SPECIFICATIONPolyester film for magnetic recording mediaThis invention relates to biaxially orientated polyester films suitable for manufacturing magnetic recording media such as audio or video magnetic tapes, magnetic video discs and floppy discs. More particularly, it relates to polyester films for magnetic recording media provided with excellent flatness, slipperiness, sliding durability and adhesion of magnetic layers thereto. Further, the invention relates to polyester films provided with excellent mechanical properties in addition to the above-mentioned properties.
Biaxially orientated polyester films have excellent mechanical, thermal and electrical properties and chemical resistance, and are used in various fields. Particularly, they are suitably used as base films for magnetic tapes. As the characteristics of the base film for magnetic tapes strongly influence the quality of the magnetic tape manufactured therefrom, the demand for base films of higher quality has been increasing steadily in recent years along with progress in magnetic tape technology. Base films provided with further improved flatness are required in order to improve such electromagnetic characteristics as video S/N, chroma S/N, dropout and generation of envelope of video magnetic tapes, for example.
On the other hand, base films which are simultaneously provided with an improved slipperiness, durability and surface flatness are desired for improving the appearance of wound tapes and their sliding property, as well as wear resistance.
In conventional base films, however, there has been a problem that if the number of minute proturbances formed on the surface of films and the height thereof are increased in order to improve the sliding property and durability of films, the surface flatness is impared thus resulting in a deterioation in the electromagnetic characteristics. In order to overcome this problem, an attempt has been made to provide the film surface with a roughness formed by protuberances accompanied by recesses. (Refer to Japanese Laid-Open PatentPublication Nos. 66936/82,167215/82, 167216/82, 189822/82, and 116173/74.) However, it has been pointed out that these films are inferior in durability in uses where the tapes are subject to repeated sliding under severe hot and humid conditions. (Japanese Laid-Open Patent Publication No. 167,216/82).
With the intention of solving the above-mentioned problems, we made an intensive study and found that films provided with excellent sliding properties and durability as well as surface flatness can be manufactured if certain conditions are satisfied in the relation between the center line average roughness (pm) and the number of secondary interference fringes measured by the multiple interference method H2 (number/mm2).
Meanwhile, thinner magnetic tapes required, since magnetic recording apparatuses which are more lightweight and compact, and that have longer recording times are desired. As thin base films, polyester films with high longitudinal tensile strength have been used. These films are produced by stretching ordinary biaxially oriented films further longitudinally or transversely. However, there is a tendency that when biaxially oriented films are further stretched longitudinally or transversely, the film surface is flattened and thus the slipperiness is remarkably impaired.We have conducted intensive research in this respect, too, and have found the above problem is solved by stretching a film longitudinally in multiple stages under conditions which restrict the degree of orientation, then stretching the thus obtained longitudinally stretched film transversely, and further stretching the thus obtained biaxially oriented film longitudinally and/or transversely.
Accordingly, the present invention provides a polyester film for magnetic recording media having minute protuberances on the surface and satisfying the following numerical formula (1) in the relation between the center line average roughness Ra (pom) and the number of the secondary interference fringes measured by the multiple interference method H2 (number/mm2): Ra 2 x 10-4-H2 + 0.008 (r) wherein H2 is not more than 200.
Further this invention provides, in a process for preparing polyester films having high longitudinal or transverse tensile strength by a restretching process, the improved process comprising stretching a raw polyester film so that the index of birefringence An becomes 0.015 - 0.055 (this stage will, hereinafter, be called the first stretching stage), further stretching the film in the same direction at a temperature between 95 - 1500C inclusive so that the index of birefringence An is prevented from exceeding 0.08 (this stage will hereinafter be called the second stretching stage), tranversely stretching the thus longitudinally stretched film by 2.5 - 5 times in width, and finally stretching the thus obtained biaxially stretched film longitudinally and/or transversely by at least 1.1 times.
By index of birefringence (an) is meant the difference between the maximum value and the minimum value of the refractive indices in the film surface.
