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Plastic and glass materials 1. Standard-index plastic materials (1.48 ≤ n < 1.54) CR39® Introduced at the turn of the millenium, Trivex®(**), available After several unsuccessful attempts to develop lenses in from PPG Industries Inc. – and marketed under various lens thermoplastic material (Igard® in PMMA or Plexiglas®, around names – is a material said to be “quasi-thermosetting”, 1940) and in thermoset material (Orma® 500 lenses, around Trivex® 1950), it was CR39®(*) that proved to be the plastic material of combining the qualities of thermosetting and thermoplastic choice in ophthalmic optics. resins. Originally developed for visors on army helmets, it comes Diethylene glycol bis (allyl carbonate), known by the name of as a thermoset resin in the form of a polymerizable liquid resin. CR39®, is the basic material used in the manufacture of the On the other hand, its special chemical structure allows control majority of plastic lenses. Discovered during the 2nd World War of the level of inter-connection of the molecules during & TREATMENTS Glass), its name came from that fact that it was Columbia Resin Trivex® combines three qualities demanded by wearers of polymerization, giving it qualities close to those of a by chemists at the Columbia Corporation (a division of the thermoplastic resin. American manufacturing company PPG or Pittsburgh Plate ophthalmic lenses: optical quality, light weight and safety (hence no. 39 in a series of monomers being studied by chemists for the name Trivex®). The optical clarity comes from the purity of the US Air Force. It was used in the manufacture of corrective the monomer, the transparency and low chromatism of the lenses between 1955 and 1960 (by LOR or Lentilles material (Abbe number ν = 43 to 45) and the ability of the Ophtalmiques Rationnelles, one of the original companies material to be treated against scratches and reflections. The light behind Essilor) and enabled the introduction of the Orma® 1000 weight comes from the material’s very low density (d = 1.11), lens (from Organic Material, i.e. plastic, and today simply known combined with a higher refractive index than CR39® as Orma®), the first lenses that were both light and impact- MATERIALS resistant. (ne = 1.533, nd = 1.530) and an ability to be surfaced to a minimum thickness of 1.0 mm at the center of minus lenses. CR39® is a thermoset resin, i.e. it comes in the form of a liquid Finally, safety is provided by the material’s high resistance to monomer that can be poured into molds and hardened (i.e. impact and good natural protection against ultraviolet radiation polymerized) under the effect of heat and a catalyst. The refining (UV cut-off at 395 nm). Trivex® is a material that is vulnerable to and control of the manufacturing procedure required many years scratching, so requiring systematic anti-scratch treatment on of research. both faces. It can be tinted but, for this, requires the use of For ophthalmic optics, CR39® has several characteristics that make it successful at the expense of glass materials: a refractive requires the use of specific functions on grinding machines. Its index of 1.5 (close to that of the traditional glass lens), a density drilling and fitting are relatively simple. of 1.32 (virtually half that of glass), an Abbe number of 58-59 appropriate techniques. Its grinding and grooving is special and (therefore, low chromatism), good resistance to impact, excellent transparency and multiple possibilities for tinting and treatments. Although it can be used uncoated, CR39® is sensitive to scratching and a surface-hardening treatment is recommended. Its anti-reflective treatment was the subject of very advanced technical developments (see part II of this file). Its use by opticians for grinding and mounting is very easy. © Essilor International © Essilor International Figure 6: CR39® molecule. Figure 7: Chemical structure of Trivex® (Source PPG). (*) CR39® is a registered trademark of PPG Industries Ohio, Inc (**) Trivex is a registered trademark of PPG industries 10 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.