Cold flow refers to two different phenomena depending on context. In materials science, it describes the slow, permanent deformation of a solid material under constant stress, even at room temperature. In the fuel industry, it describes how diesel and other fuels lose their ability to flow as temperatures drop and wax crystals form. Both meanings share a core idea: a material gradually stops behaving the way you need it to under specific conditions.
Cold Flow in Materials Science
In engineering and materials science, cold flow is another name for creep: the tendency of a solid material to slowly deform under persistent mechanical stress. Unlike bending or breaking from a sudden force, cold flow happens gradually over days, months, or years. A rubber gasket compressed between two metal flanges, for example, will slowly thin out and lose its seal. The material isn’t melting or being heated. It’s just responding to sustained pressure by slowly changing shape.
The term “cold” distinguishes this from high-temperature creep in metals, which requires significant heat to occur. Polymers and soft materials like rubber, PTFE (Teflon), and certain plastics experience cold flow at or well below room temperature. In polymers, the long molecular chains can slide past one another under load, gradually redistributing material away from the stressed area. This process is not recoverable: once the material has flowed, it doesn’t spring back to its original shape when the load is removed.
Where Cold Flow Causes Problems
Cold flow is a major concern in seals, gaskets, and valve seats. PTFE is widely used in valves because of its excellent chemical resistance and low friction, but it is particularly prone to cold flow. Under the constant compressive force of a pipeline, a PTFE valve seat will slowly deform, creating gaps that cause leaks. This can happen even in applications where temperatures and pressures seem modest.
Engineers address this in several ways. One common solution is reinforced PTFE, where filler materials like glass fiber (typically around 15%) are blended into the PTFE resin before molding. These fillers physically block the polymer chains from sliding past one another, significantly improving resistance to cold flow compared to pure PTFE. In valve design, trunnion-mounted ball valves use springs to gently press the seats against the ball, so the seats never bear the full crushing force of pipeline pressure. This makes cold flow practically nonexistent in those applications.
When pressure and temperature demands exceed what even reinforced PTFE can handle, engineers switch to harder thermoplastics that are inherently resistant to cold flow. PEEK (polyether ether ketone) can handle temperatures up to 260°C at extremely high pressures. Specialized crystalline nylon compounds are also used in oil and gas applications where cold flow resistance and wear resistance are critical. Chemical additives called blocking agents can also be used to inhibit cold flow in plastics during storage and handling.
Cold Flow in Diesel Fuel
In the fuel industry, “cold flow” refers to something entirely different: how well diesel fuel moves through lines, filters, and injectors as temperatures drop. Diesel fuel contains paraffin wax compounds that are dissolved and invisible at warm temperatures but begin to crystallize as the fuel cools. These wax crystals can clog fuel filters, block injector lines, and eventually cause the fuel to gel into a semi-solid that won’t flow at all.
Cold flow properties are measured by three key temperatures. The cloud point is the temperature at which the first tiny, visible wax crystals appear in the fuel. The cold filter plugging point (CFPP) is the temperature at which those crystals have grown and clumped together enough to actually block a fuel filter. The pour point is the temperature at which the fuel has solidified so much it stops flowing entirely. For practical purposes, the CFPP is the most useful number because it predicts the lowest temperature at which a fuel will give trouble-free flow in a real engine’s fuel system.
Industry testing follows standardized methods. ASTM D6371 is the standard test for determining CFPP in diesel and heating fuels. For European light-duty trucks, the CFPP result typically matches the real-world failure temperature closely, though certain fuel system designs (like paper filters exposed to weather) can cause problems at slightly higher temperatures than the test predicts. A general guideline is that if the CFPP is more than 12°C below the cloud point, the test may not fully reflect real-world performance.
How Cold Flow Improvers Work
Cold flow improvers are fuel additives designed to keep diesel usable in cold weather. They don’t prevent wax crystals from forming, but they change how those crystals grow and behave. Untreated wax crystals form flat, plate-like shapes that stack together and quickly clog filters. Cold flow improvers transform the crystal shape from plates to thin needles, which pass through fuel filters and injectors without causing blockages.
The most common additive chemistry uses ethylene vinyl acetate (EVA) copolymers, though different crude oil sources produce diesel with different hydrocarbon compositions, so a range of low molecular weight polymers with varying structures are used to match specific fuels. Other formulations combine olefin-ester copolymers with dispersants that keep modified crystals from clumping back together. In most cases, these additives depress the pour point and delay the agglomeration of wax crystals, buying significant extra degrees of cold-weather operability. They typically have no measurable effect on engine performance itself, only on the fuel’s ability to reach the engine.
Which Meaning Applies to You
If you’re dealing with seals, gaskets, O-rings, or any plastic component under constant load, cold flow means your material is slowly deforming and you need a more resistant material or a design that reduces sustained pressure. If you’re dealing with diesel engines, fuel storage, or winter driving, cold flow means your fuel is at risk of gelling and you need to know its cloud point and CFPP relative to the temperatures you expect. The two phenomena are unrelated physically, but both describe a material gradually failing to do its job under conditions that push it past its limits.