JPH0156654B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a polyester film having a flat surface and a low coefficient of friction. Polyester films have a variety of uses, including magnetic tape applications and electrical applications. In magnetic tape applications, particularly video tape applications, a smooth film surface is required to improve electromagnetic conversion characteristics, and a smooth film surface is required to improve the tape's runnability on a deck, abrasion resistance, and durability. A low coefficient is required. Conventionally, as a technique for reducing the coefficient of friction of a film, there has been known a method of imparting irregularities to the surface of the film by forming into a film a polymer to which inorganic particles have been added or a polymer in which insoluble catalyst residue particles have been generated. It is being This means provides protrusions on the film surface to reduce the contact area between the film and the object with which it comes into contact, thereby reducing frictional resistance. All of these methods actively create protrusions on the surface of the film, and it is effective to create a large number of high protrusions on the film surface in order to reduce the coefficient of friction. However, in this case, although the coefficient of friction can be lowered as the number of tall protrusions increases, there is a strong possibility that when magnetically coated, the protrusions will have an effect on the coated surface, deteriorating the electromagnetic conversion characteristics. As a result of extensive research into a base film that has excellent electromagnetic conversion characteristics and is suitable for slippery magnetic tapes with a low film friction coefficient, the present inventor has developed a polyester film having convex portions and concave portions on its surface. The present invention was achieved based on the finding that this problem can be solved by forming a large number of fine concavo-convex units and giving the concave-convex units directionality. That is, the present invention provides a polyester film having a large number of uneven units consisting of protrusions and depressions with the protrusions as cores on the film surface, and the protrusions are caused by particles existing inside the film. , the depression has a pseudo-elliptical shape with a major axis along the longitudinal direction (machine direction) of the film, and the major axis of the depression is in the range of 2 μm to 50 μm, and the major axis D is in the range of 2 μm to 50 μm.
(μm) and the occurrence frequency N (pieces/mm ² ) of unevenness units: 2≦D<5 200≦N<3500, 5≦D<10 150≦N<2000, 10≦D <30: 50≦N<800;30≦D<50: 0≦N≦5. The present invention will be explained. In order to obtain a slippery film in the prior art, inert inorganic compound particles (e.g. silica, clay, titania, etc.) or organic compound particles (e.g. calcium terephthalate, high melting point polyester) are added or (and) catalyst residues are added. Protrusions (protrusions) are created on the film surface using particles from
In contrast, the present invention is characterized in that a pseudo-elliptical uneven unit consisting of convex portions and concave portions is formed on the film surface, and the major axis of the pseudo-ellipse is oriented in the longitudinal direction of the film. A film having protrusions and depressions as in the present invention has the advantage that it has a significantly lower coefficient of friction and a smoother slipping effect than a film obtained by the prior art having only protrusions on its surface. The polyester to which the present invention can be applied is a combination of an aromatic dibasic acid component such as terephthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, etc. and a glycol component such as ethylene glycol, tetramethylene glycol, neopentyl glycol, etc. A polymer or copolymer obtained by condensation polymerization. Representative polymers include homoprimers such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalene dicarboxylate, partially modified copolymers of these, and polyethylene terephthalate [ethylene terephthalate/polyethylene glycol].
Examples include polymer blends to which block copolymers are added. Of course, fillers, pigments, colorants, antioxidants, light stabilizers, etc. can also be added to the polymers and copolymers. Films obtained from these materials are included in the polyester film of the present invention. The protrusions formed on the surface of the film of the present invention are due to the presence of particles of an inorganic compound added to the polymer; particles based on insoluble catalyst residues produced during polymerization of the polymer; or particles of both. The depressions having the protrusions as the core according to the present invention are not depressions formed by conventional mechanical stamps such as embossing, but are produced by deformation of the film itself during the process of stretching the film. When an unstretched film containing particles is stretched in the width direction using a conventional technique, the polymer is plastically deformed without deforming the particles, so that voids are generated at the boundary between the polymer and the particles during large deformation (stretching). When this film containing voids is then stretched in a direction approximately perpendicular to the first axis stretching direction (machine direction) to form a biaxially oriented film, the voids that were generated during the first axis stretching are further deformed in the machine direction. As shown in FIG. 1-1, a void 22 is formed around the protrusion 21 in a quasi-circular shape. In this case, as shown in the cross-sectional view of Figure 1-2, the particles existing in the shallow part near the film surface and the voids around them produce protrusions with the particles as the core.
