WO2023003180A1 - Polyester film having enhanced durability and method for evaluating durability of same - Google Patents

Abstract

The present application pertains to a method for evaluating the durability of a polyester film. According to the present application, inefficiencies in the long-term reliability evaluation can be reduced and quality control can be made easier by simplifying the method for evaluating long-term reliability, without having to conduct conventional long-term reliability evaluation that requires a long time.

Description

Polyester film having improved durability and durability evaluation method thereof

Mutual Citation with Related Applications

This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0094249 dated July 19, 2021, and all contents disclosed in the literature of the Korean patent application are included as part of this specification.

technology field

This application relates to a polyester film and its durability evaluation method.

Films used in external environmental conditions, such as biaxially oriented polyester films used in photovoltaic products or building exterior products, or in products requiring shielding, insulation or heat dissipation, are greatly affected by external environments such as temperature, humidity and solar radiation. get affected. Therefore, an evaluation method that reproduces conditions in which damage may occur, such as leaving the film under harsher conditions (eg, high temperature and/or high humidity) for a long time, is used to predict the lifespan of these films. For example, an evaluation method such as an autoclave PCT (pressure cooker test) or a damp-heat test is used to predict the lifetime of a film or to evaluate the degree of durability of a film.

Conventional evaluation methods have the disadvantage of consuming a lot of time and money for the evaluation. For example, the Damp-Heat Test, which is one of the long-term reliability evaluation methods, usually takes 500 hours, and the Damp-Heat evaluation for films for photovoltaic power generation parts that require a long lifespan is evaluated up to 4000 hours or more. , causes inefficiencies in time, cost and manpower management. Considering the number of evaluations to be performed or the types of products requiring evaluation, the above inefficiency can be seen as a risk for quality control.

Therefore, there is a need for a technique capable of simplifying the long-term reliability evaluation method, replacing the conventional long-term reliability evaluation, improving inefficiency related to long-term reliability evaluation, and increasing the ease of quality control.

One object of the present application is to provide a polyester film capable of simplifying long-term reliability evaluation and an evaluation method thereof.

Another object of the present application is to improve inefficiency related to long-term reliability evaluation of the film and to provide a polyester film and an evaluation method thereof that increase the ease of quality control.

Another object of the present application is to provide a polyester film having excellent long-term reliability.

The above and other objects of the present application can all be solved by the present application described in detail below.

Depending on process conditions (e.g. temperature, time, pressure, stretching conditions, etc.) or polymer properties (e.g. molecular weight, viscosity, end groups, additives, etc.), the crystal structure, orientation, size, uniformity, and molecular weight of the polymer change, Accordingly, physical properties or characteristics of the film may be changed. The inventors of the present application experimentally identified factors that directly affect long-term reliability among the characteristics of the polyester film, and completed the invention of the present application.

According to the present application described later, an evaluation method capable of simply predicting long-term reliability without performing a known long-term reliability evaluation method that has been inefficient in terms of time and cost, and a polyester film having excellent long-term reliability are provided. .

In the present specification, "durability" or "long-term reliability" in relation to the evaluation method or the characteristics of a film means a characteristic related to the residual elongation of the film after leaving the film in harsh conditions for a predetermined period of time. At this time, the harsh conditions may mean a state including high temperature and / or high humidity conditions, and the time when the film is left in the harsh conditions is, for example, 50 hours or more, 100 hours or more, 500 hours or more, 1000 hours or more, 2000 hours or more, 3000 hours or more, or 4000 hours or more. Specifically, the high temperature is a temperature of 80 ° C or higher, more specifically, for example, 121 ° C or higher at which an autoclave pressure cooker test (PCT) can be performed, 85 ° C or higher at which a Damp-Heat Test can be performed It can mean higher temperatures. Further, high humidity may mean a relative humidity of 80% or more, specifically, for example, a relative humidity of 85% at which a Damp-Heat Test can be performed. Although not particularly limited, the harsh conditions may include atmospheric pressure conditions that are higher or lower than atmospheric pressure.

Hereinafter, the invention of the present application will be described in more detail.

In one example pertaining to the present application, the present application relates to a method for evaluating or predicting the durability or long-term reliability of a polyester film.

Specifically, the method includes measuring the plane orientation coefficient and modulus of the polyester film, and the measured plane orientation coefficient (A) and modulus (B) are used to evaluate the reliability of the polyester film. It can be an evaluation method.

