Polyethylene Terephthalate (PET)

The present product scope covers PET fibre forms and their end uses across textile, industrial and hygiene segments. Fibre-grade polyethylene terephthalate is polymerised from purified terephthalic acid and ethylene glycol in continuous melt-phase reactors, with solid-state polycondensation used where intrinsic viscosity must exceed 0.80 dL/g. The principal fibre forms include staple fibre, partially oriented yarn, fully drawn yarn, draw-textured yarn, high-tenacity filament, spunbond nonwoven, and meltblown nonwoven. Intrinsic viscosity is measured according to ASTM D4603-18 in a 60:40 phenol/1,1,2,2-tetrachloroethane solvent at 30 °C. Textile-grade PET is commonly released at 0.62–0.68 dL/g, while high-tenacity industrial filament is specified at 0.85–1.00 dL/g. A change of 0.02 dL/g in intrinsic viscosity is sufficient to shift melt pressure, draw tension, and package tensile uniformity on production-scale spinning equipment.

On single-screw extruders with L/D 24:1–30:1 and barrier screw geometry, fibre-grade PET chips are dried to residual moisture below 30 ppm in desiccant dryers operating at 170–180 °C and dew point below −40 °C. Hopper dwell time is set between 4 h and 6 h. Over-drying above 190 °C can generate acetaldehyde and yellowing, while under-drying produces hydrolytic chain scission at melt temperatures above 280 °C. Filtration before the spin pack uses elements rated at 20–40 µm to remove gel bodies and agglomerated titanium dioxide delustrant. If the melt temperature at the die exceeds 295 °C for more than 10 min, intrinsic viscosity loss becomes measurable in package tensile data, particularly when residence time distribution broadens in hot-runner channels.

Why Does Hydrolytic Degradation Narrow the Melt-Spinning Window?

Hydrolytic degradation during PET melt spinning follows pseudo-first-order kinetics with respect to residual moisture concentration and accelerates as the melt passes through the extruder and spin beam. Chain scission lowers number-average molecular weight, reduces melt viscosity, and widens molecular weight distribution. For virgin textile-grade chip with intrinsic viscosity 0.63 dL/g, uncontrolled moisture above 50 ppm has been observed on continuous polycondensation spinning lines to cause package-to-package yarn tenacity variation of ±5 cN/tex at constant draw ratio. The deficiency is not limited to tensile loss: lower melt viscosity destabilises the spinline, increases draw resonance, and produces filament denier variation outside ±2.0%. Correction requires reducing throughput or raising quench air velocity, both of which shift as-spun crystallinity. Melt temperature is therefore maintained at 280–290 °C for textile POY and 290–300 °C for high-tenacity filament, with spin beam temperature uniformity of ±2 °C across the die width. The acceptable moisture limit is confirmed by differential scanning calorimetry of dried chips and by pressure measurement before the metering pump.

Quench air conditions for staple fibre lines typically operate at 15–25 °C and 50–70% relative humidity, with cross-flow velocity from 0.4 m/s to 0.8 m/s. Because the crystallisation rate of PET is highest in the 150–170 °C interval, filaments must be cooled below the glass transition before reaching the take-up roll. In staple production, undrawn tow is gathered from multiple spin positions, drawn in a two-stage hot-water and steam bath between 70 °C and 95 °C, crimped mechanically, and cut to lengths of 32 mm, 38 mm, or 51 mm for cotton, polyester, and woollen spinning systems respectively. Linear density for ring-spun textile blends is usually 1.2–1.5 dtex, while open-end rotor spinning uses 1.3–1.7 dtex with higher crimp stability. Yarn tenacity is reported according to ISO 2062 or ASTM D2256/D2256M-21. A typical 50/50 PET/cotton ring-spun yarn at twist factor 3.8–4.2 reaches 18–22 cN/tex.

Staple Fibre Geometries and Yarn Intermixing

Cut-length and linear density must be matched to spinning technology because PET staple has lower fibre-to-fibre friction than cotton and lacks natural crimp. In open-end rotor spinning, short fibre content above 3% by weight increases end-break rate because the rotor groove accumulates finish and fibre fragments. Cut-length distribution is therefore controlled by single-fibre measurement according to ISO 6989. Spin finish is applied at 0.08–0.15% by weight based on fibre weight to reduce static and control fibre-to-metal friction. The finish must not form deposits on draw rolls at 180 °C and must be compatible with downstream dyeing. Textile PET staple is usually supplied as semi-dull round cross-section with 0.30% titanium dioxide, but trilobal and hollow cross-sections are extruded through shaped spinnerets for moisture management and insulation. Woven apparel fabric is checked for dimensional stability according to ISO 5077, with residual shrinkage below 1.5% in both warp and weft for heat-set constructions.

Cationic-dyeable PET staple is produced by copolymerising 2–5 mol% 5-sulfoisophthalic acid dimethyl ester before polycondensation. The sulphonate groups shift dye uptake to basic dyes and lower the melting point by 3–8 °C. This complicates drying because the copolymer hydrolyses faster than homopolymer PET; drying temperature is reduced to 160 °C with dew point below −40 °C. Low-pill PET staple is produced at lower intrinsic viscosity of 0.48–0.55 dL/g, which reduces pilling but also lowers tenacity and requires adjustment of draw ratio. These variants are used in knitwear and upholstery where pilling is evaluated according to ISO 12945-2 and colour fastness is tested by ISO 105-C06.

