TotalEnergies Corbion supplies Luminy L175 High Heat High Viscosity PLA Homopolymer as a poly(L-lactide) extrusion-grade resin. The grade is polymerized with a low D-lactide content, which supports a melt endotherm near 175°C and differentiates it from lower-optical-purity PLA copolymers that melt below 160°C. The high-viscosity designation is controlled by melt mass-flow rate rather than by direct molecular weight specification. When tested at 210°C under a 2.16 kg load in accordance with ISO 1133-1:2022, the supplier-documented typical value is 4 g/10 min, with lot-specific acceptance ranges applied. This melt mass-flow rate is lower than conventional PLA injection-molding grades and is intended for sheet, film, profile, and blow-molding processes that require high melt strength. The homopolymer is also characterized by a density of 1.24 g/cm³ under ISO 1183-1:2019, a glass transition near 60°C, and a crystalline melting peak at 175°C under ISO 11357-3:2018. Residual monomer, moisture, and viscosity stability are monitored on each lot. Because the polymer is hygroscopic, converter handling must include closed desiccant drying before melt processing. The product is used in thick-gauge thermoformed trays, rigid sheet, oriented films, and other applications where thermal resistance after crystallization is required.

What Melt Rheology Boundaries Govern L175 Extrusion and Thermoforming?

The melt processing band is constrained by two failure modes: cold-melt unmelt and thermal chain scission. At melt temperatures below 180°C, incomplete melting of high-optical-purity crystallites produces gel-like domains and raises back-pressure instability on a 30:1 L/D single-screw extruder. Above 230°C, lactide reformation and random chain scission accelerate molecular weight loss; the melt mass-flow rate increases detectably after 15 min of residence time at 220°C. The recommended barrel profile for flat-sheet extrusion is therefore 180°C, 200°C, 210°C, 215°C, 215°C from feed to die, with the die held at 210°C to 220°C. Actual setpoints vary with screw speed and screw design. A barrier screw with a compression ratio of 2.5:1 to 3.0:1 and a mixing section is used to avoid excessive shear heating. Back pressure at the breaker plate is typically maintained between 50 bar and 100 bar; higher pressures indicate screen blockage or insufficient melting.

Drying constitutes a critical pre-processing operation. PLAs are hydrolytically sensitive at processing temperature; moisture above 250 ppm reduces molecular weight and produces splay, bubbling, and lower melt strength. The resin is dried in desiccant dryers with a dew point of -40°C or lower. A drying temperature of 80°C for 4 h to 6 h is used for cold resin, with time extended at ambient relative humidity above 60%. The dryer hopper should be closed-loop and insulated; residence time at drying temperature beyond 24 h should be avoided because thermal exposure can cause yellowing and lactide outgassing.

Capillary rheometry shows shear-thinning behavior across 100 s⁻¹ to 1000 s⁻¹. The high-viscosity grade retains higher melt strength than a standard PLA injection grade, which is measured indirectly by sag resistance in thick sheet and by neck-in reduction in extrusion coating trials. This characteristic is the primary rheological difference from high-flow PLA grades: L175 resists draw-down collapse but also generates higher screw torque and die pressure.

Differential scanning calorimetry at 10°C/min heating rate records the glass transition at approximately 60°C, a cold-crystallization exotherm between 95°C and 120°C, and a melt endotherm at 175°C. Cooling from the melt at 10°C/min without a nucleating agent produces limited crystallinity; therefore, the high-heat designation is realized only after a crystallization step. In thick-gauge thermoforming, sheet is heated to 95°C to 110°C for annealing before forming, which induces spherulitic growth and raises service temperature. The low D-lactide content allows crystallite formation with a higher equilibrium melting point than lower-optical-purity PLA grades. This difference explains why L175 can be annealed to a higher heat-distortion plateau than high-D PLA copolymers with a melt endotherm below 160°C.

