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"I am trying to track down information on lubricants that could be used on equipment that will come in direct contact with river water. For instance, what lubricants are used to lubricate dredging equipment or other types of machinery that will come in direct contact with river or lake water? I am thinking some sort of food-grade grease or oil would be a good place to start, but I'm wondering if there are any guidelines that can be followed or any sort of products that are specifically used for these types of projects."
There are guidelines for the selection of products that you know will be coming in contact with the environment. Keep in mind that food-grade and eco-friendly lubricants are not the same. They share some common traits, but there are some subtle differences.
It sounds as if you are wanting a lubricant that can be classified by the ISO 6743-4 standard. This is the classification for biodegradable hydraulic fluids, which are used in most equipment that comes in contact with a river or lake. There are three main classifications under ISO 6743-4. They are triglycerides (vegetable oils), synthetic esters and polyglycols.
Triglycerides are unsaturated ester oils from vegetables. Overall, they have adequate lubricating properties such as an extremely high viscosity index and high flash points. While they outperform mineral oils in a few properties, they fail miserably at others. In their natural form, they have almost no oxidative resistance and must be modified with additives to have any hope of lasting. They also have very poor (high) pour points, so their use in cold environments is very limited without using additives.
Synthetic esters are made from organic acids and alcohols. Originally formulated as a replacement for triglycerides, they perform better in nearly every performance criteria. They work better at both higher and lower temperatures, have low volatility and a higher lubricity all while maintaining decent biodegradability.
Polyglycols offer many of the same benefits as synthetic esters but do not react with water as readily as the esters. They have excellent lubricity and high viscosity indexes, and perform well at both high and low temperatures. One specific property of polyglycols that is beneficial in terms of the environment is that they are nearly nontoxic in aquatic environments like rivers and lakes. Their one downfall is that they are not compatible with hydrocarbon-based fluids.