The replacement of nylon 6 and nylon 66 multifilament in chlorinated water service with melt-spun polybutylene terephthalate fibre is driven by the oxidative susceptibility of the polyamide amide group and the lower equilibrium moisture uptake of aromatic polyester. Municipal potable water distribution systems, swimming pool filtration media, cooling tower drift fabrics, and pressure vessel internals routinely expose fibre-based components to free chlorine residuals between 0.2 mg/L and 4.0 mg/L as Cl₂, or combined chloramine residuals between 1.0 mg/L and 3.0 mg/L. In these environments, nylon 6 and nylon 66 progressively lose tensile tenacity and elongation at break through N-chlorination and subsequent chain scission, producing surface fibrillation, particle shedding, and premature fibre failure. Polybutylene terephthalate lacks the reactive amide N–H bond responsible for chlorine-induced cleavage and absorbs substantially less water; saturated water uptake at 23°C according to ISO 62:2008 is typically 0.3–0.5% for PBT, compared with 8.5–10.5% for nylon 6 and 7.0–8.5% for nylon 66. This reduced water absorption limits the aqueous transport path for chlorine species into the amorphous interphase and preserves residual tensile properties under long-term oxidative exposure.

Material selection for chlorinated water service begins with the exact water chemistry, including free chlorine, combined chlorine, chlorine dioxide, pH, temperature, periodic shock oxidants, and suspended solids. Fibre geometry is equally significant. A melt-spun PBT filament with linear density between 1.0 dtex and 3.0 dtex and draw ratio between 2.8 and 4.0 provides a balance of tenacity, strain at break, and surface area suitable for woven mesh, braided sleeve, needlefelt, and melt-blown nonwoven conversion. Where the fibre is intended for potable water contact, compliance with NSF/ANSI/CAN 61 for contact surfaces and, where applicable, Regulation (EU) 10/2011 for food-contact migration is mandatory. Published data for exact lifetime service of PBT fibre under continuous chlorination in potable water is limited; qualification therefore relies on accelerated immersion screening at elevated oxidant concentration and temperature, followed by tensile retention and molecular-weight measurement.

Table 1 provides comparative technical data for fibre-grade nylon 6, nylon 66, and melt-spun PBT using standardized material and fibre test methods. The values represent typical commercial resin and fibre ranges, not a single formulation or production lot.

How Does Chloramine Chemistry Degrade Polyamide Fibre at Residual Concentrations Below 4 mg/L?

Free chlorine in water exists as hypochlorous acid and hypochlorite ion, with distribution controlled by pH and temperature. At pH 7.2–7.6 and 20–25°C, the hypochlorous acid fraction is approximately 50–70% of total free chlorine. Hypochlorous acid is the stronger electrophilic oxidant and attacks the amide nitrogen in nylon by N-chlorination. The resulting N-chloroamide undergoes subsequent hydrolytic or thermal elimination, cleaving the carbon-nitrogen bond, reducing molecular weight, and generating aldehyde and amine end groups. The reaction is concentrated in amorphous regions because chlorine transport through crystalline lamellae is low. Nylon absorbs sufficient water to deliver hypochlorous acid into the amorphous amide-rich phase, sustaining degradation throughout the fibre cross-section. Polybutylene terephthalate contains no amide N–H group and its aromatic terephthalate ester carbonyl is far less susceptible to electrophilic chlorination under potable water conditions. The low moisture regain of PBT further reduces the local aqueous oxidant concentration at hydrolytically susceptible ester linkages. This does not make PBT immune to oxidation; radical attack at the butylene segment can still produce slow chain scission, but the kinetic advantage over nylon is large enough to justify replacement in continuous chlorinated service.

Chloramines, formed when ammonia is added to chlorinated distribution water, are less aggressive to polyamide than free chlorine but still degrade nylon over longer timescales. Continuous monochloramine residual between 1.0 mg/L and 3.0 mg/L total chlorine can reduce nylon toughness over months rather than days. Melt-spun PBT fibre shows improved resistance to monochloramine because the aromatic terephthalate unit is resistant to N-chlorination and the low equilibrium water uptake slows diffusion of the aqueous oxidant. However, PBT is not resistant to strongly alkaline hypochlorite cleaning solutions at elevated temperature. Ester hydrolysis accelerates above pH 9 and 60°C, which is relevant where chlorine is used for shock disinfection at 50–100 mg/L free chlorine in membrane cleaning or tank disinfection. In those short-term shock conditions, pH control below 8.5 and temperature below 40°C preserves PBT molecular weight. Nylon 6 and nylon 66 degrade more rapidly under acidic oxidative conditions but tolerate brief alkaline pH better than PBT. The replacement decision therefore requires consideration of the entire pH-temperature-oxidant envelope rather than the nominal service residual alone.

