Polycarbonate Sheet: It Punishes Heat Twice

Heat attacks polycarbonate from two directions

Polycarbonate is a condensation polymer, like PET, and it hydrolyses. Water at melt temperature breaks the carbonate linkages, the molecular weight falls, and the sheet loses impact strength in a way that is not visible until something hits it. That is the first punishment, and it comes from moisture.

The second punishment is oxidation. Polycarbonate at 320 C and above reacts with oxygen, and the reaction produces coloured species that show up as yellow. The yellowness is cumulative with time and temperature, so a resin that passes at 300 C for two minutes will fail at 340 C for two minutes, and the same resin will fail at 300 C if the residence time is long enough.

Both failures are permanent and both are invisible in the finished sheet until the customer looks at it in daylight or drops it. That combination is what makes polycarbonate a material that rewards discipline and punishes shortcuts more than most.

Twenty degrees roughly doubles the yellow

Yellowness index against melt temperature: 1.2 at 280 C, 2.0 at 300, 3.8 at 320, 7.5 at 340 and 14.0 at 360. A yellowness index of about 3 is where a clear sheet starts to read as slightly warm rather than clear, so the practical ceiling is around 310 C.

The rise is faster than linear and it is roughly a doubling per twenty degrees, which is the same Arrhenius behaviour that governs the hydrolysis. The two mechanisms are not independent: wet resin hydrolyses faster at higher temperature, and the same heat that drives hydrolysis also drives oxidation.

The trap is residence time. A barrel set at 320 C with a short residence time can produce less yellow than a barrel at 300 C with a long one, because the reaction integrates over time. That is why a small machine running a large shot, or a line stopped with the barrel full, produces yellow sheet that nobody can explain from the temperature setting alone.

Drying is a time-temperature trade with a ceiling

Time to reach 200 ppm against drying temperature: 8 hours at 100 C, 5 at 110, 3 at 120 and 2 at 130. The trade looks free until you notice that polycarbonate softens and sticks at around 140 C, and that a hopper drier running at 130 C is close enough to that ceiling to be a risk on a hot day.

The usual compromise is 120 C for 3 to 4 hours, which reaches the moisture target with margin and stays clear of the sticking range. Below 100 C the drying takes most of a shift and the resin may never reach 200 ppm, because the diffusion rate through a pellet falls faster than the temperature does.

Table 1 — Drying polycarbonate: time against temperature, and the risk at each level

The amber edge is a measurement, not a defect

A cut edge of polycarbonate looks amber even when the face is clear, and the effect is stronger on a thick sheet. It is not contamination. It is the light path: looking at the edge, you are looking through 3 or 4 mm of material rather than 0.5 mm, so the intrinsic absorption of the polymer is multiplied by the distance.

That makes the edge a free thickness gauge and a free quality indicator. Two sheets of the same nominal thickness with visibly different edge colour have seen different heat histories, and the darker one is the one that has lost more molecular weight. It is a crude measurement and it is available to anyone on the shop floor.

The same effect explains why a formed part looks different at a corner than on a flat face. The corner is thicker after forming, so it reads darker. A customer who compares a corner to a face and reports a colour mismatch is describing geometry, not a batch problem.

The window between too wet and too hot

Polycarbonate's process window is narrower than its data sheet suggests. Too wet and the chain breaks; too hot and it yellows; too long at temperature and it does both. The window that works is roughly 280 to 310 C melt, 200 ppm moisture or below, and a residence time under five minutes.

Table 2 — PC sheet defects and the setting behind each

The most common mistake is fixing a yellow sheet by lowering the temperature without checking the residence time. If the barrel is oversized for the shot, the material is sitting in it for eight minutes and no temperature will save it. The fix is a smaller barrel or a larger shot, not a colder one.

What to specify

A polycarbonate sheet specification has to carry the numbers that predict the two failures.

• Yellowness index, with a maximum, measured on the sheet rather than on the resin.
• Moisture at the hopper, in ppm, with the drying schedule that achieves it.
• Melt temperature and residence time, as a pair, because neither is meaningful alone.
• Impact strength after forming, since hydrolysis shows up there before it shows anywhere else.
• Which face carries the protective film, and whether the film survives the forming temperature.

The impact test is the one that catches the invisible failure. A sheet that has lost 30 percent of its molecular weight still looks clear, still measures the right thickness, and fails on the first impact in service.