Ingeo™ Biopolymer 6400D Continuous Filament Fiber-Grade PLA is a poly(L-lactic acid) (PLLA) extrusion resin supplied by NatureWorks LLC for melt-spinning continuous filament yarn, partially oriented yarn, and spunbond nonwoven fabric. The grade is specified with a melt flow rate of 15–30 g/10 min at 210 °C/2.16 kg when tested to ASTM D1238 or ISO 1133-1:2022, a specific gravity of 1.24 when measured by ASTM D792, and a glass transition temperature near 55–60 °C with a melting endotherm in the 150–170 °C range by ASTM D3418 differential scanning calorimetry. The D-lactide content is controlled below 1.5% by weight to preserve stereoregularity; this is lower than many general-purpose PLA film grades and supports stress-induced crystallization during drawing. Before processing, pellet moisture must be reduced to 250 ppm or less, verified by ASTM D6980 Karl Fischer titration, because molten PLA undergoes hydrolytic degradation at rates that increase sharply above 200 °C. Drying in a desiccant wheel dryer at 80 °C for 4–6 h with a supply dew point of −40 °C or lower is the standard preparation for continuous filament lines. Melt residence time at temperatures above 230 °C should not exceed 15 min, and the feed throat should be water-cooled to prevent bridging of softened pellets. These boundaries distinguish 6400D from lower-melt-flow PLA grades used in injection molding, which are not rheologically configured for high-speed filament attenuation.

Why Does Drying Control Dominate Melt-Spinning Performance?

Hydrolytic chain scission in PLA follows pseudo-first-order kinetics with respect to water concentration in the melt; at 250 ppm residual moisture, the number-average molecular weight can decline by 10–15% during a 10 min hold at 230 °C, resulting in a viscosity reduction that destabilizes the spinline. At 400 ppm moisture, filament breaks become frequent and extrudate may show surface roughness. Desiccant dryers are therefore configured for an air flow of at least 0.05 m³/min per kg/h throughput and an inlet air dew point below −40 °C. Return air temperature should be monitored; values below 65 °C indicate insufficient regeneration or overloaded desiccant beds. Hopper dryers without desiccant beds are not suitable because equilibrium moisture in PLA at 50% relative humidity is approximately 0.25–0.50% by weight, which is an order of magnitude above the melt-processing limit. In practice, multi-day production campaigns on single-screw extruders with 24:1 L/D have shown that moisture excursions above 300 ppm in regrind feedstocks produce measurable increases in spin pack pressure fluctuation and resulting denier variability. Processors should avoid blending more than 20% regrind without recalibrating drying time because regrind particle size reduces heat transfer and increases the effective diffusion path length.

Melt delivery for 6400D is carried out on single-screw extruders with 24:1 to 30:1 L/D and a barrier or double-flighted screw designed for low shear heating. The melt temperature profile is typically set from 190 °C at the feed section to 220–235 °C at the die, with adapters and spin beam maintained within ±2 °C to avoid local gel formation. A positive-displacement melt pump is required between the extruder and the spin pack to damp pressure pulsations; inlet pressure should be maintained at 70–120 bar and pump discharge pressure should not exceed 200 bar with standard spinneret packs. Filtration of 20–40 µm absolute rating is installed before the pump to remove char and gel bodies, and spin packs use breaker plates with 40–60 mesh screens. Spinneret capillaries of 0.25–0.50 mm diameter and 2:1 to 4:1 L/D are used for continuous filament yarns; smaller diameters favor higher draw-down but increase shear heating. Quench air is supplied at 15–25 °C and 60–80% relative humidity with a velocity of 0.3–0.6 m/s. Cross-flow quench cabinets with adjustable distribution plates are preferred over radial quench when filament counts exceed 144 because they reduce filament-to-filament temperature variation. The melt strength of 6400D allows spinline speeds up to 3,000–4,500 m/min for partially oriented yarn, while lower-D PLA grades may exhibit draw resonance above 2,500 m/min. During extended campaigns beyond 72 h, oligomer deposits can accumulate on the quench screen and spinneret face; cleaning cycles are typically scheduled every 48–72 h to maintain denier uniformity. Deposition is accelerated by melt temperatures above 240 °C and by high regrind content.

In false-twist texturing, 6400D partially oriented yarn can be textured at heater temperatures of 120–140 °C with a draw ratio of 1.6:1 to 2.2:1 and a D/Y ratio of 1.5–2.5. Textured yarns from this grade show crimp contraction values of 20–35% when tested by DIN 53840 or ISO 8160. The lower melting temperature relative to PET requires heater temperature reductions of 30–50 °C to avoid fusion at the spindle.

