Ingeo™ Biopolymer 7032D Heat Seal Thermoforming PLA is a semicrystalline polylactide resin designed for the sealant layer in coextruded rigid sheet and thermoformed packaging. The product is supplied as cylindrical pellets with a nominal density of 1.24 g/cm³ when evaluated under ASTM D792-20, and its melt flow is controlled for sheet extrusion rather than injection molding. Typical melt mass-flow rate values for commercial heat seal PLA grades are reported in the range of 5 g/10 min to 15 g/10 min at 210 °C and 2.16 kg under ISO 1133-1:2022; the grade-specific certificate of analysis remains the controlling document. The material is intended for cups, trays, clamshells, and lidding-sealed dairy containers where a lower seal initiation temperature reduces dwell time on form-fill-seal equipment and broadens the operating window against polymeric or coated lidding membranes.

The principal molecular distinction between 7032D and general-purpose extrusion PLA is a controlled D-lactide content that suppresses crystallinity and shifts the seal initiation threshold downward. In PLA, increasing the D-isomer fraction decreases the equilibrium melting point and retards spherulitic crystallization, so the sealant layer can flow and coalesce at jaw temperatures lower than those required for structural PLA core layers. Differential scanning calorimetry according to ASTM D3418-21 or ISO 11357-3:2018 is used to verify glass transition, cold crystallization, and melting endotherm positions. For semicrystalline PLA heat seal grades, the glass transition is typically observed between 55 °C and 60 °C, while melting endotherms are commonly detected in the 145 °C to 165 °C interval; product-specific thermograms should be obtained from the supplier.

Because the grade is normally coextruded as a thin cap layer over a higher-stiffness PLA core or over a non-PLA structural layer, sealant layer thickness is commonly maintained between 5% and 15% of total sheet thickness. At those thicknesses, the sealant resin contributes sufficient surface amorphous content for fusion while avoiding domination of the flexural modulus of the formed part. Production-scale coextruders fitted with barrier screws having an L/D ratio of 24:1 to 30:1 are typically used, and die lip adjustments are set to maintain a uniform cap layer because layer-to-layer variation above ±10% can produce erratic heat seal strength in finished containers.

What Limits Heat Seal Initiation and Hot-Tack Performance in 7032D-Based Structures?

Seal initiation temperature is not a single resin property; it is a combined result of resin composition, sealant layer thickness, jaw pressure, dwell time, and lidding film coating chemistry. The most common tensile-peel evaluation is conducted under ASTM F88/F88M-21, in which a 25.4 mm or 15 mm wide specimen is peeled at a defined separation rate, commonly 200 mm/min to 300 mm/min. Heat sealability curves are generated under ASTM F2029-16 following seal dwell times of 0.5 s to 1.0 s and jaw pressures of 2 bar to 4 bar; the reported value is the temperature at which peel force crosses a defined threshold, often 0.4 N/mm to 1.0 N/mm depending on specification. For heat seal PLA grades, the target seal initiation band generally lies between 80 °C and 100 °C, whereas a general-purpose extrusion PLA may not initiate an acceptable seal until the interface reaches 110 °C or higher. Published data for 7032D in every lidding film configuration is limited; converter trials using the specific lidding structure are required.

Hot-tack evaluation under ASTM F1921/F1921M-12(2018) measures the seal’s resistance to failure while the sealant is still molten. This property is critical on vertical form-fill-seal lines where product loading occurs immediately after sealing. Heat seal PLA grades achieve a useful hot-tack plateau by balancing D-lactide content and melt viscosity. If the D-isomer content is too high, hot-tack strength collapses because the molten film lacks cohesive strength; if it is too low, seal initiation temperature rises and the seal window narrows. The 7032D grade is therefore controlled within a narrower D-lactide tolerance than standard extrusion PLA so that both seal initiation and hot-tack remain stable across batch-to-batch conversion. The controlling analytical method is typically high-performance liquid chromatography or polarimetry after alkaline depolymerization, with the D-lactic acid fraction reported on a percent basis.

Before sheet extrusion, the resin must be dried in a desiccant dryer with a supply-air dew point of -40 °C or lower and an air volume sufficient to maintain pellet bed temperature at 70 °C to 80 °C for 3 h to 4 h. Target final moisture content is below 250 ppm (0.025%) by Karl Fischer titration. Hydrolytic degradation during melt processing is the dominant failure mode when drying is skipped. At typical melt temperatures, residual moisture above 0.05% reduces molecular weight, lowers melt viscosity, and produces edge tear or pinhole defects in formed parts. On a single-screw extruder with a 30:1 L/D barrier screw, the melt temperature measured at the die should be maintained between 190 °C and 210 °C for this grade unless the supplier’s grade-specific processing guide provides a different upper limit. Melt temperatures exceeding 220 °C increase lactide formation and may cause yellowing or seal strength loss.

When process air humidity exceeds 60% RH, pellet storage after drying should be in sealed moisture-barrier packaging, because PLA re-absorbs moisture quickly and can exceed 0.05% within 30 min in uncontrolled ambient conditions. Hydrolytic degradation is autocatalytic because carboxylic acid end groups generated by chain scission accelerate further hydrolysis. Extruders running 7032D should be purged with a viscosity-matched PLA purge before introducing the heat seal grade, and barrel residence time should be kept below 5 min at melt temperature above 200 °C. Screw speed should be selected to limit specific energy below approximately 0.25 kWh/kg, because excessive shear heating can raise melt temperature beyond the set barrel profile and produce lactide fuming. The hydrolysis kinetics of PLA are measurable by parallel-plate rheometry in the linear viscoelastic region, with complex viscosity followed at 210 °C for 30 min under nitrogen; a drop in complex viscosity exceeding 20% indicates moisture-induced degradation or excessive thermal history.

