JPS6119415B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of making a film in which a thermoplastic polymer is melt extruded and quenched onto a rotating cooling surface. More specifically, the rotary cooling body is used as a counter electrode,
This technology relates to an improved film manufacturing method in which a high potential electrostatic charge is applied to a metal wire, and the film is electrostatically adhered to the surface of a rotating cooling body and rapidly cooled. This invention relates to a method of manufacturing a polymer film in which an auxiliary electrode is provided to tightly adhere the film to the surface of a rotating cooling body. When producing a film by melt extruding a thermoplastic polymer, a wire or knife-shaped electrode is provided between the die and the rotating cooling body, and the rotating cooling body is used as a counter electrode to extrude the molten sheet material (film). A so-called electrostatic pinning method in which an electrostatic charge is deposited on the surface, the film is uniformly brought into close contact with the surface of the rotary cooling body, and the film is rapidly cooled is known, for example, from Japanese Patent Publication No. 37-6142. However, if the circumferential speed of the rotary cooling body is increased in order to improve productivity, the adhesion effect of this electrostatic pinning method will be drastically reduced. That is, as the circumferential speed of the rotary cooling body increases,
The amount of precipitated charge per unit area of the sheet on the surface of the cooling body decreases, resulting in a decrease in adhesion.
Air is caught between the sheet-like material and the surface of the cooling body. In this condition, the trapped air layer causes
It becomes difficult for heat to be efficiently transferred to the cooling body, and cooling efficiency decreases. Furthermore, trapped air bubbles are formed between the sheet-like material and the cooling body, which is not acceptable as a quality characteristic required for the product. Even if attempts are made to increase the voltage applied to the wire in order to increase adhesion, spark discharge tends to occur between the wire and the cooling body, and there is a limit to the applied voltage, so sufficient pinning cannot be achieved. No effect has been obtained. When a spark discharge occurs, pinholes are created in the sheet-like material and the product cannot be produced. In sheet materials formed by the electrostatic pinning method, unevenness in film thickness occurs, which is presumed to be caused by non-uniform discharge, and this unevenness in thickness often accompanies periodicity. Various proposals have been made to compensate for the drawbacks of such electrostatic pinning methods. For example, British patent no.
No. 1309644 discloses an electrostatic fixing device having two electrodes, the first electrode being a metal wire held at a high potential, the second electrode being grounded, and the first electrode being a metal wire held at a high potential. An insulating layer is provided between the electrodes, and an electric field is created between the electrodes. Furthermore, in order to extend the life of the wire, JP-A-51-114466 discloses a method in which the part on the die side is shielded with an insulating material or metal to prevent sublimate from adhering to the wire from the molten polymer. be. These techniques are still insufficient from the viewpoint of increasing the charge density to enhance the adhesion effect in electrostatic pinning. The present invention uses a metal wire as a main electrode, a rotary cooling body as a counter electrode, and further provides an auxiliary electrode on the opposite side of the metal wire to the rotary cooling body to increase charge density in a polymer film forming process. It is. The auxiliary electrode is made of a conductive material such as aluminum or copper, has a flat or curved shape, and is grounded. The present invention provides an improved electrostatic pinning method, which is a technique that makes it possible to increase the circumferential speed of a rotary cooling body and to obtain a film of uniform thickness. The present invention will be explained. Figure 1 shows the wire electrode and rotary cooling body (opposed flat plate electrode) when electrostatic pinning was applied during film formation.
This is a measurement of the current flowing between the two. Straight line 1 is the conventional method that does not use an auxiliary electrode, curve 2 is the auxiliary electrode made of an aluminum flat plate, curve 3 is the auxiliary electrode made of an aluminum cylindrical umbrella (see Figure 2 A), and curve 4 is the auxiliary electrode made of aluminum. This is the case when an auxiliary electrode with a triangular cross section (see FIG. 2B) is used. In both cases, the distance between the metal wire and the counter electrode was kept constant at 6 mm, and the distance between the metal wire and the auxiliary electrode was varied to measure the discharge current. From Figure 1, it can be seen that when a grounded auxiliary electrode is installed on the opposite side of the main electrode to the main electrode, the electric field density increases and the discharge current increases; (b) The auxiliary electrode has a flat, square or triangular cross section It can be seen that both materials, such as those with a circular cross section and those with an arcuate cross section, are effective. FIG. 2 is an example of a typical auxiliary electrode that can be used in the present invention. FIG. 2A is a side view showing a state in which the auxiliary electrode 11 having an arcuate cross section is provided so as to cover the metal wire main electrode 12. Also, Figure 2
A' is a perspective view (frog view) seen from the direction of the counter electrode (rotary cooling body), in which both ends of the main electrode 12 are covered with an insulating material 13, and the auxiliary electrode 1 has an umbrella-like semi-cylindrical shape.
