A taillight lens that held its shape at 95 °C in a lab oven still warped after three months in service, clamped against a hot housing. The datasheet for the acrylic said the glass transition temperature was 105 °C, so the design team assumed they had plenty of margin. They had misunderstood what that number actually promises.
That mistake is the most common one we see with PMMA. Engineers treat a 105 °C Tg as “safe up to 105 °C,” then watch parts soften, creep, or lose optical flatness far below it. The truth is more useful than the headline figure: the PMMA glass transition temperature describes when the polymer’s molecular backbone begins to move, not when a part can safely work.
This guide explains what Tg controls for an amorphous transparent plastic like acrylic, how it differs from heat deflection temperature and continuous-service limits, when a heat-resistant grade is the real fix, and what to verify on a Certificate of Analysis when you source resin. If you need the short version before a design review, our engineering plastic temperature guide charts Tg, HDT, and service temperature across the plastics we supply, and our technical team can pull the exact manufacturer datasheet for the grade you’re evaluating. Request a quote and we respond within 24 hours.
What Is the Glass Transition Temperature of PMMA?
Standard atactic PMMA has a glass transition temperature (Tg) of approximately 105 °C. Because PMMA is amorphous, it has no true melting point. Commercial molding grades typically report a Tg between about 90 °C and 110 °C, and heat-resistant grades extend to roughly 120 °C.
That bold number is the honest consensus behind most of the searches for “PMMA glass transition temperature” or “acrylic glass transition temperature.” Raw-polymer suppliers such as Polysciences list 105 °C for their standard PMMA, and thermal-analysis references converge on the same figure. To use it correctly, three facts matter.
- It is a property of the polymer chain, not of the part. Tg marks where the amorphous backbone gains enough thermal energy to begin large-scale segmental motion. Below Tg, PMMA is glassy and rigid. Above it, the modulus drops sharply and the material becomes rubbery and easily deformed.
- It sits far below where you process the material. Because PMMA has no crystalline melt, it is molded at 200 to 250 °C by melt viscosity, not by melting. Tg tells you nothing about barrel temperature.
- It is not the safe operating ceiling. For an optical or aesthetic part under even light load, useful service stops well before 105 °C. More on that in the next section.
Why PMMA Has No True Melting Point
PMMA, or poly(methyl methacrylate), is an amorphous thermoplastic with no melting transition. Its bulky methyl and ester side groups attach to the chain in random positions, so the chains cannot pack into the ordered crystalline regions that give materials like POM, PA66, or HDPE a sharp, well-defined melting point. Heat a semi-crystalline polymer and you’ll get a distinct melt endotherm at Tm. Heat PMMA in a DSC and you see a step change in heat flow at Tg, but no melting peak.
Instead, acrylic softens gradually as it passes through its glass transition. Above roughly 170 °C its melt viscosity is low enough to flow into a mold, and processors rely on that viscosity window rather than a phase change. Push the temperature too far, above about 250 to 270 °C, and PMMA begins to depolymerize back toward its methyl methacrylate monomer, which shows up as bubbles and silver streaks in the part.
The Tacticity Effect on Tg
If you compare sources, you’ll see PMMA Tg values from about 45 °C up to 130 °C. Tacticity, the arrangement of side groups along the chain, drives most of that spread.
- Atactic PMMA, the form commercial grades use, clusters near 105 °C.
- Isotactic PMMA, with side groups on one side of the chain, has a Tg near 45 °C.
- Syndiotactic PMMA, with alternating side groups, runs higher, roughly 115 to 130 °C.
Commercial molding resins are atactic because that configuration gives the best balance of clarity, strength, and processability. So when a supplier or database quotes a single “PMMA glass transition temperature” near 105 °C, it is describing the standard atactic molding resin, and that is the value you should carry into a design review.
Tg vs. HDT vs. Service vs. Processing: The Four Temperatures Engineers Confuse
Most thermal specification problems with acrylic come from conflating four different temperatures that serve four different purposes. Here is the ladder, from the molecular property up to the practical process.
