Batch-to-Batch Colour Shift in LLDPE Film: Root Cause and Recovery is treated as a colourimetric and thermal stability investigation rather than a simple pigment adjustment. Linear low-density polyethylene (LLDPE) film produced on single-screw blown film lines is normally specified by CIELAB coordinates L*, a*, b* under D65/10° conditions, with yellowness index measured according to ASTM E313-20 and pellet colour screened according to ASTM D6290-19. A commercial release limit commonly applied to unprinted film is ΔE*ab of 1.5 against the retained control; some laminating and medical packaging specifications reduce this to 1.0. Because colour shift can occur without measurable loss of tensile properties, the investigation must separate resin-lot variation, additive package oxidation, melt-residence history, and masterbatch feeder error. Film-grade LLDPE typically has melt mass-flow rate from 0.5 to 2.0 g/10 min at 190 °C under 2.16 kg load per ASTM D1238-20, and density from 0.916 to 0.940 g/cm³ per ASTM D792-20. Batch-to-batch drift in b* or yellowness index without a corresponding haze increase is frequently the first indication of stabiliser oxidation or thermal degradation.

When ΔE*ab Exceeds 1.5: Segregation and Spectrocolorimetric Triage

Film rolls exceeding the ΔE*ab threshold are quarantined, and the corresponding pellet lot, regrind silo, and extrusion log are retained. Measurement is performed on film specimens conditioned at 23 ± 2 °C and 50 ± 10 % RH for 40 h according to ISO 291:2008, using a sphere spectrophotometer with specular component excluded. Readings are taken over white and black backing to detect translucency-induced lightness distortion; b* is more reliable than a* for oxidative yellowing in polyolefins. The same retained control film and suspect film are measured on the same instrument, and ΔE*ab is decomposed into ΔL*, Δa*, and Δb*. Colourimeter-only yellowness index measurements are insufficient because they do not provide a full colour-difference decomposition. Extrusion log records for melt temperature, die pressure, screw speed, barrel zones, frost line height, blow-up ratio, and reclaim ratio are pulled for all suspect shifts.

Table 1. Colour and oxidative stability test matrix for LLDPE batch-shift investigations.

Resin-lot colour drift in LLDPE frequently originates from the stabilisation package rather than the polymer backbone itself. Hindered phenolic primary antioxidants such as pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) terminate peroxy radicals but can form yellow quinone methide and stilbenequinone oxidation products when the melt is over-oxidised. Phosphite secondary antioxidants such as tris(2,4-di-tert-butylphenyl) phosphite are hygroscopic; hydrolysis generates 2,4-di-tert-butylphenol and phosphorus acid species. In silo storage or open hopper systems, moisture above 300 mg/kg can initiate phosphite hydrolysis before extrusion. The resulting acidic species may react with metal stearate acid scavengers, especially zinc stearate, producing colour bodies that appear as batch-to-batch yellowing without a change in film haze. Calcium stearate and hydrotalcite are less likely to produce the same zinc-mediated discoloration, but hydrotalcite can release water at extrusion temperature and must be matched to the base resin.

Catalyst residues from Ziegler-Natta LLDPE typically contain titanium and aluminum species at low mg/kg concentrations. Residual titanium in oxidised form can contribute a yellow-brown tint; metallocene LLDPE grades generally have lower ash and narrower molecular weight distribution, but they are not immune to additive colour drift. Published correlation data between pigment-free LLDPE yellowness and individual catalyst residues is limited, so root-cause assignment should rely on paired pellet and film OIT and colour comparisons rather than ash content alone. Film-grade LLDPE at 200 °C and 100 s⁻¹ typically exhibits apparent shear viscosity in the 800 to 1500 Pa·s range; a lot that falls outside this window can alter residence time and shear heating in the extruder.

