Polycarbonate (PC) is an amorphous thermoplastic produced by interfacial phosgenation of bisphenol A or by melt transesterification with diphenyl carbonate. Lexan is a registered trademark of SABIC Global Technologies B.V.; it designates a specific family of polycarbonate resins, extruded sheets, films and compounded materials, not a generic polymer class. A transparent 3 mm sheet sold as “Lexan” may be unverified generic polycarbonate, a regrind-containing extruded sheet, or a different transparent thermoplastic such as poly(methyl methacrylate) or polyethylene terephthalate glycol. The purchaser should therefore combine supplier documentation with independent physicochemical testing before payment. The five checks below target the lowest-cost discriminating parameters for unfilled, transparent, UV-stabilized polycarbonate sheet.

Is Your Lexan Genuine? Five Checks Before You Pay for a Polycarbonate Sheet

Misbranding occurs when a distributor applies a premium-brand designation to a generic 1.20 g/cm³ aromatic polycarbonate sheet or to a sheet containing recycled content. Because Lexan grades are produced to controlled molecular weight, stabilizer packages, release-agent chemistry and UV-additive loadings, the certificate of analysis is the first discriminator. A valid supplier invoice is not sufficient; lot-level traceability to the manufacturer’s batch records must be available on request. Packaging labels should display ISO 11469 marking “>PC<” for unfilled polycarbonate. The UL Yellow Card should be current and searchable in UL Product iQ for the exact grade, colour and minimum thickness. A missing or non-current Yellow Card listing is sufficient to reject the lot before destructive testing is conducted.

The next documentation layer concerns food-contact and regulatory certificates. A genuine Lexan sheet intended for food-contact use should reference FDA 21 CFR 177.1580 and, where relevant, EU Regulation 10/2011. RoHS and REACH statements are not brand-verification tools, although they may accompany the lot. Production-scale traceability systems used by legitimate Lexan distributors tie box labels to extrusion-line records, including die position, roll-stack temperature and online thickness gauging. A distributor that cannot provide the lot number, production date or chain-of-custody record should not be accepted merely because the sheet “looks like polycarbonate.”

Documentation Traceability and Lot-Level Compliance Records

The certificate of analysis should identify the Lexan grade, lot number, production date, melt mass-flow rate, colour code, density and the applicable UL file number. If the supplied certificate lists only “polycarbonate” without a SABIC grade designation or UL reference, the material has not been verified as Lexan. Lot-level conformance data should include the actual melt flow rate measured by the producer, not a generic marketing datasheet value. For unfilled clear sheet, the density certificate should fall at 1.20 g/cm³ ± 0.01 per ISO 1183-1:2019 or ASTM D792. A certificate that omits density, melt flow rate and lot number is incomplete regardless of the distributor’s reputation.

Optical and spectral verification is performed on a specimen conditioned at 23 °C and 50 % relative humidity per ASTM D618. Total luminous transmittance and haze for a clear 3 mm sheet measured per ASTM D1003 should fall near 88–91 % and <1.0 %, respectively. Yellowness index per ASTM E313 should remain below 1.5 for a UV-stabilized grade. ATR-FTIR spectra of polycarbonate display a strong carbonyl absorbance near 1770 cm⁻¹, aromatic C=C stretching near 1505 cm⁻¹, and the characteristic C-O-C doublet near 1220 cm⁻¹ and 1190 cm⁻¹. A carbonyl near 1715 cm⁻¹ indicates polyethylene terephthalate, while a single strong carbonyl near 1730 cm⁻¹ suggests poly(methyl methacrylate); neither is a polycarbonate. Refractive index measured per ASTM D542 should be 1.585 ± 0.002 for unfilled PC, and specific gravity per ASTM D792 separates PC from PETG at 1.27 g/cm³ and PVC at 1.40 g/cm³. Density measured by water displacement or gas pycnometry has uncertainty below 0.002 g/cm³; a value above 1.22 g/cm³ suggests a filled material or blend.

Spectroscopic testing should also interrogate the sheet edge and core rather than only the gloss surface. Coextruded sheet may carry a thin UV cap layer, but the bulk carbonate absorbance remains visible through the thickness. Handheld light meters are not acceptable substitutes for an integrating-sphere transmission measurement. If the measured yellowness index exceeds 3 on a supposedly new clear sheet, thermal degradation, overdrying or excess regrind should be suspected. Spectral identification is fast, but it cannot confirm brand; it confirms polymer family. The remaining checks therefore target molecular-weight, thermal and mechanical attributes that distinguish controlled Lexan sheet from lower-grade or contaminated polycarbonate.

A Mismatched Melt Flow Rate Signals Regrind or Substitute Resin

Melt volume-flow rate or melt mass-flow rate is measured per ISO 1133-1:2022 at 300 °C and 1.2 kg. Extrusion-grade unfilled Lexan sheet typically exhibits an MFR between 5 g/10 min and 12 g/10 min, but the grade-specific CoA value is the acceptance limit. A measured MFR above the certified value by 50 % or more suggests regrind, hydrolytic chain scission from improper drying, or lower-molecular-weight substitute resin. MFR alone can be gamed by using a high-flow polycarbonate; therefore melt-flow verification should be combined with mechanical impact and thermal softening data. Capillary rheometry is recommended when available because it reveals the shear-thinning curve. Unfilled PC at 300 °C is shear-thinning at processing-relevant shear rates from 10 s⁻¹ to 1000 s⁻¹, but exact published viscosity data for every Lexan sheet grade are limited.