Glass Transition Temperature (Tg): ~105 °C
Tg is measured by differential scanning calorimetry (ASTM D3418 / ISO 11357) or dynamic mechanical analysis, and it reflects a change in the polymer’s molecular mobility. It is the right frame for understanding behavior, but it is not a load-bearing service limit. For a heat-resistant grade the value can reach roughly 120 °C, which is why grade selection matters.
Heat Deflection Temperature (HDT): ~85 to 95 °C @ 1.8 MPa
HDT is measured with the part under load, typically 1.8 MPa (264 psi) or 0.45 MPa (66 psi), per ASTM D648 / ISO 75. Under 1.8 MPa, standard PMMA deflects around 85 to 95 °C; under the lighter 0.45 MPa load it runs about 95 to 105 °C. Materials databases such as MakeItFrom list the same range, roughly 96 °C at 1.82 MPa. Because real parts work under stress, HDT is the practical loaded limit and it always sits below Tg.
Continuous Service Temperature: ~70 to 77 °C
This is the real design rule for sustained service. Below the glass transition, PMMA still creeps under sustained load, and its surface will mark or soften on contact long before the backbone mobilizes. That is why standard grades are rated for continuous service around 70 to 77 °C, roughly 30 °C below the 105 °C Tg. Optical and aesthetic parts are derated further, because a light guide that creeps 0.1 mm or a lens that picks up a permanent fingerprint mark has failed even though it never “melted.”
Processing Temperature: 170 to 275 °C
PMMA must be molded at 200 to 250 °C despite a 105 °C Tg, and that confuses newcomers every time. The reason is simple: without a melt transition, the processor relies on melt viscosity, and acrylic only flows freely well above Tg. Mold temperature typically runs 50 to 80 °C, with the higher end used for optical surfaces. Avoid long residence near or above 250 to 270 °C, where depolymerization begins.
PMMA Temperature Metrics at a Glance
| Metric | Typical Value | Test / Note |
|---|---|---|
| Glass transition temperature (Tg) | ~105 °C (90 to 123 °C by grade and method) | DSC, ASTM D3418 / ISO 11357 |
| Heat deflection temperature (HDT) | ~85 to 95 °C @ 1.8 MPa; ~95 to 105 °C @ 0.45 MPa | ASTM D648 / ISO 75 |
| Vicat softening temperature | ~92 to 108 °C, grade and load dependent | ASTM D1525 / ISO 306 |
| Continuous service temperature | ~70 to 77 °C (heat-resistant grades higher) | Supplier datasheet; design well below Tg |
| Melt / processing temperature | 170 to 275 °C (200 to 250 °C typical for injection molding) | Set by melt viscosity, not a melt transition |
| Mold temperature | 50 to 80 °C | Higher end for optical surfaces |
| Drying | 70 to 80 °C for 2 to 4 h; residual moisture ≤0.02% | Hygroscopic; pre-dry before molding |
| Beta (secondary) transition | ~40 °C | Source of room-temperature toughness |
Values are grade dependent, so always confirm the specific grade’s datasheet. When you are comparing acrylic against other candidates for a hot environment, the engineering plastic temperature guide gives you the same four-temperature ladder for every material we stock.
How PMMA’s ~105 °C Tg Compares to Other Plastics
Putting PMMA next to its common alternatives makes the transparent-material decision much clearer.
| Material | Type | Tg (°C) | HDT @ 1.8 MPa (°C) | Continuous Service (°C) |
|---|---|---|---|---|
| PMMA (acrylic) | Amorphous | ~105 | ~85 to 95 | ~70 to 77 |
| Polycarbonate (PC) | Amorphous | ~147 | ~128 to 140 | ~115 to 130 |
| ABS | Amorphous | ~105 | ~85 to 100 | ~60 to 80 |
| Polystyrene (GPPS) | Amorphous | ~95 to 100 | ~70 to 95 | ~50 to 70 |
| POM (copolymer) | Semi-crystalline | ~-60 | ~95 to 110 | ~82 to 115 |
| PA66 | Semi-crystalline | ~50 to 70 (dry) | ~70 to 105 (dry) | ~95 to 120 |
For transparent parts near heat, the decision rule is PC versus PMMA. Polycarbonate’s glass transition sits near 147 °C, roughly 40 °C above acrylic, which is why PC dominates applications like headlamp lenses and transparent housings that bake in service. We explain the physics behind that gap in our guide to the polycarbonate glass transition temperature. But PC gives back what it gains on heat: it is softer, scratches more easily, yellows faster under UV without a coating, and costs more than PMMA.
