Esters in oil

Both car owners and many workers in the field of car service have heard about ester oils, but rarely anyone can correctly explain what they are. Moreover, some firmly believe in the usefulness and irreplaceability of ester-based oils, while others consider it another marketing ploy. Let's try to figure it out.

In short, esters are esters - products of neutralization of carboxylic acids with alcohols. However, it would be wrong to start the article in such a "reasonable" way. To begin with, it is necessary to remember what the base oils are for the production of motor and transmission oils.

Base oils, or in other words - bases of motor or transmission oil, are produced:

• by oil distillation;
• by synthesis from gas or organic acids.

The first are traditionally called mineral, and the second - synthetic base oils. 

According to the classification of the American Petroleum Institute (API), base oils are divided into five categories:

Group I - base oils obtained by the method of selective purification and deparaffinization with solvents (ordinary mineral). Typical characteristics: viscosity index: 80-100; flash point: 190-205°С.

Group II - highly refined base oils, with a low content of aromatic compounds and paraffins, with increased oxidative stability (hydrotreated oils - improved mineral oils). Typical parameters: viscosity index: 115-125, flash point - 205-215°С.

Group III – base oils with a high viscosity index, obtained by the method of catalytic hydrocracking (HC technology). In the course of special processing, the molecular structure of the oil is improved, bringing its properties closer to group III base oils to group IV synthetic base oils. It is no accident that the oils of this group are classified as semi-synthetic (and some companies even as synthetic base oils). Typical parameters: viscosity index: 125-160, flash point - 210-225°С.

Group IV – synthetic base oils based on polyalphaolefins (PAO). Polyalphaolefins obtained as a result of a chemical process have the characteristics of a uniform composition, very high oxidative stability, high viscosity index and do not have paraffin molecules in their composition. Typical parameters: viscosity index: 140, flash point - 250°С.

Group V – other base oils not included in the previous groups. This group includes other synthetic base oils and vegetable base oils. Typical parameters: viscosity index: 180-200; flash point: 250-330°C.

The chemical composition of mineral bases depends on the quality of the oil, the boiling points of the selected oil fractions, as well as the methods and degree of their purification. Mineral base is the cheapest. It is a product of direct distillation of oil, consisting of molecules of different lengths and different structures. Due to this heterogeneity - instability of viscosity-temperature properties, high evaporation, low resistance to oxidation. Mineral base is the most common motor oil in the world.

1. Raw oil products distilled in a special vacuum distillation column are divided into distillates and vacuum gas oils.

2. Molecules of waxy vacuum gas oils enter the hydrocracking unit for hydrogenation.

3. At extremely high values of pressure and temperature (300 kPa, 540°C), oil molecules become chemically active. 

4. Hydrogen is added to the molecules to completely eliminate impurities. A rearrangement of molecules takes place, resulting in an exceptional base component for finished lubricants.

5. The converted component is clearly lighter in color because it is cleaner.

6. Unwanted paraffinic hydrocarbons are molecularly rearranged, which gives the base oil resistance to gel formation and excellent pumping even at extremely low temperatures.

7. Hydrogen is used again to remove the remaining aromatic hydrocarbons and light impurities. It also helps to stabilize the molecular structure of the newly formed base component to provide increased oxidation resistance and longer grease life.

Improvement of mineral base oils is carried out in two main directions. The first, in which the oil is purified only to such an extent that it remains the optimal content of resins, acids, sulfur compounds, nitrogen and, additionally, additives are introduced to improve some functional properties. This method does not allow obtaining oils of a sufficiently high level of quality. The second direction, in which the base oil is completely cleaned of all impurities and molecular modification is carried out by the method of hydrocracking. The result is an oil that has valuable properties for heavy duty operation (high resistance to shear deformations at high speeds, loads and temperatures, high viscosity index and stability of parameters).

To what class should these oils be classified? In terms of price, "hydrocracking" is closer to "mineral", and in terms of quality, as the seller assures, it is no worse than "synthetics". But we understand that if this were the case, such an expensive pleasure as synthetic oil would die out as a class... Hydrorecking oil is closer to mineral oil not only in terms of price, but also in terms of production, because it is also made from petroleum. What makes it better then? As the name implies, it undergoes deeper processing using hydrocracking. And in the first stages, its production is no different from the production of mineral oil. Unwanted impurities such as sulfur or nitrogen compounds, asphaltene (bituminous) substances and aromatic polycyclic compounds, which increase coking and viscosity dependence on temperature, are removed from ordinary mineral oil by various physicochemical methods. Deparaffinization removes paraffins, which increase the solidification temperature of oils. However, it is clear that it is impossible to remove all unnecessary impurities with this method - roughly speaking, this is the reason for the worst properties of "mineral". Oil processing can continue in the future. After all, there are still unsaturated hydrocarbons, which accelerate the aging of oil due to oxidation, and impurities are also left. Hydrotreating (exposure to hydrogen at high temperature and pressure) converts unsaturated and aromatic hydrocarbons into marginal hydrocarbons, which increases the oil's resistance to oxidation. Thus, hydrotreated oil has an added advantage.

