Polybutylene terephthalate (PBT) and poly(butylene adipate-co-terephthalate) (PBAT) are different polymers: adipate and biodegradability distinguish the two materials even though both use 1,4-butanediol as a diol monomer. PBT is a semicrystalline thermoplastic polyester synthesized from terephthalic acid or dimethyl terephthalate and 1,4-butanediol; the backbone contains no adipate units. PBAT is a random copolyester of adipic acid, terephthalic acid, and 1,4-butanediol. The aliphatic adipate sequences in PBAT lower crystallinity, reduce the melting peak to 110–120 °C per ISO 11357-3, and create ester carbonyls that are accessible to microbial extracellular hydrolases. PBT does not share this hydrolysable aliphatic block structure; its melting peak is 223–225 °C and its density is 1.31 g/cm³ per ISO 1183-1.

Unfilled PBT is specified for injection-moulded electrical connectors, relay housings, and automotive under-hood components where dimensional stability under load is required. Its tensile stress at yield is typically 55 MPa when measured according to ISO 527-2/1A. Unfilled elongation at break is 50–200%; glass-fibre reinforcement at 30% loading raises tensile stress to approximately 130 MPa but reduces elongation to 2–3%. PBAT film grades, by contrast, show tensile stress at break of 20–35 MPa and elongation at break of 600–800% under ISO 527-3. The difference is monomer-derived and cannot be compensated simply by molecular weight adjustment.

Commercial PBT grades are available in intrinsic viscosity ranges from 0.60 dL/g to 1.20 dL/g measured in phenol/tetrachloroethane. Injection moulding grades commonly use 0.80–0.95 dL/g, while extrusion grades use higher values to maintain melt strength. PBAT film grades are typically characterized by melt flow index, with commercial values of 3–8 g/10 min at 190 °C and 2.16 kg per ISO 1133-1:2022. These rheological descriptors are not interchangeable because the polymers crystallize, flow, and degrade by different mechanisms.

Thermal and Electrical Benchmarks Span Two Distinct Polymer Families

Heat deflection temperature of unreinforced PBT under 1.8 MPa is 50–60 °C per ISO 75-2:2013; a 30% glass-fibre reinforced PBT can reach 205 °C under the same stress. Flexural modulus of unfilled PBT is approximately 2500 MPa per ISO 178, increasing to 9000 MPa with 30% glass-fibre reinforcement. PBAT film modulus is typically 80 MPa, which places it in a different application envelope. Electrical properties further separate the two: unfilled PBT volume resistivity is approximately 1.0×10¹⁵ Ω·cm per ASTM D257, and dielectric strength is 17–25 kV/mm per IEC 60243-1.

Water absorption of PBT after 24 h immersion is 0.08% per ISO 62; glass-reinforced PBT absorbs about 0.04% under the same conditions. This low moisture uptake supports dimensional stability for electrical connectors exposed to condensation. PBT tensile creep modulus at 23 °C after 1000 h at 20 MPa is typically above 2000 MPa per ISO 899-1. Glass-transition temperature of PBT is 45–60 °C per ISO 11357-2, while PBAT glass transition is near −30 °C. Published creep data for PBAT are limited, but its low flexural modulus and crystallinity indicate deformation under much lower continuous stress.

The compostability divergence is not a test-duration artefact. PBT lacks the adipate sequences required for productive binding by cutinase- and lipase-type hydrolases secreted by compost microorganisms. In a compost environment at 58 °C under ISO 14855-1, PBT mineralization remains below 90% over 180 days. PBAT certified under EN 13432 disintegrates and mineralizes within the same test. The aromatic terephthalate segments in PBAT remain, but random adipate incorporation suppresses lamellar crystallinity and increases water accessibility of ester linkages.

What Role Does Adipate Play in Enzymatic Hydrolysis?

