July 1965 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
If you like pronouncing long,
complex chemical names, you'll really enjoy this "Plastic Film Capacitors" article
from a 1967 issue of Electronics World magazine. It was written by Walter
Lamphier of Sprague Electric,
a long-time manufacturer of capacitors of all sorts. As of 1992, Vishay has owned
Sprague (founded in 1926 by Robert Sprague), but has strategically retained the
very familiar Sprague name as part of theirs. Anyway, a lot of information is provided
about the relatively new (at the time) plastic-film construction. A contemporary
article on the same subject would no doubt include a whole host of new chemical
compounds not even invented in 1967. This particular issue of Electronics World
reports on a few other kinds of capacitor constructions popular at the time, including
paper, ceramic, glass, and electrolytic.
Fig. 1 - Internal details of a metal-encased film capacitor.
By Walter C. Lamphier / Senior Product Specialist, Sprague Electric Co.
Advances in plastic chemistry have produced an open-ended list of capacitor dielectrics
whose electrical characteristics can be tailored to suit almost any particular circuit
Among the plastic-film dielectrics presently used in capacitors are polystyrene,
polyester, polycarbonate. cellulose tri-acetate,
polyparaxylene. The basic construction of capacitors using these materials is
quite similar. They may be considered as parallel-plate capacitors which have been
rolled into a coil. For the most part, the conducting plates extend from opposite
ends of the coil and have leads attached. The capacitor roll is protected from the
external environment by a metal case, a plastic housing, or a plastic encapsulation.
Fig. 1 shows the basic construction of a typical capacitor which might use any of
the these dielectrics.
The photograph (next page) shows a number of capacitors which demonstrate the
numerous variety of external housings and terminal arrangements in which plastic-film
dielectric capacitors are furnished.
The dielectric materials mentioned may be used to produce extremely thin sheets
which are employed as the separators in plastic-film capacitors. Each material is
used in capacitors because some particular characteristic or combination of characteristics
of the resulting capacitors is unique and most advantageous for specific circuit
applications. Table 1 covers some of the more important physical and electrical
properties of these materials.
Table I - Characteristics of the various dielectrics which are
used in the manufacture of plastic capacitors.
Some of the different methods of packaging plastic-film units.
Fig. 2 - Capacitance change vs. temperature for various plastics.
Fig. 3 - Dissipation factor vs. temperature for various plastics.
Polystyrene is a polymer of the aromatic styrene monomer and possesses outstanding
electrical characteristics. Using this material, capacitors may be made to capacitive
tolerances as close as 0.1 %. They will retain this precise value almost indefinitely,
even following moderate temperature changes. The high resistivity and small negative,
but linear, temperature coefficient of capacitance make them very useful in analog
computers. The two chief limitations to the use of polystyrene capacitors are the
85°C maximum operating temperature and their relatively large size.
Polypropylene is an aliphatic polymer of the propylene monomer and possesses
outstanding electrical characteristics similar to polystyrene. It is available from
high-volume production equipment and hence has the lowest price per pound of any
of the organic materials mentioned. Unfortunately, it has some serious limitations
because of its physical properties. It has low tensile strength and thus is limited
to a minimum thickness of 1.0 mil (0.001") at the present time. Its lack of physical
stability prevents it from having the outstanding electrical stability that is characteristic
of properly made polystyrene capacitors.
Polytetrafluoroethylene (PTFE) is an aliphatic polymer of the fluorinated carbon
atom. Its intrinsic insulating properties are the best of ail plastic materials
presently used. The highly symmetrical fluorinated carbon atom, as well as a lack
of impurities in the material as manufactured. result in the highest resistivity
and lowest loss factor of all organic dielectric materials used in capacitors. It
also has a small linear negative temperature coefficient of capacitance. PTFE does
not melt but may be sintered above 300°C, indicating high bond strength which may
be capitalized upon to produce capacitors rated for reliable operation at 200°C.
