JPH0673944B2 - Polyester film for magnetic recording media - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polyester film used as a base film for a magnetic recording medium, and more specifically, a sulfonic acid on a surface of a base film made of a polyester resin. A thin layer made of a specific polyester having a metal base is formed, the adhesion to the magnetic recording layer or back coat layer formed thereon is improved, and the magnetic recording has good smoothness and excellent electromagnetic conversion characteristics. The present invention relates to a polyester film for a magnetic recording medium which gives a medium.

[Prior Art] A biaxially stretched film made of a polyester resin is widely used as a film base material for a magnetic recording medium because it is excellent in heat resistance, dimensional stability, strength and the like. However, since the biaxially stretched polyester film has a high degree of crystallographic orientation and has poor adhesiveness to the magnetic recording layer and the back coat layer, when it is used as a base film for a magnetic recording medium, the surface of the film is improved in adhesiveness. To provide a primer layer for this, corona discharge treatment in various gas atmospheres, UV irradiation treatment, plasma treatment, flame treatment, etc., or acid, alkali, amine aqueous solution, trichloroacetic acid, phenols on the substrate surface A method of increasing the adhesiveness is employed, such as a method of performing chemical etching treatment by the above, and a method of using the above treatments in combination. Among these, the method of providing the primer layer on the polyester film substrate is easy to process, safe in work,
It has been widely put to practical use because it is excellent in all of cost, adhesion improving effect and the like. Many compositions have been proposed as the primer treatment agent used here, and among them, the primer treatment agent containing a polyester resin as a main component has applicability to a polyester film substrate,
It has attracted attention as an excellent adhesiveness and blocking resistance of the coating layer.

However, the conventional primer treatment agent has a drawback that it lacks general versatility, for example, even if it shows a certain effect on a specific magnetic paint, the effect cannot be obtained when the kind of the paint changes. Moreover, it has been pointed out that when the primer layer is formed on the substrate film and then wound into a roll, the films tend to cause blocking.

[Problems to be Solved by the Invention] The present invention has been made in view of the circumstances as described above, and its purpose is to provide a magnetic recording layer or a back coat layer without causing problems such as blocking. It is intended to provide a polyester film for a magnetic recording medium, which exhibits excellent adhesiveness.

[Means for Solving the Problems] A polyester film for a magnetic recording medium according to the present invention, which has been able to solve the above problems, has a structure in which at least one surface of a base film made of a polyester resin has sulfonic acid in a molecule. A water-dispersible but water-insoluble sulfonic acid metal base-containing polyester having a metal base, wherein the number of coarse projections obtained by the following method when formed into a film is 10 or less per 100 cm ² The gist is that it is coated with a polyester containing a metal base.

(Measurement method of the number of coarse projections) In a clean room, form a 0.1 μm thick film made of polyester containing sulfonate metal base on a smooth glass plate, and overlay a 10 μm thick smooth film on it. After closely contacting with each other, a D ray of Na is irradiated from above, and the number of interference rings formed by protrusions due to undispersed substances or foreign matters contained in the polyester having an order of 3 or more is determined as the number of coarse protrusions.

[Function] Examples of the base film used in the present invention include thermoplastic polyester resin films such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. Of these, particularly preferable is a homo- or copolymer polyester film in which 80% or more of the resin component is polyethylene terephthalate, or 80% of polyethylene terephthalate.
The polyester blend film contains the above proportions.

There is no particular limitation on the polyester component other than the polyethylene terephthalate component in the above-mentioned copolymerized polyester or polyester blend, and any polyester component can be combined. As the dicarboxylic acid component constituting such a polyester, any of aromatic, aliphatic and alicyclic dicarboxylic acids can be used. Aromatic dicarboxylic acids include isophthalic acid, orthophthalic acid, and 2,6-naphthalenedicarboxylic acid, and aliphatic dicarboxylic acids include succinic acid, adipic acid, sebacic acid, oxalic acid, and alicyclic dicarboxylic acids. Examples of the acid include 1,3-cyclopentanedicarboxylic acid and 1,4-cyclohexanedicarboxylic acid. It is also effective to use an oxycarboxylic acid such as p-hydroxybenzoic acid in combination with part of the above dicarboxylic acid.

