Answering a Carbon Footprint Questionnaire for Polyethylene Supply is a unit-process inventory exercise that must resolve feedstock attribution, process energy, transport modes, and co-product allocation before the final carbon dioxide equivalent value can be reported. The questionnaire response for polyethylene (PE) carries different boundary obligations than a general polymer datasheet because the product system spans naphtha or ethane extraction, steam cracking, catalytic polymerization, pelletization, and in some cases mechanical recycling or bio-based feedstock substitution. A supplier that submits a single emitted mass per tonne without declaring the source database, the allocation procedure, and the geographical electricity mix creates an inventory that fails the data-quality requirements of ISO 14067:2018 and ISO 14044:2006/A1:2018.

What Constitutes a Defensible Polyethylene Carbon Footprint Inventory?

A defensible inventory begins with a declared functional unit of 1 kg of polyethylene pellets at the production gate. The unit process map for conventional high-density polyethylene (HDPE) includes feedstock extraction, pipeline transport, steam cracking with associated furnace stacks, catalytic polymerization in a slurry loop or gas-phase fluidized-bed reactor, polymer degassing, extrusion compounding, and underwater pelletization. The steam cracker operates at furnace outlet temperatures commonly between 820 °C and 860 °C. The radiant section efficiency and coil outlet pressure govern the direct CO₂ emissions, while the emission intensity is allocated among ethylene, propylene, butadiene, pyrolysis gasoline, and other co-products. ISO 14067:2018 requires the selected allocation procedure to be justified and applied consistently; mass allocation is widely used for olefin co-products, but economic allocation changes the burden assigned to ethylene when naphtha prices diverge from propylene values.

Low-density polyethylene (LDPE) produced in a high-pressure tubular reactor operates at pressures from 150 MPa to 300 MPa and at initiation temperatures determined by peroxide selection. The autoclave alternative operates at lower pressure with a back-mixed residence time distribution, which produces a different short-chain branching distribution and a different specific energy profile. For the questionnaire, the primary compressor and hypercompressor electricity demand are to be taken from plant metering where available. If actual compressor power data cannot be retrieved, secondary datasets from life cycle inventory databases are used; those datasets report cradle-to-gate global warming potential values of approximately 1.8 kg CO₂e/kg to 2.5 kg CO₂e/kg for HDPE and 2.0 kg CO₂e/kg to 2.7 kg CO₂e/kg for LDPE, depending on feedstock slate, grid intensity, and steam cracker technology.

Gas-phase fluidized-bed HDPE plants, using chromium or Ziegler-Natta catalysts, typically operate at 85 °C to 110 °C and at reactor pressures near 2.4 MPa. The gas-phase route lowers solvent recovery energy but increases monomer recovery load because the produced resin must be degassed before pelletization. The questionnaire response should separate direct emissions from the polymerization loop, indirect emissions from purchased electricity, and upstream emissions from ethylene. A site with heat integration that sends recovered low-pressure steam to the cracker's demethanizer reboiler will show lower total energy use than a non-integrated site; the difference is frequently in the range of 0.2 kWh/kg to 0.5 kWh/kg for equivalent PE grades.

Data quality fields in a carbon footprint questionnaire must include the life cycle inventory database version, the geographical scope of the electricity grid, the share of purchased steam, and the grid connection voltage. For Scope 1, direct CO₂, CH₄, and N₂O from the steam cracker stack and from the polymerization exotherm control system are assigned separately from Scope 2 emissions for purchased electricity and imported steam. The exothermic polymerization reaction releases heat that can be recovered, but the net thermal energy balance varies with residence time, catalyst productivity, and reactor temperature. If stack monitoring data are not available, the respondent should use emission factors from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 2, Chapter 4, and state the oxidation factor and the global warming potential set, such as AR5 or AR6.

Questionnaire responses that combine multiple crackers or multiple polymerization trains should not report a single arithmetic average without weighted allocation. A mixed ethylene supply from an ethane cracker and a naphtha cracker requires weighted averaging by ethylene mass and by the allocation factor used for the cracker co-products. A cradle-to-gate ethylene value can differ by 0.3 kg CO₂e/kg to 0.7 kg CO₂e/kg between a pure ethane reference and a naphtha-heavy slate in comparative life cycle assessments. If the receiving plant purchases ethylene on a merchant basis, the questionnaire should use the supplier's product carbon footprint rather than an internal estimate based on a different feedstock assumption.

Scope 3 fields in the questionnaire often include purchased goods and services, upstream fuel and energy-related activities not included in Scope 2, and transport of feedstock and pellets to the customer. The respondent should not double-count steam cracker combustion emissions by placing them in both Scope 1 and Scope 3 category 3. For polyethylene supply, the dominant Scope 3 categories are usually purchased ethylene, purchased additives, and upstream extraction emissions. Additives such as antioxidants, processing stabilizers, and slip agents are typically less than 0.5 wt% of the formulation, but their cradle-to-gate burden can be high relative to their mass if they are amine-based or phosphorus-based fine chemicals; the questionnaire should request the additive masterbatch composition and the carrier polymer grade.

