JPH0331329B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a polyoxymethylene polymer film, and particularly to a biaxially oriented polyoxymethylene polymer film that is highly oriented and crystallized, has high rigidity and high dimensional stability. [Prior Art] In recent years, with the increasing density and miniaturization of electronic devices, and the speed of signal processing increasing year by year, films used as magnetic recording media films such as video tapes, audio tapes, and floppy disks have become increasingly popular. There is a growing demand for extremely thin walls and high dimensional stability.
To meet this demand, it is essential to develop high-performance films with superior rigidity and dimensional stability. The present invention focuses on a polyoxymethylene polymer film as a film that satisfies such requirements, and has a high rigidity and high dimensional stability.
An axially oriented polyoxymethylene polymer film is provided. Polyoxymethylene films are generally formed by melt pressing or melt extrusion followed by rapid cooling, but the films obtained by these methods are not only opaque or translucent, but also have poor mechanical properties. Inferior. Therefore, various methods have been tried in the past to improve the transparency and mechanical properties, and as a result, it has become possible to obtain films with improved transparency and mechanical properties to a certain extent. There is. However, since polyoxymethylene film is extremely difficult to handle compared to ordinary film, the excellent properties of this film, namely the excellent rigidity and dimensional stability of this film as a biaxially oriented film, have not been discovered until now. It was because it had reached . One of the reasons why polyoxymethylene film is difficult to handle is said to be due to the relatively large spherulites present in its crystal structure. Tokuko Showa 40-21994
A crystallized polyoxymethylene polymer film, which is translucent to opaque due to the presence of spherulites, is processed until the film becomes substantially transparent and few spherulites can be detected by visible light microscopy.
By rolling at a roll temperature of 120°C or less, and then stretching the rolled film by 25% or more in at least one axis at a temperature in the range of 120 to 180°C,
A method for making polyoxymethylene films with improved transparency and mechanical properties is disclosed. This method is characterized by improving the transparency of the film and making it easier to orient the crystals by rolling, and as a result, the film is better than that obtained by the conventional method. , improved transparency and mechanical properties. Generally, the rigidity of a film is said to be proportional to its tensile modulus, and the higher the modulus of elasticity, the greater the rigidity. In the case of polyoxymethylene film, for example, the tensile modulus described in the examples of the above-mentioned known technology was about 530 Kg/mm ² at most. However, this level of improvement in rigidity is extremely insufficient for a high-rigidity, high-performance film, and is not the purpose of the present invention.
This known technique does not suggest the possibility of further increasing this value or any measures or means for further increasing it. [Problems to be Solved by the Invention] In the case of polyoxymethylene polymer films, it is difficult to highly orient the crystals by ordinary orientation means due to the spherulites present in the crystal structure as described above. , was considered extremely difficult. Although this has been made possible to a certain extent by the above-mentioned known technology, it has not been possible to achieve the characteristics that the present inventors are aiming for. This was considered to be the limit of its characteristics. [Means and effects for solving the problems] The present inventors have focused on the high crystallinity property of polyoxymethylene polymers, and have conducted extensive research to make the most of this property, and have developed various orientation conditions, etc. The present invention was achieved as a result of repeated numerous studies including more precise control of processing conditions. That is, we have completed a polyoxymethylene polymer film with a higher degree of crystallinity and a highly oriented film, and have also demonstrated that the film has a high tensile modulus and excellent dimensional stability. I found it. As a result, they were able to obtain a polyoxymethylene polymer film that has high rigidity and high dimensional stability, far exceeding the properties that were conventionally considered to be the limits. The present invention is a biaxially oriented film made of a polyoxymethylene polymer in which the main part of the main chain is substantially composed of repeating units of oxymethylene groups (-CH ₂ -O)-, and The degree of crystallinity measured by X-ray diffraction method is 75 to 95%, and the degree of crystal orientation measured from the two directions of end and edge by X-ray diffraction method is 85 to 95%. This is a characteristic polyoxymethylene polymer film. In the film of the present invention, its crystallinity and crystal orientation must satisfy the following conditions as its structural parameters. First of all, the degree of crystallinity is determined by the formula D=dc(d-da)/d(dc-da)×100(%) from the value of film density (d) measured by density gradient tube method. It is therefore necessary that the calculated crystallization (D) is between 75 and 95%. Here dc is 1.506g/cc,
