The present invention relates to a static eliminator and a static eliminating method for eliminating charges from an insulating sheet. Furthermore, the present invention relates to a method for producing an insulating sheet using said static eliminator or said static eliminating method, and also to an insulating sheet.

The charges of an insulating sheet such as a plastic film can prevent the sheet from being processed as desired. As a result, it can happen that the quality of the processed sheet does not come up to the expected level. For example, in the case where a sheet having locally strong charges and discharge marks called static marks caused by electrostatic discharge is printed or coated with a coating material, the processed sheet has irregularity of the ink or coating material. In a process for producing a metallized film to be used, for example, in a capacitor or for packaging, the processed sheet can have static marks after completion of film processing such as vacuum evaporation or sputtering. The strong charges such as static marks cause the film to adhere to another member due to electrostatic force, hence causing such various problems as miscarriage, positioning failure and disarrangement of cut sheets.

The conventional static eliminators used to obviate such problems include the following: a self-discharge type static eliminator in which a grounded conductor shaped like a brush is brought close to the insulating sheet, to cause corona discharge at the tip of the brush for eliminating charges, and an AC or DC voltage application type static eliminator in which a power-frequency high voltage or DC high voltage is applied to a needle electrode to cause corona discharge for eliminating charges.

A conventional static eliminating method using corona discharge is described below. Fig. 1 is a drawing showing the principle of a conventional static eliminating method for an insulating sheet. In Fig. 1, a static eliminator 1 causes corona discharge by means of an ion generating electrode 1b connected to an AC power supply 1a and an earth electrode 1c, for generating positive ions 301 and negative ions 302 near the ion generating electrode 1b. Of the positive and negative ions, the positive ions 301 are attracted by an insulating sheet S due to the Coulomb force 700 acting between the positive ions 301 and the negative charges 102 of the sheet, to be balanced by the negative charges 102. As a result, the negative charges 102 of the insulating sheet S are eliminated.

However, actually, it is not rare that the charges of the sheet S are not eliminated according to the principle. The surface resistivities and volume resistivities of insulating sheets such as polyethylene terephthalate films, polypropylene films and aramid films used as photographic films, capacitor films and magnetic tape films are high. Therefore, the charges once generated in the sheet S can little migrate in the in-plane direction or in the thickness direction of the sheet. For this reason, if the potential of the sheet S rises with a large amount of negative charges accumulated, discharge can be caused between the sheet S and a grounded component used for carrying the sheet S or the like existing near the sheet S. In a sheet with a high surface resistivity and a high volume resistivity, since the migration of charges due to discharge is confined within local sites, it can happen that when discharge occurs, the local negative charges are excessively taken away to form sites having positive charges.

The discharge marks that are the marks of this discharge are static marks. If static marks are formed, there occurs a situation where positive charges 101 and negative charges 102 exist together in the sheet S. As shown in Fig. 2, if charges of positive polarity (positive charges 101) and charges of negative polarity (negative charges 102) are alternately formed at a small pitch, that is, if two kinds of charges with relatively high charge densities but opposite to each other in polarity exist close to each other, there occurs a phenomenon that the lines of electric force 500 attributable to the charges of the sheet S are closed between the respectively adjacent charged sites opposite to each other in polarity. Therefore, there occurs a situation where the Coulomb force 700 little acts on the ions near the static eliminator located a little away from the sheet S. As a result, ions are little attracted by the sheet S, and the charges 101 and 102 in the sheet S are little eliminated.

As shown in Fig. 3, there can be a case where positive charges 101, 201 and negative charges 102, 202 exist in both the surfaces of the sheet S. For example, in the case where a large amount of negative charges 102 exist in the first surface 100 of the sheet S, it can happen that discharge occurs between the sheet S and a grounded component (for example, a carrier roll) located close to the second surface 200 of the sheet. In this case, the negative charges 102 in the first surface 100 of the sheet remain also after discharge as they are, and the discharge causes sites having positive charges 201 to be formed in the second surface 200 of the sheet S. If such discharge occurs on both the first surface 100 and the second surface 200 of the sheet S, there occurs a situation where positively charged sites and negatively charged sites exist together in both the first surface 100 and the second surface 200 of the sheet S as shown in Fig. 3. Also in this case, the lines of electric force 500 attributable to the charges of the sheet S are closed between the negative charges 102 in the first surface 100 and the positive charges 201 in the second surface 200. So, Coulomb force does not act on the ions existing near the static eliminator either, and necessary ions cannot be attracted.

That is, in the case of a sheet having a fine charge pattern, i.e., a sheet where positively charged sites and negatively charged sites are alternately formed at a small pitch in one surface or where they exist together in both the surfaces, the lines of electric force 500 are closed near the sheet S. As a result, the Coulomb force 700 acting on the ions 301 and 302 located a little apart from the sheet S (near the static eliminator) is small, and the ions cannot be attracted toward the sheet S.

Measured charge densities of sheets having positively charged sites and negatively charged sites existing together in both the surfaces are stated in "Transactions on Fundamentals and Materials A (in Japanese), Vol. 112, No. 8, pages 735-740, The Institute of Electrical Engineers of Japan, August 1992 (hereinafter called document DS1)." According to the measured charge densities stated in document DS1, the charge densities in the first surface of a film as an insulating sheet are about -23 µC/m², and the charge densities in the second surface of the sheet are about +23 µC/m². In document DS1, the charges of such a film are called "both-side bipolar charges."

On the other hand, the inventors confirmed the local charge densities at sites of sheets having a fine charge pattern such as static marks according to the method described later. As a result, it was found that there exist local sites having charge densities of about several to about 500 µC/m² in absolute value in the respective surfaces, and that there exist some local sites in which the sums of the local charge densities of both the surfaces at the same sites in the in-plane direction of the sheet (apparent charge densities) were about 1 to about 40 µC/m² in absolute value. These values are very large compared with the average charge densities generated due to the frictional electrification in an ordinary sheet production process. The average charge densities are said to be usually in a range from about 0.1 to about 1 µC/m².

Especially it was found that in a fine charge pattern such as static marks, there were sites where the charge densities of the respective surfaces (for example, the charge density on the first surface 100 of a sheet was +500 µC/m², while the charge density on the second surface 200 at the same position was -480 µC/m²) were far larger than the apparent charge densities (+20 µC/m² in the above example) (usually about 1 to about 40 µC/m² in absolute value). In the invention, the distribution of the quantities of charges in a sheet is mainly evaluated using the distribution of local charge densities. Unless otherwise stated, a charge density means the value of a local charge density of a sheet. As described above, in a sheet with a charge pattern such as static marks, the sums of charge densities of both the surfaces at the same site in the in-plane direction of the sheet (the apparent charge densities) are greatly different from the values of the charge densities of the respective surfaces at the same site.

In this specification, the sum of the (local) charge densities of both the surfaces at the same site in the in-plane direction of a sheet means the apparent charge density (the charge density identified without considering the distribution in the thickness direction) of the sheet at the site. This definition is important in the invention.

In the case where the apparent charge densities at the respective sites in the in-plane direction of a sheet are zero, the sheet appears to be non-charged, and in the case where they are not zero, the sheet appears to be charged. As described in document DS1, it has been known that an insulating sheet such as a film is bipolar-charged in both the surfaces. However, there is no report that has locally examined charge densities, and the description concerning static elimination relates to the apparent charges of a sheet. On the contrary, in discussing the statically eliminated state of an insulating sheet, the inventors have clarified that it is essentially important to examine both the apparent charge densities and the charge densities of the each surface.

For eliminating charges from an insulating sheet having such a charge pattern, usually a large quantity of the ions from a static eliminator are applied near to the sheet S without resorting to the Coulomb force acting due to the charges of the sheet.

As a technique for eliminating charges from an insulating sheet having such a charge pattern, a static eliminator as shown in Fig. 4 is known. The static eliminator 2 is disclosed in JP 2651476 C (hereinafter called document DS2). In Fig. 4, the static eliminator 2 consists of plural positive and negative ion-generating electrodes 2b connected with an AC power supply 2a and a planarly spread ion-attracting electrode 2d connected with an AC power supply 2c, and the positive and negative ion-generating electrodes 2b and the ion-attracting electrode 2d are installed to face each other through a traveling insulating sheet S. In the static eliminator 2, the positive and negative ion-generating electrodes 2b generate positive and negative ions, while high voltages opposite to the positive and negative ion-generating electrodes 2b in polarity are alternately applied to the ion-attracting electrode 2d, so that the positive and negative ions generated by the positive and negative ion-generating electrodes 2b can be attracted by the ion-attracting electrode 2d, to be forcibly irradiated to the sheet S.

As a result, positive and negative potentials are alternately induced in the sheet S, and the positive and negative ions from the positive and negative ion-generating electrodes 2b are forcibly attracted by the surface of the sheet S. So, it is said that even a sheet with a fine charge pattern can undergo static elimination. It is said that the statically eliminating action of the static eliminator 2 can be confirmed with a negative toner powder (black fine powder) used in a copier or the like to be electrostatically deposited on the sheet.

In this case, since the sheet is a thin insulator, the toner powder is deposited on the sites where the apparent charge densities are high. That is, a site where no toner powder is deposited means a site where the sheet is apparently non-charged (where the apparent charge density is almost zero).

However, the inventors confirmed that even if an insulating sheet is apparently non-charged by such static elimination, the sheet reveals its original charge pattern when it is processed to have a metalized film or to be coated. That is, it was found that the static eliminator 2 of document DS2 could not provide a sufficient static elimination effect. These can be actually confirmed since such defects as the irregularities of ink or coating material, static marks formed after such film processing as vacuum evaporation or sputtering, and disarrangement of cut sheets due to sliding failure actually occur. This is an essential problem, since the static eliminator of document DS2 can eliminate only the apparent charges described before.

This problem is described below in reference to Figs. 5 to 7. In Fig. 5 and Fig. 6, an ion-generating electrode 2b is merely described to simplify the figure. It is assumed that in the sheet undergoing static elimination, positive charges 101 and 201 and negative charges 102 and 202 exist together in the respective surfaces 100 and 200 as shown in Fig. 5. As shown in Fig. 5, when the voltage applied to the positive and negative ion-generating electrode 2b is positive while the voltage applied to the ion-attracting electrode 2d is negative, the positive ions 301 generated by the positive and negative ion-generating electrode 2b are attracted near to the sheet S along the lines of electric force 500 generated by the positive and negative ion-generating electrode 2b and the ion-attracting electrode 2d, and are deposited on the first surface 100 of the sheet S, to positively charge the sheet S.

In this case, if there sites negative charges 102 exist in the first surface 100 of the sheet S, the positive ions 301 attracted selectively more to the sites than to their surroundings, for eliminating the negative charges. The reason is that since the positive ions 301 are carried near to the sheet S and go into the space where the charges 101, 102, 201 and 202 form the lines of electric force 500 closed near the sheet S, Coulomb force 700 acts between the positive ions 301 and those charges.

As shown in Fig. 5, in the case where the positive and negative charges 101, 102, 201 and 202 exist together in the respective surfaces 100 and 200 of the sheet S, the positive ions 301 are attracted more at the sites where the apparent charge densities are negative. That is, in the case where the positive charges 101 do not exist in the first surface 100 of the sheet S at the same sites in the in-plane direction of the sheet or in the case where even if the positive charges 101 exist, their quantity is smaller than the quantity of the negative charges 102 in the second surface 200 in the in-plane direction of the sheet, the positive ions 301 are attracted not only at the sites where only the negative charges 102 exist in the first surface 100 of the sheet S but also at the sites where the negative charges 202 exist in the second surface 200 of the sheet S.

