Polycarbonate (PC) is an amorphous thermoplastic produced by the interfacial or melt polymerization of bisphenol A with phosgene or diphenyl carbonate. The repeating carbonate linkage provides high melt viscosity, high ductility, and a measurable susceptibility to hydrolytic chain scission at processing temperatures. Unfilled general-purpose PC typically has a glass transition temperature of 145–150 °C measured by ISO 11357-2, a tensile stress at yield of 60–70 MPa under ISO 527-1/2, and a notched Izod impact of 60–80 kJ/m² at 23 °C under ISO 180/A. Melt volume-flow rate for injection molding grades spans approximately 3–80 cm³/10 min at 300 °C with a 1.2 kg load per ISO 1133-1:2022. The product family designation Polycarbonate and Its Alloys: Where PC, PC/ABS and PC/PBT Sit in the Family is best understood as a performance matrix rather than a linear hierarchy, with each grade positioned by its response to heat, impact, flow, moisture, and chemical exposure.

On production-scale injection molding lines, unfilled PC is processed with a general-purpose screw having an 18:1–24:1 L/D ratio and a compression ratio of 2.0–3.0. The melt temperature at the nozzle is normally held between 280 °C and 320 °C, while the mold surface temperature is maintained at 80–110 °C for clear grades. Insufficient mold temperature produces residual stress gradients in thick sections, which can be detected by polarized-light inspection and which lower practical impact once the part is exposed to hydrocarbons. Clamp force requirements follow from shrinkage behavior: PC exhibits relatively low isotropic mold shrinkage of 0.5–0.7% under ASTM D955-08(2014), but high-viscosity grades often require hydraulic injection pressures of 70–120 MPa. This can push tonnage requirements above those for semi-crystalline resins of similar wall thickness.

Because PC is amorphous, light transmission for a 3 mm injection-molded plaque is typically 88–90% by ISO 13468-1, and haze is below 1% when polished mold surfaces are used. The refractive index is approximately 1.586 at 589 nm, which contributes to high surface gloss but also makes scratch patterns visible. Prolonged UV exposure promotes photo-Fries rearrangement and yellowing of the phenyl carbonate structure; UV-stabilized grades slow this process but do not eliminate it. PC/ABS and PC/PBT are generally opaque unless special colorants or additives are used, so they are typically textured, painted, or molded in color for exterior applications.

What Limits the Utility of Unfilled Polycarbonate in Molded Components?

Unfilled transparent PC must be dried to a residual moisture level below 0.02% before melt processing. Failure to do so initiates hydrolytic degradation of the carbonate linkage, producing a measurable decrease in molecular weight, an increase in melt volume-flow rate, and splay or silver streaking on the part surface. Desiccant dryers operating with a supply-air dew point of −40 °C or lower and a drying temperature of 120 °C for 2–4 h are the standard production method; hopper residence times exceeding 4 h at that temperature can cause yellowing in some PC grades. At the press, the processing window is narrow compared with polyolefins. Melt temperatures below 270 °C create high viscosity and injection-pressure peaks, while temperatures above 340 °C accelerate thermal degradation and color shift. For transparent parts with wall-thickness transitions, mold-filling simulation and polariscope inspection are common because orientation and packing stresses become visible fractures in the presence of acetone or toluene under ISO 22088-2 environmental stress-cracking tests.

Because the combination of high impact and clarity is gained at the cost of chemical resistance, unfilled PC is stress-cracked or attacked by aromatic hydrocarbons, ketones, esters, and some chlorinated solvents. In automotive or industrial fittings where fuel, brake fluid, or ester-based lubricants contact the surface, unprotected PC parts can fail at strain levels far below the short-term tensile limit. This limitation drives the evaluation of PC/ABS and PC/PBT alloys rather than a simple move to a lower-impact semi-crystalline resin.

Mechanical and Thermal Gradients Across PC, PC/ABS, and PC/PBT

Data in Table 1 represent representative injection molding grades from commercial sources; each value is a typical range rather than a specification minimum. Test methods are included because differences in specimen geometry, molding parameters, and moisture history can alter impact and heat-deflection values more than the formulation itself.

The lower notched impact of PC/PBT in the table reflects the contribution of the semi-crystalline polybutylene terephthalate phase and its notch sensitivity, but the same phase reduces equilibrium moisture uptake and improves resistance to hydrocarbon cracking. PC/ABS occupies the middle ground: its tensile yield strength is lower than that of PC, yet its melt processing window widens because the ABS phase lowers viscosity at typical injection temperatures from 240 °C to 280 °C. Heat deflection temperature decreases relative to PC because the styrenic phase softens below 110 °C, which excludes its use in some underhood thermal environments unless heat-resistant ABS is selected.

