In industries such as coatings, inks, paper, and printing, viscosity is a key physical parameter to measure the internal resistance of a fluid and characterize its ability to flow and deform. It directly determines the processing, construction performance and final product quality of the material, and is a core indicator that must be accurately measured and controlled.
1. Definition and physical connotation of viscosity
Viscosity, known in physics as kinetic viscosity, is essentially the ability of a fluid to resist shear flow. When there is a velocity gradient (one layer of fluid sliding relative to another) inside the fluid, there is an internal friction that hinders this relative motion, and viscosity is a quantitative description of the magnitude of that internal friction.
It follows Newton's viscosity law, that is, the shear stress is directly proportional to the shear rate (velocity gradient), and the proportional coefficient is the viscosity. It is important to distinguish that kinematic viscosity is the ratio of kinetic viscosity to fluid density and is often used in calculations involving gravitational flow. For non-Newtonian fluids, such as most coatings and inks, their viscosity values vary with the shear rate they are subjected to, which is called their rheological properties.
2. Core test methods, standards and instruments
The choice of viscosity test method is primarily based on the type of fluid being measured (Newton or non-Newton) and the accuracy requirements of the application scenario. The following is a comparison of the main methods.
Test methods and standard instruments
Capillary viscometer method, which determines kinematic viscosity by measuring the time a constant volume fluid flows through a capillary tube under gravity, is a standard method for determining Newtonian fluids (e.g., solvents, lubricants), and requires extremely high constant temperature accuracy (often ± 0.01℃).
Rotational viscometer, which calculates dynamic viscosity by measuring the viscous torque subjected to a uniform rotor in a fluid, is a reliable choice for characterizing the rheological behavior of non-Newtonian fluids and plotting viscosity-shear rate curves.
Outflow cup method - measures the time when a fixed volume of fluid flows out of the specified pore diameter from the bottom of the cup, and the result is expressed as conditional viscosity (seconds), which is suitable for rapid control of construction viscosity on the production site, such as coating-4 cups are widely used in the coating industry.
Relevant industry standard applications
In the ink industry, the standard GB/T 13217.4 specifies the method for determining the viscosity of offset printing inks. For paper printability testing, standard inks with viscosity strictly controlled within the specified range (e.g., 14.0±0.5 Pa·s at 23℃) are used to ensure the reliability and comparability of the test results.
3. Key factors affecting viscosity
Temperature is the most significant external factor affecting viscosity. For most liquids, the increase in temperature leads to an increase in molecular kinetic energy, a weakening of interaction forces, and an exponential decrease in viscosity. Therefore, any viscosity data must clearly indicate the measured temperature, and precision measurements must be performed in a constant temperature bath.
For non-Newtonian fluids, shear rate is a central influencing factor. Thinning of inks at high speeds (shear thinning) or thickening of coatings after rest (thixotropy) are typical of changes in viscosity with shear conditions.
The material composition of a fluid is an intrinsic determinant of its basic viscosity, including the molecular weight and structure of the resin, the type, concentration and shape of pigments and fillers, and the selection of additives.
4. The role of viscosity in process control
In the field of coatings, viscosity directly affects storage stability, workability (e.g., atomization effect of spraying, dragging feeling of brushing), leveling and final film thickness. Improper viscosity is the main cause of coating defects such as sagging and orange peel.
In the world of inks and printing, viscosity is at the heart of printability. It controls the transfer of ink between the ink rollers, the transfer to the substrate (such as paper), the reduction accuracy of dots, and the penetration and drying speed. Excessive viscosity may lead to poor ink transfer and paper lining; If the viscosity is too low, it is easy to cause the dots to expand and transbe printing.
5. Summary
As a well-defined and quantifiable physical parameter, viscosity runs through the whole chain of R&D, production and application of coatings, inks and other materials. In-depth understanding of its physical nature, strict measurement according to standard methods, and systematic analysis of the influence of temperature, shear history and material composition on it are the scientific basis for achieving product quality stability and process optimization. For non-Newtonian fluids, it is an inevitable requirement for modern technical analysis to go beyond a single value and comprehensively examine their rheological curves.