1094 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



The strength may actually increase as shown in Table II. The rubber tends 

 to "shorten" and ultimately ceases to be rubbery. Carried to an extreme 

 condition, oxidized rubbers can resemble hard rubber or the phenolic resins. 

 The detailed mechanisms of cross-linking are not worked out but certain 

 deductions can be made about the reaction. That it is not just a polymeriza- 

 tion of the double bonds can be shown by the fact that its rate in the absence 

 of oxygen is extremely slow. That it is probably induced by free radicals 

 can be inferred from the work on "vulcanization" of unsaturated polyesters 

 with peroxides in which it was evident that a free radical attacking a double 

 bond initiated the cross-linking reaction.^^- ^ The sequence might be as 

 follows:^ 



H H 



P— CH2— C CH2— P -^ P— CH2— C— CH2— P 



OOH O 



• + OH 



H 

 P— CH2— C— CH2— P + P— CH2— C=C— CH2— P -> 

 H H 



H 

 P— CH2— C— CH2— P 



I 

 O 



I H 

 P— CH2— C— C— CH2— P 

 H • 



Thus a link has been introduced. The new radical can react with another 

 radical, it can react with oxygen to form another peroxide group, it caii react 

 with a double bond in another chain to form an additional cross-link, or 

 when antioxidant is present it can react with antioxidant. The hydroxy 

 radical resulting in the original decomposition of the peroxide could initiate 

 a similar series of reactions resulting in one or more cross-links as follows: 



•OH + P— CH2— C=C— CH2— P -^ 

 H H 



H H 



OH OH 



P— CH2— C— C— CH2— P P— CH2— C— C— CH2— P + 

 H • H . 



H 



O H 

 P— CH2— C=C— CH2— P -> P— CH2— C— C— CH2— P 

 H H H 



H 

 P— CH2— C— C— CH2— P 

 H . 



