Grease, composed of fatty acids, esters, or hydrocarbons, is viscous or semi-solid at room temperature. It does not simply “dry out” like water evaporating. The change in its physical state, often resulting in a hard, sticky, or solid residue, is the result of irreversible chemical reactions known as degradation. This process transforms the grease’s molecular structure over time. The material’s original properties of lubricity or fluidity are lost as its component molecules break down and recombine into less desirable compounds.
The Chemical Process That Causes Grease to Harden
The primary mechanism responsible for the hardening of grease is a chain reaction called oxidation. This process begins when molecules within the grease react with oxygen present in the surrounding air, creating highly unstable molecules known as free radicals.
These free radicals are extremely reactive and immediately seek to stabilize themselves by reacting with other nearby grease molecules, propagating the degradation cycle. This leads to the second major step: polymerization. During polymerization, the broken molecular fragments link together to form much larger, heavier, and longer molecular chains.
As these molecules grow in size, they lose their ability to flow freely, which causes the viscosity of the grease to increase dramatically. This molecular linking ultimately transitions the grease from a liquid or semi-solid state into a rigid, cross-linked solid, often appearing as a hard varnish or gum-like sludge.
Variables That Accelerate or Slow Degradation
The rate at which oxidation and subsequent polymerization occur is highly dependent on environmental and compositional factors. Elevated temperatures significantly accelerate the chemical reactions responsible for degradation. For instance, for every 18-degree Fahrenheit (10-degree Celsius) increase in temperature, the rate of oxidation can roughly double.
Direct exposure to oxygen is necessary for the initial oxidation reaction. This means a grease layer spread thinly over a surface degrades much faster than a large, contained mass. Light, particularly ultraviolet (UV) radiation, acts as a catalyst by supplying the energy needed to generate the first free radicals, effectively kick-starting the process.
The inherent chemical structure of the grease components also plays a significant role. Fats and oils containing a high proportion of unsaturated fatty acids are much more susceptible to oxidation. These molecules have double bonds in their carbon chains, which are weaker points for oxygen to attack, making them less chemically stable than their saturated counterparts.
How Different Types of Grease Change State
The practical outcome of this chemical degradation differs based on the grease’s original application, whether it is for cooking or for mechanical lubrication. In the context of cooking fats and oils, oxidation leads to rancidity, which is the unpleasant change in smell and flavor caused by the formation of volatile compounds like aldehydes and ketones. The persistent, sticky film found on kitchen surfaces, such as exhaust fan filters or cabinet doors, is the solid product of polymerization, an oxidized residue known as varnish.
Lubricating greases, which are typically a base oil suspended in a thickener, degrade in a more complex manner. Oxidation first targets the base oil, causing it to darken and its viscosity to increase as heavy polymer chains begin to form. The acidic byproducts created during this chemical breakdown can then attack and damage the thickener structure, causing it to either soften and bleed oil or harden into a brittle crust.
When the base oil separates from the thickener, the grease loses its structural integrity and ability to lubricate machinery effectively, leading to mechanical failure. The hardened residue that forms in mechanical applications, often due to localized high heat, prevents the proper flow of the lubricant and causes significant wear on moving parts.