Overview
The dynamic viscosity of varnish is one of the core parameters of its rheological properties, which directly affects the construction performance, film formation quality and physical properties of the final coating. As a classic absolute measurement method, the falling ball viscometer method calculates the dynamic viscosity value according to Stokes' law and related formulas by determining the time it takes for a standard sphere to fall a fixed distance in a vertical glass tube filled with varnish to be measured. This method is intuitive to operate and reliable to use, suitable for varnish products with Newtonian or near-Newtonian fluid, and is widely used in quality control and R&D in coatings, inks and polymer materials industries.
Principle of determination
The theoretical basis of the falling ball viscometer method is Stokes' law. When a sphere falls at a lower speed and at a uniform speed in an infinitely extending uniform fluid, its viscosity resistance is related to the dynamic viscosity, the radius of the sphere and the speed of motion. In the actual measurement, a finite size vertical glass tube is used, so a wall correction factor needs to be introduced to eliminate the boundary effect. The formula for calculating the dynamic viscosity η is as follows:
η = K × (ρball - ρliquid) × t
Among them, K is the instrument constant (including gravitational acceleration, sphere radius, falling distance and pipe wall correction factor), in millimeters²/seconds²; ρballis the density of the sphere, in grams/cubic centimeter; ρliquidThe density of the varnish sample at the measured temperature, in grams/cubic centimeter; t is the time it takes for the sphere to fall to a fixed distance, in seconds.
Instruments and materials
The main equipment and materials required for the measurement are shown in the table below. All instruments are calibrated regularly to ensure measurement accuracy.
| Drop ball viscometer | It consists of a vertically calibrated glass tube, a standard sphere, a thermostatic jacket and a timing device |
| Thermostatic bathtub | The temperature control accuracy ± 0.1°C to ensure the measurement temperature is stable |
| Standard sphere group | Different diameters and materials cover the corresponding viscosity range |
| Density meter or hydrogravity bottle | Determination of varnish sample density |
| Thermometer | The index value is 0.1°C, and the sample temperature is measured |
| Sample pretreatment equipment | Used for defoaming, filtration, etc |
Assay steps
Before the measurement, the varnish sample should be fully stirred and degassed if necessary to avoid air bubbles interfering with the fall of the spheres. The specific operation process is as follows:
First, set the thermostatic bath to a standard temperature (usually 23°C or 25°C, depending on the product specifications) and install the viscometer vertically in the bath. Slowly inject a sufficient amount of varnish sample into the viscometer measuring tube along the tube wall to avoid air bubbles. Insert a standard sphere of the selected size and allow the sample to be equilibrated with the sphere in a thermostatic bath for at least 15 minutes, ensuring a uniform temperature.
The sphere is then guided above the starting line at the top of the measuring tube using a magnet or a special tool. Release the sphere to fall freely, and use a stopwatch to accurately record the time it takes for the sphere to pass the distance between the upper and lower ticks. The assay should be repeated at least three times, and the time deviation of each time should be within the allowable range.
Finally, the density of the varnish sample is measured using a density meter at the same temperature. Substituting the measured time, density and instrument constants into the calculation formula can obtain the dynamic viscosity value of the varnish at this temperature.
The result is calculated
The calculation of kinetic viscosity is expressed in millipartaz per second (mPa·s). Calculation example: If the instrument constant K is 0.085 mm²/sec², the sphere density is 7.8 g/cm3, the varnish density is 0.95 g/cm3, and the measured fall time t is 120 seconds, then the dynamic viscosity η = 0.085 × (7.8 - 0.95) × 120 = 69.87 mPa·s. The report should indicate the measurement temperature, the size of the sphere used, and the number of repeated measurements.
Influencing factors
The accuracy of the assay is affected by a variety of factors. Temperature fluctuations can significantly alter viscosity values, so thermostatic control is essential. Air bubbles or impurities in the sample can interfere with the movement of the sphere and need to be removed beforehand. The sphere should be selected so that its fall time is within the recommended range (usually greater than 30 seconds) to ensure measurement accuracy. The viscometer must be placed strictly vertically, otherwise errors will be introduced. This method may not be suitable for varnishes with non-Newtonian fluid properties, and other rheological measurement instruments should be considered.
Scope of application
This method is suitable for transparent or translucent varnishes that are Newtonian fluid or near-Newtonian fluid at the measurement temperature, and the dynamic viscosity measurement range is typically between a few millipascals and tens of thousands of millipascals, depending on the chosen sphere size and instrument design. For varnish systems with a large amount of fillers or opaque and high thixotropy, the applicability of this method is limited because it is impossible to observe the falling trajectory of the spheres. In addition, the method has high requirements for the operator's operation.
Cited Literature
GB/T 1723-1993 Coating viscosity determination method
ASTM D1545-2013 Standard Test Method for Determination of Viscosity of Clear Liquids by Bubble Time Method
ISO 2431:2019 Colours and varnishes - Determination of outflow time using overflow cups
Handbook of Testing Methods for Coatings Industry, Chemical Industry Press