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For better understanding the advantage of ZINC ALLOY metallized polypropylene for capacitors
manufactured by FILMET, we should consider the difficulties that you should face in
manufacturing high quality capacitors with normal aluminium metallized polypropylene.
To overcome such problems you have to reach a compromise, but even with the best manufacturing
techniques, it would be very difficult to pass the qualification tests.
Here-following we will consider the four most common defects that are responsible of aluminium
metallized capacitors failure.
2) Electric contact due to spraying
3) Decrease in capacitance with tension over 250 VAC
4) Inside electric shocks not due to film weakness
Here following are indicated the single points:
1) HIGH SELF HEALING
2) HIGH ELECTRIC CONTACT DUE TO SPRAYING
3) NO DECREASE OF CAPACITANCE EVEN IN THE LONG RUN
4) HEAVY DUTY METALLIZATION LAYER
1) Bad self healing
Applying a direct or alternate tension to an Al met. film made capacitor, electrical shocks
will occur between the plate corresponding to weak film points. High temperature makes the
thinner Al layer evaporate all around these specific spots enabling the winding elements to
resist higher electrical tensions. Thinner metallized layers are easier to self heal.
Thicker metallized layers need more thermal energy to evaporate and this can cause the
element's or the polypropylene films destruction. As we can see from the above mentioned, we
must reach a compromise, as we cannot reduce the metal layer under a certain limit.
This one is the most common defect in plastic film capacitors.
Contact spraying on the tops is normally made with a special pistol that overcoats with zinc.
Good contact depends on the technological system used in the spraying plant and on the
thickness of the sprayed layer.
Unfortunately, as explained on point 1, we cannot have an extreme thick layer so as to not
compromise the self healing capabilities.
Naturally a lack of a perfect contact will create a resistance in current flowing, and
therefore will create heat that will cause the elements final failure.
The same defect can occur when the spray doesn't reach each layer (particularly inner core
windings). In this case tension will flow longitudinally and not transversally creating an
increase in power factor and a local heating.
Above this tension aluminium metallized film shows a great decrease in capacitance that is
not justified by the lost plate due to the self healing process' evaporation.
This particular defect is due to aluminium oxide (Al2O3) that shows up
as transparent and isolated spots.
The corresponding decrease in capacitance is due to the partial disappearance of metallic
plate and it's promoted by specific conditions of temperature and humidity and it's worsened
in thinner metallized layers.
It's very difficult for aluminium metallized capacitors to pass a life test
(Nominal-Tension x 1,25V/85 oC/1000 h) that provides a capacitance loss of minus 3%.
During element winding, air with its average H2O content is trapped between the
coils. To stabilise the element and get rid of the air, heat treatment is normally used to
shrink the polypropylene above 90 oC.
This film shrinking cause the air expulsion, the tightening of the coils and the modification
of the superficial polypropylene film structure to improve moisture resistance.
Empty spaces between coils will also cause lack of homogeneity in film thickness thus
preventing the thermal treatment to uniformly shrink the element.
When we apply an electrical tension between these two frameworks we practically have two
capacitance in series, one with polypropylene dielectric, the second with air dielectric. As
the air has an lower dielectric constant, the higher applied tension will cause shocks that
will show up between polypropylene surface and plate and despite their low power can seriously
damage the element.
Referring to these major failure defects we can now indicate the peculiar technical
characteristic of FILMET "ZINC ALLOY" that allow you to pass the listed difficulties
and compromises.
The self healing occurs easily, because FILMET's "ZINC ALLOY" metallized plate has a
specific ohms resistivity higher than that of pure aluminium, and the low melting point
temperature needs less thermal energy flow for healing.
The process of self healing is easily facilitated from the specific surface resistance of the
metallized layer, which can be fixed to a high value (5/10
W/mm2) relative to the normal one, which is obtained in
case of metallization with a pure aluminium (3 W/mm2).
FILMET "ZINC ALLOY" is composed of three essential parts: un-metallized margin which
avoid contact between the two electrodes, a central part with metallization which constitutes
the capacity and a metallized re-enforced margin which thrusts out and constitutes the top of
the winding elements, where spraying will be effected for the contact.
The spraying is normally done with pure zinc, which solders well with the metallized margin of
FILMET "ZINC ALLOY" and this type of operation is facilitated by the higher thickness
of the re-enforced margins.
The metallized re-enforced margins (edges) are useful if some coils are not sprayed (especially
for the coils which are near to the winding core) as the electricity which runs through the
electrode longitudinally meets less resistance in the margin.
The metallized layer of FILMET "ZINC ALLOY" won't give decreasing capacitance to the
capacitors, even when major necessity of electric power and temperature occur. In fact the
oxidised composition, obtained by metallizing contemporary Aluminium and Zinc does not form
isolating spots; as Al oxide is still a good conductor and assures a continuos electrode layer.
Life tests, even the stricter one, have confirmed that with FILMET "ZINC ALLOY" don't
appear capacitance variations.
Despite today's technology improvements in winding (modern machine introduce less air into the
element), in base film (the average good tolerance on film planarity) and in metallization
(heavy duty metallization layer), we still recommended to apply the thermal treatment for the
capacitor. The best advantage of the treatment is no more to get the air out of the element
but to stabilise the element to be used at high performance level (85 oC).