JPH0149610B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a biaxially oriented thermoplastic polyester film, which has near-infrared absorption ability. More specifically, the present invention provides an isotropic balanced film in which there is almost no difference in roughness between the front and back surfaces of the film, and which are oriented approximately equally in any direction on the plane of the film. This invention relates to a polyester film endowed with infrared absorbing properties. Biaxially oriented polyester films are used in various industrial applications. Among these uses, for magnetic disks and masking for copying, it is desired that the film has the same and uniform physical properties in all directions; It is preferable that there is no such thing. Furthermore, in the case of magnetic disks, it is necessary to provide the film itself with a light-shielding property so that the light does not leak except through the index hole. However, a film that satisfies all of these quality requirements has not yet been obtained. As a result of intensive research to obtain a film with light-shielding properties that has substantially homogeneous physical properties in all directions, the inventors of the present invention have found that it absorbs near-infrared rays, and there is virtually no difference between the front and back sides regarding unevenness on the film surface. The present invention was achieved based on the finding that a film that does not have any negative effects and has a small difference in expansion coefficient with respect to temperature becomes the optimal film for magnetic disks and the like. That is, in the present invention, polyester containing a pigment with low brightness is melted, passed through a filter having an average opening of 15 to 30μ, extruded to form an unstretched film, and the unstretched film is stretched in biaxial directions. This can be obtained by heating both sides of the film to substantially eliminate the temperature difference between the front and back sides of the film, making the degree of stretching orientation uniform in the biaxial directions, and then applying heat treatment to homogenize the thermal properties in the biaxial directions. 800~
A biaxially oriented near-infrared absorbing polyester film with an absorption rate of 900 mμ near-infrared rays of 90% or more, a difference in center line average roughness between the front and back surfaces of 0.005 μ or less, and an isotropy index of the film plane within 10%. It is. Here, the isotropy index I means, when there is a direction showing the maximum value A of the thermal expansion coefficient and a direction showing the minimum value B of the thermal expansion coefficient on the film plane, I=A-B/A×100 (%) The present invention will be explained. In the present invention, polyesters forming the polyester film include polyethylene tereftate, polybutylene tereftate, polyethylene tereftate, and polyethylene tereftate.
2,6-naphthalene dicarboxylate, etc. can be applied. In addition to these, copolymerized polyesters are also included, and the above-mentioned polyesters include those to which 15% by weight or less of an organic or inorganic compound or other polymer is added. Regarding the near-infrared absorption rate, when the film of the present invention is used in a magnetic disk, it is important that near-infrared rays do not pass through areas other than the index hole.
When measuring with a visible/infrared spectrometer, it is sufficient that the absorbed light of 800 to 900 mμ is 90% or more. This absorption rate is 93
It is even better if it is % or more. In order to obtain such characteristics, it is necessary to incorporate pigments with low brightness into the film. good. Since near-infrared absorption varies depending on the thickness, the amount added may be small if the film is thick. An appropriate near-infrared absorption rate can be selected through trial and error. The smaller the surface protrusions of the film, the better, and the smaller the difference in roughness between the front and back surfaces, the better. In the present invention, it is expressed as center line average roughness, which is JIS
According to the measurement method described in B0601-1976. For example, using a stylus type surface roughness meter (SURFCOM 3B) manufactured by Tokyo Seimitsu Co., Ltd., the radius of the needle tip is 2μ.
A chart of the film surface roughness curve is drawn under the condition of a load of 70 mg, and the measured length L is measured in the direction of the center line.
(0.25 mm), the center line of the part is the x-axis, the direction perpendicular to the x-axis is the Y-axis, and the roughness curve is expressed as Y=f(x), then the roughness average is: =1/ L∫ ^L _O f(x)dx, and the center line average roughness R _CLA can be expressed as RCLA=1/L∫ ^L _O |f(x)−|dx. This measurement is performed on 8 samples, and the average value of the 5 measured values excluding the 3 largest measured values is defined as R _CLA . In the present invention, the difference in center line average roughness between the front and back surfaces of the film, which has been subjected to stretching heat treatment while containing a pigment, is set to be 0.005 μm or less, more preferably 0.003 μm or less. In order to achieve this condition, it is necessary to sufficiently knead the pigment-containing polyester and to heat the front and back surfaces of the film to substantially the same temperature during the film-forming and stretching heat treatment. is necessary. In addition, it is possible to use fine particles for pigments.
