An adhesive joint that fails above its rated service temperature rarely announces which failure mechanism is actually responsible — the visual symptoms of thermal cleavage, oxidative attack, and hydrolytic breakdown overlap enough that guessing wrong sends an engineering team chasing the wrong fix for months. Start With the Failure Symptom, Not the Chemistry Before opening a materials textbook, look at what the failed joint actually shows. A bond that has gone soft and rubbery but still shows intact adhesion at the interface points toward network damage in the bulk material rather than a surface or interfacial problem. A bond that has become chalky, discolored, or shows surface crazing points toward oxidative attack concentrated near an air-exposed surface. A bond that failed only in a region where moisture could reach the joint — near a seal, a vent, or an exposed edge — points toward hydrolytic attack rather than pure thermal cleavage. This symptom-first triage narrows the diagnostic path before any lab testing begins. Branch One: Uniform Softening Points to Thermal Cleavage When crosslinks fail from heat alone, the damage is uniform throughout the material rather than concentrated at a surface. Covalent crosslink bonds have a finite thermal stability, and once the local temperature exceeds the activation energy for a given bond type, cleavage begins throughout the bulk simultaneously. Ester crosslinks common in anhydride-cured epoxies tend to be the first to go; carbon-carbon crosslinks in polyimide or BMI chemistries hold out considerably longer. The diagnostic signature is a measurable drop in glass transition temperature that tracks evenly through a cross-section — a joint originally rated near 200°C can show an effective Tg in the 150°C range after sustained exposure, with no localized hot spot required to explain it. Branch Two: Surface-Concentrated Damage Points to Oxidation Oxidative crosslink failure behaves differently — it starts at any surface exposed to air and works inward, rather than occurring uniformly. Oxygen attacks the same reactive sites that crosslinks occupy, and the resulting free-radical reactions run measurably faster than thermal cleavage alone, with damage rate roughly doubling for every 10°C above a chemistry-specific threshold. That nonlinearity is the reason a joint that ran acceptably at 180°C for years can fail within months at 200°C — the oxidative rate isn't 10% higher, it can be two to four times higher. Email Us if a failure investigation needs help separating a genuine chemistry limitation from a service temperature that simply exceeded the formulation's design margin. Branch Three: Failure Localized Near Moisture Ingress Points to Hydrolysis Hydrolytic crosslink failure requires both heat and water, and its diagnostic signature is spatial: damage concentrates wherever moisture actually penetrated the joint rather than spreading evenly. Ester and urethane crosslink types are the most vulnerable, and the reaction is irreversible — a hydrolyzed crosslink converts to hydroxyl and carboxyl end groups that will not reform a network bond without external catalysis. A joint that fails first at a vent hole, a gasket interface, or an unsealed edge, while the interior of the same…

Every adhesive bond has a thermal ceiling — a temperature above which the polymer chemistry that gives it strength begins to unravel. For engineers designing assemblies that must perform in sustained heat, understanding how and why polymer breakdown occurs is not optional background knowledge. It is the basis for selecting materials that will last. The Nature of Polymer Breakdown Polymer breakdown in adhesives refers to the chemical degradation of the macromolecular network that constitutes the cured adhesive film. This degradation takes several distinct forms depending on temperature, time, environment, and the specific chemistry involved. It is not the same as simple softening at the glass transition — it represents permanent chemical change that reduces molecular weight, destroys crosslinks, or generates volatile byproducts. The distinction matters because softening from exceeding the Tg can, in principle, be reversed by cooling — a mechanism covered separately in why high-temperature adhesives lose strength above their Tg. Polymer breakdown cannot be reversed. Once polymer chains are cleaved, crosslinks severed, or the network oxidized, the original mechanical properties are gone. Mechanisms of Polymer Breakdown Thermal Chain Scission At sufficiently high temperatures, the covalent bonds within polymer chains absorb enough thermal energy to break. This process — called thermal chain scission — reduces the average molecular weight of the polymer and disrupts the load-bearing network. The onset temperature for chain scission depends on the polymer chemistry. Aliphatic