Plastics that can be used in prescription and safety glasses are known as CR39 and polycarbonate. Plastic lenses are also known as Hard Resin, CR39 plastic or just plastic. CR39 is the registered trade name for diallyl diglycol carbonate or DADC, CR represents Columbia Resin and the 39 represents that it was the 39th batch that was made by the company.
CR39 is used in safety glasses and prescription glasses as it is more durable, has a higher scratch resistance then polycarbonate, and is able to survive welding sparks along with minor splashes from chemicals. CR39 plastic is a thermo-set plastic which cannot be molded or bent when heat is applied; they are also unlikely to fog. However, to maintain a longer life of the CR39 or polycarbonate lenses more maintenance is required then a glass lens.
Compared to glass lenses, plastic lenses are thinner which makes them a more comfortable wear for the wearer. In the event that a glass lens breaks, they will shatter into small pieces. While polycarbonate lenses can still break, they will break into large pieces, not small shattered pieces which could potentially get into the wearers eye, CR39 may break into smaller pieces but will not shatter.
CR39 lenses are able to have scratch resistant coating applied to the lens along with an ultraviolet coating and anti-reflective coating. Glasses that have been made with CR39 plastic will provide better ultraviolet protection to the user. CR39 lenses have a higher scratch resistance over polycarbonate and also present the wearer with more choice when it comes to tints and coatings. It is cheaper to create safety and prescription safety glasses using plastic rather then glass, which means the cost for these lenses is usually less.
Plastic safety lenses are often recommended for use in over glass safety lenses as they are lower in cost and lighter to use in safety glasses.