Engineering Tools

Adhesion Failure Troubleshooter for Plastics

Answer five questions about how your bond, print or coating failed. The troubleshooter ranks the most likely causes and tells you what to check first.

Diagnose the failure

Answer what you know and skip what you do not. The ranking starts from the order in which these causes turn up in practice - insufficient or decayed treatment first by a wide margin, then mold release and silicone, then additive migration - and adjusts it to your answers.

1. Where did the failure happen? Look at both faces of the failed joint or the lifted print - a 10-40x stereo microscope settles it in minutes.
2. What is the plastic?
3. When did it fail?
4. Was the surface pretreated?
5. What did you notice? (tick all that apply)

Common causes of adhesion failure on plastics

Each cause lists the signs that point to it, how to confirm it and how to fix it. Change one variable at a time, and the strongest proof of a cause is being able to switch the defect off and back on.

Surface energy too low (untreated or under-treated)

The adhesive, ink or coating never wetted the surface, so it has almost nothing to hold on to. The most common single cause on polyolefins, acetal, fluoropolymers and TPEs.

Typical signs

  • Clean peel - the plastic looks untouched
  • Liquids bead, crawl or fish-eye
  • A low-energy plastic: PP, PE, TPO, acetal, PTFE, silicone

How to confirm

How to fix it

  • Pretreat: flame, corona, atmospheric or cold gas plasma, or Pyrosil® for fluoropolymers and silicone
  • Typical targets are 38-44 dyn/cm for inks and adhesives; structural bonds need more (nylon 50-54)
  • Verify the treatment level every shift

Read more: Surface pretreatment · Surface energy explained

Treatment decayed before processing

The part was treated, but the added surface energy faded before printing or bonding. Treated surfaces lose energy as polymer chains reorient and additives surface - on polyolefins mostly in the first 24 hours, and faster in warm storage.

Typical signs

  • Parts treated in batches and stored
  • Good results on fresh parts, poor on older stock
  • A line that works in winter and fails in summer

How to confirm

  • Treat a fresh part and coat or bond it immediately - if that works, the problem lies between treatment and process
  • Measure a decay curve: 0, 1, 4 and 24 hours; 3, 7, 14 and 28 days

How to fix it

  • Write a maximum treat-to-bond interval into the process specification
  • Treat inline, just before the process; keep treated parts cool
  • A primer applied right after treatment locks in the activation; Pyrosil® lasts weeks to months

Read more: How long plasma treatment lasts

Mold release, silicone or other contamination

Something on the surface blocks contact: silicone-bearing mold release, hand oils, airborne overspray, compressor oil or cleaning-solvent residue. Pretreatment activates whatever is on the surface, so a contaminated part can still pass a dyne test.

Typical signs

  • Good dyne reading, poor adhesion
  • Patchy failures (failed treatment tends to affect whole parts or runs)
  • Fish-eyes, craters or crawling

How to confirm

  • Split-lot trial with parts molded without external release
  • FTIR-ATR or XPS of the failed surface and of wipe samples
  • Blow the plant compressed air onto a clean white cloth

How to fix it

  • Never use silicone sprays in a molding facility - they travel to every machine
  • Prefer water-soluble mold releases; avoid wax and internal releases on parts to be painted
  • Clean with a solvent that dissolves the contaminant and does not attack the part - alcohol removes superficial dirt, not hydrocarbons
  • Powder-free gloves; no silicone-bearing gloves or packaging

Read more: Coating defects on molded parts · Adhesive bond failures

Additive migration (plasticizers, slip agents, blooming)

Additives in the plastic - plasticizers, slip and antistatic agents, flame retardants, internal mold releases - migrate to the surface after molding and weaken the interface. Disproportionately common in delayed failures.

