Polycarbonate (PC) is an amorphous engineering thermoplastic synthesized by interfacial or melt-phase condensation of bisphenol A and a carbonate source. Its unfilled density is 1.20 g/cm³ per ISO 1183-1, compared with 2.50 g/cm³ for soda-lime float glass. The mass per unit area of a 3 mm sheet is therefore 3.6 kg/m² for PC and 7.5 kg/m² for glass. This is the basis of the Polycarbonate vs Glass: Weight, Safety and the Scratch Resistance You Give Up evaluation. At equal thickness, the weight reduction reaches 52%, but the structural stiffness penalty is larger: the flexural modulus of PC is 2.3–2.4 GPa under ISO 178, while soda-lime glass is near 70 GPa. Consequently, flat-plate deflection at equal thickness is significantly greater for PC, and designers often compensate with greater thickness. The optical clarity of polished, UV-stabilized PC at 3 mm is 88–91% total luminous transmittance per ASTM D1003; glass is 90–91%. The immediate limitation is not transparency but surface durability: PC pencil hardness is 2B–HB per ASTM D3363, while glass exceeds 9H.

Does a 52% Density Reduction Automatically Create a Safety Advantage?

In high-speed impact scenarios, PC benefits from shear yielding and craze formation rather than the elastic strain energy release that produces brittle fracture in glass. A 3.2 mm notched specimen of general-purpose PC absorbs 600–850 J/m under ASTM D256, and many unfilled grades do not fully break in unnotched Izod testing at room temperature. Soda-lime glass has a fracture toughness near 0.7 MPa·m^1/2, so a surface flaw of a few tens of micrometres can propagate at low applied stress. PC yields at 60–70 MPa under ISO 527-2 and draws to 100–150% elongation at break in many unfilled grades. The result is that a PC window may crack but remain in the frame after impact, while annealed glass releases sharp fragments. For forced-entry security glazing, PC-based laminates are evaluated under EN 356; bullet-resistant glazing is tested separately under EN 1063. These systems are often thicker than glass but still lower in mass at equal thickness.

However, the density advantage is not a stiffness advantage. The flexural modulus of PC is 2.3–2.4 GPa under ISO 178, while glass is 70 GPa. For a simply supported rectangular plate under uniform pressure, maximum deflection scales inversely with elastic modulus and inversely with the cube of thickness. Matching the bending stiffness of a 3 mm glass sheet requires PC thickness of approximately 9.4 mm, because (70/2.3)^(1/3) ≈ 3.13. The PC panel at 9.4 mm weighs 11.3 kg/m², exceeding the 7.5 kg/m² of the glass it replaces. The weight saving therefore appears only when the design is impact-dominated, when the support spacing is reduced, or when PC is used as a thin internally protected layer within a laminated stack. In vehicle glazing under ECE R43, plastic panes must pass both impact and abrasion requirements; uncoated PC fails the abrasion portion, so hardcoated PC systems are required. The safety benefit is real, but it is conditional on managing deflection and surface degradation.

Under ASTM D1044, using CS-10F wheels and a 500 g load for 100 cycles, published supplier data for uncoated PC show a Taber delta haze of 30–45%. Soda-lime glass under the same protocol remains below 1%. The mechanism is not uniform material removal; it is the formation of shallow flow grooves and craze-like surface damage in the top few micrometres of the PC substrate. A polysiloxane hardcoat at 3–8 µm thickness raises pencil hardness from 2B–HB to 2H–4H under ASTM D3363 and lowers Taber delta haze to 2–5%. Crosshatch adhesion after cure is typically 5B under ASTM D3359, but adhesion degrades after thermal cycling if the primer is not matched to the substrate. The hardcoat is a brittle layer on a ductile base: large in-plane strains cause microcracking, and repeated abrasion can expose bare PC. Falling-sand testing under ASTM D968 ranks glass above coated PC because the coating is sacrificial and eventually wears through. The outdoor micro-abrasion environment matters: wind-borne silica and alumina particles are harder than both PC and most commercial hardcoats, so the coated sheet loses gloss and haze develops. The scratch resistance given up is therefore not a small cosmetic sacrifice; it is the primary lifespan limit in unglazed or mechanically cleaned PC. For anti-scratch coatings, plasma-enhanced chemical vapor deposition of silica-like layers is used on some headlamp lenses, but cost and thermal control restrict widespread sheet use.

If Polycarbonate Is Pressed Into Glazing Service, Which Processing Limits Apply?

Processing failures convert the material’s impact safety into brittleness before any service load is applied. PC must be dried to 0.02% moisture or less by mass before extrusion or injection molding. A desiccant dryer is operated at 120°C for 4 h, with supply air dew point below -40°C. Moisture above this threshold reduces molecular weight through hydrolysis at melt temperature and produces splay, silver streaks, and notch-sensitive parts. Melt temperature in injection molding is commonly 280–320°C; mold temperature is held at 80–120°C to allow relaxation and maintain transparency in thick sections. Injection grades have melt mass-flow rates of 6–15 g/10 min at 300°C/1.2 kg under ISO 1133-1; extrusion grades use 2–5 g/10 min to improve melt strength. A vented barrel in extrusion with a length-to-diameter ratio of 30:1 is used to strip residual volatiles. Molded-in stress must be minimized by uniform cavity cooling; annealing after machining at 120°C for 1–2 h in circulating air reduces stress concentration. Environmental stress cracking is evaluated under ISO 22088, and PC is incompatible with aromatic hydrocarbons, ketones, esters, chlorinated solvents, amines, and many alcohols. Isopropanol, commonly used for cleaning transparent parts, can craze a stressed PC guard; the cleaning specification must therefore exclude solvent-laden wipes. UV stabilization is separately required: without a UV absorber package or coextruded cap layer, PC undergoes photo-Fries rearrangement and yellows in outdoor exposure. Xenon-arc aging under ISO 4892-2 is used to compare stabilized formulations.

In transparent machine guarding, PC is often used at 8–12 mm thickness because it retains fragments after impact and can be fabricated by sawing, routing, and thermoforming without shattering. The same thickness in soda-lime glass would require tempering or lamination. At 10 mm, PC weighs 12 kg/m² whereas glass weighs 25 kg/m². But the guard must be cleaned only with mild soap and water or manufacturer-approved anti-static cleaners; dry wiping after metal-chip contact creates irreversible haze. In bus shelter glazing, a 6 mm PC sheet weighs 7.2 kg/m² versus 15 kg/m² for 6 mm glass. The outdoor surface is abraded by wind-borne particles; uncoated PC in high-dust environments can show an objectionable haze increase within 18–36 months, while glass may show only minor frosting after years. Coated PC extends service life, but published field data for specific climates and coating chemistries are limited, and the Taber test does not fully capture the effect of long-term particle impact. In vehicle headlamp lenses, PC replaced glass primarily to allow complex aerodynamic shapes and reduce mass; the lens is protected by a heat-cured or UV-cured hardcoat, and the cleaning specification is part of the service manual. When headlamp surfaces are polished aggressively to remove haze, the residual coating thickness decreases and UV protection can be lost. The substitution therefore succeeds only when surface protection is treated as a load-bearing layer, not as an optional add-on.