FI64534C - BLAOSFOERFARANDE FOER FRAMSTAELLNING AV EN POLYPROPYLENFILM ME EN STRAEV YTA - Google Patents

Description

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Patent- och rtglltentyrtlim AroOlctn utlagd och utUkrtft · * pubtk «nd 31.0o.o3 (32) (33) (31) Pyyd ^ ty« uoikmi · —feglrrf prlofitat 28.02. 75 USA (US) 55 ^ * 070 Proven-Styrkt (71) General Electric Company, Schenectady, NY, USA (US) (72) John Walker Eustance, South Glens Falls, New York, Emilie Lucille Carley, Hartford, New York, Stanley Young Hobbs, Scotia, New York, USA (US) (7¾) Oy Heinänen Ab (5M Bubble method for the production of rough-surfaced polypropylene film -

The present invention relates to a bubble process for producing a rough-surfaced polypropylene film, in which process a) molten polypropylene is extruded to be extruded into a tube, b) d) the tube is blown to provide a biaxially oriented bubble, and e) the bubble is compressed and cut into a film, the roughness being provided by a temperature difference between the surfaces of the tube.

Polypropylene film has gained widespread use as a dielectric medium in dielectric liquid impregnated capacitors used in electrical engineering. In the manufacture of such a capacitor, very thin polypropylene strips are placed on top of each other with aluminum foil strips and this combination is wound into a tight roll. The rollers are placed in a suitable housing and impregnated with a dielectric liquid absorbent. Alternatively, the aluminum foil strips can be replaced with metal coatings formed on suitable dielectric strips. The polypropylene film, which is made by a well-known method of pipe pain holding, drawing and stretching, usually has a very smooth surface throughout, which causes the surfaces to adhere very tightly to each other or to other surfaces facing them. This results in difficulties in absorbing the film when it is in a tightly wound capacitor roll. It is very difficult to make an absorbent such as oil completely penetrate the capacitor roll and especially between the surfaces of the polypropylene strips and the polypropylene and foil strips.

As a result, many attempts have been made in the past to enhance the absorption of condensate rollers by roughening the surfaces of interconnected film strips, foils, and the like by etching, grooving, grinding, or shaping. The results obtained by these methods have not been entirely satisfactory for a variety of reasons. In particular, many of these roughening methods tend to affect the physical strength of the film as well as its dielectric strength. Also, molding methods that increase the overall thickness of the film, such as corrugation and grooving, increase the overall thickness of the roll and, as a result, make the final roll too large. The blowpipe method has been found to be able to produce films with slightly rough surfaces. However, this roughness was uneven and random and thus not suitable for a controlled well fabrication process.

It is an object of the present invention to provide an improved method of making a rough-surfaced polypropylene film and is characterized in that the roughness-inducing temperature difference is obtained by bringing spherolites, wherein the roughness of the film from the tube is formed by the fibroid irregularities in the form of overlapping patterns on its surface, so that the result of the total light diffusion measurement of the film is from about 20% to about 40%.

64534

A preferred embodiment of the method according to the invention is characterized in that the tube moves over the mandrel at a speed of about 10-12 meters per minute and has a wall thickness of about 0.38-0.5 mm and that said temperature and stress provide a boiling and uniform effect on the outer surface of the tube. solidification effect so that a dense layer of type III spherolites with a crystal structure is formed on the outer surface of the tube.

The invention is described in more detail in the following and accompanying drawings, in which

Figure 1 shows the extrusion of polypropylene into a tube and the blowing into a bubble in a process for manufacturing a rough-surfaced polypropylene film,

Figure 2 is a photomicrograph of a smooth surface film,

Figure 3 is a photomicrograph of a polypropylene film having a surface roughness of 20%;

Figure 4 is a photomicrograph of a polypropylene film having a surface roughness of 30%, and

Figure 5 is a photomicrograph of a polypropylene film having a surface roughness of 40%.

In the process of Figure 1, the polypropylene resin is fed as granules 11 to a hopper 12 and then to an extruder 13 where it is heated to produce a very soft or hard polypropylene mass. This mass is extruded from the extruder 13 through a nozzle 14 in the form of a tube, after which it comes into contact with a cooling mandrel 15, where it begins to crystallize as a tube 16. From the cooling mandrel 15 a tube 16 is drawn between two rollers 17. After passing the rollers 17, the cooled and crystallized tube 16 is again heated to its softening temperature by suitable heating means 19, for example a radiant heater 11a, after which it is inflated by blowing pressurized air down the air tube 18. The blowing causes the formation of a bubble of a controlled size, i.e. a wide tube 20, whereby the polypropylene tube stretches both horizontally and vertically in a straight direction to about six times, thus forming a biaxially oriented polypropylene film. The bubble 20 is then pressed between two other pairs of press rolls (not shown) and drawn into a cutting roll where the bubble is cut into one or more strips which are then wound on a take-up roll. Typical apparatus and methods for making a film by the blow molding method are disclosed in U.S. Patent Nos. 2,720,680 (Gerow), 3,235,632 (Lemmer) and 3,223,764 (Kahn).