polymers with carbon-carbon backbones begin to degrade at relatively modest temperatures (often 200–300°C). Polymers with aromatic backbones — such as epoxies cured with aromatic amines, bismaleimides, or polyimides — are far more resistant because aromatic rings require more energy to disrupt. Silicone adhesives have a different backbone structure entirely (silicon-oxygen bonds), which provides superior thermal stability in the 200–350°C range because Si-O bonds are stronger and more stable than C-C bonds. Oxidative Degradation In the presence of oxygen, polymer breakdown accelerates significantly. Thermal oxidation is a free-radical chain reaction: oxygen attacks the polymer backbone, forming peroxide intermediates that decompose and generate additional radicals, propagating the degradation cycle. Adhesives exposed to air at high temperatures degrade much faster than those in oxygen-free environments — surface layers oxidize first, creating a brittle skin that cracks, exposes fresh polymer to further attack, and ultimately produces cohesive failure through the degraded layer. Oxidative degradation is cumulative. A material that survives a single high-temperature exposure may still show measurable degradation that shortens its remaining service life. Hydrolytic Degradation In environments that combine heat and moisture, some polymer systems are susceptible to hydrolysis — water molecules react with ester, urethane, or other hydrolytically sensitive linkages, severing chemical bonds and reducing the network's connectivity and mechanical performance. Epoxy adhesives cured with anhydride hardeners are particularly susceptible because the resulting ester linkages are vulnerable to hydrolytic attack. Polyurethane adhesives face similar risks when urethane groups hydrolyze under sustained heat and humidity. Depolymerization Some polymer systems do not simply degrade randomly — they undergo depolymerization, a process in which the polymer chain unzips back toward its monomer components. This…

A pull test run at room temperature the day an assembly leaves the line tells an engineering team almost nothing about whether that bond will still be carrying load a year later at 130°C — and that gap between qualification data and real service conditions is where most high-heat softening failures actually originate. Why Room-Temperature QC Misses This Failure Mode Standard incoming and in-process quality checks are built around room-temperature mechanical tests because they're fast and repeatable. Softening, however, is a temperature- and time-dependent phenomenon driven by glass transition behavior, plasticizer mobility, moisture uptake, and oxidative chain degradation — none of which a room-temperature snapshot test can reveal. A joint can pass every station on the line and still be softening its way toward failure the moment it reaches service temperature. Building the Test Plan Around Actual Service Conditions A useful qualification protocol starts with the assembly's real thermal profile, not a generic elevated-temperature spec pulled from a datasheet. That means recording continuous service temperature, peak excursion temperature, cycle frequency between hot and cold states, and any exposure to moisture or process chemicals the joint will see in the field. Testing against a profile that doesn't match real conditions produces a qualification result that looks solid on paper and still fails in service. DMA and DSC: Measuring the Real Tg, Not the Datasheet Number Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) measure the actual glass transition temperature of a cured sample rather than relying on a supplier-published figure achieved under idealized cure conditions. This distinction matters because incomplete cure, moisture absorption, or thermal cycling can depress a real-world effective Tg by 10 to 30°C relative to the rated value — a gap that only shows up when the actual cured material, not a reference sample, is tested. Running DMA on production-representative samples, not just supplier-provided coupons, is what actually confirms the margin an assembly has above its Tg. Accelerated Thermal Aging Protocols A single-point strength test at room temperature cannot reveal the progressive degradation that oxidative chain scission or plasticizer migration cause over months of real service. Accelerated aging — holding test samples at an elevated temperature for an extended duration, then mechanically testing at intervals — reveals whether strength is stable or trending downward well before a production batch reaches the field. Because oxidative degradation is irreversible and accelerates above roughly 120°C for many organic adhesive systems, an aging protocol that brackets the assembly's real peak temperature is the only way to catch this mechanism before it becomes a field failure. Email Us to discuss building an accelerated aging protocol matched to your specific service temperature and duty cycle. Humidity and Chemical Soak Testing Because absorbed moisture acts as a plasticizer and can depress Tg by 20°C or more in polar polymer systems such as epoxies, a qualification protocol for any humid or wet-service environment should include a humidity-chamber soak followed by mechanical retesting, not just dry-state aging. The same logic applies to process fluids, lubricants, or…

