Peel strength, lap shear, and failure mode are the three measurements that characterize elastomer-to-substrate adhesion in production settings. Each tells a different part of the story — peel strength quantifies force, failure mode reveals whether the bond or the material is the weak link, and lap shear measures resistance to the loading direction most common in assembled products. Understanding how TPU and TPE perform across these three dimensions on different substrates, and why the values vary, gives engineers a more complete picture than data sheet rankings alone provide.
Peel Strength: What the Numbers Represent
Peel strength for overmolded elastomers is typically measured by the 90-degree peel test (ASTM D1876 or ISO 11339), which measures the force per unit width required to peel the elastomer from the substrate at a 90-degree angle. Values are reported in N/mm or lb/in.
Peel strength values for elastomers on engineering substrates range widely — from below 1 N/mm for poorly bonded combinations to above 8–10 N/mm for well-bonded cohesive-failure systems. But the absolute value is less important than the failure mode it accompanies.
Cohesive failure (elastomer tears) indicates that the bond exceeded the elastomer’s tensile strength. The bond strength is bounded below by the measured peel force and is actually higher — the interface did not fail, the material did. This is the acceptance criterion for structural overmolding.
Adhesive failure (clean separation at the interface, substrate surface remains intact) indicates that the bond strength is lower than the elastomer’s cohesive strength. The measured peel force is an accurate upper bound on the bond strength. Parts that fail adhesively in peel testing are at risk for progressive delamination in service under real load conditions.
TPU Adhesion Values Across Substrates
TPU adhesion on polar engineering substrates under optimized production conditions:
TPU on ABS: Cohesive failure at 4–9 N/mm peel depending on Shore hardness (softer grades produce higher peel strength by presenting more contact area per unit width). Lap shear typically 8–15 MPa on clean ABS surfaces.
TPU on PC: Cohesive failure at 3–8 N/mm peel on properly dried, stress-relieved PC with CSC-screened TPU formulation. Softer grades again produce higher peel values.
TPU on PA6/PA66: Cohesive failure at 3–7 N/mm peel on dry-as-molded, optimally processed substrates. Moisture-conditioned PA substrates produce lower values — often 1.5–4 N/mm — demonstrating the peel strength sensitivity to substrate moisture content.
TPU on PA12: Adhesive failure without silane primer at typically 0.5–2 N/mm. With silane primer: cohesive failure at 2–4 N/mm, demonstrating the primer’s substantial impact on PA12 bond strength.
TPU on PP (plasma treated): Adhesive failure at 1–3 N/mm even with treatment. Not cohesive failure territory for structural applications.
TPE Adhesion Values Across Substrates
SEBS on ABS: Cohesive failure at 3–8 N/mm under controlled process (mold temperature >60°C). Lower than high-end TPU but adequate for most consumer product grip and soft-touch applications. Below 60°C mold temperature: adhesive failure at 1–3 N/mm — demonstrating the sensitivity of SEBS adhesion to process parameters.
COPE on PC: Cohesive failure at 3–7 N/mm with mold temperature >75°C and pre-dried substrate. Comparable to TPU on PC.
PEBA on PA6: Cohesive failure at 3–6 N/mm with mold temperature >80°C and dry substrate. Comparable to TPU on PA6.
SEBS on PA (without primer): Adhesive failure at 0.5–2 N/mm — not adequate for structural applications. With silane primer: adhesive failure at 2–4 N/mm — adequate for non-structural soft-touch only.
TPV on ABS (without treatment): Adhesive failure at 1–3 N/mm in most formulations. With coupling agent: adhesive failure at 3–5 N/mm. Cohesive failure not reliably achieved on ABS without SEBS tie-layer.
Lap Shear vs Peel: Different Load Geometries
Peel testing loads the bond in the weakest direction — peeling stress is concentrated at the peel front rather than distributed across the bond area. Lap shear loading distributes force across the entire bonded area and is more representative of how most assembled products are loaded in service — pull loading, torsion, and shear are more common than direct peel.
Lap shear strength for well-bonded cohesive-failure TPU on ABS is typically 8–15 MPa — meaning the assembly can withstand 8–15 MPa applied across the bond area in shear before failure. This value is substantially higher than peel tests might suggest in isolation.
For structural applications where the assembly is loaded primarily in shear rather than peel, lap shear testing (ASTM D1002) provides more relevant design data than peel testing. Use peel testing for process qualification (confirming failure mode and comparing between process conditions) and lap shear for structural design validation.
For adhesion test data on your specific material combination and guidance on which test method is relevant to your application, Email Us.
Factors That Shift Adhesion Values
The values above represent optimized production conditions. Several variables shift these values downward:
Substrate moisture (PA substrates): 20–40% reduction in peel strength between dry-as-molded and moisture-conditioned PA at typical ambient humidity conditions.
Mold temperature below threshold: 30–60% reduction in peel strength for SEBS on ABS below 50°C mold temperature, or PEBA on PA below 70°C. Cohesive-failure combinations become adhesive-failure combinations at inadequate mold temperature.
Internal mold release agents in elastomer compound: 15–30% reduction in peel strength from migration of release agents to the bond interface. Avoid overmolding compounds with internal release agents.
Surface contamination: Even trace mold release residue on the substrate surface can reduce peel strength by 50% or more and shift from cohesive to adhesive failure. Cleaning protocol discipline is non-negotiable.
Colorant lot variation: Some TPE and TPU colorant packages include release agents or pigment vehicles that reduce bond strength. Validate adhesion with the specific production-color compound, not with natural resin.
Incure’s adhesive and coating formulations are developed for demanding overmolding and bonding applications, including systems that extend achievable adhesion values beyond standard overmolding chemistry on difficult substrate-elastomer combinations. For technical support, Contact Our Team.
Visit www.incurelab.com for more information.