Why plastics age outdoors
Outdoor polymer failure is usually a coupled process, not a single-driver problem. UV radiation initiates free-radical chain reactions at the surface; heat accelerates every reaction already under way, including the ones UV started; ozone attacks unsaturated rubber backbones under strain through a completely different chemistry; moisture plasticizes, swells or hydrolyzes susceptible chemistries; and thermal or humidity cycling turns those slow chemical processes into a mechanical fatigue problem by repeatedly stressing a part that is also getting chemically weaker over time.
The failure that finally shows up - a cracked connector housing, a chalky enclosure, a seal gone hard and brittle - is usually the visible end of two or three of these mechanisms running together for years. The interesting engineering question is rarely "is UV bad for plastic?" but which specific mechanism dominates for which material, in which climate.
The six stressors
Climetry measures outdoor plastic ageing along six exposure channels. Each does a physically different thing to a polymer, which is why a single combined score is never the whole story for an engineer choosing a material.
UV / photo-oxidation
UV photons break C-H and C-C bonds at the surface, generating free radicals that react with oxygen. The resulting hydroperoxides decompose into more radicals, so the reaction is self-propagating once started - ending in chain scission (embrittlement, cracking) or crosslinking (hardening, discoloration).
Ozone / elastomer cracking
Ground-level ozone reacts almost exclusively with carbon-carbon double bonds. For unsaturated rubbers under even modest strain, this produces a specific signature: fine cracks running perpendicular to the strain direction. Saturated or silicon-backbone elastomers are essentially immune.
Temperature (thermal oxidation)
Heat rarely starts a new degradation pathway - it accelerates whichever ones are already running. The rate follows an Arrhenius relationship: every roughly 10 C of warming can double a given reaction rate, though the real activation energy is material- and mechanism-specific.
Temperature cycling
Repeated expansion and contraction stresses a part mechanically, independent of chemistry. It matters most where a plastic interfaces rigidly with a different material - a metal insert, a press-fit contact - because the two expand at different rates.
Humidity (moisture level)
Water attacks hydrolytically-susceptible backbone linkages - esters, carbonates, amides - cutting chains and reducing molecular weight, usually invisibly until a mechanical load reveals the lost toughness. Hygroscopic polymers like nylon also physically swell as they absorb water.
Humidity cycling
Repeated wetting and drying re-delivers fresh water to reactive sites faster than constant humidity would, cycles hygroscopic parts through swelling and shrinking, and drives the classic wet-dry blistering failure mode in coatings.
Material sensitivity table
Eleven material families used across automotive, EV, electronics, solar and HVAC applications, rated against all six stressors for typical unstabilized-to-moderately-stabilized grades. Real formulations vary by stabilizer package, filler and pigment - this is a starting map, not a specification.
| Material | UV | Ozone | Temp. | Temp. cycling | Humidity | Humidity cycling | Typical use |
|---|---|---|---|---|---|---|---|
| PE / PP | Very high | Low | Moderate | Moderate | Low | Low | Cable insulation, housings, packaging |
| PVC | High | Low | High | Moderate | Moderate | High | Cable jacketing, conduit, roofing membranes |
| PA6 / PA66 (nylon) | High | Low | High | Very high | High | Very high | Connector housings, under-hood parts, gears |
| Polycarbonate (PC) | High | Low | Moderate | Moderate | High | Moderate | Glazing, lenses, enclosures |
| ABS | High | Moderate | Moderate | Moderate | Low | Low | Automotive trim/housings, enclosures |
| PBT / PET | Moderate | Low | High | High | Very high | High | Electrical connector housings, under-hood parts |
| TPU (ester-type) | Moderate | Moderate | Moderate | Moderate | High | High | Cable jackets, flexible seals, wearables |
| EPDM rubber | Moderate | Low | Moderate | Low | Low | Low | Weatherseals, gaskets, radiator hoses |
| NBR (nitrile) | High | Very high | Moderate | Moderate | Low | Low | Fuel/oil seals, O-rings |
| Silicone rubber | Low | Low | Low | Low | Low | Low | EV battery seals, high-reliability gaskets |
| Powder coatings / paint | Very high | Low | Moderate | Moderate | High | Very high | Enclosure finishes, outdoor housings |
Material profiles
The mechanism behind each rating, and the failure it actually produces in the field.