Applying the film manufacturing process described above, it has been found that from time to time certain surface irregularities occur in the film. These irregularities have been considerably different in the different batches and have not been evenly distributed over the entire surface, i.e. uniform over the entire large surface area of the film. Typically, the non-uniformity has concerned both the distribution of irregularities on the film surface and the degree of irregularity. In addition, the unevenness pattern, i.e., the shape of the irregularities, was not the same over the entire surface of the film, and the occurrence or absence of irregularities was not easily predictable. At times, the occurrence of irregularities seemed random.

Irregularities in the film made it opaque, which in many cases was less desirable than obtaining a clear, smooth film. At that time, it was found that certain such rough films had quite good absorption properties. For example, if selected samples of such a very rough film were used to wrap the test capacitor rolls, the film would not adhere to or adhere to itself or adjacent surfaces. In addition, the roughness of the film provided a certain distance between adjacent surfaces so that the absorbent liquid could more easily penetrate this space. As a result, not only did uniform uniformity become a desirable feature, but also its predictability, controllability, and uniformity became essential in the film manufacturing process.

It has now been observed that in certain blowpipe methods, by adjusting the temperatures, it can be determined in advance whether or not irregularities will occur on the surface of the film. An important observation has also been made that irregularities can be produced uniformly on the surface of the film so that the density and degree of irregularities become greater. If the irregularities are of a certain type and their shape is uniform over the entire surface area of the film, and if the height of the protrusions is within certain limits and is also uniform over the entire surface area of the film, the film is called a rough film according to the present invention.

The amount of surface roughness or irregularity can be expressed as the degree of film roughness. The degree of roughness is determined by measuring the passage of light through a portion of the film, i. usually perpendicular to the top and bottom and not diagonally. A Gardner Laboratory Haze Meter manufactured by Gardner Laboratory Corporation, Bethesda, Maryland (catalog HG 1204) was used in the practice of the present invention. A digital photometric device (catalog PG 5500) was also used. The Haze Meter conducts light through the film and measures the intensity of the light as it passes through the film relative to the intensity of the light entering the film. The values obtained are expressed as percent film roughness. The methods used were standard methods American Society for Testing Materials, Test ASTM-D1003, ASTM-D1004 and FTMS 406, Method 3022.

Measurements are made with a roughness meter at short intervals from a film strip, for example 15 measurements from a film strip with a width of about one meter. The average of the ten largest roughness measurement results is calculated and the average of the five smallest roughness measurement results is also calculated. The degree of roughness of the whole film is shown as a combination of these two averages, for example 40/20 roughness. When a single roughness value is displayed, e.g., 30% roughness, this is a single measurement result obtained with a roughness meter.

A Gardiner Gloss Meter (catalog GG 9042) was also used, applying ASTM test D2457-70. This device measures the light reflected from the surface of the film, which also gives an image of the surface roughness.

The surface roughness can be adjusted by adjusting the temperature at a certain point in the tube in the device according to Figure 1, which is clearly indicated that if a certain small point in the tube is heated or cooled, for example by rapid jet cooling, differences in roughness can be observed. The temperature can be controlled, for example, by using a heating ring 21 in the chamber 6 64534 sa 22, as shown in Fig. 1. A suitable temperature control medium, such as air, can be circulated through one or more rings 21, with air jeting out through the openings therein and coming into contact with the tube 16, as a result of which the temperature of the tube 16 rises or falls by a certain axial length. Such heating or cooling can be provided by a variety of devices known in the art, which can raise or lower the temperature of the body passing therethrough by either direct or indirect contact using, for example, liquids or gases or radiation.

However, the temperature control must be directed to a specific point in the tube 16. The polypropylene is extruded from the die 14 in a molten state and moves towards a coaxial mandrel 15 where it continuously cools and crystallizes into a tube 16. As the tube 16 cools, a surrounding solidification limit [16 (a) J is formed at about 15 to 60 cm from the die 14. This the boundary is visible and indicates the area in which complete crystallization of the polypropylene tube has occurred. The temperature control according to the present invention, in which the tube is generally heated, must be carried out after the extruder and before the solidification limit, preferably very close to the latter.

Usually, in a film blowing method based on pipe blowing, the cooling temperatures of the mandrel, the pipe temperatures and the solidification limit are quite fixed and do not vary considerably. For example, the temperature of the polypropylene extruded from the die 14 is about 233 ° C, while the solidification limit temperature is about 92 ° C. Under these conditions, the crystallization properties of the tube are also somewhat constant and the type of crystals formed is approximately constant.

