Why does a chemical degradation analysis of a polymer part need an unexposed control?
A chemical degradation analysis of a polymer part needs an unexposed control because every measurement in that analysis is a comparison, and the finding is the difference between the exposed material and unexposed material of the same grade and ideally the same lot. A molecular weight figure, a spectrum or a melting endotherm from a failed polymer part means very little in isolation, because polymers vary between grades, between lots and between mold cavities. The need for control material is easy to state and often impossible to satisfy after the fact.
Suitable controls for a failed polymer part include unused inventory, an unexposed region of the same part, a sibling from the same production run, or retained material from the molder. Where no control material exists, a chemical degradation analysis is confined to what is chemically anomalous on its own terms, which is a materially weaker position.
Why confirm what polymer a failed part is made of before testing it for degradation?
A failure analysis of a polymer part should establish what the material is before asking what happened to it, because the polymer in hand is not always the polymer on the drawing. Parts are substituted, resins are re-sourced, and regrind is introduced. Infrared spectroscopy and thermal analysis identify the base polymer, and a mismatch between the identified polymer and the specification is a finding in its own right that reframes everything downstream in the analysis.
What does infrared spectroscopy (FTIR) show about chemical degradation in a polymer part?
Infrared spectroscopy (FTIR) shows that a chemical change occurred in a polymer part and gives the chemical character of that change, by reading the functional groups present in the material. Oxidation and hydrolysis both introduce species the virgin polymer did not have, and an FTIR spectrum from an exposed surface compared against a spectrum from unexposed control material commonly shows new absorbance where those groups appear. FTIR depth profiling across a sectioned wall shows whether the change is a surface skin or runs through the thickness.
FTIR does not establish the severity or the sequence of a chemical change in a polymer part. An FTIR spectrum does not say how much strength was lost, or whether the chemical change preceded the failure or followed it.
What does thermal analysis (DSC and TGA) detect in a degraded polymer part?
Thermal analysis detects changes in a polymer’s transitions and composition: differential scanning calorimetry (DSC) measures glass transition temperature, melting and crystallinity, and thermogravimetric analysis (TGA) measures mass loss with temperature. In DSC, a shifted glass transition is a common indicator of plasticizer loss or absorbed solvent, and a change in crystallinity can follow chain scission. TGA resolves the composition of a compound — polymer, additive, filler — which is how loss of a plasticizer or a filler fraction is quantified.
How does molecular weight testing show chain scission in a degraded polymer?
Molecular weight is the direct measure of chain scission in a polymer: hydrolysis and oxidation both cut chains, and cutting chains lowers molecular weight. Where the polymer is soluble, gel permeation chromatography (GPC) measures the molecular weight distribution directly, and a downward shift against unexposed control material 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.
Molecular weight is also the measurement that most cleanly separates a chemically degraded polymer part from a physically damaged one. Where molecular weight is unchanged, chain-scission mechanisms are largely excluded and the explanation for the failure lies elsewhere.
How does extraction testing find what is present in, and missing from, a polymer part?
Extraction with chromatographic analysis works in both directions: it identifies what is present in a polymer part and quantifies what should be there and is not. On the first count, extraction with chromatographic analysis identifies absorbed contaminants and degradation byproducts held in the material. On the second, it quantifies missing constituents such as the residual antioxidant, the plasticizer fraction and the stabilizer.
Depletion of those constituents against unexposed control material is frequently the clearest evidence of extraction as a degradation mechanism. Extraction as a mechanism leaves the base polymer’s spectrum essentially unaltered, so a polymer part degraded by extraction can read as normal on identification testing alone.
How do you connect chemical changes in a polymer part to its mechanical failure?
Chemical change in a polymer part is connected to its mechanical failure by tensile testing under ASTM D638 on specimens from exposed and unexposed control material, followed by fracture surface examination, because chemical change only matters if it changed how the part performs. In that tensile testing, 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 between chemical change and mechanical failure in a polymer part. 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.
Why reproduce the chemical exposure in the lab when proving polymer degradation?
Reproducing the chemical exposure under controlled conditions is the strongest demonstration of chemical degradation in a polymer part, because it tests whether the proposed mechanism actually produces the observed signature. Exemplar material is exposed to the suspect chemical under ASTM D543 or ISO 175, at the concentration and temperature the service condition supports, and the same analytical sequence is then run on the result.
A preserved sample of the fluid the polymer part contacted earns its keep at this stage. 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.
Does finding a chemical on a failed plastic part prove the chemical caused the failure?
No: identifying a chemical on a failed plastic part is not proving that the chemical caused the failure. A chemical detected on a part may have arrived during service, during the failure, during cleanup, or during handling. Finding the chemical is comparatively easy; establishing that it degraded the polymer, and that the degradation caused the failure, is a separate exercise with a higher evidentiary bar.
Causation requires more than detection: that the chemical is capable of the observed mechanism in that polymer, that the material shows the corresponding change relative to unexposed control material, 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 guidance on proving chemical degradation in polymer parts 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.