Basic Properties of Lubricating Oils and Fats

　　Lubricating oil is a technology-intensive product, consisting of a complex mixture of hydrocarbons, and its actual performance in service results from the combined effects of intricate physical and chemical processes. The fundamental properties of lubricating oil encompass general physicochemical characteristics, specialized physicochemical properties, and simulated bench‑test evaluations.
　　General physicochemical properties
　　Each category of lubricating grease possesses a set of common physicochemical properties that reflect the product’s intrinsic quality. For lubricating oils, these general physicochemical properties are as follows:
　　(1) Appearance (Color): The color of a lubricant often reflects its degree of refining and its stability. For base oils, the higher the refining level, the more thoroughly hydrocarbon oxides and sulfides are removed, resulting in a lighter color. However, even under identical refining conditions, base oils derived from different crude oil sources and grades may exhibit varying colors and levels of transparency. As for newly formulated finished lubricants, the use of additives has rendered color an unreliable indicator of the base oil’s refining quality.
　　(2) Density is the simplest and most commonly used physical property indicator of lubricating oils. The density of a lubricating oil increases with the amount of carbon, oxygen, and sulfur in its composition; therefore, at the same viscosity or the same relative molecular mass, lubricating oils rich in aromatics, resins, and asphaltenes have the highest density, those rich in cycloalkanes fall in the middle, and those rich in alkanes have the lowest density.
　　(3) Viscosity reflects the internal friction of a lubricant and serves as an indicator of its oiliness and fluidity. In the absence of any functional additives, higher viscosity corresponds to greater oil-film strength but poorer fluidity.
　　(4) Viscosity Index The viscosity index indicates the degree to which a lubricant’s viscosity varies with temperature. A higher viscosity index means that the lubricant’s viscosity is less affected by temperature, resulting in better viscosity‑temperature performance; conversely, a lower viscosity index reflects poorer performance.
　　(5) Flash Point The flash point is an indicator of a petroleum product’s volatility. The lighter the distillation fraction, the greater the volatility and the lower the flash point; conversely, the heavier the fraction, the lower the volatility and the higher the flash point. At the same time, the flash point serves as an index of the fire hazard associated with petroleum products. Products are classified into hazard categories based on their flash points: those with a flash point below 45°C are classified as flammable, while those above 45°C are classified as combustible. During storage and transportation, it is strictly prohibited to heat petroleum products to their flash-point temperature. For a given viscosity, a higher flash point is preferable. Therefore, when selecting a lubricant, users should take into account both the operating temperature and the service conditions. As a general guideline, a flash point that exceeds the operating temperature by 20–30°C is considered safe for use.
　　(6) Pour Point and Cloud Point The pour point is the highest temperature at which a petroleum product ceases to flow under specified cooling conditions. The solidification of a petroleum product differs significantly from that of a pure compound. Petroleum products do not have a well-defined solidification temperature; what is referred to as “solidification” merely means that, as a whole, the product loses its fluidity—its individual components do not all turn into solids. The pour point of a lubricant is an important quality indicator of its low‑temperature fluidity, with significant implications for production, transportation, and application. Lubricants with a high pour point cannot be used at low temperatures, while in regions with higher ambient temperatures there is no need to use lubricants with a low pour point. This is because the lower the pour point, the higher the production cost, leading to unnecessary waste. In general, a lubricant’s pour point should be 5–7°C below the lowest expected operating temperature. However, it is particularly important to note that when selecting a low‑temperature lubricant, one should comprehensively consider its pour point, low‑temperature viscosity, and viscosity‑temperature characteristics. Even oils with a low pour point may still fail to meet requirements for low‑temperature viscosity or viscosity‑temperature behavior. Both the pour point and the cloud point are indicators of a petroleum product’s low‑temperature fluidity; there is no fundamental difference between them, only a slight variation in the measurement method. For a given oil, the pour point and the cloud point are not identical; typically, the cloud point is 2–3°C higher than the pour point, though exceptions do occur.
　　(7) Acid value, alkalinity value, and neutralization value: The acid value is an indicator of the acidic substances present in lubricating oil, expressed in mg KOH/g. It is categorized into strong acid value and weak acid value; the sum of the two constitutes the total acid number (TAN). When we refer to “acid value” in common usage, we are actually referring to the total acid number (TAN). The alkalinity value indicates the content of basic substances in the lubricating oil, with units of mg KOH/g. It is likewise divided into strong alkalinity value and weak alkalinity value; their combined total is the total base number (TBN). In everyday parlance, “alkalinity value” typically refers to the total base number (TBN). The neutralization value encompasses both the total acid value and the total alkalinity value. However, unless otherwise specified, the term “neutralization value” generally denotes only the total acid value, also expressed in mg KOH/g.
　　(8) Water content refers to the percentage of water present in a lubricant, typically expressed as a weight percent. The presence of water in lubricants can compromise the oil film, reducing lubrication performance, accelerating the corrosive action of organic acids on metals, causing equipment corrosion, and promoting the formation of sludge. In short, the lower the water content in a lubricant, the better.