1 is provided. Further, FIGS. 2B and 2B' show an example of an auxiliary electrode having a triangular cross section, and are a side view and a perspective view, respectively. The auxiliary electrode may have an umbrella-like shape as shown in FIG. 2, or it may be a flat plate. When the angle between the main electrode metal wire and both ends of the umbrella is less than 60 degrees, the discharge current decreases. Therefore, there is a certain limit to the distance between the wire and the auxiliary electrode. When forming a film, if the space in which the metal wire is installed, that is, the space between the die and the rotary cooling body, is sufficiently wide and the ambient temperature around the wire is about 30°C, good casting cannot be expected. On the other hand, it is known from experience that casting is improved by shielding the vicinity of the wire and raising the ambient temperature to about 130°C. Figure 3 shows the relationship between wire ambient temperature and effective discharge current when corona discharge is generated between the wire and the counter electrode (rotary cooling body).
It can be seen that the effective discharge current increases exponentially as the wire ambient temperature increases. The case in FIG. 3 is also an example in which the distance between the metal wire and the counter electrode (rotary cooling body) is 6 mm, and the effective length of the wire is 50 mm. Due to the mechanism of the close contact between the molten polymer and the rotary cooling body using the electrostatic pinning method, the entrainment of air during high-speed film formation results in insufficient adhesion, which means that the precipitated charges at the point where they come into contact on the surface of the rotary cooling body and solidify after being cooled. This is due to a lack of quantity. Therefore, the increase in the discharge current results in an increase in the adhesion at the point of cooling and solidification. That is, if a grounded conductive auxiliary electrode is installed close to the side facing the rotary cooling body for one wire, corona discharge will occur even at a low voltage, and an increase in discharge current can be obtained. This means that corona discharge occurs in the entire space surrounded by the auxiliary electrodes, wires, and rotating cooling body.
This is because an increase in ion concentration can be obtained. The shape of the auxiliary electrode may be a flat plate, an umbrella-shaped one with a square or triangular cross section, or a cylindrical one. Here, in order for the effect as an auxiliary electrode to be effective, if the distance between the wire and the rotating cooling body is r ₁ and the shortest distance between the wire and the auxiliary utility pole is r ₂ , then r ₂ /r ₁ ≦5. It is necessary to place an auxiliary electrode. Furthermore, when operating at a potential just before spark discharge occurs between the wire and the rotary cooling body, it is obvious that r ₂ /r ₁ must be 1. () The higher the ambient temperature, the larger the discharge current, and therefore the wire ambient temperature needs to be higher. This mechanism is thought to be caused by an increase in ion density due to the promotion of corona ionization of the gas by thermal energy. Furthermore, when cold air is blown near the wire and the ambient temperature changes spatially or temporally, the discharge current changes, which leads to changes in the adhesion between the quick-melting polymer and the rotating cooling body. Therefore, it is necessary to stably maintain the temperature near the wire at T≧100°C. On the other hand, corona discharge in a non-uniform electric field such as a wire-to-flat or needle-to-flat electrode system is accompanied by an ion wind (also called a corona wind) that moves from the wire or needle toward the flat plate. This wind speed is 1~2m/
It was hot in sec. As mentioned above, by bringing cold air near the wire, ion wind fluctuates the discharge current due to the influence of the ambient temperature, and also suppresses the vibration of the molten polymer existing in the space between the die orifice and the contact point on the rotating cooling body. cause it to occur. As described above, it is necessary to minimize fluctuations in the atmosphere near the wire. For this reason, in order to isolate the wire atmosphere from the outside, reduce fluctuations in the atmosphere due to ion wind, etc., and maintain a high temperature, the auxiliary electrode should preferably have a shape that covers the wire, that is, an umbrella-shaped one. It goes without saying that the wire-side surface of the auxiliary electrode should be smooth to prevent electric field concentration and spark discharge. The shape of the umbrella is triangular or square in cross section.
Although a cylindrical shape may be used, preferably a coaxial cylindrical shape with the same distance from the wire to the umbrella is used to make the corona atmosphere near the wire homogeneous and stabilize the atmosphere. Furthermore, in order to heat the atmosphere and prevent the adhesion of oligomers, a heater can be embedded in the auxiliary electrode material and its temperature can be controlled.
In this case, the surface temperature T of the auxiliary electrode on the side facing the wire is controlled to be constant at 105° C. or higher. Preferably, when the shape of the auxiliary electrode for controlling the constant temperature at T≧150° C. is umbrella-shaped, it is optimal to determine the shape of the umbrella so that the wire is located at its geometric focal point. This has the effect of heating the wire with radiant heat from the umbrella and preventing the attachment of oligomers and the like to the wire. There is no particular limit to the width of the opening of the umbrella for imparting a charge to the molten polymer, but if the angle between the wire and both ends of the umbrella is less than 60°, the discharge current will decrease and the purpose will not be achieved. Do not fit. If the wire is fixed to the umbrella via an insulator and used as a corona charging contact device, it is extremely effective for adjusting the optimum position of the wire. FIG. 4 is a side view (partial view) showing the film forming state of the present invention. The melted thermoplastic polymer is transferred to die 1.