In the specific examples of the present application, the plane orientation coefficient may be calculated according to Equation 1 below.

[Equation 1]

Plane orientation factor (ΔP) = (nx+ny)/2 - nz

In Equation 1, ΔP is a plane orientation coefficient, nx is a machine direction (MD) refractive index, ny is a transverse direction (TD) refractive index, and nz is a thickness direction refractive index.

The plane orientation coefficient may be calculated or measured by, for example, an Abbe refractometer (manufactured by Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm). Although not particularly limited, when measuring the refractive index related to the plane orientation coefficient, a sample of 40 mm x 120 mm was measured in the longitudinal direction (MD) and the width direction (TD) of the film from the center of the film width direction (TD). Specimens of this size can be produced, and the refractive index in each direction can be measured.

In one example, the plane orientation coefficient may be an arithmetic average value. Specifically, the plane orientation coefficient may be an arithmetic average value obtained from a plurality of plane orientation coefficient values measured a plurality of times, for example, 2 to 10 times for one specimen (polyester film).

The plane orientation coefficient shows an almost proportional appearance with the refractive index in each stretching direction increasing as the stretching ratio increases, the more stress is applied during stretching, and the stretching temperature decreases. For example, the higher the stretching ratio and the higher the stretching stress, the more regular the alignment of the polymer chains, and the more evenly distributed the crystallization due to the heat generated at this time. As such, since the uniformity of polymer chains and the even distribution of amorphous crystals affect the mechanical properties of polymers, the plane orientation coefficient of refractive index is considered to have a correlation with mechanical properties or durability reliability. In particular, since the plane orientation coefficient expresses the length, width, and thickness directions as one, as in Equation 1, it is determined that the correlation with mechanical properties or durability reliability is high.

In an embodiment of the present application, the modulus may be measured using DMA (Dynamic Mechanical Analysis) (@25°C).

For example, the modulus may be measured under the conditions of a temperature rise in the range of -40 to 200 °C at a rate of 4 °C/min, an initial load of 1.5 N, a strain of 0.1%, and a frequency of 1 Hz. Although not particularly limited, the modulus measurement may be made for a specimen of a predetermined size, for example, a film having a width of 10 mm and a length of 50 mm.

In one example, the modulus may be an arithmetic average value. Specifically, the modulus may be an arithmetic average value obtained from a plurality of modulus values measured a plurality of times, for example, 2 to 10 times for one specimen (polyester film).

Modulus is a measure of the viscoelasticity of a sample. The high viscoelastic value of the sample against repeated mechanical force under certain conditions means that the sample has excellent heat resistance and resilience and can have a high viscosity (molecular weight), so it is considered to be highly correlated with the long-term reliability of the sample.

Although not particularly limited, the modulus may be measured in one or more directions of a machine direction and a transverse direction.

In one example, the method is performed on a polyester film having a plane orientation coefficient and modulus in a predetermined range, and the closer the measured plane orientation coefficient and modulus are to the upper limit of each range, the better the durability of the film. could be a way to do it. Specifically, the film to be evaluated (sample or specimen) may be, for example, a polyester film having a plane orientation coefficient in the range of 0.150 to 0.180 and a modulus in the range of 3.0 to 6.0 GPa. As confirmed through the following experimental examples, According to the method of the present application, it can be determined that the durability of the film is excellent as the plane orientation coefficient and the modulus are closer to the upper limit of each range. The plane orientation coefficient and modulus of the film to be evaluated are characteristic categories of a polyester film having a certain degree of marketability at the minimum stretching ratio at which normal stretching occurs and the maximum stretching ratio at which breakage does not occur during the process.

For example, when the plane orientation coefficient of a certain film (F) is close to 0.180 and the modulus is close to 6.0 GPa, the durability of the film (F) can be expected or evaluated to be excellent. Conversely, when the plane orientation coefficient of a certain film (F') is close to 0.150 and the modulus is close to 3.0 GPa, the durability of the film may be expected or evaluated to be poor. The tendency for the above evaluation or prediction is based on the measurement results of the plane orientation coefficient value and the modulus value for any one of the two films (e.g., two films obtained through the same process for the same composition) that are recognized as identical. , can be confirmed (verified) by comparing with the results of Autoclave PCT (pressure cooker test) or Damp-Heat Test for another film.

