JP5346517B2 - Flame retardant copolyester and biaxially oriented film comprising the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flame-retardant copolyester which, when formed into a film, has excellent flame retardancy and dimensional stability and is particularly suitably used as a base film for flexible electronic devices such as a flexible printed circuit board. <P>SOLUTION: The flame-retardant copolyester comprises aromatic dicarboxylic acid components represented by formula (I): HO(O)C-R<SP>2</SP>-OR<SP>1</SP>O-R<SP>2</SP>-C(O)OH (wherein R<SP>1</SP>is a 2-10C alkylene group and R<SP>2</SP>is a 2,6-naphthalenediyl group) and formula (II): HO(O)C-R<SP>3</SP>-C(O)OH (wherein R<SP>3</SP>is a phenylene group or a naphthalenediyl group) and a 2-4C alkylene glycol component. A copolymerization ratio of the aromatic dicarboxylic acid component represented by formula (I) is 5-80 mol% based on all dicarboxylic acid components, and furthermore an organic phosphorus compound is copolymerized for the phosphorus element content in the copolyester of 0.3-1.5 wt.%. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a flame retardant copolyester obtained by copolymerizing a 6,6 '-(alkylenedioxy) di-2-naphthoic acid component and an organic phosphorus compound, and a biaxially oriented film using the same.

  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 they are used in demanding applications such as flexible printed circuits and organic EL displays. Used as a base film for devices. However, the demand for mechanical characteristics and dimensional stability in flexible electronic devices in recent years is increasing, and further improvement of mechanical characteristics and reduction of temperature expansion coefficient and humidity expansion coefficient are required. .

  On the other hand, Patent Documents 1 to 4 propose a polyester comprising a 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component and an alkylene glycol, which is crystalline and has a melting point of polyethylene-6, 294 ° C. 6 ′-(ethylenedioxy) di-2-naphthoate is specifically described. Certainly, according to these polyesters, it is possible to provide a film having excellent heat resistance, mechanical strength, and excellent dimensional stability. However, in recent years, high flame retardancy is also required for use in flexible electronic devices, and therefore the above-mentioned copolyester may require further improvement.

JP-A-60-135428 JP-A-60-212420 JP 61-145724 A JP-A-6-145323

  An object of the present invention is to provide a flame-retardant copolyester having excellent flame retardancy and dimensional stability when formed into a film and the like, and particularly suitable for a base film of a flexible electronic device such as a flexible printed circuit board. There is to do.

  As a result of repeated studies to achieve the above-mentioned object, the present inventor further added a copolymer polyester obtained by copolymerizing a predetermined amount of 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component to organic compounds. It has been found that if a predetermined amount of a phosphorus compound is copolymerized, excellent flame retardancy can be imparted while maintaining excellent dimensional stability and mechanical properties by the acid component copolymerization, and the present invention has been completed.

Thus, according to the present invention, an aromatic dicarboxylic acid component represented by the following formulas (I) and (II) and an alkylene glycol component having 2 to 4 carbon atoms, the fragrance represented by the following formula (I): The copolymerization ratio of the group dicarboxylic acid component is in the range of 5 mol% or more and less than 50 mol% based on the total dicarboxylic acid component, and the organophosphorus compound has a phosphorus element content of 0. A flame-retardant copolyester is provided which is copolymerized in a proportion of 3 to 1.5% by weight.
HO (O) C—R ² —OR ¹ O—R ² —C (O) OH (I)
HO (O) C—R ³ —C (O) OH (II)
[R ¹ in Formula (I) represents an alkylene group having 2 to 10 carbon atoms, R ² represents a 2,6-naphthalenediyl group, and R ³ in Formula (II) represents a phenylene group or a naphthalenediyl group. ]
Moreover, the biaxially oriented film which consists of said flame retardant copolyester is also provided.

  The flame retardant copolyester of the present invention is excellent while having excellent characteristics such as a low temperature and humidity expansion coefficient of polyalkylene-6,6 ′-(alkylenedioxy) di-2-naphthoate. Since it exhibits flame retardancy, it is suitable as a base film for various flexible electronic devices including a base film for flexible printed circuit boards, and its industrial value is extremely high.

