Polystyrene: Its Electrical and Mechanical Characteristics
August 1939 QST
has been commonly used in capacitors since long before I came on the
electronics scene. Its widespread use in electronic and electrical components
was first adopted in Europe in the 1920s, and then later became popular
in the U.S. where scientists improved its characteristics and lowered
its cost to where it could be found just about anywhere current flowed.
In its present form, polystyrene can be easily bent, cut, turned, polished,
melted, drilled and tapped - truly a versatile material. This article
provides a brief introduction to polystyrene.
August 1939 QST
of Contents]These articles are scanned and OCRed from old editions of the
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QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
Polystyrene: Its Electrical and Mechanical Characteristics
How to Use it in Amateur Equipment
By Herbert S. Riddle, W1DSK
Polystyrene is a transparent solid dielectric, formed by the thermal
polymerization of monomeric styrene, C6H5CH:CH2.
This first sentence disposes neatly of the chemical side, and from here
on we can interest ourselves only in the physical and electrical properties
of polystyrene, and its application in amateur transmitters.
An example of how polystyrene can be worked. The switch shown
here was constructed by the author from sheet material. Note
how the upper part of the stationary member is bent over to
form a support and terminal for the plate cap of the tube.
Several years ago, in Europe, rapid strides were made in the molding
of small electrical parts from polystyrene materials, and it was not
long before a few of the radio parts manufacturers in this country were
doing the same. The polystyrene material was imported from England in
the form of crystals or coarse powder, and compression molded to the
required shapes. The pieces produced by this method were very expensive
because the cost of the polystyrene crystals of powder produced abroad
was between two and three dollars per pound, plus duty. This imported
material had other disadvantages aside from high cost, such as a very
low softening point, lack of clarity, and instability when exposed to
sunlight. The United States is now foremost in the development and use
of new plastic materials, and a domestic polystyrene is now produced
which has electrical properties equal to those of fused quartz, excellent
water resistance and dimensional stability up to 184(degree)F.
The electrical qualities vary little with frequency, as shown in
the following table:
These extremely low-loss characteristics, comparable only to
fused quartz, are maintained even under very adverse moisture conditions;
tests show 0.00 per cent water absorption after 48 hours immersion,
and 0.05 per cent after over 300 hours immersion. The dielectric strength
is about 500 volts per 0.001 inch, in thickness of 1/8 inch.
In appearance polystyrene resembles glass, being a very light (specific
gravity 1.05), non-inflammable transparent solid, the usual form being
sheets, rods, or tubes, with polished surfaces. The clarity of this
material is so great that news print may be read through a solid polystyrene
rod 24 inches long. The material also sounds like glass when dropped
or thrown upon a hard surface, but polystyrene differs from glass in
that it does not break.
With a little care, no difficulty will
be experienced in fabricating the material to any shape; however, it
may be well to outline a few general rules to follow when working it:
Sawing - Strips of sheets are easily sawed to size with a regular
hack saw, backing up the polystyrene with a piece of board to prevent
burring of the cut edges. The sheet may be scribed with a sharp point
as a guide to sawing. The sheets cannot be cracked off in a straight
line by scribing one or both sides as with glass. The sawing must be
done at a reasonably slow speed so that the saw blade will not become
hot and cause softening of the material, and consequent sticking of
Drilling - Care must be exercised in center punching,
because a heavy blow with a center punch may cause a small "star" fracture.
The drilling should be at slow speed to avoid heating of the drill and
softening of the material. Polystyrene drills are readily as Bakelite.
Tapping - No difficulty is experienced in tapping, using a hand
Turning - As with sawing and drilling, cutting must be done at slow
speed to avoid heating of tool and material.
A good way to make a link mounting. Polystyrene is easily cemented.
Polishing - Sawed
edges may be finished with a file and then buffed to regain the glass-like
surface. The buffing must be done very lightly.
Bending - Polystyrene
may be bent or formed into various shapes, but must be heated to over
200(degree)F. In the case of smaller strips to be bent into angles,
this may be done by holding that portion of the strip to be bent about
one-quarter inch from the soldering iron until the material has softened.
The strip may be readily bent to the desired shape, and sets immediately,
the bent portion being fully as strong as the original sheet. Larger
sheets may be bent to different shapes by inverting the family flat
iron, covering it with several layers of cloth and placing the polystyrene
sheet on the cloth. A pair of heavy cotton gloves is of great help in
handling the hot sheet.
Cementing - The material may be readily
cemented to itself by the use of toluol, a solvent for polystyrene.
The best procedure is to clamp the two pieces to be cemented in the
proper position and apply toluol to the joint. The toluol will run into
the joint, dissolve the material to provide a bond and then dissipate,
leaving a strong joint. A joint thus made may be handled in just a few
minutes but does not obtain its full strength for several hours.
Polystyrene is an insulating material known by various trade names,
the most familiar of which is "victron." The acknowledged leader in
point of low losses, its mechanical properties are such as to make it
ideal for some applications, unsuitable in others. Recent developments
in manufacture have raised the softening point and lowered the cost
to an extent which will encourage its wider use in amateur equipment.
Heat - No difficulty has been experienced from tube inductance heat
in actual operation. Care should be taken, however, about soldering
the lugs or other hardware in direct contact with the polystyrene, since
the high temperature will cause temporary softening of the material
under the lug.
|Polystyrene material is very adaptable to the construction
of coil supports and switches, condenser strips, and similar
insulators for medium- and high-power amplifier stages. The
photograph shows the plate cap support and plate lead switch
of the 100-watt output buffer in the writer's transmitter, switching
pre-tuned 14-Mc. and 28-Mc. tanks. This stage may be switched
when in operation.
The greatest need for a good dielectric is in
the tank circuits of our transmitters. In the writer's transmitter,
polystyrene was substituted for condenser strips and coil mounting strips
of other material, which had actually swelled and bubbled up to almost
twice its thickness because of heat developed by the power leaking through
it. Several other transmitters have been changed over to polystyrene
insulation in the final tank circuit, and the increased efficiency and
output have been surprising. In one case substitution of polystyrene
for a bakelite type insulation on the tank condenser, tank coil and
neutralizing condenser of a 14-Mc. 'phone transmitter resulted in a
power output increase of 40 per cent. No other changes were made in
the circuit or input.
This article is in no sense a technical
treatise on all the characteristics of polystyrene. The coincidence
that the writer is both an amateur and connected with a chemical company
specializing in the field of plastics gave us the opportunity to try
this material in our own outfit and those of a few friends. The results
obtained, plus the fact that polystyrene is bound to find increasing
use as a radio insulator, particularly for use at ultra-high frequencies,
justified an article summarizing the electrical characteristics of this
material and the methods of using it.
Posted October 18, 2013