The polyester employed in the invention of this application is a polyester at least 80 % by weight of which comprises ethylene terephthalate units and the remaining less than 20 % by weight comprises units of a comonomer or comonomers, or a blend of at least 80 % by weight of ethylene terephthalate polyester and less than 20 % by weight of another polymer or polymers. The polyester can contain a stabilizer such as phosphoric acid, phosphorous acid or esters thereof; an additive such as titanium dioxide, fine silica or kaoline; or a lubricant.
The minute protuberances referred to in this specification are those formed of inorganic particles added to the polymer, or particles formed from the insoluble residue of the catalyst used for preparation of the polymer, or both of these.
The film of this invention must satisfy the following numerical formula (1) in the relation between the center line average roughness Ra (m) and the number of secondary interference fringes measured by the multiple interference method H2 (number/mm2):Ra ~ 2 x 10-4-H2 + 0.008 (I) wherein H2 is not more than 200.
It is thought that H2 is closely related to the electromagnetic characteristics, and Ra is closely related to the friction coefficient of a film. When H2 is in excess of 200/mm2, the electromagnetic characteristics are deteriorated, and therefore such films are not suitable as base films for magnetic tapes. For base films for video magnetic tapes, it is preferred that H2 be not more than 150/mm2, and for base films for high quality video magnetic tapes, it is preferred that H2 be not more than 100/mm2. The number of interference fringes of the third order or higher (H3, H4, etc.) is preferably not more than 10/mm2, and most preferably is zero.
On the other hand, Ra must be not less than 2 x 10-4-H2 + 0.008. When Ra is less than 2 x 10-4-H2 + 0.008, no decrease in the friction coefficient is realized and improvement in the flatness and slipperiness cannot be expected. The preferred range of the Ra value is 2 x 104,H2 + 0.008 ' Ra ' 2 x 104H2 + 0.045, and a more preferred range thereof is 2 x 1 04,H2 + 0,011 Ra 2 x 1 0-4,H2 + 0.045. If the Ra value is too high, the electromagnetic characteristics deteriorate even when H2 is small.
Various methods can be employed to obtain films satisfying the above relation between Ra and H2.
Particles in films show a certain particle size distribution. If the H2 value, which represent the number of larger particles having bearing on the electromagnetic characteristics, is decreased, the total number of particles will also decrease so long as the distribution remains unchanged. The Ra value will also decrease along with the H2 value. Some special method is required to decrease the H2 value without lowering the Ra value. For instance, when ordinary stretching conditions are employed, inorganic particles of uniform particle size, that is, a batch of particles from which larger particles have been removed, may be added to film materials and be made to exist on the film surface separated from each other.Or otherwise, in the case where particles are deposited from insoluble catalyst residue, the particles may be formed from the catalyst residue so that the particles be of uniform size and exist separatedly from each other, whereby larger particles are removed without reducing the total number of the particles and the particle size distribution curve shows a narrow steep peak, by devising a proper polymerization process. However, it is generally more convenient to attain the above-mentioned effect by employing specific conditions in the stretching operation for both added particles and deposited particles.
For instance, when an extruded and quenched raw polyester film containing particles is stretched longitudinally, it is preferred that the degree of longitudinal orientation be controlled so that the index of birefringence An is 80 x 1 10-3 or less, and thereafter the film is stretched transversely. For base films for magnetic recording media, it is the most fundamental requirement that there be as little fluctuation as possible in thickness of a film and the film contain as few coarse particles as possible on the surface, which are the cause of dropout.Therefore it is preferred that the value RKX, which is a quotient of R (m), which is the difference of the maximum thickness and the minimum thickness when thickness is measured over 5 meters longitudinally, divided byX(m), which is the average thickness, is not more than 0.10 at the highest.
And it is preferred with respect to coarse particles that the number of interference fringes of the fourth or higher orders of film measured by the two-beam interference method be not more than 100/cm2.