No depressions are formed around the particles. In the present invention, the above voids are changed to depressions on the film surface. When stretching an unstretched film uniaxially (in the width direction), the film after the first axial stretching is Substantially no voids are formed around the particles (external particles due to inorganic additives or internal particles containing catalyst residues). Next, when the stretched film in this state is stretched in the machine direction, depressions (indentations) of the film with particles as cores are formed along the longitudinal direction. The major axis of the elliptical depression is along the longitudinal direction. Even if a slight void is formed around the particles during the first axial stretching, a depression will be formed with the particles as the nucleus. The surface of the film after biaxial stretching is in a state as shown in Figure 2-1 (top view), and if the stretching conditions are such that stress is concentrated around the particles during the second axial stretching, the depressed portions will be at the level of stress concentration. Depending on the depth, the second
The major axis tends to increase along the axial direction. Figure 2
-2 (cross-sectional view) shows a cross section of the film near the surface, in which the protrusions 21 containing particles and the depressions 24 formed around the protrusions 21 form the polyester film 23.
occurs in In the present invention, the depression formed around the protrusion includes a pseudo-elliptical shape eccentric in the second axis direction. In this case, if the most eccentric long axis of the depression is referred to as the major axis, it is necessary that the major axis of the depression be at least 2 μm from the viewpoint of improving the running properties of the magnetic tape and improving the electromagnetic conversion characteristics. Furthermore, if the major axis of the depression exceeds 50 μm, dropout of the magnetic tape increases, making it undesirable as a base film for a magnetic tape. According to the present invention, between the long diameter D (μm) of the depression of the uneven unit on the surface of the polyester film and the occurrence frequency N (pieces/mm ² ) of the uneven unit, 2≦D<5 200≦N<3500 , 5≦D<10 150≦N<2000, 10≦D<30 50≦N<800, 30≦D<50 0≦N≦5. The electromagnetic conversion characteristics are also excellent. More preferably, 2≦D<5, 350≦N<2500, 5≦D<10, 250≦N<1500, 10≦D<30, 100≦N<500, and 30≦D<50. When the condition of 0≦N≦3 is satisfied, the base film has excellent running properties and electromagnetic properties. According to the present invention, it is understood that the depressions on the film surface reduce frictional resistance by reducing the contact area. Although the reason for the directionality of the depression is not clear, it is particularly related to the frictional resistance with the metal pin, and the frictional resistance is low when the longitudinal direction of the depression substantially coincides with the longitudinal direction of the film. The concavo-convex unit in the present invention consists of one protrusion and a depression around the protrusion. The size and frequency of occurrence of these uneven units can be controlled by the type of particles, the amount present in the polymer, and the stretching conditions of the film. A specific method for stretching the polyester film of the present invention will be explained. When stretching an unstretched polyester film containing 0.01 to 2.0% by weight of fine particles such as kaolin and silica with an average particle diameter of 0.05 to 5 μm (preferably 0.1 to 2.5 μm) in the first axis direction (width direction), Preheat to 90-120°C. When this unstretched film is preheated to about 90 DEG to 120 DEG C., a hard chromium-plated roll or a ceramic roll with a matte surface is preferred. The unstretched film can reach a predetermined pre-temperature without sticking to the roll surface and without substantial crystallization. Of course, the unstretched film can be preheated in a non-contact manner. The unstretched film is stretched at a temperature of 100 to 135°C at a stretching ratio of 3.5 times or less (preferably 2.6 to 3.4 times). Next, in the second stretching, the uniaxially oriented film is first cooled below the glass transition point, or preheated to a temperature of 100 to 150°C without cooling, and then stretched in the second axial direction at approximately the same temperature. Stretch 3.0 to 4.0 times (preferably 3.2 to 3.8 times). When the temperature of the second axis stretching is high, the boundaries between the concave portions of the concave and convex units become clear, but at low temperatures, the boundaries are often not clear. The stretching ratio in the second axis direction does not significantly affect the frequency of occurrence of uneven units. However, if a second axis stretching ratio of 3.8 times or more is selected, the mechanical strength in the second axis direction (Young's modulus) will be higher than the mechanical strength in the first axis direction, resulting in a so-called tensilized film. There is a tendency for the slipperiness to decrease. In the present invention, it is most preferable that the direction in which the first axial stretching is performed is substantially perpendicular to the longitudinal direction of the film, and the direction in which the second axial stretching is performed is in the longitudinal direction of the film. However, before and after the first axial stretching