Polycarbonate’s combination of transparency, toughness, and thermal stability makes it the substrate of choice for enclosures and housings across electronics, medical, and automotive applications. When those designs call for a flexible layer — grip zones, protective overmolds, integrated seals — thermoplastic elastomers are a natural consideration. The problem is that TPE is not a single material. Which sub-class is specified determines whether the bond to PC is strong and durable or weak and prone to field failure. Understanding this distinction before tooling is committed saves considerable time and cost.
What Works: COPE on Polycarbonate
Copolyester elastomers (COPE) are the TPE sub-class with the strongest natural affinity for polycarbonate. The mechanism is ester-to-ester chemical compatibility: PC’s carbonate linkages and COPE’s polyester chemistry create a compatible interface that supports molecular-level adhesion without adhesion promoters in properly executed overmolding.
In two-shot molding applications on PC, COPE achieves cohesive failure at the interface — the elastomer tears before the bond separates. This is the benchmark for structural overmolding and is reproducible on standard PC grades when mold temperature and substrate condition are controlled. COPE is available in Shore hardness ranges appropriate for both soft-touch grip surfaces and structurally integrated flexible zones.
Key requirements for COPE on PC:
– Mold temperature above 70°C to maintain the interface region above the activation threshold for ester-ester interaction
– Pre-dried PC substrate (120°C, four to six hours) to eliminate moisture-induced surface defects
– COPE compounds without internal release agents, which migrate to the bond interface and reduce adhesion
What Works Conditionally: SEBS on Polycarbonate
SEBS-based TPEs can bond to polycarbonate, but the adhesion is less consistent than on ABS substrates. SEBS’s styrenic end-blocks have less chemical affinity for PC’s ester-dominated surface than they do for ABS’s styrene phase. Adhesion varies significantly by SEBS compound formulation, PC grade, and processing conditions.
Where SEBS on PC has been made to work reliably, the common factors are:
– Adhesion-promoting tie-layer compounds between the PC substrate and SEBS overmold
– Silane-based coupling agents applied to the PC surface before overmolding
– High mold temperatures (80–90°C) and extended dwell time to maximize contact at the interface
For new product programs where SEBS is preferred for cost reasons, plan for adhesion promoter incorporation and validate bond strength under thermal cycling before finalizing the process.
What Doesn’t Work Well: SEBS Without Treatment
Standard SEBS compounds without adhesion promoters produce inconsistent results on PC in production environments. The bond may appear adequate initially but fails under peel testing at lower loads than cohesive failure would require, and delamination in service is common — particularly at elevated temperatures or after thermal cycling.
The variability is the real problem. Parts that pass initial inspection may fail in the field because the bond strength scatter is wide enough that low-end samples fall below structural requirements.
What Doesn’t Work: TPV on PC
Thermoplastic vulcanizates bond poorly to polycarbonate without significant surface preparation. The crosslinked rubber phase in TPV limits molecular mobility at the interface, preventing the interdiffusion and chemical interaction that drives adhesion. TPV-on-PC applications require either plasma surface treatment of the PC substrate, silane-based coupling agents, or a COPE tie-layer to achieve acceptable bond strength.
If an application genuinely requires TPV’s compression set performance on a PC substrate, these additional process steps are manageable. If the application does not specifically need TPV’s properties, COPE or a TPU is the more efficient choice.
What Doesn’t Work: SBS on PC
SBS-based TPEs bond to PC through styrenic end-block interaction, but the bond strength on PC is lower than on ABS due to reduced chemical affinity, and the material’s UV and thermal stability limitations make it unsuitable for most PC applications. PC parts appear in applications where service life and environmental resistance matter — consumer electronics, automotive components, medical devices. None of these environments are appropriate for SBS.
Chemical Stress Cracking: A Risk for All TPE Types on PC
Polycarbonate is susceptible to chemical stress cracking (CSC) — crazing or fracture initiated at the substrate surface by chemical exposure under mechanical stress. TPE compounds, like TPU, can trigger CSC on PC if their additive packages contain plasticizers, solvents, or aromatic compounds that interact with the polymer chains under load.
Before specifying any TPE compound on PC, obtain the full additive formulation from the supplier and evaluate CSC risk. Test bonded assemblies under sustained mechanical load — static stress combined with chemical exposure is the most aggressive condition for CSC initiation. Any crazing or whitening at the bond line indicates CSC and disqualifies the material combination.
For guidance on evaluating CSC risk for your specific TPE and PC grade combination, Email Us.
Design and Process Requirements for TPE on PC
When COPE on PC is the selected approach, process execution determines whether the material compatibility translates to production-grade bonds:
Substrate preparation. PC parts used as inserts must be stress-relieved before overmolding to minimize the CSC risk from the overmold compound. Residual molding stress in the insert combined with chemical exposure from the TPE is the most common trigger for delayed CSC in production.
Surface cleanliness. Mold release agents on the PC surface reduce surface energy below the threshold for reliable COPE adhesion. Clean PC insert surfaces with isopropyl alcohol before loading into the overmold tool; avoid solvents that stress-craze PC.
Gate location. Position the COPE gate to flow across the bond surface rather than along it. Weld lines in the elastomer that coincide with the bond zone reduce local bond strength and create initiation points for peel failure.
Thermal cycling validation. COPE and PC have different coefficients of thermal expansion. Validate bond integrity across the full service temperature range, not just at room temperature. Interface stresses from thermal cycling are cumulative and can trigger delayed delamination that initial testing does not predict.
Incure’s adhesive and coating systems support demanding multi-material assemblies, including applications where TPE adhesion to polycarbonate requires surface preparation solutions or adhesion-promotion formulations. For application-specific technical guidance, Contact Our Team.
Visit www.incurelab.com for more information.