What Lexan Means Today: A PC Trade Name That Became Generic

The designation Lexan is not a formal material grade under ISO 21305-1:2019 but a trade name owned by SABIC. In many procurement documents, maintenance manuals, and verbal specifications, “Lexan” is used as a shorthand for any transparent, impact-resistant bisphenol A polycarbonate. That generic use creates a specification risk because the term alone does not identify melt volume-flow rate, UV stabilizer package, mold-release chemistry, flame-retardant classification, color tolerance, regrind allowance, or lot traceability. A safety glazing, an electrical enclosure, and a medical device housing can all be BPA-based polycarbonate and require different property profiles.

Bisphenol A polycarbonate is an amorphous thermoplastic composed of carbonate ester linkages and 2,2-bis(4-hydroxyphenyl)propane. The repeat unit includes two aromatic rings joined by a quaternary carbon and a carbonate group. This structure produces a glass transition near 147 °C, high optical clarity, and relatively high tensile elongation at break. A drawing that lists only “Lexan” does not establish minimum molecular weight, does not prevent use of regrind, and does not define drying history. Generic polycarbonate of similar density and notch impact may still differ in low-molecular-weight tail, residual BPA, and hydrolytic stability.

Polycarbonate is commercialized through two principal routes. Interfacial phosgenation reacts bisphenol A disodium salt with phosgene in a methylene chloride–water system at pH 10–11, with a monofunctional phenol chain stopper such as p-tert-butylphenol added to control molecular weight. The resulting polymer is washed, precipitated, and dried. Melt transesterification uses bisphenol A and diphenyl carbonate at high temperature and reduced pressure, with phenol removed as a by-product. This route avoids chlorinated solvents but can leave different residual carbonate end groups. Gel-permeation chromatography per ISO 16014-1, 16014-2, and 16014-4 is used to characterize weight-average molecular weight and molecular weight distribution. Standard extrusion and injection grades typically fall within 24,000–36,000 g/mol weight-average molecular weight. A narrower distribution and a low oligomer tail reduce environmental stress cracking sensitivity, but increase cost.

Which Molecular Variables Control Polycarbonate Ductility and Notched Impact?

Polycarbonate impact is controlled by molecular weight, specimen thickness, temperature, and notch radius. A low melt volume-flow rate grade of 3–10 cm³/10 min at 300 °C and 1.2 kg under ISO 1133-1:2022 generally has higher weight-average molecular weight and better notched impact than a high-flow grade at 20–30 cm³/10 min. However, the same molecular weight that improves impact increases injection pressure and reduces flow length in thin-wall sections. The ductile-to-brittle transition is sharp in thick sections; a notched Izod value at 23 °C does not guarantee performance at −20 °C or in sections greater than 6.4 mm. Data generated according to ASTM D256-23e1 and ISO 180:2023 must state specimen thickness, notch preparation, and conditioning. High-speed puncture tests such as ISO 6603-2:2023 are used where tensile impact is more relevant than Izod. Thin-wall electrical enclosures and medical devices often require high-flow grades; this flow improvement can reduce notched impact and increase sensitivity to aromatic hydrocarbons during cleaning. Process validation therefore includes both fill simulation and post-molding impact on parts taken from the gate area, where shear and residual stress are highest.

Pre-drying is a critical processing step. Hydrolysis of the carbonate linkage at melt temperature occurs rapidly when pellet moisture exceeds 0.02%. On injection molding production lines, wet polycarbonate produces splay, silver streaks, screw torque drift, and a measurable reduction in melt viscosity because molecular weight is being lost. Dehumidifying dryers with a dew point at or below −40 °C are set to 120 °C for 2–4 h; residence time is adjusted for bulk silo storage in humid environments. Residual moisture is confirmed by Karl Fischer titration per ISO 15512:2019 or ASTM D6869-17. Hot-air dryers without desiccant beds generally cannot deliver the low dew point required for optical or impact-critical parts. Overdrying is less common but can produce pellet sticking and feed instability in single-screw extruders.

Injection molding typically uses barrel zones from 260 °C rear to 290–315 °C metering/nozzle, mold temperatures of 80–120 °C, and a holding pressure sufficient to maintain sink-free surfaces. Clamp force planning commonly uses 0.5–0.8 tonnes/cm² of projected area; published data for this specific configuration is limited. Screws with L/D 20:1–24:1 and compression ratio 2.0:1–2.5:1 are typical. Back pressure is held at 0.5–1.0 MPa. Cumulative residence time at melt temperature should not exceed 10 min; longer exposure causes yellowing, carbon specks, and impact loss. Twin-screw compounding uses L/D 32:1–44:1, vacuum venting at −0.08 MPa, and melt temperature of 280–320 °C for fiber-reinforced or flame-retardant grades. In sheet extrusion, polish roll temperatures of 120–140 °C reduce surface haze and residual stress.

