Lubricant Additives

Antioxidants 25

each other into a chain termination step. As depicted in Figure 1.10, a hydrogen atom is donated
from one phenoxy radical to the other, thus regenerating one BHT molecule and one methylene
cyclohexadienone.
Under higher-temperature oxidation conditions, the cyclohexadienone alkyl peroxide previ-
ously formed is no longer stable. As illustrated in Figure 1.11, it will decompose to form an alkoxy
radical, an alkyl radical, and a 2,6-di-t-butyl-1,4-benzoquinone. As can be expected, the generation
of new radicals will deteriorate the overall effectiveness of the BHT under high-temperature oxida-
tion conditions.

R• +O 2 k 2 >> k 1

R• +

OH O•

k 1

k (^2) ROO•

• RH
  FIGURE 1.8 Reactivity of BHT with alkyl radical.
  OH
  CH 3
  O•
  CH 3


• ROOH


• ROO•


• ROO•
  O
  H 3 C OOR
  O
  H 3 C •
  FIGURE 1.9 Hydrogen donation and peroxy radical trapping mechanisms of BHT.


• 
  

  O•
  CH 3
  OH
  CH 3

  
  


• 
  

  O
  CH 3 CH 2
  O•
  FIGURE 1.10 Termination reaction of phenoxy radicals.