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Precautions for Light-Curing Vacuum Coating Coatings
Release time:
2014-06-11 10:36
In the formulation of UV-curable primers, it is advisable to minimize or even avoid using monofunctional reactive diluents, as the actual conversion rate of the UV curing reaction can never reach 100%. For polyfunctional resins or monomers, as long as a single active group participates in the crosslinking reaction, the molecule can be firmly incorporated into the crosslinked network. In contrast, monofunctional reactive diluents themselves have relatively low photopolymerization rates (compared to polyfunctional reactive diluents). After the UV curing process is complete, residual amounts of monofunctional reactive diluents may remain substantial. Moreover, given their relatively high volatility, these diluents tend to migrate and escape during vacuum coating processes, thereby compromising coating quality and hindering precise vacuum control. Particularly in sputter coating processes, large quantities of escaped monomers, when subjected to prolonged plasma bombardment, may undergo further reactions—either forming carbon particles or generating oily oligomers. If such substances deposit onto the primer along with coating material particles, they will also adversely affect both the quality and aesthetic appearance of the vacuum-coated film.
The formulation of light-curing primers should not contain highly effective surface additives, including leveling agents, defoamers, and surface gloss enhancers. Due to the limited compatibility of these additives with organic coating systems, they tend to concentrate predominantly at the coating surface during film formation, thereby reducing the surface energy of the coating. As a result, the plating particles, which possess relatively high energy, may exhibit poor adhesion to the low-surface-energy coating. This not only leads to reduced adhesion but can also cause unevenness in the plated layer.
To enhance the adhesion between the primer and the plastic substrate as well as the metal coating (which is typically a metallic coating, though occasionally a metal oxide or inorganic nonmetallic oxide), an adhesion promoter is sometimes added to the formulation of UV-curable primers. However, it is crucial to ensure that the adhesion promoter used does not cause corrosion on the metal coating. For example, adhesion promoters based on phosphate esters and carboxylic acids may induce acid corrosion on highly reactive metal coatings such as aluminum, leading to black spots or discoloration. Therefore, their dosage must be strictly controlled. Moreover, the adhesion promoters in UV-curable primer formulations should not excessively “bite into” the plastic substrate; otherwise, this could easily result in unevenness or delamination of the coating base—a defect that would be fatal in optical plastic processing.
The UV-curable primer should exhibit excellent wettability on plastic substrates to ensure adequate leveling and sufficient adhesion to the substrate. This is particularly important for controlling defects such as craters. Vacuum coating imposes higher requirements on the adhesion of primers; generally, the surface tension of a UV-curable primer should ideally be 10 mN/m (dyne/cm) lower than that of the plastic substrate.
Immediately after vacuum coating is completed, the surface should be coated with a protective layer to prevent mechanical damage or contamination of the coated surface. The UV-curable topcoat must exhibit excellent adhesion to the underlying coating and should not generate significant shrinkage stress during curing and film formation. Otherwise, interlayer delamination is likely to occur, causing the topcoat to lift off the underlying coating. Proper use of low-shrinkage reactive diluents can help alleviate the accumulation of shrinkage stress. Adjusting the proportions of polyfunctional diluents, photoinitiators, and other formulation components, as well as appropriately reducing the intensity of the UV light or increasing the conveyor speed, can all help mitigate the buildup of curing-induced shrinkage stress.
The mechanical properties of light-cured topcoats, such as scratch resistance, are particularly critical. Regarding the enhancement of scratch resistance, it is still important to pay attention to how wear-resistant measures affect the transparency of the coating and the integrity of the plating layer. Inorganic fillers used for wear resistance often reduce the transparency of the coating, and due to their inherently high hardness, it is difficult during the coating process to avoid scratching the “delicate” thin plating layer.
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