This is advantageous in obtaining a film with few surface protrusions. From this point of view, when melt-extruding polyester containing a pigment, it is necessary to use a filter with an average opening of about 15 to 30 microns. Here, the average aperture means that when a filter is submerged in ethanol and bubbles are generated through compressed air, the following equation holds between the pressure H and the aperture d in the region where the air pressure is constant with respect to the air flow rate. Refers to cases where d=4r/g (PoH-Ph) where r: surface tension of ethanol h: depth of filter in ethanol Po: density of liquid used in manometer P: density of ethanol g: gravitational acceleration Eye opening is 30μ If it exceeds 50%, coarse particles will be mixed into the polyester, which tends to cause differences in the protrusions on the front and back sides of the film. Moreover, if a filter with an opening smaller than 15 μm is selected, it is inconvenient because the filter becomes clogged and the service life is shortened. As a means for reducing the temperature difference between the front and back sides of the film during stretching of the film, it is also effective to provide a large number of preheating rolls and arrange them so that the front and back sides alternately contact the roll surfaces. Items used as magnetic disks or copying masking must have a uniform expansion rate with respect to temperature and humidity. In particular, it is important for the thermal shrinkage rate to have a low absolute value and no directionality. One way to obtain such a film is to reduce the difference in physical properties in the width direction during manufacturing. For example, when stretching in the longitudinal direction, in the case of polyethylene terephthalate, selecting a stretching temperature of 95 to 100°C can reduce differences in physical properties. In the present invention, polyester containing a pigment with low brightness, such as carbon black, absorbs near infrared rays, so by heating the stretched portion using an infrared heater having a surface temperature of 800°C or higher,
Longitudinal stretching can be achieved under suitable conditions. As is well known, as the surface temperature of the heating element increases, the wavelength of the radiant energy peak becomes shorter and shifts from the far infrared to the near infrared to the visible region, but many thermoplastic polyesters Transmits a considerable amount of light in the near-infrared region. Therefore, even if the surface temperature of the infrared heater is increased, the thermal efficiency tends to decrease. However, the polyester used in the present invention has good near-infrared and visible heat absorption, so it can be rapidly heated with an infrared heater, the drawn wire is stable, and the stretched portion can be limited to a narrow range. The subsequent stretchability in the width direction is not impaired. When the temperature of the film is increased by heating the rolls, etc., the film tends to have uneven thickness due to adhesion between the rolls and the film, the drawing line tends to move easily, causing width fluctuations, and process stability is often poor. As mentioned above, when a film that has been stretched in the longitudinal direction at a relatively high temperature is stretched in the width direction, the temperature conditions and the stretching ratio are selected so as to obtain a degree of orientation that is approximately the same as in the longitudinal direction. Measurement of thermal expansion coefficient is effective as an index of the degree of orientation. When the degree of orientation is biased in one direction, the coefficient of linear (temperature) expansion in the direction where the degree of orientation is high decreases, and the coefficient of linear expansion in the direction perpendicular to the direction increases. The value obtained by dividing the difference between the maximum and minimum values of each position and each direction of the film with these coefficients of linear expansion by the maximum value is 10%.