  The copolymer polyester of the present invention is one in which the acid component is composed of the above formulas (I) and (II), the glycol component is composed of alkylene glycol having 2 to 4 carbon atoms, Copolymerized.

The acid component represented by the formula (I) is one in which R ¹ is an alkylene group having 2 to 10 carbon atoms, and R ² is a 2,6-naphthalenediyl group. 6,6 ′-(ethylenedioxy) di-2-naphthoic acid component, 6,6 ′-(trimethylenedioxy) di-2-naphthoic acid component and 6,6 ′-(tetramethylenedioxy) di- Examples include 2-naphthoic acid component. Among these, from the viewpoint of the effect of the present invention, those having an even number of carbon atoms of R ₁ are preferable, and a 6,6 ′-(ethylenedioxy) di-2-naphthoic acid component is particularly preferable.

  Examples of the acid component represented by the formula (II) include a terephthalic acid component, an isophthalic acid component, a 2,6-naphthalenedicarboxylic acid component, and a 2,7-naphthalenedicarboxylic acid component. Among these, a terephthalic acid component and a 2,6-naphthalenedicarboxylic acid component are preferable from the viewpoint of mechanical properties, and a 2,6-naphthalenedicarboxylic acid component is particularly preferable.

  Examples of the alkylene glycol having 2 to 4 carbon atoms include ethylene glycol, trimethylene glycol, tetramethylene glycol, and 90 mol% or more of the glycol component is preferably an ethylene glycol component from the viewpoint of mechanical properties, In particular, 95 to 100 mol% is preferably ethylene glycol.

  By the way, one of the features of the present invention is that 6,6 ′-(alkylene di) represented by the above formula (I) in a range of 5 to 80 mol% based on the number of moles of all acid components of the copolymer polyester. Oxy) di-2-naphthoic acid component is copolymerized. 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 is difficult to be exhibited. On the other hand, the upper limit is preferably 80 mol% or less, and more preferably less than 50 mol% from the viewpoint of moldability. Surprisingly, the 6,6 '-(alkylenedioxy) di-2-naphthoic acid component reduces the coefficient of humidity expansion very efficiently in a small amount and is less than 50 mol%. In addition, a humidity expansion coefficient equivalent to or lower than that of the film described in the example of Patent Document 3 is achieved, and the effect from the viewpoint of the humidity expansion coefficient is saturated even when added in an amount of 50 mol% or more. It can be said that. From such a viewpoint, the upper limit of the copolymerization amount of the preferred 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component is 45 mol% or less, further 40 mol% or less, and further 35 mol% or less. In particular, it is 30 mol% or less, and the other lower limit is 5 mol% or more, further 7 mol% or more, still more 10 mol% or more, particularly 15 mol% or more.

  In the present invention, it is necessary that an organic phosphorus compound is further copolymerized with the copolymer polyester comprising the above components in a proportion of 0.3 to 1.5% by weight as the phosphorus element content in the copolymer polyester. The preferable phosphorus element content is 0.8 to 1.3% by weight, more preferably 0.9 to 1.2% by weight. If the amount of copolymerization of the organophosphorus compound is too small, the flame retardancy of the resulting copolymerized polyester will be insufficient. On the other hand, when the copolymerization amount of the organophosphorus compound is too large, not only the dimensional stability of the resulting molded product becomes insufficient, but also the mechanical properties are lowered, which is not preferable.

  The organophosphorus compound used here is not particularly limited as long as it has an ester-forming functional group that can be copolymerized with the above-mentioned copolymerized polyester. In particular, a carboxyphosphinic acid-based organic compound represented by the following formula (III) A phosphorus compound or a phosphaphenanthrene-based organic phosphorus compound represented by the following formula (IV) is preferable because a molded article having excellent flame retardancy can be obtained.

