Polycarbonate (PC) recyclate evaluation begins with the relationship between weight-average molecular weight, melt viscosity, and notch-sensitive failure. How many times can polycarbonate be reprocessed before impact strength collapses is not a fixed integer; it is a threshold controlled by initial molar mass, residual moisture before each pass, barrel residence time, venting efficiency, and the severity of the notch in the qualification test. Unfilled general-purpose PC resins are typically supplied with a weight-average molecular weight in the region of 22,000–60,000 g/mol, an ISO 1133-1:2022 melt volume-flow rate at 300 °C/1.2 kg of approximately 9–25 cm³/10 min, and an ISO 180/1A notched Izod impact value at 23 °C commonly between 60 kJ/m² and 80 kJ/m². Notched Izod response tracks molar mass loss more sharply than tensile yield strength or flexural modulus, making it the usual control property when recyclate is admitted into safety glazing, electrical enclosure, or medical housing applications.

How many times can polycarbonate be reprocessed before the notched Izod impact value drops below a 70% retention specification?

An application-specific collapse point is commonly established at 70% of the virgin lot notched Izod value because below that level brittle fracture modes begin to diverge from ductile yielding and the standard deviation of ISO 180/1A results widens. In closed-loop post-industrial regrind trials using dried, unfilled PC, a single pass at 280–300 °C with moisture held below 0.020 wt% frequently preserves more than 85% of virgin notched Izod response. By the third pass, retained impact values in laboratory twin-screw reprocessing often fall into the 70–85% band. By the fifth pass, losses exceeding 30% are common when no virgin dilution or chain-repair additive is used. Published data for this specific configuration is limited because industrial recyclers rarely run identical lot-to-lot formulations through more than three passes without blending. A conservative operational boundary is therefore 2–3 full reprocessing cycles at 100% regrind for products requiring high ductility.

Molecular Weight Collapse, Hydrolysis, and Melt-Flow Shift in Twin-Screw Reprocessing

The dominant degradation chemistry in repetitive PC melt processing is random scission of carbonate linkages. Hydrolysis is accelerated by residual moisture; even 0.05 wt% water at 300 °C can reduce molar mass sufficiently to shift melt volume-flow rate upward and notch sensitivity downward. Desiccant drying before reprocessing is mandatory, typically at 120 °C for 4 h in a dryer with a dew point no higher than −35 °C. In a corotating twin-screw extruder with an L/D ratio of 40:1, a vacuum vent at −0.08 MPa assists volatile removal, but the high shear zones between kneading blocks can generate local melt-temperature overshoots of 15–30 °C above the set barrel temperature, accelerating chain scission. A 40% increase in ISO 1133-1:2022 MVR relative to the virgin lot is a practical warning signal that weight-average molecular weight has shifted into a region where notched Izod values may lose an additional 15–25% on the next pass.

On a production-scale injection molding line, degradation depends on the ratio of shot weight to barrel capacity. A molding machine with a 120 mm screw and a 3,000 kN clamp force running a thin-wall PC cover may keep shot size below 25% of barrel capacity, producing residence times above 15 min at 290 °C. Under these conditions, the difference between second-pass and third-pass impact retention is often larger than laboratory capillary rheometry predicts. Sprues and runners from such molds should be dried again before reuse; if not, absorbed moisture can combine with prolonged residence time to reduce ISO 180/1A impact values by 20% in a single loop. This is one reason closed-loop PC regrind systems commonly restrict hot-runner sprue returns to 15–25 wt% in virgin material unless melt-flow and notched Izod data are collected for every batch.

If recyclate MVR exceeds the virgin lot by 40%, subsequent impact retention becomes nonlinear and specification failure accelerates

The threshold-based control logic used in recycling plants employs melt-flow shift as a surrogate for molar mass and impact. In an unfilled PC lot with an initial MVR of 15 cm³/10 min, reprocessing that raises MVR to 21 cm³/10 min corresponds to a 40% increase. Below this point, notched Izod values often remain above 70% retention. Above this point, further cycles can produce a cliff-edge in impact, where one additional pass may reduce ISO 180/1A values from 50 kJ/m² to 35 kJ/m² or lower. The cliff-edge arises because PC transitions from ductile shear yielding toward brittle craze-driven failure as entanglement density decreases with molar mass. Operators should not infer linear retention from the first two passes. ISO 180 data on molded test plaques should be required after every lot when MVR shift exceeds 40%.

Contamination in Post-Consumer PC Streams Introduces Extraneous Impact Failure Modes Independent of Mw

PC recyclate impact values are strongly influenced by polymer impurities. Aromatic polycarbonate is partially miscible with some polyesters, but contamination with ABS, polyolefins, or PMMA at 2–5 wt% can reduce notched impact by more than 20% even when molar mass remains intact. For post-consumer PC from water bottles, optical media, or automotive glazing, melt filtration through screens at 100–200 µm and removal of labels, coatings, and metal fragments are required. In a coextruded PC sheet plant, edge trim and skeleton scrap can be reintroduced at 30–50 wt% if continuous loss-in-weight dosing maintains blend ratio. Batch-to-batch fluctuations above ±3 wt% in regrind content create impact variability that can exceed the uncertainty of the Izod test itself. The application of ISO 294-3:2020 molding conditions for test plaques is critical because molded-in stress and thickness differences alter notch sensitivity independently of recyclate quality.

For electrical enclosures and data-storage components requiring UL 94 V-0 ratings, recycled PC blends often carry brominated or phosphorus-based flame retardants. Reprocessing flame-retardant PC can trigger acid-catalyzed hydrolysis at elevated temperature; notched Izod values may fall below 35 kJ/m² after 2–3 cycles despite stable MVR. Production-scale recycling of flame-retardant PC therefore requires predrying at the lower end of the melt temperature window, generally 260–280 °C, and shorter residence times than unfilled general-purpose resin. If the flame-retardant package is unknown, recyclers should perform Fourier-transform infrared spectroscopy and an ISO 180 multi-cycle qualification check before approving the stream.

Setting Drying and Process Safeguards for Repetitive PC Reprocessing

The critical processing window for PC regrind is narrower than for polyolefins. Barrel set points below 260 °C can generate excessive screw torque and unmelted granules; set points above 320 °C increase yellowing and chain scission, while local melt temperatures above 360 °C can produce gaseous degradation products and black specks. A corotating twin-screw extruder with an L/D ratio of 36:1–44:1 and a vacuum vent at −0.09 MPa is often used for reclaiming filled or mixed PC scrap. The degassing port should be located after the first mixing zone but before the final pressure-generation zone. Screw speed and feed rate should be balanced to keep specific mechanical energy input below 0.20–0.25 kWh/kg; higher energy input accelerates molar mass loss even if the melt thermocouple does not show bulk overheating because shear heating in narrow kneading-block clearances can create short-lived temperature spikes.

In compounding of heavily degraded PC recyclate, reactive chain extenders and epoxy-functional additives have been evaluated to rebuild molar mass. The regain in ISO 180/1A impact from chain extension is usually smaller than the loss caused by hydrolysis, and the resulting melt viscosity rise can complicate injection molding. Anti-hydrolysis additives based on carbodiimide or epoxidized soybean oil show retention effects in polyester recycling, but their use in PC is limited by transparency loss and plate-out. Published data for this specific configuration is limited; qualification of these additives requires multi-cycle ISO 180 and ISO 1133-1:2022 testing rather than reliance on a single pass.