Lithium grease is a highly common lubricant used extensively in automotive, industrial, and household applications. It is prized for its excellent water resistance and ability to operate under a wide range of temperatures. The substance is a thickened oil, typically consisting of 75% to 98% base oil, which is held in a semi-solid state by a lithium-based soap thickener. Its composition makes it combustible, but only under specific, extreme conditions.
Flammability and Ignition Thresholds
Lithium grease is classified as a combustible material because it contains a large percentage of petroleum-based or synthetic base oil, which is the component that can burn. The lithium soap thickener, which provides the structure, is not the primary fuel source. Under normal operating conditions, the grease presents a very low risk of fire and will not ignite easily.
Ignition requires the grease to be heated far beyond typical operating temperatures. The flash point is the minimum temperature at which the liquid gives off enough flammable vapor to ignite briefly when exposed to an external spark or flame. For standard lithium greases, this point is typically well over \(302^{\circ}\text{F}\) (\(150^{\circ}\text{C}\)) and can be as high as \(482^{\circ}\text{F}\) (\(250^{\circ}\text{C}\)).
To burn without an external ignition source, the grease must reach its autoignition temperature, which is significantly higher. This spontaneous ignition temperature is generally above \(599^{\circ}\text{F}\) (\(315^{\circ}\text{C}\)) and sometimes higher than \(550^{\circ}\text{C}\) (\(1022^{\circ}\text{F}\)). These high thresholds mean that a simple spark or brief application of heat will not start a sustained fire, and the grease is not considered a high fire risk during routine use.
Preventing Accidental Ignition
Preventing a fire involves controlling the grease’s exposure to heat and incompatible materials so it never reaches high ignition thresholds. Proper storage is a preventative measure, requiring containers to be kept in a cool, dry place away from direct sunlight or any heat source. Storage temperatures should ideally be maintained below \(122^{\circ}\text{F}\) (\(50^{\circ}\text{C}\)) to maintain stability.
In application, the grease must be kept away from high-temperature machinery, open flames, and potential spark sources, such as welding operations. Heat generation can also occur due to mechanical friction when moving parts are improperly lubricated or a bearing fails. Applying the correct amount of grease and regularly inspecting equipment helps prevent excessive friction that could generate enough internal heat to approach the autoignition point.
Contamination should also be avoided, as strong oxidizing agents can react with the base oil and additives, potentially lowering the required ignition temperature. Focusing on thermal management and cleanliness effectively mitigates the risk of accidental ignition.
Responding to a Lithium Grease Fire
Should a fire involving lithium grease occur, it is categorized as a Class B fire because the fuel source is the flammable base oil. Correct extinguishing agents are those designed for liquid hydrocarbon fires, including carbon dioxide, dry chemical powder, or fire-fighting foam. These agents work by smothering the fire or interrupting the chemical reaction of combustion.
It is important to avoid using a heavy stream of water, as this can cause the burning oil to splatter and spread the fire. While water fog or spray may be used to cool surrounding containers, direct application to burning grease can result in frothing and fire expansion. For small fires, dry chemical or carbon dioxide extinguishers are often the most accessible and effective tools.
A lithium grease fire is chemically distinct from a lithium-ion battery fire, which involves thermal runaway and requires specialized methods. Since the grease fire is fueled by the base oil, standard Class B protocols are appropriate. For any fire that cannot be easily contained, the immediate priority is to evacuate the area and contact emergency services.