1. Lubricant Additive Detergent Types
Detergents are the metal salts of organic acids. The acids normally used to synthesize these compounds include arylsulfonic acids such as alkylbenzenesulfonic acids and alkylnaphthalenesulfonic acids; alkylphenols; carboxylic acids such as fatty carboxylic acids, naphthenic acids, and petroleum oxidates; and alkenylphosphonic and alkenylthiophosphonic acids. Sometimes, a mixture of different types of acids is also employed. The reaction of these acids with inorganic bases, such as metal oxides, metal hydroxides, and metal carbonates, results in the formation of salts.
The quantity of the metal used may be equal to (stoichiometric amount) or in excess of the exact amount necessary to completely neutralize the acid functionality. The presence of metal in stoichiometric amount results in the formation of the neutral salt, often referred to as a neutral detergent or soap. If the metal is present in excess, the detergents are called basic, overbased, or superbased. It is important to note that basic detergents appear as clear homogeneous fluids, the same as neutral detergents, because the excess metal is present in a colloidal form. The general formulas for metal sulfonates, metal phenates, and metal carboxylates are presented in Figure.
The excess base in basic detergents may be present as metal hydroxide, metal carbonate, or both. For neutral detergents, x and y in the formulas in Figure are zero. For low overbased detergents, such as those with a base number of about 50 or less, x may be zero and y may be a low number, or both x and y may be low numbers. This implies that slightly overbased detergents are either carbonate-free or contain a mixture of both the hydroxide and the carbonate. Highly overbased detergents invariably have a large amount of carbonate as the reserve base. That is, in their case, y is low and x is very high. In some cases, x can be as high as 20, or more. In summary, the excess base per equivalent of acid in metal hydroxide–containing detergents is generally lower than that in metal carbonate–containing detergents.
2. Lubricant Additive Detergent Substrates
Various organic acids are used to synthesize detergents. These include alkylaromatic sulfonic acids, alkylphenols, alkylsalicylic acids, fatty carboxylic acids, and alkenylphosphonic acids. Alkylaromatic sulfonic acids, such as alkylbenzenesulfonic acids and alkylnaphthalenesulfonic acids, are made by reacting the respective alkylbenzenes and alkylnaphthalenes with a sulfonating agent. The alkylaromatic starting materials are made through alkylation of aromatics—benzene and naphthalene. To make synthetic sulfonates, benzene or naphthalene is frst alkylated and then sulfonated. The alkylating agent is either an alkyl halide or an olefn. The olefns can be α-olefns, internal olefns, or olefn oligomers such as polypropylene and polyisobutylene.
An acid catalyst is usually required. One may choose from many acids. These include mineral acids such as sulfuric acid and phosphoric acid, Lewis acids such as aluminum chloride and boron trifluoride, organic acids such as methanesulfonic acid, and mixtures thereof . Some of the inorganic acids such as sulfuric acid are also available on a solid support, such as Fuller’s earth or silica. Unlike other catalysts that require neutralization at the end of the reaction, these catalysts just require fltration to remove them. Zeolites and related mixed-metal oxides also enjoy the same advantage as the solid alkylation catalysts. Another class of catalysts, exemplifed by Amberlysts®, is aromatic polymer-derived sulfonic acids. Although they have the advantages of being insoluble, hence easier to remove, and of multiple use, they have the disadvantage of being expensive. It is important to note that not all catalysts have equal effectiveness in all alkylations.
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