According to the invention, the surface roughness of the film is obtained by forming a certain type of crystal layer in a controlled manner in the tube. It is a layer on the outer surface of the pipe in which the density of type III spherulites with a crystal structure has increased considerably. This crystal layer is the factor by which, when the tube is blown into a biaxial oriented bubble, the outer surface of the tube becomes rough according to the invention. One explanation associated with type III crystals in terms of roughness is that type III crystals are transformation crystals in the well manufacturing process. Experiments have shown that type III polypropylene crystals melt in the preheating process at 19 (Figure 1) just before the bubble is stretched. Reheating temperatures are in the range of approximately 140-150 ° C. The density of the type III polypropylene crystals in the tube is about 0.8, and after the tube has passed through the reheating device 19, they become type I and II crystals of the order of 0.9. As a result, the elongation of the bubble together with the change in the density of the crystals creates a discontinuity, i.e. a crater-like effect, resulting in a surface roughness.

The crystal layer which causes the formation of the rough film can be obtained by arranging a certain temperature difference between the surface of the polypropylene tube against the mandrel and the opposite outer surface of the tube. This temperature difference is also related to the fact that the tube is under tension when it is pressed between the rollers 17. As a result, additional cooling or heating of the pipe changes its stress state. Raising the temperature causes frictional stresses and promotes the formation of crystal embryos on the outer surface of the tube, resulting in an increase in the amount of type III spherulites with a crystal structure. This increased density of crystals on the outer surface determines what the crystal layer will ultimately be. One important factor in a rough film is the formation of said crystal layer only on one surface of the tube when the tube is treated to bring it into the shape of the final film using a certain temperature control. The formation of the crystal layer is a physical event and s e can be clearly seen in the microscopic images taken of the tube body.

According to one embodiment of the present invention, the formation of the crystal layer occurs by raising the temperature of the outer surface of the tube by the heating means 22 just before the solidification limit very rapidly without significantly disturbing the solidification limit and without disturbing substantially balanced heat flow conditions in the mandrel and tube. It is to be expected that under these conditions a favorable temperature gradient or temperature conditions for the formation of crystal embryos will form in the tube. In addition, the sudden addition of heat to a particular point on the outer surface of the tube or the rapid inhibition of heat transfer therefrom allows the tension acting on the tube to stretch this point in the tube and this stretching or grinding also promotes the formation of crystal embryos. The use of heat and the resulting frictional stresses at a certain point before and in the vicinity of the solidification limit cause the formation of a rough-surfaced film according to the present invention. The amount of heat introduced into the outer surface of the tube is best determined empirically and by visual observations, since the crystal layer and the consequent roughness are clearly physical properties that are easily measurable.

The temperature control according to the present invention can be achieved without a separate addition of heat. For example, the temperature distribution or gradient from the inner surface of the tube to the outer surface and the higher transformation temperature on the outer surface are the properties that promote the described crystal layer formation. The best results are obtained when the distribution curve is steep and a much higher temperature is right on the outer surface of the pipe. For example, according to one embodiment of the present invention, corresponding to the apparatus of Figure 1, the polypropylene resin was isotactic polypropylene, commercially available from Dart Industries, which is usually extruded at a die temperature of about 230 ° C. However, in this example, the temperature was lowered to about 220 ° C. The tubes were passed over the mandrel so that the surface of the tube slid with a certain friction along the surface of the mandrel. The diameter of the tube was about 15 cm, the wall thickness was about 0.4 mm, and the axial propagation along the mandrel was about 10 m per minute. A more uniform elongation of the tube occurred as a result, and the resulting frictional stresses promoted the formation of crystal embryos. At this point, the cooling of the mandrel was reduced in its degree, so that the temperature of the tube rose without this fact adversely affecting the solidification limit. Cooling may be slightly reduced in the longitudinal direction of the mandrel or at a more specific point, resulting in a higher temperature on the outer surface of the tube. At this point, the radiation of heat to the environment can also be controlled using suitable shades or heat reflectors located in the control zone 22. Also in this way said crystal layer is formed in the tube and a coarse film is formed in the bubble. Measurement of the roughness of the film prepared according to the above example showed that the roughness was in the range of 20 to 40% depending on the control of the temperature of the tube at 22.

9 64534

The polypropylene film made according to the present invention has a high degree of roughness and the roughness is very uniform and uniform throughout the film. The structure of roughness in different cases is shown in Figures 2-5.

Figure 2 is a photomicrograph of a portion of a polypropylene film having no roughness. This film has a thickness of 0.018 millimeters and an image magnification of 75 times. There are a few straight scratch lines and several small curved lines that can be ridges or small cavities or depressions.