Therefore, it is preferred to carry out the longitudinal stretching in at least two stages at lower temperatures so that the above mentioned relationship between H2 and Ra is satisfied without causing enhancement of the sticking of the films to the hot drawing rolls and deterioration in the longitudinal thickness uniformity of films. When the longitudinal stretching of films is carried out in more than two stages, at least the last stage stretching should be effected at a higher temperature. in order to reduce the number of coarse particles, a filter of smaller mesh should preferably be used at the stage of discharge from the polymerization reactor or of extrusion.
Then the thus longitudinally stretched low oriented film is stretched transversely. And the thus biaxially oriented film is further stretched longitudinally and/ortransversely, if desired, and finally the film is heat-set.
As stated above, a film in which the relation H2 =' 200, Ra ~ 2 x 1 0-4,H2 + 0.008, and the fundamental conditions such that thickness fluctuation is low and number of coarse particles is small, etc. are satisfied, can be obtained by various methods. However, there are variations among the products. The number of roughness units each consisting of a minute protuberance of the film surface and a recess extending from said protuberance on both sides generally perpendicular to the longitudinal direction varies from nil to several thousand per square millimeter from product to product.
These roughness units are formed by deformation of the polymer material in the proximity of included particles in the course of stretching. It has been pointed out that flatness and slipperiness of films are poor when the number of these roughness units is small.
in this invention, however, it was found that if the above mentioned relation of Ra and H2 is satisfied, the flatness and slipperiness of films with fewer roughness units are not inferior to those with a larger number of roughness units and may even be superior thereto.
That is to say, it has been revealed that among films which satisfy the relation H2 200, and Ra > 2 x 104H2 + 0.008, films having up to 2500 roughness units per mm2 which respectively comprise a protuberance and a recess extending therefrom 3 Fm in longer diameter (films in which the number of such units A is O < A ~ 2500) are provided with both excellent flatness and slipperiness and durability.
If the number of the roughness units A is in excess of 2500/mm2, the film becomes inferior in durability.
The number A is preferably 5 < 1500, more preferably 5 < A ~ 800. When there are roughness units, that is when A = 0, the film is a little inferior in slipperiness.
In the foregoing, flatness and slipperiness are considered in combination. However, there are cases where the flatness must be considered perferentially and other cases where slipperiness must be considered preferentially. Magnetic tape manufacturers has recently been demanding drastically improved slipperiness without sacrifice of flatness in order to increase productivity. This invention provides films which meet such a demand.
As mentioned before, the Ra value is related to the friction coefficient. However, the friction coefficient differs depending on the number of particles and the distribution in height of particles even with the same Ra value. The friction coefficient ( > ) of films should preferably satisfy the relation 0.10 ' > ~ 0.39 - 6 Too low ffi values are undesirable since the films will be irregularly wound in rapid winding, or cinching will occur. In order to avoid these phenomena, it is insufficent merely to restrict the number of protuberances having recesses therearound. Instead the ratio B (%) of the number of such protuberances to the total number of protuberances should preferably satisfy the relation 0 B' B 50.
The total number of protuberances referred to in this specification means the sum of the number of protuberances which have no recess therearound and whose diameter at the foot thereof is not less than 2 Fm and the number of protuberances which have recesses therearound per mm2.
When the B value is in excess of 50 %, the effect of enhancing slipperiness is not exhibited. If the B value is too large, the area of roughness units which contact a magnetic head or other film decreases, and the part other than the roughness units becomes to flat. Therefore, the contact area as the whole does not decrease but increases and the friction coefficient does not decrease.
When the number of the roughness units is too large, there is a tendency that other particles are buried under the surface of the polymer and the decrease in the friction coefficient is impaired.
The sliding property of the films the B value of which is in excess of 50 % largely depend upon the particles of the roughness units, the particles of which are liable to come off. The friction coefficient of such a film after it has been used many times markedly increases. Therefore, the preferred range of the B value is 0.1 < B 40.
Thus base films especially suitable for magnetic recording media provided with excellent flatness, slipperiness, slidability, durability, slidability after repeated use can be provided by arranging for the relations H2 200, Ra ~ 2 x 10-4-H2 + 0.008 to be satisfied, the number of roughness units consisting of a protuberance and a recess formed around said protuberance A to be not more than 2500/mm2, for the ratio of the number of said roughness units to the total number of protuberances B to be not more than 50 %.
