Quick answer. Corona treatment is usually the first choice for clean, continuous PE, PP, BOPP, PET, and other polymer films because it raises surface wettability without depositing another material. Use a compatible primer when corona cannot reach the required bond retention, when treatment decays before lamination, or when bonding rubber, plasticized PVC, coated metal, glass, composites, or molded low-surface-energy parts. Approve either process by aged peel or lap-shear results and failure mode—not by a dyne reading alone.
Substrate pretreatment for polyurethane adhesive bonding controls whether the liquid adhesive wets the surface, forms chemical interactions, and remains attached after heat, water, plasticizer migration, or repeated flexing. A PU formulation with acceptable bulk strength can still delaminate if release agent, oil, oxide, dust, or a weak polymer boundary layer remains under the bond.
What Pretreatment Must Accomplish Before PU Bonding
Polyurethane adhesives contain polar urethane groups and, in reactive grades, isocyanate-functional components. These systems generally wet clean polar surfaces more readily than untreated polyethylene or polypropylene, whose surface energy is commonly around 29–33 mN/m. Poor wetting produces islands, edge retraction, uncoated pinholes, and low initial peel.
Surface energy is only one control variable. ASTM D2578 measures the wetting tension of polyethylene and polypropylene films with test solutions, but ASTM explicitly states that wetting tension alone is not a complete measure of adhesive adhesion. The standard cites 35 dyn/cm as a level often associated with treated polyethylene for flexographic printing; it does not establish 35 dyn/cm as a universal PU bonding specification.
For PU lamination trials on polyolefin films, 38–42 mN/m is a practical screening window, subject to confirmation with the actual film, adhesive coat weight, cure schedule, and ageing cycle. PET, PA, TPU, coated foil, and printed webs require separate limits because surface chemistry, additives, and topcoats differ even when two rolls show the same dyne value.
Cleaning must precede activation. Corona treatment does not remove silicone release agent, compressor oil, wax, plasticizer, or hand contamination; it may oxidize the contamination and leave an activated contaminant layer. A clean but insufficiently activated surface tends to show uniform adhesive failure, while a contaminated surface often produces irregular bare patches, fisheyes, or large scatter between specimens.
Record the failure location after every test. Clean separation at the substrate–adhesive interface indicates an interfacial problem. Cohesive splitting inside the PU layer, fiber tear, foam tear, or film deformation usually shows that the interface has exceeded another part of the assembly, although the measured force still has to meet the buyer’s specification.
Corona vs Primer: Selection Table
| Decision dimension | Corona treatment | Chemical primer | Purchasing implication |
|---|---|---|---|
| Typical substrates | PE, PP, BOPP, PET, PA, TPU, and thin coated webs that can pass through a controlled electrode gap | PP or TPO parts, rubber, plasticized PVC, coated or oxidized metal, glass, composites, leather, and difficult multilayer surfaces | Name the exact resin, grade, coating, filler, and additive package; “plastic” or “metal” is insufficient |
| How the interface changes | Electrical discharge introduces polar groups and raises wettability without adding a separate layer | Deposits a coupling or tie layer selected for both the substrate and PU chemistry | A primer creates another controlled material in the bill of process and requires lot traceability |
| Process measurement | Generator power, line speed, treated width, electrode gap, electrode condition, side treated, and wetting tension | Primer identity, solids content, wet pickup, calculated dry coat weight, flash-off temperature and time, and application-to-bond interval | The purchase specification should contain measurable windows rather than the phrase “surface treated” |
| Geometry | Best suited to flat film, sheet, foil, and continuous web | Can reach recesses, edges, textile structures, molded parts, and selected three-dimensional surfaces | Complex shapes may require spray, wipe, roll, or plasma equipment plus masking controls |
| Time sensitivity | Treatment can decay through molecular reorientation, additive bloom, storage heat, or contact with the untreated roll face | Open interval may be limited by contamination, moisture uptake, under-drying, over-drying, or primer ageing | Specify maximum storage or application interval from trial data; do not accept an undefined shelf window |
| Typical defect | Untreated lanes, backside treatment, low dose at high speed, arcing, pinholes, or a weak over-treated layer | Wrong primer chemistry, excessive film thickness, trapped solvent or water, incomplete flash-off, and primer cohesive failure | Incoming inspection should include retained panels or roll samples from the production lot |
| Best approval test | Dyne or contact-angle screening followed by peel testing after cure and ageing | Dry coat verification followed by peel or lap-shear testing after cure and ageing | Release production only after both process data and mechanical results pass |
Corona and primer are not mutually exclusive. A converter may corona-treat a polyolefin web immediately before applying a thin primer, then laminate with PU adhesive. This sequence can stabilize primer coverage, but every additional interface creates another possible failure plane and must be included in the ageing program.
