Polybutylene terephthalate (PBT) is specified for optical fibre loose tubes principally because its flexural modulus of approximately 2.5 GPa according to ISO 178 provides crush protection, while its crystallisation kinetics are fast enough for high-speed tube extrusion. The performance envelope discussed here is PBT in Optical Fibre Loose Tubes: Extrusion, Shrinkage and Long-Term Hydrolysis. A high-viscosity PBT compound with a melt volume-flow rate of 8–12 cm³/10 min at 250 °C and 2.16 kg under ISO 1133-1:2022 is dried in a desiccant hopper dryer at 120–130 °C for 4–6 h to a residual moisture content below 0.02 wt% by Karl Fischer titration. Production lines commonly use a 30–45 mm single-screw extruder with an L/D ratio of 24:1–30:1, a barrier screw compression ratio of 2.2:1–2.8:1, and a melt pump to hold die pressure at 80–150 bar. The die-entry melt temperature is held between 250 °C and 265 °C. Above 270 °C, thermal degradation drops the intrinsic viscosity from 0.95–1.05 dL/g to below 0.85 dL/g and generates volatile by-products that appear as surface roughness or interfacial voids. The resulting practical processing window is no wider than ±5 °C around the target melt temperature.

What narrows the extrusion window for unfilled PBT loose-tube compounds?

Rheological constraints at the crosshead die are often the limiting factor. PBT melts are strongly shear thinning; at 250 °C the apparent viscosity at 1000 s^-1 is generally between 150 Pa·s and 400 Pa·s depending on molecular weight. The flow channel in a loose-tube crosshead is thin, and small changes in die temperature produce large changes in melt flow. If screw speed is increased above 120 rpm on a 30 mm barrier screw to raise line speed, viscous dissipation can add 5–8 °C to the melt temperature above the barrel set points. The resulting melt-temperature overshoot can exceed the degradation threshold even when the barrel zones remain stable. Production lines therefore use a gear pump with a suction pressure of 20–40 bar to decouple extruder output from die pressure, and a melt filter with 30–50 µm screen pack to remove carbonised particles. The barrel profile is normally set as 240/245/250/255/260 °C from feed to metering. If the die-entry melt temperature falls outside 250–265 °C, wall-thickness variation increases and post-extrusion shrinkage becomes erratic.

Production-scale failure modes include screw wear, carbonised PBT deposits in the crosshead, and melt-pressure oscillation from granulate bridging. A 30 mm screw with hard-facing treatment is common because PBT compounds can be abrasive at processing temperatures. Carbonised deposits form if melt stagnates in the die for more than 20–30 min; purging with a low-viscosity polyolefin is not always effective, so dedicated purging compounds are required. Screw torque should be monitored because a progressive rise at constant speed often indicates filter blinding from gels or carbon particles; a screen pressure drop above 30 bar is a typical changeout criterion.

Post-extrusion shrinkage is not intrinsic to the material alone; it is generated by frozen orientation and incomplete crystallisation during quenching. When the tube emerges from the die, the draw-down ratio, expressed as the ratio of die annulus to final tube annulus, is usually kept between 1.05 and 1.15. If the first water trough is operated at 20–30 °C, the outer skin vitrifies rapidly and traps molecular orientation. Later exposure to 85 °C in cable qualification permits relaxation and additional crystallisation, producing free length change. A second trough at 55–70 °C acts as an in-line annealer and reduces residual shrinkage to below 0.3% for a 12-fibre loose tube of 2.5 mm outer diameter and 1.6 mm inner diameter. The acceptance test for many loose-tube designs is conducted at 85 °C for 2 h according to IEC 60794-1-2:2017, with maximum length change of 0.5%. Because cooling time changes with output, line-speed changes require rebalancing the quench and annealing stages.

