 ppm / ° C. It is preferable because it can exhibit excellent dimensional stability against dimensional changes due to changes in the temperature and humidity of the atmosphere.

  The biaxially oriented polyester film of the present invention has a humidity expansion coefficient in the range of 3 to 7 ppm /% RH and 3 to 6 ppm /% RH in the direction satisfying the relationship of the temperature expansion coefficient in at least one direction, preferably the width direction of the film. It is preferable from the viewpoint of dimensional stability when a magnetic recording tape is used. In particular, when used as a base film for a magnetic recording tape, it is preferable that the direction satisfying the above-mentioned formula (1) is the width direction of the biaxially oriented polyester film because track deviation can be extremely suppressed. In the present invention, the width direction of the film is a direction orthogonal to the film forming direction of the film (also referred to as a longitudinal direction or a longitudinal direction), and is sometimes referred to as a lateral direction.

  Further, for the direction in which the temperature expansion coefficient is 10 ppm / ° C. or less, at least one direction, preferably as described above, the width direction should be satisfied, but the direction orthogonal thereto is also dimensional stability point, It is preferable that the same temperature expansion coefficient, humidity expansion coefficient, and Young's modulus are satisfied.

<Method for producing aromatic polyester resin>
Next, a method for producing the aromatic polyester in the present invention will be described in detail.
First, 6,6 ′-(alkylenedioxy) di-2-naphthoic acid or its ester-forming derivative is reacted with, for example, 2,6-naphthalenedicarboxylic acid or terephthalic acid or its ester-forming derivative with, for example, ethylene glycol. To produce a polyester precursor. And it can manufacture by superposing | polymerizing the polyester precursor obtained in this way in presence of a polymerization catalyst, You may give a solid phase polymerization etc. as needed. The intrinsic viscosity measured at 35 ° C. using a mixed solvent of aromatic polyester P-chlorophenol / 1,1,2,2-tetrachloroethane (weight ratio 40/60) thus obtained is 0.4. From the viewpoint of the effect of the present invention, it is preferably in the range of -1.5 dl / g, more preferably in the range of 0.5-1.3 dl / g.

  Further, in the step of producing the above-mentioned polyester precursor, the ethylene glycol component is used in an amount of 1.1 to 6 times, further 2 to 5 times, particularly 3 to 5 times the number of moles of the total acid component. From the point of view, it is preferable.

  The reaction temperature for producing the polyester precursor is preferably not less than the boiling point of ethylene glycol, particularly preferably in the range of 190 to 250 ° C. When the temperature is lower than 190 ° C., the reaction does not proceed sufficiently, and when the temperature is higher than 250 ° C., side reaction products such as diethylene glycol are likely to be generated. In addition, the reaction can be performed under normal pressure, but the reaction may be performed under pressure in order to further increase productivity. More specifically, the reaction pressure is 10 to 200 kPa in absolute pressure, the reaction temperature is usually 150 to 250 ° C., preferably 180 to 230 ° C., the reaction time is 10 to 10 hours, preferably 30 to 7 minutes. It is preferable to be performed for less than an hour. A reaction product as a polyester precursor is obtained by this esterification reaction.

  In the reaction step for producing the polyester precursor, a known esterification or transesterification reaction catalyst may be used. For example, an alkali metal compound, an alkaline earth metal compound, a titanium compound, and the like can be given.

  Next, the polycondensation reaction will be described. First, the polycondensation temperature is preferably in the range from the melting point of the polymer to be obtained and 230 to 280 ° C. or less, further 5 ° C. or more higher than the melting point to 30 ° C. higher than the melting point. The polycondensation reaction is usually preferably carried out under reduced pressure of 50 Pa or less. If it is higher than 50 Pa, the time required for the polycondensation reaction becomes long, and it tends to be difficult to obtain a copolymerized aromatic polyester resin having a high degree of polymerization.