And hydrocracking is an even deeper type of processing, when several reactions take place at the same time. Whose? The same hated sulfur and nitrogenous compounds are removed, long chains are broken (cracking, literally translated as breaking) shorter ones with a uniform structure, places of breaks in new shortened molecules are saturated with hydrogen (hydrogenation). Hence the name "hydrocracking". Thus, during hydrocracking, all the signs of synthesis are obvious - the creation of a new compound with a new structure and properties from the raw material. Therefore, hydrocracking is often called HC synthesis.

Transportation of the hydrocracking reactor to the Achinsk Refinery (Russia). The weight of the installation is 1,300 tons, plus the road train, which is intended for its transportation, is another one and a half thousand tons. This is, in general, the weight of a good five-story building.

But not everything is so simple. Some components of oil, which are generally considered harmful, can be valuable in places. For example, resins, fatty and naphthenic acids improve the stickiness and stability of the adsorption film of the oil and thereby improve the lubricating ability of the oil. Some compounds of sulfur and nitrogen have antioxidant properties. Thus, with deep cleaning of the oil, some of its lubricating, antioxidant and anti-corrosion properties may deteriorate. This nuisance is corrected by special additives, which are already added at oil blending plants.

So, hydrocracked oils are products of distillation and deep refining of oil. Hydroracking rejects everything "unnecessary", and if something "useful" is captured, the necessary properties are provided with the help of additives. But it is difficult to clearly filter out unnecessary impurities, so there is more soot formation and "facilitation" of corrosion in hydrocracked oils compared to "synthetics". Hydrorecking oil is close in quality to "synthetics", but it ages faster and loses its properties. But it has a high viscosity index, antioxidant resistance and resistance to shear deformations, and can often protect against wear even better than synthetic. On the other hand, "synthetics" are more homogeneous in the sense of linearity of hydrocarbon chains, which gives advantages, for example, in freezing temperatures.

There is another aspect. Hydrocracking is a catalytic process, just like synthesis. But if the first goes, for example, on nickel, then the second - on carbon. It is clear that carbon is better in this sense, so the oil will be free of unwanted impurities of catalyst compounds.

The most interesting thing is that the vast majority of motor oils positioned as semi-synthetic, and even fully synthetic, are nothing but hydrocracked oils. This is the general trend of the largest olive producers. The BP program (except Visco 7000), Shell (except 0W-40), most Castrol, Mobil, Esso, Chevron, Fuchs oils are built on hydrocracking. Many very well-known brands with a full range of oils that only use hydrocracking.

Semi-synthetics is a mixture of mineral and synthetic base oils and can contain from 20 to 40 percent "synthetics". There are no special requirements for manufacturers of semi-synthetic lubricants regarding the amount of synthetic base oil (synthetic component) in the finished motor oil. There are also no prescriptions for which synthetic component (group III or group IV base oil) to use for the manufacture of semi-synthetic lubricant. According to their characteristics, these oils occupy an intermediate position between mineral and synthetic oils, that is, their properties are better than ordinary mineral oils, but worse than synthetic oils. In terms of price, these oils are much cheaper than synthetic oils.

Synthetic oils have exceptionally good viscosity-temperature characteristics. First of all, it has a much lower solidification temperature (-50 ° C, -60 ° C) and a very high viscosity index, which makes it easier to start the engine in frosty weather. Secondly, they have a higher viscosity at operating temperatures above 100 ° C - thanks to this, the oil film that separates the friction surfaces does not break down in extreme thermal conditions. Other advantages of synthetic oils include increased resistance to shear deformations (due to the homogeneity of the structure), high thermo-oxidative stability, i.e. low tendency to form deposits and varnishes (varnishes are transparent, very strong, practically insoluble films consisting of oxidation products, deposited on hot surfaces), as well as small evaporation and carbon consumption compared to mineral oils. It is also important that synthetics require the introduction of a minimum amount of thickening additives, and especially high-class varieties do not require such additives at all, therefore, these oils are very stable - because it is the additives that are destroyed first of all. All these properties of synthetic oils help to reduce the overall mechanical losses in the engine and reduce the wear of parts. In addition, their resource exceeds the mineral resource by 5 or more times. The main factor limiting the use of synthetic oils is their high cost. They are 3-5 times more expensive than mineral ones.

