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Plastic and glass materials In order to respond even better to the demand for thin, lightweight lenses, research into the chemistry of materials continues. This has Plastic and glass materials enabled the use of new materials to be developed and, in the space of a few decades, has profoundly transformed the ophthalmic optics industry. Above all, it has brought wearers a reduction of almost half in the thickness of corrective lenses. The properties of these materials are considered below. A Plastic materials Used in ophthalmic optics since the 1960s, plastics have Thermoplastic materials have the property of softening under progressively replaced glass lenses and now make up 90% of the action of heat and being able to be hot-formed or molded the materials used. In addition to their natural qualities of light by injection. The transformation being mechanical and not weight and impact-resistance, the level of their development chemical, is reversible and makes materials recyclable. have been gradually increased: improvement in their resistance While thermoplastic materials are widely used in industry, only to scratching thanks to hardening coatings, reduced thickness polycarbonate has been used successfully in the manufacture of because of materials with a higher index, better reliability of anti- ophthalmic lenses. reflective treatments through new vacuum depositing technologies, the availability of photochromic versions by surface addition, etc. Today, they have become the benchmark materials in ophthalmic optics. Plastic materials are traditionally divided into two groups: & TREATMENTS - Thermoset materials Thermoset materials are products whose chemical transformation, under the effects of heat or UV, produces hard, rigid, three- dimensional macro-molecular compounds. They are made of relatively short and highly reactive molecular chains which are chemically linked. Under the effects of heat, a chemical reaction occurs called “reticulation” or “curing”, creating rigid links between all the molecules present to form a three-dimensional network; the structure is then said to be “reticulated” and gives the material MATERIALS particular chemical stability and mechanical strength properties. The basic molecule or “monomer” occurs in liquid form and has the property of being able to be “polymerized” under the action of heat © Essilor International or ultraviolet light and/or a catalyst. This polymerization reaction consists of linking together the monomer’s identical molecules. It creates a new molecule, the polymer, of a different nature, size and properties: the material changes from a liquid monomer to a solid Figure 5: “Thermoset” and “Thermoplastic” materials. polymer. This transformation is chemical and therefore irreversible: once the monomer is cast and polymerized, the material is hard, infusible, insoluble, resistant to impacts and chemicals and dimensionally stable. Most of the materials used in ophthalmic optics belong to this group of thermoset materials, and CR39® is the most popular. Certain more recent materials combine the characteristics of thermoset and thermoplastic resins. - Thermoplastic materials Thermoplastic materials are formed by the agglomeration of long molecular chains, linear or slightly branched, that are intertwined but not joined. It is only their tangling and inter- molecular forces that give these materials the appearance of solidity; the chains are not chemically cross-linked in any way. This free molecular structure gives them excellent impact resistance qualities, since the chains can move in relation to each other and so absorb the energy of impacts. 9 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.