Then, as shown in Fig. 6, if the voltage applied to the positive and negative ion-generating electrode 2b is switched to be negative (the voltage applied to the ion-attracting electrode 2d is positive), the negative ions 302 generated by the positive and negative ion-generating electrode 2b are attracted near to the sheet S along the lines of electric force 500 generated between the positive and negative ion-generating electrode 2b and the ion-attracting electrode 2d, and are deposited on the first surface 100 of the sheet S, to negatively charge the sheet S.

In this case, if there are sites having positive charges 101 in the first surface 100 of the sheet S, the negative ions 302 are attracted selectively more to the sites than to their surroundings, for eliminating the positive charges. Also in this case, the negative ions 302 are attracted more at the sites where the apparent charge densities of sheet S are positive.

Therefore, in the case where the negative charges 102 do not exist in the first surface 100 at the same sites in the in-plane direction of the sheet or in the case where even if the negative charges 102 exist, their quantity is smaller than the quantity of the positive charges 201 existing in the second surface 200 in the in-plane direction of the sheet, the negative ions 302 are attracted not only at the sites where the positive charges 101 exist in the first surface 100 of the sheet S but also at the sites where the positive charges 201 exist in the second surface 200 of the sheet S.

Since plural positive and negative ion-generating electrode 2b are installed in the traveling direction of the sheet, these actions are alternated, and the first surface 100 (the top surface in Figs. 5 and 6) of the sheet S is alternately irradiated with positive and negative ions 301 and 302, to be positively and negatively charged, and accordingly the ions which are opposite in polarity to the apparent charges are selectively attracted, and eliminated apparently.

Since the irradiation quantities of positive and negative ions 301 and 302 depend, for example, on the capabilities of individual positive and negative ion-generating electrodes 2b and the phase of applied voltage, the total irradiation quantities of the positive and negative ions at the respective sites of the sheet S are different, and macroscopic positive and negative charge irregularity occurs in the sheet S (see Fig. 18 of document DS2). The macroscopic charge irregularity is the apparent charge irregularity and its state can be confirmed using a toner powder as apparent charges.

This occurs since the positive (or negative) ions 301 (or 302) are forcibly applied to the sheet S along the lines of electric force 500 generated by the positive and negative ion-generating electrodes 2b and the ion-attracting electrode 2d. Since the voltage applied to the positive and negative ion-generating electrodes 2b changes alternately, the cyclic irregularity of positive and negative charges occurs in the sheet S. The cycles of the charge irregularity are decided, for example, by the cycles of the applied voltage and the traveling speed of the sheet. The charge irregularity appears in the first surface 100 only of the sheet S. The reason is that the first surface 100 only of the sheet S is irradiated with the positive and negative ions 301 and 302, and this state shows that the sheet is apparently charged.

To eliminate the macroscopic charge irregularity, the static eliminator 2 of document DS2 must include DC and AC static eliminating members 2e and 2f shown in Fig. 4. The macroscopic charge irregularity can be eliminated if such conditions as the applied voltage and installation positions of the DC and AC static eliminating members are optimized. If the sheet is wound without the DC and AC static eliminating members, the charges are so strong that discharge may occur on the sheet. Since the static eliminator 2 of document DS2 requires such DC and AC static eliminating members, the entire eliminator is large-sized and very costly, and it is difficult to add the eliminator to an existing sheet producing apparatus.

On the other hand, the charged state of the sheet treated to be free from the macroscopic charge irregularity by the DC and AC static eliminating members 2e and 2f is as shown in Fig. 7. Fig. 7 shows a case where such conditions as the voltage and arrangement of the DC and AC static eliminating members 2e and 2f are optimized and where the macroscopic positive and negative charge irregularity in the sheet is eliminated. As shown in Fig. 7, the charges in the sheet S are balanced in both the surfaces, and the sheet S is apparently non-charged. However, in the respective surfaces of the sheet S, almost equal quantities of positive and negative charges remain.

The reason why this occurs is that the positive and negative ion-generating electrodes 2b are disposed only on the side of the first surface 100 (top surface in Fig. 5) of the sheet S, and hence that at every moment during static elimination, the charges in the second surface 200 (bottom surface in Fig. 5) of the sheet S cannot be decreased. This phenomenon occurs also in the case where the DC and AC static eliminating members 2e and 2f are used. As a result, the charge densities in the first surface 100 of the sheet S can be eliminated only to such an extent that the charge densities balance the charge densities prevailing in the second surface 200 since before static elimination, i.e., to such an extent that the apparent charge densities become zero.

The inventors measured, according to the method described later, the charge densities remaining in the respective surfaces of the sheet static eliminated by the conventional static eliminator 2. The charge densities at the static mark sites of the second surface 200 were virtually the same as those prevailing before static elimination, i.e., tens of microcoulombs per square meter to about 500 µC/m² in absolute value. The charge densities of the first surface 100 at the same sites (static mark sites) were almost equal to those of the second surface 200 in absolute value, though opposite in polarity, i.e., tens of microcoulombs per square meter to about 500 µC/m² in absolute value though opposite in polarity.

In view of the effect of decreasing the charge densities in the respective surfaces, the static elimination is achieved only to such an extent that the apparent charge densities (several microcoulombs per square meter to 10 µC/m² in absolute value) are made zero. So, it can be said that the static elimination effect is only up to less than 10% of the charge densities of the first surface 100. Rather, such a phenomenon was also confirmed that at a site where the charge density of the second surface 200 was larger than the charge density of the first surface 100 before static elimination in absolute value, the charge density of the first surface 100 increased to such a level that it became equal to the charge density of the second surface 200 after static elimination. It was found that the charges remaining in the first and second surfaces 100 and 200 were the causes of such defects as the irregularity of the coating material, static marks formed after film processing and sliding failure.

This problem is an essential problem peculiar to the static elimination performed only from one surface of a sheet, and even if such conditions as the voltage and arrangement of the DC and AC static eliminating members 2e and 2f are optimized, the problem cannot be solved. The DC and AC static eliminating members 2e and 2f are provided only for making the macroscopic charge irregularity appear to be zero.

For example, two static eliminators of document DS2 (static eliminators 2 of Fig. 4) can be installed in the sheet traveling direction, and the two sets, each consisting of the positive and negative ion-generating electrodes 2b and the ion-attracting electrode 2d, can be arranged at positions facing each other, with the sheet kept between the electrodes 2b and the electrode 2d, and with one set reversed to the other set in position, in order that the first surface 100 of the sheet is irradiated with ions, and subsequently that the second surface 200 of the sheet is irradiated with ions. Even in this case, there is no effect of decreasing the charges existing in the respective surfaces. The reason is that the static eliminator of document DS2 (static eliminator 2 shown in Fig. 4) is a static eliminator intended for "apparent static elimination" only for eliminating apparent charges as described before. Even if static elimination is carried out for the second surface 200 after the "apparent static elimination" has been completed by the static elimination carried out for the first surface 100, the operation is quite meaningless.

On the contrary, as shown in Fig. 8, known is a static eliminator, in which ion irradiation devices, each consisting of an ion-generating electrode and an ion-accelerating electrode disposed to face each other, are installed reversely to each other in position on the first surface 100 side and the second surface 200 side of an insulating sheet. This static eliminator is disclosed in JP 2002-313596 A (hereinafter called document DS3).

The conventional static eliminator 3 includes an ion-generating electrode 3b connected with an AC power supply 3a and installed above the first surface 100 of a traveling insulating sheet S and an ion-accelerating electrode 3d connected with an AC power supply 3c and installed below the second surface 200 of the traveling insulating sheet S. The ion-generating electrode 3b and the ion-accelerating electrode 3d are installed to face each other with the insulating sheet S kept between them.

The next ion-generating electrode 3f connected with an AC power supply 3e and installed beside the ion-accelerating electrode 3d below the second surface 200 of the sheet S and the next ion-accelerating electrode 3h connected with an AC power supply 3g and installed beside the ion-generating electrode 3b above the first surface 100 of the sheet S, face each other.

In this static eliminator, an AC high voltage is applied to the ion-generating electrode 3b, to generate ions, and an AC high voltage opposite in polarity to the voltage applied to the ion-generating electrode 3b is applied to the ion-accelerating electrode 3d. The ions generated by the ion-generating electrode 3b are accelerated and attracted by the ion-accelerating electrode 3d, and as a result, the first surface 100 of the sheet S is forcibly irradiated with the ions. Then, an AC high voltage opposite in polarity to that applied to the ion- generating electrode 3b is applied to the ion-generating electrode 3f to generates the ions, while a high voltage opposite in polarity to that applied to the ion-generating electrode 3f is applied to the ion-accelerating electrode 3h. The ions generated by the ion-generating electrode 3f are accelerated and attracted by the ion-accelerating electrode 3h, and as a result, the second surface 200 of the sheet S is forcibly irradiated with the ions. According to this technique, since both the surfaces of the insulating sheet are forcibly irradiated with ions, it is said that the sheet can undergo static elimination even if the sheet has a fine charge pattern.

In this static eliminator, high voltages opposite in polarity to those applied to the ion-generating electrodes 3b and 3f disposed to face the ion-accelerating electrodes 3d and 3h respectively are applied to the ion-accelerating electrodes 3d and 3h restively. However, as shown in document DS3 (Figs. 4 and 5 show examples of the shape of the ion-accelerating electrodes and Fig. 9 shows the behavior of ions) , since the ion-accelerating electrodes are not shaped to allow ion generation, they do not generate ions. This is the reason why the electrodes are called "ion-accelerating electrodes" in document DS3. In this constitution, the irradiation of the first surface 100 and the second surface 200 with ions is carried out alternately, not simultaneously.

According to the inventors' finding, since both the surfaces of the insulating sheet are irradiated with ions alternately, the static eliminator of document DS3 is basically equivalent to the case where two static eliminators of document DS2 described before (static eliminators 2 of Fig. 4) are disposed in the sheet traveling direction, to be reverse to each other in the static elimination side and the non-static elimination side. That is, even in the best mode, quantities of positive and negative ions necessary to make the apparent charge densities zero are merely supplied without greatly affecting the distributions of charge densities existing in the respective surfaces before start of static elimination. In other words, at sites where a fine charge pattern such as static marks exists, a charge pattern opposite in polarity to the static marks of the first surface is merely formed in the second surface for apparent static elimination. That is, even if the static eliminator of document DS3 is used, an effect of greatly decreasing the charges in the respective surfaces where fine charge patterns are formed cannot be obtained.

This is described below in more detail. With regard to the capability of the static eliminator of document DS3 (static eliminator 3 of Fig. 8) to eliminate the charges in the respective surfaces of the sheet S (locally strong charges such as static marks, especially the charges opposite each other in polarity in both the surfaces of the sheet), the following can be said.

It is considered that a case where static elimination is performed at a site of a sheet where a large quantity of positive charges 101 in the first surface 100 and a large quantity of negative charges 202 in the second surface 200 exist as shown in Fig. 9. If the first ion-generating electrode 3b close to the first surface 100 of the sheet S generates the negative ions 302 to be sufficiently irradiated to the first surface 100 of the sheet S, and subsequently the second ion-generating electrode 3f close to the second surface 200 generates the positive ions 301 to be sufficiently irradiated to the second surface 200 of the sheet S, then the charges in the respective surfaces of the sheet S can be eliminated.

