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Plastic and glass materials 2. Mid-index plastic materials Polycarbonate has advantages that make it particularly (1.54 ≤ n < 1.64) interesting for ophthalmic optics: excellent impact-resistance (the highest of all ophthalmic materials) and the material of choice for safety, a high refractive index (ne=1.591 / nd=1.586), Nowadays, mid-index plastics are enjoying great success. extremely light weight (density = 1.20), the ability to be Compared with traditional CR39®, they make it possible to surfaced to minimum thickness (as little as 1.0 mm at the center manufacture thinner, lighter lenses. Usually, they have a slightly of minus lenses), efficient protection against ultraviolet radiation lower density than CR39® (between 1.20 and 1.32), exhibit (when using an additive giving a UV cut-off at 385 nm) and high higher chromatism (Abbe number between 31 and 42) and a resistance to heat (softening point – or vitreous transition Tg – greater sensitivity to heat and they provide better protection higher than 140°C). As with all mid-index plastics, against ultraviolet radiation. These materials are very vulnerable polycarbonate is a material that is vulnerable to scratching, to scratching and require systematic treatment and hardening making coating with an anti-scratch coating absolutely essential. of their surfaces. They can be tinted or made photochromic, Its Abbe number is relatively low (νe = 31, νd = 31) but this has most often by the deposition of a special layer. Anti-reflective no effect on the majority of prescriptions. Today, its tinting and treatment is especially recommended for them. treatment possibilities are close to those of other plastic Most of these materials are “thermoset”; only polycarbonate is materials. Since polycarbonate is by nature difficult to surface a “thermoplastic”. Let us first look at the latter and then at the tint, tinting is essentially obtained either by impregnating color into a coating which is deposited on the rear surface of the lens, family of high-index thermoset materials. or by UV attack on the surface, allowing the distribution of tints & TREATMENTS into the material. Anti-reflective treatment is applied using a similar technique to that used on other plastic materials. Thermoplastic resins: polycarbonate The cutting/fitting of polycarbonate lenses is special: it requires dry grinding, the use of suitable cycles and the polishing of the Used in the 1950s in the manufacture of the first plastic lenses, edge of the lenses. thermoplastic materials – like PMMA and Plexiglas® – proved to be insufficiently abrasion-resistant and were quickly replaced by CR39®. They saw renewed popularity between 1995 and 2000 with the development of polycarbonate, and Airwear® in particular. MATERIALS Polycarbonate is a relatively old material – having first appeared around 1955 – but it was not really used in ophthalmic optics until the 1990s. Because of the numerous improvements which it underwent – in particular for use in the compact disc industry – it offers an optical quality quite comparable with that of other plastic materials. From a chemical point of view, polycarbonate belongs to the family of poly-(aromatic carbonates); it is an amorphously structured linear polymer, whose carbon skeleton is made up of a succession of carbonate (-O-CO-O-) and phenol © Essilor International (-C6H5OH) units. It is most often manufactured by means of the following chemical reaction, called “polycondensation”: Figure 8: Thermoplastic resin: polycarbonate molecule. CH 3 CH 3 n HO C OH COC O CH 3 CH 3 n 11 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.