Polylactic Acid Fiber

Drying of semicrystalline PLA fibre grade containing 1.4 mol% D-lactide and a melt flow rate of 30 g/10 min at 210°C/2.16 kg per ISO 1133-1:2022 precedes meltblown conversion on a 30:1 L/D single-screw extruder with barrel temperature zones set at 180°C, 210°C, 230°C, and 235°C, followed by a 200-mesh screen changer. Residual moisture above 250 ppm hydrolyses the aliphatic ester linkages, lowering intrinsic viscosity below 0.45 dL/g and producing melt droplets rather than continuous filaments at the die. The meltblown die body is held at 235°C ± 2°C, spinneret orifices of 0.25 mm diameter are spaced at 30 holes/cm, and compressed air at 0.45 MPa and 250°C attenuates the extrudate to median fibre diameters of 2–5 μm at a die-to-collector distance of 180 mm. For a 20 g/m² PLA meltblown layer laminated between two spunbond PLA carrier layers for Type IIR medical face masks under EN 14683:2019, differential pressure measured according to EN 14683:2019 Annex C typically falls between 40 Pa/cm² and 60 Pa/cm². Submicron particle capture is not an inherent property of uncharged PLA meltblown media; without corona charging or hydrocharging, 0.3 μm particle filtration efficiency remains below 50% at a continuous airflow of 85 L/min. Inline electrostatic charging of the dry web raises initial efficiency above 80%, but charge decay accelerates above 50% RH, and sustained exposure at 40°C and 85% RH leads to surface hydrolysis, web embrittlement, and measurable tensile loss within 72 h. This filter configuration is therefore specified for low-moisture, single-shift disposable mask production rather than long-duration respirator cartridges.

What Limits Spunbond Line Speed When PLA Replaces Polypropylene?

A spunbond line configured for polypropylene cannot transfer directly to PLA without adjustment of quench air temperature and draw-jet pressure. PLA spunbond resin with nominal melt flow rate of 70–85 g/10 min at 210°C/2.16 kg per ISO 1133-1:2022 is extruded at 230–240°C through a spinneret with 0.35 mm orifices and 6,000 holes/m, then quenched with air at 18°C and 1.2 m/s before slot-jet attenuation at 3,500–5,000 m/min. Filament breaks increase above 5,500 m/min because PLA elongational viscosity is lower than isotactic polypropylene at equivalent melt temperature, and the window between melt fracture and capillary break is narrower than 15°C. Hydrophilic finishing is mandatory for hygiene applications: a sorbitan monooleate ethoxylate or phosphate ester spin finish at 0.3–0.6 wt% on fibre mass reduces liquid strike-through time to <3 s per ISO 9073-8:1995 and improves downstream carding and thermal bonding. The resulting spunbond nonwoven of 40 g/m² is tested for tensile strength using ASTM D5034-21, for tear propagation using ISO 9073-4, and for synthetic blood penetration using ISO 16604:2004 Class 2 at 1.75 kPa. Terminal product specifications for surgical gown reinforcement and isolation gown sleeves require a minimum machine-direction tensile strength of 45 N/50 mm and a hydrostatic head above 300 mm H₂O per ISO 811. Drying prior to extrusion is critical at 80°C for 4 h to below 250 ppm moisture; uncontrolled resin exposed to 60% RH in an open hopper can reach 2,000 ppm within 30 min, causing hydrolysative molecular weight loss and a drop in web tensile strength of more than 20% on a single production shift.

Staple Fibre Carding, Drafting, and Rotor Spinning Window

In staple fibre conversion, the processing window is defined by fibre-to-metal friction and heat generation in high-speed carding. A 1.7 dtex, 38 mm PLA staple fibre with crimp frequency of 12–16 crimps/25 mm and a tenacity of 32 cN/tex is processed on a flat-top card with cylinder speed 360 rpm, flat speed 120 mm/min, and licker-in speed 800 rpm. Drafting on a single-zone drawframe uses break draft of 1.7, total draft of 8.0, and top roller shore hardness 63 Shore A to limit fibre lapping. Roving twist is set at 60 turns/m; ring spinning of 30/1 Ne yarn uses a twist factor α_e of 3.4–3.8 and spindle speed 10,000–12,000 rpm. Yarn evenness measured by ISO 16549:2021 on a capacitance tester typically shows Uster CV_m of 11–13% for optimised PLA/cotton blends at 50/50 by mass. Dyeing is performed with disperse dyes at 110°C and pH controlled between 5.0 and 6.0 for 30 min; alkaline scouring above pH 8.0 at 100°C for more than 20 min causes surface pitting and tenacity loss exceeding 15%. Wash fastness is rated according to ISO 105-C06:2010 test A1S, with typical values for dark disperse dyeings of grade 4 or better on PLA-rich substrates. Terminal goods include workwear, athletic apparel, and sock yarns where mild industrial laundering below 60°C is specified.

