Acrylic Properties — Optical, Mechanical & Thermal

Acrylic (PMMA — polymethyl methacrylate) transmits 92% of visible light, resists UV degradation for 10+ years outdoors, and achieves a tensile strength of 10,000 psi with a Rockwell M hardness between M85 and M105. Understanding the full property profile — optical, mechanical, thermal, and weatherability — lets you match cast or extruded grades to your precise application requirements without over-specifying or underbuilding.

At a glance:

• 92% total visible light transmission across 400–700 nm; refractive index 1.49
• Tensile strength 10,000 psi; flexural strength 14,000 psi; flexural modulus 400,000 psi
• Rockwell M hardness M85–M105 — harder than most engineering thermoplastics
• Continuous service temperature: 200°F (cast) / 180°F (extruded)
• Water absorption 0.3% in 24 hours — excellent dimensional stability
• No measurable UV yellowing after 10+ years of outdoor exposure
• Density 1.19 g/cc — roughly half the weight of glass at equal thickness

Optical Properties

Acrylic's optical performance is its defining characteristic. No commercial solid polymer exceeds its combination of visible light transmission, low haze, and color neutrality at commodity pricing.

Light Transmission and Haze

General-purpose clear acrylic transmits 92% of visible light (ASTM D1003). Haze — the scattered light fraction — measures below 1% for cast sheet and 1–2% for extruded sheet. Both values are better than standard float glass (91% transmission, haze ~0.1–0.5%) on transmission, though glass has lower haze.

For comparison: polycarbonate transmits ~88% with haze around 1–2%; PETG transmits ~87%. The 4-percentage-point advantage of acrylic over polycarbonate is visible in applications where color rendering matters — museum framing, artwork protection, instrument covers.

Refractive Index and Clarity

The refractive index of 1.49 places acrylic close to crown glass (1.52) and above polycarbonate (1.586). This means acrylic sheet produces less prismatic distortion than polycarbonate in optical applications. It is used for camera lens elements, aquarium panels, and instrument covers precisely because the viewing distortion is minimal.

UV-Filtering Grades

Standard acrylic transmits UV radiation below ~360 nm, which can cause fading in displayed artwork and fabrics. UV-filtering grades — available from Plexiglas (UF-3, UF-4) and Acrylite (OP-2) — add UV absorbers that block radiation below ~390 nm while maintaining full visible clarity. These grades are specified for museum display cases, fine art framing, and archival storage.

Mechanical Properties

Acrylic is a stiff, moderately strong amorphous thermoplastic. Its stiffness (flexural modulus ~400,000–450,000 psi) is comparable to rigid PVC and polycarbonate, but its notched Izod impact strength is low — 0.3–0.5 ft-lb/in — making it susceptible to cracking from point loads or sharp-edged impacts.

Strength and Stiffness

The high compressive strength relative to tensile strength means acrylic performs well in structural glazing and display shelving under compressive load. Avoid designs that subject the material to sustained tensile stress: acrylic is susceptible to stress crazing at stress concentrations (drilled holes, notches, sharp bends) over time, especially in the presence of solvents or cleaning agents.

Hardness and Scratch Resistance

Rockwell M hardness of M85–M105 makes acrylic one of the harder commodity thermoplastics. It is harder than polycarbonate (M70), ABS (M75–M100), and PETG (M80–M90). Despite this, acrylic scratches visibly because the scratches reflect light and are obvious in transparent material. Fine scratches can be polished out with plastic polishing compound; deeper scratches require wet sanding (400–800–1500 grit progression) followed by buffing.

Impact Resistance

Acrylic's notched Izod impact value of 0.3–0.5 ft-lb/in is low. For comparison, polycarbonate measures 12–16 ft-lb/in — roughly 30 times higher. In applications where sheet may be struck, kicked, or used as a machine guard, polycarbonate (Lexan) is the correct choice. Acrylic is not suitable for safety glazing applications.

That said, acrylic is not fragile in normal service. Under static load and moderate handling, it is robust. The brittleness is most apparent under sharp, concentrated impact — which is why it cracks at drill breakthrough if the wrong bit geometry is used.

Thermal Properties

Continuous Service Temperature

Cast acrylic has a continuous-use rating of 200°F (93°C). Extruded acrylic is rated to 180°F (82°C). These represent the upper limit for sustained load-bearing service. Brief excursions above these temperatures (during laser cutting, for example) are handled by the material without damage, but sustained heat above the service rating causes creep, distortion, or softening.