Meanwhile, good adhesion of the magnetic layer to the polyester base film is required in addition to the above requirements in the application of the base film for magnetic tapes. We have found that films in which the ratio of the peak value of the (1T0) face determined by the X-ray diffraction method to that of the (1 0 0) face is not less than 0.1 provide outstandingly good adhesion to magnetic layers in comparison to ordinary polyester films, in addition to excellent flatness, slipperiness and durability. When the ratio is less than 0.10, no enhancement of adhesion is observed.
It is surmized that the inclination of crystal faces from the film surface permits easy penetration of the solvent of the magnetic coating compositions. Such films can be obtained by controlling the orientation in the longitudinal stretching, or relaxing the stretched film longitudinally or transversely, or heat-setting the stetched film at a higher temperature close to its melting point.
As described above, polyester films, in which certain specific surface conditions are satisfied, and specific relations between the center line average roughness Ra (sum), the H2 (number/mm2) determined by the multiple interference method, and the peak value of the (1 1 0) face determined by the X-ray diffraction method are satisfied, are most suitable for magnetic recording media provided with flatness, slipperiness, especially slidability in repeated use, and adhesion of magnetic layers.
This invention can be applied not only to balanced films (films requiring 9.0 - 12.0 kg/mm2 for 5 % longitudinal and transverse elongation, for instance) but also to longitudinal semitensilized films (which require more than 12.0 kg/mm2 to 14 kg/mm2 for 5 % longitudinal elongation), longitudinal tensilized films (which require more than 14 kg/mm2 for 5 % longitudinal elongation), transverse semitensilized films (which require more than 12.0 kg/mm2 to 14 kg/mm2 for 5% transverse elongation), and transverse tensilized films (which require more than 14 kg/mm2 for 5 % transverse elongation), as well as films having these tensile properties in combination.
In manufacturing longitudinal and transverse semitensilized and tensilized films by the restretching process, high stretch polyester films for magnetic media having expecially excellent properties can be prepared by employing a specific longitudinal stretching process as described below.
Specifically, a polyester film is first stretched longitudinally so that the An is 0.015 - 0.055, preferably 0.025 - 0.055 in one step or a plurality of steps. If the An value is less than 0.015, fluctuation in thickness increases. If the An value is less than 0.025, flat and slippery films can be manufactured, but orientation and crystallization of the polyester film hardly occurs and thus there is a tendency that the film sticks to the drawing rolls at the later longitudinal stretching stage. If sticking occurs, not only does uniform longitudinal stretching become impossible, but irregular protuberances are formed in the portions where sticking occurs, which sometimes constitutes a fatal defect for the magnetic tape base.If the stretching is carried out at lower temperatures in the first stretching stage in order to avoid sticking, films in which An is less than 0.015 develop irregularities in thickness in the longitudinal stretching stage after the second stage stretching.
When An is 0.025 or higher, polyester films are oriented and crystallized and are uniform in thickness and sticking to the drawing rolls does not occur. In the range of 0.015 - 0.025 of the An value, sticking occurs with the ordinary rolls, but thickness is uniform. Sticking can be avoided by using special rolls. If the An is in excess of 0.055, irregularity in thickness in the longitudinal direction is remarkable in the films after completion of the second stage longitudinal stretching, and the flatness of the film after the biaxial stretching is insufficient.
The first stage stretching, in which the An value of 0.015 - 0.055 is acieved, should preferably be carried out in one to three steps. The first stage stretching is carried out in the temperature range of 80-120 C, preferably in the range of 85 - 110 C.
The ratio of stretching in the first stage stretching is 2.0 - 4.0 when it is effected in one step, and higher ratios can be employed if it is effected in 2 - 3 steps if a one or more heat treatments for orientation relaxation are inserted between the steps. The heat treatment for orientation relaxation can be effected at a temperature higher than the stretching temperature and lower than the melting point of the film in a short period of time.
