After ice-cream and sunscreen, this week we continue our “chemistry in the summer” theme, with photochromic sunglasses.
These are not your straightforward sunglasses (no chemistry there!), but those that can darken when exposed to sunlight and return to their clear state in the shade. These are becoming an increasingly popular choice in the summer for those of us unfortunate enough to need glasses.
How do they work with glass lenses?
The original spectacles were made with glass lenses and relied on a reaction between embedded silver halide crystals and the glass. In the presence of UV light (particularly within the 320-400 nm region), colourless silver cations react with electrons from the glass to form visible silver, thus darkening the glass. Back indoors, the reverse reaction occurs and the glass becomes clear again. Despite their initial popularity, over the last few years, these have been gradually replaced with plastic photochromic lenses, which rely on a totally different reaction.
What about plastic lenses?
The first generation of plastic photochromic lenses used pyridobenzoxazines, a family of organic blue dyes. Subsequent improvements to produce a faster reaction started using naphthopyrans and today most manufacturers have adopted indenonaphthopyrans. All these follow a similar type of reaction – when exposed to UV light, a chemical bond is broken. The new molecule has to re-arrange itself to become stable again, and in the process, starts absorbing at longer wavelengths causing the lens to get darker.
The plastic is also important
Although the type of reaction is essential, the sunglasses would not work in normal plastic. To overcome this problem, manufacturers have developed a special type of plastic which can support a layer of the photochromic dyes. Even some types of plastic used for prescription glasses, such as those containing thiourea or polycarbonate, are not appropriate or would react to UV light very slowly. To get around these issues, the photochromic dyes are protected by layers of polyurethane and “sandwiched” between polycarbonate or thiourea lenses.
Which is better?
Most people prefer plastic lenses, but there are still some hard-core devotees for real glass lenses. These defend glass offers better resistance to scratches and lasts longer. This is because glass lenses use inorganic silver, whereas plastic lenses use organic chemicals, which are more prone to degradation over the years.
Why do they not work in the car?
Unfortunately, neither type of lenses can work in the car. This is because windshields are protected with a layer of polyvinyl butyral that absorbs UV light. This means the reaction to darken the lenses cannot be activated. Realising the importance of safe driving in the sun, many companies are trying to develop dyes that can absorb in the visible light (rather than UV), so that they would darken inside the car. However, for the time being, it’s back to old style sunglasses!