Ultra-thin low moisture content polyester film and its applicati

Details Ultra-thin low moisture content polyester film and its applicati Ultra-thin low moisture content polyester film and its applicati

1995-09-15

1998-06-09

Chen, Vivian

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

428458, 428480, 427 81, 427124, 427250, 427296, 427316, 427541, B32B 1508, B32B 2736, C23C 1402, C23C 1424

Patent

active

057630739

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This application is a 371 of PCT/US93/03562, filed Apr. 21, 1993.
The present invention relates to an ultra-thin low moisture content or substantially dry polyester film. It also relates to the applications of such a film and a method of processing during which outgassing takes place, and notably a metallization process, and finally, it relates to metallized films and multi-layer capacitors obtained from such film.
Polyester films, for example, in polyethylene terephthalate (PET) such as Mylar.RTM. (DuPont), or in polyethylene naphthalate (PEN) such as those produced by Teijin, are known, along with their applications. Such applications include the use of these polyester films as dielectric material in multi-layer capicators where their use has increased considerably. In such capacitors, a film of metal deposited on the polyester acts as the foil of the capacitor.
There are a very large number of manufacturing processes available. The common link between them is that they involve a step carried out under vacuum in which the virgin polyester film introduced in roll form is unwound, metallized and then wound again onto another core.
In the majority of present-day processes, narrow strips are masked during this metallization step carried out under vacuum, in order to define non-conducting regions referred to as free margins.
The roll of metallized film obtained can then, generally speaking, be treated in two different ways which, this time, are not done under vacuum. A first of such processes consists of unwinding the metallized film, slitting it in the middle of the free margins and then rewinding it in the form of flat rolls referred to as pancakes. Such pancakes are then, in their turn, unwound once again enabling either wound capacitors or what are commonly known as stack capacitors, to be obtained.
A second process consists in unwinding the film which, in this case, is generally metallized on both sides, then covering it with an insulating varnish on one side and then rewinding it onto a drum. Stack capacitors can then be obtained by slitting or guillotining the stack at the middle of the free margins and in the perpendicular direction as well.
The electrical capacity, expressed in Farads, of a multi-layer capacitor is, for a given volume, inversely proportional to the square of the polyester film thickness. This is the reason why there is a heavy demand for ultra-thin film which enable ever-increasing miniaturization to be achieved. There are currently available on the market films which can be as thin as 0.6 microns, and which typically are of the order of one micron thick. Polyester films having a thickness of around 1 micron are however poorly adapted to being treated under vacuum, in a metallizing station, unlike films which are thicker.
It has in fact been noted, somewhat surprisingly, that during unwinding under vacuum, the outer turns of the roll behave in a particular way: they are able to shift sideways with respect to each other in the direction of the axis of the roll causing an offset between layers; this is known as telescoping; the film also suffers from a tendency to "swim", i.e., oscillate laterally about the offset position and, finally, the film tends to expand radially and form wrinkles running in the direction of travel. By way of illustration, four rolls of film 0.9 micron thick and 500 mm wide were observed to suffer from telescoping by an amount of from 1 to 30 mm, from "swimming" with an amplitude of 1 to 10 mm and from formation of wrinkles of a size of from 0.5 to 5 mm.
This behavior with instability and formation of wrinkles is encountered again and/or made more pronounced when the film passes over the transport rolls and over the metallization drum. There are several harmful consequences of such behavior.
Firstly, the roll of metallized film obtained at the end of the process includes wrinkles. Moreover, the turns do not strictly lie one on top of the other but rather are offset axially. Then at the time of slitting, which as has been said, is not

REFERENCES:
patent: 4708905 (1987-11-01), Yoshii et al.
patent: 4756064 (1988-07-01), Yoshii et al.
patent: 4814221 (1989-03-01), Utsumi

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