In one example, the method may be a method of measuring plane orientation coefficient and modulus in the same way for each of a plurality of different films and evaluating durability of each film.

In another example, the method may be a method of measuring plane orientation coefficient and modulus in the same way for each of a plurality of different films, and comparing and evaluating durability of each film. For example, when the plane orientation coefficient and modulus are measured for each of the two types of films F1 and F2, when one film F1 has a higher plane orientation coefficient and higher modulus than the other film F2 , it can be predicted or evaluated that the durability of the film (F1) is more excellent than that of the film (F2).

In an embodiment of the present application, the method may further include evaluating or predicting durability of the film according to the measured or calculated plane orientation coefficient and modulus, and selecting a film of a similar or equivalent level.

For example, the method may further include selecting a polyester film having a plane orientation coefficient in the range of 0.169 to 0.180 and a modulus in the range of 4.8 to 6.0 Gpa.

For example, the method may further include selecting a polyester film having a plane orientation coefficient in the range of 0.167 to 0.169 and a modulus in the range of 4.3 to 4.8 Gpa.

For example, the method may further include selecting a polyester film having a plane orientation coefficient in the range of 0.165 to 0.167 and a modulus in the range of 4.1 to 4.3 Gpa.

For example, the method may further include selecting a polyester film having a plane orientation coefficient in the range of 0.155 to 0.165 and a modulus in the range of 3.5 to 4.1 Gpa.

For example, the method may further include selecting a polyester film having a plane orientation coefficient in the range of 0.150 to 0.155 and a modulus in the range of 3.0 to 3.5 Gpa.

The numerical ranges described in relation to the selection of the plane orientation coefficient and the modulus described above are boundary values confirmed through experiments, and the boundary values of each range may have a predetermined error range or tolerance. For example, in the case of plane orientation coefficient, an error range of ± 0.0001 to 0.002 or ± 0.0001 to 0.001 may exist, and in the case of modulus (Gpa), an error range of ± 0.01 to 0.2 or ± 0.01 to 0.1 may exist. . Although not particularly limited, the error range as described above may be applied to the upper and/or lower limit of the range described in relation to the film.

The thickness of the polyester film to be evaluated in the method of the present application is not particularly limited, but, for example, the thickness of the polyester film may range from 50 to 300 μm. Specifically, the lower limit of the thickness may be, for example, 60 μm or more, 70 μm or more, 80 μm or more, 90 μm or more, or 100 μm or more, and the upper limit may be, for example, 250 μm or less, 200 μm or less, 150 μm or less. It may be less than a micron or less than 100 microns.

Although not particularly limited, the film may be monolayer or multilayer. In the case of a multilayer film, it may be a multilayer film including at least one film selected from among films selected according to plane orientation coefficient and modulus.

Regarding the above method, the kind of polyester film to be evaluated is not particularly limited. For example, the polyester film may be manufactured by extruding a composition or chip containing a known polyester resin.

The polyester resin included in the composition or chip may be one obtained by an ester bond between a known component, for example, a dicarboxylic acid component and a glycol component. Components used to prepare the resin are not particularly limited.

The film may further include other components in addition to the polyester resin. That is, the composition or chip used to manufacture the polyester film may contain other components in addition to the polyester resin. Other components may be, for example, various known additives or fine particles.

In one example, the polyester film may include a light stabilizer. The type of light stabilizer is not particularly limited, but examples thereof include benzophenone-based compounds, benzotriazole-based compounds, benzooxazinone-based compounds, benzoate-based compounds, phenyl salicylate-based compounds, or hindered amine-based compounds. Light stabilizers may be used.

In one example, the polyester film may include particles. The particles may be organic, inorganic, or hybrid particles. Hybrid particles may refer to particles in which, for example, two different components form a core and a shell (coating component), respectively. Although not particularly limited, for example, inorganic particles such as titanium dioxide, barium sulfate, calcium carbonate, magnesium carbonate, calcium phosphate, silica, alumina, talc, and kaolin may be used. Further, the particles used may have (average) particle diameters of 0.1 μm or more, 0.2 μm or more or 0.3 μm or more, and 0.6 μm or less, 0.5 μm or less, 0.4 μm or less or 0.3 μm or less.

