Spotting Off-Spec LDPE by Grade Code: Five Checks Before You Pay

Spotting off-spec low-density polyethylene (LDPE) before payment releases requires more than matching the grade code on a consignment note to a purchase order. The grade code is a producer-side commercial label that signals nominal density, melt flow class, and reactor platform, but it does not guarantee that a particular lot will survive the thermal and shear history of a blown film tower, extrusion coating line, or injection moulding cell. A high-pressure autoclave lot and a tubular lot assigned to the same film code can differ in long-chain branching distribution, die swell, gel count, and elongation at break while appearing identical on a certificate of analysis that reports only melt flow rate and density. The five checks described below are therefore executed before payment is authorised, because off-spec material becomes more expensive to return after it has been conveyed into a silo, blended into a feed system, or converted into film.

Two constraints govern the verification sequence. First, high-pressure LDPE is inherently less uniform than linear low-density polyethylene (LLDPE) or high-density polyethylene (HDPE) because the free-radical process produces a broad molecular weight distribution with short-chain and long-chain branching. Second, many converters do not have immediate access to a full rheological laboratory; therefore the checks are arranged from documentary to equipment-intensive. The first check interrogates the grade code itself, the second checks melt viscosity, the third checks density and thermal behaviour, the fourth checks gels and additive integrity, and the fifth checks mechanical response on converted film or moulded specimens.

What Does the Grade Code Actually Encode in a High-Pressure LDPE Lot?

The grade code is not a harmonised specification; it is an alphanumeric label into which producers pack density class, melt flow class, and sometimes additive package information. A film-grade LDPE code may carry a nominal density of 0.921 g/cm³ to 0.925 g/cm³ and a melt flow rate of 0.3 g/10 min to 0.7 g/10 min, but the same two digits may refer to a different additive package in another producer’s system. The first check compares the lot number, grade code, and current supplier technical data sheet revision. The incoming technician verifies that the code string marked on the packaging or consignment note appears in the current edition of the supplier’s technical data sheet, that the production date is within the agreed shelf life, and that the declared antioxidant, slip, and antiblock packages match the purchase specification. A mismatch at this stage is a documentary off-spec condition regardless of physical properties. The packaging label should also be checked for the correct legal identification: PE-LD under ISO 1043-1, or LDPE under ASTM terminology, with the appropriate food-contact statement when required under EU Regulation 10/2011 or 21 CFR 177.1520. If the grade code cannot be traced to a valid supplier document, the lot is quarantined without further testing.

At the melt flow rate station, a semi-automatic plastometer with a corrosion-resistant barrel and a tungsten carbide die is used to measure the time-melt index at 190 °C under 2.16 kg. For LDPE film grades, the measurement is performed according to ISO 1133-1, Procedure A, or ASTM D1238. The cut-off interval is set so that each extrudate segment weighs between 0.05 g and 0.15 g for low-MFR resins. The measured value is compared against the supplier lot value, not simply the general grade range. A deviation of more than 10% is a trigger for quarantine. On production-scale blown film extruders with a 90 mm single screw and an L/D 30 grooved-feed section, a shift from 0.4 g/10 min to 0.55 g/10 min can reduce backpressure by 5–10%, change bubble cooling demand, and alter film haze. Conversely, a melt flow rate lower than the lot value can indicate contamination with a higher molecular weight grade, crosslinked gel fraction from a degraded reactor zone, or incomplete homogenisation. For extrusion coating grades with MFR values of 4 g/10 min to 12 g/10 min, the same 10% deviation is enough to shift neck-in behaviour and substrate adhesion. When the MFR result is ambiguous, a capillary rheometer fitted with a 20:1 L/D die and an apparent shear rate sweep from 100 s⁻¹ to 1,000 s⁻¹ provides a viscosity ratio. Autoclave LDPE generally shows a high viscosity ratio; a lot that appears Newtonian at high shear is suspect and may be a linear resin.

