Polycarbonate is the plastic engineers grab when they need something clear, tough, and easy to mold. You will find it in car parts, electrical housings, medical devices, and plenty of large structural components. It has one weakness that ruins a lot of good ideas, though. Standard grades hate heat. Their strength is only so-so to begin with, and once you push them into hot injection molding they yellow, go brittle, and crack. That is why you rarely see plain polycarbonate in big, complicated parts.
This piece lays out a formulation route for a high-temperature, high-strength polycarbonate. It comes from bench work that screened a lubricant, a UV absorber, and antioxidants, then pinned down the right amounts and the right processing window. If you compound polymers or pick materials for a living, it should spare you a few wasted batches.
Why ordinary polycarbonate breaks down when it gets hot
Polycarbonate (PC) is a family of polymers whose chains carry the carbonate group. Most of it is made by reacting bisphenol A with phosgene at an interface, and that route gives better clarity and easier control over molecular weight than the phosgene-free route. Clarity and molecular weight tuning are exactly what high-end parts need, so the method matters.
The enemy is heat. During injection molding the melt can sit at 280 to 350 degrees Celsius. Ordinary polycarbonate starts to degrade in that range. The chains break, the color drifts yellow, and the impact resistance everybody loves disappears. A part can look fine leaving the mold and then fail under load, or it can show stress cracks on the spot. When resin is expensive and the market is tight, you cannot ship a weaker, yellowing part.
The target is simple to state and hard to hit: keep polycarbonate’s clarity and processability, but raise both the temperature ceiling and the strength. You get there with a better additive package and a tighter process.
What actually holds a heat-resistant PC together
Four additive types do the work, and each one fixes a different failure.
A lubricant keeps the melt moving so the cavity fills and the structure stays even. A UV absorber slows the sunlight-driven oxidation that yellows and embrittles the surface. A primary antioxidant catches the heat-formed free radicals before they shred the chains. A secondary antioxidant mops up the peroxides the primary one leaves behind, and the pair beats either alone.
The amounts are the whole game. Too little and nothing changes. Too much and the additive blooms to the surface or makes the part brittle in a new way.
The lubricant: more flow, more bend resistance, but watch the surface
Pentaerythritol stearate, PETS for short, is the usual lubricant here because it stays stable at high temperature and mixes well with the resin and everything else in the blend. Add more of it and two helpful things happen. The melt flow rate rises, so the material fills the mold more easily. The flexural strength, its resistance to bending, climbs too.
In the side-by-side samples, flexural strength kept climbing with PETS content and peaked at 0.4 percent added. The other mechanical numbers barely moved, which is what you want. Past that point the trouble started. Too much and the lubricant migrates out and leaves a haze or powder on the surface, the classic blooming problem. Balancing the strength gain against the blooming risk, the work settled on 0.3 percent PETS.
The UV absorber: good for color, bad in excess
Out in sunlight, polycarbonate oxidizes and turns yellow and brittle while the light transmission drops. A UV absorber takes the damaging wavelengths before they reach the molecular structure. That keeps the color and the strength, and it also helps with stress cracking and hydrolysis.
The dose needs a careful hand. More UV absorber does improve yellowing resistance, but it also pulls down the melt flow rate, the impact strength, and the transparency. Higher levels also raise the share of samples that fracture in a brittle way during molding. The data landed on 0.2 percent as the level that gives real UV protection without handing back the brittleness it was meant to remove.
Antioxidants are where the heat resistance comes from
High-temperature processing is where polycarbonate suffers most, and antioxidants are the main defense. A primary antioxidant intercepts the alkyl radicals that heat creates. A secondary one breaks down the peroxides those radicals form with oxygen. Together they slow degradation far better than either alone.
Four antioxidants were tested at 320 degrees Celsius, holding the samples and watching how fast they yellowed. One pairing won: primary antioxidant A with secondary antioxidant B2 gave the best yellowing resistance. A follow-up ratio test put the sweet spot at 1 part primary to 3 parts secondary.
The thermal data backs the formulation
Thermogravimetric analysis shows how fast and how soon a material loses weight as it heats. In the samples with additives, the temperature of maximum weight-loss rate rose clearly above untreated polycarbonate. A higher peak means the structure stays stable longer before it breaks down, which lowers the chance of brittle fracture during molding. That is the direct line from the additive package to the claim of high temperature and high strength.
Temperature control is half the job
Even a perfect recipe fails with the wrong extrusion. Too cold and the material does not plasticize evenly, so the properties are inconsistent. Too hot and the yellow index climbs, brittle fracture shows up, and impact strength falls.
The comfortable window looked like this. Extrusion ran at 250 to 280 degrees Celsius for pelletizing, and molding used about 280 degrees Celsius. For the optimized grade, the recommended extrusion settings were a screw length-to-diameter ratio of 40 to 1, a screw speed of 500 to 600 revolutions per minute, and a processing temperature band of 280 to 300 degrees Celsius.
The finished recipe and what it earns
Putting the screening together, the optimized heat-resistant, high-strength polycarbonate uses a combined package of a primary antioxidant, a secondary antioxidant, a UV absorber, and a lubricant, with the levels tuned to the findings above. Under that process window the material improves across the board: better resistance to heat-driven yellowing, maintained light transmission, and higher mechanical strength.
The contrast with standard polycarbonate makes the point. Ordinary grades begin to show impact brittle fracture at 320 degrees Celsius and the failure rate jumps by 350. The improved material holds off until 350 degrees, and even there only about 10 percent of samples fracture. At 350 degrees, standard polycarbonate also behaves inconsistently, with tensile break strength and notched impact strength swinging widely shot to shot. That scatter is a genuine headache for customers who need predictable parts. The modified grade takes most of it away.
What to carry into your own lab
If you compound or specify polycarbonate for hot, large, or demanding parts, a few things are worth testing yourself.
Keep the lubricant modest. Around 0.3 percent PETS buys better flow and bend resistance without blooming. Treat the UV absorber as a trade, not a free win. At 0.2 percent it protects color, but more starts to hurt clarity and toughness. Build the heat resistance on a primary plus secondary antioxidant pair, and run the ratio study instead of guessing, because the 1 to 3 balance is what makes them work together. Then respect the temperature window, because a good recipe still needs clean plasticization and a controlled melt.
You end up with a polycarbonate that keeps its clarity and toughness while shrugging off the heat that normally tears ordinary grades down. For anyone trying to move polycarbonate into bigger, hotter applications, that difference is the line between a part that survives and a part that ships.