On needlepunch lines producing biodegradable crop protection mats, PLA staple fibre is blended with jute or hemp at 30/70 by mass to balance initial tensile strength and soil degradation rate. The carded cross-lapped web is pre-needled at 120 punches/cm² and final-needled at 220 punches/cm² on boards fitted with 15,000 needles/m of 0.5 mm gauge. Basis weight ranges from 300 g/m² to 800 g/m², and tensile strength is measured according to ASTM D4595-17 wide-width strip method. A 500 g/m² PLA/hemp mat typically exhibits machine-direction tensile strength of 8–12 kN/m and elongation at break of 25–35%. In neutral to acidic soil at 25°C and 55% RH, aerobic biodegradation under ISO 17556:2019 proceeds through hydrolysis of the PLA component followed by microbial assimilation; visible loss of mechanical integrity occurs between 12 months and 24 months at 20°C soil temperature. Alkaline soils above pH 7.5 accelerate surface erosion and reduce service life below one growing season. The terminal product is used as a weed-suppressing mulch mat in perennial planting rows and as a temporary erosion-control blanket on slopes up to 2:1; installation must avoid dry summer air above 60°C surface temperature because PLA fibre shrinkage initiates at 65°C under zero load.

When PLA Bicomponent Sheath-Core Fibre Enters Thermobonded Hygiene Materials

When PLA bicomponent sheath-core fibre is calendered into a hygiene acquisition layer, the sheath polymer is a low-D-lactide copolymer with a melting point of 130°C and the core is a higher-melting PLA homopolymer at 160°C, coextruded at a 50/50 sheath/core ratio and a fibre fineness of 2.2 dtex. Carded webs at 18 g/m² are bonded on a two-roll calender at 135°C ± 3°C, nip pressure 60 N/mm, and dwell time 0.02 s, producing bond points that retain core orientation and prevent web embrittlement. Liquid strike-through time is measured by ISO 9073-13:2006 in repeated insult mode; the third insult strike-through time remains below 2.5 s when a migration-resistant surfactant finish is applied at 0.4 wt%. Cross-direction tensile strength at 18 g/m² is typically 4–6 N/25 mm per ISO 9073-3, and elongation at break is 25–35%. The moisture regain of PLA is 0.5%, which causes static build-up during carding above 40% RH unless antistatic finish is applied; surface resistivity before finishing is above 10¹² Ω and after finishing below 10⁹ Ω per ASTM D257-14. Terminal product is an acquisition distribution layer for baby diapers and adult incontinence pads, where PLA replaces polyethylene/polypropylene bicomponent fibre at up to 100% of the nonwoven but requires storage below 50°C to prevent thermal shrinkage of the low-melting sheath.

Compression moulding of carded, needled PLA/hemp mats uses PLA staple fibre as a melt-binder phase at 30 wt% with hemp fibre at 70 wt%. The needled mat is preheated in a contact oven at 190°C for 120 s, then transferred to a hydraulic press and compressed at 4 MPa for 60 s in a mould held at 25°C. Cooling below the PLA glass transition of 55–60°C occurs before demoulding to prevent springback. Flexural modulus measured according to ISO 14125 on a 2.5 mm thick trimmed plaque is typically 3.5–4.5 GPa, and tensile strength by ISO 527-4 is 25–35 MPa. Volatile organic compound emission is measured according to VDA 277:2017 by headspace gas chromatography; total VOC values above 50 μg C/g require post-moulding aeration at 60°C for 4 h before interior release. The terminal component is a door panel insert or rear parcel shelf substrate, but continuous service temperature must remain below 50°C to avoid creep and dimensional distortion. Moisture content of the hemp phase must be dried to 2 wt% or lower before moulding; higher moisture causes steam pockets, delamination at the PLA/hemp interface, and flexural modulus loss greater than 20%.

Hydrolytic Degradation Windows for Bioresorbable PLA Sutures

Melt-spun PLA monofilament for resorbable sutures is produced from medical-grade resin with residual monomer below 0.25 wt% and tin catalyst below 10 ppm. The dried resin is extruded at 190–210°C through a 0.5 mm capillary die, quenched in a water bath at 25°C, drawn in two stages to a total draw ratio of 4:1 at 80°C, and annealed under nitrogen at 100°C for 12 h. Straight tensile strength per ISO 527-1 on conditioned monofilament is typically 410 MPa with Young’s modulus of 3.2 GPa; knot pull strength is determined using USP <881> and is typically 55–65% of straight tensile strength due to brittle notch sensitivity. Hydrolysis is bulk-phase; tensile strength loss becomes measurable before mass loss. For a semicrystalline 0.35 mm monofilament in phosphate-buffered saline at pH 7.4 and 37°C, published in vitro data show that tensile strength retention falls below 50% after 12–20 weeks depending on initial crystallinity, while mass loss occurs later as oligomers diffuse from the fibre core. Sterilization by gamma irradiation above 25 kGy causes radical-induced chain scission and reduces tensile strength by 30–40%; ethylene oxide sterilization according to ISO 11135:2014 is preferred, with maximum residual ethylene oxide below 4 mg/m² and residual ethylene chlorohydrin below 9 mg/m² per ISO 10993-7:2008. Cytotoxicity is assessed with ISO 10993-5:2009 using L929 cells and must yield a grade of 0 or 1; skin sensitisation is evaluated according to ISO 10993-10:2021. Terminal formats include USP 3-0 and USP 4-0 monofilament sutures and knitted surgical meshes; clinical handling is limited by fibre stiffness and shape memory that resists flattening during knot tying.