Density, Long-Chain Branching, and the Thermal Trace

Density verification is performed on compression-moulded plaques conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for at least 40 h according to ASTM D618. The density is then measured by an isopropanol-water or ethanol-water density gradient column per ASTM D1505 or by liquid displacement per ISO 1183-1. LDPE film grades typically occupy a density band of 0.917 g/cm³ to 0.930 g/cm³. A deviation of 0.002 g/cm³ from the supplier lot value is substantial because it indicates a shift in short-chain branching concentration or contamination with LLDPE, HDPE, or a nucleated masterbatch. Such a shift is not visible in the pellets, but it changes tensile modulus, puncture resistance, and water vapour transmission rate in the final film. Density alone, however, is insufficient to distinguish autoclave from tubular LDPE or to detect partially degraded material. The thermal trace by differential scanning calorimetry provides additional confirmation. Compression-moulded specimens of 5 mg to 8 mg are heated from 30 °C to 180 °C at 10 K/min under a 50 mL/min nitrogen purge according to ASTM D3418 or ISO 11357-3. A typical LDPE lot shows a broad melting endotherm peaking between 104 °C and 113 °C. A peak above 115 °C suggests HDPE contamination or a linear resin; a significantly broadened or double-peak trace may indicate a mixed lot or reactor instability. The cooling exotherm crystallisation temperature also responds to branching; an unusually sharp crystallisation peak may indicate reduced long-chain branching.

When Contamination and Additive Failure Appear as Gels and Black Specks

Many off-spec cases are discovered only after the grade code, MFR, and density are within tolerance. The lot fails on a blown film line because of gels, black specks, or additive inhomogeneity. A practical screening procedure is to extrude a 50 µm cast film or 25 µm blown film from the suspect lot and inspect it under polarized light at 50× magnification. Gel particles larger than 0.2 mm are counted over a 100 cm² area; for optical-grade film applications, counts above 20 are often rejected, although the limit varies by end market. On a production film line with a 25 µm screen pack and a melt temperature of 180 °C to 200 °C, a pressure rise of more than 1.5 MPa across the screen pack over 20 minutes indicates excessive gel or particulate contamination. Black specks may be charred polymer or metal wear particles; they can be isolated by ashing at 600 °C and analysed by energy-dispersive X-ray spectroscopy to identify Fe, Cr, or Al. Volatile and moisture contamination is assessed by heating pellets at 105 °C in a forced-air oven for 1 h or by Karl Fischer titration; LDPE is not hygroscopic, so moisture above 0.05% by mass suggests surface contamination or condensation from faulty bulk handling.

Tensile Yield, Tear Initiation, and Dart Drop Are the Last Gate

Mechanical testing is performed on film produced from the suspect lot under controlled conditions, not on compression-moulded plaques alone. For blown film, the line parameters are recorded: die diameter, die gap, blow-up ratio, frost-line height, and draw speed. Typical pilot film lines use a 50 mm single-screw extruder, a 100 mm annular die with 1.5 mm die gap, a blow-up ratio of 2.5:1, and a film thickness of 25 µm to 50 µm. Tensile properties are tested according to ASTM D638 or ISO 527-3 at 50 mm/min; LDPE film typically shows a yield stress between 8 MPa and 12 MPa, a tensile strength at break between 12 MPa and 20 MPa, and elongation at break between 200% and 600% depending on orientation. The purchaser should compare machine direction and transverse direction values against the supplier lot data. A lot that is outside the agreed tolerance band may indicate excessive long-chain branching, incorrect molecular weight distribution, or contamination with a stiffer polymer. Dart impact is measured according to ASTM D1709, Method A, on film of the specified gauge; a 50 µm LDPE film often falls between 80 g and 150 g, but these values are formulation-dependent and should be taken from the supplier’s published data. Elmendorf tear strength is tested according to ASTM D1922; for LDPE film, machine-direction tear values are often lower than transverse-direction values, and a reversal of this relationship or a drop below the supplier’s lot tolerance suggests off-spec branching or process-oxidation during the resin manufacturing or the film conversion step. If the film cannot be produced without melt fracture, bubble instability, or low impact, the lot is rejected even when the COA shows acceptable flow and density.

For injection moulding LDPE grades, the final gate is a spiral flow test on a 100-tonne injection moulding machine at 220 °C melt temperature and 60 °C mould temperature. The spiral flow length is compared with the supplier lot value; a reduction greater than 10% or a shot-weight variation greater than 0.5% indicates the wrong grade or contamination. Fourier-transform infrared spectroscopy can be used as a spot check for the additive package: absorbances for primary antioxidant, slip agent, and antiblock agent are compared against a reference spectrum. If the spectral fingerprint does not match the grade code’s declared formulation, the lot is rejected even when the mechanical values are acceptable.