Polyethylene & polypropylene (PE / PP)
Polypropylene's tertiary carbon-hydrogen bonds are unusually reactive to UV-initiated radicals, which is why unstabilized PP embrittles outdoors faster than almost any other commodity plastic. PE is somewhat more resistant but follows the same chain-scission pathway. Both are essentially immune to ozone cracking (fully saturated backbone) and don't hydrolyze. Field failure looks like surface chalking, a fine crack network, and a sharp drop in impact strength, usually starting at the sun-facing surface.
PVC
PVC degrades by losing hydrogen chloride from its backbone - a self-accelerating reaction, since the released HCl catalyzes further breakdown, and it can proceed thermally without any UV at all (UV accelerates it further). The visible signature is progressive discoloration - pale yellow, then pink or amber, then brown - well before mechanical properties collapse. Flexible, plasticized PVC cable jacketing has a second failure mode: repeated wet-dry cycling leaches the plasticizer out, and the material stiffens and cracks at flex points years before the base polymer would fail.
Polyamide - PA6 / PA66 (nylon)
Nylon is unusual: it is semi-crystalline, so temperature cycling drives real warping, and it is strongly hygroscopic - some grades absorb several percent water by weight, which physically swells the part and lowers its stiffness. Sustained heat and humidity together also cause real hydrolytic chain scission ("nylon reversion"). The combination matters most where nylon is rigidly interfaced to metal - a connector housing around metal pins experiences both thermal-cycling mismatch and humidity-cycling dimensional swings on the same interface, which is exactly the kind of contact micro-motion that can drive fretting corrosion at the electrical connection itself.
Polycarbonate (PC)
PC is tough and thermally stable relative to commodity plastics, which is why it is chosen for glazing and lenses - but it yellows and loses impact strength under UV faster than its reputation suggests, which is exactly why virtually all outdoor PC ships with a UV-protective hardcoat. The carbonate linkage also hydrolyzes under combined heat and humidity, cutting chain length and causing brittleness that often is not visible until an impact reveals it.
ABS
ABS is a two-phase material - a rigid matrix around rubbery polybutadiene particles for impact toughness - and the polybutadiene phase is what UV attacks, because it retains backbone unsaturation. Unpainted outdoor ABS shows a recognizable pattern: surface yellowing or chalking that tracks sun-exposure geometry, with gloss loss and embrittlement following. Painted or UV-stabilized ABS is largely protected - the failure is really a coating/stabilizer story, not an intrinsic ABS limitation.
PBT / PET
Both are polyesters, and the ester linkage hydrolyzes - slowly at room temperature, much faster as temperature rises, which makes hot-and-humid climates specifically damaging rather than either variable alone. This is a well-documented field-failure pathway for glass-filled PBT electrical connector housings: the housing looks fine, then cracks under an entirely normal mechanical load after the polymer's molecular weight has quietly dropped for years. UV and ozone are largely irrelevant here - humidity and temperature do almost all of the explanatory work.
TPU - thermoplastic polyurethane
TPU is really two families under one name. Polyester-based TPU hydrolyzes readily and should be treated like PBT for humid climates; polyether-based TPU is far more hydrolysis-resistant, at a cost premium. Separately, aromatic TPU (cheaper, common in cable jackets) yellows visibly under UV, while aliphatic TPU stays optically stable but costs more - a genuinely actionable distinction, since "TPU" alone says almost nothing about which failure mode to expect.
EPDM rubber
EPDM's defining feature is what it does not do: its ethylene-propylene backbone is saturated, with unsaturation confined to a small pendant diene group rather than the main chain. That single structural fact is why EPDM largely resists both ozone cracking and UV embrittlement, and is the specific reason EPDM displaced natural rubber and SBR in outdoor automotive door seals and window weatherstripping. Long-term failure, when it happens, is gradual hardening and compression-set loss, not the sudden ozone-crack signature seen in unsaturated rubbers.
NBR - nitrile rubber
NBR is the textbook ozone-cracking material: it retains backbone unsaturation for the oil and fuel resistance that makes it useful, and that same unsaturation is exactly what ozone attacks. Under even modest strain, ozone exposure produces the classic pattern - fine cracks perpendicular to the strain direction, visible within weeks to months outdoors without protection. In practice this rarely fails products, because NBR is almost always specified for sealed, UV-and-ozone-shielded locations precisely because its weatherability is poor.
