DE69702796T2 - Polyester film with low electrostatic properties and good adhesion - Google Patents

Description

Detailed description of the invention Field of the invention:

This invention relates to a polyester film having low electrostatic and high adhesion properties, and more particularly to a polyester film having a primer layer formed on at least one side thereof, which has excellent heat resistance and coating properties and has low electrostatic and high adhesion properties.

State of the art:

Polyester films are used in a wide range of applications (such as magnetic recording media, graphic materials, printing materials, packaging materials, electronic materials, etc.) due to their excellent chemical and physical properties. However, the polyester film has a high crystallinity and must be provided with surface characteristics to suit the application in order to develop sufficient film properties.

As a means of imparting adhesive properties to the polyester film, the formation of an adhesive layer (primary layer) by applying an aqueous solution such as an aqueous dispersion on an adherent polyester resin has been proposed and carried out. For example, it is known that the primer layer can be formed by applying an aqueous solution of a hydrophilic polyester resin having not less than 0.5 mol% of a sulfonic acid salt group introduced into the polymer molecule. However, this hydrophilic polyester resin has disadvantages of a low secondary transition point (Tg) and poor resistance to moist heat. Furthermore, a surfactant is added to stabilize the aqueous solution or a coating step, but the heat resistance is deteriorated by this surfactant and as a consequence, the polyester resin has poor coating properties.

It is effective to introduce a rigid component to increase the secondary transition point (Tg) of the polyester resin. In this case, the adhesion properties slightly decrease and the hydrophilic nature becomes low, with the result of a decrease in the solubility of the resin.

JP-A-63-115747 (the term "JP-A" in the present application means an "unexamined published Japanese patent application") discloses a transparent thermoplastic resin composite film having excellent antistatic properties, which has on at least one side thereof a base layer comprising a mixture of (A) a water-insoluble polyester copolymer comprising a mixed dicarboxylic acid component containing a dicarboxylic acid containing a sulfonic acid metal salt in an amount of 0.5 to 15 mol% based on the total dicarboxylic acid component and a glycol component and (B) a copolymer represented by the following formula

HO (R1 O)n - (R2 O)m -

wherein n and m are an integer of 6 to 1,200, R₁ and R₂ are an alkylene group having 1 to 4 carbon atoms or an arylene group, with the proviso that R₁ and R₂ are not the same and one of them is an alkylene group having 3 or more carbon atoms or an arylene group.

However, it cannot be claimed that this composite film provides satisfactory antistatic properties, and the above copolymer has slightly low adhesion properties due to low HLB (hydrophilic-lipophilic balance).

JP-A-7-144361 describes an easy-binding polyester film having a base layer coating comprising a water-insoluble polyester containing less than 0.5 mol% of a dicarboxyl component containing a sulfonate group and a polyoxyethylene type wetting agent as main components. JP-A-7-227948 discloses a polyester film having an easy-binding base layer made from a composition comprising (A) 75 to 96 wt% of a polyester resin containing less than 0.5 mol% of sulfonic salt groups and (B) 4 to 25 wt% of an ethylene oxide-propylene oxide copolymer. Such a base layer does not have satisfactory coating properties.

It is a first object of the present invention to provide a polyester film with a base layer having low electrostatic and high adhesion properties.

It is a second object of the present invention to provide a polyester film having a base layer which is excellent in heat resistance. It is a third object of the present invention to provide a base layer which can form a uniform coating film on the surface of a polyester film and has excellent coating processability.

It is a further object of the present invention to provide a polyester film having a base layer which has excellent running properties in contact with a heating roller.

It is still another object of the present invention to provide an adhesive polyester film suitable for use in a magnetic recording medium is suitable.

According to the investigations conducted by the inventors of the present invention, it was found that the above objects of the present invention can be achieved by a polyester film having low electrostatic and high adhesion properties, which has on at least one side thereof a base layer having a softening point of 145°C to 165°C and consists of component A and component B, wherein component A is a water-dispersible polyester resin containing as a copolymer component a compound containing a sulfonic acid metal salt group in an amount of 0.04 to 0.3 mol% of the total dicarboxylic acid compound, and component B is a mixture of a polyethylene oxide monoalkyl ether (B-1) and a polyethylene oxide-polypropylene oxide block copolymer (B-2), wherein the weight ratio of components A, (B-1) and (B-2) is 90 to 80%, 7 to 3% and 10 to 20%, respectively. 13 to 7%.

The present invention will be described in more detail below.

