A PC housing that looks perfectly molded and passes initial inspection can still develop hairline cracks weeks later, radiating outward from exactly where an elastomeric overmold or gasket contacts it. This is chemical stress cracking, and unlike a simple adhesion failure, it destroys the substrate itself — which means the diagnosis has to happen before production, not after a field return.
Why PC Fails Differently Than Other Substrates
Most substrate-compatibility problems show up as delamination: the elastomer lets go, the parts come apart, and the fix is a different chemistry or better surface prep. Polycarbonate under sustained mechanical stress and exposed to an incompatible compound doesn’t just delaminate — its own molecular structure develops micro-fractures that can propagate over weeks or months, sometimes well after the part has shipped. This makes CSC risk assessment a different exercise than ordinary adhesion screening: a compound that bonds PC beautifully in a peel test can still be actively cracking it from the inside.
Recognizing the Warning Signs Before Failure
CSC on PC rarely announces itself immediately. The earliest indicator is often a faint whitening or crazing pattern at the edge of the elastomer-to-PC interface, visible under raking light or low magnification before any crack is large enough to see with the naked eye. Parts under sustained clamping load, snap-fit stress, or residual molding stress from the PC side are the most susceptible, since CSC requires both an aggressive chemical agent and a stressed substrate to develop — a compound that’s perfectly benign on unstressed PC can still crack a part with residual molding stress at the same interface.
Root Cause 1: Incompatible Additives in the Compound, Not the Base Polymer
CSC risk on PC is not an inherent property of TPU or TPE as a material category — it’s a function of the specific additive package in a given compound. Processing oils, certain plasticizers, and some flame-retardant packages are common CSC triggers regardless of whether the base elastomer is TPU, COPE, or SEBS. This is why two TPU compounds at the same hardness and from the same general chemistry family can produce completely different CSC outcomes on the same PC grade — the risk lives in the compound formulation, not the headline material name on a datasheet.
Root Cause 2: Insufficient PC-Specific Grade Documentation
TPU has the deepest bench of PC-specific, pre-screened grades among the common elastomer options, with urethane-to-carbonate ester chemistry that bonds consistently and supplier documentation that typically includes PC compatibility data directly. COPE can match TPU’s adhesion performance through ester-to-ester chemistry and often provides better elevated-temperature capability, but fewer COPE grades carry the same depth of PC-specific CSC screening data, which pushes more of the qualification burden onto the molder. SEBS lacks a natural chemical affinity for PC’s ester-dominated surface at all without adhesion promotion, and skipping that promotion step to save cost is a frequent, avoidable source of both adhesion failure and CSC risk together.
Root Cause 3: Validation Under Sample Conditions Instead of Production Conditions
A compound that passes CSC screening on a flat, unstressed test coupon can still crack a production part with snap-fit stress, assembly-induced clamping load, or molding-induced residual stress that the test coupon never experienced. Validating under a load and stress condition representative of the actual production part — not just a generic compatibility panel — is the step most frequently skipped under schedule pressure, and it’s the step most likely to catch a CSC risk that a standard screening protocol would miss entirely.
A Practical Screening Sequence Before Committing to Production
- Request PC-specific grade documentation from the elastomer supplier first — a compound with no PC compatibility data on file should not be the default choice for a PC application regardless of cost or hardness fit.
- Confirm low aromatic and solvent content in the specific compound’s additive package, since this is the single most common CSC trigger across TPU, COPE, and SEBS alike.
- Test under sustained load on an actual molded part geometry, not a flat coupon, for a duration that reflects real field exposure rather than a quick pass/fail dip test.
- Re-screen after any compound, colorant, or supplier change — CSC risk is compound-specific, and a seemingly minor formulation change can reintroduce a risk that was cleared on the original material.
For applications where the joint is a rigid adhesive bond to PC rather than an elastomer overmold — PC-to-PC or PC-to-glass, for instance — UV-cure adhesive vs. epoxy for transparent bonding covers that separate bonding decision, and how CTE mismatch causes adhesive bond failure is relevant background for any PC assembly that will also see thermal cycling on top of the CSC risk discussed here. Email Us for help screening a specific compound and PC grade combination before committing to a molding trial.
When the Answer Is “Don’t Use an Elastomer Overmold at All”
For applications where PC-specific documentation is thin, sustained load is high, and CSC consequences would be severe — a structural housing rather than a cosmetic grip surface, for example — the more conservative answer is sometimes a mechanically retained elastomer component rather than a direct overmold, removing the chemical-compatibility question from the equation entirely. Incure’s adhesive and coating formulations, including the Uni-Weld™ plastic bonder line, support PC and PC/ABS bonding programs where a direct overmold isn’t the right answer for a given part’s stress profile. Contact Our Team to review a specific PC application’s CSC risk before finalizing a material decision.
Visit www.incurelab.com for more information.