3.15 Surface Morphology and Chemistry of Reaction Layers
3.1.4 Description of the Engine Oil Additives
An engine oil additive is defined as a material designed to enhance or to impart the performance properties o f the base stock. Usually they are materials that have been chemically synthesized to supply the desired performance features.
Individual additives are used at concentration levels ranging from several parts per m illion to greater than 10 volume percent (1).
Different additives can assist each other, resulting in a synergistic effect or they can lead to antagonistic effects. Typical commercial additives perform more that one function, and all the engine oil functions usually can be represented by only a few separate additives. A ll engine oil additives used in higher concen
tration contain hydrocarbon groups which are required to make them soluble or dipersible in base stocks. High molecular weight hydrocarbon groups are mostly derived from olefin polymers. The size, specific structure, and location of the hydrocarbon groups profoundly affect the ways in which the additives function (3).
The development and commercialization o f new additives is a long, complex and expensive process involving many technical disciplines.
According to (4) the sequence o f events generally includes;
- the definition of a need by the marketing personnel or by liason representa
tives to the engine manufacturers;
- the review of the need by the technical staff in an attempt to arrive at a fundamental understanding o f the chemistry and physics involved;
- a hypothesis of the kinds of molecular structures likely to correct the problem;
- synthesis in the laboratory new materials fittin g the hypothesis;
- bench and laboratory engine testing.
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Development which follows these steps is shown in Table 3.1.1.
Table 3.1.1: Steps in Additive Development (4)
A 1. D efinition o f Need
2. Proposal o f Chemical Structure 3. Laboratory Preparation 4. Bench Testing
5. Laboratory Engine Testing 6. Decision - Go
B. 1. Optimization and Laboratory Process Development 2. Preliminary Economic Evaluation
3. Formulation fo r Application 4. Storage and C om patibility Testing 5. Expanded Laboratory Engine Testing 6. T oxicity Testing
7. Premanufacturing N otification to EPR 8. Decision — Go
C. 1. Pilot Plant Preparation
2. Manufacturing Process Design Preparation 3. Advanced Economic Evaluation
4. Formulation fo r Application 5. Field Testing
6. Decision - Go
D. 1. Plant Construction or Adaptation 2. Initial Manufacture
3. Confirmational Engine Testing 4. Final Economic Evaluation 5. Limited Marketing
3 .1.4.2 Additives Responsible fo r Formation of Deposits
Engine oil oxidation products form varnish and sludge. Thus, the oxidation process o f the oil must be retarded. This can be achieved by using oxidation inhibitors. To control buildup o f varnish and sludge detergents have to be applied. Furthermore, dispersants are responsible fo r keeping the formed sludge from agglomerating and depositing in the engine. Since the oxidation process is catalyzed by some metals, an application o f metal deactivators provides fu r
ther reduction o f the deposit form ation. Therefore, oxidation inhibitors, metal deactivators, detergents and dispersants are prim arily responsible fo r keeping the engine mating elements clean.
3.1.4.2.1 Oxidation Inhibitors
Oxidation is the major process of engine oil deterioration. The mechanism c f this process involves free radical reactions which are catalyzed by metals and accelerated by heat. In engine oils composed o f hydrocarbons, free radicals react w ith oxygen to form peroxy free radicals and hydroperoxides. The latter undergo further reactions to form alcohols, ketones, aldehydes, carboxylic acids, and other oxygen containing compounds.
These products usually have molecular weights close to the base stock and remain in solution. As the oxidation process proceeds, the oxygenated com
pounds polymerize to form viscous materials which, at a particular point, become oil-insoluble.
Consequently, the oxidation process generates viscous soluble materials which thicken the oil and insoluble materials form ing deposits.
Two factors are o f importance
(i) Free radicals are formed faster than they are used and the rate of oxida
tion increases;
(ii) Nitrogen oxides formed in the combustion process are oxidizing agents and can result in unique oxidation products, which include nitrate esters and other nitrocompounds (5).
Some of the oxygenated compounds are active, polar materials, e.g. acids, that accelerate corrosion and rust.
