Blister film: Sealants, substrates and lidding – aluminum foil, blister packs, laminates

Whether a blister cover film is suitable is not determined by its designation as aluminum, plastic, or paper cover. What matters are the specific sealing layer, the surface of the form film that actually needs to be sealed, and the available process window for temperature, pressure, and contact time. Only this pairing of seals indicates whether a functional connection can be achieved under the specified conditions.

Equally important: sealability, tightness and opening are three different functional objectives. High seam integrity does not prove a leak-free package, a defined peel behavior or an appropriate tear function. Lidding films should therefore be specified based on their complete material structure and separate testing criteria.

Material structure: the support, barrier and sealing layer are distinct

Blister film sheets usually fulfill several functions at the same time. The backing provides stability to the lid and, depending on the design, allows for pressure or processing. Barrier layers limit the passage of certain substances. The sealing layer, on the other hand, establishes direct contact with the form film. It is crucial in determining whether the sealing seam forms and how it opens.

These functions do not have to be located in a single location. A documented structure of an aluminum cover film includes, for example, an external pressure primer, aluminum underneath, and an internal polymeric hot-seal layer. This inner layer can be implemented as a coating or as a film layer. Aluminum thus describes a supporting or barrier-relevant component, but does not automatically describe the seal-active surface. ^[1]

Plastic constructions are not necessarily homogeneous either. In a documented system, the cover and form sheets are coextruded from PP-based material. The term “monomaterial” therefore does not mean that only one layer is present. What remains decisive is which layer is located at the sealing point and which function the other layers perform. This example does not describe a general standard for all plastic covers. ^[2]

With paper-based lidding films, it would also be wrong to assume that they are made from uncoated paper. A patented design approach combines base paper, a pre-barrier layer and a polymeric dispersion coating; an additional sealing layer can be added. The coatings are intended to reduce, among other things, the porosity and water vapor permeability of the paper. This is an illustrated example of a structured design, not a complete typology of paper-based lids. ^[3]

Sealing varnish is not every polymer sealing layer

A sealing varnish is an applied coating that enables the connection to the backing material. Under heat, pressure, and contact time, the seal-active layer comes into contact with the mold film; upon cooling, the bond solidifies. The substrate, the varnish chemistry, and the process conditions all work together to influence adhesion and opening behavior. An extruded sealing layer can perform the same basic function, but is not automatically a seal-based varnish by design. ^[4]

For the preliminary selection, a clear consequence follows: carrier material and barrier construction describe important properties, but do not replace the indication of the seal-active surface. Anyone who only requests „aluminum cover foil“ or „plastic lid“ leaves the crucial part for the connection open.

Compatibility with form films: The specific seal pairing is specified

Compatibility with film forms occurs at the interface between the cover film and the forming web. Therefore, it is not sufficient to simply refer to the film as a plastic material in general. What is relevant is the surface on which the seal layer contacts: for example PVC, PVDC-coated PVC, PCTFE/PVC or PP. However, these designations do not imply universal approval for a specific paint or film. The complete pairing and process must be considered together. ^[5]

A technically usable request or specification must include at least the following information:

• Form foil: complete construction, thickness, and the actual sealing surface; for coated or multi-layer tracks, the basic material designation is not sufficient.
• Cover film: Carrier material, layer structure, thickness, and type of seal-active layer.
• Target opening date: Peelable, permanently sealed, or intended for piercing. This objective determines the type of connection that must be evaluated.
• Process window: The available sealing temperature, The pressure, the contact time of the intended system, and the material itself can all react in the laboratory and still be outside the usable window of the production system.
• Package geometry: Cavity shape, seal edge and design, because they can affect the load in the seam and the subsequent opening.

If the actual seal surface is missing, a paint assignment remains speculative. If the opening target is missing, it is not possible to assess whether high adhesion is desired or actually problematic. And without process data, it is unclear whether the material pairing can be processed reproducibly within the intended timeframe.

Test criteria: Sealability, tightness and opening evaluated separately

Tests are not a formal attachment to material selection. They answer different questions and must not be exchanged against each other. A laboratory seal curve helps to compare material pairings under defined conditions. However, it is not a production certificate. ASTM F2029 describes the manufacture and evaluation of laboratory seals; the derived conditions are starting points for further development, not the proof of validated machine parameters or consistently leak-free production packages. Overall build-up and thickness additionally influence sealability. ^[6]

The tear strength describes the mechanical breaking force and, if applicable, the type of failure of a seal-tight strip. ASTM F88/F88M is therefore relevant for peelable protective films. For comparable force values, the test geometry and the holder must remain constant within a comparison series. However, such a strip test does not simulate how a complete blister opens in the hand. ^[8]

Tightness is a separate issue. In the vacuum drop test according to ASTM F2338, gas or vapor escaping into an evacuated test chamber can cause an increase in pressure. The method allows for nondestructive leak testing. However, the maximum allowable limits must be determined for the specific product packaging system with dense reference packaging and for non-tight or simulated leak patterns. The detection sensitivity is not a general rule for every blister. ^[7]

For puncture blisters, a peel test is all the more unnecessary. Here, the puncture force must be assessed against the actual blister, because the cavity, product position, lid construction, and opening path all interact. Conversely, a successful puncture test does not answer the question of whether the seam is airtight under the intended conditions.

Which gap leads to which consequence?

• No reliable laboratory comparison basis: The selection starts without any clear indication of which pairing of seals actually reacts in the examined window.
• No leak test: A mechanically fixed seam may have local imperfections; the functional reliability of the closed package remains unknown.
• No opening test: The package may be airtight, but it fails to provide the intended peeling or tearing action.
• No proof of production: Laboratory data cannot easily be transferred to real assets, tools, processes, and fluctuations.

Hypothetical decision case: Fixed seam, but unsuitable packaging

Suppose a defined, controlled opening is provided for a peelable blister. Two lidding films are first compared with laboratory seals on the same specifically described form film. A variant shows high seam integrity in a strip test. The result appears convincing at first glance.

However, this variant fails the subsequent leak test. Therefore, it is not a suitable candidate for the intended use: High seam integrity does not compensate for the lack of proof of sealing. The selection must be further examined regarding the material pairing, the sealing edge, and the process conditions; a single strength value alone does not resolve this cause.

If a revised pairing subsequently displays a dense pack, the evaluation is not yet complete. Now, the previously determined opening target is also taken into account. For Peel, the opening must behave appropriately within the complete blister; in the case of penetration, the actual penetration function would be decisive. The example makes the goal conflict visible: A cover foil is not suitable because a single value looks good, but because it fulfills all the relevant functions of the specific package.

If the tear strength in the tear test after the leak has been repaired exceeds the previously set range for the peel target, the variant remains unsuitable despite the dense packaging. The consequence is not a one-size-fits-all correction based on a single value, but a re-evaluation of the sealing pair and the intended process conditions. At the same time, the tear test must not be equated with the complete opening by the user: The actual opening of the blister must also meet the defined target.

Conclusion: Select functions

A justified selection of blister packaging films begins with the material structure, continues with the specific sealing pairs and ends only with separate proofs for process, tightness and opening. Lidding films are not interchangeable covers, but functional layer systems.

The next logical step is a complete material and requirements profile: the actual seal surface of the form film, the structure of the cover film, the process window, the desired opening mechanism, and the corresponding testing criteria. As long as one of these functional objectives remains open, the suitability of the material combination remains open as well.