Almost every chemical compatibility dispute begins with a chart. A resin supplier, a distributor or a handbook lists a polymer against a chemical and marks it resistant, limited or unsuitable, and a designer relies on that rating. The rating is usually accurate for the conditions under which it was produced. The difficulty is that those conditions are narrow, often unstated, and rarely match the part in service. Understanding how the underlying data is generated is what turns a disagreement about a chart into a question that can be answered.
What a resistance rating actually reports
A compatibility rating is a summary of an immersion test. A specimen of a specific grade of resin is placed in a specific reagent at a specific concentration and temperature for a defined period, then removed and measured for change in weight, dimension, appearance and mechanical properties. The rating is a compression of those measurements into one word.
That compression is the problem. Two ratings that both read resistant may rest on different concentrations, different temperatures, different durations and different property thresholds. The word carries none of that, and a chart is not a specification.
Concentration and temperature are not footnotes
Chemical attack rates generally rise with temperature, and for hydrolytic and oxidative mechanisms they rise steeply. A material rated acceptable at ambient temperature may not be acceptable in a line that runs warm, near a heat source, or through summer in an uninsulated enclosure.
Concentration behaves less predictably. Some agents are more aggressive dilute than neat, and mixtures can attack materials that neither component attacks alone. A rating for a pure reagent tells you little about a process stream, a cleaning solution as actually diluted on site, or a fluid that has been in service and accumulated degradation products.
Stress is the variable the test does not apply
Standard immersion specimens are unstressed. Real parts carry residual stress from moulding, assembly stress from interference fits and fasteners, thermal stress from constrained expansion, and service loads. Chemical attack and applied stress do not simply add; a chemically softened or partly degraded material behaves differently under load than an unexposed one.
This is the single largest gap between a chart and a service condition, and it is where a well-founded specification decision can still produce a failed part. It is also where the separate question of environmental stress cracking enters, which is addressed on its own terms elsewhere.
Duration and the meaning of resistant
Immersion tests run for days or weeks. Service life runs for years. Extrapolating a short exposure across a long one assumes the degradation rate is constant, and for mechanisms with an induction period — oxidation in particular, where an antioxidant package is consumed before degradation becomes measurable — it is not. A test that ends inside the induction period reports no change, correctly, and says nothing about year five.
What the immersion standards do
ASTM D543, the practice for evaluating the resistance of plastics to chemical reagents, and ISO 175, its international counterpart for the effects of immersion in liquid chemicals, both define how the exposure is conducted: specimen preparation, reagent handling, temperature control, exposure period, and what is measured afterwards. They are practices for producing comparable data, not pass-or-fail criteria.
That distinction is frequently lost. Neither standard declares a material compatible with a chemical. They describe how to measure what changed, and the acceptance threshold is set by whoever specified the part.
Water is a chemical exposure
ASTM D570 covers water absorption in plastics, and the number it produces is often treated as a moulding or dimensional concern rather than a chemical one. For hydrolysable polymers it is both. Absorbed water is the reagent for hydrolysis, so an absorption figure indicates how much reagent the part will hold at equilibrium, and dimensional change from absorption is itself a functional failure mode in sealing and close-tolerance applications.
Retained properties rather than a verdict
The useful output of a compatibility programme is retained property data: tensile strength, elongation and modulus measured under ASTM D638 on exposed specimens and on unexposed controls from the same lot. Retained elongation is often the more sensitive indicator, since a material can hold most of its strength while losing most of its ductility.
Percentages against a control are arguable in a way that a one-word rating is not. They also make the acceptance criterion explicit, which is what a specification needs and a chart cannot supply.
The resin is not the compound and the compound is not the part
Ratings are published for generic resin families, but parts are made from specific compounds with specific fillers, pigments, plasticisers, flame retardants and stabiliser packages, any of which can change chemical behaviour. Processing adds more variation: residual stress, degree of crystallinity, weld lines and thermal history all affect how a part responds to the same fluid.
A rating for a resin family is therefore an upper bound at best. Where the exposure is severe or the consequence of failure is high, testing the actual compound, and preferably the actual part geometry, is the difference between an assumption and a finding.
What the specification record shows
In a dispute, the documents usually matter as much as the chemistry: what fluid was specified, what was actually used, whether concentration or temperature drifted, whether a cleaning agent or process chemical was substituted, and what the material supplier was told about the service environment. Chemical resistance data is only as good as the conditions it was selected against, and those conditions are recorded in the specification, the datasheet and the correspondence rather than in the part.
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.