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Plastic and glass materials 2. Mid-index plastic materials Polycarbonate has advantages that make it particularly interesting for ophthalmic optics: excellent impact-resistance (1.54 ≤ n < 1.64) (the highest of all ophthalmic materials), a high refractive index (ne=1.591 / nd=1.586), extremely light weight (density = 1.20), Nowadays, mid-index plastics are enjoying great success. the ability to be surfaced to minimum thickness (as little as 1.0 Compared with traditional CR39®, they make it possible to mm at the centre of minus lenses), efficient protection against manufacture thinner, lighter lenses. Usually, they have a slightly ultraviolet radiation (when using an additive giving a UV cut-off lower density than CR39® (between 1.20 and 1.32), exhibit at 385 nm) and high resistance to heat (softening point – or higher chromatism (Abbe number between 31 and 42) and a vitreous transition Tg – higher than 140°C). As with all mid- greater sensitivity to heat and they provide better protection index plastics, polycarbonate is a material that is vulnerable to against ultraviolet radiation. These materials are very vulnerabe scratching, making coating with an anti-scratch varnish to scratching and require systematic treatment and hardening absolutely essential. Its Abbe number is relatively low (νe = 31, of their surfaces. They can be coloured or made photochromic, νd = 31) but this has no effect on the majority of prescriptions. Today, its colouring and treatment possibilities are close to those most often by the deposition of a special layer. Anti-reflective of other plastic materials. Since polycarbonate is by nature treatment is especially recommended for them. difficult to surface tint, colouring is essentially obtained either Most of these materials are “thermosetting”; only polycarbonate by impregnating colour into a varnish which is deposited on the is a “thermoplastic”. Let us first look at the latter and then at the rear surface of the lens, or by UV attack on the surface, allowing family of high-index thermosetting materials. the distribution of colorants into the material. Anti-reflective & TREATMENTS 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=C-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.