On twin-screw extruders with an L/D ratio of 36:1–44:1, PC/ABS and PC/PBT compounding is preferred because back-venting and dispersion are more controllable than in single-screw systems. For PC/ABS, typical barrel temperatures are set from 240 °C at the feed throat to 260 °C at the die; vacuum devolatilization at −0.08 MPa removes residual moisture and oligomer volatiles. The ABS phase is usually incorporated at 20–35 wt% in standard automotive interior grades, although high-flow grades can approach 50 wt%. For PC/PBT, barrel temperatures are held at 245–270 °C to limit PBT degradation; transesterification between PC carbonate groups and PBT ester groups is controlled by addition of phosphite-based stabilizers and by minimizing aqueous acidity. The screw profile typically contains two or three kneading blocks after the feed zone to disperse the PBT phase, but excessive specific energy input can reduce impact retention. Published data for specific PC/PBT transesterification rates on production equipment is limited, and laboratory values should not be extrapolated without pilot confirmation.

When the melt cushion on an injection molding machine varies by more than 2% shot-to-shot, PC/ABS interior panels often show gloss variation and flow-line defects. The rubber phase in ABS must be uniformly dispersed, and shortened screw recovery times can produce non-uniform melt temperature. For PC/PBT connectors, mold deposits from PBT oligomers can build up on cavity surfaces after several thousand cycles, requiring periodic cleaning. These process interactions affect lot-to-lot quality more than laboratory tensile data alone.

When Chemical Exposure Crosses the Threshold That PC Alone Cannot Tolerate

PC/PBT is selected for fuel-system, electrical-connector, and exterior body applications when the part is exposed to gasoline, diesel, engine oil, or ester-based coolants under sustained load. The PBT phase crystallizes partially, and the resulting semi-crystalline network reduces solvent penetration and cracking that occur in amorphous PC. Molders must accept a wider processing window with consequences: mold shrinkage in PC/PBT is anisotropic, typically 0.8–1.4% in the flow direction and 1.0–1.8% transverse, so tooling dimensions require pre-compensation. Weld-line strength is also lower in PC/PBT than in PC/ABS because the semi-crystalline phase crystallizes against the flow front, and the resulting weld line may retain only 30–50% of the unfilled PC impact value.

When low-temperature impact retention is the primary requirement, PC/ABS is preferred. At −30 °C, notched impact values for some automotive PC/ABS grades remain above 30 kJ/m² under ISO 180-1A, whereas unmodified PC can undergo a ductile-to-brittle transition depending on thickness and notch radius. The ABS phase also provides better electroplatability, lower density, and reduced melt viscosity than PC. The trade-off is inherent flammability and UV stability: unpainted PC/ABS exposed to outdoor weathering under ISO 4892-2 can show measurable color shift and impact loss after UV exposure, requiring UV stabilization or coating.

Drying Temperature Alone Does Not Define the Processing Window

Different moisture sensitivity is the most operationally significant boundary between these materials. PC and PC/ABS are both hygroscopic enough that equilibrium moisture can exceed 0.2% in a humid warehouse; at melt temperatures above 240 °C, this moisture reacts with carbonate and ester groups. Hydrolytic degradation of PC is autocatalytic in that the reaction reduces molecular weight and raises carboxylic acid end-group concentration, which further promotes chain scission. A moisture level of 0.03% in PC entering the barrel can produce a measurable increase in MVR and a loss of 15–25% in notched Izod impact after molding compared with resin dried to 0.01%. For this reason, desiccant-bed dryers with a process-air inlet dew point of −40 °C are specified rather than hot-air hopper dryers. Drying temperatures of 120 °C for PC and 110–120 °C for PC/PBT are common; PC/ABS is dried at 80–95 °C to avoid caking of the styrenic phase and discoloration.

Although PBT absorbs less moisture than PC, PC/PBT is not exempt from drying. The ester group is hydrolytically sensitive at elevated temperature, and residual moisture above 0.01–0.02% can cause embrittlement and viscosity shifts. Overdrying is also an operational boundary: exposing PC/ABS to hopper temperatures above 105 °C for extended periods can produce yellow centers and surface defects, while prolonged drying of PC at 130 °C can oxidize light-stabilized grades. Manufacturers publishing drying parameters therefore specify maximum allowed hot-air residence time and minimum dew point rather than simple time-temperature values.

For compliance-driven applications, no single alloy family is universally acceptable simply because the base resin is PC. FDA 21 CFR 177.1580 covers polycarbonate resins for food-contact use and imposes limits on residual bisphenol A migration; alloys containing ABS or PBT must be evaluated separately because the additive resin and impact modifiers may not meet the same migration criteria. Under Regulation (EU) No 10/2011, specific migration of bisphenol A from polycarbonate food-contact articles is further restricted by Regulation (EU) 2018/213, which establishes a migration limit of 0.05 mg/kg food. For electrical and electronic parts, PC, PC/ABS, and PC/PBT are commonly assessed against IEC 62321-1 methods for RoHS compliance and may be used with halogen-free flame-retardant systems to meet IEC 61249-2-21 halogen content limits. In practice, a resin supplier’s letter of compliance is insufficient for finished articles because molding additives, UV stabilizers, and colorants can alter the final migration or extractables profile.