PBT Fibre Explained: The Ester Linkage Behind Swimwear and Stretch Performance

Polybutylene terephthalate (PBT) is a semicrystalline thermoplastic polyester produced by melt polycondensation of terephthalic acid or dimethyl terephthalate with 1,4-butanediol. The repeating unit contains two ester carbonyl groups separated by a tetramethylene sequence; this aliphatic spacer distinguishes PBT from polyethylene terephthalate (PET) and governs the fibre’s low glass transition temperature of 45–60 °C. Fibre-grade PBT typically exhibits an intrinsic viscosity of 0.85–1.05 dL/g when measured in phenol/1,2-dichlorobenzene according to ISO 1628-1:2021 and a melt density of approximately 1.31 g/cm³ at 250 °C. The ester linkage dominates the processing and end-use chemistry: it is polar enough to provide interchain association and disperse-dye uptake, yet susceptible to hydrolysis in hot aqueous or alkaline conditions. For swimwear, the practical consequence is that PBT combines low wet weight gain, resistance to chlorine-induced chain scission relative to polyamide, and elastic recovery governed by a crystal–crystal phase transition.

In melt-spun textile filaments, PBT crystallises rapidly into a triclinic α-phase. On extension to 10–15%, the α-phase transforms reversibly to a β-phase as the tetramethylene segments straighten and the ester dipoles reorient. This crystal–crystal transition provides much of the recovery observed in swimwear stretch cycling, rather than entropic amorphous relaxation alone. Fibre tensile values reported under ISO 2062:2009 for drawn PBT multifilament typically range from 2.8 cN/dtex to 4.0 cN/dtex tenacity, with elongation at break between 24% and 35%. Elastic recovery after 10% extension, measured on knitted fabric according to ASTM D3107-07(2011), commonly lies between 90% and 98% depending on heat-setting temperature and yarn texturing. The ester group contributes dipole interactions that limit segmental creep under load, but the same linkage defines the hydrolysis boundary that melt spinners and dyers must control.

Dynamic fatigue testing of PBT/elastane swimwear fabric under repeated extension to 30% for 5,000 cycles on a servohydraulic tensile fatigue tester typically shows residual fabric growth below 5%. The β-phase reversion is not fully elastic above 15% strain; therefore PBT alone is not used for high-recovery panels, and elastane is included for extensions above 20%. Stress relaxation data at constant 30% extension show that heat-set PBT retains a higher proportion of initial stress than PET, but published values vary with yarn linear density and false-twist texturing conditions.

What Limits Hydrolytic Stability of the Ester Linkage in Chlorinated Pool Water?

Ester hydrolysis in PBT proceeds by chain scission at the carbonyl carbon, generating hydroxyl and carboxyl end groups; the carboxyl end group autocatalyses further hydrolysis by lowering local pH. In pool-water exposure, free chlorine at 1–3 mg/L and pH 7.2–7.8 attacks the ester bond less directly than it attacks polyamide amide groups, but hypochlorous acid, alkaline shock treatments, and elevated water temperature can accelerate surface hydrolysis and reduce tensile retention. Because hydrolysis generates carboxyl end groups, the local pH at the fibre surface can fall below bulk pH in stagnant water layers within swimwear fabric. The resulting autocatalytic degradation is why low-carboxyl-end-group grades and carbodiimide stabilisers are specified.

Published data for specific PBT swimwear-fibre configurations under pool-water simulation is limited; validation therefore relies on control of initial carboxyl end-group concentration, hydrolysis stabiliser addition, and testing according to AATCC TM162 and ISO 105-E03:2010. Unstabilised PBT autoclaved in saturated steam at 121 °C for 48 h can show tensile strength loss of 20–40%, whereas carbodiimide-stabilised grades may retain more than 85% of original tenacity. The ester linkage also imposes a pH boundary: continuous exposure to alkaline perspiration or detergent residues above pH 9 can saponify surface ester groups and produce surface pilling after repeated swimwear laundering. Hypochlorite-induced degradation of polyamide 6 proceeds by N-chlorination and subsequent backbone cleavage; PBT has no amide hydrogen, so chlorine attack is confined to ester hydrolysis and surface oxidation.

Pre-drying represents the first operational boundary in PBT fibre extrusion. A desiccant dryer with a dew point of -40 °C is standard; pre-drying at 120–130 °C for 4–6 h reduces residual moisture to below 0.02%. If residual moisture exceeds 0.03%, hydrolytic molecular-weight loss during melting lowers intrinsic viscosity by 0.05–0.10 dL/g in a single pass, which appears downstream as filament breaks and increased spin-pack pressure. Melt temperature is maintained at 250–265 °C across extruder zones, with total residence time below 15 min to limit thermal degradation. Single-screw extruders with L/D 24:1 to L/D 32:1 and barrier screws are used; spin packs are equipped with sintered-metal filtration of 20–40 μm absolute rating. In production-scale campaigns, a filter pressure rise above 0.8–1.2 MPa/h typically indicates insufficient drying, excessive melt temperature, or stabiliser agglomeration, and triggers pack changeover. Draw ratios for PBT filament are typically 3.0:1 to 3.5:1; higher draw ratios above 3.7:1 can reduce elongation at break below the range required for texturising and circular knitting.