A differential scanning calorimetry trace per ISO 11357-2 shows a glass transition at 147–150 °C and no crystalline melting endotherm. The first heating scan may show enthalpy relaxation or processing orientation; a second heating scan after controlled cooling gives the more reproducible glass transition value. Vicat softening temperature per ISO 306 method B120 should be 145–151 °C for general-purpose unfilled Lexan sheet, and heat distortion temperature under 1.8 MPa per ASTM D648 should be 126–132 °C. If the Vicat temperature falls below 135 °C, a PC blend, a lower-molecular-weight resin or a non-PC transparent sheet should be suspected. Processing conditions can also alter residual stress and gauge distribution. On a single-screw sheet extrusion line with a polished chrome roll stack at 80–110 °C and melt temperature 270–310 °C, overdrying and uncontrolled regrind produce microgel, thickness variation and local haze. Pre-drying in a desiccant dryer at 120 °C for 4 h to a moisture content below 0.02 % is mandatory before extrusion or injection molding. Moisture above 0.05 % generates splay bubbles and reduces elongation at break.

Tensile properties are measured per ASTM D638-14 on Type I specimens cut parallel to the extrusion direction. For clear unfilled Lexan sheet, tensile stress at yield is typically 60–65 MPa, tensile modulus 2300–2400 MPa, and elongation at break 100–150 %. ISO 527-2/1A values may differ slightly because of specimen geometry and testing speed. Flexural modulus per ASTM D790-17 should be near 2300 MPa. The most sensitive mechanical discriminator is notched Izod impact per ASTM D256 at 3.2 mm thickness. General-purpose Lexan sheet commonly yields 600–850 J/m, while heavily contaminated, overdried, or high-regrind sheet can fall below 200 J/m. ISO 180/A notched impact is an alternative for ISO-based purchasers, with published values near 65–75 kJ/m² for controlled unfilled grades.

Because polycarbonate is notch-sensitive and thickness-dependent, comparisons must be made at identical thickness and conditioning state. A 4 mm sheet will not directly match a 3.2 mm Izod bar. The ductile-brittle transition shifts with molecular weight and temperature; lower-molecular-weight reprocessed material loses impact resistance before tensile yield or flexural modulus changes enough to trigger a simple QC limit. Tensile specimens cut at 0°, 45° and 90° to the extrusion direction may show orientation-induced anisotropy. A difference greater than 10 % in yield stress between directions can indicate stressed sheet or poor roll-stack temperature control. Injection-moulded specimens cannot be substituted for extruded sheet specimens without normalising for orientation and skin-core morphology.

When a Seller Cannot Provide Burn-Test and Chemical-Resistance Data, Rejection Is Justified

Flammability testing per UL 94 is thickness-dependent. General-purpose unfilled Lexan sheet is typically rated HB at 1.5 mm and V-2 at 3.0 mm; V-0 ratings require flame-retardant additives and are not typical of clear general-purpose polycarbonate unless specifically designated. Limiting oxygen index per ISO 4589-2:2017 for unfilled polycarbonate is approximately 25–27 %. A sheet that ignites readily, drips flaming particles immediately, and continues to burn after the flame is removed may be a PC/ABS blend, acrylic-modified material, or lower-molecular-weight regrind. The vertical burn test should be conducted at the exact thickness supplied, because a rating at 3.0 mm does not transfer automatically to 1.5 mm sheet.

Chemical stress-cracking response is more discriminating. Polycarbonate under a constant flexural strain of 0.5 % will craze rapidly when wetted with toluene, xylene, methyl ethyl ketone, or chlorinated solvents. This response is characteristic of aromatic polycarbonate and separates it from some transparent materials, though the cracking pattern alone is not a quantitative brand identifier. Aromatic solvents and amine-based cleaners should be avoided on Lexan sheet because they cause environmental stress cracking or aminolysis of the carbonate linkage. Alkaline paint strippers above pH 10 attack polycarbonate surfaces; a 24 h immersion in 10 % sodium hydroxide at 23 °C produces blushing and localised surface degradation. These chemical exposures are destructive and should be performed on cut specimens, never on installed sheet or material still under warranty inspection.

For UV-stabilised sheet, the purchaser should request weathering evidence. A clear UV-stabilised Lexan sheet retains higher notched impact after ISO 4892-2 exposure than unstabilised generic PC, but accelerated weathering data are grade-specific and cannot be extrapolated to service life without field validation. If the seller cannot supply the UL Yellow Card, lot certificate, density, melt flow rate, optical values, notched Izod data and burn-test classification, the material should be rejected before payment. Published data for every possible reprocessed or blended configuration are limited; therefore a multi-method verification strategy is the only objective basis for confirming that a polycarbonate sheet is genuinely a SABIC Lexan product.