Polycarbonate Glass Transition Temperature: Why 147°C Matters for Grade Selection

In 2023, a product engineer at a German electronics firm selected polycarbonate for the enclosure of a product that was going to be under a constant temperature of 135°C. The engineer made the choice to use PC when he noticed that the polycarbonate glass transition temperature from the datasheet was 147°C. 6 months after the field test commenced, the enclosures started to sag, which was not because the material was problematic, but because of his misinterpretation of a number.

If you were to navigate the rocky road of constant disappointments and finger-pointing and would miserably come across the polycarbonate datasheet with an assumed connotation that 147°C denotes, “I can use up to 147°C,” count yourself not alone. Some engineers derive reasons wrong from Tg, supposing that it specifies the service temperature or melting point, to material failures, reworks, and production schedule delays.

This article explains what PC’s 147°C glass transition temperature actually means, how it differs from heat deflection temperature (HDT) and processing temperature, and which Covestro Makrolon grades are built for high-thermal applications. By the end, you’ll know how to read PC thermal data accurately and which grade to request for your operating conditions.

[Need help selecting the right PC grade? Contact Yifuhui for a grade recommendation based on your thermal requirements →]

What Is the Glass Transition Temperature of Polycarbonate?

The glass transition temperature Tg of polycarbonate is about 147–150°C. At Tg, the polymer in the amorphous state transition from a rigid, glassy state to a relatively softer, rubbery state. Polycarbonate does not have strict melting points as POM and PA66 do. But it just softens as Tg is crossed.

Upon Tg, the molecular chains of polycarbonate suddenly find that they have sufficient mobility to slide past one another. The consequences are essentially reduced stiffness and modulus. Below Tg, the chain is locked in place such that PC exhibits outstanding rigidity, clarity, and resistance to falling.

In the system above and at 147°C and beyond, this is the manner in which a few parameters change:

• Decrease in modulus: The young modulus is likely to drop by as much as 80% or even more as Tg is crossed.
• Dimensional stability and some creepage of parts would now set in under load.
• Impact strength: The material starts to plastically deform hence becomes ductile.

This is a very fundamental trait exhibited by amorphous thermoplastics; this means that predicting real-life service limits using only Tg is not a very reliable method. When deciding at what point a part made of polycarbonate begins to sag under load, engineers should always look for the HDT, not the Tg.

Tg vs. HDT vs. Processing Temperature: Three Numbers That Confuse Engineers

Polycarbonate material safety data sheets provide multiple temperature values, and treating these values as interchangeable is one of the most common sources of specification errors that we have encountered. The following list gives the definition of three critical temperatures:

Glass Transition Temperature, Tg- Morphology Transformation Point (~147°C)

Tg is a material science phrase for the temperature at which a material goes from a glassy state to a rubbery one; or our concern here is the point at which the material transitions out of any molded condition, and this can be found encased in a range on the technical data sheets. It is normally measured utilizing the differential scanning calorimetry (DSC); this feature is identical through all standard PC grades. A true value of Tg would let the molecular level start becoming plastic in nature rather than failure of a molded component.

Heat Distortion Temperature, HDT-Maximum Service Temp (~128–140°C at 1.8 MPa)

HDT is defined under a mechanical load. At 1.8 MPa pressure–a standard test load of engineering plastics–polycarbonate should fail approximately from 128 °C to 140 °C.deflection attenuation property exhibits a significant decrease under dynamically stressed condition because real parts, bearing the load, are subject to stress, and stress accelerates softening.

HDT is important for third limit of service temperature range due to the loading of some parts, such as brackets, housings, or structural clips. An unloaded PC part could sustain temperatures higher than Tg, but any sustained mechanical load limits this effective point.

Processing Temperature — Why PC Must Be Molded at 260–320°C

Polycarbonate must be processed between 260-320°C by the injection molder-since it seems unfathomably high when compared with a Tg of 147°C. Somewhat simply put, the processing temperature must surpass the viscosity of the material to force-flow it inside thin-walled molds with complex geometries. PC “melts” in a distinctly crystalline manner, it must be taken to considerably higher temperatures over the Tg to have its very low viscosity necessary to effectively mold.

However, there is still an upper limit. PC starts thermally to degrade at temperatures above ~320°C, and it degrades further into yellowing, molecular weight reduction, and lower mechanical properties of the molded part.

When Marc, employed as a mold process engineer at a Portuguese automotive supplier, first ran Makrolon 2805 by Covestro, he set his barrel at 280°C and assumed the 147°C Tg would allow molded parts to resist 140°C of underhood service. He passed the first checking phase, but parts began warping after 500 hours at 130°C under clamp load. The problem wasn’t the incorrect one, as it was not molded temperature. The problem was service temperature that was anticipated initially, and the actual HDT. The parts post-molded received a decent annealing any time one switched to a little increased-grade HDT and then got them validated. The message was clear: Tg sets the cap, but it is HDT that defines that point, where the failure will start.