  Examples of the polycondensation catalyst include metal compounds containing at least one metal element. The polycondensation catalyst can also be used in the esterification reaction. Examples of the metal element include titanium, germanium, antimony, aluminum, nickel, zinc, tin, cobalt, rhodium, iridium, zirconium, hafnium, lithium, calcium, and magnesium. More preferable metals are titanium, germanium, antimony, aluminum, tin, etc. Among them, a titanium compound is particularly preferable because it exhibits high activity in both the esterification reaction and the polycondensation reaction.

  These catalysts may be used alone or in combination. The amount of the catalyst is preferably 0.001 to 0.5 mol%, more preferably 0.005 to 0.2 mol%, based on the number of moles of the repeating unit of the copolymerized aromatic polyester.

  Specific examples of the titanium compound as the polycondensation catalyst include tetra-n-propyl titanate, tetraisopropyl titanate, tetra-n-butyl titanate, tetraisobutyl titanate, tetra-tert-butyl titanate, tetracyclohexyl titanate, tetraphenyl titanate, and the like. Eltitanate, tetrabenzyl titanate, lithium oxalate titanate, potassium oxalate titanate, ammonium oxalate titanate, titanium oxide, titanium orthoester or condensed orthoester, titanium orthoester or condensed orthoester and hydroxycarboxylic acid A reaction product comprising an orthoester or condensed orthoester of titanium, a hydroxycarboxylic acid and a phosphorus compound, an orthoester of titanium or a condensed orthoester and at least 2 Polyhydric alcohols having a hydroxyl group, 2-hydroxy carboxylic acid, or a reaction product comprising a base and the like.

  In the aromatic polyester in the present invention, other thermoplastic polymers, stabilizers such as ultraviolet absorbers, antioxidants, plasticizers, lubricants, flame retardants, mold release agents, pigments, as long as the effects of the present invention are not impaired. Further, a nucleating agent, a filler or glass fiber, carbon fiber, layered silicate, etc. may be blended as necessary. Examples of other thermoplastic polymers include aliphatic polyester resins, polyamide resins, polycarbonates, ABS resins, polymethyl methacrylate, polyamide elastomers, polyester elastomers, polyether imides, polyimides, and the like.

<Film production method>
The biaxially oriented polyester film of the present invention is one in which the molecular orientation in each direction is enhanced by stretching in the film forming direction and the width direction. For example, the film forming property can be maintained by the following method. However, it is preferable because the Young's modulus is easily improved.

  First, the above aromatic polyester is used as a raw material, dried, and then supplied to an extruder heated to a temperature of the melting point (Tm: ° C.) to (Tm + 50) ° C. of the aromatic polyester resin. Extrude into a sheet from the die. The extruded sheet is rapidly cooled and solidified with a rotating cooling drum or the like to form an unstretched film, and the unstretched film is further biaxially stretched.

  In order to satisfy the above-mentioned physical properties such as Young's modulus, αt and αh, it is preferable to facilitate the subsequent stretching, and from such a viewpoint, it is preferable to perform cooling with a cooling drum very quickly. Specifically, it is preferable to carry out at a low temperature of 20 to 60 ° C. instead of a high temperature of 80 ° C. described in Patent Document 2. By performing at such a low temperature, crystallization in the state of an unstretched film is suppressed, and subsequent stretching can be performed more smoothly.

Biaxial stretching may be sequential biaxial stretching or simultaneous biaxial stretching.
Here, a manufacturing method in which longitudinal stretching, lateral stretching, and heat treatment are performed in this order by sequential biaxial stretching will be described as an example. First, in the first longitudinal stretching, the glass transition temperature (Tg: ° C.) to (Tg + 40) ° C. of the copolymerized aromatic polyester is stretched 3 to 8 times, and then in the transverse direction at a higher temperature than the previous longitudinal stretching. The film is stretched 3 to 8 times at a temperature of (Tg + 10) to (Tg + 50) ° C., and further treated as a heat treatment at a temperature not higher than the melting point of the polymer and at a temperature of (Tg + 50) to (Tg + 150) ° C. for 1 to 20 seconds, and further 1 to 15 It is preferable to carry out a second heat fixing treatment.

