Luminy® Polylactic Acid LX975 is a poly(L-lactic acid) (PLLA) injection-moulding grade supplied by TotalEnergies Corbion. The grade is designed for semi-crystalline parts in which elevated heat resistance and flexural stiffness are required after crystallisation. Unlike transparent PLA such as Luminy LX175, LX975 contains a nucleating system that reduces the characteristic cold-crystallisation temperature and increases spherulitic nucleation density during cooling or annealing. The final properties therefore depend on thermal history: parts formed in cold tools retain a largely amorphous state, while mould temperatures above 100 °C or post-moulding annealing produce a crystalline fraction sufficient to lift the heat deflection temperature. The product is supplied as cylindrical granules, and the manufacturer’s technical documentation identifies injection moulding as the principal conversion route.

Why Does Melt Handling Differ from Standard PLA Grades?

The melt rheology of LX975 places it in the medium-flow injection-moulding window. Under ISO 1133-1:2022 at 210 °C with a 2.16 kg load, the melt mass-flow rate is reported as 6 g/10 min. This value is lower than that of high-flow packaging grades and higher than that of extrusion grades, giving sufficient cavity-filling capability while preserving pack pressure. In practice, the melt is pseudoplastic: apparent viscosity decreases with increasing shear rate in a manner comparable to other linear PLA resins, but the nucleating package does not create a yield stress or thixotropic structure. On injection-moulding machines with 18:1 to 20:1 L/D barrier screws, the recommended melt temperature is 190–210 °C. Barrel settings above 230 °C are not recommended because lactide regeneration and chain scission become measurable within normal residence times.

Gate and runner design must account for rapid solidification. For direct sprue or tunnel gates, the gate diameter should be 60–80% of the wall thickness to allow packing before the gate freezes. In hot-runner systems, externally heated manifolds with streamlined channels are preferred over internally insulated designs that can retain stagnant melt. Typical injection pressure for wall sections of 1.2–2.5 mm is between 800 bar and 1,200 bar, with pack pressure held at 50–70% of injection pressure for 2–4 s to compensate volumetric shrinkage during crystallisation.

Predrying is mandatory before melt processing. A desiccant dryer set at 80 °C with a dew point below −40 °C reduces moisture to below 250 ppm by mass within 4–6 h, with moisture determined by Karl Fischer titration according to ISO 15512:2019. At ambient relative humidity above 60%, pellets can exceed 500 ppm moisture within 30–60 min; hoppers must be closed and supplied with dry air. In PLA systems, residual moisture above 500 ppm promotes hydrolytic chain scission, producing gas streaks and a measurable drop in melt viscosity. Practical experience on 120-tonne hydraulic injection presses with four-cavity tools shows that stable production requires hopper residence times less than 10 min when the plant relative humidity exceeds 70%.

Rheological and crystallisation kinetics interact directly in LX975. In a cold mould at 25 °C, the cooling rate through the crystallisation window between 100 °C and 80 °C is so rapid that crystallisation remains incomplete. In a hot mould at 100–110 °C, the part remains in the crystallisation window long enough for spherulites to form, but shrinkage increases and ejection can become problematic if the part is not sufficiently rigid. This processing conflict narrows the practical mould-temperature window to ±5 °C around the crystallisation optimum for some geometries. For a beaker with 1.2 mm wall thickness, a mould temperature of 100 °C may yield HDT-B values of 85–95 °C, while a drop to 95 °C can reduce HDT-B by 10–15 K because the part cools too quickly. This cliff-edge behaviour is common in nucleation-controlled PLA and should be mapped by differential scanning calorimetry and HDT-B testing on actual parts.

Measuring Crystallinity Gain Under Controlled Annealing

The amorphous glass transition of LX975 is observed near 55–60 °C by differential scanning calorimetry according to ISO 11357-2:2020. The melting endotherm maximum appears near 170–180 °C under ISO 11357-3:2018. As-moulded specimens produced in cold tools at 25–40 °C typically show HDT-B values of 55–65 °C under ISO 75-2:2013 method B at 0.45 MPa. Post-moulding annealing at 100–110 °C for 20–30 min in a recirculating air oven raises the HDT-B to 90–105 °C. This increase is attributed to crystallinity development and densification; the test itself does not distinguish between crystallinity and stress relaxation, so the value must be paired with differential scanning calorimetry to verify crystalline fraction. Cooling rate during annealing should be controlled below 10 K/min to avoid thermal stress that can warp flat plaques.

