JPH0362134B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Field The present invention relates to a polyester film that has little in-plane anisotropy and is a suitable support for flexible disk applications. Prior Art and its Problems Magnetic recording materials, such as magnetic recording tapes or magnetic recording disks, are produced by coating the surface of a base film with magnetic particles together with a binder, or by applying a magnetic metal to the surface of a base film by vacuum evaporation, sputtering, plating, or other methods. This is a stack of magnetic layers that are fixed together. These magnetic recording materials have recently become required for high-density recording and reproduction, and they must have high mechanical strength to prevent distortion in recording, and be resistant to dimensional changes due to unexpected heat during recording and reproduction. Heat-resistant dimensional stability is required.
Moreover, as a result of increasing the density of magnetic recording, the magnetic layer has to be made thinner, resulting in the problem that electromagnetic conversion characteristics deteriorate unless the base film serving as the support is smooth. Further, in the case of a magnetic recording disk, unlike a magnetic recording tape, the disk surface is required to have uniform physical properties in all directions. However, there is no method that can simultaneously satisfy these required characteristics. For example, if polyethylene naphthalene dicarboxylate is selected as the material for the base film that serves as the support, it will have excellent mechanical strength and good heat-resistant dimensional properties (Japanese Patent Application Laid-Open No. 59-1989-1).
Publication No. 127730). However, when a base film with extremely low in-plane anisotropy is required as a high-density recording material, further homogenization is required. In addition, magnetic recording materials tend to be exposed to relatively harsh operating conditions, ranging from traditional indoor environments with controlled air conditioning and clean air to outdoor environments and environments without temperature/humidity conditioning. It is changing to the state where it is used. Under these circumstances, magnetic recording disks are required to have low in-plane anisotropy so that there is no distortion in recording and reproduction even under changes in temperature and humidity. The present inventor conducted research on a polyester film that is suitable for magnetic recording materials, has low in-plane anisotropy, and is a support with a smooth surface, and found that the track density is 96 TPI or more, and the recording density is The present invention has been achieved by developing a base film suitable for high-density magnetic recording disks having a BPI of 9000 BPI or more. Purpose of the Invention The present invention has the advantage that there is virtually no dimensional change even when placed in a high humidity atmosphere from room temperature to about 60°C.
It is an object of the present invention to provide a polyester film suitable for use in magnetic recording disks, which has a small difference in expansion coefficient with respect to in-plane temperature and humidity. Structure of the Invention The present invention has an in-plane anisotropy of thermal expansion coefficient (Δαt) of 8×10 ^-6 /
℃ or less, and the in-plane anisotropy of the humidity expansion coefficient (Δαh)
is less than 4×10 ^-6 /%RH, and the surface roughness (Ra)
is in the range of 0.003 to 0.04μm and 60℃, 80%
A polyester film that can be used for flexible disks with a dimensional change rate of 0.02% or less when left under RH temperature and humidity conditions for 72 hours. It is. The present invention will be explained. The polyesters targeted by the present invention are aromatic polyesters such as polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate, and polycyclohexylene dimethylene terephthalate. In addition, the film as a support is
It is biaxially stretched and is balanced in the machine direction and width direction. The film thickness is approximately 30 to 80 μm. The in-plane anisotropy of the coefficient of thermal expansion is the maximum value of the coefficient of thermal expansion of the support film (when the coefficient of humidity expansion differs in the direction of the film surface, the coefficient of thermal expansion in the direction showing the largest value) and the minimum value. The difference between the two is called the coefficient of thermal expansion. The in-plane anisotropy of this thermal expansion coefficient is 8×
Unless the temperature is below 10 ^-6 /°C, high-density magnetic recording disks will experience off-track. In the case of a magnetic recording disk of approximately 96 TPI, the anisotropy of the thermal expansion coefficient must be controlled within the above limits. In addition, the in-plane anisotropy of the humidity expansion coefficient refers to the maximum value of the humidity expansion coefficient when the support film expands and contracts with changes in humidity. This refers to the expansion rate of the difference between the minimum value (value obtained in the direction showing the maximum) and the minimum value.
The in-plane anisotropy of this humidity expansion coefficient is 4×10 ^-6 /%RH
Unless it is below, it is not suitable for high-density magnetic recording disks. In this condition, the track density is the same as above.
This is based on high-density recording materials with a recording density of approximately 9600 BPI and approximately 96 TPI. The small in-plane anisotropy of temperature expansion coefficient and humidity expansion coefficient means that a high quality magnetic recording disk with small modulation can be obtained.