However, actually in the sheet S having the respective surfaces strongly charged opposite to each other in polarity, in the case where the negative ions 302 are irradiated to the first surface 100 of the sheet S as shown in Fig. 9, the positive charges 101 of the first surface 100 are eliminated. As a result, as shown in Fig. 10, the quantity of the negative charges 202 in the second surface 200 is excessive compared with the quantity of the positive charges 101 in the first surface 100.

In the case where a site of the sheet at which the absolute value of negative charge density of the second surface 200 is slightly larger, for example, 1 µC/m² larger than the absolute value of positive charge density of the first surface 100 is placed in the space between the first ion-generating electrode 3b and the ion-accelerating electrode 3d, the potential is calculated to be in a range from -10 to -100 kV. This value range refers to a value range in the case where the electrostatic capacity of the sheet S placed in the space between the first ion-generating electrode 3b and the ion-accelerating electrode 3d is in a range from 10 to 100 pF.

Because of the excessively existing negative charges, the Coulomb force 700 in the direction to shove away the negative ions 302 from the sheet S acts on the negative ions 302, and the negative ions 302 cannot sufficiently reach the sites of the sheet S where the positive charges 101 still exist. Also in the case where the second ion-generating electrode 3f generates the positive ions 301 to be irradiated to the second surface 200 of the sheet S, the same phenomenon occurs. As a result, the positive charges 101 of the first surface become excessive, and the positive ions 301 reaching the sheet S decrease.

Even if the respective surfaces of the sheet S are charged to have charge densities of tens of microcoulombs per square meter to about 500 µC/m² in absolute value, the quantity of ions per square meter that can reach the sheet S is as small as less than about 1 µC/m², and can little eliminate the charges of the respective surfaces of the sheet S so strongly charged as to have static marks. However, at each site where the apparent charge densities of the sheet are not zero, the charges can be eliminated to such an extent that the apparent charge densities can be made zero.

As a mode of the static eliminator of document DS3, the following constitution is described in Fig. 2 of document DS3. Ion irradiation devices, each consisting of the ion-generating electrode 3b and the ion-accelerating electrode 3d facing each other, are arranged on both the surface sides of the sheet S, with the electrodes disposed alternately in reverse positions, and on the downstream side, two ion-generating electrodes are arranged to face each other on both the surface sides of the sheet S, one on the first surface 100 side and the other on the second surface 200 side. The ion-generating electrodes disposed downstream to face each other are disposed to eliminate the residual charges (same as the charges of macroscopic charge irregularities of static eliminator 2 of Fig. 4.) However, for example, the dimensions and applied voltages of the ion-generating electrodes disposed downstream to face each other are not disclosed at all in document DS3.

Even if a voltage considered to be appropriate is applied to the ion-generating electrodes disposed to face each other, based on the inventors' finding, it is difficult to obtain a sufficient static elimination effect. For example, if the ion-generating electrode placed on the first surface 100 side of the sheet S generates positive ions to be irradiated to the first surface 100, and the ion-generating electrode placed on the second surface 200 side generates negative ions to be irradiated to the second surface 200, then a static elimination effect can be obtained at sites where the first surface 100 is charged negatively while the second surface 200 is charged positively. However, no static elimination effect can be obtained at the sites where the first surface 100 is charged positively while the second surface 200 is charged negatively.

Since positive charges and negative charges exist together in the respective surfaces of the sheet S in most cases, the charges at all the sites in the respective surfaces of the sheet S cannot be decreased. There are sites where charges can be eliminated and sites where charges cannot be eliminated. Rather, it can happen that in the case where the polarity of charges of the respective surfaces of the sheet S is the same as the polarity of the ions irradiated to the respective surfaces, charges are increased. In the case where the voltages applied to ion-generating electrodes are AC voltages with a low frequency, static elimination effect irregularity and ion irradiation irregularity appear in the traveling direction of the sheet S. On the other hand, in the case where the voltages applied to ion-generating electrodes are AC voltages with a high frequency, the static elimination effect irregularity in the traveling direction of the sheet S is small.

However, in the case where the voltages applied to ion-generating electrodes are AC voltages with a high frequency, as in the case of a static eliminator for a copier described later, since the positive and negative ions generated from ion-generating electrodes are mixed and re-combined with each other before they reach the sheet S, the quantity of ions reaching the sheet S is remarkably decreased. Therefore, the static elimination effect per se is small. So, even if, for example, the dimensions of respective parts and the applied voltage are adjusted based on the inventors' finding, it is difficult to eliminate the positive charges and negative charges existing together in both the surfaces without the irregularity due to the positions in the traveling direction of the sheet S, if one set of ion-generating electrodes, one on the first surface 100 side of the sheet S and the other on the second surface 200 side, are merely disposed.

On the other hand, as a constitution in which static eliminators are disposed to face each other with a charged material positioned between them, a transfer sheet-carrying sheet and a transfer sheet (paper) static eliminator 4 of a copier shown in Fig. 11 is known. The static eliminator 4 is disclosed in JP 03-87885 A (hereinafter called document DS4) or JP 02-13977 A (hereinafter called document DS5).

Fig. 11 is a drawing showing the copier shown in document DS4, as a whole. In Fig. 11, A indicates a section for forming a toner image onto a photosensitive drum; B indicates a section for supplying a transfer sheet 4a; C indicates a section for transferring a toner image onto the transfer sheet 4a on a transfer sheet-carrying sheet 4b wound around a transfer drum; and D indicates a section where the transfer sheet 4a having the toner image transferred from the transfer sheet-carrying sheet 4b is separated. The description of the details is not made here since it is not concerned with the present invention at all.

In the static eliminator 4 of Fig. 11, wire corotron electrodes positioned outside as corona dischargers 4c and 4d and wire corotron electrodes positioned inside as corona dischargers 4e and 4f are installed to face each other on both sides of the transfer sheet 4a as a charged material and the transfer sheet-carrying sheet 4b. The first purpose of the static eliminator 4 is to more easily separate the transfer sheet 4a from the transfer sheet-carrying sheet 4b, and the second purpose is to initialize the potential of the transfer sheet-carrying sheet 4b.

To achieve the first purpose, an AC voltage (500 Hz, 9.6 kV) is applied to the corona dischargers 4c and 4d, and a DC voltage (-4 kV) is applied as pulses to the corona discharger 4e, while a voltage different by 180° phase from that of the corona dischargers 4c and 4d is applied to the corona discharger 4f. The reason why a DC voltage is applied to the corona discharger 4e is that instead of superimpose a DC voltage as a bias on the AC voltage applied to the corona discharger 4f in opposite, it is intended to use two independent corona dischargers 4f and 4e.

With this constitution, the average potentials of the transfer sheet 4a and the transfer sheet-carrying sheet 4b can be decreased. Since the transfer sheet 4a is positively charged in the previous step, a negative voltage is used as the DC voltage to allow easier separation of the transfer sheet-carrying sheet 4b. To achieve the second purpose, an AC voltage only is applied to the corona dischargers 4d and 4f. With regard to the charges of the transfer sheet-carrying sheet 4b, it is not necessary to eliminate the charges of both the outer surface and the inner surface. If the charges of the outer surface balance the charges of the inner surface to reduce the apparent potential to almost zero, the purpose can be achieved.

As can be seen from the above description, the technique described in document DS4 is not intended to eliminate charges from a sheet having positively charged sites and negatively charged sites alternately formed at a small pitch in the same plane or a sheet having fine patterns with such sites existing together in both the surfaces. In the paper as a transfer sheet of a copier, such charge patterns are unlikely to be formed.

In the case where such a high frequency is used, the electric field between the top and bottom electrodes little has the capability of forcibly irradiating the sheet with ions. The positive and negative ions 301 and 302 generated by the corona dischargers 4d and 4f are mixed in the gap between the corona discharger 4d and the corona discharger 4f. The size of the gap is not clearly stated in document DS4, but according to other documents and the like relating to static eliminators of copiers, it is usually about 20 mm. According to document DS5, it is 22 mm.

Since an AC voltage with a high frequency of 500 Hz is applied in an electrode gap of about 20 mm as described above, a monopolar ion cloud cannot be formed. Since the frequency is high, the positive and negative ions 301 and 302 are mixed with each other, before they reach the first surface 100 and the second surface 200 of the sheet. For this reason, though the sheet is seldom forcibly charged positively or negatively, most of the positive and negative ions 301 and 302 are recombined with each other and vanish, and the quantity of the ions capable of contributing to static elimination becomes very small. That is, in the static eliminators shown in documents DS4 and DS5, though the corona discharger 4d and the corona discharger 4f are disposed to face each other with a sheet kept between them, a large quantity of ions can be little forcibly irradiated near to the sheet.

As a result, these static eliminators of copiers, like the static eliminator 1 shown in Figs. 2 and 3, are very low in the capability of eliminating the charges of the respective surfaces of a sheet having positively charged sites and negatively charged sites alternately formed at a small pitch in the same plane or a sheet having such sites existing together on both the surfaces. The techniques can be applied in the case where the sheet traveling speed is as low as several to 10-odd m/min and can be applied to a transfer sheet or paper fromwhich it is not required to eliminate the fine charge patterns in either of the surfaces. The static elimination techniques cannot be applied as techniques for eliminating charges from an insulating sheet such as a film that travels at a high speed of about 50 to about 500 m/min and from which it is necessary to eliminate fine charge patterns in both the surfaces.

Furthermore, in the static eliminators for copiers shown in documents DS4 and DS5, the width of the transfer sheet or paper undergoing static elimination is about 500 mm at the largest, and it is not necessary to consider, for example, the vibration, strength and sagging of electrodes. For this reason, a high voltage is applied to wire electrodes extending in the in-plane direction perpendicular to the traveling direction of the sheet, for causing discharge to generate ions. However, in the case where an insulating sheet such as a film undergoes static elimination, its width is about 1 m at the smallest, and there is even an insulating sheet with a width of about 7 m. When wire electrodes are used for such a wide sheet, the vibration of the electrode and the sagging of the electrode between both the ends cause discharge strength irregularity in the sheet width direction.

For example, in the case where it is intended to increase the ion irradiation dose for the sheet undergoing static elimination, for example, by further shortening the distance between the corona discharger 4d and the corona discharger 4f, or raising the voltage to be applied, or using a lower frequency, the vibration of the wires increases, and discharge is concentrated at the portion where the distance between the wires facing each other is shortest due to inaccurate parallelism or loosening of wires. As a result, a static elimination effect stable over the entire width of the material undergoing static elimination cannot be obtained. Furthermore, in the case where the voltage is raised, spark discharge occurs between the discharge electrodes (wire electrodes) of the corona dischargers 4d and 4f or between a discharge electrode and a shield electrode, not allowing a sufficient static elimination capability to be obtained.

In the static eliminators for copiers shown in documents DS4 and DS5, corona dischargers are disposed to face each other, but the principle of static elimination is quite different from the principle that a strong electric field in the direction normal to the insulating sheet is used to forcibly irradiate ions onto the sheet. Therefore, the static elimination irregularity in the traveling direction of the sheet is hard to occur, and no countermeasure against it is discussed at all. For example, in the static eliminator shown in document DS4 (the static eliminator 4 of Fig. 11), two sets of corona dischargers facing each other are installed one after another in the traveling direction of the material undergoing static elimination (transfer sheet or paper) , but as described before, this constitution is intended to provide different functions of easier separation and potential initialization, and is not employed to give any effect, for example, against the static elimination effect irregularity in the traveling direction of the sheet.