（式中、Ｒ_１は炭素数１〜１８のアルキル基またはアリール基、Ｒ_２およびＲ_３は炭素数１〜１８のアルキル基、アリール基、モノヒドロキシアルキル基または水素原子、Ｘは炭素数１〜１８の２価の炭化水素基をそれぞれ表わし、Ｒ_１〜Ｒ_３はそれぞれ同一でも異なっていてもよい。）

Wherein R ₁ is an alkyl group or aryl group having 1 to 18 carbon atoms, R ₂ and R ₃ are alkyl groups, aryl groups, monohydroxyalkyl groups or hydrogen atoms having 1 to 18 carbon atoms, and X is 1 carbon atom. To 18 divalent hydrocarbon groups, and R _{1 to} R ₃ may be the same or different.)

（式中、Ｒ_４は１価のエステル形成性官能基、Ａは２価または３価の有機残基を表し、ｎは１〜２の整数を表わす。）

(Wherein R ₄ represents a monovalent ester-forming functional group, A represents a divalent or trivalent organic residue, and n represents an integer of 1 to 2).

  Examples of the organic phosphorus compound represented by the formula (III) include carboxymethylphenylphosphinic acid, (2-carboxyethyl) phenylphosphinic acid, (2-carboxyethyl) toluylphosphinic acid, (2-carboxyethyl) 2,5- Dimethylphenylphosphinic acid, (2-carboxyethyl) cyclohexylphosphinic acid, (carboxypropyl) phenylphosphinic acid, (4-carboxyphenyl) phenylphosphinic acid, (3-carboxyphenyl) phenylphosphinic acid, (2-carboxyethyl) methyl Examples include phosphinic acid, (2-carboxyethyl) ethylphosphinic acid, and lower alcohol (1 to 4 carbon atoms) esters and ethylene glycol esters thereof. Among these, (2-carboxyethyl) phenylphosphinic acid, (carboxypropyl) phenylphosphinic acid, (2-carboxyethyl) methylphosphinic acid, and lower alcohol esters or ethylene glycol esters thereof are preferable.

  Examples of the organic phosphorus compound represented by the formula (IV) include organic phosphorus compounds represented by the following formulas (a) to (c) and their lower alcohol esters or ethylene glycol esters.

In addition, two or more of the above organic phosphorus compounds may be used in combination.
The copolyester of the present invention comprising the above components has an intrinsic viscosity of 0 measured at 35 ° C. using a mixed solvent of P-chlorophenol / 1,1,2,2-tetrachloroethane (weight ratio 40/60). The range of 0.4 to 3 dl / g is preferable, 0.4 to 1.5 dl / g, more preferably 0.5 to 1.2 dl / g.

  Furthermore, it is preferable from the point of film forming property that melting | fusing point measured by DSC exists in the range of 200-260 degreeC, the range of 210-255 degreeC, especially the range of 220-253 degreeC. When the melting point exceeds the above upper limit, when molding by melt extrusion, it is necessary to make the temperature higher to increase the fluidity, and it tends to be thermally deteriorated, and on the other hand, if the melting temperature is lowered, the fluidity is inferior, Discharge is likely to be non-uniform. On the other hand, when the value is less than the above lower limit, the film forming property is excellent, but the mechanical properties and the like of the copolyester are easily impaired. Usually, when other components are copolymerized to lower the melting point, mechanical properties and the like are likely to be lowered at the same time, but the copolymerized polyester of the present invention is excellent in mechanical properties or the like because the film-forming property is improved. Can be.

  The copolyester of the present invention has a glass transition temperature (hereinafter sometimes referred to as Tg) measured by DSC in the range of 90 to 125 ° C., more preferably in the range of 95 to 123 ° C., particularly 100 to 120. It is preferable that it is in the range of ° C. 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.

  By the way, in the copolymer polyester of the present invention, other copolymer components known per se to the obtained copolymer polyester, for example, 10 mol% or less, based on the total dicarboxylic acid component, within a range not impairing the effects of the present invention. In particular, it may be further copolymerized within a range of 5 mol% or less.

