JPH0156655B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a polyester film having a smooth 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 reduced as the number of tall protrusions increases, when magnetic coating is applied, the effect of the protrusions appears on the coated surface, and there is a strong possibility that the electromagnetic conversion characteristics will be deteriorated. 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. They discovered that this problem could be solved by forming a large number of fine uneven units, and further discovered the relationship between the intrinsic viscosity of the film and the formation of unevenness on the film surface, and arrived at the present invention. That is, in the present invention, the intrinsic viscosity of the film is from 0.42 to
A polyester in which unevenness units consisting of protrusions and depressions with a major diameter of at least 4 μm around the protrusions are formed on the film surface at a frequency of 400 pieces/mm ² or more in the range of 0.55 dl/gr. It's a film. The present invention will be explained. The polyester to which the present invention can be applied refers to the condensation polymerization of aromatic dibasic acids such as terephthalic acid, isophthalic acid, and naphthalene-2,6-dicarboxylic acid with glycols such as ethylene glycol, tetramethylene glycol, neopentyl glycol, etc. refers to the polymer or copolymer obtained in this manner. Representative polymers include homopolymers such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalene dicarboxylate, partially modified copolymers thereof,
An example is a polymer blend in which a block copolymer (polyethylene terephthalate, polyethylene glycol) is added to polyethylene terephthalate. 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 inorganic particles added to the polymer; particles based on insoluble catalyst residues produced during polymerization of the polymer; or particles of both. In the present invention, the depressions that can be generated around the protrusions with the protrusions as the nucleus are not concave-shaped depressions caused by conventional mechanical stamps such as embossing, but are caused by the deformation of the film itself during the process of stretching the film. This is what happens because of this. When an unstretched film containing particles is stretched in a uniaxial direction, the particles cannot be deformed and the polymer is plastically deformed, 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 uniaxial stretching direction (second axial direction) to form a biaxially oriented film, the voids that were generated during uniaxial stretching are further deformed in the second axial direction. , as shown in Figure 1-1, the protrusion 2
A void 22 is formed around 1 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, but no depressions are formed around the particles. . In the present invention, the above voids are changed to depressions on the film surface. When an unstretched film is uniaxially stretched, by preheating the film before stretching to a high temperature or (and) by setting a low stretching ratio, the film that has undergone the first axial stretching has particles (inorganic Voids are substantially not formed around external particles due to additives or internal particles containing catalyst residues. Next, when the stretched film in this state is stretched in the second axial direction, depressed portions (depressions) of the film with particles as cores are formed along the second axial 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 the more stress is concentrated around the grains during the second axial stretching, the deeper the recesses become. It tends to become longer along the biaxial directions. Figure 2-2 (cross section) shows a cross section of the film near the surface, and shows projections 2 containing particles.
1 and a depression 24 formed around the polyester film 23. In the present invention, the depressions formed around the protrusions include pseudo-ellipsoidal depressions that are eccentric in the second axis direction. In this case, if the most eccentric long axis of the depression is referred to as the long axis, it is necessary that the long axis of the depression be at least 2 μm from the viewpoint of improving the electromagnetic conversion characteristics and running properties of the magnetic tape. According to the present invention, the depressions on the film surface are understood to reduce frictional resistance by reducing the contact area. The film stretching conditions that form such surface depressions have the effect of making the film surface relatively smooth, resulting in improved electromagnetic characteristics as a magnetic tape. The polyester film of the present invention has protrusions on its surface that provide slipperiness. Furthermore, since these protrusions have depressions around them, they tend not to cause dropouts or color noise when a magnetic recording layer is provided on the surface of the film as a base film for magnetic tape. Another advantage of this tape is that the contact area with the magnetic head and other films is further reduced, and even with low protrusions, the sliding effect is enhanced. In the present invention, the direction in which the first axial stretching is applied may be either the machine direction or the width direction of the film. Further, the second axial stretching direction is preferably approximately perpendicular to the first axial direction. Of course, high-stage (multi-stage) stretching in which stretching is further performed in the first axial direction and/or the second axial direction can be performed. In this case as well, the protrusions and depressions on the film surface are
Even if the shape of the uneven unit is slightly deformed, it remains as it is, so that the electromagnetic conversion characteristics of the magnetic tape and the runnability (low coefficient of friction) of the film (tape) are maintained. 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. In order to maintain running properties as a magnetic tape, the size of the recessed portion must be at least 4 μm in major axis (see FIG. 2), preferably at least 8 μm. Furthermore, considering the size of the concave portion, it is preferable that the size of the concave portion is as small as possible, in consideration of signal dropout (dropout) in the video tape. The number of uneven units is required to be 400 units/mm ² or more, preferably 800 units/mm ² or more. The size of these depressions and the frequency of occurrence of uneven units can be observed by depositing a thin layer of aluminum on the surface of the stretched film and then taking a photograph using a differential interference microscope (for example, Nikon differential interference microscope R type, magnification: 900x). be able to. On the other hand, the intrinsic viscosity of the film that satisfies the preferred frequency of occurrence of surface unevenness units of the present invention is 0.42 to 0.42 when measured at 25°C with an orthochlorophenol solvent.
It is in the range of 0.55 dl/g, preferably in the range of 0.46 to 0.53 dl/g. As the intrinsic viscosity of the film is lowered, when unstretched film is uniaxially stretched, the stretching tension decreases at the same stretching ratio, stretching temperature, and stretching speed, so that voids are formed around the particles in the film after first axial stretching. is substantially less likely to be formed, and then on the surface of a biaxially stretched film obtained by stretching this uniaxial film in the second axial direction,
The number of uneven units having depressions with a major axis of 4 μm or more increases. At the same time, the center line roughness (CLA) of the surface is reduced and the coefficient of friction is also reduced, thus providing good properties as a base film for magnetic tape. If the intrinsic viscosity of the film is lower than 0.42 dl/g, the mechanical properties of the film in the longitudinal direction (Young's modulus, strength at 5% elongation) will decrease, so when used as a magnetic tape, This may cause the tape to stretch, resulting in defects in practical properties. When the intrinsic viscosity of the film exceeds 0.55 dl/g,
It becomes difficult to obtain a film characterized by a flat surface and a low coefficient of friction as used in the present invention. In order to avoid this, there are measures to reduce the stretching speed and increase the stretching temperature in the first axial stretching.
It is undeniable that both are disadvantageous from the perspective of industrial productivity. In this way, the intrinsic viscosity of the film can be adjusted to 0.42~0.55.
By setting the dl/g range, it is possible to set a high drawing speed, and it is possible to significantly improve industrial productivity. The present invention will be specifically explained below using Examples.
The method for measuring physical properties in the present invention is as follows. (1) Measuring method for uneven parts A photograph of a film surface with a thin layer of aluminum vapor deposited is taken using a Nikon differential interference microscope (R type), and its size is measured using a scale. (2) Surface roughness CLA Surface roughness CLA (Center Line
Average) value is measured by the method below. 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. SaL
(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 However, it represents the center line and is determined by =1/L∫ ^L _O f(x)dx. This measurement was performed on 8 samples,
The three highest values are excluded and the average value of the five values is expressed. Note that the measurement is performed in the vertical direction and the horizontal direction, and the average value of both directions is used. (3) Coefficient of friction As shown in Figure 4, a fixed rod of SUS304 with an outer diameter of 5 mmφ (a film cut into a 1/2 inch width with a surface roughness CLA = 0.030 μm) was installed at a room temperature of 25°C and a relative humidity of 60%. They are brought into contact at a winding angle of π radian, and moved and rubbed at a speed of 3.3 cm/sec.The exit 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 dynamic friction coefficient μk is calculated from the tension T ₂ g (detected by the exit tension detector 10) using the following formula. In the present invention, the dynamic friction coefficient when traveling 90 m is defined as μk. μk = 1/πlnT ₂ /T ₁ implementation Examples 1 and 2, Comparative Examples 1 and 2 Containing 0.3% by weight of kaolin with an average particle size of 0.7 μm, intrinsic viscosity 0.53 dl/g (value measured at 25°C using orthochlorophenol as a solvent)
After drying the polyethylene terephthalate at 160℃, it was melt extruded at 280℃ and rapidly solidified on a casting drum kept at 50℃.
An unstretched film of m was obtained. Subsequently, this unstretched film was passed through four heating rollers 31, 32, 33 and 34 as shown in FIG.
The film was further heated with an infrared heater, and then stretched one step in the longitudinal direction between rollers 34 and 35. This film was then heated to 120℃.
Stretched 3.6 times in the transverse direction at a temperature of 210
Heat treatment was performed at ℃. In addition, the stretching speed at this time is
It was 200m/min. Further, the intrinsic viscosity of the film after film formation was 0.51 dl/g. Here, while changing the preheating temperature of the heating rollers 31 to 34 during longitudinal stretching and the conditions of the infrared heater 38, film forming and stretching was performed while changing the film temperature immediately before the roller 35 and the stretching ratio. I got a film like this.

