MolaKule
Staff member
Extreme Pressure Additives in Automotive Lubricants
By MolaKule
Early EP additives were simply calcium fats, lead fats or lead naphthenates, which worked only at low speeds and low loads. High loads and speeds would shear the lead film so a new lubricant was needed for the more modern gear drives. In the late 1950's new Sulfur-Phosphorus additives as well as chlorine additives were introduced.
We are all aware of the majority additives in gear lubes and greases and that these additives are called "Extreme-Pressure" or EP additives. But how do they function and what materials are used to construct these special additives? And how do these EP additives differ from the AW/EP additives in engine oils?
This Tech Brief describes how the EP additives function at the surface to prevent wear. We will also discuss how these additives are made. For an introduction, see:
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000225
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000226
While important, we will not deal with the topics of viscosity or absorption, but rather examine the adsorption characteristics of the new EP additives such as the Borate and Calcium families of additives.
For a review of Anti-Wear (AW) and Friction Modifiers (FM) for engine oils, see:
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000089#000000
Extreme Pressure additives ARE anti-wear additives that function under high contact loads and pressures not found in normal engine operation. Tooth loads in differentials and transmissions can exceed 3,000 psi and the tooth surface temperatures (due to high pressures) can exceed 750 F for short periods of time. In differentials and transmissions, the bearings are highly loaded and need lubrication and cooling as well.
The term "EP" is normally reserved for gear oils and greases while the term Anti-Wear (AW) is usually applied to engine oils. Friction Modifiers are used for both gear lubes and engine oils.
We find that there are two classes of EP additives:
1. The film-forming type that creates surface films as a result of being forced into a small contact region where pressure and heat decompose the additive molecules into their individual elemental constituents and forms a one-, two-, or three-layer film.
2. The "dispersed" additives that coat gear teeth and bearings due to their "attractive" charges, or affinities toward metals.
EP lubrication is a "boundary lubrication" phenomenon in which the full fluid film is absent, and in which additives of the film-forming EP additive types chemically combine with the metal surfaces, thereby preventing welding and galling of those surfaces. Sulfur-Phosphorus, the Chlorines, ZDDP, MoTDC, and SbDTC all fall under this category.
EP lubrication for the dispersed additives involves an electrostatic attraction of the dispersed molecules to the metal surface. These materials, such as borates, calcium complexes, or molybdenum disulfide (powder) are suspended in the oil carrier and literally coat the metal surfaces after attraction.
Friction Modifiers, Extreme Pressure additives, and Anti-Wear additives are all friction modifiers to one extent or another, but anti-wear additive films do not provide protection under long-lasting or sustained pressures, due to attrition. However, they will quickly reform providing enough additive reserve was included. So at times the delineation between AW and EP additives is not always clear and somewhat arbitrary. EP films, under extreme pressure, stay intact at higher temperatures and form/reform these films rather quickly after the pressure has passed. Most EP films also form at room temperature, unlike engine oil AW or FM additives. So we can say that the major differences between AW and EP films are that EP films:
a.) Form/reform more quickly
b.) Act or get to the base metal at room temperature, i.e., does not require
elevated pressures/temperatures to start the film reaction (the exception is the phosphate esters)
c.) Act as anti-galling, anti-welding additives
d.) Act/react when the pressures or loads are above what the AW additives can handle.
e.) AW additives form plastic films which "glide," whereas EP additives form films which shear or "slide."
The most probable film forming effects are those of elemental atoms and the iron substrate. The chlorine additives such as the chlorinated parrafins (or sulfur monochloride), which are being phased out, formed FeCl2 or FeCl3 films. The sulfur additive most likely forms an FeS or FeS2 film. The sulfur-chlorine add forms FeSClx films, the phosphates form FePO4 films (iron phosphide films), and the borates form FeO + B2O3 films. Any oxide on the metal surface forms an anti-weld film by itself and affects the mechanism of the chemical reactions in involved with the EP lubricant.
The chlorine and sulfur containing films usually stains steel parts and can attack copper-based alloys such as brass, bronze, etc. Some GL-4 fluids and Marine gear lubes make sure the chlorine content is less than 500 ppm of chorine or contain metal deactivators to reduce yellow metal attack from both the chlorine and the sulfur. The sulfur contained in gear lubes is on the order of 2.5% for GL-5 lubes, and 1.0 % for GL-4 lubes. Chlorine is the lowest temperature EP additive, with the exception of the borates, calcium complexes, and the molys.
