Ashless Antiwear and Extreme-Pressure Additives 229
The most common additive, methylene bis-dibutyl dithiocarbamate, is prepared from sodium dibutyl
dithiocarbamate and methylene chloride.
2(C 4 H 9 ) 2 NC(=S)S–Na + CH 2 Cl 2 ⇒ [(C 4 H 9 ) 2 NC(=S)S] 2 CH 2 + 2NaCl (8.29)
8.2.4.1.2 Applications and Performance Characteristics
Un l i ke met a l l ic d it h ioca rba mates t hat have be en widely use d i n lubr ica nt s, ash less d it h ioca rba mates
have only been gaining more attention recently. Relatively high cost is certainly a major factor
in limiting wider use. The success of metallic dithiocarbamates also overshadows their ashless
counterpart. Certain metallic dithiocarbamates, such as molybdenum dithiocarbamates, offer
exceptionally good frictional properties that cannot be matched by their ashless analogues also.
However, ashless dithiocarbamates have been found to be versatile, multifunctional additives in
a few areas. They can be effective antiwear/EP additives as well as good antioxidants and metal
deactivators [52–55], (Structures Ga and Gb). They tend to generate less sludge or deposits than
mostly metallic additives and they are very compatible with various base oils.
N SS
R′RS NS
R′R
OHSTRUCTURE Ga
S NS
R′R
OHR′′ SSTRUCTURE Gb
8.2.4.2 Dimercaptothiadiazole and Mercaptobenzothiazole Additives
Additives derived from DMTD and 2-mercaptobenzo-thiazole (MBT) are well documented in the
literature. Owing to strong ring stability (partial aromaticity and resonance delocalization), balanced
sulfur–nitrogen distributions, and reactive mercaptan groups, both heterocyclic compounds can be
versatile core molecules to make many useful additives with many benefi cial characteristics, such as
improved thermal/oxidative stability and reduced corrosivity. Unfortunately, some potentially good
reactions are hampered by the limited solubility of DMTD and MBT in common petrochemical sol-
vents. Therefore, a suitable sample preparation procedure is very critical to help achieve desirable
antiwear/EP additives.
8.2.4.2.1 Chemistry and Manufacture
Many differing organic reactions can be applied to functionalize the mercaptan groups of DMTD
and MBT. Oxidative coupling reactions involving other alkyl mercaptans can bring in additional sul-
fur for EP performance and additional alkyl chains for improved solubility [56]. (Addition reactions
with organic compounds containing activated double bonds can link DMTD or MBT heterocyclic
core molecules with long chain esters, ketones, ethers, amides, and acids together [57–60]). Like-
wise, ring opening with epoxides to generate alcohol derivatives is also known [61]. Direct amine
salts formation and linking alkyl amines through Mannich base condensation are also extensively
studied [62–64]. A number of examples are listed in reactions 8.30 through 8.33, where TD is the
abbreviation for the thiadiazole moiety and BT is for the benzothiazole moiety.