As the glycol component constituting the polyester,
Aliphatic glycol having 2 to 8 carbon atoms or 6 to 12 carbon atoms
Preferred are alicyclic glycols. Examples of such glycol include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol,
Neopentyl glycol, 1,6-hexanediol, 1,2
-Cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, p
-Xylene glycol, diethylene glycol, triethylene glycol and the like. In addition, it is also possible to use polyethylene glycol, polypropylene glycol, polytetramethylene glycol, or other polyether glycol as the aliphatic glycol.

The above acid component and glycol component are polycondensed by a usual method to obtain polyester. This polyester may be used alone or in combination of two or more kinds, if necessary. Further, a small amount of other copolymerizable components may be copolymerized in the production process of the polyester, or a small amount of a resin other than polyester may be blended. Further, if necessary, an antioxidant, a heat stabilizer, a lubricant, a pigment, an ultraviolet absorber and the like may be added in an appropriate amount. These resins are usually melted
A substrate film is formed by an extrusion method or a casting method in which the material is dissolved in a solvent and cast. This base film may be used in an unstretched state, or may be uniaxially or biaxially stretched as needed.

Next, the metal sulfonate-containing polyester formed as an adhesion improving layer on at least one surface of the base film has a metal sulfonate group in the molecule and is water-dispersible but water-insoluble metal sulfonate. It is a base-containing polyester.

Here, "water-insoluble" refers to the property that the polymer is not dissolved in the hot water even when the target metal sulfonate-containing polyester is immersed in hot water at 80 ° C and stirred, and more specifically Specifically, when the metal sulfonate group-containing polyester is made into chips, which are placed in a large excess of hot water (80 ° C) and stirred for 24 hours, the weight loss due to dissolution of the polymer is 5% by weight. % Or less is water-insoluble.

Such a metal sulfonate group-containing polyester can be obtained by copolymerizing a dicarboxylic acid having a metal sulfonate group in the molecule as a dicarboxylic acid component. Examples of the dicarboxylic acid containing a sulfonic acid metal base used here include sulfoterephthalic acid, 5-sulfoisophthalic acid, 4-sulfophthalic acid, 4-sulfonaphthalene-2,7-dicarboxylic acid, 5- [4-sulfophenoxy] Metal salts such as isophthalic acid may be mentioned. Especially 5-
Preferred are sodium sulfoisophthalic acid and sodium sulfoterephthalic acid.

These sulfonic acid metal group-containing dicarboxylic acids are preferably used in the range of 0.5 to 15 mol%, more preferably 0.5 to 10 mol%, based on the total dicarboxylic acids used.
When it is less than 15 mol%, the dispersibility of the obtained polyester containing a metal sulfonate group in water is not sufficiently improved, so that it is difficult to form a uniform coating layer. On the contrary, when it exceeds 15 mol%, the blocking resistance of the obtained coating layer is high. Sex decreases.

As the dicarboxylic acid other than the dicarboxylic acid having a sulfonic acid metal base, any of aromatic, aliphatic, and alicyclic dicarboxylic acids can be used, but in the present invention, other than the dicarboxylic acid containing a sulfonic acid metal base. As the dicarboxylic acid, the aromatic dicarboxylic acid is preferably used in a proportion of 50 mol% or more, preferably 70 mol% or more. Examples of preferred aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, 2-6-naphthalenedicarboxylic acid and the like. When the amount of the aromatic dicarboxylic acid is small, the mechanical strength of the coating film formed from the obtained polyester containing a metal sulfonate group is slightly low, and the blocking resistance tends to be relatively poor. Examples of the metal in the sulfonic acid metal base include alkali metals such as sodium, potassium and lithium, alkaline earth metals such as calcium and barium, and the most common are sodium and potassium.