The two most striking drawbacks of TFE-fluorocarbon capacitors are their large size
and high cost. The size is greater than that of polystyrene capacitors and the cost
is five times as great. Because of these limitations, PTFE capacitors are used as
a last resort when the highest insulation resistance possible, a linear temperature
coefficient above 85°C, or an operating temperature above 150°C are key
parameters. The material is available from several sources here and abroad under
such names as Teflon-TFE, Fluon, Halon, and other trademarks. It also has widespread
uses in such applications as coating for chemical ware and cooking utensils because
it is inert to most chemicals and materials do not adhere to it.
Polyester capacitors are presently the most widely used of all plastic-film capacitors.
The most common polyester which is employed is a product of the reaction of
and ethylene glycol. The latter may be familiar as the standard non-boiling automotive
antifreeze material. Polyethylene terephthalate was originally developed abroad
and its electrical characteristics were first published in 1949. It is not only
used as a capacitor dielectric but it is also one of the most widely used synthetic
materials for making fibers for weaving cloth. Imperial Chemical Industries, which
originated PETP (to use the common abbreviation), markets its capacitor film under
the trademark Melinex and its fiber as Terylene. E. I. DuPont de Nemours, which
introduced the material into the U. S., calls it Mylar or Dacron for the corresponding
applications. A heavier gauge film for photographic purposes is sold under the trademark
Cronar by Dupont. Other vendors have recently come on the market with PETP as well,
selling it simply as an XX-brand polyester or under other trademarks, such as Celanar,
etc. A somewhat similar polyester material with slightly different qualities,
polycyclohexylene dimethylene terephthalate (abbreviated as PCHMTP or PMPT),
is sold by Eastman Kodak under the trademark Kodar.
Polyester, as made by Dupont, was the first dielectric to effectively challenge
the use of kraft paper in an important segment of the capacitor field. The material
is physically very tough. Moreover, it may be obtained in near-perfect sheets as
thin as 0.15 mil (15 gauge), 1/20 the thickness of a typical brunette hair. It exhibits
a much greater tolerance toward atmospheric contamination, such as from moisture,
than does kraft paper. For this reason, non-hermetically sealed polyester-film capacitors
have substantially replaced the conventional "paper tubular." Polyester capacitors
must be operated below the corona, or flash-over voltage of air, since voids are
always present. As a consequence, alternating voltages above 250 volts r.m.s. and
direct voltages above 2500 volts are frequently detrimental to dry-wound polyester
capacitors. A further limitation in PETP film is the second-order transition in
the material which may occur between 85° and 125°C. Indications are a substantial
increase in capacitance, a peak in the loss factor, and a pronounced increase in
failure rate cinder d.c. operation.
Fig. 4 - Insulation resistance vs. temperature for the plastics.
A combination dual dielectric of polyester film and kraft paper with various
impregnants, either "solid" or liquid, overcomes some of these problems and increases
reliability. These dual dielectric capacitors are very widely employed.
Polycarbonate, while also a polyester, is usually referred to by this generic
name. It is formed by reacting bisphenol-A and phosgene. The latter may be remembered
as a poisonous gas used in World War I.
The KG-polycarbonate material offered by the German firm Farbenfabriken Bayer
A. G. has proven to be superior to polycarbonates from other sources when used as
a capacitor dielectric. The thickness gauges are approximately the same as those
of Mylar-brand PETP. Its handling properties are quite similar; however, polycarbonate
does not exhibit the second-order transition characteristic of PETP and retains
its excellent electrical qualities at temperatures as high as 140°C. Both the
resistivity and loss factor approach the values for polystyrene. The capacitance
of polycarbonate units will change about 1% from room temperature over the operating
temperature range. The curve or capacitance change with temperature is crescent-shaped
with its maximum at about room temperature . Capacitance stability with time is
very good but not quite as excellent as that exhibited by polystyrene capacitors.
At this time, the two factors that have prevented its use in a major section of
the capacitor field are the physical condition of the sheet and the cost of the
film, which is more expensive than either paper of PETP-polyester.