So the honest supplier answer, and we sell both, is this. If a transparent part will see sustained heat above roughly 80 °C, specify PC or a heat-resistant PMMA grade, not standard acrylic. If the priority is scratch resistance, UV stability, optical clarity, or cost, PMMA wins, and its 105 °C Tg is more than adequate for ambient and interior duty. Our Covestro Makrolon polycarbonate resin page covers the PC side of that trade.
Against ABS, PMMA and ABS share a Tg near 105 °C, but ABS’s value belongs to the styrene-acrylonitrile phase and the material is not transparent. Where clarity and weatherability matter, PMMA is the choice; where impact and cost dominate, ABS pulls ahead.
How PMMA’s Glass Transition Is Measured (and Why Values Differ)
You will see 105 °C, 110 °C, and 112 °C from credible sources, and none of them is wrong. Method and grade explain the differences.
- Differential scanning calorimetry (DSC). The standard technique per ASTM D3418 / ISO 11357. It measures the step change in heat flow as the polymer passes through Tg. Röhm’s PLEXIGLAS AG 100, an optical grade, reports a Tg of 112 °C by ISO 11357, while general-purpose grades like PLEXIGLAS 7N or 8N sit around 104 to 110 °C.
- Dynamic mechanical analysis (DMA). DMA tracks the modulus drop under a small oscillating stress and often reports a slightly different, sometimes higher, value than DSC because it probes a different molecular response.
- The beta relaxation near 40 °C. Below the main Tg, PMMA has a secondary transition driven by motion of the methyl and ester side groups. It is the reason acrylic is tough and scratch tolerant at room temperature despite being a glassy polymer, and it explains why thin PMMA parts rarely shatter the way polystyrene does.
Beyond method, four variables shift the measured value: tacticity, molecular weight, copolymer or impact-modifier content, and plasticization. Absorbed moisture, residual monomer, and solvents all depress Tg. A wet or freshly polymerized sample reads lower than a dry, fully degassed one.
So which number do you use? The grade’s own ISO 11357 DSC value, taken from the manufacturer datasheet, is the defensible answer. Then design your service margin below the continuous-service line, not up against the Tg line. If you are comparing datasheets during sourcing, our PMMA material properties guide explains how to read the thermal fields alongside the mechanical and optical ones.
Heat-Resistant PMMA Grades: When 105 °C Is Not the Final Answer
Standard general-purpose grades such as PLEXIGLAS 7N/8N or Mitsubishi Acrypet MD carry a Tg around 104 to 112 °C. If your part must survive sustained proximity to an LED, a lamp, or an automotive heat source, that may genuinely not be enough. Heat-resistant acrylic families exist for exactly this situation.
Heat-resistant PMMA grades, which include Röhm’s PLEXIGLAS Heatresist FT15 at roughly 121 °C and the “HT” and heat-resistant acrylic families from Mitsubishi and LG Chem, lift both Tg and the practical HDT and service window. They achieve this with styrene or maleic-anhydride modifications or higher syndiotacticity, and they make LED light guides, backlit displays, and interior automotive parts viable where standard acrylic would creep or distort.
Those gains cost something. Heat-resistant grades typically trade a little optical clarity and melt flow for the higher thermal ceiling, and they are more expensive per kilogram. The rule of thumb is simple: if the ambient temperature near the part exceeds roughly 75 to 80 °C in sustained service, step up to a heat-resistant grade rather than assuming standard acrylic will survive.
A lighting manufacturer we supply learned this the hard way. Their first LED badge for a commercial refrigerator used a standard acrylic diffuser, because the LED itself stayed below 60 °C. What they missed was the enclosure’s internal ambient temperature near 80 °C, amplified by poor airflow.
