Reinforced PC for Outdoor Parts: Which Weather Resistance Metrics Actually Matter

Yellowing is only the visible part

UV exposure triggers photo-oxidative rearrangement of the carbonate linkage in PC — the Photo-Fries rearrangement — producing yellow chromophores. That is why transparent PC visibly yellows outdoors. UV absorbers slow it but do not stop it.

The dangerous change happens below the surface: photo-oxidation also cleaves chains, dropping molecular weight in the skin layer and forming microcracks. Colour barely shifts while impact strength can lose more than half.

Which tests to specify

Standard practice is xenon-arc ageing per ISO 4892-2 or ASTM G155 for 1000 or 2000 hours, then measuring colour shift ΔE and retained notched impact. Many OEMs demand ΔE below 3 and impact retention above 70%.

Where salt spray or acid rain is present — coastal or industrial areas — add chemical resistance testing, since combined stress and media accelerate cracking.

How to improve it

Higher ABS content means worse weathering; that is the main limit on outdoor PC/ABS. Prefer neat PC systems, or better-weathering materials such as PEI or ASA.

On the PC side, hindered amine light stabilisers combined with benzotriazole UV absorbers, plus a UV-cured hard coat on the surface, currently gives the best cost-performance package.