JPH035981B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a synthetic resin film with a textured or rough surface, particularly an excellent hazy polypropylene film for use primarily as a dielectric film in capacitors. Eustance U.S. Patent No. 1, Assigned to Applicant
No. 4,255,381 discloses a bubble forming process for producing a textured or rough surface film called hazy polypropylene film (trademark of General Electric Company). This bubble formation process involves extruding hot melt polypropylene through a die into a tube shape, cooling the tube on a mandrel, reheating the tube, and then inflating the tube into a biaxially oriented bubble. The bubble is then cut open to obtain a film strip, which is particularly useful as a dielectric material for capacitors. The use of the cloudy film produced by the above method in capacitors has been assigned to the applicant.
Disclosed in DiNicola, US Pat. No. 4,228,481. The continued use and increasing use of cloudy films as dielectric films for capacitors has resulted in the need for improved cloudy films, particularly those with defined and even improved surface texture properties, such as predictable uniform space factor or roughness. There is a need for a better method of producing foggy films with low manufacturing costs. It has been determined that such improvements can be achieved by irradiating a polypropylene material with an electron beam and then biaxially stretching the material into a thin film. BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention better, the invention will be explained below with reference to the drawings. Figure 1 shows electron beam radiation and film processing equipment 1.
0 diagrammatically. This device 10 includes a supply roll 11 of a flat or smooth synthetic resin film 12.
and a winding roll 13 for the processed film.
Film strip 12 can be considered an intermediate product and can be a tubular stock coming out of a tube blowing device such as that disclosed in the Eustance patent. film strip 12
The strip may be a strip coming out of an extruder of a draft-tentter film forming process, or a strip obtained from other known polypropylene strip manufacturing processes, such as other extrusion and casting processes. Roll 11 film strip 12
The electron beam is directly sent out directly below the electron beam irradiation device 14. The irradiation device 14 includes suitable shielding means 15, an electron beam focusing structure 16 and an electron gun 17.
The electron beam source 18 is typically a refractory metal, such as tungsten, which is electrically heated to induce electrons. Electrons are emitted from an electron beam source 18 through a protective slit or slot in a curtain-like structure 19 and impinge on a film strip 12 which moves longitudinally in one direction with respect to the irradiation device 14.
Examples can be found in US Pat. No. 3,702,412 and US Pat. No. 3,780,308. However, the filmstrip 12 can be moved in multiple directions, for example both longitudinally and laterally, and the illumination device 14 can be moved. This movement imparts a predetermined pattern to the film. For example, a predetermined geometric pattern is applied to the film. Additionally, the radiation can be focused or directed to create a pattern in a predetermined area of the film. For example, a pattern may be applied only to the edges of the final film strip. Depending on the type and extent of radiation, certain degrading effects are exerted on the surface of the film via surface penetration. In this deterioration process,
The crystalline integrity of the surface layer changes and some degree of crosslinking occurs. After subjecting the film strip 12 to appropriate electronic bombardment, the strip is placed on a simple film stretching machine.
While warming, the film is stretched in both the longitudinal and transverse directions to give the film a permanent stretch. Such a process is called biaxial stretching. The stretching process is diagrammatically shown in FIG. In the stretching machine 20 shown in FIG.
It is gripped by a pair or more of clamps 22 and 23 and one or more pairs of clamps 24 and 25 arranged opposite to each other in the machine direction. The film strip 21 is permanently stretched by moving the clamp in the direction of the arrow. During the stretching operation, the irradiated surface layer of the film is
Due to crystal defects and cross-linking, it is stretched differently than the rest of the film, which causes irregularities in the film surface. These irregularities appear as protrusions from the original film surface and are usually randomly located on the surface, creating a textured surface similar to that disclosed in the Eustance patent. In other words, the film surface is covered with fibrillar elements in the form of loose circles or raised reliefs, giving the appearance of craters or depressions on the film surface. When the irradiated and textured surface polypropylene film of the present invention was analyzed by infrared spectroscopy, no change in chemical composition was observed. Surface morphology analysis by scanning electron microscopy
A cloudy surface similar to the cloudy film described in connection with the Eustance patent was observed. However, films exposed to higher doses (20 megarads) exhibit deformed or degenerated oblong fibril morphology, and these fibrils are discontinuous and
They formed close islands. Representative examples of these forms are shown in FIGS. 3 and 4. The photomicrograph of FIG. 3 shows one form of the textured surface film of the present invention. Third
The photograph in the figure, taken at 500x magnification and a 30° angle, shows the fibrils, along with various protrusions and depressions, which give the film a pebble or stony appearance. The height (or depth) of the projections is controlled by the irradiance and the stretching method and parameters. The photomicrograph of FIG. 4 shows another form of the textured surface of the present invention. This form was generated at a higher irradiance than that shown in FIG. The photograph in Figure 4, taken at 500x magnification and a 30° angle, shows the array or island effect of protrusions from the plane of the film. A method for producing a surface-textured polypropylene film in accordance with the present invention is illustrated in the following examples. Example 1 In accordance with an exemplary embodiment of the present invention, a sample of polypropylene tubing from a tube blowing apparatus was irradiated as diagrammatically shown in FIG. Irradiation was carried out under a nitrogen blanket or atmosphere. The irradiation amount is 5-20 at an accelerating voltage of 175-200KV.