  Normally, when the draw ratio is increased, the film-forming stability is impaired. However, in the present invention, the 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component is copolymerized, so that the drawability is increased. Is very high, and there is no such problem, and in particular, the draw ratio can be further increased. Therefore, it is particularly useful for a thin film having a thickness of 10 μm or less, more preferably 8 μm or less. The lower limit of the thickness of the film is not particularly limited, but is usually 1 μm, preferably 3 μm.

  Although the above description has been described for sequential biaxial stretching, the biaxially oriented polyester film of the present invention can be produced by simultaneous biaxial stretching in which longitudinal stretching and lateral stretching are simultaneously performed, for example, the stretching ratio and stretching temperature described above, etc. Should be referred to.

  Further, when the biaxially oriented polyester film of the present invention is a laminated film, two or more types of molten polyester are laminated in a die and then extruded into a film, preferably the melting point (Tm: ° C.) to (Tm + 70) of each polyester. ) Extrude at a temperature of ° C. or extrude two or more types of molten polyester from a die, laminate them, quench and solidify them to form a laminated unstretched film, and then perform biaxial stretching and Heat treatment may be performed. Further, when providing the above-mentioned coating layer, a desired coating solution is applied to one or both sides of the unstretched film or the uniaxially stretched film, and then the biaxial stretching and the same method as in the case of the single-layer film are performed. It is preferable to perform a heat treatment.

  Using the biaxially oriented polyester film of the present invention as a base film, a nonmagnetic layer and a magnetic layer are formed in this order on one side, and a backcoat layer is formed on the other side to form a magnetic recording tape. Can do.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In the present invention, the characteristics were measured and evaluated by the following methods.

(1) Intrinsic viscosity The intrinsic viscosity of the obtained polyester was measured at 35 ° C by dissolving the polymer using a mixed solvent of P-chlorophenol / 1,1,2,2-tetrachloroethane (40/60 weight ratio). And asked.

(2) Glass transition point and melting point Glass transition point and melting point were measured by DSC (TA Instruments Co., Ltd., trade name: Thermal lyst 2100) at a heating rate of 20 ° C / min.

(3) Copolymerization amount For the glycol component, 10 mg of a sample was dissolved in 0.5 ml of a mixed solution of p-chlorophenol: heavy tetrachloroethane = 3: 1 (volume ratio) at 80 ° C. After adding isopropylamine and mixing well, it was measured at 80 ° C. with 600 M ¹ H-NMR (JEOL A600, manufactured by Hitachi Electronics), and the amount of each glycol component was measured.
As for the aromatic dicarboxylic acid component, 50 mg of a sample was dissolved in 0.5 ml of a mixed solution of p-chlorophenol: heavy tetrachloroethane = 3: 1 at 140 ° C., and 400 M ¹³ C-NMR (Hitachi Electronics JEOL A600) It measured at 140 degreeC and measured the amount of each acid component.

(4) Young's modulus The obtained film was cut out with a sample width of 10 mm and a length of 15 cm, and a universal tensile testing device (product name: Toyo Baldwin, trade name: 100 mm between chucks, tensile speed 10 mm / min, chart speed 500 mm / min). Pull with Tensilon). The Young's modulus is calculated from the tangent of the rising portion of the obtained load-elongation curve.