An adhesive rated for high-temperature service can still fail if it exceeds one specific threshold: its glass transition temperature. Understanding what happens to an adhesive polymer above this point is essential for engineers who need bonds that hold under thermal stress, not just at room temperature. The related question of what actually softens at that threshold — and why — is covered in more mechanistic detail in what causes adhesive softening in high-heat applications. What the Glass Transition Temperature Actually Means The glass transition temperature (Tg) is not a melting point. It is the temperature range at which an amorphous polymer transitions from a hard, glassy state to a soft, rubbery state. Below the Tg, polymer chains are locked in place and the adhesive is rigid, strong, and capable of bearing load. Above the Tg, those chains gain enough thermal energy to move freely — and mechanical properties drop sharply. For a cured epoxy adhesive with a Tg of 150°C, operating at 160°C means the material is no longer behaving as an engineering solid. It is behaving as a viscoelastic fluid with dramatically reduced modulus, shear strength, and creep resistance. Why Strength Drops So Rapidly Segmental Chain Mobility Below the Tg, polymer chain segments are essentially frozen and cannot rotate or translate in response to applied stress, which lets the crosslinked network carry load efficiently. Above the Tg, chain segments become mobile, applied stress causes viscous flow rather than elastic deformation, and load-bearing capacity decreases by an order of magnitude or more. Loss of Elastic Modulus The storage modulus (E') of a thermoset adhesive can drop by a factor of 100 to 1,000 across the glass transition region. This means a material that was rigid and stiff at 25°C becomes compliant and soft at temperatures approaching or exceeding its Tg. For joints under shear or peel loading, this dramatic modulus drop translates directly into loss of bond integrity. Creep and Stress Relaxation Above the Tg, adhesives become susceptible to creep — time-dependent deformation under sustained load. Even if the joint does not fail immediately, bond lines shift and load paths change over time, and failure can occur far below the short-term strength limit measured at room temperature. How Crosslink Density Influences the Tg The Tg of a thermoset adhesive is directly related to its crosslink density. More crosslinks restrict chain mobility and raise the Tg. Formulations with higher crosslink density resist the glass transition at higher temperatures, which is why high-temperature adhesives are engineered with tight, dense crosslinked networks. However, crosslink density alone does not guarantee high Tg performance. The chemical nature of the polymer backbone matters equally. Aromatic backbone chemistries — as found in high-performance epoxies, bismaleimides, and polyimides — maintain rigidity at elevated temperatures because their ring structures resist chain movement even at high thermal energy levels. The crosslinks themselves are also a failure point in their own right at sustained high heat — see how crosslink failure impacts adhesive performance at high heat for the chemistry behind…

Replacing a bonded assembly on a fixed calendar schedule wastes good parts and misses bad ones in roughly equal measure, because thermal aging doesn't run on a calendar — it runs on cumulative temperature exposure, which varies unit to unit even in an identical fleet. A condition-based approach catches the joints that actually need attention while leaving the rest in service. Why Time-Based Replacement Alone Doesn't Work Two identical assemblies installed on the same day can age at meaningfully different rates if one sits in a hotter ambient location, runs a higher duty cycle, or experiences more direct sun exposure. Thermal aging mechanisms — post-cure crosslinking, oxidative embrittlement, physical aging, and volatile loss from plasticizers — all accelerate with temperature, often following something close to an Arrhenius relationship where a relatively modest temperature difference produces a large difference in aging rate. Treating every unit in a fleet as if it ages identically means over-servicing the cooler-running units and under-servicing the hotter ones, right when the hotter ones are the actual risk. Building