Pbt Fibre Vs Pet Ptt And Nylon The Elasticity Trade Offs
Polybutylene Terephthalate (PBT) in Fibre Applications
Polybutylene terephthalate (PBT) is produced by melt polycondensation of terephthalic acid or dimethyl terephthalate with 1,4-butanediol. In fibre, monofilament, and engineering thermoplastic forms, PBT occupies a narrow position between polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polyamide 6/66. The comparison commonly specified as PBT Fibre vs PET, PTT and Nylon: The Elasticity Trade-Offs is controlled by methylene sequence length, crystal polymorphism, moisture response, and processing temperature. Unreinforced PBT exhibits a melting endotherm near 223 °C under ISO 11357-3 and a glass transition temperature between 45 °C and 60 °C under ISO 6721-1. Its equilibrium moisture uptake at 23 °C and 50 % RH is typically 0.1 % to 0.2 % by ISO 62, substantially below the 2.5 % to 3.5 % range for PA6 under identical exposure. That low moisture absorption limits swelling-induced dimensional variance and preserves dry-state stiffness in humid service, but it does not provide the high immediate elastic recovery of conditioned polyamide or PTT at elevated strain.
Why Does the Tetramethylene Segment Lower the Alpha-to-Beta Crystal Strain?
The PBT unit cell can adopt alpha and beta crystal forms. Under tensile stress, the alpha-to-beta transformation occurs at comparatively low strain, commonly reported between 6 % and 12 %. This reversible solid-state change acts as a strain-energy dissipation mechanism and contributes to PBT’s higher elastic recovery from low-to-moderate extension than PET, whose ethylene segment yields a stiffer chain and a glass transition near 70–80 °C. Dry-as-moulded tensile modulus for unreinforced PBT is commonly 2.4–2.8 GPa by ISO 527-2, while PET grades range from 2.8–3.5 GPa. PET’s higher modulus is accompanied by lower elongation at break and lower immediate recovery from 10 % extension. Published fibre testing under ASTM D1774 for PBT staple indicates recovery from 10 % strain in the 70–80 % range, whereas PET frequently falls below 60 %. The trade-off is thermal: PBT’s 223 °C melting peak is approximately 32 °C below PET’s 255 °C endotherm, reducing PBT suitability for high-temperature tyre cord, industrial belting, and under-bonnet textile reinforcements where PET retains greater dimensional stability.
Impact response follows a similar pattern. Unnotched and notched Izod values for unreinforced PBT under ISO 180/A are typically 4–6 kJ/m², higher than unfilled PET at 3–4 kJ/m² but lower than conditioned PA6, which can exceed 20 kJ/m² at 50 % RH because of moisture plasticisation. The elasticity difference therefore depends on whether the application experiences dry cyclic strain, humid cyclic strain, or constant load at elevated temperature.
| Signal | Test method | PBT | PET | PTT | PA6 | PA66 |
|---|---|---|---|---|---|---|
| Glass transition temperature | ISO 6721-1 | 45–60 °C | 70–80 °C | 45–55 °C | 50–60 °C | 60–70 °C |
| Melting peak | ISO 11357-3 | 223 °C | 255 °C | 228 °C | 220 °C | 260 °C |
| Tensile modulus | ISO 527-2 | 2.4–2.8 GPa | 2.8–3.5 GPa | 2.0–2.4 GPa | 2.8–3.2 GPa | 3.0–3.5 GPa |
| Tensile strength | ISO 527-2 | 50–60 MPa | 50–70 MPa | 50–60 MPa | 70–80 MPa | 80–90 MPa |
| Elongation at break | ISO 527-2 | 30–300 % | 30–70 % | 20–120 % | 30–70 % | 20–50 % |
| Notched Izod impact | ISO 180/A | 4–6 kJ/m² | 3–4 kJ/m² | 4–6 kJ/m² | 4–6 kJ/m² | 5–7 kJ/m² |
| Moisture absorption, 23 °C/50 % RH | ISO 62 | 0.1–0.2 % | 0.2–0.4 % | 0.15–0.3 % | 2.5–3.5 % | 2.5–3.0 % |
In high-recovery stretch filament, PTT introduces an odd number of methylene groups in the diol segment. That three-methylene repeat produces a helical conformation with higher molecular compliance under extension and more complete recovery after unloading. PTT’s elastic recovery after 20 % strain is routinely cited in fibre literature as 80–85 %, exceeding PBT at equivalent elongation. PTT also has a lower tensile modulus of 2.0–2.4 GPa and a melting point of 228 °C. PBT retains faster crystallisation and better resistance to aliphatic hydrocarbon and chlorinated fluid exposure, while PTT is selected for softness and recovered stretch in hosiery, intimate apparel, and upholstery-grade draw-textured yarn. The trade-off is therefore not purely elastic but also chemical and thermal: PBT suits low-moisture industrial monofilament and high-cycle brush fibre, whereas PTT suits high-recovery textile filament where immediate recovery from extension dominates.