(5) Temperature expansion coefficient (αt)
The obtained film was cut into a length of 15 mm and a width of 5 mm so that the longitudinal direction and the width direction of the film were the measurement directions, set in TMA3000 manufactured by Vacuum Riko, and at 60 ° C. in a nitrogen atmosphere (0% RH). Pre-treat for 30 minutes and then allow to cool to room temperature. Thereafter, the temperature is raised from 25 ° C. to 70 ° C. at 2 ° C./min, the sample length at each temperature is measured, and the temperature expansion coefficient (αt) is calculated from the following equation. In addition, the measurement direction is the longitudinal direction of the sample cut out, the measurement was performed 5 times, and the average value was used.
αt = {(L ₆₀ −L ₄₀ )} / (L ₄₀ × ΔT)} + 0.5
Here, L ₄₀ in the above formula is the sample length (mm) at 40 ° C., L ₆₀ is the sample length (mm) at 60 ° C., ΔT is 20 (= 60-40) ° C., 0.5 Is the temperature expansion coefficient (ppm / ° C.) of quartz glass.

(6) Humidity expansion coefficient (αh)
The obtained film was cut into a length of 15 mm and a width of 5 mm so that the longitudinal direction and the width direction of the film were the measurement directions, set in TMA3000 manufactured by Vacuum Riko, and a humidity of 30% RH in a nitrogen atmosphere at 30 ° C. The length of each sample at a humidity of 70% RH is measured, and the humidity expansion coefficient is calculated by the following equation. In addition, the measurement direction is the longitudinal direction of the cut out sample, the measurement was performed 5 times, and the average value was αh.
αh = (L ₇₀ −L ₃₀ ) / (L ₃₀ × ΔH)
Here, L ₃₀ in the above formula is a sample length (mm) when 30% RH, L ₇₀ is a sample length (mm) when 70% RH, ΔH: 40 (= 70-30)% RH is there.

(7) Center plane average roughness (Ra)
Surface measured with a non-contact three-dimensional surface structure analysis microscope (NewView 5022) manufactured by Zygo under the conditions of a measurement magnification of 25 times and a measurement area of 283 μm × 213 μm (= 0.0603 mm ² ). The center plane average roughness Ra was determined from the following equation using analysis software.

Ｚｊｋは測定方法（２８３μｍ）、それと直行する方法（２１３μｍ）をそれぞれＭ分割、Ｎ分割したときの各方向のｊ番目、ｋ番目の位置における２次元粗さチャート上の高さである。

Zjk is the height on the two-dimensional roughness chart at the j-th and k-th positions in each direction when the measurement method (283 μm) and the direct method (213 μm) are divided into M and N, respectively.

(8) Winding property The film roll which was rolled up at 8000 m was observed at a slit speed of 50 m / min and a slit width of 1000 mm, and the winding property was evaluated according to the following criteria.
◎: Wrinkles are not seen ○: Wrinkles are seen slightly, but there is no problem in practical use ×: Many wrinkles occur

(9) Electromagnetic conversion characteristics For measuring the electromagnetic conversion characteristics, a 1/2 inch linear system with a fixed head was used. Recording was performed using an electromagnetic induction head (track width 25 μm, gap 0.1 μm), and reproduction was performed using an MR head (8 μm). The relative speed of the head / tape is 10 m / sec, a signal with a recording wavelength of 0.2 μm is recorded, the reproduced signal is analyzed with a spectrum analyzer, the output C of the carrier signal (wavelength 0.2 μm), and the integration over the entire spectrum. The relative value with the noise N ratio as C / N ratio and Example 1 as 0 dB was determined and evaluated according to the following criteria.
◎: +1 dB or more ○: −1 dB or more, less than +1 dB ×: less than −1 dB

In addition, the magnetic recording tape used for electromagnetic conversion characteristic measurement was prepared by the following method.
First, a back coat layer paint having the following composition was applied to one surface of the film obtained in each Example and Comparative Example with a die coater and dried. The paint is applied by changing the film thickness simultaneously with a die coater, magnetically oriented and dried. Further, after calendering with a small test calender (steel roll / nylon roll, 5 stages) at a temperature of 70 ° C. and a linear pressure of 200 kg / cm, curing is performed at 70 ° C. for 48 hours. The tape was slit to 12.65 mm and incorporated into a cassette to obtain a magnetic recording tape. The dried back coat layer, nonmagnetic layer, and magnetic layer had thicknesses of 0.5 μm, 1.2 μm, and 0.1 μm, respectively.

