Finding a chemical on a failed plastic part is comparatively easy. Establishing that the chemical degraded the polymer, and that the degradation caused the failure, is a separate exercise with a higher evidentiary bar. Between those two points sits a sequence of laboratory work, each step answering a narrower question than the last, and all of it resting on one requirement that is easy to state and often impossible to satisfy after the fact: something to compare the failed part against. The order of the work matters as much as the methods.
The control governs everything after it
Every measurement described below is a comparison. A molecular weight figure, a spectrum or a melting endotherm from a failed part means very little in isolation, because polymers vary between grades, between lots and between mould cavities. The finding is the difference between the exposed material and unexposed material of the same grade and ideally the same lot.
Suitable controls include unused inventory, an unexposed region of the same part, a sibling from the same production run, or retained material from the moulder. Where none exists, the analysis is confined to what is chemically anomalous on its own terms, which is a materially weaker position.
Confirm what the polymer is first
Before asking what happened to a material, establish what the material is. Parts are substituted, resins are re-sourced, regrind is introduced, and the polymer in hand is not always the polymer on the drawing. Infrared spectroscopy and thermal analysis identify the base polymer, and a mismatch against specification is a finding in its own right that reframes everything downstream.
What infrared spectroscopy shows
FTIR reads the functional groups present in the material. Oxidation and hydrolysis both introduce species the virgin polymer did not have, and a spectrum from an exposed surface compared against a control commonly shows new absorbance where those groups appear. Depth profiling across a sectioned wall shows whether the change is a surface skin or runs through the thickness.
What FTIR does not do is establish severity or sequence. It shows that a change occurred and gives the chemical character of that change. It does not say how much strength was lost, or whether the change preceded the failure or followed it.
Thermal analysis and what it detects
Differential scanning calorimetry measures transitions: glass transition temperature, melting and crystallinity. A shifted glass transition is a common indicator of plasticiser loss or absorbed solvent, and a change in crystallinity can follow chain scission. Thermogravimetric analysis measures mass loss with temperature and resolves the composition of a compound — polymer, additive, filler — which is how loss of a plasticiser or a filler fraction is quantified.
Molecular weight is the direct measure of scission
Hydrolysis and oxidation both cut chains, and cutting chains lowers molecular weight. Gel permeation chromatography, where the polymer is soluble, measures the distribution directly, and a downward shift against a control is about as direct a demonstration of chain scission as the discipline offers. For polymers that resist dissolution, solution or melt viscosity methods stand in as proxies.
This is also the measurement that most cleanly separates a chemically degraded part from a physically damaged one. Where molecular weight is unchanged, chain-scission mechanisms are largely excluded and the explanation lies elsewhere.
Finding what is present and what is missing
Extraction with chromatographic analysis works in both directions. It identifies absorbed contaminants and degradation byproducts held in the material, and it quantifies what should be there and is not — the residual antioxidant, the plasticiser fraction, the stabiliser. Depletion against a control is frequently the clearest evidence of extraction, a mechanism that leaves the base polymer's spectrum essentially unaltered and can therefore read as normal on identification testing alone.
Connecting chemistry to mechanical consequence
Chemical change only matters if it changed how the part performs. Tensile testing under ASTM D638 on specimens from exposed and control material closes that gap, and retained elongation is usually the more sensitive measure, since embrittled material can hold much of its strength while losing most of its ductility.
Fracture surface examination completes the connection. A brittle fracture morphology in a material specified as ductile, with an initiation site at a chemically altered surface, links the chemistry to the mechanical event rather than leaving the two as parallel observations.
Reproducing the exposure
The strongest demonstration is a controlled one. Exposing exemplar material to the suspect chemical under ASTM D543 or ISO 175, at the concentration and temperature the service condition supports, and then running the same analytical sequence on the result, tests whether the proposed mechanism actually produces the observed signature.
This is where the preserved fluid sample earns its keep. Reproducing an exposure from a product name and an assumed concentration is an approximation; reproducing it from the actual fluid the part contacted is not.
Identifying a chemical is not proving causation
A chemical detected on a part may have arrived during service, during the failure, during cleanup, or during handling. Causation requires more: that the chemical is capable of the observed mechanism in that polymer, that the material shows the corresponding change relative to a control, that the change is sufficient to explain the mechanical failure, and that the exposure history is consistent with the extent and distribution of damage observed. Analyses that stop at detection tend to be met with an alternative source for the same chemical, and that objection is usually available.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.