Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
2001-04-17
2003-03-25
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C264S173190, C264S210700, C264S466000, C428S461000, C428S516000, C428S910000
Reexamination Certificate
active
06537652
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a polypropylene film having improved properties for use as electrical insulation film in capacitors.
2. Description of Related Art
EP-A-0 645 426 describes a biaxially oriented polypropylene film whose n-heptane-insoluble content has a chain isotacticity index, measured by C-NMR spectroscopy, of at least 95%. The base layer has essentially no resin. The modulus of elasticity of the film in the longitudinal direction is greater than 2500 N/mm
2
. The modulus of elasticity of the film in the transverse direction is greater than 4000 N/mm
2
. The modulus of elasticity of the film in the transverse direction is greater than 4000 N/mm
2
. According to the object of EP-A-0 645 426, these polypropylene films have low residual shrinkage. The description contains no information on the shrinkage values.
EP-A-776 926 describes a biaxially oriented film for use in capacitors which has a total shrinkage of 1-4% in the longitudinal and transverse directions at 120° C. The film consists of a highly isotactic polypropylene and has an isotacticity of 98-99.5% and an isotactic pentad fraction of >99%. No information is given on the shrinkage behavior of the film at temperatures above 120° C.
WO 97/05634 describes a capacitor whose dielectric consists of a polypropylene film having a modulus of elasticity of greater than 1500 MPa, a melting point above 110° C. and a Vicat softening point of above 150° C. The film has, inter alia, improved shrinkage behavior in the longitudinal direction between 100° C. and 150° C. The shrinkage behavior in the transverse direction is not described.
The object of the present invention is to provide a biaxially oriented polypropylene film which contributes to improved service properties of the capacitors produced therefrom. In particular, improvement in the electrical dissipation factors and the heat resistance of the capacitors are constantly needed in the expert world.
For the production of film capacitors, the films are generally firstly metallized and then coiled. Even the metallization of the film includes heating, which the film has to withstand, i.e. there must be no dimensional changes of the film due to shrinkage during metallization.
The film is thus frequently subjected to elevated temperatures both during conversion into the metallized coil and also during use. It has been found that, in particular in the case of so-called power capacitors for high-voltage applications, extreme temperatures in local regions, so-called hot-spot temperatures, can occur locally in the interior of the capacitor due to flowing currents or discharge processes between film layers. According to available studies, temperatures significantly above 120° C. up to 140° C. are reached in places.
For the above-mentioned reasons, the heat resistance of the film is of particular importance when it is used in capacitors. In particular, the film should have the lowest possible shrinkage under the given loads due to elevated temperatures, while at the same time the other advantageous properties must not be impaired.
After production of the coil, its face sides are vapor-deposited with a metal spray, generally zinc, in order to ensure good contact between the metallized film layers and the contact wires to be applied. The present invention is based on the knowledge that shrinkage occurring during use of the capacitor due to temperature loads impairs this contact between the wire and the metallized film. The consequence is impaired dissipation factors and capacitance drift.
In particular in the case of sudden changes in the temperature, there is a risk of the contacts tearing due to dimensional changes in the coil, which are themselves caused by shrinkage. This causes a further increase in the electrical dissipation factor (tan &dgr;); the capacitor becomes hotter ever more quickly, which in extreme cases can result in total failure of the capacitor. It has been found that the service properties of the capacitor are closely correlated with the heat resistance of the film at elevated temperatures. In particular, the electrical dissipation factor of the capacitors is improved by low shrinkage of the film at elevated temperatures.
In connection with the present invention, it has been found that longitudinal and transverse shrinkage of the film should be considered differently here. Besides low longitudinal shrinkage of the film, even greater importance is attached to transverse shrinkage, in particular, with not only the absolute shrinkage values, but also the relative temperature dependence of the transverse shrinkage in the range 120-140° C. having an effect. Surprisingly, the capacitor s exhibit improved heat resistance of the dissipation factor and improved capacitance drift with time if the transverse shrinkage in said temperature range has a particularly low increase with rising temperature.
SUMMARY OF THE INVENTION
This object formulated above is achieved in accordance with the invention by a biaxially oriented polypropylene film whose characterizing features including the fact that the shrinkage of the film at elevated temperatures is low and the transverse shrinkage curve is flatter and the dissipation factors, in particular after storage, are lower.
In accordance with the invention, the film may be single-layered, in which case it consists only of the base layer described below. In a preferred embodiment, the film has at least one top layer, if desired top layers on both sides, on its base layer.
DETAILED DESCRIPTION OF THE INVENTION
The base layer of the film generally comprises at least 95% by weight, from 99 to 100% by weight, in each case based on the base layer, of a highly isotactic propylene homopolymer described below.
This propylene homopolymer comprises from at least 98 to 100% by weight, in particular from 99 to 100% by weight, of propylene. The corresponding comonomer content of at most 2% by weight or in particular from 0 to 1 % by weight generally consists, if present, of ethylene. The data in % by weight are in each case based on the propylene homopolymer.
The propylene homopolymer of the base layer has a melting point of from 160 to 170° C., preferably from 162 to 167° C., and a melt flow index (measurement in accordance with DIN 53 735 at a load of 21.6 N and 230° C.) of from 0.5 to 10 g/10 min, preferably from 0.7 to 3.5 g/10 min. The n-heptane-soluble content of the polymer is generally from 1 to 3% by weight, preferably from 1 to 2% by weight, based on the starting polymer, corresponding to an isotactic content of from 97 to 99%, preferably from 98 to 99%. The chain isotacticity index (triad method) of the propylene homopolymer, determined by
13
C-NMR spectroscopy, is from at least 95 to 99%, preferably from 95.5 to 98%. The isotactic pentad fractions, which are likewise determined from the
13
C-NMR spectrum of the polymer, are in the range from 85 to 95%, preferably from 87 to 92%. The molecular weight distribution (determined by GPC) of the propylene homopolymer, expressed as the ratio of the weight average molecular weight M
w
to the number average molecular weight M
n
, is generally from 2 to 8, preferably from 4 to 7. The mean molecular weight M
w
(weight average), determined by GPC, is in the range from 200,000 to 500,000, preferably from 250,000 to 400,000.
In view of the proposed application of the film according to the invention as capacitor film, the propylene homopolymer should have an ash content of <35 ppm, preferably from 10 to 30 ppm, and a maximum chlorine content of 5 ppm.
The shrinkage behavior of a film is determined by several factors. Crucial factors for the shrinkage are the raw materials used, the production process and the film thickness. Basically, thinner films tend toward higher shrinkage both in the longitudinal and in the transverse directions, which is connected with the different degrees of orientation of the molecular chains in very thin films. This effect is more pronounced the higher the temperature to which the film is subjected, i.e., the s
Kochem Karl-Heinz
Muller-Nagel Kerstin
Hoescht Trespaphan GmbH
Nakarani D. S.
Powell Goldstein Frazer & Murphy LLP
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