When Feedstock Origin Shifts from Ethane to Naphtha, Inventory Outcomes Change

Ethane-based steam crackers produce a high ethylene yield per unit of feed and therefore lower co-product allocation per tonne of PE. Naphtha crackers yield more propylene, butadiene, and pyrolysis gasoline, and the allocation choice materially moves the carbon footprint of ethylene. The ethane-to-naphtha ratio should be reported as a weighted average over the production campaign because a simple annual average may obscure batch-to-batch variance in furnace severity and coil coking. The transport of naphtha by marine vessel and of ethane by pipeline or cryogenic carrier should be separated in the questionnaire; road transport per tonne-kilometre is normally reported between 60 g CO₂e/t·km and 120 g CO₂e/t·km, while rail remains below 30 g CO₂e/t·km in many national inventories.

Furnace severity, measured by the propylene-to-ethylene ratio, also changes the specific energy and the stack emissions per tonne of high-value olefin. A questionnaire that fails to request the furnace severity and the feedstock density cannot detect when the same reported ethylene value is being applied to different cracker operating periods. For supply chain scope, the product system boundary for PE supply typically terminates at pelletization; downstream film extrusion or injection molding should not be included unless the questionnaire explicitly requests cradle-to-grave rather than cradle-to-gate. The respondent must state this boundary in the data field for system boundary description, otherwise the customer may incorrectly compare a cradle-to-gate PE value against a cradle-to-gate compound value.

On a twin-screw extruder with an L/D ratio of 30:1 to 40:1, specific mechanical energy for neat PE pelletization is typically 0.05 kWh/kg to 0.12 kWh/kg, while filled or compounded PE can reach 0.15 kWh/kg to 0.25 kWh/kg. The questionnaire should use the metered electrical input at the drive cabinet rather than the mechanical output at the screw shaft because motor and gearbox losses are not negligible. Production-scale gas-phase plants may show 5% to 15% carbon footprint underestimation if grade-change periods and part-load compressor operation are excluded from the annual utility totals.

Biopolymer and Recyclate Attribution Boundaries in Carbon Footprint Disclosure

Bio-based polyethylene made from sugarcane ethanol is chemically identical to fossil PE, but its carbon footprint questionnaire cannot use the same biogenic carbon treatment without declaration. The respondent must state whether the carbon dioxide absorbed during sugarcane growth is credited at the crop stage or accounted as a biogenic removal in the polyethylene product. ISO 14067:2018 requires biogenic carbon to be reported separately from fossil carbon; the questionnaire fields should separate fossil, biogenic, and land-use-change emissions. If the bio-based PE is certified under ISCC PLUS, the mass balance allocation follows the chain-of-custody method, but the physical carbon footprint may not equal the credited mass balance value. Published data for this specific configuration is limited; the reported fossil greenhouse gas component depends on the efficiency of fermentation, ethylene dehydration, and alcohol purification.

For mechanically recycled PE, the carbon footprint of the recyclate includes collection, sorting, washing, regrinding, and re-pelletization. The cut-off point must be declared, particularly when scrap from post-consumer rigid containers is processed through an extruder with melt filtration. If the questionnaire uses recycled content allowance, the product footprint can be reduced by the avoided burden of virgin polymer production only when an explicit substitution factor is declared and the recycled stream can be shown to displace virgin material. In Europe, the reporting of recycled content under EN 15343:2007 supports the traceability of recycled material but does not by itself provide a carbon footprint.

The questionnaire calculation must derive the final polyethylene carbon footprint from the formula: net greenhouse gas emissions divided by net polyethylene production, expressed in kg CO₂e/kg. Direct emissions from the cracker furnace and polymer plant, purchased electricity, imported steam, auxiliary fuel, and allocated co-product burdens are summed over the reporting period. Inventory data should be collected over a full campaign to account for grade changes, shutdowns, and catalyst activation cycles. For sites operating multiple PE grades, the questionnaire should request grade-specific energy consumption rather than plant-wide utility totals.

Cut-off criteria of 1% of total mass and 0.1% of primary energy are common, but the respondent should state whether the questionnaire follows ISO 14044:2006 Section 4.2.3.3 or a customer-specific cut-off. High-impact material flows such as ethylene and electricity should not be excluded by the cut-off rule. The data quality score should be at least good under ISO 14044:2006 Section 4.2.3.6, and the uncertainty of the inventory should be quantified if the data source is older than five years. For polyethylene produced in regions with high grid variability, the electricity emission factor should be the market-based or location-based value declared in the questionnaire response; the selection must be consistent across Scope 2 and Scope 3 energy categories.

The questionnaire should not accept a single carbon footprint value for polyethylene without these data fields because the value cannot be reproduced or audited. If the supplier does not have primary polymerization energy data, the response must state the secondary dataset and the year of publication. Polyethylene facilities that operate with mixed virgin and recycled pellets should provide separate declarations rather than a single weighted blend value unless the downstream customer has specified a blend ratio. Polyethylene pellets do not require pre-drying before extrusion unless surface moisture exceeds 0.1%; therefore a cradle-to-gate questionnaire should not include drying energy for standard PE conversion. The operational boundary excludes downstream converting, but if the questionnaire requests a cradle-to-gate plus conversion value, the injection molding, film blowing, or blow molding energy must be reported separately.