The selection among unreinforced polybutylene terephthalate (PBT), polycarbonate (PC), and melt-compounded PBT/PC alloys is governed by three coupled criteria: notched impact energy, low-temperature toughness, and volume-based part cost. The comparative framework designated PBT vs PC and PBT/PC Alloy: Impact, Low-Temperature Toughness and Cost cannot be reduced to a single property ranking because the measured values shift with specimen thickness, moisture uptake, alloy phase morphology, moulded-in stress, and impact-modifier loading. Neat PBT is a semicrystalline polyester with a melting endotherm near 223 °C and a glass transition typically reported by differential scanning calorimetry according to ASTM D3418-21 in the 30–50 °C band. Neat PC is an amorphous aromatic carbonate with a glass transition near 145–150 °C and no melting endotherm. Both resins require pre-drying to below 0.02 % moisture before melt processing, and both respond differently to low-temperature notched impact because the deformation mechanisms available below their respective glass transitions are not equivalent.

Crystalline Morphology, Notched Izod Response, and Moisture Sensitivity in Neat PBT

Notched Izod data for unreinforced PBT according to ISO 180/A:2023 ordinarily fall in the 3–5 kJ/m² range at 23 °C and decline to 2–3 kJ/m² at -30 °C. The low-temperature loss is not caused by a secondary glass transition but by restriction of segmental motion in the amorphous interlamellar phase. At -40 °C, most unfilled PBT grades exhibit brittle fracture at the notch root before measurable plastic deformation. This notch sensitivity is compounded by spherulitic structure: crack propagation along spherulite boundaries is rapid when amorphous tie-molecule density is low. In production, fast crystallisation permits mould temperatures as low as 20–50 °C, but a cold mould raises freeze-off rate at the gate and can reduce bulk crystallinity from the commonly reported 35–40 % range to below 25 %.

Injection moulding of neat PBT on a typical 100-tonne hydraulic press with a general-purpose 20:1 L/D screw is run at a melt temperature of 245–260 °C. Residence time above 260 °C should be limited to less than 5 min because ester pyrolysis increases free terephthalic acid concentration and hydrolytic chain scission can reduce molecular weight. Pre-drying at 120 °C for 4 h in a desiccant dryer with a dew point below -40 °C is the standard boundary; at ambient relative humidity above 60 %, air exposure longer than 30 min can raise moisture to problematic levels even in a properly dried feedstock.

Representative values compiled from supplier technical datasheets for unfilled, non-flame-retardant injection grades. Ranges reflect molecular weight and impact-modifier differences. Exact values vary by specimen thickness, gate orientation, and moulding conditions.

Why Does Polycarbonate Retain Low-Temperature Ductility Beyond the PBT Glass Transition?

Neat polycarbonate retains a broader low-temperature impact window than neat PBT because its secondary β-relaxation, detected by dynamic mechanical analysis under ASTM D4065-20 near -100 °C, permits local carbonate segmental motion even when the primary glass transition is far above service temperature. The notched Izod response of PC at 23 °C commonly lies in the 60–80 kJ/m² range for 4 mm ISO bars. At -30 °C, PC can still return 40–60 kJ/m² in thin sections, but the transition is thickness-dependent: 6.4 mm ASTM bars can drop into the 10–20 kJ/m² range because plane-strain constraint suppresses shear yielding. This thickness sensitivity is a critical boundary when comparing PBT vs PC.

PBT does not show the same ductile plateau below its glass transition. Its low-temperature performance is controlled by crystallinity and tie-molecule density, not by a strong sub-Tg relaxation. Unfilled PBT is therefore seldom used for notched load-bearing applications below 0 °C without impact modification. The trade-off is chemical resistance: PC exposed to aggressive solvents under moulded-in stress can show craze initiation at tensile strains as low as 0.5–1.0 % when tested according to chemical resistance protocols such as ASTM D543-21. PBT is more resistant to many organic solvents, which is why alloying is used instead of direct substitution.

When PBT/PC Alloys Cross the Phase Inversion Threshold During Twin-Screw Compounding

Compounding of PBT/PC alloys is typically run on co-rotating twin-screw extruders with an L/D ratio of 40:1 to 48:1 and segmented screws containing kneading blocks downstream of the feed throat. The melt-temperature envelope is narrower than that of either neat resin: 250–280 °C at the die. Below 250 °C, the PC phase may not fully plasticate; above 280 °C, ester-carbonate interchange is accelerated, producing random copolyester-carbonates that reduce crystallinity and long-term impact retention. In practice, die setpoints are commonly limited to 275 °C to allow for shear heating in the kneading zones.

