Within high-speed extrusion coating lines running low-density polyethylene at melt temperatures of 315 °C to 330 °C, oxidative chain scission and macroradical recombination compete as parallel degradation routes. The base polymer is a low-density polyethylene grade with a melt index of 7.5 g/10 min when measured per ASTM D1238-20 at 190 °C/2.16 kg and a density of 0.918 g/cm³ per ISO 1183-1:2019. In a single-screw extruder with a 120 mm screw diameter, 33:1 L/D, barrier flight, and Maddock shear mixer, the residence time distribution under 850 kg/h throughput typically has a mean residence time near 90 s and a tail extending beyond 240 s; the tail fraction is the main contributor to gel formation because local oxygen depletion converts peroxy radicals into carbon-centered radicals that terminate by combination. Oxidation begins with homolytic scission of carbon-carbon and carbon-hydrogen bonds at 325 °C, generating primary alkyl radicals. Oxygen dissolved in the melt at the feed throat and oxygen entrained at the screw tip react with these radicals at diffusion-limited rates to produce peroxy radicals. Hydrogen abstraction from tertiary carbon atoms in the branched LDPE backbone yields hydroperoxides. The hydroperoxides decompose with an activation energy of 90–120 kJ/mol in non-isothermal kinetic analysis per ASTM E1641-18 and liberate alkoxy and hydroxyl radicals. Beta-scission of alkoxy radicals forms chain-end aldehydes and internal ketones, while disproportionation of secondary alkyl radicals produces vinylidene unsaturation. Carbonyl absorption at 1715 cm⁻¹ in attenuated total reflectance FTIR spectra increases relative to the 1465 cm⁻¹ C–H bending reference; a carbonyl index above 0.25 for a 20 g/m² coating is generally associated with reduced heat-seal strength and odor transfer in dairy packaging. Low-molecular-weight oxidized fragments migrate to the melt surface during the 180–250 mm air gap and create a weak boundary layer at the paperboard interface. Adhesion to the clay-coated liquid packaging board then becomes dependent on the removal of this oxidized layer by the melt curtain impingement onto the substrate and on the penetration of molten polymer into the paperboard pore structure before the chill roll quenches the interface. The quench temperature of 12–18 °C freezes the adhesive interface and preserves the concentration of oxidized species. In a coextruded structure of LDPE/tie/EVOH/tie/LDPE with a total coating weight of 25 g/m², the outer LDPE layer is the oxygen-exposed surface; its degraded boundary layer can reduce the oxygen barrier of the adjacent EVOH by generating volatile carbonyl compounds that condense between layers. Compliance for food contact is evaluated under FDA 21 CFR 177.1520 and Regulation (EU) No 10/2011 with an overall migration limit of 10 mg/dm². Table 1 lists the principal degradation indicators used on production lines.

How Does Melt Temperature Influence Lactone and Aldehyde Evolution in LDPE Extrusion Coating?

The rate of lactone and aldehyde evolution in LDPE extrusion coating follows an Arrhenius dependence on melt temperature in the range 290 °C to 340 °C, but the response is not linear. At 290 °C, hydroperoxide formation is limited and adhesion to clay-coated board is poor because the melt viscosity exceeds 120 Pa·s at 100 s⁻¹ when measured by parallel-plate rheometry per ISO 6721-10:2021, preventing rapid penetration into the substrate pore structure. At 340 °C, chain scission accelerates and the concentration of low-molecular-weight aldehydes increases sharply, causing detectable odor in the finished package at coating levels above 15 ppm total volatile carbonyls as determined by headspace gas chromatography. The production window for a conventional LDPE extrusion coating grade with melt index 7.5 g/10 min is therefore ≤ ±5 °C around a set point of 315–320 °C. Barrel zones are typically profiled from 180 °C at the feed throat to 320 °C at the adapter, with the die maintained at 315 °C. At the higher end of the window, lactone formation becomes measurable as absorbance at 1780 cm⁻¹. Lactones are cyclic esters formed by intramolecular esterification of hydroperoxide-derived carboxylic acids with adjacent hydroxyl groups. Their concentration is low but increases with the third power of oxygen partial pressure in the vent port and air gap. In contrast, the concentration of aliphatic aldehydes such as nonanal and decanal is controlled by hydrogen abstraction followed by beta-scission at tertiary alkoxy radicals. Production-scale sensory failure is generally observed when the aldehyde concentration in the coating exceeds 0.5 µg/dm² in the 55–65 °C headspace of filled aseptic packages. The analytical sequence includes purge-and-trap gas chromatography with flame ionization detection and confirmation by mass spectrometry, following DIN 10955 sensory testing of packaging materials.

