Diesel fuel is a complex mixture of petroleum-derived hydrocarbon chains and a non-polar solvent. Its interaction with plastic is often misunderstood; it does not “melt” plastic through heat, which is a physical phase change. Instead, the interaction is a chemical process of solvency and degradation. The outcome is highly variable and depends entirely on the specific type of polymer the plastic container is made from. Understanding the safety and compatibility of storing diesel requires looking into the molecular reality of how these substances interact.
The Chemical Reality Diesel Is Not Melting Plastic
The relationship between diesel and plastic is governed by the concept that “like dissolves like.” Diesel is a non-polar organic solvent that readily interacts with and degrades other non-polar substances, including many types of plastics. This process is chemical degradation, not melting, which is a structural breakdown occurring at a material’s thermal transition temperature.
When non-polar diesel molecules encounter the polymer chains of plastic, they penetrate the material’s matrix. This penetration causes swelling, where the plastic absorbs the solvent and expands, softening its structure. The material’s mechanical integrity is compromised because the diesel weakens the intermolecular forces holding the polymer chains together.
This chemical attack can lead to stress cracking, where internal or external forces on the swollen, weakened plastic cause microscopic fissures to form and propagate. In vulnerable plastics, the diesel acts as a plasticizer leach, pulling out small molecules added to increase flexibility. The removal of these plasticizers causes the material to become brittle, leading to structural failure.
Understanding Plastic Compatibility with Diesel Fuel
A plastic’s ability to resist diesel depends heavily on its molecular structure, specifically its density and degree of crystallinity. Highly crystalline plastics have tightly packed, ordered polymer chains that are difficult for diesel molecules to penetrate, offering strong chemical resistance. Conversely, amorphous plastics have a looser, random structure that allows the solvent to permeate and degrade the material quickly.
Highly Resistant Plastics
High-Density Polyethylene (HDPE) is the most common plastic accepted for diesel storage due to its high crystallinity and low branching. Containers made from HDPE (resin code “2”) exhibit excellent resistance to swelling and chemical attack at ambient temperatures. Other highly resistant plastics include:
- Polytetrafluoroethylene (PTFE), commonly known as Teflon, possesses high chemical resistance due to its strong carbon-fluorine bonds, making it impervious to diesel.
- Nylon demonstrates excellent resistance to oils and fuels and is often used for fuel lines and fittings, as its chemical structure is unaffected by non-polar hydrocarbons.
Low Resistance/Vulnerable Plastics
Vulnerable plastics typically have a high amorphous content or rely on plasticizers for flexibility.
- Polyvinyl Chloride (PVC) is vulnerable because it often contains plasticizers that diesel can dissolve and leach out, causing the remaining polymer to become brittle and crack.
- Polystyrene (PS) is a highly amorphous plastic quickly attacked by non-polar solvents like diesel, resulting in rapid softening, swelling, and dissolution.
- Acrylics are susceptible to chemical attack from hydrocarbons, which can cause crazing—the formation of a network of fine cracks—and structural failure when exposed to diesel.
Safe Handling and Storage of Diesel
To ensure safety and prevent material failure, diesel must only be stored in containers explicitly approved for petroleum products. Approved containers are typically constructed of robust, high-grade HDPE or steel and carry specific regulatory markings. Look for containers with clear Department of Transportation (DOT) or United Nations (UN) certification codes, confirming they have passed tests for hazardous material transport. Diesel containers are typically coded yellow to prevent mistaking the fuel for gasoline, which is stored in red cans.
Avoid using unapproved household plastics, such as common water bottles or food-grade storage bins, even for temporary storage. These items are not designed to resist hydrocarbon penetration, and the diesel will quickly degrade them, potentially leaking fuel and contaminating the engine with leached chemicals. Even in approved HDPE drums, the fuel can react with the polymer over time, so the container’s seal must be regularly checked for signs of swelling or cracking. For long-term storage, experts advise keeping diesel for no more than six months, regardless of the container material, to mitigate degradation.