Viscosity

Viscosity depends strongly on temperature. Most liquids become thinner when warmed and thicker when cooled, so the SDS value should come with a temperature. It can be reported as dynamic viscosity or kinematic viscosity, which use different units, and suppliers vary in how they present it. Do not compare numbers without checking the type and the units.

For safety, viscosity affects spills and exposure. A thin liquid spreads over a large area in seconds and soaks into porous materials, reaching drains and cracks. A thick liquid stays put and is easier to contain, though it can be slow to clean. It also influences skin contact: a runny liquid soaks through clothing quickly.

There is a specific health reason to care about it. For hydrocarbon liquids, low viscosity is one of the criteria used when assessing Aspiration hazard. If a thin liquid is swallowed and then enters the airways during vomiting, it can spread deep into the lungs and cause serious damage, even from a small amount.

Viscosity also matters in processes: pump selection, hose sizing, transfer rates and how long it takes a container to drain. Heated viscous materials may become thin enough to splash or leak from seals that held when cold.