Acrylonitrile butadiene styrene is a two-phase polymer system in which polybutadiene rubber particles are dispersed in a styrene-acrylonitrile copolymer matrix. The product is specified by melt volume-flow rate under ISO 1133-1:2022, tensile properties under ISO 527-2:2012 or ASTM D638-14, and notched impact resistance under ISO 179-1:2010 or ASTM D256-10. General-purpose ABS grades typically exhibit a tensile yield stress of 40–50 MPa and notched Izod impact values of 200–400 J/m at 23 °C, but these values are not intrinsic. They are sensitive to melt moisture, shear history, and barrel residence time. In PC/ABS blends, the polycarbonate phase raises heat deflection temperature and impact strength, yet it introduces a chemical vulnerability: the carbonate linkage undergoes hydrolytic chain scission when the resin is moulded with absorbed moisture. That hydrolysis, together with thermal degradation from overheat, is a primary cause of brittle failure in PC/ABS components that otherwise meet dimensional and visual inspection.

What Causes Hydrolytic Chain Scission in PC/ABS Blends?

Polycarbonate in PC/ABS reacts with water at melt processing temperatures through cleavage of the carbonate group, yielding phenol chain ends and carbon dioxide. The reaction is both temperature- and time-dependent; at hopper moisture levels above 0.02%, measurable molar-mass reduction occurs within normal injection moulding residence times. A reduction in average molecular weight from hydrolysis corresponds to an increase in melt volume-flow rate and a drop in ductility. In practice, a PC/ABS part moulded with granulate moisture above 0.05% may lose more than half of its dry-baseline notched impact strength, with failure occurring before yield in ISO 6603-2 instrumented puncture tests. The carbon dioxide generated during hydrolysis also appears as splay, silver streaks, and low-gloss patches, but chemical degradation is present even in parts with acceptable surface finish. Equilibrium moisture absorption for PC/ABS at 23 °C, 50% RH is commonly reported near 0.15%, while moulding requires less than 0.02%; the margin for error is therefore narrow.

Desiccant dryer performance is defined by the dew point of the process air, not the hopper set-point temperature. For PC/ABS, a closed-loop desiccant dryer supplying air at a dew point of -40 °C or lower and an inlet temperature of 100–110 °C for 4–6 h is required to bring granulate moisture below 0.02%. Drying of standard ABS is less demanding: 80 °C for 2–4 h to a moisture level below 0.05% is usually sufficient, though glass-reinforced or high-impact grades may require longer residence in the hopper. In production, the dryer must be sized for the granulate consumption rate; a machine consuming 8 kg/h of PC/ABS with a drying time of 5 h needs a hopper capacity of at least 40 kg, and return-air moisture traps must be maintained. A dew-point sensor reading above -30 °C during a production run indicates silica or molecular sieve saturation or a leaking manifold; under such conditions, the melt may be hydrolytically degraded even when the hopper temperature controller displays the correct value.

Melt residence time, shear heating, and the butadiene degradation window

Thermal degradation in PC/ABS proceeds by a different route than hydrolysis. The polybutadiene rubber phase contains residual unsaturation that is vulnerable to oxidation and crosslinking at elevated melt temperatures. At barrel temperatures above 270 °C, or at lower set-points with high screw shear, the rubber phase can form gel particles and lose its ability to craze and absorb impact energy. Simultaneously, the styrene-acrylonitrile matrix may yellow, and the polycarbonate phase can undergo Fries rearrangement or transesterification at overheated melt interfaces. The practical processing window for PC/ABS is therefore often stated as 240–270 °C for the melt, with the rear zone kept below 240 °C and the nozzle not exceeding 270 °C except for short cycles. A residence time above 5 min at 270 °C is frequently associated with brown streaks and a measurable loss in notched impact. For ABS alone, melt temperatures are typically 210–240 °C, and overheating above 250 °C can produce acrid gases and a visible colour shift from cream to tan.

At the injection unit, the melt temperature is not solely the barrel set-point; shear heating from screw recovery and injection speed can add 10–30 °C to the actual melt temperature. This is especially relevant in small-diameter screws and hot-runner systems where local shear rates exceed 1,000 s⁻¹. Equipment manufacturer data for a 120-tonne hydraulic injection moulding machine with a 28:1 L/D general-purpose screw indicate that increasing recovery speed from 120 rpm to 180 rpm raises measured nozzle melt temperature by 8–12 °C, which can move a PC/ABS grade from ductile to brittle if the barrel profile is already at the upper limit. The use of a hot runner requires separate control of manifold and drop temperatures; published data for this specific configuration is limited, but equipment suppliers recommend setting the hot-runner manifold 10–20 °C below the nozzle temperature to compensate for additional shear heating during gate filling.

When the Barrel Zone Profile Crosses 280 °C

At a barrel front-zone set-point of 280 °C or higher, PC/ABS can degrade within a short residence time, and the resulting moulded article may be brittle even when feed moisture is below 0.02%. The degradation indicators are not always visible: the part can be colour-stable yet the notched impact strength may fall below 20 kJ/m² in ISO 179-1:2010 Charpy testing, from a dry, correctly heated baseline above 40 kJ/m². Overheated PC/ABS often exhibits a lower ductile-to-brittle transition temperature and may fail under low-speed bending at 23 °C even when a control part remains ductile at -10 °C. The mechanism involves simultaneous polybutadiene degradation and polycarbonate chain rearrangement; the SAN matrix may also undergo thermal yellowing. When such parts are recycled into virgin material, the degraded fraction acts as a contaminant, reducing impact resistance even at regrind levels of 10–15%.

In failure analysis, the fracture surface of a hydrolytically degraded PC/ABS part tends to be smooth and featureless through the skin, while the core may contain voids from carbon dioxide expansion. Thermal overheat, by contrast, often leaves visible brown or yellow streaks, but low-level exposure may produce no colour shift. Moulders attempting to separate the two causes should first verify hopper moisture and dew point before changing barrel settings, because thermal damage is frequently secondary to a cycle-time reduction that increases melt residence time at the front zone. Production audits on an electric injection moulding machine with 140-tonne clamp force have documented that reducing cooling time from 22 s to 16 s increases barrel residence time from 3.5 min to 4.8 min; notched impact values then dropped by 25% despite unchanged temperature settings. Published data for this specific configuration is limited, but the pattern is consistent with cumulative thermal degradation of the polybutadiene phase at the front zone.

Detection of the two failure mechanisms is performed by melt flow, mechanical testing, and thermal or spectroscopic analysis. A shift in MVR under ISO 1133-1:2022 at 260 °C/5 kg from 12 cm³/10 min to 18 cm³/10 min or higher indicates polycarbonate hydrolysis, while a drop in notched impact at 23 °C with no MVR shift suggests thermal damage to the rubber phase. Fourier-transform infrared spectroscopy of hydrolytically degraded PC/ABS shows a reduction in carbonate carbonyl absorbance near 1770 cm⁻¹ and an increase in hydroxyl absorption near 3400 cm⁻¹. Differential scanning calorimetry under ISO 11357-1 may show a broadened glass transition or a change in the polycarbonate-specific heat step, but the most reliable production-floor indicator remains the combination of dryer dew point, actual melt temperature at the nozzle, and notched impact testing of cold-runner samples.