Plastic

Plastic
ZEISS plastic lenses with refractive indices ranging from 1.5 to 1.67

Materials used for Plastic Lenses

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.

What is organic glass?

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.

Where does the name CR 39 come from?

A structural model of CR 39
A structural model of CR 39

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.

Glass or plastic?

Tips and advice

Glass Lenses Mineral 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

Examples for lens types

Materials for plastic lenses Examples of lenses in which used Mean refractive index nd Abbe number νe  

CR 39

SV Sph 1.5

1.501

58.0

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