Lubricant Additives

Antioxidants 11

To make ADPAs, diphenylamine is reacted with an appropriate alkylating agent such as alcohol,
alkyl halide, aliphatic carbonyl compound, or an olefi n. The olefi ns are preferred for economic rea-
son. The most commonly used are isobutylene (C 4 ), diisobutylene (C 8 ), nonenes (C 9 ), styrene, and
propylene tetramer (C 12 ). Depending on the acidic catalyst, olefi n, and other reaction conditions, for
instance, the temperature, the degree of alkylation will vary from mono- to di-alkylation.
Mono-ADPA is generally more effective than the corresponding disubstituted on a weight
basis because additional alkylation substantially reduces the number of moles of diphenylamine
per weight unit. However, in practice, obtaining monosubstituted diphenylamine in relatively
pure format is diffi cult because as soon as the diphenylamine is monoalkylated, it quickly pro-
ceeds to dialkylation. Attempt in the preparation of high content of mono-ADPAs has led to
the use of novel clay catalyst with greater selectivity in alkylation reactions and C 6 –C 18 linear
olefi ns to produce high levels (at least 50 wt%) of mono-ADPAs with lower levels of dialkyl
diphenylamines and undesirable unsubstituted diphenylamine [110]. Alkyl groups of six or more
carbon of mono-ADPA tend to render the material lower yellow color and higher resistance to
d iscolor at ion [111].
It was found that monosubstituted diphenylamines more readily oligomerize under various
conditions to produce higher-molecular weight, linear oligomers. Oligomers with 2–10 degrees of
polymerization are desirable antioxidants especially for high-temperature applications. Disubsti-
tuted and polysubstituted diphenylamines, however, are more restricted from forming oligomers
higher than dimers. Oligomeric versions of monosubstituted diphenylamine prepared from reacting
diphenylmine with C 4 –C 16 olefi ns have been described for use in ester lubricants [112]. The prod-
ucts are claimed to be more effective than simple diphenylamines for extremely high-temperature
applications. Homo-oligomers of alkylated (C 4 –C 8 ) diphenylamines, styryenated diphenylamines,

FIGURE 1.4 Synthesis routes of ADPA antioxidants.

N

H

N

H C

C 8 , St 8 , St

N

H C

C 4 , C 8 4 , C 8

N

H

N

H C

C (^99)
N
H C
C 8 8
C 4 + C 8 olefin
C 8 olefin + styrene
N
H
C 9 olefin
C 8 olefin

• HNO 3 NO 2 + H 2 Catalyst NH 2 N
  H
  Benzene Nitrobenzene Aniline Diphenylamine
  Catalyst