In the present invention, the polyester constituting the polyester film as a base film is a polyester comprising an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. The polyester is substantially linear and has a film-forming property, particularly a property of film formation by melt molding.

Illustrative examples of the aromatic dicarboxylic acid constituting the polyester include terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenoxyethanedicarboxylic acid, diphenyldicarboxylic acid, diphenyl ether dicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenyl ketone dicarboxylic acid and anthracene dicarboxylic acid.

Illustrative examples of the aliphatic glycol component include Polymethylene glycol with 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and decamethylene glycol, alicyclic diols such as cyclohexanedimethanol.

In the present invention, polyesters comprising alkylene terephthalate and/or alkylene naphthalate as main component(s) are preferably used as polyesters.

Of these polyesters, polyethylene terephthalate, polyethylene-2,6-naphthalate and a copolyester comprising terephthalic acid and/or 2,6-naphthalenedicarboxylate in an amount of not less than 80 mol% of the total dicarboxylic acid compound and ethylene glycol in an amount of not less than 80 mol% of the total glycol component are particularly preferred. Not more than 20 mol% of the total acid component may be the above other aromatic dicarboxylic acids other than terephthalic acid and/or 2,6-naphthalenedicarboxylic acid, aliphatic dicarboxylic acids such as adipic acid and sebacic acid, alicyclic dicarboxylic acids such as cyclohexane-1,4-dicarboxylate and the like. Not more than 20 mol% of the total glycol component may be the above other glycols other than ethylene glycol, aromatic diols such as hydroquinone, resorcinol and 2,2-bis(4-hydroxyphenyl)propane, aliphatic diols having an aromatic ring such as 1,4-dihydroxydimethylbenzene, polyalkylene glycol (polyoxyalkylene glycol) such as polyethylene glycol, polypropylene glycol and polytetramethylene glycol.

The polyester used in the present invention includes copolymers copolymerized with an oxycarboxylic acid component such as an aromatic oxyacid, for example, hydroxybenzoic acid, or an aliphatic oxyacid, typically ω-hydroxycaproic acid, in an amount of not more than 20 mol% of the total amount of the carboxylic acid components and the oxycarboxylic acid components.

The polyester used in the present invention also includes a copolymer containing a polycarboxylic acid component or a Polyhydroxy compound component having 3 or more functional groups, such as trimellitic acid or pentaerythritol, in an amount such that it is substantially linear, e.g. in an amount of not more than 2 mole percent of the total acid component.

The polyester is known per se and can be prepared by a process known per se. The polyester preferably has an intrinsic viscosity, measured at 35°C in an o-chlorophenol solution, of about 0.4 to 0.9.

Inert inorganic particles or crosslinked polymer particles, alone or in a desired combination, may be contained in the polyester as a lubricant in the present invention. Preferred examples of the inert inorganic particles include kaolin, calcium carbonate, silica, alumina and the like. Preferred examples of the crosslinked polymer particles include crosslinked silicone resin, crosslinked polystyrene resin, crosslinked acrylic resin and the like. These particles are preferably added to a reaction system as a slurry in a glycol when a polyester is produced, for example, in a desired step during an ester exchange reaction or during a polycondensation reaction when an ester exchange method is used, or in a desired step when a direct polymerization method is used. The average particle size of the lubricant is preferably 0.005 to 2 µm, more preferably 0.01 to 1.8 µm. The amount of the lubricant added is preferably 0.01 to 2 wt%, more preferably 0.03 to 1.5 wt%.

The polyester film of the present invention is preferably a biaxially oriented film. The thickness of the film is preferably 3 to 20 µm. Further, the polyester film preferably has an elastic modulus in biaxial directions of not less than 450 kg/mm² and not more than 2,000 kg/mm².

Although the manufacturing process for the polyester film is not particularly limited, the polyester film can be produced by any commonly known process.

According to the invention, the base layer formed as a base film on one side or both sides of the polyester film consists essentially of the following components A and B:

Component A: Water-dispersible polyester resin containing a compound containing a sulfonic acid metal salt group in an amount of 0.04 mol% to 0.3 mol% of the total dicarboxylic acid compound.

Component B: Mixture of a polyethylene oxide monoalkyl ether (component B-1) and a polyethylene oxide-propylene oxide block copolymer (component B-2).