How PC’s 147°C Tg Compares to Other Engineering Plastics

Precision components for the engineers usually came as a choice, where they select PC, POM, PA66, ABS, and PMMA as per requirement. The transparency by Tg and practical service temperature helps making the choice clearer.

When Should PC be a Material of Choice for Heat Performance?

• Applications in which optical clarity and moderate heat resistance are necessary (LED lenses, display covers, light guides)
• Components that are occasionally subjected to thermal spikes above 120°C for short times, but are otherwise free of any load
• Electronic housings to be similar strength-wise but require dielectric strength and flame retardancy and, additionally, need dimensional stabilization

When Should PC NOT be a Material of Choice?

• Service at temperatures exceeding 125°C (use of PPS or high-heat PA66)
• Regular exposure to strong bases, ketones, or aromatic hydrocarbons (use of POM or PTFE)
• Scratch-prone settings that benefit more from PMMA or coated PC

For a deeper look at how polycarbonate compares to acetal in precision mechanical applications, see our guide on POM vs. PC material selection.

Covestro Makrolon Grades and High-Temperature Performance

Not all polycarbonates exhibit the same behavior at high temperatures. Covestro’s Makrolon product line includes general-purpose, flame-retardant, UV-stabilized, and high-heat grades — each designed to fit the thermal profile required for the particular application.

General-purpose grades: Makrolon 2407 and 2805

• Makrolon 2407: This high-flowing, UV-stabilized general-purpose grade has excellent optical quality. Having a standard Tg (~147°C) and an HDT of around 128°C at 1.8 MPa, it is perfect for outdoor lighting lenses; transparent covers, and display applications.
• Makrolon 2805: This medium-grade relative-viscosity general-purpose grade has an HDT slightly higher than 2407; it shows very good dimensional stability. It is widely used in industrial housings, medical device components, and automotive interior trims.

Both grades exhibit the standard PC commodity properties: high impact strength, 88-90% transmission of light, and good processability.

Flame-Retardant Grades: Makrolon 6555 and 6557

• Makrolon 6555: UL94 V-0 at 1.5 mm remains in keeping with opticality. The Tg and HDT have an offset close to that of standard grades because of flame-retardant additives—which is a prerequisite for electronics enclosures and electrical insulators.
• Makrolon 6557: In addition to the flame-retardant property, it offers outdoor weathering resistance through ultraviolet stabilization. The Tg will still be about ~145°C. However, the real-world temperature limit will be determined by HDT (~130°C at 1.8 MPa) and the degradation profile of the FR additives.

Applications:y-yal connector housings, switch components, and battery enclosures which also require heat resistance and fire safety.

High-Heat and Reinforced Grades: 8025, 8035, 8325, and 9415

High-heat and reinforced grades usually push the practical thermal envelope when Tg is not enough to meet the requirements:

• Makrolon 8025 / 8035: Glass-fiber-filled grades (usually 20% GF), featuring stiffer than the basic grade, with HDT reaching 145°C and more. These are the grades to go for with structural brackets, components under the hood or sub-parts, and components under prolonged service loads at elevated temperatures.
• Makrolon 8325: A high-flow glass-filled grade specifically made for thin-wall molding in demanding heat-related applications.
• Makrolon 9415: A mineral-filled high-heat grade with low CLTE and extremely good dimensional stability. It is the preferred grade for precision parts which may be subjected to thermal cycling without cutting down the holding of tolerances.

At Yifuhui, we maintain full manufacturer COAs for Makrolon 2407, 2805, 6555, and 6557. High-heat grades such as 8025, 8035, 8325, 9415 are available for an order and come with lot-specific data sheets and traceability records.

[Request a quote for Covestro Makrolon PC resin — response within 24 hours with full COA documentation →]

Design and Processing Implications of PC’s Tg

Knowing about Tg is hardly an academic exercise; it really impacts mold design, processing parameters, and post-mold treatments.

Mold Temp Ranges: 80–120°C

Optical polycarbonate is best molded with mold temps at 80–120°C. This is relevant to Tg because rapid cooling through this temperature could lock in residual stresses. A higher mold temperature gives relaxation time for the polymer chains before solidifying below Tg, minimizing internal stress and enhancing dimensional stability.

For optical parts, mold temps as high as 120°C would further minimize birefringence and stress cracking.

Avoiding Thermal Degradation Above ~320°C

Molders usually melt 10°C above Tg; but, care is also needed in the upper limits. Prolonged residence time at 260-320°C or above causes:

• Yellowing and color change
• Decreased molecular weight that leads to a brittle part
• Release of degradation byproducts

Addenda: The molder should attempt to use the lowest barrel temperature that still permits complete cavity fill, or else minimize time spent in the barrel–all in an effort to restore the thermal properties of the product, including Tg and HDT.

