Polybutylene terephthalate (PBT) is a semicrystalline polyester produced from 1,4-butanediol and dimethyl terephthalate or terephthalic acid. The homopolymer’s rapid crystallization, low equilibrium moisture uptake of 0.08–0.10% after 24 h per ASTM D570, and melt temperature of 220–225°C support high-volume injection molding. Unfilled PBT typically exhibits tensile strength of 50–60 MPa under ISO 527-2, flexural modulus of 2.5–2.9 GPa under ISO 178, and notched Izod impact of 30–50 J/m at 23°C per ASTM D256. Nevertheless, notch sensitivity, low-temperature brittleness, high differential shrinkage, and limited outdoor weather resistance drive the comparison of PBT Alloy Options: PC, ASA and Copolyester Blends Compared by the Problem They Solve.

Before compounding, PBT and its alloys are dried to <0.02% moisture using a desiccant dryer with a -40°C dew point. Hydrolysis during melt processing is a time-temperature-moisture failure mode; an intrinsic viscosity drop above 0.05 dL/g is sufficient to reduce tensile strength and weld-line integrity. The processing window for unfilled PBT is comparatively narrow in melt temperature, 240–270°C, and mold temperature, 30–80°C. For fast-crystallizing PBT, low mold temperature produces a frozen amorphous skin before the core crystallizes, causing differential shrinkage and post-mold warpage measured under ISO 294-4. This is the baseline problem that alloying targets.

Moisture Uptake, Crystallization Rate, and the Unfilled PBT Baseline

Differential scanning calorimetry under ISO 11357-3 shows sharp non-isothermal crystallization exotherms for unfilled PBT; this indicates a fast crystallization rate and a short escape time from the melt to a dimensionally stable solid. At mold temperatures below 40°C, the skin freezes before shrinkage compensation is complete, producing anisotropic mold shrinkage. Neat PBT mold shrinkage is commonly reported at 1.5–2.0% per ISO 294-4; the rapid crystallization creates differential shrinkage between flow and transverse directions. These values explain why unfilled PBT is seldom selected for large flat housings unless alloying reduces crystallinity or adds a second phase.

Why Does Notched Impact Fail at Low Temperatures Without a Second Phase?

Unmodified PBT is notch-sensitive because the semicrystalline spherulitic morphology offers limited energy dissipation under plane-strain conditions. At subzero temperatures, notched Izod per ASTM D256 often falls below 50 J/m, while tensile elongation at break under ISO 527-2 drops sharply. The introduction of PC in the melt creates a two-phase system in which the amorphous PC-rich phase redistributes stress and promotes shear yielding. Commercial PC/PBT grades with PC mass fraction 0.40–0.60 are commonly reported to raise 23°C notched Izod above 600 J/m; low-temperature values at -30°C vary widely from 100 J/m to 500 J/m depending on PC molecular weight and impact modification. The heat deflection temperature at 1.82 MPa per ASTM D648 increases from the neat PBT range of 50–60°C to 90–130°C.

Processing PC/PBT is not a simple proportionality exercise. Melt compounding on a 40 mm co-rotating twin-screw extruder with an L/D 40:1 configuration requires strict residence-time control because transesterification between the ester groups of PBT and the carbonate groups of PC is temperature-dependent. At melt temperatures above 270°C, residence times exceeding 2 min generate copolyester sequences that reduce PBT crystallinity, slow crystallization rate, and lower chemical resistance. On production lines, this appears as batch-to-batch variation in post-mold shrinkage and occasional black specks at the hot runner tip. Phosphorus-based stabilizers at 0.1–0.3 wt% are used to suppress ester exchange, but they do not eliminate the need for temperature control.

