The question of which elastomer performs better on polycarbonate does not have a universal answer — it has a conditional one. TPU and the right TPE sub-class (COPE) both bond reliably to PC when the process is executed correctly. Where they diverge is in chemical stress cracking risk management, processing discipline requirements, material availability, and long-term durability under specific service conditions. Evaluating these differences systematically gives engineers a basis for choosing rather than guessing.
Bond Strength Comparison on PC
TPU bonds to PC through urethane-to-ester group interactions, a polar mechanism that produces consistent adhesion across the TPU family. Cohesive failure — elastomer tears before bond line separates — is achievable under standard overmolding conditions without adhesion promoters on standard PC grades.
COPE bonds to PC through ester-to-ester chemical compatibility, a mechanism equally strong and similarly able to achieve cohesive failure on PC without primers. COPE is the one TPE sub-class that matches TPU’s adhesion performance on polycarbonate without requiring process modifications.
SEBS-based TPEs bond inconsistently to PC without adhesion promoters. Adhesion varies by compound formulation and process conditions, and cohesive failure is not reliably achieved in standard production environments. Other TPE sub-classes — TPV, SBS, PEBA — are not appropriate for PC without tie-layer materials or surface treatment.
Verdict on bond strength: TPU and COPE are equivalent on PC under optimized conditions. SEBS and other TPE types require adhesion promotion to be competitive.
Chemical Stress Cracking Risk
Both TPU and COPE can trigger chemical stress cracking (CSC) on PC if the compound formulation contains incompatible additives — plasticizers, processing oils, aromatic solvents, or residual monomers — that migrate to the PC surface under mechanical load.
The difference is in the available grade ecosystem. TPU suppliers have been formulating for PC compatibility longer and across a broader product range. PC-specific TPU grades with documented CSC test results are available from major suppliers; requesting this documentation before material evaluation substantially reduces risk.
COPE suppliers offer PC-compatible grades, but the product range is narrower and documentation depth varies. Evaluating CSC risk for a specific COPE-PC combination requires more compound-level investigation than for a well-characterized TPU grade.
Verdict on CSC risk: Manageable for both, but documented PC-compatible TPU grades are more widely available and better characterized. COPE requires more diligent compound-level evaluation.
Processing Comparison
Moisture management. Both TPU and COPE must be thoroughly dried before processing. PC substrate drying requirements (120°C, four to six hours) apply regardless of the elastomer selected.
Processing temperature window. Both TPU and COPE process at 190–240°C — similar windows that require the same attention to barrel temperature management relative to PC’s 260–310°C substrate processing range.
Mold temperature sensitivity. TPU on PC performs well at mold temperatures of 80–100°C. COPE on PC requires a minimum of 70–75°C, with 85–95°C producing more consistent bond strength. Both are more demanding than SEBS on ABS (60°C minimum), but the requirement is equivalent between TPU and COPE on PC.
Verdict on processing: Equivalent requirements between TPU and COPE. Both require greater process discipline than ABS overmolding applications.
Service Temperature Performance
COPE offers a higher service temperature capability than equivalent-hardness TPU formulations. At sustained temperatures above 80°C — relevant in automotive interior applications during peak solar loading — COPE maintains dimensional stability and bond integrity where some TPU grades begin to soften and creep.
For applications at or below typical consumer product service temperatures (below 70°C sustained), this distinction is not practically significant. For automotive, industrial, or high-power electronics applications where component temperatures regularly exceed 80°C, COPE’s higher softening point is a meaningful advantage.
Verdict on service temperature: COPE leads for high-temperature applications. TPU is adequate for standard consumer and medical applications.
Moisture and Chemical Resistance
Ether-based TPU outperforms COPE in hydrolysis resistance. COPE’s polyester backbone is susceptible to hydrolytic degradation under sustained moisture exposure — bond strength and elastomer mechanical properties both degrade over time in humid service environments. Applications involving repeated aqueous cleaning, perspiration exposure, or high-humidity storage favor ether-based TPU.
COPE performs adequately in low-moisture service environments and has better fuel resistance than TPU in some formulations. For chemical-resistant sealing applications on PC substrates in industrial environments involving fuels or oils, COPE is worth evaluating with chemical resistance data specific to the exposure.
Verdict on moisture resistance: Ether-based TPU leads clearly. COPE requires careful service environment matching.
Material Availability and Cost
TPU grades for PC applications are available from a broad supplier base including major global compounders with documented PC compatibility test results. Lead times, minimum order quantities, and customization options are more favorable for TPU than for engineering COPE grades.
COPE grades appropriate for PC overmolding come from a smaller number of suppliers at comparable material cost per kilogram. Custom formulations for specific PC grades or application requirements have longer development timelines.
For straightforward overmolding applications where lead time and supply chain flexibility matter, TPU has a practical advantage.
For guidance on COPE versus TPU selection for your specific PC application and service environment, Email Us.
The Verdict
TPU is the more broadly applicable choice for PC overmolding — wider grade availability, better documentation for CSC risk evaluation, stronger moisture resistance in ether-based formulations, and a broader supplier base. It should be the default starting point for engineers evaluating elastomers on PC for the first time.
COPE earns its place in PC applications that operate at sustained elevated temperatures, that require specific ester-chemistry performance, or where the ester-to-PC affinity provides a processing advantage in tightly controlled two-shot operations.
Incure’s specialty adhesive and coating formulations are developed for demanding PC bonding applications, including systems where TPU or COPE on PC requires adhesion promotion or CSC risk mitigation. For technical support, Contact Our Team.
Visit www.incurelab.com for more information.