How does a chemical compatibility dispute usually begin?
Almost every chemical compatibility dispute begins with a compatibility chart: a resin supplier, a distributor or a handbook lists a polymer against a chemical and marks the polymer resistant, limited or unsuitable, and a designer relies on that rating. The rating on a compatibility chart is usually accurate for the conditions under which it was produced. The difficulty is that those test conditions are narrow, often unstated, and rarely match the part in service. Understanding how the underlying chemical resistance data is generated is what turns a disagreement about a compatibility chart into a question that can be answered.
What does a chemical resistance rating actually report?
A chemical resistance rating reports a summary of an immersion test, compressed into one word. In that 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 on a compatibility chart is a compression of those measurements into one word.
That compression is the problem. Two chemical resistance 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 compatibility chart is not a specification.
How do concentration and temperature affect whether a chemical resistance rating applies?
A chemical resistance rating produced at one concentration and one temperature may not hold at another, so concentration and temperature are not footnotes to the rating. 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 than temperature. Some agents are more aggressive dilute than neat, and mixtures can attack materials that neither component attacks alone. A chemical resistance 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.
Why is stress the biggest gap between a compatibility chart and real service conditions?
Stress is the biggest gap between a compatibility chart and a service condition because standard immersion specimens are unstressed, while real parts are not. Real plastic parts carry residual stress from molding, 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.
Stress is the single largest gap between a compatibility chart and a service condition, and it is where a well-founded specification decision can still produce a failed part. Stress is also where the separate question of environmental stress cracking enters, and environmental stress cracking is addressed on its own terms elsewhere.
Does a “resistant” rating from a short immersion test predict years of service?
Not by itself: chemical immersion tests run for days or weeks while service life runs for years, and extrapolating a short exposure across a long one assumes the degradation rate is constant. For mechanisms with an induction period the degradation rate is not constant, oxidation in particular, where an antioxidant package is consumed before degradation becomes measurable. An immersion test that ends inside the induction period reports no change, correctly, and says nothing about year five.
What do the immersion standards ASTM D543 and ISO 175 actually do?
ASTM D543 and ISO 175 define how a chemical immersion exposure is conducted and measured; they are practices for producing comparable data, not pass-or-fail criteria. ASTM D543 is the practice for evaluating the resistance of plastics to chemical reagents, and ISO 175 is its international counterpart for the effects of immersion in liquid chemicals. Both standards define specimen preparation, reagent handling, temperature control, exposure period, and what is measured afterward.
That distinction is frequently lost. Neither ASTM D543 nor ISO 175 declares a material compatible with a chemical. ASTM D543 and ISO 175 describe how to measure what changed, and the acceptance threshold is set by whoever specified the part.
Is water absorption a chemical exposure for plastics?
Water is a chemical exposure, and for hydrolyzable polymers the water absorption figure measured under ASTM D570 is both a chemical concern and a molding or dimensional one. ASTM D570 covers water absorption in plastics, and the number ASTM D570 produces is often treated as a molding or dimensional concern rather than a chemical one. Absorbed water is the reagent for hydrolysis, so an absorption figure indicates how much reagent a hydrolyzable part will hold at equilibrium, and dimensional change from water absorption is itself a functional failure mode in sealing and close-tolerance applications.
What is the useful output of a chemical compatibility testing program?
The useful output of a chemical compatibility program is retained property data rather than a verdict: 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 compatibility rating is not. Retained property percentages also make the acceptance criterion explicit, which is what a specification needs and a compatibility chart cannot supply.
Does a chemical resistance rating for a resin apply to the finished plastic part?
A chemical resistance rating for a resin family is an upper bound at best for a finished part, because ratings are published for generic resin families while parts are made from specific compounds and shaped by specific processing. Plastic parts are made from specific compounds with specific fillers, pigments, plasticizers, flame retardants and stabilizer packages, any of which can change chemical behavior. Processing adds more variation: residual stress, degree of crystallinity, weld lines and thermal history all affect how a part responds to the same fluid.
Where the chemical 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 records matter in a chemical compatibility dispute?
In a chemical compatibility 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.