LLDPE COA Says One Thing, the Film Says Another: How to Resolve a Dispute

When a converter observes tensile elongation, dart impact, or tear values outside the agreed range while the supplier certificate of analysis reports acceptable pellet properties, the discrepancy is generally caused by one of four mechanisms: non-equivalent test geometry, processing-induced morphology change, sampling error, or unintended dilution with a different resin. Linear low-density polyethylene (LLDPE) groups copolymers of ethylene with 1-butene, 1-hexene, or 1-octene; ASTM D4976-23 classifies LLDPE by density from 0.915 g/cm³ to 0.940 g/cm³. The COA records pellet-state values under normalized thermal and mechanical conditions, not film performance across a specific blown-film line. The initial step in dispute resolution is to translate the disagreement into measurable terms: the exact test standard, the sample conditioning state, and the conversion history.

Raw resin testing and film testing are often performed under different loading geometries. A COA tensile yield value cannot be compared with a blown-film secant modulus unless both methods state the specimen thickness, the extension rate, the jaw separation, and the conditioning time. Similarly, the melt mass-flow rate on the COA is not a direct predictor of film tear because it contains no information on molecular weight distribution, short-chain branch distribution, or the presence of ultra-high-molecular-weight species. A dispute that begins with “the COA says” but ends with “the film says” is therefore a protocol collision before it is a material defect.

Why Does a COA Report Pellet-State Rheology Instead of Blown-Film Failure Envelopes?

The COA is a conformity document, not a downstream processing guarantee. A standard LLDPE COA normally includes melt mass-flow rate determined by ASTM D1238-23 or ISO 1133-1:2022 at 190°C with 2.16 kg, density measured by ASTM D1505-18, ASTM D792-20, or ISO 1183-1:2019, and sometimes additive levels, ash, or gel count. Melt mass-flow rate is a single-point capillary flow result; it cannot distinguish a bimodal high-molecular-weight tail, long-chain branching, or comonomer sequence distribution. Two LLDPE lots with identical 1.0 g/10 min MFR can diverge in blown-film tear and dart impact if one lot contains high-molecular-weight gels or a different comonomer type. The COA value that passes the sales specification may therefore be insufficient for a film purchase specification that includes Elmendorf tear, dart drop, and optical clarity.

For a complaint involving low machine-direction tear, the first root-cause check is to measure MFR and density on pellets sampled from the actual hopper, not from the original railcar sample. Silo segregation, pellet regrind return, or incorrect silo selection can create an in-plant resin blend that no longer matches the COA. A hopper sample with 0.922 g/cm³ density instead of the certified 0.918 g/cm³ may raise film secant modulus but reduce down-gauge puncture resistance. A statistically valid composite should be taken from multiple hopper locations, not from a single purge shot, because pellet size and shape can segregate with additives and regrind particles.

The COA Enumerates Pellet-State Density, but Blown Film Orientation Rewrites the Tensile Curve.

ASTM D882-18 and ISO 527-3:2018 measure tensile properties on film specimens with thickness below 1.0 mm. The tensile curve of an oriented film is not the same curve recorded on a compression-molded plaque under ASTM D638-14 or ISO 527-2:2012. Blown LLDPE film possesses a balance of machine-direction and transverse-direction orientation governed by blow-up ratio, frost line height, and draw-down ratio. At a blow-up ratio of 2.0:1 to 3.0:1, the transverse direction may display lower elongation to break than the pellet-state tensile curve would suggest. If the dispute compares film tensile values to a COA tensile value that was generated on a compression-molded plaque, the comparison is invalid unless the purchase specification expressly permits it. Film tensile testing must condition samples at 23°C ± 2°C and 50% ± 10% relative humidity for a minimum of 40 h under ISO 291 or ASTM D618-21, and the specimen jaw separation and extension rate must be recorded because the apparent modulus changes with strain rate.

When the Dispute Centers on Dart Impact, Reconstruct the Melt Thermal History and Die Gap Before Challenging the COA.

Dart impact testing under ASTM D1709-22 or ISO 7765-1 compares falling-mass failure energy at specified striker geometry and velocity. Method A uses a 38 mm dart and higher drop height; Method B uses a 51 mm dart and lower drop height. A film that passes Method A may fail Method B if the purchase specification does not state the method. More importantly, LLDPE dart impact is sensitive to melt temperature, output rate, die gap, frost line height, and back pressure. If a COA reports an acceptable 1.0 g/10 min MFR but the line runs at 230°C melt temperature with a 1.0 mm die gap, polymer oxidation and melt stress can create gels or reduce molecular weight in a way that pellet-state MFR cannot reveal. A film audit should record extruder barrel profile, adapter temperature, die temperature, screw speed, pressure, output, frost line height, and take-off speed before a complaint is filed. If the film thickness is below 25 µm, small gauge variations dominate dart impact variability.

