Batch colour drift and claims handling for polyethylene deliveries is governed less by visual perception than by instrumented colorimetry, sample preparation, and lot-level traceability. Polyethylene (PE) is supplied across a wide viscosity and density range under ISO 17855-1:2020 and ISO 1872-1:2018 classifications, yet colour stability is not a single intrinsic property; it arises from polymerization catalyst residues, thermal stabiliser package, pigment dispersion state, additive interactions, and the thermal-mechanical history of the converting step. A delivered pellet or pulverised grade may satisfy melt flow rate under ISO 1133-1:2022 and density under ISO 1183-1:2019 while still producing a visible colour discrepancy after sheet extrusion or injection moulding. Purchase specifications therefore increasingly define colour drift in CIELAB coordinates, not in descriptive terms such as “natural” or “off-white.”

In a claims investigation, the first technical distinction is between a true lot-to-lot pigment or base-resin colour shift and a processing-induced colour change. The agreed measurement condition is a compression-moulded plaque prepared according to ISO 293:2004, because pellet surface roughness, internal voids, and particle size distribution can alter spectral reflectance. For low-density PE grades, plaques are typically moulded at 160 °C ± 5 °C; for high-density PE grades, at 180 °C ± 5 °C, with a cooling rate of 15 °C/min ± 5 °C/min to a plate temperature of 40 °C or below. Without this preparation, colorimetric differences can be generated by pellet size alone, not by the delivered chemical composition.

How Is Batch Colour Drift Quantified for Delivered Polyethylene?

Colorimetric repeatability requires a defined instrument geometry. The recommended configuration for opaque polyolefin plaques is a sphere-based spectrophotometer with d/8° geometry and specular component excluded, operated under D65/10° illuminant and observer conditions according to ISO 11664-4:2008 and CIE 15:2004. The CIELAB coordinates L*, a*, and b* are recorded after calibration against a white tile traceable to a national metrology institute. Total colour difference ΔE*ab is calculated as the Euclidean distance between the delivery sample and a supplier-retained reference plaque or an agreed target colour. Yellowness index is measured according to ASTM E313-20, and pellet colour may be evaluated according to ASTM D6290-19 for compact pellet cells. Claims should not be accepted on the basis of ΔE*ab from a single unreplicated reading; a minimum of 5 readings per plaque and 3 plaques per lot is a common internal requirement. A reported standard deviation above 0.10 CIELAB units indicates surface preparation variability rather than a colour drift conclusion.

Thermo-oxidative degradation during compounding and extrusion modifies the b* axis before any pigment-related drift is detectable. In LDPE and LLDPE film extrusion at melt temperatures between 190 °C and 250 °C, phenolic antioxidants and phosphite stabilisers are consumed; residual peroxide species can convert hindered phenols to quinoid chromophores. This mechanism produces a positive yellowness shift even when the delivered pellet colour remains unchanged. Injection moulding of HDPE caps and closures at barrel temperatures of 220 °C to 260 °C, especially with hot-runner residence times above 30 s, can induce similar oxidation in the presence of dissolved oxygen. Measurement of the as-delivered pellet cannot capture this effect. Therefore a processor claim must be separated into two branches: colour of the dried pellet as received, and colour of a standard moulded or extruded test specimen. The latter determines whether the polymer responds identically to the agreed thermal history.

Several optical artefacts are capable of generating false colour drift. First, gloss differences alter specular reflection at the surface. Although the instrument’s specular-excluded geometry reduces gloss effects, the mould surface must be polished against the same release film or metal plate. Second, plaque thickness affects opacity and reflectance. A clear PE plaque of 2.0 mm is not equivalent to a 4.0 mm plaque because residual crystallinity and internal scattering differ. Third, ultraviolet content in the measurement source cannot excite most unpigmented PE; if optical brighteners are present, the UV component of the D65 simulator must be calibrated to CIE 15:2004. Without these controls, a batch-to-batch ΔE*ab of 1.2 can arise from sample geometry, not from resin drift.

Sample Preparation Disciplines That Determine the Dispute Record

Discrepancies in polyethylene colour claims commonly arise because supplier and purchaser have used different specimen preparation routes. A compression-moulded plaque prepared to ISO 293:2004 from virgin pellets is less affected by shear, degassing, and screw contamination than an injection-moulded plaque, but it is not representative of all converted articles. If a claim concerns extrusion film colour, the parties should define whether the decisive specimen is a pellet cell measurement, a compression-moulded plaque, or an extruded film sample produced on a specified cast-film or blown-film line. The specification should state granulate drying conditions: for hygroscopic pigment masterbatches and filled compounds, drying at 80 °C for 4 h is sometimes required, but unfilled PE homopolymers are generally not predried unless ambient relative humidity exceeds 70%. Conditioning before measurement is fixed at 23 °C ± 2 °C and 50% ± 10% relative humidity per ISO 291:2008. Failure to control plaque thickness, surface gloss, or thermal history makes any reported ΔE*ab technically non-actionable.