Changed within the range of Megarad. At doses lower than this, fogging was not sufficient. The tubular material was passed through the irradiation area at approximately 236 cm/sec (50 ft/min).
After this, the tubular material is placed in a stretching machine frame at approximately 140 to 160℃.
Biaxial stretching was carried out at a ratio of 6:6. The results obtained are shown in the table below.

[Table] "10 + 10 megarads" in the irradiation dose column in the table above is 10 each.
This means that it was treated with Megarad twice. As can be seen from the above data, the irradiated biaxially stretched film exhibits sufficient haze and a space factor of about 7-10%, making it an excellent solid dielectric film for capacitors. Increasing the dose causes crosslinking of the polypropylene film, a desirable result for many capacitors. The reason is that crosslinked polypropylene is more resistant to swelling and dissolution when in contact with certain insulating oils. In this regard,
Lawton describes irradiation and subsequent heating as a crosslinking process in US Pat. No. 2,948,666. The film of the above example was subjected to electrical tests as shown in the following table.

[Table] The irradiation method of the present invention is a method that allows control of irradiation intensity and irradiation focus convergence. Thus, the textured surface of the film of the present invention allows for control in two important respects that have heretofore been difficult to control. That is, it is possible to control (1) uniformity over a continuous and identically extended surface and (2) a predetermined uniform roughness. Both of these factors are easily controlled by the intensity of irradiation and the speed of movement of the film through the irradiated area. An important factor of the present invention is that the processing of the film is in the form of solid particle processing. It is the bombardment of the film by electron particles, rather than light energy, that rapidly transforms the crystals in the surface layer, alters the depth, and promotes crosslinking. Particle bombardment is generally
Defined as an atom or subatomic whose equivalent wavelength is significantly lower than the wavelength of ultraviolet (UV) light. Other particle bombardment methods include particles such as protons and neutrons.
The solid particles operate at extremely high energy levels to quickly and deeply penetrate the surface of the film, changing the crystalline structure to more than an amorphous structure. By comparison, the shortest wavelength of light, ie, the wavelength of ultraviolet (UV) radiation, is about 250 cm or more, while the equivalent wavelength of an electron beam at about 100-150 KV is about 0.004 cm. In a preferred embodiment of the invention, the film is heated to a temperature above 100 DEG C. and close to its melting point, such as about 150 DEG-160 DEG C. for polypropylene, before stretching. These three steps can be performed continuously. The irradiation dose can be varied from 5 to 20 megarads or more, and the electron beam acceleration voltage can be varied from 100 to
Can be converted to 200KV or more.
The film exposure speed is approximately 12.7 to 50.8 cm/sec (25 to
100 feet/minute), with good results obtained at feed rates of about 50 feet/minute or more. The surface textured film of the present invention can be used extensively in the dielectric material of a capacitor as a means to facilitate impregnation of the capacitor with an insulating oil. The strip can be metallized with a thin metal layer, which can be applied to either surface of the strip. The film of the present invention is optimally used in a capacitor using only the film as a dielectric material. For capacitor applications, two measurements are made on the surface textured film, viz.
(1) Cloudiness and (2) Space factor are important. Haze is determined by measuring light passing through a cross-section of the film, ie, light passing approximately perpendicular to the top-to-bottom plane of the film, rather than in an edge-to-edge direction.
Actually, the Gardner Laboratory Haze Meter
(Gardner, Maryland, USA)
A commercially available instrument from Laboratory Corporation, catalog number HG1204) can be used. A Gardner digital photometer, catalog number PG5500, may also be used. A haze meter shines light onto a film and measures the intensity of the light that passes through the film.
The resulting values are given as percent haze of the film. The test methods used are American Society for Testing and Materials test methods ASTM-D1003 and ASTM-D1044.
and FTMS-406, Method 3022. Space factor is an important measurement value for capacitor dielectric films.