When Secondary Operations Expose Molded PC to Environmental Stress Cracking

Environmental stress cracking in polycarbonate is evaluated by constant-strain immersion using ASTM D543-21 or ISO 22088-2:2016. The test measures whether a pre-strained specimen crazes or fractures when exposed to a chemical. Molded parts are rarely stress-free; gate vestiges, sharp internal corners, ultrasonic weld joints, insert bosses, and overtorqued screws can concentrate residual stress. A threshold strain as low as 0.5% may initiate cracking in incompatible fluids. Toluene, acetone, methyl ethyl ketone, ethyl acetate, chlorinated degreasers, and amine-based cleaners are documented stress-cracking agents for BPA polycarbonate. Isopropanol and ethanol are not considered severe solvents, but repeated wiping of stressed bosses can produce cracks; published data for this specific configuration is limited. A controlled secondary operation requires replacing the cleaning solvent with a tested compatibility fluid, annealing parts at 120–130 °C for 1–2 h, or reducing stress through design changes. Flexible PVC packaging can release plasticizer that migrates to stressed PC surfaces; production lines therefore use polyethylene or barrier-film packaging to avoid contact.

Elevated temperature and weathering impose separate service boundaries. The heat deflection temperature under 1.82 MPa spans 121–132 °C per ASTM D648-18; continuous structural load above approximately 120 °C is generally not recommended for unmodified grades because creep modulus declines sharply near T_g. Unstabilized polycarbonate exposed to ultraviolet radiation develops yellowing and surface microcracks. UV-stabilized formulations reduce the degradation rate but do not eliminate the need for hardcoating or replacement intervals. Weathering programs often use ISO 4892-2:2013 and ASTM G155-21; the acceptable yellowness index shift is application-specific.

Sterilization compatibility is grade-specific. EtO sterilization is typically performed at 50–60 °C and relative humidity of 30–70%; post-process aeration reduces residual ethylene oxide below device-specific limits such as those in ANSI/AAMI/ISO 10993-7:2017. Gamma sterilization at 25–50 kGy can cause yellowing, loss of notched impact, and radical-induced chain scission. Radiation-stabilized polycarbonate grades use additives to minimize color shift. Steam autoclaving at 121 °C or 134 °C is generally unsuitable for unmodified BPA polycarbonate because repeated cycles accelerate hydrolysis and haze development. For medical devices, ISO 10993-1, ISO 10993-5, and ISO 10993-10 data are generated on the final device or on specified material-process combinations. USP Class VI does not apply automatically to every PC grade. A change from a specific PC grade to a generic “Lexan-equivalent” requires review of sterilization validation, impact retention, and extractables profile.

Additive packages alter the performance envelope. Carbon black and titanium dioxide reduce light transmission and can lower low-temperature impact when loaded above a few weight percent. Internal mold release agents such as pentaerythritol tetrastearate reduce demolding forces but can affect painting and bonding. Phosphorus-based flame retardants, often used to achieve UL 94 V-0, may reduce notched impact and increase melt viscosity; mineral or glass reinforcements raise modulus but reduce transparency. Each formulation is therefore qualified at the final additive loading using ISO 179-1, ASTM D256, and ISO 6603-2 rather than being assumed from neat resin values.

Flame Retardant and Optical Grade Selection Criteria

Unmodified BPA polycarbonate has a limiting oxygen index of 25–27% under ISO 4589-2:2017, and its UL rating is generally thickness-dependent. Flame-retardant PC grades using phosphorus-based or silicone-based packages achieve UL 94 V-0 at 1.5 mm or 3.0 mm, but the additives can reduce clarity. Optical grades are characterized by total luminous transmittance of 89–91% and haze below 1.0% at 3.0 mm per ASTM D1003-21, with refractive index around 1.585 per ISO 489:2022. Polycarbonate glazing for vehicles or machinery must retain optical quality and impact after weathering. Accelerated weathering tests use ISO 4892-2:2013 or ASTM G155-21, with yellowness index change commonly limited to a delta YI of 2–4. In automotive glazing contexts, hardcoated polycarbonate is subject to regulatory abrasion requirements such as ECE R43 or ANSI/SAE Z26.1; uncoated PC is not suited to windshield applications.

Compliance is not conferred by the trade name or by the polymer class. Each fabricated article must be validated against the relevant standard for the final material, color, additive package, and process conditions. Table 2 lists common compliance anchors; it is not exhaustive for aerospace, potable water, or infant food applications.