<Composition of non-magnetic paint>
-Titanium dioxide fine particles: 100 parts by weight-SREC A (vinyl chloride / vinyl acetate copolymer made by Sekisui Chemical: 10 parts by weight) Nipponran 2304 (polyurethane elastomer made by Nippon Polyurethane): 10 parts by weight-Coronate L (polyisocyanate made by Nippon Polyurethane ): 5 parts by weight-lecithin: 1 part by weight-methyl ethyl ketone: 75 parts by weight-methyl isobutyl ketone: 75 parts by weight-toluene: 75 parts by weight-carbon black: 2 parts by weight-lauric acid: 1.5 parts by weight

<Composition of magnetic paint>
Iron (length: 0.3 μm, needle ratio: 10/1, 1800 oersted): 100 parts by weight Eslek A (vinyl chloride / vinyl acetate copolymer made by Sekisui Chemical: 10 parts by weight) Nipponan 2304 (Nippon Polyurethane Polyurethane elastomer): 10 parts by weight, Coronate L (polyisocyanate made by Nippon Polyurethane): 5 parts by weight, lecithin: 1 part by weight, methyl ethyl ketone: 75 parts by weight, methyl isobutyl ketone: 75 parts by weight, toluene: 75 parts by weight, carbon Black: 2 parts by weight ・ Lauric acid: 1.5 parts by weight

<Composition of back coat layer paint:>
Carbon black: 100 parts by weight Thermoplastic polyurethane resin: 60 parts by weight Isocyanate compound: 18 parts by weight (Coronate L manufactured by Nippon Polyurethane Industry Co., Ltd.)
Silicone oil: 0.5 parts by weight Methyl ethyl ketone: 250 parts by weight Toluene: 50 parts by weight

[Example 1]
Dimethyl 2,6-naphthalenedicarboxylate, 6,6 ′-(ethylenedioxy) di-2-naphthoic acid and ethylene glycol were subjected to esterification and transesterification in the presence of titanium tetrabutoxide, and then further The polycondensation reaction was carried out to give an intrinsic viscosity of 0.66 dl / g, 73 mol% of the acid component being 2,6-naphthalenedicarboxylic acid component, and 27 mol% of the acid component being 6,6 ′-(alkylenedioxy). An aromatic polyester in which 98 mol% of the di-2-naphthoic acid component and glycol component was an ethylene glycol component and 2 mol% was a diethylene glycol component was obtained. The aromatic polyester contained silica particles having an average particle size of 0.3 μm before the polycondensation reaction so as to be 0.15% by weight based on the weight of the obtained resin composition. The aromatic polyester had a melting point of 240 ° C. and a glass transition temperature of 117 ° C.

The aromatic polyester thus obtained was supplied to an extruder and extruded from a die at 290 ° C. onto a cooling drum having a rotating temperature of 50 ° C. in a molten state to form an unstretched film. Then, between two sets of rollers having different rotational speeds along the film forming direction, the film surface temperature is heated from above by an IR heater so that the film surface temperature becomes 135 ° C., and stretching in the machine direction (film forming direction) is performed. A uniaxially stretched film was obtained at a magnification of 6.2. Then, this uniaxially stretched film is guided to a stenter, stretched at 140 ° C. in the transverse direction (width direction) at a stretching ratio of 6.3 times, then heat-set at 200 ° C. for 10 seconds, and biaxially stretched with a thickness of 5 μm. A film was obtained.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Example 2]
In Example 1, the stretching temperature in the film forming direction is 135 ° C., the stretching ratio in the film forming direction is 5.3 times, the stretching temperature in the width direction is 135 ° C., and the stretching ratio in the width direction is 5.8 times. The same operation was repeated except that the heat setting treatment temperature was changed to 210 ° C. to obtain a biaxially stretched film having a thickness of 5 μm.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Example 3]
Dimethyl 2,6-naphthalenedicarboxylate, 6,6 ′-(ethylenedioxy) di-2-naphthoic acid and ethylene glycol were subjected to esterification and transesterification in the presence of titanium tetrabutoxide, and then further The polycondensation reaction was carried out, the intrinsic viscosity was 0.72 dl / g, 94 mol% of the acid component was 2,6-naphthalenedicarboxylic acid component, and 6 mol% of the acid component was 6,6 ′-(ethylenedioxy). An aromatic polyester in which 99 mol% of the di-2-naphthoic acid component and glycol component was an ethylene glycol component and 1 mol% of a diethylene glycol component was obtained. In the aromatic polyester, silica particles having an average particle size of 0.15 μm were contained before the polycondensation reaction so as to be 0.1% by weight based on the weight of the resin composition to be obtained. The aromatic polyester had a melting point of 255 ° C. and a glass transition temperature of 119 ° C.