Annealed Heat-Distortion Response and Nucleated Crystallization

The heat-distortion temperature of as-molded PLA homopolymer is limited by the glass transition and rarely exceeds 55°C at 0.45 MPa. In annealed or nucleated formulations, the crystalline phase maintains stiffness above the glass transition. L175 processed with a nucleating masterbatch and annealed at 100°C to 120°C for 30 min to 60 min can exhibit a heat-distortion temperature of 90°C to 150°C under ISO 75-2:2013 method B, depending on part thickness, degree of crystallinity, and annealing uniformity. The actual response is controlled by the final crystallinity measured by differential scanning calorimetry or density correlation. Thin-gauge parts crystallize faster; thick sections develop crystallinity gradients because of slow heat transfer. In extruded sheet, edge-to-center crystallinity variation can cause distortion during post-annealing.

The crystallization half-time for high-optical-purity PLA is shortest at approximately 105°C; at this temperature, isothermal DSC data show half-times in the range of 2 min to 5 min for nucleated formulations. Unnucleated L175 is slower, with half-times exceeding 10 min. The use of talc, poly(D-lactide) stereocomplex nucleants, or sulfonate nucleants modifies kinetics. Combination with amine-based additives should be avoided because residual alkaline species can catalyze ester hydrolysis at melt temperature and degrade molecular weight.

When L175 replaces a lower-viscosity PLA homopolymer in corotating twin-screw extrusion, screw torque and melt pressure are expected to rise. A corotating twin-screw extruder with 30:1 L/D and a vacuum vent at the penultimate barrel is used for compounding. The high-viscosity grade transfers more viscous dissipation into the melt, so screw speed should be reduced by 10% to 20% relative to a high-flow PLA grade until die pressure stabilizes. The grade does not require a compatibilizer in homopolymer applications; however, when blended with polybutylene succinate or polycaprolactone, a reactive chain extender or peroxide masterbatch is added only under controlled shear to avoid excessive gel formation. The high melt strength supports blown-film bubble stability and sheet production, but it reduces molded flow length in thin-wall injection. Therefore, L175 is not specified for injection molding of parts with wall thickness below 1.0 mm unless a hot runner with high gate shear is used.

If a Converter Switches from General-Purpose PLA to L175 in Flat-Sheet Extrusion

The substitution alters the extrusion profile, thermoforming window, and sheet crystallinity. General-purpose PLA grades with higher melt flow index require lower melt temperature and less drying; L175 demands a narrower melt temperature band and stricter moisture control. The flat-sheet line should be fitted with a horizontal three-roll stack. Roll temperatures are set in three zones: 40°C, 50°C, and 30°C from top to bottom for a 0.8 mm sheet. A lower roll temperature freezes the surface and reduces blocking; a higher middle roll promotes release. The die gap is set 10% to 15% wider than the target sheet thickness because the high-viscosity melt exhibits higher die swell than low-viscosity PLA. Edge trim is reground at up to 20 wt% with virgin resin, provided the regrind has been dried to below 250 ppm moisture and the particle size is uniform. Higher regrind levels reduce melt strength and increase gel formation.

Thermoforming of L175 sheet requires plug assist for deep-draw parts because the high-viscosity material resists uniform stretching. The sheet surface temperature is brought to 95°C to 110°C and held for 20 s to 40 s before forming. At temperatures below 90°C, the sheet tears; above 120°C, it develops surface stickiness and loses orientation. The mold temperature is set at 90°C to 110°C to induce crystallinity during contact. Cycle time increases relative to amorphous PLA because crystallization must be completed in the mold; ejection before full crystallization causes warpage after demolding.

Regulatory documentation for Luminy L175 addresses food-contact use under relevant EU Regulation (EU) No 10/2011 and U.S. FDA food-additive clearances for poly(lactic acid) where applicable. Converters must verify migration limits against the intended food simulant and end-use time/temperature conditions because final compliance is formulation- and process-dependent. The resin is not a drop-in replacement for petroleum-based high-heat polymers without revalidation of drying, screw configuration, and annealing. Published data for this specific configuration in high-viscosity Luminy grades is limited for some niche applications; pilot-line validation is advised before production-scale commitment. The grade should not be combined with amine-based processing aids, unneutralized acidic fillers, or high-moisture regrind streams, because such combinations accelerate hydrolytic degradation and reduce molecular weight stability during extrusion.