Draw Ratio Effects on Filament Tenacity and Shrinkage

When the as-spun 6400D filament is drawn at 2:1 to 4:1 over a heated godet set at 80–100 °C, the amorphous PLLA chains orient along the fiber axis and undergo strain-induced crystallization. The drawn yarn typically reaches tenacity values of 3.0–4.5 cN/dtex when tested by ASTM D3822 or ISO 5079:2020, with elongation at break of 20–40% and hot-air shrinkage at 100 °C for 10 min of 5–12%. Lower draw ratios produce lower tenacity and higher elongation, while draw ratios above 4:1 reduce elongation and increase the incidence of filament wraps on the draw rolls. The crystalline melting point shifts upward by 2–4 °C after full drawing, and the cold crystallization exotherm area decreases when measured by ASTM D3418. Heat-setting at 120–140 °C for 0.5–2.0 s under controlled tension increases the crystalline fraction and reduces hot-air shrinkage to 2–6%. This dimensional stability is critical for texturing and for downstream fabric finishing above 110 °C. Published fiber tensile data for 6400D at high draw ratios are limited; the ranges reported here derive from manufacturer processing guides for PLA continuous filament and must be confirmed with line-specific trials.

The principal differences between 6400D and other PLA processing grades are rheological and compositional. The melt flow band of 15–30 g/10 min is wider than high-viscosity fiber grades used for staple fiber, but narrower than high-flow injection molding grades. Injection molding grades typically have melt flow rates above 30 g/10 min and do not provide the melt strength required for spinline draw ratios of 2:1 to 4:1. Amorphous film grades with D-lactide above 4% show limited strain-induced crystallization and therefore lower fiber thermal stability. The additive package in 6400D is formulated for fiber contact surfaces and must be verified against the downstream article compliance checklist in this document.

When Higher Crystallinity Is Required for Dimensional Stability

In applications such as automotive interior textiles or hot-fill nonwoven filters where exposure exceeds 80 °C, the as-drawn 6400D fiber can be heat-set to increase crystallinity. The process is performed on a multi-stage drawing line with heated godets at 100–140 °C and a relaxation stage of 5–10% between the final draw roll and the winder. The resulting crystal fraction, measured by DSC, is frequently reported in the 35–45% range, depending on draw ratio and residence time. This reduces boiling-water shrinkage to below 5% and raises the onset of thermal deformation to 120–140 °C. However, over-crystallization at temperatures above 150 °C causes embrittlement because large spherulites form at the expense of oriented amorphous tie chains. For this reason, the heat-setting tunnel temperature should not exceed 145 °C for continuous filament yarns with titer below 3 dtex.

Fiber finishes for 6400D should be selected from nonionic or weakly anionic lubricants with a pH between 6.0 and 7.5; strongly alkaline stock solutions above pH 9 attack the ester backbone and reduce fiber tensile strength during storage. Dyeing with disperse dyes is performed at 110 °C rather than the 130 °C used for polyester to limit hydrolysis; depth of shade may be lower than PET because of lower dye uptake. Washing fastness after reduction clearing can be evaluated by ISO 105-C06. Textile applications requiring high color yield usually require an additional heat-setting step before dyeing to prevent dimensional change.

Spunbond Process Conditions Diverge from Continuous Filament Yarn Spinning

When 6400D is used in spunbond nonwovens, the extruder and spin beam configuration differ from continuous filament yarn lines. The melt is distributed to spinnerets spanning the web width, and the filaments are attenuated by high-velocity air rather than mechanical godets. Cabin pressure is typically 10–30 kPa, and air temperature at the attenuator is held at 20–40 °C. Web formation speeds are commonly 100–400 m/min, but filament speed in the venturi can exceed 3,000 m/min. Bonding of the web is performed by calendering at roll temperatures of 120–150 °C and nip pressures of 50–90 N/mm. The lower quench air velocity relative to continuous filament yarn prevents excessive web loft collapse. The main operational boundary is that PLA webs cannot be bonded at the higher temperatures used for polypropylene; exceeding 160 °C produces sticking and holes in the web.

Compliance verification is required on the final article, not on the pellet alone. The following checklist identifies the standards commonly applied to PLA fiber products; supplier certificates should be obtained for each production lot.