In addition to general physical and chemical properties, each type of lubricating grease should also possess specific physical and chemical properties that characterize its application. The higher the quality requirements or the more specialized the oil, the more prominent its specific physical and chemical properties will be.
1. Oxidation Stability
Oxidation stability indicates the anti-aging performance of lubricating greases. This indicator is required for many long-service industrial lubricating greases, making it a specific performance requirement for these types of oils. There are many methods for determining the oxidation stability of oils. Basically, a certain amount of oil is oxidized for a certain period of time at a certain temperature in the presence of air (or oxygen) and a metal catalyst, and then the acid value, viscosity change, and precipitation formation are measured. All lubricating greases have different auto-oxidation tendencies depending on their chemical composition and external conditions. Oxidation occurs during use, gradually forming substances such as aldehydes, ketones, acids, gums, and asphaltenes. Oxidation stability is the property of inhibiting the formation of these substances that are detrimental to the use of the oil.
2. Hydrolytic Stability
Hydrolytic stability characterizes the stability of oils under the influence of water and metals (mainly copper). When the acid value of the oil is high, or when it contains additives that easily decompose into acidic substances in water, this indicator often fails to meet the requirements. The testing method involves adding a certain amount of water to the test oil, mixing and stirring it for a certain time under a copper sheet and at a specific temperature, and then measuring the acid value of the water layer and the weight loss of the copper sheet.
3. Demulsibility
Industrial lubricating greases often inevitably mix with some cooling water during use. If the grease has poor demulsibility, it will form an emulsion with the mixed water, making it difficult for the water to drain from the bottom of the circulating oil tank, potentially causing poor lubrication. Therefore, demulsibility is a very important physicochemical property of industrial lubricating greases. For general oils, 40 ml of test oil and 40 ml of distilled water are vigorously stirred at a certain temperature for a certain time, and then the time it takes for the oil layer, water layer, and emulsion layer to separate into 40-37-3 ml is observed. For industrial gear oils, the test oil and water are mixed, stirred for 5 minutes at a certain temperature and 6000 rpm, left for 5 hours, and then the milliliters of oil, water, and emulsion layers are measured.
4. Thermal Stability
Thermal stability indicates the oil’s ability to withstand high temperatures, specifically its resistance to thermal decomposition, i.e., its thermal decomposition temperature. High-quality anti-wear hydraulic oils and compressor oils have specific thermal stability requirements. The thermal stability of an oil primarily depends on the composition of its base oil. Many additives with low decomposition temperatures often have a negative impact on oil stability; antioxidants also do not significantly improve thermal stability.
5. Anti-foaming Properties
During operation, grease often produces foam due to the presence of air, especially when the oil contains surface-active additives. This foam is also more likely to form and is difficult to eliminate. Foaming during grease use can damage the oil film, causing sintering or increased wear on friction surfaces, promoting grease oxidation and deterioration, and creating air resistance in the lubrication system, affecting its circulation. Therefore, anti-foaming properties are an important quality indicator for greases.
6. Air Release Value
This requirement is included in hydraulic oil standards because if dissolved air in the hydraulic system cannot be released in time, it will affect the accuracy and sensitivity of hydraulic transmission, and in severe cases, fail to meet the operating requirements of the hydraulic system. The method for measuring this property is similar to that for anti-foaming properties, but it measures the time it takes for dissolved air (mist) to be released from the oil.
7. Rubber Sealing Performance
Rubber is commonly used as a seal in hydraulic systems. In machinery, the oil inevitably comes into contact with some seals. Oil with poor rubber sealing performance can cause the rubber to swell, shrink, harden, and crack, affecting its sealing performance. Therefore, good compatibility between the oil and rubber is required. Hydraulic oil standards require a rubber sealing performance index, which is measured by the change in a rubber ring of a certain size after immersion in oil for a certain period of time.
8. Oiliness and Extreme Pressure Properties
Oily properties refer to the formation of a robust physicochemical adsorption film by the polar components of the grease on the metal surface of the friction points, thus providing resistance to high loads and frictional wear. Extreme pressure properties, on the other hand, refer to the decomposition of the polar components of the grease on the metal surface of the friction points under high temperature and high load conditions, resulting in a tribochemical reaction with the surface metal to form a low-melting-point, soft (or malleable) extreme pressure film, thus providing impact resistance and high-load, high-temperature lubrication.
9. Corrosion and Rust
Due to oxidation or the action of additives in oils, corrosion of steel and other non-ferrous metals is common. Corrosion tests typically involve immersing a copper strip in oil at 100°C for 3 hours and then observing the changes in the copper. Rust tests, conducted under the influence of water and moisture, cause rust to form on the steel surface. To determine rust resistance, 30 ml of distilled water or artificial seawater is added to 300 ml of test oil, and a steel rod is placed inside, stirred at 54°C for 24 hours, and then the presence or absence of rust is observed. Oils should have the ability to resist metal corrosion and prevent rust. In industrial grease standards, these two items are usually mandatory tests.
