Atomization mechanism and parameters
During the spraying process of water-based paint, the atomization effect directly determines the uniformity and surface quality of the coating film. By measuring rheological parameters such as shear viscosity, energy storage modulus (G'), loss modulus (G'') and complex viscosity (η*), the rotary rheometer can accurately evaluate the flow and deformation behavior of water-based paints at different shear rates. The atomization process involves two core stages: first, the liquid is torn into droplets by the high-speed airflow at the outlet of the spray gun; Second, the droplets are further broken into smaller particles. In the rheological parameters, the zero shear viscosity reflects the static resistance of the paint in the low shear region. High shear viscosity determines the rupture efficiency of droplets at the nozzle. Generally speaking, paints with high shear viscosity and moderate viscosity are more likely to form fine and uniform droplets.
Test method setting
Referring to international standard methods (e.g. relevant rotary rheometer specifications), the test is conducted under the following conditions:
| Test type | Set parameters |
| Shear rate scan | 0.1 s⁻¹ to 1 000 s⁻¹ |
| Oscillation frequency scanning | 0.1 Hz to 100 Hz, 1% strain |
| Temperature control | 25 ℃ ± 0.1 ℃ |
| geometric way | The parallel plate spacing is 1 mm, and the cone plate angle is 1° |
The paint sample should be left to stand for defoaming in advance, and be balanced on the board for 30 seconds before testing. There are no less than 50 data collection points to ensure curve continuity.
Rheological parameter characterization
Key parameters can be obtained by rheological experiments:
(1) Shear thinning index n: Describes the rate at which viscosity decreases with shear rate. The smaller the n-value, the more significant the shear thinning behavior. The atomization process requires n between 0.6 and 0.8, too high a droplet is not easy to refine, and too low a droplet is easy to break too much and cause splashing.
(2) Energy storage modulus G' and loss modulus G'': reflect the elasticity and viscosity of the paint. In the low frequency region, G'' is greater than G' to indicate viscosity dominance; In the high frequency region, if the G' rises rapidly, it indicates that the elastic structure resists breaking during stretching. Atomization suitability requires a G' between 0.1 Pa and 10 Pa (frequency 1 Hz), too high elasticity leads to droplet retraction, and too low a droplet instability.
(3) Compound viscosity η*: Measured in oscillation mode, similar to the shear viscosity η trend. The atomization stability requirements are η* in the range of 0.1 Pa·s to 1.0 Pa·s (frequency 10 Hz), if the value is too high, the droplets will be thick, and if the value is too low, the droplets will be too fine and easy to drift.
Analysis and evaluation of results
The following is an example of a typical paint sample comparing the rheological data and atomization effect:
| Rheological parameters | Corresponding atomization performance |
| Zero shear viscosity 5 Pa·s | Static flow is stable and there is no settlement |
| High Shear Viscosity 0.8 Pa·s (1000 s⁻¹) | The atomized droplets are fine and have a diameter of about 30 μm |
| Shear thinning index n=0.72 | The atomization process is smooth and there is no splashing |
| G'=2.3 Pa（1 Hz） | The droplets are stretch-resistant and non-sticking |
| η*=0.45 Pa·s（10 Hz） | Narrow droplet size distribution and high uniformity |
If the G' is too high (such as more than 15 Pa), the paint is easy to form filaments at the nozzle, and the wire drawing phenomenon will occur after atomization. If the η* is too low (less than 0.05 Pa·s), the droplets are small and easy to drift away during spraying, affecting the adhesion.