Polyoxymethylene (POM), designated POM-H or POM-C under ISO 9988-1, is specified for outdoor mechanisms, exterior window guides, fuel-system components, and snap-fit clips where dimensional stability under load is required. In these applications, the phrase UV-Stabilised POM Outdoors: What Black Pigment Does and Does Not Fix defines a material-selection boundary rather than a broad performance guarantee. The black pigment used in commercial UV-stabilised grades is predominantly furnace carbon black at loadings between 0.5 wt% and 2.5 wt%. Its function is primarily ultraviolet screening and radical scavenging, not bulk protection against acidolysis, thermal oxidation, or processing-derived defects. A moulded part can therefore retain its black colour while losing surface molecular weight. Because the term UV-stabilised is not a single ISO definition, outdoor claims must be tied to a defined test protocol, specimen geometry, and weathering regime.

Why Does Solar Ultraviolet Exposure Propagate Chain Scission in Polyoxymethylene?

Absorption of terrestrial solar UV by the polyoxymethylene backbone begins not with strong intrinsic chromophores but with chromophoric impurities, hydroperoxide groups, and thermal oxidation products generated during melt processing. Excitation at the methylene ether linkage produces alkoxy and methyl radicals by α-scission. Subsequent autoxidation forms hydroperoxides that decompose to formaldehyde and formic acid. Formic acid can protonate the acetal oxygen, initiating acid-catalysed hydrolysis of the main chain from the surface inward. The measured consequence is a loss of tensile elongation at break per ISO 527-2 or ASTM D638 before a large loss of tensile strength, because localised chain scission reduces fracture energy while retaining some short-term load-bearing capacity. Surface chalking is the visible result of oligomeric fragments and freed pigment particles remaining after volatile formaldehyde escapes.

Accelerated weathering of black UV-stabilised POM under ASTM G155 with daylight filters typically shows the split between pigment shielding and bulk stabilisation. A grade with a well-dispersed 1.5 wt% furnace black can maintain notched Charpy impact strength above 70% of its initial value after 2000 h, whereas a natural UV-stabilised grade of similar melt flow rate may fall below 50%. These values are strongly sample-thickness-dependent. Thin sections with a high surface-to-volume ratio degrade faster because the embrittled surface layer occupies a larger fraction of the load-bearing cross-section. Retention data generated on 4.0 mm plaques is therefore not transferable to snap-fit arms with a wall thickness of 1.2 mm. Published data for specific carbon black grades in this exact configuration is limited because most manufacturer literature reports colour and gloss stability rather than fracture mechanics after weathering.

Furnace Black Screening, Surface Temperature Rise, and Radical Termination in a Semi-Crystalline Polyether

Carbon black functions in POM by absorbing UV across the 300 nm–400 nm range and converting the energy to heat within the first few micrometres of the exposed surface. Primary particle size, aggregate structure, and dispersion quality define the screening depth. Typical furnace blacks for POM have primary particle diameters of 18 nm–25 nm and require high-shear dispersive mixing to prevent agglomerates larger than 20 μm. At 2.0 wt%, the pigment can reduce UV transmission through a 3.0 mm plaque to below practical measurement limits. This does not mean the surface itself is stabilised. The black surface is also a thermal radiator; under full solar load, black POM can reach surface temperatures 10 K–20 K higher than an equivalent white or natural surface. The higher temperature accelerates hydroperoxide decomposition and formaldehyde evolution in the exposed layer. Carbon black partially compensates by terminating peroxy radicals at quinone and phenolic surface groups, but that radical inhibition is confined to the immediate pigment-polymer interface.

The tabulated ranges indicate why carbon black is specified for outdoor POM. Gloss retention per ASTM D523 at 60° and tensile elongation retention per ISO 527-2 improve with carbon black loading, but the improvement is not linear. Beyond 2.0 wt%, viscosity rise and weld-line strength reduction become more significant than additional UV screening. Carbon black also reduces melt flow rate by 10%–30% relative to natural polymer, depending on aggregate structure and dispersive mixing. In extrusion and injection moulding, the screening effect can lower the apparent degradation onset temperature by increasing viscous dissipation; actual melt temperature must be monitored with immersion depth thermocouples, not barrel set-point alone.