[Table] As shown in Table 1, the lower the longitudinal stretching ratio and the higher the film temperature during longitudinal stretching, the greater the number of depressions (uneven units) and the smoother the surface becomes. I understand. Examples 1 and 2 are Comparative Example 1
-2, it is characterized by a low coefficient of friction despite the low surface roughness CLA. A surface photograph of Example 1 (magnification: 900 times) and a surface photograph of Comparative Example 1 (magnification: 900 times) are shown in FIG. 6 and FIG. 5, respectively. It was observed that the surface of the film of the present invention had uneven units as shown in Figure 6. Examples 3-4, Comparative Examples 3-6 Polyethylene terephthalate containing 0.2% by weight of kaolin with an average particle size of 0.6 μm was heated at 160°C.
After drying, the film 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. After preheating this unstretched film with the four heating rollers 31, 32, 33, and 34 shown in Figure 3, the film is heated with an infrared heater, and then the film is heated by 3.2 mm in the longitudinal direction between rollers 34 and 35.
Stretched once to double. At this time, the heating roller 31~
While changing the temperature of 34 and the conditions of the infrared heater 38, the temperature of the film immediately before the roller 35 is 125.
℃. Furthermore, the film was stretched 3.6 times in the transverse direction at a temperature of 120°C, and then heat-treated at 210°C. Here, the intrinsic viscosity of polyethylene terephthalate (using orthochlorophenol as a solvent,
Film-forming and stretching were carried out by changing the value measured at 25° C.) and the stretching speed (speed of the roller 35) to obtain films as shown in Table 2.

