1082 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



theraioset materials, of which the phenolic resins are typical examples, are 

 so highly interconnected that the molecular weight can be considered to be 

 infinite. Each molding, for example, may consist of a single molecule. Be- 

 cause of their extensive internal cross-bracing their deterioration is usually a 

 surface phenomenon.^ It will be discussed later in this memorandum. The 

 paragraphs which follow immediately will refer to Hnear polymers. 



Any material chosen for an engineering apphcation obviously must pos- 

 sess desirable characteristics and "corrosion" or deterioration changes these 

 characteristics in some undesirable way. There are three ways in which a 

 system of chain-like molecules can change: 1) the chains may be cut into 

 smaller pieces, 2) the chains may be tied together by cross-links, and 3) 

 the nature of any side groups along the chain may be modified. All of these 

 changes have been found to occur during normal weathering of polymers 

 and the properties of the product are determined by the extent of each 

 change.® 



The first type, chain scission, is usually the most serious because it cuts 

 at the very essence of polymeric nature which is high molecular weight. As 

 molecular weight is lowered, strength is lowered and ultimately is lost com- 

 pletely. To continue the analogy to a cotton thread, the individual fibers 

 become so short that they cease to overlap each other adequately. Tough 

 horny polyethylene, for example, deteriorates to something akin to paraffin 

 wax. If chain scission occurs extensively in rubbers, portions of chains are 

 cut loose from the relatively few cross-links and the product will appear to 

 have become unvulcanized. This phenomenon is well known with natural 

 rubber and is called "reversion".'^ (Fig. 7) 



The second type of change caused by aging, the introduction of ties or 

 cross-Hnks, is not usually of great importance in plastics unless carried to an 

 extreme when the rigidity and brittleness of thermoset polymers might 

 result. As a matter of fact, the introduction of a few cross-links in a thermo- 

 plastic, without accompanying chain scission, probably serves to toughen 

 the material. In rubbers, however, where high elongation is a desired prop- 

 erty and is derived from the uncoiling of the molecules under stress, in- 

 troduction of cross-links beyond those necessary for vulcanization tends to 

 ''shorten" the material and can eventually stiffen it to the point that it 

 loses serviceabiHty. The introduction of cross-hnks increases the density, 

 and frequently when the surface of a plastic or rubber has been cross-linked 

 extensively it develops an "alligator" or "mud crack" pattern resulting 

 from excessive shrinkage. 



The third type of change, the modification of side groups, normally has 

 little effect on the strength of a polymer, but may have a pronounced effect 

 on the dielectric properties, solubility, moisture absorption, etc., depending 