The manufacturing method of the polyester film to be evaluated is not particularly limited, but the polyester film may be manufactured by (melting) extruding and stretching a composition or chip containing a polyester resin. For example, extrusion may be performed by melt extrusion using a T-die widely used in the prior art. And, for the melt-extruded unstretched sheet, stretching may be performed in the machine direction (MD) and/or the width direction (TD). At this time, stretching in each direction may be performed simultaneously or at different times.

Although not particularly limited, the stretching ratio in the machine direction (MD) and/or the transverse direction (TD) may be, for example, 2.0 to 4.0 times. And, the stretching may be made in the range of 100 to 150 ℃, for example.

After the stretching as described above, a so-called heat setting treatment may be performed. Heat setting conditions are not particularly limited, but heat setting may be performed at, for example, 180 to 240 ° C for about 5 to 30 seconds.

In another example related to the present application, the present application relates to a polyester film having excellent long-term reliability. Description of film formation components, thickness, etc. is the same as described above.

Even when films are manufactured from chips or compositions containing the same polyester-forming component, films having different properties such as long-term reliability may be provided depending on film manufacturing conditions and the like. According to the present application, a polyester film that can be evaluated as having excellent long-term reliability can be provided.

For example, according to the specific examples of the present application, a polyester film with excellent long-term reliability having a plane orientation coefficient in the range of 0.169 to 0.180 and a modulus in the range of 4.8 to 6.0 Gpa can be provided. According to another embodiment of the present application, a polyester film having a plane orientation coefficient in the range of 0.167 to 0.169 and a modulus in the range of 4.3 to 4.8 Gpa may be provided. In this case, the above error range may be applied.

In one example, the film may be monolayer or multilayer. The single-layer film may be a film expected to have excellent long-term reliability according to the above-described evaluation method, and the multi-layer film may be a multi-layer film in which at least one film expected to have excellent long-term reliability is laminated (or included).

According to a specific embodiment of the present application, a method for evaluating the durability of a polyester film capable of simplifying long-term reliability evaluation, improving inefficiencies in terms of time and cost that occur when performing existing long-term reliability evaluation, and enhancing the ease of quality control is provided. Provided. In addition, this application has the effect of the invention to provide a polyester film with excellent durability.

Hereinafter, the action and effect of the invention will be described in more detail through specific examples of the invention. However, this is presented as an example of the invention, and thereby the scope of the invention is not limited in any sense.

Preparation of specimen film

Psalm 1

A co-polyester chip having an intrinsic viscosity of about 0.63 dl/g was prepared using a dicarboxylic acid component such as terephthalic acid and a diol component such as ethylene glycol or neopentyl glycol.

After melting it at 280 ° C. in an extruder and extruding it through a T-die, it was rapidly cooled and solidified on a cooling roller having a surface temperature of 20 ° C., and at the same time, an amorphous unstretched sheet was obtained while adhering to the cooling roller using an electrostatic application method.

The unstretched film is stretched 3.0 times at about 110 ° C. in the machine direction (MD) of the film through a preheating roller, and the uniaxially stretched film is guided to a tenter and preheated at 108 ° C. for about 2 seconds, and then the width In the direction (TD; Transverse Direction), it was stretched 3.5 times at about 140 ℃.

Subsequently, a heat setting treatment was performed on the biaxially stretched film at about 210° C. for 15 seconds, followed by a relaxation treatment at about 210° C., and then a biaxially stretched polyester film having a thickness of about 70 μm was obtained.

Psalms 2 to 12

Polyester films were prepared according to the manufacturing process of specimen 1, but films of specimens 2 to 12 were prepared by varying one or more of the stretching ratio, stretching temperature, heat setting time, heat setting temperature, and film thickness from specimen 1.

Evaluation and comparison of specimen films 1

The plane orientation coefficient and modulus of each sample film were measured according to the method described above (arithmetic average over 5 measurements). In addition, an autoclave PCT (pressure cooker test) or a damp-heat test was also performed on the same specimen film.