When line speed exceeds 300 m/min, quench dynamics control the dimensional outcome

At take-up speeds above 300 m/min on a 12-fibre loose tube, a 10 m water trough gives less than 2 s of cooling. The tube can leave the trough above the glass transition of PBT, which is in the range 45–60 °C, allowing molecular relaxation before winding. This creates online diameter oscillation and higher post-extrusion shrinkage. Extended troughs of 15–20 m or vacuum sizers with water at 10–20 °C are used to recover dimensional stability. However, water colder than 15 °C can microcrack the inner wall where the tube contacts the fibre bundle. Laser micrometers sampling at 25 Hz are standard on production lines; ovality above 0.05 mm on a 2.5 mm tube is typically rejected. The conflict between high speed and low shrinkage is managed by increasing annealing-trough temperature rather than by reducing line speed alone.

Shrinkage interacts directly with fibre strain in the completed cable. The loose tube is extruded with a controlled fibre overlength, typically 0.1–0.3% for stranded loose-tube cables. If the PBT tube shrinks after cabling, it reduces the overlength and can produce fibre attenuation at low temperature. Cable qualification therefore includes temperature cycling from -40 °C to +70 °C under IEC 60794-1-2. PBT loose-tube grades with post-extrusion shrinkage below 0.3% are preferred because they retain overlength even after the 85 °C shrinkage test.

Hydrolytic ageing and end-group autocatalysis in outdoor loose tubes

Under moisture ingress, PBT hydrolyses at the terephthalate ester bond. The reaction is pseudo-first-order in water concentration at low water uptake and is autocatalysed by carboxylic acid end groups. A hydrolysis-resistant grade therefore starts with a low carboxyl end-group content, typically below 30 mmol/kg by potentiometric titration, and contains acid-scavenging additives such as aromatic polycarbodiimide or epoxy-functional chain extenders at 0.5–1.5 wt%. These additives neutralise carboxyl groups and delay autocatalysis. The standard ageing condition for outdoor cable materials is 85 °C and 85% relative humidity for 1000 h. A retention of at least 70% of the initial intrinsic viscosity, measured by ISO 1628-2, is a common batch criterion. At 60 °C water immersion, unstabilised PBT can lose a substantial fraction of its elongation, while stabilised compounds with an initial intrinsic viscosity of 0.95–1.05 dL/g generally retain 70–80% of original elongation, depending on tube wall thickness and fibre count. Published data for specific cable configurations is limited.

Water uptake in PBT at 23 °C immersion is around 0.4–0.5 wt% at saturation. At 85 °C and 85% RH, the equilibrium water concentration is higher, and diffusion through a 0.3–0.5 mm loose-tube wall approaches equilibrium in days rather than years. Because the tube wall is thin, water reaches the inner surface relatively quickly. This is why hydrolysis-resistant grades are required even though PBT has lower water absorption than polyamide. The apparent activation energy for hydrolytic chain scission is commonly reported between 70 kJ/mol and 100 kJ/mol. A temperature rise from 40 °C to 50 °C in a splice closure or sun-exposed duct can therefore double or triple the degradation rate. Continuous service above 60 °C under wet conditions should be avoided unless a hydrolysis-resistant grade and an additional cable sheath moisture barrier are specified.

Hydrolysis-stabiliser loading is not a linear variable. At 0.5 wt% of a polymeric carbodiimide, the melt viscosity may increase by 5–10% due to chain extension; at 2.0 wt% and above, die lip plate-out and gel formation are observed on long runs. The compounding step must achieve dispersive mixing without local overheating. Twin-screw compounding with a screw temperature profile between 230 °C and 250 °C and vacuum venting at ≤50 mbar absolute is used to remove residual moisture and volatile stabiliser by-products. The stabilised compound is then re-dried before tube extrusion.

Moisture regain after drying is a field issue. At shop-floor relative humidity above 60%, dried PBT granulate can pick up surface moisture within minutes if open handling is used. Closed conveying and hopper dryers with a dew point of -40 °C or lower are therefore required. Batch-to-batch variation in intrinsic viscosity of more than ±0.05 dL/g changes melt pressure by 20–30 bar at constant screw speed and forces barrel-temperature compensation of 3–5 °C. Without correction, the line produces periodic diameter oscillation or melt fracture at the die lip. Contact with high-pH cementitious grouts or calcium hydroxide-containing filling compounds must also be avoided because alkaline conditions accelerate ester hydrolysis well beyond neutral moisture. The operational boundary for long-term wet service is approximately pH 4–8.