Recently, along with electronization of automobile-related equipment and digital equipment, energy saving has advanced, and there is a demand for impedance reduction and lifetime elongation of parts to be mounted thereon. In addition, due to the impedance reduction of parts to be mounted thereon, there are attained many merits such as reduction in power loss, response to voltage reduction of semiconductor operating power and increase in operating speed, and enhancement of frequency characteristics.
Furthermore, due to an energy serving policy and an alternative energy policy to oil by not only Japan but also by every country in the world, the utilization of inverter circuits or the like each having good energy efficiency has been continuously expanded in every field related to environments such as wind power generation, solar cell, hybrid vehicle, electric vehicle, and various types of energy saving devices. Also in home appliances, a large number of equipment such as air conditioner, refrigerator, laundry machine and lighting equipment all of which are for saving energy include the invertor circuit.
In the invertor circuit, an aluminum electrolytic capacitor is used for smoothing of varied component included in a direct current that is output from a rectifier.
Since the aluminum electrolytic capacitor occupies a large volume ratio in the components of the invertor circuit, further miniaturization of the aluminum electrolytic capacitor is strongly demanded.
When a ripple current is applied to the aluminum electrolytic capacitor, the capacitor is self-heated due to loss of power. It is possible to suppress heat generation by the ripple current due to reduction in impedance of the aluminum electrolytic capacitor. The heat generation in the aluminum electrolytic capacitor is a factor that directly affects the lifetime, and since the reduction in the heat generation results in prolongation of the lifetime, the requirement for the impedance reduction becomes higher.
In general, an electrolytic capacitor such as an aluminum electrolytic capacitor is fabricated by interposing a separator between an anode aluminum foil and a cathode aluminum foil, winding these to thereby form a capacitor element, impregnating the capacitor element with an electrolytic solution, inserting into a case, and then sealing an opening.
It is known that the electrolytic solution and the separator give a large influence on the impedance of the aluminum electrolytic capacitor. Accordingly, in order to improve the impedance of the aluminum electrolytic capacitor, the reduction in the impedance of the electrolytic solution and the separator is highly required.
Furthermore, it is effective to stake the separator thin for down-sizing the aluminum electrolytic capacitor.
In a case of using aluminum foils having the same area, a capacitor element having a smaller outer diameter can be fabricated by the use of the capacitor element formed of the thinner separator. Therefore, a thin separator is required.
The main role of the separator in the aluminum electrolytic capacitor is isolation of the electrode foils and retaining of the electrolyte. Electrical insulation is required for the material of the separator, and hydrophilic properties and lipophilic properties are required for retaining various types of electrolytes. Therefore, there is used a separator made from cellulose having these properties at the same time.
Examples of the cellulose materials for the separator to be usually used include: a natural cellulose fiber such as a conifer craft pulp, a manila hemp pulp or an esparto pulp; and a regenerated cellulose fiber such as a solvent-spun cellulose fiber.
Effective techniques for reducing the impedance of the separator are to reduce the basis weight of the separator, to reduce the density, and to make the thickness thin.
However, there are caused various problems in mere reduction of the basis weight of the separator, reduction of the density, and making the thickness thin.
The denseness of the separator is also reduced by reduction of the basis weight of the separator, reduction of the density, and making the thickness thin, the denseness of the separator is also reduced. Accordingly, in case of the use in the aluminum electrolytic capacitor, there is a problem that an element short circuit defect rate and an aging short circuit defect rate are increased, and even if short-circuiting does not occur, the short circuit defect rates of products after coming onto the market are increased.
In addition, the value of tearing strength of the separator is also lowered, in a case of reducing the basis weight of the separator, reducing the density, and making the thickness thin. As a result, during the manufacturing step of the aluminum electrolytic capacitor, the separator is broken to lower productivity and yield.
For these reasons, even if the separator has a low basis weight, low density and is thin, the separator is required to have high denseness so as not to increase the short circuit defect rats and strength so as to avoid paper breaking in each step.
In order to enhance the denseness of the separator and to reduce the short circuit defect rate of the aluminum electrolytic capacitor, there have been known methods in which the thickness of the separator is made large, and a value of CSF (Canadian Standard Freeness) in accordance with JIS P 8121 which indicates a degree of beating of pulp being a raw material is made smaller, thereby enhancing the density.
However, when the thickness of the separator is made large and the density is increased, the impedance is worsened.
Furthermore, at the time of winding the aluminum electrolytic capacitor element, a stress is applied to the separator, the anode foil and the cathode foil mainly in the longitudinal direction. However, by shifting of the position of the separator from side to side on a transporting foil, there is a case where a stress of the separator is added also in the width direction. At that time, when the tearing strength of the separator is weak, there is a case where the separator is broken to lower the yield.
Therefore, the separator for the aluminum electrolytic capacitor is also required to have strong tearing strength.
As mentioned above, as to the separator for the aluminum electrolytic capacitor, there is required a thin separator having excellent impedance and being capable of improving short circuit defect rate and enhancing yield.
In the separator for the aluminum electrolytic capacitor, there have been proposed various configurations in order to enhance properties (for example, refer to Patent Literature 1 to Patent Literature 7).