JPH0670851B2 - Polyester film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polyester film having excellent adhesiveness to a magnetic layer formed by an electron beam curing method. More specifically, when an electron beam curable resin composition and magnetic powder are applied to a support and irradiated with an electron beam to polymerize and cure to form a magnetic recording layer, the support has a specific average refractive index and a specific surface area. It relates to using a polyester film having a degree of orientation.

2. Description of the Related Art At present, a method of manufacturing a magnetic recording medium that is widely used, for example, a magnetic tape, a magnetic disk, a magnetic card, etc.
Using vinyl chloride resin, cellulose resin, acetal resin, urethane resin, acrylic resin, etc. dissolved in a solvent as a binder, a composition prepared by dispersing magnetic powder in these solutions is applied on a support, and hot air is applied. It is a method of drying.

In this case, in order to compensate for the lack of wear resistance of the magnetic layer of the obtained magnetic recording medium, it is known to add a crosslinkable binder by a chemical reaction, such as an isocyanate compound or an epoxy compound, to the thermoplastic resin. And is widely used industrially.

However, the use of these crosslinkable binders has the drawbacks that the storage stability of the coating composition is poor, a large-scale heat treatment step is required for curing the coating film, and aging requires a long time. Is also well known.

Further, when such a method is used, a great deal of labor is required to recover the solvent that volatilizes during coating and drying, which increases the cost of the product.

Therefore, in order to prevent these drawbacks, a compound having a carbon-carbon unsaturated bond capable of being polymerized by an electron beam, for example, one or more of acrylic acid derivatives is used as a binder, and is polymerized and cured by electron beam irradiation after drying. Studies on the formation of a magnetic recording layer by an electron beam curing method are being conducted. For example, JP-B-47-12423 and JP-A-50-774.
33, JP-A-57-127926, JP-A-57-130230, JP-A-57-130231, JP-A-58-35728, JP-A-59-58623.
JP-A-59-129267 and the like disclose techniques relating to these.

Problems to be Solved by the Invention However, one of the problems that cannot be solved even when the magnetic recording layer is formed by this electron beam curing method is adhesiveness to the polyester film as a support. That is, even in the electron beam curing method, various organic compounds are used as the binder of the magnetic powder, but the adhesive strength to the support is not always sufficient, and improvement in this respect is desired.

As one of the methods for this purpose, Japanese Patent Application Laid-Open No. 54-124709 discloses a technique of subjecting a film base material to ultraviolet irradiation treatment in advance. However, the addition of such a pretreatment step complicates the production line and increases the cost. Therefore, it has been desired to improve the adhesiveness by changing the characteristics peculiar to the film.

Means for Solving the Problems An object of the present invention is to solve the above-mentioned drawbacks and to provide a polyester film excellent in adhesion to a magnetic recording layer by an electron beam curing method.

The inventors of the present invention have characteristics of the polyester film as the support, such as elastic modulus and strength, type and amount of slipping agent contained, surface roughness based on these, diethylene glycol bond or carboxyl group content, crystallinity, 3 As a result of extensive studies on the adhesive force between each factor such as the refractive index in each dimension and the magnetic recording layer, the average refractive index of the film, the degree of plane orientation, and the carboxyl group content are strong against the adhesive force. The inventors have found that it has an influence, and have completed the present invention.

That is, the present invention is a support for a magnetic recording medium, wherein one side or both sides are coated with an electron beam-curable resin composition and a magnetic powder, and irradiated with an electron beam to polymerize and cure to laminate a magnetic recording layer. A biaxially stretched polyester film having an average refractive index n and a plane orientation degree ΔP satisfying the following formulas (1) and (2), and a terminal carboxyl group concentration of 35 equivalents / ton or more.

≧ 1.600 (1) ΔP ≦ 1.43 · −2.128 (2) A preferable example of the electron beam curable resin composition according to the present invention is
It is a relatively low molecular weight compound having one or more carbon-carbon unsaturated bonds at the terminal or side chain of the molecule, and specific examples include the following compounds.

(1) Unsaturated polyester oligomer: For example, a resin composition obtained by reacting maleic anhydride, tetrahydrophthalic anhydride, etc. with diethylene glycol, 1,1,1-trimethylolpropane, etc.

(2) Polyester acrylate: A reaction product of a polyester oligomer having a terminal hydroxyl group with a repeating unit of about 2 to 30 and a acrylic acid or methacrylic acid, which is obtained by reacting a dibasic acid or a tribasic acid with a glycol.

For example, a resin composition obtained by a combination of adipic acid-1,6 hexanediol-acrylic acid, phthalic anhydride-propylene oxide-acrylic acid, trimellitic acid-diethylene glycol-acrylic acid and the like.

(3) Epoxy acrylate: A compound obtained by acryloylating the terminal of an epoxy resin.

Examples thereof include polyglycidyl ethers derived from a polyhydric alcohol or a hydroxyl group-containing compound such as bisphenol A and epichlorohydrin, and polyglycidyl esters derived from a polycarboxylic acid and epichlorohydrin.

