JPH0146304B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a polyamide film, in which a polyamide film melt-extruded from a die is rapidly cooled by bringing it into close contact with cooling rolls by electrostatic pinning, which improves work safety. The purpose of this is to improve the homogeneity of the film obtained. (Prior art) A method of rapidly cooling and forming a polyamide film by electrostatic pinning has been known for a long time. A method is described in which a corona discharge is performed and a high current is applied to the film to bring it into close contact with a cooling roll. (Problems to be Solved by the Invention) However, this method requires a high current, which poses a major problem in work safety. Tokuko Akira
In the examples of Japanese Patent No. 59-23270, the current value when forming a polyamide film by electrostatic pinning using a 90 mm extruder at a film forming speed of 50 m/min is exemplified as 25 mA. Therefore, on a larger production scale, for example when producing a wide film using a 150 to 250 mm extruder, which is commonly used in production equipment for industrial polyamide biaxially oriented films, the amount of current is approximately Since it is proportional to the film width, a larger current value is required, for example, about 100 mA. The electrostatic pinning electrode is installed near the T-die, and a worker often approaches this location to operate the adjustment bolt of the T-die lip, for example, to adjust the thickness. Therefore, there is a risk of electric shock if the terminal of the high voltage lead wire comes off due to accidental contact with the electrode or due to some kind of trouble. DC high voltage electric shock is extremely dangerous and
It is known that if the current exceeds mA, it will cause a serious disaster, and if it exceeds 100mA, it will sometimes lead to a fatal accident. Naturally, various safety measures are absolutely necessary to prevent these accidents. For example, there is a method of turning off the power when the operator approaches the electrode, and a method of remotely controlling the adjustment bolt of the T-die lip. However, since the behavior of the film between the T-die and the cooling roll is the most delicate and important in the film manufacturing process, these restrictions pose a serious obstacle to operation and make it impossible to obtain a high-quality film. Furthermore, no matter how strict safety measures are taken, there is still the possibility of serious accidents due to operator error or equipment failure. The second problem with the method of forming a polyamide film by electrostatic pinning is that the thickness of the film is disturbed at the portion where the streamer corona is generated. In order to prevent this, JP-A-56-105929 discloses that the ratio S/N between the applied current S [mA] and the number N of discharge points of the electrode that discharges the streamer corona is kept low, and the local strength of the streamer corona is reduced. A method to weaken it is described. However, this method requires very strict control of the accuracy of the electrode, and if the film forming speed is slow or the specific resistance is around 10 ⁴ Ω-cm, such as with nylon 6 mixed with an antistatic agent, the This is an impossible method. That is, when cooling a polyamide film, the conventional method of applying a high current to the film by performing corona discharge in a streamer corona state had serious problems such as operational safety problems and disturbances in the thickness of the streamer discharge point. The present invention relates to a method for solving these problems. (Means for solving the problem) When applying an electrostatic charge to a polyamide film whose specific resistance in the molten state is 3×10 ⁵ Ω・cm or less, a large amount of current leaks from the film to the cooling roll. It is believed that it would be necessary to impart more charge to the film than is necessary to create an electrical attraction between the film and the chill roll, thus requiring a high current discharge, i.e., a streamer corona discharge. As a result of research into a method of reducing leakage current by providing an electrical resistor between the electrode and the cooling roll, the present invention found that by providing an electrically insulating coating on the surface of the cooling roll, a streamer state is prevented. The present invention was achieved by learning that electrostatic pinning of polyamide films is possible with low current discharge. The electrically insulating coating here refers to organic polymer materials such as fluororesin, acrylic resin, and polyester resin, or electrically insulating ceramic materials such as Al ₂ O ₃ , and usually a combination of these is most suitable. ing. The coating layer has a dielectric breakdown voltage of 5KV or more,
More preferably, the material and thickness are selected by first considering the condition of having a withstand voltage of 10 KV or more. This is because it is necessary to avoid damage to the coating layer if arc discharge occurs due to some trouble during the film forming operation. However, since electrically insulating coating materials generally have low thermal conductivity, there is a problem in that if the coating layer is made too thick, the cooling efficiency of the cooling roll will deteriorate. For this reason, the coating layer has a dielectric breakdown voltage
A range of 5KV or more and 15KV or less is generally suitable, and the optimum one is selected from this range depending on the film forming conditions such as the specific resistance of the polymer, film thickness, speed, and cooling roll temperature. At this time, setting the apparent applied density to 2 μC/cm ² is the second criterion for selecting the material and thickness of the electrically insulating coating. The apparent applied density here is the value obtained by dividing the current consumed by the electrode by the product of the film width and velocity.
For example, a film with a width of 1 m (100 cm) can be filmed at a speed of 40
If the current value when forming a film at m/min (66.7 cm/sec) is 4 mA, the apparent applied density is
