Short answer
Plastics yellow when light, heat and oxygen create colour-absorbing structures in the polymer or its additives. Which plastics yellow depends on chemistry: PVC loses hydrogen chloride and forms conjugated double bonds that absorb blue; aromatic polymers such as polystyrene, polycarbonate and styrene-based blends oxidise into coloured products; polyethylene and polypropylene mostly crack and chalk instead. Stabilisers slow all of this, but they are consumed or lost over the years, which is why old plastics are most at risk.
A review of weathering studies sets out the pattern. Fluoropolymers, silicones and acrylic (PMMA) absorb little UV and resist oxidation, so they stay clear for a long time. Polyolefins such as polyethylene and polypropylene degrade by oxidation that introduces carbonyl groups and breaks chains, but they do not generally yellow: they crack, craze and chalk at the surface. Polyvinyl chloride behaves differently: it loses hydrogen chloride and forms sequences of double bonds, which absorb blue and violet light, so PVC turns yellow, then brown, and becomes brittle. Aromatic polymers — polystyrene, polyesters, polycarbonate — absorb UV in their aromatic rings, which oxidise to coloured products, and in these materials UV absorbers and stabilisers are less effective than in aliphatic polymers.
Many cream and white casings of household appliances, computers and games consoles were moulded from styrene-based plastics such as ABS (acrylonitrile–butadiene–styrene) or high-impact polystyrene. Their aromatic styrene units put them in the group that forms coloured oxidation products, and the rubbery butadiene phase that makes ABS tough is itself vulnerable to oxidation. Casings that sat in sunny rooms often yellow unevenly, darkest on the faces that received most light and paler where a label or another object shaded them — a sign that light, not age alone, did the work. Colour of the original matters too: a study of 3D-printed ABS-type material aged under a xenon lamp found that mechanical and optical changes differed from one pigment colour to another.
Manufacturers add stabilisers to slow ageing: UV absorbers that turn UV into heat, hindered-amine light stabilisers (HALS) that trap radicals and break the oxidation chain, and antioxidants for processing and service. Pigments help too; carbon black, titanium dioxide and iron oxides absorb or scatter UV near the surface, reducing colour loss. But additives are consumed as they work and some evaporate or migrate out. The Canadian Conservation Institute notes that old plastics are more at risk because their stabilisers may have been lost, and that some plastics — cellulose esters and PVC among them — are prone to losing plasticisers as well. Stabiliser breakdown products can themselves be coloured; in recycled polyethylene, spent antioxidants leave a faint yellow tint.
For museum objects CCI recommends keeping illuminance as low as possible and never above 150 lux, with UV below 75 µW/lm, because UV is damaging and for some plastics visible light also causes deterioration. Heat speeds every reaction, so keep plastics away from radiators, lamps and sunny windowsills; some rubbers and plastics benefit from low-oxygen storage. For everyday objects the same logic applies at home: shade from direct sun and keep them cool. Once a plastic has yellowed, the colour change is chemical and cannot be reversed without further chemistry; home bleaching treatments act on the surface, can weaken the plastic and may leave it prone to yellowing again, so they are not recommended here for anything of value.
| Plastic | Typical colour change | Why |
|---|---|---|
| PMMA (acrylic), fluoropolymers, silicones | Little | Absorb little UV; resist oxidation |
| Polyethylene, polypropylene | Little yellowing; surface cracking and chalking | Oxidation introduces carbonyls and breaks chains |
| PVC | Yellow to brown; brittle | Loss of HCl forms conjugated double bonds |
| Polystyrene, ABS and other styrenics | Yellowing, especially where lit | Oxidation of aromatic rings to coloured products |
| Polycarbonate, polyesters | Yellowing | Aromatic groups absorb UV and oxidise |
| Cellulose esters (nitrate, acetate) | Discolouration with other decay | Loss of plasticiser and chemical instability |
Why: Light-driven oxidation of a styrene-based plastic; the sticker shaded the area beneath.
Fix: Keep it out of sunlight; the change cannot be undone without treatments that risk the plastic.
Why: PVC dehydrochlorination under sun and heat, as its stabilisers are used up.
Fix: Cleaning removes dirt but not the chemical change; frames with better stabilisation or coatings resist it longer.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Thermoplastic polyolefins do not undergo yellowing on weathering but their surfaces crack; PVC loses HCl and forms double bonds, resulting in yellowing and brittleness; in aromatic polymers such as polystyrene, polyesters and polycarbonates, UV absorption leads to oxidation of the aromatic rings and coloured products.
FactStrong evidence
Hindered amine light stabilisers and UV absorbers slow polymer photodegradation, and UV-absorbing pigments such as carbon black, titanium dioxide and iron oxides also provide protection; UV absorbers and HALS are less effective in aromatic polymers.
ConventionStrong evidence
CCI recommends illumination for plastics and rubbers as low as possible and never above 150 lux, with UV below 75 µW/lm, and notes that old plastics are more at risk because stabilising additives may have evaporated.
Source: Care of Objects Made from Rubber and Plastic — CCI Notes 15/1
FactLimited evidence
In 3D-printed ABS-type material aged under a xenon arc lamp for up to 450 hours, mechanical and optical properties changed differently depending on the colour of the material.
Caveat: One 3D-printing material in five colours under accelerated ageing; not injection-moulded appliance plastics.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.