Elastomeric substrates — vulcanized rubber, thermoplastic rubber compounds, and silicone — present bonding challenges that differ fundamentally from engineering thermoplastics. The absence of rigid structural rigidity, the surface energy extremes at both ends (silicone is among the lowest surface energy materials; some rubbers have moderate surface energy), and the presence of vulcanizing agents and release compounds at rubber surfaces all influence how TPU and TPE bond to these substrates. Products that integrate thermoplastic elastomers with rubber or silicone components appear in sealing systems, medical devices, footwear, and industrial gaskets — and the bonding approach for each material type follows distinct principles.
Vulcanized Rubber: Moderate Affinity With Process Limitations
Vulcanized (thermoset) rubber — EPDM, SBR, NBR, NR — has surface energy in the 30–38 mN/m range, depending on the formulation and vulcanizing system. This is higher than polyolefins but lower than most engineering plastics, creating a bonding situation that requires more careful management than ABS or PC but is more tractable than PTFE or silicone.
TPU on vulcanized rubber. TPU bonds to EPDM and SBR rubbers through polar interaction, producing adhesive-mode bonds in adhesive bonding applications with polyurethane adhesive systems. Overmolding TPU directly onto vulcanized rubber inserts is less common but is used in shoe manufacturing and industrial seal assembly — the rubber insert is placed in the injection mold and TPU is injected over it.
Bond strength on vulcanized rubber depends heavily on surface preparation. Mold release agents used in rubber vulcanization contaminate the rubber surface and must be removed before bonding. Buffing or abrading the rubber surface followed by IPA cleaning, then applying a PU or isocyanate-based adhesion promoter, is the standard surface preparation sequence for structural TPU-to-rubber bonding.
SEBS-based TPE on vulcanized rubber. SEBS bonds to SBR and natural rubber better than to many engineering plastics, because the styrenic and polyolefin soft segments in SEBS have some affinity for rubber’s hydrocarbon backbone. In footwear applications, SEBS-TPE compounds are bonded to natural rubber outsoles using PU adhesive systems with modest surface preparation.
TPV on EPDM. TPV compounds with EPDM rubber phase have natural affinity for EPDM rubber substrates — a polyolefin-to-EPDM compatibility through the shared rubber chemistry. This combination appears in automotive weather-strip systems where TPV overmolded sections connect to EPDM continuous extrusion profiles.
Mold Release Contamination: The Primary Surface Barrier
Vulcanized rubber parts are removed from compression or transfer molds using internal and external mold release agents. Internal release agents migrate to the rubber surface during vulcanization; external release agents are applied directly to the mold surface. Both deposit on the rubber part surface and dramatically reduce surface energy — from 35+ mN/m on a clean rubber surface to below 25 mN/m on a release-contaminated surface.
No bonding approach works reliably on release-contaminated rubber without surface preparation. The preparation sequence for rubber bonding:
1. Buff or grind the bond surface with abrasive to remove the release-contaminated surface layer
2. Clean with IPA or MEK, wipe dry with lint-free cloth
3. Apply adhesion promoter (isocyanate-based or specialized rubber adhesive primer)
4. Apply adhesive or overmold within the primer’s open time
This sequence is standard in industrial rubber bonding and footwear manufacturing. Skipping any step — particularly the primer application — produces adhesive failure at low loads.
Silicone: The Most Challenging Substrate
Silicone rubber’s surface energy (20–22 mN/m) is the lowest of any common material encountered in product manufacturing. This extremely low surface energy creates a surface that repels virtually all standard adhesives and elastomers — silicone’s non-stick properties are the same properties that make it difficult to bond.
TPU on silicone. Without surface modification, TPU does not bond to cured silicone. Adhesive failure occurs at trace loads. No standard overmolding or adhesive bonding process produces structural bonds without specialized surface preparation.
Effective surface preparation for silicone bonding:
– Plasma treatment at high power (atmospheric or vacuum plasma with extended exposure time) introduces polar functional groups to the silicone surface — silanol groups and other oxidized species — that increase surface energy to 60+ mN/m transiently
– UV/ozone treatment produces similar surface oxidation through UV-generated ozone
– Primer systems — specifically silane-based primers formulated for silicone bonding — create reactive coupling sites on the silicone surface that coordinate with both the silicone substrate and the adhesive
Even with optimal surface preparation, bonds between TPU or TPE and cured silicone are weaker than bonds to engineering thermoplastics. Peel strength of 1–3 N/mm is achievable on plasma-activated and primed silicone — adequate for sealing and retention applications with appropriate mechanical design but not equivalent to cohesive failure bonds on ABS or PC.
Liquid silicone rubber (LSR) co-molding. A different approach to TPU-silicone integration is co-molding LSR with a rigid thermoplastic substrate in a two-shot process. In this approach, the LSR is cured against the solid substrate rather than bonding to it mechanically — adhesion promoters applied to the substrate before LSR molding create covalent bonds between the curing LSR and the substrate. This produces stronger, more consistent bonds than adhesive bonding of pre-formed silicone parts.
For bonding systems and primer recommendations for rubber and silicone substrates, Email Us.
Practical Guidance by Substrate Type
EPDM/SBR vulcanized rubber: Buff, clean, apply isocyanate primer, then PU adhesive or TPU overmold. TPV with EPDM-phase chemistry provides the best overmolding compatibility without primer.
Natural rubber: Similar to SBR preparation. SEBS-based TPE with compatible styrenic chemistry bonds adequately for footwear applications with proper surface prep.
Silicone rubber: Plasma or UV/ozone treatment required. Silane-based adhesion promoter recommended after treatment. Expect lower bond strength than on engineering thermoplastics; supplement with mechanical retention design.
TPU on TPU, SEBS on SEBS: Compatible through shared chemistry. PU adhesive or thermal bonding (ultrasonic, heat press) produces strong bonds between compatible elastomeric substrates.
Incure’s specialty adhesive and coating formulations include isocyanate-based adhesion promoters for vulcanized rubber bonding and primer systems for silicone applications where surface activation alone is insufficient for the required bond strength. For formulation support and application guidance, Contact Our Team.
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