The aromatic polyester thus obtained was supplied to an extruder and extruded from a die at 290 ° C. in a molten state onto a cooling drum having a rotating temperature of 60 ° C. to form an unstretched film. Then, between the two sets of rollers having different rotation speeds along the film forming direction, the film surface temperature is heated from above by an IR heater so that the film surface temperature becomes 140 ° C., and stretching in the machine direction (film forming direction) is performed. A uniaxially stretched film was obtained at a magnification of 5.3 times. Then, this uniaxially stretched film is guided to a stenter, stretched at 140 ° C. in the transverse direction (width direction) at a stretch ratio of 4.0 times, and then heat-set at 200 ° C. for 10 seconds, and biaxially stretched with a thickness of 5 μm. A film was obtained.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Example 4]
In Example 3, the stretching temperature in the film forming direction is 135 ° C., the stretching ratio in the film forming direction is 3.0 times, the stretching temperature in the width direction is 135 ° C., and the stretching ratio in the width direction is 5.0 times. The same operation was repeated except that the heat setting treatment temperature was changed to 210 ° C. to obtain a biaxially stretched film having a thickness of 5 μm.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Example 5]
Dimethyl 2,6-naphthalenedicarboxylate, 6,6 ′-(ethylenedioxy) di-2-naphthoic acid and ethylene glycol were subjected to esterification and transesterification in the presence of titanium tetrabutoxide, and then further The polycondensation reaction was carried out, the intrinsic viscosity was 0.77 dl / g, 80 mol% of the acid component was 2,6-naphthalenedicarboxylic acid component, and 20 mol% of the acid component was 6,6 ′-(ethylenedioxy). An aromatic polyester in which 99 mol% of the di-2-naphthoic acid component and glycol component was an ethylene glycol component and 1 mol% of a diethylene glycol component was obtained. The aromatic polyester contains silica particles having an average particle size of 0.5 μm and silica particles having an average particle size of 0.15 μm before the polycondensation reaction, based on the weight of the obtained resin composition. The content was 02 wt% and 0.30 wt%. The aromatic polyester had a melting point of 252 ° C. and a glass transition temperature of 116 ° C.

The aromatic polyester thus obtained was supplied to an extruder and extruded from a die at 290 ° C. onto a cooling drum having a rotating temperature of 50 ° C. in a molten state to form an unstretched film. Then, between two sets of rollers having different rotational speeds along the film forming direction, the film surface temperature is heated from above by an IR heater so that the film surface temperature becomes 135 ° C., and stretching in the machine direction (film forming direction) is performed. A uniaxially stretched film was obtained at a magnification of 5.5. And this uniaxially stretched film is led to a stenter, stretched at a stretching ratio of 4.3 times in the transverse direction (width direction) at 140 ° C., and then heat-set at 210 ° C. for 10 seconds, and biaxially stretched with a thickness of 5 μm. A film was obtained.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Example 6]
Dimethyl 2,6-naphthalenedicarboxylate, 6,6 ′-(ethylenedioxy) di-2-naphthoic acid and ethylene glycol were subjected to esterification and transesterification in the presence of titanium tetrabutoxide, and then further The polycondensation reaction was performed, and the intrinsic viscosity was 0.77 dl / g, 65 mol% of the acid component was 2,6-naphthalenedicarboxylic acid component, and 35 mol% of the acid component was 6,6 ′-(ethylenedioxy). An aromatic polyester in which 98 mol% of the di-2-naphthoic acid component and glycol component was an ethylene glycol component and 2 mol% was a diethylene glycol component was obtained. The aromatic polyester contains silica particles having an average particle size of 0.5 μm and silica particles having an average particle size of 0.15 μm before the polycondensation reaction, based on the weight of the obtained resin composition. The content was 02 wt% and 0.30 wt%. The aromatic polyester had a melting point of 247 ° C. and a glass transition temperature of 116 ° C.

