A DISCUSSION OF PETROLEUM SULFONATE IN LUBRICANTS - PART I: CALCIUM SULFONATES

Petroleum Sulfonates are the ubiquitous lubricant additives that can be used in many  lubricating applications such as engine oils, rust preventative oils, metalworking fluids, greases, and some fields that needs to used as oil-soluble emulsifiers. There are many types of Sulfonate depends on the alkyl groups of Petroleum Sulfonic Acid, counter ions, TBN content, metal contents,… Each type will play several important roles in particular applications., and you probably are involved in those industries. However, a question is that do you really fully understand the Sulfonate additives? If not, please stay beside me to discuss on this topic.

 

I. WHAT ARE PETROLEUM SULFONATES?

 

Sulfonates are salt of petroleum sulfonic acids. The counter ions can be Na, Mg, Ca, Ba or some alkanolamine. These products as far as I know should be diluted in base oil (~ 60%) since its super high viscosity at room temperature. An example of this is the neutralization of 25% n-Dodecyl Benzene Sulfonic Acid (DBSA) and NaOH solution that I made as economic detergent for my family laundry store, showing a viscous liquid just @ 25% Sulfonic Acid without any thickeners (i.e., Hydroxypropyl Methycellulose, sodium sulfate). I never synthesize those Na-neutralized detergents at high percentage of DBSA (~60%) in water since this is exothermic reaction, and the high concentration of DBSA and NaOH could make water boiling. Nevertheless, I strongly believe that the viscosity will be very high at that concentration since the hydration of Sodium Sulfonate increases the molecular volume and the interaction between those molecules.

 

II. CLASSIFICATION OF SULFONATES

 

There are 2 types Sulfonates: one is synthetic and the other is natural.  Normally, synthetic type is more cost-effective than natural one.  Natural Sulfonates are the by-products of acid-treating process of petroleum oil and synthetic sulfonates are manufactured through the sulfonation of long-chained alkyl benzene or dialkylated naphthalene. The carbon chain of alkyl benzene can starting from C12 to C40, while the dialkylated naphthalene possesses shorter chain lengths (C9-C10). The typical naphthalene is Ba dinonyl naphthalene sulfonate (BaDNNS).

 

The (synthetic and natural) sulfonic acids are formed in the reaction of SO3 with a feedstock. Then, it will be neutralized to form salts of Na, Ba, Mg, Ca, Alkanolamine. In addition, the neutral salts can also be over-based by addition of excess base and CO2 depending on the applications.

 

- Na Sulfonate: its MW is from 400 to 700 depending on the lengths of the carbon chain and its chemical structure of sulfonic acid. Na Sulfonates are a common additive in metalworking fluids application (i.e., soluble oils). The chemical structures and the MW are quite important to evaluate the performance of these additives.

 

- Ca Sulfonate: CaO and Ca(OH)2 are used to neutralized the sulfonic acids. These Sulfonic derivations can be classified into 3 types: low base (TBN 20-70), medium base (TBN 200-300) and over base (TBN 400-500). These additives are common used for engine oil, rust preventative oil, grease, oil-soluble emulsifiers for crop care. 

 

- Mg Sulfonates: they are produced by neutralization of sulfonic acis and MgO or Mg(OH)2.  It is common used as additive for engine oil and fuel oil. 

 

- Ba Sulfonates: they are formed when sulfonic acids are neutralized with BaO or Ba(OH)2.  They are available in low-based and over-based types. Rust Preventative are the main application of Ba sulfonates.

 

- Alkanolamine Sulfonates: Neutralization of Sulfonic Acid by alkanolamine will produce alkanolamine sulfonates. They are also used in metalworking industries.

 

III. CALCIUM SULFONATES

 

Ca Sulfonates can be used as detergents in engine oils, film-forming additives for rust preventative, EP additive for neat cutting oil in lubricant industries. Normally, it is prepared by the synthetic sulfonic acids due to its cheaper price. They are available in low-based and over-based types. In the over-based type, calcium sulfonate are existed in micell structure as below. 

Figure 1: Micelle structure of over-based Ca sulfonate

In this structure, the micelle core is Cacium hydroxide -encapsulated Calcite (CaCO3) absorbed by the Ca sulfonate chains. This structure provides the stability of the colloidal system through steric repulsion. Because this is the colloidal system, the Calcite nanoparticales can be deposited at the bottom in some situation. 

