Polycarbonate (PC) sheet subjected to outdoor weathering undergoes photochemical degradation initiated primarily by near-UV radiation between 290 nm and 350 nm. The carbonate linkage participates in photo-Fries rearrangement and chain scission; the observable responses are yellowing, gloss loss, microcracking, and retention loss in notched Izod impact measured by ASTM D256-23 or tensile elongation measured by ASTM D638-22. A co-extruded UV-protective cap is therefore specified on the weather-facing side of solid, twin-wall, multiwall, and corrugated PC sheets. The cap consists of a thin PC-based layer containing a high loading of benzotriazole or hydroxyphenyl-triazine UV absorber; the additive is not simply dispersed at low concentration through the bulk. In production, this layer is melt-fused to the substrate during sheet extrusion rather than applied as a secondary coating. The specification question for architectural and transportation glazing is UV-protected polycarbonate: what the co-extruded layer is and how thick it should be to survive the intended exposure period. Yellowness index is evaluated under ASTM E313-20; haze and luminous transmittance are evaluated under ASTM D1003-21; artificial weathering is conducted under ISO 4892-2:2013 or ASTM G155-21. Those standards do not themselves prescribe a cap thickness; they provide the degradation endpoints against which a given cap thickness is judged.

How Is the Co-Extruded Cap Layer Formed and Why Is It Not a Coating?

Two principal co-extrusion configurations are used: a feedblock attached to a single manifold die, or a multi-manifold die that maintains separate melt channels until the final die lip. For UV-capped PC sheet, the multi-manifold approach is preferred when the cap layer is very thin because it reduces residence-time differences between the cap and core melts. The cap layer melt is typically produced from the same PC base resin as the core to maintain a matching refractive index and melt rheology. The UV absorber masterbatch is compounded into the cap stream at high concentration; exact additive loadings are proprietary, but published studies on lamp-aged PC sheet indicate cap-layer absorber concentrations are an order of magnitude higher than whole-thickness bulk loadings. The cap is therefore an integral polymer phase, not a lacquer or silicone hardcoat; interlayer adhesion is governed by autoadhesion of two PC melts at 280 °C to 310 °C.

Commercial thickness ranges for this cap vary by product class. For multiwall and corrugated sheet, published technical literature frequently lists cap thicknesses between 30 µm and 60 µm; for solid flat sheet used in overhead glazing, caps between 50 µm and 80 µm are common. These values are not universal; they reflect the need to maintain a minimum absorbance across the entire weather surface after forming. Because UV absorber absorbance follows the Beer-Lambert relation, reducing cap thickness from 50 µm to 25 µm halves the UV-absorbing optical density only if absorber concentration is constant. In practice, suppliers may compensate with higher absorber concentration, but concentration is bounded by solubility limits, migration, plate-out on polishing rolls, and haze development. Benzotriazole absorbers operate by excited-state intramolecular proton transfer and convert absorbed UV into heat; triazine absorbers offer high molar extinction coefficients in the 300 nm to 400 nm range. Some suppliers specify that a given cap formulation reduces UV transmission at 350 nm to below 1%, but this target is not standardized and must be verified by spectrophotometry on the formed part.

Process capability also bounds the lower limit. A 30 µm cap layer extruded at a total sheet thickness of 10 mm represents only 0.3% of the cross-section. Maintaining uniform distribution across a 2,100 mm-wide sheet requires gear pumps or melt pumps on both extruders, tight die-bolt adjustment, and infrared or beta-transmission thickness gauging. Local cap-thickness thinning to 10 µm or less can occur at web edges or over ribs in multiwall profiles; these zones remain vulnerable even if the average cap thickness is within specification.

Accelerated weathering of unprotected PC under xenon-arc conditions with daylight filters produces measurable yellowness increase within 1,000 h to 2,000 h of exposure; exact values depend on irradiance level, black-standard temperature, and specimen backing. UV-capped PC in the same test can maintain lower yellowness, but published data for this specific configuration is limited because cap formulations and test cycles vary among suppliers. The critical evaluation is not only initial cap thickness but post-forming thickness distribution: thermoformed corner radii, ribs, and clamp areas can stretch the cap significantly. For a thermoformed part with 30% local draw ratio, a nominal 50 µm cap can thin to approximately 35 µm if deformation is uniform; sharp radii can produce greater local thinning. For high-exposure parts, forming simulations and cross-sectional microscopy are therefore required rather than reliance on average sheet thickness. Natural weathering under ASTM G7-15 or ISO 877-1:2009 may be specified for validation, but the correlation between xenon-arc acceleration and a given climate is material-specific.

Thermal and Rheological Constraints in Cap Layer Extrusion

Polycarbonate requires pre-drying to a residual moisture content below 0.02%. Extrusion dryers typically supply air at 120 °C with a dew point of -40 °C for 4 h to 6 h if ambient RH exceeds 60%. Moisture above this threshold causes hydrolysis in the melt, visible as bubbles, splay, or local haze at the cap-core interface. Melt temperatures are held between 270 °C and 310 °C; die temperatures are typically 290 °C to 310 °C for multiwall sheet. At the lower end, viscosity is high and melt fracture may appear; at the upper end, the UV absorber can degrade and plate out on die lips or polishing rolls.