Continuous filament product forms include POY, FDY, and DTY. POY with intrinsic viscosity 0.62–0.65 dL/g is spun at 280–290 °C and wound at 2,800–3,500 m/min. The partially oriented structure is then draw-textured at heater temperatures of 180–210 °C with draw ratio 1.6–1.8 for apparel-grade DTY. Fully drawn yarn for sportswear and woven linings is produced on spin-draw equipment with draw ratio 2.5–3.5 and hot-roll temperatures of 80–90 °C and 130–160 °C. Tensile testing uses ISO 2062 at 500 mm gauge length and 250 mm/min extension rate. A typical 167 dtex/48 filament FDY has breaking tenacity of 35–45 cN/tex and elongation at break of 20–35%. Draw pan temperature deviation greater than ±2 °C produces package-to-package dye uptake differences visible as barré in warp knitting.

When Filament Drawing Shifts from Textile POY to High-Tenacity Industrial Cord

High-tenacity PET filament is produced from higher-IV chip and drawn at significantly higher ratios. Melt spinning uses extruder temperatures of 295–305 °C and slower cooling to reduce orientation before solid-state drawing. Undrawn yarn is then drawn in two or three stages to total draw ratio 5.0–6.0. A 1100 dtex/192 filament high-tenacity yarn commonly has tenacity of 70–85 cN/tex, elongation at break 8–12%, and hot-air shrinkage below 3% at 177 °C for 2 min when tested according to ASTM D885/D885M-23. The high draw ratio creates a fibrillar morphology with high amorphous orientation, which provides dimensional stability for tyre cord and conveyor belts but makes the yarn notch-sensitive during warping. Production lines use controlled-tension creels and avoid ceramic guide damage because a surface scratch of 5 µm depth can reduce cord fatigue life in rubber by more than 50%. Rubber adhesion is achieved with a resorcinol-formaldehyde-latex dip, and dipped cord is tested for H-adhesion according to ASTM D4776-24. Published data for specific cord constructions is limited, but tensile rating must be matched to cord twist factor and end count.

For ropes and slings, high-tenacity PET filament is preferred over polyamide when low elongation under load is required. A 12-strand braided rope from 2,200 dtex high-tenacity yarn can show breaking load of 70–85 kN at 24 mm diameter, with elongation below 5% at 30% of break when tested according to ISO 2307. The limitation is wet abrasion: when operated over steel sheaves with surface roughness above Ra 1.6 µm, PET rope loses strength faster than polyamide because of internal heat generation under cyclic loading. Supplier technical bulletins commonly list maximum continuous working temperature of 80 °C, short excursions to 120 °C, and strength loss of 10–20% above 150 °C. These values should be verified by rope assembly testing under the intended load cycle.

For geotextiles and industrial fabrics, PET staple or filament is needlepunched or woven into structures with mass per unit area from 150 g/m² to 1,200 g/m². The primary technical advantage over polypropylene is higher modulus and creep resistance, but the boundary condition is hydrolytic instability in alkaline soils. Geotextile-grade PET is therefore specified with carboxyl end-group content below 20 mmol/kg because carboxyl groups catalyse hydrolysis; titration is performed according to ASTM D7409-15. Installation in contact with cement-stabilised soils requires a sacrificial alkalinity barrier. Tensile properties are reported according to ISO 10319. A woven geotextile of 250 g/m² may show machine-direction tensile strength of 40–60 kN/m, while a needlepunched nonwoven of the same mass is typically 15–25 kN/m. The difference is attributed to fibre orientation and entanglement.

Meltblown PET Requires Tighter Melt Temperature Control Than Polypropylene

In meltblown PET, the polymer is extruded through a die with 25–35 holes per inch and attenuated by high-velocity hot air at 300–320 °C. Compared with polypropylene meltblown, PET requires greater attention to crystallisation because as-spun filaments can solidify before reaching the collector if the air gap is excessive or the melt temperature drops below 280 °C. Production equipment reports a usable processing window of approximately ±5 °C when targeting mean fibre diameter of 2–5 µm. Larger deviations increase fibre diameter distribution and reduce filtration efficiency. Die pressure is maintained at 35–70 bar depending on throughput, and screen packs of 60–80 mesh protect the spinneret. Because PET meltblown nonwovens have a narrower bonding window than polypropylene, collector distance is set to 200–500 mm and calender bonding is operated at 120–150 °C for hygiene-compatible grades. Nonwoven tensile properties are tested according to ISO 9073-3, and air permeability according to ISO 9073-15.

In hygiene applications, PET fibre forms are used selectively because of higher stiffness, thermal stability, and lower absorbency compared with polypropylene or cellulose. Spunbond PET and PET/polyethylene bicomponent filaments are converted into acquisition distribution layers and coverstock at basis weights of 10–30 g/m². Fibre denier is often 1.5–4.0 dtex for softness, while 6.0–9.0 dtex is used in the acquisition layer to maintain compression resistance under load. Liquid strike-through is evaluated according to ISO 9073-8, and rewet is limited by through-air bonding temperature and fibre surface finish. PET staple fibre is also used in air-through-bonded nonwovens for hygiene cores when the finish composition is selected to avoid leaching into synthetic urine. For skin-contact certification under ISO 10993-10, the finish formulation must pass irritation and sensitisation testing; however, many hygiene converters rely on supplier declarations rather than final-device ISO 10993 certification. A known boundary is that PET fibre does not provide the same bulk softness as polypropylene at equivalent denier, so blend ratios must be validated by panel testing and laboratory abrasion protocols.

The operational boundary for PET in hygiene nonwovens is set by the conflict between melt processability, fibre diameter control, and skin-contact finish durability. Spunbond lines with 1.5–4.0 dtex filaments can maintain continuous operation only when chip moisture remains below 30 ppm, die temperature uniformity is held within ±2 °C, and calender bonding pressure is limited to avoid film formation that would reduce strike-through performance. Meltblown PET used in hygiene composites is restricted to constructions where fibre diameter distribution does not control the primary fluid-handling function; published data for this specific configuration is limited, and each hygiene laminate must be validated on the converting line against the final absorbent article specification.