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An oil is a bundle of performance properties and its requirements vary by application (Figure 1). Degraded properties compromise the lubricant’s ability to minimize friction, wear and corrosion, thus placing machinery reliability at risk. A lubricant requires properly designed and executed maintenance responses.
Reasons to change
Oil doesn’t last forever. It must be changed or otherwise maintained when it loses the properties that protect the machine, the application and the environment. Oxidative, thermal and hydrolytic degradation permanently change the base oil’s chemical and physical properties, thus altering the lubricant’s performance. In other cases, the additive package becomes depleted or the oil becomes contaminated with material that can’t be removed.
Base oil degradation
Oxidation occurs when oxygen reacts with the base oil, typically a hydrocarbon. Oxidized hydrocarbon molecules are transformed into acid and sludge. The degree to which oil is aerated and the presence of water and reactive metals, such as iron and copper, influences the oxidation rate. Oxidation-inhibiting additives sacrifice themselves to protect the base oil from oxidation.
Thermal degradation occurs when oil comes into contact with very hot machine surfaces or compressed bubbles, such as in hydraulic systems. Thermal failure leaves carbon-rich sludge and deposits. Thermal failure doesn’t produce acid, but it generates deposits that affect the oil’s performance. In some cases, the hydrocarbon chain is cracked into smaller units, which reduces the average molecular weight and its viscosity.
Hydrolysis is the reaction of the oil with water, which permanently modifies its molecular structure. Ester-based lubricating oils, including dibasic acid ester, polyol ester and phosphate ester, are most susceptible to hydrolysis. When exposed to water, esters readily hydrolyze back to the alcohols and acids from which they were synthesized. Many lubricants and hydraulic fluids use esters as the primary base oil component or as a co-base oil to improve the solubility and seal performance of highly refined mineral or synthetic oils.
Oil additives can enhance desirable performance properties, suppress undesirable properties or impart new properties. With use, additives become depleted and must be restored by oil change, sweetening with a partial drain-and-fill or by lubricant reclamation, which ostensibly restores the oil to like-new condition. The rate at which additives deplete depends on its type and environmental conditions, particularly temperature and presence of water. Some additives separate from the base oil at low temperatures. Many additives are susceptible to hydrolysis in the presence of water.
One way to extend life is to select lubes formulated using premium base oils, premium additives or both. The American Petroleum Institute’s (API) standard classification for base oils, called groupings, is intended to reflect oil quality. Groups I, II and III are refined mineral base oils. Group IV covers synthesized hydrocarbon base oils, such as polyalphaolefin (PAO), the most common example.
Group V includes everything that’s not in Groups I, II, III or IV, and includes dibasic acid ester, polyol ester, poly glycol, phosphate ester and a host of other base oils. With a range of base oils included in Group V, API set no specific requirements for this group except that the oil can’t be included in Groups I-IV.
Viscosity index (VI) relates oil viscosity to temperature. A lube with a high viscosity index is favored because it functions across a greater range of temperatures. Generally, high-VI base oil exhibits lower viscosity at cold start and higher viscosity at full operating temperature than low-VI base oils. API Group I has the lowest viscosity index, Group IV the greatest, with Groups II and III in between. Group IV (PAO) base oils generally have a higher viscosity index than Groups I, II or III. The viscosity index in Group V varies.
A mineral base oil that’s highly refined to reduce or eliminate unsaturated molecules resists oxidation and thermal degradation more effectively than a less-refined base oil. This is why the oxidative and thermal life of Group III base oil is superior to that of Group II, which is superior to that of Group I. Refining, however, has a cost. Highly unsaturated base oils don’t dissolve additives effectively and they tend to shrink elastomeric seals. Many Group II, III and IV base oils are formulated with co-base oil, such as diester or polyol ester to improve additive solubility and offset seal shrinkage problems.
Most mineral base oils contain sulfur. Group I has more than Group II or III. Group IV PAO is sulfur-free. Sulfur improves oxidation resistance and natural lubricity. Formulators prefer to start with low-sulfur base oil and add sulfur for the concentration and chemical form they believe is appropriate for the application.
Users often assume premium lubricant means a synthetic base oil. Not every synthetic base oil possesses each advantageous property and, moreover, it might not be required. For instance, Group IV base oil isn’t required for a machine that operates 24 hours a day, seven days a week at a constant temperature. Likewise, there are detrimental aspects associated with synthetic base oil that must be considered in the decision. If you can’t make the decision yourself, seek expert advice.
Base oil is only one relevant variable and additives aren’t all created equal. Some are better than others and these cost more. Many suppliers use additives to formulate specialty lubricants. Naturally, expensive additives, small batches, special sales and application engineering services add cost.