Thus, a longitudinally stretched film having a An value of 0.015 - 0.055 is obtained in the first stage longitudinal stretching stage. The film is stretched in the second stage at 95 - 1 500C maintaining the An value so as not to exceed 0.08 If the second stage longitudinal stretching is carried out at a temperature lower than 95 C, the biaxially stretched film is insufficient in flatness, and slipperiness. If a temperature higher than 1500C is employed, crystallization proceeds and transverse stretching is impaired. And it is essential for theAn value not to exceed 0.08 at the second stage stretching. This is a considerably small value in light of the ordinary film-making technique, especially that used in manufacturing films for magnetic tapes. This means that longitudinal orientation is restricted.If the An value is greater than 0.08, the flatness of the biaxially stretched film is insufficient, and if it is far greater than this value, longitudinal splitting occurs frequently in the course of transverse stretching. The ratio of stretching in the second stage is 1.05 - 1.7, preferably 1.1 1.6. The second stage stretching is preferably carried out as the final step of the longitudinal stretching in one step in a short period of time.
The thus longitudinally stretched film is then stretched transversely at 80 - 1 600C at a ratio of 2 - 5, preferably at a ratio of 3 - 4.5. Finally the film is heat-set at 100 - 20000 to obtain a biaxially oriented film. The thus biaxially stretched film is further stretched longitudinally and/or transversely.
A large variety of magnetic recording tape products are manufactured today, and the strength required of base films varies in accordance with the intended use. However, for high tensile strength films, it is preferred that the tensile strength for 5 /O elongation F5 in either the longitudinal or the transverse direction be 13 kg/mm2or more. In orderto provide a film with an F5 value of at least 13 kg/mm2 in the longitudinal direction, it is required to restretch the film by 1.1 times in the longitudinal direction at a temperature between 10000 and 20000.In order to provide a film with F5 values of not less than 13 kg/mm2 in the longitudinal and transverse directions, the film is first stretched longitudinally by at least 1.1 times, preferably 1.2 times or more at the above-mentioned temperature, and then is stretched transversely by 1.1 times in width or more.
It is possible for a film to be stretched simultaneously longitudinally and transversely by 1.1 times or more.
The ratio of the restretching is 1.1 - 1.7, preferably 1.2 - 1.5.
After restretching, the film is subjected to heat-setting at a temperature between 170 - 240 C. When biaxially oriented films prepared by the conventional film-making process are restretched, the films are flattened but are remarkably deteriorated in slipperiness. In accordance with the above-described process, however, the deterioration in slipperiness by restretching is extremely small. Therefore, high tensile strength films for magnetic recording tapes having especially excellent flatness and slipperiness can be provided.
Now the invention will be illustrated by way of working examples. The methods employed for measuring various properties of films are as follows: (1) Friction coefficient (p) A strip of film was brought into contact with the surface of a hard chromium plated roll having a diameter of 6 mm and a surface finish of 2.0S over an arc of 135 (0), a load (T2) of 53 9 was applied to one end thereof, the strip was allowed to slide over the surface of the roll, at the rate of 1 m/min and the supporting force T (g) on the other end of the strip was measured. The kinetic friction coefficient ,a was determined in accordance with the following equation: 1 T = 0.424n T2 L =wn ''' (2) Intrinsic viscosity ( [ Til) To 200 mg of a sample, 20 ml of phenol/tetrachloroethane (50:50 by weight) was added and the sample was dissolved by heating at 110 C. The viscosity of the solution was measured at 30 C.
(3) F5 value Speciment 1/2 inch in width and 50 mm in length (length between chucks) were drawn by a tensile tester,Tensilon UTM-III, marketed to Toyo Measuring Instruments Co., Ltd, at a rate of 50 mm/min in an environment of 20 C and 65 % . The measured 5 % off-set load was divided by the initial cross-sectional area of the specimen and the value is given in kg/mm2.
