1.What is the positive effect of coating thickness on fade resistance?
Increase the thickness of the UV "barrier layer"
Ultraviolet rays are the core factor that causes pigment decomposition and resin degradation. Thicker coatings can provide more sufficient UV absorbers and pigment particles, extend the path of UV penetration into the deep layer of the coating, and reduce direct damage to the underlying pigments and resins.
Slow down the "consumption" rate of pigments
The pigments in the coating will gradually decompose with aging. Thicker coatings contain more pigment reserves. Even if the surface pigment fades due to UV damage, the deep pigment can still maintain color stability and delay the overall appearance deterioration.
Thicker coatings can better resist microcracks caused by hot and cold cycles and mechanical stress. If the coating is too thin, it is easy for UV rays to directly penetrate the bottom layer due to local cracking, resulting in "local fading" or "mottled degradation", thereby expanding the overall fading range.
2.What are the possible negative effects of coating too thick?
The internal stress of the coating is too large and it is easy to crack.
During the curing process, the coating will shrink due to the evaporation of the solvent. When the thickness is too thick, the shrinkage stress cannot be released evenly, and fine cracks are easily formed. These cracks will become "channels" for ultraviolet rays and water vapor, accelerating the aging of the underlying pigments and resins, resulting in increased local fading.
Insufficient drying and residual defects
When the overly thick coating is baked and cured, the surface layer dries first to form a film, which hinders the evaporation of the internal solvent and may form bubbles or pinholes. These defects will reduce the density of the coating, making it easier for ultraviolet rays to penetrate, and at the same time accelerate the oxidation of local pigments, resulting in uneven fading.
Decrease in cost-effectiveness and diminishing marginal benefits
When the coating thickness reaches a certain critical value, the increase in thickness will significantly reduce the improvement in anti-fading performance, but the cost of raw materials and the difficulty of the process will increase significantly, and the cost-effectiveness will be unbalanced.
3.How sensitive are highly weather-resistant coatings to thickness?
The resin itself has strong UV resistance and good pigment encapsulation, and the positive impact of increased thickness is more significant. For example, when the fluorocarbon coating is increased from 20μm to 30μm, the ΔE*ab can be reduced from 2.2 to 1.5 after 3000 hours of xenon lamp aging, and the anti-fading property is significantly improved.
4.How "sensitive" are common coatings (like common polyester) to thickness?
Resin weather resistance is limited, and even if the thickness is increased, it is difficult to offset the degradation rate of the resin itself. For example, if the thickness of ordinary polyester coating increases from 20μm to 30μm, the ΔE*ab may only drop from 5.5 to 5.0 after aging, which is a slight improvement. In addition, if the thickness is too thick, the resin degradation is accelerated, resulting in overall powdering, which in turn exacerbates the fading.
5.What are the reference suggestions in practical application?
Choose a reasonable thickness based on the use environment:
Outdoor strong sunlight environment: give priority to high weather-resistant coatings, and the recommended topcoat thickness is 25-30μm;
Indoor or weak UV environment: ordinary polyester coatings are sufficient, and the topcoat thickness of 15-20μm is sufficient to meet the anti-fading requirements.
Pay attention to thickness uniformity: the coating thickness deviation of the same batch of color-coated coils should be controlled within ±2μm to avoid "early fading points" caused by local excessive thinness.
Combined with other indicators for comprehensive evaluation: anti-fading is the result of the combined effect of multiple factors such as coating thickness, resin type, pigment stability, and additive addition.