Silicone rubber
Silicone's backbone is silicon-oxygen, not carbon-carbon, so it has no UV chromophore and no ozone-reactive double bond to attack - both mechanisms that dominate every carbon-backbone material above simply do not apply. Combined with wide-range thermal stability, this is why silicone is the default choice for the highest-reliability outdoor seals: EV battery pack gaskets and long-service electronics enclosures. When silicone fails outdoors, it is almost always mechanical (tear, compression set) rather than an environmental-exposure story.
Powder coatings & paint systems
"Chalking" - a dull, powdery surface that rubs off on a finger - is literally the visible residue of UV-degraded binder resin left behind after the surrounding polymer has eroded away, exposing pigment particles. Separately, coatings fail by blistering and adhesion loss, which is fundamentally a wet-dry cycling phenomenon rather than a constant-humidity one - a coating in a climate with dramatic day/night or seasonal RH swings can blister faster than one in a climate that is simply humid all the time.
Synergy effects
The combinations that make climate ageing worse than any single-variable exposure test predicts.
UV-generated surface microcracks give water direct access to the bulk polymer - part of why ASTM G154 cycles UV with condensation rather than testing UV dry.
The 85 C / 85% RH condition used across electronics and PV qualification exists because hydrolysis and diffusion both accelerate with temperature - hot-humid climates disproportionately damage hydrolysis-sensitive materials.
Both peak together on hot, sunny, low-wind days - ground-level ozone is itself a UV-driven photochemical product - so a summer smog season delivers worst UV and worst ozone in the same weeks.
A part that has lost molecular weight to UV, heat or hydrolysis is mechanically weaker exactly when thermal or humidity cycling is loading it - it can pass a cycling test early in service and fail the identical load later purely because the chemistry moved first.
Climetry's scoring model
Climetry Plastic Ageing Exposure combines four scored channels - UV, ozone, thermal oxidation and moisture/hydrolysis - each 0-100, weighted 40/20/25/15, into one Plastic Ageing Exposure (PAE) class from PAE-1 (Mild) to PAE-5 (Severe). Temperature and humidity cycling are measured separately in Climetry's engineering-statistics reporting today and are the next planned channel; see Limitations below.
Thermal-oxidation and moisture/hydrolysis are computed from real local temperature and humidity data where the required data products are available for a location: hourly CEY data where Climetry has hourly coverage, and the monthly ERA5 climatology as a coarser fallback otherwise. The thermal channel is scored the right way for an exponential (Arrhenius) process - averaging the acceleration factor across the full temperature series before scoring, not averaging already-capped scores, which would silently discard the disproportionate contribution of the hottest hours.
Worked example: reading the channel breakdown
These are real Plastic Ageing Exposure scores, computed by Climetry's production model for two locations in the Baden-Wuerttemberg free-Pro demo region.
Both locations land at the same overall class, PAE-3 (Moderate) - but the combined class alone would not tell an engineer which failure mechanism actually matters here. The per-channel breakdown does: UV is clearly the dominant driver at both locations, with a real but secondary ozone signal and comparatively low thermal-oxidation and moisture exposure. Cross-referencing that pattern against the material table above points toward UV-sensitive materials - unstabilized PE/PP, ABS, PVC, unprotected polycarbonate - as the components most worth reviewing for these two locations, while ozone-sensitive-only materials like NBR are a lower relative priority here.
Plastic ageing FAQ
What causes plastic to crack or become brittle outdoors?
Most outdoor plastic cracking comes from UV-initiated chain scission (breaking polymer chains into shorter, more brittle fragments) combined with thermal oxidation. Ozone causes a specific crack pattern in unsaturated rubbers, and hydrolysis-sensitive plastics like PBT, PC and ester-based polyurethane can also embrittle from heat and humidity alone.
Does ozone damage all types of rubber?
No. Ozone only attacks carbon-carbon double bonds in a rubber's backbone. Nitrile (NBR) and natural rubber retain backbone unsaturation and crack readily under strain. EPDM's backbone is mostly saturated, and silicone's backbone is silicon-oxygen with no reactive double bonds at all - both are highly ozone resistant, which is why they dominate outdoor automotive seals.
Why does PVC turn yellow, pink or brown in sunlight?
