Acrylic vs Polycarbonate Sheet: Impact, Clarity, Weathering and Price Side by Side

Polycarbonate (PC) sheet is an amorphous engineering thermoplastic produced from bisphenol A through interfacial phosgenation or melt transesterification with diphenyl carbonate. Poly(methyl methacrylate) (PMMA), commonly referred to as acrylic, is a methacrylate ester homopolymer available in cell-cast and extruded sheet form. The side-by-side evaluation below is limited to clear, uncoated sheet except where coextruded UV cap layers or hard coats are specifically noted. Laminated, multiwall, surface-textured, and bullet-resistant grades shift the comparison because they introduce additional interlayers, geometry, or coating chemistry.

What Does ASTM D256 Reveal About Notched Izod Impact Disparity?

Notched Izod impact testing per ASTM D256-23 on 3.2 mm specimens conditioned at 23 ±2 °C and 50 ±5% RH per ASTM D618 places general-purpose PC at 12–17 ft-lb/in (640–907 J/m), compared with 0.3–0.4 ft-lb/in (16–21 J/m) for continuous cast PMMA. The difference is also evident in tensile elongation at break per ASTM D638-22: PC typically reports 100–150%, while PMMA reports 2–7%. Drop-weight impact testing per ASTM D5420 shows the same ranking, but absolute energy values are fixture-dependent; striker geometry, support span, and sheet conditioning must be reported for side-by-side comparison. In glazing and machine-guarding service, PC is specified not only for higher Izod numbers but also for stable crack arrest under dynamic loading. PMMA fails in a brittle mode at stress concentrations such as drilled holes, notches, and edge chips, whereas PC undergoes local yielding that limits crack propagation. At −20 °C, PMMA becomes increasingly notch-sensitive, particularly in thicknesses above 10 mm, while PC retains a larger fraction of room-temperature toughness, although sub-zero impact data for specific sheet formulations should be requested from the sheet producer.

Optical Transmission, Haze, and Surface Hardness Benchmarks

Total luminous transmittance measured per ASTM D1003-21 using an integrating sphere is 92% for clear PMMA and 88–90% for UV-stabilized PC at 3 mm thickness. Haze for cast PMMA is generally 0.5–1.0%; polished-roll PC sheet is 0.5–1.5%. The higher refractive index of PC, 1.586, compared with PMMA at 1.490 per ASTM D542-13, increases surface reflection and lowers total transmission at off-normal angles. Pencil hardness per ASTM D3363 is 2H–3H for PMMA and HB–F for uncoated PC. Taber abrasion per ASTM D1044-19 using CS-10F wheels and 500 g load for 100 cycles produces a larger haze increase on uncoated PC unless a polysiloxane or polyurethane hard coat is applied. These optical benchmarks explain why PMMA is commonly specified for edge-lit sign panels, display covers, and low-contact light diffusers, while PC is selected where optical clarity must coexist with impact resistance, elevated temperature, or thinner-wall safety glazing.

Thermoforming production data from a single-station shuttle machine with quartz top and bottom heaters show distinct processing windows. PC sheet of 4.5 mm reaches uniform sag at a surface temperature of 177–204 °C, measured by contacting pyrometer; PMMA reaches its forming range at 143–177 °C. Aluminum tooling for PC is held at 71–93 °C to reduce moulded-in stress and post-form dimensional recovery, while PMMA tools are commonly set at 60–82 °C. Sheet extrusion of PC on a vented twin-screw line with L/D 32:1 requires melt temperatures of 270–310 °C after desiccant drying at 120 °C for 4 h to reach moisture below 0.02%. PMMA extrusion runs at 230–260 °C and is less hydrolysis-sensitive but still requires drying to prevent bubble formation. PC sheet extrusion grades typically exhibit melt volume-flow rate of 3–10 cm³/10 min at 300 °C/1.2 kg per ISO 1133-1:2022. The corresponding acrylic extrusion grades are tested under different temperature and load conditions, so direct flow comparison without pressure-volume-temperature data is not valid. The higher melt temperature and drying requirement raise energy consumption and tooling cost for PC; however, PMMA sheet below 3 mm often shows higher downstream breakage losses because edge cracks propagate during trimming, drilling, and handling.

When Xenon-Arc and Outdoor Weathering Protocols Expose Yellowing Divergence

Accelerated weathering per ISO 4892-2:2013 method A with daylight filters at 0.51 W/m²·nm at 340 nm and black-standard temperature 65 °C separates the two materials on yellowing index shift. PMMA without UV stabilizers typically remains below ΔYI 2 after 3000 h, while unstabilized PC can exceed ΔYI 10 before 1000 h. UV-stabilized PC with a coextruded cap layer or benzotriazole-based additive package can limit ΔYI to 3–5 after 3000 h, but cap thickness, additive concentration, and exposure orientation influence the result. Outdoor exposure under ASTM D1435 at a 45 ° south-facing Florida mount shows the same relative ranking over 5 years; published data for specific sheet configurations is limited to supplier technical bulletins. PC is sensitive to alkaline cleaners, ammonia, and some amine-containing sealants, while PMMA is attacked by ketones, esters, chlorinated solvents, and aromatic hydrocarbons but tolerates dilute mineral acids and aliphatic hydrocarbons.

