Polycarbonate appears in products where failure is measured in lives, not units — medical device housings, automotive lamp lenses, aircraft interior components, and protective equipment all rely on PC’s combination of optical clarity, impact resistance, and thermal stability. When engineers add flexible seals, grip zones, or vibration-damping layers to these assemblies, thermoplastic polyurethane is frequently the first material evaluated. Understanding how TPU behaves on polycarbonate — and where the interaction can go wrong — determines whether that evaluation leads to a production-ready bond.
Why TPU Bonds to Polycarbonate
Polycarbonate is a polar engineering thermoplastic with surface energy typically in the 42–46 mN/m range — higher than ABS and substantially higher than polyolefins. The carbonate linkages in PC introduce ester groups to the surface, and these groups interact with TPU’s urethane chemistry through dipole-dipole interactions and hydrogen bonding at the interface. The result is a chemical affinity that supports strong adhesion without adhesion promoters in most injection overmolding applications.
In well-executed two-shot overmolding on PC, TPU achieves cohesive failure — the elastomer tears before the bond line separates. This is the target result for structural overmolding and is reproducible on standard PC grades under controlled processing conditions.
Chemical Stress Cracking: The Critical Risk
The defining complication in TPU-on-PC bonding is chemical stress cracking (CSC). Polycarbonate is susceptible to crazing and cracking when exposed to certain chemicals while under mechanical stress — a phenomenon that does not require high chemical concentrations to trigger. Some TPU formulations contain residual solvents, plasticizers, or processing aids that can induce CSC at the PC interface, particularly in parts that carry mechanical load.
To avoid this failure mode:
– Specify TPU grades formulated with low solvent content and no aggressive plasticizers
– Avoid TPU compounds with aromatic content that could interact with PC’s carbonate groups
– Validate bond strength under sustained load — static stress combined with chemical exposure is the most damaging combination for PC
– Test for crazing at the bond line after thermal cycling, not just immediately after processing
Chemical stress cracking is most common in adhesive bonding applications where solvent-based adhesives or primers contact the PC substrate, but it can also occur in overmolding if the TPU compound’s additive package is not compatible.
Selecting the Right TPU Grade for PC
Grade selection for TPU on polycarbonate involves the same parameters as ABS applications, with additional attention to chemical compatibility:
Base chemistry. Ether-based TPU is preferred for PC applications in humid or chemical environments — hydrolysis resistance protects bond integrity over time. Ester-based grades offer higher initial bond strength but degrade in moisture-exposed applications and may present a higher CSC risk on PC.
Shore hardness. Softer grades (Shore 60A–85A) conform more readily to the PC surface during injection, increasing molecular contact area. Harder grades require tighter process control to achieve equivalent interfacial bonding.
Additive compatibility. Request full formulation disclosure from the TPU supplier for any compound being evaluated on PC. Internal mold release agents, UV stabilizers, and flame retardant packages must be screened for CSC risk on polycarbonate.
Flame retardancy. PC applications in electronics and automotive frequently require UL94 V-0 or equivalent flame ratings. Flame-retardant TPU compounds are available but must be validated for adhesion performance — some FR packages reduce bond strength or pose CSC risk.
For application-specific TPU grade guidance and CSC risk evaluation, Email Us.
Processing Requirements for TPU Overmolding on PC
Temperature matching. PC typically processes at 260–310°C, and TPU at 190–240°C. The melt temperature differential is larger than in ABS applications and requires careful process window management. The PC substrate must be at an appropriate temperature to support interfacial bonding without degrading at the high end or solidifying the interface prematurely at the low end.
Pre-drying. PC is hygroscopic and must be dried at 120°C for four to six hours before processing. Moisture in PC at melt temperatures causes hydrolytic chain scission, reducing molecular weight and generating CO₂ bubbles that appear as silver streaks in the finished part. TPU must also be dried separately at 80–100°C before injection.
Mold temperature. Maintain mold temperature at 80–100°C for TPU overmolding on PC to support interfacial bonding and control the residual stress state at the interface. Lower mold temperatures increase the risk of both poor adhesion and residual stress that contributes to long-term CSC.
Substrate integrity. PC parts used as inserts must be stress-relieved before overmolding. Residual molding stress in the PC insert combined with TPU compound chemistry dramatically increases the CSC risk.
Applications Where TPU on PC Delivers Value
Medical device housings. PC is a primary material for diagnostic equipment, surgical instrument housings, and infusion pump enclosures. TPU overmolds add soft-grip surfaces, protective bumpers, and flexible cable strain reliefs. Ether-based, medical-grade TPU on PC is the standard combination, with ISO 10993 biocompatibility validation required for skin-contact zones.
Consumer electronics. PC and PC/ABS blends dominate portable electronics housings. TPU provides drop protection, corner reinforcement, and tactile grip surfaces while maintaining the rigid structural function of the PC shell.
Automotive lighting and optics. PC lamp lenses and light guide housings sometimes incorporate TPU sealing gaskets integrated through insert molding, providing weather sealing without separate assembly operations.
Adhesive Bonding of TPU to PC
When overmolding is not the joining method, polyurethane-based adhesives provide strong bonds on both substrates. Surface preparation is critical: clean the PC surface with isopropyl alcohol (not solvent-based cleaners that can stress-craze PC) and allow complete evaporation before adhesive application. Light mechanical abrasion with fine abrasive — 320 grit or finer on PC to avoid visible scratching — increases bond area without creating stress concentrations.
Avoid cyanoacrylate and solvent-cement adhesives on loaded PC parts. These adhesive types present a high CSC risk on mechanically stressed polycarbonate.
Incure’s adhesive and coating formulations are developed for demanding overmolding and bonding applications, including TPU on PC in medical, electronics, and automotive programs. For technical support on material selection and CSC risk management, Contact Our Team.
Visit www.incurelab.com for more information.