Flexible switches and electrets

Electrical applications can provide new, high value markets, in contrast with the less profitable commodity plastics. The three products described here are related to the phenomena discussed earlier in the chapter. The low cost processing of polymer into thin film has allowed the radical redesign of many such electrical devices. 

Four layer polycarbonate film switch for a keyboard (from Modern Plastics International, McGraw-Hill, 1983). 

If sound vibrations move an FEP diaphragm relative to another electrode, it acts as a microphone. In a telephone handset (Fig. 12.18), the metallised surface of an electret is exposed to the sound, while the other electrode is a metallised ABS moulding containing holes. Such microphones are insensitive to mechanical shocks and electromagnetic radiation, and are cheaper than condenser microphones. 

Quartz and piezoelectric ceramic crystals have more temperature independent constants than PVDF, so they are used for force and acceleration transducers. However, PVDF films can be used for large area flexible transducers. Their sensitivity to stress or strain allows the construction of pressure sensors (using the J33 coefficient), and accelerometers by mounting a seismic mass on the film. PVDF electrets are particularly suited for large area hydrophones (Fig. 12.21) that detect underwater signals. Their 

Typical piezoelectric constants of a PVDF electret vs. temperature (from Mort, J., Ed., Electronic Properties of Polymers, Wiley, 1982). 

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