Polyoxymethylene (POM) incoming lots are designated as either POM-C copolymer or POM-H homopolymer under ISO 9988-1. The two granule types are visually indistinguishable in natural, unfilled form, yet their melt-temperature windows, crystallisation rates, alkaline hydrolysis resistance, and dimensional stability are not interchangeable. The receiving-quarantine task is therefore separating POM-C from POM-H on incoming material: tests that decide it are not limited to a single physical-property check; they require a thermally based primary screen supported by density and flow data. On a production line with a 40:1 L/D co-rotating twin-screw extruder, a homopolymer lot incorrectly accepted as copolymer can produce melt-pressure fluctuation, elevated specific mechanical energy, and strand pelletizer output containing unmelted granules because the barrel setpoint for POM-C, commonly 170°C to 180°C, is below the complete melting range of POM-H. The consequence is not a theoretical modelling exercise; it appears as torque alarms, burnt deposits on screw elements after extended runs, and dimensional non-conformity in thin-walled moulded parts.

Why Does Differential Scanning Calorimetry Provide the Highest Specificity for Unmodified Incoming Lots?

Because the melt-peak separation between POM-C and POM-H is larger than the uncertainty of a calibrated instrument, differential scanning calorimetry is the first decision gate. A specimen mass of 5 mg to 10 mg is cut from a single granule or from a cryomilled powder and sealed in an aluminium pan with a pierced lid. The thermal cycle under ISO 11357-3 is typically a first heating at 10 K/min from 25°C to 210°C, held isothermally for 3 min to erase processing history, cooled at 10 K/min, and then reheated at 10 K/min. Purge gas flow of 50 mL/min nitrogen and an empty-pan baseline subtraction are applied to minimise oxidation during heating. The second-heat melting endotherm is read as the peak maximum. POM-H homopolymer grades generally produce a single endotherm from 175°C to 181°C; POM-C copolymer grades generally produce a single endotherm from 162°C to 173°C depending on comonomer content and nucleating additives. The minimum separation of 8°C to 18°C exceeds the ±1.0°C calibration tolerance required by ISO 11357-1, so a natural lot can be assigned on the second heat without statistical interpretation.

The cooling exotherm provides a second independent signal. POM-H crystallises on controlled cooling at a higher onset temperature than POM-C because the homopolymer chain packs with higher linear regularity and lacks the oxyethylene interruptions present in POM-C. Published data for specific cooling-rate comparisons on filled grades is limited; therefore, the cooling exotherm is used only as a confirmation, not as a release criterion for mineral-filled, impact-modified, or nucleated materials. A sample pan that loses seal integrity due to purge gas expansion will show an exothermic baseline deviation before the melt; such a run is invalid and must be repeated. North American laboratories may use ASTM D3418-21 as an alternative thermal method, but the specimen mass, heating rate, and gas flow recommendations are not identical to ISO 11357-3 and should not be mixed within a single incoming lot-release protocol.

Density screening under ISO 1183-1 is run on as-received granules or a hot-pressed film after conditioning at 23°C and 50% RH. The Archimedes displacement in ethanol or water must be free of air bubbles because the density offset between the two polymer families is only 0.01 g/cm³ to 0.02 g/cm³. Unfilled POM-H typically measures 1.42 g/cm³ to 1.43 g/cm³, while unfilled POM-C typically measures 1.41 g/cm³. The method is fast but not definitive once pigment, glass reinforcement, or mineral filler is present, and it is therefore used as a screening gate rather than a release gate. Melt-flow-rate determination per ISO 1133-1:2022 at 190°C and a 2.16 kg load is also used at incoming inspection because it detects rheological shifts from reground POM-C/POM-H mixtures. However, both polymer families are produced across a broad MFR range; the test only separates chemistries when the approved vendor specification defines a narrow MFR window for a named commercial grade. On a capillary rheometer, the apparent shear viscosity at 100 s⁻¹ and 190°C shows proportionally higher values for high-molecular-weight POM-H, but this is not an incoming lot-release test for every facility.

When a False Negative From DSC Is Suspected, Tensile and Flexural Arbitration Becomes Necessary

If an incoming lot contains a nucleated or glass-filled POM-C with a melting peak that falls within the POM-H window, a secondary tensile screen is performed on injection-moulded type 1A specimens per ISO 527-2. The specimens are conditioned for 48 h at 23°C and 50% RH under ISO 291. Unfilled POM-H typically shows a tensile modulus of 3000 MPa to 3200 MPa and a yield stress of 68 MPa to 73 MPa; unfilled POM-C typically shows 2700 MPa to 2900 MPa and 62 MPa to 68 MPa. The notched Charpy impact strength under ISO 179-1/1eA is not a separation criterion because standard grades of both chemistries often measure 6 kJ/m² to 8 kJ/m². Flexural modulus under ISO 178 at 2 mm/min shows a similar offset: 2800 MPa for many unfilled POM-C grades and 3100 MPa for many POM-H grades. The mechanical screen requires enough material to mould a family of five specimens, so it is reserved for cases where DSC is contaminated by additives, where the vendor certificate is challenged, or where a mixed regrind lot is under quarantine.

