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Identifying an unknown polymer: what each method delivers and where the conclusion ends
The usual starting point: a part or a bag of pellets with no documents. The supplier is gone, the specification is lost, production has to continue. The question comes in short form — "identify the grade".
From a single spectrum or a standard set of analyses, a specific commercial grade generally cannot be confirmed unambiguously. Analysis establishes the material class and a set of measurable characteristics. Matching against a specific grade becomes realistic only with an authenticated reference and a sufficient body of comparative data — and even then it remains a match, not proof of origin: formulations change without the trade name changing.
The practical value of analysis lies elsewhere. It produces an analytical profile sufficient to compile a list of candidate grades and a plan for verifying them.
On the scope of this text. The general logic of identification applies across polymers. Some of the methods and examples below — viscosity number, conditioning, drying before rheological testing — are given primarily for polyamide compounds, since those are what most substitution requests involve.
Representativeness: where the error starts
The most frequent cause of a wrong conclusion is neither the instrument nor the method, but the place the sample was taken from.
An unknown part may be multilayer, pigmented, lacquered, contaminated with lubricant, made from a polymer blend, carry a local masterbatch, be partially degraded in a heated zone, be produced from heterogeneous recycled feedstock, or be assembled from different materials. Near the gate the material is packed and oriented differently than at the periphery.
Analysis of one small fragment will honestly describe that fragment — and present it as the whole material.
So before methods are chosen, the record fixes: where the sample was cut from, whether surface or bulk is analysed, how many locations were examined, whether visible layers, inclusions or colour differences exist, whether coating was removed, whether repeats were run. These lines in the report weigh no less than the results themselves.
What each method delivers
| Method | What it delivers | Where the limit lies |
|---|---|---|
| Visual inspection, density | Product type, presence of filler, approximate class | Nothing quantitative |
| Ash content (ISO 3451-1; for polyamides conditions per ISO 3451-4) | Mass of inorganic residue under the specified conditions of the method | Not equal to the original filler mass: some mineral substances change composition on ignition. Does not distinguish glass, talc, chalk |
| TGA | Mass loss distribution under a defined temperature programme and atmosphere; with sequential inert and oxidative stages it separates the polymer fraction, carbon residue and ash | The decomposition profile supports matrix identification but is rarely conclusive without a reference library or other methods |
| FTIR | Polymer class from functional groups, sometimes the copolymer type | Gives neither grade nor molar mass; stabilisers at typical concentrations fall below the threshold of confident detection |
| DSC | Melting and crystallisation temperatures and enthalpies, Tg where a measurable transition exists, the influence of thermal history | The first heating retains processing and storage history, the second shows behaviour after controlled cooling. Crystallinity is calculated only after the matrix is identified, a reference enthalpy chosen and a filler correction applied |
| Viscosity number (ISO 307) — for soluble polyamides | A parameter correlated with molar mass for a defined polymer–solvent–temperature system | Not applicable to insoluble, crosslinked and many other classes. Gives no molar mass distribution |
| SEC/GPC (ISO 16014) | Average molar masses and the molar mass distribution of the soluble polymer fraction | Requires a suitable solvent and calibration; filler and the insoluble fraction stay outside the analysis |
| Microscopy | Filler morphology, fibre length, dispersion, layer structure | Chemical composition |
| EDX/EDS | Elemental composition of a selected area with spatial resolution | Limited sensitivity to trace and light elements; the result depends on surface and specimen geometry. Gives no molecular form |
| ICP-OES / ICP-MS | Bulk elemental composition after dissolution or digestion, high sensitivity | Does not show where in the structure the element was; does not establish molecular form; requires demanding preparation |
| Chromatography with mass spectrometry | Identification and, under a validated method, quantification of individual extracted or pyrolysis components | Does not reconstruct the full formulation and does not cover substances that are not extracted or that decompose during analysis |
| Capillary rheometry (ISO 11443) | Viscosity curve over a shear rate range close to processing conditions | Nothing about composition |
The table reads in pairs: each method answers the question the previous one left open. Ash content says inorganic material is present and does not say which. Microscopy and elemental analysis say which, and do not say how it is bonded to the matrix. DSC confirms the matrix and stays silent on additives.
