Chemistry of sunglasses

Chemistry of sunglasses

Glass or plastic, for example, polycarbonates or Poly-Allyl-Diglycol Carbonate, or PADC, are the manufactured materials of sunglasses lenses. PADC is a plastic polymer, which is resistant to gamma radiation. In order to lower the amount of ultraviolet radiation transmitted, metal oxides are used, such as silicon dioxide, also known as Silica, in which it is known for its hardness. Oxides of most metals adopt polymeric structure. 

The chemical formula of silicon dioxide is SiO₂. In silica, each Silicon is bonded to four Oxygens, but each Oxygen is shared with two Silicons. Silicon does not contain a Carbon atom; therefore, it is a synthetic polymer. Its heat-resistance is high compared with the natural polymer, in which carbon is the main component of the structure. 

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When exposing to high-frequency light, especially UV radiation, a lens becomes darker, which called a photochromic lens. This lens works by a process known as temperature dependency, and a chemical reaction that depends on the nature of the manufactured materials. If a non-organic compound is used, commonly silver halide, a redox reaction occurs. At the manufacturing step, a silver halide and a copper are evenly added over the molten glass.

Cl⁻→ Cl + e⁻

Ag⁺ + e⁻ → Ag

AgCl ⇌ Ag + Cl

Cu⁺⁺ + Ag → Cu⁺ + Ag⁺

Chloride is oxidized, resulting in a chlorine atom and a released electron, conversely silver ion accepts the free electron; thus, a reduction occurs converting silver ion into a silver atom. Therefore, light transmittance is prevented by the silver atom, and the lens color is changed, becoming darker. Once the source of light is unavailable, the reaction is reversed, so the lens is lighter in color. As a result, copper ion, which is a reducing agent, reduces chloride atom to chloride ion. Thus, chloride molecule cannot bound by silver atom, and copper ion (II) is reduced by silver atom, resulting in a copper ion (I) and a silver ionic state. Gradually, the lens becomes clear.

Molecules that are used in manufacturing the plastic photochromic lens, such as pyridobenzoxazines, naphthopyrans, and indenonaphthopyrans are organic. A weak chemical bond is broken due to the exposure of UV radiation; therefore, the organic molecule rearranges into a species that absorbs light in a longer wavelength. Then, the lens is darkened, and it is sensitive to light. However, most plastic photochromic lens loses its ability to change its color, after two years of using. The lens surface is plated rather than combined with the compound, in reverse to glass lens manufacture.