Scratch Resistant Coatings Technology (Hard Coating) provide lenses with durability and increased longevity. A high quality thermally cured dip hardcoat provides the best scratch resistance and provides for the most durable ingredient for an Anti-Reflective coating application. Plastic lens materials scratch easily than glass materials, and therefore benefit greatly with application of a durable hardcoat.
Two of the more common types of hard coats are:
Thermally Cured Coatings — Thermally cured coatings were traditionally used only by lens manufacturers and referred to as factory coatings or the front side coating on a semi-finished lens blank. They have good to excellent abrasion resistance and AR compatibility. They allow the option of using primers to achieve better adhesion, tintability and impact enhancement. Most thermally cured coatings are designed for adhesion to a single substrate. This makes them ideal in the lens manufacturing environment. The use of a primer, however, allows the same coating to be used on multiple substrates or even as an overcoat (more details below). This has resulted in increased usage of thermally cured coatings at labs and retailers. Thermally cured coatings are generally thought to be more compatible with AR and mirror coatings. Thermally cured coatings can be spin or dip coated. Thermally cured coatings
have longer cure times (typically one to four hours) and have limited substrate compatibility.
UV Cured Coatings — UV cured coatings are traditionally used in the laboratory environment and at some lens manufacturers. The advantages of UV cure coatings are quick cure time and multiple substrate compatibility. This makes them ideal for the laboratory or retailer where multiple substrates are used and quick turnaround times are expected. UV cured coatings can be applied by spin coating, dip coating or in-mold coating. Spin coating is the most typical method of application. UV cured coatings are typically lower in abrasion resistance and often less compatible with AR and mirror coatings. UV cure coatings often have good steel wool abrasion resistance, but do not perform as well as thermally cured coatings in the Bayer Abrasion Test (more on test methods below).
One of the most important things to understand related to this topic is that lenses have what is scientifically referred to as a coefficient of thermal expansion. This is a fancy way of saying that the lenses expand and contract with temperature. When the lens expands and contracts at a different rate than the AR coating, the AR coating can, and very often will, crack. (Keep in mind the AR coating is very, very thin relative to the lens.) The highest coefficient of thermal expansion in lenses belongs to Hi Index, then CR39 and Mid Index. Poly and Trivex have relatively little thermal expansion. If we are going to see temperature related cracking, we will typically see it first in Hi Index and then follow it down. One hardly ever sees temperature related cracking in Poly.