To retard and reduce the oxidation product form ation from hydrocarbons o x i
dation inhibitors are used. They w ork by reducing organic peroxides. The re
duction o f organic peroxides terminates the oxidation process (oxidation chain) and thus, minimizes the form ation of:
- Acids
- Resins/Polymers - Varnish
- Sludge.
The form ation o f oxidation products is complicated by the nature or the base stock, the presence and influence o f the additives used, and the environment.
A ll these factors can change the specific compounds formed and their rates o f form ation (5).
Petroleum based oils may contain some natural inhibitors. Their nature and amount depends on the crude oil type and the mode and degree of refining.
However, the great m ajority o f oxidation inhibitors is provided by synthetic materials. These materials encompass:
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- Hindered phenols
- Zinc dialkyldithiophosphates - Metal dithiocarbamates - Aromatic amines
- Sulfurized fats and hydrocarbons - Metal phenol sulfides
- Phospho-sulfurized fats and olefins
- Metal salicylates and many other compounds.
In many engine oils zinc dialkyldithiophosphates are the only antioxidants used, while in other oils they are supplement by other types of antioxidants, such as amines, hindered phenols, sulfides, etc. (6).
Sometimes metal deactivators, i.e. additives which react w ith metal ions and surfaces to reduct their catalytic activity, are also incorporated into oxidation inhibitors. Taking this into account, one can say that that antioxidants may function by one o f the follow ing three mechanisms:
- Free radical inhibition - Peroxide decomposition - Metal deactivation.
Hindered phenols are effective free radical (or radical scavenging) antioxidants as they react w ith free radicals to form nonfree-radical compounds. Some sulfur containing antioxidants decompose peroxides into stable compounds.
Metal deactivators/passivators inh ibit catalytic activity o f metal ions and sur
faces.
The action mechanism o f zinc dialkyldithiophosphates (ZnDDP) seems to be very complex since it may involve all o f the three mechanisms. Laboratory investigations o f individual ZnDDP compounds have shown that they act as radical scavenging oxidation inhibitors (7, 8), peroxide decomposers (9, 10), and metal passivators (11). Further investigations (12, 13), combining results of laboratory and engine fleet testing w ith a commercial, synthetic engine oil containing a combination o f ZnDDP and ashless oxidation inhibitors, have shown that in the early stages o f engine operation:
(i) antioxidant agents are consumed by their reactions w ith combustion- derived free radicals in the piston-cylinder area
(ii) ZnDDP species are consumed at a higher rate than ashless antioxidant additives.
These findings lead to a conclusion that in a new engine oil the free radical scavenging reactions o f ZnDDP are important. As an engine oil ages it w ill accumulate an ever increasing amount of products from combustion-derived free radical reactions and subsequent reactions in the oil sump; thus, in an aged oil the other mechanisms of ZnDDP consumption, such as peroxide decompo
sition, w ill become increasingly im portant (6). Additional work on antioxidant behavior of pure neutral and basic zinc dialkyldithiophosphates, dialkyldithio- phosphoric acid, tetraalkylthioperoxydithiophosphate (disulfide), and o f neutral zinc dialkyldithiophosphate in combination w ith a hindered phenol antioxi
dant was performed (6). This w ork included studies of the effect of hydro
peroxides and hydrocarbon oxidation products on the radical scavenging activity of the above compounds. Among other things, it was found that only neutral ZnDDP and dialkyldithiphosphoric acid as such are radical scavenging antioxi
dants. Basic ZnDDP scavenge peroxy radicals only after interactions w ith o x i
dation products and disulfide does not scavenge peroxy radicals at all.
Detailed inform ation on a model fo r oxidation o f engine oils in internal combustion engines and the assessment o f high temperature antioxidant capabilities are provided in (14 ,15 ).
3.1.4.2.2 Heavy D u ty (HD) Additives
HD additives encompass detergents and dispersants. Both additives provide a cleaning function. The purpose o f these additives in engine oils is:
- to keep o il-in so lu b le combustion products in suspension and
- to prevent resinous and asphalt-like oxidation products from agglomeration into solid particles.