Within four months, the diffusers had crept and developed stress whitening around the mounting screws. Switching to a heat-resistant acrylic grade with a documented Tg near 118 °C solved the field failures without a redesign. That one spec change was cheaper than the warranty batch.
Design and Processing Implications of PMMA’s Tg
Understanding Tg is useful only if it changes how you design and run the part. Four practical rules cover most of it.
Keep Service Well Below the Tg Line
The design rule for PMMA is to keep continuous service roughly 40 to 50 °C below Tg, which lands standard grades in the 60 to 77 °C band. Creep under sustained load, surface marking from contact, and optical distortion under clamp or mounting stress all begin well below the molecular transition. A part that holds its shape at 95 °C unloaded can still mark or distort under a clamp load at 75 °C.
Anneal Below Tg to Relieve Molded-In Stress
Injection-molded acrylic retains internal stress from non-uniform cooling. Parts that will see heat should be annealed below Tg, typically at 80 to 95 °C, to relax that stress before service. Skipping this step invites stress whitening and cracking later, especially around inserts, gates, and sharp corners.
Dry Thoroughly, Then Mold in the Right Window
PMMA is hygroscopic. Dry the pellets at 70 to 80 °C for 2 to 4 hours in a desiccant dryer and hold residual moisture at or below 0.02%. Moisture above that limit flashes to steam in the barrel and produces splay, bubbles, and haze, defects that are fatal in an optical part.
Mold at 200 to 250 °C with a 50 to 80 °C mold, using the higher mold temperature when surface finish and low residual stress matter. Don’t let the melt linger near or above 250 to 270 °C, where depolymerization back to MMA monomer begins.
Respect That Processing and Tg Are Unrelated
We see buyers ask for a grade “with a low glass transition temperature so it molds easier.” That logic is backwards. Processing temperature is governed by melt viscosity, not Tg. If you need easier flow at lower barrel temperatures, look at a high-flow PMMA grade or a different material, not a lower-Tg one.
Sourcing Consistency: Why Thermal and Optical Properties Go Together for Acrylic
For optical and aesthetic parts, thermal and optical consistency are two sides of the same verification problem, because both trace back to the same polymer quality.
Off-spec, regrind-contaminated, or counterfeit PMMA reveals itself in predictable ways: reduced Tg, HDT, and Vicat values, haze, yellowing, and lot-to-lot swings in melt flow index. In an optical part, recycled or regrind content is invisible at first, then shows up as haze, color shift, or premature yellowing after months of UV or heat exposure. That is why virgin prime resin is non-negotiable for lenses, light guides, and display covers. The contamination is only discovered when parts start failing in the field.
When you source acrylic, verify these fields on the Certificate of Analysis:
- Manufacturer and exact grade name, with a traceable lot or batch number
- Melt flow index and density, the two best lot-to-lot consistency proxies
- Light transmission above 92% and haze below 1% for premium optical grades
- Yellowness index, tracked against the grade baseline
- HDT and Vicat values for thermal specification
Red flags include a generic description like “acrylic pellets” with no manufacturer or grade, missing MFI and density, or a COA that changes shape between shipments. A procurement manager at an automotive tier-one supplier once caught a counterfeit batch this way: the “PMMA” pellets arrived with an MFI that matched nothing Mitsubishi had ever published for the Acrypet grade on the paperwork. The full container was quarantined before a single lens was molded. That kind of check is why branded prime acrylic with full documentation matters more than a low price.
At Suzhou Yifuhui, we supply certified branded PMMA, including Mitsubishi Acrypet and LG Chem grades with PLEXIGLAS and ALTUGLAS cross-references, each shipment backed by a manufacturer-issued COA, MSDS, commercial invoice, and packing list. We are located in Suzhou near the Port of Shanghai, standard lead time to major international ports is 7 to 14 days, and our minimum order is 25 kg, which makes a qualification trial practical before you commit to a container.
Need a certified PMMA grade for a part that runs near heat? Contact Yifuhui’s technical team with your operating temperature and optical requirements, and we’ll recommend a standard or heat-resistant grade with COA verification.