Rheological matching is the principal processing conflict. The cap layer melt and core layer should have similar viscosity at the shear rates experienced in the co-extrusion feedblock or multi-manifold die. If the cap-layer melt is substantially more viscous, it may not spread evenly; if it is less viscous, it can encapsulate the core or migrate to the edges. A typical PC sheet extrusion resin has a melt volume-flow rate of roughly 6 cm³/10 min to 12 cm³/10 min at 300 °C under 1.2 kg, measured by ISO 1133-1:2022. Adding UVA masterbatch can alter this value depending on carrier resin and additive melting point; a higher MVR cap layer may be easier to spread but may also tear under draw. Extruder L/D ratios of 32:1 or 36:1 are typical, with vented barrels and downstream melt filtration. The UVA masterbatch is usually produced in a co-rotating twin-screw extruder with an L/D ratio of 40:1 before letdown into the cap melt stream; the sheet extrusion line itself may use single-screw extruders with separate melt pumps.

Interfacial instability is observed in production as a wavy or grainy appearance at the cap-core boundary when the melt flow ratios are incorrectly set. The condition is checked by coupling online melt pressure transducers before the feedblock and after the melt pumps. If the cap layer viscosity is too low, the cap material can encapsulate the core at the die lips and form an uneven edge bead; if too high, it can produce lens-like inclusions or transverse thickness bands at the central web. Corrective action is not always a temperature change; it may require changing the cap-layer melt pump speed or selecting a masterbatch with a lower letdown viscosity. Production records commonly describe die-lip oxidation of UVA-rich cap melt as a source of die lines that appear as haze bands. Alkaline or basic additives are incompatible with PC because they can accelerate hydrolysis. Hindered amine light stabilizers, common in polyolefins, are generally avoided in PC cap layers for this reason. UV absorbers used in PC cap layers are therefore selected from non-basic chemistries, mainly benzotriazole and triazine derivatives.

When the Cap Layer Must Exceed 50 µm for Architectural Glazing

Thickness selection is not governed by an absolute universal value but by exposure category, sheet geometry, and expected service life. In high-UV environments—high-altitude glazing, equatorial roofs, or coastal locations with reflected UV—a cap at the upper end of the 50 µm to 80 µm range is used. In low-UV, shaded, or interior applications, a 30 µm cap may be sufficient. The rationale is not only optical density but mechanical robustness: thin cap layers are more sensitive to surface erosion, scratches, cleaning abrasion, and thermoforming stretch. A surface flaw that penetrates a 50 µm cap allows UV to reach the bulk only if the flaw depth exceeds 50 µm; the same flaw penetrates a 30 µm cap more easily. Cap layer thickness is controlled in production by gravimetric feeders, melt pumps, and haul-off speed. The ratio of cap extruder output to core extruder output establishes average cap thickness; for a PC density of 1.2 g/cm³, a 50 µm cap adds approximately 60 g/m² of cap material.

The ranges in the table are compiled from manufacturer technical literature for PC sheet; they are typical rather than universal. Project specifications should require a demonstrated cap thickness distribution on the formed part rather than a nominal extruder setpoint. Building code requirements do not routinely mandate cap thickness; they address fire performance, wind load, and impact classification. For example, ASTM D635-20 or EN 13501-1 may be cited for fire response, but these test methods do not measure cap durability. Cap thickness is selected through supplier technical data, natural weathering databases, and accelerated weathering verification.

Quality control of the co-extruded cap layer on a production line is performed by cutting cross-sections from the sheet edges and center, mounting them in resin, polishing, and imaging under transmitted light at magnifications sufficient to resolve a 10 µm layer. The total sheet thickness is monitored continuously with beta or X-ray gauges, but this method cannot resolve the cap layer because the cap and core are both polycarbonate. Optical microscopy or confocal laser scanning microscopy is therefore required; some formulations include a low level of a non-migrating optical tracer to increase contrast between cap and core, but tracer use must not affect haze per ASTM D1003-21. Suppliers of UV-capped PC sheet typically mark the weather-facing side with a peel-off film or print that indicates which surface carries the cap. Installing the sheet with the cap facing inward is a field failure mode because the bulk PC is then directly exposed to solar UV. This orientation error can be detected after exposure by yellowness index measurement per ASTM E313-20, but remediation requires replacement.

The cap layer does not significantly modify the basic recycling stream, although the UV absorber can be diluted in mechanical recycling and may reduce the value of clear recyclate. The cap layer protects against solar UV, but it does not eliminate moisture-induced hydrolysis, thermal oxidation above 120 °C, or surface damage from incompatible cleaning agents. Aromatic solvents, ketones, and strongly alkaline cleaners can attack the PC surface and penetrate the cap, leading to stress cracking. For exterior PC sheet, cleaning specifications should avoid abrasive pads that erode the cap; published field data on erosion rates for this specific configuration is limited, but surface roughness increase after cleaning can be measured by profilometry and is an indirect indicator of cap loss.