In recent years, insulating sheets such as polyester films are used in many applications as magnetic recording materials, various photographic materials, insulating materials and various process materials, since they have excellent properties such as heat resistance, chemicals resistance and mechanical properties. For this reason, they are required to have surface properties suitable for respective applications, and they are covered with various materials. For example, the sheets are thinly coated on their surfaces with a magnetic paint, ink-like paint, lubricating paint, releasing paint, or hard coating material, to form a coating layer.

For the coating process for forming such a coating layer, it is proposed to install a static eliminator in any of various coaters such as roll coater or gravure coater, for eliminating the charges from an insulating sheet before start of coating, or to eliminate charges from the sheet and a coating solution simultaneously before the coating solution applied as a paint is dried after coating. These proposals are described in JP 08-334735 A (hereinafter called document DS6) and JP 10-259328 A (hereinafter called document DS7). As the quantity of charges of a sheet for obviating the occurrence of coating irregularities, document DS6 states it is preferred that the surface potentials of the sheet are in a range from 0 to 80 V, and document DS7 states it is preferred that the surface potentials of the sheet are in a range from 0 to 2 kV.

In these conventional techniques, the surface potential refers to a value measured while the sheet is carried in air. Hereinafter this surface potential is called an aerial potential. In the state where a sheet is carried in air, since the thickness of the sheet is sufficiently small compared with the distance between the sheet and a grounded component, the surface potential corresponding to the sum of charges is measured without discriminating the charges of the first surface of the sheet from the charges of the second surface. That is, in these conventional techniques, the aerial potential relates to apparent charges (the apparent charge densities). Therefore, in the conventional techniques, the charge densities of the respective surfaces of a sheet are not taken into account at all.

The visual field of a general electrostatic voltmeter used for measuring the aerial potential is usually a virtually circular area portion having a diameter of tens of millimeters to tens of centimeters, and the value of the measured potential is detected as an average value of potentials in the visual field. This matter is described in the catalogue (in Japanese) for Digital Low Potential Measuring Instrument KSD-0202 produced by Kasuga Electric Works Ltd (hereinafter called document DS8). In a dense charge pattern having positive and negative charges existing together peculiar to an insulating sheet, the positive and negative charges are averaged within the range of the visual field, and the aerial potential appears to be almost zero. With these as causes, even in a sheet having a low aerial potential according to the conventional techniques, it can happen that numerous positive and negative changes exist in the sheet actually, and in this case, coating irregularity occurs in the coating layer.

As described above, even if the above-mentioned sheet having positively and negatively charged sites alternately formed at a small pitch or having such sites existing together in both the surfaces has its charges controlled in reference to the aerial potential, the control is not sufficient. Much less, the coating irregularity can never be prevented.

The following describes why an apparently non-charged sheet having both the surfaces equally charged though opposite in polarity (in this case, the aerial potential is also zero) poses a problem and why coating irregularity occurs.

In a coating process, for example, when a die coater is used, the sheet travels, for example, with its second surface kept in contact with a backup roll. In this state, a coater roll is used to coat the first surface of the sheet. Since the sheet is kept in contact with the backup roll, stable traveling is assured to stabilize coating work, and a coating layer having uniform thickness can be formed. As the material of the backup roll, a metallic material is often used since the roll is required to be mechanically precise and to have durability such as wear resistance. Therefore, one surface of the sheet is kept in contact with the metallic surface of the backup roll, and the other surface is coated to have a coating film.

It is considered that a sheet having the first surface and the second surface charged equally though opposite in polarity (apparently non-charged sheet). The charges of the second surface in contact with the metallic surface induce an equal quantity of charges opposite in polarity in the surface of the metal that is a conductor. The induced charges opposite in polarity apparently cancel out the charges in the second surface. On the other hand, the charges in coating surface (the first surface) also induce charges opposite in polarity in the surface of the metal. However, since the surface of the metal is far in this case, the quantity of charges induced is smaller. Therefore, the induced charges opposite in polarity do not perfectly cancel out the charges of the first surface, and the charges actively exist in the coating surface (the first surface).

In this way, "the apparently non-charged" sheet have charges actively existing in the first surface above the backup roll during coating. Therefore, coating irregularity occurs. That is, even in an apparently non-charged sheet, as far as charges exist in the respective surfaces of the sheet, coating irregularity can occur. This phenomenon occurs also similarly in the carrier roll or drying roll used after coating.

As described above, even if the aerial potential of a sheet is kept low as in the prior art, and furthermore, even if apparent charges are used for control, the prior art cannot prevent coating irregularity.

An object of the invention is to solve the above-mentioned problems of the prior art by providing a static eliminator and a static eliminating method for easily eliminating the positively and negatively charged sites alternately formed at a small pitch in either surface or both the surfaces of a sheet. Another object of the invention is to provide a method for producing an insulating sheet liberated from the positively and negatively charged sites alternately formed at a small pitch in the surfaces of the sheet to such an extent that no problem occurs at least in the processing of the surfaces of the sheet or in the processed sheet, and also to provide an insulating sheet with such surface properties. When the insulating sheet is coated with a coating material on a surface to form a coating layer, coating irregularity or repellent coating is hard to occur. Furthermore, a sheet having a metallic layer formed on a surface of the insulating sheet is hard to cause the problem of disarrangement of cut sheets.

These and other objects of the present invention are achieved by the present invention described below.

In accordance with the present invention, there is provided a static eliminator for an insulating sheet, in which at least two static eliminating units are provided in the traveling path of an insulating sheet with an interval kept between them in the traveling direction of the sheet; each of the respective static eliminating units has a first electrode unit and a second electrode unit disposed to face each other through the sheet; the first electrode unit has a first ion-generating electrode and a first shield electrode having an opening near the pointed ends of the first ion-generating electrode; and the second electrode unit has a second ion-generating electrode and a second shield electrode having an opening near the pointed ends of the second ion-generating electrode, characterized in that at each of the respective static eliminating units,

• (a) the voltage applied to the first ion-generating electrode and the voltage applied to the second ion-generating electrode are substantially opposite to each other in polarity, and
• (b) at each position in the width direction of the sheet, if the interval between the pointed end of the first ion-generating electrode and the pointed end of the second ion-generating electrode in the traveling direction of the sheet is d₀ (in mm) , the distance between the pointed end of the first ion-generating electrode and the pointed end of the second ion-generating electrode in the direction normal to the sheet is d₁ (in mm) , the shortest distance between the first shield electrode and the second shield electrode in the direction normal to the sheet is d₃ (in mm), and the average value of the widths of the opening of the first shield electrode and the opening of the second shield electrode in the traveling direction is d₄ (in mm), then the following formula (I) d₀ < 1.5 x d₁²/(d₃ x d₄) is satisfied. This static eliminator is called a first static eliminator.

In the first static eliminator, it is preferable that the voltages applied to the first ion-generating electrodes of the respective static eliminating units and the voltages applied to the second ion-generating electrodes of the respective static eliminating units are supplied from respective single AC power supplies, or from respective groups of plural AC power supplies synchronous with each other in the group with a zero or predetermined potential difference. This static eliminator is called a second static eliminator.

In the first static eliminator, it is preferable that the first ion-generating electrode and the second ion-generating electrode of each of the respective static eliminating units are arrays of needle electrodes. This static eliminator is called a third static eliminator.

In the first static eliminator, it is preferable that the first shield electrode comprises a first rear shield electrode disposed on the rear side of the first ion-generating electrode, and the second shield electrode comprises a second rear shield electrode disposed on the rear side of the second ion-generating electrode. This static eliminator is called a fourth static eliminator.

In the fourth static eliminator, it is preferable that in the first shield electrode, a first insulating member is provided between the first ion-generating electrode and the first rear shield electrode, and/or in the second shield electrode, a second insulating member is provided between the second ion-generating electrode and the second rear shield electrode. This static eliminator is called a fifth static eliminator.

In the first static eliminator, it is preferable that at each position in the width direction of the sheet, at any two adjacent static eliminating units, if the static eliminating unit interval between the middle point of the line segment connecting the pointed end of the first ion-generating electrode with the corresponding pointed end of the second ion-generating electrode of one of the two adjacent static eliminating units, and the corresponding middle point of the other static eliminating unit in the traveling direction of the sheet is d₂ (in mm) , the following formula (II) d₂ < 12 x d₁²/(d₃ x d₄) is satisfied. This static eliminator is called a sixth static eliminator.

In accordance with the present invention, there is provided a static eliminator for an insulating sheet, in which at least two static eliminating units are provided in relation with a virtual plane, with an interval kept between them in a predetermined direction along the virtual plane; each of the static eliminating units has a first electrode unit and a second electrode unit disposed to face each other through the plane; the first electrode unit has a first ion-generating electrode and a first shield electrode having an opening near the pointed ends of the first ion-generating electrode; and the second electrode unit has a second ion-generating electrode and a second shield electrode having an opening near the pointed ends of the second ion-generating electrode, characterized in that at each of the static eliminating units, the first ion-generating electrode and the second ion-generating electrode are disposed to face each other through the plane substantially symmetrically with the virtual plane, and the voltage applied to the first ion-generating electrode and the voltage applied to the second ion-generating electrode are substantially opposite to each other in polarity. This static eliminator is called a seventh static eliminator.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, comprising the step of simultaneously irradiating the first surface and the second surface of an insulating sheet with respective monopolar ion clouds substantially opposite to each other in polarity at respective sites of the sheet, and the step of simultaneously irradiating the first and second surfaces with respective monopolar ion clouds reverse in polarity to those applied before at said site of the sheet. This static eliminating method is called a first static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, in which the first surface of an insulating sheet is irradiated with a monopolar first ion cloud reversing in polarity with the lapse of time while the sheet travels, and the second surface of the sheet is irradiated with a monopolar second ion cloud reversing in polarity with the lapse of time, but substantially opposite in polarity to the first ion cloud, simultaneously with the first ion cloud, characterized in that the first and second ion clouds are reversed in polarity so that while respective sites of the sheet in the traveling direction pass through the region irradiated with the first and second ion clouds, the first and second ion clouds are reversed in polarity once or more. This static eliminating method is called a second static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, in which the first surface and the second surface of an insulating sheet are simultaneously irradiated with a pair of monopolar ion clouds substantially opposite to each other in polarity by a predetermined number of times, while the sheet travels, characterized in that the pair of ion clouds are applied so that the respective numbers of times of irradiating the first and second surfaces with a positive ion cloud and a negative ion cloud are not less than 1/4 of said predetermined number of times at respective sites of the sheet. This static eliminating method is called a third static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, in which the first surface of an insulating sheet is irradiated with a group of first monopolar ion clouds smoothly reversing in polarity with the lapse of time, and the second surface of the sheet is simultaneously irradiated with a group of second monopolar ion clouds smoothly reversing in polarity with the lapse of time but substantially opposite in polarity to the first group of ion clouds, characterized in that in sites of 2/3 or more at all the sites in the traveling direction of the sheet, the respective groups of ion clouds are irradiated in such a manner that the polarity of the ion clouds corresponding to 1/4 or more of the ion clouds in each of the first and second groups of ion clouds can be opposite to the polarity of the other ion clouds in the group. This static eliminating method is called a fourth static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, in which an insulating sheet is made to travel between the first and second ion-generating electrodes of the respective static eliminating units in the static eliminator for an insulating sheet as set forth in claim 6, while both the surfaces of the sheet are irradiated with the positive and negative ions generated from the first and second ion-generating electrodes, characterized in that where respective AC voltages of the same phase are applied to the first and second ion-generating electrodes of the respective static eliminating units, and if the frequency of the AC voltages is f (in Hz) and an effective value of the potential difference between the first and second ion-generating electrodes is 2V (in V) , then the following formulae (III) and (IV) 90d₁ ≤ V ≤ 530d₁ 0.0425 x d₁² x f ≤ V ≤ 0.085 x d₁² x f are satisfied. This static eliminating method is called a fifth static eliminating method.