For more info on metal deactivators, see: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=21;t=000029
Phosphate esters form phosphate or pyrophosphate salts of iron at the surface under extreme pressure conditions. These are often added, in addition to the Sulfur compounds, to provide a higher temperature/pressure EP environment. One such P-additive is tricresyl phosphate, an ester. The phosphor content of gear lubes is around 0.12% for all service classifications of gear lubes.
The EP films for the most part form, on the gear tooth or rack for example, as easily sheared films. The theory behind this is that is better to lay down a sacrificial film and have it sheared, than to have the base metal sheared, galled, or welded. The sheared film then reforms for the next "film-shearing" cycle.
The calcium complexes (such as calcium carbonates), the borates, and the molysulfides
provide a coating that not only shears at the top layer, but also fills depressions and voids, providing a very hard anti-scuffing surface.
We have already spoken about the phosphorus additives such as the esters, like tricresyl phosphate. The phosphorus esters are formed by reacting the appropriate kinds and amounts of alcohols to phosphorus pentoxide, or phosphoric acid. The phosphorus additive forms films at low temperatures and thus starts as an anti-wear additive even before things "heat-up" and the requirement for EP films start to occur.
The most often used additive in gear oils is the sulfur additive or the sulfurized fat, also named synthetic polysulfides. The natural fats or synthetic esters provide a good measure of Friction Modification, while the addition of sulfur and/or phosphorus provides the EP activity. But how are the sulfurized additives manufactured? Well, much the same way as phosphor esters. The ester or natural fat is heated and the elemental sulfur or phosphor is added to the hot ester (or fat) until dissolved. This way, a high percentage of sulfur can be liberated in the gear oil or grease. In another process, the phosphor or sulfur is reacted at the time of ester creation, forming a single and controlled molecule that is complete with let's say, the complete S-P level of additives. In some cases the complete ester is a sulfur-phosphorus-chlorine ester (sulfur-monochloride-phosphate) that contains all three molecules.
So you can now see how the EP additives provide some FM and AW protection as well.
In some cases, only the sulfurized-phosphated-chlorinated fats or esters are included with the base oils and moly sulfide powder is added, along with rust inhibitors, metal deactivators, and Viscosity Index Improvers.
For some gear lubes that work in limited slip or positive traction differentials, a friction modifier is added, which may consist of additional natural fats or synthetic esters.
[ October 13, 2003, 05:07 PM: Message edited by: MolaKule ]
By MolaKule
Early EP additives were simply calcium fats, lead fats or lead naphthenates, which worked only at low speeds and low loads. High loads and speeds would shear the lead film so a new lubricant was needed for the more modern gear drives. In the late 1950's new Sulfur-Phosphorus additives as well as chlorine additives were introduced.
We are all aware of the majority additives in gear lubes and greases and that these additives are called "Extreme-Pressure" or EP additives. But how do they function and what materials are used to construct these special additives? And how do these EP additives differ from the AW/EP additives in engine oils?
This Tech Brief describes how the EP additives function at the surface to prevent wear. We will also discuss how these additives are made. For an introduction, see:
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000225
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000226
While important, we will not deal with the topics of viscosity or absorption, but rather examine the adsorption characteristics of the new EP additives such as the Borate and Calcium families of additives.
For a review of Anti-Wear (AW) and Friction Modifiers (FM) for engine oils, see:
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000089#000000
Extreme Pressure additives ARE anti-wear additives that function under high contact loads and pressures not found in normal engine operation. Tooth loads in differentials and transmissions can exceed 3,000 psi and the tooth surface temperatures (due to high pressures) can exceed 750 F for short periods of time. In differentials and transmissions, the bearings are highly loaded and need lubrication and cooling as well.
The term "EP" is normally reserved for gear oils and greases while the term Anti-Wear (AW) is usually applied to engine oils. Friction Modifiers are used for both gear lubes and engine oils.
We find that there are two classes of EP additives:
1. The film-forming type that creates surface films as a result of being forced into a small contact region where pressure and heat decompose the additive molecules into their individual elemental constituents and forms a one-, two-, or three-layer film.
2. The "dispersed" additives that coat gear teeth and bearings due to their "attractive" charges, or affinities toward metals.
EP lubrication is a "boundary lubrication" phenomenon in which the full fluid film is absent, and in which additives of the film-forming EP additive types chemically combine with the metal surfaces, thereby preventing welding and galling of those surfaces. Sulfur-Phosphorus, the Chlorines, ZDDP, MoTDC, and SbDTC all fall under this category.
EP lubrication for the dispersed additives involves an electrostatic attraction of the dispersed molecules to the metal surface. These materials, such as borates, calcium complexes, or molybdenum disulfide (powder) are suspended in the oil carrier and literally coat the metal surfaces after attraction.
