Is Your Delrin Genuine? Five Checks Before You Pay the Premium

Polyoxymethylene (POM) is classified as POM-H for homopolymer or POM-C for copolymer under ISO 1043-1. Delrin is the DuPont de Nemours, Inc. trade designation for POM-H. The price premium for Delrin homopolymer over generic acetal copolymer is only justifiable when the application exploits the higher crystallinity, short-term stiffness, or long-term creep resistance of POM-H. Because both resin families share similar appearance and machinability, mislabelled POM-C is occasionally supplied as Delrin homopolymer. The five checks below form a pre-payment screening matrix for unfilled general-purpose grades. Filled grades, impact-modified grades, and electrically conductive grades require additional reference data.

What Does Melt Volume-flow Rate Reveal About Homopolymer Identity?

Melt mass-flow rate (MFR) or melt volume-flow rate (MVR) is measured at 190 °C under 2.16 kg using a capillary melt indexer with a 2.095 mm bore and 8.000 mm die length according to ISO 1133-1:2022. General-purpose unfilled POM-H injection grades typically fall between 7 g/10 min and 22 g/10 min, while POM-C extrusion grades are often produced from 2 g/10 min to 9 g/10 min; POM-C injection grades overlap POM-H. A grade-specific certificate of analysis is required. A measured value outside ±10 % of the declared lot value should trigger thermal identification. Pre-drying is not generally required for MFR, but when ambient relative humidity exceeds 60 %, surface moisture on granules can produce splay and apparent melt-viscosity drift; drying at 80 °C for 3 h to 4 h in a desiccant dryer is recommended. Melt flow alone cannot distinguish a POM-H/POM-C blend from POM-H because a blend can be formulated to match the target MFR.

Batch-to-batch die melt pressure data on a 25-mm co-rotating twin-screw extruder with 40:1 L/D at 40 kg/h and a 210 °C die set temperature have shown that similar-MFR POM-H and POM-C lots may differ by less than 0.5 MPa; this pressure difference is not a reliable identification criterion. Published data for this specific configuration is limited, and pressure transducers should not replace standardized rheological testing. Melt volume-flow rate is preferred over melt mass-flow rate when the resin contains releasing agents or pigments because density assumptions introduce unnecessary error.

Differential scanning calorimetry (DSC) per ISO 11357-3:2018 is performed on an 8 mg to 12 mg specimen cut from a dried granule or machined chip. The specimen is heated from 25 °C to 210 °C at 10 K/min under 50 mL/min nitrogen purge. Genuine unfilled POM-H shows a sharp endothermic melting peak at 178 °C ± 2 °C; unfilled POM-C shows a broader endotherm between 165 °C and 172 °C. Nucleated POM-C can approach 174 °C, so peak position alone is not decisive. The full width at half maximum is more reliable: POM-H lots generally fall below 6 K, whereas POM-C lots exceed 8 K. A low-temperature shoulder between 168 °C and 172 °C on the main homopolymer peak indicates a POM-H/POM-C blend. Heat of fusion is not a primary identification property because it shifts with nucleating agents, mold cooling rate, and additive content.

Because machining and processing history can alter crystallinity, the first heating run should be followed by controlled cooling at 10 K/min to 25 °C and a second heating run. The second heating peak is used for identification; the first heating peak reflects processing history. The DSC cell should be calibrated with indium according to ISO 11357-1 before the run. A specimen mass below 8 mg can exaggerate baseline curvature and broaden the apparent melting peak.

Thermogravimetric Isothermal Weight Loss at 220°C Under Nitrogen

Unstabilized POM-H depolymerizes by formaldehyde unzipping from chain ends above 200 °C. Heat-stabilized commercial homopolymer grades resist this autocatalytic degradation, but their isothermal weight loss signature still differs from POM-C. Thermogravimetric analysis is performed according to ASTM E1131-20 or ISO 11358-1:2022. A 5 mg to 10 mg specimen is dried at 105 °C for 2 h under vacuum, then held at 220 °C for 60 min in nitrogen at 40 mL/min. Homopolymer lots typically show 1.0 % to 2.5 % mass loss; heat-stabilized POM-C grades often show 0.3 % to 0.8 %. Some highly stabilized POM-H grades can fall below 1.0 %, but a lot that loses less than 0.3 % and fails the DSC peak-shape criterion is likely POM-C. The pan material must be recorded: residual formaldehyde can interact with aluminum pans above 200 °C, so platinum or alumina pans are preferred. An abrupt mass-loss event before 30 min at 220 °C suggests insufficient stabilization, reclaimed material, or copolymer contamination. The isothermal mass-loss test is not valid below 5 mg because purge-gas flow and pan effects dominate the signal.

