Hard coat is an indispensable materials in consumer electronics, building, automobile, medical, and optics industries to add on more multifunctions to the base materials, e.g. anti-reflection (AF), anti-glare (AG), Anti-smudge, Anti-fingerprint and more.
The general formulation of a hard coat includes oligomers, monomers, additives, photo initiators, et.al. as shown in the following figure.
Some oligomer examples:
Monomer examples:
Hard coat can be coated by micro gravure, slot die methods on substrate film followed by drying, photo curing. To evaluate the performances of a hard coat, the following items are the main foci: Transmittance, Haze, Gloss, Anti-abrasion, water contact angle (WCA), curling, rainbow effects or fringe intensity, an light interference effect. Among them, rainbow effect is one of the toughest topics.
Fundamentals of rainbow effect
In nature, it is light interference at thin film interface. The optical path and interference can be shown in the following figure. One thing needs to notice is that once light enters into higher n media from lower n media, there is a 180 degree phase difference. However, no phase difference the other way around.
How to reduce rainbow effects?
- Match HC’s refractive index,n1, to that of substrate (e.g. PET), n2. If the n1=n2, in the eye of light, the two media are same, no reflection at the interface and no interference at all. However it is difficult to find such HC materials.
- Multilayer structure. Insertion of another layer, with suitable refractive index n, would help reducing the interference. In the internet, it is suggested to select a material with refractive index between n1 and n2. In this scenario, this inserted layer enhance transmission, reduce reflection and interference accordingly. The thickness can be calculated by the follow equation. In practice, a primer is often indeed needed to enhance the adhesion between hard coat layer and substrate anyway. Prudent selection of primer materials would kill two birds by one stone. However, In theory, I would believe it should still work even if the 2nd layer’s refractive index is larger than that of substrate.
- Make the HC layer thin. Severe rainbow effects are often observed when the HC thickness is about 2-8um. Making the film with thickness NOT in this range would be helpful. However, Thinner film may have less hardness than needed while thicker film may have strong tensile stress causing curling. After all, by advanced formulation, the conflicting issues can be balanced.
- Make the HC super flat. Although the interference is inevitable, if HC is flat enough, interferences are same at all locations on the film surface. The inference pattern may not be too noisy, still uniform; Customers may not even notice the interference. Color may be slightly different when HC has different thickness.