Gauge variation is a frequent hidden cause in LLDPE film property disputes. A film targeted at 50 µm may exhibit local thickness from 45 µm to 55 µm across the web because of die lip inconsistency or uneven air-ring cooling. Tear and tensile results are normalized to thickness, but puncture and dart impact are not linearly normalized in the same manner. A transverse-direction thickness map produced with a capacitance gauge or beta gauge should accompany any film sample sent to the supplier. If the converter’s gauge scanner is not calibrated according to its manufacturer interval, the measured film property may fall outside specification even though the resin has not changed. This is an equipment verification issue, not a COA error.

Gel Count, Additive Bloom, and the Limits of Pellet-State Quality Indicators

A film complaint attributed to resin quality is often caused by gels recorded as visual defects. Gel counting by optical surface inspection with a detection threshold of 50 µm or 20 µm according to the film specification can reveal two classes: unmelted high-molecular-weight resin particles and oxidized crosslinked material. Polarized light microscopy with a hot stage can separate a clear unmelting gel from a degraded oxidised gel. LLDPE extrusion at excessive temperature may produce oxidative gels even though the original pellet lot meets the COA MFR and density. Additive migration can also produce surface haze that optical scanners identify as gels. If slip or antiblock masterbatch is added at the press side, the COA of the base resin does not account for the dispersion quality of the masterbatch. A dispute should therefore include the complete let-down ratio, mixing screw type, and whether the additive was pre-blended by the converter.

Developing a Multi-Stage Testing Program to Reconcile COA and Film Data

The resolution of a COA-to-film conflict requires a staged testing matrix. The first stage verifies resin identity and purity on retained pellets: melt mass-flow rate under ASTM D1238-23 or ISO 1133-1:2022, density under ASTM D1505-18 or ISO 1183-1:2019, Fourier-transform infrared spectroscopy for comonomer type, and differential scanning calorimetry for melting peak shape. The second stage verifies film fabrication consistency: thickness profile, gel count, tensile properties under ASTM D882-18 or ISO 527-3:2018, Elmendorf tear under ASTM D1922-23 or ISO 6383-2, and dart drop under ASTM D1709-22 or ISO 7765-1. The third stage performs comparative testing on a reference lot, because the same converter may generate different results with the same resin on different dies or air-ring configurations. Without a reference lot and a calibrated line audit, no single film measurement is sufficient to prove that the COA is wrong.

Fourier-transform infrared spectroscopy on LLDPE film can be confounded by erucamide slip and silica antiblock additives. Additive bands overlap with comonomer absorption regions in the spectrum. When comonomer identity is part of a dispute, solution-state carbon-13 NMR is the more authoritative method because it provides branch concentration and distribution data. However, the analysis cost is significantly higher than FTIR, so it is normally reserved for formal complaints. For routine hold-and-check investigations, a pellet spectrum from retained resin is compared with the COA spectrum, and the film spectrum is used only as a secondary indicator.

If the Retained Pellets Meet the COA but the Film Fails, the Claim Moves to Process Capability.

This boundary separates material conformity from conversion performance. The technical file should contain the extruder screw type and L/D ratio, die diameter, die gap, air-ring type, output rate, melt temperature, frost line height, and gauge map. For LLDPE blown-film lines, melt temperature is commonly reported between 190°C and 240°C, but the optimum profile depends on comonomer type and stabilization package. Higher melt temperatures increase gel formation risk, while lower melt temperatures can leave high-molecular-weight species unmelten and raise melt pressure. Neither condition is visible on a pellet COA. The process audit should also record screw speed and back pressure because high shear can generate local melt-temperature gradients that are not captured by a barrel thermocouple. If the film failure follows a press-side additive change, the resin COA should be removed from the complaint until the additive masterbatch has been tested separately.

A formal dispute should never proceed on the basis of a single COA parameter or a single film roll. The retained samples should be split into three portions: one for the supplier, one for an ISO/IEC 17025:2017 accredited independent laboratory, and one for the converter. Chain-of-custody records must note the date of sampling, silo or hopper location, roll number, and extrusion log. If the independent laboratory confirms that the retained pellets meet the COA but the film remains outside specification, the claim shifts from resin conformity to process capability. Under those conditions, the technical file should include the extruder screw type and L/D ratio, die diameter and die gap, air ring type, output rate, melt temperature, frost line height, and gauge map. Published data for a direct correlation between a specific COA value and a specific blown-film failure is limited because the converting line’s thermal and orientation history acts as a second, independent specification layer.