The heat deflection temperature (HDT) under 264 psi load is 185–210°F for cast grades, slightly lower for extruded. For comparison, polycarbonate's HDT is ~270°F, making it the better choice for enclosures near hot components.

Thermal Expansion

The coefficient of thermal expansion (CTE) for acrylic is 3.8 × 10⁻⁵ in/in/°F — about twice that of glass. In large-panel glazing installations, expansion gaps must account for this movement. A 4′ × 8′ sheet experiencing a 50°F temperature swing expands approximately 0.18″ along its 8′ length. Failure to provide clearance gaps in framing leads to buckling or cracking.

Electrical Properties

Although not a primary use case, acrylic has useful electrical properties for some fixture and enclosure applications:

Acrylic can accumulate static charge, which attracts dust. Anti-static grades and anti-static cleaners are available to mitigate this in dust-sensitive display environments.

Weatherability and Chemical Resistance

Outdoor UV Performance

Acrylic is one of the best-performing transparent plastics in outdoor UV exposure. Standard (non-UV-filtering) clear acrylic retains >90% of its original light transmission and shows no significant yellowing after 10 years of outdoor exposure in most climates. This makes it the material of choice for outdoor signage, greenhouse glazing, and pool enclosures over polycarbonate (which yellows in 3–5 years without a UV cap sheet or coating).

Chemical Resistance

Acrylic has moderate chemical resistance. It is attacked by aromatic and chlorinated solvents (toluene, methylene chloride) — in fact, solvent bonding with Weld-On 4 works precisely because these solvents dissolve the surface. In service, avoid prolonged contact with:

• Ketones (acetone, MEK)
• Aromatics (toluene, xylene, benzene)
• Chlorinated solvents (MEK, methylene chloride)
• Strong acids (concentrated sulfuric, nitric)
• Ammonia-based cleaners (crazing)

Acrylic is resistant to dilute acids, dilute bases, aliphatic hydrocarbons (hexane, heptane), alcohols at room temperature, and most aqueous solutions. See the grades and specifications pages for grade-specific resistance data.

For applications requiring better solvent resistance, PET and PETG offers improved resistance to alcohols and mild solvents, though at lower UV resistance.

Flammability and Fire Ratings

Standard acrylic is a flammable thermoplastic. It carries a UL 94 HB (horizontal burn) rating — the lowest classification, meaning the material burns slowly when exposed to flame but is not self-extinguishing. Acrylic is not appropriate for applications requiring a V-0, V-1, or V-2 UL 94 rating.

Fire-retardant (FR) acrylic grades are available from specialty suppliers. These achieve HB or V-2 ratings by incorporating flame-retardant additives. However, FR acrylic is less optically clear than standard grades and is significantly more expensive. For fire-rated glazing applications, polycarbonate (UL 94 V-2 in standard grade, V-0 in FR) or specialized materials are typically specified instead.

In practice, acrylic's flammability is a consideration in:

• Commercial building code compliance for large-format glazing (check local codes)
• Aircraft and marine interiors (acrylic is prohibited in many aviation fire zones)
• Any application governed by NFPA or IBC occupancy type fire ratings

For display fixtures, signage, and retail environments, UL 94 HB is typically acceptable. Confirm with your local authority having jurisdiction (AHJ) when in doubt.

Acrylic vs. Polycarbonate on Flammability

Polycarbonate has a UL 94 V-2 rating in standard grades — it self-extinguishes within 30 seconds of flame removal. This is a significant advantage for machine guards and enclosures in regulated environments. If a V-2 or better rating is needed, polycarbonate is the correct choice. See the cross-material comparison for a full property matrix.

Cast vs. Extruded — Property Differences

The manufacturing process creates measurable property differences between cast and extruded acrylic:

For the detailed grade-by-grade breakdown including trade names, see the acrylic grades guide.

Compare acrylic against impact-resistant alternatives on the acrylic comparisons page, or browse stock and request a quote at the acrylic line card.

More related guides

Cross-cluster suggestions to help shoppers and engineers explore adjacent topics:

Applications

• Machine Guards

Compare to other materials

• Polycarbonate Vs. Acrylic
• Acrylic Vs. Pet Petg
• Acrylic Vs. Abs

Frequently asked questions — Acrylic FAQ