Polycarbonate (PC) is an amorphous engineering thermoplastic produced by interfacial polymerization of bisphenol-A and phosgene or by melt transesterification with diphenyl carbonate; the repeating unit contains two aromatic rings, two methyl groups, and a carbonate linkage. Its density is 1.20 g/cm³ per ISO 1183-1, light transmission through a 3 mm plaque is 88–91% per ASTM D1003, and refractive index is 1.585 per ASTM D542. The glass transition temperature measured by differential scanning calorimetry is approximately 147°C; no sharp crystalline melting endotherm is present. The working descriptor Polycarbonate Explained: The Transparent Plastic That Bends Before It Breaks follows from the coexistence of high optical transmission and ductile tensile overstrain before fracture. Water absorption at 23°C for 24 h is 0.15% per ISO 62, low relative to polyamide but sufficient to require drying before melt processing.

Why does polycarbonate bend before fracture while retaining optical clarity?

Under uniaxial tension at 23°C and a crosshead speed of 50 mm/min per ASTM D638, general-purpose injection-moulding grades exhibit tensile stress at yield of 60–65 MPa, elongation at break of 100–150%, and flexural modulus of 2.30–2.40 GPa per ISO 178. Notched Izod impact from ASTM D256 on 3.18 mm specimens is commonly 650–850 J/m at 23°C. The ductile response is attributed to segmental mobility of the bisphenol-A backbone at room temperature, permitting shear yielding and craze formation before crack propagation. Under triaxial constraint, however, polycarbonate can transition to brittle behaviour below its grade-dependent ductile-brittle transition; thick-section transitions are observed near −20°C for some unfilled grades, but published data for other molecular-weight distributions are limited.

On production-scale injection moulding lines equipped with general-purpose screws of 20:1 to 24:1 length-to-diameter ratio and clamp force from 500 kN to 40,000 kN, melt temperature measured at the nozzle is held between 280°C and 320°C. Barrel zone profiles are typically rear 260°C, centre 280°C, front 300°C, and nozzle 290°C; these settings are grade-dependent and shift with screw recovery and back pressure. Mould surface temperature is maintained at 80–100°C through pressurised-water or oil thermolator circuits to reduce residual stress and orientational anisotropy in gate areas. Before melting, pellets must be dried to less than 0.02 wt% moisture by weight. A desiccant-bed dryer with process-air dew point below −40°C and drying temperature of 120°C for 3–4 h is standard; hopper residence should be limited when ambient relative humidity exceeds 60%. Moisture above 0.02 wt% hydrolyzes the carbonate linkage, reducing molecular weight and producing silver streaks, gate splay, and notched impact loss. A documented failure mode on 1,500 kN presses is delamination near rib intersections when regrind fraction exceeds 20 wt% and is fed without secondary drying.

Melt viscosity and hydrolytic degradation thresholds

Melt viscosity follows a pseudoplastic curve; at 300°C and an apparent shear rate of 1,000 s⁻¹, a 10 g/10 min MFR grade typically falls between 100 Pa·s and 300 Pa·s, with higher molecular weight grades exceeding this range. The available melt-flow-rate window spans 3–80 g/10 min per ISO 1133-1 at 300°C under 1.2 kg. High-MFR grades below 15 g/10 min are selected for thin-wall optical parts and optical disc substrates because they permit short filling times but exhibit lower notched impact than high-viscosity grades above 40 g/10 min. Above 320°C carbonate linkages undergo measurable thermal scission; at 350°C residence times beyond 10 min generate yellow-brown degradation products, liberated phenol-related species, and molecular weight reduction. Hot-runner manifolds should be designed with streamlined flow channels and no dead spots; black specks in transparent mouldings are commonly traced to stagnant melt in poorly purged valve gates or runner drops.

