Motor Oils. PAG (polyalkylene glycols): Next step. Article ENG/UK/RU

PAG: Next step

PAG: Next step

As the progress in automobile motor construction and increased load on internal combustion piston engines increased, the requirements for lubricants also increased. Mineral bases were replaced by hydrocracking and various synthetic technologies. However, the search for the ideal motor oil continues. And today, technology based on polyalkylene glycols (PAG), which have the most outstanding characteristics at the moment and are environmentally friendly at the same time, has been implemented for internal combustion engines.

Base oils, obtained in one way or another from crude oil, in many ways no longer meet the increased requirements for lubricants, and are more often being replaced by high-quality synthetic oils. This leads to both an increase in the temperature regimes of engine operation in combination with an increase in the specific pressure in the friction pairs, as well as a desire to increase service intervals.

For those who actively use a passenger car, driving up to hundreds of thousands of kilometers per hour, or even more, oil quality is especially important. After all, they require several oil changes for every year of operation, which means that the engine runs on fairly used oil for several tens of thousands of kilometers. Unlike those who change the oil once an year, active drivers take the risk that after three years they will remain with a new-looking and still quite modern car, but with a badly worn out engine.

The quality of engine oil is manifested during long-term operation in the engine - it must maintain stable lubrication characteristics and not oxidize prematurely. Being a mixture of a base base and a package of additives that give the necessary detergent-dispersing, anti-wear and anti-seize, viscous and other properties, petroleum-based oil is highly dependent on the production of these additives. However, for many years, it was petroleum hydrocarbons that remained an almost non-alternative source of base oil, despite the existence of bases that are much more suitable in terms of their initial properties for work in engines.

PAG: Next step
PAG oils have been working in various compressors for decades without losing their original properties - both in refrigerators and under the hoods of cars.
New - unused old
In fact, PAG oils, i.e. polyalkylene glycols, were not discovered yesterday. Active work with them began in the 1930s. Even then, experts drew attention to their unique characteristics: high viscosity index, resistance to shear and mechanical loads, as well as temperature stability.

Actually, the use of PAG as motor oil is not new. Before the Second World War, the creators of aviation engines, which were exclusively piston engines, drew attention to them. During the war itself, they were used by the USAF in the engines of various types of aircraft: P-38s ("Lightning" is a heavy two-engine fighter of the Lockheed company), P-47s fighter-bombers (P-47 Thunderbolt produced by Republic), and B-bombers 25s (North American B-25 Mitchell).

Huge distances and, accordingly, large engine flight hours, forced designers to look for a solution that would extend their service life and service interval. The fact is that earlier air battles were fought near front-line aviation bases, the life time of planes in the air was short, and there was no special need to take care of the resource - usually the plane was shot down before it wears out. But in the Pacific Ocean, it was necessary to fly up to a thousand kilometers before entering the battle, and the mass of sorties passed in vain - the opponents simply did not meet. This is where the unique properties of PAG come in handy.

PAG was also used by the German Luftwaffe - on planes hunting Allied convoys in the conditions of the far north. The high viscosity index came in handy here. After all, the engines had to be started "cold" at temperatures up to -60°C, and the oil warmed up to very high temperatures in the running engine. Aviation piston engines are highly boosted. The speed of the rotating parts reaches 2,800 rpm (shaft) and even 25,000 rpm (separate parts), the specific loads on the rubbing surfaces are up to 1,200 kg cm. In the engine, the oil is exposed to various temperature conditions. For example, the temperature in the combustion chamber is 1500-2500°С, the temperature of the bottom of the piston is up to 300°С.
PAG: Next step
During the Second World War, synthetic lubricants, including those based on PAG, were actively used by the US Air Force and the German Luftwaffe.
After the war, polyalkylene glycol oils "came to the fore". In the USA, against the background of the conversion of the industry, which had greatly strengthened during the war years, a boom in the field of household appliances was just beginning. "A refrigerator in every home" was the slogan then. At the same time, none of the manufacturers of household refrigerators were smiling at the perspective of annual servicing of tens of millions of household refrigerators. Yes, and consumers wanted to buy a refrigerator once so that it worked for twenty years. In addition, changing the oil in the compressor of the refrigeration equipment requires replacing the freon, which is expensive. And here PAG came to the rescue again - they have been working in compressors for decades without losing their original properties. Actually, PAG oils also work in the compressors of car air conditioners.