and after the biaxial stretching, it is permissible to stretch at a low magnification in a direction different from the above stretching direction as long as the direction of the depressions is not disturbed. The film stretching conditions that form such surface depressions tend to make the film surface relatively flat and smooth, resulting in improved electromagnetic characteristics as a magnetic tape. That is, the polyester film of the present invention has the ability to be used as a base film for magnetic tapes, since the film surface is relatively flat, so that it does not cause dropouts or color noise when a magnetic recording layer is provided, and it also has the ability to prevent protrusions. Since the circumference has a depression, the contact area of the tape with the magnetic head, guide roll, and other films is further reduced, and the major axis of the depression runs along the longitudinal direction of the film, which is perpendicular to the axial direction of the guide roll. Since they are equivalent, there is an advantage that the slipping effect becomes even higher. These slippery effects are limited to those with only normal protrusions,
Alternatively, a pseudo-elliptical shape in which the major axis of the depression runs along the width direction of the film cannot be obtained even if it is provided with a concavo-convex unit. The method for measuring physical properties in the present invention is as follows. (1) Measuring method for uneven parts A film surface with a thin layer of aluminum vapor deposited is photographed using a differential interference microscope device (for example, Nikon differential interference microscope R type), and its size is measured on a scale. (2) Surface roughness CLA Surface roughness CLA (Center Line
Average) value is measured by the following method. Using a stylus-type surface roughness meter (SURFCOM3B) made by Tokyo Seimitsu Co., Ltd., for example, obtain the film roughness curve of the roughened film under the conditions of a needle radius of 2 μm and a load of 70 mg, and use this to determine the measurement length. L
(Reference length 2 mm) is extracted, the center line of this extracted portion is the X-ray, the vertical magnification direction is the Y axis, and the roughness curve is expressed as Y=f(x).
The value given by the following formula is expressed in μ. CLA=1/L∫ ^L _O f(x) dx This measurement is performed on eight samples, the three with the largest values are excluded, and the average value of the five is expressed. Note that measurements are made in the vertical and horizontal directions, and the average value of both is used. (3) Coefficient of friction As shown in Figure 3, 18-8 stainless steel with an outer diameter of 5 mmφ was measured at a room temperature of 25°C and a relative humidity of 60%.
1/ to SUS304 fixing rod (surface roughness CLA=0.030)
Wrap the film cut to 2 inches wide at an angle of π
They are brought into contact in radians and moved at a speed of 3.3cm/sec. When the tension controller 2 is adjusted so that the inlet tension T ₁ (detected by the inlet tension detector 5) is 30 g, the outlet tension T ₂ g (detected by the outlet tension detector 10)
The dynamic friction coefficient μk is calculated using the following formula. In the present invention, the coefficient of dynamic friction when traveling 90 m is defined as μk. μk=1/πlnT ₂ /T ₁ (4) Chroma S/N The magnetic coating tape is measured by the following method. A signal obtained by superimposing a 100% chroma level signal on a 50% white level signal is recorded using a commercially available home VTR, and the reproduced signal is measured using a Shibasoku noise meter 925C. In addition, Chroma S/
The definition of N is as follows according to Shibasoku's definition. Chroma S/N = 20log ES (p-p) / EN (rms) (dB) However, ES (p-p) = 0.714V (p-p) EN (rms) = AM noise effective value voltage (V) Also magnetic The powder coating is created in the following way. The magnetic powder coating shown below is applied using a gravure roll, and the magnetic coating layer is smoothed using a doctor knife to form a magnetic layer of about 5 μm. While the magnetic paint is still dry, it is magnetically oriented by a conventional method and then introduced into an oven for dry curing. Further, it is calendered to make the coating surface uniform and create a 1/2 inch wide tape. Composition of magnetic paint γ-Fe ₂ O ₃ powder 100 parts by weight Eslec A (Sekisui Chemical Co., Ltd., vinyl chloride/vinyl acetate copolymer) 16 〃 Hiker 1432J (Nippon Zeon Co., Ltd., butadiene acrylonitrile copolymer) 11 〃 Lecithin 1 〃 Carbon 8 MEK 100 MIBK 100 Additive (lubricant, silicone resin) 0.15 The present invention will be explained in more detail with reference to Examples below. Examples 1 to 4 After drying polyethylene terephthalate with an intrinsic viscosity of 0.62 dl/g (value measured at 35°C using orthochlorophenol as a solvent) at 160°C and containing 0.24% clay with an average particle size of 0.7 μm.
The mixture was melt-extruded at 280°C and rapidly solidified on a casting drum kept at 50°C to obtain an unstretched film of 160 μm. Subsequently, this unstretched film was preheated to 80°C and then subjected to one step of stretching in the transverse direction using a tenter in a conventional manner. Furthermore, this uniaxially stretched film was again preheated to 90°C using two preheating rollers, then stretched longitudinally to 3.3 times between two rollers while being heated with an infrared heater, and then stretched to 190°C.
