Polystyrene: Its Electrical and Mechanical Characteristics
August 1939 QST

August 1939 QST Table of Contents These articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list of the QST articles I have already posted. All copyrights are hereby acknowledged.

Polystyrene 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.

Polystyrene: Its Electrical and Mechanical Characteristics

How to Use it in Amateur Equipment

By Herbert S. Riddle, W1DSK

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.

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.

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 the saw.

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 wrench.

A good way to make a link mounting. Polystyrene is easily cemented.

Turning - As with sawing and drilling, cutting must be done at slow speed to avoid heating of tool and material.

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.

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.

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.

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