　　(9) Mechanical Impurities Mechanical impurities refer to insoluble precipitates or colloidal suspensions present in lubricating oils, which do not dissolve in solvents such as gasoline, ethanol, and benzene. These impurities are predominantly composed of sand, gravel, iron filings, and certain poorly soluble organometallic salts derived from additives. Typically, the level of mechanical impurities in base oils is kept below 0.005% (impurities at or below 0.005% are considered negligible).
　　(10) Ash and Sulfated Ash Ash refers to the non‑combustible residue remaining after incineration under specified conditions. Its composition is generally considered to consist of certain metallic elements and their salts. The meaning of ash varies depending on the type of lubricant: for base oils or oils without additives, ash can be used to assess the degree of refining; for oils containing metal‑salt additives (new oils), ash serves as a quantitative measure for controlling the amount of additive added. Abroad, sulfated ash is often used in place of total ash. This method involves adding a small quantity of concentrated sulfuric acid to the oil sample prior to incineration and ashing, thereby converting the metallic elements of the additives into their corresponding sulfates.
　　(11) Under specified test conditions, the dark‑brown carbonaceous residue formed after thermal evaporation and combustion of a petroleum product is referred to as residual carbon. Residual carbon is an important quality indicator for lubricant base oils and serves as a parameter for assessing the oil’s properties and the extent of its refining. In lubricant base oils, the amount of residual carbon depends not only on the oil’s chemical composition but also on its degree of refining; the primary constituents responsible for residual carbon formation are gums, asphaltenes, and polycyclic aromatic hydrocarbons. Under conditions of insufficient oxygen, these substances undergo intense thermal decomposition and condensation, yielding residual carbon. The deeper the refining process, the lower the residual carbon value. Generally speaking, a lower residual carbon value is preferable for blank base oils. However, many modern lubricants contain additives containing metals, sulfur, phosphorus, and nitrogen, which impart high residual carbon values; consequently, residual carbon measurements for such additive‑containing oils no longer reflect their original intended purpose. Mechanical impurities, water, ash content, and residual carbon are all quality indicators that assess the purity of the oil and indicate the degree of refining of the lubricant base oil.
　　Special physicochemical properties
　　In addition to the general physicochemical properties mentioned above, each lubricant should also possess specific physicochemical characteristics that define its service performance. The higher the quality requirements or the more specialized the application, the more pronounced these special physicochemical properties become. A brief overview of the test methods used to evaluate these specific properties is provided below:
　　(1) Oxidative Stability Oxidative stability characterizes the aging resistance of a lubricant. Many industrial lubricants with extended service lives are required to meet this specification, making it a distinctive performance criterion for such products. Numerous methods exist for determining oxidative stability; essentially, a defined quantity of lubricant is exposed to air (or oxygen) in the presence of metal catalysts, oxidized at a specified temperature for a set duration, and then analyzed for changes in acid value, viscosity, and the formation of precipitates. All lubricants exhibit varying degrees of auto‑oxidation propensity, depending on their chemical composition and the external conditions they encounter. During service, oxidation occurs, gradually producing aldehydes, ketones, acids, as well as gums and asphaltenes. Oxidative stability refers to the ability to inhibit the formation of these undesirable degradation products that can compromise lubricant performance.
　　(2) Thermal Stability Thermal stability refers to the oil’s ability to withstand high temperatures, that is, its resistance to thermal decomposition, expressed as the thermal decomposition temperature. Certain high‑quality anti‑wear hydraulic oils and compressor oils, among others, specify requirements for thermal stability. The thermal stability of a lubricant depends primarily on the composition of its base oil; many additives with low decomposition temperatures can adversely affect oil stability, and antioxidants do not significantly enhance thermal stability either.
　　(3) Oiliness and extreme-pressure properties: Oiliness refers to the formation of a robust physicochemical adsorption film on the metal surfaces at the friction interface, enabled by polar additives in the lubricant, thereby providing resistance to high loads and reducing friction‑induced wear. Extreme-pressure (EP) performance, on the other hand, arises when the polar components of the lubricant undergo tribochemical decomposition under conditions of high temperature and heavy load, reacting with the surface metal to form a low‑melting‑point, soft (or plastic) EP film that delivers superior lubrication under impact, high‑load, and elevated‑temperature conditions.
　　(4) Corrosion and rusting: Due to oxidation of the lubricant or the action of additives, steel and other nonferrous metals are often subject to corrosion. Corrosion testing typically involves immersing a copper strip in the oil at 100°C for 3 hours, followed by visual inspection of the copper surface. Rust‑prevention testing, on the other hand, assesses the tendency of steel to corrode under the influence of water and moisture; this is done by adding 30 mL of distilled water or artificial seawater to 300 mL of the test oil, then placing a steel rod in the mixture and stirring at 54°C for 24 hours, after which the presence or absence of rust on the steel rod is evaluated. Lubricants should exhibit both metal‑corrosion resistance and rust‑prevention properties; in industrial lubricant standards, these two parameters are generally mandatory test items.