When Black Pigmentation Fails to Arrest Acid-Catalysed Unzipping or Interior Oxidation

The central limitation of carbon black in POM is that it cannot neutralise acidic species that initiate acetal unzipping. Outdoor atmospheres containing sulphur dioxide, nitrogen oxides, or chloride salts can deposit reactive species on the surface. In wet conditions these form acids that attack the polymer independently of UV exposure. A black part may show no visible colour change while the surface molecular weight drops, because carbon particles remain in place on the eroding surface. Carbon black also does not prevent formaldehyde emission from the bulk after the part reaches service temperatures above 60°C. Residual hydroperoxides formed during processing continue to decompose, and the emitted formaldehyde can oxidise to formic acid in the presence of moisture. If the part is under mechanical load, stress accelerates propagation of surface cracks. This is why weathering tests using unstressed black plaques under ASTM G155 or ISO 4892-2 can fail to reproduce cracking observed in press-fit, snap-fit, or bolted assemblies.

A production-scale field failure in an exterior window-lift guide moulded from UV-stabilised POM-C with 1.5 wt% furnace black illustrates the boundary. The part was moulded on a 1300 kN injection machine with a hot-runner valve gate, melt temperature 205°C, and mould temperature 90°C. After 18 months in a high-UV, high-rainfall location, longitudinal cracks originated at the gate. Microtome sections showed an undispersed carbon black agglomerate band along the flow front and a high degree of molecular orientation at the gate. The black pigment had blocked UV transmission into the bulk; the crack initiation site was not a UV-limiting surface. It was a shear-induced weakness created before weathering began. The failure demonstrates that black pigment cannot fix oriented weak regions, weld lines, or carbon black agglomerates. Those require dispersive screw elements, gate sizing, and low-shear melt delivery rather than higher carbon black content.

Accelerated Wet-and-Dry Cycling Is More Discriminating Than Continuous Xenon Exposure for Black POM

Moisture condensation cycles in ISO 4892-2 and ASTM G154 expose a different failure mechanism than continuous dry UV exposure. During the wet phase, water extracts formaldehyde and formic acid from the degraded surface. The acidic extract can autocatalyse further surface hydrolysis, and repeated swelling and desorption of the surface layer produces microcracking. Carbon black does not change the water absorption of the polyoxymethylene matrix, which is low but not zero. It also does not prevent acid-extract recycling under condensation. For outdoor qualification, black UV-stabilised POM should be tested with at least one wet/dry exposure protocol, not continuous xenon-only methods, because xenon-only protocols overestimate pigment screening. UL 746C outdoor suitability evaluations for electrical enclosures similarly assess UV exposure, water immersion or spray, and mechanical impact at low temperature; the standard addresses a material system, not a single pigment performance. Impact values after weathering should be reported as notched Charpy per ISO 179-1/1eA or Izod per ASTM D256, because unnotched data can remain high even after severe surface embrittlement.

Compounding black UV-stabilised POM requires moisture control. POM pellets are pre-dried at 80°C for 3–4 h to below 0.1% moisture before entering a 40:1 L/D co-rotating twin-screw extruder. Barrel set-points are normally held between 190°C and 210°C; excursions above 230°C produce rapid formaldehyde evolution, even in black grades. Carbon black cannot scavenge this gas; it can amplify vent and vacuum-pump fouling because carbon particles collect decomposing oligomers. Atmospheric venting followed by vacuum venting at -0.08 MPa is used to remove volatiles, but vent blockage remains a known production limit. The melt is screened through a 60 μm or finer filter to control agglomerates. These process controls are required before outdoor performance can be meaningfully assessed, because weathering data on poorly dispersed black POM reflects agglomerate flaws rather than UV resistance.