Conclusion
The PMMA glass transition temperature of approximately 105 °C is a genuine molecular property, but it is not a license to run a part at 105 °C. The correct design picture is the four-temperature ladder: Tg near 105 °C describes the polymer chain, HDT near 85 to 95 °C at 1.8 MPa sets the loaded limit, continuous service runs around 70 to 77 °C for standard grades, and processing happens at 200 to 250 °C by melt viscosity.
Three points will save you from the mistake most people make with acrylic:
- Keep sustained service roughly 40 to 50 °C below Tg, which means 60 to 77 °C for standard grades.
- For transparent parts near sustained heat above about 80 °C, step up to a heat-resistant PMMA grade or move to polycarbonate.
- Verify thermal and optical consistency on a traceable COA, because in optical parts the contamination is invisible until it fails.
When you source PMMA for a demanding application, work from verified numbers, not assumptions about the datasheet. Request a quote for certified PMMA resin from Yifuhui with a 25 kg minimum order, full export documentation, and a technical team that responds within 24 hours. To gain a deeper understanding of ABS Melting Temperature, Polycarbonate Glass Transition Temperature, Engineering Plastic Processing Temperature Guide, and heat deflection temperature vs. melting point, please refer to our accompanying guide.
Frequently Asked Questions
What is the glass transition temperature of PMMA?
Standard atactic PMMA has a Tg of approximately 105 °C. Commercial molding grades typically report between 90 °C and 110 °C, and heat-resistant grades reach about 120 °C. Values vary with tacticity, grade, and test method.
What temperature does acrylic soften at?
Acrylic begins to soften around its glass transition near 90 to 110 °C, but for practical purposes it should not run above about 70 to 77 °C in continuous service. Under load, heat deflection occurs at roughly 85 to 95 °C.
Does PMMA have a melting point?
No. PMMA is amorphous, so it has no true melting point. It softens gradually through its glass transition, and it is processed by melt viscosity above about 170 °C rather than by melting.
What is the difference between PMMA Tg and heat deflection temperature?
Tg is a molecular property measured unloaded by DSC, near 105 °C. HDT is measured with the part under a 1.8 MPa load, near 85 to 95 °C. Because real parts work under stress, HDT is the practical loaded limit.
What is the maximum service temperature for acrylic?
Standard PMMA grades are rated for continuous service around 70 to 77 °C. Heat-resistant grades extend the practical window to roughly 90 to 100 °C. Short excursions above the service line are possible, but sustained exposure causes creep and surface marking.
Is PMMA or polycarbonate more heat resistant?
Polycarbonate is more heat resistant. PC has a Tg near 147 °C versus about 105 °C for PMMA, and a higher HDT and service temperature. PMMA beats PC on scratch resistance, UV stability, clarity, and cost.
Why is PMMA Tg different from PC Tg?
The two chains move differently at the molecular level. PC’s stiff aromatic backbone and strong intermolecular forces require much more thermal energy to mobilize than PMMA’s simpler chain, so PC’s Tg sits about 40 °C higher.
What temperature should I mold PMMA at?
Mold PMMA at 200 to 250 °C with a mold temperature of 50 to 80 °C. Use the higher mold temperature for optical surfaces. This window is set by melt viscosity, not by the 105 °C glass transition.
How is PMMA glass transition temperature measured?
Most commonly by differential scanning calorimetry per ASTM D3418 / ISO 11357, which detects a step change in heat flow at Tg. Dynamic mechanical analysis measures the modulus drop and may report a slightly different value.
Are there heat-resistant PMMA grades?
Yes. Grades such as PLEXIGLAS Heatresist FT15, near 121 °C, and HT acrylic families from Mitsubishi and LG Chem raise Tg and HDT for LED and automotive proximity applications. They cost more and can trade a little optical clarity and flow.
Does PMMA get brittle at low temperature?
PMMA stays glassy but not brittle at ordinary low temperatures because its beta transition near 40 °C provides room-temperature toughness, and its main Tg is well above ambient. Impact-modified grades add further low-temperature robustness where needed.