Although haze measurements based on light transmission give some indication of roughness, haze measurements are unduly influenced by internal coloration and irregularities in the film that affect light transmission. Furthermore, the light transmission method only provides an approximate value of the space factor. For example, if a film surface has only a few protrusions protruding from the surface, it will show a low degree of haze when measured by light transmission. However, in reality, film has a high space factor. A large number of small protrusions on a film will give a high haze reading in light transmission, even though the film actually has a low fill factor. For example, a high haze reading correlates with a high fill factor only if certain process conditions are met. In practicing the present invention, polypropylene films are manufactured to a fill factor of about 11% or less, typically in the range of about 7-11%. Increasing the irradiation also increases the space factor. The measurement of the occupancy factor can be performed by a simplified direct calculation. The reason for this is that the space factor is a term that describes the relationship between the theoretical volume, or actual volume, of a polypropylene film strip, for example, compared to the total volume occupied by the film strip. For example, a film strip with a textured surface can be measured for its total thickness to obtain a dimension from which the first
Calculate the volume of However, this volume includes intervening spaces, such as the valleys between the protrusions of the textured surface. A second calculation can be made for the actual volume of solid material in the strip.
The quotient obtained by dividing the difference between these calculated values by the weight average thickness is the space factor given as the ratio (%) of the space obtained in the textured surface film to the theoretical actual volume. For example, a fill factor of 10% means that 10% of the measured volume is not occupied by solid material. Weight average thickness is determined by weighing a film sample of known length, width and density and calculating the thickness. The volume average thickness is determined by measuring the film thickness with a micrometer. The quotient obtained by dividing the difference between these two thicknesses by the weight average thickness is the space factor. A typical capacitor using the film of the present invention is shown in FIGS. 5 and 6. FIG. 5 shows the capacitor roll 26 in a partially unwound state. roll 2
6 comprises a pair of spaced apart metal foil strip electrodes 27 and 28 and intermediate polypropylene film strips 29 and 30. The roll is completed with additional polypropylene film and foil strips so that pairs of polypropylene film strips appear between the metal foil electrode strips throughout the roll. Taps or tabs 31 and 32 are inserted into the roll 26 adjacent the electrode strips to serve as electrical connections for the electrodes. As an assembled capacitor, one or more capacitor rolls 26 are placed in a suitable case, the case is filled with insulating oil,
Insulating oil is infiltrated into the roll, filling the spaces between turns of the roll and infiltrating the polypropylene material itself. An example of a capacitor using multiple rolls called a power capacitor is shown in FIG. The capacitor 33 of FIG. 6 houses a plurality of rolls 26 arranged in two rows, one above the other, in a two-pack design, all of which are immersed in an insulating oil 34.
Case 35 can be over 65.0 cm in height and roll 26
can be approximately 25.4-30.5 cm (10-12 inches) long. Roll 26 has taps 31 and 32
, which are electrically combined and connected to terminals 36 and 37, respectively. Single pack design (1
If a group of rolls 26) is employed, the rolls have a length of about 60 cm or more and are referred to as wide rolls. By adopting a structure that exposes the foil, the taps 31 and 32 can be eliminated. In this case, electrode foils are made to protrude from each end of the roll;
Solder the foil wraps together at each end. Connections to terminals 36 and 37 are then made. Because of the tightness of the roll 26 and the fact that the metal foil strip electrodes constitute a substantially impermeable lateral barrier, the insulating oil must enter through the roll ends 38 and 39 (FIG. 5). No. At this point, the vacuum drying of the rolls and the high temperatures used in the impregnation process cause the edges of the polypropylene film strips, which overlap at the ends of the rolls, to seal together and adhere to the electrode strips.
When absorbing the insulating oil, it swells densely with the adjacent strips, thus preventing the insulating oil from penetrating into the roll. In the present invention, the use of a textured polypropylene film strip provides a unique and economical spacer means of controlled permeability and oil penetration path from the roll end into the foil 26. Form by method. The film strip of the present invention may be deposited with a metal layer, e.g. aluminum, to form a metallized capacitor structure, either in the roll form as previously described or in the stack form in which the layers are stacked in flat, intimate relationship. It can be used in In practicing the present invention, the method and apparatus of FIG. 1 can be suitably combined with known methods of forming polypropylene films. For example, FIG. 7 diagrammatically depicts a tube blowing device 40 as described in Eustance, US Pat. No. 4,255,381. In the tube blowing device 40 shown in FIG.