What is modulation here? JISC 6291 (1983)
It has been standardized by , and it is possible to guarantee a modulation of 10% or less for dimensional changes within the limit of the in-plane anisotropy of the present invention. Furthermore, when a tracking servo is applied to a product with a track density of 192 TPI, there is a problem that the head cannot follow the disk if the in-plane anisotropy is large. Next, the polyester film of the present invention has a center line average surface roughness (Ra) in the range of 0.003 to 0.04 μm. With high-density magnetic recording materials, the magnetic layer is thinner than that of standard disks, so the electromagnetic conversion characteristics
In order to obtain high quality in drop-out and the like, it is predicted that the lower the surface roughness of the support polyester film (in other words, the smoother the surface), the better the result. However, a support film with a surface that is too smooth has poor slip properties and is extremely difficult to handle. Therefore, in order to suppress dropouts and the like to below a predetermined value, Ra must be 0.04 μm or less. Further, when Ra is less than 0.003 μm, the film is smooth and a blocking phenomenon occurs, and this blocking causes wrinkles, so the appropriate range of the surface roughness (Ra) of the film is determined. Of course, when the surface roughness is adjusted to about 0.005 to 0.025 μm, the support becomes suitable for magnetic recording materials with a good balance of slipperiness and electromagnetic conversion characteristics. Note that when the surface roughness (Ra) exceeds 0.04 μm, spacing loss becomes large in high-density recording materials, resulting in insufficient output. Furthermore, an extremely smooth support cannot pass a durability test of 10 million passes on the same track, considering the abrasion resistance of the polyester film. In order for the magnetic recording material to be able to follow even slight environmental changes and to satisfy the requirement that there is almost no distortion during recording and reproduction, it is further required that dimensional changes be small at temperatures close to the glass transition point. In other words, from ambient room temperature (260℃, 65%RH) to 60℃, 80%RH
An indicator of dimensional stability (60°C) is that the in-plane dimensional change (shrinkage or elongation) of the support is 0.02% or less when left in an environment of 3 days (72 hours).
For magnetic recording disks, expansion and contraction within about 0.02% will not result in modulation failure of 10% or more. The method for obtaining the polyester film of the present invention is as follows:
For example, fine inert particles (silica, alumina, alumina silicate, titania, glass powder, etc.) with an average particle size of about 0.2 μm to 0.01 μm are mixed into 0.008 to 0.3 μm.
By mixing it with weight% polyester, forming a film by a conventional method, and producing a stretched film by a conventional method, a film having a surface roughness (Ra) in the range of 0.003 to 0.04 μm can be obtained. If the surface is too rough, it is advisable to take measures such as making the particle size of the inert particles even smaller or reducing the amount added. The surface roughness changes depending on the stretching conditions (stretching temperature) or the stretching ratio. Then 60℃, 80%RH (72
A film having good low-temperature dimensional stability with a dimensional change within 0.02% over time can be achieved by subjecting the film to heat treatment or aging treatment after stretching. The heat treatment is about 150 to 230°C in the case of polyethylene terephthalate, and about 150 to 240°C in the case of polyethylene-2,6-naphthalene dicarboxylate and polycyclohexylene dimethylene terephthalate. During the heat treatment, it is preferable to reduce the tension applied to the film as much as possible, and the film may be shrunk to the extent that its flatness can be maintained. In addition, the aging process is carried out under low tension for a long time (10 to 200 hours) at a temperature of about 40 to 75°C, as a film roll, ideally as a slit film (sheet) close to a disk. be.
The conditions for these heat treatments and aging treatments are preferably selected while comparing the low-temperature dimensional stability of the film. A high-quality support with low in-plane anisotropy in temperature expansion coefficient and humidity expansion coefficient can be achieved by suppressing bowing during the stretching heat treatment of the film. Further, in the case of a wide film, since it is predicted that the central portion has less plane anisotropy, it is also possible to use only the central portion of the film. Effects of the Invention 1 A 5.25-inch disk with 96 TPI (tracks/inch) does not generate missing pulses due to track deviation even without a servo mechanism. 2 No deterioration of modulation (10% or more) due to thermal history up to 60℃. 3. No spacing loss or coating failure occurs even at linear recording density of 9000 BPI (bits/inch) or higher. 4. Does not reduce the durability of the paint film. 5. When attempting to achieve high track density using the tracking servo, the head has good followability.
This results in a base film with less head vibration (due to less anisotropy) and no reduction in service life. Effects such as this can be achieved. Example 1 Polyethylene terephthalate containing 0.3% titanium oxide with an average particle size of 0.3 μm was melt-extruded from a T-die and cooled and solidified on a quenching drum to obtain an unstretched film (thickness: 1000 μm). This film was stretched 3.6 times in the longitudinal (machine) direction, then gripped at both ends with clips, stretched 3.7 times in the transverse (width) direction in a tenter oven, and heat-set under tension at 220°C. and removed both ends. Subsequently, this stretched film was brought into contact with a roll at 80°C for 1 second and then a roll at 110°C for 1 second, and the tension of the film was set at 6.5 kg/kg.
cm ² and was slightly shrunk in the longitudinal direction to obtain a product film with a thickness of 75 μm. Approximately 1/2 of the total width of this product film was sampled from the center to form the center section (referred to as center section). Other parts of the film have at>8×10 ^-6 /°C, and the coefficient of thermal expansion and anisotropy thereof are too large. The expansion coefficient of the obtained film is Δαt≦7.5×10 ^-6 /
It only exhibited anisotropy of °C and Δαh≦3.5×10 ^-6 /%RH. Also, this film can be used at 60℃ and 80%RH.