The above water-dispersible polyester resin (component A) in the base layer of the present invention contains, as a copolymer component, a compound containing a sulfonic acid metal salt group in an amount of 0.04 to 0.3 mol% of the total dicarboxylic acid compound. If the copolymerization ratio of this compound is less than 0.01 mol%, the dispersibility of the resin and the stability of a coating solution deteriorate extremely. If the ratio is 0.5 mol% or more, the resulting film has high electrostatic properties. Therefore, low electrostatic properties, which is the characteristic feature of the polyester film of the present invention, cannot be obtained.

Preferred examples of the compound containing a sulfonic acid metal salt group include dicarboxylic acids having a sulfonic acid salt group (such as sodium sulfonate group, lithium sulfonate group, potassium sulfonate group, ammonium sulfonate group, polyalkylphosphinesulfonate group, and the like). Specific examples thereof include 5-sodium sulfoisophthalate, 5-potassium sulfoisophthalate, 5-sodium sulfoterephthalate, 5-potassium sulfoterephthalate, and the like.

Preferred examples of the dicarboxylic acid compound having the above water-dispersible polyester resin (A) include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, hexahydroterephthalic acid, 4,4'-diphenyldicarboxylic acid, phenylindanedicarboxylic acid, adipic acid, sebacic acid, trimellitic acid and the like. The water-dispersible polyester resin containing 2,6-naphthalenedicarboxylic acid in an amount of at least 30 mol%, preferably from 35 to 90 mol%, of the total dicarboxylic acid compound imparts high heat resistance to the base layer.

Preferred examples of the glycol component constituting the polyester resin as component A include ethylene glycol, diethylene glycol, neopentyl glycol, 1,4-butanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, dimethylolpropionic acid, glycerin, trimethylolpropane, an adduct of bisphenol A with alkylene oxide, and the like. They may be used in a combination of two or more.

The above water-dispersible polyester resin (component A) can be prepared by a conventionally known method. The number-average molecular weight of the water-dispersible polyester resin (component A) is preferably 3,000 to 30,000, more preferably 5,000 to 15,000. Although the water-dispersible polyester resin (A) is used as a dispersion in water to prepare a coating solution, its average particle size is preferably several tens of nm.

Preferred examples of component A include water-dispersible polyesters which comprise dicarboxylic acid compounds consisting of 30 to 99.06 mol% of 2,6-naphthalenedicarboxylic acid, 0 to 69.7 mol% of isophthalic acid and 0.04 to 0.3 mol% of 5-sodium sulfoisophthalate, and glycol components consisting of 40 to 100 mol% of ethylene glycol and 0 to 60 mol% of an adduct of bisphenol A with propylene oxide (1 to 10 mols of adduct).

Component B, which forms the base layer according to the invention, is a mixture of component B-1 and component B-2.

Component B-1 is a polyethylene oxide monoalkyl ether, preferably a polyethylene oxide monoalkyl ether in which the monoalkyl ether has 8 to 18 carbon atoms in its alkyl portion. Component B-1 preferably has 5 to 15 ethylene oxide repeating units. Component B- 1 is an ether compound represented by the following formula (1):

RO-(C₂H₄O)q-H (1),

where R is an alkyl radical having 8 to 18 carbon atoms and q is a number from 3 to 15 .

In the above formula (1), the alkyl group represented by R is preferably a primary or secondary alkyl group having 8 to 18 carbon atoms. Preferred examples of such alkyl include octyl, lauryl, myristyl, pentadecyl, stearyl and the like, of which an alkyl group having 12 to 14 carbon atoms is particularly preferred. Preferably, q is a number of 5 to 9. In the above formula (1), the alkyl group represented by R serves as a component providing a lipophilic nature, and the ethylene oxide unit [-(C₂H₄O)q-H] serves as a component providing a hydrophilic nature. Therefore, depending on the number of carbon atoms in an alkyl group, in general, an ethylene oxide having a smaller degree of addition polymerization (in other words, q is a small number) has a higher lipophilic nature and an ethylene oxide having a larger degree of addition polymerization (in other words, q is a large number) has a higher hydrophilic nature. An ether compound in which R is an alkyl group having 12 to 14 carbon atoms and q is a number of 5 to 9 is particularly preferred in view of the balance between these natures.

The component B-1 is used in admixture with a polyethylene oxide-polypropylene oxide block copolymer (component B-2). The polyethylene oxide-polypropylene oxide block copolymer as the component B-2 is preferably a block copolymer having polyethylene oxide units represented by the following general formula (2) at both ends:

HO - (C2 H4 O)n - (C3 H6 O)m - (C2 H4 O)p - H (2)

In the above formula (2), m is 20 to 80, (n+p) is preferably 80 to 600, more preferably 100 to 600, and n and p are each preferably 20 or greater.