(4) Urethane acrylate: A compound obtained by reacting a polyisocyanate compound with an acrylate or methacrylate having a hydroxyl group.

For example, the reaction product of tolylene diisocyanate and 2-hydroxyethyl acrylate.

(5) Acrylate monomer: a cross-linking agent for polymers and oligomers, acrylic acid mainly used as a viscosity modifier,
A derivative of methacrylic acid.

For example, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl acryloyl phosphate, 1,3-butanediol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol. Diacrylate, diethylene glycol diacrylate,
Examples include neopentyl glycol diacrylate, polyethylene glycol diacrylate, tetrahydrofurfuryl acrylate, and the like. Of course, these acrylic acid derivatives can be used by replacing them with methacrylic acid derivatives.

In the present invention, these electron beam-curable resin compositions are used as the main constituents of the binder, but in order to control the physical performance of the coating film, the composition is appropriately free of electron beam-curable or poor thermoplasticity. Resins can also be mixed.

Further, in order to balance the physical properties of the coating film and the viscosity of the composition, if necessary, a normal organic solvent such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, methanol, ethanol, isopropanol, butanol, tetrahydrofuran, dioxane, dimethyl. Formamide, toluene, xylene and the like can also be used in combination.

Of course, the coating composition contains magnetic powder in addition to these resin compositions and solvents, but may further contain an antistatic agent, a dispersant, an abrasive and the like.

In the present invention, the coating film is cured by applying a composition containing these electron beam curable resin and magnetic powder onto one or both sides of a polyester film as a support and irradiating with an electron beam.

As the electron beam to be irradiated, ionizing radiation such as neutron beam and γ-ray can be used in addition to the electron beam, but the electron beam is industrially preferable. When using an electron beam accelerator for irradiation, set the accelerating voltage
It is good to set it to 100 to 1000 KV, preferably 150 to 400 KV.
The absorbed dose is 0.3 to 20 megarads, preferably 1 to 10 megarads.

A feature of the present invention is that, when forming the magnetic recording layer by the electron beam curing method, a biaxially stretched polyester film having a specific average refractive index and a certain degree of plane orientation is used as the polyester film of the support.

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

First, the average refractive index in the present invention is given by the following equation, where n _α is the refractive index in the thickness direction, n _{γ is} the refractive index in the main alignment direction, and n _β is the refractive index in the direction perpendicular to the main alignment direction. .

Further, the plane orientation degree ΔP is obtained by using the above n _α , n _β , and n _γ , Given in.

The polyester film in the present invention is intended for a biaxially stretched polyester film having an ethylene terephthalate unit of 80 mol% or more, and the average refractive index of the film needs to be 1.600 or more, preferably 1.605 or more. . If this value is less than 1.600, shrinkage may occur during electron beam irradiation after coating the magnetic layer, or when a small amount of solvent is used in combination, which is not preferable.
Also, when the calendering process is performed, heat loss is likely to occur and the quality is inferior. The upper limit of the average refractive index is not particularly limited, but if it is too high, the mechanical strength of the film becomes poor, and the film surface becomes brittle. Therefore, it is usually preferable to keep this value up to 1.620.

On the other hand, the degree of plane orientation ΔP and the average refractive index must satisfy the following relationship.

△ P ≦ 1.43 ・ −2.128 …… (2) Usually, a biaxially stretched polyester film is melt extruded by a conventional method and then subjected to sequential or simultaneous biaxial stretching, and then again in the longitudinal or transverse direction as required. The polyester film satisfying the formula (2) can be obtained by stretching and then heat-setting at a temperature of 130 to 240 ° C. It is necessary to take various conditions. For example, in longitudinal-transverse sequential biaxial stretching, for example, the longitudinal stretching temperature is set to about 100 to 120 ° C., which is 5 to 30 ° C. higher than the normal stretching temperature, or before the heat setting and after the heat setting. It can be obtained by relaxing the vertical or horizontal direction or both.

Of course, when these methods are adopted, not all of them satisfy the requirements of the average refractive index and the plane orientation degree of the present invention,
Moreover, since these values are influenced by the conditions of the film forming machine, such as the film forming speed and stretching width, or the conditions based on the polyester raw material, such as the amount of the catalyst and the addition aid, the crystallization speed, etc. The requirements of the present invention must be satisfied by appropriately selecting the conditions of.

In order to maximize the effects of the present invention, in addition to these two requirements, the amount of terminal carboxyl groups of the polyester film as the support is further 35 equivalents / ton or more, preferably 40 equivalents / ton or more. is necessary.

The amount of the carboxyl group is, for example, the method of A. Conix (Makromol.
chem., 26 , 226 (1958)),
When this value is 35 equivalents / ton or more, the adhesiveness to the magnetic recording layer cured by the electron beam becomes more excellent.

As described above, the characteristic of the present invention is that a polyester film having a specific average refractive index and plane orientation degree and a specific amount of a carboxyl group is used as a support. It has a remarkable effect of being excellent in adhesiveness to the magnetic recording layer by the crosslinking method.