The combustion products mostly include carbon-like deposits formed by pyrolysis o f deteriorated oil products which accumulate in the ring-belt area.
These products include soot and coke-like materials and can amount to 10 percent in the case o f diesel oil.
It is reasonable to include to HD additives also alkaline agents which neutralize and buffer the offending acids. This group of additives prevent:
- deposits to be formed on metal surfaces
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— sludge deposition in the engine
— corrosive wear by the neutralization o f acidic combustion products.
3.1.4.2.2.1 Detergents
Usually all detergents contain:
— Polar groups, e.g. sulfonate, carboxylic;
— Aliphatic, cycloaliphatic o r alkylaromatic radicals;
— One or several metal ions or amino-groups.
The most im portant agents providing detergency — which is a surface phenomenon o f cleaning surface deposits — include:
— Sulfonates
— Phenates
— Sulfurized phenates
— Salicylates
— Thiophosphates.
Detergents work by liftin g deposits from the surfaces to which they adhere.
Extensive literature (1 6 -2 2 ) associated w ith various kinds o f detergent additives describes in detail their chemistry, manufacturing, application and action mechanism. Problems related to the engine oil form ulation are also discussed (23).
3.1.4.2.2.2 Dispersants
Dispersants are nonmetallic materials characterized by a nitrogen or oxygen con
taining polar group attached to a high molecular weight hydrocarbon chain which solubilizes the additive in hydrocarbon base stocks. The most im portant agents providing dispersancy — which is a bulk lubricant phenomenon of keeping contaminants suspended in the o il - include:
— Copolymers (polymethacrylates, styrenemaleinic ester copolymers, etc.);
— Substituted succinamides;
— Polyamine succinamides;
- Polyhydroxy succinic esters;
- Polybutene hydroxy benzyl polyamine.
Dispersants have a strong a ffin ity fo r d irt particles and surround themselves w ith oil soluble molecules which keep the sludge from agglomerating and depositing in the engine.
Dispersants and detergents each perform both dispersancy and detergency functions and differ in their relative ability to function in the bulk of lubricant or at the engine surface.
Usually, many papers associated w ith dipersants (16,18, 21—23) also relate to the detergent additives.
3.1.4.2.2.3 Alkali Agents
These additives are formed by incorporating calcium or magnesium carbonates into sulfonate or phenate soaps, in a dispersed form in which a tin y core of the metal carbonate is solubilized by the attached soap (16, 17). By the over
basing technology the soap molecules are able to incorporate 10 to 20 times the stoichiometric soap equivalent o f alkaline earth metal; overbased sulfonates containing 50 percent active ingredient may have a total base number as high as 500 mg KOH/g (4). Micellar structure o f alkaline agents was recently studied in (20).
3.1.4.3 Additives Modifying Oil Properties
The major additives o f this group include viscosity index inprovers and pour point depressants. They have been used by the lubricating oil industry not only when the nature o f the engine oil makes it impractical to obtain a product w ith the needed high viscosity index and low pour point by the refining process but also in applications where extremely wide temperature variations are involved.
Usually, engine oils w ith high viscosity index and low pour p oint, i.e. the oils which function e fficiently, are obtained by a combination o f solvent refining procedures and application o f these additives. Broad literature (22, 24 — 31) discusses all specific features relating to these additives.
3.1.4.3.1 Viscosity index improvers
They are oil soluble polymers in the 50,000 to 1,000,000 molecular weight range such as:
- Polyisobutylenes
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- Poly methacrylates
- Ethylene/Propylene copolymers
■+ Polyacrylates
- Styrene/Maleic ester copolymers
- Hydrogenated Styrene/Butadiene copolymers.
Dispersant viscosity index improvers incorporate polar groups as do dispersants.
Viscosity index (V I) improvers reduce the rate o f change o f viscosity w ith temperature, i.e. they cause minimal increase in engine oil viscosity at low temperature but considerable increase at high temperature. This is due to the fact that the polymer molecule assumes a compact curled form in a cold base stock — which is poorly solvent — and an uncurled large surface area in a hot base stock that is better solvent. The uncurled form thickens the oil.