Although the proportions of polyethylene oxide units and polypropylene oxide units in the polyethylene oxide-polypropylene oxide block copolymer (component B-2) can be adjusted to desired values, it is preferred that the proportion of polyethylene oxide units is in the range of 50 to 90% by weight, more preferably 70 to 86% by weight. The number average molecular weight of the block copolymer is preferably 5,000 to 30,000.

The contents of the component A and the component B in the base layer of the present invention are 90 to 80% by weight and 10 to 20% by weight, respectively. If the amount of the component A is more than 93% by weight, the coating film-forming properties are poor, while if it is less than 70% by weight, the adhesion properties of the base layer are lowered. If the amount of the component B (i.e., component B-1 and component B-2) is less than 7% by weight, the coating film-forming properties are lowered and the electrostatic properties become insufficient. On the other hand, if the amount of the component is more than 30% by weight, the adhesion, anti-blocking properties and heat resistance of the resulting film are disadvantageously deteriorated.

The contents of component B-1 and component B-2 are 7 to 3 wt.% and 13 to 7 wt.%, respectively.

In the present invention, the base layer formed on the surface of a polyester base film has a thickness of 0.005 to 1 µm, preferably 0.01 to 0.7 µm. The surface roughness of the base layer is 0.002 to 0.02 µm, preferably 0.004 to 0.01 µm in terms of center line roughness (Ra).

If the center line roughness (Ra) of the surface of the base layer is less than 0.002 µm, the handling properties in the film processing step are poor, while if it is more than 0.02 µm, the electromagnetic conversion properties disadvantageously deteriorate when the film is used as a magnetic recording medium.

Investigations conducted by the inventors have revealed that there are slight differences in the characteristics of the adhesive polyester films obtained depending on the ratio of component A to component B. Desirable properties of processability or heat resistance can be achieved by utilizing these differences when the film is used as a magnetic recording medium.

In general, however, the obtained base layer exhibits excellent adhesion and coating processability.

The base layer of the present invention is provided by applying a coating solution to a non-stretched polyester film, a stretched polyester film or a biaxially oriented film and then drying the film. This coating is applied to one side or both sides of a film. Of the above films, the coating is preferably carried out on a monoaxially oriented film which has been stretched in the longitudinal direction. In this case, the film after coating may be stretched in the transverse direction and further stretched again in the longitudinal direction or transverse direction, as the case may be. The stretching itself may be carried out under conventionally known conditions by a conventionally known method. For example, the film is stretched at a stretch ratio of 2.5 or more, preferably 3.0 or more, in biaxial directions, or at an areal stretch ratio of 8 or more, preferably 8 to 30.

The solid content of the coating solution (base coating solution) is preferably 0.5 to 30 wt.%, more preferably 1 to 15 wt.%. This Coating solution may contain other components such as inert fine particles, heat stabilizers, etc., as desired, in addition to water-dispersible polyester resin (component A), polyethylene oxide monoalkyl ether (component B-1) and polyethylene oxide-polypropylene oxide block copolymer (component B-2).

The polyester composite film with a base layer thus obtained has excellent coating and adhesion properties, heat resistance and low electrostatic properties and is particularly useful as a base material for a magnetic recording medium:

Examples:

The following examples are given to further illustrate the present invention. The values of the characteristic properties were measured according to the following methods.

1. Adhesion properties

The following coating for evaluation is applied to a polyester film (the surface of a base layer) with a Meyer bar to ensure that the thickness of the film after drying is about 4 µm, and then dried at 100ºC for 3 minutes. After that, the film is aged or cured at 60ºC for 24 hours, and then a 12.7 mm wide and 15 cm long piece of Scotch tape No. 600 (a product of 3M Co.) is attached to the film in a manner that no air enters between the tape and the film. A manual load roller defined in JIS C2701 (1975) is rolled over the tape to adhere it firmly to the film, and then it is cut into a width of the tape. The film is peeled off from the tape at an angle of 180º to measure its adhesion or peel strength.

