How to evaluate lamination bond strength
How to evaluate lamination bond strength
Bond strength numbers are only comparable when structure, curing window and failure mode travel with them. Age the laminate to the recommended window, peel at a fixed speed and temperature, and read where the peel travels — the layer that fails tells you more than the force.
This guide covers solvent-free lamination of flexible packaging: what the peel test measures, why curing time changes the result, how to classify failure modes, and which test conditions keep your numbers comparable between trials and against your customer’s test.
What is the peel test actually measuring?
A peel test measures the force required to separate two webs joined by an adhesive, but the number is a system property, not an adhesive property. The same adhesive can peel differently on a well-treated outer film than on a marginal one, and differently again when a printed ink layer sits between film and adhesive. Quote the structure whenever you quote the value: outer film, ink, adhesive, sealant web.
The layer stack also explains why the test must be run on production film rather than on a convenient lab film. A structure that passes on an easy-to-bond film can fail on the actual outer web, and the failure will be diagnosed as an adhesive problem when it is really a film or ink problem.
Reading the structure behind the peel value
In flexible packaging, bond strength is tested through the printed outer film and the sealant web. A weak ink layer or a marginal film surface fails long before the adhesive itself does — which is why the peel face must be inspected, not just the force recorded.
Why does curing time change the result?
Solvent-free polyurethane adhesives build molecular weight after lamination: bond strength climbs from a low green state toward a plateau as curing proceeds. Testing too early reads a failure that later disappears; testing only after long storage reads aged performance that may not match the window your customer actually receives.
A typical evaluation ages samples at controlled temperature and measures at set intervals — commonly 24 hours, 72 hours and 7 days, with temperature and humidity recorded — until the curve flattens. Confirm the recommended window on the adhesive technical data sheet, and map the curve for a new adhesive–film combination before you set an internal release test.
Which failure modes matter, and what do they mean?
Read the peeled faces, not just the force. Adhesive failure at one interface means the bond broke where adhesion was weakest — often a film-treatment or ink problem. Cohesive failure inside the adhesive layer indicates the adhesive itself is the limiting link. Delamination inside the film, or ink lifting from the printed film, means the failure belongs to the converting step rather than the adhesive.
A common mistake is to treat every low peel as an adhesive fault. If the peel travels through the ink layer or a coated layer, changing adhesive supplier will not help — the fix is in ink adhesion or film treatment, which is why the film adhesion screening order is a useful companion to this guide.
Making peel results comparable
Fix these elements before comparing any two trials — each one changes the number independently of the adhesive.
Typical method elements for solvent-free lamination bond evaluation.
| Test element | Typical method | What it indicates |
|---|---|---|
| Sample preparation | Laminate on production film; condition at controlled temperature and humidity | Comparable starting state between trials |
| Curing window | Age at the recommended temperature; test at 24 h, 72 h and 7 days | Bond build-up curve and release-test timing |
| Peel geometry | T-peel or 180° peel at a fixed speed, for example 100–300 mm/min | Relative bond strength under a controlled rate |
| Failure mode | Inspect both peeled faces and classify the failing layer | Root cause: interface, cohesive, ink or film failure |
| End-use correlation | Compare lab peel with the converting or customer test | Confirms the lab method predicts the line |
Speed, conditioning and pass levels are system-specific — fix your method, record it, and compare like with like. Values from different labs are not interchangeable without identical conditions.
When is a low bond a real problem?
A laminate is a problem when it fails the end-use test — seal integrity, leak resistance or a customer-specified peel — not when a lab number looks low in isolation. Some structures intentionally use lower-peel sealant webs, and others pass at low peel because the end-use stress is small. Define the pass condition from the application before chasing a bigger number.
For the process side of the same line — foam in water-based gravure inks and the water-based versus solvent-based ink decision — the related guides cover the press conditions that feed this lamination step. Confirm your method and targets with an application engineer so the numbers you collect stay comparable across trials.
Get a bond-evaluation method matched to your structure.
Send the film structure, adhesive grade and end-use test, and we will propose the curing and peel protocol plus the failure-mode checklist for your line. Reply within 2 business days.