Accelerated immersion screening for fibre degradation under chlorinated water conditions uses a dark, stirred reactor with a buffered hypochlorite solution maintained at pH 7.2±0.2 and 40±2°C. Total chlorine is held at 2.0±0.2 mg/L free chlorine or 3.0±0.2 mg/L combined chlorine, with verification by the DPD method of ISO 7393-2:2017. Specimens are withdrawn at 168 h, 500 h, 1000 h, and 2000 h, rinsed with deionized water, conditioned, and tested for tensile retention. Published data for exact retention values in this specific configuration are limited; comparative laboratory studies generally show PBT retaining a higher proportion of original tenacity than nylon after identical chlorinated immersion. The test protocol is an industry screening method, not a consensus standard, and should not be represented as such.

Drying, Venting, and Residence-Time Controls Are Not Interchangeable in PBT Melt Spinning.

Unlike polyamide, which can be processed after extended post-condensation and water washes, PBT is hydrolytically sensitive during melt processing. Fibre-grade PBT with an intrinsic viscosity of 0.85–1.05 dL/g, measured in phenol/1,2-dichlorobenzene at 25°C according to ISO 1628-5:2015, must be dried to a moisture content below 50 ppm before extrusion. Desiccant-wheel dryers with process-air dew point no higher than -40°C and air temperature of 120–140°C for 4–6 h are typical for fibre-grade chip. Residual moisture above 80 ppm produces hydrolytic chain scission in the extruder, reducing intrinsic viscosity by 0.05–0.15 dL/g and narrowing the molecular-weight distribution. On a 36:1 L/D counter-rotating twin-screw compounder or a 24:1 single-screw spinning extruder, the result is lower melt strength, increased spinneret drip, and reduced drawability. The dryer-to-throat transfer line should be closed-loop and blanketed with dried air because PBT reabsorbs surface moisture rapidly when exposed to ambient air at relative humidity above 60%.

For melt spinning, barrel temperatures from feed to metering generally range from 230–255°C, with melt temperature at the spin pack held between 245–265°C. The processing window is intentionally narrow. At melt temperatures below 240°C, PBT viscosity is too high for stable filtration through 20–40 µm screen packs, while at melt temperatures above 265°C, β-scission of the butylene segment accelerates, releasing tetrahydrofuran and increasing carboxyl end-group concentration. Residence time in the melt train should not exceed 15 min and is preferably below 10 min. Processing at 265°C for 20 min can reduce intrinsic viscosity by more than 0.10 dL/g, producing a measurable loss in as-spun tenacity and an increase in oligomer deposits on spinneret faces. The spin pack should use breaker plates and sintered-metal filtration with an absolute rating of 20–40 µm to minimise gel interruptions. Pack pressure is commonly 8–12 MPa at 250°C.

Spinneret hole diameter of 0.25–0.45 mm and capillary length-to-diameter ratio between 2 and 4 are typical for round PBT filament, with mass throughput per hole between 0.8 g/min and 2.5 g/min. Cross-flow quench air at 18–25°C and 0.35–0.60 m/s cools the filament. The as-spun threadline must then be drawn in a heated zone. A total draw ratio of 2.8–4.0 applied in two stages at 120–170°C increases orientation and crystallinity. Heat setting under tension at 170–200°C for 0.3–1.0 s reduces boiling-water shrinkage to less than 4%, measured per ASTM D2259-10(2018). If heat-setting temperature falls below 165°C, residual shrinkage in hot chlorinated water can exceed 6% and create dimensional instability in woven filter fabrics. If heat-setting exceeds 205°C, fibre tenacity declines due to over-crystallization and loss of amorphous tie chains. Table 2 summarises the melt-spinning limits for fibre-grade PBT.

When Chlorine Dioxide or Alkaline Shock Oxidants Reverse the Expected Service Ranking

Chlorine dioxide is used in potable water at residuals between 0.1 mg/L and 0.8 mg/L and in industrial process water at higher doses. Unlike free chlorine, chlorine dioxide is a selective radical oxidant and does not produce the same N-chlorination products, but it can still oxidise polyamide fibre over time. Melt-spun PBT fibre generally shows higher retention than nylon in chlorine dioxide service because the aromatic ester is less susceptible to oxidation, and the low water uptake limits transport of the oxidant to reactive sites. However, chlorine dioxide can generate acidic by-products that lower pH locally; PBT resists short-term acid exposure better than nylon, which undergoes acid-catalysed amide hydrolysis. If the system uses acid shock for scale removal at pH 2.0–3.0, nylon 6 and nylon 66 are likely to lose toughness more rapidly than PBT. Conversely, if alkaline shock cleaning at pH 11–12 with 50–200 mg/L hypochlorite is used, PBT ester hydrolysis may be more severe than nylon degradation. In that situation, PBT replacement should be specified only if the pH and temperature are controlled or if the cleaning step is isolated from the fibre component.