Examples of the aliphatic dicarboxylic acid used in combination with the aromatic dicarboxylic acid include succinic acid, adipic acid, sebacic acid, and the alicyclic dicarboxylic acid includes 1,3-cyclopentanedicarboxylic acid and 1,2-cyclohexanedicarboxylic acid. Acid, 1,3
-Cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid and the like can be mentioned. These non-aromatic dicarboxylic acid components may be effective in enhancing the water dispersibility of the obtained sulfonate metal group-containing polyester and the adhesiveness of the coating layer formed by the polyester, but the amount used is large. If it is too much, the mechanical strength and blocking resistance of the coating layer may be insufficient, so the amount used should be kept low.

As the glycol component to be combined with the dicarboxylic acid component, an aliphatic glycol having 2 to 8 carbon atoms or an alicyclic glycol having 6 to 12 carbon atoms is used. Examples of such glycols are ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, Examples include 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, p-xylylene glycol, bisphenol compounds, diethylene glycol and triethylene glycol.

When polycondensation is carried out by a melt polycondensation method, a solution polycondensation method, an interfacial polycondensation method or the like using the above-mentioned dicarboxylic acid containing a sulfonic acid metal base and the above-mentioned other dicarboxylic acid component and polyhydric alcohol component, A metal sulfonate-containing polyester having For example, each component is directly reacted to distill off water for esterification, and a direct esterification method in which polycondensation is performed, or a dialkyl ester of the dicarboxylic acid component is reacted with a glycol component to distill an alcohol. While transesterification and polycondensation are performed, a transesterification method or the like is adopted.

The water-insoluble but water-dispersible sulfonic acid metal group-containing polyester thus obtained preferably has a glass transition temperature (Tg) of 100 ° C. or lower, more preferably a glass transition temperature of 80 ° C. or lower, More preferably 20
~ 80 ° C. When the glass transition temperature is higher than 100 ° C., the film-forming property when the composition containing the metal sulfonate-containing polyester is applied to the base film is poor,
The adhesiveness between the obtained coating layer and the base film is not sufficiently improved, and as a result, the adhesiveness between the obtained coating film and the magnetic recording layer or back coat layer laminated thereon is also insufficient. On the other hand, if the glass transition temperature is too low, blocking resistance deteriorates, so it is desirable to use one having a temperature of 20 ° C or higher.

The metal sulfonate group-containing polyester can be used alone or in combination with other various water-dispersible resins to further improve smoothness, adhesiveness, blocking resistance and the like. Is.

Water-dispersible resins preferably used in combination include vinyl-based resins such as polyvinyl acetate, polyvinyl chloride, polyvinylidene chloride and copolymers thereof: acrylic acid, methyl methacrylate, hydroxymethyl acrylate, styrene,
Acrylic resin prepared using glycidyl methacrylate, methyl acrylate, ethyl acrylate, etc .: Isophthalic acid, adipic acid, sebacic acid, ethylene glycol, diethylene glycol, neopentyl glycol, polyethylene glycol, polytetramethylene glycol, and other glycols, There are polyurethane-based resins prepared from diisocyanates.

As the water-dispersible resin, in addition to this, it has dispersibility in an aqueous solvent because it has a hydrophilic group in the molecule, or by adding a small amount of a dispersion aid because it has a hydrophilic group in the molecule, an aqueous solvent. In, a polymer compound exhibiting dispersibility is included. As the hydrophilic group, a phosphonic acid group,
Examples thereof include a phosphinic acid group, a sulfonic acid group, an ammonium group and a carboxyl group. Examples of the resin having such a hydrophilic group include polyurethane, polyamide, acrylic resin, epoxy resin, and polyolefin resin.

In particular, by using an appropriate amount of polyurethane and / or an acrylic resin in combination, not only the adhesion and blocking resistance are improved, but also the solvent resistance, antistatic property and the like are improved.

Other additives to the polyester containing a metal sulfonate base are, for example, various other water-dispersible resins and inorganic inert particles such as silica, calcium carbonate, kaolinite, alumina, talc, and barium sulfate; benzoguanamine resins; Organic inert particles such as polystyrene-based resin (all having a particle size of about 0.001 to 2 μm) can be added, and by adding these, slipperiness and blocking resistance can be further improved. Further, if necessary, a pigment; an organic or inorganic antistatic agent; an antiseptic agent; an antifoaming agent; an ultraviolet absorber or the like can be used in combination.