The aromatic polyester thus obtained was supplied to an extruder and extruded from a die at 290 ° C. onto a cooling drum having a rotating temperature of 50 ° C. in a molten state to form an unstretched film. Then, between the two sets of rollers having different rotation speeds along the film forming direction, the film surface temperature is heated from above by an IR heater so that the film surface temperature becomes 140 ° C., and stretching in the machine direction (film forming direction) is performed. A uniaxially stretched film was obtained at a magnification of 5.5. Then, this uniaxially stretched film is guided to a stenter, stretched at 140 ° C. in the transverse direction (width direction) at a stretch ratio of 6.0 times, and then heat-set at 210 ° C. for 10 seconds, and biaxially stretched with a thickness of 5 μm. A film was obtained.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Example 7]
In Example 1, the stretching temperature in the film forming direction is 135 ° C., the stretching ratio in the film forming direction is 4.8 times, the stretching temperature in the width direction is 135 ° C., and the stretching ratio in the width direction is 6.7 times. A biaxially stretched film having a thickness of 5 μm was obtained by repeating the same operation except that the heat setting treatment temperature was changed to 190 ° C.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Comparative Example 1]
Dimethyl 2,6-naphthalenedicarboxylate and ethylene glycol are subjected to an esterification reaction and a transesterification reaction in the presence of titanium tetrabutoxide, followed by a polycondensation reaction to obtain an intrinsic viscosity of 0.62 dl / g. Polyethylene-2,6-naphthalate in which 1.5 mol% of the glycol component was a diethylene glycol component was obtained. The polyethylene-2,6-naphthalate contains silica particles having an average particle size of 0.3 μm before the polycondensation reaction so that the amount is 0.15% by weight based on the weight of the resin composition obtained. Contained. This polyethylene-2,6-naphthalate had a melting point of 270 ° C. and a glass transition temperature of 120 ° C.

The polyethylene-2,6-naphthalate thus obtained was supplied to an extruder and extruded from a die at 300 ° C. in a molten state onto a cooling drum at a temperature of 60 ° C. during rotation to form an unstretched film. . Then, between the two sets of rollers having different rotation speeds along the film forming direction, the film surface temperature is heated from above by an IR heater so that the film surface temperature becomes 140 ° C., and stretching in the machine direction (film forming direction) is performed. A uniaxially stretched film was obtained at a magnification of 3.0. And this uniaxially stretched film is led to a stenter, stretched at a stretching ratio of 4.3 times in the transverse direction (width direction) at 140 ° C., and then heat-set at 200 ° C. for 10 seconds to be biaxially stretched with a thickness of 10 μm. A film was obtained.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Comparative Example 2]
In Comparative Example 1, the stretching temperature in the film forming direction is 140 ° C., the stretching ratio in the film forming direction is 4.0 times, the stretching temperature in the width direction is 140 ° C., and the stretching ratio in the width direction is 4.0 times. The same operation was repeated except that the heat setting treatment temperature was changed to 200 ° C. to obtain a biaxially stretched film having a thickness of 10 μm.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Comparative Example 3]
In Comparative Example 1, the stretching temperature in the film forming direction is 140 ° C., the stretching ratio in the film forming direction is 4.5 times, the stretching temperature in the width direction is 140 ° C., and the stretching ratio in the width direction is 3.4 times. The same operation was repeated except that the heat setting treatment temperature was changed to 200 ° C. to obtain a biaxially stretched film having a thickness of 10 μm.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Comparative Example 4]
Dimethyl 2,6-naphthalenedicarboxylate and ethylene glycol are subjected to an esterification reaction and a transesterification reaction in the presence of titanium tetrabutoxide, followed by a polycondensation reaction to obtain an intrinsic viscosity of 0.62 dl / g. Polyethylene-2,6-naphthalate in which 1.5 mol% of the glycol component was a diethylene glycol component was obtained. A thermoplastic resin composition was prepared by uniformly blending 15% by weight of syndiotactic polystyrene (manufactured by Idemitsu Petrochemical Co., Ltd., grade: 130ZC) with 85% by weight of the polyethylene-2,6-naphthalate. The thermoplastic resin composition had a melting point of 270 ° C. and a glass transition temperature of 120 ° C.