is the theoretical density of a completely crystallized polymer,
da is 1.25 g/cc, the density of the completely amorphous polymer. When the degree of crystallinity is less than 75%, even if the degree of orientation of the crystals is sufficiently high, the influence of the amorphous portion becomes noticeable, reducing the rigidity and dimensional stability of the film. Furthermore, it is extremely difficult to form a film exceeding 95% due to the properties of polyoxymethylene polymer. Next is the second parameter, the degree of crystal orientation, which is determined by the (100) Surface (2θ=
22° to 23.5°) can be determined from the intensity distribution when scanning in the azimuth direction. The end direction of the film is a direction parallel to the film surface and parallel to the film machine direction (extrusion direction), and the edge direction is a direction parallel to the film surface and the width of the film. A direction parallel to the direction. In the present invention, the half width (W) of the diffraction peak centered at an azimuth angle of 90° is measured, and the value (A) calculated by the formula A=180−W/180×100(%) is used for the crystallization. It was defined as the degree of orientation. In the present invention, it is necessary that the degree of crystal orientation measured from two directions, end and edge, is 85 to 95%. When the degree of crystal orientation is less than 85%, the crystal orientation is insufficient, and even if the film has a degree of crystallinity that satisfies the above conditions, it cannot be expected to have high rigidity as a biaxially oriented film. Furthermore, films exceeding 95% are extremely difficult to form due to the properties of polyoxymethylene polymers. It is an essential requirement of the present invention to simultaneously satisfy the above two structural parameters, and a polyoxymethylene polymer film that satisfies these requirements has excellent characteristics of high rigidity and high dimensional stability. Specifically, its tensile modulus in each axial direction is 550 Kg/mm ² or more, which is a high modulus that exceeds conventional limits. Furthermore, in order to obtain a film with more excellent properties, the crystallinity degree is preferably 78 to 95% and the crystal orientation degree is preferably 90 to 95%. At this time, the tensile modulus in each axial direction becomes an extremely high value of 700 Kg/mm ² or more. The polyoxymethylene film of the present invention has a relatively high molecular weight, for example, a number average molecular weight of 35,000 to 35,000.
It is derived from polyoxymethylene of 100,000 or more. In the case of a film having high rigidity and high dimensional stability as in the present invention, polyoxymethylene having a relatively high molecular weight is preferred. Also, acetalized polyoxymethylene, or its reaction product with isocyanate, or, for example, a small amount of tertiary
It can also be applied to polyoxymethylene polymers in which the main chain of the main chain is substantially composed of repeating units of oxymethylene groups ( _-CH2 -O)-, such as copolymers obtained by copolymerizing components. Next, a specific method for producing a film that satisfies each parameter of the present invention will be exemplified below. First, a sheet or film (hereinafter referred to as "original fabric") of the polymer is formed by conventional means, such as melt pressing or melt extrusion followed by rapid cooling. The crystallinity of the original fabric at this time varies depending on the format conditions such as the cooling conditions after melting, but it is usually 60 to 72%.
It is. Next, this original fabric is stretched at least 5 times or more in each axial direction. That is, the film of the present invention could only be discovered by stretching the original film to a high magnification of at least 5 times or more. More preferably, more excellent properties can be obtained by stretching the film by a factor of 8 or more. During this stretching, particular attention must be paid to the stretching temperature. The stretching temperature of the original fabric according to the present invention is determined using a differential calorimeter (Differential scanning).
The temperature range is at least 25° C. lower than the peak temperature of the crystal melting curve of the original fabric measured by calorimetry/DSC) and at most 5° C. higher than the peak temperature. At temperatures lower than this temperature range, not only is it difficult to orient the crystal molecules, but there is also a risk that the film will whiten or break. Furthermore, at temperatures higher than this temperature range, the melting of crystal molecules becomes dominant, and the effect of crystal orientation becomes almost impossible to expect, and in some cases, there is a risk that the film will melt and break. And preferably 15°C below the peak temperature.
The temperature range is above the low temperature and below the peak temperature, and a more uniform stretched film can be obtained at this time. Note that the peak temperature of the crystal melting curve measured by a differential calorimeter varies depending on the molecular weight, crystallinity, formation conditions, etc. of the original fabric, but is usually 173 to 178°C. Furthermore, in the case of the film of the present invention, since the stress during stretching is relatively large compared to ordinary films, it is preferable that the stretching be performed at a relatively low speed. Specifically, it is 50 to 3000%/min, preferably 100 to 3000%/min, relative to the length of the sample to be stretched before stretching.