This sulfonic acid metal group-containing polyester is, for example,
A method of adding the fine particles of the metal sulfonate group-containing polyester to hot water and dispersing by vigorous stirring; a method of mixing the water-insoluble polyester with a water-soluble organic compound and dispersing the mixture in water; It is said that At this time, water-soluble organic compound (usually organic solvent)
It is advantageous to use in combination because it is easy to obtain a uniform aqueous dispersion. The organic compound used in combination has a boiling point of 60 to
It is preferable that the amount of 1 dissolved in water at 20 ° C. at 200 ° C. is 20 g or more, and specific examples thereof include aliphatic and alicyclic alcohols, ethers, esters and ketones. Examples of alcohols include monohydric alcohols such as methanol, ethanol, isopropanol, and n-butanol; glycols such as ethylene glycol and propylene glycol; and glycol derivatives such as methyl cellosolve, ethyl cellosolve, and n-butyl cellosolve. Examples of ethers include dioxane and tetrahydrofuran; examples of esters include
Ethyl acetate and the like; examples of the ketones include methyl ethyl ketone and the like. These water-soluble organic compounds,
They can be added alone or in combination of two or more. Of the above compounds, butyl cellosolve and ethyl cellosolve are most preferable in view of dispersibility in water and coatability on a film.

In order to prepare an aqueous dispersion of a sulfonate metal base-containing polyester using such a water-soluble organic compound, the sulfonate metal base-containing polyester chip and the water-soluble organic compound are mixed at 50 to 200 ° C., A method in which water is added to this and dispersed by stirring;
A method in which a mixture obtained by mixing at 200 ° C. is added to water and dispersed by stirring; or, the above-mentioned polyester containing a metal sulfonate group, a water-soluble organic compound and water
A method of stirring and dispersing at ~ 120 ° C is adopted.
It is also possible to use an emulsifier for promoting dispersion within a range that does not deteriorate the blocking resistance and surface properties of the coating layer.

By the way, the resin layer made of the sulfonic acid metal group-containing polyester formed as a coating layer on the base film,
Coarse protrusions that increase the adhesiveness to the magnetic recording layer or the back coat layer, but are caused by insoluble foreign substances that may be mixed in this coating layer, such as dropouts when a magnetic layer is formed on it. Cause a signal defect of. Therefore, the number of coarse protrusions present in the coating layer must be minimized. Therefore, in the present invention, the number of coarse protrusions shown below is specified as a criterion for ensuring the smoothness required as a film for a magnetic recording medium. .

That is, in a clean room, a 0.1 μm-thick film made of the above polyester containing a metal sulfonate group on a smooth glass plate.
Film is formed, a smooth film having a thickness of 10 μm is superposed on the surface of the film, and the film is adhered to the film. And the order of the interference ring formed by coarse protrusions caused by foreign matter or foreign matter is 3 or more.
It should be 10 or less, more preferably 3 or less per 0 cm ² . If the number of coarse projections is larger than this, defects such as dropouts increase when used as a magnetic recording medium, and the object of the present invention cannot be met. In order to obtain such an aqueous dispersion having high cleanliness, the coarse insoluble matter may be removed by passing the aqueous dispersion through a filter having a pore size of about 3 μm or less, more preferably about 1.5 μm or less before coating. It should be noted that the fine protrusions having the secondary or lower interference ring have almost no adverse effect on the magnetic characteristics, so that there is no particular problem in the present invention, but it is preferable that the fine protrusions are also small. Of course.

The polyester film for a magnetic recording medium of the present invention thus obtained has both the adhesiveness between the substrate and the coating layer and the adhesiveness between the coating layer and the magnetic recording layer or the back coat layer formed on the surface thereof. It is good and does not cause blocking even when wound in a roll. Moreover, since the film has few coarse projections and high surface smoothness, a magnetic recording medium having excellent electromagnetic conversion characteristics can be obtained by using it as a base film.

[Examples] Hereinafter, the present invention will be specifically described with reference to Examples.

The performance evaluation method used in the examples is as follows. In the following examples, "parts" means "parts by weight" unless otherwise specified.