an Arrhenius-Based Acceleration Model A practical starting point is an accelerated-aging study run at two or three elevated temperatures above normal service, with samples pulled at intervals to track modulus rise and elongation loss. Fitting that data to an Arrhenius model produces an activation energy figure for the specific adhesive system, which then lets you convert a fleet's actual temperature logs into an effective aging rate per unit — a joint that runs 20°C hotter than a sibling unit for a meaningful fraction of its service life may be accumulating aging damage at two to three times the rate, even though both units are the same age in calendar terms. This turns "how old is it" into "how much thermal dose has it actually absorbed," which is the number that actually predicts remaining life. Choosing Sentinel Samples Rather Than Testing Every Joint Destructive testing — dynamic mechanical analysis, elongation-at-break, peel testing — can't be run on every joint in a fleet without taking the fleet apart. A sentinel-sample strategy solves this: identify a subset of units representing the hottest-running, most heavily duty-cycled conditions in the fleet, and pull those for periodic destructive characterization while leaving the rest in service under non-destructive monitoring. Because the sentinel units run hotter than the fleet average, they age faster and give an early warning of approaching embrittlement before the broader population reaches the same state — effectively turning a handful of units into a leading indicator for the whole fleet. Setting Retirement-for-Cause Thresholds Rather than retiring a joint at an arbitrary age, define thresholds tied to the actual failure-relevant properties: a storage modulus rise beyond a set percentage over the as-cured baseline, an elongation-at-break drop below a minimum floor needed to survive the assembly's expected thermal-cycling or vibration environment, or a peel-strength drop even while lap-shear strength still reads acceptable — the classic sign that a joint has crossed from tough to brittle without the standard strength test catching it. Setting these…

Fillers are integral to many high-performance adhesive formulations, added to control CTE, thermal conductivity, viscosity, and mechanical stiffness. The filler-matrix interface — the boundary between inorganic filler particle and organic polymer matrix — is not passive; it's a chemically and mechanically active zone particularly vulnerable to thermal stress. When that interface breaks down at elevated temperatures, the composite properties the filler was selected to provide degrade, often in ways difficult to predict from either component alone. Why the Filler-Matrix Interface Matters In a well-formulated filled adhesive, the filler particles are dispersed throughout the polymer matrix and bonded to it — sometimes physically, sometimes chemically through coupling agents such as silanes. Load applied to the adhesive transfers between matrix and filler at this interface, and thermal properties like conductivity and CTE are also governed by the quality of that bonding. When the interface is intact, the filled adhesive behaves as a composite with properties determined by both components combined. When the interface fails — through debonding, coupling agent degradation, or differential thermal expansion — the filler particles become disbonded inclusions. Rather than reinforcing the matrix, they become stress concentrators that initiate cracking at far lower stresses than the unfilled matrix would exhibit. Mechanisms of Filler-Matrix Interface Degradation at High Temperatures Differential Thermal Expansion Organic polymer matrices have high CTEs, typically 50–150 ppm/°C, while inorganic fillers run far lower — alumina around 8 ppm/°C, silica roughly 0.5–7 ppm/°C, silicon carbide about 4 ppm/°C. When a filled adhesive is heated, the matrix expands far more than the filler particles, stressing the interface as the matrix tries to move while the filler resists; cooling reverses the stress, and repeated thermal cycles progressively fatigue and debond the bond. As debonding progresses, voids form and grow around filler particles with each cycle — a damage pattern distinguishable from other void formation mechanisms by its uniform distribution correlated with filler locations. Silane Coupling Agent Degradation Silane coupling agents are routinely used to chemically bond inorganic fillers (which carry silanol groups on their surfaces) to organic polymer matrices. The silane is applied to the filler surface, where it hydrolyzes and bonds through Si-O-Si linkages on one end and reacts with the polymer matrix on the other. At elevated temperatures, silane coupling agents are vulnerable to three degradation paths: hydrolysis, where humid environments reverse the Si-O-Si linkages; thermal