1000%/min. By stretching the original fabric at least 5 times or more in each axial direction at the above stretching temperature and stretching speed, the polyoxymethylene molecules are highly crystallized and highly oriented. An oriented polyoxymethylene polymer film can be obtained. The film thus obtained has a high tensile modulus of elasticity in each axial direction of 550 Kg/mm ² or more, and also has excellent dimensional stability due to its high degree of crystallinity. The film of the present invention is characterized by its structural parameters, crystallinity and crystal orientation, and the relationship between its characteristics, tensile modulus and stretching ratio, depends on the crystallinity of the original film or the stretching ratio. In order to obtain a film with a higher elastic modulus, it is sufficient to perform stretching at a higher magnification under the above-mentioned stretching conditions, although it is not necessarily determined uniquely depending on the conditions and the like. For example, if the stretching ratio in each axial direction is 8 times or more, the tensile modulus will be 700 kg/
It is possible to obtain a film with an extremely high value of mm ² or more. This value is more than three times that of a normal original film, and it becomes an extremely high rigidity film. Further, if necessary, stretching can be performed with anisotropy in the elastic modulus in each axial direction. That is, this can be easily achieved by providing different stretching ratios in each axial direction. For example, stretching can be performed at a stretching ratio of 10 times or more in the longitudinal direction and 5 times or more in the transverse direction. Further, the film obtained here is subjected to heat treatment, if necessary. The temperature and time in this case are not particularly limited, but it is carried out at a temperature near the stretching temperature for a relatively short time, for example, 1 to 120 seconds. In the above film manufacturing method, ordinary means can be applied to the stretching method. For example, a longitudinal-horizontal or transverse-longitudinal sequential biaxial stretching method or a simultaneous biaxial stretching method can be applied. In the case of polyoxymethylene polymers, when an attempt is made to reorient crystals that have been oriented in a particular direction in a different direction, due to the characteristics of the crystal structure, some of the crystal molecules break, resulting in
There is a risk that the film will break during stretching. Therefore, a simultaneous biaxial stretching method in which stretching is performed simultaneously in two axial directions is a more preferable method, but when adopting a sequential biaxial stretching method, for example, first stretching is performed in the lateral direction using a tenter, and then stretching is performed in a relatively easy roll direction. A preferred method is a transverse-longitudinal sequential biaxial stretching method in which stretching is performed in the longitudinal direction. In obtaining the film of the present invention, some of the above-mentioned known techniques can also be used in combination in order to more easily orient the crystals in the raw film. That is, prior to stretching, the original fabric is rolled using a roll mill or the like, and then stretched under the above-mentioned stretching conditions. In this case, rolling is performed in at least one axial direction, that is, in the roll direction, until the length becomes about 1.1 to 3 times the length of the original fabric.
In the subsequent stretching process, in order to make it easier to orient the crystals in both directions of the two axes,
If this rolling operation is performed both in the roll direction and in the direction perpendicular thereto, that is, in the width direction, a more uniform stretched film can be obtained in the stretching process. In addition, when rolling is performed prior to stretching in this way, the original fabric is stretched to a certain extent during rolling, so
It is considered that the stretching ratio in the stretching process is already included in the stretching ratio during rolling. For example, when obtaining a film stretched 10 times, if the film is stretched up to 2 times in the rolling process, it may be stretched 5 times in the actual stretching process. That is, the stretching ratio in the present invention is based on the original fabric. Furthermore, the method exemplified below can also be applied as a method for making it easier to orient the crystals in the original fabric. First, the original fabric is preheated to an appropriate temperature of 100 to 150°C, and then the original fabric is pressurized. The pressurizing operation in this case can be easily performed using, for example, a compression molding machine. By this operation, the crystals in the original fabric are preliminarily oriented in the biaxial directions to some extent.
This is a method in which the preliminarily oriented original fabric is then highly oriented and crystallized under the stretching conditions according to the present invention. In the above pressurizing operation, a suitable resin with better fluidity than the polyoxymethylene polymer,
For example, by using a resin such as polymethyl methacrylate (PMMA), sandwiching the original fabric between the resins and pressurizing the original fabric together with the resin, the original fabric can be more easily oriented. In obtaining the film of the present invention as described above,
Various methods can be adopted. The film thus obtained has excellent properties of high rigidity and high dimensional stability. Therefore, the film of the present invention can be applied to various application fields that utilize such characteristics, such as as a base film for magnetic recording materials such as magnetic tapes and floppy disks. The density of the film in the present invention is a value measured using a density gradient tube at 23° C. using a solution consisting of normal heptane/carbon tetrachloride. Next, a method for measuring the degree of crystal orientation using X-ray diffraction will be described. For each sample, a rectangular molded product with a thickness of 2 mm, a width of 1 mm, and a length of 10 mm was produced with the stretching directions aligned. Cyanoacrylate adhesive was used to fix each film during molding. Next, this molded product was attached to a universal sample stand (rotating sample stand) manufactured by Shimadzu Corporation so that X-rays could be incident from the edge or end direction of the film. The X-ray generator is an XD-3A model manufactured by Shimadzu Corporation.
