Paint viscosity

Viscosity is the main physical property of fluids. The fluid flows and deforms under the action of external force . Viscosity is an item that expresses the rheological properties of the fluid. It is a measure of the internal resistance of the fluid against flow, so it is also called the internal friction coefficient. It is expressed as the ratio of the external force applied to the fluid to the flow velocity gradient generated. The external force includes shear force and tensile force. Usually, the ratio of shear force to shear velocity gradient is called shear viscosity, commonly known as dynamic viscosity, and the international unit is Pa second (Pa. s) [the conventional unit is P ( Poise), cP (centipoise), 1Pa. s=10P, 1mPa ●s= 1cP]. The ratio of dynamic viscosity to density is called kinematic viscosity, and its international unit is square meter per second (m2/s) [common units are st (St), cSt (centis), 1cSt= 1mm2/s].

Fluids can be divided into Newtonian fluids and non-Newtonian fluids. Newtonian fluids are fluids whose viscosity remains constant in a wide range of shear For non-Newtonian fluids, the shear stress is not proportional to the velocity, and its viscosity value changes with the shear stress. With the increase of shear stress, the fluid whose viscosity value decreases is called pseudoplastic flow; the shear stress increases and the viscosity value also increases is called dilatant flow; if it is necessary to apply Shear stress can flow, below this yield value, the fluid can only be deformed called plastic flow. The viscosity of non- Newtonian fluids is usually called apparent viscosity to distinguish it from that of Newtonian fluids. The apparent viscosity is that the viscosity value is only related to one shear rate, and a fluid can exhibit different apparent viscosity values ​​at different shear rates.

Except for solvent-based varnishes and low-viscosity paints that belong to Newtonian flow in liquid coatings, most paints belong to non-Newtonian pseudoplastic flow or plastic flow, so their viscosity values ​​are actually their apparent viscosity . For thick paste paints such as putty, it is customary to call its viscosity as consistency, which also means its fluidity .

The thixotropy of certain varieties of paint is also a rheological property, that is, the viscosity of these varieties decreases when they are subjected to external force, and the property of returning to the original viscosity quickly after resting, this property is beneficial to the construction of the paint. Liquid paints, especially paints containing pigments with high density, usually maintain commonly known as the original viscosity of the paint. During construction, it is necessary to adjust to a lower viscosity with a diluent to suit the needs of different construction methods. The viscosity at this time is commonly known as the construction viscosity. The original viscosity of the paint varies from species to species. For example, 150~300mm2/s for general varnish, 200~300mm2/s for general enamel, nitro varnish is thicker than alkyd paint, and some thick build types can reach tens of thousands of centistokes. The construction viscosity of brush coating is relatively high, about 250mm2/s. The construction viscosity of air spraying is usually required to be around 50mm/s. The construction viscosity of airless spraying, shower coating or dip coating is different. As mentioned above , the original viscosity and construction viscosity of the paint change their values ​​with the rise and fall of temperature, so they can only be measured under the same temperature conditions .