Definition
Pour point is the lowest temperature at which an oil can flow under gravity. It is measured by ASTM D97 (conventional method) or ASTM D5985 (rotational method) and is expressed in °C. Pour point is the primary low-temperature pumpability limit for lubricants in applications where cold starts are expected.
What Causes High Pour Point?
Paraffinic mineral base oils contain long-chain wax molecules (n-alkanes) that crystallise as temperature drops. These wax crystals form an interlocking network that immobilises the oil — even when the bulk liquid phase is still present. The temperature at which this network sets is the pour point.
Naphthenic and severely hydrotreated Group III base oils have fewer wax components and inherently lower pour points. Synthetic PAO and ester base oils are inherently wax-free, with pour points as low as -50°C or below.
Pour Point Depressants (PPDs)
Pour point depressants are polymer additives — typically alkylated naphthalenes, polymethacrylates (PMA), or polyacrylates — that co-crystallise with wax and modify the crystal morphology. Instead of large interlocking platelets, the wax forms small, rounded crystals that do not gel the oil. A good PPD can depress pour point by 20–40°C at treat rates of 0.1–1.0%.
CheMost supplies pour point depressants for mineral base oil (paraffinic) applications, as well as for heavy and residual fuel oil treatment.
Pour Point vs. Cold Crank Viscosity
Pour point alone is not sufficient for engine cold-start performance. An oil can have a low pour point but still be too viscous to crank an engine. SAE J300 specifies Cold Crank Simulator (CCS) viscosity at low temperature as the definitive cold-start viscosity limit. The “W” (winter) grade (0W, 5W, 10W) is defined by the CCS viscosity at -35°C, -30°C, and -25°C respectively.