Extrusion coating of unbleached kraft cupstock with INZEA F16C on a single-flight screw line with 30:1 L/D ratio begins with resin drying to a moisture ceiling of 250 ppm. Desiccant hopper dryers with a dew point of -40 °C are operated at 80 °C for 4 h minimum, because PLA hydrolyzes rapidly above 300 ppm moisture and melt viscosity loss becomes visible as die-lip bead rupture. Melt temperature at the flat die is held within ±5 °C of the setpoint, typically between 200 °C and 220 °C, verified by an infrared melt probe. Lot-specific melt flow rate is verified under ISO 1133-1:2022 at 190 °C and 2.16 kg; melt temperature setpoints are adjusted when melt flow rate deviates more than ±2 g/10 min from the qualified value. A gear pump after the discharge zone reduces pressure fluctuation below 0.3 MPa across the die, which is required to maintain coating weight uniformity below ±1.5 g/m² at line speeds of 80–120 m/min. The die gap is set at 0.6–0.8 mm, the air gap is limited to 150–250 mm, and the chill roll is kept at 15–25 °C with a mirror-finish surface. Corona pre-treatment of the paper web to 42–48 dyn/cm immediately before the nip raises surface energy and reduces interfacial delamination during cup side-seam sealing. Heat seal initiation occurs in the range of 90–110 °C; seam strength is measured according to ASTM F88 on sealed strips conditioned at 23 °C and 50% RH. The PLA coating provides grease resistance for warm beverage service but is not a high moisture barrier; water vapour transmission through a 25 µm PLA film commonly exceeds 100 g/m²·day at 38 °C and 90% RH under ASTM F1249. Consequently, hot cup stock made with PLA extrusion coating is typically limited to service temperatures below 60 °C and is specified for filled-product distribution rather than prolonged steam-table exposure. Industrial compostability must be confirmed against EN 13432 or ASTM D6400; disintegration tests are conducted at 58 °C, with 50–60% moisture, for 12 weeks, and biodegradation of 90% organic carbon conversion must occur within 180 days. Food-contact status for the coated paper is evaluated under FDA 21 CFR 176.170 for aqueous and fatty food categories and under EU 10/2011 with migration testing specific to the coating weight and substrate.

What Limits Extrusion Curtain Stability on Clay-Coated Board Above 120 m/min?

Clay-coated board increases coating weight sensitivity because the mineral coating absorbs heat and changes the solidification point of the melt curtain. At line speeds above 120 m/min, PLA melt curtains exhibit neck-in and draw resonance earlier than LDPE due to lower elongational viscosity. Maintaining a die-to-chill-roll air gap below 200 mm reduces the time available for edge cooling; however, an air gap below 120 mm can increase web oxidation and gel formation. Slitting edge trim is typically set at 10–15 mm per side to remove necked edges. The die exit temperature must be held within 205–225 °C. A chill roll temperature of 18–25 °C is used for clay-coated board, but higher backing roll pressure of 3–5 N/mm linear force is required because clay-coated surfaces have lower compressibility than uncoated kraft. Pinhole density is assessed by TAPPI T437 and should remain below 2 holes/m² for liquid barrier performance; however, published data for INZEA F16C on specific clay-coated grades is limited, and runnability at the upper speed boundary must be validated on the target substrate. Corona treatment is adjusted to 44–50 dyn/cm because clay coatings can exhibit uneven surface energy after drying. The melt pump and internal deckle configuration must be calibrated for the narrower die width; otherwise transverse coating weight variation exceeds ±1.0 g/m².

Bakery Wrap Fold Retention and Moisture Vapour Conflict

For bakery wrap structures, INZEA F16C must satisfy dead-fold and fold retention without the plastic memory of LDPE. Fold retention is evaluated by repeated crease testing on a 20 µm coating. The dead-fold behaviour of PLA-coated paper is distinct from LDPE-coated paper; PLA has lower elastic recovery and can retain a crease after folding, but the coating may develop microcracks at the fold line when coating weight exceeds 25 g/m². Coating weight for bakery wrap is typically set at 12–18 g/m². The moisture vapour conflict is unresolved by PLA alone: sorption of water from fresh bread accelerates PLA hydrolysis on the inner surface, especially in closed bags at 4–10 °C refrigeration. Barrier properties are measured under ASTM E96 at 38 °C and 90% RH. PLA-coated paper generally shows higher water vapour transmission than LDPE-coated paper, which means shelf life for crisp bakery products is reduced unless a secondary outer wrap is used. Grease resistance is tested by ISO 16532 at 23 °C for 24 h; PLA coatings resist vegetable oil penetration when the coat weight is uniform and pinhole-free. Print adhesion on the coated surface requires corona treatment above 42 dyn/cm and water-based flexo inks with surface tension below 38 mN/m.

