JP2937693B2 - Biaxially oriented laminated polyester film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biaxially oriented laminated polyester film, and more particularly to a tape used as a base film of a digital recording type audio cassette tape using a fixed magnetic head such as a digital compact cassette tape (DCC). The present invention relates to a biaxially oriented laminated polyester film capable of recording at high density with very little elongation deformation of a tape when used.

[0002]

2. Description of the Related Art In recent years, a digital audio tape recorder (D) of a rotary magnetic head system has been used as an audio cassette tape recorder of a digital recording system having excellent sound quality.
AT) and digital compact type with fixed magnetic head
Cassette tape recorders (DCCs) and the like have been developed. In particular, DCC using a fixed magnetic head method is described in, for example, Kinya Murata's "DCC / MD Guidebook".
As described in (Denpa Shimbun), "Azimuth (azimuth) stabilizing tape guide" and "Azimuth lock pin" are installed on both sides of the tape pad to stably run the tape above and below the magnetic head. , Ensuring more stability. Of these, in particular, the tape guide has an inclined guide wall, on which one side edge of the tape is forced to pass, thereby pressing the tape against the opposite edge in the width direction, and pressing the tape in the width direction. The structure is such that the tape runs stably without shaking, and the tape is reliably brought into contact with the magnetic head by the lock pin. For this reason, one side edge of the tape passing through the inclined wall of the tape guide is forcibly extended by several percent. This allows
One edge of the tape may be stretched or bent, causing the tape and the magnetic head to break loose (head touch), causing loss of recording signals during recording or insufficient output during playback, A trouble occurs in which the running becomes unstable and the reproduced signal cannot be read accurately. For this reason, even if the base film of the long-strength high-strength analog recording tape is used as the base for DCC, the elongation and deformation of the tape due to the tension fluctuation at the start and stop of the tape cannot be suppressed, or The film may not have sufficient strength in the width direction, and thus cannot prevent the edge damage as described above.

In addition, since the DCC is recorded at a high density corresponding to digital reproduction and recording, the surface of the magnetic layer needs to be flatter. The "DCC / MD Guidebook"
In this case, the tape used is equivalent to a standard type VTR tape, and the surface of the base film is required to be flat. In digital audio cassette tapes, if a base film with very low surface roughness is used for high-density recording, the slippage between the film surfaces is poor, and the escape of air between the films is poor. It becomes very difficult to wind the film on a roll. Therefore, it is required that the base film has a flat surface and excellent running properties.

Therefore, the surface of the base film is flattened, and a coating of an organic polymer containing a lubricant is applied to the surface on the opposite side of the magnetic layer to provide lubricity. Although it is possible to improve the slipperiness on the opposite side of the layer, there is a problem that the productivity is increased by applying the back coating agent.

[0005] Conventional analog audio cassette tapes usually have a rough polyester film surface,
There is no product that satisfies the above-mentioned required characteristics and can be put to practical use.

[0006] Audio cassette tapes are also being used outdoors in summer or in cars, and are often used under severe high-temperature environments. Because of this, it is excellent in heat resistance etc., and the tape characteristics do not change even under more severe environmental changes,
For example, it is desired that the tape does not deteriorate even when the tape is left at a high temperature such as 90 to 100 ° C.

Therefore, as a base film of a digital audio cassette tape of a fixed magnetic head type, a film which does not cause these problems, in other words, is excellent in resilience to elongation, excellent in flatness and lubricity, and excellent in high temperature. There is a need for a film that does not cause deterioration in characteristics.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problem and, when a digital audio tape of a fixed magnetic head type is used, to prevent the tape guide of the magnetic head from stretching the tape edge by the inclined wall at the tape guide. To provide a biaxially oriented laminated polyester film for digital audio cassette tapes that is excellent in restorability, has low recording distortion and output fluctuation, has good tape running properties and audio characteristics (electromagnetic conversion characteristics), and is less expensive. is there.

[0009]

The present invention has the following arrangement to attain the object.

