Qualifying a Polybutylene Terephthalate Supplier: Nine Checks Before a Trial Order is a technical gate that examines melt rheology, moisture, thermal stability, mechanical properties, crystallisation, electrical safety, regulatory status, dimensional stability, and contamination. PBT is a fast-crystallising semicrystalline polyester whose moulding behaviour is governed by melt viscosity, residual moisture, and thermal history. A trial lot is not adequately screened by density and melting point alone. The incoming material is tested against ISO 1133-1:2022, ISO 527-1/-2, ISO 178, ISO 179-1/1eA, ISO 180/A, ISO 75-1/-2, IEC 60112:2020, IEC 62631-3-1, and ISO 11357-3:2018. The sequence moves from rheology to thermal degradation, then to application-specific mechanical and electrical data, and finally to lot-level documentation and contamination.

The first check quantifies melt volume-flow rate consistency and shear-viscosity offset. The resin is dried to 0.02% maximum moisture and tested at 250 °C under a 2.16 kg piston load in accordance with ISO 1133-1:2022. For unfilled PBT moulding grades, acceptance is generally 15 cm³/10 min to 25 cm³/10 min; for 30% glass-filled grades, values typically fall from 8 cm³/10 min to 18 cm³/10 min because the fibre network raises viscosity. The supplier’s certificate must report the exact method condition, not a converted melt index. When the incoming value differs from the certificate by more than ±2.5 cm³/10 min or ±15% relative, the lot requires a capillary viscosity scan with a die of L/D 20:1 at 250 °C across apparent shear rates of 100 s⁻¹ to 5000 s⁻¹. For unfilled injection-grade PBT at 1000 s⁻¹, apparent melt viscosity is typically 80 Pa·s to 200 Pa·s. A value above 300 Pa·s can indicate high molecular weight or filler contamination, while a value below 60 Pa·s can indicate regrind dilution or hydrolytic chain scission. A deviation above 8% at 1000 s⁻¹ from the retained reference sample is a rejection signal. On production twin-screw compounding lines with 40 mm screw diameter and L/D 40, pellet-to-pellet rheology differences are observed when feeder segregation or shear-heating variation is present; the trial lot should therefore be blended and sampled from multiple bags before testing.

Residual moisture is then quantified by coulometric Karl Fischer titration at 160 °C using ISO 15512:2019. As-received PBT must be below 0.03% (300 ppm). A lot at 0.05% or higher is not automatically rejected, but it must be dried in a desiccant dryer with −40 °C dew point at 120 °C for 4 h before processing. The critical boundary is melt hydrolysis: above 260 °C, residual water attacks the ester linkage, lowering molecular weight and reducing notched impact strength. Published data for this specific configuration is limited, but production batches processed at 265 °C with 0.04% residual moisture have shown reductions in ISO 179-1/1eA notched impact exceeding 20% relative to dry material. Drying must not be performed in a hopper dryer with ambient air when relative humidity exceeds 60%, because the achieved equilibrium moisture may remain too high. The supplier’s packaging must include sealed, moisture-barrier bags; opened bags stored at 50% relative humidity for more than 6 h require re-drying. Karl Fischer samples should be taken within 30 min of bag opening to limit false moisture pickup.

Thermal degradation thresholds and ash-based filler accountability

The third check is thermogravimetric analysis under nitrogen at 10 K/min in accordance with ISO 11358-1:2014. Unmodified PBT should show no significant mass loss below 360 °C; a 2% mass loss temperature below 340 °C indicates contamination, regrind, or insufficient stabilisation. The residue at 600 °C in nitrogen is used to estimate filler and inorganic content. For an unfilled grade, ash should be below 0.5%. For a 30% glass-filled grade, ash after pyrolysis should align with the nominal glass content within ±2% absolute. If the ash value is higher than the certificate, filler substitution or recycled content is probable. A complementary Fourier transform infrared spectrum of the ash or residual filler should match the supplier’s reference for glass fibre or mineral type; a mismatch in the 1200 cm⁻¹ to 900 cm⁻¹ region can indicate talc or calcium carbonate substitution. Thermogravimetric curves with a secondary decomposition step near 300 °C may also point to flame-retardant additive decomposition; the supplier must disclose whether the grade is halogenated or non-halogenated before the trial, because the decomposition chemistry changes mould deposit behaviour and vent emissions.

The fourth check covers mechanical test data on dry-as-moulded specimens prepared to ISO 294-1. Tensile modulus and stress at break are measured on ISO 527-2 type 1A specimens at 23 °C and 5 mm/min. Unfilled PBT should fall near 2500 MPa to 2800 MPa tensile modulus and 50 MPa to 60 MPa tensile stress at break. Flexural modulus by ISO 178 should be 2400 MPa to 2600 MPa for unfilled material. Notched impact is evaluated at 23 °C; for unfilled PBT, notched Izod by ISO 180/A is typically 3 kJ/m² to 5 kJ/m², and notched Charpy by ISO 179-1/1eA is similar. Temperature control of the impact laboratory is critical because unfilled PBT passes through a brittle-ductile transition near room temperature; a shift from 23 °C to 18 °C can lower impact values by more than 10%. The mechanical certificate must include actual specimen conditioning state; values obtained after conditioning to 50% relative humidity are not directly comparable to dry-as-moulded datasheet limits. A batch that fits MVR but fails impact by more than 15% may indicate moisture-induced degradation during the supplier’s compounding or a broader molecular weight distribution.