Simultaneously, oxidative degradation in coextruded tie layers and ethylene-acrylic acid copolymers is initiated by residual hydroperoxides in the paperboard fiber and by corona pre-treatment; the resulting carboxylic acid functionality accelerates acid-catalyzed chain scission and corrodes chrome-plated screw surfaces. Ethylene-acrylic acid tie resins with acrylic acid comonomer contents of 6.5–9.7 wt% and melt indices of 5.0 g/10 min at 190 °C/2.16 kg are particularly sensitive to moisture released from the paperboard during extrusion. The acid groups catalyze esterification and transesterification reactions with oxidized LDPE fragments, forming low-molecular-weight ester oligomers that migrate to the food-contact surface. This degradation pathway is distinct from free-radical oxidation and can proceed even when the melt temperature remains below 280 °C. The specific migration limit for acrylic acid under Regulation (EU) No 10/2011 Annex I Table 1 is 6 mg/kg food simulant. In production, tie-layer degradation is monitored by Fourier transform infrared spectroscopy of the carbonyl ester band at 1740 cm⁻¹ and by melt flow rate shift per ISO 1133-1:2022. An increase in melt flow rate above 25% relative to the virgin tie resin indicates excessive chain scission and predicts delamination in the downstream converting process. The presence of acetic acid released from ethylene-vinyl alcohol copolymer layers under thermal load further accelerates corrosion of the adapter and die lip surfaces, producing iron carboxylates that are visible as brown specks in the coated board.

When Recycled LDPE Fractions Exceed 25 wt% in Coextruded Barrier Structures

At recycled LDPE fractions exceeding 25 wt%, contaminant polypropylene domains with melting points near 165 °C persist as discrete inclusions in the 315 °C LDPE melt because shear stress in the single-screw extruder is insufficient to disperse the higher-viscosity PP phase below the 5 µm size required for smooth 20 g/m² coatings. These inclusions act as stress concentrators during melt drawing and cause edge-tear propagation in the finished liquid packaging board. The gelatinous material formed from crosslinked oxidized LDPE also increases melt filtration pressure. On a 120 mm extruder equipped with a 200 mesh screen pack, the pressure rise across the screen pack is commonly observed to increase from 0.1 MPa/h for virgin LDPE to 0.3–0.8 MPa/h at recycled contents above 25 wt%, indicating rapid gel accumulation on the filtration media. The melt curtain neck-in per edge increases from 35 mm to 55 mm at the same recycled fraction because low-molecular-weight fractions produced by prior thermal histories reduce average melt elasticity. This neck-in instability narrows the usable coating width and increases edge trim generation. In coextruded structures, recycled LDPE is typically confined to the outer heat-seal layer to avoid direct contact with EVOH; however, volatile aldehydes and carboxylic acids from the recycled fraction can still migrate through the tie layer and attack the EVOH interlayer. The resulting EVOH phase separation produces optically visible haze above 8% when measured per ASTM D1003-21. Published data for this specific configuration is limited, but production trials indicate that gel count per ASTM D7310-20 exceeds 10 gels/m² at recycled fractions above 25 wt% unless additional melt filtration and antioxidant supplementation are used. The operational boundary is therefore set at 25 wt% for linear low-density and low-density polyethylene blends in high-speed extrusion coating of aseptic board. Above this threshold, the frequency of die lip plate-out events increases significantly, requiring line stoppages of 15–30 min for lip cleaning at intervals below 8 h. The degradation chemistry is compounded by paper fiber char from post-industrial board trim; char particles above 40 µm produce streak defects and reduce the wetting tension of the coated surface below 38 mN/m measured per ISO 8296.