Polyester A Layer and Polyester B Used for Base Film of Digital Audio Cassette Tape
A biaxially oriented laminated polyester film comprising at least two layers each of which constitutes an outermost layer, wherein the laminated film has a stress at 5% elongation in the longitudinal direction of 12 kg / mm ² or more, and a residual in the longitudinal and lateral directions. Elongation 0.2 each
%, The heat shrinkage in the vertical direction when heat-treated at 105 ° C. for 30 minutes under no load is 1.5% or less, the total thickness of the film is 12 μm or less, and the surface of layer A The surface of the layer A has a surface roughness Ra of 1 to 8 n.
m, and the surface roughness Ra of the surface of the layer B is 10 to 40.
A biaxially oriented laminated polyester film having a thickness of 1.5 nm.

In the present invention, the polyester constituting the polyester A layer and the polyester B layer is a polymer containing polyethylene terephthalate as a main component, and not only a polyethylene terephthalate homopolymer but also 80% by mole or more of an acid component is composed of terephthalic acid. In addition, copolymerized polyethylene terephthalate in which 90% by mole or more of the glycol component is ethylene glycol, or polyethylene terephthalate accounts for 85% by weight or more (preferably 90% by weight or more), and another polymer accounts for 15% by weight or less (preferably 10% by weight). %).

Examples of the acid component of the copolymer include aromatic dicarboxylic acids such as naphthalenedicarboxylic acid, isophthalic acid, diphenylsulfone dicarboxylic acid, and benzophenone dicarboxylic acid, and aliphatic acids such as succinic acid, adipic acid, sebacic acid and dodecane dicarboxylic acid. Examples thereof include alicyclic dicarboxylic acids such as dicarboxylic acid, hexahydroterephthalic acid, and 1,3-adamantanedicarboxylic acid.
Examples of the glycol component include 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, and P-xylylene glycol. Can be. Examples of other polymers that can be blended include polyamides, polyolefins, and other polyesters.

The polyester may contain a lubricant, a matting agent, a coloring agent, a stabilizer, an antioxidant, and the like, if necessary.

Such a polyester is usually produced by a known method by a melt polymerization method. At that time, additives such as a catalyst can be optionally used as needed.

In the biaxially oriented laminated polyester film of the present invention, the polymers constituting the polyester A layer and the polyester B layer may be the same or different, but are preferably the same polymer. Further, the biaxially oriented laminated polyester film may have a laminated structure in which one or more polyester C layers exist between the A layer and the B layer.
The polymer constituting the polyester C layer may be the same as or different from the polymer of the A layer or the B layer, but is preferably the same polymer. Especially, it is preferable that each layer is the same polymer.

The biaxially oriented laminated polyester film of the present invention can be basically produced by a known film-forming method which has been conventionally accumulated. For example, a method in which the dried polyester forming each layer is melted and co-extruded from a die, cooled on a casting drum to obtain an unstretched laminated film, and the unstretched laminated film is simultaneously or sequentially biaxially stretched and heat-set. Can be manufactured. Instead of the co-extrusion method,
For example, an extrusion lamination method or the like can be used.

The biaxially oriented laminated polyester film of the present invention has a stress at the time of 5% elongation in the longitudinal direction of the laminated film of 12 k.
g / mm ² or more. The stress at 5% elongation in the longitudinal direction is preferably changed depending on the recording / reproducing time of the tape. When the recording / reproducing time is 60 minutes or less, it is preferably 12 to 15 kg / mm ² , and the recording / reproducing time is 6 times.
In the case of more than 0 minutes and 90 minutes or less, 15 to 20 kg / mm
² is preferable, and the recording / reproducing time is more than 90 minutes and 15
In the case of 0 minutes or less, the pressure is preferably 20 kg / mm ² or more.
Thus, expansion and contraction and permanent deformation of the tape at the start and stop of the tape and in electronic editing can be prevented, and the distortion of the tape can be quickly recovered. In addition, permanent deformation of one edge of the tape by the inclined fixing pin of the tape width guide can be prevented. The stress at 5% elongation in the longitudinal direction of the film is 12 kg /
If it is less than mm ² , the tape will expand and contract and deform due to tension fluctuations during tape running and start / stop, resulting in large output fluctuations and poor sound quality. Furthermore, one side edge of the tape is subjected to permanent elongation deformation by the inclined fixing pin, and the head touch is also deteriorated, so that the electromagnetic conversion characteristics are also poor.