[Table] As shown in Table 2, the lower the intrinsic viscosity of the film, the greater the number of depressions (uneven units) on the surface.
It can be seen that the surface becomes smoother as a result. Examples 3 and 4 have a low surface roughness CLA and a low coefficient of friction, so they have excellent quality as base films for magnetic tapes, and can be formed at relatively high stretching temperatures, resulting in high productivity. can ensure sex. On the other hand, Comparative Example 3 has the characteristics of low surface roughness CLA and low coefficient of friction, but is not suitable as a base film for magnetic tape because of its low F-5 value in the longitudinal direction of the film. Moreover, Comparative Examples 4 and 6 have a high intrinsic viscosity, so compared to Example 3, there are no surface characteristics. Comparative example 5
Compared to Comparative Example 4, if the stretching speed is lowered, the surface roughness CLA is lower and the coefficient of friction is lower.
This is not desirable because productivity decreases. Example 5 and Comparative Example 7 A conventional method using 200 parts of calcium acetate, 150 parts of lithium acetate as a transesterification catalyst, 450 parts of antimony trioxide as a polymerization catalyst, and 1450 parts of trimethyl phosphate as a stabilizer per ⁶ parts of polyethylene terephthalate 10 Polyethylene terephthalate was polymerized. This polymer contained many internally precipitated particles and had an intrinsic viscosity of 0.52 dl/g in an orthochlorophenol solution at 25°C. This polyethylene terephthalate was dried at 160°C, then melt-extruded at 280°C, and then rapidly cooled and solidified on a casting drum kept at 50°C.
An unstretched film of 160 μm was obtained. Subsequently, the unstretched film was stretched one step in the longitudinal direction using the stretching equipment shown in FIG. Subsequently, the film was stretched 3.5 times in the transverse direction at a temperature of 125°C, and then heat treated at 210°C. In addition,
The stretching speed at this time was 200 m/min. Also, the intrinsic viscosity of the film in this example is
It was 0.50dl/g. At this point, film-forming stretching was carried out while changing the longitudinal stretching ratio, and films as shown in Table 3 were obtained.

[Table] As shown in Table 3, although Example 5 has a lower surface roughness CLA than Comparative Example 7,
Features a low coefficient of friction. Comparative Example 8 In Comparative Example 1, kaolin with an average particle size of 0.6 μm was used.
The frequency of unevenness units with a length of 4 μm or more in the film produced under the same conditions except that the content was 0.18% by weight was 0 units/ ^mm2 , and the surface roughness CLA was
It was 0.027. The friction coefficient μk of this film is
Although the surface roughness is as high as 0.50, which is almost the same as in Example 1, there is no smoothing effect.

[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. FIG. 3 is a schematic diagram of a stretching machine used in an example of the present invention. Figure-
4 is a schematic diagram of a tape base inspection machine that measures the dynamic friction coefficient μk of a film rough surface. FIG. 5 is a microscopic photograph showing the surface of a polyester film according to the prior art, and FIG. 6 is a microscopic photograph of the surface of the polyester film of the present invention (both magnifications are 900 times).

Claims (1)

[Claims] 1 The intrinsic viscosity of the film (value measured at 25°C using orthochlorophenol as a solvent) is 0.42
~0.55 dl/gr, there is an uneven unit on the film surface consisting of a protrusion and a depression with the protrusion as a nucleus, and the long axis of the depression in the uneven unit is at least 4 μm.
A polyester film with 400 or more uneven units/ ^mm2 .