  Furthermore, the copolymer polyester of the present invention includes other thermoplastic polymers, stabilizers such as ultraviolet absorbers, antioxidants, plasticizers, lubricants, flame retardants, mold release agents, as long as the effects of the present invention are not impaired. , Pigments, nucleating agents, fillers or glass fibers, carbon fibers, layered silicates, etc. may be blended as needed, and making such polyester compositions will give additional properties to the resulting molded article It is preferable because it is possible. Other thermoplastic polymers include polyamide resins, polycarbonates, ABS resins, polymethyl methacrylate, polyamide elastomers, polyester elastomers, polyether imides, liquid crystalline resins, and 6,6 ′-(alkylenedioxy). ) Other polyester resins in which the copolymerization amount of di-2-naphthoic acid deviates.

  The flame-retardant copolyester of the present invention described above can be produced according to a conventionally known polyester production method. Hereinafter, a preferable method will be described by taking the case where the alkylene glycol is ethylene glycol as an example, but other alkylene glycols can be produced by the same method. That is, 6,6 ′-(alkylenedioxy) di-2-naphthoic acid represented by the formula (I) or a lower alkyl ester thereof, and, for example, 2,6-naphthalene represented by the formula (II) A polyester precursor is first produced by transesterification or esterification of dicarboxylic acid, terephthalic acid or their lower alkyl ester with ethylene glycol, and then the resulting polyester precursor is present in the presence of a polycondensation reaction catalyst. It can be produced by polycondensation below and solid-phase polymerization if necessary.

  The organophosphorus compound is added at any time during the production of the polyester, but a more preferred addition time is obtained after the completion of the first stage reaction for producing the polyester precursor by esterification or transesterification. This is until the start of the second stage in which the polycondensation reaction of the precursor is carried out, and it may be added all at once or in several portions.

  In the step of producing the polyester precursor, it is possible to use ethylene glycol in an amount of 1.1 to 6 times, more preferably 2 to 5 times, particularly 3 to 5 times the number of moles of the total acid component. It is preferable from the point.

  Moreover, as reaction temperature at the time of manufacturing a polyester precursor, it is preferable to carry out above the boiling point of ethylene glycol, and it is preferable to carry out especially in the range of 190-250 degreeC. When the temperature is lower than 190 ° C., the reaction does not proceed sufficiently. When the temperature is higher than 250 ° C., diethylene glycol as a side reaction product is likely to be generated. In addition, the reaction can be performed under normal pressure, but may be performed under pressure to further increase the productivity.

  In the step of producing this 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 temperature is in the range of a temperature not lower than the melting point of the obtained copolyester and not higher than 230 to 280 ° C., more preferably not lower than 5 ° C. and higher than the melting point by 30 ° C. The polycondensation reaction is usually preferably performed under a 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 becomes difficult to obtain a copolyester having a high degree of polymerization.

  Examples of the polycondensation catalyst include metal compounds containing at least one metal element. In addition, you may use together a polycondensation catalyst as a catalyst of esterification reaction or transesterification. 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 preferable because it exhibits high activity in both the esterification reaction, the transesterification 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 repeating unit of the flame retardant copolyester.

  Next, a method for producing a biaxially oriented film using the flame retardant copolyester of the present invention will be described. First, the copolymer polyester chip is dried, then supplied to an extruder heated to a melting point (Tm: ° C.) to (Tm + 50) ° C., and extruded from a die such as a T die into a sheet. The copolymer polyester resin to be used is not limited to one type, and for example, a polymer having a large proportion of the aforementioned formula (I) and a polymer having a large amount of the aforementioned formula (II) are prepared. Melting and kneading may be performed so that the ratio of (II) is within the target range, and by adopting such a method, the ratio of the above formulas (I) and (II) is arbitrarily and easily changed. be able to. In this case, one polyester that does not contain the aromatic dicarboxylic acid component of the formula (I) may be used.

