Polycarbonate (PC) produced from bisphenol A by interfacial phosgenation or melt transesterification with diphenyl carbonate is specified for transparent sheet where initial luminous transmittance is approximately 89 % for a 3.0 mm plaque and initial yellowness index is below 2.0 under ASTM E313-20. The aromatic carbonate chromophore absorbs strongly in the 290–315 nm region at the earth’s surface, and solar UV-B photons in this band drive a non-oxidative photorearrangement that generates phenolic and ketonic chromophores. A failed sheet therefore exhibits yellowness that is measurable as an increase in YI, typically from 1.2–1.5 to 12–20 after prolonged xenon-arc or Florida outdoor exposure, depending on stabilizer package and cap layer. The following sections examine the photochemistry responsible for this failure mode in transparent PC sheet.

What Photochemistry Occurs in the Solar UV-B Window?

The carbonate linkage in bisphenol A polycarbonate has an n→π* and π→π* absorption envelope with λmax near 260–280 nm and a tail that overlaps solar radiation in the 295–315 nm interval. Absorption of a UV-B photon at 300 nm corresponds to approximately 400 kJ/mol, which exceeds the C–O bond dissociation energy of the aryl carbonate linkage. Excited-state cleavage followed by in-cage radical recombination produces phenyl salicylate and 2,2′-dihydroxybenzophenone as primary photo-Fries products. Phenyl salicylate absorbs with a maximum near 310–320 nm; the dihydroxybenzophenone absorbs near 355 nm. Both photoproducts absorb more strongly than the original carbonate in the 300–360 nm window, shifting attenuation deeper into the UV-A region and creating a yellow-brown appearance as higher-order oxidation products such as quinones extend absorption to 420–450 nm. Because light below 310 nm is attenuated by the polymer itself, the primary photorearrangement is concentrated in the outermost 10–50 µm of a transparent sheet. This surface-localized chromophore accumulation explains why YI rises before tensile strength or elongation at break changes significantly in the bulk polymer. Field inspection of failed twin-wall PC sheet after 4 years in a high-UV location typically reveals exposed crowns with YI above 20 while shaded flutes below remain below 3.0; this gradient matches the 10–50 µm absorption depth and confirms that bulk mechanical properties are not necessarily the first indicator of photochemical failure.

Hydroperoxide-Mediated Oxidation Versus Primary Photorearrangement Under Humid UVA

At wavelengths longer than 340 nm, the aromatic carbonate unit absorbs weakly, and secondary chromophores formed by the initial photo-Fries reaction absorb UV-A and sensitise formation of singlet oxygen or abstract hydrogen from the methyl groups of bisphenol A. The isopropylidene bridge is the most vulnerable aliphatic site. Under humid service conditions, hydroperoxide formation on the tertiary carbon and subsequent decomposition generate alkoxy and hydroxyl radicals that attack the carbonate linkage and the aromatic rings. Phenoxy radical coupling and quinone formation are accelerated when relative humidity exceeds 60 %, because water plasticises the surface layer, increases oxygen diffusion, and hydrolyses a fraction of carbonate linkages to free bisphenol A. Free bisphenol A is readily photooxidised to quinoid species with absorbance in the 400–500 nm range. Hydrolytic chain scission is measurable by melt flow rate increase according to ISO 1133-1:2022; a PC grade with an initial MFR of 10 g/10 min at 300 °C under 1.2 kg may exceed 25 g/10 min after extensive humid UV exposure. This secondary oxidation pathway becomes dominant in greenhouse and roofing panels that see both condensation and UV-A through the full thickness of a translucent sheet.

During melt processing on a 75 mm twin-screw compounding line with 36:1 L/D and a gear pump feeding a 1,800 mm coat-hanger sheet die, barrel temperatures are maintained between 260 °C and 290 °C; melt temperatures above 310 °C introduce thermal oxidation chromophores that mimic and amplify subsequent photoyellowing. Desiccant drying with a dew point below -40 °C at 120 °C for 4 h is required when ambient relative humidity exceeds 55 %. Residual moisture above 0.02 wt% in the feed throat hydrolyzes carbonate bonds, releasing bisphenol A and generating melt-flow instability, surface silver streaking, and a yellow colour that is already present before weathering. On the same line, vent vacuum of at least -0.08 MPa and screw speed below 450 rpm are typical to limit shear heating and residence-time degradation; higher speed or a worn screw tip can reduce intrinsic viscosity and increase yellowness after only 500 h of subsequent UV exposure.