As a means for obtaining the stress at 5% elongation in the longitudinal direction, a general stretching roll or a stenter is used.
The film may be stretched simultaneously in the vertical and horizontal directions, sequentially stretched in the vertical and horizontal directions, or may be stretched in two or more stages in the vertical and horizontal directions. Specifically, the stretching method may be a known method, the stretching temperature is usually 80 to 140 ° C., and the stretching ratio is 3.0 to 5.0 times, preferably 3.3 to 5.0 in the longitudinal direction.
4.3 times, 3.0 to 5.0 times in the horizontal direction, preferably 3.0 times.
Select 5-5.0 times. The biaxially oriented laminated polyester film can be obtained by heat-setting the obtained biaxially stretched laminated film at 150 to 260 ° C, preferably 180 to 250 ° C for 1 to 100 seconds.

In the biaxially oriented laminated polyester film of the present invention, the laminated film must have a residual elongation of 0.2% or less in both the longitudinal and lateral directions. The residual elongation in the machine direction is 0.15% or less, more preferably 0.10% or less, particularly 0.0%.
It is preferably at most 5%. The residual elongation in the horizontal direction is preferably equal to or larger than the residual elongation in the vertical direction. As a result, even if one side edge is forcibly extended by the tape running on the inclined wall of the tape guide of the magnetic head, the resilience immediately after passing the head is good, and particularly, the residual elongation in the lateral direction is reduced. When it is larger than the vertical direction, it is particularly excellent in resilience, there is no occurrence of residual elongation or permanent deformation, the familiarity of the tape with the head (head touch) is good, and the output characteristics and sound quality characteristics are excellent. If the film has a residual elongation of more than 0.2% in the longitudinal or lateral direction or both directions, the tape runs on the inclined wall of the tape guide of the magnetic head, so that one side edge is elongated or bent, and the tape and the tape are bent. Since the familiarity with the magnetic head (head touch) is deteriorated, the recording signal is lost during recording and the output during reproduction is not sufficiently obtained.

Since the residual elongation greatly changes depending on the heat setting temperature when the biaxial stretching and the heat setting are performed by the means for obtaining the 5% elongation stress, the above-mentioned heat setting temperature range is set in advance. By measuring the residual elongation in the vertical and horizontal directions simultaneously with the stress at 5% elongation in the longitudinal direction of the film obtained by changing the heat setting temperature in the inside, the optimum heat setting temperature is selected while comparing the measured values. It is preferable to obtain.

In the biaxially oriented laminated polyester film of the present invention, the surface roughness Ra of the surface of the layer A on which the magnetic layer is provided is 1 to 8
It must be in the range of nm, preferably in the range of 3-7 nm. If the surface roughness Ra is larger than 8 nm, the gap between the magnetic head and the tape becomes large and the electromagnetic conversion characteristics are reduced, which is not preferable for achieving the purpose of high density and high sensitivity. On the other hand, if Ra is less than 1 nm, the coefficient of friction of the base film becomes large, it becomes extremely difficult to handle the film and it is very difficult to wind it up on a roll, and the slipperiness of the magnetic layer deteriorates, causing the magnetic layer to be scraped. Is not preferred.

In the biaxially oriented laminated polyester film, the surface roughness Ra of the surface of the layer B opposite to the surface of the layer A on which the magnetic layer is provided is in the range of 10 to 40 nm, preferably 15 to 30 nm.
It needs to be in the range of nm. This surface roughness Ra
Is larger than 40 nm, the show-through phenomenon on the magnetic surface occurs, and the electromagnetic conversion characteristics are deteriorated due to the roughening of the magnetic surface, and the dropout occurs. On the other hand, if Ra is less than 10 nm, the runnability as a tape is deteriorated, and the effect as a smooth surface (running surface) is not obtained, which is not preferable.