  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 biaxially stretched to form a biaxially oriented film. In addition, from the viewpoint of facilitating the biaxial stretching, the cooling by the cooling drum is preferably performed very quickly, and is preferably performed at a low temperature of 20 to 60 ° C. 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, the first longitudinal stretching is performed at a glass transition temperature (Tg: ° C.) to (Tg + 40) ° C. of polyester at 3 to 8 times, and then at a higher temperature than the previous longitudinal stretching (Tg + 10) in the transverse direction. It is preferable that the film is stretched 3 to 8 times at a temperature of (Tg + 50) ° C., and further heat-treated 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. The heat setting time is preferably 1 to 15 seconds.

  Normally, when the stretching ratio is increased, the film-forming stability is impaired. However, since the copolymerized polyester resin composition of the present invention has high stretchability, there is no such problem, and in particular, the stretching ratio is higher. Therefore, even a thin film having a thickness of 10 μm or less, particularly 8 μm or less can be stably formed. The lower limit of the film thickness is not particularly limited, but is usually about 1 μm, preferably 3 μm.

On the other hand, simultaneous biaxial stretching in which longitudinal stretching and lateral stretching are simultaneously performed can be similarly performed, and the stretching ratio, stretching temperature, and the like described in the sequential biaxial stretching may be referred to.
In addition, a coating layer may be provided on the surface of the biaxially oriented film. In that case, a desired coating solution is applied to one or both sides of the unstretched film or the uniaxially stretched film described above, and then the same method as described above. Biaxial stretching and heat treatment may be performed.

  Furthermore, the biaxially oriented film of the present invention has a Young's modulus in the film forming direction (MD direction) and the width direction (TD direction) of preferably 4.5 GPa or more, more preferably 5 GPa or more. This is preferable from the viewpoint of suppressing elongation during processing.

  Hereinafter, the present invention will be described in more detail 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 polymer was dissolved using a mixed solvent of P-chlorophenol / 1,1,2,2-tetrachloroethane (40/60 weight ratio) and measured at a temperature of 35 ° C.

(2) Flame retardance After the polymer was dried at 175 ° C. for 4 hours, it was melt-pressed under conditions of a molding temperature of 300 ° C., a press pressure of 30 MPa, and a molding time of 1 minute to obtain an unstretched sheet. This unstretched sheet was simultaneously biaxially stretched 3.5 times in the longitudinal direction and in the width direction at 150 ° C., and then heat-set at 210 ° C. for 15 seconds to form a film having a thickness of 75 μm. The UL94VTM test ( The vertical burn test for thin plastic films was conducted according to the vertical burn test for thin plastic films.

(3) Phosphorus element concentration in polyester Polyester was melt-molded and analyzed with a fluorescent X-ray analyzer (ZSX100e) manufactured by Rigaku Corporation.

(4) Glass transition point and melting point It measured with DSC (TA Instruments Co., Ltd. make, brand name: Q100) at a heating rate of 20 ° C / min.

[ Reference Example 1]
30 kg (74.6 mol) of 6,6 ′-(ethylenedioxy) di-2-naphthoic acid, 7.8 kg (32.0 mol) of dimethyl ester of 2,6-naphthalenedicarboxylic acid and 25 kg of ethylene glycol were stirred. Then, the mixture was charged into a pressure vessel equipped with a rectifying column and a cooling tube, and the temperature was raised to 150 ° C. At that time, 2.3 g equivalent of titanium trimellitic acid as titanium element was added, the whole reactor was pressurized to 0.25 MPa with nitrogen, and the pressure vessel internal temperature was raised to 240 ° C. The pressure was always controlled to 0.25 MPa, and the total reflux was reached when the top temperature of the rectifying column reached 200 ° C., and the reaction was continued at a reflux ratio of 1 below 200 ° C. As the reaction progressed, the inside of the container gradually became transparent, and finally the internal temperature was raised to 250 ° C. After confirming that the liquid was transparent, the reaction was completed.

  Subsequently, the pressure was returned to normal pressure, 3.78 kg of 2-carboxyethylphenylphosphinic acid (abbreviated as CEPPA manufactured by Daihachi Chemical Co., Ltd.) represented by the following formula was added, and the internal temperature was raised again to 255 ° C. After distilling ethylene glycol, 6.2 g of antimony trioxide was added as a polymerization catalyst, and then the reaction solution was transferred to a polycondensation vessel through a wire mesh filter having an average opening of 30 μm.

