Silicone is a versatile synthetic polymer, often chosen for applications requiring high thermal stability, flexibility, and general inertness. Chemically known as polysiloxanes, these materials have a backbone of alternating silicon and oxygen atoms, providing resilience across a wide temperature spectrum. Determining if silicone is oil-resistant requires understanding chemistry and material science, as the answer depends heavily on the specific type of oil involved.
Silicone’s General Resistance to Hydrocarbon Oils
Standard silicone rubber (VMQ) exhibits poor resistance when exposed to non-polar, hydrocarbon-based oils. Fluids such as petroleum-based motor oils, mineral oils, gasoline, and diesel fuel cause material degradation. This incompatibility makes standard silicone unsuitable for seals in automotive engines or industrial machinery where contact with these common oils is expected.
The polymer demonstrates excellent resistance to polar fluids, including water, steam, and high-molecular-weight alcohols. Silicone also resists other silicone-based oils and greases because they are chemically identical. However, exposure to typical lubricating or motor oils, which are non-polar hydrocarbons, leads to rapid failure in standard silicone components. The practical consequence is significant softening and swelling, rendering the part mechanically useless for sealing or structural purposes.
The Mechanism of Swelling and Degradation
The failure of standard silicone elastomers in hydrocarbon oils is explained by the chemical principle of “like dissolves like.” Silicone’s polysiloxane backbone is bonded to non-polar organic side groups, typically methyl groups. This non-polar character makes the silicone matrix chemically similar to the non-polar hydrocarbon molecules found in petroleum-based oils.
When the silicone elastomer is immersed, oil molecules are drawn into the polymer network. The oil acts as a solvent, permeating the material and breaking the weak intermolecular bonds holding the polymer chains together. This process, known as swelling, causes the elastomer’s volume to increase significantly, often by 50% or more.
As the oil is absorbed, the material’s structural integrity is compromised, leading to a reduction in mechanical properties such as tensile strength and hardness. This loss of physical strength means the material can no longer maintain a seal or withstand mechanical stress. The degree of swelling is directly related to the chemical similarity between the oil and the silicone’s side groups, confirming the incompatibility.
Specialized Grades and Alternative Materials
For applications requiring silicone’s benefits, such as its wide temperature range and flexibility, alongside oil resistance, Fluorosilicone (FVMQ) is used. This specialized grade is manufactured by replacing some standard methyl side groups with trifluoropropyl groups, incorporating fluorine atoms into the polymer structure. The introduction of fluorine significantly alters the material’s solubility, making it highly resistant to non-polar hydrocarbon fuels and oils.
Fluorosilicone is specified for use in aerospace and automotive fuel systems because it combines silicone’s excellent temperature stability with enhanced fluid resistance. However, this chemical modification involves trade-offs compared to standard silicone. FVMQ is typically more expensive and exhibits lower tear strength and abrasion resistance, making it less mechanically robust.
When superior oil resistance is needed without silicone’s extreme temperature range, non-silicone elastomers offer practical alternatives. Nitrile Rubber (NBR), or Buna-N, is a cost-effective solution widely used for general petroleum-based oil and fuel resistance at moderate temperatures. For environments involving high temperatures or exposure to harsh chemicals, Fluoroelastomers, such as FKM (Viton), provide a higher level of resistance.