decomposition, where the organic component of the silane degrades at high enough temperature and severs the connection outright; and oxidative degradation, where the organic moiety oxidizes in air, reducing effectiveness. When coupling agent integrity is lost by any of these paths, the filler-matrix bond reverts from a chemical connection to a purely physical one — weaker and more susceptible to interface failure under load and thermal cycling. Email Us to discuss filler selection and coupling agent compatibility for your high-temperature adhesive requirements. Matrix Degradation and Filler Reaction at the Interface Filler surface chemistry can also influence the local cure and degradation behavior of the adjacent matrix. Transition metal impurities in some…

The question of which plastics bond to TPU and TPE is answered differently depending on which failure mode is acceptable, which process method is available, and how much adhesion preparation the manufacturing operation can support. A substrate that bonds “adequately” with a primer system may be rated “incompatible” without one. This guide provides a practical compatibility assessment across the plastics engineers most commonly encounter in multi-material product design, structured around what is achievable in production rather than under laboratory conditions. Category 1: Easy Substrates — Bond Without Treatment These plastics offer high-to-moderate surface energy and chemical groups that engage TPU and selected TPE chemistries directly. Cohesive failure is achievable without adhesion promoters under standard overmolding conditions. ABS. The most compatible substrate for both TPU and SEBS-based TPE. TPU bonds through urethane-to-nitrile interaction; SEBS bonds through styrenic end-block affinity with ABS’s styrene phase. Both can achieve cohesive failure without primers with correct process parameters. Default overmolding substrate for soft-touch consumer and industrial product design. Polycarbonate (PC). TPU bonds reliably through urethane-to-ester group interaction. COPE bonds through ester-to-ester compatibility. Both can achieve cohesive failure without primers when appropriate grades are selected and PC-specific precautions (CSC risk screening, substrate stress relief) are followed. SEBS does not bond naturally to PC without adhesion promotion. PA6 and PA66. TPU bonds through urethane-to-amide interaction; PEBA bonds through amide-to-amide compatibility. Both achieve cohesive failure on PA6 and PA66 without primers with controlled mold temperature and dry substrates. Moisture management is a process requirement, not a material limitation. PET. TPU and COPE both bond well to PET through urethane-to-ester and ester-to-ester mechanisms. PET is less commonly used as an injection overmolding substrate than ABS or PC but appears in packaging, consumer-goods, and electronic applications. Rigid PVC. TPU bonds to rigid PVC through polar interaction. Standard overmolding conditions produce adequate adhesion. Flexible PVC is more complex — plasticizer migration from the PVC formulation can contaminate the bond interface over time. Category 2: Moderate Substrates — Achievable With Process Control These substrates have chemical groups that support elastomer adhesion but require tighter process control or specific elastomer sub-classes to achieve structural bonds. PC/ABS blends. Behave similarly to ABS toward TPU and SEBS, with the ABS phase providing the adhesion surface. Surface energy is between pure ABS and pure PC. Compatible with the same elastomers as ABS, using the same process parameters. PMMA (Acrylic). Moderately polar substrate with surface energy in the 36–39 mN/m range. TPU bonds adequately; SEBS requires adhesion promotion. Less commonly overmolded than ABS or PC. Acrylic assemblies more often pair a TPU or TPE gasket against a separate glass or metal component rather than another plastic, an interface covered separately in matching adhesive grade to viscosity and tensile requirement for glass and metal bonding. PA12. Chemically similar to other polyamides but with reduced amide group density. TPU and PEBA both bond, but at lower strength than on PA6. Mechanical interlocks and silane primers are required for structural bond strength. Not a direct substitute for PA6 in overmolding…

