polymer weathering
Metals rust; plastics rot in their own way. Leave a plastic chair, a garden hose, or a car dashboard out in the sun and after a year or two it fades, chalks, stiffens, and cracks when you flex it. This is polymer weathering: the slow degradation of plastics and rubbers by sunlight, oxygen, heat, and moisture. It is the non-metal branch of corrosion — different chemistry, same story of a material being destroyed by its environment, and just as costly in failed pipes, cracked seals, and crumbling coatings.
The main villain outdoors is ultraviolet light. A polymer is a long chain of thousands of repeating units, and a UV photon carries enough energy to snap a chemical bond in that chain — this is chain scission (bond rupture). Broken chains are shorter, so the material loses strength and flexibility and turns brittle; the freed bond fragments react with oxygen (photo-oxidation) to form new groups that discolor the surface (yellowing) and generate a powdery degraded layer (chalking). Other environments attack differently: some solvents cause swelling as they soak in and force chains apart, some cause outright dissolution, and heat plus oxygen drives thermo-oxidative aging even in the dark. Unlike metal corrosion there is no electrode potential and no anode — the damage is direct bond-breaking chemistry, not an electrochemical cell.
The defenses are additives blended into the plastic. UV absorbers and, most effectively, hindered-amine light stabilizers (HALS) intercept the harmful reactions before they cut chains; antioxidants mop up the reactive fragments; and plain carbon black is a cheap, superb UV screen, which is why outdoor pipes, tires, and cable jackets are so often black. Choosing an inherently stable polymer (fluoropolymers, acrylics) and keeping parts out of direct sun also help. Two honest notes: 'plastics last forever' is only half true — they are hard to biodegrade yet they weather and embrittle within years outdoors; and the same UV vulnerability that ruins litter is deliberately engineered into some products so they break down faster.
A cheap unstabilized polypropylene rope left on a boat deck can lose most of its strength in a single summer: sunlight breaks its chains, the surface chalks white, and the fibers snap under a light pull — while an identical rope loaded with UV stabilizers and carbon black stays strong for years in the same sun.
UV stabilizers and carbon black are why outdoor plastics are so often black and why cheap clear ones fail fast.
Polymer weathering is not electrochemical corrosion — there is no anode, cathode, or electrode potential; the damage is direct bond-breaking by UV, oxygen, heat, or solvents, so metal-corrosion cures like cathodic protection do nothing and the fixes are stabilizer additives and material choice instead.