Mechanical abrasion is a separate operation, not a substitute label for either method. On steel, aluminum, rubber, or fiber-reinforced composites, controlled abrasion can remove weak oxide, glaze, or mold skin and increase area. Loose debris must then be extracted and the surface cleaned with an approved material that does not leave surfactant or lint.
How to Specify and Control Corona Treatment
Control energy input instead of copying a generator setting
Generator power alone cannot define treatment. A 2 kW setting applied to a 1 m web at 100 m/min gives a nominal treatment dose of 20 W·min/m²: 2,000 W divided by 100 m²/min. Doubling line speed halves that nominal dose unless power or the number of treatment stations changes.
Use dose as a process comparison, then qualify the result by surface measurement and bond testing. Electrode design, air gap, polymer thickness, ambient humidity, roll temperature, additives, and prior treatment can cause two lines at the same calculated dose to produce different surfaces.
An operating sheet should record power in kW, speed in m/min, treated width in m, calculated dose in W·min/m², electrode gap in mm, treatment side, roll ID, time of treatment, and time of adhesive application. Install a treatment-side marker or use a verified test pen procedure; bonding the reverse side is a common cause of apparently random delamination.
Measure promptly and recognize treatment decay
Use one documented method across supplier and customer plants. ASTM D2578 is limited to PE and PP film, while ISO 15989:2004 specifies water-contact-angle measurement for corona-treated polymer films and is applicable to most films unless the surface has a chemical affinity for water.
Dyne pens are fast production screens, but liquid age, evaporation, contamination, operator judgment, and dwell time affect the result. For an approval study, use controlled test solutions or a calibrated contact-angle instrument, take at least five readings across the web, and report the minimum and range rather than the average alone.
Measure after treatment and again immediately before adhesive application. A roll that reads 42 mN/m at the treater can fall below its validated limit after storage as low-molecular-weight additives migrate and polar groups reorient into the polymer. No universal “corona lasts six months” rule is defensible across resin grades and storage conditions.
Over-treatment is also a failure mode. A powdery oxidized layer may wet readily but detach under peel load, producing residue on both separated surfaces. Other warning signs include film blocking, heat distortion, pinholing, odor, and a plateau in wetting tension despite additional power.
Run the PU adhesive inside its real process window
Surface activation cannot compensate for missed adhesive conditions. For 2K PU, document component mass ratio, mixing accuracy, pot life, coat weight in g/m², open time, press pressure, press temperature, and full cure interval. For 1K moisture-curing PU, record substrate moisture, ambient relative humidity, bond-line thickness, and whether both adherends block moisture diffusion.
A useful line trial changes one variable at a time: untreated control, low corona dose, target dose, and higher dose. Apply the same adhesive batch and coat weight to all four groups. Testing only treated specimens can hide the true process gain and may approve a line whose result came from a different film lot.
How to Specify and Control Primer Application
A primer must bond to the substrate and remain compatible with the polyurethane adhesive. Chlorinated polyolefin-type tie layers are commonly evaluated for PP and TPO; organofunctional silanes are evaluated for glass and mineral surfaces; metal systems may combine cleaning, oxide control, coupling chemistry, and corrosion protection. Rubber and shoe components may require chemistry matched to the elastomer, vulcanization system, surface bloom, and intended PU cement.