Figure 3 is a photomicrograph of a 0.018 mm thick polypropylene film having a surface roughness of about 20% measured as described above at 70x magnification. The curved lines delimiting the craters or recesses are now clearly visible. Upward elevations, i.e., fibroids, can rise 2-3 microns from the flat surface of the membrane and are uniform and continuous. By uniformity and continuity is meant that a continuous film body, at least several meters long, or a film on a take-up reel, has the roughness of Figure 3 uniformly distributed on one surface of the film.

Figure 4 shows a piece of polypropylene film having a thickness of 0.018 mm at a magnification of 75 times and a roughness of 30% as measured by the light measurement method described above. In this figure, a large number of fibroids or craters are intertwined and superimposed and are uniformly and evenly distributed over the entire surface of the membrane. The image appears to be a very compressed mat of thin loose fibers. This surface structure conducts ifflation fluid well, is non-sticky, and is a good example of a rough film made in accordance with this invention.

Figure 5 shows a piece of 0.018 mm thick polypropylene film magnified 75 times. The roughness of the film, measured as described above, was greater than 40%. Figure 5 shows a large number of crater-like depressions, which can be seen as fiber-like protruding lines which overlap and are intertwined. The appearance is roughly similar to a cracked glass. Figure 5 shows a very good roughness shape obtained according to the present invention.

The roughness of the film obtained according to the present invention is assumed to be due to the crystal layer on the surface of the tube described above, which is obtained by means of crystal elements provided by heat and tension. The increased roughness can be adjusted by these factors so that the roughness can be achieved completely and uniformly on large film surfaces, such as, for example, films stored in coils from which capacitor rolls can be wound. Rough films made in accordance with the present invention can be used, for example, in capacitor structures such as those disclosed in U.S. Patent RE-27,824 (Cox), 3,754,173 (Eustance) and 3,724,043 (Eustance).

A rough-surfaced film made in accordance with the present invention may have an electrically conductive coating on either its rough surface or another surface. Metallic coatings, such as aluminum, do not adhere as tightly as would be desired to a smooth polypropylene surface. The irregularities in the film surface in the rough film made in accordance with the present invention provide better contact between the metal layer and the film, as well as better bonding. When the coating is on the smooth surface of the film, a better combination is formed, i. electrode, dielectric film and rough surface. These metallized strips have an important use in capacitors, eliminating the need for separate electrode foils.

The film made in accordance with the present invention provides the considerable advantage of facilitating absorption. Since the roughness is uniform over the entire surface area of the film, the absorption step can be programmed to take this advantage into account. Because of the remarkably high degree of roughness, it is easier for the liquid to penetrate between the rough surface of the polypropylene strip and its adjacent surface, whether it is rough polypropylene or another material.

The invention can be applied to crystallizable polypropylene materials, the manufacturing properties of which generally correspond to those of isotactic polypropylene. These materials include 11,645,334 polypropylene grades such as syndiotactic polypropylene and polypropylene copolymers and homopolymers, as well as blends of polypropylene with other synthetic plastics.

The rough film made in accordance with the present invention can be advantageously used as an electrical insulating material and product, especially for purposes where roll winding and roll opening are used or where impregnation or drying steps are required. The rough surface not only facilitates winding and winding on the roll, as it does not have sticky properties, but also forms passageways for the removal of vapors during the drying stage. The rough film of the present invention is also very suitable for printing on it. One previous dilemma associated with smooth-surfaced films is that plain printing ink does not adhere well to it and rubs off easily. The uneven surface of the rough film provides a better surface for adhesion of inks and printing inks.

Claims (3)

12 64534 A bubble process for producing a rough-surfaced polypropylene film, comprising a) melting molten polypropylene with a die (14) into a tube (16), b) conducting a tube (16) coaxially over a cooling mandrel (15) to crystallize and solidify the tube, subjecting the tension to biaxially oriented bubbles, and e) causing the bubble to collapse and cut into a film, the roughness being provided by a temperature difference between the surfaces of the tube, characterized in that the temperature difference is obtained by causing the tube to a droplet surface at a certain point between the nozzle (14) and the solidification limit to solidify the outer surface of the tube uniformly so that spherulites of crystalline type III are formed in the outer layer of the tube, the roughness of the film being formed by ibroid irregularities such that the result of the total light diffusion measurement of the well is from about 20% to about 40%. A method according to claim 1, characterized in that said point where the tube (16) is heated is spaced from the nozzle (14) and very close to the solidification limit. A method according to claim 1 or 2, characterized in that the tube (16) moves over the mandrel (15) at a speed of about 10-12 meters per minute and has a wall thickness of about 0.36-0.5 mm and that said temperature and the tension provides a rotating effect and a uniform solidifying effect on the outer surface of the tube so that a dense layer of type III spherolites with a crystal structure is formed on the outer surface of the tube. !