It is 0.6 μC/cm ² . A lower apparent applied density is preferable from the viewpoint of work safety, but if it is too low, film formation becomes difficult. A range of 2 μC/cm ² or less is suitable from the viewpoint of the balance between safety, film formability, and difficulty in forming the coating layer. The electrode is generally a steel wire or a tungsten wire with a diameter of 0.1 to 0.3 mm and is disposed at a position of 1 to 10 mm from the film surface above the position where the film contacts the cooling roll. A positive or negative DC voltage of 5 to 15 KV is applied to the electrodes by a DC high voltage generator.
Although the polarity may generally be either positive or negative, it is often more preferable to apply a polarity voltage to the electrode that is opposite to the charging polarity of the coating material determined from the charging series of the insulating coating material and the film. In addition, if there is uneven charging on the insulating coating material just before it comes into contact with the film, the electric field will be disturbed and the uniform adhesion between the molten polyamide and the cooling roll will be inhibited. It is necessary to reduce the surface potential to 300V or less, more preferably 100V or less. (Function) By applying the method of the present invention, it becomes possible to rapidly cool and form a polyamide film by bringing it into close contact with a cooling roll at a low current that does not reach the streamer corona discharge region. As a result, operational safety is improved and film uniformity is improved, with no thickness disturbances occurring at streamer corona discharge points. Polyamide films to which the present invention is applied include films made of homopolymers such as nylon 6, nylon 66, nylon 6-10, and nanlon 4-6, or mixtures thereof, and films that do not change the basic properties of these polyamides. A film made of a copolymer or the like, containing additives such as a plasticizer, a lubricant, and an antioxidant, and having a specific resistance of 3×10 ⁵ Ω·cm or less in a molten state. Furthermore, by the method of the present invention, an antistatic agent is added to a low resistivity polymer, especially a polyamide film, which has been difficult to achieve with conventional methods, and the resistivity in the molten state can be reduced to 10 ⁴ Ω.
It becomes possible to produce films using the electrostatic pinning method down to the cm level. When the polyamide film produced by the method of the present invention is further biaxially stretched, a biaxially stretched polyamide film with improved thickness accuracy, optical uniformity, etc. is obtained, and the effects of the method of the present invention are further enhanced. Examples 1 to 4 A 150μ nylon 6 film (melting ratio measured at 260°C) was produced using a film forming apparatus consisting of a 90mm extruder, a 400mm width T-die, a 900mm diameter cooling roll, and a 600W (15KV x 20mA) DC high pressure generator. Resistance 1.7×10 ⁵ Ω・cm)
A film was formed. The cooling roll has a chrome-plated surface (comparative example) and a ceramic (Al ₂ O ₃ ) surface.
Two types were used: one coated with 0.15 mm thick (Example). The breakdown voltage of the ceramic coating layer is
It was 8.5KV. In addition, carbon brushes with a length equal to the width of the cooling roll were arranged in two rows 30 cm upstream from the point where the film contacts the surface of the coating layer, and were placed in contact with the cooling roll, and the carbon brush holder was grounded. , the surface potential of the ceramic coating layer was neutralized to below 100V. The electrode is made of brass and has needles with a diameter of 0.8 mm and a length of 8 mm embedded in a brass plate with a pitch of 2.5 mm (comparative example).
and a tungsten wire with a diameter of 0.2 mm (example). The effective widths of the electrodes are both 270 mm, the distance between the electrode and the film is 7 mm in the case of the comparative example, the distance between the electrode and the cooling roll is 15 mm, and the distance between the electrode and the cooling roll is 6 mm in the case of the example.
The electrodes were placed at a distance of 8 mm. Table 1 shows the film forming conditions and the variation in film thickness (difference between maximum and minimum thickness in the width direction). (Effects of the Invention) As is clear from the above explanation, the method of the present invention enables the electrostatic pinning formation of polyamide films with a low current that is safe for work, and the uniformity of the obtained film is improved by the film formation speed. It is excellent in a wide range of conditions. As a result, even if the electrostatic pinning film forming method is applied to a biaxially stretched film manufacturing process where the speed at the film forming stage is relatively slow, a film with excellent uniformity can be obtained, and the film forming speed can be increased. Even when the conventional method requires a dangerous amount of current, the method of the present invention makes it possible to form a film with a safe low current.

【table】

[Table] Area speed = Film forming speed x Film width Applied density = Current / Area speed

Claims (1)

[Claims] 1 Extrude a molten polyamide resin with a specific resistance of 3×10 ⁵ Ω・cm or less through a die as a film,
When casting onto a cooling roll, an electrostatic charge is applied to the film by an electrode placed above the film, and an electric attraction between the film and the grounded cooling roll causes the film to be cast onto the cooling roll. In the method of closely contacting and cooling, an electrically insulating coating with a dielectric breakdown voltage of 5 KV or more is provided on the surface of the cooling roll, and the surface potential of the coating is removed to 300 V or less just before contacting the film, and the electrode and molten polyamide are removed. A method for rapidly cooling and forming a polyamide film, characterized by applying a non-streamer corona discharge between the film and the film to impart a charge with an apparent applied density of 2 μC/cm ² or less to the film.