[Boatanchors] Re: Polyester vs. Polypropylene Orange-Drops? [Also component lifetimes]

[Barry L. Ornitz]

Cliff Bond, WA4MWM, asked:

> 1.    Can someone enlighten me as to the difference
> between Polyester Film versus Polypropylene Film Orange-
> Drop capacitors?  I know the Polyesters are slightly
> smaller and are 10% while the Polypropylenes are larger,
> 5% tolerance and slightly less expensive, but is there a
> functional difference between them or a preference of one
> over the other?

Orange-Drop was a tradename of Sprague at one time. but
Sprague was bought by Vishay, an Indian company, so the
idea of Orange-Drops being American-made no longer
applies.  Forget what the folks on the R-390 list say, any
quality capacitor can be used.

Polyester is the generic name for a class of polymers;
DuPont's tradename is Mylar.  Polypropylene is another
generic name; it is for a different type of polymer.
Capacitors made from polyester have a positive temperature
coefficient, while those of polypropylene have a negative
temperature coefficient.  The dielectric losses of
polypropylene are much less than for polyester, however,
and these losses increase for both types of capacitors with
frequency.  Also, the higher dielectric constant of
polyester means capacitors made with it can be physically
smaller than those made with polypropylene.  The upper
temperature limit for polyester capacitors is slightly
higher than that for polypropylene.  So a choice between
these is always a tradeoff.

At audio, either capacitor would be suitable.  At radio
frequencies, the polypropylene capacitors will be slightly
better for bypassing or coupling applications.  Neither
should be used in radio frequency resonant circuits.

Also note that film capacitors can be made with metal foil
as the electrodes, or by metallizing the insulating film
directly.  The metallized film type has a slightly longer
lifetime and it is generally considered self-healing (a
high voltage puncture vaporizes the metallization near the
short thereby clearing it).

As for sources, I would look to Digi-Key's line of
Panasonic capacitors, or Mouser's Vishay or Xicon
capacitors.

I am also copying a post that I wrote for the Old Tube
Radios list a while back on the lifetime of capacitors and
other components.  It goes into a little more detail.

Please note I consider these posts copyrighted, and I ask
that they NOT be re-posted in other mailing lists or
websites without permission.

        Dr. Barry L. Ornitz    WA4VZQ    ornitz@tricon.net


[Article originally posted to Old Tube Radios mailing list
on March 29, 2002]

From: "Barry L. Ornitz" <ornitz@tricon.net>
To: "Old Tube Radios" <boatanchors@theporch.com>
Subject: Re: Capacitor lifetime & a little info on new materials
Date: Wednesday, May 29, 2002 07:34 PM

John Gibson <gibsonj@mindspring.com> initially asked:

> ......   I have always replaced the leaky waxed paper
> caps in vintage radios with polyester caps thinking that
> they will last forever. But is this true?  Has any
> estimate been made of their lifetime?

Arden Allen <gumbear@pacbell.net> then answered:

> Does anything last forever?

and continued with anecdotal evidence of the longevity of
modern film capacitors.  He then concluded:

> Beings the film is a relatively pure substance, free from
> internal degradation, I believe only extreme heat would
> lead to destruction.  They would essentially have to melt
> and then short out as opposing plates come in contact
> with each other or the melted plastic shorted through
> from electrostatic stress and physical distortion.  I
> think your leakage worries are over for the next
> millenium or two.

Having spent most of my career involved in manufacturing
polyethylene terephthalate polyester (PET, Mylar� is
DuPon'ts variety), polypropylene (PP) and
polytetrafluoroethylene (PTFE, Teflon� is the most common
variety), and been involved in a few research projects on
other newer materials occasionally used in capacitors such
as polyphenylene sulfide (PPS), polyethylene naphthalate
(PEN), and cyclohexane dimethanol modified polyester
(PETG), I think I can address this issue.  I also am
familiar with polystyrene (PS) and polyphenylene oxide
(PPO) materials.

Of these, the PS and PTFE capacitors tend to have
specialized uses and are not often found.  The PPS and PEN
materials are relatively new and I expect to see more of
them in high temperature applications.  The PETG material
was investigated for film capacitors at least 30 years
ago and found to have some advantages over conventional
PET, but Eastman management decisions prevented further
development.  It is being looked at again today.

So this leaves the conventional PET and PP materials for
most of the film capacitors we can find today.
Polypropylene is a good dielectric material with quite low
losses at higher frequencies, buts its low temperature
rating limits its application in many circuits.  The
lifetime of polypropylene capacitors should be exceptional
as long as they are not overheated.

Polyester capacitors have greater dielectric losses,
especially at high frequencies, but having a higher
dielectric constant than PP and a higher temperature rating
they are probably the most popular film capacitor material
today.  Their lifetime is probably somewhat less than
polypropylene, but the term "forever" means different
things to different people.  Moisture will eventually react
with the polyester structure to decrease its polymer chain
length.  It will also react with the heavy metal catalysts
(typically antimony) to form ions which will increase the
leakage in capacitor applications.  The effect will be seen
with wrapped foil capacitors long before it is seen with
those whose electrodes are metallized onto the film (the
aluminum metallization retards the diffusion of moisture
into the polyester).

>From a practical viewpoint, even wrapped foil polyester
capacitors will certainly outlive those reading this
message - that is, if they are kept relatively dry and not
overheated.  Metallized polyester capacitors will last much
longer.  My guess is at least a few hundred years.  Somehow
I cannot be convinced to worry about what might happen to a
Boatanchor a millennia from now!

Electrolytic capacitors are an entirely different story.
They need moisture to function (normal room relative
humidity is fine, but storage under exceptionally dry
conditions will shorten their life).  The so-called dry
electrolytics are not really dry, they contain a paste that
needs some moisture to remain electrolytically conductive.

Carbon composition resistors certainly do age, but the
modern thin-film resistors have good lifetimes.  Tubes do
lose vacuum, especially the softer glass receiving tube
varieties.  But these generally do not need quite as hard a
vacuum as transmitting tubes.  Most large transmitting
tubes use borosilicate glass which is better than the soda-
lime glass found in small tubes.

Probably the fastest aging things in our old radios are the
transformers with paper insulation, and the vinyl
insulation on wires.  Keeping everything cool will help
more to prolong the life of this old gear more than
anything.

      73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net