O: 50 g or more

Δ: 30 g or more but less than 50 g

X: less than 30 g

(Coating for evaluation)

Expressed as solids content,

(1) 25 parts by weight of a urethane resin (Nipporan 2304 from Nippon Polyurethane Co.),

(2) 50 parts by weight of a vinyl chloride-vinyl acetate resin (S-Lec A from Sekisui Chemical Co. Ltd.),

(3) 1 part by weight of a dispersant (Resion P from Riken Vitamin Co. Ltd.), and

(4) 500 parts by weight of a magnetic agent (CTX-860 from Toda Chemical Co. Ltd.)

dissolved in a mixed solvent of methyl ethyl ketone, toluene and cyclohexanone to prepare a 40 wt% solution and dispersed with a sand mill for 2 hours. Thereafter, 25 parts by weight (expressed as solid content) of a cross-linking agent (Coronate L) is added and stirred well to obtain a magnetic coating.

2. Elastic modulus

A film is cut into a 10 mm wide and 150 mm long specimen, which is then pulled at tensile intervals of 100 mm using an Instron type universal tensile tester, at a tensile rate of 19 mm/min and a recorder feed rate of 500 mm/min. A modulus of elasticity is calculated from a tangent to an upstream part of the resulting stress-strain curve.

3. Centerline roughness (Ra)

A record (film surface roughness curve) is stretched under such conditions as a needle radius of 2 µm and a probe load of 30 mg using a probe type surface roughness meter (SE-3 FAT) of Kosaka Laboratories Ltd. A portion of the measured length L in the direction of a center line thereof is extracted from the present surface roughness curve, and when the center line of the extracted portion is taken as the X-axis, the direction of vertical magnification as the Y-axis and the roughness curve is expressed by Y = f(X), Ra [µm] given in the following equation is defined as the center line average roughness of the surface of a film:

Ra = f(x) dx

4. Running friction coefficient (uk) at the time of heating

This is measured in the following manner using a device shown in Fig. 1. In Fig. 1, reference numeral 1 denotes a supply roller, 2 a tension regulator, 3, 5, 6, 8, 9 and 11 free rollers, 4 a tension meter (input), 7 a fixed pin made of stainless steel SUS304 (outer diameter: 5 mmφ), 10 a tension meter (output), 12 a guide roller and 13 a take-up roller.

The base layer side of a ½ inch wide section of a polyester film is moved (friction) with the fixed pin 7 (surface roughness: 10 nm) heated by a heater to 65ºC or 105ºC at an angle of 90º [θ = (90/180)π (radian)] and at a speed of 2 m/min. A tension T₁ at the entrance is set to 30 g with the tension controller 2 and the tension (T₁: g) at the exit after the film is run 40 m is detected. When the maximum tension is represented by T2 (g), the running friction coefficient (uk) is calculated from the following equation.

uk = 1/? · log T2 /30

5. Electrostatic properties

A polyester film (10 µm x 500 mm x 3,000 mm) is rewound at a speed of 150 m/min in an atmosphere of 23ºC x 75ºC RH, and the amount of electrostatic charge when the resulting roll is peeled off is measured using a digital electrostatic meter Model-203 manufactured by Hugle Electronics Co. Ltd.

O: less than 2.5 kV

Δ: 2.5 to 5.0 kV

X: more than 5.0 kV

6. Softening point (ring and ball method)

A dry solid product of a coating solution for forming a base layer is prepared and measured for its softening point according to JIS K-3531-1960. Glycerin is used as a heat bath and heated at a temperature increase rate of about 5ºC/min.

7. Heat resistance

A polyester film is run such that its base layer surface is in contact with a heating roller heated to 110ºC and having a rotating surface speed of 4 m/min at a roller pressure of 2 kg/cm2, and the surface of the roller is evaluated after 40 minutes.

O: No foreign material is observed on the surface of the roller.

Δ: A small amount of foreign material is observed on the surface of the roller.

X: A large amount of foreign material is found on the surface of the roller observed.

8. Evaluation of coating properties

When a primary or base coating solution is applied to a film, the coated surface of the film is examined to visually evaluate whether the coating is evenly applied.

O: No spots or stripes are observed.

Δ: A small number of dots or stripes are observed.

X: A large number of dots or stripes are observed.

9. Coating solution stability

A base coating solution is kept at 60ºC for 3 days and it is observed whether a precipitate is produced.

O: No precipitate is observed.

Δ: A small amount of precipitate is observed.