Because polycarbonate is amorphous and susceptible to environmental stress cracking under imposed strain, chemical contact must be evaluated using ASTM D543 or ISO 22088-3 bent-strip fixtures at 0.5% to 1.0% outer-fibre strain. Aromatics such as benzene, ketones including acetone and methyl ethyl ketone, chlorinated solvents such as methylene chloride, and amines including triethylamine cause crazing and fracture at stresses far below the short-term yield point. Production failures have been observed when mould-release sprays containing petroleum distillates or brake-cleaner overspray contact unannealed PC brackets under residual moulded-in stress. Strong alkaline solutions above pH 10 attack the carbonate linkage, particularly above 50°C. In contrast, water, aliphatic alcohols, dilute mineral acids, and many aqueous salt solutions are generally compatible at room temperature provided stress levels remain below the critical crazing strain. Amine-based additives should be eliminated from PC compounds because they induce premature chain scission during melt processing.

To meet the low birefringence and pit-replication demands of optical data storage, optical-disc grades are produced with melt flow rates of 60–80 g/10 min and are processed in servo-driven injection-compression machines with clamp force from 1,000 kN to 4,000 kN and automatic vacuum venturi handling. Mould cavities for 120 mm diameter discs require surface replication below 0.05 µm pit depth and tilt control within 0.2° across the disc. The high flow required for pit filling reduces molecular weight, so published data for cyclic mechanical durability of optical-disc regrind in load-bearing applications is limited.

If sheet surface temperature falls below 160°C during thermoforming

Polycarbonate sheet extruders with 32:1 to 40:1 length-to-diameter ratio and atmospheric venting operate with barrel temperatures between 260°C and 315°C; polished chrome rolls at 90–130°C control sheet surface quality and thickness tolerance. Thermoforming requires sheet surface temperature between 160°C and 180°C. Below 160°C the sheet is too stiff for uniform drape, and corner thinning in female tools increases; above 190°C retained moisture can generate micro-bubbles and surface splay. The forming window is narrow because polycarbonate has a glass transition near 147°C, but sheet surface temperature is the process-control variable, not oven set point. Radiant-quartz sandwich ovens with zone control on both sheet faces are used, and sag bands or photocell sag control are required to compensate for sheet density and roll-stress variations. Free-forming draw ratios up to 3:1 to 4:1 in plug-assisted pressure forming are attainable with blank holders and heated plugs, but published data for larger ratios is limited and depends on sheet starting molecular weight.

Outdoor glazing applications couple ultraviolet exposure with impact demands. Unstabilised polycarbonate develops measurable yellowness index increase under ISO 4892-2 xenon-arc exposure, with the rate dependent on grade, stabiliser package, and light source; coextruded UV-cap layers containing benzotriazole or triazine absorbers are used on multiwall and solid sheet rather than bulk-only stabilisation. Impact retention after weathering is evaluated with ASTM D256 notched Izod and ISO 6603-1 puncture impact. Clear unfilled PC without UV stabilisation is not recommended for long-term external glazing where yellowness index must remain below 2 units after 2,000 h; published data for specific weatherable grades should be obtained from the sheet manufacturer because additive loadings are not reflected by melt-flow-rate data alone.

Regulatory compliance matrix for food-contact and medical device grades

Steam autoclaving of PC medical components at 121°C and 1.0 bar overpressure is feasible for limited cycles, but hydrolytic chain scission accumulates with each cycle and reduces impact resistance beyond the first 25–50 cycles depending on part thickness and molecular weight. Dry heat sterilisation below 125°C avoids saturated-steam hydrolysis but imparts greater oxidative exposure after repeated cycles. Gamma and electron-beam sterilisation at 25–50 kGy can induce yellowing in unfilled grades; radiation-stabilised medical grades are required when colour stability is a device acceptance criterion. The operational boundary is therefore application-specific: polycarbonate is suitable for transparent, impact-resistant devices that do not require indefinite autoclave reuse or exposure to amine-based disinfectants and strong alkaline cleaning agents.