Introduction Overview
As a printing material that cures rapidly under ultraviolet light, the rheological characteristics of UV ink directly affect the uniformity of coating, transfer effect and final printing quality. Accurate determination of non-Newtonian fluid behavior of UV inks is critical in formulation development and quality control. With its precise shear rate control and trace sample requirements, the cone plate viscometer is the right tool for studying the rheological curves of such inks. This article focuses on the application of cone plate viscometer in this field, and analyzes the test principle, operation points and data interpretation methods.
Rheological Theory Basis
UV inks typically appear as non-Newtonian fluids, and their viscosity fluctuates with shear rates. Such behavior can be expressed by the power law model:
η = K · γ(n-1)
Among them η is the apparent viscosity, K is the consistency coefficient, γ is the shear rate, and n is the rheological index. n less than 1 means shear thinning, and n greater than 1 means shear thickening. The cone plate viscometer produces a uniform shear field by rotating between the cone and the plate at a fixed angle to obtain a continuous rheological curve.
Cone plate system structure
The core components of the cone plate viscometer are a conical rotor and a flat plate stator. Typical parameters include cone angle (typically 0.5° to 4°) and rotor radius (typically 10 to 30 mm). The shear rate is inversely proportional to the cone angle, and the relationship is as follows:
γ = ω / tan α
where ω is the angular velocity, α is the cone angle. This design ensures that the shear rate is uniform across the sample, making it suitable for non-Newtonian fluid analysis.
Sample preparation points
UV inks should be restored to room temperature under dark conditions before testing to avoid premature reaction of photoinitiators. When sampling, pay attention to exclude air bubbles, as bubbles can cause fluctuations in readings. The sample volume is typically about 0.5 to 2 mL and the cone plate gap needs to be completely filled. Disposable pipettes or plastic sampling spoons are recommended to prevent metal contamination from affecting ink properties.
Test parameter setting
The shear rate range can be set according to the actual ink process, e.g. from 0.1 to 1000 s-1。 Temperature control is important, it is recommended to maintain 25°C±0.1°C, with the help of a constant temperature circulating water bath. The settling time for each rate point is approximately 5 to 15 seconds, depending on the ink thixotropic recovery time. The data acquisition frequency should be 5 to 20 points per second to capture curve details.
Rheological curve analysis
The curves obtained by the test are usually displayed in logarithmic coordinates, with the horizontal axis as the shear rate and the vertical axis as viscosity or shear stress. For shear thinning UV inks, the viscosity in the low shear region is high, and the viscosity gradually decreases with the increase of rate. The following table lists common data types:
| Viscosity (Pa·s) | Corresponding shear rate (s-1） |
| 12.3 | 10 |
| 5.6 | 50 |
| 2.1 | 200 |
| 0.9 | 500 |
The data points can fit the power law equation to obtain n values. If n is between 0.3 and 0.7, the ink has good leveling and transferability, and if n is low, the resin or filler ratio needs to be adjusted.
Common influencing factors
Temperature fluctuations can cause viscosity shifts, and system stability needs to be ensured. Undispersed pigment aggregates in the sample can produce noise and pre-dispersion is recommended. Cone surface cleanliness is important, and residual solvents or carbon deposits can affect measurement repeatability. In addition, at higher shear rates, the viscosity of the ink may mutate due to local activation of the photoinitiator, and operation under strong UV light sources should be avoided.
Application case reference
A printing plant uses a cone plate viscometer to test two UV ink formulations. Recipe A at a shear rate of 100 s-1The viscosity is 4.8 Pa·s and the viscosity is 2.2 Pa·s for Formulation B. The rheological curve shows that the n value of Formula A is 0.55 and that of Formula B is 0.72. The former shows better dot clarity on high-speed presses, while the latter has better transfer uniformity. The entire curve scan from low to high speed can be completed in a single test, taking about 3 minutes, significantly improving the efficiency of recipe adjustment.
Measurement considerations
After each test, the cone plate should be cleaned immediately with a solvent such as ethyl acetate, and then dried with a soft cloth to prevent the ink from curing. Calibrate the instrument regularly with standard viscosity silicone oil, and the deviation should be less than 2%. Avoid excessive bubbles when filling the sample, and use centrifugal defoaming aid. For highly thixotropic inks, a pre-shear step is recommended to eliminate historical effects.
Data reproducibility guarantee
The three-point repeated test mode was adopted, and the data with a relative standard deviation of less than 5% was taken as the valid value. Record test environment temperature, humidity and sample batch number for easy traceability. If abnormal data is found, check whether the cone plate clearance setting is correct, the typical value is about 0.05 mm.
Standard reference notes
The test method here refers to the non-Newtonian fluid viscosity determination section of the ASTM D2196 standard and the general specification for rotational viscometers in the ISO 3219 standard. The relevant technical analysis is synthesized from the technical report of the International Symposium on Printing Rheology.