In the fifth static eliminating method, it is preferable that if the traveling speed of the sheet is u (in mm/sec) and at each position in the width direction of the sheet, the interval between the middle point of the line segment connecting the pointed end of the first ion-generating electrode with the corresponding pointed end of the second ion-generating electrode of the most upstream static eliminating unit, and the corresponding middle point of the most downstream static eliminating unit in the traveling direction of the sheet, i.e., the sum of all the static eliminating unit intervals d₂ from the most upstream static eliminating unit to the most downstream static eliminating unit is D₂ (in mm) , the following formula (V) D₂ > u/f is satisfied. This static eliminating method is called a sixth static eliminating method.

In the fifth static eliminating method, it is preferable that at sites of 2/3 or more of all the sites in the traveling direction of the sheet, said AC voltages are applied to the respective first and second ion-generating electrodes of n static eliminating units, where n is the total number of static eliminating units, in such a manner that the polarity of the potentials of the ion-generating electrodes of static eliminating units as many as not smaller than the number obtained from formula (n - 0.0006/d_f)/2 {where d_f (in m) is the thickness of the sheet }and not smaller than 0, said potentials working while the each of said sites passes directly under the ion-generating electrodes of said specified number of static eliminating units, can be opposite to the polarity of the potentials of the other ion-generating electrodes of the static eliminating units concerned, said potentials working while the said portion passes directly under the ion-generating electrodes of the other static eliminating units. This static eliminating method is called a seventh static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, in which while an insulating sheet is made to travel between the first and second ion-generating electrodes of the respective static eliminating units in the static eliminator for an insulating sheet as set forth in claim 1, both the surfaces of the sheet are irradiated with the positive and negative ions generated from the first and second ion-generating electrodes of the respective static eliminating units, characterized in that in the case where a voltage is applied to each of the respective first and second ion-generating electrodes of the respective static eliminating units, if the frequency of the voltage is f (in Hz) and the one-side peak voltage is Vp (in V), then the following formulae (VI) and (VII) 130 x d₁ ≤ Vp ≤ 750 x d₁ 0.120 x d₁² x f ≤ Vp are satisfied and the voltage is applied to each of the respective ion-generating electrodes in such a manner that in the case where a portion of the sheet is considered, the polarity of the potentials of the ion-generating electrodes of static eliminating units corresponding to 1/4 or more of static eliminating units, said potentials working while the said portion passes directly under the ion-generating electrodes of the specified number of static eliminating units can be opposite to the polarity of the potentials of the ion-generating electrodes of the other static eliminating units concerned, said potentials working while the said portion passes directly under the ion-generating electrodes of the other static eliminating units. This static eliminating method is called an eighth static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, in which while an insulating sheet is made to travel between the first and second ion-generating electrodes of the respective static eliminating units in the first static eliminator, both the surfaces of the sheet are irradiated with the positive and negative ions generated from the first and second ion-generating electrodes of the respective static eliminating units, characterized in that in the case where AC voltages smoothly changing in polarity are applied to the respective first and second ion-generating electrodes of the respective static eliminating units, if the frequency of the AC voltages is f (in Hz) and an effective value of the potential difference between the first and second ion-generating electrodes is 2V (in V) , then the following formulae (VIII) and (IX) 90 x d₁ ≤ V ≤ 530 x d₁ 0.085 x d₁² x f ≤ V are satisfied and in the case where a portion of 2/3 or more is considered in the traveling direction of the sheet, the AC voltages are applied to the respective first and second ion-generating electrodes in such a manner that the polarity of the potentials of the ion-generating electrodes of static eliminating units corresponding to 1/4 or more of the static eliminating units, said potentials working while the said portion passes directly under the ion-generating electrodes of the specified number of static eliminating units can be opposite to the polarity of the potentials of other ion-generating electrodes of the static eliminating unit concerned, said potentials working while the said portion passes directly under the ion-generating electrodes of the other static eliminating units. This static eliminating method is called a ninth static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, in which while an insulating sheet is made to travel between the first and second ion-generating electrodes of the respective static eliminating units in the first static eliminator, both the surfaces of the sheet are irradiated with the positive and negative ions generated from the first and second ion-generating electrodes of the respective static eliminating units, characterized in that where AC voltages smoothly changing in polarity are applied to the respective first and second ion-generating electrodes of the respective static eliminating units, if the frequency of the AC voltages is f (in Hz) and an effective value of the potential difference between the first and second ion-generating electrodes is 2V (in V) , then the following formulae (X) and (XI) 90 x d₁ ≤ V ≤ 530 x d₁ 0.085 x d₁² x f ≤ V are satisfied and in the case where a portion of 2/3 or more is considered in the traveling direction of the sheet, the AC voltages are applied to the respective first and second ion-generating electrodes of n static eliminating units (where n is the total number of static eliminating units) in such a manner that the polarity of potentials of the ion-generating electrodes of static eliminating units as many as not smaller than the number obtained from formula (n - 0.003/d_f)/2, where d_f (in m) is the thickness of the insulating sheet, and not smaller than 1, said potentials working while the said portion passes directly under the ion-generating electrodes of the specified number of static eliminating units, can be opposite to the polarity of the potentials of the other ion-generating electrodes of the static eliminating units concerned, said potentials working while the said portion passes directly under the ion-generating electrodes of the other static eliminating units. This static eliminating method is called a tenth static eliminating method.

In the ninth static eliminating method, it is preferable that at each position in the width direction of the sheet, if the any interval between the middle point of the line segment connecting any of the pointed ends of the first ion-generating electrodes with the corresponding pointed ends of the second ion-generating electrodes of one of any two adjacent static eliminating units, and the corresponding middle point of the other static eliminating unit is constant value, i.e., the any eliminating unit intervals d₂ is constant value d₂₀ (in mm) , and the AC voltages substantially identical in phase are applied respectively to the first and second ion-generating electrodes of the respective static eliminating units, in such a manner that if the traveling speed of the sheet is u (in mm/sec), the frequency of the AC voltages is f (in Hz) and the total number of the static eliminating units is n, then the value of X is represented by the following formula (XII) X = |sin(nπfd₂₀/u)/{n·sin(πfd₂₀/u)}| (ku ≠ fd₂₀, where k = 1, 2, 3, ...) = 1 (ku = fd₂₀) and the value of X satisfies 0 ≤ X < 0.5. This static eliminating method is called an eleventh static eliminating method.

In accordance with the present invention, there is provided a static eliminating method for an insulating sheet, characterized in that in the predetermined period of starting and/or ending the traveling of an insulating sheet, the second or fifth static eliminating method is used for eliminating charges from the sheet, and in the steady traveling state of the sheet, the third, fourth, ninth or tenth static eliminating method is used for eliminating charges from the sheet. This static eliminating method is called a twelfth static eliminating method.

In the fifth, eighth or tenth static eliminating method, it is preferable that in the case where a DC potential difference is established between the first and second shield electrodes of the respective static eliminating units, if the DC potential difference is Vs (in V), the following formula (XIII) |Vs|/d₃ < 5 is satisfied. This static eliminating method is called a thirteenth static eliminating method.

In any one of the first through fifth, eighth and tenth static eliminating method, it is preferable static elimination is carried out so that the rear side equilibrium potentials of the first surface and the rear side equilibrium potentials of the second surface at the respective sites in the plane of the insulating sheet may be respectively in a range from -340 V to 340 V. This static eliminating method is called a fourteenth static eliminating method.

In the fourteenth static eliminating method, it is preferable static elimination is carried out so that the rear side equilibrium potentials of the first surface and the rear side equilibrium potentials of the second surface may be respectively in a range from -200 V to 200 V. This static eliminating method is called a fifteenth static eliminating method.

In accordance with the present invention, there is provided a method for producing a charge-eliminated insulating sheet, comprising the step of eliminating charges from an insulating sheet by any one of the first through fifth, eighth, ninth and tenth static eliminating method.

In accordance with the present invention, there is provided a charge-eliminated insulating sheet, characterized in that both the charge densities of the first surface of the sheet and the charge densities of the second surface change smoothly cyclically in the longitudinal direction of the sheet; the amplitudes in the change of the respective charge densities are in a range from 1 to 150 µC/m²; and the charges of the first surface and the charges of the second surface at respective sites in the in-plane direction of the sheet are opposite to each other in polarity. This sheer is called a first sheet.

In the first sheet, it is preferable that the amplitudes are in a range from 2 to 30 µC/m². This sheet is called a second sheet.

In the first sheet, it is preferable that both the charge densities of the first surface and the charge densities of the second surface change in cycles of 10 to 100 mm. This sheet is called a third sheet.

In accordance with the present invention, there is provided a charge-eliminated insulating sheet, characterized in that the rear side equilibrium potentials of the first surface and the rear side equilibrium potentials of the second surface at respective sites of an insulating sheet are respectively in a range from -340 V to 340 V, and that the charges of the first surface and the charges of the second surface at respective sites in the in-plane direction of the sheet are opposite to each other in polarity. This sheet is called a fourth sheet.

In the fourth sheet, it is preferable that the rear side equilibrium potentials of the first surface and the rear side equilibrium potentials of the second surface are respectively in a range from -200 V to 200 V. This sheet is called a fifth sheet.

In the first sheet, it is preferable that the sums of the charge densities of the first surface and the charge densities of the second surface at respective sites in the in-plane direction of the sheet, i.e., apparent charge densities at respective sites of the sheet, are in a range from -2 to 2 µC/m². This sheet is called a sixth sheet.

In the fourth sheet, it is preferable that the sums of the charge densities of the first surface and the charge densities of the second surface at respective sites in the in-plane direction of the sheet, i.e., apparent charge densities at respective sites of the sheet, are in a range from -2 to 2 µC/m². This sheet is called a seventh sheet.

Typical examples of the insulating sheet include a plastic film, fabric and paper. The sheet can be fed from a long sheet wound as a roll or sheet by sheet. Examples of the plastic film include a polyethylene terephthalate film, polyethylene naphthalate film, polypropylene film, polystyrene film, polycarbonate film, polyimide film, polyphenylene sulfide film, nylon film, aramid film, polyethylene film, etc. In general a plastic film has high insulation performance compared with sheets of other materials. The static elimination technique provided by the invention can be effectively used for eliminating charges from a plastic film, especially for eliminating the positively and negatively charged sites alternately formed at a small pitch in the surfaces of the film.

In the invention, "the traveling path of an insulating sheet" means a space through which the insulating sheet passes for being liberated from charges.

In the invention, "the direction normal to an insulating sheet" means the direction normal to the plane free from sagging in the width direction, which plane is assumed to be the insulating sheet traveling in the traveling path.

In the invention, "the virtual plane" means a predetermined plane virtually assumed between first and second ion-generating electrodes. In the case where the insulating sheet traveling in the traveling path is assumed to be a plane free from sagging in the width direction, and where the position of the insulating sheet in the direction normal to the sheet varies with the traveling of the sheet, it can happen that the plane of the sheet assumed to be in the temporally averaged position agrees with the virtual plane.