−Kα radiation was used as the X-ray source. The goniometer used was also a model VG-108R manufactured by Shimadzu Corporation, and was attached to the above rotating sample stage. The slit type has a receiving slit of 2mmφ and a scattering slit of 1mmφ.
It was adopted. Next, the diffraction angle was set to the (100) plane of polyoxymethylene (2θ = 22° to 23.5°), the X-rays were incident from the edge direction and the end direction, and the rotation speed was 4°/min. The sample stage was rotated twice and the diffraction intensity of the (100) plane was scanned in the azimuthal direction. The chart speed of the recorder was 10 mm/min. An example of the results obtained by this measurement is shown in FIG. Next, a method for calculating the degree of crystal orientation will be explained with reference to the same figure. For a peak at an azimuth angle of 90°, the peak intensity I from the background is determined, and then the width (half width) W of the peak, which is the intensity I/2, is determined. Note that the background should be determined prior to measurement. Next, the degree of crystal orientation A was calculated using the following formula. A=180-W/180×100(%) Also, the differential calorimeter (DSC) is Seiko Electronics Co., Ltd.
Measured using DSC-20 model manufactured by Co., Ltd. with a sample weight of 10 mg and a heating rate of 10°C/min, and a chart speed of 2 cm/min.
It was recorded at. An example of measurement at this time is shown in FIG. [Examples] The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to these Examples. The various properties of the films shown in the following examples were measured using the following test methods. (1) Tensile properties For a test piece with a length of 100 mm and a width of 10 mm, the chuck distance was 60 mm and the chuck speed was 30 mm/min.
Measured at a temperature of 23°C and humidity of 50%. The elastic modulus was calculated from the initial slope of the stress-strain curve obtained at this time. (2) Dimensional stability According to JIS C-2318, for a sample with a length of 150 mm, a width of 20 mm, and a gauge line spacing of 100 mm, the thermal shrinkage rate (%) after 150°C/2 hours was determined, and this value was calculated as the thermal dimensional stability (%). ). Example 1 Pelletized polyoxymethylene (Tenac 3010 manufactured by Asahi Kasei Corporation) with a number average molecular weight of approximately 63,000 was
The material was melted and pressed at a temperature of 200° C. for 2 minutes under a pressure of 200 Kg/cm ² , and then immediately cooled in water at a temperature of about 10° C. to obtain a raw sheet with a thickness of 300 μm. The crystallinity of this original fabric was 67% by the density method, and the peak temperature of the crystal melting curve by DSC was 175°C. Next, this original fabric was stretched using a biaxial stretching machine (tentter method) manufactured by Iwamoto Seisakusho at a temperature of 175°C and a stretching speed of 170%/
Simultaneous biaxial stretching was performed at min. Although this stretching resulted in netting stretching and some non-uniform stretched areas were generated, a biaxially stretched film with a size of 6×6 times was obtained. Table 1 shows the results of comparison with the original film regarding the tensile properties of this film, the degree of crystallinity determined by the density method, and the degree of crystal orientation determined by the X-ray diffraction method. The biaxially stretched film obtained here had equivalent physical properties in each direction, and Table 1 shows the results in one direction. This result shows that this film is highly oriented and crystallized, and also shows that it has higher rigidity than conventional polyoxymethylene films. Example 2 The same polyoxymethylene as in Example 1 was extruded from a slit die at a temperature of 200°C, rapidly cooled on a casting roll heated to 120°C, and a thickness of 500μ was obtained.
An original fabric sheet was obtained. The crystallinity of this original fabric was 71% as determined by the density method, and the peak temperature of the crystal melting curve as determined by DSC was 176°C. Next, this original fabric was sandwiched between two sheets of polymethyl methacrylate resin having a thickness of 10 mm, and after being preheated to a temperature of 130° C., a pressure of about 1000 Kg/cm ² was applied, and then it was cooled. Through this operation, the original fabric was stretched 2×2 times. Next, the original fabric stretched to 2×2 times was stretched at a stretching temperature of 160° using a biaxial stretching machine as in Example 1.