And what were automakers doing all this time? Did they not pay attention to polyalkylene glycol oils? Of course, they chose - in the 50s, when car owners in the USA, which became rich after the war, began to drive more and further, not skimping on cheap gasoline by the tons, car manufacturers noted the unique properties of PAG in terms of engine protection. Experiments with PAG were conducted until the 70s. From the point of view of the main function of the oil, the results were good, but the minuses of the operational plan were revealed.

Unlike base oils of petroleum origin, PAG turned out to be hydrophilic. That is, they absorb moisture and dissolve in water. In the closed environment of refrigeration compressors, this did not matter. Yes, it was also uncritical in aviation engines, because the working hours of an aircraft engine in any case, and even more so for a war, are not the same as in a car, the engine of which can work up to one to two thousand hours per year (3-5 hours per day), not to mention taxi cars. In addition, the temperature regime of the automobile engine of those years did not contribute to the rapid evaporation of water. And it was not made of aluminum, as it is today - that is, it simply rusted from the presence of moisture in the oil.

In principle, it can be assumed that, if desired, chemists could deal with the hydrophilicity of PAG even then. However, one more feature became clear - polyalkylene glycol oils in their original form do not dissolve in other hydrocarbon bases. And now let's imagine a situation when it is necessary to top up the oil, which was often performed by whoever and whenever: the owner in the garage, the refueler at the gas station. It is impossible to control that the owner or mechanic did not accidentally add ordinary mineral water to the PAG oil.

It is obvious that the presence of two immiscible bases in the oil will lead to permanent ruptures of the oil film and a very quick failure of the engine - it will "knock" almost after the first turn. And no matter how much we write a warning on the cover of the oil filler neck, even if we screw a plate the size of a candy box next to it - there will still be cases all the time. And for them, since the USA is a country of lawyers and case law, there are lawsuits and endless lawsuits: the plate is not the same color, it is not written in all languages, there is no illumination of the plate to change the oil in the dark, the mechanic is illiterate... Therefore, they decided to refuse to introduce the idea into mass production.

Nevertheless, experiments with PAG continued - their properties were already very attractive. Thus, in 1998, Renault began developing the so-called "maintenance-free" ELLYPSE car. The prototype had to travel at least 100,000 km without any repairs or maintenance. That's when they remembered PAG oils again. Fortunately for the car service industry, further development of the project was refused - ELLYPSE was shown at several exhibitions in 2002, and that was the end of it. Although the topic of PAG was not completely abandoned - RENAULT and FIAT continued research. We did not manage to find out how their research ended, but one way or another, the researchers of the chemical concerns did not stop trying to get everything possible from PAG, and finally they succeeded.

PAG: Next stepA bit of PAG chemistry
Polyalkylene glycols are formed from gases such as ethylene oxide and propylene oxide, which, in turn, are obtained by cracking longer hydrocarbon chains from oil or natural gas. It is possible to synthesize ethylene and propylene oxides from renewable raw materials such as ethanol or glycerin.

Polyalkylene glycols are extremely important for the production of special lubricants, brake fluids, hydraulic fluids and coolants. They are widely used as coolants in automobile cooling systems, when processing metals, as brake and hydraulic fluids. They are used as high-temperature lubricants and coolants in the paper, ceramic, glass and other industries.

To obtain oils with high anti-caking properties, various OH end groups, alkoxy and alkylamino groups are introduced into the structure of polyalkylene glycols. They are used as special purpose lubricants (for example, for electric motors, ice cream production equipment) and as lubricants for compressors, pumps, bearings and transmissions operating at high temperatures, as well as for rubber production.

They are obtained as a result of the interaction of compounds containing OH groups or other active hydrogen atoms (water, alcohols, diols, polyols, carboxylic acids, amines) with alkylene oxides (epoxides, cyclic radicals) in the presence of special alkaline catalysts. That is, polyalkylene glycols are polymers of alkylene oxides.

They can be used in a wide range of temperatures, are not prone to the formation of deposits and are able to stabilize the products of their own decomposition. They have excellent viscosity-temperature properties and do not leave any deposits on the parts when heated. Polyglycols do not show straight lines on the viscosity-temperature graph. The fact that the molecular weight and, consequently, the viscosity of polyalkylene glycols can be regulated within narrow limits during their production is very important.