Temperature limits also differ between polyamide and polyester in wet oxidative service. Nylon 6 retains some ductility in hot water up to 90–100°C for short-term exposure, but PBT long-term hydrolytic stability under continuous chlorinated water is usually specified for service below 70–75°C. Above that boundary, hydrolysis of the ester group becomes kinetically significant even at neutral pH. In hot chlorinated pools or spas operating at 35–40°C, the margin is acceptable, but in steam-sterilised filter media at 121°C, melt-spun PBT fibre is not suitable unless the steam cycles are short and dry. Published data for continuous-pressure hot chlorinated water at 80°C for melt-spun PBT fibre is limited; conservative design should use PVDF or PTFE fibre beyond that boundary. The expression chlorinated water service therefore does not imply universal oxidant compatibility. Process engineers must bound the maximum alkaline oxidant exposure, the maximum temperature, and the frequency of shock-cleaning events before replacing nylon with PBT.

Qualification Testing and Standards for Chlorinated Water Contact Fibre

Fibre tensile property testing should be performed according to ISO 5079:2020 for single fibres or ISO 2062:2009 for multifilament yarns, after conditioning at 20±2°C and 65±4% relative humidity according to ISO 139:2005. Linear density is determined by ISO 1973:2021, and moisture regain by ASTM D2654-22. The melt mass-flow rate of the PBT resin can be measured according to ISO 1133-1:2022 at 250°C with a 2.16 kg load. Intrinsic viscosity is monitored by ISO 1628-5:2015 in phenol/1,2-dichlorobenzene at 25°C. Thermal transitions are measured by ISO 11357-3:2018. Colour fastness to chlorinated water can be characterised by ISO 105-E02:2013, but that method does not provide tensile durability data. These standards establish the baseline for batch-to-batch comparison and do not themselves define chlorine resistance.

Chlorinated immersion screening is performed in a dark, stirred reactor using a buffer of pH 7.2±0.2 and sodium hypochlorite solution maintained at 2.0±0.2 mg/L free chlorine or 3.0±0.2 mg/L total combined chlorine, with temperature at 40±2°C. Free chlorine is verified by the DPD method of ISO 7393-2:2017 at intervals not exceeding 24 h. Specimens are withdrawn at 168 h, 500 h, 1000 h, and 2000 h, rinsed with deionized water, conditioned, and tested for tensile retention and elongation retention. Acceptance depends on service safety factor. A fibre that retains less than 80% of original tenacity at 1000 h under this accelerated screen may not provide sufficient margin for a 10-year service interval in potable equipment unless additional Arrhenius prediction supports long-term durability. This protocol is an industry screening method and not a consensus standard.

For potable water contact, the finished fibre and any spin finish or lubricant must meet NSF/ANSI/CAN 61 extraction limits, and metallic additives must comply with NSF/ANSI/CAN 372 for lead content where applicable. In Europe, Regulation (EU) 10/2011 and EC 1935/2004 apply to food-contact uses; for potable water in Germany, KTW-BWGL guidelines may be required. These certifications address migration of organic and inorganic contaminants and do not measure fibre tensile durability; both must be evaluated in parallel.

On industrial nonwoven carding lines, melt-spun PBT staple with cut length between 38 mm and 60 mm and fibre linear density between 1.2 dtex and 2.2 dtex exhibits low moisture regain and can develop triboelectric charge during high-speed carding at low ambient relative humidity. Antistatic spin finishes are normally applied at 0.3–0.6% by mass during fibre production, but facilities with relative humidity below 35% often require additional ionization bars on the card and crosslapper. Calender bonding of PBT filter media requires roll surface temperatures in the range 215–230°C; at 235°C and above, surface fusion can reduce air permeability by more than 50% as measured by ISO 9237:1995 or ASTM D737-18. PBT fibre is not compatible with prolonged exposure to strong alkaline detergents, molten alkali, or hot concentrated phosphoric acid. It is also sensitive to UV ageing in outdoor chlorinated water channels unless stabilised with carbon black or a weathering package. Where the fibre is co-mingled with nylon in a hybrid felt, differential moisture swell and different thermal shrinkage can produce curl and filter bag distortion after wet-dry cycling. The safe operating envelope for melt-spun PBT fibre in chlorinated water service is therefore bounded by continuous pH below 8.5, temperature below 75°C, free chlorine below 5 mg/L, and short-term clean-in-place excursions held below 40°C when hypochlorite is present.