PMMA has been the standard optical thermoplastic since 1933. With 92 % light transmission (vs 88 % for PC) and outstanding UV stability, PMMA outperforms all other plastics for visible-light applications. The drawback is brittleness — PMMA shatters on hard impact, which is why PC is preferred for safety glazing despite lower transparency.
PMMA Property Values
Property values for standard cast (PMMA-GS) and extruded (PMMA-XT) acrylic. Cast PMMA has higher molecular weight and better mechanical properties; extruded is cheaper and easier to thermoform.
Property
PMMA standard
PMMA-GS cast
PMMA impact-mod
Density
1.17–1.20 g/cm³
1.18–1.19 g/cm³
1.16–1.18 g/cm³
Tensile strength
70–80 MPa
75–85 MPa
50–65 MPa
Tensile modulus
3,000–3,300 MPa
3,300 MPa
2,200–2,800 MPa
Glass transition Tg
105 °C
115 °C
100 °C
HDT/A (1.8 MPa)
85–100 °C
95–105 °C
80–95 °C
Charpy notched (ISO 179)
1.5–2.5 kJ/m²
1.5 kJ/m²
6–10 kJ/m²
Light transmission (3 mm)
92 %
92 %
90 %
UV stability
excellent
excellent
excellent
Pencil hardness
HB-2H
2H
HB
Water absorption (24 h)
0.3 %
0.3 %
0.3 %
PMMA Processing
PMMA is processed by injection molding, extrusion, thermoforming and cell-casting. Injection molding: melt temperature 220–250 °C, mold temperature 60–90 °C. Drying: 80 °C / 3–4 h to less than 0.04 % moisture (PMMA is slightly hygroscopic). For optical clarity, use polished tools and dust-free processing. Cell-cast PMMA sheets (Plexiglass GS) are made by polymerization between glass plates and have superior optical properties to extruded sheets.
PMMA vs PC for Optical Applications
PMMA: 92 % transmission, UV-stable, scratch-resistant (HB-2H pencil), brittle. PC: 88–90 % transmission, UV-yellowing without protection, scratch-prone, very impact-resistant. Choose PMMA for: outdoor signage, optical lenses without impact requirement, lighting covers, aquariums. Choose PC for: safety helmets, riot shields, vandal-resistant glazing, automotive headlamps that may encounter stones.
FAQ — Frequently Asked Questions
Why is PMMA so brittle?
PMMA has a Tg of 105 °C and a relatively rigid backbone with bulky methyl methacrylate side groups. At room temperature, the polymer chains cannot dissipate impact energy through deformation, leading to brittle fracture. Impact-modified PMMA grades contain dispersed rubber particles (similar to ABS structure) and reach Charpy notched 6–10 kJ/m² but with reduced transparency.
How does PMMA compare to glass?
PMMA: 30 % lighter than glass, 17 times more impact-resistant, easier to fabricate (drilling, cutting, gluing), but softer (scratches more easily than glass). Glass: harder surface, higher temperature resistance (>500 °C), better UV cutoff, but heavier and shatters dangerously. PMMA outdoor lifetime: 25–30 years without significant transmission loss. Glass: essentially permanent.
Can PMMA be polished?
Yes, PMMA can be polished to optical clarity from machined or sandblasted surfaces. Methods: (1) Flame polishing — quick gas flame pass, melts surface to glossy finish (skill-dependent, can create stress). (2) Diamond polishing — multi-step abrasive polish, best results for thick edges. (3) Chemical polishing — solvent vapor (dichloromethane) for delicate surfaces. (4) Vapor polishing for production. Always work in well-ventilated environment.
PMMA gluing — which adhesives?
Best results with reactive solvents that partially dissolve and rebond PMMA: dichloromethane (DCM, gives clear glue line, fast), Acrifix 192 (acrylic monomer-based, polymerizes within PMMA, structural strength). For invisible bonds: capillary application of DCM between mating surfaces. For structural bonds: 2-component acrylic adhesives (Acrifix 1R 0190, Plexus MA300) cured by polymerization. Avoid epoxy and superglue (cyanoacrylate) — show poor adhesion to PMMA.
Is PMMA food-contact safe?
Standard PMMA grades comply with EU 10/2011 and FDA 21 CFR 177.1010 for food contact. Common applications: salad bowls, display cases, juice dispensers, beverage tubes. Migration values are very low for typical food matrices. Not suitable for hot-fill applications above 80 °C due to softening. For dishwasher applications, only specific impact-modified grades are recommended (cycle temperature 70 °C).