The surface roughness of the polyester A and B layers is as follows:
For example, it is preferable to adjust by adding inert particles to the polyester of each layer. Examples of the inert particles include (1) silicon dioxide; (2) alumina;
(3) oxides of Mg, Zn, Zr, and Ti; (4) C
(5) Li, Ba, and Ca phosphates (including mono- and di-hydrogen salts); (6) Li, N
(7) Ca, Ba, Zn, and Mn terephthalates; (8) Mg, Ca, Ba, Z
(9) titanates of n, Cd, Pb, Sr, Fe, Co, and Ni; (9) chromates of Ba and Pb; (10) carbon (eg, carbon black, graphite, etc.);
Glass (eg, glass powder, glass beads, etc.); (12) C
a and Mg carbonates, more preferably silicic anhydride, hydrous silicic acid, alumina, aluminum silicate (including calcined and hydrated), monolithium phosphate, trilithium phosphate, sodium phosphate, calcium phosphate , Barium phosphate,
Examples include titanium oxide, lithium benzoate, calcium carbonate, and the like, and fine particles made of a heat-resistant polymer such as a silicone resin and cross-linked polystyrene. One or more of such inert particles can be used, and the surface roughness can be obtained by selecting and adjusting the particle size, amount, and the like.

Examples of the method for incorporating the inert particles into the polyester include a method of adding the inert particles before or during the polymerization of the polyester, a method of kneading the inert particles at the time of pelletizing after the polymerization, and a sheet. For example, a method of kneading the inert particles at the time of melt-extrusion into a shape can be used. Not only these addition methods, but also a method of adding a phosphorus component or other necessary additives during polymerization to precipitate particles, and a method of blending the polymer obtained by this method and the polymer obtained by the above addition method. It can also be used.

The biaxially oriented laminated polyester film of the present invention must have a heat shrinkage in the longitudinal direction of 1.5% or less when heat-treated at 105 ° C. for 30 minutes without load. Preferably it is 1.2% or less, more preferably 1.0% or less. If the heat shrinkage is too large, the magnetic tape is greatly deformed during use or storage in a high-temperature atmosphere such as in summer, and the properties of the tape are impaired.

The heat shrinkage at 105 ° C. for 30 minutes is usually achieved by appropriately selecting the heat treatment conditions within the above heat setting conditions. As another means, it is achieved by passing the film between two rolls having a difference in speed after the above-mentioned heat setting treatment, and applying a temperature higher than the glass transition temperature (Tg) of the polyester to carry out a relaxation treatment. However, the present invention is not particularly limited to these methods.

In the biaxially oriented laminated polyester film of the present invention, the total thickness of the laminated film must be 12 μm or less. The preferable total thickness varies depending on the recording / reproducing time, and is 9 to 12 μm when the recording / reproducing time is 60 minutes or less.
When the recording / reproducing time is more than 60 minutes and 90 minutes or less, the thickness is preferably 6 to 9 μm, and when the recording / reproducing time is more than 90 minutes and 150 minutes or less, 4 to 6 μm is preferable. If the total thickness of the laminated film is larger than 12 μm, the function as a small and lightweight digital audio cassette tape cannot be achieved. The lower limit of the total thickness of the laminated film is not particularly limited.
Even when the stress at the time of% elongation is 12 kg / mm ² or more, the stiffness of the tape is insufficient, and the running property of the tape becomes unstable due to the weakness of the waist of the tape. Trouble starts to occur. Further, when the thickness is reduced, the residual elongation is extremely reduced,
It is difficult to maintain the residual elongation at 0.2% or less at 0.2 μm or less, and the tape end surface of the tape guide of the magnetic head is partially stretched at the inclined wall. Therefore, the lower limit of the total thickness of the laminated film is preferably 4 μm or more. .

In the biaxially oriented laminated polyester film of the present invention, the thickness of the polyester B layer is preferably in the range of 1/20 to 1/2 of the total thickness of the laminated film. If the polyester B layer is too thin, the number of inert particles contained in the B layer is reduced, the protrusion to the surface layer is reduced, and the small protrusions on the surface background are reduced, so the surface roughness is extremely reduced, and the tape The runnability is deteriorated, and coarse particles stick out to the surface to cause shaving of the film surface, which is a cause of dropout, which is not preferable. On the other hand polyester B
If the layer is too thick, the push-up of the coarse particles contained in the layer B affects the surface of the layer A, and the surface of the layer A is roughened. This is not preferable for achieving the purpose of high density and high sensitivity. On the other hand, if the polyester B layer is too thick, the amount of particles in the laminated film increases, the production cost increases, and it is economically undesirable.

