ABS, PC, and nylon dominate most discussions of TPU overmolding substrates — but engineering practice regularly encounters PET, polypropylene, and PVC in product assemblies where flexible zones are needed. Each of these three substrates presents a distinct compatibility profile with TPU, ranging from strong natural affinity (PET) to challenging non-polar surfaces (PP) to a nuanced situation driven by formulation variation (PVC). Understanding where TPU bonds well, where it requires intervention, and where alternatives may be more practical saves development time and prevents production failures on substrates outside the most common set.

TPU on PET (Polyethylene Terephthalate)

PET is a moderately polar polyester with surface energy in the 40–44 mN/m range, comparable to PA6 and ABS. Its ester linkages in the polymer backbone interact with TPU’s urethane groups through urethane-to-ester interaction — the same mechanism that drives TPU adhesion on PC. The result is a genuinely compatible pairing that produces cohesive failure in overmolding without adhesion promoters under controlled process conditions.

PET applications where TPU overmolding appears include:
– Packaging equipment components with flexible sealing zones
– Electronic device substrates where PET’s optical and dielectric properties are required
– Injection-molded PET structural components with integrated flexible grips

Processing considerations for TPU on PET. PET must be dried aggressively before processing — at 150°C for four to six hours in a desiccant dryer — because it is more hygroscopic than ABS and less forgiving of residual moisture than PC. Undried PET at melt temperature undergoes hydrolytic chain scission, dramatically reducing molecular weight and producing severe viscosity reduction and part surface defects. TPU adhesion on improperly dried PET is unreliable because surface degradation changes the substrate’s bonding chemistry.

PET processing temperatures (260–290°C) are higher than TPU’s preferred processing window (190–240°C), requiring careful management of substrate temperature at the moment of TPU contact in two-shot applications.

TPU on Polypropylene (PP)

Polypropylene is a non-polar, semi-crystalline thermoplastic with surface energy in the 29–31 mN/m range — below the threshold where TPU’s polar urethane mechanism can develop adequate adhesion. The mismatch between TPU’s polar chemistry and PP’s non-polar surface means that standard overmolding without surface preparation produces adhesive failure at low peel loads, regardless of process parameters.

This does not mean TPU on PP is impossible — it means that surface activation is required:

Plasma treatment. Atmospheric or vacuum plasma treatment oxidizes the PP surface, introducing polar functional groups (carbonyl, hydroxyl, peroxide) that increase surface energy to 60+ mN/m transiently. TPU overmolding must occur within 4–24 hours of plasma treatment before surface energy relaxes back toward baseline. Plasma treatment is effective but requires capital investment in plasma equipment and careful process timing.

Flame treatment. Open-flame treatment oxidizes the PP surface through combustion products, producing similar surface energy improvement to plasma. More capital-accessible than plasma equipment; harder to control uniformly on complex three-dimensional geometries.

Corona treatment. Used for film and sheet applications; less applicable to three-dimensional injection-molded PP substrates.

Even with surface activation, TPU on PP typically produces adhesive failure rather than cohesive failure — structural overmolding of TPU on PP is not a standard production process. For PP substrates requiring flexible zones, polyolefin-backbone TPE compounds (TPO) are a more reliable technical solution than TPU with surface activation.

TPU on Rigid PVC

Rigid PVC (unplasticized PVC, uPVC) is a polar substrate with surface energy in the 38–42 mN/m range, comparable to ABS. TPU bonds to rigid PVC through polar interaction — urethane groups engaging the polar chlorine-bearing PVC backbone — producing adequate adhesion in overmolding and adhesive bonding without primers on clean surfaces.

Applications include flexible seals and grips overmolded on rigid PVC profiles in construction, plumbing, and electrical equipment.

Flexible PVC is more complex. Flexible PVC contains significant plasticizer additions (typically 20–40% by weight) to achieve its flexibility. These plasticizers migrate to the substrate surface continuously throughout the PVC’s service life, progressively lowering surface energy. TPU bonded to flexible PVC initially may develop adequate adhesion, but plasticizer migration at the bond interface over weeks and months reduces adhesion over time — producing progressive delamination in service that does not appear in initial qualification testing.

Evaluating TPU adhesion to flexible PVC requires long-term testing (accelerated aging at elevated temperature to model plasticizer migration rate) rather than immediate peel testing. The specific plasticizer system in the PVC formulation determines the migration rate and the long-term compatibility with TPU; this information should be requested from the PVC compounder before finalizing the design.

For TPU grade selection and compatibility assessment for PET, PP, and PVC substrates, Email Us.

Summary

PET: Naturally compatible with TPU through urethane-to-ester interaction. Aggressive pre-drying (150°C, 4–6 hours) is mandatory. Cohesive failure achievable under controlled conditions.

PP: Not naturally compatible with TPU. Surface activation (plasma or flame) required. Even with treatment, adhesive failure is the typical outcome. Polyolefin-matched TPE compounds are the more reliable alternative for PP substrates.

Rigid PVC: Compatible with TPU through polar interaction. Clean surfaces required. Adequate for structural bonds in appropriate applications.

Flexible PVC: Initial TPU adhesion may be adequate, but plasticizer migration reduces adhesion over time. Long-term testing required; application may not be suitable for TPU overmolding without plasticizer compatibility screening.

Incure’s specialty adhesive and coating formulations address challenging substrate-elastomer combinations including PET bonding applications, PP surface activation systems, and PVC formulations where plasticizer compatibility is a design requirement. For technical support, Contact Our Team.

Visit www.incurelab.com for more information.