Polyethylene (PE) grades are converted in single-screw and twin-screw extrusion, injection moulding, blow moulding, and film casting at melt temperatures that normally range from 160 °C for low-density PE to 240 °C for high-density PE. When barrel wall temperature, screw friction, or die restriction carries the melt above the stabilizer-limited threshold, the condition described as Overheating Polyethylene: Degradation, Odour and Gel Formation develops. The primary processes are hydroperoxide decomposition, β-scission, free-radical recombination, volatile carbonyl release, and microgel aggregation. Melt flow rate is measured according to ISO 1133-1:2022 at 190 °C under 2.16 kg or 21.6 kg load. Oxidative stability is assessed by ISO 11357-6:2018 or ASTM D3895-19 oxidation induction time. The processing window is narrow; once the induction time is consumed, degradation accelerates sharply.

Production-scale blown-film lines with 30:1 L/D single-screw extruders are sensitive to overheating at the screw tip and die lip. A die zone set at 220 °C can generate local melt slugs above 250 °C when screw speed exceeds 120 rpm or when backpressure rises above 35 MPa. Gel counts on thin film may shift from a baseline below 2 gels/m² to more than 20 gels/m², measured by online optical surface inspection. Such gels are crosslinked polyethylene domains insoluble in hot xylene according to ASTM D2765-16. The same condition produces volatile aldehydes and ketones that mark the beginning of odour formation before gels become visible.

When the Melt Temperature Exceeds 240°C

Thermal oxidation in PE follows a free-radical chain. Alkyl radicals formed by heat and shear react with dissolved oxygen to form peroxy radicals; abstraction of hydrogen yields hydroperoxides, which decompose to alkoxy and hydroxyl radicals. β-scission then generates carbonyl compounds and terminal vinyl unsaturation, while radical recombination produces long-chain branching and crosslinks. Reported activation energies for polyethylene thermo-oxidation fall between 80 kJ/mol and 120 kJ/mol, implying that the degradation rate increases sharply over a 10 °C to 20 °C temperature excursion. Carbonyl accumulation is monitored by infrared absorbance at 1715 cm⁻¹ for ketones and aldehydes, and at 1740 cm⁻¹ for esters and lactones. Unsaturation increases at 909 cm⁻¹ for vinyl groups and 965 cm⁻¹ for trans-vinylene groups.

Stabilizer consumption is the controlling variable. A hindered phenolic primary antioxidant combined with a tris(2,4-di-tert-butylphenyl) phosphite secondary antioxidant is consumed as oxygen diffuses into the melt. Oxidation induction time measured by ASTM D3895-19 at 200 °C in oxygen declines below 5 min when phosphite protection is exhausted. Unstabilized PE can show values below 1 min. A melt-temperature excursion above 240 °C therefore cannot be tolerated if residence time extends beyond the measured induction period. The local melt temperature, not only the barrel set point, must be controlled at the screw tip and die entry.

An accumulator-head blow moulding line operating with HDPE at melt temperatures above 240 °C produces parison sag and odour complaints in containers. Headspace GC-MS of such containers can show total aldehyde and ketone loads above 50 µg/g. Acetaldehyde from overheated PE accumulates in closure systems; sensitive mineral-water applications often reject acetaldehyde above 1 mg/kg. The odour is described as sour, waxy, or apple-like. Residual volatile load is quantified by VDA 278 with thermal desorption at 90 °C for 30 min, followed by GC-MS. Sensory grading is performed according to VDA 270, with grades from 1 to 6; many automotive and packaging converters require grade 3 or better. Once headspace aldehydes exceed the specification, melt temperature and screw speed must be reduced.

Does Oxidation Drive Chain Scission or Crosslinking in the Melt?