In the biaxially oriented laminated polyester film of the present invention, it is preferable to select a stress at 5% elongation, a residual elongation, a film thickness and the like as described below in accordance with the recording / reproducing time. When running the tape or when starting
Edge damage during stopping, tape elongation and deformation can be suppressed.

When the recording / reproducing time is 60 minutes or less, the stress at 5% elongation in the longitudinal direction of the film is 12 to 15 kg / mm ^2.
The residual elongation in the longitudinal direction is 0.15% or less, more preferably 0.10% or less, particularly preferably 0.05% or less, and the residual elongation in the transverse direction is 0.2% or less. It is preferably at most 0.15%, particularly preferably at most 0.1%, and the total thickness of the film is from 9 to 12 μm.

When the recording / reproducing time is more than 60 minutes and 90 minutes or less, the stress at 5% elongation in the longitudinal direction of the film is 15 to 20.
kg / mm ² , the residual elongation in the longitudinal direction is 0.15% or less, more preferably 0.10% or less, particularly preferably 0.05% or less, and the residual elongation in the lateral direction is 0.2% or less. %, More preferably 0.15% or less, and the total thickness of the film is 6 to 9 μm.

When the recording / reproducing time exceeds 90 minutes, the stress at 5% elongation in the longitudinal direction of the film is 20 kg / mm ² or more, and the residual elongation in the longitudinal direction is 0.15% or less, more preferably 0.15% or less. 10% or less, particularly preferably 0.05% or less, the residual elongation in the transverse direction is 0.2% or less, more preferably 0.15% or less, and the total thickness of the film is 4 to 6 μm. .

[0033]

The present invention will be further described below with reference to examples. The various physical properties and properties in the present invention are measured and defined as follows.

(1) Stress at 5% elongation A film was cut into a sample having a width of 10 mm and a length of 150 mm. The distance between the chucks was set to 100 mm, and the tensile speed was 100 mm / min and the chart speed was 100 mm / min. Pull. The obtained stress at the time of 5% tension of the load-elongation curve is converted into a unit area (mm ² ).

(2) Residual elongation A film sample having a width of 3/20 inches and a length of 600 mm is placed on a measuring table of a precision universal projector PJ321F manufactured by Santoyo Seisakusho, one side in the longitudinal direction is fixed, and the other side is a sample table. The upper part of the roll, which is installed horizontally in parallel, passes over the free roll at the same height as the horizontal plane of the sample table, and hangs vertically. After accurately measuring the mark of about 200 mm attached to the film sample on the sample stand, and then applying a 200 g load to the hanging film end on the free roll side for 10 minutes, the 200 g
After removing the load, the sample was left for 10 minutes, and the dimensional change was read. The residual elongation is determined by the following equation (Equation 1) from the length under no load (S0) and the dimensional change (ΔS) under a load of 200 g.

[0036]

(Equation 1)

(3) Film Surface Roughness (Ra) Using a stylus type surface roughness meter (Surfcoder 30C) manufactured by Kosaka Laboratory Co., Ltd. under the conditions of a needle radius of 2 μm and a stylus pressure of 30 mg. A chart (film surface roughness curve) is applied below. From the film surface roughness curve, a portion of the measured length L is extracted in the direction of the center line, the center line of the extracted portion is set as the X axis, the direction of the vertical magnification is set as the Y axis, and the roughness curve is expressed as Y = f.
When represented by (x), R given by the following equation (Equation 2)
a (μm) is defined as the film surface roughness.

[0038]

(Equation 2)

(4) Heat Shrinkage A film having a length of about 30 cm and a width of 1 cm, which has been measured accurately in advance, is placed in an oven set at 105 ° C. without any load, and heat-treated for 30 minutes. Remove the sample, bring it to room temperature and read the change in its dimensions. From the length (L0) before the heat treatment and the dimensional change (ΔL) due to the heat treatment, the heat shrinkage is determined by the following equation (Equation 3).