The dimensional change rate when left for 72 hours is 0.01 in the vertical direction.
%, and 0.02% in the horizontal direction. When this film was used as a magnetic disk, the modulation was within 2%. The results are summarized in Table 1. Example 2 A polyethylene-2,6-naphthalene dicarboxylate resin containing 0.25% silica with an average particle size of 0.12 μm was melted in an extruder, quantitatively extruded through a T-die using a gear pump, and placed on a quenching drum. The mixture is cooled and solidified to obtain a 920 μm unstretched film. This unstretched film was stretched 3.4 times in the machine direction, 3.7 times in the width direction, and 1% toe-in (shrinkage treatment) in the width direction in the same manner as in Example 1, then heat-set at 160°C and rolled once. I took it. Furthermore, the film roll that had undergone the stretching heat treatment was rewound while being heat treated at 230°C. In this heat treatment step, heating is performed in the opposite direction to the stretching heat treatment, so that the in-plane anisotropy of the film is reduced, and Δαt≦4×10 ⁻⁶ /° C. over the entire width. A homogeneous isotropic film was obtained by winding the film at a tension of 7 kg/cm ² while subjecting it to relaxation at about 115° C. in the machine direction. Example 3 Poly(1,4-cyclohexylene dimethylene terephthalate) 85 parts, poly(1,
Using 15 parts of a copolymer (abbreviated as PCHDT) of 4-cyclohexylene dimethylene isophthalate, 0.30 wt% of titanium oxide with an average particle size of 0.3 μm was dispersed, and casting was performed under the same conditions as in Example 1. After biaxial stretching (machine direction stretch ratio: 3.6 times, width direction stretch ratio: 3.9 times), heat treatment was performed twice in the forward direction and in the reverse direction in the same manner as in Example 2. The first heat treatment was at 140℃, and the second was at 225℃.
2% toe-in (limited shrinkage) in the width direction at °C
was applied. As a result, Δαt≦5×10 ^-6 /℃, Δαh
A film with isotropy of ≦2×10 ⁻⁶ /%RH was obtained. Furthermore, leave the film roll at 65℃ (50%
When left in an atmosphere of 60℃ and 80% RH for 48 hours, the dimensional change rate was 0.02% (shrinkage) in the vertical direction and 0.02% (shrinkage) in the horizontal direction.
It was 0.01% (growth). It was found that this copolymer film exhibits isotropy and is excellent as a magnetic disk. The results of these tests are shown in Table 1. Comparative Example 1 0.3 wt% of kaolin with an average particle size of 1.0 μm was added to the polyethylene terephthalate used in Example 1, and after kissing, biaxial stretching (machine direction stretch ratio 3.6; width direction stretch ratio 3.8) was performed. Heat setting was performed at 220°C under the condition of 3% toe-in (limited shrinkage in the width direction). The obtained film is for the center Δαt≦8×10 ^-6 /℃, Δαh≦4×10 ^-6 /%RH, and for the both sides Δαt＞8×10 ^-6 /℃, Δαh＞6×10 ^{- 6} /%RH. Both have high anisotropy (expansion coefficient), and the dimensional change in the longitudinal direction at 60℃ and 80%RH for 72 hours is 0.09.
% and 0.04% (elongation) in the lateral direction, indicating poor dimensional stability. Comparative example 2 Polyethylene terephthalate (intrinsic viscosity 0.58)
Δαt≒12 without using any additives.
In addition to the high anisotropy of ×10 ^-6 /℃, the dimensional stability (60℃, 80%RH, after 72 hours) is 0.9 in the longitudinal direction.
% (shrinkage) and 0.04% (elongation) in the width direction.
The surface of this film is extremely smooth, so although it had good electromagnetic properties when tested as a magnetic disk, its durability was low, and the surface became rough and became defective before it reached 10 million passes (a sudden drop in electromagnetic conversion properties).
did. 【table】

Claims (1)

[Claims] 1 The in-plane anisotropy of the coefficient of thermal expansion is 8×10 ^-6 /℃ or less, and the in-plane anisotropy of the coefficient of thermal expansion is 4×10 ^-6 /%
RH or less, and the center line average surface roughness (Ra) is
In the range of 0.003~0.04μm and 60℃, 80%RH
The dimensional change rate when left for 72 hours under the temperature and humidity conditions of
A polyester film that can be used for flexible disks with a content of 0.02% or less.