Adhesiveness Evaluation Paint-1 (Magnetic paint) Co-γ-Fe ₂ O ₃ 100 parts Vinyl chloride / vinyl acetate copolymer 15 parts Urethane modified saturated polyester resin 15 parts α-alumina 3 parts Carbon black 1 part Toluene 105 Parts Methyl ethyl ketone 105 parts Cyclohexanone 105 parts After mixing and dispersing with the above raw material by a ball mill for about 12 hours, palmitic acid 1 part dodecyl stearate 2 parts oleic acid 2 parts is added and mixed for another 30 minutes.

Next, 10 parts of a 75% solid content ethyl acetate solution of a triisocyanate compound is added and mixed for another 1 hour to obtain a magnetic paint.

Evaluation paint-2 (Backcoat paint) Carbon black 50 parts Vinyl chloride / vinyl acetate copolymer 30 parts Triisocyanate 30 parts Polyurethane elastomer 30 parts Nitrocellulose 30 parts Stearamide 5 parts Toluene 120 parts Methyl ethyl ketone 120 parts in a ball mill Mix uniformly and use as back coat paint.

The magnetic paint has a thickness of 3μ after drying on the test film.
It is coated so as to have a thickness of m, subjected to magnetic field orientation treatment, dried at 60 ° C., calendered at 100 ° C., and further cured at 60 ° C. for 72 hours.

The back coat paint is applied on the test film so that the thickness after drying becomes 1 μm, dried at 60 ° C., calendered at 100 ° C., and cured at 60 ° C. for 72 hours.

Evaluation of paint adhesion After coating the test film with the above-mentioned magnetic paint or backcoat paint under specified conditions, the coated surface of the obtained film is coated with Scotch tape (3M) under an atmosphere of 23 ° C x 50% RH. "No.610" made by the company) (width 10 mm x length 10 cm)
Paste so that there are no bubbles, and press the roll on it to smooth it. Then, using Tensilon manufactured by Toyo Baldwin Co., Ltd., 360 ° peeling is performed at a pulling speed of 1000 cm / min. The distance between chucks is 18 cm, and the adhesive area is 5 cm.

Vapor deposition adhesion evaluation A 2000-Å-thick Co-Ni alloy thin film is formed on the surface of the sample polyester film by vacuum vapor deposition. On the vapor deposition surface of this vapor deposition film, under the atmosphere of 23 ° C x 50% RH, Scotch tape ("No.610" manufactured by 3M) (width 18mm x length 15cm)
Is attached so as not to contain air bubbles, pressure is applied by a roll on this, and then it is quickly peeled off by hand in 180 ° direction, and the vapor deposition layer remaining on the film is visually judged.

◯: No peeling at all Δ: Some peeling was observed ×: One-third or more peeling blocking resistance Two films coated with a sulfonate metal base-containing polyester on one surface ( 3cm x 3cm), the coated side and non-coated side are overlapped and sandwiched from both sides with a 6cm x 6cm silicone rubber and a glass plate with the same dimensions and a thickness of 7mm. A load of 2 kg is applied to this, and it is left in an atmosphere of 50 ° C. and 85% RH for 48 hours, and then the adhesive state when the film is peeled off is observed.

◯: No tackiness was observed at all Δ: Film tackiness occurred in some places X: Stickiness occurred on the entire surface Evaluation of coarse protrusions A bar coater was used to classify an aqueous dispersion of a sulfonate metal group-containing polyester into a class. In a clean room of 1000, coat and dry on a smooth glass plate to a dry film thickness of 0.1 μm. Then, on the coating film, a polyester film having a thickness of 10 μm in which foreign matter is minimized is superposed, and air in the gap is pushed out by a roller to bring them into close contact, and then a D line of Na is applied from above to remove the interference ring generated. Observe. Then, the number of fringes of the interference ring which is 3 or more is checked and the number per 100 cm ² is obtained.

Electromagnetic conversion characteristics After forming a magnetic coating film with a thickness of 3 μm on one side of the test film, slit it to 1/2 inch width to obtain a VHS standard video tape. Chroma S / N of this tape with respect to the standard tape (Comparative Example 1) was measured using a TG-7 NTCSTV test signal generator manufactured by Shibasoku Co., Ltd. and a 925D / 1NTCS color video noise measuring instrument, and the following criteria were used. Ranked.

