Plastic weathering

Why does a plastic chair left outdoors crumble while the same plastic indoors lasts decades?

Two chairs come out of the same mould on the same afternoon. One goes into a garage, the other onto a patio. Twelve years later the garage chair is indistinguishable from new, and the patio chair has faded to a powdery pale grey and snaps at the leg when someone heavy sits down.

The usual account is that sunlight breaks the bonds, and there is a real fact underneath it: a photon at 300 nanometres — about the shortest wavelength that survives the ozone layer — carries roughly 400 kilojoules per mole, comfortably more than the 350 or so holding a carbon-carbon bond together. So the energy is there. But notice what the account does not explain. Why the powder? Why does a chair hold up for years and then fail over one season rather than weakening smoothly from the first week?

Start with an awkward observation. A chemically clean polypropylene chain has nothing in it that absorbs sunlight. Carbon-carbon and carbon-hydrogen bonds absorb far out in the vacuum ultraviolet, at wavelengths that never reach the ground. A perfectly pure polyolefin should be nearly transparent to the sun. So if the photon has the energy but the polymer cannot catch it, what does the catching?

Impurities do. Traces of catalyst left from polymerisation, and — more importantly — the carbonyl groups and hydroperoxides created while the resin was molten in the extruder. The chair was born with the defects that will kill it. Each of those absorbs in the near ultraviolet, and each absorption can knock a hydrogen off a chain and leave a carbon radical behind.

Now ask what a radical does next in air. It meets oxygen almost immediately and becomes a peroxy radical, which is not very reactive but is patient; it wanders until it pulls a hydrogen off a neighbouring chain, making a hydroperoxide and a fresh radical. That is already a chain reaction — one initiation event keeps consuming chains. But the important step is what happens to the hydroperoxide, because it is itself unstable to light and heat, and when it comes apart it gives two radicals where there was one.

Sit with that arithmetic. The damage does not accumulate in proportion to the dose; it compounds. Sunlight is not the destructive agent so much as the ignition. The destructive agent is oxygen, and the reaction feeds itself.

Two more things follow. First, oxygen has to get there. It diffuses into the solid slowly and is consumed as it goes, so the oxidation is confined to a thin outer layer while the interior stays chemically untouched. That is the powder: the surface loses so much chain length that it no longer coheres, and rubs off as chalk. Second, the chair does not fail because the whole thing degraded. It fails because a brittle skin over sound material cracks, and a crack is a stress concentrator — it runs into the good plastic underneath.

And the delay? Manufacturers put stabilisers in: ultraviolet absorbers, antioxidants that intercept peroxy radicals, hindered amines that keep regenerating as they mop up. These do not slow the reaction proportionally; they suppress it, until they are used up or washed out. Then the autocatalysis, which had been held at zero, starts from a stock of hydroperoxides that has been quietly building. Hence the shape everyone reports: fine, fine, fine, gone.

The three factors that get listed separately — ultraviolet, oxygen, heat — are not three independent causes. Ultraviolet supplies the initiating events and splits the hydroperoxides; oxygen does the actual cutting; heat sets the rate of every step in between, which is why a dark chair sitting in the sun at sixty degrees degrades far faster than the air temperature would suggest. Remove any one and the others largely stall. Window glass works precisely because removing the short wavelengths starves the initiation step, and indoor light is orders of magnitude weaker besides.

It is also worth being clear about what "crumbles" means mechanically. A polymer's toughness comes from long chains entangling and tying crystalline regions together; scission shortens the average chain, and below a threshold the entanglement network stops carrying load. Elongation at break collapses long before tensile strength does, which is why a weathered chair feels stiff right up to the moment it shatters instead of bending.

Two honest caveats. The relative weight of catalyst residue, processing defects, and direct photolysis differs between resins and manufacturers and is not settled in general. And accelerated weathering chambers correlate with real outdoor service far less reliably than their reports imply.

• Why some polymers crosslink and stiffen under the same exposure that makes others break apart.
• How carbon black manages to be both a screen and a radical scavenger.

Every term the collection defines is gathered in the glossary.