PC/PBT should be dried to <0.02% moisture at 110–120°C for 4–6 h. Mold temperature is typically held at 60–90°C to minimize molded-in stress and preserve impact. On an injection molding machine of 1000 kN clamp force, nozzle temperature should not exceed 270°C, and screw rotation should be adjusted to keep melt residence below 5 min. The main limitation is chemical resistance: continuous exposure to hot hydrocarbon fluids or glycol-water mixtures reduces molecular weight faster than neat PBT.

Rheological comparison under ISO 1133-1 is complicated by PC content; melt volume-flow rate at 250°C and 2.16 kg is typically lower for PC/PBT than for neat PBT because PC contributes higher melt viscosity. Capillary rheometry at 100–1000 s⁻¹ shows higher shear sensitivity; therefore injection pressure and gate size must be increased compared with neat PBT. Published data for exact PC/PBT viscosity curves are commercial and grade-specific.

In outdoor applications, the failure mode shifts from low-temperature notch sensitivity to ultraviolet degradation, gloss loss, and impact retention after weathering. ASA-modified PBT is formulated for this condition because the acrylic ester elastomer in ASA has no photo-oxidatively sensitive unsaturation. Stabilized PBT/ASA grades are evaluated under ISO 4892-2 xenon-arc exposure with daylight filters or SAE J2527; OEM acceptance thresholds commonly specify a color change ΔE of <3 after 1500 kJ/m² and 60° gloss retention above 70% per ASTM D523. Published data for a specific grade should be verified, because ΔE also depends on carbon black and pigment loading.

Notched Izod of ASA/PBT at 23°C is typically 100–300 J/m, and after xenon-arc weathering the impact retention is higher than unmodified PBT. The blend is processed at a melt temperature of 240–260°C and mold temperature of 40–80°C. Drying at 100–110°C for 4 h is sufficient for <0.02% moisture. Avoid melt temperatures above 260°C because the acrylate rubber phase can degrade and generate splay. Part warpage in ASA/PBT cowl vent grilles is controlled by maintaining cavity pressure uniformity; hot runner valve-gate systems with independent tip temperature control are used to avoid localized overheating and color streaking.

How Copolyester Blends Reduce Differential Shrinkage in Thin-Wall Housings

Copolyester modification addresses a separate problem: anisotropic mold shrinkage and surface finish. Neat PBT mold shrinkage is typically 1.5–2.0% per ISO 294-4; the fast crystallization creates differential shrinkage between flow and transverse directions. Amorphous copolyesters slow crystallization and reduce the degree of crystallinity, lowering shrinkage to 0.8–1.2% in blends. This is measurable by ASTM D955 and is relevant to large thin-wall housings where post-mold warpage exceeds flatness tolerance under OEM metrology.

Surface finishing also improves. Copolyester/PBT blends are tested for paint adhesion using cross-cut ISO 2409; class 0–1 is commonly achieved without primer, whereas neat PBT may require flame or plasma pretreatments. Gloss at 60° under ASTM D523 can be adjusted from low-gloss to high-gloss by mold surface finish and additive package. Copolyester/PBT is processed at 230–260°C melt temperature and 30–60°C mold temperature. Drying at 100–120°C to <0.02% moisture is still mandatory. The trade-off is thermal performance: continuous service above 100°C is limited, and chemical resistance to aromatic solvents declines with increasing copolyester content.

The following matrix consolidates alloy selection by the problem solved and verification pathway.

General injection molding parameters for the four systems are given below; exact set points require machine size, part wall thickness, and gate configuration.

On the injection molding floor, transfer position is set by short-shot study; for PC/PBT, hold pressure is commonly adjusted between 60–80 MPa and hold time is extended by 20–30% relative to unfilled PBT because the amorphous PC phase slows gate sealing. Cavity pressure uniformity is monitored with piezoelectric sensors. In plants where relative humidity exceeds 60%, dried pellets can regain moisture above 0.02% within 30 min if exposed to open air; central vacuum loading is therefore used between dryer and feed throat. PBT alloys should not be processed after PVC or acetal without purging, because acidic or formaldehyde residues accelerate ester hydrolysis.