Oxidation in PE can simultaneously reduce molecular weight by β-scission and raise high-molecular-weight fractions by radical combination. The balance depends on comonomer content, branching density, initial molecular weight distribution, oxygen concentration, and temperature. High-density ethylene homopolymer at high temperature tends to form long-chain branches and gels; low-density PE with high tertiary carbon content undergoes more chain scission. Melt mass-flow rate is a coarse indicator: crosslinking lowers MFR, while chain scission raises MFR. In overheated HDPE pipe extrusion, MFR at 190 °C under 21.6 kg can fall by 15 % to 40 %, while gel content rises. Size-exclusion chromatography according to ISO 16014-2:2019 shows the high-molecular-weight shoulder increasing, sometimes producing a bimodal distribution. Parallel-plate rheology at 0.1 rad/s shows storage modulus G′ increasing as long-chain branching and crosslink density rise. Capillary rheometry reveals a low-shear viscosity increase and a reduction in the shear thinning slope.

Gel content is determined directly by ASTM D2765-16 as xylene-insoluble material after solvent extraction. In blown and cast film, acceptable gel content is often below 0.1 wt%; overheating can raise it to 0.5 wt% or higher. Gel particles are not uniformly distributed. They accumulate on screen packs, die lips, and downstream cooling rolls, then release as specks. This explains why gel count can be intermittent rather than uniform across a production batch. Pressure differential across a melt screen pack is a practical proxy: an increase of 2 MPa to 5 MPa above baseline often indicates gel accumulation before visible film defects appear.

Odour-Active Volatile Profile Generated Above 220°C

At melt temperatures above 220 °C, oxidative chain scission releases a homologous series of aldehydes, ketones, carboxylic acids, and short-chain hydrocarbons. Headspace GC-MS following VDA 278 can identify acetaldehyde, propanal, butanal, hexanal, octanal, 2-butanone, acetic acid, and formic acid. Acetaldehyde has a low odour threshold and can dominate sensory complaints even when other volatiles are present at higher concentrations. Overheated LDPE film can show acetaldehyde above 5 mg/kg by headspace GC-MS, leading to sour or apple-like notes. Long-chain aldehydes such as nonanal can produce waxy or citrus-like odours; short-chain carboxylic acids contribute rancid and acidic notes.

Sensory evaluation methods include VDA 270 and ISO 12219-2:2012. Odour failure often occurs before gel formation is visible, because volatile carbonyls form at an earlier degradation stage than macroscopic crosslinked domains. The analytical table below summarises markers used to distinguish overheated PE from correctly stabilised PE.

Processing Stabilizer Consumption Limits Are Reached Before Pressure Rise

In twin-screw compounding, shear heating in kneading blocks can create local melt temperatures 15 °C to 25 °C higher than barrel set points. With co-rotating screws of 40:1 L/D and 30 mm diameter, the residence-time distribution may expose a small fraction of melt to high shear for more than 60 s. If phosphite secondary antioxidant is already depleted, the remaining phenolic antioxidant cannot suppress radical flux. Melt pressure rises as crosslinked gel accumulates on screen packs; a pressure differential above 5 MPa is a production signal of gel build-up. The melt temperature measured at the die by infrared should be maintained at or below 210 °C for LDPE and 230 °C for HDPE in standard film and extrusion coating lines. Higher temperatures are possible only where residence time is short and stabilizer doses are high, but published data for prolonged operation above 240 °C without property loss are limited.

Operational boundaries are material-specific. Melt pumps, screen changers, and dies must be designed to minimise dead zones where stagnant melt oxidises. A dead zone with residence time above 5 min at 240 °C can generate carbonized material that periodically releases black specks. Pre-drying is not normally required for PE at relative humidity below 60 %; however, external moisture and high oxygen ingress accelerate carbonyl formation. Combining overheating with metal residues from worn barrels or screw surfaces may further catalyse decomposition, increasing gel and odour output.

Film extrusion for food packaging using 40 µm LLDPE blown film is a sensitive indicator of thermal history. When die lip temperatures drift above 235 °C, surface oxidation produces a wax-like odour and visible gel specks. Surface energy rises as oxygenated polar groups are formed; water contact angle measured per ISO 19403-2:2020 can fall from 100° toward 80°. This change in surface polarity affects printability and lamination adhesion, but it also confirms oxidation at the film surface. The specification for such film is not only optical clarity but also low odour, low gel count, and stable surface energy. Adjusting melt temperature, screw speed, and stabilizer package together is the main restraint on overheating-induced degradation.