What happens first when a chemical attacks a polymer?
Absorption happens first: solvation, additive extraction, hydrolysis and oxidation of a polymer all start with a small molecule getting into the polymer. Polymers are not dense the way metals are; there is free volume between and along the polymer chains, and a liquid or vapor of the right size and polarity will occupy it. What happens after that absorption is what separates the chemical attack mechanisms from one another.
Because chemical attack on a polymer begins with absorption, exposure duration and temperature matter more than any single contact event. Diffusion into a thick polymer section is slow, and a part can carry an advancing attack front through its wall for months while the outer surface looks unremarkable.
What does solvation and swelling do to a polymer part?
Solvation makes a polymer part swell, soften, and lose modulus and strength: a solvent chemically similar to the polymer is absorbed, forces the polymer chains apart, and weakens the secondary forces holding them together. Nothing in the polymer backbone has been broken by solvation, and molecular weight is essentially where it started.
Early on, solvation and swelling are partly reversible. Solvation 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.
What is plasticizer and additive extraction, and why is it easy to miss?
Plasticizer and additive extraction is the removal of unbound additives from a polymer compound by a fluid that dissolves them, and it is the chemical attack mechanism most easily missed because the base resin is untouched. Flexible polymer compounds owe their flexibility to low-molecular-weight additives that are not chemically bound to the polymer 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.
Stabilizer packages in a polymer compound behave the same way. Antioxidants are both consumable and extractable, and a polymer compound stripped of its stabilizer has lost its resistance to degradation mechanisms such as hydrolysis and oxidation without yet having failed. Additive extraction is often the first stage of a two-stage polymer failure rather than the failure itself.
How does hydrolysis degrade a polymer?
Hydrolysis degrades a polymer when water cleaves bonds in its backbone, which polymers built with ester, amide or urethane linkages are susceptible to; the chains are cut and molecular weight falls. The hydrolysis reaction is slow at ambient temperature and neutral pH and accelerates sharply with heat and with movement away from neutral pH in either direction. Acids and bases do not simply attack ester, amide and urethane polymers; they catalyze the hydrolysis reaction.
Hydrolysis cuts polymer chains throughout the water-penetrated region rather than only at the surface. The consequence of hydrolysis is embrittlement in a part that still meets its dimensional drawing and shows no discoloration, and that then fractures in a brittle manner well below its rated load.
How does oxidative chain scission degrade a polymer?
Oxidative chain scission degrades a polymer through a radical chain reaction that heat accelerates and that certain metal ions catalyze strongly, and oxidation attacks even polymers that have no hydrolyzable linkage at all. Oxidation consumes the polymer’s antioxidant package first, which is why the induction period can be long and the degradation that follows comparatively fast.
Because oxidation introduces oxygen-containing groups the virgin polymer did not have, oxidative chain scission is among the more legible polymer degradation mechanisms spectroscopically. Oxidation also tends to be surface-weighted, since it depends on oxygen availability, producing a degraded skin over a largely intact core.
How does etching or surface attack damage a polymer part?
Etching and surface attack damage a polymer part at its surface: strong acids, strong bases and oxidizing agents can degrade a polymer surface without penetrating far into it, producing roughening, pitting or discoloration while bulk properties remain close to specification. Where the etched surface was a barrier layer, its loss matters chemically. Where the polymer 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.
Can contamination speed up the degradation of a polymer part?
Yes: residual catalyst, metal-ion pickup from processing equipment, or an incidental contaminant can raise the rate of an otherwise tolerable polymer degradation process considerably, accelerating the degradation rather than causing it. In that case the base resin, tested clean, would have performed as specified; the polymer compound as actually produced did not. Contamination that accelerates degradation is a distinct category from both misuse and material selection, and it points at the manufacturing record rather than the design record.
Is environmental stress cracking a form of chemical attack?
No: environmental stress cracking belongs to a different category from chemical attack entirely, because the mechanism is not chemical. Environmental stress cracking requires sustained tensile stress, the agent need not degrade the polymer, and the polymer’s chemistry is left largely unchanged. Environmental stress cracking is the standing alternative explanation to chemical attack, and any chemical-attack finding has to survive comparison against it.
What does identifying the chemical attack mechanism reveal about how a polymer part was exposed?
Identifying the chemical attack mechanism substantially narrows what can have happened to a polymer part, because each mechanism implies particular 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. Additive extraction implies a fluid selective for the additive rather than the resin.
That narrowing of the exposure is reached from the material rather than from the account of events. Where the mechanism identified in the material and the account of events 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.