　　(5) Anti‑foaming properties: During operation, lubricating oils often generate foam due to the presence of air, particularly when the oil contains surface‑active additives, which further promote foaming and make the foam more persistent. Foam formation in lubricants can disrupt the oil film, leading to scuffing or increased wear on friction surfaces, accelerate oxidation and degradation of the lubricant, and cause air lock in the lubrication system, thereby impairing oil circulation. Consequently, anti‑foaming performance is an important quality indicator for lubricants and related products.
　　(6) Hydrolytic Stability Hydrolytic stability characterizes the stability of a lubricant in the presence of water and metals (primarily copper). When the oil has a high acid value or contains additives that readily decompose in water to form acidic substances, this parameter often fails to meet specifications. The test procedure involves adding a specified amount of water to the sample oil, mixing and stirring it with a copper strip at a set temperature for a defined period, and then determining the acid value of the aqueous layer and the weight loss of the copper strip.
　　(7) Anti-emulsification: During service, industrial lubricating oils inevitably become contaminated with cooling water. If the oil exhibits poor anti-emulsification properties, it will form an emulsion with the entrained water, making it difficult to separate the water from the bottom of the circulating oil tank and potentially leading to inadequate lubrication. Therefore, anti-emulsification is a critical physicochemical property of industrial lubricants. For general lubricants, 40 mL of the test oil is vigorously stirred with 40 mL of distilled water at a specified temperature for a set duration, after which the time required for the oil–water–emulsion layers to separate into 40 mL, 37 mL, and 3 mL, respectively, is recorded. For industrial gear oils, the test oil is mixed with water, agitated at a specified temperature and at 6,000 rpm for 5 minutes, allowed to stand for 5 hours, and then the volumes of the oil, water, and emulsion layers are measured.
　　(8) The air‑release value is a requirement specified in the hydraulic‑oil standard, because in hydraulic systems, if the air dissolved in the oil cannot be released promptly, it will compromise the accuracy and responsiveness of hydraulic transmission; in severe cases, this may render the system unable to meet its operational requirements. The method for determining this property is similar to that used for measuring foam resistance, except that it measures the time required for air (in the form of fine bubbles) dissolved in the oil to be released.
　　(9) In hydraulic systems, rubber is predominantly used as a sealing material. In machinery, lubricants inevitably come into contact with various seals; oils with poor rubber compatibility can cause rubber to swell, shrink, harden, or crack, thereby compromising its sealing performance. Consequently, lubricants are required to exhibit good compatibility with rubber. Hydraulic oil standards specify a rubber‑sealability index, which is determined by measuring the dimensional changes of a rubber ring after immersion in the oil for a prescribed period.
　　(10) Shear Stability: During service, the high‑molecular‑weight polymers in oils containing viscosity‑index improvers are subjected to mechanical shear, leading to chain scission and a reduction in oil viscosity, which can impair normal lubrication. Consequently, shear stability is a critical, specially required physicochemical property for such oils. Numerous methods exist for determining shear stability, including ultrasonic shear, nozzle shear, Wicks pump shear, and FZG gear‑test shear; ultimately, all these methods measure the rate of viscosity loss in the oil.
　　(11) Dissolving Power: Dissolving power is typically expressed in terms of the aniline point. The aniline point—the threshold at which a lubricant can dissolve a composite additive—varies with the grade of the oil: low‑ash oils exhibit higher values than overbased oils, and single‑grade oils have higher values than multigrade oils.
　　(12) The volatility of base oils affects fuel consumption, viscosity stability, and oxidation stability. These properties are particularly important for multigrade and fuel‑saving oils.
　　(13) Rust‑prevention performance refers specifically to the unique physicochemical properties that rust‑preventive greases must possess. Test methods include the humidity test, the salt‑spray test, the stacking test, and the water‑displacement test; additional tests comprise the louvre‑box test and the long‑term storage test, among others.
　　(14) Electrical Properties Electrical properties are characteristic of insulating oils and include the dielectric loss tangent, dielectric constant, breakdown voltage, and impulse voltage, among others. The degree of refining of the base oil, as well as impurities and moisture content, all significantly affect the electrical performance of the oil.
　　(15) Special Physicochemical Properties of Greases In addition to their general physicochemical properties, specialty greases also exhibit specific characteristics tailored to particular applications. For example, greases with excellent water resistance must undergo water‑immersion testing; low‑temperature greases are evaluated for low‑temperature torque; multi‑purpose greases are tested for extreme‑pressure and anti‑wear performance as well as rust‑prevention; and long‑life greases are subjected to bearing‑life tests. Corresponding test methods have been established for assessing these properties.
　　(16) Other Special Physicochemical Properties In addition to their general properties, each type of lubricant should possess its own unique special characteristics. For example, quenching oils must be tested for cooling rate; emulsifiable oils must be evaluated for emulsion stability; hydraulic guide‑rail oils must be assessed for creep‑resistance; spray lubricants must be measured for oil‑mist dispersion; refrigeration compressor oils must be tested for cloud point; and low‑temperature gear oils must be evaluated for pour point, among others. These specific properties typically require either a distinctive chemical composition of the base oil or the addition of particular additives to ensure they are met.