Polyethylene terephthalate (PET) fibre competes directly with polyamide 6/66 (nylon) and isotactic polypropylene (PP) in staple, tow, and spunbond applications where cost per tonne, post-industrial/post-consumer recovery, and tactile hand are specified in purchase contracts. The keyword set PET vs Nylon vs Polypropylene Fibre: Cost, Recovery and Hand Feel maps onto three distinct technical decision boundaries: raw material and conversion economics, mechanical or chemical recyclability, and instrumentally measured fabric surface and bending behaviour. PET is a semicrystalline polyester with a terephthalate ester repeat unit, nylon is a semicrystalline polyamide with interchain hydrogen bonding, and PP is a semicrystalline polyolefin with a methyl side group on alternating backbone carbons. These structural differences produce measurable gaps in moisture regain, melt rheology, fibre tenacity retention after recycling, and low-stress mechanical properties relevant to hand feel.

PET vs Nylon vs Polypropylene Fibre: Cost, Recovery and Hand Feel

The comparative decision is governed by three independent variables: feedstock and conversion cost per kilogram of fibre, the available recovery route under EN 15343:2007 traceability rules, and the Kawabata KES-FB low-stress mechanical signature of finished fabric. For PET, the reference fibre-grade intrinsic viscosity is typically 0.60–0.68 dL/g measured per ISO 1628-5; for nylon 6, relative viscosity is typically 2.4–2.8 in 96% sulfuric acid per ISO 307; for PP, the melt flow index is commonly 18–35 g/10 min per ISO 1133-1:2022 at 230 °C/2.16 kg. These specifications set spinning temperature and draw ratio boundaries.

Bulk fibre-grade spot prices for 2024 place bottle/flake-derived PET resin in the range of €0.95/kg to €1.45/kg, polyamide 6 fibre-grade between €2.10/kg and €2.85/kg, and polypropylene fibre-grade between €0.98/kg and €1.30/kg, with PA66 fibre-grade frequently €0.70/kg to €1.20/kg above PA6 due to adiponitrile and hexamethylene diamine supply constraints. Conversion cost differentials are driven by drying energy, spin pack life, and downstream draw/winding speeds. PET requires hot-air hopper drying at 160 °C for 4–6 h to reduce moisture below 50 ppm, while PA6 requires vacuum drying at 80 °C to below 0.15 wt% moisture; PP generally requires no pre-drying unless surface condensation is present. These requirements add roughly 0.04–0.06 kWh/kg for PET and 0.05–0.08 kWh/kg for PA6 depending on ambient relative humidity and hopper configuration.

On a twin-screw extruder with L/D ratio of 36:1 to 48:1 and gravimetric feeding, PET melt viscosity at 285 °C and 100 s⁻¹ typically ranges from 120 Pa·s to 250 Pa·s, PA6 at 260 °C from 80 Pa·s to 180 Pa·s, and PP at 230 °C from 60 Pa·s to 150 Pa·s. The lower viscosity of PP and PA6 permits higher spin-pump throughput, but PET's narrower processing window between crystallisation and degradation requires more precise melt temperature control; sustained residence above 295 °C leads to acetaldehyde generation above 10 ppm in bottle-grade feedstock, measured by headspace GC per ASTM F2013. For fibre extrusion, spinneret hole diameter is commonly 0.2–0.6 mm, with melt draw ratio selected according to crystallisation orientation. Nylon 6 exhibits die-swell and higher moisture sensitivity, whereas PP shows greater thermal-oxidative chain scission unless stabilised with hindered phenolic/phosphite packages at 0.05–0.15 wt%.

What Constrains Mechanical Recovery Yields in Post-Consumer PET Fibre?

Mechanical recycling of PET fibre into polyester staple is constrained by intrinsic viscosity loss during melt reprocessing, contamination with polyolefin or polyamide, and hydrolytic chain scission from residual moisture. Post-consumer bottle flake may contain PVC, EVOH, aluminium, and soluble contaminants that exceed the 50 ppm PVC threshold for fibre-grade melt filtration. Filtration through 20–40 µm screen packs on a single-screw extruder with vacuum venting removes many gels, but polyamide contamination above 0.5 wt% produces unmelted domains and filament breaks. Solid-state polycondensation at 205–215 °C under vacuum restores intrinsic viscosity to 0.70–0.80 dL/g for spun-dyed PET, while nylon 6 can be depolymerised to caprolactam by acid or steam hydrolysis, and PP is commonly recovered by washing, extrusion, and additive re-stabilisation. Recovery yields are typically 65–75% for mechanically recycled PET fibre under EN 15343:2007 controlled blending, 70–85% for PA6 depolymerisation depending on feedstock purity, and 75–85% for PP re-extrusion with stabilizer top-up. These values should be treated as process-specific; published data for mixed-fibre post-consumer waste streams is limited by municipality collection quality.