FTIR and the Oxyethylene Comonomer Absorption: A Supplementary Molecular Signature

Mid-infrared spectroscopy can identify POM-C by the presence of the oxyethylene unit introduced during copolymerisation with ethylene oxide or 1,3-dioxolane. The C-O-C asymmetric and symmetric stretching absorptions of both POM chemistries occur in the 1100 cm⁻¹ to 900 cm⁻¹ region, and the methylene rocking and bending modes are strongly overlapped. In POM-C, a weak band from the ethylene oxide segment appears in the 1450 cm⁻¹ to 1470 cm⁻¹ region and in the 2850 cm⁻¹ to 2930 cm⁻¹ C-H envelope, but the signal is easily masked by fatty acid lubricants, pigments, or thermal stabilisers. An ATR-FTIR diamond plate with a single-reflection accessory and 32 scans at 4 cm⁻¹ resolution can be used on a hot-pressed film or a solvent-cast film from hexafluoro isopropanol. Published reference libraries for separating POM-C from POM-H by mid-FTIR are limited, and the method is not recommended as a sole release test. Nuclear magnetic resonance spectroscopy provides the unambiguous comonomer signal but requires solution preparation and is rarely used at incoming quarantine.

Consequences of Melt-Point Error in a 40:1 L/D Twin-Screw Line

When POM-H is processed on a POM-C temperature profile, the melt-temperature error is not limited to the heater setpoint. The barrel zones of an injection moulding machine set at 170°C to 180°C for POM-C leave the interior of a POM-H granule at a temperature below its 175°C to 181°C melting peak. The screw must deliver additional viscous work to fuse the crystal domains. In a 40:1 L/D co-rotating twin-screw extruder with vacuum devolatilisation, the resulting torque increase and residence-time extension can initiate depolymerisation at hot spots and generate formaldehyde. The gas is drawn into the vacuum port but can also bypass a partially sealed port and vent to the operator area. On an injection moulding machine, the lower melt temperature produces a shorter flow length in thin sections and a higher risk of weld-line fracture at part features because the material does not have the same enthalpy of melting. The correct melt-temperature window for POM-H is generally above 190°C to 210°C at the nozzle, whereas POM-C can be moulded at 180°C to 210°C. The overlap zone does not remove the need for material identity because the barrel residence time and screw recovery speed must be adjusted for the higher-viscosity homopolymer.

Water uptake is not the main separation test because unfilled POM-C and POM-H absorb roughly 0.2 % to 0.3 % moisture at 23°C and 50 % RH per ISO 62. However, the hydrolysis resistance in hot alkaline environments is different. The homopolymer is more susceptible to oxidative and alkaline chain scission at terminal hemiacetal groups, while the copolymer contains oxyethylene units that interrupt unzipping. A 72 h immersion screen in 5 % NaOH at 80°C followed by a change in tensile yield or MFR can therefore separate the two if the laboratory has the required safety controls. This test is not a routine incoming release method because the exposure time is too long; it is applied only when post-industrial regrind is suspected of containing a mixture of POM-C and POM-H. In that case, the increase in MFR after alkaline exposure is measured by ISO 1133-1:2022; published data for specific mixed-regrind numerical pass/fail thresholds is limited, so the test is interpreted against a retained reference sample. Thermogravimetric analysis under ISO 11358-1 in nitrogen at 10 K/min can sometimes separate POM-C from POM-H because the homopolymer initial degradation onset is often lower than the copolymer due to end-group unzipping; however, the onset value is highly sensitive to the stabiliser package and cannot be used alone for a universal release criterion.

The incoming inspection sequence used in a high-volume moulding operation therefore begins with a 5 mg to 10 mg DSC specimen, followed by ISO 1183-1 density and ISO 1133-1:2022 MFR only when the vendor certificate lacks a melt-peak value. If the second-heat endotherm is between 175°C and 181°C, the lot is quarantined as POM-H and the barrel/nozzle profile is raised before first use. If the endotherm is between 162°C and 173°C, the lot is released as POM-C under the normal profile. When the melt peak falls in the overlap region 173°C to 175°C, the lot is not released by DSC alone; a density check and a moulded tensile modulus are added. The same three-test logic applies to mixed regrind lots received from a recycler, where a single POM-H granule in a POM-C feedstock can remain visible as a hard spot in a thin-walled moulded part.