  Thereafter, while gradually raising the temperature inside the reaction vessel, the inside of the vessel was slowly depressurized and the polycondensation reaction was continued until a predetermined stirring power was reached at 290 ° C. and 50 Pa to produce a copolyester. Table 1 shows the evaluation results of the flame retardancy of the copolyester resin.

[ Reference Example 2]
Reference Example 1 except that 3.78 kg of ethylene glycol ester of 2-carboxyethylmethylphosphinic acid represented by the following formula (manufactured by Clariant: Phosphorane) was used instead of 2-carboxyethylphenylphosphinic acid of Reference Example 1. It carried out like. The results are shown in Table 1.

[ Reference Example 3]
Instead of 6,6 ′-(ethylenedioxy) di-2-naphthoic acid in Reference Example 1, 32.1 kg (74.6 mol) of dimethyl ester 6,6 ′-(ethylenedioxy) di-2-naphthoic acid In addition, instead of 2-carboxyethylphenylphosphinic acid, a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-based organophosphorus compound represented by the following formula (Pionin ZB- manufactured by Takemoto Yushi) 101) was used in the same manner as in Reference Example 1 except that 7.96 kg was used. The results are shown in Table 1.

[Examples 4 to 5]
The ratio of 6,6 '-(ethylenedioxy) di-2-naphthoic acid and 2,6-naphthalenedicarboxylic acid dimethyl ester was changed so that 6,6'-(ethylenedioxy) di-2-naphthoic acid The same procedure as in Reference Example 1 was performed except that the charging ratio was changed so that the polymerization ratio and phosphorus element content were as shown in Table 1. The results are shown in Table 1.

[Comparative Example 1]
The same procedure as in Reference Example 1 was performed except that 14.9 g of trimethyl phosphate was added instead of 2-carboxyethylphenylphosphinic acid in Reference Example 1. The results are shown in Table 1.

  The flame-retardant copolyester of the present invention maintains excellent characteristics such as good dimensional stability against temperature and humidity possessed by polyalkylene-6,6 ′-(alkylenedioxy) di-2-naphthoate. However, since it also has excellent flame retardancy, it can be suitably used as a base film for various flexible electronic devices.

Claims (4)

Copolymerization of an aromatic dicarboxylic acid component represented by the following formula (I) comprising an aromatic dicarboxylic acid component represented by the following formulas (I) and (II) and an alkylene glycol component having 2 to 4 carbon atoms. The ratio is in the range of 5 mol% or more and less than 50 mol% based on the total dicarboxylic acid component, and the organic phosphorus compound is 0.3 to 1.5 wt% as the content of phosphorus element in the copolymerized polyester. A flame-retardant copolyester characterized by being copolymerized at a ratio of
HO (O) C—R ² —OR ¹ O—R ² —C (O) OH (I)
HO (O) C—R ³ —C (O) OH (II)
[R1 in Formula (I) represents an alkylene group having 2 to 10 carbon atoms, R2 represents a 2,6-naphthalenediyl group, and R3 in Formula (II) represents a phenylene group or a naphthalenediyl group. ] The flame retardant copolymer polyester according to claim 1, wherein the organic phosphorus compound is a phosphorus compound represented by the following formula (III) or (IV).

Wherein R ₁ is an alkyl group or aryl group having 1 to 18 carbon atoms, R ₂ and R ₃ are alkyl groups, aryl groups, monohydroxyalkyl groups or hydrogen atoms having 1 to 18 carbon atoms, and X is 1 carbon atom. To 18 divalent hydrocarbon groups, and R _{1 to} R ₃ may be the same or different.)

(Wherein R ₄ represents a monovalent ester-forming functional group, A represents a divalent or trivalent organic residue, and n represents an integer of 1 to 2).   A flame retardant biaxially oriented film comprising the flame retardant copolyester according to claim 1.   The flame-retardant biaxially oriented film according to claim 3, which is used as a base film for a flexible printed board.