When asked to describe a lubricant, people typically refer to its brand or product name. More precisely, a lubricant, whether it is an oil or grease, is a bundle of performance properties such as oxidative life, resistance to thermal or hydrolytic degradation, antiwear or antiscuffing characteristics and air and water separability. Required performance characteristics vary by application (Figure 1). When performance properties are compromised, the lubricant's ability to minimize friction, wear and corrosion, control heat and contamination and transmit force and motion in hydraulic systems deteriorates. To ensure machine reliability, the offending lubricant requires maintenance actions that are properly designed and executed. This article discusses how oil degrades, proactive ways to extend the lubricant's life and proper disposal methods once the oil has been changed.
Contrary to popular belief, oil doesn't last forever. The lubricant in a machine must be changed or at least maintained; otherwise it will no longer possess the required performance properties to carry out the demands of the machine, application and operating environment. In some instances, the oil must be changed because the lubricant's base oil becomes degraded and is no longer fit for service. Oxidative, thermal and hydrolytic degradation will change the base oil's chemical and physical properties, which then alters the lubricant's performance properties. In other cases, the lubricant's additive package becomes depleted. Unfortunately, the lubricant may also become contaminated with foreign material that cannot easily be removed.
Base Oil Degradation
Oxidation. One of the most common forms of base oil degradation is oxidation. It occurs when oxygen reacts with the lubricant's base oil, which is typically a hydrocarbon. When the oil becomes oxidized, some hydrocarbon molecules are transformed into acid and sludge, which affect the performance properties of the oil. Some molecules are better equipped to resist oxidation than others. Therefore, some base oils have better oxidation stability than others. Oxygen is a necessary component for oxidation; consequently the degree to which the lubricant is aerated affects the oxidation rate. The presence of water and reactive metals, such as iron and copper, also influences the rate of oxidation. Oxidation-inhibiting additives sacrifice themselves to protect the base oil from oxidation.
Thermal Degradation. Unlike oxidation, thermal degradation does not require oxygen to occur. Thermal failure takes place when the oil comes in contact with hot surfaces inside the machine, such as combustion or exhaust areas, or when coming in contact with compressed bubbles, such as in hydraulic systems. Thermal failure results in the loss of hydrogen, leaving carbon-rich particles behind in the form of sludge and deposits. Thermal failure does not produce acid, however it does produce deposits that affect the performance properties of the oil. In some cases, the hydrocarbon's carbon chain cracks into smaller subsets of itself, reducing the average molecular weight and the viscosity of the resultant molecules.
Hydrolysis. Hydrolysis is the direct reaction of the base oil mixing with water, which permanently modifies the base oil's molecular structure. Ester-based lubricating oils, including dibasic acid ester, polyol ester and phosphate ester, are the most susceptible to hydrolysis. Esterification of alcohol and acid, the process that creates ester base oils, produces ester and water as its by-products. When exposed to water, esters readily hydrolyze back into alcohol and acid. Hydrolysis affects the performance properties of the base oils that utilize esters. Many lubricants and hydraulic fluids employ esters as their primary base oil component or as a co-base oil to improve the solubility and seal performance of highly refined mineral or synthetic oils.
Additives are formulated into the lubricant to enhance performance properties such as separability from air or water, and to suppress undesirable properties such as the tendency to form wax at low operating temperatures. Additives are also included to impart new properties such as reducing wear under boundary contact conditions. Over time, additives become depleted and the lubricant requires service to restore the performance properties. This can occur either in the form of an oil change, additive sweetening with a partial drain and fill or lubricant reclamation, where the lubricant is seemingly restored to like-new conditions. The rate at which additives deplete depends on the additive type as well as environmental conditions, particularly temperature and presence of water. Some additives condense and separate from the base oil at low temperature; therefore, the additive depletion rate increases as temperature increases. Many additives are susceptible to hydrolysis, and the presence of water usually damages the additive system. Numerous additive depletion mechanisms influence additives to varying degrees.
