Page 12 - Cahier-Materials-and-Treatments-USA.indd

Plastic and glass materials Thermoset resins a Most high-index plastics available today are thermoset resins. When perfecting them, chemists come up against an unavoidable law of physics which links the refractive index, chromatic dispersion and density of the material: generally speaking, the higher the index, the stronger the chromatic dispersion and the heavier the material. When perfecting a new material, chemists always look for the best compromise between the three characteristics, in combination with other essential properties of the material such as sensitivity to heat, vulnerability to yellowing, treatment possibilities and suitability for grinding, grooving and drilling. © Essilor International & TREATMENTS obtained as follows: b An increase in the refractive index of a plastic material can be - either by modifying the structure of the initial material, for example, by introducing aromatic structures - or by introducing heavy atoms such as, sulphur into an initial molecule. We should note that the introduction of metal and halogen atoms, which was used originally, was abandoned, since it gave rise to excessive yellowing of the material. MATERIALS The first high-index plastic materials appeared between 1980 and 1990; they belonged to the allylic family. The increase in the index was obtained through the addition of cyclical functions – benzene-type aromatic groups – to the starting CR39® molecule. This process gave birth to a family of mid-index lenses, n = 1.54 to 1.57, with an Abbe value between 36 and 43 and a density in the order of 1.20. The material Ormex® (ne=1.561 / nd=1.558, νe = 37 / νd = 37, d = 1.23) belonged to this category. Since this technique only allowed a limited increase in the refractive index, chemists then became interested in the © Essilor International thiourethanes family and the chemistry of sulphur. From the 1990s onwards, the association of the functions of thiols and isocyanates enabled the creation of materials with an index of between 1.58 and 1.61, with an Abbe value varying between 30 and 40 and a density between 1.30 and 1.40. Materials Figure 9: Mid-index thermoset resin: examples of the Ormex® such as Ormil®, later replaced by Ormix® / Thin&Lite 1.60 are (a) and Ormil®/Thin&Light® 1.60 (b) molecules. examples. 12 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.