In conclusion, the CaCO3 content has significant effect on V, when provided surfactant is available to ensure a good contact between the base and the acid species. In a very high BN sulfonate such as A5, the major part of the surfactant is used to stabilize the nanoparticles of CaCO3 and little "free" soap is available. This limits the speed of the reaction at a relatively low level. In the case of BN300 and BN400 sulfonates, more "free" soap can be available because the amount of CaC03 to stabilize is lower. Data have shown that to obtain the best level of V, an optimized ratio between the CaCO3 content and the "free" soap content probably exists.
4.2.2 - Lime content effect
The overbasing process consists firstly of neutralizing an alkyl benzene sulfonic acid with lime (Ca(OH)2) to obtain a sulfonate. Secondly, an excess of lime is carbonated with carbon dioxide to obtain CaCO3, base species that is stabilized by the sulfonate. During this process, it is possible to control the amount of unreacted non-carbonated lime, which is also stabilized in the oil medium by the sulfonate surfactant. As Ca(OH)2 is also a base species, it potentially reacts when acid is introduced in a lubricant that contains an overbased sulfonate.
In order to see if remaining lime should have an influence on the neutralization speed, samples including various amounts of Ca(OH)2 have bcen compared: A4c, A4a and A4b that respectively contain 0%, 1% and 2.3% of lime. As it can be seen on Fig. 6, the remaining lime has no influence on V. For the three samples, the three parameters, V, d and BNu are in the same range, without very significant variations.
Fig. 6 - Neutralization speed vs lime content
4.2.3 - Metal salt nature and form effect
The overbased sulfonates used in marine lubricants are generally calcium salts, but some available and potentially useful overbased sulfonates are magnesium salts.
The effect of a metal change and the effect of the form of the salt, e,g. crystallized or amorphous, on V have been investigated. The samples A4h and A4i that contain respectively a crystallized complex of magnesium salts and an amorphous form of magnesium salts, have been compared. As it can be seen on Fig.7, the magnesium yields a severe drop of V. This decrease is more important when the salt is crystallized. Thus the base species is less available than when it is calcium salt: the level of BNu confirms this.
The initiation delay remains logically unchanged, as d depends only on the surfactant nature.
Fig. 7 - Neutralization speed vs metal salt
4.2.4 - Surfactant type and allylate ckenastry effects
It is now clear that the most influencing parameter for the neutralization ability, except the base species, is the surfactant.
The comparison between the "classical" BN400 overbased alkyl benzene sulfonate, A4a, and a BN400 detergent made with a fully different raw material, A4f shows that the alkyl benzene chemistry is definitely the most adapted as V drops from 7.59 mbar/s to 3.07 mbar/s.
The alkyl beazene molecules used to make the sulfonate surfactants are typically from two sources. The first one is call "natural" (NAT) because the alkylate materials are by-products of the petroleum refining industry. These alkylates have generally a wide molecular mass distribution. The second one is call "synthetic" because alkylates are made by chemically controlled processes. Synthetic products can be linear monoalkyl benzene (MAB) or dialkyl benzene (DAB) alkylates. They can also be branched alkyl benzene alkylates (BAB). Generally the synthetic alkylates, contrary to natural alkylates, have a well-controlled molecular mass distribution, which depends on the raw materials used and on the process conditions and catalysts.
The surfactant properties of the overbased sulfonates are fully dependent on the chemistry of the detergents, therefore on the type of alkylates. This is why additives of the same BN range produced by different suppliers can have very different properties. For this reason, the effect of the type of alkylates on the neutralization speed has been investigated.
The sample A4a is made with a blend of MAB, DAB and NAT alkylates in which the synthetic products, and more particularly DAB, are predominant.
On Fig. 8, the neutralization speed of A4a is compared to the neutralization speed of :
・A4d in which NAT alkylates are predominant;
・A4e in which BAB alkylate is predominant;
・A4g made only with DAB and MAB, in which DAB represents 80% of the surfactant content.
On Fig 9, A4a is compared to:
・A4l made with a major part of MAB and obtained with a process using AlCl3 catalyst;
・B4 made with the same kind of MAB but obtained with a process using HF catalyst;
・A4j, a sample of A4a in which an additive used to enhanced the carbonation step during the overbasing process has been added;
・A4k, same kind of sample as A4j in which the rate of previously described additive is higher.
