In our last post, we traced the unlikely path of plastic resin compound CR-39 from a wartime aviation innovation to a promising material for a very different purpose: the production of eyeglasses.
This time, we’ll look at some of the technical challenges Dr. Robert Graham had to overcome before he was able to perfect the use of CR-39 for optical lenses for eyeglasses. The lessons Dr. Graham learned serve as an inspiration for the plastics industry to this day.
The incentives for perfecting CR-39 were strong: the material was thirty times as resistant to scratching as Plexiglas or Lucite, the two primary competitors at that time. It was also lightweight and relatively cheap to manufacture.
Despite these benefits, the technical challenges were immense.
The Sound of Cracking Glass
One of the biggest problems with the CR-39 resin was one of its original strengths – its adhesive properties and ability to laminate with other materials. This property, combined with its propensity to shrink by 14% when curing and cooling, made it difficult to remove the lenses from the moulds once they were formed. This problem was finally solved by using glass moulds (the process of converting plastic into lenses is shown in detail in this video). However, a significant challenge remained: because the lenses were not symmetrical, differential shrinkage would occur at different parts of the lens, effectively ruining the optical properties. After months of experimentation, Dr. Graham and his team learned that the best way to produce lenses with minimal distortion was to use a thick mould with parallel front and back surfaces and to then grind and polish the plastic to its final shape and thickness.
One Major Disadvantage Remains
Armorlite, the company started by Dr. Graham, successfully sold plastic lenses starting in the late 1940s. However, despite the many advantages of plastics their sales were limited by one fatal flaw – the lenses scratched too easily, despite their great scratch-resistance compared to other leading plastics of the day.
By the 1970s, company 3M had the answer. After much trial and error, they were able to produce distortion-free anti-scratch coatings by applying proprietary compounds in a clean-room like environment. The basic anti-scratch coatings still in use today only exist because of these innovations.
Lenses of the Future
We hope you’ve found this two part series interesting. As plastics continue to advance, new materials with even more incredible properties are sure to be discovered! We’ll keep you up to date on the most exciting innovations on this blog. Until next time.
For more resources about the history of CR-39, take a look at this website.