The weathering performance of UV-stabilized PC depends on a coextruded cap layer typically 25–75 µm thick that contains UV absorber and quencher chemistry. Cut edges expose unstabilized core and can yellow preferentially within the first 12–24 months of outdoor exposure unless edge sealants or post-finishing restore the cap. PMMA does not require a cap layer because the methacrylate backbone is less susceptible to photo-Fries rearrangement and its UV absorbance below 400 nm is low. For skylights and exterior sign faces with unsealed cut edges, this edge-yellowing mechanism is an operational boundary for PC that is not always visible in flat-sheet weathering coupons.

Price Positioning and Unit-Area Cost Multipliers for Fabricators

Clear PMMA sheet serves as the lower-cost baseline in most regional markets. Distributor price sheets for 4.5 mm sheet list UV-stabilized PC at 1.5–2.2 times the unit-area cost of cast PMMA; the multiplier varies with thickness, surface texture, UV package, and monomer market conditions. PC multiwall configurations may approach 1.4, while abrasion-resistant PC with a silicone hard coat can exceed 2.5. The raw-material cost drivers differ: PMMA is linked to methanol and acetone cyanohydrin supply, and PC tracks bisphenol A plus phenol/acetone value chain. Fabrication cost also includes scrap, tooling, and rework. CNC routing of PMMA at thicknesses below 3 mm produces brittle edge chips unless feed rates and tool geometry are matched to the sheet grade; PC routing uses polished diamond tooling at 18,000–24,000 RPM spindle speed with air or mist coolant to limit edge haze. These secondary costs can narrow the installed price gap when breakage losses and edge finishing are included.

Building and safety glazing standards shift the threshold toward PC. Under CPSC 16 CFR 1201 Category II, a glazing panel is subjected to an impact of 400 ft-lb without creating a hazardous opening. Glazing-grade PC sheet of 6 mm in a tested frame assembly can meet Category II perforation limits, while monolithic PMMA of the same thickness generally does not. Wind load and deflection calculations per ASTM E1300 use a flexural modulus of 2.3–2.4 GPa for PC and 3.0–3.3 GPa for PMMA. The lower PC modulus increases deflection under continuous load, but its 100–150% tensile elongation allows local stress redistribution before fracture. PMMA is stiffer and more dimensionally stable up to failure; however, sustained edge loads or uneven clamping can initiate crazing at stress concentrations below the short-term tensile strength. For overhead glazing, snow load and wind pressure calculations must also include the higher thermal expansion coefficient of PC, 65–70 ⁻⁶ m/m/°C, compared with PMMA at 50–60 ⁻⁶ m/m/°C per ISO 11359-2.

Heat-deflection temperature under 1.82 MPa load per ASTM D648-18 is approximately 126–134 °C for PC and 88–100 °C for PMMA. Continuous service temperature ratings in supplier datasheets are 115–130 °C for PC and 65–80 °C for PMMA. In lighting fixtures and glazing above heat sources, PMMA can soften and sag at lower ceiling temperatures, while PC retains modulus but requires slotted mounting holes because of higher thermal expansion. Regrind handling also separates the two materials. PC edge trim and scrap from sheet extrusion can be reprocessed into multi-layer sheet with a virgin cap layer; melt flow rate per ISO 1133-1:2022 can be maintained within supplier specification at 20–30% regrind use when drying and filtration are controlled. PMMA cast sheet scrap is not simply returned to the cell-cast process without depolymerization or downgrade to extruded regrind. On vented single-screw lines with 30:1 L/D, PC regrind is melt filtered through 200–400 mesh screens to avoid optical defects, while PMMA regrind is filtered at 100–200 mesh and is more sensitive to moisture from ambient storage above 60% RH.

Transparent PC sheet has a higher stress-optical coefficient than PMMA, producing visible birefringence under polarised light after aggressive forming or clamping. For instrument windows and display covers between crossed polarizers, PMMA is selected; for impact-rated covers, PC may require annealing or a low-stress grade. The stress-optical coefficient of PC is approximately 70–80 brewsters (1 brewster = 10⁻¹² m²/N), while PMMA is in the range of −4 to −6 brewsters; published data for specific sheet grades is limited to optical supplier bulletins. This optical behaviour can produce coloured fringes in PC when viewed through polarised light or liquid-crystal displays, an effect that is less pronounced in PMMA.

If Chemical Compatibility and Flame Ratings Constrain the Selection

PC sheet is generally resistant to dilute mineral acids, aliphatic hydrocarbons, and many alcohols but degrades in contact with concentrated alkalis, ammonia, and certain amine-containing sealants. PMMA resists dilute acids and alkalis but is attacked by ketones such as acetone and methyl ethyl ketone, chlorinated solvents, esters, and aromatic hydrocarbons. Flame performance differs by additive package: general-purpose PC sheet may achieve UL 94 V-2 at 3.0 mm, while PMMA is typically classified as HB unless modified. This difference matters in electrical enclosures, interior glazing, and transit applications where flammability requirements reference UL 94 or IEC 60695-11-10. In smoke-sensitive compartments, PC may require low-smoke grades; acrylic combustion produces lower smoke than some aromatic polymers but is not inherently a low-smoke engineering sheet. These chemical and fire limits must be checked against manufacturer certification for the specific sheet stock, thickness, and formed-part geometry before substitution.