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Basic Performance Requirements for Vacuum Coating Coatings

Release time:

2014-06-13 11:03

Basic Performance Requirements for Vacuum Coating Coatings

Many injection-molded parts often exhibit microscopic protrusions or air bubbles on their surfaces due to impurities and porosity. Moreover, the plastic mold itself has a surface roughness of approximately 0.5 μm, which inevitably results in some degree of surface roughness in extruded products. Since vacuum coating typically achieves a thickness of no more than 0.2 μm, the resulting coating cannot effectively mask the irregularities of the substrate, making it impossible to achieve the desired mirror-like finish after coating. Additionally, during the thermal processing and molding of plastic products, surface stress cracks and other defects may occur. When vacuum coating is applied onto surfaces that are both rough and have stress cracks, the coating will inevitably lose its luster. By employing a primer-based leveling technique, with a coating thickness of about 10–20 μm, the substrate’s roughness can be completely concealed. Furthermore, the surface smoothness of the coating itself generally falls below 0.1 μm. As a result, vacuum coating can fully deliver the ideal mirror-like finish, significantly enhancing the brightness of the coated surface.

Volatile small molecules contained in the plastic substrate escape to the surface under vacuum heating conditions, severely compromising the adhesion of the coating to the substrate. After sealing with a primer, these volatile small molecules are effectively blocked from escaping, thereby helping to improve the adhesion of the coating.

The study also found that pre-coating the substrate with a primer can itself enhance the barrier performance of the thin film against moisture and oxygen—though this barrier effect is relatively limited. Moreover, without prior primer treatment, it is difficult to achieve a vacuum-deposited coating thickness exceeding 35 nm on PET film substrates. This is mainly because the thermal expansion coefficients of the plastic substrate and the deposited coating differ by an order of magnitude. During the heating and cooling cycles in vacuum deposition, excessively thick coatings tend to accumulate significant stress, making them prone to cracking, which in turn degrades adhesion and leads to coating failure, such as cracking and flaking. However, if a suitably performing primer is applied as a pretreatment layer onto the film substrate, the difference in thermal expansion coefficients can be reduced. Without compromising the adhesion of the deposited coating, this approach allows for an increase in the thickness of the vacuum-deposited coating, thereby significantly enhancing the barrier properties of the coated product.

The performance of the primer should simultaneously meet the requirements of both plastic substrates and vacuum coating processes. Generally, it should include the following aspects:

(1) The primer exhibits good adhesion not only to the plastic substrate but also to the vacuum-deposited coating. This is the most fundamental property that a vacuum-coating primer should possess. For further information on the adhesion between coatings and plastic substrates, please refer to the preceding article titled "Overview of UV Plastic Coatings." Organic coatings contain a relatively high proportion of polar components. After curing, the surface of the cured coating generally possesses sufficient polarity, enabling it to achieve strong adhesion to metal or metal oxide coatings. Moreover, in the vacuum chamber, the high-energy particles generated by the coating material—such as vaporized atoms, molecules, and plasmas—due to their inherent energy, tend to partially embed themselves into the shallow layers of the coating, blurring the interface between the coating and the substrate. Additionally, the temperature rise induced by the coating process further enhances the bonding between the coating and the primer. Therefore, in most cases, achieving adequate adhesion between the primer and the vacuum-deposited coating is not difficult. If necessary, polar components can be appropriately incorporated into the coating formulation, or metal adhesion promoters can be added. Furthermore, controlling the curing process of the primer also has an impact on bidirectional adhesion.

(2) The primer should have excellent leveling properties, which are crucial for determining whether the vacuum-deposited coating achieves high gloss and a mirror-like finish. Since vacuum coating itself is extremely thin, the smoothness of the coating largely depends on the surface smoothness of the primer.

(3) The curing primer should not contain volatile small molecules, primarily referring to residual monomers, volatile additives, volatile impurities, and high-boiling-point solvents. Under normal temperature and pressure, the adverse effects of these substances on the coating may not be immediately apparent; however, in a vacuum environment with temperature rise, they will gradually volatilize and, from multiple angles, degrade the quality and performance of the coating. Therefore, coatings used as primers for vacuum coating are best those that are solvent-free and conversion-type, such as powder coatings, UV-curable coatings, or waterborne coatings. Given the high-temperature melting and curing process required for powder coatings, traditional powder coatings are still not suitable for use as vacuum-coating primers. By contrast, UV-curable coatings and waterborne coatings—and especially waterborne UV-curable coatings—offer significant advantages when used as vacuum-coating primers.

(4) The primer used for curing should possess a certain degree of heat resistance, which is relatively easy to achieve in both convertible and thermosetting coatings. During vacuum coating, as mentioned earlier, the process involves a temperature rise. As the substrate and coating undergo thermal expansion and contraction during heating and cooling, their thermal expansion characteristics must be compatible with each other. Moreover, the thermal expansion behavior of the primer itself must also be well-matched with the thermal expansion and contraction properties of the vacuum-deposited coating. Therefore, the primer’s heat resistance, resistance to thermal deformation, and thermal decomposition performance must meet the requirements of the vacuum coating process.

(5) For flexible plastic substrates, the primer should also possess sufficient flexibility. The vacuum-aluminized film is very thin and inherently capable of bending without issue. However, if the primer has poor flexibility, it can similarly cause the coating layer to crack or even peel off entirely.

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