The results are shown in Table 1 below.

	film properties		Long-term reliability evaluation under known harsh conditions
	face orientation factor	modulus (MD direction)	50 PCT	Damp-Heat 500	Damp-Heat 1000	Damp-Heat 2000	Damp-Heat 3000	Damp-Heat 4000
Psalm 1	0.142	2.0	21	42	28	11	8	5
Psalm 2	0.148	2.4	24	48	36	18	13	8
Psalm 3	0.151	3.1	36	51	44	19	18	11
Psalm 4	0.155	3.1	54	56	51	21	21	16
Psalm 5	0.162	3.5	66	77	55	25	25	21
Psalm 6	0.163	4.1	71	79	74	35	37	32
Psalm 7	0.165	4.0	71	85	88	54	48	41
Psalm 8	0.167	4.3	78	91	98	58	55	48
Psalm 9	0.169	4.8	84	98	99	70	63	51
Psalm 10	0.170	4.8	81	98	98	71	71	63
Psalm 11	0.172	5.2	99	100	99	73	72	68
Psalm 12	0.175	5.5	100	100	100	85	79	75
* PCT 50: Residual elongation measured after leaving the specimen film in the harsh conditions of 121 ℃ and 1.4 bar for 50 hours according to the known auto clave PCT * Damp-Heat 500: Residual elongation measured after leaving the specimen film in harsh conditions of 85 ℃ and 85 % RH (relative humidity) for 500 hours according to the known Damp-Heat Test * Damp-Heat 1000: Residual elongation measured after leaving the specimen film in harsh conditions of 85 ℃ and 85 % RH (relative humidity) for 1000 hours according to the known Damp-Heat Test * Damp-Heat 2000: Residual elongation measured after leaving the specimen film in harsh conditions of 85 ℃ and 85 % RH (relative humidity) for 2000 hours according to the known Damp-Heat Test * Damp-Heat 3000: Residual elongation measured after leaving the specimen film in harsh conditions of 85 ℃ and 85 % RH (relative humidity) for 3000 hours according to the known Damp-Heat Test * Damp-Heat 4000: Residual elongation measured after leaving the specimen film under severe conditions of 85 ℃ and 85 % RH (relative humidity) for 4000 hours according to the known Damp-Heat Test * The elongation retention rate described above means the elongation of the film after being left under each of the harsh conditions. * Elongation related to the elongation retention rate is the breaking elongation in the machine direction (MD) measured using a universal tensile tester (Instron Tensile Test Machine) under predetermined conditions, and means the arithmetic average value after 10 measurements (measurement conditions: Measurement sample width 15mm, sample length 50mm, cross head-up speed 500mm/min)

Regarding auto clave PCT or Damp-Heat Test, if the elongation retention rate is high even though the film is left or stored in harsh conditions for a long time, it means that the film has excellent long-term reliability.

Summarizing the contents of Table 1, in a polyester film having a plane orientation coefficient in the range of 0.150 to 0.180 and a modulus in the range of 3.0 to 6.0 GPa, the closer the plane orientation coefficient and modulus are to the upper limit of each range, the traditional long-term reliability test It is confirmed that the elongation retention rate according to the law (Auto clave PCT or Damp-Heat Test) is also high. That is, the method of the present application can replace the conventional long-term reliability evaluation.

Evaluation of specimen film 2

For specimens 1 to 12, the items listed in Table 2 were measured or evaluated. The evaluation items are as follows.

*crystal size

After obtaining the wavelength and peak value of the target by X-ray diffraction, it can be obtained by substituting into the equation below.

[Equation] Crystal size (D) = 0.9 (target wavelength) / line width (radian) cos (peak / 2)

*Degree of crystallinity

The diffraction peak can be obtained by X-ray diffraction and can be obtained using the formula below.

[Formula] relative crystallinity = Aa x 100/(Aa + Ac)

(Aa = area of the part recognized as amorphous peak, Ac = area of crystalline peak, Aa + Ac = total area)

*burglar

This is the arithmetic average value obtained after measuring the breaking elongation in the machine direction (MD) 10 times using a universal tensile tester (Instron Tensile Test Machine) (conditions: measurement sample width 15 mm, sample length 50 mm, Cross head-up speed 500 mm/min)

* Shinto

This is the arithmetic average value obtained after measuring the breaking elongation in the machine direction (MD) 10 times using a universal tensile tester (Instron Tensile Test Machine) (Conditions: Measurement sample width 15 mm, sample length 50 mm, Cross head-up speed 500mm/min)

* Dielectric breakdown strength

Under the conditions of a speed of 500 V/sec and a maximum leakage current of 5 mA, the voltage indicator at which insulation is destroyed when more than 5 mA is conducted was measured by reading the indicator. The sample used at this time has a size of at least 5 cm x 5 cm.