Fabric hand is measured under low-stress deformation using the Kawabata KES-FB system, which records bending rigidity, shear stiffness, surface friction, compression, and tensile elongation. PET fibre typically produces higher bending rigidity and lower moisture regain than nylon, resulting in a crisper hand unless texturising or microfibre deniers below 1 dtex are used. Nylon 6 with moisture regain of 3.5–4.5 wt% at 65% RH per ISO 62:2008 exhibits a softer, cooler hand and better drape recovery after laundering; PP with regain below 0.1 wt% can feel waxier and shows higher static charge accumulation, measurable as surface resistivity above 10¹³ Ω/sq per ASTM D257. Circular bend stiffness per ASTM D4032 is commonly 1.5–3.5 N for PP nonwovens, 2.0–4.5 N for PET spunbond, and 1.0–2.5 N for nylon knitted structures, depending on basis weight.

When Moisture Regain Drops Below 0.5 wt%, Static Dissipation and Drape Shift

Moisture regain below 0.5 wt% in PET and below 0.1 wt% in PP reduces fibre conductivity, shifting static decay time beyond 2 s at 50% RH per MIL-PRF-81705D or ASTM D257, unless external antistatic finishes are applied at 0.1–0.3% by weight. This creates operational boundaries in carding and needlepunching: speeds above 150 m/min on a card with a 1.2 m working width can generate static cling, web transfer faults, and needle loading in PP and PET, while PA6's higher moisture regain suppresses charge accumulation. Drape coefficient measured by Cusick drape tester per BS 5058:1973 or image analysis per ISO 9073-9 shifts upward when fibre bending modulus exceeds 2 GPa; PET filament at 3.3 dtex typically gives a higher drape coefficient than PA6 at equivalent denier and fabric tightness.

Drawing and heat-setting define the semicrystalline morphology and therefore hand. PET filament yarn is typically drawn at draw ratio 3.0–4.5 at 80–130 °C; PA6 is drawn at 2.5–4.0 at 20–60 °C; PP slit film or multifilament is drawn at 4.0–7.0 at 80–120 °C. The resulting crystallinity measured by density gradient column per ISO 1183-1:2019 or DSC per ISO 11357-3:2018 is 30–50% for PET, 20–35% for PA6, and 40–60% for PP in oriented fibres. These values determine dye uptake, dimensional stability, and residual shrinkage. PET requires disperse dyeing above 130 °C under pressure, PA6 can be acid or disperse dyed below 100 °C, and PP fibre is mass-pigmented because it lacks dye sites. This difference contributes to recovery economics: PET post-consumer fibre is more difficult to overdye than PA6, while PP nonwovens are often mechanically recycled into black or mixed-colour applications.

Pre-drying and masterbatch compatibility are decisive in fibre-grade PET and PA6. PET must not be co-extruded with polyamide-contaminated regrind above 0.25 wt% because melt-phase ester-amide exchange causes yellowing and viscosity instability; PET also degrades in contact with alkaline lubricants above pH 9 at temperatures above 60 °C. PA6 is incompatible with acidic spin finishes below pH 3 and exhibits post-spinning moisture absorption that changes package weight by 1.0–1.5% within 48 h at 65% RH. PP requires antioxidant re-stabilisation after any thermal cycle exceeding 200 °C; without re-stabilisation, melt flow index measured by ISO 1133-1:2022 can increase by more than 30% after three extrusions. These boundaries limit allowable regrind ratios to 15–25 wt% for PET fibre, 10–20 wt% for PA6 depending on relative viscosity, and 20–40 wt% for PP with stabilizer masterbatch.

Comparative Property Matrix for Fibre-Grade PET, PA6, and PP

Recovery Route Compliance and Feedstock Certification Boundaries

Recovery route selection is not governed solely by yield. Under ISO 14021:2016 self-declared environmental claims, recycled-content declarations require traceable mass balance; under ISO 22095:2020, chain-of-custody boundaries must define whether post-consumer or post-industrial material is allocated through batch segregation or controlled blending. Mechanical PET recycling typically operates with solid-state polycondensation to raise intrinsic viscosity, while depolymerisation of PA6 to caprolactam allows virgin-equivalent polymer claims only when caprolactam purity exceeds 99.5% and downstream polymerisation is controlled per ISO 307. PP mechanical recycling is constrained by oxidative degradation, so melt-flow shift and stabilizer depletion are monitored with ISO 1133-1:2022 and oxidative induction time per ISO 11357-6:2018.

Selection for hygiene, filtration, and automotive textile applications is constrained by the above recovery and hand-feel data. In needlepunched geotextiles, PET staple of 6.7 dtex and 60 mm length provides tensile strength per ISO 10319 above 15 kN/m at 250 g/m², while PP equivalents are specified where density below 0.91 g/cm³ reduces transport mass. In apparel linings, PA6 filament of 2.2 dtex is selected when Kawabata KES-FB bending rigidity below 0.08 cN·cm²/cm and shear stiffness below 0.7 N/m·deg are required. In automotive carpets, PET staple dominates due to lower cost per 10,000 km of abrasion resistance measured by Taber abrasion per ISO 5470-1, but PP needlefelt is preferred where chemical resistance to battery acid splashes is required. Processing of recycled PET into fibre below 1.7 dtex is limited when contaminant gel counts exceed 10 particles/g on a 20 µm screen test.