Why do we drive on a parkway and park in a driveway?
Why do inflammable and flammable mean the same thing?
And why are eyeglasses made of – plastic?
Ok, we have an answer for that last one. And some history too.
The history starts with the Second World War. Natural materials, like metal and rubber, were in short supply – so the search for substitutes was a high priority.
One of those “searches”, was in the labs of the Columbia Southern Chemical Company, and on their 39th try, the chemists there created a plastic resin – strong, easy to shape and simple to combine with other reinforcing materials. They called it CR-39 and it was made from a reaction of three chemicals derived from ethylene and propylene, two fundamental petrochemical building blocks.
During the war, CR-39 was reinforced with glass fibers to make fuel tanks for bombers. That made the planes lighter, which meant they could fly longer and farther. And they used CR-39 to replace glass fuel lines on those planes (safer, since when the glass lines got hit, they shattered, and leaked fuel.)
But when the war ended, the company had a rail car filled with 38,000 pounds of expensive CR-39 – but no need now for new bombers and fuel tanks. So, it was back to the lab – and eventually the discovery that some of the same qualities that made CR-39 a good choice for fuel tanks – made it a good choice for eyeglass lenses.
Like those glass fuel lines, if your glass lenses shattered – that was dangerous for your eyes. Plastic lenses were safer – and definitely lighter and thinner. (There’s a reason they used to call them “Coke-bottle glasses”, back in the day.)
Today, plastic lenses today are made from a variety of materials (like polycarbonate made from propylene and benzene) not just CR-39 – though 50 years later, that’s still in use too.
Oh, and If you’re wearing contacts instead of glasses, those are plastic lenses too now (in fact, there wouldn’t even be soft contacts without plastic).
And happily for all of us who wear contacts or glasses – plastic lenses make excellent lenses – so we can have our cake and see it too.