Do not approve a primer solely because it makes liquid adhesive spread. A primer can give uniform wetting while failing cohesively after heat or hydrolysis. Plasticized PVC creates another risk: plasticizer can migrate through the primer and soften the bond line, so aged peel and heat exposure are mandatory.
Specify deposited dry mass
“One wipe” or “a thin coat” cannot be audited. Measure wet pickup by mass over a known area, then calculate dry coat weight from the primer solids fraction. A pickup of 4 g/m² from a primer containing 25% nonvolatile solids produces 1 g/m² theoretical dry deposit, before accounting for transfer loss.
A screening design may examine 0.5, 1.0, and 2.0 g/m² dry primer, but these are experimental points rather than universal limits. Too little primer leaves discontinuous coverage; too much can trap carrier, slow flash-off, crack during flexing, or move the failure plane into a mechanically weak primer layer.
Record ambient temperature and relative humidity, substrate temperature, flash-off time, oven temperature where used, and the interval before adhesive coating. Visible dryness does not prove that water or solvent has left a porous textile, leather, foam, or micro-roughened surface.
Match preparation to the substrate
Polyolefin parts should be checked for mold release, slip agent, talc, glass fiber, recycled content, and paintable skin layers. Two PP grades with the same melt-flow rate can bond differently because the surface additive package is different. Require the molder to notify the adhesive buyer before changing resin grade, masterbatch, release agent, or mold-cleaning chemical.
Metals need an agreed cleanliness and oxide condition. Abrasion with a controlled 80–120 grit range can serve as a starting comparison for many fabricated parts, followed by dust extraction and a two-cloth cleaning method. The final procedure must be qualified on the actual alloy, coating, galvanizing layer, and service environment because excessive abrasion can remove a functional conversion coating.
Shoe rubber and molded elastomers may carry wax bloom, silicone release, sulfur-containing residues, or a dense molded skin. Record buffing media, wheel speed, pressure, number of passes, dust removal, primer quantity, and time to cement application. Aggressive buffing that exposes loose filler can raise roughness while lowering peel retention.
Porous wood, foam, fabric, and leather can absorb primer or PU adhesive below the bonding plane. Control application in g/m² and inspect for strike-in. If the dry surface shows starved regions, adjust viscosity, solids, or application sequence with the adhesive manufacturer rather than increasing corona power.
Qualification, Purchasing, and Factory Documentation
A B2B buyer should purchase an approved bonding process, not a drum described only by viscosity and color. Send the adhesive manufacturer representative substrates from normal production, including aged film, printed or coated variants, recycled-content grades, and parts carrying the actual release system.
For flexible laminates, use peel testing such as ASTM D1876, which covers relative T-peel resistance between flexible adherends. For rigid assemblies, ISO 4587:2003 covers tensile lap-shear testing of rigid-to-rigid bonded assemblies; ISO cautions that this test does not provide design values.
A defensible qualification plan uses at least three independent adhesive or substrate lots and at least five specimens per condition. Report every result, mean, minimum, standard deviation, failure mode, specimen width, crosshead speed, bond-line thickness, cure time, and conditioning history. An average that passes while several specimens fail adhesively at one edge should trigger investigation.
Build ageing around the exported product’s service. A screening matrix could include 7 days at 23°C in water, 7 days at 70°C dry heat, five cycles between −20°C and 60°C, and testing at both 24 hours and 7 days after bonding. These are example development conditions; the buyer’s drawing, OEM standard, or regulatory requirement must define the final temperatures, durations, and pass values.
For every aged group, retain an unaged control made from the same materials. Compare retained strength as a percentage of that control and inspect failure-mode movement. A fall from substrate tear to clean interfacial release can expose primer hydrolysis or treatment decay even if the force remains temporarily above a low acceptance limit.