[0040]

(Equation 3)

(5) Running Durability of Magnetic Tape 70 wt% of γ-Fe ₂ O ₃ and 30 wt.
A paint consisting of a wt% binder is applied. The binder is 5 wt% urethane rubber, 3.5 wt% nitrocellulose, 1.5 wt% vinyl chloride, 90 wt%
% Of methyl ethyl ketone and 15 wt% of an isocyanate compound as a curing agent with respect to the resin. By this coating, a magnetic layer coating material having a width of 600 mm was obtained, the coating material was subjected to a micro slit of 3/20 inch using a shear type slitter, and the obtained magnetic tape was used to make Matsushita Electric Industrial Co., Ltd.
Start running on the digital cassette deck RS-DC10
The magnetic tape is run for 100 hours while stopping repeatedly, and the running durability of the magnetic tape at this time is determined as follows. <Three-step judgment> ：: The tape edge does not stretch or bend. Also, there is no scraping and no white powder adhesion. Δ: A slight amount of wakame is generated due to elongation of the edge of the tape, bending occurs, and a small amount of white powder adheres. ×: One side edge of the tape is elongated or bent, and wakame or breakage is remarkable. Further, the tape is sharply scraped and a large amount of white powder is generated.

(6) Non-defective winding rate The film is 500 mm wide and 4000 m long, and 100
The number of non-defective products obtained when this winding was performed is shown as a percentage.
At this time, the non-defective product means the following. The film is rolled up in a cylindrical shape and is not angular or sagging. There are no wrinkles on the film roll.

[0043]

Examples 1 and 2, Comparative Examples 1 to 3 Intrinsic viscosity (value measured at 25 ° C. using orthochlorophenol as a solvent) containing inactive particles shown in polyester A layer of Table 1 for A layer 0 After drying 0.65 dl / g of polyethylene terephthalate and 0.65 dl / g of intrinsic viscosity containing inactive particles shown in the polyester B layer of Table 1 for the B layer at 160 ° C., 28
It was melted at 0 ° C., co-extruded from a multi-manifold type die at a B layer thickness ratio shown in Table 1, and quenched and solidified on a casting drum kept at 40 ° C. to obtain an unstretched laminated film.

This unstretched laminated film is stretched longitudinally at a magnification shown in Table 1 between two rolls having a speed difference, and further stretched in a transverse direction at a magnification shown in Table 1 by a tenter. After heat treatment at the heat setting temperature shown in 9
It was cooled at 0 ° C. and wound up. Thus, the thickness of 10 μm
The biaxially oriented laminated polyethylene terephthalate film was obtained. At this time, the thicknesses of the layer A and the layer B were adjusted by changing the discharge amounts of the two extruders. The measurement of the thickness of each layer was obtained by using both a fluorescent X-ray method and a method of cutting a film into thin slices and searching for an interface with a transmission electron microscope.

Subsequently, a magnetic paint having the composition described in (5) Running durability of the magnetic tape was applied to the surface of the layer A of the biaxially oriented laminated polyethylene terephthalate film thus obtained, and the recording / reproducing time was measured. A 60-minute audio cassette tape was obtained.

Table 1 shows the properties of the obtained films and tapes.
Shown in

As is clear from the table, the films of Examples 1 and 2 have good handleability of the base film and excellent running durability of the tape. In contrast, Comparative Examples 1 to 3
Film has low stress at 5% elongation in the longitudinal direction,
Due to the high residual elongation, tape elongation occurs during running, and wakame and breakage are observed at the tape edge. Further, since the film of Comparative Example 1 has a low roughness of the B layer surface,
The running property was poor, and the coarse projections caused shaving during tape running. Since the film of Comparative Example 2 had high surface roughness of the A layer and the B layer, a show-through phenomenon of roughness on the magnetic surface of the tape was observed. Further, since the film of Comparative Example 3 has low surface roughness,
The handling workability was poor, and the rate of non-defective products was extremely deteriorated. For this reason, the films of Comparative Examples 1 to 3 are not preferable as base films for digital audio tape.

[0048]

Example 3, Comparative Examples 4 and 5
To obtain an unstretched laminated film in which the inert particles shown in Table 1 are contained in the polyester A layer and the B layer. The unstretched laminated film is stretched and heat-set under the conditions shown in Table 1, and cooled in the same manner as in Example 1. Thus, a biaxially oriented laminated polyethylene terephthalate film having a thickness of 8 μm and an audio cassette tape having a recording and reproducing time of 90 minutes were obtained. Table 1 shows the results. The film of Example 3 gave good results as in Example 1. On the other hand, the films of Comparative Examples 4 and 5 had
% Elongation stress is low, and the residual elongation is high, so that tape elongation occurs during running, wakame and breakage occur at the tape edge, and the film of Comparative Example 4 has a large heat yield, especially in summer. When used and stored in a high-temperature atmosphere, the tape was greatly deformed, which was not preferable as a digital audio tape.