× ... less than -1 dB △ ... -1 dB or more and less than +1 dB ○ ... + 1 dB or more Example 1 (A) Preparation of sulfonate metal base-containing polyester and aqueous dispersion: First, sulfonate metal base-containing polyester (a) was prepared by the following method. Prepared. 49 mol% of dimethyl terephthalate, 49 mol% of dimethyl isophthalate and 2 mol% of sodium 5-sulfoisophthalate as dicarboxylic acid components
Ethylene glycol 50 as the glycol component
A transesterification reaction and a polycondensation reaction were carried out by a conventional method using mol% and neopentyl glycol of 50 mol%. The glass transition temperature of the obtained polyester containing a metal sulfonate group was 69 ° C. 300 parts of this sulfonic acid metal group-containing polyester and 150 parts of n-butyl cellosolve
And stirring to form a viscous solution.
550 parts were gradually added to obtain a uniform pale white aqueous dispersion having a solid content of 30%.

This dispersion was further added to an equal volume mixture of water and isopropanol to prepare an aqueous polyester dispersion containing a metal sulfonate having a solid content of 5% (coating solution).

(B) Preparation of substrate film: 280 to 300 polyethylene terephthalate in which calcium carbonate fine particles having an average particle size of 0.8 μm are dispersed at a concentration of 2500 ppm.
Melt extruded at ℃, cooled by chill roll at 15 ℃, thickness about
An unstretched film of 150 μm was obtained. This unstretched film was stretched 3.5 times in the machine direction between a pair of rolls heated to 85 ° C. and having different peripheral speeds to obtain a substrate film.

(C) Preparation of coated polyester film (base film for magnetic recording medium): The sulfonic acid metal group-containing polyester aqueous dispersion obtained in (A) above was passed through a filter having a pore size of 1.0 μm and then obtained in (B) above. It is coated on the surface of the base film by a roll coater method and dried with hot air at 70 ° C. Then, it was stretched 3.5 times in the transverse direction at 98 ° C. by a tenter and then heat set at 200 to 210 ° C. to obtain a biaxially stretched polyester film having a thickness of 12 μm. The final coating amount (polyester containing metal sulfonate) on the film was about 0.04 g / m ²
Met.

Table 1 shows the blocking resistance, adhesiveness, number of coarse projections, and electromagnetic conversion characteristics of the obtained film.

Example 2 45 mol% of dimethyl terephthalate, 45 mol% of dimethyl isophthalate and 10 mol% of sodium 5-sulfoisophthalate were used as dicarboxylic acid components, and 50% of ethylene glycol and 40 mol% of neopentyl glycol were used as glycol components, and diethylene glycol. 10 mol%
In the same manner as in Example 1 except that was used, a sulfonic acid metal base-containing polyester aqueous dispersion was prepared, and coating stretching was performed on the base film to obtain a biaxially stretch-coated polyester.

Table 1 shows the blocking resistance, adhesiveness, number of coarse projections, and electromagnetic conversion characteristics of the obtained film.

Example 3 A biaxially stretch-coated polyester film was obtained in exactly the same manner as in Example 1 except that the pore size of the filter used for the filtration treatment of the aqueous dispersion of a metal sulfonate-containing polyester was changed to 2.5 μm.

Table 1 shows the blocking resistance, adhesiveness, number of coarse projections, and electromagnetic conversion characteristics of the obtained film.

Comparative Example 1 Using polyethylene terephthalate in which CaCO ₃ fine particles having an average particle diameter of 0.8 μm were dispersed at a concentration of 2500 ppm, melt extruded into a film at 290 ° C., then cooled by a cooling roll at 25 ° C., and unstretched to a thickness of 150 μm. I got a film.

This unstretched film was stretched 3.5 times in the machine direction between a pair of rolls with different peripheral speeds heated to 90 ° C., then stretched 3.5 times in the transverse direction at 98 ° C. using a tenter, then 200-210 ° C.
Then, it was heat-fixed to obtain a biaxially oriented polyester film having a thickness of 12 μm.