and then to an extruder 43 where it is heated to form a very soft or molten mass of polypropylene. This softened or melted material is extruded from an extruder 43 through a die 44 into a tube and brought into contact with a cooling mandrel 45 where the tubular material begins to crystallize into a tube (stalk) 46. The tube 46 is pulled from the cooling mandrel 45 through a pair of rolls 47, which press it in intimate relation against an air tube 48 passing between the rolls within the grooves of the rolls. After passing through roll 47,
The cooled crystallized tube 46 is heated by suitable heating means 49;
For example, it is reheated to a softening point using a radiant heater, and then inflated by introducing pressurized air from the air tube 48. An inflated controlled size bubble or thick tube 50 is formed which stretches the polypropylene tube approximately 6 times both horizontally and vertically to form a biaxially oriented polypropylene film. The bubble 50 is then moved to the frame roller 51
The bubbles are crushed between a pair of nip rolls 52 via a slitter 53, where the flattened bubbles are cut into one or more strips, and the strips are then wound onto a take-up roll 54. Representative configurations and methods for producing film by tube blowing are described in U.S. Pat. No. 2,720,680 to Gerow;
See U.S. Pat. No. 3,235,632 to Lemmer. As the tube 46 passes upwardly through the cooling mandrel 45, the tube 46 moves at a speed of about 25-50 feet per minute and absorbs electrons from one or more irradiators 14 as shown in FIG. exposed to radiation. Continuing to move upward, the irradiated tube 46 reaches approximately
It is reheated to a temperature above 125° C. and then inflated into the shape of a bubble 50 as shown. The process of stretching from a tube shape to a bubble shape achieves biaxial stretching of the bubble polypropylene film. Biaxial stretching is the result of the bubble 50 being stretched lengthwise by the nip roll 51 at the top of the bubble, while the expansion stretches the bubble laterally. In the final state, the cross section of the bubble is stretched by a ratio of approximately 6:6. That is, it is stretched six times in the length direction and six times in the transverse direction. The same irradiation method can also be applied to the draft-tentter method, for example as disclosed in US Pat. No. 2,412,182. In this method, a thick flat strip or slab of synthetic resin emerges from a rectangular die. Grab the strip with multiple clamps,
These stretch the strip both longitudinally and laterally. The apparatus and method of FIG. 1 is configured to irradiate a thick strip prior to the drawing step. The films of the present invention can be used in numerous other applications, such as packaging applications, bandaging applications, and labeling applications where printing or writing on the surface of the film is desired. Although the invention has been described with respect to specific embodiments thereof,
Those skilled in the art will be able to imagine various modifications without departing from the spirit of the invention. Therefore, the present invention includes all changes and modifications that fall within the scope of the claims.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of a simplified method and apparatus for irradiating polypropylene strips with electrons;
Figure 3 is an explanatory diagram of a simplified apparatus and method for drawing polypropylene strips; Figure 3 is a photomicrograph of an example of a polypropylene film with a textured surface obtained from the method and apparatus of Figure 2; Figure 4; 5 is a micrograph of another example of a polypropylene film with a textured surface.
The figure is a perspective view showing a capacitor roll using a film with a textured surface according to the present invention, FIG. 6 is a perspective view showing a power capacitor using the roll of FIG. 5, and FIG. 7 is a perspective view showing a process for producing a polypropylene film It is a diagram showing a tube blowing method. DESCRIPTION OF SYMBOLS 10... Irradiation processing device, 12... Film strip, 20... Stretching machine, 21... Strip, 22-2
5... Clamp, 26... Capacitor roll, 27,
28... Electrode, 29, 30... Film strip.

Claims (1)

[Scope of Claims] 1 (a) Polypropylene film in solid state 5
(b) exposing the film to a bombardment of high-energy particles in the range of ~20 megarads and 100-200 kilovolts at a radiation rate of 25-100 feet/minute; (b) bringing the film to a temperature close to its melting point; A method for producing a polypropylene film having a rough textured surface, comprising the steps of: heating, and then (c) stretching the film to roughen and texture the impact surface of the film. 2. The method according to claim 1, wherein the stretching is biaxial stretching. 3. The method according to claim 1, wherein both sides of the film are textured. 4. A method according to claim 1, in which a beam of electrons is generated as high-energy particles from a high-temperature metal source. 5. The method of claim 4, wherein the source is an electrically heated refractory metal. 6. The method of claim 1, wherein the film is moved relative to the high energy particles. 7. The method according to claim 6, wherein the movement is performed in multiple directions. 8. The method according to claim 7, wherein the movement is performed in a plurality of directions in a horizontal plane. 9 Claim 6 in which the movement is performed in one direction
The method described in section. 10. The method according to claim 1, wherein the film having a rough textured surface has a space factor of 5% or more. 11. The method of claim 1, wherein both surfaces of the roughly textured surface film are textured. 12. The method of claim 1, wherein the polypropylene is crosslinked. 13. The method of claim 1, wherein the rough textured surface polypropylene film is manufactured as a cloudy polypropylene film dielectric used in capacitors.