All additives are solutions of metals (calcium, zinc, etc.) diluted in a mineral base. Additives are ALWAYS diluted in mineral base oil because it combines best with all types of additives. The number of additives in engine oil varies, depending on the purpose of the oil, from 20 to 45%. Thus, absolutely all motor oils, even "fully synthetic" are actually mixed!

Polyalphaolefins (PAT) or esters or their mixture usually act as a synthetic base.

PATs are hydrocarbons with a chain length of about 10...12 atoms. It is obtained by polymerization (in simple words - compounds) of short hydrocarbon chains - monomers of 3...5 atoms. The raw materials for this are usually petroleum gases - butylene and ethylene.

Esters are complex esters - products of neutralization of carboxylic acids with alcohols. Raw materials for production are vegetable oils, usually rapeseed or coconut. Esters have a number of advantages over all other known bases. Firstly, ester molecules are polar, that is, the electric charge is distributed among them, so the molecule itself "sticks" to the metal. Secondly, the viscosity of esters can be set at the base production stage: the heavier alcohols are used, the higher the viscosity. You can do without any thickening additives that "burn out" during operation in the engine and lead to "aging" of the oil. Modern technology makes it possible to create fully biodegradable oils based on esters, as esters are environmentally friendly products and are easily disposed of. However, all these advantages may seem like too expensive a pleasure. The ester base is 5...10 times more expensive than the mineral base!

So, let's describe in more detail what the use of ester oils means in practice?

With what do we start the daily operation of the car? Of course, from starting the engine. It is at this moment that many "sores" appear. For example, a dead battery, frozen sensors, etc. But all these are visible problems. And there are problems hidden from our eyes and our senses. The main one is oil starvation during a cold engine start. It consists in the fact that when the oil is at rest, it flows into the crankcase, and when the engine is started, it runs without lubrication for the first few seconds. And only when the oil is distributed throughout the system, the dry friction of metal against metal stops. Accordingly, during each start-up, the engine parts receive significant wear in friction pairs, which significantly reduces the engine life of the unit.

Compared to polyalphaolefins, which are simple hydrocarbon chains, ether molecules are polar - the electron density is shifted to the oxygen atom of the carbonyl group. Hence, the most important advantage of ether-based oils: a negatively ionized oxygen atom will definitely be attracted to the metal surface of lubricated parts, since the crystal lattice of any metal or alloy consists only of positively ionized and neutral atoms.

The cost of an ether base is 5-10 times higher than a mineral base, because its production includes several stages. The raw material is usually copra oil from coconuts or rapeseed, which is subjected to hydrolysis, separating glycerin and obtaining the necessary carboxylic acids.

The final stage is esterification, that is, the interaction of acid with alcohol. Of course, not guilty, whose molecule has only two carbon atoms, but heavier, where there are from 4 to 22, because the larger the R1 radical in the ether molecule, the higher its viscosity. By the way, this is the second main advantage of esters over polyalphaolefins: here you can easily change the viscosity of the final product by using one or another alcohol.

You can additionally vary the properties of the oil by changing the acid radical R, which increases the cost even more - then carboxylic acids have to be synthesized. That is why vegetable oils are used more often, the benefit of coconut trees on Earth is enough for motor oils.

With certain combinations of radicals, environmentally friendly biodegradable esters are obtained: they are 15 times more expensive than "mineral oil". The oil made on their basis, after getting into the soil, is decomposed by bacteria to 85% in 21 days, although 66 percent decomposition is enough to obtain an ecological certificate.

The second important property is the stability of ester oils at different temperatures, which allows you to protect the engine in all temperature ranges. But what does viscosity in any other oil depend on?

Special thickeners are included in engine oil additive packages to maintain viscosity. They are spiral-shaped molecules, which act exactly according to the principle of a spiral spring. When the oil is exposed to high temperatures, the spiral expands, but to a certain level that keeps the viscosity of the oil within acceptable limits. Under the influence of low temperatures, the thickener molecules do not allow the oil to thicken much, acting in the opposite direction. This technology works perfectly as long as the thickeners are not triggered by mechanical effects on them. After that, the stability of the oil's viscosity will depend solely on its base. In addition, it should be remembered that the more additives in the oil, the more slag formation, which is always bad for the engine.