The resin is not intended as the primary layer in cast film or biaxially oriented film operations; its melt rheology is optimized for sheet extrusion, polishing roll contact, and downstream thermoforming. When coextruding 7032D as a cap layer, die temperatures should be profiled to avoid stagnant melt at the edge of the manifold. Polished roll temperatures in the range of 40 °C to 60 °C are typical for PLA sheet. Higher roll temperatures can accelerate cold crystallization and increase sheet haze, while lower roll temperatures can create residual stress that later manifests as corner cracking during plug-assist thermoforming.

When Coextruded Sheet Replaces Laminated Sealant Films in Dairy and Produce Packaging

In dairy packaging lines, cups formed from 7032D-capped sheet are sealed against coated paperboard, PET/aluminum foil, or PLA-based lidding films. The sealant layer is intended to reduce dependence on lacquer or extrusion-coated lidding sealants because the cup flange itself participates in fusion. Actual seal strength depends on flange flatness, seal bar temperature profile, and anvil pressure. Production lines often operate with seal bar temperatures between 90 °C and 130 °C and dwell times from 0.6 s to 1.8 s. For films with low-temperature heat-seal coatings, the lower end of this range may be used; for aluminum foil lidding without a low-melt coating, the upper end may be needed. Dwell time and pressure should be mapped against peel force per ASTM F88 to avoid a false seal where surface tack is present but cohesive bond depth is insufficient.

A key operational boundary is the thermoforming sheet surface temperature. For plug-assist pressure forming, sheet surface temperatures between 85 °C and 110 °C are typical, with aluminum tooling maintained at 25 °C to 45 °C. Above the upper temperature, PLA sheet can sag and thin unpredictably; below the lower temperature, stress whitening and microcracking can occur. Tool design must account for PLA’s relatively low elongation at break and notch sensitivity. Unlike amorphous PETG, semicrystalline PLA exhibits a sharper transition from rigid to extensible; therefore, plug speed and plug temperature must be tuned to avoid contact-webbing in square or rectangular cavities with draw ratios above 2:1.

Compared with a higher-crystallinity biaxially oriented PLA film grade, 7032D is not supplied for high-orientation processes where strain-hardening and low D-lactide crystallinity are required for blown film or tenter-frame film. Compared with a general-purpose extrusion grade, 7032D provides a lower seal initiation temperature, but its lower crystallinity in the formed part may reduce the upper-use temperature of the sealant side. The material’s flexural modulus and heat deflection temperature are not the primary selection criteria for a cap layer; the structural design should rely on a higher-stiffness core layer, with 7032D limited to the sealing surface. Published comparative data for all mechanical properties in the same tool geometry is limited; selections should be based on application-specific coextrusion pilot trials.

Comparative Seal Performance Framework Across PLA Grade Classes

Regulatory compliance is application-dependent and must be confirmed against current supplier documentation. The following standard designations are used in qualifying PLA for food-contact thermoformed packaging.

Heat sealing is a molecular diffusion process across the interface. The sealant surface must reach a temperature above its melting onset but below its degradation threshold. Under jaw pressure, polymer chains at the interface interdiffuse, and on cooling they either crystallize partially or vitrify. For PLA, seal strength depends on whether the lidding film coating and 7032D layer form a miscible or compatible interface. Seal strength measured after 24 h aging can differ from immediate seal strength because secondary crystallization in PLA can embrittle the seal. Testing should therefore include both immediate and aged peel force measurements, especially for refrigerated distribution. Package integrity can be checked by bubble leak testing under ASTM F2096; however, that method is not specific to the sealant resin formulation.

The sealant layer should not be blended with PVC, PVDC, or recycled PLA from unknown sources. Chlorinated polymer contamination can release hydrogen chloride during melt processing, which autocatalyzes PLA hydrolysis and produces surface defects. High-pH cleaning agents should not be applied to formed cups before seal integrity testing, because alkaline hydrolysis can reduce seal peel force and confound data. D-lactic acid assay should be monitored per batch because shifts in D-isomer content can move seal initiation temperature by several degrees Celsius. Incorporation of in-house regrind is possible up to 20 wt% to 30 wt% when the regrind is dry and generated from identical coextruded sheet, but higher regrind levels raise crystallinity due to repeated heat history and may narrow the thermoforming window. Regrind must be ground to a uniform granule size and passed through a metal separator to protect screw and die. The melt flow rate of reclaimed material should be checked under ISO 1133-1:2022 at 210 °C; a shift of more than 2 g/10 min relative to virgin resin suggests hydrolysis and warrants troubleshooting of drying or screw temperature.

Film lidding incompatibility appears when the coating chemistry requires seal temperatures above 150 °C; such films can damage the PLA flange before an adequate seal is formed. Sealing 7032D caps to uncoated aluminum foil typically requires surface temperatures at the upper end of the forming window, which can cause flange softening, thinning, and false seal failure. Converter trials should include peel-force mapping across seal bar temperature gradients, flange thickness, and dwell time. Shelf-life studies under refrigerated distribution at 2 °C to 8 °C are generally used to confirm seal integrity and caustic-stress-cracking performance; published data for this specific configuration is limited and must be generated for each package.