(4) Center line average roughness (Ra)Surface roughness was measured using a surface roughness measuring apparatus SE-3FK manufactured by Kosaka Kenkyusho as explained below. The radius of curvature of the tip of the contact needle was 0.5 pi and the needle pressure was 30 mg. Center line average value of surface roughness Ra was determined in accordance with the procedure prescribed in JiS (Japanese Industrial Standards) B0601. The standard lengthL (2.5 mm of the film) is taken from the film profile curve along the center line. The center line of the portion is taken on an abscissa and the roughness is taken on an ordinate. Then the roughness curve is expressed as y = f(x). The Ra value was obtained as an average of 5 points along the longitudinal line and 5 points along the transverse line.The waves longer than 80 Fmwerecutoff. Ra is given in Fm as:
(5) Number of roughness units consisting of a protuberance and a recess therearound A and the ratio thereofBThe surface of a film coated with vaporized aluminium was photographed under a Carl Zeiss differential interference microscope at a magnification factor of 750 and the number of protuberances in 1 mm2 area was counted. The term "number of protuberances" means the sum of the number of protuberances withoutrecess therearound at least 2 Fm in diameter at the foot thereof and the number of roughness units consisting of a protuberance having a recess around said protuberance at least 3 m in the longer diameterA among the protuberances formed by included particles.The ratio of the number of the roughness units tothe number of protuberances B is indicated in %.
(6) Measurement of surface roughness by the multiple interference methodThe surface of a film was coated with vaporized aluminiun. The coated surface was placed under a surface finish microscope made by Nippon Kogaku K. K. and interference fringes formed at the wavelength of 0.54 m were photographed. Number of interference fringes of n-th order was counted and is indicated as the number per mm2 area. The reflectivity of the mirror used was 65 % and the magnification (factor) was 200.
(7) X-ray diffractometryThe peak value of the (1 0 0) face near 2 #=28 and peak value of the (1 1 0) face near 2 # =23 of a film sample were read by means of an automatic X-ray diffractometer and the ratio of the two values was taken.
X-ray output was 30 KV x 15 mA.
(8) Chroma S/N and adhesion of magnetic layerChroma S/N was measured by a Shibasoku's NTSC type video noise determinator Q25R using a commercially available home VTR. A magnetic tape made of a conventional base film which has no protuberances having recesses around them was used as a reference and its noise was taken as 0 dB.
Meanwhile, adhesion measured as follows. The coated surface of a magnetic tape was bonded to the surface of a stainless steel plate by means of a double-coated adhesive tape. The film was peeled off at an angle of 180 and the tensile force in peeling was taken as the index of adhesion strength. The adhesion strength of the reference tape was taken as unity (1.0).
The magnetic coating was prepared as follows.
A magnetic powder coating composition indicated below was applied on the surface of a cleaned pre-treated film by a gravure roll and was smoothed with a doctor knife so as to form a magnetic layer about 6 Fm in thickness. Before the coating was completely dried the magnetic layer was magnetically oriented and the dried film was cured at 80 C for 20 hours, and thereafter calendered. The thus prepared magnetic film was slitted into tapes 1/2 inch in width.
Ingredients Parts by wt.
Ferromagnetic powder principally 250comprised of -Fe203 Polyurethane resin 50Vinyl chloride-vinyl acetate copolymer 30Nitrocellulose 20Lecithin 3Carbon black 15MEK 900Polyisocyanate compound 15 (9) Index of birefringence (An)Retardation (R) was measured using a Carl Zeiss polarization microscope, and the index of birefringence was calculated in accordance with the following equation:An = whereinR: retardationd: thickness of film (10) Temperature of the filmTemperature of a film portion being stretched was measured using an IR radiation thermometer made byBarnes Engineering Company.
(11) Surface defects caused by stickingThe surface of a film was coated with evaporated aluminium and was observed by a Carl Zeiss differential interference microscope. Films having defects are indicated by x and those without defects are indicated by ".
