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Plastic and glass materials 3. High- (1.64 ≤ n < 1.74) and ultra- To sum up, we note that it is essentially through the introduction of sulphur atoms into the different molecular families that an high-index (n ≥ 1.74) plastic materials increase in the refractive index of plastic materials is obtained. So, as the materials chemical composition table below shows, To obtain a higher refractive index through the chemistry of the higher the proportion of sulphur, the higher is the material’s thiourethanes, thiols richer in sulphur but still associated with refractive index. isocyanate functions were used. It was therefore possible to raise We should note that it is the presence of sulphur in the the refractive index to n = 1.67 and the material Thin&Lite® composition of plastic materials with a high index that explains 1.67 was produced. the particular smell released during lens grinding. We should note that, given their special chemical composition, materials resulting from the chemistry of thiourethanes (Thin&Lite® 1.60 and Thin&Lite® 1.67) proved particularly well suited to grooving and drilling. Thin&Lite® Thin&Lite® Thin&Lite® Orma® 1.6 1.67 1.74 Finally, to raise the refractive index still further, chemists began to explore the chemistry of episulphides, allowing the Carbon % 65 54 48 36 introduction of sulphur atoms in a greater concentration. So it Oxygen % 25 8 10 1 was materials with a very high index n ≥ 1.74, such as Nitrogen % -78- Thin&Lite® 1.74, that made an appearance. However, it should & TREATMENTS be noted that, although these materials allowed extremely thin Sulphur % -242958 lenses to be manufactured, they also proved to be more Hydrogen % 10 7 5 5 sensitive to heat, easier to break and more difficult to tint. Index 1.5 1.6 1.67 1.74 Abbe number 58 41 32 33 Density 1.32 1.31 1.36 1.47 a Tg (Vitreous transi- 80°C 115°C 85°C 80°C tion temperature) Figure 11: Chemical composition of plastic materials. MATERIALS The perfecting of a new material is a complex exercise since it must seek not only to optimise the basic characteristics – refractive index, Abbe number and density – but also to ensure that all their other physical and chemical properties are controlled, in particular the ease with which they can be © Essilor International given photochromic, tinted, polarized, given anti-scratch and surfaced (using traditional and digital surfacing technology), anti-reflective treatments and finally, edged, grooved, drilled and slotted for fitting. It goes without saying that with the increased knowledge and progress in chemistry, materials have seen constant changes and improvements. Thus research work in ophthalmic optics is, to a large extent, devoted to the chemistry of materials and ophthalmic lens manufacturers b have become at least as much specialists in chemistry as they are in optics! © Essilor International Figure 10: High- and very-high-index thermosetting resins: a) Thin&Lite® 1,67 b) Thin&Lite® 1,74. 13 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.