126 Lubricant Additives: Chemistry and Applications
(ash) on combustion, a formulator must know the metal content of a formulation to offset any prob-
lem that might occur. This is because the lubricant travels past piston rings into areas that experi-
ence fl ame and high temperatures, such as the top land and the groove behind the top ring; it burns
to produce ash. Ash is undesired because it is believed to initiate deposit formation. Sulfated ash is
one of the methods used to assess the metal content of a lubricant, and the methods to determine
this are described in the ASTM Standards D 482 and D 874 [27].
The degree of overbasing is the number of equivalents of the metal base per equivalent of the
acid substrate. This is usually expressed as conversion, which indicates the amount of inorganic
material relative to that of organic material. Conversion is expressed as the number of equivalents of
base per equivalent of acid times 100 [7]. The soap content is the amount of neutral salt as a percent
of detergent composition.
The TBN of the detergent refl ects its ability to neutralize acids. For basic sulfonate and phos-
phonate detergents, only the overbased portion of the detergent, that is, the carbonate and the
hydroxide (see Figure 4.2), possesses this capability. The neutral metal sulfonates and phospho-
nates, that is, soaps, lack this ability. However, for basic carboxylates, salicylates, and phenates,
soaps also possess the acid-neutralizing ability. This is because, unlike sulfonates and phospho-
nates that are strong acid–strong base salts, metal carboxylates, metal salicylates, and metal phe-
nates are strong base–weak acid salts. This makes them Lewis bases, hence the acid-neutralizing
ability.
Let us try to calculate the detergent parameters for a detergent of a hypothetical molecular
formula (RSO 3 )vCaw(CO 3 )x(OH)y.* I n t h is for mu la , v, w, x, and y denote the number of sulfonate
groups, the number of calcium atoms, the number of carbonate groups, and the number of hydroxyl
groups, respectively. The metal ratio, the total equivalents of metal per equivalent of acid, for such
a detergent equals to 2w/v. The coeffi cient 2 signifi es the divalent nature of calcium. For metals
such as sodium and potassium, which are monovalent, the ratio equals to w/v. The degree of over-
basing or conversion, which is metal ratio times 100, is (w × 100)/v for monovalent metals and
(2w × 100)/v for divalent metals. Neutral detergents, or soaps, have a conversion of 100 because the
ratio of equivalents of base to the equivalents of acid is 1. Soap content for such a detergent can be
calculated using the following equation:
Percent soapformula weight [(RSO Ca] 100
effective form^3 )^2
uula weight (4.1)The effective formula weight is the weight of all the atoms that make up the formula (RSO 3 )v
Caw(CO 3 )x(OH)y plus the diluent, if present. The diluent can be the incidental alkylate that does not
get sulfonated or the diluent oil that is intentionally added. If one must add oil, most of it is added
to reactants at the beginning of the reaction, especially during the manufacture of basic detergents.
The presence of diluent is believed to facilitate micelle formation, thereby making the process more
effi cient. Adding oil after the reaction is not as effective.
The TBN indicates a detergent’s ability to neutralize acids. In additives and formulated lubri-
cants, the TBN is expressed as mg KOH/g of additive [27]. The method to determine base numbers
is described in the ASTM Standard D 974 [28]. For sulfonate and phosphonate detergents, it can be
calculated by using the number of equivalents of excess metal after salting the acid, that is (2w – v),
according to Equation 4.2.
TBN (mg KOH/g)
effective formula weight
()2wv , 56 100
(4.2)- The correct formula for such a detergent is (RSO 3 )Ca.xCaCO 3 .yCa(OH) 2.