Coating processes – With stretching or tensioning
Reexamination Certificate
1996-10-18
2003-04-22
Rickman, Holly (Department: 1773)
Coating processes
With stretching or tensioning
C427S314000, C427S250000, C427S525000, C427S531000, C427S566000, C204S192140, C204S192170
Reexamination Certificate
active
06551653
ABSTRACT:
The invention relates to metalized, in particular two-sided-metalized films, which are highly suitable as dielectric in capacitors. The novel films are polyolefin films, to be precise made from cycloolefin polymers, which, surprisingly, should not be subjected to a process for increasing the surface tension or surface energy (such as corona treatment) before metalization, unlike the usual case for polyolefins.
For use of polymer films as dielectrics in capacitors, the dissipation factor tan &dgr;, the heat resistance, i.e. the stability of the mechanical (for example shrinkage) and electrical film properties at elevated temperature, and the metalizability are of considerable importance.
Low dissipation factors are of particular interest in high-frequency alternating-current applications, since dissipation performance is also low at low tan &dgr;. Increased dissipation performance—and thus increased tan &dgr;—means warming, so that the heat resistance of the film material can ultimately be exceeded and the capacitor can be damaged or destroyed. Accordingly, an ideal capacitor dielectric has a low dissipation factor coupled with high heat resistance.
With respect to metalizability, it is known that polyester films are easier to metalize than polyolefin films, since the latter must be subjected to surface treatment before metalization in order to achieve adhesion of the metal to the film. Metalization of thin films for use in capacitors is the subject of intensive research efforts. There is a fundamental difference in this respect between the polyethylene terephthalate (PET) predominantly employed at present as dielectric and polypropylene (PP). Owing to the polar polymer structure, PET has a critical surface tension of about 43 mN/m, which is sufficient to ensure adhesion to the metal, for example aluminum. By contrast, the critical surface tension of polyolefin films is, at from 30 to 33 mN/m, in a range which is not sufficient to ensure adhesion to the vapor-deposited metal layer. For this reason, the surfaces of polyolefin films must be treated by various methods in order to increase the surface tension and to achieve wettability, bondability and metalizability.
The most frequently used method is treatment with a high-frequency alternating voltage (10-60 kHz, 10-20 kV), known as corona treatment. This allows the surface tension to be increased to up to 50 mN/m. In the case of polyolefin films, in particular biaxially oriented films made from polypropylene, surface tensions of from 36 to 42 mN/m are usually established by means of corona discharge. However, the disadvantages of corona treatment are that, for example, the surface tension is time-dependent, and that treatment results in the formation of low-molecular-weight fragments of the polymer chain which can result in weakening of the bond between the polymer surface and a vapor-deposited metal layer.
For economic reasons, it is desirable to construct a capacitor from a two-sided-metalized film and an unmetalized film. As described in U.S. Pat. No. 3,900,775 this is possible, for example, by using a two-sided-metalized polyethylene terephthalate film and an unmetalized polypropylene film. However, the disadvantage of this structure is the greatly increased tan &dgr; value of polyethylene terephthalate compared with polypropylene. For reasons of the better dissipation factor, polyolefin films are preferred over polyester films in alternating current applications. However, economical production of polypropylene films vapor-deposited (metalized) on both sides is significantly more difficult and is hitherto not performed industrially. A problem is corona treatment which must be carried out on both sides of the film before metalization. The electrostatic charges arising mean that this results in sticking (blocking) of the film to the reel. The adhesion forces occurring during winding up in turn produce more charges, which prevent subsequent uniform vapor deposition with the metal. According to DE-A-28 02 769, this problem can be circumvented by dissipating the charge on the film before vapor deposition. However, this is again an additional process step and consequently an additional source of error and is therefore uneconomical.
There therefore continues to be a need for a metalizable, preferably two-sided-metalized polyolefin film in which the disadvantages of the prior art are avoided and which has a low dissipation factor and high heat resistance.
A further object of the present invention was to provide a process for the production of a metalized (if possible on both sides) polyolefin film which avoids the disadvantages of the prior art, in particular the additional process step for increasing the surface tension.
Surprisingly, it has now been found that, from the multiplicity of polyolefins, cycloolefin polymers—against all expectations—can be metalized without pretreatment which increases the surface tension.
Accordingly, the object set is achieved by a one- or two-sided-metalized, single- or multilayer polyolefin film in which at least one outermost layer of the unmetalized polyolefin film essentially consists of a cycloolefin polymer which has not been subjected to a process for increasing the surface tension before metalization.
‘One- or two-sided-metalized’ means that the film carries a metal layer on one or both surfaces.
Single- or multilayer means that the unmetalized film is either a monofilm, i.e. consisting of only one layer, or is a multilayer structure and accordingly can be constructed from two, three, four, five or even more layers. It is essential to the invention here that the monofilm or at least one outermost layer of the multilayer film essentially consists of a cycloolefin polymer.
The expression ‘essentially consists of a cycloolefin polymer’ means that the monofilm or at least one outermost layer of the multilayer film consists of at least 90-100% by weight, preferably at least 95-100% by weight, in particular at least 98-99% by weight (based on the weight of the monofilm or the outermost layer of the multilayer film) of cycloolefin polymer. If desired, the single-layer film or the outermost layer may additionally include additives which are usually employed in the production of films.
The expression ‘not subjected to any process for increasing the surface tension before metalization’ means that the film is not subjected to additional treatment resulting in an increase in the surface tension after its conventional production process, which usually involves extrusion, stretching and heat-setting. This is taken to mean conventional processes, such as corona or flame treatment. It is essential to the invention that the metalization can take place without the film having been subjected beforehand to such a process.
Cycloolefin polymers are materials which are distinguished by high heat deflection temperatures, high moduli of elasticity, low water absorption and good dielectric properties.
DD-A-224 538 describes the production of films from norbornene-ethylene copolymers by a film casting process. The production of cycloolefin polymer films by melt extrusion is described in EP-A0 384 694, EP-A0 610 814, EP-A-0 610 815 and EP-A-0 610 816. The improvement in the mechanical properties of the films by monoaxial or biaxial stretching is likewise described in these specifications.
DD-241 971 and DD-224538 state that films made from cycloolefin polymers are distinguished by low dissipation factors (tan &dgr;). The values given for tan &dgr; of up to 1,2·10
−5
are below the values found for polymer materials used, in accordance with the current prior art, as dielectrics in capacitors. Only polystyrene has similar low values. As further stated in DD-241 971, low values of tan &dgr; are of particular interest for high-frequency alternating-current applications, since electrical dissipation in the film can result in warming. The combination of high heat resistance (stability of the mechanical and electrical properties of the film at elevated temperatures) and low tan &dgr; makes cycloolefin polymers highly su
Grosse Kreul Theo
Hatke Wilfried
Kochem Karl-Heinz
Rickman Holly
Ticona GmbH
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