Blown-film conversion of INZEA F16C for pre-cut organic waste caddy liners uses an extruder with 28:1 to 30:1 L/D and a barrier screw optimized for PLA rather than polyethylene. Drying limits mirror paper coating: moisture must be below 250 ppm with desiccant air at -40 °C, otherwise bubble instability and hydrolytic viscosity loss produce gauge variation greater than ±8%. The die gap is set wider than for LDPE, generally 1.5–2.5 mm, because PLA has lower melt strength and requires lower drawdown. Blow-up ratio is held between 2.0:1 and 3.0:1; higher ratios create bubble flutter and reduce dart impact resistance. Frost line height is controlled at 2–4 die diameters to avoid excessive crystalline orientation. Melt temperature at the die is set between 180 °C and 200 °C, lower than extrusion coating because film blowing is more sensitive to thermal degradation. Film impact resistance is measured by ASTM D1709, tear resistance by ASTM D1922, and tensile properties by ASTM D882. Compostable waste bags made from this material are certified under EN 13432 and must disintegrate after 12 weeks under industrial composting conditions. The primary process bottleneck is bubble stability during ambient air humidity above 60% RH; exposed resin absorbs moisture quickly, and hopper residence time must not exceed 30 min when ambient humidity is high. Sealing on side-seal bag machines requires controlled jaw temperature between 110 °C and 130 °C with a dwell time of 0.3–0.8 s; seal strength measured by ASTM F88 declines above 140 °C due to film shrinkage and thinning at the seal interface.

If Coating Weight Falls Below 18 g/m² on Single-Face Board, Pinholing and Seal Integrity Diverge

Thin extrusion coatings below 18 g/m² shift failure mode from interfacial adhesion loss to pinhole-linked seal leakage. At 12 g/m², pinhole density can exceed 5 holes/m² on single-face board unless melt temperature is raised to 220–230 °C and the air gap is shortened to 100–150 mm. Higher melt temperature increases oxidation risk and can generate low-molecular-weight fractions that migrate to the coating surface and reduce heat seal strength. Seal strength measured according to ASTM F88 at 15 g/m² typically remains below 3 N/15 mm when pinhole density is above 2 holes/m². Coating weight uniformity is measured by beta-gauge scanning across the web and controlled within ±0.8 g/m². For single-face board with roughness above 3.5 µm Ra, a primer or pre-coating may be required to close surface voids. Published data for INZEA F16C at coat weights below 15 g/m² on high-roughness board is limited; pilot trials are required to establish the threshold pinhole defect rate for the specific substrate. Liquid barrier performance is evaluated by TAPPI T559 kit rating, and water absorption is measured by ISO 535 after 60 s contact. The process window for thin coatings is narrow because low coat weight reduces thermal mass, causing premature solidification at the chill roll and insufficient fibre penetration for mechanical adhesion.

At Recovered-Fiber Content Above 40%, Backing Roll Hardness and Melt Curtain Geometry Shift

Recycled paperboard introduces porosity, surface roughness, and variable caliper that disrupt melt curtain contact uniformity. When the recovered-fiber content exceeds 40%, the board surface is more compressible but less homogeneous; backing roll hardness is reduced to 70–80 Shore A to increase nip width and compensate for caliper variation. A lower hardness backing roll reduces channeling across the web but increases the risk of roll marking on the coated surface if the backing roll is not cleaned continuously. The melt curtain geometry must be adjusted with a longer air gap of 200–250 mm to allow molten polymer to penetrate surface voids before solidification. Preheating of the paperboard to 35–45 °C using infrared banks is required to reduce moisture flash and prevent steam blisters at the coating interface. Paperboard moisture content is controlled between 5% and 7%, measured by a near-infrared gauge before the nip. Low moisture below 4% reduces fibre flexibility and adhesion; high moisture above 8% creates steam at the hot polymer interface and generates pinholes. Adhesion is tested by TAPPI T540 using 25 mm strips conditioned at 23 °C and 50% RH. Compostability certification must account for the paperboard fraction as well as the coating; the entire structure is tested under EN 13432 or ASTM D6868 depending on the target market. For food-contact recycled board, the presence of mineral oil residues may require migration testing under EU 10/2011 and FDA 21 CFR 176.260, because PLA coatings can absorb lipophilic substances and transfer them to the food contact surface over time.