It is important to make adjustments so that it is within this range. This adjustment is completed by measuring the thermal expansion coefficient in each direction of the film while appropriately changing the stretching ratio and stretching temperature in the longitudinal direction and the width direction, and setting suitable conditions. Note that in order to obtain a film with little difference in expansion coefficient in each direction, it is not sufficient to have the same degree of orientation in the longitudinal direction and in the width direction; it is necessary to reduce the difference in physical properties in the width direction. . As a means for this purpose, there is a method in which the temperature of the film after being stretched in the width direction is once cooled to below the glass transition point and then subjected to tension heat treatment, or
It is possible to choose a method in which the film, which has been subjected to tension heat treatment at a relatively low temperature of .degree. C. and passed through a tenter, is further heat treated using a heating roll. When an imaginary straight line is drawn in the width direction of the film stretched in the longitudinal direction, and this is stretched in the lateral (width) direction and then subjected to tension heat treatment, the imaginary curve bends, resulting in so-called bowing. There is a tendency to exhibit In the present invention, by reducing this bowing, a film with small differences in physical properties in the width direction is obtained. As described above, if the stretching temperature is set high during stretching in the longitudinal direction, bowing will be reduced, making it relatively easy to obtain the film of the present invention. In addition, since it is complicated to measure physical properties (refractive index, shrinkage rate, temperature expansion coefficient, humidity expansion coefficient, etc.) at every position and every direction, in reality, measurements are taken along the width direction of the film, near the center of the film, and so on. Differences in physical properties can generally be estimated by measuring 6 to 12 points at intervals of 15 to 30 degrees as representative values for each direction at several locations, including locations near both ends. Isotropy index is 1≦10 near the center
%, there is a tendency for it to increase monotonically as it shifts to both sides. The film of the present invention has extremely small differences in physical properties on the front and back sides of the film, in its position, and in its direction. Therefore, it is suitably used for films that require isotropy and have near-infrared absorption ability. A typical application for this is magnetic disks, which are characterized by excellent performance and high dimensional stability even under environmental changes. Next, the invention will be further explained with reference to examples. [Example 1] Polyethylene terephthalate chips without carbon black and polyethylene terephthalate chips containing 4% by weight of carbon black were mixed, and the total amount of carbon black was 0.5.
% by weight, melted in an extruder, and made into Eugene's 10G grade (average opening).
It is extruded through a T-die while passing through a 25μ) filter, formed into a film on the surface of a quenching drum, heated with a preheated roll at 75°C, and placed between this roll and another roll with a different circumferential speed. The film was rapidly heated from both sides with a silicon carbide heating element heated to a surface temperature of 1000°C, stretched in the longitudinal direction at a stretching ratio of 3.6 times, and then stretched 3.9 times in the width direction at a temperature of 105°C to obtain a 75μ film. A biaxially stretched film was obtained. After that, the surface temperature of the film immediately after width stretching was cooled to 80℃ by blowing cold air, and then heat treated in an atmosphere of 230℃, and then brought to an atmosphere of 215℃ and stretched by 10% in the width direction. Heat treated at 100℃
A biaxially stretched film is obtained by applying relaxation heat treatment in an atmosphere of
The sample was run at a relatively low tension while being reheated, and then cooled to near room temperature using rolls and wound up to provide a sample for an example. The resulting physical properties were as shown. [Comparative Example 1] In Example 1, the surface temperature of the infrared heater was
A 75μ biaxially stretched film was obtained in the same manner as in Example 1 except that the temperature was 750°C and the longitudinal stretching ratio was 3.4 times. [Comparative Example 2] In Example 1, fine silica powder (average particle size 1.0μ) was used without adding carbon black.
A 75μ biaxially stretched film was obtained in the same manner as in Example 1, except that 0.3% by weight was added and a Eugene 15G filter (average opening 40μ) was used. A comparison of these physical properties is shown in the table below. The near-infrared (800 to 900 mμ) absorption rates of the films of Example 1 and Comparative Examples 1 and 2 were both 95%.
It was hot.

[Table] The film of Example 1 of the present invention had a small isotropy index of 4%, and there was almost no difference in roughness between the front and back surfaces, and was found to be of high quality as a magnetic disk.

Claims (1)

[Claims] 1. Polyester containing a pigment with low brightness is melted, passed through a filter with an average opening of 15 to 30μ, extruded to form an unstretched film, and the unstretched film is stretched in biaxial directions. By heating both sides of the film to substantially eliminate the temperature difference between the front and back sides of the film, further equalizing the degree of stretching orientation in the biaxial directions, and then applying heat treatment to homogenize the thermal properties in the biaxial directions. The obtained near-infrared absorption rate of 800 to 900 mμ is 90% or more, the difference in center line average roughness between the front and back surfaces is 0.005μ or less, and the isotropy index of the film plane is
Biaxially oriented near-infrared absorbing polyester film within 10%. Here, the isotropy index I means, when there is a direction showing the maximum value A of the thermal expansion coefficient and a direction showing the minimum value B of the thermal expansion coefficient on the film plane, I=A-B/A×100 (%)