Colorimetric databases for polyethylene deliveries are only reproducible when the measurement geometry and specimen type are fixed. The matrix below summarises variables that should be stated in a purchase specification before any claim is opened.

A sphere instrument with d/8° geometry should report specular-excluded data for colour comparison; specular-included data are useful for opacity but not for perceived colour. The instrument must be calibrated with a black card and a white standard certified by a national metrology institute. A diagnostic check of long-term repeatability on the white tile with ΔE*ab from 0.03 to 0.05 is typical for benchtop sphere spectrophotometers such as the HunterLab UltraScan VIS or X-Rite Ci7600. Wavelength calibration is verified with a didymium filter per manufacturer procedure.

When a Colorimetric Non-Conformance Survives Retesting

If split-sample testing with the agreed specimen preparation still produces a ΔE*ab above the threshold specified in the purchase contract, the claim proceeds from colorimetry to compositional attribution. The purchaser should retain the original pallet, unopened liner, and delivery documents; the supplier should produce the retained pre-shipment control sample and its certificate of analysis. A documented chain of custody is mandatory for both portions. The independent laboratory verifies the identity of the polymer using melt flow rate per ISO 1133-1:2022 at the grade-specific condition, density per ISO 1183-1:2019, and ash per ISO 3451-1:2019 for filled or pigmented material. If these metrics are within the agreed specification, the colour deviation is classified as pigment-related or additive-related, not base-resin substitution. For blow-moulded bottles and films where post-consumer recycled content is present, the claim must also record the percentage of recycled input, because mixed regrind raises spectral noise and may cause batch-to-batch b* scatter without a change in virgin resin chemistry.

A frequent root cause of colour drift in pigmented polyethylene is not the base resin but a shift in masterbatch let-down ratio caused by feeder drift on a high-throughput compounding line. Single-screw feeding or volumetric dosing zones that drift by 0.3% to 0.5% can alter the final pigment concentration enough to shift ΔE*ab in low-saturation colours. Gravimetric feeders operating at 2% let-down of a TiO₂-based white masterbatch in LDPE film must maintain short-term feed accuracy of ±0.25% of set rate. Simultaneously, the carrier resin of the masterbatch must match the base resin rheology; a high-MI carrier in a low-MI high-density PE can produce localised macrodispersion defects rather than a uniform colour, particularly when the processing window is narrow. For carbon black concentrates in pressure pipes, pigment dispersion is assessed by the method described in ISO 18553:2002 using melt-extruded tape or compression-moulded plaques; poorly dispersed agglomerates above the standard’s class limit are considered a performance issue independent of total colour difference.

Additive packages further complicate attribution. Hindered-amine light stabilisers, phenolic antioxidants, phosphites, slip agents, and antiblocks can all shift initial pellet colour or ageing colour. Erucamide migration to the surface, for example, can scatter reflectance and lower L* on film without indicating pigment failure. If the delivered product is intended for food contact, any replacement of a colour masterbatch during a claim resolution must respect the positive list requirements of Regulation (EU) No 10/2011 and its amendments, and in the United States the relevant food additive status under 21 CFR Part 177 for polyolefins. A claim cannot be resolved by adding unapproved colour masterbatch or by increasing regrind content without a new conformity assessment.

The physical state of delivered granulate also requires control. Moisture uptake in PE is normally low, but condensation on cold pellets, anti-static coatings, and external contamination from silo or road tanker lines can produce surface reflectance changes that are measured as colour drift. Before any colour claim is opened, the sample should be sieved or air-knife-cleaned when surface dust exceeds 0.05% by mass; the cleaning step itself must be recorded. Density and molecular architecture differences also influence perceived colour. A high-density HDPE plaque with high crystallinity scatters more light than an LDPE plaque; when a purchaser compares films of equal gauge, differences in density or orientation can shift lightness even if the pellet colorimeter reading is constant. Therefore, the specification for colour should include the exact converting route—cast film, blown film, injection moulding, blow moulding, or compression moulding—because the spectral reflectance of a monolayer PE article is a function of the physical surface and crystalline morphology, not solely the absorption coefficient of the resin.

Lot Traceability, Melt Index Drift, and the Limits of Visual Assessment

In any formal claim for polyethylene colour, visual assessment under a D65 cabinet is treated as a screening tool, not as a binding measurement, because human colour matching varies with observer metamerism and panel intensity. A lot may show no perceptible difference to one assessor and a clear mismatch to another, particularly in blue-grey or translucent film. The objective record must be the CIELAB values, not the verbal descriptor. If the base polymer has drifted in melt flow rate, the supplier’s certificate of analysis can reveal whether the converter’s machine was adjusted to compensate; high shear rates in the melt can alter the dispersion of pigments and the apparent opacity of the article. Consequently, claims handling requires joint review of conversion conditions, not merely the delivered pellet. Published data for a single universal ΔE*ab limit applicable to all PE grades and all converting lines is limited; action thresholds are contract-specific and typically range between 0.8 and 2.0 CIELAB units for pigmented polyolefins. The absence of an agreed threshold at the time of order removes the technical basis for an automatic rejection.