The phase morphology shifts from PC domains dispersed in a PBT matrix to PBT domains dispersed in a PC matrix across a composition-dependent phase inversion band. In viscosity-matched systems processed at 250–270 °C, the inversion commonly lies in the 40–60 wt% PBT interval, but the exact boundary moves with screw shear rate, viscosity ratio, and impact-modifier loading. At low PC content, the alloy retains PBT-like chemical resistance and fast crystallisation, but low-temperature notched Izod remains weak. At high PC content, the alloy approaches PC-like low-temperature ductility but becomes more sensitive to solvent stress cracking and requires higher mould temperature.

Impact-modified PBT/PC grades using core-shell acrylic or methacrylate-butadiene-styrene particles can shift the low-temperature failure mode from brittle to ductile. The modifier particles act as stress concentrators and induce cavitation; low-temperature Izod can be raised from the unmodified 8–15 kJ/m² range to 25–45 kJ/m² at -30 °C, but flexural modulus decreases by 15–25 %. Instrumented puncture testing according to ISO 6603-2:2023 at -30 °C is often a better discriminator than Izod because it records the ductile-to-brittle failure mode transition under biaxial loading.

A direct injection-moulding cost comparison must normalise for density, cycle time, mould temperature, and regrind stability. PC’s lower density of 1.20 g/cm³ yields approximately 8–10 % more parts per tonne than PBT at 1.31 g/cm³ for the same part volume. However, neat PBT crystallises rapidly, and an unfilled PBT housing can be moulded with a cooling time of 5–10 s on an 80–120 tonne press, whereas an amorphous PC housing of the same wall thickness may require 10–20 s cooling and a mould temperature of 80–120 °C to avoid sink marks. PBT/PC alloys occupy an intermediate position: mould temperature is commonly set at 60–90 °C, and cooling times are 10–20 % longer than neat PBT but shorter than PC.

Pre-drying energy is a hidden cost. Both PC and PBT require desiccant drying to below 0.02 % moisture: PC at 120 °C for 3–4 h, PBT at 120 °C for 4 h, and PBT/PC alloy at 100–120 °C for 3–4 h. The alloy’s ester-carbonate interchange risk above 280 °C reduces the safe purge-temperature window and can force lower melt throughput than neat PC, increasing specific energy consumption. Absolute bulk resin price indices are grade-dependent and volatile; published data for a fixed PBT/PC alloy price is therefore of limited use. A cost model should use density-adjusted part weight, cycle time, scrap rate, and regrind retention rather than a fixed price per kilogram.

Regrind Retention, Melt Viscosity, and Part Cost Reshape the Cross-Grade Decision

On production-scale injection moulding lines, regrind loops change the practical cost position. PC regrind dried to below 0.02 % moisture can be reintroduced at 20–30 % loading with minimal shift in notched Izod in uncritical applications, but repeated extrusion through a hot runner with high shear can reduce melt strength. PBT regrind is more sensitive to hydrolysis; if sprues and runners are stored at ambient relative humidity above 60 %, a subsequent drying step at 120 °C may not fully restore original molecular weight, and the -30 °C Izod value can fall by 10–20 % after three regrind cycles.

PBT/PC alloy regrind carries an additional risk: the mixed ester-carbonate structure generated during the first heat history can nucleate different crystal sizes and shift phase morphology. Moulders therefore avoid loading more than 30 % regrind in impact-modified PBT/PC alloy unless the part is nonstructural. Melt viscosity also affects cost: PC at 300 °C and 100 s⁻¹ typically has a shear viscosity in the 250–400 Pa·s range, while PBT at 250 °C under the same shear rate is commonly 80–150 Pa·s. The lower PBT viscosity permits thinner walls and lower clamp force, whereas higher PC viscosity contributes to impact strength through higher molecular weight retention.

When low-temperature toughness at -30 °C is the primary requirement, PC-rich PBT/PC alloy or neat PC is usually selected. When chemical resistance plus modest low-temperature impact is needed, a PBT-rich alloy may be adequate, and the faster cycle time can offset raw material cost. The comparison of PBT vs PC and PBT/PC alloy under impact, low-temperature toughness and cost cannot be completed from Izod data alone. Tensile elongation at break, notched Izod at the minimum service temperature, instrumented puncture energy, chemical exposure, and regrind fraction must be evaluated on the actual moulded part geometry.