How the method sequence is built
There is no ready universal protocol. The plan follows from the hypothesis, the sample type and the level of conclusion required; cheap screening methods go first only when they genuinely eliminate a large share of the options.
For a transparent unfilled pellet, FTIR belongs at the start: it settles the class question immediately. For a black filled compound it is wiser to plan TGA or pyrolysis in advance, because carbon black will suppress the spectrum. A multilayer part starts with microscopy and layer separation, otherwise a mixture is analysed. A contaminated surface calls for a section taken from the bulk. Where a crosslinked or thermoset material is suspected, the solution-viscometry route drops out entirely. And if the task is comparison against a known reference, the scheme differs fundamentally from blind identification.
Where the result is distorted
Carbon black. It absorbs broadly and suppresses the spectrum. Black filled samples without surface preparation or pyrolysis yield little usable information.
Oxidation and service life. A sample that has worked in the field carries carbonyl bands from degradation. Reading them as evidence of a copolymer is a classic interpretation error.
Glass fibre. It shifts the baseline and introduces bands in the region that matters for matrix identification.
Moisture. It shifts DSC results and makes melt flow measurement on polyamide meaningless without controlled drying.
A specimen cut from a part. It carries fibre orientation and residual stresses. Mechanical values from such a specimen are not comparable with datasheet figures: the datasheet describes a standard specimen moulded under normative conditions. Part mechanics are measured to compare part with part.
When the answer stays ambiguous
Not every investigation ends in a clear conclusion, and an honest report shows this.
A realistic wording looks like this: the evidence is consistent with PA6 or PA6/66 containing an inorganic filler; unambiguous discrimination is not possible with the data set obtained. The melting points of these two systems lie close together, the spectra in the characteristic region are similar, and ash content is the same for glass and for part of the mineral fillers.
What follows from that: either extend the method set, or take the decision at class level and build a larger verification scope into trial moulding. Both routes are legitimate. The illegitimate third is naming a grade so that the report looks finished.
What is needed from the customer
The quality of the conclusion depends on input data more than on the list of instruments.
A pellet is often more informative than a finished part, since it has not gone through secondary processing in a tool or through service ageing. It does already carry the thermal and shear history of compounding — a pellet is not an untouched sample.
If only the part is available, the manufacturing route, service life and environment need to be known.
Then come the part function and the failure criterion. Without them the analysis answers the abstract question "what is this", while the real question is usually different: "what can replace this so that the part works". Different scope of work.
Any surviving documentation matters, even partial: an old specification, a photograph of the marking on the bag, a supplier mentioned in correspondence.
From result to decision
The conclusion of an identification is an analytical profile: matrix class, type and content of the inorganic component, indications of the additive package, parameters available for the identified class, a list of candidate grades and a verification plan. Not confirmation of a grade.
From there the logic of substitution levels applies, set out in the article on functional grade substitution. Identification brings you to level one — candidate on documentation. Reaching a qualified part requires a CTQ list, trial moulding and a pilot lot. Analysis replaces none of those steps.
The role of Material Wizard
Part of the methods is performed in-house, part in partner laboratories. The report states which method was performed where, to which standard and under what conditions, so that the result can be reproduced or challenged.
The customer receives an analytical profile of the material, a list of candidate grades and a verification plan proportionate to the function of the part.
What we do not do: we do not name a commercial grade as an established fact on the basis of spectral analysis, we do not issue conclusions on the suitability of a finished product, and we do not replace the approval holder's procedure where one applies.
Checklist before sending a sample
- Sample in pellet form where available
- Recorded where the fragment was cut from and whether the part has layers, inclusions or different colours
- Manufacturing route, service life and environment known
- Part function and failure criterion stated
- All surviving documentation collected, even partial
- Agreed what the output should be: class identification or a candidate list for substitution
Sources and limits of applicability
The methods are covered by ISO 3451 (ash content, general part and the part for polyamides), ISO 307 (viscosity number of polyamides), ISO 16014 (SEC/GPC), ISO 11357 (DSC), ISO 11358 (TGA), ISO 11443 (capillary rheometry). Current editions are to be verified against the ISO catalogue at the date of publication.
The description of capabilities and limits is general and is not the specification of any particular laboratory. The result of identification is an analytical profile for finding candidates, not confirmation of a grade. The scope of further verification is determined by the function of the part and the level of failure risk.