In the invention, "the width direction" means the direction corresponding to the in-plane direction of the virtual plane, perpendicular to the traveling direction of the insulating sheet or perpendicular to the direction of predetermined row direction of disposed static eliminating units.

In the invention, "the pointed end of ion-generating electrode" means the region that forms an electric field capable of generating ions, among respective portions of the ion-generating electrode and that is nearest to the virtual plane. The ion-generating electrode is often extended in the width direction. In this case, "the pointed ends" are determined at the respective positions in the width direction.

For example, in the case where the ion-generating electrode is substituted by a wire electrode formed by a wire extending in the width direction of the sheet, the regions among the wire nearest to the virtual plane at the respective positions in the width direction correspond the regions. In the case where the ion-generating electrode is an array of needle electrodes installed at predetermined intervals in the width direction and extending in the direction normal to the insulating sheet, the region among respective portions of the respective needle nearest to the virtual plane (the tips of the respective needle electrodes) correspond to the regions at those position in the width direction. At positions in the width direction where no tip of needle exist, "the pointed ends of the ion-generating electrodes" are defined at the respective positions on a polygonal line 5dL connecting the respective tips of the needle electrodes provided at predetermined intervals in the width direction as shown in Fig. 18A. The polygonal line 5dL is called the virtual line of the pointed ends of the ion-generating electrodes. At positions in the width direction where the tips of the needle electrodes exist, the positions on the virtual line of the pointed ends of the ion-generating electrodes agree with the tips of the needle electrodes.

In the case where two or more electrodes capable of generating ions exist in the traveling direction of the sheet within the opening of one shield electrode, for example, in the case where two wires are extended, the average position of the pointed ends of the two or more ion-generating electrodes at each position in the width direction is considered as the pointed end of the ion-generating electrode at the position in the width direction.

In the invention, "first and second ion-generating electrodes are disposed to face each other" means that the first and second ion-generating electrodes face each other through the sheet traveling path or the virtual plane, and that at each position in the width direction there exists no conductor such as a shield electrode between the position of the feet of the perpendiculars from the pointed end of the first ion-generating electrode to the plane including the position of the pointed end of the second ion-generating electrode and parallel to the virtual plane, and the position of the pointed end of the second ion-generating electrode.

In the invention, "ions" mean various charge carriers such as electrons, atoms gaining or losing electrons, molecules having charges, molecular clusters and suspended particles.

In the invention, "an ion cloud" means a group of ions generated by ion-generating electrode, which spreads and floats in a certain space like a cloud without staying in a specific place.

In the invention, "a monopolar ion cloud" means an ion cloud in which the quantity of positive or negative ions is overwhelmingly larger the quantity of the ions opposite in polarity. Usually when the ion-generating electrode is positive in potential, a positive monopolar ion cloud is formed near the ion-generating electrode, and when ion-generating electrode is negative in potential, a negative monopolar ion cloud is formed near the ion-generating electrode. However, if the polarity of the voltage of the ion-generating electrode is reversed twice or more till the ions generated near the ion-generating electrode reach the insulating sheet, there occurs such a phenomenon that positive and negative ions exist together between the ion-generating electrode and the insulating sheet. In this case, the positive and negative ions are recombined with each other to lower the concentrations of ions, and whenever the polarity is reversed, the direction of Coulomb force to the ions is also reversed. As a result, the ion cloud irradiated to the insulating sheet cannot be monopolar any more.

In the invention, "an ion-generating electrode" means an electrode capable of generating ions in the air near the pointed ends of the electrode due to, for example, the corona discharge caused by application of a high voltage.

In the invention, "a shield electrode" means an electrode disposed near ion-generating electrode, to give an adequate potential difference between the shield electrode and the ion-generating electrode, for assisting the corona discharge at the pointed ends of the ion-generating electrode.

In the invention, "first and second ion-generating electrodes are disposed to face each other substantially symmetrically with virtual plane" means that the first and second ion-generating electrodes face each other through the virtual plane and that at each position in the width direction, the distance between the positions of the feet of the perpendiculars from the pointed ends of the first and second ion-generating electrodes to the virtual plane is shorter than the distance between the positions of the feet of the perpendicular from the pointed end of the first ion-generating electrode and the second shield electrode to the virtual plane, and also shorter than the distance between the positions of the feet of the perpendiculars from the pointed end of the second ion-generating electrode and the first shield electrode to the virtual plane.

In the invention, "a charge pattern" means a state where at least a part of the insulating sheet is locally positively and/or negatively charged. This state can be referred to a pattern formed by a fine powder (toner) or the like owing to the charged state by the method disclosed, for example, in JP 09-119956 A (hereinafter called document DS9) or JP 2001-59033 A (hereinafter called DS10).

In the invention, "apparent charge density" means the sum of the local charge density of both the surfaces at the same site in the in-plane direction of insulating sheet. The local charge density means the charge density of circular area portion having a diameter about 6 mm or less, more preferably a diameter 2 mm or less.

In the invention, "being apparently non-charged" means a state where the apparent charge densities at respective sites in the in-plane direction of an insulating sheet are substantially zero (-2 to 2 µC/m²).

In the invention, "charges are apparently eliminated" means a state where sites of a sheet substantially non-zero (less than -2µC/m² or more than +2 µC/m²) in the apparent charge densities are made apparently non-charged by means of static elimination.

In the invention, "the rear side equilibrium potential" of the first surface of an insulating sheet means the potential of the first surface measured when the measuring probe of a electrostatic voltmeter is sufficiently kept as close as keeping a clearance of about 0.5 to about 2 mm to the first surface in such a condition that a grounded conductor is kept in contact with the second surface to induce the charges in the grounded conductor to ensure that the potential of the second surface may be substantially kept at zero. The measuring probe of the electrostatic voltmeter has as small as less than two millimeters in the diameter of the opening for measurement. The probe can be, for example, probe 1017 (opening diameter 1.75 mm) or 1017EH (opening diameter 0.5 mm) produced by Monroe Electronics, Inc.

In the invention, keeping the rear surface (second surface) of the insulating sheet in contact with a grounded conductor means that both of them are kept in tight contact with each other in such a state that there is no clear air layer between the insulating sheet and the metallic roll. This state means that the thickness of the air layer remaining between both of them is 20% or less of the thickness of the sheet and 10 µm or less.

To obtain the distribution of the rear side equilibrium potential in the first surface, either the probe of the electrostatic voltmeter or the sheet having the grounded conductor kept in contact with its rear surface (second surface) is made to travel at a low speed (about 5 mm/sec) using a moving means capable of being adjusted in position such as an XY stage, to measure the rear side equilibrium potential one after another, and the obtained data are one-dimensionally or two-dimensionally mapped. The rear side equilibrium potential of the second surface can also be measured similarly.

In the invention, each potential is a potential from a grounded point, unless otherwise stated.

In the invention, "synchronization" means that the respective static eliminating unit intervals of two adjacent static eliminating units are integer times of the traveling distance of the insulating sheet per one cycle of the applied AC voltage. Furthermore, "superimposition" means that at a certain site of the insulating sheet, the ions irradiated by respective static eliminating units are superimposed.

In the invention, "synchronous superimposition" means that all the static eliminating unit intervals are integer times of the traveling distance of an insulating sheet per cycle of the applied AC voltage. In this case, when a certain site of the insulating sheet passes directly under the electrodes of respective static eliminating unit, all the ion-generating electrodes on one side generate ions of the same polarity, and charges of the same polarity are superimposed at the site.

In the invention, "synchronous superimposition intensity" expresses the intensity of polar concentrated degree of the ion clouds irradiated from respective static eliminating units to respective site of an insulating sheet, as a relative value with the value in the case of synchronous superimposition as one.

In the invention, parameters d₀, d₁, d₂, d₃, d₄, and D₂ expressing the positional relations of the respective electrodes and respective static eliminating units are defined as each position in the width direction as shown in Figs. 17, 18A and 18B. In Fig. 18A and 18B, the first static eliminating unit is shown as the typical unit. As symbol for distinguishing the positions of the static eliminating units, suffix is used. Suffix "1" in Fig. 18A and 18B signifies that that belongs to the first static eliminating unit. To express the ion-generating electrode facing the first surface of the sheet, symbol d is used, and to express the ion-generating electrode facing the second surface of the sheet, symbol f is used. Furthermore, to express the shield electrode facing the first surface of the sheet, symbol g is used, and to express the shield electrode facing the second surface of the sheet, symbol h is used.

In the invention, "electrode discrepancy d₀-1" of first static eliminating unit means a gap between the pointed end of the first ion-generating electrode 5d-1 and the pointed end of the second ion-generating electrode 5f-1 in the traveling direction of the sheet.

In the invention, "normal direction inter-electrode distance d₁-1" of first static eliminating unit means the distance between the pointed end of the first ion-generating electrode 5d-1 and the pointed end of the second ion-generating electrode 5f-1 in the direction normal to the insulating sheet.

In the invention, "static eliminating unit interval d₂-1" means the interval between the middle point 5x-1 of the line segment connecting the pointed end of the first ion-generating electrode 5d-1 of first static eliminating unit with the pointed end of the second ion-generating electrode 5f-1 of first static eliminating unit, and the middle point 5x-2 (not shown in the drawing) of the line segment connecting the pointed end of the first ion-generating electrode 5d-2 (not shown in the drawing) of the static eliminating unit adjacent to said static eliminating unit (second static eliminating unit) with the pointed end of the second ion-generating electrode 5f-2 (not shown in the drawing) of the static eliminating unit adjacent to said static eliminating unit (second static eliminating unit) , in the traveling direction of the sheet.

In the invention, "the normal direction inter-shield-electrode distance d₃-1" of first static eliminating unit means the shortest distance between the first shield electrode 5g-1 and the second shield electrode 5h-1 in the direction normal to the sheet. In this case, in the case where the shortest distance between the first and second shield electrodes d_3l-1 on the upstream side in the sheet traveling direction is different from d_3r-1 that on the downstream side, the average value (d_3l-1 + d_3r-1)/2 between the upstream shortest distance d_3l-1 and the downstream shortest distance d_3r-1 is used as the "normal direction inter-shield-electrode distance d₃ -1".

In the invention, "shield electrode opening width d₄-1" of first static eliminating unit means the opening width of the first and second shield electrodes in the traveling direction of the sheet. In this case, in the case where the width d_41-1 of the opening of the first shield electrode in the traveling direction of the sheet is different from the width d_42-1 of the opening of the second shield electrode in the traveling direction of the sheet, the average value (d_41-1 + d_42-1)/2 of them is used as the "shield electrode opening width d₄-1".

In the invention, "static eliminating gate length D₂" means the distance between the middle point 5x-1 of the line segment connecting the pointed ends of the first and second ion-generating electrodes 5d-1 and 5f-1 of the most upstream static eliminating unit (the first static eliminating unit) and the middle point 5x-n of the of the line segment connecting the pointed ends of the first and second ion-generating electrodes 5d-n and 5f-n of the most downstream (n-th) static eliminating unit in the traveling direction of the sheet. As can be seen from this definition, the static eliminating gate length D₂ agrees with the sum of all the inter-static-eliminating-unit intervals d₂-k (k=1,2,...,n-1) ranging from the most upstream static eliminating unit to the most downstream static eliminating unit.

