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Polystyrene: Its Electrical and Mechanical Characteristics
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See all available vintage QST articles. Polystyrene: Its Electrical and Mechanical CharacteristicsHow to Use it in Amateur EquipmentBy Herbert S. Riddle, W1DSK
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.
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.
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 |
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