The short answer is yes — but polycarbonate introduces a failure mode that does not appear on ABS or most other engineering substrates, and ignoring it is the most expensive mistake engineers make when evaluating TPU on PC for the first time. The bond chemistry is favorable. The risk is not the bond itself; it is what certain TPU formulations do to the PC substrate under mechanical load. A full compatibility breakdown requires understanding both the adhesion mechanism and the stress cracking risk before committing to a material and process.

The Adhesion Chemistry

Polycarbonate is a polar substrate with surface energy in the 42–46 mN/m range. The carbonate linkages in PC introduce ester groups to the surface, and these groups interact with TPU’s urethane chemistry through dipole-dipole forces and hydrogen bonding at the interface during overmolding.

This interaction is genuine chemical adhesion — not surface-only mechanical interlocking — and it produces bond strength that, under optimized process conditions, exceeds the cohesive strength of the TPU itself. Cohesive failure in peel testing, where the elastomer tears before the interface separates, is achievable with TPU on PC and represents the target outcome for structural overmolding.

The chemistry does not require primers or surface activation for standard PC grades under standard overmolding conditions. The compatibility is inherent to the material pairing.

Chemical Stress Cracking: What It Is and Why It Matters

Polycarbonate stress cracking occurs when the polymer chains at the surface are exposed to a chemical agent while under mechanical stress. The combination of stress and chemical exposure causes chain-level degradation that appears as surface crazing, whitening, or fracture — often well below the stress levels that would cause failure in the absence of chemical exposure.

The relevant chemical agents in TPU overmolding are not harsh industrial chemicals — they are residual solvents, plasticizers, processing oils, and aromatic compounds present in standard TPU compound formulations. These migrate to the interface under thermal and mechanical loading and can trigger CSC at the PC surface, particularly in parts that carry structural loads.

CSC does not always appear immediately. Parts can pass initial bond strength testing, ship to customers, and develop craze patterns at the interface after weeks or months of mechanical loading combined with the slow migration of TPU additives. Field CSC failure is significantly more costly than identifying the risk during material selection.

How to Evaluate TPU-PC Compatibility for CSC Risk

The standard compatibility test (ASTM peel or lap shear on a freshly overmolded part) does not detect CSC risk. A more appropriate evaluation includes:

• Sustained load testing: Apply a static stress of 50–75% of PC’s tensile strength to a bonded assembly and observe for crazing at the bond line after 24, 72, and 168 hours
• Thermal cycling under load: Cycle between -30°C and 80°C with sustained mechanical stress applied during the test
• Chemical immersion control: Expose unstressed PC substrate to the TPU compound (dissolved in IPA if necessary to create a test solution) and observe for surface attack
• Fractography: Examine failed bond samples under magnification to distinguish adhesive failure (clean PC surface) from CSC-induced failure (crazing visible on PC substrate)

TPU grades formulated for PC overmolding are available from major suppliers and are characterized with low aromatic content, screened plasticizer packages, and documented CSC test results on standard PC grades. Requesting this documentation before evaluation eliminates most of the CSC risk before the first part is shot.

For help selecting a TPU grade with documented PC compatibility and CSC screening data, Email Us.

Grade Selection Parameters for TPU on PC

Base chemistry. Ether-based TPU is preferred for PC applications in humid or chemically exposed environments. Ether-based grades resist hydrolysis, and their additive packages tend to contain fewer aggressive plasticizers than commodity ester-based compounds. Ester-based TPU provides higher initial bond strength but is more susceptible to hydrolysis and may present higher CSC risk depending on formulation.

Shore hardness. Softer grades (Shore 60A–80A) flow more readily into surface features at the PC interface, increasing contact area and bond strength. Harder grades require tighter injection parameters to achieve equivalent adhesion.

Flame retardancy. PC applications in electronics and automotive frequently require UL94 V-0 ratings. Flame-retardant packages must be evaluated for CSC risk independently — some halogen-free FR systems have been associated with PC surface attack under sustained load.

Colorants and additives. Each production colorant added to a base TPU compound is a new potential source of CSC risk. Validate adhesion and CSC performance with the specific production-color compound, not only with the natural base resin.

Processing Requirements

Drying. PC requires drying at 120°C for four to six hours. Moisture in PC at processing temperature causes hydrolytic chain scission and produces silver streaks, which reduce surface quality and may introduce stress at the interface. TPU requires separate drying at 80–100°C for two to four hours.

Substrate stress relief. PC parts used as inserts for overmolding should be annealed to relieve residual molding stress before overmolding. Residual stress in the insert increases the PC’s susceptibility to CSC from TPU compound additives.

Mold temperature. Maintain mold temperature at 80–100°C for TPU on PC. This supports interfacial bonding and reduces the rate of TPU solidification at the bond surface, allowing adequate contact time for the urethane-ester interaction to develop.

Substrate surface condition. Clean PC insert surfaces with isopropyl alcohol before loading. Do not use chlorinated solvents or ketone-based cleaners on PC — these cause immediate stress cracking on contact with loaded or stressed substrates.

When Adhesive Bonding Is the Joining Method

For applications where overmolding is not feasible, polyurethane-based adhesives provide strong bonds on both TPU and PC surfaces. The same chemical affinity that drives overmolding performance applies in adhesive applications.

Surface preparation for PC in adhesive bonding: clean with IPA, allow complete evaporation, and apply adhesive promptly. Avoid abrasion with coarse abrasives on transparent PC — 320 grit or finer preserves optical properties. For frosted or textured surfaces, standard 220-grit abrasion is appropriate.

Avoid cyanoacrylate adhesives on mechanically loaded PC parts. CSC risk from cyanoacrylate on stressed polycarbonate is well-documented and produces rapid catastrophic failure.

Incure’s adhesive and coating formulations are developed for demanding bonding environments including TPU-to-PC applications where CSC risk management is part of the material specification requirement. For technical support on material selection and process development, Contact Our Team.

Visit www.incurelab.com for more information.