Factory-direct sourcing allows the formulation and pretreatment window to be developed together. For example, a producer can adjust PU viscosity, open time, initial tack, cure speed, and application solids against the customer’s corona-treated film or primer-coated component instead of asking a distributor to select from a warehouse list. See the PU lamination adhesive application page for a related foam-to-textile process.
Customization still requires change control. The approved sample, commercial shipment, TDS, SDS, certificate of analysis, label, component ratio, and packaging must refer to the same formulation revision. Ask the manufacturer to retain a batch sample and notify purchasing before changing raw-material source, catalyst level, solvent package, or production site.
Certification claims must identify the tested adhesive grade, complete assembly, laboratory, report number, specimen construction, and test edition. A certificate for an adhesive alone may not cover the buyer’s primer, film, coating, foam core, or cure schedule. Request REACH, RoHS, VOC, food-contact, flame, or automotive documents only where the application requires them, and verify their scope before placing them in a customer submission.
For EU industrial or professional use, check whether the supplied PU system falls within REACH Annex XVII Entry 74. The ECHA restriction text requires training before use after 24 August 2023 unless the combined diisocyanate concentration is below 0.1% by weight. SDS review, ventilation, skin protection, and task-specific exposure assessment remain necessary because isocyanates can cause respiratory and dermal sensitization.
Include the following fields in an RFQ so a PU adhesive manufacturer can reproduce the intended interface:
- Exact substrates, grades, thicknesses, coatings, recycled content, and treatment side
- Corona target in mN/m, measurement method, minimum value, and maximum treatment-to-bond interval
- Primer chemistry or approved reference, solids percentage, dry coat weight, flash-off window, and application method
- PU type—1K, 2K, solvent-borne, water-borne, or reactive hot melt—plus coat weight and cure conditions
- Required peel in N/25 mm or lap shear in MPa, specimen geometry, test speed, conditioning, and failure-mode requirement
- Production speed in m/min, operating temperature in °C, monthly consumption in kg or tonnes, package size, target MOQ, and required lead time in days
- Required TDS, SDS, COA, retained sample, change notification, and application-specific test reports
FAQ
Q: Is corona treatment enough for every polyurethane adhesive bond?
No. Corona is most effective on clean continuous polymer films. Primer, abrasion, atmospheric plasma, flame treatment, or another validated process may be needed for molded PP, rubber, plasticized PVC, glass, coated metal, composite parts, and surfaces carrying release contamination.
Q: What dyne level should PE or PP have before PU adhesive application?
Use 38–42 mN/m as an initial trial range for many film-lamination projects, then establish the production minimum from aged peel data. Polymer grade, slip additives, film age, adhesive chemistry, coat weight, and service exposure can move the acceptable limit above or below that range.
Q: Can a higher dyne reading guarantee higher peel strength?
No. Wetting tension indicates how a test liquid behaves on the surface; it does not measure cohesive strength, contamination, primer hydrolysis, plasticizer migration, or cured adhesive performance. ASTM D2578 specifically warns that wetting tension is not a complete adhesion measure.
Q: How soon should PU adhesive be applied after corona treatment?
Apply it inside a validated maximum interval, preferably inline where the process allows. Measure the surface immediately after treatment and again before coating because storage temperature, additive bloom, roll contact, and molecular reorientation can reduce treatment without a visible change.
Q: Should corona treatment be applied before primer?
For low-surface-energy polyolefin films, corona before primer can improve primer coverage and anchorage. Confirm the sequence with untreated, corona-only, primer-only, and corona-plus-primer controls because an incompatible or excessively thick primer can reduce aged peel even when initial wetting improves.
Q: What information should be supplied for a custom PU adhesive recommendation?
Provide actual substrate samples, treatment method, dyne or contact-angle data, primer dry coat, application weight, line speed, cure conditions, service temperature, water or chemical exposure, required peel or shear value, and failure-mode criterion. These inputs allow a factory formulation trial to target the production line rather than a generic resin description.