According to the invention, as can be seen from the comparison between examples and comparative examples described later, an insulating sheet having positively and negatively charged sites alternately formed at a small pitch in the same plane or having such charged sites existing together in both the surfaces can be balanced between positive and negative charges and can be liberated from charges in both the surfaces substantially to a harmless level. Not only such an insulating sheet made apparently non-charged but also an insulating sheet made substantially non-charged can be produced by a very simple static eliminating method and eliminator.

That is, even from an insulating sheet having positively charged sites and negatively charged sites existing together within the same plane and/or in both the surfaces, the static charges can be effectively eliminated, and charge patterns can be eliminated. When the insulating sheet produced by the static eliminator or the static eliminating method of the invention, or the insulating sheet of the invention in post-process, such disadvantages as vacuum evaporation failure or coating irregularities are hard to occur, since the insulating sheet has few locally strongly charged portions such as static marks.

• Fig. 1 is a schematic drawing for illustrating the static eliminating action by the prior art.
• Fig. 2 is a schematic drawing for illustrating the static eliminating action by the prior art.
• Fig. 3 is a schematic drawing for illustrating the static eliminating action by the prior art.
• Fig. 4 is a schematic front view showing a conventional static eliminator.
• Fig. 5 is a schematic drawing for illustrating the static eliminating action by the eliminator shown in Fig. 4.
• Fig. 6 is a schematic drawing for illustrating the static eliminating action by the eliminator shown in Fig. 4.
• Fig. 7 is a schematic drawing for illustrating the charged state of a sheet that underwent the static elimination by the static eliminator shown in Fig. 4.
• Fig. 8 is a schematic front view showing another conventional static eliminator.
• Fig. 9 is a schematic drawing for illustrating the static eliminating action by the static eliminator shown in Fig. 8.
• Fig. 10 is a schematic drawing for illustrating the static eliminating action by the static eliminator shown in Fig. 8.
• Fig. 11 is a schematic front view showing a further other static eliminator.
• Fig. 12 is a schematic drawing showing the charged state of an insulating film that is apparently charged.
• Fig. 13 is a schematic front view showing a coating section of a die head coater.
• Fig. 14 is a schematic drawing showing a state where a conductive layer is kept in contact with one surface of an insulating film.
• Figs. 15A and 15B are schematic drawings showing relations of the film thickness to the charge densities of the first surfaces and the rear side equilibrium potentials of the first surfaces.
• Fig. 16 is a graph for illustrating relation among the charge density, the rear side equilibrium potential and occurrence of coating irregularity.
• Fig. 17 is a schematic vertical sectional view showing an embodiment of the static eliminator of the invention.
• Fig. 18A is an enlarged perspective view showing a static eliminating unit of the static eliminator shown in Fig. 17.
• Fig. 18B is a front view for illustrating the positional relation of the electrodes of the static eliminator shown in Fig. 17.
• Fig. 19 is a schematic drawing for illustrating the static eliminating action by the static eliminator shown in Fig. 17.
• Fig. 20 is a schematic drawing for illustrating the static eliminating action by the static eliminator shown in Fig. 17.
• Fig. 21 is a schematic drawing for illustrating the static eliminating action by the static eliminator shown in Fig. 17.
• Fig. 22 is a schematic drawing for illustrating the static eliminating action by the static eliminator shown in Fig. 17.
• Fig. 23 is a schematic drawing for illustrating the charged state of the sheet that underwent the static elimination by the static eliminator shown in Fig. 17.
• Fig. 24 is a graph for illustrating the relation among the normal direction inter-electrode distance, applied voltage and charging mode.
• Fig. 25 is a schematic drawing for illustrating the static eliminating action in the weakly charging mode by the static eliminator shown in Fig. 17.
• Fig. 26 is a graph for illustrating an example of the synchronous superimposition intensity by the eliminator shown in Fig. 17.
• Fig. 27 is a schematic drawing for illustrating a phenomenon in which the potential of a wound sheet roll rises due to the electric double layer.
• Fig. 28 is a schematic drawing for illustrating the state of the potentials of a wound sheet roll formed by winding a sheet that underwent the static elimination of the invention.
• Fig. 29 is a schematic front sectional view showing a mode of an electrode unit in the static eliminator of the invention.
• Fig. 30 is a schematic front sectional view showing another mode of an electrode unit in the static eliminator of the invention.
• Fig. 31 is a schematic front sectional view showing an electrode unit showing in Fig. 29 in the static eliminator of the invention.
• Fig. 32 is a schematic front view showing another embodiment of the static eliminator of the invention.
• Fig. 33 is a graph for illustrating the relation among the traveling speed, synchronous superimposition intensity and charge density amplitude, of the sheet that underwent static elimination using the static eliminator shown in Fig. 17.
• Fig. 34 is a graph showing an example of the measured distribution of rear side equilibrium potentials of a film that did not undergo static elimination.
• Fig. 35 is a graph showing an example of the measured distribution of rear side equilibrium potentials of a film that underwent static elimination.
• Fig. 36A and 36B are graph showing another example of the measured distribution of rear side equilibrium potentials of a film that did not undergo static elimination.
• Fig. 37A and 37B are graph showing another example of the measured distribution of rear side equilibrium potentials of a film that underwent static elimination.

1 ... static eliminator 1a ... AC power supply 1b ... ion-generating electrode 1c ... earth electrode 2 ... static eliminator 2a ... AC power supply 2b ... positive and negative ion-generating electrode 2c ... AC power supply 2d ... ion-attracting electrode 2e ... DC static eliminating member 2f ... AC static eliminating member 3 ... static eliminator 3a ... AC power supply 3b ... ion-generating electrode 3c ... AC power supply 3d ... ion-accelerating electrode 3e ... AC power supply 3f ... ion-generating electrode 3g ... AC power supply 3h ... ion-accelerating electrode 4 ... static eliminator 4a ... transfer sheet 4b ... transfer-sheet carrying sheet 4c ... corona discharger 4d ... corona discharger 4e ... corona discharger 4f ... corona discharger 5 ... static eliminator 5a ... guide roll 5b ... guide roll 5c ... first AC power supply 5d ... first ion-generating electrode 5e ... second AC power supply 5f ... second ion-generating electrode 5g ... first shield electrode 5h ... second shield electrode 5i ... insulating component 5j ... insulating component 5k ... vertical direction 51 ...traveling direction of insulating sheet 6 ... core 7 ... discharge electrode 7a ... ion-generating electrode 7b ... shield electrode 7c ... high voltage core wire 7d ... insulating component 8 ... discharge electrode 8a ... ion-generating electrode 8b ... shield electrode 8c ... high voltage core wire 8d ... insulating component 10 ... support of electric conductor 12 ... coated surface 13 ... die head coating section 14 ... backup roll 15 ... carrier roll 16 ... die 100 ... first surface (of sheet) 200 ... second surface (of sheet) 101 ... positive charge (of first surface of sheet) 102 ... negative charge (of first surface of sheet) 201 ... positive charge (of second surface of sheet) 202 ... negative charge (of second surface of sheet) 301 ... positive ion 302 ... negative ion 400 ... induced charge 500 ... line of electric force 700 ... Coulomb force S ... sheet &thetas; ... angle formed between 5k and 5l

Examples of the invention in the case of using a plastic film (hereinafter simply called a film) as an insulating sheet are described below in reference to drawings. The invention is not limited thereto or thereby.

For judging the effect of static elimination in the invention, a case where the absolute values of the charge densities of the respective surfaces (front surface and rear surface, or first surface and second surface) of a film that underwent static elimination declined by 10 µC/m² or more in absolute value compared with the absolute values of charge densities of the respective surfaces before static elimination is judged to be high in the effect of "eliminating the charges of the respective bipolarly charged surfaces."

As another method, a case where the absolute values of the charge densities of the respective surfaces of a film that underwent static elimination became 1/3 or less of the values of the charge densities of the respective surfaces before static elimination is judged to be high in the effect of "eliminating the charges of the respective bipolarly charged surfaces."

The reason is that in the "apparent static elimination" that is static elimination by the conventional static elimination techniques, the decline in the charge densities in absolute value of both-side bipolar charges is zero or 1 µC/m² at the highest. Furthermore, if the charge densities of the respective surfaces of a film that underwent static elimination are respectively in a range from -30 to +30 µC/m², the state can be judged to be "substantially non-charged," not to be "apparently non-charged. "

The existence of charges in the first surface 100 of a film can be confirmed, for example, according to the following methods. The existence of charges in the second surface 200 can also be confirmed similarly, as a matter of course.

The second surface 200 of a film is brought into contact with the grounded conductor, and in this state, the rear side equilibrium potential V_f of the first surface 100 is measured. Between the measured rear side equilibrium potential V_f and the charge density σ, a relation of σ = C x V_f holds, where C is the electrostatic capacity per unit area. If a sensor of electrostatic voltmeter is brought sufficiently close to about 2 mm from the film, the measured V_f is almost from the local charge right under the sensor in the first surface 100.

In the case where the thickness of the film is thin, the electrostatic capacity C per unit area can be obtained as the electrostatic capacity per unit area of a plane parallel plate, from C = ε₀ε_r/d_f, where ε₀ is the dielectric constant in vacuum = 8.854 x 10^-12 F/m; ε_r is the relative dielectric constant of the film; and d_f is the thickness of the film. Therefore, the local charge density in directly under sensor of the first surface 100 of file can be obtained. Since this method is a non-destructive charge confirmation method, keeping the reverse surface in contact with the conductor allows the charge density of the other surface of the film to be also confirmed.

In this case, if the film kept in contact with the conductor and the electrostatic voltmeter sensor are moved relatively to each other in the in-plane direction of the film with the clearance between them kept as it is, the distribution of the charge densities of the first surface 100 of the film can be measured.

The second surface 200 of a film is kept in contact with a conductor, and in this state, a toner powder is sprinkled over the first surface 100. The conductor can be used a metallic plate, metallic roll, etc. In the case where the film is not so firm that it is difficult to keep the film in contact with a metallic plate due to wrinkling or the like, it is desirable to use a cloth, paper or the like impregnated with a conductive liquid. In this method, since a toner powder is sprinkled, the film is destroyed. However, for confirming the effect of static elimination, it is a simple method. As the toner powder, a negative toner powder only can be used, but positive and negative toners with respective colors can also be used.

Only the charges of the second surface 200 of a film are treated for neutralization, and subsequently a toner powder is sprinkled over the first surface, to confirm the charges of the first surface 100. For neutralizing only the charges of the second surface 200, the following two methods can be exemplified. The first charge neutralization method is to form a conductive film on the second surface 200, for example, by vacuum evaporation. As the second neutralization method, the first surface 100 of the film is kept in contact with a conductor, and in this state, the second surface 200 is coated with a polar solvent. The coated surface is then dried to neutralize only the charges of the second surface 200. As for the neutralization of charges using a polar solvent, the action of isopropyl alcohol or the like is known, for example, as disclosed in document proceeding for 17^th symposium on Ultra clean technology, pages 361 - 363, ultra clean society, February 1993 (hereinafter called document DS14).

In the state where the first surface 100 of a film is kept in contact with a conductor, while the second surface 200 is coated with a polar solvent. In this state, the charges of the first surface 100 of the film balance the charges of opposite polarity induced in the conductor, and the charges of the second surface 200 of the film balance the charges of opposite polarity induced in the polar solvent. Then the coated surface is dried, the charges of the second surface 200 are neutralized. If the film is separated from the conductor after completion of neutralization treatment, the charges of opposite polarity induced in the conductor vanish. As a result, the film has charges left only in the first surface 100. The inventors have developed this method as a simple method for preparing a film having charges on one side only.

