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The material CR 39 was the first plastic suitable for spectacle lens production. It was launched on the market by the company PPG (Pittsburgh Plate Glass) in 1947.
Initially, the low-index CR 39 remained the only material used for plastic lenses. It was not until the mid-1980s that the development of plastic materials with a higher refractive index began.
New developments are being focused on creating materials not only with a higher refractive index, but also with good workability, mechanical hardness, low dispersion and good tinting properties. The aim is to provide even people with high prescriptions with thin, flat plastic lenses.
Organic glass is a fully synthetic plastic material available in a vitreous state. It consists of macromolecular organic compounds which do not follow any principle of periodic arrangement and are hence amorphous.
In most cases, duromers are used to produce plastic lenses made of organic glass. Once they have been thermally treated after production, their shape can no longer be changed.
A typical feature of the production process is that, when subjected to heat, many molecules (monomers) combine to form giant molecular chains (polymers) as a result of chemical reaction.
The well-known plastic CR 39 is one of the organic materials used for plastic lenses.
Due to the scarcity of natural raw materials at the beginning of World War II the plastic industry became a source of outstanding substitute materials.
At that time, a subsidiary of PPG Industries – Columbia Southern Chemical Company – began to research into ways of obtaining non-thermoplastic materials. The project was given the name "Columbia Resins" by the scientists involved.
In May 1940 the research work resulted in the monomer allyl diglycol carbonate (ADC). In the following years more than 180 different compounds of this monomer were investigated and examined.
The 39th tested turned out to be the most significant due to its unique properties and was later used for the production of plastic lenses.
|Very high refractive indices allow the production of thin lenses, even for high prescriptions||Large range of refractive indices from n = 1.5 to n = 1.9
|Resistant to scratches, hence greater durability and longer lens life||Good surface hardness
|Fewer colour fringes than plastic lenses with same refractive index||Low dispersion, even with high refractive index
|No palpable edges in bifocal and trifocal lenses||Good fusability of different materials|
|Unproblematic disposal of by-products resulting from manufacturing process||Good environmental compatibility of manufacturing process|
|No deformation and therefore no impairment of optical properties at high temperatures||High thermal resistance
|Equitint lenses and cemented segments possible, e. g. with different prismatic powers in near and distance portions||Good cementing properties of the materials
|Plastic Lenses||Organic Lenses|
|High refractive indices allow the production of thin lenses, even for higher prescriptions||Range of refractive indices from n = 1.5 to n = 1.74
|Lightweight spectacles which are comfortable to wear||Low density
|Very suitable for sports and children’s spectacles||High resistance to breakage
|Tinting using dipping process, irrespective of prescription, in whatever colour the wearer requires||Extensive tinting possibilities
|Uniform darkening of plastic photochromic lenses, irrespective of power||Incorporation of photochromic substances in lens surface
|No damage to the lens in welding or grinding work||Very resistant to sparks|
|A hard coating is necessary to achieve a similar hardness to that of glass lenses||Low surface hardness
|Materials for plastic lenses
||Examples of lenses in which used
||Mean refractive index nd
||Abbe number νe
||SV Sph 1.5
||First plastic used to produce spectacle lenses
|MR 8||SV AS 1.6||1.598||41.0||High-index plastic, used for ZEISS lenses since 1993|
|MR 7||SV AS 1.67||1.665||32.0||Plastic with super-high index, used for ZEISS lenses since 1997|
|CR 330||Photolet||1.501||57.0||Plastic for photochromic lenses, used for ZEISS lenses since 1995|
|MR 174||SV AS 1.74||1.738||32.0||Plastic with super high-index|