When the Cap Layer Carries a Benzotriazole Absorber During Coextrusion

A UV-shielding cap layer is coextruded on the exposed surface of PC sheet to absorb the 290–380 nm band before it reaches the bulk. A 50 µm cap layer containing 5 wt% of a benzotriazole such as 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol attenuates more than 99 % of incident radiation below 380 nm. Triazine UV absorbers with molecular weights above 600 g/mol are used where low migration is required under glazing or food-contact specifications. In a coextrusion line, a secondary extruder with 32:1 L/D and 40–50 mm screw diameter delivers the cap polymer at a melt temperature of 270–285 °C into a feedblock and multi-manifold sheet die; the cap thickness is controlled to ±5 µm through layer-adjusting restrictor bars. If the cap layer drops below 35 µm or the absorber concentration falls below 3 wt%, accelerated weathering under ISO 4892-2:2013 can produce YI increases above 5.0 within 1,000 h. The cap layer itself may yellow slightly due to sacrificial absorption, but this is confined to a superficial region and does not impair the bulk sheet until the absorber is depleted. In polycarbonate, hindered amine light stabilizers are generally avoided at processing temperatures above 280 °C because basic amine species accelerate hydrolysis of carbonate linkages; benzotriazole and triazine UV absorbers are the dominant stabilizer classes for transparent PC sheet.

Representative accelerated weathering data for 3.0 mm PC sheet under ISO 4892-2:2013, xenon arc, irradiance 0.51 W/m²/nm at 340 nm, black panel 65 °C, chamber dry bulb 38 °C, cycle 102 min light / 18 min light plus water spray are shown in Table 1. Exact values vary with grade, light source calibration, and humidity.

Yellowness Index Methods, Failure Thresholds, and Cyclic Xenon Exposure

Yellowness index is calculated from CIE tristimulus values measured with a spectrophotometer or colorimeter under ASTM E313-20 or the withdrawn but still cited ASTM D1925-70. The two methods use different illuminant/observer conditions; ASTM D1925-70 uses CIE illuminant C and the 2° observer, whereas ASTM E313-20 uses D65 and 10°. A change in YI of 2.0 units is typically visible to a trained observer in transparent sheet; building glazing specifications often set post-weathering YI limits between 3.0 and 5.0 after 2,000 h of ISO 4892-2:2013. Luminous transmittance measured per ASTM D1003-21 may remain above 85 % even as YI rises, because yellowness selectively attenuates the blue region (400–480 nm) while the total visible light transmission remains high. Haze is not a primary indicator of photoyellowing; surface microcracking and chalking introduce haze independently, and measurement per ASTM D1003-21 separates haze from yellowness. For outdoor PC sheet, a yellowing failure is therefore defined as a YI increase exceeding 3.0 units from the original value, combined with a loss of blue-edge transmission below 70 % at 450 nm.

Injection-moulded headlamp lenses and transparent covers are processed on 300-ton hydraulic clamping machines with 45 mm screw and 500 g shot capacity. Barrel set points from 270 °C to 300 °C, injection velocity of 80–120 mm/s, and mould temperature of 80–110 °C are typical. If melt residence time exceeds 15 min at 300 °C, yellowing at the gate and knit lines becomes visible under ASTM E313-20. Low-molecular-weight benzotriazole absorbers can volatilise during long residence at 300 °C; high-molecular-weight triazine absorbers are preferred for moulded parts requiring extended weatherability.

Table 2 lists the primary test designations used in qualification of PC sheet for weathering and optical properties.

Operational boundaries for PC sheet production and service require simultaneous control of moisture, melt temperature, UV-stabilizer dispersion, and cap-layer integrity. A specification that permits regrind addition may reduce initial resin cost but can increase yellowness variability if the regrind contains partially photorearranged polycarbonate from edge trim; the recycled fraction should be characterisable by MFR under ISO 1133-1:2022 and limited to 15 wt% when outdoor weathering resistance is critical. Polycarbonate sheet is incompatible with cleaning agents containing aromatic hydrocarbons or ketones; environmental stress cracking can occur at surface residual strain above 0.5 %, and the resultant microcrazes scatter light and accelerate moisture ingress. Under continuous condensation with high UV, a bare PC sheet without cap layer shows rapid yellowing, whereas a capped sheet retains YI below 5.0 after 2,000 h of ISO 4892-2:2013 in laboratory screening. For food-contact PC sheet, migration of UV stabilizers is regulated under FDA 21 CFR 177.1580 and EU 10/2011; benzotriazole absorbers must meet specific migration limits, which restricts loading levels in thin sheet. The photochemistry of the failed sheet is a competition between primary photo-Fries rearrangement, secondary hydroperoxide oxidation, and hydrolytic chain scission; each pathway has a distinct wavelength, moisture, and temperature dependence that stabilizer and cap-layer design must address.