Proactive Management of Lubricant Life Selecting Premium Base Oil
One strategy for extending lubricant life is to select premium lubricants formulated with premium base oils, premium additive systems or a combination of both. The American Petroleum Institute (API) has provided a standard classification for base oils, called groupings, to summarize the quality of the oil. The API categories include Group I, II, III, IV and V oils. Groups I, II and III are mineral base oils refined to varying degrees. Group IV oils are specifically synthesized hydrocarbon base oils such as polyalphaolefin (PAO), the most common synthetic base oil. The API also indicates viscosity index (VI), percent saturated hydrocarbon and percent sulfur requirements for Groups I, II and III. Group V oils include everything not in Groups I, II, III or IV such as dibasic acid ester, polyol ester, poly glycol, phosphate ester and numerous other base oils that possess special properties. Due to the wide range of Group V base oils, specific requirements for this group have not been made.
The VI is an indication of the base oil's relative change in viscosity for a given change in temperature. A high VI is generally considered a favorable characteristic because lubricants with this quality are operable across a greater range of temperatures. Compared to a low VI base oil of the same viscosity grade, a high VI base oil has comparatively lower viscosity at cold start. Therefore, its flow characteristics are superior and it maintains a higher viscosity at full operating temperature, thus providing a thicker oil film to protect the oil. Group I has the lowest VI requirement and Group IV has the highest requirement set by the API, and Groups II and III fall in between. Group IV (PAO) base oils generally possess a higher VI than either Groups I, II or III. The VI of Group V base oils varies depending upon type.
The percentage of unsaturated hydrocarbons in oil indicates the base oil's ability to resist oxidation and thermal failure. Base oil that has been highly refined to reduce or eliminate unsaturated molecules will resist oxidation and thermal failure more effectively than base oil with a comparatively high percentage of unsaturated hydrocarbon molecules. Group I base oils possess a higher percentage of unsaturated molecules than Groups II or III, which generally means that the oxidative and thermal life of Group III base oil is superior to Group II, which is superior to that of Group I. However, improving resistance to oxidative and thermal failure by refining the base oil to reduce or eliminate unsaturated hydrocarbons can have negative side effects. Base oils with a low percentage of unsaturated molecules have trouble dissolving additives and they tend to cause elastomer shrinkage. To counter this, many Group II, III and IV base oils are formulated with co-base oil, such as diester to polyol ester to improve additive solubility and offset seal shrinkage tendencies.
Sulfur occurs naturally in most mineral base oils. The API has designated maximum sulfur levels for Group I, II and III base oils, with Group I having a higher sulfur allowance than Groups II or III. Group IV PAO, which is a synthesized hydrocarbon, is sulfur-free. Surprisingly, sulfur improves the base oil's lubricity (the oil's ability to lubricate under boundary metal-to-metal contact conditions) and natural resistance to oxidation. In fact, sulfur is a component in many additive formulations, including antioxidants, antiwear (AW) agents and antiscuffing or extreme pressure (EP) agents. Why, then, is lower sulfur associated with higher base oil grades? Modern lubricant formulators prefer to control the chemical context where the sulfur resides in the finished lubricant. Therefore, they prefer to start with a base oil containing a low concentration of naturally occurring sulfur so it can be added back into the concentration and chemical form believed to be appropriate for the application.
Synthetic Base Oil
End users often presume that specifying a premium lubricant by definition means selecting a lubricant formulated with synthetic base oil. In some instances, synthetic base oil is appropriate, but not always. Synthetic base oil, depending on its type, offers several possible advantages (Table 1). However, not all synthetic base oils offer these properties and, moreover, they may not be required. For instance, high VI base oil isn't required for a machine which operates 24/7 at a constant temperature. Likewise, detrimental aspects associated with synthetic base oil must be considered. If you can't make the decision yourself, consult expert advice.
When selecting a premium lubricant, choosing a base oil is not the only decision an end user must make. A small number of providers supply additives to the lubricant formulators and marketers who then incorporate the additive technology into their products to achieve the desired performance characteristics for the targeted application. As one might conclude, not all additives are created equal. Some additive technologies are better or more modern than others and may be more costly. Additives may also be supplied as complete systems that need to be blended with the base oil to produce standard finished products to serve specific applications. However, many lubricant suppliers purchase additive components and formulate specialty lubricants that possess specific performance characteristics. These custom-formulated products are more expensive than standard products, which reflect the use of expensive additive components and the engineering required to formulate them. They are often blended in small batches due to their low demand and require special sales and application engineering services, adding further to the cost.
