Manufacturing process of a capacitor with delayed ageing and a capacitor manufactured according to this process

The manufacturing process of a capacitor with a polypropylene dielectric film by introducing pre-cross-linking elements of the polypropylene at the time of manufacturing. When the capacitor is used, the electrical field and/or temperature generated trigger the cross-linking process of the polypropylene, delaying and slowing down the ageing phenomena, i.e. the decrease of the dielectric strength. The pre-cross-linking agent is a polyfunctional monomer to which an activation accelerator and/or a stabilizing agent can be added.

The invention relates to a manufacturing process of a capacitor comprising 
one or more windings having two metal armatures separated by one or more 
polypropylene films. 
In power capacitors, polypropylene is mainly used as dielectric material, 
inserted between metal armatures. The armatures can be formed by sheets of 
aluminium foil wound with the polypropylene film or films or be formed by 
a metal coating of one or both faces of the polypropylene film. The 
winding formed by the coil of the armatures and polypropylene is generally 
impregnated with a dielectric liquid, but the invention is applicable to 
dry capacitors in which the winding is arranged in air or in a high 
dielectric strength gas, notably sulphur hexafluoride. 
The coil can also be located in a solid medium, in particular in greases. 
It has been noted that the above-mentioned capacitors present ageing in the 
course of use resulting in a decreased dielectric strength, and in 
internal breakdowns which quickly make the capacitor inoperational. 
The object of the present invention is to achieve a manufacturing process 
of a capacitor whose ageing is delayed. 
The manufacturing process according to the invention is characterized in 
that, in the manufacturing stage, pre-crosslinking elements of the 
polypropylene are introduced comprising a polyfunctional monomer having 
chain functions which once they have been activated, make a chemical bond 
with macro-molecular polypropylene chains and that cross-linking of the 
polypropylene is generated by the action of an electrical field and/or 
temperature in the course of use of the capacitor, so as to reduce the 
degradation of the dielectric strength of the polypropylene film. 
Ageing of the capacitor is essentially due to degradation of the 
polypropylene which gives rise to a decrease of the dielectric strength. 
By generating according to the invention cross-linking of the 
polypropylene in the course of use of the capacitor, a notable decrease of 
the ageing phenomenon and of breakdowns due to this ageing is observed. 
The object of cross-linking of the polypropylene is to create chemical 
bonds between the macromolecular chains to obtain a three-dimensional 
lattice which improves the properties of the polypropylene. Polyfunctional 
monomers, which are small organic chains having chemical functions which 
once activated can make chemical bonds with the macromolecular chains of 
the polypropylene, are incorporated in the polypropylene. The 
cross-linking mechanism requires in addition an energy input to create 
these new chemical bonds and according to the invention this energy is 
produced by the electrical field and/or the temperature generated in the 
course of use of the capacitor. The invention consists in using a 
non-cross-linked polypropylene and in creating the conditions favorable 
for cross-linking by adding polyfunctional monomers. A pre-cross-linked 
polypropylene is thus created, cross-linking of which is instigated and 
then maintained by an energy input supplied when the capacitor is used. 
The dielectric strength of the polypropylene is thus stabilized in the 
course of operation of the capacitor whose lifetime is thus notably 
improved. 
The polyfunctional monomers or bonding agents used are molecules or 
chemical compounds which have one or more chemical functions able to 
create a chemical bond with the polypropylene in the presence of an 
activating energy. The polyfunctional monomer has one or more of the 
following characteristics: 
it is an organic macromolecule of low molecular weight, with an olefinic 
structure comprising double bonds such as polybutadiene for example, 
it comprises a vinyl group such as trimethoxysilane vinyl for example, 
it comprises an acrylyl group or an acrylate group (acrylic acid) such as 
for example: tetramethylolmethane tetraacrylate, trimethylolpropane 
triacrylate, 1.6 hexaneoglycoldiacrylate, 
it comprises an isocyanato group or a cyanato group or a cyanuric cycle or 
a cyanurate cycle (cyanuric acid) such as for example: triallyl 
isocyanurate, triallyl cyanurate, 
it comprises an ether with an allyl group such as allyl ether of glycerine 
for example, 
it comprises a cyclic ketone or a biketone (quinone) such as p-benzoquinone 
for example, 
it comprises a phenoxy group such as phenol for example, 
it comprises a phenylenedioxy group such as hydroquinone for example, 
it comprises an epoxy cycle such as for example: bisphenol A 
diglycidylether, 3-4 epoxy cyclo hexyl methyl-3-4-epoxy cyclohexane 
carboxylate. 
According to a development of the invention, the cross-linking mechanism is 
enhanced by the use of activation accelerators which are chemical 
molecules easy to activate and to form a radical which will transfer its 
activation to the bonding zones. 
The activation accelerator is advantageously chosen from the following 
products: 
benzoyl peroxide 
di-tert-butyl peroxide 
dicumyl peroxide 
1.4-di-tert-butylperoxy diisopropyl benzene 
2.5-dimethyl- 2.5-di-tert-butyl peroxyhexyne 
tert-butyl perbenzoate. 
In order to guide all these activated species towards a cross-linking 
process, it is advantageous to add an antioxidant product, notably 
molecules or chemical compounds having properties inhibiting degradation 
of the polypropylene. These products moreover have the advantage of 
stabilizing the cross-linked polymer and they are notably chosen from the 
following products: 
2.6-di-tert-butyl-4 methyl phenol 
methylene 3-(3',5'-di-tert-butyl-4-hydroxyphenyl) - propionate 
hydroxybenzophenone. 
The invention relates to a capacitor achieved according to the process 
described above, this capacitor being characterized in that it 
incorporates pre-cross-linking elements and that the cross-linking 
operation of the polypropylene is instigated and maintained by the action 
of the electrical field and/or the temperature. 
The pre-cross-linking agents or elements can be incorporated in the 
capacitor at different stages of its manufacture. When the capacitor 
comprises a dielectric impregnation liquid, the pre-cross-linking elements 
are preferably mixed with this dielectric liquid which is in contact with 
the polypropylene. The pre-cross-linking elements can also be incorporated 
at any time in the polypropylene transformation chain from its production 
in the form of basic resin through to its implementation to constitute a 
capacitive element of the capacitor. A polypropylene film is thus achieved 
containing all the elements for cross-linking when used in a capacitor, 
and this polypropylene film can be used in all types of capacitors, with 
or without impregnation liquid, with gas or solid insulation. The 
pre-cross-linking elements can also be incorporated when winding of the 
capacitor element is carried out, for example by powdering or any other 
operative means, bringing these pre-cross-linking elements into contact 
with the polypropylene.