The question of which has better adhesion — TPU or TPE — does not have a universal answer because adhesion is not a material property in isolation; it is a property of a material pair. Asking “which has better adhesion” without specifying the substrate is like asking “which paint dries faster” without specifying the environment. The more useful question is: for a given substrate, which elastomer produces the strongest, most durable bond — and why? How Adhesion Develops in Overmolding When a molten elastomer contacts a solid substrate in an injection mold, adhesion develops through two mechanisms: Chemical bonding: Functional groups in the elastomer’s surface interact with compatible functional groups on the substrate through hydrogen bonding, dipole interaction, or covalent bonds. This is the primary mechanism for structural bonds and is governed by surface energy and chemical compatibility. Physical entanglement: Polymer chains in the molten elastomer interpenetrate with the polymer chains at the substrate surface. This requires that both materials be partially mobile at the bond interface during the bonding period — which is why mold temperature, substrate pre-heating, and injection speed affect bond strength. The combination of chemical and physical bonding produces the interface strength. When chemical compatibility is high, both mechanisms contribute and cohesive failure bonds result. When chemical compatibility is low, physical entanglement alone provides limited bond strength and adhesive failure mode results. On Polar Engineering Plastics: Comparable Performance With Chemistry-Dependent Differences ABS substrates. Both SEBS and TPU bond to ABS through different but compatible mechanisms. SEBS through styrenic affinity; TPU through urethane-nitrile interaction. Both achieve cohesive failure. Comparative peel strength on ABS: SEBS often produces slightly higher peel values (3–6 N/mm) due to the direct styrenic match; TPU typically 2.5–5 N/mm. The practical difference is small — both are structural bonds. SEBS has a slight adhesion advantage on ABS; TPU has an abrasion resistance advantage that matters in wear-exposed applications. PC substrates. COPE and TPU both bond to PC through ester-group interaction. COPE’s direct ester-to-ester match sometimes produces higher initial peel strength than TPU’s urethane-carbonate mechanism, but both achieve cohesive failure under good process conditions. The practical distinction is service temperature: COPE maintains bond strength at higher sustained temperatures than standard TPU. PA6 and PA66 substrates. PEBA and TPU compete on PA substrates. PEBA’s direct amide-to-amide match provides robust adhesion with good process latitude — bond strength is reliable across a reasonable mold temperature range. TPU bonds well to PA but shows more sensitivity to mold temperature drops below 75°C. On PA, PEBA has a slight process reliability advantage; TPU provides broader grade availability. PET and PBT substrates. COPE and TPU both bond through ester chemistry. Performance is comparable; aggressive pre-drying of both substrates is required for either elastomer. On Polypropylene: TPO Wins Clearly On PP, TPO’s polyolefin backbone chemistry produces cohesive failure bonds without surface treatment. TPU on PP — even with plasma or flame surface activation — produces adhesive failure at 1–3 N/mm. This is not a marginal difference; it is a fundamental gap…

The answer is substrate-dependent — but the pattern is consistent enough to provide a useful framework. TPU bonds better than most individual TPE sub-classes across the widest range of engineering plastic substrates, because its polar urethane chemistry finds compatible bonding partners in ABS, PC, PA, and PET without requiring sub-class reformulation for each substrate. TPE sub-classes, when correctly matched to their target substrate (SEBS to ABS, COPE to PC/PET, PEBA to PA), match or approach TPU’s bond strength on those specific substrates — but the wrong sub-class on the wrong substrate fails regardless of process execution. The comparison is more nuanced than a simple ranking. Engineering Plastics Where TPU Leads Nylon (PA12, glass-filled PA): On the difficult polyamide grades — PA12 and fiber-reinforced variants — TPU has a broader documented performance record and more available grade options with silane primer compatibility data than PEBA on the same grades. Both materials require primers and mechanical interlocks on PA12 and GF-PA, but TPU’s wider grade ecosystem means more options for engineering around the adhesion challenge. Cross-substrate programs: Products with multiple rigid substrates — an ABS housing with a PA connector, or a PC body with an ABS component — can use the same ether-based TPU on all interfaces. No single TPE sub-class spans ABS, PC, and PA adhesion; using TPU avoids the need to manage multiple elastomer specifications and supplier relationships for a single product. Applications with uncertain substrate specification: When the rigid substrate is still being finalized during development, TPU’s broader substrate compatibility reduces the risk that a substrate change (ABS to PC, ABS to PA) will invalidate the elastomer specification. Prototype work on TPU transfers more reliably between substrate candidates than work on SEBS or COPE. Engineering Plastics Where TPE Sub-Classes Match TPU ABS: SEBS