What is the isothermal crystallisation half-time at 200 °C?

The fifth check is crystallisation behaviour. Differential scanning calorimetry is performed under nitrogen at 10 K/min heating and cooling using ISO 11357-3:2018. The melting peak of standard PBT should fall at 223 °C to 227 °C; a lower or split melting peak may indicate copolymer modification or contamination. On cooling, the crystallisation peak temperature is typically 185 °C to 200 °C depending on nucleation. For an isothermal test at 200 °C, nucleated moulding grades finish most crystallisation within 1 min to 3 min; non-nucleated grades may require longer. Published data for this specific configuration is limited, but a half-time above 5 min at 200 °C is generally unsuitable for fast-cycle injection moulding because it extends required mould residence time and increases post-mould shrinkage. The processing window itself is narrow: melt temperature should be held between 250 °C and 265 °C, and mould temperature between 40 °C and 80 °C. Above 270 °C, thermal degradation accelerates and tetrahydrofuran evolution becomes measurable; below 245 °C, the viscosity increase produces fill deficits in thin-wall tools. A supplier lot that has acceptable MVR but abnormal crystallisation half-time may nucleate differently and produce warpage even if mechanical tests pass.

If the grade is specified for electrical connectors, lot-level CTI, volume resistivity, and UL 94 evidence must be present

For electrified applications, the sixth check is electrical and flammability documentation. Comparative tracking index is measured under IEC 60112:2020 using solution A; unreinforced PBT typically gives CTI 400 V to 600 V, while some flame-retardant grades fall to CTI 250 V to 300 V because the additive package increases tracking susceptibility. Volume resistivity at 23 °C is tested by IEC 62631-3-1:2016; unfilled PBT usually exceeds 10¹⁴ Ω·m, and glass-filled grades remain above 10¹³ Ω·m. After conditioning for 24 h at 50% relative humidity, values commonly drop by one to two decades, so the certificate must state the conditioning environment. Any lot offered for a connector or circuit breaker must carry a valid yellow card from UL Solutions listing the exact grade name, minimum thickness, and UL 94 rating, commonly V-0 at 0.8 mm to 1.6 mm for flame-retarded PBT. The certificate must align with the formulation actually supplied; a shift from a brominated to a non-halogen flame-retardant package can alter CTI by more than 100 V and change mould deposit behaviour.

The seventh check demands regulatory evidence that is current for the shipping jurisdiction. Each lot must be accompanied by a signed lot-level declaration covering EU REACH Candidate List substances above 0.1% by weight, RoHS 2011/65/EU recast limits for lead, mercury, cadmium, hexavalent chromium, PBB, and PBDE, and the specific food-contact requirements if the moulded part is used under FDA 21 CFR 177.166 for polyesters. The RoHS 2011/65/EU thresholds are 0.1% by weight for lead, mercury, hexavalent chromium, PBB, and PBDE, and 0.01% for cadmium in homogeneous material. For electrical waste, the supplier must provide the IPC 1752 material declaration or equivalent, listing intentionally added substances. The absence of a lot-level declaration is not resolved by a general product datasheet; the trial order should be blocked until the exact production plant and tolling site are named. If the part is intended for food-contact use, migration testing data under FDA 21 CFR 177.166 or EU 10/2011 must cover total non-volatile residues and specific migration limits for any colorants or processing aids.

The eighth check is dimensional stability under the intended mould temperature. PBT develops crystallinity rapidly, but final post-mould shrinkage is determined by mould temperature, hold pressure, and filler content. Unfilled PBT moulded at 60 °C mould temperature has parallel shrinkage in the range of 1.5% to 2.0% and perpendicular shrinkage near 1.7% to 2.2% by ISO 294-4; a 30% glass-filled grade shrinks substantially less, commonly 0.2% to 0.5% in the flow direction. Mould temperature drift from 60 °C to 80 °C can change shrinkage by 0.1% to 0.3% and alter warpage in flat connectors. The supplier lot should be checked on a shrinkage plate before the full trial tool is used; local thickness variation changes cooling rate and therefore crystalline skin thickness. Post-mould shrinkage following 1 h at 150 °C should not exceed 0.1% additional movement for filled grades intended for underhood use. A batch that produces shrinkage outside these bands on the trial tool indicates filler level drift or molecular weight shift, even when other checks pass.

The ninth check is contamination, colour stability, and packaging integrity. Visual inspection of pellet samples under 10× magnification is acceptable for gross contamination, but reliable batch acceptance requires a 25 kg sample tested for black speck count, metal contamination, and colour difference. A compounder may use a laboratory-scale twin-screw extruder with 25 mm to 40 mm screw diameter and L/D 40 to compare filter pressure rise across a 20 µm screen pack; a pressure rise above 5 bar in 10 min indicates gels or foreign particles. Colour difference by DIN EN ISO 11664-4 should be below ΔE*ab 0.5 against the retained masterbatch reference for natural and pre-coloured grades. The supplier’s bag must be sealed with desiccant and purge gas, and any punctured, oil-stained, or heat-seal-failed packaging invalidates the lot regardless of laboratory results. Metal contamination is screened with inline magnets and X-ray inspection; pellets showing magnetic capture of ferrous particles above 0.05% by mass are rejected.

Nine-point pre-trial acceptance matrix