Process conflict arises between melt temperature, output, and thermal stability. LLDPE blown film lines commonly operate with barrel profiles from 170 to 230 °C and adapter/die temperatures from 220 to 240 °C. At die temperatures above 230 °C, oxidation accelerates sharply if oxygen is not excluded; at temperatures below 190 °C, high-molecular-weight LLDPE may develop melt fracture or inadequate homogenisation. The practical thermal window for high-speed film production is therefore narrow, and batch-to-batch colour shift can be the first visible sign of residence-time drift. A barrier screw with L/D 30:1 and a Maddock mixing section generates specific energy input of approximately 0.20 to 0.28 kWh/kg; worn screw clearances above 0.15 mm at the metering zone increase local shear heating and stagnate melt, producing yellowed degraded pockets that intermittently release into the film stream.

Reclaim addition above 20 wt% without antioxidant top-up decreases oxidation induction time and can raise film yellowness index by two to three units after multiple heat histories. Oxidation induction time measured at 200 °C by ISO 11357-6:2018 is a sensitive batch discriminator; values below 20 min indicate antioxidant depletion for many film-grade LLDPE formulations, while values above 40 min may indicate over-stabilisation that can contribute to colour bodies during processing. Published data for exact OIT limits are resin-specific, and each film structure should have a control-lot baseline established before a shift investigation. Masterbatch dispersion also matters. In unpigmented LLDPE film, additive masterbatches are typically let down at 2 to 5 wt%. If the carrier resin has a melt index more than 2.0 g/10 min different from the base LLDPE, dispersion in a single-screw extruder can be incomplete, producing local antioxidant-rich zones that yellow preferentially. Screen pack differential pressure across a 100 μm screen should be monitored; a rise above 2.0 MPa indicates gel or agglomerate accumulation that may accompany colour bodies.

Table 2. Root-cause indicator matrix for LLDPE blown film colour shift.

Can Feeder Drift Alone Account for Yellowing in Unpigmented Film?

The calculation is linear for masterbatch-fed additives. Final additive concentration equals masterbatch feed rate multiplied by masterbatch active concentration divided by total throughput. On a gravimetric loss-in-weight feeder running a 5 wt% additive masterbatch, a drift of 0.2 wt% absolute changes final additive loading by 0.010 wt%. For a phosphite or phenolic additive masterbatch, this concentration change is generally below the threshold required to shift ΔE*ab by 1.0 if the masterbatch itself is in specification. Feeder drift therefore becomes a plausible root cause only when compounded with an already oxidised masterbatch, when the masterbatch contains a colour-sensitive component such as high-concentration optical brightener, or when feed interruption creates an unmixed pocket of stabiliser that degrades at the die lip. Two loss-in-weight feeders with refill cycles can create sinusoidal error; if refill intervals synchronise, short-term colour drift appears periodically. In pigmented film the relationship is direct, but in unpigmented LLDPE the film colour response is nonlinear because the colour bodies arise from degradation products, not from a uniformly dispersed pigment concentration.

Recovery on the blown film line requires mechanical cleaning before formulation adjustment. The extruder is purged with a high-viscosity purging compound or an abrasive purging grade, and the die lips are inspected for carbonised polymer. Screen packs are replaced after differential pressure exceeds 2.0 MPa; the breaker plate and die body are cleaned to remove oxidised stagnant layers. Melt temperature is reduced in 5 °C increments while maintaining die pressure above 20 MPa to preserve gauge uniformity. If oxygen ingress is suspected, nitrogen blanketing of the hopper and feed throat is introduced with residual oxygen concentration kept below 2 vol%. Antioxidant top-up is allowed only after OIT and colour measurements confirm depletion; a typical corrective addition is 0.05 to 0.10 wt% of a phosphite/phenolic masterbatch, validated by a 3:1 control-to-trial extrusion run. If pellet YI is unchanged but film YI remains elevated, the screw and barrel are measured for wear; for 90 mm single-screw extruders, screw manufacturers often specify metering-zone radial clearance below 0.15 mm and feed-zone clearance below 0.20 mm. Re-qualification uses three full-width rolls from the suspect lot and three control rolls, each measured at five points across the web; batch release is restored only when mean ΔE*ab is ≤ 1.0 and no single point exceeds 1.5.