Environmental Stress Cracking

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

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

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

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.

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