	crystal size (Å)	crystallinity (%)	IV (dl/g)	-COOH (eq/ton)	burglar (kgf/cm²)	Shinto	dielectric breakdown strength
Psalm 1	52	42	0.63	32.0	987.4	56.8	351
Psalm 2	51	41	0.68	24.0	1037.3	72.5	289
Psalm 3	52	41	0.75	20.0	1286.3	62.0	322
Psalm 4	51	43	0.63	32.0	1083.7	48.7	287
Psalm 5	51	41	0.68	24.0	1309.8	69.0	266
Psalm 6	42	48	0.75	20.0	1286.3	62.5	324
Psalm 7	50	48	0.63	32.0	1127.4	56.9	311
Psalm 8	47	47	0.68	24.0	1202.9	65.4	298
Psalm 9	46	51	0.75	20.0	1316.8	43.2	311
Psalm 10	48	51	0.63	32.0	1437.5	65.9	378
Psalm 11	51	51	0.68	24.0	1287.4	78.4	297
Psalm 12	46	49	0.75	20.0	1316.7	58.9	255
* IV Means intrinsic viscosity. * -COOH: Carboxyl terminal, which means the end of a polymer chain.

The items listed in [Table 2] are generally used as indicators for evaluating the durability of polymers. For example, crystallinity or crystal size is an index for evaluating mechanical strength, and intrinsic viscosity (I.V.) has a high correlation with molecular weight, modulus, and viscosity. In addition, -COOH, which means a carboxyl terminal, promotes decomposition due to thermal oxidation decomposition, so its content (eq / ton) can be considered important when determining durability. In the case of dielectric breakdown strength, it is considered to be related to durability because it is lowered when the polymer is decomposed or foreign matter is generated inside.

As such, the above indicators are things that can be considered in relation to securing durability of PET or PET film, but since the range of change in each sample is small and the trend of change is not clear, the tendency of long-term reliability after being processed into an actual film It is not suitable for judging overall.

As a result, the present invention simplifies the long-term reliability evaluation method, improves inefficiency related to long-term reliability evaluation, and manages quality, without checking each of the indicators identified in [Table 2] or conducting the conventional long-term reliability evaluation that takes a long time. ease of use can be increased.

Claims (10)

1. Measuring the plane orientation coefficient and modulus of the polyester film,

   A method for evaluating the durability of a polyester film using plane orientation coefficient (A) and modulus (B) for reliability evaluation of the polyester film.
2. According to claim 1,

   The polyester film has a thickness of 50 to 300 ㎛ range, durability evaluation method of the polyester film.
3. According to claim 1,

   The polyester film is a method for evaluating the durability of a polyester film comprising a light stabilizer or particles.
4. According to claim 1,

   For a polyester film having a plane orientation coefficient within the range of 0.150 to 0.180 and a modulus within the range of 3.0 to 6.0 GPa,

   A method for evaluating the durability of a polyester film, in which the plane orientation coefficient and the modulus are closer to the upper limit of each range, the durability of the film is judged to be excellent.
5. According to claim 4,

   A method for evaluating the durability of a polyester film by measuring the plane orientation coefficient and modulus of a plurality of films that are different from each other and comparing and evaluating the plane orientation coefficient and modulus of each film.
6. According to claim 4,

   A method for evaluating durability of a polyester film, further comprising selecting a polyester film having a plane orientation coefficient in the range of 0.169 to 0.180 and a modulus in the range of 4.8 to 6.0 Gpa.
7. According to claim 4,

   A method for evaluating durability of a polyester film, further comprising selecting a polyester film having a plane orientation coefficient in the range of 0.167 to 0.169 and a modulus in the range of 4.3 to 4.8 Gpa.
8. According to claim 4,

   A method for evaluating the durability of a polyester film, further comprising selecting a polyester film having a plane orientation coefficient in the range of 0.165 to 0.167 and a modulus in the range of 4.1 to 4.3 Gpa.
9. According to claim 4,

   A method for evaluating durability of a polyester film, further comprising selecting a polyester film having a plane orientation coefficient in the range of 0.155 to 0.165 and a modulus in the range of 3.5 to 4.1 Gpa.
10. According to claim 4,

    A method for evaluating durability of a polyester film, further comprising selecting a polyester film having a plane orientation coefficient in the range of 0.150 to 0.155 and a modulus in the range of 3.0 to 3.5 Gpa.