Comparative Examples 1 and 2 (Preparation of polyester)One hundred (100) parts by weight dimethyl terephthalate, 70 parts by weight ethylenegiycol 0.10 part by weight calcium monohydrate and 0.17 part by weight lithium acetate dihydrate were placed in a reactor and heated. As the temperature rose, ester interchange proceeded and ethanol was distilled away. After about 4 hours, the temperature reached 2300C and the ester interchange reaction was substantially finished. Triethyl phosphate (0.35 parts by weight) was added to the reaction product, and 0.05 parts by weight antimony trioxide was further added as a condensation polymerization catalyst. Thus the reaction product was polymerized into a polyester by the conventional process.A number of uniformly dispersed deposited fine particles 0.5 - 1 ym in particle size including particles containing calcium and lithium and elemental phosphorous were observed in the polyester. The intrinsic viscosity [ X ] of this polyester was 0.65. A polyester without these particles was prepared separately and was mixed with the above-described polyester at a ratio of 1:1 by weight, and the mixture was used for making films, too.
(Film making)A raw film ( [ N ] = 0.62, which was prepared by melt-extrusion and quenching by the electrostatic pinning method, was stretched longitudinally by a factor of 3.7 at 900C, then transversely by factor of 3.5 at 110 C and finally was heat-set at 220 C. Thus a biaxially oriented film 15 pm in thickness was obtained (ComparativeExample 1). Meanwhile, another portion of the biaxially stretched film was heat-set at 1500C instead of at 220 C, and the resulting film was further longitudinally stretched by factor of 1.1 at 1 300C and was heat-set at 220 C. Thus another biaxially oriented film 15 Fm in thickness was obtained (Comparative Example 2).
Various properties of the obtained films are indicated in Table 1 together with those of the films of theExamples described below.
Example 1-4The same raw film as used in Comparative Example 1 was first stretched longitudinally by a factor of 2.4 at 85 C and further stretched longitudinally by a factor of 1.2 at 110 C by means of drawing roll pairs rotating at different rotation speeds and, the thus longitudinally stretched film was stretched transversely by factor of 3.5 at 1 40 C using a tenter and finally was heat-set at 220 C. Thus a biaxially oriented film 15 pbm in thickness was obtained (Example 1).
Instead of being heat-set at 220 C, the film was heat-set at 150 C and was further longitudinally stretched by a factor of 1.2 (Example 2), 1.4. (Example 3) and 1.5 (Example 4) and were then heat-set at 2200C to obtain films 15 Fm in thickness.
Examples 5 and 6The same raw film as used in Comparative Example 1 was stretched longitudinally by factor of 2.4 at 850C in the same way as Example 1, that is, by using roll pairs rotating at different rotation speeds, and was further stretched in the same direction by a factor of 1.25 (Example 5) and 1.3 (Example 8) at 110 C, and the longitudinally stretched films were then stretched transversely by a factor of 3.5 at 140 C and finally were heat-set at 220 C so as to give finished films 15 m in thickness.
Example 7The same raw film as used in Comparative Example 1 was stretched longitudinally by a factor of 2.2 at 85 C by using roll pairs rotating at different rotation speeds, and further stretched in the same direction by a factor of 1.3 at 110 C, and thereafter the film was stretched transversely by factor of 3.5 at 140 C by means of a tenter and finally heat-set at 220 C. A 15 m thick biaxially orientated film was obtained.
Example 8The same raw film as used in Comparative Example 1 was stretched by a factor of 2.7 at 88 C using roll pairs rotating at different rotation speed and futher stretched in the same direction by factor of 1.2 at 110 C, then was stretched transversely by a factor of 3.5 at 140 C, and finelly was heat-set at 220 C. A 15 m thick film was obtained.
Properties of the obtained films are summarized in Table 1.