Annealing: Appearance Considerations for High-Temperature Service Types

When working at a temperature close to the HdT level, annealing makes excellent examples of thermal stability enhancement for the final part. The process involves heat treating the section to a shade below Tg (120-130°C, typically), cooling it slowly. By so doing, one is able to relieve stresses introduced during molding and reduce the chances of climatic warping or creep.

Sourcing Consistency: Why Batch Quality Affects Thermal Performance

The thermal properties, such as Tg and HDT, are no theoretical constants; they depend on the molecular weight of the polymer, the additive package, and the processing history of the polymer. Off-spec or degraded PC can reveal a reduced Tg, lower HDT, and underperforming thermal aging under long-range conditions.

During 2022, a purchasing manager of an Italian manufacturer of lighting material changed low-cost supplier for Makrolon 2805 grade equivalent polycarbonate. The first two lots were quite good. The third one was a nightmare, though — a COA that was issued by the producer indicated that the MFI is good, but the pellets revealed a visibly yellowish shade. Upon testing internally, parts showed reduced HDT, following which the supplier was seeking degraded or improperly stored material. Production was stopped, and they returned directly to a known distributor with original stale traceability.

So what is the importance of a COA to thermal properties rather than simple appearance or MFI? A proper COA regarding polycarbonate should provide the following:

• Full Material grade and brand name
• Full Lot Number, perhaps including manufacturer name.
• melt index rate (MIR) or melt flow rate (MFR)
• Thermal characterization data primary references (Tg, HDT where possible)
• Compliance allowance declarations (RoHS, REACH, UL94 where applicable)

Yifuhui’s modus operandi is (as follows): Every batch of Covestro Makrolon inventory that is sent out has a factory-issued COA that comes back to the production lot. We do not mix lots, substitute for heat, or send out material with expired or in unmentioned thermal attributes.

Conclusion

Polycarbonate boasts a 147°C glass transition temperature that is one of the defining factors for it — although it is not the only figure that matters in choosing the material. In working components, HDT (128–140°C) is the practical service range. For molders, processing at 260–3200°C is standard despite the much lower Tg. And for engineers deciding between materials, the amorphous nature of the PC is such that it softens gradually rather than behaves as a sharp newtonian fluid, which is both a plus and a design constraint.

The key takeaway is this: grade selection depends on the full thermal and regulatory profile, not Tg alone. General-purpose grades like Makrolon 2407 and 2805 serve optical and structural applications. Flame-retardant grades like 6555 and 6557 meet UL94 V-0 requirements. High-heat reinforced grades like 8025, 8035, and 9415 extend the practical thermal envelope for demanding mechanical applications.

If you are looking at polycarbonates for a high-temperature application, make sure you select the proper grade-and source it from a supplier who can give a traceable COA for its thermal properties.

[Request a quote for Covestro Makrolon PC resin — response within 24 hours with full COA documentation →]

Frequently Asked Questions

The maximum continuous service temperature for polycarbonate is 147 degrees Celcius?

No. Illumination at about 147°C corresponds to the glass transition temperature, Tg, but in practical loading conditions, a better measure appropriate for practical hot-load conditions is HDT, which varies some 128-140°C at 1.8 MPa load. Continuous loading to extremes should be avoided with PC at ~115-125°C.

What is the difference between Tg and Melting Point in polycarbonate?

Polycarbonate, being an amorphous polymer, does not have a sharp melting point like the crystalline polymers (POM or PA66); rather, it undergoes a gradual thickening at a point it passes through the glass transition temperature of approximately 147°C.

Which Covestro’s polycarbonate Makrolon is most preferable for high-thermal applications?

Any glass fibre reinforced grade, like the Makrolon 8025 or 8035 will offer highest thermal performance from good effect of high HDT and stiffness…over an unfilled grade. Surely Makrolon 6555 and 6557 would retain standard thermal properties after imparting the flame-retardation, having UL 94 V-0 rating.

How can the polycarbonate be processed at temperatures up to 260°C-320°C if its transition temperature is just 147°C?

To fill tricky mold shapes, it is necessary to decrease the polymer’s viscosity level to a considerable minimum. Even at slightly above the Tg the PC is still too viscous to be brightly molded. A temperature range of 260 to about 320°C allows for both mobility and small enough to avoid decomposition.

Is there any way I can verify the thermal properties of a batch of PC resin?

Obtain the manufacturer’s Certificate of Analysis (COA) for the particular lot. Check the MVR/MFI along with some of the HDT values with the official material data sheet (MDS) of Covestro posted in the bulletin. Well-preserved, correctly produced Makrolons vary very little in their thermal properties among different batches.