A chemically attacked polymer has been changed, not merely damaged, and the change is measurable. But chemical attack is not one process. A solvent need not react with the polymer at all. Hydrolysis needs water and a susceptible linkage. Oxidation needs oxygen and usually heat. Extraction removes something that was never bonded to the chain in the first place. Each leaves a different analytical signature and each implies a different set of service conditions, which is why naming the mechanism is generally more informative than describing the damage.
Absorption comes before anything else
Every one of these mechanisms starts with a small molecule getting into the polymer. Polymers are not dense the way metals are; there is free volume between and along the chains, and a liquid or vapour of the right size and polarity will occupy it. What happens after that is what separates the mechanisms from one another.
This is also why exposure duration and temperature matter more than any single contact event. Diffusion into a thick section is slow, and a part can carry an advancing attack front through its wall for months while the outer surface looks unremarkable.
Solvation and swelling
A solvent chemically similar to the polymer is absorbed, forces the chains apart, and weakens the secondary forces holding them together. The part swells, softens, and loses modulus and strength. Nothing in the backbone has been broken, and molecular weight is essentially where it started.
Early on this is partly reversible. It stops being reversible once the swollen state has permitted something else — dimensional change that unseats a seal, stress relaxation in a loaded part, or the loss of low-molecular-weight components carried out with the solvent as it evaporates.
Plasticiser and additive extraction
Extraction is the mechanism most easily missed, because the base resin is untouched. Flexible compounds owe their flexibility to low-molecular-weight additives that are not chemically bound to the chain. A fluid that dissolves those additives removes them, leaving a material that is still correctly identified as the specified resin and is progressively brittle.
Stabiliser packages behave the same way. Antioxidants are both consumable and extractable, and a compound stripped of its stabiliser has lost its resistance to the mechanisms below without yet having failed. Extraction is often the first stage of a two-stage failure rather than the failure itself.
Hydrolysis of the backbone
Polymers built with ester, amide or urethane linkages carry bonds that water can cleave. The reaction is slow at ambient temperature and neutral pH and accelerates sharply with heat and with movement away from neutral in either direction. Acids and bases do not simply attack these materials; they catalyse the reaction.
Hydrolysis cuts chains, so molecular weight falls, and it does so throughout the water-penetrated region rather than only at the surface. The consequence is embrittlement in a part that still meets its dimensional drawing and shows no discolouration, then fractures in a brittle manner well below its rated load.
Oxidative chain scission
Oxidation attacks polymers that have no hydrolysable linkage at all. It runs as a radical chain reaction that heat accelerates and that certain metal ions catalyse strongly. It consumes the antioxidant package first, which is why the induction period can be long and the degradation that follows comparatively fast.
Because it introduces oxygen-containing groups the virgin polymer did not have, oxidation is among the more legible mechanisms spectroscopically. It also tends to be surface-weighted, since it depends on oxygen availability, producing a degraded skin over a largely intact core.
Etching and surface attack
Strong acids, strong bases and oxidising agents can degrade a surface without penetrating far into it, producing roughening, pitting or discolouration while bulk properties remain close to specification. Where that surface was a barrier layer, its loss matters chemically. Where the part carries tensile load, a pitted surface is a crack initiation site, and the failure that follows is mechanical even though its origin is chemical.
Contamination that accelerates rather than causes
Residual catalyst, metal-ion pickup from processing equipment, or an incidental contaminant can raise the rate of an otherwise tolerable degradation process considerably. The base resin, tested clean, would have performed as specified; the compound as actually produced did not. This is a distinct category from both misuse and material selection, and it points at the manufacturing record rather than the design record.
The mechanism that is not chemical
Environmental stress cracking is the standing alternative explanation and belongs to a different category entirely: it requires sustained tensile stress, the agent need not degrade the polymer, and the chemistry is left largely unchanged. It is treated separately. The point here is only that any chemical-attack finding has to survive comparison against it.
Why the mechanism narrows the exposure
Each mechanism implies conditions. Hydrolysis implies water and usually elevated temperature or a pH excursion. Oxidation implies heat, time and oxygen availability. Solvation implies a solvent within a particular polarity range. Extraction implies a fluid selective for the additive rather than the resin.
That is a substantial narrowing of what can have happened, reached from the material rather than from the account of events. Where the two agree, the finding is well supported. Where they conflict, the conflict is itself the useful result.
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.