Environmental Stress Cracking
Key Takeaways
- ESC needs stress and a chemical agent together — neither alone would cause the failure.
- It is widely cited as the largest single cause of in-service plastic part failure.
- The agent need not attack the polymer chemically. Many are benign in a stress-free part.
- Amorphous polymers are the most vulnerable, polycarbonate and ABS notably so.
Environmental Stress Cracking
Environmental stress cracking (ESC) is the brittle failure of a polymer under the combined action of mechanical stress and contact with a chemical agent, at stress levels and chemical concentrations that would each be harmless on their own. The failure is physical rather than chemical: no significant degradation of the polymer occurs, which is precisely why it catches people out.
The agent penetrates the polymer surface and plasticises the region under stress, lowering
the local resistance to crazing. Crazes form, coalesce into cracks, and the part fails in a
brittle manner even though the material is nominally ductile. Analysis of the failed part
often shows the polymer to be entirely within specification, which is what makes ESC failures
so persistently misdiagnosed as material defects.
The Three Requirements
| Requirement | Where it comes from | What reduces it |
|---|---|---|
| Tensile stress | Applied load in service, residual moulding stress, assembly strain from press fits, snap fits and over-torqued fasteners, or thermal expansion mismatch | Annealing, gate and cooling optimisation, generous radii, designed-in compliance at joints |
| A chemical agent | Cleaning fluids, lubricants, inks, adhesives, plasticisers migrating from adjacent parts, sunscreens, foods, disinfectants | Material and agent compatibility screening across everything the part will actually contact |
| A susceptible polymer | Amorphous polymers most of all; semi-crystalline polymers are more resistant but not immune | Material substitution, or higher molecular weight grades of the same polymer |
Remove any one of the three and the failure does not occur. That is the practical basis of
every prevention strategy, and it is usually cheapest to attack the stress.
Susceptibility by Polymer
| Polymer | Susceptibility | Common triggering agents |
|---|---|---|
| Polycarbonate | High | Many solvents, some inks and adhesives, certain cleaning agents, some greases |
| ABS | High | Alcohols including isopropanol, ketones, many solvents |
| Acrylic (PMMA) | High | Alcohols, solvents, some cleaning products |
| Polystyrene | High | A wide range of organics; among the most susceptible in common use |
| Polyethylene | Moderate — the classic historical case | Detergents and surfactants; the failure mode that drove the original ESCR test methods |
| Polypropylene | Lower | Some solvents at elevated temperature |
| PPS, PEEK and high-temperature aromatics | Low | Highly resistant; part of why they are specified |
Why It Matters in Decorating and Marking
ESC is a recurring and under-recognised risk in exactly the operations that follow
moulding, because those operations introduce both stress and chemistry:
- Laser marking adds local thermal stress on top of residual moulding
stress. On polycarbonate and acrylic, crazing may appear hours or days after marking rather
than immediately — so a part that passes inspection at the machine can fail the
following shift. - Inks, adhesives, primers and their solvents are chemical agents applied
directly to a stressed surface. A cross-hatch adhesion test passing says nothing about
whether the ink will initiate cracking in service. - Cleaning steps before decorating are a frequent culprit. Isopropanol is
the standard pre-bond wipe and is also a well-known ESC agent for ABS and acrylic. - Marking across a gate, weld line or rib root places the thermal input
exactly where residual stress concentrates. - Assembly strain compounds it. A part decorated without incident can
crack once it is snap-fitted or screwed down, because assembly supplies the missing stress.
Prevention and Testing
- Reduce residual stress. Anneal where the application warrants it,
optimise gate location and cooling, and avoid sharp internal corners. This is usually the
cheapest of the three levers to pull. - Screen every chemical the part will meet — not only the intended
ones, but cleaning agents, hand creams, lubricants and plasticisers migrating from adjacent
components. - Test under stress, not on flat coupons. This is the single most common
testing mistake. Bent strip and elliptical jig methods apply a known strain during chemical
exposure, which is the only way the interaction shows up. ASTM D543 covers chemical
resistance, and ASTM D1693 the classic polyethylene ESCR method. - Use a solvent stress test to reveal residual stress during process
development — controlled exposure makes stress concentrations visible as crazing before
they cause field failures. - Inspect after a delay. Build a 24-hour or longer re-inspection into
qualification, since crazing frequently develops after the part has left the cell.
Recognising an ESC Failure
| Evidence | What it points to |
|---|---|
| Brittle fracture in a material that should be ductile | The signature observation. A ductile polymer failing without yielding is the first indicator. |
| Multiple crazes near the fracture origin | Crazing is the precursor mechanism; its presence separates ESC from simple overload. |
| Cracks originating at a gate, weld line or rib root | Residual stress concentration supplying the mechanical half of the requirement. |
| Failure at loads well below the design limit | Suggests the chemical contribution rather than a load calculation error. |
| Material testing entirely within specification | Characteristic of ESC and the reason it is so often misattributed. |
| Failures clustering after a cleaning or assembly change | A new agent or a new strain has completed the combination. Change history is often the fastest route to the cause. |
| Cracks perpendicular to the stress direction | Consistent with tensile-driven crazing rather than impact or fatigue. |
When investigating, resist concluding from the material test alone. An ESC investigation
has to reconstruct the stress state and the chemical exposure history together, because the
material data on its own will look unremarkable.
Related Terms and Reading
- Polycarbonate
- Adhesion failures
- Failure analysis and engineering services
- Plastics laser marking solutions
- Accelerated aging
Applying this in production
The Sabreen Group provides independent engineering support for failure analysis, environmental stress cracking investigation and materials compatibility screening. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
What causes environmental stress cracking?
The simultaneous presence of tensile stress and a chemical agent, in a susceptible polymer. Neither factor alone would cause failure at the levels involved, which is what makes ESC so easy to miss — the stress is within design limits and the chemical is one the material datasheet lists as compatible. It is their interaction that lowers the resistance to crazing.
Why do failure analyses often show the material was within specification?
Because ESC is a physical failure rather than a chemical one. The agent plasticises the stressed region and promotes crazing without significantly degrading the polymer, so testing the failed part finds molecular weight, composition and mechanical properties all normal. That result frequently leads to the wrong conclusion that the material was fine and the load was excessive.
Which plastics are most vulnerable?
Amorphous polymers, particularly polystyrene, acrylic, ABS and polycarbonate. Semi-crystalline polymers such as polypropylene are more resistant because the crystalline regions impede agent penetration, and the high-temperature aromatics like PPS and PEEK are highly resistant — which is part of why they are specified for demanding chemical environments despite their cost.
How should ESC resistance be tested?
Under stress. Testing a flat unstressed coupon in the chemical is the most common mistake, because it removes the very interaction being investigated and almost always passes. Bent strip and elliptical jig methods apply a known strain during exposure; ASTM D543 covers chemical resistance and ASTM D1693 the classic polyethylene method. Screen every chemical the part will actually contact, including cleaning agents.
Can laser marking trigger stress cracking?
It can contribute, by adding local thermal stress on top of residual moulding stress — particularly on polycarbonate and acrylic, and particularly when the mark crosses a gate, weld line or rib root where stress already concentrates. Crazing often appears hours or days later rather than at the machine, so qualification should include a delayed re-inspection rather than relying on an immediate visual check.
