500 BELL SYSTEM TECHNICAL JOURNAL 



The trend in the design of most apparatus has been toward smaller 

 lighter parts and at the same time toward more rapid assembly. 

 Synthetic resin adhesives are aiding this trend by avoiding in various 

 places the dependence upon bolts, screws and similar mechanical 

 locking devices. Proprietary resin-cellulosic lacquer adhesives and 

 vinyl and acrylate polymers are proving of value because they give 

 strong tough joints that are affected but little by moisture and are not 

 apt to give trouble from corrosion or growth of mildew. The use of 

 these materials for assembling parts in a thermoplastic manner looks 

 particularly encouraging. When the surfaces which are to be joined 

 are carefully cleaned, then primed with an air-dried coat of a suitable 

 thermoplastic resinous adhesive and finally molded together under 

 heat and pressure, tensile strengths of several tons per square inch 

 are possible between the joined parts. Synthetic resin cements and 

 adhesives are employed in the construction of the handset transmitter, 

 moving coil microphones, loud speakers, switchboard lamps, vacuum 

 tubes and the wood veneer of telephone booths. 



Conclusion 



Many important applications of plastics have been made in the 

 telephone field. These have sprung from the economies of design, 

 methods of fabrication, as well as from the excellent serviceability of 

 the molded plastic products. It might be well to emphasize again the 

 chief limitations of present day plastics which have prevented wider 

 use. When exposed to outdoor conditions which involve the effect of 

 temperature and sunlight, many plastics, particularly the newer 

 thermoplastic materials, are revealed to be insufficiently permanent 

 for telephone use with respect to the physical and chemical character- 

 istics associated with color, distortion at elevated temperatures, 

 surface deterioration due to action of sunlight and brittleness at 

 moderately low temperatures. 



Modern trends in stylized designs make it necessary to take ad- 

 vantage of the molding art in order to achieve good ornamentation. 

 This will probably result in the use of plastics on surfaces exposed to 

 light. The effect of light is not confined to direct sunlight for it has 

 been found that daylight filtered by ordinary window glass for long 

 periods will cause reduction in surface electrical resistance of plastics. 

 In fact, the effect of light seems to be over a rather broad range of the 

 spectrum, becoming intensified as the ultra-violet range is approached. 



