UltraOptics

SCRATCH-RESISTANT COATINGS AND AR

Posted from a submission in Optical Lab Products (OLP), a publication of First Vision Media Group

An AR coating is only as good as the hardcoat that it adheres to, but a brief history of polysiloxane can help explain the chemistry.

Jim Grootegoed

Look up scratch-resistant coating on Wikipedia and you'll find: "An anti-scratch or scratch-resistant coating... film or coating that can be applied to optical surfaces, such as the faces of a lens... (acting)... as a protective layer thus making the lenses more durable." The hardcoat performs an equally important function providing a sort of "primer" layer for anti-reflective (AR) enhancing adhesion and scratch resistance. 

Polysiloxane is the key component in today's scratch-resistant (SR) coatings, but it has a disadvantage: poor stability in a solvent-free system. Ultra Optics Company (UOC) recently introduced an option, AST-1, to address this problem: a very high polysiloxane content, solvent-free, stable, UV-cured SR coating optimized for all lens materials (pending on 1.74) which can be used in all existing Ultra Optic systems. The admitted downside is that it is lower in viscosity so it does not hide surfacing defects as well as their standard UVNV.

COVERING THE BACKSIDE

A brief history of SR coatings and AR treatments would help put this advance in context. When CR-39® became the first practical lightweight ophthalmic lens material, lens manufacturers in the early '80s developed thermally cured polysiloxane-based SR coatings that greatly increased the durability of these lenses.

Then along came polycarbonate with acceptable optical properties, a higher index of refraction, thinner CT's and virtually unbreakable durability. However, polycarbonate is soft and scratches very easily. The thermal-cured polysiloxanes worked well with polycarbonate, allowing for the lens material to enter the market in the mid '80's.

But all SR coatings were factory applied, and surfacing obviously removed the backside coating. This was acceptable for CR-39, but not for polycarbonate or all other ensuing high-index materials, so developing backside coating systems for labs was imperative.

REAPPLYING HARDCOAT

Ultra Optics developed the first viable unit for labs to reapply a hardcoat after surfacing by using a thermally cured polysiloxane. While possessing excellent performance, this process was difficult to manage. Lens Technology International (LTI) soon followed with a UV-curable acrylate-based SR coating that was much more user-friendly and Ultra Optics followed with a UV-cured acrylate as well. These easily reapplied SR coatings enabled the growth of all higher-index plastic materials.

Tinting became the next issue. Initial coatings were essentially non-tintable and demand was high for a coating that would tint to sunwear shades. When tintable coatings were introduced, they came with a tradeoff: the more tintable the coating, the less scratch-resistant. Because the backside of a lens does not get as much abuse

as the front, a coating with less scratch resistance could be utilized.

ENTER AR COATINGS

AR coatings adhere quite well to glass as it is chemically attracted to the base layer of most AR stacks, silicone dioxide (SiO2 or silica, a component of glass). But disaster ensued when the highly successful glass-based European AR process was introduced into the United States in the late 80's. The process was essentially unworkable with "naked" CR-39 and, to a somewhat lesser degree, acrylic-based hardcoats. The AR coatings cracked, peeled, crazed, and simply fell off.

However, AR coatings demonstrated excellent adhesion to polysiloxane coatings, and AR coaters soon started using them to "prime" the lens for better performance. Manufacturer-applied SR coatings all contained polysiloxane and the suppliers of backside coatings responded by changing their formulations to include polysiloxane to increase adhesion, theoretically providing performance equal to that of the manufacturers. However, labs still insisted upon a tintable coating despite its lesser grade of scratch resistance. Another compromise is that the SR coating was required to adhere to all materials and be solvent-free. Simply, it's a tradeoff that negated optimal performance.