On a 40-mm single-screw extruder processing unfilled POM-H at 40 kg/h, the melt temperature measured by an immersion probe should remain below 220 °C. Above 230 °C, formaldehyde evolution increases rapidly, and residence time above 220 °C should be kept under 10 min. This processing boundary is the reason that a lot failing the isothermal weight-loss check is not suitable for hot-runner molding or long residence-time operations. Twin-screw compounding of POM-H should use low-shear screw elements because high-shear mixing can generate local melt temperatures above the degradation threshold even when barrel set points are controlled.

A diamond attenuated total reflectance (ATR) accessory coupled to a Fourier-transform infrared spectrometer is used to compare the 4000 cm⁻¹ to 650 cm⁻¹ absorbance spectrum against a certified POM-H reference lot. The POM backbone produces strong C-O-C stretching bands between 900 cm⁻¹ and 1150 cm⁻¹. Homopolymer and copolymer differ mainly in the comonomer-derived ethylene oxide sequences, which introduce a broad absorbance envelope in the 1050 cm⁻¹ to 1150 cm⁻¹ region that is less dominant in POM-H. Second-derivative or ratio-based comparisons against a certified reference are required because ATR band intensity varies with crystal contact and specimen surface finish. The diamond ATR crystal should be cleaned with anhydrous ethanol or isopropanol before each spectrum; the specimen surface should be flat and free of machining coolant residue. At least 32 co-added scans at 4 cm⁻¹ resolution are recommended. Spectral subtraction of the POM-H reference from the unknown is more informative than visual overlay; a positive residual band near 1090 cm⁻¹ indicates comonomer-derived absorbances. The method is semi-quantitative; low-level POM-C contamination below 10 % may not be reliably detected unless calibration sets are prepared from the same grades and processing conditions. ASTM E1252-98(2021) provides general qualitative infrared practice; no public standard gives a universal acceptance limit for acetal homopolymer identity.

When the Molded Test Bar Does Not Reproduce Homopolymer Shrinkage in a 48-Hour Post-Mold Interval

Unfilled POM-H has a density of 1.420 g/cm³ to 1.430 g/cm³ measured by ISO 1183-1:2019 at 23 °C; unfilled POM-C is typically 1.405 g/cm³ to 1.415 g/cm³. A density below 1.415 g/cm³ after correction for glass or mineral fillers indicates a copolymer or blend. Density alone is insufficient because mineral-filled POM-C can exceed 1.45 g/cm³. Mold shrinkage is therefore paired with density. Injection-molded tensile bars per ISO 294-3 and ISO 527-2:2012 are measured after 48 h at 23 °C and 50 % RH. Unfilled POM-H typically records parallel shrinkage of 1.8 % to 2.2 % and perpendicular shrinkage of 1.5 % to 1.9 %; POM-C records 1.6 % to 1.9 % parallel and 1.4 % to 1.7 % perpendicular. These windows are grade-specific and should be treated as advisory rather than definitive. Shrinkage measurement should use a calibrated dial micrometer or a 3D optical scanner because the difference between the resin families can be below 0.3 %. The molded specimen must be conditioned in the same orientation and gate location; injection rate, melt temperature, and mold temperature affect frozen-in stress and shrinkage.

When thermal identity is uncertain, tensile yield stress provides a supplementary material property. Unfilled POM-H tested at 50 mm/min per ISO 527-2:2012 gives a tensile yield stress between 66 MPa and 72 MPa and tensile modulus between 3100 MPa and 3500 MPa. Unfilled POM-C typically gives 57 MPa to 65 MPa and 2700 MPa to 3000 MPa. Notched Charpy impact per ISO 179-1:2010 is not diagnostic because both families can be formulated to similar values. ASTM D638-14 Type I data are not interchangeable with ISO 527-2:2012 data because specimen thickness and strain measurement differences produce offset values.

Concentrated mineral acids, strong oxidizing media, and alkaline cleaning baths above pH 10 at elevated temperature can attack POM; chlorinated solvents at elevated temperature can induce stress cracking. Amine-based additive masterbatches should not be introduced into POM-H melts without prior screening because basic amine residues can accelerate chain scission. These operational boundaries apply after the identity check because a mislabelled POM-C lot may fail earlier in the same chemical environment than POM-H.