Mechanical testing of LX975 is usually performed on injection-moulded ISO multipurpose test specimens after conditioning at 23 °C and 50% RH for at least 40 h according to ISO 291:2008. The values in the table are representative of annealed specimens; cold-tool specimens will fall in the lower portion of the stated ranges.

The notched impact values are low in absolute terms, which limits LX975 in snap-fit closures and high-strain demoulding features without radiused transitions. Sharp internal corners below 0.5 mm radius are known stress concentrators and should be avoided in mould design.

LX975 is normally introduced directly from pellet into the injection-moulding machine without compounding. If masterbatch addition is required, the carrier resin should be PLA-compatible; ethylene-based carriers above 2 wt% can produce visible delamination because of incompatibility at the interface. In twin-screw compounding trials on a 25 mm corotating extruder with L/D 40, PLA-compatible masterbatches dispersed well at barrel temperatures between 180 °C and 200 °C, whereas polyethylene-based masterbatches generated melt-pressure variation exceeding 15% and surface defects. Therefore, masterbatch suppliers should confirm PLA carrier chemistry before use. The addition of mineral fillers is not required for stiffness because the base resin is already high-modulus; filler above 10 wt% raises viscosity and reduces impact properties without proportional thermal gain.

If the Melt Residence Time Exceeds Fifteen Minutes

LX975 is susceptible to thermal degradation when the melt residence time in the barrel exceeds 15 min at 210 °C. The failure mode is not abrupt but manifests as a progressive reduction in melt viscosity and yellowing. In a shut-down or interruption, the barrel temperature should be reduced to 150 °C or the screw should be purged with a recommended purging compound before a restart. Machines with accumulators that hold melt at processing temperature for longer than 5 min require shot-size discipline. If a hot-runner system is used, the manifold residence time must be added to the barrel residence time; a hot runner volume larger than 30–40% of the shot volume frequently produces degradation in the runner vestige and can contaminate subsequent cavities. The maximum processing temperature of 230 °C should not be interpreted as a target; it is an upper excursion limit for short residence times below 5 min.

LX975 should not be melt-compounded with unneutralized acidic additives or exposed to polyethylene terephthalate processing temperatures above 240 °C. The polyester backbone is susceptible to hydrolysis; therefore, regrind should be dried under the same conditions as virgin pellets and limited to 20–30% by mass when dimensional tolerances are critical. Interaction with certain colorant masterbatches can shift crystallisation kinetics; masterbatches based on linear polyesters are preferred over polyethylene-based carriers to avoid phase separation. If volatile organic lubricants are used, they must be selected for compatibility with PLA and should not generate formaldehyde-releasing species at processing temperatures.

Regulatory Compliance Matrix and Food-Contact Conditions

LX975 is covered by the manufacturer’s compliance documentation for European food-contact regulation and United States food-contact prerequisites. Final compliance is condition-of-use dependent and must be assessed on the finished article. The following matrix summarises typical starting points; it does not replace a migration study for the specific part geometry and fill temperature.

In industrial composting, LX975 is expected to hydrolyse and mineralise under thermophilic conditions with sufficient moisture and oxygen. However, the rate depends on thickness; parts thicker than 3 mm may not fully disintegrate within the 12-week biodegradation window specified under EN 13432:2000. Anaerobic landfill conditions or home composting are not equivalent process conditions and are not covered by the industrial compostability certification.

In conversion trials on transparent and opaque Luminy grades, LX975 is selected when the part is exposed to hot beverages, heated oil, or forced-air dishwashing. The difference relative to LX175 is primarily thermal: LX175 remains transparent and exhibits lower HDT-B under identical annealing protocols, while LX975 sacrifices transparency for crystallinity. The difference relative to LX575 is more subtle and often appears as higher flexural modulus and slightly higher HDT-B in annealed specimens; both grades can be considered for hot-fill tableware, but LX975 is preferred when mechanical stiffness is the deciding requirement. In comparison with impact-modified PLA compounds, LX975 has higher modulus but lower elongation at break, making it unsuitable for snap-fit lids that require repeated flexure. All comparative statements should be verified on production tools because crystallinity development in thick sections is slower than in thin sections, and the effective thermal history of a moulded article is geometry-dependent. Published data for complex three-dimensional articles is limited; testing on flat plaques does not fully predict part performance under asymmetric loads.