Meyer Rod Coating is a precision coating technology widely used in coatings, inks, adhesives, etc., which controls the wet film thickness by metering the wire rod (winding rod or scraper), and theoretically should obtain a uniform coating. However, in actual production, the phenomenon of uniform physical thickness but uneven color difference often occurs, which affects the appearance and performance of the product. In this paper, we will analyze the causes from five dimensions: drying kinetics, material system, process parameters, substrate properties, and optical effects, and provide systematic solutions.
1. Non-uniformity effect in the drying process
1.1 Thermodynamic gradient effects
When there is a temperature fluctuation of more than ±3°C in the Drying Oven, it will lead to a difference in the local evaporation rate of the solvent. Experimental data show that Bénard cells are induced when the drying rate difference is more than 15%, especially in low viscosity (<500 cP) systems, which can cause radial migration of pigment particles.
1.2 Solvent volatilization kinetics
For mixed solvent systems (e.g., xylene/n-butanol = 7/3), differences in the evaporation rate of each component result in a change in interfacial surface tension. Studies have shown that when the solvent volatilization gradient reaches 0.25 g/m²·s, the Marangoni effect will occur, resulting in local enrichment of pigments.
Second, the stability factors of the material system
2.1 Pigment dispersion state
If the pigment is not fully dispersed (D50>5μm, PDI >0.3), it is easy to flocculate or settle after coating. For example:Carbon black (surface energy > 50mN/m) is easy to agglomerate, and a dispersant (such as Disperbyk-161) needs to be added.If the pearlescent powder (flake structure) is not sufficiently wetted, it can lead to inconsistent orientation and uneven glitter.
The laser particle size analyzer found that when the pigment particle size D50>5μm or the dispersion PDI was >0.3, the coating film would have visible stains. In particular, organic pigments such as phthalocyanine blue often have a surface energy of more than 50mN/m, and an appropriate amount of dispersant (such as Disperbyk-161, with an addition amount of 0.5-1.2%) needs to be added.
2.2 Compatibility of additives
Leveling agents (e.g., BYK-333) can migrate excessively at concentrations above 0.3%, resulting in a difference in surface energy (Δγ>5 mN/m). Defoamers, such as Tego Foamex 810, can form micron-sized defects that alter the light scattering path if they are not fully compatible.
3. Sensitivity of process parameters
3.1 Wire rod shear effect
With #10 wire rods (wet film thickness 25 μm), shear rates of up to 5000 s⁻¹ can be achieved, which reduces the viscosity of thixotropic systems by more than 30%. Comparative tests have shown that switching to #20 wire rods (shear rate approx. 2000 s⁻¹) can reduce the chromatic aberration ΔE*ab value by 60%.
Wire rod types (e.g., #10, #20) affect the shear rate, and high shear may lead to thixotropic architecture destruction and uneven color after drying. furthermoreVariations in wet film shear force due to fluctuations in coating speed inevitably affect pigment orientation (e.g., metallic glitter paints are particularly sensitive).
3.2 Fluctuations in environmental parameters
When the ambient humidity changes ± 10%, the drying time of the aqueous system will fluctuate by ±15%. Laboratory data confirms that at 23°C, a humidity increase from 50% to 70% results in a 2-3°C reduction in coating surface temperature, significantly affecting leveling. moreoverHigh humidity can cause water-soluble resins, such as PVA, to absorb water locally, altering the refractive index.
Fourth, substrate-coating interaction
4.1 Substrate absorption anisotropy
The difference in liquid absorption rate of porous substrates can be more than 20%, and if the contact angle difference is > 10° when measured with a surface energy Tester, corona treatment is required (dyne value needs to be increased to more than 38 mN/m).
4.2 Micromorphological influence
When the Ra value of the surface roughness of the substrate changes > 0.2 μm, it will cause the scattering angle difference of more than 5° in the incident light. Atomic force microscopy (AFM) observations have shown that this microstructure causes a fluctuation of 3-5 units in the chromaticity value L*.
5. Optical interference effect
5.1 Thin film optical properties
When the thickness of the coating reaches 1/4 of the wavelength of visible light (about 100-150 nm), there is a significant interference effect. Spectrophotometer measurements show that the chromatic aberration ΔE*ab can reach more than 2.0 at this time, even if the thickness deviation is only ± 5nm.
5.2 Pigment orientation arrangement
For flake aluminum powder (particle size 20-50μm), the orientation degree can reach 80% under the shearing action of the wire rod, but if the coating speed fluctuates ± 10%, it will cause the orientation angle distribution to be broadened by 15°, resulting in obvious visual difference.
The problem of uniform thickness but uneven color difference after wire rod coating is mainly due to the pigment migration (Bernard vortex effect) caused by uneven temperature/wind speed during the drying process, poor pigment dispersion or uneven distribution of additives in the material system, fluctuation of process parameters (such as wire rod shear force, coating speed), differences in substrate absorption or roughness, and optical interference and pigment orientation.
So here comes the problem, how to solve it? Nanbeichao is looking forward to discussing with you!