Why Ca hydroxide exist in the micelle? === When Ca Sulfonates are synthesized through the neutralization of Ca hydroxide and sulfonic acid and CO2 is blown into the process to create over-based Ca sulfonates, the reaction is usually terminated before all the base is converted into Ca carbonate. This explains why Ca hydroxide and Ca carbonate always appeared in the over-based Ca sulfonates, but the percentage of Ca carbonate is much higher. If CO2 is overblown, the amorphous calcium carbonate will be converted into crystal Ca carbonate.

Which parameters are importants for Ca Sulfonates? To answer this question, please kindly see the Figure 2 for Lanxess products before discussion!

Figure 2: Typical properties of Ca Sulfonates - reference of Lanxess documents

- Chemical structure: this is a very important parameter when the manufacturers choose to design their products. The performance will be different between liner- and branch- chained alkylbenzen sulfonic acid, the mono- and di-alkyl benzene sulfonic acid. In addition, the MW makes a contribution to the performance of Ca Sulfonates as well. The higher the MW, the better the oil solubility in base oil. However, the manufacturers will not show those information on their technical data sheets. At this moment, my knowledge is not that strong to recommend which methods to analyze the chemical structures of sulfonates, so you just need to know the basic chemical structure of the petroleum sulfonic acid an its synthesis process to have an overview on their chemical structures.

- Ca Sulfonate percentage: since the chemical structure is a secret,  we can pay attention to the Ca Sulfonate content to select additives before going to the formulation. This value uses to evaluate the detergency power and rust protection ability of the Ca Sulfonate additive. 

Normally low-based Ca sulfonate shows higher Sulfonate content than over-based ones shown in Figure 2. So, they have better detergency and corrosion protection since it can form a thin film Sulfonates absorbed on the metal surface to prevent them from oxydizing agents (e.g., Acids)

As a result, beside over-based Ca Sulfonates, low-based ones also are used in engine oils to improve detergency beside the dispersants provided by PIBSI and others Phenate and Salicilates. Moreover, low-base Sulfonates are also good for rust preventative oil. Following ASTM B1748, Calcinate NC and C-300R shows outstanding rust protection in humidity cabinet. However, if you just pay attention to rust protection only, I highy recommend Calcinate NC because Calcinate C-300R contains high percentage of Calcite colloids which have potential to deposit at the bottom of the drums during storage condition.

Figure 3: Diameters of particles shown in TEM and DLS 

- Micell particles size: I do not know which methods Lanxess is using to analyze this parameter. However, if this is the result from TEM/SEM, then it is a dry diameter of the particles. Furthermore, TEM/SEM cannot distinguish the clusters (few or many particles gathering together, but they are still dispersable) and single particles unless they use very low concentration. If, the addiitve is diluted with some suitable solvents and run  Dynamic Light Scattering (DLS), then that value is the hydrodynamic diemeters. Noting that: DLS also cannot give you whether the particles in the additive are single or clusters. Alright, please be back to this property. In lubricant industry, soft deposits are quite sentitive. It can stem from many causes, but the particle size of Calcite in Sulfonate is also a reason. In some situation, the colloids can be unstable due to water absorption or contaiminants in blending process or additive storage condition. Therefore, If I work for neat cutting oil, I will prefer Caicinate C-300R, but if I work for just RP oil, off course, Calcinate NC will be my first choice.

- TBN: this value is not strange to those are working in automotive oil. Basic agents in Ca Sulfonate as mentioned before coming from the hydroxide and carbonate Calcium, and it can neutralize the forming acids in the oil to protect the engine from corrosion. Acids sterm from the oxidation of the oil and some Sulfur in gasoline or diesel fuels. 

Other parameters such as Welding Load/Scar Diameter is for MWF, and if I design a formula for neat cutting or forming oil, I will not choose this Sulfonate systems. Off course, some blenders can also apply them for MWF, but in my perspective, esters and other EP addiitve can work better Sulfonates. Because when we use Sulfonates in MWF, we may need to deal with the colloid stability in the complex systems of MWF.

To summary, Lanxess has given some recommendation for us when we would like to use their products in Figure 4. Off course, there will be other Sulfonates manufacturers such as Lubrizol, King Industry,... you can also consider to use from them. Noting that lubricants are a complex systems, any adding new additives can enhance or deteriorate the quality of the system. Therefore, after blending, test results are more reliable than some theoricaly discussion here. 

Figure 4: Application Recommendation for some Lanxess's Sulfonates

Thanks for your reading. If you want to discuss further, please give me a comment or send me a email at ntphuong2153@gmail.com.

Written by Steven Nguyen