It should be noted that dual-dielectric capacitors, which balance the characteristics
of PETP and polystyrene to achieve an essentially zero temperature coefficient of
capacitance over a wide temperature range, in many cases may be replaced by polycarbonate
Cellulose tri-acetate (CTA), which is a nearly completely acetylated cellulose,
has been well known for many years. Straight cellulose acetate and cellulose acetate
butyrate have also been used as capacitor dielectrics but have been substantially
replaced by polyester. Cellulose tri-acetate, however, may be manufactured in film
as thin as 4 gauge or 0.00004". The smallest capacitor which can be manufactured
from CTA film is 1/3 the volume of the smallest equivalent PETP capacitor. Electrical
properties of the cellulose tri-acetate capacitors are adequate for most requirements
at temperatures up to 85°C but are a limitation on their use at higher temperatures.
Polypyromellitimide film is made from a resin resulting from the condensation
reaction of pyromellitic dianhydride and an aromatic diamine. This unique organic
polymer, sold by DuPont under the trademark Kapton-H polyimide, is capable of being
used over a very wide temperature range and is highly stable. Its electrical characteristics
approach those of TFE-fluorocarbon except that the temperature coefficient of capacitance
is positive and not precisely linear. The material presents a number of exciting
possibilities to the capacitor designer and user; but its ultimate value will best
be determined after some history in capacitor production when it has passed the
pilot-plant stage, production problems have been solved, and prices have been firmed.
This should not be too far in the future.
Table 2 - MIL-Specs covering some of the plastic dielectrics.
Polyparaxylene polymer may be deposited on electrode in extremely thin films.
It is consistent with the other highly priced plastics mentioned previously in yielding
a dielectric strength of several thousand volts per mil. Capacitor data to this
time is quite limited because the material is still in the final stages of development
for this application. Available information does hold out the prospect of a substantial
reduction in capacitor sizes for uses above 125°C.
Because this film can be vapor deposited directly onto the metal electrodes as
a pure uniform polymer in thicknesses one-third that of free film dielectrics, stable
devices having less than one-fifth the size of polystyrene capacitors of equivalent
electrical rating can be produced.
Capacitors produced from these materials are compared in a number of ways when
selecting the proper type to best meet circuit needs. The curves given in Figs.
2, 3, and 4, showing variation in capacitance, dissipation factor, and insulation
resistance to temperature, illustrate the type of data used for this purpose.
In addition to these fundamentals, two other features of organic-film capacitors
must be taken into consideration. Of the eight materials mentioned, polystyrene
has not been extensively available or used in the metallized form. Thin sheets of
polystyrene must have very low voltage ratings, and the basic size problem which
exists with capacitors made from this material becomes amplified. Both TFE-fluorocarbon
and polyparaxylene can be deposited on electrodes by vacuum techniques which permit
film thicknesses of less than 1000 Angstrom units. These techniques are still in
the embryonic stages but do present the possibility of low-voltage film capacitors
in the future being minified to approach the sizes, in some cases, of aluminum electrolytic
capacitors. As previously mentioned, two different films may be used together in
a capacitor winding, either in series or in parallel within the same roll. The resultant
compromise in electrical characteristics can be forecast from curves of Figs. 2,
3, and 4. The PS/PETP combination, which has substantially zero capacitance change
from 0°C to 80°C, is the best in this respect of any known capacitor, including
the so-called NPO ceramics.
One measure of the acceptance of a new type of capacitor in industry is found
in the status of its Military Specification. MIL Specs are written only after a
capacitor has had some usage by suppliers of military electronic equipment. A rule
of thumb might be that a MIL Spec is generated after two to three years of significant
and successful usage. Keeping this in mind, Table 2 may be of interest, setting
forth as it does the present specification status in the Department of Defense of
capacitors which are currently manufactured from the organic plastic materials discussed
in this article.
Posted September 7, 2022