Adding polymer into grease can significantly enhance its properties like consistency, water resistance and shear stability. In this work, a selection of polymers with different structures was studied with the objective of in improving the water spray off properties of grease. Two types of lithium grease were used: mineral oil-based and vegetable oil-based. It is found that by adding 1% of a high molecular weight polyisoprene in vegetable oil-based grease, the weight loss percentage of the grease in water spray test decreased from 83% to 17%; and by adding of 1% of an ethylene/propylene copolymer in mineral oil-based grease, the weight loss percentage decreased from 73% to 15%. The water resistance improvement after adding polymer is due to the formation of the interpenetration networks (IPNs) by polymer and soap thickener. The polymer network can be formed by the physical crosslinking via crystalline phase, less soluble hard phase, hydrogen bonding, or by the long chain entanglement. Due to the chemistry difference between mineral oils and vegetable oils, the compatibility of polymer with base fluid varies as a result of its chemical structure. Therefore, their performance in grease varies with the base fluid of the grease.
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