In this context it has to be remembered that:
- The addition of the viscosity index improver additive alters the flo w be
havior o f the base stock; dynamic viscosity o f the formulated oil changes w ith the rate share;
- The sensitivity o f the viscosity index improver additives towards mechanical stresses increases w ith increasing molecular weight o f the additives;
- Shear stresses, as they occur, e.g. between piston and cylinder walls in the engine, lead to irreversible breakdown o f the polymer molecules into smaller fragments; this results in a drop o f viscosity.
Effect o f viscosity index improvers as function o f the viscosity index o f the base oils w ill be discussed during the conference presentation.
3.1.4.3.2 Pour Point Depressants
Normal paraffin hydrocarbons tend to form waxy crystals at moderately low temperature. Usually, they are not present in engine oil base stocks. Other components such as isoparaffins, alkylnaphthenes, alkylaromatics and alkyl- naphthene-aromatics which are present in the petroleum engine base stocks show much less tendency to crystallize.
Pour point depressants are organic compounds which lower pour point o f the oil by retarding the form ation o f full-size wax crystals by coating or co-crystal
lization w ith the wax. The pour point depressants have no effect on the precipi
tation temperature (cloud p oint), the amount and the crystal lattice o f the
separated wax. They only change the external shape and size o f the crystals.
Spherical crystals are formed instead o f needles and thin platelets. Such change diminishes the a b ility o f the wax crystals to overlap and interlock to form large conglomerates o f wax which would impede the flo w o f the oil.
The m ajority of depressant additives include polymerization and condensation products. Some o f them act the same time as viscosity index improvers. The main products used fo r this application encompass:
- A lk y l methacrylate polymers and copolymers - Alpha-olefin polymers and copolymers
- V inyl carboxylate-dialkyl fumarate copolymers.
The molecular weight range of polymers effective as pour point depressants is generally below that o f polymers used as VI improvers and is usually in the range of 5,000 to 100,000.
Wax alkylated naphthalene and long chain alkyl-phenols and phthalic-acid dialkylaryI esters have also been used as depressants.
In paraffinic oils depressants show a better effect than in napththenic oils. This item w ill be discussed in detail during the conference presentation.
3.1.4.4 Corrosion Inhibitors
The function of an oxidation inh ibito r is to minimize the form ation of organic peroxides, acids and other oxygenated materials which deteriorate engine oils.
Thus, it also acts as a corrosion inhibitor. Consequently one can say that corrosion inhibitors just enhance the function o f antioxidants.
These additives protect bearing and other metal surfaces from corrosion, which also applies to the decomposition o f nonferrous alloys. For example, engine copper-lead bearings are structured so that small pockets o f lead occur dis
persed throughout the continuous copper phase and the surface. Oxidative corrosion o f the bearing occurs in tw o steps (4):
- In the first step the lead at the surface o f the bearing is oxidized by the peroxides formed in the fuel and oil;
— In the second step the oxidation products, such as organic acids, dissolve away the lead oxide renewing the surface fo r reoxidation.
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High end-point and unleaded gasolines aggravate copper-lead bearing corrosion, suggesting that the fuel hydrocarbons are the preliminary instrument o f corro
sion, modern engines have bearings provided w ith a very light tin overlay on the surface.
Corrosion inhibitors form an adsorbed protective film on metal surfaces which prevents contact between corrosive agents:
- acids - peroxides - others
and base metal. The adsorbed protective film stops also the catalytic effect o f metals on oxidation.
The film formed by corrosion inhibitors must adhere tig h tly to bearing surface to avoid it be removed by dispersants or detergents and expose the underlying metal surface to attack by acidic components in the engine oil.
Corrosion inhibitors include:
- Metal dithiophosphates
— Sulfonates
- Metal dithiocarbonates - Sulfurized terpenes
— Sulfurized olefins
— Many other compounds.
Sinoe metal corrosion and oil oxidation inhibitors are closely related to one another they are sometimes discussed together (32).