[0049]

Example 4 and Comparative Example 6 In the same manner as in Example 1, an unstretched laminated film containing the inactive particles shown in Table 1 in the polyester A layer and the B layer was obtained.
The film was stretched and heat-set under the following conditions to obtain a 6 μm-thick biaxially oriented laminated polyethylene terephthalate film and an audio cassette tape having a recording and reproduction time of 120 minutes. At that time, in Example 4, the heat-set biaxially oriented laminated film was heated to 100 ° C. by a heating roll and then adjusted in tension with a cooling roll to reduce the heat shrinkage in the vertical direction by 0.5.
Adjusted to%. On the other hand, in Comparative Example 6, this adjustment was not performed. Table 1 shows the results. In Example 4, similar to Example 1, good results were obtained. In Comparative Example 6, since the film had insufficient stress at 5% elongation in the machine direction and had a high residual elongation, tape elongation occurred during running, wakame and folds were observed at the tape edge, and A Since the surface of the layer was rough and the heat shrinkage was high, it was not preferable as a digital audio tape.

[0050]

[Table 1]

[0051]

According to the present invention, the base film has good workability, and when a digital audio tape of a fixed magnetic head type is used, the tape forcibly stretched near the magnetic head is excellent in restorability, A biaxially oriented laminated polyester film useful for digital audio cassette tapes, having small recording distortion and small output fluctuation, excellent tape running properties and audio characteristics (electromagnetic conversion characteristics), can be provided.

────────────────────────────────────────────────── (5) References JP-A-4-259910 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B32B 1/00-35/00 G11B 5 / 62-5/82

Claims (5)

(57) [Claims] 1. A biaxially oriented laminated polyester film composed of at least two outermost layers each of which is a polyester A layer and a polyester B layer used for a base film of a digital audio cassette tape. % Elongation stress is 12 kg / mm ² or more, and the residual elongation in the longitudinal direction and the transverse elongation are each 0.
2% or less, the heat shrinkage in the vertical direction when heat-treated at 105 ° C. for 30 minutes under no load is 1.5% or less, the total thickness of the film is 12 μm or less, and the surface of layer A is a magnetic layer. And the surface roughness Ra of the surface of the layer A is 1 to 8
nm, and the surface roughness Ra of the B layer surface is 10 to 4
A biaxially oriented laminated polyester film having a thickness of 0 nm. 2. The biaxially oriented laminated polyester film according to claim 1, wherein the thickness of the polyester B layer is in the range of 1/20 to 1/2 of the total thickness of the laminated film. 3. A biaxially oriented laminated polyester film used as a base film of a digital audio cassette tape having a recording and reproduction time of 60 minutes or less, wherein the laminated film has a stress at 5% elongation in the longitudinal direction of 12 to 15 kg / mm. ^Two
The biaxially oriented laminated polyester film according to claim 1, wherein the residual elongation in the longitudinal direction is 0.15% or less, and the total thickness of the film is 9 to 12 µm. 4. A biaxially oriented laminated polyester film used as a base film of a digital audio cassette tape having a recording / reproducing time of more than 60 minutes and 90 minutes or less,
The 5% elongation stress in the longitudinal direction of the laminated film is 15 to 20.
kg / mm ^2, the residual elongation in the longitudinal direction is 0.15% or less, and the biaxially oriented laminated polyester film of claim 1, wherein the total thickness of 6~9μm film. 5. A biaxially oriented laminated polyester film used as a base film of a digital audio cassette tape having a recording / reproducing time of more than 90 minutes and not more than 150 minutes, wherein the laminated film has a stress at 5% elongation in the longitudinal direction of 20 k.
g / mm ² or more, and the residual elongation in the longitudinal direction is 0.15%
2. The biaxially oriented laminated polyester film according to claim 1, wherein the total thickness of the film is 4 to 6 [mu] m.