PVC degrades by losing hydrogen chloride (HCl) from its backbone, a reaction that is self-accelerating because the released HCl catalyzes further breakdown. It can happen from heat alone, though UV accelerates it. The visible color change - pale yellow, then pink or amber, then brown - is an early-warning sign that appears well before mechanical properties collapse.
What is hydrolysis in plastics?
Hydrolysis is a chemical reaction where water breaks a susceptible backbone linkage - the ester bond in polyesters (PET, PBT) and some polyurethanes, the carbonate bond in polycarbonate, or the amide bond in nylon. It cuts polymer chains, reduces molecular weight, and typically shows up as brittleness or cracking under normal mechanical loads, often with no visible warning beforehand.
Which plastics are most resistant to UV and outdoor weathering?
Among common engineering plastics, none are fully UV-immune without stabilization, but silicone rubber has essentially no UV-reactive chemistry due to its silicon-oxygen backbone. EPDM rubber also weathers well. For rigid thermoplastics, UV resistance depends heavily on the stabilizer package (HALS, UV absorbers) and pigmentation (carbon black in particular) rather than the base polymer alone.
Is humidity alone enough to degrade plastic, or does it need heat too?
For hydrolysis-sensitive materials like PBT, PC and ester-type polyurethane, temperature matters as much as humidity - the reaction rate roughly follows an Arrhenius relationship, so hot-humid climates are disproportionately damaging compared to what either variable predicts alone. This is why electronics and photovoltaic qualification testing uses combined damp-heat conditions (commonly 85 C / 85% RH) rather than humidity alone.
What is a Plastic Ageing Exposure (PAE) class?
PAE is Climetry's screening classification (PAE-1 Mild to PAE-5 Severe) for how aggressively a location's climate ages outdoor polymers and elastomers. It combines UV, ozone, thermal-oxidation and moisture/hydrolysis exposure into one index, weighted 40/20/25/15 respectively. It is a screening tool for material selection and qualification planning, not a service-life prediction or a certified laboratory-equivalent rating.
What is the difference between EPDM and NBR rubber for outdoor seals?
EPDM's ethylene-propylene backbone is mostly saturated, so it strongly resists both UV embrittlement and ozone cracking, which is why it is the standard choice for outdoor weatherseals and gaskets. NBR (nitrile) retains backbone unsaturation for oil and fuel resistance, but that same unsaturation makes it a classic ozone-cracking material - it is normally specified only for sealed, non-UV-exposed locations like fuel lines and internal O-rings.
Limitations
- PAE classes are screening indicators for material selection and qualification planning, not service-life predictions or failure probabilities.
- PAE classes are not equivalent to ISO 4892, ASTM G154 or ASTM D1149 laboratory test results, and do not replace material-specific qualification testing.
- The weighting (UV 40%, ozone 20%, thermal 25%, moisture 15%) is a screening default, not material-specific - the material table above is a starting point for future material-specific weighting.
- Thermal-oxidation and moisture/hydrolysis prefer hourly CEY data (today: the Baden-Wuerttemberg free-Pro region) and fall back to the coarser monthly ERA5 climatology where that is available instead; both channels stay unavailable for a location outside the UV/ozone dataset's own coverage. The score's resolution is disclosed per location.
- Temperature cycling and humidity cycling are not yet scored channels in the PAE index, even though Climetry already computes both from hourly CEY data for its engineering-statistics reporting - a concrete next step, not a hidden gap.
References
- ISO 4892-1:2024, Plastics - Methods of exposure to laboratory light sources.
- ASTM G154, Standard Practice for Operating Fluorescent UV Lamp Apparatus for Exposure of Nonmetallic Materials.
- ASTM D1149, Standard Test Methods for Rubber Deterioration - Cracking in an Ozone Controlled Environment.
- Wypych, G., Handbook of Material Weathering, ChemTec Publishing.
- Hawkins, W. L., Polymer Degradation and Stabilization, Springer.
- Layer, R. W. & Lattimer, R. P., "Protection of rubber against ozone," Rubber Chemistry and Technology.
- Climetry Plastic Ageing Exposure methodology, internal reference (four-channel model).
Screen a deployment location for plastic ageing risk.
Get UV, ozone, thermal-oxidation and moisture/hydrolysis scores, a PAE class, monthly profiles and material-selection context.