The thermoplastic resin composition thus obtained was supplied to an extruder and extruded from a die at 300 ° C. in a molten state onto a cooling drum at a temperature of 60 ° C. during rotation to obtain an unstretched film. Then, between the two sets of rollers having different rotation speeds along the film forming direction, the film surface temperature is heated from above by an IR heater so that the film surface temperature becomes 140 ° C., and stretching in the machine direction (film forming direction) is performed. A uniaxially stretched film was obtained at a magnification of 3.0. And this uniaxially stretched film is guided to a stenter, stretched at a stretching ratio of 5.0 times in the transverse direction (width direction) at 140 ° C., and then heat-fixed at 210 ° C. for 10 seconds, and biaxially stretched with a thickness of 5 μm. A film was obtained.
The characteristics of the obtained biaxially oriented polyester film are shown in Table 1.

[Reference Example 1]
As a reference example, Table 1 shows the results of Young's modulus, temperature expansion coefficient, and humidity expansion coefficient described in Example 1 of Patent Document 2.

  In Table 1, NA is 2,6-naphthalenedicarboxylic acid component, ENA is 6,6 '-(ethylenedioxy) di-2-naphthoic acid component, EG is ethylene glycol component, DEG is diethylene glycol component, and Tg is glass transition. Temperature and MD indicate the film forming direction, and TD indicates the width direction of the film. In addition, the Young's modulus of the reference example 1 shows what was converted into GPa.

  The biaxially oriented polyester film of the present invention is excellent as cannot be achieved with conventional polyethylene terephthalate, polyethylene-2,6-naphthalate or polyalkylene-6,6 ′-(alkylenedioxy) di-2-naphthoate. It has dimensional stability and surface flatness, and can be suitably used as a base film for applications in which they are required, particularly for high-density magnetic recording media.

Claims (4)

A biaxially oriented polyester film comprising an aromatic polyester of an aromatic dicarboxylic acid component and a glycol component,
5 to 80 mol% of the aromatic dicarboxylic acid component is represented by the following formula (I)

(R in the structural formula (I) represents an alkylene group having 1 to 10 carbon atoms.)
A 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component represented by:
The biaxially oriented polyester film characterized in that at least one surface has a surface roughness (Ra) of 10 nm or less. The biaxially oriented polyester film according to claim 1, wherein at least one direction in the film surface direction has a relationship represented by the following formula (1).
αh <−1.2Y + 17 (1)
(In the above general formula (1), αh represents a humidity expansion coefficient (ppm /% RH), and Y represents a Young's modulus (GPa).)   The biaxially oriented polyester film according to claim 1, wherein the aromatic polyester has a melting point in the range of 200 to 260 ° C.   The biaxially oriented polyester film according to claim 1, wherein the biaxially oriented polyester film is used as a base film of a magnetic recording medium.