matches TPU’s bond strength on ABS under controlled conditions at lower material cost. For high-volume consumer products where cost efficiency matters and mold temperature can be maintained above 60°C, SEBS on ABS is the equal of TPU on ABS in bond strength and more cost-effective in total part economics. SEBS does not match TPU’s mechanical durability or abrasion resistance, but for tactile grip and soft-touch applications where the overmold is not load-bearing, these properties are not the limiting factor. PC (with COPE): COPE on PC through ester-to-ester chemistry matches TPU’s adhesion performance on PC under optimized conditions and surpasses TPU in service temperature capability for applications above 85°C sustained. For automotive interior PC components that reach high temperatures, COPE on PC is a technically superior specification to standard TPU. PA6 and PA66 (with PEBA): PEBA’s amide-to-amide mechanism on PA6 and PA66 produces cohesive failure at conditions competitive with TPU. On these specific substrates, PEBA is not an inferior alternative to TPU — it is a matched alternative, with different trade-offs in Shore hardness range, supplier availability, and cost. PET (with COPE): COPE bonds to PET through ester-to-ester compatibility more strongly than SEBS or PEBA. TPU also bonds well to PET. Either is appropriate; the selection…

Every multi-material product design reaches the moment when the elastomer must be specified. At that moment, two parallel questions exist: which material is chemically compatible with the substrate, and which material delivers the functional properties the design requires. The best selection approach treats these as sequential filters — compatibility first, function second — rather than simultaneous trade-offs. A material that cannot bond to the substrate is not a candidate regardless of its Shore hardness or cost. The Compatibility Filter: What Eliminates Candidates Immediately The substrate material determines which elastomers remain in consideration. Before evaluating any functional properties, apply the compatibility filter: Does the substrate have polar surface chemistry? Polar substrates (ABS, PC, PA, PET, PBT, rigid PVC) are compatible with polar elastomers: TPU, SEBS (on ABS/ABS-PC), COPE (on PC/PET/PBT), PEBA (on PA). Multiple candidates remain after the first filter. Non-polar substrates (PP, HDPE, LDPE) are not compatible with polar elastomers in standard overmolding. The filter eliminates TPU and most TPE sub-classes except polyolefin-compatible compounds. For PP: TPO. For HDPE/LDPE: adhesive bonding with CPO primer, or specialty polyolefin-matrix TPE. Does the substrate have a specific chemistry match with one TPE sub-class? Some substrates have a chemically direct match with a specific TPE sub-class: - ABS ↔ SEBS (styrenic chemistry) - PA ↔ PEBA (amide chemistry) - PET/PBT ↔ COPE (ester chemistry) - PP ↔ TPO (polyolefin chemistry) When such a match exists, the matched TPE sub-class achieves cohesive failure through direct chemistry and is the natural first candidate. TPU also bonds to ABS, PA, and PET through its urethane mechanism — TPU is not eliminated, but the chemistry-matched TPE has a structural compatibility advantage. After the Compatibility Filter: Applying Functional Requirements Once the compatible elastomers are identified, functional requirements narrow the choice: Mechanical durability. TPU excels here. Abrasion resistance, tensile strength, and tear resistance are higher for TPU than SEBS or COPE at equivalent Shore hardness. For applications where the elastomeric zone will experience sustained mechanical stress — tool handles under repetitive grip, footwear soles under cyclic loading, cable jacketing under flex fatigue — TPU’s mechanical properties are the differentiating factor. Service temperature. This filter eliminates standard SEBS and TPU from high-temperature applications: - Up to ~80°C: SEBS, standard TPU, PEBA, TPO are all viable - Up to ~100°C: COPE in select grades; high-performance TPU grades - Up to ~140°C: COPE specifically; other sub-classes not applicable - Below -30°C: PEBA and low-temperature TPU maintain flexibility; standard SEBS and COPE may stiffen Selecting for the temperature extremes alone is not sufficient — a material that stays flexible across the full range can still lose bond strength from repeated thermal cycling, the mechanism covered in how CTE mismatch causes adhesive bond failure. Moisture and hydrolysis resistance. Ether-based TPU is the specification for sustained moisture exposure, sweat contact, or aqueous fluid contact. Ester-based TPU is not appropriate for these environments. SEBS has adequate moisture resistance for most non-immersion applications. COPE’s ester chemistry is susceptible to hydrolysis. Chemical resistance (specific fluids). This filter depends entirely…