The highest viscosity index of ester oils is directly related to the alcohol component of esters – its density affects the viscosity of the final product. Thus, using more or less dense alcohols in the production of the ester base, the developers, as we have already said, first set the oil viscosity parameters. And unreliable thickeners are no longer needed. This means that ester-based oils do not depend on the presence of thickeners, and their viscosity will be stable from the beginning to the end of operation.

The ester base also has a high flash point, which dramatically reduces the consumption of oil per soot. Its indicators of high-temperature shift of the oil film significantly exceed the indicators of any traditional oils, including those created with PAT synthetic bases in mind.

One of the most important requirements when operating units that require lubrication is the strength of the oil film. Protection of friction pairs from wear depends on how strong it is. To do this, we will give the figures of the maximum load that oil films can withstand (with vertical impacts):

• mineral base - 900 kg/cm2;
• synthetics (PJSC) - 6500 kg/cm2;
• synthetics (esters) - 22,000 kg/cm2.

It is clearly visible that the oil film of the ester base is about three times stronger compared to the synthetic PAO base. That is why oils based on esters are so popular in professional motor sports - they are ideal at peak engine loads!

And, in addition to all that has been said, ester oils showed the best resistance to oxidation, which is inevitable with the use of low-quality fuel (that is, almost any fuel from gas stations in Ukraine).

In summary, we can say that ester oils are really very different from their "brothers". Let's briefly list their main properties and positive "consequences":

1. The effect of sticking to metal - safe engine start.

2. Constant oil viscosity – constant oil pressure and engine protection.

3. The strongest oil film - increased power and protection against wear.

4. The highest flash point is a decrease in oil consumption.

5. The best resistance to oxidation - preservation of the main properties of the oil during the entire service interval.

Well, we will once again remind you about the shortcomings, one of which has turned into a myth. Namely, the loss of basic properties and fluidity when ester compounds interact with water. At the same time, the ester oil turns into jelly. Shocking pictures and warnings appear in the forums, and scare motorists with "insidious" ester oils, which fail when a drop of water hits them. But we will disappoint fans of the "detective genre". In order to bring the ester oil to this state, you will need a volume of water equal to the volume of the oil. It seems absurd that such an amount of moisture would accidentally enter the system. And a small amount of condensate that can form in the system due to temperature changes is completely harmless. It quickly evaporates when the working temperature of the oil is reached and is removed through the crankcase ventilation system.

And, as mentioned, the main disadvantage of the ester base is its cost. 100% ester base is theory, not practice. But even a small content of this component endows the oil with all the properties that are manifested to one degree or another, which we talked about above. The content of esters in motor oils is usually limited to a few percent (rarely more than 10%), and they are used only in the most advanced products, which usually represent the top of the product line of leading companies.

Take note! The word "ester" in chemistry is as broad a concept as "alcohol". And alcohol can be both edible ethyl alcohol and poisonous methyl alcohol (aka wood alcohol). A typical ester in an oil is nothing more than a carboxylic acid base with some hydrocarbon group. Children's favorite toy is nitroglycerin, also an ester, but based on nitrogen, not carbon. In general, you can Google the name of most well-known brands of oils along with the word ester and make sure that esters are used by almost all of them. The only thing that can profitably distinguish an expensive racing oil from a cheaper ordinary one is which esters are mixed there, because their lubricating and film-forming properties can differ significantly.

Let's give examples. A prominent representative of the producers of ester oils is the Belgian company XENUM. Five of the company's products: Xenum WRX 7.5W40, VX 5W30 and the X1 Ester Hybrid oil series of viscosities 5W-30, 5W-40, 5W-50 contain esters.

If you look at the "dry numbers", what you see is unlikely to impress the imagination. Freezing and flash points, viscosity indicators at the main control points (40 and 100 ° C), HTHS indicators (high-temperature viscosity at 150 ° C) - all this can be within the same limits as those of PAT-sisters. The main indicator is the stability of properties! And this can be found out only with the help of analysis of used oil.

But no numbers will show you the "sticking effect". Except for the increased motor resource of the engine, which you will be able to see much later. Do not forget that the more esters in the oil, the more stable its viscosity remains! It is the presence of esters that allows the aforementioned Xenum oils to contribute to fuel economy (6-19%), increase engine power (3-6%), reduce noise and wear of parts.

So, should you buy expensive ester oils? The question is personal for everyone. But, based on all of the above, we can say without a doubt that ester oils are for those who bought a car not for 2-3 years with the prospect of resale. They are needed more by those who take the car for a long time and see in the purchase and replacement of oil not just another maintenance, but a reliable investment in the longevity of the engine.