EXAMPLES Next, the present invention will be described in more detail based on Examples and Comparative Examples, but the present invention is not limited thereto.

Comparative Example 1 and Comparative Example 2 First, a coating composition containing magnetic powder was obtained as follows. That is, a urethane acrylate obtained by reacting a reaction product of caprolactone polyol and isophorone diisocyanate with 2-hydroxyethyl acrylate.
42 parts, tetraethylene glycol diacrylate 13 parts,
Urethane resin 25 parts, vinyl chloride-vinyl acetate copolymer 15
Parts and 5 parts of lecithin, 100 parts of a resin composition, 900 parts of methyl ethyl ketone, and 340 parts of gamma-ferric oxide were kneaded for 24 hours using a ball mill to obtain a viscous dispersion composition.

On the other hand, a biaxially stretched polyester film was obtained based on one of the methods usually adopted. That is, polyethylene terephthalate having a terminal carboxyl group concentration of 32 equivalents / ton is used as 28
After melt-extruding at 5 ℃, it is stretched 3.5 times at 84 ℃ in the longitudinal direction and then 3.5 times at 107 ℃ in the transverse direction, heat treated at 220 ℃ for 3 seconds, and then cooled to form a biaxially stretched film with a thickness of 15μ. Obtained.

When the refractive index of the film was measured, it was found that n _α , n _β and n
_γ was 1.4885, 1,6495 and 1.6753, respectively. Therefore, the average refractive index of the film is 1.6044 and the plane orientation degree is 0.1739.

Next, the dispersion composition was applied to the film so that the dry film thickness was 8 μm. The methyl ethyl ketone was removed from the coating film by preliminary drying, and then the coating film was cured by using an electron beam accelerator with an accelerating voltage of 180 KV and a beam current of 9 mA so that the absorbed dose was 10 megarads.

The magnetic recording medium thus obtained was slit into 1/2 inch, and the adhesive force between the support and the magnetic recording layer was measured by the 180 ° tape peeling method to find that it was 10 g, which was an unsatisfactory value. (Comparative example 1).

A polyester film was obtained in the same manner as in Comparative Example 1 except that the same polyester raw material as in Comparative Example 1 was used and the stretching temperature in the machine direction was 107 ° C.

Table 1 shows the physical properties of the obtained film and a magnetic coating film formed on the film in the same manner as in Comparative Example 1 and the adhesive strength was measured (Comparative Example 2).

Example 1 and Comparative Example 3 Polyethylene terephthalate having a terminal carboxyl group concentration of 40 equivalents / ton was used in the same manner as in Comparative Example 2, that is, 3.5 times at 107 ° C. in the longitudinal direction and then 3.5 times at 107 ° C. in the lateral direction. The film was stretched and heat-treated at 220 ° C. for 3 seconds to obtain a biaxially stretched film having a thickness of 15 μm.

Next, in the same manner as in Comparative Example 1, a magnetic coating film was formed on the film and the adhesive force was measured (Example 1). The results are shown in Table 1, and as is clear from comparison with Comparative Example 2, even if the average refractive index and the degree of plane orientation are the same, the effect of the present invention is obtained when a film having a higher carboxyl group concentration is used. It can be seen that is exhibited more significantly.

Next, using the polyethylene terephthalate used in Example 1 as a raw material, a biaxially stretched film was obtained in the same manner as in Comparative Example 1. However, in this case, the heat treatment condition was 190 ° C. for 5 seconds.

A magnetic coating film was formed on the obtained film in the same manner as in Comparative Example 1 and the adhesive strength was measured (Comparative Example 3).

The results are shown in Table 1.

The measuring methods of each physical property used in the present invention are as follows.

Refractive index Abbe's refractometer (manufactured by Atago Co., Ltd.) at 25 ° C
It is a measured value for the Na-D line.

The refractive index of the film is 3 at the center of the film.
Direction, the refractive index is measured, and n _γ is the refractive index in the main orientation direction of the film n _{β is} the refractive index in the direction perpendicular to the main orientation direction of the film n _{α is} the refractive index in the thickness direction of the film Average refractive index and plane orientation The degree ΔP is obtained according to the following equation.

Carboxyl group 0.1 g of polyethylene terephthalate was taken, dissolved in heated benzyl alcohol, chloroform was added, and titration was performed with a 1/10 N sodium hydroxide / methanol solution using a microburet.

Claims (1)

[Claims] 1. A support for a magnetic recording medium, wherein one side or both sides are coated with an electron beam-curable resin composition and magnetic powder, and irradiated with an electron beam to polymerize and cure to laminate a magnetic recording layer. A biaxially stretched polyester film having an average refractive index n and a plane orientation degree ΔP satisfying the following formulas (1) and (2), and a terminal carboxyl group concentration of 35 equivalents / ton or more. ≧ 1.6000 …… (1) △ P ≦ 1.43 ・ −2.128 …… (2)