Adipate introduces a four-carbon aliphatic segment between ester groups. The carbonyls in this segment are less sterically hindered than terephthalate carbonyls, allowing microbial extracellular hydrolases to cleave the polymer backbone. Gel permeation chromatography of PBAT during composting shows molecular weight decline from a weight-average molar mass range of 8.0×10⁴ to 1.2×10⁵ g/mol to below 1.0×10⁴ g/mol before complete mineralization. PBT crystallinity after slow cooling is commonly 35–45%; PBAT crystallinity is significantly lower, often 10–15%, because adipate incorporation disrupts chain folding and crystal growth.

PBT hydrolysis is predominantly a hydrothermal failure mode rather than a composting pathway. At melt temperatures above 250 °C, residual moisture above 0.02% causes ester cleavage and intrinsic viscosity reduction. A decline from 0.85 dL/g to 0.65 dL/g in intrinsic viscosity is sufficient to reduce tensile elongation and impact strength in moulded parts. This failure mode must be distinguished from the controlled enzymatic hydrolysis of PBAT that occurs in moist solid-state composting at 58 °C.

Differential scanning calorimetry under 10 K/min cooling shows a PBT crystallization exotherm between 190 °C and 200 °C; PBAT crystallizes slowly and exhibits a much lower crystallization enthalpy. These thermal differences explain why PBT can be moulded with short cycle times but PBAT is better suited to blown film and sheet extrusion where rapid crystallization is not required.

When Moisture Content Exceeds 0.02% in Melt Processing

Processing PBT on a co-rotating twin-screw extruder with an L/D ratio of 40:1 requires barrel temperatures from 240 °C to 270 °C. Residence time above 270 °C should be kept below 5 min. Prolonged thermal exposure causes yellowing, tetrahydrofuran evolution, and impact strength loss. Desiccant drying at 120 °C for 4 h to a dew point of −40 °C is mandatory; residual moisture is measured by ISO 15512 and must be below 0.02% before feeding. Glass-fibre reinforced PBT is routinely compounded on a 40:1 L/D twin-screw extruder with side-feeding of fibre at 30% loading. Screw torque excursions are observed when barrel zone temperatures in the latter zones exceed 270 °C, particularly with lower-viscosity PBT grades and narrow die gaps.

Injection moulding of unfilled PBT uses melt temperatures of 250–260 °C and mould temperatures of 40–80 °C to balance crystallization and surface appearance. Glass-reinforced PBT uses mould temperatures of 80–120 °C. On a 120-ton injection moulding machine with an eight-cavity connector tool, a mould temperature below 40 °C can produce jetting and surface delamination because the flow front freezes before full packing. Hydraulic injection pressure is typically 70–100 MPa; for a projected part area of 250 cm², required clamp force is 1750–2500 kN. Surface-mount PBT connectors exposed to lead-free reflow peak temperatures of 260 °C for 30–60 s require desiccant drying and sealed moisture-barrier packaging to prevent blistering.

Regulatory, Electrical Safety, and Migration Compliance Matrix

Flame-retarded PBT can achieve V-0 classification at 0.75 mm thickness under UL 94. Comparative tracking index of unfilled PBT may exceed 600 V per IEC 60112. These values support use in appliance and electrical enclosures, but the additive package must be selected for the specific wall thickness and colour. Migration and food-contact compliance for PBT are evaluated under FDA 21 CFR 177.1660 in the United States and EU 10/2011 in the European Union. PBAT used in food-contact biodegradable films is frequently tested under EU 10/2011 for overall migration, while compostability certification remains governed by EN 13432 and ASTM D6400.

Operational boundaries of PBT include sensitivity to hot aqueous environments. Continuous exposure to water above 80 °C can hydrolytically degrade unreinforced PBT over extended periods; glass-fibre reinforced grades may lose surface gloss and dimensional stability. Strong alkaline cleaning agents, especially at elevated temperatures, are not recommended; amine-based additives can accelerate ester cleavage. PBAT is not a drop-in replacement for PBT in rigid electrical or under-hood applications because its heat deflection temperature, modulus, and creep resistance are far lower. PBT is not suitable for compostable packaging because it fails ISO 14855-1 and EN 13432 mineralization criteria.