Antioxidant depletion in aseptic board follows a dual mechanism: the primary hindered phenolic stabilizer donates hydrogen to peroxy radicals, while the phosphite secondary stabilizer reduces hydroperoxides to alcohols and is oxidized to phosphate. In a coextruded LDPE/tie/EVOH/tie/LDPE structure, the phenolic antioxidant concentration drops below 300 ppm after two extrusion heat histories when measured by high-performance liquid chromatography per ASTM D6953-18. Below this concentration, the oxidative induction time at 200 °C per ISO 11357-6:2018 declines from 22 min to 8 min, and the coating becomes vulnerable to autocatalytic oxidation during subsequent hot-air drying in printing and converting. The phosphite secondary antioxidant concentration falls to undetectable levels more rapidly because phosphite oxidation is stoichiometric with hydroperoxide concentration rather than catalytic. A combined stabilizer package of 800 ppm phenolic antioxidant and 1200 ppm phosphite antioxidant maintains a melt index shift below 12% after multiple passes. Increasing the phenolic antioxidant above 1500 ppm provides diminishing oxidative protection and raises the concentration of extractable degradation products, which can exceed the overall migration limit of 10 mg/dm² under Regulation (EU) No 10/2011 when tested with 10% ethanol simulant for 10 days at 40 °C. The operational boundary is that amine-based antioxidants are avoided in these structures because they cause discoloration when combined with EVOH and with epoxy-coated paperboard. Gel permeation chromatography of the aged LDPE layer shows a bimodal molecular weight distribution: a low-molecular-weight tail below 10,000 Da from chain scission and a high-molecular-weight shoulder above 500,000 Da from long-chain branching and crosslinking. This bimodality explains why melt index alone is an incomplete degradation indicator, because simultaneous scission and crosslinking can produce a cancelling effect on average molecular weight while severely degrading coating toughness and heat-seal strength.

Thermal Scission and Crosslinking in Maleic Anhydride-Grafted Tie Resins

Maleic anhydride-grafted linear low-density polyethylene tie resins with grafted maleic anhydride levels of 0.5–1.0 wt% and melt indices from 2.0 g/10 min to 6.0 g/10 min are used at layer thicknesses of 3–7 µm between LDPE and EVOH in coextruded liquid packaging board. The grafted anhydride groups are thermally labile above 290 °C and undergo ring-opening hydrolysis in the presence of moisture released from paperboard; the resulting dicarboxylic acid groups form intermolecular hydrogen bonds that increase zero-shear viscosity and promote gel formation. In a 90 mm, 30:1 L/D single-screw extruder running at 120 rpm, barrel temperatures above 310 °C cause the melt index to drop by 20–40% after 30 min residence. Crosslinking in the tie resin is attributed to esterification between opened anhydride rings and hydroxyl groups present in the EVOH melt interlayer; this reaction accelerates when EVOH moisture content exceeds 0.2 wt%. Pre-drying of EVOH at 80 °C for 4 h is mandatory when relative humidity exceeds 60% to prevent foam and acid-catalyzed degradation. Adhesion testing per ASTM D1876-08 on coextruded board typically shows peel strengths of 8–15 N/15 mm; below 5 N/15 mm, delamination occurs during converting. The degradation products include maleic acid, succinic acid, and low-molecular-weight anhydride esters that can migrate into liquid simulants and raise polar extractables. Under Regulation (EU) No 10/2011, maleic acid has a specific migration limit of 30 mg/kg food; production-scale extraction testing is performed with 3% acetic acid simulant under the allocated worst-case contact conditions.

During extrusion coating of liquid packaging board with a nominal coating weight of 20 g/m² and line speed of 400 m/min, the melt curtain enters a 200 mm air gap at 320 °C and oxidizes on both surfaces before contact with the paperboard and the chill roll. The surface oxidation creates a boundary layer of lower molecular weight polymer with higher carboxylic acid content; when this layer is not displaced by the impingement pressure and the paperboard surface energy is below 38 mN/m, measured by ISO 8296, the adhesion drops below target. Moisture in the paperboard at 5–8 wt% is flashed to steam during molten polymer contact and creates a vapor film that interferes with wetting; boards above 8 wt% moisture exhibit blisters and low peel adhesion. To retain liquid resistance, the coating is tested for water vapor transmission rate per ASTM F1249-20 at 38 °C and 90% RH and for oxygen transmission rate per ASTM F1927-20 at 23 °C and 50% RH. A 20 g/m² LDPE coating on paperboard typically produces a WVTR of 8–15 g/(m²·day) and an OTR of 1500–2500 cm³/(m²·day·atm), but oxidized coatings with carbonyl index above 0.30 exhibit higher transmission by 10–20% due to microcracking and polar diffusion paths. Liquid resistance is further assessed by TAPPI T 559 grease resistance and by contact angle measurements with test liquids; a contact angle below 70° for water on the coated surface indicates excessive polar oxidation. Process conditions are constrained by the need to avoid exposure times longer than 1.5 s in the air gap and to maintain the chill roll surface free of condensed oligomers. Production-scale failure modes include curl from differential thermal contraction, edge bead oxidation, and die lip plate-out from degraded polymer residues. The degradation pathways in the outer LDPE layer are therefore inseparable from the adhesion, barrier, and organoleptic performance of the final aseptic carton.