484 BELL SYSTEM TECHNICAL JOURNAL 



in which molecules join together to give a larger complex but during the 

 reaction there is a separation of a small amount of water, alcohol or 

 some other substance, so that the final chemical composition is not 

 quite the same as at the start. 



Those materials which are thermoplastic and readily soluble owe 

 these characteristics to the fact that the molecules are linked essentially 

 in chain-like fashion. The forces holding the chains together are of a 

 secondary valence character, that is, the chains are free to move apart 

 when heat is applied or a solvent is present. Vinyl, acryl and styryl 

 resins are typical thermoplastics and incidentally each is formed from 

 a monomeric material containing the characteristic ethylene grouping 

 CH2 = C<. The properties of synthetic materials of the thermo- 

 plastic type vary with the average chain length and distribution, and 

 with the nature of any side groups which may be attached to the main 

 hydrocarbon chains. The materials do not have sharp melting points 

 as do more simple organic substances but soften gradually when they 

 are warmed. Usually on further heating decomposition occurs to 

 the monomeric form before any rapid flow point is reached. 



The molecules of thermosetting plastics are initially in chain-like 

 form also, although the chains are generally much shorter in length 

 than those of the thermoplastics. On heating, the material fuses and 

 the chains become cross-linked sufhciently to give a permanent and 

 rigid three-dimensional structure. In the case of a phenol-for- 

 maldehyde resin the linkage between chains is directly through CH2 

 groups (or according to some investigators through — O — CH2 or 

 — CH2 — O — CH2-groups) and the force necessary to separate the 

 chains is therefore high and of an order equal to that needed to break 

 up any complex organic molecule. Infusibility and inability to go 

 into solution are consequently the prominent characteristics of a 

 phenolic resin in the heat-hardened form. In other materials belong- 

 ing in this class the cross-linkage may be brought about through oxygen 

 or sulphur atoms though the hardening action sometimes occurs 

 more slowly. Certain oxygen convertible alkyd resins for example are 

 particularly useful as organic finishes because they may be greatly 

 hardened and in this case very much toughened by a baking process, 

 the necessary oxygen for cross-linkage of the polymeric resin being 

 absorbed from the surrounding air. 



The properties of the more modern materials including the phenol 

 plastics, the cellulose derivatives and the ethenoid (vinyl, acryl and 

 styryl) type plastics, have yet to be fully evaluated but certain elec- 

 trical and mechanical characteristics of these materials have already 

 resulted in their adoption to a|'greater or lesser extent in the telephone 