Simultaneous 2 at ~177℃, stretching speed 100~400%/min
Axial stretching was performed to obtain a uniform stretched film. Table 2 also lists the stretching conditions, stretching ratio, and physical properties of the film after stretching. In addition, the stretching ratios in the table indicate the ratios based on the original fabric, and those with anisotropy in the stretching ratio are those obtained by varying the stretching speed in each direction. In experiment number 7, the temperature was 170℃/ after stretching.
It was heat treated for 1 minute. The results in Table 2 show that the film of the present invention has extremely high stiffness, and also shows that it has excellent dimensional stability. Example 3 The raw sheet obtained in Example 2 was cut into a sheet with a length of 300 mm.
Rolling was performed using a two-high rolling mill consisting of two rolls with a diameter of 250 mmφ. Rolling was performed at a roll temperature of 70°C and a rolling speed of 1.2 m/min. Rolling was repeated alternately in the direction perpendicular to the . Next, this original film stretched to 2×2 times was subjected to simultaneous biaxial stretching using a biaxial stretching machine in the same manner as in Example 1 to obtain a uniform stretched film. Table 3 summarizes the stretching conditions, stretching ratio, and physical properties of the film after stretching. Note that the stretching ratios in the table are based on the original fabric. Example 4 The raw sheet obtained in Example 2 was roll-rolled using the rolling mill used in Example 3 at a roll temperature of 70°C and a rolling speed of 1.2 m/min. , 1×3 times and 1.5×2 times stretching ratios were obtained. These stretched original fabrics were then stretched using a biaxial stretching machine. As the stretching method at this time, a sequential biaxial stretching method was adopted in which stretching was first carried out in the direction of a low stretching ratio and then stretching was carried out in the direction of a high stretching ratio. Table 4 lists the stretching conditions, stretching ratio, and physical properties of the film after stretching for each rolled product. Note that the stretching ratios in the table are based on the original fabric. Comparative Example 1 In Example 3, the original fabric stretched 2×2 times by rolling was heated to
100% in one direction at 170℃, stretching speed 300%/min
A stretched film having a stretching ratio of 2×4 times that of the original film was obtained. The physical properties of this stretched film are shown in Table 5. This comparative example was made based on the example described in Japanese Patent Publication No. 40-21994. The results in Table 5 show that this film is highly biaxially oriented and not crystallized, and the obtained physical properties are also at an extremely low level compared to the film of the present invention. That is, in the present invention, a polyoxymethylene polymer film having excellent rigidity and dimensional stability has been discovered for the first time as a biaxially oriented film. Comparative Example 2 In Example 3, the original fabric stretched 2×2 times by rolling was heated to a temperature of 173 using the same stretching machine as in Example.
Simultaneous biaxial stretching was performed in two directions at a stretching rate of 300%/min to obtain a stretched film with a stretching ratio of 3×3 times that of the original film. The physical properties of this stretched film are shown in Table 5. This result shows that although the crystallinity of this film satisfies the requirements of the present invention, sufficient physical properties cannot be obtained due to the low crystal orientation. Comparative Example 3 The original film stretched to 2×2 times by the pressure operation in Example 2 was further repeated in the same manner to obtain a biaxially oriented film with a stretching ratio of 5×5 times compared to the original film. The physical properties of this oriented film are also shown in Table 5. This result shows that even if the degree of crystal orientation satisfies the requirements of the present invention, a film with a low degree of crystallinity
This indicates that sufficient physical properties cannot be expected.

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[Table] [Effects of the Invention] The polyoxymethylene film of the present invention is highly oriented and crystallized, has high rigidity and high dimensional stability, and can be made extremely thin. Therefore,
The film of the present invention can be applied to various applications that utilize such characteristics, such as as a base film for magnetic recording materials such as magnetic tapes and floppy disks, and has great industrial significance.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing the direction of the sample for measuring the degree of crystal orientation by X-ray diffraction, Figure 2 is an explanatory diagram showing the method for calculating the degree of crystal orientation, and Figure 3 is an explanatory diagram showing the direction of the sample for measuring the degree of crystal orientation using the X-ray diffraction method. 3 is a graph showing a DSC curve of a measurement example of DSC).

Claims (1)

[Claims] 1 The main part of the main chain is substantially oxymethylene group (-
It is a biaxially oriented film made of a polyoxymethylene polymer composed of repeating units of CH ₂ -O)-, and has a crystallinity of 75 to 75 as measured by the density method.
95%, and the degree of crystal orientation measured from two directions, end and edge, by X-ray diffraction method is both 85 to 95%.