7 and casted on the surface 15 of the rotary cooling body 14 as a sheet-like material 16. During casting, high-voltage static electricity is applied to the metal wire 12 by a high-voltage power supply 18 so that static charges are deposited on the sheet-like material, and corona discharge occurs between the metal wire 12 and the rotating cooling body 14 serving as a counter electrode. An auxiliary electrode 11 is placed on the opposite side of the rotary cooling body with a metal wire in between.
is provided. The auxiliary electrode 11 and the rotary cooling body 14 are grounded. The auxiliary electrode is provided so as to cover the metal wire at an angle of at least 60 degrees. The thickness of the film produced by this method was more uniform than that produced by the conventional electrostatic pinning method. This, along with the effect of stabilizing the atmosphere,
Another possible effect is that the discharge between the auxiliary electrodes forms a corona atmosphere in the entire space around the wire, making the discharge uniform. This invention covers any thermoplastic polymeric material that can be formed into a smooth film by extrusion and cooling;
For example, it is applicable to polystyrene, polyamide, vinyl chloride polymers and copolymers, olefin polymers and copolymers such as polycarbonate and polypropylene, and polyesters condensed from dibasic aromatic carboxylic acids and dihydric alcohols. can. This invention is particularly suitable for forming molten polyethylene terephthalate films. Therefore, examples of the present invention are shown below regarding polyethylene terephthalate films. Comparative Examples A to E Using a binding device substantially similar to that shown in Japanese Patent Publication No. 37-6142, a polyethylene terephthalate film was extruded and electrostatically placed on a cooling drum (rotary cooling body). I was tied up. The voltage applied to the restraint devices was unified at +60KV. This result is the first
As shown in the table, when the circumferential speed of the cooling drum exceeded 45 m/min, air was trapped between the polyethylene terephthalate film and the surface of the cooling drum due to insufficient adhesion, causing bubbles to form. Examples 1 to 5 Molten polymer binding device according to the present invention (Figure 4)
The polyethylene terephthalate was extruded into a film and electrostatically bound onto a rotating cooling body.
At this time, the voltage applied to the metal wire was +6.0 KV as in the comparative example, and the auxiliary electrode had a cylindrical shape. The ratio of the distance r ₁ between the wire and the conductive auxiliary electrode to the distance r ₂ between the wire and the rotating cooling body is r ₂ /r ₁ =1.5. The results are shown in Table 2. Bubbles were generated when the circumferential speed of the rotary cooling body was 50 m/min or more. Examples 6 to 10 Using the same apparatus as Examples 1 to 5 (Fig. 4),
The surface temperature of the auxiliary electrode was maintained at 280°C using a heater provided in the auxiliary electrode, and the polyethylene phthalate film was bound to the surface of the rotary cooling body. At this time, the voltage applied to the metal wire was +6.0 KV, and the shape of the auxiliary electrode was cylindrical. Further, r ₂ /r ₁ was set to 2.0. The results are shown in Table 3. No bubbles were observed until the circumferential speed of the rotary cooling body was 55 m/min, and extremely fine bubbles were observed for the first time at 60 m/min or higher. Comparative Example F and Example 11 Raw films produced using an apparatus substantially similar to the apparatus shown in Japanese Patent Publication No. 37-6142 and using an apparatus according to the present invention Table 4 shows a comparison of the thickness unevenness. The thickness unevenness of the film produced according to the present invention is significantly improved.

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[Table] Average vertical thickness

[Brief explanation of the drawing]

FIG. 1 is a graph showing the amount of discharge current when an auxiliary electrode is provided (the present invention) and when the auxiliary electrode is not provided (the conventional method). FIG. 2 is a side view and a perspective view showing an auxiliary electrode used in the present invention. FIG. 3 is a graph showing the metal wire ambient temperature and discharge current amount according to an embodiment of the present invention. FIG. 4 is a side view (partial view) showing an embodiment of the present invention. In the drawings, 11 is an auxiliary electrode, 12 is a metal wire, 14 is a rotating cooling body, and 18 is a power source.

Claims (1)

[Claims] 1. A method for producing a film in which a molten thermoplastic polymer is extruded from a die onto a rotating cooling body, and is cooled and solidified while electrostatic pinning is performed using a metal wire stretched parallel to the rotating cooling body. a main electrode, and a main electrode provided on a side of the wire opposite to the rotary cooling body and grounded;
The film is made using a flat auxiliary electrode made of a conductive material or an auxiliary electrode made of a conductive material having a shape that covers the wire and has an open opening in the direction of the position where the extruded film contacts the rotary cooling body. 1. A method for producing a polymer film, which comprises electrostatically bringing the film into close contact with the rotary cooling body.