Two examples of capacitors whose ageing has been improved by the process 
according to the invention are given below: 
EXAMPLE 1 
The power capacitor is formed by a set of wound elements which comprise two 
electric armatures in the form of metal strips between which a dielectric 
insulator is inserted in the form of polypropylene films. The wound 
elements are impregnated with a dielectric liquid, notably with a mixture 
of mono and dibenzyl-toluene. To limit degradation of the polypropylene 
films by the electrical field, the pre-cross-linking elements are 
incorporated in the capacitor at the time of impregnation. The 
impregnation liquid contains the following additives: 
dicumyl peroxide, i.e. bis (.alpha., .alpha.- dimethyl benzyl) peroxide at 
2% weight 
vinyl trimethoxysilone at 4% weight 
2.6-di-tert-butyl-4-methyl phenol at 1% weight 
EXAMPLE 2 
The power capacitor is formed by a set of wound elements which comprise two 
alternately wound metallized polypropylene films. To limit degradation of 
the polypropylene films by the electrical field, the following additives 
are incorporated homogeneously in the material at the time the 
polypropylene film is achieved: 
benzoyl peroxide at 2% weight 
1.4 benzoquinone at 1% weight 
The invention is not limited to the process more particularly described 
herein, and can be applied to low voltage or medium and high voltage 
capacitors.