According to this method, the charged state of a film can be identified simply and quickly in an atmosphere of room temperature and atmospheric pressure. This method is recommended since the sensitivity of the toner to be deposited on the surface having charges is high. Polar solvents easy to handle and quick to dry include ethanol, isopropyl alcohol, etc. It is preferred that a polar solvent is coated as if wiping using cloth or the like and then is dried.

On the other hand, the film having a conductive material such as a metal vapor-deposited can be used as it is as a sample for evaluating the charged state of the non-vapor-deposited surface.

Also in these cases, for identifying the charged state, a negative toner powder or positive and negative toners with respective colors can be used.

The inventors confirmed charged states of films using these methods for identifying the charged states of films, and examined mechanisms working in such problems that when a film is coated with a coating material, coating irregularity occurs, that a coating material is partially repelled without being deposited in some places, and that when plural films are overlaid, the edges of the films cannot be neatly arranged due to cling films together (disarrangement of overlaid films). As a result, they found a preferred charged state of the film capable of obviating the problems otherwise caused by charges in the post-processes. Modes of charged states of films are described below.

The state, the charges in both the surfaces of a film balance (almost same in quantities, polarities opposite) each other, and the film is in an apparently non-charged. That is, the state, in the evaluation of charge densities by the first confirmation method, the sums of the charge densities of both the surfaces at the respective sites in the in-plane direction (apparent charge density in the respective sites) of a film are in a range from - 2 to +2 µC/m², or the toner powder is not deposited.

In this state, the charge densities existing in the respective surfaces of a film are sufficiently small. The state, the evaluation of charge densities by the first confirmation method, the charge densities of the respective surfaces of the film are respectively in a range from -150 to + 150 µC/m². In the state, it is preferable that the charge densities of the respective surfaces of the film are respectively in a range from - 30 to +30 µC/m². This state is defined to be "substantially non-charged."

The charge densities existing in the respective surfaces of a film are sufficiently small, and when the film is kept in tight contact with a conductor, the potentials of the surface not kept in contact with the conductor, i.e., rear side equilibrium potentials in a range from -340 to 340 V in this state. The state preferred that the rear side equilibrium potentials are in a range from -200 to +200 V.

In this state, neither the sites at which the charge density changes sharply in each surface of the film nor the local sites where the charge densities are high exist. It is preferred that the charge densities change smoothly and cyclically in cycles of about 10 to about 100 mm in the respective surfaces of the film.

In most cases where a conductive material is formed on one surface of a film in post-processing, for example, by vacuum evaporation or bonding of a metallic foil such as an aluminum foil, the film is only required to satisfy the modes A and B, though depending on the post-processing of the film. For example, in the case of a film having a conductor on one surface, disarrangement of overlaid films can occur. In this case, the Coulomb force proportional to the quantity of charges in the surface not having a conductive film affects the disarrangement of overlaid films (slipperiness). Therefore, it is preferred to control the charged state of the film by means of charge densities.

In the case where coating is performed as post processing and where it is desired to inhibit coating irregularity, a film with a thickness of about 1 µm to about 60 µm is only required to satisfy the modes A and B. If the film is thicker than the range, it is preferred to satisfy the rear side equilibrium potentials of mode C, instead of mode B. The reason is that both the apparent charges of the film and the rear side equilibrium potentials of the coated surface caused by the charge densities of the coated surface affect the coating irregularity defect. Also for inhibiting other defects, it is preferred to satisfy the modes B and C.

The inventors examined and found that the coating irregularity defects come in the following two modes.

As shown in Fig. 12, the apparent charge densities in absolute value of the film S are large in this mode. The apparent charge densities are less than -2µC/m² or more than +2µC/m², and the film is apparently charged. The coating irregularity of this mode occurs when the film is held in air.

As shown in Fig. 7, the rear side equilibrium potentials of the coated surface of the film S are large in absolute value in this mode. The rear side equilibrium potentials are less than - 340 V or more than +340V. The coating irregularity of this mode occurs above a conductive backup roll.

The following describes the mechanisms in which the above-mentioned coating irregularity defects clarified by the inventors occur, and the charged states of the film for inhibiting them.

In the film S having the charged state shown in Fig. 12 referred to for the first mode of coating irregularity defects, in the state where the film S is held in air, a strong electric field is formed near outside the coating surface of the film S. This electric field occurs since the apparent charge densities of the film S are not zero. This electric field lets such actions as electrophoresis and dielectrophoresis work on the applied coating solution, to cause coating irregularity.

On the contrary, in a film satisfying the charged state A, for example, in the film S in the charged state as shown in Fig. 7, in the state where the film is held in air, the electric field due to the charges of opposite polarity existing in both the surfaces of the film is closed in the film. So, a strong electric field little works near outside the coated surface. For this reason, such actions as electrophoresis and dielectrophoresis little work on the applied coating solution, and coating irregularity is hard to occur.

If a charge pattern with positive and negative charges existing together exists in the coating surface, the electric field formed between respectively adjacent positive and negative charges is slightly formed near outside the coating surface, but the influence of the electric field on the applied coating solution is small. The reason is that the distances between positive and negative charges existing in the respective surfaces of the film are small. The distances correspond to the thickness of the film and are in a range from several micrometers to hundreds of micrometers at the longest. In a site where the distances between the positive and negative charges existing in the plane of the film are sufficiently longer than the range, the electric field is closed in the film, and a strong electric field does not work near outside the coating surface. In a sole case if a distance of adjacent positively charged site and a negatively charged site in the plane of the film with a distance almost equivalent to the thickness of the film, an electric field in the in-plane direction of the film works near outside the coated surface.

However, this electric field is in a very limited microscopic region, i.e., a region of several micrometers to hundreds of micrometers at the largest, and the migration area of the coating solution is very small. Furthermore, the quantity of the solution capable of migrating in proportion to the region is also very slight. So, even if irregularity occurs, the irregularity cannot be visually observed. This explanation is concerning the relation between charges and coating irregularity in the case where a film held in air is coated.

On the other hand, though a film can be coated while it is held in air, a film can also be coated while it travels on a roll. The roll can be, for example, a backup roll of a die head coater, or a carrier roll for changing the traveling direction of the film. In this case, if the film is "apparently non-charged," with the both the surfaces charged equally in quantity but opposite in polarity, apparent charge density is zero, that is, if the film is the film S as shown in Fig. 7, there is a large problem that coating irregularity defects of the second mode occur. The mechanism in which the coating irregularity of this mode occurs is described below in detail.

Fig. 13 is a schematic drawing showing a part of the coating process using a die head coater. In Fig. 13, the film S is continuously unwound from a film package (not shown in the drawing) wound up as a roll and reaches a coating section 13. The coating section 13 is provided with two carrier rolls 15a and 15b, a backup roll 14 positioned between them, and a die head 16. The film S reaching the coating section 13 travels in contact with the carrier roll 15a, the backup roll 14 and the carrier roll 15b, in the direction indicated by the arrow 17, being changed in traveling direction. The coating solution put out from the die head 16 is applied to the film S, to form the coating surface 12 formed by coating layer on the film S. The film S coated with the coating solution gets the solvent of the coating solution evaporated and dried in a drying section (not shown in the drawing) , and finally wound as a roll in a winding section (not illustrated) .

In the state where the film S travels while being kept in contact with the backup roll 14, the film S is coated with a predetermined coating material (coating solution) put out from the die head 16. The backup roll 14 is installed for allowing the film S to travel stably and for keeping the clearance between the film S and the die head 16 constant. The backup roll 14 is, for example, a metallic roll plated with hard chromium, or a metallic roll covered with an elastic substance. As the elastic substance, a conductive rubber is often used.

The conductive rubber is used for the purpose of preventing the electrification of the backup roll 14, and prevents the firing of the organic solvent by electrostatic discharge. As described here, the backup roll 14 is made of a conductive material in most cases. Furthermore, in other coating methods using a roll coater or gravure coater, similarly a backup roll is often used. The charged state of the film S on the conductive roll is as shown in Fig. 14.

In Fig. 14, in the state where the film S is kept in contact with the conductive backup roll 14, the second surface 200 of the film S is kept in contact with the conductor, and the first surface 100 is on the coater side (die head 16 side) and becomes the surface coat with the coating solution (hereinafter called the coated surface 12). In this case, in response to the positive charges 201 and the negative charges 202 of the second surface 200, charges 400 of opposite polarity are induced in the backup roll 14. As a result, the potentials of the second surface 200 become zero.

On the other hand, since the positive charges 101 and the negative charges 102 of the first surface 100 as the coated surface 12 cannot induce sufficient charges 400 in the backup roll 14, because of the distance corresponding to the thickness of the film S from the surface of the backup roll 14. As a result, the charges of the first surface 100 actively exist. As a result, in the coating surface 12, the positive and negative charges 101 and 102 of the first surface 100 form an electric field. Because of the phenomenon in which the charges actively exist, even if the apparent charge density of film is zero, the electric field acts on the applied coating solution, causing coating irregularity.

The above description covers a phenomenon on the backup roll 14 of a die head coater, but also in the following case, an electric field acts on an applied coating solution in a similar mechanism. That is, a film S uniformly coated with a coating solution is carried into a drying step for evaporating and drying the solvent contained in the coating solution. In this case, it is practiced that the film S coated with the coating solution not yet dried is passed on the surface of a metallic roll, or that for better thermal conduction to the film S, the film is kept in contact with a metallic roll for drying. Even on the metallic roll, the same phenomenon as occurring in the case of the backup roll 14 occurs, and coating irregularity occurs in the film S.

The inventors found that the coating irregularity by charges occurs if a strong electric field of more than a certain level acts on a thin coating solution layer. The reason is considered to be that the coating solution migrates according to the electric field, for forming an uneven distribution of the coating solution. If the coating solution can be charged, the migration of the coating solution occurs due to electrophoresis. The electrophoresis causes the coating solution to be collected in the site of the film charged in the polarity opposite to that of the charges of the coating solution. As a result, the thickness of the coating layer in the portion becomes larger than the thickness of the coating layer in the surrounding, to cause coating irregularity. On the other hand, if the coating solution cannot be charged, the migration of the coating solution occurs due to dielectrophoresis, and the coating solution is collected in a site of the film with a strong electric field, and the thickness of the coating layer in the portion becomes larger than the thickness of the coating layer in the surrounding, to cause coating irregularity.

With regard to the occurrence of coating irregularity on an "apparently non-charged" film S above a metallic roll, since the intensity of an electric field is decided in relation with the charge densities of the film S, smaller charge densities result in a weaker electric field if the thickness of the film S is constant. As a result, coating irregularity is hard to occur. However, the coating irregularity occurring above a metallic roll is not decided by the charge densities only, and the inventors found that the intensity of the electric field near outside the first surface 100 formed the coated surface, that is, the magnitude of "the rear side equilibrium potentials" in the first surface 100 greatly affect the coating irregularity.

In the case where the surface (second surface 200) reverse to the coating surface of an apparently non-charged film S is kept in contact with a metallic plate, the electric field intensity near outside the first surface 100 in the direction normal to the film S is proportional to the rear side equilibrium potentials. That is, it is proportional to the distance between the conductor (metallic plate) and the first surface 100, in other words, the thickness of the film S. For example, if the number of charges is the same, i.e., if the same charge density exists, the rear side equilibrium potentials of thin