Typical signs

  • Passed initially, failed days, weeks or months later
  • Waxy or greasy film on aged parts
  • Flexible PVC, TPE, or film and closure grades with slip additives

How to confirm

  • Compare adhesion on fresh and aged parts
  • FTIR or XPS of the failed interface
  • Trial a different lot or grade as a control

How to fix it

  • Change the grade or additive package; avoid internal mold releases
  • Treat and bond or coat soon after molding
  • Use a migration-resistant adhesive or a barrier primer

Read more: Why paint and ink delaminate · Preparing plastics for painting

Ink, coating or adhesive not suited to the plastic

On a clean, properly treated surface, some chemistries still do not bond some resins. The product was qualified on another substrate or for a different process window.

Typical signs

  • Clean peel despite good dyne levels
  • Works on one plastic or color, fails on another

How to confirm

  • Side-by-side trial with a product qualified for this exact grade

How to fix it

  • Select a chemistry qualified for the resin, or add a primer or adhesion promoter

Read more: Adhesive bonding & joining · Engineering services

Incomplete cure or poor application

The adhesive or coating never developed full strength: wrong mix ratio, too little time or heat, low UV dose, or too much of it. Applying extra adhesive is a leading cause of bonding failures.

Typical signs

  • Adhesive or coating split in two (cohesive failure)
  • Soft or tacky material
  • Thick or uneven bond lines

How to confirm

  • Check mix ratio, cure time and temperature, and UV dose
  • Hardness or solvent-rub test of the cured film

How to fix it

  • Keep bond lines thin and uniform - typically 3-5 mil for maximum shear strength
  • Two-part epoxies need about 7 days at 75°F for full cure unless heat-accelerated
  • Pre-measured cartridges remove mixing errors

Read more: Adhesive bonding & joining

Molded-in stress and solvent attack

Solvents or monomers in the adhesive, ink, coating or cleaner attack a stressed amorphous plastic - polycarbonate, acrylic, polystyrene, ABS - and crack it at the bond line, sometimes hours later.

Typical signs

  • Crazing or cracks near the bond or print
  • Amorphous plastics: PC, PMMA, PS, ABS
  • Failures concentrated in high-stress areas

How to confirm

  • Solvent-stress immersion test (glacial acetic acid for ABS)
  • Anneal a set of parts below the heat deflection temperature and compare

How to fix it

  • Choose a chemistry compatible with the resin
  • Reduce molded-in stress through molding conditions or annealing

Read more: Environmental stress cracking

Joint design and thermal stress

Cohesive failures in the field are typically caused by poor joint design: too little overlap, peel or cleavage loads, or plastics bonded to metal or glass that expand at very different rates as temperature changes.

Typical signs

  • Failure after thermal cycling
  • Starts at edges or corners
  • Plastic bonded to metal or glass

How to confirm

  • Thermal cycling test of the assembly
  • Compare expansion of both materials with the adhesive's elongation

How to fix it

  • Design the joint for shear, not peel; increase overlap
  • Use a tougher, more flexible adhesive and control bond-line thickness

Read more: Adhesive bonding & joining

Weak boundary layer in the plastic skin

The bond is stronger than the outer skin of the part. Material degraded during molding, a bloomed additive layer, a resin-poor or glazed skin, or haze left by excess solvent tears away with the adhesive.

Typical signs

  • A skin of plastic on the adhesive or coating side
  • Filled grades or heavy regrind
  • Good dyne readings, poor peel strength

How to confirm

  • FTIR of the failed adhesive face
  • Trial a virgin-resin lot as a control
  • Review melt temperature and residence time

How to fix it

  • Correct molding conditions; limit regrind
  • De-glaze a hard molded skin by tumbling or blasting
  • Use only as much cleaning solvent as needed

Read more: Why paint and ink delaminate

Over-treatment

Past the optimum, treatment fragments the polymer surface into a weakly bound layer that fails under load. More treatment is not better.