TABLE 1Ra H2 A B F4 1 200 I(110) Chroma Relative( m) (number/mm) (number/mm) (%) Longitudinal I(110) S/N Adhesion(Kg/mm2) (dB) StrengthComparative 0.021 100 0 0 10.7 0.32 0.35 0.05 0 1.0Example 1Example 2 0.020 80 0 0 14.0 0.34 0.37 0.07 0 0.9Example 1 0.015 15 800 15 11.3 0.26 0.30 0.21 +1.2 2.2Example 2 0.014 12 800 13 13.0 0.28 0.32 0.20 +1.2 2.0Example 3 0.013 10 850 16 14.2 0.27 0.32 0.18 +1.4 1.9Example 4 0.013 g 900 14 17.0 0.28 0.34 0.18 +1.4 1.8Example 5 0.016 20 300 5 11.5 0.25 0.29 0.20 +1.0 2.0Example 6 0.017 30 10 0.15 11.8 0.22 0.26 0.18 +0.8 2.1Example 7 0.014 10 1400 30 10.5 0.27 0.32 0.22 +1.8 2.3Example 8 0.019 40 0 0 11.5 0.30 0.34 0.16 +0.4 1.8 As seen in Table 1, films in which the relation Ra ~ 2 x 10-4.H2 + 0.008 is not satisfied, are inferior in flatness and slipperiness.
Comparative Examples 3 - 5 and Examples 9 and 10The same raw film as used in Comparative Example 1 was stretched first longitudinally by a factor of 1.9 at 85"C, further stretched in the same direction by factor of 1.7 at 110 C, and thereafter stretched transversely by a factor of 3.5 at 140 C by means of a tenter, and was heat-set at 150 C. The thus obtained biaxially oriented film was further longitudinally stretched by a factor of 1.3 and was finally heat-set at 2000C (Example 9). In the same way, three other restretched films were prepared under the conditions indicated in Table 2 asComparative Examples 4 and 5 and Example 10. All were 15 clam in thickness.The film of ComparativeExample 3 in Table 2 was made by the conventional film-making process. That is, a raw film (1Q ] = 0.62) was first stretched longitudinally by a factor of 3.7 at 90 C, then transversely by a factor of 3.5 at 110 C, heat-set at 150 C, restretched longitudinally by factor of 1.1 at 1 300C and was finally heat-set at 200"C.
Example 1 7 and Comparative Example 6The same raw film was first stretched longitudinally by a factor of 2.3 at 85 C, further stretched in the same direction by a factor of 1.3 at 110 C, stretched transversely by factor of 3.5 at 140 C and was heat-set at 1 500C and thus a biaxially oriented film was obtained. The film was further longitudinally stretched by a factor of 1.3 at 1 30aC, and then transversely by a factor of 1.15 at 140"C and finally was heat-set at 200"C. The obtained film was 15 Fm in thickness (Example 11).
Separately, the same raw film was stretched longitudinally by a factor of 3.7 at 90 C, the tranversely by a factor of 3.5 at 100 C, and was heat-set at 150 C. The resulting film was again stretched longitudinally by a factor of 1.1 at 130 C and transversely by a factor of 1.18 at l40C and was finally heat-set at 2000C (Comparative Example 6). When properties of products of Example 11 and Comparative Example 6 shown inTable 2 are compared, it is apparent that the film of this invention is superior in slipperiness.
TABLE 2Ratio of # oflongi- longitudinal tudinally Finished filmstretching stretched Ratio of Ratio of Surfacefilm Longi- Transverse defect F5 (kg/mm)First Second First Second tudinal Stretching due to Ra Longi- H2Stage Stage Stage Stage Stretching Sticking ( ) tudinal Transverse No./mmEx.9 1.9 1.7 0.023 0.065 1.3 - x1) 0.017 0.30 14.1 11.0 15Comp.
Ex.4 2.1 1.7 0.032 0.083 1.2 - 0 0.020 0.32 14.2 11.2 60Ex.10 2.3 1.3 0.040 0.065 1.3 - 0 0.016 0.29 14.2 11.0 12Comp.
Ex.5 2.5 1.1 0.058 0.065 1.3 - 0 0.019 0.33 14.0 11.3 56Comp.
Ex.3 3.7 - 0.105 - 1.1 - 0 0.016 0.36 14.2 11.2 80Ex.11 2.3 1.3 0.040 0.065 1.3 1.15 0 0.015 0.30 13.2 13.1 10Comp.
Ex.6 3.7 - 0.105 - 1.1 1.18 0 0.015 0.38 13.0 13.1 50 1) Slipperiness is excellent, although surface defect is observed.