Key Takeaways
- ESC requires stress and chemical exposure at the same time — either alone is usually harmless, which is why materials data sheets do not predict it.
- Assembly supplies both. Adhesives, solvents, coatings and the clamping used to apply them introduce agent and stress simultaneously.
- Amorphous polymers are the vulnerable class — polycarbonate, ABS, acrylic, polysulfone and PPO fail this way far more than semi-crystallines.
- Residual molded-in stress is invisible and frequently larger than service load; annealing and polarised-light inspection are cheap ways to find it.
Environmental stress cracking is the failure mode that most often surprises engineering teams, because it defeats the way plastics are normally specified. The material meets its tensile strength. The part is loaded well below that strength. The chemical it contacts is listed as compatible. And the assembly cracks anyway, weeks after it shipped, in a brittle manner completely unlike the material’s data sheet behavior.
The explanation is that ESC is not a property of the material alone. It is a property of the combination of material, stress state and chemical environment — and the specification process usually evaluates those three separately.
The Mechanism, Briefly
Amorphous polymers under tensile stress form crazes: microscopic regions of voids bridged by drawn polymer fibrils. Under normal conditions crazing absorbs energy and is largely benign. Introduce a chemical agent with the right solubility relationship to the polymer, and it penetrates the craze, plasticises the fibrils locally, and lowers the stress required for the craze to develop into a crack. The polymer is not dissolved or chemically degraded — conventional chemical compatibility testing on unstressed samples finds nothing wrong — but the energy barrier to crack growth has collapsed.
Two consequences follow. First, both ingredients are strictly necessary: an unstressed part in the same chemical is fine, and a stressed part in clean air is fine. Second, the process is time-dependent and threshold-based, so a part can survive months and then fail, and a small increase in stress or concentration can move the failure time by an order of magnitude.
Why Assembly Is the Usual Culprit
Bonding, coating and decorating operations are unusually efficient at supplying both required conditions at the same place.
- Solvents. Solvent-based adhesives, primers, cleaning wipes and inks bring the chemical agent into direct contact with the polymer, often in the highest-stress region of the part because that is where the joint is.
- Cure shrinkage. Adhesives and coatings shrink as they cure, and a rigid film bonded to a rigid substrate transmits that shrinkage as tensile stress in the substrate surface — precisely where crazes initiate.
- Assembly stress. Press fits, snap fits, self-tapping screws, clamping during cure and mismatched thermal expansion all impose sustained stress. Unlike service loads, these never go away.
- Residual molded-in stress. Frequently the largest term of all, and invisible. A part can leave the tool with internal stress comparable to or greater than anything service will apply, concentrated near gates, thickness transitions and knit lines.
An investigation that treats service load as the stress term and ignores the other three will conclude that the part is loaded well within limits and that ESC is impossible. That conclusion is wrong often enough to be worth checking every time.
Finding the Stress You Cannot See
Residual stress measurement does not require sophisticated equipment. Transparent and translucent parts placed between crossed polarising filters display birefringence patterns that map stress distribution directly; the technique takes minutes and immediately shows whether gate regions and knit lines are carrying high stress. For opaque parts, controlled immersion in a defined reagent for a defined time produces crazing preferentially in stressed regions, giving a qualitative map. Annealing a sample just below its glass transition and measuring dimensional change quantifies how much stress was frozen in.
Where residual stress is confirmed as significant, the corrective actions are molding actions: higher mold temperature, slower fill, adjusted packing, gate relocation, thicker-section redesign to avoid abrupt transitions, and in some cases a post-mold anneal. These are usually cheaper than changing the material and always cheaper than a field campaign.
Designing the Chemical Exposure Out
Where the agent is introduced by the process, substitution is often available and underused. Solvent-free or water-based cleaners in place of aggressive solvents; UV-cure or hot-melt adhesives in place of solvent-cemented joints; low-shrinkage adhesive formulations; and, for polycarbonate and acrylic in particular, adhesive chemistries specifically qualified as non-crazing on those substrates.