Processing Boundaries for PBT Staple and Draw-Textured Yarn
Compounding and spinning of PBT require pre-drying at 120 °C for 4 h in a desiccant dryer with dew point below -40 °C. Residual moisture must remain below 0.02 %. Hydrolysis at melt temperatures above 260 °C can reduce intrinsic viscosity from 0.8 dL/g to below 0.6 dL/g within 10–15 min of residence time, generating tetrahydrofuran and carboxylic acid end groups. On production-scale co-rotating twin-screw extruders with L/D 40:1 and vacuum devolatilisation at approximately -0.08 MPa gauge, the melt temperature is normally maintained at 240–260 °C. Spin pumps deliver polymer through filtration media rated at 20–40 μm. Take-up speed for partially oriented yarn spans 1,000–3,000 m/min, followed by draw ratios of 1.5–3.0 on heated godets at 80–120 °C. PBT crystallises more rapidly than PET, so spinline orientation can be locked at lower draw ratios, but the processing window at 250 °C is narrower than for PET at 285 °C. In injection-moulded connectors and sensor housings, mould clamping force of 1,100–1,800 kN is typical for multi-cavity tooling, with barrel profiles flat around 240–260 °C. Standard PBT melt volume-flow rate under ISO 1133-1 at 250 °C/2.16 kg is commonly 10–20 cm³/10 min. Processing above 280 °C is not recommended for standard unfilled PBT because tetrahydrofuran evolution accelerates sharply; published data for extended residence time at 280 °C remain limited.
| Process variable | PBT | PET | PTT | PA6 |
|---|---|---|---|---|
| Predrying | 120 °C, 4 h | 160–170 °C, 4–6 h | 130 °C, 4 h | 80 °C, 8 h vacuum |
| Melt spinning temperature | 240–270 °C | 285–295 °C | 260–270 °C | 260–280 °C |
| Typical draw ratio | 1.5–3.0 | 3.0–5.0 | 1.5–3.0 | 2.5–4.0 |
| Residual moisture target | <0.02 % | <0.02 % | <0.02 % | <0.05 % |
| Elastic recovery after 10 % extension | 70–80 % | 50–60 % | 80–90 % | 90–100 % |
Direct substitution of PA6 or PA66 into PBT fibre and monofilament applications introduces moisture-dependent property shifts not present in PBT. At 23 °C and 50 % RH, PA6 absorbs 2.5–3.5 % moisture. Its dry tensile modulus of 2.8–3.2 GPa can fall below 1.5 GPa when conditioned, and its glass transition may shift from 50–60 °C to near room temperature. Dimensional change from dry-as-moulded to equilibrium at 50 % RH can reach 1.5–2.5 % in unfilled sections. Nylon’s elastic recovery from 10 % extension is nevertheless higher than PBT, typically 90–100 % in the conditioned state, which makes it suitable for snap-fit fingers, cable ties, and resilient textile components where moisture plasticisation is accepted or controlled. PBT’s response is the inverse: lower moisture uptake preserves stiffness and dimensions, but recovery from cyclic strain remains below conditioned polyamide. The substitution decision therefore depends on whether the service environment is dry or humid and whether the load is cyclic or constant. Published data comparing PBT and conditioned PA66 under hot-wet cyclic extension remain sparse, particularly for monofilament above 0.30 mm diameter.
When PTT’s Odd-Methylene Helix Outperforms PBT in Cyclic Strain Recovery
When the specification imposes repeated extension above 15 % strain, PTT’s three-methylene helix often outperforms PBT in immediate elastic recovery. In cyclic tensile tests under ISO 20932-1 or equivalent internal fabric stretch testing, PTT-rich yarns show lower permanent set and higher recovery after 20 % elongation than PBT-rich yarns. PBT’s reversible alpha-to-beta transformation remains active below roughly 12 % strain; beyond this threshold, irreversible crystal slip and lamellar rotation introduce larger residual set. PA6 can provide superior recovery, but its conditioned modulus is variable and its fibre processing requires moisture-controlled spinning. PBT therefore occupies a moderate-elongation niche: low moisture uptake, fast cycle times, high chemical resistance, and adequate springback below 12 % strain. Published data comparing PBT and PTT under hot-wet cyclic loading are limited, especially in melt-spun staple with draw ratios above 3.0. Extrapolation from dry filament tests should be treated with caution.
In paper machine clothing and screen-printing mesh, PBT monofilament is specified where flexural fatigue resistance under low strain is critical. Compared with PET monofilament, PBT offers lower bending stiffness and greater resistance to cracking at loop edges, but its heat deflection temperature under 0.45 MPa is close to 150 °C and its continuous-use temperature in dry air is typically 120–140 °C. PET can withstand 150 °C short-term excursions with less deformation. In polyamide-dominated bristle stock, PBT is specified when exposure to chlorinated swimming-pool water, aromatic solvents, or acidic process fluids excludes PA6/66, because polyester linkages resist hydrolysis in acid baths and do not swell at 50 % RH. The trade-off is lower wet tensile recovery than conditioned nylon; the exact recovery loss depends on draw ratio, annealing time at 160 °C, and filament denier.