Heat treatment was performed at ℃. Here, the infrared heater conditions and stretching ratio during transverse stretching were changed to form a film and the film as shown in the table was obtained. The depressions present on the surface of the film of Example 2 are the fourth depressions.
As shown in the figure, the film had a pseudo-elliptic shape with its major axis along the longitudinal direction of the film, that is, the longitudinal stretching direction. Comparative Examples 1 to 4 Obtained using the same raw materials and the same method as Examples 1 to 4
An unstretched film of 160 μm was preheated to 80° C. and then stretched one step in the longitudinal direction using a tenter in a conventional manner. Furthermore, this film was heated horizontally at a temperature of 120℃.
It was stretched 3.3 times and then heat treated at 190°C. Films were formed by changing the heating conditions of the infrared heater and the stretching ratio during transverse stretching, and the films shown in the table were obtained. The depressions present on the surface of these films were pseudo-ellipsoidal with a major axis along a direction substantially perpendicular to the longitudinal direction of the film, that is, along the lateral stretching direction. Comparing the table and table types, it can be seen that the table type has a lower coefficient of friction against metal. The chroma S/N standard is based on Example 4.

【table】

[Table] Comparative Example 5 Obtained using the same raw materials and the same method as Examples 1 to 4
An unstretched film of 210 μm was preheated to 80° C. using two preheating rollers, and then stretched in one step to 3.6 times in the longitudinal direction by a conventional method while heating the film to 90° C. using an infrared heater. Furthermore, this film was stretched 3.9 times in the transverse direction at a temperature of 105°C, and then stretched at 190°C.
Heat treatment was performed. On the surface of this film, there were no depressions as in Examples 1 to 4, and most of the film had only convex protrusions, and there were approximately circular voids around the additive particles as shown in Figure 1. Ta. The surface roughness CLA of this film was 0.035 μm, which was rougher than in Examples 1-4, but the friction coefficient μk was 0.42, which was considerably higher than in Examples 1-4. Comparative Example 6 Obtained using the same raw materials and the same method as Examples 1 to 4
An unstretched film of 310 μm was preheated to 80° C. using two preheating rollers, and then stretched once in the longitudinal direction by a factor of 3.0 in a conventional manner while heating the film to 90° C. using an infrared heater. Furthermore, this film was stretched 3.4 times in the transverse direction at a temperature of 105°C, and then further stretched 2 times.
The film was preheated to 80°C using a book roller, then stretched 2.0 times in the machine direction while being heated to 100°C using an infrared heater, and then heat treated at 190°C. On the surface of this film, there are no depressions as recognized in the present invention, and there are mostly only convex protrusions, and there are pseudo-elliptical voids around the additive particles in the longitudinal direction of the film with the additive particles as the center. exists,
Therefore, the shape of the protrusions on the surface of the film formed a convex portion having directionality along the longitudinal direction of the film. The surface roughness CLA of this film was 0.029 μm, which made the surface rougher than in Examples 1-4, but the coefficient of friction was 0.44, which was considerably higher than in Examples 1-4. Comparative Examples 7 to 9 Films were formed under the same conditions as Examples 1 to 4, except for the preheating stretching temperature and stretching ratio during lateral stretching, and the results shown in the table were obtained. It was found that the friction coefficient and/or chroma S/N were inferior compared to Examples 1 to 4.

【table】 [Brief explanation of drawings]

Figure 1 shows the state of voids created around particles when stretched using the conventional method, and Figure 1-1 is a plan view;
1-2 is a cross-sectional view. FIG. 2 shows a polyester film of the present invention, in which projections containing particles and depressions are formed around the projections, FIG. 2-1 is a plan view, and FIG. 2-2 is a cross-sectional view. Figure 3 is a schematic diagram of a tape-based inspection machine that measures the coefficient of dynamic friction μk of a rough film surface. Moreover, FIG. 4 is a micrograph of the surface of the polyester film of Example 2 of the present invention (magnification: 900 times).

Claims (1)

[Scope of Claims] 1. A polyester film in which a large number of uneven units consisting of protrusions and depressions with the protrusions as cores are formed on the film surface, and the protrusions are caused by particles existing inside the film. The depression has a pseudo-elliptical shape with a major axis along the longitudinal direction of the film, and the major axis of the depression is 2 μm to 50 μm.
In the range of 2≦D<5, 200≦N<3500, and 5≦D<10, 150 between the major axis D (μm) and the occurrence frequency N (pieces/mm ² ) of unevenness units. An easily slippery polyester film characterized by satisfying the following relationships: ≦N<2000, 10≦D<30 50≦N<800, 30≦D<50 0≦N≦5.