Due to the content of oxygen atoms, they have a better dissolving ability compared to hydrocarbons, as well as a strong electric charge, which determines the "magnetization" of the oil film to lubricated metal surfaces. When using PAG, the development of fatigue wear in rolling bearings and gearboxes is reduced, their properties can be improved by the introduction of anti-seize additives. In terms of carrying capacity, these oils are superior to mineral oils.

When using polyethylene glycols as motor oils, their high dissolving ability prevents the formation of deposits due to the dissolution of sediments formed in them. Low ash content and lack of tendency to coke formation allow polyglycols to be used as base oils for lubricants containing graphite and molybdenum disulfide. In some cases, it can be used without detergents and dispersants at all.

PAG: Next step
New fully synthetic motor oil based on polyalkylene glycol, which is biodegradable, can provide fuel savings of up to 3% in cars. Scientist Mathias Woydt from the BAM research group of the Federal Institute for Materials Research and Testing (Germany, Berlin) calculated that polyalkylene glycol (PAG) can reduce carbon dioxide (CO2) emissions by approximately five grams per kilometer. The advantages of PAHs in fuel economy are especially pronounced in urban traffic flow. In addition, the vehicle will have to change the oil only approximately every 30,000 km.

The revolution is over!
Finally, chemists found a way to synthesize poylalkylene glycols, in which the structure of their molecules ensures mixing with hydrocarbon oils, both petroleum and synthetic. And also reduces hydrophilicity to values that are insignificant from the point of view of using PAG as motor oils. At the same time, all the advantages of PAG are preserved, and there are many of them. We have already mentioned their main ones above.

PAG: Next step

Once again, we note that PAGs are characterized by a very high viscosity index, that is, their viscosity changes less over a wide range of temperatures than other base oils. And this means that it is possible to use fewer additives-thickeners. Special molecules, which are very long chains of atoms, are added to petroleum base oils. In cold oil, they float, twisted into microscopic lumps, and do not affect the viscosity of the oil in any way. When the oil is heated, the thickeners straighten and begin to stick together, preventing the oil from becoming too thin.

PAG: Next step

The problem with thickeners is that they are subject to mechanical destruction - they simply break during the use of the oil, so its viscosity index drops. There is no such problem with PAG – these oils by themselves are able to maintain sufficient viscosity even in a heated engine. At the same time, PAG also provides a gentler cold start - this happens precisely due to polarization. Charged molecules are attracted to metal, so a minimal oil film remains on friction pairs even after a long stop, and in the first moments after starting, the engine no longer runs "dry", which is usually one of the main factors of wear.

PAG: Next stepThe low ash content of PAG and the lack of tendency to coke significantly reduce soot formation in the engine. This is especially noticeable on diesel engines, in which, in the presence of a particulate filter, an oil change interval of 30,000 km is declared. km The practice of operating the Volkswagen Transporter T5 under the control of the editors of the autoExpert magazine on XENUM XPG PAG oil shows that the sulfur removal mode of the particulate filter is activated twice as often. This means not only that the filter itself will last longer. It is much more important that less soot is formed in the engine itself, which is not equipped with a self-cleaning mode.

PAG oil has a higher heat capacity than PAO, so it cools the engine much more efficiently. The high stability and strength (carrying capacity and resistance to shear) of the oil film reduces the load and energy loss in the piston system, which is reflected in the reduction of fuel consumption. Which, taking into account its constantly growing cost, allows to compensate to a certain extent the higher cost of PAG oils.

For comparison, the cost of 4 liters of XENUM XPG oil is equal to the cost of 5 liters of PAO oil of the same brand. And after all, it is also worth taking into account the savings on the extension of the engine resource, although it is difficult to calculate it for a specific car.

XENUM XPG PAG is designed to protect the engine in the most extreme operating modes, and in normal daily use, it works, accordingly, to increase the replacement interval that is safe for the engine. And since, as already mentioned, complete solubility with other hydrocarbon bases is ensured, the oil can be poured into the engine after the second synthetic or semi-synthetic oil - its remainings will not cause any damage beyond the usual (remainings are contaminated, etc.). When replacing the XENUM XPG again, you can confidently count on the maximum possible cleanliness and protection of the motor.

The author is Denys Petrov.

Published in the magazine autoExpert #2`2020.
Use of materials is possible only with reference to the source.

A unique special edition of the autoExpert magazine about the XENUM company and products.

PAG: Next step
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