Sunlight Ages Whatever You Leave Beside A Window

Position in a room is a maintenance decision. Ultraviolet light carries enough energy to break chemical bonds in common materials, and the damage accumulates whether the item is used or not.

Ultraviolet breaks bonds

Visible light mostly passes through or reflects off a material. Ultraviolet photons carry more energy, enough to sever the long molecular chains that give polymers their strength.

Once a chain is broken the fragments can re-link in disordered ways, which is why aged plastic is brittle rather than merely faded.

Oxygen participates in the reaction, so the process is fastest at the exposed surface and slows with depth into the material.

The visible signs arrive after the mechanical ones

Colour change comes from dye and pigment molecules breaking down, which happens at a different rate from the degradation of the polymer itself.

A component can therefore look acceptable while having lost much of its impact strength, and it fails the first time it is dropped or flexed.

Conversely a badly faded item may be structurally fine, if the pigment was simply less stable than the plastic carrying it.

Stabilisers are a consumable ingredient

Manufacturers add ultraviolet absorbers and antioxidants that intercept the reaction. These additives are used up as they work, at a rate set by exposure.

Once the stabiliser package is exhausted, degradation proceeds at the unprotected rate, which is why aged plastics often deteriorate suddenly after years of looking fine.

Carbon black is one of the most effective absorbers, which is part of why outdoor components are so often black rather than a chosen colour.

Heat and light arrive together

A sunlit surface also runs hot, and every chemical degradation process roughly doubles in rate for each modest rise in temperature.

Dark items in sunlight reach substantially higher surface temperatures than pale ones, so colour affects the thermal half of the damage as well as the optical half.

Adhesives and foams are especially sensitive to this combination, softening in the heat and oxidising in the light at the same time.

Glass changes the exposure

Ordinary window glass blocks the shortest and most damaging ultraviolet wavelengths but passes a great deal of the longer ones.

Indoor fading is therefore slower than outdoor fading but not absent, and a south-facing sill delivers a serious dose over a few years.

Moving an item a short distance out of direct sun, or drawing a blind during the brightest hours, changes its ageing rate more than any product applied to its surface.