Table 1 shows the blocking resistance, adhesiveness, number of coarse projections, and electromagnetic conversion characteristics of the obtained film.

Comparative Example 2 A biaxially stretched coated polyester film was obtained in exactly the same manner as in Example 1 except that the pore size of the filter used in Example 1 was changed to 5 μm.

Table 1 shows the blocking resistance, adhesiveness, number of coarse projections, and electromagnetic conversion characteristics of the obtained film.

Comparative Example 3 A biaxially stretch-coated polyester film was obtained in exactly the same manner as in Example 2 except that the pore size of the filter used in Example 2 was changed to 5 μm.

Table 1 shows the blocking resistance, adhesiveness, number of coarse projections, and electromagnetic conversion characteristics of the obtained film.

Comparative Example 4 As the dicarboxylic acid component, 40 mol% of dimethyl terephthalate, 40 mol% of dimethyl isophthalate and 20 mol% of sodium 5-sulfoisophthalate were used, and as the glycol component, 50 mol% of ethylene glycol and 49.5 mol% of neopentyl glycol. And average molecular weight of about 1000
Polyester glycol (0.5 mol%) was subjected to transesterification and polycondensation by a conventional method to obtain a metal sulfonate group-containing polyester. The obtained metal sulfonate-containing polyester was heated and stirred with water to prepare an aqueous solution coating solution having a solid content of 5%. A biaxially stretched polyester film was obtained in the same manner as in Example 1 except that this coating solution was used.

Table 1 shows the blocking resistance, adhesiveness, number of coarse projections, and electromagnetic conversion characteristics of the obtained film.

The following can be considered from Table 1.

Examples 1 to 3: Satisfies the specified requirements of the present invention, the adhesion of the magnetic coating film and the backcoat coating film and the adhesion with the vapor deposition layer and the blocking resistance are both good, and moreover, the coating layer is coarse. No protrusion is seen and it has excellent electromagnetic conversion characteristics.

Comparative Example 1: This is a normal base film having no coating layer formed, and has poor adhesion to the magnetic coating film and the backcoat coating film and poor adhesion to the vapor deposition layer.

Comparative Examples 2 and 3: Coated using an aqueous dispersion of a polyester containing a metal sulfonate group, but the number of coarse projections was large due to the large pore size of the filter, and electromagnetic conversion characteristics were lacking.

Comparative Example 4: A water-soluble metal sulfonate group-containing polyester was used, and the adhesion was good, but the blocking resistance was poor, and the electromagnetic conversion characteristics were poor despite the small number of coarse projections.

[Advantages of the Invention] The present invention is configured as described above, and by coating the surface of a substrate film made of a polyester resin with a specific sulfonate metal base-containing polyester having a sulfonate metal base introduced. It has become possible to provide a base film for a magnetic recording medium, which has excellent adhesion to the magnetic recording layer, the back coat layer and the vapor-deposited metal layer and has good blocking resistance. Moreover, since this film has a high surface smoothness and very few coarse projections, by using this film as a base film, it is possible to obtain a magnetic recording medium having further excellent electromagnetic conversion characteristics.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Haruo Matsumoto 10-24 Toyo-cho, Tsuruga City, Fukui Prefecture Toyobo Co., Ltd. General Research Institute Tsuruga Branch Office (56) Reference JP-A-1-236247 (JP, A) )

Claims (1)

[Claims] 1. A water-dispersible but water-insoluble metal sulfonate-containing polyester having a sulfonate metal base in the molecule on at least one side of a base film made of a polyester resin, which is in the form of a film. A polyester film for a magnetic recording medium, characterized in that it is coated with a polyester containing a metal sulfonate group having a number of coarse projections of 10 or less per 100 cm ² when determined by the following method. (Measurement method of the number of coarse projections) In a clean room, form a 0.1 μm thick film made of polyester containing sulfonate metal base on a smooth glass plate, and overlay a 10 μm thick smooth film on it. After closely contacting with each other, a D ray of Na is irradiated from above, and the number of interference rings formed by protrusions due to undispersed substances or foreign matters contained in the polyester having an order of 3 or more is determined as the number of coarse protrusions.