X: A large amount of precipitate is observed.

example 1 Preparation of the base coating solution:

There were added 82.5% by weight of a water-dispersible polyester resin (component A) having a number average molecular weight of about 10,000 and comprising 40 mol% of a 2,6-naphthalenedicarboxylic acid component, 59.95 mol% of an isophthalic acid component, 0.05 mol% of a 5-Na-sulfoisophthalate component, 70 mol% of an ethylene glycol component and 30 mol% of an adduct of bisphenol A with 4 mol of propylene oxide,

5% by weight polyethylene oxide monoalkyl ether (number average molecular weight: about 500, alkyl radical: mixed alkyl with 12 to 14 carbon atoms) (component B-1) and

12.5% by weight of a block copolymer comprising polyethylene oxide (80% by weight) and polypropylene oxide (80% by weight) and polypropylene oxide (20% by weight) (number average molecular weight: about 23,500) (component B-2),

and fully mixed with a water medium to prepare an aqueous base coating solution having a solid content of 10 wt%.

Production of an adhesive polyester film:

A non-stretched sheet of polyethylene terephthalate having an intrinsic viscosity of 0.62 was stretched 2.3 times in the longitudinal direction, and the above aqueous base coating solution was applied to one side of the above sheet with a roll coater. Thereafter, the sheet was stretched 3.7 times in the transverse direction while drying, and further stretched 2.3 times in the longitudinal direction again, and then heat-set at 220°C to obtain a 10 µm thick film. The film thickness of the base layer was 0.015 µm. The characteristic properties of the film are shown in Table 1.

Examples 2 to 4 and comparative examples 1 to 3

The procedure of Example 4 was repeated except that the amount of the polyester resin component was changed to 85 wt% and the ratio of the coating composition was changed as shown in Table 1. The characteristic properties of the obtained films are shown in Table 1. Table 1

Example: Example

Comparative example: Comparative example

Footnote 1) EC: ethylene glycol BPA-P: addition product of bisphenol A with 4 moles of propylene oxide

Effect of the invention:

The adhesive polyester film of the present invention is characterized in that a composition having a specific amount of a sulfonic acid metal salt group in a polyester resin and comprising a mixture of a polyethylene oxide monoalkyl ether and a polyethylene oxide-polypropylene oxide block copolymer in a specific ratio is used as a base layer. This provides a polyester film having excellent heat resistance, high adhesion and low electrostatic properties and improved coating processability. Therefore, this film has excellent properties as a base film for a magnetic recording medium.

Short explanation of the drawings:

Fig. 1 is a diagram showing a typical device for measuring the running friction coefficient (uk) of the surface of a film.

1: Settlement role

2: Voltage regulator

3, 5, 6, 8, 9 and 11: Free reels

4: Voltmeter (input)

7: Fixed pin made of stainless steel SUS304

10: Voltmeter (output)

12: Guide roller

13: Take-up roll.

Claims (10)

1. Polyester film having low electrostatic and high adhesion properties and having a base layer formed on at least one side thereof which has a softening point of 145°C to 165°C and consists of component A and component B, wherein component A is a water-dispersible polyester resin containing as a copolymer component a compound containing a sulfonic acid metal salt group in an amount of 0.04 to 0.3 mol% of the total dicarboxylic acid compound, and wherein component B is a mixture of a polyethylene oxide monoalkyl ether (B-1) and a polyethylene oxide- polypropylene oxide block copolymer (B-2), wherein the weight ratio of components A, (B-1) and (B-2) is 90 to 80%, 7 to 3% and 13 to 7%, respectively. 2. Polyester film according to claim 1, wherein the monoalkyl ether of component B-1 in the base layer has 8 to 18 carbon atoms in its alkyl moiety. 3. Polyester film according to claim 1 or 2, wherein component B-1 in the base layer has 3 to 15 ethylene oxide repeat units. 4. The polyester film according to claim 1, wherein component B-2 in the base layer is a block copolymer comprising polyethylene oxide units and polypropylene oxide units in a weight ratio of 50:50 to 90:10. 5. The polyester film according to claim 1, wherein component B-2 in the base layer is a polyethylene oxide-polypropylene oxide block copolymer having a number average molecular weight of 5,000 to 30,000. 6. A polyester film according to claim 1, wherein component A in the base layer is a water-dispersible polyester resin containing a 2,6-naphthalenedicarboxylic acid component in an amount of at least 30 mol% of the total dicarboxylic acid compound. 7. The polyester film according to claim 1, wherein the base layer has a thickness of 0.005 to 1 µm. 8. The polyester film according to claim 1, wherein the base layer has a center line average roughness (Ra) of 0.002 to 0.02 µm. 9. The polyester film according to claim 1, wherein the polyester film has a thickness of 3 to 20 µm. 10. Use of the polyester film according to claim 1 as a base film for a magnetic recording medium.