Where the agent arrives in service — cleaning products, sunscreens, insect repellent, fuels, plasticizers leaching from adjacent parts, disinfectants in medical settings — the design response is to reduce sustained tensile stress at the exposed surface, place joints away from exposure, or move to a semi-crystalline material. Sunscreen and hand sanitiser deserve particular mention because they cause a steady stream of field failures in consumer and medical products and are almost never in the original requirements document.
Testing That Actually Predicts
Standard chemical compatibility tables are compiled on unstressed specimens and therefore systematically understate ESC risk. Meaningful testing applies stress and chemical simultaneously. Bent-strip testing holds specimens at a defined strain while exposed to the agent; a constant tensile load test measures time to failure at a series of stress levels; and testing on the actual assembled part, with the real adhesive and the real clamping, is the only method that captures the stress contribution from assembly itself.
Run the actual service chemicals rather than surrogates, including the ones nobody specified but everyone uses. Test at the highest realistic temperature, because ESC accelerates strongly with temperature. And run to failure rather than to a fixed duration where possible, because knowing the margin is worth more than knowing the part survived an arbitrary interval.
Recognizing It in the Field
ESC has a distinctive appearance that separates it from the other ways a plastic part breaks. The fracture is brittle even in a material that is normally ductile, with little or no drawing or stress whitening at the break. The crack surface is typically smooth and glassy near its origin, and often shows a mirror-like region where the crack initiated surrounded by rougher material where it accelerated. Multiple parallel cracks, or a network of fine crazes near the fracture, are common and rarely seen in simple overload failure.
Location is equally diagnostic. ESC cracks initiate where tensile stress is highest and where chemical contact occurred, so they cluster at gates, knit lines, wall transitions, around inserts and screw bosses, and immediately adjacent to bond lines and coated edges. A crack that starts at the edge of an adhesive bead and runs into the part is nearly a signature.
Where the diagnosis is in doubt, the confirmation is a reproduction test: take unstressed parts from the same lot, apply a known strain, expose them to the suspected agent, and see whether the same fracture appearance develops. If it does, the mechanism is settled and the corrective action — reduce stress, change the agent, or change the material — can be chosen on cost rather than on speculation.
Related Reading
- Adhesive Bonding and Joining
- Root-Cause Analysis Methods
- Polycarbonate
- Knit Lines and Weld Lines as Hidden Causes
- Best Practices for Bonding Semi-Crystalline Thermoplastics
Need help with this?
The Sabreen Group provides independent engineering support for diagnosing and designing out environmental stress cracking in assembled plastics. Our engineering services team works with manufacturers on process development, material qualification and production troubleshooting. Contact us to discuss your application.
Frequently Asked Questions
What exactly is environmental stress cracking?
ESC is brittle cracking of a polymer under the combined action of mechanical stress and a chemical agent, at stress levels and chemical concentrations that neither would cause failure on its own. The agent does not chemically attack the polymer; it accelerates crazing and crack growth by reducing the energy needed to form new surfaces.
Which plastics are most susceptible?
Amorphous thermoplastics are the vulnerable class: polycarbonate, ABS, acrylic, polystyrene, polysulfone and modified PPO. Semi-crystalline materials such as polypropylene, acetal, nylon and PBT are considerably more resistant because their crystalline regions impede craze propagation, though they are not immune under aggressive conditions.
Can an adhesive cause stress cracking?
Yes, and it is a common mechanism. Solvent-based adhesives and cleaners supply the chemical agent directly, uncured components of reactive systems can act as agents, and cure shrinkage supplies stress. A joint can therefore create both required conditions at the same point at the same moment, which is why bonded polycarbonate and acrylic assemblies fail this way so often.
How do we detect residual stress in a molded part?
Polarised light shows stress patterns in transparent parts directly. For opaque parts, a solvent immersion test using a controlled reagent reveals stressed regions as crazing. Annealing a sample below its glass transition and re-measuring dimensions indicates how much frozen-in stress was present. All three are inexpensive relative to a field failure.
Does ESC appear immediately or over time?
Usually over time, which is what makes it dangerous. Crack initiation can take weeks or months at service stress levels, so parts pass every production test and fail in the field. Accelerated testing with the actual service chemicals under applied stress is the only reliable way to expose it before customers do.