Rheological behaviour of PBT fibre grades is sensitive to both temperature and moisture. Melt flow index measured at 250 °C with 2.16 kg per ISO 1133-1:2022 typically falls between 18 g/10 min and 35 g/10 min for fibre-grade resins. A shift of -5 °C in melt temperature can increase apparent viscosity by 8–12%, which in spin drawing changes denier uniformity. Spin pumps are therefore controlled within ±0.5 °C per zone. Unlike polyamide 6, where residual water reduces melt viscosity temporarily, moisture in PBT predominantly causes permanent molecular-weight loss and a non-recoverable drop in melt strength.

When PBT Replaces Polyamide 6 in Warp-Knit and Circular-Knit Swimwear Structures

Compared with polyamide 6, PBT fibre shows an equilibrium moisture regain of approximately 0.4% versus 4.0% at 20 °C and 65% RH according to ASTM D1909-13. The lower regain reduces wet fabric weight, limits dimensional growth, and preserves modulus when wet. In warp-knit constructions using 44–58 dtex PBT filament with 22–44 dtex elastane, the PBT yarn contributes surface stability and chlorine resistance while the elastane supplies recovery from high extension. Knitting tensions are lower than those used for PET because PBT’s lower initial modulus reduces needle wear; however, heat-setting must be controlled at 160–180 °C for 30–60 s on a stenter. Overheating above 190 °C can embrittle the ester-containing amorphous phase and reduce tear strength. Fabric growth after 1 h at 30% extension is typically below 5% when tested by ASTM D2594-20, but the result depends on elastane denier, knit density, and heat-set temperature.

Coloration of PBT swimwear is performed with disperse dyes at atmospheric boil or at 105–115 °C in jet dyeing equipment. The lower glass transition permits carrier-free dyeing, which reduces residual auxiliaries that can plasticise the ester phase and lower chlorine resistance. Colourfastness to chlorinated pool water by AATCC TM162 and to Xenon arc light by ISO 105-B02:2014 should be specified at grade 4 or higher for swimwear. Amine-based softeners and alkaline detergent residues above pH 9 should be avoided because they can accelerate ester aminolysis or saponification and reduce surface abrasion resistance.

Comparative fibre data illustrate the position of PBT relative to PET and polyamide 6 in swimwear evaluations.

Spin-Pack Pressure Spikes Can Be Traced to Stabiliser Dispersion Failure

Addition of polymeric carbodiimide hydrolysis stabiliser at 0.3–0.7 wt% in a twin-screw compounding step lowers carboxyl end-group concentration from 25–35 meq/kg to below 10 meq/kg. Higher loadings above 1.0 wt% can elevate melt viscosity and spin-pack pressure, and may migrate to filament surfaces causing dye uptake variability. Trials on a 75 mm twin-screw extruder with L/D 40:1 at 240–260 °C indicate that stabiliser dispersion quality correlates with residence time distribution; poor dispersion produces periodic hard spots that survive 20 μm filtration and cause breakouts at draw ratios above 3.2:1. Melt temperature deviation exceeding ±5 °C from set point can shift melt viscosity enough to alter filament denier by more than 3% and change draw resonance. The compounding step therefore determines whether a stabilised PBT grade can spin continuously for 24 h without a spin-pack pressure rise above 0.8 MPa/h.

Fabric-level certification for swimwear stretch performance should specify recovery and growth under ASTM D2594-20 or ISO 20932-1:2018. In PBT/elastane warp knits, load and unload cycling at 30% extension is used to measure fabric growth after 1 h; growth values below 5% are typical for correctly heat-set constructions. Chlorine resistance is normally assessed by colour change and breaking strength retention after 25–50 h exposure to 5–10 mg/L hypochlorite solution at 25 °C. PBT surfaces can show fibrillation under repeated abrasion if heat-setting is too low or if oligomer deposits remain from dyeing; an alkaline reduction clearing step is avoided in favour of dispersant-compatible acidic clearing agents because alkaline saponification of surface ester groups increases pilling.

Operational boundaries include pre-drying below 0.02% moisture, melt residence below 15 min, and avoidance of amine-based additives, alkaline finishing above pH 9, and steam exposure above 121 °C unless hydrolysis stabiliser is present. PBT fibre is not recommended for continuous immersion in strongly acidic media below pH 4 or strongly alkaline media above pH 10 at elevated temperature because ester cleavage accelerates under those conditions. In swimwear, the ester linkage is both the source of elastic recovery and the primary chemical vulnerability; processing and finishing controls define whether the fibre meets the chlorine-resistance and dimensional-stability requirements of production swimwear.