Friction Modifiers, Extreme Pressure additives, and Anti-Wear additives are all friction modifiers to one extent or another, but anti-wear additive films do not provide protection under long-lasting or sustained pressures, due to attrition. However, they will quickly reform providing enough additive reserve was included. So at times the delineation between AW and EP additives is not always clear and somewhat arbitrary. EP films, under extreme pressure, stay intact at higher temperatures and form/reform these films rather quickly after the pressure has passed. Most EP films also form at room temperature, unlike engine oil AW or FM additives. So we can say that the major differences between AW and EP films are that EP films:
a.) Form/reform more quickly
b.) Act or get to the base metal at room temperature, i.e., does not require
elevated pressures/temperatures to start the film reaction (the exception is the phosphate esters)
c.) Act as anti-galling, anti-welding additives
d.) Act/react when the pressures or loads are above what the AW additives can handle.
e.) AW additives form plastic films which "glide," whereas EP additives form films which shear or "slide."
The most probable film forming effects are those of elemental atoms and the iron substrate. The chlorine additives such as the chlorinated parrafins (or sulfur monochloride), which are being phased out, formed FeCl2 or FeCl3 films. The sulfur additive most likely forms an FeS or FeS2 film. The sulfur-chlorine add forms FeSClx films, the phosphates form FePO4 films (iron phosphide films), and the borates form FeO + B2O3 films. Any oxide on the metal surface forms an anti-weld film by itself and affects the mechanism of the chemical reactions in involved with the EP lubricant.
The chlorine and sulfur containing films usually stains steel parts and can attack copper-based alloys such as brass, bronze, etc. Some GL-4 fluids and Marine gear lubes make sure the chlorine content is less than 500 ppm of chorine or contain metal deactivators to reduce yellow metal attack from both the chlorine and the sulfur. The sulfur contained in gear lubes is on the order of 2.5% for GL-5 lubes, and 1.0 % for GL-4 lubes. Chlorine is the lowest temperature EP additive, with the exception of the borates, calcium complexes, and the molys.
For more info on metal deactivators, see: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=21;t=000029
Phosphate esters form phosphate or pyrophosphate salts of iron at the surface under extreme pressure conditions. These are often added, in addition to the Sulfur compounds, to provide a higher temperature/pressure EP environment. One such P-additive is tricresyl phosphate, an ester. The phosphor content of gear lubes is around 0.12% for all service classifications of gear lubes.
The EP films for the most part form, on the gear tooth or rack for example, as easily sheared films. The theory behind this is that is better to lay down a sacrificial film and have it sheared, than to have the base metal sheared, galled, or welded. The sheared film then reforms for the next "film-shearing" cycle.
The calcium complexes (such as calcium carbonates), the borates, and the molysulfides
provide a coating that not only shears at the top layer, but also fills depressions and voids, providing a very hard anti-scuffing surface.
We have already spoken about the phosphorus additives such as the esters, like tricresyl phosphate. The phosphorus esters are formed by reacting the appropriate kinds and amounts of alcohols to phosphorus pentoxide, or phosphoric acid. The phosphorus additive forms films at low temperatures and thus starts as an anti-wear additive even before things "heat-up" and the requirement for EP films start to occur.
The most often used additive in gear oils is the sulfur additive or the sulfurized fat, also named synthetic polysulfides. The natural fats or synthetic esters provide a good measure of Friction Modification, while the addition of sulfur and/or phosphorus provides the EP activity. But how are the sulfurized additives manufactured? Well, much the same way as phosphor esters. The ester or natural fat is heated and the elemental sulfur or phosphor is added to the hot ester (or fat) until dissolved. This way, a high percentage of sulfur can be liberated in the gear oil or grease. In another process, the phosphor or sulfur is reacted at the time of ester creation, forming a single and controlled molecule that is complete with let's say, the complete S-P level of additives. In some cases the complete ester is a sulfur-phosphorus-chlorine ester (sulfur-monochloride-phosphate) that contains all three molecules.
So you can now see how the EP additives provide some FM and AW protection as well.
In some cases, only the sulfurized-phosphated-chlorinated fats or esters are included with the base oils and moly sulfide powder is added, along with rust inhibitors, metal deactivators, and Viscosity Index Improvers.
For some gear lubes that work in limited slip or positive traction differentials, a friction modifier is added, which may consist of additional natural fats or synthetic esters.
[ October 13, 2003, 05:07 PM: Message edited by: MolaKule ]
