1. The important significance of the water resistance of lubricating grease

Lubricating grease with good water resistance ensures that it can still provide good lubrication in the presence of water, while lubricating grease with poor water resistance is not suitable for parts in contact with water. For example, the continuous steel casting process in the steel rolling industry is characterized by a large amount of cooling water directly sprayed onto the hot steel billets. At this time, not only does a large amount of water enter the bearings, but also a large amount of water vapor is generated and exists on the surface of the bearings. The lubricating grease is always in a high temperature and high humidity state. If the water resistance of the grease is poor, it is easy to be washed away and lose its lubricity.

For the vast majority of usage areas, lubricating grease is required to have good water resistance. But some parts require the opposite, such as the lubricating grease used on knitting machines and sewing machines, which often hope to have water solubility. When grease splashes onto fabric, it can be easily washed away through a rinsing process to remove oil stains, while lubricating grease with good water resistance is not easy to remove oil stains. Therefore, when selecting lubricating grease, it is necessary to be flexible.

2. Moisture content and water resistance of lubricating grease

Lubricating grease itself contains water, which can be divided into two types. One is structural water, where water acts as a stabilizer in the grease and has a significant impact on the formation and properties of the grease structure. Another type is free water, which is unwanted in lubricating grease and must be restricted. But the water resistance of lubricating grease refers to the object, and water comes from the outside. Water resistance refers to the ability of lubricating grease to remain insoluble, non-emulsified, not easily absorb moisture from surrounding media, not be washed away by water, and not significantly change its own properties when in contact with water.

The water resistance of lubricating grease mainly depends on the thickener, followed by the base oil. Generally speaking, lubricating grease based on silicone oil has better water resistance, followed by mineral oil and ester oil. Polyether oils have poor water resistance. For thickening agents, urea based lipids and hydrocarbon based lipids have the best water resistance, followed by aluminum, barium, calcium and composite based lipids. Sodium based lipids and composite sodium based lipids have the worst water resistance.

3. Summary of Evaluation Methods for Water Resistance of Lubricating Greases

Method for determining the water resistance of lubricating grease: SH/T0109. This standard refers to the ASTM D1264 method for determining the water resistance of lubricating grease. The plan of the water immersion testing machine is shown in Figure 1.

During the experiment, 4g ± 0.5g of the sample was loaded into a 204 type ball bearing and rotated at a speed of 600r/min ± 30r/min. The water temperature was controlled at 38 ℃ ± 2 ℃, and sprayed onto the protective plate of the bearing sleeve at a flow rate of 5ml/s ± 0.5ml. The water resistance of the lubricating grease was measured by the amount of grease washed away by water within 1 hour.

4. Other evaluation methods for the water resistance of lubricating grease

Simplified judgment method. Apply a thin layer of lubricating grease onto a 1mm thick glass plate, then immerse it in distilled water at 25 ℃, 50 ℃, and 90 ℃ for 5 hours, observe the changes in the lubricating grease coating and distilled water, and make a judgment.

Water addition shear test method. Add 10% distilled water to the sample for 10000 or 100000 shearing cycles, and finally measure the cone penetration of the lubricating grease after shearing. The test results are expressed as the cone penetration value after shearing. For example, the water resistance of General Motors lithium grease is specified to have a cone penetration value of no more than 375 after water addition and shearing.

Stability of lubricating grease in hot water FS791B3463.1. Put about 500ml of distilled water in a beaker, heat it to slight boiling, dip a small piece of sample about 5g with one end of a stirring rod and soak it in boiling water for 10 minutes, then check the water and record whether the water is turbid or if there are other signs of sample emulsification.

Determination method for water resistance and water ethanol solution resistance of lubricating grease FS791B5415. Put 200ml of distilled water in one container and 200ml of ethanol distilled water in the other container. Place two small lipid clusters into each container, plug them tightly with stoppers, let them stand for one week, shake each container once or twice, and then visually observe the disintegration of the lipid clusters in the container.