Contrary to popular belief, specially formulated lubricants do not always employ synthetic base oil or highly refined mineral oils. Base oil selection contributes to the performance characteristics of the finished lubricant; however, the lubricant's performance characteristics depend on the base oil selection, additive selection and formulation engineering. A formulator may prefer to employ a Group I or II base oil to formulate a specialty or high-performance product. It is important to understand the required performance properties for the application and to match the performance characteristics of the finished lubricant accordingly.
Lubricant Condition Control
Regardless of the lubricant selected, the end user has a great deal of influence over the actual life of the lubricant by managing system contamination and refreshing the additive system. Contamination control is the easiest and most widely applicable method for extending lubricant life.
Contamination includes all foreign and unwanted forms of matter and energy, including particles, moisture, heat, air, chemicals and radiation.
Heat is the lubricant's worst enemy. The oxidative life of a lubricant relative to temperature generally follows the Arrhenius Law; that the rate of a chemical reaction increases exponentially with the absolute temperature. A rule of thumb is that the oxidative life of oil is halved for every 10ºC increase in temperature. For example, if the oxidative life of the lubricant is 1,000 hours at 100ºC bulk oil temperature, a useful life of 500 hours could be projected at 110ºC, 250 hours at 120ºC and so forth. Managing temperature is critical to managing lubricant life. If a cool temperature cannot be maintained, a premium lubricant may be required. Bulk oil temperature (for example, tank or sump temperature) influences the rate of oxidation. However, transient contact with hot surfaces can result in thermal degradation, as previously discussed.
Air is another factor that influences both the rate of oxidation and thermal degradation. It is the primary source of oxygen required in the oxidation process and all lubricants contain some dissolved and/or entrained air. Increasing the amount of dissolved and entrained air increases the rate of oxidation. The relationship is approximately one to one, so doubling the concentration of air roughly doubles the rate of oxidation. Hot compressed bubbles are also a primary cause for thermal failure, especially in high-pressure hydraulic machines. Managing air contamination should be an important component of any plan to extend lubricant life. Interfacial tension between the oil and air bubbles, which is influenced by both the base oil and additive system, determines how air can be entrained in the lubricant. Where interfacial tension is high, air bubbles dissipate and separate readily. Where interfacial tension is low, air is more readily entrained. Tank design and volume, lubricant delivery mechanism, and numerous other factors also influence the air contamination level.
Moisture is the enemy of most lubricant components. It results in de-esterification of ester base oil components, reduces additives to acid and/or sludge and promotes base oil oxidation, especially in the presence of catalytic metals such as iron or copper. Water enters the machine where it interfaces with the environment, including contaminated new oil sources, breathers and vents, shaft seals, etc. Humid environments where the machine operates intermittently and where machines are subjected to water spray hold the highest risk. The best way to control water contamination is by using premium seals, desiccant or other water-excluding breathers. Dehydrating methods can also be employed to remove excess water.
The influence particles have on lubricant degradation depends on the particle type. Suspended particles can increase air entrainment, which indirectly increases the rate of oxidation. However, some particles may catalyze oxidation. The catalytic influence depends on the metallurgy and the presence of water. Silicon, which is the primary element found in the earth's crust, is not highly catalytic to lubricant oxidation. On the other hand, iron and copper, the primary elements found in machine metallurgy, are highly catalytic to lubricant oxidation. The degree to which iron and copper particles influence the oxidation rate depends on the presence of water. The water reacts with the metal, forming peroxides and free radicals, which causes oxidation. Fortunately, the ingress of particle contamination is typically managed in the same manner as water contamination because it enters at the points where the machine interfaces with the machine. Similar to water, particles should be excluded. Unfortunately, because the machine generates its own particles, removal is required to maintain material balance. Numerous particle removal devices are available for use in industry, most notably filters. Filter quality and the decision to incorporate other particle removal technologies is application-specific.
An abridged version of this article called "Slick Lubrication Tips" was published in the February 2005 issue of Plant Services magazine.
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