Every material pairing decision on polycarbonate carries consequences that extend beyond the initial bond strength test. PC is used where dimensional precision, optical properties, or impact performance cannot be compromised — which means an elastomeric layer added through overmolding or adhesive bonding must not degrade the substrate, must survive the same service conditions as the PC housing, and must remain bonded through the product’s full service life. Comparing TPU and TPE performance on polycarbonate across these dimensions gives engineers a basis for a defensible material decision.

Bond Strength: Which Achieves Stronger Adhesion

TPU bonds to PC through urethane-to-ester group interactions — a polar mechanism that produces consistent adhesion across the TPU family without requiring specific sub-class matching to the substrate. In well-executed overmolding, TPU on PC achieves cohesive failure: the elastomer material tears before the bond separates, indicating that the interface is stronger than the elastomer itself.

Within the TPE family, COPE (copolyester elastomer) achieves equivalent results on PC through ester-to-ester chemical compatibility. SEBS bonds to PC less consistently and requires adhesion promoters or tie-layer materials to reach cohesive failure. TPV, PEBA, and SBS are not appropriate for PC without significant surface intervention.

For applications where the full TPE family is in consideration, the comparison effectively becomes TPU versus COPE — the only sub-class that matches TPU’s performance on PC without additional process steps.

Bond strength verdict: TPU and COPE are comparable on PC under optimized conditions. COPE’s ester-to-ester mechanism is specific to PC and compatible substrates; TPU’s mechanism is broader and more consistent across process variation.

Chemical Stress Cracking Risk

This is where the comparison diverges most significantly. Polycarbonate is susceptible to chemical stress cracking — crazing or fracture at the surface triggered by chemical exposure under mechanical load. The risk is not from the elastomer’s bulk chemistry alone but from plasticizers, solvents, residual monomers, and processing additives in the compound formulation.

TPU compounds formulated for overmolding applications are typically screened for CSC risk on PC. Grade selection from a supplier with PC-specific product offerings substantially reduces this risk. The risk is higher with adhesive bonding if solvent-based systems are used on loaded PC parts.

COPE compounds can also trigger CSC if the formulation contains incompatible additives, and SEBS compounds with certain processing oils present a documented CSC risk on PC. Evaluating CSC risk requires full formulation disclosure from the compound supplier and validation testing under sustained load — not just initial peel testing.

Both TPU and COPE require CSC validation on polycarbonate. The risk is manageable with appropriate grade selection and process design, but it cannot be ignored.

Processing Compatibility

Temperature window. PC processes at 260–310°C. TPU’s processing window of 190–240°C creates a meaningful temperature differential in two-shot molding applications. The substrate must be at a sufficient temperature to support interfacial bonding without degrading; the TPU must not see barrel temperatures that cause degradation while waiting in the runner. This requires careful process design and temperature balance between the two stations.

COPE processes at 200–240°C, a similar window to TPU on PC. Processing temperature compatibility on its own does not differentiate the two.

Moisture sensitivity. Both TPU and COPE require drying before processing. PC requires drying at 120°C for four to six hours — more demanding than ABS. The combined drying requirements for substrate and elastomer make multi-station handling discipline more critical in PC overmolding than in ABS operations.

Mold temperature. TPU on PC performs well with mold temperatures of 80–100°C. COPE on PC similarly requires elevated mold temperature — 70°C minimum, with 80–90°C producing more consistent bond strength. Both materials require attention to mold temperature control that is less critical in polyolefin overmolding.

For technical guidance on process parameter optimization for your specific PC and elastomer combination, Email Us.

Performance in Service Conditions

Thermal cycling. Both TPU and COPE maintain bond integrity through thermal cycling relevant to consumer and industrial applications, but the coefficient of thermal expansion (CTE) mismatch between elastomer and PC creates cumulative interfacial stress. COPE’s CTE may be better matched to PC in certain formulations due to the compatible ester chemistry; this is grade-specific and must be validated rather than assumed.

UV stability. PC without UV stabilizers yellows under UV exposure. Both TPU and COPE are available with UV stabilizer packages. SEBS, if a tie-layer is used to support COPE or another TPE on PC, has inherent UV stability from its hydrogenated mid-block that reduces the stabilizer burden.

Chemical and moisture resistance. Ether-based TPU outperforms COPE in hydrolysis resistance. Applications with sustained moisture, perspiration, or aqueous cleaning agent contact favor ether-based TPU over COPE, which contains hydrolytically susceptible ester linkages.

Cost and Material Availability

COPE grades appropriate for PC overmolding are available from a narrower set of suppliers than commodity SEBS or standard TPU grades. Material cost for engineering-grade COPE is comparable to mid-tier TPU formulations. The cost comparison between the two narrows further when process steps are accounted for — COPE on PC requires similar process control rigor to TPU.

SBS and standard SEBS are less expensive alternatives, but neither bonds reliably to PC without adhesion promotion, and the process cost of adding that step typically eliminates the material cost advantage.

Which to Specify on Polycarbonate

TPU is the more widely available, better-documented choice for PC overmolding and adhesive bonding applications. It is appropriate for applications where bond reliability across process variation is the priority, where moisture resistance is required, or where the engineering requirements exceed what COPE can deliver.

COPE is appropriate when the specific ester-to-PC affinity delivers performance advantages for the application — particularly in demanding high-temperature applications where COPE’s higher softening point than equivalent-hardness TPU provides service temperature headroom.

Incure’s adhesive and coating formulations are developed for demanding PC bonding applications, including both TPU and TPE systems for electronics, medical device, and automotive programs. For application-specific selection support, Contact Our Team.

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