Lubricant Additives

24 Lubricant Additives: Chemistry and Applications

TABLE 1.4

Metal Deactivators for Lubricants

Surface Passivators Basic Structure

Triazole derivative

N

N

N

CH 2 NR 2

Benzotriazole

N

N

N

H

2-Mercaptobenzothiazole

N

S

SH

Tolyltriazole derivative N

N

N

R 1

R 3

R 4

CH 2 N

Chelators

N,N′-disalicylidene-1,2-diaminopropane

OH HO

C

H

CH 2 CH

CH 3

N

H

NC

1.10.6 MECHANISMS OF PRIMARY ANTIOXIDANTS

1.10.6.1 Hindered Phenolics

A representative example of HP antioxidant is 3,5-di-t-butyl-4-hydroxytoluene (2,6-di-t-butyl-4-
methylphenol), also known as BHT. Figure 1.8 compares the reaction of an alkyl radical with BHT
versus oxygen. The reaction rate constant (k 2 ) of alkyl radical with oxygen to form alkyl peroxy
radicals is much greater than that (k 1 ) of alkyl radical with BHT [179]. Hence with an ample supply
of oxygen, the probability of BHT to react with alkyl radicals is low. As oxidation proceeds with
more alkyl radicals being converted to alkyl peroxy radicals, BHT starts to react by donating a
hydrogen atom to the peroxy radical as shown in Figure 1.9. In this reaction, the peroxy radical is
reduced to hydroperoxide, whereas the BHT is converted into a phenoxy radical that is stabilized
through steric hindrance and resonance structures. The steric hindrance provided by the two butyl
moieties on the ortho positions effectively prevent the phenoxy radical from attacking other hydro-
carbons. The cyclohexadienone radical resonance structure can further combine with a second alkyl
peroxy radical to form the alkyl peroxide, which is stable at temperatures <120°C [200]. Without
resonance transformation, an alternative reaction pathway for phenoxy radicals is to combine with