3.1.4.S Other Additives But Tribological Ones 3.1.4.5.1 Rust Inhibitors
The term is used to designate materials which protect ferrous metals against rust. Mostly it relates to the form ation o f hydrated iron oxide. Rust inhibitors prevent water from penetrating the protective oil film . This is achieved by application o f polar molecules which are adsorbed preferentially on the metal surface and serve as barrier against water. To be effective, the additive mole
cules have to adsorb tig htly on the iron surface and form a very stable film . Rust inhibitors used in engine oils include polar compounds such as sulfonates, amine phosphates, esters, ethers, and derivatives o f dibasic acids. Calcium and magnesium sulfonate used as detergents also provide antirust characteristics.
Additionally, the overbased detergents neutralize the acids which catalyze rusting.
The effectiveness o f rust inhibitors is controlled by the alkyl chain length of the additives. This relates to the fact that decrease in the size o f the alkyl groups increases the tendency o f the additive molecules to come out of solution and adhere on the iron surface.
3.1.4.5.2 Foam Inhibitors
Strong foaming affects the lubricating properties o f engine oils and decreases their oxidation stability due to the intensive mixing w ith air. Strong foaming which relates to the splashing action o f the crankcase and connecting rods can also lead to the oil transport in circulation systems.
The most universally used foam inhibitors are liquid silicones, especially polydimethylsiloxanes. In order to achieve a maximum effect, the silicones must be insoluble in the oil. They have to be finely dispersed in order to be sufficiently stable and must have a lower surface tension than the oil. In order to obtain stable dispersions of silicones, the droplet size must not exceed 10 jLim.
They function by attacking the oil film surrounding each bubble and thereby reducing interfacial tension so that the film breaks. Consequently, the small bubbles liberated combine to form large ones which flo at to the surface.
3.1.4.6 Tribological Additives 3.1.4.6.1 Introduction
In the regime of flu id film lubrication there is no contact between solids. The thickness o f the film that supports the load is governed by the lubricant viscosity.
However, when the severity of operating conditions increases (high load, low speed, high surface roughness), a point is eventually reached where the load can not longer be carried completely by the flu id film . Asperities o f the solids have to share w ith the flu id film in load support. The lubrication flu id film regime shifts to mixed film and then to complete boundary lubrication. The situation is presented in Fig. 3.1.1. The contact o f the solids involves wear, increased friction and welding o f asperities. To reduce friction and wear and pre
vent damage o f the mating surfaces tribological additives were developed.
Tribological additives present an extremely im portant group o f chemicals incorporated into engine oils. They are also called boundary additives. The tribological additives encompass organic, metal-organic (organometallics), and inorganic compounds. They may function as fric tio n m odifying (FM), antiwear (AW), and extreme pressure (EP) additives. EP additives are also called load- carrying agents. Performance o f these additives depends on chemical structure o f the additives and the composition o f the base stock used.
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w Figure 3 ,1 ,1 : Lubrication Regimes
3.1.4.6.2 F riction M odifying Additives
Friction m odifying additives can be described as chemicals that allow to reduce coefficient o f fric tio n and achieve smooth sliding or to increase coefficient of friction and achieve no sliding. Usually, they increase oil film strength and thereby keep metal surface apart and prevent oil film breakdown. FM additives that reduce coeeficient o f fric tio n conserve energy. They are mostly applied in engine oils and automotive engine drive-train gear oils. These additives provide 3-4 percent improvement o f fuel economy in automotive vehicles. Generally, they are used when smooth sliding w ith no vibration and minimum coefficient o f fric tio n is needed.
Fig. 3.1.2 presents the effect o f FM additive on gasoline engine frictio n losses in a motored engine. This figure clearly shows that at high-crankcase tempe
rature, low viscosity o f the lubricant increases engine fric tio n losses and the FM additive reduces these losses effectively. However, at low-crankcase temperature the FM has little effect; it is due to higher lubricant viscosity that reduces boundary lubrication. Screening tests described in (34) can provide guidance in developing fuel-efficient oils. Problems connected w ith fuel economy improvement by m odification o f oils have been broadly discussed in many papers (35 - 48). It is also to note that engine power loss can be reduced through improved filtra tio n o f m otor oil (49 — 50).