Typical signs

  • High dyne level, weak or powdery surface
  • Pinholes; failure just inside the plastic
  • Heat sealing or welding affected downstream

How to confirm

  • Treatment ladder: test adhesion at several treatment levels

How to fix it

  • Reduce corona watt density or flame and plasma dose
  • On cold gas plasma, reduce time before reducing power
  • Specify an upper as well as a lower treatment limit

Read more: Surface pretreatment

Moisture in the plastic

Hygroscopic resins - nylon, polycarbonate, ABS - absorb water. Heated during cure, they release it into the bond line or coating as bubbles and blisters. Nylon can absorb more than 3% of its weight.

Typical signs

  • Bubbles, voids or blisters after a heated cure
  • Nylon, PC or ABS parts
  • Worse in humid seasons

How to confirm

  • Dry a sample of parts and compare

How to fix it

  • Bond or coat soon after molding
  • Store parts sealed with desiccant; dry before processing

Read more: Bonding nylon

Environmental attack (humidity, chemicals, UV)

The joint was fine until it met heat and humidity, cleaners, fuels, sweat or sunlight. A bond that passes on the day it is made can fail months later through hydrolytic attack at the interface.

Typical signs

  • Passed dry testing, failed after exposure
  • Softened, swollen or discolored adhesive or coating

How to confirm

  • Accelerated aging - for example 5 days at 158°F dry and 5 days at 90°F / 90% RH against an ambient control, then peel or tape testing

How to fix it

  • Select an adhesive or coating rated for the exposure
  • Pair every immediate adhesion test with an aged test in the specification

Read more: Accelerated aging · ASTM D3359 tape test

The bond held - the part broke

When the plastic itself fractures and the bond stays intact, the bond is stronger than the material: substrate failure is the optimal result of a bonding process. If the part broke at too low a load, the problem is the part, not the adhesion.

Typical signs

  • The plastic fractured away from the bond line
  • Adhesive intact on both parts

How to confirm

  • Compare the failure load with the design requirement

How to fix it

  • Strengthen the part: geometry, grade or molding conditions

Read more: Adhesive bond failures

Frequently asked questions

What is the difference between adhesive, cohesive and substrate failure?

In adhesive (interfacial) failure the adhesive separates cleanly and remains on only one surface - usually a surface energy, contamination or migration problem. In cohesive failure the adhesive itself fractures and remains on both surfaces - look at cure, application and joint design. In substrate failure the plastic breaks rather than the bond, which is the optimal bond strength. Mixed-mode failure shows characteristics of both adhesive and cohesive failure.

What is the most common cause of paint or adhesive failure on plastics?

Insufficient or decayed surface treatment, by a wide margin. Mold release and silicone contamination come second, and additive migration third - the last is disproportionately common in failures that appear weeks after production.

Why does adhesive not stick to polypropylene?

Polypropylene has a low surface energy - about 29-31 dyn/cm - and no polar groups, so most adhesives cannot wet it. Flame, corona or plasma treatment raises the surface energy to the 38-44 dyn/cm range and adds polar groups the adhesive can bond to.

The dyne level is good but the bond still fails. Why?

Almost always contamination. Pretreatment activates whatever is on the surface, and a dyne solution will read an activated contaminant quite happily. Raising the treatment level does not help; trace the contaminant - silicone mold release, oils, overspray, compressed air - to its source.

Does wiping with isopropyl alcohol improve adhesion?

Only a little. Alcohol is a weak solvent: it removes superficial dirt but not hydrocarbon contaminants, and it does not raise the surface energy of a low-energy plastic. Use a solvent that dissolves the actual contaminant without attacking the part, and pretreat low-energy plastics.

Go deeper

Need a root cause, not a shortlist?

Sabreen investigates adhesion failures on plastic parts - surface analysis, process audit and a corrective plan. Tell us what failed and how.

Talk to Scott Sabreen

More free tools: Surface Energy & Dyne Level Calculator · Laser Marking Plastics Chart · All engineering tools

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