A heat seal is the last thing standing between a product and the outside world. It is also the thing nobody inspects until it fails. Two application families ask more of it than any other, and they do not ask for the same thing.
What a lid foil is actually being asked to do
A sealed lid has to satisfy two requirements that pull in opposite directions. It must hold absolutely: a hermetic bond that survives filling, palletising, a distribution chain and a consumer’s handbag. And it must then open easily, in one motion, without tearing, without stringing, and without leaving lacquer on the flange.
That tension is the whole engineering problem. Push the seal strength up for security and you make a lid a customer cannot open. Tune it down for comfort and you get leakers. The coating is what holds those two demands in balance, and the property that describes it is peel force, quoted per unit width.
Two behaviours sit at opposite ends of that scale. An easy-peel seal is designed to separate cleanly at the interface at a low, controlled force. A lock seal is designed not to peel at all; it holds until the material itself gives. Both are legitimate, and specifying the wrong one is one of the more expensive mistakes in lidding.
What a blister foil is being asked to do instead
Pharmaceutical blisters change the question. Barrier performance stops being a quality attribute and becomes a stability requirement: moisture and oxygen ingress are what set the product’s shelf life, and the seal is part of that barrier.
Three things follow that have no equivalent in food lidding:
- Controlled push-through. The foil must protect the dose through distribution, then yield predictably when a patient pushes. Too strong and the tablet crumbles or the patient cannot get it out; too weak and it releases in a bag.
- Tamper evidence. A disturbed blister has to show it. This is a design requirement of the seal, not a label.
- Validation. The seal is not merely tested, it is validated and documented under GMP, and its integrity has to be demonstrated over the product’s stated shelf life rather than at production.
Adhesion targets differ too. Blister lacquers are specified against PVC, PVDC and high-barrier laminates, where food lidding is specified against PP, PS, PET, PVC and aluminium containers.
Why one grade rarely serves both
The chart above is the short version. Typical seal initiation temperatures for food lid foils sit in a broad band, while blister foils sit high and narrow. The two overlap only at the top of the food range.
A coating formulated to seal low, so a food line can run fast and cool, will not necessarily develop the bond a blister needs at 200 °C. A coating built for blister performance, run on a food lidding line at 130 °C, may not have initiated at all. This is why “a heat seal lacquer” is not a product category you can buy from, and why the useful question is always which application, on which substrate, at which temperature.
The property that quietly sets your line speed
Of the nine properties in the panel above, one determines throughput more than the rest: the sealing window, meaning the range of jaw temperature that still produces an acceptable seal.
A narrow window means every ordinary variation on the line becomes a reject. Jaw temperature drifts, film gauge varies between reels, dwell changes when the machine speeds up or slows down. With a wide window those variations are absorbed. With a narrow one they become scrap, and the operator’s response is to slow the line down to keep conditions stable.
So a coating with a wide window can be worth more than a coating with a better peak seal strength, because the wide window is what you actually run in. Ask for the range, not the value at one temperature.
Hot tack: the failure that lab testing hides
Seal strength is measured after the seal has cooled. On the line, the pack is loaded while the seam is still molten, and the property holding it in that moment is hot tack.
The two are not interchangeable and the second cannot be inferred from the first. A coating can produce excellent cooled seal strength under ASTM F88 and still fail on the machine, because by the time a specimen has been conditioned and pulled, the hot tack failure mode has disappeared. Hot tack has its own method, ASTM F1921, and it needs to be asked for by name.
Resistance, and being specific about it
Durability claims mean nothing until the stress is named. Three matter here and they are different problems:
- Chemical resistance. Whether the seal survives contact with the product. Fats and oils are the usual aggressor in food; actives and excipients in pharma.
- Sterilisation resistance. Whether the seal survives the process, not just the product. Retort, pasteurisation and medical sterilisation each subject the seam to conditions well beyond the sealing step.
- Ageing stability. Whether the seal strength you measured at production is still there at the end of shelf life. This one is routinely assumed and rarely requested, and it is the one that surfaces as a complaint eighteen months later.
There is also cohesive strength, which is the coating’s resistance to splitting within itself. When a seal fails cohesively the lacquer tears through its own thickness and leaves residue on both faces. It looks like an adhesion failure and it is not, which is why a failed seal is worth examining rather than just re-specifying.
Odour and taste: the requirement that only shows up late
For water, dairy and beverage lidding there is a requirement that no mechanical test will catch. The coating has to be organoleptically neutral: it must not transfer any odour or taste to the contents.
This is not covered by migration compliance. A coating can be fully compliant with every applicable migration limit and still taint a mineral water, because the threshold at which a human detects an off-note is far below any regulatory figure. It is a sensory panel question, and for low-flavour products it should be specified explicitly rather than assumed.
The compliance side
Heat seal coatings for food contact are developed against the same frameworks as the rest of the pack.
- Regulation (EC) No 1935/2004 is the EU framework; Regulation (EU) No 10/2011 governs plastics with specific and overall migration limits, and Regulation (EC) No 2023/2006 covers good manufacturing practice.
- Regulation (EU) 2025/351 has applied since 16 March 2025 and tightens what the declaration of compliance must carry. The transition for material placed on the market under the previous rules closes on 16 September 2026.
- In the United States, coatings sit under 21 CFR 175.300, with 175.320 covering coatings applied to polyolefin films.
- For blister applications, GMP validation sits on top of all of that, and the declaration has to match the structure rather than the product family.
What to send us
A grade recommendation is only as good as the four things behind it: the container or substrate and its full structure, the line and its real running speed, the jaw temperature and dwell you can offer, and the opening behaviour you need, easy-peel or lock seal, with a target peel force if you have one.
With those, we will tell you which grade to trial and why. Without them, any recommendation is a guess in the shape of a specification.
Frequently asked questions
What is the difference between an easy-peel and a lock seal?
An easy-peel seal is designed to separate cleanly at the interface at a low, controlled peel force, so a consumer can open it in one motion without tearing or leaving residue. A lock seal is designed not to peel at all: it holds until the material itself gives. Both are legitimate specifications, and choosing the wrong one is an expensive mistake in lidding.
Why do food lid foils and pharmaceutical blister foils need different heat seal lacquers?
Because their sealing temperatures barely overlap and their requirements differ. Food lid foils typically initiate across a broad, lower band and must run on PP, PS, PET, PVC and aluminium. Blister foils initiate high and narrow, need strong adhesion to PVC, PVDC and high-barrier films, and add controlled push-through, tamper evidence and GMP validation.
What is a sealing window and why does it matter more than peak seal strength?
The sealing window is the range of jaw temperature that still produces an acceptable seal. A narrow window turns ordinary line variation, in jaw temperature, film gauge and dwell, into rejects, and operators respond by slowing the line. A wide window absorbs that variation, so it often delivers more real throughput than a coating with a better figure at one temperature.
Can a heat seal pass laboratory testing and still fail in production?
Yes. Seal strength under ASTM F88 is measured after the seam has cooled, while a filling line loads the pack when it is still molten. The property that holds at that moment is hot tack, measured separately under ASTM F1921. A cooled seal test cannot detect a hot tack failure because the failure mode is gone by the time the specimen is conditioned.
What does organoleptic neutrality mean for lid foils?
It means the coating transfers no odour or taste to the contents, which matters most for water, dairy and beverage lidding. It is not the same as migration compliance: a coating can meet every applicable migration limit and still taint a low-flavour product, because human detection thresholds sit far below regulatory ones. It should be specified explicitly, not assumed.